Abstract:
A method for compensating for a wavelength shift in a wavelength selective switch (WSS), and a device therefor. The device comprises a fixed seat ( 301 ) as well as a rotation beam ( 304 ) and a compensation block ( 302 ) that have different thermal expansion amounts, the rotation beam ( 304 ) and the compensation block ( 302 ) being fixedly adhered to the fixed seat ( 301 ). In the method, a combined structure of the rotation beam ( 304 ) and the compensation block ( 302 ) with different thermal expansion amounts is adopted; the combined structure rotates by means of different expansion amounts generated by the rotation beam ( 304 ) and the compensation block ( 302 ) at the same external temperature, and further drives an optical element of the WSS to rotate, hence compensating for a wavelength shift of the WSS. The method is safe and reliable; the device has a simple structure, and is convenient to encapsulate, is applicable to various WSS optical paths, and does not affect advantages of the optical path structure of the WSS.

Description:
BACKGROUND 
       [0001]    According to the invention hereof, there is provided a compensation method and its apparatus for wavelength shift in Wavelength Selective Switch (hereinafter referred to as WSS). This invention is categorized into the photo-communication. 
         [0002]    The development of wavelength division system and higher demand on its flexibility have facilitated the popularization of WSS, which can enable adding or dropping the wavelength arbitrarily at a random port and render a necessary technological platform for the flexible networking of optical networks. 
         [0003]    WSS is often disseminated at different nodes of optical networks, thereby being required for reliable working at a large temperature range. Glass elements are often chosen for optical elements in WSS, which will be subjected to thermal expansion effect, leading to the change in their optical property. Besides, the glue through which the aforementioned optical elements and optical baseplate are connected is also under the thermal expansion effect, which might lead to the migration of optical elements. All this will cause the deviation of wavelength from the original direction, affecting ITU-T wavelength alignment in WSS, which will result in wavelength shift. 
         [0004]    Capella Company put forward a compensation method for WSS wavelength shift through MEMS micromirror or LC array in its American patent application US2009/0028503A1 (publication date of Jan. 29, 2009). The patent claims to dispose the light path deflection element MEMS micromirror or LC array behind the collimator array to deflect light path. However, this method requires inserting the light path deflection element into the original light path, in which case the possible MEMS encapsulation is complex whilst LC array installation increasing difficulty in temperature controlling, polarization element disposing at the light path, the consequent light path optical alignment, and size expanding for whole apparatus. 
         [0005]    JDSU Company came up with a compensation method for WSS wavelength shift through mechanical structure design and assembly method in its American patent application U.S. Pat. No. 8,036,502B2 (publication date of Dec. 22, 2009). The patent claims to adopt special design of optical baseplate and matching buffers, fixed buffer ensuring alignment position invariance between light line and MEMS chip, while moving buffer decreasing thermal stress caused by mismatching expansion coefficient between optical baseplate and optical shell and compensating for related wavelength shift. However, this method is not suitable for active compensation for wavelength shift and asks for a high standard for module encapsulation consistency. 
       SUMMARY 
       [0006]    The object of this invention is to overcome the existing technological deficiency, solve the wavelength shift problem of WSS, and put forward a compensation method for WSS wavelength shift by leveraging compensation apparatus herein described. 
         [0007]    This invention adopts the following principle: in the WSS light path structure, a compensation apparatus can be used to change the angle of incidence of incident beam at beam split element, after which the diffraction angle of every wavelength will also be altered. Hereupon, every wavelength will, through focusing lens, has a translation along the direction of the switch and attenuation unit on the switch and attenuation array so that every incident wavelength on the switch and attenuation array will have a migration relative to ITU-T wavelength. Or the incidence angle might be kept unchanged, in which case a compensation apparatus will be added to the back of the folding reflector on the light path after beam split by diffraction grating so as to have a whole regulation over the light path deflecting direction of every wavelength, change the center position of every wavelength on the switch and attenuation array, and cause a migration relative to ITU-T wavelength. While the center position of every wavelength is adjusted, an angle is introduced into the direction of switch and attenuation unit. However, due to the wavelength sensitivity of the WSS light path, the angle is small. Moreover, the orientation of switch and attenuation is a small light spot which is not sensitive to angle. Therefore, introducing an angle together with adding a compensation apparatus to the back of light path folding reflector will not cause any change of intersection loss of WSS light path. The more the angle of WSS light path introduced by compensation apparatus varies, the more translation every wavelength along the orientation of the switch and attenuation array will have, the higher the wavelength compensation will gain. 
