Variable optical attenuator and optical filter

A variable optical attenuator includes an optical fiber on an incident side and an optical fiber on an emission side; a polarization rotator element that includes a liquid crystal layer and a pair of substrates that are arranged in such a manner that the liquid crystal layer is sandwiched between the substrates; and a Savart plate that is arranged between the optical fibers and the polarization rotator element. In the polarization rotator element, rotation of a polarizing direction is controlled by controlling a birefringence characteristic of liquid crystal, which is controlled by controlling alignment of the liquid crystal layer.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a variable optical attenuator and an optical filter that includes the variable optical attenuator.

2) Description of the Related Art

In recent years, large-capacity high-speed optical communications have been promoted. Accordingly, bandwidth extension is in increasing demand. To respond such demands, various optical devices have been developed and introduced. For example, a tunable filter technology such as a wavelength selection switch and a dynamic gain equalizer for Wavelength Division Multiplexing (WDM) in optical communications is an important technology in a dynamic optical network.

To achieve a tunable filter with high resolution, a free-space optical system has been suggested, which includes a lens, a diffraction grating, and an optical device with pixelized optical elements formed in an array. In such a free-space optical system, a light beam incident from outside via an optical fiber on an incident side (hereinafter, “incident-side optical fiber”) is branched to a predetermined optical element pixel of the optical device, and the light branched is reflected from the optical element pixel. The light reflected is then multiplexed to enter (be coupled to) an optical fiber on an emission side (hereinafter, “emission-side optical fiber”), and is then emitted to outside.

In the free-space optical system, an amount of light entering the emission-side optical fiber can be adjusted by the optical device. In this example, the optical device corresponds to a variable optical attenuator that adjusts an amount of attenuation of incident light so as to adjust the amount of light. For such variable optical attenuator, which is formed in an array, for example, a technology of micro-electro-mechanical systems (MEMS) has been developed and applied, such as MEMS of an analog tilt mirror scheme or of a diffraction type, and a digital micromirror device (DMD).

There is another type of a variable optical attenuator that uses a liquid crystal technology besides the technology of MEMS. Such variable optical attenuator is one of important options for achieving a tunable device that meets the demands for the dynamic optical network, because of such advantages as large birefringence of liquid crystal, easy integration with a standard complementary metal oxide semiconductor (CMOS) circuit, and a high degree of maturity of liquid crystal technology derived from a technology for a liquid-crystal display.

Unlike the variable optical attenuator based on MEMS technology, the variable optical attenuator based on the liquid crystal technology does not require a movable component. In the MEMS, due to the movable component, such as a mirror, internal or external disturbances, such as friction, stiction (a phenomenon in which a movable component is attached to another component), vibration, and electrification are caused. Such troubles may be a factor of a failure in the variable optical attenuator. Therefore, there is a high possibility of failures in such variable optical attenuator based on the MEMS. On the other hand, in the variable optical attenuator based on the liquid crystal technology, the movable component is not required. Therefore, there is no possibility of failures due to such problems, and thus, the variable optical attenuator based on the liquid crystal technology has higher reliability.

Moreover, the variable optical attenuator based on the MEMS generally requires a higher driving voltage than that in the variable optical attenuator based on the liquid crystal technology. Furthermore, because of the movable component, a manufacturing process tends to be complex in the variable optical attenuator based on the MEMS. Compare to the variable optical attenuator based on the MEMS, the variable optical attenuator based on the liquid crystal technology requires a lower driving voltage and lower manufacturing cost. In addition, the variable optical attenuator based on the liquid crystal technology can be formed in a reduced size, and is suitable for integration with the standard CMOS circuit.

FIG. 28is a schematic of a conventional variable optical attenuator that includes a liquid crystal element (for example, refer to Japanese Patent Application Laid-Open Publication No. 2001-13477). As shown inFIG. 28, an incident light L271emitted through a fiber collimator271enters a polarized beam splitter272on an incident-side. The polarized beam splitter272is a polarizing and separating unit, and separates the incident light L271into a p-polarized light beam and an s-polarized light beam. Both the p-polarized light beam and s-polarized light beam enter a liquid crystal element276, which is a polarization rotator element.

The liquid crystal element276is formed with two pieces of 45-degree-twisted nematic liquid crystal cells (hereinafter, simply “liquid crystal cells”)274and275that are laminated, and that are formed such that each of the liquid crystal cells274and275adjusts an applied voltage independently. In a process where the p-polarized light beam and the s-polarized light beam entering the liquid crystal element276pass through the liquid crystal cells274and275, the amount of light emitted from the liquid crystal element276is adjusted.

The light beams emitted from the liquid crystal element276further enter a polarized beam splitter273on an emission side. The polarized beam splitter273is a polarized-light combining unit, in which the light beams are multiplexed. The light beams multiplexed are emitted as emitted light beams L272and L273from side surfaces273aand273bof the polarized beam splitter273via fiber collimators278and279, respectively.

In the conventional variable optical attenuator described above, the polarized beam splitter272and the polarized beam splitter273are required to be provided separately on the incident side and the emission side. Therefore, the number of components increases, thereby making it difficult to reduce the size of the variable optical attenuator. Furthermore, such structure increases the manufacturing cost, and makes the manufacturing process complex.

In addition, in the process in which the p-polarized light beam and the s-polarized light beam pass through the polarized beam splitter272and the polarized beam splitter273, a difference in optical path length occurs between the polarized beams. As a result, polarization mode dispersion (PMD) of the variable optical attenuator is deteriorated.

Moreover, if a polarization axis of each of the polarized beam splitter272and the polarized beam splitter273is not arranged at an appropriate angle with respect to a liquid crystal director of the liquid crystal element276, polarization dependent loss (PDL) occurs.

SUMMARY OF THE INVENTION

A variable optical attenuator according to one aspect of the present invention includes an input and output unit that inputs light from outside and that outputs light to outside; a polarization rotator element that includes a liquid crystal, that reflects, toward the light input and output unit, the light input while controlling rotation of a polarizing direction, the liquid crystal sandwiched between a pair of electrodes and having a birefringence characteristic controlled by controlling a voltage applied between the electrodes, the rotation controlled by controlling the birefringence characteristic; and an optical-path-length correction plate that arranged between the input and output unit and the polarization rotator element, and that includes a birefringent plate for polarizing the light input and for combining the light polarized and reflected from the polarization rotator element, the birefringent plate guiding the light combined to the input and output unit. The optical-path-length correction plate is structured in such a manner that a difference in an optical path length between optical paths of the light polarized is correctable, the optical paths in the optical-path-length correction plate, and an amount of light output from the input and output unit is controlled by controlling the rotation of the polarizing direction.

An optical filter according to another aspect of the present invention includes a variable optical attenuator according to the above aspects.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. As an example of a variable optical attenuator according to embodiments of the present invention, a reflective variable optical attenuator having a liquid-crystal polarization rotator element is described. Also, as an example of the optical filter according to the present invention, a tunable filter having the variable optical attenuator is described.

FIG. 1is a schematic of a variable optical attenuator (VOA) according to a first embodiment of the present invention.FIG. 2is a front view of a Savart plate in the variable optical attenuator shown inFIG. 1.FIGS. 3 and 4are schematics of the variable optical attenuator.

As shown inFIG. 1, the variable optical attenuator includes, as main components, an incident-side optical fiber101having an end at which a fiber collimator102is placed, a lens103, a Savart plate104, a liquid-crystal polarization rotator element105, and an emission-side optical fiber107having an end at which a fiber collimator106is placed. Here, the incident-side and emission-side optical fibers101and107are assumed to be single mode fibers (SMFs).

In this structure, the incident-side optical fiber101, the fiber collimator102, the lens103, the emission-side optical fiber107, and the fiber collimator106correspond to a light input and output unit, and the Savart plate104corresponds to a birefringent plate for correction of a difference in optical path length.

The components101to107are arranged so as to form an incident route and an emission route for light. Specifically, on the light incident route, light L1entering the incident-side optical fiber101passes through the fiber collimator102and the lens103to enter the Savart plate104. The light L1passing through the Savart plate104further enters the liquid-crystal polarization rotator element105, from which the light is reflected. On the light emission route, reflected light L2passes, as emitted light, through the lens103again and the fiber collimator106to enter the emission-side optical fiber107, and is then emitted to the outside.

The end of the incident-side optical fiber101at the fiber collimator102side and the end of the emission-side optical fiber107at the fiber collimator106side are arranged so as to be positioned at focuses of a front surface side of the lens103. Also, a reflecting electrode65(seeFIG. 6) of the liquid-crystal polarization rotator element105is placed so as to be positioned at a focus of a back surface side of the lens103.

In the above, to describe the arrangement of the components101to107of the variable optical attenuator, the light incident and emission routes in the variable optical attenuator have been described in a simplified manner. In practice, however, as for the incident light L1and the emitted light L2, separation of polarized components at the Savart plate104and rotation of the polarized light at the liquid-crystal polarization rotator element105have to be taken into consideration.

The Savart plate104is a birefringent plate formed by laminating two birefringent plates104aand104b. These birefringent plates104aand104bare both made of rutile (TiO2), which is an anisotropic material. As shown inFIG. 2, the plates are laminated so that an optical axis104a′ of the birefringent plate104aand an optical axis104b′ of the birefringent plate104bare orthogonal to each other.

Here, the Savart plate104and the liquid-crystal polarization rotator element105are arranged so that the optical axes104a′ and104b′ of the birefringent plates104aand104beach form an appropriate angle with respect to a direction of a liquid crystal director when the liquid-crystal polarization rotator element105functions as a ¼ wave plate as described further below.

The material forming the birefringent plates104aand104bis not meant to be restricted to rutile, and may be another material. For example, the birefringent plates104aand104bmay be made of yttrium vanadium oxide (YVO4).

FIGS. 5 to 13are schematics for explaining the structure and operation of the liquid-crystal polarization rotator element105. Specifically,FIG. 5is a characteristic plot of birefringence of a liquid crystal used in the liquid-crystal polarization rotator element105, wherein the horizontal axis represents a wavelength of light entering the liquid crystal and the vertical axis represents an effective birefringence ratio of the light.

FIGS. 6 to 9are schematics for explaining the operation of the liquid crystal having a structure in which an electric field is formed on the liquid-crystal polarization rotator element105in a direction of thickness.FIGS. 10 to 13are schematics for explaining the operation of the liquid crystal having a structure in which an electric field is formed on the liquid-crystal polarization rotator element105in a lateral direction (width direction).

For example, as shown inFIG. 6, the liquid-crystal polarization rotator element105(seeFIG. 3) has a structure in which a liquid crystal layer105cformed of liquid crystal molecules105d(properly speaking,105ddenotes liquid crystal directors) is placed between a front substrate105aand a back substrate105b. The front substrate105ais formed with an indium-tin oxide (ITO) film being formed as a transparent electrode63on a transparent substrate61and with an alignment layer64being formed on the ITO film. Here, as the transparent substrate61, a glass substrate is used.

