Semiconductor Optical Amplifier Gain Research Articles

Accelerating gain and phase recovery in Quantum-Dot reflective Semiconductor optical Amplifier: Unraveling the influence of inhomogeneous broadening and pumping power for enhanced performance

This investigation examines the complex interaction between inhomogeneous broadening (IHB) and pumping power concerning the gain and phase recovery dynamics of Quantum-Dot (QD) Reflective Semiconductor Optical Amplifiers (RSOAs). An improved QD-RSOA model, employing coupled rate equations and incorporating IHB effects and amplified spontaneous emission is used to model and compare ultrafast gain and phase recovery responses in QD-SOAs and QD-RSOAs for an electrical and two single-color optical pumping schemes. Analysis of the contributions from QD states and Quantum Well (QW) carrier reservoirs to phase changes provides valuable insights. A reduction in IHB leads to an accelerated phase response by diminishing the slow phase recovery components for the QD ground and excited states, and so reducing the contributions from slow phase recovery in carrier reservoirs. Across all pumping schemes, the QD-RSOA exhibits more pronounced phase recovery acceleration. Despite achieving ultrafast gain and phase recovery responses observed in QD-RSOAs when subjected to electrical pumping, a significant overshoot in the phase recovery response is observed. This overshoot leads to a potentially significant chirping and distortion in amplified optical pulses. The study suggests that in long QD-SOAs and QD-RSOAs with higher gain, optical pumping schemes offer a promising strategy for achieving phase recovery acceleration without introducing distortion, which is important for applications of QD-SOAs.

High Dynamic Range 100G PON Enabled by SOA Preamplifier and Recurrent Neural Networks

In recent years the PON research community has focused on future systems targeting 100 Gb/s/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> and beyond, with digital signal processing seen as a key enabling technology. Spectrally efficient 4-level pulse amplitude modulation (PAM4) is seen as a cost-effective solution that exploits the ready availability of cheaper, low-bandwidth devices, and Semiconductor Optical Amplifiers (SOA) are being investigated as receiver preamplifiers to compensate PAM4's high signal-to-noise ratio requirements and meet the demanding 29 dB PON loss budget. However, SOA gain saturation-induced patterning distortion is a concern in the context of PON burst-mode signalling, and the 19.5 dB loud-soft packet dynamic range expected by the most recent ITU-T 50G standards. In this paper we propose a recurrent neural network equalisation technique based on gated recurrent units (GRU-RNN) to not only mitigate SOA patterning affecting loud packet bursts, but to also exploit their remarkable effectiveness at compensating non-linear impairments to unlock the SOA gain saturated regime. Using such an equaliser we demonstrate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &gt; 28$</tex-math></inline-formula> dB system dynamic range in 100 Gb/s PAM4 system by using SOA gain compression in conjunction with GRU-RNN equalisation. We find that our proposed GRU-RNN has similar equalisation capabilities as non-linear Volterra, fully connected neural network, and long short-term memory based equalisers, but observe that feedback-based RNN equalisers are more suited to the varying levels of impairment inherent to PON burst-mode signalling due to their low input tap requirements. Recognising issues surrounding hardware implementation of RNNs, we investigate a multi-symbol equalisation scheme to lower the feedback latency requirements of our proposed GRU-RNN. Finally, we compare equaliser complexities and performances according to trainable parameters and real valued multiplication operations, finding that the proposed GRU-RNN equaliser is more efficient than those based on Volterra, fully connected neural networks or long short-term memory units proposed elsewhere.

