Fundamental noise limitations on supercontinua generated in microstructure fiber determined by NIST.NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. recently completed a thorough study of the amplitude amplitude (ăm`plĭt  d'), in physics, maximum displacement from a zero value or rest position. noise on supercontinua generated by injecting femtosecond laser pulses into microstructure mi·cro·struc·ture n. The structure of an organism or object as revealed through microscopic examination. microstructure Noun a structure on a microscopic scale, such as that of a metal or a cell fiber. Supercontinuum generation in microstructure fiber has the potential of providing an extremely broadband, very bright, spatially and phase coherent source; these supercontinua already have revolutionized the field of optical frequency metrology metrology Science of measurement. Measuring a quantity means establishing its ratio to another fixed quantity of the same kind, known as the unit of that kind of quantity. . Unfortunately, the supercontinuum can exhibit substantial amplitude noise that can limit its applications. Work at NIST has succeeded in identifying the origins of this amplitude noise and characterizing its dependence on the experimental parameters. This work will be useful for researchers seeking to exploit this new optical source for metrology and other applications. Through measurement and simulations, NIST researchers identified two components to the amplitude noise: a low-frequency component, which arises from amplification amplification /am·pli·fi·ca·tion/ (33000) (am?pli-fi-ka´shun) the process of making larger, such as the increase of an auditory stimulus, as a means of improving its perception. of the input laser's technical noise; and a broadband component, which arises primarily from amplification of the shot noise on the input laser pulse. Because the low-frequency component of the noise is simply an amplified version of the technical noise on the input laser, it can be reduced experimentally by using a quieter laser. The broadband component is much more fundamental, since it originates from the basic quantum shot noise on the input laser pulse. Interestingly, as a result of non-linear modulation modulation, in communications modulation, in communications, process in which some characteristic of a wave (the carrier wave) is made to vary in accordance with an information-bearing signal wave (the modulating wave); demodulation is the process by which instability effects in the fiber, the initially weak shot noise is amplified by a much larger factor than the lower-frequency technical noise. Amplification of the shot noise of up to 90 dB during supercontinuum generation has been measured. In fact, under the wrong conditions, the broadband noise is high enough that the pulse-to-pulse amplitude fluctuations exceed 50%, rendering the supercontinuum essentially useless for most applications. Fortunately, one result of this work is the determination that amplification of the shot noise is extremely dependent on the input pulse duration In radar, measurement of pulse transmission time in microseconds; that is, the time the radar's transmitter is energized during each cycle. Also called pulse length and pulse width. ; for very short pulses of [approximately equal to] 30 fs or less, the broadband amplitude noise is reduced dramatically. CONTACT: Nathan Newbury, (303) 497-4227; nnewbury@boulder.nist.gov. |
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