JPH07175026A - Laser beam phase modulator - Google Patents

Abstract

PURPOSE:To efficiently execute phase modulation of a laser beam even when a low external applied voltage. CONSTITUTION:This laser beam phase modulator 1 is constituted by mounting paired electrodes 4 on an electro-optic crystal 2 so as to face each other and executes phase modulation of a laser beam by an electric resonance circuit 9 which is partly composed of the electrodes 4. This laser beam modulator is provided with a means for increasing the optical path length of the laser beam within the electro-optic crystal 2 without changing the length of the electro-optic crystal 2. This means is formed by subjecting the opposite surfaces of the electro-optic crystal 2 to mirror finishing to make the vertex of the opposed surfaces nearly a right angle so that the laser beam is totally reflected on the opposed surfaces.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light phase modulator, and more particularly to a laser light phase modulator capable of performing stable and efficient phase modulation of laser light.

[0002]

2. Description of the Related Art Phase modulation of laser light is widely used not only in the field of communication technology but also in the field of laser spectroscopy. For example, phase modulation of single mode oscillation dye laser light,
It is also possible to scan the wavelength rapidly with time or to electronically control the desired spectral width.

By the way, an electro-optic crystal utilizing the electro-optic effect is used in a typical conventional laser light phase modulator. In this phase modulator, the electric field (voltage) is applied to the electro-optic crystal from the outside, and the phase of the laser light passing through the inside is modulated.

On the other hand, an electro-optic crystal used as a dielectric in a laser light phase modulator changes the refractive index of light in the electric field spectrum direction by applying a voltage (electro-optic effect). In order to effectively apply a voltage to the body, it is necessary to form an electric resonance circuit (tank circuit).

The resonance frequency f of the electric resonance circuit is determined by the circuit constants of the capacitance Cc of the electro-optic crystal, the capacitance Cv other than the electro-optic crystal, and the inductance L of the inductance coil.

[Equation 1] Becomes The voltage of the electro-optic crystal has a resonance frequency f
Heavily depends on. Here, the index δ representing the magnitude of the phase modulation can be represented by the following equation in the case of a typical LiTaO ₃ crystal, for example.

[0006]

[Equation 2] From the constants of the LiTaO ₃ crystal, n _e and γ ₃₃ depend on the optical path length 1 in the LiTaO ₃ crystal of the laser light and the externally applied voltage V when d is constant, and in general, The optical path length l of the laser light in the crystal is constant because it is equal to the length of the LiTaO ₃ crystal, and the index δ representing the magnitude of phase modulation depends on the externally applied voltage.

[0007]

In the conventional laser optical phase modulator, an electro-optic crystal having excellent responsiveness is used, and an external voltage is applied to the electro-optic crystal through an electric resonance circuit to utilize the electro-optic effect. Then, phase modulation is performed.

However, when a high voltage of, for example, about 1 KV / mm is applied in order to increase the magnitude of modulation, ultrasonic waves are generated in the electro-optic crystal, and the dissipation wave of the laser light is generated by the standing wave generated by the ultrasonic waves. To do. In order to avoid the laser light dissipation phenomenon, a means for increasing the length of the electro-optic crystal has been taken, but from the standpoint of electro-optic crystal production, a limit (about 100 mm) occurs.

Experiments have shown that this is particularly remarkable in low frequency (-10 MHz) modulation.

The present invention has been made in consideration of the above-mentioned circumstances, and in the low frequency region, a high voltage is not applied,
It is an object of the present invention to provide a laser light phase modulator capable of efficiently performing phase modulation of laser light.

[0011]

In order to achieve the above object, in a laser light phase modulator according to the present invention, as described in claim 1, a pair of electrodes are opposed to each other on an electro-optic crystal. In a laser light phase modulator in which these electrodes form a part of one electric resonance circuit and perform phase modulation of the laser light, the laser light in the electro-optic crystal is changed without changing the length of the electro-optic crystal. A means for increasing the optical path length of the laser light is provided, and the means for increasing the optical path length of the laser light is formed by mirror-finishing the facing surface of the electro-optic crystal so that the apex angle of the facing surface is substantially right angle so that the laser light is totally reflected. Is.

[0012]

According to the present invention constructed as described above, by providing means for increasing the optical path length of the laser light in the crystal without changing the length of the electro-optic crystal, even when the externally applied voltage is low. It is designed to obtain a sufficiently large modulation effect.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the laser light phase modulator according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows an embodiment of a laser light phase modulator according to the present invention. This phase modulator 1 has an electro-optical structure that is thin in the electric field application direction and long in the direction orthogonal to the electric field application direction. A crystal 2 is provided. This electro-optic crystal 2
Are covered with a case (not shown) or the like.

