JP3517408B2 - Variable optical attenuator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable optical attenuator capable of varying the attenuation amount of light having a specific wavelength.

[0002]

2. Description of the Related Art As a conventional optical attenuator capable of changing the attenuation rate of light, for example, as shown in FIG.
An optical attenuator in which a metal thin film 102 is vapor-deposited on 01 is used. The film thickness of this metal thin film is vapor-deposited with a continuous gradient along a certain direction. This optical attenuator is movable by a moving stage 103 in the direction of the arrow whose thickness changes. In this way, when light is incident so as to pass through the variable optical attenuator and transmitted light is obtained, if the irradiation position is continuously changed by the moving stage 103, the corresponding transmittance as shown in FIG. 8 is obtained. can get. For example, by changing the irradiation position of the incident light beam from X1 to X2, the transmittance can be varied between T1 and T2, and an arbitrary amount of attenuation can be obtained for the incident light signal. The metal thin film has low dependency on wavelength, and particularly low at wavelength around 1500 nm used in optical communication. Therefore, it can be used without considering the wavelength dependence.

[0003]

In the field of optical communication, an optical wavelength division multiplexing system (WDM) for multiplexing a large number of wavelength components is used.
Communication method) is being adopted. In this method, there are cases where it is desired to adjust the intensity of an optical signal of a specific wavelength component with respect to a multi-wavelength signal propagating in one optical fiber or to make the optical intensities of respective wavelengths uniform. Since a conventional optical attenuator does not have a wavelength selective function, only a desired wavelength component is branched and extracted (dropped) from a multi-wavelength signal transmitted in one optical fiber to obtain a desired optical intensity. It must be attenuated to a level and then added back to the original optical fiber.

However, in order to attenuate the light of a certain wavelength by using the conventional optical attenuator with an arbitrary amount of attenuation, there were the following problems. (1) Since a strip-shaped dielectric multilayer filter element having a film thickness gradient in the longitudinal direction is position-controlled by an operating stage to realize wavelength variability, the device must be upsized. (2) Long-term reliability is poor because it is necessary to mechanically move the dielectric multilayer filter element. (3) The variable wavelength speed is limited by the driving speed of the operating stage manual micrometer or motor.
The response characteristic is slow, and it takes more than 1 second to change the wavelength.

The present invention has been made by paying attention to such a conventional problem, and there is no mechanical movable part, and the variable light capable of adjusting the light intensity level of a specific wavelength component at high speed. The purpose is to provide an attenuator.

[0006]

The invention according to claim 1 of the present application is a dielectric multilayer filter formed by alternately laminating at least two kinds of dielectric thin films having different refractive indexes on a transparent optical substrate. An element, a piezoelectric actuator for pressing the dielectric multilayer filter element, a holding portion that sandwiches the dielectric multilayer filter element and the piezoelectric actuator, and a voltage source that supplies DC power to the piezoelectric actuator. By providing a voltage to the piezoelectric actuator, pressing the dielectric multilayer film by the piezoelectric effect, and changing the physical film thickness thereof, the attenuation rate for light of a specific wavelength is changed.

According to a second aspect of the present invention, in the variable optical attenuator according to the first aspect, the dielectric multilayer filter element is
A dielectric multilayer filter element having a bandpass filter structure having at least one cavity is characterized.

The invention of claim 2 of the present application is the invention of claim 1 or 2.
In the variable optical attenuator of, the piezoelectric actuator is
The dielectric multilayer filter element is configured to press the multilayer film in a vertical direction.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, in order to realize a variable optical attenuator, the wavelength is changed by uniformly changing the film thickness of the dielectric multilayer film using a piezoelectric actuator. First, the deformation due to the elastic effect will be described with reference to FIG. Now, according to Hooke's law in an isotropic three-dimensional material, strain components ε _X , ε _Y , ε _Z generated in each of the X, Y and Z directions can be expressed by the following equations. _{ε X = αT + [σ X} -ν (σ Y + σ Z)] / E (1) ε Y = αT + [σ Y -ν (σ Z + σ X)] / E (2) ε Z = αT + [σ Z - ν (σ _X + σ _Y )] / E (3) where α is the coefficient of thermal expansion, T is the temperature, σ _X , σ _Y , σ _Z
Is a stress component. ν is the Poisson's ratio (Poisso
n's ratio), E is Young's modulus (Elasticity of Modulu
s).

