Abstract:
A wavelength selective device including an arrayed waveguide grating is disclosed. The wavelength selective device includes a MEMS mirror, which couples light from an input port to an elongate aperture of an input star coupler or slab of the arrayed waveguide grating. A controller tilts the MEMS mirror in response to a sensed temperature change of the arrayed waveguide grating, thereby lessening a sensitivity of the arrayed waveguide grating to the temperature change. The MEMS mirror can also be tilted to shift wavelengths of the wavelength channels of the arrayed waveguide grating by pre-defined amounts upon receiving a corresponding remote command.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. patent application Ser. No. 61/717,424 filed Oct. 23, 2012, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to wavelength selective devices, and in particular to devices and methods for lessening thermal drift of arrayed waveguide gratings. 
     BACKGROUND OF THE INVENTION 
     Arrayed waveguide gratings (AWGs) are planar lightwave circuit (PLC) devices used for demultiplexing optical signals into individual wavelength channels. An AWG includes two slab-type star couplers, coupled to each other back-to-back via an array of planar waveguides of gradually increasing length. The gradually increasing waveguide length creates a gradually increasing optical delay on an inner surface of the output star coupler, which causes light at different wavelengths to couple into different output waveguides of the output star coupler. An AWG is a reciprocal device, that is, when used in a reverse direction, it can also combine wavelength channels into a common multiplexed signal. Thus, AWGs can be used for both multiplexing and demultiplexing of optical wavelength channels. Due to compactness and scalability of manufacturing, AWGs have found a wide application in optical networks. 
     One well-known drawback of AWGs is their thermal sensitivity. In an AWG, the optical path difference is created in the waveguide array, the refractive index of which depends on temperature. Because of this, center wavelengths of individual wavelength channels at the output of a silicon-based AWG drift with temperature, unless this drift is mitigated by some external means. 
     A common method to reduce AWG thermal drift is to stabilize the temperature of the PLC chip in which the AWG is formed. A heater and a temperature sensor are attached to the PLC chip. The temperature sensor is used to sense the PLC temperature. A temperature controller provides a signal to the heater to keep the temperature of the PLC chip constant. The temperature of the PLC chip is usually selected to be at the top of the required temperature range of the AWG device. 
     Temperature stabilization of AWGs has several drawbacks. One drawback is high electrical power consumption. Heaters having a power rating of at least several Watts are usually required to uniformly heat an AWG PLC chip. Another drawback is related to integration of thermally stabilized AWGs into a larger optical system. Heat released by the AWG heaters increases the overall system heat dissipation requirement, which calls for providing additional cooling means for the system. Furthermore, a time constant required for temperature stabilization and temperature tuning of heated AWGs is relatively large, typically ranging from few tens of seconds to few minutes. 
     Dragone in U.S. Pat. No. 5,920,663 discloses a method to reduce thermal drift of wavelength of an AWG by controllably deforming the PLC chip. The deformation stretches or compresses the optical lengths of the arrayed waveguides. Such changes give rise to birefringence effects that produce different propagation constants for the TE and TM waveguide modes. The deformation also provides some tuning of the transmission characteristics of the AWG, to correct for manufacturing tolerances. However, stress-induced birefringence increases polarization-dependent loss and polarization mode dispersion. 
     It has been recognized that an AWG can be tuned in wavelength by translating the input waveguide relative to the input star coupler of the AWG. Samiec et al. in U.S. Pat. No. 6,865,323 disclose an AWG device, in which an input waveguide is mounted on an expansion arm fixed on one end to a frame and having a holder on the other end. The expansion arm has a coefficient of thermal expansion (CTE) different from that of the frame. To restrict a movement of the input waveguide out of the PLC plane, a pair of flexible arms connect the holder to the frame. Detrimentally, the movable input waveguide in the Samiec device can cause the optical throughput of the AWG to be susceptible to shock and vibration, especially if the shock or vibration occurs in the PLC plane. 
     Delisle et al. in U.S. Pat. Nos. 6,701,043 and 6,798,048 disclose an AWG having a reflective input that permits variable coupling to compensate for AWG temperature drift. Referring to  FIG. 1 , an athermal reflective coupling  60  of a Delisle AWG  22  includes a thermally actuated pivot mechanism for supporting a mirror  32 . The athermal coupling  60  includes a first arm  62  of a material having a first coefficient of thermal expansion and a second arm  64  of a different material having a second coefficient of thermal expansion. Each arm  62 ,  64  abuts a substrate edge  19 . The first arm  62  supports a mirror frame  66 , which is coupled to the second arm  64  at one side, and which carries a mirror  32  on another side of the first arm  62 . A flex or pivot point  68  at the first arm  62  forms a rotation center, about which the mirror frame  66  pivots as shown by an arrow  21  in response to a differential thermal expansion of the first and second arms  62 ,  64 . Light emitted by an input optical fiber  10  mounted to a holder  26  is collimated by a lens  30  and impinges on the mirror  32  as a collimated beam. The collimated beam is reflected back into the lens  30  at an angle determined by the pivot of the athermal coupling  60 . The angle is translated by the lens  30  as an offset, thus shifting the input point at the input plane  20  of an input slab  12 , and thereby at least partially compensating the thermal drift of the AWG  22 . 
