JP2840708B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an optical system such as optical communication or optical recording,
The present invention relates to an optical isolator attached to prevent return light.

[Conventional technology]

Conventionally, a semiconductor laser is a coherent light source, and when light having substantially the same wavelength and the same polarization direction enters the light source as return light, they interfere with each other, resulting in a slight change in intensity and a signal component. Noise.

Therefore, an optical isolator is used to remove the return light.

FIG. 5 shows a conventional optical isolator of this type.

As shown in the figure, a conventional optical isolator sandwiches a Faraday rotator 83 between two polarizing beam splitters (PBS) 81 and 82, and has a cylindrical shape around the outer circumference of the polarizing beam splitters 81 and 82 and the Faraday rotator 83. Shaped magnet 8
It is configured with 4 attached.

Here, the polarization plane of the polarization beam splitter 81 and the polarization plane of the polarization beam splitter 82 are arranged at positions shifted by 45 ° around the optical axis. The magnet 84 is connected to the Faraday rotator 83.
Is configured to apply a magnetic field substantially in the direction of the optical axis.

The light incident from the direction of arrow E shown in FIG.
The light enters the polarization beam splitter 81, and only predetermined linearly polarized light passes therethrough.

Next, this linearly polarized light enters the Faraday rotator 83,
Here, the plane of polarization rotates by 45 °.

Next, the linearly polarized light whose polarization plane is rotated by 45 ° passes through the polarization beam splitter 82 as it is.

On the other hand, the light that has entered from the direction of arrow F is transmitted through the polarization beam splitter 82 with only predetermined linearly polarized light, and enters the Faraday rotator 83. The light is now rotated by another 45 ° in its plane of polarization.

As a result, the plane of polarization of the linearly polarized light incident on the polarization beam splitter 81 is orthogonal to the plane of polarization of the polarization beam splitter 81, so that it is reflected there and the transmitted return light is blocked.

[Problems to be solved by the invention]

By the way, the Faraday rotator 83 used in the conventional optical isolator as described above is usually made of YIG (yttrium iron garnet), and therefore has a large shape. A high-rotation type Faraday rotator has been developed, and the thickness of the Faraday rotator can be increased to about 200 to 500 μm.

However, even if the thickness of the Faraday rotator becomes thick, the conventional optical isolator still needs two polarization beam splitters 81 and 82 and a magnet 84 as separate components, and the number of components is large. Or polarizing beam splitters 81 and 82 are very expensive,
As a result, not only can the size of the entire apparatus not be reduced, but also the apparatus becomes considerably expensive.

The present invention has been made in view of the above points, and it is an object of the present invention to provide an optical isolator having a simple structure that can be reduced in size by reducing the number of parts and can be manufactured at low cost.

[Means for solving the problem]

In order to solve the above problems, the present invention provides an optical isolator including a thick-film Faraday rotator, and a polarizing beam splitter layer on either one of the surfaces of the Faraday rotator directly or via a flat substrate. Is formed, the Faraday rotator is arranged to be inclined at a predetermined angle with respect to the optical axis, and a cylindrical magnet for applying a magnetic field in the direction of the optical axis is attached to the Faraday rotator. A groove or a hole is provided in a direction in which the light incident in the axial direction is reflected by the Faraday rotator, and the thickness of the Faraday rotator is adjusted to approximately 45 degrees by changing the polarization plane of the light incident from the optical axis direction.
し た Configured as the thickness required for rotation.

[Action]

By configuring the optical isolator as described above, 1
One polarizing beam splitter can be integrated with the Faraday rotator. Therefore, the number of parts can be reduced and the structure is simplified.

Further, since the Faraday rotator is installed obliquely with respect to the optical axis, unnecessary light reflected by the Faraday rotator or the like does not become return light.

Also, since the Faraday rotator and the substrate are provided inclined with respect to the optical axis, there is an optical path difference in the light passing through them, and this light becomes astigmatic light, and this light is detected by servo detection. It can be used as a means.

〔Example〕

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a side sectional view showing an optical isolator 1 according to the present invention.

As shown in FIG. 1, the optical isolator 1 includes a thick-film Faraday rotator 3, and a polarization beam splitter layer 4 is formed on one surface of the Faraday rotator 3 via a flat substrate 2. Further, the Faraday rotator 3 is inserted into the cylindrical magnet 5, and is arranged so as to be inclined at about 45 ° with respect to the optical axis ab.

 Next, each component of the optical isolator 1 will be described.

The Faraday rotator 3 is formed on the substrate 2 and is made of, for example, a bismuth-substituted garnet film.

The substrate 2 is constituted by a GGG substrate (abbreviation of Gd.Ga. garnet substrate).

Normally, the Faraday rotator 3 is formed on the GGG substrate, and in this embodiment,
G. Although the substrate is used as it is, the substrate 2 is not always necessary. If not necessary, the substrate 2 may be scraped off from the Faraday rotator 3.

