JPH01241502A - Polarizing element for optical isolator - Google Patents

Abstract

PURPOSE:To improve the performance of the polarizing element as an optical device by forming a dielectric multilayered film at the joint part of birefringent prisms for the polarizing element. CONSTITUTION:One polarizing element consists of two birefringent prisms 1 and 2 are the prism angles of both birefringent prisms are so set that incident light and projection light are parallel to each other. The other polarizing element consists of one birefringent prism 4 and an optically equivalent glass prism 3 and the prism angle of the glass prism is so set that its optical axis is perpendicular to the incident light and the incident light and projection light are parallel to each other. Dielectric films M and N consisting of at least six layers are formed at the joint part of the 1st and 2nd prisms. Consequently, the quenching characteristics are improved and split light is inhibited from entering an optical path.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention uses calcite or rutile (TiOz) in optical circuit devices such as optical isolators, optical circulators, and optical switches, and converts the S-wave component into an optical axis path. This invention relates to the structure of a polarizing element that improves the extinction ratio of divergence.

[Prior Art] Optical isolators are used in high-speed optical communication using semiconductor lasers, high-precision optical measurements, etc., to prevent malfunctions in the semiconductor laser due to reflected return light from the optical circuit system. < is an optical device that is placed in front of the semiconductor laser and has the function of allowing optical signals from the forward direction (incidence side) to pass through without loss, and blocking reflected return light from the reverse direction (output side). In particular, its importance has come to be recognized.

Its operating principle is that the incident light is linearly polarized by a polarizer, then the plane of polarization is rotated by 45 degrees by placing a Faraday rotator, and the rotated linearly polarized light is sent to the next analyzer. Introduce and make it permeable. At this time, since the optical axes of the polarizer and analyzer are rotated by 45 degrees, only light emitted after being rotated by 45 degrees from the forward direction can be transmitted. Conversely, in the case of returning light, only the light that matches the analyzer surface can pass through the analyzer, and the plane of polarization is roughly rotated by 45 degrees again by the Faraday rotator, so when it finally reaches the polarizer, the initial plane of polarization is Since the light is shifted by 90 degrees from the polarizer, the light cannot pass through the polarizer. This is called extinction property.

[Problems to be Solved by the Invention] Various types of optical isolators have been proposed so far, but they have been limited by the following points. In order to prevent reflections from the near end of the optical isolator from returning to the laser, optical isolators that are tilted are used because the incident light and the outgoing light are not on the same straight line, nor are they parallel. It is difficult to align the axis U.

Rochon prisms and Wollaston prisms have high performance as polarizing elements compared to inexpensive polarizing beam splitters (PBS) or rutile prisms, but when incorporated into optical isolators, the separation angle of extraordinary rays cannot be set large. It invades the effective optical path and becomes a factor that deteriorates the extinction characteristics of the optical isolator.

On the other hand, PBS has a price advantage over birefringent polarizing prisms using calcite or rutile, but the process of forming a dielectric multilayer film of 30 or more layers is complicated, and the
A higher extinction characteristic cannot be expected. Furthermore, among birefringent polarizing elements, the Glan-Thompson prism completely refracts the ordinary ray and diffracts it out of the optical path, so it has a high extinction characteristic of 50 dB or more19, and even when used as an optical isolator, the ordinary ray does not fall within the effective optical path. Since there is no intrusion, high isolation can be expected, but as the dimensions become longer and the cost of the element itself also increases, it cannot be accommodated in terms of cost considering the use of many products in future optical communications and the like.

The present inventors previously disclosed in Japanese Patent Application No. 62-185662,
In particular, a polarizing prism that has excellent effects as a polarizing element for an optical isolator has been disclosed. The polarizing prism based on the above patent application was able to significantly improve the drawbacks of the conventional prism described above. In other words, in the case of one Wollaston prism shown in FIG. 2(a), the first birefringent prism 1
By adjusting the apex angle β of
Even if the angle is more than °, it is still a sufficient angle to capture the S-polarized light at the time of incidence.
Since the thickness of the polarizing element can be reduced in the direction of the light beam, the thickness of the polarizing element can be reduced. Furthermore, in order to obtain a cheaper polarizing element, the first prism 3 of the John type prism (1) of (b) is made of isotropic glass, and the second prism 4 is made of birefringent material. 1
If this is the case, only a small amount of expensive birefringent material can be used, resulting in an economical element. The above configuration suggests a method for greatly improving the prior art described above. However, all of these configurations of polarizing elements were proposed in order to shift the unnecessary polarized light component from the optical axis so that it would not be reflected to the semiconductor laser, and the separated unnecessary polarized light component constitutes an optical isolator. There is a possibility that the light will be diffracted from the parts (for example, the four holes of the holder case) and returned to the semiconductor laser.