Now, consider the case where the stress σ _Y is applied in the Y direction, excluding the thermal expansion effect (αT). In this case, equations (1), (2), and (3) can be rewritten as the following equation. _{ε X = - (ν / E} ) σ Y (4) ε Y = (1 / E) σ Y (5) ε Z = - (ν / E) σ Y (6) These equations are further, epsilon _X = Ε _Z = −νε _Y (7) Therefore, as shown in FIG. 2, it can be seen that the compressive strain of ε _Y appears in the X and Z directions as an expansive strain of a magnitude that can be expressed by equation (7).

We decided to utilize this elastic effect in a variable optical attenuator. The piezoelectric effect causes the piezoelectric actuator to expand in micron units in proportion to an electric applied voltage. Until now, piezoelectric actuators have been used for dot printer heads, inkjet printer heads, valve drives,
It has been used as a linear motor for autofocus of cameras. The piezoelectric actuator is, for example, 100 kg / c
It has the feature of generating a large stress of m ² or more with an electromechanical conversion efficiency of 50% or more. Further, in driving, there is an advantage that it is noiseless and is very small (about 1 cm ³ ).

Therefore, in this embodiment, as shown in FIG. 1, a multilayer film 2 is formed by laminating two kinds of different high refractive index dielectric thin films and low refractive index dielectric thin films on a transparent optical substrate 1. The dielectric multilayer filter element 3 is used.

Next, the optical bandpass filter having the dielectric multilayer film structure will be described in more detail with reference to FIG.
3A is a side view, and FIG. 3B is an enlarged cross-sectional view of the multilayer film portion. An optical filter having a dielectric multilayer film structure is
Similar to the conventional one, the dielectric multilayer film 2 is laminated on the transparent optical substrate 1 such as glass or silicon. The dielectric multilayer film 2 is formed by alternately laminating a high refractive index film H which is a first dielectric film and a low refractive index film L which is a second dielectric film. When the refractive indices of the high refractive index film H and the low refractive index film L are n _H and n _L , respectively, the film structure of the dielectric multilayer filter having a high reflectance around a certain design wavelength λ ₀ is given by the following formula. To be Substrate / HLHL ... HL / Medium (air) (8) Here, the film thicknesses of the high refractive index film H and the low refractive index film L are d _H and d _L , respectively. These are called λ physical film thickness of 4 minutes and are given by the following equation. d _H = λ ₀ / 4n _H (9) d _L = λ ₀ / 4n _L (10) The lowermost high refractive index film H is in contact with the substrate 1, and the uppermost low refractive index film L is in contact with the medium. ing. Generally, the medium may be air or the like. (8) indicates that the stacking thickness d _H and d 2 kinds of dielectric thin film H of _L, and L are alternately located on the substrate. Expression (8) can be simply expressed as in the following expression. Substrate / (HL) ⁿ / medium (air) (11) Here, the expression (11) means that layers of HL pairs are repeatedly laminated n times. That is, substrate / HLHLHL / medium is equivalent to substrate / (HL) ³ / medium.

By the way, in the wavelength tunable optical filter device, a narrow band pass filter is often used in order to extract and analyze a signal by separating only a certain optical wavelength component. The film structure of the bandpass filter can be expressed by the following equation. Substrate / A / Medium (air) (12) A = [(HL) ⁿ HS _i LH (LH) ⁿ L] ^m (13) where m, n, S _i (i = 1, 2, ...,) m) is a positive integer (= 1, 2, 3, ...).

The film structure of A is called a cavity structure, and a spacer layer SiL having a thickness Sid _L is arranged next to the high reflection mirror portion (HL) ⁿ H. When m is 1, 2 and 3, they are called single cavity, double cavity and triple cavity, respectively. The larger the order of m, the steeper the characteristics of the bandpass filter. Moreover, the transmission bandwidth tends to become narrower as the thickness of the spacer layer increases. Therefore, in designing a bandpass filter, these parameters m, n, and Si are selected in consideration of the pass width and the separability from other signals.

