U.S. patent application number 09/804828 was filed with the patent office on 2003-02-13 for apparatus and method of remote gas trace detection.
Invention is credited to Cooper, David E., Vujkovic-Cvijin, Pajo.
Application Number | 20030030001 09/804828 |
Document ID | / |
Family ID | 23939745 |
Filed Date | 2003-02-13 |
United States Patent
Application |
20030030001 |
Kind Code |
A1 |
Cooper, David E. ; et
al. |
February 13, 2003 |
APPARATUS AND METHOD OF REMOTE GAS TRACE DETECTION
Abstract
This specification discloses a method and apparatus for the
mobile and remote detection of a gas, such as methane, in the
atmosphere. The apparatus includes a TDL based Light Detection and
Ranging (LIDAR) driven at carrier frequency lying within the
absorption line of the gas. The apparatus also drives the TDL with
a modulation frequency to generate upper and lower sidebands in the
output of the TDL and with a low ramp frequency to sweep the output
of the TDL across twice the width of the pressure-broadened
absorption line of the gas, preferably the first overtone
absorption line in the case of methane detection. Suitable power
for remote detection through use of the TDL is provided by a master
oscillator/fiber amplifier transmitter has no moving or adjustable
parts at all. An all-solid-state monolithic and integrated
amplifier is achieved, which leads to a compact and virtually
maintenance-free LIDAR system. The remote detection apparatus
includes reference and calibration cells or chambers, and includes
a light collector and detectors to detect the quantity and
modulation of the light that passes the reference or calibration
cells and that is received by the apparatus after reflection back
toward the apparatus from an uncooperative target. The apparatus
further includes a signal processor that applies a derivative
spectroscopy technique, such as frequency modulation spectroscopy
or wavelength modulation spectroscopy, to determine the presence of
the gas in the atmosphere.
Inventors: |
Cooper, David E.; (Palo
Alto, CA) ; Vujkovic-Cvijin, Pajo; (Los Altos,
CA) |
Correspondence
Address: |
MARK E. FEJER
GAS TECHNOLOGY INSTITUTE
1700 SOUTH MOUNT PROSPECT ROAD
DES PLAINES
IL
60018
US
|
Family ID: |
23939745 |
Appl. No.: |
09/804828 |
Filed: |
March 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09804828 |
Mar 13, 2001 |
|
|
|
09488453 |
Jan 20, 2000 |
|
|
|
Current U.S.
Class: |
250/338.5 ;
359/341.1 |
Current CPC
Class: |
G01J 3/433 20130101;
G01N 21/538 20130101; G01N 21/39 20130101; G01N 2021/1793
20130101 |
Class at
Publication: |
250/338.5 ;
359/341.1 |
International
Class: |
G01N 021/35; G02F
001/01 |
Claims
We claim:
1. A laser amplifier system for a laser source comprising: a. a
doped, active, first section of optical fiber; b. pump lasers at a
first wavelength for inducing lasing in the doped fiber section at
a second wavelength; c. means for coupling the pump lasers to the
doped fiber section; d. a narrowband laser lasing in a third
wavelength suitable for absorption in a fundamental absorption band
or overtone sideband of the gas to be detected; e. a second section
of amplifying, nonlinear Raman fiber coupled to the doped section
of fiber for amplifying the second wavelength; f. means for
coupling the narrowband laser to the combined nonlinear and doped
fiber sections; and g. means for outputting the third wavelength
from the combined fiber sections.
2. The laser amplifier system for a laser source of claim 1 wherein
the doped fiber section is a rare earth doped section.
3. The laser amplifier system for a laser source of claim 2 wherein
the doping of the doped fiber section comprises Erbium.
4. The laser amplifier system for a laser source of claim 2 wherein
the doping comprises Ytterbium.
5. The laser amplifier system for a laser source of claim 1 wherein
the first fiber section comprises a double clad fiber.
6. The laser amplifier system for a laser source of claim 1 wherein
the pump diodes are high power, broad area, laser diodes emitting
at about a 980 nanometer wavelength.