[Means for Solving the Problems] The present invention provides at least six dielectric films at the junction between the first prism and the second prism of the birefringent polarizing element shown in FIGS. 2(a) and 2(b). The unnecessary polarized light components are reflected at the junction, thereby preventing them from entering the optical circuit. In other words, the main purpose of the present invention is to improve the performance of optical devices such as optical isolake by combining the properties of PBS that utilizes birefringence effect and Brewster reflection, and improving the extinction property on the surface. .

The present invention provides a Faraday rotator and a polarizing element disposed in the central part of a permanent magnet for generating Ia field light for magnetically saturating the Faraday rotator, and on both sides of the Faraday rotator, polarizing elements whose optical axes are shifted by 45 degrees from each other. In the optical isolator, one is composed of an EVA birefringent prism as a polarizing element, and the optical axes of the first N-refractive prism and the second N-refractive prism are arranged. are both perpendicular to the direction of light entry, and the angle between the optical axis of the first birefringent prism and the optical axis of the second birefringent prism is a right angle, and the incident light is roughly A dielectric multilayer film is formed at the joint part of a birefringent prism for a polarizing element, in which the prism angles of both birefringent prisms are set at an angle such that the output light becomes parallel to the other one. One is that the polarizing element consists of a birefringent prism and an optically isotropic glass prism, and the birefringent prism has its optical axis perpendicular to the incident light, and the incident light and the output light are parallel. A dielectric multilayer film is formed at the joint portion of the prism for a polarizing element, in which the prism angle of the glass prism is set so that Further, it is preferable that the dielectric multilayer film has at least six layers that have low absorption at the center wavelength.

Generally, light entering the boundary between two types of dielectrics at an angle θ has an amplitude reflection coefficient of Rp and R for P-polarized light and S-polarized light, respectively.
s, and if ψ is the refraction angle in the second dielectric, then jan(θ-ψ) 5in(θ-Φ)Rp-
-111 rows -100 meters 1 piece S Therefore, when (θ+ψ)=90', Rp=O, and only R3 is reflected, making it possible to separate two types of polarized light. θ under this condition is called Brewster's angle. In order to actually separate the polarized light components using this type of film, if a dielectric material having two types of high and low refractive indexes is used, the films are alternately laminated with a film thickness of λ/4, where λ is the wavelength. High refractive index layer 1-1. Rp=0, where L is the low refractive index layer
To do this, l-Lm (Ll-1) , (+-1> , (Assuming that m is 3 or more, the removal band for S polarization is '5
It is known that the reflectance of the S-polarized light component is 90% or more, which is F (for example, Kozo Ishiguro's ``Optical Hakumei'', published by Kozo).

[Example 1] As shown in FIG. 1(a), a first prism 1 of a Ruston type polarizing element was prepared. Aj20z on the 2nd nib rhythm 2nd joint part M
The laminated films of 1r02 and 1r02 were laminated in the following pattern by vacuum evaporation.

(L H shi) ([-HL) (L H shi) →
1- FI L HL l-('-'2"'l"
2Y'Z7 'l''2' where H: 1rO2 and film thickness λ/4. L: AJ203 and film thickness λ/4, L/
2: AJ 703 with a film thickness of λ/8. After forming this vapor-deposited multilayer film on the bonding surface of one of the first and second prisms, the first and second prisms were bonded and the polarization characteristics were measured by spectroscopy. When measured with a photometer, the extinction characteristics at wavelength 13IIIR were improved by 3 dB: 38 dB without multilayer film formation and 41 dB with 7-layer multilayer film formation, compared to an element in which no multilayer film was formed on the bonding surface. Since the electric shock measured by the spectrophotometer of the present invention was at -L limit of 40 dB, it is impossible to measure more than that, so it is considered that the extinction characteristic is actually higher. Next, when an optical isolator was assembled using these polarizing elements, the extinction ratio was significantly improved to 38 dB.

