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Supercontinuum Q-switched Yb fiber laser using an intracavity microstructured fiber J. Cascante-Vindas, A. Díez,* J. L. Cruz, and M. V. Andrés Departamento de Física Aplicada—ICMUV, Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Spain *Corresponding author:
[email protected] Received July 24, 2009; revised October 16, 2009; accepted October 26, 2009; posted November 2, 2009 (Doc. ID 114728); published November 19, 2009 We report on an intracavity configuration for supercontinuum generation in a Q-switched Yb fiber laser. The supercontinuum laser includes a section of microstructured fiber within the Q-switched laser cavity. With 380 mW of pump power, the supercontinuum laser can emit broadband pulses of 6 J energy and 10 ns temporal width, at repetition rates from few hertz up to 2 kHz. The supercontinuum spectrum spans over a wavelength range in excess of 1.4 m. © 2009 Optical Society of America OCIS codes: 060.4005, 060.4370, 060.3510.
The most common scheme for supercontinuum (SC) generation is based on the use of a nonlinear optical fiber pumped near its zero-dispersion wavelength (ZDW) by short light pulses. The advent of the microstructured optical fibers (MOFs) [1] opened new possibilities, since silica MOFs with small effective area and ZDW well below 1.3 m can be manufactured, which facilitates the extension of the spectrum toward visible wavelengths [2]. At present, low-cost supercontinuum sources rely on an MOF pumped by a subnanosecond microchip laser [2,3], with the common disadvantages of integrating a bulk laser and fiber components. In addition to alignment and mechanical stability issues, an important limitation arises from the damage threshold of the input facet of the MOF that limits the peak power that can be launched into the fiber, thus limiting the spectral broadening and power density available at the output of the fiber. An interesting alternative is the use of fiber lasers combined with intracavity SC generation. The intracavity technique for broadband generation in the context of fiber lasers was first demonstrated in [4] where IR radiation due to cascaded Raman scattering was reported within the multiwatt optical pumping scale. Recently, a similar configuration of a SC laser based on an Yb-doped nonlinear MOF was demonstrated [5], though the reported results were modest and only light in the short IR 共1000– 1200 nm兲 and the visible 共650– 750 nm兲 domains was generated. Here, we report our investigations on a diode-pumped Q-switched Yb-doped fiber laser emitting over a broadband spectrum using pump powers in the order of few hundreds of milliwatts. Intracavity SC generation was achieved by the insertion of an MOF section with proper chromatic dispersion properties within the Q-switched laser cavity. In contrast to the previous reports, in our configuration the gain and the nonlinear media are different fibers. Figure 1 shows the experimental arrangement of the SC Q-switched Yb fiber laser. A Fabry–Perot laser cavity was arranged using 2.5 m of single-mode single-clad Yb-doped fiber (Nufern SM-YSF-HI) and a pair of fiber Bragg gratings (FBGs) whose Bragg wavelength was 1064 nm. The bandwidth of the 0146-9592/09/233628-3/$15.00
FBGs was 0.1 nm, and they were chosen with high reflectivity, ⬎99%, to enhance the energy of the Q-switched pulses inside the laser cavity. The pair of gratings provides feedback for the 1064 nm signal, which acts as a pump for the nonlinear processes, while other spectral components generated within the cavity will exit the laser with full transmission. The Yb-doped fiber was pumped through a wavelength-division multiplexer (WDM) with a pigtailed laser diode emitting at 976 nm, which provided a maximum pump power of 600 mW. Q-switching was achieved by using a pigtailed acousto-optic modulator (3 dB insertion loss, 25 ns rise time). The inset of Fig. 1 shows a scanning electron microscope image of the MOF used in the experiments. The structural parameters of the MOF are ⌳ = 5.7 m, d / ⌳ = 0.9, and 4.5 m core diameter, where ⌳ is the pitch and d is the hole diameter. The MOF supported several modes at 1064 nm. The ZDW of the fundamental mode was 1030 nm, so its dispersion was anomalous at 1064 nm. The MOF section 共3 m兲 was inserted between the acousto-optic modulator (AOM) and the output FBG. Because of the insertion loss of the AOM, placing the MOF in the opposite side of the AOM, i.e., between the gain fiber and the AOM, would lead to stronger pulses in the MOF. However, the bandpass response of the AOM would filter most of the spectral components of the generated SC. The setup shown in Fig. 1 has another advantage; the rather narrow bandpass filter response of the AOM in conjunction with the wavelength response of the WDM prevents SC and amplified spontaneous emission signals feeding back to the pump laser, which is
