In this work we present results on supercontinuum (SC) generation in a photonic crystal fiber (PCF) fabricated from lead-bismuth-gallium-oxide glass (PBG-08). Due to high refractive index, high nonlinearity and high transmittance, the PBG-08 glass-based fibers seem to be excellent media for broad supercontinuum generation in the infrared spectral region. In our experiment, a short-length piece of PCF (5-6 cm) is pumped by a femtosecond chirped pulse amplification (CPA) setup, which may be seeded by two different fiber-based oscillators. This compact and cost-effective system allows to generate SC spanning from 900 to 2400 nm. The paper describes in detail the fabrication process of the fiber, as well as the SC generation results.
© 2013 Optical Society of America
Broadband, compact and cost-effective infrared (IR) supercontinuum (SC) sources are currently on demand of many applications in the field of spectroscopy and sensing. The combination of high spectral brightness and broadband wavelength coverage of SC  enables detection of spectral signatures of multiple species simultaneously. It has been already shown, that SC sources might be used for trace gas sensing using classical absorption spectroscopy [2,3], cavity-enhanced absorption spectroscopy [4,5] and Cavity Ring-Down Spectroscopy (CRDS) . For all those applications, SC spanning into the mid-infrared region is particularly desirable, because of the presence of vibrational spectra of many molecules above 2 μm. Broadband SC with spectral widths exceeding 1000 nm can be conveniently generated by launching ultrashort pulses into highly-nonlinear fibers. Recently, the development of photonic crystal fibers (PCFs) brought new opportunities in generation of SC with low pump power threshold, high conversion efficiency and extremely broad bandwidth . Up till now usually silica-based fibers were used as nonlinear media for visible and near-IR SC generation [7–9]. However, the intrinsic transmission window of silica is strongly limited to around 2.3 µm. In order to meet the necessary criteria of most spectroscopic applications, it is demanded to find new, alternative materials with broader transmission windows and stronger nonlinearities. Recently, tellurite [10–12], chalcogenide [13–15], heavy oxide [16,17] or fluoride (ZBLAN) glasses [18,19] have been considered as host materials for PCFs, as an alternative to conventional fused silica. For example, Domachuk et al.  demonstrated generation of very broad SC, spanning up to 4500 nm in a tellurite PCF. However, the experiment required very short (100 fs) pulses, delivered from a complex optical parametric oscillator (OPO) system pumped by a Ti:Sapphire laser. Similarly, in  Savelli et al. have shown efficient SC generation in a tellurite PCF, but also requiring the combination of an OPO + Ti:Sapphire for pumping. In practical applications, the compactness and cost effectiveness of the whole supercontinuum generator is required. Femtosecond, mode-locked fiber lasers seem to be a promising alternative for complicated OPOs, thanks to their compactness and flexibility. Additionally, the radiation from fiber lasers might be easily amplified in chirped pulse amplifiers (CPA). Octave-spanning supercontinuum generation by femtosecond radiation from a fiber laser was demonstrated using tellurite  as well as silica  fibers. However, in both cases the PCFs were pumped with Yb-doped lasers, operating at 1 μm. Such pumping wavelength is inconvenient for mid-IR supercontinuum generation and the obtained spectrum barely reaches 1800 nm [21,22]. Erbium-doped fiber lasers operating at 1560 nm seem to be more suitable pumping sources for mid-IR SC generation. It was nicely proven in the experiment performed by Liao et al., where both Er and Yb-doped lasers were used to pump a nonlinear tellurite fiber. Pumping at 1557 nm allowed SC generation up to 2300 nm, while the 1064 nm pump supported spectrum broadening only to 1600 nm . Using Er-doped fiber-based pump lasers, SC up to 2400 nm has been generated in both tellurite and silica fibers [24,25]. Recent works have shown, that heavy-oxide-based soft-glass PCFs seem to be an interesting alternative for silica fibers thanks to their fabrication flexibility for the creation of mid-IR SC sources [16,17,26].
In this paper we report on supercontinuum generation in a single-mode regular-lattice, soft glass photonic crystal fiber, pumped by a fiber-based CPA setup at 1560 nm. Microstructure of fiber has been designed for efficient pumping at wavelengths over 1500 nm in the anomalous dispersion region. The PCF was drawn from lead-bismuth-gallium-oxide glass, which features high nonlinearity and very good mechanical properties. The paper describes in detail the fabrication process of the photonic crystal fiber, its characterization and the experimental results of supercontinuum generation, together with its numerical representation, performed with Nonlinear Schrödinger Equation (NLSE) approach.
