Bomse D S, Stanton A C, Silver J A
Appl Opt. 1992 Feb 20;31(6):718-31. doi: 10.1364/AO.31.000718.
Wavelength modulation spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS) are compared by measuring the minimum detectable absorbances achieved using a mid-IR lead-salt diode laser. The range of modulation and detection frequencies spans over 5 orders of magnitude. The best results, absorbances in the low-to-mid 10(-7) range in a 1-Hz bandwidth, are obtained by using high-frequency WMS (10-MHz detection frequency) and are limited by detector thermal noise. This sensitivity can provide species detection limits well below 1 part per billion for molecules with moderate line strengths if multiple-pass cells are used. High-frequency WMS is also tested by measuring the absorbance due to tropospheric N(2)O at 1243.795 cm(-1). WMS at frequencies < 100 kHz is limited by laser excess (1/f) noise. Both of the FMS methods, which require modulating the laser at frequencies >/= 150 MHz, give relatively poor results due to inefficient coupling of the modulation waveform to the laser current. The re ults obtained agree well with theory. We also discuss the sensitivity limitations due to interference fringes from unintentional étalons and the effectiveness of étalon reduction schemes.
通过测量使用中红外铅盐二极管激光器实现的最小可检测吸光度，对波长调制光谱法（WMS）以及单音和双音频率调制光谱法（FMS）进行了比较。调制和检测频率范围跨越5个数量级。通过使用高频WMS（10 MHz检测频率）可获得最佳结果，即在1 Hz带宽内吸光度处于低至中10^(-7)范围，且受探测器热噪声限制。如果使用多程池，这种灵敏度可为具有中等谱线强度的分子提供远低于十亿分之一的物种检测限。还通过测量1243.795 cm^(-1)处对流层N₂O的吸光度对高频WMS进行了测试。频率<100 kHz的WMS受激光过量（1/f）噪声限制。两种FMS方法都需要在频率≥150 MHz下调制激光器，由于调制波形与激光电流的耦合效率低下，结果相对较差。获得的结果与理论非常吻合。我们还讨论了由于无意形成的标准具产生的干涉条纹导致的灵敏度限制以及标准具减少方案的有效性。
