All-fiber wavelength swept ring laser based on F-P filter for optical frequency domain imaging

DOI:10.1007/s00340-007-2600-3Appl.Phys.B 87,293–296(2007)Lasers and OpticsApplied Physics Bk.s.lee 1,u c.s.kim 1r.k.kim 1g.patterson 2m.kolesik 2j.v.moloney 2n.peyghambarian 2Dual-wavelength external cavity laser with a sampled grating formed in a silica PLCwaveguide for terahertz beat signal generation1School of Information and Communication Engineering,SungKyunKwan University,Suwon 440-746,Korea2College of Optical Sciences,The University of Arizona,Tucson,AZ 85721,USAReceived:2September 2006/Revised version:9January 2007Published online:22March 2007•©Springer-Verlag 2007ABSTRACT We propose and demonstrate a dual-wavelength ex-ternal cavity laser (ECL).In our design,a Fabry–P´e rot laser diode (FP-LD)is hybrid-integrated with a sampled Bragg grat-ing written in a silica planar lightwave circuit (PLC).The grating selects two laser wavelengths that share the same laser cavity.The dual-wavelength oscillation with a side mode sup-pression ratio >32dB has been demonstrated experimentally.Experimental results indicate that the hybrid-integrated dual-wavelength ECL exhibits strong dual-wavelength emission cor-responding to beat frequency of 1THz.Simulations of this system also indicate good mutual coherence of the two modes and stability of the 1THz beat signal.PACS 42.79.Dj;42.60.By;42.55.Px1IntroductionDual-wavelength lasers (or two-color lasers)havebeen investigated by many researchers [1–6]because of vari-ous applications such as terahertz frequency generation [1–3],dual wavelength interferometry [7],THz-imaging [8]and wavelength multiplexing or switching.One way to achieve dual-wavelength laser operation is to combine a single laser with one or two external cavities.In these dual-wavelength lasers,one of the critical devices is the grating which acts as an external cavity mirror.Gratings that have successfully served this purpose include bulky gratings [1–3],distributed Bragg gratings [4,5]and fiber Bragg gratings [6].The dis-tributed Bragg gratings [4,5]were grown directly in the ver-tical cavity surface emitting lasers (VCSELs);therefore,the dual-wavelength lasers based on VCSELs with two mono-lithic gratings are more compact and stable than the dual-wavelength lasers with the other types of gratings.Espe-cially,for terahertz signal generation,the VCSEL-based dual-wavelength laser is advantageous because it can be operated in the two-color (or dual-wavelength)coherent regime [1],which is practically possible using a very short external cavity with high enough frequency selectivity.However,this kind of VCSEL-based dual-wavelength laser is expensive to develop because of its complex structure.u Fax:+82-31-290-7204,E-mail:kslee@ece.skku.ac.krIn this paper,we present a hybrid-integrated dual-wave-length laser using a sampled Bragg grating formed in a planar lightwave circuit (PLC).This new kind of hybrid-integrated dual-wavelength laser is a good candidate for the dual-wavelength source for terahertz beat signal generation be-cause it is cost-effective and has an external cavity,which is short and compact enough for the two-color coherent lasing.Since a good THz beat signal is necessary starting point for THz radiation generation,it is extremely important to demon-strate a stable hybrid-integrated dual-wavelength laser.We will experimentally demonstrate that this hybrid-integrated ECL [9–11]combined with a sampled Bragg grating written in a silica PLC exhibits strong dual-wavelength laser oscilla-tion with terahertz beat frequency.In addition,using computer simulation,we will also predict the stable terahertz beat signal generation.2Device fabricationFigure 1shows the schematic of the proposed hy-brid integrated dual-wavelength ECL in which an FP-LD and a sampled Bragg grating are integrated on a silica PLC plat-form.The FP-LD was a spot-size converter integrated laser diode having low coupling loss.The front and rear facets of the FP-LD were coated with anti-reflection film (with reflec-tivity R ≤1%)and high reflection film (R =∼80%),respec-tively [11].The length of the FP-LD was 300µm .The center wavelength of the FP-LD was about 1290nm ,and the longi-tudinal mode spacing was 0.8nm .The sampled Bragg grating was written by exposing the PLC waveguide to a KrF