Abstract:
A probe of a Raman spectroscopy system has a wavelength and/or amplitude referencing system for determining a wavelength of the excitation signal. Preferably, this referencing system is near an output aperture, through which the excitation signal is transmitted to the sample. In this way, any birefringence or polarization dependent loss (PDL) that may be introduced by optical elements in the system can be compensated for since the wavelength reference system will detect the effect or impact of these elements.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Raman spectroscopy is similar to infrared (IR), including near infrared (NIR), spectroscopy but has several advantages. The Raman effect is highly sensitive to slight differences in chemical composition and crystallographic structure. These characteristics make it very useful for substance identification such as the investigation of illegal drugs as it enables distinguishing between legal and illicit compounds, even when the compounds have a similar elemental composition. In other applications, taggants, with known Raman signatures, are used as markers for goods.  
         [0002]     Raman spectroscopy has additional advantages. When using IR spectroscopy on aqueous samples, a large proportion of the vibrational spectrum can be masked by the intense water signal. In contrast, with Raman spectroscopy, aqueous samples can be more readily analyzed since the Raman signature from water is relatively weak. Also, because of the poor water signature, Raman spectroscopy is often useful when analyzing biological and inorganic systems, and in studies dealing with water pollution problems.  
         [0003]     Raman scattering may be regarded as an inelastic collision of an incident photon with a molecule. The photon may be scattered elastically, that is without any change in its wavelength, and this is known as Rayleigh scattering. Conversely the photon may be scattered inelastically resulting in the Raman effect.  
         [0004]     There are two types of Raman transitions. Upon collision with a molecule, a photon may lose some of its energy. This is known as Stokes radiation. Or, the photon may gain some energy—this is known as anti-Stokes radiation. This happens when the incident photon is scattered by a vibrationally excited molecule—there is gain in energy and the scattered photon has a higher frequency or shorter wavelength.  
         [0005]     When viewed with a spectrometer, both the Stokes and anti-Stokes radiation are composed of lines that correspond to molecular vibrations of the substance under investigation. Each compound has its own unique Raman spectrum, which can be used as a fingerprint for identification.  
         [0006]     The Raman process is nonlinear. When incident photons have a low intensity, only spontaneous Raman scattering will occur. As the intensity of the incident light wave is increased, an enhancement of the scattered Raman field can occur in which initially scattered Stokes photons can promote further scattering of additional incident photons. With this process, the Stokes field grows exponentially and is known as stimulated Raman scattering (SRS).  
         [0007]     One disadvantage associated with Raman spectroscopy, however, is fluorescence from the sample or impurities in the sample. In many cases, the fluorescence response can overwhelm the typically much weaker Raman signature. This can make detection of small peaks in the Raman signature difficult. Often, fluorescence can be mitigated by moving to a longer wavelength excitation. This can create other problems, however.  
         [0008]     One robust solution to the fluorescence response is using excitation signals at multiple wavelengths. Specifically, in the past others have suggested to use excitation signals that comprise two excitation wavelengths, generated by two different single frequency lasers. This is sometimes referred to as Shifted Excitation Raman Difference Spectroscopy (SERDS). Then, by looking at the spectrums generated by each of the wavelengths, the fluorescence signal can be identified since it changes very little with excitation wavelength, whereas the Raman signal changes as a direct function of the excitation wavelength. In the simplest example, the spectra at two excitation wavelengths are subtracted to remove the highly stationary fluorescence response. Recently, this solution has been further enhanced by using a continuously tunable semiconductor diode laser system. In these systems, the spectral response of the sample is monitored as the excitation signals wavelength is scanned over a scan range. By looking at how the spectral response changes with the tuning of the excitation signal and how it does not change, the Raman response can be separated from the fluorescence response of the sample.  
       SUMMARY OF THE INVENTION  
       [0009]     The use of a tunable laser excitation signal, however, creates other problems. Specifically, in older single frequency systems, or dual frequency systems, the wavelength of the excitation signal was static or drifted only by a small amount due to ambient temperature changes. In contrast, with the newer tunable laser systems, the instantaneous wavelength of the excitation signal must be compared to the instantaneous spectral response from the sample as the excitation source is tuned through the scan range. Moreover, it is often important to know the instantaneous power of excitation signal.  
         [0010]     Similar these problems have been confronted by providing some sort of wavelength and/or power monitoring in the tunable laser system. In tunable Raman systems, the accurate detection of the instantaneous wavelength and/or amplitude of the tunable excitation signal is made complex by the inherent, differing optical characteristics of optical elements at different wavelengths. One example is the changing birefringence as a function of wavelength in the optical elements. A further issue concerns polarization. In these newer tunable laser excitation source systems, semiconductor diode lasers are used to generate the excitation signal. This class of lasers, however, produces highly polarized light, generating light along only one axis of the device. This can give rise to polarization dependent loss (PDL) due to changes in polarization or how the polarization is changed at different wavelengths of the excitation signal as it is tuned across the scan range and in different ambient environments.  