Wideband SOA fiber-to-fiber gain and saturation output power in the C-band: impact of characteristic parameters

Semiconductor Optical Amplifiers (SOAs) have numerous attractive characteristics; for example: minimum power usage, broadband, flexible wavelength, wide dynamic range and significant nonlinearities. In this study, we present a comprehensive model of output saturation power and fiber-to-fiber gain of wideband SOAs. Furthermore, we investigate the impact of optical confinement factor, bias current, input signal power, temperature, and amplifier length on the fiber-to-fiber gain spectrum in the C-band. The obtained numerical results are approved by comparison with simulation results showing a fair agreement. The results reveal that output saturation power is improved by decreasing the optical confinement factor. Additionally, when input signal power rises, the maximum fiber-to-fiber gain value declines. Furthermore, as the temperature rises, the fiber-to-fiber gain decreases. Moreover, the maximum fiber-to-fiber gain is obtained at shorter amplifier lengths. Additionally, the effect of carrier density on the spectrum of coefficient of material gain is investigated, showing an increase in its value when increasing the carrier density, with a peak near 1550 nm.

Brillouin Amplifier Noise Limiting With a Semiconductor Optical Amplifier for Coherent Communication Applications

Suppression of Brillouin amplifier intensity noise is demonstrated by use of a semiconductor optical amplifier (SOA) at its output to enhance application of its narrowband gain for boosting the carrier to noise ratio (CNR) of spectral lines in coherent communications. This utilizes the fast recovery time of the SOA gain in saturation to limit distortions induced when Brillouin gain is reduced from its maximum by detuning of the input frequency or state of polarization relative to the backward propagating pump. Although the SOA carrier dynamics associated with the varying gain also converts intensity noise to phase distortion, the similarly low bandwidth in the tens of megahertz range as for Brillouin gain permits its removal in communication applications by the same digital signal processing used to compensate the similarly slow phase error from coherent detection with a local oscillator laser. When applied to a spectral line with initial poor CNR of 19.5 dB/0.1 nm that is then boosted by Brillouin gain but at detuned input frequency of ∼±10 MHz from optimum (or 67% of a 30 MHz gain bandwidth), the SOA improves use of the SBA output as a carrier of 72 Gb/s 64-QAM signals with up to 2 dB higher <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -factor at the receiver. Overall, SBA's benefit for noise suppression in combination with the SOA is extended to more flexible and wider ranging input parameters.

Conventional/linear/Lorentzian material gain semiconductor optical amplifiers performance signature with four wave mixing (FWM) nonlinearity in optical fiber communication systems

Abstract This study demonstrates the FWM nonlinearity effects on fiber systems based on conventional/linear/Lorentzian material gain wide band traveling wave semiconductor optical amplifiers (WBTWSOAs) at data rates of 10 Gbps. Max signal power (MSP) and noise power (NP) levels are illustrated versus time and SW after WDM multiplexer. The MSP and min NP are clarified against time and SW based conventional, linear, Lorentzian material gain WBTWSOAs. The total optical power after WDM multiplexer, and the total optical power based conventional, linear, Lorentzian material gain WBTWSOAs are measured. Max Q versus time after receiver based WDM multiplexer, and max Q versus time after receiver based conventional, linear, Lorentzian material gain WBTWSOAs are reported clearly in this study. Besides the max Q against CS is studied with/without FWM effects. Nonlinear coefficient, conversion efficiency is analyzed clearly against fiber length without/with FWM effects.

320 Gb/s all-optical logic operations based on two-photon absorption in carrier reservoir semiconductor optical amplifiers

When a train of optical pulses is injected, two-photon absorption (TPA)-induced pumping results in significant and fast gain and phase shifts in the carrier reservoir semiconductor optical amplifier (CR-SOA). Therefore, in this work, the effect of TPA on all-optical (AO) exclusive-OR (XOR), AND, NOT-OR (NOR), OR, NOT-AND (NAND), and exclusive-NOR (XNOR) logic operations using CR-SOAs is theoretically implemented for the first time at 320 Gb/s. The XOR, AND, NOR, NAND, and XNOR logic gates are implemented using the Mach-Zehnder interferometer, which has two symmetrical CR-SOAs in each of its two arms, whereas the OR logic gate is produced by combining a CR-SOA with a delayed interferometer. The quality factor (QF) and the associated bit error rate (BER) metrics are employed to evaluate the operations’ performance. The obtained simulation results indicate that when exploiting TPA in CR-SOAs the target Boolean functions can be executed with QF and BER that are more than acceptable than without TPA at a high speed up to 320 Gb/s. Various circuits of enhanced logic functionality, such as AO half-adder, latches, comparator, half-subtractor, and encoder can be implemented by combing the presented AO gates and hence exploiting the ultrafast TPA-induced gain and phase changes in CR-SOAs. Furthermore, the potential of CR-SOAs as nonlinear elements is systematically and thoroughly established for a wide suite of AO logic operations.