7. The laser amplifier system for a laser source of claim 1 wherein
the second wavelength is about 1530-1560 nanometers.
8. The laser amplifier system for a laser source of claim 1 wherein
the third wavelength is about 1651 nanometers.
9. The laser amplifier system for a laser source of claim 1 wherein
the narrowband laser is a tunable diode laser (TDL).
10. The laser amplifier system for a laser source of claim 1
wherein the narrowband laser is a tunable diode laser (TDL) having
temperature compensation in its packaging.
11. The laser amplifier system for a laser source of claim 1
wherein the narrowband laser is a tunable diode laser (TDL) having
a distributed feedback (DFB) configuration.
12. The laser amplifier system for a laser source of claim 1
wherein the gas to be detected is methane.
13. The laser amplifier system for a laser source of claim 1
wherein the first and second fiber sections are connected together
in a ring laser configuration.
14. The laser amplifier system for a laser source of claim 1
wherein the fiber sections are connected together in a linear laser
configuration.
15. The laser amplifier system for a laser source of claim 1
wherein the pump diodes are connected to the first fiber section
via a wavelength division multiplexer.
16. The laser amplifier system for a laser source of claim 1
wherein the pump diodes are connected to the fiber sections with a
dichroic beam splitter and a lens and a mirror.
17. The laser amplifier system for a laser source of claim 1
wherein the second wavelength is produced by a Ytterbium doped
fiber laser.
18. The laser amplifier system for a laser source of claim 1
wherein the amplifying fiber produces Stokes shifts.
19. The laser amplifier system for a laser source of claim 1
wherein the narrowband laser is coupled to the fiber via Wavelength
Division Multiplexer.
20. The laser amplifier system for a laser source of claim 1
wherein the narrowband laser is coupled out of the fiber via
Wavelength Division Multiplexer.
21. The laser amplifier system for a laser source of claim 13
wherein a Bragg fiber grating is located on the first fiber section
to double pump the pump diode power therethrough.
22. The laser amplifier system for a laser source of claim 13
further including an isolator for protecting the narrowband laser
from high power radiation.
23. The laser amplifier system for a laser source of claim 14
wherein a Bragg fiber grating is located on the second fiber
section to form one end of the laser cavity.
24. The laser amplifier system for a laser source of claim 14
wherein mirror is located on the first fiber section to form the
other end of the laser cavity.
25. A single stage fiber Raman amplifier pumped by an erbium-doped
fiber laser amplifying the output of a 1651 nm DFB diode laser.
26. An all-solid-state monolithic and integrated laser amplifier
with a master oscillator section and a fiber amplifier transmitter
section, the amplifier having no moving or adjustable parts.
27. A method of remotely detecting a particular gas dispersed in
the atmosphere, the dispersed gas being of the type that absorbs
light at frequencies within an absorption line range for the
dispersed gas in the atmosphere, the method comprising the steps
of: a. driving a source of generally monochromatic light by a laser
drive to provide a carrier output at a predetermined frequency with
an all-solid-state monolithic and integrated laser amplifier with a
master oscillator section and a fiber amplifier transmitter
section, the amplifier having no moving or adjustable parts; b.
frequency modulating the laser drive with a modulation frequency to
generate at least one frequency modulated sideband signal in the
output of the light source; and c. adding to the laser drive a ramp
frequency to scan the light source and the sideband signal across a
scan range including at least a portion of the absorption line
range; d. directing at least a test portion of the ramped and
modulated light toward an uncooperative target; e. collecting light
reflected from the uncooperative target and directing it toward a
first detector to generate at least one test signal based on the
degree of attenuation of the sideband signal by the dispersed gas;
and f. generating an output indicative of an amount of the
dispersed gas in the atmosphere, if any, based on the at least one
test signal.
