JP2000338361A - Fiber stub and optical module using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber stub having less internal reflection, a stable optically coupled state and small size. SOLUTION: The fiber stub 1 with a ferrule 5 mounted on the outer periphery of an optical fiber 4 is constituted such that a groove 24 for bisecting the optical fiber 4 is formed on the side, that an optical isolator 2 held between two lenses 3a, 3b is arranged in the groove 24, and that the bisected optical fibers 4, 4 are optically connected to each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber stub having a built-in optical isolator for blocking return light from the outside of an optical module suitably used for optical communication equipment and sensors, and an optical module using the same. About.

[0002]

2. Description of the Related Art A laser diode (hereinafter, also referred to as an LD) used as a light source for optical communication reflects emitted light, and when it returns to an active layer of the LD again, its starting state is disturbed, and fluctuations in emission power and wavelength shift. Etc., which degrade the signal. In order to prevent this, an LD and an optical isolator that normally transmits light in only one direction are provided in the same package to constitute an LD module as one of the optical modules. In this LD module, particularly when an analog signal is used, the signal is easily degraded by the reflected return light, and the higher the density of the signal, the more likely the signal is to be affected by the reflected return light. With increasing speed and speed, optical isolators have become indispensable components.

Hereinafter, the operation of a general optical isolator will be described. The optical isolator is as shown in FIG.
The Faraday rotator 17 is sandwiched between the two polarizers 16a and 16b. As shown in FIG. 6A, the forward light 18 incident on the first polarizer 16a is used.
Is linearly polarized light in a specific polarization direction (19a in the figure).
The polarization direction 20 of the forward light 18 is rotated by 45 degrees in the Faraday rotator 17 (clockwise in the drawing: clockwise in a state where the light travels in the direction), and the polarization direction of the first polarizer is previously determined. The light enters the second polarizer 16b installed so as to be polarized at an angle of 45 degrees with 19a, and is transmitted as it is while maintaining the polarization direction.

On the other hand, the backward light 21 becomes linearly polarized light by the second polarizer 16b, and the polarization direction 20 is further rotated by 45 degrees by the Faraday rotator 17 as shown in FIG. Clockwise: clockwise with the light traveling in the direction of travel). As a result, the polarization direction forms an angle of 90 degrees with the polarization direction 19a of the first polarizer 16a. The light is blocked.

Such an optical isolator is used by being mounted on an LD module as described above. An example of a conventional LD module is shown in FIG. The LD 12, the lens 3, the optical isolator 2, the end of the optical fiber 4, and the like are housed in the package. Light emitted from the LD 12 is collimated by the lens 3, passes through the optical isolator 2, is collected by the lens 3, and enters the optical fiber 4. The whole is built into the package to isolate it from the outside environment. In the figure, reference numeral 26 denotes a rubber boot for protecting the extra length of the optical fiber.

[0006] Further, in order to reduce the size and facilitate the alignment, a subassembly component in which a semi-finished product of a lens, an optical isolator, and an optical fiber is formed first has been proposed. That is, as shown in FIG. 8, the first polarizer 16a, the Faraday rotator 17, the second polarizer 16b, and the magnet 9 for applying a magnetic field to the Faraday rotator and the optical fiber 4 are combined with the lens and the polarization function. That is, the light 18 from the laser diode 12 is made to enter the optical fiber 4 (see Japanese Patent Application Laid-Open No. 9-90282).

Further, as shown in FIG. 10, there has been proposed a device in which an optical fiber and an optical isolator are combined without using a lens. This is a so-called core-expanded fiber 25 in which the core of the optical fiber is enlarged to improve optical coupling.
Are used to divide the elements constituting the optical isolator, and insert the elements obliquely with respect to the optical axis to prevent reflection (see Japanese Patent Application Laid-Open No. 9-105886).

