Measurements and analysis of diode laser modulation wavelength at high accuracy and response rate

It is a key procedure of measuring the diode laser wavelength in the wavelength modulation spectroscopy (WMS) technique since it determines the selection of specific modulation amplitude and frequency and thus the overall accuracy of the WMS technique. However, the wavelength modulation frequency of lasers is usually from tens of kHz to hundreds of kHz, which makes the traditional methods difficult to measure the wavelength with the sufficient accuracy and time response rate. Therefore, in this paper, we developed a method to measure the modulated wavelength with improved accuracy and time response rate by using a customized long fiber ring etalon. In the method, the free spectral range (FSR) of the etalon was determined by using two adjacent absorption lines of water. And the amplitude of the laser wavelength and its phase relative to the driving voltage were determined by means of interference peak identification and sinusoidal fitting. Finally, we used the developed method to measure dynamic wavelengths as well as phases of a distribute feedback (DFB) diode laser with the modulation frequency from 1 to 500 kHz and the modulation voltage from 0.2 to 1 V. Based on the measurements, the response characteristics of both the linear and nonlinear wavelengths as well as phases with modulation frequency and amplitude were obtained, which provide necessary data for the application of quantitative and high-repetition WMS technique in combustion diagnostics.