Abstract:
An agile optical sensor based on scanning optical interferometry is proposed. The preferred embodiment uses a retroreflective sensing design while another embodiment uses a transmissive sensing design. The basic invention uses wavelength tuning to enable an optical scanning beam and a wavelength dispersive element like a grating to act as a beam splitter and beam combiner to create the two beams required for interferometry. A compact and environmentally robust version of the sensor is an all-fiber in-line low noise delivery design using a fiber circulator, optical fiber, and fiber lens connected to a Grating-optic and reflective sensor chip.

Description:
SPECIFIC DATA RELATED TO THE INVENTION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/498,558 filed on Aug. 28, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Scanning optical interferometry is the field of invention. It is well known that optical interferometry can be used to detect very small changes in optical properties of a material (e.g., refractive index, material thickness). These changes can be man-made such as on a phase-encoded optical security card or environmentally induced such as by temperature changes in a jet engine.  
         [0003]     Earlier, for example, acousto-optic devices or Bragg cells have been used to form scanning interferometers such as in N. A. Riza, “Scanning heterodyne acousto-optical interferometers,” U.S. Pat. No. 5,694,216, Dec. 2, 1997; N. A. Riza, “In-Line Acousto-Optic Architectures for Holographic Interferometry and Sensing,”  OSA Topical Meeting on Holography Digest,  pp. 13-16, Boston, May, 1996; N. A. Riza, “Scanning heterodyne optical interferometers,”  Review of Scientific Instruments,  American Institute of Physics Journal, Vol. 67, pp. 2466-2476 7 Jul. 1996; and N. A. Riza and Muzamil A. Arain, “Angstrom-range optical path-length measurement with a high-speed scanning heterodyne optical interferometer,” Applied Optics, OT, Vo. 42, No. 13, pp. 2341-2345, 1 May 2003. These interferometers use the changing RF (radio frequency) of the Bragg cell drive to cause a one dimensional (1-D) scanning beam. The limitations of this design include the temperature dependence, bulky size, high drive power requirements of the Bragg cell, limiting this scanning interferometer&#39;s use for optical sensing in hostile remote settings. Moreover, these are not passive optical sensors, i.e., they require electrical power delivery at the sensor front end (in this case, RF power to the Bragg cell) for sensor operations. This power delivery means requiring extra remote cabling to the sensor, adding to the bulk and complexity of the sensor frontend that engages the sensing zone.  
         [0004]     Hence, the goal of this invention is to form a robust ultra-compact passive frontend interferometric optical sensor with remoting and optical beam scan capabilities so as to act as a remote time multiplexed sampling head.  
       SUMMARY OF THE INVENTION  
       [0005]     An agile optical sensor based on scanning optical interferometry in one embodiment uses a retroreflective sensing design while another embodiment uses a transmissive sensing design. The basic invention uses wavelength tuning to enable an optical scanning beam and a wavelength dispersive element like a grating to act as a beam splitter and beam combiner to create the two beams required for interferometry. A compact version of the sensor is an all-fiber delivery design using a fiber circulator, optical fiber, and fiber lens connected to a Grating-optic and reflective sensor chip. An all-fiber design is also possible using a transmissive sensor chip and two fiber segments with related Grating-optics and fiber lens optics. Freespace optic designs are also possible for this sensor using bulk-optics. Another embodiment of the sensor using two fibers in the remoting cable includes a two receive-channel interferometric optical sensor design for lower noise sensing with improved signal processing. The sensor chip can be any optically sensitive material that changes optical properties due to effects such as temperature, pressure, material composition, and electronic states. Applications for the proposed invention include industrial sensing, security systems, optical and material characterizations, biological sensing, ultrasonic sensing, RF/antenna field sensing. It is also possible to not use a sensor chip, but to directly engage the sensing zone (e.g., human tissue) via the freespace beam used for capturing the sensing signature while the other beam (not entering the sensing zone) is used as a reference beam. Another option can include differential sensing where both beams are present in the sensing zone (e.g., tissue).  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates one embodiment of a single remote fiber, all-passive frontend, optical scanning, reflective sensing mode interferometric sensor;  
