Contents I. Introduction
FABRY–PÉROT (FP) lasers as the simplest kind of semiconductor lasers are routinely fabricated in the laboratory and in industry. As the FP cavity generally supports multiple longitudinal modes, FP lasers normally behave as multimode lasers. This is not ideal in many applications especially in the source for optical communications. However, they are preferable in cases where the cost is critical and the bandwidth of the emission spectrum does not have a dominating negative impact. Recently, there has been strong interest in using FP lasers as cheap wavelength-division-multiplexing sources in fiber-to-the-home applications [1]. In the laboratory, FP lasers are also routinely made in order to find the material and device information such as the modal gain, internal loss, injection efficiency, etc. Such measurements are an important diagnostic tool in the semiconductor device research field. It is not surprising that various methods have been developed to extract these parameters from the fabricated FP lasers. A typical method is the cutback method [2], which uses FP lasers with different cavity lengths. By measuring the relationship between the output slope efficiency and the cavity length, the internal loss and injection efficiency can be estimated. The net modal gain can be extracted from the amplified spontaneous emission (ASE) spectrum when the laser is operated below threshold. Several methods to extract the modal gain are in use such as the Hakki–Paoli method [3], the method proposed by Cassidy [4], the Fourier transform method [5], [6], and the Fourier series expansion (FSE) method [7]. Parameter extraction from the below-threshold ASE spectrum is specific to individual devices and the measurement of the below-threshold ASE spectrum is rather easy. It would be ideal if as many parameters as possible could be extracted from the below-threshold ASE spectrum in as simple a manner as possible. This paper introduces a series of procedures to achieve this goal based on the FSE method. The net modal gain, internal loss, quasi-Fermi level separation, linewidth enhancement factor, injection efficiency, and geometric spontaneous emission factor can all be estimated for below threshold based on the procedures introduced.