     The Samiec and Delisle AWG devices have drawbacks of vibration sensitivity and a relatively slow response to an abrupt temperature change. When the temperature changes quickly, thermal gradients between the PLC and the thermally expanding beams can cause time-varying wavelength drift. Furthermore, each PLC chip possesses slightly different thermal wavelength drift characteristics, requiring individual mechanical tuning of thermal response of each device, e.g. by adjusting individual lengths of the arms  62 ,  64 . This makes the Samiec and Delisle AWG devices more difficult to mass produce. 
     SUMMARY OF THE INVENTION 
     It is a goal of the invention to provide a quickly tunable and manufacturable temperature-compensated AWG capable of operating at a moderate electrical power consumption. 
     The present invention advantageously utilizes a micro-electro-mechanical system (MEMS) having a tiltable mirror for reflecting input light onto an input slab of an AWG. A MEMS controller is used to controllably tilt the MEMS mirror to compensate for temperature drifts of the AWG and/or to introduce controllable wavelength/frequency shifts. Low power consumption MEMS controllers are preferred, because they can be powered by an incoming light signal converted into electricity. Utilization of MEMS mirrors in AWG-based wavelength selective devices enables construction of manufacturable, tunable AWG devices with a reduced sensitivity to temperature drifts and mechanical vibration. 
     In accordance with the invention, there is provided a wavelength selective device comprising: 
     an input port for inputting an optical beam; 
     a tiltable MEMS mirror optically coupled to the input port, for reflecting the optical beam; 
     a focusing element optically coupled to the MEMS mirror, for focusing the reflected optical beam into a focal spot displaceable by varying an angle of tilt of the MEMS mirror; 
     an arrayed waveguide grating comprising an input slab having an elongate aperture for receiving the focal spot, and a plurality of output waveguides for outputting wavelength sub-beams of the optical beam, wherein the input slab is disposed so that when the angle of tilt of the MEMS mirror is varied, the focal spot is displaced along the elongate aperture; 
     a temperature sensor thermally coupled to the arrayed waveguide grating, for sensing a change of temperature thereof; and 
     a controller electrically coupled to the temperature sensor and the tiltable MEMS mirror, and configured for varying the angle of tilt upon sensing the arrayed waveguide temperature change by the temperature sensor, so as to lessen a wavelength drift of the wavelength sub-beams induced by the temperature change. 
     Preferably, the MEMS controller has a low electrical power consumption, for example 10 mW or less, or even 1 mW or less. In one embodiment, an optical splitter splits a portion of the input optical beam to a photoelectric generator such as a photovoltaic cell, which powers the MEMS controller. A few milliwatt of incoming optical power can be sufficient to power the MEMS controller, thus providing thermal stabilization of the AWG without requiring a dedicated power line or an internal battery. 
     In one embodiment, the controller is configured to vary the angle of tilt of the tiltable MEMS mirror, so as to shift wavelengths of the wavelength sub-beams by a controllable amount, upon receiving a corresponding external command. 
     In accordance with the invention, there is further provided a method of thermal stabilization of an AWG having an input slab having an elongate aperture for free-space coupling of an optical beam, and a plurality of output waveguides for outputting wavelength sub-beams of the optical beam, the method comprising: 
     (a) coupling the optical beam to the elongate aperture of the input slab by 
     (I) coupling the optical beam to a tiltable MEMS mirror for reflecting the optical beam; and 
     (II) directing the reflected optical beam to a focusing element for focusing the reflected optical beam into a focal spot on the elongate aperture, so that when the MEMS mirror is tilted by a first angle, the focal spot is displaced by a first displacement along the elongate aperture; 
     (b) sensing a change of temperature of the arrayed waveguide grating; and 
     (c) varying the first angle so as to lessen a wavelength drift of the wavelength sub-beams due to the change of temperature sensed in step (b). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1  is a plan view of a prior-art temperature-compensated AWG; 
         FIG. 2  is a schematic view of a wavelength selective device of the invention; 
         FIG. 3  is a schematic view of a wavelength selective device of the invention powered by an optical signal; 
         FIG. 4  is a combined optical frequency spectrum of remotely tunable wavelength channels of the wavelength selective device of  FIG. 3 ; 
         FIG. 5  is a block diagram of an embodiment of the wavelength selective device of the invention; and 
         FIG. 6  is a flow chart of an exemplary method of thermal stabilization of an arrayed waveguide grating according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIG. 2 , a wavelength selective device  200  of the invention includes an input port  202  for inputting an optical beam  204 . A tiltable MEMS mirror  206  is optically coupled to the input port  202 . The tiltable MEMS mirror  206  reflects the optical beam  204  and redirects it towards a lens  208  optically coupled to the MEMS mirror  206 . The lens  208  focuses the reflected optical beam  204  into a focal spot  210 , which is displaceable by varying an angle of tilt α of the MEMS mirror  206 . Another focusing element, such as a concave mirror, can be used in place of the lens  208 . 
