Abstract:
The present invention provides a photoacoustic spectrometer that is field portable and capable of speciating complex organic molecules in the gas phase. The spectrometer has a tunable light source that has the ability to resolve the fine structure of these molecules over a large wavelength range. The inventive light source includes an optical parametric oscillator (OPO) having combined fine and coarse tuning. By pumping the OPO with the output from a doped-fiber optical amplifier pumped by a diode seed laser, the inventive spectrometer is able to speciate mixtures having parts per billion of organic compounds, with a light source that has a high efficiency and small size, allowing for portability. In an alternative embodiment, the spectrometer is scanned by controlling the laser wavelength, thus resulting in an even more compact and efficient design.

Description:
[0001] This invention was made with Government support under contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to photoacoustic spectrometers and, in particular, to photoacoustic spectrometers having compact, mid-range infrared light sources.  
         BACKGROUND OF THE INVENTION  
         [0003]    The rapid identification of molecular species has many applications in the areas of science and technology. The determination and measurement of harmful pollutants in the environment also has gained increasing importance as government agencies require industries to meet pollution control standards based on the best available testing technologies. The development of inexpensive equipment that can provide a rapid measurement of chemical species in environmental samples can thus have a wide-ranging application.  
           [0004]    Various spectroscopic techniques monitor the interaction of laser light with a sample by measuring either transmitted or absorbed laser light as a function of wavelength. Many absorption techniques such as frequency modulation and wavelength modulation spectroscopy estimate species according to the derivative of the spectra. These techniques are best suited to detecting small molecules with well defined spectral features as they are not capable of discriminating the broad spectral features of large molecules. The difference between the spectra of a large molecule, such as toluene, and a small molecule, such as NO 2 , are illustrated in FIG. 1. In comparison with small molecules, the spectral features of large molecules generally include fine spectral features over a broad spectral range. It is difficult or impossible for many existing laser-based spectroscopic techniques to quantitatively speciate mixtures of such large molecules.  
           [0005]    Photoacoustic spectrometers, in contrast to most other techniques, analyze a sample according to heat absorption and the resulting pressure waves generated within the sample. Photoacoustic spectrometers are described, for example, in U.S. Pat. No. 3,948,345 to Rosencwaig, incorporated herein by reference. In photoacoustic spectroscopy, a tunable light source is passed through a sample contained in an enclosed cell. As the wavelength of the light source is varied, the sample absorbs light according to it absorption spectra. Absorbed light is converted into heat within the sample that is detectable as an increase in pressure of the contained sample. The photoacoustic spectrum of the sample is the variation of pressure oscillations in a sample with the wavelength of light from the light source. The ability to speciate mixtures of complex molecules requires a light source having an output that is both tunable over the absorption wavelength range of the molecules and narrow enough to capture fine spectroscopic features of the particular molecules. In addition, sufficient power must be available to produce measurable pressure oscillations or pulses in the sample and distinguish these pulses from background noise. Photoacoustic spectrometers are capable of measuring concentrations of complex molecular species at concentrations of parts per billion, and thus have great potential for the rapid speciation of complex toxic compounds in the air.  
           [0006]    Of concern for environmental measurements is the detection of volatile organic compounds (VOCs). The optimum wavelength ranges for detecting VOCs is generally 3-5 μm and 8-12 μm, where atmospheric transmission is good and where functional organic groups, such as the fundamental stretch mode of C—H, strongly absorb. At present there are several promising sources in the mid-range infrared range of 3-5 μm. The most promising sources in the 8-12 μm range are the CO 2  lasers and the quantum cascade diode lasers. The former, however, is only tunable over about 40 discrete lines in the 9 to 11 μm range. The latter are only tunable over about 10 cm −1  per device.  
           [0007]    Tunable light in the mid-range infrared can be generated with available light sources through the interaction of laser light with non-linear optical materials. Typically, the output wavelength is varied by changing some physical property of the non-linear material, such as its temperature or orientation. This technique for generating tunable light is particularly promising for environmental uses, since it has the potential to be robust and relatively maintenance-free. Higher output powers and stable output wavelength can be generated non-linear materials by incorporating them into an optical oscillator.  
           [0008]    A non-linear material that is particularly useful for spectroscopy and chemical sensing is periodically poled lithium niobate (LiNbO 3 ), or PPLN. U.S. Pat. No. 5,434,700 to Yoo, incorporated herein by reference, describes the operation of optical wavelength converters constructed of materials having non-linear optical properties. The non-linear properties of a PPLN crystal can be changed by changing the material temperature or by adjusting the orientation of light relative to the non-linear material structure, such as by rotating the material relative to the incident light path, or by having a material with varying structures and by moving the material so that different portions of these varying structures intercept the incident light.  
           [0009]    While strides have been made in the development of photoacoustic spectrometers, prior art systems have limitations that hinder their use for environmental applications. One of the major limitations is the inability of prior art systems to conduct real-time measurements of mixtures of complex organic compounds. To accomplish this, the light source must be narrow and finely tunable (either continuously, or in steps of a fraction of a wave number) over a broad range (hundreds of wavenumbers). In addition, it must be capable of being used at the place where the environmental measurement is to be made that is it must be portable so that is useful in the field.  
           [0010]    Prior art systems typically use lasers having an output in the several watt range to drive non-linear materials. For example, such systems have used neodymium-vanadate (Nd:Vanadate) pump lasers operating at about 1 μm and generating sufficient power to induce non-linear effects in non-linear materials, such as PPLN. Typically the non-linear material in located in an optical parametric oscillator (OPO) that is tuned to produce light of a wavelength different from the pump laser. While these systems produce usable IR light, there are many problems in adapting them for portable applications, such as real-time environmental measurements. Prior art systems typically have limited tuning capabilities and require large amounts of external power, making it difficult to include them in portable photoacoustic spectrometers.  
           [0011]    What is needed is an improved photoacoustic spectrometer which has a laser system that operates at high efficiency and generates light with a beam profile that efficiently couples into an OPO, which is be capable of speciating gaseous mixtures of complex organic molecules, and which is robust and portable.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention solves the above-identified problems with photoacoustic spectrometers by providing a compact and efficient solid state laser system to drive a PPLN crystal in an OPO.  
           [0013]    It is one aspect of the present invention to provide photoacoustic spectrometers that is portable and rugged for use in the field.  
           [0014]    It is another aspect of the present invention to provide a photoacoustic spectrometer that can speciate mixtures of volatile organic compounds.  
           [0015]    It is one aspect of the present invention to provide a photoacoustic spectrometer for analyzing a sample including a light source, a photoacoustic cell, and a controller, where the light source has a laser and an OPO for generating a beam of an adjustable wavelength light from the laser. The OPO has a light path and a material with non-linear optical properties within the light path, a first tuner to vary the adjustable wavelength by modifying said non-linear optical properties within the light path, and a second tuner to vary said adjustable wavelength by modifying the oscillating frequency of the OPO. The photoacoustic cell is adapted to contain the sample and has at least one window to accept the generated beam and irradiate a sample, and a pressure transducer adapted to provide an indication of the pressure of the sample; and a controller to scan said adjustable wavelength. In one embodiment, the non-linear material is a PPLN crystal.  
