Abstract:
A wavelength tuning system adjusts an external cavity of a laser about two orthogonal axes for aligning features of a diffractive optic with a third orthogonal axis about which the diffractive optic is pivoted for providing selected wavelength feedback to the laser. A laser mount supports the laser, and a mounting arm supports both the diffractive optic and a reflective optic in a fixed orientation with respect to each other. A flexural member forms at least part of a connection between the mounting arm and the laser mount in a relative orientation for optically coupling the diffractive optic to the laser within the external cavity. The mounting arm is pivoted with respect to the laser mount about the pivot axis by an actuator for varying a wavelength of a diffracted portion of the output beam returned to the laser without significantly varying an angular orientation of the output beam with respect to the laser mount as reflected from the reflective optic.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of prior filed U.S. Provisional Application 60/384,058, filed May 29, 2002, and U.S. Provisional Application 60/392,960, filed Jul. 1, 2002. Both Provisional Applications are hereby incorporated by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The invention relates to tunable laser systems, particularly of the type having adjustable external cavities for varying the wavelength of laser output beams. One end of such cavities is formed by diffractive optics that are inclined to the laser beams under the control of adjustment mechanisms to provide controlled wavelength feedback.  
         BACKGROUND  
         [0003]    External cavity lasers are well known in the art. The external cavity returns a portion of the radiation generated by the laser back into a primary laser cavity as a form of optical feedback that alters the laser radiation being amplified in the primary laser cavity. Modifications can be made to the external cavity to control properties of the light amplified in the primary laser cavity. This procedure has been used for several years to stabilize and tune the frequency output of dye lasers and is described in  Spectrally Narrow Pulsed Dye Laser Without Beam Expander,  Applied Optics, Vol. 17, No. 14, Jul. 15, 1978.  
           [0004]    External cavities are also commonly used with diode lasers and can be used to create narrowband, wavelength tunable diode laser systems for such applications as telecommunications, spectroscopy, and metrology.  
           [0005]    A primary laser has a defined output beam direction and lasing wavelength. The lasing wavelength, which is the wavelength that exhibits maximum gain, is set by properties of the laser gain medium and the laser cavity. While it is possible for the primary laser to operate at any of a range of wavelengths, a single wavelength can be favored by prevailing conditions for which gain is maximized.  
           [0006]    The external cavity allows for the alteration of the lasing wavelength of the primary laser by providing feedback to the laser in the form of a selected wavelength other than the one initially favored by the primary laser. In this manner, the primary laser is artificially caused to favor another wavelength, thus creating a means to tune the wavelength of the laser system.  
           [0007]    Two types of external laser cavities are commonly used—the Littman cavity and the Littrow cavity. Specific configurations of Littman cavities have been the subject of U.S. Pat. Nos. 5,319,668; 5,802,085; and 5,867,512. An article in the March 1998 Review of Scientific Instruments, Volume 69, Number 3, pages 1236-1239, by Arnold et al. entitled “A Simple Extended-cavity Diode Laser” describes an exemplary Littrow cavity configuration. These patents and the above referenced articles are incorporated by reference.  
           [0008]    Although both the Littman cavity and Littrow cavity configurations provide effective wavelength tuning, the Littrow cavity configuration is preferred for many commercial applications because it has a lower component cost. According to the Littrow configuration, a collimated output beam from the primary laser is directed toward a reflective diffraction grating that is oriented so that a first diffracted order of the output beam is returned to the primary laser on a path of retroreflection. The wavelength of the first diffracted order provides feedback to the laser to influence the lasing wavelength. The zero diffracted order of the output beam reflects from the grating for exiting the external cavity.  
           [0009]    For returning a particular wavelength “λ”, a Littrow orientation angle “φ pitch ” of the grating normal with respect to the beam direction of the primary laser is given by  
         φ   pitch     =       sin     -   1            (     λ     2                 d       )                             
 
           [0010]    where “d” is the grating period. The external cavity thus provides feedback at wavelength “λ”; and if this feedback is such that “λ” becomes the wavelength for which the primary laser has maximum gain, then the laser system will lase at the feedback wavelength “λ”.  
           [0011]    A difficulty with the Littrow configuration, as shown in FIG. 1, is that the orientation of an output beam  18  from the external cavity  16  varies as the pitch angle φ pitch  of the diffraction grating  14  is changed. Thus, as the output beam  18  is tuned to different wavelengths, the orientation of the output beam changes, which produces significant alignment problems for optical devices intended for operation over multiple wavelengths.  
