JPS6010293B2 - Optical heterodyne detection device - Google Patents

Description

[Detailed description of the invention] The present invention relates to an optical heterodyne detection device.

In recent years, the quality of optical semiconductor elements and optical fibers has improved, and the practical application of optical communications has progressed rapidly. An optical detection device is indispensable for the practical application of this optical communication. Optical detection methods include the direct detection method and the heterodyne detection method.The latter allows the signal to be detected regardless of thermal noise, bright current noise, background optical noise, etc. by making the local oscillation light strong enough; The device has excellent features such as not needing to have any particular culturing effect, and is considered an ideal method for optical communications, especially optical fiber communications. However, in realizing this heterodyne detection method, a half mirror or the like was used for the part that combines the signal light and the local oscillation light, so the heterodyne detection device is large in size and dusty. , moisture, mechanical vibration, etc.

The purpose of the present invention is to improve the above-mentioned drawbacks by using a combination of a back-refractive material and an analyzer in the part that combines the signal light and the local oscillation light, and is small and less susceptible to mechanical vibrations and surrounding influences. An object of the present invention is to provide an optical heterodyne detection device.

According to the present invention, it has a local oscillation light source and parallel input and output end faces for combining the signal light and the local oscillation light, and the direction of the principal axis with respect to the refractive index does not coincide with the perpendicular direction of the input end face. An optical heterodyne detection device is obtained that includes a birefringent material, an analyzer, and a photodetector.

For the following explanation, we will first discuss the birefringence of light when the light is perpendicularly incident on a birefringent material.

Generally, when light is perpendicularly incident on a birefringent material, the light separates into two rays and travels inside the material.

Furthermore, these lights are refracted and propagated at the incident end face. When the birefringent substance is optically tactile, the two rays are divided into an ordinary ray that travels straight and an abnormal ray that travels by being refracted. If the birefringent material is uniaxial (the direction perpendicular to the incident end surface is the Z axis), its optical axis lies within the pin plane and makes an angle of 0 with the Z axis, and one of the principal axes is aligned with the y axis. , the light incident perpendicularly to the incident end plane parallel to the xy plane is an ordinary ray parallel to Z
The beam is separated into extraordinary rays that differ by an angle J from the Z axis in the X plane.
Between the small angle and the angle 8, there is a nre=harmful poisonous return dream.
There is the relationship (1).

Here m=n. /ne, n. and ne are the principal refractive indices for the ordinary and extraordinary rays, respectively. The small angle becomes maximum when the angle 8 satisfies the relationship tana=1/m, and its value can be found from the equation (2).

Under this condition, if the output end face is parallel to the input end face and the distance between them is d, the separation width D between the ordinary ray and the extraordinary ray is D=
It is represented by d (3).

Conversely, when two linearly polarized lights whose polarization planes are perpendicular to each other are incident at a distance D, the two light beams are combined at the output end face and emitted as one light beam. As the birefringent substance, a liquid crystal or a uniaxial or biaxial birefringent crystal can be used.

For example, in the case of calcite (n.=1650 ne=1485), when the angle 0 is rubbed by 4〆2M, the small angle becomes the maximum (J=6
o23). Therefore, if the thickness d is 1 mm, the separation width D is 1.0 mm. Next, the present invention will be explained using the drawings.

FIG. 1 shows a first embodiment of the present invention, in which 1 is a local oscillation light source that generates linearly polarized light, 2 has parallel input and output end faces, and the direction of the principal axis relative to the refractive index is perpendicular to the entrance end face. 3 is a birefringent material in which the direction of the polarization plane of the linearly polarized light that can be transmitted does not match with the polarization plane of the light propagating inside the birefringent material 2; 4 is a photodetector; 5
is a linearly polarized signal light emitted from an optical fiber.

In the above configuration, the signal light 5 corresponding to the ordinary ray of light propagating inside the birefringent material 2 travels straight through the birefringent material 2 to the analyzer 3.

