JPH07301734A - Optical coupling device - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to an optical coupling device for optically coupling a semiconductor lens and an optical fiber, and an object thereof is to facilitate adjustment work. [Structure] A semiconductor laser 1, a first lens 2, and a second lens
Lens 3, optical isolator 10, and wedge prism 2
0 and the optical fiber 50 having the inclined incident end face 51 are arranged in this order.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an optical coupling device for optically coupling a semiconductor laser (laser diode; LD) and an optical fiber, and more specifically, an optical axis adjustment in which an optical isolator is incorporated. To an easy optical coupling device.

[0002]

2. Description of the Related Art A transmitter of an optical communication system usually includes an optical coupling device for optically coupling a semiconductor laser as a light source and an optical fiber as a transmission line. When the reflected light from the surfaces of optical components such as lenses, optical fibers and connectors inserted in the optical circuit returns to the semiconductor laser, the oscillation of the semiconductor laser becomes unstable and, as a result, the S / N deteriorates. Therefore, such an optical coupling device usually incorporates an optical isolator for suppressing reflected feedback light.

Conventionally, as an optical coupling device for optically coupling a semiconductor laser and an optical fiber, a first lens for collimating an emission beam of a semiconductor laser and emitting the collimated beam, and a collimated beam emitted from the first lens are optically coupled. It is known to have a second lens that converges on the incident end face of the fiber and an optical isolator that is arranged in the optical path formed by the first and second lenses.

Although the optical isolator can suppress the reflected feedback light, it cannot completely block it. That is, the optical isolator has a finite extinction ratio (for example, 30
dB).

In order to provide a higher performance optical coupling device using such an optical isolator having a finite extinction ratio, it is desirable to suppress the generation of reflected feedback light itself. Therefore, for example, the incident end face (excitation end) of the optical fiber is tilted so that the reflected light on this face does not go backward in the original optical path.

[0006]

In an optical coupling device to which an optical fiber having such an inclined incident end face is applied, the position of each optical component is set in a state where the incident beam to the optical fiber is inclined. However, there is a problem that the adjustment is complicated.

Therefore, an object of the present invention is to provide an optical coupling device in which the degree of optical coupling between a semiconductor laser and an optical fiber is high and the tilt angle of a light beam can be easily adjusted.

[0008]

According to the present invention, there is provided an optical coupling device for optically coupling a semiconductor laser and an optical fiber, which comprises a first lens for collimating an emission beam of the semiconductor laser and emitting the collimated beam. A second lens for converging a parallel beam emitted from the first lens to an incident end face of the optical fiber, and an optical isolator and a wedge prism arranged in an optical path formed by the first and second lenses. , The incident end face of the optical fiber is inclined by a predetermined inclination angle with respect to the optical axis of the optical fiber,
When the tilt angle is φ, the refractive index of the core of the optical fiber is n, and the deviation angle of the wedge prism is θ, the expression n · sin is obtained.
An optical coupling device that satisfies φ = sin (φ + θ) is provided.

Preferably, an optical film is formed on at least one surface of the wedge prism through which light passes.

[0010]

According to the present invention, the optical coupling is maximized only by fixing the optical isolator and the wedge prism and moving and adjusting the optical fiber in the direction in which the optical axis of the optical fiber deviates from the optical axis of the optical isolator. Becomes That is, it is possible to provide an optical coupling device that can be easily adjusted.

According to a preferred embodiment of the present invention, an optical film is formed on the surface of the wedge prism. By using this optical film as a band-pass filter having a narrow band, for example, when a semiconductor laser oscillates in multiple modes, one oscillation peak can be selected from a large number of oscillation spectrum peaks, which is generally expensive. It is possible to realize an optical transmitter that oscillates in a single mode without using a DFB type (distributed feedback type) semiconductor laser.

