Abstract:
Apparatus and method of providing an easily aligned directly pivotable grating suitable for high power agile wavelength laser tuners. A mounting and adjustment plane ( 26 ) is located on a rotational axis ( 28 ) and on a grating ( 27 ). An external adjustment mechanism and thermal bonding ( 40 ) reduce rotational moment and enable high power applications. The implementation is capable of utilizing grating blank materials that resist tapping or drilling. Low cost is achieved by construction in a matrix of ruling blanks ( 52 ) thereby complimenting the characteristics of ruling engines.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of Invention  
           [0002]    This invention relates to tunable laser systems and in particular to high power and highly agile directly driven grating tuned devices.  
           [0003]    2. Prior Art  
           [0004]    Laser radar (LIDAR) systems, utilizing tunable lasers, can be used to transmit different wavelengths of light into airborne suspensions (such as smog or poison gasses) which have differing reflectance&#39;s or absorption to different wavelengths. The reflected light intensity is then measured for remote spectrographic analysis of suspension samples. It is advantageous to maximize the stability and repeatability of the output at each different wavelength. It is also advantageous to minimize intervals between transmitting wavelengths in order to reduce measurement interference by relative motion between the LIDAR unit, the intervening atmosphere and the suspension sample. Maximum accuracy is achieved by successively transmitting different wavelengths with constant power at the laser&#39;s maximum cyclic rate.  
           [0005]    Tunable lasers typically include an intra-cavity diffraction grating. The wavelength of such lasers is tuned by adjusting the angle of incidence of the laser cavity beam against the diffraction grating. Such intra-cavity tuning requires very high accuracy and stability. Tuned CO 2  lasers, for instance, require a grating angular range of typically 0.2 radians and an accuracy of 10 or 20 μradians. Output laser power is a sensitive function of the tuning angle near a particular wavelength.  
           [0006]    Laser tuners utilizing gratings typically have a tuning axis and a non-tuning axis. Tuning is accomplished either by rotating a grating about the tuning axis directly in the path of the beam or by rotating a mirror against a fixed grating. The subject of this invention and disclosure is the case where the tuning element is a grating directly mounted on a rotational shaft. For proper tuning, the beam or cavity being tuned must strike the grating with the same accuracy in both the tuning and non-tuning directions or axes. The stability and accuracy of the tuning axis positioning is determined by the corresponding accuracy of the device or system rotating the tuning element about the tuning axis and is not a subject of this disclosure.  
           [0007]    As the tuning element is rotated about the tuning axis, the beam must maintain perpendicularity with the grating lines. Any departure from said perpendicularity represents motion about the non-tuning axis and therefore an error in tuning. A grating behaves about the non-tuning axis as a simple mirror. No errors will occur if the grating rotational axis, including bearing or flexure translations, run outs and tilts, is parallel to the grating lines. With practical and economical machining, significant and typically excessive errors in the non-tuning axis will occur.  
           [0008]    In the prior art, a number of approaches to correct this error have been used with varying degrees of success. At very low speeds, straightforward dual axis adjustment schemes are effective. These methods use fine threads, balls and grooves to iteratively adjust the rotational axis to the grating line parallel. For these techniques, alignment is a time consuming process and attempts to increase tuning rate are hampered by excessive moment of inertia, difficult balance, drive complexities, vibration injection, backlash and general lack of robustness.  
           [0009]    McNeil et al in U.S. Pat. No. 4,815,820 proposes a one-axis adjustment scheme. While only one adjustment on the rotating mechanism is required, the approach as disclosed is still mechanically large and complex. The alignment process is improved but is still an iterative one. While the scheme is more agile, its response time is measured in seconds rather than milliseconds and it suffers the same difficulties noted above when increasing tuning rate.  
           [0010]    Other, more efficient dual axis approaches, use simpler and therefore usually lighter configurations with bending types of adjustment. Tuning rate, balance and robustness are enhanced significantly. Contrary to the assertion in the referenced patent that dual adjustment schemes are iterative and require knowledge of the value of the two components of error, non-iterative dual axis adjustment schemes can be configured. Agreeably, the hardware for two adjustments on the rotating axis is undesirable.  
           [0011]    The above schemes must deal with the tradeoff between stiffness for stability and the loss of adjustment control due to static friction. All perform poorly when trying to adjust with robustness on a rotational axis down to angles of a few μ radians. Further, material incompatibilities, thermal conductivities, thermal expansion coefficients and other issues related to the tuning of high speed and high power lasers place difficult requirements on grating blank material. Grating blanks of ceramic like or crystalline structure may in some cases be preferable or required. These materials do not always machine well and, in some cases, may not be tapped or drilled. This often prevents the integration of the mechanical adjustment with the grating, thereby increasing complexity.  
