doc. Ing. Matěj Komanec, Ph.D.

We propose an approach to interconnect a hollow-core fiber (HCF) of arbitrary core size with standard single-mode fiber with perfect mode-field size adaptation and experimentally achieve for the first time insertion loss agreeing with that predicted by simulations. We demonstrate this using three low-loss HCFs, including 1st window nested antiresonant nodeless fiber (NANF), 2nd window NANF and the state-of-the-art double NANF (DNANF). The connection with a minimum achieved insertion loss of 0.079 dB was permanently secured via gluing and did not degrade during 4 weeks of continuous measurement. To the best of our knowledge, this is the lowest reported value and is comparable to or lower than the connection between dissimilar single-mode fibers (e.g., standard single-mode fiber and dispersion-compensating fiber). We also show that such connection leads to excellent suppression of higher-order modes coupling, of importance to all applications sensitive to multi-path interference. Importantly, obtaining agreement between simulations and experiments validates for the first time the accuracy of the simulations and opens the door to further optimization via simulations with the ability to subsequently achieve the same result experimentally.

We propose a hollow-core fiber (HCF) end-cap that incorporates a short segment of a single-mode fiber (SMF) that serves as a modal filter. To adapt the end-cap input and output beam to the desired size, the SMF was fusion spliced with short segments of mode-field adapting graded index (GRIN) fibers on both sides. The end-cap is anti-reflective coated to minimize insertion loss and parasitic reflections. The presented proof-of-concept experiments show its ability to suppress coupling into HCFs' higher-order modes. For example, without any end-cap, the extinction ratio between the LP 11 and the fundamental mode was found to be as low as 9 dB when coupling light from a free-space beam that was misaligned by as little as 1.1 ∘ . This was improved to 23 dB when inserting the developed end-cap. Such small angle misalignment often exists when aligning the input beam with 3-axis (x,y,z) stages only (rather than 5-axis that also include pitch and yaw). Finally, we glued the end-cap with the HCF, providing hermetic sealing to the HCF input/output. This is of interest for stable operation in applications that use free-space light launch or require HCF output into free space.

The attenuation of hollow-core fibers (HCFs) is predicted to surpass the minimum intrinsic attenuation of standard single-mode fibers (SMFs) in the near future. Recent advances in HCF performance and drawing technology have motivated their application not only in telecommunications but also in sensing and high-power delivery. Among HCFs, nested antiresonant nodeless fibers (NANFs) have shown the lowest attenuation values with 0.28 dB/km at 1550 nm and 0.22 dB/km at 1625 nm. Furthermore, the latest generation of NANFs effectively mitigates higher-order modes, which in some applications introduces a significantly limiting factor. As HCFs are becoming more available, their incorporation into standard SMF-based systems needs to be efficiently addressed. Various solutions to the HCF-SMF interconnection have already been proposed, such as the commonly employed fusion splicing with bridge fibers, using tapers to match the mode-fields, employing micro-optics, or using the fiber-array approach. Based on the fiber-array approach we have recently demonstrated losses of only 0.16 dB per interconnection and back reflection below -60 dB. But what if the interconnection itself can provide some additional functionality beyond low loss and low back reflection? Such an approach was already proposed in the micro-optics interconnection providing a function as an optical isolator or a wavelength-division multiplexer. Still, the relatively high complexity of such a device might limit its wider application. In this talk, I will overview current trends in HCF-SMF interconnection techniques which are enabling their incorporation into current SMF-based fiber-optic systems. I will present a future outlook of providing additional functionality to the HCF-SMF interconnection. I will focus on an interconnection technique we developed, based on the fiber-array approach. I will show how components such as an optical filter, a gas cell, or a Fabry-Perot cavity can be easily formed by simple tailoring of the HCF-SMF interconnection.

We demonstrate design of fiber F abry-Perot c avities b ased o n large-core graded-index multimode fibers. Smallest full width at half maximum along with maximum transmission is reached for core diameters over 200mm core and reflectivity above 99%.

By modifying the design of the interconnection between standard and hollowcore fibers, we achieved low insertion loss while creating a gap in between. This will allow for insertion of thin optical elements such as filters.

We report simultaneous low coupling loss (below 0.2 dB at 1550 nm) and low back-reflection (below −60 dB in the 1200-1600 nm range) between a hollow core fiber and standard single mode optical fiber obtained through the combination of an angled interface and an anti-reflective coating. We perform experimental optimization of the interface angle to achieve the best combination of performance in terms of the coupling loss and back-reflection suppression. Furthermore, we examine parasitic cross-coupling to the higher-order modes and show that it does not degrade compared to the case of a flat interface, keeping it below −30 dB and below −20 dB for LP11 and LP02 modes, respectively.

Today’s lowest-loss hollow core fibers are based on antiresonance guidance. They have been shown both theoretically and experimentally to have very low levels of backscattering arising from the fiber structure – 45 dB below that of traditional optical fibers with a solid silica glass core. This makes their longitudinal characterization using conventional reflectometric techniques very challenging. However, it was recently estimated that when filled with air, their backscattering coefficient increases to about 30 dB below that of standard solid core fibers. This level should be measurable with commercially available high performance optical time domain reflectometers (OTDR). Here we demonstrate – for the first time to the best of our knowledge – the measurement of backscattering from the air inside a hollow core fiber. We show that the characterization of multi-km long hollow core fibers with 15 m spatial resolution is possible using a commercial OTDR instrument. To benefit from its full dynamic range, we strongly suppress the 4% back-reflections that ordinarily occur at the OTDR’s standard fiber output when directly-connected to a hollow core fiber. Furthermore, low coupling loss into the hollow core fiber (0.3 dB in our experiment) also helps to maximize the achievable OTDR signal-to-noise ratio. This approach enables distributed characterization and fault-finding in low-loss hollow core fibers, a topic of increasing importance as these fibers are now starting to be installed in commercial optical communication networks.

With the onset of 5G networks and growing demands on data capacity, fiber-optics and fiber-optic systems are drawing more attention. Their fundamental element being the optical fiber. The conventional optical fiber is a result from decades of technological evolution. Up to now, optical fibers have been used not only in telcom but also for sensing, fiber lasers and high-power delivery. Still conventional optical fibers are made of glass, mostly silica, which limits their performance, e.g. in achieving zero attenuation. Using hollow-core optical fibers brings many advantages as light propagates in air. Currently we are on the verge of commercial application of hollow-core optical fibers which will have significant impact on telecommunications but also on sensory networks, interferometry and lasers from visible to mid-infrared regions.

This letter presents results of an experimental measurement campaign involving the deployment of combined radio over the fiber and radio over free space optics (FSO) technology in the cloud-based fifth generation (5G) fronthaul network operating in the millimeter wave (mmW) area. For this purpose, we have used 10 km of optical fiber, 50 m long outdoor FSO link and 1 m long antenna seamless radio frequency transmission at 39 GHz. The results show excellent performance in terms of the phase noise and the signal-to-noise (SNR) ratio. The error vector magnitude performance depends on the modulation format and are below the standard limits for the 5G new radio signals, with 400 MHz QPSK and 64-QAM showing almost identical results for SNR of up to ~19 dB whereas 256-QAM signal offering the best spectral efficiency. Moreover, we investigate the received mmW signal deterioration due to the atmospheric conditions in the FSO channel.

We demonstrate a 3-way interconnection device (hollow-core fiber, standard single-mode fiber and gas inlet) that is compact, low-loss, and easy-to-use.We demonstrate its performance on fibre purging, observing water vapor via infrared spectroscopy

We present a novel concept of visible light communication (VLC) uplink by introducing a distributed receiver (Rx) formed by an illuminating optical fiber (IOF) connected to the photodetector. The major functionality of IOF-distributed Rx is to sense/detect the light signal along the fiber span at a range of modulation frequencies coming from an on-off keying modulated light-emitting diode (LED) transmitter. We show that our proposed proof-of-concept of distributed Rx performs below the forward error correction limit of 3.8⋅10−3 at frequencies up to 400kHz. We unveil the signal-to-noise ratio (SNR) limit of 19dB must be maintained for successful data reception. Higher data rates are predicted once the concept is optimized, especially in terms of SNR. The proposed IOF-based scheme can be considered as a solution to the current challenges of VLC uplink.