           [0016]    It is another aspect of the present invention to provide a photoacoustic spectrometer that has a light source that includes an Yb-fiber pumped OPO having a PPLN crystal, where the OPO is finely tuned by continuous or mode-hopped tuning of the OPO cavity and is coarsely tuned by moving a fan-shaped PPLN crystal in the optical cavity of the OPO.  
           [0017]    It is yet another aspect of the present invention to provide a photoacoustic spectrometer for analyzing a sample including a light source, a photoacoustic cell, and a controller, where the light source has a laser system including a laser and an optical-fiber amplifier adapted to amplify light from said laser, and an OPO having a non-linear optical material for generating a beam of an adjustable wavelength light from said amplified laser. The photoacoustic cell is adapted to contain the sample and has at least one window to accept the generated beam and irradiate a sample, and a pressure transducer adapted to provide an indication of the pressure of the sample; and a controller to scan said adjustable wavelength.  
           [0018]    It is an aspect of the present invention to provide a photoacoustic spectrometer that has a light source that includes a neodymium-yttrium aluminum garnet (Nd:YAG) laser, amplified by a Yb-fiber amplifier, to drive an OPO having a PPLN crystal, where the OPO is finely tuned by continuous or mode-hopped tuning of the OPO cavity and is coarsely tuned by moving a fan-shaped PPLN crystal in the OPO cavity.  
           [0019]    It is yet another aspect of the present invention to provide a photoacoustic spectrometer for analyzing a sample including a light source, a photoacoustic cell, and a controller, where the light source has a laser system with a laser having a wavelength adjustable output that is adjustable within the range from approximately 750 to approximately 900 nm. The amplifier output is provided to an OPO for generating a beam of an adjustable wavelength light from the amplified laser, which has a fixed light path and a fixed non-linear material. The spectrometer also includes a photoacoustic cell to contain a sample and has at least one window to accept said generated beam and irradiate a sample, and a pressure transducer adapted to provide an indication of the pressure of the sample. A controller is provided to control the wavelength of the pump laser.  
           [0020]    A further understanding of the invention can be had from the detailed discussion of the specific embodiment below. For purposes of clarity, this discussion refers to devices, methods, and concepts in terms of specific examples. However, the method of the present invention may be used to connect a wide variety of types of devices. It is therefore intended that the invention not be limited by the discussion of specific embodiments.  
           [0021]    Additional objects, advantages, aspects and features of the present invention will become apparent from the description of preferred embodiment set forth below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0022]    The foregoing aspects and the attendant advantages of the present invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0023]    [0023]FIG. 1 is a comparison of the absorption spectra for (a) toluene (a large molecule) and (b) NO 2  (a small molecule);  
         [0024]    [0024]FIG. 2 is a schematic of an embodiment of the photoacoustic spectrometer of the present invention;  
         [0025]    [0025]FIG. 3 is an optical layout of a preferred embodiment photoacoustic spectrometer of the present invention;  
         [0026]    FIGS.  4 A-C are optical layouts of OPO embodiments, where FIG. 4A is an optical layout of a preferred OPO embodiment having one coarse tuning mechanism that uses a non-linear material and two fine tuning mechanisms, one that uses an etalon and one that translates a mirror of the OPO cavity; FIG. 4B is an optical layout of another preferred OPO embodiment having one coarse tuning mechanism that uses a non-linear material and one fine tuning mechanisms that uses an etalon; and FIG. 4C is an optical layout of another embodiment having one course tuning mechanism that uses a non-linear material and one fine tuning mechanisms that translates a mirror of the OPO cavity;  
         [0027]    [0027]FIG. 5 is a perspective view of a periodically poled lithium niobate nonlinear material of the preferred embodiment;  
         [0028]    [0028]FIG. 6 is a schematic diagram of the doped-fiber amplifier of the preferred embodiment;  
         [0029]    [0029]FIGS. 7A and 7B are schematic diagrams of an air-spaced etalon of the preferred embodiment and a solid rotating etalon of the preferred embodiment, respectively;  
         [0030]    [0030]FIG. 8 is a graph showing the sensitivity of the preferred embodiment photoacoustic spectroscopy cell for ethane and pentane using an unamplified, 6 W, SLM, 1.06 μm Nd:Vanadate laser manufactured by Coherent Inc (5100 Patrick Henry Drive, Santa Clara, Calif. 95054)(the “Coherent light source”);  
         [0031]    [0031]FIG. 9 is a graph of the photoacoustic spectrum of the methane Q branch as obtained with the preferred embodiment OPO pumped with the Coherent light source and the theoretical spectrum;  
         [0032]    [0032]FIG. 10 is a graph showing the scanning characteristics of an air-spaced etalon as the output wavelength of beam as a function of the etalon displacement;  
         [0033]    [0033]FIG. 11 is an alternative embodiment laser system and OPO; and  
         [0034]    [0034]FIG. 12 is a side view of a PPLN crystal in an oven.  
         [0035]    Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    The present invention will now be described with reference to the Figures. The description that follows will first describe several embodiments of the photoacoustic spectrometer of the present invention, and is followed with detailed descriptions of the OPO and of the tuning of the OPO. A description of cell calibration and data acquisition are then presented, followed by alternative embodiments.  
         [0037]    [0037]FIG. 2 is a schematic of an embodiment of a photoacoustic spectrometer  200  of the present invention. Photoacoustic spectrometer  200  includes a tunable light source  210 , a photoacoustic cell  260  adapted for receiving a gas sample and accepting light from the light source, and a control and data acquisition system  270 . The gas sample is admitted into cell  260  from a source G that can be from the environment or from a sampling container, and can be admitted either continuously or as a fixed volume. Cell  260  has windows (not shown) that allow for the transmission of light beam D into the gas sample and a pressure transducer or microphone (not shown) to monitor variations in the pressure P of the sample.  
         [0038]    Light source  210  produces a beam D of light having a narrow spectral distribution about a tunable wavelength λ D , and provides the light to the sample within cell  260  with intensity I. Wavelength λ D  of beam D is adjustable and, preferably, is capable of being modulated so that the intensity I of beam D may be pulsed. In one embodiment, the inventive spectrometer provides for fine tuning of light source  210 , with steps of less than 0.1 cm −1  over a broad spectral range of from approximately 100 cm −1  to approximately 300 cm −1  or more.  
         [0039]    System  270  sends control signals S λ  and receives signals S data  from light source  210  and cell  260  to control light source  210 , and obtain data from the light source and cell  260 . Specifically, system  270  controls, through signal S λ , the wavelength λ D , and receives information regarding the intensity I and pressure P as signals S data . System  270  can include a computer having appropriate interfaces for sending and receiving signals as well as specialized data acquisition components, such as lock-in amplifiers, and controllers, such a stepper motor controllers for adjusting experimental parameters such as laser wavelength, power or control gas into out of cell  260 , and data analysis and display devices.  
