JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 20, OCTOBER 15, 2009 4455 Dual-Wavelength Single-Longitudinal-Mode Polarization-Maintaining Fiber Laser and Its Application in Microwave Generation Weisheng Liu, Meng Jiang, Daru Chen, and Sailing He, Senior Member, IEEE Abstract A novel approach for generating high-frequency microwave signals is proposed and experimentally demonstrated. With a pair of wavelength matched fiber Bragg gratings written directly in a polarization-maintaining erbium-doped fiber, a stable short cavity dual-wavelength single-longitudinal-mode (DW-SLM) distributed-bragg-reflector fiber laser with orthogonal oscillation modes is realized at room temperature. The wavelength spacing between the two lasing modes is 0.374 nm. By heterodyning the two wavelengths of the DW-SLM fiber laser at a photodetector, microwave signal at over 46 GHz is achieved. Index Terms Fiber laser, microwave generation, polarizationmaintaining, single-longitudinal-mode. I. INTRODUCTION P HOTONIC generation of microwave signals has attracted much research interest due to their great application potentials on radio-over-fiber networks, broadband wireless access, radar, sensor networks and so on [1] [4]. Compared with electronic solutions, photonic generation of microwave has a lot of advantages such as free of speed limitations (caused by electronic elements), low power consumption, low cost and high reliability [5]. Several techniques have been developed to meet this challenge and optical heterodyning has been considered to be an effective and promising approach for photonic generation of microwave signals. One of the optical heterodyning techniques is to beat two laser beams from two different laser sources at a photodetector (PD). The main advantage of this approach is that the wavelength and power are easy to control since the two laser sources are independent. However, in order to generate high Manuscript received December 14, 2008; revised April 22, 2009. First published June 02, 2009; current version published August 28, 2009. This work was supported in part by the National Natural Science Foundation of China (Grant 60707020) and in part by the Science and Technology Department of Zhejiang Province of China (Grant 2007C21159). W. Liu and M. Jiang are with the Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou, 310058, China (e-mail: wsliu@coer. zju.edu.cn; jiangmeng@coer.zju.edu.cn). D. Chen is with the Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou, 310058, China, and also with the Joint Research Laboratory of Optics of Zhejiang Normal University and Zhejiang University (e-mail: daru@coer.zju.edu.cn). S. He is with the Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou, 310058, China, and also with the Division of Electromagnetic Engineering, School of Electrical Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden. (e-mail: sailing@kth.se). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2009.2024172 quality microwave signals with low phase noise and high stability, the phases of the two laser sources must be locked [6], [7] which is difficulty to realize in many applications. Another optical heterodyning technique for photonic generation of microwave signals is beating of a dual-wavelength single-longitudinal-mode (DW-SLM) fiber laser. With this approach, the two beating laser modes share a common cavity and gain medium, so that most noise processes which are originated with the same laser cavity and gain can be canceled out. To realized SLM oscillation, ultra-narrow filter ( ) is usually required. Chen et al. developed an ultra-narrow bandpass filter with two transmission peaks using an equivalent phase-shifted fiber Bragg grating (FBG) [8], [9]. Another ultra-narrow filter based on high finesse Fabry-Perot etalon formed by FBG pair has also been used in SLM laser and microwave signals generation recently [10], [11]. However, these ultra-narrow bandpass filters must be specially designed and difficult to fabricate especially for generating microwave signals with frequency higher than 40 GHz. In addition, because of strong homogeneous broadening in the erbium-doped fiber (EDF), these schemes do not work stably for DW-SLM