We have recently presented a novel concept of optical camera communications (OCC) based on illuminating optical fibers (IOF) as transmitter (Tx). In this paper, we further investigate this concept of IOF-based OCC system by replacing a straight IOF by an L-shaped IOF as the transmitter. An intensity modulated light from a white light emitting diode in the on-off keying format at frequencies of 1800 and 2400Hz is launched into the IOF. We experimentally investigate the impact of IOF bending as well as horizontal and vertical orientations of camera-based receiver on the performance of the IOF-based OCC system. The results depict 100% success of reception is achieved over a transmission distance of 50cm for the IOF bending angle in the range of 0 to 90°, and horizontal and vertical orientations of the camera.

We present angled interconnection between standard single-mode fiber and nested nodeless antiresonant fibers achieving an insertion loss of 0.45 dB and return loss below -60 dB over a wide (1450-1650 nm) spectral range.

In this paper, we study the novel idea of fiber optic lighting - optical camera communications. We carry out analyses of the proposed scheme employing a light emitting diode (LED) illuminated plastic optical fiber (POF) and a rolling-shutter-based camera as the transmitter and the receiver (Rx), respectively in an indoor static environment. We also provide optical and electrical characterization of the LED and LED coupled POF illumination sources. The experiment results demonstrate that, despite the small diameter of the illuminating POF, flicker-free wireless communications at modulation frequencies of 300 and 600 Hz over the transmission distances of 50 and 75 cm can be achieved. Consequently, we show that a 100 % reception success rate is achieved for the camera-based optical Rx with the exposure time and the gain of 400 µs and 25 dB, respectively.

The telecommunication world is experiencing the 5th generation (5G) networks deployment including the use of millimeter wave (mmW) frequency bands to satisfy capacity demands. This leads to the extensive use of optical communications, especially the optical fiber connectivity at the last mile access and the edge networks. In this paper we outline fiber and free space optics (FSO) technologies for use as part of the 5G optical fronthaul network. We investigate two different mmW transmission schemes based on (i) the conventional analog radio over fiber transmission using one Mach-Zehnder modulator (MZM) with double sideband (DSB) optical modulation, and (ii) an optical-based frequency doubling with one MZM biased at the null point to introduce carrier suppression DSB (CS DSB) transmission and second MZM used for data modulation. Both systems are assessed in terms of the error vector magnitude, signal-to-noise ratio, dynamic range and phase noise. We consider a configuration for the fronthaul network in the frequency range 2 (FR2) at 27 and 39 GHz with the scale of bandwidth up to 400 MHz with M-quadrature amplitude modulation and quadrature phase shift keying. Results are also shown for FR1 at 3.5 GHz. Moreover, we investigate for the first time the 5G new radio signal transmission under strong turbulence conditions and show the turbulence-induced FSO link impairment. We finally demonstrate the CS DSB scheme performs well under chromatic dispersion-induced fading for the frequency up to 40 GHz and single mode fiber length of 30 km, whereas the DSB format seems more appropriate for an antenna seamless transmission.

We present a hybrid radiofrequency and microwave photonic link at 25 GHz using the chromatic dispersion of an optical fiber to steer the beam of a three-element planar dipole-based phased antenna array (PAA). Our team has designed and developed an in-house built PAA, experimentally verified its parameters, and successfully demonstrated optically controlled beam steering as measured in an anechoic chamber.Moreover, a detailed analysis of the optically based beam steering in the proposed microwave photonics system has been carried out, with data transmission achieving an error vector magnitude as low as 5.6% for the frequency of 25 GHz and 20 MHz bandwidth.

A monolithic fiber laser operating in the short wavelength infrared that is suitable for CO2 gas sensing applications is proposed and presented. The current study reports a laser design based on the direct inscription of a monolithic Fabry–Perot (FP) cavity in a thulium-doped optical fiber using the femtosecond laser (FsL) plane-by-plane inscription method to produce the cavity mirrors. The FP cavity was inscribed directly into the active fiber using two wavelength-identical fiber Bragg gratings (FBGs), one with high and one with low reflectivity. Initially the effective length of the fiber was defined using a single high reflectivity FBG and subsequently a very weak FBG was inscribed at the other end of the fiber in order to demonstrate a fully monolithic fiber laser. All fiber lasers were designed for continuous wave operation at 1950 nm and characterized with respect to the power output, slope efficiency, stability, and effective resonator length. The performance of the presented monolithic laser cavities was evaluated using the same active fiber as a reference fiber spliced to FBGs inscribed in passive fiber; an improvement exceeding 12% slope efficiency is reported for the presented monolithic laser.

Low-loss hollow-core fiber interconnection to standard optical fibers paves the way for high-finesse resonators, low-noise sensors, high-power delivery and next-generation fiber-optic communications. We will present main interconnection principles, discuss how back-reflection and higher-order mode excitation can be mitigated, and provide application perspectives.

In this paper, we present results of long-term stability tests of a low-loss (<0.55 dB) hollow core fiber (HCF) to standard optical fiber interconnection prepared by modified gluing-based fiber-array technology. We measured insertion loss of three interconnected HCF samples over a period of 100 days at room temperature, observing a variation in insertion loss of less than 0.02 dB. Subsequently, we placed the HCF samples in a climatic chamber and heated to +85°C in four cycles. Maximum insertion loss variation of 0.10 dB was observed for HCF samples with angled 8° interconnections and only 0.02 dB for a HCF sample with a flat interconnection.

We demonstrate halving the record-low loss of interconnection between a nested antiresonant nodeless type hollow-core fiber (NANF) and standard single-mode fiber (SMF). The achieved interconnection loss of 0.15 dB is only 0.07 dB above the theoretically-expected minimum loss. We also optimized the interconnection in terms of unwanted cross-coupling into the higher-order modes of the NANF. We achieved cross-coupling as low as −35 dB into the LP11 mode (the lowest-loss higher-order mode and thus the most important to eliminate). With the help of simulations, we show that the measured LP11 mode coupling is most likely limited by the slightly imperfect symmetry of the manufactured NANF. The coupling cross-talk into the highly-lossy LP02 mode (>2000 dB/km in our fiber) was measured to be below −22 dB. Furthermore, we show experimentally that the anti-reflective coating applied to the interconnect interface reduces the insertion loss by 0.15 dB while simultaneously reducing the back-reflection below −40 dB over a 60 nm bandwidth. Finally, we also demonstrated an alternative mode-field adapter to adapt the mode-field size between SMF and NANF, based on thermally-expanded core fibers. This approach enabled us to achieve an interconnection loss of 0.21 dB and cross-coupling of −35 dB into the LP11 mode.

In this Letter, we propose and demonstrate a novel wireless communications link using an illuminating optical fiber as a transmitter (Tx) in optical camera communications. We demonstrate an indoor proof-of-concept system using an illuminating plastic optical fiber coupled with a light-emitting diode and a commercial camera as the Tx and the receiver, respectively. For the first time, to the best of our knowledge, we experimentally demonstrate flicker-free wireless transmission within the off-axis camera rotation angle range of 0–45° and the modulation frequencies of 300 and 500 Hz. We also show that a reception success rate of 100% is achieved for the camera exposure and gain of 200 µs and 25 dB, respectively.

One of the key functionalities in microwave photonics is to be able to define controllable time delays during the signal processing. Optical fibers are often used to achieve this functionality, especially when a long delay or a widely-tunable delay is needed. However, the stability of this delay in the presence of environmental changes (e.g., temperature) has not, to the best of our knowledge, been reviewed yet. Here, we firstly discuss the impact of temperature-induced variations on the signal propagation time in optical fibers and its implications in microwave photonics. We compare the impact of the thermal sensitivity of various delay lines for applications in which the signal is transported from point A to point B, as well as for applications in which the propagation time through a fiber or the fiber dispersion is used to create a fixed or tunable delay. In the second part of the article we show the impact of fiber thermal sensitivity on a narrow-band microwave photonics filter made of standard single mode fiber (SSMF) and a hollow core fiber (HCF), which has significantly lower thermal sensitivity of propagation time to temperature. The central frequency of the band-pass filter changes almost 16 times more in the filter made of SSMF as compared to that of HCF, dictating very tight (0.05 °C) temperature stabilization for SSMF-based filters. On the basis of our thermal sensitivity analysis we conclude that HCFs are very promising for environmentally stable microwave photonics applications.