The thickness of the Faraday rotator 3 is such that the polarization plane of light incident on the Faraday rotator 3 at an angle of approximately 45 ° is approximately 45 °.
゜ The thickness is necessary to rotate.

The Faraday rotator 3 has an AR (anti-
Reflect) coat is applied.

On the other hand, the polarization beam splitter layer 4 is formed by forming a dielectric multilayer film on the surface on the opposite side of the substrate 2 or forming a relief grating.

The next magnet 5 is formed in a cylindrical shape, and applies a magnetic field substantially in the direction of the optical axis ab to the Faraday rotator 3 installed therein. That is, the center axis of the magnet 5 coincides with the optical axis ab.

A groove 6 is provided at a predetermined position on the side surface of the magnet 5 (the lower side in the figure).

 Next, the operation of the optical isolator 1 will be described.

First, the case where light is incident from the direction of arrow A shown in FIG. 1 is a case where this optical isolator 1 is used as a transmission type.

That is, when the predetermined linearly polarized light coming from the direction of arrow A passes through the Faraday rotator 3, its polarization plane is rotated by about 45 °.

Next, this linearly polarized light enters the polarization beam splitter layer 4 after passing through the substrate 2.

At this time, since the polarization plane of the polarization beam splitter layer 4 is oriented so as to transmit the linearly polarized light rotated by 45 °, the linearly polarized light passes through the polarization beam splitter layer 4 as it is.

On the other hand, the return light from the direction of arrow B passes through the polarization beam splitter layer 4 and the substrate 2 and then returns to the Faraday rotator 3.
This causes the plane of polarization to be further rotated by 45 °. As a result, the polarization plane of the return light is orthogonal to the polarization plane of the linearly polarized light incident from the direction of arrow A.

 Therefore, this return light does not interfere with the incident light.

If the analyzer 7 is attached to the incident light side of the optical isolator 1, the return light can be reliably removed.

When the optical isolator 1 is used as a transmission type as described above, the groove 6 shown in the figure is used for removing reflected light.

Next, the case where light is incident from the direction of arrow C shown in the figure is a case where the optical isolator 1 is used as a reflection type.

That is, the predetermined linearly polarized light incident from the direction of the arrow C passes through the Faraday rotator 3, but at this time, since the direction of the light is substantially orthogonal to the direction of the magnetic field, the plane of polarization does not rotate.

Next, this linearly polarized light enters the polarization beam splitter layer 4 after passing through the substrate 2.

At this time, the direction of the polarization plane of the polarization beam splitter layer 4 is set so as to substantially totally reflect the plane of polarization of the incident linearly polarized light. Almost totally reflected.

The linearly polarized light totally reflected by the polarization beam splitter layer 4 enters the Faraday rotator 3 again.

At this time, since the direction of the linearly polarized light and the direction of the magnetic field are substantially the same, the plane of polarization of the linearly polarized light is rotated by approximately 45 °.
This light is emitted rightward as shown in FIG.

On the other hand, the return light of this linearly polarized light is incident on the Faraday rotator 3 from the direction of arrow A, where its polarization plane is further rotated by 45 °, and eventually turned as a whole by 90 °. Therefore, the linearly polarized light is not reflected by the polarization beam splitter layer 4, is transmitted, and hardly returns to the side of the incident light that enters from the direction of the arrow C. Therefore, the return light does not interfere with the incident light. It will be.

At this time, if the analyzer 7 is attached at the same position as that of the transmission type, the return light can be surely removed.

In this optical isolator 1, the Faraday rotator 3 is installed at a position inclined at approximately 45 ° with respect to the optical axis, so that the reflected light due to the reflection on these surfaces is removed from the optical axis direction, and the return light Does not.

 The groove 6 may be a hole.

FIG. 2 is a view showing an application example when the optical isolator 1 is used as a reflection type in an optical system for optical recording / reproduction.

As shown in the figure, in this application example, a laser diode 8 is inserted into the groove 6 of the optical isolator 1.

A collimator lens 9 is attached to the end of the optical isolator 1 on the side facing the disk 11.
An objective lens 10 is mounted near 11.

On the other hand, a photodetector 12 is arranged at a predetermined position on the opposite side of the optical isolator 1 from the disk 11.

Almost all of the laser light emitted from the laser diode 8 is reflected by the polarization beam splitter layer 4 and its polarization plane is rotated by 45 ° by the Faraday rotator 3 as described above, and the collimator lens 9 and the objective lens After passing through 10, the light is focused on the disk 1.

Next, the return light reflected by the disk 11 passes through the objective lens 10 and the collimator lens 9 again, and the polarization plane thereof is further rotated by 45 ° by the Faraday rotator 3.

Therefore, since this linearly polarized light is eventually rotated by 90 °, it passes through the polarization beam splitter layer 4 and is condensed on the photodetector 12, and its signal is read.

