JPH0836157A - Variable wavelength filter - Google Patents

Abstract

PURPOSE:To provide a variable wavelength filter which is capable of selecting plural light signals of arbitrary wavelengths and frequencies and arbitrarily setting transmission spectral shapes. CONSTITUTION:This variable wavelength filter is formed by providing both surfaces of a double refractive plate 32 with quarter-wave plates 33, 34 and further, providing one thereof with a reflection surface 35 and the other with a Fabry-Perot interference type filter 36 having plural transmission spectra and a polarized light control reflection element array 37 capable of arbitrarily controlling the angle of the plane of polarization. The rotating angle of the plane of polarization given by the polarized light control reflection element array 37 for the light of every transmission spectrum of the Fabry-Perot interference type filter 36 is controlled, by which the wavelength of the light returning to one end side of the light entering from one end of the double refractive plate 32 and its ratio are changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable wavelength filter capable of selectively extracting optical signals of arbitrary plural wavelengths / frequencies from wavelength / frequency multiplexed optical signals and arbitrarily setting the shape of transmission spectrum. It is about.

[0002]

2. Description of the Related Art Optical communication using an optical fiber has been rapidly put into practical use in recent years because it can transmit a large amount of information at high speed. At present, only optical pulses of a specific wavelength / frequency are transmitted, but if a large number of optical pulses of different wavelength / frequency can be transmitted, it is possible to transmit a larger amount of information. . This is called wavelength division multiplexing (WDM) / frequency division multiplexing (FDM) communication and is currently being actively researched. In the wavelength / frequency multiplex communication, a variable wavelength filter serves as a key device for selectively extracting only optical pulses of arbitrary wavelength / frequency from a large number of optical pulses of different wavelength / frequency. Therefore, various variable wavelength filters have been studied and put to practical use.

However, most of these have been able to extract only one wavelength / frequency optical signal from the wavelength / frequency multiplexed optical signals. Further, the shape of the transmission spectrum is determined by the principle of the filter, and it is impossible to form a light filter having an arbitrary transmission spectrum like an electric filter. In the future wavelength / frequency multiplex communication, a variable wavelength filter that can extract a plurality of optical signals of arbitrary wavelengths and frequencies and set an arbitrary transmission spectrum becomes very important. For example, it is expected that a future exchange will be an optical exchange that directly exchanges light, but among them, a multi-channel filter that can simultaneously select optical signals of a plurality of arbitrary wavelengths and frequencies becomes important.

FIG. 1 shows the concept of wavelength optical switching, in which 1 is a plurality of input optical fibers, 2 is a plurality of optical branching devices,
3 is a plurality of variable wavelength filters, 4 and 7 are a plurality of wavelength conversion elements, 5 is a plurality of optical couplers, 6 is a plurality of multi-channel variable wavelength filters, 8 is a plurality of optical multiplexers, and 9 is a plurality of output optical fibers. Is.

The wavelength-multiplexed optical signal sent from the subscriber via the input optical fiber 1 is demultiplexed by the optical branching device 2, selected by the variable wavelength filter 3, and the wavelength conversion element 4 is used by each subscriber. Is converted into light having a wavelength corresponding to, and wavelength-multiplexed by the optical coupler 5. Optical switching in the wavelength region is realized by selecting a plurality of wavelengths from the wavelength-multiplexed optical signal with the multi-channel variable wavelength filter 6 and repeating the next wavelength conversion with the wavelength conversion element 7.

A filter whose transmission spectrum shape can be arbitrarily controlled is very important for correcting the wavelength dependence of optical components. For example, an Er-doped optical fiber amplifier has a large wavelength dependence in its optical amplification characteristic. Therefore, there is only a band of 2 nm at 1.536 μm, and
There is only a band of 4 nm at 1.552 μm. By placing an optical filter that corrects this amplification characteristic in the subsequent stage of the optical fiber amplifier, if the band can be expanded to several tens of nm, it becomes possible to multiplex more wavelengths.

[0007]

A photoacoustic filter is known as a conventional filter capable of simultaneously extracting optical signals of arbitrary plural wavelengths and frequencies. The photoacoustic filter can be selected by changing the wavelength by changing the frequency of the applied high-frequency voltage, and can simultaneously select the plurality of wavelengths by applying a plurality of high-frequency voltages at once. However,
When the applied voltage becomes multiple, the heat generation of the element increases,
The number of wavelengths that can actually be selected was only about 5 (for example, ECOC, 89, Gothenburg, Sweeden, Sept.,
1989, KW Cheung et a1., Pp10-14).

Further, the electrodes of the liquid crystal variable wavelength filter in which the liquid crystal is filled in the Fabry-Perot etalon are finely separated,
There is also a report that the light beam is expanded so as to pass through these and a plurality of wavelengths are selected, but the number of selected wavelengths is about four at most, and there is a problem that the loss increases as the number of wavelengths increases ( For example, IEEE Phon. Tech. Lett.,
vol.3, 1991, JSPatel and MWMaeda, pp.643-64
Four).

Further, a multi-channel filter in which a Mach-Zehnder interferometer with an optical waveguide and a phase shifter are combined has been reported (see, for example, Technical Report, pst9.
1-46, Sasayama, Okuno, Habara) had a problem that the loss increased as the number of selected frequencies increased and the driving of the element became complicated.