         [0008]    The invention hereof adopts the following technological solutions: A compensation method for wavelength shift of WSS comprises the following steps: Step 1. Measure the direction and amount of wavelength shift of WSS; Step 2. Adopt a composite structure of rotating beam ( 304 ) and compensation blocks ( 302 ) with different thermal strokes; Step 3. Glue the composite structure to the side of collimating array of WSS or the back of the reflecting surface of the reflector of WSS; neutralize the wavelength shift direction with the rotating direction of the composite structure, and keep equal the wavelength compensation dosage of the composite structure and the practical wavelength shift amount of WSS. 
         [0009]    The stated different thermal strokes of the composite structure in Step 2 is realized by setting the size of the compensation blocks with a width and the different thermal expansion coefficients of compensation blocks and rotating beam at the same external temperature. 
         [0010]    The stated different thermal strokes of the composite structure in Step 2 is realized by electrical heating of compensation blocks and rotating beam with the same expansion coefficient to reach different temperatures. 
         [0011]    A compensation apparatus for WSS wavelength shift comprises a fixing seat, a rotating beam and compensation blocks. The compensation blocks and the rotating beam are connected to the fixing seat. And the compensation blocks have larger thermal expansion stroke than the rotating beam. 
         [0012]    The stated compensation blocks have the same height as the rotating beam; a rotating arm is connected and fixed on the compensation blocks and the rotating beam. 
         [0013]    The stated fixing seat is in the shape of L; compensation blocks are on the inner side of the fixing seat. 
         [0014]    The stated compensation blocks are at one side of the rotating beam; and the compensation blocks have a larger thermal expansion coefficient than the rotating beam. 
         [0015]    The sated rotating beam and the fixing seat are a composite structure which is shaped through integral machining. 
         [0016]    The compensation block of stated rotating beam and the compensation blocks are fixed to the fixing seat parallelly and symmetrically, both of which are disposed with electronic controlling heating apparatus. 
         [0017]    The stated rotating arm is mounted with a boss, which contacts with the rotating beam and compensation blocks. 
         [0018]    The stated rotating beam and compensation blocks are metal blocks, the electronic controlling apparatus on which is a heating resistor. 
         [0019]    The stated rotating beam and compensation blocks are piezoelectric ceramics. 
         [0020]    This invention has the following advantages:
       1. The compensation apparatus of this invention is applicable to various WSS light paths without changing the light path structure of WSS;   2. The compensation apparatus of this invention is simple in structure and convenient in encapsulation without complicating the processing.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Drawing  1 . Structure Chart of the WSS Light Path in the Invention; 
           [0024]    Drawing  2 . Structure Chart of the Passive compensation apparatus in the Invention; 
           [0025]    Drawing  3 . Connecting Chart of WSS Optical Element, Optical Baseplate and compensation apparatus; 
           [0026]    Drawing  4 . Light Path Graph of Wavelength Shift Compensation by installing the compensation apparatus on Collimator Array in the Invention; 
           [0027]    Drawing  5 . Schematic Diagram of the Relation between the deflection angle of Collimator Array and Wavelength Shift; 
           [0028]    Drawings  6   a ˜ 6   c.  Schematic Diagram of the Working Condition of the Passive compensation apparatus; 
           [0029]    Drawing  7 . Structure Chart of the Active compensation apparatus in the Invention; 
           [0030]    Drawing  8 . Structure Chart of the Improved Active compensation apparatus for Decreasing Arm of Force; 
           [0031]    Drawing  9 . Schematic Diagram of the Working Arm of Force of the Improved Active compensation apparatus in Drawing  8 ; 
           [0032]    Drawing  10 . Improved Active compensation apparatus for Increasing Wavelength Compensation Dosage via Increasing the Height of Compensation Blocks; 
           [0033]    Drawing  11 . Improved Passive compensation apparatus for Increasing Wavelength Compensation Dosage via Increasing the Height of Compensation Blocks; 
       
    
    
