IEEE PHOTONICS TECHNOLOGY LETTERS 1

Transcription

1 PHOTONICS TECHNOLOGY LETTERS 1 Simultaneous Dual-Band Wavelength-Swept Fiber Laser Based on Ative Mode Loking Hwi Don Lee, Zhongping Chen, Myung Yung Jeong, and Chang-Seok Kim Abstrat We report a simultaneous dual-band wavelengthswept laser based on the ative mode loking method. By applying a single modulation signal, synhronized sweeping of two lasing-wavelengths is demonstrated without the use of a mehanial wavelength-seleting filter. Two free spetral ranges are independently ontrolled with a dual path-length onfiguration of a laser avity. The stati and dynami performanes of a dual-band wavelength-swept ative mode loking fiber laser are haraterized in both the time and wavelength regions. Two lasing wavelengths were swept simultaneously from to nm for the 1310 nm band and from 1493 to nm for the 1550 nm band. The appliation of a dual-band wavelengthswept fiber laser was also demonstrated with a dual-band optial oherene tomography imaging system. Index Terms Fiber lasers, laser mode loking, optial imaging. 17 I. INTRODUCTION OVER the last deade, wavelength-swept lasers have 19 been widely investigated for various sensing and 20 imaging appliations [1] [6]. Conventional wavelength-swept 21 lasers onsist of a wide-band gain medium and a wavelength- 22 seleting filter in the avity. The sweeping mehanism 23 of a wavelength-swept laser is determined mainly by the 24 mehanial-tuning property of the filter, suh as a fiber 25 Fabry-Perot tunable filter (FFP-TF) [2], [3] and a polygon 26 mirror-sanning filter [4]. Reently, we reported on a novel 27 filter-less wavelength-swept laser based on ative mode 28 loking (AML) and showed the feasibility of produing an 29 OCT image [5]. In-vivo OCT imaging was also reported 30 based on this AML wavelength-swept laser [6]. Beause there 31 is no mehanial wavelength-tunable filter, the mehanial 32 limitation on their imaging performane an be suessfully 33 eliminated [5]. 34 Speially, dual-wavelength lasers have been widely 35 developed for differential absorption light detetion and 36 ranging (LIDAR) [7], [8], mirowave generation [9], [10] Manusript reeived July 2, 2013; revised Otober 15, 2013; aepted November 9, Date of publiation November 22, This work was supported by the Tehnology Innovation Program funded by the Ministry of Trade, Industry and Energy, Korea, under Grant H. D. Lee, M. Y. Jeong, and C.-S. Kim are with the Department of Cogno-Mehatronis Engineering, WCU Program, Pusan National University, Busan , Korea ( rahido@pusan.a.kr; myjeong@pusan.a.kr; kim@pusan.a.kr). Z. Chen is with the Department of Cogno-Mehatronis Engineering, WCU Program, Pusan National University, Busan , Korea, and also with the Bekman Laser Institute, Department of Biomedial Engineering, University of California, Irvine, CA USA ( z2hen@ui.edu). Color versions of one or more of the figures in this letter are available online at Digital Objet Identifier /LPT and spetrosopi dual-band OCT imaging [11] [13]. Many 37 different kinds of dual-band lasers have been proposed, but 38 most of their shemes onsist of two gain regions and a 39 wavelength-seleting filter in the laser avity [11] [13]. For 40 example, in the ase of a dual-band wavelength-swept FDML 41 fiber laser, the FFP-TF is inserted into the avity to sweep 42 eah wavelength-band [11], [12]. It is not easy to tune a 43 dual hannel driver for the two FFP-TFs for simultaneous 44 osillation in the ring avities of both bands [11]. In the 45 ase of a dual-band wavelength-swept soure based on a 46 polygon mirror sanning filter, two narrowband intra-avity 47 wavelength filters with a single high-speed polygonal sanner 48 have been used [13]. However, these mehanial filter auses 49 disadvantages, suh as high ost, limited stability and bulk 50 volume, needed to tune the enter wavelength. 51 In this letter, we desribe novel type of simultaneous and 1550 nm-bands wavelength-swept fiber lasers based 53 on the AML method. The priniple of dual-wavelength lasing 54 and simultaneous tuning is theoretially analyzed, and the 55 experimental harateristis of a dual-band wavelength-swept 56 AML fiber laser is presented in both the time-domain and 57 wavelength-domain. A single modulation signal is applied to 58 sweep both 1310 and 1550 nm-bands without a mehanial 59 wavelength-seleting filter. Furthermore, we report simultane- 60 ous dual-band OCT imaging results using this novel swept 61 soure (SS) laser II. PRINCIPLE OF DUAL-WAVELENGTH 63 TUNING BASED ON AML 64 In a harmoni mode loking ondition in a fiber laser avity, 65 the lasing wavelength, λ m of a stable short pulse train an 66 be determined by modulating the injetion urrent into the 67 gain with a radio frequeny (RF) signal at frequeny, f m, 68 that is an integer (N) times the free spetral range (FSR); i.e. 69 f m = N FSR. The basis FSR, FSR 0, is simply given by the 70 avity length and the speed of light in the fiber avity where 71 FSR 0 = /nl. However, in a hromati dispersive medium, 72 the FSR beomes a funtion of wavelength. This means that 73 the lasing wavelength an be determined simply, as desribed 74 in [5] 75 λ m = λ 0 S ( f m f m0 ) (1) 76 where λ 0 is the basis output wavelength at a basis 77 RF frequeny of f m0,ands is a sensitivity parameter given 78 by 79 S = n2 L 2 DN (2) 80

