JPS6130247B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a small and simple variable branching circuit capable of branching light from a single mode fiber at an arbitrary division ratio.

Conventionally, there has been no known variable branching circuit capable of branching light from a single mode fiber at an arbitrary splitting ratio. However, the configuration shown in FIG. 1 can be considered as a variable branch circuit. That is, the light beam emitted from the core 2 of the input single mode fiber 1 is made into a parallel beam by the lens 3, and then made into linearly polarized light along the optical axis indicated by the arrow 5 by the polarizer 4. The linearly polarized light 6 passes through the polarization separation prism 7 with its polarization direction parallel to the optical axis of the polarizer 4. At this time, if the optical axis of the prism 7 forms an angle θ with the optical axis 5 of the polarizer 4 as shown by the arrow 8, the light is separated into a polarization direction 9 parallel to the optical axis of the prism 7 and a polarization direction 11 perpendicular to the optical axis of the prism 7. Ru. At this time, the branching ratio in the parallel direction and the perpendicular direction is cos ² θ:sin ² θ. Therefore, by changing θ, any branching ratio can be obtained. These branched lights are condensed by lenses 12 and 13 and convergently incident on single mode fibers 14 and 15 for coupling.

This branch circuit has the following problems. Normally, the light emitted from a single mode fiber becomes non-polarized while propagating through the fiber 1 even if polarized light from a semiconductor laser is input. Therefore, about half of the light is lost in the polarizer 4, and a branch circuit with low loss cannot be achieved. Second, the light exits the fiber 1 and enters the fibers 14, 15 through the entrance and exit surfaces of the lens 3, polarizer 4, prism 7, lens 12 or 13, and the exit surface of the fiber 1, the fiber 14 or It receives a total of 10 Fresnel reflections from 15 incident surfaces, so approximately
There is a loss of 2dB. Therefore, all of these fibers and optical elements require antireflection coatings. Thirdly, this branching circuit must rotate prism 7 and simultaneously rotate lenses 12, 13 and fibers 14, 15 in order to change the branching ratio.
Therefore, unless these elements are integrated, the optical axes incident on the fibers 14 and 15 will be shifted and a stable coupling cannot be obtained. However, prism 7, lenses 12 and 1
Since the three fibers 14 and 15 are separated in the direction of light propagation, it is difficult to rotate them together.

In order to eliminate these drawbacks, this invention uses a converging lens with a flat end surface that can be integrated with other optical elements, and uses a polarization splitting prism and a wave plate to produce highly stable variable light. A branch circuit is obtained. The drawings will be explained in detail below.

FIG. 2 shows an embodiment of the present invention, in which an input single mode fiber 16 having an elliptical core 17 as shown in FIG. 3 is used. This elliptical core fiber 16 is the long axis (or short axis) of the core 17.
When linearly polarized light 18 parallel to the elliptical core fiber 16 is incident, the polarization state is maintained within the elliptical core fiber 16. That is, the output light from the fiber 16 becomes linearly polarized light in the direction of the long axis (or short axis) of the core 17. In Fig. 1, a polarizer 4 is required to obtain linearly polarized light, resulting in a 3 dB loss, but in this example, a polarizer is not required to obtain linearly polarized light, and there is no 3 dB loss. The entrance surface of the focusing lens 19 is bonded to the exit surface of the fiber 16 using an optical adhesive or the like to be integrated. The converging lens 19 is cylindrical and has a refractive index distribution of n(r)=n ₀ (1−ar ² /2), where r is the distance from the central axis in a cross section perpendicular to the central axis. It is. Here, n ₀ is the refractive index of the central axis,
a is a constant indicating the degree of change in refractive index. The property of this focusing lens 19 is that when a light beam in a direction perpendicular to its end surface is incident at a position x ₀ away from the central axis, the light beam will cross the central axis with a periodic length (pitch) expressed as P=2π/√. It undulates as an axis of symmetry. At a length of P/2 (1/2 pitch), the light beam comes to the position -x ₀ . On the other hand, the diameter of the light beam is repeatedly increased and decreased within the lens 19 at a period of P/2. Therefore, when the lens 19 has a length of P/2, the light beam diameter is the same as when it entered the lens, that is, it returns to the same size as the output beam diameter of the fiber 16, but the positional deviation is reversed with respect to the central axis. . In FIG. 2, the lens 19 integrated with the entrance fiber 16 is made slightly shorter than 1/2 pitch, and the central axes of the fiber 16 and lens 19 are aligned.

