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April, 2004 Fundamental Phase Noise Limitations on Supercontinua Generated in Microstructure Fiber Analyzed by EEEL Researchers |
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Nathan Newbury and Brian Washburn of the EEEL Optoelectronics Division have recently completed a thorough study of the fundamental phase noise and timing jitter on the optical supercontinuum pulses generated in nonlinear optical fiber. This analysis extends earlier work that explored the amplitude noise on the pulses. A supercontinuum (i.e., optical continua spanning more than a factor of two in frequency) can be generated by injecting femtosecond pulses from a Ti:Sapphire laser into highly nonlinear microstructure fiber. Ideally, the supercontinuum provides an extremely broadband, very bright, spatially and phase coherent source; it has already revolutionized the field of optical frequency metrology by providing the basis for a self-referenced optical frequency comb. Unfortunately, the supercontinuum can exhibit substantial excess phase and amplitude noise that can potentially limit its applications. The most fundamental cause of this excess noise is the initial quantum vacuum fluctuations on the input laser pulse. Normally, these vacuum fluctuations will give rise to the well-known shot noise, which is low at the high pulse energies considered here. However, during the supercontinuum formation, the vacuum fluctuations on the input laser pulse are effectively amplified and give rise to much larger amplitude and phase noise across the supercontinuum. Using numerical simulations, the dependence of this fundamental noise on various parameters such as laser pulse energy, pulse width, and fiber length was explored. They found that for long pulses and very high pulse energies, this fundamental phase and amplitude noise can reach significant levels. However, under carefully chosen experimental conditions, the noise can be minimized so as not to interfere with optical frequency metrology experiments. The mathematical framework developed for this work should apply equally well to an analysis of other noise sources on the supercontinuum. This work will be useful for researchers seeking to exploit this new optical source for metrology and other applications.
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