JPH04265536A - Optical waveguide recording medium reproducing device - Google Patents

Abstract

PURPOSE:To reduce the size of a reference light generation optical system by adopting an acoustooptical modulation element, in an optical waveguide recording medium reproducing device provided with a heterodyne detection optical system. CONSTITUTION:An ultrasonic wave generation element 101 is an electrostriction element or a piezoelectric element provided with an opening 101a through which a laser beam entering via a beam splitter is made incident, an acoustooptical medium block is the crystal of quarts, TeO2, etc., and the ultrasonic generation element 101 is formed on the incident end face with the electrostriction element composed of barium titanate or the piezoelectric element composed of crystal, etc. By applying intermittent high frequency electricity to the element 101, grid stripes whose refractive index changes according to the wavelength of the ultrasonic wave are generated within the block 100. The laser beam going through the block 100 is given phase delay and frequency displacement due to the Doppler effect of the ultrasonic wave, a part of it is reflected to return, and the reflection light is used as the reference light.

Description

[Detailed description of the invention]

0001

[Technical Field] The present invention provides an optical storage medium, in particular, a refractive index discontinuity portion that generates reflected guided light having a plurality of different amplitudes and phase lags by guiding a laser beam (generally referred to including low-coherence light). Using an optical waveguide storage medium having an optical waveguide, a laser beam is guided through the medium, and a part of the reflected guided light and the laser beam with shifted Doppler frequency are optically heterodyne detected, and the recorded information is converted into a time-series signal. The present invention relates to an optical waveguide storage medium reproducing device that reproduces waveforms.

0002

BACKGROUND OF THE INVENTION Conventional optical storage media include optical discs in which a plurality of low light reflectance recesses are arranged in a line as recorded information on a flat reflective film with high light reflectance formed on the surface of a disk substrate as a recording film. In this optical storage medium, a laser beam is focused and irradiated onto a row of recesses, and the difference in the amount of light reflected from the reflective film and the recesses is detected as recorded information. Other optical storage media include
There is also a magneto-optical disk in which information is recorded by forming a plurality of minute magnetization reversal regions in an array on a uniaxial magnetic anisotropic recording film. In this optical storage medium, the difference in rotation angle of the plane of polarization of the reflected light from the magnetization reversal region array is detected as recorded information.

[0003] In these optical storage media, since reproduction is performed using reflected light from a recessed portion or a row of magnetization reversal regions serving as a recording portion, there is a limit to the areal density of such recording portions. When reproducing these optical storage media, the focal point of the laser beam is moved in the optical axis direction in order to follow the surface deflection of the optical storage medium, but this requires focusing for each recording section. Furthermore, since the optical reflectance of the reflected light and the rotation angle of the polarization plane are very small, the signal-to-noise ratio of the detected optical signal is low. Furthermore, since the time-series signal is reproduced only by moving the column of recording units, the access time for reproduction and recording is limited by the moving speed of the optical storage medium.

The optical waveguide storage medium and its reproducing apparatus disclosed in Japanese Patent Laid-Open No. 2-210627 have been developed to solve these problems. Furthermore, as a reproducing device for an optical waveguide storage medium, one having a Michelson interferometer type optical heterodyne detection optical system has been proposed. Such a reproducing device includes a collimation lens that converts the emitted laser beam from a light source into a parallel beam, a half mirror that splits the laser beam guided onto the optical waveguide storage medium, and a cup of one of the split laser beams on the optical waveguide storage waveguide. An objective lens is used to make the laser beam ring, a movable mirror is used to give a phase shift and a frequency shift to the other part of the split laser beam and use it as a reference beam, and the beam is reflected by the refractive index discontinuity created on the optical waveguide and returns again. The sensor is composed of a photodetector that interferes with the signal light and the reference light and heterodyne detects the optical output.

In such an optical waveguide storage medium reproducing device, a movable mirror is used as a means for imparting a phase shift and frequency modulation to the split laser beam for heterodyne detection, so there is a limit to the modulation frequency and the information density is limited. Improvement has been hindered. Furthermore, due to the presence of the mirror drive section, it is difficult to miniaturize and reliability of the reproduction optical system.

[0006]

OBJECTS OF THE INVENTION The present invention has been made in view of this difficulty, and it is an object of the present invention to provide a miniaturized optical waveguide storage medium reproducing device that can widen the modulation frequency of reference light.

