We report the fabrication of photonic crystal fibers with a continuously-decreasing zero-dispersion wavelength along their length. These tapered fibers are designed to extend the generation of supercontinuum spectra from the visible into the ultraviolet. We report on their performance when pumped with both nanosecond and picosecond sources at 1.064 µm. The supercontinuum spectra have a spectral width (measured at the 10 dB points) extending from 0.372 µm to beyond 1.75 µm. In an optimal configuration a flat (3 dB) spectrum from 395 to 850 nm, with a minimum spectral power density of 2 mW/nm was achieved, with a total continuum output power of 3.5 W. We believe that the shortest wavelengths were generated by cascaded four-wave mixing: the continuous decrease of the zero dispersion wavelength along the fiber length enables the phase-matching condition to be satisfied for a wide range of wavelengths into the ultraviolet, while simultaneously increasing the nonlinear coefficient of the fiber.
©2006 Optical Society of America
The generation of broadband supercontinua by focusing intense light in a nonlinear medium has been well known since the 1970’s , but has recently attracted renewed attention due to the emergence of photonic crystal fibers (PCFs) . Our highly nonlinear PCFs consist of a regular array of microscopic air holes surrounding a solid silica core. By adjusting the geometry of the microstructure and the size of the core, it is possible to profoundly alter the chromatic dispersion of the guided mode [3, 4] and to dramatically increase the nonlinear coefficient when compared to conventional fibers . Simple designs of PCFs with small cores can cause the zero-dispersion wavelength λ0 to be shifted to wavelengths significantly shorter than 1.27 µm [6, 7], which is impossible with standard single-mode silica fibers. At the same time the relatively low nonlinearity of silica is offset by the long interaction lengths in fibers, which when combined with the small effective area of PCFs gives a very high nonlinear response for modest pump powers. These particular properties of PCFs have been extensively investigated for supercontinuum generation with pulsed pump sources in the femtosecond to nanosecond regimes [6, 8, 9, 10]. However the supercontinua generated using inexpensive and compact Neodymium and Ytterbium pump lasers operating at 1060–1080 nm do not extend to wavelengths much shorter than ~500 nm. Indeed generating wavelengths shorter than half the pump wavelength requires multiple-order processes and the generation of such short (blue, ultraviolet) wavelengths typically requires a short λ0 to phase-match the nonlinear processes. On the other hand, initiating the supercontinuum generation at the pump wavelengths beyond 1000 nm requires λ0 at a similar wavelength, implying a relatively large core diameter (~5 µm). Our intention in this work is to use a tapered fiber design which will efficiently initiate supercontinuum generation around the pump wavelength, and then extend it into the blue and ultraviolet spectral region through continuously decreasing core size and λ0.
Tapered fibers with a ~1 cm transition have been investigated with a similar aim [11, 12]. In one experiment a standard telecom fiber was tapered and then pumped by a Nd:YVO 4 picosecond laser, generating a supercontinuum from 460 nm to 1000 nm . In another, a tapered PCF with a 6 mm-long transition pumped by a femtosecond Ti:Sa laser has been used to generate blue-violet femtosecond coherent pulses . Travers et al. have proposed an alternative method which consists of using cascaded PCFs with reducing λ0 spliced together . This enabled generation of a 1.2 W supercontinuum extending from 0.44 µm to 1.89 µm with a picosecond ytterbium pump laser operating at 1064 nm, with particularly high spectral power in the blue. In this paper we report the fabrication of PCFs with a continuously decreasing λ 0 over longer lengths (1–12 m), on a fiber drawing tower. The fibers have been used to generate supercontinua spanning more than two octaves, and extending from the ultraviolet to beyond 1.75 µm with both nanosecond and picosecond pump pulses. It is the first time to our best knowledge that a supercontinuum pumped around 1.064 µm has been extended to wavelengths below 400 nm.
