JP3099851B2 - Variable wavelength filter device - 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 signal transmitted through an optical fiber,
The present invention relates to a variable wavelength filter device capable of selectively and variably extracting an optical signal of a frequency.

[0002]

2. Description of the Related Art Optical communication using optical fibers has recently been rapidly put into practical use because of the ability to transmit a large amount of information at high speed. However, at present, only an optical pulse having a specific wavelength and frequency is transmitted. If a large number of optical pulses having different wavelengths and frequencies can be transmitted, a larger amount of information can be transmitted.
Generally, such a transmission system is called wavelength division multiplexing (WDM) / frequency division multiplexing communication (FDM), and is being actively studied at present.

In wavelength / frequency multiplexing communication, a key device is a variable wavelength filter that selectively selects only light of an arbitrary wavelength and frequency from optical pulses of many wavelengths and frequencies. For this reason, various tunable filters have been studied and put to practical use. For example, there is a filter in which an etalon cavity is filled with liquid crystal and a voltage is applied to change the refractive index of the liquid crystal and change the optical gap of the etalon (Stephen R. Ma).
llinson, Applied Optics Vo
l. 23, no. 3 (1987) p430).

Therefore, preferable conditions for a variable wavelength filter will be described.

[0005] A transmission spectrum curve drawn by taking the wavelength of light on the horizontal axis and the transmittance on the vertical axis shows that the transmittance becomes maximum at the resonance wavelength, and that the transmittance changes gradually near the resonance wavelength. It is desirable that the curve be such that the transmittance changes steeply as the distance from the resonance wavelength increases (hereinafter, such a transmission spectrum curve shape is generally referred to as a rectangular wave shape). This is because, when the transmission spectrum curve shape is a substantially rectangular wave shape, control for improving the selection accuracy of incident light having a certain wavelength width becomes easy.

[0006] On the other hand, a transmittance spectrum curve (hereinafter, referred to as a curve) in which the change in transmittance is sharp near the resonance wavelength and the change in transmittance becomes gentler as the distance from the resonance wavelength increases.
When such a transmission spectrum curve shape is referred to as a roughly lambda-like shape), in order to obtain a high transmittance, it is necessary to select a specific wavelength within a specific wavelength width. It is necessary to increase the wavelength selection accuracy.

Furthermore, for light of a non-selected wavelength, it is possible to increase the extinction ratio by using a variable wavelength filter whose transmission spectrum curve has a substantially rectangular wave shape. .

In general, in a filter used in an electric circuit, the shape of a transmission spectrum curve can be made to be a substantially rectangular wave shape using various digital techniques. It is difficult to make the shape to be drawn a substantially rectangular wave shape. Therefore, it is conceivable to connect variable wavelength filters in multiple stages in order to make the shape drawn by the transmission spectrum curve into a substantially rectangular wave shape.

An example will be described below.

[0010] The transmission spectrum of light passing through a single Fabry-Perot filter is

[0011]

(Equation 1)

It is assumed that Here, λ is the wavelength shifted from the resonance wavelength, FWHM is the half-value width of the transmission spectrum, and T ₀ is the transmittance at the resonance wavelength. Therefore, the transmission spectrum when two sheets are superimposed is

[0013]

(Equation 2)

## EQU1 ##

As a specific example, FWHM = 0.4 nm
FWHM = 1.33 nm filter and 2 filters
The comparison is made for the case where the sheets are stacked.

The cavity gap is 15 μm, the reflectivity of the mirror is 99% and 93%, the variable width is 50 nm,
And each T ₀ is 60% and 90%.

FIG. 5 shows a transmission spectrum curve of the filter under such conditions.

The horizontal axis represents the wavelength, and the resonance wavelength is set to 0. The vertical axis represents transmittance. The maximum transmittance at the resonance wavelength was set to 1. Reference symbol A′1 is a transmission spectrum curve when only one filter is used, and A′2 is a transmission spectrum curve when two filters are used. The maximum transmittance of one filter is 60%, and the transmittance when two filters are overlapped is 90%. Both filters have an extinction ratio of 20 dB when separated from the resonance wavelength by 2 nm. However, when two sheets are overlapped, the shape of the transmission spectrum curve is closer to a substantially rectangular wave shape, and at a wavelength separated by 2 nm or more, the extinction ratio becomes higher when two sheets overlap.

[0019]

However, when the conventional variable wavelength filters are connected in multiple stages, it is necessary to control the wavelength of each filter. Therefore, the control of the entire variable wavelength filters connected in multiple stages is very difficult. The problem of complication arises.

