Gain measurements of Fabry-Pe/spl acute/rot semiconductor lasers using a nonlinear least-squares fitting method

A method for the measurement of the gain-reflectance product of Fabry-Pe/spl acute/rot (F-P) semiconductor lasers is proposed and compared to other techniques. The method is based on a nonlinear, least-squares fitting of the F-P modes to an Airy function. A separate fitting is performed over each mode, as measured with an optical spectrum analyzer (OSA), so that the gain-reflectance parameters are extracted. The influence of the OSAs response function is considered by convolution of the Airy function with the response function of the OSA. By comparing with the Hakki-Paoli method, the mode sum/min method, and the Fourier series expansion method, we find that the nonlinear fitting method is the least sensitive to noise. However, owing to a broadening of the F-P modes of the semiconductor laser, the mode sum/min method combined with a deconvolution technique gives the least underestimated gain above threshold.

I. Introduction

The optical gain of a semiconductor laser is an important parameter for device performance and design, especially for widely-tunable lasers [1] and high-speed lasers [2]. The techniques for measurement of gain have been investigated intensively for more than two decades. Theoretically, the gain spectrum can be obtained by the definition: illuminate one facet of the laser with some probe light and measure the powers of the input light and of the output light from another facet. The ratio of the output to input power gives the single-pass modal gain [3]. However, this method requires a widely tunable light source as the probe, a wideband antireflection coating of the laser facets, and an accurate estimate of the coupling efficiency. All of these are difficult to obtain. Therefore, people usually use other methods to measure the gain.