       [0034]    Thereinto: 
         [0035]      201 : collimator array;  202 : focusing lens;  203 : collimating lens;  204 : diffraction grating;  205 : focusing lens;  210 : switch and attenuation array;  208 A: a 1 st  reflector;  208 B: a 2 nd  reflector;  301 : fixing seat;  302 : compensation blocks;  303 : rotating arm;  304 : rotating beam;  305 A: a 1 st  electronic controlling apparatus;  305 B: a 2 nd  electronic controlling apparatus;  300 : compensation apparatus;  100 : optical baseplate;  200 : WSS optical element; g: space between compensation blocks  302  and rotating beam  304 ; H: height of compensation blocks  302 ; b: width of compensation blocks  302 ; a: width of rotating beam  304 ; 
       DETAILED DESCRIPTION 
       [0036]    Hereinafter, a specified embodiment to which the present invention is applied is described in detail with reference to the drawings. 
         [0037]    The light path structure of WSS applicable to the invention at the wavelength spectral plane is as the details in Drawing  1  that collimator array  201 , focusing lens  202 , collimating lens  203 , diffraction grating  204 , focusing lens  205  and switch and attenuation array  210  are set in sequence. The light path is as follows: after alignment by the collimator array, the input optical signal is expanded by the beam expander composed of a focusing lens  202  and a collimator lens  203 . The collimated light beam through diffraction grating  204  falls into single channel optical signals in sequence, which will be focalized by a focusing lens  205  on a switch and attenuation array  210 . In addition, to decrease the size of WSS module, a first reflector  208 A could be added to the space between collimator lens  203  and diffraction grating  204 , or a second reflector  208 B could be added to the space between collimator lens  205  and attenuation reflection array  210  to fold the light path, or both a 1 st  reflector  208 A and a 2 nd  reflector  208 B could be added at the same time to fold the light path. The compensation apparatus could be connected to the collimator array  201 , or the back of the reflecting surface of the 1 st  reflector or the 2 nd  reflector. 
         [0038]    The compensation apparatus pursuant to the invention hereof has two structures. One is passive compensation apparatus, and the other is active compensation apparatus. As in drawing  2 , the passive compensation apparatus comprises a fixing seat  301 , compensation blocks  302 , a rotating arm  303 , and a rotating beam  304 . The rotating beam  304  and fixing seat  301  are connected and fixed as a whole. fixing seat  301  and rotating beam  304  could also be shaped through integral processing. In this case, rotating beam  304  is a boss set on the fixing seat  301 . Compensation blocks  302  and rotating beam  304  are fixed to the fixing seat  301 , compensation blocks  302  are on one side of the rotating beam  304 , and compensation blocks  302  and rotating beam  304  should be at the same height. When compensation blocks  302  are installed at the right side of the rotating beam  304 , the compensation blocks  302  will drive the rotating arm  303  to move anticlockwise in conditions of high temperature and move clockwise in conditions of low temperature. When compensation blocks  302  are installed at the left side of the rotating beam  304 , compensation blocks  302  will drive the rotating arm  303  to move clockwise in conditions of high temperature and move anticlockwise in conditions of low temperature. fixing seat  301  and rotating beam  304  could be glass, and compensation blocks  302  should use metal. To get enough wavelength compensation dosage, compensation blocks  302  should choose metal with big thermal expansion coefficient, such as aluminum. Due to difference in thermal expansion coefficients of such two materials as glass and metal, when environment temperature changes, the rotating beam  304  and compensation blocks  302  have different strokes. The rotating arm  303  of the compensation apparatus could be connected in parallel to a side face of WSS collimator array  201 , or to the back of the 1 st  reflector  208 A, or the back of the 2 nd  reflector  208 B. The rotating arm  303  of the compensation apparatus could be connected to one of the three elements in WSS module so as to realize common rotating with the connecting element, and reach the goal of wavelength compensation. 
