JPS5993417A - Optical directional coupler - Google Patents

Abstract

PURPOSE:To maintain the polarization characteristic and distribution ratio of output light of a 3dB coupler against the disturbance such as temperature fluctuation, torsion, and bending by using polarization holding optical fibers for input and output. CONSTITUTION:Polarization holding optical fibers 9-11 are birefringent index optical fibers which have strain generation parts around a core and hold linear polarization in directions (X, long axis) connecting the core and the strain generation part and in orthogonal direction (Y, short axis). The linear polarization direction of the fiber 9 is in parallel (P polarization) to the paper surface and the polarization of the whole system is made coincident with P by setting the X axes of the fibers 10 and 11 in parallel to the paper surface; arrows indicate polarization directions. Output light from the fiber 9 while diverging in a convergent rod lens 4 enters a half-reflecting mirror 5 as a plane wave, so that crosstalk between reflected light and transmitted light is hardly changed. The reflected light and transmitted light converge in convergent rod lenses 7 and 8 in a linear polarization state and propagate to the fibers 10 and 11 as X-axial polarized waves.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polarization-maintaining focusing rod lens 8 CIB directional coupler in which polarization and distribution ratio are accurately maintained.

The configuration of a conventional 8 dB directional condenser with a focusing rod lens is shown in FIG. In FIG. 1, 112+8 is a circular core optical fiber, 4 is a focusing rod lens, 5 is a semi-reflecting mirror, and 6+7+8 is a spread of the light beam. optical fiber 1
The output light beam from the beam spreads inside the convergent rod lens 4 as shown in 6, and the reflected light and transmitted light from the semi-reflective mirror 6 are as follows.
Optical fibers 2 and 8 are focused at 7°8 respectively.
incident on the In this configuration, since the optical fiber used is a true circular core optical fiber and does not maintain the same wave, the incident light to the optical fiber 1 may be affected by disturbances such as temperature fluctuations, twisting, and external pressure applied to the optical fiber. Even if the light is linearly polarized, the light incident on the semi-reflecting mirror 5 becomes elliptically polarized, and its polarization state varies greatly. For this reason, there was a drawback that the polarization states of reflected light and transmitted light also varied greatly due to disturbances. Moreover, the current focusing rod lens 8 dB
In the configuration of the directional coupler, the light emitted from the optical fiber 1 is not perpendicularly incident on the semi-reflecting mirror 5, and the reflectance and transmittance differ depending on the polarization state of the incident light. Therefore, as the power of the reflected light and transmitted light from the semi-reflecting mirror 5 changes according to the change in the polarization state of the incident light due to disturbance, the distribution ratio of reflected light and transmitted light can only be within the range of this fluctuation. The drawback was that it could not be controlled.

In order to solve these drawbacks, the present invention uses a polarization-maintaining optical fiber as an input/output optical fiber to maintain the polarization and distribution ratio of the entire directional device by 8 dB.

The present invention will be explained in detail below with reference to the drawings.

FIG. 2 shows an embodiment of the present invention, in which reference numerals 9, 10, and 11 denote polarization-maintaining optical fibers, which in this embodiment have a structure with a stress-applying part around the core. The direction connecting the parts (y-axis, long axis) and the direction perpendicular to it (y-axis, long axis)
PANDA optical fiber is a birefringent optical fiber that maintains linearly polarized light (axis, short axis).
on -maintaining anclAbsor
ption-reduc±ng, O,FC'
82 THOO6) is used.

8th m (FL>, (b), (0), (d)
Lt shows enlarged cross-sections of the polarization-maintaining optical fiber at A, B, (1, and D in Fig. 2, respectively. Fig. 8 shows the long axis of the core birefringence at the input end and the core birefringence at the output end. The long axes of the ratios are parallel.

The linear polarization direction of the light incident on the polarization-maintaining optical fiber 9 is parallel to the plane of the paper, that is, as a PI wave, and the side waves of each part are maintained so that the polarization of the entire system matches the P polarization. The direction of the y-axis, which holds one wave of the optical fibers 10 and 11, is parallel to the plane of the paper. The arrow ↓ in FIG. 2 indicates the polarization direction of light. The output light from the polarization-maintaining optical fiber 9 is diagonally incident on the semi-reflective mirror 5 while expanding inside the converging rod lens 4 as shown in 6 as linearly polarized light parallel to the plane of the drawing.