         SUMMARY OF THE INVENTION  
         [0012]    The invention contemplates a wavelength tuning system for adjusting the external cavity of lasers and is intended to be particularly effective for varying output beam wavelength while maintaining the output beam from the external cavity at a fixed angular orientation. Improvements are made to the adjustment and alignment of components as well as to the components themselves, resulting in a robust structure that is easy to align and to maintain in alignment. The invention is especially useful for providing a tunable external cavity laser that operates optimally for various applications, such as multi-wavelength interferometry.  
           [0013]    An exemplary wavelength tuning system for adjusting the external cavity of a laser in accordance with the invention includes a laser mount for supporting the laser that emits a wavelength-tunable output beam from the external cavity. A mounting arm supports both a diffractive optic and a reflective optic in a fixed orientation with respect to each other. A flexural member forms at least part of a connection between the mounting arm and the laser mount in a relative orientation that optically couples the diffractive optic to the laser within the external cavity. An actuator pivots the mounting arm with respect to the laser mount about a pivot axis for varying a wavelength of a diffracted portion of the output beam returned to the laser without significantly varying an angular orientation of the output beam with respect to the laser mount as reflected from the reflective optic. The flexural member supports a limited rotation of the mounting arm with respect to the laser mount about the pivot axis while providing resistance to a similar limited rotation of the mounting arm with respect to the laser mount about an orthogonal rotational axis.  
           [0014]    Preferably, the tuning system also includes an adjuster to adjust the relative angular orientation of the mounting arm with respect to the laser mount about the orthogonal rotational axis. One such adjuster provides an adjustable mounting for the flexural member that adjusts the mounting of the flexural member between the mounting arm and the laser mount about the orthogonal rotational axis.  
           [0015]    The orthogonal rotational axis is preferably a first of two orthogonal rotational axes that are orthogonal to the pivot axis, and the adjuster is preferably a first of two adjusters that adjust the relative angular orientation of the mounting arm with respect to the laser mount. A second of the adjusters can be used to adjust the relative angular orientation of the mounting arm with respect to the laser mount about the second of the orthogonal rotational axes. For example, the second adjuster can provide an adjustment of the flexural member between the mounting arm and the laser mount about the second orthogonal rotational axis. Similar adjustments can be made between two portions of the mounting arm, a first portion being connected to the flexural member and a second portion supporting both the diffractive optic and the reflective optic.  
           [0016]    Alternatively, the second adjuster could engage the mounting arm remote from the flexural member for imparting a limited angular adjustment of the mounting arm about the second orthogonal rotational axis with respect to the laser mount through the flexural member. The mounting arm of this alternative preferably includes first and second ends—the first end being connected to the flexural member and a second end being connected to the second adjuster. The second adjuster permits motion of the mounting arm about the pivot axis while restricting further motion about the second orthogonal rotational axis from an established adjustment position. The flexural member preferably exerts a first preload torque on the mounting arm about the pivot axis for imparting a preload force on the actuator and preferably exerts a second preload torque on the mounting arm about the second orthogonal rotational axis for imparting a preload force on the second adjuster.  
           [0017]    The first and second adjusters provide for adjusting the diffractive optic to return the diffracted portion of the output beam to the laser along a path of retroreflection. The diffractive optic preferably has a planar surface with rulings, and the first and second adjusters provide for relatively adjusting the rulings substantially parallel to the pivot axis. In this regard, the first adjuster preferably provides for relatively adjusting the rulings about the first orthogonal rotational axis in an orientation extending substantially perpendicular to the rulings, and the second adjuster provides for relatively adjusting the rulings about the second orthogonal rotational axis in an orientation extending substantially normal to the planar surface of the diffractive optic.  
           [0018]    For defining the pivot axis, the flexural member is preferably mounted between nominally parallel mounting surfaces of the mounting arm and the laser mount. The pivot axis extends parallel to the nominally parallel mounting surfaces of the mounting arm and the laser mount. However, the first adjuster can provide for making small angular adjustments between the nominally parallel mounting surfaces of the mounting arm and the laser mount about the first orthogonal axis to better align the rulings of the diffractive optic substantially parallel to the pivot axis.  
