Abstract:
External cavity lasers apparatus and methods that allow fast tuning, high wavelength stability, low cavity losses, and form factors that are comparable to solid state, fixed wavelength lasers. The apparatus comprise a gain medium emitting a light beam, a tunable wavelength selection element positioned in the light beam and configured feed back light of a selected wavelength to the gain medium, and a microelectromechanical systems (MEMS) actuator element operatively coupled to the tunable wavelength selection element. The MEMS actuator element may be configured to actuate the tunable wavelength selection element according to a first degree of freedom to select the wavelength of the feedback to the gain medium, and to actuate the actuate the tunable wavelength selection element according to a second degree of freedom to provide phase control of the feedback. The MEMS actuator element and tunable wavelength selection element may additionally be configured such that actuation of the tunable wavelength selection element with respect to a third degree of freedom provides a selectable level of attenuation of the feedback to the gain medium.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 09/900,373, filed on Jul. 6, 2001, and incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The demand for increased bandwidth in fiberoptic telecommunications has driven the development of sophisticated transmitter lasers usable for dense wavelength division multiplexing (DWDM) systems wherein multiple separate data streams propagate concurrently in a single optical fiber. Each data stream is created by the modulated output of a semiconductor laser at a specific channel frequency or wavelength, and the multiple modulated outputs are combined onto the single fiber. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz, which allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Greater bandwidth requirements will likely result in smaller channel separation in the future. 
     Telecom DWDM systems have largely been based on distributed feedback (DFB) lasers. DFB lasers are stabilized by a wavelength selective grating that is predetermined at an early step of manufacture. Unfortunately, statistical variation associated with the manufacture of individual DFB lasers results in a distribution of (wavelength) channel centers. Hence, to meet the demands for operation on the fixed grid of telecom wavelengths (the ITU grid), DFBs have been augmented by external reference etalons and require feedback control loops. Variations in DFB operating temperature permit a range of operating wavelengths enabling servo control; however, conflicting demands for high optical power, long lifetime, and low electrical power dissipation have prevented use in applications that require more than a single channel or a small number of adjacent channels. 
     Continuously tunable external cavity lasers have been developed to overcome the limitations of individual DFB devices. Tunable external cavity lasers, in order to provide effective side mode suppression and wavelength stability, require very stringent manufacturing tolerances. In order to meet these tolerances, expensive custom-made components are typically required for the external cavity lasers. Tuning has relied on the use of stepper motors to mechanically components, which reduces form factor, introduces vibration and shock sensitivity, reduces useful lifetime due to motor component wear, and increases the overall size and complexity of the external cavity lasers. 
     There is accordingly a need for an external cavity laser that is compact in size and has a small form factor, that is of simple, inexpensive construction, that provides for effective side mode suppression and wavelength stability during operation, that has reduced cavity loss and increased output power, and which has loose machining tolerances. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art. 
     SUMMARY 
     The invention provides external cavity lasers apparatus and methods that allow fast tuning, high wavelength stability, low cavity losses, and form factors that are comparable to solid state, fixed wavelength lasers. The apparatus of the invention comprises a gain medium emitting a light beam, a tunable element positioned in the light beam and configured feed back light of a selected wavelength to the gain medium, and a microelectromechanical systems (MEMS) actuator element operatively coupled to the tunable element. The MEMS actuator element may be configured to actuate the tunable element according to a first degree of freedom of movement to select the wavelength of the feedback to the gain medium, and to actuate the tunable element according to a second degree of freedom of movement to provide phase control of the feedback. The MEMS actuator element and tunable element may additionally be configured such that actuation of the tunable element with respect to a third degree of freedom of movement provides a selectable level of attenuation of the feedback to the gain medium. 
     The tunable element and MEMS actuator are configured to provide orthogonalized wavelength selection control and phase control of the feedback from the tunable element to the gain medium according to independent orthogonalized positional adjustments to the tunable element. In other words, wavelength tuning is uncoupled or decoupled from tuning of the external cavity tuning, such that the tuning mechanisms for wavelength selection and external cavity length adjustment operate independently or orthogonally with respect to each other. The adjustment of the wavelength passband thus has minimal effect on the effective cavity length, and adjusting the effective cavity length has minimal effect on the passband of the tunable element. 
