Semiconductor optical amplifiers for all transmission wavelength bands

Semiconductor optical amplifiers (SOAs) as nonlinear devices always work in the gain saturation region. Therefore, the saturation gain is an important parameter of SOAs. However, the existing theoretical gain formula greatly deviates from the experimental measurements. We find experimentally that the effect of the absorption loss on the SOA gain cannot be neglected, especially when the SOA is biased into the saturation region. In this paper, considering the influence of the absorption loss, we propose a modification to the SOA gain formula in common use. We are able to validate the revised formula against the experimental measurements. Thus the theoretical formula after modification can be accurately applied to evaluate the SOA gain in practice.

We have developed a theory of high-speed operation of quantum dot (QD) semiconductor optical amplifiers (SOAs), and showed that pattern-effect-free amplification of pulse trains, cross gain modulation (XGM) and cross phase modulation (XPM) can take place in QD SOA in the regime with maximum gain. Formulas, which relate the maximum bit-rate for the pattern-effect-free operation and the average SOA output power to the SOA pump current density, were derived. XGM without pattern effect can be realized in the regime with maximum gain due to spectral hole burning effects. Possibility of ultrafast frequency conversion and demultiplexing of data pulse streams through this nonlinearity is illustrated. Expression for the nonlinear refractive index η nl due to spectral hole burning in QD structure was obtained. The value of η nl in QD SOAs can be by 4-5 orders larger than η nl in silica; and efficient ultrafast XPM without pattern effects can be carried out in QD SOA through this nonlinearity. In whole, usage of the regime with maximum gain in QD SOAs can lead to development of new generation of high-speed devices for ultrafast optical processing and communications.

During the last few years, large-scale efforts towards realizing high-photonic inte‐ gration densities have put SOAs in the spotlight once again. Hence, the need to de‐ velop a complete framework for SOA-induced signal distortion to accurately evaluate a system’s performance has now become evident. To cope with this de‐ mand, we present a detailed theoretical and experimental investigation of the deter‐ ministic timing jitter and the pulse peak power equalization of SOA-amplified intensity-modulated optical pulses. The deterministic timing jitter model relies on the pulse mean arrival time estimation and its analytic formula reveals an approxi‐ mate linear relationship between the deterministic timing jitter and the logarithmic values of intensity modulation when the SOA gain recovery time is faster than the pulse period. The theoretical analysis also arrives at an analytic expression for the intensity modulation reduction (IMR), which clearly elucidates the pulse peak pow‐ er equalization mechanism of SOA. The IMR analysis shows that the output intensi‐ ty modulation depth is linearly related to the respective input modulation depth of the optical pulses when the gain recovery time is faster than the pulse period. This novel theoretical platform provides a qualitative and quantitative insight into the SOA performance in case of intensity-modulated optical pulses.

The random walk coefficient (RWC) is the index of white noise of FOG, which determines the minimum detection sensitivity of FOG. The white noise of FOG mainly focuses on optical or photoelectric detection, which can be divided into three categories: detector thermal noise, shot noise and relative intensity noise (RIN). In practice, the RWC of FOG is determined by the signal-to-noise ratio (SNR) in the phase detection process rather than the white noise. When the photocurrent is small, the SNR of FOG is determined by the shot noise. When the photocurrent is large, the signal-tonoise ratio of FOG is determined by the RIN. Therefore, the suppression of RIN is the key to improve the precision of FOG. Through the analysis of the macro input and output characteristics of semiconductor optical amplifier (SOA) and the noise spectrum analysis of saturated SOA, it is demonstrated that the gain saturation effect of SOA can effectively suppress the RIN of wide spectrum light source. Through calculation, the theoretical noise rejection ratio is more than 20dB. Therefore, an erbium-doped fiber source with low RIN is designed. The verification is carried out at the light source level and gyro level respectively. In the light source level verification, the RIN suppression effect of 10dB is achieved. In the gyro level verification, the noise suppression effect under different modulation depth is analyzed, and the random walk reduction of gyro achieves 30% - 60%.