28. A method of remotely detecting a particular gas dispersed in
the atmosphere, the dispersed gas being of the type that absorbs
light at frequencies within an absorption line range for the
dispersed gas in the atmosphere, the method comprising the steps
of: a. driving a source of generally monochromatic light by a laser
drive to provide a carrier output at a predetermined frequency with
a laser amplifier system for a laser source comprising: a doped,
active, first section of optical fiber; pump lasers at a first
wavelength for inducing lasing in the doped fiber section at a
second wavelength; means for coupling the pump lasers to the doped
fiber section; a narrowband laser lasing in a third wavelength
suitable for absorption in a fundamental absorption band or
overtone sideband of the gas to be detected; a second section of
amplifying, nonlinear Raman fiber coupled to the doped section of
fiber for amplifying the second wavelength; means for coupling the
narrowband laser to the combined nonlinear and doped fiber
sections; and means for outputting the third wavelength from the
combined fiber sections; b. frequency modulating the laser drive
with a modulation frequency to generate at least one frequency
modulated sideband signal in the output of the light source; and c.
adding to the laser drive a ramp frequency to scan the light source
and the sideband signal across a scan range including at least a
portion of the absorption line range; d. directing at least a test
portion of the ramped and modulated light toward an uncooperative
target; e. collecting light reflected from the uncooperative target
and directing it toward a first detector to generate at least one
test signal based on the degree of attenuation of the sideband
signal by the dispersed gas; and f. generating an output indicative
of an amount of the dispersed gas in the atmosphere, if any, based
on the at least one test signal.
Description
[0001] This is a continuation of U.S. patent application Ser. No.
09/488,453, filed January 2000.
FIELD OF THE INVENTION
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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 CO2 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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
[0017] 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 current is also superimposed on the TDL drive current to sweep
the carrier and its associated sidebands over a range, and the
range preferably is twice as wide as, and centered on, the
atmosphere-pressure broadened absorption line width of the gas. The
resulting TDL light is directed at an uncooperative target and
collected by a detector. An uncooperative target is one which is
undefined, such as a methane plume, rather than a defined target
such as a retroreflector. The collected sideband laser light and
carrier signal are then fed to an FMS processor to generate a
derivative signature, preferably a second derivative signature
(derived from the mixing of upper and lower sidebands) that
indicates whether the gas is present in the atmosphere. A closely
related technique, WMS, or wavelength modulation spectroscopy, can
achieve comparable results in certain situations and may be used in
some instances in place of FMS in the present invention.
[0018] The present method and apparatus preferably includes a
reference WMS gas detection technique. The reference provides a
baseline for comparison of the atmospheric derivative signature
with the reference derivative signature and confirmation that any
apparent detection of the gas in issue from the atmosphere
derivative signature is consistent with the reference signature and
not likely to be the result of misalignment, anomalous performance
of the apparatus, or gas other than sought to be detected in the
atmosphere.
[0019] It is to be understood that this is a brief summary of
aspects of the invention. There are other aspects of the invention
that will become apparent as the specification proceeds.
OBJECTS OR ADVANTAGES OF THE PRESENT INVENTION
[0020] It is therefore an object and an advantage of the present
invention to provide an apparatus and method for mobile detection
of gas leaks in the atmosphere, particularly methane gas leaks.
[0021] It is an advantage of the present invention to provide a
LIDAR (and method of using it) that is relatively sensitive,
mobile, and economical.
[0022] It is yet another advantage of the present invention to
provide a LIDAR that is relatively stable and rugged, and can
operate unattended or with relatively minimal attention by an
operator.
[0023] It is a further advantage of the present invention to
provide a relatively compact LIDAR for detection of gas plumes.
[0024] It is a still further advantage of the present invention to
incorporate commercially available TDL's to achieve the mobile
detection of gas plumes in the atmosphere.
[0025] It is an additional advantage of the present invention to
provide a mobile gas detection LIDAR that utilizes a single source
emitter.
[0026] Another advantage is that the present invention provides
continuous wave operation, reducing the likelihood of missing the
detection of a remote gas line leak when the apparatus is
moving.
[0027] It is yet another advantage of the present invention to
provide a gas detection technique that not only is mobile but also
provides for self-calibration through the gas detection
process.
