Abstract:
An apparatus for sensing the position of an object includes an irradiation portion for irradiating the object with continuous electromagnetic radiation, a detection portion for detecting electromagnetic radiation reflected by the object, and an output portion for supplying a change in an amplitude intensity or a phase of the electromagnetic radiation based on information obtained by the detection portion. The position of the object is detected based on information supplied from the output portion.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus and method for sensing the position of an object by propagating electromagnetic radiation in a space, and particularly to an apparatus and method for sensing the position of an object by propagating terahertz (THz) electromagnetic radiation in a space. 
   2. Description of the Related Background Art 
   In this specification, the terminology “the terahertz (THz) electromagnetic radiation” or “terahertz (THz) radiation” or “terahertz (THz)” is used for radiation in a frequency range between about 30 GHz and about 30 THz. Conventionally, terahertz (THz) radiation pulses are used for position sensing using terahertz (THz) radiation, as disclosed in “Time domain terahertz impulse ranging studies (Applied Physics Letters, Vol. 67, p. 1960, 1995)”. A femtosecond laser used for generation of the terahertz (THz) radiation pulses is, however, typically expensive and large in size. 
   Accordingly, a relatively low-cost position sensing apparatus is strongly desired. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an apparatus and method for sensing the position of an object without necessarily having to use a femtosecond laser. 
   According to one aspect of the present invention, there is provided an apparatus for sensing the position of an object, which includes an irradiation portion for irradiating the object with continuous electromagnetic radiation, a detection portion for detecting electromagnetic radiation reflected by the object, and an output portion for supplying a change in an amplitude intensity, or a phase of the electromagnetic radiation based on information obtained by the detection portion, and in which the position of the object is detected based on information supplied from the output portion. In the present invention, the change in the amplitude intensity of the reflected electromagnetic radiation can be handled equivalently to the change in the phase of the reflected electromagnetic radiation since the detection portion detects the electromagnetic radiation reflected by the object. 
   According to another aspect of the present invention, there is provided a method of sensing the position of an object, in which the above position sensing apparatus is used, and a time delay corresponding to the change in an amplitude intensity or a phase of the electromagnetic radiation is calculated by time-delaying the electromagnetic radiation by a time delaying system, so that the position of the object can be detected. 
   According to the present invention, it is possible to achieve position sensing without necessarily having to use a relatively high-cost femtosecond laser, thus providing a device and method for sensing the position of an object at relatively low cost. 
   These advantages, as well as others, will be more readily understood in connection with the following detailed description of the preferred embodiments and examples of the invention in connection with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view illustrating the structure of an embodiment of the present invention. 
       FIG. 2  is a view illustrating an example of a normalized signal of a signal detected by a lock-in amplifier. 
       FIG. 3  is a view illustrating plotted integral values of absolute values of outputs from the lock-in amplifier. 