On the other hand, on the back substrate105b, an aluminum (Al) film serving as an electrode and also a reflective film is formed on the surface of a silicon (Si) substrate62to the reflecting electrode65. As such, the liquid-crystal polarization rotator element105is a reflective element. On the surface of the reflecting electrode65, an alignment layer64is further formed. The alignment layers64formed on the front and back substrates105aand105bare polyimide alignment layers subjected to a rubbing process. The front substrate105aand the back substrate105bare spaced a predetermined distance apart from each other so that their alignment layers64are opposed to each other.

Here, although detailed representations are omitted, the transparent electrode63and the reflecting electrode65are connected to a driving circuit. For example, in this case, the driving circuit is implemented by a complementary metal oxide semiconductor (CMOS) circuit.

The liquid crystal layer105csandwiched between the front substrate105aand the back substrate105bis implemented by a nematic liquid crystal with homogeneous alignment. In such a nematic liquid crystal, as shown inFIG. 5, an effective birefringence ratio Δn has a wavelength dependency characteristic.

Here, the effective birefringence ratio Δn of the liquid crystal represents a difference between a refractive index ne in the longer axis (major axis) direction and a refractive index no in the shorter axis direction of the liquid crystal director, and is expressed by the following equation (1).
Δn=ne−no(1)

The effective birefringence ratio Δn of the liquid crystal forming the liquid crystal layer105cof the liquid-crystal polarization rotator element105is calculated, for example, by the following equation (2) from a phase difference Δφ found by a transmission spectroscopic ellipsometry scheme.
Δn=λ/(2π·d)×Δφ  (2)

In equation (2), λ represents a wavelength of light entering the liquid crystal layer105c, drepresents a thickness of the liquid crystal layer10λ5c, and Δφ represents a phase difference of light emitted from the liquid crystal layer105c.

Next, a polarization rotating operation in the liquid-crystal polarization rotator element105is described. This polarization rotating operation directly contributes to an optical attenuation operation of the variable optical attenuator, which will be described further below.

A summary of the polarization rotating operation in the liquid-crystal polarization rotator element105is briefly described. In the liquid-crystal polarization rotator element105, when no voltage or a low voltage is applied to the liquid crystal layer105c, the liquid crystal molecules105din the liquid crystal layer105care aligned along the alignment layer64, thereby regulating nematic liquid crystal alignment. Therefore, in this case, the liquid crystal layer105cexhibits a birefringence characteristic, thereby causing the liquid-crystal polarization rotator element105to serve as a uniaxial wave plate.

Here, the liquid-crystal polarization rotator element105is designed to serve as a ¼ wave plate when no voltage or a low voltage is applied. Also at this time, as shown inFIGS. 7 and 11described further below, the liquid-crystal polarization rotator element105is designed so that the liquid crystal molecules105dand incident light L71form an angle of 45 degrees.

On the other hand, when a high voltage is applied to the liquid crystal layer105c, the liquid crystal molecules105dreceives an influence of an electric field formed by the voltage and becomes in a state of nematic liquid crystal alignment. Therefore, the liquid-crystal polarization rotator element105does not server as a wave plate. As such, in the liquid-crystal polarization rotator element105, with the magnitude of the applied voltage being adjusted, the alignment state of the liquid crystal molecules105dand the birefringence characteristic of the liquid crystal layer105ccan be controlled. Thus, here, the liquid-crystal polarization rotator element105serves as a uniaxial variable wave plate.

Polarization rotation of the liquid-crystal polarization rotator element105is influenced by the magnitude of the voltage applied to the liquid crystal layer105cand the orientation of the electric field formed by the voltage. In the following, the case in which an electric field in the direction of thickness (depth direction) is formed in the liquid crystal layer105cand the case in which an electric field in the lateral direction (width direction) is formed therein are separately described.

First, the case in which an electric field in the direction of thickness is formed in the liquid crystal layer105cis described. Initially, as shown inFIG. 6, when no voltage is applied to the liquid crystal layer105cor when a low voltage is applied to the liquid crystal layer105cto form a weak electric field in the direction of thickness, the liquid crystal molecules105dof the liquid crystal layer105care in a state such that the longer axis (major axis) is approximately parallel to the surfaces of the front and back substrates105aand105b, that is, the major axis lies along those surfaces. Thus, the liquid crystal layer105cexhibits a birefringence characteristic.

Here, the voltage applied to the liquid crystal layer105cis an effective voltage determined by a voltage applied to the transparent electrode63of the front substrate105aand a voltage applied to the reflecting electrode65of the back substrate105b.

With the liquid crystal molecules105dbeing in the state described above, as shown inFIG. 7, when light having a wavelength of λ (that is, the incident light L71) enters from the front substrate105aside and then passes through the front substrate105ato enter the liquid crystal layer105c, a polarizing direction is rotated in the process where the incident light L71is passing through the liquid crystal layer105c. With this, retardation (Δn·d) of the incident light L71emitted from the liquid crystal layer105cbecomes λ/2.

The incident light L71emitted from the liquid crystal layer105cthen reaches the reflecting electrode65(seeFIG. 6) of the back substrate105b, where the incident light L71is reflected. The reflected light passes again through the liquid crystal layer105cas the emitted light L72in a direction opposite to the direction of the incident light L71, and is then emitted to the front substrate105a. While the emitted light L72is passing through the liquid crystal layer105c, the polarizing direction is further rotated. As a result, retardation (Δn·d) of the emitted light L72from the transparent substrate61of the front substrate105ato the outside becomes λ/4. Also, a polarizing direction74of the emitted light L72is rotated by 90 degrees with respect to a polarizing direction73of the incident light L71. As such, in this case, the liquid-crystal polarization rotator element105functions as a ¼ wave plate.

On the other hand, as shown inFIG. 8, when a high voltage is applied to the liquid crystal layer105cto form a strong electric field in the direction of thickness of the liquid crystal layer105c, the liquid crystal molecules105dof the liquid crystal layer105care in a state such that the major axis is approximately perpendicular to the surfaces of the front and back substrates105aand105b, that is, the major axis lies between these substrates. Thus, in this case, the liquid crystal layer105closes a birefringence characteristic.

Therefore, with the liquid crystal molecules105dbeing in the state described above, as shown inFIG. 9, when light having a wavelength of λ (that is, the incident light L71) enters from the front substrate105aside and then passes through the front substrate105ato enter the liquid crystal layer105c, a polarizing direction is not changed in the process where the incident light L71is passing through the liquid crystal layer105c. Thus, retardation (Δn·d) of the incident light L71emitted from the liquid crystal layer105cis 0.

The incident light L71emitted from the liquid crystal layer105cthen reaches the reflecting electrode65(seeFIG. 8) of the back substrate105b, where the incident light L71is reflected. The reflected light passes again through the liquid crystal layer105cas the emitted light L72in a direction opposite to the direction of the incident light L71, and is then emitted to the front substrate105a. Also while the emitted light L72is passing through the liquid crystal layer105c, the polarizing direction is not rotated. As a result, in this case, retardation (Δn·d) of the emitted light L72from the transparent substrate61of the front substrate105ato the outside is 0. The polarizing direction74of the emitted light L72is identical to the polarizing direction73of the incident light L71. As such, in this case, the liquid-crystal polarization rotator element105does not function as a wave plate.

Next described is the case in which an electric field is formed in the lateral direction of the liquid crystal layer105cof the liquid-crystal polarization rotator element105, that is, a direction orthogonal to the direction of thickness described above, in other words, an extending direction of the surfaces of the front substrate105aand the back substrate105b.

First, as shown inFIG. 10, when no voltage is applied to the liquid crystal layer105cor when a low voltage is applied to the liquid crystal layer105cto form a weak electric field in the lateral direction, similarly to the case inFIG. 6, the liquid crystal molecules105dof the liquid crystal layer105care in a state such that the major axis is approximately parallel to the surfaces of the front and back substrates105aand105b, that is, the major axis lies along those surfaces.

With the liquid crystal molecules105dof the liquid crystal layer105cbeing in the state described above, as shown inFIG. 11, when the incident light L71having a wavelength of λ enters from the side of the transparent substrate61of the front substrate105aand then passes through the front substrate105ato enter the liquid crystal layer105c, a polarizing direction is rotated in the process where the incident light L71is passing through the liquid crystal layer105c.

The incident light L71emitted from the liquid crystal layer105cthen reaches the reflecting electrode65(seeFIG. 10) of the back substrate105b, where the incident light L71is reflected. The reflected light passes again through the liquid crystal layer105cas the emitted light L72in a direction opposite to the direction of the incident light L71, and is then emitted to the front substrate105a. While the emitted light L72is passing through the liquid crystal layer105c, the polarizing direction is further rotated. As a result, the polarizing direction74of the emitted light L72is rotated by 90 degrees with respect to the polarizing direction73of the incident light L71.

On the other hand, as shown inFIGS. 12 and 13, when a high voltage is applied to the liquid crystal layer105cto form a strong electric field in the lateral direction of the liquid crystal layer105c, the liquid crystal molecules105dof the liquid crystal layer105care in a state such that, on a virtual plane parallel to the surfaces of the front and back substrates105aand105b, the major axis is parallel to an end side of each side surface of these substrates. As such, with an electric field being formed in the lateral direction, the direction of placing the major axis of the liquid crystal molecules105dcan be controlled.

With the liquid crystal molecules being in the state described above, as shown inFIG. 13, when the incident light L71having a wavelength of λ enters from the front substrate105aside and then passes through the front substrate105ato enter the liquid crystal layer105c, a polarizing direction is not changed in the process where the incident light L71is passing through the liquid crystal layer105c.

The incident light L71emitted from the liquid crystal layer105cthen reaches the reflecting electrode65(seeFIG. 12) of the back substrate105b, where the incident light L71is reflected. The reflected light passes again through the liquid crystal layer105cas the emitted light L72in a direction opposite to the direction of the incident light L71, and is then emitted to the front substrate105a. Also while the emitted light L72is passing through the liquid crystal layer105c, the polarizing direction is not rotated. Therefore, in this case, the polarizing direction74of the emitted light L72from the transparent substrate61of the front substrate105ato the outside is identical to the polarizing direction73of the incident light L71.

Here, as will be described further below, the light having a wavelength of λ entering the liquid-crystal polarization rotator element105is separated by the Savart plate104(seeFIG. 1) placed on the front surface side of the liquid-crystal polarization rotator element105into orthogonal two linear polarization components, that is, a p-polarized light beam and an s-polarized light beam. Thus, in the liquid-crystal polarization rotator element105, the incident light L71may be a p-polarized light beam or an s-polarized light beam.

In the liquid-crystal polarization rotator element105, the polarization rotating operation described above with reference toFIGS. 6 to 13is performed with the incident light L71being any one of a p-polarized light beam or an s-polarized light beam.

Next, in consideration of the operation of the liquid-crystal polarization rotator element105as described above, the operation of the variable optical attenuator show inFIG. 1is described.

As shown inFIG. 1, the incident light introduced to the incident-side optical fiber101becomes parallel light by the fiber collimator102, and then enters the lens103. The light L1after passing through the lens103further enters the Savart plate104.

