NASA Technical Reports Server (NTRS) 20100001356: Vacuum-Compatible Wideband White Light and Laser Combiner Source System

0.12 MB·English

by  NASA Technical Reports Server (NTRS)

Vacuum-Compatible Wideband White Light and Laser
Combiner Source System
NASA’s Jet Propulsion Laboratory, Pasadena, California
For the Space Interferometry Mission to coarse align the optical paths, a laser gives a broad spectrum of about 525 to
(SIM) Spectrum Calibration Develop - light is sent into the system to allow track- 800 nm full width at half maximum
ment Unit (SCDU) testbed, wideband ing of fringes because a narrow band (FWHM), with about 1.4 mW of power
white light is used to simulate starlight. laser has a great range of interference. at 630 nm. A custom-made beam splitter
The white light source mount requires The design requirements forced the window with special coating for broad-
extremely stable pointing accuracy (<3.2 innovators to use a new type of optical band wavelengths is used with the white
microradians). To meet this and other fiber, and to take a large amount of care light input via a 50-mm multi-mode
needs, the laser light from a single-mode in aligning the input sources. The test- fiber. The large mode area PCF is an
fiber was combined, through a beam bed required better than 1% through- LMA-8 made by Crystal Fibre (core di-
splitter window with special coating from put, or enough output power on the ameter of 8.5 mm, mode field diameter
broadband wavelengths, with light from lowest spectrum to be detectable by the of 6 mm, and numerical aperture at 625
multimode fiber. Both lights were cou- CCD camera (6 nW at camera). The sys- nm of 0.083). Any science interferome-
pled to a photonic crystal fiber (PCF). tem needed to be vacuum-compatible ter that needs a tracking laser fringe to
In many optical systems, simulating a and to have the capability for combin- assist in alignment can use this system.
point star with broadband spectrum with ing a visible laser light at any time for This work was done by Alireza Azizi, Daniel
stability of microradians for white light calibration purposes. J. Ryan, Hong Tang, Richard T. Demers, Hi-
interferometry is a challenge. In this The red laser is a commercially pro- roshi Kadogawa, Xin An, and George Y. Sun
case, the cameras use the white light in- duced 635-nm laser 5-mW diode, and of Caltech for NASA’s Jet Propulsion Labora-
terference to balance two optical paths, the white light source is a commercially tory. Further information is contained in a
and to maintain close tracking. In order produced tungsten halogen lamp that TSP (see page 1).NPO-46165
Optical Tapers as White-Light WGM Resonators
Such resonators could be attractive for broad-band optical processing applications.
NASA’s Jet Propulsion Laboratory, Pasadena, California
A theoretical analysis has revealed that The theoretical analysis was per- critical radius Rc at a classical turn-
tapered optical waveguides could be use- formed for a multimode, axisymmetric, around axial position zc, the group delay
ful as white-light whispering-gallery- circular-cross-section waveguide having a of a WGM increases and the electromag-
mode (WGM) optical resonators. The taper sufficiently smooth and gradual to netic field becomes increasingly concen-
compactness and the fixed-narrow-fre- justify the approximation of adiabaticity. trated, albeit in an effective mode vol-
quency-band nature of the resonances In this approximation, the equation for ume typically much larger than the
of prior microdisk and microsphere the dependence of the electromagnetic mode volume in a prior microdisk or mi-
WGM resonators are advantageous in field upon the axial (longitudinal) wave- crosphere WGM resonator. Thus, it was
low-power, fixed-narrow-frequency-band guide coordinate (z) can be separated found that the power density of the elec-
applications. However for optical-pro- from the equation for the dependence tromagnetic field is much less than in a
cessing applications in which there are upon the radius (r) and the azimuthal prior microdisk or microsphere WGM
requirements for power levels higher angle (ϕ). Electromagnetic modes char- resonator and the onset of undesired
and/or spectral responses broader than acterized by high angular momentum nonlinearities is shifted to a significantly
those of prior microdisk and micros- (equivalently, large values of the ϕ-de- higher power level. It was found that in
phere WGM resonators, white-light pendence quantum number) were con- the special case of a linear taper, the
WGM resonators in the form of optical sidered. The solution of the equation for turning point varies linearly with the fre-
tapers would be preferable. the axial dependence was found to be an quency of the electromagnetic field,
In a typical prior microdisk or micros- amplitude that varies gradually with z. while the resonance quality factor and
phere WGM resonator, the optical power For a given axial location z, where the dispersion remain fixed to first order.
is concentrated mostly in a small WGM outer surface of the waveguide has a ra- A resonator having these characteris-
volume, making it necessary to limit the dius R(z), the solutions for the radial tics can be considered a white-light res-
power to a low level in order to minimize and azimuthal dependences were found onator in that it exhibits resonance over
undesired nonlinear optical and thermo- to be WGM modes equivalent to those a continuous frequency range.
optical effects. If one could construct a for a cylinder of radius R(z). This work was done by Dmitry V. Strekalov
WGM resonator in which the optical In effect, it was found that the tapered and Andrey B. Matsko of Caltech and Ana-
power were spread over a larger volume, waveguide can be considered to support toliy A. Savchenkov of OEWaves for NASA’s
then the threshold power level for the WGMs propagating along the waveguide Jet Propulsion Laboratory. For more informa-
onset of undesired nonlinear optical and axis. It was further found that as the ra- tion, contact [email protected]. NPO-
thermo-optical effects would be higher. dius tapers down toward the classical 45842
NASA Tech Briefs, January 2010 33