[0028] A further advantage is that the present invention includes a
reference gas detection signature to compare against an apparent
detection of gas in the atmosphere and ensure that the apparent
detection is correct.
[0029] A still further advantage of the invention is that it can
use an TDL laser that emits light in other than the primary
absorption band of the gas under study and yet detect the gas.
[0030] A still further advantage of the invention is that it can
use an TDL laser in conjunction with a master oscillator/fiber
amplifier transmitter that has no moving or adjustable parts at
all. An all-solid-state monolithic and integrated device can be
achieved, which leads to a compact and virtually maintenance-free
LIDAR system.
[0031] There are other objects and advantages of the present
invention. They will become apparent as the specification
proceeds.
[0032] In this regard, it is to be understood that, although the
applicants' believe that their preferred embodiment described
herein meets the objects and provide the advantages recited herein,
the scope of the invention is to be determined by reference to the
claims and not necessarily by whether any given embodiment achieves
all objects and advantages stated herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The applicants' preferred embodiment of the present
invention is shown in the accompanying drawings in which:
[0034] FIG. 1 is a functional block diagram of the preferred mobile
TDL/FMS gas detection apparatus with a reference cell;
[0035] FIG. 2 is the video data display screen for remote methane
leak detection by the preferred detection apparatus shown in FIG.
1; and
[0036] FIG. 3 is the calibration monitor display screen of the
preferred detection apparatus shown in FIG. 1.
[0037] FIG. 4 is a schematic of a ring laser diode amplifier
suitable for use with the present invention.
[0038] FIG. 5 is a schematic of a linear laser diode amplifier
suitable for use with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0039] With reference to FIG. 1, the preferred detection apparatus,
generally 10, is used to detect the presence of a remote methane
gas plume 12 in the atmosphere, generally 14. The apparatus 10
includes a source of generally monochomatic light issued by a
distributed feedback tunable diode 16. The diode 16 is an
InGaAs/InP type that emits light in the 1.65 .mu.m band, which is
the first overtone band of methane. The diode 16 is manufactured by
Sensors Unlimited, Inc. This type of diode 16, although not
emitting in the fundamental absorption band of methane (3.3 .mu.m)
provides significantly greater power than the lead salt TDL's
available for use in the 3.3 .mu.m band.
[0040] Basic control of the diode 16 is provided by a Melles Griot
laser diode/thermoelectric cooler controller 18. The controller 18
provides DC laser drive current and temperature control of the TDL
to set and maintain the diode 16 emissions at the desired
wavelength, 1.65 .mu.m.
[0041] A ramp generator 20, manufactured by Stanford Research
Systems, is connected to the controller 18 and thus adds to the
drive current provided by the controller 18 a low current saw-tooth
ramp signal (0.5 volt) at a relatively low repetition frequency of
1 kHz. This ramp signal sweeps the TDL 16 over a wavelength range
of about 3 GHz, which is approximately twice the width of the
atmospheric pressure-broadened absorption line width of the first
overtone band of methane. As a result, the ramp signal sweeps the
output of the TDL 16 so that a significant portion of the sweep
occurs within the absorption line and a substantial portion of the
sweep takes places outside the line. Sweeping at 1 kHz means that
this type of sweep occurs 1,000 times per second.
[0042] The ramped drive current from the controller 18 is then fed
into the methane detector drive assembly (MDDA) 22. The design and
operation of such an MDDA 22 is well known to those of skill in the
art. The MDDA 22 modulates the drive current with a 4 MHZ radio
frequency signal. The MDDA 22 is then connected to the TDL 16 to
drive it with the ramped and rf modulated drive current to yield
sidebands displaced from the carrier frequency of the ramped drive
current. The laser output from the TDL 16, including the carrier
frequency and the upper and lower sideband signals displaced from
the TDL 16 carrier wavelength by the modulation frequency, are thus
swept across the absorption line of methane. As a result, an
unbalance in the sideband signals will occur if an absorption is
present. This unbalance yields a derivative line shape signature
with an amplitude proportional to the gas concentration. As
explained in the '511 patent, the amplitude of these sidebands is a
Bessel function.
