Bismuth-doped Fibre Amplifiers for Multi-band Optical Networks

Fibre-optic networks are the backbone of the global communications infrastructure that made possible modern Internet, providing a multitude of online services and a digital economy. The development of novel approaches for further increasing capacity of optical communication systems is in the focus of research around the world due to the constantly growing data traffic and the corresponding bandwidth demand. Arguably, the most practical technique is multi-band transmission which utilises a huge spectral bandwidth of the existing fibre base that has not previously been used. Unlike spatial division multiplexing, multi-band transmission does not require a new fibre deployment. However, it involves a significant upgrade of current networks with novel amplifiers in the O-, E-, S-, and U- optical bands that are yet to be developed and optimised. In this thesis, E- and S-band bismuth-doped fibre amplifiers (BDFAs) are demonstrated. The following record characteristics of BDFAs are achieved: 40 dB gain, 4.5 dB noise figure, and 38% power conversion efficiency. In total, three BDFAs have been developed, characterised and optimised using pump laser diodes at different wavelengths. Two modelling techniques of BDFAs are proposed: one based on conventional rate equations, and another one based on a neural network "black box" approach. Both of these methods are analysed and their challenges are discussed. A big part of the thesis is devoted to data transmission demonstrations supported by developed BDFAs in E- and S-bands. The experiments include both IM/DD and coherent signal transmissions through various lengths of single mode fibre including record E-band transmission through 160 km of single mode fibre. In addition, a multi-band transmission experiment in E-, S-, C-, and L-band is performed with an in-line amplifier based on combined bismuth-doped fibre and discrete Raman amplification. The total signal bandwidth is 195 nm and the total number of transmitted channels is 143. The obtained results pave the way towards commercial implementation of multi-band transmission enabled by BDFAs in E- and S- optical communication bands

Intensity-only-measurement mode decomposition in few-mode fibers

Recovery of optical phases using direct intensity detection methods is an ill-posed problem and some prior information is required to regularize it. In the case of multi-mode fibers, the known structure of eigenmodes is used to recover optical field and find mode decomposition by measuring intensity distribution. Here we demonstrate numerically and experimentally a mode decomposition technique that outperforms the fastest previously published method in terms of the number of modes while showing the same decomposition speed. This technique improves signal-to-noise ratio by 10 dB for a 3-mode fiber and by 7.5 dB for a 5-mode fiber

Numerical model of hybrid mode-locked Tm-doped all-fibre laser

Abstract: Ultrafast Tm-doped fibre lasers have been actively studied for the last decade due to their potential applications in precise mid-IR spectroscopy, LIDARs, material processing and more. The majority of research papers is devoted to the comparison between a numerical modelling and experimental results; however, little attention is being paid to the comprehensive description of the mathematical models and parameters of the active and passive components forming cavities of Tm-doped all-fibre lasers. Thus, here we report a numerical model of a stretched-pulsed Tm-doped fibre laser with hybrid mode-locking and compare it with experimental results. The key feature of the developed numerical model is employment of the experimentally measured dispersion coefficients and optimisation of some model parameters, such as the bandwidth of the spectral filter spectral filtering and the saturation power of the active fibre, for a conformity with the experiment. The developed laser emits 331.7 fs pulses with a 23.8 MHz repetition rate, 6 mW of average power, 0.25 nJ of pulse energy, and a 21.66 nm spectral bandwidth at a peak wavelength of 1899.5 nm. The numerical model characteristics coincide with experimentally achieved spectral width, pulse duration, and average power with inaccuracy of 4.7%, 5.4%, and 22.9%, respectively. Moreover, in the discussion of the work the main possible reasons influencing this inaccuracy are highlighted. Elimination of those factors might allow to increase accuracy even more. We show that numerical model has a good agreement with the experiment and can be used for development of ultrafast Tm-doped fibre laser systems

Ultra-wideband discrete Raman amplifier optimization for single-span S-C-L-band coherent transmission systems

We experimentally compare the performance of two key ultra-wideband discrete Raman amplifier structures, a cascaded dual-stage structure and an in-parallel dual-band structure, in fully loaded S-C-L band coherent transmission systems over 70 km of single-mode fiber. Our results show that dual-band discrete Raman amplifier with minimized backreflections can effectively avoid unstable random distributed feedback lasing, reduce the noise figure, and therefore improve the transmission performance for signals at shorter wavelengths, versus the cascaded dual-stage structure. The average noise figure for S-band signals is 6.8 dB and 7.2 dB for the dual-band structure and cascaded dual-stage structure, respectively, while the average S-band Q2 factor is similarly improved by 0.6 dB. Moreover, the cascaded dual-stage discrete Raman amplifier requires guard bands around the 1485-nm and 1508-nm pumps as the signal and pump wavelengths overlap, which results in a bandwidth loss of ∼10 nm and reduces the potential net data throughput to 28.6 Tb/s for 30-GBaud DP-16QAM signals. However, the dual-band structure can utilize the bandwidth more effectively, which leads to a higher estimated net data throughput of 31.2 Tb/s

Octave-Spanning Supercontinuum Generation in As2S3–Silica Hybrid Waveguides Pumped by Thulium-Doped Fiber Laser