[Example 2] Next, in the structure of a glass Rochon prism as shown in FIG. 1(b), an upper layer film of Aj203 and 7rO2 was formed on the joint surface N with the glass prism 3 in the same manner as in Example 1. The extinction characteristics of the element without a film and the polarizing element formed thereon were 36 dB without multilayer film formation and 40 dB with seven-layer multilayer film formation. Next, when an optical isolator was assembled using these polarizing elements, the extinction ratio was significantly improved to 35 dB.

In other words, it is considered that sufficient reflection separation from the optical path within the optical isolator has been realized. As shown in (b), more than 90% of the S-polarized light is cut off at the bonding surface N, resulting in high extinction characteristics. The film structure shown in FIG. 1 is given in Table 1 (the following is a blank space).A multilayer film was formed on the calcite prism 4 according to the structure shown in Table 1, and was bonded to the glass prism 3 with resin. In order to confirm the extinction characteristics and S-polarization separation effect of this element, a light beam was incident from 8 B.

When the intensity of the emitted light in the C and D directions was measured, the transmittance of the P component in the B direction was 99.98%, and the reflection intensity in the C direction was 99.98%.
712%, and the polarization separation power in the D direction was 2.8%. That is, from the configuration shown in Table 1, 97% of unnecessary polarizing valves can be separated.

[Efficacy of invention! ! ! ] Semiconductor lasers for optical communication and measurement, which generally require an optical isolator, exhibit fairly strong linear polarization, so S
Although the polarization component itself is small, if the two polarization cables built into the optical isolator have a multilayer film as in the present invention, a PB that utilizes the birefringence effect and Brewster reflection can be used.
By combining the properties of S, the extinction property is made on the surface, so it is possible to almost prevent the separated light from entering the optical path, and maintain stable operation of the semiconductor laser.
The polarizing element of the present invention has made it possible to significantly improve the reliability of isolators as future optical communication devices.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a polarizing element of the present invention. FIG. 2 is a configuration diagram of a conventional polarizing element. 1.2.4: Birefringent prism 3; Glass prism patent applicant Namiki Precision Jewel Co., Ltd. (a) ” (b) Figure 1 Figure 2

Claims (4)

[Claims] (1) A Faraday rotator and a permanent magnet for generating a magnetic field to magnetically saturate the Faraday rotator are arranged in the center, and the optical axes are set at 45 degrees on both sides of the Faraday rotator.
In an optical isolator configured with an optical system consisting of shifted polarization probes, the polarizing element is configured of two birefringent prisms, a first birefringent prism and a second birefringent prism. are both perpendicular to the direction of incidence of the light, and the angle between the optical axis of the first birefringent prism and the optical axis of the second birefringent prism is perpendicular, and An optical isolator characterized in that a dielectric multilayer film is formed at the junction of a birefringent prism for a polarizing element, in which the prism angles of both birefringent prisms are set at an angle such that the output light becomes parallel to the birefringent prism. Polarizing element for use. (2) A polarizing element for an optical isolator according to claim (1), comprising at least six dielectric multilayer films. (3) A Faraday rotator and a permanent magnet for generating a magnetic field for magnetically saturating the Faraday rotator are arranged in the center, and the optical axes are set at 45 degrees on both sides of the Faraday rotator.
In an optical isolator configured with an optical system consisting of shifted polarizing elements, the polarizing element consists of a birefringent prism and an optically isotropic glass prism, and the birefringent prism A dielectric multilayer film is formed at the joint part of a prism for a polarizing element, whose optical axis is perpendicular to the polarizing element, and the prism angle of the glass prism is set so that the incident light and the outgoing light are parallel to each other. A polarizing element for optical isolators featuring: (4) A polarizing element for an optical isolator according to claim (3), comprising at least six dielectric multilayer films.