Abstract:
A consolidated laser alignment and test station. In exemplary embodiments, equipment sufficient to perform complete dynamic testing and alignment of a laser transceiver unit is provided in one compact arrangement. As a result, cavity-box efficiency testing, dynamic open-interferometer alignment, dynamic open-case alignment, closed-case laser boresighting, and complete laser functionality and diagnostic testing can be carried out efficiently at a single location. Real-time diagnostic feedback relating to beam quality, radiometry, and temporal behavior is provided so that high-precision laser alignments and repairs can be made quickly and cost effectively. Customized test fixtures provide easy access to every level of the transceiver unit under test, and two cameras provide far-field, near-field, wide-field and receiver-field beam viewing. One camera is combined with a pin-hole lens and a quad step-filter optic attenuator to provide a wide-field beamfinder assembly enabling an operator to quickly align the laser under test to the narrower field of the second (diagnostic) camera. The second camera provides near-field and far-field beam viewing, while a radiometer and a pulse detector provide additional diagnostic information. The beamfinder assembly also provides receiver-field laser viewing for receiver-path boresight adjustments. In an exemplary embodiment, the beamfinder assembly includes a quad step-filter constructed from circular wedge filters.

Description:
This application is a divisional, of application Ser. No. 08/931,289, filed Sep. 16 1997, now U.S. Pat. No. 5,872,626. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to laser transceivers, and more particularly to methods and apparatus for testing, aligning, and refurbishing laser transceivers. 
     Today, laser radar (LADAR) and other systems incorporating laser transmit-and-receive devices are in widespread use. For example, laser transceivers are routinely employed in military applications for such purposes as target detection, acquisition, identification and tracking. However, due to the inherent high-precision nature of laser systems, testing and alignment of laser transceiver units can be difficult. Indeed, except for the simple adjustment of transceiver mounting screws or the wholesale replacement of peripheral receiver assemblies, failed laser transceivers are conventionally returned to the transceiver supplier for repair and maintenance. The supplier can provide both the facilities and the skill required to carry out high-precision laser alignment and testing. 
     However, because the delays associated with shipping a laser transceiver unit to and from an appropriate supplier can be quite long, and because laser system suppliers typically have significant repair backlogs, conventional laser transceiver repair and maintenance is extremely costly in terms of both time and money. As a result, laser system users often keep a large number of laser transceiver spares on hand. The cost of doing so, however, can be prohibitive. Thus, there is a need for improved methods and apparatus for testing, aligning and refurbishing laser transceivers. In particular, there is a need for methods and apparatus allowing a relatively unskilled technician, who can be located nearer the system user, to perform advanced laser maintenance and refurbishment. Such methods and apparatus will yield significant improvements in terms of laser repair cost and turnaround time and will greatly enhance user self-sufficiency. 
     SUMMARY OF THE INVENTION 
     The present invention fulfills the above-described and other needs by providing a consolidated laser alignment and test station which enables a technician having relatively little specialized training to quickly perform advanced laser repair and maintenance procedures. The test station utilizes recent developments in electro-optic technologies to put all of the resources necessary for dynamically testing and aligning a laser transceiver unit within arm&#39;s reach of a single operator. Thus, the test station provides a high level of system repair through-put at significantly reduced cost. 
     In exemplary embodiments, equipment sufficient to dynamically test and align NdYAG laser transceiver units is provided on a single compact bench. Thus; according to the present invention, cavity-box efficiency testing, dynamic open-interferometer alignment, dynamic open-case alignment, closed-case laser boresighting, and complete laser functionality and diagnostic testing can be carried out at a single station. Real-time user feedback is provided so that high-precision laser alignments and repairs can be made quickly and cost effectively. 
     According to the present invention, a customized test fixture is used to mount the cavity box and interfrometer of a laser transceiver unit under test to the test station bench. Thus, the cavity box and interferometer are mounted external to the laser transceiver unit case so that full-access laser-optics adjustments can be made. Additionally, a virtual laser bed fixture can be mounted in place of the laser transceiver unit interferometer to permit direct testing of the cavity box assembly itself. Furthermore, the entire closed-case laser transceiver unit can be mounted on the test station bench for final adjustments and boresighting of the laser transmitter and receiver. Generally, laser transceiver unit laser-optics adjustments are made while critical laser functions are simultaneously monitored via discrete test station readouts relating to beam quality, radiometry, and temporal behavior. 
     In exemplary embodiments, two cameras provide far-field, near-field, wide-field and receiver-field beam viewing. The novel two-camera combination allows a test station operator to efficiently view, measure, analyze and adjust a laser transceiver unit at all levels of operation. One of the cameras is combined with a pin-hole lens and a quad step-filter optic attenuator to form a wide-field beamfinder assembly. Using the beamfinder assembly in conjunction with self-contained alignment optics included with the text fixtures described above, a test station operator can quickly align the laser to the second camera. The second camera then provides both near-field and far-field beam viewing, while a radiometer and a pulse detector provide additional diagnostics. In exemplary embodiments, the beamfinder assembly also provides receiver-field laser viewing to allow for rapid boresight adjustments. 
