Saturated Semiconductor Optical Amplifier Research Articles

Non-orthogonal PS-OOK transmission for FSO communications

This paper proposes a non-orthogonal polarization shift on-off keying (NOPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, NOPS-OOK signal is modulated with the non-orthogonal polarizations of bits ‘1’ and ‘0’ characteristics employing a single Mach-Zehnder modulator (MZM). At the receiver end, NOPS-OOK signal is detected using the cascaded polarization-insensitive and gain saturated semiconductor optical amplifier (SOA) and sloped-linear polarizer (SLP) with the polarization perpendicular to bit ‘0.’ The proposed technique is verified in experiments. The experimental results showed that the bit-error-rate (BER) performance of the proposed technique was close to the orthogonal polarization shift OOK (OPS-OOK) signal transmission.

Investigation of amplitude and phase evolution of a short-pulse propagating through a gain saturated semiconductor optical amplifier

Investigation of amplitude and phase evolution of a short-pulse propagating through a gain saturated semiconductor optical amplifier

Enhancement of SOA-Based Scintillation Mitigation by PS-OOK Transmission in FSO Communication

This paper proposes an enhancement of semiconductor optical amplifier (SOA)-based scintillation mitigation by polarization shift on-off keying (PS-OOK) transmission in free-space optical (FSO) communication. The scintillation effect is a critical issue in a vertical FSO link. The issue of the extinction ratio (ER) degradation of the received OOK signal caused by the high dynamic gain frequency of the SOA limits the performance of the turbulence-induced scintillation effect mitigation using a gain-saturated and polarization-independent SOA. Therefore, a PS-OOK signal, which has the characteristics of orthogonal polarization of bits “1” and “0,” is transmitted at the transmitter end and a linear polarizer is deployed after the gain saturated SOA at the receiver end. Based on the polarization orthogonality of PS-OOK, the performance of the scintillation-effect suppression is significantly improved by the mitigation of the ER degradation by the blocking of the polarization of bit “0” of the received optical PS-OOK signal using the linear polarizer. The proposed scheme is experimentally verified utilizing the Mach-Zehnder modulator (MZM)-based fading simulator accommodated various turbulence channels. The experimental results illustrate that the scintillation effect is effectively suppressed by the proposed technique compared to the SOA-based conventional single-polarized OOK detection and balanced PS-OOK detection.

On the Phase Noise Enhancement of a Continuous Wave in Saturated SOA Used for RIN Reduction

We report on the spectral characteristics of the phase noise enhancement of continuous-wave (CW) optical signals passing through a semiconductor optical amplifier (SOA) operating in the saturation regime and used for relative intensity noise (RIN) reduction. We show that the known effect of phase noise enhancement, attributed to the linewidth enhancement factor of the device, happens only at a limited band of the phase noise spectrum, and the actual measurable linewidth of the output CW signal may not be affected. While this phase noise enhancement is not shown as an increase of spectral linewidth, it can still affect system performance when coherent detection is used, especially in applications with relatively low symbol rates. Numerical simulations and experimental results are used to support the observation. A single spectral line from a quantum-dot mode-locked laser is used as the light source, which is known to have relatively high RIN (> −120 dB/Hz in the low frequency region). Experimental transmission of 16-QAM modulation with coherent detection has been performed at 5 GBd to assess the implication on system performance.

Time- and Frequency-Domain Characterization of the Relative Intensity Noise of a Quantum-Dot Frequency Comb Source Laser

The intensity noise of a quantum dot, coherent comb source laser is characterized using time- and frequency-domain measurements. The time-domain measurements yield the average relative intensity noise (RIN) over the measurement bandwidth, and by calculation, the correlation time and power spectral density (frequency resolved RIN). The frequency-domain measurements yield the RIN spectrum and, by calculation, the integrated RIN. The short duration (50 $\mu$ s) time-domain results exhibit a line-to-line variability in the properties of the intensity noise that is not revealed by the frequency-domain measurements. It is shown that the RIN of a comb line can be reduced by over 13 dB using a saturated semiconductor optical amplifier (SOA). The performance of a low-complexity, low-latency 40 Gb/s intensity modulation, direct detection system using a comb line as a CW source, without and with SOA-based noise suppression, is assessed in terms of the bit error ratio.

