We demonstrate optical frequency combs using the fluorite whispering gallery mode resonator as a nonlinear Kerr medium. Two regimes of generation are observed, giving the record low repetition rate of 13 GHz, equal to the cavity's free spectral range (FSR) or high repetition rates of multiples of cavity FSR. An intermediate regime was also observed. Raman lasing spectrum similar to modulation instability in fibers was observed for the first time to the best of our knowledge

Grudinin, Ivan S.

Maleki, Lute

Yu, Nan

English

Caltech Authors - Main

878 OPTICS LETTERS / Vol. 34, No. 7 / April 1, 2009Generation of optical frequency combs
with a CaF2 resonator
Ivan S. Grudinin,1,2,* Nan Yu,1 and Lute Maleki1
1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive,
Pasadena, California 91109, USA
2Current address: California Institute of Technology, 1200 East California Boulevard,
Pasadena, California 91125, USA
*Corresponding author: grudinin@caltech.edu
Received December 10, 2008; revised January 27, 2009; accepted January 28, 2009;
posted February 12, 2009 (Doc. ID 105172); published March 17, 2009
We demonstrate optical frequency combs using the fluorite whispering gallery mode resonator as a nonlinear
Kerr medium. Two regimes of generation are observed, giving the record low repetition rate of 13 GHz,
equal to the cavity’s free spectral range (FSR) or high repetition rates of multiples of cavity FSR. An inter-
mediate regime was also observed. Raman lasing spectrum similar to modulation instability in fibers was
observed for the first time to the best of our knowledge. © 2009 Optical Society of America
OCIS codes: 190.2620, 140.4780, 190.4380.Optical frequency combs are new and powerful spec-
troscopic and metrology tools [1,2]. Conventionally,
evenly spaced spectral components of a comb are gen-
erated by a mode-locked laser [3]. It was, however, re-
cently demonstrated that combs may also be gener-
ated by means of a four-wave-mixing process
enhanced in a high-Q whispering gallery mode reso-
nator (WGMR) made of silica [4]. Crystalline
WGMRs have a generally higher Q factor [5] and
may more easily be fabricated with a larger diameter,
providing frequency combs with repetition rates as
low as 24 GHz, which are easier to detect [6,7]. It has
been possible to control the repetition rate and offset
frequency of a WGMR-generated comb through opti-
cal power and other experimental parameters [8]. Di-
rect detection of the comb’s pulses is thus necessary
to realize fine tuning. While commercial frequency
combs have repetition rates as low as 0.1 GHz, the
high repetition rate of WGMR-based combs compli-
cates implementation of a practical generator.
In this Letter we demonstrate a monolithic optical
comb generator with the record low repetition rate of
13 GHz based on an optical resonator with the high-
est quality factor (as compared to previous combs) of
6109. We also show that the comb may be gener-
ated in full, selective, and mixed regimes, where the
comb components may have spectral separations of
multiples of the cavity free spectral range (FSR). The
selection of the generating regime in our experiments
appears to be defined by coupling conditions. In addi-
tion, we present what we believe to be the first obser-
vation of a Raman Stokes spectrum suggestive of
modulation instability.
We used an excimer grade calcium fluorite CaF2
from Corning to fabricate a WGMR that is 5 mm in
diameter and has a 0.1 mm perimeter curvature ra-
dius. The resonator axis is coincident with the crys-
talline [111] direction. We used diamond abrasives to
shape and polish the resonator. A very simple setup
was used to pump the fluorite resonator with up to
50 mW of optical power at the wavelength of
1560 nm. The Koheras Adjustik fiber laser with
0146-9592/09/070878-3/$15.00 ©kilohertz-range linewidth was used. As schematically
shown in Fig. 1, the resonator transmission was
monitored by a Thorlabs DET10c photodetector and
by a Yokogawa AQ6319 spectrum analyzer. Angle-
polished single-mode fiber couplers mounted onto
Thorlabs Nanomax stages were used to guide light in
and out of the resonator with a coupling efficiency of
up to 80%. The setup was enclosed in a dust-free en-
vironment that also provided good temperature sta-
bility. No direct locking of the laser to a whispering
gallery mode (WGM) was employed in this work.
Each spectrum was generated by scanning the laser
frequency in the vicinity of the WGM having the best
coupling efficiency among other modes. The laser fre-
quency was typically scanned through several widths
of the WGM resonance by application of a linearly
changing voltage to the laser’s controller. The WGM