However, since most of the light is incident on the semi-reflecting mirror 5 as a plane wave, the crosstalk between the reflected light and the transmitted light from the semi-reflecting mirror 5 hardly changes (-20 dB or less).

Then, the reflected light and the transmitted light are linearly polarized parallel to the plane of the paper and converged in the focusing rod lenses as shown in 7 and 8, respectively, and sent to polarization-maintaining optical fibers IO and 11 as polarized waves in the X-axis direction. propagate. With this structure, (1) the polarization is maintained against external disturbances, and the output light is maintained as linearly polarized light when it is incident in a straight line; ePI wave or S polarized wave (here P
The @ wave indicates a straight lunar wave 2 in which the electric field vector is within the plane of incidence, and the S-polarized wave indicates a linearly polarized wave in which the electric field vector is perpendicular to the plane of incidence. ).

(11) Since the wave of the light incident on the semi-reflecting mirror is retained (PI wave or S-wave), the thickness, refractive index, and number of layers of the multilayer thin film for semi-reflecting light are optimally designed. What to do,
It becomes possible to match the power ratio of reflected light and transmitted light with an accuracy of five decimal places.

FIG. 4 shows an embodiment of the pond of the present invention. Here, the y-axes of the respective end faces (B', 0') of the input optical fiber 9 and the output optical fiber 10 at the point of the focusing rod lens 4 are shown in FIG. As shown, the directions are perpendicular to each other. With this structure, the light that propagates in the optical fiber 9 as a linearly polarized wave in the It becomes possible to propagate it as polarized light.

Figures 6 (a), (b), (03, (d)) show enlarged cross sections of the polarization maintaining optical fiber at A/, B/, Q/, and D/ in Figure 4, respectively. In this case, the long axis of the core birefringence at the input end and the short axis of the core birefringence at the output end are arranged in parallel.

Hereinafter, the above (1) and (11) will be explained in detail.

(1) The electric field of reflected light and transmitted light from a semi-reflecting mirror is expressed as follows (M, Born, E, WO1f
'Pr1noiples Of 0ptios'IP
P, as ~% 48].

In FIG. 6, 12 is the surface of the semi-reflecting mirror 5, 18 is the incident surface, 14 is the incident light, 15 is the reflected light, 16 is the transmitted light, and n in the figure is the normal direction of the semi-reflecting mirror 5. Indicates A/
, A shows the components of the incident electric field vector parallel and perpendicular to the plane of incidence, R/.

R construction and T7 + T soil show respective components of reflected light and transmitted light. At this time, θt is the angle of incidence on the semi-reflecting mirror, and θt is the angle between the traveling direction of the transmitted light and the normal n.

In the implementation situation of the present invention shown in FIG. 2, the polarization direction of the incident light is within the plane of incidence, so it is A-0, (11,
From (2), on T = on R = 0, and the deterioration of crosstalk between reflected light and transmitted light can be ignored. Therefore, it is possible to make the direction of the linear peak of the reflected light and the transmitted light coincide with the direction of the main axis of the polarization-maintaining optical fiber, so that the directly polarized light propagates through the entire system.

(11) When P□□□ wave and S polarized wave (7) having the same intensity are incident, the ratio pL /p/ of the intensity of each S polarized wave and P polarized wave of transmitted light is (1 ) in the formula, A on = A/T, and P''/P' = (: cos(θ soil - θt)
]2...(1)′T. The length of the converging rod lens used is 3 as, and the optical fibers in Fig. 1) < 1 and 2
Since the distance between the cores is approximately 600 μm, the incident angle θi to the semi-reflecting mirror 5 is approximately
°5 (“) / 8 (engineering) = 0-16 radians 1
Therefore, using 7Renell's law regarding reflected light and transmitted light, it becomes θ1.;0.1 radian, and (1)l
Therefore, P/P/P is 0.996 T, which means that there is a difference of about 0.4%.

