DOI: 10.1007/s00340-003-1177-8 Appl. Phys. B 77, 279–284 (2003) Lasers and Optics Applied Physics B j.n. ames 1 s. ghosh 2 r.s. windeler 3 a.l. gaeta 2 s.t. cundiff 1, ✉ Excess noise generation during spectral broadening in a microstructured fiber 1 JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309–0440, USA 2 School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA 3 OFS Fitel Laboratories, Murray Hill, NJ 07974, USA Received: 19 February 2003 Published online: 16 July 2003 • © Springer-Verlag 2003 ABSTRACT We observe that nanojoule femtosecond pulses that are spectrally broadened in a microstructured fiber acquire ex- cess noise. The excess noise is manifested as an increase in the noise floor of the rf spectrum of the photocurrent from a photodetector illuminated by the pulse train from the laser os- cillator. Measurements are made of the intensity dependence of the excess noise for both 100 fs and sub-10 fs pulses. The excess noise is very strong for 100 fs pulses, but barely measurable for sub-10 fs pulses. A rigorous quantum treatment of the nonlinear propagation of ultrashort pulses predicts that, for a fixed gener- ated bandwidth, the amount of excess noise decreases with pulse duration, in agreement with the experimental results. PACS 42.65.Re; 42.62.Eh; 06.20.-f 1 Introduction The discovery that continuum generation occurs in microstructured fibers with nanojoule femtosecond pulses [1] has led to a wealth of applications. Perhaps the largest impact has been in optical frequency metrology and optical atomic clocks. The underlying scheme of these applications is based on using a mode-locked laser as a “comb generator”. The abil- ity to generate a spectrum that spans an octave in frequency allows the offset frequency f 0 of the comb to be determined using the self-referencing technique [2, 3] or more complex chains [4,5]. Together, the rf frequencies f rep , the repetition rate, and f 0 determine the optical frequency of any comb line via ν n = nf rep + f 0 . This yields a simple connection be- tween the microwave frequency produced by a cesium clock and optical frequencies, enabling a significant improvement in absolute optical frequency measurements, including the Rb two-photon transition [3, 4], HeNe [4, 6], cold calcium [7, 8], single Hg + [8], and Yb + [9] ions. Optical atomic clocks use the femtosecond comb in the inverse fashion to derive a mi- crowave output based on an optical frequency atomic or ionic transition. To date, implementations of optical atomic clocks based on a single Hg + ion [10] and molecular iodine [11] have been demonstrated. Several reviews of these applications have recently been published [12]. ✉ Fax: +1-303/735-0101, E-mail: cundiffs@jila.colorado.edu Although the initial demonstration of continuum gen- eration used 100 fs duration input pulses [1], all published metrology results have used lasers generating pulses of ∼ 30 fs duration or less. The reason for this is demonstrated in Fig. 1, which shows a prominent noise floor that grows with increasing pulse energy for 100 fs pulses. Metrology ex- periments require the detection of a heterodyne beat between either the femtosecond comb and a cw laser and/or between a portion of the comb and the second harmonic of the comb at half the frequency. These beat signals are weak and are over- whelmed by the generated noise for sufficiently high pulse energies, at which an octave spanning spectrum is produced. In contrast, for shorter duration pulses, the generated noise is smaller; for ∼ 10 fs pulses it is almost undetectable. In this paper, we present measurements and theoretical calculations of the excess noise generated during supercon- tinuum generation in a microstructured fiber. Excess noise is defined to be the noise level above the shot noise. To gain in- sight into the generation mechanism, we measured the excess noise for both 100 fs and 10 fs pulses. For the 100 fs pulses, we also measured it at several laser wavelengths, specifically focusing on wavelengths close to the zero group velocity dis- persion (GVD) wavelength of the microstructured fiber. FIGURE 1 rf spectra of the output of a microstructured fiber for 100 fs input pulses. Several pulse energies are shown. The peaks correspond to mul- tiples of the laser repetition rate and are normalized in amplitude. Note the relative increase in the noise between the peaks. For the lowest pulse energy spectrum, the noise floor is due to the electronics