Cryogen-free heterodyne-enhanced mid-infrared Faraday rotation spectrometer

Abstract

A new detection method for Faraday rotation spectra of paramagnetic molecular species is presented. Near shot-noise limited performance in the mid-infrared is demonstrated using a heterodyne enhanced Faraday rotation spectroscopy (H-FRS) system without any cryogenic cooling. Theoretical analysis is performed to estimate the ultimate sensitivity to polarization rotation for both heterodyne and conventional FRS. Sensing of nitric oxide (NO) has been performed with an H-FRS system based on thermoelectrically cooled 5.24 μm quantum cascade laser (QCL) and a mercury-cadmium-telluride photodetector. The QCL relative intensity noise that dominates at low frequencies is largely avoided by performing the heterodyne detection in radio frequency range. H-FRS exhibits a total noise level of only 3.7 times the fundamental shot noise. The achieved sensitivity to polarization rotation of 1.8 × 10(-8) rad/Hz(1/2) is only 5.6 times higher than the ultimate theoretical sensitivity limit estimated for this system. The path- and bandwidth-normalized NO detection limit of 3.1 ppbv-m/Hz(1/2) was achieved using the R(17/2) transition of NO at 1906.73 cm(-1).

Shot-noise Limited Faraday Rotation Spectroscopy for Detection of Nitric Oxide Isotopes in Breath, Urine and Blood

“…In an attempt of suppression of these unwanted effects we have already demonstrated that at high frequencies (>10 kHz) FRS can be performed with sensitivities dominated by the fundamental quantum noise without the need for custom laser drivers (for reduction of laser noise) or cryogenic cooling (for reduction of thermal detector noise). However to achieve that goal a rather complex heterodyne enhanced FRS detection (H-FRS) has been developed 30 , which would be difficult to use outside specialized laboratories. In the new DM-FRS method developed here, a signal-to-noise ratio (SNR) comparable to H-FRS has been achieved with a significantly simpler and robust optical layout, which was the key to enable truly mobile sensor systems.…”

“…The laser RIN and the photodetector noise equivalent power (NEP) at signal frequency of ~100 kHz were measured to be σ(ω) = 1.78 × 10 −7 Hz −1/2 and NEP(ω) = 1.39 × 10 −12 W/Hz 1/2 , respectively. An optimum performance (maximum SNR) of FRS spectrometer is achieved at analyzer offset angle that provides laser noise equal to the detector noise 30 . For this DM-FRS system the θ opt is set to 1.8°.…”

“…A noise-equivalent Faraday rotation angle Θ NEA = 7.11 × 10 −9 rad/Hz 1/2 has been determined for this DM-FRS system. Θ NEA is a figure-of-merit that can be conveniently used to compare the performance of different FRS systems 30 .…”

A Review of Signal Enhancement and Noise Reduction Techniques for Tunable Diode Laser Absorption Spectroscopy

“…The isotope ratio (δ 15 N) has been determined with a precision (1σ ) of 0.52 at 800-s averaging time for 100 ppm NO gas with a time resolution of 2 s. In 2011, a Faraday modulation spectrometer for sensitive and fast detection of nitric oxide at 5.33 μm utilizing a room temperature CW distributed feedback QCL was presented by Kluczynski et al (114). The spectrometer provides a detection limit of 4.5 ppb for a response time of 1 s. Most related work on the use of diode laser (VCSEL and QCL)-based Faraday rotation spectroscopy for trace gases sensing (O 2 , NO, and OH radicals) was carried out by Wysocki's group at Princeton University and their collaborators (115)(116)(117). More information can be found on their homepages at http://www.princeton.edu/∼gwysocki/index.htm.…”

Dual modulation Faraday rotation spectroscopy of HO_2 in a flow reactor

“…(13) in [13]} has been used after prior multiplication by a factor of 0.62, which accounts for the effects of the actual wavelength modulation depth used in the experiment and for the losses associated with harmonic detection in DM-FRS. The reduction in 1∕f laser intensity noise measured at 51 kHz for DM-FRS results in a NEA DM-FRS of 8.9 × 10 −9 rad Hz −1∕2 .…”