JPH08160359A - Optical isolator and its production - Google Patents

Abstract

PURPOSE: To facilitate adjustment and to improve an extinction ratio characteristic by providing a case enclosing a Faraday rotator with a reference plane and adjusting the polarization direction of incident light past this Faraday rotator in such a manner this direction is made perpendicular to this reference plane. CONSTITUTION: The case 8 consisting of the metal enclosing the Faraday rotator 6 of the optical isolator 10 consisting of the 45 deg. Faraday rotator 6 and an analyzer 4 arranged on one side thereof is provided with the reference plane A. The polarization direction of the light made incident from the analyzer 4 side and past the Faraday rotator 6 is so adjusted that the direction is made perpendicular to the reference plane A. The polarization direction of the light made incident from the analyzer 4 side and past the Faraday rotator 6 is preferably so adjusted that the direction is kept within 0±1.8 deg. when the axial direction perpendicular to the reference plane A is assumed to be 0 deg.. Then, the polarization directions of the return light to a semiconductor laser diode(LD) 1 and the exit light of LD 1 are intersected orthogonally with high accuracy if the reference plane A and the front surface of the substrate 11 are fixed in tight contact with each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator used in optical communication using a semiconductor laser or the like, optical information processing, an optical measurement system and the like.

[0002]

2. Description of the Related Art An optical system using a semiconductor laser has problems such as fluctuations in oscillation power of the semiconductor laser, fluctuations in wavelength, increase in noise, etc., due to returning light due to reflection from optical components such as fibers. Therefore, an optical isolator is used so that the returning light does not return to the semiconductor laser. At present, a bulk type optical isolator as shown in FIG. 5 has been put into practical use as an optical isolator.

In FIG. 5, 1 is a semiconductor laser diode (hereinafter referred to as LD), 2 is a coupling lens, 3 is a polarizer, and the incident light S1 is linearly polarized. 6 is a Faraday rotator, which is formed of a magneto-optical crystal having a Faraday effect,
Incident light S when combined with cylindrical magnet 5
1. The polarization plane of the return light S3 is rotated non-reciprocally by 45 degrees. Reference numeral 4 is an analyzer, which has a transmission polarization plane of 45 with respect to the polarizer 3.
It is arranged to form an angle of degrees.

The incident light S1 is linearly polarized by the polarizer 3, the plane of polarization of which is rotated by 45 degrees by the Faraday rotator 6, and passes through the analyzer 4. On the other hand, the component of the return light S3 that has passed through the internal analyzer 4 is further rotated by 45 degrees by the Faraday rotator 6 and becomes polarized light perpendicular to the polarizer 3, so that the light does not return to the LD1 side.

Here, the light emitted from a normal LD is linearly polarized in the X-axis direction in the TE mode. Therefore, the transmission axis direction of the polarizer 3 is parallel to the X-axis and is opposite to the polarizer 3. The return light incident from is polarized in the Y-axis direction.

Therefore, even if the polarizer 3 is removed, the return light to the LD is only the light polarized in the Y-axis direction and does not affect the stable oscillation of the LD oscillating in the TE mode. Therefore, an optical isolator (hereinafter referred to as 0.5-stage type optical isolator) in which the polarizer 3 is omitted and the cost is reduced can be considered.

In order to incorporate the 0.5-stage type optical isolator into the LD module, as shown in FIG. 6, the light from the LD 1 is made incident on the optical isolator 10 in the forward direction, and the optical isolator 10 is rotated around the optical axis (Z axis). Optical isolator
The optical isolator 10 is fixed at an angle at which the light emitted from 10 becomes maximum.

The change in the amount of light emitted from the optical isolator 10 during this rotation adjustment is as shown in the graph of FIG. 7, so that the angle at which the amount of light emitted is maximum (0 in the case of this graph).
The optical isolator may be fixed at a position of 180 degrees or 180 degrees.

However, at this position, the change in the amount of light is very small, and it is difficult to assemble it accurately. 8 is shown in FIG.
This is an expansion of the light intensity change near 0 degrees in the middle, but even if the allowable range of light intensity change during assembly is kept within 0.03 dB, the fixed position of the optical isolator will vary within ± 4.8 degrees. As for this angle deviation, the polarization direction of the light returning to the LD from the opposite direction varies within ± 4.8 degrees from the Y-axis direction.

FIG. 9 shows the component ratio in the X-axis direction when the polarization direction of the return light is deviated from the Y-axis direction. From this figure, when the fixed position of the optical isolator is deviated by 4.8 degrees, the X-axis of the return light is shown.
It can be seen that the extinction ratio of the axial component is 21 dB.