           [0019]    The flexural member in its preferred form has a plate-shaped body, and the pivot axis is defined by the bending of the plate-shaped body. For exerting the desired preload torques, the plate-shaped body of the flexural member is preferably made of a resilient material. A first fastening system preferably connects the plate-shaped body of the flexural member to the laser mount, and a second fastening system preferably connects the plate-shaped body of the flexural member to the mounting arm. One of these first and second fastening systems is preferably adjustable for adjusting the relative angular orientation of the mounting arm with respect to the laser mount about the first orthogonal rotational axis. The pivot axis extends substantially parallel to a plane of the plate-shaped body of the flexural member, and the first orthogonal rotational axis extends normal to the plane of the plate-shaped body of the flexural member.  
           [0020]    An adjustable external cavity arrangement for a wavelength tunable laser according to the invention includes a laser mount for supporting the wavelength tunable laser, which emits an output beam whose peak wavelength is adjustable by wavelength feedback within the external cavity. A mounting arm supports both a diffractive optic and a reflective optic for movement with the mounting arm. A pivot supports relative rotation between the laser mount and the mounting arm for orienting the diffractive optic with respect to the laser through a range of pitch angles. The diffractive optic is movable with respect to the laser through the range of pitch angles for providing feedback to the laser as a corresponding range of wavelengths. The reflective optic is movable together with the diffractive optic for maintaining a substantially fixed angular orientation of the output beam with respect to the laser mount as reflected from the reflective optic. The diffractive optic and the reflective optic have front and back surfaces—the front surface of the diffractive optic being an operative surface for diffracting the output beam, and the front surface of the reflective optic being an operative surface for reflecting the output beam. The mounting arm has first and second mounting surfaces. The front surface of the diffractive optic is mounted against the first mounting surface of the mounting arm, and the front surface of the reflective optic is mounted against the second mounting surface of the mounting arm for mounting the diffractive optic and the reflective optic for movement together with the mounting arm.  
           [0021]    As so mounted, the first and second mounting surfaces preferably have a fixed relation to each other and a fixed relation to the mounting arm. Preferably, the first and second mounting surfaces are fixed parallel to each other. In a preferred form of the mounting arm, a common aperture is formed through the first and second mounting surfaces for conveying the output beam between the diffractive optic and the reflective optic. The common aperture extends beyond one side of the reflective optic to permit the output beam inside the external cavity to reach the diffractive optic, and the common aperture extends beyond one side of the diffractive optic to permit the output beam outside the external cavity to propagate past the diffractive optic.  
           [0022]    An alignment system for an external cavity of a laser in accordance with the invention includes a laser mount for the laser, which emits a wavelength-tunable output beam from the external cavity, and a mounting arm carrying a diffractive optic for providing wavelength feedback to the laser. A pivot permits a range of relative rotations between the mounting arm and the laser mount about a pivot axis for varying the wavelength of feedback to the laser. An adjuster adjusts a relative angular orientation of the mounting arm with respect to the laser mount about an adjustment axis that is oblique to the pivot axis. The mounting arm has first and second junctures that are spaced apart along a length of the mounting arm on opposite sides of the diffractive optic. The first juncture is connected to the pivot for supporting the range of relative motions between the mounting arm and the laser mount about the pivot axis. The second juncture is connected to the adjuster for adjusting the relative angular orientation of the diffractive optic for aligning the wavelength feedback to the laser.  
           [0023]    The adjuster is preferably a second of two adjusters, and the adjustment axis is preferably a second of two mutually oblique adjustment axes. The first of the two adjusters provides for adjusting the relative angular orientation of the mounting arm with respect to the laser mount about the first of the two mutually oblique adjustment axes. Preferably, the first adjuster is connected to the first juncture of the mounting arm for further adjusting the relative angular orientation of the diffractive optic for aligning the wavelength feedback to the laser.  
           [0024]    The diffractive optic preferably has a planar surface with rulings, and the first and second adjusters provide for relatively adjusting the rulings substantially parallel to the pivot axis. The adjustments of the two adjusters can be made independently by orienting the oblique adjustment axes mutually orthogonal. The first adjuster preferably provides for relatively adjusting the rulings about the first adjustment axis in an orientation extending substantially perpendicular to the rulings, and the second adjuster preferably provides for relatively adjusting the rulings about the second adjustment axis in an orientation extending substantially normal to the planar surface of the diffractive optic.  