excimer laser beam (λ=248nm )through a combination of a phase mask and an amplitude mask.The PLC waveguide was Ge-doped silica waveguide and hydrogenated at room tempera-ture and 100atm pressure for more than 4days to enhance photosensitivity.According to the diffusion theory,4days is enough time [9]to saturate the waveguide core with hydrogen molecules at 100atm pressure.The PLC sampled grating should be designed such that two strong reflection peaks occur within the gain bandwidth of the FP-LD for the operation of the dual-wavelength laser.The separation of the two lasing wavelengths is determined by ∆λ=λ2B /2n eff Λs ,where λB is the Bragg wavelength,n eff is the effective refractive index of the silica PLC waveguide and Λs is the sampling period of the PLC sampled grating.294Applied Physics B –Lasers andOpticsFIGURE 1Configuration of a dual-wavelength external cavity laser with a sampled Bragg grating formed in a silica PLC waveguideThus,for n eff =1.45,the required sampling period Λs for gen-eration of 1THz beating signal is ∼103µm .The length of the sampled grating was adjusted to ∼10mm by using a shut-ter placed between the KrF excimer laser and the phase mask.One can also vary the strength of the grating by controlling the exposure time of the excimer laser beam because the strength is proportional to the UV exposure time.A number of PLC gratings with a pitch,Λ,of 450.91nm and different sampling periods Λs were written with the laser beam of fluence of 680mJ /cm 2at the repetition rate of 5Hz .In an ideal case,when the gain experienced by the two frequencies fed back in the laser by the sampled grating is ap-proximately equal,and the two modes are perfectly mutually coherent,the output intensity I (t )is modulated as [1]I (t )=I 1+I 2+2I 1I 2cos (2πf b t ),(1)where I 1and I 2are the laser mode intensities for the two dif-ferent wavelengths,and f b is the beat frequency of the two wavelengths.In reality,the lasing modes are only partially mutually coherent,and the amplitude of the modulation term in (1)decreases accordingly.Thus,a good dual-wavelength design must exhibit high mutual coherence between the lasing modes in order to achieve strong THz radiation generation.3Experimental resultsThe PLC gratings were grown after 900s-irradia-tion.Figure 2shows the transmission spectra of thePLCFIGURE 2Transmission spectra of the PLC sampled gratings used in the dual-wavelength external cavity laser with two different sam-pling periods,(a )Λs =∼100µm and (b )Λs =∼150µmsampled gratings used in the dual-wavelength external cavity lasers for two different sampling periods Λs =∼100µm and Λs =∼150µm .The two reflection peaks in Fig.2indicate that the PLC gratings act as an external cavity mirror feed-ing back two laser wavelengths.The two peak wavelengths of the PLC gratings with Λs =∼100µm are approximately at 1312.02nm and 1318.86nm ,as shown in Fig.2a.Those of the PLC gratings with Λs =∼150µm are 1313.08nm and 1317.02nm as shown in Fig.2b.The bandwidths of the reflec-tion peaks of the PLC gratings are measured to be ∼0.1nm ,which is limited to the resolution of the optical spectrum analyzer.The reflectivity of the reflection peaks ranges be-tween 80%and 94%.The wavelength spacing between the two peaks for Λs =∼100µm is ∼5.84nm ,while the spacing for Λs =∼150µm is ∼3.94nm .We demonstrated two different dual-wavelength ECLs using the PLC sampled gratings.The optical spectra of the dual-wavelength external cavity lasers with the PLC sam-pled gratings corresponding to the Fig.2a and b are shown in Fig.3a and b,respectively.Note that the FP-LD,which runs multi-mode without the external cavity feedback,becomes stabilized at two laser modes with sidemode suppression ratio (SMSR)>32dB after the hybrid integration on the silica PLC platform.The oscillation spectra of the PLC-based dual-wavelength ECL were measured at 25◦and at an injection current of 60mA .Figure 3a shows that the dual-wavelength ECL (with the grating with Λs =∼100µm )emits laser out-puts with equal intensity (i.e.,I 1=I 2)at 1313.04nm and 1318.74nm .Figure 3b shows that the dual-wavelength ECL with Λs =∼150µm oscillates with unequal intensity (i.e.,I 1=I 2)at 1313.08nm and 1317.14nm .This indicates that the laser output of the first ECL (Λs =∼100µm )results in a beat signal with f b ∼1THz and that