JPH0358090B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an acousto-optic modulator, particularly an acousto-optic modulator suitable for stabilizing laser light intensity.

This type of acousto-optic modulator uses an optical method, for example.
Used for PCM audio disc recording, etc.
By stabilizing the intensity of the laser beam, the diameter of the pits on the disk can be controlled correctly, allowing for the input of regular signals. In the conventional acousto-optic modulator, as shown in FIG. 1, a laser beam 1 first enters an acousto-optic medium 2 at a Bragg angle θ _B with respect to the wavefront of an ultrasonic signal, which will be described later.
A transducer 3 is installed in the acousto-optic medium 2 to constitute an acousto-optic modulator, and the transducer 3 is excited by a high-frequency signal from a high-frequency modulator 11, which will be described later, to generate ultraviolet light in the medium 2. Since a sound wave signal is being propagated, after the laser beam 1 described above enters the acousto-optic medium 2, the zero-order beam 4 that travels straight,
The wavefront of the ultrasonic signal is separated into diffracted light 5 polarized at an angle 2θ _B and sent out. Next, the zero-order light 4 is separated by a beam splitter 6, which is an optical separation means, into a passing light 7 that travels straight through the main light intensity and a detection light 8 that monitors the remaining light intensity. It is converted into voltage by the detector. Next, as is well known, this converted voltage is compared with a reference voltage in a differential amplifier 10 serving as a comparing means, and if a voltage difference occurs between the two, that voltage difference is used for the variable high frequency modulation. 11, and the high frequency modulator 11 responds to the voltage difference so that the reference light amount corresponding to the reference voltage and the light amount of the transmitted light 7 that passes through the beam splitter 6 are substantially equal. Control the electrical input energy of high frequency signals. In this way, the light intensity of the transmitted light 7 sent out from the beam splitter 6 is stabilized.

In an acousto-optic modulator having such a configuration,
For example, when the light intensity of the laser beam 1 becomes larger than the reference light intensity corresponding to the reference voltage due to the influence of ripples, noise, etc., when the voltage converted by the photodetector and the reference voltage are compared in the differential amplifier 10, The converted voltage becomes larger with respect to the reference voltage.

At this time, the resulting voltage difference between the two voltages is supplied to the high frequency modulator 11, and the electrical input power of the high frequency signal is controlled to increase. As a result, the diffraction efficiency of the ultrasonic wave propagating through the acousto-optic medium 2 increases, thereby reducing the amount of zero-order light and making it possible to make the reference light amount and the amount of zero-order light substantially equal.

On the other hand, when the amount of laser light decreases, the amount of diffracted light is reduced and the amount of 0th order light is increased by acting in the opposite manner to the above, and the amount of light of the 0th order light and the reference amount of light are substantially equal to each other. can be made equal.

By the way, the diffraction efficiency of an acousto-optic modulator, that is, the value of the ratio of the incident light intensity I ₀ to the deflected light intensity I ₁ , is calculated by the formula
Indicated by (1).

η=I ₁ /I ₀ = sin ² (π/λ ₀ )√・・2 (1) Here, π: Pi λ ₀ : Wavelength of light Pa: Input acoustic power L: Length of sound field H : Height of the sound field Next, Me is a constant determined from the physical constants of the acousto-optic medium, and is defined by equation (2).

Me=n ⁶ P ² /ρV ³ (2) where, n: refractive index P: photoelastic constant ρ: density V: sound velocity Since the acousto-optic medium 2 is generally not isotropic, its photoelastic constant P and The refractive index n has different values depending on the slope of the polarization plane of the incident laser beam 1,
The value of Me is not constant. When polarized light such as laser light is incident on an acousto-optic modulator, the following disadvantages occur.