oscillation at room temperature. Hence, a semiconductor optical amplifier is usually needed. In this paper, we proposed and demonstrated a DW-SLM distributed-bragg-reflector (DBR) fiber laser with orthogonal linear polarizations by using polarization-maintaining erbiumdoped fiber (PM-EDF). Due to the polarization hole burning (PHB) effect and spatial-hole burning (SHB) effect, stable DW oscillation with wavelength spacing of 0.374 nm at room temperature is achieved. By beating the two wavelengths of the laser at a PD, microwave signals at a frequency of more than 46 GHz can be generated, which is measured by an electrical spectrum analyzer (ESA) with a measurement range up to 25.65 GHz through frequency up/down conversion technique. The optical signal-to-noise ratio (OSNR) of the DW-SLM fiber laser and the signal-to-noise ratio (SNR) of the generated microwave signal are db and over 35 db, respectively. Additionally, the reflectivity of each polarization mode of the PM-FBG is exactly measured for the first time with a homemade linear-polarized light source in this paper. II. FABRICATION AND OPERATION OF SLM FIBER LASER The cavity of the DW-SLM fiber laser proposed consists of a pair of wavelength matched FBGs written on a segment of PM-EDF with high erbium concentration (with a peak absorption of 55 db/m at 1530 nm), as shown in Fig. 1. 0733-8724/$26.00 2009 IEEE
4456 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 20, OCTOBER 15, 2009 Fig. 1. Configuration of the DW-SLM distributed-bragg-reflector fiber laser. WDM is wavelength division multiplexer; SMF is single mode fiber. The Bragg wavelength of the FBG is (1) where is the effective index of the fiber core, is the period of the FBG. Since the refractive indexes along the fast and slow axes of the polarization-maintaining fiber (PMF) are different, an FBG written on a PMF exhibits two reflection peaks with orthogonal polarization modes. The Bragg wavelength difference between the two polarization modes of the PM-FBG is where ( are the modal refractive indexes) is the birefringence of the PMF, is the period of the PM-FBG. As shown in Fig. 1, two PM-FBGs as well as the PM-EDF between them composes the laser cavity. The length, peak reflectivity, 3 db bandwidth of PM-FBG1 is 12 mm,, and for both polarization modes, while 8 mm, and for both polarization modes of PM-FBG2, respectively. The interval between the two PM-FBGs is 10 mm. The PM-FBGs here act both as cavity mirrors and mode discriminators introducing different losses (by different reflectivity) to different modes within the same polarization state. By presetting the reflectivity and bandwidth of the PM-FBGs and the cavity length of the laser, only the dominant mode is allowed to operate above the lasing threshold [12]. Since the two lasing wavelengths corresponding to the two Bragg wavelengths are in two linearly orthogonal polarization modes, the PHB effect is enhanced in the cavity [13]. In addition, the round-trip loss of the short linear cavity is relatively low, so a well-defined standing wave will be formed between the two PM-FBGs and thus SHB occurs [14]. The combination of PHB and SHB effects will reduce the homogeneous linewidth of the EDF and suppress the competition between different modes. Therefore, stable DW oscillation with orthogonal polarization modes can be achieved at room temperature. In our experiment, the FBGs are fabricated by using a KrF excimer laser (TuiLaser Ltd., Germany) with a phase-mask grating-writing technique. A phase mask with a pitch of 1070.6 nm is employed to achieve a grating period of 535.3 nm. And the PM-EDF we use is panda-style with a birefringence of the. The transmission spectra of PM-FBG2 measured by an optical spectrum analyzer (OSA, ANDO 6317) with 0.01 nm resolution are shown in Fig. 2. When seeded with an un-polarized amplified spontaneous emission (ASE) source, two transmission notches both with depth of about db (i.e., 50% of (2) Fig. 2. Measured transmission spectra of PM-FBG2 when it is illuminated by an un-polarized ASE source (solid line), and by a homemade linear-polarized source with polarization state along fast axis (dot line) and slow axis (dash line) of the PM-EDF, respectively. the