U.S. patent application number 10/145209 was filed with the patent office on 2003-11-13 for system and method for controlling a light source for cavity ring-down spectroscopy.
Invention is credited to Augustine, Robert, Krusen, Calvin R., Wang, Chuji, Yan, Wen-Bin.
Application Number | 20030210398 10/145209 |
Document ID | / |
Family ID | 29400423 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030210398 |
Kind Code |
A1 |
Augustine, Robert ; et
al. |
November 13, 2003 |
System and method for controlling a light source for cavity
ring-down spectroscopy
Abstract
An apparatus and method for controlling a light source used in
Cavity Ring-Down Spectroscopy. The apparatus comprises a controller
that generates a control signal to activate and deactivate the
light source based on a comparison of an energy signal from a
resonant cavity and a threshold. The light source is activated for
a predetermined period based on the stabilization time of the light
source and the time necessary to provide sufficient energy to the
resonant cavity. Thereafter the controller deactivates the light
source for a predetermined time period by interrupting its current
source so that the light energy in the cavity ring downs and so
that the presence of analyte can be measured. The light energy from
the light source is directly coupled to the resonant cavity from
the light source.
Inventors: |
Augustine, Robert; (Willow
Grove, PA) ; Krusen, Calvin R.; (Richboro, PA)
; Wang, Chuji; (Hatboro, PA) ; Yan, Wen-Bin;
(Cranbury, NJ) |
Correspondence
Address: |
RATNERPRESTIA
P O BOX 980
VALLEY FORGE
PA
19482-0980
US
|
Family ID: |
29400423 |
Appl. No.: |
10/145209 |
Filed: |
May 13, 2002 |
Current U.S.
Class: |
356/432 |
Current CPC
Class: |
G01J 3/10 20130101; G01N
21/39 20130101 |
Class at
Publication: |
356/432 |
International
Class: |
G01N 021/59 |
Claims
What is claimed is:
1. An apparatus for controlling a light source for use with a
resonant cavity, the apparatus comprising: a controller for
receiving a comparison of a detection signal and a predetermined
threshold, the comparator generating a control signal to at least
one of activate and deactivate the light source based on the
comparison; a first delay circuit coupled to the controller for
generating a first delay signal to the controller; and a second
delay circuit coupled to the comparator and the controller for
generating a second delay signal to the controller based on the
comparison of the detection signal and the predetermined
threshold.
2. The apparatus according to claim 1, wherein the first delay
circuit is initialized by an initialization signal.
3. The apparatus according to claim 1, wherein the light source
provides light as an input to the resonant cavity used to measure
the presence of an analyte in the resonant cavity.
4. The apparatus according to claim 1, wherein light from the
source is coupled to the resonant cavity by an optical fiber.
5. The apparatus according to claim 4, further comprising a fiber
collimator coupled between the optical fiber and an input of the
resonant cavity.
6. The apparatus according to claim 1, further comprising a
comparator to generate an output signal to the controller based on
the comparison of the detection signal and the predetermined
threshold.
7. The apparatus according to claim 6, further comprising a
detector coupled between an output of the resonant cavity and the
comparator, the detector generating the detector signal.
8. The apparatus according to claim 1, wherein the light source is
deactivated based on a time period of the first delay circuit.
9. The apparatus according to claim 8, wherein the first delay
period is based on a ring down time period of the resonant
cavity.
10. The apparatus according to claim 9, wherein the first delay
period is about 10 times the ring down period.
11. The apparatus according to claim 8, wherein the light source is
activated after an end of the first delay period.
12. The apparatus according to claim 8, wherein an analyte level
present in the resonant cavity is measured during the first delay
period.
13. The apparatus according to claim 1, wherein a period of the
second delay circuit is based on a stabilization time of the light
source.
14. The apparatus according to claim 13, wherein the second delay
period is about 100 msec.
15. The apparatus according to claim 1, wherein the first delay
signal is generated after a third delay period.
16. The apparatus according to claim 15, wherein the third delay
period is based on a modulation frequency of the light source.
17. The apparatus according to claim 16, wherein the third delay
period is an inverse of the modulation frequency.
18. The apparatus according to claim 15, wherein the third delay
period follows the second delay period.
