Comparative study of supercontinuum generation using standard and high-nonlinearity fibres pumped by noise-like pulses

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1 Laser Physics PAPER Comparative study of supercontinuum generation using standard and high-nonlinearity fibres pumped by noise-like pulses To cite this article: 2017 Laser Phys View the article online for updates and enhancements. Related content - Numerical study of supercontinuum generation using noise-like pulses in standard fibre J P Lauterio-Cruz, J C Hernandez-Garcia, O Pottiez et al. - Generation and characterization of erbium- Raman noise-like pulses from a figureeight fibre laser H Santiago-Hernandez, O Pottiez, R Paez- Aguirre et al. - Flat supercontinuum generation by a F8L in high-energy harmonic noise-like pulsing regime J C Hernandez-Garcia, J M Estudillo- Ayala, O Pottiez et al. Recent citations - Noise-like pulses generated from a passively mode-locked fiber laser with a WS 2 saturable absorber on microfiber Zhenhong Wang et al - Numerical study of supercontinuum generation using noise-like pulses in standard fibre - Numerical study of multiple noise-like pulsing in a dispersion-managed figureeight fibre laser O Pottiez et al This content was downloaded from IP address on 14/09/2018 at 04:06

2 Astro Ltd Laser Physics Laser Phys. 27 (2017) (9pp) Comparative study of supercontinuum generation using standard and high-nonlinearity fibres pumped by noise-like pulses J P Lauterio-Cruz 1, O Pottiez 1, Y E Bracamontes-Rodríguez 1, J C Hernández-García 2,3, E García-Sánchez 4, M Bello-Jimenez 5 and E A Kuzin 6 1 Centro de Investigaciones en Óptica (CIO), Loma del Bosque 115, Col. Lomas del Campestre, León, Gto , Mexico 2 Departamento de Electrónica, División de Ingenierías CIS, Universidad de Guanajuato, Carr. Salamanca-Valle de Santiago Km Km, Comunidad de Palo Blanco, Salamanca, Gto , Mexico 3 Consejo Nacional de Ciencia y Tecnología, Av. Insurgentes Sur 1582, Col. Crédito Constructor, Del. Benito Juárez, , Mexico 4 Instituto Tecnológico Superior de Guanajuato, Carr. Estatal Guanajuato-Puentecillas, Km 10.5, Predio el Carmen, Guanajuato, Gto , Mexico 5 Instituto de Investigación en Comunicación Óptica, Universidad Autónoma de San Luis Potosí, Av. Karakorum 1470, Lomas 4ta Secc., San Luis Potosí, S.L.P , Mexico 6 Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), L. E. Erro 1, Sta. Ma. Tonantzintla, Pue , Mexico jplauterio@hotmail.com Received 5 April 2017 Accepted for publication 21 April 2017 Published 10 May 2017 Abstract We present a systematic experimental study of supercontinuum (SC) spectra produced by propagating noise-like pulses (NLPs) from an erbium-doped figure-eight laser through sections of different lengths of standard single-mode fibre (SMF) and of high-nonlinearity fibre (HNLF), as well as their combinations. With an average input power that does not exceed 35 mw, very broad and smooth SC spectra extending over several hundreds of nm are typically observed, even when only SMF is used. However, maximal broadening and flatness are obtained through a section of about 300 m of SMF followed by 50 m of HNLF; the spectrum then spans from ~1260 nm up to ~2070 nm, with a bandwidth of 431 nm at 5.7 db from the maximum (over 800 nm at 20 db). Our analysis indicates that nonlinear processes are still operating even after propagation through several hundreds of meters of fibre, extending and flattening the spectrum, its maximal bandwidth being eventually limited by the strong fibre attenuation in the 2 µm region. Keywords: noise-like pulses, stimulated Raman scattering, ultrafast processes, nonlinear waveguides (Some figures may appear in colour only in the online journal) /17/ $ Astro Ltd Printed in the UK

3 1. Introduction Supercontinuum (SC) generation has proved to be a fertile area of research involving multiple nonlinear effects and their interactions. Since it was first reported in 1970 [1], SC generation has been the subject of intense research activities and has led to various applications. First obtained in a bulk glass and then in a wide variety of nonlinear media including optical fibres, SC generation refers to the process through which relatively narrowband optical signals suffer extreme spectral broadening as a result of the combination and interaction of nonlinear optical phenomena such as self-phase modulation (SPM), cross-phase modulation (XPM), stimulated Raman scattering (SRS), four-wave mixing (FWM), modulation instability (MI) and fission of higher-order solitons (HOS), as it has been published on the subject [2, 3]. The interest to continue developing supercontinuum light sources relies mainly on their potential applications, which include telecommunications [4], optical pulse compression [5], sensing and microscopy [6], and imaging [7]. Typically, SC is generated