2 2 PHOTONICS TECHNOLOGY LETTERS Fig. 2. (a) Conept of modulation frequeny and repetition rate of dual-band wavelength swept laser. (b) Stati output spetra of the dual-band wavelengthswept AML laser for various modulation frequenies Fig. 1. Configuration of dual-band wavelength-swept ative mode loking laser. (SOA: semiondutor optial amplifier, DCF: dispersion ompensation fiber, DF: delay fiber, PC: polarization ontroller, OC: optial oupler). where n is the refrative index of the fiber at λ, L is the avity length, is the veloity of light in a vauum, D is a dispersion parameter, and N is an integer of the mode-loking order. From Eq. (1) and (2), we note that the differene between the lasing wavelength (λ m ) and basis wavelength (λ 0 ) is proportional to the differene between the modulation frequeny of the RF signal ( f m ) and the basis RF frequeny ( f m0 ) by a fator of sensitivity parameter, S. Based on this priniple of shifting wavelength via RF signal modulation, it is possible to expand the tuning of a single lasing wavelength into the tuning of multiple lasing wavelengths. For example as shown in Fig. 1, when we add two different delay path-lengths, l 1 and l 2, into the original path-length of L (suh as L + l 1 and L +l 2 ), these two path-lengths an easily define two basis FSR values, FSR 01 and FSR 02. Then the two orresponding basis RF frequenies of f m01 and f m02 an be expressed, respetively, by the following equations; f m01 = N FSR m01 = N n(l + l 1 ), (3) f m02 = N FSR m02 = N n(l + l 2 ), (4) By substituting Eq (3) and (4) to Eq (1) an be expressed as ( λ m1 = λ 0 S λ m2 = λ 0 S ) f m N, (5) n (L + l 1 ) ( ) f m N, (6) n (L + l 2 ) From Eq. (5) and (6), we expet that dual-wavelength outputs of λ m1 and λ m2 an be generated by a single modulation signal with a frequeny of f m. The separation of dual wavelength positions is determined from basis RF frequenies, f m01 and f m02. By hanging the modulation frequeny, the dual-wavelength outputs an be swept simultaneously. As the modulation frequeny is hanged repeatedly, the sweeping of dual-wavelength outputs are be repeated aordingly. More than two wavelength outputs an be available by adding more than two delay path-lengths. III. EXPERIMENTAL SETUP AND CHARACTERISTICS 113 A. Design of Dual-Band Wavelength-Swept AML Fiber Laser 114 Figure 1 shows the experimental set-up of the dual-band 115 wavelength-swept AML fiber laser. The avity onsists of two 116 semiondutor optial amplifiers (SOAs) for gain media, a 117 polarization ontroller (PC), a dispersion ompensation fiber 118 (DCF) of length L for the high dispersive avity, and two 119 broadband 50:50 optial ouplers (OCs). To demonstrate the 120 dual-band wavelength-swept laser, two broadband SOAs with 121 enter wavelengths of 1310nm (ISPAD-1301, Inphenix) and nm (ISPAD-1501, Inphenix), respetively, were used as 123 gain media. The two SOAs at the 1310 nm band (O-band) and nm band (C-band) used in this experiment an reah a db bandwidth of 45 nm when urrents of 250 and 350 ma, 126 respetively, are applied. We inserted two different delay paths, 127 l 1 and l 2, for these two SOAs for the dual-wavelength tuning 128 of λ m1 and λ m2, due to the two basis FSR values, FSR and FSR 02, and the two basis RF frequenies, f m01 and f m02, 130 respetively. It is important to selet two broadband OCs to 131 share a ommon path of the avity for both the 1310 and nm optial soures without insertion loss for the broad 133 bands. PCs were used to adjust the polarization states. The 134 dispersion oeffiient, D, of the DCF is 80 ps/nm/km at nm and 90 ps/nm/km at 1550 nm. The total avity 136 lengths of L+l 1 and L+l 2 are 75 m and 76 m, respetively. 137 B. Other Reommendations 138 As we already have been shown in our previous study [5], 139 there are some trade-off between instantaneous linewidth and 140 overall bandwidth aording to the mode-loking order (N). 141 Therefore, the appropriated modulation frequeny ( f m0 ) has 142 been hosen for around 570 MHz regions by the optimal 143 seletion for OCT imaging. Figure 2 shows the stati output 144 spetra of the dual-band wavelength-swept AML laser in the 145 modulation frequeny, f m, around the 570 MHz region. As 146 the f m was hanged from to MHz, the lasing 147 wavelength at the 1310 nm band, λ m1, shifted from to nm, respetively. As f m was hanged from to MHz, the lasing wavelength at the 1550 nm band, 150 λ m2, shifted from to nm, respetively. During MHz to MHz, 1310 nm band is not lasing 152 beause of limited gain bandwidth of SOA. A tuning range of for the 1310 nm band and 70 nm for the 1550 nm band 154

3 LEE et al.: SIMULTANEOUS DUAL-BAND WAVELENGTH-SWEPT FIBER LASER Fig. 3. (a) OSA peak-hold mode spetra. (b) Time domain traing of the dual-band wavelength-swept AML laser output. was obtained. The total bandwidth of the wavelength-swept AML laser an be determined simultaneously from the tuning range of FSR and the gain range of SOA. From the experiment, the sensitivity parameter, S, shows different tuning sensitivities of 97.6 nm/mhz for the 1310 nm band and 70.0 nm/mhz for the 1550 nm band beause the tuning sensitivity depends on the dispersion oeffiient, mode-loking order (N) and FSR at two different wavelength bands. The 3 db linewidth of the lasing peak was measured to be 0.69 nm at 1308 nm and 0.78 nm at 1553 nm. Aording to mode-loking theory, the linewidth an be further redued through optimal use of