[0012]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Prior to the description of the embodiments, the outline of the conventional technique and its problems will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram showing a configuration of a conventional optical coupling device, and FIGS. 2A and 2B show optical paths in the conventional device.

In the figure, 1 is a semiconductor laser, 50 is an optical fiber for transmitting the emitted light of the semiconductor laser 1, and 2 is a first optical beam for making the light beam emitted by the semiconductor laser 1 a parallel beam.
Lens 4 is an optical isolator for passing a light beam in the forward direction and blocking passage of a light beam in the reverse direction, and 3 is a second lens for converging the light on the incident end face of the optical fiber 50.

The optical isolator 4 cannot completely block the passage of light in the opposite direction. Semiconductor laser 1, first lens 2, optical isolator 4, second
The lens 3 and the optical fiber 50 are arranged in this order, whereby the semiconductor laser 1 and the optical fiber 50 are optically coupled.

By the way, the emitted light of the semiconductor laser 1 which is converged by the second lens 3 is incident on the optical fiber 5 at an incident angle of 0 °.
When incident on the end face of 0, the reflected light on the incident end face returns to the optical isolator 4 side.

In order to prevent this and to obtain an optical coupling device with higher isolation, conventionally, as shown in the drawing, the incident end face of the optical fiber 50 is set in a direction perpendicular to the optical axis 50A of the optical fiber 50. The inclined incident end face 51 is inclined at a predetermined inclination angle (Φ) so that the reflected light at the end face 51 does not return to the optical isolator 4 side.

Of course, the larger the tilt angle Φ, the more the reflected light does not return to the optical isolator side. However, if the tilt angle Φ is too large, the transmission coefficient becomes small, and the optical power traveling to the optical fiber 50 becomes small. .

Therefore, the inclination angle φ of the end face 51 is set in consideration of the reflection coefficient and the transmission coefficient at the inclined incidence end face 51. Now, in order to make the light from the semiconductor laser 1 enter the optical fiber 50 having the inclined end face 51 as described above most efficiently, the light refracted at the inclined incident end face 51 is relative to the optical axis 50A of the optical fiber 50. It should be parallel.

The condition for this parallelism is, as shown in FIG. 2A, n.sinΦ = sin (Φ + Θ).

Where Θ is the emission angle of the light emitted from the second lens (that is, the angle formed by the light emitted from the second lens and the optical axis of the optical isolator), n is the refractive index of the core of the optical fiber, Φ is the inclination angle of the inclined end face.

That is, in order to efficiently couple light to an optical fiber having an inclined end face, it is necessary to inject the light beam that has passed through the optical isolator into the optical fiber by inclining it by Θ shown in the above equation. It

The second lens 3 is passed through the optical isolator 4.
In order to tilt the light beam emitted from, the conventional method is as shown in FIG. An optical fiber 50 is arranged behind the second lens 3 with a space L. Optical axis 4A of optical isolator 4 and optical axis 50A of optical fiber 50
Letting t be the deviation from and, Θ is adjusted so as to satisfy the relationship of tan Θ = t / L.

In practice, the optical fiber 50 is arranged so that the optical axis 50A of the optical fiber 50 is displaced from the optical axis 4A of the optical isolator 4 by approximately t centers.
Is connected to an optical power meter (for example, an optical power measuring device equipped with a light receiving element).

Then, the second lens 3 is finely moved so that the output of the optical fiber 50 is maximized, and the second lens 3 is moved perpendicularly to the optical axis of the optical isolator 4 in the incident surface of the optical fiber 50. 50 is finely moved to make adjustments.

However, many semiconductor lasers for optical communication use infrared light having a wavelength band of 1.3 μm and a wavelength band of 1.55 μm, and this light cannot be visually inspected.
There is a problem that adjusting the emission angle Θ of the light beam emitted from the second lens is complicated and time-consuming.

