JP3263501B2 - Polarization separation element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarized light separating element for separating specific polarized light, and more particularly to a polarized light separating element suitable for use in a waveguide type optical component.

[0002]

2. Description of the Related Art Waveguide-type optical components using a polarization splitting element are known, and are applied to optical switches, circulators, isolators, and the like for optical communication using optical fibers. Optical devices such as optical switches, circulators, and isolators are usually configured by combining individual bulk optical elements such as a polarizer, a lens, a Faraday rotator, a birefringent element, and a half-wave plate. However, there are problems such as the increase in the size and the size, and the complicated adjustment of the mutual relationship between the elements. On the other hand, a waveguide type optical component incorporates a waveguide and a Faraday element or a half-wave plate or the like on a single substrate using thin film technology, so that a lens is not required and the entire size is reduced. Very difficult to manufacture. Therefore, a waveguide such as an optical fiber is used or a waveguide is formed on a substrate by thin film technology, or an optical fiber is embedded in the substrate to form a waveguide, and a slit is formed in a part of the waveguide, and a predetermined slit is formed therein. It has been proposed to insert a polarization separation element, a Faraday rotator, a half-wave plate, and the like. Such a technique is disclosed in, for example, JP-A-2-18525 and JP-A-3-171103.

[0003] Japanese Patent Application Laid-Open No. Hei 2-18525 discloses that two
An input and output four-terminal waveguide is formed, a slit is formed at the intersection of the two input-side waveguides, and a polarization separation film for separating polarized light is inserted. A slit is also formed at the intersection, and a polarization separation film for synthesizing polarized light is inserted. A slit is formed between the two polarization separation films in the middle of the two waveguides to form a magneto-optical element (Faraday rotator) /
An optical switch is described in which a two-wave plate is inserted and a magnetic field is inverted to couple an arbitrary input to an arbitrary output. on the other hand,
Japanese Patent Application Laid-Open No. Hei 3-171103 realizes a polarization splitting element by controlling the effective refractive index at the intersection of waveguides instead of the polarization splitting film of Japanese Patent Application Laid-Open No. Hei 2-18525.

[0004]

The waveguide type optical component disclosed in the above-mentioned document can be downsized and the optical axis can be easily adjusted as compared with the conventional bulk type optical component. The characteristics are stabilized and mass production becomes possible. However, the polarization separation elements disclosed in these documents require an intersection of waveguides, and it has been extremely difficult to manufacture this intersection with high accuracy. Accordingly, it is an object of the present invention to provide a polarization splitting element that does not require an intersection in a waveguide.

[0005]

SUMMARY OF THE INVENTION The present invention provides at least two
A birefringent plate is inserted obliquely into the parallel straight waveguides of the book,
A polarization splitting element characterized in that when light passes through a birefringent plate, extraordinary light goes straight, and ordinary light goes off-axis and goes to another waveguide. The number of waveguides on the incident side may be one less than that on the exit side.

[0006]

The basic principle of the present invention will be described with reference to FIG.
In FIG. 1, the birefringent plate 1 is arranged so that the optical axis is in the plane of the paper. When the birefringent plate is arranged so that the incident light is perpendicularly incident on the surface of the birefringent plate as shown in FIG.
Travels straight, and the extraordinary light E causes an axis shift. Therefore, when the ordinary light O and the extraordinary light E enter in a mixed state, the polarization is separated. However, there is a problem that reflected light is generated by reflection on the surface of the birefringent plate. Therefore, when the birefringent plate is arranged obliquely as shown in FIG. 1B, there is a certain incident angle θ _{in at} which the extraordinary light E goes straight. At this time, the ordinary light O causes an axis shift due to the law of refraction. Therefore, in this arrangement, when the ordinary light O and the extraordinary light E enter in a mixed state, they are separated into the extraordinary light that goes straight and the ordinary light whose axis is shifted, but the reflected light does not return in the incident direction. The present invention takes advantage of this phenomenon. That is, a birefringent plate is obliquely inserted between the two parallel waveguides so as to satisfy the condition of FIG. 1B, and the distance between the two waveguides is the ordinary light shown in FIG. Set to the amount of axis deviation. With this configuration, the extraordinary light travels straight, and the ordinary light travels to another waveguide. Therefore, the extraordinary light functions as a polarization splitting element, and the light reflected on the entrance surface and the exit surface of the birefringent plate in which the birefringent plate is arranged diagonally. Does not return to the original waveguide.