the ontrolling dispersion parameter and mode-loking order [5]. The measured optial output powers are 600 μw and 550 μw for 1310nm and 1550nm bands, respetively. To sweep the wavelength dynamially, we used an external frequeny sweeper to generate a triangular signal as a frequeny modulation (FM) funtion. The frequeny of this FM funtion means the repetition rate, f s, of the dual-band wavelength-swept AML laser soure. The repetition rate and tuning range are easily ontrolled using the FM funtion. Figure 3 (a) shows peak-hold mode spetra (deteted using an optial spetrum analyzer) of the dual-band wavelengthswept laser output at a 1 khz sweep rate for f m value of 0.71 MHz, 0.88 MHz, and 1 MHz. Figure 3 (b) shows time traking spetra at eah f m. Beause the tuning sensitivity is higher at the 1310 nm band than at the 1550 nm band, the operation ondition an be analyzed for full range tuning of both the 1310 and 1550 nm bands. For example, when f m is 0.71 MHz and 0.88 MHz, as shown in Figs. 3 (a) and (b), the 1550 nm band is not fully tuned, but the 1310 nm band is under full range operation. Therefore, the ondition of f m = 1 MHz is seleted for the full-range tuning. For this experiment, a perfetly synhronized dual-band wavelengthswept laser was easily obtained due to simple operation using the single modulation signal. In addition, beause the final output of the dual-band laser soure omes out from a single output port, it is not neessary to employ additional effort to ombine two beams of dual bands into one position [11], [13]. Fig. 4. (a) Shemati diagram of our dual-band swept-soure OCT system (DB-SS: dual-band swept-soure, OC: optial oupler, WDMC: wavelengthdivision-multiplexing oupler, PD: photo detetor). (b), () Point spread funtion of the 1310 nm band and 1550 nm band outputs, respetively. As we reported previously [5], at the high repetition 194 rate, the instantaneous linewidth beome broader due to the 195 respetively long avity length of DCF avity media. Even 196 at the higher repetition rate above 700 khz, the laser annot 197 sweep for the broad wavelength ranges beause this value 198 of repetition rate meets the half of FSR [5]. In this letter, 199 therefore, a slower repetition rate ( f s ) of 1 khz has been 200 hosen for optimal operation of reliable OCT imaging. How- 201 ever, we expet this disadvantage at high repetition rate an 202 be easily overome using a high-dispersion and short length 203 medium suh as hirped fiber Bragg grating (CFBG) [14]. The 204 onventional DCF -used in this experiment- was not optimized 205 for the shorter length and higher dispersion parameter, but it 206 still has the advantage of a broad dispersive spetrum with 207 whih to apply the dispersion effet for the 1310 and 1550 nm 208 bands simultaneously. 209 C. Optial Coherene Tomography Based on Dual-Band 210 Wavelength-Swept AML Fiber 211 Figure 4 (a) shows the experimental setup of the dual-band 212 SS-OCT system. The swept-soure OCT system is basially 213 a Mihelson interferometer omposed of a broadband 70/ fiber OC. Beause the dual-band output omes from a single 215 output port, there is no need to make an additional sample 216 arm (or probe) to expose two beams (both 1310 and 1550 nm 217 bands) into the ommon sample simultaneously, unlike the 218 other dual-band SS-OCT system [10], [12]. Therefore, after 219 induing most of the optial interferene signals, the final stage 220 of the optial signals are simply divided using a wavelength 221 division multiplexing oupler (WDMC) and deteted by two 222 photo-detetors (1817-FC, New Fous) simultaneously. For a 223 stable mode-loking ondition, the wavelength-sweeping was 224 operated at 1 khz for axial diretion tomography imaging. 225 Figure 4 (b) and () show the measured point spread fun- 226 tion (PSF) under various path length differene onditions. 227 The signal-to-noise ratio (SNR) of the system at a posi- 228 tion of 200 μm from the path-mathed depth was mea- 229 sured to be 38.3 and db for the 1310 and 1550 nm 230 bands, respetively. Due to eah integrated bandwidth of and 70.0 nm for this dual-band wavelength-swept 232 AML laser soure, the theoretial axial resolutions were 233

4 4 PHOTONICS TECHNOLOGY LETTERS design of the laser avity with a shorter length and higher 278 dispersion medium in the laser avity. 279 IV. CONCLUSION Fig. 5. (a) OCT image of three over glasses using 1310 nm and (b) OCT image of three over glasses using 1550 nm. () 1-D depth profiles of the three over glasses. OCT image of a fish eye using (d) 1310 nm, (e) 1550 nm and (f) ombined 1310/1550 nm. alulated to be 10.7 and 15.1 μm for the 1310 and 1550 nm bands, respetively. The measured axial resolutions were 22.8 μm and 27.8 μm for 1310 and 1550 nm bands, respetively. Beause the axial resolution is proportional to the square of the enter wavelength, the 1310nm band shows better resolution. Furthermore, the linewidth of the lasing peak is related to the mode-loking order (N) and the dispersion oeffiient (D) [5]. To obtain a similar mode-loking order N, the linewidth an be mainly determined using the dispersion oeffiient D. Beause the measured linewidth at the 1310 nm band is relatively narrower than that of the 1550 nm band, we note that the PSF sensitivity of the 1310 nm band dropped more slowly than that of the 1550 nm band. Using the proposed dual-band wavelength-swept AML laser, various