         [0011]     The present invention is directed to a probe of a Raman spectroscopy system. It has a wavelength and/or amplitude reference system for determining a wavelength and/or amplitude of the excitation signal. Preferably, this wavelength reference system is near an output aperture, through which the excitation signal is transmitted to the sample. In this way, any birefringence or PDL that may be introduced by optical elements in the system can be compensated for since the wavelength reference system will detect the effect or impact of these elements. Moreover, in the preferred embodiment, both the wavelength and the amplitude of the excitation signal are detected.  
         [0012]     In general, according to one aspect, the invention features a probe subsystem for a Raman spectroscopy system. This probe subsystem comprises a wavelength reference system for determining a wavelength of an excitation signal and an output aperture through which the excitation signal is transmitted to the sample.  
         [0013]     In the preferred embodiment, the wavelength reference system comprises at least one reference detector. In the preferred embodiment, a first reference detector and a second reference detector are used. A wavelength reference element filters the excitation signal received by the wavelength reference detector or detectors. This wavelength reference renders the response at the reference detector dependent upon the wavelength of the excitation signal, allowing a controller, for example, to determine the instantaneous wavelength of the excitation signal.  
         [0014]     In specific embodiments, a partially reflective mirror is placed in a path of the excitation signal to tap a portion of the excitation signal received by the wavelength reference system. Excitation optical elements are also preferably provided for shaping a beam of the excitation signal after detection of a portion of the excitation signal by the wavelength reference system. In a preferred embodiment, separation optics is used for enabling transmission of the excitation beam to the sample and receipt of the spectroscopic response from the sample along a common axis through a common aperture. A separation system comprising a mirror with a hole or optical port of the excitation signal is preferably used.  
         [0015]     In the preferred embodiment, the probe subsystem receives the excitation signal from a semiconductor tunable laser subsystem. Specifically, the excitation signal is generated by one or more semiconductor diode lasers. The excitation signal is transmitted from these diode lasers to the probe subsystem through an optical fiber. Preferably, polarization-controlling system such as polarization-maintaining fiber is used to provide polarization control such that the polarization is stable in spite of any mechanical shock or other perturbations to the system. To further address polarization issues, one or more polarizers are preferably provided in the probe subsystem. Typically, these are free space optical elements that improve the polarization of the beam transmitted through the probe subsystem. Further, at least one amplified spontaneous emission filter is provided for attenuating amplified spontaneous emission in the excitation signal after receipt from the optical fiber.  
         [0016]     In general, according to another aspect, the invention features a Raman spectroscopy system. This system comprises a tunable laser excitation subsystem comprising at least one tunable semiconductor diode for generating an excitation signal. A probe subsystem is also provided comprising a wavelength reference for determining a wavelength of the excitation signal and an output aperture through which the excitation signal is transmitted to the sample. A spectrometer subsystem is provided for resolving a spectrum of light returning from the sample. A controller then determines a Raman spectral response of the sample in response to the spectrum of light resolved by the spectrometer subsystem and the wavelength of the excitation signal from the wavelength reference system.  
         [0017]     In general, according to another aspect, the invention features a spectroscopy method. This method comprises generating an excitation signal having a varying wavelength within a scan range. This excitation signal is transmitted to a probe. In the probe, the wavelength and/or amplitude of the excitation signal is determined, and then the excitation signal is transmitted from the probe to the sample. Light is detected from the sample. A Raman spectral response of the sample is determined in response to the detected light from the sample and the wavelength and amplitude of the excitation signal.  
         [0018]     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:  
         [0020]      FIG. 1  is a schematic view showing a Raman spectroscopy system according to the present invention comprising a semiconductor tunable laser subsystem, the inventive Raman probe subsystem, and a spectroscopy subsystem;  
         [0021]      FIG. 2  is a plot of normalized signal as a function of wavelength illustrating the operation of the wavelength reference system according to the present invention; and  
         [0022]      FIG. 3  is a plan, scale view of the probe subsystem according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]      FIG. 1  shows a Raman spectroscopy system  10 , which has been constructed according to the principles of the present invention.  