of the second ECL (Λs =∼150µm )yields to the beat signal with f b ∼0.7THz .However,the dual-wavelength laser with equal intensity is ideal because it gives the modulation depth of unity (see (1)).Therefore,the first dual-wavelength ECL shown in Fig.3a is better than the second dual-wavelength ECL for the terahertz beat signal generation.The oscillation wavelengths of the dual-wavelength ECL were extremely stable and consistent with the Bragg wave-lengths of the PLC gratings.This is because the thermal sta-LEE et al.Dual-wavelength external cavity laser for terahertz beat signal generation295FIGURE3Optical spectra of the dual-wavelength external cavity laser with different grating sampling periods for two different bias conditions,(a)Λs=∼100µm and(b)Λs=∼150µmbility of the silica-based PLC gratings is excellent and the PLC gratings act as the external mirror of the PLC-based dual-wavelength ECLs.Also,the silica PLC waveguide was formed to be operating under the single-mode condition above 1300nm.Therefore,the two modes should be oscillating in the same fundamental spatial mode such that the mode overlapping is excellent.In this section,we experimentally demonstrated that our external cavity lasers,consisting of a FP-LD hybrid-integrated with a sampled Bragg grating writ-ten in a PLC,exhibit strong dual-wavelength emission whose corresponding beat frequency is about1THz.The beat fre-quency can be directly measured by an intensity autocorre-lator[2].But in this work we do not attempt to measure it directly;instead,we use computer simulation to demonstrate the terahertz beat signal generation from the hybrid-integrated dual-wavelength ECLs in the next section.4Simulated results and discussionSeveral two-color lasing regimes exists,among which the coherent two-color lasing regime is the most promising one for THz radiation generation[1].In order to operate the laser systems in this regime,it requires very short feedback loops and very narrow spectralfilters,preferentially integrated on a chip[1]as shown in Fig.1.To complement our experimental results,we use realistic computer simulations to assess the coherence properties and stability of our two-color source.We utilize the same simulation technology as in[1], with the core of the model based on a broad-band laser model capable of resolving semiconductor laser dynamics on very fast time scales while properly capturing all properties of the active structure[12].Our model consists essentially of two parts.Thefirst is the waveguide with the sampled grating,and the second is a Fabry–P´e rot semiconductor laser.Since we do not have access to details of the laser active structure,we re-sort to using an active structure model that should have similar properties,though the gain-maximum wavelength is slightly different from the lasing wavelength of the experimental laser. However,from the point of view of THz generation,details of the active structure are less important.In fact,the behav-ior observed here can be reproduced on a qualitative level with any similar active structure.On the other hand,the laser cav-ity and feedback properties are crucial[1].These parameters are,therefore,taken directly from the experiment.These pa-rameters are taken directly from the ly,we reproduce the mode spacing of the laser and both facet reflec-tivities.Also,we use experimental parameters for the sampled grating.We choose coupling coefficient that reproduces the experimental reflectivity spectrum by shifting the central fre-quency to match the gain peak of the modellaser.FIGURE4Simulation results of the dual-wavelength ECL with a sampled grating withΛs=∼100µm.(a)Optical spectrum and(b)laser output power exhibiting deep modulation(f b=∼1THz)296Applied Physics B–Lasers and OpticsFigure4a shows the simulated spectrum in the two-color operation regime.The two peaks correspond to two frequen-cies enforced by the sampled-grating feedback.The almost equal height of the two peaks means roughly equal output powers of the two lasing modes.Simulation shows that their relative intensitiesfluctuate only slightly on a nano-second time scale.To check the long-term stability,we have simu-lated the system over hundreds of microseconds.Indeed,the simulation showed that the two-color regime is stable and ex-hibits only smallfluctuations.The mutual coherence