samples of over glasses and fish eyes were prepared to ompare 2-D OCT images aording to both spetral regions. In the organi sample, a redued OCT penetration depth is observed at the 1550 nm ompared with 1310 nm beause of the effet of water absorption. However in ase of inorgani sample with low water ontent, the 1550 nm system exhibited better penetration [15]. As shown in Figs. 5(a) and (b), eah layer of the three over glasses an be distinguished learly in both the 1310 and 1550 nm bands and the last layer is more lear at the 1550 nm band. Figure 5 () shows 1-D depth profiles of the three over glasses from the OCT ross setions. Figure 5 (d)-(f) show OCT images of a fish eye measured by 1310 nm band light, 1550 nm band light, and 1310/1550 ombined light, respetively. The Fig. 5 (f) was obtained after post-proessing. Before ombining two images, the both images are resaled by using referene depth information suh as mirror positions. After post-proessing, we an rearrange both images in the same position and sale. Beause of the high water ontent, we an monitor that the deeper image of 1310 nm band is relatively learer than that of 1550 nm band. Beause the spetral features of the different samples appeared at different wavelength bands, the optimized dual-band wavelength-swept laser will enhane the imaging ontrast due to its spetral harateristis. For a pratial SS-OCT system using the proposed dual band wavelength-swept AML fiber laser, we are modifying it for the deeper imaging, higher resolution and a higher sweeping rate beause the narrower linewidth, broader bandwidth and larger FSR, respetively, an be implemented by proper A novel simultaneous dual-band wavelength-swept laser 281 soure was developed based on the ative mode loking 282 method. Aording to the dual FSR onfiguration of the 283 dispersive laser avity inluding two delayed path-lengths, 284 we generated two synhronized lasing outputs at the and 1550 nm bands by applying a single modulation signal. 286 The feasibility of the proposed dual-band wavelength-swept 287 laser soure for dual-band SS-OCT imaging was demonstrated 288 using two kinds of transparent samples. Although urrent 289 report fouses on the dual wavelength at 1310 nm and nm, the priniple and design report in this letter an 291 be extend to other dual wavelength regions for measurement 292 of biologial important parameters suh as oxygen saturation 293 for 800 nm band. 294 REFERENCES 295 [1] S. H. Yun, D. J. Rihardson, D. O. Culverhouse, and B. Y. Kim, 296 Wavelength-swept fiber laser with frequeny shifted feedbak and res- 297 onantly swept intraavity aoustoopti tunable filter, J. Quantum 298 Eletron., vol. 3, no. 4, pp , Aug [2] R. Huber, D. C. Adler, andj. G. Fujimoto, Buffered Fourier domain 300 mode loking: Unidiretional swept laser soures for optial oherene 301 tomography imaging at 370,000 lines/s, Opt. Lett., vol. 31, no. 20, 302 pp , [3] E. J. Jung, et al., Charaterization of FBG sensor interrogation based 304 on a FDML wavelength swept laser, Opt. Express, vol. 16, no. 21, 305 pp , [4] W. Y. Oh, B. J. Vako, M. Shishkov, G. J. Tearney, and B. E. Bouma, 307 >400 khz repetition rate wavelength-swept laser and appliation to 308 high-speed optial frequeny domain imaging, Opt. Lett., vol. 35, 309 no. 17, pp , [5] H. D. Lee, J. H. Lee, M. Y. Jeong, and C. S. Kim, Charaterization 311 of wavelength-swept ative mode loking fiber laser based on refle- 312 tive semiondutor optial amplifier, Opt. Express, vol. 19, no. 15, 313 pp , [6] Y. Takubo and S. Yamashita, In vivo OCT imaging using wavelength- 315 swept fiber laser based on dispersion tuning, Photon. Tehnol. 316 Lett., vol. 24, no. 12, pp , Jun. 15, [7] U. Sharma, C. S. Kim, andj. U. Kang, Highly stable tunable dual- 318 wavelength Q-swithed fiber laser for DIAL appliations, Photon. 319 Tehnol. Lett., vol. 16, no. 5, pp , May [8] P. J. Moore, Z. J. Chaboyer, and G. Das, Tunable dual-wavelength fiber 321 laser, Opt. Fiber Tehnol., vol. 15, pp , Jan [9] Y. Yao, X. F. Chen, Y. T. Dai, and S. Z. Xie, Dual-wavelength erbium- 323 doped fiber laser with a simple linear avity and its appliation in 324 mirowave generation, Photon. Tehnol. Lett., vol. 18, no. 1, 325 pp , Jan. 1, [10] J. L. Zhou, L. Xia, X. P. Cheng, X. P. Dong, and P. Shum, Photoni 327 generation of tunable mirowave signals by beating a dual-wavelength 328 single longitudinal mode fiber ring laser, Appl. Phys. B, vol. 91, no. 1, 329 pp , [11] R. Zhu, J. Xu, E. Y. Lam, and K. K. Y. Wong, High-speed dual-band- 331 swept Fourier domain mode loking laser, in Pro. 12th Photon. 332 So. Postgraduate Conf., De. 2011, pp [12] R. Zhu, et al., Dual-band time-multiplexing swept-soure OCT based 334 on optial parametri amplifiation,. J. Sel. Topis Quantum 335 Eleton., vol. 18, no. 1, pp , Feb [13] Y. Mao, S. Chang, E. Murdok, and C. Flueraru, Simultaneous 337 dual-wavelength-band ommon-path swept-soure optial oherene 338 tomography with single polygon mirror sanner, Opt. Lett., vol. 36, 339 pp , Jun [14] Y. Takubo and S. Yamashita, High-speed dispersion-tuned wavelength- 341 swept fiber laser using a refletive SOA and a hirped FBG, Opt. 342 Express, vol. 21, no. 4, pp , [15] B. R. Biedermann, W. Wieser, C. M. Eigenwillig, and R. Huber, 344 Reent developments in fourier domain mode loked lasers for optial 345 oherene tomography: Imaging at 1310 nm vs nm wavelength, 346 J. Biophoton., vol. 2, nos. 6 4, pp ,