[0043] The frequency-swept and frequency-modulated laser output of
the TDL 16 is then fed through a laser collimating lens, 24, and a
helium neon alignment laser 26, and then through a brewster window
28. The helium neon alignment laser 26 emits light in the visible
spectrum to provide a visible light source for alignment of the
apparatus in a fashion well known in the art. The brewster window
28 directs most of the laser light toward the laser output mirror
30, which directs the laser light toward the target in the
atmosphere, e.g., the sand ground 32. The brewster window 28 also
directs a small amount of the laser light through methane gas
reference cell or chamber 34. The laser light thus passes through
the reference cell 34 to an InGaAs PIN photodetector/receiver 36.
The PIN receiver 36 detects the laser light and outputs two
signals. One signal is proportional to the quantity of the light
incident on the detector and the other signal is proportional to
the modulation frequency of the laser light incident on the
detector. The method of arranging and operating the PIN detector 36
is well known to those skilled in the art.
[0044] The outputs 38, 40 of the PIN receiver 36 are connected to
the reference channel signal inputs (not shown in detail) of a
methane detector signal processor (MDSP) 42. The MDSP 42 contains
demodulation circuits that are not shown and are well known to
those skilled in the art. The MDSP 42 circuits (not shown) provide
either first derivative (2 MHZ) or second derivative (4 MHZ) FMS
signature information (derived from the first sideband or second
sideband, respectively, as explained the '511 patent). Although not
shown in FIG. 1, this information can be line-locked with the
Melles Griot controller 18 in order to lock in the sweep of the TDL
16 of the desired absorption line of methane as described above.
This wavelength control circuit logic can be installed in MDSP 42
and eliminate the need for the Melles Griot controller 18 and the
ramp generator 20 by performing the functions of the controller 18
and ramp generator within the MDSP 42.
[0045] A seven inch F2.5 telescope 44 serves as the reflected light
collector for the apparatus 10. The collector 44 is focused on the
target 32, and the laser output mirror 32 is mounted adjacent the
light entry-way 46 of the collector so that the laser emitted
toward the target 32 is in alignment with the laser light back from
the target 32 and received by the collector 44.
[0046] The telescope or collector 44 thus receives and focuses the
reflected laser light on an InGaAs Avalanche Photo Diode (APD)
detector 48. The APD photodetector 48 provides noise equivalent
power (NEP) performance of approximately 0.1 pW-Hz.sup.1/2. The APD
photodetector 48 detects the RF (modulation) 50 and DC
(photocurrent) 52 components of the reflected laser light in a
fashion well known to those skilled in the art and provides two
current outputs proportional to the modulation and photocurrent
components. These outputs 50, 52 of the APD photodetector 48 are
connected to the main signal inputs (not shown in detail) of the
methane detector signal processor (MDSP) 42 in a fashion well known
to those skilled in the art.
[0047] Accordingly, the sampling rate(s) of the detectors 48, 36 is
the same as the ramp or saw-tooth frequency, 1 KHz. The detectors
48, 36 also have a bandwidth of 20 MHZ. The amount of modulation
component detected by the PIN receiver 36 and the APD photodetector
48 are directly proportional to the total absorption along the
laser beam path through the reference cell 34 and target 32
respectively. In addition, the amount of modulation component
detected by the APD photodetector 48 is calibrated by the MDSP 42
circuitry based upon the distance of the apparatus 10 and
particularly its output mirror 30 to the target 32. The MDSP 42
then combines the detected back scattered signal modulation
component 50 with the reference modulation component from the MDSP
42, which cancel each other out when no absorption takes place
along the path to or from the target 32. As explained in the '511
patent, when methane absorption does take place along the path to
or from the target 32, the back scatter and reference modulation no
longer cancel each other out, producing a derivative FMS signature
or signal, thus indicating the presence of methane in the
atmosphere along the path to or from the target 32.
