Detailed model and investigation of gain saturation and carrier spatial hole burning for a semiconductor optical amplifier with gain clamping by a vertical laser field

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.     

Noise figure of saturated 1310-nm polarization insensitive multiple-quantum-well optical amplifiers

The noise figure degradation due to saturation by amplified spontaneous emission of high-gain 1310-nm polarization insensitive multiple-quantum-well optical amplifiers has been investigated systematically by studying amplifiers of different lengths. It is found that the intrinsic noise figure of 4 dB of a device with 20-dB single-pass gain is degraded to about 5 dB for a device having 36-dB single-pass gain. This degradation in noise figure is very modest, compared to results reported earlier for semiconductor optical amplifiers with bulk-gain media, illustrating the benefits of using a quantum-well gain medium in semiconductor optical amplifiers.     

1.5 µm GaInAsP traveling-wave semiconductor laser amplifier

This paper presents a theoretical and experimental study in terms of small-signal gain, signal gain saturation, and noise characteristics of a 1.5 μm GaInAsP traveling-wave amplifier (TWA), realized through the application of SiOxfilm antireflection coatings. This TWA, having a residual facet reflectivity of 0.04 percent, exhibits a wide, flat signal gain spectrum and a saturation output power of +7 dBm at a 20 dB signal gain. The TWA also has a noise figure of 5.2 dB, which is the smallest value reported for semiconductor laser amplifiers. The experimental results are confirmed to be in good agreement with the theoretical predictions based on the multimode traveling-wave rate equations in conjunction with the photon statistic master equation analysis, which takes into account the amplifier material and device structural parameters. Signal gain undulation, saturation output power, and noise figure are also theoretically evaluated as functions of the facet reflectivity. The superior performance of the TWA demonstrates that the device is favorable for use in linear optical repeaters in fiber transmission systems.     

Novel Closed-Form Model for Multiple-State Quantum-Dot Semiconductor Optical Amplifiers

Novel closed-form model for multiple-state quantum-dot semiconductor optical amplifiers (QD-SOAs) is derived. The model takes into account the effect of the ground state, excited state and the wetting layer. The model is simple, accurate and exhibits negligible computational time compared with numerical simulation. In addition, the derived model is valid for arbitrary applied current and input photon density and is interesting for device design and optimization. Analytical expressions for the optical gain, effective saturation density, maximum output density and the transparency current are also derived. Our model revealed that the effective saturation density of QD-SOAs strongly depends on the photon density and the applied current.     

基于两只半导体光放大器级联结构的多波长光纤激光器

通过数值模拟对两只半导体光放大器(SOA)级联结构的静态增益饱和特性进行了理论研究.在不考虑自发辐射的情况下,分析了注入电流对两只SOA级联结构增益的影响.实验上构建了一种基于两只SOA级联结构的多波长光纤激光器,观测并分析了半导体光放大器的驱动电流和增益带宽对多波长输出结果的影响.在室温下,获得了基本符合ITU-T标准100 GHz的27个波长以上的稳定多波长输出,各信道输出功率不平坦度小于±3 dB,线宽小于0.102 nm,信噪比大于25 dB,总输出功率为1.94 mW,并且与由单只SOA构成的多波长光纤激光器进行了对比.     

Static Gain, Optical Modulation Response, and Nonlinear Phase Noise in Saturated Quantum-Dot Semiconductor Optical Amplifiers

A rate equation model preserving charge neutrality for quantum-dot semiconductor optical amplifiers (QD-SOAs) is established to investigate the nonlinear gain dynamics in the saturation regime. The static gain of QD-SOA is calculated assuming overall charge neutrality and compared with that without overall charge neutrality. Optical modulation response and nonlinear phase fluctuation through saturated QD-SOAs are calculated numerically based on a small-signal analysis. The gain dynamics of QD-SOAs are strongly dependent on the current injection level. The carrier reservoir in the wetting layer and continuum state is necessary for QD-SOAs to operate with high gain, high saturation power, and ultrafast gain recovery.     

Acceleration of gain recovery in semiconductor optical amplifiersby optical injection near transparency wavelength

By using optical injection near the transparency wavelength of semiconductor optical amplifiers, we show experimentally that both the saturation output power and the gain recovery can be greatly improved. By injecting 80 mW of pump power, we observe a 3-dB increase in saturation output power. For 73 mW of pump power, we find a reduction in gain recovery time from over 200 ps down to below 40 ps, while maintaining 14 dB of fiber-to-fiber gain at 1555-nm wavelength     

