Ultrafast Laser Physics

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Solid-state vs fiber lasers

The high-Q cavities of diode-pumped solid-state lasers give much better noise performance of frequency combs from modelocked lasers.

In collaboration with the group of Prof. Pierre Thomann, we have demonstrated the first full stabilization of an optical frequency comb generated from a femtosecond diode-pumped solid-state laser (DPSSL) operating in the 1.5-μm spectral region - the ERGO laser:

[320] S. Schilt, N. Bucalovic, V. Dolgovskiy, C. Schori, M. C. Stumpf, G. Di Domenico, S. Pekarek, A. E. H. Oehler, T. Südmeyer, U. Keller, P. Thomann,
“Fully stabilized optical frequency comb with sub-radian CEO phase noise from a SESAM-modelocked 1.5-µm solid-state laser” (PDF, 1.5 MB)
Optics Express, vol. 19, pp. 24171-24181, 2011 Download Download (PDF, 1.6 MB)

The stability of the comb is characterized in free-running and in phase-locked operation by measuring the noise properties of the carrier-envelope offset (CEO) beat, of the repetition rate, and of a comb line at 1558 nm. The high Q-factor of the semiconductor saturable absorber mirror (SESAM)-modelocked 1.5-μm DPSSL (i.e. ERGO laser) results in a low-noise CEO-beat, for which a tight phase lock can be much more easily realized than for a fiber comb.

Using a moderate feedback bandwidth of only 5.5 kHz, we achieved a residual integrated phase noise of 0.72 rad rms for the locked CEO, which is one of the smallest values reported for a frequency comb system operating in this spectral region. The fractional frequency stability of the CEO-beat is 20-fold better than measured in a standard self-referenced commercial fiber comb system and contributes only 10−15 to the optical carrier frequency instability at 1 s averaging time.

Allan deviation of the CEO frequency
Fig 1. (from Ref. Optics Express 19, 24171, 2011; Fig. 5b): Allan deviation of the CEO frequency in the ERGO comb and in a commercial self-referenced Er-fiber comb for comparison. The fractional stability is shown with respect to the CEO frequency fCEO = 20 MHz on the left scale and with respect to the optical frequency νN = 200 THz on the right scale.
Enlarged view: Stabilized CEO peak
Fig 2. (adapted from Ref. Optics Express 19, 24171, 2011; inset of Fig. 4): Coherent stabilized CEO peak from stabilized ERGO laser with only 5 kHz feedback bandwidth. One of the lowest residual integrated phase noise (0.72 rad rms in 0-100 kHz) for a 1.5-µm comb.