As shown inFIGS. 3 and 4, in the birefringent plate104aplaced on the front surface of the Savart plate104, the incident light is separated into two different light beams with their polarizing directions having an orthogonal relation to each other, that is, a p-polarized light beam31and an s-polarized light beam32obtained through linear polarization. Here, inFIGS. 3 and 4, for the convenience of description, it seems that the incident light has already been separated into the p-polarized light beam31and the s-polarized light beam32when the incident light enters the birefringent plate104a. In practice, however, the incident light first enters the birefringent plate104aand is then separated.

For example, the p-polarized light beam31obtained through separation proceeds through the birefringent plate104aas being tilted according to the direction of the optical axis104a′ (seeFIG. 2) of the birefringent plate104a. On the other hand, the s-polarized light beam32proceeds horizontally through the birefringent plate104a. As such, the p-polarized light beam31and the s-polarized light beam32proceeding through the birefringent plate104aalong different optical paths further enter the birefringent plate104b.

Here, the birefringent plate104ais made of rutile. Therefore, a distance k between the p-polarized light beam31and the s-polarized light beam32emitted from the birefringent plate104ato the birefringent plate104bis represented by k=j/10 using a thickness j of the birefringent plate104a. As such, the p-polarized light beam31and the s-polarized light beam32proceed through the birefringent plate104aalong different optical paths, and therefore a difference occurs in both optical path lengths in the birefringent plate104a. Here, the optical path length of the p-polarized light beam31is longer than the optical path length of the s-polarized light beam32.

The p-polarized light beam31and the s-polarized light beam32entering the birefringent plate104bproceed through the birefringent plate104balong different optical paths. Here, in contrast to the case of the birefringent plate104adescribed above, the p-polarized light beam31proceeds horizontally through the birefringent plate104b, and the s-polarized light beam32proceeds through the birefringent plate104bas being tilted according to the optical axis104b′ (seeFIG. 2) of the birefringent plate104b. As such, the p-polarized light beam31and the s-polarized light beam32proceed through the birefringent plate104balong different optical paths, a difference occurs in both optical path lengths in the birefringent plate104b. Here, the optical path length of the s-polarized light beam32is longer than the optical path length of the p-polarized light beam31.

Here, in the Savart plate104, the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the process where these beams are passing through the birefringent plate104ais equal to the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the process where these beams are passing through the birefringent plate104b. Therefore, when viewed as the entire Savart plate104, the differences in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the birefringent plates104aand104bcancel out each other. Therefore, the optical path lengths of the p-polarized light beam31and the s-polarized light beam32are equal to each other.

The p-polarized light beam31and the s-polarized light beam32passing through the Savart plate104in the manner described above further enter the liquid-crystal polarization element105from the front substrate105aside. Then, these beams pass through the liquid crystal layer105c, and are then reflected by the reflecting electrode65(seeFIG. 6) of the back substrate105b. The reflected p-polarized light beam31and s-polarized light beam32pass again through the liquid crystal layer105c, and are then emitted from the front substrate105ato the Savart plate104.

Here, such behaviors of the p-polarized light beam31and the s-polarized light beam32in the liquid-crystal polarization rotator element105vary, as described with reference toFIGS. 6 to 13, depending on the state of the liquid crystal molecules105dof the liquid crystal layer105cdetermined by the voltage applied to the liquid crystal layer105c. Therefore, in the following, the case in which no voltage or a low voltage is applied to the liquid-crystal polarization rotator element105and the case in which a high voltage is applied thereto are separately described.

Here, a voltage applied to the liquid crystal layer105cmay form an electric field in the direction of thickness of the liquid crystal layer105cas shown inFIGS. 6 to 9, an electric field in the lateral direction thereof as shown inFIGS. 10 to 13, or both of these electric fields.

First, when no voltage or a low voltage is applied to the liquid crystal layer105cof the liquid-crystal polarization rotator element105, as described with reference to either or both ofFIGS. 6 and 7andFIGS. 10 and 11, the polarizing direction of the p-polarized light beam31and the s-polarized light beam32reflected and emitted from the liquid-crystal polarization rotator element105is rotated by 90 degrees with respect to the polarizing direction of the p-polarized light beam31and the s-polarized light beam32entering the liquid-crystal polarization rotator element105.

As shown inFIG. 3, the p-polarized light beam31and the s-polarized light beam32emitted from the liquid-crystal polarization rotator element105with their polarizing directions being rotated again enter the birefringent plate104bof the Savart plate104. At this time, with both of the polarizing directions of the p-polarized light beam31and the s-polarized light beam32being rotated by 90 degrees by the liquid-crystal polarization rotator element105, the incident optical paths of the p-polarized light beam31and the s-polarized light beam32to the birefringent plate104bare different from the emission optical paths thereof from the birefringent plate104bas described above to the liquid-crystal polarization rotator element105.

The p-polarized light beam31entering the birefringent plate104bhorizontally proceeds through the birefringent plate104bto reach the birefringent plate104a. Then, after proceeding through the birefringent plate104aalong the optical axis104a′ (seeFIG. 2) as being tilted, the p-polarized light beam31is emitted from the birefringent plate104ato the outside. Here, as described above, the p-polarized light beam31enters the birefringent plate104bvia an optical path different from the optical path when entering. Therefore, the optical path along which the light beam sequentially passes through the birefringent plates104band104ais different from that when first entering.

On the other hand, the s-polarized light beam32entering the birefringent plate104bproceeds through the birefringent plate104balong the optical axis104b′ (seeFIG. 2) as being tilted to reach the birefringent plate104a. Then, after horizontally proceeding through the birefringent plate104a, the s-polarized light beam32is emitted from the birefringent plate104ato the outside. Here, as described above, the s-polarized light beam32enters the birefringent plate104bvia an optical path different from the optical path when entering. Therefore, the optical path along which the light beam sequentially passes the birefringent plates104band104ais different from that when first entering.

As described above, when the p-polarized light beam31and the s-polarized light beam32sequentially pass through the birefringent plates104band104a, similarly to the case of first entering as described above, the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104band the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104acancel out each other. Therefore, when viewed as the entire Savart plate104, the difference in optical path length between the p-polarized light beam31and the s-polarized light beam32has been solved.

The p-polarized light beam31and the s-polarized light beam32emitted from the Savart plate104pass through the lens103, as shown inFIG. 1. In this process, the p-polarized light beam31and the s-polarized light beam32are re-combined. Here, the p-polarized light beam31and the s-polarized light beam32to be combined each have passed along the emission optical path different from the optical path at the time of first entering with the polarizing direction being rotated by the liquid-crystal polarization rotator element105as described above. Therefore, an optical path of the emitted light L2obtained through combing both light beams is shifted from a position through which the emitted light L2can enter the emission-side optical fiber107.

Therefore, in this case, the emitted light L2does not enter the emission-side optical fiber107. Thus, the variable optical attenuator is a normally-off (-close)-type variable optical attenuator in which the emitted light L2to the outside is 0 when no voltage is applied to the liquid-crystal polarization rotator element105.

On the other hand, when a high voltage is applied to the liquid crystal layer105cof the liquid-crystal polarization rotator element105, as described with reference to either or both ofFIGS. 8 and 9andFIGS. 12 and 13, the polarizing directions of the p-polarized light beam31and the s-polarized light beam32reflected and emitted from the liquid-crystal polarization rotator element105are not rotated, and are identical to those at the time of first entering the liquid-crystal polarization rotator element.

As shown inFIG. 4, the p-polarized light beam31and the s-polarized light beam32emitted from the liquid-crystal polarization rotator element105with their polarizing directions being retained as those at the time of first entering again enter the birefringent plate104bof the Savart plate104. At this time, the p-polarized light beam31and the s-polarized light beam32are in the same polarizing directions as those at the time of first entering. Therefore, the incident optical paths of the p-polarized light beam31and the s-polarized light beam32to the birefringent plate104bare identical to the emission optical paths from the birefringent plate104bto the liquid-crystal polarization rotator element105.

The p-polarized light beam31entering the birefringent plate104bhorizontally proceeds through the birefringent plate104bto reach the birefringent plate104a. Then, after proceeding through the birefringent plate104aalong the optical axis104a′ (seeFIG. 2) as being tilted, the p-polarized light beam31is emitted from the birefringent plate104a. Here, the p-polarized light beam31enters the birefringent plate104balong the same optical path as that at the time of first entering as described above. Therefore, the optical path along which the light beam sequentially passes through the birefringent plates104band104ais identical to that at the time of entering.

On the other hand, the s-polarized light beam32entering the birefringent plate104bproceeds through the birefringent plate104balong the optical axis104′b(seeFIG. 2) as being tilted to reach the birefringent plate104a. Then, after horizontally proceeding through the birefringent plate104a, the s-polarized light beam32is emitted from the birefringent plate104ato the outside. The s-polarized light beam32enters the birefringent plate104balong the same optical path as that at the time of first entering as described above. Therefore, the optical path along which the light beam sequentially passes through the birefringent plates104band104aare identical to that at the time of first entering.

As described above, when the p-polarized light beam31and the s-polarized light beam32sequentially pass through the birefringent plates104band104a, similarly to the case of first entering as described above, the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104band the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104acancel out each other. Therefore, when viewed as the entire Savart plate104, the difference in optical path length between the emitted p-polarized light beam31and the s-polarized light beam32has been solved.

The p-polarized light beam31and the s-polarized light beam32emitted from the Savart plate104pass, as shown inFIG. 1, through the lens103. In this process, the p-polarized light beam31and the s-polarized light beam32are recombined. Here, the p-polarized light beam31and the s-polarized light beam32to be combined pass along the optical path identical to the optical path at the time of first entering as described above. Therefore, an optical path of the emitted light L2obtained by combing both light beams coincides with the position through which the emitted light L2can enter the emission-side optical fiber107. Therefore, in this case, the structure is such that the emitted light L2enters the emission-side optical fiber107.

Furthermore, in the above, the case in which the liquid-crystal polarization rotator element105functions as a ¼ wave plate, thereby not allowing the emitted light to be introduced to the emission-side optical fiber107(seeFIG. 3) and the case in which the liquid-crystal polarization rotator element105does not function as a wave plate, thereby allowing a maximum amount of the emitted light to be introduced to the emission-side optical fiber107(seeFIG. 4) have been described. By adjusting the magnitude of voltage to be applied to the liquid crystal layer105cof the liquid-crystal polarization rotator element105to either one of the low voltage and the high voltage to adjust the alignment state of the liquid crystal molecules105dto either one of those shown inFIGS. 3 and 4, thereby making it possible to adjust the amount of light to be introduced to the emission-side optical fiber107.

That is, by adjusting the voltage applied to the liquid-crystal polarization rotator element105, the amount of light to be emitted from the variable optical attenuator can be changed from 0 to a maximum value. Here, the maximum amount of the emitted light of the variable optical attenuator is an amount of light when incident light is emitted without being attenuated, that is, an amount equal to the amount of incident light to the variable optical attenuator.

In the above description, an angle formed by an incident plane formed by the incident light L1and the emitted light L2at the Savart plate104side of the lens103and a plane formed by two optical axes in the Savart plate104has not been specifically mentioned. However, if the incident angle of the light L1is large, the incident plane and the plane formed by two optical axes are preferably orthogonal to each other because the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32due to tilted entering can be reduced.