Quantum-Dot Semiconductor Optical Amplifiers

This paper reviews the recent progress of quantum-dot semiconductor optical amplifiers developed as ultrawideband polarization-insensitive high-power amplifiers, high-speed signal regenerators, and wideband wavelength converters. A semiconductor optical amplifier having a gain of > 25 dB, noise figure of < 5 dB, and 3-dB saturation output power of > 20 dBm, over the record widest bandwidth of 90 nm among all kinds of optical amplifiers, and also having a penalty-free output power of 23 dBm, the record highest among all the semiconductor optical amplifiers, was realized by using quantum dots. By utilizing isotropically shaped quantum dots, the TM gain, which is absent in the standard Stranski-Krastanow QDs, has been drastically enhanced, and nearly polarization-insensitive SOAs have been realized for the first time. With an ultrafast gain response unique to quantum dots, an optical regenerator having receiver-sensitivity improving capability of 4 dB at a BER of 10^-9 and operating speed of > 40 Gb/s has been successfully realized with an SOA chip. This performance achieved together with simplicity of structure suggests a potential for low-cost realization of regenerative transmission systems.     

Modeling of noise in erbium-doped fiber amplifiers in the saturatedregime

We have made a theoretical study of the noise figure of erbium-doped fiber amplifiers in the saturated regime. The noise figures of amplifiers subjected to specific gain and gain compression requirements were calculated for various amplifier lengths. The resulting noise figures together with the required pump and input signal powers map out all possible solutions given constraints on gain, compression, pump power, output signal power, and noise figure. In some cases, requirements on the output signal power prohibit any solutions. A way to solve this problem is the introduction of a post-amplifier loss. For this configuration, two possible solutions exist, which collapse into one solution at a certain critical loss, under which there exist no solutions. When the impact of amplified spontaneous emission is neglected in the model, only one solution is obtained, and the critical loss is much smaller than when the amplified spontaneous emission is included in the model. We conclude that amplified spontaneous emission generally has to be taken into account, even when the gain is as low as 10 dB, to accurately predict the noise performance of erbium-doped fiber amplifiers     

An ultrawide-band semiconductor optical amplifier having an extremely high penalty-free output power of 23 dBm achieved with quantum dots

A semiconductor optical amplifier (SOA) having a gain of >25 dB, noise figure of <5 dB, and 3-dB saturation output power of >19 dBm, over the record widest bandwidth of 90 nm among all kinds of optical amplifiers, and also having a penalty-free output power of 23 dBm, the record highest among all the SOAs, was realized by using quantum dots.     

Semiconductor laser optical amplifiers for multichannel coherentoptical transmission

The application of semiconductor laser optical amplifiers in multichannel coherent optical transmission systems is investigated. The amplifiers considered (λ=1.3 μm) exhibit a gain of 24 dB at a grain ripple <2 dB and a 3-dB bandwidth of about 4000 GHz. The characteristics of these amplifiers and transmission experiments with these amplifiers are described. The investigations concern noise accumulation in an amplifier chain, generation of echoes due to backward gain in cascaded amplifiers, crosstalk in multichannel transmission, and the effect of gain saturation due to spontaneous emission. A good fit is shown between the advantages of multichannel coherent optical transmission systems and the properties of semiconductor laser optical amplifiers, which are very promising for future long-haul optical transmission systems     

Noise analysis of conventional and gain-clamped semiconductoroptical amplifiers

We present a numerical study of the noise of conventional and gain-clamped semiconductor optical amplifiers (SOAs), using a detailed device model. The model makes use of a density-matrix gain calculation, and takes into account the forward and backward amplified spontaneous emission (ASE) spectra and the spatial carrier hole-burning. The device is longitudinally divided into M sections and a rate equation for averaged photon and carrier densities is used for each section. We demonstrate that the accuracy on the calculated noise figure strictly depends on the number of sections M. We obtain a good tradeoff between the results accuracy and the computational complexity with M=8. The model is then applied to study the noise in a distributed Bragg reflector (DBR)-type gain-clamped SOA for varying signal power, pump current, and lasing wavelength. We show that changes in the spatial carrier profile caused by the input signal significantly affect the noise figure, even when the gain is constant. A slight dependence of the noise figure on lasing wavelength is also foreseen, while the dependence on the pump current is negligible. A new method for gain-clamped SOA noise figure reduction is proposed, based on unbalanced Bragg reflectors. An improvement of noise figure (NF) as large as 2 dB is devised     

Measuring gain and noise figure of erbium-doped fiber amplifiers using a broad-band source and a transmission filter

We report a unique low-cost technique for measuring gain and noise figure (NF) in erbium-doped fiber amplifiers using a broad-band amplified spontaneous emission source and a transmission filter, by extracting the slope and intercept of output power spectral density versus input power spectral density at filter edges. The required filter depth is 30 dB. Discrepancies of /spl les/0.3 dB are found between gain or NF obtained from this technique and those measured using conventional spectral-interpolation technique employing multiple lasers at 100-GHz spacing across the C-band.     

