JP2009122495A - Wavelength selective optical attenuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the temperature characteristics of a wavelength selective optical attenuator used for optical wavelength multiplex communication and the like. <P>SOLUTION: Wavelength multiplexed light is separated for every wavelength by a diffraction grating 15, and a liquid crystal plate 20 is irradiated with the light via a λ/2 wavelength plate 18 and a polarizer 19. The liquid crystal plate 20 transmits the light by rotating the polarization direction by the optical rotatory power of the liquid crystal in an off state where voltage is not applied to each electrode. Also, the liquid crystal plate 20 transmits the light as it is by aligning the liquid crystal molecules in an electric field direction in an on state where the voltage is applied to each electrode of the liquid crystal plate. In this case, temperature dependence disappears and therefore, the wavelength selective optical attenuator reduced in temperature dependence together with the blocking and transmission of the light can be provided. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a wavelength-selective optical attenuator that is used for optical wavelength division multiplexing communication and the like and selectively attenuates light having a desired wavelength.

  In order to meet the rapidly increasing demand for communication capacity, optical networks using optical wavelength division multiplexing technology (WDM communication) have been put into practical use. In the WDM transmission system, a plurality of light beams having wavelength intervals determined by ITU (International Telecommunication Union) are multiplexed to increase the transmission capacity. WDM communication was initially introduced for the purpose of high-capacity and high-speed communication between two long distances using optical fibers, but it has also been applied to networks in metro access areas as the amount of information has increased in recent years. ing. In particular, in a WDM transmission system, a transmission apparatus having an add / drop function for each wavelength is mainly used at each node of a ring network.

  As shown in FIG. 1, a WDM transmission apparatus that realizes an add / drop function is configured such that a WDM signal is incident on a coupler 101, a part of the WDM signal is separated and taken out as a drop signal, and the rest is used as a wavelength selective optical attenuator ( (Also referred to as a wavelength blocker) 102. Then, a part of the wavelength components of the drop signal is extracted, a signal having the same wavelength component as the extracted wavelength is newly generated, and supplied to the coupler 103 as an add signal. The wavelength selective optical attenuator 102 blocks the extracted wavelength component of the optical signal branched by the coupler 101 and transmits other wavelength components. The coupler 103 combines the signal that has passed through the wavelength-selective optical attenuator 102 and the add signal into a new WDM signal. Further, the wavelength selective optical attenuator 102 has an attenuation function for adjusting the WDM signal intensity in order to completely block the inserted wavelength and to make the dispersion of the light intensity uniform for each WDM signal.

  Patent Document 1 shows an example of a wavelength selector used as a wavelength-selective optical attenuator. In this document, as shown in FIG. 2, WDM signals are aligned in the same polarization direction, for example, S polarization. Then, the light is separated for each wavelength of light, enters the lens 110 as a plurality of light beams A1 to An, and enters the liquid crystal 112 that rotates the polarized light disposed at the focal position of the lens 110 through the polarizer 111 by 90 °. A polarizer 113 is disposed on the emission side of the liquid crystal 112. Further, the polarization direction through which the light of the polarizers 111 and 113 is transmitted is made the same, and the exit lens 114 is arranged at a position symmetrical to the lens 110.

The liquid crystal 112 is formed with an electrode at a position where it is incident on each wavelength, and a voltage is applied to the electrode corresponding to the wavelength to pass through, and no voltage is applied to the electrode at the position where light is shielded. When a voltage is applied as shown in FIG. 3A, the liquid crystal 112 transmits the incident light as it is without rotating the polarization plane of the incident light. Further, when no voltage is applied, there is optical rotation, and as shown in FIG. 3B, the polarization angle of incident light is rotated by 90 ° and emitted as P-polarized light. As a result, as shown in FIG. 2A, the light separated for each wavelength passes through the pair of polarizers 111 and 113 and the liquid crystal electrode when passing through the electrode position to which the voltage is applied. Further, as shown in FIG. 2B, light can be shielded when passing through an electrode position to which no voltage is applied.
Special Table 2007-519010