As shown in the overall structure of the variable optical attenuator in FIG. 1, the dielectric multilayer filter element 3 is sandwiched between a pair of holding portions 4 and 5 and a piezoelectric actuator 6. At this time, the surface of the dielectric multilayer film 2 is arranged so as to face laterally so that light can enter and exit. An arbitrary voltage can be applied from the variable voltage source 7 to the positive electrode of the piezoelectric actuator 6, and the negative electrode 8 is grounded. The holding portions 4 and 5 are fixed members made of metal or the like.

Next, the operation of this variable optical attenuator will be described. As shown in FIG. 1, incident light enters from the dielectric thin film side. When no voltage is applied to the piezoelectric actuator 6, it is an optical filter using an ordinary dielectric multilayer film, which allows only light of a design wavelength to pass through and reflects light of other wavelengths. Then, when a direct current voltage is applied to the piezoelectric actuator 6, the piezoelectric actuator is driven according to the applied voltage.
It is deformed as shown in (3), and thereby the dielectric multilayer filter is pressed in the direction of arrow B. Therefore, the film thickness of each dielectric multilayer film is slightly deformed and expanded. Since such a change occurs equally for all the dielectric multilayer films, the selected wavelength itself can be changed. Since the piezoelectric effect is generated in proportion to the applied voltage, the selected wavelength can be continuously changed by changing the applied voltage, and the attenuation rate for a predetermined wavelength can be changed.

Further, since the physical film thickness of the dielectric multilayer film is controlled by causing an elastic effect in the dielectric multilayer film filter element 3 from the lateral direction, it does not depend on the film design of the dielectric multilayer film filter used therein. Therefore, it is possible to use an arbitrary dielectric multi-layer film filter element having a chip shape of several mm square having no in-plane film thickness distribution as a variable optical attenuator.

[0019]

Example 1 In Example 1, as a variable optical attenuator using a dielectric multilayer film filter element, a dielectric thin film Nb was formed on a glass substrate having a width of 1.3 mm and a thickness of 1.0 mm.
_{The 2} O ₅ film (refractive index 2.22) is used as the high refractive index film H, and Si
An O ₂ film (refractive index 1.45) was laminated as a low refractive index film L, and a film having a narrow bandpass filter structure having the following film structure was used. A = [(HL) ⁿ Hs _i LH (LH)
ⁿ L] ^m However, design wavelength λ ₀ = 1560.7 nm, n =
6, m = 3, s1 = s2 = s3 = 4. For the piezoelectric actuator 6, a piezoelectric actuator manufactured by Tokin Co., Ltd. (model number: AE0203D04-01) with dimensions of 2 mm × 3 mm and a thickness of 4 m
m was used. A variable optical attenuator was realized by sandwiching these between holding parts 4 and 5 made of stainless steel.

Next, the dependence of the elastic effect due to the piezoelectric effect in this example was examined. As a result, as shown in the wavelength tunable characteristic with respect to voltage in FIG. 4, the wavelength to the long wavelength side changes due to the expansion strain due to the compressive stress, and the wavelength tunable characteristic of 0.2 nm is realized at the applied DC voltage of 100V. It was Therefore, when the incident light is, for example, 1560 nm, the transmittance is −16 dB unless voltage is applied, and −22 dB for 100 V.
During that time, the attenuation rate can be continuously changed according to the voltage. In order to expand the attenuation range, the driving voltage may be increased or a piezoelectric actuator having a higher piezoelectric effect may be used.

(Embodiment 2) In the variable optical attenuator using the wavelength tunable optical filter device of Embodiment 2, a dielectric multilayer film filter element having a double cavity type narrow bandpass filter structure is used. Two different dielectric thin films are alternately laminated on a glass substrate, a high refractive index dielectric film H is an Nb ₂ O ₅ film (refractive index 2.22), and a low refractive index dielectric film L is SiO.
_Two films (refractive index 1.45) were used. The constructed film structure was as follows. A = [(HL) ⁿ Hs _i LH (LH) ⁿ L] ^m where design wavelength λ ₀ = 1550 nm, n = 8, m = 2,
s1 = s2 = 6.

FIG. 5 shows the filter characteristics. The transmission bandwidth of this filter was 0.12 nm. As shown in FIG. 5, by changing the applied voltage to V0 = 0V, V1 = 10V, V2 = 20V, the peak wavelengths corresponding to each are λ ₀ = 1550 nm, λ = 1550.2 nm, λ ₂ =
It changed to 1550.4 nm. Therefore, for example, 1550n
For a wavelength of m, the voltage is 0 V and the transmittance is 0 dB, 1
It becomes about -21 dB at 0 V and about -33 dB at 20 V, and the attenuation characteristics can be changed according to the applied voltage.

