JP2610174B2 - Optical isolator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator used for optical communication and optical information processing.

[Background Art] In optical communication and optical information processing, a part of light emitted from a light source may be reflected by an optical component or the like and return to the light source. When a semiconductor laser is used as a light source, this return light has caused unstable oscillation characteristics of the semiconductor laser and increased noise. In general, an optical isolator is used to prevent deterioration of various characteristics of the semiconductor laser due to the return light.

Conventionally, there is an optical isolator of this type as shown in FIG. 8, but a polarizing beam splitter (hereinafter referred to as PBS
) Of the polarization separation film was not controlled at all. FIG. 8 shows that an angle formed by a vector obtained by projecting a normal vector of the film surface of the polarization separation film of two PBSs onto a plane perpendicular to the light incident on the optical isolator is 135.
FIG. 4 is a configuration diagram of an optical isolator in the case of ゜. FIG. 8 is disclosed in "Publication Patent Publication No. 61-113,026".

In the figure, 1a and 1b are PBS, and 2 is the polarization plane of the incident light.
フ ァ Rotating Faraday rotator, 3 denotes a permanent magnet for applying a magnetic field to Faraday rotator 2, 4 denotes the optical axis of light incident on the optical isolator, 5 denotes the direction of rotation of the polarization plane of light by Faraday rotator arrows, 6a, 6b is PBS1a viewed from the left side of the optical axis 4, the normal vector n _a membrane surface of the polarization separation film 1b, n
_b . Further, 7a of FIG. 9, 7b is a vector 6a, vector n _a obtained by projecting 6b to a plane perpendicular to the optical axis 4 ', n _b'.

Since the conventional optical isolator is configured in this manner, light incident in the forward direction, that is, in the direction from right to left in FIG. 8, transmits only the linearly polarized light component by the PBS 1b, and the Faraday rotator 2 The polarization plane rotates 45 ° in the direction indicated by arrow 5 and transmits through PBS1a. Where PBS1a
The transmission polarization directions are adjusted so that the light transmitted through the Faraday rotator 2 is transmitted with little loss. On the other hand, the reflected return light enters in the opposite direction, that is, from left to right in FIG. 8, passes through the PBS 1a, passes through the Faraday rotator 2, and further rotates the polarization plane by 45 °. Becomes a polarization component orthogonal to the transmission polarization direction of
Completely shut off.

However, the transmission polarization direction of the PBS depends on the ray vector 1 of the incident light and the normal vectors n _a and n _b of the polarization separation film of the PBS.
Is determined by the plane containing. Therefore, if the incident direction of the light beam changes, the transmitted polarization direction of the PBS changes, as if P
The BS is caused to have a rotational displacement around the optical axis, and the above-described blocking characteristic (isolation) is deteriorated.

As shown in FIG. 10, changing the ray vector of the incident light from the vector l ₁ into vector l _2, PBSIa, transmitting polarization direction 1b is rotated in the reverse direction as indicated by arrow 8a, 8b, 2 two PBS
The angle formed by the transmitted polarized light direction greatly changes, and the isolation deteriorates. The calculation of the degradation of the isolation is described below.

Iso = Iso = −10 log (sin ² θder) using the deviation angle θdef of the angle θ between the transmission polarization directions of the PBSs 1a and 1b from 135 °, where θdef = θ−135 (2) ) It is expressed as

As shown in FIG. 11, the results of the isolation calculation performed by changing the incident angle ξ and the azimuth η of the incident light are shown in FIG.
In FIG. 12, 11 is an optical isolator in FIG. 11,
Reference numeral 15 denotes a light beam vector of incident light, and 16 denotes an arrow indicating the forward direction of the optical isolator 11. By the way, when an optical isolator is used for optical communication, “M. SIKADA et al,“ Evaluation of
Power Penalties Caused by Feedback Noise of Distri
buted Feedback Laser Diodes ", J. Light wave Technol.
vol. 6, pp. 655-659, 1988 ", an isolation of 25 dB or more is required. As shown in FIG. 9, when the vector formed by projecting the normal vectors n _a and n _b of the polarization separation film of PBS onto a plane perpendicular to the light incident on the optical isolator has an obtuse angle, 25 dB The incident angle for ensuring the above isolation is as small as 3 °.