Generally, the rotation angle of a Faraday element is proportional to its thickness. In the case of a normal Faraday element (bismuth substitution garnet by the LP method), the Faraday rotation angle is 1
The thickness of the Faraday element per degree is about 5 μm, and it is difficult to accurately finish the Faraday rotation angle to a thickness equivalent to 45 degrees, so there is usually a variation of about ± 2 degrees. Further, when the assembling method shown in FIG. 6 is used, the deviation of 2 degrees is 4 degrees between the polarization direction of the return light to the LD and the Y-axis direction.

After all, the permissible range of the light amount change at the time of assembly is 0.03.
The polarization direction of the return light varies up to ± 8.8 degrees with respect to the Y-axis even if it is kept below dB, so the extinction ratio characteristic that can be expected as a 0.5-stage optical isolator is about 16 dB from Fig. 9 assuming the worst case. However, there is a problem that the extinction ratio characteristic (40 dB or more) of an ordinary one-stage type optical isolator is remarkably low and it is not practical.

[Object] In view of the above problems, an object of the present invention is to provide a 0.5-stage type optical isolator which has a simple structure and is easy to adjust and has an excellent extinction ratio characteristic (30 dB or more).

[0014]

The present invention has been made to solve the above-mentioned problems of the prior art. An optical isolator according to the present invention comprises a 45 degree Faraday rotator 6 and a detector arranged on one side thereof. In the optical isolator 10 composed of the photons 4, the reference plane A is provided in the case 8 made of metal that encloses the Faraday rotator 4, and the polarization direction of the light incident from the analyzer 4 side and passing through the Faraday rotator 6 is the reference plane. It is fixed so that it is perpendicular to A.

In the optical isolator 10, when the axial direction perpendicular to the reference plane A is set to 0 degree, the polarization direction of the light incident from the analyzer side and passing through the Faraday rotator is 0 degree ± 1.8 degree. It is adjusted and fixed within.

Further, the optical isolator has a TE mode LD 1 and a coupling lens 2 installed, and is fixed by bringing a reference plane A into contact with the upper surface of a base substrate 11 parallel to the polarization direction of the emitted light of the LD 1. .

[0017]

In the optical isolator and the method of manufacturing the same according to the present invention, since it is configured as described above, the polarization direction of the light incident from the analyzer side and passing through the Faraday rotator is perpendicular to the reference plane A with high accuracy. Since it is possible to manufacture and provide a 0.5-stage type optical isolator, if the reference plane A and the upper surface of the substrate are closely attached and fixed, the polarization direction of the return light to the LD and the LD
The polarization direction of the light emitted from the can be made orthogonal to each other with high precision, stable oscillation of the LD is possible, and the same level of characteristics as the one-stage optical isolator, with one polarizer less one less cost.
It can be provided with a 5-stage optical isolator.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram for explaining a 0.5-stage optical isolator showing an embodiment of the present invention. FIG. 2 is a diagram showing a method for adjusting an optical isolator showing an embodiment of the present invention. Figure 3,
FIG. 4 is a diagram for explaining the adjustment angle of the analyzer showing the characteristics of the optical isolator showing the embodiment of the present invention and the detector output (extinction ratio dB in the reverse direction).

The optical isolator according to the present invention is devised so that the polarization direction of the light returning to the LD and the polarization direction of the light emitted from the LD are orthogonal to each other with high precision.
In the case of an LD that oscillates in the mode, the polarization direction of the return light is Y
This can be achieved by accurately paralleling the axial direction. This makes it possible to minimize the polarization component ratio in the same direction as the light emitted from the LD among the polarization components of the light returned to the LD, and thus it is possible to stabilize the LD oscillation.

An ordinary LD module has a structure in which an LD module and an optical isolator are placed on a table called a substrate. Referring to FIG. 1, the polarization direction of the light emitted from the LD1 is parallel to the upper surface of the substrate 11, which is indicated by Y in the figure.
It is a plane perpendicular to the axis.

First, the reference plane A is provided on the metal case 8 or the like which contains the Faraday rotator 6. Next, it consists of Faraday rotator 6, magnet 5, metal case 8 and analyzer 4.
When the 0.5-stage optical isolator 10 is incorporated in the LD module, the reference plane A is set to be perpendicular to the Y axis. In this case, the optical isolator 10 and the substrate 11 can be fixed by bringing the reference surface A and the upper surface of the substrate 11 into contact with each other, and workability is improved.

Next, in the system as shown in FIG. 2, the analyzer 4 is adjusted with respect to the reference plane A to complete the 0.5-stage type optical isolator 10. Described in detail below, the transmission axis direction C of the polarizer 3 is parallel to the reference plane A, that is, the X axis in the drawing. The Faraday rotation is 45 degrees in the direction of arrow F.

Here, the adjusting beam light can use two kinds of polarization states, that is, circular polarization and linear polarization.
The rotation adjustment angle of the analyzer 4 is represented by the angle between the transmission axis direction and the Y axis, and the Faraday rotation direction is defined as the plus direction.