FIG. 3 is a diagram showing a basic configuration of the optical coupling device of the present invention, and FIG. 4 is a diagram showing an optical path in the device of FIG.
This optical coupling device optically couples the semiconductor laser 1 and the optical fiber 50, and emits a first laser beam from the first lens 2 and a first lens 2 which emits a laser beam emitted from the semiconductor laser 1 into a parallel beam. The parallel beam to the optical fiber 50
The second lens 3 that converges on the incident end surface 51 of the optical path, the optical isolator 10 and the wedge prism 20 that are arranged in the optical path formed by the first lens 2 and the second lens 3.

The incident end surface 51 of the optical fiber 50 is inclined by a predetermined inclination angle with respect to the optical axis 50A of the optical fiber 50. This inclination angle is φ, and the optical fiber 5 is
When the refractive index of the core of 0 is n and the deviation angle of the wedge-shaped prism is θ, the equation n · sin θ = sin (φ + θ) is satisfied.

In the illustrated example, the first lens 2, the second lens 3, the optical isolator 10 and the wedge prism 20 are arranged in this order between the semiconductor laser 1 and the optical fiber 50. The optical isolator 10 and / or the wedge prism 20 may be arranged between the first lens 2 and the second lens 3.

Although wedge-shaped prism 20 is shown as a separate body from optical isolator 10, wedge-shaped prism 20 is shown.
The incident side surface of the optical isolator may be fixed to the outgoing side surface of the optical isolator to integrate them.

Desirably, the index of refraction of wedge prism 20 is made equal to the index of refraction of the core of optical fiber 50. As a result, the wedge angle of the wedge prism 20 becomes equal to the inclination angle of the inclined incident end face 51 of the optical fiber 50, and the wedge prism 2
Design and manufacture of 0 becomes easy.

FIG. 5 is a cutaway perspective view showing an example of the optical isolator applicable to the present invention, and FIG. 6 is a view showing the second polarization beam splitter shown in FIG. In this example, the optical isolator 10 utilizes a Faraday effect and a prism in a magneto-optical crystal.

This optical isolator comprises a first polarization beam splitter 12, a Faraday rotator 13 for rotating the polarization plane by 45 °, and a second polarization plane whose polarization plane is inclined at 45 ° with respect to the first polarization beam splitter 12. The polarization beam splitter 14 and the polarization beam splitter 14 are arranged in this order on the optical path, and a cylindrical magnet 11 that applies a magnetic field to the Faraday rotator 13 is provided around the Faraday rotator 13.

The first polarization beam splitter 12 and the second
The polarization beam splitter 14 is provided with the polarization separation films 12A and 14A on the slopes of the right-angled prism, and the slopes of the other right-angle prisms are closely contacted with the polarization separation film to form a rectangular parallelepiped shape.

Then, the second polarization beam splitter 14
The wedge-shaped prism 20 is fixed to the exit surface of, for example, by an optical adhesive. It goes without saying that the larger the inclination angle φ of the inclined incidence end face 51 of the optical fiber, the more the reflected light does not return to the optical isolator side. However, if the inclination angle φ is too large, the transmission coefficient becomes small, and the light traveling to the optical fiber 50 is reduced. Less power. Therefore, the inclination angle φ is set in consideration of the reflection coefficient and the transmission coefficient at the inclined incidence end face 51.

In order to most efficiently couple the light of the semiconductor laser 1 to the optical fiber 50 having the inclined end face 51, the light refracted at the inclined incident end face 51 becomes parallel to the optical axis 50A of the optical fiber 50. It is good to do so.

Therefore, as shown in FIG. 6, the deviation angle θ of the wedge prism 20 (the angle formed by the incident ray and the outgoing ray).
The shape of the wedge-shaped prism 20 is set so as to satisfy n · sin φ = sin (φ + θ).

