JPH09113859A - Optical filter module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily make it a polarization independent type and to reduce a cost without precisely adjusting a variable wavelength filter. SOLUTION: This module is constituted so that one side polarization Lp of incident light L1 separated by an inlet polarizing beam splitter 10 transmits through a variable wavelength filter 1 obliquely arranged by 45 deg. after transmitting through a λ/2 wavelength plate 12, and the other side S polarization Ls transmits through a second λ/2 wavelength plate 13 after transmitting through the same position as the transmission position of the variable wavelength filter 1, and the P and S polarizations Lp, Ls are provided with the same optical path length, and are multiplexed by an outlet polarizing beam splitter 11. Thus, outgoing light L2 with a transmission spectrum of a single peak coincident with both polarizations Lp, Ls with each other is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical filter module capable of selectively extracting an optical signal having an arbitrary desired wavelength from a wavelength-multiplexed optical signal propagating in an optical fiber and varying the wavelength. More specifically, the present invention relates to a polarization-independent optical filter module.

[0002]

2. Description of the Related Art As an optical filter module of this type, there is one disclosed in Japanese Patent Application Laid-Open No. 4-140714.

In this optical filter module 100, as shown in FIG. 5, incident light L ₁ emitted from an optical fiber 101 through a collimating lens 102 becomes substantially parallel and enters a birefringent prism 103, and this birefringent prism is used. 1
Due to the P-polarized light and the S-polarized light in 03, the light is split into two lights L _P and L _S , respectively, and only the light L _S passes through the ½ wavelength plate 104 to become P-polarized light, resulting in two P-polarized lights. Is incident on the variable wavelength filter 105 filled with liquid crystal. Then, the two lights L _P and L _S emitted from the variable wavelength filter 105 become one light L2 by the ½ wavelength plate 104 and the birefringent prism 103 which are arranged symmetrically with respect to the incident side, and further the condensing lens. The optical fiber 107 on the emission side is guided through 106.

The optical filter module 100 having the above-described structure is capable of transmitting the light L transmitted through the variable wavelength filter 105.
_P and L _S have the same polarization state (both have P polarization)
Therefore, no matter what polarization light L ₁ is incident on the incident side, the wavelength is tuned according to the voltage from the electronic circuit 108 without changing the characteristics of the tunable wavelength filter 105. It is a polarization-independent type that can perform.

[0005]

However, in the conventional optical filter module 100, the lights L _P and L _S passing through the variable wavelength filter 105 pass through different optical paths as shown in FIG. If the gap d in the optical path (the distance d between the reflection films in the tunable wavelength filter 105) is not completely the same, for example, as shown in FIG.
Two peaks due to _P and L _S appear, and a shift occurs in the selected wavelength for each optical path, which does not result in a polarization independent type.

Therefore, in the conventional optical filter module 100, in order to make the mutual gaps d in the different optical paths the same, it is necessary to adjust the tunable wavelength filter with high accuracy, and the substrate forming the filter has high accuracy. However, there is a problem in that the flatness is required and the substrate cost increases.

The present invention has been made to solve the above-mentioned problems, and an object thereof is to make it possible to easily make a polarization-independent type without requiring highly accurate adjustment of a tunable wavelength filter. An object of the present invention is to provide an optical filter module that can reduce cost.

[0008]

In order to achieve the above-mentioned object, the invention according to claim 1 is 45 with respect to the optical axis of incident light.
A tunable wavelength filter set at an inclination angle of °, entrance and exit polarization beam splitters provided corresponding to the other two sides forming a right triangle whose oblique side is the opposite side of incident light of the tunable wavelength filter, The first and second λ / 2 wave plates provided in correspondence with the other two sides forming a right triangle having the opposite side surface of the opposite side surface of the variable wavelength filter as the hypotenuse, and the entrance polarization beam splitter. One of the P and S polarizations of the separated incident light is λ
After passing through the 1/2 wavelength plate, the variable wavelength filter is transmitted, and the other polarization is transmitted through the second λ / 2 wavelength plate after passing through the same portion as the transmission portion of the variable wavelength filter, In addition, the P and S polarized lights have the same optical path length and are arranged so as to be combined by the exit polarization beam splitter.

Therefore, according to the first aspect of the present invention, the P and S polarized lights of the incident light separated by the entrance polarization beam splitter enter and exit the same position of the variable wavelength filter from the front and back in a crossed manner. At this time, one of the P and S polarizations is transmitted through the first λ / 2 wave plate and rotated by 90 degrees to become the same polarization component as the other polarization, and then enters the variable wavelength filter. There is no loss due to polarization difference.