Further, as shown in FIG. 9, there has been a proposal to mount an optical isolator on a fiber stub with the miniaturization of the module itself (see Japanese Patent Application Laid-Open No. H10-68909). That is, the optical isolator 2 surrounded by the magnet 9 is inserted into the ferrule 5 holding the spherical fiber 6 at the center, and the whole is fixed in the sleeve 10. In this case, since the ferrule is manufactured with high coaxial accuracy, no axial deviation occurs. Further, a module with an optical isolator can be assembled with the same man-hour as a module without an optical isolator.

[0009]

As an indispensable element for widespread use of optical modules, there is a demand for downsizing and cost reduction of optical modules, and at the same time, downsizing of optical isolators, simplification of alignment, and low cost. It is also a request for conversion.

In the above conventional example, the optical isolator is basically composed of two polarizers and a Faraday rotator, and although the mounting method is different, all the structures are the same.

In the case of the module shown in FIG. 7, since the optical isolator 2, the lens 3 and the like are independent components and each are separately fixed to the holder and then aligned, the number of components is large, adjustment is complicated, and the size is large. There was a problem of becoming.

Further, in the case of the sub-assembled parts shown in FIG. 8, although the final alignment is simplified, the number of parts as a whole is large and the polarizer with lens function is provided regardless of whether one or two lenses are used. When the positional relationship between the LD 16a and the LD 12 changes, the state of the beam between the optical isolator and the optical fiber changes. Although the number of assembling steps is reduced, tolerance in alignment is small, and adjustment is difficult in a stable state.

In the structure shown in FIG. 10, although the characteristics of the combination with the optical fiber 4 are good, since the elements constituting the optical isolator are completely divided, the grooves 24 having different angles are used.
In addition to the trouble of forming a plurality of, a difficult step of aligning the polarization planes of the microelements with each other is required.

Although the optical isolator blocks reflected return light from farther away, it is necessary to prevent reflection from the optical isolator itself. 0.2% reflection even with high performance anti-reflective coating
(21 dB), but since the return loss required for the LD module is severe, at 50 dB, it is necessary to install the element obliquely so that reflected light does not return directly. However, if the element is inclined, the light beam will have an axial offset.

In the example shown in FIG. 10, the Faraday rotator 1
Although 7 is arranged obliquely, 16a, which is the element on the incident side where the influence of reflection is the greatest, remains perpendicular to the optical path, and the reflection from here cannot be removed. Further, in order to correct the offset of the light beam (displacement in the parallel direction from the axis) due to the oblique installation of the Faraday rotator 17, the Faraday rotator 17 is divided and inclined so as to face each other. Since the light beam is eventually offset between the divided Faraday rotators 17, a loss occurs here. Further, since the angle of the groove 24 is different, the length of the element becomes longer.

On the other hand, as shown in FIG. 9, in the case of mounting in a fiber stub, it is necessary to use an optical isolator having a shorter light space propagation portion and a smaller effective diameter as compared with the examples shown in FIGS. The advantage is that the mode field is stabilized while propagating through the optical fiber.Therefore, the light emitted from the optical fiber always has a constant beam diameter and shape. Has the advantage that the coupling state is always stable even if the positional relationship changes. Further, since the collectively aligned optical isolator is cut into a desired size and used, the complexity of assembly as in the example of FIG. 10 is released. However, even in this case, as in the example of FIG. 10, the optical isolator 2 is inclined to cause an offset of the light beam. In particular, since the coaxial is very good in the fiber stub, the influence of the offset is rather large.

Therefore, the present invention solves the above-mentioned conventional problems, and uses an excellent fiber stub which has a simple structure, has no internal reflection, and can eliminate a loss due to offset of a light beam, and an excellent fiber stub. An object is to provide an optical module.

[0018]

In order to achieve the above object, a fiber stub according to the present invention is characterized in that the optical fiber is divided into two parts at the side of the fiber stub having a ferrule attached to the outer periphery of the optical fiber. A groove is formed, and an optical isolator sandwiched between two lenses is provided in the groove, and the two divided optical fibers are optically connected to each other.

