Noise figure of vertical-cavity semiconductor optical amplifiers

The noise figure of vertical-cavity semiconductor optical amplifiers (VCSOAs) is investigated theoretically and experimentally. Limitations on the noise figure set by the reflectivity of the mirrors are studied. Highly reflective mirrors lead to increased output noise as well as lasing at moderate carrier densities, which imposes a limit on the obtainable population inversion. Expressions for the excess noise coefficient, which governs signal-spontaneous beat noise enhancement due to finite mirror reflectivity, are presented for transmission and reflection-mode operation. Experimental results from a VCSOA operating in the reflection mode at 1.3 μm are presented. The results, from optical as well as electrical measurement techniques, are analyzed and compared to theoretical values     

Noise in semiconductor laser amplifiers with quantum box structure

The low-noise properties of a quantum-box traveling-wave semiconductor laser amplifier (QB-TW-SLA) are discussed. The gain and population-inversion parameter of a quantum-box structure are precisely expressed using density matrix theory. Due to sharp gain characteristics as well as a small population inversion parameter, dominant two-beat noise is significantly reduced, even in a solitary device without a narrow bandpass filter. The noise figure can be reduced to 3.5 dB     

Noise characteristics of semiconductor laser amplifiers

Noise characteristics of Fabry-Perot (FP) cavity type AlGaAs laser amplifiers are studied theoretically and experimentally. Noise power dependencies on pumping rate and input signal level are measured. Multimode rate equations with McCumber's fluctuation operator are solved for the parabolic band and the exponential bandtail models, both with the no k-selection rule. Experimental results are in good agreement with theoretical predictions.     

Theory of four-wave mixing wavelength conversion in quantum dot semiconductor optical amplifiers

Detailed theoretical analysis of four-wave mixing (FWM) wavelength conversion in quantum dot semiconductor optical amplifier (QD-SOA) is presented. The model takes into account the effect of the multidiscrete QD energy levels and the wetting layer. Good agreement between calculated and experimental data is obtained. Because of the discreteness of the energy levels, QD-SOAs demonstrate high FWM conversion efficiencies at high detuning frequency. Our calculations show that carrier escape from the ground state significantly affects the performance of the amplifier.     

Modeling broad-area semiconductor optical amplifiers

The authors report on the wave optics modeling of broad-area semiconductor amplifiers used in a double-pass reflective configuration. The results show excellent agreement with recent reports of 2.5 and 12 W of nearly diffraction-limited output from such systems. Several design issues that must be considered in a practical laser communication system based on this class of amplifier are discussed     

Noise and error rate performance of semiconductor laser amplifiers in PCM-IM optical transmission systems

Applications of semiconductor laser amplifiers in intensity modulated digital optical transmission systems were studied theoretically. An optical linear amplifier repeater between electronically regenerating terminal repeaters and an optical linear preamplifier in front of a photodetector in an electronically regenerating repeater are discussed. Both traveling-wave type and Fabry-Perot cavity type laser amplifiers are considered. The noise and error rate performance in these systems are evaluated using formulations for semiconductor laser amplifiers. The mean and variance in the optical amplifier output photons calculated by the photon master equation [1] is used to obtain the worst case variance in the equalized output voltage [2] for these systems. The required receiving power reduction from direct detection scheme by a preamplifier system and the repeater spacing expansion between two electronically regenerating terminals by an optical linear amplifier repeater system are delineated.     

Noise figure improvement in semiconductor optical amplifiers by holding beam at transparency scheme

We have studied the noise properties of a semiconductor optical amplifier (SOA) under the injection of a pump laser. The laser wavelength is set within the transparency region of SOA. The noise figure (NF) is found to decrease, strongly depending on the relative injection direction of signal and pump (i.e., copropagating or counterpropagating beams). In the copropagating scheme, an improvement in NF is obtained, ranging from 2.5 dB in the low current regime (40 mA, 13-dB chip gain) to 0.5 dB at high currents (150 mA, 23-dB chip gain). In the counterpropagating scheme, NF improvement is less pronounced. We explain the effect by an advantageous carrier redistribution along the SOA under the presence of the pump beam at transparency. A detailed simulation reproduces the observed results.     