Further, in order to increase the optical path length of the laser light 3 in the electro-optic crystal 2, prism-shaped mirror surface processing is performed so that the apex angle of the two surfaces facing each other becomes exactly 90 degrees as shown in FIG. The zigzag shape is applied, and the incident laser light 3 is refracted at each prism-shaped apex angle, scanned so as to change the direction by 180 degrees, and is advanced.

Further, LiTaO ₃ or LiNbO ₃ usually used as the electro-optic crystal 2 has a refractive index n of about 2.
Therefore, the laser light 3 incident on the electro-optic crystal 2 at an incident angle of about 45 degrees is totally reflected and has almost no loss.

Laser light 3 incident on the electro-optical optical path 2
Is a large number of times 18 in the electro-optic crystal 2 with almost no loss.
It is reflected in the 0 degree direction. Therefore, the optical path length of the laser light 3 is dramatically increased as compared with the single long crystal. Electrodes 4 are attached to the electro-optic crystal 2 at positions facing each other by a method such as vapor deposition. In this electrode 4,
An electric resonance circuit 9 to which the lead wire 5 is connected and which has a variable capacitor 6, an inductance coil 7, and an externally applied voltage 8.
To form. At this time, the resonance condition of the electric resonance circuit 9 is expressed by the above-described formula (1), and the index indicating the magnitude of the phase modulation is expressed by the formula (2).

Next, the operation of the laser light phase modulator will be described.

Electro-optic crystal 2 of laser light phase modulator 1
For example,

## EQU3 ## When an ideal sinusoidal voltage is applied as in V (t) = asin 2πft (3), phase modulation of laser light occurs in proportion to the sinusoidal wave at time t.

Generally, the electric field intensity ψ (t) of laser light is

[Equation 4] It is represented by.

Substituting equation (3) into equation (4),

[Equation 5] And the instantaneous frequency of the electric field strength ψ (t) is

[Equation 6] Becomes

From the equation (6), the frequency of the incident laser light of ω ₀ is the frequency deviation (fluctuation) represented by the cosine function of the time t.
Cause

From the equation (2), if the electric field of the electro-optic crystal is constant (V / d = constant), the modulation magnitude δ is proportional to the optical path length 1 of the laser light in the electro-optic crystal 2. .

In the conventional type laser light phase modulator, the optical path length 1 of the laser light in the electro-optic crystal 2 is equal to the length of the electro-optic crystal 2. Therefore, when the length of the electro-optic crystal 2 is determined, The magnitude of modulation has been increased by increasing the applied voltage 8.

However, in the case of modulation in the low frequency (-10 MHz) range, the laser beam deviation phenomenon increases in proportion to the externally applied voltage 8, and there is a problem only by increasing the externally applied voltage 8. However, since the optical path length 1 in the electro-optic crystal can be remarkably increased by using the above-mentioned embodiment, the modulation magnitude δ can be made large, and sufficient modulation can be obtained even with a low external applied voltage 8. You can

[0026]

As described above, according to the laser light phase modulator of the present invention, even when the magnitude of modulation is increased in the low frequency region, the electro-optic crystal can be formed without increasing the externally applied voltage. By providing the means for increasing the optical path length of the laser light, there is an effect that the magnitude of the modulation can be increased even with a low externally applied voltage, and the phase modulation of the laser light can be efficiently performed.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a laser light phase modulator according to the present invention.

FIG. 2 is a diagram showing a scanning state of laser light in an electro-optic crystal of a laser light phase modulator according to the present invention.

[Explanation of symbols]

 1 Laser Light Phase Modulator 2 Electro Optical Crystal 3 Laser Light 4 Electrode 5 Lead Wire 6 Variable Capacitor 7 Inductance Coil 8 Externally Applied Voltage 9 Electrical Resonance Circuit

Claims (1)

[Claims] 1. A laser light phase modulator for mounting a pair of electrodes on an electro-optic crystal so as to face each other and forming a part of one electric resonance circuit to perform phase modulation of laser light. , Means for increasing the optical path length of the laser light in the electro-optic crystal without changing the length of the electro-optic crystal is provided. A laser light phase modulator characterized in that the apex angle of the opposing surface is formed by mirror-finishing so as to be almost perpendicular so as to be reflected.