NASA Technical Reports Server (NTRS) 20120006588: High-Speed Digital Interferometry

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by  NASA Technical Reports Server (NTRS)

the output waveguides accommodates higher effective refractive index. For the Thus, in the two-mode input waveguide
one mode only. At the input, the wave- light originating from the star, if the in- the starlight excites the fundamental
guide structure is symmetric and, there- terferometer is perfectly balanced, the mode, while the planet light excites the
fore, the fundamental mode of the input field in the focal plane of the fo- second, anti-symmetric, mode.
structure at the input is symmetric and cusing optic at the input of the device is This work was done by Alexander Ksendzov
the other mode is anti-symmetric. At the symmetric, while for the light field orig- of Caltech for NASA’s Jet Propulsion Labora-
output, one of the waveguides is wider inating from the planet (assuming the tory. Further information is contained in a
than the other, and therefore has a exact π phase shift) it is anti-symmetric. TSP (see page 1).NPO-47834
High-Speed Digital Interferometry
Optical decoding eliminates the need for high-speed detectors and digital signal processing.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Digitally enhanced heterodyne inter- negating the need for high-speed detec- for the first time, optical demodulation of
ferometry (DI) is a laser metrology tech- tors and digital signal processing. This re- the encoded laser beams (as opposed to
nique employing pseudo-random noise duced bandwidth also reduces the power the more traditional/common electronic
(PRN) codes phase-modulated onto an consumption of the entire system. demodulation). This technique is en-
optical carrier. Combined with hetero- The heterodyne signal is created by tirely implemented in software via hard-
dyne interferometry, the PRN code is off-set phase-locking two lasers with a ware that would already exist onboard a
used to select individual signals, return- digital phase-locked loop. The error- spacecraft. This reduces complexity,
ing the inherent interferometric sensi- point is monitored on a dedicated power consumption, volume, and risk of
tivity determined by the optical wave- phase-locking photoreceiver. PRN codes failure.
length. The signal isolation arises from are phase-modulated by waveguide mod- There are many proposed missions
the autocorrelation properties of the ulators onto each laser beam. These are that will employ lasers and require ex-
PRN code, enabling both rejection of subsequently interfered via a fiber beam- tremely high-resolution metrology. Digi-
spurious signals (e.g., from scattered splitter, thus optically demodulating the tal interferometry can be implemented
light) and multiplexing capability using laser signals, before being detected on a and achieve sub-10-pm resolution. With
a single metrology system. The mini- signal photoreceiver. One PRN code is this new technique, the metrology can be
mum separation of optical components digitally delayed with respect to the performed on optical components sepa-
is determined by the wavelength of the other in order to align the codes with re- rated by centimeters. This allows meas-
PRN code. spect to the reflected light from an optic urements of optics on a single optical
A variation of DI has 100 times reduc- under interrogation, thus optically de- bench within a single spacecraft, in addi-
tion in the minimum component separa- modulating the signal for that specific tion to inter-spacecraft metrology meas-
tion, allowing measurements of optical mirror. The delay is altered (controlled urements.
components only a few centimeters apart. digitally) to pick out any one of the op- This work was done by Glenn De Vine,
Instead of the usual electronic decoding, tics under interrogation. Daniel A. Shaddock, Brent Ware, Robert E.
the DI signal is interfered with an appro- At the time of this reporting, this is the Spero, Danielle M. Wuchenich, William M.
priately delayed, identically PRN-en- first known time that DI has been em- Klipstein, and Kirk McKenzie of Caltech for
coded, local oscillator beam. Optical de- ployed to measure optics separated by NASA’s Jet Propulsion Laboratory. Further in-
coding allows the use of a low-bandwidth less than meters, down to a few centime- formation is contained in a TSP (see page 1).
signal processing chain with GHz codes, ters. This was achieved by implementing, NPO-47886
Ultra-Miniature Lidar Scanner for Launch Range
Data Collection
×
New scanning technology promises at least a 10 performance improvement.
John F. Kennedy Space Center, Florida
The most critical component in lidar is ning speed, with an ultra-miniature size (e.g. 5–20 kHz, in contrast to several hun-
its laser scanner, which delivers pulsed or and much lighter weight. This technology dred Hz in existing scanners); structure
CW laser to target with desirable field of promises at least a 10× performance im- design to meet stringent requirements on
view (FOV). Most existing lidars use a ro- provement in these areas over existing size, weight, power, and compactness for
tating or oscillating mirror for scanning, lidar scanners. Features of the proposed various applications; and the scanning
resulting in several drawbacks. ultra-miniature lidar scanner include the speed and FOV can be altered for obtain-
A lidar scanning technology was devel- ability to make the entire scanner <2 mm ing high image resolutions of targeted
oped that could achieve very high scan- in diameter; very high scanning speed areas and for diversified uses.
NASA Tech Briefs, January 2012 31