of the two lasing modes can be seen as a beat signal in the output intensity shown in Fig.4b.The depth of the modulation is almost100%indicating both equal-ized power and high mutual coherence of the lasing modes. Long-time simulation showed that the mutual coherence is stable and exhibits only smallfluctuations.Among the various two-color lasing regimes classified by Matus et al.in[1],our regime can be characterized as a coherent two-color regime, with a small multi-mode contribution.Thus,our simulations strongly indicate that the proposed design is highly suitable for THz beat signal generation.5ConclusionWe demonstrated experimentally and theoretically a hybrid-integrated dual-wavelength laser consisting of an FP-LD which is hybrid-integrated with a sampled Bragg grat-ing written in a silica PLC for terahertz beat signal gen-eration.The PLC sampled grating in this new design acts as an external mirror that forces the two laser wavelengths to share the same composite cavity,unlike the conventional dual-wavelength laser which often used two ing the PLC sampled gratings written with sampling periods of 100µm and150µm,we demonstrated two ECLs exhibit-ing simultaneous dual-wavelength emission corresponding to the beat signals of f b≈1THz and≈0.7THz,respectively. The dual-wavelength oscillation with a side-mode suppres-sion ratio>32dB has been demonstrated.Our simulation results also indicate that the two-color operation is stable and produces mutually highly coherent modes suitable for THz beat signal generation.Thus,our dual-wavelength ECL de-sign combines favorable manufacturing aspects with the de-sired high quality dual-wavelength operation.ACKNOWLEDGEMENTS This work was supported by the Ko-rea Science and Engineering Foundation(KOSEF)grant funded by the Korea government(MOST)(No.R01-2005-000-10252-0)and by AFOSR MRI grant F49620-02-1-0380.JVM acknowledges support from the Alexander von Humboldt Foundation.REFERENCES1M.Matus,M.Kolesik,J.Moloney,M.Hofmann,S.Koch,J.Opt.Soc.Am.B21,1758(2004)2C.Wang,C.Pan,Opt.Lett.20,1292(1995)3M.Tani,P.Gu,M.Hyodo,K.Sakai,T.Hidaka,Opt.Quantum Electron.30,503(2000)4P.Pellandini,R.Stanley,R.Houdre,U.Oesterle,M.Ilegems,C.Weis-buch,Appl.Phys.Lett.71,864(1997)5M.Brunner,K.Gulden,R.Hovel,M.Moser,J.Carlin,R.Stanley, M.Ilegens,IEEE Photon.Technol.Lett.12,1316(2000)6W.Wang,M.Cada,J.Seregelyi,S.Paquet,S.Mihailov,P.Lu,IEEE Photon.Technol.Lett.17,2436(2005)7C.Tilford,Appl.Opt.16,1857(1977)8B.Hu,M.Nuss,Opt.Lett.20,1716(1995)9J.H.Lim,G.Lim,K.S.Lee,J.H.Song,Y.K.Oh,S.T.Jung,T.Kim,Fiber Integ.Opt.24,73(2005)10J.H.Lim,J.H.Song,R.K.Kim,K.S.Lee,J.R.Kim,IEEE Photon.Tech-nol.Lett.17,2430(2005)11R.K.Kim,J.H.Lim,J.H.Song,K.S.Lee,IEEE Photon.Technol.Lett.18,580(2006)12M.Kolesik,J.V.Moloney,IEEE J.Quantum Electron.QE-37,936 (2001)。

第46卷第7期2016年7月激光与红外L A S E R&IN F R A R E DV o l.46,N o.7July, 2016文章编号:1001-5078(2016)07-0809-05•激光器技术■基于全光纤滤波技术的多波长掺铥光纤激光器王大博，祝连庆，何巍，辛璟焘(北京信息科技大学光电信息与仪器北京市工程研究中心，光电测试技术北京市重点实验室，生物医学检测技术及仪器北京实验室，北京100192)摘要:设计了一种基于马赫-曾德和光纤光栅滤波结构的掺铥光纤激光器，实现了 2 p m波段多波长激光输出。

马赫-曾德滤波器由2个3d B耦合器构成，光纤光栅反射波长为1950.35 nm,滤波器的波长间隔为16nm,激光器阈值为70m W。

通过实验证明了采用马赫-曾德结合光纤光栅进行滤波能够有效提高波长稳定性，实验中通过调节偏振控制器能够实现稳定的单波长、双波长及三波长激光输出。

1892.2 nm单波长激光的波长漂移和功率漂移分别小于0.6 nm和0.969 dB，边模抑制比为49. 75 dB; 1902. 8 nm和1932. 0 nm双波长激光的波长漂移均小于0.4 nm，功率漂移分别小于1.021 dB和2.583 dB;1895.7 nm、1902.5 nm和1931.9 nm三波长激光的波长漂移分别小于0.4 nm、0.3 nm和1.0 nm,功率漂移分别小于2.548 dB、1.441 dB 和 0.809 dB。

输出激光 3 dB 线宽均小于 0.8 nm。

关键词：掺铥光纤激光器;2^m多波长光纤激光器;马赫-曾德滤波器;光纤光栅中图分类号：TN248 文献标识码：A DOI：10.3969/j.i s s n.1001-5078.2016.07.007Multi-wavelengtli Tm-doped fiber laser based onall fiber filtering technologyWANG Da-bo,ZHU Lian-qing,HE Wei,XIN Jing-tao(Beijing Engineering Research Center of Optoelectronic Information a n d Instruments,Beijing K e y Laboratory forOptoelectronics M e a s u r e m e n t T e c h n o l o g y,Beijing Laboratory for Biomedical Detection Technology a n d Instrument,Beijing Information Science a n d Technology University,Beijing 100192，China)Abstra c t：A T m-d o p e d fiber l aser based o n M a c h-Z e h n d e r a n d F B G filter w a s designed,w h i c h achieved the multi­wavelength laser output around 2jxm waveband. T h e M a c h-Z e h n d e r filter consisted of t*wo 3 d B couplers,and reflec­tion wavelength of the F B G is 1950. 35 nm. T h e wavelength interval of filter is 1. 