     In certain embodiments, the tunable element may comprise a movable grating that is positioned in the light beam and configured to selectively feed back light to the gain medium according to positioning of the grating by the MEMS actuator. The grating is rotatable about a first axis to provide wavelength selection of the light fed back to the gain medium, and is translatable along a second axis to provide phase control of the light fed back to the gain medium. The rotational adjustment about the first axis to control wavelength selection is orthogonalized with respect to the translational adjustment along the second axis to control external cavity length, such that adjusting the grating for wavelength selection does not effect, or minimally effects, phase adjustment. Similarly, translatational adjustment of the grating along the second axis to provide phase control does not effect, or minimally effects, wavelength selection. The grating may additionally be rotatable about a third axis to provide attenuation control to the light fed back to the gain medium. The first axis may be parallel or substantially parallel to the grating face of the movable grating, and the second axis may be perpendicular or substantially perpendicular to the grating face and first axis. The third axis may be substantially parallel to the grating face, and substantially perpendicular to the first and second axes. 
     In some embodiments, the movable grating is a reflective grating and, together with a reflective facet of the gain medium, defines an external laser cavity. The grating may be etched or otherwise formed onto a MEMS mirror surface. The grating may be positioned with respect to the reflective facet of the gain medium such that external laser cavity is dimensioned to suppress lasing modes at wavelengths other than a selected wavelength. Specifically, the grating and gain medium may be positioned such that the external laser cavity is of sufficiently short length that the external cavity axial modes are spaced sufficiently far apart such that unwanted mode hopping from a selected wavelength to an external cavity mode will not occur during laser operation. The apparatus may, in certain embodiments, also comprise a mode filtering element positioned in the optical path, which may be in the form of an etalon configured to define a plurality of transmission peaks corresponding to selectable feedback wavelengths. 
     In other embodiments, the tunable element may comprise a movable mirror together with a stationary grating, with the movable mirror operatively coupled to the MEMS actuator element. The mirror is rotatable about a first rotational axis to control feedback wavelength and translatable along a second axis to control feedback phase. In certain embodiments, the mirror may additionally be rotatable about a third axis to control level of feedback attenuation. 
     The methods of the invention comprise emitting a light beam by a gain medium, positioning a tunable element in the light beam, coupling the tunable element to a microelectromechanical (MEMS) actuator, feeding back light to the gain medium by the tunable element, and positionally adjusting the tunable element with respect to a first degree of freedom of movement, via the MEMS actuator, to select wavelength of the light fed back to the gain medium. The methods may additionally comprise positionally adjusting the tunable element with respect to a second degree of freedom of motion to adjust phase of the light fed back to the gain medium. The positional adjusting with respect to the first and second degrees of freedom may be carried out orthogonally, such that positional adjustment of the tunable element to adjust wavelength does not affect phase adjustment provided by the tunable element. The methods may further comprise positionally adjusting the tunable element with respect to a third degree of freedom of movement to control attenuation of the light fed back to the gain medium. 
     The positioning of the tunable element in the light beam may in certain embodiments comprise positioning a reflective grating in the light beam. The grating may be etched, engraved or embossed or otherwise formed, using photolithographic or other technique, onto the surface of a MEMS-movable mirror that is coupled to a MEMS actuator. Positionally adjusting the grating to select wavelength of the feedback light may comprise rotatably actuating the grating with respect to a first axis that is parallel or substantially parallel to the grating face. Positionally adjusting the grating to select or adjust the phase of the feedback light may comprise translating the grating with respect to a second axis that is perpendicular or substantially perpendicular to a grating face thereof. The first and second axes are configured such that wavelength selection and phase selection are orthogonalized. Positionally adjusting the grating to control attenuation of or otherwise control the optical power of the feedback light may additionally comprise rotatably actuating the grating with respect to a third axis that is substantially perpendicular to the first and second axes. 
     In certain embodiments, the positioning of the grating in the light beam may comprise positioning the grating such that the grating and an reflective facet of the gain medium define an external laser cavity that is dimensioned to suppress lasing modes at wavelengths other than a selected wavelength. The grating and gain medium may be positioned such that the external laser cavity is of sufficiently short length that the external cavity axial modes are spaced sufficiently far apart such that unwanted mode hopping from a selected wavelength to an external cavity mode will not occur during laser operation. In some embodiments, the method may comprise positioning a mode filtering element in the light beam, and suppressing feedback at unselected wavelengths with the mode filtering element. 
