JPH04221921A - Optical isolator for diagonal incidence - Google Patents

Abstract

PURPOSE:To use polarization beam splitters as polarizers and to prevent the degradation in transmittance generated when an isolator is disposed to incline with an optical axis in order to prevent the return of the reflected light from the plane of the polarizers to a light source. CONSTITUTION:The two polarizers 21, 23 constituting the optical isolator are constituted of the polarization beam splitters. The incident angles of the light source light on the respective incident surfaces of a Faraday rotor 25 and the respective polarizers are so set as to incline in the direction there the transmittance of this incident light does not degrade.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator for oblique incidence, and more particularly to an optical isolator for oblique incidence that prevents light reflected by elements constituting the optical isolator from returning to the light source side.

0002

[Prior Art] In an optical transmission circuit using a laser diode or the like as a light source, the light source is often reflected by a circuit element placed in the transmission circuit, such as an optical component or a light receiving element, and the reflected light is transmitted to the oscillator of the laser diode. If the light returns to the inside and enters the laser diode again, the oscillation operation of the laser diode becomes unstable, resulting in a problem in that noise is more likely to occur and transmission quality deteriorates.

In order to solve this problem, conventionally, an optical isolator having a non-reciprocal transmission function has been used as a means for reducing the reflected return light.

FIG. 3 is a diagram showing the basic configuration of a conventionally used optical isolator, in which a polarizer 1 and a polarizer (analyzer) 3 are arranged so that their transmitted polarization directions differ by 45 degrees from each other. a Faraday element 5 made of a magneto-optical crystal that provides a Faraday rotation angle of 45 degrees between the two; and a permanent magnet 7 for saturating the magnetization of the Faraday element 5.
It consists of

In the optical isolator constructed in this manner, the light emitted from the laser diode 9 becomes linearly polarized light parallel to the X axis by passing through the polarizer 1 as shown in FIG. 5, the polarization plane of the linearly polarized light is perpendicular to the light traveling direction (z-axis), that is,
It is rotated 45 degrees in the y-axis direction and enters the polarizer 3 at the rear.

[0006] The linearly polarized light incident on the polarizer 3 is transmitted without loss because its polarization direction coincides with the easily transmitted polarization direction of the polarizer 3.

On the other hand, as shown in FIG. 4, of the return light incident on the optical isolator from the opposite direction, only the linearly polarized light of the component matching the easily transmitted polarization direction of the polarizer 3 passes through and enters the Faraday element.

Since the Faraday element 5 is non-reciprocal,
The incident light from the opposite direction is also rotated by 45 degrees in the same direction as the forward direction described above, and as a result, linearly polarized light having a polarization direction parallel to the y-axis is emitted, and is easily transmitted through the polarizer 1. Since it is perpendicular to the polarization direction, the reflected light does not pass through the polarizer and does not return to the laser diode 9, which is the light source. By the way, since the optical components such as the above-mentioned elements existing on the transmission path of the optical isolator themselves reflect the signal light,
When this reflected light returns to the light source, the oscillation of the laser diode serving as the light source becomes unstable, similar to the above case.

In order to solve this problem, anti-reflection coating (AR) has conventionally been applied to the surface of elements to suppress reflection, but the problem has been that the cost required for the coating increases.

From this point of view, conventionally, as shown in FIG. 5, each reflected light R from each element 1, 3, 5 is
, R, and R do not return to the laser diode 9 and thereby avoid the influence of reflected light at low cost.

[0011] The above-mentioned measures eliminate the influence of the returned light and the reflected light from the internal elements of the isolator, and each achieves a certain degree of effect.

By the way, rutile crystal is generally used as a polarizer used in conventional optical isolators, and as long as this rutile crystal is used as a polarizer, there will be no problem in terms of characteristics even if the entire isolator is installed tilted. However, rutile crystals have the disadvantage of being expensive.

Therefore, as a cost reduction measure, an inexpensive polarizing beam splitter (Polariz
ed Beam Splitter. Below, “PB
It's called "S". ) is preferably used. As is well known, a polarizing beam splitter is a device in which the end surfaces of two right-angled prisms are joined together so that the overall shape is a cube or a rectangular parallelepiped, and a polarizing film is interposed on the joined surface, but rutile crystal and Since the direction of the polarization plane that is transmitted is uniquely determined, the transmittance will decrease if the relationship between the polarization direction of the incident light and the direction of the transmitted polarization plane of the polarizing beam splitter is twisted even slightly and is no longer parallel. It has the following characteristics.

