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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 19, OCTOBER 1, 2013
Dispersion-Tuned Harmonically Mode-Locked Fiber Laser Chunshu Zhang, Peicheng Liao, Bryan Burgoyne, Youngjae Kim, Nicolas Godbout, Alain Villeneuve, Member, IEEE, and Odile Liboiron-Ladouceur, Member, IEEE
Abstract— A dispersion-tuned harmonically mode-locked fiber laser is demonstrated using a reflective semiconductor optical amplifier (RSOA) and a chirped fiber Bragg grating. The laser is controlled by modulating the RSOA with a RF signal. The laser performance in terms of tunable bandwidth, pulsewidth, spectral width, output power, and optical signal to noise ratio (OSNR) is studied with respect to their dependence on the injected current levels to the RSOA. This letter indicates that lasing at higher harmonic order results in broaden temporal pulse width, larger spectral width, and lower OSNR. Narrower spectral width is obtained with lower modulation current causing less longitudinal modes to lase. A second synchronously gated RSOA is used to further amplify the laser with peak power up to 0 dBm with a constant output spectrum maintained at all wavelengths and reduced noise between pulses. Index Terms— Mode-locked laser, tunable, reflective semiconductor optical amplifier, chirped fiber Bragg grating.
I. I NTRODUCTION
M
ANY types of wavelength tunable lasers have been proposed, such as dye lasers, diode lasers, and optical fiber lasers, which have been used as light sources for applications in optical communications, imaging, medicine, materials processing and sensing applications. Conventional wavelengthtunable laser comprises a tunable filter or a diffraction grating and a wide gain medium in the cavity [1], [2]. The tunable property of the laser is determined by the tunable optical filter or grating, whose mechanical parts are the limiting factor in tuning speed. Fast sweep speed and wide wavelength tunable range are required in some applications, such as optical coherence tomography (OCT) and real time optical sensors. Several alternative tuning methods have been presented to overcome the limits of the mechanically tunable wavelength filter. Kim et al. applied a lithium niobate polarization controller to realize the wavelength swept fiber laser [3]. Villeneuve et al. Manuscript received June 29, 2013; revised August 2, 2013; accepted August 9, 2013. Date of publication August 15, 2013; date of current version September 12, 2013. This work was supported by the Natural Science and Engineering Research Council (NSERC) through the Engage Partnership Program. C. S. Zhang, P. C. Liao, and O. Liboiron-Ladouceur are with the Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 2A7, Canada (e-mail:
[email protected];
[email protected];
[email protected]). B. Burgoyne, Y. Kim, and A. Villeneuve are with Genia Photonics, Laval, QC H7V 5B7, Canada (e-mail:
[email protected];
[email protected];
[email protected]). N. Godbout is with Department of Engineering Physics, Polytechnique Montréal, Montreal, QC H3T 1J4, Canada (e-mail: nicolas.godbout@ polymtl.ca). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2278312
described tunable mode-locked lasers using fiber Bragg gratings (FBGs) and chirped FBGs (CFBGs) as the reflectors of the cavity [4]. Dispersion tuning technique was also used to implement a tunable wavelength laser [5]–[10]. Huber et al. demonstrated a Fourier domain mode locking technique for frequency-swept fiber laser with a higher Q factor for the cavity [11]. Another limiting factor of the tunable speed is the cavity length, which is inversely proportional to the photon lifetime in the cavity. Burgoyne et al. [5] and Takubo et al. [6] introduced a CFBG into the laser cavity as the dispersive element, which enables shorter cavity length and enhances the laser performance. In many cases, semiconductor optical amplifier (SOA) is selected as the gain medium because of various optical characteristics, such as multiple wavelength bands, large gain bandwidth, high gain saturation, and low noise figure [5]–[8]. Moreover, the SOA enables wide and fast wavelength tunability in the mode-locked laser through its fast gating property in conjunction with a dispersive element. Integrating the SOA and CFBG in the laser cavity enhances the tuning speed while providing pulses with short pulse width, high output power, narrow spectral width and thus long coherence length. Takubo et