[0048] The MDSP 42 provides signal conditioning electronics in
order to optimize detection response or derivative FMS signal. The
conditioning electronics include phase control, bandpass filtering,
amplifiers, and signal level control circuits. Digital signal
processing, DSP, such as Kalman filtering or matched filtering may
be used, as further explained below. These components and their use
are well known in the art.
[0049] The MDSP 42 then forwards the field and reference FMS
signatures to a digital oscilloscope 54 for monitoring and
digitizing the waveforms for observation and further processing. In
the arrangement shown in FIG. 1, the top trace 56 on the
oscilloscope represents the second derivative signature (4 MHZ) for
the reference cell methane concentration, and the bottom trace 58
represents that second derivative signature for the field methane
concentration detected by the laser light traveling through a
methane plume 12 to and from the target 32. The difference in
temporal shape between the reference and field FMS signatures is
due to the relatively low pressure of the methane gas in the
reference cell and its relatively narrow methane absorption line as
compared to the atmospheric-pressure-broadening of the methane
absorption line in the atmosphere.
[0050] The oscilloscope 54 provides signature data output, which
then is input into an IBM-compatible personal computer (PC) 60 via
a General Purpose Instrument Bus (GPIB) bus cable interconnecting
GPIB ports in the oscilloscope 54 and the PC 60. Alternatively, the
PC 60 may be a portable laptop computer with a standard,
off-the-shelf digitizing card mounted inside the laptop.
[0051] The PC 60 has a video display 62. MICROSOFT LAB WINDOWS
(TRADEMARK) software running on the PC 60 in a fashion well known
in the art processes the signature data from the oscilloscope and
provides a display of data such as shown in FIG. 2. With reference
now to FIG. 2, the data display 62 includes a graph of time series
data 64, which shows the concentration of methane in the atmosphere
over time. The display also includes a real-time graph of the
absorption line detection, if any is taking place, as is the case
in the example shown in FIG. 2. The data display 62 further
includes (I) a real-time target area 68 display being interrogated
by the LIDAR apparatus 10 of FIG. 1, and (ii) a global positioning
satellite (GPS) controlled presentation 70 of the location shown in
the real-time target area display 68.
[0052] Referring back to FIG. 1, the apparatus 10 is preferably
mounted on a mobile platform 99 such as a small truck or sports
utility vehicle. The apparatus weighs about 100 lbs. and occupies
about 6 cu. ft. within the vehicle. The apparatus is thus uniquely
quite mobile and useable for the remote detection of plumes of
methane in the atmosphere.
EXAMPLES
[0053] In order to test the apparatus 10, remote gas leak
simulation was effected by disseminating methane through a manifold
72 over a 3 sq. inch area under a paper filter 74 underlying, and
separating the manifold 72 from, a sand target 76 about one half
inches deep. The methane needle valve 78 was opened for varying
periods of time in order to simulate large and small gas leaks in
the 10 to 300 p.p.m. range. The display shown in FIG. 2 is an
example of the data collected during one such simulated gas
leak.
[0054] In order to calibrate the response of the apparatus 10, a 10
inch diameter, 1 meter long sample chamber or reference cell 80 can
be placed in the path of the laser light emitted from the output
mirror 30. A pre-calibrated RKI EAGLE (TRADEMARK) methane monitor
82 is connected to the chamber in a manner well known to those
skilled in the art. The methane monitor 82 provides an independent
measurement of methane concentration in the cell as function of
time during the test.
[0055] The PC 60 output to a standard color printer (not shown)
provides a calibration graph such as shown in FIG. 3, including
both raw 84 and Kalman-filtered 86 TDL LIDAR data. This output and
printout such as shown in FIG. 3 is generated through LAB WINDOWS
(TRADEMARK) software running on the PC in a fashion well known to
those skilled in the art.