SOA Assisted Wavelength Reusing for 25G Colorless PON With Low-Cost 10G EAM

This paper demonstrates a C-band 25G colorless passive optical network (PON) based on semiconductor optical amplifier (SOA) and 10G electro-absorption modulator (EAM). The experimental results show that the SOA in the optical network unit (ONU) can erase the downlink signal by taking advantage of the gain saturation feature, realizing the reuse of downstream wavelength. Meanwhile, the SOA gain can compensate for the EAM loss by optimizing the current and injection power. Apart from this, equalization algorithms at the optical line terminal are investigated for linear and nonlinear impairments compensation. Besides, the system robustness to the ASE and the maximum transmission speed are also evaluated. Finally, the system capacity can reach up to 25 Gbps with NRZ format at a bit-error ratio threshold of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> , proving that this system is a promising technology for future high-speed colorless 2 × 25G or 4 × 25G Wavelength division Multiplexing (WDM) PON system.

Weight adjustable photonic synapse by nonlinear gain in a vertical cavity semiconductor optical amplifier

In this paper, we report a high-speed and tunable photonic synaptic element based on a vertical cavity semiconductor optical amplifier (VCSOA) operating with short (150 ps-long) and low-energy (μW peak power) light pulses. By exploiting nonlinear gain properties of VCSOAs when subject to external optical injection, our system permits full weight tunability of sub-ns input light pulses, just by varying the VCSOA's applied bias current. Not only is the VCSOA-based synapse able to adjust the strength of incoming optical pulses, but it can also provide gain (applied weight factors &amp;gt;1). Moreover, we show that this simple approach permits dynamical weight tuning at high-speed (ns rates) with up to an 11.6-bit precision. These results are realized with commercially sourced, inexpensive vertical cavity surface emitting lasers, operating at the key telecom wavelengths of 1300 and 1550 nm and hence making our approach compatible with optical network and data center technologies. This VCSOA-based system, therefore, offers a hardware friendly, low-energy, and high-speed solution for photonic synaptic links with high potential for use in future neuromorphic photonic systems.

Small-Signal Analysis in Two Calculation Sections and Experimental Validation of Up-Converted Coherent Population Oscillations in Semiconductor Optical Amplifiers

This paper presents theoretical and experimental analysis of up-converted coherent population oscillations (Up-CPO) in semiconductor optical amplifiers (SOAs) for tunable optical phase shift applications at high frequencies. The study of Up-CPO frequency response is carried out by a small-signal analysis of a SOA modeled by two calculation sections in order to better take into account the nonlinear effects such as the SOA gain saturation. We demonstrate that we can fully determine the dynamic parameters of Up-CPO from the experimental frequency response of an optical probe signal obtained by cross-gain modulation mechanism. The Up-CPO frequency responses are measured for three SOAs with different cut-off frequencies up to 18.45 GHz. We finally validate experimentally the obtained analytical frequency responses for the three SOAs over different operating points, each characterized by the corresponding optical gain.

