JP3502132B2 - Optical circulator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical prism applicable to a wide range of optical fields such as an optical communication device and an optical measuring device, and an optical circulator including the optical prism. 2. Description of the Related Art In an optical device used for an optical communication device or the like, an optical transmission technology equipped with an optical amplification system in a long-distance optical communication network is rapidly developed, and a large-capacity signal transmission is instantaneously performed. To be distributed worldwide. Although the technical difficulties in the early stages have already been almost resolved on a laboratory scale, the polarization-independent optical isolator placed between the optical fibers and the multiplexer / demultiplexer of the pump light source considering actual mass production. When mounting optical passive components such as optical circulators required for bidirectional optical communication and the like, the degree of finish and the shape tolerance of each component are directly reflected on assembly accuracy and ease of assembly. Therefore, it is a necessary condition that the optical components have a structure that can be easily assembled with high precision. For example, when a device having a certain function is inserted between optical fibers and optical coupling is performed via a lens, the light propagation direction is the Z axis with respect to the light axis, and the X and Y axes are orthogonal to the light axis. Even if the axis is determined, the propagating ray coupling axis pacing (hereinafter referred to as "axis alignment") is performed not only in these Z, X, and Y directions, but also in a direction perpendicular to the meridian with a meridian direction with the Z axis as a central axis. It is also necessary to adjust the angle with respect to the φ axis. That is, ideally, the axis alignment must be spatially adjusted in five directions. However, in the field of a communication switch or the like in which a related system must be condensed in a limited space, an optical passive device such as an optical isolator is used. Approximately 8m thickness assigned to parts
It is limited to m or 8 mm or less, and it is not possible to expect a space in which individual components can be freely adjusted and optical axis alignment can be performed. Therefore, it is required to reduce the degree of freedom of the components in advance. On the other hand, in an optical circulator, for example, FIG.
A schematic structure as shown in JP-A-55-93120 has been invented, and is currently commercially available as the most general structure.
In this configuration, since each component constitutes an independent serial optical path, the number of components increases in the entire system, and the facility must be economically expensive. For example,
In FIG. 2, a signal input / output port is provided, a pair of polarizing beam splitters (PBS) and a reflecting mirror (M) are opposed to each other, and a light non-reciprocal part 5 and a light reciprocal part 6 are provided in a gap therebetween to propagate light. The direction is →, →, →,
→ It is designed to be In this configuration, all four input / output ports function efficiently, and optical components are integrated. However, when the conventional technology is put to practical use on an industrial scale, there are some drawbacks from the viewpoint of optical alignment of a manufacturing surface and long-term reliability in use. That is, the opposite P
The parallelism of the BS degrades the perpendicularity of the emitted light beams, and adds an angle adjustment. Moreover, the PBS has a large angle dependency, and the angle with respect to the input / output port axis must be adjusted within a predetermined tolerance. In addition, since the bonding surface of the PBS portion is fixed with resin, there is a question from the viewpoint of reliability, such as damage or deterioration when the light intensity increases. Further, the wavelength change and the temperature change of the reciprocal portion optical element 6 are large and there is a problem in widening the band. Of course, there is a flat-plate type PBS instead of a matching prism, but it is difficult to align the optical axis because the parts are separated. In order to further increase the optical non-reciprocal level, the non-reciprocal portion 5 including the polarizer portion is generally realized in a two-stage series. However, in the configuration shown in FIG. Impractical to shape. On the other hand, it is conceivable to make the PBS a parallel plate type to improve the reliability deterioration due to the bonding, but there are problems such as an increase in the number of parts and difficulty in optical coupling. SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned current problems. An optical circulator according to the present invention uses a conventional birefringent crystal plate for an optical non-reciprocal portion. A polarization-independent optical isolator is adopted, and the right-angle turning part of the light beam is arranged with a pair of pentagonal optical prisms at rotationally symmetric positions around the optical isolator placed in the middle, and the inclination of each pentagonal optical prism In this configuration, one or two birefringent crystal plates are arranged at positions where the orthogonal light rays reflected from the surface enter and exit. FIG. 1 is an example of a four-terminal optical circulator according to the present invention, and FIG. 3 is a perspective view of a pentagonal prism optical prism required for the present invention. FIGS. 4, 5 (a) and 5 (b) are a schematic structural diagram and an operation principle diagram of a circulator when the optical circulator of the present invention is coupled to an optical fiber. 