JPS58220254A - Optical modulator - Google Patents

Abstract

PURPOSE:To ensure constitution of an optical system in a simple way and with a low cost, by modulating the light by means of an acoustic-optical modulator and a stepped digital multi-value signal of >=3 ternary value, and controlling the intensity of output light with high accuracy. CONSTITUTION:The laser light LO is supplied to an acoustic-optical modulator 11 from a laser light source 12. A digital multi-value signal SI is supplied to an amplitude modulator 14 through a terminal 13. At the same time, a high frequency signal is supplied to the modulator 14 as a carrier SC from a high frequency oscillator 15. The modulator 14 produces a signal SI' which is smoothed and modulated by an information signal SI. This signal SI' is supplied to the modulator 11 as a modulated signal via an amplitude modulator 16. The modulator 16 applies further modulation of amplitude to the signal SI' on the basis of a comparison error signal SCa obtained from a comparator 21. Therefore the primary diffracted light D1 delivered from the modulator 11 is set at a prescribed level with control of the light intensity.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical modulation device suitable for use in, for example, an optical recording device that records digital signals of three or more values on a video disc or a digital audio disc.

Figure 1 shows the configuration of a commonly used optical recording device. In the figure, a laser beam from a recording laser light source (1) passes through a mirror (2) to an optical modulator (3).
), where it is modulated by the information signal to be recorded, and is further supplied to the beam slittor (4), shutter (5), lens (6), mirror (7) and objective lens (8). ) and other lens systems (9)
An information signal is provided as a micro-diameter spora L on a photosensitive layer (for example, a photoresistor) and is recorded on a recording disk (9) that is rotated by a motor 00.

In this case, if the information signal is a digital signal of three or more values, it is advantageous because the recorded information density can be increased. However, in order to record a step-like digital multilevel signal on the recording disk (9) using the optical recording device shown in FIG.
The light can be sufficiently modulated with this high-speed step-like digital multilevel signal, and the exposure level, that is, the output light intensity of the optical modulator (3), can be adjusted precisely for each value in accordance with the sensitivity characteristics of the photosensitive layer. It is necessary to control the temperature to a predetermined level.

Conventionally, optical modulators (3) include electro-optic optical modulators that utilize the so-called electro-optic effect, in which the refractive index of a crystal changes when an electric field is applied to the crystal, or so-called electro-optic optical modulators, in which the refractive index in a medium changes by sound waves. Acousto-optic modulators that utilize the acousto-optic effect are used.

Electro-optic light modulators can perform high-speed modulation without distorting the laser beam (2), but the modulation bias point changes due to the expansion of the crystal due to temperature changes, and the bias point changes depending on the sensitivity characteristics of the photosensitive layer. Exposure level, i.e. light modulator (3
) There is a difficulty in precisely controlling the output light intensity of each individual device to a predetermined level.

It is conceivable to use a separate electro-optic light modulator to control the output light intensity, but this electro-optic light modulator is expensive, and using two of them means that the overall It becomes expensive.

Furthermore, the optical system becomes complicated.

The present invention has been made in view of this point, and is configured using one acousto-optic modulator, which can sufficiently modulate light with the above-mentioned high-speed step-like digital multilevel signal, and whose output The present invention aims to propose a light modulation device in which the light intensity is precisely controlled to a predetermined level for each individual light modulator.

Hereinafter, with reference to FIG. 2, a case will be described in which an embodiment of the optical modulation device according to the present invention is applied to an optical recording device.

In Fig. 2, α is an acousto-optic light modulator, and this optical modulator α is supplied with laser light L from a laser light source (6).
O (argon laser light, helium cadmium laser light) is supplied.

Further, in FIG. 2, θ] is a terminal to which an information signal SI to be recorded (shown in FIG. 3B) is supplied. There are three or more information signals SI, in this example,
It is a three-value digital signal of now, Itljl, and Pa2''. This information signal SI is supplied as a modulation signal to an amplitude modulator 04, a balanced modulator in this example. This amplitude modulator α→ High frequency oscillator (c) For example 1
A 10 MHz high frequency signal is supplied as carrier 8C.