In view of the above-mentioned conventional problems, the present invention can select a plurality of optical signals of arbitrary wavelengths and frequencies without loss,
Moreover, it is an object of the present invention to provide a tunable wavelength filter whose transmission spectrum shape can be arbitrarily set.

[0011]

In the present invention, in order to achieve the above object, in a variable wavelength filter for selectively extracting an optical signal of an arbitrary wavelength from an optical signal, a birefringent plate serving as a transmission medium of the optical signal, Polarization rotating means for sandwiching the birefringent plate and rotating the plane of polarization of the optical signal by a predetermined angle, a filter for selecting an optical signal of an arbitrary wavelength from the optical signal, and a polarization capable of controlling the polarization rotation angle of an arbitrary portion. We propose a variable wavelength filter that includes a control element and a reflecting surface that is arranged so as to sandwich the entire element.

[0012]

When a linearly polarized light beam is vertically incident on the birefringent plate, the optical path of the light beam in the birefringent plate differs depending on the polarization direction of the light beam.

For example, as shown in FIG. 2, the birefringent plate 10
(10a is the direction of the optical axis), the direction perpendicular to the paper surface (in the figure,
Shown with ●. When a light beam 12 having an electric field vibration is incident on the light) 11, the light beam 12 travels straight in the birefringent plate 10.
This light beam 12 is called an ordinary ray (a ray that follows the law of refraction). Further, the light beam 14 having electric field oscillation in a direction (indicated by an arrow in the figure) 13 parallel to the paper surface is the birefringent plate 1.
Go diagonally through 0. This light beam 14 is called an extraordinary ray (a ray that does not follow the law of refraction). Therefore, by controlling the polarization direction of the incident light beam, the optical path of the light in the birefringent plate can be selected to be straight or oblique.

As shown in FIG. 3, when polarization rotation reflecting means 21 and 22 for rotating and reflecting the polarized light of light by 90 ° are provided on both sides of the birefringent plate 20, the light incident from one end of the birefringent plate 20 is provided. A light transmission mechanism that guides the light to the other end can be realized. That is, when a normal ray of light 24 having electric field vibration is incident from a point A on one surface in a direction (indicated by ● in the figure) 23 perpendicular to the paper surface,
The light 24 goes straight through the birefringent plate 20 and reaches point B on the other surface. Then, the light 24 is reflected by the polarized light rotating / reflecting means 21, is returned to the inside of the birefringent plate 20, and is rotated by 90 ° in the plane of polarization.

As a result, the light 24 is converted into an extraordinary ray having electric field oscillation in a direction (indicated by an arrow in the figure) 25 parallel to the paper surface (since the planes of polarization of the ordinary ray and the extraordinary ray are orthogonal to each other). ), As described above, the inside of the birefringent plate 20 is obliquely advanced to proceed from the point B to the point C. Further, here, the light is reflected by the polarization rotating / reflecting means 22 in the same manner as described above and is rotated by 90 ° in the plane of polarization, and becomes an ordinary ray again. Therefore, this time, go straight on the birefringent plate 20 toward the point D. Similarly, light is sequentially reflected on both surfaces of the birefringent plate 20 and travels rightward on the paper surface to be output (26).

The above-mentioned reflection mechanism can be realized by two mirrors facing each other, but if the parallelism of the mirrors is perfect and the distance between the mirrors is not controlled to accurately control the angle of incident light, the reflection repetition pitch is changed. It cannot be controlled accurately.
It is difficult to perform such three-dimensional alignment. In this respect, since the birefringent plate is a single crystal, the crystal plane can be utilized, it is easy to realize perfect parallelism, and the repeating pitch is determined by its thickness, but it is necessary to control the thickness. It's easy. It is easy because the angle of incident light may be perpendicular to the birefringent plate. in this way,
By adopting this structure, alignment becomes easy.

Next, in the light transmission mechanism in which the birefringence plate and the polarization rotation reflection means are combined, as shown in FIG. 4, between the birefringence plate 20 and the polarization rotation reflection means 21,
A polarization rotation means 27 and a filter, for example, a Fabry-Perot interference type filter 28 are arranged, and the rotation angle of the polarization plane of a part of the polarization rotation reflection means 21, here the part 21a corresponding to point D is changed by some method to 0 °. Or 180 °
Is an integral multiple of.

In this case, the light incident on the point D is converted into circularly polarized light by the polarization rotating means 27 and reaches the Fabry-Perot interference type filter 28. When the incident light coincides with the transmission wavelength of the Fabry-Perot interference type filter 28, the light passes through the Fabry-Perot interference type filter 28 and reaches the polarization rotation reflection means 21.

Here, the portion 21 of the polarized light rotating / reflecting means 21.
Since the rotation angle of the polarization plane of a is 0 ° or an integral multiple of 180 °, the polarization state of the light reflected by the polarization rotation reflection means 21 does not change, and is incident on the Fabry-Perot interference filter 28 again. Will be done. As previously mentioned,
Since this light matches the transmission wavelength of the Fabry-Perot interference type filter 28, the Fabry-Perot interference type filter 2
After passing through 8, the polarized light is converted into linear polarized light by the polarization rotation means 27, but since the direction of the polarized light is orthogonal to the polarized light when it is first incident on the point D, it becomes abnormal light, Proceed toward point E.