5 PHOTONICS TECHNOLOGY LETTERS 1 Simultaneous Dual-Band Wavelength-Swept Fiber Laser Based on Ative Mode Loking Hwi Don Lee, Zhongping Chen, Myung Yung Jeong, and Chang-Seok Kim Abstrat We report a simultaneous dual-band wavelengthswept laser based on the ative mode loking method. By applying a single modulation signal, synhronized sweeping of two lasing-wavelengths is demonstrated without the use of a mehanial wavelength-seleting filter. Two free spetral ranges are independently ontrolled with a dual path-length onfiguration of a laser avity. The stati and dynami performanes of a dual-band wavelength-swept ative mode loking fiber laser are haraterized in both the time and wavelength regions. Two lasing wavelengths were swept simultaneously from to nm for the 1310 nm band and from 1493 to nm for the 1550 nm band. The appliation of a dual-band wavelengthswept fiber laser was also demonstrated with a dual-band optial oherene tomography imaging system. Index Terms Fiber lasers, laser mode loking, optial imaging. 17 I. INTRODUCTION OVER the last deade, wavelength-swept lasers have 19 been widely investigated for various sensing and 20 imaging appliations [1] [6]. Conventional wavelength-swept 21 lasers onsist of a wide-band gain medium and a wavelength- 22 seleting filter in the avity. The sweeping mehanism 23 of a wavelength-swept laser is determined mainly by the 24 mehanial-tuning property of the filter, suh as a fiber 25 Fabry-Perot tunable filter (FFP-TF) [2], [3] and a polygon 26 mirror-sanning filter [4]. Reently, we reported on a novel 27 filter-less wavelength-swept laser based on ative mode 28 loking (AML) and showed the feasibility of produing an 29 OCT image [5]. In-vivo OCT imaging was also reported 30 based on this AML wavelength-swept laser [6]. Beause there 31 is no mehanial wavelength-tunable filter, the mehanial 32 limitation on their imaging performane an be suessfully 33 eliminated [5]. 34 Speially, dual-wavelength lasers have been widely 35 developed for differential absorption light detetion and 36 ranging (LIDAR) [7], [8], mirowave generation [9], [10] Manusript reeived July 2, 2013; revised Otober 15, 2013; aepted November 9, Date of publiation November 22, This work was supported by the Tehnology Innovation Program funded by the Ministry of Trade, Industry and Energy, Korea, under Grant H. D. Lee, M. Y. Jeong, and C.-S. Kim are with the Department of Cogno-Mehatronis Engineering, WCU Program, Pusan National University, Busan , Korea ( rahido@pusan.a.kr; myjeong@pusan.a.kr; kim@pusan.a.kr). Z. Chen is with the Department of Cogno-Mehatronis Engineering, WCU Program, Pusan National University, Busan , Korea, and also with the Bekman Laser Institute, Department of Biomedial Engineering, University of California, Irvine, CA USA ( z2hen@ui.edu). Color versions of one or more of the figures in this letter are available online at Digital Objet Identifier /LPT and spetrosopi dual-band OCT imaging [11] [13]. Many 37 different kinds of dual-band lasers have been proposed, but 38 most of their shemes onsist of two gain regions and a 39 wavelength-seleting filter in the laser avity [11] [13]. For 40 example, in the ase of a dual-band wavelength-swept FDML 41 fiber laser, the FFP-TF is inserted into the avity to sweep 42 eah wavelength-band [11], [12]. It is not easy to tune a 43 dual hannel driver for the two FFP-TFs for simultaneous 44 osillation in the ring avities of both bands [11]. In the 45 ase of a dual-band wavelength-swept soure based on a 46 polygon mirror sanning filter, two narrowband intra-avity 47 wavelength filters with a single high-speed polygonal sanner 48 have been used [13]. However, these mehanial filter auses 49 disadvantages, suh as high ost, limited stability and bulk 50 volume, needed to tune the enter wavelength. 51 In this letter, we desribe novel type of simultaneous and 1550 nm-bands wavelength-swept fiber lasers based 53 on the AML method. The priniple of dual-wavelength lasing 54 and simultaneous tuning is theoretially analyzed, and the 55 experimental harateristis of a dual-band wavelength-swept 56 AML fiber laser is presented in both the time-domain and 57 wavelength-domain. A single modulation signal is applied to 58 sweep both 1310 and 1550 nm-bands without a mehanial 59 wavelength-seleting filter. Furthermore, we report simultane- 60 ous dual-band OCT imaging results using this novel swept 61 soure (SS) laser II. PRINCIPLE OF DUAL-WAVELENGTH 63 TUNING BASED ON AML 64 In a harmoni mode loking ondition in a fiber laser avity, 65 the lasing wavelength, λ m of a stable short pulse train an 66 be determined by modulating the injetion urrent into the 67 gain with a radio frequeny (RF) signal at frequeny, f m, 68 that is an integer (N) times the free spetral range (FSR); i.e. 69 f m = N FSR. The basis FSR, FSR 0, is simply given by the 70 avity length and the speed of light in the fiber avity where 71 FSR 0 = /nl. However, in a hromati dispersive medium, 72 the FSR beomes a funtion of wavelength. This means that 73 the lasing wavelength an be determined simply, as desribed 74 in [5] 75 λ m = λ 0 S ( f m f m0 ) (1) 76 where λ 0 is the basis output wavelength at a basis 77 RF frequeny of f m0,ands is a sensitivity parameter given 78 by 79 S = n2 L 2 DN (2) 80