           [0012]    There is therefore a critical need, heretofore unsatisfied, for a structure and alignment methodology for producing a simple, low moment, balanced and easily aligned rotary grating with both mechanical and thermal robustness.  
         SUMMARY OF INVENTION  
         [0013]    Preferred embodiments of a directly pivotable grating for agile laser tuners according to the present invention preferably include a rotary shaft supported by bearings or flexures with a machined adjustment flat or plane, a mounting means for receiving a removable grating adjuster, a grating blank with ruled grating lines on a front face and an adjustment plane on a back face for receiving the shaft at its adjustment plane and a bonding means for attaching the grating to the shaft. The grating is preferably fabricated as one in a matrix of gratings and maintains a constant optical and mounting cross-section for efficient machining and ruling. Virtually any grating blank material can be used that is compatible with the lasers optical, thermal, mechanical and dissimilar materials requirements. A one-axis adjustment mechanism preloads the grating onto the shaft&#39;s adjustment plane and performs the alignment under the most favorable condition of low friction before bonding and removal. Filled bonding materials with high thermal conductivities and high shear modulus are available with suitable pre-cure times for alignment. The resulting assembly has high stiffness and, depending on the blank material itself, low thermal gradients even in very high power applications. Because of the inherent simple nature of the components, from a machining point of view, very little movement during adjustment is required meaning that in most cases balance-by-design is adequate.  
         OBJECT AND ADVANTAGES  
         [0014]    It is a primary objective of the present invention to provide a simple low moment rotary grating suitable for wavelength tuning of high power agile lasers.  
           [0015]    It is another objective to enable the use of lightweight grating blank materials, which cannot be drilled or tapped.  
           [0016]    It is another objective to simplify grating blank machining and ruling operations.  
           [0017]    It is another objective to improve thermal conductivity, reduce thermal drops and increase laser power capability.  
           [0018]    It is another objective to eliminate the grating adjustment mechanism from the rotary assembly.  
           [0019]    It is another objective to enable simplified grating alignment under low friction conditions while providing a robust final grating assembly.  
           [0020]    It is another objective to enable the use of a wider range of materials for optical elements in a wavelength tuner.  
           [0021]    The foregoing and other object features and advantages will become more apparent from a reading of the following description of the preferred embodiments as shown in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]    In the drawing:  
         [0023]    [0023]FIG. 1 is a diagram of a preferred embodiment of the present invention showing separated rotor and grating with adjustment planes at a zero degree angle;  
         [0024]    [0024]FIG. 2 a  is a diagram of a cross-section of the rotor and grating of the preferred embodiment with the components in contact;  
         [0025]    [0025]FIG. 2 b  is a diagram of a cross-section of the rotor and grating of a modified embodiment with the components in contact and having an adjustment plane at substantially the mid tuning band incidence angle;  
         [0026]    [0026]FIG. 3 a  is a diagram showing a rotational misalignment between the tuning axis and the ruled lines as viewed along the beam axis;  
         [0027]    [0027]FIG. 3 b  is a diagram showing a tilt misalignment between the tuning axis and the ruled lines as viewed perpendicular to the beam and tuning axes;  
         [0028]    [0028]FIG. 4 is a diagram of an apparatus suitable for the alignment of the preferred embodiment;  
         [0029]    [0029]FIG. 5 is an optical layout of an alignment scheme suitable for the alignment of the preferred embodiment;  
         [0030]    [0030]FIG. 6 is a diagram of the preferred embodiment of the present invention showing a separated rotor and a grating as a component of a matrix of gratings. 
     
    
     DETAILED DESCRIPTION  
       [0031]    DESCRIPTION OF A PREFERRED EMBODIMENT  
         [0032]    Referring to FIG. 1 and FIG. 2 a , a directly pivotable grating according to the present invention in a preferred embodiment includes a rotor means  20 , a grating means  30  and a bonding means  40 .  