Cross-polarization modulation in semiconductor optical amplifiers

The polarization sensitivity of semiconductor optical amplifiers can be assessed in terms of gain or in terms of induced phase shift. Although the former aspect has received a lot of attention, the latter is rarely mentioned in the literature. Nevertheless, this birefringence leading to a rotation of the lightwave polarization at the output of the device may give rise to some interesting or unwanted effects. An optical control of the birefringence can be applied to wavelength conversion, signal regeneration, all-optical switching or gating. In this letter, the variation of the birefringence with input polarization and input power is measured     

Long wavelength vertical-cavity semiconductor optical amplifiers

This paper overviews the properties and possible applications of long wavelength vertical-cavity semiconductor optical amplifiers (VCSOAs). A VCSOA operating in the 1.3-μm wavelength region is presented. The device was fabricated using wafer bonding; it was optically pumped and operated in reflection mode. The reflectivity of the VCSOA top mirror was varied in the characterization of the device. Results are presented for 13 and 12 top mirror periods. By reducing the top mirror reflectivity, the amplifier gain, optical bandwidth, and saturation output power were simultaneously improved. For the case of 12 top mirror periods, rye demonstrate 13-dB fiber-to-fiber gain, 0.6 nm (100 GHz) optical bandwidth, a saturation output power of -3.5 dBm and a noise figure of 8.3 dB. The switching properties of the VCSOA are also briefly investigated. By modulating the pump laser, we have obtained a 46-dB extinction ratio in the output power, with the maximum output power corresponding to 7-dB fiber-to-fiber gain. All results are for continuous wave operation at room temperature     

Sampling pulses with semiconductor optical amplifiers

We demonstrate three techniques to measure the instantaneous frequency and intensity of optical pulses using semiconductor optical amplifiers (SOAs). Four-wave mixing, gain-saturation, and interferometric switching through a nonlinear optical loop mirror are three mechanisms by which sampling is done. We have experimentally measured the intensity and chirp profiles of pulses with energies as low as 10 fJ. Since the nonlinearity in the SOA is relatively slow, these measurement techniques are most appropriate for picosecond pulses often found in telecommunication applications. The temporal resolution of these methods are limited by timing jitter, which was ≈0.5 ps for the mode-locked laser diodes we used in our experiments, and by the width of the switching window     

Wavelength conversion using semiconductor optical amplifiers

This paper reports a detailed theoretical study of the dynamics of wavelength conversion using cross-gain and cross-phase modulation in semiconductor optical amplifiers (SOA's) involving a large signal, multisection rate equation model. Using this model, recently reported experimental results have been correctly predicted and the effects of electrical and optical pumping on the conversion speed, modulation index, and phase variation of the converted signal have been considered. The model predicts, in agreement with experimental data, that recovery rates as low as 12 ps are possible if signal and pump powers in excess of 14 dBm are used. It also indicates that conversion speeds up to 40 Gb/s may be achieved with less than 3 dB dynamic penalty. The employment of cross-phase modulation increases the speed allowing, for example, an improvement to 60 Gb/s with an excess loss penalty less than 1 dB     

Gain recovery of bulk semiconductor optical amplifiers

The gain recovery time of 1.55-μm bulk semiconductor optical amplifiers (SOA's) with lengths from 500 to 1500 μm has been measured with a continuous-wave (CW) probe in the time domain. It is shown to decrease with increasing length down to 60 ps for the longest SOA. This behavior is theoretically explained. A lower limit for the recovery time is observed and explained     

Theoretical study of GaInNAs-GaAs-based semiconductor optical amplifiers

We have calculated the basic properties of multiple-quantum-well semiconductor optical amplifiers (SOAs) with active regions based on the newly proposed material GaInNAs. The band structure is modeled using k/spl middot/p theory and accounting for strain effects and the material gain in the context of free-carrier theory. The performance of structures with different nitrogen composition that emit at the same wavelength is modeled using a multisection approach accounting for spontaneous emission. The trends in the SOA performance related parameters are identified and explained.     

High accuracy numerical solutions for band structures in strained quantum well semiconductor optical amplifiers

In this paper,we have calculated the band structure of strained quantum well (QW) semiconductor optical amplifiers (SOAs) by using plane wave expansion method (PWEM) and finite difference method (FDM),respectively.The difference between these two numerical methods is presented.First,the solution of Schr(o)dinger's equation in a conduction band for parabolic potential well is used to check the validity and accuracy of these two numerical methods.For the PWEM,its stability and computational speed are investigated as a function of the number of plane waves and the period of QW.For FDM,effects of mesh size and QW width on its accuracy and calculation time are discussed.Finally,we find that the computational speed of FDM generally is faster than that of PWEM.However,the PWEM is more efficient than the FDM when wider SOAs are needed to be calculated.Therefore,to obtain high accuracy and efficient numerical solutions for band structures,numerical methods should be selected depending on required accuracy,device structure and further applications.     