An optical module according to the present invention is characterized in that an optical element for receiving or emitting light and the above-mentioned fiber stub are provided on a substrate, and the optical element and the fiber stub are optically connected.

More specifically, the optical isolator is configured such that a pair of polarizers whose polarization planes are inclined at 45 degrees to each other sandwiches a Faraday rotator that rotates the polarization plane by 45 degrees.
In this case, if the Faraday rotator has spontaneous magnetization, the magnet can be omitted. The optical isolator is sandwiched between a pair of collimator lenses. Further, in the groove in which the optical isolator and the lens are provided, the wall surface is formed so that the end faces of the opposing optical fibers are parallel to each other, and the optical isolator is also inclined in the same direction as the angle of the end face of the optical fiber. Further, it is particularly preferable to use a fiber type polarizer for at least one of the polarizers.

According to the above configuration, it is possible to obtain an excellent fiber stub which is small in size, easy to align, has little reflection from the device itself, and does not change the coupling state of the optical isolator section due to alignment with the LD. By tilting the integrated optical isolator sandwiched between the pair of collimator lens systems in the same direction as the angle of the end surface of the optical fiber, the optical axis offset of the light beam can be corrected.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the optical module according to the present invention will be described in detail with reference to the drawings.

As shown in FIG. 2, a groove 24 is formed substantially at the center of the fiber stub 1 holding the optical fiber 4 in the ferrule 5 and oblique to the direction perpendicular to the optical axis. On the wall surface of the groove 24, the optical fiber 4
Are also exposed at the same angle. In the groove 24, the optical isolator 2 manufactured by cutting the lenses 3a and 3b, the polarizers 16a and 16b, and the Faraday rotator 17 after integrally molding them is installed in the same direction as the end face of the optical fiber 4. In general, a Faraday rotator requires a magnet for applying a magnetic field, but some have a spontaneous magnetization and do not require a magnet. In FIG. 2, such a type of Faraday rotator is used. .

The state of light passing through the groove will be described with reference to FIG. The light beam from the obliquely polished fiber has an optical axis L
The light is emitted at an angle of 0 to θo. Since the light beam is collimated by the lens 3a, the light beam becomes parallel to the optical axis L0 and is offset by δ. The offset amount δ of the light beam is determined by the angle θi of the end face of the optical fiber and the distance L between the end face of the optical fiber on the axis of the optical fiber and the main surface of the lens. Assuming that the refractive index is n1, sin (θi + θo) = n1 × sin (θi) δ = L × tan (θo) (Equation 1)

The oblique installation of the optical isolator 2 has two functions of preventing direct reflection of light from the end face and canceling the offset (offset in the opposite direction). In this case, assuming that the inclination of the element is θ2, the refractive index of the element is n2, and the thickness of the element is d, δc = d × sin {θ2-sin ⁻¹ (sin θ2 / n2)}. (Equation 2) The above Equation 2 is the offset amount of each element, and the offset amount of the entire element is the sum of these. The optical isolator is composed of a plurality of elements having different refractive indices. The sum of δc calculated by adding the values of the respective elements to n2 and d is the offset amount of the entire optical isolator.

Thus, according to the fiber stub of the present invention, since the direction in which the optical isolator 2 is inclined is the same as the direction of the end face of the optical fiber 4, it is easy to adjust and the configuration is compact in the longitudinal direction. Moreover, it is not necessary to tilt some of the elements separately, and only the tilt of the inserted optical isolator 2 needs to be adjusted. As a result, there is no internal reflection regardless of the compact configuration implemented in the fiber stub,
In addition, a loss due to a beam offset can be removed in principle, and a fiber stub and an optical module using the same can be realized in which the coupling state of the isolator is stable.

[0027]

EXAMPLES Specific examples will be described below.

