JPS60173429A - Method and device for measuring dispersion of polarized wave - Google Patents

Abstract

PURPOSE:To offer the method and device for measuring the dispersion of a polarized wave easily with high precision by using a light source with short coherent length and detecting difference in propagation speed due to the polarization dispersion in a polarized wave holding optical fiber to be measured through a Michelson interferometer as the movement extent of a reflecting mirror. CONSTITUTION:Light from a semiconductor laser 1 is admitted in the polarized wave holding optical fiber 6 to be measured and waves in two orthogonal modes of the polarized wave holding optical fiber, i.e. modes H11X and HE11 are excited almost to the same intensity and multiplexed at the projection terminal of the fiber 6 to obtain linear polarized light, which is split by a half-mirror 9 in two orthogonal directions, reflected totally by reflecting mirrors 10 and 11 respectively, and multiplexed by the half-mirror 9 again. One of the reflecting mirrors 10 and 11, i.e. 11 is moved and the polarized wave dispersion of the optical fiber 6 is calculated from the distance between two positions of the reflecting mirrors which cause interference.

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a method for easily and highly accurately measuring the group delay time difference (polarization dispersion) between two polarization-maintaining modes in a polarization-maintaining optical fiber; It is related to the device.

(Prior Art) Conventional polarization dispersion measuring devices for polarization-maintaining optical fibers include devices that use optical pulses and devices that use a Twyman-Green interferometer. The former has the advantage of being able to easily measure the polarization dispersion of long fibers in real time, but has the disadvantage of requiring a light source to generate ultrashort optical pulses and a high-speed photodetector to receive the pulses. be.

-b) With a small scale device, the polarization dispersion of a polarization-maintaining optical fiber of several meters can usually be measured with an accuracy of several percent, but the measurement procedure is extremely complicated. J Before measuring,
1. Adjust the positions of the two total reflection mirrors so that the optical path lengths of the two arms of the Wyman-Green interferometer match.Next, set up the polarization-maintaining optical fiber to be measured, and l-1 [: , ,
, and i-I E 1 , one total inversion so that the light of the modes interferes maximally! )11N, and the polarization dispersion of the polarization-maintaining optical fiber to be measured can be determined from the moving distance of this total reflection mirror.

Furthermore, although a TV camera is used to find the position where the interference between the two modes is maximum, the light source used has a short coherent beam and requires a high output light source. .

(Object of the invention) In order to solve these drawbacks, the present invention aims to
Using a low-coherence light source with a coheren i~ length on the order of μm, the propagation velocity difference due to polarization separation in the polarization-maintaining optical fiber to be measured is calculated by Michelson interference, and the movement of the reflecting mirror ω
The purpose is to provide a method and apparatus that can easily and accurately measure the polarization dispersion of a polarization-maintaining optical fiber. The present invention will be explained in detail below with reference to the drawings.

(Structure and operation of light) Fig. 1 (A> shows one embodiment of the present invention, 1 is a semiconductor laser (wavelength 1.3 μm), 2 is an objective lens, 3 is 1/4
Wave plate; 4 is a polarizer; 5 is an objective lens; 6 is a stress-applied polarization-maintaining optical fiber (PANDA optical fiber) to be measured;
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Vol, 19. No, “19.1) L792-794
(1983)), 7 is an objective lens, 8 is an analyzer, 9 is a semi-transparent mirror, 10.11 is a total reflection mirror, 12 is a Ge-photodiode, 13 is an amplifier, 14 is an AC voltmeter, 15 is a It is. Note that FIG. 1(B) shows the linearly polarized light (21-d) entering the input side of the polarization-maintaining optical fiber to be measured in FIG. To IE Mort and 1-(E y mode 11 1
1 direction, and FIG. 1(C) is a diagram showing the analyzer of FIG. 1(A) in the direction of 456 with respect to the principal axis that maintains the polarization of the polarization-maintaining optical fiber to be measured. It is a diagram showing the direction of time.

To operate this, first, the injection current of the semiconductor laser is set above the threshold value to cause the laser to oscillate, and the system alignment 1 to 1 is performed.
I take the. In other words, the output light from the laser is passed through the objective lens 2.
to parallel light, 1/4 wavelength plate 3 to circularly polarize light, polarizer 4
-C is used to create linearly polarized light in a specific direction, condensing it with the objective lens 5, and directing it to the main axis that maintains the polarization of the polarization-maintaining optical fiber to be measured, as shown in Figure 1 (B). 45° to
direction, and simultaneously excite the HE mode and the IFy mode. At the output end of the optical fiber, as shown in FIG. After propagation, the l-I E and 1-(E,,1 modes of light are combined at rIQ with matching polarizations.

The combined light is directed to the Michelson interference IT, is split into two by a half mirror 9, and is split into two by a total reflection mirror 10 in each arm. Il'r is irradiated and the beam is combined again by half mirror 9C.

After alignment is performed in this way, the injection current of the semiconductor laser is lowered to below the threshold, and with the output light from the semiconductor laser incident on the Noiba under test, total reflection is performed on one arm of the Michelson interferometer. The mirror 11 is moved at a constant speed of about 10 μm/sec, the interference intensity is received by the Ge-photodiode 12C, and the AC component at this time is
Measure with meter 4 and record this on meter 15.

