1. Field of the Invention
The present invention relates to an optoacoustic convolver for detecting a correlation between two signals by converting two signals to be correlated to acoustic compressional waves which propagate through light-transmissive acoustic media and then observing interference light between the two light beams which have passed through the light-transmissive acoustic media.
2. Description of the Related Art
Communication systems using radio waves and ultrasonic waves are currently incorporated and used in various sensors. One of the most important requirements of these sensors is the system-wise smallness, i.e., the small load on the system in which the sensor is incorporated, not to mention the physical smallness.
Some communication systems use signals that are modulated/demodulated with spreading codes for increasing the functionality, e.g., ensuring the communication quality and increasing the efficiency of information transfer. In the transmitter/receiver (primarily the receiver) of such a communication system, a convolver for detecting a correlation between a signal received (received signal) and a signal generated by the transmitter/receiver based on a spreading code (reference signal) is an indispensable component.
Typically, the correlation process in the receiver using such a communication scheme is performed by a digital filter or a convolver implemented on a computer as a program, after the received radio or ultrasonic signal is converted to a digital signal by an analog/digital converter. The amount of computation required for these processes is large, and a high-speed computer is required in order to keep the signal delay to a minimum. Therefore, it is an objective for realizing a high-functionality communication device to satisfy both this and the requirement of a sensor described above.
Regarding this objective, Japanese Patent Application Laid-Open Publication No. S59-201020 (see FIG. 3, in particular) for example discloses an optoacoustic convolver which utilizes light and acoustic waves, which is small system-wise, and which is capable of high-speed operations. The optoacoustic convolver first converts two signals to be correlated to amplitude-modulated signals corresponding to carrier waves (sinusoidal waves) each having an intended frequency. That is, two signals are upconverted to carrier wave frequencies as amplitude-modulated signals. These amplitude-modulated signals are converted to elastic waves (compressional waves) and propagated through different acoustic media, thereby producing two gradient refractive index diffraction gratings (GRIN gratings). The intensities of diffracted light which have been diffracted through the diffraction gratings are measured, thereby obtaining a correlation signal between the two signals.
FIG. 37 shows a configuration of a conventional optoacoustic convolver described in Japanese Patent Application Laid-Open Publication No. S59-201020. In FIG. 37, carrier wave generation circuits 151a and 151b generate sinusoidal waves (carrier waves) having frequencies that coincide with resonance frequencies of piezoelectric oscillators 1510a and 1510b. The two signals generated by the signal generation circuits 152a and 152b are converted by modulators 153a and 153b, respectively, so that time variations of amplitude values of the sinusoidal waves are in proportion to the time waveforms of the two signals. That is, the two signals are upconverted to amplitude-modulated signals whose frequencies are equal to the carrier wave frequencies.
The amplitude-modulated carrier waves are amplified by amplifiers 154a and 154b and then input to the piezoelectric oscillators 1510a and 1510b, respectively. The piezoelectric oscillators 1510a and 1510b propagate elastic waves based on the input signals through acoustic media 159a and 159b, respectively. These elastic waves induce refractive index distributions in the acoustic media 159a and 159b. The elastic waves generated in the acoustic media 159a and 159b are sinusoidal waves whose frequencies are equal to the resonance frequencies of the piezoelectric oscillators 1510a and 1510b, respectively, and whose amplitude values vary over time based on the input signal. Therefore, the refractive index distributions induced by the acoustic media 159a and 159b each become a gradient refractive index diffraction grating which has a grating pitch equivalent to the one wavelength of the elastic wave and which propagates at an elastic wave propagation velocity.
A laser light source 151 emits laser light toward two gradient refractive index diffraction gratings through which elastic waves propagate in opposite directions. The laser light emitted from the laser light source 151 is enlarged by an optical system 155 to a sufficient beam diameter, and enters the acoustic media 159a and 159b. As a result, a diffracted light beam having various orders of diffraction is generated. The diffracted light beam is condensed through a condensing optical system 156 to form a plurality of bright spots on a space filter 158. Only a bright spot corresponding to diffracted light of an intended order is extracted by the space filter 158, and the optical intensity thereof is output by a light-receiving element 157 as an electric signal.