Example 1 This will be described with reference to FIG. The optical fiber 4 is fixed in the zirconia ferrule 5. The ferrule 5 had a diameter of 2.5 mm and a length of 12 mm. An eight-degree groove 24 is formed obliquely (in the direction perpendicular to the optical axis (core axis of the optical fiber 4)) substantially at the center of the fiber stub 1 thus configured with a dicing saw having a width of 2.2 mm. Formed.

At this time, the end face of the internal optical fiber 4 is also exposed at the same angle. Here, a ball lens 3 having a diameter of 600 μm and an optical isolator 2 produced by integrally molding and cutting a polarizer and a Faraday rotator were installed at an angle of 8 ° in the same direction as the surface of the optical fiber.

This optical isolator 2 includes polarizers 16a, b
(Thickness 200 μm, refractive index 1.5), Faraday rotator 17 (magnetic garnet, thickness 300 μm, refractive index 2.
After forming an anti-reflection film of 2), they are joined with an epoxy-based translucent adhesive. In addition, this element is collectively aligned with a large element having a size of 10 mm square or more, and after being bonded, each is cut into a thickness of 500 μm.
The thickness becomes 700 μm.

The relationship between the angle of the end face of the fiber, the distance between the end face of the fiber and the main surface of the lens, and the beam offset amount will be described below. Since the diameter of the ball lens is 600 μm, it is 300 μm up to the main surface. Table 1 shows the beam shift amount (offset amount) of the optical fiber.

[0032]

[Table 1]

On the other hand, the angles of the polarizers 16a and 16b and the Faraday rotator 17 and the beam offset amount are shown. The beam offset amount of the entire optical isolator corresponds to the sum of two polarizers and one Faraday rotator. Table 2 shows the beam shift amount (offset amount) of each element (polarizer and Faraday rotator).

[0034]

[Table 2]

Therefore, when the angle of the fiber end face is 8 degrees, the offset can be canceled by setting the optical isolator to 8 degrees, and when the fiber end face is 6 degrees, setting the optical isolator to 6 degrees. However, the beam offset amount of the optical isolator changes depending on the thickness and the refractive index of the Faraday rotator.

As shown in FIG. 1, the light beam from the 8-degree polished fiber is emitted at about 4 degrees from the optical axis. Since the light beam is collimated by the lens 3, the light beam becomes parallel to the optical axis.
Offset by μm. If the optical isolator 2 is installed at an angle of 8 degrees, it is offset by about -42 μm and enters the ball lens. Here, the beam is deflected and condensed at an angle of about 4 degrees and enters the fiber 4. Since it is an 8-degree polished fiber, there is almost no loss of angle shift.

The ball lens 3 and the optical isolator 2
Can be joined into the groove 24 of the ferrule 5 with an adhesive, solder or the like. Further, in the fiber stub, when mounting on an LD module, an end face corresponding to the LD is used to prevent reflection.
As shown in FIG. 4, slanting polishing 7 or spherical polishing (generally PC
Polishing 8) is recommended. Further, the spherical fiber 6 may be used as a method for preventing reflection and improving the coupling efficiency at the same time.

Conventionally, the effective diameter of the optical isolator was required to be about 1 mm in order to align separate elements, but 500 μm
The following effective diameter is sufficient, and the material cost of the isolator can be reduced to 1/4 or less.

Example 2 An optical fiber in a fiber stub may have a polarizer function. This will be described with reference to FIG.
A part of the outer peripheral portion of the optical fiber 4 is polished until the distance from the fiber core (core diameter 6 to 8 μm) becomes 3 μm, and Al is deposited to a thickness of 12 nm. The method of vapor deposition is not limited to vacuum vapor deposition, sputtering and the like. This results in a loss of .0.
A polarizer having an extinction ratio of 3 dB or less and an extinction ratio of 35 dB or more can be formed. The two fiber polarizers are inserted and fixed from both end surfaces of the ferrule so as to hit each other at the center. At that time, the polarization planes (the same as the vapor deposition planes) are rotated so that they are at 45 degrees to each other.
Thereafter, similarly to Example 1, a diagonal saw formed a groove of 1.7 mm inclined at 6 degrees, and only the Faraday rotator was inserted and fixed in the groove with a ball lens having a diameter of 600 μm. The Faraday rotator is fixed at about 10 degrees. Since the function of the polarizer is imparted to the fiber, the number of components is reduced, and since there is no polarizer, the groove can be narrowed, so that the coupling efficiency increases, and the insertion loss of the entire A optical isolator can be reduced.