A similar estimation can be made for the reflection gear C. Therefore, when using a normal circular core optical fiber, the optical power of reflected light and transmitted light will fluctuate by about 0.4% depending on the polarization state that changes due to disturbance!・b(8) On the other hand, when using a polarization-maintaining optical fiber, it is possible to maintain the incident light to the semi-reflector as P polarization with -20 dB crosstalk, so 0.4 %x O,01=
It is possible to suppress fluctuations in the power of reflected light and transmitted light with an accuracy of 0.004%, or 5 digits, and the maximum im design of the semi-reflector makes it possible to match the distribution ratio with an accuracy of 5 digits. Become.

Figure 7 is an explanatory diagram of P polarization and S polarization, where 17 is P polarization, 1
8 indicates S polarization.

Using a long optical fiber, the rotational angular velocity of the system (>t fiber drum) is measured by detecting the phase difference between the left and right propagating light within the optical fiber, which occurs when the optical fiber drum rotates. In an optical fiber rotation sensor, in order to achieve a rotation speed detection sensitivity of lo-80/hr, it is necessary to match the optical power of the left and right rotation lights in the optical fiber with an accuracy of 8 (I).
It is also necessary to suppress fluctuations in the distribution ratio of the B-directional condenser to within an accuracy of 10-4. From the above, it is expected that detection sensitivity will be improved by using the focusing rod lens type 8 dB directional coupler of the present invention.

In the embodiment of the present invention, PAN is used as the polarization maintaining optical fiber.
Although DA optical fiber was used, the present invention is not limited to PANDA optical fiber, but can also be applied to polarization-maintaining optical fiber (so-called twisted optical fiber) consisting of a twisted core fiber. It is valid.

As explained above, the optical directional coupler of the present invention uses a harmonic-maintaining optical fiber as an optical fiber for optical input/output of a focusing rod lens 8 dB directional coupler to maintain polarization of the entire system. This has the advantage that the polarization characteristics (mainly crosstalk) and distribution ratio of the output light of the BaB coupler can be maintained against disturbances such as temperature fluctuations, twisting, and bending to the optical fiber. Therefore, the detection sensitivity decreases due to variations in the plane of polarization. This is expected to significantly improve detection sensitivity.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional focusing rod lens 8 CIB directional coupler, Fig. 2 is a block diagram of an embodiment of the present invention, and Fig. 8 (a), (b), (C1, (dl are the second
An enlarged cross-sectional view of the polarization maintaining optical fiber at points A, B, O, and D in the figure; FIG. 4 is a configuration diagram of another embodiment of the present invention; FIG. 5(a),
(b), (0), ((1) are respectively A/ in Figure 4,
Enlarged cross-sectional view i at B/, Ql, D/ (Figure 6 is a diagram to explain the crosstalk of reflected light and transmitted light on a semi-reflective surface, Figure 7 is an explanatory diagram of Pm wave and S polarized wave 1.2.8...Perfect circular core/optical 7 eyeper, 4...Focusing rod lens, 5...Semi-reflection fR, 61718...
- Spread of optical beam %9110111...Polarization-maintaining optical fiber, 12...Semi-reflecting mirror, 18...Incidence surface, 14...Incoming light, 15...Reflected light, 16...・Transmitted light, 17...P polarization, 18...S @ wave, A/ *
R/ + T/... component parallel to the plane of incidence of incident light, reflected light, and transmitted light, A soil, '(11) On R, on T... plane of incidence of incident light, reflected light, and transmitted light component perpendicular to . Patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. 8 dB light direction with an optical fiber at the input and output ends, with the axes of two focusing rod lenses aligned on -l1fl, and a semi-reflecting mirror sandwiched between the focusing rod lenses. In the directional coupler, by using a polarization maintaining optical fiber for the optical fiber at the protruding end, the linearly polarized light incident on the 8 dB optical directional coupler maintains a linearly polarized state until the output end. An optical directional coupler characterized by: a In the optical directional coupler according to claim 1, a birefringence optical fiber is used as the polarization maintaining optical fiber, and the long axis of the core birefringence at the input end and the core birefringence at the output end are aligned. Optical directional coupling characterized by two characteristics: the long axis of the refractive index is arranged in parallel, or the long axis of the core birefringence at the input end and the short axis of the core birefringence at the output end are arranged in parallel. vessel.