2. Fiber fabrication and characterization
The nonlinear photonic crystal fiber was developed from lead-bismuth-gallium-oxide glass (PBG-08) with the following composition [mol%]: 40% SiO2, 30% PbO, 10% Bi2O3, 13% Ga2O3, 7% CdO. Glass selection was motivated by its resistance to recrystallization in multiple thermal processing, which enabled stack-and-draw procedure for fiber drawing [27,28]. The nonlinear refractive index of the PBG-08 glass is equal to 4.3 × 10−19 m2/W (measured using the z-scan method at 1240 nm wavelength ) and is very high for oxide soft glasses, much higher than that reported for any other oxide glass used in mid-IR supercontinuum generation. The measured transmission characteristic of the glass is shown in Fig. 1.The transmission window covers the most of the visible spectrum, and the near/mid infrared up to around 2800 nm, where presently onset of OH¯ impurity absorption in glass is observed. This is a technological issue, not caused by the glass itself, which has a multi-phonon cut-off after 5000 nm. Inset in Fig. 1 shows attenuation of the drawn PCF, which was measured using cut-back method. In later numerical modeling of supercontinuum generation, wavelength dependent loss of fiber was used, which was based on the absorption profile measured for the bulk glass (most importantly enabling to account for the OH¯ peak), which was upscaled to reflect attenuation level measured in the 1500-1600 nm range.
The preform was fabricated from circular glass capillaries with 1 mm diameter ordered in hexagonal lattice. Standard stack-and-draw technique was employed for the development of the fiber. The fabricated PCF, shown on SEM images in Fig. 2, consists of 8 rings of air holes with a lattice constant typically at Λ = 2.4 μm and relative hole size d/Λ = 0.73 in the first ring and d/Λ = 0.5 in the remaining seven rings. In order to preserve different diameters of the air holes in the first and subsequent rings, low-speed drawing of the sub-preform and a relatively low pulling temperature of 720°C was used. Relative hole size of the outer rings is decreased in order to increase modal losses of the higher order modes, though it is still larger than 0.42, hence the fiber is not strictly single mode. We verified numerically, that the fiber should be guiding the fundamental mode with negligible modal loss, and just a few higher order modes with significantly increased modal losses. We also measured near-filed output profile of the PCF, which was very close to pure Gaussian. Based on this, we consider the fiber to be effectively single-mode. The diameter of the core of the PCF was 3.1 µm with a photonic cladding diameter of 39.4 µm. The total fiber diameter was 159.8 μm. In practice, it is very difficult to maintain a design with varying d/pitch during drawing. As a result, some non-uniformity in air-hole diameters can be seen in the SEM images in Fig. 2. We used this image to calculate fiber parameters like dispersion and waveguide losses for the fundamental and several higher order modes. Comparison of these parameters with parameters of an ideal, designed structure revealed differences below 10%, which allows to believe that influence of hole size non-uniformity of global parameters of fiber is not excessive.
The calculated value of the nonlinear coefficient of the fiber at the pump wavelength is γ = 376 km−1W−1. The effective mode area for the fundamental mode of the fiber at the central pulse wavelength used in experiments (1560 nm) is Aeff = 4.64 μm2, as calculated with a FDTD mode solver.
Figure 3(a) shows the calculated and measured chromatic dispersion of the fabricated fiber. Both are in very good agreement. The zero-dispersion wavelength (ZDW) is located at 1465 nm. The group velocity dispersion characteristic (GVD) calculated from dispersion characteristic is shown in Fig. 3(b) along with its Taylor series fit. Taylor series expansion coefficients up to β22 were included in later modeling, due to relatively broad spectral range of considered dispersion.
