February 15, 2014 / Vol. 39, No. 4 / OPTICS LETTERS
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Ultrashort pulses from an all-fiber ring laser incorporating a pair of chirped fiber Bragg gratings Simon Duval,1,* Michel Olivier,1,2 Martin Bernier,1 Réal Vallée,1 and Michel Piché1 1
Centre d’optique, photonique et laser (COPL), Université Laval, Québec G1V 0A6, Canada 2
Département de physique, Cégep Garneau, Québec G1S 4S3, Canada *Corresponding author:
[email protected]
Received November 26, 2013; accepted December 24, 2013; posted January 3, 2014 (Doc. ID 201626); published February 11, 2014 By incorporating two linearly chirped ultrabroadband fiber Bragg gratings of opposite dispersion in an all-fiber ring laser, we demonstrate a mode-locking regime in which a femtosecond pulse evolving in the normal dispersion gain segment is locally transformed into a highly chirped picosecond pulse that propagates in the remaining section of the cavity. By minimizing nonlinear effects and avoiding soliton pulse shaping in this anomalous-dispersion section, low repetition rate fiber lasers can be made to produce high-energy ultrashort pulses. Using this approach, 98 fs pulses with 0.96 nJ of energy are obtained from an erbium-doped fiber laser operated in the highly anomalous dispersion regime at a repetition rate of 9.4 MHz. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (190.5530) Pulse propagation and temporal solitons; (320.7090) Ultrafast lasers; (060.3735) Fiber Bragg gratings. http://dx.doi.org/10.1364/OL.39.000989
In recent years, much attention has been paid to ultrafast fiber lasers owing to their compactness, reliability, and easy turn-key operation. These passively mode-locked lasers have become an alternative to solid-state lasers in several commercial and scientific applications such as micromachining, metrology, and terahertz wave generation [1]. Passive mode locking can be achieved via different mechanisms inducing nonlinear loss modulation. Whatever the mechanism being used, the energy per pulse in fiber laser cavities is always limited, if not by the pump power available or the power handling of the cavity components, then by the amount of nonlinear phase accumulated by the pulse during each roundtrip. This situation caused femtosecond fiber oscillators to lag behind their solid-state counterparts in terms of pulse energy [2]. Different cavity designs have been proposed to remedy this situation; the more relevant schemes are the stretched-pulse [3], the similariton [4], the all-normal dispersion [5], and the soliton–similariton [6] lasers. For all designs, the global group-velocity dispersion (GVD) of the cavity greatly affects the performance of the laser. Higher-energy pulses are produced at large normal dispersion, while shorter pulses are obtained at near-zero dispersion [7]. The use of diffractive gratings or linearly chirped fiber Bragg gratings (CFBGs) for dispersion management in fiber laser cavities is attractive since these dispersive elements present weak if not negligible nonlinearity [8]. For instance, a compressor and a stretcher based on diffraction gratings were incorporated in a linear-cavity ytterbium fiber laser to reduce the peak power in the fiber [9]. CFBGs were integrated in linear cavities either to produce high-energy pulses [10,11] or to bring the cavity dispersion close to zero for compact picosecond and femtosecond sources [12,13]. Recently, the impact of a single intracavity CFBG in mode-locked fiber lasers was investigated for different dispersion regimes [8,14]. The main drawback in the use of a single CFBG is that its dispersion usually overcompensates that of the fibers in0146-9592/14/040989-04$15.00/0
side the cavity and dominates the global cavity dispersion [13]. Wang et al. [15] proposed an all-fiber ytterbium laser that employs two CFBGs for dispersion management; 15 nJ chirped pulses of nanosecond to picosecond duration were observed at different positions within the cavity. The generated pulses were compressed to a minimal duration of 340 ps. Picosecond pulses were also obtained from a fiber ring laser based on a semiconductor optical amplifier that used one CFBG in both directions to reduce the pulse peak power in the gain medium [16]. The generation of stable femtosecond pulses has not yet been reported with CFBG-based ring lasers. In this Letter, we demonstrate both theoretically and experimentally that 98 fs pulses with an energy of 0.96 nJ can be generated at 1.55 μm from an all-fiber modelocked ring cavity that includes two ultrabroadband CFBGs with opposite dispersion. With this design, one section of the cavity in which the circulating pulse is strongly chirped and stretched acts essentially like a linear dispersive delay line. Fiber-induced nonlinearity can thus be minimized while fixing the net cavity dispersion to a desired value. This approach can be used as a simple tool for the construction of longer ring cavities with higher energy pulses for a given pump power and net cavity dispersion. The experimental setup, along with the conceptual model of the mode-locked fiber laser, is presented in Figs. 1 and 2, respectively. It should be noted that the conceptual model is a simplified version of the experimental laser that highlights the principal features of the steady-state pulse evolution. As for the experimental cavity that will be described subsequently, mode-locking operation is initiated and maintained by nonlinear polarization evolution (NPE) [17]. The simplified cavity includes single-mode fibers (SMFs), an erbium-doped gain fiber (EDF), and a 50% rejection port output coupler (50/50 OC). The novelty arises from two CFBGs that have opposite group delay dispersion combined with circulators arranged in such a way as to force unidirectional © 2014 Optical Society of America
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Fig. 1. Schematic of the laser cavity. WDM/ISO, wavelength division multiplexer and isolator hybrid; 50/50 OC and 90/10 OC, 50% and 10% rejection port output couplers; CIRC, circulator; CFBG, chirped fiber Bragg grating; PC, polarization controller; POL, polarizer; EDF, erbium-doped fiber.