The modulation characteristics of this amplitude modulator (14) are as shown in FIG. 4, for example. This amplitude modulator α→ has the modulation characteristics as shown in FIG. By adjusting the sensitivity, a modulation characteristic as shown in FIG. 4 was obtained.

As a result, in this amplitude modulator 04, a signal SI' (shown in FIG. 3C) is obtained in which the carrier SC supplied by the oscillator is balanced-modulated with the information signal SI. This signal SI' is supplied as a modulation signal to the optical modulator 01) via the amplitude modulator 0→.

The optical modulator a is an amplitude modulator C14 as shown in FIG.
a converter (l1m) that converts the signal SI' supplied from the amplifier via the amplitude modulator Q into an ultrasonic signal;
A medium (1 lb) that causes optical modulation by mutual interference with
and a sound-absorbing material (ll) that absorbs ultrasound signals and converts them into heat.
c). Signal S supplied from amplitude modulator 04
I' is supplied to a transducer (l1m) and converted into an ultrasound signal. Then, the refractive index of the medium (llb) is locally changed, a diffraction grating is formed with a pitch corresponding to the wavelength of the sound wave, and light passing through the grating is diffracted.

There are various ways in which this diffracted light beam is generated depending on the wavelength of the sound wave and light wave and the shape of the sound beam, and typical examples include Ramannus scattering and Brillian scattering. The distinction between Ramannus scattering and Brillouin scattering is based on the relationship between the wavelength 20/n of light in a medium with a refractive index n, the wavelength A of the sound wave, and the width W of the sound beam extending in the direction of propagation of the light beam (5). It is determined. That is, if Klein constant Q is defined by the following formula, Q =
2xWλ0/nA2... (1) Ramannus scattering occurs between 4π>Q>00, and Brilliant scattering occurs between Q>4
Happens at π.

Diffraction due to Ramannus scattering is similar to the scattering of light from a diffraction grating made in the shape of a sinusoidal distribution, and is symmetrical in the direction of sound wave propagation with the 0th-order diffracted light traveling straight in the direction of propagation of the incident light as the center. Higher order diffracted light such as ±1st order, ±2nd order, etc. is generated. On the other hand, diffraction due to Brilliant scattering is
As shown in FIG. 3, in addition to the 0th-order diffracted light, only one 1st-order diffracted light is generated.

When performing optical modulation recording, it is inappropriate to use high-order diffracted light because it will degrade the efficiency when considering the aim of improving laser light efficiency. is good. Therefore, in this example, the 0th optical modulator is used under conditions that cause Brilliant scattering, and the 1st-order diffracted light is generated in addition to the O-order diffracted light from the α power of this optical modulator, and this 1st-order diffracted light (6 ) Light is used.

This first-order diffraction light intensity I is expressed by the following equation.

I, = IoRsdn2(η town (2) this (2)
In the equation, ■o is the incident light intensity, R is the loss rate of the amount of light on the surface and inside of the transformation element, and η is a value given by the following equation.

In this equation (3), W is the width of the sound bundle (cross-sectional width of the ultrasound beam), h is the height of the sound bundle (height in the direction perpendicular to the width of the ultrasound beam cross-section), Ps is the sound wave input, Me is the deflection efficiency figure of merit of the medium. This Ma is also expressed by the following formula.

In this equation (4), n is the refractive index of the medium, p is the photoelastic constant, ρ is the density of the medium, and V is the speed of sound. This Me is used to compare the deflection intensity depending on the type of medium when the wavelength of the light, the cross-sectional shape of the ultrasonic beam, and the sound wave input are constant.The larger the value of Me, the stronger the diffracted light, resulting in a modulation effect It's also big.

According to the following relational expression 1, the intensity of the first-order diffracted light is the sonic power P8
The intensity is modulated by This sound wave input Ps is a signal supplied to the converter (lla) of the optical modulator θ°C, that is, a signal 8I' (see FIG. 3C) supplied via the amplitude modulator 04 and the amplitude modulator αQ. (as shown) is determined by the magnitude of the amplitude. Therefore, the intensity I of the first-order diffracted light is modulated by the information signal SI.

As explained above, the optical modulator 01) outputs the 0th-order diffracted light and the 1st-order diffracted light whose intensity is modulated by the information signal SI (shown in FIG. 3G). This first-order diffracted light is detected by a photodetector a'/).