profiles were highly nonlinear at the power levels in-
volved in this work. The periodic excitation of a WGM
leads to heating of the resonator and to shifting of its
spectrum. The laser was manually tuned until the
heating of the cavity was balanced by convective and
diffusive cooling. This arrangement may be called
“thermal lock” and has been successfully used by
other researchers [8,9]. To obtain the Q factor of a
mode the width of the resonance was measured at
low pump power, where the resonator’s frequency re-
sponse is a Lorentzian. The intrinsic Q factor of our
resonator was estimated to be 6±1109 and half as
much for the measurement conditions where the
resonator was close to being critically coupled.
When optical power concentrated in a WGM is high
enough, the hyperparametric oscillations mediated
by the Kerr nonlinearity [10,11] generate optical
Fig. 1. Experimental scheme. FC, input and output angle
polished fiber couplers; WGMR, fluorite resonator.
2009 Optical Society of America
April 1, 2009 / Vol. 34, No. 7 / OPTICS LETTERS 879sidebands in the resonator. These sidebands are gen-
erally located at multiples of cavity FSR away from
the pump frequency. A cascaded four-wave-mixing
process leads to generation of the wide spectrum of
evenly spaced components, as shown in Fig. 2. We
also changed the pump power level and the coupling
conditions, which led to observation of three interme-
diate regimes of spectral comb generation. It was pre-
viously observed [6] that a comb may be generated at
frequency intervals corresponding to multiples of the
FSR. Here we observe combs with repetition rates of
mFSR, where m can be 1 or some whole number,
but also a combination when two combs are gener-
ated simultaneously. Figure 2 shows generation for
m=1. The comb spacing was used to estimate the
resonator diameter D from FSR=c /nD. Figure 3 is
an example of the regime where the system selects
specific multiples of FSR as preferential for sideband
generation. The spectrum’s energy distribution
around the carrier is uneven. Weak, asymmetrical
generation at the FSR offset is still present around
the pump and sidebands. This regime is interesting,
as it provides a single repetition rate comb with fre-
quencies extending into hundreds of gigahertz. Fi-
nally, Fig. 4 shows an example of mixed generation,
where the frequency comb is generated for m=1 and
26 simultaneously. Recent work [6] has emphasized
that it is possible to lock the laser to the cavity mode
and control selective generation of combs for different
multiples of FSR by changing the laser detuning
from the cavity WGM center. However, no direct lock
was implemented in this work, and the only differ-
ence between the circumstances under which the
spectra were obtained were the coupling conditions
and small variations in the pump power level. It is
unclear why these particular offsets are enhanced
and what defines them. We speculate that in this
regime coupling to more than one WGM is realized,
leading to interference of various comb components.
A monolithic comb generator based on a single-
Fig. 2. Optical frequency comb generated in a fluorite
resonator. Comb lines are spaced 0.112 nm apart, which
translates into cavity FSR of 13.81 GHz and resonator di-
ameter of 4.85 mm. The input optical power at 1560 nm is
25 mW.mode resonator geometry [12] may resolve these
complications.
The Raman-active phonon branch of fluorite has a
wave vector k=2322 cm−1, which for the 1560 nm
pump corresponds to the Stokes component at
1643 nm and the anti-Stokes at 1485 nm. While the
Stokes components have been extensively observed
[13,14], to our knowledge no evidence of the anti-
Stokes lasing has ever been seen in WGMRs. In ther-
mal equilibrium the population of the excited vibra-
tional level responsible for Raman transition is less
than the ground level population by a Boltzmann fac-
tor of exp−R/kT0.21, where R is the Raman
frequency shift in fluorite. As long as there is no
population inversion between the levels involved in
Raman transition, the anti-Stokes beam will experi-
ence damping. In our experiments, generation of a
comb was accompanied by Raman Stokes generation.
Under high pump power, some weak anti-Stokes
components are visible, which may be explained by
nonlinear four-wave mixing between pump and Ra-
man beams. Under high pump power the Raman
spectrum has developed the sidebands that were not
previously observed in WGMRs. One possible expla-
Fig. 3. Frequency comb generated by a fluorite resonator
in a selective regime. Generation is enhanced at frequency
offsets equal to 15 cavity FSRs.
Fig. 4. Frequency comb generated in a mixed regime. Be-
sides components regularly spaced by cavity FSR, selec-