Example 3 FIG. 5 shows an example in which an LD module is constructed using the optical isolators formed in Examples 1 and 2. LD in V-shaped groove of Si platform
A fiber stub whose side end face is a spherical fiber is fixed. All the optical adjustment of the LD module is completed only by aligning the LD 12 and the tip fiber. In addition, since all of the optical system is in the ferrule, not only a small size but also an extremely stable LD module with little change over time can be provided.

[0041]

As described above, according to the fiber stub and the optical module of the present invention, the following effects can be obtained.

Since the optical isolator is constituted by a small space inside the ferrule, the effective diameter of the optical isolator can be small, and the material efficiency is excellent, so that the cost can be reduced.

Even if the positions of the fiber stub and the LD change, the coupling state of the optical isolator does not change. Also L
The optical adjustment of the LD module is possible only by adjusting the D and the fiber stub, and the assembling is easy.

The effect of the beam offset can be removed and the internal reflection can be prevented even though the optical isolator is built-in.

Since the angles of the internal element and the end face of the optical fiber are almost the same, the length is short, and the processing is easy and practical.

If a fiber polarizer is used, the number of components can be reduced, the groove of the isolator section is narrow, and a configuration with high coupling efficiency can be realized.

It is possible to construct an excellent LD module which is small, easy to manufacture, inexpensive and has little change over time.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view schematically showing an optical isolator section according to a first embodiment of the present invention.

FIG. 2 is a plan view showing a first embodiment of the present invention.

FIGS. 3A to 3C are perspective views schematically showing examples of manufacturing an optical fiber type polarizer.

FIGS. 4 (a) and (b) show L of a fiber stub, respectively.
It is a side view which shows the example of the D side end surface.

FIG. 5 is a sectional view showing the inside of the optical module according to the embodiment of the present invention.

FIGS. 6A and 6B are perspective views schematically explaining the operation of the optical isolator.

FIG. 7 is a cross-sectional view showing the inside of a conventional optical module.

FIG. 8 is a sectional view showing a conventional lens-integrated optical isolator.

FIG. 9 is a sectional view showing a conventional optical isolator for an optical module.

FIG. 10 is a cross-sectional view showing an example in which an optical isolator is mounted on an optical fiber.

[Explanation of symbols]

 1: Fiber stub 2: Optical isolator 3: Lens 4: Optical fiber 5: Ferrule 6: Spherical fiber 7: Obliquely polished ferrule end face 8: PC polished ferrule end face 9: Magnet 10: Sleeve 11: Platform 12: LD 13: PD 14: Peltier cooler 15: Package 16a, b: Polarizer 17: Faraday rotator 18: Forward incident light 19: Polarizing direction of polarizer 20: Polarizing surface 21: Reverse incident light 22: Polished surface 23: Metal thin film 24 ： Groove 25 ： Core expanded fiber 26 ： Rubber boot

Claims (2)

[Claims] 1. A fiber stub having a ferrule attached to an outer periphery of an optical fiber, the optical fiber being attached to a side of the fiber stub.
A groove for dividing the optical fiber is formed, an optical isolator sandwiched between two lenses is disposed in the groove, and the two divided optical fibers are optically connected to each other. Fiber stub. 2. An optical device comprising: an optical element for receiving or emitting light; and a fiber stub according to claim 1 disposed on a substrate, and optically connecting the optical element and the fiber stub. module.