6 n m a n d the threshold value of laseris 70 m W.T h e experimental results s h o w that the M a c h-Z e h n d e r filter c o m b i n e d with F B G can improve the stability ofwavelength efficiently. In t he experiment, laser output of single-wavvlength，dual-wavelength a n d three-wavelengthwer e obtained b y adjusting the polarization controller. For 1892. 2n m single-wavelength laser，the wavelength drifta n d p o w e r shift are less than 0. 6n m a n d 0. 969d B，respectively，the side-mode suppression ratio is 49.75d B；for1902. 8 n m a n d 1932. 0 n m dual-wavelength laser，the wavelength shift are both less than 0. 4 n m，the p o w e r shift areless than 1. 021d B a n d 2. 583d B，respectively；for 1895. 7n m，1902. 5 n m a n d1931. 9n m three-'wavelength laser，the wavelength shift are less than 0. 4 n m，0. 3 n m a n d 1. 0 n m，respectively,the p o w e r shift are less than 2. 548 d B，1. 441d B a n d0. 809d B，respectively. T h e 3 d B linewidth of output laser is lesKey wo r ds：T m-d o p e d fiber laser ；2jxm multi-'wavelength fiber laser ；M a c h-Z e h n d e r filter ；F B G基金项目：教育部“长江学者和创新团队”发展计划(No. I R T1212);北京市重大科技计划项目（No. Z151100003615010)资助。

C-band wavelength-swept single-longitudinal- mode erbium-doped fiber ring laserKang Zhang* and Jin U. KangDepartment of Electrical and Computer Engineering, The Johns Hopkins University,3400 N. Charles St., Baltimore, MD 21218, USA*Corresponding author: kzhang8@Abstract: A wavelength-swept single-longitudinal-mode erbium-dopedfiber ring laser capable of operating at sweeping frequency in the order of afew kHz is designed and demonstrated by using a fiber Fabry-Perot tunablefilter and a Sagnac loop incorporated with a 3.5-meter unpumpederbium-doped fiber. The laser operates in continuous-wave (CW) mode andcan sweep approximately 45 nm over the entire C-band (1520nm-1570nm)window with linewidth less than 0.7 kHz. The optimum wavelengthsweeping frequency in order to achieve the best output power stability wasfound to be ~20Hz with sweeping-induced power fluctuation of only 0.1%.©2008 Optical Society of AmericaOCIS codes: (060.3510) Lasers, fiber; (140.3570) Lasers, single-mode.References and links1. C. S. Kim, F. N. Farokhrooz, and J. U. Kang, “Electro-optic wavelength-tunable fiber ring laser based oncascaded composite Sagnac loop filters,” Opt. Lett. 29, 1677-1679 (2004).2.N. Libatique, L. Wang, and R. Jain, “Single-longitudinal-mode tunable WDM-channel-selectable fiberlaser,” Opt. Express 10, 1503-1507 (2002),/abstract.cfm?uri=OE-10-25-1503.3.M. Choma, M. Sarunic, C. Yang, and J. Izatt, “Sensitivity advantage of swept source and Fourier domainoptical coherence tomography,” Opt. Express 11, 2183-2189 (2003),/abstract.cfm?URI=OPEX-11-18-2183.4.Y. W. Song, S. A. Havstad, D. Starodubov, Y. Xie, A. E. and W. J. Feinberg, “40-nm-wide tunable fiberring laser with single-mode operation using a highly stretchable FBG,” IEEE Photon. Technol. Lett. 13,1167-1169 (2001).5.H. Chen, F. Babin, M. Leblanc, and G. W. Schinn, “Widely tunable single-frequency Erbium-doped fiberlasers,” IEEE Photon. Technol. Lett. 15, 185-187 (2003).6. C. H. Yeh, T. T. Huang, H. C. Chien, C. H. Ko, and S. Chi, “Tunable S-band erbium-doped triple-ring laserwith single-longitudinal-mode operation,” Opt. Express 15, 382-386 (2007),/abstract.cfm?uri=oe-15-2-382.7.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 laser based on high finesse fiber Bragg grating Fabry-Perot Etalon,” IEEE Photon. Technol. Lett. 20, 976-978 (2008).8. B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, “Nonlinear wave mixing and inducedgratings in erbium-doped fiber amplifiers,” Opt. Lett. 18, 2108-2110 (1993).9.S. J. Frisken, “Transient Bragg reflection gratings in erbium-doped fiber amplifiers,” Opt. Lett. 17,1776-1778 (1992).10.J. Liu, J. P. Yao, J. Yao, and T. H Yeap, “Single-longitudinal-mode multiwavelength fiber ring laser,” IEEEPhoton. Technol. Lett. 16, 1020-1022 (2004).11. A. Yariv, Optical electronics in modern communications (Oxford University Press, 1997).12.S. Fleming and T. Whitley, “Measurement and analysis of pump dependent refractive index and dispersioneffects in erbium-doped fiber amplifiers,” IEEE J. Quantum Electron. 