seeded light is reflected at the Bragg wavelengths) are observed from the OSA. And then we use a homemade linear-polarized light source with tunable polarization state to illuminate the PM-FBG. When the polarization state of the light source is along the fast axis of the PMF, a single transmission notch presents at 1550.892 nm with a peak depth of db (i.e., with a calculated reflectivity of 98.6%). A transmission notch at 1551.270 nm with a peak depth of db (i.e., with a calculated reflectivity of 98.7%) exhibits when the polarization state of the incident light is tuned to be along the slow axis of the PMF. The wavelength difference between the two orthogonal modes is 0.378 nm which is accordant very well with the calculated result of (2). The DW-SLM fiber laser is pumped by a 980 nm laser diode through a 980/1550 nm wavelength division multiplexer (WDM). Fig. 3 shows the spectra of the dual wavelength laser out. The two orthogonal lasing wavelengths are measured to be 1550.916 nm and 1551.290 nm with a wavelength spacing of 0.374 nm. And the OSNR is measured to be nearly 50 db. The output power of the DW-SLM fiber laser is 0.8 mw. When the output of the laser is injected to a PD (Anritsu MN4765A) with a bandwidth of 65 GHz which is cascaded by an ESA (Agilent E4440A) with a measurement range from 3 Hz to 26.5 GHz, there is no beating signal observed from the ESA within its whole range (as shown in the insert of Fig. 3) which indicates that the DW laser is in a SLM operation, noticing that the effective cavity length of our laser is 2 cm, and the corresponding longitudinal-mode spacing is GHz. III. MICROWAVE GENERATION If the two wavelengths of the DW-SLM fiber are turned to be with the same polarization state through a polarizer, by beating the two wavelengths at a PD, microwave signal can be generated, and the beat frequency can be expressed as where is the mean of the lasing wavelengths. (3)
LIU et al.: DUAL-WAVELENGTH SINGLE-LONGITUDINAL-MODE POLARIZATION-MAINTAINING FIBER LASER 4457 Fig. 3. Spectra of the dual-wavelength fiber laser. The insert shows the electrical spectrum observed at the output of the photodetector which indicates the laser is in single-longitudinal-mode operation. As introduced in Section II, the wavelength spacing of our DW-SLM fiber laser is 0.374 nm. Thus, if we beat the two wavelengths at a PD, the frequency of the generated microwave signal will be over 46 GHz, which is out of the measurement range (up to 26.5 GHz) of the ESA we use. Here we detect the generated microwave signal by employing a frequency up/down conversion technique. The schematic configuration of the microwave frequency up/down conversion system is shown in Fig. 4. After amplified by an erbium doped fiber amplifier (EDFA), the output light of the DW-SLM laser is modulated in an optical intensity modulator (IM) which is driven by an analog signal generator (ASG, Agilent E8257D) with a frequency of. The IM here also acts as a polarizer because of its polarization relative characteristic. The modulated light is divided into two split through a 90 : 10 coupler. One part is monitored by an OSA, and the other is injected into the PD which is cascaded by the ESA. The output field of the IM can be expressed as [15] where is the normalized driving voltage of the IM, is the normalized voltage bias point and is the lasing frequency of the DW-SLM fiber laser. Here, and denote the two orthogonal polarizations of the lasing modes of the DW-SLM fiber laser. The Bessel function expansion of (4) is From (5) we can find that when, even order sidebands will be suppressed. Considering the higher order sidebands are relatively small, the -order sidebands will be dominant. Because of the polarization relative characteristic of the IM, (4) (5) Fig. 4. Experiment setup of the microwave generation and frequency up/down conversion system: ISO is isolator; EDFA is erbium doped fiber amplifier; PC is polarization controller; ASG is analog signal generator; IM is intensity modulator; C is coupler; PD is photodetector; OSA is optical spectrum analyzer; ESA is electrical spectrum analyzer. and cannot be achieved simultaneously, and, thus, the fundamental modes of the two lasing polarizations