2. Fabrication and properties of the fibers
In our highly nonlinear PCFs, the strong waveguide dispersion makes it possible to shift the λ0 from infrared to visible wavelengths , by decreasing the core diameter while keeping the air-filling fraction constant. Our tapered fibers were drawn directly from a small (3 mm diameter) preform, by adjusting the drawing parameters so as to decrease the outer diameter, while keeping the air-filling fraction in the cladding almost constant. As a result the core diameter decreased by the same fraction as the outer diameter, so that the outer diameter measurements acquired during fiber fabrication give an accurate description of the variation of core diameter along the fiber length. We drew a series of tapered fibers, under slightly different drawing conditions. Here we report results obtained with two different fibers referred as fiber 1 and 2. The fiber 1 outer diameter was decreased from 190 µm to 85 µm with a 11 m-long transition, resulting in a core size decreasing from 5.2 µm to 2.3 µm within the same length. The nonlinear coefficient γ=2πn 2/(λA eff) has been evaluated by using the core area as an estimate of the effective area A eff and by taking the value of n 2=3×10-20 m2/W for the nonlinear refractive index of silica. The nonlinear coefficient γ increases from ~8 W-1.km-1 to ~40 W-1.km-1 at a wavelength of 1.064 µm. The structure of the fiber was preserved along the transition: in particular the air filling fraction is roughly constant. A scanning electron micrograph (SEM) of the output of fiber 1 (small core end) is displayed in Fig. 1. The evolution of the core size as a function of the fiber length is shown in Fig. 2(a) for fiber 1. As shown by this curve, the fiber consists of a 1 m-long portion with a constant core size (5.2 µm) followed by the 11-m long transition from 5.2 µm to 2.3 µm. As the air holes surrounding the core are large, the propagation constant of the guided mode (from which the group-velocity dispersion (GVD) can be calculated) can be approximated by that of a plain strand of silica surrounded by air . Following this procedure, we have deduced the λ0 as a function of fiber length. This curve is shown in the inset of Fig. 2(b). The λ0 evolves from 0.99 µm at the input end of the tapered fiber to 0.76 µm at the output end.
The core diameter of fiber 2 decreases from 4.8 µm to 1.1 µm (outer diameter from 320 µm to 82 µm) with a 12 m-long transition. The shape of the transition is shown in Fig. 2(b). It is similar to fiber 1, but with no uniform fiber at the input end and a flatter change in dispersion in the last 8 meters. The air filling fraction in the cladding is slightly smaller than in fiber 1, but has likewise been observed to be nearly constant along the transition. The λ 0 of this taper decreases from 0.98 µm to 0.58 µm, with a similar shape to the change in core size.
3. Supercontinuum generation experiments
3.1. Nanosecond pumping regime
Fiber 1 was pumped with a Nd:YAG microchip laser source operating at 1.064 µm (JDS Uniphase). The laser emits passively Q-switched 600 ps pulses (FWHM) at a repetition rate of 7.25 kHz. The peak power from the laser was ~15 kW and the beam was launched into the fiber with a 20× microscope objective, with a coupling efficiency of 60%. Output spectra were recorded using an optical spectrum analyzer (Ando AO-6315B) with a spectral resolution of 5 nm. The spectrum recorded under these conditions with fiber 1 is displayed in Fig. 3 (black line) between 350 nm and 1750 nm. To investigate the modal properties of the generated supercontinuum the output pattern was passed through 10 nm bandpass filters and examined by imaging the fiber end with either a CCD or a Vidicon camera. Despite the fact that theoretically, the fiber itself was not single mode at the pump wavelength, the continuum was observed to be generated in the fundamental mode in the entire 350–1750 nm spectral range.
The development of the supercontinuum along the length of fiber 1 was studied by cutting the fiber back at a fixed pump power. The fiber was progressively cut from the output end (smaller core size) and the spectrum was recorded. The results of these measurements are summarized in Fig. 3. Blue and green curves were recorded respectively for fiber lengths of 0.2 m and 1 m. As can be seen in Fig. 2, this corresponds to the part of the fiber 1 with a constant core diameter and thus with a constant λ0 of 0.99 µm. The pump wavelength offset is thus +65 nm, and the GVD at the pump wavelength is around D=17 ps/nm.km. As expected from pumping in the low anomalous dispersion regime with nanosecond pulses, the process of continuum generation is initially driven by MI , self-phase modulation (SPM) being negligible in the nanosecond regime [8, 9]. This can be clearly seen for the shortest piece of fiber (0.2 m) corresponding to the blue curve in Fig. 3, where symmetrically-located MI peaks are visible on either side of the pump wavelength. Higher-order MI sidebands then appear yielding a symmetrical broadening of the spectrum . Thereafter the spectral power generated in the normal dispersion regime allows shorter wavelengths to be generated via four-wave mixing (FWM), which can be phase-matched as demonstrated in Ref. . At the same time, the continuum keeps broadening towards longer wavelengths, aided by Raman scattering. These phenomena are observable in the green curve in Fig. 3, corresponding to a 1 m-long piece of fiber with a constant λ 0. The continuum at this point extends to ~600 nm in the visible region and goes as far as ~1.7 µm into the near infrared. For increasing lengths of fiber, the spectrum keeps extending towards the blue until a length of 5.1 m (red curve of Fig. 3). At this point the spectral broadening almost ceases, but the shape of the spectrum is slightly modified. The spectrum measured for the 5.1 m-long fiber is the best result achieved with nanosecond pumping in terms of both spectral range, broadness and flatness. It spans the whole optical spectrum analyzer range (from 0.35 to 1.75 µm) with less than 10 dB intensity variation.