19. The apparatus according to claim 15, wherein light energy
builds up within the resonant cavity during the third delay
period.
20. The system according to claim 1, wherein the light source is a
laser.
21. A system for use with a light source to measure the presence of
an analyte in a resonant cavity, the system comprising: a detector
coupled to an output of the resonant cavity to generate a detection
signal based on a light output from the resonant cavity; a
controller coupled to the light source, the controller activating
and deactivating the light source based on the detection signal;
and a processor coupled to the controller to process the detection
signal and determine a level of the analyte present in the resonant
cavity.
22. The system according to claim 21, wherein the controller
deactivates the light source by shunting a supply of current for
the light source.
23. The system according to claim 21, further comprising an optical
fiber coupling light energy from the light source to the resonant
cavity.
24. The system according to claim 23, further comprising a fiber
collimator coupled between an end of the optical fiber and the
resonant cavity.
25. The system according to claim 21, wherein the controller
activates the light source for a first predetermined time period
and deactivates the light source for a second predetermined time
period.
26. The system according to claim 25, wherein the first
predetermined period is about based on a stabilization time of the
light source.
27. The system according to claim 26, wherein the first
predetermined period is further based on a modulation frequency of
the light source.
28. The system according to claim 21, wherein the light source is a
laser.
29. The system according to claim 21, wherein the energy is light
energy.
30. A method for measuring the presence of an analyte in a resonant
cavity, the method comprising the steps of: detecting a light
energy signal output from the resonant cavity; comparing the
detected signal with a predetermined threshold; generating a
control signal to control the light source based on the comparison;
generating a first delay signal to the controller; generating a
second delay signal after an end of a first delay period; and
measuring a level of the analyte after an end of a second delay
period.
31. The method according to claim 30, further comprising the steps
of: activating the light source following generation of the second
delay signal; and deactivating the light source during at least
said first delay period.
32. The method according to claim 30, further comprising the step
of providing an initialization signal to initialize the first delay
signal.
33. A system for measuring the presence of an analyte in a resonant
cavity, the system comprising of: detecting means for detecting a
light energy signal output from the resonant cavity; comparison
means for comparing the detected signal with a predetermined
threshold; control means for generating a control signal to control
the light source based on the comparison; first delay means for
generating a first delay signal to the controller and initiating a
first delay period; second delay means for generating a second
delay signal and initiating a second delay period after an end of
the first delay period; and processing means for measuring a level
of the analyte after an end of the second delay period.
34. The system according to claim 33, wherein said processing means
measures the level of analyte during said first delay period.
35. An apparatus for controlling a light source for use in cavity
ring-down spectroscopy, the apparatus comprising: a controller for
receiving a comparison of a detection signal and a predetermined
threshold, the comparator generating a control signal to at least
one of activate and deactivate the light source based on the
comparison; a first delay circuit coupled to the controller for
generating a first delay signal to the controller; and a second
delay circuit coupled to the comparator and the controller for
generating a second delay signal to the controller based on the
comparison of the detection signal and the predetermined threshold.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to absorption spectroscopy
and, in particular, is directed to the activation and deactivation
of a light source for use with an optical resonator for cavity
ring-down spectroscopy.
BACKGROUND OF THE INVENTION
[0002] Referring now to the drawing, wherein like reference
numerals refer to like elements throughout, FIG. 1 illustrates the
electromagnetic spectrum on a logarithmic scale. The science of
spectroscopy studies spectra. In contrast with sciences concerned
with other parts of the spectrum, optics particularly involves
visible and near-visible light--a very narrow part of the available
spectrum which extends in wavelength from about 1 mm to about 1 nm.
Near visible light includes colors redder than red (infrared) and
colors more violet than violet (ultraviolet). The range extends
just far enough to either side of visibility that the light can
still be handled by most lenses and mirrors made of the usual
materials. The wavelength dependence of optical properties of
materials must often be considered.
[0003] Absorption-type spectroscopy offers high sensitivity,
response times on the order of microseconds, immunity from
poisoning, and limited interference from molecular species other
than the species under study. Various molecular species can be
detected or identified by absorption spectroscopy. Thus, absorption
spectroscopy provides a general method of detecting important trace
species. In the gas phase, the sensitivity and selectivity of this
method is optimized because the species have their absorption
strength concentrated in a set of sharp spectral lines. The narrow
lines in the spectrum can be used to discriminate against most
interfering species.
