JPS62218905A - Wedge like phase plate - Google Patents

Abstract

PURPOSE:To obtain a required phase difference while observing the phase difference of transmitted light by forming a wedge like plate by a plane obtained by inclining a double refractive crystal from an optical axis and displacing a light transmitting position in a thickness changing direction. CONSTITUTION:Incident light from an optical fiber 12 is converted to a linearly polarized light by a polarizer 13 and then converted to 9 circularly polarized light by a 1/4 wavelength plate 11. The light senses a refractive index proportional to a voltage impressed via an electrode 15 in its passage through a photoelectric crystal 14 and generates a phase difference so as to form an elliptically polarized light. The elliptically polarized light is converted into a change in light quantity through an analyzer 16. Since the optical phase bias of pi/2 is applied by the 1/4 wavelength plate 11, light is positively and negatively changed with high linearity in accordance with the positive/negative change of the impressed voltage. Thereby, the phase difference can be easily adjusted so as to obtain the phase difference of 1/4 wavelength by displacing the wedge-like phase plate 11 in the thickness changing direction and finely adjusting the thickness of a position where light is transmitted.

Description

[Detailed description of the invention] Industrial applications The present invention relates to an optical phase plate.

2. Description of the Related Art Conventionally, a wave plate is a parallel plane plate obtained by cutting a birefringent crystal perpendicular to the optical axis to a predetermined thickness. When light is incident on this, after passing through the crystal, there is a phase difference δ between the two orthogonal components.
will occur. Now, let the thickness of the phase plate be d, and the refractive index for the orthogonal component be no s n.

Then, the phase difference δ is is given by

Problems to be Solved by the Invention In the conventional phase plate, a phase difference δ is given by the wavelength λ of the light source used and the thickness d of the phase plate, as shown in equation (1). Therefore, if there are variations in the wavelength of the light source, it is inevitable that the obtained phase difference will vary depending on the wavelength of the light source. Therefore, in order to obtain the desired phase difference for each light source, it is necessary to adjust the thickness of the phase plate according to the variation in the wavelength of the light source, but this is extremely difficult and reduces workability. The problem is that it gets worse.

Means for Solving the Problems The present invention has been made to solve the above problems.
This is a wedge-shaped phase plate characterized by cutting a birefringent crystal with a plane inclined to the optical axis.

Function: Since the wedge-shaped phase plate of the present invention is made by cutting a birefringent crystal with a plane inclined with respect to the optical axis, the thickness in the light transmission direction changes depending on the inclination of the plane. The phase difference of transmitted light changes. Therefore, a desired phase difference can be obtained by moving the position where light passes through the phase plate while observing the phase difference of transmitted light.

Example Hereinafter, details of the present invention will be explained along with examples.

FIG. 1 is a perspective view of a wedge-shaped phase plate of the present invention. At least one surface is a wedge-shaped flat plate cut by a plane inclined with respect to the two optical axes of the birefringent crystal. This plane includes an X-axis (y-axis) with fa@ as the axis and a y-axis (X-axis) with the Illow axis. When light is incident on this, the two orthogonal lights have different refractive indexes n! , nA (nx←nA), which produces a phase difference δ. Since it is a wedge-shaped flat plate, the thickness d changes within the plane, and the phase difference δ changes according to the thickness d.

FIG. 2 is a configuration diagram of an optical voltage sensor using the wedge-shaped phase plate of the present invention as a quarter-wave plate. The light incident on the optical fiber 12 becomes linearly polarized light by the polarizer 13 and further becomes circularly polarized light by the quarter-wave plate 1. When this light passes through the photoelectric crystal 14, it senses a refractive index proportional to the voltage applied through the electrode 16, causing a phase difference and becoming elliptical. This is passed through an analyzer 16 and converted into a change in light amount. Reference numeral 17 is an optical fiber for receiving light, and reference numerals 18 and 19 are lenses for converting light into parallel light and efficiently combining the light.