We report the fabrication of a hollow core fiber (HCF) coupled Faraday rotator mirror component with a total insertion loss below 1.1 dB and demonstrate its use in a polarization-stable all-fiber Michelson interferometer. The signal split into the two interferometer arms is performed using a standard 2×2 fused fiber coupler. One of the coupler output ports is permanently and directly connected to the HCF with a low return loss (below −40 dB) and negligible signal coupling into higher-order modes. The demonstrated polarization-insensitive interferometer shows residual peak-to-peak polarization-induced power variations as low as 0.04 dB. The device will be of interest in interferometric applications requiring low thermal sensitivity as the phase drift of light propagating through an HCF is over an order of magnitude less sensitive to temperature than in standard fiber. Moreover, the fiber nonlinearity is almost three orders of magnitude lower in HCFs as compared to standard solid optical fibers.

Multi-element antenna beam steering in freelicence higher frequency bands is one of the crucial features of 5G networks enabling better tracking of the users. In this paper, we present an experimental microwave photonics transmission system operating at 26 GHz where beamforming is fully realized in the optical domain. The system is designed to be deployed as a part of the mobile fronthaul network with an optical fiber span of 15 km. As a proof of concept, a planar 3-element antenna array has been developed and radiation patterns were measured in an anechoic chamber with high agreement between experimental and simulation results.

We present a simple but highly accurate modeling technique for real photonic crystal fibers (PCFs) characterization. We determine the influence caused by idealized model parameters. Our technique can be applied to arbitrary PCF air-hole structures, as it takes into account all structural distortions. It requires only an image of the PCF cross-section to create an accurate PCF model. Model outputs are presented in comparison with the measurement of chromatic dispersion curve and the effective mode area. We provide a study on the impact of imprecise determination of glass refractive index on the PCF model accuracy. We demonstrate how the simplification of the air-hole deformations can influence the chromatic dispersion curve. Finally, we show the effect of precise PCF modeling on example of supercontinuum generation.

Radio over fiber (RoF) transmission systems have been developing rapidly, especially for applications in 5G networks. In scenarios unsuitable for fiber-optics, radio over free-space optics (RoFSO) presents a suitable solution. Nevertheless, free-space optics (FSO) suffers from atmospheric conditions. The use of the 2000 nm band offers several advantages over the commonly used 1550 nm region. We focus on proof-of-concept evaluation of such a 2000 nm RoFSO transmission system. Measured characteristics are compared with a similar 1550 nm RoFSO system. We demonstrate both systems for QPSK and 64-QAM LTE formats, at 5 GHz and 10 GHz with a 20 MHz bandwidth.

We propose a highly sensitive motion detection based on fiber Fabry-Perot interferometry with a fast solution achieved the resolution of 2.08 nm by using a simple analysis. Two configurations are compared based on experimental results.

Today hollow-core optical fibers (HCF) are on the verge of surpassing the attenuation benchmark of silica single-mode optical fibers used in optical communication. Compared to solid-core optical fibers, HCFs exhibit ultra-low nonlinearity, high damage threshold, low latency and temperature insensitivity, making them ideal candidates for high-speed data communication, high-resolution sensing, high-power delivery and precise interferometry. The main challenges of low insertion loss, suppressed back-reflections and fundamental mode coupling must be addressed to incorporate HCFs into existing fiber-optic systems to fully exploit their potential. This paper provides an overview of the HCF history, from early papers in the 1980s, over the invention of photonic-bandgap HCFs, to the recent achievements with antiresonant HCFs. Then light guiding mechanisms are presented and key HCF properties are discussed. Interconnection techniques to standard optical fibers are compared with respect to possible HCF applications. Fusion splicing results are presented with an~alternative interconnection solution based on a modified fiber-array technique newly developed by our team. Finally, cutting-edge HCF applications that take advantage of our HCF interconnection, are discussed.

Microwave photonics is a promising solution to transmit millimeter wave (mmW) signals for the 5th generation (5G) mobile communications as part of a centralized radio access network (C-RAN). In this paper, we experimentally evaluate the impact of turbulent free space optics links on photonically generated mmW signals in the frequency range of 26−40 GHz . We analyze the remote generation of mmW signals over hybrid links based on free-space optics (FSO) and standard single mode optical fiber (SSMF) with −39.97dBm received electrical power and phase noise level at 100kHz as low as −95.92dBcHz at 26GHz . Different thermal distributions along the FSO link have been implemented and Gamma-Gamma model has been employed to estimate the thermally induced turbulence. The results show high electrical power decrease and fluctuation of the generated mmW signal according to the particular level of the turbulence in terms of refractive index structure parameter and thermal distribution along the FSO link. 8Gb/s 16-quadrature amplitude modulation (QAM) data transmission at 42GHz has been demonstrated over the hybrid link with minimal error vector magnitude (EVM) value of 5% whereas turbulent FSO link introduced up to 5dB power penalty.

We made and characterized two Fabry-Perot interferometer samples made of the latest-generation hollow core fiber with sub-1-dB/km loss. Thanks to this low transmission loss, we achieved a finesse of over 140 and 120, for interferometer lengths of 5 and 23 m, respectively. This resulted in transmission peaks as narrow as 47 kHz. Our all-fiber Fabry-Perot interferometers have standard single-mode fiber pigtails (for easy integration in conventional fiber optic systems) and employ fiber mode field adapters to enable low-loss coupling between the pigtails and the low-loss hollow core fiber. The high-reflectivity mirrors (>98%) were deposited directly on the fiber mode field adapters, which were glued to the hollow core fiber, resulting in permanently-aligned Fabry-Perot interferometers. We also measured how the position of the transmission peaks change with temperature (an important performance metrics for most applications, e.g., when used as a narrow-band band-pass filter) and found that it changed 14.5 times less in our Fabry-Perot interferometer relative to a similar device made of standard single mode fiber.

A new hybrid microwave photonic link based on a polarization division multiplexing Mach-Zehnder modulator (PDM-MZM) is proposed. The link enables co-transmission of millimeter-wave (mmW) and sub-6 GHz wireless signals over a seamless single-mode fiber (SMF) and free-space optics (FSO) channels. Optimization of the chromatic dispersion (CD)-induced power fading regardless of the power fading due to the non-deterministic atmospheric turbulence (AT) is simultaneously demonstrated. Extensive simulation analysis is first presented to examine (i) the impact of CD on mmW (25 GHz) and sub-6 GHz (2.6 GHz) signals, envisioned for the 5th generation networks, and (ii) optimization of CD-induced power fading by changing the phase relations between the optical carrier and optical sidebands in each polarization channel using single tunable polarization controller. A proof-of-concept experiment is finally performed to simultaneously deliver 25 GHz and 2.6 GHz signals with 4/16/64-quadrature amplitude modulation over (i) 20 km SMF and 2 m radio wireless link and (ii) 20 km SMF, 4.2 m FSO (with AT) and 2 m radio wireless links. The optimization of the CD-induced power fading is experimentally verified and link performance shows high tolerance to CD with no power penalties and the measured error vector magnitudes well below the required limits. The predicted bit error rates are also below the forward error correction threshold of 2×10−4

In the modern wireless networks, millimeter-wave radio-frequency (RF) bands are becoming more attractive as they provide larger bandwidth and higher data rates than the today-used systems operating at frequencies below 6 GHz. In addition, according to the fact that coaxial cables exhibit extremely high attenuation for millimeter-wave RF signals, analog radio over fiber techniques (RoF) form a promising technology for delivering unaltered radio waveform to a remote antenna. This paper experimentally analyzes three types of RoF modulations, namely a directly modulated laser, an electro-absorption modulator, and a Mach-Zehnder Modulator. The primary focus is on the implementation of each RoF transmitter in an RoF system, such as those in 5G networks. The experimental study includes a detailed characterization of an RoF system with a 50-m long outdoor free-space RF channel operating in the frequency band of 25 GHz. Frequency response (S-parameters) and third-order nonlinear distortion are investigated in detail. Tests of EVM performance were conducted using an orthogonal frequency division multiplexing signal modulated with 16-quadrature amplitude modulation (16-QAM) with a long-term evolution signal. It is demonstrated that the transmitters studied can operate under a 13.5% EVM limit given for 16-QAM. Apart from the detailed system performance, the considerable power fluctuations in the 25 GHz free-space RF outdoor channel are reported.