Intensity pattern types in broadband Fourier domain mode-locked (FDML) lasers operating beyond the ultra-stable regime

We report on the formation of various intensity pattern types in detuned Fourier domain mode-locked (FDML) lasers and identify the corresponding operating conditions. Such patterns are a result of the complex laser dynamics and serve as an ideal tool for the study of the underlying physical processes as well as for model verification. By numerical simulation we deduce that the formation of patterns is related to the spectral position of the instantaneous laser lineshape with respect to the transmission window of the swept bandpass filter. The spectral properties of the lineshape are determined by a long-term accumulation of phase-offsets, resulting in rapid high-amplitude intensity fluctuations in the time domain due to the narrow intra-cavity bandpass filter and the fast response time of the semiconductor optical amplifier gain medium. Furthermore, we present the distribution of the duration of dips in the intensity trace by running the laser in the regime in which dominantly dips form, and give insight into their evolution over a large number of roundtrips.

Hybrid dual-gain tunable integrated InP-Si3N4 external cavity laser.

We present a hybrid dual-gain integrated external cavity laser with full C-band wavelength tunability. Two parallel reflective semiconductor optical amplifier gain channels are combined by a Y-branch in the Si3N4 photonic circuit to increase the optical gain. A Vernier ring filter is integrated in the Si3N4 photonic circuit to select a single longitudinal mode and meanwhile reduce the laser linewidth. The side-mode suppression ratio is ∼67 dB with a pump current of 75 mA. The linewidth of the unpackaged laser is 6.6 kHz under on-chip output power of 23.5 mW. The dual-gain operation of the laser gives higher output power and narrower linewidth compared to the single gain operation. It is promising for applications in optical communications and light detection and ranging systems.

MEMS-SOA Spectrum-Sliced Auto-Equalized Source Enabling Uniformly Tunable Microwave Photonic Filter

In this work, we report a Micro-Electro-Mechanical System (MEMS)-Semiconductor Optical Amplifier (SOA) active interferometer as a low cost and miniaturized tunable broadband sliced source and optical loss auto-equalizer for single pass band microwave photonic filter (MPF). A low finesse Fabry-Perot MEMS interferometer is formed from a movable deeply etched micro-mirror in front of a cleaved fiber facet. The interferometer slices the amplified spontaneous emission (ASE) output of the SOA, while the output is feedback to the SOA again for amplification and optical loss auto-equalization through the SOA gain saturation effect. This configuration improved the uniformity of the MPF passband attenuation along the tuning range down to 3.72 dB only. The filter center frequency can be finely tuned electrically for ±241.6 MHz with a MEMS electrostatic comb actuator driving the mirror. The center frequency is controlled by the distance between fiber facet and micro-mirror. The operation of the proposed MEMS-SOA based MPF is experimentally verified as a proof of concept from 1.2 to 4.5 GHz and can be easily extended to higher frequencies. A tuning resolution of 6.04 MHz/ $\mu \text{m}$ is achieved with stopband attenuation larger than 20 dB and 3-dB bandwidth mean value of 141 MHz.

Scalable SiPh-InP Hybrid Switch Based on Low-Loss Building Blocks for Lossless Operation

We design and experimentally demonstrate scalable 2 × 2, 4 × 4 and 8 × 8 silicon photonic (SiPh) thermo-optic switch exhibiting low loss, low crosstalk, low power penalty, and BER below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> for payload data transmission. Less than 3.13 dB insertion loss (IL) and approximately 20.5 dB crosstalk is measured in the 8 × 8 SiPh banyan switch with thermal phase shifters. We also report on a semiconductor optical amplifier (SOA) in an indium phosphide (InP) technology platform with 25 dB gain and 7 dB noise figure enabling to transmit optical signals with large OSNR. Combining SiPh and InP technologies, we propose a lossless hybrid switch matrix with distributed SOA-based gain capable of transmitting data with near zero loss and low crosstalk over a large switching matrix. In hybrid SiPh/InP switches, the SOA gain compensates for the SiPh switch loss at the cost of amplified spontaneous emission (ASE) noise but mitigated by bandpass optical filters. Lower IL from the SiPh switch requires less gain from the SOAs leading to less OSNR degradation. Experimentally validated building blocks confirmed scalability up to 64 × 64 in SiPh-InP hybrid platform.

