JPH04242716A - Laser beam phase modulator - Google Patents

Abstract

PURPOSE:To provide a laser beam phase modulator capable of performing phase modulation of a laser beam for a wide frequency range stably and smoothly. CONSTITUTION:In a laser beam phase modulator 1, a long electro-optic crystal 2 is used for performing laser beam phase modulation, where a plural electrodes 3a-3d forming pairs with the crystal 2 are installed facing each other, and electric resonance circuits 7a-7d are formed separately for the electrodes 3a-3d respectively, and the electric resonance circuits 7a-7d are operated in different frequency characteristic ranges.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam phase modulator, and more particularly to a laser beam phase modulator that has improved frequency characteristics and is capable of continuous and efficient phase modulation of a laser beam.

0003

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, a desired spectrum is produced by phase modulating a single mode oscillation dye laser beam to form a so-called chirped spectrum.

By the way, an electro-optic crystal that utilizes the electro-optic effect is used in a typical conventional laser beam phase modulator. This phase modulator modulates the phase of a laser beam passing through the electro-optic crystal while applying an electric field from the outside. Phase modulation of laser light depends on the magnitude of the electric field (voltage) and the modulation waveform.

On the other hand, electro-optic crystals used in laser optical phase modulators change the refractive index of light in the electric field spectrum direction by applying voltage, but in order to effectively apply voltage to this derivative, it is necessary to It is necessary to assemble an electrical resonant circuit (tank circuit). However, in an electric resonant circuit, the resonance frequency f

[0007]

[Math 1]

##EQU1## is determined. The voltage of the electro-optic crystal largely depends on the resonance frequency f.

[0009] Therefore, a wide frequency range (up to several GHz
), due to the frequency characteristics determined by the circuit constants of the electrical resonant circuit, the phase modulation efficiency of the laser beam is poor outside a specific frequency range, and the degree of phase modulation changes depending on the frequency.

0010

[Problems to be Solved by the Invention] Conventional laser beam phase modulators use an electro-optic crystal with excellent responsiveness, and phase modulation is performed by applying an external voltage to this electro-optic crystal through an electric resonant circuit. ing.

However, when an electrical resonant circuit is constructed using an electro-optic crystal, it is currently extremely difficult to perform uniform phase modulation over a wide frequency band due to its frequency dependence. there were.

The present invention was made in consideration of the above-mentioned circumstances, and it is possible to stabilize the phase modulation of laser light over a wide frequency band even when an electric resonant circuit is constructed using an electro-optic crystal. It is an object of the present invention to provide a laser beam phase modulator that can perform a laser beam phase modulation process accurately and smoothly. [Structure of the invention]

[0013]

[Means for Solving the Problems] In order to achieve the above object, a laser beam phase modulator according to the present invention uses an electro-optic crystal to perform phase modulation of a laser beam. An electro-optic crystal is used, and a plurality of pairs of electrodes are attached to the electro-optic crystal facing each other, and an independent electric resonant circuit is formed for each pair of electrodes, and each electric resonant circuit has a different frequency characteristic. It is set to operate in this area.

[0014]

[Operation] According to the present invention constructed as described above, a long electro-optic crystal is used, a plurality of pairs of electrodes are provided on the electro-optic crystal, and a separate electric resonant circuit is provided for each pair of electrodes. By forming and operating in different frequency ranges,
Each electrical resonant circuit is made to cover a wide frequency band, and uniform phase modulation can be performed efficiently over a wide frequency band.

[0015]

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

FIG. 1 shows a first embodiment of a laser beam phase modulator, and this phase modulator 1 is equipped with one long rectangular bar-shaped electro-optic crystal 2. As shown in FIG. This electro-optic crystal 2 is covered with a case (not shown) or the like, and has a laser beam entrance 2a on one end face and a phase-modulated laser beam exit 2b on the other side. The incident laser light passes through the inside of the electro-optic crystal 2 and is output from the laser light exit port 2b.

A plurality of pairs of electrodes 3a to 3d, for example, four pairs, are formed on the electro-optic crystal 2 at positions facing each other by vapor deposition of an electrode material. Each of these electrodes 3a to 3d
Lead wires 4a to 4d are connected to each pair of electrodes 4.
Electric resonant circuit 7a having variable capacitors 5a to 5d and inductance coils 6a to 6d between a to 4d
~7d are formed independently.

The electrical resonance circuits 7a to 7d are installed to have different resonance frequencies f1 to f4, respectively. That is, the resonance frequency fi (i=1 to 4) in each of the electrical resonance circuits 7a to 7d can be expressed by the following equation.