     Advantageously, the modular and highly flexible design of the test station allows it to be rapidly reconfigured to align and test a wide variety of laser transceiver units. For example, with appropriate optic interface fixtures and suitable cameras, the test station can accommodate NdYAG lasers operating at 1.064 μm, 0.532 μm, or 1.54 μm (Raman or wavelength shifted 1.064 μm) wave-lengths. Additionally, dual wavelength lasers (i.e., 1.5/1.064 μm) can be aligned and tested using an appropriate collimator mirror and bolt-in dual-camera modules. Radiometry and image processing components of the test station automatically reconfigure to the laser transceiver unit under test. Because virtually any dynamic adjustment of a laser transceiver unit under test can be performed at a single test station, laser testing and alignment can be conducted in a quick and cost-effective manner. Additionally, because the test station diagnostics provide straightforward real-time feedback, laser repair and maintenance can be achieved by a relatively inexperienced technician. 
     The above described and additional features of the present invention are explained in greater detail hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those skilled in the art will appreciate that the described embodiments are provided for purposes of illustration and understanding and that numerous equivalent embodiments are contemplated herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a high-level block diagram of an exemplary interferometer of the type found within laser transceiver units that can be tested and aligned using the methods and apparatus of the present invention. 
     FIG. 2 is a conceptual diagram of an exemplary laser repair facility, incorporating a consolidated laser alignment and test station, constructed in accordance with one embodiment of the present invention. 
     FIG. 3 is a conceptual diagram of an exemplary test station constructed in accordance with the teachings of the present invention. 
     FIG. 4 is a block diagram of the basic components of an exemplary test station constructed in accordance with the teachings of the present invention. 
     FIG. 5 depicts an exemplary reference plate, constructed in accordance with the teachings of the present invention, which may be used to obtain a boresight reference for laser transceiver alignment. 
     FIG. 6 is a conceptual diagram of a reference laser beam following a far-field folded collimator path within a light-tight box of an exemplary test station constructed in accordance with the teachings of the present invention. 
     FIG. 7 is a conceptual diagram of a test laser beam following a far-field folded collimator path within a light-tight box of an exemplary test station constructed in accordance with the teachings of the present invention. 
     FIGS.  8 (A),  8 (B) and  8 (C) are front, side and bottom views, respectively, of an exemplary test station constructed in accordance with the teachings of the present invention. 
     FIGS.  9 (A),  9 (B) and  9 (C) are top, side and front views, respectively, of an exemplary interferometer test fixture constructed in accordance with the teachings of the present invention. 
     FIGS.  10 (A),  10 (B),  10 (C) and  10 (D) are side cross-section, front, bottom and perspective views, respectively, of an exemplary cavity-box test fixture constructed in accordance with the teachings of the present invention. 
     FIGS.  11 (A),  11 (B) and  11 (C) are front, side and perspective views, respectively, of an exemplary beamfinder assembly constructed in accordance with the teachings of the present invention. 
     FIG. 12 is a diagram of an exemplary method for constructing a step-wise adjustable beam attenuator as taught by the present invention. 
     FIG. 13 is a diagram of a second exemplary method for constructing a step-wise adjustable beam attenuator as taught by the present invention. 
     FIGS.  14 (A),  14 (B),  14 (C) and  14 (D) are left-side, front, right-side and bottom views, respectively, of an exemplary four-step variable beam attenuator constructed in accordance with the teachings of the present invention. 
     FIG. 15 depicts an exemplary beamfinder assembly constructed in accordance with the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A typical laser transceiver unit comprises three major subsystems including a) a laser electronics unit, b) a laser interferometer and c) a cooling system. Generally speaking, the laser electronics unit provides power-supply and timing-control signals to the interferometer which in turn establishes the pulse shape, pulse width, beam spread and spectral content of the beam transmitted by the laser transceiver unit. The cooling system dissipates heat which is generated within the laser transceiver unit, for example by the power supply or by a triggered light source within the interferometer. The basic layout and operating characteristics of a laser interferometer are described below. Laser electronics units and cooling units are well known in the art and a detailed description of such systems is not necessary for an understanding of the present invention. 
     FIG. 1 is a high level block diagram of an interferometer  100  of the type found in laser transceiver units which may be tested and aligned according to the teachings of the present invention. As shown, the exemplary interferometer  100  includes a switch  110  for controlling a Pockels cell  120 , a first terminating mirror  115 , a first wave plate  140 , a cavity box  155  (including a flashlamp  125  and a laser rod  130 ), a beam splitter  145 , a pair of lenses  150 , a second wave plate  135 , a corner reflector  160  and a second terminating mirror  185 . In practice, each component of the interferometer  100  is mounted on a rigid base (not shown) to form a self-contained interferometer sub-assembly which can be mounted within a laser transceiver unit. 
     In FIG. 1, a first interferometer input  165  (labeled Trigger Pulse in the figure) is coupled to a first input of the Pockels cell switch  110  and to an input of the cavity box  155 . Additionally, a second interferometer input  170  (labeled High Voltage Pulse) is coupled to an input of the flashlamp  125 , and a third interferometer input  175  (labeled DC Supply Voltage) is coupled to a second input of the Pockels cell switch  110 . An output of the cavity box  155  serves as an interferometer output  180  (labeled Cavity Box Temperature). As shown, the first terminating mirror  115 , the first wave plate  140 , the laser rod  130 , the beam splitter  145  and the pair of lenses  150  are arranged to form a first optical path  190 . At the same time, the first terminating mirror  115 , the first wave plate  140 , the laser rod  130 , the beam splitter  145 , the corner reflector  160 , the Pockels cell  120 , the second wave plate  135  and the second terminating mirror  185  are arranged to form a second optical path  195 . 
