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Mary
Rowe and William Swann of the EEEL Optoelectronics Division have developed
a multiple-wavelength calibration reference that incorporates the wavelength
flexibility of fiber Bragg grating artifact references and the stability
of fundamental molecular absorption references. A prototype wavelength
calibration unit was demonstrated at the Navy Primary Standards Laboratory,
San Diego, in December 2002.
Wavelength calibration references are needed to calibrate wavelength-measuring
instruments such as optical spectrum analyzers. Atomic and molecular absorption
lines are very stable under changing environmental conditions and are
excellent references for this purpose. Unfortunately, good atomic or molecular
candidates are not available in all wavelength regions. Customized multiple
wavelength reflectors can be generated by writing multiple superimposed
fiber Bragg grating (FBG) reflectors into optical fiber using ultraviolet
light. Each grating is a reflector for a specific wavelength of light;
the wavelengths are selected during the grating fabrication process. Strain
and temperature changes, however, can cause the center wavelength of these
reflectors to change; the temperature sensitivity is about 0.01 nm/ C.
If one of the FBG reflectors is located near an atomic or molecular absorption
line, it can be actively stabilized to that line. This stability is then
transferred to the other gratings, because they are superimposed at the
same location in the fiber. Once the wavelength of each FBG reflection
peak is measured at NIST, all of the peaks can be used as calibration
references. The NIST prototype uses interleaved sampled FBGs to produce
multiple peaks in the 1300 and 1550 nm regions. The unit provides 8 calibration
references between 1297 and 1306 nm and 12 references between 1531 and
1550 nm, each with a stability of a few picometers. Rowe and Swann are
now investigating methods to incorporate calibration references in the
850 nm region.
Contact:
Sarah
L. Gilbert, (303) 497-3120
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