     The apparatus and methods of the invention provide external cavity lasers that can be manufactured and assembled with relaxed tolerances and inexpensive components than is presently possible. The use of a MEMS actuator for positioning of a tunable element as provided by the invention allows shorter external cavity dimensions and smaller package sizes than have previously been achieved. In certain embodiments, the external cavity may be of sufficiently small dimension that effective suppression of unwanted wavelengths is achieved without the use of an intracavity filter or mode suppression element. The multiple degrees of freedom of movement of the tunable element allow wavelength selection, phase control and output power control during laser operation by appropriate actuation of the tunable wavelength reflection element. Adjustment to provide wavelength selection and phase control may be carried out independently or orthogonally, such that adjustments to a tunable element to provide wavelength selection minimally affect external cavity length or phase control adjustment. The short external cavity length and use of MEMS actuation also allows dynamic provisioning and rapid tuning or adjustment of output wavelength, feedback phase, and output power during laser operation. These and other objects and advantages of the invention will be apparent from the detailed description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more fully understood by reference to the following drawings, which are for illustrative purposes only. 
     FIG. 1 is a schematic illustration of a tunable external cavity laser apparatus in accordance with the invention. 
     FIG. 2 is a top plan view of the tunable external cavity laser apparatus of FIG. 1 shown mounted on a sled in a hermetically sealed container. 
     FIG. 3 is a perspective view of the tunable external cavity laser apparatus of FIG.  2 . 
     FIG. 4 is a top plan view of the grating and MEMS actuator of the tunable external cavity laser apparatus of FIG.  2 . 
     FIG. 5 is a schematic illustration of another embodiment of a tunable external cavity laser apparatus in accordance with the invention. 
     FIG. 6 is a schematic illustration of another embodiment of a tunable external cavity laser apparatus in accordance with the invention. 
     FIG. 7 is a top plan view of the tunable external cavity laser apparatus of FIG. 6 shown mounted on a sled in a hermetically sealed container. 
     FIG. 8 is a top plan view of the tunable external cavity laser apparatus of FIG. 7 shown with the grating oriented such that the grating face is substantially perpendicular to the face of the movable mirror. 
     FIG. 9 is a top plan view of an external cavity laser apparatus in accordance with the invention with a shortened external cavity length and without an intracavity mode suppression filter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus shown in FIG.  1  through FIG.  9 . It will be appreciated that the apparatus may vary as to configuration and as to details of the parts, and that the method may vary as to details and the order of the acts, without departing from the basic concepts as disclosed herein. The invention is disclosed primarily in terms of use with an external cavity laser. The invention, however, may be used with various types of laser devices and optical systems. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. The relative sizes of components and distances therebetween as shown in the drawings are in many instances exaggerated for reason of clarity, and should not be considered limiting. 
     Referring now to FIG. 1, there is shown an external cavity laser apparatus  10  in accordance with the invention. The apparatus  10  includes a gain medium  12  and a tunable wavelength selection element that is shown in FIG. 1 as a reflective grating  14 . Gain medium  12  may comprise a conventional Fabry-Perot diode emitter chip and has an anti-reflection (AR) coated front facet  16  and a reflective or partially reflective rear facet  18 . Grating  14  is reflective, and an external laser cavity  22  is defined by rear facet  18  and the surface or face  20  of grating  14 . Gain medium  12  emits a coherent light beam  24  from front facet  16  that is collimated by lens  26  to define an optical path  28 . Grating  14  in other embodiments may comprise a transmissive grating, a prism, an interference filter, or other tunable element capable of providing wavelength selection. 
     Gain medium  12  emits an output beam  30  from facet  18 , which is collimated by lens  32  along an output path  34 . The beam  30  from path  34  is focused by lens  36  into an optical fiber  38 . An optical isolator  40  is positioned in optical path  34  between lenses  32 ,  26  to prevent return of light from fiber  38  into gain medium  12 . A coarse spectrometer (not shown) may also be positioned in output path  34  between lenses  32 ,  26  to provide monitoring of output wavelength during the operation of the apparatus  10 . 
     Grating  14  is operatively coupled to a microelectromechanical system or MEMS actuator element  29  that provides for positional adjustment of grating  14  during the operation of laser apparatus  10  as described further below. The terms “microelectromechanical system actuator” and “MEMS actuator” as used herein refer to actuator devices based on microsystems technology that are fabricated using micromachining techniques such as those used in the semiconductor industry. Numerous MEMS actuator devices that can provide selective actuation of components are known and commercially available. The use of MEMS actuation of a tunable wavelength selection element in an external cavity laser in accordance with the invention offers several advantages, also discussed more fully below, that have heretofore not been available in external cavity lasers. In certain embodiments, other actuator systems, such as stepper motors, voice coil actuators and the like, may be used to position grating  14  in accordance with the invention. 
