Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
2000-07-13
2003-10-28
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
C356S440000
Reexamination Certificate
active
06639678
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to gas sensing using tunable diode laser absorption spectroscopy, and more particularly, is related to an apparatus and method for nondestructive monitoring of gases in sealed optically transparent containers, using tunable diode laser absorption spectroscopy, while reducing or eliminating etalon interference fringes.
2. Background and Material Information
Tunable diode laser absorption spectroscopy (TDLAS) is a highly selective and versatile technique for measuring many trace atmospheric constituents with detection sensitivities in the sub-parts-per-billion (ppbv) concentration range. Other spectroscopic techniques such as Fourier-transform infrared (FTIR) spectroscopy, which record the interaction of infrared radiation (IR) with experimental samples, measuring the frequencies at which the sample absorbs the radiation and the intensities of the absorptions, do not have as high a sensitivity as TDLAS in the near infrared (NIR) region of the spectrum. Specifically, FTIR spectroscopy can only achieve minimum detectable absorbencies of 10
−3
-10
−4
in a 1 Hz detection bandwidth, whereas TDLAS can achieve 10
−6
-10
−7
in the same detection bandwidth. Thus it is desirable to use TDLAS over FTIR in the NIR spectral region where many atmospherically important constituents have weak molecular overtone and combination absorption bands.
TDLAS utilizes the wavelength response of matter to probe physical and chemical properties. Also, TDLAS determines the concentration of a gas by measuring the amount of light absorbed at a particular wavelength. The intensity of light absorbed is directly related to gas concentration through Beer's law:
ln
(
I
x
/I
o
)=
−n&sgr;x
where I
x
is the intensity of the transmitted light, I
o
is the intensity of incident light, n is the concentration gas, &sgr; is the absorption cross section, and x is the pathlength.
Applications for laser absorption spectroscopy range from basic chemical kinetics research and environmental monitoring to medical diagnostics and industrial process monitoring. When trace gas concentrations need to be measured, a system must be designed to minimize noise and background signals.
The amplitude of tunable diode laser absorption signals is proportional to the distance, x, (or pathlength) over which a target gas is sampled. Multipass configurations can increase the pathlength, but require the use of collimated laser beams to ensure a laser beam of sufficient power reaches the detection circuitry. In situations where the target gas resides in a sealed optically transparent container, the laser beam must pass through the container walls to sample the target gas. U.S. Pat. No. 5,317,156 which issued to Cooper et al. on May 31, 1994, shows a typical TDLAS system using a multi-pass sample cell.
Most TDLAS systems are limited in sensitivity not by laser or detector noise but by optical fringes superimposed on the measured spectrum. These result from unwanted optical artifacts, or etalons, formed by reflections and scattering in the optical system.
FIG. 1
shows a collimated laser beam
11
having a width defined by edges A and B passing through an optically transparent material, such as a glass container wall W. Partial reflected rays, shown as B′, occur at the air/container interface. The partial rays B′ from one ray are reflected within the wall W and overlap with adjacent incident rays, shown as AB′ in
FIG. 1
, causing interference. These overlapping rays AB′ are phase shifted with respect to each other due to the different optical pathlengths traversed and cause constructive interference when the phase shift is 0, and destructive interference when the phase shift is &pgr;. The interference effect from these overlapping laser beams creates the unwanted etalons.
When the interfering beams are incident on a detector (e.g. a photodetector or square law detector) and the laser frequency is scanned, the phase relationship varies, producing a periodic intensity variation in the photo current. The interference pattern in most cases limits the sensitivity of the measurement and obscures small absorption signals.
FIG. 2
shows a molecular absorption graph of a TDLAS signal S using a collimator lens. The signal is recovered in a single pass through a sample container and with no signal averaging. The distorting effects of the etalons are evident; the amplitude and frequency of gas absorption feature G is comparable to the amplitude and frequency outside OFQ the gas absorption feature G as a result of interference fringes F, which may obscure the reading, especially when analyzing small concentrations of gas.
The art is replete with devices and techniques that attempt to reduce etalons. These techniques can be categorized as follows; (I) mechanical modulation or dithering of the etalon spacing (ii) modified modulation schemes (iii) background subtraction and (iv) post-detection signal processing.
If the etalon pathlength-difference is mechanically modulated then the fringes will shift relative to the absorption spectrum. As the spectrum is averaged, the fringes will average to zero, provided the modulation amplitude corresponds either to an integral or a large number of fringes. One method of accomplishing this modulation has been demonstrated by Webster, in U.S. Pat. No. 4,684,258, which issued on Aug. 4, 1987. This patent discloses the interposing of an oscillating Brewster plate into the beam at a point between the two surfaces forming the etalon. Oscillating the plate (by typically 1°) varies the optical pathlength through the plate. Because the plate is at the Brewster angle, reflection losses are minimized. One disadvantage of this method that it is difficult to apply to a multi-pass cell without causing significant attenuation of the beam.
U.S. Pat. No. 4,934,816 which issued to Silver et al. On Jun. 19, 1990, discloses a method using a piezoelectric transducer (PZT) to vibrate the mirror or other component which forms one surface of the etalon. However, both the Webster and Silver techniques are difficult to implement when the sample containing the gas to be analyzed is a sealed product container that must be sampled in-situ.
In TDLAS systems using wavelength modulation spectroscopy (WMS), the fringes can be averaged to zero by applying an additional low-frequency wavelength modulation to the diode laser, with an amplitude equal to an integral number of periods of the etalon fringe. As a result, the technique is effective only at removing fringes with a period less than the absorption line width since the modulation amplitude needed to remove longer period fringes would also smear the absorption line shape and reduce its peak height.
For a perfectly stable system a background spectrum, obtained by supplying zero air to the instrument inlet, would display the same etalon fringes as the sample spectrum. Subtraction of this background spectrum would then remove the fringes. Real systems however are subject to thermal drift, so that in the time between taking the sample spectrum and the background spectrum the fringes will have drifted and cancellation will not be perfect. Thus, the success of background subtraction depends firstly on the thermal and mechanical stability of the system and secondly on the rapidity with which sample and background spectra can be alternated.
Post-detection signal processing can take the form either of analog processing of the signal from the look-in amplifier or demodulator, or digital processing of the signal acquired by the signal averager. Both take advantage of the periodic nature of the optical fringes. A simple low-pass filter following the lock-in amplifier can dramatically reduce fine-pitch fringes. In known TDLAS systems, a combination of background subtraction with some form of post-detection processing is most commonly used.
The minimum detectable absorption is given by the smallest variation in ln(I
x
/I
o
) that can be distinguished from noise
Greenblum & Bernstein P.L.C.
Lighthouse Instruments LLC
Rosenberger Richard A.
LandOfFree
Apparatus and method for nondestructive monitoring of gases... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method for nondestructive monitoring of gases..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method for nondestructive monitoring of gases... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3157826