     The wavelength selective device  200  includes an AWG  211  implemented in a PLC chip  212 . The AWG  211  has an input slab  214 , a waveguide array  216 , an output slab  218 , and a plurality of output waveguides  220  for outputting wavelength sub-beams  205  of the optical beam  204 . In the embodiment shown, the output waveguides  220  are coupled, via a fiber array  219  joined to a rear side of the PLC chip  212 , to a plurality of output optical fibers  221 . 
     The input slab  214  has an elongate aperture  222  for receiving the focal spot  210 . The input slab  214  is disposed so that when the angle of tilt α of the MEMS mirror  206  is varied, the focal spot  210  is displaced along the elongate aperture  222 . For example, tilting the MEMS mirror  206  by the angle α results in a shift of the optical beam  204  to a position  204 ′ shown with a dashed line, causing the focal spot  210  to shift to a position  210 ′ on the aperture  222 . The shift is exaggerated in  FIG. 2  for clarity. 
     A temperature sensor  224  is thermally coupled to the PLC chip  212  for sensing a change of temperature of the PLC chip  212  and the AWG  211 . A controller  226  is electrically coupled to the temperature sensor  224  and the tiltable MEMS mirror  206 , and configured for varying the angle of tilt α upon sensing the PLC chip  212  temperature change, so as to compensate, or at least lessen, a wavelength drift of the wavelength sub-beams  205  induced by the temperature change. 
     In the embodiment shown, the wavelength selective device  200  includes an electrical power source  228 , such as an internal battery or a photoelectric power generator. Using MEMS technology and a low-power controller  226  allows utilization of a low-power electrical power source  228 . To conserve electrical power, the controller can have a power rating of 10 mW or less, or even 1 mW or less. 
     Turning to  FIG. 3  with further reference to  FIG. 2 , a wavelength selective device  300  is similar to the wavelength selective device  200  of  FIG. 2 . The wavelength selective device  300  of  FIG. 3  further includes an input waveguide  302  disposed on or within the PLC  212 . The input waveguide  302  has an input end  304  optically coupled to the input port  202  via a beam splitter  306 , and an output end  308  optically coupled to the tiltable MEMS mirror  206 . In the embodiment shown, the output end  308  is coupled to the tiltable MEMS mirror  206  via the lens  208 , resulting in a more compact construction. In operation, the splitter  306  splits a portion  310  of the optical beam  204 , for example 5% of optical power or less, and directs it to a photovoltaic cell  312  coupled to the splitter  306  via an optical fiber  314 . The photovoltaic cell  312  receives the split portion  310  of the optical beam  204  and converts the received portion  310  into electrical power supplied via a cable  316  to a controller  326 . Another type of a photoelectric current generating device, such as a photodiode, may be used in place of the photovoltaic cell  312 . 
     Preferably, the photoelectric generator has a maximum power rating of 10 mW or less, and more preferably 1 mW or less. This allows one to split only a small portion of the input beam  204  for powering purposes. For instance, at the input optical power of 100 mW and the power consumption by the controller  326  of 1 mW or less, only 1%-2% of the input light energy needs to be used to power the controller  326  and the MEMS  206 . 2% of optical power loss corresponds to an extra insertion loss for the optical signal  204  of less than 0.1 dB, which is quite acceptable in view of a typical AWG loss of 2-3 dB. 
     In one embodiment, the splitter  306  is wavelength-selective, so that the split portion  310  has a different wavelength than the wavelength sub-beams  205 . This allows one to add the “powering light” to the optical beam  204  at a remote location, and substantially not spend the energy of the wavelength sub-beams  205  of the optical beam  204  to power the controller  326  powering the MEMS  206 . This can relax the electrical power consumption requirement for the controller  326 . 