ここでｕは次式で示される。

ｄ：液晶セルギャップ
Δｎ：複屈折率
λ：波長
ここで液晶素子は入力する偏光により屈折率が異なり、その屈折率差がΔｎとなっている。Δｎは温度依存性を持つため、式（１），（２）より透過率Ｔも温度依存性を持ち、透過率Ｔが最も低い波長、即ち最大消光が得られる波長も温度により変化する。図４はこの波長選択性光減衰器の電圧を印加としたときの異なった温度状態における挿入損失を示すものであり、電圧を印加し、光を遮断状態とするときに温度による波長の変動が大きい。従って使用する波長の範囲内で消光特性を安定させるために、正確な温度制御を行ったり、その周囲温度に合わせて印加する電圧レベルを調整する必要があった。 Now, in such a wavelength selective optical attenuator having the configuration of Patent Document 1, the light transmittance T is expressed by the following equation.

Here, u is expressed by the following equation.

d: Liquid crystal cell gap Δn: Birefringence index λ: Wavelength Here, the liquid crystal element has a different refractive index depending on the input polarized light, and its refractive index difference is Δn. Since Δn has temperature dependence, the transmittance T also has temperature dependence according to the equations (1) and (2), and the wavelength at which the transmittance T is the lowest, that is, the wavelength at which maximum extinction can be obtained varies with temperature. FIG. 4 shows the insertion loss in different temperature states when the voltage of the wavelength selective optical attenuator is applied. When the voltage is applied and the light is cut off, the variation of the wavelength due to the temperature is shown. large. Therefore, in order to stabilize the extinction characteristic within the wavelength range to be used, it is necessary to perform accurate temperature control or to adjust the voltage level to be applied according to the ambient temperature.

  The present invention has been made paying attention to such problems, and an object thereof is to provide a wavelength-selective optical attenuator that eliminates the influence of temperature and stably obtains optical attenuation characteristics.

  In order to solve this problem, the wavelength-selective optical attenuator of the present application separates incident light into two light beams according to the polarization state, and rotates the polarization direction of one of the separated polarizations. An incident-side polarization diversity unit that emits two light beams having the same polarization direction, and a wavelength that separates the light beams of the two light beams that have passed through the incident-side polarization diversity unit and reflects them in different directions. The light separated by the dispersion element and the wavelength dispersion element is collected by a lens, and a plurality of electrodes corresponding to each wavelength are arranged in parallel, and a voltage applied to an arbitrary electrode among the plurality of electrodes A first liquid crystal plate that transmits the light with the polarization direction of the incident light transmitted when the voltage is not applied to the electrode while maintaining the polarization direction of the incident light at the time of application; LCD panel A voltage source that applies a voltage to an electrode corresponding to a wavelength that attenuates light and is connected to a pole, and light that has passed through the first liquid crystal plate is collected by a lens, and light incident from different directions is synthesized. Then, the wavelength combining element that reflects in the same direction as two light beams and the polarization direction of one of the light beams that have passed through the wavelength combining element are rotated to combine the two light beams. And an exit-side polarization diversity unit that emits light in the same direction.

  In order to solve this problem, the wavelength-selective optical attenuator of the present application separates incident light into two light beams according to the polarization state, and rotates the polarization direction of one of the separated polarizations. The incident-side polarization diversity unit that emits two light beams having the same polarization direction and the light beams of the two light beams that have passed through the incident-side polarization diversity unit are separated for each wavelength and reflected in different directions. A chromatic dispersion combining element that combines light incident from different directions and reflects them in the same direction as two light beams, and the light separated by the chromatic dispersion combining element is condensed by a lens, for each wavelength. A plurality of electrodes corresponding to each of the electrodes are arranged in parallel, and when a voltage is applied to any of the plurality of electrodes, the polarization direction of incident light is maintained as it is reflected, and the voltage to the electrodes is applied. When not applied, a voltage is applied to the first liquid crystal plate that reflects the light whose polarization direction of incident light is rotated and the electrode corresponding to the wavelength that attenuates the light, connected to each electrode of the first liquid crystal plate And a light-emitting-side polarization diversity unit that rotates the polarization direction of one of the light beams combined by the wavelength dispersion combining element, combines the two light beams, and emits them in the same direction. Are provided.