           [0025]    An actuator pivots the mounting arm with respect to the laser mount about the pivot axis for varying the orientation of the diffractive optic with respect to the laser. The pivot is preferably formed by a flexural member that exerts a preload force on the actuator. The flexural member also preferably exerts a preload force on the second adjuster. The flexural member preferably has a plate-shaped body, and both the pivot axis and the second adjustment axis preferably lie substantially in a plane of the plate-shaped body of the flexural member. In addition, the flexural member preferably provides resistance to relative rotation between the mounting arm and the laser mount about the first adjustment axis that extends normal to the plane of the plate-shaped body of the flexural member. The preferred mounting arm has a home position for tuning the laser to a predetermined wavelength, and the flexural member exerts a preload force between the mounting arm and the actuator at the home position.  
           [0026]    The alignment system also includes a base supporting both the laser mount and the adjuster. The second adjuster adjusts the second juncture of the mounting arm with respect to the base for varying the relative angular orientation of the diffractive optic with respect to the base. The second adjuster preferably includes a guideway that restricts motion of the mounting arm with respect to the laser mount in a direction parallel to the pivot axis while permitting motion of the mounting arm with respect to the laser mount in a plane normal to the pivot axis. The second adjuster also preferably provides for adjusting the guideway parallel to the pivot axis. The mounting arm is preferably preloaded against the guideway.  
           [0027]    The actuator referred to above is preferably an electrical device such as a solenoid or an electric motor. Preferably, associated with the actuator are a position transducer and a position encoder for determining the position of actuator arm. A microprocessor responsive to commands from a computer system preferably controls the actuator for adjusting the wavelength output of the external cavity laser as required for its intended use. Generally, the external cavity contemplated for the invention has a Littrow configuration in which the first diffraction order is returned directly to the laser. However, the invention can also provide benefits for other cavity configurations. The descriptions of the invention are exemplary in nature for calling attention to specific forms of the invention to aid in its practice and understanding. 
       
    
    
     DRAWINGS  
       [0028]    [0028]FIG. 1 is a perspective view, diagrammatically illustrating a tunable laser system having a laser and a diffractive optic and referencing the axes about which the diffractive optic is adjustable.  
         [0029]    [0029]FIG. 2 is a plan view diagrammatically depicting a light path through an external cavity laser system in accordance with the invention showing a reflective optic in relation to the diffractive optic for maintaining a fixed orientation of a laser output beam.  
         [0030]    [0030]FIG. 3 is a perspective view of an assembly including a mounting arm for carrying both the diffractive optic and the reflective optic connected to a laser mount through a flexure member.  
         [0031]    [0031]FIG. 4 is different perspective view of the assembly of FIG. 3 showing an adjustment mechanism at a remote end of the mounting arm.  
         [0032]    [0032]FIG. 5 is a perspective view of the mounting arm showing additional details.  
         [0033]    [0033]FIG. 6 is a plan view of the assembly mounted together with an actuator on a common base.  
         [0034]    [0034]FIG. 7 is a block diagram showing the electrical/electronic components of the tunable laser system. 
     
    
     DETAILED DESCRIPTION  
       [0035]    As shown in FIGS. 1 and 2, a tunable laser system  10  includes a primary laser  12  coupled to a diffractive optic  14  in the form of a reflective diffraction grating at one end of an external cavity  16 . The output beam  18  of the laser system  10  is wavelength tunable by rotating (i.e., pivoting) the diffractive optic  14  about a pitch axis  20  through a range of pitch angles φ pitch .  
         [0036]    To carry out wavelength tuning, the diffractive optic  14  must be initially oriented properly, and the orientation must be maintained while the pitch angle φ pitch  is varied. For example, the diffractive optic  14  has rulings  26  that are formed in a reflective planar surface  28  and that extend parallel to the pitch axis  20 . For achieving the proper initial orientation, the diffractive optic  14  is angularly adjustable about two oblique adjustment axes  22  and  24 , which are preferably orthogonal to each other and to the pitch axis  20 . The adjustment axis  22 , which can be referred to as a “roll” axis, extends perpendicular to the rulings  26  in plane parallel to the planar surface  28  of the diffractive optic  14 . The adjustment axis  24 , which can be referred to as a “yaw” axis, extends normal to the planer surface  28  of the diffractive optic  14 .  
