1.55-μm polarization-insensitive optical amplifier with strain-balanced superlattice active layer

A polarization insensitive (sensitivity < 1 dB) semi- conductor optical amplifier has been realized at 1.55 pm. The active layer consists of a strain-balanced superlattice structure. Gain polarization insensitivity on a large bandwidth (60 nm) together with a 22.5-dB signal gain and a 11-dBm polarization- insensitive saturation output power are obtained. EMICONDUCTOR optical amplifiers (SOA's) are a key S element for optical communication systems. Their main advantages compared to fiber optical amplifiers (FOA's) are the continuous choice of wavelength (from 1.3 to 1.55 pm), the low driving power, and its integrability in photonic integrated circuits. However, a disadvantage of standard SOA-based on bulk active layer is the polarization sensitivity of the gain due to difference of the TE and TM mode confinement factors, favoring the TE mode gain (l?TE/l?TM M 3 dB). A first solution to reduce this polarization sensitivity is to use a nearly square section active waveguide geometry (l). In addition to technological difficulties to fabricate waveguide with cross sections as small as 0.3 pm x 0.5 pm, this solution leads to strong coupling losses (which requires the use of tapers to reduce these losses) and difficulties in photonic integration with other components. Another solution is to keep the usual waveguide geometry (hence reducing the coupling losses), and to use tensile-strained materials in the active layer so as to favor the TM mode gain and to compensate the confinement factor difference. Three types of heterostructures have been proposed so far: low tensile-strained multiquantum well (MQW) (2), MQW comprising tensile and compressively strained wells (3) and strained-layer superlattices (SL's) al- temating tensile-strained and lattice-matched thin layers (4). We focused on this last approach to be able to simultaneously control the material bandgap and the Tm gain ratio, in contrast to the compressive/tensile MQW approach. We report here the results for a SOA based on a strain- balanced superlattice alternating tensile-strained GaInAs (-0.5%) and compressive-strained GaInAsP (1%). The zero net strain allows to avoid plastic relaxation problems. Therefore the thickness of the active layer can be varied without limitation, which is not the case with the strained-layer SL active layer (4). A low polarization sensitivity (51 dB) as well as a high signal gain (22 dB) and high saturation