A procedure now under development is intended to enable the precise alignment of sheet arrays of microscopic lenses with the end faces of a coherent bundle of as many as 1,000 single-mode optical fibers packed closely in a regular array (see Figure 1). In the original application that prompted this development, the precise assembly of lenses and optical fibers serves as a single-mode spatial filter for a visible-light nulling interferometer. The precision of alignment must be sufficient to limit any remaining wavefront error to a root-mean-square value of less than 1/10 of a wavelength of light. This wavefront-error limit translates to requirements to (1) ensure uniformity of both the lens and fiber arrays, (2) ensure that the lateral distance from the central axis of each lens and the corresponding optical fiber is no more than a fraction of a micron, (3) angularly align the lens-sheet planes and the fiber-bundle end faces to within a few arc seconds, and (4) axially align the lenses and the fiber-bundle end faces to within tens of microns of the focal distance.
Figure 2 depicts the apparatus used in the alignment procedure. The beam of light from a Zygo (or equivalent) interferometer is first compressed by a ratio of 20:1 so that upon its return to the interferometer, the beam will be magnified enough to enable measurement of wavefront quality. The apparatus includes relay lenses that enable imaging of the arrays of microscopic lenses in a charge-coupled-device (CCD) camera that is part of the interferometer. One of the arrays of microscopic lenses is mounted on a 6-axis stage, in proximity to the front face of the bundle of optical fibers. The bundle is mounted on a separate stage. A mirror is attached to the back face of the bundle of optical fibers for retroreflection of light. When a microscopic lens and a fiber are aligned with each other, the affected portion of the light is reflected back by the mirror, recollimated by the microscopic lens, transmitted through the relay lenses and the beam compressor/expander, then split so that half goes to a detector and half to the interferometer. The output of the detector is used as a feedback control signal for the six-axis stage to effect alignment.
The alignment procedure, which is rather complex, can be summarized as follows:
- In the absence of a sheet array of microscopic lenses, the longitudinal axis of the fiber-optic bundle is aligned with the optical axis of the interferometer by use of the reflection from the front face of the bundle.
- One of the sheet arrays of microscopic lenses is placed in front of the fiber-optic bundle and similarly aligned with the interferometer optical axis by use of the reflection from its front face. As a result, the optical axes of the lens array and the fiber-optic bundle are parallel with each other.
- The axial position of the lens sheet is adjusted until the interferometric image of light reflected from the front face of the fiber-optic bundle indicates that the lenses are at the proper focal distance.
- The lateral (relative to the optical axis) position of the lens sheet is adjusted until the interferometric image shows that at least one lens is centered on the end of at least one optical fiber. The lateral coordinates of the six-axis positioner are measured. The lateral position of the lens sheet is further adjusted until another lens/fiber pair is thus centered, and the corresponding coordinates are measured. The two sets of coordinates are used to compute the translation and rotation needed to effect the lateral alignment of the remaining lens/fiber pairs.
- Guided by the foregoing coordinate measurements, the final adjustments of the lens sheet are made.
- The lens sheet is bonded to the fiber-optic bundle.
- The fiber-optic bundle is turned around so that what was previously the back face is now the front face.
- The retroreflecting mirror is aligned with the optical axis of the interferometer.
- Steps 1 through 7 are repeated to effect the alignment and bonding of the second lens sheet to what is now the front face of the fiber-optic bundle.
This work was done by Duncan Liu of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-40021.