32, 1113-1121 (1996).13. E. Desurvire, Erbium-doped fiber amplifiers: Principles and Applications (John Wiley& Sons, 1994).14.X. W. Shu, S. Jiang, and D. Huang, “Fiber Grating Sagnac Loop and Its Multiwavelength-LaserApplication,” IEEE Photon. Technol. Lett. 20, 980-982 (2000).15. A. Othonos and K. Kalli, Fiber Bragg gratings: Fundamentals and Applications in Telecommunicationsand sensing (Artech House, 1999).#98508 - $15.00 USD Received 9 Jul 2008; revised 19 Aug 2008; accepted 19 Aug 2008; published 26 Aug 2008 (C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 1417316. R. N. Liu, I. A. Kostko, R. Kashyap, K. Wu, and P. Kiiveri, “Inband-pumped, broadband bleaching ofabsorption and refractive index changes in erbium-doped fiber,” Opt. Commun. 255, 65-71 (2005). 17. D. Chen, C. Shu, and S. He, “Multiple fiber Bragg grating interrogation based on a spectrum-limitedFourier domain mode-locking fiber laser,” Opt. Lett. 33, 1395-1397 (2008).18. S. H. Yun, G. J. Tearney, J. F. de Boer, N. Iftimia, and B. E. Bouma, “High-speed opticalfrequency-domain imaging,” Opt. Express 11, 2953-2963 (2003), /abstract.cfm?URI=oe-11-22-29531. IntroductionTunable 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 [1-3]. 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 [4,5]. 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 [6], a tunable fiber Bragg grating (FBG) Fabry-Perot etalon [7], and a saturable absorber with a tunable FBG [4]. 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 [8,9].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%. 2. Experimental setupFig. 1. Schematic layout of the tunable SLM EDF ring laserFFP-TF3.5-meter Unpumped EDFPC#98508 - $15.00 USD Received 9 Jul 2008; revised 19 Aug 2008; accepted 19 Aug 2008; published 26 Aug 2008(C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 14174A schematic layout of the tunable SLM EDF ring laser is shown in Fig. 1. The erbium doped fiber amplifier (EDFA) section consists of a 15-meter EDF as the gain medium, pumped by a 980nm laser diode (LD) through a 1550nm / 980nm wavelength division multiplexer (WDM) coupler. Two optical isolators are placed in the input and output of EDFA to ensure the unidirectional lasing operation and to prevent any feedback that could cause instabilities in the cavity. An electrically driven intra-cavity FFP-TF with a bandwidth of 25GHz provides a coarse first-stage wavelength selection. Using a 3-dB fiber coupler, a Sagnac loop is formed and an unpumped 3.5-meter EDF is inserted into the loop. The unpumped EDF functions as a saturable absorber and forms a self-induced FBG with an ultra-narrow bandwidth and dynamically tracks the wavelength of the FFP-TF. Theoretical modeling in Section 3 shows that this structure can provides a fine second-stage filtering which ensures SLM lasing. Polarization controllers (PC) are inserted in both the main fiber cavity and the Sagnac loop to control the cavity birefringence. A 3-dB coupler was used as an output coupler of the laser. The total ring cavity was measured to be approximately 23m, which corresponds to a longitudinal mode space of 9 MHz. All components were fusion spliced to reduce back reflection and high transmission between components.3. Modeling for mode selecting by Sagnac loop incorporated with unpumped EDF As shown in Fig. 1, the incident light to the Sagnac loop shown as a solid arrow on the left side of the loop is splitted into two identical waves by the 50:50 coupler forming the Sagnac loop. These two waves subsequently counter-propagate in the unpumped EDF. A standing wave (white wave line inside the EDF) is thus formed [10], and the spatial period of light intensity distribution can be easily determined to be λ/2n eff from the standing wave theory, where λ is the central wavelength and n eff is the effective refraction index of the EDF. Consider the two-level system consists of energy levels 4I 15/2 and 4I 13/2 of erbium ion, where saturable absorption coefficient can be written as [11]: 0()1()/satz I z I αα=+ (1)where I sat is the saturation intensity. As the result of the spatial periodical distribution of the light intensity I(z), the absorption coefficient α(z) also varies periodically. This results in periodical spatial variation of refraction index, given by the Kramers-Kronig relation [12]: 21(,')(,)..'