Here, n is the refractive index of the core of the optical fiber 50, and φ is the inclination angle of the inclined incident end face 51 of the optical fiber 50. For example, the refractive index n of the core of the optical fiber 50 is 1.
In the case of 46, considering the reflection coefficient and the transmission coefficient at the inclined incident end face 51, the inclination angle φ is preferably in the range of 4 ° to 15 °. In this case, the deviation angle θ of the wedge prism 20 is 1.85 ° ≦ θ ≦ 7 °. Therefore, the wedge prism 20 has such a deviation angle θ.
Is preferably designed to have

The light emitted from the semiconductor laser 1 travels through the first lens 2 to the second lens 3 and is polarized in the light emitted from the second lens 3 so as to match the polarization plane of the first polarization beam splitter 12. Only the component (P wave) passes through the first polarization beam splitter 12, becomes linearly polarized light, and advances to the Faraday rotator 13.

The polarization component (S wave) orthogonal to the polarization plane of the first polarization beam splitter 12 is the polarization separation film 12
The reflection is reflected at the portion A and the progress is stopped. For P waves,
While advancing through the Faraday rotator 13, the plane of polarization is rotated clockwise by 45 ° due to the Faraday effect, passes through the second polarization beam splitter 14 and advances to the wedge prism 20, and exits at an angle of deviation θ to output the optical fiber 50. It is converged on the inclined incident end face 51 and advances in the optical fiber 50 in the forward direction.

A part of the P wave projected on the inclined incident end face 51 is reflected by the end face 51, but this reflected light does not return to the wedge prism 20. On the other hand, the light traveling backward through the optical fiber 50 is refracted at the inclined incident end face 51 and is incident on the wedge prism 20, and is refracted at the wedge prism 20 to proceed to the second polarization beam splitter 14.

Then, the second polarization beam splitter 14
Only the polarization component that coincides with the polarization plane of (1) passes through the second polarization beam splitter 14 and advances to the Faraday rotator 13. This polarization component is rotated in the counterclockwise direction (clockwise when viewed from the semiconductor laser 1 side) by 45 ° while traveling through the Faraday rotator 13, and proceeds to the first polarization beam splitter 12. That is, the retrograde light has a polarization plane that is rotated by 90 ° with respect to the polarization plane of the forward light. Therefore, the retrograde light is prevented from traveling by the first polarization beam splitter 12, and the semiconductor laser 1 Does not enter.

In the basic structure of the present invention, since n, φ and θ satisfy the above-mentioned relation, the optical isolator 1
0 and the wedge prism 20 are fixed, and the optical coupling is maximized only by moving and adjusting the optical fiber 50 in the direction in which the optical axis 50A of the optical fiber 50 is offset from the isolator optical axis 10A. As described above, according to the present invention, it is possible to provide an optical coupling device that can be easily adjusted.

FIG. 7 is a sectional view of the essential parts of an optical coupling device showing an embodiment of the present invention. Reference numeral 55 denotes a ferrule manufactured by processing stainless steel, ceramics, etc. The ferrule 55 includes a shaft portion 55A having a rectangular cross section or a circular cross section, and a collar 55B integrally formed at one end of the shaft portion 55A. Have

The end of the optical fiber 50 (the end from which the coating is peeled off) is attached to the fine hole at the axial center of the ferrule 55, and the end face of the shaft portion is polished and finished together with the end face of the optical fiber 50. The inclined incident end face has a predetermined inclination angle φ.

In the case of the shaft portion 55A having a circular cross section, a slide key is embedded in the shaft portion 55A, and the slide key shafts in a key groove provided on an inner peripheral surface of an axis shift hole 62 of an outer cylinder 60 described later. It is desirable to allow sliding movement in the axial direction.

The material of the wedge prism 20 is selected so that the refractive index of the wedge prism 20 becomes equal to the refractive index of the core of the optical fiber 50. Therefore, the wedge prism 2
The wedge angle of 0 is equal to the tilt angle φ of the tilted incident end face of the optical fiber 50.