  Here, a voltage may be applied to the liquid crystal plate, and the state in which the polarization direction is maintained and transmitted is used as the light attenuation state.

  Here, an optical switch may be further provided which is provided in the light passage path, transmits light when power is supplied, and blocks light when power is not supplied.

  Here, a second liquid crystal plate that is provided in the light passage path and transmits light while maintaining the polarization direction of light when a voltage is applied, and changes the polarization direction of incident light when no voltage is applied. You may make it have further.

  Here, in the first liquid crystal plate, electrodes for each wavelength may be arranged in parallel so that the distance between the electrodes is continuously different.

  According to the present invention having such a feature, when light is transmitted, no voltage is applied to the first liquid crystal plate from the voltage source, and the first liquid crystal plate is transmitted by rotating the polarization direction of the light. Further, if a voltage is applied to the first liquid crystal plate when the light is attenuated, the light application property is lowered, and the light can be attenuated without temperature characteristics. For this reason, the effect that the temperature characteristic of a wavelength selective optical attenuator can be improved significantly is acquired.

(First embodiment)
A wavelength-selective optical attenuator according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 5 is a diagram showing an incident portion of light on the XY plane, FIG. 6 is a diagram showing an arrangement of main optical elements of the wavelength selective optical attenuator on the XZ plane viewed from the Y-axis direction, and FIG. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator on XY plane seen from the Z-axis direction. 5 and 6, WDM signal light having wavelengths λ1, λ2,... Λn is incident on the optical fiber 11 from one end. A birefringent crystal 13 is disposed on the other end of the optical fiber 11 via a collimator 12. The birefringent crystal 13 separates incident light by going straight or refracting depending on the polarization component, and emits the incident light as two light beams of S-polarized light indicated by a circle and P-polarized light indicated by a vertical arrow in the figure. It is a polarization separation element. For example, rutile is used as the birefringent crystal. Among them, a λ / 2 wavelength plate 14 is disposed on the emission side of the P-polarized light beam. The λ / 2 wavelength plate 14 is an S-polarized light by rotating the polarization direction of a P-polarized light beam by 90 °, and the two light beams B1 and B2 are both an S-polarized light beam. These constitute the incident-side polarization diversity section, and can be converted into two collimated lights B1 and B2 having a constant linear polarization regardless of the polarization state of the incident light.

  Now, the outputs of the two light beams B1 and B2 from this polarization diversity section are given to the diffraction grating 16 through the prism 15 as shown in FIG. The prism 15 functions as a beam expander that expands the beam diameter of light. A plurality of prisms can be used. Since it is shown on the XZ plane in FIG. 6, the two light beams B1 and B2 are displayed as one light beam. The diffraction grating 16 is a wavelength dispersion element that reflects WDM light in different directions depending on the wavelengths of light λ1 to λn. A lens 17 is provided on the emission side of each separated light beam. The lens 17 focuses light reflected by the diffraction grating 16 in different directions for each wavelength, and a λ / 2 wavelength plate 18 is disposed on the exit side. The λ / 2 wavelength plate 18 converts incident light from S-polarized light indicated by a circle in the figure to P-polarized light indicated by a vertical arrow by rotating the polarization direction by 90 °. A polarizer 19 is provided on the emission side of the λ / 2 wavelength plate 18. The polarizer 19 transmits the P-polarized component light as it is to the liquid crystal plate 20 and shields the S-polarized component to improve the polarization purity of the light, and is not necessarily required if the polarization purity is sufficiently high. A liquid crystal plate 20 is disposed at the focal position of the lens 17. The liquid crystal plate 20 uses, for example, TN liquid crystal. The liquid crystal plate 20 rotates the polarization direction by 90 ° when no voltage is applied. When the voltage is applied, the liquid crystal molecules are aligned in the electric field direction, so there is no birefringence and the light is transmitted while maintaining the polarization direction. is there.