by ultrashort pulses interacting with a nonlinear medium [2, 8, 9], and can also be produced easily by long (ns) pulses [10 13], and even by continuouswave beams [14]. Nonetheless, in recent years the use of noise-like pulses (NLPs) as the seed to generate wider and flatter spectra has been promoted by several researchers, motivated by the need to improve SC sources in terms of efficiency and low cost. NLPs are ~ns-long wave packets produced in passively mode-locked fibre lasers (PML-FL) containing thousands of sub-pulses (fs ps), exhibiting a fairly stable overall behaviour on large time scales but displaying a very rich and complex internal dynamics at short time scale [15]. As typical signature [16], this regime presents a broad and smooth average spectrum and a double-scale autocorrelation trace with an ultrashort coherence spur (reflecting the internal temporal granularity of the main packet) sitting on a wide pedestal (representing the entire temporal extension of the pulse burst). Since the fundamental mode locking operation was first reported in 1997 [15], several applications have been developed; for instance, low-coherence spectral interferometry [17], micromachining [18], nonlinear frequency conversion [19], and optical coherence tomography (OCT) [20]. In the frame of SC generation, NLPs offer many advantages over ultrashort pulses. First of all, NLPs are robust against dispersion. In a dispersive medium, an ultrashort pulse quickly disperses away and its peak power vanishes, provoking the extinction of nonlinear effects. Hence, SC generation with this type of pulses requires nonlinear spectral broadening to take place over a short initial distance in the fibre. Therefore, fibres with high nonlinear coefficient and specially tailored dispersion properties must be employed, such as photonic crystal fibres (PCFs) [8, 21], which are very expensive. In contrast, in spite of their fine sub-ps inner structure, NLPs are very long, so that they do not vanish quickly and their peak power remains high over long distances. Hence, nonlinear effects integrate over much longer lengths of fibre, allowing SC generation even in fibres whose nonlinear and dispersive properties are not optimized for this purpose, as it will be observed in the present work. Spectral broadening in such media could also be strongly enhanced by the nonlinear interaction between the NLP components (inner sub-pulses); although the underlying physical mechanisms are not clear at this stage, the effects of pulse interaction were illustrated in a recent experimental work [22], in which the spectrum produced by a pair of pulses after travelling through a segment of conventional fibre was much broader than a single pulse. Another advantage of NLPs is a high energy per pulse, up to 100s of nj (~1000 times that of a soliton) [23 25], because a NLP is not a simple pulse but a large bunch of radiation. Also in the frame of SC generation, high pump pulse energy is important, because a SC once generated can hardly be amplified without suffering drastic bandwidth reduction. In addition, because the NLP spectrum is broad (more than solitons, typically tens of nm), one can expect that further broadening will be reached more easily in comparison with conventional pulses; actually, some papers have reported NLP spectra that exceed 100 nm directly at the laser output [25 28]. Finally, as a consequence of the diversity of the sub-pulses forming the NLPs in terms of intensity and duration, the resulting SC is typically more uniform than the spectrum obtained from ultrashort pulses with fixed properties, which usually displays a spiky aspect, as can be seen in [16]. On the other hand, NLPs also have some advantages over conventional ns pump pulses. In the case of ns pulses, spectral broadening is initially driven by MI that progressively converts the long pulse into a packet of ultrashort pulses; broadly speaking, the fragmentation stage of the long pulse is skipped when using NLPs, since from the beginning we already have a packet of ultrashort pulses with a large spectral width. Besides, although some works report the generation of a broad SC (even spanning from 1 µm to beyond 2 µm in short pieces of conventional fibre) using ns pulses and simple setups, its flatness is usually affected by the presence of a pronounced peak of residual pump, as can be seen in [11 13]. Furthermore, the required input peak power usually scales as a few kw [11 13] (in comparison, in the present work the NLP envelope does not reach 100 W). Some researchers have reported SC generation using NLPs produced in several fibre laser designs, demonstrating the benefits of this type of pump pulses as mentioned above. In [16] the authors present a comprehensive review on NLPs; they also include several SC spectra obtained using NLPs from a ring