Further, since the wavelengths of P-polarized light and S-polarized light can be selected at the same position of the variable wavelength filter, the distance between the reflection films in the filter at the wavelength selection position is necessarily the same for both polarized lights, and the distance is It is not necessary to perform high-precision adjustment including high flatness of the filter substrate to make both polarizations the same.

Further, since the P and S polarized lights respectively pass through the same part of the variable wavelength filter, there is no deviation of the optical axis upon combining at the exit polarization beam splitter, and both polarized lights are Since the optical path lengths are the same, there is no phase shift between both polarized lights.

The invention according to claim 2 is the optical filter module according to claim 1, wherein the prism is provided outside the entrance and exit polarization beam splitters and the first and second λ / 2 wave plates. It is characterized by comprising a plurality of triangular prisms arranged in close contact with each other.

Therefore, according to the second aspect of the present invention, a plurality of triangular prisms can be brought into close contact with each other to form a compact optical path with little optical loss.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be specifically described based on the illustrated embodiments.

FIG. 1 shows an optical filter module 20 as one embodiment.

[0016] The optical filter module 20 includes a variable wavelength filter 1 inclination θ is set to be inclined so as to be 45 ° with respect to the optical axis L _O of the incident light L _I, of the incident light L _I of the variable wavelength filter 1 Inlet and outlet polarization beam splitters 10 and 11 provided on the side face 1a opposite to each other, first and second λ / 2 wave plates 12 and 13 provided on the side face 1b opposite to the side face 1a opposite to the variable wavelength filter 1, Inlet and outlet polarization beam splitters 10, 11 and first and second λ /
Prism 1 provided around the two wavelength plates 12 and 13
4, 15, 16, and 17, the output light L ₂ in a direction orthogonal to the incident direction of the incident light L _I can be obtained.

In the tunable wavelength filter 1, as shown in FIG. 2, transparent electrodes 3 and 3 are formed on the opposing surfaces of a pair of glass substrates 2, and reflective films 4 and 4 are arranged on the transparent electrodes 3 and 3, and further, Alignment films 5 and 5 for liquid crystals are arranged on the reflection films 4 and 4, and a pair of glass substrates 2 and 2 are arranged in parallel with a spacer 9 maintaining an appropriate interval. A liquid crystal 7 is sealed between the glass substrates 2, and the antireflection films 6 and 6 are arranged on the outer surfaces of the pair of glass substrates 2 and 2 opposite to the facing surfaces.

In FIG. 1, reference numeral 8 is an electronic circuit for controlling the tunable wavelength filter 1.
The applied voltage between 3 is controlled.

The variable wavelength filter 1 has an incident angle of 45 °.
The reflection films 4 and 4 of the substrates 2 and 2 facing each other are designed so that the reflectances of the P polarized light and the S polarized light are the same, and the transparent electrode 3 and the reflection film 4, the alignment film 5 for liquid crystal, and the antireflection film 6 are formed by a conventional method, and the liquid crystal 7 is a nematic liquid crystal as in the conventional case.

For example, the transparent electrode 3 is formed by sputtering indium tin oxide on the opposite surface of the glass substrate 2, and the reflective film 4 is formed of TiO ₂ formed on the voltage application portion of the transparent electrode 3 by a vapor deposition method. The alignment film 5 for liquid crystal is formed of a multi-layered film of SiO ₂ , and is formed by an appropriate alignment treatment agent which is rubbed on the reflection film 4.

Variable wavelength filter 1 formed in this way
Is installed at an angle of 45 ° on the extension line of the optical axis L _O of the incident light L _I, with one side surface thereof serving as the opposite side surface 1 a for the incident light L _I.

The entrance and exit polarization beam splitters 10 and 11 are provided so as to correspond to the other two sides forming a right triangle whose oblique side is the facing side surface 1a of the tunable wavelength filter 1. At this time, the entrance polarization beam splitter 10 is provided on the incident light L _I side (on the left side of the variable wavelength filter in FIG. 1), and the exit polarization beam splitter 1 is provided.
1 is provided on the output light L ₂ side (the upper side of the tunable wavelength filter 1 in FIG. 1).

Further, the first and second λ / 2 wave plates 12 and 13 respectively correspond to the other two sides forming a right triangle having the opposite side surface 1b of the tunable wavelength filter 1 opposite to the opposite side surface 1a. It is provided. At this time, the first λ / 2
The wave plate 12 is provided below the tunable wavelength filter 1 in FIG. 1, and the second λ / 2 wave plate 13 is provided on the right side of the tunable wavelength filter 1 in FIG.

Further, the prisms 14, 15, 16 and 1
Reference numeral 7 denotes one of the P and S polarizations of the incident light L _I separated by the entrance polarization beam splitter 10 after passing through the first λ / 2 wave plate 12 and then the tunable wavelength filter 1 and the other. The polarized light passes through the same position as the transmission part of the variable wavelength filter 1 and then through the second λ / 2 wave plate 13, and both polarizations have the same optical path length, and the exit polarization beam splitter 11 It is arranged so as to be multiplexed.

