James J. Scherer

cw Integrated cavity output spectroscopy

Abstract A new approach is described in which continuous, narrow band laser sources are employed with the recently developed integrated cavity output spectroscopy technique to obtain sensitive, quantitative absorption spectra in a simple experimental configuration. Absorption data obtained with cw-ICOS are related to the classical Fabry–Perot intracavity absorption model, which describes why the intracavity absorption is enhanced. A method of continuously injecting cw laser light into the cavity is described, as is a simple means of interpreting the ICOS data to extract accurate absorption intensities. Absorption spectra of vibrational combination bands of CO 2 and H 2 O in the 1.3 μm region are presented.

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.

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.

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.

CW integrated cavity output spectroscopy

An approach is described that enables continuous, narrow band laser sources to be used in conjunction with the recently developed integrated cavity output spectroscopy (ICOS) technique to obtain sensitive, quantitative absorption spectra.