We numerically demonstrate ultra-wideband mid-infrared supercontinuum (SC) generation in a liquid-filled arsenic–selenide circular photonic crystal fiber (C-PCF). The inner ring air-holes are filled with two different nonlinear liquids: chloroform (CHCl3) and carbon disulfide (CS2). Based on simulation results, we show that ultra-wideband SC spectra spanning from 1 to 20  μm can be achieved using only 5-mm long CHCl3 liquid filling with a pump optical pulse of 10-kW peak power at the wavelength of 2.55  μm. In addition, using 5-kW peak power, SC spectrum spanning up to 12  μm is obtained in the case of the CS2 liquid-filled C-PCF.

In this chapter, an all-optical FSO relay-assisted system is proposed to mitigate the destructive effects of the distance-dependent AT-induced fading. Relays are inserted directly in the link in order to reduce the AT-induced link loss, thus extending the link span and ensuring higher link availability as well as improving the overall system performance. Two all-optical relaying schemes are proposed and investigated, namely all-optical amplify-and-forward (AOAF) and all-optical regenerate-and-forward (AORF) FSO relay-assisted approaches. For the AOAF approach, the performance analysis of triple-hop AOAF FSO communications is done under the impact of nonhomogeneous atmospheric turbulence. The AORF relaying approach is then proposed to overcome the limitation imposed by AOAF system, where the signal and noise are accumulated at each relay, thus limiting the number of relay nodes that can be used.

This paper presents experimental results for an all-optical free-space optical (FSO) relay-assisted system by employing an all-optical regenerate and forward (AORF) scheme in order to increase the transmission link span. The ultra-short pulse (i.e., 2 ps) regeneration technique based on Mamyshev method is adopted. We have developed a dedicated experimental test-bed composed of optical fiber components and FSO links to demonstrate the proposed scheme and evaluate its performance in terms of the Q-factor and bit error rate (BER) under turbulence regimes for both single and dual-hop network architectures. We show that, using the AORF a hundred times improvement in the BER performance is achieved compared to the amplify-and-forward scheme for a fixed signal-to-noise ratio under turbulence conditions.

This paper outlines experimental investigation of the performance of the wireless optical network based on an all-optical triple-hop free space optical (FSO) communications employing amplify-and-forward relaying under the influence of atmospheric turbulence. We present new results for the bit error rate (BER) performance for seven possible turbulence network scenarios for relay-assisted FSO link and validate them with numerical simulations based on Gamma-Gamma turbulence model showing a good agreement between them. We also show results, which elucidate the impact of non-homogeneous turbulence along the entire transmission link span for the multiple-hop relay-assisted FSO system. More specifically we show that the BER performance considerably deteriorates for the case where turbulence is near to the receiver end. We outline that for a target BER of 10−4 the signal-to-noise ratio penalty can be as high as 9 dB compared to the case with no turbulence.

We show results on adopting our new technique, developed for connecting solid-core fibers with hollow-core photonic bandgap fibers, to connect solid-core with antiresonant hollow core fibers. We achieved insertion loss below 0.5 dB per connection.

We present a new approach to permanently inter-connect hollow-core fiber (HCF) to solid-core fiber, which does not involve fusion splicing. Our approach is based on a modification of the glue-based fiber-array technology routinely used for fiber pigtailing of planar lightwave circuits. The resulting interconnection provides for a low insertion loss due to the fact that the HCF microstructure is not deformed during the gluing (low temperature) process that is almost impossible to achieve with the standard (high temperature) fusion splicing method. Furthermore, this low-temperature technique enables the deposition and preservation of thin films deposited at the solid-to-hollow core fiber interface, allowing for additional functionality without the introduction of extra losses or any increase in complexity. To demonstrate this, we have applied an anti-reflection (AR) coating. A further feature of our approach is the ability to control very precisely the length of the graded-index (GRIN) fiber mode field (MF) adapter inserted in between the standard single-mode fiber (SMF-28) and the HCF. We show experimentally how the length of the GRIN fiber MF adapter influences the coupling between the SMF-28 and the fundamental as well as the higher-order modes of the HCF. We coupled between SMF-28 [10μm mode field diameter (MFD)] and the fundamental mode of a 19-cell hollow-core photonic bandgap fiber (HC-PBGF, 21.1μm MFD) with the lowest-ever reported insertion loss of 0.30dB per interface.

Two experimental configurations of a hybrid K-band (25 GHz) microwave photonic link (MPL) are investigated for seamless broadband wireless access networks. Experimental configurations consist of optical fiber, free-space optics (FSO) and radio frequency (RF) wireless channels. We analyze in detail the effects of channel impairments, namely fiber chromatic dispersion, atmospheric turbulence and multipath-induced fading on the transmission performance. In the first configuration, transmission of the 64-quadrature amplitude modulation (QAM) signal with 5, 20 and 50 MHz bandwidths over 5 km standard single-mode fiber (SSMF), 2 m turbulent FSO and 3 m RF wireless channels is investigated. We show that, for QAM with a high bandwidth, the link performance is being affected more by atmospheric turbulence. In the second configuration, the 20 MHz 4/16/64-QAM signals over a 50 km SSMF and 40 m FSO/RF wireless links are successfully transmitted with the measured error vector magnitude (EVM) values of 12, 9 and 7.9%, respectively. It is shown that, for all transmitted microwave vector signals, the bit error rate is lower than the hard-decision forward-error-correction limit of 3.8×10−3. Moreover, an extended FSO link span of 500 m for 25 GHz hybrid MPL with 16-QAM at 10 Gb/s under the weak and strong turbulence regimes is evaluated via simulation analysis to mimic a practical outdoor system.

We experimentally demonstrate a seamless hybrid transmission of radio-over-fiber (RoF), radio-over-free-space optics (RoFSO) and millimeter wave wireless link using long-term evolution (LTE) 4/16/64-QAM signals for broadband wireless access applications. The signals are transmitted and evaluated over the hybrid channel consisting of 5 km of standard single-mode fiber (SMF), 2 m FSO under the turbulence level up to 3.2×10-11 m-2/3 and 3.3 m 25 GHz wireless channel. The performance shows that, the error vector magnitudes (EVM) are below the 3GPP standard for 4/16/64-QAM signals. We show that, the bit error rate (BER) for all signals is below the forward error correction limit of 10-3. We also perform simulation analysis at higher bit rates of 10 and 15 Gb/s for the hybrid link of SMF (5 km) and FSO (extended up to 500) m under the turbulence level of 1.3×10-15 m-2/3 for the practical outdoor scenario. We achieved acceptable performance for 4- and 16-QAM at 10 and 15 Gb/s.

In this paper we present experimental results from precise measurements of mode field distribution in various commercial multimode (MM) fiber types. We characterize the mode field distribution by the encircled flux (EF) parameter. EF determines optimal launch conditions in the core of the MM fiber, which is crucial especially for accurate characterizing of insertion losses at fiber connections, e.g. in data centers. For the purpose of EF measurement, a universal testbed enabling mode field characterization, integral optical power estimation or fiber connector cleanness evaluation has been proposed and experimentally verified. The uniqueness of the proposed testbed lies in backlighting of the tested MM fiber. This backlighting enables accurate positioning of the MM fiber end-face with the collimating lens to measure in the near-field area. The testbed allows the deployment of variable MM fibers and is also wavelength independent. Therefore it has the high potential within growing usage of plastic or large core fibers for short-range communications, e.g. in automotive or avionics. The obtained results are discussed within a selected real application. Moreover, since the mode field distribution is evaluated even for fibers, which do not have available any standards for EF characterization, we present new recommendations for optimal launch conditions in such fibers.

