Noise in vertical-cavity semiconductor optical amplifiers

We have investigated the noise properties of an optically pumped reflection mode 1.3 μm VCSOA. The device consisted of a stacked multiple quantum well InGaAsP/InP active region wafer bonded to two GaAs/Al(Ga)As distributed Bragg reflectors (DBRs). The bottom and top DBR had 25 and 13.5 periods, respectively. A 980 nm laser diode was used to pump the VCSOA through the bottom DBR and the undoped GaAs substrate. The diameter of the active region in the planar VCSOA structure was defined by the pump beam and measured to be about 8 μm. The reflectivity of the mirrors in the device was calculated in order to estimate the best obtainable noise figure. The bottom and top mirror reflectivity was determined to be 0.999 and 0.955, respectively. This R _b value gives an excess noise coefficient of 1 and a population inversion parameter at lasing threshold of about 3. This gives a minimum intrinsic noise figure of 7.7 dB, clearly limited by the early onset of lasing. A better n_sp can be obtained with more transmissive top mirror that would allow stronger pumping. Assuming an input coupling efficiency of <1 dB, the minimum fiber-to-fiber noise figure (at threshold) for the present device is 8.7 dB. For operation below lasing threshold a slightly higher noise figure is expected