     In operation, a pulse forming network (not shown) is used to convert a relatively low-current, high-voltage (on the order of 1 kV) supply to a much higher current pulse which is used to supply the flashlamp  125  via the second interferometer input  170 . At the same time, a trigger generator (not shown) is used to control the flashlamp  125  via the first interferometer input  165  so that the flashlamp  125  emits triggered pulses of non-coherent radiation which in turn stimulate pulsed laser emissions within the laser rod  130 . A portion of the emitted laser beam follows the first optical path  190  and serves as the laser output for the laser transceiver unit in which the interferometer  100  is included. Another portion of the resulting laser beam follows the second optical path  195  and causes the laser to resonate (as the beam is reflected back and forth between the terminating mirrors  115 ,  185 ), and to thereby maximize lasing efficiency, as is well known in the art. The interferometer output  180  provides a measure of cavity box temperature, for example to control a cooling unit. 
     Generally, the wave plates  140 , 135  are used to control polarization of the internal beam, and the pair of lenses  150  is used to collimate and focus the output beam. Additionally, a pair of wedged lenses or Risleys (not shown), which are typically mounted inside the laser transceiver unit casing and external to the interferometer itself, are used to sub-tune the direction of the laser output. The terminating mirrors  115 ,  185  allow the second optical path  195  to resonate, as noted above, and thereby intensify the resulting laser output. The optional Pockels cell  120  provides Q-switching for the resonator if it is desired. In other words, the controllable birefringence of the Pockels cell  120  is set to block the second optical path  195  and to thereby prevent the laser from resonating so that maximum energy is stored in the laser rod  130  prior to excitation of a laser pulse. Switching of the Pockels cell  120  is coordinated with the triggering of the flashlamp  125  via the Pockels cell switch  110 . As shown, the Pockels cell switch  110  receives the flashlamp trigger signal via the first interferometer input  165  as well as a DC supply voltage via the third interferometer input  175 . 
     According to the present invention, a consolidated laser alignment and test station is used to assess and adjust the optics components of laser transceiver units containing interferometers such as that depicted in FIG.  1 . Those skilled in the art will appreciate, however, that the exemplary interferometer  100  of FIG. 1 is provided merely to aid explanation of the present invention and that the test station of the present invention can be used to test and align a wide range of laser transceiver units containing various interferometer configurations. Additionally, though the detailed description below makes reference to particular physical embodiments of components of the present invention which are tailored to accommodate a particular type of laser transceiver unit containing a particular type of interferometer, those skilled in the art will recognize that equivalent embodiments can be constructed to accommodate any laser transceiver/interferometer combination of interest. 
     FIG. 2 provides an overhead view of an exemplary laser repair facility  200  incorporating a test station  210  such as that taught by the present invention. In the exemplary laser repair facility  200 , the test station  210  is situated proximate other laser transceiver unit repair equipment so that a laser transceiver unit (including the interferometer, laser electronics unit, cooling unit, outer chassis, etc.) can be fully tested and refurbished as necessary. As noted above, it may be advantageous to locate a laser repair facility close to the ultimate laser transceiver unit user, for example near troops in the field for military applications. Thus, the laser repair facility  200  of FIG. 2 may be constructed to fit within, for example, a mobile trailer. 
     As shown, the exemplary laser repair facility  200  comprises three rooms or work areas including first and second clean rooms  205 ,  215  and a cooling unit refurbishment area  225 . The first clean room  205  includes an exemplary test station  210 , and the second clean room  215  includes a tear-down bench  220  and a static alignment bench  230 . Generally, the first clean-room  205  is used to dynamically test and align the optics portions of the laser transceiver unit, and the second clean room  215  is used for laser transceiver unit tear-down, optics inspection, optics cleaning and bonding, and static alignment of the interferometer. The cooling unit refurbishment area  225  is used to decontaminate and refurbish the laser transceiver unit cavity box and cooling system sub-assemblies as necessary. 
     In operation, an incoming laser transceiver unit is first tested on the test station  210  while it is operating to obtain data on the laser transceiver unit performance and to perform a preliminary diagnosis of any detected failures. Following confirmation of a laser failure, the laser transceiver unit is moved to the tear-down bench  220  where the cavity box and cooling unit assemblies are removed and taken to the cooling unit refurbishment area  225 . There the cooling unit and cavity box assemblies are tested and refurbished as necessary (e.g., replacement of faulty cavity box reflectors, cleaning and replacement of the laser rod, replacement of liquid coolant, etc.). Preferably at the same time, the interferometer optics are cleaned and any unserviceable optics components are replaced at the tear-down bench  220 . Thereafter, the interferometer is statically realigned at the static alignment bench  230 , and laser electronics unit repairs and any upgrades or retrofit incorporations are performed as necessary. Once cleaning, refurbishment and static alignment are complete, the laser transceiver unit sub-assemblies are re-integrated, dynamically aligned, boresighted, and final-tested using the test station  210 . 
