An all-fiber, single-frequency, erbium-doped ring laser has been frequency locked to a resonance peak of an external fiber Fabry-Perot resonator by the Pound-Drever technique. In addition, feedback to the mode selection filter in the laser resonator eliminates occasional mode hopping completely, resulting in frequency-locked, stable, single-frequency operation of the laser for periods of several hours

Dawson, Jay W.

Park, Namkyoo

Vahala, Kerry J.

English

Caltech Authors - Main

June 1, 1993 / Vol. 18, No. 11 / OPTICS LETTERS 879
Frequency locking of an erbium-doped fiber ring laser to an
external fiber Fabry-Perot resonator
Namkyoo Park, Jay W. Dawson, and Kerry J. Vahala
Department of Applied Physics, Mail Stop 128-95, California Institute of Technology, Pasadena, California 91125
Received December 23, 1992
An all-fiber, single-frequency, erbium-doped ring laser has been frequency locked to a resonance peak of an
external fiber Fabry-Perot resonator by the Pound-Drever technique. In addition, feedback to the mode selection
filter in the laser resonator eliminates occasional mode hopping completely, resulting in frequency-locked, stable,
single-frequency operation of the laser for periods of several hours.
There have been several demonstrations of fre-
quency stabilized lasers operating in the important
1.5-,ttm telecommunications window. Techniques in-
clude absolute frequency stabilization using atomic
lines' and frequency locking to the resonance peak
of a reference cavity using either the transmitted
laser field2 or the reflected laser field.3'4 For appli-
cation to wavelength-division-multiplexed systems,
the Pound-Drever approach (i.e., the reflected field
method) is attractive since it provides evenly spaced
multiple resonance peaks that can be shared by
several lasers and since the reflected wave approach
provides response to laser phase fluctuations that is
not limited by the reference resonator lifetime.3'4
We have recently demonstrated an all-fiber erbium-
doped ring laser whose characteristics include
quantum-limited intensity noise operations narrow
linewidth (<4 kHz),6 large sidemode suppression
(>60 dB), and large tuning range.' This device is
based on a novel tandem fiber Fabry-Perot (FFP) con-
cept in which a first, narrow-bandwidth Fabry-Perot
(free spectral range 6 GHz, bandwidth 50 MHz)
provides mode selection while a second, broad-
band device (free spectral range 4 THz, bandwidth
26 GHz) provides tuning (see Ref. 8 for a discussion
of FFP technology). Although it is quite stable, this
device does exhibit periodic mode hops on a time
scale of minutes and experiences slow frequency
shifts because of thermal drift of the resonator. In
this Letter, we demonstrate an all-fiber stabilization
scheme based on the Pound-Drever approach, which
eliminates mode hopping altogether and locks the
lasing frequency to an external FFP resonator. The
resulting device is stable for periods of several hours
and opens up the possibility of locking multiple fiber
ring lasers to a single reference. Such a system is
inherently compatible with optical fiber and could be
of potential use in a fiber link employing wavelength-
division multiplexing, in a fiber sensing network, or
as a spectroscopic source.
The laser setup and the electrical schematic for
the error signal generation are shown in Fig. 1. The
laser includes broadband and narrow-band FFP's for
tuning and mode selection, a Corning FiberGain mod-
ule with a 980-nm pump diode, polarizers and a phase
modulator with a polarization controller, metal-clad
fiber (MCF) for laser cavity-length control, and iso-
lators for unidirectional operation and isolation from
external reflections. All the components were fiber
pigtailed and assembled by using either a mechani-
cal splicer or a fusion splicer. Lasing emission was
coupled out by a 3-dB fiber coupler. The laser cav-
ity length was 65 m (free spectral range 3 MHz).
The laser was housed in a styrofoam box to provide
shielding from room microphonics, which cause mode
hopping.
The stabilization system consists of two indepen-
dent control circuits. The first causes the internal
mode-selection FFP filter to track the lasing fre-
quency as it drifts or shifts owing to tracking provided
by the second control circuit. This first circuit elim-
inates mode hopping. The second circuit causes the
lasing frequency to track a particular resonance of
the external FFP reference. Both circuits utilize the
Pound-Drever method.
For the first circuit, we obtained the frequency
deviation error signal between the lasing mode and
300 MHz
FFP
(Ref)
OUTPUT
Fig. 1. Erbium-doped fiber ring laser with stabilization
setup. ISO, isolator; MS, mode selection; BB, broadband;
PC, polarization controller; D1, D2, photodiodes.
0146-9592/93/110879-03$6.00/0 © 1993 Optical Society of America
880 OPTICS LETTERS / Vol. 18, No. 11 / June 1, 1993
the internal mode-selection FFP filter by placing
an electro-optic phase modulator inside the ring,
immediately before the output coupler and inter-
nal mode-selection FFP filter as illustrated. The
phase modulator was driven at 300 MHz. At this
frequency the phase modulation sidebands were well
outside the FFP bandwidth of 50 MHz, ensuring
nearly full reflection of the sidebands. Full reflec-
tion also prevents any possibility of unintentional
mode locking by the phase modulator. The fully
reflected sidebands of the lasing field and the
partially reflected lasing field were coupled out of
