JPS62134619A - Optical isolator - Google Patents

Abstract

PURPOSE:To make the optical coupling constitution between a semiconductor laser and an optical fiber small-sized and improve the efficiency of optical coupling by combining a Farady rotator and a polarizing prism to separate a return light and using the polarizing prism as a lens to constitute a confocal system. CONSTITUTION:Polarizing prisms 2 and 3 are arranged on both sides of a Farady rotator 1 in a uniform magnetic field H indicated by an arrow, and a semiconductor laser 4 is provided on the side of the prism 2, and an optical fiber 5 is provided on the side of the prism 3. The rotator 1 is rotated at a rotation angle proportional to its thickness (d) and Verdet's constant, and at this time, distance from prisms 2 and 3 to the rotator 1 are set to d1 and d2 and the radius of curvature of prism faces opposite to the rotator 1 is R. That is, prisms 2 and 3 are so arranged that their crystal axes are shifted from each other by the rotation angle of the rotator 1, and the lens is constituted by curved surfaces to constitute the confocal system. Thus, distance d1 and d2 are narrowed to separate the return light easily.

Description

[Detailed Description of the Invention] [(Already required)] A confocal system is constructed by combining a Faraday rotator and a polarizing prism to separate returned light, and using the polarizing prism as a lens. It is possible to achieve miniaturization in the optical coupling configuration with the fiber.

[Industrial application field]

The present invention relates to an optical isolator for separating light returning to a semiconductor laser and stabilizing the operation of the semiconductor laser.

In an optical signal transmission system, an optical signal is transmitted by inputting the output light of a semiconductor laser into an optical fiber via a lens system. In this case, as semiconductor lasers are increasingly driven in single mode excitation, the semiconductor lasers are affected by return light due to reflections from end faces of optical fibers, connectors, etc., and noise is generated. Therefore,
An optical isolator is used to separate the returned light so that it does not enter the semiconductor laser.

[Conventional technology]

A lens system is used to input the output light of a semiconductor laser into an optical fiber, as shown in Figure 4.
This constitutes a confocal compound lens system. That is, the output light of the semiconductor laser 11 placed at the focal position of the lens 12 is focused by the lens 12 and transmitted to the optical fiber 1 via the lens 13.
, 14, and the focal length of the lens 12 is f.
When the focal length of the lens 13 is fl, the distance between the lenses 12 and 13 is (f, +f2).

With only such a confocal compound lens system, the semiconductor laser 1
In the configuration in which the output light of 1 is input to the optical fiber 14, the reflected light from the end face of the optical fiber 14, the reflected light from the connector (not shown), etc. are transmitted to the lens 13.12.
The single mode laser light is input to the semiconductor laser 11 through the laser beam, and the noise component in the output of the single mode laser light increases. Therefore, in order to separate such returned light, an LD module is generally employed in which an optical inverter 15 is arranged between lenses 12 and 13, as shown in FIG.

As shown in FIG. 6, this optical isolator 15 has polarizing prisms 17 and 18 arranged on both sides of a Faraday rotator 16, and the Faraday rotator 16 has, for example,
It is composed of YIC, (yttrium iron gas, t)N crystal, and is placed in a uniform magnetic field H in the direction of light propagation shown by the arrow. The polarizing prisms 16 and 17 are made of, for example, birefringent rutile crystals, and their crystal axes are configured to differ from each other by the Faraday rotation angle by the Faraday rotator 16.

The forward light shown by the solid line that enters from the polarizing prism 17 side is polarized by the polarizing prism 17, its polarization direction is rotated by the Faraday rotator 16, and the polarizing prism 17
．． Since the crystal axes of 1'8 are selected to differ by the Faraday rotation angle, the polarization state for the two prisms does not change, and therefore the refractive index for the two prisms also does not change. The light is transmitted and emitted with low loss in a direction parallel to the direction of the incident light.

On the other hand, light in the opposite direction shown by the dotted line that enters from the polarizing prism 18 side is polarized by the polarizing prism 18 , the polarization direction is rotated by the Faraday rotator 16 , and the light is incident on the polarizing prism 17 . In this case, since the polarization state with respect to the two prisms changes and the refractive index with respect to the two prisms changes, the light is separated and emitted in a direction that is not parallel to the incident direction of the light in the opposite direction. Therefore, if the light in the opposite direction is used as return light, the light in the opposite direction shown by the dotted line will not be incident on the semiconductor laser, which is the light source, and the operation of the semiconductor laser can be stabilized.

