Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2001-01-05
2004-02-17
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical waveguide sensor
C385S037000, C385S145000
Reexamination Certificate
active
06694067
ABSTRACT:
TECHNICAL FIELD
The present invention relates to chemical sensing to detect the presence or measure the amount or concentration of specific chemicals or classes of chemicals in the environment surrounding a sensor. The invention relates in particular to fiberoptic or optical waveguide chemical sensors, especially that class of chemical sensors which uses changes in optical transmission or absorption in the fiber or waveguide or evanescent wave emanating therefrom to sense the chemicals.
BACKGROUND ART
Detection and measuring of atomic and molecular trace chemical species using lasers dates to the 1980s. Because the various chemical species have unique absorption spectra, several forms of absorption spectroscopy have been developed to detect them at low levels (on the order of parts per billion) with short response times (on the order of microseconds) and with limited interference from the other chemical species that may be present. One such technique, ring-down cavity (RDC) spectroscopy, measures the absorption rate of a light pulse confined within a stable, low loss optical cavity. The cavity may be formed from a pair of highly reflective (R≧. 9999) mirrors in stable resonator configurations. Light from a pulsed laser tuned to match an absorption frequency of a chemical species of interest is coupled into the optical cavity through one mirror. If the pulse length is less than the cavity's round trip time, a small (~10
−5
) but stable fraction of the incident light enters t:he cavity and “rings” back and forth between the cavity mirrors. The number of photons trapped in the cavity slowly decays (rings down) due to the combined loss through the cavity mirrors and the presence of a chemical absorber and/or scattering in the cavity. For an empty cell, the decay time constant is &tgr;=d/[c(1−R)], where d is the cavity length, c is the speed of light, and R is the average mirror reflectivity. When an atomic or molecular absorber is present in the cavity, the decay rate will be increased, with strong absorption resulting in a faster decay then weak absorption. At each round trip of the injected pulse, a small fraction (~10
−5
) of the intracavity light is transmitted through the back mirror and is detected by a sensitive photodetector such as a photomultiplier tube, with the time constant of the detector set long compared to the cavity round trip time, its output follows a smooth exponential decay that can be analyzed in the time domain to obtain the absorption coefficient of the chemical species in the cavity, and indirectly its concentration. An advantage of this ring down cavity technique is that it is highly sensitive, limited largely by the reflectivity of the cavity mirrors, and is insensitive to amplitude fluctuation between laser pulses. However, the laser wavelength must be matched to an absorption line of the chemical of interest. In practice, wavelength dependent ringdown signals are determined in a pointwise manner to determine the corresponding absorption spectrum for the species within the cavity. This data is then converted into species concentration using standard analytical methods.
The ring-down cavity spectroscopy technique can be modified to use a continuous-wave (cw) laser source in place of a pulsed laser source. Examples of such cw-laser-based spectroscopy instruments are described in U.S. Pat. No. 5,528,040 to Lehmann and U.S. Pat. No. 5,903,358 to Zare et al., as well as by Anthony O'Keefe et al. in Chemical Physics Letters 307, pp. 343-349 (Jul. 9, 1999). In one such cw laser technique, the laser frequency is scanned and ringdown decay events recorded to provide an absorption spectrum as a function of laser frequency. The radiation intensity in the cavity will build up when excited at a resonance frequency coinciding with a cavity mode and will ring down at a decay rate corresponding to the absorption by the sample gas when the optical frequency is modulated off of the cavity mode. In another cw laser technique, the optical frequency of the laser source is maintained at the resonance frequency of a cavity mode. The intracavity intensity builds up to a saturation value determined by the mirror reflectivity and sample gas absorption. An advantage of this technique is that there is higher intracavity power, but it requires that the cavity be stabilized. A third cw technique, described in the aforementioned paper of O'Keefe et al. in Chemical Physics Letters, uses dithering to avoid having to stabilize the cavity. The laser output is modulated rapidly over a frequency spacing containing several cavity modes, or one of the cavity mirrors is vibrated using a piezoelectric transducer to rapidly modulate the cavity modes, or both. The average transmission through the cavity is measured to obtain the effective absorption for the mirrors and chemical sample. The light trapped in the optical cavity passes through the absorbing sample many times, effectively amplifying the absorption signal. The resulting average change in the signal transmitted through the cavity is greatly enhanced over what would result from a single pass through the same sample pathlength. In a simple system, this enhancement is given by the inverse of the fractional mirror transmission. For a system comprised of two mirrors of identical reflectivity, R, this fractional transmission is T=1−R, and the absorption enhancement is 1/T. By addition to this simple system of a weak fractional absorption per cavity length, equal to k, the observed average absorption is equal to k divided by T. Because T is typically very small, in the range of 0.01 to 0.00001, the observed amplification of the absorption signal is large, ranging from 100 to 10000. This makes this very useful in measuring weak absorption signals. The observed transmission signal is recorded as a function of light source frequency, with and without sample to provide a complete quantitative spectrum.
All of these optical cavity techniques are very sensitive because they effectively multiply the sample path length by large factors. However, they are limited in their practical application to very clean gas samples only, because strong absorption or other attenuation mechanisms drive the system to saturation rapidly. Anything that scatters light, such as glass flow tubes or cells, turbulent samples (e.g. flames), or liquid samples, must be avoided, because any amount of scattering tends to interfere with accurate absorption measurement.
A different class of sensor is used to detect trace chemical species of interest in liquid and gas samples. In particular, sensors that employ fiber optics and optical waveguides are becoming increasingly common for applications where a chemical sensor needs to be introduced into a liquid medium, such as a storage tank or well. Fiber optics and optical waveguides make it possible to place a chemical sensor device in contact with the medium without needing to also insert the entire optical source and detector system. An optical fiber or waveguide
13
transmits light from a source
11
to the sample
15
and returns the optical signal to the detection system
17
, as shown in FIG.
1
. Such sensors employ a variety of means for producing a chemical detection signal.
Some devices employ a two-fiber or two-waveguide geometry made up of a source fiber or waveguide and a signal fiber or waveguide. For example, the source fiber may send a excitation optical signal to an end fiber piece containing an optical phosphor, producing a fluorescence signal that is sensitive to the presence of certain chemicals. The signal fiber is located relative to the source fiber and phosphor such that the resulting signal is partially collected and transmitted to a detector for measurement and analysis. One drawback of this scheme is that the quenching of the fluorescence signal is effected by many types of chemicals, so the number of chemical species that can be unambiguously sensed is limited.
Another fiber or waveguide-based approach uses evanescent wave effects to provi
O'Keefe Anthony
Scherer James J.
Bovernick Rodney
Los Gatos Research
Protsik Mark
Schneck Thomas
Song Sarah U
LandOfFree
Cavity enhanced fiber optic and waveguide chemical sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Cavity enhanced fiber optic and waveguide chemical sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cavity enhanced fiber optic and waveguide chemical sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3287140