Abstract:
A microelectromechanical wavelength monitor includes a first wafer that includes a first movable layer. A first chevron is a thermal actuator that is connected to the first movable layer by a first tether. A second chevron is a thermal actuator that is connected to the first movable layer by a second tether. A second wafer is bonded to the first wafer and includes a trench defining a second stationary layer that is flat or curved. The first and second chevrons adjust a distance between the first movable layer and the second stationary layer to vary a resonated wavelength between the first and second stationary layers. The first movable layer includes an antireflective coating formed on an outer surface thereof. The first and second movable layers include a highly reflective coating formed on an inner surface thereof.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to wavelength monitors, and more particularly to microelectromechanical tunable Fabry-Perot wavelength monitors with thermal actuators.  
         BACKGROUND OF THE INVENTION  
         [0002]    Microelectromechanical (MEMS) wavelength monitors or interferometers are important components in wavelength division multiplexing (WDM) telecommunications systems. For example, optical lasers employ tunable wavelength monitors that generate wavelength error feedback signals. MEMS-based wavelength monitors are currently produced by companies such as Coretek and Axsun. When compared with bulk optical interferometers, such as the bulk optical interferometer that is produced by SDL-Queensgate, the MEMS-based wavelength monitors are easier to fabricate, require less alignment, and are smaller and lower-cost.  
           [0003]    Conventional MEMS-based wavelength monitors, however, have some significant performance issues that need to be addressed. For example, the Coretek MEMS tunable filter is temperature sensitive and cannot handle significant power levels. The Axsun MEMS tunable filter is capable of handling high power levels but has problems associated with fabrication and repeatability.  
           [0004]    Conventional MEMS-based wavelength monitors employ either parallel flat mirrors or a curved mirror that forms a confocal or semi-confocal cavity. The parallel flat mirrors are extremely sensitive to parallelism and have beam walk-off problems. Therefore, some form of dynamic adjustment for parallelism must be incorporated into designs incorporating parallel flat mirrors. One MEMS-based interferometer, for example, includes four electrostatic actuators and four sensing capacitors. The sensing capacitors provide feedback that is used to continually adjust for parallelism. The Coretek and Axsun wavelength monitors use a single curved mirror to eliminate diffraction losses and to lower the sensitivity to misalignment of the surfaces. Generally, the confocal cavity approach produces a higher finesse than can be obtained with wavelength monitors using parallel flat mirrors.  
           [0005]    Generally, conventional MEMS wavelength monitors are fabricated from two or more dissimilar materials that have temperature stability problems. The Coretek device uses a suspended membrane that is fabricated out of silicon nitride and aluminum. The temperature of the chip varies due to environmental effects and/or absorbed power. The membrane has a different thermal expansion coefficient as compared with the underlying semiconductor substrate. As the temperature of the chip varies, the stress in the tethers changes and varies the transmission properties of the wavelength monitor.  
           [0006]    The surface micromachined wavelength monitors must maintain precise control over the stress and uniformity of the film for proper operation. Wavelength monitors fabricated out of bulk silicon or semiconductor wafers have fewer problems with stress and uniformity. The bulk silicon wavelength monitors have a higher thermal mass and improved dissipation that protects against temperature variation due to the absorbed power. Additionally, there is no variation in the built-in stress with temperature because the temperature expansion coefficient is the same for the two sides of the wavelength monitor.  
           [0007]    All of the conventional MEMS wavelength monitors employ electrostatic actuators for tuning the cavity. The electrostatic force is generally extremely weak. As a result, the reactive elements, such as the springs and tethers, must have very small spring constants to provide sufficient movement. During fabrication, the fragile reactive elements are easily damaged, which increases the complexity of the fabrication process. This is particularly true when passing a high-rate flow of water across the wafer during the dicing process. Wavelength monitors with electrostatic actuators are also more sensitive to shock, vibration and environmental effects. For example, the passband of these devices is often unstable in real-world environments. To compensate for variation and drift, some devices require a light source and an etalon for periodic calibration. These devices increase the cost and complexity of the wavelength monitor. While electrostatic actuation consumes relatively low-power, it requires a relatively high operating voltage.  
