GB2117132A

GB2117132A - Interferometer

Info

Publication number: GB2117132A
Application number: GB8307262A
Authority: GB
Inventors: Kiyofumi Mochizuki
Original assignee: Kokusai Denshin Denwa KK
Current assignee: KDDI Corp
Priority date: 1982-03-19
Filing date: 1983-03-16
Publication date: 1983-10-05
Also published as: GB2117132B; JPS6352694B2; JPS58160848A; GB8307262D0

Abstract

An interferometer, characterized by a polarization-maintaining optical fiber (312) for transmitting incident light while preserving planes of polarization in two perpendicularly crossing principal axes, a constant temperature bath (311) for maintaining the polarization- maintaining optical fiber at an arbitrary temperature, and combining means (313) for receiving light emitted from the polarization- maintaining optical fiber to combine components along the two perpendicularly crossing principal axes. An optical path difference is introduced between the 2 polarization states by passage through the fibre, which is proportional to the temp. of the fibre, and therefore the 2 states can be considered to follow 2 independent optical paths, the difference between which can be varied by changing the temp. of the bath. <IMAGE>

Description

SPECIFICATION Interferometer The present invention relates to an interferometer.

An example of a known interferometer is a Mach-Zehnder interferometer. However, high precision measurement is impossible with such a conventional interferometer.

It is an object of the present invention to provide an interferometer which permits high precision variatios of the optical path lengths of two light waves by using an optical fiber and changing its atmosphere temperature.

In accordance with the present invention, there is provided an interferometer, comprising a polarization-maintaining optical fiber for transmitting incident light while preserving planes of polarization in two perpendicularly crossing principal axes, a constant temperature bath for maintaining the polarization-maintaining optical fiber at an arbitrary temperature, and combining means for receiving light emitted from the polarization-maintaining optical fiber to combine components along the two perpendicularly crossing principal axes, wherein two optical waveguides extending along the two perpendicularly crossing principal axes are used as two independent optical paths, and a difference in length between the optical paths is varied by changing atmosphere temperature in the constant temperature bath.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a system diagram showing the arrangement of an example of a known interferometer; Fig. 2 is a perspective view illustrating a basic arrangement of a part of an interferometer of the present invention; Fig. 3 is a system diagram illustrating an embodiment of the present invention; Fig. 4 is a system diagram showing an arrangement of a partly modified portion of the embodiment of Fig. 3; and Fig. 5 is a graph showing, by way of example, output variations of an analyzer with respect to temperature changes of the atmosphere of a polarization-maintaining optical fiber for use in the present invention.

To distinguish the present invention from the prior art, an example of a known interferometers will first be described.

An interferometer heretofore employed is such as shown in Fig. 1. The illustrated interferometer is commonly referred to as a Mach-Zehnder interferometer. In Fig. 1, light waves 11 are branched by a beam splitter 12 into light waves 1 3 and 14. These two light waves are, respectively, reflected by a mirror 1 5 and an optical delay line 19 and a mirror 16, and then combined again by a beam splitter 1 7 into a composite wave 18. In this interferometer, one of the optical paths is used as a test optical path while the other a reference optical path. In this case, since adjustment of the delay time of the optical delay line 19 is carried out mechanically, high precision measurement is impossible.

A detailed description will now be given of embodiments of the present invention with reference to Figures 2 to 5.

Fig. 2 shows a basic arrangement of the present invention. In Fig. 2, light waves rendered by a polarizer 21 into linearly polarized light are rotated, as they are, by a half-wave plate 23 so that they may lie intermediate between two perpendicularly crossing principal axes 25 and 26 of a stress-induced polarization-maintaining optical fiber 29 which transmits light while preserving the plane of polarization in the two perpendicularly crossing principal axes. The polarization-maintaining optical fiber 29 is an optical fiber designed so that two perpendicularly crossing linearly polarized light waves 27 and 28 may propagate therethrough independently without mode coupling to each other; it can be said that two independent optical waveguides along the principal axes 25 and 26 are formed by one optical fiber.Upon entering the polarizationmaintaining optical fiber 29, the light waves 24, rotated by the halfwave plate 23 so that they may lie intermediate between the two principal axes 25 and 26, are split into light waves 27 and 28 and propagate independently in the optical fiber.

At the output side of the optical fiber, the light waves 27 and 28 are partly combined by an analyzer 210 into light waves 211.