         [0037]    The angular orientation of the diffractive optic  14  about the three referenced axes  20 ,  22 , and  24  are designated as φ pitch , φ roll , and φ yaw . Preferably, the diffractive optic  14  is oriented to a pitch angle φ pitch  near the so-called “Littrow angle” with values of the roll angle φ roll  and the yaw angle φ yaw  set to zero degrees (i.e., where the rulings  26  extend parallel to the pitch axis  20 ). The pitch angle φ pitch  is adjusted while maintaining the zero values for the roll and yaw angles φ roll  and φ yaw . The required alignment returns a first order diffracted portion of the output beam  18  to the primary laser  12  for tuning the laser  12  to a particular wavelength and advances a zero order diffracted portion of the output beam  18  beyond the external cavity  16  to a reflective optic  30 , which can take the form of a folding mirror or prism.  
         [0038]    In accordance with the illustrated embodiment of the invention, the diffractive optic  14  is mounted together with the reflective optic  30  on a rotatable mounting arm  32  shown in FIGS.  3 - 6 . The reflective optic  30  is preferably mounted in a fixed orientation in the mounting arm  32  parallel to the diffractive optic  14 . Both the diffractive optic  14  and the reflective optic  30  are preferably permanently affixed to the mounting arm  32  by an adhesive or other fastening system.  
         [0039]    In one example, the primary laser  12  in the form of a laser diode has a collimated nominal output at 785 nanometers (nm), and the diffractive optic  14  in the form of a reflective diffraction grating has 1800 rulings (e.g., grooves) per millimeter (mm). Under these conditions, the nominal Littrow pitch angle is approximately 45 degrees. Thus, the mounting arm  32  could be initially oriented in a so-called “home” position at a pitch angle φ pitch  equal to approximately 45 degrees and at respective roll and yaw angles φ roll  and φ yaw  equal to zero degrees.  
         [0040]    A flexure member  34  preferably attaches the mounting arm  32  to a laser mount  36 , which mounts the primary laser  12 . This flexure member  34  is resiliently flexible and can be constructed, for example, from sheet steel that is 0.01 inches thick. As shown in FIGS. 3, 4, and  6 , the laser mount  36  is formed with a mounting surface  38  that is aligned with a mounting surface  40  at an end junction of the mounting arm  32  for attaching the flexure member  34  to both the mounting arm  32  and the laser mount  36 .  
         [0041]    Preferably, the flexure member  34  orients the diffractive optic  14  at respective roll and yaw angles φ roll  and φ yaw  equal to zero degrees. In practice, however, the mounting of the diffractive optic  14  may not be perfect. Thus, it is desirable to incorporate the ability to adjust the orientation of the mounting arm  32  in order to correct for any deviation in the initial roll and yaw angles φ roll  and φ yaw .  
         [0042]    Precise adjustment of the roll angle φ roll  can be made by a roll angle adjuster  42  between the mounting arm  32  and the flexure member  34 . The adjuster  42  includes oversized through-holes  44  in the flexure member  34  together with screws  46  that pass through these holes  42  into threaded engagements with the mounting arm  32 . To adjust the roll angle φ roll , the screws  44  are set so that they are slightly loose. A set screw  48  in the mounting arm  32  is turned against the laser mount  36  for angularly adjusting the mounting arm  32  with respect to the laser mount  36  within the plane of the flexure member  34 . The adjustment of the roll angle φ roll  is made while monitoring the position of the output beam  18 . When the zero roll angle φ roll  position is identified, the screws  46  are tightened so that the mounting arm  32  is connected to the laser mount  36  at the zero roll angle φ roll  position of the diffractive optic  14 . A similar roll angle φ roll  adjustment could be made between the flexure member  34  and the laser mount  36  or between angularly adjustable portions of the mounting arm  32 .  
         [0043]    The yaw angle φ yaw  of the diffractive optic  14  can also be adjusted in a variety of ways. For example, the mounting arm  32  is divided into two portions  50  and  52  separated by flexible portion  54 . The screws  46  or others like them can be arranged to pass through the portion  50  of the mounting arm  32  into threaded openings  56  within the portion  52  of the mounting arm  32 . The threaded openings  56  are located on opposite sides of the flexible portion  54  so that the two portions  50  and  52  of the mounting arm can be tilted with respect to each other by differentially tightening the screws  46 . The flexible portion  54  is shaped and oriented so that the tilt between mounting portions  50  and  52  takes place about an adjustment axis parallel to the yaw reference axis  24 .  