         [0040]    Unlike prior art photoacoustic spectrometers, photoacoustic spectrometer  200  is small, efficient, and has a low power consumption rate. As such, spectrometer  200  can be provided in a self-contained package that is portable and rugged enough for field use. Spectrometer  200  thus also includes one or more batteries  210  to power the spectrometer, including but not limited to light source  210  and system  270 .  
         [0041]    The photoacoustic spectrum of a sample is determined by measuring pressure waves in a contained sample as a function of the wavelength of absorbing light as follows. Absorption of light of wavelength λ D  by the gas in cell  260  both locally and nearly instantaneously raises the temperature of the absorbing gas, and is quickly converted into a localized pressure increase. When beam D is pulsed, the absorption of light results is thus converted into localized pressure pulses in the gas. As the wavelength XD is varied, pressure pulses are generated that have an amplitude that varies with the absorption coefficient of the sample.  
         [0042]    In general, photoacoustic cell  260  has the following characteristics that result in a spectrometer that is well-suited for use in the field. The size of spectrometer  200  is decreased and the sensitivity in increased by amplifying the laser-induced pressure oscillations. Acoustic amplification of laser-induced pressure oscillations are provided by having a photoacoustic cell that is acoustically resonant at a modulation frequency of the laser, and that allows access of the laser to regions in the cell where the pressure oscillations are greatest. In some instances there is an interaction of the laser light with the photoacoustic cell windows through which it passes. Acoustic disturbances from this interaction are reduced by isolating the resonant chamber from the windows with a cell cavity enlargement near the windows. A large photoacoustic cell mass is also desirable to dampen external acoustic noise. Also, rapid analysis of samples is facilitated by having a photoacoustic cell volume that is small, permitting rapid exchange of the gas volume.  
         [0043]    Acoustic amplification of the pressure oscillations in photoacoustic cell  260  results from the interaction of beam D and the gas contained within cell  260 . The gas contained in cell  260  has numerous acoustic modes at which it can resonate. These acoustic modes are determined by shape standing acoustic waves in the volume of gas in cell  260 . For example, a cylindrical volume of finite length can support an infinite number of discrete modes combining pressure distributions in the shape of radially dependent Bessel&#39;s functions and longitudinal sine or cosine waves. The lowest radial and longitudinal frequency modes have periodic pressure waves whose amplitude varies monotonically from the center to the edge of the cylinder. The acoustic oscillation frequency of the individual modes is proportional to the speed of sound of the gas in cell  260 . The sound speed is a thermodynamic property of the gas that depends on the gas constituents, pressure and temperature.  
         [0044]    When the pulsing of beam D occurs at the frequency of an acoustic mode of the gas in cell  260 , and when the absorbed energy is deposited in time with the oscillations such that energy is locally deposited near pressure maximums, the energy of the absorbed light is then acoustically coupled to the resonating gas, amplifying the pressure waves. This amplification of pressure waves through this process is analogous to the timed pushing of a pendulum, where the pushes are timed to the oscillation of the pendulum and the energy input occurs when the potential energy is greatest.  
         [0045]    The proper timing of the position and pulsation frequency of beam D thus increases the pressure oscillations for a given amount of light absorption, effectively increasing the sensitivity of the spectrometer. Acoustic amplification by a factor, Q, of greater than 100 is possible, increasing the sensitivity of the photoacoustic spectrometer. Acoustic amplification is exploited by varying the intensity I between a high value and low value, preferably zero. Measurements of P and λ D  can then be used by processor  270 , or are transmitted to another system to determine the photoacoustic spectra, P(λ D ). It is known that the pressure P increases linearly with the intensity I, and thus the intensity I can be used to normalize by the pressure P to obtain intensity independent spectra, P′(λ D )=P(λ D )/I(λ D ).  
         [0046]    [0046]FIG. 3 shows a preferred embodiment of a photoacoustic spectrometer  300 . Photoacoustic spectrometer  300  includes a tunable light source  310  that is capable of generating a periodically modulated light beam D of adjustable wavelength to probe a sample of gas G in a photoacoustic cell  360 . Spectrometer  300  also includes a control and data acquisition system  370  that controls and/or monitors light source  310  and acquires a photoacoustic spectrum of sample G.  
         [0047]    Light source  310  has optical and mechanical elements that cooperatively adjust the wavelength λ D  of light beam D and provides light beam D to cell  360 . Specifically, light source  310  includes a laser system  320 , an OPO  330 , a modulator  311 , and a reflector  317 . Light source  310  also includes a beam splitter  313 , a lens  314 , and a light detector  315  that are used maintain the intensity of beam D.  
         [0048]    As shown in FIG. 6, laser system  320  includes a laser  321 , a Faraday isolator  323 , an optical-fiber amplifier  325 , and a fiber port  329 . In one embodiment, laser  321  is a cw diode seed laser, such as an Nd-based laser, having a narrow spectral output in the mid-IR of several to a few hundred milliwatts and a linewidth of less than about 100 MHz.  
         [0049]    Light from laser  321  is amplified by optical-fiber amplifier (OFA)  325  which includes a doped fiber  326 , and one or more pump lasers  327  that are each coupled to fiber  326  through coupling fibers  328 . Faraday isolator  323  is provided between laser  321  and optical-fiber amplifier  325  to isolate the laser from upstream reflections. OFA  325  is similar to doped fiber amplifiers that are known and used in the telecommunications industry, such as an erbium-doped fiber amplifier. It is preferred that fiber  326  is an Ytterbium (Yb) doped fiber, as this type of fiber amplifier is well-suited to amplifying light at 1.06 μm. Pump laser  327  supplies light at 980 nm and is mixed with the output of laser  321 , causing the incident light at 1.06 μm to be amplified within fiber  326 . The amplified laser output in fiber  326  passes from OFA  325  through fiber port  329  as beam A with sufficient power for use by OPO  330  to generate beam B. In a preferred embodiment, laser  321  is an Nd:YAG laser that is amplified by Yb-doped OFA  325  to a power of from 4 to 6 watts at λ A =1.06 μm. In a particularly preferred embodiment, laser  321  has a power of about 50 to about 100 milliwatts.  
         [0050]    OPO  330  includes one or more non-linear elements that accept light of one wavelength (beam A of wavelength λ A ) and can tunably generate light of two different wavelengths. In general, optical parametric oscillators operate more stably in a continuous mode and at a single wavelength, and thus it is preferred that laser  321  is a continuous wave (cw) laser that oscillates in a single-longitudinal-mode (SLM), and that OPO  330  be singly resonant.  
