JPH01205121A - Optical demultiplexer - Google Patents

Abstract

PURPOSE:To obtain a multi-wavelength optical demultiplexer which is small in size and less in loss without using any precise working parts by turning the polarizing direction of linearly polarized light to a prescribed direction in accordance with the wavelength of wavelength-multiplexed light and demultiplexing the linearly polarized light by using a biaxial optical crystal. CONSTITUTION:Wavelength-multiplexed light becomes linearly polarized light having the same polarizing direction after passing through a polarizer 5. When each of the linearly-polarized constituent light is passed through an optical rotator 6, the polarizing direction of each constituent light is turned by an angle corresponding to the wavelength. When each constituent light which is turned by the angle corresponding to the wavelength is passed through a biaxial optical crystal 7, each light is emitted from a different position depending upon the polarizing direction, and thus, the wavelength-multiplexed light is separated into each constituent light. Therefore, a multi-wavelength optical demultiplexer which is small in size and less in loss can be obtained without using any precise working parts.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a multi-wavelength optical demultiplexer that does not require sophisticated processed parts, is small in size, and has low loss.

(Prior art) Conventionally, in an optical demultiplexer that applies polarization multiplexing, the polarization directions of the multiplexed light beams are made orthogonal using birefringence or optical rotation dispersion, and then birefringence elements, polarization beam splitters, etc. There is a configuration that separates the polarized light using a polarization splitting element.

(Problems to be Solved by the Invention) In the conventional technology, after the polarization directions of wavelength-multiplexed linearly polarized light are orthogonalized, a polarization separation element with inherent polarization is used to
Since the light is separated into light of each wavelength, in principle only two waves can be separated. Furthermore, even if conventional optical demultiplexers are cascaded to create multiple wavelengths, in the first stage of demultiplexing, the wavelength-multiplexed light is group-demultiplexed into two groups. The polarization direction of the light must be parallel,
From now on, there will be restrictions on the wavelength arrangement, and - for boat fishing, there will be a problem that wavelengths will have to be arranged at equal intervals.

SUMMARY OF THE INVENTION An object of the present invention is to provide a multi-wavelength optical demultiplexer that solves the problems of the prior art described above, does not require sophisticated processed parts, is small, and has low loss.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides an optical demultiplexer that separates input wavelength-multiplexed light into constituent lights with different wavelengths and outputs the separated wavelength-multiplexed light. A polarizer that makes the constituent lights linearly polarized with the same direction of polarization, an optical rotator that rotates the polarization direction of each constituent light linearly polarized by the polarizer by different angles corresponding to the difference in wavelength, and the optical rotator. It is characterized by being constructed from a biaxial optical crystal that is rotated by a mirror and outputs from different positions depending on the polarization direction of the incident linearly polarized light.

(Function) In the present invention, by passing wavelength-multiplexed light through a polarizer, the polarization direction thereof becomes linearly polarized light, and by passing each of the linearly polarized constituent lights through an optical rotator,
The polarization direction of each component light is rotated by a different angle corresponding to the difference in wavelength, and finally, each component light rotated by a different angle passes through a two-wheeled optical crystal and is emitted from a different position depending on its polarization direction. and separates each component light.

(Embodiment) FIG. 1 is a configuration diagram of an optical demultiplexer in the case of three-wave demultiplexing, showing the first embodiment of the present invention, in which 1 is an input optical fiber, 2-1
~2-3 is the output optical fiber, 3 is the collimating lens,
4-1 to 4-3 are focusing lenses, 5 is a polarizer, 6 is an optical rotator, and 7 is a biaxial optical crystal. Also, X* yr
z indicates a coordinate axis of a temporarily determined orthogonal system, A and B indicate a light incident surface and a light exit surface of the optical rotator 6, respectively, and C indicates a light exit surface of the biaxial optical crystal 7.

The input optical fiber 1 is set so that its optical axis coincides with that of the collimating lens 3, and its end face is located at the focal point of the collimating lens 3. Similarly, output optical fiber 2-
1 to 2-3 are focusing lenses 4-1 to 4-3, respectively.
and the optical axis is aligned with the end face of the focusing lens 4-1 to 4-4.
-3 focal point. Polarizer 5
is designed to pass light vibrating in the X-axis direction.