In order to operate as a convolver, it is necessary to observe a bright spot of diffracted light that is not corresponding to the 0th-order diffraction in the gradient refractive index diffraction gratings. The diffracted light intensity is in proportion to the square of the contrast of the refractive index distribution in the gradient refractive index diffraction grating (∞ refractive index variation range/average refractive index). Therefore, the intensity of the electric signal output from the light-receiving element 157 is in proportion to the square of the product between the contrasts of the gradient refractive index diffraction gratings produced in the acoustic media 159a and 159b. The contrasts of the gradient refractive index diffraction gratings are generally in proportion to the intensities of the signals generated in the signal generation circuits 152a and 152b. Therefore, the intensity of the electric signal output from the light-receiving element 157 at a certain point in time is in proportion to the square of the product between the intensities of the two signals at the point in time, ignoring the time delay in signal processes.
Therefore, by obtaining the square root of the intensity of the electric signal, it is possible to obtain the correlation signal between the signals (corresponding to the “product” of these signals in this case). This is the operation of the conventional optoacoustic convolver described in Japanese Patent Application Laid-Open Publication No. S59-201020.
As described above, the conventional optoacoustic convolver described in Japanese Patent Application Laid-Open Publication No. S59-201020 has no analog/digital conversion means and no digital signal processing means, and is small system-wise. In addition, the convolver is advantageous in that it is fast and has a small influence due to delay because all correlation processes are passively performed. Thus, a device such as an ultrasonic sensor having an optoacoustic convolver disclosed in Japanese Patent Application Laid-Open Publication No. S59-201020 is advantageous in that it has a small load on the whole system in which it is incorporated (e.g., a robot having an ultrasonic sensor).
However, with the configuration disclosed in Japanese Patent Application Laid-Open Publication No. 559-201020, a physically large optical system is needed in order to desirably capture diffracted light of a necessary order, and an ultrasonic sensor having the same will have a large physical size.
For example, where a carrier wave whose frequency is 100 MHz is used with an acoustic medium having an elastic wave propagation velocity of 1000 m/s (which corresponds generally to a propagation velocity of a dynamically hard substance with a high refractive index which is necessary for obtaining a sufficient diffracted light intensity), the elastic wave wavelength is 10 μm. Assuming that the extent of a bright spot of scattered light attenuates sufficiently over a radius of 2.5 mm, a bright spot of 1st-order diffraction needs to appear at a distance of 5 mm or more from a bright spot of 0th-order diffraction. Therefore, when laser light having a wavelength of 600 nm is output, the distance from an acoustic medium 159b to the space filter 158 that is necessary for a bright spot of 1st-order diffraction through one gradient refractive index diffraction grating to appear at a distance of 5 mm from a bright spot of 0th-order diffraction therethrough is about 83.3 mm. Therefore, the size of a device such as an ultrasonic sensor incorporating therein an optoacoustic convolver of the conventional configuration inevitably increases.
With current communication systems using radio waves and ultrasonic waves, code spreading is sometimes used as the signal modulation/demodulation scheme with the aim of increasing the functionality, as described above. In this case, the correlation process between two signals is not performed for the instantaneous values of the signals, but it is necessary to measure the correlation between the time waveforms of the two signals for a certain period of time.
However, with the conventional configuration, in order to measure the correlation between the time waveforms of the two signals for a certain period of time, the sizes of the acoustic media 159a and 159b need to be very large.
For example, in order to perform a correlation process for a signal whose time interval is 0.07 msec using an acoustic medium having an elastic wave propagation velocity of 1000 m/s, at least an elastic wave with an amplitude variation that corresponds to the time interval's worth of a signal needs to be entirely present in the acoustic media 159a and 159b. In this case, the length necessary for the acoustic media 159a and 159b is 70 mm. Since the opening diameters of the optical system 155 and the condensing optical system 156 need to be about the same as the length of the acoustic media 159a and 159b, a device incorporating the conventional optoacoustic convolver will be large.
The present invention has been made to solve the problems described above, and an object thereof is to provide an optoacoustic convolver that not only is small system-wise but also is capable of high-speed operations and physically small.