al. presented a wavelength-swept fiber laser using a reflective SOA (RSOA) and a CFBG, and successfully applied it to OCT system [6]. However, to the best of our knowledge, there is no reported detailed study analyzing the dependence of RSOA and CFBG-based laser performance, such as optical signal to noise ratio (OSNR), optical spectral width, pulse width, and output power on the injected gating current characteristics to the SOA. In this letter, we experimentally demonstrate a dispersiontuned harmonically mode-locked laser based on a RSOA and a CFBG. The laser is controlled by modulating the RSOA with a RF signal, which is generated by a pulse pattern generation (PPG). The CFBG is used to control the cavity dispersion. The performance of the mode-locked laser in terms of tunable bandwidth, pulse width, optical spectral width, output optical power, and OSNR is studied with respect to their dependence on the injected modulation and DC currents of the RSOA. A second synchronously gated RSOA is used to further amplify the laser for a smoother output spectrum, and to reduce the noise accumulation effect between pulses. II. E XPERIMENTAL D EMONSTRATION Fig. 1 shows the experimental setup of the harmonically mode-locked fiber laser based on the RSOA and CFBG, which essentially consists of a linear resonator configuration. Current
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ZHANG et al.: DISPERSION-TUNED HARMONICALLY MODE-LOCKED FIBER LASER
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drivers and TECs control both RSOAs (CIP SOA-R-C-7SFCA). The RSOA1 is used as the gain medium in the cavity. There is an anti-reflective coating on one end, and a high reflective coating on the other end. The polarization dependent gain (PDG) of the RSOAs is 20 dB, so PCs is used to control the polarization and maintain the maximum output power. The CFBG is used to reflect the light and controls the dispersion in the cavity, which has a 10-nm bandwidth, a reflectivity of 93% and a dispersion of +100 ps/nm. RSOA2 is controlled by the gated injection current to amplify the laser output. The synchronization of the optical output pulses of the modelocked laser with the RF gating signal to RSOA2 is achieved by tuning an optical delay line. The RSOAs have a 50 ohm terminated SMA port with a bandwidth of 1.2 GHz leading to relatively fast gating. The optical spectra of the laser are measured with an optical spectrum analyzer (Agilent 86142B). A photodetector (HP 11982A, optical bandwidth of 15 GHz) is used for the O/E conversion, and the electrical signals are measured using a sampling oscilloscope (Agilent 86100C, module 86116C Opt 040 with RF bandwidth of 30 GHz). A square-wave signal is generated using a PPG (Anritsu MU181020A) and used to directly modulate the RSOA1 to achieve the mode locking condition. The injected current to RSOA1 causes an increase of the carrier density in the active region, and then results in optical gain. The high reflective coated facet of the RSOA1 and CFBG compose a typical laser oscillator. When the modulation frequency exactly becomes an integer multiple m of the cavity free spectral range (FSR), mode locking condition is satisfied. Once the modulated injection current exceeds the transparency threshold, some longitudinal modes can be locked experiencing more gain as the phase condition is met, and results in a pulsed lasing output from the cavity. The pulse shape has a steady state retaining an identical shape since the mode locking is a dynamic steady-state process. It is worth to note that this mode locking mechanism is different from a pulse carver [12], [13]. While pulse carver can also generate repetitive pulses by applying periodic modulation current, the optical pulses develop from cavity noise and each pulse is independent from each other leading to uneven pulse amplitude. In addition, beating between lasing modes results in a noisier spectrum profile. The pulsed laser we present in this letter is obtained