The return light from the disk 11 is transmitted to the Faraday rotator 3
And the substrate 2, the Faraday rotator 3 and the substrate 2 are inclined at about 45 °, so that an optical path difference is generated in the return light transmitted through the Faraday rotator 3 and the substrate 2, whereby this light is converted into astigmatic light. Become. Therefore, a servo detection signal can be obtained from this light.

As described above, according to this application example, the optical isolator 1 not only separates and blocks light, but also has a function as a servo detection unit.

3 and 4 are views showing an application example in which the optical isolator 1 according to the present invention is applied to a laser diode module for optical communication. FIG. 3 shows a case where the optical isolator 1 is used as a transmission type. FIG. 4 shows a case where the optical isolator 1 is used as a reflection type.

First, in the application example shown in FIG. 3, a spherical lens 13 and a laser diode 14 are installed on the Faraday rotator 3 side of the optical isolator 1.

On the other hand, on the side of the polarization beam splitter layer 4 of the optical isolator 1, a spherical lens 15 and a stopper 17 of a ferrule are attached. 16 has a ferrule inside (not shown)
This is a ferrule insertion unit for inserting a.

Then, the linearly polarized light emitted from the laser diode 14 is converted into parallel light by the spherical lens 13 and transmitted through the polarization beam splitter layer 4 after its polarization plane is rotated by 45 ° by the Faraday rotator 3.

Next, the linearly polarized light is condensed and incident on an end face of an optical fiber (not shown) in the ferrule inserted into the ferrule insertion portion 16 by the spherical lens 15.

On the other hand, the return light reflected by the end face of the optical fiber or the like passes through the spherical lens 15 and then passes through the polarization beam splitter layer 4 and further rotates its polarization plane by 45 ° by the Faraday rotator 3. The emitted linearly polarized light is light rotated by 90 °.

Therefore, this return light does not interfere with the outgoing light emitted from the laser diode 14 and does not become noise.

Next, in the application example shown in FIG. 4, a spherical lens 20 and a laser diode 21 are provided on the right side of the optical isolator 1.

On the other hand, a ball lens 22 and a ferrule stopper 23 are attached to the lower side of the optical isolator 1 shown in FIG.

Then, the linearly polarized light emitted from the laser diode 21 is converted into parallel light by the spherical lens 20, the polarization plane thereof is rotated by 45 ° by the Faraday rotator 3, and then almost completely downward by the polarization beam splitter layer 4. Is reflected.

After passing through the hole 6 ', the linearly polarized light is condensed and incident on the end face of an optical fiber (not shown) in the ferrule inserted into the ferrule insertion portion 24 by the spherical lens 22.

On the other hand, the return light reflected by the end face of the optical fiber and the like is transmitted through the spherical lens 22, reflected by the polarization beam splitter layer 4, is further rotated by 45 ° by the Faraday rotator 3, and finally emitted from the laser diode 21. The linearly polarized light is a light rotated by 90 °.

Therefore, this return light does not interfere with the outgoing light emitted from the laser diode 21 and does not become noise.

〔The invention's effect〕

As described in detail above, the optical isolator according to the present invention has the following excellent effects.

(1) Since one polarizing beam splitter can be integrated with the Faraday rotator, not only the number of parts can be reduced but also the structure can be simplified, the size of the optical isolator can be reduced, and the cost of the device can be reduced.

(2) If a groove or a hole through which reflected light passes is provided on the side surface of the cylindrical magnet, this optical isolator can be used for both the transmission type and the reflection type.

(3) Since the Faraday rotator is installed inclined with respect to the optical axis, unnecessary reflected light reflected by the surface of the Faraday rotator or the like does not become return light.

(4) Since the Faraday rotator is provided obliquely with respect to the optical axis, it also has a servo detection function when used in an optical recording / reproducing system. Therefore, the number of components such as a lens and a beam splitter can be reduced.

[Brief description of the drawings]

FIG. 1 is a side sectional view showing an optical isolator 1 according to the present invention, and FIG.
FIG. 3 shows an application example when used in a reproduction optical system, FIG.
FIG. 4 is a diagram showing an application example in which the optical isolator 1 according to the present invention is applied to a laser diode module for optical communication;
FIG. 5 shows a conventional optical isolator. In the figure, 1 ... optical isolator, 2 ... substrate, 3 ... Faraday rotator, 4 ... polarizing beam splitter layer, 5 ...
Magnets, 6 grooves, 6 'holes.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-57-44110 (JP, A) JP-A-64-74524 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02B 27/28

Claims (1)

(57) [Claims] 1. A Faraday rotator comprising a thick film-like Faraday rotator, and a polarizing beam splitter layer formed on one surface of the Faraday rotator directly or via a flat plate-like substrate. The Faraday rotator is provided with a cylindrical magnet that applies a magnetic field in the optical axis direction to the Faraday rotator, and light incident in the optical axis direction is applied to the side of the magnet. A groove or hole is provided in the direction reflected by the rotator, and the thickness of the Faraday rotator is set to a thickness necessary to rotate the plane of polarization of light incident from the optical axis direction by approximately 45 °. Isolator.