[0007]

[Structure of the Invention] The optical waveguide recording medium reproducing device of the present invention includes:
It has an optical waveguide having an optical coupling part for introducing a laser beam, and a plurality of refractive index discontinuities arranged in the optical waveguide, and the shape and relative position of the refractive index discontinuities are information to be recorded. An apparatus for reproducing recorded information from a variable optical waveguide recording medium, the apparatus comprising: a light emitting means for generating a laser beam; a dividing means for dividing the laser beam into two to generate first and second light beams; a reference light generating means that receives one light beam and modulates it by applying a frequency shift to generate a reference light; an irradiation means that guides a second light beam to the optical coupling section; and a second light beam that is reflected by the refractive index discontinuity section. a light superimposing means for superimposing the reference light on the reflected signal light whose amplitude and phase are modulated and returns through the optical coupling section to form interference light; and a light detection means for photoelectrically converting the interference light to generate an electrical output. and a transparent acousto-optic medium block extending along the optical axis of the first light beam, and the reference light generating means disposed on an end surface of the acousto-optic medium block on the incident side of the first light beam. and an acousto-optic optical delay reflection element having an ultrasonic wave generating element that generates an acoustic wave in the light propagation direction of the acousto-optic medium block, and supplying intermittent high-frequency power to the ultrasonic wave generating element. Features.

[0008]

According to the present invention, there can be obtained an optical waveguide storage medium reproducing device having a heterodyne detection optical system in which the modulation frequency of the reference light is widened.

[0009]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating the principle of the present invention. First, in FIG. 1(a), the optical waveguide storage medium 1 has a structure in which a core optical waveguide 31 through which light is guided is formed on a substrate 32 forming a cladding having a refractive index lower than that of the core. Air or other cladding is present on the upper boundary surface of the core 31. The end face of the core 31 is an optical coupling part 30 that introduces the laser beam into the core. A plurality of refractive index discontinuities 34 are arranged and recorded in the elongation direction on the upper boundary surface of the inner surface of the core. The refractive index discontinuity portion 34 generates reflected guided light having various amplitudes and phases (i.e., amplitude and phase) based on a complex reflectance based on the relative position and shape from the optical coupling portion 30 on the end face with respect to the incident guided light of the laser beam. This is a minute recess that generates a modulated signal light. The shape and position of the refractive index discontinuities 34 are recorded to obtain a predetermined complex reflectance depending on the information to be stored. The concave portion of the refractive index discontinuity portion 34 may be an embedded portion. If air or cladding is used, the refractive index of the embedded portion being smaller than the refractive index of the core, the shape of the refractive index discontinuity portion 34 may be, for example, a semicircle or a cladding. It is an anti-elliptical embedded type, and its size is a fraction of the wavelength of light to several times the wavelength of light. As for the constituent materials, for example, the core 31 is made of polycarbonate which is transparent to light, and the cladding is made of a polymeric material such as polymethyl methacrylate having a lower refractive index. In this way, the optical waveguide storage medium 1 has at least the optical coupling part 3
0, a core 31, a substrate 32, and a refractive index discontinuous portion 34.

Specifically, as shown in FIG. 2, in the optical waveguide storage medium 1, a plurality of channel type ridge optical waveguides 31 each having an end face of an optical coupling part 30 are arranged in parallel on a substrate 32, and the optical waveguides are arranged in parallel on a substrate 32. There is also one in which a plurality of minute refractive index discontinuities 34, in which a plurality of reflected guided light beams with different amplitudes and phases are generated within the core, are arranged in accordance with the information to be recorded. Although this embodiment is explained as a ridge type waveguide, in the case of a channel type waveguide such as a strip type or a buried type, such a refractive index discontinuity portion 34 may be provided in the core or cladding of the optical waveguide. The same effect can be obtained.