         [0044]    Another way of adjusting the yaw angle φ yaw  of the diffractive optic  14  is by tilting the entire mounting arm  32  with respect to the laser mount  36  through the flexure member  34 . An adjuster  62 , which is shown in FIGS. 4 and 6, includes a guideway  64  that engages a pin  66  at a remote end junction of the mounting arm  32 . The guideway  64  is formed in a slide  68  that is moveable on a guide mount  70  with respect to the laser mount  36 . In fact, both the guide mount  70  and the laser mount  36  can be mounted together on a common base  74  as shown in FIG. 6.  
         [0045]    The guide mount  70  permits adjustment of the slide  68  in the direction of the pitch pivot axis  20  via screws  72  extending through the guide mount  70  into threaded engagements with the slide  68 . The pin  66  translates with the slide  68 , twisting the flexure member  34  for tilting the mounting arm  32  about an adjustment axis parallel with the yaw reference axis  24 . After reaching a zero yaw angle φ yaw  of the diffractive optic  14 , the screws  72  are tightened to restrict further movement of the mounting arm  32  with respect to the laser mount  36  in a direction parallel to the pitch pivot axis  20 . However, the guideway  64  permits movement of the mounting arm  32  with respect to the laser mount  36  in a plane normal to the pitch pivot axis  20 . Although the pin  66  projects from the remote junction end of the mounting arm  32  and the guideway  64  is formed in the slide  68 , the pin  66  could project from the slide  68  and the guideway  64  could be formed in the remote junction end of the mounting arm  32 .  
         [0046]    Once the initial alignment of the roll and yaw angles roll φ roll  and φ yaw  is accomplished and the alignment positions are fixed, an actuator  76  is used to push or pull the remote end of the mounting arm  32  about the pitch pivot axis  20  as defined by the flexural member  34  through a range of pitch angles φ pitch  to tune the laser wavelength. The laser can be calibrated so that the initial wavelength corresponds to the home position of the mounting arm  32 .  
         [0047]    Preferably, the actuator  76  is an electronically controlled actuator incorporating an electronically readable encoder  78  (a) to measure the output of a position transducer  79  for providing information concerning the position of the mounting arm  32  and (b) to indicate the laser system&#39;s operating wavelength. The geometry of the flexure member  34  can be selected to change the location of the pitch pivot axis  20  for the mounting arm  32 . The pitch pivot axis  20  axis can be located remote from the diffractive optic  14  to provide for inclining as well as displacing the diffractive optic  14  to minimize the amount of mode-hopping exhibited by the laser  12  as the wavelength is tuned.  
         [0048]    At the home position of the mounting arm  32  and actuator  76 , the flexural member  34  is preferably (a) bent to exert a first preload torque on the mounting arm  32  about the pitch pivot axis  20  for imparting a preload force on the actuator  76  and (b) twisted to exert a second preload torque on the mounting arm  32  about the yaw adjustment axis  24  for imparting a preload force on the adjuster  62 . Both preload forces are preferably maintained in their original directions throughout a range of pitch angles φ pitch  required to tune the laser wavelength to ensure more repeatable and robust tuning operations. For example, the first preload force preferably exerts a compressive force against the actuator  76  throughout the intended range of pitch angles φ pitch . Similarly, the second preload torque preferably maintains the pin  66  engaged with one side or the other of the guideway  64  throughout the range of adjustment positions about the yaw reference axis  24 .  