Changes in the amount of emitted light in such a variable optical attenuator are described as follows by using equation (3) indicative of an amount of optical attenuation in the variable optical attenuator.
ATT(V)=−10 log(cos2[2πΔn·d/λ])  (3)

First, the case in which no voltage or a low voltage is applied to the liquid-crystal polarization rotator element105(seeFIG. 3) is described. In this case, the liquid-crystal polarization rotator element105functions as a ¼ wave plate, and therefore the retardation Δn·d of the light emitted from the liquid-crystal polarization rotator element105(specifically, the p-polarized light beam31and the s-polarized light beam32) is λ/4. Therefore, when Δn·d=λ/4 is substituted into equation (3), an amount of optical attenuation ATT of the variable optical attenuator in this case can be calculated.

Specifically, when Δn·d=λ/4 is substituted into equation (3), the following equation (3′) is obtained.
ATT(V)=−10 log cos2(π/2)  (3′)

cos2(π/2)=0, therefore, based on the equation (3′), the amount of optical attenuation ATT of the variable optical attenuator is ∞. The case in which the amount of optical attenuation ATT of the variable optical attenuator is ∞ means that the amount of emitted light from the variable optical attenuator is 0.

On the other hand, when a high voltage is applied to the liquid-crystal polarization rotator element105(seeFIG. 4), as described above, the liquid-crystal polarization rotator element105does not rotate the polarizing direction. Therefore, the retardation Δn·d of the light emitted from the liquid-crystal polarization rotator element105(specifically, the p-polarized light beam31and the s-polarized light beam32) is 0. Therefore, when Δn·d=0 is substituted into equation (3), the amount of optical attenuation ATT of the variable optical attenuator in this case can be calculated.

Specifically, when Δn·d=0 is substituted into equation (3), the following equation (3″) is obtained.
ATT(V)=−10 log cos20  (3″)

Here, cos20=1. Therefore, based on equation (3″), the amount of optical attenuation ATT of the variable optical attenuator is 0. The case in which the amount of optical attenuation ATT of the variable optical attenuator is 0 means that the amount of emitted light from the variable optical attenuator is at maximum and, more specifically, is equal to the amount of incident light.

Furthermore, when the voltage applied to the liquid-crystal polarization rotator element105is changed from the low voltage at which the amount of optical attenuation ATT of the variable optical attenuator is ∞ to the high voltage at which the amount of optical attenuation ATT is 0, the retardation Δn·d of the light emitted from the liquid-crystal polarization rotator element105takes a value in a range from 0 to λ/4. Therefore, by substituting a value in those range into equation (3), the amount of optical attenuation ATT takes a value in a range from 0 to ∞. Thus, it is evident from equation (3) above that, by adjusting the voltage applied to the liquid-crystal polarization rotator element105in the manner described above, the amount of optical attenuation of the variable optical attenuator can be arbitrarily adjusted.

As described above, according to the variable optical attenuator of the present embodiment, with the Savart plate104being placed, the difference in optical path length occurring between the separated p-polarized light beam31and s-polarized light beam32can be reduced. As a result, reduction in PDL and PMD can be attained. Therefore, light having a desired wavelength can be adjusted with excellent accuracy so as to have a desired amount of light.

Also, in this case, such an effect is achieved by a simple structure, that is, only by placing a Savart-plate. Therefore, a small-sized variable optical attenuator having a small number of components and easy to manufacture at low cost can be achieved.

Furthermore, unlike the variable optical attenuator using MEMS, no movable component is required. Therefore, high reliability can be achieved. Still further, driving at a low voltage is possible.

The variable optical attenuator according to the present embodiment described above is an exemplary attenuator operating as a normally-off-type attenuator. According to this structure, when a high voltage is applied to the liquid crystal layer105cof the liquid-crystal polarization rotator element105for changing the polarizing direction of the incident polarized light, the amount of emitted light can be maximized. However, attenuation caused by residual birefringence occurs. To improve this attenuation, a residual birefringence correction plate (not shown) having a crystallographic axis orthogonal to the liquid crystal molecules105dof the liquid crystal layer105cis preferably provided between the front substrate105aforming the liquid-crystal polarization rotator element105and the Savart plate104shown inFIG. 1. More preferably, this residual birefringence correction plate is provided so as to adhere to a surface opposite to the liquid crystal layer105cof the front substrate105a. With this structure, the attenuation of the amount of emitted light described above can be improved.

Also, in place of the normally-off-type variable optical attenuator according to the present embodiment described above, a normally-on (-open)-type variable optical attenuator can be used to achieve the effect of the present invention, wherein the amount of emitted light from the liquid-crystal polarization rotator element105is minimized when a low voltage is applied to the liquid crystal layer105cof the liquid-crystal polarization rotator element105; the amount of emitted light is maximized when a high voltage is applied thereto; However, the amount of emitted light is maximized when a low voltage is applied thereto; and the amount of emitted light is minimized when a predetermined high voltage is applied thereto.

When such a normally-on-type variable optical attenuator is used, a switching-mode reversing wave plate (not shown), such as a crystal plate, having a crystallographic axis in a direction of canceling the retardation of the liquid-crystal polarization rotator element105is further placed between the front substrate105aforming the liquid-crystal polarization rotator element105and the Savart plate104shown inFIG. 1. With this, the variable optical attenuator has a structure in which combined birefringence of this switching-mode reversing wave plate and the liquid-crystal polarization rotator element105is minimized when the low voltage is applied. Thus, a normally-on-type variable optical attenuator can be achieved wherein the amount of emitted light is maximized when the low voltage is applied and the amount of emitted light is minimized when the predetermined high voltage is applied.

Furthermore, in place of the structure described above, the normally-on-type variable optical attenuator may have a structure in which an alignment layer64(seeFIG. 6) is formed through an orthorhombic vapor deposition scheme by using, for example, silicon oxide, so that a pretilt angle, which represents a direction of initial alignment, is almost vertical and homeotropic molecular alignment using a negative-type liquid crystal is used for the liquid crystal layer105c.

Still further, in place of the structure described above, the normally-on-type variable optical attenuator may have a structure in which the direction of I the liquid crystal molecules105dof the liquid crystal layer105cis set so that the incident-light polarizing direction73(seeFIG. 11) is parallel to the liquid crystal layer105cwhen a low voltage is applied.

FIG. 14is a schematic of the structure of a variable optical attenuator according to a second embodiment of the present invention. As shown inFIG. 14, in the variable optical attenuator according to the present embodiment, an input of light to the variable optical attenuator and an output of light from the variable optical attenuator are provided via a common optical fiber1401. Also, the optical fiber1401is connected to an optical circulator1402that separates the incident light and the emitted light.

Also, a compound lens1403that collimates and gathers the incident light from the optical fiber1401at the time of first entering and the emitted light entering the optical fiber1401at the time of emission is placed between an end of the optical fiber1401and the Savart plate104. Other than that, the second embodiment is similar to the first embodiment.

In the operation of the variable optical attenuator according to the present embodiment, light enters the variable optical attenuator via the optical fiber1401. After passing through the compound lens1403, this incident light reaches the Savart plate104and then the liquid-crystal polarization rotator element105in sequence. Then, light reflected on the liquid-crystal polarization rotator element105proceeds, as emitted light, along a path opposite to the path at the time of first entering.

With this, as described in the first embodiment, the incident light is attenuated as appropriate by the operation of the liquid-crystal polarization rotator element105, and the amount of emitted light is adjusted to be a desired amount of light. When the emitted light is emitted via the optical fiber1401to the outside, the optical circulator1402switches the optical path in the optical fiber1401for separating the incident light and the emitted light.

Also in the variable optical attenuator having the structure as described above according to the present embodiment, an effect similar to the effect described in the first embodiment is achieved.

Furthermore, in the variable optical attenuator according to the present embodiment, a common optical fiber is used for light entering and emission. Therefore, the structure can be further simplified.

FIG. 15is a schematic of a variable optical attenuator according to a third embodiment of the present invention. As shown inFIG. 15, in the variable optical attenuator according to the present embodiment, the fiber collimator102provided at the end of the incident-side optical fiber101is structured so as to cause a beam diameter of the light L1emitted through the incident-side optical fiber101to be equal to or smaller than 50 micrometers (μm). Also, the fiber collimator106provided at the end of the emission-side optical fiber107is structured so as to cause a beam diameter of the light L2entering the emission-side optical fiber107to be equal to or smaller than 50 μm.

As such, according to the present embodiment unlike the first embodiment, with the use of the fiber collimators102and106capable of reducing the beam diameter of the light, an optical system, such as a lens, is not required to be placed in front of the Savart plate. Therefore, the number of components can be reduced, thereby achieving downsizing. Here, if the Savart plate104is allowed to have a large thickness, the beam diameter does not have to be reduced. In this case, for example, although not shown, the incident-side optical fiber101and the emission-side optical fiber can have a structure in which these fibers are connected to the same two-core fiber collimator.

Also in the variable optical attenuator having the structure as described above according to the present embodiment, an effect similar to the effect described in the first embodiment is achieved.

In the first to third embodiments, the case in which the transparent electrode63made of an ITO film is formed on the front substrate105aof the liquid-crystal polarization rotator element105have been described. In a fourth embodiment according to the present invention, a variable optical attenuator having a structure in which, in place of an ITO film, an ITiO film having indium doped with titanium (Ti) is included in the liquid-crystal polarization rotator element105, is described below. Here, in the present embodiment, the structure may take the structure of any one of the first to third embodiments except for the ITiO film, and therefore is not described herein.

As shown inFIG. 6, in the operation of the variable optical attenuator on the light in a wavelength band of the near-infrared region, the transmittance of a transparent conductive film provided as the transparent electrode63on the front substrate105aof the liquid-crystal polarization rotator element105is important. In general, in liquid crystal elements for use in a liquid crystal display, an ITO film having low resistance is used as the transparent conductive film forming the transparent electrode.

The ITO film generally exhibits a high reflectivity, which is caused by plasma reflection, in the near-infrared region. Therefore, when the ITO film is used as a transparent electrode of the liquid crystal element, electron concentration of the ITO film has to be reduced with a sufficient conductivity being maintained.

In general, ITO can be regarded as degenerate n-type indium oxide excessively doped with tin. Both of tin dopant and oxygen deficiency act as a donor which contributes to conductivity of ITO. For this reason, to mitigate oxygen deficiency to an intermediate level, an ITO film for use as a transparent electrode of a liquid crystal element is formed at a partial pressure higher than that at which a normal low-resistance ITO film is formed. Alternatively, the ITO film is made thin for increasing the transmittance.

For example, according to the first to third embodiments, to achieve a much higher transmittance as the transparent electrode63, an ITO film having a thin thickness is formed on the transparent substrate61formed of a glass substrate. In this case, an ITO film having a film thickness in a range of 15 nanometers to 30 nanometers and a sheet resistance of 500 Ω/□ to 1000Ω/□ is formed through electron-beam vapor deposition.

