Abstract:
A near infrared (NIR) semiconductor laser system is shown for gas sensing. An embodiment is centered on the use of a system with a much wider tunable laser, which today has a scan band of more than 150 nanometers (nm) to as much as 250 nm or more. In some cases the scan band is about 400 nm or more. This is achieved in the current embodiment through the use of a widely tunable microelectromechanical system (MEMS) based Fabry-Perot filter as an integral part of the laser cavity. Using this technology, these systems are capable of capturing a variety of gases in the any of the well-known spectroscopic scan bands, such as the OH, NH or CH. For example, a single laser with a 250 nm scan band window between 1550-1800 nm can capture ten or as many as twenty hydrocarbon-based gases simultaneously.

Description:
RELATED APPLICATIONS 
     This application is a Continuation-in-Part of U.S. application Ser. No. 11/948,520, filed on Nov. 30, 2007, which claims the benefit under 35 USC 119( e ) of U.S. Provisional Application No. 60/867,849 filed on Nov. 30, 2006, both of which are incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Combustible gas detectors can be divided into two general categories. The first category includes a variety of “passive” technologies of which the electrocatalytic (catalytic bead) type is the most common. The second category is based on technology that uses infrared absorption as the detection modality. 
     Infrared (IR) absorption is considered “active” since an IR source emits a signal, usually many times a second, and the amount of energy falling on the detector serves as an active measure of the gas concentration at that moment. Any failure of the source or detector, or blockage of the signal by dirt, is detected immediately as a malfunction. For this reason, IR detectors are also considered to be fail-to-safe. IR gas detectors can be used for “point” (single location) or “open path” (line of sight) applications. 
     Electrocatalytic or “catalytic” detectors have been used for over 30 years and are widely deployed in a variety of industries as single-point detectors for combustible gases. They function on the relatively simple and reliable principle that a combustible gas can be oxidized to produce heat. The resulting temperature change can be converted, via a standard Wheatstone bridge, to a sensor signal. That signal can then be used to activate alarms and initiate fire preventative action. Even though these detectors can be manufactured to be very low cost, their primary drawback is that they can be contaminated or “poisoned” and they hence require dedicated user attention. 
     An alternative method of measuring gas concentration is based on absorption of infrared (IR) radiation at certain wavelengths as the optical signal passes through a volume of gas. Devices using this technology have a light source and a light detector and measure the light intensity at two specific wavelengths, one at an absorption (active) wavelength and one outside of the absorption (reference) wavelength. If a volume of gas passes between the source and detector, the amount of light in the active wavelength falling on the detector is reduced, while the amount of light in the reference wavelength remains unchanged. Much like the catalytic detectors, the gas concentration is determined from the relative difference between the two signals. 
     Generally, there are several key advantages to using IR-based detectors: immunity from chemical poisons; does not need oxygen or air to detect gas; can work in continuous exposure gas environments; fail-to-safe technology; and internal compensation can eliminate span drift. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention concern the use of Near Infrared (NIR) semiconductor lasers for gas sensing. This technology is expected to address some of the limitations of the current IR detectors. Such limitations include the following: 1) interference from other gases or moisture, due to the mid-IR operation as well as the fact that these IR detectors are typically low resolution and cover a limited spectral range, which prevents them from being able to identify and correct for the interfering gases; 2) inability to detect multiple gases with a single device, since these detectors use fairly coarse optical filters over a specific spectral range and therefore cannot detect gases with spectral signatures outside that window; and 3) limited sensitivity due to low signal-to-noise ratio, which is primarily attributed to the use of mid-IR sensitive detectors (pyroelectric), which are typically not as sensitive as InGaAs detectors. 
     The key elements of embodiments of the invention often include a number of aspects. 
     The use of a widely-tuned semiconductor laser is important. Currently there are laser-based gas detectors that use a technology called Tunable Diode Laser Absorption Spectroscopy (TDLAS), but these devices are very narrow in spectral range (10-60 nanometers (nm)) and they use a dithering technique to monitor individual gases. It is not possible to use these devices to monitor multiple gases. An embodiment of the invention is centered on the use of a system with a much wider tunable laser, which today has a scan band of more than 150 nm to as much as 250 nm or more. In some cases the scan band is about 400 nm or more. This is achieved in the current embodiment through the use of a widely tunable microelectromechanical system (MEMS) based Fabry-Perot filter as an integral part of the laser cavity. Using this technology, these systems are capable of capturing a variety of gases in the any of the well-known spectroscopic scan bands, such as the OH, NH or CH. For example, a single laser with a 250 nm scan band window between 1550-1800 nm can capture ten or as many as twenty hydrocarbon-based gases simultaneously. 
