JP3108344B2 - Optical filter module - Google Patents

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 which can selectively and variably extract an optical signal having a desired wavelength from a wavelength-multiplexed optical signal propagating in an optical fiber. More particularly, the present invention relates to a polarization independent optical filter module.

[0002]

2. Description of the Related Art An optical filter module of this type is disclosed in Japanese Patent Laid-Open Publication No. Hei 4-140714.

[0003] The optical filter module 100, as shown in FIG. 5, the optical fiber 101 is almost the incident light L ₁ emitted through the collimator lens 102 parallel incident on the birefringent Purisumu 103, the birefringent prism 1
In FIG. 03, due to the P-polarized light and the S-polarized light, the light is split into two lights L _P and L _S , respectively, and only the light L _S passes through the half-wave plate 104 to become P-polarized light. Is incident on the variable wavelength filter 105 filled with liquid crystal. The two light L _P and L _S is a single optical L2 next by a half-wave plate 104 disposed on the incident side and symmetric birefringent prism 103, further condensing lens emitted from the variable wavelength filters 105 The light is guided to the optical fiber 107 on the emission side through the light 106.

[0004] The optical filter module 100 having the above-described configuration provides the light L transmitted through the tunable wavelength filter 105.
_P and L _S are in the same polarization state (both are P-polarized)
Because it is light, even when any light L ₁ polarization is incident on the incident side, without the changing the characteristics of the variable wavelength filter 105, thereby varying the wavelength in response to the voltage from the electronic circuit 108 It is a polarization-free type that can be used.

[0005]

[SUMMARY OF THE INVENTION However, in the conventional optical filter module 100, the light L _P and L _S is transmitted through the tunable wavelength filter 105, for passing the separate optical paths, as shown in FIG. 5, each If the gap d (the distance d between the reflection films in the tunable wavelength filter 105) in the optical path is not exactly the same, for example, as shown in FIG.
Two peaks due to _P and L _S appear, causing a shift in the selected wavelength for each optical path, and the polarization independent type is not obtained.

For this reason, in the conventional optical filter module 100, in order to make the mutual gap d in different optical paths equal to each other, high-precision adjustment of the tunable wavelength filter is required, and high-precision adjustment of the substrate constituting the filter is required. And the substrate cost increases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to make it possible to easily make a polarization independent type without requiring high-precision adjustment of a variable wavelength filter. An object of the present invention is to provide an optical filter module capable of reducing costs.

[0008]

According to the first aspect of the present invention, there is provided an image forming apparatus, comprising:
° variable wavelength filter set at an inclination angle of °, the entrance and exit polarization beam splitters provided respectively corresponding to the other two sides constituting a right-angled triangle with the opposite side of the incident light of the variable wavelength filter as the hypotenuse, A first and a second λ / 2 wavelength plate provided corresponding to the other two sides forming a right triangle having the opposite side of the opposing side of the variable wavelength filter as an oblique side, and the entrance polarization beam splitter. One of the P and S polarized lights of the separated incident light is
A second wavelength plate after passing through the λ / 2 wavelength plate, the other polarized light passing through the same portion as the transmission portion of the variable wavelength filter, and then passing through the second λ / 2 wavelength plate; And a prism disposed so that the P and S polarized lights have the same optical path length and are combined by the exit polarizing beam splitter.

For this reason, in the first aspect of the invention, the P and S polarized lights of the incident light split by the entrance polarization beam splitter enter and exit the same portion of the variable wavelength filter in a crosswise manner from both sides. At this time, one of the P and S polarized lights is transmitted through the first λ / 2 wavelength plate, rotated by 90 degrees, becomes the same polarization component as the other polarized light, and then enters the variable wavelength filter. Loss due to polarization differences is eliminated.

Further, since the respective wavelengths of the P and S polarizations 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 inevitably the same between the two polarizations. It is not necessary to perform high-precision adjustment including high flatness of the filter substrate to make the same between both polarized lights.

Further, since the P and S polarized lights are transmitted through the same portion of the tunable wavelength filter, there is no shift of the optical axis at the time of multiplexing at the exit polarizing beam splitter. Since the optical path lengths are the same, there is no phase shift between the two polarized lights.

According to a second aspect of the present invention, in the optical filter module according to the first aspect, the prism is provided outside the entrance and exit polarization beam splitters and the first and second λ / 2 wavelength plates. It is characterized by being composed of a plurality of triangular prisms arranged closely and mutually closely.

Therefore, according to the second aspect of the present invention, a plurality of triangular prisms can be brought into close contact to form a compact optical path with less light 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 according to 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 Entrance and exit polarization beam splitters 10 and 11 provided on the opposite side 1a side, first and second λ / 2 wavelength plates 12 and 13 provided on the opposite side 1b side of the opposite side 1a of the variable wavelength filter 1, The entrance and exit polarization beam splitters 10 and 11 and the first and second λ /
Prism 1 provided around two wavelength plates 12 and 13
4,15,16,17 Prefecture are largely constituted from, so that it is possible to obtain output light L ₂ in a direction perpendicular to the incident direction of the incident light L _I.