cavity and different types of fibres, including conventional single-mode fibre (SMF) and high-nonlinearity fibre (HNLF). These spectra, which extend over a few hundreds of nm, contrast in terms of flatness with those generated in the same fibres by ultrashort pulses. The potential of NLPs for SC generation in SMF has been demonstrated by several authors; spectral broadening is then mainly observed towards longer wavelengths. In [29], for an average pump power as small as 20 mw, a spectrum extending over 200 nm at least was obtained after the propagation of NLPs through 750 m of SMF. In [30], by propagating NLPs through 100 m of SMF in the normal dispersion regime, the authors achieved a flat, broadband spectrum of about 200 nm with an average pump power of 3 W. If high laser pump powers are available, flat, broadband 2

4 Figure 1. Scheme of the laser under study: (a) figure-eight laser (F8L); (b) amplifier and nonlinear element for SC generation. spectra covering 200 nm can also be obtained directly at the laser output if a long section of SMF is inserted in the cavity, as recently shown by our group [25, 31]. On the other hand, in order to further improve the SC extension (in particular, to the short-wavelength side) and flatness, some authors proposed using HNLF. In [26], the use of a piece of HNLF inside a ring laser cavity allowed the generation of NLPs with a very flat spectrum over a bandwidth of 135 nm (at 3 db points); a similar result was obtained in [27]. In [32], by inserting at the laser output a 1 km long HNLF with near-zero dispersion at the pump wavelength, and for a moderate NLP power (72 mw), a very broad spectrum of about 950 nm was achieved, extending in both directions and reaching the 2 µm region on the long-wavelength side. In [33], again using a HNLF at the laser output, for ~200 mw average power a spectrum with a similar extension, spanning from 1200 to 2100 nm was produced, although with a reduced flatness. Spectral broadening over several hundreds of nm on the short-wavelength side was also achieved in [34] through a piece of HNLF, yielding a flat spectrum as broad as 500 nm. Finally, in a recent work [25], we used NLPs from a figure-eight laser (F8L), presenting a very broad spectrum covering 200 nm directly at the laser output, for ~300 mw average pump power; then, launching these pulses into a 100 m piece of HNLF, the spectrum was extended to both sides and flattened significantly, reaching up to 450 nm. In spite of the relatively abundant literature on SC generation using NLPs in different types of fibre, most authors do not justify their particular choice of the fibre length (which can be as long as 1 km [32] or as short as 1 m [33]), nor do they perform a comparative study or optimize this length. Besides, combining different types of fibres in an attempt to improve the SC properties is rarely considered. In order to fill these gaps, in the present work, we perform a systematic experimental study of SC generation by launching NLPs through different lengths of standard and HNLF and their combinations. NLPs of low pump power which are used as the seed are amplified using an erbium-doped fibre (EDF) amplifier, up to an input power level that does not exceed a few tens of mw (corresponding to a few tens of watts for the NLP envelope). We obtain very flat and extended spectra on both sides of the 1550 nm region by combining specific lengths of fibres, and demonstrating that the nonlinear spectral broadening is still at play even after hundreds of meters. 2. Experimental setup The scheme under study, depicted in figure 1, consists of 3 stages: a figure-eight laser as a source of NLPs, an amplification stage and a nonlinear element for the generation of very broad spectra, consisting of different sections of SMF and/or HNLF of various lengths and their different 2-by-2 combinations. The PML F8L with a total cavity length of about 218 m is formed by a unidirectional ring cavity and a nonlinear optical loop mirror (NOLM), as shown in figure 1(a). The ring section includes two pieces of erbium-doped fibre (EDF1 is 3 m long, and EDF2 is 2 m long; 30 db m 1 absorption at 1530 nm), pumped at 980 nm through wavelength-division-multiplexing (WDM) couplers. It also includes a fiberized optical isolator (ISO1) to ensure unidirectional laser operation, a piece of 100 m of dispersion compensating fibre (DCF, D = 3 ps/nm/km), a polarizer (POL) to ensure linear polarization, and a polarization controller (PC). The PC consists of a half-wave retarder (HWR1) and a quarter-wave retarder (QWR1), used to maximize the power transmission through the polarizer. To control the angle of linear polarization at the NOLM input, a second 3