As shown in FIG. 1(a), the optical waveguide storage medium reproducing apparatus of the present invention uses an SLD (Super Luminescensor) as a light emitting means for generating a laser beam.
A light emitting element 35 such as a broadband wavelength oscillation laser diode or a broadband wavelength oscillation laser diode is used to divide the laser beam into two parts.
and a half mirror (beam splitter) 36 as a splitting means for generating a second light beam, and an acoustic mirror as a reference light generating means for receiving the first laser beam and modulating it by applying a Doppler frequency shift to generate a reference light. The optical delay reflection element 37, the objective lens 42 as an irradiation means for guiding the second laser beam to the optical coupling section 30, and the reflected signal light and reference light returned by the refractive index discontinuity section 34 are superimposed to produce interference light. a half mirror 36 as a light superimposing means,
It has a photodetector 39 as a photodetection means that photoelectrically converts the interference light and generates an electrical output.

A laser beam emitted from a light emitting element 35 disposed opposite to the optical coupling section 30 of the optical waveguide storage medium 1 is converted into a substantially parallel beam by a collimation lens 41 and divided into two by a half mirror 36 . One of the first laser beams traveling straight is focused by the objective lens 42 and guided from the optical coupling section 30, and a part of it is transmitted through the plurality of refractive index discontinuities 34.
This results in a plurality of reflected waveguide lights having different amplitudes and phases, which become signal lights returning from the optical coupling section 30. Figure 1 (
As shown in a), the refractive index discontinuities 34 are a, b, c,
When the information to be recorded is recorded at the position d, the shape and relative position of each refractive index discontinuity portion 34 are determined according to the information to be recorded (explained as analog information in the figure). By arranging a plurality of refractive index discontinuities 34 in the refractive index discontinuous portion 31, it is possible to create modulated signal light having different amplitude and phase information as a function of its shape and propagation distance. The other second laser beam split and reflected by the half mirror 36 is passed through the acousto-optic delay reflection element 37 by the lens 43.
The light becomes a reference light that is focused into the optical coupling part of the light beam, undergoes a Doppler frequency shift, and returns. These signal beams and reference beams are multiplexed by a half mirror 36, condensed by a lens 44, and input into a photodetector 39 through optical heterodyne interference. The input light is photoelectrically converted and becomes an electric signal through a frequency filter 40, and as shown in FIGS. An electrical output I(t) of the signal waveform is obtained.

As shown in FIG. 3, the acousto-optic delay reflection element 37 includes a transparent acousto-optic medium block 100 which extends along the optical axis of the first light beam using a half mirror 36, and a transparent acousto-optic medium block 100 that extends along the optical axis of the first light beam. The ultrasonic generating element 101 is disposed on the laser beam incident end face and generates ultrasonic waves in the light propagation direction of the acousto-optic medium block. The ultrasonic wave generating element 101, as shown in FIGS. 4(a) and 4(b),
The ultrasonic wave generating element 101 is preferably an electrostrictive element or a piezoelectric element having an opening 101a through which a laser beam is incident, and may be formed from an electrostrictive element or a piezoelectric element that is transparent to the incident laser beam. For example, the acousto-optic delay reflection element 37 is made of an acousto-optic medium block 100 made of bulk crystal such as quartz, glass, or TeO2, and has an electric current made of barium titanate or lead zirconate titanate-based dielectric material on the incident end surface. A strain element or a piezoelectric element made of crystal or the like is created as the ultrasonic wave generating element 101. When intermittent high-frequency power is supplied to these ultrasonic generation elements, the propagation of ultrasonic waves (concentration traveling waves in the medium) creates lattice-like stripes with a refractive index that changes at intervals of the wavelength of the ultrasonic waves in the acousto-optic medium block. As this wave packet further propagates through the medium, the laser beam in the acousto-optic media block is given a phase delay and frequency shift due to the Doppler effect of the ultrasound, and a part of it is reflected along the acousto-optic media block. and return. This reflected light is used as reference light.

As described above, in this embodiment, the conventional movable mirror for generating reference light is abolished, and instead, an ultrasonic wave generating element, which is an acousto-optic modulating element driven by a burst-shaped modulation signal, is used as a light wave delay reflection device. is used. This acousto-optic modulation element has a structure in which a laser beam entrance window is provided on the surface of the ultrasonic generation element, and the direction of incidence of the laser beam is parallel to the propagation direction of the elastic waves generated within the acousto-optic modulation element. have. A propagating compressional wave is formed within the acousto-optic modulation element driven by the burst modulation signal. When the incident laser beam passes through this traveling compression wave, it interacts with the diffraction grating created by the compression wave to produce reflected light, which is frequency modulated by the Doppler effect, and furthermore, the location where this compression wave is coordinated is Since it is moving, it undergoes a phase shift. Therefore, this laser beam can be used as a reference beam. After all, this is equivalent to moving a conventional movable mirror at a certain speed and reflecting the laser beam as a reference wave.