  FIG. 8A shows a front view of the liquid crystal plate 20 viewed from the X-axis direction. On the front surface of the liquid crystal plate 20, a number of transparent electrodes 21-1 to 21-n corresponding to the number n of channels of the WDM signal light are formed at regular intervals in parallel with the Y-axis direction. The electrode is made of, for example, ITO. A voltage source 22 is connected to each electrode of the liquid crystal plate 20. The voltage source 22 applies a voltage independently between each transparent electrode 21-1 to 21-n of the liquid crystal plate 20 and the entire transparent electrode on the back surface, thereby applying a voltage to each region where the electrode is formed. It can be controlled. Here, when a voltage is applied, a rectangular wave having a constant frequency is used.

  Further, a polarizer 23 and a lens 24 similar to the polarizer 19 are provided along the light traveling direction. The polarizer 23 transmits S-polarized light. The lens 24 is a symmetric lens having the same characteristics as the lens 17, and collects the light transmitted through the liquid crystal plate 20 to produce collimated light. Similarly to the incident side, a diffraction grating 25, a prism 26, a λ / 2 wavelength plate 27 and a birefringent crystal 28 are provided, and are further connected to an optical fiber 30 through a collimator 29. Here, the diffraction grating 25 is a wavelength synthesizing element that synthesizes light incident from different directions and reflects them in the same direction as two light beams, and the birefringent crystal 28 synthesizes polarized light that synthesizes two light beams. Functions as an element. Further, the λ / 2 wavelength plate 27, the birefringent crystal 28, and the collimator 29 constitute an output-side polarization diversity section. The pair of polarizers 19 and 23 are for improving the purity of the extinction ratio, and are not necessarily required when the polarization is sufficiently high.

  Next, the operation of the wavelength selective optical attenuator according to the first embodiment will be described. First, when all the wavelength components of the WDM light applied to the optical fiber 11 are transmitted, no voltage is applied from the voltage source 22 to any electrode of the liquid crystal plate 20, or a weak voltage at which the liquid crystal molecules do not rotate. Apply. FIG. 6 shows this case. When WDM light is incident on the collimator 12 from the optical fiber 11, the WDM signal usually contains various polarization components, so that two light beams of S-polarized light and P-polarized light are emitted from the birefringent crystal 13 according to the polarization direction. . Among them, the P-polarized light becomes S-polarized light through the λ / 2 wavelength plate 14, and becomes two S-polarized light beams B1 and B2. The beam width is expanded by the prism 15 and added to the diffraction grating 16. In the diffraction grating 16, light is separated for each wavelength and applied to the lens 17. Light that has passed through the lens 17 is applied to the liquid crystal plate 20, while the λ / 2 wavelength plate 18 rotates the polarization direction by 90 ° to become P-polarized light, and further passes through the polarizing plate 19 and is applied to the liquid crystal plate 20. FIG. 8A shows a state where a light beam dispersed for each wavelength enters each of the electrodes 21-1 to 21-n. Here, since no voltage is applied to any electrode of the liquid crystal plate 20, the liquid crystal plate has an optical rotation, and when passing through the liquid crystal, the polarization direction of the light is rotated by 90 ° and becomes S-polarized light again. . Then, the light passes through the polarizer 23 as it is, is condensed again by the lens 24, and is added to the diffraction grating 25. The diffraction grating 25 is a wavelength synthesizing element that synthesizes wavelengths by using light incident from different directions for each wavelength as two light beams, and the reflected light is given to the prism 26. These light beams are reduced in beam diameter via the prism 26. Furthermore, the polarization direction of one of the light beams is converted from S-polarized light to P-polarized light by the λ / 2 wavelength plate 27 by the polarization diversity unit similar to the incident side, and the birefringent crystal 28 forms one light beam. Through the optical fiber 30. As a result, all wavelength components of the WDM light incident from the optical fiber 11 are once decomposed, further integrated, and output from the optical fiber 30 as they are.