         [0039]    Drawing  3  is the side view of connection between the optical baseplate  100  with compensation apparatus and WSS optical element  200 . On this occasion, WSS optical element  200  is collimator array  201  or 1 st  reflector  208 A or 2 nd  reflector  208 B. Compensation blocks  302 , rotating beam  304  and fixing seat  301  are glued together, fixing seat  301  and optical baseplate  100  are glued together, and rotating arm  303  is glued to WSS optical element  200 . In the compensation apparatus of this structure, rotating arm  303  is connected with WSS optical element so that the connecting surface between the compensation apparatus and WSS optical element is flat and fixed area increases, which creates a better result of wavelength shift compensation. The invention may not use a rotating arm  303 . both compensation blocks  302  and rotating beam  304  could be glued to the WSS optical element. The compensation apparatus of this structure could also drive the connected elements to rotate to reach the technological goal of wavelength shift compensation. 
         [0040]    When adopting the compensation apparatus in the invention to have wavelength shift compensation, the direction and shift amount of the WSS wavelength shift before disposing compensation blocks  302  can be measured. With compensation apparatus  300  on the collimator array  201  as in drawing  4 , if WSS wavelength is measured to drift toward long wave direction at high temperature and toward short wave direction at low temperature, to compensate the stated wavelength shift, compensation blocks  302  at the right side of rotating beam  304  is to be installed so that at high temperature compensation apparatus  300  will make the angle of collimator array  201  to bend down to resist the WSS shift toward long wave at high temperature. If the practical WSS module wavelength shift are measured at a negative direction, compensation blocks  302  must be disposed at the left side of rotating beam  304 , after which adjustment must be made on the space between compensation blocks  302  and rotating beam  304  according to wavelength shift amount necessary to be compensated. Finally the compensation blocks  302 , fixing seat  301  and rotating arm  303  are to be glued together. Besides, the glue could be UV-curved glue. Relatively soft UV-curved glue should be adopted to glue rotating arm  303  and fixing seat  301  together. 
         [0041]    Via compensation apparatus  300 , the deflecting direction of collimator array or reflector decides the wavelength deflecting direction. Its process of wavelength shift compensation is as follows: As in drawing  4 , with compensation apparatus  300  glued to one side surface of collimator array  201 , then temperature varies, compensation apparatus  300  will drive collimator array  201  to rotate horizontally so that light path angle through collimator array  201  is changed. That the deflected light path passes through beam expansion system including focusing lens  202  and collimating lens  203 , together with the change of incidence angle of light path into diffraction grating will realize compensation for wavelength shift. As in drawing  5 , when angle of collimator array  201  moves upward, the incidence angle at diffraction grating will decrease, and diffraction angle will increase, which means wavelength will drift toward the left of switch and attenuation array  210 , or say, the long wave direction. On the contrary, if the angle of collimator array  201  bents down, wavelength will drift toward the short wave direction. Therefore, by adopting compensation apparatus  300  to make WSS optical element drift upward or downward, it could make wavelength drift to long wave or short wave direction so as to compensate the WSS wavelength shift to different directions. 
         [0042]    By installing the compensation apparatus at the place or 1 st  reflector  208 A or 2 nd  reflector  208 B, the compensation apparatus will drive reflector to rotate horizontally making every wavelength light path change so as to realize the wavelength shift compensation. Collimator array  201  and reflector  208  adopt identically equipped compensation apparatus, yet they have different optical effects. Because the focusing lens  202  and collimator lens  203  of beam expansion system reduces the light path angle deflection through collimator array  201 , the compensation result of compensation apparatus at reflector  208  for wavelength shift compensation is more obvious. And wavelength compensation dosage at reflector  208  is times of that at collimator array  201 . Compensation times equal the expansion ratio of the WSS expansion system. 