3. Experimental setup for SC generation
The experimental setup is shown in Fig. 4. The PCF is pumped with a high-power, high-repetition rate femtosecond system, which consists of a mode-locked oscillator and a chirped pulse amplifier. The pulses are temporally broadened in a grating stretcher based on two reflective 900 l/mm gratings in Martinez-type configuration. The pulses are afterwards amplified in a two-stage fiber amplifier based on Er-doped (1st stage) and Er/Yb-doped double-clad (2nd stage) fibers. After amplification, the pulses are compressed in a Treacy-type compressor based on two gold-coated reflection gratings with 950 l/mm line density. The CPA can be seeded with two different oscillators. The first one is an Er-doped fiber laser mode-locked due to nonlinear polarization rotation mechanism (NPR). This seed delivers 100 fs pulses centered at 1560 nm with 200 MHz repetition frequency . After amplification, with this seed the system is capable of generating 2.5 W of average power with sub-400 fs pulse duration. The maximum pulse energy is 12 nJ and the peak power exceeds 30 kW. The details of the system design are described in . In the second case, the system is seeded by a graphene mode-locked, Er-doped soliton laser, which delivers 440 fs pulses at 50 MHz repetition rate. After amplification, at the output of the system we can obtain 1 W of average power with 800 fs pulse duration . The pulse energy is equal to 20 nJ and the peak power reaches 25 kW. In both cases, the fiber amplifier contains only single-mode fibers, so the output beam is truly single transverse mode with nearly-diffraction limited quality (M2 < 1.17). The light from the pumping system is coupled into a short piece of PCF through a 40x microscope objective. The generated SC is collected with a multimode fiber and delivered to the optical spectrum analyzer (OSA). In order to provide the analysis in the range from 800 to 2400 nm, 2400 nm, two optical spectrum analyzers were used (Yokogawa AQ6370B and AQ6370C).
4. Experimental results and discussion
4.1. SC generation using sub-400 fs pulses
The measured supercontinuum spectra at different input pulse energies, generated using the configuration with NPR-based oscillator are shown in Fig. 5. The length of the PCF in the experiment was 5 cm. Initial numerical simulations, based on a model described later, were performed for the established fiber parameters (dispersion and attenuation) in order to assume a sample length for the supercontinuum generation experiment. Calculated spectra did not show broader bandwidth than obtained experimentally (Fig. 5), and a gradual decreasing of signal due to fiber attenuation could be observed numerically for lengths closer to 10 cm. Since soliton fission was not expected to play the main role in the broadening based on calculated fission length (discussed later), we preferred to reduce PCF sample length in order to avoid attenuation. Hence the final length was chosen at 5 cm, which was in part a cutting convenience, as well.
Considering a 20 dB dynamic range, the supercontinuum spans over an octave, from 1000 nm to 2200 nm. The measured supercontinuum power was at the level of 600 mW. Numerical trace in Fig. 5 (dashed line corresponding to pump pulse energy of 12 nJ and measured 18% coupling efficiency into the fiber) was obtained using a numerical representation based on model proposed by Travers, Dudley and Frosz , which was extended to include measured, wavelength-dependent attenuation of the nonlinear fiber. The model reconstructs the experimental results with generally good agreement. Series of small peaks around the pump wavelength visible in experimental spectrum for the highest used pump pulse energy (12 nJ) stems from instability of the pump laser, present for this energy level, as well as from short averaging time of the optical spectrum analyzer. Instability of laser intensity at highest pulse energy was difficult to reproduce numerically, although pump noise including one-photon-per-mode background and limited laser linedwidth was included in the simulations, as proposed by Frosz . Numerical spectrum exhibited fine structure across assumed spectral window, which was beyond spectral resolution of our apparatus. For this reason, numerical spectrum was smoothed with Savitzky-Golay filter. The inconsistency at longer wavelength part of the spectrum, where model predicted more supercontinuum light, that was actually measured, stems from some inaccuracy in dispersion characteristic calculation from the SEM image of fiber’s photonic structure. Accuracy of an SEM image of the photonic structure is generally limited, and an inaccuracy of relative hole size reading will influence dispersion profile at longer wavelengths to a bigger extent, than in the shorter wavelengths. Limit of calculation window at short wavelengths was well below the limit of supercontinuum short-wavelength edge, and also a wavelength-dependent numerical absorbing window used in the model assured, that no cyclic wrap-over effect occurred in the simulation. We verified experimentally Raman gain spectrum in the glass used for drawing of our fiber (first order Raman shift at 29 THz) and took Raman response parameters of τ1 = 5.5 fs and τ2 = 32 fs, while Raman fractional contribution to nonlinearity in the modeling was set to fR = 0.05. These parameters were generally in line with those reported for other soft glasses by e.g. Price et. al. . Numerical evolution of spectral broadening and corresponding spectrogram are shown in Fig. 6. General features present in the evolving spectrum are self-phase modulation and four-wave mixing (FWM). Soliton fission should not play significant role in this particular case, which is evidenced by calculated soliton order of over 60 and soliton fission length exceeding 7 cm, while nonlinear fiber sample length in the experiment and simulation was 5 cm. At 3-4 cm of propagation energy is transferred to new sidebands centered at about 1350 nm and 1900 nm. We calculated FWM phase-matching condition according to κ = 2·γ·P0·(1-fR) + 2·Ʃ[ω2m·β2m/(2·m)!] = 0, and the resulting phase-matching curves are shown in Fig. 7.The curves show, that center wavelengths of these two new sidebands coincide with parametric wavelengths of a degenerate FWM process, when the pump wavelengths are in the vicinity of the supercontinuum pump wavelength of 1560 nm.