A polarization splitting prism 7 is used as an element for splitting light. The prism 7 uses a uniaxial birefringent crystal, and its optical axis (C-axis) 21 lies within a plane that includes the direction of travel of light (optical axis) and forms an angle α with the direction of travel. The light incident on this prism 7 has a polarized component (extraordinary light) parallel to the plane containing the C-axis and the optical axis, which is refracted by the prism and focused within the plane containing the C-axis, as shown in the light beam 22 in the figure. The light is incident perpendicularly to a position y ₀ away from the central axis of the sexual lens 23 . On the other hand, the polarized light component perpendicular to the above plane (ordinary light) travels straight through the prism 7 like the light beam 24 in the figure and coincides with the central axis of the lens 23. If the length of the lens 23 is selected to be an integral multiple of 1/2 pitch, the light beams of both polarizations will pass through the lens 23 at the output end of the lens 23.
The diameter of the beam will be the same as that when it enters the beam, and the two positions will be separated by y ₀ . Therefore, if two single mode coupling fibers 14 and 15 are placed in close contact with the output end face of the lens 23 so that the distance between their core centers is y ₀ , the extraordinary light 22 and the ordinary light 24 can be transmitted without loss. Join.

Therefore, the polarization separation prism 7, the focusing lens 2
3. The single mode fibers 14 and 15 are integrated in advance with their optical axes aligned, and the converging lens 1
9 and 23, the angle θ between the polarized light incident parallel or perpendicular to the long axis of the elliptical fiber 16 and the plane formed by the C-axis and the optical axis of the polarization separation prism 7 will be Can be changed arbitrarily.
Therefore, the ratio between the extraordinary light 22 and the ordinary light 24 is cos ² θ:
sin ² θ, and a variable branch circuit is realized.

Here, the conditions for the lenses 19 and 23 and the prism 7 will be further explained. Prism 7 is an extraordinary light,
The ordinary light must be separated by at least the diameter of the fibers 14 and 15. When calcite is used for the prism 7, the separation becomes maximum when α=42.06°, and if the thickness of the prism 7 is t, then y ₀ /t=0.1. When y ₀ is 125 μm or more, t≧1.25 mm. In the lens 23, the light beam 2
2 and 24 are incident perpendicularly. Therefore, the beam emitted vertically from the lens 23 has a length of 1/
This is obtained when the pitch is an integral multiple of 2 pitches, and the beam diameter is the same as the beam diameter at the incident surface.
In order to efficiently couple the single mode fibers 14 and 15, the length of the lens 23 must be an integral multiple of 1/2 pitch. Furthermore, lens 23
The beam diameter entering the fibers 14 and 15 must be made the same as the beam diameter of the fibers 14 and 15. The diameter of the beam incident on lens 23 depends on the length of lens 19. In a multi-component rod lens with a diameter of 1.8 mm, the parameter a representing a change in refractive index is about 0.12 mm ^-2 , so the 1/2 pitch length is 9.2 mm. If the length of the lens 19 is set to an appropriate value close to the length obtained by subtracting the thickness of the prism 7, approximately 1.3 mm, from this length, the fiber 1
The beam emitted from the lens 19 passes through the fiber 16 again at the contact surface between the prism 7 and the lens 23.
The beam is focused to the same size as the mode.
Therefore, the length of the lens 19 is the value obtained by subtracting the thickness of the prism 7 from the 1/2 pitch length and adding an integral multiple (including 0 times) of the 1/2 pitch length. The key is lens 1
The length from the entrance surface of lens 9 to the exit surface of lens 23 is
It may be an integral multiple (twice or more) of 1/2 pitch, but in that case, for ease of design, the lens 23 is preferably an integral multiple of 1/2 pitch. In the example shown in FIG. 2, the fiber 16 and the lens 19 are integrated, and the prism 7, lens 23, and fibers 14 and 15 can also be integrated, so the contact surface between the optical element and the air is between the lens 19 and the prism 7. Reduced to 2 sides. Therefore, Fresnel reflection loss is reduced to about 0.4 dB.

FIG. 4 shows another embodiment of the invention. The basic components are almost the same as those shown in Fig. 2, but there is a
A two-wavelength plate 25 is inserted. 1/2 wavelength plate 2
5, the polarization direction is rotated by 2, where the angle between the fast axis and the incident linearly polarized light is taken as the angle. In advance, the optical fibers 14, 15 and 16 and the lens 1 are
9 and 23 and the polarization separation prism 7 are integrated with their optical axes aligned. Here, the fiber 16 is arranged so that the long axis or short axis of the ellipse of the core 17 coincides with the plane formed by the C axis and the optical axis of the prism 7, and the 1/2 wavelength plate 2
5, the beam incident on the prism 7 is made to be ordinary light or extraordinary light. In this state, when inserting the 1/2 wavelength plate 25 and rotating it around the optical axis, the fast axis of the 1/2 wavelength plate 25 and the lens 1
Since the polarization direction incident on the prism 7 changes by 2 depending on the angle with the linear polarization direction incident through the prism 9, the prism 7, the lens 23, and the lens 23 shown in FIG.
This is equivalent to rotating the integrated fibers 14 and 15 twice, and a variable branch circuit is realized.