         [0033]    Rotor means  20  is preferably the rotor of a galvanometric actuator including a shaft  22 , an actuator rotor  21   a , one or a plurality of pivots  24  defining an equivalent rotational or tuning axis  28 , a rotor adjustment plane  26   a  substantially parallel to the rotational axis  28  and preferably one or a plurality of bonding planes  26 . Pivots  24  are preferably rotational flexures but could be bearings, bushings or other types of rotation controlling members. Said pivots are preferably located outboard of the adjustment plane and the actuator rotor as shown in FIG. 1, but may lie in any functional combination, outboard or between said plane and actuator rotor. Actuator stator  21   b  and actuator rotor  21   a  are magnetically coupled and comprise an actuator means to control and rotate said rotor means to stop at positions significant to the tuning function. Said actuator means, within the scope of this invention, could also be a separate and external actuator mechanically coupled to said rotor means.  
         [0034]    Grating means  30  comprises an optical surface  32 , an oppositely positioned grating adjustment plane  26   b  at a predetermined angle relative to the surface  32  and preferably one or a plurality of thermal ridges, said optical surface being a ruled grating. Optical surface  32  and adjustment plane  26   b  comprise grating  27 . Grating adjustment plane  26   b  is configured to contact the rotor adjustment plane  26   a  of rotor means  20  for rotational adjustment of the ruled lines of grating means  30  relative to tuning axis  28 . Grating means  30  preferably also includes one or a plurality of thermal ridges  34 . Said thermal ridges  34  being arranged a predetermined spacing from said bonding planes  26  to allow for the rotational alignment of said ruled lines of grating means  30  about a perpendicular to the adjustment planes  26   a  and  26   b.    
         [0035]    Bonding means  40  is shown in FIG. 2 a , a cross-section through grating means  30  and rotor means  20 . Bonding means  40  comprises a thermal and mechanical connection between rotor adjustment plane  26   a  and grating adjustment plane  26   b  and between bonding planes  26  and thermal ridges  34 .  
         [0036]    In operation, pivots  24  are attached to a housing and constrain the movement of rotor means  20  and grating means  30  to rotation about the tuning axis  28 . Rotation angle θ  25  of rotor  20  is responsive to magnetic coupling from actuator stator  21   b  in turn controlled by a positioning system not a part of this invention or disclosure. Arbitrarily, for this disclosure, the angle θ will be regarded as having the same zero reference as the incidence angle of an input beam  36  to the plane of the grating. Grating means  30 , after adjustment, is bonded to adjusting plane  26   a  and bonding planes  26 , said planes being integral parts of rotor means  20 . The grating optical surface thus rotates in unison with rotor tuning angle θ  25 . The input beam  36 , typically a laser or the cavity beam of a tuned laser, impinges the grating optical surface  32  as shown in FIG. 2 a , a cross-section of the rotor and grating. A resulting plurality of output beams, in the tuning direction about the tuning axis  28 , is typically represented by beams  38   a  to  38   d . Responsive to the angle θ  25  of said input beam  36 , the wavelength λ of the beam and a grating ruled line spacing d, the angles, α, of said plurality of output beams is:  
         α=sin −1 ( n λ/d −sin θ)  
         [0037]    where n is the grating reflection order and is a positive number or zero. This equation is typically applicable for alignment of the grating and rotor or other functions where the wavelength of the beam is predetermined.  
         [0038]    In the typical laser tuning application, the input θ and output α beam angles are equal to each other and to the angle of the cavity. This equation then reduces to the Littrow reflection equation:  
         θ=sin −1 ( n λ/ 2 d )  
         [0039]    where θ is the input and output beam angle.  
         [0040]    In a non-tuning direction about axis  29  in FIG. 2 a , the grating has no periodic nature and operates as a simple mirror. Therefore, for the tuning of lasers where the accuracy in the non-tuning direction is as important as that in the tuning direction, the perpendicularity of the laser cavity with the grating lines must be maintained as the rotor and grating are rotated through the tuning range.  
       Alignment  
       [0041]    If the tuning axis  28  is parallel to the ruled lines of the grating, the non-tuning axis angle will be invariant with tuning angle  25  and no rotation about the non-tuning axis will occur. Typical manufacturing tolerances and economics make this unlikely without specific and sometimes complicated alignment. Proper alignment of the non-tuning axis can be achieved with a simple non-iterative single adjustment by considering the following line of reasoning and FIGS. 3 a  and  3   b . The angle between tuning axis  28  and the grating ruled lines  70  could be broken down into two orthogonal angles φ R  and φ T .  
