Abstract:
A chemical sensing system and method. The system (10) includes a transmitter having a laser for providing a collimated beam of electromagnetic energy at a first frequency and a Q switch in optical alignment with the beam. The system further includes a crystal for shifting the frequency of the beam from the first frequency to a second frequency. A mechanism is included for shifting the beam from the second frequency to a third frequency in the range of 8-12 microns. The system includes a mechanism for switching the polarization state of the second beam and providing third and fourth beams therefrom. The third beam has a first polarization and the fourth beam has a second polarization. The second polarization is orthogonal relative to the first polarization. The frequency shifted third and fourth beams are combined to provide an output beam in the range of 8-12 microns. The output beam is transmitted and a return signal is detected by a receiver in the illustrative chemical sensing application.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/478,229; entitled MONOLITHIC SERIAL OPTICAL PARAMETRIC OSCILLATOR filed Jan. 6, 2000, by J. M. Fukumoto now U.S. Pat. No. 6,344,920. In addition, this application relates to copending applications Ser. Nos. 09/556,216 and 09/563,073 entitled SYSTEM AND METHOD FOR PROVIDING COLLIMATED ELECTROMAGNETIC ENERGY IN THE 8-12 MICRON RANGE, filed Apr. 24, 2000 by J. M. Fukumoto and VARIABLE PATH LENGTH PASSIVE Q SWITCH, filed Apr. 24, 2000 by J. M. Fukumoto now U.S. Pat. No. 6,466,593. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to solid state lasers. More specifically, the present invention relates to systems and methods for atmospheric sensing using solid state lasers. 
     2. Description of the Related Art 
     Lasers are currently widely used for communication, research and development, manufacturing, directed energy and numerous other applications. For many applications, the energy efficiency, power and lightweight of solid state lasers makes these devices particularly useful. Solid state lasers currently lase in the range of one to three microns. 
     For certain applications, there is a need to reach higher laser operating frequencies. In particular, there is interest in the 8-12 micron (μm) region. The 8-12 micron region provides an ‘open window’ to the atmosphere making it useful for many applications. The window is ‘open’ in the sense that there is little atmospheric attenuation of the energy in the beam in this region of the electromagnetic spectrum. Hence, the 8-12 micron window allows for a probing of the atmosphere. 
     One such application, for which there is a need to probe the atmosphere, is that of remote sensing of chemical agents. Remote detection of toxic chemical agents is of current interest to both military and civilian defense agencies due to the growing availability and use of these compounds by terrorist groups and rogue nations. The 8-12 μm spectral region of the atmosphere offers an opportunity to remotely detect commonly used chemical agents since these species typically have distinct band structure in this wavelength range, and there is relatively low atmospheric attenuation in this region. 
     Wavelength conversion to this region has been demonstrated using various solid-state lasers, or with optical parametric oscillators (OPOs) as pump sources for longer wavelength OPOs and difference frequency generation crystals. See for example: 1) S. Chandra, T. H. Allik, G. Catella, R. Utano, J. A. Hutchinson, “Continuously tunable 6-14 μm silver gallium selenide optical parametric oscillator pumped at 1.57 μm,” Appl. Phys. Lett. 71, 584-586 (1997): 2) T AIlik, S. Chandra. D. M. Rines, P. G. Schunemann, J. A. Hutchinson, and R. Utano, “7-12 μm generation using a Cr, Er:YSGG pump laser and CdSe and ZnGeP2 OPOs,” in  Advanced Solid State Lasers, OSA Trends in Optics and Photonics  (Optical Society of America, Washington, D.C., 1997), Vol. 10, pp. 265-266; and 3) R. Utano and M. J. Ferry, in  Advanced Solid State Lasers, OSA Trends in Optics and Photonics  (Optical Society of America, Washington, D.C., 1997), Vol. 10, pp. 267-269[WJB1]. 