In the tunable wavelength filter 1, as shown in FIG. 2, transparent electrodes 3 and 3 are formed on respective opposing surfaces of a pair of glass substrates 2, and reflection films 4 and 4 are disposed thereon. The alignment films for liquid crystal 5 and 5 are arranged on the reflection films 4 and 4, and the pair of glass substrates 2 and 2 are arranged in parallel with the spacers 9 being maintained at an appropriate distance from each other. A liquid crystal 7 is sealed in between, and antireflection films 6 and 6 are arranged on the outer surfaces of the pair of glass substrates 2 and 2 opposite to the opposing surfaces.

In FIG. 1, reference numeral 8 denotes an electronic circuit for controlling the tunable filter 1, and the circuit 8 controls both transparent electrodes 3 and 3.
The applied voltage between the three is controlled.

This variable wavelength filter 1 has an incident angle of 45 °.
The reflective films 4 and 4 of the substrates 2 and 2 facing each other are designed so that the reflectivity is the same for both P-polarized light and S-polarized light, and the transparent electrode 3 and the reflective film described above. 4, an alignment film 5 for liquid crystal and an anti-reflection film 6 are formed by a conventional method, and a nematic liquid crystal is used as the liquid crystal 7 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 reflection film 4 is formed of TiO ₂ formed on the voltage applying portion of the transparent electrode 3 by vapor deposition. The alignment film 5 for liquid crystal is formed of a multilayered film of SiO ₂ , and is formed of an appropriate alignment treatment agent that is rubbed on the reflection film 4.

The tunable wavelength filter 1 thus formed
Is, 45 ° inclined on the extension of the optical axis L _O of the incident light L _I are provided as opposite sides 1a against the one side incident light L _I.

The entrance and exit polarization beam splitters 10 and 11 are provided respectively corresponding to the other two sides forming a right triangle having the opposite side surface 1a of the variable wavelength filter 1 as an oblique side. In this case the inlet polarizing beam splitter 10 is incident light L _I side (in FIG. 1, the left side of the variable wavelength filter) with provided, the outlet polarizing beam splitter 1
1 is provided on the outgoing light L ₂ side (in FIG. 1, the upper variable-wavelength filter 1).

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

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

More specifically, as shown in FIG. 1, the prisms 14 and 17 are composed of large triangular prisms having the same size.
The prisms 15 and 16 are configured as small triangular prisms having the same size, and the prism 14 has one side 1 of its orthogonal sides.
4a is attached to the lower surface 10a of the entrance polarizing beam splitter 10 by bonding, and the prism 15 is formed by joining one side 15a of the orthogonal side to the other side 14b of the orthogonal side of the prism 14.
And the other side 15b is attached to the lower surface 12a of the first λ / 2 wavelength plate 12 by attaching the other side 15b.
The prism 16 defines one side 16a of the orthogonal sides as a second λ
The prism 17 is attached to the outer surface 13a of the half-wave plate 13 by bonding, and the prism 17 is attached by bonding one side 17a of the orthogonal side to the other side 16b of the orthogonal side of the prism 16. The adhesive used here has the same refractive index as the prisms 14, 15, 16, 17 optically.

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

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

The incident light L _I is input to the entrance polarization beam splitter 1.
0, where it is separated into both polarized lights by reflecting P-polarized light and transmitting S-polarized light. Enters the exit polarizing beam splitter 11 via the optical path P polarized light L _P and S-polarized light L _S after separation shown in FIGS. 3 (a) and (b), both polarizations L _P, L _S are multiplexed here to emitted as outgoing light L _2. The incident light L _I is, for example, is configured to emit from the collimator lens connected to the end of the optical fiber (not shown) incident side, and the emitted light L ₂ is, for example, not shown condenser lens is condensed It is configured to be guided to the optical fiber on the output side.

[0029] That is, P-polarized light L _P is the entrance polarization through the prism 15 after being reflected by the beam splitter 10, the first lambda / 2 wavelength plate 12 as shown in FIG. 3 (a) 9
After being rotated by 0 degrees, the light reaches the exit polarization beam splitter 11 via the variable wavelength filter 1 and passes through the exit polarization beam splitter 11 to be emitted. Here the first
The λ / 2 wavelength 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, as shown in FIG. 3B, the S-polarized light L _S passes through the entrance polarization beam splitter 10 and then passes through the variable wavelength filter 1 to the second λ / 2 wave plate 13 where
After that, the light passes through the prisms 16 and 17 and reaches the exit polarizing beam splitter 11, where the light is reflected and emitted by the exit polarizing beam splitter 11. Here, the second λ / 2 wavelength plate 13 functions to convert the S-polarized light L _S into the same polarization component as the P-polarized light L _P to be reflected by the exit polarization beam splitter 11.