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by coupling the longitudinal modes and locking their phase together. The longitudinal mode interval in the cavity is inversely proportional to the cavity length. Compared with the cases where dispersion compensation fiber (DCF) is usually used as the dispersive element [6]–[11], [14], such a CFBG is able to shorten the cavity of the fiber-based laser and enhance the stability of the laser output. With the CFBG acting as the dispersive element in the cavity, the laser wavelength λ can be tuned continuously over a group of longitudinal modes by modulating the injected current to the RSOA with a RF pulse signal at frequency f . The pulse repetition rate f is equal to the modulation frequency, whose relationship with the laser wavelength, λ, can be expressed as [5], [7]: mc (1) f = 2n (λ) L where n(λ) is the wavelength-dependent refractive index of the cavity, L is the cavity length, m is the harmonic order, and c is the speed of the light in vacuum. Assuming that the dispersion characteristic of the CFBG is a quadratic function of the wavelength [5], and taking the inverse of the derivative of Eq. (1), the harmonic mode-locked condition is obtained [10], [14]: m dλ =− 2 (2) df f DL where D is the dispersion coefficient in ps/nm/km. From Eq. (2), the laser wavelength can be changed by tuning the modulation frequency f , referred as the dispersion tuning technique [14]. The reflection point of the resonant modes changes in the CFBG when the laser wavelength is tuned, which corresponds to the slight change of the cavity length. The output pulse train of the mode-locked laser is a chirped Gaussian pulse due to the intracavity dispersion [9]. Consequently, the spectral width, λ, defined as the full-width at half-maximum (FWHM), is given by [10] 1/ 2 λ2 8πcM 1/ 4 f (3) λ = |D| L 2c πλ where M is the modulation index, which indicates the optical carrier’s amplitude variation due to modulation. The equation indicates that the spectral width can be narrower by reducing the modulation frequency for a given dispersion and length L. III. R ESULTS AND D ISCUSSIONS The cavity length in this experimental demonstration is 3.63 m, which can be shortened by removing the RSOA and CFBG pigtails. According to Eq. (1), the calculated FSR of this laser cavity is 28.5 MHz at 1550 nm. Many longitudinal modes experience gain as current is injected to the RSOA. A certain fraction of photons generated by stimulated emission are lost in the cavity. If the optical gain is not large enough to compensate for the cavity loss, the photons cannot build up and achieve oscillation. Thus, the injected current must reach a certain threshold level, the laser threshold, for lasing. The spacing interval ν between the longitudinal modes is determined by ν = c/ 2nL. The corresponding mode spacing of the cavity is approximately 28.5 MHz. Many longitudinal
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 19, OCTOBER 1, 2013
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modes of the cavity simultaneously experience gain due to large bandwith of the RSOA (30-nm bandwidth at 50-mA bias current). Consequently, the mode-locked laser supports a certain number of the modes. The neighboring modes on both sides of the main mode carry a significant portion of the optical power [2]. The wavelength tunability of the mode-locked laser is first investigated. The PPG is used to generate a digital pattern of 21 bits with a 19% duty-cycle (4 ones followed by 17 zeros). A modulation current is directly injected into RSOA1 with modulated (ON) and DC currents of 66 mA and 20 mA, respectively. The synchronously modulated and DC currents to RSOA2 are 75 mA and 1 mA, respectively, with a 28.5% duty cycle (6 ones followed by 15 zeros). Fig. 2(a) presents the output spectra of the laser before (at Point A in Fig. 1) and after being amplified by RSOA2. The mode-locked frequency varies from 476.0 to 487.7 MHz. The figure indicates that the laser wavelength can be tuned continuously, and shows a red shift as the mode-locked frequency is increased. The tuning range of this specific cavity is 8.5 nm (1548.30)–(1556.8 nm) as the frequency varies over 11.7 MHz. The unamplified lasing spectra at the top of Fig. 2(a) are exhibiting slight non-uniformity with a 5 dB difference in the average power over the CFBG spectrum caused mainly by the varying reflectivity of the CFBG at different wavelengths (black curve in Fig. 2(a)). The CFBG has higher transmission near its edges causing larger lasing output power. The bottom figure in Fig. 2(a) presents more uniform and symmetric spectra after amplification. Rising and falling edges of the modulation current results in transient refractive index changes leading to high-frequency photons oscillating and lasing according to Eq.(1) and (2). Consequently, the laser output has more spectral components at higher frequency (shorter wavelength), leading to some asymmetry and red shift in the unamplified spectra (top of Fig. 2(a)). Fig. 2(b) shows that the amplification through RSOA2 provides a flatter lasing output power (± 0.5 dB) over the 8.5-nm tunable range. The black curve in Fig. 2(c) shows the laser wavelength