     These approaches generally involve the use of a flashlamp pumped (Cr, Er:YSGG) laser emitting at 2.79 microns to pump a cadmium-selenide (CdSe) laser. This method has been represented as being effective to yield a tunable 8-12 micron output. Unfortunately, the laser is too large and inefficient to be feasible in the field. That is, the poor overall electrical efficiency of the Cr, Er:YSGG pump laser, together with its fairly long (50 ns) output pulse width, result in a less than optimal CdSe OPO pump source. 
     On the other hand, carbon-dioxide (CO 2 ) lasers lase at 10 microns. However, these devices are not tunable and not sufficiently portable to be feasible for use in the field. 
     Hence, a need remains in the art for an efficient, feasible, portable, tunable system or method for converting the output of a typical 1-3 μm laser to the 8-12 μm range. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the system and method of the present invention. The system includes a transmitter having a laser for providing a collimated beam of electromagnetic energy at a first frequency and a Q switch in optical alignment with the beam. The system further includes a crystal for shifting the frequency of the beam from the first frequency to a second frequency. A mechanism is included for shifting the beam from the second frequency to a third frequency. 
     In the particular implementation, the third frequency is in the range of 8-12 microns. Ideally, the input beam is provided by a neodymium-YAG laser and the Q switch is a passive Q switch. The crystal is x-cut potassium titanyl arsenate. 
     In the best mode, the system includes a mechanism for switching the polarization state of the second beam and providing third and fourth beams therefrom. The third beam has a first polarization and the fourth beam has a second polarization. The second polarization is orthogonal relative to the first polarization. The mechanism for shifting the beam from the second frequency to the third frequency includes first and second optical parametric oscillators, each optical parametric oscillator including a cadmium selenide crystal. The frequency shifted third and fourth beams are combined to provide an output beam in the range of 8-12 microns. The output beam is transmitted and a return signal therefrom is detected by a receiver in the illustrative chemical sensing application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an illustrative implementation of a remote chemical sensing system incorporating the teachings of the present invention. 
     FIG. 2 is a block diagram of the laser transmitter of FIG.  1 . 
     FIGS. 3 a-c  are diagrams which illustrate the design and operation of the passive Q switch utilized in preferred embodiment of the transmitter of FIG.  2 . FIG. 3 a  show a side view of the switch. 
     FIGS. 3 b  and  3   c  depict side and top views, respectively, of a wedge of the passive Q switch of the present invention. 
     FIG. 4 is a simplified diagram illustrative of the operation of the solid state lasers utilized in the transmitter of the present invention. 
     FIG. 5 is a diagram showing the receiver of the system of FIG. 1 in greater detail. 
    
    
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1 is a block diagram of an illustrative implementation of a remote chemical sensing system incorporating the teachings of the present invention. The system  10  includes a laser transmitter  20  which outputs a reference beam and a probe beam as discussed more fully below. 
     FIG. 2 is a block diagram of the laser transmitter of FIG.  1 . The transmitter includes a diode pumped laser  200  having a rear high reflector  202 . The gain medium for the laser is a neodymium YAG (Nd:YAG) slab  210 . The oscillating beam  211  output by the slab  210  is directed to the rear high reflector  202  by a turning mirror  208 . 
     As shown in FIG. 1, an electro-optic Q switch  204  and a linear polarizer  206  may be positioned between the rear high reflector  202  and the turning mirror  208 . However, as discussed more fully below, in the best mode, a novel passive Q switch  230  can be used as an alternative. Accordingly, the electro-optic Q switch  204  and a linear polarizer  206  are shown in phantom in FIG. 1 to indicate that these elements are associated with an optional alternative implementation. 
