JPH02130517A - Polarized wave coupler - Google Patents

Abstract

PURPOSE:To reduce loss due to absorption and to prevent the output level of monitor light, etc., from depending upon the plane of polarization of incident light by arranging a 1/4-wavelength plate between a polarized wave coupling prism and a branch prism and making circular polarized light incident on the branch prism. CONSTITUTION:The polarized wave coupling prism 11, 1/4-wavelength plate 15, and branch prism 16 are fixed mutually and united by using an adhesive. The optical axis, etc., of the 1/4-wavelength plate 15 is so set that two planes of polarization of the incident light are at 45 deg. to the plane of polarization is light which becomes an ordinary light beam and an extraordinary light beam as to birefringent crystal forming the 1/4-wavelength plate 15, so the two linearly polarized light beams which are incident on the 1/4-wavelength plate 15 on the same optical axis are converted into circular polarized light beams respectively. Consequently, even when a dielectric multi-layered film is used for the branch film 18 of the branch prism, the branch ratio does not depend upon the direction of the polarization. Consequently, transmitted light power or reflected light power does not have polarization dependency and the absorption loss of the branch film is reduced.

Description

[Detailed Description of the Invention] Overview This invention relates to a polarization coupler that combines lights whose polarization planes are orthogonal to each other, which has low loss due to absorption, the output level of monitor light, etc. does not depend on the incident polarization plane, and has a simple configuration. In order to provide a simple polarization coupler, we have developed a polarization coupling prism that outputs incident lights whose polarization planes are orthogonal to each other onto the same optical glaze, and a monitor light that is placed on the output optical axis of the polarization coupling prism. In a polarization coupler configured with a branching prism for extraction, a branching prism equipped with a dielectric multilayer film is used as the branching prism,
1/2 between the polarization coupling prism and the branching prism.
A four-wavelength plate is arranged so that the light incident on the branching prism becomes circularly polarized light.

INDUSTRIAL APPLICATION FIELD The present invention relates to a polarization coupler that combines lights whose polarization planes are orthogonal to each other.

When duplicating light sources on the transmitting side in order to construct a highly reliable optical communication system, wipe each light source in advance so that the emitted light is coupled to the optical transmission line.
When the system starts operating, only one light source is used, and when that light source breaks down, the other light source is used to prevent the system from going down. Polarization couplers are used in this type of highly reliable system to combine the output light from two light sources into a common optical transmission path. By the way, for the transmitting light source, it is necessary to split a part of the emitted light and monitor it, for example in order to monitor its deterioration over time, and there is a demand for a polarization coupler that can perform such monitoring well. has been done.

BACKGROUND OF THE INVENTION FIG. 5 is a block diagram of a conventional polarization coupler. 41 is a polarization coupling prism having a polarization separation (synthesis) film 41a made of a dielectric multilayer film, etc. By using such a prism, incident lights whose polarization planes are orthogonal to each other can be output on the same optical axis. can do. That is, the polarization plane (vibration plane of the electric field vector, the same applies hereinafter) is the polarization separation (synthesis) film 41a.
P (para
llel) wave passes through the polarization separation (synthesis) film 41a,
The 3 (senkrecht) waves from the input boat whose plane of polarization is perpendicular to the plane of incidence to the polarization separation (synthesis) film 41a are reflected by the polarization separation (synthesis) film 41a, and these transmitted and reflected lights are are emitted on the same optical axis.

42 is a branching prism for extracting monitor light from the light emitted from the polarization coupling prism 41 emitted on the same optical axis, and this branching prism 42 is usually made of metal and 5j02 in order to suppress polarization dependence. The structure includes a branch film 42a formed by alternately laminating layers. branch prism 4
Of the light incident on the branching prism 4, the light that has passed through the branching film 42a is guided from the main signal output boat to the optical transmission path, and then passes through the branching prism 4.
The light reflected by the branching film 42a among the light incident on the optical fiber 2 is guided from the monitor light output port to a suitable light receiver, and is used for monitoring the output light level, feedback control, and the like.

However, if the branch film 42a includes a metal film,
There is a problem in that the absorption loss in the branching film 42a is large, and the level of the main signal coupled to the optical transmission path decreases when monitor light of a predetermined level is extracted. In view of this point, Aj! 20. It is conceivable to use a dielectric multilayer film consisting of alternating layers of TiO2 and TiO2, but in this case, unlike the case where the branch film includes a metal film,
As shown in FIG. 6, when the incident angle to the branching film 42a is 45°, which is convenient for the device configuration, the polarization dependence of the power of the light transmitted through the branching film 42a becomes significantly large.

Therefore, when a dielectric multilayer film is used as the branch film, a configuration as shown in FIG. 7 is adopted.