         [0043]    Passive compensation apparatus works As in drawings  6   a  to  6   c,  among which  6   a  is the compensation apparatus in the room temperature condition,  6   b  the high temperature and  6   c  the low temperature. At room temperature, compensation blocks  302  keep the same height; at high temperature, rotating beam  304  and compensation blocks  302  will expand in height due to thermal expansion. Thermal expansion coefficient of metal is an order of magnitudes larger than that of glass. So compensation blocks  302  will have a larger elongation than rotating beam  304 . If the height of compensation blocks  302  at room temperature is H, thermal expansion coefficient difference between glass and metal is Δα, environment temperature difference between high and low temperatures is ΔT, then ΔH=H×Δα×ΔT. ΔH is the height difference between compensation blocks  302  and rotating beam  304  due to temperature effect. If the arm of force in various conditions is L, and the rotation angle of rotating arm  303  is θ, then 
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         [0000]    If the width of parallel direction of rotating beam  304  along fixing seat  301  is a, the width of parallel direction of blocks  302  along fixing seat  301  is b, the space between rotating beam  304  and compensation blocks  302  is g, then rotating arm of force at high temperature is L=a+g, and rotating arm of force at low temperature is L=g+b. As implied by the above formulas, to increase wavelength compensation dosage, θ value is to be increased through adding the compensation block height and reducing the rotating arm of force L of compensation apparatus. 
         [0044]    When increasing the height of compensation blocks to increase wavelength compensation dosage, heights of rotating beam  304  and compensation blocks  302  increase which increases the WSS volume. To maintain the size of compensation apparatus in light path, it is to decrease the thickness of fixing seat  301  to keep the whole size of WSS unchanged. When the thickness of fixing seat  301  decreases, to ensure the bonding strength between fixing seat  301  and WSS baseplate  100  the fixing seat  301  could be designed in the form of L so as to keep unchanged the contacting area between the fixing seat  301  and the WSS baseplate  100 . As in drawing  11 , compensation blocks are installed inside the L fixing seat  301 . Compensation blocks  302 , vertical to the optical baseplate, are glued to fixing seat  301 . This technological method could keep unchanged the contacting area between the fixing seat  301  and optical baseplate in the situation of increasing the height of compensation blocks, and decreasing the thickness of fixing seat  301  so as to increase wavelength compensation dosage. 
         [0045]    According to focal length of every lens, space between lenses and the position of WSS optical element as compensation apparatus in light path, it is known that the relations amongst the collimator array  201 , the angle of reflector  208  as well as the wavelength shift, so as to identify the relation between the angle of compensation apparatus and wavelength shift. Usually it is started by identifying the space g between compensation blocks  302  and rotating beam  304  according to the measured WSS wavelength shift amount at high temperature. However, from room temperature to high and low temperatures, WSS wavelength shift amounts are different at the same range or temperature. To have complete compensation, compensation apparatus at the stated temperature variations should have different wavelength compensation dosages. According to the stated compensation apparatus&#39;s angle formula, to have different compensation dosages from room temperature to high and low temperatures, the ratio 
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         [0000]    of the rotation angle θ of rotating arm  303  to temperature range ΔT should be different. When the whole compensation size and component materials are decided, H and Δα are determined value. In this case, width b of compensation blocks  302  is to be changed so that the arm of force L will be different at high and low temperatures. Then the high temperature and low temperature variations have different proportionality coefficients 
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         [0000]    at high or low temperature is decided by the wavelength compensation dosage goal. Appropriate width b of compensation blocks  302  will be defined to keep the wavelength compensation dosage of compensation apparatus the same as the measured WSS wavelength shift amount so as to realize complete compensation for wavelength shift. 
         [0046]    When using the first passive compensation apparatus to realize wavelength shift compensation, you may only use the stated compensation apparatus to finish WSS wavelength shift compensation rather than additionally install control circuit to control over compensation apparatus. Every component part of the compensation apparatus is simple in structure and easy to get. But the passive compensation apparatus is not easy to have continuous and adjustable compensation for WSS module wavelength shift. Therefore passive compensation apparatus is applicable to situation which does not require much wavelength shift compensation accuracy. If the bandwidth margin of WSS optical module is adequate, passive compensation apparatus might be adopted to compensate for the WSS module wavelength shift. 