         [0049]    To maintain the laser output beam  18  exiting the external cavity  16  in the same angular orientation despite variations in the angular orientation of the diffractive optic  14  throughout the range of pitch angles φ pitch , the reflective optic  30  is mounted together with the diffractive optic  14  in a fixed parallel orientation on the mounting arm  32 . Accurate mounting of the diffractive optic  14  and the reflective optic  30  is achieved in part by forming parallel mounting surfaces  80  and  82  in opposite sides of the mounting arm  32 . The diffractive optic  14  includes front and back surfaces  84  and  85 , and the reflective optic  30  includes front and back surfaces  86  and  87 . The front surface  84  of the diffractive optic  14  is an operative surface for diffracting the output beam  18 , and the front surface  86  of the reflective optic  30  is an operative surface for reflecting the output beam  18 . The front surface  84  of the diffractive optic  14  is mounted against the parallel mounting surface  80  of the mounting arm  32 , and the front surface  86  of the reflective optic  30  is mounted against the parallel mounting surface  82  of the mounting arm  32 . By mounting the operative surfaces  84  and  86  of the diffractive optic  14  and the reflective optic  30  against the parallel mounting surfaces  80  and  82 , a higher degree of accuracy is achieved. Mounting errors associated with differences between the front and back surfaces  84  and  85  or  86  and  87  of the diffractive optic  14  and the reflective optic  30  are avoided.  
         [0050]    To permit the propagation of the output beam  18  through the mounting arm  32 , a common aperture  88  is formed through the parallel mounting surfaces  80  and  82  for conveying the output beam  18  between the diffractive optic  14  and the reflective optic  30 . The common aperture  88  extends beyond one side of the reflective optic  30  to permit the output beam  18  inside the external cavity  16  to reach the diffractive optic  14 . The common aperture  88  extends beyond one side of the diffractive optic  14  to permit the output beam  18  outside the external cavity  16  to propagate past the diffractive optic  14  for its intended use.  
         [0051]    Referring to FIG. 7, laser tuning system  10  is shown having a computer  90  that controls a microprocessor  92  providing a computer controller on an electronics/control board  94 . The control board  94  has an actuator controller  96  in which the wavelength selection or tuning data generated in the microprocessor  92  is applied and converted into an electrical signal which drives the actuator  76 , the actuator being a DC motor, stepping motor, or another type of actuator. For example, the microprocessor  92  can send signal(s) to the actuator controller  96  to drive the actuator  76  forward or backward one or more steps. The actuator controller  96  then applies current or voltage signals to drive the actuator  76  accordingly.  
         [0052]    A laser power supply  98  is electrically interlocked via a switch  99 . This interlock is generally required as an emergency switch for laser safety. The microprocessor  92  also senses the state of the interlock and can report the status to the computer  90 . A home position detection switch  100  detects when the mounting arm  32  is at the desired home position.  
         [0053]    In operation, the computer  90  issues commands to the microprocessor  92 . For example, such commands include: a) Turn laser power on (microprocesser enables laser)—microprocessor checks interlock status and reports the status to the computer; b) Turn laser power off (microprocessor disables laser); c) Move actuator forward by N steps to tune the laser  12  to a new wavelength that is separated from the previous wavelength by an amount proportional to N. This is done via actuator controller; d) Return actuator home—move actuator backward until home position switch is triggered.  
         [0054]    The microprocessor  92  can operate in accordance with programming in the memory of the microprocessor  92 , or in other memory (e.g., EEPROM, FLASH, or ROM) on the board  94 . The computer  90  can represent a computer system, or a computer controller typical of tunable lasers, interfaced to the board  94 . The interface between the computer  90  and the board  94  can use typical data communication (e.g., hardware, software, and cables) to communicate between a computer system and a microprocessor. The board  94  and the laser  12  can be located in a housing (not shown) having a window or port to output the laser beam  18 . The computer  90  can be programmed to convert user selected or automatically selected wavelength or frequency into a number of actuator steps (with direction forward or backward) in a command to the microprocessor  92 , in accordance with curves, look-up table, or formula in the memory of computer  90 , to tune laser  12 . The system can be calibrated at manufacture to associate different actuator positions to output characteristics of the laser (e.g., wavelength or frequency).  
         [0055]    Variations and modifications in the described tunable laser system and alignment apparatus will undoubtedly suggest themselves to those skilled in the art. For example, a variety of angular and linear adjustment mechanisms could be used for making the prescribed adjustments along the adjustment axes  22  and  24 . The adjustment axes themselves could be located in a variety of positions and other orientations for aligning the diffractive optic  14  with the laser output beam  18  and the pivot axis  20 . Various manual or automatic adjustment mechanisms could be used to pivot the diffractive optic  14  about the pivot axis  20 , and the location of the pivot axis  20  could be varied for such purposes as reducing mode hopping. Although the pivot axis  20  is preferably formed by a flexure member in the form of a resilient plate to provide a robust tuning system, other types of flexure members could be used as well as other types of pivot axes.