In this configuration example, the branching prism 43 is configured such that the incident angle to the branching film 43a is sufficiently small, about 10°, so that the polarization dependence of the transmitted light power of the branching film 43a is small. The reflected and extracted monitor light is further reflected by the total reflection surface 43b of the branching prism and is extracted to the outside. The reason why the monitor light is reflected by the total reflection surface 43b is that it is almost perpendicularly incident on the branching film 43a, and if it is not reflected by the total reflection surface 43b, it will be blocked by the polarization coupling prism 41 etc. and will not be monitored by the light receiver. This is because the light cannot be monitored.

Problems to be Solved by the Invention With the conventional configuration as described above, there was a problem in that when the branching film of the branching prism included a metal film, the loss due to absorption was large. In addition, simply changing the branching film containing a metal film to a dielectric multilayer film poses the problem that the output level of the monitor light or the output level of the main signal guided to the optical transmission line depends on the plane of polarization of the incident light. there were. moreover,
When adopting the configuration shown in Figure 7 to eliminate this polarization plane dependence, there were problems in that the configuration became complicated, especially for the branching prism, and complicated optical axis adjustment was required. .

The present invention was created in view of these circumstances, and provides a polarization coupler that has a small loss due to absorption, the output level of monitor light, etc. does not depend on the polarization plane of the incident light, and has a simple configuration. intended to provide.

Means to solve problems FIG. 1 is a diagram showing the principle of the present invention.

Reference numeral 2 denotes a polarization coupling prism, which emits incident lights whose polarization planes are orthogonal to each other onto the same optical axis 1.

Reference numeral 4 denotes a branching prism for extracting monitor light, which is disposed on the output optical axis l of the polarization coupling prism 2, and is provided with a dielectric multilayer film 3 as a branching film.

Reference numeral 5 denotes a quarter-wave plate disposed between the polarization coupling prism 2 and the branching prism 4, which converts the light emitted from the polarization coupling prism 2 into circularly polarized light and makes it enter the branching prism 4.

FIG. 2 is an auxiliary diagram for explaining the principle of the operation of the quarter-wave plate 50, and for convenience, it is assumed that the birefringent crystal constituting the quarter-wave plate 5 is a positive axle crystal. It shows the index ellipsoid at the time.

Now, the refractive index of the quarter-wave plate 5 for ordinary rays is no.
and the maximum value of the refractive index for the extraordinary ray is n# (
n o < n s ). Assume that light is propagating in the direction of arrow S from the origin 0 of the orthogonal three-dimensional coordinate axes whose two axes are the optical axes of the quarter-wave plate 5, and that the projection of arrow S onto the xy plane coincides with the y-axis. Suppose that

At this time, the refractive index ellipsoid is
22/ne”=1.

The refractive index no for ordinary rays is always constant, and the origin up to the point P where the circle of the refractive index ellipsoid intersected by the xy plane and the ellipse B cut by the plane perpendicular to the propagation direction S at the origin ○ intersect. It is expressed as the distance Jtop from 0. In addition, the refractive index 16' for the extraordinary ray changes depending on the angle θ between the propagation direction S and the two axes, and is expressed as the distance All0Q from the origin O to the point Q where the ellipse B and the yz plane intersect. Ru.

In other words, the refractive index n,' for the extraordinary ray changes continuously from no to n depending on the propagation direction S of the light. In this way, the refractive index for extraordinary rays changes depending on the propagation direction S of the light, so for example, the optical axis of the birefringent crystal is set so that the propagation direction S coincides with the y-axis (θ = 90°). The difference between the refractive index for the ordinary ray and the refractive index for the extraordinary ray is maximized, and the polarization plane of the output light of the polarization coupling prism 2 whose polarization planes are orthogonal to each other is set to the polarization direction OP of the ordinary ray and the polarization direction OP of the extraordinary ray. 45° to polarization direction OQ
The birefringent crystal is tilted so that the amplitudes of the ordinary ray component and the extraordinary ray component are equal, and the thickness of the birefringent crystal is set so that the phase difference between the ordinary ray component and the extraordinary ray component is π/2. By configuring the quarter wavelength plate 5, the light incident on the branching prism 4 can be converted into circularly polarized light.

At this time, unless the light incident on the quarter-wave plate is linearly polarized, it cannot be converted into completely circularly polarized light. In reality, the polarized light that enters the polarization coupling prism 2 is somewhat elliptically polarized, but when it passes through the polarization separation (synthesis) film, it becomes linearly polarized light with a high polarization extinction ratio (20 dB or more), so this configuration is suitable. Accordingly, circularly polarized light can be incident on the branching prism 4 regardless of the polarization state before it is incident on the polarization coupling prism 2.

When the incident light on the branching prism 4 is converted into circularly polarized light, the P wave component and the S wave component incident on the dielectric multilayer film 3 have the same phase difference but the amplitudes are the same. Even if the incident angle is set to about 45 degrees, there is no fear that polarization dependence will occur in the transmitted light power or the reflected light power, and there is no need for the branching prism 4 to have a complicated configuration.