         [0042]    [0042]FIG. 3 a  is a diagram of a grating rotationally misaligned from the direction of the input beam. The angle φ R    74  is defined as the rotational error between axis  28  and ruled lines  70  as viewed along said input beam  50 . The input beam  50  is perpendicular to grating lines  70  at a value of θ defined as θ C , preferably at the tuning range center. As the grating rotates about the tuning angle θ, the non-tuning axis perpendicularity error angle ε follows the sinusoid:  
         ε=φ R  sin(θ−θ C )  
         [0043]    This function, for an angular misalignment error φ R  of 4000 μradians, has a value of +350 μradians for +5 degrees rotation from θ C  and −350 μradians for −5 degrees rotation from θ C . Peak to peak error is 700 μradians over a typically useful angular tuning range of 10 degrees. Viewed differently, adjustment to reduce φ R  to 230 μradians is necessary for an acceptable non-tuning axis peak error of 20 μradians.  
         [0044]    [0044]FIG. 3 b  is a diagram of a grating misaligned in tilt from the direction of the input beam. The angle φ T    76  is the tilt error between axis  28  and ruled lines  70  as viewed perpendicular to the input beam axis  50  and the tuning axis  28 . The angles φ R  and φ T  are orthogonal to each other and therefore constitute two independent components making up the totality of the angular error between the tuning axis and the grating lines. In FIG. 3 b , the input beam  50  is perpendicular to ruled lines  70  at the tuning angle θ C . As the tuning angle θ rotates, the non-tuning axis perpendicularity error angle ε follows the sinusoid:  
         ε=φ T (1−cos(θ−θ C )  
         [0045]    This function, for an angular misalignment error φ T  of 4000 μradians, has a value of +15 μradians for +5 degrees rotation from θ C  and also for −5 degrees rotation from θ C . Peak to peak error is 15 μradians over the same angular tuning range of 10 degrees. Viewed differently, a value for the tilt angle φ T  of 10500 μradians is acceptable for a non-tuning axis peak error of 20 μradians. No adjustment for the tilt error would be necessary for the same φ T  of 4000 μradians. It is obvious that although the sine function for the non-tuning axis perpendicularity error ε caused by angles φ R  and φ T  have the same nominal amplitude and shape, the tuning range is located at the peak with small first derivative for φ T  while at the maximum first derivative for φ R .  
         [0046]    The foregoing example shows that satisfactory alignment can be achieved by adjusting for φ R  alone and accepting reasonable manufacturing errors on tilt φ T . For the configuration shown in FIG. 2 a  where the grating adjustment axis  31  is not parallel to the input beam  36 , some cross coupling between φ R  and φ T  will occur as the necessary adjustment of φ R  is performed. In the example above, even doubling the value of the tilt while adjusting the rotational misalignment is still acceptable because of the greatly reduced sensitivity to the tilt misalignment.  
         [0047]    [0047]FIG. 2 b  is a diagram of a modification of the first embodiment whereby the perpendicular of the grating adjustment plane  26   b  is nominally set to the input beam  36  incidence angle for an angle within the tuning range, preferably at its center. In this case the grating adjustment axis  31  is substantially parallel to the input beam axis  36  and the φ R    74  adjustment cross coupling into the tilt angular error φ T    76  is nominally zero.  
       Alignment Mechanism  
       [0048]    The adjustment mechanism is preferably removed after alignment for reduction of inertial moment, simplification of balance, improved robustness and other reasons. Additionally, because it is not a part of the rotating assembly, the design of the alignment mechanism can be satisfied by a variety of designs. One such design satisfying the adjustment requirements is depicted in FIG. 4. Grating  27  is held with light pressure against shaft  22  by preload clamps  100  and screws  102  thereby mating the rotor adjustment plane  26   a  and grating adjustment plane  26   b  of FIG. 1 and allowing restricted rotation of grating  27 . Adjustment base  104  is firmly attached to the opposing side of shaft  22  by screws  106 . Counter opposing screws  108   a  and  108   b  in tapped holes of adjustment base  104  contact grating  27  on opposite sides and at a first end of said grating. Under low friction loading, coordinated tightening and loosening of screws  108   a  and  108   b  accurately and with high resolution, move the first end of the grating relative to shaft  22  thereby rotating grating lines  70  relative to axis  28  as required. Preferably, screws  110   a  and  110   b  perform a complimentary function at a second end of grating  27 . Preload clamps  100  and adjustment base  104  are preferably removed after alignment and bonding.  