[Problems to be Solved by the Invention] Since the conventional optical isolator is configured as described above, the allowable incident angle is small and cannot be used for a converging optical system having various incident angles. There were also restrictions on use in the system.

The present invention has been made to solve the above problems, and can be used for a converging optical system.
An object of the present invention is to obtain an optical isolator that is easy to use even in a parallel optical system.

[Means for Solving the Problems] An optical isolator according to the present invention includes a projection vector 7a obtained by projecting a normal vector 6a on an incident direction side of a polarization splitting film surface of a first polarization beam splitter 1a onto a plane perpendicular to incident light.
And the projection vector 7b obtained by projecting the incident-direction-side normal vector 6b with respect to the polarization separation film surface of the second polarization beam splitter 1b onto a plane perpendicular to the incident light so as to form an acute angle.
1. The second polarization beam splitters 1a and 1b are connected to the optical axis 4 of the incident light.
And the projection vector 7a and the projection vector when the angle between the optical axis of the incident light and the optical axis of the optical isolator changes.
In order to reduce the change in the angle between the first and second
The polarization beam splitters 1a and 1b are tilted with respect to the optical axis of the incident light to change the projection vector 7a and the projection vector 7b in the same direction.

[Operation] In this optical isolator, the first and second polarization beam splitters 1a and 1b are arranged on the optical axis 4 so that the angle between the projection vector 7a and the projection vector 7b is an acute angle, and the light of the incident light is The first and second polarizing beam splitters 1a and 1b are used to reduce the change in the angle between the projection vector 7a and the projection vector 7b when the angle between the axis and the optical axis of the optical isolator changes. Since the projection vector 7a and the projection vector 7b were changed in the same direction by tilting with respect to the optical axis of the first, the change directions of the transmission polarization directions of the first and second polarization beam splitters 1a and 1b become the same. 1. The angle between the transmission polarization directions of the second polarization beam splitters 1a and 1b does not change much.

[Embodiment of the Invention] FIG. 1 is a configuration diagram of an optical isolator according to an embodiment of the present invention. Components corresponding to the components shown in FIG. I do. This embodiment is characterized in that a projection vector 7a (FIG. 2) in which the normal vector 6a on the incident direction side with respect to the polarization splitting film surface of the first polarization beam splitter 1a is projected on a plane perpendicular to the incident light, The angle formed between the normal vector 6b on the incident direction side of the polarization beam splitter 1b of the polarization beam splitter 1b and the projection vector 7b (FIG. 2) projected on a plane perpendicular to the incident light is an acute angle (45 °). First, the second polarization beam splitters 1a and 1b are arranged on the optical axis 4 of the incident light.

Next, the operation of this embodiment will be described. The light incident in the forward direction, that is, in the direction from right to left in FIG.
Only the linearly polarized light component is transmitted by the PBS 1b, the polarization plane is rotated by 45 degrees in the direction indicated by the arrow 5 by the Faraday rotator 2, and transmits through the PBS 1a. Here, the transmission polarization direction of the PBS 1a is adjusted so that the light transmitted through the Faraday rotator 2 is transmitted with little loss. On the other hand, the reflected return light enters in the opposite direction, that is, from right to left in FIG. 1, passes through the PBS 1a, passes through the Faraday rotator 2, and is further rotated by 45 °, so that The polarization component becomes orthogonal to the transmission polarization direction of PBS1b, and is completely cut off.

In order to prevent reflection from the optical isolator itself, as shown in FIG. 3, the person who arranges the optical isolator tilts the PBS1a and PBS1b with respect to the optical axis of the incident light, and changes the ray vector of the incident light from l1. l2, the transmission polarization directions of the PBSs 1a and 1b are rotated in the same direction as shown by arrows 8a and 8b. Become smaller.

