Abstract:
An apparatus for testing an optical test piece comprising an interferometer for emitting an incident light beam. The apparatus includes a first reflective optic that receives the incident beam and produces a first reflected beam by focusing and expanding the received incident beam. The apparatus also includes a second reflective optic that receives and collimates the first reflected beam and outputs the collimated beam toward the optical test piece. Both the first and the second reflective optics are fixed to their respective positions relative to a thermally insensitive platform and the optical test piece is docked to the thermally insensitive platform and can be removed.

Description:
BACKGROUND 
     Interferometry techniques are used for testing surfaces of optical elements. In one interferometry test configuration, known as a Fizeau interferometer, a plane parallel wavefront is used to test flat optics. There are several such devices available from manufacturers and most commonly have a 4 inch diameter aperture. When a measurement is required on a larger flat, it is often accomplished by coupling one of these common 4 inch Fizeau interferometers to a beam expander. 
     Commercially available beam expanders are typically made from large refractive elements. One of the purposes of using refractive optics in interferometry techniques is to guide light through the interferometer system. However, refractive optical elements tend to produce beam scattering which degrades the quality of the interferometer output measurements. Moreover, functionalities of refractive optics are dependent on the wavelength of operation. As such, the interferometer system needs to be adjusted for a specific wavelength every time the operational wavelength is changed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawings, with like elements having the same reference numerals. When a plurality of similar elements are present, a single reference numeral may be assigned to the plurality of similar elements with a small letter designation referring to specific elements. When referring to the elements collectively or to a non-specific one or more of the elements, the small letter designation may be dropped. The letter “n” may represent a non-specific number of elements. Also, lines without arrows connecting components may represent a bi-directional exchange between these components. According to common practice, the various features of the drawings are not drawn to the scale. Also, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures: 
         FIG. 1  is a perspective diagram illustrating an embodiment of the present invention showing a test setup for wavefront measurements. 
         FIG. 2  is a top-plan view showing a beam expander in accordance with an embodiment of the present invention. 
         FIG. 3  is a perspective illustrating a beam expander mount platform sitting on a test bed in accordance with an embodiment of the present invention. 
         FIG. 4 . is a perspective diagram showing a non-adjustable mount for mounting a primary mirror according to an embodiment of the present invention. 
         FIG. 5  is a perspective diagram depicting a setup of a secondary mirror with an auxiliary alignment device in accordance with an embodiment of the present invention. 
         FIG. 6  is a perspective diagram of a transmission flat mounted on a tip/tilt stage according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In recent times interferometer systems have been employed for testing optical surfaces. However, testing of optical surfaces with large dimensions has been a challenge. One way to overcome such a challenge is to include a device, such as a beam expander, in the interferometer system which can expand a narrow incident beam emanating from a light source to a wide beam in order to match the large dimensions of the optics under test. Typically these devices use large refractive optics which facilitate enlarging the beam. For example, in a Fizeau Interferometer including a beam expander with refractive optics may be used to expand a beam from 4″ to 12″-32″. Moreover, the refractive optic components in the beam expander typically have mechanically adjustable features to enable alignment of the system. 
     However, refractive optics are inherently dispersive. That is, their optical responses change with respect to wavelengths. As such, the operation of these large beam expanders is limited to a specific wavelength for a given alignment resulting in additional and frequent alignments for different wavelengths of operation. Large refractive optics also often have issues with polarization-dependence and birefringence, which can cause problems in the interferometric measurements. Additional problems may arise with drift in alignments, for example, when large test optics with different weights are introduced on a test bed in the interferometer system. Drifts in alignments also occur when the adjustable mounts for the refractive optics, required for adjusting the system alignment for different wavelengths, are thermally sensitive and tend to drift in position over time due to temperature variations. Moreover, typically each of the refractive optical components has its own adjustable mount leading to an increase of the overall cost of the system. 
     The present invention overcomes the issue of wavelength sensitivity and polarization sensitivity by replacing refractive optics with reflective optics in the beam expander of the interferometer system. The present technology also addresses the alignment issues described above by using a beam expander on a thermally insensitive platform. The platform interfaces kinematically with the test bed on which the interferometer system resides mitigating issues with weight variations of the optical test piece. Lastly, concerns related to cost and drifts with adjustment stages are addressed by eliminating adjustments in the final interferometer setup. 
     Thus, the present invention lends itself to instantaneous and highly precise interferometry by including reflective optics and optical mounts with novel features. 