(')-c z n z P V d ωωαωωωπωωΔΔ=∫ (2)where P. V. is the principal value of the integral derived over the frequency range ω1<ω<ω2. For this laser system, we used low-doped EDF (doping concentration ρ<1019 Er 3+/cm 3) and the maximum intra-cavity power was limited under 20 mW. Therefore we can easily deduce induced refraction index change which is estimated to be Δn<3×10-7 [13]. Therefore, a weakly coupled FBG with a period of Λ=λ/2n eff is induced by the incident light itself inside the unpumped EDF. The reflected light is then combined and transmitted through the coupler (dotted arrows in Fig. 1). The transmission of the Sagnac loop incorporated with an FBG canbe then written as [14]:2[(1-2)cos()]K L T β+Δ=(3)where β is the mode propagation constant, δ=β-π/λ is the detuning, κ is the coupling coefficient of the grating, L g is the grating length, K is coupling coefficient of the coupler,#98508 - $15.00 USDReceived 9 Jul 2008; revised 19 Aug 2008; accepted 19 Aug 2008; published 26 Aug 2008(C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 14175ΔL=L 1-L 2 is the difference of the fiber lengths shown in Fig. 1. For K=0.5 and ΔL =0, we getthe same transmission as a typical FBG:2sinh )T =(4)The full-width-half maximum (FWHM) bandwidth of the induced FBG can be written as [15]:f Δ= (5) where N=Lg /Λ is the total number of grating periods, and λ is the central wavelength at which maximum reflectivity occurs. The coupling coefficient of the induced grating κ can be estimated from Δn as [16]:2eff n n κλΔ=(6)If we take the values: λ=1540nm, L g =3.5m, n eff =1.45 and Δn<3×10-7, from Eq. (5) and Eq. (6) we can estimate the FWHM of Sagnac loop with self-induced FBG as Δf<5.6 MHz. Considering the 9 MHz longitudinal mode space of the fiber ring cavity, the SLM operation condition is well satisfied.4. Experimental results and discussionO p t i c a l P o w e r [d B m ]O u t p u t P o w e r [m W ](b) #98508 - $15.00 USD (C) 2008 OSABy changing the voltage applied to the FFP-TF, while maintaining a constant pump power of 100mW we measured the output spectrum of the laser using an optical spectrum analyzer (OSA), as shown as Fig. 2(a). The laser tuning range achieved was more than 45nm, the full width of the C band (1520nm-1570nm), and the power variation over the entire tuning range was less than one dB if we neglect the wavelength below 1520 nm. Figure 2(b) shows the measured output laser power at 1550 nm as a function of the 980nm LD input pump power. The laser threshold was 45mW and the conversion efficiency was 5%. The low conversion efficiency is mainly due to the high absorption loss in the Sagnac loop. Figure 2(c) shows a delayed self-heterodyne RF spectrum of the laser output which was used to measuring the spectral linewidth of the laser output. A 20-km-long single mode fiber was used as the delay line and an acoustic optical modulator operating at 15MHz was used in one arm of the delayed self-heterodyne set-up to shift the beat frequency away from DC. The RF spectral was fitted and the spectral linewidth of the laser was deduced to be less than 0.7 kHz.5101520250.000.040.080.120.16(a)Frequency [MHz]P o w e r [m W ]5101520250.000.040.080.120.16Frequency [MHz](b)P o w e r [m W ]Fig. 3. Noise spectrum of output laser (a) with 3.5-meter EDF and (b) with 0.3-meter EDF.To verify the SLM operation condition of our laser, we measured the RF spectrum of thelaser output by a high speed photodetector and RF spectrum analyzer, as show in Fig. 3(a). As expected there are no beat and high frequency RF signal caused by adjacent modes since the laser operates in SLM. As a comparison, we replaced the 3-meter EDF with a 0.3-meter one, which according to Eq. (5) gave a much larger FWHM of Δf<65 MHz. In this case the SLM status is not guaranteed and as a result we can see strong beat signal around 9 MHz in Fig. 3(b).Wavelength [nm]O p t i c a l P o w e r [d B m ](a)1548.401548.42 1548.44 1548.46 1548.48 1548.50 W a v e l e n g t h [n m ]Time [min](b)Fig. 4. (a) Measured output spectrum at fixed wavelength of 1548.45nm every 3 minutes for 60minutes. (b) Fluctuation of wavelength during 60 minutes.Figure 4(a) shows the measured output spectrum at fixed wavelength of 1548.45nm for 60 minutes under the pump power of 200mW. The output power is 8.1 dBm and the signal-to-noise ratio is ~50dB. No significant variation in the spectrum shape is observed from#98508 - $15.00 USDReceived 9 Jul 2008; revised 