JP2500346B2 - Polarization plane rotation speed multiplier - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the field of light wave control using an optical device that rotates the plane of polarization of laser light.

[0002]

2. Description of the Related Art Generally, a light source that emits linearly polarized laser light has a Brewster window arranged at the exit end, and a light wave having an electric field oscillation direction parallel to the plane of incidence at the Brewster window is generated. Is emitted. The plane of polarization of linearly polarized light is specified as the plane perpendicular to the vibration direction of the electric field component of the light wave, and as a method of rotating this, the Brewster window alone or the method of rotating the entire device is the principle. However, this is not generally done because of output stability problems and convenience. As a general method for rotating the plane of polarization, as shown in FIG. 1, a half-wave plate is rotated about a normal vector passing through the center of the wave plate as an axis, and linearly polarized laser light is propagated with this rotation axis. There is a method of making the light incident in the same direction. In the half-wave plate, a phase difference of π is given between the electric field component in the direction parallel to the crystal main axis of the half-wave plate and the electric field component in the direction perpendicular to the electric field vector of the incident light to transmit the light. As a result, the direction of the electric field vector of the light transmitted through the half-wave plate is twice the angle θ between the direction of the electric field vector of the incident light and the crystal main axis of the half-wave plate with respect to the direction of the electric field vector of the incident light. Angle 2
form θ. Therefore, when the half-wave plate is continuously rotated, the polarization plane of the emitted light rotates in the same direction by a rotation angle that is twice the rotation angle of the half-wave plate. When the linearly polarized light is transmitted through a polarizer such as a polarizing plate or a polarization beam splitter, the intensity of the transmitted light can be controlled by the direction of the plane of polarization of the incident light. Therefore, by continuously rotating the direction of the polarization plane and transmitting the light through the polarizer, it is possible to modulate the light intensity with a modulation degree of almost 100%. By the way, in order to continuously rotate the half-wave plate, a method of rotating a jig provided with the half-wave plate using a power source such as a motor is adopted. For example, a configuration as shown in FIG. 2 can be considered. The rotation speed of the half-wave plate at this time is the specifications of the power source such as the motor, the configuration of the power transmission mechanism such as gears, pulleys, and belts, the rotation torque of the jig with the half-wave plate installed, and the allowable rotation speed. Limited by the conditions of. For this reason, the rotation speed of the polarization plane is also limited. Therefore, in order to realize a faster rotation of the polarization plane, it is necessary to increase the allowable rotation speed of the half-wave plate rotating device.

[0003]

The problem to be solved is to rotate the plane of polarization of laser light at a higher speed.

[0004]

[Means for Solving the Problems] Two half-wave plate rotating devices for rotating a half-wave plate about the propagation direction of the laser light with respect to a linearly polarized laser light are sequentially arranged. Rotate the stage rotation device in the opposite direction,
Allows laser light to pass through.

[0005]

[Operation] The polarization plane of the light emitted from the first half-wave plate rotation device is
It rotates at twice the rotation speed of the first-stage rotation device.
The half-wave plate of the next stage is rotated in the direction opposite to the direction of rotation of the polarization plane, and the relative rotation angle between the polarization plane of the incident light and the crystal plate main axis of the next stage is increased. Then, the polarization plane of the light transmitted through the two wave plate rotating devices can be rotated at a rotation speed twice as high as the sum of the rotation speeds of the first stage and the second stage.

[0006]

EXAMPLE As a general construction of a polarization plane rotation speed multiplier, as shown in FIG. 3, a half-wave plate is rotated about a laser light propagation direction with respect to linearly polarized laser light. The two wave plate rotating devices to be arranged are sequentially arranged, and the rotation directions of the first-stage rotating device and the second-stage rotating device are reversed.

FIG. 4 shows how the electric field vector of the light wave changes in the first half-wave plate. Here, for simplicity, the electric field vector of the incident light is taken in the vertical direction. This electric field vector is expressed as Equation 1 as Bertrel components Ey and Ex, where the vertical direction is the Y axis and the horizontal direction is the X axis. Where I
Is the electric field strength of the incident light.

[0008]

[Equation 1] Ey = I Ex = 0

The crystal major axis of the wave plate lies in the plane of the wave plate,
As shown in FIG. 4, the angle formed by the crystal main axis and the electric field vector of the incident light is set to θ. In the half-wave plate, as described above, of the electric field vector of the incident light, a phase difference of π is given between the electric field component Ey1i in the direction parallel to the crystal main axis and the electric field component Ex1i in the direction perpendicular to the crystal main axis to transmit the light. The positive / negative of this phase difference π differs depending on each half-wave plate. Ey1i and Ex1i can be expressed by Equation 2, and the electric field component Ey1o in the direction parallel to the crystal principal axis of the transmitted light and the electric field component Ex1o in the direction perpendicular to the crystal main axis are Equation 3. The electric field vectors of the incident light and the transmitted light are as shown in FIG. 4, and since these electric field vectors form an angle of 2θ, the polarization planes of the incident light and the transmitted light form an angle of 2θ. This angle θ changes with the rotation of the wave plate, and the rotation speed of the plane of polarization of the transmitted light is twice the rotation of the wave plate.