Depending on whether the plane of polarization of the incident laser beam 1 is parallel or perpendicular to the direction of ultrasound propagation within the acousto-optic medium 2 as shown in FIG. 2A, the electric field shown in FIG. The diffraction efficiency characteristics with respect to the input power are different as shown by curve a when the input power is parallel, and curve b when the input power is perpendicular. For example, the diffraction efficiency η of curves a and b is when the electrical input power is 0.2 (watt): η _a = 0.46 and η _b =
0.35, when the electrical input power is 0.6 (watt): η _a =
0.9 and η _b =0.75. This phenomenon shows that the inclination of the polarization plane of the incident laser beam 1 with respect to the ultrasonic propagation direction depends on the diffraction efficiency depending on the electric input power, and this means that the inclination of the polarization plane of the incident laser beam 1 with respect to the ultrasonic propagation direction depends on the diffraction efficiency. When the polarization plane is incident on the incidence plane of the acousto-optic medium at an angle α, the laser polarization plane of the diffracted light 5 becomes an angle γ different from the angle α, and the zero-order light 4
The laser polarization plane also has an angle β different from the angle α. The angles γ and β also differ depending on the value of the electrical input power. Furthermore, by applying a high frequency signal to the transducer 3, approximately half of its electrical input power is substantially converted into heat, which is transmitted to the acousto-optic medium 2, causing thermal distortion and thermal birefringence. As a result, the polarization plane of the laser beam rotates after passing through the acousto-optic medium 2.

These phenomena affect the separation ratio between the passing light 7 and the detection light 8 by the beam splitter 6, and appear as a drawback in the acousto-optic modulator by reducing the stability of the output light amount of the passing light 7.

The present invention has been made to eliminate such drawbacks, and is an acousto-optic device in which laser light intensity is highly stabilized by inserting a polarizer between an acousto-optic modulator and an optical separation means. The purpose of the present invention is to provide a modulation device.

Embodiments according to the present invention will be described below with reference to the drawings.

FIG. 3 shows an embodiment of the acousto-optic modulator according to the present invention, and the same reference numerals as those shown in FIG. 1 indicate the same functional parts.

In this example. The laser beam 1 is a He-Ne laser beam (wavelength: 6328 Å), the acousto-optic medium 2 is a lead molybdate (PbMoO ₄ ) single crystal, the transducer 3 is a lithium niobate (LiNbO ₃ ) 36 degree Y plate, The beam splitter 6 is made of optical glass BK-7, the photodetector 9 is made of a silicon photodiode, and the slit 12 is made of optical glass BK-7.
The polarizer 13 uses a black-painted aluminum plate with a small hole of 2 mmφ, which is twice the beam diameter, and a polarizing beam splitter. still,
Since a lithium niobate (LiNbO ₃ ) 36 degree Y plate is used as the transducer 3,
Longitudinal ultrasound waves propagate within the acousto-optic medium 2.

The diffracted laser beam 5 diffracted by the acousto-optic modulator is absorbed and blocked by the black painted portion of the slit 12, and the zero-order laser beam 4 passes through the small hole of the slit 12 and enters the polarizer 13. This polarizer 13
As shown in Figure 4, the slope of the plane of polarization is 0-order light 4
The reference slope of the selected plane of polarization, that is, Y
It is aligned with the axis. This Y-axis is shown in Figure 2 C.
As shown in , it is also the inclination that serves as a reference for the plane of polarization of the laser beam 1 that enters the acousto-optic medium 2 . As a result, the zero-order light 4 emitted from this polarizer 13 is
The light is emitted with the slope of the polarization plane defined by the selected polarization plane. As a result, the inclination of the plane of polarization of the zero-order light 4 incident on the beam splitter 6 is always kept constant, so that the separation ratio by the beam splitter 6 is also always constant.

