SLMS athermal technology has been demonstrated in the small 4-foot helium cryogenic test chamber located at the NASA/MSFC X-Ray Calibration Facility (XRCF). A SLMS Ultraviolet Demonstrator Mirror (UVDM) produced by Schafer under a NASA/MSFC Phase I SBIR was helium cryo tested both free standing and bonded to a Schafer designed prototype carbon fiber reinforced silicon carbide (Cesic) mount. Surface figure data was obtained with a test measurement system that featured an Instantaneous Phase Interferometer (IPI) by ADE Phase Shift. The test measurement system's minimum resolvable differential figure deformation and possible contributions from test chamber ambient to cryo window deformation are under investigation. The free standing results showed differential figure deformation of 10.4 nm rms from 295K to 27K and 3.9 nm rms after one cryo cycle. The surface figure of the UVDM degraded by lambda/70 rms HeNe once it was bonded to the prototype Cesic mount. The change was due to a small astigmatic aberration in the prototype Cesic mount due to lack of finish machining and not the bonding technique. This effect was seen in SLMS optical assembly results, which showed differential figure deformation of 46.5 nm rms from 294K to 27K, 42.9 nm rms from 294K to 77K, 28.0 nm rms from 294K to 193K and 6.2 nm rms after one cryo cycle.
This effort describes a design method used to develop a binary-phase Fourier grating that generates an incoherent array of output source points with nonuniform user-defined intensities, symmetric about the zeroth order. Like the Dammann fanout grating approach, the binary-phase Fourier grating uses only two phase levels in its grating surface profile to generate the output array. Unlike the Dammann fanout grating approach, this method provides for the generation of nonuniform, user-defined intensities within the final fanout pattern. The process employs both simulated annealing and nonlinear optimization algorithms to locate solutions to the specified grating design problem. Because the desired grating output is incoherent, each source point of the grating response is assessed in terms of intensity, from which an overall efficiency is calculated. Efficiencies are calculated for each solution and are used to evaluate the relative value of each solution. A final design solution that produces an incoherent, symmetric, user-defined nine spot array is presented.
We describe a multilayered dielectric stack configuration designed specifically for use as a transmissive phase modulator for broadband optical signals. Applications for this device range from full aperture wavefront correction to nonmechanical beam steering arrays for free space optical communication links. Our implementation employs alternating GaAs and AlAs layers of varying thickness on a GaAs substrate to create a bandpass region of high average transmission in the near infrared. Within this transmission bandpass, the phase component of the complex transmission coefficient varies in a near-linear fashion with respect to wavelength. The transmission bandpass is designed to have a bandwidth of 21.0 nm (or 6.3THz frequency bandwidth) and to have an edge-to-edge relative phase change of greater than 4p radians. Modification of the stack materials' optical properties causes the transmission profile to shift spectrally, resulting in a phase modulation for specific bands of transmitted frequencies. Our broadband phase modulator imparts nearly a full-cycle of phase modulation with low loss and low group velocity dispersion. A sample comprising 91 alternating layers has been fabricated to exhibit the bandpass properties required for optical signal phase modulation. We experimentally characterize the sample using an interferometer and spectrometer to measure the spectral transmission and relative phase profiles and to assess the relative phase modulation in response to a variable angle of incidence. We compare the experimental data to computational predictions and discuss the results.
We present an optical delay line structure incorporating InxGa1-xAs quantum wells in the GaAs quarter- wave layers of a GaAs/AlAs distributed Bragg reflector. Applying an electric field across the quantum wells shifts and broadens the e1-hh1 exciton peak via the quantum- confined Stark effect. Resultant changes in the index of refraction thereby provide a means for altering the group delay of an incident laser pulse. Theoretical results predict tunable delays on the order of 50 fs for a 30-period structure incorporating 3 quantum wells per GaAs layer. Structure design, growth and fabrication are detailed. Preliminary group delay measurements on large-area samples with no applied bias are presented.
We describe a design for an agile, electronically- configurable, optical beam steering array to be used in directional free-space transmission of optical signals. The proposed device employs a 1D array of tunable resonant transmissive modulators constructed from customized multi- layered stacks of dielectric materials. Each modulator may be individually configured to transmit an optical signal with a known amount of phase and group velocity modulation. Proper configuration of each individual modulator results in diffractive interactions between multiple modulator outputs, providing a method for directional optical signal transmission. Of particular focus within this paper is the design of the individual modulator. We generate custom transmission functions by varying the parameters describing the modulator's specific construction, such as number of layers within the multi-layer stack, refractive indices of stack materials, layer thickness, and combinations of periodic versus non-periodic layer repetitions. A computational optimization of the variables describing the stack's construction strives to maximize the amount of optical signal modulation obtainable within defined limits. Our optimization is based largely on maximizing transmitted phase delay. We discuss trade-offs between methods of increasing device performance versus practical limitations of fabrication technologies.
Conference Committee Involvement (3)
Free-Space Laser Communications VII
28 August 2007 | San Diego, California, United States
Free-Space Laser Communications VI
15 August 2006 | San Diego, California, United States
Free-Space Laser Communications V
31 July 2005 | San Diego, California, United States