Abstract:
Optical delay line system that includes a retro-reflection mirror which is displaced along a circular path while being maintained in angular alignment with launch and return sources of light subject the components of the system to minimum levels of unbalanced linear acceleration. A retroreflector is pivotally mounted on a rotating element such that the optical axis of the retroreflector&#39;s motion is mobile such that its angle or position changes relative to a fixed observer. There is no linear stopping and starting of the retroreflector and all acceleration of the retroreflector is rotational acceleration with small angles so the required forces in the optical delay line are greatly reduced. Both large displacement and high repetition rates are achieved. The system can be configured so that optical fibers serve as launch and return optics. Alternatively, free space beam paths deliver light to the optical delay and return the reflected light from the retroreflector.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to optical delay line apparatuses that include a retro-reflection mirror, which is displaced along a circular path while being maintained in angular alignment with the launch and return sources of light such as optical fibers. With this arrangement, the component parts of the apparatus are subject to minimum levels of unbalanced linear acceleration. The apparatus achieves both large displacement and high repetition rates. 
     BACKGROUND OF THE INVENTION 
     Optical delay lines are an essential part of most time-resolved optical experiments, including time-domain terahertz technology, ultrafast optics research, time resolved detection, interferometric spectroscopy, optical coherence tomography, most optical pump/probe experiments, and other applications. Optical delay lines generally employ beam splitting optics to duplicate a pulse of light whereby one copy of the pulse is sent via a first optical path through one part of a system and the second copy is sent via a second optical path through a second part of the system that incorporates an optical delay arrangement such that the length of the second optical path can be changed in a controlled manner. A common optical delay technique reflects pulses of light off a moving retro-reflector mirror that is mounted on a motorized translation stage, such as a linear screw type translation stage, or on voice coils. Another technique is to simply stretch the optical fiber through which the pulses of light travel. 
     U.S. Pat. No. 5,220,463 to Edelstein et al. describes an optical delay line with opposite-facing hollow front surface retroreflectors that are offset to each other. A standard mechanical translating device that is connected to one of the retroreflectors adjusts the distance between the retroreflectors along a line of movement that is parallel to the reflected light beam as it enters and exits the retroreflectors. In one variation, a movable retroreflector is mounted on a linear slide that is constrained for movement in a straight line on a stage. A motor driven drive wheel links an eccentric pivot on the drive wheel with a pivot on the movable retroreflector. As the wheel rotates, the retroreflector moves back and forth in a generally sinusoidal fashion with respect to the stage so that the rotational motion of the wheel is translated into a linear motion. This optical delay line arrangement, which requires a relatively massive mirror to constantly stop and accelerate, is not suitable for applications that require both high amplitude and frequency. 
     One such application involves online measurements using terahertz (T-ray or THz) radiation, which lies on the boundary of electronics (millimeter waves) and photonics (infrared). The terahertz spectrum encompasses the wavelengths approximately in the range of 3 mm to 15 μm. Terahertz radiation exhibits a large range of modifications on passage through varying materials or on reflection from materials. Such changes include attenuation or partial attenuation of different frequencies of the waveform and other alteration of the waveform depending upon the material through which the radiation or pulses pass. Terahertz radiation interacts strongly with polar molecules, a prime example being water. Water molecules absorb terahertz waves, on the one hand limiting penetration of the radiation in moist substances, and on the other hand making it readily detectable even in very low concentrations. It can be used for detecting low concentrations of polar gases. However, terahertz radiation will penetrate non-polar substances such as fats, cardboard, cloth and plastics with little attenuation. Materials including organic materials have varying transmission, reflection and absorption characteristics to terahertz radiation. Accordingly, use of terahertz radiation can indicate the presence of different materials. 
     Typically, a terahertz time-domain spectroscopy setup has three major categories of components: optics components include the laser and optical-delay line; terahertz components include the emitter and detector; and control components that are used to modulate terahertz generation, synchronize the delay line, and perform data acquisition. Both the optical-delay and the optical modulator impose limits on the overall speed of the system. In a delay line used in terahertz time domain spectroscopy, the magnitude of the path length change affects the frequency range over which a measurement can be obtained and the repetition rate generally governs the time it takes to scan a frequency window. Higher repetition rates lead to more measurements per time period. 
     Since most moving displacement designs (other than fiber stretching) as exemplified by U.S. Pat. No. 5,220,463 operate on the principle of linear displacement of a mirror, conventional optical delay arrangements do not generate both high repetition rates and large displacements due to the high acceleration required. The art is in need of an optical delay system that affords both large amplitude and high frequency. In particular, commercial online scanning measurement systems would benefit from an optical delay configuration which can provide large displacement with a repetition rate that is faster than that which is currently available. 
