Rapid centering of optics



Traditionally a rotary table is used for optical centering because the table creates an axis as a reference. Previously, we showed that a Bessel beam also creates an axis useful for centering. The Bessel beam axis and a center of curvature of a surface makes it possible to center an optic simultaneously in tilt and decenter. We also showed that simultaneously sampling two arbitrary points along the Bessel beam also permits full adjustment of tilt and decenter of a powered optic. This makes centering possible without either a rotary table or a precision linear stage. In most common instances, however, sampling the beam at two points is unnecessary because of the inability to correct for both tilt and decenter. We discuss an alternative, simpler method using a Bessel beam.

Practical considerations for using grating produced Bessel beams for alignment purposes



Bessel beams are useful for alignment because they create a small diameter, bright, straight line image in space perpendicular to the Axicon, or Axicon grating, producing the beam that is an exact analog of a single ray in a ray tracing program. Here we limit our discussion to Bessel beams produced by plane gratings whose pattern is evenly spaced concentric circles that are illuminated by a point source of light on the grating axis. The gratings produce a more nearly ideal Bessel beam than a lens type Axicon, and the plane grating serves as a plane mirror as well in an alignment setup so the combination define four degrees of freedom in space rather than the usual two.

Most discussions of Bessel beams assume illumination with collimated light. We have found it advantageous to use a point source for illumination because it is easy and less expensive to use a single mode fiber as a source than a precision collimating lens the diameter of the Axicon. Besides, collimated illumination produces a Bessel beam of finite length in transmission while in theory a beam of infinite length is created using a point source.

With these assumptions about how the beams are produced and details about the grating diameter and line spacing it is easy to calculate the useful length of the Bessel beam in reflection from the grating, the usual matter of concern when using the grating for alignment purposes in a double pass test setup. Other practical matters are also discussed such as lens centering with a test apparatus with no moving parts.

Precision cementing of doublets without using a rotary table



Methods of centering without using a precision rotary table to establish a reference axis in space are several times faster than with a rotary table. However, finding an optimum method of establishing an alternative reference axis is challenging. We look at the small class of centering situations involving the precision cementing of doublets to illustrate the advantages of using a Bessel beam as the reference axis. Two approaches to centering illustrate the method; one involving first aligning the meniscus element and then adding the positive element, and the other, cementing the two elements and aligning the pair.

Aligning reflecting optics with Bessel beams



Bessel beams have found use in the alignment of transmissive optics for some time. They are also used for the alignment of reflecting optics when used in the imaging mode, that is, when the wavefront is near spherical. However, there are cases where it would be useful to use the Bessel beam for alignment of far-off axis aspheres to order to get the asphere aligned close enough to its final position that light will go through the system in the imaging mode. In another mode, the Bessel beam is used to determine the normal to a free form surface.

Design for Alignment


The premise of this paper is that the only remaining way to improve optical system performance is with better alignment techniques. We feel optical design is a mature field and that little can be done to improve the design of optical systems by improvements to lens design software. The software may become easier or more convenient to use but the optical designs produced are near optimum given the design constraints specifying the system.

The same holds for the manufacture of optical elements. Between computer controlled manufacturing methods and interferometric testing of the manufactured elements and the many improvements in optical glass quality, not many avenues are open to improved quality of the optical components themselves. The only area left for improvement in performance of precision, or high quality, optical systems is the assembly and alignment of the glass elements and mirrors into mechanical cells, and lens benches, for more complex system geometries.

Based on this premise we will first define our concept of what precision optical alignment means and why traditional methods of alignment have not kept up with the improvements in lens design and the manufacture of high quality optical elements. We contrast traditional methods with more modern methods of optical alignment that make use of optical datums rather than mechanical datums and show the advantages of the optical methods.

Next, we show some advances in the optical methods of alignment including newer optical alignment tools and tooling including gratings that define axes in 5 degrees of freedom and how these make alignment easier. Finally, we look at the implications of these newer methods on how the opto-mechanics of cell and lens bench design are impacted so that tolerances can be loosened while achieving improved optical system performance. While this applies largely to precision optics manufacture, there are aspects of this approach that are applicable to production assembly as well.

Prism alignment using a Point Source Microscope



Spherically mounted retroreflectors (SMRs) are an essential part of spatial metrology when using a laser tracker. However, the precision of the laser tracker measurement is no better than the precision with which the cube corner retroreflector is mounted in the spherical ball. Thus measuring the position of the apex of the cube corner with respect to the center of the ball is a critical part of both the assembly and inspection of SMRs. This leads to cost implications because the better centered the cube corner in the ball, the more the SMR costs.

We begin by explaining the use of SMRs with a laser tracker, and then explain how the question arose of whether an autostigmatic is useful in measuring the apex location of the cube corner. We follow this with the theory of the measurement based on a two-dimensional argument, but the same argument applies to the three-dimensional case. The two-dimensional case is much easier to explain and understand.

Finally, we look at how real hardware must be fixtured to make the measurements. At first it looks difficult because a sphere has no axis and the SMR must rotate about orthogonal axes to measure the apex location. However, a very simple fixture is satisfactory. We give some measured results to demonstrate the precision of the method.