In FIG. 1B, the angle between the normal vector of the plane of incidence of the birefringent parallel plate and the incident light is θ _in , and the angle between the optical axis of the birefringent parallel plate and the incident light is θ _C (see FIG. 1B). , The refractive index of the region on the incident side is n _in , the refractive index of ordinary light of the birefringent plate is n _O , and the refractive index of extraordinary light of the birefringent plate is When the refractive index is n _E , the condition that the extraordinary light travels straight as shown in FIG. 1B is expressed by the following equation.

Tan θ _in = A / [√B (n _in -√B)] where A = (n _O ² −n _E ² ) cos θ _C sin θ _C B = n _E ² sin ² θ _C + n _O ² cos θ _{C As} an example, the refractive index of the waveguide is set to 1.45, a rutile parallel plate is used as the birefringent plate, and the refractive index of ordinary light n _O =
Assuming that 2.44, the refractive index of extraordinary light n _E = 2.69, and the direction of the optical axis θ _C = 32.7 °, the incident angle θ _in =
When the angle is 12.3 °, the extraordinary light goes straight as shown in FIG. At this time, the optical axis of the rutile is inclined by θ _in + θ _C = 45 ° with respect to the normal vector of the incident surface of the rutile plate.
On the other hand, ordinary light is θ _in −θ with respect to incident light according to the law of refraction.
Proceed at the _O angle. Where θ _O = sin ⁻¹ [(n _in / n
_O ) sin θ _in ]. Assuming that the thickness of the birefringent parallel plate is t, the axis deviation of the emitted light with respect to the incident light is

(6) sin (θ _in −θ _O ) t / cos θ _{O If the} waveguides are arranged at this interval, the incident light is guided to the output side waveguide. For example, in the above example, ordinary light travels at an angle of 5 ° with respect to incident light, and 88 ° per mm of rutile plate.
An axis shift of μm occurs. By forming two waveguides at an interval of 88 μm and inserting a 1 mm thick rutile plate between them with an oblique angle θ _in = 12.3 ° and an optical axis θ _C = 32.7 °, extraordinary light is generated. Functions to travel straight, and the ordinary light is shifted in axis and travels through another waveguide.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. Embodiment 1 FIG. 2 illustrates a polarization beam splitting device according to a first embodiment of the present invention. Reference numeral 10 denotes a substrate on which linear first waveguides 11 and second waveguides 12 are formed by thin film technology. A slit 14 for cutting the waveguides 11 and 12 is formed in the substrate 10 at an angle. As described with reference to FIG. 1, the slit 14 has an angle that satisfies the condition that the extraordinary light travels straight. Ordinary light is misaligned and the other waveguide 12
Is inserted and fixed.

In operation, of the light incident on the waveguide 11, the extraordinary light E travels straight through the birefringent plate 13 and is guided to the waveguide 11, and the ordinary light O is deviated from the axis by the birefringent plate 13 and is guided by the waveguide. 1
2 is guided. Since each surface of the birefringent plate is inclined with respect to the waveguide, the reflected light does not return to the waveguide on the incident side. When applied to an optical isolator, the waveguide 12 on the incident side is unnecessary, but can be used according to the application.

Embodiment 2 FIG. 3 illustrates a polarization beam splitter according to a second embodiment of the present invention. In this example, a first optical fiber 21 and a second optical fiber 22 embedded in a substrate are used as waveguides.
Slits 24 are formed in the waveguides 21 and 22 to cut them obliquely. These slits have an angle, as shown in FIG. 1, which satisfies the condition that the extraordinary light E travels straight.
Further, a birefringent plate 23 having a thickness such that the ordinary light O is guided to the other optical fiber 22 with the axis shifted is inserted and fixed. Since the cores 25 of the optical fibers 21 and 22 have the cores 26 expanded on both sides of the birefringent plate 23, loss of light can be prevented. The polarization beam splitter of this embodiment operates in the same manner as that of the first embodiment.

Embodiment 3 FIG. 4 shows a third embodiment of the present invention. In this example, three or more waveguides are used, and first, second,..., Fifth waveguides 31, 32,. The substrate 30 includes waveguides 31, 32,.
Are formed, and the slits 34 are formed in the slits as shown in FIG.
An angle that satisfies the condition for the extraordinary light E incident on 35 to travel straight, and the waveguides 32,... Adjacent to each other with the ordinary light O incident on the waveguides 31,.
A birefringent plate 23 having a thickness guided to 35 is inserted and fixed. The waveguide may be one using an optical fiber as shown in FIG.