         [0051]    The selection of a laser  321  and an OFA  325  is governed by the necessity to efficiently generate a pump beam A that is both spectrally narrow and has sufficient power to induce non-linear light generation in OPO  330  at low cost. The specifications on wavelength and power are to be understood in conjunction with the operation of the OPO. Since the amount of power required to generate beam B will depend on wavelength λ A , different combinations of lasers and amplifiers are within the scope of the present invention. For example, optical-fiber amplifiers contain different dopants depending on wavelength λ A , and the amount of power required to drive the non-linear material of the OPO decreases with decreasing wavelength λ A . For example, 1.55 μm light is best amplified with an erbium-doped fiber amplifier, but such systems require higher powers to operate an OPO.  
         [0052]    The combination of laser  321  and OFA  325  thus has several features that make it advantageous for use in photoacoustic spectrometers and particularly advantageous for use in a field portable photoacoustic spectrometer. First, the amount of power required to operate preferred laser system  320  is much less than that required for operating an Nd:Vanadate laser having similar output characteristics. An Nd:Vanadate laser having 6 W of output power at 1.06 consumes only approximately 60 W of electrical power. The reduced power consumption allows for use of battery power for the various lasers, as well as the data acquisition and control system and ancillary electronics. Further, the preferred laser system is much less expensive than prior art systems. For example, currently the cost of a 6 W Nd:Vanadate laser is $70,000, while the combined Nd:YAG/Yb-doped fiber amplifier having 6 W of output power costs $20,000. Third, the inventive system is easily tunable. This allows for tuning the wavelength of the light source through tuning of laser  321 , or OPO  330 , or a combination thereof. Laser tuning allows use of more advanced techniques for acquiring photoacoustic spectra, such as by dithering the excitation frequency to provide differencing measurements.  
         [0053]    After exiting OPO  330 , beam B is periodically interrupted by modulator  311  to produce periodic beam D at a wavelength λ D  that is the same wavelength as beam B (λ D =λ B ). Modulator  311  also generates a data signal S ref  that provides a chopping frequency reference that is useful for data analysis. In one embodiment, modulator  311  includes a rotating disk that periodically allows beam B to pass through, thus generating a periodic beam D according to the rotation rate of the disk and the pattern of openings on the disk. Alternatively, modulator  311  could be a rotating prism or a solid state device, such as an acousto-optic modulator.  
         [0054]    The intensity of beam D is monitored by extracting a small portion of the beam with beam splitter  313 , though lens  314 , to light detector  315 . It is preferable that beam D is monitored by sampling a small portion of the beam, such as 1-5% of the incident beam. Lens  314  tightly focuses the sampled light onto the face of detector  315 , which responds to the temporal variation of the intensity I of beam D by generating a data signal S I . Infrared detectors, such as detector  315  are well known in the art. It is preferred that detector  315  has a flat spectral response over the spectral range of beam D and that there are no windows to cause etaloning of the sampled beam. A preferred brand of detector is a pyrometer manufactured by Molectron Detector, Inc. (7470 SW Bridgeport Road, Portland, Oreg. 97224) with a detector area of approximately 5 mm 2 .  
         [0055]    The portion of beam D that passes undeflected by beam splitter  313  continues onto cell  360  and is reflected back towards the cell by reflector  317 , resulting in a double-pass through the sample gas.  
         [0056]    Photoacoustic cell  360  accepts a sample G and has a pressure transducer  365  that produces a pressure-level proportional signal S P . In one embodiment, transducer  365  is a hearing aid microphone. The pressure levels generated in cell  360  when determining the photoacoustic spectra are typically acoustic waves at a frequency on the order of about 1 kHz. The measurement of acoustic pressures is well known in the art, and there are many pressure transducers that are capable of accurately measuring these pressures.  
         [0057]    Signal S P , which is indicative of the pressure of the sample in cell  360  depends on a number of factors: the overlap of the laser beam and those areas of the acoustic mode having large pressure oscillations, the intensity of the laser beam, the excitation or chopping frequency, the volume and acoustic amplification, Q, of the cell and the absorption properties of the gas. While a large Q would appear to be desirable, it was found that such a cell is also prone to picking up background noise and is sensitive to environmental factors, such as changes in temperature. After testing several cells, it was found that a cell with a Q of about 10 produced good photoacoustic sensitivity and low noise. As an example of such a cell is shown schematically in FIG. 3. Specifically, cell  360  has a cylindrical volume  361 , a pair of acoustic filters  367  at the cylinder ends, and a window  363  near each filter. Windows  363  are transparent to beam D, and can be manufactured, for example, from ZnSe tilted at Brewster&#39;s angle to reduce reflection losses and to avoid stray reflections which could raise the acoustic background level.  
         [0058]    One cylindrical volume  361  that was found to produce good results when illuminated by the Coherent light source has a length of 15 cm and a diameter of 9 mm, resulting in a lowest acoustic resonance frequency corresponding to the first longitudinal acoustic mode. This volume has an oscillation frequency, when filled with an atmospheric sample, of approximately 1,600 Hz. The small volume allows for quick gas exchange and thus quick data acquisition. Acoustic filters  367  are enlarged cavity volumes that acoustically dampen noise generated by absorption of the laser beam at the surface of the windows from reaching transducer  365 . Cell  360  has a lowest frequency mode with pressure waves that vary sinusoidal in time and that have a peak pressure along the cylinder centerline. Acoustic coupling of light absorption of a wavelength λ D  into a cylindrical sample can thus be accomplished by pulsing beam D along the cylinder centerline at a frequency corresponding to that acoustic mode.  
         [0059]    Windows  363  and volume  361  are aligned with beam D, including reflector  317 , to provide a double pass of beam D through cell  360 . Since only a small portion of beam D is absorbed by the sample in cell  360 , the amount of energy absorbed by the sample, and thus the pressure P, increases with the number of passes of light through the cell. However, it has been found that each pass through cell  360  also increases the noise in signal Sp due to scattering at the windows. For configurations with more than two passes, an off-axis beam geometry is required that makes is more difficult to aim the beam through the cell. The effects, coupled with beam profile changes that were observed with etalon mode hops, produced a noticeable modulation in the photoacoustic signal when more than two passes were used. Although filters  367  reduce the amount of noise, it is preferred that a two-pass configuration be used to increase the signal S P  without unduly increasing the complexity of the cell or increase noise in the system.  
         [0060]    The sensitivity of cell  360  as determined for ethane and pentane using the Coherent light source is shown in FIG. 8. The measurements shown in FIG. 8 were made with the sample gas diluted in pure nitrogen and at atmospheric pressure, and indicate extrapolated sensitivities of approximately 15 ppb for ethane, and approximately 22 ppb for pentane. Cell  360  is thus seen to have the sensitivity required to detect small quantities of organic compounds.  
         [0061]    System  370  preferably includes processor  373 , an amplifier  371  that is preferably a lock-in amplifier, and a display unit  375 . Processor  373  controls the adjustment of wavelength λ B . Amplifier  371  receives reference signal S ref , intensity signal S I , and pressure signal S P , and effectively amplifies those components of the intensity and pressure having a component occurring at the chopping frequency. In one embodiment, amplifier  371  includes two separate lock-in amplifiers, one amplifier which accepts reference signal S ref  and intensity signal S I , and the other amplifier accepts reference signal S ref  and pressure signal S P .  
