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Single-and dual-wavelength switchable erbium-doped fiber ring laser based on intracavity polarization selective tilted fiber gratings

Single-and dual-wavelength switchable erbium-doped fiber ring laser based on intracavity polarization selective tilted fiber gratings
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   1 Single- and Dual-Wavelength Switchable Erbium-DopedFiber Ring Laser based on Intra-Cavity PolarizationSelective Tilted Fiber Gratings Chengbo Mou, 1,* Pouneh Saffari, 1 Hongyan Fu, 2 Kaiming Zhou, 1 Lin Zhang, 1 and IanBennion 1   1 Photonics Research Group, School of Engineering and Applied Science, Aston University, Birmingham, B4 7ET, UK  2 Center for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentation, Zijingang Campus, Zhejiang University, Hangzhou, 310058, P.R.China * Corresponding author: mouc@aston.ac.uk  We have proposed and demonstrated a single- and dual-wavelength switchable Erbiumdoped fiber laser (EDFL) by utilizing intra-cavity polarization selective filters based ontilted fiber gratings (TFGs). In the cavity, one 45°-TFG is functioning as an in-fiberpolarizer and the other 77°-TFG is used as a fiber polarization dependent loss (PDL) filter.The combined polarization effect from these two TFGs enables the laser to switch betweenthe single- and dual-wavelength operation with single polarization state at roomtemperature. The laser output at each wavelength shows an optical signal to noise ratio(OSNR) of > 60dB and a side mode suppression ratio (SMSR) >50dB and a polarizationextinction ratio of ~35dB. The proposed EDFL can give stable output under laboratorycondition.   2© 2009 Optical Society of America OCIS codes: 060.2410, 060.3510, 060.3735, 140.3500, 140.3510. 1. Introduction Optical fiber lasers with switchable multi-wavelength output are useful in manyapplications, such as wavelength division multiplexed (WDM) optical fiber communicationsystems, fiber sensors, optical instrument and system diagnostics and so on. Fiber Bragg gratings(FBGs) are ideal wavelength selective components for fiber lasers due to their advantages of intrinsic fiber compatibility, ease of use, and low cost etc. Erbium-doped fiber (EDF) has beendeveloped and widely used for commercial fiber lasers and amplifiers owing to its high opticalgain and low noise figure in 1550nm region. Because of its relatively broad homogeneousexcitation, it is difficult to obtain stable and relatively close wavelength spacing oscillations inEDF lasers (EDFLs) at room temperature. Various techniques have been developed to suppressthe mode competition induced by the homogeneous broadening of EDF, such as cooling downEDF in liquid nitrogen [1], incorporating a frequency shifter in the cavity [2], employing ahybrid gain medium [3] and utilizing spatial hole burning by inserting a multi-phase shift FBG ina linear cavity fiber laser [4]. Special laser cavity configurations for multi-wavelength operationby incorporating a segment of highly nonlinear photonic crystal fiber or dispersion-shifted fiberhave also been reported [5-7]. In recent years, multi-wavelength fiber lasers operating at roomtemperature by utilizing polarization hole burning (PHB) effect from polarization maintaining(PM) FBGs have been studied extensively and various setups have been demonstrated [8-10].However, in all these setups, the PHB effect has only been studied with PM fiber based devices.We report here a stable, single- and dual-wavelength switchable fiber laser by utilizing two   3special tilted fiber gratings (TFGs) in an EDF ring laser cavity without any PM fiber baseddevice. The two TFGs include one with the structure tilted at 45 º , which is used as an in-fiberpolarizer [11] and the other at 77 º as a polarization dependent loss filter. The optical signal tonoise ratio of >60dB has been obtained in such an EDFL system, which is higher than that in theprevious reports [8-10]. In this configuration, the separation between the switchable wavelengthscan be more flexibly designed with potential tuning capability. 