JPH03287113A - Optical isolator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical isolator that is easy to manufacture and to couple with an optical fiber.

<Prior Art> Optical isolators have the function of restricting the propagation of light in one direction, and are used in optical transmission networks using optical fibers.

By the way, the polarization direction of light traveling through an optical fiber changes as it travels a long distance, so even if linearly polarized light in a fixed direction is incident on one end of the optical fiber, random polarization will be observed at the other end. . For this reason, an optical transmission network using optical fibers requires a so-called polarization-independent optical isolator, which has a transmittance that does not depend on the direction of polarization.

As such optical isolators, those shown in FIGS. 3 to 5, for example, are known.

The optical isolator shown in FIG. 3 is disclosed in Japanese Patent Publication No. 6158809, and is a combination of two wedge-shaped rutile crystals 31a, 31b and a Faraday rotator 32 disposed between them. . The light emitted from the optical fiber 33 and made into parallel light by the lens 34 is
It is separated into ordinary light and extraordinary light by passing through the first rutile crystal 31a, but the plane of polarization is rotated by the Faraday rotator 32 so that it corresponds to the ordinary light and extraordinary light respectively inside the second rutile crystal 31b. Therefore, after passing through the second rutile crystal 31b, the ordinary light and the extraordinary light become parallel to each other.

Therefore, all the light beams can be focused onto the optical fiber 36 by the lens 35. On the other hand, the light emitted from the optical fiber 36 is separated into ordinary light and extraordinary light by the second rutile crystal 31b, and then further divided into extraordinary light and ordinary light inside the first rutile crystal 31a by the Faraday rotator 32. Since the light is converted, the light is not focused onto the optical fiber 33 and acts as an optical isolator.

The optical isolator shown in FIG. 4 is disclosed in Japanese Patent Application Laid-Open No. 5522729, and includes two flat rutile crystals 41a and 41b, a Faraday rotator 42 and an optically active crystal 43 disposed between them. It is a combination of

The light emitted from the optical fiber 44 is transmitted through the lens 4
5, the light is focused onto an optical fiber 46.

In addition, the optical isolator shown in Fig. 5 is also
This is shown in Japanese Patent No. 729, and is a combination of three rutile crystals 51a, 51b, 51c and a Faraday rotator 52. Also in this case, the light emitted from the optical fiber 53 is condensed onto the optical fiber 55 by the lens 54.

In the configurations shown in FIGS. 4 and 5, as well as in the configuration shown in FIG. 3, all the light emitted from the optical fibers 44° 53 enters the respective optical fibers 46° 55, but the light in the opposite direction It is designed so that it does not enter the optical fiber 44.53.

<Problems to be Solved by the Invention> As mentioned above, conventional polarization-independent isolators as shown in FIGS. 3 to 5 are manufactured by cutting out anisotropic crystals and arranging them in combination. There were difficulties in cutting along the crystal axis, cutting out with the same thickness, and arranging the crystal with the axes aligned.

In addition, such coupling between an optical isolator and an optical fiber is usually performed using a gradient index rod lens (product name: SELFOC Micro Lens), but the alignment of the optical system is delicate and the coupling between the optical fiber and the optical fiber is difficult. binding was not necessarily high. Furthermore, since it passes through a lens, the deterioration of characteristics due to reflection and scattering on the surface cannot be ignored.
child.

In view of these circumstances, it is an object of the present invention to provide a polarization-independent optical isolator that is easy to manufacture, inexpensive, and has high coupling properties with optical fibers.

Means for Solving the Problems> An optical isolator according to the present invention that achieves the above object is made by arranging two single mode fibers whose claddings have different refractive indexes and fusing and stretching their central parts. It is characterized by

Effect> According to the above configuration, light incident from the first optical fiber whose cladding has a relatively lower refractive index than the other optical fiber leaks to the other second optical fiber at the fused portion. On the other hand, this second
Even if light enters the other optical fiber from the optical fiber whose cladding has a relatively higher refractive index than the other, it will not leak out to the other optical fiber at the fusion part, that is, the first optical fiber.

Therefore, it becomes an optical isolator that allows light to pass from the first optical fiber to the second optical fiber, but not vice versa.

Note that since no polarizer is used, it is a polarization-independent type.

Practical example Hereinafter, the present invention will be explained based on examples.

FIG. 1(,1) shows the external appearance of an optical isolator according to one embodiment, and FIG. 1(bl) shows the cross section of the fused portion and the refractive index profile. This is a fused biconical taper fiber coupler shape in which the central parts of two optical fibers 11 and 12 are fused and stretched to form a small diameter fused part 13.The manufacturing method is the usual one. It is similar to a coupler, but the optical fiber used is different.

The optical fibers 11 and 12 of this embodiment are single-mode optical fibers that have the same refractive index profile but different absolute values of the refractive index; in this case, the refractive index of the optical fiber 11 is larger than that of the optical fiber 12. ing. That is, the relative refractive index difference and magnification between the cladding lla and the cores 11 and b of the optical fiber 11 are as follows:
The relative refractive index difference and magnification between the optical fiber 2a and the core 12b are equal to each other, but the cladding ll of the optical fiber 11
The refractive index of a and the refractive index of the core 12b of the optical fiber 12 are equal.