On the other hand, when the rotation angle of the polarization plane of the polarization rotation reflection means 21 is an integral multiple of 90 °, the light passing through the Fabry-Perot interference filter 28 and the polarization rotation means 27 is converted into linearly polarized light as described above. However, the direction of the polarization is D
It becomes parallel to the polarized light when it is first incident on the point, that is, it becomes ordinary light, and this time it advances toward point C and exits at the incident point A (2
9) It will be done.

At this time, when the spectrum of the input light 24 is white light (24a), the spectrum of the output light 29 becomes the same spectrum (29a) as that which has passed through the Fabry-Perot interference type filter 28 twice, resulting in a sharp transmission. A spectrum is obtained. Therefore, the filter characteristic is improved as compared with the case of single pass.

When the wavelength of the light incident on the point D does not match the transmission wavelength of the Fabry-Perot interference filter 28, the Fabry-Perot interference filter 28 simply functions as a mirror and reflects the light. As a result, it becomes extraordinary light, and the light travels in the direction of point E.

When the wavelength of the light incident on the point D coincides with the transmission wavelength of the Fabry-Perot interference filter 28,
The rotation angle of the polarization plane of the polarization rotation reflection means 21 is 0 °, 90
If it is set to a value other than an integral multiple of 90 ° such as ° or 180 °, ordinary light and extraordinary light are mixed, and the ratio of light emitted to points C and E can be arbitrarily changed. Therefore,
The transmittance of the transmission spectrum of the light obtained on the emission side can be set arbitrarily.

Further, by changing the cavity gap or cavity loss of the Fabry-Perot interference filter 28 at the points B, D, ... Or the reflectance of the mirror, its filter characteristics (transmission spectrum wavelength, transmittance,
Bandwidth) can be changed. Also, by repeating the above-mentioned operation at points B, C, D, E, ...
By controlling the polarized light rotating / reflecting means at each point, an arbitrary sum of the filter characteristics at each point can be obtained at the emission point. Hereinafter, a specific description will be given according to examples.

[0025]

5 and 6 show a first embodiment of the present invention, in which 30 is a polarization beam splitter, 31 is a Faraday rotator, 32 is a birefringent plate, and 33 and 34 are 1/4. A wave plate (λ / 4 plate), 35 is a reflecting surface, 36 is a Fabry-Perot interference type filter, 37 is a polarization control reflecting element array, 41
Is input light, 42 is output light, and 43 is a line indicating the path of the light beam. Further, A to J are light beam entrance / exit points on the surface of the birefringent plate 32, and B ′ to J ′ are λ / 4 plates 33 and 3.
4, points B ″ to J ″ indicate points advanced to the Fabry-Perot interferometer filter 36. Further, X is an incident point of the light beam, Y is an emission point of the light beam, a solid arrow is an arrow indicating the propagation direction of the forward light beam, and a broken arrow is an arrow indicating the propagation direction of the return light beam. Calcite is used as the birefringent plate 32, the thickness thereof is 4.6 mm, and the beam pitch is 0.5 mm.

FIG. 7 shows the structure of the Fabry-Perot interference filter 36 and its transmission spectrum at points B'-J '. In the figure, 361 is a thin glass substrate, which is polished in a tapered shape. 362 and 363 are dielectric mirrors. Assuming that the thickness of the glass substrate 361 at the point B'is L and the refractive index is n, the transmission peak wavelength at the point B'of the present filter 36 is λ _m = 2 nL / m (1). However, m is an integer, and a plurality of transmission peaks corresponding to m appear.

The peak interval is called FSR.
Is FSR _m = λ _m / (m + 1) (2) When the glass substrate 361 is tapered, the transmission spectrum differs depending on the points B ′ to J ′, as shown in the figure. It can be seen that as the cavity gap increases, the transmission spectrum wavelength gradually shifts to longer wavelengths.

For example, L = 70 μm, n = 1.5, m = 13
In case of 5, λ ₁₃₅ is 1.55μm and FSR is about 11n
m. Here, if the cavity gap of the Fabry-Perot etalon is not perfectly parallel and has a taper of 1 μm / cm, the transmission peak wavelength increases by FSR / 5, that is, 2.2 nm when the cavity gap increases.
The bandwidth of the spectrum depends on the reflectance of the mirror and the surface accuracy of the substrate, but normally, a mirror having a reflectance of about 99%, λ
With a surface accuracy of / 20, the spectral width is approximately 0.036 nm.
(4.5 GHz), finesse (= FSR / spectral width) of about 300, and transmittance of 90%.

In FIG. 5, the p-polarized input light 41 passes through the polarization beam splitter 30 and passes through the Faraday rotator 31.
Incident on. The polarization plane of the light is rotated by 45 ° by the Faraday rotator 31, and enters the birefringent plate 32. At the time of incidence on the birefringent plate 32, the polarization direction of the incident light is a direction perpendicular to the paper surface, and since it behaves as an ordinary ray, it travels straight through the birefringent plate 32 and enters the λ / 4 plate 33 from point B. ,
Reach point B '.

