# Design of a Laser Driver and Its Application in Gas Sensing
###### Abstract  
A laser driver which features high stability and a graphical user-interface was designed and used in trace gas sensing. The running of the laser driver was managed by an ARM processor which was embedded with a real-time operating system (RTOS). Through clicking on the touch screen that was configured with an emWin graphical user-interface (GUI), the parameters of the driving current can be graphically set and monitored. The circuit model of the distributed feedback laser diode (DFBL) was introduced into a TINA-SPICE simulation to evaluate the performance of the current source. Through simulation, the potential self-oscillation can be visually predicted, and the feedback loop can be appropriately compensated. To validate the applicability, the laser driver was used for driving a carefully selected DFB-LD and was employed in wavelength modulation spectroscopy (WMS) for CH\({}_{4}\) detection at \(R(3)\) absorption line of the 2\(\
u\)3 overtone. Under the conditions of room temperature, normal pressure and an effective absorption path of 15.4 cm, repetitive experiments were conducted using gas samples, with their concentrations ranging from 400 ppm to 1%, and the detection limit derived from the signal-to-noise ratio (SNR) was 7.2 ppm. The promising result indicates the high potential of this laser driver for use in absorption spectrum-based sensing applications. laser driver; TINA-SPICE simulation; gas sensing; WMS +
Footnote †: journal: _Sci. still exists an evident drawback in direct absorption spectroscopy: because the emitting wavelength of the laser source employed in TDLAS corresponds mainly to the overtone or combination vibrational transitions, the absorption strength is several orders weaker than those absorption strengths corresponding to the transitions of fundamental vibrational bands [14]. WMS accompanying harmonic detection is therefore introduced into TDLAS to obtain higher detection sensitivity than direct absorption spectroscopy [15]. In TDLAS-WMS, the injection current is usually ramped at a moderate frequency (in the order of Hz) and a sinusoid or \"dither\" with a higher frequency (in the order of KHz) is superimposed upon the ramp to modulate the lasing wavelength. As a result of wavelength modulation and phase sensitive detection, the 1/f noise and laser noise are largely suppressed and thus lead to an enhancement in SNR [16; 17; 18]. Various constant current sources [19; 20; 21] had been developed for driving the LDs. Although the structures of these circuits differ from each other, the basic working principle which can be briefly summarized is roughly the same: The impedance between the collector and emitter of a transistor is automatically adjusted through negative feedback, so that the current flowing through the loop in which it is located tracks the preset value. Compared with other designs, the schematic given in ref. [21] is easier to understand and to fabricate with most inexpensive electronic components. However, it should be pointed out that the stability analysis of the feedback loop and the compensation method had not been mentioned in all of the aforementioned studies, which may not have an impact on the temporary use but rather pose a hidden safety hazard in the future. In this work, the design of a laser driver that is characterized with high stability, easy operation, and AC current output, is simulated using TINA-SPICE [22] and verified by absorption spectrum-based trace gas detection. The outline of this paper is as follows: In Section 2, a brief introduction to the structure of our laser driver is given, and then each building block is described, including the working principle of the current source, the operation of the user-interface, and the composition of the macro-model of DFB-LD. In Section 3, the experimental setup and basic theory of TDLAS-WMS based gas sensors are explained in detail. The advantages of employing a WMS-2f strategy have been detailed previously.