       FIG. 1  provides a view of an embodiment of the invention illustrating an interferometer test setup  100 . The setup  100  includes an interferometer  102 , a secondary mirror  104 , a primary mirror  106 ; and a reference flat  109  and a transmission flat  108  mounted on tip/tilt stages  114 . The secondary mirror  104  and the primary mirror are inside an enclosure  112 , whereas, the interferometer  102 , and the reference and transmission flats and the tip/tilt stages  114  are outside the enclosure  112 . The interferometer  102 , the secondary mirror  104 , the primary mirror  106  and the enclosure  112  sit on a beam expander mount platform  116 . The secondary mirror  104  and the primary mirror  106  constitute a beam expander  200 . All the optical components, stages, platforms both inside and outside the enclosure  112  reside on a test bed  110 . 
     The interferometer  102 , at the input of the beam expander may be a Fizeau interferometer, for example. The interferometer  102  may be other interferometers (but not limited to), such as Twyman-Green interferometer, or Shack-Hartman wavefront sensor, for example. In the discussion that follows, implementation of a Fizeau interferometer will be considered but the same principles apply to the implementations of the other interferometers mentioned above. During a test operation, the interferometer  102  in conjunction with the beam expander  200  using the test setup  100  collect results of large aperture optics. The beam expander  200  receives a narrow incident beam from the interferometer  102  through a cut-out window  118  in the enclosure  112 . The incident beam interacts with the reflective optics (secondary mirror  104  and the primary mirror  106 ) of the beam expander  200  and expands before it exits through another cut-out window  120  to reach the reflecting reference and test pieces such as the reference/transmission flats  108  along a path. The enclosure  112  may provide thermal isolation for the beam expander  200  from a variation in the atmospheric temperature. This enclosure also limits air turbulence within the path of the beam expander. Reflected beams coming back from the reference/transmission flats  108  along the same path are then processed by the interferometer  102 . 
     Briefly, in a Fizeau interferometry technique, light reflected from two reflecting surfaces combines constructively and destructively to form interference fringes. One of the reflecting surfaces is a reference surface whereas the other one is a test surface (e.g., see reference and transmission flats  109  and  108  in  FIG. 1 ). The fringes produced from the reflected light may be used to measure the surface profile of the optical test piece. An incident beam directed towards the reflecting surfaces may be expanded through a beam expander in order to match the dimensions of the optical test surface (e.g. see interferometer  102  and beam expander  200  in  FIG. 1 ). Details of an exemplary beam expander  200  are given below. 
     As detailed above, one of the major concerns of a beam expander using refractive optics is that it tends to have inherent problems such as birefringence. To overcome such issues, the present invention implements a beam expander comprised of reflective optics (mirrors).  FIG. 2  is a top-plan view of the beam expander  200 , interferometer  102  and the transmission flat  109 . For clarity, enclosure  112  is not shown. The interferometer  102  is configured to direct incident beam or rays  202  toward the secondary mirror  104  included in the beam expander  200 . The incident beam  202  may be a coherent and collimated beam for example. Alternatively, the incident beam may be a diverging beam, for example. Additional adjustments may be required in the positioning of the reflective optics to account for a diverged incident beam. The incident beam may also have different wavelengths at different times, for example. The present setup is configured to operate in a narrow band of wavelengths centered at 632 nm. For example, a Helium-Neon (HeNe) laser may be used to emit an incident beam. The setup is configured to accommodate wavelengths from 400 nm-1064 nm, for example, without any additional changes in the configuration. The secondary mirror  104  receives the incident beam  202  and is configured to bring the beam to a focus as it reflects it towards the primary mirror. Prior to impinging on the primary mirror  106  the beam is focused and expanded to a size larger than it was at the secondary mirror  104 . In this configuration the secondary mirror may be a concave mirror which may have an off-axis paraboloid shape. Alternatively, the secondary mirror  104  may be a convex off-axis paraboloidal mirror, for example. As such, the reflected beam  204  simply diverges or expands when it is reflected to the primary mirror  106 . The primary mirror  106 , also included in the beam expander  200 , is configured to receive the diverging beam  204  from the secondary mirror  104 . The primary mirror  106  may be a concave off-axis paraboloidal mirror that collimates the diverging beam into a collimated beam  206 , for example. The collimated beam  206  is reflected toward the transmission flat  108 . The beam width of the collimated beam  206  exiting the beam expander is substantially larger than the incident beam  202 . Thus, the beam expander  200  through the reflecting mirrors expands an incident beam. The collimated beam  206  may exit the beam expander  200  along an axis (not shown) which is parallel to another axis (not shown) of the incident beam  202 , for example. The primary and the secondary mirrors are aligned in a way such that the beam  206  impinging on the transmission flat  108  is collimated and the wavefront error of the beam  206  is minimized. 