Specifically, as shown in FIG. 1, the prisms 14 and 17 are large triangular prisms having the same size.
The prisms 15 and 16 are small triangular prisms having the same size, and the prism 14 is one of the orthogonal sides.
4a is attached to the lower surface 10a of the entrance polarization beam splitter 10 by adhesion, and the prism 15 has one side 15a of the orthogonal sides of the prism 15 which is the other side 14b of the orthogonal side of the prism 14.
Is attached to the lower surface 12a of the first λ / 2 wave plate 12, and the other side 15b is attached.
The prism 16 has one side 16a of the orthogonal sides of the second λ
The prism 17 is attached to the outer side surface 13a of the / 2 wave plate 13 by adhering, and the prism 17 is attached by adhering one side 17a of the orthogonal sides thereof to the other side 16b of the orthogonal side of the prism 16. The adhesive used at this time has the same refractive index as that of the prisms 14, 15, 16 and 17 optically.

The tunable wavelength filter 1, the beam splitters 10 and 11, the λ / 2 wavelength plates 12 and 13, and the prisms 14, 15, 16 and 17 described above are optically precise while maintaining the mutual positional relationship described above. It is installed in.

Next, the operation of the optical filter module 20 will be described with reference to FIG.

The incident light L _I is transmitted through the entrance polarization beam splitter 1
It is incident on 0, where P-polarized light is reflected and S-polarized light is transmitted to be separated into both polarized lights. The separated P-polarized light L _P and S-polarized light L _S enter the exit polarization beam splitter 11 via the optical paths shown in FIGS. 3 (a) and 3 (b), respectively, where both polarizations L _P and L _S are combined. The emitted light L ₂ is emitted. The incident light L _I is configured to be emitted from, for example, a collimator lens connected to the end of an incident-side optical fiber (not shown), and the emitted light L ₂ is condensed by, for example, a condenser lens (not shown). It is configured to be guided to the optical fiber on the output side.

That is, the P-polarized light L _P passes through the prisms 14 and 15 after being reflected by the entrance polarization beam splitter 10 as shown in FIG.
After being rotated by 0 °, it reaches the exit polarization beam splitter 11 via the variable wavelength filter 1, and passes through the exit polarization beam splitter 11 and is emitted. Here the first
The λ / 2 wave plate 12 functions to convert the P-polarized light L _P into the same polarization component as the S-polarized light L _S.

On the other hand, the S-polarized light L _S passes through the entrance polarization beam splitter 10 and then passes through the tunable wavelength filter 1 as shown in FIG.
It is rotated by a degree, then passes through the prisms 16 and 17, reaches the exit polarization beam splitter 11, and is reflected and emitted by the exit polarization beam splitter 11. Here, the second λ / 2 wave plate 13 functions to convert the S-polarized light L _S into the same polarization component as the P-polarized light L _{P in} order to be reflected by the exit polarization beam splitter 11.

P reaching the exit polarization beam splitter 11
The polarized light L _P and the S polarized light L _S are combined there, and the emitted light L
Emits as ₂ . The optical paths of both polarizations L _P and L _S at this time are the prisms 14 and 15 and the prism 1 respectively.
6 and 17 are formed to have the same optical path length.

In such an optical path, the P-polarized light L _P and the S-polarized light L _S enter the entrance 1c of the tunable wavelength filter 1 from the back and front directions, respectively, and exit from the exit 1d of the tunable wavelength filter 1 respectively. And emits to the back direction. That is, the P-polarized light L _P and the S-polarized light L _S are incident on the same location 1c of the tunable wavelength filter 1 in a cross shape from the front and back, and are emitted from the same location 1d. Therefore, since the wavelengths of the P-polarized light L _P and the S-polarized light L _S can be selected at the same position of the tunable wavelength filter 1, the distance d between the reflecting films (see FIG. 2) in the filter 1 at the wavelength selection position is equal to that of the polarized light L. Inevitably the same between _P and L _S.

Further, the P-polarized light L _P is rotated through 90 ° by passing through the first λ / 2 wave plate 12 to become the same polarization component as the S-polarized light L _S and then the tunable wavelength filter 1
Since it is incident on, the loss due to the difference in polarization is eliminated.

Further, since the P-polarized light L _P and the S-polarized light L _S are transmitted through the same portions 1c and 1d of the tunable wavelength filter 1, respectively, the optical axes are deviated when they are combined by the exit polarization beam splitter 11. Is not generated and the optical path lengths of both polarizations L _P and L _S are the same, so both polarizations L _P and L S
There is no phase shift between _S.