Detailed numerical investigations are undertaken of wavelength reused bidirectional transmission of adaptively modulated optical OFDM (AMOOFDM) signals over a single SMF in a colorless WDM-PON incorporating a semiconductor optical amplifier (SOA) intensity modulator and a reflective SOA (RSOA) intensity modulator in the optical line termination and optical network unit, respectively. A comprehensive theoretical model describing the performance of such network scenarios is, for the first time, developed, taking into account dynamic optical characteristics of SOA and RSOA intensity modulators as well as the effects of Rayleigh backscattering (RB) and residual downstream signal-induced crosstalk. The developed model is rigorously verified experimentally in RSOA-based real-time end-to-end OOFDM systems at 7.5 Gb/s. It is shown that the RB noise and crosstalk effects are dominant factors limiting the maximum achievable downstream and upstream transmission performance. Under optimum SOA and RSOA operating conditions as well as practical downstream and upstream optical launch powers, 10 Gb/s downstream and 6 Gb/s upstream over 40 km SMF transmissions of conventional double sideband AMOOFDM signals are feasible without utilizing in-line optical amplification and chromatic dispersion compensation. In particular, the aforementioned transmission performance can be improved to 23 Gb/s downstream and 8 Gb/s upstream over 40 km SMFs when single sideband subcarrier modulation is adopted in the downstream systems.

The switching performance improvement of an electro-optical space switch based on a semiconductor optical amplifier (SOA) through the implementation of feed-forward filtering is experimentally evaluated. The acquired and post-processed data encompass the SOA operating under practical conditions, ranging its bias current, switching technique, and optical link. The implemented feed-forward filtering method was based on the recursive least squares (RLS) algorithm, aiming to evaluate its capacity to adapt to the SOA operational conditions, mitigating both the nonlinear SOA dynamic behavior and signal-to-noise degradation. The results showed a strong dependency on the quantity of symbols per switching cycle and the possibility to achieve a modest improvement in the system overall bit error rate, specially due to noise degradation mitigation. Considering the small computational power requirement of this approach, its application could be a possible solution to improve the commutation rates of networks that already include post-processing stages.

This paper presents the effects of the input signal on the gain and carrier density response of a semiconductor optical amplifier (SOA). The SOA is modeled using a segmentation method. The input bias current, input power, wavelength and the length of SOA required for amplification and switching functions are investigated. Moreover, the effect of the temperature on SOA operation is investigated as well as the thermo-optical effect. The operation principle is simulated and the results show the input boundary conditions and requirements in which the SOA can be used as an amplifier and as switch.

The performance of semiconductor optical amplifiers (SOAs) employing regions of high and low optical confinement designed for high saturation power and high gain using a novel quantum well intermixing and MOCVD regrowth fabrication scheme is reported. The scheme enables the monolithic integration of high performance SOAs with high gain laser diodes, high efficiency electroabsorption modulators, and high saturation power photodiodes. The SOAs presented exhibit saturation powers in the 19 to 20 dBm range with nearly 15 dB of gain.

Four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) has several important features, such as, high speed and high FWM conversion efficiency as well as optical demultiplexing (DEMUX) (Mecozzi et al., 1995; Mecozzi & Mrk, 1997; Das et al., 2000). The are several applications of FWM in SOAs for all-optical devices, such as, wavelength converters (Vahala et al., 1996), optical samplers (Inoue & Kawaguchi, 1998), optical phase conjugators (Kikuchi & Matsumura, 1998) and optical multiplexers/demultiplexers (Kawanishi et al., 1997; Kawanishi et al., 1994; Morioka et al., 1996; Uchiyama et al., 1998; Tomkos et al., 1999; Kirita et al., 1998; Buxens et al., 2000) have been demonstrated for optical communication systems. When a pulse of a time-multiplexed signal train (for example, a probe pulse) and a pump pulse are injected simultaneously into an SOA, gain and refractive index in the SOA are modulated and an FWM signal is generated by the modulations. Thus, we can obtain a demultiplexed signal as an FWM signal at the output of SOA. All-optical demultiplexing has been experimentally demonstrated up to 200 Gbit/s (Morioka et al., 1996). Tomkos et al., (Tomkos et al., 1999) suggested a number of ways to improve the performance of the dual-pump demultiplexer at 40 Gbit/s as follows; adjustment of the input wavelengths at the peak gain wavelength of the SOA under saturation conditions, the use of higher pump power at the input of the device, or/and the use of pulsed pumps with short pulsewidths. For the higher bit-rate, the overlap of the input to the FWM signal pulses may appear both in the time and spectral domain. The pattern effect may also appear in the FWM signal due to the slow components of the optical nonlinearities in SOAs (Saleh & Habbab, 1990). These effects degrade the usefulness of the FWM in SOAs as a practical DEMUX device in optical network/ communication systems. Therefore, it is very important to analysis the optical DEMUX characteristics based on FWM in SOAs for the ultrafast multi-bit input optical pulses.

