JPH07218740A - Optical fiber polarization device and measuring instrument using same - Google Patents

Abstract

PURPOSE:To provide the device which has a high-performance polarizing function or optical fiber transmission or measuring instrument with simple constitution wherein a periodic refractive index distribution is generated in an optical fiber having light induced refraction effect. CONSTITUTION:A periodic refractive index layer 3 is formed in the core 2 of the single-mode fiber which has light induced refractive index variation effect by projecting many interference fringes, which slant by 45 deg. or nearly 45 deg. to the length direction of the fiber and have optical intervals that are an integral multiple of one wavelength in the length direction of the fiber. Consequently, the (s)-polarized component of monochromatic light passing through this refractive index distribution layer is all radiated out of the fiber by interference effect, but the (p)-polarized component is passed as it is, so the polarization device of simple constitution which has a high degree of polarization is obtained.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber type polarization device used when performing optical communication or optical measurement using an optical fiber. [0002] Conventionally, there is known a method of realizing a fiber-type polarizer by polarization dependency of transmission loss caused by bending an optical fiber or by coating a metal thin film on a cross section of the optical fiber. There is. However, these methods require a very long fiber or require a polarizing element to be inserted into the fiber.
There was a problem with stability and insertion loss, so there was a limit to the applicable applications. The present invention creates a polarization function at an arbitrary position in a fiber by creating a periodic refractive index distribution in the fiber by a photo-induced refractive index effect. It is a thing. When a single-mode fiber is irradiated with interference fringes formed by light rays such as ultraviolet rays to cause a chemical reaction in the fiber core according to the light intensity, the core is weakened depending on the brightness of the interference fringes. Different refractive index changes are induced and recorded as a periodic refractive index distribution. When a monochromatic ray is incident on a periodic refractive index distribution, it is reflected only in a specific direction determined by the Bragg condition. As a special case, if the interference fringes are formed with an angle θ = 45 ° with respect to the fiber axis, the reflection direction determined by the Bragg condition becomes a direction perpendicular to the fiber axis. Generally, scattered light is based on the emission of electric dipoles induced by incident light. Since light is a transverse wave, the traveling direction and deviation of the light rays are at right angles. Therefore, the polarization of the light scattered in the direction perpendicular to the incident light is only the polarized component (s-polarized component) that is polarized perpendicular to the plane including the incident light and the scattered light, and includes the incident light and the scattered light. A polarization component (p-polarization component) that is polarized parallel to the plane is not included. As a result, when an unpolarized light beam is incident on the gradient index layer relating to
The polarized component is reflected at right angles and is emitted out of the fiber. However, the p-polarized component is not reflected and goes straight through the fiber. As a result, the recorded interference fringes act as a polarizing element because the light rays passing through are only p-polarized light components. The fiber polarizing device and its application according to the present invention will be described in detail below. FIG. 1A is a configuration diagram of a fiber polarizing element for showing an embodiment of the present invention. The periodic refractive index distribution layer 3 is created by projecting interference fringes inside the core 2 having the photoinduced refraction effect of the single mode fiber 1. Refractive index peaks 4 and valleys 5 are generated according to the lightness and darkness of the interference fringes. FIG. 1B shows the spatial distribution of the refractive index along the transverse direction aa ′ of the interference fringes. FIG. 2 is a configuration diagram showing the generation of reflected partial waves in the refractive index distribution region. Induced iso-refractive surface (refractive index peaks or valleys) in the ray traveling in the fiber 3
The p-polarized component 31 (the component in which the electric field component of the light wave is biased parallel to the incident surface) passes through 0a, 30b, 30c, ... Without being reflected. On the other hand, the s-polarized component 32
(Component in which the electric field component of the light wave is biased perpendicular to the incident surface)
Causes partial reflection at the adjacent refractive index surface. A large number of s-polarized partially reflected waves 1,
2, 3, 4, 5, 6 ... Interfere with each other, but the interval d of the interference fringes is √2nd = mλ, where n is the average refractive index of the fiber core, m is a positive integer, and λ is Since the wavelength of the light beam propagating through the fiber is defined as the wavelength in vacuum, the relative phases of all partial waves are 360 °, that is, the same phase, and therefore each partial wave causes mutual constructive interference (Bragg reflection). As a result, all the s-polarized light components are radiated out of the fiber, and the transmitted light has only the p-polarized light components. FIG. 3 is a diagram showing an example of the construction for producing the fiber polarizing device according to the present invention. For example, a germanium-doped quartz fiber is used as the fiber 6 exhibiting the photo-induced refractive index effect. A part of the fiber 6 is put in a transparent container 7 and placed in a liquid 8 having a refractive index n ′ close to the refractive index n of the fiber. Two light beams 9 and 10 having good coherence emitted from the same light source are made incident from the outside of the container 7, and an interference fringe 11 having an inclination of 45 ° with respect to the fiber axis is formed inside the fiber. As the light source laser, for example, an excimer laser emitting an ultraviolet ray or an argon laser is used. In the place where light is strongly irradiated (bright line of interference fringe), the substance changes due to photochemical reaction and the refractive index becomes different from that before irradiation. On the other hand, at the dark line of the interference fringe, no chemical reaction occurs, so the refractive index is the same as before irradiation. A sufficient polarization effect can be obtained from a sufficient number of interference fringes. The number of interference fringes can be freely adjusted by increasing the beam width of the laser beam to be irradiated. In that case, the angle φ formed by the two irradiation rays is set so that the interference fringes 11 in the fiber core are inclined at 45 ° with respect to the fiber axis. When the angle is not exactly 45 ° with respect to the optical axis of the interference fringe, the transmitted light contains a small amount of the s-polarized component, so that the performance as a polarizer is somewhat deteriorated, but a strong polarization function is obtained. Is guaranteed. FIG. 4 is a block diagram showing an embodiment in which the polarizing device according to the present invention is applied to a current sensor. The fiber polarization device 1 according to the present invention is provided at two locations A and B, which are separated by an appropriate distance from the single mode fiber 12.
3 and 14 are prepared by inclining their polarization planes by 45 °. When a monochromatic light 15 of wavelength λ is incident on the fiber, the first polarizer 1
When the polarization plane of the light beam linearly polarized by 3 passes through the sensor fiber portion 16, the polarization plane of the passing light is slightly rotated by the Faraday effect due to the magnetic field H induced by the measured current 17. Therefore, the intensity of the light 19 which passes through the second polarization device 14 and reaches the detector 18 outside the fiber changes in proportion to the magnitude of the current I. From now on, the current can be measured. This method is a compact and mechanically stable current / magnetic field measuring method because it is an all-fiber type structure as compared with the conventional fiber current sensor which requires an external polarization element. FIG. 5 is a block diagram showing an embodiment for transmitting a light beam having a high degree of polarization in a polarization maintaining fiber. A polarization device 21 according to the present invention is formed inside one polarization maintaining fiber 20 so as to match the polarization maintaining fiber and the polarization plane. When a linearly polarized light ray 23 is incident on the fiber 20 so as to match the polarization plane-maintaining fiber and the polarization plane, the polarization is usually disturbed at the entrance 24 of the fiber, and the polarization degree of the fiber transmission light 25 is reduced. . However, by using this device 21, the disorder of polarization is corrected, and the degree of polarization of transmitted light can be improved. FIG. 6 is a block diagram showing an embodiment in which the polarizing device according to the present invention is applied to a fiber gyro. In the fiber gyro, a ring-shaped interferometer 27 in which a light beam emitted from a light source 26 is composed of a single mode fiber is used.
, And run clockwise and counterclockwise light in the ring. When the ring rotates, a phase difference occurs between the two lights, and as a result of the interference, the amount of light reaching the detector 28 changes, so that the rotation speed Ω can be detected. At this time, a measurement error occurs unless the clockwise light and the counterclockwise light have the same polarization in the fiber. Therefore, a polarizer is required, and a long fiber polarizer is usually used, but the degree of polarization is insufficient and the size becomes large. Therefore, by using the polarizing device 29 according to the present invention as a polarizer, a compact and highly accurate rotation sensor can be configured. According to the present invention, inside the optical fiber,
The polarization function can be created with an extremely simple structure. Such a polarization device has not been realized by the conventional fiber technology, and has a wide application range as a new device such as optical communication and optical measurement technology.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram showing an embodiment of a fiber polarization device according to the present invention. FIG. 2 is a configuration diagram for explaining the principle of a fiber polarization device according to the present invention. FIG. 3 is a configuration diagram showing an embodiment of an optical system for producing a fiber polarizing device according to the present invention. FIG. 4 is a configuration diagram showing an embodiment in which the fiber polarization device according to the present invention is applied to a fiber magnetic field sensor. FIG. 5 is a configuration diagram showing an embodiment for applying the fiber polarizing device according to the present invention to optical transmission. FIG. 6 is a configuration diagram showing an embodiment in which the fiber polarization device according to the present invention is applied to a fiber gyro. [Explanation of reference numerals] 1 fiber 2 fiber core 3 gradient index layer 4 high refractive index surface 5 low refractive index surface 6 fiber 7 transparent container 8 refractive index matching liquid 9 light ray 1 10 light ray 2 11 interference fringe 12 fiber 13 refractive index Distribution layer 1 14 Refractive index distribution layer 2 15 Incident light 16 Sensor section 17 Current 18 Detector 19 Exit light 20 Fiber 21 Refractive index distribution layer 22 Lens 23 Incident light ray 24 Fiber entrance 25 Fiber transmission light 26 Light source 27 Fiber ring interferometer 28 Detector 29 Fiber Polarizers 30a, 30b, 30c Isorefractive Index Surface 31 p Polarized Component 32 s Polarized Component

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication G02F 1/09

Claims (1)

Claim: What is claimed is: 1. A single-mode optical fiber core is provided with a periodic refractive index distribution surface inclined with respect to the optical axis, of a monochromatic light beam which propagates through the fiber at an optical pitch along the fiber axis. Create it as one wavelength, and use Bragg reflection due to this periodic refractive index distribution,
A device characterized by causing a polarization effect. 2. An optical transmission and measurement device using the device according to claim 1.