It is assumed therein that, for example, an IT0film having a thickness of 17 nanometers is formed so that the transmittance is equal to or higher than 90 percent (%), and the sheet resistance of this ITO film is 1000Ω/□. When light having a wavelength of 1550 nanometers enters this ITO film, a complex refractive index n˜ of the IT0film at this time is represented by the following equation (4) with n=0.56 and k=0.95 being substituted thereinto.
n˜=n+ik(4)

Here, n is an effective refractive index, k is an effective extinction coefficient, and i is an imaginary unit.

When such an ITO film is placed on a glass substrate having a refractive index n of 1.52 and then light enters the ITO film from the glass substrate side, a reflection on the order of 1/100 of the amount of incident light occurs at an interface between the ITO film and the glass substrate.

As shown inFIG. 6, in the variable optical attenuator according to any one of the first to third embodiments, the front substrate105aof the liquid-crystal polarization rotator element105has a structure in which an ITO film (corresponding to the transparent electrode63) is placed on the glass substrate as described above (corresponding to the transparent substrate61). Therefore, such a reflection occurs between the glass substrate and the ITO film. As a result, optical loss occurs in the variable optical attenuator, which may become noise in an optical attenuation operation.

To get around this problem, according to the present embodiment, an ITiO film is formed on the transparent substrate61of the liquid-crystal polarization rotator element105in place of an ITO film to form the transparent electrode63. The ITiO film is formed by doping indium oxide with titanium (Ti) in place of tin.

By using such an ITiO film, according to the present embodiment, conductivity similar to that in the case of using an ITO film can be ensured. Furthermore, the reflection at the interface between the transparent substrate61and the transparent electrode63on the front substrate105aof the liquid-crystal polarization rotator element105, that is, the reflection at the interface between the glass substrate and the ITiO film, can be suppressed.

For example, when an ITiO film having a thickness of 60 nanometers is formed so that the transmittance is equal to or larger than 90%, the birefringence ratio n˜ of the film when light having a wavelength of 1550 nanometers enters the ITiO film is represented by equation (4) above with n=1.6 and k=0.02 being substituted thereinto. With the ITiO film, compared with the ITO film, reduction in extinction coefficient can be reduced. As a result, when an ITiO film is formed on the glass substrate, the refractive index at the interface between the glass substrate and the ITiO film can be reduced by the order of 1/1000.

Therefore, in the variable optical attenuator including the liquid-crystal polarization rotator element105that has the front substrate105awith an ITiO film being formed as the transparent electrode63on the transparent substrate61made of a glass substrate, optical loss caused by the reflection mentioned above can be suppressed at the liquid-crystal polarization rotator element105, thereby reducing noise. Also, the sheet resistance of the ITiO film is 250 Ω/□, and therefore an ITiO film having a low electron concentration and a high electron mobility can be achieved.

As described above, the ITiO film is an optimal film as a transparent conductive film forming the transparent electrode63of the liquid-crystal polarization rotator element105of the variable optical attenuator. With the use of this ITiO film, a variable optical attenuator having a further excellent characteristic can be achieved.

A variable optical attenuator according to a fifth embodiment of the present invention has a structure in which the liquid-crystal polarization rotator element105of the variable optical attenuator according to any one of the first to fourth embodiments includes a temperature adjusting unit. Other than that, the structure may take the structure of any one of the first to fourth embodiments. Therefore, the structure except for the liquid-crystal polarization rotator element105is not described herein.

FIG. 16is a graph of temperature dependency in the characteristic of the liquid-crystal polarization rotator element. Specifically,FIG. 16is a graph of temperature dependency of a voltage applied to the liquid-crystal polarization rotator element when the liquid-crystal polarization rotator element is in a normally-off state, in other words, temperature dependency of a voltage applied to the liquid-crystal polarization rotator element when the amount of optical attenuation in the variable optical attenuator is at maximum.

FIG. 17is a schematic of the liquid-crystal polarization rotator element of the variable optical attenuator according to the present embodiment.

As shown inFIG. 17, in the variable optical attenuator according to the present embodiment, one surface of a Peltier element1701, which is the temperature adjusting unit, is bonded onto the back surface side of the liquid-crystal polarization rotator element105(that is, the back surface side of the back substrate105b). The other surface of the Peltier element1701is bonded onto a side plate1702′ of a supporting member1702.

Particularly, in the liquid crystal molecules105dforming the liquid crystal layer105cof the liquid-crystal polarization rotator element105shown inFIG. 6, a response speed is delayed at low temperatures. Due to such changes in response speed, as shown inFIG. 16, the voltage applied to the liquid-crystal polarization rotator element105when the amount of optical attenuation is at maximum is changed according to temperature. Such temperature dependency of the applied voltage may cause the optical attenuation characteristic in the variable optical attenuator to deteriorate.

To get around this problem, in the present embodiment, the Peltier element1701(seeFIG. 17) is used to keep the temperature of the liquid-crystal polarization rotator element105constant irrespectively of temperature changes in the external environment. With this, even if the temperature in the external environment is changed, a stable response speed can be achieved in the liquid crystal molecule105d(seeFIG. 6) of liquid-crystal polarization rotator element105. Thus, temperature dependency of the voltage applied to the liquid-crystal polarization rotator element105as shown inFIG. 16can be solved. For this reason, a stable optical attenuation characteristic is achievable in the variable optical attenuator.

In the above, the case in which the Peltier element1701is used as the temperature adjusting unit has been described. Alternatively, a temperature adjusting unit other than the Peltier element may be placed. For example, a heater may be used. When a heater is used, cost can be further reduced compared to the case of using a Peltier element.

In a method of driving the variable optical attenuator according to a sixth embodiment of the present invention, a pulse width modulation (PWM) scheme is applied. In the variable optical attenuator driven by the PWM scheme, a driving circuit (specifically, a CMOS circuit) of the liquid-crystal polarization rotator element105can be downsized, and also an easy driving control is possible. Furthermore, a multi-tone driving circuit can be formed exclusively with digital circuits. As such, by driving the variable optical attenuator through the PWM scheme, an advantageous effect is attained compared with the case of driving through a pulse height modulation (PHM) scheme requiring an analog circuit.

The method of driving the variable optical attenuator through the PWM scheme is specifically described with reference toFIGS. 18 to 20.FIG. 18is a schematic of waveforms of voltages applied to the transparent electrode63of the front substrate105aand the reflecting electrode65of the back substrate105bin the liquid-crystal polarization rotator element105shown inFIG. 6. A solid-line waveform A shown inFIG. 18represents a voltage applied to the transparent electrode63, and a dotted-line waveform B shown inFIG. 18represents a voltage to the reflecting electrode65. The vertical axis represents the magnitude of the applied voltage, and the horizontal axis represents time.

As shown inFIG. 18, a voltage of 5 volts (V) is applied to the transparent electrode63of the liquid-crystal polarization rotator element105at predetermined cycles for a predetermined period of time, and has a waveform of a rectangular pulse (refer to the solid-line waveform A in the drawing). On the other hand, a voltage of 5 V is applied to the reflecting electrode65at the same cycle as that for the transparent electrode63with a timing being shifted from the timing of applying the voltage to the transparent electrode63(refer to the dotted-line waveform B inFIG. 18). Therefore, the voltage waveform B of the voltage applied to the reflecting electrode65has a waveform of a rectangular pulse similar to that of the voltage waveform A for the transparent electrode63. Also, phases of both of the voltage waveforms are shifted from each other, thereby causing a phase difference.

Here, such application of the voltage to the transparent electrode63and the reflecting electrode65is controlled by, for example, the CMOS circuit.

FIG. 19is a schematic of a waveform of an effective voltage applied to the liquid crystal layer105c(see a box shown inFIG. 18) by the voltage application to the transparent electrode63and the reflecting electrode65shown inFIG. 18. The vertical axis shown inFIG. 18represents a magnitude of the effective voltage, and the horizontal axis shown inFIG. 18represents time.

As shown inFIG. 19, in the liquid-crystal polarization rotator element105, a difference between the voltages applied to the transparent electrode63and the reflecting electrode65(see the box shown inFIG. 18for both electrodes) represents an effective voltage to be applied to the liquid crystal layer105c(see the box shown inFIG. 18). Therefore, the effective voltage has a waveform of a rectangular pulse, and a phase difference between the voltage waveforms A and B of both of the electrodes63and65(seeFIG. 18) represents a pulse width W of the effective voltage waveform.

Specifically, it is assumed herein that a value obtained by subtracting the voltage applied to the reflecting electrode65from the voltage applied to the transparent electrode63is taken as an effective voltage for the liquid crystal layer105c(seeFIG. 6). The effective voltage is positive when the voltage applied to the transparent electrode63is higher than the voltage applied to the reflecting electrode65, and the effective voltage is negative when the voltage applied to the reflecting electrode65is higher than the voltage applied to the transparent electrode63.

Also, the cycle of the effective voltage, that is, a driving cycle R1of the liquid-crystal polarization rotator element105coincides with a voltage waveform cycle of the voltage applied to the transparent electrode63. Here, a half cycle of this driving cycle R1is defined as a refresh cycle R2.

As such, according to the present embodiment, in the variable optical attenuator, the timing of application of a voltage to the transparent electrode63and the reflecting electrode65of the liquid-crystal polarization rotator element105is adjusted as shown inFIG. 18, thereby making it possible to adjust the pulse width of the effective voltage applied to the liquid crystal layer105c. With this, a driving control through a PMW scheme is attained.

FIG. 20is a schematic of intensity variations of light emitted from the variable optical attenuator when the effective voltage is applied in the manner as shown inFIG. 19. Here, the vertical axis represents relative intensity of light emitted from the variable optical attenuator, and the horizontal axis represents time.

As shown inFIG. 20, when the effective voltage is applied (that is, during periods t1and t3inFIG. 19), as described above, rotation of the polarizing direction is suppressed at the liquid-crystal polarization rotator element105thereby suppressing optical attenuation. Therefore, the intensity of the light emitted from the variable optical attenuator is increased.

On the other hand, when no effective voltage is applied (that is, during periods t2and t4inFIG. 19), as described above, the polarizing direction is rotated at the liquid-crystal polarization rotator element105, thereby increasing optical attenuation. Therefore, the intensity of the emitted light is decreased.

Thus, the intensity of the light emitted from the variable optical attenuator cyclically changes according to the driving cycle R1(seeFIG. 19) of the effective voltage. In such changes in emitted light intensity, one cycle is equal to the refresh cycle described above.

Here, when the liquid crystal layer105c(refer to the box inFIG. 18) of the liquid-crystal polarization rotator element105is made of nematic liquid crystal having a low response speed, the liquid crystal operates in response to the effective value of the voltage waveform. However, when the liquid crystal layer105cis made of nematic liquid crystal having a high response speed, the liquid crystal tries to operate in response to the pulse waveform itself. This is due to the fact that the response time of the liquid crystal is shorter than the refresh cycle R2(in other words, a pulse-train interval).