       FIG. 4  is a view illustrating plotted integral values of squares of outputs from the lock-in amplifier. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will hereinafter be described with reference to the drawings. 
     FIG. 1  shows a first embodiment of the present invention. In the first embodiment, as illustrated in  FIG. 1 , laser light emitted from two tunable laser diodes  1  is mixed at a ratio of 50:50 using a beam splitter  10 . Two mixture beams, each composed of two laser lights, are thus generated. A frequency of each of the thus-generated mixture beams is equal to a difference between frequencies of the two laser diodes  1 . Frequencies of the laser diodes  1  are selected so that the difference frequency falls within a THz range. For example, wavelengths of the laser diodes  1  are changeable around 830 nm by about 10 nm. 
   One of the two mixture beams is converged onto a photoconductive device  2  for emitting electromagnetic radiation, while the other is converged onto a photoconductive device  3  for detecting electromagnetic radiation. With respect to the mixture beam converged on the photoconductive device  2 , the beam is converged thereon through an optical chopper  4  for chopping the beam at a given chopping frequency. The optical chopper  4  is used to modulate the beam (pumping radiation) so that lock-in detection (described below) can be carried out. As for the mixture beam converged on the photoconductive device  3 , the beam is converged thereon through a mirror  12  and a time delay is given thereto by a time delaying system  7 . 
   As illustrated in  FIG. 1 , parabolic mirrors  11  are arranged so that THz radiation generated by the photoconductive device  2  can be directed to an object  9  and THz radiation reflected by the object  9  can be condensed on the photoconductive device  3  for detecting the electromagnetic radiation. 
   A current generated in the photoconductive device  3  by the reflected THz radiation is supplied to a lock-in amplifier  5  through a current amplifier  8 . The current amplifier  8  is used to amplify a small signal detected by the photoconductive device  3  for detecting the electromagnetic radiation. The THz radiation is thus detected by a lock-in amplifiers  5 , and a signal detected thereby is supplied to a computer  6 . The computer  6  controls a movable stage of the time delaying system  7  based on the detected signal. Here, a chopping signal at the chopping frequency is also supplied to the lock-in amplifier  5  from the optical chopper  4 . 
     FIG. 2  illustrates a normalized signal of the signal detected by the lock-in amplifier  5 . The abscissa of  FIG. 2  represents time or delay time. In the normalized signal, the detected signal is normalized so that a maximum of the signal is equal to one (1), a minimum of the signal is equal to minus one (−1), and a central value of the amplitude is equal to zero (0). 
   When such a signal as illustrated in  FIG. 2  is obtained, it is assumed that the object  9  stays at a position X 1 , and the movable stage of the time delaying system  7  is controlled and reposed at a position x 1  so that a maximum output (for example, a point A shown in  FIG. 2 ) can be obtained from the lock-in amplifier  5 . 
   Under the above condition, if the object  9  moves by a distance ΔX and reaches a position X 2  in a small time Δt, the detected signal decreases. Accordingly, it is assumed that the computer  6  controls and moves the movable stage of the time delaying system  7  by Δx so that the maximum output can be obtained by the lock-in amplifier  5 . Then, the amount Δx of movement of the movable stage provides a time delay τ corresponding to a change in the signal due to the movement of the object  9 . 
   In a case where the movable stage of the time delaying system  7  has only a single round-trip optical path, the relationship between the time delay τ and the amount Δx of movement of the movable stage is given by
 