We propose and experimentally demonstrate a photonics-assisted converged radio-over-fiber (RoF), radio-over-free-space optics (RoFSO) and millimeter-wave (MMW) wireless transmission system for use in broadband wireless access (BWA) networks. The focus is at the emerging frequency band of 25 GHz, as recommended for fifth-generation networks. As a proof-of-concept demonstration, all-optical up-converted long-term evolution test models with 4-, 16- and 64-quadrature amplitude modulation (QAM) are transmitted and evaluated over the proposed hybrid link under weak-to-strong atmospheric turbulence regimes. Link performance shows that, the error vector magnitudes are below the 3GPP standard for 4-, 16- and 64-QAM. We also show that, for all QAM signals under turbulence conditions, the bit error rate performance is below the forward error correction limit of ${10^{ - 3}}$ . Simulation analysis is also performed for the 10 Gb/s hybrid systems under turbulence for an extended FSO link up to 500 m to emulate a practical outdoor environment. Furthermore, we analytically estimate the attainable MMW wireless range for different rain rates in Prague, Czech Republic. The obtained experimental and simulation results confirm the feasibility and potential of the proposed hybrid system for next-generation last mile BWA networks.

We present an improvement of tapered optical fiber (TOF) sensor’s response for the detection of liquids, which is achieved by TOF surface structural modification. Our TOF sensors utilize the refractometric principle with enhanced evanescent-wave overlap due to the wavelength of 1550 nm and TOF waist diameters of 4–6 μm. The structural modification is achieved by long-term TOF exposition to hygroscopic liquid analytes and ambient atmosphere. To analyze the structural modification process, long-term as well as proof-of-principle tests have been carried out to evaluate TOF sensors stability in terms of sensitivity and resolution. Maximum sensitivity of over 2100 dB/RIU has been reached when TOF is used to detect a liquid analyte with refractive index of 1.415. Increase of more than 1400 dB/RIU is attributed to the enhanced sensitivity. Sample TOF sensors were then linearly calibrated and have been tested in more than one year-long continuous measurement campaign. Resolution better than 7x10^-4 for a refractive index range of from 1.405 to 1.425 with working point drift below 2x10^-4 over 12 months period has been achieved. Our results and observations are suitable for reliable, low-cost and application-tailored TOF sensor development.

This Letter outlines radio-over-fiber combined with radio-over-free-space optics (RoFSO) and radio frequency free-space transmission, which is of particular relevance for fifth-generation networks. Here, the frequency band of 24–26 GHz is adopted to demonstrate a low-cost, compact, and high-energy-efficient solution based on the direct intensity modulation and direct detection scheme. For our proof-of-concept demonstration, we use 64 quadrature amplitude modulation with a 100 MHz bandwidth. We assess the link performance by exposing the RoFSO section to atmospheric turbulence conditions. Further, we show that the measured minimum error vector magnitude (EVM) is 4.7% and also verify that the proposed system with the free-space-optics link span of 100 m under strong turbulence can deliver an acceptable EVM of <9% with signal to noise ratio levels of 22 dB and 10 dB with and without turbulence, respectively.

The paper discusses conventional telecommunication silica single-mode optical fiber tapers acting as sensors of liquids based on the refractometric principle. Wavelength of 1550 nm is exploited not only due to the facile system integration but especially for the increased evanescent-wave overlap. Based on the simulation results, numerous sensor samples with waist diameters ranging from 4 to 6 um were prepared in precisely controlled conditions. Proof-of-principle tests were carried out to prove sensitivity enhancement of more than 1400 dB/RIU. Sensitivity enhancement was achieved by a structural modification of the tapered fiber surface. A maximum sensitivity of over 2000 dB/RIU was reached. Our results are essential for reliable, low-cost application of developed sensors, in particular, those used for monitoring of gasoline quality. Furthermore the observed long-term attenuation increase can also be critical for many telecommunication components, especially for those exposed to varying ambient atmosphere.

A generalized model for the detection of liquids within suspended-core microstructured optical fibers has been experimentally and theoretically derived. The sensor detection is based on the refractometric principle of transmission losses due to the overlap of the evanescent field with the liquid analyte. A number of parameters, including fiber core diameter and filling length, have been included in the general model. Specially tailored suspended-core fibers were manufactured with the core diameters within the range of 2.4 um to 4.0 um. Five selected liquid analytes were used to cover the refractive index range of 1.35 to 1.42. Based on experiments, the characteristics of the parameters of the semi-empirical model have been determined by a genetic algorithm using 283 measurement data sets. The model can be used to design sensors for the detection of liquid analytes as it provides a set of parameters allowing to optimize the sensor’s sensitivity for a wide scale of applications. Finally, numerical simulations of the system were carried out by an eigenmode routine to support the results of the generalized model.

n this paper we investigate several silica-based suspended core microstructured fibers optical (SC-MOFs) in regards to providing the high efficiency coupling for broadband dispersion measurement. We present free-space optic and butt coupling setup capable of coupling signal from standard single mode fibers (SMF) into SC-MOF with core diameter of less than 4 μm and coupling efficiency over 50%. We then investigate SC-MOF’s effective mode area, nonlinearity coefficient and chromatic dispersion curve, using both modeling of the fibers and measurements. Lastly, we have investigated the effects of the aspheric lens on broadband coupling for the chromatic dispersion measurement.

We present an experimental hybrid radio over fiber (RoF) and radio over free space optics (RoFSO) system employing a directly modulated laser (DML) for a radio frequency band of 24 – 26 GHz. Moreover, a 3.6 m long radio free space link is combined with a pair of wideband antennas. We outline the specific settings of the proposed high-frequency system for deployment in the centralized fronthaul networks as part of the 5th generation wireless technologies. We show a potential extension in terms of the added optical loss when using a 100 MHz signal bandwidth. In order to determine the optimum system performance, the setting of directly-modulated laser bias current is investigated. Measured results illustrate that the performance of the proposed system is less susceptible for up to medium turbulence conditions by maintaining an error vector magnitude (EVM) value below the limit of 9 %, given for 64- quadrature amplitude modulation (64-QAM). The lowest recorded EVM of the whole system for 64-QAM and a 100 MHz bandwidth at a frequency 24 GHz is 4.1 % while using a band pass filter to reduce amplified spontaneous emission noise in the system.

Multi-mode fiber (MMF) networks are under higher pressure with the wide deployment of high data-rate transmission systems. Reachable distance and system reliability in particular is essential for MM fiber usage in strategic areas like avionics or big data centers. Therefore, knowledge of the mode-field distribution within MMFs is crucial for proper design and optimization of such MMF systems. An optimally excited mode-field in MMFs leads to lower insertion losses (IL) at MMF connections and also contributes to achieving a higher effective modal bandwidth (EMB). To properly measure IL in a MM network and characterize the network for high data rates, the encircled flux (EF) parameter determines an ideal mode distribution in a MMF. This paper studies all above-mentioned aspects, to fully describe mode behavior in MM fiber connections. Moreover, potential accidents in terms of MM fiber offset in adaptors are tested and completed with simulations. Behavior of different optical sources on EF is presented.

A modified octagonal photonic crystal fiber (MO-PCF) is proposed and numerically investigated for the purpose of residual dispersion compensation in the optical transmission link. The results show that the proposed fiber with optimized parameters exhibits ultraflattened negative dispersion over the 300 nm band (1380 nm to 1680 nm) with an average dispersion of –506 ps/(nm·km) and an absolute dispersion variation of 11.3 ps/(nm·km). In addition to large negative dispersion, the proposed MO-PCF also exhibits high birefringence in the order of 0.0207 at the 1550 nm wavelength. The proposed MO-PCF can be advantageously used especially for residual chromatic dispersion compensation in the wavelength-division-multiplexing optical fiber transmission system. The proposed fiber design is easy to draw and is tolerant to manufacturing imperfections.

