Development of an IR tunable diode laser absorption spectrometer for trace humidity measurements at atmospheric pressure.
The development of a laser diode absorption spectrometer that uses a strong water vapor absorption at 1393 nm is reported. Three spectroscopic techniques were compared in approximately 0.4 m of laboratory air, namely, frequency modulation, wavelength modulation, and two-tone frequency modulation spectroscopy. The first two techniques use a single-frequency modulation at 9.2 GHz and 1 kHz, respectively, generated either by a phase modulator operating at 9.2 GHz or injection current modulation at 1 kHz. The two-tone method requires modulation at two frequencies, in this case 9.19 and 9.21 GHz. It is shown that the two-tone method should provide the highest sensitivity for a trace moisture detection system.
Edwards
C S
CS
Centre for Length Metrology, National Physical Laboratory, Queens Road, Teddington, Middlesex TW11 0LW, UK.
Barwood
G P
GP
Gill
P
P
Schirmer
B
B
Venzke
H
H
Melling
A
A