     Grating  14  is positioned in optical path  28  and provides optical feed back to gain medium  12  along path  28 . Reflective grating face  20  includes a plurality of ridges, grooves or other diffractive features (not referenced) thereon that are dimensioned and configured to diffract light beam  24 . In the embodiment shown in FIG. 1, grating  14  and gain medium  12  are positioned in a “Littrow” configuration, although other configurations or arrangements of grating  14  and gain medium  12  may alternatively be used with the invention. Grating  14  is movable, and may be positionally adjusted to select the wavelength of light that is fed back to gain medium  12  by grating  14 , the phase of the feedback light to gain medium  12 , and the amount or optical power of the feedback to gain medium  12  by positional adjustment grating  14 . The positional adjustment of grating  14  in accordance with the invention allows wavelength selection and phase adjustment of the feedback to gain medium  12 , by selective positional adjustment of grating, to be carried out orthogonally or independently, such that wavelength selection adjustment does not affect or minimally effects phase adjustment, and vice versa. 
     Grating  14  is movable with respect to a first degree of freedom of motion to provide wavelength selection. In the embodiment shown, grating  14  is rotatable about a first axis A (normal to page) that is parallel to or substantially parallel to grating face  20 , in order to provide for wavelength selection. The diffractive nature of grating  14  imparts a spatial separation to light of different wavelengths in beam  24 , and rotatable adjustment of grating  14  about axis A controls the particular diffraction, and hence the particular wavelength, that is returned or fed back to gain medium  12 . 
     Positional adjustment of grating  14  with respect to a second degree of freedom of movement can provide for adjustment of the phase of the light fed back to gain medium  12  by grating  14 . In the embodiment of FIG. 1, grating  14  is translatably adjustable with respect to a second axis B. Translation of grating  14  along axis B or in the direction of axis B alters the distance between grating face  20  and gain medium facet  18 , and thus alters the length of external cavity  22 . Axis B as shown is perpendicular or substantially perpendicular or normal to axis A and to grating face  14 . 
     External cavity  22  defines a plurality of cavity modes (not shown) that result in transmission maxima that are periodically spaced apart in wavelength. During laser operation at a selected wavelength, lasing may jump or “hop” to an external cavity mode that is adjacent to the selected wavelength. Such “mode hops” are generally undesirable, and adjustment of the length of external cavity  22  are made during laser operation to optimally position the external cavity modes with respect to the selected wavelength and thus avoid unwanted mode hopping. Selective translational actuation of grating  14  along axis B adjusts the length of external cavity  22  and provides a phase adjustment to the light fed back to gain medium  12  from grating  14 , and serves to position the external cavity modes so that mode hopping is avoided. This phase adjustment can also be used to provide fine tuning of the selected wavelength. 
     Translation of grating  14  along axis B to provide phase control for positioning of external cavity modes can be carried out without varying the rotation of grating  14  with respect to axis A. Phase adjustment and wavelength adjustment thus can be easily orthogonalized or made independent of each other in the apparatus  10 , and wavelength adjustment for the laser apparatus  10  by angle tuning of grating  14  can be made without effecting, or minimally effecting, external cavity length. Similarly, translation of grating  14  along axis B to adjust external cavity length does not affect the orientation of grating with respect to axis A. The orthogonal tuning of a wavelength selection element and an external cavity length tuning element in an external cavity laser is also described in U.S. patent application Ser. No. 09/900,373 filed on Jul. 6, 2001, the disclosure of which is incorporated herein by reference. 
     Grating  14  may also be positionally adjustable with respect to a third degree of freedom of motion to control the amount or level of feedback to gain medium  12  from grating  14 , and hence control the output power level of the apparatus  10 . In the apparatus  10 , rotatable motion of grating  14  about axis C provides this third degree of freedom of motion. Rotation of grating  14  about axis C alters the alignment of the return beam (not shown) towards gain medium and thus can control the amount of level of light fed back to gain medium  12 . Rotation of grating  14  about axis C also changes the orientation of grating  14  with respect to the polarization of light beam  24 , which also effects the level of feedback. Rotation of grating  14  about axis C does not effect or alter the orientation or angle of grating with respect to axis A, and thus does not effect wavelength selection. Rotation of grating  14  with respect to axis C during laser operation provides several advantages, which are discussed further below. 
     The apparatus  10  includes a filter element that provides for suppression of lasing by the apparatus  10  at unwanted wavelengths. In the embodiment of FIG. 1, the filter element is shown as a Fabry-Perot etalon  41  positioned in optical path  28 . Etalon  41  includes partially reflective faces  42 ,  44  that, together with the refractive index of etalon  41 , are configured to define a plurality of transmission peaks (not shown) for light beam  24 . In this regard, etalon  41  serves as a wavelength locker or grid generator, with the plurality of transmission peaks of etalon  41  corresponding to discrete selectable wavelengths that may be chosen or selected by rotation of grating about axis A as related above. The transmission peaks of etalon  41  thus, for example, may correspond in wavelength to the wavelengths of the International Telecommunications Union (ITU) grid channels. Lasing at wavelengths other than wavelengths corresponding to the transmission peaks of etalon  41  are suppressed. Etalon  41  thus provides for suppression of external cavity modes at unwanted wavelengths. 