In the optical filter module 20 formed in this way, the emitted light L ₂ having the resonance wavelength selected by the variable wavelength filter 1 is emitted in the direction orthogonal to the incident light L ₁ . Moreover, the transmission spectrum of the emitted light is a polarization-independent type in which both polarizations L _P and L _S are perfectly matched as shown in FIG.

The optical filter module 20
The resonant wavelength can be tuned by changing the refractive index of the liquid crystal 7 by applying a voltage between the transparent electrodes 3 and 3 by the control electronic circuit 8 in the tunable wavelength filter 1. Can be selected.

As described above, the optical filter module 20 can be easily made into the polarization independent type without the need for highly accurate adjustment of the distance d between the reflection films of the tunable wavelength filter 1, and the wavelength filter 1 can be used. Since the glass substrate 2 constituting the glass substrate 2 is not required to have a high degree of flatness, it is possible to reduce the cost by reducing the substrate cost.

Moreover, this optical filter module 20
Since a plurality of triangular prisms 14, 15, 16 and 17 are brought into close contact with each other to form a compact optical path with little optical loss, it is possible to improve the wavelength selection accuracy with a sufficient amount of light and to reduce the size of the entire measure device. Can be planned.

[0039]

As described in detail above, according to the present invention, the following effects can be obtained.

That is, according to the first aspect of the present invention, one of the P and S polarizations separated by the entrance polarization beam splitter is transmitted through the first λ / 2 wave plate and the
After being rotated by a degree to become the same polarization component as the other polarization, it enters the tunable wavelength filter, so there is no loss due to the difference in polarization. P and S polarization wavelength selection is the same position of the tunable wavelength filter. Therefore, the distance between the reflection films in the filter at the wavelength selection position is necessarily the same for both polarizations, and high precision adjustment including high flatness of the filter substrate for making the distance the same for both polarizations. Is unnecessary, and since the P and S polarizations respectively pass through the same part of the tunable wavelength filter, there is no deviation of the optical axis when combining at the exit polarization beam splitter, and both polarizations are Since the optical path length of is the same, there is no phase shift between the two polarizations, which makes it possible to easily make a polarization independent type without requiring highly accurate adjustment of the tunable wavelength filter. It is possible to provide an optical filter module can also be achieved strike reduced.

According to the second aspect of the present invention, a plurality of triangular prisms are brought into close contact with each other to form a compact optical path with little optical loss. As a result, the effect of the first aspect of the invention is provided. In addition to this, it is possible to provide an optical filter module capable of improving the wavelength selection accuracy with a sufficient amount of light and downsizing the entire apparatus.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an optical filter module as an embodiment of the present invention.

FIG. 2 is a sectional view of a variable wavelength filter used in the optical filter module of FIG.

3 shows the optical path of the optical filter module of FIG.
(A) is an optical path diagram for P-polarized light, and (b) is an optical path diagram for S-polarized light.

FIG. 4 is a transmission spectrum diagram of the optical filter module of FIG.

FIG. 5 is a schematic configuration diagram of a conventional optical filter module.

FIG. 6 is a transmission spectrum diagram showing a problem of the optical filter module of FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Variable wavelength filter 1a Opposite side surface (of variable wavelength filter) 1b Opposite side surface (of variable wavelength filter) 10 Entrance polarization beam splitter 11 Exit polarization beam splitter 12 First λ / 2 wave plate 13 Second λ / 2 wave plate 14, 15, 16, 17 Prism 20 Optical filter module L ₁ Incident light L ₂ Outgoing light L _P P polarized light L _S S polarized light θ tilt angle (of variable wavelength filter)

Claims (2)

[Claims] 1. A tunable wavelength filter set at an angle of inclination of 45 ° with respect to the optical axis of incident light, and two other sides forming a right triangle whose oblique side is the side facing the incident light of the tunable wavelength filter. Inlet and outlet polarization beam splitters respectively provided correspondingly, and first and second first and second sides provided respectively corresponding to other two sides forming a right triangle having an opposite side surface of the opposite side surface of the variable wavelength filter as a hypotenuse. Λ / 2 wave plate of P and the P of the incident light split by the entrance polarization beam splitter.
And one of the S-polarized light is transmitted through the λ / 2 wavelength plate and then through the variable wavelength filter, and the other polarized light is transmitted through the same portion as the transmission portion of the variable wavelength filter and then the second And a prism arranged so that the P and S polarized lights have the same optical path length and are combined by the exit polarization beam splitter. Filter module. 2. The optical filter module according to claim 1, wherein the prism is adhered to the outsides of the entrance and exit polarization beam splitters and the first and second λ / 2 wave plates and is in close contact with each other. An optical filter module comprising a plurality of triangular prisms arranged as a unit.