May 08 2013

10.05.2013 Talk of Felix Rohde in the Ctrl-Q seminar

Felix Rohde (PTB Braunschweig) gives a talk in the Ctrl-Q seminar on "Characterization of the tuning properties of single-frequency optical frequency synthesizers" in room E.11 at 09:30.

Precise and agile manipulation of frequency and phase of output
signals of single-frequency optical frequency synthesizers based on a novel
technique for frequency shifting of frequency combs is investigated by
relative measurements between identical synthesizers.

Optical frequency comb generators based on mode locked lasers, common in
precision-laser spectroscopy and metrology, are often referred to as
optical frequency synthesizers (OFS) [1]. They provide a broadband
frequency grid with fixed frequency and phase relations between the
individual comb lines. In analogy to an electrical synthesizer, a
single-frequency OFS provides a single-frequency optical field whose
frequency and phase can be arbitrarily adjusted within a certain tuning
range and resolution while they are related to a reference signal in a
phase-coherent fashion. The most advanced single-frequency OFS approaches
up to date are based on optical frequency combs combined with phase-locking
of a single-frequency "clean-up" laser to an individual comb line [2-4].
These implementations suffer from an ambiguity at those optical
frequencies, where the frequency of the beat-note from a specific comb line
becomes zero or coincides with that from an adjacent comb line, which poses
limitations on the agility of the OFS and in the worst case can lead to a
loss of the phase lock.
In this talk I will present the first characterization of a
single-frequency OFS based on a novel technique for frequency shifting
lines of an optical frequency comb [5]. The method is based on optical
carrier phase shifting between subsequent pulses emitted by the comb
generator, which corresponds to shifting of the carrier frequency. This
approach does not require any intrusion into the comb generator. Using a
single line selector, which selects one of the frequency-shifted comb lines
enables the implementation of a single-frequency OFS with outstanding
degree of agile tunability (10 THz/s), wide optical bandwidth (many THz)
and high resolution while it circumvents the previously mentioned
I will discuss advantages and drawbacks of different types of single-line
selectors, such as phase locking of clean up lasers, selective
amplification by stimulated Brillouin scattering or feed forward schemes,
with respect to our frequency shifting method. Moreover, we demonstrate the
ability to precisely manipulate the frequency and phase of our
single-frequency OFS by comparison of two independent synthesizers sharing
the same frequency comb generator. Thus, frequency and phase fluctuations
of this ?master oscillator? are ruled out via common-mode rejection.


[1] R. Holzwarth, Th. Udem, T.W. Hänsch, J. C. Knight, W. J. Wadsworth, and
P.St.J. Russell, ?Optical Frequency Synthesizer for Precision
Spectroscopy,? Phys. Rev. Lett. 85, 2264-2267 (2000).

[2] J.D. Jost, J.L. Hall, and J. Ye, ?Continuously tunable, precise, single
frequency optical signal generator,? Opt. Expr. 10, 515-520 (2002).

[3] T.R. Schibli, K. Minoshima, F.-L. Hong, H. Inaba, Y. Bitou, A. Onae,
and H. Matsumoto, ?Phase-locked widely tunable optical single-frequency
generator based on a femtosecond comb,? Opt. Lett. 30, 2323-2325 (2005).

[4] V. Ahtee, M. Merimaa, and K. Nyholm, ?Single-frequency synthesis at
telecommunication wavelengths,? Opt. Expr. 17, 4890-4896 (2009).

[5] E. Benkler, F. Rohde, and H. R. Telle, "Endless frequency shifting of
optical frequency comb lines", Opt. Expr. 21, Issue 5, pp. 5793-5802 (2013)

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