Further, the biaxial optical crystal 7 is cut out with a thickness d so that the surface perpendicular to the main optical axis is the surface, and on one surface C side, the output optical fibers 2-1 to 2-2- 3 and focusing lenses 4-1 to 4-3 are arranged such that their respective optical axes are located on the circumference of a diameter a. The diameter a of the circle and the thickness d of the biaxial optical crystal 7 are the refractive index of the biaxial optical crystal 7, nl,
n2. When n3, a2-d21 (n2 /nl)2-11(1-(n
2/n3) 2). However, nl<n2
<n3.

Now, the wavelength λ1. which is emitted from the input optical fiber 1. λ2
．． The wavelength-multiplexed light of λ3 is converted into a parallel light beam by the collimating lens 3, and then passes through the polarizer 5, thereby becoming linearly polarized light vibrating in the X-axis direction. This linearly polarized light is
However, the polarization direction of the linearly polarized light that passes therethrough differs depending on the wavelength due to the dispersion of the optical rotatory power of the optical rotator 6.

Here, the wavelengths λl, λ2 . The polarization direction of the light of λ3 after passing through the optical rotator 6 is aP counterclockwise with respect to the X axis.
1, respectively. Let θ2°θ3.

FIG. 2 shows the wavelength λ1. λ2. Before and after the light of λ3 passes through the optical rotator 6, that is, the plane A.

FIG. 3 is a diagram schematically showing the polarization direction at B. FIG. The light that has passed through the optical rotator 6 then enters a biaxial optical crystal 7. As mentioned above, if the biaxial optical crystal 7 is cut by a plane perpendicular to the main optical axis and light is incident perpendicularly to this plane, the passing straight line On the surface C, light is emitted from different points on the circumference of a diameter a depending on the polarization direction of the polarized light. Here, the center of the circle with diameter a is 01
If the point on the circumference when light travels straight through the biaxial optical crystal 7 is D, and the position where the linearly polarized light with the polarization direction θ is emitted is expressed as an angle α measured counterclockwise with respect to the straight line OD, then α -w2θ, wavelengths λl, λ2. The light of λ3 is
αl-2θ11α2-2θ2 on the circumference of center 01 diameter a
, the light is emitted from the position α3-2θ3.

Wavelength λ1. from this plane C. λ2. FIG. 3 schematically shows how light of λ3 is emitted. Therefore, if the focusing lens 4-i and the output optical fiber 2-i are set at the position of the angle α1, the light with the wavelength λl will be
Each of the beams is focused by the focusing lens 4-1 and coupled to the output optical fiber 2-1, whereupon demultiplexing is performed.

In the above explanation, we explained the case of splitting three waves.
The rotation of the polarization direction corresponding to the wavelength of the passing light by the optical rotator 6 is analogous, and furthermore, the function of the biaxial optical crystal 7 to convert the polarization direction that differs depending on the wavelength into a difference in position is also analogous. It will be obvious that the optical demultiplexer of the present invention can be similarly applied to cases where the number of wavelengths multiplexed is even larger.

In the explanation so far, it has been assumed that the input optical fiber 1 is a normal optical fiber. Therefore, the light emitted from the input optical fiber 1 is considered to be non-polarized light, and passing through the polarizer 5 causes a loss of 3 dB. If a polarization maintaining fiber is used as the input optical fiber 1, the emitted light can be linearly polarized, so the polarizer 5 can be omitted and the 3 dB loss mentioned above can also be eliminated.

Next, FIG. 4 shows a configuration in another embodiment of the present invention in which the number of wavelengths multiplexed is three. In this configuration, even if the input optical fiber 1 is a normal optical fiber, an excess loss of 3 dB does not occur. In Fig. 4, 8 is a polarizing beam splitter, 9 is a mirror, 1o-1 to 1o-3 are optical fibers, 11-1 to 11-
3 is a connecting optical fiber, 12-1 to 12-3 are focusing lenses, 13-1 to 13-3 are focusing lenses, 14-1 to 1
4-3 is a collimating lens, 15-1 to 15-3 are collimating lenses, and 16 is a polarizing beam splitter, with the symbols 1.2-1 to 2-3.3.4-1 to 4-3.6. ．．
7 shows the same thing as in FIG. Connection optical fiber 1
o-1 to 10-3. 11-1 to 11-3 require the use of polarization-maintaining optical fibers. Furthermore, x, y, and z indicate coordinate axes of a temporarily determined orthogonal system.

