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 time 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 rings down 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.

Description:
[0001]     This application is a Continuation-in-Part of application Ser. No. 10/145,209 filed on May 13, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     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  
       [0003]     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.  
         [0004]     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.  
         [0005]     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 2 , O 2 , H 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.  
         [0006]     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.  
         [0007]     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.  
         [0008]     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.  
         [0009]      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  206   a . 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 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 .  
         [0010]     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 .  
         [0011]     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).  
         [0012]     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  206   a  due to these losses. Furthermore, these conventional systems are costly and the AOM requires additional space and AOM driver (not shown) within the system.  
         [0013]     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  
       [0014]     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.  
         [0015]     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.  
         [0016]     According to a further aspect of the invention, light from the source is coupled to the resonant cavity by an optical fiber.  
         [0017]     According to yet another aspect of the invention, a collimator couples the light into the resonant cavity.  
         [0018]     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.  
         [0019]     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.  
         [0020]     According to another aspect of the invention, the light source is deactivated once the signal generated from the detector exceeds the level of the threshold voltage.  
         [0021]     According to yet another aspect of the invention, the first delay circuit is activated on the deactivation of the light source.  
         [0022]     According to yet another aspect of the invention, the second delay circuit allows for the stabilization of the light source after re-energizing prior to a new set of data being examined.  
         [0023]     According to yet another aspect of the invention, the light source is activated after an end of the first delay period.  
         [0024]     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.  
         [0025]     According to still another aspect of the invention, an analyte level present in the resonant cavity is measured during the first delay period.  
         [0026]     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.  
         [0027]     According to yet another aspect of the invention, the light source is a laser.  
         [0028]     According to still a further aspect of the invention, an algorithm is used to set the threshold voltage through the use of a digital to analog converter. This algorithm is used to establish the best cavity signal to noise ratio.  
         [0029]     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.  
         [0030]     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  
       [0031]     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:  
         [0032]      FIG. 1  illustrates the electromagnetic spectrum on a logarithmic scale;  
         [0033]      FIG. 2  illustrates a prior art CW-CRDS system;  
         [0034]      FIG. 3A  illustrates an exemplary embodiment of the present invention;  
         [0035]      FIG. 3B  illustrates another exemplary embodiment of the present invention;  
         [0036]      FIG. 4  is an illustration of an exemplary controller of the present invention;  
         [0037]      FIG. 5A  is a graph illustrating various delay timing according to an exemplary embodiment of the present invention;  
         [0038]      FIG. 5B  is a partial timing diagram of certain signals according to an exemplary embodiment of the present invention; and  
         [0039]      FIG. 6  is a flow chart according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]      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 to the output of optical cavity  318 . In turn, detector  312  generates an output signal  313  and provides this signal to controller  314  and data analysis system  316 . 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  313  received from optical detector  312 , in accordance with commands from controller  314 , in order to determine the level of trace species (analyte) in optical resonator  318 .  
         [0041]     Desirably, 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.  
         [0042]     Resonant cavity  318  desirably 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 .  
         [0043]     Detector  312  is desirably a photovoltaic detector, such as photodiodes or photo-multiplier tubes (PMT), for example.  
         [0044]     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 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  generated by data analysis system  316  which, in one exemplary embodiment, is converted from a digital signal to an analog signal by threshold DAC  405 . In operation, threshold signal  404  is incremented upward or downward to obtain the maximum signal level from detector  312 . An exemplary process for this is illustrated in  FIG. 6 . As a result, threshold signal  404  is based on the level of the ring down signal which has the greatest signal to noise ratio. The output of comparator  406  is provided as an input to control circuit  408 .  
         [0045]     Referring now to  FIG. 6 , an exemplary flow chart fro threshold control is illustrated. At Step  600 , threshold control is initialized. This may be accomplished as part of system initialization or under control of Data Analysis System  316 , for example. At Step  602 , an initial threshold value is set. At Step  604 , a determination is made whether a ring-down occurred within a predetermined time period, such as about one second, for example. If a ring-down occurred Step  608  is entered, otherwise Step  606  is entered. At Step  606 , because a ring-down did not occur, the threshold voltage is decremented and Step  604  is re-entered. At Step  608 , because a ring-down did occur, the threshold voltage is incremented and Step  604  is re-entered. This process is repeated as desired. In this way, an optimum signal to noise ratio is obtained.  
         [0046]     At time t 0   + , control circuit  408  generates control signal  408   a , based on the rise of the ring down signal crossing the threshold level, in order to activate first delay circuit  412  (via control signal  408   a ) while simultaneously turning off light source  302  through switch circuit  410  and driver  416  (via control signal  408   c ). At the end of the first delay period t 1  (at subsequent time t 0  as shown in  FIG. 5A ), signal  412   a  is generated by first delay circuit  412  and provided to control circuit  408 . In turn, control circuit  408  generates signal  408   b  to activate second delay circuit  414 , and provides an active signal  408   c  (previously deactivated at the beginning of the first delay period) 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 2  (shown in  FIG. 5A ), second delay circuit  414  generates signal  414   a  and provides it to control circuit  408  to indicate that light source  302  has stabilized and to begin a third time period t 3  (shown in  FIG. 5A ). Time period t 3  (described in detail below with respect to  FIG. 5A ) 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 3 , which it should be noted is a time period such that cell  318  is sufficiently charged with light energy, control signal  408   c  is deactivated, which in turn is used by switch circuit  410  (and, in one exemplary embodiment, driver  416 ) 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 .  
         [0047]     It should be noted that although terms such as active, inactive, activate, and/or deactivate as used, one of skill in that art will readily recognize and appreciate that the exemplary signal levels are arbitrary and may for example be inverted from those discussed. Further, although certain signals may be shown as maintaining a particular level throughout a particular time period, it is also possible that a level transition is all that may be required (such as a pulse) to accomplish the desired result.  
         [0048]     Coincident with the deactivation of signal  408   c , signal  408   d  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  408   c  and  408   d  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  408   c  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 .  
         [0049]     Signal  408   d  (in the two-signal  408   c / 408   d  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. In other words, during the period that control signal  408   d  is inactive data analysis system  316  is prevented from accepting new data represented by signal  313 , At this point, the process repeats itself to measure successive ring downs by once again initializing first delay circuit  412  through control circuit  408 .  FIG. 5B  illustrates a exemplary timing diagram for various ones of the aforementioned control signals.  
         [0050]     Table 1 lists system status at various times set forth in  FIG. 5A .  
                   TABLE 1                       TIME   STATUS                   Initial t 0     Light source ON; Delay circuits OFF; Wait State       t 0   +     Sufficient energy build up in resonant cavity;           Activate first delay circuit; Turn off light source;       Subsequent t 0     End of t 1  delay period; Turn on Light Source; Begin           time delay t 2 ; (Cycle repeats)                  
 