     In some embodiments of the invention, the length of external cavity  22  may be sufficiently short such that effective external cavity mode suppression is achieved without the presence of etalon  41 . In certain embodiments, etalon  41  may be actively tuned during laser operation to vary the free spectral range of etalon  41 , and hence the wavelength location of transmission peaks defined by the etalon  41 . Tuning of etalon may be carried out mechanically via tilt adjustment or via thermo-optic, electro-optic, acousto-optic or other mechanism where the material of etalon  41  has a refractive index that is responsive to temperature, voltage or other controllable property. The active tuning of a grid generator etalon is described more fully in U.S. patent application Ser. No. 09/900,474 filed on Jul. 6, 2001, the disclosure of which is incorporated herein by reference. 
     Referring to FIG.  2  and FIG. 3 as well as FIG. 1, the laser apparatus  10  is shown embodied in the apparatus  46 , wherein like parts are denoted with like reference numbers. In the apparatus  46 , the external cavity laser  10  is enclosed in a hermetically sealable container  48 . The lid (not shown) of container  48  is omitted for clarity. Container  48  allows the laser  10  to be sealed within an inert atmosphere to prevent contamination and/or degradation of optical surfaces on the various components of laser  10 , and particularly the anti-reflection coating on facet  16  (not shown in FIG.  2  and FIG. 3) of gain medium. A tubular support  50  holds an optical fiber (not shown) and allows the optical fiber to communicate with the hermetically sealed interior of container  48 . A ferrule  52  is provided to position the end of the optical fiber so that output from the laser apparatus  10  may be focused by lens  36  into the fiber. The use of an external cavity laser in a hermetically sealable enclosure is also described in U.S. patent application Ser. No. 09/900,423 filed on Jul. 6, 2001, the disclosure of which is incorporated herein by reference. 
     As shown in FIG.  2  and FIG. 3, gain medium  12  and optical isolator  40  are configured for selective thermal control independently from etalon  41  and grating  14 . Gain medium  12  and optical isolator  40  are mounted on a thermally conductive platform or pad  54 . Lenses  26 ,  36  and ferrule  52  may also be mounted on platform  54  as shown. Platform  54  in turn is mounted on a thermal control element  56 , which may comprise a conventional thermoelectric controller or TEC. Thermal control element  56  allows selective thermal control of gain medium  12  and optical isolator during laser operation. The use of selective thermal control of a gain medium and optical isolator in an external cavity laser is also described in U.S. patent application Ser. No. 09/900,429 filed on Jul. 6, 2001, the disclosure of which is incorporated herein by reference. Etalon  41  is mounted on a thermally conductive platform  58 , which in turn is mounted on a separate thermal control element  60 , to allow active thermal control of etalon  41  independently from the thermal control of gain medium  12  and isolator  40 . 
     A plurality of electrical leads communicate  62  with the interior of container  48  to provide power to gain medium  12 , to MEMS actuator  19 , to thermoelectric controllers  56 ,  61 , and to logic elements and/or circuitry (not shown) that is associated with controlling the current delivered to gain medium, control of thermoelectric controllers  56 ,  60 , and control of the degrees of freedom of motion with respect to axes A and B provided by MEMS actuator  19 . Container  48  includes flanges  64  to allow mounting of the apparatus  46  onto suitable surfaces (not shown). 
     The MEMS actuator  19  provides for rotational motion of grating  14  about axis A, translation motion with respect to axis B, and rotational motion about axis C as noted above. MEMS rotational and translational actuation may be carried out through a variety of mechanisms, including mechanical, electrostatic, piezoresistive, piezoelectric, thermoelectric electromagnetic and/or other interaction of micromachined parts. Micromachining may involve, for example, conventional photolithographic, material deposition, etching, polishing, plating and other techniques used in semiconductor device manufacture, to form actuator components. The fabrication and use of various types of MEMS devices are known in the art and are described, for example, in “Introduction to Microelectromechanical Systems Engineering” by Nadim Maluf and published by Artech House, Inc., Norwood, Mass. (2000), the disclosure of which is incorporated herein by reference. 