Similarly to the embodiment of FIG. 1, the input optical fiber 1 and the collimating lens 3 or the connecting optical fibers 10-1 to 1
0-3, focusing lenses 12-1 to 12-3, collimating lenses 14-1 to 14-3, and connecting optical fiber 11-1.
11-3, focusing lenses 13-1 to 1B-3, and collimating lenses 15-1 to 15-3 are each set so that their optical axes lie on one straight line. In addition, the focusing lenses 12-1 to 12-3 and the connecting optical fiber 10-1
~10-3 system or focusing lens 13-1~1
3-3 and a system of connecting optical fibers 11-1 to 11-3 are each set on the circumference of a diameter a on one surface side of the biaxial optical crystal 7.

The wavelength radiated from the input optical fiber 1 is λ1°λ2. λ
The wavelength-multiplexed light of No. 3 is converted into a parallel light beam by the lens 3, and divided by the polarization beam splitter 8 into linearly polarized light vibrating in the y-axis direction and linearly polarized light vibrating in the X-axis direction. The linearly polarized light vibrating in the y-axis direction passes through the polarizing beam splitter 8,
The light is incident on the optical rotator 6. On the other hand, the linearly polarized light vibrating in the X-axis direction is reflected by the polarization beam splitter 8, further reflected by the mirror 9, and then enters the optical rotator 6 at a different position from the linearly polarized light vibrating in the y-axis direction. Here, the optical rotator 6 has wavelengths λl, λ2 . Assuming that the polarization direction of the light of λ3 is rotated by the same angle as shown in the explanation of FIG. λ2. The polarization direction of the light of λ3 is π/2+θ11π at the output point of the optical rotator 6, respectively.
/20θ2, π/2+θ3. Similarly, wavelength λ1. λ2. The polarization directions of the light of λ3 are θl, θ2° and θ3 at the output point of the optical rotator 6, respectively. These lights further enter the biaxial optical crystal 7 and are internally conically refracted. Therefore, in the same way as explained in FIG.
The light incident on the biaxial optical crystal 7 exits from different points on the circumference having a diameter a. Here, the polarizing beam splitter 8
Let us consider the light that passes through and enters the optical rotator 6. In this case, the center of the circle with diameter a is P1, and the point on the circumference when the light travels straight through the biaxial optical crystal 7 is E, and the position where the light exits is the angle measured counterclockwise with the straight line PE as a reference. Expressed as wavelength λ1゜λ2. For light of λ3, π+
2θ1°π+2θ2. It becomes π+2θ3. therefore,
Focusing lenses 12-1 to 12-3 and connecting optical fiber 10
If the system of -1 to 10-3 is set as shown in FIG. 5(a), light of wavelength λl is focused by the focusing lens 12-1 and coupled to the connecting optical fiber 10-1. Next, consider the light reflected by the polarizing beam splitter 8 and the mirror 9 and incident on the optical rotator 6. In this case, the center of the circle with diameter a is defined as the point on the circumference when the Q1 light travels straight through the biaxial optical crystal 7, and the position where the light exits is the angle measured counterclockwise with the straight line QF as a reference. When expressed as wavelength λ1. λ2゜λ3
2θ1, 2θ2, and 2θ3 for each light. Therefore, if the system of connecting optical fibers 11-1 to 11-3 of the focusing lenses 13-1 to 13-3 is set as shown in FIG.
3-1 and coupled to a connecting optical fiber 11-1.