         [0007]      FIG. 2  illustrates an embodiment of the internal design of the scan front-end of a z-scan interferometric sensor;  
         [0008]      FIG. 3  illustrates an embodiment of a starring-mode single remote fiber. passive front-end, optical interferometric sensor that allows simultaneous sensing of different spatial points in the reflective sensing zone;  
         [0009]      FIG. 4  illustrates an embodiment of a dual remote fiber, all-passive frontend, optical scanning, transmissive sensing mode interferometric sensor; and  
         [0010]      FIG. 5  illustrates an embodiment of a multi-fiber, all-passive frontend, optical scanning, reflective sensing mode interferometric sensor with dual-signal pair receive signals for low noise signal processing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     It is well known that changes of wavelength coupled with a wavelength dispersive optic can lead to one-dimensional (“1-D”) beam scans in freespace. This idea dates back to the 1970s, and has been explored to make optical scanners, optical radar, optical microscopy, optical printing, and optical memory system for holographic data recording. More recently, this wavelength tuning along with wavelength selection has been proposed for wide coverage optical laser scanners and optical data reading devices. In addition, wavelength tuning combined with traditional fiber-optics such as 2×2 couplers have been used to form interferometers. All these works are described in the following references: R. L. Forward, U.S. Pat. No. 3,612,659, Oct. 12, 1971; R. S. Hughes, et.al., U.S. Pat. No. 4,184,767, Jan. 22, 1980; K. G. Leib, U.S. Pat. No. 4,250,465, Feb. 10, 1981; K. G. Leib, U.S. Pat. No. 4,735,486, Apr. 5, 1988; T. Inagaki, U.S. Pat. No. 4,938,550, Jul. 3, 1990; B. Picard, U.S. Pat. No. 4,965,441, Oct. 23, 1990; G. Li, P. C. Sun, P. C. Lin, Y. Fainman, Optics Letters, Vol. 25, pp. 1505-1507, 2000; J. R. Andrews, U.S. Pat. No. 5,204,694, Apr. 20, 1993; N. A. Riza, “Photonically controlled ultrasonic probes,” U.S. Pat. No. 5,718,226, Feb. 17, 1998; N. A. Riza, “Photonically controlled ultrasonic arrays: Scenarios and systems,”  IEEE Ultrasonic Symposium , Vol. 2, pp. 1545-1550, November 1996; N. A. Riza, “Wavelength Switched Fiber-Optically Controlled Ultrasonic Intracavity Probes,”  IEEE LEOS Ann. Mtg . Digest, pp. 31-36, Boston, 1996; G. J. Tearney, R. H. Webb, and B. E. Bouma, “Spectrally encoded confocal microscopy,”  Optics Letters,  Vol. 23, No. 15, pp. 1152-1154, August 1998; G. J. Tearney, et.al., U.S. Pat. No. 6,134,003, Oct. 17, 2000; N. A. Riza and Y. Huang, “High speed optical scanner for multi-dimensional beam pointing and acquisition,”  IEEE-LEOS Annual Meeting,  San Francisco, Calif., pp. 184-185, November 1999; N. A. Riza and Z. Yaqoob, “High Speed Fiber-optic Probe for Dynamic Blood Analysis Measurements,”  EBIOS  2000 : EOS/SPIE European Biomedical Optics Week,  SPIE Proc. vol. 4613, Amsterdam, July 2000; N. A. Riza, “Multiplexed optical scanner technology (MOST),” IEEE LEOS Annual Meeting, paper ThP5, Pueto Rico, USA, Nov. 12, 2000; N. A. Riza and Z. Yaqoob, “Ultra-high speed scanner for data handling,” IEEE LEOS Annual Meeting, paper ThP2, Pueto Rico, USA, Nov. 12, 2000; Z. Yaqoob and N. A. Riza, “High-speed scanning probes for internal and external cavity biomedical optics,” OSA Biomedical Topical Meetings, pp. 381-383, Miami, Fla., USA, Apr. 7-10 2002; Z. Yaqoob and N. A. Riza, “Free-Space Wavelength-Multiplexed Optical Scanner Demonstration,” Applied Optics-IP, Vol. 41, Issue 26, Page 5568 (September 2002; Z. Yaqoob and N. A. Riza, “Low-loss wavelength-multiplexed optical scanner for broadband transmit-receive lasercom systems using volume Bragg gratings,” SPIE Conference on Free-Space Laser Communication and Active Laser Illumination III, SPIE Proc. Vol. 5160, No. 47, 6 Aug. 2003, San Diego, Calif. USA.  