     High resolution spectroscopy is provided by use of the MEMS Fabry-Perot filter, which enables the potential for operation at very high resolution (as high as 0.1 cm −1 ), which allows the system to identify fine spectral features of the interfering gases (such as moisture) and correct for them, providing a much more accurate reading. 
     High sensitivity measurements are further enabled, providing applicability beyond the detection of explosive or flammable gases, to be used for any gas, liquid or solid spectral measurement, assuming that the gas, liquid or solid in question absorbs infrared radiation. 
     The preferred embodiment also provides for on-board referencing. Also the higher SNR of embodiments leads to higher measurement speed and hence there is a tradeoff option between laser power, detector sensitivity and measurement speed depending on the application requirements. Further small explosion proof packaging is preferably provided: The use of miniaturized packages allows for reduced requirements on the size of the explosion proof housing. Also, the highly efficient NIR semiconductor lasers allow for minimal power consumption and potentially improving battery life, if used. In the preferred embodiment, the system includes wireless communication. 
     In general, according to one aspect, the invention features a spectroscopy system. The system comprises a hermetic package with at least one optical port for coupling an optical beam from the hermetic package to a sample and back into the hermetic package. An optical bench is installed within the hermetic package, and a tunable laser is installed on the optical bench for generating the optical beam. A signal detector is further provided on the bench for detecting the optical beam received in through the optical port from the sample. Also, a controller spectrally analyzes the sample from a response of the signal detector. 
     In embodiments, a housing contains the hermetic package, the housing comprises a sample port for receiving the sample and the optical beam from the hermetic package into the sample port. A mirror is preferably installed in the sample port for reflecting the optical beam back into the optical port. 
     Additional possible features include a battery for powering the controller and the tunable laser and a wireless interface for providing wireless communication of spectral analysis information from the controller. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIG. 1  is a perspective view of a system for gas detection and analysis according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of the system for gas detection and analysis showing the optical bench layout according to an embodiment of the present invention; 
         FIG. 3  is a block diagram view of the system for gas detection and analysis showing relationship between the optical bench layout and system electronics according to an embodiment of the present invention; 
         FIGS. 4A ,  4 B, and  4 C show three variations for optical interface to the gas sample port, single bounce, double bounce and triple bounce, according to embodiments of the present invention; 
         FIG. 5  is a plot of absorbance as a function of wavelength in nanometers for 100% methane measured by the system; 
         FIG. 6  is a plot of absorbance as a function of wavelength in nanometers for 0.4% methane measured by the system; 
         FIG. 7  is a plot of absorbance as a function of wavelength in nanometers for 0.2% ethylene measured by the system; 
         FIG. 8  is a plot of concentration as a function of time in measurement cycle for a methane sample measured by the system; and 
         FIG. 9  illustrates is a schematic cross-sectional view of the gas sensor system  100  illustrating the thermal dissipation scheme according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a system  100  for gas detection and analysis, which has been constructed according to the principles of the present invention. 
     Generally, detection of several gases that have spectroscopic signatures in the near-infrared zone of 1.3-2 micrometers are detected by using a single-chip tunable source covering 200-300 nanometer (nm) scan band in this zone. Such gases include most of the common hydrocarbons, such as methane, ethane, ethylene, acetylene and other types, such as ammonia. The detection limits for the system are preferably in the 10% of lower explosion limit (LEL) levels. 
     The gas sensor system  100  preferably comprises three basic functional components: an optical module  110 , an electronics board  250 , and a gas sample optical interface. 
     The optical module  110  comprises a single hermetically sealed butterfly package  112 , which contains all of the active optical functions of the system  100  assembled on a single micro-optical bench  105 : a widely tunable semiconductor laser, optical amplitude and wavelength reference detectors, and the signal detector. 
     The optical module  110  produces a collimated optical beam output  16  into the gas sample optical interface. Specifically, this beam  16  interacts with the sample gas in an optical sample interface port  210  in a housing  200 , using one or more bounces in an external mirror arrangement in order to accumulate a sufficient optical absorption path length. 
     The returning beam  18  from the gas sample optical interface is detected by the signal detector inside the optical module  110 . 