     SUMMARY OF THE INVENTION 
     The present invention is based in part on the recognition that optical delay lines exhibiting large amplitude (displacement) and high frequency (repetition rate) can be developed by designing the retroreflector to be displaced along a circular path, rather than along a linear one, while being held in angular alignment with launch and return sources of light such as optical fibers. In particular, the retroreflector is pivotally mounted on a rotating element such that the optical axis of the retroreflector&#39;s motion is mobile so that its angle or position changes relative to a fixed observer. There is no linear stopping and starting of the retroreflector and all acceleration of retroreflector is rotational acceleration within small angles so that the required forces needed to operate the optical delay line are greatly reduced. 
     In one aspect, the invention is directed to an optical delay line that includes:
         an elongated member having a retroreflector that is slidably mounted thereon, wherein the elongated member has a linear slot that defines a path through which the retroreflector moves and wherein the elongated member has on its proximal end a fixed pivot axis;   a launch optical fiber having a first end for introducing a light beam toward the retroreflector;   a return optical fiber having a first end for receiving a light beam that is reflected from the retroreflector;   a rotatable disk having a central rotation axis that is parallel to the fixed pivot axis of the elongated member and having a second pivot axis, that is parallel to the fixed pivot axis of the elongated member, wherein the retroreflector is attached to the rotatable disk such that rotation of the rotatable disk translates the retroreflector between a first end and a second end of the linear slot and rotation of the disk defines a circular path through which the second pivot axis travels; and   means for rotating the rotatable disk.       

     In another aspect, the invention is directed to a dual rotating element optical delay line that includes:
         an elongated member having a retroreflector that is slidably mounted thereon, wherein the elongated member has a linear slot that defines a path through which the retroreflector moves;   a launch optical fiber having a first end, for introducing a light beam toward the retroreflector, that is positioned at an proximal end of the elongated member;   a return optical fiber having a first end, for receiving a light beam that is reflected from the retroreflector, that is positioned at the proximal end of the elongated member;   a first rotatable disk having a first central rotation axis and a first pivot axis onto which the retroreflector is attached so that rotation of the first rotatable disk translates the retroreflector between a first end and a second end of the linear slot and rotation of the first disk defines a first circular path through which the first pivot axis travels;   a second rotatable disk having a second central rotation axis that is parallel to the first central rotation axis and a second pivot axis that is parallel to the second central rotation axis, wherein the second rotatable disk has the same diameter as that of the first rotatable disk and is offset by one disk diameter, wherein the first end of the launch optical fiber and the first end of the return optical fiber are positioned at the second pivot axis and wherein the first rotatable disk is coupled to the second rotatable disk for synchronized movement of the first and second rotatable disks and the retroreflector is oriented such that the reflected light beam is directed back along a direction, that is parallel to the longitudinal axis of the linear slot, toward the fixed pivot axis; and   means for rotating the two rotatable disks such that the first rotatable disk rotates in a rotational direction that is opposite to that of the second rotatable disk.       

     In a further aspect, the invention is directed to an optical delay line, which employs free beam paths between the launch and return optics and the retroreflector, that includes:
         an elongated member having a retroreflector that is slidably mounted thereon, wherein the elongated member has a linear slot that defines a path through which the retroreflector moves;   a pivotally mounted mirror that is positioned to reflect an input light beam towards the retroreflector and to reflect a return light beam from the retroreflector;       

     means for transmitting an input light beam towards the mirror along a first optical path;
         means for receiving the return light beam that is reflected from the mirror along a second optical path;   a rotatable disk having a central rotation axis and a pivot axis wherein the retroreflector is attached to the rotatable disk such that rotation of the rotatable disk translates the retroreflector between a first end and a second end of the linear slot and rotation of the disk defines a circular path through which the pivot axis travels;   means for rotating the rotatable disk; and   means for rotating the pivotally mounted mirror such that input light is reflected from a first area on the mirror and towards the retroreflector and the return light is reflected from a second area on the mirror and towards the means for receiving the return light.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate an optical delay line for fiber optic systems with a single rotating element; 
         FIGS. 2A and 2B  illustrate an optical delay line for fiber optic systems with dual rotating elements; 
         FIGS. 3A ,  3 B, and  3 C illustrate an optical delay line that is characterized by free space light beams; and 
         FIG. 4  illustrates a scanning terahertz sensor system employing an optical delay line. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1A and 1B  illustrate a rapid, cyclically variable, long delay length optical delay line apparatus  11  with reduced driving torque requirements. The apparatus employs optical fibers for source and return light paths to allow a retroreflection mirror on the apparatus to be displaced along a circular path, rather than on a linear one, while the mirror is held in angular alignment with the source and return optical fibers. Apparatus  11  includes an elongated alignment member (or alignment linkage)  8  with a linear guide slot  16  formed at the distal end. A stage  5 , onto which retroreflector  10  is mounted, is constrained to only freely translate along the longitudinal axis (path) of linear guide slot  16 . Retroreflector  10  has reflective surfaces  12  and  14 . The opening of the linear guide slot  16  is preferably located in the plane that is defined by the two longest dimensions of elongated alignment member  8 . 