         [0051]     Because 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  may be provided as an input to control circuit  408 . Upon activation of initialization signal  420 , such as through a button, control signal from data analysis system  316 , or an automatic reset at power-up, for example, delay time to begins. The process then follows the procedure outlined above.  
         [0052]     In one exemplary embodiment, switch circuit  410  functions as a current switch/shunt for enabling/disabling current drive to light source  302 .  
         [0053]     As a result, controller  314  energizes light source  302  to generate energy into resonant cavity  318 , employs a first delay to allow light energy from light source  302  to completely ring down and be captured by data analysis system  316 . A second delay then allows light source  302  to stabilize before looking for new data. Once sufficient energy is built up in resonant cavity  318  the process is 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. 5A .  
         [0054]     As shown in  FIG. 5A , at time to, light source  302  is energized by providing operating current I, which is above the light source&#39;s threshold current I 0 , Threshold current I 0  varies based on the type of light source used. Delay time t 2  represents the delay to allow the light source to stabilize. In one exemplary embodiment, time delay t 2  is set to about 100 msec. Wait time t 3  represents the time to allow the current modulation to build up within resonant cavity  318 . It should be noted that the actual time required for the current modulation to build up within resonant cavity  318  is &lt;&lt;t3.  
         [0055]     In an exemplary embodiment, wait time t 3  is based on the modulation frequency f of light source  302 , and is desirably equal to about 1/f. In another exemplary embodiment, t3 is equal to about 1/f plus the time needed to exceed the threshold level in the resonant cavity for a ring-down to occur. Time delay t 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 1  is desirably set to about ten (10) times the ring down time of the cavity.  
         [0056]     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.  
         [0057]     Various advantages are realized from the present invention, such as: 
        Allowing use of almost 100% of the beam power generated by light source  302  (there may be negligible albeit undetectable losses within optical fiber  304  and collimator  308 ). Higher intra-cavity 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˜80% of light power goes to the first order when light passes through an AOM.     Cost savings are realized from eliminating the AOM. A typically commercially available AOM costs approximately $2,000.     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.        
 
         [0061]     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.