Performance enhancement of gain saturated SOA based free space optical link using dual-wavelength transmission

This paper proposes the dual-wavelength transmission technique for improving the performance of the gain-saturated semiconductor optical amplifier (SOA)-based free-space optical (FSO) communication link. When the gain saturated SOA is used to suppress turbulence-induced scintillation, extinction ratio (ER) degradation effect occurs owing to the high-dynamic gain frequency characteristics for intensity modulation and direct detection (IM/DD) system. The proposed method uses small-spacing dual-wavelength (signal and supplemental wavelengths) transmission to compensate ER degradation by mitigating the gain difference between the bits ‘1’ and ‘0’. Regarding the supplemental wavelength, continuous wave (CW) with various average powers and differential signal (DS) transmissions are experimentally investigated. The proposed technique is evaluated under various turbulence channel strengths. The experimental results show that the bit-error rate (BER) performance is significantly improved via effective ER degradation compensation and scintillation suppression.

A Low-RIN Spectrally Pure Whispering-Gallery-Mode Resonator-Based Semiconductor Laser

We demonstrate a high-spectral-purity self-injection locked distributed-feedback semiconductor laser with relative intensity noise approaching −143 dB/Hz at 1 kHz and −160 dB/Hz at 100-kHz frequency offset. The frequency noise of the laser is 2 Hz/Hz1/2 at 1 kHz and 1 Hz/Hz1/2 at 100-kHz frequency offset. The fiber-pigtailed output power exceeds 70 mW. The low-intensity-noise performance is achieved due to the utilization of a low-noise current source feeding the laser chip and a saturated semiconductor optical amplifier reducing the technical amplitude noise of the laser. Reduction in the laser frequency noise results from the self-injection locking of the laser chip via a high-quality-factor whispering-gallery-mode resonator. The laser linewidth is less than 100 Hz.

Further Investigation of Intensity Noise Reduction on an Incoherent Light Source using a Gain Saturated Semiconductor Optical Amplifier in a Spectrum-sliced Channel at 2.5 Gb\\s

The “spectrum-slicing” technique, employing incoherent light, has been shown to be a highly practical, cheap and hence very attractive proposal for future all-optical networks. In this study, the use of semiconductor optical amplifier gain saturation for intensity noise reduction on incoherent light is further studied in terms of the Relative-Intensity-Noise (RIN), with a view to obtaining the optimum SOA injection current and input power conditions to achieve the best possible intensity noise reduction, in conjunction with OSNR, BER and Q-factor results. The results reported herein give designers knowledge of the best SOA operating conditions to enhance overall system performance, whilst still obtaining signal gain from the SOA.

First Monolithically Integrated Dual-Pumped Phase-Sensitive Amplifier Chip Based on a Saturated Semiconductor Optical Amplifier

For the first time, a monolithically integrated photonic phase-sensitive amplification chip is fabricated and demonstrated based on an InP/InGaAsP platform. Different semiconductor optical amplifiers have been fabricated as well for characterization. On the chip, two tunable laser pumps that are coherently injection-locked, respectively, from two first-order sidebands of an externally modulated tone are generated to enable signal-degenerate dual-pumped phase-sensitive amplification in a saturated semiconductor optical amplifier. Experiments on different chips are conducted to successfully demonstrate phase-sensitive amplification with approximately 6.3 and 7.8 dB extinction of phase-sensitive on-chip gain. Theoretical simulations are performed and agree well with experimental results. The additive noise properties of the phase-sensitive amplification chip are also investigated.

Phase-Preserving Amplitude Noise Compression of 40 Gb/s DPSK Signals in a Single SOA

Wavelength-preserving and wavelength-converting amplitude regeneration of 40 Gb/s DPSK data in a saturated semiconductor optical amplifier (SOA) with limited excess phase noise feature is reported and investigated. By injecting simultaneously in the amplifier the noisy data and a CW beam with proper power levels, both the pass-through (PT) signal and the wavelength-converted data by four-wave-mixing (FWM) can be regenerated. The strongest regenerative effect is observed for the converted FWM data. The effect of input power variations on the regenerative performance of the device is thoroughly investigated in terms of Bit-Error Rate (BER) versus threshold margin improvement. We observe that regenerative behavior is attained for different levels of the data and CW power levels at SOA input. Experimental investigation of small-signal amplitude and phase SOA response reveals indeed the role of data/CW power balance for reducing the impact of excess phase noise from the amplifier. The analysis also confirms the stronger regenerative capability of the FWM process due to the strong amplitude-limiting SOA response at lower input data power in respect to the PT signal. The scheme offers the advantages of simplicity, compactness, and low operating power levels, making it a practical candidate for all-optical regeneration of high-speed DPSK signals.