For this reason, when a polarizing beam splitter is used as a polarizer instead of a rutile crystal to reduce costs, the entire isolator is used as a transmission path within the above-mentioned allowable angle range in order to prevent return due to element reflected light. It is impossible to install the device at a predetermined angle.

The reason for this is that the normal lines of the two polarizing beam splitters arranged in the isolator are twisted by 45 degrees to each other as described above, so the normal line of the polarizing beam splitter 1 in front and laser diode 9
Even if the entire isolator is tilted within a range that can maintain parallelism with the polarization plane of After the polarization plane of the transmitted light is rotated by 45 degrees by the Faraday element 5, the polarization plane of the transmitted light is not parallel to the transmitted polarization direction of the rear polarizing beam splitter 3. Therefore, light transmission loss becomes extremely large.

[0016]

OBJECTS OF THE INVENTION The present invention has been made in view of the above, and uses a polarizing beam splitter as a polarizer to realize cost reduction, and also reduces reflection from the internal element surface of the beam splitter without increasing transmission loss. It is an object of the present invention to provide an optical isolator for oblique incidence that can prevent light from returning.

[0017]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an optical isolator including a Faraday rotator disposed on the optical path of light from a light source, and polarizers disposed in front and rear positions of the Faraday rotator. The two polarizers are constituted by polarizing beam splitters, and the angle of incidence of the light source light with respect to each incident surface of the Faraday rotator and the two polarizers is tilted in a direction that does not reduce the transmittance of the incident light. It is characterized by having been set. That is, the present invention is characterized in that the isolator internal elements are individually tilted instead of tilting the entire isolator as in the conventional case.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on preferred embodiments shown in the accompanying drawings.

FIG. 1 is an explanatory diagram showing a schematic configuration of an embodiment of the present invention. This optical isolator 20 includes polarizing beam splitters (polarizers) 21 arranged so that the transmitted polarization directions differ by 45 degrees from each other. The faraday element 25 is composed of a magneto-optical crystal that provides a Faraday rotation angle of 45 degrees between

In this embodiment, each polarizing beam splitter 21, 23 for the light source light L from the laser diode 29
The configuration is characterized in that the postures of each of the incident surfaces are arranged to be inclined by a predetermined angle θ1 relative to the posture of orthogonal incidence.

In the figure, the chain line in FIG.
The solid line shows the arrangement of elements in the conventional example, and the solid line shows the arrangement in the embodiment of the present invention.

As is clear from this figure, conventionally the entrance plane 2
Whereas the polarizing beam splitters 1a and 23a are arranged so that they are perpendicular to the light source light L, in the present invention, each polarizing beam splitter 21 and 23 is arranged so that the polarized beams are easily transmitted by a predetermined angle θ1 than the orthogonal orientation. It is tilted parallel to the direction.

FIG. 2(a) is a front view of the entrance surface 21a of the polarizing beam splitter 21 of FIG. 1, and the dotted line is an imaginary line indicating the easily transmitted polarization direction. This polarizing beam splitter 21 is tilted parallel to the easily transmitted polarization direction shown by the dotted line so that the incident surface 21a is upward than the output surface on the rear surface, so that it can split the incident light having a polarization plane in the x-axis direction. It can be transmitted without reducing the transmittance.

Furthermore, the easily transmitted polarization direction of the rear polarizing beam splitter 23 is set to be rotated by 45 degrees with respect to the easily transmitted polarization direction of the front polarizing beam splitter 21, as shown in FIG. 2(b). It is also tilted by θ1 parallel to the direction of easily transmitted polarization shown by the dotted line. Therefore, the incident light having a plane of polarization rotated by 45 degrees can be transmitted through the Faraday element 25 without reducing the transmittance.

Note that since the Faraday element 25 has no directionality, it may be tilted in any direction.

In the optical isolator configured as described above, the light component emitted from the laser diode 29 and transmitted through the polarizer 21 becomes linearly polarized light parallel to the X axis, and the polarization plane of the linearly polarized light is changed in the Faraday element 25 at the next stage. The light is rotated 45 degrees in the y-axis direction and enters the rear polarizer 23.

The linearly polarized light incident on the polarizer 23 is transmitted without loss because its polarization direction coincides with the easily transmitted polarization direction of the polarizer 23.

On the other hand, as for the return light incident on the optical isolator from the opposite direction, only the linearly polarized light of the component matching the easily transmitted polarization direction of the polarizer 23 passes through the Faraday element 25, as described in the conventional example. incident on .