         SUMMARY OF THE INVENTION  
         [0008]    A microelectromechanical wavelength monitor according to the invention includes a first wafer that includes a first movable layer. A first chevron is connected to the first movable layer. A second chevron is connected to the first movable layer. A second wafer is bonded to the first wafer and includes a trench defining a second stationary layer. The first and second chevrons are thermal actuators that adjust a first distance between the first movable layer and the second stationary layer.  
           [0009]    In other features of the invention, the first and second surfaces are connected to the first and second chevrons using first and second tethers. The first movable layer includes an antireflective coating formed on an outer surface thereof. The first movable layer includes a highly reflective coating formed on an inner surface thereof. The first movable layer is patterned in a first semiconductor layer of the first wafer.  
           [0010]    In other features, the first chevron includes a first out-of-plane actuator and the second chevron includes a second out-of-plane actuator. The second stationary layer is flat or curved. The second stationary layer has a highly reflective coating formed thereon.  
           [0011]    In yet other features, a third chevron is connected to the first movable layer by a third tether. A fourth chevron is connected to the first movable layer by a fourth tether. The first movable layer is generally rectangular and the first, second, third and fourth tethers are connected to mid-portions of first, second, third and fourth edges of the first movable layer. Alternately, the first movable layer is generally circular and the first, second and third tethers are approximately equally spaced around the first movable layer.  
           [0012]    In still other features, the first movable layer, the first and second tethers and the first and second chevrons are patterned in a single semiconductor layer. The first and second chevrons are partially released from a substrate and the first movable layer is fully released from the substrate.  
           [0013]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 illustrates a Fabry-Perot etalon according to the prior art;  
         [0016]    [0016]FIG. 2 illustrates waveforms generated by the Fabry-Perot etalon of FIG. 1;  
         [0017]    [0017]FIG. 3 illustrates the Fabry-Perot etalon of FIG. 1 in an exemplary tunable laser embodiment;  
         [0018]    [0018]FIG. 4 illustrates a MEMS tunable Fabry-Perot wavelength monitor according to the present invention that includes thermal actuators;  
         [0019]    [0019]FIG. 5 is a perspective view illustrating a movable mirror structure of the wavelength monitor of FIG. 4 in a planar position;  
         [0020]    [0020]FIG. 6 is a perspective view illustrates the movable mirror structure of the wavelength monitor of FIG. 4 in an extended position;  
         [0021]    [0021]FIG. 7 illustrates a trench that is etched in a silicon wafer;  
         [0022]    [0022]FIG. 8 illustrates an exemplary movable structure including a silicon layer formed on a silicon on insulator (SIO) wafer;  
         [0023]    [0023]FIG. 9 illustrates a plan view of the movable mirror of FIG. 6 patterned in the silicon (Si) layer of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0025]    Referring now to FIGS. 1 and 2, a Fabry-Perot etalon  10  according to the prior art is illustrated. The Fabry-Perot etalon  10  includes two spaced, partially-reflecting mirrors  12  and  14 . The partially-reflecting mirror  14  typically includes an antireflective coating  16  on one surface and a highly reflective coating  18  on an opposite surface. The partially-reflecting mirror  12  typically includes a highly reflective (HR) coating  20  on one surface and an antireflective (AR) coating on an opposite surface  21 . An input light beam of light  22  is directed onto the partially-reflecting mirror  12 .  
         [0026]    Approximately 99% of incoming light is reflected by the mirror  12  and approximately 1% passes through the mirror  12 . Resonation occurs between the partially reflecting mirrors  12  and  14 . The particular wavelength of the resonation depends upon a distance d between the partially reflecting mirrors  12  and  14  and the free spectral range (FSR) is proportional 1/d. An output beam of light  24  that is resonated by the etalon  10  passes through the partially reflecting mirror  14  and is incident upon a detector  26 . For a high Q etalon  10 , an output signal  28  has a plurality of peaks that are separated by the FSR. For a low Q etalon  10 , the output signal is sinusoidally-shaped and also has a plurality of peaks that are separated by the FSR.  