6 (a) and 6 (b) are operation principle diagrams as an optical circulator showing another embodiment of the present invention. To explain the structure of the optical circulator, a pair of pentagonal prism structures required for the present invention will be described. In FIG. 3, a light-transmitting window is provided substantially at the center of the surface 31 (portion A), and the other is a reflecting mirror. Is applied. In the manufacturing method, for example, an anti-reflection film relating to the light wavelength used in the vacuum evaporation method is formed on the surface 31 while shielding portions other than the portion A. Next, the portion A is shielded, and a reflecting mirror is formed in the remaining portion, and a reflecting mirror portion having a light-transmitting window at substantially the center required for the present invention is formed. Similarly, the 45 ° side slope 32 is also a reflecting mirror, and the other surfaces 33, 34 and 35 are formed with antireflection films as required. Then, the pentagonal prism is arranged in the configuration of FIG. That is, when a light beam is transmitted through the central window of the surface 31 of the pentagonal prism at an appropriate angle, the light beam is emitted from the surface 33 in the same direction as the incident light beam, and propagates to the polarization independent isolator arranged in the forward direction. . In this configuration, the light beam emitted from the opposite direction through the optical isolator is split into ordinary light and extraordinary light, and enters the surface 33 of the pentagonal prism. At this time, if the size of the light transmission window at the center of the surface 31 is sufficiently small and the Gaussian beam light flux divided into two from the opposite direction is large enough to prevent leakage from the window, the distance between the ordinary light and the extraordinary light is large. Is reflected by the surface 31. At this time, if the optical isolator is arranged at an appropriate angle with respect to the optical isolator, the ordinary light and the extraordinary light reflected on the pentagonal prism surface 31 are reflected again on the surface 32 and emitted from the surface 34 to the outside of the prism. You. The opposing pentagonal prisms have the same structure. Here, if a birefringent crystal plate having an appropriate thickness is arranged in an azimuth where two rays recombine as shown in FIG. 1, it functions as an optical circulator. That is, FIG.
Light input / output end ports P1 to P4 are provided on the four side surfaces of the rectangular parallelepiped holder 1 as abbreviated as above, the polarization independent optical isolator 2 is provided at the center, and the light transmission windows on both sides face the light port side, The pentagonal prisms 3 and 4 are arranged at rotationally symmetric positions around the optical isolator, and a birefringent crystal plate is arranged so that ordinary light and extraordinary light of light propagating to the ports P3 and P4 are coupled. At this time, if the direction of propagation of the optical isolator from P1 to P2 is the forward direction,
As shown in FIG. 1, between the ports through which the light propagates, P1 to P2, P2 to P3, P3 to P4, and P4 to P1.
Is a four-terminal circulator that propagates to an optical fiber that can couple the four types of light. FIG. 5 is a diagram illustrating the principle of tracking the relationship of FIG. 1 from the state of polarization separation and coupling. 1 and 5 are examples of an optical circulator based on the present invention. For example, when the light propagates from P to P, the light enters from the light transmission window A of the first prism PR1,
After being recombined from the B-plane to the F-plane of the optical isolator, the light propagates from the light transmitting portion G of the second prism PR2 to P.
The light incident from P is emitted from the optical isolator D
Section has the same behavior as the P → P ray action, but F
When passing through R, ordinary light and extraordinary light are reversed, and when passing through B1, the light is projected onto PR1 while being separated at a point symmetrical position about the ray propagation axis of P → P. Does not return and is repeatedly reflected by the reflecting surfaces A and M1 and guided to the birefringent crystal plate B4. Since B4 recombines separated ordinary light and extraordinary light, it can be transmitted to P3 without loss of light intensity. The ray incident from P is P
→ Return the same path as the ray propagation of P to the FR part, and
When transmitted through, ordinary light and extraordinary light are inverted. Therefore B2,
When transmitted after passing through B3, the light is separated at a point symmetric position about the ray propagation axis of P → P and projected to the second prism,
The light is reflected on the G plane and the M2 plane, is recombined after passing through B5 and B6, and propagates to P. For a ray incident from P, the ray propagates from P to P and FR to the FR section through the same path.