     Numerous micromachined MEMS rotational and translational driver configurations may be used with the invention. Referring more particularly to FIG. 4, as well as FIG.  1  through FIG. 3, MEMS actuator  19  is shown as includes one or more drive elements  66  which are contained or housed in a carrier  68 . Drive element  66  is coupled to carrier  68  by one or more spring elements, hinge elements, gimbal elements, other movable elements (not shown) that are internal to carrier  68  and which movably connect drive element  66  to carrier  68 . Drive element  66 , carrier  68 , and the movable connecting elements therebetween may be fabricated from the same substrate or from different components. Drive element  66  is movable with respect to carrier via actuation according to mechanical, electrostatic, piezoresistive, piezoelectric, thermoelectric electromagnetic and/or other effect as noted above. 
     Drive element  66  and carrier  68  are configured to provide a range of movement to grating  14  that includes rotation about axis A, translation along axis B, and rotation about axis C as noted above. In this regard, drive element  66  is operatively coupled to a transmitter substrate  70 , and transmitter substrate  70  is operatively coupled to a MEMS engine element  72 . Grating  14  is provided by the reflective surface  20  of MEMS engine  72 , which may comprise a polished surface of semiconductor substrate material. A plurality of grating lines, grooves or other diffractive features (not referenced) are included on surface  20  to define grating  14 , and may be formed on surface  20  via conventional photolithographic techniques or other methods. The grating lines as shown in FIG.  4  and the other FIGs. herein are only illustrative and are not shown to scale, and are not necessarily indicative of grating orientations that would be used during laser operation. 
     MEMS actuator  19  as described above provides positional adjustment to grating along three degrees of freedom of movement using a single actuator device. In certain embodiments, multiple MEMS actuators may be utilized, with each actuator configured to provide a desired positional degree of freedom to grating  14 . Thus, three separate MEMS actuator devices may be used in place of the single device  19 , with one MEMS actuator configured to provide rotational adjustment of grating  14  about axis A, another MEMS actuator configured to provide translational adjustment of grating  14  along axis B, and a third MEMS actuator used to provide rotational adjustment to grating  14  about axis C. Various arrangements and configurations of MEMS actuators for positioning of grating  14  in accordance with the invention are possible, and will suggest themselves to those skilled in the art. 
     Referring again to FIG. 1 in particular, in the operation of laser apparatus  10 , gain medium  12  is current-pumped in a conventional manner, and emits beam  24  from anti-reflection coated facet  16  which collimated along path  28  and directed to grating  14 . Grating  14  returns or feeds back light of a selected wavelength (according to the angle of grating with respect to axis A) along path  28  to gain medium  12  to provide lasing at the selected wavelength. Etalon  41  creates a plurality of transmission peaks, and the selected wavelength corresponds with one of these transmission peaks. Lasing at other wavelengths, which may arise from the presence of external cavity modes as described above, is suppressed by etalon  41 . During lasing, a portion of the optical output from gain medium  12  exits partially reflective facet  18 . This output is collimated and directed along output path  34  through optical isolator  40 , and then focused into fiber  38  for use. 
     When a change in the lasing wavelength is desired, MEMS actuator  19  drives grating  14  rotatably with respect to axis A to change the diffraction, and hence the selected wavelength, that is returned to gain medium. A controller element (not shown) may be used in association with MEMS actuator  19  to provide control signals thereto to drive or actuate grating  14  to positions corresponding to desired or selected wavelengths. The relation of grating position with selectable wavelengths may be embodied in a stored lookup table that is consulted by the controller when a change in wavelength selection is made. The relatively small size of MEMS actuator  19  allows rapid wavelength tuning. Using the apparatus of FIG. 1, wavelength tuning across a 40 nanometer channel spacing at sub-microsecond times, and channel switching on the order of milliseconds, are achievable. 
     MEMS actuator  19  may additionally translate grating  14  along axis B to “trim” or otherwise adjust the length of external cavity  28  to provide fine tuning of the selected wavelength and to prevent mode hopping to external cavity modes adjacent to the selected wavelength as described above. Translational adjustment of grating  14  along axis B may be made via use of a servo system (not shown). In such a servo system, for example, a sensor is used to monitor the output power of the apparatus  10 , either by optically monitoring output or by electrically monitoring of voltage across gain medium  12  during laser operation. Detection of non-optimal output power gives rise to error signals, which are then used by the servo system to drive MEMS actuator  19  to translate grating  14  along axis B until optimal output power is detected. The use of servo systems to control external cavity length are disclosed in U.S. patent application Ser. Nos. 09/900,426 and 09/09/900,443, both filed on Jul. 6, 2001, the disclosures of which are incorporated herein by reference. 