4.2. SC generation using sub-900 fs pulses
In the second variant of the setup, the supercontinuum was generated using CPA pump source seeded by a graphene-based fiber laser. The measured supercontinuum spectra at different input pulse energies are shown in Fig. 8.The length of the PCF in this experiment was 6 cm. The 20 dB flatness of the supercontinuum generated at maximum available power was exceeding a bandwidth of 1400 nm. The average output power of the SC under 20 nJ pulses was at the level of 130 mW.
Spectra recorded for 8 and 10 nJ pulses show a sharp cutoff at around 1050 nm, which was a sensitivity issue of the optical spectrum analyzer at this level of optical power. For the experiment with sub-900 fs pulses, a new PCF sample was used, which was cut at 6 cm out of convenience. The PCF sample used for the sub-400 fs pumping experiment broke off, which necessitated use of a new sample, which was from a fiber drawn from the same preform, but in a separate drawing run at the drawing tower. A difference in evolution of spectrum with increasing pump pulse energy can be observed between experimental results presented in Fig. 5 and 8. It can be noticed, that the wavelength spacing between the FWM-attributable sidebands and the pump wavelength in Fig. 8 is smaller than in Fig. 5. This is suggestive of larger anomalous dispersion at the pump wavelength in PCF sample used in the sub-900 fs experiment, than in fiber used in the sub-400 fs experiment. An increase of anomalous dispersion at the pump wavelength of 1560 nm most certainly is a result of the 900 fs fiber’s zero dispersion point moved to even shorter wavelengths, than in case of the 400 fs fiber. This in turn could be the result of smaller core diameter, as drawing conditions could have been somewhat different. We realize this is a crude analysis, however since the measured supercontinuum bandwidth covers well over an octave (and this measurement was limited by sensitivity range of disposed spectrometer) under almost a picosecond pump pulse, the demonstrated fiber design encourages further optimization.
We have demonstrated generation of infrared supercontinuum in lead-bismuth-gallium-oxide glass photonic crystal fiber, in which intended dispersion profile and effectively single mode propagation was achieved through careful design of photonic structure. Supercontinuum bandwidths covering over an octave were generated using two types of fiber lasers, both operating at a wavelength of 1560 nm. One laser employed nonlinear polarization rotation mode-locking mechanism and generated sub-400 fs pulses (12 nJ) and the second one was mode-locked with a graphene saturabe absorber, which enabled generation of 900 fs pulses (20 nJ). Experimental results were analyzed with a numerical model for one of the cases (sub-400 fs) with very good agreement between simulation and measured spectrum. Dispersion uncertainty related to a possible irregularity in relative hole size and core diameter in fiber sample used for the second experiment (900 fs), prevented meaningful reconstruction of experimental data with a numerical model. This particular case however covered a broader bandwidth from 900 nm to over 2400 nm (limited detector range). Accurate control of fiber geometry and its physical dispersion profile would contribute to extending supercontinuum bandwidth towards mid-infrared in this soft glass PCF design under sub-picosecond pumping.
This work was in part supported by the Polish Ministry of Science and Higher Education under the project entitled “Amplification of femtosecond pulses from fiber lasers utilizing graphene” (project no. IP2012 056772) and in part supported by the project operated within the Foundation for Polish Science Team Programme co-financed by the European Regional Development Fund, Operational Program Innovative Economy 2007-2013. The scholarship of one of the authors (K. Krzempek) is co-financed by the European Union as part of the European Social Fund.
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