Here, the thickness of the 1/2 wavelength plate 25 varies depending on the birefringence value of the birefringent crystal used, but it is approximately 100 mm thick.
Since it is very thin, less than .mu.m, the imaging conditions, that is, the optimum length of the lens 19, do not differ much from the case of FIG. 2. In this example, all the optical elements shown in FIG. 4 are integrated, and only the 1/2 wavelength plate 25 needs to be able to rotate from 0 to 45 degrees, and the central axis of rotation may be anywhere. Therefore, the optical axis (center axis of lens 19 and lens 23) due to rotation is a problem in the embodiment shown in FIG.
There is no fluctuation in the coupling efficiency to the fibers 14, 15 due to wobbling, and the coupling is always performed under the initially set conditions, resulting in even higher reliability. Note that the phase difference of the half-wave plate 25 does not need to be completely 180 degrees, and may be shifted by ±11 degrees assuming that the maximum attenuation of the variable optical branch circuit is 20 dB.

In addition, instead of changing the branching ratio from 1:0 to 0:1, it may be applied to monitor the light intensity of other coupling fibers by changing the branching ratio from 1:0 to about 0.9:0.1 and using a branching fiber that couples several percent. For example, instead of the 1/2 wavelength plate 25, a wavelength plate having a slightly different phase difference may be used. Further, as the monitoring fiber, a step type or graded type fiber with a large core diameter and NA may be used. Among the components used in the branch circuit of the present invention, the focusing lens, polarization separation prism, and wavelength plate that come into contact with air may be coated with an antireflection film using a dielectric multilayer film or the like in advance.

As explained above, in this invention, the lens system and the fiber can be integrated with an optical adhesive or the like, so that unnecessary Fresnel reflection loss can be reduced. Further, although a movable part is required to change the branching ratio, since the prism, lens, and branching fiber are integrated, optical axis deviation due to movement can be reduced, and increase in extra coupling loss due to movement can be reduced. In particular, in the embodiment shown in Fig. 4, which uses rotation of the wave plate, the components that require optical axis alignment can remain fixed, so there is no optical axis misalignment caused by changing the branching ratio, resulting in extremely stable and low A variable loss branch circuit can be constructed. Furthermore, when linearly polarized light from a semiconductor laser or the like is used and the polarization characteristics are maintained with an elliptical fiber, there is no extra loss due to linearly polarized light.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the principle of an unimproved variable optical branching circuit for a single mode fiber using polarization, Fig. 2 is a diagram showing the principle of an embodiment of a variable optical branching circuit according to the present invention, and Fig. 3 4 is a sectional view of the fiber 16, and FIG. 4 is a diagram showing the principle of another embodiment of the variable optical branch circuit according to the present invention. 1: Single mode fiber for incidence, 2: Core portion, 3: Lens, 4: Polarizer, 7: Polarization separation prism, 14, 15: Branching fiber, 16: Single mode elliptical core fiber, 17: Elliptical core, 1
9, 23: Focusing lens, 22: Extraordinary light, 24:
Ordinary light, 25:λ/2 wavelength plate.

Claims (1)

[Claims] 1. The output light from one single mode fiber is
In a variable optical branching circuit that couples to a main coupling fiber at an arbitrary branching ratio, an elliptical core fiber is used as the input single mode fiber, and the output end face of the elliptical core fiber is slightly shorter than an integral multiple of 1/2 pitch. A first converging lens with a length of 1/2 pitch is attached and integrated, and two of the coupling fibers are arranged in an array, and a second converging lens with a length of an integral multiple of 1/2 pitch is attached to the incident end face. attaching and integrating a polarization separation prism to the incident surface of the second converging lens, and making the polarization separation prism face the first convergence lens;
The entire coupling fiber with a prism lens or the entire input single mode fiber with a lens can be rotated about the optical axis formed by the coupling fiber and the input single mode fiber as a central axis. Variable optical branch circuit for single mode fiber. 2 The output light from one single mode fiber is
In a variable optical branching circuit that couples to a main coupling fiber at an arbitrary branching ratio, an elliptical core fiber is used as the input single mode fiber, and the output end face of the elliptical core fiber is slightly shorter than an integral multiple of 1/2 pitch. A first focusing lens having a length of 1/2 pitch is attached and integrated, and two of the above-mentioned coupling fibers are arranged in an array, and a second focusing lens having a length that is an integral multiple of 1/2 pitch is attached to the incident end face. A polarization separation prism is attached to the incident surface of the second converging lens, and a wavelength plate is inserted between the polarization separation prism and the output surface of the first condensing lens. 1. A variable optical branching circuit for a single mode fiber, characterized in that the wavelength plate can be rotated in a plane perpendicular to the optical axis while the components other than the plate are fixed with their optical axes aligned.