     The beam exits and enters the hermetic package  112  via at least one optical port  116 ,  118 , comprising at least one window being transparent to the optical beam. This window is formed as part of an outer wall of the hermetic package  112 . 
     The electronics board  250  contains analog and digital components for controlling tunable laser scanning, detecting and digitally processing the return signal, and outputting the gas sensing data in the digital format. In some examples, external system control and data upload are provided via a wireless interface  254 . Also, in examples, the system  100  includes a battery  252  for main system power and/or backup power. 
     In more detail, the system housing  200  comprises a base plate  216 . A frame unit  214  is installed and sealed to this base plate  216 . Generally, the frame unit  214  of the housing  200  provides a hollow internal area in which the optical module  110  and specifically the butterfly package  112  is installed. The hollow internal area further houses the wireless interface  254 , the battery  252  and the digital signal processing electronics board  250 . A top cap  212  is sealed to the frame member  214  to seal the internal area and thus the electronic and optical components. 
     A central port  212  is formed in the housing  200  and specifically the frame member  214 , base plate  216  and the top member  212 . The optical sample interface port  210  mechanically interfaces with the output optical port  216  and the input optical port  218 , which are formed on the side wall of the butterfly package  212 . At the far lateral end of the optical sample interface port  210 , a mirror  114  is provided to couple the output beam  116  as the input beam  118  between the output optical port  116  and the input optical port  118 . 
     In the preferred embodiment, the output optical port  116  and the input optical port  118  comprise optically transparent windows. In some examples, these windows are bandpass filters that are limited to the scan band of the system  100  to thereby to prevent interference from external light sources. 
     While the applicability of the system is described in the context of gas analysis, the system  100  also has application in fluid and solid analysis. In some of these applications, the system uses the mirror  114  where the gas, liquid or solid sample is transmissive. When it is opaque to the wavelengths within the scan band, system collects the diffusely reflected light from the sample for analysis. 
       FIG. 2  illustrates the internal optical components  111  of the gas sensor optical module  110  within the hermetic package  112  including the micro-optical bench layout. 
     The widely tunable external cavity semiconductor laser comprises a semiconductor gain chip  120  which is installed on the optical bench  105  via a submount substrate  122 . A micro-electro-mechanical (MEMS) tunable optical filter  126  provides narrowband tunable feedback into the gain chip  120 . An intervening intracavity lens component  124  couples light between the chip  120  and the MEMS filter  126 . 
     In a current embodiment, the laser is as described in US 20060215713 A1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by Flanders, et al. This application is incorporated herein in its entirety by this reference. 
     A sample of the output beam  16  is provided from a collimating lens  128  to two taps  130 ,  132  to an amplitude detector  140  and a wavelength reference detector  138 . For wavelength referencing, a fixed wavelength etalon  134  and a possible a cut-off filter  136  are placed in front of the wavelength detector  138  to provide wavelength calibration reference for the wavelength scanning output laser signal, along with the amplitude information provided by the amplitude detector  140 . 
     A combination of two series output lens  142  and  144  are used for both collimating the output beam  16 , received from the collimating lens component  128  and the first tap  130 , at the proper beam diameter and for finely steering the beam  16  for alignment to the optical gas sampling interface outside the optical package  112 . Specifically, the beam is steered to be coupled back into the package  112  via input port  118  and detected by signal detector  150 . 
     The return optical signal detector  150  is integrated on the same micro-optical bench  105 . 
     In the preferred embodiment, the optical bench  105  is temperature controlled. This is provided by a thermoelectric cooler  106 . 
     The requirements for explosive gas monitoring are: that the systems are capable of detecting down to 10% of the Lower Explosive Limit (LEL) for each gas. For methane, this limit is 5%, so the detectors must be accurate to 0.5%. However, regulatory and safety requirements are pushing these limits down, and there is a need for higher sensitivity devices. Operation at the near infrared (NIR) regime for the present system  100  allows for the use of more sensitive detectors, such as InGaAs rather than pyroelectric, as it is currently the case with existing IR sensors. In addition, the use of semiconductor, high power lasers allows for higher signal-to-noise ratio measurements, which directly translates to lower sensitivity limits. 
       FIG. 3  illustrates the relationship between the electronics and the optical bench  105  in a current embodiment of the invention. Specifically, a digital signal processing board  150  controls power to the gain chip  120  and the tuning of the tunable filter  126 . Digital signal board  250  receives the response of the signal detector  150 . 