     In exemplary embodiments, a depot repair form is generated (e.g., within a laser repair facility computerized data base) when a laser transceiver unit first enters the laser repair facility  200 . The depot repair form is then continually updated throughout the laser transceiver unit repair process to document the repair and maintenance history of the particular laser transceiver unit. If appropriate, the completed depot repair form is downloaded to a central computerized laser transceiver unit tracking system (not shown). Those skilled in the art will appreciate that the brief description of the exemplary laser repair facility  200  provided above is intended to further understanding of the present invention and that the beneficial aspects of the test station embodiments described in detail below are equally applicable to any form of laser repair facility. 
     FIG. 3 is a conceptual diagram showing the basic components of the exemplary test station  210 . As shown, the exemplary test station  210  comprises a hood  305  and a test bench  370 . The test bench  370  includes a test platform  390  and a light-tight beam housing  380 . In FIG. 3, a laser electronics unit  310 , a laser run box  320 , a waveform display unit  330 , a display monitor  340  and a radiometer display unit  350  are positioned on the bench  370 . Additionally, a laser transceiver unit under test  360  is mounted on the test platform  390 . 
     Generally, the laser electronics unit  310  and the laser run box  320  are used to provide power and control signals to the laser transceiver unit  360  just as they would be provided to the laser transceiver unit  360  during operation in the field. The laser transceiver unit  360 , or a sub-assembly of the laser transceiver unit  360  (e.g., the interferometer), is fired through apertures in the testing platform  390  and the top portion of the test bench  370  and into the beam housing  380 . As described below, laser measurement equipment attached to the beam housing  380  is used to provide a test station operator with real-time diagnostic information relating to the beam emitted by the laser transceiver unit  360 . The diagnostic information is provided to the test station operator via the waveform display unit  330 , the display monitor  340  and the radiometer display unit  350  as described below. Using the real-time diagnostic feedback, the test station operator can quickly test and align the laser transceiver unit  360  (and the laser transceiver unit sub-assemblies) as necessary. 
     FIG. 4 is a high-level schematic diagram showing electric and optic interconnections between components of the exemplary test station  210 . As shown, the test station  210  includes the laser electronics unit  310 , a pulse detector  430 , the light-tight beam housing  380 , an adjustable optical attenuator  450 , a near/far field camera  460 , a beamfinder camera  499 , a beam processor  470 , a laser source  498 , the display monitor  340 , a detector head  475 , an attenuator controller  455 , a radiometer  465 , a data bus  445 , a computer interface  485  and an input keyboard  495 . As shown, the laser electronics unit  310  is bi-directionally coupled to a first input of the computer interface  485  and also bi-directionally coupled to an input of the transceiver unit under test  360 . An optical output of the laser transceiver unit  360  is coupled to a first input of the beam housing  380 . 
     First, second and third outputs of the beam housing  380  are optically coupled to inputs of the pulse detector  430 , the detector head  475 , and the camera  460  respectively. An electric output of the camera  460  is coupled to an input of the beam processor  470 , and the beamfinder camera  499  is selectively positionable between the first camera  460  and the beam processor  470 . An electric output of the attenuator controller  455  is coupled to an input of the adjustable optic attenuator  450 , and an optical output of the laser source  498  is coupled to a second optical input of the beam housing  380  via a fiber optic cable  497 . An electric output of the detector head  475  is coupled to an input of the radiometer  465 , and an electric output of the pulse detector  430  is coupled to a second input of the computer interface  485 . An output of the computer interface is coupled to an input of the display monitor  340 , and an output of the keyboard  495  is coupled to a third input of the computer interface  485 . The beam processor  470 , the attenuator controller  455 , the radiometer  465  and the computer interface  485  are all coupled via the data bus  445  which may be, for example, an IEEE  488  bus. 
     As described above, optical energy from the transceiver unit under test  360  is fired into the light-tight beam housing  380  and real-time diagnostic feedback is provided so that the test station operator can efficiently perform testing and alignment as necessary. The beam housing  380  contains two folded collimator paths which are integrated with (i.e., aligned with) the variable attenuator  450 , the camera  460 , the pulse detector  430 , the detector head  475 , the radiometer  465 , and the laser source  498  (which may provide a boresight reference source and a receiver range testing source as described below). Generally, the camera  460  and the beam processor  470  provide detailed beam viewing, for example via the display monitor  340 . Both near-field and far-field views can be displayed by switching between the two folded collimator paths within the beam housing  380  as described below. 
     The detector head  475  and the radiometer  465  provide beam energy diagnostics (e.g., via the radiometer display unit  350 ), and the pulse detector  430  provides laser pulse waveform diagnostics (e.g., via the waveform display unit  330  which may comprise an oscilloscope). The variable attenuator  450  is used to set saturation levels for the camera  460 . The computer interface  485  provides central control of the test station components, user prompting at appropriate points in the test and alignment process, and detailed data, processing and management. 