Centering Steep Aspheric Surfaces



Finding the optical axis of an aspheric surface is an essential part of making an aspheric lens because the center of curvature, or optical axis, of the second side must lie on, or be coincident with, respectively, the optical axis of the first side for maximum optical performance. Looking at the center of an aspheric surface and measuring the tilt and coma as a function of decenter is an obvious means of determining centration, but many aspheric surfaces are relatively spherical over the part of the lens aperture that can be viewed with commercially available optics and there is too little coma to make a useful measurement of decenter.

We describe an alternative method of viewing a small patch of the aspheric surface near the edge of the clear aperture where the asphericity is greatest while rotating the lens about an axis close to coincident with the optical axis of the surface. By also tracking the reflected light image when using an autostigmatic microscope (ASM), or using an interferometer to measure the low order Zernike coefficients, as the lens is rotated, both the tilt and decenter of the surface can be determined.

The relationship between the image motion and Zernike coefficients is described for both tilt and decenter of the surface as well as the means of separating the relative amounts of tilt and decenter are given. These methods of determining tilt and decenter seem to work for all aspheric surfaces we have tried to date.

We begin by reviewing the thinking that led to the method described for finding the optical axis of an aspheric surface. The discussion moves through successive attempts to find a method of locating the optical axis for aspheric surfaces that works for all surfaces that have enough asphericity to qualify as aspheres. Results are shown for both the interferometer and the autostigmatic microscope method*. Finally, we conclude by describing a method that may work even better for many aspheric lenses that makes use of a computer generated hologram.

*After submitting the Abstract for this talk it came to the author’s attention that there is a European Patent #EP 1918 787 A1 covering the autostigmatic microscope method described in this paper for locating the optical axis of the first surface of an aspheric lens.

Computer Generated Holograms as Fixtures for Testing Optical Elements


1. Introduction

It is common to think of computer generated holograms (CGH) as artifacts for testing aspheres but they can also be used as general calibration artifacts and fixtures for the alignment and test other more conventional optics. We show how simple Fresnel zone patterns can be created to simulate centers of curvature or axes in space with dimensional precision associated with microlithography. These centers of curvature and axes can then be located in space to similar sorts of precision with an autostigmatic microscope (ASM) or an interferometer.

Once the ASM is centered on the center of curvature, or axis, of the Fresnal zone pattern, a ball, or cylinder, respectively, of matching radius can be aligned to the ASM or interferometer to similar sorts of precision and physically attached to the CGH to serve as kinematic datums against or on which to mount other optical or mechanical components.

We then give an example of the fixture for the mounting of a rectangular lens element into a kinematically located frame so the two can be cemented together prior to inserting the bonded pair into an optical bench.

Use of the Surface PSD to Investigate Near Specular Scatter from Smooth Surfaces


The Rayleigh Rice vector perturbation theory has been successfully used for several decades to relate the surface power spectrum of optically smooth reflectors to the angular resolved scatter resulting from light sources of known wavelength, incident angle and polarization. While measuring low frequency roughness is relatively easy, the corresponding near specular scatter can be difficult to measure. This paper discusses using high incident angle near specular measurements along with profile generated surface power spectrums as a means of checking a near specular scatter requirement. The specification in question, a BRDF of 1.0 sr-1 at 2 mrad from the specular direction and at a wavelength of 1μm, is very difficult to verify by conventional scatter measurements. In fact, it is impractical to directly measure surface scatter from uncoated Zerodur because of its high bulk scatter. This paper presents profilometer and scatterometer data obtained from coated and uncoated flats at several wavelengths and outlines the analysis technique used to check this tight specification.

Case Studies & Testimonials

  • Sometimes good news goes unnoticed for the longest time. We just stumbled on this unattributed, but complimentary note about the PSM.
    See  https://www.idex-hs.com/wp-content/uploads/2018/05/Enabling-Micron-Level-Mounting-Accuracy.pdf   

    Notice the PSM in the top center of Fig. 9.  Optical Perspectives Group would like to express "thank you, IDEX".

  • "We are enjoying our Point Source Microscope and finding it invaluable in alignment and diagnostic tasks."

    Dr. John Mitchell
    Senior Optical Metrologist
    Glyndwr Innovations Ltd., St. Asaph, Wales, UK


  • "Just wanted to share a recent success aligning an adaptive optics test bed with the PSM. We used to use a traditional alignment telescope in the past, but the PSM made the whole process really easy and fast. The main requirements were to quickly determine the quality of beam collimation and pupil conjugates since there are several beam expanders and compressors with multiple pupil and focal planes."

    Suresh Sivanandam
    Dunlap Institute for Astronomy and Astrophysics
    University of Toronto


  • "You are always responsive and give us lots of useful information!!"

    Dr. Shaojie Chen
    Dunlap Institute for Astronomy and Astrophysics
    University of Toronto


  • "As always we are very much loving the instrument, I personally love the camera upgrade from what I'm used to!"

    Weslin Pullen
    Hart Scientific Consulting International, LLC
    Tucson, Arizona


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