Optimization of Intensity Noise Reduction from an Incoherent Light Source Using Gain Saturated Semiconductor Optical Amplifier in a Spectrum-sliced Channel at 2.5 Gb\\S

The “spectrum-slicing” technique employing incoherent light has been shown to be a highly practical, cheap and hence very attractive proposal for future all-optical networks. In this study, the use of Semiconductor Optical Amplifier (SOA) gain saturation for intensity noise reduction of incoherent light is studied with a view to obtaining the optimum SOA injection current and input power conditions to achieve the best possible intensity noise reduction-in terms of OSNR, BER, noise power and Q-factor. The results reported herein give designers knowledge of the best SOA operating conditions to enhance overall system performance, while still obtaining signal gain from the SOA.

Monolithically Integrated All-Optical Regenerator for Constant Envelope WDM Signals

All-optical regeneration has attracted growing interest due to the possibility to increase the all-optical reach. However, to be really competitive with optoelectronic regenerators, all-optical regenerators should process an entire wavelength division multiplexed (WDM) signal comb at once and significantly reduce the footprint and the power consumption, as possible, in principle, by exploiting photonic-integrated circuits (PICs). Moreover, since constant envelope modulation format signals have come into play in recent years, all-optical regenerators dealing with a multiplex of these signals are of particular interest. This paper presents a monolithically integrated indium-phosphide-based PIC acting as an all-optical regenerator for constant envelope WDM signals. The regeneration scheme is based on hard-limiting amplification in saturated semiconductor optical amplifiers (SOAs), which removes the signal intensity noise. The presented PIC, designed and fabricated within the JePPIX technology platform, can handle up to four WDM signals, which are demultiplexed and enter an array of SOAs to undergo regeneration before being multiplexed again. The channel-by-channel regeneration of both polarization shift keying and frequency shift keying signals at 10 Gb/s is experimentally demonstrated in terms of Q-factor improvement.

Multichannel All-Optical RZ-PSK Amplitude Regeneration Based on the XPM Effect in a Single SOA

We propose and demonstrate simultaneous amplitude regeneration for return-to-zero phase shift keying (RZ-PSK) signals. Parallel 6 × 40 Gb/s operation can be achieved using the cross-phase modulation effect in a single saturated semiconductor optical amplifier and subsequent narrow filtering. The spectrum of the distorted signal can be broadened due to the phase modulation induced by the synchronous optical clock signal. A narrow bandpass filter is utilized to extract part of the broadened spectrum and remove the amplitude noise, while preserving the phase information. Numerical simulation and experimental demonstration show that the proposed scheme can work well for RZ-PSK signals with amplitude noise. The bit error ratio measurements show that an average -1.3 dB power penalty can be achieved for all the channels. Furthermore, the tolerance of the amplitude regeneration is investigated.

Regeneration of DPSK Signals in a Saturated SOA

We investigate and experimentally demonstrate a simple method for phase-preserving amplitude regeneration of constant envelope phase-coded signals. This scheme exploits nonlinear interaction between noisy data and a continuous wave beam at a different wavelength in a saturated semiconductor optical amplifier (SOA). We show that proper balancing of the input signals' power allows us to exploit the amplitude limiting effect of SOA saturated gain without introducing significant excess phase noise due to suppression of the α-factor in the amplifier. In a 10-Gb/s nonreturn-to-zero differential phase-shift-keying experiment, both four-wave-mixing (FWM) and pass-through signals are remarkably improved with respect to input data in terms of Q-factor and bit error ratio threshold margin, demonstrating wavelength preserving and wavelength converting regeneration. In particular, the FWM signal exhibits better regenerative performance over a broader range of degraded input data and for lower input overall power levels.