[0004] In many industrial processes, the concentration of trace
species in flowing gas streams and liquids must be measured and
analyzed with a high degree of speed and accuracy. Such measurement
and analysis is required because the concentration of contaminants
is often critical to the quality of the end product. Gases such as
N.sub.2, O.sub.2, H.sub.2, Ar, and He are used to manufacture
integrated circuits, for example, and the presence in those gases
of impurities--even at parts per billion (ppb) levels--is damaging
and reduces the yield of operational circuits. Therefore, the
relatively high sensitivity with which water can be
spectroscopically monitored is important to manufacturers of
high-purity gases used in the semiconductor industry. Various
impurities must be detected in other industrial applications.
Further, the presence of impurities, either inherent or
deliberately placed, in liquids have become of particular concern
of late.
[0005] Spectroscopy has obtained parts per million (ppm) level
detection for gaseous contaminants in high-purity gases. Detection
sensitivities at the ppb level are attainable in some cases.
Accordingly, several spectroscopic methods have been applied to
such applications as quantitative contamination monitoring in
gases, including: absorption measurements in traditional long
pathlength cells, photoacoustic spectroscopy, frequency modulation
spectroscopy, and intracavity laser absorption spectroscopy. These
methods have several features, discussed in U.S. Pat. No. 5,528,040
issued to Lehmann, which make them difficult to use and impractical
for industrial applications. They have been largely confined,
therefore, to laboratory investigations.
[0006] In contrast, continuous wave-cavity ring-down spectroscopy
(CW-CRDS) has become an important spectroscopic technique with
applications to science, industrial process control, and
atmospheric trace gas detection. CW-CRDS has been demonstrated as a
technique for the measurement of optical absorption that excels in
the low-absorbance regime where conventional methods have
inadequate sensitivity. CW-CRDS utilizes the mean lifetime of
photons in a high-finesse optical resonator as the
absorption-sensitive observable.
[0007] Typically, the resonator is formed from a pair of nominally
equivalent, narrow band, ultra-high reflectivity dielectric
mirrors, configured appropriately to form a stable optical
resonator. A laser pulse is injected into the resonator through a
mirror to experience a mean lifetime which depends upon the photon
round-trip transit time, the length of the resonator, the
absorption cross section and number density of the species, and a
factor accounting for intrinsic resonator losses (which arise
largely from the frequency-dependent mirror reflectivities when
diffraction losses are negligible). The determination of optical
absorption is transformed, therefore, from the conventional
power-ratio measurement to a measurement of decay time. The
ultimate sensitivity of CW-CRDS is determined by the magnitude of
the intrinsic resonator losses, which can be minimized with
techniques such as superpolishing that permit the fabrication of
ultra-low-loss optics.
[0008] FIG. 2 illustrates a conventional CW-CRDS apparatus 200. As
shown in FIG. 2, light is generated from a narrow band, tunable,
continuous wave diode laser 202. Laser 202 is temperature tuned by
a temperature controller (not shown) to put its wavelength on the
desired spectral line of the analyte. An acousto-optic modulator
(AOM) 204 is positioned in front of and in line with the radiation
emitted from laser 202. AOM 204 provides a means for providing
light 206 from laser 202 along the optical axis 219 of resonant
cavity 218. Light 206 exits AOM 204 and is directed by mirrors 208,
210 to cavity mirror 220 as light 206a. Light travels along optical
axis 219 and exponentially decays between cavity mirrors 220 and
222. The measure of this decay is indicative of the presence or
lack thereof of a t trace species. Detector 212 is coupled between
the output of optical cavity 218 and controller 214. Controller 214
is coupled to laser 202, processor 216, and AOM 204. Processor 216
processes signals from optical detector 212 in order to determine
the level of trace species in optical resonator 218.
