Comb stabilization

Prof. Keller’s group, in collaboration with Dr. Telle (PTB, Braunschweig), introduced and made first feasibility demonstrations for several novel techniques to measure and stabilize the CEO phase (or shorter also referred to as CEP) fluctuations in the frequency domain using the frequency comb generated by the modelocked oscillator. For example Keller and Telle proposed both the f-to-2f and the 2f-to-3f heterodyne technique which are being used today.

First paper in 1999: Carrier-envelope offset phase control: A novel concept for absolute optical frequency measurement and ultrashort pulse generation: Download Ref. [113] (PDF, 258 KB)

Our pioneering contribution to this field was also recognized by the Nobel committee for the Physics Nobel Prize in 2005 (see page 11, Ref. 37, "Advanced Information" for the Nobel Prize in Physics 2005,
Download Advance information for Physics Nobel Prize 2005

Enlarged view: comb stabilization — Principle frequency comb stabilization: CEO beat signal is detected with an f-to-2f interferometer technique (i.e. direct SHG/DFG in Ref. 113). This CEO beat signal is used to stabilize the frequency comb offset (which is typically controlled with the pump power of the modelocked laser oscillator). The frequency comb spacing is stabilized with the cavity length (i.e. timing jitter)

Principle of CEO beat signal detection in an f-to-2f interferometer:
The harmonic generated (SHG) frequency comb is overlapping with the fundamental for an octave-spanning spectrum. The frequency beat
2f1 −f2 (i.e. difference frequency generation, DFG) between the fundamental f1 and the SHG f2 results in the carrier-envelope frequency beat signal:

This CEO frequency signal is clearly detectable with simple photodetector and a microwave spectrum analyzer (according to Download Ref. [113] (PDF, 258 KB) & Download Ref. [300] review (PDF, 750 KB))
Both intensity and beam pointing fluctuations in the laser can introduce CEP changes. We have discussed the physical origin of the CEP fluctuations in passively modelocked Ti:sapphire lasers and optimized the laser cavity to stabilize the carrier envelope-phase with a long-term residual jitter corresponding to only 10 attoseconds in a (100 kHz – 0.01 Hz) bandwidth.

Carrier-envelope offset dynamics and stabilization of femtosecond pulses:
Download Ref. [167] (PDF, 221 KB)

Carrier-envelope offset phase-locking with attosecond timing jitter:
Download Ref. [188] (PDF, 695 KB)

CEP-controlled pulses in the few-optical-cycle regime have had a major impact in many new research areas ranging from precision frequency metrology to high field physics and attosecond pulse generation.

Frequency comb stabilization – a historical perspective: two different research communities that traditionally did not interact much, solved an important problem in their specific fields with a cross-disciplinary approach based on the frequency comb.
For a personal review see Section V in Download Ref. [300] (PDF, 750 KB)