When the wavelength of the incident light does not match the transmission wavelength of the Fabry-Perot interference type filter 36 at the point B ', the Fabry-Perot type interference filter 36 is 100%.
It acts as a reflecting surface with a close reflectivity and the light is reflected to point B. At this time, since it passes through the λ / 4 plate twice, it is equivalent to passing once through the ½ wavelength plate (λ / 2 plate), and the polarization plane of the light rotates by 90 °. Therefore, the light incident on the birefringent plate 32 becomes extraordinary light and reaches the point C. Similarly at point C, it is reflected by the λ / 4 plate 34 and the reflecting surface 35,
After that, the reflection is repeated from point D, point E, point F, ....

However, when the wavelength of the incident light coincides with the transmission wavelength of the Fabry-Perot interference type filter 36,
Not limited to this. For example, when the wavelength of the light matches the wavelength of the Fabry-Perot interference type filter 36 at the point D ′, the light passes through the Fabry-Perot interference type filter 36, and D ″
After reaching the point, the light enters the polarization control reflective element array 37.

The structure of the polarization control reflection element array 37 is shown in FIG.
Shown in The polarization control reflective element array 37 is a one-dimensional array of a plurality of polarization controlled reflective elements. The pitch of each polarization control reflective element is 500 μm, which matches the reflection repetition pitch of the light beam. The polarization control reflective element array 37 has its polarization rotation characteristic changed according to a control signal (for example, an electric field or a magnetic field) applied from the outside. As a specific example, consider a polarization control reflective element using liquid crystal.

In FIG. 8, 371 is a homogeneously aligned nematic liquid crystal (oriented at an angle of 45 ° with respect to the incident polarized light), 372 is a reflective film electrode, 373 is a transparent electrode, and 374 and 375 are glass plates. , 376 and 377 are spacers. In this element, when a voltage is applied between the electrodes, its birefringence changes. The transparent electrode 373 or the reflective film electrode 372 is divided into stripes, the voltage applied to each can be controlled individually, and each birefringence can be controlled at a pitch of 500 μm.

FIG. 9 shows the relationship between the applied voltage of the polarization control reflection element and the amount of phase change. The amount of phase change shown here is a value for one pass of light.

When a voltage giving a phase change amount of π / 4 is applied and light is reflected at the point B ″, it is equivalent to passing through the λ / 2 plate. As a result, passing through the λ / 4 plate 33. Sometimes it returns to the same polarization (ordinary light) as the first incident, and the light is A
Reflected in a point. On the other hand, when a voltage that gives an amount of phase change of π / 2 is applied and light is reflected at the point B ″, the polarization returns to the original at the point B ′, and this time it is incident on the λ / 4 plate 33 and B
It becomes extraordinary light at a point and travels diagonally to reach point C. When a voltage that gives another phase is applied, it is possible to arbitrarily adjust the reflected and returned components and the advancing component.

When the spectrum of the input light 41 is white light,
The spectrum obtained on the output side is shown in FIG. Figure (a)
Is incident white light, and Fig. 7 (b) shows the phase change amount of all polarization control reflective elements set to π / 4. Fig. 6 (c) shows the phase change amount of polarization control reflective elements π / 4 or When set to π / 2, FIG. 6D shows the case where an arbitrary voltage is applied to the polarization control reflective element.

In FIG. 7B, all the light incident on the Fabry-Perot interference filter 36 is reflected and returned. as a result,
The total spectrum of the transmission spectra at each point is obtained.
In Figure (c), only the wavelengths of the selected part are returned, and Figure (d)
Then, the transmittance of the transmission spectrum can be arbitrarily set by adjusting each voltage.

FIG. 10 shows the transmission spectrum in the case where the reflectance of the mirror of the Fabry-Perot interference filter is increased to increase the finesse and the transmission spectrum peaks at respective points are set so as not to overlap each other. FIG. 11 shows a transmission spectrum in the case where the finesse is lowered by lowering the reflectance of No. 3 and the transmission peaks at respective points are overlapped.

(A) and (a) 'in the figure are transmission spectra when the phase change amounts of the polarization control reflection element are all adjusted to π / 4. The same figure (a) shows the transmission spectrum of each point separately, and the same figure (a) 'is the total spectrum. In addition, in FIGS. 2 (b) and 2 (b) ', the polarization control element is adjusted to
It is a transmission spectrum when the wavelength dependence of the optical amplification characteristic of the r-doped optical fiber amplifier is corrected. Same figure
(b) shows the transmission spectrum of each point separately, and (b) 'in the same figure is the summed spectrum. Since an optical filter having an arbitrary transmission spectrum can be realized in this manner, the wavelength dependence of the optical fiber amplifier can be corrected and the band of the optical fiber amplifier can be expanded to several tens of nm.

Although the Fabry-Perot interference type filter in which the cavity gap changes in the plane is used in the above-mentioned embodiment, the same effect can be obtained by using the Fabry-Perot type interference filter in which the cavity loss and the reflectance of the mirror are different in the plane. There is.

In the first embodiment, no lens is included in the element. Usually collimated from optical fiber (parallel)
The beam diameter of light spreads with the propagation distance. Therefore, as the number of reflections increases, the beam size increases and the characteristics deteriorate. Also, when the beam diameter of the emitted light is large,
This causes a large loss when coupling to the optical fiber on the output side.

