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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 104 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. [1] S. V. Chernikov, J. R. Taylor, P. V. Mamyshev, and E.M. Dianov, “Generation of Soliton Pulse Train in Optical Fiber Using Two CW Single-mode Diode Lasers”, Electron. Lett. vol. 28, pp. 931, 1992 [2] E. Gelmini, U. Minoni, and F. Docchio, “Tunable, doublewavelength heterodyne detection interferometer for absolutedistance measurements”,Opt. Lett. vol. 19, pp.213, 1994 [3] O. Hadeler, E. Ronnekleiv, M. Ibsen and R. I. laming, “Polarimetric fiber distributed feedback laser sensor for simultaneous strain and temperature measurements”, Appl. Opt. vol. 38, pp.1953, 1999 [4] M. Alouini, M. Brunel, F. Bretenaker, M. Vallet and A. Le Floch, “Dual tunable wavelength Er :Yb : glass laser for terahertz beat frequency generation”, IEEE J. Photo. Technol. Lett, vol. 10, pp. 1554, 1998 [5] W. Wang, M. Cada, J. Seregelyi, S. Paquet, S. J. Mihailov and P. Lu, “A Beat-Frequency tunable dual-mode fiber-Bragggrating external-cavity laser”, IEEE J. Photo. Technol. Lett, vol. 17, pp.2436, 2005 [6] G. E. Town, S. Pradhan and K. Grant, “Dual wavelength DBR fibre laser”, Proceedings 29th Australian Conference on Optical Fibre Technology (ACOFT2004), Mon 10:45, Canberra, July 2004 [7] G. Bonfrate, F. Vaninetti and F. Negrisolo, “Single-frequency MOPA Er/sup 3+/ DBR fiber laser for WDM digital telecommunication system”, IEEE J. Photo. Technol. Lett., vol. 10, pp. 1109-11, 1998 [8] V. Jayaraman, Z. Chuang, and L. Coldren, IEEE J. Quant. Electron. vol. 29, pp.1824, 1993 105