This paper presents results from the development of a fiber-optic gyroscope based exclusively on polarization-maintaining (PM) components. The main element represents the gyroscope coil, which consists of a 400m-long depressed-cladding PM fiber. To reduce temperature and stress effects, the gyroscope coil is wound in a quadrupolar fashion, which significantly reduces the inhomogeneous distribution of temperature and mechanical stress over the entire fiber coil. This results in mitigation of the so-called Shupe effect. We have employed a super-fluorescent fiber source to overcome the limiting effect of nonlinearities in order to improve the gyroscope performance. We compared the performance of a commercial super-luminescent LED and two types of in-house built super-fluorescent fiber sources realized by PM Erbium-doped fibers. We studied the effect of fiber doping, pumping powers and fiber length on the performance of the fiber-optic gyroscope. Using a digital feedback loop a gyroscope’s dynamic range can be dramatically increased and precision improved as well. In addition, Kalman filtering is proposed to further reduce parasitic drift effects. Conventional tools such as Allan variance are presented, depicting our long-term fiber-optic gyroscope development.

Over the last two decades, a large amount of optical fiber (OF) cables has been deployed as part of the global communication networks. Both the aging of OFs as well as the need to increase transmission data rates, particularly in the backbone, have become hot topics. We present the study of the aged OF deployment in various optical networks including free space optics (FSO) link as a part of modern optical communication networks. Here, we show extended results obtained using a dedicated OF testbed focusing on the long-term monitoring of polarization mode dispersion (PMD) because of its time-varying nature. The adaptation of polarization multiplexed radio over fiber (RoF) and radio over FSO (RoFSO) systems as well as 10 Gbps on-off-keying (OOK) non-return-to-zero (NRZ) intensity modulation with the direct detection system, which is common cost-effective transmission system in passive networks, are demonstrated. Moreover, simulation of 100 and 200 Gbps return-to-zero (RZ) differential quadrature phase shift keying (DQPSK) with direct detection is outlined to verify the impact of aged OF network connected with FSO under turbulence conditions. Results reveal more than 6 dB of power penalty with the aged OF route for 100 Gbps systems. In addition, there is a 0.8 dB power penalty due to the strong seasonal induced PMD fluctuations. The influence of scintillations in terms of Rytov variance for the FSO link is also investigated for weak to moderate turbulence. Finally, we derive an expression for the long-term mean PMD value determined over one-month measured frequency response.

Due to the high demands for the reliability of data centers, the determination of properly excited optical field inside the multimode (MM) fiber core belongs to the key parameters while designing such a MM optical system architecture for data centers. Appropriately excited mode field of the MM fiber provides optimal power budget in connections, leads to the decrease of insertion losses (IL) and achieves effective modal bandwidth (EMB), which is essential for high data rates. Crucial is especially the encircled flux (EF) parameter, which should be properly defined for combinations of variable optical sources and MM fiber infrastructure to provide proper mode-field distribution. In this paper, we present detailed investigation and measurements of the mode field distribution for short MM optical data center links with the emphasis on their reliability. Such characterization is essential for the design of large MM networks. Various scenarios were tested in terms of IL and mode-field distribution to reveal potential problematic scenarios. Furthermore, we focused via simulations and experiments on the estimation of particular defects and errors, which can realistically occur like eccentricity or connector misalignment. Their dependence on EF statistics and functionality of data center infrastructure was evaluated. Finally, we provide recommendations for data center systems and networks, using OM3 MM fiber connections

We present a detailed chromatic dispersion characterization of heavy-metal oxide (HMO) glass photonic crystal fibers (PCFs) suitable for mid-infrared applications. Based on previous work with hexagonal and suspended-core fibers the focus was placed on determination of the chromatic dispersion curve to reach precise correlation between simulation model and real fiber based on both a post-draw model correction and broadband chromatic dispersion measurement. The paper covers the fiber design, discusses fiber manufacturing, presents measurements of fiber chromatic dispersion, provides the simulation model correction and finally proposes further applications. Selected fiber designs from simulation model were fabricated by the stack-and-draw technique. The dispersion measurement setup was based on an unbalanced Mach-Zehnder interferometer. The influence of optical elements on the measurement results and broadband coupling is discussed. We have proved that the critical factor represents the accuracy of the refractive index equation of the HMO glass and real fiber structure. By improved technique we reached the zero-dispersion wavelength with a reasonable precision of less than 30 nm.

This paper presents novel experimental results for a 10 Gbps triple-hop relay-based all-optical free space optical (FSO) system by employing the amplify-and-forward (AF) relaying scheme. We provide a mathematical framework for the end-end signal-to-noise ratio (SNR) and the bit-error rate (BER) performance and confirm that the derived analytical results reasonably match experimental results especially at relatively high SNR. The evaluated BER performances under different atmospheric turbulence regimes (modeled by the Gamma-Gamma distribution) show that the considered relay-assisted FSO system offers a significant performance improvement for weak to strong turbulence regimes, even without knowledge of the channel state information. More precisely, at a target BER of 10-5 the proposed scheme offers ~ 5 dB and ~4 dB of SNR gain compared to the direct transmission for turbulence strengths C_n^2 of 3.8×10^(-10) m-2/3 and 5.4 ×10^(-12) m-2/3, respectively.

Today's optical networks are composed of thousands of kilometers of aging optical cables. Many of these cables are located in harsh environments which contribute to induced birefringence of the fibers and a corresponding increase of polarization mode dispersion (PMD). This paper introduces derived statistics from the longest-known running evaluation of a PMD measuring campaign and an investigation into how higher optical power affects these aging systems. Results indicate strong seasonal dependence of PMD on temperature for an optical cable testbed exposed to atmospheric changes, leading to a 16 % increase of mean PMD value in summer. This fluctuation causes bit error rate (BER) limits to be exceeded for 10 Gbps and 40 Gbps non-return-to-zero (NRZ) signals which is a critical issue for applications where high reliability is required. Moreover, due to the high optical power load within old optical infrastructures, a more than 0.15 dB increase of relative loss per year in tested routes, compared to reference routes, has been observed.

Evanescent wave spectroscopy in the mid-infrared (MIR) is a powerful tool for remote real-time sensing. Chalcogenide fibers transparent in MIR are considered as a base for creation of a fiber-optical platform for the MIR sensing. In this paper, a rigorous theoretical approach has been applied for analysis of evanescent modes propagation in a multimode chalcogenide fiber surrounded by an absorbing medium. A role of particular evanescent mode in power delivering through the fiber has been revealed. Strong absorption of water in this spectral range has been shown to be a main factor limiting sensitivity of the evanescent wave sensor. Possibilities of sensitivity enhancement by using waveguiding properties of the fiber have been discussed. The analysis is supported with an experimental measurement of a [GeSe 4 ] 95 I 5 glass fiber partially immersed in an aqueous acetone solution, in the wavelength range of 2-5 μ m.

We present a detailed procedure of precise chromatic dispersion and mode field characterization of unknown photonics crystal fibers for their further utilization in nonlinear applications. To verify our methods an Endlessly single-mode photonic crystal fiber (ESM-PCF) was used. A finite-element method simulation software was utilized for mode field distribution and chromatic dispersion simulation based on the particular fiber structure. Subsequently, a measurement setup for dispersion characterization was proposed, based on the Mach-Zehnder interferometer. Experiment results include as well the influence of various optical elements in the configuration on the measured fiber parameters. Second experimental setup was realized with the possibility of parallel mode-field diameter measurement and visualization of the fiber core structure. Effective area and dispersion measurements results were compared to the simulation outputs. We achieved up to 5 nm accuracy of zero-dispersion wavelength for a 28 cm long ESM-PCF and maximum mismatch of 2 μm2 with effective mode-field area determination. Our procedures and characterization outputs could be applied to specialty optical fiber design and characterization mainly for nonlinear applications such as supercontinuum generation.

Detecting explosive, flammable or toxic industrial liquids reliably and accurately is a matter of civic responsibility which cannot be treated lightly. Tapered optical fibers (TOFs) and suspended core microstructured optical fibers (SC MOFs) were separately used as sensors of liquids without being compared to each other. We present a highly-sensitive time-stable TOF sensor incorporated in the pipeline system for in-line regime of measurement. The paper is furthermore focused on the comparison of this TOF and SC MOF of similar parameters for detection of selected liquids. A validated method that incorporates TOF and SC MOF of small core (waist) diameter for refractometric detection is presented. The principle of detection is based on the overlap of an enhanced evanescent wave with a liquid analyte which either fills the cladding holes of the SC MOF, or surrounds the waist area of the TOF. Optical power within the evanescent wave for both sensing structures and selected liquid analytes is analyzed. Measurement results concerning TOF and SC MOF are compared. Calculations to ascertain the limit of detection (LOD) for each sensor and the sensitivity (S) to refractive indices of liquid analytes in the range of 1.4269 to 1.4361 were performed at a wavelength of 1550 nm with the lowest refractive index step of 0.0007. Results affirming that S = 600.96 dB/RIU and LOD = 0.0733 RIU for the SC MOF and S = 1143.2 dB/RIU and LOD of 0.0026 RIU for the TOF sensor were achieved, clearly illustrating that TOF based sensors can reach close to two times greater sensitivity and 30-times higher limit of detection. The paper extends the comparison of the fiber sensors by discussing the potential applications.