Here, since the quarter-wave plate 1 provides an optical phase bias of π/2, the amount of light P is given by the following equation (2) with respect to the applied voltage V.

Therefore, the amount of light changes positively or negatively with good linearity in response to a positive or negative change in the applied voltage. However, if the phase plate does not provide a strictly 1/4 wavelength phase difference, the optical phase bias will deviate from π/2, so the relationship between the amount of light P and the applied voltage is P oc I + sin (V+a ) ・−
(3) α: Changes like a deviation of the phase bias from π/2, and the range in which the amount of light changes with good linearity becomes smaller with respect to changes in the applied voltage. Therefore, if the wedge-shaped phase plate of the present invention is used, it can be easily adjusted to obtain a phase difference of 1/4 wavelength by displacing it in the direction of thickness change and finely adjusting the thickness of the area through which light passes. be able to.

FIG. 3 is a perspective view showing a schematic configuration of a polarization-maintaining optical fiber connector using the wedge-shaped phase plate of the present invention as a half-wave plate. In the figure, 21° and 22 are the first connectors to be connected now. The second polarization maintaining optical fiber (hereinafter referred to as SPF), 23.24 has both 5PF21.22
The lens 26 for efficiently combining the two is a wedge-shaped half-wave plate of the present invention, which can be displaced in the direction of thickness change and can also rotate within one plane. When connecting two SPFs with connectors, it is necessary to align the emitted linearly polarized light with the polarization axis of the receiving SPF.In particular, in order to obtain a high extinction ratio of -3o dB or more, it is necessary to align the linearly polarized light to within approximately 2 degrees. It is necessary to match. To adjust this angle, the rotatable 1/2
Use a wave plate. In other words, when the optical axis of the 1/2 wave plate forms an angle θ with the linearly polarized direction of the incident light, the 1/2 wavelength plate has an angle of 2θ in the linearly polarized direction of the output light.
Regardless of the positional relationship of the main axis of the second SPF to be combined with the first SPF by rotating the crucible,
The polarization directions can be matched.

By the way, it is inevitable that the wavelength of light transmitted within this SPF varies depending on the light source, so if the phase plate has a constant thickness, the phase difference may deviate from 1/2 wavelength. Therefore, a problem arises in that the light transmitted through the phase plate becomes elliptical and the extinction ratio deteriorates.

Therefore, if the wedge-shaped 1/2 wavelength plate of the present invention is used, it can be easily adjusted to give a phase difference of exactly 1/2 wavelength by displacing it in the direction of thickness change, so the extinction ratio can be adjusted. Two SPFs can be connected to each other without dropping them.

Although the explanation has been given with reference to two embodiments, the wedge-shaped phase plate of the present invention can be used not only for the purpose of strictly obtaining phase differences of 1/2 wavelength and 1/4 wavelength, but also for any other purpose. The phase difference can also be freely selected, and of course it can be applied to various other usage methods.

Effects of the Invention As described above, according to the present invention, a wedge-shaped flat plate is made of a birefringent crystal with a plane inclined with respect to the optical axis, and the light transmission position is displaced in the direction of thickness change. By doing this, the phase difference can be easily changed.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a wedge-shaped phase plate according to an embodiment of the present invention;
Figure 2 is a plan view of an optical voltage sensor using the wedge-shaped phase plate of this example as a 1/4 wavelength plate, and Figure 3 shows polarization plane preservation using the wedge-shaped phase plate as a 1/2 wavelength plate. FIG. 1 is a perspective view showing a schematic configuration of an optical fiber connector. 1... Wedge-shaped phase plate, 11... Wedge-shaped 1/4 wavelength plate, 26... Wedge-shaped 1/2 wavelength plate.

Claims (1)

[Claims] A wedge-shaped phase plate characterized by cutting a birefringent crystal with a plane inclined to the optical axis.