         [0012]     It has been proposed that an interferometric optical sensor with a no-moving parts scanning arm can be formed using a traditional Michelson interferometer design with a 2×2 fiber-optic coupler component and physically separated fiber arms. In effect, one fiber arm contains a wavelength tuned freespace optical scanner based on a Grating optic and another completely separate fiber arm forms a reference arm with a mirror. Although this design forms an interferometric sensor, the design uses many components and separate fiber arms, making it less robust to noise such as from fiber stresses and strains and other component vibrations such as vibration of the Grating optic in the scanning arm. Moreover, the fiber-optics is not ultra-compact to form a single remote sensing head and so cannot be deployed where space is premium.  
         [0013]      FIG. 1  is a simplified representation of one design  10  according to the present invention of a noise tolerant, single remote fiber, all-passive frontend, optical scanning interferometric sensor. Light from a tunable laser (TL) 12  is (if required) electrically modulated in phase/frequency/amplitude via an electrical-to-optical modulator  14  in response to a modulation signal S n , where n is the nth wavelength transmit modulation. A single mode fiber  16  couples light between the elements of  FIG. 1 . One segment of fiber  16  couples light from modulator  14  to a 3-port fiber-optic circulator  18  that directs the light via another fiber  16  segment to the compact remote head optics  20 . Light from fiber  16  is collimated by a tiny fiber lens  22  and then incident at the required (e.g. Bragg) angle of a highly dispersive optic device  24 , such as a diffraction Grating, photonic crystal superprism, or any other in-line wavelength dispersive 1:2 beam splitting single optic. For example, optic device  24  can be a holographic grating such as a thin grating with a wide spectral response with high diffraction efficiency (e.g., 90%) for the first diffracted order. Specifically, the ultra-compact optic device  24  acts as a tiny beam splitter creating the un-deflected or stationary beam  26  and the +1 order or deflected scan beam  28 . The two beams  26 ,  28  are directed onto optical sensors  30 ,  32 , respectively, by a focusing lens  34  having a focal length F 1 . The ratio of optical power between the two beams  26 ,  28  depends on the diffractive optic device  24  and can be tailored to match requirements of sensors  30 ,  32 . Similarly, the polarization properties of the device  24  can be designed to match sensor needs. For instance, the Dickson grating is well known for its low (&lt;0.2 dB) polarization dependence and hence works well with regular single mode fibers. The device  24  must also simultaneously act as a wavelength dispersive element so a wavelength encoded scan beam can be generated. Hence the device  24  is a beam splitter/beam combiner plus a dispersive prism effect component. It turns out that a grating such as the holographic phase grating makes an excellent dispersive optical device  24 , and is preferred in this application.  
         [0014]     When the laser wavelength is changed or tuned, the scan beam  28  moves along in one-dimension on the sensor chip  32  while the fixed reference beam  26  stays fixed on the reference position of sensor chip  30 . The sensor chips  30 ,  32  are designed to be reflective in nature, so light reflected from both the stationary beam  26  and the scan beam  28  trace back their paths to enter the fiber  16  again. Hence, now two optical beams as required for interferometric sensing travel back the fiber path  16  and exit the circulator  18  to be detected by a photodetector  36 . Based on the relative phase and amplitude of the two received beams, photodetector  36  will produce a sensing signal corresponding to the sensing parameters present at the remote sensor chip. Note that the lens  34  with focal length F 1  acts to create a one-dimension point scan region on the sensor chip  32 . Note that because an in-line, self-aligning design is formed after the fiber  16  tip in the remote head  20 , all of the light suffers similar noise effects until it reaches the sensor chips  30 ,  32 . In addition, both beams  26 ,  28  share the same fiber cable  16  and hence the same stresses and strains. Hence, both beams carry correlated noise that later cancels out on interferometric detection, providing a low noise compact remote head design. Intelligent RF modulation of the laser  12  can be deployed to add enhanced signal processing features to the sensor head  20 . Note that all the remote head optics can be extremely small in size (e.g., 1 mm diameter), hence making an ultra-compact sensor head  20 .  