[0056] Since the sample chamber 80 is 1 meter in length, the FMS
signal detected by the ACP photodetector 48 is a one meter, path
integrated measurement. Accordingly, the signal levels recorded
when the methane concentration in the chamber 80 is at equilibrium
(several minutes after dispersion of the gas into the chamber 80)
correspond to the known concentration of the methane in the chamber
80 as indicated by the methane monitor 82 (210 p.p.m.-m in this
instance). A linear CL division scale is constructed and provided
on the graph shown in FIG. 3 between the limits of the reference
path integrated concentration level (CL, 210 p.p.m.-m) and the
background CL level measured prior to injection of the methane into
the chamber 80. With Kalman filtering of the data, the minimum
detection is in the range of less than 10 p.p.m.-m, and without
Kalman filtering, the minimum detection is in the range of 10-20
p.p.m.-m. Another DSP technique that can improve the detection
performance significantly is the use of matched filtering. Matched
filtering requires that a detection technique be used such as FMS
derivative detection, that provides a well characterized temporal
signature of the absorption line feature being detected. The FMS
detection scheme in the present design provides second derivative
absorption line features that are ideally suited for this
application. Matched filtering works by correlating the noisy
signal with a template representing the time signal shapes. It is a
statical approach for finding weak signals under fairly general
assumptions about the noise statistics.
[0057] In the calibration test shown in FIGS. 1 and 3, at first the
sample chamber 80 contained no methane. After recording several
minutes of background data, a trace amount of methane was injected
into the chamber 80 by a calibration methane source 88. As the gas
dispersed into and reached equilibrium within the chamber 80, the
ADP photodetector 48 detected the absorption and the results are
indicated in the first peak 90 shown in the calibration graph 90 in
FIG. 3. Since the methane monitor 82 indicated a concentration of
210 p.p.m. of methane in the sample chamber 80 at the time of the
occurrence of the first peak 90 of FIG. 3, the operator could see
that the output of the display screen 62 correlated quite well with
the methane concentration measured by the methane monitor 82.
[0058] In the example shown in FIG. 3, the operator performed a
second test after several minutes, and the display screen 62
indicated the results by the second peak 92. Once again, the
display screen 62 indicated a high degree of correlation between
the concentration of methane measured in the sample chamber 80 (200
p.p.m.) and that shown in the display screen 62.
[0059] It can thus be seen that the applicants' preferred
embodiments provide a particularly effective and yet economical and
mobile method and apparatus for the remote detection of gas,
particularly methane, in the atmosphere. Once set up, the apparatus
can run unattended as the vehicle containing the apparatus moves at
typical operational speeds of 20 to 30 m.p.h. The preferred LIDAR
apparatus 10 uses a single TDL, and it provides continuous wave
operation. It also provides significant self-calibration by means
of the integral reference cell 80.
[0060] Referencing FIG. 4, a laser radiation amplifier 111 for
LIDAR application of the desired wavelength laser source 115, is
based on the master oscillator/power amplifier concept. This system
provides both the spectral purity of a master oscillator and the
high output power of an amplifier. In addition to providing high
output power, the amplifier portion of the transmitter can be used
in the receiving channel as well, where it can provide selective
narrowband optical amplification of the return signal, prior to the
detection by the LIDAR photodetector.
[0061] The upper part of the diagram on FIG. 4 contains a rare
earth, such as Erbium, doped optical fiber section 119, while the
lower part represents the Raman shift/amplifier section 127
comprising nonlinear fiber. The feedback for the rare earth ion
lasing is provided by the fiber loop itself; that is, the rare
earth ion laser oscillates as a unidirectional ring in the circular
fiber loop in FIG. 4. The radiation of a high-power broad-area pump
diode, or diodes, 121 for example at 980 nm, is coupled into the
system with a wavelength division multiplexer (WDM) 123, which
separates the pump wavelength, 980 nm, from the rare earth laser
wavelength of 1550 nm. The pump diode power is preferably
double-passed through the active fiber 119 by incorporating a high
reflectivity Bragg grating 125 at the pump wavelength. The Raman
fiber amplifier section 127 is pumped by the circulating
intracavity power of the rare earth ion laser within the ring. The
Stokes-shifted stimulated Raman lines of this section amplify the
radiation of a low power, narrowband laser 115, e.g., at the
methane sideband 1651 nm wavelength. The narrowband laser is
preferably a tunable diode laser (TDL), and preferably in the
distributed feedback (DFB) configuration. This narrowband laser 115
is coupled into the fiber cavity by a 1651/1550 nm WDM 129. A
second 1651 nm WDM 131 passes the amplified 1651 nm radiation out
of the ring. An isolator 130 may be provided to prevent high power
radiation from reaching and damaging the tunable diode laser 115.