τ=2Δ x/c  ( c : velocity of light)
 
The amount ΔX of movement of the object  9  is represented by
 
ΔX=2Δx=τc
 
   Where the movable stage of the, time delaying system  7  has n round-trip optical paths, the relationship between the time delay τ′ and the amount Δx of movement of the movable stage is represented by
 
τ′= nτ= 2 nΔx/c  ( c : velocity of light)
 
The amount ΔX of movement of the object  9  is written as
 
Δ X= 2 nΔx=τ′c  
 
   Upon calculation of the amount ΔX of movement of the object  9  by the computer  6  every small time Δt, the location of the object  9  can be detected every small time Δt. Since ΔX needs to be smaller than a wavelength of the graph in  FIG. 2 , a small time Δt is selected so that this condition can be satisfied. It should be noted that the abscissa of  FIG. 2  represents the time delay, and it can be seen from  FIG. 2  that one period of the graph is three (3) picoseconds, i.e., 3×10 −12  seconds. The velocity of the radiation (THz radiation) can be assumed to be equal to 3×10 11  mm/s, and hence the distance the radiation travels for three picoseconds is 3×10 −12  (s)×3×10 11  (mm/s)=0.9 (mm) Thus, the wavelength of the graph in  FIG. 2  is equal to 0.9 mm. 
   A second embodiment will now be described. In a sensing method of the second embodiment, the position of the object  9  is detected without controlling the movable stage of the time delaying system  7  every small time Δt. 
   In the second embodiment, when the movable stage of the time delaying system  7  moves a distance larger than the value corresponding to the wavelength of the signal illustrated in  FIG. 2 , the amount ΔX of movement of the movable stage is obtained by calculating the number of pulses of an output supplied from the lock-in amplifier  5 . The structure of the second embodiment is substantially the same as that of the first embodiment illustrated in  FIG. 1 . 
   The second embodiment is different from the first embodiment in a method of detecting the position. In the first embodiment, the movable stage of the time delaying system  7  is controlled every small time Δt, and the amount ΔX of movement of the object  9  is obtained from the amount Δx of movement of the movable stage. By contrast, in the sensing method of the second embodiment, the one-to-one correspondence relationship is established beforehand between the amount ΔX of movement of the movable stage and the output from the lock-in amplifier  5 . This relationship is stored in the computer  6  as a data base. The position of the object  9  is acquired by comparison of the output of the lock-in amplifier  5  with the data base. 
   It is assumed that the object  9  stays at a position X 1  and the output of the lock-in amplifier  5  is obtained as illustrated in  FIG. 2 . The ordinate of  FIG. 2  represents the signal detected by the lock-in amplifier  5 , which is normalized so that its maximum is one, its minimum is minus one, and the central value of the amplitude is zero. 
   Under such a condition the movable stage of the time delaying system  7  is moved to equalize the output of the lock-in amplifier  5  with zero (0) (for example, a point B in  FIG. 2 ). The position of the movable stage at this moment is assumed to be x 1 . Here, the movable stage is moved in a direction, and an integral value of absolute values of outputs from the lock-in amplifier  5  during this movement is recorded. Thus, the one-to-one correspondence relationship between the time delay and the output from the lock-in amplifier  5  can be established, as illustrated in  FIG. 3  in which the abscissa indicates the time delay. Since a proportional relationship exists between the delay time and the amount Δx of movement of the movable stage,  FIG. 3  shows that the one-to-one correspondence relationship is established between the amount Δx of movement of the movable stage and the output from the lock-in amplifier  5 . The ordinate in  FIG. 3  can be coordinates-transformed according to necessity. 
   Under a condition that the relationship between the amount Δx of movement of the movable stage and the integral value of absolute values of outputs from the lock-in amplifier  5  is stored beforehand in the computer  6  as the database, the computer  6  can calculate the integral value of absolute values of outputs from the lock-in amplifier  5 , which varies as the object  9  moves. The amplifier can also acquire the amount Δx of movement of the movable stage by comparison of the thus-calculated integral value with the stored data base. When the amount Δx of movement of the movable stage is known, the amount Δx of movement of the object  9  can be obtained from the following relationship. 
   Where the movable stage of the time delaying system  7  has only a single round-trip optical path, the amount ΔX of movement of the object  9  can be obtained from ΔX=2Δx. Where the movable stage of the time delaying system  7  has n round-trip optical paths, the amount ΔX of movement of the object  9  can be obtained from ΔX=2nΔx. 
   The amount ΔX of movement of the object  9  can be thus obtained based on a change in the output from the lock-in amplifier  5  due to the movement of the object  6 . In the second embodiment there is typically no need to make the amount ΔX of movement of the object  9  smaller than the value corresponding to the wavelength of the curve in  FIG. 2 . 
   In the second embodiment, the integral value of absolute values of outputs from the lock-in amplifier  5  is used to obtain the one-to-one correspondence relationship between the amount Δx of movement of the movable stage and the output from the lock-in amplifier  5 . It is, however, possible to use an integral value of squares of outputs from the lock-in amplifier  5  instead of the integral value of absolute values of outputs from the lock-in amplifier  5 .  FIG. 4  shows plotted integral values of squares of outputs from the lock-in amplifier  5 . The ordinate in  FIG. 4  can be coordinates-transformed according to necessity. 
   According to the present invention, it is possible to achieve position sensing by means of generation of continuous electromagnetic radiation that is obtained by using two relatively-low-cost and small-sized laser diodes without using a relatively-high-cost femtosecond laser, for example. It is hence possible to provide an inexpensive small-sized apparatus and method for sensing the position of an object. 
   While the present invention has been described with respect to what is presently considered to be the preferred embodiments and examples, it is to be understood that the invention is not so limited. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and the scope of the appended claims. 
   This application claims priority from Japanese Patent Application No. 2005-98172, filed Mar. 30, 2005, which is hereby incorporated by reference.