(Embodiment 3) A dielectric multilayer filter element having a single cavity type narrow bandpass filter structure is used in the variable optical attenuator using the variable wavelength optical filter device of the third embodiment. Two different dielectric thin films are alternately laminated on a glass substrate, a high refractive index dielectric film H is an Nb ₂ O ₅ film (refractive index 2.22), and a low refractive index dielectric film L is SiO.
_Two films (refractive index 1.45) were used. The constructed film structure was as follows. A = [(HL) ⁿ Hs _i LH (LH) ⁿ L] ^m where design wavelength λ ₀ = 155.35 nm, n = 6, m
= 1 and s1 = 8.

FIG. 6 shows the filter characteristics. The transmission bandwidth of this filter was 1.0 nm. As shown in FIG. 6B, applied voltages are V0 = 0V, V1 = 5V, V2 = 10V.
The peak wavelengths are λ ₀ = 155.35 nm and λ ₁ = 155.45 corresponding to
nm, λ ₂ = 155.55 nm. Also in this case, for example, the transmittance becomes about 87% to 62% for the wavelength of 1550.2 nm, and the attenuation rate can be continuously changed according to the voltage.

[0025]

As described above in detail, according to the inventions of claims 1 to 3 of the present application, an excellent effect that the intensity level can be easily adjusted at high speed for light of a predetermined wavelength is obtained. To be Several meters compared to the conventional linear slide system
An m-square chip-shaped dielectric multilayer filter element can be used, and a slide mechanism is unnecessary. Since a small piezoelectric actuator is used, the size can be reduced. Further, a usual dielectric multilayer film filter element having an arbitrary film structure can be used. Therefore, a multi-cavity structure can be used as needed, and a higher performance narrow band characteristic can be obtained. It is not necessary to form a film thickness gradient on the substrate in the film forming process, and the yield is improved. Since it has no mechanical moving parts, it has excellent reliability over a long period of time. Piezoelectric actuators have excellent high-speed response,
No noise such as mechanical vibration is generated, and a faster response time of about a millisecond can be obtained. Further, since the device structure is simple, it is possible to obtain the effect of being inexpensive.

[Brief description of drawings]

FIG. 1 is a schematic view showing a variable optical attenuator using a variable wavelength optical filter device according to the present invention.

FIG. 2 is a diagram showing deformation of a piezoelectric actuator due to an elastic effect.

FIG. 3 is a diagram showing a variable wavelength optical filter element.

FIG. 4 is a diagram showing wavelength tunable characteristics of the first embodiment.

FIG. 5 is a diagram showing a wavelength tunable characteristic of the second embodiment.

FIG. 6 is a diagram showing wavelength tunable characteristics of the third embodiment.

FIG. 7 is a schematic diagram showing an example of a conventional variable attenuation rate optical attenuator.

FIG. 8 is a graph showing the relationship between the irradiation position and the transmittance in the conventional variable attenuation rate optical attenuator.

[Explanation of symbols]

1 substrate 2 Dielectric multilayer film 3 Dielectric multilayer filter element 4,5 holding part 6 Piezoelectric actuator 7 Variable voltage source

Claims (3)

(57) [Claims] 1. A dielectric multilayer filter element formed by alternately laminating at least two kinds of dielectric thin films having different refractive indexes on a transparent optical substrate, and pressing the dielectric multilayer filter element. A piezoelectric actuator, a holding portion that sandwiches the dielectric multilayer film filter element and the piezoelectric actuator, and a voltage source that supplies a direct-current power source to the piezoelectric actuator. A variable optical attenuator that presses the dielectric multilayer film by the effect and changes the physical film thickness to change the attenuation rate for light of a specific wavelength. 2. The variable optical attenuator according to claim 1, wherein the dielectric multilayer filter element is a dielectric multilayer filter element having a bandpass filter structure having at least one cavity. 3. The variable optical attenuator according to claim 1, wherein the piezoelectric actuator is configured to press a multilayer film of the dielectric multilayer filter element in a vertical direction.