|
July, 2003 Novel System to Accurately Measure Optical Frequencies in the Near Infrared |
|
Staff of EEEL’s Optoelectronics Division, in collaboration with staff of PL’s Time and Frequency Division, have recently developed the capability to accurately measure optical frequencies across the near infrared region from 1100 to 2000 nm, including the important telecommunication window from 1300 to 1600 nm. The basic technique compares an unknown optical source against a calibrated infrared frequency comb, or ruler, that is itself referenced to established frequency standards. This technique, which is an extension of one already successfully employed in the visible region of the spectrum, is expected to serve as a very general tool to calibrate infrared optical frequencies or wavelengths. In order to demonstrate the future potential of this system, researchers have used it to make calibrated measurements of several frequency references in the telecommunication window. Researchers have also used it to calibrate the physical length of a prototype compact length standard.
The system is based on a stabilized infrared frequency comb, which comprises a mode-locked Cr:forsterite laser whose output is spectrally broadened in highly nonlinear fiber to generate a supercontinuum spanning from 1100 nm to beyond 2000 nm. Since the laser operates at a repetition rate of 430 MHz, the supercontinuum is actually a comb of individual frequency lines with a 430 MHz spacing. By locking the frequency comb to both a hydrogen maser and the calcium optical frequency standard, an accurate frequency ruler is formed covering this entire near infrared region. Any unknown optical frequency can then be measured by simply comparing its frequency to that of the nearest tooth of the stabilized frequency comb. Researchers have used this technique to make measurements of telecommunication optical frequency references, including three methane absorption lines in the 1300 nm region and a 1560 nm laser stabilized to a rubidium line.
Contact:
|