     The test setup  100  further includes beam expander mount platform  116  on which the beam expander  200  resides. Details of an example platform  116  are provided below. 
     As discussed above, an interferometer setup may suffer from alignment drift primarily due to adjustable mounts that are thermally sensitive and tend to drift over time. To address such problems, the example interferometer setup  100  includes platform  116  which is thermally insensitive. As such, the beam expander  200  sitting on the platform  116  is not subject to misalignments with the interferometer  102  or with the transmission/reference flats  108 / 109  due to a temperature variation. The temperature variation may also arise from an atmospheric change in the test environment. For example, the room temperature where the experiment is carried out may vary throughout the day. To compensate for these variations, the materials used in the example platform  116  have a low coefficient of thermal expansion (CTE) enabling the test setup  100  to be completely functional in an environment with temperature variation. In one example, the material used for platform  116  may be Invar™/Carbon Fiber. 
     Moreover, the platform  116  may be further configured to interface with the test bed  110  kinematically via interface plate  302 . This kinematic interface allows the two surfaces ( 110  and  116 ) to have different rates of expansion without inducing any strain into the beam expander mount platform. Strain may cause bending of the platform and therefore induce a misalignment within the beam expander. Thus this feature of the platform allows the beam expander to maintain its internal alignment as well as its alignment with the interferometer  102  and the transmission/reference flats  108 . As shown in  FIG. 3 , the platform is mounted on the test bed  110  by three sets of large hardened Vee locators and receivers  304  (only two are shown). These are doweled to face a center point of the platform  116  In other words, it is contemplated that the virtual long axis of each vee connects to the center point of platform  116 . Thus, due to this configuration any stress/strain that may be imparted on the optics due to material growth (e.g. thermal expansion) can be eliminated. Hence, the operation of the test setup  100  tends to be repeatable without any additional adjustments. 
       FIG. 4  is a schematic of the primary mirror  106  mounted in a non-adjustable mount  402 . The non-adjustable mount  402  is a rigid fixture that keeps the primary mirror  106  in a fixed position and is configured to sit on the platform  116 . The fixed positioning of the non-adjustable mount  402  reduces a potential drift in position of the primary mirror  106 , thus minimizing the possibility of a misalignment. This may be advantageous compared to a primary mirror  106  being mounted on a flexible frame that may experience relatively larger drift in position. For example, vibrations of the test bed  110  may cause a shift in position of the primary mirror  106 . However, the non-adjustable mount  402  ensures that the positioning of the primary mirror is minimally affected by the vibration. Thus, in the test setup  100 , the primary mirror  106  mounted on the non-adjustable mount  402  may be used as a reference point for the alignment process. 
     Furthermore, the non-adjustable mount  402  may be used to mount the primary mirror  106  at the time of testing the mirror during the manufacturing process. The same non-adjustable mount  402  may be used in the setup of the beam expander  200 . By doing so, for example, any effects of strain created in the mirror by the mount during the manufacturing process are the same as during the test setup  100 . Thus, all the effects of the strain can be compensated during the final configuration of the mirror. Whereas, if different mounts were to be used for the same primary mirror, additional strain may be introduced to the mirror resulting in uncompensated strain. 
     A fixed mount, such as the non-adjustable mount  402 , may also cut down the cost of the overall setup by eliminating expensive adjustable mounts and stages for the primary mirror  106 . 
     Another feature of the primary mirror  106 , as shown in  FIG. 4 , may be that it is bonded to mount pads  404   a - c . Although three mount pads are shown, it is contemplated that fewer or more mount pads can be bonded to the primary mirror  106 . The mount pads  404   a - c  interface between the primary mirror  106  and the mount  402  allowing the primary mirror  106  to be mounted in the non-adjustable mount  402 , and also dismounted from mount  402 , with ease in a repeatable fashion. Specifically, mount pads  404   a - c  ensure that additional strain is not introduced during the mounting and dismounting process. 
     The mounting pads  404  may be, for example, made from Invar™ (64FeNi) combined with silicone-rubber, for example, room temperature vulcanizing silicone (RTV) such as RTV  566 . The advantage of using such composite materials for the mounting pads  404  is briefly explained as follows. Invar™ has a small CTE, results in substantially small dimensional changes in these composite materials. On the other hand, RTV  566  is physically soft. As such, upon combining together these two different materials, any minute dimensional change in the Invar™ can be absorbed, and thus compensated, by the soft RTV  566  material. Thus, mounting pads made out of the above mentioned composite can minimize thermal strain on the mirror. 