FIG. 4 shows the result of obtaining the relationship between the incident direction of light and the isolation in the optical isolator having the configuration shown in FIG. 1 in the same manner as in the conventional example. In the configuration of FIG. 1, the second expression is replaced by the following expression.

θdef = θ-45 normal vector n _a membrane surface of the polarization separation film of two PBS as shown in FIG. 2, n _b, and vector 7a projected onto a plane perpendicular to the incident light to the optical isolator, 7b When the angle is made to be an acute angle, the incident angle for securing the isolation of 25 dB or more is as large as 6 °.

FIG. 5 is a configuration diagram of a semiconductor laser module of an optical communication device using the optical isolator of this embodiment for optical communication. In the figure, 9 is a semiconductor laser, 10a and 10b are condenser lenses, 11 is an optical isolator, 12 is an optical fiber, 13 is light emitted from the semiconductor laser 9, and 14 is light reflected by the optical isolator 11.

By the way, in a semiconductor laser / D module with a built-in optical isolator used in optical communication or the like as shown in FIG. 5, the incident light of the optical isolator 11 is set so that the reflected light from the optical isolator 11 is not coupled to the semiconductor laser 9. It is configured such that light rays enter the surface obliquely. Further, in the optical isolator 11, the maximum isolation is obtained for the light beam incident parallel to the optical axis of the optical isolator 11. The relative angle θ _PBS between the normal vectors of the polarization separation films of the two _PBSs is θ _PBS = 90 ° −θ _FR, where the Faraday rotation angle is θ _FR . However, shifting the relative angle of the normal vectors of the polarization separation films of the two PBSs from θ _PBS by a small angle Δ from 90 ° -θ _{FR has} the following effects. For example, FIG. 6 shows the result of calculating the isolation by changing the incident angle and the azimuth of the incident light when Δ = 2 °. As shown in FIG. 6, for the large incident angle of 6 °, as shown in FIG. The azimuth angle of the incident light which can obtain a high isolation of 40 dB or more is as wide as 130 °. Also, for example, FIG. 7 shows the result of calculating the isolation by changing Δ and the azimuth angle of the incident light when the incident angle is 3 °. As shown in FIG. 7, the incident light with high isolation is obtained. Azimuth range can be changed. Thus, the optical isolator
When a light beam enters obliquely with respect to the optical axis of 11, two PBs
The relative angle ΔPBS of the normal vector of the polarization separation film of S is _calculated as 90 ° −
By slightly deviating from θ _{FR, a} high isolator 11 that is easy to use with high isolation can be easily obtained.

[Effect of the Invention] As described above, according to the first aspect of the present invention, the first projection vector obtained by projecting the normal vector on the incident direction side of the polarization splitting film of the first polarization beam splitter onto a plane perpendicular to the incident light. And a second projection vector obtained by projecting a normal vector on the incident direction side of the polarization splitting film of the second polarization beam splitter onto a plane perpendicular to the incident light so as to form an acute angle.
1. A first projection vector and a second projection vector when the angle between the optical axis of the incident light and the optical axis of the optical isolator is changed by disposing the second polarizing beam splitter on the optical axis of the incident light. So that the change in the angle between the first and second
Since the first and second projection vectors are changed in the same direction by tilting the polarizing beam splitter with respect to the optical axis of the incident light, it is possible to prevent the isolation from deteriorating. As a result, it is possible to obtain an effect that an optical isolator that can be used in a converging optical system and is easy to use in a parallel optical system can be provided.

Further, according to the second aspect of the present invention, θ _PBS is an angle formed by the first projection vector and the second projection vector, θ
Rotation angle of the polarization plane of the incident light _FR by the Faraday rotator, ray vector of the incident light l, the incident direction side normal vector of the polarization separation film of the n _a the first polarization beam splitter, a n _b When a normal vector on the incident direction side of the polarization splitting film of the second polarization beam splitter is used, the angle between the ray vector of the incident light and the optical axis is set so as to widen the range of the azimuth with high isolation. Corresponding constant angle Δ And θ _PBS = 90 ° −θ _FR ± Δ, so that there is an effect of obtaining high isolation characteristics over a wide range of azimuth angles. As a result, a high-performance optical isolator can be obtained for a specific incident angle.