Note that this first-order diffracted light D1 is transmitted to a recording signal by a beam sinter (corresponding to the beam sinter (4) in FIG. 1) disposed between the optical modulator 0η and the photodetector aη (not shown). It is then taken out as a recording medium and further supplied to a recording medium via an optical system.

The first-order diffracted light detected by the photodetector is converted into an electrical signal according to its intensity. After this electrical signal is amplified by an amplifier M, it is supplied to a sample and hold circuit α. This sample hold circuit OI is supplied with a sample hold Ps as shown in the third diagram across the terminal. This sample t4 Lus P Hatake is the information signal SI mentioned above.
(shown in FIG. 3B). Therefore, in this sample and hold circuit (to), the first-order diffracted light corresponding to the intermediate level "1" of the information signal SI is , only the electrical signal corresponding to the light intensity of the information signal SI is sampled and its value is held.
The reason why the part corresponding to level "1" is sampled is because this level "1#" appears most frequently. This held value is supplied to the comparator circuit 0! and
Output from terminal (A): reference voltage vr6f
compared to This reference voltage vrof is set to a value such that the light intensity of the first-order diffracted light is at an optimum level taking into consideration the sensitivity of the recording material and the like.

The comparison error signal Sea obtained by this comparison circuit is
loop gain control circuit) and a phase compensation circuit (c) for compensating for the phase shift between the information signal SI and the first-order diffracted light.
(9) is supplied as a modulation signal to the amplitude modulator αQ via This amplitude modulator edge is a balanced modulator,
Its modulation characteristics are, for example, as shown in FIG. This amplitude modulator a→ is similar to the above-mentioned amplitude modulator α→, for example, in an amplitude modulator having a modulation characteristic as shown in FIG. 5, the applied bias and sensitivity are adjusted, The modulation characteristics are obtained.

Eventually, in this amplitude modulator 4, the amplitude modulator Q4
The signal SI' (shown in FIG. 3C) provided by the SI' is further amplitude modulated based on the comparison error signal Sea. This means that the light intensity of the first-order diffracted light output from the optical modulator α is controlled so that it reaches a predetermined level.

Here, consider a case where the laser light LO supplied to the optical modulator α is accompanied by a fluctuation amount Δf as shown in FIG. In this case, the comparison error signal SCa obtained from the comparator (c) is as shown in E of the same figure, and the signal as shown in F of the same figure is obtained from the amplitude modulator 0, and as a result, From the optical modulator a9, an information signal S is output as shown in G in the same figure.
A first-order diffracted light D0 having a light intensity of (10) at a predetermined level is obtained for each of I.

As is clear from the embodiments described above, the optical modulation device according to the present invention is configured using one acousto-optic modulator, and the step-like digital multivalue signal of three or more values is sufficient to provide light. can be modulated, and its light intensity can be precisely controlled to a predetermined level. Therefore, it is suitable for use in optical recording devices such as those described above.

According to the above embodiment, amplitude modulator 04→amplitude modulator (
Although they are arranged in the order of 1→→optical modulator αη, they may be arranged in the order of amplitude modulator OQ→amplitude modulator Q4→optical modulator αη. Further, in the above-described embodiments, the case where the present invention is applied to an optical recording device has been described, but it goes without saying that the present invention can be similarly applied to other devices having such a function.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of a conventional optical recording device, FIG. 2 is a block diagram showing an embodiment of a light modulation device according to the present invention, and FIGS. 2 is a diagram for explaining the example in FIG. 2, and FIG. 6 is a configuration diagram showing a specific example of the acousto-optic modulator used in the example in FIG. 0◇ is an acousto-optic optical modulator, (6) is a laser light source, 04
and αQ are respectively amplitude modulators, and αυ is a high frequency oscillator. 31

Claims (1)

[Claims] An amplitude modulator that modulates a carrier with a signal of three or more values, an acousto-optic modulator that modulates light with the output of this amplitude modulator, and an acousto-optic modulator that converts the output of this acousto-optic modulator into an electrical signal and uses it as a reference signal. An optical modulation device comprising a comparator for comparison and an amplitude modulator for modulating the carrier with a comparison error signal from the comparator.