     In exemplary embodiments, the digital image processor  470  provides real-time far-field divergence optimization and measurement over a range of 250 μR to 2.0 mR, radiation outside the main beam, near-field beam viewing including profile top-hat and isometric, and boresight alignment and measurement. Additionally, the radiometer  465  output provides energy per pulse with 2% accuracy, average power from 20 mW to 30 W, missing pulses, pulse frequency and time jitter, pulse width, and secondary pulses. 
     As described in detail below, the present invention teaches that customized fixtures can be utilized to allow the test station operator to test the laser transceiver unit  360  at every level of laser operation. In other words, in addition to mounting the entire laser transceiver unit  360  to the test platform  390 , the test station operator can use specialized test fixtures to mount sub-assemblies to the test platform  390  so that they may be tested independently of other laser transceiver unit components. For example, the present invention provides an interferometer test fixture whereby an interferometer which has been removed from its laser transceiver unit housing can be mounted on the test platform  390  for open-case testing and alignment. Additionally, the present invention provides a cavity box test fixture whereby a cavity box which has been removed from its interferometer assembly can be mounted on the test platform  390  and tested directly. 
     Advantageously, in addition to providing for diagnostics with respect to beams emitted from the laser transceiver unit, the test station  210  also provides for boresight referencing and receiver testing as described below. Throughout the testing and alignment process, the test station operator utilizes a novel beamfinder assembly, also described in detail below, to quickly align the various beams to the diagnostic camera  460 . A more detailed explanation of the various aspects of the exemplary test station  210  is next provided. 
     In exemplary embodiments, a boresight reference source (e.g., a laser diode generating a HeNe-type laser output 635 nm) is used to achieve precision laser boresight alignment. Advantageously, the HeNe-type reference beam is integrated with and afocal to the far-field collimator path, thereby providing a collimated HeNe-type output which is autocollimated from a novel reference fixture mounted on the test platform  390 . To boresight a transceiver unit under test, the test station operator mounts the reference fixture on the test platform  390  in place of the unit under test. As described below, the reference fixture includes a partially reflecting reference plate and a reflecting corner cube which provide two reflections of an impinging laser beam. The test station operator then fires the boresight reference source into the beam housing  380  such that a beam emitted by the reference source exits a test beam entry port of the beam housing and reflects back from the reference fixture to provide two reference beam spots on the diagnostic image provided by the diagnostic camera  460 . 
     As described in more detail below, the test station operator can then adjust the position of the boresight reference source so that the two reference beam spots are aligned (e.g., on the display monitor  340 ). Next, the test station operator records the position of the aligned reference beam spots, removes the reference fixture from the testing platform  390 , mounts the laser transceiver unit under test in its place, and fires the laser transceiver unit into the beam housing. By viewing the laser transceiver unit beam spot via the diagnostic camera  460 , the test station operator can precisely boresight the laser transceiver unit by adjusting the direction of the laser transceiver unit beam (e.g., by using a pair of Risley&#39;s included in the laser transceiver unit) until the laser transceiver unit beam is positioned at the recorded reference beam spot position. 
     FIG. 5 depicts an exemplary reference fixture  500  constructed in accordance with the teachings of the present invention. As shown, the exemplary reference assembly  500  comprises a reference plate  530  including a front reflective surface  540 , a reflective corner cube  520 , and a protective housing  510 . The reference fixture  500  provides two reflections  550 ,  560 . The first  560  is from the front surface  540  of the reference plate  530 , which provides the laser mounting plane reference. The second  550  is from the corner cube  520 , which defines the boresight reference axis. As described above, the two reflections  550 ,  560  project back through the collimator within the beam housing  380  to the diagnostic camera  460 . While viewing the camera&#39;s monitor  340  and controlling a motorized gimbal connected to the boresight reference source (as described below), the boresight reference axis is aligned to coincide with the laser mounting plane reflection  560 , and the digital image (beam) processor  470  stores that location in memory for laser alignment referencing as described above. 
     FIG. 6 is a conceptual top view of a reference laser beam following a far-field folded collimator path within a light-tight beam box  625  which is included in the beam housing  380  of the exemplary test station  210  as described below. As shown, an exemplary reference configuration  600  comprises the laser source  498 , the fiber optic cable  497 , a fiber optic attenuator/collimator  645 , the camera  460 , the radiometer  465 , a gimballed mirror  640 , the light-tight beam box  625 , two beam splitters  650 , 680  and five folding mirrors  655 ,  660 ,  665 ,  670 ,  675 . The laser source  498  includes a receiver range testing source  697 , a boresight reference source  698  and an optical combiner  699 . The receiver range testing source  697  can be, for example, a 1.064 μm source, and the boresight reference source can be, for example, a 635 nm wavelength diode laser. 
     In the figure, a reference beam  615  emitted by the laser source  498  passes through the fiber optic cable  497 , the fiber optic collimator  645  and the beam splitter  650 . The reference beam  615  may be a boresight reference generated by the 635 nm source  698  or a receiver range testing reference generated by the 1.064 μm source  698 , depending on which testing function is being performed. The test operator powers up either the 1.064 μm source  697  or the 635 nm source  698  as appropriate, and the combiner  699  couples the resulting reference beam  615  to the fiber optic cable  497 . 