[0015] As described above, by using an acousto-optic light wave delay reflection element, it is possible to downsize the optical delay reflection device using a conventional movable mirror, and it is possible to reduce the size of the optical waveguide recording medium more than the conventional optical delay reflection device using a movable mirror. It is also possible to read information from waveguides as long as several millimeters. The principle of the playback device will be explained below. Each frequency component of the laser beam from the light-emitting element 35 is simultaneously reflected by one refractive index discontinuity and receives a constant amplitude and a phase delay proportional to the propagation distance, and the reference beam and Doppler frequency shifted corresponding to the phase delay are generated. have a finger in the pie. At that time, each frequency component becomes in phase, forming one pulse having an amplitude that is the sum of the amplitudes of each frequency component. The reflected component from the next refractive index discontinuity whose phase is further delayed interferes with the reference light whose Doppler frequency has been shifted with a time delay, and forms the next pulse. Here, in the orthogonal coordinate xyz shown in FIG. 1(a), if the z-direction of extension of the optical waveguide 31 is vertical and the y-direction perpendicular to this is horizontal, each pulse width is equal to the z-axis direction of the refractive index discontinuity. Each amplitude depends on the size of the refractive index discontinuity in the x-axis and y-axis directions, respectively.

For example, the center frequency ν of the SDL of the light emitting element
Let us consider the case where the effective spectral width of this light source is δν, the propagation constant of the optical waveguide is βο, the higher-order one is β1, and the effective Doppler velocity of the acousto-optic light wave delay reflection element is v. If the electric field of the reference light is Eref and the electric field of the reflected light from the p-th refractive index discontinuity is Ep, then the intensity I(z, t) of the detection signal at the photodetector is expressed by the following formula 1.

[
0017

[Math 1]

It can be described as follows. A detection signal expressed by Equation 1 is detected by a photodetector. As is clear from Equation 1, the beat output signal is an incoherent superposition of a wide spectrum, and is detected as a pulse train subjected to exponential amplitude modulation. In this way, the configuration of the optical waveguide recording medium reproducing apparatus allows a plurality of refractive index discontinuities (a in FIG. 1(a),
b, c, d) correspond to time-series signal electrical outputs (in Fig. 1(c), a', b', c', d'
corresponding to each other), are played. Each pulse is si
It has a Nyquist sampling type of n(F)/F, and the signal waveform is a series of these. F is a light emitting element 35
It is a function of the emission spectrum width of the optical waveguide, the acoustic modulation frequency or the speed of the movable mirror, the refractive index dispersion of the optical waveguide, and the length and position of the refractive index discontinuity in the optical waveguide medium. By selecting these values appropriately, the range from several KHz to several tens of M
Signal modulation frequencies can be set up to Hz. The width of each pulse can also be set from several tens of milliseconds to several tens of nanoseconds. The light emitting element 35 does not necessarily require a semiconductor laser with a narrow spectrum width; rather, a semiconductor light emitting element with a wide spectrum width close to incoherent light can set the modulation frequency higher. In addition, by optical heterodyne interferometry,
These time-series signal waveforms can be reproduced with a high signal-to-noise ratio of 102 to 104.

FIG. 5 specifically shows an optical waveguide storage medium and a reproducing device. The optical waveguide storage medium 50 is a laminated drum-type three-dimensional optical disk made by arranging a large number of plate-like bodies 51 of the channel-type ridge optical waveguide storage medium 1 described above and winding them into a drum shape. In addition to the above-mentioned components, the optical waveguide storage medium reproducing device further includes a condenser lens 42 for coupling to the optical waveguide installed in the focus actuator, a beam splitter 52 for taking out a part of the reflected light beam for tracking, and a tracking lens. a concave lens 53 that separates the laser beam for use, and a photodetector 54 for tracking.
, 55. Both ends in the long axis direction of the elliptical cross section of the laser beam from the light emitting element 35 are applied to the clad end face with the core end face optical coupling part in between, and the reflected light is used as a tracking laser beam. The photodetectors 54 and 55 capture the tracking laser beam with high sensitivity and high signal-to-noise ratio by optical heterodyne detection. Further, in accordance with error detection between the two, the focusing microlens 42 and the like are moved in the optical axis direction to focus on the optical coupling portion of the optical waveguide, and the optical reproducing device head 56 is caused to follow the surface deflection of the three-dimensional optical disk 50. .