6 2 PHOTONICS TECHNOLOGY LETTERS Fig. 2. (a) Conept of modulation frequeny and repetition rate of dual-band wavelength swept laser. (b) Stati output spetra of the dual-band wavelengthswept AML laser for various modulation frequenies Fig. 1. Configuration of dual-band wavelength-swept ative mode loking laser. (SOA: semiondutor optial amplifier, DCF: dispersion ompensation fiber, DF: delay fiber, PC: polarization ontroller, OC: optial oupler). where n is the refrative index of the fiber at λ, L is the avity length, is the veloity of light in a vauum, D is a dispersion parameter, and N is an integer of the mode-loking order. From Eq. (1) and (2), we note that the differene between the lasing wavelength (λ m ) and basis wavelength (λ 0 ) is proportional to the differene between the modulation frequeny of the RF signal ( f m ) and the basis RF frequeny ( f m0 ) by a fator of sensitivity parameter, S. Based on this priniple of shifting wavelength via RF signal modulation, it is possible to expand the tuning of a single lasing wavelength into the tuning of multiple lasing wavelengths. For example as shown in Fig. 1, when we add two different delay path-lengths, l 1 and l 2, into the original path-length of L (suh as L + l 1 and L +l 2 ), these two path-lengths an easily define two basis FSR values, FSR 01 and FSR 02. Then the two orresponding basis RF frequenies of f m01 and f m02 an be expressed, respetively, by the following equations; f m01 = N FSR m01 = N n(l + l 1 ), (3) f m02 = N FSR m02 = N n(l + l 2 ), (4) By substituting Eq (3) and (4) to Eq (1) an be expressed as ( λ m1 = λ 0 S λ m2 = λ 0 S ) f m N, (5) n (L + l 1 ) ( ) f m N, (6) n (L + l 2 ) From Eq. (5) and (6), we expet that dual-wavelength outputs of λ m1 and λ m2 an be generated by a single modulation signal with a frequeny of f m. The separation of dual wavelength positions is determined from basis RF frequenies, f m01 and f m02. By hanging the modulation frequeny, the dual-wavelength outputs an be swept simultaneously. As the modulation frequeny is hanged repeatedly, the sweeping of dual-wavelength outputs are be repeated aordingly. More than two wavelength outputs an be available by adding more than two delay path-lengths. III. EXPERIMENTAL SETUP AND CHARACTERISTICS 113 A. Design of Dual-Band Wavelength-Swept AML Fiber Laser 114 Figure 1 shows the experimental set-up of the dual-band 115 wavelength-swept AML fiber laser. The avity onsists of two 116 semiondutor optial amplifiers (SOAs) for gain media, a 117 polarization ontroller (PC), a dispersion ompensation fiber 118 (DCF) of length L for the high dispersive avity, and two 119 broadband 50:50 optial ouplers (OCs). To demonstrate the 120 dual-band wavelength-swept laser, two broadband SOAs with 121 enter wavelengths of 1310nm (ISPAD-1301, Inphenix) and nm (ISPAD-1501, Inphenix), respetively, were used as 123 gain media. The two SOAs at the 1310 nm band (O-band) and nm band (C-band) used in this experiment an reah a db bandwidth of 45 nm when urrents of 250 and 350 ma, 126 respetively, are applied. We inserted two different delay paths, 127 l 1 and l 2, for these two SOAs for the dual-wavelength tuning 128 of λ m1 and λ m2, due to the two basis FSR values, FSR and FSR 02, and the two basis RF frequenies, f m01 and f m02, 130 respetively. It is important to selet two broadband OCs to 131 share a ommon path of the avity for both the 1310 and nm optial soures without insertion loss for the broad 133 bands. PCs were used to adjust the polarization states. The 134 dispersion oeffiient, D, of the DCF is 80 ps/nm/km at nm and 90 ps/nm/km at 1550 nm. The total avity 136 lengths of L+l 1 and L+l 2 are 75 m and 76 m, respetively. 137 B. Other Reommendations 138 As we already have been shown in our previous study [5], 139 there are some trade-off between instantaneous linewidth and 140 overall bandwidth aording to the mode-loking order (N). 141 Therefore, the appropriated modulation frequeny ( f m0 ) has 142 been hosen for around 570 MHz regions by the optimal 143 seletion for OCT imaging. Figure 2 shows the stati output 144 spetra of the dual-band wavelength-swept AML laser in the 145 modulation frequeny, f m, around the 570 MHz region. As 146 the f m was hanged from to MHz, the lasing 147 wavelength at the 1310 nm band, λ m1, shifted from to nm, respetively. As f m was hanged from to MHz, the lasing wavelength at the 1550 nm band, 150 λ m2, shifted from to nm, respetively. During MHz to MHz, 1310 nm band is not lasing 152 beause of limited gain bandwidth of SOA. A tuning range of for the 1310 nm band and 70 nm for the 1550 nm band 154

7 LEE et al.: SIMULTANEOUS DUAL-BAND WAVELENGTH-SWEPT FIBER LASER Fig. 3. (a) OSA peak-hold mode spetra. (b) Time domain traing of the dual-band wavelength-swept AML laser output. was obtained. The total bandwidth of the wavelength-swept AML laser an be determined simultaneously from the tuning range of FSR and the gain range of SOA. From the experiment, the sensitivity parameter, S, shows different tuning sensitivities of 97.6 nm/mhz for the 1310 nm band and 70.0 nm/mhz for the 1550 nm band beause the tuning sensitivity depends on the dispersion oeffiient, mode-loking order (N) and FSR at two different wavelength bands. The 3 db linewidth of the lasing peak was measured to be 0.69 nm at 1308 nm and 0.78 nm at 1553 nm. Aording to mode-loking theory, the linewidth an be further redued through optimal use of the ontrolling dispersion