         [0024]     Specifically, it comprises a semiconductor tunable laser subsystem  50 . The tunable laser subsystem  50  comprises a semiconductor diode module  52 . In the illustrated example, this module  52  is a hermetic package such as a butterfly hermetic package. The diode laser module  52  holds a semiconductor gain element  56 . In the present embodiment, this gain element  56  is a semiconductor optical amplifier, and specifically, a reflective semiconductor optical amplifier. These semiconductor reflective optical amplifiers  56  comprise a reflective back facet  68  and an antireflection coated (AR) coated front facet  60 . They are useful in the construction of external cavity tunable semiconductor lasers.  
         [0025]     In the illustrated embodiment, the external cavity tunable laser configuration provided by a wavelength tunable element module  66 , which provides tunable narrow band feedback into the semiconductor gain element  56 . In the preferred embodiment, this is a Bragg grating tuning system. Specifically, it comprises a fiber Bragg grating  67  that is mechanically stretched by a stretcher system. Specifically, a first half of the stretcher  70  and a second half of the stretcher  72  are moved toward and away from each other in the direction of arrow  74 . In the current embodiment, a cam system  76  is used in order to mechanically stretch the fiber Bragg grating  67 .  
         [0026]     An optical fiber pigtail  78  transmits the excitation signal from the semiconductor tunable laser subsystem  50  to the probe subsystem  100 . In the preferred embodiment, the fiber pigtail  78  is polarization controlling fiber that controls the polarization of the light transmitted through it. Specifically, polarization controlling fiber is used between the gain element  56  and the grating  67  and between the grating and the probe subsystem  100 . In the preferred embodiment, it is polarization maintaining fiber, although other polarization controlling systems could be used such as polarization stripping systems or polarizing fiber.  
         [0027]     The light from the semiconductor chip  56  is coupled into a fiber pigtail  78  via a facet  62 . This fiber goes through the hermetic package  52  via a fiber feedthrough  64  in one example.  
         [0028]     The probe subsystem  100  comprises a first collimating lens  112 . This receives the excitation signal from the fiber pigtail  78  and forms a collimated beam from the typically diverging beam that exits the fiber  78 . The excitation signal  102  is preferably filtered to remove amplified spontaneous emission—a spectrally continuous emission from the semiconductor laser that is broadband in spectrum. In the preferred embodiment, two spectral notch or bandpass filters are provided as spontaneous emission filters  114  and  116 . These suppress the ASE emission by reflecting any light that is outside the scan range of the excitation signal  102 .  
         [0029]     The probe subsystem  100  also preferably provides for polarization filtering of the excitation signal  102 . In a preferred embodiment, two polarizers  118  and  120  are used. These filters  118 ,  120  ensure that the excitation signal  102  has substantially only a single polarization. The single polarization of the excitation signal is important because of polarization dependent loss (PDL) in the taps and other polarization changes due to ambient changes, for example, that lead to tracking errors of the wavelength and/or amplitude of the excitation signal  102  that can not be effectively addressed with calibration.  
         [0030]     A partially reflective excitation mirror  122  is provided in the path of the excitation signal  102 . This reflects a portion  102 ′ of the excitation signal to a wavelength/amplitude reference system  130 .  
         [0031]     The wavelength/amplitude reference system  130  in the preferred embodiment detects both the instantaneous wavelength of the excitation signal  102  along with its amplitude or power. In the current implementation, this is achieved by using a partially reflective reference mirror  132 . This reflects the excitation signal received by the wavelength reference system  130  through a wavelength reference element  134 . In the preferred embodiment, this is a fixed wavelength Fabry Perot Etalon. A slope filter could also be used. As such, the reference has an Airy transmission function to pass light at specific wavelengths and reflect wavelengths outside those ranges. The light transmitted through the wavelength reference element  134  is detected by a first photodetector pd 1 . Light reflected by the wavelength element  134  is transmitted back through the partially reflective reference mirror  132  to a second photodiode pd 2 .  
         [0032]      FIG. 2  is a plot showing the relationship between the transmitted etalon signal reference element  134 , the signal detected by photodetector pd 1 , see reference numeral  210 , the reflective Etalon signal, specifically the signal detected by the diode pd 2 , reference numeral  212  and the reflective divided by transmitted signal  214 , which is the division of these two responses formed by combining the response of pd 1  and pd 2 .  
         [0033]     Specifically, in the illustrated embodiment, the excitation signal  102  is scanned over a scan range of approximately 976 to 982 nanometers (nm) in one embodiment. In another embodiment, the range of 988 to 994 nm is used. The wavelength reference element etalon  134  has a free spectral range (FSR) of approximately 0.5 nanometers. As a result, the transmitted and reflective signals vary with a periodicity of approximately 0.5 nanometers over this range. From this information, both the amplitude of the excitation signal  102  and its wavelength can be determined according the following equations:  
         [0034]     Incident Light I 0  is split with ˜4% being sent to the Etalon and the rest discarded by partially reflective reference mirror  132 .  
