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Objective
EDF ring lasers
usually suffer from multimode oscillation, mode hopping and
competing due to their long cavity length and thus narrow
longitudinal mode spacing. Therefore narrow-bandwidth mode
selecting mechanism is required to ensure SLM operation, and
the mode selecting mechanism should also be
wavelength-tunable over a wide spectral range. We developed
an simpler but efficient way to achieve wavelength-swept
single-longitudinal-mode EDF ring laser.
Introduction
Tunable and wavelength-swept
single-longitudinal-mode (SLM) lasers have many important
applications such as high-resolution spectroscopy, gas
sensors, WDM optical communication technology, and swept
source Fourier domain optical coherent tomography. Tunable
SLM erbium-doped fiber (EDF) ring lasers are good candidates
for those applications due to their broad tuning range, high
output power, and narrow linewidth. However, EDF ring lasers
usually suffer from multimode oscillation, mode hopping and
competing due to their long cavity length and thus narrow
longitudinal mode spacing. Therefore narrow-bandwidth mode
selecting mechanism is required to ensure SLM operation, and
the mode selecting mechanism should also be
wavelength-tunable over a wide spectral range. Various
schemes have been demonstrated to show both SLM and
tunability simultaneously, for example, using such schemes
as a multi-ring cavity with a band pass filter, a tunable
fiber Bragg grating (FBG) Fabry-Perot etalon, and a
saturable absorber with a tunable FBG. It has been shown in
prior works that a section of unpumped EDF in a Sagnac loop
can be used as a saturable absorber in which two
counter-propagating waves form a standing wave and induce
spatial-hole-burning (SHB). The refraction index of the
unpumped EDF changes spatially due to SHB and this results
in an ultra-narrow bandwidth self-induced FBG.
In this work, a Sagnac loop with a 3.5-meter unpumped EDF is
used as a self-induced FBG filter, which dynamically tracks
the wavelength tuning of a fiber Fabry-Perot tunable filter
(FFP-TF). Theoretical modeling brought forth valid SLM
condition and then a series of experiments proved SLM laser
operation. The laser can be wavelength-swept over the entire
C-band (1520nm-1570nm) window with linewidth less than 0.7
kHz. The optimum sweeping frequency in order to achieve the
best output power stability was experimentally found to be
20 Hz with swept-induced high frequency power fluctuation of
only 0.1%.
Experimental setup and results
Fig. 1
Schematic layout of the tunable SLM EDF ring laser
Fig. 2. (a) OSA
measured output spectrum of the laser under the pump power
of 100mW. (b) Measured output laser power as a function of
the 980nm LD input pump power. (c) RF spectrum of delayed
self-heterodyne signal.
Fig.
3. Noise spectrum of output laser (a) with 3.5-meter EDF and
(b) with 0.3-meter EDF.
Fig. 4. (a) Measured
output spectrum at fixed wavelength of 1548.45nm every 3
minutes for 60 minutes. (b) Fluctuation of wavelength during
60 minutes.
Fig.
5. (a)~(c) Output power intensity as function of wavelength
at sweeping frequency of 0.2Hz, 20 Hz and 100Hz
respectively. (d) RMS of output power as a function of
sweeping frequency. (e) Noise spectrum of output laser at
sweeping frequency of 20 Hz. (f) Peak-hold (persistence
mode) spectrums of the lasing modes when triangle waveforms
are applied to the FFP-TF, with frequencies of 2.0 kHz
(solid line) and 3.0 kHz (dot line) respectively.
References
K. Zhang and J. U.
Kang, "C-band wavelength-swept single-longitudinalmode
erbium-doped fiber ring laser,"
Opt. Express 16, 14173-14179
(2008)
http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-18-14173 |