Saturated semiconductor optical amplifier phase modulation for long range laser radar applications

We investigate the use of a semiconductor optical amplifier operated in the saturation regime as a phase modulator for long range laser radar applications. The nature of the phase and amplitude modulation resulting from a high peak power Gaussian pulse, and the impact this has on the ideal pulse response of a laser radar system, is explored. We also present results of a proof-of-concept laboratory demonstration using phase-modulated pulses to interrogate a stationary target.

A SOA-Based Scheme for MAI Suppression in OCDMA

A noise suppression scheme based on semiconductor optical amplifier (SOA) is investigated in this paper. It aims to inhibit the Multiple-Access interference (MAI) which is the major noise in incoherent optical code division multiple access (OCDMA) system. The saturated SOA is employed as optical limiting amplifier here. The performance of proposed MAI suppression system is analyzed theoretically and experimentally.

Simulation on semiconductor optical amplifier intensity noise reduction for future spectrum-sliced optical networks

Spectrum-slicing techniques employing incoherent light are an economic, practical and therefore attractive solution for future all-optical networks, especially for wavelength-division multiplexing (WDM) transmission systems in local area networks (LAN). However, spectrum-sliced methods exhibit a large excess intensity noise factor that limits the performance of the system. In this paper, we investigate noise suppression of spectrum-sliced incoherent light using a saturated semiconductor optical amplifier (SOA). The system incorporating the noise reducing SOA is modeled and simulated using OptSim software, and the results are compared to practical schemes from the literature. Performance comparisons are made with two different broadband sources test-beds. The characteristics of the SOA gain saturation are also presented. In both cases, it is found that a high degree of intensity noise is suppressed by the use of the non-linear gain saturation characteristics of the SOA so as to achieve better system performance. The position of a modulator in the system is also investigated in order to greatly reduce the excess intensity noise.

All-Optical Regeneration of 40 Gb/s Constant Envelope Alternative Modulation Formats

We experimentally investigate the all-optical regeneration of constant envelope alternative modulation format signals at 40 Gb/s. The regeneration scheme is based on hard-limiting amplification in a saturated semiconductor optical amplifier (SOA) that can remove the signal intensity noise. Indeed a significant improvement of the signal quality is reported for either polarization shift keying (PolSK) and frequency shift keying (FSK) signal formats, both detected using simple schemes based on direct detection. The regeneration solution, however, is found to be quite limited for non-return-to-zero (NRZ) differential phase shift keying (DPSK) (demodulated using a 1-bit delay interferometer). Indeed, SOA saturation generates a detrimental nonlinear phase noise that can override the reduction of intensity noise.

Low-Complexity Compensation of SOA Nonlinearity for Single-Channel PSK and OOK

Carrier density fluctuations in semiconductor optical amplifiers (SOAs) impose penalties on phase-shift keying (PSK) signals due to nonlinear phase noise (NLPN), and on-off keying (OOK) signals due to self-gain modulation. In this paper, we propose a simple scheme to equalize the impairments induced by SOA nonlinearities, derived from the small signal analysis of carrier density fluctuations. We demonstrate via simulation almost complete cancelation of the NLPN added by a saturated SOA on a differential PSK signal. We demonstrate via both simulations and experiment the effectiveness of the method for mitigation of nonlinear distortions imposed by SOAs on an OOK signal.

Filter Design for SOA-Assisted SS-WDM Systems Using Parallel Multicanonical Monte Carlo

We address design and optimization of optical filters for spectrum-sliced wavelength division multiplexed (SS-WDM) systems employing saturated semiconductor optical amplifiers (SOAs) to suppress intensity noise. We study the impact of the shape of both slicing and channel selecting optical filters vis-a-vis two important impairments: the filtering effect and the crosstalk. The quantification of bit error rate (BER) is made possible by a parallel implementation of the multicanonical Monte Carlo algorithm. The intensity noise suppression by the SOA and signal degradation by subsequent optical filtering are studied both numerically and experimentally. We find optical filter shape and bandwidth that minimizes BER. By varying channel spacing and width, we estimate the achievable spectral efficiency when using both noise-cleaning SOA and forward error correction. We show that when constrained to use a symmetric architecture, i.e., identical filters for both slicing and channel selecting filters, there is a degradation in achievable spectral efficiency. We show that noise suppression is robust to variations in relative channel powers in multichannel systems. Our numerical simulations, vetted experimentally, provide accurate and quantitative results on optimized system performance.