  Next, when blocking the wavelength λi of the WDM light applied to the optical fiber 11, a voltage is applied only to the electrode 21-i of the liquid crystal plate 20 corresponding to the blocked wavelength, and no voltage is applied to the other electrodes. . In the electrode 21-i of the liquid crystal plate 20 to which a voltage is applied, the liquid crystal molecules are arranged in the voltage application direction, so that the optical rotation is lost and the light is transmitted as P-polarized light. Therefore, light is shielded by the polarizing plate 23 and is not added after the lens 24. Thereby, the light of this wavelength can be blocked. The light that cannot be shielded by the polarizing plate 23 is shielded by the polarization diversity section on the emission side.

  When light is blocked in this way, a voltage is applied to a desired electrode of the liquid crystal, whereby the liquid crystal molecules are aligned in the direction of the electric field to which the voltage is applied. At this time, since the molecular arrangement is horizontal in the light traveling direction, the birefringence disappears with respect to the input light when an appropriate voltage is applied. For this reason, there is almost no temperature characteristic, and the insertion loss with respect to the wavelength becomes a substantially constant level as shown by a curve A in FIG. 9A. Curve B shown in FIG. 9A shows a comparison of the insertion loss of a conventional wavelength selective optical attenuator that blocks light when no voltage is applied, and is not only highly wavelength dependent but also as described above. The change in wavelength due to temperature characteristics is large.

  On the other hand, when no voltage is applied, light is transmitted due to optical rotation, but in this case as well, the dependence on wavelength does not change much as shown in FIG. 9B. In this way, the temperature dependence of the wavelength of the wavelength selective optical attenuator can be reduced.

(Second Embodiment)
Next, a wavelength-selective optical attenuator according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 10 is a view showing an incident part of light on the XY plane, FIG. 11 is a view showing the arrangement of the main optical elements of the wavelength selective optical attenuator on the XZ plane viewed from the Y-axis direction, and FIG. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator on the YZ plane seen from the X-axis direction. In FIG. 10, WDM signal light having wavelengths λ1, λ2,. A birefringent crystal 43 is disposed on the other end of the optical fiber 41 via a collimator 42. The birefringent crystal 43 is a polarization separation element that separates incident light by traveling or refracting light according to the polarization component, and emits it as two light beams of S-polarized light and P-polarized light. For example, rutile is used. Among them, a λ / 2 wavelength plate 44 is disposed on the emission side of the P-polarized light beam. The λ / 2 wavelength plate 44 is an S-polarized light by rotating the polarization direction of a P-polarized light beam by 90 °, and the two light beams B1 and B2 are both an S-polarized light beam. These constitute the incident-side polarization diversity section, and can be converted into two collimated lights B1 and B2 having a constant linear polarization regardless of the polarization state of the incident light.