[0020]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a tunable wavelength filter device having a liquid crystal tunable wavelength filter and a liquid crystal tunable wavelength filter close to the surface of the liquid crystal tunable wavelength filter.
And input light to the liquid crystal variable wavelength filter.
An incident optical fiber for emitting light, and the liquid crystal variable wavelength
The liquid crystal variable wave arranged in contact with the back surface of the filter;
The light in a predetermined wavelength range that passes through the long filter in the first optical path is
And then re-enter the liquid crystal variable wavelength filter.
Reflecting means for reflecting the light reflected by the reflecting means
Light of a predetermined wavelength range is again transmitted to the liquid crystal variable wavelength filter by another
Transmitted in the optical path and emitted from the liquid crystal variable wavelength filter
An output optical fiber for inputting light is provided on the front side.
The liquid crystal variable wavelength fiber is provided separately from the
On the surface of the filter, the liquid crystal tunable wavelength filter
The extra light emitted does not enter the output optical fiber.
It is characterized by having slits
is there.

[0021]

According to the present invention, of the light emitted from the incident optical fiber, only light in a predetermined wavelength range passes through the liquid crystal variable wavelength filter and is reflected by the reflection means, and the reflected light is returned to the liquid crystal. As the light passes through the tunable wavelength filter and is captured by the output optical fiber as output light, the wavelength control of light is facilitated, and the transmittance of the tunable wavelength filter is improved, and as the shape of the transmission spectrum curve moves away from the resonance wavelength, The curve is such that the change in transmittance becomes steep.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an embodiment of a variable wavelength filter device according to the present invention.

Reference numerals in the figure represent the following, respectively. That is, reference numeral 1 is an optical fiber with a collimating lens for incidence, 2 is an optical fiber with a collimating lens for emission, 3 is a polarizing beam splitter, 4 is a half-wave plate, 5
Is a slit, 6 is a non-reflective coating film, 7 is a glass substrate, 8
Is a transparent electrode, 9 is a high reflection mirror, 10 is an alignment film for liquid crystal,
Reference numeral 11 denotes a liquid crystal that is homogeneously aligned, and reference numeral 12 denotes a high reflection mirror. In particular, 9 is a dielectric mirror, and 12 is a 98% gold metal mirror or a dielectric mirror having a reflectance close to 100%. Reference numerals 6 to 11 are portions corresponding to the conventional liquid crystal variable wavelength filter 14. The liquid crystal variable wavelength filter 14 is disclosed in Japanese Patent Application No.
13811 is a tunable wavelength filter disclosed in the aforementioned application for a detailed description of this filter.

Light B ₁ emitted from the incident optical fiber ₁
Is incident on the surface 13a of the liquid crystal variable wavelength filter 14 with an inclination of several degrees. In this embodiment, the angle is set to 2.4 degrees. The diameter of the light beams B ₁ incident from the incident optical fiber 1 to the liquid crystal variable wavelength filter 14 is 400 [mu] m, whereas the opening width of the slit 5 provided on the surface of the liquid crystal tunable filter 14 is 500 [mu] m. Incident light B
Reference numeral ₁ denotes a polarization beam splitter 3 which separates the polarized light into two polarized lights composed of polarized light parallel to and perpendicular to the alignment direction of liquid crystal molecules. Then, the vertically polarized light 4 is converted into a polarized light parallel to the liquid crystal molecules by a 板 plate, and is incident on the liquid crystal variable wavelength filter 14. After these two beams are selected in wavelength by the liquid crystal variable wavelength filter 14, they are reflected by the gold mirror 12.
The two reflected beams are again applied to the liquid crystal variable wavelength filter 1.
4 and the polarization beam splitter 3 as in the incident side.
It is incident on the light-emitting optical fiber 2 as outgoing light B ₂ through.

FIG. 2 shows a transmission spectrum curve of the tunable wavelength filter device according to the present embodiment having such a configuration.

The horizontal axis represents the wavelength, and the resonance wavelength is set to 0. The vertical axis represents transmittance. The maximum transmittance at the resonance wavelength was set to 100%. A1 in the figure
Shows a transmission spectrum curve when incident light passes through the liquid crystal variable wavelength filter used in the present embodiment only once, while reference symbol A2 denotes a liquid crystal variable wavelength filter. 9 shows a transmission spectrum curve when the apparatus of the present embodiment is used, which exhibits the same effect as when sheets are stacked.

The transmission spectrum curve of the variable wavelength filter device of the present embodiment is gentler in the vicinity of the resonance wavelength as compared with the conventional one, and the transmission spectrum becomes steeper as the distance from the resonance wavelength increases. The conventional filter had a maximum transmittance of 60%, while the filter of the present invention had a maximum transmittance of 75%.

As described above, in the variable wavelength filter device according to the present embodiment, the light beam passes through one liquid crystal variable wavelength filter 14 twice, so that the same effect as when two filters are stacked can be obtained. Is possible. Since the filters are the same, the number of voltage systems to be controlled is one and control can be performed very easily.

Further, in this embodiment, the slit 5 is provided in the liquid crystal variable wavelength filter, and the role of this slit 5 is that extra light B ₃ reflected by the liquid crystal variable wavelength filter 14 is used as shown in FIG. This is to prevent the light from entering the emission side optical fiber 2.

In the present invention, the polarization beam splitter and the 1 /
The polarization independence is achieved by the two-wave plate, but the same effect can be obtained by the birefringent prism and the half-wave plate.