As a result of such a pulse waveform response, the intensity of the emitted light exhibits an optical response influenced by the pulse waveform, thereby causing a “waveform response” phenomenon. Specifically, the intensity of the emitted light is not only changed according to the driving cycle R1(seeFIG. 19), but also, as shown inFIG. 20, always fluctuated to cause flickers. Such occurrence of flickers is not preferable when the variable optical attenuator is used for optical communications.

To get around this problem, according to the present embodiment, for the purpose of suppressing the occurrence of such flickers, a refresh frequency having a length that does not cause a pulse waveform response of the liquid crystal is set. Specifically, for example, when MJ011580 (Merck Ltd., Japan) is used as the nematic liquid crystal, a critical refresh frequency that does not cause a waveform response is determined at 40 kilohertz, and the frequency is set so as to be below this critical refresh frequency. This can suppress the occurrence of flicker, thereby achieving an excellent optical attenuation characteristic in the variable optical attenuator.

Here, the case in which an effective voltage of 5 V is applied has been described. The effective voltage is not meant to be restricted to the above, and is appropriately set in consideration of an influence the driving scheme may have on an electrical-optical response of the liquid crystal. Also, the driving cycle of the effective voltage is not meant to be restricted to the above.

FIGS. 21 and 22depict a structure of a variable optical attenuator according to a seventh embodiment of the present invention.FIG. 21is a plan view of the variable optical attenuator andFIG. 22is a side view thereof.FIG. 23is a schematic of arrayed liquid-crystal polarization rotator elements shown inFIGS. 21 and 22.

As shown inFIGS. 21 and 22, the variable optical attenuator according to the present embodiment is implemented by a plurality of variable optical attenuator units2100each having the structure according to the first embodiment shown inFIG. 1, these units being integrally arrayed. Here, the variable optical attenuator unit2100may have the structure of any one of the second to sixth embodiments excluding that of the first embodiment as a basic structure.

In this structure, each variable optical attenuator unit2100forms one channel in an optical attenuation operation. Therefore, according to the present embodiment, a multichannel variable optical attenuator is achieved. Although a four-channel variable optical attenuator having four variable optical attenuator units2100is shown in the drawing, the number of channels is not meant to be restricted to the above. As shown inFIG. 23, which will be described further below, the variable optical attenuator for use in optical communications often has an extremely large number of channels.

In the multichannel variable optical attenuator having the structure described above, the variable optical attenuator unit2100includes, as described in the first embodiment, the incident-side optical fiber101having an end at which the fiber collimator102is placed, the emission-side optical fiber107having an end at which the fiber collimator106is placed, the lens103, and the Savart plate104.

Here, as shown inFIG. 21, the incident-side optical fiber101and the emission-side optical fiber107are arranged so as to be overlap with each other when viewed from front. Also, the Savart plate104is commonly shared by the variable optical attenuator units2100.

In the above description, an angle formed by an incident plane formed by the incident light L1and the emitted light L2at the Savart plate104side of the lens103and a plane formed by two optical axes in the Savart plate104has not been specifically mentioned. However, if the incident angle of the light L1is large, the incident plane and the plane formed by two optical axes are preferably orthogonal to each other because the difference in optical path occurring between the p-polarized light beam31and the s-polarized light beam32due to tilted entering can be reduced.

Also, the variable optical attenuator according to the present embodiment includes an arrayed liquid-crystal polarization rotator element2101commonly provided for the variable optical attenuator units2100. The arrayed liquid-crystal polarization rotator element2101has a structure in which a plurality of liquid-crystal polarization rotator element pixels2102are arrayed on the same Si integrated substrate2103. The liquid-crystal polarization rotator element pixels2102and the variable optical attenuator units2100are placed so as to have a one-to-one correspondence. That is, the structure is such that the liquid-crystal polarization rotator elements of the variable optical attenuator for each channel are integrated.

As shown inFIG. 23, in an arrayed liquid-crystal polarization rotator element2101, liquid-crystal polarization rotator element pixels2102each having the structure ofFIG. 6are one-dimensionally arranged and arrayed on a Si integrated substrate2103having a circuit, such as a CMOS circuit, with the Si integrated substrate2103serving as the back substrate105bcommonly shared.

Specifically, on the Si integrated circuit2103, a flexible print circuit (FPC)2301, a latch circuit2302, an upper driver area2303having a driving circuit (not shown), and a lower driver area2305. Between the upper and lower driver areas2303and2305are the liquid-crystal polarization rotator element pixels2102arrayed one-dimensionally (in a row in the horizontal direction) to form a liquid-crystal polarization rotator element pixel array area2304.

Although not shown, on the Si integrated circuit2103, a connection pad for connection to the flexible print circuit2301and wirings for connection are provided.

Also, for example, the upper and lower driver areas2303and2305each include a driving circuit (not shown) that can achieve 12-bit tone through a PWM scheme. With the driving circuits, the CMOS circuits (not shown) of the liquid-crystal polarization rotator element pixels2102are separately controlled. Thus, the voltages applied to the liquid-crystal polarization rotator element pixels2102can be separately controlled.

As shown in a partial enlarged view shown inFIG. 23, for example, in the liquid-crystal polarization rotator element pixel array area2304, liquid-crystal polarization rotator element pixels2102each having 380 μm in height×23.8 μm in width are arranged with a pitch of 1.2 μm. A cell gap in the liquid-crystal polarization rotator element pixels2102is 5 μm. To widen a pitch between the driving circuits (not shown) of the upper and lower driver areas2303and2305, the liquid-crystal polarization rotator element pixels2102are connected to the upper and lower driving circuits alternately for each adjacent pixel.

In the arrayed liquid-crystal polarization rotator element2101having the structure described above, one liquid-crystal polarization rotator element pixel2102corresponds to one channel of the variable optical attenuator. Therefore, in the arrayed liquid-crystal polarization rotator element2101having 512 pixels, 512 channels can be achieved.

In the arrayed liquid-crystal polarization rotator element2101described above, the liquid-crystal polarization rotator element pixel array area2304and the upper and lower driver areas2303and2305are separated from each other. Therefore, the liquid-crystal polarization rotator element pixel array area2304does not have to be planarized. In a normal arrayed liquid-crystal polarization rotator element2101where the liquid-crystal polarization rotator element pixels2102are arranged in a matrix, each liquid-crystal polarization rotator element pixel2102is formed on the driving circuit of the driver area, and therefore a process of planarizing the liquid-crystal polarization rotator element pixel array area2304is essential. By contrast, in the structure according to the present embodiment, such a planarizing process is not required, thereby simplifying a manufacturing process. As a result, a small-sized, multichannel arrayed liquid-crystal polarization rotator element2101can be manufactured through a simple process.

In the multichannel variable optical attenuator including the arrayed liquid-crystal polarization rotator element2101, with the operation of each variable optical attenuator unit2100, the optical attenuation operation described in the first embodiment is performed independently for each channel. With this, an effect similar to that according to the first embodiment is achieved in each channel.

From the above, according to the variable optical attenuator of the present embodiment, reduction in PDL and PMD is achieved in each channel, thereby making it possible to attain a small-sized, multichannel variable optical attenuator that is easy to manufacture.

In the above, as shown inFIG. 21, the case in which the channels of the variable optical attenuator and the liquid-crystal polarization rotator element pixels2102have a one-to-one correspondence has been described. Alternatively, as an example of modification of the present embodiment, the channels and the liquid-crystal polarization rotator element pixels2102may not have a one-to-one correspondence. Specifically, one channel may include a plurality of liquid-crystal polarization rotator element pixels2102, or a plurality of channels may include a single one of the liquid-crystal polarization rotator element pixels2102.

For example, in a multichannel variable optical attenuator for use in a gain equalizer, to reduce ripples occurring in an optical characteristic due to a space among the liquid-crystal polarization rotator element pixels2102(seeFIG. 21), one channel include a plurality of liquid-crystal polarization rotator element pixels2102. In this case, in the variable optical attenuator unit2100forming one channel (seeFIG. 21), the number of placement of incident-side and emission-side optical fibers101and107(seeFIG. 21) is different from the number of placement of the liquid-crystal polarization rotator element pixels2102in the arrayed liquid-crystal polarization rotator element2101(seeFIG. 21). Such a structure is achieved by placing an optical element, such as a prism, as appropriate between the incident-side and emission-side optical fibers101and107and the arrayed liquid-crystal polarization rotator element2101.

In an eighth embodiment according to the present invention, a tunable filter of a free-space optical system including the multichannel variable optical attenuator according to the seventh embodiment is described. Here, a tunable filter for use in a wavelength division multiplexing (WDM) system is described.

FIG. 24is a schematic of the structure of the tunable filter according to the eighth embodiment of the present invention. As shown inFIG. 24, the tunable filter according to the present embodiment includes a variable optical attenuator2400, a compound lens2403, and a spectroscope2404. The variable optical attenuator2400includes the arrayed liquid-crystal polarization rotator element2101forming the structure described above in the seventh embodiment, the Savart plate104, and an optical fiber for incident and emitted light2402that is connected to an optical circulator2401.

Here, each of the liquid-crystal polarization rotator element pixels2102forming the arrayed liquid-crystal polarization rotator element2101forms on channel of the variable optical attenuator2400. At each channel, an attenuation operation for the light for each wavelength is performed. Also, the single optical fiber for incident and emitted light2402and the single Savart plate104are shared in common among the channels. The variable optical attenuator2400having the structure described above is placed on a Fourier plane2410of an optical system of the tunable filter.

In the tunable filter having the structure described above, wavelength-multiplexed light (beam1) entering the optical fiber for incident and emitted light2402of the variable optical attenuator2400is emitted toward the compound lens2403from an end of the optical fiber for incident and emitted light2402. Here, as the compound lens2403, a Littrow lens is used.

Also, for convenience of description of optical behaviors in the tunable filter, the optical fiber for incident and emitted light2402is embedded in the arrayed liquid-crystal polarization rotator element2101and the Savart plate104of the variable optical attenuator2400. In practice, however, the optical fiber for incident and emitted light2402is spaced apart from the arrayed liquid-crystal polarization rotator element2101and the Savart plate104.

Therefore, when the light (beam1) is emitted from the optical fiber for incident and emitted light2402toward the compound lens2403and when the light (beam1) enters the optical fiber for incident and emitted light2402from the compound lens2403, the light (beam1) proceeds along an optical path not via the Savart plate104. Also, at the time of entrance and emission of light beams (beams2and3) in the arrayed liquid-crystal polarization rotator element2101, which will be described further below, the optical fiber for incident and emitted light2402does not hinder the light beams (beams2and3).

The light (beam1) passing through the compound lens2403enters the spectroscope2404, where the light is separated by wavelength and reflected. The light beams (beams2and3) obtained through separation by wavelength again enter the compound lens2403and then the variable optical attenuator2400. At the variable optical attenuator2400, each of the light beams (beams2and3) for the relevant wavelength enters the arrayed liquid-crystal polarization rotator element2101via the Savart plate104, and is then reflected on the liquid-crystal polarization rotator element pixel2102corresponding to the relevant wavelength.