[0009] In AOM 204, a pressure transducer (not shown) creates a
sound wave that modulates the index of refraction in an active
nonlinear crystal (not shown), through a photoelastic effect. The
sound wave produces a Bragg diffraction grating that disperses
incoming light into multiple orders, such as zero order and first
order. Different orders have different light beam energy and follow
different beam directions. In CW-CRDS, typically, a first order
light beam 206 is aligned along with optical axis 219 of cavity 218
incident on the cavity in-coupling mirror 220, and a zero order
beam 224 is idled with a different optical path (other higher order
beams are very weak and thus not addressed). Thus, AOM 204 controls
the direction of beams 206, 224.
[0010] When AOM 204 is on, most light power (typically, up to 80%,
depending on size of the beam, crystals within AOM 204, alignment,
etc.) goes to the first order along optical axis 219 of resonant
cavity 218 as light 206. The remaining beam power goes to the zero
order (light 224), or other higher orders. The first order beam 206
is used for the input coupling light source; the zero order beam
224 is typically idled or used for diagnostic components. Once
light energy is built up within the cavity, AOM 204 is turned off.
This results in all the beam power going to the zero order as light
224, and no light 206 is coupled into resonant cavity 218. The
stored light energy inside the cavity follows an exponential decay
(ring down).
[0011] In order to "turn off" the laser light to optical cavity
218, and thus allow for energy within optical cavity 218 to "ring
down," AOM 204, under control of controller 214 and through control
line 224, redirects (deflects) light from laser 204 along path 224
and, thus, away from optical path 219 of optical resonator 218.
This conventional approach has drawbacks, however, in that there
are losses of light energy primarily through the redirecting means
contained within the AOM. Other losses may also be present due to
mirrors 208, 210 used to direct light from AOM 204 to optical
cavity 218. It is estimated that only 50%-80% of light emitted by
laser 202 eventually reaches optical resonator 218 as light 206a
due to these losses. Furthermore, these conventional systems are
costly and the AOM requires additional space and AOM driver (not
shown) within the system.
[0012] To overcome the shortcomings of conventional systems, an
improved system and method for providing and controlling laser
light to a resonant cavity is provided. An object of the present
invention is to replace the conventional AOM/control system with a
simplified and cost effective control system.
SUMMARY OF THE INVENTION
[0013] To achieve that and other objects, and in view of its
purposes, the present invention provides an improved apparatus and
method for controlling a light source for use with a resonant
cavity. The apparatus includes a controller for receiving a
comparison of a detection signal and a predetermined threshold, the
comparator generating a control signal to one of activate and
deactivate the light source based on the comparison; a first delay
circuit coupled to the controller for generating a first delay
signal to the controller; and a second delay circuit coupled to the
comparator and the controller for generating a second delay signal
to the controller based on the comparison of the detection signal
and the predetermined threshold.
[0014] According to another aspect of the invention, the light
source provides light as an input to the resonant cavity to measure
the presence of an analyte in the resonant cavity.
[0015] According to a further aspect of the invention, light from
the source is coupled to the resonant cavity by an optical
fiber.
[0016] According to yet another aspect of the invention, a
collimator couples the light into the resonant cavity.
[0017] According to still another aspect of the invention, a
comparator generates an output signal to the controller based on a
comparison of the detection signal and a predetermined
threshold.
[0018] According to yet a further aspect of the invention, a
detector is coupled between the output of the resonant cavity and
the comparator, and generates a signal based on the light output
from the resonant cavity.
[0019] According to another aspect of the invention, the light
source is deactivated based on the first delay signal.
[0020] According to yet another aspect of the invention, the light
source is activated after an end of the first delay period.
[0021] According to yet another aspect of the invention, after an
end of the first delay period, the light source is activated and
energy builds up within the cavity through the current
modulation.
[0022] According to still another aspect of the invention, an
analyte level present in the resonant cavity is measured during the
first delay period.
[0023] According to yet a further aspect of the invention, the
controller deactivates the light source by shunting a supply of
current for the light source.
[0024] According to yet another aspect of the invention, the light
source is a laser.
[0025] The method includes the steps of, detecting a light energy
signal output from the resonant cavity; comparing the detected
signal with a predetermined threshold; generating a control signal
to control the light source based on the comparison; generating a
first delay signal to the controller; generating a second delay
signal after the end of the first delay signal; providing a current
modulation; and measuring a level of the analyte after an end of
the second delay signal.