     During the operation of the laser apparatus  10 , grating  14  will typically be oriented to maximize the level of feed back to gain medium  12  at the selected wavelength. In many instances, however, it may be desirable to vary the orientation of grating  14  with respect to the polarization orientation of light beam  24  during laser operation to control the amount of level of feedback to gain medium  12  from grating  14 , and hence control the output power level of the apparatus  10 . This is achieved by rotation of grating  14  about axis C by MEMS actuator  19 . Active control of the pitch of grating  14  by rotation of grating about axis C provides rapid, accurate control of the output power level of the apparatus. 
     Rotation of grating  14  with respect to axis B allows the output from apparatus  10  to be “turned off” without actually powering down the diode gain medium  12 . The output of laser apparatus  10  can thus be briefly interrupted, by pitch adjustment of grating with respect to axis C, without power down of gain medium, while wavelength selection adjustment is made by rotational adjustment of grating  14  with respect to axis A. Due to the small size of MEMS actuator  19  and rapid rotational movement that it can impart to grating  14 , temporary “power downs” for the apparatus on the order microsecond duration or less can be achieved. 
     Active pitch control of grating  14  by rotation of grating  14  with respect to axis B allows the apparatus  10  to provide a steady level of output power in situations where environmental fluctuation may causes unwanted variation in laser output power. One such source of environmental fluctuation is current fluctuation when the current supply to gain medium  12  is uneven or not “clean”. During external cavity laser operation, spurious fluctuation in the level of current delivered to the laser gain medium causes unwanted fluctuation in the level of output power from the laser. Current fluctuation can be controlled using a “roll off” filter to provide a “clean” current to the gain medium. The use of such a filter, however, prevents active control of the current delivered to the laser gain medium. The active pitch control of grating  14  by MEMS actuator  19  allows steady laser output power without the use of a filter to provide a clean current. 
     Pitch control of grating  14  as described above may also be used to maintain a steady or even level of power output over the lifetime of a laser apparatus. The pitch of grating  14  with respect to axis C may be initially adjusted to provide a sub-maximum output power. As the diode gain medium  12  and antireflection coating  14  on facet  16  age and deteriorate due to repeated use, the level of output power achievable from the apparatus  10  will drop off. Period adjustment of the pitch of grating  14  to maintain a constant output power level avoids this effect. Pitch control of grating  14  with respect to axis C also may be used during manufacture or assembly of the apparatus. 
     Active pitch control of grating  14  to provide a constant output power may involve use of a servo mechanism (not shown) to monitor laser output power and make corresponding adjustments to the pitch of grating  14 . The servo mechanism may involve monitoring of output power of the apparatus  10 , with detection of non-optimal output power resulting in error signals that are then used by the servo system to drive MEMS actuator  19  to rotate grating  14  about axis C until optimal output power is again detected. Monitoring of output power may be carried optically, or electrically by monitoring voltage across gain medium  12 , as related above. Servoing of grating pitch to output power may also be carried out by introducing a frequency dither or modulation to the pitch of grating, and then monitoring the frequency dither. The use of servo systems using introduction of a frequency dither to an optical element to control external cavity length are disclosed in U.S. patent application Ser. Nos. 09/900,426 and 09/09/900,443, filed on Jul. 6, 2001 and incorporated herein by reference. 
     Reference is now made to FIG. 5, wherein another embodiment of an external cavity laser apparatus  74  is shown, with like reference numbers used to denote like parts. In the apparatus  74 , etalon  41  external to cavity  22 , and is positioned in output beam  40 . Etalon  41  may be located before isolator  40 , or elsewhere in output path  34 . Grating  14  is operative coupled to a MEMS actuator  19  as described above, and is positionable by MEMS actuator  19  to control wavelength selection, external cavity length and power level as described above. Etalon  41  operates in the same was when in an intracavity location as shown in FIG. 1, by defining selectable transmission bands or peaks, and providing for suppression of lasing at wavelengths other than the selectable transmission bands. In other respects, the operation of laser apparatus  74  is substantially the same as described above for the apparatus  10  of FIG.  1 . 
     Referring now to FIG. 6, there is shown still another embodiment of an external cavity laser apparatus  76  in accordance with the invention, with like reference numbers used to denote like parts. In the apparatus  76 , a movable reflective element or mirror  78  is operatively coupled to MEMS actuator  19 . Movable mirror  78  reflects beam  24  to a stationary grating element  80 , such that optical path  28  extends from mirror  78  to grating  80 . Grating  80  includes a reflective surface  82  with a plurality of grating lines (not referenced) etched thereon to provide for diffraction of light beam  24  in the manner described above. The external cavity (not referenced) in the apparatus  76  is defined or delineated by grating surface  80  and facet  18  of gain medium  12 , such that the external cavity is “folded” about mirror  78 . 