         [0062]    Display unit  375  receives wavelength, pressure and intensity information that is used to generate a visual display of the photoacoustic spectra. Preferably, amplifier  371  provides a normalized pressure output to display unit  375 , such as the ratio of the pressure to intensity.  
         [0063]    Optical Parametric Oscillator  
         [0064]    FIGS.  4 A-C are optical layouts three preferred embodiments of OPO  330 , shown an OPO  330 ′, and OPO  330 ″, and an OPO  330 ′″, respectively. The embodiments of FIG. 4 differ according by their fine tuning mechanisms. FIG. 4A is an optical layout of a preferred embodiment of OPO  330 ′ having one coarse tuning mechanism that uses a non-linear material and two fine tuning mechanisms as described below, one that uses an etalon and one that translates a mirror of the OPO cavity. FIGS. 4B and 4C each have the same general optical layout as OPO  330 ′, but each has only one of the fine tuning mechanisms of OPO  330 ′. Specifically, FIG. 4B is an optical layout showing the fine tuning portion of OPO  330 ″ that uses an etalon; and FIG. 4C is an optical layout showing the fine tuning portion of OPO  330 ′″ that translates a mirror of the OPO cavity. The following discussion of FIG. 4A thus applies to the embodiments of FIGS. 4B and 4C with respect to their respective tuning mechanism.  
         [0065]    [0065]FIG. 4A shows a schematic of a preferred embodiment an OPO system  330 ′, which accepts beam A from laser system  320 , oscillates a beam C, and provides an output beam B. OPO  330  includes a pair of plano-concave mirrors  331  and  337 , a pair of planar mirrors  339  and  343 , a non-linear optical material  333 , an intra-cavity etalon  341 , a first beam splitter  347 , a diagnostic etalon  349 , a second beam splitter  351 , a beam dump  353 , and a lens  355 .  
         [0066]    Mirrors  331 ,  337 ,  339 , and  343  form an optical cavity, as shown by the path of beam C. Beams A and B pass out of the optical cavity through mirror  337 . Preferably, a small portion of beam B is sampled by beam splitter  347  to diagnostic etalon  349  to monitor the wavelength of beam A, and the remaining beam A is separated by beam splitter  351  into beam dump  353 , allowing beam B to exit OPO  330  after being collimated by lens  355 . OPO  330  also includes a coarse tuning mechanism and at least one fine tuning mechanism, described subsequently.  
         [0067]    As described subsequently, non-linear optical material  333  interacts with a beam A to generate a beam B and a beam C. Specifically, non-linear material  333  within the path of beam A generates two coaxial beams: a beam B having a wavelength λ A  and beam C. (These beams are shown schematically in FIG. 3 as being laterally displaced.) Beams A, B, and C are reflected and/or transmitted by planar mirrors  339  and  343  and mirrors  331  and  337 , along with concave surfaces  331   a  and  337   a  of respectively, as follows. Mirrors  331  and  337  have high transmissivities for the wavelength range of beam A, allowing beam A to substantially pass once through OPO  330 . Mirror  337  also has a high transmissivity for the wavelength range of beam B, allowing beam B to substantially exit OPO  330  after being generated by non-linear optical material  333 . Planar mirrors  339  and  343  and mirrors  331  and  337  are highly reflectivity at the wavelength range of beam C. The high reflectivity of mirrors  339 ,  343 ,  331 , and  337  and the curvature of concave surfaces  331   a  and  337   a  allow a substantial portion of beam C to recirculate through OPO  330 , in a “bow-tie” configuration, and in particular to make multiple passes through non-linear optical material  333 .  
         [0068]    The “bow-tie” configuration of OPO  330  provides better frequency stability, single mode operation and more space for intra-cavity tuning elements than other configurations. Specifically, the geometry of OPO  330  supports single mode or single frequency operation, without intra-cavity tuning elements. This is not the case with linear resonators, which suffer from random mode hopping and multi-mode operation.  
         [0069]    In one embodiment of OPO  330 , curved surfaces  331   a  and  337   a  have a radius of curvature of 10 cm with non-linear optical material  333  centered between mirrors  331  and  337  and mirrors  339  and  343 . An example of acceptable coatings for beam A wavelength of 1.064 μm, beam B wavelength of 3.3 μm, and beam C wavelength of 1.57 μm is as follows. Mirrors  331  and  337  are coated on both sides for high transmission (&gt;98%) of the beam A at 1.064 μm and for high reflectivity (&gt;99.5%) on the curved surfaces for beam C at 1.57 μm. The reflectivity of mirror  337  at the wavelength range of beam B (3.3 μm) is as low as possible (&lt;10% for curved surfaces  331   a  and  337   a  and &lt;0.1% for planar mirrors  339  and  343 ) to couple as much 3.3 μm light out of the cavity of OPO  330  as possible and to avoid feedback from beam C, since feedback of 10 −4  or greater per roundtrip can result in double resonance. OPO  330  thus supports resonating beam C and allows beams A and B to pass through mirror  337 .  
         [0070]    Since the spectra of beam B is a function of the spectra of beam A, it is preferable to operate laser  321  in a single-longitudinal-mode to achieve single frequency operation of OPO  330 . In general, a multi-mode laser  321  could be used if the idler wave (beam B) were resonated inside the OPO cavity instead of the signal wave (beam C). However, this is difficult due to mirror coating considerations. Beam A is focused to approximately 100 μm in intensity diameter inside the PPLN crystal. The oscillation threshold of OPO  330  operated as a cw, singly resonant OPO is approximately 3 watts and when pumped at 6.5 watts, the OPO depletes beam A by 85-90%.  
         [0071]    [0071]FIG. 5 shows a preferred non-linear material  333  as a periodically poled lithium niobate (PPLN) crystal  533  that converts beam A into beams B and C. Beam A drives the non-linear material  333 , and is usually called the “pump beam.” The two output beams have different photon energies (wavelengths). Beam B has the lower photon energy (longer wavelength), and is commonly called the “idler beam,” and beam C has the higher photon energy (shorter wavelength), and is commonly called the “signal beam.” The wavelengths of the signal and idler beams are adjustable according to the nonlinearities of the non-linear material and the resonant modes of the cavity, as well as the wavelength of the pump beam. The energy of the generated beams B and C equals the energy of the converted portion of beam A, and the sum of the frequency of beams B and C equals the frequency of beam A.  
         [0072]    PPLN crystal  533  is used to tunably convert light from beam A into beam B over a wavelength range that is useful for spectroscopic measurements of organic compounds. One embodiment PPLN crystal  553  is the fan-type crystal shown in FIG. 5, and described in U.S. Pat. No. 6,359,914 and incorporated herein by reference. The preferred embodiment PPLN crystal  533  has the following dimensions along the x, y, and z axis, respectfully: 50 mm long, 20 mm wide, and 0.5 mm thick. Crystal  533  has a 1° wedge between the input and output facets (the faces perpendicular to the x axis) to help eliminate idler feedback in OPO  330 . The faces of PPLN crystal  533  have anti-reflection coatings at both 1.064 μm and at 1.57 μm. PPLN crystal  533  has a theoretical tuning range of about 350 cm −1  at 180° C., and can convert pump beam A having a wavelength λ A  of 1.06 μm into a signal beam (beam C) having a wavelength λ C  that is adjustable from 1.53 to 1.62 μm and an idler beam (beam B) having a wavelength λ B  that is related to wavelength λ B  and is adjustable from 3.1 to 3.5 μm.  