P reaching the exit polarizing beam splitter 11
The polarized light L _P and the S polarized light L _S are multiplexed there and output light L
Emitted as ₂ . At this time, the optical paths of the two polarized lights L _P and L _S correspond to the prisms 14 and 15 and the prism 1 respectively.
6 and 17 are formed so as 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 are incident on the incident point 1 c of the tunable wavelength filter 1 from the back and front, respectively, and from the outgoing point 1 d of the tunable wavelength filter 1. And exits in the reverse direction. That, P-polarized light L _P and S-polarized light L _S is incident on the cross-like from the front and back at the same position 1c of the variable wavelength filter 1 and is emitted from the same point 1d. Therefore, since the wavelength selection of the P-polarized light L _P and S-polarized light L _S is performed at the same position of the variable wavelength filter 1, (see FIG. 2) between the reflective films distance d in the filter 1 at a wavelength selected position both polarizations L It is inevitably the same between _P and L _S.

Further, P-polarized light L _P is a variable wavelength filter 1 after becoming a rotated 90 degrees S-polarized light L _S of the same polarization component by passing through the first lambda / 2 wavelength plate 12
, The loss due to the difference in polarization is eliminated.

Furthermore, P-polarized light L _P and S-polarized light L _S is, deviation of the optical axis when combined at the exit polarizing beam splitter 11 because as each passes through the same portion 1c and 1d of the variable wavelength filter 1 without causing, and both polarizations L _P, since the same optical path length of L _S both polarized light L _P, L
There is no phase shift between _S.

The optical filter module 20 thus formed emits the light L ₂ having the resonance wavelength selected by the variable wavelength filter 1 in the direction orthogonal to the incident light L ₁ . Further, as shown in FIG. 4, the transmission spectrum of the emitted light is of a polarization independent type in which the two polarized lights L _P and L _S are perfectly matched.

The optical filter module 20
The resonance wavelength can be varied 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 easily be of a polarization-independent type without requiring high-precision adjustment of the distance d between the reflection films of the tunable wavelength filter 1, and the wavelength filter 1 can be used. Since high precision flatness of the glass substrate 2 is not required, the cost can be reduced by reducing the substrate cost.

In addition, the optical filter module 20
Since a plurality of triangular prisms 14, 15, 16 and 17 are closely connected to each other to form a compact optical path with less light loss, it is possible to improve the wavelength selection accuracy by a sufficient amount of light and to make the entire device compact. Can be planned.

[0039]

According to the present invention, as described in detail above, the following effects can be obtained.

That is, according to the first aspect of the present invention, one of the P-polarized light and the S-polarized light separated by the entrance polarizing beam splitter is transmitted through the first λ / 2 wave plate and is reflected by the first λ / 2 wave plate.
After being rotated by an angle and turned into the same polarization component as the other polarized light, the light enters the variable wavelength filter, so that loss due to the difference in polarization is eliminated. Therefore, the distance between the reflection films in the filter at the wavelength selection position is inevitably the same between the two polarized lights, and the precision adjustment including the high flatness of the filter substrate for making the distance the same between the two polarized lights is performed. Is unnecessary, and the P and S polarized lights pass through the same portion of the tunable wavelength filter, respectively, so that the optical axis does not shift when multiplexed by the exit polarizing beam splitter, and both polarizations are not changed. Since the optical path lengths are the same, there is no phase shift between the two polarizations, so that the polarization-independent type can be easily achieved without the need for high-precision adjustment of the variable wavelength filter. It is possible to provide an optical filter module can also be achieved strike reduced.

Further, according to the second aspect of the present invention, a plurality of triangular prisms can be brought into close contact to form a compact optical path with less light loss. As a result, the effect of the first aspect of the present invention can be obtained. In addition, it is possible to provide an optical filter module capable of improving the wavelength selection accuracy by a sufficient amount of light and reducing the size of the entire apparatus.

[Brief description of the drawings]

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

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

FIG. 3 shows an optical path of the optical filter module of FIG. 1,
(A) is an optical path diagram of P-polarized light, and (b) is an optical path diagram of S-polarized light.

FIG. 4 is a transmission spectrum diagram of the optical filter module of FIG. 1;

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. 5;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Variable wavelength filter 1a Opposite side surface (of a variable wavelength filter) 1b Opposite side surface (of a variable wavelength filter) 10 Inlet polarization beam splitter 11 Outlet polarization beam splitter 12 First λ / 2 wave plate 13 Second λ / 2 wave plate 14, 15, 16, 17 prism 20 an optical filter module L ₁ incident light L ₂ emitted light L _P P-polarized light L _S S polarization θ inclination angle (of the variable wavelength filter)

Claims (2)

(57) [Claims] 1. A variable wavelength filter which is set to be inclined at an angle of 45 ° with respect to the optical axis of incident light, and two other sides forming a right-angled triangle having oblique sides on opposite sides of the incident light of the variable wavelength filter. The entrance and exit polarization beam splitters provided correspondingly, and the first and second polarization beam splitters provided respectively corresponding to the other two sides forming a right-angled triangle having an oblique side opposite to the opposite side of the variable wavelength filter. Λ / 2 wavelength plate, and P of incident light separated by the entrance polarization beam splitter.
And one of the S-polarized light passes through the variable wavelength filter after passing through the λ / 2 wavelength plate, and the other polarized light passes through the same portion of the variable wavelength filter as the transmission portion, and And a prism disposed 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 outside 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 in parallel.