being linearly proportional to the mode-locked frequency with a tuning sensitivity of 0.72 nm/MHz. The wavelength tuning range is limited here by the 10-nm bandwidth of the CFBG, shown as the black line in Fig. 2(a). The blue and red curves in Fig. 2(c) are the spectral width before and after amplification through RSOA2 as a function of the mode-locked frequency. It can be observed that the spectral width is not affected by the amplifier. Fig. 2(d) shows that the laser pulse width in time is significantly reduced from approximately 350 ps to 150 ps by the amplifier (RSOA2) with optimized gating current. Inset figure of Fig. 2(d) presents a typical steady output in time of laser pulses. There is a pulse compression effect due to the carrier dynamic. The gating current to RSOA2 leads to a finite accumulation of carrier concentration due to lower carrier density in the active region during the period when the RSOA2 is not gated between pulses to reduce noise accumulation. Fig. 3 shows the performances of the mode-locked laser for different harmonic orders. The modulation current gate duration of RSOA is maintained to 400 ps with a varying duty cycle from 1.1% for the fundamental mode to 21% for the 23rd harmonic at a fixed lasing wavelength at 1552.5 nm. The modulated and DC current are fixed at 66.5 mA and 19 mA, respectively. Fig. 3(a) presents the temporal waveform of the fundamental-, ninth-, and twenty third- harmonic order, respectively. The inset figure indicates that the pulse width has a modest broadening from 305 ps to 355 ps as the harmonic mode-locked order increases from 1 to 23. Fig. 3(b) shows that the laser performance drops by increasing the harmonic order. Indeed, the OSNR degrades by approximately 4.5 dB from 39 to 34.5 dB, and the spectral width increases from 0.22 nm to 0.26 nm when the harmonic order is changed from fundamental to the 23rd . These orders correspond to a mode-locked frequency varying from 28.5 MHz to 655 MHz. The noise background increases at larger mode-locked frequency, as observed in Fig. 3(a), which degrades the OSNR. Furthermore, Eq. (2) indicates that smaller modulation frequency f increases the instability of the laser wavelength and causes spectral width broadening. While Eq. (3) indicates that the spectral width will become narrower as the modulation frequency f decreases for a given dispersion and cavity length. These two effects in the mode-locked laser counteract each other causing the observed trends of the spectral width. To optimize the performance of the mode-locked laser, the effect of the modulation currents injected into RSOA1 is also studied. In the experiment, the PPG is used to generate a digital
ZHANG et al.: DISPERSION-TUNED HARMONICALLY MODE-LOCKED FIBER LASER
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pattern of 21 bits with a 19 % duty-cycle (4 ones followed by 17 zeros), and the laser wavelength is maintained to 1552.5 nm by setting the clock frequency to 10.132 GHz. The DC current to RSOA1 is fixed at 13.5 mA, and the modulation current is varied from 38 mA to 60 mA. The RSOA2 is removed to study the modulation current effect on the lasing output. Fig. 4(a) presents the evolution of the pulse waveform with modulation current. The insert figure shows a linear relationship between the pulse width and modulation current. Increasing the modulation current can augment the carrier density in the active region of the RSOA1 and, consequently, decrease the carrier lifetime leading to faster pulse generation. As such, the pulse width can be larger by increasing the injected modulation current to the RSOA1. The pulse width increases from 78 ps to 260 ps as the modulation current varies from from 38 mA to 60 mA. Fig. 4(b) shows the variance of the optical spectra with injected modulation current. The effective refractive index of the active medium in RSOA1 becomes slightly larger when the modulation current increases, which leads to the main longitudinal modes lasing at longer wavelength (red shift) according to Eq. (1) and (2). Fig. 4(c) presents the evolution of the 3-dB, 6-dB and 10-dB spectral width with the modulation current. More longitudinal modes are locked and lased when