     The reference beam  615  is deflected by the folding mirrors  655 ,  665 ,  675  and beam splitter  680  to the gimballed mirror  640  where it is deflected to the reference fixture  500  mounted on the testing platform  390  (e.g., the gimballed mirror  640  deflects the beam out of the page in FIG. 6 so that it passes through an aperture in the top of the test bench  370  and toward the test platform  390 ). As shown, the beam(s) reflected back from the reference fixture  500  deflect off of the gimballed mirror  640 , the beam splitter  680 , the folding mirrors  675 , 665 , 655  and the beam splitter  650  to the diagnostic camera  460  as desired. The adjustable attenuator  450  is used to control the saturation level of the camera  460 . 
     FIG. 7 is a conceptual top view of a test laser beam  705  following a far-field folded collimator path within the light-tight box  625  included in the beam housing  380  of the exemplary test station  210 . As shown, an exemplary test configuration  700  comprises the camera  460 , the radiometer  465 , the gimballed mirror  640 , the light-tight beam box  625 , the two beam splitters  650 , 680  and the five folding mirrors  655 ,  660 ,  665 ,  670 ,  675 . Such a configuration would be used, for example, during the latter stages of the boresighting procedure or during general diagnostic assessment of a laser transceiver unit and its sub-assemblies. 
     In the figure, a test beam  705  emitted from a laser transceiver unit or laser transceiver unit sub-assembly mounted to the test platform  390  (e.g., passing through an aperture in the top of the test bench  370 ) deflects off of the gimballed mirror  640 , the beam splitter  680 , the folding mirrors  675 , 665 , 655  and the beam splitter  650  to the diagnostic camera  460  as desired. As described above, the camera  460  provides an image to the digital beam processor  470  which in turn provides real-time beam diagnostics to the test station operator via the display monitor  340 . As before, the adjustable attenuator  450  is used to control the saturation level of the camera  460 . 
     Additionally, a portion of the beam  705  passes through the beam splitter  680  and impinges on the detector head (not shown) of the radiometer  465  to provide radiometry information to the test station operator. The additional folding mirrors  660 , 670  are used to selectively provide a near-field collimator path as described below. 
     FIGS.  8 (A),  8 (B) and  8 (C) show front, side and bottom views, respectively, of the exemplary test station  210 . As shown, the exemplary test station  210  comprises the hood  305 , the test bench  370 , a cavity box test fixture  805 , an interferometer test fixture  810 , the test platform  390 . a beamfinder assembly  815  (housing the beam findercamera  499 ) and the gimballed mirror  640 . The test station  210  also comprises a first sub-housing  820 , a second sub-housing  830 , a light tube  835  and the beam box  625 , which collectively form the light-tight beam housing  380 . As shown in FIG.  8 (C), the test station  210  also includes first and second corner reflectors  840 ,  845 , the camera  460 , a camera cable  850 , the variable attenuator  450 , a field selector  860 , the fiber optic attenuator  645 , and the radiometer  465 . 
     During testing and alignment, the test station operator mounts a laser transceiver unit under test, or the reference fixture  500 , directly to the test platform  390 . Alternatively, the test station operator can use the interferometer test fixture  810  to mount an interferometer standing alone. Additionally, the cavity box test fixture  805  can be used in combination with the interferometer test fixture  810  to mount a cavity box standing alone, for example for cavity box efficiency testing. The test laser transceiver unit, the test interferometer, or the test cavity box is then fired through an aperture in the test platform  390 , through an aperture in the bench top and into the beam housing  380 . Within the beam housing  380 , the test beam is deflected from the gimballed mirror  640 , through the light tube  835 , off the mirrors  840 , 845  and into the beam box  625 . Within the beam box  625 , the test beam is directed to the various diagnostic components as described above. In exemplary embodiments, the beam box  625  is constructed as described above with reference to FIG.  6 . Such a beam box is available from Coherent, Inc. of Santa Clara, Calif. 
     As described in more detail below, the beamfinder assembly  815  of FIGS.  8 (A) and  8 (B) is used to align the test beam to the diagnostic camera  460 . In other words, because the field of view of the diagnostic camera  460  is narrow (tailored to provide diagnostics for a pin-point beam), the relatively wide field-of-view beamfinder assembly  815  is used so that the test station operator can quickly position the beam within the viewing range of the diagnostic camera  460 . When a device under test is first fired into the beam housing  380 , the beamfinder assembly  815  is positioned between the device under test and the top of the test bench  370 . The test station operator then views an image of the test beam which is provided by the wide-field beamfinder camera  499  within the beamfinder assembly (e.g., via the display monitor  340 ). Next, the operator makes coarse adjustments (e.g., using the mirrors or the Risleys provided on the device under test) to bring the test beam within the field of view of the diagnostic camera  460  (e.g., by matching the beam spot provided by the beamfinder camera  499  to a cross-hair provided on the display monitor  340 ). Finally, the operator removes the beamfinder assembly  815  and proceeds with laser testing and alignment using the primary diagnostic camera  460 . 