In operation, of light incident on a specific waveguide, extraordinary light goes straight, and ordinary light is guided to an adjacent waveguide. That is, among the light incident on any of the input terminals 1A to 5A in FIG.
B, the ordinary light is guided to the output terminals 2B to 6B, respectively. On the other hand, the reflected light on the surface of the birefringent plate 23 does not return to the incident side. This example can be used for an optical circulator, an optical switch, and the like.

[0013]

According to the present invention, there is no need for an intersection as in the prior art, and the waveguide can be constituted by parallel linear waveguides, so that the manufacture is very easy. Further, since the birefringent plate is disposed obliquely, the return loss is large (the amount of reflected return light is small).

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram illustrating a polarization splitting device according to a first embodiment of the present invention.

FIG. 3 is a view illustrating a polarization beam splitter according to a second embodiment of the present invention.

FIG. 4 is a view illustrating a polarization beam splitter according to a third embodiment of the present invention.

[Explanation of symbols]

10, 30: substrate 11, 12, 22, 23, 31, 32, 33, 34, 3
5, 36: waveguide 13, 23, 33: birefringent plate 14, 24, 34: slit

Continuation of front page (56) References JP-A-5-34650 (JP, A) JP-A-2-219003 (JP, A) JP-A-4-307512 (JP, A) JP-A-5-232322 (JP) JP-A-5-93924 (JP, A) JP-A-6-175070 (JP, A) JP-A-6-18940 (JP, A) JP-A-5-142498 (JP, A) JP-A-4-349421 (JP, A) JP-A-5-151634 (JP, A) JP-A-5-66363 (JP, A) JP-A-4-264516 (JP, A) Jikken Sho 53-50448 (JP, A Y2) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 6/12-6/14 G02B 5/30 G02B 27/28

Claims (5)

(57) [Claims] A birefringent parallel plate is obliquely inserted into at least two parallel linear optical waveguides, and a plane formed by the waveguide is parallel to a normal vector of an incident surface of the birefringent parallel plate. And the optical axis of the birefringent plate is also parallel to the plane, and the following relational expression: tan θ _in = A / [√B (n _in -√B)] (where A = (n _O ² − n _E ² ) cos θ _C sin θ _C B = n _E ² sin ² θ _C + n _O ² cos θ _C n _in : refractive index of waveguide n _O : refractive index of ordinary light of birefringent parallel plate n _E : refractive index of birefringent parallel plate Refractive index of extraordinary light θ _in : Angle between waveguide and normal vector of incident surface of birefringent parallel plate θ _C : Angle between waveguide and optical axis of birefringent parallel plate (θ
_in a polarization separating element opposite to the positive)) almost satisfied. 2. The distance d between two waveguides is expressed as follows: d = sin (θ _in −θ _O ) t / cos θ _O (where θ _O = sin ⁻¹ [(n _in / n _O ) sin θ) _in ]
2. The polarization splitting element according to claim 1, wherein t = thickness of the birefringent parallel plate is substantially satisfied. 3. A parallel birefringent flat plate obliquely inserted into at least one parallel linear optical waveguide aligned with the incident side on the incident side and one more than the incident side on the exit side, The plane formed by the waveguide is parallel to the normal vector of the plane of incidence of the birefringent parallel plate, the optical axis of the birefringent plate is also parallel to the plane, and the following relation: tan θ _in = A / [ √B (n _in −√B)] (where A = (n _O ² −n _E ² ) cos θ _C sin θ _C B = n _E ² sin ² θ _C + n _O ² cos θ _{C in} : the refractive index of the waveguide n _O : refractive index of ordinary light of the birefringent parallel plate n _E : refractive index of extraordinary light of the birefringent parallel plate θ _in : angle formed between the waveguide and the normal vector of the incident surface of the birefringent parallel plate θ _C : conductivity The angle (θ) between the waveguide and the optical axis of the birefringent parallel plate
_in a polarization separating element opposite to the positive)) almost satisfied. The distance d between the two waveguides is expressed as follows: d = sin (θ _in −θ _O ) t / cos θ _O (where θ _O = sin ⁻¹ [(n _in / n _O ) sin θ) _in ]
4. The polarization beam splitter according to claim 3, wherein t is substantially satisfied. 5. The optical waveguide according to claim 1, wherein the waveguide comprises an optical fiber.
5. The polarization beam splitter according to any one of items 1 to 4.