Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
2003-02-28
2004-08-10
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
C356S432000, C356S433000, C356S435000
Reexamination Certificate
active
06775001
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a laser-based spectrometer that is used to measure the absorption spectrum and other physical parameters of trace gases and other substances. Specifically, it provides a way to use room-temperature quantum cascade lasers and other pulsed and unstable wavelength laser sources for sensitive absorption measurements. This invention can be used in the field of laser spectroscopy, industrial processing, environmental monitoring, medicine, biophotonics, and related fields and in particular in the mid-infrared range (3 &mgr;m to 15 &mgr;m) with room-temperature quantum cascade lasers for spectroscopic measurements.
BACKGROUND OF THE INVENTION
Gaseous, solid, molecular, chemical, or biological substances can be identified and much information about their surrounding environment—temperature, pressure, neighboring material constituents, completed or ongoing chemical and biological activities, for example—can be obtained by measuring the optical spectrum of light that the substances emit or absorb. The making of such measurements is spectroscopy, a wide and diverse field with applications in industry, environmental monitoring, process control, research and development, combustion control, forensics, and a variety of other fields. Applied to biological tissues, materials, or molecules, spectroscopy is a component of biophotonics, a rapidly growing field embracing the life sciences and optical measurement technologies. Applied to medicine, spectroscopy is a component of the rapidly growing field of medical photonics.
Absorption spectroscopy, where the amount of light absorbed in a substance is determined at different wavelengths, is one of the most important of the spectroscopic measurements. Absorption measurements using narrowband tunable lasers having a bandwidth narrower than the absorption features of the substance to be measured have become particularly important, as they allow increased sensitivity and can be done with simple apparatus. In particular, they can be carried out without using monochromators or spectrometers that are otherwise required to provide narrowband optical signals or high resolutions. Furthermore, narrowband laser sources provide much more intense concentrations of light in the required measurement bandwidth, greatly increasing the ease of detection and reducing measurement times.
The sensitivity of an absorption measurement is determined by the absorption cross section of the substance to be measured at the measurement wavelength, by the distance the measurement beam travels through the medium containing the substance, and by the ability of the photodetection apparatus to detect small changes in power transmitted through the substance. Absorption cross sections are usually larger at longer wavelengths, so measurement sensitivities can be increased by operating in the mid-infrared wavelength region (3 &mgr;m to 15 &mgr;m) as opposed to the near-infrared wavelength region (0.8 &mgr;m to 3 &mgr;m) or the visible wavelength region (0.4 &mgr;m to 0.8 &mgr;m). For example, moisture can be detected at 2.0 parts per billion (ppb) sensitivity at 5.94 &mgr;m, but only 60 ppb sensitivity at 1.39 &mgr;m using typical measurement parameters. For some substances, the sensitivities are vastly different. For carbon dioxide, for example, the detection sensitivities are 0.13 ppb at 4.23 &mgr;m versus detection sensitivities of 3,000 ppb at 1.96 &mgr;m. For carbon monoxide, detection sensitivities are 0.75 ppb at 4.60 &mgr;m versus 30,000 ppb at 1.570 &mgr;m. These numbers show that measurement sensitivities can be a thousand or even ten thousand times more sensitive at mid-infrared wavelengths. A consequence is that measurements at longer wavelengths can be carried out using much shorter path lengths and lower detection efficiencies provided that appropriate lasers are available. This lowers costs considerably.
The usual method for carrying out a spectroscopic absorption measurement of the concentration of a trace gas or other substance using a laser is outlined in FIG.
1
. Light from a semiconductor laser
11
is collimated into a parallel beam using collimator
12
consisting of a telescope and spatial filter. The collimated parallel beam of light propagates to beamsplitter
13
which sends part of the beam through lens
14
to photodetector
15
where it is detected and the resulting photocurrent sent to the electronics processor
16
. The remainder of the collimated parallel beam propagates through gas cell
17
containing the gas or other substance to be measured. The gas cell may be a single pass cell where light passes through once, it may be a multi-pass cell where there are reflecting mirrors that reflect the beam back and forth several times, or the gas cell may be simply an area of free space that the beam passes through. After passing through the gas or other substance to be measured, the beam is focused through lens
18
onto photodetector
19
where it is detected and the resulting photocurrent sent to the electronics.
FIG. 2
shows a representative absorption spectrum for nitric oxide calculated assuming a concentration of 1 part per billion, a pressure of 0.08 atm, and an absorbing path length of 5000 meters.
FIG. 3
show details of the absorption spectrum of FIG.
2
.
Spectroscopy, like many other photonics technologies, is moving out of the laboratory and into the workplace. The growth in the number of applications, combined with the advances in photonics achieved in the related field of optical telecommunications, has both increased the market size for spectroscopic devices and has created strong interest in the development of low-cost, small, easy to use, reliable, robust, and sensitive spectroscopic devices. In particular, the commercialization of tunable semiconductor lasers for optical telecommunications applications has lead to the ready availability of low-cost infrared laser sources that, combined with modern optical and electronics design, have the potential to open broad new markets for spectroscopic instruments. Infrared diode lasers offer the possibility of building spectroscopic absorption measurement devices that have increased sensitivity, including the capability to measure trace gas concentrations in the part per billion range; excellent selectivity for a particular substances without interference from other substances, robustness, maintenance-free operation, long laser lifetimes, speed, simple control and data acquisition mechanisms, small, compact designs, and low costs.
Semiconductor laser sources currently in use or under development in the near-infrared include inexpensive high-quality easily-tunable DFB and DBR lasers that operate at room temperature that are ideal for spectroscopic applications. In the mid-infrared where absorption cross sections are large and the potential for sensitive, low-cost devices is the greatest, the choices are lead-salt semiconductor lasers and unipolar quantum well lasers (quantum cascade lasers). However, lead-salt lasers require cryogenic cooling, have narrow tuning ranges, and have mode hops where the operating wavelength changes abruptly. Quantum cascade lasers have excellent properties when cooled to cryogenic temperatures, but operate at room temperature only in a pulsed mode that produces a sequence of pulses where the wavelength changes in time and the pulses vary in average wavelength. This rules out sensitive absorption measurements with lasers operating at room temperature using conventional measurement techniques in the mid-infrared.
Quantum cascade lasers, although not able to operate continuously at room temperature, are otherwise ideal for mid-infrared spectroscopic measurements in general and trace gas measurements in particular. They are fabricated from well-understood materials (for example, indium gallium arsenide and aluminum indium arsenide) and can operate at nearly all wavelengths between 3.5 &mgr;m and 24 &mgr;m. Single-mode operation with narrow line widths (100 kHz or smaller) is easily possible, and continuous tunability
Friberg Stephen R.
Harb Charles C.
Dorsey & Whitney LLP
Lambda Control, Inc.
Nguyen Sang H.
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