On the other hand, the light from the local oscillation light source 1, which corresponds to the extraordinary ray, is refracted through the back-refractive material 2 and propagated.
coincident with its propagation axis and proceed to the analyzer 3. In optical heterodyne detection, the polarization planes of local oscillation light and signal light must match. The analyzer 3 extracts components with matching planes of polarization from the light from the local oscillation light source 1 and the signal light 5. This is because the planes of polarization of the ordinary ray and the extraordinary ray form an angle of 90 degrees to each other. "By extracting polarization components at angles of 45 degrees with respect to the polarization planes of the two lights, components with equal polarization planes are extracted from the two lights.Light from the local oscillation light source 1 from the analyzer 3 Some light components of the signal light 5 and signal light 5 are extracted and incident on the photodetector 4.The photodetector 4 receives an interference light beam consisting of these two light beams.As a result, the amplitude, frequency, or phase is modulated. The signal light 5 is converted into intensity modulated light and an information signal is extracted. Fig. 2 shows a second embodiment of the present invention, in which 1 is a local oscillation light source, 2 is a local oscillation light source,
3 is an analyzer, 4 is a photodetector, and 5 is a signal light emitted from an optical fiber.

Here, the light from the local oscillation light source 1 and the signal light are vertically polarized lights whose polarization planes are perpendicular to each other, and they correspond to the ordinary ray and extraordinary ray of light propagating inside the birefringent material 2, respectively. In this configuration, the light was passed through the analyzer 3 as in the first embodiment. The signal light 5 and the light from the local oscillation light source 1 interfere with each other and reach the photodetector 4. As a result, an intensity modulation component corresponding to the modulated signal is extracted from the signal light 5 whose amplitude, frequency, or phase is modulated.

According to the first and second embodiments, even if the intensity of the signal light 5 is weak, a high-power light is used as the light from the local oscillation light source 1, and the polarization plane of the linearly polarized light transmitted by the analyzer 3 is changed. By appropriately selecting the direction, a low-noise, high-quality signal can be easily extracted from the signal light 5.

Further, since the main components have flat and parallel end faces, it is easy to integrate the entire device. In the first and second embodiments, in order to simplify the explanation, the two lights incident on the birefringent material 2 are linearly polarized lights, but the invention is not limited to this. Of course, it is also possible to use light that is not biased.

Further, in the first and second embodiments, the signal light 5 and the light from the local oscillation light source 1 may be passed through an object having a lens effect.

A transparent body or the like that allows light to pass through in the axial direction and whose refractive index gradually decreases from the center toward the periphery in a cross section perpendicular to the axial direction can be used as the lens object. Further, in the above embodiment, the photodetector 4 may be placed in close contact with the analyzer 3,
It is clear that a lens object may also be provided between them.

It is also clear that the signal light 5 may be obtained by bringing the output end face of the optical fiber close to the birefringent material 2.

Further, in the above embodiment, the case where the signal light 5 is light emitted from an optical fiber has been described, but the signal light 5 is not limited to this, and may be light that propagates in space or light that propagates through a lens guide. Of course.

Finally, to enumerate the features of the present invention, it is possible to obtain an optical heterodyne detection device that is small, less susceptible to surrounding influences, and easy to integrate as a whole, and has low noise in the case of directly polarized signal light. These include the ability to extract high-quality signals with extremely high efficiency.

[Brief Description of the Drawings] Figures 1 and 2 show first and second embodiments of the present invention, respectively, in which 1 is a local oscillation light source, 2 is a birefringent material,
3 is an analyzer, 4 is a photodetector, and 5 is a signal light. Oh figure juice 2 figure

Claims (1)

[Claims] 1 having a local oscillation light source and parallel input and output end faces for combining the local oscillation light and signal light from this light source;
An optical heterodyne detection device including a birefringent material whose principal axis direction with respect to the refractive index does not match the vertical direction of the incident end face, an analyzer, and a photodetector.