  Now, the outputs of the two light beams B1 and B2 from the polarization diversity section are given to the diffraction grating 46 through the prism 45 as shown in FIG. The prism 45 functions as a beam expander that expands the beam diameter of light. A plurality of prisms can be used. Since it is shown on the XZ plane in FIG. 11, the two light beams B1 and B2 are displayed as one light beam. The diffraction grating 46 is a wavelength dispersion / synthesizing element that reflects WDM light in different directions according to light and reflects light having wavelengths λ1 to λn from different directions in the same direction. A λ / 2 wavelength plate 47 is disposed on the exit side of the diffraction grating 46. The λ / 2 wavelength plate 47 converts incident light from S-polarized light indicated by a circle in the figure to P-polarized light indicated by a vertical arrow by rotating the polarization direction by 90 °. A polarizer 48 is provided on the emission side of the λ / 2 wavelength plate 47. The polarizer 48 transmits the P-polarized light component as it is to the liquid crystal plate 50 and shields the S-polarized light component to increase the polarization purity of the light, and is not necessarily required if the polarization purity is sufficiently high. A lens 49 is provided on the emission side of each separated light beam. The lens 49 focuses the light diffracted by the diffraction grating 46 in different directions for each wavelength. A liquid crystal plate 50 is disposed at the focal position of the lens 49. The liquid crystal plate 50 uses, for example, TN liquid crystal, and a large number of transparent electrodes are arranged on the lens 49 side surface in parallel with the Y-axis direction. The configuration of this electrode is the same as that of the above-described embodiment. A metal electrode that reflects light, for example, a metal electrode such as Al, Au, or Ag, is deposited on the back surface. When no voltage is applied, the liquid crystal plate 50 is reflected by rotating the polarization direction of incident light by 90 °, and when a voltage is applied, the liquid crystal molecules are aligned in the electric field direction so that there is no birefringence and the polarization direction is maintained. It reflects light. The liquid crystal plate 50 is the same as the liquid crystal plate 20 described above except that the back surface is a reflective surface.

  The liquid crystal plate 50 is provided with a voltage source 51. Similar to the voltage source 22 of the first embodiment, the voltage source 51 applies a rectangular wave voltage to an electrode having a wavelength corresponding to shielding light, and corresponds to the wavelength when transmitting light. A voltage is not applied to the electrodes, or a weak voltage that does not rotate the liquid crystal molecules is applied.

  Further, as shown in FIG. 12, a polarizer 52 and an optical path length adjusting plate 53 similar to the polarizer 48 described above are provided via a lens 49 on the reflection side of the liquid crystal plate 50. The lens 49 condenses the light transmitted through the liquid crystal plate 50 to produce collimated light. The optical path length adjusting plate 53 is an element having an optical length corresponding to the λ / 2 wavelength plate 47. The same diffraction grating 46, prism 45, λ / 2 wavelength plate 54 and birefringent crystal 55 as those on the incident side are provided, and are further connected to an optical fiber 57 via a collimator 56. Here, the λ / 2 wavelength plate 54, the birefringent crystal 55, and the collimator 56 constitute a polarization diversity section on the output side. The pair of polarizers 48 and 52 is for improving the purity of the extinction ratio, and is not necessarily required when the polarization purity is sufficiently high.

  In the present embodiment, since the back surface of the liquid crystal plate 50 is a metal electrode, light is reflected by this surface, and a reflective wavelength selective optical attenuator can be obtained. The diffraction grating 46 has a function of dispersing the wavelength and synthesizing the wavelength. Since other configurations are the same as those of the first embodiment described above, detailed description thereof is omitted.

(Third embodiment)
Next, a wavelength selective optical attenuator according to a third embodiment of the present invention will be described. In a WDM transmission system, the wavelength-selective optical attenuator arranged at each node has an optical signal of a new wavelength and a wavelength-selective optical attenuation that are added from the add port when it falls out of control due to a power failure. The wavelength-selective optical attenuator is required to have a function of blocking (quenching) all channels of all WDM signals so as not to compete with the signal that has passed through the filter. In this case, it is required to completely block light over the entire operating temperature range without applying a voltage to the internal temperature or liquid crystal, or controlling the temperature. In this embodiment, light is shielded when no power is supplied between the collimator 12 and the birefringent crystal 13 as shown in FIG. An optical switch 61 is provided. As the optical switch, for example, a non-latch type mechanical switch using a MEMS shutter or an electromagnetic solenoid can be used. Such an optical switch allows light to pass when in a power supply state and mechanically blocks the light when power supply is stopped. The position of the switch 61 can be provided at an arbitrary position on the light passage path of the wavelength selective optical attenuator. The function of the switch 61 can be provided separately from the wavelength selective optical attenuator.