Fig. 1. SC Q-switched laser arrangement. © 2009 Optical Society of America
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essential to ensure good pulse-to-pulse and long-term stability of the emission. The MOF was fusion spliced to the fiber pigtails of the output FBG and the AOM, both made of HI1060 fiber, using a commercial arc fusion splicer (Fujikura FSM-20CSII). The discharge parameters, i.e., arc power and arc duration, were set to minimize the holes’ collapse. The discharge current was 11.5 mA, which is approximately one third of the typical arc power values used for splicing SMF-28 fibers, and the arc duration was 300 ms. Once the two fibers were fused together, several arcs were discharged on the splice with the aim of reducing the holes’ diameter, and so reducing the mode field diameter mismatch. Especial attention was paid when doing those splices in order to favor the excitation of the fundamental mode in the MOF. The total insertion loss was about 1 dB, including the MOF transmission loss and the splice losses. Figure 2 shows the spectral and temporal characteristics of the output radiation when the laser was running at 1 kHz repetition rate. At the pump power of 380 mW, the output pulse energy was 6.4 J and the temporal width was 10 ns, with an estimated intracavity peak power of about 2.5 kW. The pulse-topulse amplitude fluctuation remained below 5%. The generated spectrum spans from 400 nm to 2000 nm, showing a very flat region from 1.2 to 1.6 m, where the spectral flatness is ±0.5 dB. Output pulses with similar energy and spectral characteristics than shown in Fig. 2 were achieved at repetition rates from few hertz up to 2 kHz. Of course, a change in repetition rate required adjusting the pump power in
Fig. 2. (Color online) (a) SC output spectrum for different pump powers of 280, 320, and 380 mW. Black and gray (red online) traces were measured with different optical spectrum analyzers [ANDO AQ6314A (10 nm resolution) and AQ6375 (2 nm resolution), respectively]. (b) Output pulse measured with a 1 GHz bandwidth InGaAs photodetector. Pump power, 380 mW.
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order to optimize the performance of the system. Higher repetition rates required higher pump powers. For example, at 2 kHz repetition rate, the pump power required to obtain 6 J energy pulses was 420 mW. The output of the SC radiation was emitted mostly in the fundamental mode. Although the output radiation was emitted from a conventional fiber (HI-1060) whose cutoff wavelength is about 920 nm, the modedness of the SC radiation did not change significantly with respect to the generated in the MOF, since the length of the HI-1060 fiber was short (⬍50 cm) and bending was avoided. The observation of the visible components showed that only the blue component— corresponding to the peak centered at 478 nm, see Fig. 2—was generated in a higher-order mode, as a result of multimode phase-matching in the MOF. Figure 3(a) shows the short and long wavelength edges of the SC spectrum measured at −20 dB, as a function of the pump power of the Q-switched laser. The corresponding pulse energy measured with a piroelectric power meter is shown in Fig. 3(b). An SC spectrum spanning almost two octaves was generated with 380 mW of pump power. Broader SC could not be generated, since higher pump powers led to unstable operation of the SC Q-switched laser. We believe that at higher pump powers, the high cavity gain in conjunction with unwanted small backreflections—for example, from splices between dissimilar fibers—led the laser to reach the cw emission threshold even when the AOM was off. The spectral features observed in the SC spectrum at different stages of its development agree with the typical nonlinear mechanisms reported for extracavity SC arrangements where an MOF is pumped with nanosecond duration pulses in anomalous dispersion regime and near the ZDW [3,6,7]. At low pump powers, modulation instability (MI) bands were observed. At 280 mW of pump power [see Fig. 2(a)], the MI bands are separated about 22 nm from the 1064 nm signal. At higher pump powers, it has been shown that the development of MI leads to the formation of solitons [8] that propagate in the fiber experiencing self-frequency shift (SFS), owing to Raman scattering [7–9], and generating dispersive waves (DWs) [10,11]. SFS of solitons is responsible for the spread-
Fig. 3. (a) SC wavelength edges measured at −20 dB and (b) pulse energy, as a function of pump power.