FIG. 12 shows a second embodiment of the present invention which solves the above-mentioned problems. In the figure, reference numeral 38 denotes a microlens array, and it is effective to insert a lens so as to suppress the spread of the beam. Generally, a lens having such a function is called a relay lens. Here, how to spread the beam is schematically shown.

FIG. 13 shows the equivalent optical circuit of FIG. By using a lens having a focal length f corresponding to the total thickness of the birefringent plate 32, the λ / 4 plate 33, the Fabry-Perot interference filter 36, and the polarization control reflective element array 37, the beam waist is always in the middle of the lens. The beam waist does not spread and becomes constant even if the propagation distance of light becomes long.

In the first and second embodiments, there is a limitation that only linearly polarized light having a polarization plane in a specific direction can be handled as input light.

FIG. 14 is a third embodiment of the present invention which solves the above-mentioned point, that is, polarization independent that can be used for any light (linearly polarized light in any direction, elliptically polarized light, unpolarized light). 2 shows a variable wavelength filter of. In the figure, 50 is an input / output separating means in the first embodiment, that is, a portion excluding the polarization beam splitter 30 and the Faraday rotator 31 (hereinafter referred to as a filter body), 51 is a λ / 2 plate, and 52 is a compound. The refraction plate 53 is an input / output separating means.

In this embodiment, in order to make the light independent of polarization, the light component is divided into two light beams for each polarization direction with respect to the incident surface of the birefringent plate, and the light beams are independently applied to the filter main body 50. Pass through. However, since the characteristics of the filter change depending on the location, the light beams divided into two are brought close to each other (1 mm or less). The details will be described below with reference to the drawings.

The input light 41 having an arbitrary polarization state passes through the input / output separating means 53 and enters the birefringent plate 52. The birefringent plate 52 separates the input light 41 into ordinary light and extraordinary light. The extraordinary light is converted into ordinary light by the λ / 2 plate 51,
It is input to the filter body 50. The two ordinary rays input to the birefringent plate 52 are parallel to each other and close to each other, and are repeatedly reflected as described in the first and second embodiments to perform a predetermined filter operation, and then are output to the emission side. . On the emission side, similarly to the incidence side, the light passes through the λ / 2 plate 51, the birefringent plate 52 and the input / output separation means 53, and is output to the point Y.

As the input / output separating means 53, one having no polarization dependency is required, and for example, a half mirror can be used, but a theoretical loss of 3 dB occurs. To remove such a loss, a known circulator may be used.

FIG. 15 shows an example of a circulator. An input light 41 having an arbitrary plane of polarization is split into two polarization components orthogonal to each other by a polarization beam splitter 61, and they are independently Faraday rotators 62 and 1 /. A two-wave plate (λ / 2 plate) 63 is passed. Each polarization component is a Faraday rotator (rotating the polarization plane by 45 °) 62 and λ /
When passing through the second plate 63, the plane of polarization is rotated by 90 °, so that the p polarization is converted into the s polarization and the s polarization is converted into the p polarization. Further, these are combined again by the polarization beam splitter 64 and made incident on the birefringent plate 52 (not shown) from the point V.

On the other hand, the emitted light enters the polarization beam splitter 64 from point V, is separated into p-polarized light and s-polarized light, and has a wavelength of λ / 2.
It passes through the plate 63 and the Faraday rotator 62. At this time, the Faraday rotator 62 rotates the polarization plane of 45 ° in the same direction as the forward path with respect to the traveling direction, while the λ / 2 plate 63 rotates the polarization plane of 45 ° in the reverse direction of the forward direction with respect to the traveling direction. Rotates. Therefore, the Faraday rotator 62 and the λ / 2 plate 63 are
The polarization rotation due to
The s-polarized light as it is and the s-polarized light as it is are incident on the polarization beam splitter 61 and are emitted from the point Y. In this way, the incident light and the outgoing light can be separated. 6
Reference numerals 5 and 66 are right-angle prisms.

FIG. 16 shows still another example of the lossless input / output separating means. FIG. 16 (a) is a plan view, FIG. 16 (b) is a front view, and FIG. 16 (c) is a side view. is there.

The input light 41 having an arbitrary polarization plane is separated by the birefringent plate 71 into p-polarized light (ordinary ray here) and s-polarized light (extraordinary light). In the birefringent plate 72 arranged at a right angle to the birefringent plate 71, p-polarized light is oblique and s-polarized light is straight. P polarized light is a split 1/2 wavelength plate (split λ / 2 plate) 73
The plane of polarization is rotated by 90 ° by the portion of the λ / 2 plate inside, and is converted into s-polarized light. The s-polarized light in the birefringent plate 72 passes through the transparent medium portion of the split λ / 2 plate 73 as it is as the s-polarized light.
These are sent to the Faraday rotator 76 by the polarization beam splitters 74 and 75. When passing through the Faraday rotator 76, the polarization planes of these lights are rotated by 45 °, and these are incident on the birefringent plate 52 (not shown) from the points V and W.

On the other hand, the emitted light enters the Faraday rotator 76 from points V and W. At that time, the polarization plane is 45
Rotated by °, both become p-polarized, and the polarization beam splitter 7
Pass 5. Further, these are combined by the birefringent plates 72 and 71 (in the reverse procedure of the outward path) and emitted from the point Y. In this way, the incident light and the outgoing light can be separated.