         [0027]    Referring now to FIG. 3, a tunable laser  30  includes a laser  32  and a wavelength locker  34 . A controller  38  may be packaged with the tunable laser  30  and/or the wavelength locker  34  or packaged separately. The laser  32  generates a primary beam of light  40  at an output  42  onto fiber  44  and a secondary beam of light  46  having relatively low power at a tap  48 . The primary and secondary beams of light  40  and  46  have a wavelength (λ). Using the secondary beam of light  46 , the wavelength locker  34  and detector(s) (not shown) generate sensing signal(s)  50  that are output to the controller  38 . The controller  38  determines an error signal based on a difference between the wavelength (λ) of the laser  32  and a desired wavelength (λ d ) using the sensing signals  50 . The controller  38  generates a control signal  52  that adjusts the wavelength (λ) to the desired wavelength (λ d ). Conventional wavelength lockers  14  are typically fabricated using Fabry-Perot etalons, such as electrostatically actuated MEMS devices. Skilled artisans can appreciate that the wavelength monitor has a wide variety of other applications in addition to tunable lasers.  
         [0028]    Referring now to FIG. 4, a MEMS tunable Fabry-Perot wavelength monitor with thermal actuators according to the present invention is illustrated and is generally designated  60 . The wavelength monitor  60  includes a mirror structure  62  that is suspended and movable relative to a first wafer  64 . The wavelength monitor  60  further includes a trench  66  that is formed in a second wafer  68 .  
         [0029]    As with other Fabry-Perot devices, an input beam of light  70  is directed at the suspended mirror structure  62 . Some of the light passes through the suspended mirror structure  62 . The wavelength of light that resonates between the trench  66  and the suspended mirror structure  62  depends upon a distance between the trench  66  and the suspended mirror structure  62 . Some of the light that passes through the trench  66  forms an output beam of light  72  that is received by a detector. Skilled artisans will appreciate that the distance between the wafers  64  and  68  is exaggerated in the partial assembly view of FIG. 4 to illustrate the structure of the suspended mirror structure  62  and the trench  66 .  
         [0030]    Referring now to FIG. 8, the trench  66  is preferably etched into the substrate  68  using an etch-stop layer for a flat partially reflecting mirror or a reflowed photoresist process for a curved or spherical partially reflecting mirror. As previously discussed, the curved or spherical partially reflecting mirror produces a stable cavity that is capable of producing a higher finesse and is less sensitive to alignment errors. The depth of the trench  66  determines the cavity length, free spectral range (FSR), and “off” state transmission wavelength of the wavelength monitor  60 . The trench  66  is preferably coated with a highly reflective (HR) coating  72  on an inner surface thereof and an anti-reflective (AR) coating  74  on an outer surface thereof.  
         [0031]    Referring now to FIG. 5, the suspended mirror structure  62  is shown in further detail. The suspended mirror structure  62  includes a partially reflecting mirror  80  that is connected by tethers  82 - 1 ,  82 - 2 , . . . ,  82 -n to a plurality of chevrons  84 - 1 ,  84 - 2 , . . . ,  84 -n. Preferably, the chevrons  84  are thermal actuators  86 - 1 ,  86 - 2 , . . . ,  86 -n that move in an out-of-plane direction. In other words, the chevrons  84  move in the z axis when the mirror lies in the x-y axis. In a preferred embodiment, the mirror  80  has a square shape with four tethers located at mid-points of side surfaces  90 ,  92 ,  94  and  96  of the mirror  80 .  
         [0032]    Referring now to FIGS. 6 and 7, the suspended mirror structure  62  is shown in an extended position. When current is passed through the thermal actuators  86 , the thermal actuators  86  heat and expand. A plurality of notches  100  may be formed in the actuators  86  on a side opposite to the direction of intended movement to facilitate bending. To make the actuator  86  preferentially buckle in the out-of-plane direction, the beams forming the out-of-plane actuator  86  are made much thicker in the in-plane direction than in the out-of-plane direction. Best performance is achieved when the actuator thickness out-of-plane is tapered linearly from the anchored ends to the center of the beam. This can be performed using grayscale photoresist technology, by etching trenches of equal or varying depth into the beam or by other similar techniques.  