Therefore, when the zero-order laser beam 4 changes, it appears as a change in the light intensity |E| and a change in the slope θ of the polarization plane of the laser beam, but when this zero-order laser beam 4 enters the polarizer 13, As shown in FIG. 4, the amount of transmitted light becomes |E|cosθ and is rectified and detected. As a result, even if the zero-order laser beam 4 emitted from the polarizer 13 is tilted by θ with respect to the polarization plane of the reference zero-order light at the time it is emitted from the acousto-optic modulator, the polarization is By passing through the beam splitter 13, when the light is incident on the beam splitter 6, it is defined by the inclination of the plane of polarization, which is the reference for the zero-order light. That is, the zero-order light that has passed through the polarizer 13 enters the beam splitter 6 with substantially the same inclination as the tilt of the plane of polarization that is the reference for the zero-order light. In this way, the zero-order laser beam 4 transmitted through the polarizer 13 is separated into the transmitted light 7 and the detection light 8 at the beam splitter 6, and the detection light 8 is transmitted to the photodetector 9 as described above |E|
COSθ is detected as a variable light amount and converted.
This voltage and a reference voltage are compared by a differential amplifier 10, and the resulting voltage difference is supplied to a high frequency modulator 11. Since the diffraction efficiency depends on the electrical input energy as shown in FIG. 2B, in the high frequency modulator 11, the high frequency signal is Control input energy. As a result, it becomes possible to stabilize the light intensity of the transmitted light 7, and the phenomenon of difference in diffraction efficiency due to the inclination of the polarization plane of the incident laser light, as shown by curves a and b in FIG. 2B, which was previously seen, can be avoided. can be removed, and the same diffraction efficiency characteristics can be obtained, as shown, for example, by curve c in FIG. 2B.

The present invention is not limited to the above embodiments, and the polarizer may include a polarizing multilayer film (for example, a thin film with a relatively high refractive index (TiO ₂ , Ta ₂ O ₅ , ZrO ₂ ) and thin films with relatively low refractive index (SiO ₂ , MgO _{2 ,} etc.) may be attached, or Glan-Thompson prisms, Rossillon prisms, etc. made of calcite may be attached. A polarizing prism may also be used.

Further, as the laser light, a random laser light whose polarization plane changes over time may be used.

The inclination of the plane of polarization of the polarizer is usually selected to be substantially equal to the inclination of the plane of polarization that is the reference for zero-order light, but
The plane of polarization of the emitted laser beam may be selected by selecting a specific inclination.

Since the beam splitter is a means having an optical detection function that transmits the main light amount (for example, 96%) and detects the remaining light amount as a monitor, it may be replaced with various prisms.

As described above, according to the present invention, the amount of laser light and the inclination of its polarization plane are comprehensively detected and controlled, thereby achieving higher noise reduction and stabilizing the light intensity. A modulation device can be realized.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional acousto-optic modulator. Figure 2 shows the operation of a conventional acousto-optic modulator, with Figure A showing the traveling direction of the incident laser beam and ultrasonic signal in the acousto-optic modulator, and Figure B showing the inclination of the polarization plane of the incident laser beam. A graph showing a diffraction efficiency characteristic versus electric input power when used as a parameter is shown, and C in the figure shows the inclination of the polarization plane of the incident laser beam and the inclination of the polarization plane of the zero-order light and the diffracted light after being emitted. Third
The figure is a diagram showing an embodiment of an acousto-optic modulator according to the present invention. FIG. 4 is a diagram showing the action of the polarizer according to the present invention. 1... Incident laser beam, 2... Acousto-optic medium, 3
... Transducer, 4 ... 0th order light, 5 ... Diffracted light, 7 ... Transmitted light, 8 ... Detection light, 9 ... Photodetector, 10 ... Differential amplifier, 11 ... High frequency modulation Vessel, 12...Slit, 13...Polarizer.

Claims (1)

[Claims] 1. A transducer is installed in an acousto-optic medium,
A variable high frequency signal is applied to the transducer from a high frequency modulator to propagate an ultrasonic signal to the acousto-optic medium, and a laser beam is generated at a substantially Bragg angle with respect to the wavefront of the ultrasonic signal. an acousto-optic modulator for transmitting a zero-order light and a diffracted light by making the zero-order light incident on the acousto-optic medium; a polarizer having a plane of polarization with an inclination substantially equal to a reference inclination of a plane of polarization of a laser beam incident on an acousto-optic medium; and a polarizer that converts zero-order light emitted from the polarizer into transmitted light and detection light. an optical separating means for separating; a photodetector for converting the amount of the detected light into a voltage; and comparing the voltage obtained by the photodetector and a reference voltage, and as a result, a voltage difference between the two voltages is determined. and comparing means for supplying the voltage difference to the high frequency modulator when the voltage difference occurs, the voltage difference is adjusted so that the reference light amount corresponding to the reference voltage and the light amount of the transmitted light are substantially equal. An acousto-optic modulator, characterized in that the electrical input energy of the high frequency signal is controlled in response to.