Optical feedback cavity enhanced absorption spectroscopy with diode lasers

Abstract

An optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) apparatus has been developed, using a distributed feedback diode laser centred close to a wavelength of 1596 nm. Light from a V-shaped optical cavity is allowed to feed back to the laser diode, injection seeding the laser at the cavity mode frequencies, and bringing about linewidth narrowing and frequency locking to the cavity resonance. The OF-CEAS technique's expediency and sensitivity have been demonstrated, first on measurements of aerial water absorptions close to 6260.8 cm(-)(1), and then on two carbon dioxide absorptions close to 6261.7 cm(-)(1) which may be used to measure the concentrations of (13)CO(2) and (12)CO(2) isotopologues in a CO(2) sample. Employing a cavity of moderate finesse (F approximately 3500), an acquisition time-reduced minimum detectable absorption coefficient of 5.8 x 10(-)(9) cm(-)(1) s(1/2) has been determined.

Detection of HO2in an atmospheric pressure plasma jet using optical feedback cavity-enhanced absorption spectroscopy

“…Consequently, the optical cavity is used to both greatly increase the optical pathlength through the sample and enhance the measured signal intensity, resulting in very low detection limits being achievable [18,24]. Initially, NIR diode lasers were used as sources [25,26], but recently OF-CEAS instruments based on quantum and interband cascade lasers have also been reported [27][28][29][30][31].…”

Optical-feedback cavity-enhanced absorption spectroscopy in a linear cavity: model and experiments

“…If the cavity has a high finesse, this returning field will have a narrow linewidth (~10-100 s kHz) relative to the free-running linewidth of the laser (~few MHz for QCLs, typically determined by the quality of the current controller). Thus, a resonant cavity field in phase with the laser will induce a narrowing of the emitted linewidth and stimulate emission at the resonant wavelength [6,8,23]. The laser becomes "locked" to the cavity frequency; we note that this term can be misleading as it applies in this context to the condition when the laser frequency is being tuned very slowly due to feedback rather than remaining stationary at a single value.…”

“…Optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) is a variant that uses the optical cavity to reduce the linewidth of the laser source emission and thereby increase lasercavity coupling [5][6][7][8][9][10][11][12]. In essence, the resonant cavity field retraces back to the laser facet and acts as an external source of injection seeding.…”

“…Most work has been conducted using V-shaped optical cavities where the output of the laser is injected at the central mirror. In this set-up, any light not injected into the cavity is reflected away from the incident laser propagation direction ensuring that only light leaking out of the resonant optical cavity can return to the laser facet [6,8,28]. Using this geometry, Maisons et al reported a minimum detectable absorption coefficient (α min ) of 3 × 10 −9 cm −1 for 1-s averaging measurements of N 2 O and CO 2 with a 4.46 µm QCL, while Hamilton et al went to longer wavelengths to probe CH 4 and N 2 O at 7.84 µm and achieved an α min of 5.5 × 10 −8 cm −1 for 1-s averaging [9,10].…”

Cavity-based absorption spectroscopy techniques

“…Such OF-CRDS approaches have been used to measure absorption spectra (Morville et al , 2004;Motto-Ros et al , 2007;van Helden et al , 2009;Tan et al , 2010), aerosol-particle extinction (Butler et al , 2007(Butler et al , , 2009) and ultra-high refl ectivity (Gong et al , 2008). In addition to timedomain OF-CRDS measurements, its closely related amplitude-domain analogue, CEAS/ICOS (as in Section 6.4.1 ), has also been applied in the OF mode to absorption spectroscopy (Morville et al , 2005;Courtillot et al , 2006;Kerstel et al , 2006;Romanini et al , 2006;Motto-Ros et al , 2008;Wehr et al , 2008;Baran et al , 2009;van Helden et al , 2009;Ventrillard-Courtillot et al , 2009;Hamilton et al , 2010;Habig, et al , 2012). We note that analogous passive feedback between an AR-coated diode laser and a simple three-mirror V-shaped optical build-up cavity was demonstrated in early frequency-locking work by King and Pittaro (1998), providing an excitation source for measurements of spontaneous Raman spectra.…”