The efficiency of optical communication system operation is limited by the low operation rate and relatively complicated realization of electronic components. The transparent and fast optical signal processing becomes very important for the modern optical communication networks in order to overcome the electronic bottleneck. In this work, we consider two examples of all-optical signal processing based on the unique nonlinear optical characteristics of semiconductor optical amplifiers (SOAs). We developed a theoretical model of all-optical logic gates based on quantum dot SOAs (QD-SOAs), which are the most promising candidates for up to 100 Gb/s optical networks. We also proposed a novel method of the all-optical automatic gain control (AGC) of erbium doped fiber amplifiers (EDFA) based on the application of semiconductor optical amplifiers (SOAs).

Polarization insensitive 1310 nm InGaAsP–InP multi-quantum-well (MQW) semiconductor optical amplifiers (SOAs), with 7° tilted ridge waveguide and buried-window end facets, have been fabricated and fully characterized on chip and module level. SOAs chips with an optimized complex strained MQW active region exhibited less than 1 dB polarization dependence of amplified spontaneous power in the drive current range of 50–200 mA. The amplifier module, having a residual facet reflectivity of 2.8 × 10−5, achieved 25 dB fibre-to-fibre unsaturated gain, for both transverse electric and transverse magnetic polarization states, 11.2 dBm saturation output power, and 7.6 dB noise figure at 1310 nm. The polarization dependence of gain was less than 0.6 dB in the 3 dB gain bandwidth of 56 nm. Coupling efficiency played a significant role in the gain, saturation output power and noise figure of a SOA module. Spot-size-converter integrated SOAs with buried heterostructures are expected to exhibit further improved performances.

A detailed model for semiconductor linear optical amplifiers (LOAs) with gain clamping by a vertical laser field is presented, which accounts the carrier and photon density distribution in the longitudinal direction as well as the facet reflectivity. The photon iterative method is used in the simulation with output amplified spontaneous emission spectrum in the wide band as iterative variables. The gain saturation behaviors and the noise figure are numerically simulated, and the variation of longitudinal carrier density with the input power is presented which is associated with the on-off state of the vertical lasers. The results show that the LOA can have a gain spectrum clamped in a wide wavelength range and have almost the same value of noise figure as that of conventional semiconductor optical amplifiers (SOAs). Numerical results also show that an LOA can have a noise figure about 2 dB less than that of the SOA gain clamped by a distributed Bragg reflector laser.

A mode-adapted semiconductor optical amplifier (SOA) has been fabricated and packaged. At the gain peak, 1500 nm, the fiber to fiber gain was measured to be 32.5 dB. Statistics for eight packaged devices indicate that a fiber-to-fiber gain of 26.3 dB /spl plusmn/ 1.3 dB and a saturation output power of 12.4 dBm /spl plusmn/ 0.4 dBm are typical at a bias of 500 mA for /spl lambda/ = 1550 nm. Polarization sensitivity at 1550 nm was measured to be 1.1 dB /spl plusmn/ 0.4 dB and the transverse electric (TE) polarization state noise figure (NF) was determined to be 7.0 dB /spl plusmn/ 0.5 dB. The coupling loss was 1.3 dB /spl plusmn/ 0.1 dB per facet. This SOA, with a 1.3-nm filter, was used as an optical preamplifier in a 10-Gb/s return-to-zero (RZ) system testbed with a pseudorandom binary sequence (PRBS) of 2/sup 31/ -1. A 14.5-dB improvement in receiver sensitivity was observed at a bit error rate (BER) of 10/sup -11/.

An all-optical clock recovery (CR) scheme with wide locking range is demonstrated, based on a silicon microring resonator (MRR) and a semiconductor optical amplifier (SOA). The MRR with a free spectral range of 40 GHz works as a comb filter, which removes the modulation information, whereas the SOA operating in the saturation regime performs amplitude equalization to achieve a good optical clock signal. The design of the MRR is investigated according to the tolerance of the subsequent equalizer. Based on the fabricated MRR, single and dual channels CR from 40 Gb/s input data streams can be achieved successfully with root mean square time jitter of ~ 900 fs. Because of the moderate finesse of the MRR, the CR can be achieved with a wide range of several gigahertz.

By using gain nonlinearity characteristics of semiconductor optical amplifier, an all-optical binary half adder at 10 Gbps is demonstrated. The half adder operates in single mechanism, which is XGM.