         [0035]     Light Transmitted by the Etalon is sensed by Photodiode pd 1 , I PD1 .  
         [0036]     96% of Light reflected by the etalon is sensed by Photodiode pd 2 , I PD2 .  
         [0037]     The amplitude of I 0  can be written as:
 
 I   0   =K   1 ·( I   PD1   +K   2   ·I   PD2 )
 
         [0038]     Wavelength is:  
       λ   =     f   ⁡     (       I     PD   ⁢           ⁢   2         I     PD   ⁢           ⁢   1         )           
 
 wherein K 1  and K 2  are experimentally determined constants. 
 
         [0039]     Returning to  FIG. 1 , the excitation signal  102  that is not reflected by the partially reflective excitation mirror  122  passes through excitation optics. Specifically, the excitation optics comprise a focusing lens  124  and a diverging lens or concave lens  126 . This has the effect of focusing the excitation signal down to a small diameter and increasing its working distance. Specifically, in the illustrated embodiment, a separation mirror device  128  is used. Specifically, this is a mirror that is angled relative to the axis of the excitation signal  102  and a collection axis  142  that passes through an input/output aperture  144  to the sample  20 . The angled mirror  128  has a pinhole aperture  140  through which the excitation signal  102  passes to the output aperture  144 . This configuration has advantages in easing alignment between the excitation and collection paths.  
         [0040]     The sample  20 , responding to the excitation signal  102  produces a Raman response. This is collected by a high numerical aperture (NA) system. Specifically, two focusing lenses  146  and  148  are used to collect the light from the sample  20  while improving the working distance. They are transmitted to a third focusing lens  150  that collimates the light from the sample  20 . This light is then filtered by a first excitation filter  152  and a second excitation filter  154 . Each of these filters stops or blocks the wavelengths in the scan band of the excitation signal  102 . The two filters  152 ,  154  are used to fully suppress the response of the excitation signal  102  in the detected light. A fold mirror  156  is used to bend the light from the sample to a focusing lens  158  that couples the light into the spectroscopy subsystem  80 .  
         [0041]     The spectroscopy subsystem  80  detects and resolves the spectrum of the light returning from the sample  20 . In the preferred embodiment, it comprises a spectrometer that resolves the spectrum of the sample light. Typically, these spectroscopy systems use gratings to disperse the spectrum over a detector array. In other examples, tunable filters are used along with single detector elements.  
         [0042]     In the present embodiment, a combination of an etalon filter  82  and a grating  84  are used to produce a hybrid system. Specifically, preferred spectrometer configuration is as described in described in U.S. patent application Ser. No. 10/967,075, filed on Oct. 15, 2004 (US 2005-0264808 A1), which claims the benefit of U.S. Provisional Application Nos. 60/550,761, filed Mar. 5, 2004, and 60/512,146, filed Oct. 17, 2003, all of which are incorporated herein by this reference in their entirety. One or more lenses such as lenses  86  and  88  are used to collimate and relay the dispersed spectrum to the array detector  90 .  
         [0043]     The spectroscopy system controller  95  controls the power to the diode semiconductor chip  56  and the tuner  66  to thereby generate the tunable excitation signal  102  and scan this signal over the scan range, such as about 988 to 992 nanometers. Controller  95  further monitors the response of the first photodiode pd 1  and the second photodiode pd 2  in order to calculate both the wavelength and the amplitude of the excitation signal  102 . With this information, including the response of the spectroscopy subsystem and specifically its detector array  90 , the spectroscopy controller determines the Raman response of the sample  20 .  
         [0044]     In one example, the spectroscopy system controller  95  is implemented in electronics including possibly a field programmable gate array and a signal processor. This connects to a host computer  5 , such as a standard personal computer. The spectroscopy system controller  95  loads the Raman spectral information to the host  5 .  
         [0045]      FIG. 3  is a scale perspective view of the probe system  100 . Specifically, it illustrates the mechanical construction. Specifically, the probe  100  is manufactured in two parts. Specifically, a first substrate  302  is used to contain the optical elements associated with the excitation signal. Specifically, the collimating lens  112 , the ASE filters  114 ,  116  and the polarizers  118 ,  120  are connected in the excitation portion  302 . In the illustrated example, light traps such as  310  are provided to attenuate reflected light. The excitation substrate is connected to the signal collection substrate  304 . Specifically, this allows different excitation and collection optics to be used by simply detaching the excitation substrate  302  from the collection substrate  304 .  
         [0046]     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.