parameter and mode-loking order [5]. The measured optial output powers are 600 μw and 550 μw for 1310nm and 1550nm bands, respetively. To sweep the wavelength dynamially, we used an external frequeny sweeper to generate a triangular signal as a frequeny modulation (FM) funtion. The frequeny of this FM funtion means the repetition rate, f s, of the dual-band wavelength-swept AML laser soure. The repetition rate and tuning range are easily ontrolled using the FM funtion. Figure 3 (a) shows peak-hold mode spetra (deteted using an optial spetrum analyzer) of the dual-band wavelengthswept laser output at a 1 khz sweep rate for f m value of 0.71 MHz, 0.88 MHz, and 1 MHz. Figure 3 (b) shows time traking spetra at eah f m. Beause the tuning sensitivity is higher at the 1310 nm band than at the 1550 nm band, the operation ondition an be analyzed for full range tuning of both the 1310 and 1550 nm bands. For example, when f m is 0.71 MHz and 0.88 MHz, as shown in Figs. 3 (a) and (b), the 1550 nm band is not fully tuned, but the 1310 nm band is under full range operation. Therefore, the ondition of f m = 1 MHz is seleted for the full-range tuning. For this experiment, a perfetly synhronized dual-band wavelengthswept laser was easily obtained due to simple operation using the single modulation signal. In addition, beause the final output of the dual-band laser soure omes out from a single output port, it is not neessary to employ additional effort to ombine two beams of dual bands into one position [11], [13]. Fig. 4. (a) Shemati diagram of our dual-band swept-soure OCT system (DB-SS: dual-band swept-soure, OC: optial oupler, WDMC: wavelengthdivision-multiplexing oupler, PD: photo detetor). (b), () Point spread funtion of the 1310 nm band and 1550 nm band outputs, respetively. As we reported previously [5], at the high repetition 194 rate, the instantaneous linewidth beome broader due to the 195 respetively long avity length of DCF avity media. Even 196 at the higher repetition rate above 700 khz, the laser annot 197 sweep for the broad wavelength ranges beause this value 198 of repetition rate meets the half of FSR [5]. In this letter, 199 therefore, a slower repetition rate ( f s ) of 1 khz has been 200 hosen for optimal operation of reliable OCT imaging. How- 201 ever, we expet this disadvantage at high repetition rate an 202 be easily overome using a high-dispersion and short length 203 medium suh as hirped fiber Bragg grating (CFBG) [14]. The 204 onventional DCF -used in this experiment- was not optimized 205 for the shorter length and higher dispersion parameter, but it 206 still has the advantage of a broad dispersive spetrum with 207 whih to apply the dispersion effet for the 1310 and 1550 nm 208 bands simultaneously. 209 C. Optial Coherene Tomography Based on Dual-Band 210 Wavelength-Swept AML Fiber 211 Figure 4 (a) shows the experimental setup of the dual-band 212 SS-OCT system. The swept-soure OCT system is basially 213 a Mihelson interferometer omposed of a broadband 70/ fiber OC. Beause the dual-band output omes from a single 215 output port, there is no need to make an additional sample 216 arm (or probe) to expose two beams (both 1310 and 1550 nm 217 bands) into the ommon sample simultaneously, unlike the 218 other dual-band SS-OCT system [10], [12]. Therefore, after 219 induing most of the optial interferene signals, the final stage 220 of the optial signals are simply divided using a wavelength 221 division multiplexing oupler (WDMC) and deteted by two 222 photo-detetors (1817-FC, New Fous) simultaneously. For a 223 stable mode-loking ondition, the wavelength-sweeping was 224 operated at 1 khz for axial diretion tomography imaging. 225 Figure 4 (b) and () show the measured point spread fun- 226 tion (PSF) under various path length differene onditions. 227 The signal-to-noise ratio (SNR) of the system at a posi- 228 tion of 200 μm from the path-mathed depth was mea- 229 sured to be 38.3 and db for the 1310 and 1550 nm 230 bands, respetively. Due to eah integrated bandwidth of and 70.0 nm for this dual-band wavelength-swept 232 AML laser soure, the theoretial axial resolutions were 233

8 4 PHOTONICS TECHNOLOGY LETTERS design of the laser avity with a shorter length and higher 278 dispersion medium in the laser avity. 279 IV. CONCLUSION Fig. 5. (a) OCT image of three over glasses using 1310 nm and (b) OCT image of three over glasses using 1550 nm. () 1-D depth profiles of the three over glasses. OCT image of a fish eye using (d) 1310 nm, (e) 1550 nm and (f) ombined 1310/1550 nm. alulated to be 10.7 and 15.1 μm for the 1310 and 1550 nm bands, respetively. The measured axial resolutions were 22.8 μm and 27.8 μm for 1310 and 1550 nm bands, respetively. Beause the axial resolution is proportional to the square of the enter wavelength, the 1310nm band shows better resolution. Furthermore, the linewidth of the lasing peak is related to the mode-loking order (N) and the dispersion oeffiient (D) [5]. To obtain a similar mode-loking order N, the linewidth an be mainly determined using the dispersion oeffiient D. Beause the measured linewidth at the 1310 nm band is relatively narrower than that of the 1550 nm band, we note that the PSF sensitivity of the 1310 nm band dropped more slowly than that of the 1550 nm band. Using the proposed dual-band wavelength-swept AML laser, various samples of over glasses