         [0073]    The temperature of PPLN crystal  533  is controlled as shown in FIG. 12, which shows the PPLN crystal in an oven  1200  having an upper portion  1201 , an upper portion heater  1205 , a lower portion  1203 , and a lower portion heater  1207 . The two planes of PPLN crystal  533  bound by surfaces parallel to the x-y plane, shown in FIG. 5, are in thermal contact with portions  1201  and  1203 , respectively. Oven  1200  also includes a temperature sensor  1209  and a control system  1215 . Control system  1215  receives oven temperature information from sensor  1209  through an electric connection  1211  and provides power to heaters  1205  and  1207  through connections  1213 . It is preferred that portions  1201  and  1203  are highly thermally conductive materials, such as copper, and that heaters  1205  and  1207  are electric resistance heaters. Control system  1215  is instructed to maintain a prescribed temperature of crystal  533  and supplies power accordingly to heaters  1205  and  1207  to maintain this temperature. temperature. While it is preferred that control system  1215  is a stand-alone system with a non-changing prescribed temperature, control system  1215  is alternatively a controller programmed by  370 .  
         [0074]    The prescribed temperature must meet two requirements. First, the optical properties of PPLN crystal  533  are temperature dependent, with thermally-induced changes in the refractive index having a large impact on the wavelengths λ B  and λ C  To maintain control of the wavelengths of light generated by crystal  533  to the degree required for detailed spectroscopic analysis, the prescribed temperature should be maintained to within 0.01° C. Second, PPLN crystals are known to suffer from photorefractive damage. This damage is mitigated by heating the PPLN crystal  533  to a temperature high enough to allow the crystal to anneal. It is believed that a prescribed temperature of 180° C. is sufficient to anneal the crystal, though other temperatures may achieve the same effect. It is preferred that oven  1200  maintain the temperature of PPLN crystal  533  to 180.0±0.1° C.  
         [0075]    Three axes of crystal  533  are shown in FIG. 5 as x, y, and z. The optical properties of crystal  533  are constant in the z direction, and are periodic for a beam propagating perpendicular to the z axis. Specifically, crystal  533  has periodic properties that depend on the y position, with periods that vary from Λ=29.3 to Λ=30.1 μm at increasing values of y. Incident beam A is thus subject to periodically changing optical properties as it propagates through crystal  533  in the x direction It is important that the polarization of beam A, as indicated in FIG. 5, is aligned in the z-direction to undergo conversion of similarly polarized beams B and C in crystal  533 .  
         [0076]    PPLN crystal  553 , and in particular the period of the crystal, can be adjusted by moving the crystal along the y-axis and relative to pump beam A, producing non-linear interactions that generate two beams of different wavelengths that vary as a function of the position of the crystal along the y-axis. The use of a fan-type crystal for coarse tuning of OPO is described below.  
         [0077]    While the previous description refers to pumping OPO  330  from laser system  320 , the combined coarse and fine tuning capabilities of OPO  330  can produce tunable output using other pumping lasers or laser systems having sufficient output and at a proper wavelength to enable the OPO to generate beam B. Thus, for example, beam A of FIGS.  4 A- 4 C can be a beam from a different light source with 1 μ output having that is spectrally narrow and has an output in the watt range that is polarized as previously described with respect to the PPLN crystal.  
         [0078]    Tuning the Optical Parametric Oscillator  
         [0079]    Preferred OPO  330  combines coarse tuning and fine tuning to scan a large range of wavelength λ B  with high resolution. Coarse and fine tuning are individually and collectively controlled by processor  373  to scan wavelength λ B  through the combined commands of control signals controls S λ coarse  and S λ fine , respectively. One scanning technique is to repeatedly scan the fine tuning range while the coarse tuning range is increased stepwise at the beginning of each fine tuning range. The fine tuning can be either continuous or discrete depending on the technique used, as described below. Non-monotonic scanning can be corrected by sorting the spectra according to a measurement of wavelength λ B .  
         [0080]    In general, preferred coarse tuning for OPO  330  is accomplished through changes to the non-linear material  333  within the optical cavity in response to a control signal S λ coarse . Preferred fine tuning alters the optical cavity of OPO  330  through one or both of the following techniques. The first alters the optical cavity in response to a control signal S λ fine−1  by adjusting elements within the cavity (such as etalon  341 ), allowing the oscillations to jump from one mode to another. This results in discrete changes in the output wavelength during tuning and is termed “mode-hop” tuning. The second alters the optical cavity in response to a control signal S λ fine−2  by increasing or decreasing the cavity length through the movement of mirror  343 , allowing the oscillating frequency can adjust accordingly, and is termed “continuous” tuning. FIG. 4A shows OPO  330 ′ with coarse tuning and two fine tuning mechanisms-mode hop tuning using etalon  341  and continuous tuning through the translation of mirror  343 . FIG. 4B shows details of the fine tuning mechanism of OPO  330 ″ using only mode hop fine tuning by etalon  341 . FIG. 4C shows details of the fine tuning mechanism of OPO  330 ′″ using only continuous fine tuning by translation of mirror  343 . The coarse and fine tuning techniques are described subsequently.  
         [0081]    Coarse tuning through the movement of crystal  533  is achieved as follows. As noted above, crystal  553  is aligned for propagation of pump beam A along the x-axis, with periods varying along the y-axis from Λ=29.3 to Λ=30.1 μm. PPLN crystal  553 , and in particular the period of the crystal, can be adjusted by moving the crystal along the y-axis and relative to pump beam A, producing non-linear interactions that generate two beams of different wavelengths that vary as a function of the position of the crystal along the y-axis. Coarse tuning using the fan-shaped PPLN crystal  533  is accomplished by moving the crystal in the “y” direction shown in FIG. 5 by first translator  335  in response to control signal S λ coarse . Translator  335  can be a stepper motor or any other mechanism for repeatably and controllably translating crystal  533 . PPLN crystal  533  has a theoretical tuning range of about 350 cm −1  at 180° C., and can convert pump beam A having a wavelength λ A  of 1.06 μm into a signal beam (beam C) having a wavelength λ C  that is adjustable from 1.53 to 1.62 μm and an idler beam (beam B) having a wavelength λ B  that is related to wavelength λ B  and is adjustable from 3.1 to 3.5 μm. Translating crystal  533  approximately 0.04 mm moves the OPO gain peak approximately 4 cm −1 .  