cannot be totally suppressed at the same time. However, by carefully adjusting the polarization controller (PC) and the driving voltage bias point of the IM, both polarization modes can be modulated with high efficiency. By heterodyning the modulated light at the PD, several new microwave signals can be generated including one at frequency [16], where is the frequency of the microwave signal generated by beating the two original wavelengths of the DW-SLM laser before modulating. When is set to a suitable value, can be deduced by measuring which is much lower than. Thus, we can measure a microwave of higher than 46 GHz by using the ESA with a limited measurement range When the output frequency of the ASG is set to Hz, we can see the optical spectrum after the IM monitored by the OSA is similar to the original spectrum of the DW-SLM fiber laser which is shown in Fig. 3. And there is no beating signal observed from the ESA. Then we tune the output frequency of the ASG up to GHz, and -order sidebands of the two fundamental frequencies of the DW-SLM fiber laser are generated. By carefully adjusting the PC and the driving voltage bias point of the IM, both polarization modes can be modulated with high efficiency and the power of the -order sidebands are nearly at the same level as the 0-order (see Fig. 5). Fig. 6 shows the microwave signals measured by the ESA. As shown in Fig. 6(a), within the whole measurement range of the ESA, three signals at 10.50254 GHz ( ), 21.00511 GHz ( ) and 25.65674 GHz ( ) are observed as expected. Fig. 6 (b) shows the detail of the microwave signal which is at a frequency of 25.65674 Hz. The SNR of is over 35 db, and the db bandwidth is which indicates that line width of each laser line in the DW-SLM is less than 50 khz. The phase noise of the generated microwave signal is measured to be dbc/hz at 1 MHz by using the ESA. The frequency drift is observed to be MHz within 15 minutes in the free-running mode at room temperature. Thus, we can deduce the frequency of the original microwave signal generated by beating two wavelengths of the proposed DW-SLM laser is
4458 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 20, OCTOBER 15, 2009 IV. CONCLUSION We have proposed and demonstrated a novel DW-SLM fiber laser with orthogonal linear polarizations at room temperature. By beating the two wavelengths of the laser, microwave signal at frequency as high as 46.66 GHz (determined by the wavelength spacing of the DW-SLM fiber laser) has been generated. By employing a frequency up/down conversion technique, the frequency of the microwave signal is converted down to 25.65674 GHz and measured by a 26.5 GHz ESA. The frequency of the microwave signal can be designed by writing the grating into PM fiber with proper birefringence. The birefringence of the PMF is sensitive to the lateral pressure, which indicates that the frequency of the microwave signal can be tuned by staving the short laser cavity. And this makes it potential to be a high sensitivity pressure sensor. Fig. 5. Spectrum of the dual-wavelength fiber laser output after modulated by a 10.5 GHz microwave signal. 61-order sidebands of the two fundamental lasing frequencies can be observed obviously. ACKNOWLEDGMENT The authors would like to thank Dr. H. Y. Ou and Y. Gao for their helpful discussion. Fig. 6. Electrical spectra of the beating signals observed at the output of the photodetecter (a) with a span of 26.5 GHz and (b) the detail of the microwave signal at 25.65674 GHz with a span of 3.5 MHz. GHz, which is accordant very well with the calculated result from (3). REFERENCES [1] A. J. Seeds and K. J. Williams, Microwave photonics, J. Lightw. Technol., vol. 24, pp. 4628 4641, 2006. [2] J. Capmany and D. Novak, Microwave photonics combines two worlds, Nature Photon., vol. 1, pp. 319 330, Jun. 2007. [3] R. C. Williamson, RF Photonics, J. Lightw. Technol., vol. 26, pp. 1145 1153, 2008. [4] P. O. Hedekvist, B. E. Olsson, and A. Wiberg, Microwave harmonic frequency generation utilizing the properties of an optical phase modulator, J. Lightw. Technol., vol. 22, pp. 882 886, 2004. [5] S. Baunel, O. Brox, J. Kreissl, G. Sahin, and B. Sartorius, Optical microwave source, Electron. Lett., vol. 