Two-wave competition in ultralong semiconductor optical amplifiers

The copropagation of two waves in an ultralong semiconductor optical amplifier (SOA) is considered in theory and experiment. One wave is a modulated signal, whereas the other one is unmodulated (continuous wave). The theory bases on a comprehensive traveling-wave model and predicts an exponential improvement of the signal extinction ratio (ER) of the modulated signal, caused by the presence of the unmodulated signal. Conditions for achieving this two-wave competition (TWC) effect are as follows. The SOA is operated under saturation, both waves are copolarized, they have comparable gain and their spectral correlation is between certain limits. The TWC effect is due to nondegenerate four-wave mixing (FWM) in the saturated part of a long SOA and is expected to have a high-speed potential. In order to check the theoretical predictions, 4-mm-long SOAs are developed and experimentally investigated under the given conditions. The measured ER improves by 1.3 dB for a 5-GHz sinusoidal signal, which compares well with the 2 dB theoretically predicted for this configuration. FWM is identified also experimentally as the basic mechanism. Variation of wavelength detuning, pump current, modulation frequency and ER of the injected signal are used to determine optimum conditions for the given device.     

Ultra-low-reflectivity semiconductor optical amplifiers without antireflection coatings

High-performance, GaInAsP/InP tilted stripe ridge waveguide semiconductor optical amplifiers which require no antireflection coating to achieve reflectivities of less than -40 dB are demonstrated. This very low reflectivity is found to be both largely independent of polarisation and wavelength, and also easily reproducible from wafer to wafer.<>     

Relative intensity noise in semiconductor optical amplifiers

The spontaneous noise spectrum of high-gain semiconductor optical amplifiers is normally assumed to be dominated by spontaneous-spontaneous and signal-spontaneous beat noise, which is white over the frequency range important to fiber-optic systems. Recent measurements have shown that a strong resonance peak in the spontaneous noise spectrum appears well below the threshold current, indicating the existence of relative intensity noise. This noise term has important implications for system design, and its effect on several transmission systems is described. Relative intensity noise in semiconductor optical amplifiers is compared to the similar relative intensity noise found in semiconductor lasers     

Nonlinear gain in vertical-cavity semiconductor optical amplifiers

We report on the wavelength-dependent nonlinear-gain properties of a vertical-cavity semiconductor optical amplifier. A step-like output versus input transfer curve and bistability are found on the long wavelength side of the resonant peak. The characteristics are caused by a dispersive nonlinearity arising from gain saturation in the device. The nonlinear transfer of the amplifier could be used for logic regeneration and all-optical logic operations, while the optical injection power required to switch the device is an order of magnitude lower than previously reported with edge-emitting devices.     

Numerical modeling of the emission characteristics of semiconductor quantum dash materials for lasers and optical amplifiers

This paper deals with the simulation of the emission characteristics of self-assembled semiconductor quantum dash (QDash) active materials, characterized by high length-to-width and width-to-height ratios of the dash size and by a wide spreading of the dash dimensions. This significant size fluctuation requires to compute numerically the corresponding energy distribution of the electron and hole confined states. Furthermore, due to the long dash length, it is necessary to take into account the many longitudinal confined states that contribute to the emission spectrum. To implement a model that does not require excessive computation time, some simplifying assumptions have been introduced and validated numerically. Starting from good knowledge of the dash size, material composition, and optical waveguide dimensions, we have been able to simulate the amplified spontaneous emission and gain spectra of a quantum dash semiconductor optical amplifier with a good quantitative agreement with the measured data. As an application example, the model is used to predict the gain properties of different QDash ensembles having various size distributions.     

Gain saturation in traveling-wave semiconductor optical amplifiers

The gain saturation behavior of semiconductor traveling-wave optical amplifiers has been analyzed using a model that includes the specific dependence of gain on carrier concentration. Under the condition of a specific gain at a particular current, it is found that the saturation power strongly depends on the choice between quantum well (QW) or bulk amplifying medium but weakly on the detailed design of the device such as the number of QW's or the thickness of the bulk layer. The higher saturation power of the QW-based amplifier is caused by its logarithmic gain-current relation rather than its low optical confinement factor. Also, when the unsaturated device gain is specified, the designed saturation power can be obtained with the lowest drive current by using the highest optical confinement