We present results from the development of an pulsed thulium laser for near-infrared and mid-infrared nonlinear applications utilizing specialty optical fibers. The laser is based on a gain-switched seed-laser and three amplification stages, where output pulse peak powers in order of kilowatts maximum are expected. The laser is at first analyzed in a simulation software. Pre-amplification stage is discussed, with focus on low-noise signal amplification. Following the pre-amplification stage setting the second gain stage is evaluated incorporating a thulium-fiber of 3m length with increased rare-earth dopant ratio. Finally, a power-booster stage is analyzed in simulation environment to reach peak power of kilowatts. Based on the developed thulium pulsed laser, results from nonlinear phenomena modelling and experiments in the vicinity of 2000 nm are presented. Specialty optical fibers based on fluoride and chalcogenide glass are evaluated.

In recent years, organic small molecule and polymer light-emitting diodes (LEDs) and photodetectors (PDs) have been used as optoelectronic components in visible light communications (VLC). The first study appeared in [1] which demonstrated data transmission rates in the hundreds of kb/s region is possible. This was further improved by using advanced modulation formats such as orthogonal frequency division multiplexing (OFDM) [2]. Ethernet transmission speeds were reported for the first time in [3] and was achieved using the multilayer perceptron artificial neural network equalization technique. The current state-of-the-art transmission speeds available in organic VLC (OVLC) transmission is 55 Mb/s using aggregated wavelength multiplexed data streams [4]. This chapter gives an overview of organic based VLC focusing on the LED technology trends, organic LED (OLED) based devices, the organic semiconductors, and visible light photodetectors. To enhance the OLED-based VLC links blue filtering and a number of equalization schemes including artificial neural network equalizer, decision feedback equalizer, and linear equalizer are discussed and their performance are compared and contrasted. Finally an experimental allorganic VLC system employing both OLED and organic photodetectors employing artificial neural network base equalizer is introduced and its performance is evaluated. The chapter is completed with concluding remarks.

The 100-kHz picosecond 100-W thin-disk laser has been developed. The application potential of the laser is extended by wavelength conversion which spans from deep-ultraviolet into mid-infrared. The fundamental wavelength of 1030 nm was converted into second (515 nm), third (343 nm), fourth (257.5 nm), and fifth (206) harmonics by cascaded second harmonic and sum frequency generation. Generation of longer wavelengths by optical parametric generation and amplification covers the region from 1.8 μm to 2.4 μm.

This letter introduces specific design features of a proposed chalcogenide photonic crystal fiber for ultra-wideband supercontinuum generation in the mid-infrared region. The fiber is optimized to include a circular photonic crystal lattice having tremendous potential in the fields of spectroscopy, food quality control, pulse compression, gas sensing and various nonlinear applications. By tailoring the zero-dispersion wavelength up to 2.0 μm and 2.5 μm, we have reached supercontinuum generation in the anomalous dispersion regime with the entire design hinging upon fibers based on two types of chalcogenide glass - arsenic-selenide and arsenic-sulfide, where supercontinuum broadening from 1.2 μm to 9.3 μm is made possible.

This paper reports on our recent progress with lead-silicate suspended-core microstructured optical fibers (SC- MOFs) for the detection of liquids. Simulations for SC-MOFs with one micrometer core diameter and of two different glass compositions with refractive indices of 1.67 and 1.89 at 1550 nm are discussed. Mode-field area study is performed for liquid analytes within refractive index range of 1.65-1.75. Experimental results of evanescent-wave refractometry are presented, with an impact study of uneven analyte-filling impact on the sensor performance. Studied SC-MOFs can find their application as monitoring sensors of various liquid quality especially when spectroscopic approach is pursued.

We present theoretical and experimental results for the fiber optic refractometric sensor employing a semiellipsoidal sensing element made of polymethyl methacrylate. The double internal reflection of light inside the element provides sensitivity to the refractive index of the external analyte. We demonstrate that the developed sensor, operating at a wavelength of 632 nm, is capable of measurement within a wide range of refractive indices from n=1.00 to n=1.47 with sensitivity over 500 dB/RIU. A comparison of the developed sensor with two more complex refractometric sensors, one based on tapered optical fiber and the other based on suspended-core microstructure optical fiber, is presented.

The paper reports our progress on soft-glass fibers for optical switching purposes based on four-wave mixing. Specific development of connectorized nonlinear modules for switching application is presented. For the arsenic-selenide fiber we present a novel solid joint technology, with connection losses of only 0.25 dB, which is the lowest value presented up-to-date. We have carried out conversion efficiency simulations, conversion efficiency of -16.1 dB was obtained with arsenic-selenide fiber of length reduced to 5 m. Finally experimental tests are included employing our developed optical switch testbed. Measurement results with a 26 m and a 4.5 m long arsenic-selenide nonlinear modules are presented.

Tapered optical fibers were prepared by heating of a section of the single-mode optical fiber using the tapering apparatus equipped by hydrogen-oxygen flame. Prepared samples of taper waist diameter of 6.1 μm and 3.4 μm were characterized and used as sensors working on refractometric principle of detection at a wavelength of 1550 nm. The repeatability of sensitivity measurement for selected liquids (hydrocarbons) was successfully tested with maximum of deviation of 0.1 dB from the mean value. The samples returned to baseline after purifying very well. Maximum of deviation of 0.05 dB from the mean value was calculated from measured data. OCIS codes: 280.0280

We report development of suspended-core silica and lead-silicate microstructured optical fibers for detection of liquids and supercontinuum generation. Theoretical analysis of effective mode area, dispersion curve, nonlinear coefficient and mode-field overlap is presented. For a specific lead-silicate fiber we determinated the zero dispersion wavelength at 1113 nm with a nonlinear coefficient of 1321 W-1km-1. For detection of liquids both silica and lead-silicate fibers are found to be suitable in different refractive index ranges 1.38-1.44 and 1.68-1.74 respectively. Coupling efficiency into all studied fibers over 40%; fiber attenuation was measured by the cut-back technique and is approximately 2 dB/m for silica glass fibers and over 3 dB/m for lead-silicate fibers.

Relay-assisted free space optics (FSO) communications becomes one of the promising solutions to improve the FSO link capabilities by implicitly reducing the transmission distance and exploiting distance-dependent fading variance of turbulence. Highly motivated by the capabilities of the system, this paper presents bit error rate (BER) performance of an all-optical 10-Gbps FSO relay based system using amplify-andforward signaling is investigated through numerical simulations and experimental implementation. Results show that BER improves up to several orders of magnitudes when using relay based links over the same turbulent distance.

This Letter presents original measurement results from an all-optical 10 Gbit/s free-space optics (FSO) relay link involving two FSO links and an all-optical switch. Considering the fact that reported analyses of relay links are dominated by analytical findings, the experimental results represent a vital resource for evaluating the performance of relay FSO links in the presence of atmospheric turbulence. Bit-error-rate (BER) performance of the relay system is tested for single and dual-hop links under several turbulence regimes. Furthermore, results from this measurement are used to ascertain real parameters of the outdoor links and to improve the accuracy of simulation results. Results show that using a dual-hop FSO link against a single FSO link could result in up to four orders of magnitude improvement in BER in the presence of atmospheric turbulence.

Liquid filling into silica suspended-core microstructured optical fiber capillaries is studied for the application as a liquid sensor. Liquid analytes with various refractive index are theoretically and experimentally compared with respect to their density, viscosity and repeatable measurement. Maximum filling lengths are derived and their dependence on filling time is analyzed. Degradation effects on suspended-core microstructured optical fiber are observed for particular liquid analytes.

