JPS63304214A - Laser diode module containing optical isolator - Google Patents

Abstract

PURPOSE:To simplify constitution and to decrease the loss of forward transmitted light by disposing an LD, a Faraday rotor which optically rotates exist light of the LD by 45 deg. and emits the same, a polarizer which allows the transmission of the exit light thereof and couples the exit light to an optical fiber and a lens system on a optical axis. CONSTITUTION:The laser diode LD 24 which emits practically the linearly polarized light, the Faraday rotor 30 which optically rotates the exit light of the LD 24 by 45 deg. and emits the light, the polarizer which allows the transmission of the exit light of the Faraday rotor 30 and couples the same optically to the optical fiber 38 and the lens system are disposed on the optical axis. The polarizer is a double refractive crystal 32 having the optical axis perpendicular to or parallel with the plane of polarization of the exit light of the Faraday rotor 30. The reflective feedback light component having the same plane of polarization as the plane of polarization of the exit light of the LD which is practically the linearly polarized light is thereby removed and the miniaturization and simplification of the module are attained.

Description

Detailed Description of the Invention: A 1-ler laser diode (hereinafter abbreviated as LD), a Faraday rotator that rotates the output light of the LD by 45 degrees and outputs it, and a A polarizer and a lens system that are optically coupled to the fiber are arranged on the optical axis.
Configure the D module. According to this configuration, it is possible to remove the reflected feedback light component having the same polarization plane as the polarization plane of the emitted light from the LD, which is the practical 11JQ light, and it is possible to miniaturize the module.

INDUSTRIAL APPLICATION FIELD The present invention relates to an LD module having a built-in optical isolator for eliminating the influence of reflected feedback light in a light emitting section of a device such as an optical communication device.

In optical communication systems that use optical fiber as the transmission path,
A portion of the light that enters the optical fiber from the light source may be reflected at the joints between the optical fibers and return to the light source. When such reflected feedback light occurs, especially when the light source is an LD, its operation becomes unstable and deterioration of transmission quality is unavoidable. For this reason, there is a need for an optical isolator that can eliminate the influence of reflected feedback light, and there is a demand for a simplified configuration of the optical isolator.

2. Description of the Related Art This type of optical isolator is generally used as an LD module by being integrated with an LD serving as a light source. FIG. 4 is a cross-sectional configuration diagram of a conventional LD module. This LD module is configured by arranging an LD 4 and a lens 6 provided in a package 2, an optical isolator 8, and a lens 10 in this order in a casing (not shown), and the output light of the LD 4 is transmitted to a terminal 12a of an optical fiber 12. It is designed to be optically coupled to. Optical isolator 8
is constructed as follows. In the figure, reference numeral 14 denotes a first polarizing prism made of a birefringent axle crystal having a crystal axis (corresponding to the main axis, the same applies hereinafter) 14a in a direction perpendicular to the plane of the paper;
Reference numeral 16 denotes a second polarizing prism made of a birefringent center-axis crystal having a crystal axis 168 tilted 45 degrees clockwise when viewed from the forward direction with respect to the crystal axis 14a, and 18 a second polarizing prism formed between these polarizing prisms 14 and 16. A Faraday rotator is inserted and applied with a uniform magnetic field H to rotate transmitted light by 45 degrees.

Now, assuming that the polarizing prisms 14 and 16 are made of the same material, the refractive index for this ordinary ray is n. , when the refractive index for the extraordinary ray is ne, the ordinary ray component for the first polarizing prism 14 of the light from the light emitting source, that is, the polarized component perpendicular to the crystal axis 14a is
4, the refractive index n. In the bond bent at 2 polarizing prism 1
6, the refractive index n as an ordinary ray for the second polarizing prism 16. It is refracted and emitted. In addition, an extraordinary ray component of the light from the light source toward the first polarizing prism 14,
That is, the polarized light component parallel to the crystal axis 14a has a refractive index n in the first polarizing prism 14. After being refracted at , the Faraday rotator 18 rotates the same 45 degrees, and the second polarizing prism 16 has a refractive index n as an extraordinary ray for the second polarizing prism 16 . It is refracted and emitted. The light emitted from the second polarizing prism 16 is condensed by an appropriate lens and guided to an optical fiber. In addition, D
When applying this optical isolator to an LD module that uses an FB-LD as a light emitting source, the polarization plane (the plane containing the ?t2 field vector) of the DFB-LD that emits linearly polarized light is set to the first polarizing prism 14. The light from the DFB-LD is guided to the optical fiber through a single optical path so as to match the direction of the ordinary ray component, thereby increasing the coupling efficiency.

On the other hand, the ordinary ray component toward the second polarizing prism 16 of the reflected feedback light that has been reflected and returned from the end surface of the optical fiber, etc.
That is, the polarized light component perpendicular to the crystal axis 16a has a refractive index n in the second polarizing prism 16.