5 Figure 2. NLPs fundamental mode locking: (a) time-domain envelope measured using a scope in average mode (inset: pulse train); (b) optical spectrum in mode locking regime and ASE emission below lasing threshold; (c) intensity autocorrelation trace of pulses (inset: close-up on central spur). half-wave retarder (HWR2) is implemented. The NOLM is a power-symmetric, polarization-imbalanced scheme, whose switching relies on nonlinear polarization evolution (NPE) in the twisted loop [35]. It is made of a 50/50 coupler, 100 m of SMF (D = 17 ps/nm/km) twisted at a rate of 5 turns per meter, and a quarter-wave retarder (QWR2) to break the polarization symmetry [36]. Finally, the output ports are provided by two 90/10 couplers, being the output 2 to monitor the pulses. The laser output 1 is spliced to the second and third stages as shown in figure 1(b). A second isolator (ISO2) was placed to protect the laser cavity from the amplified spontaneous emission (ASE) and avoid destabilizing the NLP operation. The amplifier includes a piece of 4 m long EDF3 (same type as in the laser) pumped at 980 nm through a WDM coupler; this length was the optimal one to achieve the results of the present work. To monitor the pulses at the EDF3 input and output, two ports are provided by 99/1 couplers. Finally, as the nonlinear media to generate SC, several combinations of spools of different lengths of SMF and HNLF (high-index core surrounded by a deeply depressed ring yielding a mode field diameter of 3.9 µm, nonlinear coefficient of γ = 10.8/W/ km, 1550 nm zero-dispersion wavelength, dispersion slope of ps/nm 2 /km) are used. 3. Experimental results For proper configuration of the wave retarders, self-starting fundamental mode locking operation was obtained. With some adjustments, we obtained a nearly stationary NLP regime. Figure 2(a) presents the NLP envelope measured at output 2 with a 25 GHz photodetector (New Focus 1414) and a 16 GHz real-time oscilloscope (Tektronix DPO71604C), which allows estimating a mean full-width at half-maximum (FWHM) pulse duration of 350 ps (average of 1000 traces). The pulse train was fairly stable (inset) with a repetition rate of 904 khz (which corresponds to a period of T = µs). To verify that the obtained pulses were NLPs, besides their temporal profile, we measured the optical spectrum and the autocorrelation trace. The spectrum was measured with an optical spectrum analyzer (OSA, Anritsu MS9740A) at output 2, being smooth and wide with a maximum at 1567 nm (figure 2(b), solid red line), with 13.4 nm 3 db bandwidth and 53.4 nm 20 db bandwidth. In this work we also present values of bandwidth measured at 20 db from the maximum, because values at 3 db do not always provide enough information to correctly describe the actual extension of SC spectra, as we shall see below. At 1531 nm, a small shoulder attributed to ASE appeared; this attribution was confirmed by the presence of a peak at the same wavelength in the spectrum measured below the lasing threshold (figure 2(b), marked black line). Besides, a small peak corresponding to the maximum of the Raman gain appeared at 1661 nm. Intrapulse Raman scattering is also present, as evidenced by the slight asymmetry of the main spectrum at 1567 nm and its red shift with respect to the gain maximum at 1557 nm. Finally, the autocorrelation trace is shown in figure 2(c). It exhibits a typical doublescale structure, with an ultrashort coherence spur of fs (inset) riding a broad pedestal of ps (corresponding to the measurement window of the autocorrelator, narrower than the duration of the whole packet). The level of the pedestal 4

6 Figure 3. (a) SC spectra at the output of the nonlinear element formed with SMF (inset: spectra with extended span including the residual pump); (b) evolution of bandwidths at 3 and 20 db from the maximum, for the different lengths of SMF. is related to the density of the sub-pulses in the bunch and the statistical distribution of their intensities. Due to imperfect alignment of the autocorrelator (FR-103XL), the trace shows a slight left-right asymmetry. Once the stationary NLP operation was confirmed, the pulses at output 1 were amplified using the scheme of figure 1(b) with 290 mw pump power, and the resulting pulses were launched into the nonlinear fibre. As reference, the spectrum was measured directly at the output of the amplifier (output 4, figure 1(b)). With 25 mw of average power at the EDF3 output, the spectrum (figures 3(a) and 4(a), cyan crosses line) presented a maximum at ~1563 nm and 21 nm 3 db bandwidth (64 nm at 20 db). The presence of residual pump at the amplifier output was attested by a narrow peak at 976 nm flanked by a pair of lobes (~ nm), as can be seen in the inset of figure 3(a). This structure had a replica: its second-order diffraction at 1952 nm, with a similar pair of lobes (~ nm). Due to the extension of the SC spectra discussed below, measurements required the simultaneous use of two Yokogawa