(Fourth embodiment)
Next, a wavelength selective optical attenuator according to a fourth embodiment of the present invention will be described. In this embodiment, instead of the mechanical optical switch in the third embodiment, a light switching function is realized by using a liquid crystal plate. In this embodiment, in addition to the configuration of the first embodiment, a liquid crystal plate 62 is disposed between the prism 15 and the diffraction grating 16 as shown in FIG. The liquid crystal plate 62 is a second liquid crystal plate that uses TN liquid crystal and transmits polarized light without rotating in an energized state, and rotates polarized light in an unenergized state.

  In this way, since the polarization does not rotate if a predetermined voltage is applied during energization, the operation of a normal wavelength-selective optical attenuator is performed as described in the first embodiment. When the supply of power is stopped for some reason, the polarized light rotates, so that it is extinguished by the diffraction gratings 16 and 25, and does not pass through the polarizers 19 and 23, so that the light is blocked. Therefore, light can be shielded in the event of a power failure or the like. Further, the extinction function can be achieved by the polarization diversity on the output side. In addition to this, if a liquid crystal plate 63 is further provided between the prism 26 and the diffraction grating 25, the polarization component that could not be extinguished is rotated by 90 °, so that better extinction characteristics can be expected when the power is stopped. .

  In the first and second embodiments described above, the electrodes of the liquid crystal plate 20 are parallel as shown in FIG. 8A. On the other hand, each electrode may be slightly tilted as shown in FIG. 8B, and the electrodes may be arranged at different intervals along the Y-axis direction. If formed in this way, the position of the liquid crystal plate 20 is finely adjusted in the Y-axis direction even when the beam spacing for each wavelength slightly changes at the passage position of the liquid crystal due to the change in the focal length of the lens 17 or the like. An optimum position can be set just by adjusting. Similarly, in the second embodiment, the liquid crystal plate 50 can have the electrode structure shown in FIG. 8B.

  In the polarization diversity unit of the above-described embodiment, rutile is used to disperse light depending on the polarization direction. However, any other birefringent crystal may be used, or a polarization beam splitter may be used.

  In this embodiment, the diffraction grating is used as the wavelength dispersion element, the wavelength synthesis element, and the wavelength dispersion synthesis element. However, an element such as a prism or VIPA may be used instead of the diffraction grating.

  Furthermore, the wavelength selective optical attenuator of the present invention can be applied not only to a node of a WDM optical communication system but also to other fields. For example, by appropriately controlling the voltage level applied to each electrode of the liquid crystal plate, the attenuation factor at that wavelength changes. Therefore, by adjusting the voltage level of each electrode, the spectrum of incident light can be changed to obtain light having an arbitrary spectrum.

It is a figure which shows the wavelength selective optical attenuator used for a WDM transmission apparatus. It is a figure which shows the principal part of the conventional wavelength selective optical attenuator. It is a figure which shows the application of the voltage with respect to a liquid crystal, and the change of a polarization direction. It is a figure which shows an example of the insertion loss of the conventional wavelength selective optical attenuator. It is a figure which shows the incident side polarization diversity part of the wavelength selective optical attenuator by embodiment of this invention. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator by this Embodiment. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator by this Embodiment. It is a front view which shows the liquid crystal plate by this Embodiment. It is a front view which shows other embodiment of the liquid crystal plate by this Embodiment. It is a graph which shows the relationship between the insertion loss at the time of not applying a voltage of this Embodiment and a prior art example, and a wavelength. It is a graph which shows the relationship between the insertion loss at the time of applying the voltage of this Embodiment, and a wavelength. It is a figure which shows the polarization diversity part of the wavelength selective optical attenuator by the 2nd Embodiment of this invention. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator by this Embodiment. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator by this Embodiment. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator by the 3rd Embodiment of this invention. It is a figure which shows arrangement | positioning of the main optical elements of the wavelength selective optical attenuator by the 4th Embodiment of this invention.