After being refracted by the Faraday rotator 18, it is rotated by 45 degrees counterclockwise with respect to the traveling direction (from right to left in the figure), and then refracted by the first polarizing prism 14 as an extraordinary ray toward the first polarizing prism 14. Rate n. It is refracted and emitted. Further, among the reflected feedback light, an extraordinary ray component to the second polarizing prism 16, that is, a polarized component parallel to the crystal axis 16a, has a refractive index n in the second polarizing prism 16.
.

After being refracted at , the Faraday rotator 18 rotates the same counterclockwise by 45 degrees, and the first polarizing prism 14 has a refractive index n as an ordinary ray for the first polarizing prism 14 . It is refracted and emitted.

In this way, the forward light from the light source to the optical fiber undergoes refraction at each polarizing prism 14.16 (combination of n and n or combination of n and n8) 0 0
e and the reflected feedback light that is reflected from the end face of the optical fiber and returns to the polarizing prism 16.
．． Since the refraction (combination of n and n8) received by LD 4 is different, if the optical coupling structure is designed to guide the light from LD 4 to the optical fiber, the reflected return light will be
The light is separated into two polarized light components and emitted from the polarizing prism 14, and does not return to the LD 4.

Problems to be Solved by the Invention However, in the above-mentioned conventional LD module, the three optical components constituting the optical isolator 8 are
Since the lenses 6 and 10 are arranged in series on the optical axis, the separation distance between the lenses 6 and 10 cannot be made small, and there is a problem in that the coupling efficiency is reduced due to lens aberration.

Furthermore, the reality is that the optical crystals that are the materials for the polarizers 14 and 16 and the Faraday rotator 18 are generally expensive.

The present invention was created in view of these circumstances,
It is an object of the present invention to simplify the configuration of an optical isolator and provide an inexpensive LD module with small loss of forward transmitted light.

In general, the emitted light of an LD is practically linearly polarized light, and the polarized light component having a polarization plane perpendicular to this polarization plane has a low intensity compared to the intensity of the polarized light component of the emitted light. , is known to be about 20 dB smaller. Furthermore, if the reflected feedback light returning to the LD is linearly polarized light having a polarization plane perpendicular to the polarization plane of the emitted light, that is, if the reflected feedback light does not have the polarization component of the emitted light, the reflected feedback light It is known that there is little effect on

The present invention has been made with attention to these facts, and an optical isolator is configured to exclude only the polarization component having the same polarization plane as the polarization plane of the light emitted from the light source in the reflected feedback light. This is intended to simplify the configuration of the LD module.

That is, the LD module with a built-in optical isolator of the present invention practically includes an LD that emits linearly polarized light, a Faraday rotator that rotates the output light of the LD by 45 degrees and outputs it, and a Faraday rotator that transmits the output light of the Faraday rotator to generate light. A polarizer and a lens system that are optically coupled to a fiber are arranged on the optical axis.

Function: As described above, in the LD module of the present invention, the optical isolator is configured to exclude only the polarization component having the same polarization plane as the polarization plane of the light emitted from the light source in the reflected return light. , only one polarizer is sufficient,
Therefore, it is possible to reduce the thickness of the optical isolator in the optical axis direction, so it is possible to provide an LD module with high coupling efficiency.

Friend-English-1 Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 1 is a cross-sectional configuration diagram of an LD module to which the present invention is applied. This LD module 22 includes an LD assembly 23 in which an optical isolator part is integrated with an LD 24.
and a lens 34 for optically coupling this emitted light to an optical fiber 38. 36.40 is
This is a ferrule for fixing the end of the optical fiber 38. The reason why the end surface 36a of the ferrule 36 on the lens 34 side is formed obliquely is to prevent reflected light from occurring.

The LD 24 is fixed to the mount 26 so that its joint surface is perpendicular to the plane of FIG. 1. For convenience, the y-axis is the direction perpendicular to the paper surface of the drawing and passes through the paper from the back side to the front side.
The propagation direction of the light emitted from the LD24 is 2 axes and y in 2 directions.
An orthogonal three-dimensional coordinate system is set in which the X axis is the direction obtained by rotating the axis by 90 degrees clockwise. At this time, the light emitted from the LD 24 is substantially in the TE mode, and its polarization plane is y
It is perpendicular to the z plane.

The mount 26 is closed by a housing 28 having an opening provided with a Faraday rotator 30 and a birefringent crystal 32. The Faraday rotator 30 is made of, for example, Bi
It is a normal one made of a magneto-optic crystal such as a substituted magnetic garnet, and its length is set so that a predetermined saturation magnetic field is applied from a magnet 33 to rotate transmitted light by 45 degrees.

The birefringent crystal 32 is a uniaxial crystal such as rutile (Tie, single crystal), and its optical axis is perpendicular to the propagation direction of the transmitted light and perpendicular or parallel to the polarization plane of the transmitted light. (vertical in this embodiment).