[Problem that the invention seeks to solve]

As shown in FIG. 7, the above-mentioned optical isolator 15 is arranged between the lenses 12 and 13, and transmits the output light of the semiconductor laser 11 as forward light, and prevents the output light from returning due to reflection from the end face of the optical fiber 14, etc. The light is separated and emitted as light in the opposite direction as shown by the dotted lines. The return light separated by the optical isolator 15 will not return to the semiconductor laser 11 unless it enters the lens 12. Therefore, the angle at which the returned light is separated and emitted from the optical isolator 15 and the diameter of the lens 12 are It is necessary to provide a distance p corresponding to the relationship so that the return light separated and emitted from the optical isolator 15 does not enter the lens 12. This interval! For example, if the semiconductor laser 11 is configured to operate in a single mode, it is necessary to set it to 5 mm or more.

In this way, the distance p between the optical isolator 15 and the lens 12 must be increased, and for this reason, the distance p between the optical isolator 15 and the lens 12 must be increased.
13 becomes large, for example 10 mm or more, thereby causing a deviation from the confocal system. As described above, the coupling efficiency of light to the optical fiber 14 decreases due to the deviation from the confocal system. Therefore, it would be better to select a lens with a large focal length.
In that case, the light collecting ability of the lens is reduced, so the coupling efficiency of light to the optical fiber 14 is reduced, and furthermore, the distance between the lenses is large, resulting in an increase in the size of the LD module.

An object of the present invention is to separate returned light with a relatively simple configuration, and to achieve miniaturization without reducing coupling efficiency.

[Means for solving problems]

The optical isolator of the present invention has a structure in which a polarizing prism is also used as a lens. To explain with reference to FIG. The polarizing prisms 2.3 are arranged so that the crystal axes of the respective polarizing prisms 2.3 are mutually inclined by the Faraday rotation angle by the Faraday rotator 1, so that light is incident on the curved surface of the polarizing prism. It is composed of

[Effect]

The Faraday rotator 1 and the polarizing prism 2.3 separate the returned light, and the curved end surfaces of the polarizing prisms 2, 3 create a lens effect, making it possible to configure a confocal system, allowing optical coupling. This means that it can be made smaller without reducing efficiency.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a detailed explanatory diagram of the present invention, in which 1 is a Faraday rotator, 2 and 3 are polarizing prisms, 4 is a semiconductor laser, and 5 is a polarizing prism.
is an optical fiber. The Faraday rotator 1 is placed in a uniform magnetic field H in the direction of the arrow, and the polarization direction of light propagating in the direction of the magnetic field is rotated. In a sufficiently large magnetic field H, the Faraday rotation angle θ is proportional to the thickness d of the Faraday rotator 1 through which the light passes and a proportionality constant (Verdet constant). Further, the polarizing prism 2.3 is a Faraday rotator 1.
are placed at predetermined intervals d, d2 before and after the polarizing prism 2.3, and the opposite side to the Faraday rotator 1 is a curved surface with a radius of curvature R, and the crystal axis of the polarizing prism 2.3 is set by the Faraday rotation angle of the Faraday rotator 1. only to be arranged differently from each other. In this way, since a lens is formed by the curved surface of the polarizing prism 2.3, a confocal system can be constructed by the polarizing prism 2.3.

When the semiconductor laser 4 outputs laser light by single mode excitation, the optical fiber 5 is a single mode optical fiber with a core diameter of several μm.

FIG. 2 is an explanatory diagram of light propagation in the forward direction. Similar to the conventional optical isolake, the light incident on the polarizing prism 2 is polarized and incident on the Faraday rotator 1. Since the magnetic field H is applied to the propagation direction of the light, the direction of polarization is rotated by the Faraday rotation angle, and the light enters the polarizing prism 3 and is emitted in a direction parallel to the direction of incidence on the polarizing prism 2.