         [0082]    In fine mode-hop tuning, an etalon  341  in the optical cavity alters the effective length of the optical of OPO  330  by adjusting the spacing of the etalon with a motor controlled by signal S λfine−1 . Although the etalon may be continuously varied, the optical cavity of the OPO prefers to oscillate at discrete frequencies, and changes in etalon  341  result in discrete changes in the tuned frequency of the OPO. For the embodiment of FIGS. 4A and 4B, etalon  341  provides fine-frequency steps on the order of 0.6 to 1.2 GHz. The longitudinal mode spacing of OPO  330  is on the order of approximately 570 MHz, and thus the frequency changes during mode hoping correspond to 1 to 2 cavity modes. Uncontrollable perturbations of the OPO can result in mode hopping, and thus it can be difficult to achieve control of the mode hops to within a mode or two.  
         [0083]    Since OPO  330  tends to oscillate in a single mode without intra-cavity elements, etalon  341  has to constrain only the oscillating mode, which allows the use of weakly frequency selective (or “low-finesse”), low-loss etalons. This is important since the OPO can only tolerate cavity losses on the order of 5% or less. Although fine tuning has been demonstrated in many laser systems, there are some subtle yet important differences in both OPO tuning and in the use of PPLN.  
         [0084]    Several types of etalons  341  can be used as in inter-cavity etalon with an OPO as shown in FIGS. 4A and 4B, for example, the etalon can be either an air-spaced etalon  341 ′, as shown in FIG. 7A, or a rotating solid etalon  341 ″, as shown in FIG. 7B. It is important that the reflectivity or spectral rejection of the etalon be quite low—on the order of a few percent or so, since there is a tradeoff between reflectivity and required pump power.  
         [0085]    Rotating solid etalon  341 ″ includes a solid etalon material  711  and rotation stage (not shown) that rotates material  711  through an angle γ in the plane of FIG. 7B in response to control signal S″ λ fine−1 , as indicated by arrow  713 . Rotation through an angle γ of a few degrees with etalon  341  ″ in the path of beam C tunes OPO  330 ′ or  330 ″ over a few wavenumbers. A preferred rotating solid etalon  341 ″ is a 400 μm thick, uncoated YAG substrate. Measurements using the Coherent light source with OPO  330  indicate that this etalon gives the best combination of mode hop step size, tuning range (several hundred wave numbers), and power (approximately 120 mW maximum in the idler), with a pump depletion typically in the range of 40-50% for 6 W of pump power. Although the rotation is nearly continuous, the frequency steps are discrete on the order of 0.02-0.1 cm −1 , depending on the number of cavity modes jumped. Various performance factors limit the solid etalon mode-hop tuning to the range of approximately 4 cm −1 .  
         [0086]    To illustrate the use of a mode-hop-tuned PPLN OPO in spectroscopic applications, FIG. 9 shows the photoacoustic spectrum of the methane Q branch as obtained with OPO  330  pumped with the Coherent light source, along with the theoretical spectrum. This spectrum was acquired at atmospheric pressure where pressure broadening is large. The scan of FIG. 9 was acquired by simultaneously tuning the PPLN crystal  533  combined with rotation of the solid etalon  341 ″. Approximately four etalon scans were necessary to cover the 10 cm −1  spanned by the methane Q branch, resulting in a broad and finely resolved spectrum.  
         [0087]    There are several drawbacks however, of using a rotating solid etalon. First, the scan rate depends nonlinearly (quadratically) on etalon angle which requires software to linearize the scan and furthermore, the intra-cavity loss also depends nonlinearly with angle.  
         [0088]    Air-spaced etalon  341 ′ overcomes some of the problems encountered with solid etalons by having a constant tuning rate and a constant insertion loss which reduces the possibility of etalon mode hops. Air-spaced etalon  341 ′ is shown in FIG. 7A includes of two wedged fused, UV-grade silica substrates,  701  and  703 . Each substrate has a pair of sides that approximately perpendicular to beam C: a pair  701   a  and  701   b , and  703   a  and  703   b , respectively. Each pair of sides forms an angle, α, of approximately 30′. Substrates  701  and  703  are oriented with adjacent thick and thin portions, spaced apart by a distance ε of approximately 0.5 to 1.5 mm. One side of each substrate  701  and  703  has a 1.5 μm AR coating, and the other side of each substrate has no coating, yielding a reflectivity of approximately 5% and reducing misalignment. Air-spaced etalon  341  was inserted into OPO  330  at an angle, β, approximately 0.5° off of normal incidence of beam A to avoid optical feedback. A piezoelectric element  705  responds to control signal S λ fine−1  to tune the distance between the substrates of the air-spaced etalon as indicated by arrow  707 . Piezoelectric element  705  is preferably an annular element adapted to tune the etalon spacing over approximately 3 μm, resulting in a tuning range on the order of 10-50 cm −1 , depending on the etalon mirror spacing.  
         [0089]    An example of a scan obtained with the air-spaced etalon is shown in FIG. 10, which shows, in arbitrary units, the output wavelength of beam D as a function of the etalon displacement,  6 , and displays a mode-hop scan over 20 cm −1  obtained with a scanning air-spaced etalon and synchronized with the tuning of crystal  533 . For this scan, the etalon displacement is scanned by approximately 0.1 μm at an average spacing of 1.5 mm, yielding frequency steps on the order of 0.1 cm −1 . Scan non-linearities result, in part, from differential tuning between the etalon transmission peaks and the PPLN gain peak, and also by nonlinearities inherent in the piezo, especially at higher driving voltages. Also, while an air-spaced etalon has the advantage over the solid etalon of a constant insertion loss, the oscillation threshold is somewhat higher (approximately 4 W when pumped with the 6 W Nd:YAG laser), with a corresponding reduced output power (approximately 80 mW of idler power).  
         [0090]    Since both the solid and air-spaced etalons used in OPO  330  are of low finesse, any secondary eltaoning or wavelength-dependent absorption or reflection can influence tuning. These effects include intra-cavity absorption by a gas, such as CO 2 , etaloning in crystal  333 , and mirror reflectivity at 3.3 μm. Thus for example, residual reflectivity of the cavity mirrors at 3.3 μm can cause OPO  330  to become doubly resonant, causing instabilities. Also, idler feedback as small as 10 −4  can affect stability. These effects can be eliminated through better multiband coatings on the flat cavity mirrors.  
         [0091]    For continuous tuning, the cavity length of OPO  330 ′ or OPO  330 ′″ is adjusted by moving mirror  343  with second translator  345  in response to control signal S λ fine−2 . A reliable method of translation on this scale is through the use of piezo-electric transducers  345 . The OPO cavity used a multiple stack piezo-electric transducer which was capable of translations on the order of 40 μm. The effective tuning is twice this since the optical cavity length changes by twice the translation amount. As the cavity length shortened, the cavity modes shift to shorter wavelengths. For OPO  330 ′, etalon  341  is then controlled by a lock-loop to track the peak of a cavity mode as the cavity is tuned. Tuning is accomplished by keeping etalon  341  locked to the cavity mode as the cavity length is tuned.  
