- Generation of a broadband supercontinuum with Er3+-fiber oscillator and nonlinear fiber
- Difference frequency generation between the lower and upper spectral parts of the supercontinuum in a nonlinear crystal
- Both spectral parts have identical fCEO
- The resulting comb is carrier envelope phase stabilized and fCEO is fixed to zero
- TOPTICA patent (DE102004022037)
The operating principle of the offset-free Difference Frequency Comb (DFC) relies on generating a broadband supercontinuum from the output of a low noise Er-fiber mode-locked oscillator and subsequent optical difference frequency generation (DFG) between the low- and high-frequency parts of the octave spanning spectrum in a nonlinear crystal. The most important features are an improved stability and a more simple and reliable, all passive frequency offset stabilization. The comb is free from fluctuations of the offset-phase and offset-frequency due to the common mode suppression of the two parts of the original spectrum. Additionally, the carrier envelope offset frequency fCEO of the DFC is fixed to zero.
The passive all optical fCEO stabilization has distinct advantages with respect to the conventional f-to-2f-interferometer approach. This has been pointed out by several major players in the field of frequency metrology (Krauss et al., Opt. Lett, 36, 540 (2011)). The biggest advantage of the passive stabilization is that it is not limited by electronic noise. Additionally, no carrier envelope offset beat alignment or complicated drift compensation are required and the absence of carrier envelope offset locking electronics reduces the complexity of the electronics package significantly. The only adjustable parameter is the repetition rate of the DFC. It can easily be stabilized to suitable GPS-disciplined radio frequency (RF) references. Several user-defined output wavelengths of the DFC can be chosen to match the experimental situation.
The DFC is well-suited as frequency reference for atom-optics applications but also fulfills the high demands for frequency stability in frequency metrology and optical clock applications. It is especially suitable for use with an optical reference like a narrow linewidth laser. If stabilized to such an optical reference the frequencies of the comb lines are tightly locked at 0 Hz through the DFG process and at a high optical frequency by the optical reference. Here, the CERO technology is at a clear advantage over the conventional f-to-2f approach where high frequency noise components of fCEO cannot be canceled.
- DFG in the literature
TOPTICAs frequency comb DFC shows quadratic scaling of the phase noise with frequency. This textbook like behavior has been shown to a remarkable degree. The DFC behaves like an elastic tape with a true fixpoint at zero. An ideal system to be combined with an optical reference. Characterization of a DFG comb showing quadratic scaling of the phase noise with frequency, T. Puppe et al., Optics Letters Vol. 41, Issue 8 (2016)
“The DFG comb possesses a vanishing carrier envelope offset frequency that permits the construction of a simple and thus potentially more stable optical clockwork.“ Zimmermann, Gohle, Holzwarth, Udem and Hänsch, Opt. Lett., 29, 310 (2004)
“This all passive approach provides extremely reliable and rigid locking of fCEO on both short and long time scales”, Krauss, Fehrenbacher, Brida, Riek, Sell, Huber, and Leitenstorfer, Opt. Lett., 36, 540 (2011)
“… in this nonlinear process the phases of the two pulses add up with different signs, leading to CEP cancellation and resulting in passive, all-optical phase stabilization.”Manzoni, Cerullo, and De Silvestri, Opt. Lett., 29, 2668 (2004)
“The unique feature … is that fCEO of the DF is zero without any servo-loop feedback system; i.e., the waveform is self-stabilized in the resultant pulse train.” “The generated phase-stable pulses may - after preamplification - be suitable for seeding high-gain Ti:sapphire amplifiers and (or) the oscillator itself.”Fuji, Apolonski and Krausz, Opt. Lett., 29, 632 (2004)