     Secured at the proximal end of alignment member  8  are collimation lens assemblies  18  and  20 , which include collimating lens  22  and  24 , respectively. Launch or light source optical fiber  26  is coupled to collimation lens assembly  20  and light return optical fiber  32  is coupled to collimation lens assembly  18 . The collimation lens assemblies  18 ,  20  are configured to rotate about fixed lens pivot axis  2  which has an axis that is perpendicular to the plane that is defined by the two longest dimensions of elongated alignment member  8 . Collimating lenses  22  and  24  are aligned so that light  28  from light source optical fiber  26  impinges upon a selected spot on reflective surface  12  of retroreflector  10  and return light  30  that is reflected from reflective surface  14  impinges on lens  22 . Retroreflector  10  is oriented such that reflected light is generally directed back along the direction of the longitudinal axis of the guide slot  16  towards lens pivot axis  2 . As is apparent, alignment linkage  8  may be replaced by electromotive devices and controls to maintain angular alignment without physical linkage of the collimation lens assemblies to the retroreflector. 
     Optical delay line apparatus  11  further includes a rotatable disk  4  that has (i) a fixed central rotational axis  34  that is parallel to fixed lens pivot axis  2  and (ii) an eccentric mirror pivot axis  3 , located near the edge of rotatable disk  4 , which is also parallel to fixed rotational axis  34 . A counter weight  6  is secured to the front surface of rotating disk  4  at a diametrically opposite point from mirror pivot axis  3  that is separated by inner diameter  36 . A rotary encoder  7  can be incorporated with rotating disk  6  for motion feedback; alternatively, a linear encoder scale can be installed on alignment member  8 . With the configuration of optical delay apparatus  11 , the length of inner diameter  36  typically ranges from 0.5 to 10 cm and preferably from 1 to 5 cm and rotatable disk  4  rotates at from 0 to 10,000 rpm and preferably from 600 to 6,000 rpm. 
     In operation as shown in  FIG. 1A , as motor  35  drives rotatable disk  4 , retroreflector  10 , which is mounted on a stage  5 , is constrained to only freely translate along the longitudinal axis of linear guide slot  16  thereby maintaining substantial angular alignment with the longitudinal axis. Simultaneously, retroreflector  10  is only free to rotate about mirror pivot axis  3  of rotating disk  4 , thereby retroreflector  10  moves in a constrained circular path, with its velocity and acceleration defined by the relative motions of linear guide slot  16  and mirror pivot axis  3 . When rotatable disk  4  rotates at a constant speed, the retroreflector  10  exhibits a symmetrical sinusoidal displacement profile. The distance from the collimation lens assembly  20  to reflective surface  12  of retroreflector  10  is approximately equal to one-half the optical delay length. In the position of the optical delay line shown in  FIG. 1A , this distance is designated “A”. 
     As rotatable disk  4  continues along a circular path to the position shown in  FIG. 1B , retroreflector  10  moves closer to collimation lens assemblies  18 ,  20  and the distance designated “B” is equal to about one-half that of the optical delay length. In one complete cycle or revolution of rotatable disk  4 , the change in delay distance is equal to the optical delay length. During the continuous circular displacement of rotatable disk  4 , the proximal end of elongated alignment member  8  rotates about fixed lens pivot axis  2  such that retroreflector  10  is held in angular alignment with light source optical fiber  26  and light return optical fiber  32 . This is possible in part because of the flexible nature of the optical fibers. 