[0019]

[Math 2]

[0020] Here, Li; inductance C in the electrical resonant circuit
iv; Variable capacitance (capacitance in the electrical resonant circuit) Cic; Capacitance of the electro-optic crystal between the electrodes Therefore, the inductances L1 to L4 of the inductance coils 6a to 6d of each of the electrical resonant circuits 7a to 7d, and the inductances of the variable capacitors 5a to 5d Capacity C1v~C4V, each electrode 3
Capacitance of electro-optic crystal 2 between a and 3d (
By changing the capacitances C1c to C4c, the resonance frequency fi (i=1 to 4) can be changed to cover a wide frequency band.

As shown in FIG. 2, each resonance frequency fi (
i=1 to 4) to each of the above parameters (Li, Civ, Ci
c) by setting each electrically resonant circuit 7a via
~7d covers a wide frequency band, and phase modulation of laser light can be efficiently and uniformly performed over a wide band. Note that in FIG. 2, the horizontal axis represents the frequency, and the vertical axis represents the quality factor of the capacitor.

Each of the inductance coils 6a to 6d is a primary coil 8 having inductances L'1 to L'4.
The power supplies 9a to 9d are electromagnetically connected to the power supplies 9a to 9d through the power supplies 9a to 9d, and the power supplies 9a to 9d are configured to apply a modulation voltage to the electro-optic crystal 2.
can be shared as one power source, or can be supplied from different power sources.

Next, the operation of the laser beam phase modulator will be explained.

For example, as shown in FIG. 3, the electro-optic crystal 2 of the laser beam phase modulator 1 has the following equation: V(t) =a(t-t0)2+1(a=
-1.6×1015 (KV/sec))


When a voltage with an ideal quadratic function is applied as shown in (2), phase modulation of the laser beam occurs in proportion to the quadratic function of time (t).

Generally, the electric field strength ψ(t) of laser light,
ψ(t)=E0 sin(ω0 t+at2)
...(
3), and the phase of this laser beam is θ=ω0 t+at2

...It is expressed as a quadratic function of time t, as shown in (4).

Therefore, the time differential of the phase θ of the laser beam is

[0027]

[Math 3]

From this equation (5), a frequency deviation (fluctuation) occurs as a linear function of time (t) with the frequency of the incident laser beam of ω0 as the origin. And the coefficient a
The magnitude of is proportional to the voltage applied to the electro-optic crystal 2, and the applied voltage has the frequency characteristics shown in FIG. 2, so that the voltage can be applied smoothly over a wide frequency band.

Conventional laser beam phase modulators have poor frequency characteristics when applied to voltages with waveforms as shown in FIG. 3, and voltages are applied smoothly only in a specific frequency band; Although the voltage application efficiency is poor in the frequency band of , by using the above embodiment, it is possible to efficiently apply a voltage to the electro-optic crystal 2 over a wide frequency band.

[0030] In one embodiment, a single long electro-optic crystal was used, but instead of this electro-optic crystal, electro-optic crystals of different crystals were connected together with an adhesive. A laser beam phase modulator may be constructed by forming one long electro-optic crystal and incorporating an electric resonant circuit into each of the electro-optic crystals in the same manner as described above.

[0031]

[Effects of the Invention] As described above, according to the laser optical phase modulator according to the present invention, a modulation voltage can be efficiently applied to an electro-optic crystal over a wide frequency band without preparing a large modulation power source. This has the effect that phase modulation of laser light can be performed over a wide band.

[Brief explanation of the drawing]

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

FIG. 2 is a diagram showing resonance characteristics in an electric resonant circuit incorporated in a laser beam phase modulator.

FIG. 3 is a diagram showing an ideal voltage applied to an electro-optic crystal.

[Explanation of symbols]

1 Laser optical phase modulator 2 Electro-optic crystal 3a-3d Electrode 4a-4d Lead wire 5a-5d Variable capacitor 6a-6d Inductance coil 7a-7d Electrical resonant circuit 9a-9d Power supply

Claims (1)

[Claims] Claim 1: A laser beam phase modulator that performs phase modulation of laser light using an electro-optic crystal, in which a long electro-optic crystal is used, and a plurality of pairs of electrodes are arranged facing each other on the electro-optic crystal. A laser beam phase modulator characterized in that when attached, an independent electric resonant circuit is formed for each pair of electrodes, and each electric resonant circuit is set to operate in a region with a different frequency characteristic.