Position sensing apparatus, and position sensing method

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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows a first embodiment of the present invention. In the first embodiment, as illustrated inFIG. 1, laser light emitted from two tunable laser diodes1is mixed at a ratio of 50:50 using a beam splitter10. 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 diodes1. Frequencies of the laser diodes1are selected so that the difference frequency falls within a THz range. For example, wavelengths of the laser diodes1are changeable around 830 nm by about 10 nm.

One of the two mixture beams is converged onto a photoconductive device2for emitting electromagnetic radiation, while the other is converged onto a photoconductive device3for detecting electromagnetic radiation. With respect to the mixture beam converged on the photoconductive device2, the beam is converged thereon through an optical chopper4for chopping the beam at a given chopping frequency. The optical chopper4is 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 device3, the beam is converged thereon through a mirror12and a time delay is given thereto by a time delaying system7.

As illustrated inFIG. 1, parabolic mirrors11are arranged so that THz radiation generated by the photoconductive device2can be directed to an object9and THz radiation reflected by the object9can be condensed on the photoconductive device3for detecting the electromagnetic radiation.

A current generated in the photoconductive device3by the reflected THz radiation is supplied to a lock-in amplifier5through a current amplifier8. The current amplifier8is used to amplify a small signal detected by the photoconductive device3for detecting the electromagnetic radiation. The THz radiation is thus detected by a lock-in amplifiers5, and a signal detected thereby is supplied to a computer6. The computer6controls a movable stage of the time delaying system7based on the detected signal. Here, a chopping signal at the chopping frequency is also supplied to the lock-in amplifier5from the optical chopper4.

FIG. 2illustrates a normalized signal of the signal detected by the lock-in amplifier5. The abscissa ofFIG. 2represents 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 inFIG. 2is obtained, it is assumed that the object9stays at a position X1, and the movable stage of the time delaying system7is controlled and reposed at a position x1so that a maximum output (for example, a point A shown inFIG. 2) can be obtained from the lock-in amplifier5.

Under the above condition, if the object9moves by a distance ΔX and reaches a position X2in a small time Δt, the detected signal decreases. Accordingly, it is assumed that the computer6controls and moves the movable stage of the time delaying system7by Δx so that the maximum output can be obtained by the lock-in amplifier5. 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 object9.

In a case where the movable stage of the time delaying system7has 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=2Δx/c(c: velocity of light)
The amount ΔX of movement of the object9is represented by
ΔX=2Δx=τc

Where the movable stage of the, time delaying system7has 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=2nΔx/c(c: velocity of light)
The amount ΔX of movement of the object9is written as
ΔX=2nΔx=τ′c

Upon calculation of the amount ΔX of movement of the object9by the computer6every small time Δt, the location of the object9can be detected every small time Δt. Since ΔX needs to be smaller than a wavelength of the graph inFIG. 2, a small time Δt is selected so that this condition can be satisfied. It should be noted that the abscissa ofFIG. 2represents the time delay, and it can be seen fromFIG. 2that one period of the graph is three (3) picoseconds, i.e., 3×10−12seconds. The velocity of the radiation (THz radiation) can be assumed to be equal to 3×1011mm/s, and hence the distance the radiation travels for three picoseconds is 3×10−12(s)×3×1011(mm/s)=0.9 (mm) Thus, the wavelength of the graph inFIG. 2is equal to 0.9 mm.

A second embodiment will now be described. In a sensing method of the second embodiment, the position of the object9is detected without controlling the movable stage of the time delaying system7every small time Δt.

In the second embodiment, when the movable stage of the time delaying system7moves a distance larger than the value corresponding to the wavelength of the signal illustrated inFIG. 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 amplifier5. The structure of the second embodiment is substantially the same as that of the first embodiment illustrated inFIG. 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 system7is controlled every small time Δt, and the amount ΔX of movement of the object9is 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 amplifier5. This relationship is stored in the computer6as a data base. The position of the object9is acquired by comparison of the output of the lock-in amplifier5with the data base.

It is assumed that the object9stays at a position X1and the output of the lock-in amplifier5is obtained as illustrated inFIG. 2. The ordinate ofFIG. 2represents the signal detected by the lock-in amplifier5, 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 system7is moved to equalize the output of the lock-in amplifier5with zero (0) (for example, a point B inFIG. 2). The position of the movable stage at this moment is assumed to be x1. Here, the movable stage is moved in a direction, and an integral value of absolute values of outputs from the lock-in amplifier5during this movement is recorded. Thus, the one-to-one correspondence relationship between the time delay and the output from the lock-in amplifier5can be established, as illustrated inFIG. 3in 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. 3shows 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 amplifier5. The ordinate inFIG. 3can 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 amplifier5is stored beforehand in the computer6as the database, the computer6can calculate the integral value of absolute values of outputs from the lock-in amplifier5, which varies as the object9moves. 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 object9can be obtained from the following relationship.

Where the movable stage of the time delaying system7has only a single round-trip optical path, the amount ΔX of movement of the object9can be obtained from ΔX=2Δx. Where the movable stage of the time delaying system7has n round-trip optical paths, the amount ΔX of movement of the object9can be obtained from ΔX=2nΔx.

The amount ΔX of movement of the object9can be thus obtained based on a change in the output from the lock-in amplifier5due to the movement of the object6. In the second embodiment there is typically no need to make the amount ΔX of movement of the object9smaller than the value corresponding to the wavelength of the curve inFIG. 2.

In the second embodiment, the integral value of absolute values of outputs from the lock-in amplifier5is 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 amplifier5. It is, however, possible to use an integral value of squares of outputs from the lock-in amplifier5instead of the integral value of absolute values of outputs from the lock-in amplifier5.FIG. 4shows plotted integral values of squares of outputs from the lock-in amplifier5. The ordinate inFIG. 4can 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.