         [0047]    To realize a continuous and adjustable compensation for WSS wavelength shift and advance wavelength compensation accuracy, the invention hereof puts forward a second structure, an active compensation apparatus, which has the same structure principle as passive compensation apparatus. They both adopt a composite structure including rotating beam and compensation blocks, which have different strokes that enables compensation apparatus to rotate and lead WSS optical element to rotate. They only differ in that rotating beam in active compensation apparatus also adopts a compensation block, implying that there are 2 compensation blocks in the apparatus. When compensation apparatus starts to work, the compensation block which is not via electric control among the two compensation blocks serves as rotating beam. Utilization of this compensation apparatus can have active compensation for wavelength shift. As in drawing  7 , an active compensation apparatus comprises a fixing seat  301 , a rotating arm  303 , compensation blocks  302 , a 1 st  electric control device  305 A, and a 2 nd  electric control device  305 B. Two compensation blocks  302  are mounted on the fixing seat  301  symmetrically. Two compensation blocks  302  are glued and fixes to the fixing seat  301 . Two compensation blocks are respectively installed with a 1 st  electric control device  305 A and a 2 nd  electric control device  305 B. Rotating arm  303  is installed above the compensation blocks  302 . The contacting part between rotating arm  303  and compensation blocks  302  could be designed into a prominent boss As in drawing  8 . The main function of boss is to decrease the arm of force L and increase wavelength compensation. As in drawing  9 , the working arm of force L is no longer related to the width of compensation blocks  302  but related to the width of the boss installed at the rotating arm  303 . In this situation, the working arm of force L decreases so as to increase the wavelength compensation dosage of the compensation apparatus. Compensation blocks could use metal such as aluminum. Compensation blocks  302  could also be piezoelectric ceramics or other elements with micrometric displacement kinetic energy. When two compensation blocks  302  are metal blocks, the 1 st  electric control device  305 A and the 2 nd  electric control device  305 B are heating resistor. If you want the compensation apparatus to realize the rotating effect as the Drawing  6   b  to make wavelength shift compensation, you could power up the heating resistor on the compensation block  302  on the right side so as to make the right compensation block  302  heat up and expand. On this occasion, the left compensation block  302  serves as rotating beam, and vice versa. When compensation blocks  302  are piezoelectric ceramics, you could power up one of the piezoelectric ceramics according to the WSS wavelength shift direction so as to make the stated piezoelectric ceramic have certain micrometric displacement. In this situation, the piezoelectric ceramic that is not powered up serves as the rotating beam and drive WSS optical element to rotate so as to reach the same rotating effect with that adopting aluminum blocks. 
         [0048]    The active compensation apparatus of the invention, just like the passive compensation apparatus, could not have a rotating arm  303 . Two compensation blocks  302  being glued and fixed to WSS optical element, compensation apparatus with such structure also can drive the connecting element to rotate so as to realize the technical goal of wavelength shift compensation. 
         [0049]    In active compensation apparatus, in order to increase compensation dosage and ensure the bonding strength between fixing seat  301  and WSS optical baseplate  100 , the fixing seat  301  could be designed in the shape of L so as to keep unchanged the contacting area between the fixing seat  301  and WSS optical baseplate. As in drawing  10 , two compensation blocks  302  are installed inside the L fixing seat  301 . Compensation blocks  302  vertical to one side of the optical baseplate are glued and fixed to the fixing seat  301 . This technological pan could keep unchanged the contacting surface between fixing seat  301  and optical baseplate while increasing the height of compensation blocks, decrease the thickness of fixing seat  301  and increasing the wavelength compensation dosage. 
         [0050]    Although a specified embodiment to which the present invention is applied is described in detail for reference, it is to be noted that, for technologists in this filed, this invention can be arbitrarily modified in both form and details without departing from the spirit and scope thereof, and the modification(s) will fall within the scope of protection of the invention stated herein.