optical spectrum analyzers (AQ6370B and AQ6375), whose data were merged. In a first set of experiments, the performances of SC generation through different lengths of SMF were analyzed. During measurements, the average power of the injected NLPs was maintained constant (25 mw). Figure 3 presents the results obtained with spools of 50 m, 100 m, 315 m, 600 m and 1000 m. The spectra are shown in figure 3(a), whereas figure 3(b) presents the evolution of the 3 db and 20 db bandwidths as fibre length is increased. It has to be stressed first that all these spectra are extremely smooth, as can be seen in figure 3(a). For a fibre span as short as 50 m, no spectral broadening is observed at the fibre output, and the resulting curve is indistinguishable from the input spectrum (labelled 0 m ). Then, starting with a length of 100 m, spectral broadening becomes noticeable, taking place exclusively towards longer wavelengths. As fibre length is increased, the spectral maximum in the 1560 nm region slightly shifts to the right. The maximal spectral extension is reached with 315 m of SMF; in this case, the spectrum spans over more than 300 nm, from the 1560 nm region to ~1850 nm, where a secondary maximum is observed (the 20 db bandwidth then reaches a maximum of 323 nm, see figure 3(b)). In spite of this, a relatively poor flatness is observed at this stage, as attested by the moderate value of the 3 db bandwidth (~25 nm). If now the fibre length is further increased, spectral components grow in the 1700 nm region, filling the gap between the two maxima, which results in an improved spectral flatness. Therefore, the 3 db bandwidth increases to ~40 nm with 600 and 1000 m of SMF. In addition to an improved flatness, extending the fibre length beyond 315 m also has the consequence of reducing the long-wavelength limit of the spectrum. This effect is attributed to higher fibre losses when the 2 µm region is approached. As a result, the 20 db bandwidth decreases to 247 nm for the 1000 m piece of fibre. Hence, although nonlinear effects are still operating after 500 m of SMF, fibre losses ultimately limit the maximal fibre length and the maximal spectral extension of the SC, as can be seen in figure 3(b). Finally, this figure is a nice illustration of the limits of using the 3 db bandwidth alone for assessing the spectral extension of the SC. In particular, when the SMF length is extended beyond 315 m, the 3 db bandwidth increases, although the 20 db bandwidth actually decreases. With the intention to further extend the spectrum, mainly towards shorter wavelengths, a piece of HNLF of 50 m of length was added to each of the previous SMF spools. During measurements, the average power of the injected NLPs was again maintained constant (25 mw). The results are presented in figure 4. Because the 50 m long piece of SMF did not produce any spectral broadening, it was omitted in this section. At the beginning of this second set of experiments, the spectrum produced by the 50 m long HNLF without SMF was studied (figure 4(a), orange triangles line); the inset shows spectra with extended span to include residual pump. In this case, the spectrum extended not only to the right but also to the left, reaching a bandwidth of 224 nm at 3 db from the maximum and 542 nm at 20 db, with the spectral maximum centred at ~1561 nm. This and the subsequent spectra overall were very smooth, although they presented two regions of water absorption bands: at nm and nm; these features were caused by the 30% humidity prevailing in the laboratory during the measurements. From ~1520 to 1550 nm, a small shoulder appeared with two humps, which can be attributed to ASE (see figure 2(b), marked black line). If now the HNLF is preceded by a 100 m piece of SMF, the spectrum is further extended in both directions, covering 647 nm at 20 db from the maximum. However this spectrum also presents a higher concentration of energy in the 1800 nm 5

7 Figure 4. (a) Extended SC spectra at the output of the nonlinear element formed with SMF plus HNLF (inset: spectra with extended span including the residual pump); (b) evolution of bandwidths at 3 and 20 db for the different lengths of SMF plus 50 m of HNLF. Figure 5. Comparison of SC spectra by combining 315 m of SMF with 50 and 100 m of HNLF (inset: spectra with extended span including the residual pump). For the sake of comparison, both curves are normalized with respect to the residual peak in the 1560 nm region. region, reducing the flatness. For the combination of 315 m of SMF plus the HNLF, the flattest and most extended spectrum is obtained (at 25 mw average power), with 234 nm 3 db bandwidth and up to 772 nm at 20 db, spanning from the 1260 nm region to 2040 nm (red squares line in figure 4(a), as well as in figures 5 and 6(a)). Finally, if the SMF piece is further extended to 600 m or 