Broadband supercontinuum sources are of interest for various applications. The near-infrared region (1–3 μ m) is specifically useful for biomedical diagnostics. One of the promising media for supercontinuum generation in the infrared region is the strongly guiding nonlinear waveguide with an arsenic trisulfide core (As 2 S 3 ) and a fused silica cladding. The geometrical and chemical properties of such a waveguide allow to finely tune the dispersion landscape and nonlinearity through the core diameter variations. Here we report the generation of octave-spanning supercontinuum in As 2 S 3 -silica hybrid nanospike waveguides pumped by a thulium-doped all-fiber femtosecond laser and amplifier system at 1.9 μ m wavelength. The widest supercontinuum was obtained in the wavelength range from 1.1 to 2.5 μ m (full width at -10 dB) in the waveguide with a core diameter of 1.7 μ m. Generation of significant dispersive waves, as well as third harmonics component, is observed. Numerical simulation shows that the generated supercontinua are coherent in the entire spectral range and can be exploited to create a self-referenced laser comb

Multi-band Transmission over E-, S-, C- and L-band with a Hybrid Raman Amplifier

Capacity enhancement by utilising the unused spectral bands of the low-loss optical window of standard single-mode fibre (SSMF) is a cost-effective solution for meeting the future demand of data traffic. The development of optical amplifiers that can operate in different spectral bands is expected to play an integral part in the establishment of multi-band networks. In this work, we perform experimental, analytical and numerical modelling of a multi-band transmission system using a hybrid distributed-discrete Raman amplifier enabling signal amplification from 1410-1605 nm. The developed amplifier was tested over 50km of SSMF using 200 Gbit/s channels, where successful transmission was achieved, well above the HD-FEC threshold of 8.5 dB. Further study on the multi-band transmission performance was carried out using a semi-analytical closed-form approximation and split-step Fourier method-based simulations for various related test cases. The analytical and numerical models are also compared with experimental results, showing reasonable agreement in terms of system performance estimation

Bismuth doped fibre amplifier operating in Eand S- optical bands

Bismuth-doped fibre amplifiers offer an attractive solution for expanding the bandwidth of fibre-optic telecommunication systems beyond the current C-band (1530-1565 nm). We report a bismuth-doped fibre amplifier in the spectral range from 1370 to 1490 nm, with a maximum gain exceeding 31 dB, and a noise figure as low as 4.75 dB. The developed system is studied for forward, backward, and bi-directional pumping schemes and three different signal power levels. The forward pumping scheme demonstrates the best performance in terms of the achieved noise figure. The developed amplifier can be potentially used as an in-line amplifier with >20dB gain in the spectral band from 1405 to 1460 nm

E-band Telecom-Compatible 40 dB Gain High-Power Bismuth-doped Fiber Amplifier with Record Power Conversion Efficiency

Multi-band transmission is one of the key practical solutions to cope with the continuously growing demand on the capacity of optical communication networks without changing the huge existing fiber base. However, ultra-broadband communication requires the development of novel power efficient optical amplifiers operating beyond C- and L-bands, and this is a major research and technical challenge comparable to the introduction of the seminal erbium-doped fiber amplifiers that dramatically changed the optical communication sector. There are several types of optical fibers operating beyond C- and L-bands that can be used for the development of such amplifiers, specifically the fibers doped with neodymium, praseodymium, thulium, and bismuth. However, among these, Bi-doped fibers are of special interest as the most promising amplification medium because, unlike the others, different Bi-associated active centers allow amplification in an enormous band of overall width of 700 nm (1100–1800 nm). Such spectral coverage can be obtained by using different host materials, such as aluminosilicate, phosphosilicate, silica, and germanosilicate glasses. Here, we report a novel Bi-doped fiber amplifier with record characteristics for E-band amplification, including the highest power conversion efficiency among telecom-compatible E-band amplifiers reported to date. This bismuth-doped fiber amplifier (BDFA) features a maximum gain of 39.8 dB and a minimal noise figure of 4.6 dB enabled by 173 m Bi-doped fiber length. The maximum achieved power conversion efficiency of 38% is higher than that of L-band Er-doped fiber amplifiers. This performance demonstrates the high potential of BDFA for becoming the amplifier of choice in modern multi-band optical communication networks

30-GBaud DP 16-QAM transmission in the E-band enabled by bismuth-doped fiber amplifiers

We report the transmission of five 30-GBaud dual polarization 16-QAM signals over 160 km of standard single-mode fiber in the E-band (1410–1460 nm). The transmission line consists of two 80-km spans and three independent bismuth -doped fiber amplifiers. The developed amplifiers feature a maximum gain of 27.3 dB, 33.8 dB, and 28.3 dB with a minimum noise figure of 4.8 dB, 4.7 dB, and 5.3 dB, respectively. The maximum signal Q2 factor penalty is 4.5 dB, and the overall performance of the system is above the pre-forward-error-correction (FEC) threshold for a 10−15 post-FEC bit error rate. To the best of our knowledge, this is the record experimentally demonstrated transmission length for a coherent detection signal in the E-ban

Experimental comparison of E-band BDFA and Raman amplifier performance over 50 km G.652.D fiber using 30 GBaud DP-16-QAM and DP-64-QAM signals

We compare the performance of three optical amplifiers in the E-band: a bismuth-doped fiber amplifier (BDFA), a distributed Raman amplifier, and a discrete Raman amplifier (RA). Data transmission performance of 30 GBaud DP-16QAM and DP-64-QAM signals transmitted over 50 km of G.652.D fiber is compared in terms of achieved signal-to-noise (SNR). In this specific case of relatively short distance, single-span transmission, the BDFA outperforms the distributed and discrete Raman amplifiers due to the impact of fiber nonlinear penalties at high input signal powers