The wedge prism 20 is fixed to the optical isolator 10 by fixing one surface sandwiching the top portion to the emission surface of the second polarization beam splitter 14 of the optical isolator 10. In this embodiment, an optical film 70 is formed on the other surface sandwiching the top of the wedge prism 20 by, for example, vapor deposition. The optical film 70 will be described later.

Reference numeral 60 denotes an outer cylinder made of stainless steel or the like, and the second lens 3, the optical isolator 10, the wedge prism 20, and the ferrule 55 are accommodated in the outer cylinder 60. In this embodiment, the first polarization beam splitter 12, the Faraday rotator 13 and the second polarization beam splitter 14 of the optical isolator 10 are inserted and fixed in the magnet 11, and the outer cylinder 60 is fixed to the magnet 1.
First hole 6 with a bottom opening on one end face side for inserting 1
1 and a bottomed second hole 63 opening to the other end face side into which the flange of the ferrule 55 is loosely inserted.

The outer tube 60 has a first hole 61 and a second hole 61.
The shaft misalignment hole 62 communicating with the hole 63 is provided. The amount of eccentricity t of the axis shift hole 62 is set to be an amount close to a predetermined amount calculated in consideration of the distance between the wedge prism 20 and the inclined end surface of the ferrule 55 and the deviation angle θ of the wedge prism 20.

The first hole 61 has a circular shape, and a key groove into which a slide key (not shown) embedded in the outer peripheral portion of the magnet 11 is fitted is provided on the inner peripheral surface of the first hole 61. In addition, the axis deviation hole 62
Is a square or a circle with a key groove into which the shaft portion 55A of the ferrule 55 is slidably fitted.

When such an outer cylinder 60 is used, when the optical isolator 10 is inserted into the first hole 61 and the shaft portion 55A of the ferrule 55 is inserted into the axis shift hole 62, the surface of the wedge prism 20 on the emitting side is exited. And the inclined end surface of the ferrule 55 become parallel.

Further, a screw hole into which a lens holder 71 described later is screwed is provided on the inner peripheral surface on the opening side of the first hole 61, and a ring screw 72 described later on the inner peripheral surface on the opening side of the second hole 63. Are provided with screw holes.

Reference numeral 71 is a ring-shaped lens holder in which a thread is threaded on the outer peripheral surface. The second lens 3 is fitted into the hollow hole provided in the axial center of the lens holder 71,
Is fastened to the screw hole of the first hole 61 to fix the first lens 2 to the outer cylinder 60.

Further, the shaft portion 55A of the ferrule 55 is inserted into the shaft shift hole 62, the end face of the collar 55B is brought into close contact with the bottom face of the second hole 63, and the ring screw 72 is fitted into the screw hole of the second hole 63. The ferrule 55 is fixed to the outer cylinder 60 by wearing it.

According to this embodiment, the output beam from the wedge prism 20 is focused on the center of the inclined incident end face of the optical fiber 50 by moving and adjusting the ferrule 55 in the axial direction.

The axial adjustment of the ferrule 55 can be easily performed by interposing a liner 65 made of tracing paper or the like between the collar of the ferrule 55 and the bottom surface of the second hole 63. . Further, the outer cylinder 60 has the first lens 2, the optical isolator 10, and the wedge prism 20.
Since the ferrule 55 is housed and modularized, it is possible to provide an optical coupling device that is easy to handle and can be downsized.

In the embodiment shown in FIG. 7, the optical film 70 is formed on at least one of the surfaces of the wedge prism 20 through which light passes. 8A and 8B are diagrams showing examples of characteristics of the optical film. In the example shown in (A), the optical film 70 is composed of a bandpass filter.

In the figure, reference numeral 101 indicates the wavelength dependence of the attenuation factor in this band pass filter, and reference numeral 102.
Shows the oscillation spectrum of the semiconductor laser 1 that oscillates in multiple modes.