In this example, a plate-like body in which optical waveguides each having a rectangular cross section of 2×2 μm and a waveguide length of 10 mm are embedded and juxtaposed in a 3.2 μm thick cladding at 2 μm intervals is used.
Rolled and laminated into a drum shape, tracking pitch width 3.2μ
m three-dimensional optical disks 50 are formed. The length of the recorded refractive index discontinuity in the z-axis direction is 10 to 30 μm, x
The depth in the axial direction is 0.1-0.5 μm, the width in the y-axis direction is approximately 0.7 μm, and an average of 500 such discontinuities are recorded for each optical waveguide. Since the optical reflectance of each of the refractive index discontinuities is at most 10-6 to 10-8, even with these reflection losses, the attenuation rate of the final returning laser beam is 1.
It is about 0%. In the figure, the cross section of each optical waveguide is opened at the lower end surface of the three-dimensional optical disk 50, and serves as an optical coupling section. A polycarbonate protective film with a thickness of 2 mm is attached to the surface of the optical coupling portion, and the degree of optical coupling is increased by matching the refractive index of the optical waveguide. In addition, the end face of the optical waveguide is similarly protected,
At the same time, the light propagating through the optical waveguide is allowed to escape. Such a three-dimensional optical disc 50 has a diameter of 8 inches, and the stored information is reproduced while rotating like a compact disc (CD).

The optical waveguide storage medium reproducing device is constructed as explained in the section [Structure of the Invention]. The laser beam from the light emitting element 35 is guided to the optical waveguide storage medium 51, a part of which returns as a signal reflection laser beam whose amplitude and phase are modulated, and the other laser beam is transmitted to the acousto-optic optical delay reflection element 37. 1 (c ) of I
An electrical output of the time-series signal waveform shown in (t) is obtained. 1
The reproduced signals from the two optical waveguide storage media are temporarily stored in a buffer memory and transferred at an arbitrary clock time. 1
After reading out the information stored in one optical waveguide, the information stored in the optical waveguide of the next channel is sequentially read out by rotating the stereoscopic optical disk 50 and tracking the optical reproducing device head 56.

The specifications of this embodiment are, for example, typically, an emission center wavelength of 1.3 μm and an emission spectrum width of approximately 2×10 1
A 2 Hz SLD was used as the light emitting element 35, an acoustic modulation frequency of 2.75 GHz was used, and an optical waveguide with a refractive index dispersion of 0.14 was provided with an average length and relative position of 20
In the case of the optical waveguide storage medium of this embodiment in which data is recorded in μm, a storage/reproduction frequency of about 30 MHz can be achieved. At this time, the minimum pulse width is about 35 nanoseconds. The light output of the light emitting element 35 is about 1 mW, and the signal-to-noise ratio of the optical heterodyne interference output based on the reflected light from each of the refractive index discontinuities and the reference light is as large as 104. The total storage capacity in terms of digital amount is 500 times that of the current compact disk (1 Gbyte: gigabyte), the mechanical access time per bit is 1/500, and the bit cycle time is approximately 14
It can be doubled.

In this embodiment, an example was explained in which the memory section of the optical waveguide storage medium is composed of long and short refractive index discontinuous parts, and an analog signal is stored and reproduced. It is clear that if the presence or absence of these refractive index discontinuities is recorded at regular intervals, it is possible to store and reproduce digital signals. Furthermore, although an embodiment has been described in which the optical waveguide storage medium is formed into a three-dimensional disk, it is also possible to realize a structure in which the optical waveguide storage medium is arranged side by side on a tape or in a card type and stacked. Further, in this embodiment, the optical waveguide length was set to 10 mm, but the optical waveguide length as a storage medium can be made longer or shorter depending on the required memory capacity.