larger modulation current is injected. Moreover, the mode competition between the main mode and its adjacent modes affects the optical spectra profile leading to more ripples in the 3-dB spectral width curve than the 10-dB curve. The relationship between the pulse peak power and modulation current is presented as the dotted curved of Fig. 4(d). The optical peak power gets larger from −13.4 to −11.1 dBm as the modulation current increases from 38 mA to 60 mA. The solid curve in Fig. 4(d) also indicates that the OSNR is maintained to approximately 35 dB with no effect introduced by changes in the modulation current. IV. C ONCLUSION In this letter, a harmonically wavelength-tunable modelocked laser with 8.5-nm tuning range is realized using a
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RSOA and a CFBG. By directly modulating the RSOA (RSOA1) in the cavity with a RF signal, laser pulses are successfully obtained in the experimental demonstration. A second RSOA (RSOA2) driven by a synchronized gate current is used to amplify the laser pulses. The performance of the mode-locked laser in terms of tunable bandwidth, pulse width, spectral width, output power, and OSNR, is also studied with respect to their dependence on the injected currents and modulated frequency. This letter indicates that lower harmonic order causes shorter temporal pulse width, narrower spectral width and higher OSNR. When the injected current exceeds the laser threshold of the cavity, lower modulation current causes less longitudinal modes to lock resulting in narrower laser spectral width with no OSNR penalty at a fixed laser wavelength. By using the gating-current driven RSOA2, a flatter lasing output power over the tuning range with over 12-dB gain is obtained. The laser pulse width is greatly reduced by RSOA2 with optimized gating current, while the spectral width remains unaffected by the amplifier. R EFERENCES [1] D. J. Jones, et al., “Carrier-envetope phase controt of femtosecond modelocked lasers and direct optical frequency synthesis,” Science, vol. 288, no. 5466, pp. 635–639, 2000. [2] C. Chong, T. Suzuki, A. Morosawa, and T. Sakai, “Spectral narrowing effect by quasi-phase continuous tuning in high-speed wavelengthswept light source,” Opt. Express, vol. 16, no. 25, pp. 21105–21118, 2008. [3] C. S. Kim, F. N. Farokhrooz, and J. U. Kang, “Electro-optic wavelengthtunable fiber ring laser based oncascaded composite Sagnac loop filters,” Opt. Lett., vol. 29, no. 14, pp. 1677–1679, 2004. [4] A. Villeneuve and N. Godbout, “Tunable mode-locked laser,” U.S. Patent 8 085 822 B2, Dec. 27, 2011. [5] B. Burgoyne and A. Villeneuve, “Programmable laser: Design and applications,” Proc. SPIE, vol. 7580, p. 758002, Feb. 2010. [6] Y. Takubo and S. Yamashita, “High-speed dispersion-tuned wavelengthswept fiber laser using a reflective SOA and a chirped FBG,” Opt. Express, vol. 21, no. 4, pp. 5130–5139, 2013. [7] S. Yamashita and M. Asano, “Wide and fast wavelength-tunable modelocked fiber laser based on dispersion tuning,” Opt. Express, vol. 14, no. 10, pp. 9299–9306, 2006. [8] H. D. Lee, J. H. Lee, M. Y. Jeong, and C. S. Kim, “Characterization of wavelength-swept active mode locking fiber laser based on reflective semiconductor optical amplifier,” Opt. Express, vol. 19, no. 15, pp. 14586–14593, 2011. [9] K. Tamura and M. Nakazawa, “Dispersion-tuned harmonically modelocked fiber ring laser for self-synchronization to an external clock,” Opt. Lett., vol. 21, no. 24, pp. 1984–1986, 1996. [10] S. Yamashita, Y. Nakazaki, R. Konishi, and O. Kusakari, “Wide and fast wavelength-swept fiber laser based on dispersion tuning for dynamic sensing,” J. Sensor, vol. 2009, pp. 1–12, Jun. 2009. [11] R. Huber, M. Wojtkowski, and J. G. Fujimoto, “Fourier domain mode locking (FDML) a new laser operating regime and applications for optical coherence tomography,” Opt. Express, vol. 14, no. 8, pp. 3225–3237, 2006. [12] S. Y. Yan, X. P. Xie, Z. Q. Hui, H. Feng, and W. Zhao, “Generation of 10 GHz, 1.9 ps optical pulse train using semiconductor optical amplifier and silica-based highly nonlinear fiber,” J. Opt., vol. 12, no. 8, p. 085401, 2010. [13] G. H. C. New and D. Wood, “Dynamics of gain-switched and mode-locked semiconductor lasers,” J. Modern Opt., vol. 38, no. 4, pp. 785–799, 1991. [14] S. Li and K. T. Chan, “Electrical wavelength tunable and multiwavelength actively mode-locked fiber ring laser,” Appl. Phys. Lett., vol. 72, no. 16, pp. 1954–1956, 1998.