A Low-Noise and Wide-Band Esaki Diode Amplifier with a Comparatively High Negative Conductance Diode at 1.3 Gc/s

This paper discusses the stability problem, output power, saturation level, and noise figure of Esaki diode amplifiers, and describes design considerations of the broadband circulator type amplifier with a large negative conductance diode. An experimental amplifier with a diode which has a negative resistance of -25 ohms is also described. The amplifier has a 3 dB bandwidth of 20 per cent, 18 dB gain, and a 3.6 dB noise figure including 0.3 dB insertion loss of the circulator. The output level for which the gain is 1 dB lower than the small signal gain is -17 dBm. These experimental results are in fair agreement with those estimated theoreticaly.     

CW injection seeding of a modelocked semiconductor laser

The authors extended the injection locking technique to control the output of a modelocked semiconductor laser with an external continuous-wave (CW) signal. With this injection seeding technique, over 8 mW of average power in 30 ps pulses with side cluster suppression of over 20 dB was obtained from an actively modelocked AlGaAs semiconductor laser. This average output power compares favorably with the 12 mW W output power of the extended resonator. The frequency spectrum of the laser is determined by the background noise level as set by the spontaneous emission. Injection seeding overrides the noise and concentrates over 99% of the available energy in a single nearly transform-limited pulse     

Facet reflectivity of a spot-size-converter integratedsemiconductor optical amplifier

Traveling-wave type semiconductor optical amplifiers (SOAs) integrated with a spot-size-converter (SSC) are extensively studied for improvement of coupling efficiency with single-mode fibers and for cost reduction in packaging. In this paper, the structural dependence of the SSC on effective facet reflectivity R_eff is investigated theoretically and experimentally. It is shown that, not only sufficient mode-conversion in the SSC region, but also the introduction of angled facets, are essential for reducing R_eff. A small gain ripple (less than 0.1 dB) in an amplified spontaneous emission (ASE) spectrum, fiber-to-fiber gain of 26 dB, and saturation output power of 7 dBm are observed for the fabricated SOA, which consists of a window length of 20 μm, facet angle of 7°, and anti-reflection coated facet of less than 1% reflectivity     

Effects of gain saturation in semiconductor laser amplifier links

The effects of gain saturation due to amplified spontaneous emission in systems comprising cascaded semiconductor laser amplifiers and attenuators are investigated. The influence of the relative spatial distribution in the link of gain and loss is analyzed numerically, and conclusions regarding noise optimal structures are drawn. It is shown that the performance of links with gain saturation can be quite different from the ideal, unsaturated case     

On crosstalk and noise in an optical amplifier with gain clamping by vertical laser field

We have calculated the transient behavior and noise figure of a semiconductor optical amplifier (SOA) with the gain clamped by a vertical cavity laser (VCL). The characteristic behavior of the more conventional gain-clamped SOAs and SOAs with no gain-clamping is also studied and compared with the vertically gain-clamped amplifier. The calculations are based on a numerical stochastic rate equation model including several forward- and backward-propagating channels that are coupled to the vertical laser field through the active medium. The noise model takes into account the input noise, randomly amplified spontaneous emission, and random gain. Numerical simulations have been carried out to study the relaxation oscillations, crosstalk, and noise in a system with a strong input signal switched on and off while observing the output signals, VCL photon density, and carrier density. Results show that the VCL field captures most of the disturbances, in agreement with available experimental data.     

Noise study of low-dimensional quantum-well semiconductor laseramplifiers

Low noise characteristics of semiconductor laser amplifiers (SLAs) consisting of low-dimensional quantum-well structures are obtained theoretically using density matrix theory. Due to a sharper gain spectrum as well as a smaller population inversion parameter in quantum-wire and quantum-box structures, predominant two beat noises of traveling-wave SLAs were found to be reduced in the lower dimensional quantum-well structures, even in solitary devices without a narrow bandpass filter. The noise figure can be reduced to 3.3 dB in a quantum-box structure, which nears the theoretical limit of 3 dB     

Intermodulation distortion in a multiple-quantum-well semiconductor optical amplifier

The measurement of intermodulation distortion (IMD) induced by carrier-density modulation in a multiple-quantum-well (MQW) semiconductor amplifier is reported. The results show that MQW amplifiers have 15 dB less IMD than conventional buried-heterostructure semiconductor amplifiers. The IMD is dependent on the output power of the amplifiers, which confirms that the carrier-density modulation is the dominant nonlinear mechanism in MQW amplifiers. In addition, the results show that, unlike conventional buried-heterostructure amplifiers, MQW amplifiers have at least two time constants (200-250 ps and <10 ps) for the gain recovery process.<>