1000 m, the SC extension reduces significantly and it becomes comparable to the case of HNLF only; the benefits of using SMF in combination with the HNLF section to enhance spectral broadening thus vanish in these cases. This is probably due to the larger attenuation in long sections of SMF, in particular towards 2 microns. Hence, similarly to the first set of experiments, it turns out that attenuation sets an upper limit to the fibre length and to the SC extension. Again, the evolution of bandwidths at 3 db and 20 db as SMF length is increased is presented (figure 4(b)). Finally, measurements were repeated with the SMF and HNLF fibres combined in the inverse order. For all lengths of SMF, the obtained spectra were similar to the case of the HNLF only, so that no benefit was obtained by using the SMF together with the HNLF in that configuration. This result could be related to losses of the taper at the HNLF output. In figure 4, the most extended spectrum, towards both longer and shorter wavelengths was obtained with the 315 m long SMF plus the 50 m long HNLF. Figure 5 shows the effect of replacing this piece of 50 m HNLF with a 100 m piece of the same fibre. The differences between the two spectra demonstrate that the nonlinear effects continue to take place beyond 50 m of propagation in the HNLF; however the main effect of lengthening the HNLF is a higher concentration of energy in the nm region, which rises above the pump peak in the 1560 nm region and reduces the SC flatness. Besides, the overall extension of the spectrum is slightly reduced on both sides, and the 20 db bandwidth reduces from 772 nm to 726 nm, again presumably due to higher losses when fibre length is increased. Next, figure 6(a) presents the evolution of the SC spectrum when the input power is varied by adjusting the pump power of the EDF3 amplifier. Again we choose the combination of 315 m of SMF and 50 m of HNLF. This figure shows how the spectrum widens roughly symmetrically on both sides of the spectral maximum and how it becomes progressively flattened as the power level increases. The maximal spectral extension is reached (grey down triangles line; reproduced in figure 6(b)) by setting the maximum pumping level of the amplifier (408 mw, which corresponds to 35 mw average power of the injected NLPs). Although the spectrum presents a substanti al reduction of its 3 db bandwidth (167 nm) 6

8 Figure 6. (a) Evolution of SC spectrum by varying the amplifier pump power in the combination 315 m of SMF plus 50 m of HNLF (inset: spectra with extended span including the residual pump); (b) maximal spectral extension with 315 m of SMF plus 50 m of HNLF (35 mw average input power). compared with the 25 mw case, its 20 db bandwidth is as broad as 808 nm, extending from 1263 to 2071 nm. Its flatness is best characterized by considering that its bandwidth extends over 431 nm at 5.7 db from the peak ( nm) and over 595 nm at 10 db, as can be seen in figure 6(b); also this spectrum presents an excellent dynamic range of 60 db. It should be noted that we are not considering the water absorption bands in these measurements since they can be eliminated by controlling the humidity of the laboratory. Finally, as a summary, the evolution at 20 db for the different lengths of SMF only and then adding the HNLFs for 25 mw and 35 mw input powers, is depicted in figure 7, where it can clearly be seen that the optimal length is an intermediate length of SMF (315 m) plus 50 m of HNLF. 4. Discussions When SMF only is used as the nonlinear medium, the spectral broadening takes place exclusively to longer wavelengths, as shown in figure 3(a). In this case, that broadening is probably due to a combination of cascaded Raman scattering and Kerr effect, the former being responsible for the extended spectral widening, while the latter is responsible of smoothening Figure 7. Evolution of bandwidths at 20 db for the different lengths of SMF and SMF plus HNLF. the generated spectrum, as discussed in [30]. It is important to note that this widening only becomes noticeable beyond 100 m of propagation through the SMF, which is not surprising considering the low nonlinearity of the fibre, and also shows that the nonlinear effects continue to be important after that length. This is a consequence of the robustness of NLPs against dispersion, which allows their application to SC 7