propagation of the signal. These elements are inserted at both ends of the active fiber segment and do not affect the net cavity dispersion. One should note that additional SMFs related to the use of circulators are not included in the conceptual model for simplicity. To model the cumulative effect of NPE along the fibers, numerical simulations of the dynamics of this laser were performed by solving the vector nonlinear Schrödinger equation using the split-step algorithm, as presented in [18]. Starting from white noise, the signal was propagated in the cavity until it converged to a steady-state pulse. The values of the parameters chosen for the SMFs (β2 −22 fs∕mm, γ 0.0011 W−1 m−1 , Lsmf 6.2 m) and the EDF (β2 58.6 fs∕mm, γ 0.0064 W−1 m−1 , Ledf 1.5 m) correspond to the estimated experimental values. Saturated gain along the EDF is represented by g g0 ∕1 E p ∕E sat , where g0 is the small-signal gain with an implicit Lorenztian profile of 5 THz bandwidth and a peak R value of 30 dB. The pulse energy is given by E p jAx tj2 jAy tj2 dt, where Ak t is the amplitude of the electric field component in the k direction k x; y. The saturation energy E sat depends on the pump power [4]. The NPE system is implemented via a polarizer, a quarter waveplate, and a half waveplate with different angles θQ and θH with respect to the polarizer axis and represented by a Jones matrix. The positions of both CFBGs, which are described as simple linear dispersive elements, are emphasized in the schematic diagram (Fig. 2). Because of the ultrabroad reflectivity band (∼300 nm centered at 1.575 μm) of the gratings used in the experimental setup [19], no filtering effect has been considered in the simulations. Thereby, one passage through such a grating is represented in the in 2 spectral domain by Aout k ω Ak ω expiB2 ω ∕2. The values of the dispersion parameter were fitted as B2 1.63 ps2 for CFBG1 and B2 −1.63 ps2 for CFBG2 , in agreement with the gratings used in the experimental laser. Considering the estimated fiber and CFBG dispersions, the net cavity dispersion is Bnet ∼ −0.049 ps2 .
Fig. 2. Top, conceptual model of the ring cavity. Bottom, simulated evolution of the temporal and spectral widths (FWHM, sum of x and y components).
Stable mode locking is obtained for various pump powers and waveplate orientations. Figure 2 shows the intracavity dynamics of the steady state pulse in a case of low energy (E sat 25 pJ, θQ π∕8, and θH −π∕16). During one roundtrip, the pulse experiences important temporal stretching when passing through CFBG1 while being compressed below 1 ps after crossing CFBG2 . Only small variations of the spectral width are observed along the cavity. The introduction of these two dispersive elements that compensate for each other forces the steadystate pulse to be short in the gain fiber while being highly chirped in the remaining section of the cavity. Figure 3 shows the temporal and spectral intensity profiles that correspond to the sum of the x and y contributions, at two different locations in the cavity that are identified by a dashed line in the top diagram of Fig. 2. The temporal shape of the 15 ps stretched pulse after its passage through the NPE system mimics almost perfectly its spectral shape. Accordingly, the temporal profile of
Fig. 3. Simulated optical spectrum and temporal intensity profile for two positions in the laser: (I) after the EDF; (II) after the NPE system.
February 15, 2014 / Vol. 39, No. 4 / OPTICS LETTERS
the chirped pulse and thus the temporal stretching ratio, which is in this case ∼25, are strongly dependent upon the spectrum of the propagating pulse for a fixed value of B2 . The 0.18 nJ pulse at the output port can be dechirped to 210 fs external to the cavity. The experimental setup built to validate the concept is presented in Fig. 1. The 1.5 m long EDF is pumped by a single-mode laser diode. Up to 935 mW pump light is delivered through a 980/1550 nm WDM/ISO hybrid. The 50% rejection port coupler is inserted after the EDF. A circulator is used for each passage through the reflective gratings. The ultrabroadband CFBGs were fabricated in SMF28 fibers using 800 nm fs pulses and a highly chirped phase mask, as described in [19]. Hydrogen loading of the SMF28 fiber was performed prior to CFBG writing in order to reach the required index modulation to obtain a high reflectivity over a large bandwidth. The phase mask has a chirp rate of 53.2 nm∕cm, providing CFBGs with a dispersion of 1.25 ps∕nm when used as reflection filters. The CFBGs were written over a length of 40 mm at an index modulation estimated to ∼5 × 10−3 , providing a reflectivity larger than 99% over a bandwidth of 300 nm, ranging from 1425 to 1725 nm. The NPE system, including two fiber squeezer PCs and a polarizer, follows the first circulator along with a second output coupler (10% rejection port) for the characterization of the highly chirped pulse. Including the back and forth propagation between the circulators and the CFBGs, the length traveled by the signal in the Corning SMF28 segments is 6.2 m. Taking into account the EDF and HI1060 fibers, the overall cavity dispersion is Bnet ∼ −0.049 ps2 . For a given adjustment of the PCs, the laser generates a stable pulse train at 24 MHz. Figure 4 presents the temporal and spectral features of the signal at two different output ports of the laser along with the numerical simulation results for an identical cavity that includes all-fiber segments and estimated losses of the real components (reflectivity of the CFBGs and insertion losses of splices,
Fig. 4. Left, measured (black solid curve) and numerically simulated (gray dashed curve) optical spectrum and autocorrelation trace of the dechirped pulse at output I. Right, measured (black solid curve) and numerically simulated (gray dashed curve) optical spectrum and temporal profile of the highly chirped pulse at output II.