By using the input / output separating means shown in FIG. 15 or FIG. 16 as the input / output separating means 53 in FIG. 14, lossless and polarization independent can be realized. FIG.
7 shows a fourth embodiment of the present invention, in which 81 is an input optical fiber with a collimator lens, 82 is an output optical fiber with a collimator lens, 83 and 84 are birefringent plates,
85, 86, 87, 88 are quarter wavelength plates (λ / 4 plates),
89, 90 are filters, 91, 92, 93 are glass plates,
Reference numerals 94 and 95 are reflection films, 96 is a liquid crystal, 97 and 98 are transparent electrodes, 99 is input light, and 100 is output light.

The filters 89 and 90 are interference film filters formed of a multi-layered film, and are formed by tilting the substrate when forming a vapor deposition film to give an in-plane distribution of the transmission wavelength. Here, a filter whose wavelength changes at about 1 nm / mm was used. The liquid crystal 96 is a homogeneously aligned nematic liquid crystal, one transparent electrode 97 is patterned in a stripe shape, and a reflective film 95 is formed on the other transparent electrode 98 whose entire surface is patterned. Although a normal nematic liquid crystal is used here, it is effective to use a ferroelectric liquid crystal for high-speed switching. That is, the nematic liquid crystal has a speed of about msec, but a ferroelectric liquid crystal can realize a high speed of about μsec.

The major difference between this embodiment and the first to third embodiments is that two stages of circuits each including a birefringent plate sandwiched between quarter-wave plates and a filter are superposed. Further, in the first to third embodiments, the input light and the output light are on the same optical path and it is necessary to separate them, but in the present embodiment, the light is input from the lower side and output to the upper side. Therefore, it is not necessary to separate the input light and the output light.

Next, the operation of this embodiment will be described.
The input light 99 input from the optical fiber 81 passes through the point A and reaches the point B. Here, when the transmission wavelength of the filter 89 at the point _B is λ _B , only the light having the wavelength of λ _B passes through the filter 89 and reaches the point C ′. Since the light reaching point C'has passed through the quarter wave plate twice (quarter wave plates 86 and 87) before and after the filter 89, the plane of polarization is rotated by 90 °, and the direction of point B'is obtained. Can be bent to. Light is 1/4 from point B '
Wave plate 88, filter 90, glass plate 92, transparent electrode 9
The light enters the liquid crystal 96 through 7 and is reflected by the reflection film 95 (note that the filter 90 has its B ′, D ′, F ′,
H '... The transmission wavelength at the point is B, D, and
It is assumed that the wavelengths are set so as to match the transmission wavelengths at points F, H ... ).

Here, by controlling the birefringence of the liquid crystal 96 by applying an appropriate voltage to the stripe-shaped transparent electrode 97 in this portion, the plane of polarization of the light from the point B ′ can be controlled. It is possible to change the traveling direction of the light reflected by and the intensity ratio of the light in the two directions.

That is, when the liquid crystal 96 has no influence on the polarization state of the light from the point B ', the reflected light goes from the point B'to the point A', is reflected at the point A ', and is the output light. It becomes 100 and is coupled to the optical fiber 82. In addition, A '
It is reflected at the output side at the point because the transmission wavelengths λ _A ′ and λ _B of the filter 89 at the point A ′ are different (that is, although the filters 89 and 90 are transparent to the transmission wavelengths). , For other wavelengths work as a mirror).
When the liquid crystal 96 acts so as to rotate the polarization plane of the light from the point B ′ by 90 °, the light returns from the point B ′ to the point C ′, passes through the filter 89 again, and returns to the input side.

On the other hand, at the first point B, the light having a wavelength other than λ _B is reflected by the filter 89 and returns to the point B.
Since the light passes through the four-wave plate 86 twice, the direction thereof is bent to reach the point C, further passes through the quarter-wave plate 85, is reflected at the point C ′ of the reflection film 94, and is again the point C. However, also in this case, since the laser beam passes through the quarter-wave plate 85 twice, it goes straight and reaches the point D. At point D, λ _D
Light having a wavelength of
Propagate as in the case of _B light.

As described above, according to this embodiment, the filter 8
The traveling direction of light passing through 9.90 and its intensity ratio can be controlled by the voltage applied to the stripe-shaped transparent electrode 97, whereby the wavelength of light coupled to the output optical fiber 82 and its intensity distribution can be controlled. It is possible to tune and take out optical signals of arbitrary wavelengths and frequencies at the same time, that is, it is possible to select multiple wavelengths at the same time, not select at all, and control the intensity of light of the selected wavelengths. Becomes Further, it becomes possible to realize a filter having an ideal transmission spectrum.

In this embodiment, an interference film filter having a transmission wavelength characteristic different depending on the place is used, but the same effect can be obtained by using a liquid crystal variable wavelength filter in which a liquid crystal is filled in a Fabry-Perot interferometer. .

FIG. 18 shows an example of the above-mentioned liquid crystal variable wavelength filter, here, IEEE Photon Technol. Lett., Vol. 3, 1
991, K.Hirabayashi, H.Tsuda and T.Kurokawa, Narrow
-Band Tunable Wavelength-Selective Filters of Fabr
y-Perot Interferometers with a Liquid Crystal Intr
The liquid crystal variable wavelength filter described in acavitu, P213 is shown, in which 101 and 102 are glass substrates and 10
3 and 104 are transparent electrodes, 105 and 106 are dielectric mirrors, 107 and 108 are alignment films for liquid crystals, and 109 is liquid crystal.