9 generation in such long fibres, with a small nonlinear coefficient and a considerable dispersion at 1550 nm. Nonlinear effects are still at play even beyond 500 m, filling up the central part of the spectrum and contributing to its flattening. But even when nonlinear effects continue to operate at such long lengths, a limit is imposed by the high attenuation of the fibre in the region of 2 µm, which erodes the spectrum at its right end. So the best result in terms of bandwidth is obtained for an intermediate length of SMF (~300 m). On the other hand, when the SMF is followed by a piece of HNLF, the high nonlinearity coefficient of the second fibre and its zero dispersion at 1550 nm allow the spectrum to extend hundreds of nm to the left, but also allow a very important widening to the right, even exceeding 2 µm, although the attenuation of the fibre again imposes an upper limit. The nearly symmetrical broadening apparent in figure 6(a) suggests that, besides the Raman and Kerr effects, parametric nonlinear phenomena such as FWM are involved and play a major role. In spite of this, the nonlinear effects in the SMF sections (Raman and Kerr) contribute substantially to such a dramatic widening as we can see in the spectrum generated by the combination of the 315 m SMF and 50 m HNLF, which was broader than that generated by the HNLF only. This particular combination yields the broadest and flattest of all the spectra obtained in this work, although in all cases very broad and smooth spectra are obtained, which did not present a structured aspect like SC generated with conventional pulses, because NLPs are packets of ultrashort sub-pulses with a wide range of parameters. In terms of performances, the results presented in this work compare favourably with those of previous works dealing with SC generation using NLPs through SMF and HNLF. Concerning SC generation through a piece of SMF, the spectra presented in figure 3, which extend over more than 300 nm, from 1550 nm to the nm region, compare with or even outperform those obtained in other works [29, 30] in terms of bandwidth, which is noticeable considering that the average pump power that was employed here lies two orders of magnitude below the one used in [30]. When a 50 m piece of HNLF only is used, the spectrum (figure 4(a), orange triangles line) is slightly less extended than those reported in other references [32, 33]; however, when the HNLF is preceded by the 315 m long SMF, the spectrum further expands by ~100 nm on each side (figure 4(a), red squares line), spanning from ~1200 nm to the 2100 nm region, and then compares with those references in terms of bandwidth. Besides, in terms of flatness, our results compare with those of [32], and actually outperform those of [33]. Again, this is a noticeable achievement considering that the average power lies two to ten times below that of other references [25, 32, 33] and in view of the relatively moderate length of HNLF. Finally, it should be mentioned that, because out experimental scheme is made up mainly of standard telecom elements, it is as simple and cheap as other schemes developed to generate SC using ultrashort pulses or ns pulses, and definitely cheaper than those that use Nd:YAG sources, for example. 5. Conclusion In this work we perform a systematic comparative study of SC generation using NLPs with low average power (25 35 mw) as the seed, using different pieces of standard and HNLF of various lengths and their combinations. When pieces of SMF of several hundreds of meters are used, the spectrum extends to longer wavelengths, covering more than 300 nm (from the 1560 nm region to ~1850 nm) for a SMF length of 315 m. As the fibre length is further extended, the spectral flatness improves whereas the long-wavelength edge of the spectrum erodes as a consequence of high fibre losses near 2 µm. Using HNLF with zero dispersion at 1550 nm, bilateral spectral broadening is achieved; however the broadest and flattest spectrum is obtained for a specific combination of SMF and HNLF. With a piece of 315 m of SMF followed by 50 m of HNLF, maximal broadening is achieved, with over 800 nm 20 db bandwidth, the spectrum then spans from the 1260 nm region to ~2070 nm. This spectrum also shows an excellent flatness over a region of 431 nm at 5.7 db and it has an outstanding dynamic range as high as 60 db, which makes it an excellent source for some applications such as OCT or spectral device testing. Our analysis indicates that nonlinear processes are still operating due to the NLPs characteristics after propagating into fibres of hundreds of meters in length, eventually limited to larger wavelengths by the strong fibre attenuation in the 2 µm region. On the basis of the experimental results presented here, and with the help of numerical simulations, future research efforts will be directed towards a precise identification of the processes involved in the production of SC spectra by NLPs in different types of fibre. Acknowledgments This work was funded by the CONACyT Fronteras de la Ciencia program (grant 471) and by CONACyT Ciencia Básica project J.P. Lauterio-Cruz was supported by CONACyT grant References [1] Alfano R R and Shapiro S L 1970 Phys. Rev. Lett [2] Zheltikov A M 2006 Phys. 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