     Returning to FIG. 2, in the preferred embodiment, the laser  200  is a neodymium YAG laser. The laser  200  can also use Nd:YLF or Nd:YVO 4  as the gain medium, depending on the particular pulse energy, pulse width, and pulse repetition rate required. A cooling block is soldered to the Nd:YAG slab  210  to provide cooling as is common in the art. A cylindrical lens  213  is positioned in optical alignment with the slab  210  to optimize the profile of the beam output thereby. The slab  210  is pumped by an array of diodes (not shown) disposed in an air-cooled package  214 . A collimated optic  216  is included for collimating and directing the output of the diodes into the laser slab  210  to achieve an optimal pump distribution. In the preferred embodiment, a collimating optic such as that disclosed and claimed in copending U.S. patent application Ser. No. 09/553,515 now U.S. Pat. No. 6,462,891 entitled SHAPING OPTIC FOR DIODE LIGHT SHEETS, filed by J. M. Fukumoto et al.., the teachings of which are incorporated herein by reference. Contacts  218  and  219  are provided for the package  214  as shown in the figure. 
     In the preferred embodiment, the oscillating beam of the slab  210  is directed by a second turning mirror  220  to an output coupler  240  via a passive Q switch  230  of novel design. The passive Q switch  230  is disclosed and claimed in copending U.S. patent application Ser. No. 09/563,073, now U.S. Pat. No. 6,466,593 entitled VARIABLE PATH LENGTH PASSIVE Q SWITCH, filed Apr. 24, 2000 by J. M. Fukumoto, the teachings of which are incorporated herein by reference. 
     FIGS. 3 a-c  are diagrams which illustrate the design and operation of the passive Q switch utilized in preferred embodiment of the transmitter of FIG.  2 . FIG. 3 a  shows a side view of the switch  230 . In the preferred embodiment, the switch  230  is implemented with first and second identical wedges  232  and  234 . In the illustrative embodiment, each wedge is made of Cr +2 :YAG). As shown in FIG. 3 a,  the first wedge has a slanted surface  236  while the second wedge has a slanted surface  238 . The wedges  232  and  234  are mounted to slide relative to each other in a plane parallel to plane of the slanted surfaces  236  and  238  thereof respectively. This sliding of the wedges has the effect of increasing the path length of the switch  230  with respect to the laser alignment axis  211  and beam. As is known in the art, the thickness of the switch  230  determines the laser&#39;s hold-off point thereof, i.e., the point at which the switch allows for the beam to pass therethrough. As is well known in the art, the Q switch  230  serves to ensure that the laser beam is output in a short duration pulse of energy. 
     The wedges  232  and  234  are translated by a suitable mechanical arrangement (not shown). For example, the wedges may be translated by solenoids (not shown) in response to a control signal from the laser controller  270  which is driven by the computer  50 . By sliding the wedges  232  and  234  relative to each other, the thickness of the switch  230  may be adjusted. By keeping the wedge separation distance ‘d’ constant when sliding the wedges, the resonator alignment axis of the beam  211  remains unchanged. This may be important for resonators using curved mirror surfaces sensitive to resonator axis translations. The arrangement of the present invention allows for maintaining precise alignment of the resonator axis while varying the path length, and hence hold-off, through the passive Q-switch material. 
     FIGS. 3 b  and  3   c  depict side and top views, respectively, of a wedge of the passive Q switch of the present invention. FIGS. 3 b  and  3   c  show dimensions of the passive Q switch  230  of the illustrative embodiment. Note that the wedge fabrication specifically calls out crystal orientations in order to provide optimum Q switch performance. The laser polarization should be parallel to the [010] axis for best performance. The wedges should be polished and anti-reflection coated in order to minimize the Fresnel losses from the four surfaces. 
     Returning to FIG. 2, those skilled in the art will recognize the laser  200  as extending from the high reflector  202  to the output coupler  240 . In the figures, the short line segment with double arrowheads  241  indicates horizontal polarization and the circle  243  indicates vertical polarization of the beam. The horizontally polarized output from the laser is rotated to the vertical by a half waveplate  242  (λ/2) so that the KTA OPO output at 2.59 μm is vertically polarized for processing by the optical parameter oscillator  250  as discussed more fully below. (This allows high reflectivity of the 2.59 μm wave and wavelength separation of the p-polarized 3.47 μm and 3.76 μm waves at the dichroic beamsplitter (DBS) 260 below.) 