[0026] It is to be understood that both the foregoing general
description and the following detailed description are exemplary,
but are not restrictive, of the invention.
BRIEF DESCRIPTION OF THE DRAWING
[0027] The invention is best understood from the following detailed
description when read in connection with the accompanying drawing.
It is emphasized that, according to common practice, the various
features of the drawing are not to scale. On the contrary, the
dimensions of the various features are arbitrarily expanded or
reduced for clarity. Included in the drawing are the following
figures:
[0028] FIG. 1 illustrates the electromagnetic spectrum on a
logarithmic scale;
[0029] FIG. 2 illustrates a prior art CW-CRDS system;
[0030] FIG. 3A illustrates an exemplary embodiment of the present
invention;
[0031] FIG. 3B illustrates another exemplary embodiment of the
present invention;
[0032] FIG. 4 is an illustration of an exemplary controller of the
present invention; and
[0033] FIG. 5 is a graph illustrating various delay timing
according to an exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0034] FIG. 3A illustrates an exemplary embodiment of the present
invention. As shown in FIG. 3A, light is generated from light
source 302, such as a narrow band, tunable, continuous wave diode
laser. Light source 302 is temperature tuned by a temperature
controller (not shown) to put its wavelength on the desired
spectral line of the analyte of interest. Light energy from light
source 302 is coupled to fiber collimator 308 through optical fiber
304. Light energy 306 is, in turn, provided by collimator 308 to
resonant cavity 318 and substantially parallel to its optical axis
319. Detector 312 is coupled between the output of optical cavity
318 and controller 314. Controller 314 is coupled to light source
302 and data analysis system 316. Data analysis system 316, such as
a personal computer or other specialized processor, processes
signals from optical detector 312, under control of controller 314,
in order to determine the level of trace species (analyte) in
optical resonator 318.
[0035] Preferably, light source 302 is a temperature and current
controlled, tunable, narrow line-width radiation, semiconductor
laser operating in the visible to near- and middle-infrared
spectrum. Alternatively, light source 302 may be an external-cavity
semiconductor diode laser.
[0036] Resonant cavity 318 preferably comprises at least a pair of
high reflectivity mirrors 320, 322 and a gas cell 321 on which the
mirrors are mounted. Cell 321 can be flow cell or vacuum cell, for
example. Alternatively, and as shown in FIG. 3B, resonant cavity
318 may be comprised of a pair of prisms 324, 326 and a
corresponding gas cell 321.
[0037] Detector 212 is preferably a photovoltaic detector, such as
photodiodes or photo-multiplier tubes (PMT), for example.
[0038] Referring now to FIG. 4, a detailed block diagram of
controller 314 is shown. As shown in FIG. 4, buffer 402 receives
signal 313 (representing the amplitude of the ring down signal)
from detector 312 (shown in FIGS. 3A-3B). Comparator 406 receives
buffered signal 313 and performs a comparison with a threshold
signal 404. In operation, threshold signal 404 is incremented
upward until the output of comparator 406 is a zero state. Then,
threshold signal 404 decrements until comparator 406 provides an
output signal. As a result, threshold signal 404 is based on the
level of the ring down signal. In this way, control circuit 408 is
able to determine when ring down signal output from detector 312
dissipates.
[0039] Control circuit 408 generates control signal 408a based on
the dissipation of the ring down signal in order to activate first
delay circuit 412. At the end of the first delay period (time
t.sub.1 as shown in FIG. 5), signal 412a is generated and provided
to control circuit 408. In turn, control circuit 408 generates
signal 408b to activate second delay circuit 414, and provides
signal 408c to switch circuit 410, which in turn activates light
source 302 (shown in phantom and described above with respect to
FIGS. 3A and 3B). At the end of delay period t.sub.2 (shown in FIG.
5), delay circuit 414 generates signal 414a to control circuit 408
to indicate that light source 302 has stabilized and to begin a
third time period t.sub.3 (shown in FIG. 5). Time period t.sub.3
(described in detail below with respect to FIG. 5) is used to
ensure that resonant cavity 318 is fully charged through current
modulation with light energy prior to measuring analyte
concentration. At the end of time period t.sub.3, control signal
408c is deactivated, which in turn is used by switch circuit 410 to
deactivate light source 302. In one embodiment of the present
invention, switch circuit 410 shunts current from light source 302
using convention power devices to deactivate light source 302.