FIG. 2 is for explaining the polarization state at each point on the optical path shown in FIG. 1, and is all viewed in the forward direction. Output light of LD24 in TE mode (a)
The polarization plane of the light is rotated by 45° clockwise in two directions by the Faraday rotator 30 (b), and the light is incident on the birefringent crystal 32. Since the optical axis of the birefringent crystal 32 (indicated by the dotted line arrow in the figure) is set perpendicular to the incident light, this incident light is directed toward the birefringent crystal 32.
It propagates as an ordinary ray of light and is coupled into an optical fiber 38 by a lens 34.

On the other hand, among the reflected feedback light, the polarized light component having the same polarization plane as that in FIG. 45 counterclockwise in one or two directions by the rotor 30
Since the light is optically rotated by °, it returns to the LD 24 as 1M polarized light, as shown in (C). Also, of the reflected return light,
The polarized light component (d) having a plane of polarization parallel to the optical axis of the birefringent crystal 32 is refracted as an extraordinary ray different from the forward transmitted light, and is further optically rotated by the Faraday rotator 30 to return to the original TE. It is emitted as polarized light (a). At this time, since the focus of the lens 34 is adjusted so that the light emitted from the LD 24 is coupled to the optical fiber 38, the reflected feedback light that has been refracted in a different direction from the forward direction is not coupled to the LD 24. . In this way, L of the reflected return light
Since the light coupled to the D24 is only 1M polarized light, the influence of reflected feedback light on the LD24 is prevented.

FIG. 3 is a cross-sectional configuration diagram of an LD module showing another embodiment of the present invention. Components that are substantially the same as those in FIG. 1 are designated by the same reference numerals. In this example, the birefringent crystal 3
2, a polarization separation film 42 is used. Polarization separation WM
42 is formed by laminating, for example, dielectric multilayer films with different refractive indexes on the Faraday rotator 30, and the material, thickness, and position of the film are determined according to the output of the LD 24, which is optically rotated by the Faraday rotator 30. It is set to transmit the emitted light and reflect the light having a polarization plane perpendicular to the polarization plane of the emitted light. The light emitted from the LD 24 is propagated in the forward direction via the lens 43, the Faraday rotator 30, the polarization separation IEm 42, and the lens 44, and is converted into an optical beam. It is coupled to IPA 38.

On the other hand, the light that has passed through the polarization separation film 42 out of the reflected feedback light is optically rotated by the Faraday rotator 30 and then
LD2 becomes a polarized light component that does not affect LD2.
The light that should return to the LD 24 and affect the LD 24 is reflected by the polarization separation film 42 and separated from the optical path.

In this way, only the components of the reflected feedback light that affect the LD 24 are cut, which simplifies the configuration of the optical isolator and shortens the optical path of the LD module, increasing coupling efficiency. It is. At this time, since the number of polarizers that conventionally required two is reduced to one, losses due to reflection and transmission are reduced accordingly.

Furthermore, since the optical isolator section was conventionally provided separately from the LD assembly, the housing 28 needed a window section made of, for example, sapphire to protect the LD 24. In addition, since the Faraday rotator 30 integrated with the birefringent crystal 32 or the polarization separation film 42 is provided directly on the housing 28, and this is used as a window, the loss is further reduced compared to the conventional method, and the required This reduces the number of parts.

1. As detailed above, according to the present invention, a simplified optical isolator is used to eliminate only the reflected feedback light having the same polarization plane as the polarization plane of the output light of the LD. , L.D.
Since it is configured as a module, its optical path length can be shortened, resulting in the effects of miniaturizing the device and increasing coupling efficiency.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional configuration diagram of an LD module showing an embodiment of the present invention. FIG. 2 is a diagram for explaining the polarization state at each point on the optical path shown in FIG. 1. FIG. FIG. 4 is a cross-sectional diagram of a conventional LD module. 4.24...LD. 6.10.34.43.44...Lens, 18.30
...Faraday rotator, 32...birefringent crystal, 42...polarization separation film. × 30; Faraday cycle 32: Osara Xi #re 4S5 hard 5 light σ) Hiroshi (column 124 giant convex 1st figure y (G) (b) to the first evil.
Figure 2 X 24' LD 30: Fallate-Giant I Town 42: Polarized Light IJ Figure 3 Figure 4 :LD 8 29 Isolator Colored Rice Row Bin Figure 4

Claims (3)

[Claims] (1) Laser diode (2) that emits linearly polarized light in practical use
4), a Faraday rotator (30) that rotates the output light of the laser diode (24) by 45 degrees and outputs it, and a Faraday rotator (30) that transmits the output light of the Faraday rotator (30) and optically connects it to the optical fiber (38). A laser diode module with a built-in optical isolator, characterized in that a polarizer and a lens system to be coupled are arranged on the optical axis. (2) Claim 1, wherein the polarizer is a birefringent crystal (32) having an optical axis perpendicular or parallel to the polarization plane of the light emitted from the Faraday rotator (30).
A laser diode module with a built-in optical isolator as described in . (3) A patent claim characterized in that the polarizer is a polarization separation film (42) that reflects and separates light having a polarization plane perpendicular to the polarization plane of the output light of the Faraday rotator (30). A laser diode module with a built-in optical isolator according to item 1.