         [0015]     There are numerous options for the sensor chips  30 ,  32  that is reflective in nature. Sensor chip  32  can be a reflection layer coated silicon carbide (SiC) sensor chip whose refractive index varies with temperature change. The fixed beam  26  can strike a fixed reflectivity mirror surface on chip  30 , while the scan beam  28  can strike physically separate reflection channels with temperature sensitive filled materials on chip  32 . For a given nth laser wavelength, a given nth sensor chip reflection channel can be accessed. Thus, the fixed beam  26  provides a fixed optical phase and amplitude reference while the scan beam  28  spatially samples the changing (e.g., temperature) scenario of the sensed zone. Since tunable lasers can tune at nanosecond speeds, very fast interferometric spatial sampling along a one-dimensional spatial direction can be implemented with the sensor system of  FIG. 1 . Temporal effects in the sensing zone of head  20  can be captured (such as Doppler flow information) using this sensor system.  
         [0016]     The principles incorporated in the system of  FIG. 1  can also be applied to sensing parameters other than temperature, such as, for example, pressure or material composition. In effect, the proposed interferometric scanning sensor  10  can be applied across any sensing zone or sensor chip mechanism as there are always two beams available—one that can act as the sensing beam and the other that can act as a given amplitude and phase reference beam. Thus, the design of  FIG. 1  provides an ultra-compact fiber-remoted interferometric sensor.  
         [0017]     An application where the sensor head  20  can have a fixed setup is an optical security card code chip that is inserted into the scan zone of the sensor beam  28  to be read. In this or other applications, the roles of the scan and fixed beams can be reversed. For example, the fixed beam can interrogate a sensing point/zone while the scanned beam can access different reference sites to implement a comparative sensing operation. In this approach, the same fixed point is exposed to all the laser wavelengths, one wavelength at a time by tuning the source  12 , allowing broadband sensing data to be generated. In another form, one of the two beams at the sensing head  20  can also be temporally modulated such as via a vibrating piston-type moving mirror (not shown) to induce a phase modulation frequency or via a shutter-type spatial light modulator (SLM), (not shown) that acts as a phase or amplitude modulator. Hence, by introducing modulation into one of the beams, heterodyne detection at the desired intermediate modulation frequency can be achieved, providing low 1/frequency noise sensor detection.  
         [0018]     Polarization effects that may be caused by polarization dependent diffraction effects of the optical device  24 , such as a holographic grating, can be reduced by positioning a 45 degree power Faraday rotator between the lens  34  and the reflective sensors  30 ,  32  to reduce polarization dependent effects in the overall sensor.  
         [0019]     While the sensor head  20  uses a device  24  that is shown as a single transmissive grating such as a holographic grating, any other type of grating such as a reflection Blazed grating made using diffractive optics technology can be used for the device  24  with appropriate alignment of the sensor beams. The device  24  design sets the diffraction efficiency and relative angles between the fixed and diffracted/deflected beams  26 ,  28 . Although  FIG. 1  discloses a system to scan the diffracted beam in one dimension, it is also possible to scan the beam  28  in three dimensions. For instance, the device  24  can be a holographic device with multiple wavelength-coded gratings stored as holograms in different x-y planes in the holographic device. By tuning the laser light source  12 , each Bragg wavelength matches to a given x-y plane grating and hence produces a given x-y diffracted beam deflection in two dimensions. One hologram with multiple tilted gratings or stacked plates each with tilted gratings can cause the wavelength tuned diffracted beam to steer in two dimensions. See, for example, U.S. Pat. No. 3,612,659 and article by Z. Yaqoob, M. Arain, N. A. Riza, “Wavelength Multiplexed Optical Scanner Using Photothermorefractive Glasses, Applied Optics, September 2003. Applying this two-dimensional (2-D) wavelength tuned scanning using multiple gratings to  FIG. 1  creates an interferometric optical sensor that can produce a 2-D scanning beam. The reference or stationary beam  26  is also produced and used with the 2-D optic device to produce a powerful 2-D scanning interferometric sensor using wavelength tuning in an ultra-compact fashion.  