The normal light and extraordinary light for B1 are reversed after transmission, and are superimposed on the P → P path described first,
The light propagates from the transmission window A of PR1 to P. Here, the amount of crosstalk between the other ports is determined by taking in the Gaussian beam foot portion taken in the transmission windows formed in the A and G planes and the elliptic component of the linear polarization generated when passing through the Faraday rotator. This is the main element. Currently manufactured Faraday rotators have an ellipticity of 40
It is more than dB, and even if it is not a two-stage configuration in which polarization independent optical isolators are connected in series, it has sufficient characteristics depending on the application. Of course, if a multi-stage connected high extinction characteristic isolator is mounted, the amount of crosstalk can be greatly improved. further,
The polarization-independent optical isolators to be mounted are the types of isolators, provided that the parallel separation effect of ordinary and extraordinary light using a birefringent crystal plate and a 45 ° polarization plane Faraday rotator is applied to the non-reciprocal part. Regardless, the circulator function disclosed by the present invention is shown. The four-terminal optical circulator according to the present invention has the following advantages over the conventional system. First, the greatest advantage of this configuration is that, when four light entrances and exits are provided from the port P to the port P as shown in FIG. 4, the parallelism and the squareness are precisely formed in a rectangular case. Alignment and fixing of the two pentagonal prisms and the polarization independent optical isolator to be arranged can achieve optical coupling between ports only by roughly adjusting the orientation. For example, when propagating from a ray P to a ray P, the ray propagates in a direction parallel to the ray, but the parallelism is preserved. Therefore, the constituent optical elements are housed in a rectangular holder as shown in FIG. 4, and there is a light beam through hole on the side of the holder. Is in the side of the holder, that is, the pentagonal prism according to the present invention has the parallelism of the transmitted light and the reflected light even if the ray angle with respect to the prism surface is displaced.
Since the perpendicularity is maintained (refer to Japanese Patent Application No. 5-261896), when the light beam is a parallel light beam or a convergent light beam that provides optical coupling equivalent to the parallel light beam, the plane on the holder side where each port is fixed. Optimal coupling is obtained from the two-axis adjustment within. That is, if the orthogonality of the light beam is adjusted on one side surface of the holder, the other three light beam ports are inevitably arranged even if they are visually measured without precise positioning of the optical components to be housed. The orthogonality between the holder surface and the light beam is preserved, and the assembly adjustment is simple as described above. Of course, the accuracy of the perpendicularity between the adjacent side, upper and lower surfaces and the side surface of the pentagonal prism, and the 45 ° inclination angle between the inclined side surface and the side surface must be suppressed to at most several tens of seconds. The accuracy of this is a level that can be sufficiently realized from the machining accuracy. Compared with the conventional pentaprism, the accuracy can be easily controlled because it can be manufactured from a square prism, and the circulator assembly is also compared with the conventional 5-axis adjustment. The difficulty in assembling is negligible. In the circulator of the present invention, the arrangement of the first and second pentagonal prisms is such that even if the installation angle with respect to each of the isolators is shifted, the azimuth of the light beam is only displaced in parallel. Sufficient optical coupling can be formed from alignment on a plane in a plane. The second advantage is that, as described in detail above, the optical isolator portion can be of any structure as long as it is a parallel separation system of ordinary light and extraordinary light. The mounting of the mold also enhances the circulator function and does not cause any structural defects. Third, since there is no optical junction such as PBS in the optical path, long-term reliability, light intensity damage and the like do not occur. Fourth, since two reflecting films are formed on a pentagonal prism, two independent reflecting mirrors can be individually adjusted. In contrast, in the present invention, optical coupling can be obtained by fixing the mirrors to a holder with almost no adjustment. Is to be done. 6 (a) and 6 (b) show a light propagation state of a four-terminal optical circulator according to another embodiment of the present invention, and show the same characteristics as those in FIG. Hereinafter, the gist of the present invention will be described in detail with reference to examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As an example of an optical circulator according to the present invention,
Description will be given based on the configuration related to FIG. In FIG. 1, the polarization independent optical isolator has a thickness ratio B in the light beam direction.
The structure includes a birefringent crystal plate using three rutiles in which 1: B2: B3 = 1: 1 / √2: 1 / √2 and a Faraday rotator obtained by an LPE film forming method as a non-reciprocal element. . The thickness of the other birefringent crystal plates B4, B5 and B6 is 2: {2:} with respect to B1.
In the case of 2, separate coupling is realized. An anti-reflection film was formed on the prism window, and after a suitable shielding film was attached, a reflection mirror film was formed on portions other than the window. As the film material, a metal film such as Au or Al or a dielectric multilayer film can be used. The parallelism and the perpendicularity of the prism and the rectangular parallelepiped holder are important, and in this embodiment, the angle error is limited to within ± 30 seconds. Since the component accuracy directly contributes to the optical coupling between the fiber ports, the smaller the component loss, the better the coupling loss. Fibers and lenses were prepared so that all the fiber ports were emitted at right angles to the holder surface. Of course, if angle adjustment is included in the manufacturing process, perpendicular emission is not an essential condition. In this embodiment, an optical coupling section with a yoke including a fiber and a lens is formed inside the side surface of the rectangular parallelepiped optical member storage holder, and the yoke is slid while being pressed against the side surface to find an optimum coupling point. The A-side window is designed to have a diameter of about 150 μm, and the light beam is about 80 μm at the beam waste position.