19 Aug 2008; accepted 19 Aug 2008; published 26 Aug 2008(C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 14177measurements at different time points. The wavelength fluctuation is less than 0.01nm which is due to thermal drift, shown as Fig. 4(b).1550 1552 1554 1556 155815601.301.351.40 1.451.50 Wavelength [nm]I n t e n s i t y [m W ]Scanning at 0.2 Hz (a)Wavelength [nm ] 1550155215541556155815601.301.351.401.451.50I n t e n s i t y [m W ]1550 1552 15541556 1558 15601.301.351.401.45 1.50Scanning at 100 Hz Wavelength [nm](c) I n t e n s i t y [m W ]0.1110100Sweeping Frequency [Hz]R M S [μW ]20 40 60 80 1000.000.04 0.08 0.12 0.16 P o w e r [m W ]Frequency [MHz](e)154015451550-40-20020I n t e n s i t y [d B m ]Wavelength [nm]3.0kHz 2.0kHz(f)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.By applying RF triangle signal from a function generator, the central wavelength of the FFP-TF can be continuously swept. Figure 5(a) ~ (c) are the measured output performance at different sweeping frequency of the FFP-TF over the spectrum range of 1550nm~1560nm. From Fig. 5(d), in respect to output power stability, we can find the optimum sweeping frequency to be around 20 Hz, which gives the minimum root-mean-square (RMS) of the output power of 1.5μW. At the optimum frequency, the laser out put is very stable with power fluctuation (the high frequency variation due to the wavelength sweeping) of only 0.1 %. In addition, we have measured the noise spectrum over 0~ 100 MHz of the output when the laser was sweeping at 20 Hz which is shown in Fig. 5(e). As can be seen from the spectrum, there are no beat and high frequency RF components which proves that the laser maintains its SLM#98508 - $15.00 USDReceived 9 Jul 2008; revised 19 Aug 2008; accepted 19 Aug 2008; published 26 Aug 2008(C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 14178operation even operating in the swept mode. The above features together with solid wavelength stability and narrow linewidth enable the laser a number of spectroscopic and sensing applications, especially in ultra-high-resolution spectroscopy and sensory of trace gases.A 23 dBm EDFA was used to amplify the laser output and the swept laser spectrum was measured under peak-hold mode of OSA, as shown in Fig. 5(f). At the frequency of 2.0 kHz, the power fluctuation is only about 1dBm and a very stable output spectrum is obtained.However, the laser sweeping frequency and range is limited by the maximum frequency response of the FFP-TF and the total laser cavity length, which is just a little above 3 kHz. The main focus of our work was to achieve stable single-longitudinal-mode wavelength sweep operation but not to achieve high frequency operation. Nevertheless as shown in Fig. 5(f), with the same triangle waveform, when the frequency is changed from 2.0 kHz (solid line) to 3.0 kHz (dot line), the laser is decreased in sweeping range but still with stable spectrum. Therefore it is possible to increase the sweeping range and frequency by using a higher speed and more stable FFP-TF, which can reach the frequency of a few tens of kHz [17], and also a shorter gain length in the cavity can improve the laser sweeping performance [18].Compared to other works which also used unpumped EDF as a saturable absorber, but with FBGs as wavelength selector [4,10], our laser scheme is simpler to build, easier to control for sweeping operation, less vulnerable to temperature perturbation than FBGs and has much narrower sub-kHz linewidth.5. SummaryWe designed and demonstrated a wavelength-swept EDF ring laser that operates in stable SLM. A Sagnac loop with a 3.5-meter unpumped EDF was used as a self-induced FBG filter, which dynamically tracks the wavelength of a fiber Fabry-Perot tunable filter (FFP-TF). Theoretical modeling was used to validate SLM condition and this was proved by a series of experiments. The laser is continuously tunable over 45nm over the entire C-band (1520nm-1570nm) with linewidth less than 0.7 kHz. The optimum sweeping frequency in order to achieve the best power stability was experimentally found to be ~20 Hz with power fluctuation of only 0.1%.#98508 - $15.00 USD Received 9 Jul 2008; revised 19 Aug 2008; accepted 19 Aug 2008; published 26 Aug 2008 (C) 2008 OSA 1 September 2008 / Vol. 16, No. 18 / OPTICS EXPRESS 14179。