In the configuration of the present invention, a dielectric multilayer film is used as the branching film of the branching prism in order to reduce absorption loss in the branching film.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 3 is a plan view of a polarization coupler showing an embodiment of the present invention. 11 is a polarization coupling prism, and triangular prism 12
．． A polarization separation (synthesis) film 14 such as a dielectric multilayer film is interposed between the slopes 13. Reference numeral 15 denotes a quarter wavelength plate, which is formed from a birefringent crystal having a predetermined thickness.

A branch prism 16 is constructed by interposing a branch film 18 made of a dielectric multilayer film between the slopes of triangular prisms 17 and 19. And polarization coupling prism 11.1/
The four-wavelength plate 15 and the branching prism 16 are integrated by fixing them to each other using an optical adhesive or the like.

The input port ■ is configured as shown in the figure, with the output light of the polarization maintaining optical fiber 20 inserted and fixed in the ferrule 21 being approximately collimated by a lens 22 such as a focusing rod lens, and the output light is emitted at an incident angle of 45, for example. The beam is incident on the polarized wave separation (synthesis) film 14 at an angle of .degree., and its plane of polarization is parallel to the plane of incidence. The input port ■ is
The output light of the polarization maintaining optical fiber 23 inserted and fixed in the ferrule 24 is configured to be approximately collimated by the lens 25, and the output light has an incident angle of 45, for example.
The polarized wave is incident on the polarization separation (synthesis) film 14, and its plane of polarization is perpendicular to the plane of incidence, and the optical axis reflected by the polarization separation (synthesis) film 14 is aligned with the transmitted light from the input port (■). It is made to match the axis.

The quarter-wave plate 15 is arranged so that the two planes of polarization of the incident light form an angle of 45° with respect to the plane of polarization of the ordinary ray and the extraordinary ray of the birefringent crystal forming the quarter-wave plate 15. Since the optical axis etc. are set to 1 on the same optical axis,
The two linearly polarized lights incident on the /4 wavelength plate 15 are each converted into circularly polarized lights. Therefore, even when a dielectric multilayer film is used as the branching film 18 of the branching prism as in this embodiment, the branching ratio does not depend on the polarization direction, and therefore,
The angle of incidence on the branching film 18 can be set to about 45°. The circularly polarized light transmitted through the branching film 18 and emitted from the branching prism 16 is focused by a lens 29 and coupled to an optical fiber 31 inserted and fixed into a ferrule 30.

On the other hand, the circularly polarized light reflected by the branching film 38 and emitted from the branching prism 16 is focused by the lens 26 and coupled to the optical fiber 28 inserted and fixed in the ferrule 27.

The light introduced into the optical fiber 31 can be sent out as a main signal to the optical transmission line, and the light introduced into the optical fiber 28 can be used as monitor light to monitor the deterioration of the light source LD over time.

FIG. 4 is a graph showing the relationship between the power of transmitted light through the branching film 18 and the angle of incidence. Almost constant optical power can be obtained for the light from the input port (2) and the light from the input port (2) regardless of the incident angle, and there is almost no difference between them. Therefore, unlike the conventional method, the angle of incidence on the branching film is reduced to 10°.
There is no need to set it as small as possible, and in particular, the configuration of the branching prism can be simplified.

Effects of the Invention As detailed above, according to the present invention, since a dielectric multilayer film is used as the branching film, loss due to absorption can be eliminated, and the light incident on the branching prism can be converted into circularly polarized light. Therefore, it is possible to prevent the main signal output level or the monitor light output level from depending on the polarization plane of the incident light without complicating the configuration of the branching prism.

[Brief explanation of drawings]

Fig. 1 is a diagram of the principle of the present invention, Fig. 2 is a supplementary diagram for explaining the principle of the present invention, Fig. 3 is a plan view of a polarization coupler showing an embodiment of the present invention, and Fig. 4 is an embodiment of the present invention. Figure 5 shows the configuration of a conventional polarization coupler, and Figure 6 shows that the branching film shown in Figure 5 is a dielectric multilayer film. FIG. 7 is a graph showing the relationship between transmitted light power and incident angle at a certain time. FIG. 7 is a configuration diagram of another conventional polarization coupler. 2.11...Polarization coupling prism, 3...Dielectric multilayer film, 4.16...Branching prism, 5.15...1/4 wavelength plate. ``To 堝-B month 0J に り concave 1st figure (λ1 kipaito ■) (Lord, I wo gate i, 1.'A o＼゛-to) pod Shukuin 1 concave 3rd figure OP: n . OQ:ne Winter Komei, /) Fig. 5 Fig. 5 Fig. 5 (λ1 line °-t■) 111 (λ cover ■) - (Monitor light output port) Figure 7

Claims (1)

[Claims] A polarization coupling prism (2) that emits incident lights whose polarization planes are orthogonal to each other on the same optical axis (1), and a polarization coupling prism (2) that emits incident lights on the same optical axis (1); In a polarization coupler configured with a branching prism for extracting monitor light disposed in Combined prism (2) and the above-mentioned branch prism (4)
A polarization coupler characterized in that a quarter wavelength plate (5) is disposed between the branching prism (4) and the light incident on the branching prism (4) to become circularly polarized light.