Dual modulation laser line-locking technique for wavelength...

Details Dual modulation laser line-locking technique for wavelength... Dual modulation laser line-locking technique for wavelength...

1994-11-30

2002-02-26

Evans, F. L. (Department: 2877)

Optics: measuring and testing

For light transmission or absorption

Of fluent material

C250S343000

Reexamination Certificate

active

06351309

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The invention has application to instruments that use optical spectroscopy with variable-wavelength light sources to monitor a known species.
2. Background Art
Optical spectroscopy is a well-established technique that allows quantitation of a known species within a sample by measuring the fraction of light intensity that is absorbed by the sample at a specific wavelength. The underlying scientific principle, known as Beer's Law, is expressed as:
I/I
0
=e
−n&sgr;l
,  (1)
where I is the light intensity after passing through the sample, I
0
is the initial light intensity, n is the species number density or concentration, &sgr; is the species optical absorption cross-section which is a fundamental property of the species and depends on wavelength, and l is the optical path length through the sample. Typically, &sgr; and l are well known, implying that measurement of absorbance, where absorbance is defined as &agr;=−log
e
(I/I
0
), is sufficient to determine n, the species number density within the sample.
Continuous monitoring of a target species concentration can be made practical through the use of continuous measurement of optical absorbance. It is instructive to focus on applications in which the light source is a wavelength tunable continuous-wave laser and the sample probed contains a gas exhibiting an absorption spectrum composed of well resolved, narrow lines. At least one of the absorption lines of the target gas is assumed to lie within the accessible wavelength tuning range of the laser. The quantity of the gas in a sample is determined by measuring the absorbance of the laser light when the laser wavelength is made coincident with one pre-selected absorption feature.
Monitoring species at low concentrations requires measuring accurately weak absorbances, i.e., &agr;<10
−3
. Signals due to weak absorbances are often obscured by laser noise. The dominant noise source is known as “1/f” noise because it decreases with increasing frequency; therefore, most strategies for improving the signal-to-noise ratio of absorbance measurements attempt to shift the detection bandwidth to high frequencies. One such approach, wavelength modulation spectroscopy, is effective for avoiding laser 1/f noise. The technique is described by Wilson (G. V. H. Wilson, “Modulation Broadening of NMR and ESR Line Shapes,”
J. Appl. Phys.
34, 3276-3285 (1963)) and by Arndt (R. Arndt, “Analytical Line Shapes for Lorentzian Signals Broadened by Modulation,”
J. Appl. Phys.
36, 2522-2524 (1965)). The laser wavelength is modulated at a frequency &OHgr; with the modulation amplitude chosen such that the wavelength excursions are comparable to the width of the absorption line being investigated. The laser beam passes through the sample and impinges on a detector that provides a voltage or a current that is linearly proportional to the laser light power or intensity. The detector output is demodulated at the modulation frequency, or some integral multiple of the modulation frequency, to produce a signal that can be related to the sample absorbance. Demodulation methods are usually identified as 1f, 2f, 4f, etc., for demodulation at frequencies &OHgr;, 2&OHgr;, 4&OHgr;, respectively. Demodulation using an odd harmonic, that is, 1f, 3f, etc. gives spectral waveforms that are typically zero when the laser wavelength is coincident with the gas absorption line center wavelength and that exhibit inversion symmetry about the line center wavelength. Detection using an even harmonic, that is 2f, 4f, etc., gives signals with extrema when the laser wavelength is at line center and these signal amplitudes are proportional to sample absorbance.
FIG. 1
includes a representative absorption line spectrum,
10
, as well as 1f, 2f, 3f and 4f spectral waveforms,
12
,
14
,
16
, and
18
, respectively.
To make practical the continuous, long term monitoring of the gas, the laser wavelength must be fixed at a wavelength within the absorption line of the gas. It is often preferred that the fixed wavelength coincide with the center of the absorption line. In the absence of active control of the laser wavelength, the laser wavelength will vary due to changes in the laser temperature, the laser gain profile, etc. Diode laser wavelengths can drift by an unacceptably large amount over time periods of less than 10 minutes. A number of schemes exists that use a selected absorption line of the target gas as a wavelength standard for controlling the laser wavelength. These techniques are known as line-locking methods and are well described by White (A. D. White, “Frequency Stabilization of Gas Lasers,”
IEEE Journal of Quantum Electronics
QE-1, 349-357 (1965)), with improvements to the art presented by Brun (Henri Brun, “Arrangement for Controlling the Frequency of a Light Source Using an Absorption Cell,” U.S. Pat. No. 3,609,583, issued Sep. 28, 1971), by Smith (Peter William Smith, “Apparatus for Stabilizing a Laser to a Gas Absorption Line,” U.S. Pat. No. 3,742,382, issued Jun. 26, 1973), by Buhrer (Carl F. Buhrer, “Frequency Stabilization System,” U.S. Pat. No. 3,593,189, issued Jul. 13, 1971) and by Kavaya (Michael J. Kavaya and Robert T. Menzies, “Spectrophone Stabilized Laser with Line Center Offset Frequency Control,” U.S. Pat. No. 4,434,490, issued Feb. 28, 1984). In each invention, a portion of the laser beam is directed through a reference cell holding a known amount of the gas being studied and then onto a detector. The laser wavelength is modulated by a small amount about its nominal wavelength and this modulation causes synchronous changes in the detector output. The usefulness of the modulation scheme is evident from
FIG. 1
which includes a representative absorption line
10
, i.e., absorbance plotted against laser wavelength. If wavelength modulation amplitude is comparable to the wavelength width of the absorption line and the detector output is processed using a phase sensitive detector, then the resulting spectral waveform looks like a 1f spectral waveform
12
. The signal is zero when the laser average wavelength matches the absorption line center and it varies linearly with small displacements in wavelength about the line center. The signal can be used as a discriminant to correct the laser average wavelength back to the center of the absorption line.
It is the intent of most laser stabilization schemes to obtain the smallest possible fluctuations in the laser wavelength and, in many cases, demonstrated root mean squared wavelength fluctuations are as small as 1 part in 10
10
to 10
12
. For example, both Brun and Smith use the method of saturated absorbance to achieve reference line widths considerably smaller than the line widths exhibited by the same reference gas in a conventional absorption measurement. These narrow line widths provide more precise control of the laser wavelength. The magnitude of the wavelength excursions required to implement wavelength modulation spectroscopy are at least as large as the absorber gas Doppler linewidth, which is larger than 1 part in 10
7
for nearly all gaseous absorbers.
The wavelength stabilization method disclosed by Cook is only applicable to lasers in which the output power as a function of wavelength exhibits the phenomenon known as a “Lamb dip.” Cook's invention is applicable only to some gas lasers in which the extent of continuous wavelength tunability is defined by the Doppler profile of an optical transition of a known gaseous component of the laser gain medium. Similarly, Fork's laser stabilization method (R. L. Fork, “Frequency Stabilized Optical Maser,” U.S. Pat. No. 3,395,365, issued Jul. 30, 1968) is also limited to lasers making use of an active medium characterized by a Doppler broadened optical emission line.
Kavaya and Mead each disclose methods for stabilizing a laser to an absorption line at a wavelength different from the laser center wavelength. These wavelength offset approaches also use only one modulation frequ

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