     Third and fourth turning mirrors  244  and  246  direct the vertically polarized beam output by the laser to the first optical parametric oscillator (OPO)  250 . As described more fully in the above-identified parent application U.S. patent application Ser. No. 09/478,229 now U.S. Pat. No. 6,344,920 entitled MONOLITHIC SERIAL OPTICAL PARAMETRIC OSCILLATOR filed Jan. 6, 2000, by J. M. Fukumoto, the OPO  250  is comprised of an x-cut crystal  251  of potassium titanyl arsenate (KTA), or other suitable material, as a non-linear medium, together with a rear high reflector  248  and output coupler  256 . In a standard configuration, the crystal  251  is placed between the high reflector  248  and the output coupler  256 . The first OPO  250  can also be operated as an intracavity element to the Nd:YAG laser (not shown) with suitable mirror coatings for the laser and OPO. The OPO serves to shift the frequency of the beam output by the laser  200  from 1.064 microns to 2.59 microns in the illustrative embodiment. 
     An OPO pump retro-reflector  258  is a high reflector mirror that reflects the unconverted 1.06 micron energy from the OPO  250  back to the OPO  250 , such that it has two passes through the crystal  250  for additional nonlinear gain, and passes energy at 2.59 microns. 
     A polarization and frequency selective dichroic beamsplitter (DBS)  260  transmits secondary emissions from the OPO  250  at 3.47 microns and 3.76 microns and reflects energy at 2.59 microns to a quarter-wave plate  262 . One of ordinary skill in the mirror manufacture art would be able to construct the DBS  260  without undue experimentation. The DBS should be highly reflective to s-polarized light at 2.59 μm at 45° incidence angle and highly transmissive to wavelengths longer than 3.1 μm for p-polarized light at 45° incidence angle. 
     The 2.59 μm wave is passed through a λ/4 plate  269  and an RTA electro-optic switch  264  in order to maintain the vertical polarization or to rotate it by 90° so that the 2.59 μm wave can be steered to either of two cadmium selenide (CdSe) optical parametric oscillators as discussed more fully below. This polarization switching can be done at near megahertz repetition rates depending on the repetition rate of the laser. Employing a fixed λ/4 plate before the switch  264  allows the switch  264  to operate at alternating + and − voltages so that the average voltage on the switch is zero. In addition, lower λ/4 voltages can be used to avoid breakdown. The switch  264  rotates the plane of polarization in response to a voltage applied by a driver circuit  268  of FIG. 1 under command of a laser controller  270 . 
     A thin film linear polarizer  266  is included to transmit horizontally polarized light and reflect vertically polarized light at 2.59 μm. This is effective to create the reference and probe beams  34  and  36 , respectively, as discussed more fully below. The thin film polarizer  266  is fabricated in such a manner as to highly reflect s-polarized 2.59 μm light and highly transmit p-polarized 2.59 μm light. Using the switch  264  and the polarizer  266 , the single wavelength converted beam from the laser  200  is used to create the reference beam  34  and the probe beam  36  and to rapidly switch therebetween. 
     The reference beam is generated by a second OPO assembly  271 , while the probe beam  36  is generated by a third OPO  273 . In combination with the first stage OPO  250 , the second stage OPOs ( 271  and  273 ) provide tunable output in the 8-12 micron range. The operation of the first and second stage OPOs are best described with reference to the drawing of FIG.  4 . 
     FIG. 4 is a simplified diagram illustrative of the operation of the first and second stage OPOs utilized in the transmitter of the present invention. At the outset, it should be noted that FIG. 4 is illustrative of the operation of the first and second stage OPOs with the exception that the desired primary beam at 8-12 microns is shown exiting the rear of the crystal  274 ′. As discussed more fully below, this arrangement is useful to provide angle tuning without beam displacement with a single crystal. Accordingly, the reflectors are numbered  275  and  277  in FIG. 4 to illustrate that the figure depicts an alternative single crystal arrangement for the first and the second stage OPOs. Hence, the function of the reflectors  275  and  277  in FIG. 4 is implemented by the reflectors  272 / 280  and  290 / 296  of FIG.  2 . 