[0040] Coincident with the deactivation of signal 408c, signal 408d
is also generated and provided to data analysis system 316 (shown
in phantom and described above with respect to FIGS. 3A and 3B).
Although signal 408c and 408d are shown as separate signals, it may
be preferable to combine them into a single control signal if
desired. In such an approach conditioning of signal 408c may be
required to provide a convenient control signal logic level (based
on digital signals, for example) to provide proper control of data
analysis system 316.
[0041] Signal 408d (in the two-signal 408c/408d approach) is used
by data analysis system 316 to indicate that light source 302 has
been deactivated and that the measurement of the analyte should
begin. At this point, the process repeats itself to measure
successive ring downs by once again initializing first delay
circuit 412 through control circuit 408.
[0042] Since the above description relates to ongoing measurement
of analytes, the circuit needs to be initialized prior to the first
measurement. To accomplish this initialization, an initialization
signal 420 is provided as an input to first delay circuit 412. Upon
activation of initialization signal 420, such as through a button,
or control signal from data analysis system 316, for example, delay
time to begins. The process then follows the procedure outlined
above.
[0043] In the exemplary embodiment, switch circuit 410 provides
three functions: 1) as a laser current driver providing laser
driving current for a desired output of laser power, 2) providing
current modulation resulting in energy build-up within cavity 318,
and 3) as a current switch/shunt for enabling/disabling current
drive to light source 302.
[0044] As a result, controller 314 energizes light source 302 to
generate energy into resonant cavity 318, employs a first delay to
allow light source 302 to stabilize before looking for new data,
utilizes a second delay to wait for a build up of sufficient light
energy in the cell, then turns off the energy to light source 302.
Another delay is employed after energy is removed from light source
302 to allow the light energy to completely ring down. This process
is then repeated for a single wavelength ring-down data at a given
temperature. Ring-down spectra are processed by the data analysis
system 316. These various delays are illustrated in FIG. 5.
[0045] As shown in FIG. 5, at time t.sub.0, light source 302 is
energized by providing operating current I, which is above the
light source's threshold current I.sub.0. Threshold current I.sub.0
varies based on the type of light source used. Delay time t.sub.2
represents the delay to allow the light source to stabilize. In one
exemplary embodiment, time delay t.sub.2 is set to about 100 msec.
Delay time t.sub.3 represents the time to allow the current
modulation to build up within resonant cavity 318. It should be
noted that the time required for the current modulation to build up
within resonant cavity 318 is <<t3.
[0046] In an exemplary embodiment, time delay t.sub.3 is based on
the modulation frequency f of light source 302, and is preferably
equal to about 1/f. Time delay t.sub.1 is based on the ring down
time of resonant cavity 318. In order to allow sufficient time for
light energy to "ring down" in resonant cavity 318, time delay
t.sub.1 is preferably set to about ten (10) times the ring down
time of the cavity.
[0047] Laser temperature driver 416, under control of convention
means (not shown), provides temperature control for light source
302 for the generation of a desired light frequency at a given
temperature. The frequency is selected based on the particular
analyte of interest.
[0048] Various advantages are realized from the present invention,
such as:
[0049] Allowing use of almost 100% of the beam power generated by
light source 202 (there may be negligible albeit undetectable
losses within optical fiber 304 and collimator 308). Higher
intracavity energy build-up provides better signal to noise ratio
and reduces shot noise. This is extremely beneficial when a light
source is weak. As mentioned above, typically, only about
50.about.80% of light power goes to the first order when light
passes through an AOM.
[0050] Cost savings are realized from eliminating the AOM. A
typically commercially available AOM costs approximately
$2,000.
[0051] Simplified CW-CRDS setup-This allows more spatial
flexibility for the setup arrangements, and eliminates the
mechanical and optical sensitivity, introduced by the AOM, to the
testing environment.
[0052] Although illustrated and described herein with reference to
certain specific embodiments, the present invention is nevertheless
not intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the spirit
of the invention.
* * * * *