         [0033]    Referring now to FIGS. 9 and 10, an exemplary method for fabricating the thermally actuated MEMS mirror structure  62  is shown. A silicon layer  110  having a desired thickness is bonded, grown or sputtered on a silicon on insulator (SOI) wafer including silicon dioxide (SiO 2 ) and silicon (Si) layers  112  and  114 . A bottom side or topside etch is performed to release selected portions of the thermally actuated MEMS mirror structure  62 . For example, the portions lying within the dotted lines  120  in FIG. 9 are released while the portions outside the dotted line  120  remain attached. After patterning, an anti-reflective (AR) coating  122  is formed on an outer surface of the mirror structure  62  and a highly reflective (HR) coating  124  is formed on an inner surface of the mirror structure  62 .  
         [0034]    While the mirror  80  has a rectangular shape in FIG. 5, other shapes may be employed. For example, a circular mirror structure or other suitable shapes may be employed. The circular mirror structure requires fewer tethers, for example, three tethers may be employed with spacing at 120° apart.  
         [0035]    The thermally actuated mirror structure  62  can be fabricated using surface or bulk micromachining processes. The presently preferred method for fabricating the thermally actuated mirror structures is the bulk micromachining process due to its inherent repeatability and fewer problems with surface micromachining. The thermally actuated mirror structure  62  can be easily fabricated using bulk micromachining with silicon wafers or bulk micromachining with SOI wafers.  
         [0036]    In either case, the structure is formed by etching the front surface with a single masking step. A metalization step defines device contacts (not shown) and the highly reflective (HR) layer on the surface of the mirror  80 . Portions of the thermally actuated mirror structure  62  are then released using backside etching. When SOI micromachining is performed, a hydrofluoric (HF) dip is used to remove the SiO 2  layer  112 . A second etching step on the front surface or a stressed film can be used to break the symmetry and cause buckling in a preferred direction.  
         [0037]    The inner surfaces of the substrates  64  and  68  are fused together. Preferably, the substrates  64  and  68  are fused together using anodic bonding. As a result, a high finesse Fabry-Perot cavity is formed between the tethered mirror and the trench. As power is adjusted to the chevrons  84 , the suspended mirror is translated along the z axis to increase or decrease the cavity and to change the transmitted wavelength. Cavity alignment is maintained by supplying unequal current and/or voltage to the chevrons  84  to cause the mirror  80  to be tipped in a desired direction.  
         [0038]    The trench  66  is etched in the silicon wafer  68 . The finesse of the flat cavity that is formed between the etched trench  66  and the mirror  80  is dependent on the parallelism of the two surfaces. In general, the parallelism between the two surfaces (after anodic bonding) should be less than approximately 10 −2  degrees. The mirror design set forth above allows for post-assembly parallelism control. This is possible because the mirror structure  62  provides both planar motion and tilt motion in any direction. The wavelength monitor  60  can be calibrated electronically for errors that occur during fabrication. In addition, the finesse of the cavity can be deliberately reduced by tilting the mirror  80  for a low resolution scan.  
         [0039]    Etching a curved or spherical trench into the Si wafer is relatively straightforward using the commercially-available photoresist reflow process. The advantage of the spherical trench  66  is that it forms a stable cavity. The maximum achievable finesse of the spherical trench is larger than the finesse of the flat cavity trench that is limited by the diffraction of the incident beam. In addition, misalignment of the mirror  80  in the stable cavity configuration leads to increased insertion loss but does not result in the degradation of the cavity finesse.  
         [0040]    While the mirror  80  may heat up due to the direct connection to the tethers  82 , the relatively low temperatures that are required for motion are likely to cause negligible stress on the silicon and the coatings. Alternately, the tethers  82  may be altered to limit thermal diffusion into the mirror  80  while providing sufficient mechanical linkage.  
         [0041]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.