     In any event, as shown in FIG. 4, the first stage OPO  250  (using x-cut KTA in the illustrative embodiment) receives the pump beam (at 1.064 micron in the illustrative embodiment) from the laser  200  and outputs a beam (at 2.59 microns) as discussed above. This beam (at 2.59 microns) serves to pump the crystal  274 ′ of the second stage OPO  271 ′ such that it emits a primary beam along with a secondary emission. In the illustrative embodiment, the crystals  273  and  291  are of cadmium selenide construction which outputs a primary beam at 8-12 microns with a secondary emission in the range 3.3 to 3.8 microns in response to a pump beam at 2.59 microns. KTA OPO mirror reflectivities must be maintained to tight specifications to generate the desired beams efficiently as will be appreciated by one of ordinary skill in the art. 
     Those skilled in the art will appreciate that in FIG. 4, the forward emission of the beam in the range of 3.3 to 3.8 microns and the backward emission of the 8-12 micron beam is a result of the coatings on the reflectors  275  and  277 . One skilled in the art would appreciate that these mirrors could be coated to output the beam desired for a given application (e.g. the 8-12 micron beam) in an optimal direction for a given application and layout without departing from the scope of the present teachings. However, the direction of the beam will affect its displacement as the crystal is tuned as discussed below. 
     That is, a tilting of the crystal, as depicted in FIG. 2, results in a corresponding change in the wavelength of the output beam. Hence, the OPO may be tuned continuously by tilting the crystal. However, the tilting of the crystal will also result in a displacement of the beam output in the forward direction, i.e. the 3.3-308 micron beam in FIG.  2 . However, the primary beam output in the reverse direction (i.e., the beam at 8-12 microns) will not be displaced because this beam is reflected by the second mirror  277  and therefore retraces its path through the crystal. Hence, the effect a displacement in one direction is countered by a corresponding displacement in the opposite direction as the beam retraces its path. 
     When it is desired to output a spatially stable beam in a forward direction, each crystal  273  and  291  may be segmented into two smaller identical crystals  274 / 276  and  292 / 294  respectively. The crystals  274 ,  276 ,  292  and  294  are pivotally mounted. The two crystals in each set  274 / 276  and  292 / 294  are tilted in opposite directions as shown in FIG.  2 . This novel arrangement provides angle tuning without beam displacement. Actuators  278  (shown) and  293  (not shown) provide angle tuning in FIG. 2 in response to the laser controller  270 . 
     Note that in FIG. 2, the first crystal set  273  is seen from a side view while the second crystal set  291  is seen from a top view. This orientation is necessary because, in the illustrative embodiment, the reference beam  34  is vertically polarized and the probe beam is horizontally polarized and the optical parametric oscillators are polarization selective. The two second stage OPO outputs are made precisely co-linear by two final tuning mirrors  282  and  284  and a beam combining prism (BCP)  286 . 
     In accordance with the present teachings, the reference beam is selected to be ‘out-of-band’ with respect to a chemical to be sensed in the atmosphere while the probe beam is ‘in-band’. That is, since the spectra of major chemical agents is known in the 8-11 rim region, probing for a specific agent first requires in-band and out-of-band wavelength setting of the second stage CdSe OPOs. This wavelength setting can be accomplished by a relatively slow and small electro-mechanical motor, or can be manually set to predetermined angular positions. 
     Once the second stage OPOs are set to their respective wavelengths (λ1, λ2), the laser  200  and RTA switch  264  can be fired to produce rapidly alternating, λ1, λ2, output wavelengths. A major advantage of this approach is that rapidly alternating wavelengths can be produced without the necessity of rapidly rotating crystals, as would be required for angle tuning using one second stage OPO. In addition, vibration isolation of a rapidly oscillating crystal(s) would be a significant concern in a platform that required interferometric stability, such as that of the transmitter laser. Finally, near megahertz switching rates, if required, would be exceeding difficult or impossible to implement mechanically, due to the mass, angular position accuracy, and angular velocity required to angle tune the CeSe crystals at these rates. 
