Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-01-20
2003-12-16
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S573000, C356S437000
Reexamination Certificate
active
06664533
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the use of Light Detection and Ranging (LIDAR) to detect elements in the atmosphere remotely. More particularly, this invention relates to mobile use of modulated tunable diode lasers in order to sweep the laser wavelength through an absorption line of a gas such as methane in order to determine the presence of the gas in the atmosphere.
BACKGROUND
LIDAR systems operate somewhat like radar. LIDAR, however, directs laser light rather than radar waves at a particular target to detect the target. The laser light may be pulsed or relatively continuously generated, and it may be focused or collimated as desired to reach the desired end. Objects, particles, and gases can scatter and/or absorb the laser light. Thus, the measurement of the reflected light can provide information about the target or atmospheric constituents along the optical path. LIDAR data is derived by collecting the scattered (reflected) light with a telescope, which focuses the collected light onto a photodetector. The amount or intensity of the light thus detected can be processed to provide information about the object being scanned and the optical path through which the laser beam passes.
LIDAR systems have been used in the past for the mobile determination of the presence of particular gases, such as the presence of methane from a gas line leak, in the atmosphere. One such LIDAR system employs an Optical Parametric Oscillator (OPO) as the laser source. OPO based LIDAR is particularly effective for determining the presence of methane because the wavelength of the OPO-based light lies in the fundamental absorption band of the methane gas.
On the other hand, OPO-based LIDAR is expensive and also requires extreme environmental controls to maintain stable long term operation. OPO systems are complex and prone to alignment problems, requiring highly trained maintenance personnel.
Also, since OPO-based LIDAR emits pulses of laser light, the pulse repetition frequency (PRF) can present a significant problem for mobile applications seeking to detect small gas plumes, such as gas leak plumes. Most currently used commercial OPO-based systems having sufficient output energy to detect such plumes operate at a PRF of 10 Hz or less. At such extremely low pulse repetition rates, the speed of the mobile measurement platform can strongly influence the measured data. The mobile platform is thus not only likely to miss some plumes entirely but also can incorrectly estimate plume concentrations as the OPO is tuned between wavelengths and the target moves relative to the OPO-based system. Although the latter, moving-target problem can be reduced by using two OPO-based LIDARs that near simultaneously transmit differential wavelength pairs, this dual-OPO laser system is not only expensive but also very complex and does not solve the former, low PRF problem.
Recently, OPO-based systems have been developed that provide higher PRF rates (in the kHz range). One such system is that developed by Sandia National Laboratory. These systems, however, produce micro-joule energies due to the high PRF, requiring long integration times to accomplish detection. For this reason, the system will likely miss small or low concentration plumes, particularly in the mobile environment. These systems are also very expensive—probably too much so for use by pipeline survey companies—and they are difficult to maintain in alignment, especially in a mobile application. This is because OPO-based systems require extreme environmental controls and stability to operate properly. Field and mobile applications generally do not allow for these types of controls.
Another prior art LIDAR technique uses frequency mixing to generate emissions in the fundamental absorption band of methane. These frequency-mixing systems use expensive lasers (such as ND-YAG and Ti:Sapphire lasers in downconverting frequency mixing schemes or CO
2
lasers in upconverting devices). Like the OPO-based systems, they also are non-linear crystal based systems that are difficult to maintain in alignment, especially in mobile applications.
There are also Tunable Diode Lasers (TDLs) that have been developed for the detection of methane gas plumes in the atmosphere. One such TDL laser has been developed by the Tokyo Gas Company. The Tokyo Gas TDL laser is reported to have sufficient sensitivity to detect gas line leaks, using low frequency wavelength modulation and lock-in (phase differential) detection. Low frequency lock-in detection, however, has several major disadvantages for mobile, remote detection operations.
First, low frequency lock-in detection requires long scanning and data averaging times to achieve sufficient sensitivity to detect small remote plumes. As a result, low frequency lock-in detection TDL LIDAR techniques are effectively limited to static line-of-sight, not mobile, applications.
Second, although there are other processing techniques such as matched filtering that can often be used in LIDAR systems to improve sensitivity, these techniques cannot be used with low frequency TDL LIDAR systems. This is because these types of processing techniques are based on the absorption line signature information which require use of much higher (RF) frequencies.
While there are lasers available, such as the OPO-based LIDARs described above, that operate within the fundamental absorption level and overtone band of gases such as methane, the applicants believe that such systems have not provided a solution to the problem of using LIDAR to economically and reliably detect gas leaks, particularly methane gas leaks, in mobile applications.
There have been TDL-based lasers in the prior art that operate in the first overtone band, but not in the fundamental absorption band, of gases such as methane, but they have not been applied to mobile detection of gases such as methane. Because such lasers operate in only the overtone band, they are not as readily absorbed by gases such as methane. Applicants believe that, as a result of this limitation and possibly other aspects of TDL-based lasers, such lasers have not been applied to the mobile detection of gases such as methane.
Frequency Modulation Spectroscopy (FMS) techniques exist in the prior art, such as those identified in U.S. Pat. No. 4,594,511 (“the '
511
Patent”), entitled “Method and Apparatus for Double Modulation Spectroscopy,” issued to one of the present inventors, and in U.S. Pat. No. 5,572,031 (“the '
031
Patent”), entitled Pressure and Temperature Compensating Oxygen Sensor, issued to two of the present inventors.
As the '
511
Patent explains, FMS can be used to detect spectral properties of a sample more economically, conveniently, and accurately than detection techniques operating strictly in the frequency domain of the information of spectroscopic interest. The '
511
Patent also states that such FMS techniques can be used to take measurements of gaseous samples.
Although the '
511
patent does suggest that FMS techniques may be used with a variety of lasers including TDL-based lasers, the '
511
patent does not teach how to apply FMS techniques to any particular TDL apparatus. The '
511
Patent also does not teach any mobile apparatus or method or use of FMS or TDL techniques to detect methane gas in particular, much less remotely detect methane gas in the atmosphere.
BRIEF SUMMARY OF ASPECTS OF THE INVENTION
The applicants have invented a method and apparatus for remote detection of gas, preferably methane, dispersed into the atmosphere. The method utilizes a TDL-based LIDAR, utilizing a TDL whose frequency can be altered by changing the TDL drive current. The TDL laser is driven by a drive current or carrier, and the carrier frequency is preferably centered in the center of the absorption line of the gas in issue. A small RF modulation current (preferably at 4 MHZ) is superimposed on the TDL carrier frequency to produce sidebands, which lie within the pressure broadened absorption line of the gas. A low frequency (about 1 KHz) sawtooth ramp curr
Carlisle Sylvie
Cooper David E.
van der Laan Jan E.
Warren Russell E.
Fejer Mark E.
Gas Research Institute
Le Que T.
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