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circulators, and couplers). All other parameters of the simulation are identical to those of the conceptual model except for the NPE system (θQ π∕8 and θH −π∕8) and the saturation energy of the EDF (E sat 150 pJ), which were adjusted to fit the experimental temporal and spectral profiles. The spectrum and the intensity autocorrelation trace of the dechirped pulse taken from output I after a 1 m long SMF are shown in the left part of Fig. 4. The output power is 12.05 mW and the pulse duration is 124 fs, assuming sech2 t shape. The corresponding pulse energy is 0.5 nJ. The temporal intensity profile of the chirped pulse at output II was measured with a 50 GHz sampling oscilloscope and is presented on the right of Fig. 4 along with the pulse spectrum. The measured pulse duration (FWHM) is 28 ps. The large spectrum (FWHM 23.6 nm) indicates that the pulse is strongly chirped at output II, as expected from theory. A stretching ratio of ∼45 was evaluated with the numerical simulation. In comparison to simulations, sharp asymmetric peaks are present in both experimental spectra. These abrupt variations probably arose from the imperfections in the reflectivity profile and the group delay induced by both CFBGs. To prove the potential benefits of this laser, the 90/10 OC in Fig. 1 was removed and replaced by 15 m of SMF, leading to a net cavity dispersion of Bnet ∼ −0.34 ps2 . Figure 5 presents the temporal and spectral features of the dechirped pulse at output I along with the numerical simulations of the laser (E sat 300 pJ, θQ π∕8 and θH −3π∕16). Modulations in the spectrum are mostly due to nonlinear compression inside the output fiber. According to simulations, the CFBG-induced stretching of the pulse in the main part of the cavity seems to prevent soliton pulse shaping and the formation of Kelly sidebands. This allows the generation of 98 fs pulses with an energy of 0.96 nJ at a repetition rate of 9.4 MHz. The experimental results confirm the essential features of this laser. First, the presence of CFBGs does not prevent mode locking even if the reflectivity and group delay curves of the gratings did show some ripples [19] that were not considered in the simulations. Second, the pulse was relatively short in the EDF section and strongly chirped and stretched (up to 200 times its dechirped duration) in the other portion of the cavity. The section where the pulse is stretched behaves essentially like an all-fiber dispersive delay line. Consequently, this section could be made much longer to reduce the repetition rate of the laser and thereby produce higher-energy pulses for a given pump power, as shown with the 9.4 MHz cavity. As emphasized by Renninger et al. [20], the highly chirped pulse generated on one side of the cavity can be used to simplify the integration of an all-fiber amplifier scheme external to the laser for the production of high peak power femtosecond pulses. Furthermore, the cavity dispersion could be controlled precisely by introducing fibers of different GVDs and lengths in the portion with negligible nonlinearity. In the case of an EDF laser, this is especially relevant because both anomalous and normal dispersion fibers are available. The possibility of changing and controlling the dispersive and reflective properties of the CFBGs also procures a great advantage over previous lasers
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This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Le Fonds de Recherche du Québec Nature et Technologies (FRQNT), and the Canada Foundation for Innovation (CFI).
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and opens vast possibilities for exploring new modelocking regimes employing controllable lumped dispersive elements. In conclusion, we have introduced an all-fiber laser in which the temporal features of the circulating pulse are governed by a pair of ultrabroadband fiber Bragg gratings of opposite dispersion. A highly chirped pulse propagates along the majority of the anomalous fiber and is subsequently compressed in the normal dispersion gain fiber segment. This concept was demonstrated experimentally and was also applied in the case of a highly anomalous dispersion cavity. The 0.96 nJ output pulses following the EDF were dechirped externally to ∼98 fs. This is, to the best of our knowledge, the first femtosecond all-fiber ring laser employing CFBGs. This approach, that leads to a reduced nonlinearity in the cavity, could be applied to other fiber lasers operated in different dispersion regimes and with other mode-locking mechanisms if one wants to increase the pulse energy.
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