Explanation of symbols

11, 30, 41, 57 Optical fiber 12, 29, 42, 56 Collimator 13, 28, 43, 55 Birefringent crystal 14, 18, 27, 44, 47, 54 λ / 2 wavelength plate 15, 26, 45 Prism 16 , 25, 46 Diffraction grating 17, 24, 49 Lens 19, 23, 48, 52 Polarizer 20, 50 Liquid crystal plate 21-1 to 21-n Transparent electrode 22, 51 Voltage source 53 Optical path length adjusting plate 61 Optical switch 62, 63 LCD panel

Claims (6)

Incident-side polarization diversity unit that divides incident light into two light beams according to the polarization state and outputs two light beams with the same polarization direction by rotating either polarization direction of the separated polarized light When,
A wavelength dispersion element that separates the light beams of the two light beams that have passed through the incident-side polarization diversity section into different wavelengths and reflects them in different directions;
A plurality of electrodes corresponding to each wavelength of light separated by the wavelength dispersion element are arranged in parallel, and the polarization direction of the incident light is maintained as it is when a voltage is applied to any of the plurality of electrodes. A first liquid crystal plate that transmits light having a polarization direction rotated when no voltage is applied to the electrodes,
A voltage source connected to each electrode of the first liquid crystal plate and applying a voltage to an electrode corresponding to a wavelength that attenuates light;
A wavelength combining element that passes through the first liquid crystal plate and combines lights incident from different directions and reflects them in the same direction as two light beams;
A wavelength selectivity comprising: an output-side polarization diversity unit that rotates the polarization direction of one of the light beams that have passed through the wavelength combining element, combines the two light beams, and emits them in the same direction. Optical attenuator. Incident-side polarization diversity unit that divides incident light into two light beams according to the polarization state and outputs two light beams with the same polarization direction by rotating either polarization direction of the separated polarized light When,
The light beams of the two light beams that have passed through the incident-side polarization diversity unit are separated for each wavelength, reflected in different directions, and combined from light incident from different directions to be the same as the two light beams. A wavelength dispersion combining element that reflects in the direction;
A plurality of electrodes corresponding to each wavelength of the light separated by the wavelength dispersion / synthesizing element are arranged in parallel, and the polarization direction of incident light is maintained as it is when a voltage is applied to any of the plurality of electrodes. And a first liquid crystal plate that reflects the light whose polarization direction is rotated when no voltage is applied to the electrodes,
A voltage source connected to each electrode of the first liquid crystal plate and applying a voltage to an electrode corresponding to a wavelength that attenuates light;
A wavelength comprising: an output-side polarization diversity unit that rotates the polarization direction of one of the light beams combined by the wavelength dispersion combining element, combines the two light beams, and emits the light beams in the same direction; Selective optical attenuator.   The wavelength-selective optical attenuator according to claim 1 or 2, wherein a voltage is applied to the liquid crystal plate and a state in which the liquid crystal plate is transmitted while maintaining the polarization direction is used as an optical attenuation state.   3. The wavelength-selective optical attenuator according to claim 1, further comprising an optical switch that is provided in a light passage path and transmits light when power is supplied and blocks light when power is not supplied.   A second liquid crystal plate that is provided in a light passage path and transmits light while maintaining a polarization direction of light when a voltage is applied, and changes a polarization direction of incident light when no voltage is applied; The wavelength selective optical attenuator according to claim 1 or 2.   The wavelength-selective optical attenuator according to claim 1 or 2, wherein the first liquid crystal plate has electrodes for each wavelength arranged in parallel so that the distance between the electrodes is continuously different.

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