the ring by the bidirectional coupler as illustrated
and then photomixed at the photodiode (D1). The
magnitude and the phase of the reflected lasing field
depend on the frequency deviation of the reflected
field relative to the transmission peak of the FFP
cavity, so that the demodulated photomix signal at
the mixer output gives the sign and the magnitude
of the deviation.3 Radio-frequency bandpass filters
were employed before the mixer to improve the
signal-to-noise ratio. Frequency tracking was then
attained by processing this error signal through a
proportional-integral-differential control circuit and
feeding it back to the controlling voltage of the mode-
selection FFP filter.
Figure 2(a) shows the error signal obtained with
and without the feedback loop in operation. With-
out the feedback (lower curve), the acoustically un-
shielded laser gives frequent mode hops, resulting in
an error signal of -10 MHz (corresponding to 3-4
longitudinal modes). Once feedback is engaged, a
significant reduction of the error signal is apparent.
We confirmed the resulting mode-hop-free operation
of the laser by monitoring the lasing spectrum, us-
ing a Newport Super-Cavity scanning interferometer.
Without any acoustical shielding of the laser and with
introduction of intentional acoustic disturbances, no
mode hops were observed over periods of several
hours.
To lock the laser frequency to the external FFP
cavity, an additional 3-dB fiber coupler and the ref-
erence FFP cavity (closely matched with the internal
mode-selection FFP) were added after the output port
of the ring laser as illustrated. Even though the
laser output was modulated at 300 MHz, after the
external 50-MHz-bandwidth FFP the sidebands at
300 MHz were 20 dB smaller than the main carrier.
The partly reflected field from the reference FFP cav-
ity was then detected at photodiode D2, processed as
described for the first servo loop, and then converted
to a current used to resistively heat the MCF (1.5 m
long, 2-4 resistance) making up part of the laser ring.
This has the effect of controlling the overall laser ring
optical path length. The tuning range possible by
resistive heating alone was approximately 200 MHz
with a corresponding maximum current of 500 mA.
This range and the MCF response time of 4 ms were
sufficient to compensate for the relative drift of the
external reference FFP cavity and laser frequency
caused by thermal variations.
Once the laser frequency was locked, it was possible
to observe laser wavelength tracking of the transmis-
sion peak of the external reference FFP cavity, either
by observing the optical spectrum, using the Super-
Cavity scanning interferometer, or by monitoring the
applied voltage on the mode-selection FFP filter in-
side the cavity. Within the limited tracking range of
200 MHz, owing to the limited applied current on the
MCF, the laser wavelength varied linearly with the
external FFP tuning voltage. Figure 2(b) shows the
error signal obtained from the external FFP cavity
servo loop with and without the feedback loop en-
gaged. Error signal reduction is again apparent for
the case of the loop engaged.
To summarize, we have locked the lasing mode of
a single-frequency, erbium-doped fiber ring laser to
an external FFP cavity by implementing the Pound-
Drever technique. Limited tracking of 200 MHz was
possible as we tuned the external reference FFP
cavity. An additional tracking circuit was also im-
plemented to control the internal Fabry-Perot mode-
selection filter, thereby eliminating mode hopping
completely. The complete system attains stable,
frequency-locked operation of the laser for several
hours. Further improvements to the system will
include the locking of multiple lasers to the same
reference FFP cavity and the possible introduction of
C
0
a)
U
c
r
a,
U-
C
U_
C
a,
..
LL
0 2 4 6 8 1(
Time ( minutes )
I I I !
- (b)
I 50 MHz
0 2 4 6
Time ( minutes
8 10
Fig. 2. (a) Error signals from the internal mode-selection
filter locking circuit: upper curve, with feedback; lower
curve, without feedback. (b) Error signals from the ex-
ternal reference cavity locking circuit with feedback acti-
vated on the internal mode-selection filter: upper curve,
with feedback; lower curve, without feedback.
- (a)
20 MHz
W _A~u~M~d
D
June 1, 1993 / Vol. 18, No. 11 / OPTICS LETTERS
an absolute frequency reference based on an atomic
reference.
The authors thank Northrop Corporation and the
U.S. Office of Naval Research (contract N00014-91-
J-1524) for their support.
References
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2. W. Vassen, C. Zimmermann, R. Kallenbach, and T. W.
Hdnsch, Opt. Commun. 75, 435 (1990).
3. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough,
G. M. Ford, A. J. Munley, and H. Ward, Appl. Phys. B.
31, 97 (1983).
4. T. Day, E. K. Gustafson, and R. L. Byer, IEEE J.
Quantum Electron. 28, 1106 (1992).
5. S. Sanders, N. K. Park, J. W. Dawson, and K. J. Vahala,
Appl. Phys. Lett. 61, 1889 (1992).
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Vahala, Appl. Phys. Lett. 53, 2369 (1991).
8. C. M. Miller and F. J. Janniello, Electron. Lett. 26,2122
(1990).
881


Frequency locking of an erbium-doped fiber ring laser to an external fiber Fabry-Perot resonator

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Frequency locking of an erbium-doped fiber ring laser to an external fiber Fabry-Perot resonator

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