According to the third aspect of the present invention, the angle Δ for setting an azimuth with high isolation to a predetermined azimuth is defined as: Since θ _PBS = 90 ° −θ _FR ± Δ, the azimuth of the incident angle at which high isolation is obtained can be adjusted in a desired direction. This results in a very easy to use optical isolator.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical isolator according to one embodiment of the present invention, FIG. 2 is a vector diagram for explaining a projection vector in this embodiment, and FIG. FIG. 4 is a graph showing the relationship between the azimuth angle of incident light and isolation in this embodiment, FIG. 5 is a configuration diagram of a semiconductor laser module including the optical isolator of this embodiment, FIG. 6 and 7 are graphs showing the relationship between the azimuth angle of incident light and isolation for explaining the operation of the semiconductor laser module of FIG. 5, and FIG. 8 is a configuration diagram of a conventional optical isolator. FIG. 9 is a vector diagram for explaining a projection vector in this conventional example, FIG. 9 is a vector diagram for explaining a change in transmitted polarization direction in this conventional example, and FIG.
FIG. 11 is a diagram for explaining the operation of this conventional example, and FIG. 12 is a graph showing the relationship between the azimuth of incident light and isolation in this conventional example. 1a: first polarizing beam splitter, 1b: second polarizing beam splitter, 2: Faraday rotator, 3 ... magnet, 4 ... optical axis of incident light, 6a, 6b ... normal vector, 7a,
7b: Projection vector, 11: Optical isolator.

Claims (3)

(57) [Claims] A first polarization beam splitter having polarization separation film surfaces arranged so that transmission polarization directions of incident light are different from each other by a fixed angle; the first polarization beam splitter and the second polarization; In an optical isolator comprising a Faraday rotator disposed between the beam splitter and rotating a polarization plane of incident light by a fixed angle, and a magnet for applying a magnetic field to the Faraday rotator, the incident light is convergent or obliquely incident. A first projection vector obtained by projecting a normal vector on the incident direction side with respect to the polarization splitting film surface of the first polarization beam splitter onto a plane perpendicular to the incident light, and a polarization separation of the second polarization beam splitter; The first and second images are formed so that the angle formed between the normal vector on the incident direction side with respect to the film surface and the second projection vector that is projected on a plane perpendicular to the incident light is an acute angle. Is disposed on the optical axis of the incident light, and the angle between the optical axis and the optical axis of the optical isolator changes when the first projection vector and the second projection vector form an angle. In order to reduce the change in the angle, the first
An optical isolator, wherein the first and second projection vectors are changed in the same direction by tilting a second polarizing beam splitter with respect to an optical axis of the incident light. Wherein theta said first projection vector and the second projection vector and angle formed with _PBS, the rotation angle of the polarization plane of the incident light theta _FR by the Faraday rotator, ray of the incident light l vector, when a n _a and the incident direction side normal vector of the first incident direction normal vectors of the polarization splitting film of the polarization beam splitter, n _b the polarization splitting film of the second polarization beam splitter, iso A constant angle Δ corresponding to the angle between the ray vector of the incident light and the optical axis so as to widen the range of the high azimuth angle 2. The optical isolator according to claim 1, wherein θ _PBS = 90 ° −θ _FR ± Δ. Wherein theta said first projection vector and the second projection vector and angle formed with _PBS, the rotation angle of the polarization plane of the incident light theta _FR by the Faraday rotator, ray of the incident light l vector, when a n _a and the incident direction side normal vector of the first incident direction normal vectors of the polarization splitting film of the polarization beam splitter, n _b the polarization splitting film of the second polarization beam splitter, iso The angle Δ that sets the high azimuth to the predetermined azimuth 2. The optical isolator according to claim 1, wherein θ _PBS = 90 ° −θ _FR ± Δ.