         [0049]    [0049]FIG. 5 is a schematic of an alignment setup suitable for grating adjustment. Many other methods are practical. Alignment laser  90  of a convenient wavelength, outputs beam  92  toward partial mirror  94  and is reflected towards grating  27  under test as beam  36 . Reflected order beams typically  38   a ,  38   b ,  38   c  and  38   d,  responsive along the tuning and non-tuning axes as previously described, pass to the plane of dual detector  98 . Non-tuning axis measurement  100  interprets the non-tuning axis position information from upper cell  98   a  and lower cell  98   b  producing a net position value in a conventional way. At particular angles of tuning angle θ  25 , one of the reflective order beams will fall on dual cell detector  98  for non-tuning axis measurement, beam  38   c  in the example of FIG. 5. Alignment laser wavelength λ is preferably selected for producing a pair of reflective orders centered within the tuning range.  
         [0050]    In operation, an alignment procedure could proceed as follows. Two non-tuning position values are collected from said pair of reflective orders. Non-tuning axis measurement value V L  is collected at a lower tuning angle θ L  for a lower reflective order and a value V H  at a higher tuning angle θ H  for a higher reflective order. An extrapolation from these two values to a final value is possible. Note that at a particular tuning angle θ, the grating adjustment axis  31  will become parallel to the input beam axis  36 . At that θ, a reflected beam is invariant with adjustment angle φ R  about axis  31 . Defining said θ as θ INV , an extrapolating calculation for the final value V F  can be made approximately as:  
           V   F   =V   H (θ INV −θ L )+ V   L (θ H −θ INV )/(θ H −θ L )  
         [0051]    Then, using either tuning angle θ H  or θ L , the grating is rotated until the measurement  100  is the value V F  from the above formula. The measurement and grating rotate cycle can be repeated if sufficient accuracy is not achieved.  
       Grating Matrix  
       [0052]    The simple machining cuts required for the disclosed grating blank format result in lower cost, higher inherent accuracies and easier alignment. Additionally the economics of ruling engines involve, among other things, the length of individual lines, the rate that the line is traversed, the engine retrace time and the blank setup time. The grating as disclosed can result in further reduced costs by fabricating and ruling a matrix of blanks as a single assembly. An example of such an assembly is shown in FIG. 6 where a matrix of nine gratings is depicted. Gratings  52   a  through  52   i  are fabricated as a single blank, enabled by the fact that the front surface to be ruled and the mounting plane on the opposing side have a constant and easily machined cross-section.  
         [0053]    To minimize the alignment function, the ruled lines on one side of the blank must match the mechanical structures on the opposing side. Fortunately, machining methods produce good parallelism between edge  66  and the side  68  of thermal ridges  34  used typically as a grating mounting constraint. Aligning the ruling engine on edge  66  from the grating side of the blank completes the parallelism requirement. After ruling, cuts  60  and  62  separate individual gratings.  
       Conclusion, Ramifications and Scope of the Invention  
       [0054]    As has been disclosed, this invention enables the operation of agile laser tuners at higher speeds and at higher power. Lower costs have been achieved with simpler alignment and grating fabrication techniques more compatible with ruling engines. The invention enables the use of a wider range of grating blank materials making tuner designs less dependent on denser, expensive and incompatible ones.  
         [0055]    It is understood that the invention is not confined to the particular embodiments set forth herein as illustrated, but embraces such modified forms thereof as come within the scope of the following claims as would be obvious to those skilled in the art to which the present invention pertains. Mechanical and optical configurations have been shown in simplified form to present ideas; curved gratings and other mechanical alternatives would similarly fall within the scope of this invention. Simplified equations have been disclosed, but improved equations or finite element analysis techniques would also fall within the scope of this invention.  
         [0056]    Although variations have been described, other and in some cases less desirable variations, would fall within the spirit and intent of this disclosure. For example:  
         [0057]    (1) The rotary grating could be used in a non-Littrow laser configuration or other optical equipment.  
         [0058]    (2) Various angles could be used between the grating adjust plane  26   b  and the optical surface  32  for mechanical or optical alignment reasons.  
         [0059]    (3) While a removable grating adjustor is preferable, a non-removable one would fall within the scope of the invention.  
         [0060]    (4) Gratings could be fabricated individually and could have holes or tapped holes.  
         [0061]    (5) Other alignment methods and setups could be used.  
         [0062]    (6) Thermal ridges reduce temperature gradients and improve alignment by restricting grating movement but could be eliminated.  
         [0063]    (7) Other shapes around the adjustment interface could be used to enhance balance or minimize moment of inertia.  
         [0064]    (8) Bearings, flexures, actuators and grating mounting areas can lie along the shaft in any order.  
         [0065]    Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.