In the case of circularly polarized light, the analyzer 4 is rotated about the optical axis (Z axis) in FIG. 2, and when the output of the photodetector 7 becomes the minimum, the Faraday rotator 6 subassembly and the analyzer 4 are fixed. To do. At this time, it is apparent that the polarization direction of the beam emitted from the Faraday rotator 6 matches the Y axis.

FIG. 3 shows the adjustment angle of the analyzer 4 on the horizontal axis and the output of the photodetector 7 on the vertical axis. From the figure, the light amount becomes minimum at -45 degrees or 135 degrees. Therefore, the subassembly of the Faraday rotator 6 and the analyzer 4 may be fixed at -45 degrees or 135 degrees. Furthermore, if it is fixed within -45 degrees ± 1.8 degrees or within 135 degrees ± 1.8 degrees, reverse insertion loss of 30 dB or more can be easily achieved.

In the case of linearly polarized light, the polarization direction of the adjusting beam light L is approximately -45 degrees from the Y axis. When the analyzer 4 is rotated about the optical axis, four positions where the output of the photodetector 7 is minimum appear per rotation. The state at this time is shown in FIG. -45 out of the four minimum values
The Faraday rotator 6 and the analyzer 4 may be fixed at a position of 135 degrees or 135 degrees. By doing so, the polarization direction of the light beam emitted from the Faraday rotator 6 coincides with the Y axis. Furthermore, if it is fixed within -45 degrees ± 1.8 degrees or within 135 degrees ± 1.8 degrees, reverse insertion loss of 30 dB or more can be easily achieved.

As described above, according to the assembling method of FIG. 2, as shown in FIGS. 3 and 4, the change in the light amount at the fixed position of the analyzer 4 is steep, and therefore the adjustment can be easily performed with high accuracy. It is possible and is not affected by the variation in the thickness of the Faraday rotator 6.

[0028]

Since the optical isolator of the present invention is configured as described above, the angle adjustment of the analyzer 4 with respect to the reference plane A is easily performed with high accuracy because the light output of the detector changes sharply. It is possible to manufacture and provide a 0.5-step type optical isolator that makes the polarization direction of the light emitted from the Faraday rotator perpendicular to the reference plane A with high accuracy. By doing so, the polarization direction of the return light to the LD and the polarization direction of the light emitted from the LD can be orthogonalized with high accuracy, stable oscillation of the LD becomes possible, and the characteristics similar to those of the one-stage type optical isolator can be obtained. It can be provided by a low-cost 0.5-stage optical isolator with one polarizer.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a 0.5-stage optical isolator showing an embodiment of the present invention.

FIG. 2 is a diagram showing a method of adjusting an optical isolator showing an embodiment of the present invention.

FIG. 3 is a diagram for explaining an adjustment angle of an analyzer and a detector output (extinction ratio in the reverse direction dB) showing characteristics of an optical isolator showing an embodiment of the present invention.

FIG. 4 is a diagram for explaining an adjustment angle of an analyzer and a detector output (extinction ratio in the reverse direction dB) showing characteristics of an optical isolator showing an embodiment of the present invention.

FIG. 5 is a diagram showing a conventional optical isolator.

FIG. 6 is a diagram showing a conventional 0.5-stage type optical isolator.

FIG. 7 is a diagram for explaining the rotation angle of an isolator showing the characteristics of a conventional 0.5-stage optical isolator and the amount of light emitted from the isolator (forward loss dB).

FIG. 8 is a diagram for explaining the rotation angle of an isolator showing the characteristics of a conventional 0.5-stage optical isolator and the amount of light emitted from the isolator (forward loss dB).

FIG. 9 is a diagram for explaining the polarization angle of return light (Y-axis reference) and the X-axis component (reverse extinction ratio dB) of the amount of return light showing the characteristics of the conventional 0.5-stage type optical isolator.

[Explanation of symbols]

 1 LD 2 Coupling lens 3 Polarizer 4 Analyzer 5 Magnet 6 Faraday rotator 7 Photodetector 8 Metal case 10 0.5 step optical isolator 11 Substrate A Reference plane

Claims (3)

[Claims] 1. An optical isolator comprising a 45 ° Faraday rotator and an analyzer arranged on one side of the Faraday rotator, wherein a reference surface is provided in a case made of resin or metal enclosing the Faraday rotator, and the light is incident from the analyzer side. An optical isolator, which is adjusted and fixed so that the polarization direction of light passing through a Faraday rotator is perpendicular to the reference plane. 2. The optical isolator according to claim 1, wherein an axial direction perpendicular to the reference plane is 0 degree,
An optical isolator characterized in that the polarization direction of the light incident from the analyzer side and passing through the Faraday rotator is adjusted and fixed within 0 ° ± 1.8 °. 3. The optical isolator according to claim 1, wherein a TE mode LD and a coupling lens are installed.
A method for manufacturing an optical isolator, characterized in that the reference plane is brought into contact with and fixed to the upper surface of the base substrate parallel to the polarization direction of the outgoing light.