     Since retroreflector  10  has two reflective surfaces  12 ,  14 , for this optical delay apparatus, the average optical delay length can be defined as the average between the maximum and minimum distances from the collimation lens assembly  20  and reflective surface  12 , multiplied by two. As is apparent, more mirrors can be employed to increase this multiplier to 4 times or more. For instance, two retroreflectors that are positioned so that a light beam is reflected between them a plurality of times parallel to the optical axis can be employed. This arrangement is described in U.S. Pat. No. 5,220,463 to Edelstein et al., which is incorporated herein by reference. Multipass optical retroreflectors with multiple reflecting surfaces are described in U.S. Pat. No. 6,979,088 to Currie, which is incorporated herein by reference. 
       FIGS. 2A and 2B  depict an optical delay line apparatus  40 , that is also particularly suited for fiber optic systems, and which employs dual rotating elements. Apparatus  40  includes rotatable disks  42  and  44  that have the same outer diameter and rotate at the same speed but in opposite directions. Each of Rotatable disks  42 ,  44  can be driven by separate motors for synchronized rotation. Preferably, each rotatable disk has linkages that mesh so that rotating one disk by one motor  47  effectively rotates the other at the same speed as well. Each rotatable disk, for example, can comprise a tooth wheel. 
     Rotatable disk  42  has a fixed central rotational axis  43  and an eccentric lens pivot axis  51 , located near the edge of rotatable disk  42 , where collimation lens assemblies  60  and  62  are pivotally mounted. A counter weight  48  is secured to the front surface of rotating disk  42  at a diametrically opposite point from lens pivot axis  47  that is separated by inner diameter  46 . Similarly, rotatable disk  44  has a fixed central rotational axis  45  and an eccentric mirror pivot axis  49 , located near the edge of rotatable disk  44 , where retroreflector  54  is pivotally mounted. A counter weight  52  is secured to the front surface of rotating disk  44  at a diametrically opposite point from mirror pivot axis  49  that is separated by inner diameter  50 , which preferably has the same as length as inner diameter  46 . 
     Optical delay apparatus  40  further includes an elongated alignment member  70  with a linear guide slot  76  onto which retroreflector  54  is slidably mounted via moveable hanger device  72  so as to be constrained to only freely translate along the longitudinal axis (path) of linear guide slot  76 . Retroreflector  54  has reflective surfaces  56  and  58 . Collimation lens assemblies  60  and  62 , which include collimating lens  66  and  64 , respectively are slidably mounted via moveable hanger device  74  at the proximal end of alignment member  70 . Launch or light source optical fiber  84  is coupled to collimation lens assembly  62  and light return optical fiber  86  is coupled to collimation lens assembly  60 . In use, the source of light pulses may be quite some distance from the optical delay apparatus so a stationary fiber optic coupling device  88  can be employed to connect source fiber optic cable  80  and return fiber optic cable  82  to light source optical fiber  84  and light return source optical fiber  84 , respectively. 
     Collimating lenses  64  and  66  are aligned so that light  68  from light source optical fiber  84  impinges upon a selected spot on reflective surface  56  of retroreflector  54  and return light  78  that is reflected from reflective surface  58  impinges on lens  66 . Retroreflector  54  is oriented such that reflected light is generally directed back along the direction of the longitudinal axis of the guide slot  76  towards lens pivot axis  47 . As is apparent, a retroreflector with more than 2 mirrors, or multiple retroreflectors, can be employed. 
     As rotatable disks  42  and  44  rotate, the optical distance between collimating lenses  64 ,  66  to reflective surfaces  58  and  58 , respectively, changes. The optical distance for the apparatus as shown in  FIG. 2B  is closer than that shown in  FIG. 2A . As a result of this synchronized movement, retroreflector  54  exhibits a symmetrical sinusoidal displacement profile. One feature of dual element optical delay apparatus  40 , in which collimation lens assemblies  60 ,  62  rotate counter-clockwise with rotatable disk  42  while retroreflector  56  simultaneously rotates clockwise with rotatable disk  44 , is that the diameters of the rotatable disks used can be smaller than the single rotatable element used in apparatus  11  ( FIG. 1A ). In other words, in order to achieve the same optical delay length, the size of each of the rotatable disks in the dual rotating element apparatus can be significantly smaller than that of the rotatable disk single the rotatable element apparatus. Each of inner diameter  46  in rotatable disk  42  and inner diameter  50  typically has a length that ranges from 0.2 to 5 cm and preferably from 0.5 to 2.5 cm and each of rotatable disks  42  and  44  rotates at from 0 to 10,000 rpm and preferably from 600 to 6,000 rpm. 
