Joshua B. Paul

Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment

A simple and easy to use method that allows high-finesse optical cavities to be used as absorption cells for spectroscopic purposes is presented. This method introduces a single-mode continuous-wave laser into the cavity by use of an off-axis cavity alignment geometry to eliminate systematically the resonances commonly associated with optical cavities, while preserving the absorption signal amplifying properties of such cavities. This considerably reduces the complexity of the apparatus compared with other high-resolution cavity-based absorption methods. Application of this technique in conjunction with either cavity ringdown spectroscopy or integrated cavity output spectroscopy produced absorption sensitivities of 1.5 x 10(-9) cm(-1) Hz(-1/2) and 1.8 x 10(-10) cm(-1) Hz(-1/2), respectively.

Broadband ringdown spectral photography

A new technique that enables frequency-resolved cavity ringdown absorption spectra to be obtained over a large optical bandwidth by a single laser shot is described. The technique, ringdown spectral photography (RSP), simultaneously employs two key principles to record the time and frequency response of an optical cavity along orthogonal axes of a CCD array detector. Previously, the principles employed in RSP were demonstrated with narrow-band laser light that was scanned in frequency [Chem. Phys. Lett. 292, 143 (1998)]. Here, the RSP method is demonstrated using single pulses of broadband visible laser light. The ability to obtain broad as well as rotationally resolved spectra over a large bandwidth with high sensitivity is demonstrated.

Infrared cavity ringdown and integrated cavity output spectroscopy for trace species monitoring

Although the ability of high finesse optical cavities to provide effective absorption path-lengths exceeding 10 km. has been known for quite some time, attempts to utilize this property for the purposes of high-resolution spectroscopy have often resulted in extremely complex experimental systems. Here, we demonstrate how off-axis optical paths through such cavities can be employed to produce relatively simple spectrometers capable of ultrasensitive absorption measurements. A proof-of-concept study using visible diode lasers has achieved a normalized absorption sensitivity of 1.8*10-10 cm-1Hz-1/2. Additionally, quantum cascade lasers have been employed to extend this method into the mid-infrared region, where sensitivities of 1.2*10-9 cm-1Hz-1/2 have been obtained.

Sensitive absorption measurements in the near-infrared region using off-axis integrated cavity output spectroscopy

A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-infrared diode lasers and absorption spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of ~1.4x10-5 over an effective optical path of 4,200 meters was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of ~3.1x10-11 cm-1 Hz-1/2. The instrument has been applied for measurements of CO, CH4, C2H2, and NH3 in the 1530-1650 nm range, and stable isotopes of CO2 (13CO2, 12CO2, 12C16O18O) near 2052 nm.

Pump-enhanced difference-frequency generation at 3.3 μ m

The demonstration of continuous wave intracavity difference-frequency generation in the mid-infrared (mid-IR) is presented. A cavity for pump laser enhancement is constructed around a periodically poled lithium niobate crystal, and the cavity length is locked to the frequency of the pump laser using the Pound-Drever-Hall technique, producing a gain of 12 in the resultant idler power compared to the single-pass case. A widely tunable single-mode 3.3 microm idler beam with a power of nearly 10 mW is available for direct absorption spectroscopy. The pump-enhancement method demonstrated here should be readily scalable to produce hundreds of milliwatts of mid-IR light by using higher power signal and pump lasers.

Broadly-tunable high-power fiber laser system for IR spectral range

Broadly tunable fiber laser system has been demonstrated at room temperature. The IR fiber laser system consists of two high peak power individual pulse fiber lasers and their difference frequency generation (DFG) in Periodically Poled Lithium Niobate (PPLN) nonlinear optical crystal. Both lasers were operating at 20 kHz Pulse Repetition Rate and 200 ns pulse duration with diffraction limited beam quality. The first laser is a CW, < 16 GHz spectral bandwidth, tunable in 1050-1081 nm spectral range and Acousto- Optically extra cavity amplitude modulated Yb fiber laser ring oscillator - high power two stage amplifier which produced 280 mW of average and 70W of Peak power, respectively. The second laser is a DFB CW, single 1556 nm wavelength, < 1MHz spectral bandwidth and extra cavity amplitude modulated Er fiber laser oscillator - high power single stage amplifier with 160 mW of output average power and 40W of peak power. Synchronized pulses from two fiber lasers have been combined and fiber coupled into a single polarization maintained fiber using a fiber WDM combiner and then fibercoupled into the NovaWave Technologies, commercial DFG laser module which employed 50 mm PPLN crystal. The DFG stage of the system produced tunable radiation in 3236.4-3545.4 nm spectral range (309 nm). The difference frequency generation has a 9 mW average power, 20 kHz pulse repetition rate and 200 ns pulse duration which corresponds to 2.25 W of peak power. The demonstrated pulse DFG conversion efficiency is 0.2 W/W2 (20%/W) which is ~ 100 times higher than that of CW operation. Further scaling of IR laser power was limited by optical damage of PPLN crystal and fiber lasers combining optics. Using a PPLN-MgO crystal and additional fiber laser amplifier stages based on Large Mode Area gain fibers is expected to allow us to achieve damage free difference frequency generation with up-to 100 mW of average power and peak power of up to 25 W.

Sensitive absorption measurements based on novel cavity enhanced spectroscopy techniques

A novel laser-based absorption diagnostic technique based on off-axis paths in high-finesse optical cavities provides high measurement sensitivities (~2×10−10 cm−1). Applications to environmental monitoring and industrial process control using near-IR semiconductor diode lasers and mid-IR quantum cascade lasers will be presented.

Broadband ringdown spectral photography for real-time environmental monitoring

Over the last decade there has been a significant increase in the implementation of cavity ringdown- based methods for spectroscopic studies in the gas phase[l]. Although these methods have demonstrated high sensitivity in numerous applications and spectral regions, real-time applications of the approach have been hindered by the need to tune the probe laser over the absorption lineshape while the ringdown time is obtained. The resultant pointwise determination of the frequency dependent intracavity loss leads to monitoring times ranging from seconds to minutes, depending on the spectral coverage required. In many cases, large optical bandwidths need be covered in order to differential between absolute absorption intensities for the target species vs. other intracavity losses. In real-world environments, large bandwidths must frequently be measured in order to identify spectral interferences from non-target species, broadband absorption due to soot, or scattering due to particulates. As real-time environmental monitoring requires the need to deal effectively with transient conditions, tuned-frequency approaches can be inadequate.