2. Fabrication and Characterization of the polarization selective tiltedgratings Two fiber gratings with structures tilted at 45 ° and 77 ° (named as 45 ° -TFG and 77 ° -TFG)were fabricated by the standard scanning UV-inscription technique using two different phasemasks. The 45 ° -TFG was UV-inscribed in the single mode B/Ge co-doped photosensitive fiberand the 77 ° -TFG was in the standard telecom fiber (SMF-28). Both fibers were photosensitizedby high pressure H 2 -loading at 100 º C for 2 days prior to the UV-inscription.In order to induce slanted index fringe structure at ~77 ° in the fiber core, we used a phasemask with 6.6 µ m period (from Edmund Optics) and oriented it at 73 ° in the UV-inscriptionsystem. According to our previous work, TFGs with excessively tilted structures exhibitpolarization dependent loss property, giving a series of paired polarization loss peaks in 1550nmregion [12]. The transmission spectrum of this 77 ° -TFG was first examined using a broadbandsource (Agilent 83437A) and an optical spectrum analyzer, which is shown in Fig.1(a). From thefigure we can see that all paired loss peaks exhibit near-3dB strength, indicating the light iscoupled almost equally to the two sets of cladding modes with orthogonal polarization states.We then examined the 77 ° -TFG under polarized lights by using another source (AFC BBS   41550A-TS) with even lower degree of polarization and inserted an in-fiber polarizer and apolarization controller between the light source and the 77 ° -TFG. As the zoomed spectra of onepaired modes shown in Fig. 1 (b), when the light is polarized at P1 state, the fast-axis loss peak isfully excited to ~12dB whereas the slow-axis peak almost disappeared, and vice versa when thelight is switched to P2 state (90 ° to the P1). This proves that although the 77 ° -TFG was made instandard telecom fiber, the excessively tilted structure makes it behaving as a PM-like device, i.e.a polarization dependent loss filter.The 45 º -TFG used as an in-fiber polarizer in the EDFL system was fabricated previouslyusing concatenation method employing a phase mask with 1.8 µ m period (from QPS) [11]. Weexamined the polarization dependent loss (PDL) of the 45 ° -TFG using a commercial EXFO PDLcharacterization kit that incorporated a tuneable laser to provide optimization measurement over100nm range. The measured PDL is shown in Fig.2 and from which we can see that the 45 ° -TFGis a near-ideal polarizer with a polarization extinction ratio close to 35dB over 100nm bandwidthfrom 1520nm to 1620nm.In addition to the two TFGs, two standard FBGs (G1 and G2 in Fig. 3), as seedingwavelength selectors, were also UV-inscribed in H 2 -loaded SMF-28 fiber with their Braggwavelengths matching the two loss peaks of the 77 ° -TFG. The reflectivities of the two FBGs are2.51dB and 2.28dB at 1547.07nm and 1553.24nm, respectively, and the bandwidths are of ~0.1nm.   5 3. Principle of the EDFL system and experimental results The set-up for the proposed EDFL is shown in Fig.3. In this configuration, the gainmedium is a 6m of highly Erbium doped fiber (from Lucent Technology), which has anabsorption coefficient of 12dB/m. A 976nm laser diode (from SDL) controlled by a set of commercial laser diode driver (Newport 505B) and temperature controller (Newport 300 Series)is used to pump the EDFL through a 980/1550 WDM coupler. An optical isolator (OIS) ensuresan anticlockwise ring cavity. The 30% arm of the coupler is used as the output port of the laser.A fiber polarization controller (PC) is placed between the 77 º -TFG and the 45 º -TFG. Twostandard FBGs (G1 and G2) functioning as seeding wavelength selectors are coupled into thelaser cavity via a circulator. The end of the FBG array is terminated by index matching gel inorder to eliminate any unwanted background ASE noise.The operation principle of this EDFL is described as follows. The intracavity 45 º -TFGhas a very high polarization extinction ratio, which can guarantee that the fiber ring laser willoscillate in single polarization regime [13]. The 77 º -TFG will induce polarization dependentloss to the ring cavity around its paired attenuation band region, thus imposing PHB effect to thegain medium in this region. The amplitude of the loss depends on the polarization state of thelight travelling in the 77 º -TFG.By adjusting the PC to control the polarization state of the light entering the 77 º -TFG,i.e. polarized in the equivalent fast- or slow-axis of the 77 º -TFG, single-polarization and single-wavelength lasing at either 1547nm or 1553nm can be realized. Fig. 4 (a) and (b) show the singlewavelength oscillation of the fiber ring laser at the two seeding wavelengths, respectively.The laser output amplitude variation was measured to be less than 0.5dB within 1 hour atlaboratory condition and the spectra were recorded every 5min and are plotted in Fig. 4(c) and
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