The transparent electrode 103 is patterned on the entire surface, but the transparent electrode 104 is patterned in a stripe shape, and the refractive index of the parallel-aligned liquid crystal 109 is adjusted by adjusting the voltage applied to each electrode. Is changed so that the wavelength of the transmission spectrum has a surface distribution. Therefore, when this liquid crystal variable wavelength filter is applied to the variable wavelength filter of the present invention, an arbitrary number of
It is possible to realize a filter capable of selecting a transmission spectrum of arbitrary intensity and arbitrary wavelength.

Further, a filter having a surface distribution at the transmission wavelength can be realized by arranging filters having different wavelengths cut into strips and arranged in an array. With such a filter, since the transmission absolute wavelength of the filter can be designated in advance, it is possible to easily realize a filter in which the wavelength changes stepwise at intervals of 1 mm, for example.

Furthermore, in the present embodiment, the path of light is changed by using a normal nematic liquid crystal, but by using the ferroelectric liquid crystal, the switching time can be increased to μsec. Therefore, the wavelength can be switched at high speed,
Furthermore, a filter whose wavelength is absolutely stable can be realized.

FIG. 19 shows a fifth embodiment of the present invention, in which 111 is an input optical fiber with a collimating lens, 112 is an output optical fiber with a collimating lens, and 1 is an optical fiber.
13, 114 are birefringent plates, 115, 116, 117, 1
18 is a quarter wavelength plate (λ / 4 plate), 119 and 120 are filters, 121 and 122 are glass plates, 123 and 124 are reflective films, 125 is a liquid crystal, 126 and 127 are transparent electrodes, 1
28 is input light and 129 is output light.

The filters 119 and 120 are multilayer interference filters having an in-plane distribution in which the wavelength changes at about 1 nm / mm, which is similar to that of the fourth embodiment. The major difference of the present embodiment from the fourth embodiment is that a polarization control unit is provided at the junction where two stages of circuits consisting of a birefringent plate sandwiched by quarter-wave plates and a filter are superposed. . Also, in the fourth embodiment, the optical path length from the input to the output differs depending on the selected wavelength, but in this embodiment the optical path length from the input to the output does not depend on the optical path, so the loss changes depending on the selected channel. There is nothing to do.

The operation of this embodiment will be described below.
The light 128 input from the optical fiber 111 passes through the point A and reaches the point B. Assuming that the transmission wavelength of the filter 119 at the point _B is λ _B , only light having a wavelength of λ _B passes through the filter 119 and reaches the point _B ′ (note that the filter 120 has its B ′ and D ′). , F ', H', ... The transmission wavelengths at the points are set so as to match the transmission wavelengths at the B, D, F, H ,.

The light reaching point B'is the glass plate 121 between the filter 119 and the filter 120, the transparent electrode 126,
Since the light has passed through the polarization control unit composed of the liquid crystal 125, the transparent electrode 127 and the glass plate 122, an appropriate voltage is applied to the stripe-shaped transparent electrode 126 in this portion, and the liquid crystal 12
When the birefringence of 5 is controlled, the plane of polarization of the light from the point B can be controlled, whereby the traveling direction of the passing light and 2
The intensity ratio of light in one direction can be changed.

That is, when the liquid crystal 125 has no influence on the polarization state of the light from the point B, the transmitted light of the wavelength λ _B passes through the quarter-wave plates 116 and 117 between the points BB ′. Therefore, the plane of polarization is rotated 90 degrees, the optical axis is bent, and C
When reaching the ′ point and reflecting at the C ′ point, the ¼ wavelength plate 118
Since it passes through twice, the plane of polarization is rotated by 90 degrees and goes straight to the point D '.

Since the transmission wavelength is different at point D ', the filter 120 functions as a mirror and the light of wavelength λ _B is reflected. The reflected light in this case is also a quarter wavelength plate 117.
, The polarization plane is rotated by 90 degrees to point _E ′, and thereafter, the light of wavelength λ _B repeats rotation and reflection of the polarization plane between the filter 120 and the reflection film 124, and Z ′. It is output from a point and is coupled to the optical fiber 112.
Further, when the liquid crystal 125 acts so as to rotate the polarization plane of the light from the point B by 90 degrees, the light reaches the point A ′ from the point B ′ and repeats reflection in the direction opposite to the output side, and the birefringent plate. 114
It goes out and is not coupled to the optical fiber 112.

On the other hand, at the first point _B , light of wavelengths other than λ _B is reflected by the filter 119 and returns to point B, but at that time, since it passes through the quarter-wave plate 116 twice, the plane of polarization is 90 degrees. It is rotated once, the optical axis is bent, and it reaches point C, where it undergoes rotation and reflection of the plane of polarization and goes to point D. At this point _D , only the light of wavelength λ _D passes through the filter 119, and thereafter propagates in the same manner as the case of light of wavelength λ _B.