         [0092]    There are many perturbations which can disrupt the tuning process, such as air currents inside the cavity caused by the PPLN oven since thermal changes in PPLN crystal  533  can change the effective optical length of the cavity. Some of the perturbations such as convection currents generated by the PPLN oven can be controlled by thermally isolating the oven. Other perturbations, like the rapid thermal fluctuations inside the PPLN crystal (caused in part by absorption of 3 μm light in the crystal) cannot be controlled. If the perturbations occur too rapidly, i.e., outside the bandwidth of the lock loop or if the perturbation was too large then the OPO may uncontrollably mode hop. To keep the insertion losses low the etalon was of relatively low-finesse making the cavity more susceptible to mode hops. The etalon also had to be of low mass so that the loop response frequency is high.  
         [0093]    The application of the continuous tuning methods described herein to tunable OPOs presents many challenges. In particular, although mirror  337  and the mirrors in etalon  341  are designed to transmit at 3.3 μm, there is enough feedback to cause a double resonance effect. Doubly resonant OPOs are in general very unstable. As the cavity length defined by the path of beam C in OPO  330  is tuned, the 1.5 μm light of beam C tunes continuously, whereas the 3.3 μm light of beam B tunes continuously in the opposite direction. To complicate matters, there are occasions when the 3.3 μm light is slightly resonant in the cavity, which raises the intra-cavity 3 μm power. This in turn raises the temperature of crystal  333  which effectively changes the optical length and causes the laser to tune uncontrollably. To mitigate this problem, alternative OPOs have optical components that are more effective in rejecting intra-cavity 3.3 μm light.  
         [0094]    Cell Calibration and Data Acquisition  
         [0095]    To obtain an interpretable photoacoustic spectrum, it is preferable that the pressure signal, S P , is normalized by intensity of the incident light, S I , by dividing the pressure signal by the incident light signal. As noted above, one embodiment includes two separate lock-in amplifiers  371 , one which accepts reference signal S ref  and intensity signal S I , and the other accepts reference signal S ref  and pressure signal S P . Since the pressure and intensity signals are modulated by a rate given by the reference signal, amplifier  371  can use S ref  to obtain an accurate indication of the pressure and intensity. The ratio of the amplified pressure and intensity signals provides an intensity normalized spectral signal. Intensity normalization compensates for intensity fluctuations, but other effects such as detector nonlinearity, detector window etaloning, detector homogeneity, and beam profile changes all can cause residual noise. It has been determined that lens  314  helps to reduce some of these sources of error.  
         [0096]    In general, the sampled gas will contain a mixture of gases having unknown concentration. Obtaining quantitative speciation of a spectrum requires that calibrated photoacoustic spectra be obtained for each species to be identified, preferably at more than one concentration. The following procedures were found to give acceptable results when using light from OPO  330  pumped with the Coherent light source. The cell responsively, R, is required to quantify the raw normalized pressure data. R has units, for example, of μVolts/(C*mW*α), where α is the absorption (1/ppm-m) and C is the concentration in ppm. If the cell is operated at a pressure other than at atmospheric pressure, it is preferable that absolute concentration units. Gases with known absorptions (α&#39;s) and concentrations are used to determine the cell responsivity. Under atmospheric conditions the calibration should be independent of the calibration gas since energy transfer from vibration/rotation to translation (heat) is nearly 100%.  
         [0097]    Calibration was obtained for several gases: methyl ethyl ketone, isopropyl acetate, n-butyl acetate and butane. Calibration constants varied from 103 (butane) to over 300 μV-m/mW. There were several reasons for the wide variations; some of the VOCs were slightly polar and therefore stuck to the surfaces of the gas bottle and photoacoustic cell, thus lowering the effective concentration, and second, the absorptions of some of the VOCs were not known accurately. For butane however, which is a nonpolar species, the calculated cell responsivity was from 150-200 μV-m/mW at high concentrations (&gt;50 ppm) but at low concentrations (5 ppm) was reduced to approximately 80. The source of this discrepancy has not yet been determined but we have found variations as large as 20% in the gas dilution system. An adequate approximate calibration constant of 200 μV-m/mW was used in measurements using the Coherent light source for a two pass configuration.  
         [0098]    Alternative Laser Embodiments  
         [0099]    An alternative embodiment laser system and OPO operating near  750  to near 900 nm is shown in FIG. 11. Specifically, FIG. 11 shows a laser source  1220  and an OPO  1230  that are alternatives to laser source  320  and OPO  330  of spectrometer  300 . Laser source  1220  includes a diode seed laser  1221 , a Faraday isolator  1223 , and a tapered waveguide amplifier  1225 . Laser source  1220  generates a beam A′ that is controllable about a wavelength in the range of from 750 to 900 nm according to control signal S λ . Lasers of this type include Ti:sapphire and diode lasers, and are generally tunable over a broad range, such as from 700 to 1000 mm, and can have a narrow band width of 1 MHz.  
         [0100]    OPO  1230  includes a pair of plano-concave mirrors  1231  and  1237 , a pair of planar mirrors  1239  and  1243 , a non-linear optical material  1233 , an etalon  1241  a beam splitter  1251 , a beam dump  1253 , and a lens  1255 . Non-linear optical material  1233 , which can be a PPLN crystal of constant poling frequency, is temperature controlled in a manner similar to crystal  533 , generates a signal beam B′ and an idler beam C′. OPO  1230  is singly resonant at the wavelength of signal beam B′. As the wavelength of beam A′ is varied, the OPO resonates at a fixed signal wavelength and wavelength of idler beam C′ varies according to changes in the pumping wavelength of beam A′. Etalon  1241  can be an air-spaced etalon, similar to etalon  341 ′ or a solid etalon, similar to etalon  341 ″, is used in OPO  1230  to hold the wavelength of signal beam B′ fixed, allowing the wavelength of idler beam C′ to follow the wavelength of pump beam A′. Mirrors  1231  and  1237  are coated on both sides for high transmission (&gt;98%) of the pump beam A′ and for high reflectivity (&gt;99.5%) on the curved surfaces at the wavelength of signal beam B′.  
         [0101]    These operating characteristics make laser  1221  are sufficient to provide sufficient range and controllability to speciate complex organic molecules. In addition, such lasers have greater efficiencies than longer wavelength lasers and can operate an OPO with less power, the total size and efficiency of a photoacoustic spectrometer system operating with a pump laser having a wavelength in the range from 750 to 900 m are reduced over those of a 1 μm laser. In addition, the laser is readily tunable, allowing for tuning of the OPO via changes in the pump wavelength. In such a system the OPO would be singly resonant at a fixed signal frequency and OPO output idler wavelength would follow changes in the pump wavelength. This would eliminate the need for intra-cavity tuning elements within the OPO.  
         [0102]    The invention has now been explained with regard to specific embodiments. Variations on these embodiments and other embodiments may be apparent to those of skill in the art. It is therefore intended that the invention not be limited by the discussion of specific embodiments. It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.