Since the Faraday element 25 is non-reciprocal, it is rotated by 45 degrees in the same direction as in the forward direction described above, and as a result, linearly polarized light having a polarization direction parallel to the y-axis is emitted. Since it is perpendicular to the direction of easily transmitted polarization of the polarizer 1, the reflected light does not pass through the polarizer and does not return to the laser diode 29, which is the light source.

Next, each reflected light that is reflected by each of the above-mentioned elements 21, 25, and 23 and returns to the laser diode 29 is transmitted when the incident surface of each element 21, 25, and 23 is at an angle of θ1 with respect to the optical axis of the light source light. Because the laser diode is tilted by 29
It will no longer be incident on .

Each of the polarizing beam splitters 21 and 23 is constructed by, for example, joining two right-angled prisms to form a cube or a rectangular parallelepiped, and an S
A multilayer stack of thin films of i and SiO 2 is used. In general, the above-mentioned elements are arranged in a cylinder and the cylinder itself is made into a permanent magnet, or a permanent magnet is added to the circumferential surface of the cylinder.

As described above, in the present invention, the entrance plane of each element constituting the optical isolator is tilted at a predetermined angle with respect to the orientation perpendicular to the optical axis direction of the light source light, and a polarizer consisting of a polarizing beam splitter is used. The direction of inclination of the polarizer is set to be a direction that maintains parallelism between the easily transmitted polarization direction of the polarizer and the plane of polarization of the light, so while preventing deterioration of the extinction ratio, the light reflected from the element is can be prevented from returning to the light source.

[0033] The reason why no one has thought of a configuration in which elements such as polarizing beam splitters themselves are tilted in conventional optical isolators is that as long as rutile crystal is used as a polarizer as in the past, there is no way to tilt the elements themselves. Even if the temperature is increased, the extinction ratio does not deteriorate.

Therefore, as shown in FIG. 5, it would have been much easier to manufacture the optical isolator by slightly tilting the entire optical isolator in which all the elements were arranged in parallel than by tilting each element.

On the other hand, the present inventor believes that the above-mentioned structure can prevent the return light from entering the light source without causing deterioration of the extinction ratio, as an advantage that is worth accepting the above-mentioned disadvantages. This effect was discovered, and the present invention was completed based on this discovery.

[0036]

As described above, according to the present invention, in order to realize cost reduction, a polarizing beam splitter is used as a polarizer, and the incident angle of the light source light with respect to the incident surface of each element is set at a predetermined angle rather than 90 degrees. Even when the polarizing beam splitter is set at an angle to prevent the reflected light from returning from the element surface, it is possible to prevent the light transmittance of the polarizing beam splitter from decreasing.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the arrangement of each element in an optical isolator according to an embodiment of the present invention.

FIG. 2(a) is a front view of the entrance surface of the front polarizing beam splitter in the present invention, and FIG. 2(b) is a front view of the entrance surface of the rear polarizing beam splitter.

FIG. 3 is an explanatory diagram of the configuration of a conventional optical isolator.

FIG. 4 is an explanatory diagram of return light entering the optical isolator in FIG. 2 from the opposite direction.

FIG. 5 is an explanatory diagram of the arrangement of another conventional optical isolator.

[Explanation of symbols]

1, 21 Polarizer (polarizing beam splitter) 3, 23
Polarizer (polarizing beam splitter) 5, 25 Faraday element 7, 27 Permanent magnet 9, 29 Laser diode

Claims (3)

[Claims] 1. An optical isolator comprising a Faraday rotator disposed on the optical path of a light source and polarizers disposed in front and rear positions of the Faraday rotator, wherein the two polarizers are configured by a polarizing beam splitter. and an oblique incidence optical isolator, characterized in that the angle of incidence of the light source light on each of the incident surfaces of the Faraday rotator and the two polarizers is set to be inclined in a direction that does not reduce the transmittance of the incident light. . 2. The polarizing beam splitter has two right-angled prisms joined together to form a hexahedron such as a cube or a rectangular parallelepiped, and Si and SiO 2 are added to the joint surfaces of each other.
Claim 1 characterized in that the thin film is laminated in multiple layers.
The optical isolator for oblique incidence described above. 3. The inclination direction of the polarizing beam splitter is such that the normal line of the prism joint surface constituting the polarizing beam splitter is parallel to the plane of polarization of the polarized light that passes through and travels straight. ) or the oblique incidence optical isolator according to claim 2.