Each of the light beams (beams2and3) reflected on the relevant liquid-crystal polarization rotator element pixel2102of the arrayed liquid-crystal polarization rotator element2101again passes through the Savart plate104and is then emitted to the compound lens2403. Then, each of the light beams (beams2and3) passing through the compound lens2403again enters the spectroscope2404, where the light is reflected. The reflected light (beam1) again enters the optical fiber for incident and emitted light2402via the compound lens2403. As such, in the tunable filter according to the present embodiment, a 4f-telecentric optical system is achieved.

Here, the arrayed liquid-crystal polarization rotator element2101of the variable optical attenuator2400, the state of liquid crystal alignment of the liquid-crystal polarization rotator element pixels2102is controlled for each channel. With this control, rotation of the polarizing direction of each of the light beams (beams2and3) distributed for each channel is controlled for each channel.

Therefore, as has been described in the first embodiment, in the arrayed liquid-crystal polarization rotator element2101, for example, the emitted light (beam1) from the channel where the state of alignment of the liquid crystal is controlled to a state shown inFIG. 6with the polarizing direction being rotated by 90 degrees proceeds along an optical path different from that when first entering. Therefore, the emitted light cannot again enter the optical fiber for incident and emitted light2402. On the other hand, the emitted light (beam1) from the channel where the state of alignment of the liquid crystal is controlled to a state shown inFIG. 8with the polarizing direction not being rotated proceeds along the same optical path as that when first entering. Therefore, the emitted light enters the optical fiber for incident and emitted light2402with the same amount of light as that when first entering.

As such, in the structure described above, the state of alignment of the liquid crystal in the liquid-crystal polarization rotator element pixels2102is controlled for each channel, thereby making it possible to adjust, as appropriate, the amount of light entering the optical fiber for incident and emitted light2402for each channel (that is, for each light beam obtained through wavelength separation).

The light (beam1) entering the optical fiber for incident and emitted light2402is emitted via the optical fiber for incident and emitted light2402. At this time, in the optical fiber for incident and emitted light2402, the incident light and the emitted light are separated from each other by the optical circulator2401.

As such, in the tunable filter according to the present embodiment, the light beams (beams2and3) of different wavelength distributed to the respective channel of the variable optical attenuator2400can be attenuated separately for each wavelength. Therefore, wavelength-multiplexed light entering the tunable filter can be separated at a desired intensity for each wavelength.

FIG. 25is a drawing of a characteristic of the tunable filter according to the present embodiment, wherein either one or both of ON and OFF are sequentially performed for each channel.

In this case, as has been described in the first embodiment, in the variable optical attenuator2400, an excellent characteristic in optical attenuation for each wavelength for each channel is achieved. Therefore, the wavelength-multiplexed light can be easily separated with such an excellent characteristic. Also, as described above, the variable optical attenuator2400is downsized. Therefore, a small-sized tunable filter having a simplified structure can be achieved.

FIG. 26is a schematic of a tunable filter according to a ninth embodiment of the present invention. As shown inFIG. 26, the tunable filter according to the present embodiment has a structure similar to that in the eighth embodiment, but the structure of the light input and output unit is different from that in the eighth embodiment.

Specifically, according to the eighth embodiment, the optical circulator2401and the single optical fiber for incident and emitted light2402(seeFIG. 24for both) form the light input and output unit. By contrast, according to the present embodiment, an incident-side optical fiber2601having an end at which a fiber collimator2602is placed, an emission-side optical fiber2604having an end at which a fiber collimator2605is placed, and a relay lens2603form the light input and output unit. That is, according to the present embodiment, a light inputting unit and a light outputting unit are separately provided in the tunable filter.

Also in the present embodiment having the structure described above, an effect similar to the effect described in the eighth embodiment is achieved.

In the above description, an angle formed by an incident plane formed by the incident light L1and the emitted light L2at the Savart plate104side of the relay lens2603and a plane formed by two optical axes in the Savart plate104has not been specifically mentioned. However, if the incident angle of the light L1is large, the incident plane and the plane formed by two optical axes are preferably orthogonal to each other because the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32due to tilted entering can be reduced.

Furthermore, in the eighth and ninth embodiments, the case in which the variable optical attenuator according to the present invention is applied to a tunable filter has been described. Alternatively, the variable optical attenuator according to the present invention can be applied for another purpose, such as a blocking filter.

Next, a tenth embodiment according to the present invention is described.FIG. 27Aincludes a side view of a modified Savart plate of the variable optical attenuator shown inFIG. 1and a schematic, viewed from the side view, of a beam position on a surface a at an incident beam side and a beam position on a surface b at emitted beam side.FIG. 27Bis a schematic of the progress of an incident beam when the modified Savart plate inFIG. 27Ais viewed from the bottom surface side.FIG. 27Cis a schematic of a crystallographic axis direction of the birefringent plates and a ½ wave plate forming the modified Savart plate viewed from the surface a inFIG. 27A.

The tenth embodiment is different from the first embodiment in the structure of the Savart plate104. The Savart plate104according to the first embodiment is a birefringent plate with two birefringent plates104aand104bbeing laminated together. By contrast, the Savart plate (modified Savart plate)104according to the tenth embodiment is a birefringent plate with two birefringent plates104aand104bbeing laminated via a ½ wave plate made of crystal, as shown inFIG. 27A. The birefringent plates104aand104bare both made of rutile (TiO2), which is an anisotropic material.

Also, as shown inFIG. 27C, the components forming the birefringent plate are laminated so that projection components on the optical axis104a′ of the birefringent plate104aand projection components on the optical axis104b′ of the birefringent plate104bare parallel to each other and an optical axis104c′ of the ½ wave plate forms an angle of 45 degrees with respect to each of the optical axes104a′ and104b′.

With this structure, as shown inFIG. 27A, the light L1(seeFIG. 1) enters the birefringent plate104aat a position denoted by a black dot on the drawing of “entering the surface a”, and is then emitted from the birefringent plate104bat positions denoted by black dots on the drawing of “emitted from the surface b. Here, the light L1having two polarizing components proceeds along different courses depending on the polarizing components, which is shown inFIG. 27B.

Here, the modified Savart plate104and the liquid-crystal polarization rotator element105are placed so that the optical axes104a′ and104b′ of the birefringent plates104aand104beach form an appropriate angle with respect to a direction of a liquid crystal director when the liquid-crystal polarization rotator element105functions as a ¼ wave plate, which will be described further below. The optical axes104a′ and104b′ are parallel to each other (seeFIGS. 2 and 3).

As shown inFIG. 3described above, the p-polarized light beam31and the s-polarized light beam32emitted from the liquid-crystal polarization rotator element105with the rotated polarizing direction again enter the birefringent plate104bof the modified Savart plate104. At this time, with the polarizing direction of each of the p-polarized light beam31and the s-polarized light beam32being rotated by 90 degrees, an incident optical route of each of the p-polarized light beam31and the s-polarized light beam32to the birefringent plate104bis a optical path different from an emission optical path from the birefringent plate104bto the liquid-crystal polarization rotator element105.

The p-polarized light beam31entering the birefringent plate104bproceeds through the birefringent plate104balong the optical axis104b′ (seeFIG. 27C) as being tilted, and after an azimuth of the polarized light is rotated by 90 degrees at the ½ wave plate104c, the p-polarized light beam31reaches the birefringent plate104a. Then, after horizontally proceeding through the birefringent plate104a, the p-polarized light beam31is emitted from the birefringent plate104ato the outside. Here, the p-polarized light beam31enters the birefringent plate104balong the optical path different from that when first entering. Therefore, the optical path along which the light beam sequentially passes the birefringent plate104b, the ½ wave plate104c, and then the birefringent plate104ais different from that when first entering.

On the other hand, the s-polarized light beam32entering the birefringent plate104bhorizontally proceeds through the birefringent plate104b, and after an azimuth of the polarized light is rotated by 90 degrees at the ½ wave plate104c, the p-polarized light beam32reaches the birefringent plate104a. Then, after proceeding through the birefringent plate104aalong the optical axis104a′ (seeFIG. 27C) as being tilted, the s-polarized light beam32is emitted from the birefringent plate104ato the outside. Here, the s-polarized light beam31enters the birefringent plate104balong the optical path different from that when first entering. Therefore, the optical path along which the light beam sequentially passes the birefringent plate104b, the ½ wave plate104c, and then the birefringent plate104ais different from that when first entering.

As described above, when the p-polarized light beam31and the s-polarized light beam32sequentially pass the birefringent plate104b, the ½ wave plate104c, and then the birefringent plate104a, similarly to the case of first entering, a difference in optical path length between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104band a difference in optical path length between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104acancel out each other. Therefore, the optical path along which the light beam sequentially passes through the birefringent plates104band104ais different from that at the time of first entering.

The p-polarized light beam31and the s-polarized light beam32emitted from the Savart plate104pass through the lens103, as shown inFIG. 1. In this process, the p-polarized light beam31and the s-polarized light beam32are re-combined. Here, the p-polarized light beam31and the s-polarized light beam32to be combined each have passed along the emission optical path different from the optical path at the time of first entering with the polarizing direction being rotated by the liquid-crystal polarization rotator element105. Therefore, an optical path of the emitted light L2obtained through combing both light beams is shifted from a position through which the emitted light L2can enter the emission-side optical fiber107.

Therefore, in this case, the emitted light L2does not enter the emission-side optical fiber107. Thus, the variable optical attenuator is a normally-off (-close)-type variable optical attenuator in which the emitted light L2to the outside is 0 when no voltage is applied to the liquid-crystal polarization rotator element105.

On the other hand, when a high voltage is applied to the liquid crystal layer105cof the liquid-crystal polarization rotator element105, as described with reference to either or both ofFIGS. 8 and 9andFIGS. 12 and 13, the polarizing directions of the p-polarized light beam31and the s-polarized light beam32reflected and emitted from the liquid-crystal polarization rotator element105are not rotated, and are identical to those at the time of first entering the liquid-crystal polarization rotator element.

As shown inFIG. 4, the p-polarized light beam31and the s-polarized light beam32emitted from the liquid-crystal polarization rotator element105with their polarizing directions being retained as those at the time of first entering again enter the birefringent plate104bof the modified Savart plate104. At this time, the p-polarized light beam31and the s-polarized light beam32are in the same polarizing directions as those at the time of first entering. Therefore, the incident optical paths of the p-polarized light beam31and the s-polarized light beam32to the birefringent plate104bare identical to the emission optical paths from the birefringent plate104bto the liquid-crystal polarization rotator element105.

The p-polarized light beam31entering the birefringent plate104bhorizontally proceeds through the birefringent plate104bto reach the birefringent plate104a. Then, after the p-polarized light beam31proceeds through the birefringent plate104aalong the optical axis104a′ (seeFIG. 27C) as being tilted, the azimuth of the polarized light is rotated by 90 degrees at the ½ wave plate, and then is emitted from the birefringent plate104a. Here, the p-polarized light beam31enters the birefringent plate104balong the same optical path as that at the time of first entering as described above. Therefore, the optical path along which the light beam sequentially passes through the birefringent plates104band104ais identical to that at the time of entering.