[0024]

[Effects of the Invention] As explained above, according to the present invention,
An acousto-optic optical delay reflection element is used as a reference light generating means in an optical waveguide storage medium reproducing device having a heterodyne detection optical system, and the acousto-optic optical delay reflection element produces a transparent acoustic wave that extends along the optical axis of the incident laser beam. It has an optical medium block and an ultrasonic generation element disposed on the end face of the acousto-optic medium block on the laser beam incident side and generates ultrasonic waves in the light propagation direction of the acousto-optic medium block, and the ultrasonic wave generating element has an intermittent Since high-frequency power is supplied, it is possible to obtain an optical waveguide storage medium reproducing device in which the modulation frequency of the reference light is widened.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram illustrating the principle of an optical waveguide storage medium reproducing device of the present invention.

FIG. 2 is a perspective view of an optical waveguide storage medium according to the present invention.

FIG. 3 is a plan view of an acousto-optic delay reflection element according to the present invention.

FIG. 4 is a perspective view of an acousto-optic delay reflection element according to the present invention.

FIG. 5 is a schematic diagram of an optical waveguide storage medium reproducing apparatus according to an embodiment of the present invention.

[Explanation of symbols]

30... Coupling portion of optical waveguide 31... Core of optical waveguide 32... Clad (substrate) of optical waveguide 34... Refractive index discontinuity portion 35... Light emitting element 36... Half mirror 37... Acousto-optic optical delay Reflection element 39... Photodetector 40... Frequency filters 41 to 44... Lens 50... Laminated drum type three-dimensional optical disk 51... Optical waveguide storage medium 52... Beam splitter 53... Concave lenses 54, 55... Tracking Photodetector 56... Optical reproducing device head 57... Electrical output terminal 58... Iris 100... Acousto-optic medium block 101... Ultrasonic wave generating elements 101a, 101b, 101c... Comb-shaped electrode 102...
…substrate

Claims (9)

[Claims] 1. An optical waveguide having an optical coupling part for introducing a laser beam, and a plurality of refractive index discontinuities arranged in the optical waveguide, and the shape and relative position of the refractive index discontinuities are different. A device for reproducing recorded information from an optical waveguide recording medium serving as a variable of information to be recorded, comprising a light emitting means for generating a laser beam, and a first for dividing the laser beam into two.
and a splitting means for generating a second light beam, a reference light generation means for receiving the first light beam and modulating it by applying a frequency shift to generate a reference light, and guiding the second light beam to the optical coupling section. a light superimposing means for superimposing the reference light on the reflected signal light that is reflected by the refractive index discontinuity portion, modulated in amplitude and phase, and returned via the optical coupling portion to form interference light; and a light detection means for photoelectrically converting light to produce an electrical output, the reference light generation means being
a transparent acousto-optic medium block extending along the optical axis of the light beam; and a transparent acousto-optic medium block disposed on the end surface of the acousto-optic medium block on the incident side of the first light beam, and generating an acoustic wave in the light propagation direction of the acousto-optic medium block. 1. An optical waveguide recording medium reproducing device comprising: an acousto-optic optical delay reflection element having an ultrasonic wave generating element, and intermittent high frequency power is supplied to the ultrasonic wave generating element. 2. The optical waveguide recording according to claim 1, wherein the amplitude and delay time of the reflected signal light are detected as a time-series electric signal by detecting a beat output component from the light detection means. Media playback device. 3. The optical waveguide recording medium reproducing apparatus according to claim 1, wherein the ultrasonic wave generating element comprises an electrostrictive element or a piezoelectric element having an aperture through which the laser beam is incident. 4. Claim 1 or 2, wherein the ultrasonic wave generating element is made of a transparent electrostrictive element or a piezoelectric element.
The optical waveguide recording medium reproducing device described above. 5. The optical waveguide recording medium reproducing apparatus according to claim 1, wherein the light emitting means includes a superluminescent diode or a broadband wavelength oscillation laser diode. 6. The optical waveguide recording medium reproducing apparatus according to claim 1, wherein the dividing means includes a half mirror or a beam splitter. 7. The optical waveguide recording medium reproducing apparatus according to claim 1, wherein the irradiation means has an objective lens. 8. The optical waveguide recording medium reproducing apparatus according to claim 1, wherein the light superimposing means includes a half mirror or a beam splitter. 9. The optical waveguide recording medium reproducing apparatus according to claim 1, wherein the photodetecting means includes a photodetector.