Polarization-insensitive fiber-to-fiber gain of semiconductor optical amplifier using closely stacked InAs/GaAs quantum dots

The gain characteristics of a semiconductor optical amplifier (SOA) that contains forty layers of closely stacked InAs/GaAs quantum dots (QDs) were studied by employing a fiber-to-fiber measurement system. The fiber-to-fiber gain values obtained for transverse-electric (TE)- and transverse-magnetic (TM)-polarized optical input signals exhibited their maxima at a wavelength of 1100 nm, which corresponds to the excited-state of the closely stacked QDs. In the wavelength range from 1060 to 1170 nm, the gain obtained for TE-polarized input signal was slightly higher than that for TM-polarized input signal and the absolute gain difference was less than 1 dB for an injection current of 80 mA. The small but non-zero difference of the gain reflects the polarization anisotropy of the optical gain provided by the closely stacked QDs. Our results suggest that closely stacked QDs are suitable for realizing polarization-insensitive SOA device with operation bandwidths of more than 100 nm.

Spectroscopic Gas Sensing Based on a MEMS-SOA Swept Fiber Laser Source

In this work, we report on real-time gas sensing using a swept fiber laser source (SFLS) based on a micro-electro-mechanical-system (MEMS) tunable filter, a semiconductor optical amplifier (SOA) gain medium and a fiber ring. The MEMS filter is implemented using deeply-etched silicon-on-insulator (SOI) silicon/air Bragg mirrors attached to a micro-actuator enabling a relatively high-speed tunability and compatibility with single-mode fibers. A theoretical analysis is presented relating the required laser output power and spectral linewidth to the gas detection limits taking into account the signal-to-noise ratio (SNR). The minimum detectable concentrations of nitrous oxide, carbon dioxide, carbon monoxide and acetylene gases were determined. The dependence of the gas spectrum in terms of its absorption line strength, spectral width and spacing on the laser source was emphasized in the analysis, revealing the limits of distinguishing between different gases. The fiber-laser source is then characterized under static and dynamic conditions. An experimental parametric study was carried out varying the fiber cavity length from 20 m to 2 km, the sweep rate up to 2 kHz and the pumping current up to 6 times the threshold current. Finally, the SFLS is exploited to measure the transmission spectrum of carbon dioxide gas – as an example – at various pressures. The effects of the pumping current, the sweeping speed, and the sweeping direction on the measured spectrum were studied in order to determine the optimal conditions. The measured spectrum is compared with the theoretical one taking into consideration the FWHM of the laser source. The comparison shows a good match.

Modeling the effects of interband and intraband transitions on phase and gain stabilities of quantum dot semiconductor optical amplifiers

In this paper, the effects of interband and intraband transitions on the gain and phase stabilities in quantum dot semiconductor optical amplifier (QD-SOA) are investigated both temporally and spectrally employing electrical and optical pumping schemes. For this purpose, the carrier rate equations in different energy states coupled to the traveling wave optical field equation have been numerically solved to derive the dynamical behavior of QD-SOA. Our results show that the gain and phase response can be stabled under optical pumping (OP) scheme because the role of the interband and intraband transitions on the dynamics of QD-SOA is reduced. This behavior leads to high-speed pattern effect-free cross-phase modulation (XPM) in QD-SOA. It is found that optically pumped QD-SOA can have high performance in phase based applications. Moreover, it is shown that under OP scheme although the QD-SOA has lower gain value and slower gain recovery time, the ultrafast cross-gain modulation (XGM) without pattern effect is possible and the phase is recovered within a shorter time compared to EP scheme. The behavior arises from the different capacity of the carrier reservoir for pumping schemes.

Performance comparisons between semiconductor and fiber amplifier gain assistance in a recirculating frequency shifter.