       FIG. 3A  illustrates an optical delay line apparatus  90  which is suitable for free space light beams, that is, where the source and return light paths are not confined within optical fibers. Apparatus  90  includes motor  109 , a motor driven rotatable disk  92  having a fixed central rotational axis  120 , and an elongated alignment member  102  with a linear guide slot  103  formed at the distal end. Stage  101  is attached to the surface of rotatable disk  92  at an eccentric mirror pivot axis located near the edge of rotatable disk  92 . Stage  101  is positioned within linear guide slot  103  so as to be constrained to only freely translate along its longitudinal axis. A retroreflector mirror  100  is mounted to stage  101 . A counter weight  118  is attached at a diametrically opposite point from stage  101 ; the distance in between is referred to as the inner diameter of rotatable disk  92 . The length of linear guide slot  103  is at least equal to that of the inner diameter so that stage  101  so that stage  101  aligned with member  102  throughout the rotation of disk  92 . The inner diameter in rotatable disk  92  typically has a length that ranges from 0.5 to 10 cm and preferably from 1 to 5 cm and rotatable disk  92  at from 0 to 10,000 rpm and preferably from 600 to 6,000 rpm. 
     Apparatus  90  further includes a plane mirror  98  that is positioned adjacent the proximal end of elongated alignment member  102  so that the reflective surface of retroreflector  100  faces plane mirror  98 . Retroreflector  100  can comprise more than two mirrors to increase the optical delay length or multiple retroreflectors can be employed. Light source beam  110  is directed towards mirror  98  such that reflected light beam  106  is redirected towards retroreflector  100 . Similarly, reflected return light beam  104  from retroreflector is redirected by mirror  98  as output beam  108 . Light source beam  110  can be irradiated towards mirror  98  through a stationary collimation lens  121  so that the path of light source beam  110  remains constant. In this arrangement, output beam  108  is directed back from mirror  98  in the same direction as the axis of light source beam  110  and is captured by collection lens  123 . 
     Plane mirror  98  is designed for coordinated movement with retroreflector  100  so the optical paths of light source beam  110  and output beam  108  remain constant during operation of optical delay apparatus  90 . In particular, the distal end of elongated alignment member  102  is operatively coupled to a 2:1 timing belt driven gear reduction apparatus so that mirror  98  pivots about the axis at one-half the angle at which retroreflector rotates. Timing belt  116  is looped around gear  112  and gear (pinion)  115 , which have a gear ratio of 2:1. Elongated alignment member  102  is affixed to and drives gear  115 . Gears  94  and  96  are the same size and have timing belt  114  being looped around them. Gear  96  is coupled to gear  112  and plane mirror  98  is pivotally mounted on gear  94  about an axis that is collinear to gear  115 . The motion of elongated alignment member  102  drives timing belt  116  in the appropriate direction in coordination with rotatable disk  92  so that the optical paths of free space light source beam  110  and free space output beam  108  remain constant. 
     In operation, as motor  109  drives rotatable disk  92  to rotate clockwise, retroreflector  100  which is mounted on stage  101  is constrained to only freely translate along the longitudinal axis (path) of linear guide slot  102  thereby maintaining substantial angular alignment with the longitudinal axis. Simultaneously, retroreflector  100  is only free to rotate about mirror pivot axis  120  of rotating disk  92 , thereby retroreflector  100  moves in a constrained circular path, with its velocity and acceleration defined by the relative motions of linear guide slot  103  and mirror pivot axis  120 . When rotatable disk  92  rotates at a constant speed, the retroreflector  110  exhibits a symmetrical sinusoidal displacement profile. The distance from the collimation lens  121  to a reflective surface of retroreflector  110  is approximately equal to one-half the optical delay length. The change in delay length is then equal to twice the change in distance from retroreflector  100  to mirror  98  as disk  92  rotates through a complete revolution, which is equivalent to the diameter at which retroreflector  100  is mounted to disk  92 . 
     As rotatable disk  92  continues along its circular path, plane mirror  98  moves in synchronized fashion so that the path of light source beam  110  remains optically aligned with mirror  98  and the paths of input beam  110  and output beam  108  do not change. In particular, as retroreflector  100  moves from its position initial position depicted in  FIG. 3A  to those shown in  FIGS. 3B and 3C , the angle of incidence light source beam  110  on plane mirror  98  increases in order to maintain the optical alignment. Once retroreflector  100  reaches the lowest point along its circular path,  FIG. 3C , and begins to rise, belt  114  reverses direction to cause mirror  98  to move in the opposite direction, decreasing angle of incident keeping optical alignment of paths  110  and  108  with retroreflector  100 . 