     FIGS.  9 (A),  9 (B) and  9 (C) show top, side and front views, respectively, of the exemplary interferometer test fixture  810 . As shown, the exemplary interferometer test fixture  810  comprises a cavity box mounting plate  920  (including screw holes  995 ), mounting bolts  905 , interferometer mounting points  935 , an electrical connector  930 , an electrical cable  915 , a pair of rotatable wedged lenses or Risleys  910 , and a beam aperture  925 . During testing and alignment, an interferometer under test is mounted to the mounting plate  920  via the screw holes  995 . The test fixture  810  is then mounted to the testing platform  390  via the mounting bolts  905 . The electrical cable  915  and the connector  930  are used to provide power supply and control signals to the interferometer under test. During testing, the interferometer fires through the beam aperture  925  which corresponds to a beam aperture in the mounting platform  390 . The Risleys  910  simulate those found within the laser transceiver unit from which the interferometer was removed and are used to adjust the direction of the beam. 
     FIG.  10 (A) is a cross-sectional side view of the exemplary cavity-box test fixture  805 . Additionally, FIGS.  10 (B),  10 (C) and  10 (D) show front, bottom and perspective views, respectively, of the exemplary cavity-box test fixture  805  shown in FIG.  10 (A). The exemplary cavity box test fixture  805  includes two terminating mirror housings  1030 , four terminating mirror control knobs  1005 , a body  1045 , a vertical mirror housing  1025 , a first handle  1020 , a second handle  1010 , a directional mirror  1035 , and two directional mirror control knobs  1015 . 
     During testing and alignment, a laser cavity box is mounted to the body  1045  of the cavity box test fixture  805 , the cavity box test fixture  805  is mounted to the interferometer test fixture  810 , and the interferometer test fixture  810  is mounted to the test platform  390 . The cavity box is then fired, and a reflecting mirror within the vertical mirror housing  1025  directs the emitted test beam up and back to the directional mirror  1035 . The directional mirror  1035  then directs the beam through the test aperture  925  of the interferometer test fixture  810  and toward the beam housing  380  for diagnostic measurement. Terminating mirrors within the terminating mirror housings  1030  are used to simulate those of the interferometer from which the cavity box under test was removed. The terminating mirror control knobs  1005  and the directional mirror control knobs  1015  are used to adjust the terminating mirrors and the directional mirror  1035 , respectively. 
     FIGS.  11 (A),  11 (B) and  11 (C) show front, side and perspective views, respectively, of the exemplary beamfinder assembly  815 . As shown, the exemplary beamfinder assembly  815  comprises a base  1105 , mounting screws  1110 , a quad-step adjustable attenuator  1115 , a filter housing  1120 , and a camera housing  1125 . During testing and alignment, the beamfinder assembly  815  is positioned beneath the testing platform  390  to provide for quick coarse adjustments as described above. The adjustable attenuator  1115  is used to set saturation levels for the wide-field beamfinder camera  499  which is contained within the camera housing  1125 . In exemplary embodiments, the adjustable attenuator  1115  is a step-filter providing four levels of attenuation. Such a step-filter can be constructed as described in detail below. The beamfinder camera  499  of the beamfinder assembly  815  is coupled to the display monitor  340  in a fashion which is well known in the art. 
     In exemplary embodiments of the test station  210 , a receiver testing source (e.g., the 1.06 μm NdYAG laser source  697 ) generating an output simulating that of a laser transceiver unit under test is used to fire a beam through the test station beam housing and into a receiver assembly associated with the laser transceiver unit under test. The receiver test equipment includes the receiver testing source  697 , the laser attenuator  645 , an EMI screen box (not shown), and a receiver interface adapter (not shown). The laser receiver is mounted in an EMI resistant metal box to reduce the possibility of false alarms due to other electronic equipment. Access holes in the box allow probing of test points and setting of trim pots for sensitivity adjustments. The receiver is mounted on the testing platform  390  via the interface adapter, which provides all electrical connections necessary for power and I/O signals. An optical port on the EMI box allows laser stimulus to enter the receiver&#39;s optic aperture. The optical port is held by machine tolerances to provide a boresighted input. 
     A noted above, the optical signal for receiver (or boresight) sensitivity testing and adjustment can be provided by a fiber optic coupled laser diode source operating at the 1.06 μM wavelength. Such devices are commercially available and are controllable via an IEEE- 488  bus. The precise output of the optical signal is determined by attenuating the pulse generated by the optical source with the fiber optic attenuator  645 . The fiber optic attenuator  645  can control the signal over 50 dB with a resolution of 1%. The output of the fiber optic attenuator  645  is sent via the fiber optic cable  497  to a collimating lens that underfills the receiver&#39;s optical aperture. The underfilling of the aperture insures that all the energy from the fiber optic attenuater  645  is sent through the receiver&#39;s field stop eliminating radiometric errors due to aperture tolerances and speckle effects. In alternative embodiments, a coiled fiber optic bundle can be attached to the beam box in place of the fiber optic attenuator for range testing. 
     As noted above, the beamfinder assembly  815  may use a multiple-step attenuator to set saturation levels for the beamfinder camera  499 . While adjustable filters are available commercially, they tend to be somewhat bulky and relatively expensive. Advantageously, the present invention teaches that a useful step-wise adjustable attenuator can be constructed inexpensively using commonly available circular wedged filters. In exemplary embodiments, the wedged filters are inexpensive 1″ diameter circular filters having an unknown degree of wedge. As is well known in the art, the wedge tends to deflect a beam passing through the filter and is not a desirable filter feature in the context of the present invention. Advantageously, the present invention teaches that a step-filter having substantially no wedge effect can be constructed using the inexpensive and imprecise circular filters. 