and fish eyes were prepared to ompare 2-D OCT images aording to both spetral regions. In the organi sample, a redued OCT penetration depth is observed at the 1550 nm ompared with 1310 nm beause of the effet of water absorption. However in ase of inorgani sample with low water ontent, the 1550 nm system exhibited better penetration [15]. As shown in Figs. 5(a) and (b), eah layer of the three over glasses an be distinguished learly in both the 1310 and 1550 nm bands and the last layer is more lear at the 1550 nm band. Figure 5 () shows 1-D depth profiles of the three over glasses from the OCT ross setions. Figure 5 (d)-(f) show OCT images of a fish eye measured by 1310 nm band light, 1550 nm band light, and 1310/1550 ombined light, respetively. The Fig. 5 (f) was obtained after post-proessing. Before ombining two images, the both images are resaled by using referene depth information suh as mirror positions. After post-proessing, we an rearrange both images in the same position and sale. Beause of the high water ontent, we an monitor that the deeper image of 1310 nm band is relatively learer than that of 1550 nm band. Beause the spetral features of the different samples appeared at different wavelength bands, the optimized dual-band wavelength-swept laser will enhane the imaging ontrast due to its spetral harateristis. For a pratial SS-OCT system using the proposed dual band wavelength-swept AML fiber laser, we are modifying it for the deeper imaging, higher resolution and a higher sweeping rate beause the narrower linewidth, broader bandwidth and larger FSR, respetively, an be implemented by proper A novel simultaneous dual-band wavelength-swept laser 281 soure was developed based on the ative mode loking 282 method. Aording to the dual FSR onfiguration of the 283 dispersive laser avity inluding two delayed path-lengths, 284 we generated two synhronized lasing outputs at the and 1550 nm bands by applying a single modulation signal. 286 The feasibility of the proposed dual-band wavelength-swept 287 laser soure for dual-band SS-OCT imaging was demonstrated 288 using two kinds of transparent samples. Although urrent 289 report fouses on the dual wavelength at 1310 nm and nm, the priniple and design report in this letter an 291 be extend to other dual wavelength regions for measurement 292 of biologial important parameters suh as oxygen saturation 293 for 800 nm band. 294 REFERENCES 295 [1] S. H. Yun, D. J. Rihardson, D. O. Culverhouse, and B. Y. Kim, 296 Wavelength-swept fiber laser with frequeny shifted feedbak and res- 297 onantly swept intraavity aoustoopti tunable filter, J. Quantum 298 Eletron., vol. 3, no. 4, pp , Aug [2] R. Huber, D. C. Adler, andj. G. Fujimoto, Buffered Fourier domain 300 mode loking: Unidiretional swept laser soures for optial oherene 301 tomography imaging at 370,000 lines/s, Opt. Lett., vol. 31, no. 20, 302 pp , [3] E. J. Jung, et al., Charaterization of FBG sensor interrogation based 304 on a FDML wavelength swept laser, Opt. Express, vol. 16, no. 21, 305 pp , [4] W. Y. Oh, B. J. Vako, M. Shishkov, G. J. Tearney, and B. E. Bouma, 307 >400 khz repetition rate wavelength-swept laser and appliation to 308 high-speed optial frequeny domain imaging, Opt. Lett., vol. 35, 309 no. 17, pp , [5] H. D. Lee, J. H. Lee, M. Y. Jeong, and C. S. Kim, Charaterization 311 of wavelength-swept ative mode loking fiber laser based on refle- 312 tive semiondutor optial amplifier, Opt. Express, vol. 19, no. 15, 313 pp , [6] Y. Takubo and S. Yamashita, In vivo OCT imaging using wavelength- 315 swept fiber laser based on dispersion tuning, Photon. Tehnol. 316 Lett., vol. 24, no. 12, pp , Jun. 15, [7] U. Sharma, C. S. Kim, andj. U. Kang, Highly stable tunable dual- 318 wavelength Q-swithed fiber laser for DIAL appliations, Photon. 319 Tehnol. Lett., vol. 16, no. 5, pp , May [8] P. J. Moore, Z. J. Chaboyer, and G. Das, Tunable dual-wavelength fiber 321 laser, Opt. Fiber Tehnol., vol. 15, pp , Jan [9] Y. Yao, X. F. Chen, Y. T. Dai, and S. Z. Xie, Dual-wavelength erbium- 323 doped fiber laser with a simple linear avity and its appliation in 324 mirowave generation, Photon. Tehnol. Lett., vol. 18, no. 1, 325 pp , Jan. 1, [10] J. L. Zhou, L. Xia, X. P. Cheng, X. P. Dong, and P. Shum, Photoni 327 generation of tunable mirowave signals by beating a dual-wavelength 328 single longitudinal mode fiber ring laser, Appl. Phys. B, vol. 91, no. 1, 329 pp , [11] R. Zhu, J. Xu, E. Y. Lam, and K. K. Y. Wong, High-speed dual-band- 331 swept Fourier domain mode loking laser, in Pro. 12th Photon. 332 So. Postgraduate Conf., De. 2011, pp [12] R. Zhu, et al., Dual-band time-multiplexing swept-soure OCT based 334 on optial parametri amplifiation,. J. Sel. Topis Quantum 335 Eleton., vol. 18, no. 1, pp , Feb [13] Y. Mao, S. Chang, E. Murdok, and C. Flueraru, Simultaneous 337 dual-wavelength-band ommon-path swept-soure optial oherene 338 tomography with single polygon mirror sanner, Opt. Lett., vol. 36, 339 pp , Jun [14] Y. Takubo and S. Yamashita, High-speed dispersion-tuned wavelength- 341 swept fiber laser using a refletive SOA and a hirped FBG, Opt. 342 Express, vol. 21, no. 4, pp , [15] B. R. Biedermann, W. Wieser, C. M. Eigenwillig, and R. Huber, 344 Reent developments in fourier domain mode loked lasers for optial 345 oherene tomography: Imaging at 1310 nm vs nm wavelength, 346 J. Biophoton., vol. 2, nos. 6 4, pp ,