         [0020]     In U.S. Pat. No. 4,965,441, it was suggested that wavelength coding of light coupled with a high chromatic dispersion lens can result in a beam with wavelength coded focal planes. In effect, wavelength tuning of light can cause beam scanning of light along the optic-axis or z-direction.  FIG. 2  shows a modification of the interferometric optical sensor head  20  of  FIG. 1  that can utilize the wavelength-coded depth scanning mechanism to realize a z-scan interferometric sensor head  40 . Sensor head  40  comprises a fiber lens  22 , a single optical separation device  42 , such as a Dickson grating, and two lenses  44  and  46 . Lens  44  is a high chromatic dispersion lens whose focal length changes with wavelength. Lens  46  is a classic achromatic lens design to have minimal focal length change with wavelength. The reference or undiffracted beam  48  from the optical device  42  passes through lens  44  and hence does not scan in a direction parallel to device  42  (indicated as the “x-direction”) when wavelength is changed. However, the beam  48  scans along a z-axis (optical axis)  50  as the wavelength is tuned producing focused points along the sensing z-axis of a sensing zone  52 . The diffracted and deflected beam  54  passes through lens  46  and generates an x-scanning beam on a reference mirror  56 . As the laser tunes, i.e., changes frequency, the path length on the reference mirror  56  stays fixed while the path length in the fixed x-y position but changing z-axis position changes as the beam scans in the z-direction  50 . This path length change in the z-direction allows sensing data collection for different z-planes of the sensing zone  52 . It is possible to temporally modulate the reference reflected beam  54  by phase-modulating the mirror via mirror piston motion at a desired modulation frequency. One can also use shutter-type amplitude modulation of the reflected reference beam  54  using a single pixel optical amplitude modulator, e.g., a liquid crystal modulator or a digital tilt-mirror modulator as the reference mirror. Hence, using modulation, one can implement heterodyne detection for the sensor head  40 .  
         [0021]      FIG. 3  illustrates an adaptation of the systems of  FIGS. 1 and 2  into an interferometric sensor that can simultaneously provide interferometric sensing data for many spatial sensing channels. The tunable laser light source of  FIG. 1  is replaced by a N-wavelength or broadband source  60 . Modulation and channel/wavelength selection is achieved by controlling the drive signal set s n  (n=1, 2, 3, . . . , N) to a tunable modulator device  62 , such as an acousto-optic tunable filter (AOTF). All light coupling is via optical fiber indicated at  64 . A circulator  66 , similar to circulator  18  of  FIG. 1 , allows transmittal light to be passed through to sensor head  68  and reflected light to be passed to photodectector/receiver  70 . Sensor head  68  may be either heads  20  or  40 . Receiver  70  is similar to head  68  and uses another optical grating  72  to separate the N sensed optical beam pairs and directs the scanning beams  74  to respective individual photodetectors within an N photo-detector array chip  76 . The non-diffracted light beam  78  strikes a single photodetector  80 , and is used to calibrate the sensor  76  for power. A lens  82  focuses the beams  74 ,  78  onto the respective sensors. A collimating lens  84  directs light from fiber  64  to device  72 .  
         [0022]      FIG. 4  illustrates an embodiment of the present invention adapted for a transmissive mode sensing device wherein the light passes through rather than being reflected from the device. The primary difference from  FIG. 1  is the use of a pair of optical fibers or cables, one for delivering light to the sensors and one for carrying light from the sensors to a detector, with each fiber having its own set of lenses and refractors. A tunable laser  90  provides light via fiber  92  to a modulator  94 , which modulator receives a transmit modulation signal from a conventional source (not shown). The modulated light is coupled from modulator  94  via fiber  92  to remote sensing head  96 . Note that the circulator is not used since the light beam return path is through another optical fiber.  