       FIG. 5  is a perspective drawing providing a detailed view of an assembly of the secondary mirror  104  consisting of a secondary mirror mount  522 , a base plate  502 , potting posts  504   a - c  and the secondary mirror  104 . Secondary mirror  104  is mounted in the secondary mirror mount  522 . Mount  522  is affixed to the base plate  502 . The base plate  502  has multiple holes (not shown) through which the potting posts  504   a - c  can be attached. 
     During an alignment process of the reflective optics, the secondary mirror assembly is separated from the base mount  508  by an auxiliary alignment device  510 . The example auxiliary alignment device  510  shown in  FIG. 5  is a hexapod, but it could be any other alignment device that allows similar adjustments. A structural member (e.g. bar)  512  is configured to be fixed to the hexapod&#39;s top part  510   a . Weights  514  are attached to the bar  512 . The weights  514  act as a counterbalance as the hexapod can only handle a specific amount of weight in an off-axis position. Examples of weights shown in  FIG. 5  is are two 12 Kg (25 lb) weights, but other weights may be used for counterbalancing. Bar  512  is configured to hold base plate  502  kinematically during alignment via cone (not shown), vee (not shown), flat mounts  518  and balls  526 . The top part of the base mount includes potting cups  506   a - c . Motion of the hexapod  510  may be controlled by a controller (not shown). The hexapod has multiple degrees of freedom of motion. As such, it may hover the secondary mirror  104  assembly over the base mount  508  and potting cups  506 . The controller may be connected to the hexapod  510  via the connector  524 . The hexapod&#39;s bottom part  510   b  and the base mount&#39;s bottom part  508   b  are configured to be attached to the platform  116 . Details of the interaction of the potting posts  504   a - c  with the potting cups  506   a - c  are provided below. 
     As mentioned earlier, during an alignment process of the beam expander  200 , the primary mirror  106  is used as the reference optical component. Thus, the secondary mirror  104  is aligned to the primary mirror  106  using the alignment assemblies as described in  FIG. 5 . The fine alignment of the mirrors is accomplished by measuring the wavefront of the beam expander. Using the large transmission flat  108  at the output of the system and a reference flat  109  (e.g. standard commercial off-the-shelf (COTS) flat) mounted to the front of the interferometer  102 , an interference pattern is measured by the interferometer. The secondary mirror  104  is then adjusted until the wavefront error is reduced to an acceptable level. 
     At first, the primary mirror  106 , mounted on the non-adjustable mount  402 , is positioned at a desired location on the platform  116 . Following that, the secondary mirror assembly is aligned with the primary mirror  106 . The secondary mirror assembly then sits misaligned on the mounts  518  on bar  512 . The bar  512  is positioned between the secondary mirror assembly and the base mount top  508   a . In order to align the secondary mirror assembly to the primary mirror  106 , the hexapod  510  is operated via a controller such that the motion of the hexapod stimulates a movement in the secondary mirror  104  via the bar  512 . The motion of the hexapod  510  helps to adjust the position of the secondary mirror  106 . Particularly, in the adjustment process the potting posts  504   a - c  can move around within the oversized potting cups  506   a - c . Once the secondary mirror  104  is properly positioned, or aligned to the primary mirror  106 , the potting cups are then filled with a bonding agent to lock the position of the secondary mirror  104 . The hexapod  510 , bar  512  and weights  514  may then be removed from the setup. Thus, by using a hexapod or other removable auxiliary alignment device  510 , the cost of the overall setup can be kept low. Moreover, because the auxiliary alignment device  510  can be removed after the alignment, no further movement of the optical components is necessary during the test measurements. Thus, the secondary mirror  104  can be aligned to the primary mirror  106  during an alignment process using a removable auxiliary alignment device. The secondary mirror  104  may be aligned when the auxiliary alignment device  510  is at operating temperature. 
       FIG. 6  shows a transmission flat  108 , or alternatively, a reference flat  109 , mounted on the mount  602 . The mount  602  is mounted on the tip/tilt stage  114 . The tip/tilt stage  114  has a hinge  604  around which the mount  602  can tip or tilt. The hinge allows one axis of tilt whereas the other axis may be enabled by a pin in the bottom (not shown), for example. The stage  114  has a docking feature  606 . The docking feature  606  is designed so that the stage can dock into the platform  116 . By docking the flats  108  and  109  in this way, the positioning of the flats with respect to the beam expander is simplified and the test measurements are repeatable, avoiding any positional drift in the stages. 
     Although the present invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.