38, pp. 334 335, 2002. [6] J. Genest, M. Chamberland, P. Tremblay, and M. Tetu, Microwave signals generated by optical heterodyne between injection-locked semiconductor lasers, IEEE J. Quant. Electron., vol. 33, pp. 989 998, 1997. [7] Z. Fan and M. Dagenais, Optical generation of a megahertz-linewidth microwave signal using semiconductor lasers and a discriminator-aided phase-locked loop, IEEE Trans. Microw. Theory Tech., vol. 45, no. 8, pp. 1296 1300, Aug. 1997. [8] X. Chen, J. Yao, and Z. Deng, Ultra-narrow dual-transmission-band fiber Bragg grating filter and its application in a dual-wavelength single-longitudinal-mode fiber ring laser, Opt. Lett., vol. 30, pp. 2068 2070, 2005. [9] X. Chen, Z. Deng, and J. Yao, Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode fiber ring laser, IEEE Trans. Microw. Theory Tech., vol. 54, pp. 804 809, 2006. [10] X. P. Cheng, P. Shum, C. H. Tse, J. L. Zhou, M. Tang, W. C. Tan, R. F. Wu, and J. Zhang, Single-longitudinal-mode erbium-doped fiber ring laser based on high finesse fiber Bragg grating Fabry-Perot etalon, IEEE Photon. Technol. Lett., vol. 20, pp. 976 978, 2008. [11] J. L. Zhou, L. Xia, X. P. Cheng, X. P. Dong, and P. Shum, Photonic generation of tunable microwave signals by beating a dual-wavelength single longitudinal mode fiber ring laser, Appl. Phys. B, vol. 91, pp. 99 103, 2008. [12] G. A. Ball, W. H. Glenn, W. W. Morey, and P. K. Chan, Modeling of short, single frequency, fiber lasers in high-gain fiber, IEEE Photon. Technol. Lett., vol. 5, pp. 649 651, 1993. [13] J. Sun, J. Qiu, and D. Huang, Multiwavelength erbium-doped fiber lasers exploiting polarization hole burning, Opt. Commun., vol. 182, pp. 193 197, 2000. [14] J. J. Zayhowski, Limits imposed by spatial hole burning on the singlemode operation of standing-wave laser cavities, Opt. Lett., vol. 15, pp. 431 433, 1990. [15] J. J. O Reilly, P. M. Lane, R. Heidemann, and R. Hofstetter, Optical generation of very narrowlinewidth millimetrewave signals, Electron. Lett., vol. 28, pp. 2309 2310, 1992. [16] J. Sun, Y. T. Dai, X. F. Chen, Y. J. Zhang, and S. Z. Xie, Stable dual-wavelength DFB fiber laser with separate resonant cavities and its application in tunable microwave generation, IEEE Photon. Technol. Lett., vol. 18, pp. 2587 2589, 2006.
LIU et al.: DUAL-WAVELENGTH SINGLE-LONGITUDINAL-MODE POLARIZATION-MAINTAINING FIBER LASER 4459 Weisheng Liu was born in Hebei, China, in 1982. He received the B.S. degree from Zhejiang University in 2006. He is currently pursuing the Ph.D. degree at the Centre for Optical and Electromagnetic Research, Zhejiang University. His research interests include fiber gratings, fiber optic sensors, and microwave photonics. Daru Chen was born in Wenzhou, China, in 1982. He received the B.S. degree from Zhejiang University, Hangzhou, China, in 2004. He is currently pursuing the Ph.D. degree at the Centre for Optical and Electromagnetic Research, Zhejiang University. He has authored/coauthored over 30 papers in refereed international journals. His current research interests include fiber lasers, fiber sensors, and photonic crystal fibers. Meng Jiang was born in Liaoning, China, in 1983. She received the B.S. degree with honors in optical engineering from Zhejiang University in 2005. She is currently pursuing the Ph.D. degree at the Center for Optical and Electromagnetic Research, Zhejiang University, China. Her main interests of research include fiber grating, fiber optical communications, and fiber sensors. Sailing He (M 92 SM 98) received the Licentiate of Technology and Ph.D. degrees from the Royal Institute of Technology, Stockholm, Sweden, in 1991 and 1992, respectively. After receiving the Ph.D. degree, he worked at the Royal Institute of Technology as an Assistant Professor, as an Associate Professor, and as a Full Professor. He has been with Zhejiang University, Hangzhou, China, since the time he was appointed by the Ministry of Education of China. He authored one monograph (Oxford University Press) and authored/coauthored over 300 papers in refereed international journals. His current research interests include photonic integration, fiber optical communication, optical sensing technologies, and microwave photonics.