[0010]

[Equation 2] Ey1i = I cos θ Ex1i = -I sin θ

[0011]

[Equation 3] Ey1o = I cos θ Ex1o = I sin θ

Next, in the half-wave plate rotating device of the next stage, the half-wave plate is rotated in the direction opposite to the direction of rotation of the polarization plane with respect to the light wave transmitted through the first stage and having the polarization plane rotated. FIG. 5 shows how the electric field vector of the light wave changes in the next half-wave plate. As shown in FIG. 5, the angle formed by the crystal main axis of the next wave plate and the vertical direction is set to φ. As in the case of the first stage, when the electric field components of the incident light and the transmitted light with respect to the crystal main axis of the second-stage wave plate are set as Ey2i, Ey2i, Ey2o, and Ex2o, respectively, they can be expressed as Formulas 4 and 5. These electric field vectors are shown in FIG. 5, and the angle formed by the electric field vector of the light transmitted through the next wave plate and the crystal main axis is φ + 2θ. Therefore, the electric field vector of the light transmitted through the second-stage wave plate forms an angle of 2φ + 2θ with respect to the electric field vector of the laser light that is incident on the first stage in the vertical direction. Therefore, when two wave plate rotating devices are rotated, φ and θ change with time, and the polarization plane of the transmitted light rotates at a speed twice the sum of the rotating speeds of the rotating devices.

[0013]

[Equation 4] Ey2i = I cos (2θ + φ) Ex2i = I sin (2θ + φ)

[0014]

[Equation 5] Ey2o = I cos (2θ + φ) Ex2o = -I sin (2θ + φ)

[0015]

The polarization plane rotation speed multiplying device of the present invention has a very simple structure including a linearly polarized laser light source and two devices for rotating about the center of a half-wave plate. The rotation speed of the polarization plane, which was limited by the permissible rotation speed of the rotation device for the wave plate, can be increased.

By applying the principle of the polarization plane rotation speed multiplying device of the present invention and using three or more half-wave plate rotation devices in multiple stages, the polarization plane can be rotated at a higher speed. The rotation direction of the rotation device of the next stage and thereafter is opposite to that of the rotation device of the previous stage, and is opposite to the rotation direction of the polarization plane of the light wave incident on each wave plate. With this setting, the polarization plane of the transmitted light at the final stage can be rotated at a rotation speed twice the sum of the rotation speeds of the devices.

As a method of using the polarization plane rotation speed multiplier, separation of linearly polarized signal light and randomly polarized background noise light can be considered. Light in which signal light and background noise light are mixed is passed through a polarization plane rotation speed multiplier, and the polarization plane of signal light is rotated at high speed. When the transmitted light is passed through the polarizer, only the signal light can be intensity-modulated at a frequency according to the rotation speed of the polarization plane. Then, after photoelectrically converting with a photodiode or the like, if only the signal of that frequency is detected, the noise of the detector in the unnecessary band can be removed and only the signal light excluding the background light can be detected. . Here, by using the polarization plane rotation speed multiplier, the rotation speed of the polarization plane can be increased to raise the detection frequency, and a desired detection frequency can be set.

This polarization plane rotation speed multiplier uses the electro-optic effect or the acousto-optic effect because it can rotate the polarization plane for a laser beam having a beam width within the range of the effective diameter of the half-wave plate. Compared to the light intensity modulation method, it can handle light with a larger beam diameter, and directly perform intensity modulation on a large beam of light, or expand the beam diameter for light with a high energy density. You can

[Brief description of drawings]

FIG. 1 is a principle diagram of rotation of a plane of polarization of light using a half-wave plate.

FIG. 2 is a perspective view of an apparatus for rotating a half-wave plate rotation.

FIG. 3 is a principle diagram of a polarization plane rotation speed multiplier.

FIG. 4 is a diagram showing electric field vectors of incident light and transmitted light at the first half-wave plate of the polarization plane rotation speed multiplier.

FIG. 5 is a diagram showing electric field vectors of incident light and transmitted light in a half-wave plate at the next stage of a polarization plane rotation speed multiplier.

[Explanation of symbols]

1 incident light 2 electric field vector of incident light 3 half-wave plate 4 electric field vector of transmitted light 5 device for rotating half-wave plate 6 first half-wave plate rotating device 7 next-stage half-wave plate rotating device 8 half-wave plate Direction of crystal principal axis 9 Electric field vector of first stage transmitted light 10 Electric field vector of second stage transmitted light θ Angle between crystal principal axis of half-wave plate and electric field vector of incident light φ Crystal principal axis of next half-wave plate is vertical Angle I Electric field intensity of incident light Ey Vertical electric field component of incident light Ex Horizontal electric field component of incident light Ey1i Wave plate of incident light at the first-stage wave plate Electric field component parallel to crystal main axis Ex1i At first-stage wave plate Electric field component of incident light in the direction perpendicular to the waveplate crystal main axis Ey1o Electric field component of transmitted light in the first-stage waveplate parallel to the waveplate crystal main axis Ex1o Perpendicular to the waveplate crystal main axis of transmitted light in the first-stage waveplate Field component in different directions Ey2i Next-stage wave Wave plate of incident light on the plate Electric field component parallel to the principal axis of the crystal Ex2i Wave plate of incident light on the second-stage wave plate Electric field component perpendicular to the crystal main axis Ey2o Wave plate of transmitted light on the second-stage wave plate Electric field component in the direction parallel to the crystal main axis Ex2o Wave plate of transmitted light in the next-stage wave plate Electric field component in the direction perpendicular to the crystal main axis

Claims (1)

(57) [Claims] 1. A linearly polarized laser beam,
Two half-wave plate rotating devices that rotate the half-wave plate about the light propagation direction are arranged in order, and the laser light is transmitted by making the rotation directions of the first-stage rotation device and the next-stage rotation device opposite to each other. A device that rotates the plane of polarization of light.