     Returning to FIG. 1, the beams output by the transmitter  20  are directed by an optical arrangement  21  including a first mirror  22 , a sampling beamsplitter  24 , a second mirror  26 , a third mirror  28 , a convex mirror  29  and a concave mirror  32 . The convex mirror  29  and concave mirror  32  comprise an off-axis parabolic expansion telescope  32   a  for the output beam. The optical arrangement  21  outputs the probe beam  34  and a reference beam  36  through an aperture  37  in a single element, off-axis paraboloid  38 . Reflections of the probe and reference beams return to the system  10  and are received and focused by the paraboloid  38  onto a detector  40 . 
     As mentioned above, the probe beam  36  is in the absorption band of chemical contaminants while the reference beam is out of band. A difference in the return signals for the two beams will indicate whether a chemical cloud is present in the path of the probe beam. That is, if a chemical contaminant cloud is present, it will selectively absorb energy from the in-band probe beam  34 , and will subsequently reduce the reflected probe beam energy sensed at the detector  40 . In contrast, the reference beam  36  which is not absorbed by the cloud, will be reflected back to the detector  40  without suffering significant attenuation. 
     In the illustrative embodiment, the detector  40  is a mercury cadmium telluride (HgCdTe) detector. Nonetheless, those skilled in the art will appreciate that the present invention is not limited to the detector technology employed. 
     FIG. 5 is a diagram showing the receiver in greater detail. As shown in FIG. 5, the receiver  30  consists of the single element, off-axis paraboloid  38  which is focused onto the cryogenically cooled detector  40 . The detector element  40  is a single 0.5 mm diameter HgCdTe element mounted in a rotary cooler. In the preferred embodiment, the microcooler is an integral Stirling engine with the detector directly mounted to the cold finger. 
     To eliminate the effects of parallax and pointing complexities with separate receiver and transmitter apertures, a hybrid coaxial design is utilized as shown in FIG. 5. A central obscuration of less than  0 . 5  percent in area is realized from this design and simplifies pointing the sensor unit. An afocal design using two off-axis parabolic sections forms the transmitter beam expander assembly  32   a.  In the preferred embodiment, the 0.5 mm diameter transmitter beam is expanded 13 times to reduce transmitter divergence to 3 mR. In the preferred embodiment, the receiver and transmitter mirror designs are fabricated from 6061-T6 aluminum alloy and coated with gold for high reflectivities at the 8-12 μm band. 
     Transmitted energy is measured by sampling a fraction of the laser output with a room temperature HgCdZnTe photodetector  25 . Sampling is accomplished by reflection from a beamsplitter surface  24  positioned upstream from the transmitter beam expander assembly  32   a.    
     Returning to FIG. 1, the temperature of the detector  40  is controlled by a conventional temperature controller  42 . Detector signals, power and command signals are routed to the sensor head through a umbilical from an instrument rack. The detector  40  outputs a number of electrical signals which are amplified by a preamplifier  44  and digitized by an analog-to-digital (A/D) converter  46 . In the illustrative implementation, the A/D conversion is implemented in a computer  50  having memory  48 . Those skilled in the art will appreciate that the present teachings are not limited to the signal processing system shown in FIG.  1 . Data acquisition is accomplished by conventional concentration-pathlength (CL) measurements of clouds using returns off of topographic targets. Intensity comparison measurements of the transmitted beam and the received energy are calculated for each laser firing. 
     Any analog, digital, optical or hybrid circuit may be used to process the signals received by the system  10  without departing from the scope of the present teachings. The computer  50  outputs to a computer monitor  52  or a television monitor  54 . The computer may be programmed to process the return signals to extract range data with respect to a cloud of contaminants. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,