Connection optical fiber 10-1 or connection optical fiber 1
The light with the wavelength λ1 coupled to the connecting optical fiber 10
-1.11-1, and is emitted as a parallel light beam from the collimating lens 14-1 or the collimating lens 15-1. At this time, the light transmitted from the focusing lens 12-1 side to the collimating lens 14-1 side through the connecting optical fiber 10-1 maintains linear polarization,
On the collimating lens 14-1 side, it is assumed that the polarized light is emitted in the X-axis direction. Similarly, focusing lens 1
The light transmitted from the 3-■ side to the collimating lens 15-1 side by the connecting optical fiber 11-I maintains linear polarization, and on the collimating lens 15-■ side, the polarization direction is emitted in the X-axis direction. shall be taken as a thing. Therefore, collimating lens 14-1. Alternatively, collimating lens 15-
Light with a wavelength λl emitted as a parallel beam from the polarizing beam splitter 16 is combined by the polarizing beam splitter 16, focused by the focusing lens 4-1, and coupled to the output optical fiber 2-1.

As explained above, in this embodiment, the input optical fiber 1
The light emitted from the is split into two linearly polarized lights whose polarization directions are orthogonal by the polarizing beam splitter 8, but since both of the lights with the wavelength λl are coupled to the same output optical fiber 2-1, as shown in FIG. There is no increase in insertion loss caused by converting unpolarized input light into linearly polarized light as described in the embodiments.

Further, the light oscillating in the X-axis direction split by the polarizing beam splitter 8 is transmitted to the connecting optical fiber 10-1 or
Since the process in which the light vibrating in the axial direction is demultiplexed to the connecting optical fiber 11-1 is exactly the same as the embodiment shown in FIG. 1, it is clear that it can be extended to an optical demultiplexer with N wavelength multiplexing. Will.

The optical rotator 6 used in the embodiment of FIG. 1 or the embodiment of FIG. 4 for rotating the polarization direction of linearly polarized light is suitably an optically active crystal such as tellurium dioxide or quartz. In some cases, magneto-optic effects can also be used. Also, a polarizing beam splitter 8, which is used to separate light into two orthogonal polarized lights, or conversely, combine them.
Similar effects can be expected by using a birefringent crystal, a Wollaston prism, a Rochon prism, a Savart plate, etc. using a birefringent crystal as the polarizing beam splitter 16.

(Effects of the Invention) As explained above, in the present invention, after directing the polarization direction of linearly polarized light in a predetermined direction using the wavelength window of the wavelength multiplexed light, a biaxial optical crystal is used to change the polarization direction. A biaxial optical crystal has no fixed intrinsic polarization, and linearly polarized light incident from its main optical axis direction is split in different directions depending on the direction of the polarized light. and emit from different points. Therefore, the incident N waves of different wavelengths are due to the dispersion of the optical rotator.

The polarization direction is N directions that differ depending on the wavelength, and the light of each wavelength is output from a separate point from the biaxial optical crystal, forming an N-wave optical demultiplexer. Therefore, the present invention has the advantage that it is possible to realize a small, low-loss, multi-wavelength optical demultiplexer without using special highly processed parts.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of the first embodiment of the present invention in the case of three-wave demultiplexing. FIG. A schematic diagram showing the polarization direction of light, FIG. 3 is a schematic diagram showing the emission position of each wavelength of light on the biaxial optical crystal exit surface of the embodiment shown in FIG. 1, and FIG. FIG. 5 is a diagram showing the arrangement of connecting optical fibers in the embodiment shown in FIG. 4. 1... Input optical fiber, 2... Output optical fiber, 3
... Collimating lens, 4... Focusing lens, 5
... Polarizer, 6... Optical rotator, 7... Biaxial optical crystal, 8... Polarizing beam splitter, 9... Mirror, 1
0... Connection optical fiber, 11... Connection optical fiber,
12... Focusing lens, 3... Focusing lens, 14
... Collimating lens, 15... Collimating lens, 16... Polarizing beam splitter. Patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] In an optical demultiplexer that separates input wavelength-multiplexed light into constituent lights with different wavelengths and outputs them, a polarizer converts each wavelength-multiplexed constituent light into linearly polarized light with the same polarization direction, and the polarized light An optical rotator that rotates the polarization direction of each constituent light linearly polarized by the rotator at different angles corresponding to the difference in wavelength, and a biaxial rotator that rotates the polarization direction of each constituent light beam by a different angle corresponding to the difference in wavelength and outputs from a different position depending on the polarization direction of the incident linearly polarized light. An optical demultiplexer comprising an optical crystal.