JPS5942857B2 - optical phase demodulator - Google Patents

Description

[Detailed description of the invention] The present invention relates to an optical phase modulator for demodulating phase modulated light.

It concerns a demodulator. With the remarkable development of optical devices including optical fibers in recent years, the practical application of optical fiber communications is rapidly progressing.

Amplitude modulation, phase modulation, and the like are used as communication methods for optical fiber communication, and phase modulation has excellent characteristics for long-distance and wideband communication. An optical phase demodulator is an indispensable device for putting this phase modulation type optical fiber communication device into practical use. Conventionally, optical resonators and the like have been used as demodulators of this type, but they are large in size and susceptible to the effects of dust, moisture, and the like.

The purpose of the present invention is to improve the above-mentioned drawbacks, and to achieve a compact and simple structure.
Another object of the present invention is to provide an optical phase demodulator that is less susceptible to surrounding influences. For the following explanation, we will discuss the birefringence of light when the light is perpendicularly incident on a birefringent crystal.

Generally, when light enters a birefringent crystal perpendicularly, the light propagates in the crystal as two rays: an ordinary ray and an extraordinary ray.
If the direction of the wavefront normal and the crystal axis are not parallel, 2
The light rays travel in spatially different directions. In this case, the two light beams are linearly polarized and their polarization planes are orthogonal to each other. If the crystal has a uniaxial orientation, ordinary rays will travel straight after incidence, but extraordinary rays will be refracted and propagated. When the output end face is parallel to the input end face, the two light beams exit perpendicularly from the output end face and proceed in parallel to each other. Conversely, if two linearly polarized lights whose polarization planes are orthogonal to each other are made parallel and perpendicularly incident on the incident end surface, 2
The light beams are combined into one light beam at the output end face of the crystal. If the phase difference between the two light beams is an integral multiple of π, the combined light becomes linearly polarized light. Next, the present invention will be explained using the drawings.

The figure shows an embodiment of the present invention, in which 1 is a birefringent crystal that has incident and exit end faces that are parallel to each other and whose principal axis of refractive index does not coincide with the vertical direction of the end faces; 2 and 3 are birefringent crystals that emit light in the length direction; A transparent body (having a focusing effect equivalent to a thick lens) that has a refractive index gradient in a cross section perpendicular to its length, and the gradient gradually decreases from the center of the cross section to the periphery. 4
is an optical phase shifter, 5 is a light source, and 6 is an analyzer.

In the above configuration, a case where phase-modulated linearly polarized light is incident on the transparent body 2 will be described.

When the phase-modulated light passes through the transparent body 2, it is focused by its lens effect, enters the birefringent crystal 2, and reaches the analyzer 6. At this time, since the phase modulated light corresponds to the ordinary ray in the birefringent crystal 1, it travels straight through the birefringent crystal 1. On the other hand, linearly polarized light from a light source 5 passes through a transparent body 3 and a phase shifter 4 and then enters a birefringent crystal 1, but since it corresponds to an extraordinary ray in the crystal, it is refracted and proceeds. Reach analyzer 6. It is assumed that the vibrations of the phase-modulated light at one certain time and the next certain time have an opposite phase relationship. If the phase shifter 6 is adjusted so that the phase difference between one light within a certain period of time and the light from the light source 5 is an integral multiple of π, the combined light on the output end face of the birefringent crystal 1 will be Corresponding to the reversal of the phase of the phase modulated light, the light becomes orthogonally linearly polarized light.
That is, by setting the output polarization plane of the analyzer 6 to be parallel to the polarization plane of one of the orthogonal linearly polarized lights, output light may or may not be output depending on the change in the phase of the phase modulated light. In this way, the phase modulated light is finally converted into intensity modulated light by passing through the analyzer 6. In the present embodiment, the phase modulated light is composed of light with vibration components having mutually opposite phases, but is not limited to this, and may be composed of vibration components with any predetermined phase difference.

Generally, the output from the analyzer 6 is intensity-modulated light. Further, in this embodiment, the phase shifter 4 is placed on the axis along which the light from the light source 5 propagates, but it may also be placed on the axis on which the phase modulated light travels, or furthermore, it may be placed on the axis of both light beams. Of course it is possible.

Furthermore, it is clear that if the phase modulated light is not linearly polarized light, it may be converted into linearly polarized light using a linear polarization circuit such as a polarizer.

Further, although the two transparent bodies 2 and 3 are used in this embodiment, it is clear that they may be provided on the output end face side of the phase shifter 4 or on the input and output end face sides of the analyzer 6.

Furthermore, although special transparent bodies 2 and 3 are used in this embodiment, it is of course possible to use lenses having so-called spherical surfaces.

Finally, the main feature of the present invention is that it is easy to integrate (the end faces of each component can be treated to be flat), making it possible to obtain an optical phase demodulator that is small and less susceptible to the effects of the surroundings. .

[Brief explanation of drawings]

The figure shows an embodiment of the present invention, in which 1 is a birefringent crystal, 2 and 3 are lens-like transparent bodies, 4 is a phase shifter, 5 is a light source,
And 6 is an analyzer.

Claims (1)

[Claims] 1. A birefringent material having parallel input and output end surfaces, the direction of the principal axis of which does not coincide with the perpendicular direction of the input and output end surfaces with respect to the refractive index, and a birefringent material provided on the entrance end surface side of the birefringent material, a lens object for focusing phase-modulated light; a light source provided on the incident end surface side of the birefringent material; a lens object for focusing the light from this light source on the exit end surface side; An optical phase demodulator including a phase shifter provided on at least one path of phase-modulated light and light from the light source, and an analyzer provided on the output end surface side of the birefringent material.