In this example, the pass band of the band pass filter is the main peak A with the highest intensity in the oscillation spectrum.
Is set so that only light corresponding to

By employing a bandpass filter as the optical film in this manner, a relatively inexpensive Fabry-Perot type semiconductor laser that oscillates in multiple modes is used, and relatively expensive to oscillate in a single mode (single spectrum). It is possible to obtain a light source equivalent to a DFB type semiconductor laser.

In the example shown in (B), the optical film includes a bandpass filter having a passband in the first wavelength region as indicated by reference numeral 103 and a first wavelength region as indicated by reference numeral 104. And a band stop filter having a stop band in the narrower second wavelength region. And this
Two peaks B and C are selected from a plurality of oscillation spectra of a semiconductor laser that oscillates in multiple modes.
Thus, according to this embodiment, it is possible to obtain an oscillation spectrum according to the application.

The band-pass filter and the band-stop filter may be formed so as to be overlapped with each other at the position shown by reference numeral 70 in FIG. 7, but the band-pass filter is provided on either one of the surfaces of the wedge prism 20 through which light passes. May be formed and a band stop filter may be formed on the other side.

[0063]

As described above, according to the present invention,
There is an effect that it is possible to provide an optical coupling device in which the degree of optical coupling between the semiconductor laser and the optical fiber is high and the tilt angle of the light beam can be easily adjusted.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a conventional optical coupling device.

FIG. 2 is a diagram showing an optical path in the conventional device of FIG.

FIG. 3 is a diagram showing a basic configuration of an optical coupling device of the present invention.

FIG. 4 is a diagram showing an optical path in the apparatus of FIG.

FIG. 5 is a cutaway perspective view showing an example of an optical isolator applicable to the present invention.

6 is a diagram showing a second polarization beam splitter shown in FIG.

FIG. 7 is a cross-sectional view of essential parts of an optical coupling device showing an embodiment of the present invention.

FIG. 8 is a diagram showing a characteristic example of an optical film in an example of the present invention.

[Explanation of symbols]

 1 Semiconductor Laser 2 First Lens 3 Second Lens 4, 10 Optical Isolator 50 Optical Fiber

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Tetsuo Ishizaka 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor Koeko Yokoi 1015, Kamedotachu, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor Manabu Komiyama 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Within Fujitsu Limited

Claims (7)

[Claims] 1. An optical coupling device for optically coupling a semiconductor laser and an optical fiber, comprising: a first lens for emitting a beam emitted from the semiconductor laser into a parallel beam and emitting the beam through the first lens. A second lens for converging a parallel beam on an incident end face of the optical fiber, an optical isolator and a wedge prism arranged in an optical path formed by the first and second lenses, and The end face is inclined by a predetermined inclination angle with respect to the optical axis of the optical fiber, the inclination angle is φ, the refractive index of the core of the optical fiber is n, and the deviation angle of the wedge prism is θ. Sometimes, an optical coupling device that satisfies the equation: n · sin φ = sin (φ + θ). 2. The optical coupling device according to claim 1, wherein a refractive index of the wedge prism is equal to a refractive index of a core of the optical fiber. 3. A ferrule into which an end portion of the optical fiber is inserted and fixed, and an outer cylinder holding the ferrule slidably in an axial direction of the optical fiber, the second lens and the optical fiber. The optical coupling device according to claim 1, wherein the isolator and the wedge prism are housed in the outer cylinder. 4. The optical coupling device according to claim 1, further comprising an optical film formed on at least one surface of the wedge prism through which light passes. 5. The optical coupling device according to claim 4, wherein the optical film comprises a bandpass filter. 6. The optical film comprises a bandpass filter having a passband in the first wavelength region and a bandstop filter having a stopband in a second wavelength region narrower than the first wavelength region. The optical coupling device according to claim 4. 7. The optical coupling device according to claim 1, wherein the first lens, the second lens, the optical isolator, and the wedge prism are arranged in this order between the semiconductor laser and the optical fiber. .