The distance between μm, P1, and P2 is about 14 mm, and the dimensions of the optical component storage holder are 12 mm in length, 12 mm in width, and 8 mm in height. The applied light wavelength is 1550 nm. When measuring the coupling loss between each port using a semiconductor laser light source and optical power meter,
The following values were obtained: Propagation direction Optical coupling loss between ports P1 → P2 0.7dB P1 → P3 53dB P1 → P4 45dB P2 → P1 39dB P2 → P3 1.2dB P2 → P4 55dB P3 → P1 41dB P3 → P2 38dB P3 → P4 1.4dB P4 → P1 1.1dB P4 → P2 40dB P4 → P3 43dB From the above results, it was confirmed that the configuration proposed by the present invention functions as a four-terminal optical circulator. The crosstalk can be further cut off by connecting an isolator, but the propagation distance between ports becomes longer and the degree of optical coupling decreases, so adjustments such as increasing the beam diameter and designing the propagation distance to a minimum are necessary. It is. FIG. 6 shows another embodiment of the present invention. The same effect as that of the embodiment shown in FIG. 5 is shown only by changing the arrangement of the rutile flat plate and the pentagonal prism. further,
The present invention is not limited by the above examples at all,
By using this pentagonal prism, various combinations of multi-terminal optical circulators are possible. The structure of the present invention does not require a resin-fixed portion, so it is highly reliable, and can be adjusted as individual polarized light at the time of assembling, so that crosstalk between ports and insertion loss can be minimized. Can control. In addition, it is easier to reduce the size of the birefringent crystal plate, the Faraday rotator, and the like, as compared with a circulator using a conventional PBS, and it is expected that the price will be reduced. In addition, it can be mounted on a part of an optical amplification module or connected to an optical fiber terminal with integrated lens to form a simple optical device. is there.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram of an example of a four-terminal optical circulator of the present invention. FIG. 2 is a schematic diagram of a conventional optical circulator. FIG. 3 is a schematic view of a pentagonal prism in the optical circulator of the present invention. FIG. 4 is an explanatory diagram of an optical circulator of the present invention. FIG. 5 is an explanatory diagram (a) of a light beam propagation state when the optical circulator of the present invention is coupled to an optical fiber, and a polarization state diagram (b) of the optical circulator. FIG. 6 is an explanatory diagram of a light propagation state when an optical circulator according to another embodiment of the present invention is coupled to an optical fiber.
(a) and a polarization state diagram (b) as an optical circulator. [Description of Signs] 1 rectangular parallelepiped holder 2 optical isolators 3 and 4 pentagonal prism 5 light non-reciprocal part 6 light reciprocal parts A and G light transmitting windows 31, 32, 33, 34, 35 pentagonal prism surfaces P1, P2, P3, P4 port FR Faraday rotator PBS Polarizing beam splitter B1, B2, B3, B4, B5, B6 Rutile plate M Reflector

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-100016 (JP, A) JP-A-55-50215 (JP, A) JP-A-7-92429 (JP, A) (58) Investigation Field (Int.Cl. ⁷ , DB name) G02B 27/28 G02F 1/09

Claims (1)

(57) [Claims 1] A light reflecting side slope in which a rectangular ridge portion of a rectangular or square prism is deleted so as to form a plane having an angle of 45 ° with a side surface of the prism. One of the two prismatic side surfaces opposed to the light reflection side slope and adjacent to each other at an angle of 90 ° is a light transmission window portion in the center of the surface, and a light beam including a reflection mirror portion other than the window portion. two pentagonal prism prism having a reflecting side is disposed the light transmission window portion symmetrically positioned on the outside, a polarization-independent optical isolator using a flat birefringent crystal is arranged between the prism, the light
Light incident on the isolator is separated into ordinary light and extraordinary light.
And emitted from the optical isolator to the prism.
Among the rays, between the ordinary light and the extraordinary light according to the propagation port
If the separation is smaller than the size of the window,
It is emitted from the window, and if it is large, it is reflected by the reflector.
And reflected by the light reflecting side slope,
A line exits the prism and the optical isolator
From the light to the prism
Sowing, and at the position where the propagating ray is incident, the propagating ray
An optical circulator comprising one or a plurality of birefringent crystal plates having a crystal orientation and a thickness necessary for separating and combining the crystals.