     Mirror  78  is movable with respect to optical path  28  according to operation of MEMS actuator  19 , and is rotatable about axis A, translatable along axis B, and rotatable about axis B in the same manner as the grating  14  in the apparatus  10  described above. Rotation of mirror about axis A by MEMS actuator  19  alters the diffraction from stationary grating  80  that is returned from grating  80  to mirror  78 , and thus provides for selection of a wavelength of light that is fed back to gain medium  12  by mirror  78  along optical path. Translation of mirror along axis B by MEMS actuator  19  adjusts the length of the external cavity and allows cavity “trimming” adjustment to avoid unwanted mode hopping and to fine tune selected feedback wavelength as described above. Rotation of mirror  78  about axis C by MEMS actuator  19  alters the amount of light that is returned from grating  80  to mirror  78 , and hence controls the amount or level of feed back that is returned from mirror  78  to gain medium  12 . 
     The apparatus  76  thus operates in a manner that is similar to the apparatus  10  and  74  described above, with the primary exception being that positional adjustment of an intracavity mirror  78  with respect to a stationary grating  80 , rather than positional adjustment of a grating, is used to control the wavelength selection, external cavity length, and power attenuation operations described above. The same degrees of freedom of movement of mirror  78  are used to perform the wavelength selection control, external cavity length control, and power attenuation control, as described above for the grating  14  of apparatus  10 . In other respects the operation of the laser apparatus  76  is substantially the same as described above for the apparatus  10 . 
     The laser apparatus  78  may also be embodied in a laser transmitter device  84  as shown in FIG. 7 wherein the apparatus  78  is enclosed within a hermetically sealable container or enclosure  48 . Like parts in FIG. 7 are denoted by like reference numbers. As in the apparatus  46  described above, gain medium  12  and isolator  40  are mounted on thermally conductive platform  54  for selective thermal control by thermoelectric controller  56 , and etalon  41  is mounted on thermally conductive platform  58  for independent thermal control by thermoelectric controller  60 . Output from laser apparatus  78  is directed into a fiber (not shown) mounted in ferrule  52  as also described above. The overall package size of the laser device  84  and container  48  of FIG. 7 may be slightly larger than that of the apparatus  46  in FIG. 2, in order to accommodate the longer external cavity of the laser apparatus  78 . 
     Referring now to FIG. 8, there is shown a laser apparatus  84 , with like numbers denoting like part. In the apparatus  84 , the stationary reflective grating  80  is changed in orientation with respect to movable mirror  78 . In the apparatus  76  and  78  discussed above, grating  80  is shown in an orientation such that grating surface  82  is substantially parallel to movable mirror  78 . In the apparatus  84  of FIG. 8, grating  80  is positioned such that grating face  82  is substantially perpendicular or normal to mirror  78 . The apparatus  84  in other respects is substantially identical to the apparatus  84  described above. 
     Referring next to FIG. 9, yet another embodiment of a laser apparatus  86 , wherein like numbers are used to denote like parts. In the apparatus  86 , the etalon filter  41  is omitted, and grating  14  is positioned proximate to collimator  26 , to shorten the length of the external laser cavity defined by grating surface  20  and rear facet  18  of gain medium. Shortening of the laser cavity in this manner increases the spacing of the external cavity modes with respect to each other. Thus, the external cavity modes may be configured such that no external cavity modes are proximate to or otherwise close in wavelength to the selectable wavelengths of the laser apparatus  86 . 
     The short external laser cavity length of the apparatus  86  eliminates the need for an additional wavelength suppression element or filter, such as the etalon  41  shown in FIG.  1  through FIG.  8  and discussed above, because unwanted lasing associated with external cavity modes will not occur due to the relatively wide spacing of the cavity modes. In some embodiments, the external laser cavity length can be adjusted such that the external cavity modes themselves define the wavelengths that are selected by positioning of grating  14 . In other words, the external cavity serves as a wavelength locker or grid generator, with the cavity mode peaks corresponding to a desired wavelength grid. Omission of the etalon filter also allows a smaller overall package for the apparatus  86 , and reduces the overall cost of the apparatus. 
     In other embodiments of the invention, a tunable wavelength selection other than a grating may be used. Thus, MEMS actuator  19  may be configured to position an etalon, interference filter, prism or other element that can provide wavelength selection according to MEMS actuated positioning. Various types of tunable wavelength selection elements that may be used with the invention in place of grating  14  are disclosed in U.S. patent application Ser. No. 09/814,464 filed on Mar. 21, 2001, the disclosure of which is incorporated herein by reference. 
     While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.