     FIG. 12 depicts an exemplary method for constructing a variable beam attenuator according to the present invention. As shown, the exemplary method comprises eight steps S 1 -S 8 . In step S 1 , a first circular wedged filter  1200  is marked with a bisecting line  1202 . In step S 2 , the wedged filter  1200  is cut along the bisecting line  1202  to create a left portion  1205  and a right portion  1210 . In step S 3 , the right portion  1210  is rotated 180° in a plane defined by an upper surface of the right portion  1210  and positioned beneath the left portion  1205 . In step S 4 , the left portion  1205  and the right portion  1210  are bonded to form a first half-filter  1215  providing a first level of optical attenuation. Because the wedge effects caused by the left and right portions will largely cancel each other, the first half-filter  1215  will include substantially no wedge effect. 
     In step S 5 , steps S 1  through S 4  are repeated using a second wedged filter to create a second half-filter  1220  providing a second level of optical attenuation and including substantially no wedge effect. In step S 6 , the first half-filter  1215  and the second half-filter  1220  are bonded edge-wise to form a first two-step optical attenuator  1225 . In step S 7 , steps S 1  through S 6  are repeated using third and fourth wedged filters to form a second two-step filter  1235 . In step S 8 , first two-step filter  1225  and second two-step filter  1235  are bonded to form a quad step-filter  1240 . Thus, the present invention teaches that a four-step variable attenuator can be constructed inexpensively using four circular wedge filters. As described below with reference to FIGS. 13 and 14, the eight pieces (i.e., the two halves of each of the four circular filters) can be configured in an alternative fashion to provide a four-step filter having very low wedge. 
     FIG. 13 depicts an exemplary method for constructing a half-filter such as that described above with reference to FIG.  12 . As shown, in step S 1 A two marks  1310  are scribed on the edge of the circular filter  1200  at opposite ends of an imaginary arbitrary center line  1315  crossing the face of the filter  1200 . At step S 2 A, the circular filter  1200  is cut along a bisecting centerline which is offset from the edge marks by 0.1 inches to form the left piece  1205  and the right piece  1210 . At step S 3 A, the right piece  1210  is rotated 180 degrees in the plane of FIG.  13 . Finally, at step S 4 A, the right piece  1210  is positioned beneath the left piece  1205  so that the scribe marks  1310  are aligned. Because some material is removed during the cutting process, the two pieces will be slightly offset from one another as shown. Rather than bonding the faces of the left and right pieces directly to one another as described above with respect to FIG. 12, the two pieces can be configured with six corresponding pieces cut from three additional circular wedge filters to provide a superior quad step-filter. 
     FIGS.  14 (A),  14 (B),  14 (C) and  14 (D) show left side, front, right side and bottom views, respectively, of such an exemplary four-step filter. As shown, the quad-step filter comprises four pairs  1410 ,  1420 ,  1430 ,  1440  of semicircular filter pieces, each pair processed in accordance with the method of FIG.  13 . In FIG. 14, the eight pieces are matched in corresponding pairs and interleaved to form a four-layer quad filter having four sections each providing a distinct level of optic attenuation with substantially no wedge effect. As shown, the pieces in each layer are alternately offset from a centerpoint of the quad filter, and adhesive is applied at the resulting recesses  1405  so that the quad filter is substantially cylindrical and mechanically sturdy. 
     FIG. 15 depicts the exemplary beamfinder assembly  815  employing a step-filter such as that shown in FIG.  14 . As shown, the beamfinder assembly  815  comprises the quad step-filter  1115 , a pin hole aperture  1520 , the camera housing  1125  and an adjustment knob  1525 . A center axis  1505  of the quad filter  1115  is positioned such that a center portion of one quadrant of the step-filter  1115  is aligned with a line of sight  1510  defined by the pin-hole aperture  1520  and a camera within the camera housing  1125 . During testing and alignment, the test station operator positions the beamfinder assembly underneath the testing platform  390  as described above. The operator then selects a desired level of filter attenuation (i.e., a desired camera saturation level) by rotating the step-filter  1115  until an appropriate section of the filter  1115  is aligned with the camera. The operator then tightens the adjustment knob  1525  to secure the filter  1115  in place. 
     As the foregoing discussion makes clear, the present invention teaches methods and apparatus which significantly improve the art of laser repair and maintenance. By placing extensive dynamic testing and alignment resources within arm&#39;s reach of a single operator, the present invention enables a technician having relatively little specialized training to obtain a high level of system repair through-put at significantly reduced cost. Empirical studies have proven that the synergies created by embodiments of the present invention yield laser repair success rates, in terms of both speed and accuracy, heretofore unheard of in the art of laser repair and maintenance. 
     Those skilled in the art will appreciate that the present invention is not limited to the specific exemplary embodiments which have been described herein for purposes of illustration. The scope of the invention, therefore, is defined by the claims which are appended hereto, rather than the foregoing description, and all equivalents which are consistent with the meaning of the claims are intended to be embraced therein.