This paper describes the refractometric detection of liquids based on silica multimode optical fibers which were tapered to increase the evanescent-wave overlap for higher sensor sensitivity. By precisely monitoring the production process, consistent sample parameters were achieved. More than 200 tapers with a taper waist diameter range from 6.0 to 76.3 μm were prepared from polymer-clad silica and gradient-index multimode fibers. U-shaped fiber taper sensitivities were analytically compared with straight tapers with resulting intensity sensitivities of over 200%/RIU. Crucial parameters for real sensor applications, such as measurement repeatability, reproducibility, and long-term stability, were further studied for polymer-clad silica straight tapers. Longterm stability was monitored showing stable measurement results over a 6 months long interval. Measurement repeatability and reproducibility with standard deviations of 0.55%/RIU and 2.26%/RIU, respectively, were achieved.

This paper presents results from the development of soft-glass fiber modules for near- and mid-infrared applications. They can be utilized especially in applications such as high-power lasers, supercontinuum generation and spectroscopy. The technology of fiber processing, preparation and connectorization has been mastered in cooperation with SQS Fiber optics in the frame of joint cooperation in several co-running projects; the high-power lasers and supercontinuum generation is prepared with the IPE Academy of Sciences of the Czech Republic. We have studied soft-glass fibers including chalcogenide arsenic-selenide, lead-silicate and zirconium-fluoride glass materials. Crucial optical signal factors for specific purposes such as operational wavelength, insertion-loss on the soft-glass-silica-glass boundaries, back-reflected optical power reduction to prevent lasing, power-induced heat generation issues and mode-field mismatch must be taken into account. Technological aspects then include bending radii, glass fragilities and glass toxicities. Results from laboratory measurements will be shown to verify the theoretical dispersion calculation. Results from technological tests of chalcogenide arsenic-selenide fiber module, lead-silicate microstructured fiber free-space optic coupling and zirconium-flouride glass processing challenges will be as well presented more in detail.

A silica suspended-core microstructured optical fiber sensor for detection of liquids, operating at 1550 nm, is analyzed. The sensing principle is based on the evanescent wave overlap into a tested analyte, which is filled via capillary forces into the cladding holes. Validations for analytes in the refractive index range of 1.35–1.43 are carried out with liquid-analyte-filling-length limits being studied both theoretically and experimentally. We prove, for the first time to our knowledge, that an extreme sensitivity of 342.86 dB/RIU and resolution of 4.4 × 10−5 can be achieved. This sensor represents a high-quality alternative for applications requiring a facile, low-cost solution.

We have developed a data transparent optical packet switch, working at optical powers conventional in telecommunications. Polarization sensitivity of only 0.6 dB was achieved. The optical packet switch exploited wavelength conversion based on four-wave mixing. To achieve higher switching efficiencies, Ge-doped silica suspended-core and chalcogenide arsenic-selenide singlemode fibers were employed and compared to conventional highly-nonlinear fibers. Improved connectorization technology has been developed for both fibers to reduce overall component insertion loss, where we achieved connection losses of 0.9 dB. For the arsenic-selenide fiber we present the lowest attenuation up-to-date of 0.58 dB/m. Pump power limits for optical packet switching were set to 23 dBm, as limited by most thin-film components. Conversion efficiency of -16 dB was obtained for the highly-nonlinear fiber utilizing pump peak powers of 16 dBm. Arsenic-selenide fiber of 26 m length was evaluated, providing conversion efficiency lower than -38 dB with component insertion loss of -16.5 dB, thus further optimization was carried out according to simulated results. Conversion efficiency of -20 dB was observed with arsenic-selenide fiber length reduced to one meter.

Polymer-clad silica fiber tapers for refractometric detection of liquids are presented. Experimental study of sensor stability and measurement reproducibility is carried out, providing stable sensitivities with respect to production imperfections.

This paper presents final results from an optical packet switch developed in a joint project of CTU in Prague, SQS Fiber optics, Czech Republic and the Academy of Sciences of the Czech Republic. Conventional highly nonlinear fibers were theoretically and experimentally evaluated and confronted with novel extremely nonlinear soft-glass nonlinear fibers and microstructured fibers developed in the framework of the project. Fiber characterization was carried out with respect to nonlinearity, attenuation and stimulated Brillouin scattering threshold limitations. Nonlinear switching technique based on four-wave mixing was evaluated with respect to switching parameters such as modulation format insensitivity, polarization insensitivity and conversion efficiency. BER tests were performed at 10Gbps, verifying optical packet switch functionality with further simulations for advanced modulation formats.

Fiber taper refractometric sensors for detection of liquids are presented. Almost one hundred tapers of a wide waist diameter range were reproducibly prepared from multimode and polymer-clad silica fibers. Sensitivity above 400%/RIU was achieved.

All-optical networks introduce the indisputable solution for future networking especially due to discarding slow and power-demanding electronic processing. Ultrafast data communications ultimately head towards all-optical packet data transfer and optical IP routing. In this paper, nonlinear switching techniques are evaluated with respects to particular types of nonlinear fibers with regards to their specific parameters. Optical packet labeling is discussed, considering label allocation between ITU-grid defined data wavelengths. Possible aspects of a highly-nonlinear fiber, untapered and tapered chalcogenide fibers integration into the optical switch were investigated both theoretically and experimentally.

Future of data networks indisputably stands in all-optical processing, components and communication. Ultrafast data transfers of enormous capacity head towards IP-routed optical packet networks. This paper focuses on the routing process and the switching matrix with respect to novel modulation formats appearing in optical data communication. Four-wave mixing switching technique is evaluated to achieve packet switching insensitive to data speed, modulation format and state of polarization. Best up to date peak-to-peak ratio for the routed signal in respect to the original was achieved exploiting the four-wave mixing effect in a highly-nonlinear fiber, with 12dB peak-to-peak ratio, whereas obtaining flat conversion profile from 1535 to 1565nm. Application of chalcogenide fibers promises even better results according to simulation models and achieved chalcogenide taper parameters. Frequency plan is presented proposing two variants with more possible outputs in a high-cost model and vice-versa.

Development of a novel optical packet switch for future all-optical packet switching, IP routing and highly-sophisticated label formats is presented. Chalcogenide-based nonlinear fabrics were analyzed for utilization in the proposed switching configuration. Particular results from simulations and measurements during research of the optical packet switch will be introduced. The future essential research challenges of such a system development are discussed as well.

Novel optical packet switch for future all-optical packet switched networks and IP routing is presented. Chalcogenide-based nonlinear glass fabrics are evaluated for the proposed switching configuration. Particular research results of the optical packet switch development will be introduced. Future major research challenges are discussed as well, focusing on sophisticated labeling techniques.

The exploitation of particular nonlinear effects in optical materials often requires higher light densities. One of attractive solutions stands in concentrating light into ultra-short pulses by utilizing fiber lasers either with active or passive mode-locking. A fiber laser was developed for the wavelengths close to the upper edge of the telecommunication C-band (around 1565 nm). The pulse regime of the fiber laser in the scale of picoseconds was achieved by the nonlinear polarization rotation effect. The proper polarization states in the ring cavity were adjusted by polarization controllers. Various laser regimes were investigated.

Broadband amplification has recently drawn attention, as higher data bit rates are to be propagated and the C-band capacity is limited. Therefore a simulation program for hybrid amplifier combining parametric and Raman effects was developed. Femtosecond passively mode locked laser was created to provide signal source for experimental part. Various delay-lines were studied to provide single-shot measurement techniques.

Several approaches of ultra-short optical pulse replication in a fiber are analyzed in the paper. Aspects of one pulse replication as well as a pulse-pair replication are discussed more in detail. Based on measurement and simulation results the specifics for replicator utilization in the regenerative resonator configuration are proposed.

An all-optical single-shot sampling oscilloscope with a picosecond resolution is developed. An innovative approach for data pulse replication using a polarization-maintaining resonator is employed. Pulses are sampled in a highly nonlinear fiber. Acquired data are used for pulse shape reconstruction. Proposed setup eliminates the need of a delay line.