On the other hand, since the two perpendicularly crossing optical waveguides of the polarization-maintaining optical fiber 29 have different refractive indexes, the two light waves 27 and 28 propagate at different different velocities. In consequence, the light waves 27 and 28 have a phase difference 5R therebetween at the output side of the polarization-maintaining optical fiber 29.The phase difference err is given by the following equation:

where A is the wavelength of the light waves 24, L is the length of the polarization-maintaining optical fiber 29, Cp is the optical elastic constant of the polarization-maintaining optical fiber 29, a is a proportional coefficient related to the Young's modulus, the Poisson's ratio and thermal expansion coefficient of the polarizationmaintaining optical fiber 29, To is the softening temperature of silica glass containing a dopant, which is about 1000 C and T is the temperature of the atmosphere in which the polarizationmaintaining optical fiber 29 is placed. Eq. (1) indicates that the phase difference r, between the light waves 27 and 28 bears a linear relation to a temperature variation.Namely, the temperature variation corresponds to movement of the optical delay line 19 of the interferometer shown in Fig. 1. The beam splitter 12 and the beam splitter 17 in Fig. 1 correspond to the end face 212 of the polarization-maintaining optical fiber 29 on the input side thereof and the analyzer 210 in Fig. 2.

The polarization-maintaining optical fiber 29 is housed in a constant temperature bath which can be set at an arbitrary temperature. Fig. 5 shows an example of measured results of output variations of the analyzer with temperature changes of the atmosphere of the polarization maintaining optical fiber. It is seen from Fig. 5 that the optical output undergoes sinusoidal variations with the temperature changes.

Fig. 3 illustrates an embodiment of the present invention. In Fig. 3, light waves 31 are split by a beam splitter 32 into light waves 33 and 34. The two light waves are respectively rendered into perpendicularly crossing linearly polarized light waves by halfwave plates or polarizers 35 and 36 and halfwave plates 38 and 39 depending on whether they are linearly polarized light waves or not, and they are combined again by a beam splitter 37. Composite waves 316 of the two perpendicularly light waves are brought by a halfwave plate 310 into alignment with two perpendicularly crossing axes of a polarization maintaining optical fiber 312 to propagate therein. The two perpendicularly crossing light waves are supplied to an analyzer 313, from which only components in its axial direction are sent out.The delay path of this interferometer is constituted by changing the temperature of the polarization-maintaining optical fiber by changing the temperature of the atmosphere of a constant temperature bath 311 the temperature of which can be set as desired. The amount of change in temperature, in the case of the light waves 31 emitted from an He-Ne laser (having a wavelength of 0.6328 m), can easily be held below 0.1 ym/C-m per unit length and unit temperature change of the polarization maintaining optical fiber, and the temperature can be changed continuously.

While the foregoing description has been given in connection with the interferometer which is a combination of the polarization-maintaining optical fiber and the Mach-Zehnder interferometer, it is possible to produce an interferometer similar to that described above from another interferometer with the polarization maintaining optical fiber. By using two optical paths of this interferometer as a test optical path and a reference optical path, respectively, the refractive index of a material or the coherence of a -light source can be measured with high accuracy.

For this measurement, it is possible to employ a method of obtaining the abovesaid refractive index from variations in the coherence through using a photo detector 314 or a method of obtaining it from displacement of interference fringes by using a screen in place of the photo detector 314.

The part of the inferometer formed between the beam splitters 32 and 37 in Fig. 3 can also be arranged as shown in Fig. 4. In Fig. 4, reference numerals 41 and 42 indicate a combination of a polarizer and a halfwave plate, or a polarizer and a 1/4 plate; in the case of the former, the output light of the halfwave plate 42 is adjusted to be inclined 45 degrees to the principal axis of a polarization beam splitter 43 and, in the case of the latter, the output of the 1/4 wave plate 42 is adjusted to be a circularly polarized light.

Reference numerals 44 and 45 designate mirrors, and 46 identifies a polarization beam splitter.

As has been described in the foregoing, the present invention employs two perpendicularly crossing transmission lines of the polarizationmaintaining optical fiber as two optical paths or a part of the optical path of the interferometer and utilizes the fact that the difference in lengths between the two optical paths is varied by temperature with high accuracy; the invention offers an optical fiber interferometer having an optical delay line which moves with high accuracy.

Claims

1. An interferometer, comprising a polarization-maintaining optical fiber for transmitting incident light while preserving planes of polarization in two perpendicularly crossing principal axes, a constant temperature bath for maintaining the polarization-maintaining optical fiber at an arbitrary temperature, combining means for receiving light emitted from the polarization-maintaining optical fiber to combine components along the two perpendicularly crossing principal axes, wherein two optical waveguides extending along the two perpendicularly crossing principal axes are used as two independent optical paths, and means are provided for controlling the atmosphere temperature in the constant temperature bath to vary a difference in length between the optical paths.

2. An interferometer substantially as herein described with reference to Figure 2 with or without reference to Figures 3 and 4 of the accompanying drawings.