Optical fibers 11 and 12 having such a relationship include Si02-GeO2 core,
5in2 clad single mode fiber, a 5IO2 core, SiO2-F clad single mode fiber may be used as the optical fiber 12.

In such an optical isolator 10, the light incident from one end 12A of the optical fiber 12 leaks into the optical fiber 11 at the fusion part 13 and exits to the other end 11B of the optical fiber 11. The light incident from the end 11B of the fiber 11 (7) leaks into the optical fiber 12 at the fusion part 13, but does not exit from the end 12A of the optical fiber 12. Therefore, it can be used as an optical isolator between the end portion 12A and the end portion 11B. Similarly, the other end 12B of the optical fiber 12 and the end 1 of the optical fiber 11 are
It can also be used as an optical isolator between 1A and 1A.

5102.GeO2 core as optical fiber 11.

5IO2 clad SM fiber, optical fiber 12 is a pure SiO2 core, 5iO2-F clad SM fiber, both have a cladding diameter of 125 μm1 a core diameter of 8.5 μm1
The optical isolator 10 having the above configuration was manufactured using an optical isolator having Δn of 0.32% and a cutoff wavelength of 1.3 μm. The stretching and fusion conditions in case (B) were adjusted so that the loss of light of 1.53 μm from the optical fiber 12 to the optical fiber 11 was minimized.

In such an optical isolator 10, the optical loss in the direction from the end 12A to the end 11B is ? Optical loss in the direction from end 11B to end 12A is 1.53
When measured with μm light, the values were 0.5 dB and 0.5 dB, respectively.
It was 30 dB.

Since the optical isolator 10 described above does not use a polarizer, it goes without saying that it is polarization independent.

Furthermore, since the optical isolator 10 is made of an optical fiber,
It has a high coupling efficiency with optical fibers, and if it is butted and fusion spliced, it can be used with, for example, S i 02-GeO2 core fiber,
Even when coupling between the SiO2 core and the 5in2-F clad fiber, a splice loss of about 0.01 can be easily achieved.

Here, the optical fiber 11 and the optical fiber 12 are not limited to those described above; for example, the refractive index profiles of the two do not have to be exactly the same, and if the claddings of the two are at least different, they can function as an optical isolator. shows. However, it is preferable that the refractive index of the cladding having a higher refractive index is set to be the same as or slightly higher than the refractive index of the core of the other optical fiber.

8 Next, an example in which the optical isolator 10 of the above embodiment is used in a fiber amplifier will be described with reference to FIG.

In Fig. 2, 21 is a DFB laser which is a signal light source with a wavelength of 1.53 μm, 22 is a laser diode for pumping light with an output of 20 mW, 23 is a multiplexing coupler, and 14 is an Er-doped fiber (Er concentration 300 ppm, length 20m) of amplification fibers, 25 and 26 each have a length of approximately several 10k.
The optical fibers 27 and 28 are fiber connectors, and the part inside the dotted line constitutes an optical amplifier. The optical isolator 10 of the above embodiment is provided such that the end 12A is coupled to the output end of the amplification fiber 24, and the end 11B is coupled to the inside of the fiber connector 28 on the output side of the optical amplifier.

In this configuration, signal light is emitted from the DFB laser 21 and introduced from the optical connector 27 through the optical fiber 25, and excitation light is emitted from the laser diode 22 in the amplifier, and both lights are combined by the optical coupler 23. The signal light is then introduced into the amplification fiber 24 and amplified. The amplified light passes through the optical isolator 1° and is emitted from the fiber connector 28 to the optical fiber 26.

Here, the optical isolator 10 prevents the reflected light from the fiber connector 28 from entering the amplification fiber 24. That is, such reflected light escapes from the end portion 11A of the optical isolator 10.

Therefore, if the optical isolator 10 is not provided, the reflected light from the fiber connector 28 will also enter the amplification fiber 24 and be amplified, causing noise when amplifying high frequency signals. In this example, such noise can be reduced to zero by the optical isolator 10.

<Effects of the Invention> As explained above, the optical isolator of the present invention, unlike the conventional polarization-independent optical isolator, uses rutile crystal,
Faraday rotator.

It is inexpensive because it uses only optical fibers without using optically active crystals, and it is extremely easy to manufacture and install during use because there is no need to cut out, adjust, or polish the crystal, and there is no need to align the optical system. Furthermore, it is free from the conventional effects of scattering and reflection on the crystal surface, and has the effect of extremely high coupling efficiency with optical fibers.

[Brief explanation of drawings]

Fig. 1 (al is an external view of an optical isolator according to an embodiment of the present invention, Fig. 1 fb) is an explanatory diagram showing its cross section and refractive index profile, and Fig. 2 shows its application to an optical fiber amplifier. The explanatory diagrams and FIGS. 3 to 5 are explanatory diagrams showing optical isolators according to the prior art, respectively. In the drawings, 10 is an optical isolator, 11 and 12 are optical fibers, 11a and 12a are claddings, 12-11b and 12b are cores, and 13 is a fused portion.

Claims (1)

[Claims] An optical isolator characterized by being made by arranging two single mode fibers whose claddings have different refractive indexes and fusing and stretching their central portions together.