FIG. 4 shows another embodiment of the variable wavelength filter according to the present invention. In this embodiment, a prism 13 is provided in the variable wavelength filter instead of the mirror 12 of the first embodiment. Using a flat mirror in the first embodiment, although the incident by tilting the incident light to the variable wavelength filter, in this second embodiment, the incident light B ₁ represents incident perpendicular to the liquid crystal tunable filter ing. After passing through the filter 14, the light beam is shifted by about 1 mm by the prism 13 so that it is reflected vertically. As described above, when the prism 13 is used as a mirror, light can be vertically incident on the liquid crystal variable wavelength filter. As a result, the wavelength selectivity fluctuates and the liquid crystal variable wavelength filter having high transmittance is obtained. It becomes a wavelength filter device.

[0033]

As described above, as described above, the liquid crystal variable wavelength filter is used.
Filter and the surface of the LCD variable wavelength filter.
To allow light to enter the liquid crystal tunable wavelength filter.
Contact the input optical fiber and the back of the liquid crystal tunable wavelength filter.
And the liquid crystal variable wavelength filter is connected to the first optical path.
Reflects light in the specified wavelength range transmitted by the
Reflecting means for entering the long filter, and reflecting means
Therefore, the reflected light in the predetermined wavelength range is again reflected by the liquid crystal variable wavelength filter.
Filter through another optical path, and
The outgoing optical fiber into which the outgoing light is incident enters the front side.
The liquid crystal variable wavelength filter is provided separately from the
On the surface of the
Slits are provided to prevent unwanted light from entering the output optical fiber.
The simple method of attaching a mirror or prism to the liquid crystal variable wavelength filter and reflecting the light once transmitted by the mirror or prism allows easy control and greatly increases the filter's transmittance and transmission spectrum shape. There is an effect that it can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a schematic configuration of a first embodiment of a variable wavelength filter device according to the present invention.

FIG. 2 is a diagram showing a transmission spectrum curve of the variable wavelength filter device according to the present invention shown in FIG. 1;

FIG. 3 is a diagram for explaining a role of a slit in the variable wavelength filter device of FIG.

FIG. 4 is a view for explaining a schematic configuration of a second embodiment of the variable wavelength filter according to the present invention.

FIG. 5 is a diagram illustrating a transmission spectrum curve of a conventional variable wavelength filter.

[Explanation of symbols]

 Reference Signs List 1 optical fiber with collimating lens for incidence 2 optical fiber with collimating lens for emission 3 polarizing beam splitter 4 1/2 wavelength plate 5 slit 6 antireflection coating film 7 glass substrate 8 transparent electrode 9 high reflection mirror 10 liquid crystal alignment film 11 liquid crystal 12 High reflection mirror 13 Prism 14 Liquid crystal variable wavelength filter

Continuation of the front page (56) References JP-A-4-140714 (JP, A) JP-A-4-307822 (JP, A) JP-A-63-29737 (JP, A) JP-A-4-142090 (JP) JP-A-4-248515 (JP, A) JP-A-4-248514 (JP, A) JP-A-4-98131 (JP, A) JP-A-6-500408 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/13 505

Claims (6)

(57) [Claims] 1. A liquid crystal variable wavelength filter, provided near a surface of the liquid crystal variable wavelength filter ,
And, an incident optical fiber for allowing light to enter the liquid crystal variable wavelength filter , and a predetermined wavelength range which is disposed in contact with the back surface of the liquid crystal variable wavelength filter and transmits the liquid crystal variable wavelength filter through a first optical path . Reflecting means for reflecting light and re-entering the liquid crystal variable wavelength filter, and transmitting the light in the predetermined wavelength range reflected by the reflecting means again to the liquid crystal variable wavelength filter through another optical path ,
An outgoing optical fiber for receiving the outgoing light from the liquid crystal variable wavelength filter , the incoming optical fiber on the surface side
Is provided separately, and the liquid crystal is provided on the surface of the liquid crystal variable wavelength filter.
Excess light reflected by the variable wavelength filter is reflected by the output optical filter.
A variable wavelength filter device , wherein a slit is provided so as not to enter the fiber . 2. The variable wavelength filter device according to claim 1, wherein said reflection means is a high reflection mirror. 3. The variable wavelength filter device according to claim 1, wherein said reflection means is a prism. 4. The variable wavelength filter device further includes a birefringent prism and a half-wave plate disposed between the incident optical fiber and the output optical fiber and the liquid crystal variable filter. The tunable wavelength filter device according to any one of claims 1 to 3, wherein: 5. The variable wavelength filter device further includes a polarizing beam splitter and a liquid crystal variable filter between the input optical fiber and the output optical fiber and the liquid crystal variable filter.
4. The tunable wavelength filter device according to claim 1, further comprising a half-wave plate. 6. The variable wavelength filter device according to claim 1, further comprising a slit on the surface of the liquid crystal variable wavelength filter, the slit transmitting only the incident light and the output light. A tunable wavelength filter device according to any one of the preceding claims.