     The controller  326  of  FIG. 3  is configured not only for thermal stabilization as the controller  226  of  FIG. 2 , but also to shift wavelengths of the wavelength sub-beams  205  by a controllable amount upon receiving a corresponding external command. The controller  226  is configured to receive a “Frequency Shift” command via a dedicated control line  318 . Upon receiving the “Frequency Shift” command, the controller  226  tilts the MEMS mirror  206  by an additional angle β, thereby shifting the focal spot on the elongate aperture  22  from a first position  310  to a second position  310 ′. The corresponding optical beams for the second position  310 ′ are shown in  FIG. 3  with dashed lines. Turning now to  FIG. 4  with further reference to  FIG. 3 , insertion loss spectra  402  of the wavelength sub-beams  205  shift by a controllable optical frequency shift Δf upon receiving the “Frequency Shift” command. The shifted spectra positions are shown in  FIG. 4  with dashed lines  404 . This frequency-shifting functionality can also be implemented in the controller  226  of  FIG. 2 . 
     The temperature dependence of central wavelengths or frequencies of the wavelength channels  205  can be calibrated with high precision, for example to 10 pm or better. As a result, the spectra  402  can be shifted in frequency with high precision. This is particularly important in applications where 50 GHz frequency grid is used to transmit 40 GBit/second and even 100 GBit/second modulated optical signals. Precise frequency positioning results in an optimum bandwidth utilization for such applications. 
     Referring to  FIG. 5 , a wavelength selective device  500  is a variant of the wavelength selective device  200  of  FIG. 2 , additionally including the input waveguide  314  ending with a launch port  501 , which is coupled to the MEMS  206  via the lens  208 . The controller  226  includes a microprocessor  502  in serial communication with an external device, not shown, for shifting the optical frequency/wavelength, and a MEMS driver  504  for generating DC voltages required to tilt the MEMS mirror  206  at a pre-defined angle. The frequency vs. temperature, and frequency shift vs. MEMS angle calibrations are performed to reduce the temperature dependence of the 96-channel AWG  211  to less than 1 GHz of the optical frequency drift, or less than about 10 pm of wavelength drift, in the working temperature range of 0° C. to 70° C. Either a local power supply rated at 100 mW or less, or a photo-generated power as explained above with reference to  FIG. 3 , can be used to power the microprocessor  502  and the MEMS driver  504 . 
     Turning to  FIG. 6  with further reference to  FIG. 3 , a method  600  of thermal stabilization of the AWG  11  includes a step  602  of generating powering light at a powering wavelength different from wavelengths of the optical beam  204 . In a step  604 , the powering light is added to the optical beam  204 , for example, by means of a wavelength division multiplexor. Steps  602  and  604  can be performed at a remote location, from which the optical beam travels (e.g. inside an optical fiber) towards the wavelength selective device  300 . In a step  606 , the portion  310  of the optical beam  204 , containing the powering light, is split from the optical beam  204  using the splitter  306 , and directed towards the photovoltaic cell  312  for conversion into electric power in a step  607 . 
     In a step  608 , the optical beam  204  is coupled to the input slab  214  of the AWG  211 . This is done by first coupling the optical beam  204  to the tiltable MEMS mirror  206  for reflecting the optical beam  204 ; and second, directing the reflected optical beam  204  to the lens  208  (or another suitable focusing element) for focusing the reflected optical beam  204  into the focal spot  310  on the elongate aperture  222  of the input slab  214 . When the MEMS mirror  206  is tilted by a predefined angle, the focal spot  310  is displaced by a first displacement along the elongate aperture  222 . For example, when the MEMS mirror  206  is tilted by the angle β ( FIG. 3 ), the focal spot  310  shifts to the position  310 ′. In a step  610 , the temperature sensor  224  senses the temperature of the AWG  211 . Finally, in a step  612 , the MEMS  206  is tilted to vary the angle β, so as to lessen a wavelength drift of the wavelength sub-beams  205  due to the change of temperature sensed by the temperature sensor  224  in step  610 . The electric power generated in step  607  is used by the controller  326  to perform steps  610  and  612 . 
     First three steps  602 ,  604 , and  606  of the method  600  are optional, and are taken in cases where the controller  326  is powered by photoelectric power. The last three steps  608 ,  610 , and  612  of the method  600  can also be used to operate the wavelength selective device  200  of  FIG. 2 . Regardless of the power source, it is preferable that the controllers  226  and  326  have low power consumption of no more than 10 mW, and more preferably no more than 1 mW. The splitting ratio of the splitter  306  is selected in accordance with the power rating of the controllers  226  or  326 . The method  600  can also include an optional step of tilting the MEMS mirror  206  by the additional angle β so as to shift optical frequencies of the wavelength sub-beams  205  by a pre-defined amount. This optional step can be performed upon receiving a command from a remote location. 
     The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.