FIG. 20 shows the light transmission spectrum obtained in this example. In the figure, the left side shows an example in which one or many spectra are selected, and the right side shows an example in which the transmittance of many spectra is controlled.
The extinction ratio is good because it passes through the interference film filter twice.
An extinction ratio of dB or more can be realized.

[0075]

As described above, according to claim 1 of the present invention, in a variable wavelength filter for selectively extracting an optical signal of an arbitrary wavelength from an optical signal, a birefringent plate serving as a transmission medium of the optical signal is provided. , A polarization rotating means for sandwiching the birefringent plate and rotating a polarization plane of an optical signal by a predetermined angle, a filter for selecting an optical signal of an arbitrary wavelength from the optical signal, and a polarization rotation angle of an arbitrary portion can be controlled Since the polarization control element and the reflection surface arranged so as to sandwich them all are provided, the transmission spectrum of the output light can be controlled by sequentially propagating the input light through the filter, and the light having a plurality of wavelengths can be selected at the same time. Further, it is possible to arbitrarily set the transmittances of these plurality of transmission spectra, or to realize a variable wavelength filter having an arbitrary transmission spectrum.

According to the second aspect of the present invention, since a plurality of lenses are provided, it is possible to prevent the beam size from increasing, and it is possible to reduce the characteristic deterioration and the coupling loss.

According to the third aspect of the present invention, the input / output separating means for separating or combining the optical signals into two polarization components orthogonal to each other at the input / output end, and the polarization components orthogonal to each other are parallel and parallel. Since the polarization control means for inverting the optical signal is provided, it is possible to separate the optical signal into two parallel polarization components that are orthogonal to each other, propagate them in the filter, and combine them at the time of emission. A variable wavelength filter that can be used for

[Brief description of drawings]

FIG. 1 is a diagram showing the concept of wavelength optical switching.

FIG. 2 is a diagram showing how ordinary and extraordinary rays travel to a birefringent plate.

FIG. 3 is a diagram showing a circuit in which a birefringent plate and a polarization rotation reflection means are integrated.

FIG. 4 is a diagram showing a circuit in which a birefringent plate, a polarization rotation reflection means, and a Fabry-Perot interference filter are integrated.

FIG. 5 is a configuration diagram showing a first embodiment of a variable wavelength filter of the present invention.

6 is an enlarged side view of the input / output separating means in FIG.

FIG. 7 is a diagram showing the structure and transmission spectrum of a Fabry-Perot interference filter.

FIG. 8 is a diagram showing a specific structure of a polarization control reflective element array.

9 is a diagram showing the relationship between the applied voltage and the amount of phase change of the polarization control reflective element array of FIG.

FIG. 10 is a diagram showing an example of spectra of input light and output light in the first embodiment.

FIG. 11 is a diagram showing another example of spectra of input light and output light in the first embodiment.

FIG. 12 is a configuration diagram showing a second embodiment of the variable wavelength filter of the present invention.

13 is a diagram showing an equivalent optical circuit of FIG.

FIG. 14 is a configuration diagram showing a third embodiment of the variable wavelength filter of the present invention.

FIG. 15 is a diagram showing an example of input / output separation means in the third embodiment.

FIG. 16 is a diagram showing another example of the input / output separation means in the third embodiment.

FIG. 17 is a block diagram showing a fourth embodiment of the variable wavelength filter of the present invention.

FIG. 18 is a diagram showing another example of the filter according to the fourth embodiment.

FIG. 19 is a configuration diagram showing a fifth embodiment of the variable wavelength filter of the present invention.

FIG. 20 is a diagram showing an example of a light transmission spectrum in the fifth embodiment.

[Explanation of symbols]

30 ... Polarizing beam splitter, 31 ... Faraday rotator, 32, 52, 83, 84, 113, 114 ... Birefringent plate, 33, 34, 85, 86, 87, 88, 115, 1
16, 117, 118 ... λ / 4 plate, 35 ... Reflecting surface, 36
Fabry-Perot interference filter, 37 ... Polarization control reflective element array, 38 ... Microlens array, 51 ... λ /
2 plates, 53 ... Input / output separating means, 89, 90, 119, 1
20 ... Filter, 94, 95, 123, 124 ... Reflective film, 96, 125 ... Liquid crystal, 97, 98, 126, 127
… Transparent electrodes.

Claims (4)

[Claims] 1. A variable wavelength filter for selectively extracting an optical signal of an arbitrary wavelength from an optical signal, wherein a birefringent plate serving as a transmission medium of the optical signal and a polarization plane of the optical signal are sandwiched between the birefringent plates. The polarization rotation means for rotating the optical signal, the filter for selecting an optical signal of an arbitrary wavelength from the optical signal, the polarization control element capable of controlling the polarization rotation angle of an arbitrary portion, and the reflection arranged so as to sandwich them all. A variable wavelength filter having a surface. 2. The variable wavelength filter according to claim 1, further comprising a plurality of lenses. 3. An input / output separating means for separating or combining an optical signal into two orthogonal polarization components at an input / output end, and a polarization control means for making these orthogonal polarization components parallel and vice versa. The variable wavelength filter according to claim 1, wherein the variable wavelength filter is provided. 4. The variable wavelength filter according to claim 1, wherein a filter having a non-uniform distribution of a transmission spectrum wavelength, a transmittance or a band width is used.