Download Microwave frequency generation using a dual

ACOFT/AOS 2006 – Proceedings
Melbourne, Australia, 10 – 13 July 2006
Microwave frequency generation using a dual-wavelength DBR fiber
laser
Shilpa Pradhan1, Graham E.Town1 and Ken J. Grant2
(1) Department of Electronics and Centre for Lasers & Applications, Macquarie University, NSW 2109,
Australia.
(2) Intelligence, Surveillance & Reconnaissance Division, DSTO West Avenue, Edinburgh, SA 5111,
Australia.
Abstract — We report the generation of microwave signals by
mixing a tunable semiconductor laser with a short-cavity dualwavelength distributed Bragg reflector (DBR) fiber laser. The
DBR laser generated two continuous-wave longitudinal modes
separated by 25GHz.
I.
INTRODUCTION
Dual-frequency lasers have many potential applications
such as soliton pulse train generation [1], heterodyne
interferometry for distance measurement [2], and optical
sensing [3]. Dual-mode lasers are also of interest for their
potential in generating radio-frequency signals for microwave
applications. Radio-frequency (RF) beat signal has been
demonstrated previously in dual wavelength lasers [4 -5].
Short-cavity DBR fiber lasers are attractive devices for these
applications due to their narrow linewidth, single longitudinal
mode operation, compact design and good reliability [6]. In
this paper we report dual-wavelength generation in a DBR
fiber laser at room temperature with 0.2 nm (approx. 25GHz)
wavelength separations between the two lasing modes, and
the simultaneous generation of two tunable microwave
signals by mixing the output of the DBR laser with a tunable
semiconductor diode laser.
II.
DBR fiber laser schematic
The output of the laser was passed through a 50:50coupler
and other end of the coupler were connected to a tunable
laser. The tunable laser was tuned to 1550.432 nm, i.e.
between the two DBR laser wavelengths, as shown in Fig. 2b.
EXPERIMENTAL SETUP
The DBR fiber laser consisted of 8 cm of highly doped
(80dB/m) erbium fiber with one end butt-coupled to a gold
mirror, and another end connected to a 12 cm dual-channel
Bragg grating comb-filter with free spectral range (FSR) 0.2
nm (25GHz) and reflectivity of 76%. The bandwidth of each
channel was 11pm.
The dual channel Bragg grating
determined the lasing wavelengths [7]. The length of the
amplifying fiber and the grating were chosen to ensure single
longitudinal mode lasing at each wavelength selected by the
grating [8].
The laser was pumped up to 136 mW by a 980 nm
semiconductor diode laser. The signal was coupled out via
980/1550 nm wavelength division multiplexer (WDM), with
a circulator to prevent back-reflection. A schematic of the
laser layout is shown in Fig. 1. The output was first measured
with an optical spectrum analyzer (OSA), as shown in Fig 2a.
There were two lasing wavelengths, at 1550.34 nm and
1550.54 nm.
ISBN 0 –9775657– 0 –X
Fig. 1.
Fig. 2a. Optical spectrum of dual-wavelength laser
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Fig. 2b. Optical spectrum including tunable laser
Fig. 4.
The combined optical output was detected using a 45 GHz
photodiode and monitored on a RF spectrum analyzer with
23GHz bandwidth. The beat signal between the two DBR
laser wavelengths could not be detected directly; however the
beat signals between each of the DBR laser wavelengths and
the tunable laser were detected. The two RF beat signals at
11.4 GHz and 13.8 GHz (sum 25.2GHz) shown in Fig. 3
confirmed only two longitudinal modes from the DBR laser.
Tuning the semiconductor diode laser tuned the two beat
frequencies in opposite directions.
Beat frequencies vs. pump power
III.
Conclusion
We have demonstrated simultaneous generation of two
tunable microwave signals using a dual-mode DBR fiber
laser. The RF beat frequencies could be tuned over a large
range by tuning the external laser source; however the sum of
the two RF frequencies was constant, even if the fiber laser
output wavelengths varied due to environmental changes.
ACKNOWLEDGEMENT
The authors wish to acknowledge the Redfern Optical
Component for supplying with gratings
REFERENCES
Fig. 3.
RF beat frequency spectrum
The frequencies of the two beat signals also varied slightly
due to environmental temperature variations, but could be
stabilized by isolating the laser from the latter perturbations.
We also measured the dependence of the beat frequencies on
laser pump power. It was found that increasing the 980 nm
pump power caused one beat signal to increase in frequency
whilst the other decreased at a rate of approximately 10
MHz/mW, as shown in Fig. 4. The latter may be explained by
simultaneous reduction in both lasing wavelengths associated
with a pump-dependent change in the refractive index and
temperature of the fiber amplifier.
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