Another isolator 132 may optionally be provided in the ring to make
laser oscillation happen only in one direction around the ring.
This isolator 132 helps make the output radiation spectrally
narrow.
[0062] The described system 111 is designed to achieve 20-30 dB of
amplification at low order Stokes wavelengths. High power, on the
order of watts, can be achieved, while retaining the spectral
purity of a single-mode DFB laser 115. The combination of
tunability, modulation bandwidth, spectral purity, beam quality and
output power makes this amplifier/transmitter system highly
suitable for LIDAR remote sensing. In addition, a remarkably
compact and practical device can be made. Unlike most other tunable
lasers, the master oscillator/fiber amplifier transmitter has no
moving or adjustable parts at all. An all-solid-state monolithic
and integrated device can be achieved, which leads to a compact and
virtually maintenance-free LIDAR system.
[0063] Referencing FIG. 5, there is shown an alternative scheme for
the high power optical amplifier of FIG. 4. Instead of the ring
laser configuration described previously, an equivalent linear
laser configuration 133 can be used.
[0064] The first fiber section 135 after the diode or diodes of the
980 nm pump source 137 comprises a rare earth-doped fiber, such as
an Er:Yb co-doped double clad fiber providing 1550 nm lasing. The
second fiber section 148 is a nonlinear fiber which represents the
Raman shift/amplifier section. Feedback for the rare earth ion
lasing is provided by a Bragg fiber grating mirror 143 at the end
of the nonlinear fiber section 148. In this case, the rare earth
ion laser oscillates in a linear cavity defined by the output
coupler 145 at the pump end and by the high reflectivity Bragg
fiber grating 143 at the other end. The 980 nm pump radiation
provided by high-power broad-area diodes 137 or diode arrays is
coupled into the system through the bulk dichroic beam splitter
147. The beam splitter 147 transmits at the pump wavelength of 980
nm and reflects at the rare earth laser wavelength of 1550 nm. In
this way, the output of the laser is separated from the pump 137 at
this end of the fiber.
[0065] The fiber laser Raman amplifier system of nonlinear fiber
148 works with reflectivity as low as 4%, i.e., the reflectivity of
a cleaved fiber, at one end of the fiber. However, better
efficiency in Raman amplification occurs if intracavity power in
the 1550 nm laser is higher. This can be achieved by increasing
that reflectivity. To achieve maximum circulating power in the
cavity, a highly reflective mirror can be used, together with the
dichroic beam splitter 147 at the pump end of the fiber. A simple
approach is the one presented in FIG. 5, where a lens 151 and a
100% reflective flat mirror 149 are used. The lens 151 is necessary
in order to produce a flat wavefront at the flat mirror 149, as
required for a stable laser cavity. The narrowband laser 141 is
coupled into and out of the amplifier 133 by a pair of 1550/1651 nm
WDMs 153,155, respectively.
[0066] The configuration of FIG. 5 is somewhat more practical,
since it does not require closing the fiber loop in order to
provide the feedback for lasing at 1550 nm. Closing a fiber loop
made of dissimilar fibers, e.g., when double clad fibers are used
for gain elements, results in large losses due to the lossy free
space transition necessary between the two sections. In all other
aspects the linear cavity configuration produces results equivalent
to the ring cavity described above.
[0067] It is to be understood that the foregoing is a detailed
description of the preferred embodiments. The scope of the
applicants' invention, however, is to be determined by reference to
the following claims.
* * * * *