In this way, the polarization dispersion can be determined by the distance between the two peaks appearing on the loader according to the principle described below. That is, the injection current I is equal to the threshold value 1
The coherent length of the output light from the semiconductor laser below th is I=0.81. h is 50 μm, which can be considered to be a collection of wave packets whose output light is 50 μm. Group delay time difference between two modes of polarization-maintaining optical fiber under test (polarization dispersion)
As a result, the light emitted from the optical fiber is composed of two wave packets temporally shifted by the polarization dispersion DL, as shown in FIG. 2(A). Here, D is the group delay time per unit length, L
is the fiber length. Therefore, by using a Michelson interferometer to pass through two arms and moving the total reflection mirror of one arm to give a variable time shift, the signal propagated through one arm as shown in Figure 2. With respect to the wave packet (A>), the wave packets propagated through the other arm (the moving arm of the total reflection mirror) are as shown in (B), (C), , (D), and the time difference is 1" = -DL, T They will interfere with each other at the positions of the respective full anti-tiJ mirrors corresponding to =O, T=DL,
Since the magnitude of this interference light is measured with an AC voltmeter and recorded on a recorder, the optical path difference between the two peaks appearing on the recorder corresponds to the polarization dispersion DL.

FIG. 3 is a diagram showing interference peaks generated in this manner. The fiber length was 13.511, and the moving speed of the total reflection mirror was 56.8 μyn/S.

In this measurement example, it can be seen that the optical path difference between the maximum peak and the peak on one side is 5.5II11, and the polarization dispersion is igps.

FIG. 4 shows the fiber length dependence of polarization dispersion measured using two types of PANDA fibers. We were able to determine polarization dispersion with an accuracy of 2% for each fiber length. The minimum detection sensitivity was o, ips.

Note that λ shown in FIG. 4 is the wavelength, and Δ is the non-refractive index difference of the PANDA fiber to be measured.

The meaning of using a semiconductor laser as a light source here is:
When the injected current exceeds the oscillation threshold and the laser oscillates, the emitted light from the semiconductor laser has a long coherent length and high coherence, making it suitable for adjusting the optical arrangement of the Michelson interferometer. Furthermore, when the injection current is lower than the oscillation threshold, the coherent length is 50 μm.
Therefore, it is suitable for the measurement method of the present invention.

After the arrangement of the optical system is adjusted, measurement can be performed using a light source whose coherence length is generally known, such as an LED or a white light source.

(Effects of the Invention) As explained above, the present invention uses a light source with a short heeling length, a Michelson interferometer, and a photodetector, so that polarization-maintaining optical fibers can be easily and precisely It has the advantage of being able to measure polarization dispersion.

[Brief explanation of the drawing]

Figure 1 (A) is an embodiment of the present invention, Figure 1 (B) is 45° to the main axis that maintains polarization at the input end of the polarization-maintaining optical fiber to be measured in Figure 1 (A>).
Figure 1 (C) shows the polarization-maintaining light incident on the analyzer in Figure 1 (A). Figure 2 (A) and (
B), (C), and <D> are explanatory diagrams of the measurement oI principle related to the present invention. Figure 3 is a diagram showing an example of measurement of polarization dispersion. Figure 4 is the dependence on fiber length in measurement of polarization dispersion. FIG. 1...Semiconductor laser 2...Objective lens 3...1
/4 wavelength plate 4...Polarizer 5...Objective lens 6.
...Polarization-maintaining optical fiber to be measured (PANDA fiber) 7...Objective lens 8...Analyzer 9...Semi-transparent mirror 10, 11...Full role O1 mirror 12...
・Ge-photodiode 13...Amplifier 14...AC voltmeter 15・
...Recorder. Figure 2-DL ODL 2DL B! fM 1st M eg! hard m)

Claims (1)

[Claims] 1. Light emitted from a light source with low coherence is introduced into a polarization-maintaining optical fiber to be measured, and the HE mode and HEy mode, which are two orthogonal modes of the polarization-maintaining optical fiber, are synchronized. Excitation U is applied to an intensity of about 11 11 , both modes are combined again at the output end of the polarization-maintaining optical fiber to form linearly polarized light, and this linearly polarized light is separated into two mutually orthogonal directions by a semi-transparent mirror. The light is totally reflected by a set of reflecting mirrors and recombined by the semi-transparent mirror, and the reflected light is calculated from the distance between the two positions of the reflecting mirrors that cause interference by moving one of the reflecting mirrors of the set of reflecting mirrors. A polarization dispersion measurement method characterized by measuring polarization dispersion of a polarization-maintaining optical fiber. 2. A light source that emits light with low coherence, a polarizer that converts the light from the light source into linearly polarized light, a lens system that introduces the linearly polarized light that has passed through the polarizer into the polarization-maintaining optical fiber to be measured, and A lens system that converts the output light from a polarization-maintaining optical fiber into parallel light, an analyzer that converts the parallel output light into linearly polarized light, and a Michelson interferometer that separates and interferes the linearly polarized light that has passed through the analyzer. 1. A polarization dispersion measuring device, characterized in that one of the reflecting mirrors constituting the Michelson interferometer has a linear movable mechanism, and is equipped with a photodetector for detecting interference light.