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Fig. 4. SC spectrum showing the residual 1064 nm signal, for (a) the intracavity SC Q-switched configuration and (b) the extracavity SC generation scheme. Both spectra were recorded with 1 nm resolution.
ing of the SC into the IR, while the generation of DWs extends the SC to the blue. Recently, it has been demonstrated experimentally that the long- and short-wavelength edges of the SC are related by the group index of the fiber [2,12]. A possible mechanism has been pointed out in [13]: the SFS of a soliton propagating in anomalous dispersion effectively traps blue radiation propagating with the same group index in a gravity-like potential and scatters the blue radiation to shorter wavelengths in a cascaded FWM-like process. We found that the intracavity configuration presents some advantages with respect to the conventional SC generation extracavity scheme using MOFs. First, provided that the MOF insertion loss is small, the pulse energy inside the laser cavity is larger than the laser’s output owing to the reflectivity of the output grating. This fact has been further enhanced in our arrangement by using highly reflective FBGs. As a result, broader SC spectrum can be obtained for a given 976 nm pump level. Second, the SC exits the system guided by a standard singlemode fiber, leading to a better beam shape quality. And third, the use of a highly reflective FBG as the laser’s output coupler reduces dramatically the presence of residual 1064 nm signal in the SC spectrum. This effect is shown in Fig. 4, where the output spectrum around 1064 nm wavelength is compared with the SC spectrum obtained with the conventional SC generation scheme. In the later case, the same piece of MOF was pumped with a subnanosecond microchip laser (Teem Photonics SNP-20F-100); the pump power was adjusted to obtain an SC spectrum with similar span to that shown in Fig. 2. The average
pump power required was 40 mW, which corresponds to a pulse peak power of about 3.4 kW. Finally, although being an intracavity arrangement, the limitation arising from the fiber end facets damage is not completely avoided because of the use of the bulk AOM for Q-switching. In summary, we have reported on the intracavity supercontinuum generation in a Q-switched Ybdoped fiber that incorporates a section of an MOF within the laser cavity. With pump powers in the order of few hundreds of miliwatts, the supercontinuum laser can emit broadband pulses of 6 J energy and 10 ns temporal width, at tunable repetition rates from few hertz up to 2 kHz. The spectrum of the emitted pulses spans over a wavelength range in excess of 1.4 m. Additionally, the residual pump signal is reduced significantly as a result of using FBGs with high reflectivity. Research is currently in progress to increase the pulse energy and hence the extension of the supercontinuum, as well as to extent the repetition rate to higher frequencies. This work has been financially supported by the Ministerio de Ciencia e Innovación and the Generalitat Valenciana of Spain (projects TEC 2008-05490 and PROMETEO/2009/077, respectively). References 1. P. St. J. Russell, Science 299, 358 (2003). 2. J. M. Stone and J. C. Knight, Opt. Express 16, 2670 (2008). 3. W. Wadsworth, N. Joly, J. Knight, T. Birks, F. Biancalana, and P. Russell, Opt. Express 12, 299 (2004). 4. S. V. Chernikov, Y. Zhu, and J. R. Taylor, V. P. Gapontsev, Opt. Lett. 22, 298 (1997). 5. A. Roy, M. Laroche, P. Roy, P. Leproux, and J. L. Auguste, Opt. Lett. 32, 3299 (2007). 6. J. M. Dudley, G. Genty, and S. Coen, Rev. Mod. Phys. 78, 1135 (2006). 7. J. M. Dudley, L. Provino, N. Grossard, H. Maillotte, R. S. Windeler, B. J. Eggleton, and S. Coen, J. Opt. Soc. Am. B 19, 765 (2002). 8. S. M. Kobtsev and S. V. Smirnov, Opt. Express 13, 6912 (2005). 9. G. Genty, M. Lehtonen, H. Ludvigsen, J. Broeng, and M. Kaivola, Opt. Express 10, 1083 (2002). 10. N. Akhemediev and M. Karlsson, Phys. Rev. A 51, 2602 (1995). 11. G. Genty, M. Lehtonen, H. Ludvigsen, and M. Kaivola, Opt. Express 12, 3471 (2004). 12. L. Fu, B. K. Thomas, and L. Dong, Opt. Express 16, 19629 (2008). 13. A. V. Gorbach and D. V. Skryabin, Nat. Photonics 1, 653 (2007).
