Sub-ppm-Level Ammonia Detection Using Photoacoustic Spectroscopy with an Optical Microphone Based on a Phase Interferometer

A sensitive optical microphone for photoacoustic spectroscopy based on the common path topology of a fibre laser Doppler vibrometer (FLDV) using phase-generated carrier demodulation and a slim diaphragm as an acoustic wave transducer was demonstrated. A resonant gas cell was adapted to enhance gas-detection performance and simultaneously provide efficient cancellation of the window background acoustic signal. Ammonia (NH3) was selected as the target gas. The absorption line was experimentally identified using a distributed feedback laser diode emitting at 1530 nm. The linearity and sensitivity of the gas sensor were measured using wavelength modulation spectroscopy with second harmonic detection. A Teflon diaphragm was used to implement the optical microphone, along with the FLDV, showing a minimum detectable pressure of 79.5 μPa/Hz1/2. The noise-equivalent absorption sensitivity for NH3 detection at the absorption line at 1531.7 nm was 1.85 × 10−8 W cm−1 Hz−1/2, and the limit of detection was 785 ppbv.

Download Full-text

Frequency Modulation Detection and its Application to Absorption Line Shape Parameters

Applied Spectroscopy ◽

10.1366/0003702824638511 ◽

1982 ◽

Vol 36 (3) ◽

pp. 227-235 ◽

Cited By ~ 11

Author(s):

J. P. M. de Vreede ◽

S. C. Mehrotra ◽

A. Tal ◽

H. A. Dijkerman

Keyword(s):

Absorption Line ◽

Frequency Modulation ◽

Second Harmonic ◽

Modulation Depth ◽

Theoretical Calculations ◽

Harmonic Signal ◽

Rotational Transition ◽

Center Frequency ◽

Line Center ◽

Harmonic Detection

The theory for frequency modulation given by Karplus has been extended to include Doppler broadening and arbitrarily large modulation depths. It is shown that the detection limit for second harmonic detection is of the same order as for first harmonic detection: the modulation depth must be approximately the full linewidth of the line profile. Procedures are discussed for extracting from experimental data the linewidth and center frequency of an absorption line with a Voigt profile. At zero detuning information about collisional line broadening can be obtained from the dependence of the second harmonic signal upon the modulation depth. Some experimental examples in the microwave region are presented as a test of the theory. Measurements on the rotational transition J = 2 → 3 (ν2 = 1) of OCS yield collisional self-shift and self-broadening parameters of −0.14 (±0.01) and 5.97 (±0.01) MHz/Torr, respectively; these values are in accordance with literature but are more accurate. For the line center at zero pressure a much more accurate value of 36 571 422.0 ± 0.7 kHz has been found, which is in agreement with recent theoretical calculations.

Download Full-text

Absolute frequency control of a 1560 nm (192 THz) DFB laser locked to a rubidium absorption line using a second-harmonic-generated signal

IEEE Transactions on Instrumentation and Measurement ◽

10.1109/19.392867 ◽

1995 ◽

Vol 44 (4) ◽

pp. 839-842

Author(s):

C. Latrasse ◽

M. Poulin ◽

M. Tetu ◽

M. Breton ◽

M. Poirier

Keyword(s):

Absorption Line ◽

Second Harmonic ◽

Frequency Control ◽

Absolute Frequency ◽

Dfb Laser

Download Full-text

Interrogating the health condition of rails using the narrowband Rayleigh waves emitted by an innovative design of non-contact laser transduction system

Structural Health Monitoring ◽

10.1177/1475921720967600 ◽

2020 ◽

pp. 147592172096760

Author(s):

Faeez Masurkar ◽

Kim Ming Ng ◽

Peter W Tse ◽

Nitesh P Yelve

Keyword(s):

Health Status ◽

Optical System ◽

Rayleigh Waves ◽

Rail Steel ◽

Second Harmonic ◽

Laser Doppler Vibrometer ◽

Health Condition ◽

Innovative Design ◽

Scanning Laser Doppler Vibrometer ◽

Contact Experiment

The article reports an innovative optical system that is designed to interrogate the health condition of macroscopically intact rail specimens by measuring its inherent nonlinearity using the narrowband Rayleigh waves. A line-arrayed pattern is developed through the optical system that generates narrowband Rayleigh waves with high power on the surface of the rail. As a result of lattice-anharmonicity, a second harmonic is produced in the wave that is sensed by a scanning laser Doppler vibrometer. The spectral amplitudes of the first and generated second harmonics are used to calculate the inherent nonlinearity using an amplitude-based nonlinearity equation. These measurements are carried out on the head, web, and foot of the rail. The performance of the non-contact experiment is also compared with that of a contact experiment carried out using wedge transducers. The experimentally evaluated nonlinearity of the rail steel is further compared with that obtained using a physics-based nonlinearity equation that relies on the higher-order elastic constants. Agreement of the results shows that the new optical system is effective in generating Rayleigh waves in rails and thereby measuring the inherent nonlinearity of the rail track. The estimation of inherent nonlinearity may help in diagnosing the health status of the macroscopically intact rail specimens in terms of their microstructural consistency and level of dissolved impurities before fixing them on a track.

Download Full-text