On the other hand, the s-polarized light beam32entering the birefringent plate104bproceeds through the birefringent plate104balong the optical axis104′b(seeFIG. 27C) as being tilted to reach the birefringent plate104a. Then, after horizontally proceeding through the birefringent plate104a, the s-polarized light beam32is emitted from the birefringent plate104ato the outside. Here, the s-polarized light beam32enters the birefringent plate104balong the same optical path as that at the time of first entering. Therefore, the optical path along which the light beam sequentially passes through the birefringent plates104band104ais identical to that at the time of first entering.

As described above, when the p-polarized light beam31and the s-polarized light beam32sequentially pass through the birefringent plate104b, the ½ wave plate104c, and then the birefringent plate104a, similarly to the case of first entering as described above, the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104band the difference in optical path length occurring between the p-polarized light beam31and the s-polarized light beam32in the birefringent plate104acancel out each other. Therefore, when viewed as the entire modified Savart plate104, the difference in optical path length between the p-polarized light beam31and the s-polarized light beam32has been solved.

Other than the above, the present embodiment is similar to the first embodiment, and therefore is provided with the same reference numerals as those in the first embodiment and is not described further. Also, as with the first embodiment, the second to ninth embodiments can be applied to the tenth embodiment.

Furthermore, when an arrayed liquid-crystal polarization rotator element is used for the variable optical attenuator according to any one of the seventh to ninth embodiment of the present invention, if a liquid crystal director is provided so as to be parallel to, be perpendicular to, or be at an angle of 45 degrees to a longitudinal direction of the array sequence, alignment of the liquid crystal director can be selectively performed when retardation in the liquid-crystal polarization rotator element pixels is large.

For example, a polarized-light-rotation ½ wave plate is placed between the modified Savart plate104, and the liquid-crystal polarization rotator element105, and the liquid crystal director is aligned so as to be parallel to the longitudinal direction of the array sequence. In this case, it is possible to prevent an operation in which, particularly when a pixel pitch is narrow, the liquid crystal director of the adjacent pixel in a fringing field of the electrode is subjected to twist deformation, thereby extending an effective area of effective pixels.

Still further, depending on the electric field applied to each pixel, it is possible to prevent an operation in which the liquid crystal director of the adjacent pixel in a fringing field of the electrode is subjected to reverse-tilt deformation when the liquid crystal direction is aligned so as to be perpendicular to the array sequence.

Still further, when the liquid crystal director is aligned so as to be at an angle of 45 degrees to the array sequence, modification is possible at each pixel without changing the orientation of the polarized light emitted from the modified Savart plate104. Therefore, a polarized-light-rotation ½ wave plate can be omitted.

Still further, the structure and operation of the variable optical attenuator and the optical filter using such a variable optical attenuator are not meant to be restricted to the first to tenth embodiments described above, and may be those other than these embodiments.

For example, the optical-path-length-difference correcting unit may be any unit other than a Savart plate. For example, such an optical-path-length-difference correcting unit may be a birefringent plate other than a Savart plate as long as the birefringent plate can correct the difference in optical path length of the polarized light components.

As has been described in the foregoing, according to the embodiments of the present invention, the optical path lengths of two polarized light components separated by the Savart plate can be made identical to each other. As a result, in the variable optical attenuator, PDL and PMD can be reduced, thereby achieving an excellent optical attenuation characteristic.

Also, according to the embodiments of the present invention, birefringent plates and a ½ wave plate are combined to allow correction of a difference in optical path length between the optical paths of the separated polarized light beams in the birefringent plates. Therefore, the optical path lengths of two polarized light components separated by the modified Savart plate can be made identical to each other. As a result, in the variable optical attenuator, PDL and PMD can be reduced, thereby achieving an excellent optical attenuation characteristic.

Furthermore, according to the embodiments of the present invention, a polarized-light-rotation ½ wave plate for rotating a polarized light beam is further placed between the optical-path-length-correction birefringent plate and the liquid-crystal polarization rotator element. Therefore, the azimuth direction of the polarized light beam emitted from the modified Savart plate can be set in an optimal direction with respect to a direction of the liquid crystal director of the liquid-crystal polarization rotator element at the subsequent stage.

Still further, according to the embodiments of the present invention, the liquid-crystal polarization rotator element is a uniaxial variable wave plate. Therefore, attenuation of the amount of emitted light from the liquid-crystal polarization rotator element can be improved. For example, such a uniaxial variable wave plate is formed by using a liquid crystal proven for a liquid crystal display, such as a nematic liquid crystal, a surface-stabilized ferroelectric liquid crystal, or an antiferroelectric liquid crystal.

Still further, according to the embodiments of the present invention, in the liquid-crystal polarization rotator element, the liquid crystal is in homogeneous alignment or vertical alignment, and an electric field is formed in the direction of thickness of the liquid crystal with application of a voltage to the liquid crystal. Alternatively, the liquid crystal is in homogeneous alignment or vertical alignment, and an electric field is formed in the lateral direction of the liquid crystal. Still alternatively, the liquid crystal is in homogeneous alignment or vertical alignment, and an electric field is formed in the direction of thickness and the lateral direction of the liquid crystal. Therefore, for example, with the electric field formed in the direction of thickness, retardation of the liquid crystal can be controlled. Alternatively, with the electric field formed in the lateral direction, the placement direction in the longitudinal direction of the liquid crystal in the lateral direction perpendicular to the direction of thickness can be controlled. Alternatively, with both of these electric fields being simultaneously formed, retardation and the placement direction in the longitudinal direction of the liquid crystal can be controlled. With these controls, rotation of the polarizing direction with the liquid crystal can be controlled.

Still further, according to the embodiments of the present invention, one of paired electrodes of the liquid-crystal polarization rotator element is formed on a Si substrate, and a driving circuit that controls the voltage is placed on that Si substrate. Therefore, it is possible to achieve a highly-functional, low-power-consumption structure with a driving circuit and a control circuit being integrated thereon.

Still further, according to the embodiments of the present invention, voltage control is performed through a pulse width modulation scheme. Therefore, the driving circuit is downsized, and the control is easily performed. Also, a multi-tone driving circuit can be formed exclusively with digital circuits.

Still further, according to the embodiments of the present invention, a pair of electrodes includes a transparent electrode formed of a transparent conductive film and a reflecting electrode formed of a reflective conductive film, and the transparent electrode is formed of an ITiO film. Therefore, the same conductivity as that of the transparent electrode can be achieved from ITO. Also, plasma reflection can be suppressed more than the case in which the transparent film is formed of ITO. Therefore, the optical attenuation characteristic is further improved.

Still further, according to the embodiments of the present invention, the light input and output unit is formed of optical fibers, and an optical fiber for light input and an optical fiber for light output are separately provided. Therefore, light input and light output are separately performed through separate optical fibers. Thus, no optical circulator is required, thereby reducing the number of components.

Still further, according to the embodiments of the present invention, the light input and output unit is formed of an optical fiber, and light input and light output is performed through the common optical fiber. Therefore, the number of placement of optical fibers can be reduced, thereby simplifying the structure.

Still further, according to the embodiments of the present invention, a fiber collimator is placed at an end of the optical fiber. Therefore, with the fiber collimator, optical coupling efficiency to the optical fiber can be improved.

Still further, according to the embodiments of the present invention, a light-gathering optical lens is placed between the optical fiber and the optical-path-length-difference correction plate. Also, the beam diameter of light entering and emitted from the fiber collimator is set to be equal to or smaller than 50 μm. Furthermore, light input and output is directly performed between the fiber collimator and the optical-path-length-difference correction plate. Therefore, even when a light-gathering optical lens is not placed between the optical fiber and the optical-path-length-difference correction plate, light gathering can be made similarly to the case in which such an optical lens is placed.

Still further, according to the embodiments of the present invention, the liquid-crystal polarization rotator element has a structure in which a plurality of liquid-crystal polarization rotator element pixels are arrayed, and the voltage can be applied to each liquid crystal of each liquid-crystal polarization rotator element pixel separately for each pixel. Therefore, a multichannel variable optical attenuator can be achieved.

Still further, according to the embodiments of the present invention, in the liquid-crystal polarization rotator element, the liquid crystal director is aligned so as to be parallel to or perpendicular to the longitudinal direction of the array sequence when retardation in the liquid-crystal polarization rotator element pixels is large. Thus, in combination of the modified Savart plate and the polarized-light-rotation ½ wave plate, the liquid crystal director is aligned so as to be parallel to the array. In this case, it is possible to prevent an operation in which, when a pixel pitch is narrow, the liquid crystal director of the adjacent pixel in a fringing field of the electrode is subjected to twist deformation, thereby extending an effective area of arrayed effective pixels.

Still further, according to the embodiments of the present invention, in the liquid-crystal polarization rotator element, the liquid-crystal polarization rotator element pixels and the driving circuit for the liquid-crystal polarization rotator element pixels are placed on the same Si substrate. Therefore, it is possible to achieve a liquid-crystal polarization rotator element having a circuit, such as a driving circuit, and a plurality of liquid-crystal polarization rotator element pixels integrated thereon. Thus, a small-sized multichannel optical attenuator with reduced power consumption can be achieved.

Still further, according to the embodiments of the present invention, drivers that drive the driving circuit are further provided. The drivers are provided on an upper portion and a lower portion of a placement area of the liquid-crystal polarization rotator element pixels. The adjacent driving circuits of the liquid-crystal polarization rotator element pixels are alternately connected to the driver on the upper portion and the driver on the lower portion. Also, the area for placing the liquid-crystal polarization rotator element pixels and the drivers are separated from each other. Therefore, the area for placing the liquid-crystal polarization rotator element pixels does not have to be planarized, thereby simplifying a manufacturing process. Also, the liquid-crystal polarization rotator element pixels are alternately connected to the upper driver and the lower driver, thereby widening a pitch of the driver circuits.

Still further, according to the embodiments of the present invention, the light input and output unit is formed of optical fibers, and the number of placement of optical fibers for light input is equal to or smaller than the number of placement of liquid-crystal polarization rotator element pixels of the liquid-crystal polarization rotator element. When the number of placement of optical fibers for light input is equal to the number of placement of liquid-crystal polarization rotator element pixels of the liquid-crystal polarization rotator element, one channel in an optical attenuation operation of the variable optical attenuator is achieved by one liquid-crystal polarization rotator element pixel. On the other hand, when the number of placement of optical fibers for light input is smaller than the number of placement of liquid-crystal polarization rotator element pixels of the liquid-crystal polarization rotator element, one channel in an optical attenuation operation of the variable optical attenuator is achieved by a plurality of liquid-crystal polarization rotator element pixels.

Still further, according to the embodiments of the present invention, the optical filter includes the variable optical attenuator described above. Therefore, since the effect can be attained in the variable optical attenuator, an excellent filter characteristic can be achieved.

According to the present invention, a small-sized variable optical attenuator having an excellent optical attenuation characteristic can be achieved. Moreover, a small-sized optical filter with an excellent filter characteristic can be achieved.

The present document incorporates by reference the entire contents of Japanese priority document, 2004-122646 filed in Japan on Apr. 19, 2004, 2004-211002 filed in Japan on Jul. 20, 2004 and 2005-070169 filed in Japan on Mar. 11, 2005.