High band-width mid-infrared frequency-modulated Faraday rotation spectrometer for time resolved measurement of the OH radical.

We present a novel mid-infrared frequency-modulated Faraday rotation spectrometer (FM-FRS) for highly sensitive and high bandwidth detection of OH radicals in a photolysis reactor. High frequency modulation (up to 150 MHz) of the probe laser using an electro-optical modulator (EOM) was used to produce a modulation sideband on the laser output. An axial magnetic field was applied to the multi-pass Herriott cell, causing the linearly polarized light to undergo Faraday rotation. OH radicals were generated in the cell by photolyzing a mixture of ozone (O3) and water (H2O) with a UV laser pulse. The detection limit of OH reaches 6.8 × 108 molecule/cm3 (1σ, 0.2 ms) after 3 and falling to 8.0 × 107 molecule/cm3 after 100 event integrations. Relying on HITRAN absorption cross section and line shape data, this corresponds to minimum detectable fractional absorption (Amin) of 1.9 × 10-5 and 2.2 × 10-6, respectively. A higher signal-to-noise ratio and better long-term stability was achieved than with conventional FMS because the approach was immune to interference from diamagnetic species and residual amplitude modulation noise. To our knowledge, this work reports the first detection of OH in a photolysis reactor by FM-FRS in the mid-infrared region, a technique that will provide a new and alternative spectroscopic approach for the kinetic study of OH and other intermediate radicals.

Portable cavity ring-down spectrometer for an HO2 radical measurement: instrument's performance and potential improvement using a narrow linewidth laser.

We report the development of a portable cavity ring-down spectrometer (CRDS) for direct and absolute measurement of HO2 radical concentration using a distributed feedback (DFB) diode laser operating at 1506 nm. The spectrometer has a compact design with all optics in a 1000 × 400 × 140 mm3 box. At a pressure of 100 mbar and a ring-down time (τ0) of 136 µs, the detection limit of the CRDS spectrometer was ∼ 7.3 × 107 molecule/cm3 (1σ, 10s). The corresponding detection sensitivity was 1.5 × 10-11 cm-1, which was close to the state-of-the-art performance. By replacing the DFB diode laser with a narrow linewidth erbium-doped fiber (EDF) laser, the amplitude fluctuation caused by the laser phase noise was reduced and the cavity mode injection efficiency was improved. The sensitivity was improved to 3.9 × 10-12 cm-1 with a short data-acquisition time of 0.2 s. Compared with the DFB laser, the improvement was nearly an order of magnitude. The use of the narrow linewidth laser is attractive. The instrument can achieve very high sensitivity without the need for a complex locking technique, ensuring simple and ease of use in future field applications.

Optical-feedback cavity-enhanced absorption spectroscopy for OH radical detection at 2.8†µm using a DFB diode laser.

We report the development of an optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) instrument for OH detection at 2.8 µm using a DFB diode laser. Two different approaches, symmetry analysis and wavelength modulation, were performed to achieve laser frequency locking to the cavity mode. Compared with the symmetry analysis method, the wavelength modulation method continuously locked the laser frequency to the cavity mode and eliminated decoupling the laser from the cavity mode. A detection sensitivity of 1.7×10-9 cm-1 was achieved in a 25 s sampling time and was about 3 times better than that of the symmetry analysis method. The corresponding OH detection limit was ∼ 2×108 molecule/cm3. Further improvement can be achieved by using higher reflectivity mirrors and other high-sensitivity approaches, such as frequency modulation spectroscopy and Faraday rotation spectroscopy.

Amplitude-Modulated Cavity-Enhanced Absorption Spectroscopy with Phase-Sensitive Detection: A New Approach Applied to the Fast and Sensitive Detection of NO2.

Accurate and sensitive measurements of NO2 play an extremely important role in atmospheric studies. Increasingly, studies require NO2 measurements with parts per trillion by volume (pptv-level) detection limits. Other desirable instrument attributes include ease of use, long-term stability, and low maintenance. In this work, we report the development of an amplitude-modulated multimode-diode-laser-based cavity-enhanced absorption spectroscopy (AM-CEAS) system operating at 406 nm that uses phase-sensitive detection for extremely sensitive NO2 detection. The laser was TTL-modulated at 35 kHz. The mirror reflectivity was determined to be 99.985% based on the ring-down time measurement. The cavity base length was 47.5 cm, giving an effective absorption pathlength of ∼3.26 km. AM-CEAS achieved a 1σ detection precision of 35 pptv in a 1 s data acquisition time (4.98 × 10-10 cm-1), over 4 times lower than that attained using a ring-down approach and the same optical system. The AM-CEAS precision improved to 8 pptv over a data acquisition time of 30 s (1.14 × 10-10 cm-1). The AM-CEAS method with the multimode diode laser integrates the advantages of high light injection efficiency like on-axis alignment cavity ring-down spectroscopy, low cavity-mode noise like off-axis alignment CEAS, and narrow-bandwidth high-sensitivity weak signal detection of modulation spectroscopy, providing a powerful, straightforward, and general method for ultrasensitive absorption and extinction measurements.

Long-term digital frequency-stabilized laser source for large-scale passive laser gyroscopes.

We report on the development of a digitally controlled long-term frequency stabilized ultrastable laser source, which serves as an injection laser to stabilize the perimeter of a 3 m × 3 m heterolithic passive resonant gyroscope. We operate the gyroscope at two different cavity modes to reduce back-scattering coupling disturbance for gyroscope locking. This scheme increases the requirement for the injection laser frequency stability since we are using the wavelength of the laser as the length standard for the heterolithic gyroscope structure. The laser source is digitally locked to an ultrastable high-finesse Fabry-Perot cavity and a femtosecond optical frequency comb referenced to an active hydrogen maser simultaneously. The fractional frequency stability of the locked laser is better than 1.2 × 10-14 for averaging times from 0.1 s to 10 000 s. The short-term frequency stability is limited by the stability of the Fabry-Perot cavity, and the long-term frequency stability is limited by the stability of the frequency comb. The digital locking system enables the laser to run autonomously for weeks and can quickly relock itself within seconds to ensure continuous running of the gyroscope. The digital frequency stabilization technique can also fulfill the requirements of space gravitational waves detection and the next generation space gravity recovery mission.