Off-Axis Parabolic (OAP) Mirrors

Application examples
  • Target simulators
  • Collimators
  • MTF measuring systems and other optics test devices
  • Spectroscopic and FTIR systems
  • Radiometers
  • Beam expanders
  • Laser divergence measuring systems
Use of off-axis optics allows optical engineers to achieve the following advantages
  • Minimize system sizes
  • Minimize system weight
  • Both “wedged” and equi-thick mirrors are available
  • Minimize system cost
A new optical system is introduced for the imaging of Coulomb crystals held in a cryogenic ion trap where there are space limitations preventing the placement of an objective close to the fluorescing ions. The optical system features an off-axis parabolic (OAP) mirror relay microscope that will serve to acquire images of a lattice of fluorescing ions confined within an ultra-high-vacuum vessel operating at temperatures below 10 K. We report that the OAP mirror relay setup can resolve features smaller than the separation between neighboring ions in Coulomb crystals. The setup presented here consists of two 90-degree OAP mirrors arranged into a relay from which standard microscope optics deliver the image to a camera. This design allows the first element in the imaging setup—an OAP mirror—to be located as close as possible to the ion trap, achieving high resolution without the need for a direct line-of-sight to the trap center or for a view port to be located in close proximity to the ion trap. Such an arrangement would not be possible with a standard microscope objective, which is the approach commonly adopted by the field. OAP mirrors represent a novel solution for delivering polychromatic images with micrometer-scale resolution over extended distances.
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According to the geometric optics, off-axis parabolic mirrors are used in terahertz time domain system for focusing the terahertz pulse. Theoretically, the focus of the parabolic reflector to eliminate the effects of spherical aberration, but a very high sensitivity to its optical axis misalignment, subtle misalignment will reduce the focal spot quality and therefore the need for precision optical axis adjustment is important. This article discusses terahertz time-domain spectroscopy system adjustment method in four off-axis parabolic mirror, after many experiments have proved feasible. In this work, two optical path adjustment methods were analyzed. The first approach is based on adjustment of spot shape from collimation and focus. In the second method, depend on the principle that the incident light path is different from reflected light path in parabolic cross section. Through testing this two methods have been proved feasible.
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Two parallel laser beams are used to align an off-axis parabolic mirror without alignment telescope and reference flat. In this method the optical axis of an off-axis parabolic mirror is made parallel to the incident laser beams, in the plane of incidence, by measuring directions of the reflected beams and by changing height and orientation of the mirror. Then, the focal point ofthe off-axis parabolic mirror is automatically found where the two reflected beams cross each other. The alignment of the optical axis to the incident beams is done without knowing focal length and off-axis distance of the mirror. Alignment sensitivity is derived both numerically and analytically. When focal length is 457 mm, off-axis distance is 127 mm, and diameter is 178 mm, the off-axis parabolic mirror is aligned to the incident beams with an angular error of less than 3 mrad.
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OAP fundamentals
A parabolic mirror takes light from a point source located at the focus and creates a collimated beam. In other words, a source with spherical wavefronts placed at the parabolic focus is converted into a beam with plane wavefronts. The reverse operation is also true; plane wavefronts incident on the parabolic mirror are focused at the focal point. This is a valuable tool for optical design because the single surface of an OAP can produce a diffraction limited image without chromatic effects. A complete parabolic mirror would focus a collimated beam at its focal point, which is often not useful because it overlaps with part of the incoming beam. Accessing the focal point can be difficult and even impossible without obstructing part of the incoming beam. However, if only a portion of the parabolic surface is used, the beam will focus off-axis at a more accessible location, as shown in Figure 1.
There are two important design considerations to remember when using OAPs:
a.The orientation matters. A collimated beam incident upon the OAP from the off-axis focal anglewill NOT produce a diffraction limited image. Similarly, a spherical wave incident on-axis willnot produce a collimated beam.
b.Only infinite conjugates. OAPs only achieve diffraction limited imaging when focusing acollimated beam. Similarly, they only produce a perfectly collimated beam from a sphericalwave. OAPs have terrible imaging quality when used at finite conjugates. They can beparticularly useful when used as part of a relay system, or similar operation to transition betweenthe focal plane and pupil plane.
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