         [0023]     The sensor head  96  incorporates an optical receiving section  96 A and an optical transmitting section  96 B. Section  96 A is substantially identical to the optical section of sensor head  20  of  FIG. 1 , i.e., each includes a collimating lens  22 , a diffraction grating  24  and a focusing lens  34 . The transmitting section  96 B is essentially a mirror image of the receiving section but adds a light block  98  to absorb non-refracted light from transmitted beam  100 . The remaining corresponding optical components use reference numbers from section  96 A but with a B suffix. Sensor  96  is appropriate when transmissive sensing is desired in a sensing zone or with a predesigned sensor chip  102 . The two lenses  34 ,  34 B implement 1:1 imaging between the gratings  24 ,  24 B. As the wavelength is tuned, the diffracted beam from the first grating  24  scans the sensing region of chip  102 . The second grating  24 B un-scans this diffracted beam via a second diffraction process, making the scanned beam and fixed or reference beams in-line so they can be fed into the fixed receive fiber  92 B that sends light to the photodetector  36 .  
         [0024]      FIG. 5  shows an alternate embodiment of the invention using a multi-fiber optical scanning interferometric optical sensor system  104  with dual-channel per wavelength signal processing capabilities that can lead to low noise in-phase (I) and quadrature (Q) signal processing. Specifically, for each nth wavelength position (or scan beam position), the sensor system  104  generates the standard in-phase sensing signal “r” via the circulator  18  and detector  36 . In addition, sensor system  104  also generates an nth sensing signal r n  (n=1, 2, . . . N) for the nth wavelength that is quadrature with the standard sensed signal “r”. Thus, for each sensor scan position on chip  32 , a pair of output electrical signals (an “r” and an “r n ”) are generated that can be used for differential detection via an operational amplifier  106  for signal noise cancellation and improved signal-to-noise ratios for the sensor. The operation of the  FIG. 5  system requires the diffracting optical device  24  (e.g. grating) to operate in a spatially symmetric way. Imaging is implemented between the sensor head N+1 fiber array  108  and the sensing zone  110  where the sensor chip  112  may be placed. The focal lengths of lenses  114  and  116  can be chosen such that appropriate compact design is implemented. Light enters via the path of tunable laser  12 , modulator  14 , circulator  18 , fiber  16  to be collimated by lens  114  to strike the diffracting device  24  (e.g., grating optic), generating a fixed reference beam  118  and a diffracted/deflected scan beam  120 . On retroreflection from the sensing zone  110 , both reference and diffracted beams return to the device  24  where both beams undergo another diffraction. Hence, two reflected beam pairs exit the device  24 , one collinear beam pair goes back through the original input fiber  16  and hence is a stationary beam pair regardless of wavelength. This beam pair travels via the fiber  16  to the circulator  18  and is then directed to the photodetector  36  to generate the standard in-phase sensing signal “r”. The diffracting optical device  24  also generates another beam pair from the retroreflection double diffraction process. This beam pair is also collinear but moves along a one-dimension direction on the N-fiber array  108  depending on the laser wavelength. Hence, for the nth-wavelength setting, this particular collinear beam pair enters the nth-fiber in the N-fiber array, traveling via the fiber to the nth photodetector on an N-element photodetector array  120 . The nth photodetector in the array  120  generates the quadrature electrical signal r n  for the nth-wavelength setting. Thus, for any given wavelength, a pair of sensing receive signals “r” and “r n ” are generated that can be then fed to the differential amplifier  106  for low noise sensing signal generation. In effect, the  FIG. 5  system uses the device  24  optic (e.g., planar grating optic) as a 2×2 coupler. The system of  FIG. 5  can be enabled for two dimension and three dimension scanning by modification in accordance with the system of  FIG. 2 .