     By analyzing the response of the signal detector in conjunction with the information from the wavelength detector  138  and the amplitude detector  140 , the digital signal processor  250  resolves the spectral response of the gas sample within the gas port. Further, the digital signal processor  250  monitors the temperature of the optical bench  105  via a temperature detector  160 , such as a thermistor, which is typically installed on the surface of the bench  105 . Using this feedback, the digital signal processor board  250  drives the thermoelectric cooler  106  to temperature stabilize the optical bench  105 . 
     In one embodiment, the system is also provided with a battery  252  for power. Further, a wireless interface  254  is used in some examples to both provide control to the spectral analysis system  110  and also provide data upload to a host system. 
       FIGS. 4A through 4C  illustrate a number of examples for configuring the beams in the gas interface port  210 .  FIG. 4A  shows one example where the output beam  16  is reflected to form the input beam  18  by the mirror  114  in a single bounce arrangement. 
     In  FIG. 4B , a double bounce arrangement is used in which a second mirror  114 ′ is added optically between the output port  118  and the input port  116  of the hermetic package  112 . Specifically, this doubles the beam path length by yielding passes  117  through port  210 . 
       FIG. 4C  illustrates a triple bounce arrangement in which the output beam  16  is reflected between mirrors  114  and  114 ′ three times before returning to the input port  118 . 
       FIGS. 5 and 6  are plots of absorbance as function of wavelength in nanometers for methane gas sensing spectra at 100% and 0.4% concentration levels using a single pass 10 centimeter (cm) absorption cell, which were obtained using the system  100  for gas detection and analysis. The methane Lower Explosive Limit (LEL) is 5.0% and the required sensing level of 10% LEL is 0.5%. High signal to noise ratios of the spectra shows that methane can be sensed to very low concentrations. 
       FIG. 7  shows a similar gas-sensing spectrum for ethylene gas in a 10 cm path cell, illustrating simple sensing of different gases using the spectroscopic gas system. Such system also potentially allows measurement of mixtures of multiple gases. 
       FIG. 8  illustrates concentration measurements of methane gas in a 10 cm path cell using gas sensor  100 , where gas concentration number is extracted from the measured spectra. Methane gas concentration is measured down to the 0.15% level, which is equivalent to the 0.03*LEL level. Gas detection sensitivity can be enhanced even further by using a very compact multi-pass gas cell allowed by the single transverse mode optical beam of the sensor. 
       FIG. 9  illustrates the thermal dissipation paths of the gas sensor system  100  that prevent the optical ports  116 ,  118  and mirror  114  from accumulating condensation. 
     In the preferred embodiment, the optical ports  116 ,  118  and/or mirror  114  are held at temperatures above ambient temperature. As a result, even in highly saturated environments, condensation will be prevented from forming on the optical ports  116 ,  118  and/or mirror  114 . 
     Elevated temperatures for the optical ports  116 ,  118  and/or mirror  114  are achieved in one embodiment by including heaters near or on the optical ports  116 ,  118  and/or the mirror  114 . Simple resistive heaters are used in some examples. 
     However, in the preferred embodiments, the thermal dissipation paths for the optical system  111  and/or electronic system  250  are judiciously designed to provide these elevated temperatures. 
     In one example, the thermal dissipation path for heat generated by the thermoelectric cooler  106  is designed to extend through to the lid  112 L of the hermetic package  112 . In more detail, the thermoelectric cooler  106  has a hot side  912  and cool side  910 . The hot side  912  is thermally coupled to the bottom  112 B of the hermetic package  112  as is conventional. However, a thermally insulating layer or air or vacuum  930  separates the bottom  112 B of the package  112  from a thermal sink such as the baseplate  216 . Instead, the thermal dissipation path is through the package  112  to the lid  112 L. In one example, a thermally conductive paste is used to create a thermal path from the lid  112 L to the cap  212 . This thermal dissipation path through the package  112  and thus the ports  116 ,  118  of the package  112  ensures a heated state of the ports  116 ,  118  during operation of the system  100 . 
     In a similar way, the mirror  114  is also included in a thermal dissipation path. In more detail, in one example, heat generated by the electronics of the DSP board  250  is dissipated through the portion of the frame member  214  that supports the mirror  114  to a thermal sink  920 . As a result, the mirror  114  is passively heated by the “waste” heat generated by the electronics  250  to ensure that the mirror  114  is hotter than the environment during operation, thereby preventing the accumulation of condensation on the mirror  114 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention.