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-16-QAM 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.

E-, S-, C- and L-band coherent transmission with a multistage discrete Raman amplifier.

We report for the first time an ultra-wideband coherent (UWB) WDM transmission over a 70 km standard single mode fibre (SSMF) solely using a multistage discrete Raman amplifier (DRA) over the E-, S-, C- and L-bands of the optical window. The amplifier is based on a split-combine approach of spectral bands enabling signal amplification from 1410-1605 nm over an optical bandwidth of 195 nm (25.8 THz). The proposed amplifier was characterized with 143 channelized amplified spontaneous emission (ASE) dummy channels in the S-, C- and L-bands and 4 laser sources in the E-band (1410-1605 nm). The amplification results show an average gain of 14 dB and a maximum noise figure (NF) of 7.5 dB over the entire bandwidth. Coherent transmission with the proposed amplifier was performed using a 30 Gbaud PM-16-QAM channel coupled with the ASE channels over a 70 km SMF. The ultra-wideband transmission using the tailored multistage DRA shows transmission bandwidth of 195 nm with a maximum Q2 penalty of ∼4 dB in E- and S-band, and ∼2 dB in C- and L-band.

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.

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-band.

All-fiber ultrafast amplifier at 1.9 µm based on thulium-doped normal dispersion fiber and LMA fiber compressor.

The duration reduction and the peak power increase of ultrashort pulses generated by all-fiber sources at a wavelength of [Formula: see text] are urgent tasks. Finding an effective and easy way to improve these characteristics of ultrafast lasers can allow a broad implementation of wideband coherent supercontinuum sources in the mid-IR range required for various applications. As an alternative approach to sub-100 fs pulse generation, we present an ultrafast all-fiber amplifier based on a normal-dispersion germanosilicate thulium-doped active fiber and a large-mode-area silica-fiber compressor. The output pulses have the following characteristics: the central wavelength of [Formula: see text], the repetition rate of 23.8 MHz, the energy per pulse period of 25 nJ, the average power of 600 mW, and a random output polarization. The pulse intensity and phase profiles were measured via the second-harmonic-generation frequency-resolved optical gating technique for a linearly polarized pulse. The linearly polarized pulse has a duration of 71 fs and a peak power of 128.7 kW. The maximum estimated peak power for all polarizations is 220 kW. The dynamics of ultrashort-pulse propagation in the amplifier were analyzed using numerical simulations.

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.

Speed characterization of p-i-n photodiode based on GaSb/GaInAsSb/GaAlAsSb heterostructure with frontal bridge contact at 1.9 µm.

We report a study of the response function parameters (amplitude and rise/fall time) of a high-speed GaSb/GaInAsSb/GaAlAsSb photodiode operating at 1.9 µm as a function of optical input power and reverse bias voltage. The experimental measurement results yield the optimal pulse energy and optimal reverse bias voltage for the photodiode. The 44 ps minimal rise time of the response function and 3.6 GHz bandwidth are achieved under a 3 V reverse bias voltage and pulse energy in the 0.27-2.5 pJ range.

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

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.

Generation of multi-solitons and noise-like pulses in a high-powered thulium-doped all-fiber ring oscillator.

We report a study on the switching of the generation regimes in a high-powered thulium-doped all-fiber ring oscillator that is passively mode-locked with nonlinear polarization evolution technique with different pumping rates and cavity dispersion values. In one experimental setup, switching was observed between the noise-like pulse and the multi-soliton (in the forms of soliton bunches and soliton rain) regimes by the adjustment of the intracavity polarization controllers. We attributed this to the crucial influence of the nonlinear polarization evolution strength determined by such key parameters as saturation (over-rotation) power, linear phase bias, and nonlinear losses on the pulse evolution and stability. So the soliton collapse effect (leading to noise-like pulse generation) or the peak power clamping effect (generating a bunch of loosely-bound solitons) may determine pulse dynamics. Both the spectrum bandwidth and coherence time were studied for noise-like pulses by varying the cavity length and pump power, as well as the duration of solitons composing bunches. As a result, both noise-like pulses (with spectrum as broad as 32 nm bandwidth) and multi-soliton formations (with individual pulse-widths ranging from 748 to 1273 fs with a cavity length increase from 12 to 53 m) with up to 730 mW average power were generated at a wavelength of around 1.9 µm. The results are important for the realization of the broadband and smooth supercontinuum which can be used as a source for mid-IR vibrational spectroscopy of gas samples for breath analysis and environmental sensing.