A simple scheme for polarization insensitive four-wave mixing insemiconductor optical amplifiers

A simple scheme for polarization insensitive four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is proposed, which utilizes just one conventional polarization-insensitive SOA and one input pump wave. The technique would be efficient in practical applications for frequency shifts about 0.5 THz for a device length of say 200 μm. The polarization insensitive operating principle of the FWM type is analyzed using an improved space-averaged model for describing FWM of copropagating and counterpropagating optical waves     

A novel method for polarization insensitive four-wave mixing in semiconductor optical amplifiers

A new method for obtaining polarization insensitive four-wave mixing with phase inversion in a semiconductor optical amplifier is proposed. The method uses the simplest configuration with co-propagating pump and signal waves. The possibility of polarization insensitive four-wave mixing is shown for a tensile quantum well. Polarization insensitivity can be obtained by the right detuning of the pump wavelength from the gain peak.     

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.     

Polarization insensitive optical amplifier consisting of twosemiconductor laser amplifiers and a polarization insensitive isolatorin series

A polarization-insensitive optical amplifier consisting of two semiconductor laser amplifiers and a polarization-insensitive isolator in series is presented. The isolator not only eliminates the cavity coupling between two amplifiers, but also rotates polarized forward light by 90°. Experiments demonstrate the elimination of cavity coupling and a polarization sensitivity of 0.6 dB with a gain of 26 dB     

Influence of buried structure on polarization sensitivity instrained bulk semiconductor optical amplifiers

In order to achieve an accurate design of polarization-insensitive semiconductor optical amplifiers based on tensile strained bulk InGaAsP, the reduction of strain in the active layer of the buried heterostructure and its influence on polarization sensitivity are analyzed numerically for the first time. The gain calculation, including the strain distribution in the active layer, is examined based on the k · p method for the different active layers. It is found that the strain introduced during the epitaxial growth is strongly reduced after regrowth of the burying layer. In an active layer having the aspect ratio of 1 : 4, the strain reduction causes more than a 0.5-dB deviation in the polarization sensitivity of the gain. From a comparison with the experimental results, it is shown that including the effect of the burying layer in the calculation gives an accurate determination of the amount of strain for the polarization independence     

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 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.     

Time-domain modeling of semiconductor optical amplifiers for OTDMapplications

An advanced time-domain dynamical model for the investigation of semiconductor optical amplifiers (SOA) is presented. The model accounts for the ultrafast gain dynamics, the gain saturation and the gain spectral profile. It is also suitable for analyzing the amplifier in a system environment. As an example the model is used to investigate the gain dynamics of an SOA as well as the characteristics of an interferometer switch semiconductor laser amplifier in a loop mirror (SLALOM). Good agreement between modeling and experiment is shown. The model can be applied to the investigation of other optically time-division multiplexed (OTDM) applications, too     

Structural design criteria for polarization insensitivesemiconductor optical amplifiers

A study is made of the possibility of realizing polarization independent semiconductor optical amplifiers by operating on waveguide parameters such as active- and cladding-layer thickness, stripe width, etc. The accurate design of antireflection coating has also been considered in a formulation that allows one to consider the stripe width. It is shown that in coated ridge waveguide structures, the carrier effects on the refractive index may allow equal TE and TM gain to be obtained for various values of the gain when the waveguide geometry is designed with accuracy. The performance of a polarization-independent structure is analyzed, showing that the cost of gain equalization is an increase of the excess coupling losses     

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     

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     

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     

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     

Angled-facet S-bend semiconductor optical amplifiers for high-gainand large-extinction ratio

The extinction ratio of an optical gate with a spot-size-converter-integrated semiconductor optical amplifier (SSC-SOA) is deteriorated following the direct coupling of unguided light between the input and output fibers. In this letter, an S-bend waveguide structure is introduced into an active waveguide to suppress such direct coupling. Angled facet structures are also introduced for obtaining low facet reflectivity. The fabricated SSC-SOA operating at 1.55-μm wavelength achieves an extinction ratio up to 70 dB and a fiber-to-fiber gain of 20 dB     

Nonlinear polarization rotation in semiconductor optical amplifiers: theory and application to all-optical flip-flop memories

We present a model for polarization-dependent gain saturation in strained bulk semiconductor optical amplifiers. We assume that the polarized optical field can be decomposed into transverse electric and transverse magnetic components that have indirect interaction with each other via the gain saturation. The gain anisotropy due to tensile strain in the amplifier is accounted for by a population imbalance factor. The model is applied to a nonlinear polarization switch, for which results are obtained, that are in excellent agreement with experimental data. Finally, we describe an all-optical flip-flop memory that is based on two coupled nonlinear polarization switches.     

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     

Multiwavelength lasers based on semiconductor optical amplifiers

Stable multiwavelength lasing is demonstrated with a novel laser, in which a semiconductor optical amplifier offers optical gain and cascaded sampled fiber gratings serve as a comb filter. Five lasing lines are obtained with a fixed wavelength spacing of 0.8 nm. Optical power fluctuations among the lasing lines are improved by adjusting the reflectivity profile through the cascade of two sampled gratings     

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.