       FIG. 4  shows the structure of a terahertz time-domain spectrometer for monitoring at least one property of the moving sheet or web of material  140 . The basic components of the spectrometer include: pulsed laser source  122 , beam splitter  126 , terahertz transmitter  142 , modulated power source  136 , terahertz receiver or detector  144 , spectroscopic analyzer  138  and an optical delay device that includes retroreflector  154 . Pulsed laser source  122 , such as a femto-second pulse laser, generates pump signals  150  that are directed toward beam splitter  126  that splits the light pulses of pump signal  150  to yield excitation light  156  and detector gating light  152 . 
     Excitation light  156  is focused by objective lens  130  and launched into and transmitted through delivery optical fiber  158 . Excitation light  156  illuminates transmitter  142  to generate terahertz radiation or T-rays  160  which are directed by mirror  146  into moving sheet  140 . Modulated power source  136  supplies an electrical input  168  into terahertz transmitter  142 . T-rays  162  which emerge from moving sheet  140  are reflected from mirror  148  and captured by detector  144 . Mirrors  146  and  148  when employed are typically off-axes parabolic mirrors. 
     Detector gating light  152  is directed to optical delay device which serves to set or modify the difference between the timing of the detector gate light  152  and the timing of the excitation light  156 . The optical delay device can comprise any of the inventive optical delay devices such as apparatus  11  depicted in  FIGS. 1A and 1B . As shown in  FIG. 4 , the device includes an elongated alignment member  155  that has a linear guide slot formed at the distal end. Collimation lens assemblies  125 ,  123  are secured at the proximal end of alignment member  155  while a retroreflector  154  is secured to a rotatable disk  170  through the linear guide slot in alignment member  155 . Collimation lens assemblies  125  and  123  are in optical alignment with retroreflector  154  so that as motor  171  drives rotatable disk  170 , changes the length of the optical path of detector gating light  152 , thereby changing and setting the difference between excitation light irradiation timing (T-ray generating timing) and the detector gating light irradiation timing (T-ray detecting timing). The optical delay device launches light into delivery optical fiber  166  and into receiver or detector  144 . The laser pulses that exit from the end of optical fiber  166  are used to effectively switch on the terahertz receiver in a synchronous detection scheme. When the arrival time of these synchronizing pulses to the terahertz receiver are varied, the terahertz pulses can be traced out. The output  164  from receiver  144  is an electrical signal that is typically amplified and digitized and then read into a computer for analysis or alternatively the electrical signal can be analyzed in a digital signal processor. The electrical signal can be amplified with a transimpedance amplifier and then fed into a lockin amplifier. If lockin detection is employed, a modulated bias voltage is typically applied to power source  136 . The lockin detector is then synchronized with this bias modulation. 
     Detector  144  generates detection signals  164  which are transmitted to spectroscopic analyzer  138 . The electrical signals generated by the detector that can be analyzed in the computer in the temporal or frequency domain. For instance, this analysis can also be done in a Field-Programmable Gate Array (FPGA) or a Digital Signal Processor (DSP). 
     While the optical delay device is positioned in optical path of detector light  152 , an optical delay device could be positioned in the optical path of excitation light  156  instead. Preferably, laser source  122 , beam splitter  126 , the optical delay device are housed in compartment  170 . In a transmission mode embodiment, terahertz transmitter  142  and mirror  144  are located in sensor head  132  whereas detector  144  and mirror  148  are located in sensor head  134 . The sensor head can be any suitable light weight structure housing the associated components. 
     If optical rectification is used to generate or detect the THz radiation, then optical fibers are preferably selected from those which can maintain the linear polarization state of the light which is injected into them since the THz transmitter and receiver are dependent upon the polarization state of the pump light. Preferred optical fibers are highly birefringent or single polarization photonic bandgap fiber which will maintain the polarization of the femto-second pulse laser generated pulses of light. It is often preferable to use a THz antenna to both generate and receive the THz radiation, in which case, using non-polarization maintaining optical fibers are preferred since the generation and detection of the THz radiation is not polarization sensitive. 
     In order to function as a scanning terahertz sensor, sensor heads  134  and  132  must be mobile which means that movement of optical fibers  158  and  166 , which are in optical communication with sensor heads  132  and  134 , respectively, must also be accommodated. Optical fibers  158  and  166  can be routed through take-up mechanisms to control the bending of the optical fibers, as further described in US Patent Application No. 20060109519 to Beselt et al., which is incorporated herein by reference. 
     The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.