Recirculating frequency shifter (RFS) loops can be used for electronically programmable, variable-spacing multiline spectrum generation, which can benefit the development of fully flexible optical communications and other frequency comb applications. Here, we report on and explain the observation of significant performance variations between chip-based gain in semiconductor optical amplifiers (SOA) and fiber-based gain in erbium-doped fiber amplifiers (EDFA) when used as the gain element in the RFS. Previously, SOAs and EDFAs have been demonstrated in different RFS experiments and studied separately from each other; thus, discussion mainly focused on the noise from amplified spontaneous emission. We show that SOA effects, including four wave mixing, can be measured, which impose limits to the wavelength spacing of the combs, and that this effect is mitigated by increasing the RF drive frequency of the RFS and operating SOA in deeper saturation.

High Modulated Output Power Over 9.0 dBm With 1570-nm Wavelength SOA Assisted Extended Reach EADFB Laser (AXEL)

With a view to realizing a light source for future access networks, we demonstrated a 1570-nm (L-band) wavelength semiconductor optical amplifier (SOA) assisted extended reach electroabsorption-modulater integrated with a distributed feedback (EADFB) laser (AXEL) for a high modulated light output power ( P avg) of more than 9.0 dBm. We first compared the static lasing characteristics of the AXEL with those of a conventional EADFB laser fabricated on the same wafer. We found that the light output power of the AXEL was significantly higher and the power conversion efficiency of the AXEL was more than double that of the EADFB laser. Second, we experimentally estimated the effect of the integrated SOA. Under typical driving conditions, a net SOA gain of 4.9 dB was obtained, and the excellent chirp compensation effect of the SOA was observed. Next, we investigated the AXEL driving condition in terms of power conversion efficiency and found that the AXEL with commonly driven laser diode and SOA sections approached the most efficient driving condition even when the electroabsorption modulator bias voltage was applied. Finally, the 10-Gbit/s transmission characteristic was examined. A sufficient P avg of 9.0 dBm was achieved, revealing the potential for access network applications. Furthermore, the transmission distance was successfully extended to 80 km thanks to the chirp compensation effect of the SOA.

Single-mode SOA-based 1kHz-linewidth dual-wavelength random fiber laser.

Narrow-linewidth multi-wavelength fiber lasers are of significant interests for fiber-optic sensors, spectroscopy, optical communications, and microwave generation. A novel narrow-linewidth dual-wavelength random fiber laser with single-mode operation, based on the semiconductor optical amplifier (SOA) gain, is achieved in this work for the first time, to the best of our knowledge. A simplified theoretical model is established to characterize such kind of random fiber laser. The inhomogeneous gain in SOA mitigates the mode competition significantly and alleviates the laser instability, which are frequently encountered in multi-wavelength fiber lasers with Erbium-doped fiber gain. The enhanced random distributed feedback from a 5km non-uniform fiber provides coherent feedback, acting as mode selection element to ensure single-mode operation with narrow linewidth of ~1kHz. The laser noises are also comprehensively investigated and studied, showing the improvements of the proposed random fiber laser with suppressed intensity and frequency noises.

1550 nm VCSEL-based 10 Gb/s optical NRZ signal transmission over 20 km SMF using RSOA gain saturation

Abstract A method for a 1550 nm vertical cavity surface emitting laser (VCSEL)-based 10 Gb/s optical non-return to zero (NRZ) signal transmission over 20 km single mode fiber (SMF) using reflective semiconductor optical laser amplifier (RSOA) gain saturation is presented. The optical signal distortion of 10 Gb/s optical NRZ signals caused by the inherent frequency chirp of the VCSEL and the chromatic dispersion of SMF were investigated through numerical simulations and experiments. For 20 km transmission over SMF, VCSEL-based 10 Gb/s optical NRZ signals were severely distorted and ‘1’ bit signals had different amplitudes with respect to their bit patterns. The gain saturation of RSOA can equalize the different amplitudes of ‘1’ bit signals and can mitigate the optical pulse distortion after 20 km transmission over SMF. The experimental results showed that VCSEL-based 10 Gb/s optical NRZ signals can achieve a bit error rate (BER) of 10 - 9 with a 2.4 dB dispersion penalty for 20 km transmission of SMF with RSOA gain saturation.