FIG. 3 is an explanatory diagram of light propagation in the opposite direction, and the polarizing prism 3
The light incident on the side is polarized and enters the Faraday rotator 1, and the polarization direction is rotated by the Faraday rotation angle and enters the polarizing prism 2. The Faraday rotator 1 rotates forward light and reverse light in the same direction with the same Faraday rotation angle, and the polarizing prism 2.3
Since the relationship between the crystal axes of is opposite to that for forward direction light, the polarization direction for the two prisms changes, and as a result, the refractive index for the two prisms changes, and the light in the opposite direction is shown as a dotted line. As shown, the light is separated from the polarizing prism 2 and emitted.

As mentioned above, the returned light can be separated as an optical isolator, and since the end face of the polarizing prism 2.3 is formed into a curved surface, the output light of the semiconductor laser 4 is focused by the polarizing prism 2. The light is incident on the polarizing prism 3 via the Faraday rotator 1, and is condensed by the polarizing prism 3 and then incident on the optical fiber 4.

The curvature of each curved surface of the polarizing prism 2.3 is R, the thickness is a, the refractive index is n, the thickness of the Faraday rotator 1 is d, the refractive index is ny, and the Faraday rotator l and the polarizing prism 2
．． 3 and the interval between dI.

d2, the matrix A of this optical system is expressed by the following formula % Formula % Here, if the focal length is f, it can be expressed as follows. The conditions for this optical system to become a confocal system are [→■
−(41, so (3
) formula becomes 1/f →Q − (5). Therefore, from equations (3) and (5), where D mid d, + d2 - (7) is obtained.

For example, using rutile crystal as the polarizing prisms 2 and 3,
Using a YIG crystal as the Faraday rotator 1, in order to rotate the polarization direction by 45 degrees with the Faraday rotator 1, the thickness d of the Faraday rotator 1 is as follows for light of wavelength λ = 1.30 μm. 2 mm, the ordinary refractive index n of the rutile crystal at this wavelength is 2,463, and the refractive index n of the YIG crystal.

= 2.20, so from (8) 2, D = 0.56
RO, 91 (mm) - (Q). Therefore, a confocal system can be constructed for a combination of D and R that satisfies equation (9).

Note that when considering increasing the light coupling efficiency and downsizing the LDD module, the smaller D is, the better; however, when considering the characteristics of the optical isolator, the larger the radius of curvature R is, the better.

Therefore, in reality, a combination of D and R is selected so that it satisfies each required condition. - As an example, if R210mm, D≧
It becomes 4.7mm. At this time, the size of the entire optical system is
It will be about 10 mrn.

〔Effect of the invention〕

As explained above, the present invention is an optical isolator that combines a Faraday rotator 1 and a polarizing prism 2.3, in which the end surface of the polarizing prism 2.3 is curved, and this curved surface provides a lens effect. By doing so, a confocal system can be constructed. Therefore, by applying the LDD module, there is an advantage that the optical coupling efficiency can be improved with a small size, and the operation of the single mode semiconductor laser can be stabilized.

[Brief explanation of drawings]

Fig. 1 is a detailed illustration of the present invention, Fig. 2 is an illustration of forward light propagation, Fig. 3 is an illustration of reverse light propagation, and Fig. 4 is an illustration of a confocal compound lens system. FIG. 5 is an explanatory diagram of a conventional LD module, FIG. 6 is an explanatory diagram of a conventional optical isolator, and FIG. 7 is an explanatory diagram of conventional problems. 1 is a Faraday rotator, 2 and 3 are polarizing prisms, 4 is a semiconductor laser, and 5 is an optical fiber. 1-+d, ÷d+d2÷a-( Fig. 2 Fig. 3 An explanatory diagram of a confocal compound lens system Fig. 4 An explanatory diagram of a conventional LD module An illustration of problems with the conventional example

Claims (1)

[Claims] Before and after a Faraday rotator (1) placed in a uniform magnetic field, polarizing prisms (2) and (3) each having a curved end face in the direction opposite to the Faraday rotator (1), The polarizing prism (
2) An optical isolator characterized in that the crystal axes of (3) are arranged such that they differ from each other by the Faraday rotation angle by the Faraday rotator (1).