Swept continuous wave cavity ring-down spectroscopy

Details Swept continuous wave cavity ring-down spectroscopy Swept continuous wave cavity ring-down spectroscopy

1999-12-23

2002-10-15

Rosenberger, Richard A. (Department: 2877)

Optics: measuring and testing

For light transmission or absorption

Of fluent material

Reexamination Certificate

active

06466322

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to absorption spectroscopy, and more particularly to a method and apparatus for performing ring-down spectroscopy using a continuous wave light source and using two polarizations of light in a cavity.
BACKGROUND ART
Cavity ring-down spectroscopy (CRDS) is a general, high sensitivity technique for measuring absorption. CRDS has been primarily applied to the study of very weakly absorbing species or dilute species concentrations. In CRDS, monochromatic light from a laser is injected into a high finesse optical resonator, called a ring-down cavity (RDC), which encloses a sample. When the light source is abruptly terminated, light trapped inside the RDC decays due to finite resonator losses and can be monitored by detecting light transmitted through a mirror of the RDC. Typically, the light exiting the RDC decays exponentially in time with a decay time constant &tgr;, called the ring-down decay constant. The rate of decay, called the ring-down rate (RDR) is directly proportional to resonator losses due to transmission, scattering, diffraction, absorption etc., and absorption by sample species at a particular wavelength. The RDR is inversely proportional to &tgr;. A spectrum of the sample species is obtained by measuring the RDR as a function of wavelength.
CRDS has been performed using both pulsed and continuous wave (CW) laser sources. Pulsed CRDS (P-CRDS) suffers from several limitations. Because the pulse duration is typically less than several RDC round-trip times, no energy buildup can occur in the optical resonator. The RDC output is therefore severely attenuated at the cavity output. This attenuation produces weak output signals with inferior signal to noise characteristics. Furthermore, many pulsed laser sources have repetition rates lower than 1 kHz, which preclude real-time spectral acquisition and extensive averaging to improve signal-to-noise ratio. Furthermore, most pulse laser sources have line-widths exceeding hundreds of kHz for nanosecond pulses (even when Fourier-transform-limited), which limits the spectral resolution of the CRDS technique.
Recently, efforts have been made to overcome most of these limitations by the use of narrow band (<10 MHz) CW lasers. By coupling the CW laser into the high finesse RDC, light inside the RDC is built up, and cavity throughput increases. In principle, cavity throughput can become close to 1.0, allowing shot-noise-limited detection, as demonstrated by Zare and co-workers in 1998. Much current laser ring-down spectroscopy is still performed with fairly costly laser sources, e.g. Ti:Sapphire lasers, optical parametric oscillators, and external cavity diode lasers. The advent of laser diodes as CW laser sources dramatically decreased the cost of CRDS based laser systems. Semiconductor laser diodes can potentially provide inexpensive laser sources for CW-CRDS as they rapidly improve in power, wavelength coverage, and reliability.
Diode lasers are CW sources with relatively weak output powers, e.g., a few milliwatts (mW). If a CW diode laser is modulated with a small duty cycle (i.e., shorter than the cavity roundtrip time), resonator interference effects and the need to frequency match the laser to a narrow cavity resonance, can be eliminated. Unfortunately, only very small cavity throughput can be achieved this way, which results in very noisy signals.
For example, a high finesse resonator constructed with 99.999% reflecting mirrors attenuates any input by about 10
−10
. The injection of a 1 mW pulse of non-mode-matched CW radiation would result in 10
−13
W of light power at the beginning of the ring-down decay waveform. Such a signal is virtually impossible to detect with a broadband photodetector, particularly in the infrared where photomultipliers are generally ineffective. To record ring-down decay transients with decay constants of order 1 microsecond, a 1 MHz bandwidth, 10
−14
W/Hz
½
noise equivalent detector is typically necessary and a detector noise of about 10
−11
is calculated. The noise, which is inherent in the detection process, is significantly larger than the ring-down signal power at any point in the waveform.
If the laser is locked to one of the cavity resonances over the course of several decay constants, and the laser linewidth is smaller than the cavity resonance, then substantial buildup of the intracavity field can occur. Consequently, strong ring-down signal can be observed after the laser beam is quickly terminated, i.e. faster than &tgr;. The cavity throughput may become close to 1.0, which allows shot-noise limited detection of the ring-down signal. Shot-noise-limited detection of several mW of light can produce high signal-to-noise ratios, on the order of 1,000,000:1.
Laser diode sources typically have linewidths broadened by high frequency jitter to about 10 MHz. This is significantly larger than the typical ring-down cavity linewidth of a few kHz. Classical error signal extraction is, therefore, extremely difficult, The problem of locking a laser and a super-cavity is illustrated in
FIGS. 1
a
and
1
b
.
FIG. 1
a
illustrates locking an ordinary low finesse cavity and laser together. In this case, the laser linewidth is much smaller than the cavity linewidth. Frequency modulation of the laser typically causes modulation of the intensity of light transmitted through the cavity. The amplitude of this intensity modulation, which represents a form of error signal, is generally proportional to the frequency detuning of the laser with respect to the cavity resonance frequency. The phase of the intensity modulation changes sign when the laser passes from a frequency less than the cavity resonance frequency to a frequency greater than the cavity resonance frequency. The error signal is zero if the laser line is centered on the cavity resonance frequency. If this error signal is demodulated, amplified and applied to the element that changes the laser frequency, the laser will be kept in resonance with the cavity.
When the same laser is locked to a very high finesse cavity, the effective laser linewidth is typically much larger than the cavity resonance frequency, as shown in
FIG. 1
b
. The instantaneous frequency changes very rapidly over a spectral range several orders of magnitude larger than the laser linewidth. The duration of the laser center frequency changes can be as short as a few microseconds, so that the laser frequency changes essentially instantaneously. Because the response time of the a ring-down cavity depends on the bandwidth of its frequency changing element, typically a piezo-electric transducer (PZT), most cavities can respond at low kHz rates. In this case, no distinct error signal will be produced from this cavity and the “simple” servo-loop won't work.
In principle, the signal error problem can be overcome using the Pound-Dever locking technique and feedback to an electro-optic or acousto-optic modulator to change the laser frequency of laser light reaching the ring-down cavity. However, these systems require extreme mechanical stability and, for practical systems, a very large locking bandwidth. Furthermore, if the laser light is extinguished, e.g., to measure the decay constant, the Pound-Dever lock would be lost. The locking servo becomes ineffective after each decay waveform measurement, which introduces long system recovery times. Furthermore, strong mechanical perturbations might cause the laser to re-lock to a cavity resonance separated in frequency from the previous resonance. At best, such a locked system would be intermittently usable for recording absorption lines of specific species in real-time, e.g. a concentration measurement every few seconds.
An alternative method for locking a laser diode (LD) to a high finesse cavity utilizes optical feedback. In this method, a small fraction of the laser radiation already accumulated inside the cavity is sent back to the diode laser. The distance between the LD and the cavity input mirror is adjusted so that the feedback radiation is in

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