tively enhanced generation is observed at 26 cavity FSRs.
880 OPTICS LETTERS / Vol. 34, No. 7 / April 1, 2009nation for the spectrum presented in Fig. 5 is that
the intensity of Stokes components was strong
enough to generate its own spectral comb. However,
the intensity envelope of this comb is significantly
different from that generated by the pump laser. The
spectrum resembles that of modulation instability
[15], which appears because of the cross-phase modu-
lation between the pump and the Raman beams [16].
Cavity dispersion, or change in FSR over adjacent
optical modes, has two contributions [4,17]. Geo-
metrical dispersion is normal and for our cavity is
FSR= m+1−m− m−m−1−600 Hz at 1550 nm.
The material dispersion changes sign near 1550 nm,
as can be found from a four-term Sellmeier equation.
The total cavity dispersion remains smaller than op-
tical mode bandwidth 64 kHz allowing for genera-
tion of a spectral comb. It is worth noting, however,
that under strong optical pump conditions, the cavity
dispersion will be modified by “mode pulling” effects
owing to cross- and self-phase modulation. In addi-
tion, studies of dispersion in real fluorite cavities [18]
show that significant deviations from theory may
take place owing to impurities.
In conclusion, we have demonstrated generation of
optical frequency combs in a fluorite WGMR in selec-
tive and nonselective regimes and also in a combined
regime. We have demonstrated the lowest repetition
rate of a WGMR-based comb of 13 GHz generated
with the WGMs having the highest Q factor of 6
109 ever used for this purpose. Our observations
Fig. 5. Intensity envelope spectrum recorded by the peri-
odic excitation of the WGM. Raman lasing Stokes intensity
spectrum exhibits two sidebands at 1635 and 1650 nm.
Pump power is 20 mW. The inset shows the resonator with
two fiber couplers. The sidebands may be explained by
modulation instability.suggest that selection of generation regimes is deter-mined by coupling conditions and pump power val-
ues. The low repetition rate of our comb in the non-
selective regime 13 GHz makes it easier to measure
and control in practical applications, and represents
an important step toward a compact monolithic fre-
quency comb generator. Higher frequencies gener-
ated in the selective regime may also be useful for op-
tical generations of microwave beatnotes. We have
also observed the Raman Stokes spectrum suggestive
of a modulation instability process.
We are grateful to D. Strekalov for help with mea-
surements and to A. Matsko for helpful discussions
and suggestions. This work was performed at the Jet
Propulsion Laboratory, California Institute of Tech-
nology, under a contract with NASA and with support
from the Defense Advanced Research Projects Agency
(DARPA).
References
1. Th. Udem, R. Holzwarth, and T. W. Haensch, Nature
416, 233 (2002).
2. S. T. Cundiff and J. Ye, Rev. Mod. Phys. 75, 325 (2003).
3. J. D. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R.
S. Windeler, J. L. Hall, and S. T. Cundiff, Science 288,
635 (2000).
4. P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R.
Holzwarth, and T. J. Kippenberg, Nature 450, 1214
(2007).
5. A. A. Savchenkov, A. B. Matsko, V. S. Ilchenko, and L.
Maleki, Opt. Express 15, 6768 (2007).
6. A. A. Savchenkov, A. B. Matsko, V. S. Ilchenko, I.
Solomatine, D. Seidel, and L. Maleki, Phys. Rev. Lett.
101, 093902 (2008).
7. D. V. Strekalov and N. Yu, arXiv:0807.1909v1.
8. P. Del’Haye, O. Arcizet, A. Schliesser, R. Holzwarth,
and T. J. Kippenberg, Phys. Rev. Lett. 101, 053903
(2008).
9. T. Carmon, L. Yang, and K. J. Vahala, Opt. Express 12,
4742 (2004).
10. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala,
Phys. Rev. Lett. 93, 083904 (2004).
11. A. A. Savchenkov, A. B. Matsko, D. Strekalov, M.
Mohageg, V. S. Ilchenko, and L. Maleki, Phys. Rev.
Lett. 93, 243905 (2004).
12. A. A. Savchenkov, I. S. Grudinin, A. B. Matsko, D.
Strekalov, M. Mohageg, V. S. Ilchenko, and L. Maleki,
Opt. Lett. 31, 1313 (2006).
13. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala,
Nature 415, 621 (2002).
14. I. S. Grudinin and L. Maleki, Opt. Lett. 32, 166 (2007).
15. S. Pitois and G. Millot, Opt. Commun. 226, 415 (2003).
16. G. P. Agrawal, P. L. Baldeck, and R. R. Alfano, Phys.
Rev. A 39, 3406 (1989).
17. L. Maleki, A. A. Savchenkov, V. S. Ilchenko, and A. B.
Matsko, Proc. SPIE 5104, 1 (2003).
18. A. A. Savchenkov, E. Rubiola, A. B. Matsko, V. S.Ilchenko, and L. Maleki, Opt. Express 16, 4130 (2008).


Generation of optical frequency combs with a CaF2 resonator

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Generation of optical frequency combs with a CaF2 resonator

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