The shortest generated wavelength is observed to decrease with increasing fiber length, i.e. with decreasing λ0. The shortest wavelength is defined by a -10 dB drop from the maximum value of the spectrum for a given fiber length. From results of Fig. 4 we can see that the shortest wavelength decreases dramatically from more than 1 µm to 372 nm. The limitation in terms of short wavelengths generation in the present work is believed to be due to the measurement (i.e. to the optical spectrum analyzer), and more precise measurements in the blue/UV region are under investigations. However, this is to our knowledge the shortest reported supercontinuum wavelength edge generated with 1064 nm nanosecond pump sources.
3.2. Picosecond pumping regime
The picosecond pump laser (IPG Photonics) used in these experiments is described in detail in . It is a completely fiber integrated, mode locked Yb pump laser, based on nonlinear polarization evolution, emitting up to 60 kW peak power pulses with an average power of 8 W. The pulses are of 3 to 4 ps duration at 51 MHz at a wavelength of 1064 nm with a 3 dB spectral width of 40 nm. This laser was used to pump the fiber 2 described above. The laser output fiber was spliced directly to a fiber-pigtailed polarization insensitive isolator to avoid back reflection. The output of the isolator was coupled into the taper input via two singlet lenses, with the pump settings used, approximately 4–5 W was coupled into the core. In principle, one could create a fully fiber-integrated setup by splicing directly to the input of the taper. The spectra shown in this section were obtained using a spectrum analyzer (Anritsu) with 5 nm spectral resolution and are normalized to the total measured supercontinuum power.
As with the nanosecond pumping we explored the dynamics of tapered fiber continuum generation by performing a cut-back experiment. Fig. 5 shows the results of cutting the fiber from the small core end. The lengths indicated are measured from the start of the large core end. In contrast to nanosecond pumping, even in just 0.1 m of taper, the picosecond pumped supercontinuum spectrum shows a considerable broadening due to the more dominant role of SPM. However, the short edge of the continuum only extends to around 700 nm. In this short length, λ0 at the end of the taper is in the region of 900 nm, and as discussed in , phase matching conditions in such a fiber do not enable any shorter wavelength generation. Even pumping extended lengths of fiber with similar λ0 and high pump powers does not overcome this fundamental limitation [10, 13]. But using longer taper lengths, and hence providing a shorter λ 0, phase matching conditions to shorter wavelengths can be satisfied. In 0.7 m of fiber 2 the spectrum extends as short as 470 nm. The shortest wavelength generated and total continuum output power is shown in Fig. 6 for increasing taper length. As the length is increased, the supercontinuum edge gets shorter and the total continuum output power decreases. This can be attributed to the considerably higher Rayleigh scattering losses at the short visible wavelengths, and due to losses at the Stokes FWM wavelength at the long end of the spectrum.
In 1 m of taper, the flattest continuum with picosecond pumping was achieved. In this length the continuum spans from 375 to 1750 nm with a total output power of 3.5 W. From 395 to 850 nm, the entire visible window, the continuum flatness is under 3 dB (Fig. 7) with an unprecedented spectral power of over 2 mW/nm, the peak of which is 5 mW/nm at 413 nm.
The modal properties of the output were checked using bandpass filters and digital imaging. The continuum was in the fundamental mode, even throughout the region from 390 to 420 nm, and was found to have no dependence on input polarization, indicating low birefringence in the tapers.
The shortest wavelength generated with picosecond pumping was 375 nm (-10 dB), which was generated in the 2 m length of taper. In the longer lengths of fiber, the red wavelength edge of the continuum is limited by confinement loss (around 1600 nm on the 12 m curve in Fig. 5) and the blue edge of the continuum also exhibits a depression around 600 nm (see the curve for the 12 and 2 m-long taper in Fig. 5). The short wavelength depression was not observed with nanosecond pumping of this fiber. One can see from Fig. 2(b) that the last 8–10m of fiber 2 have a relatively flat change in λ0, around the 600–580 nm region which leads to soliton effects in the longer tapers. Specifically the solitons split between a dispersive wave in the normal dispersion and a soliton in the anomalous  resulting in depletion at the zero dispersion wavelength. Thus the optimal taper length of 1 m, is that which maintains a continuous decrease of λ 0 from around 1 µm to 600 nm to achieve efficient transfer of power as the spectrum evolves to shorter wavelengths, but which is short enough to avoid soliton effects in the taper end.
We have reported the fabrication of tapered photonic crystal fibers with zero dispersion decreasing along the fiber length. The smooth monotonic decrease of dispersion in the fibers allowed parametric phase-matching conditions to be satisfied for progressively short anti-Stokes wavelengths. Consequently, for both nanosecond and picosecond 1.06 µm pump sources, we observed supercontinuum generation extending from at least 1750 nm to 375 nm. With high-power picosecond pumping of an optimized taper length, a supercontinuum with over 3.5 W total power and 3 dB flatness between 395 and 850 nm with 2 mW/nm spectral brightness was achieved.
The Imperial College group gratefully acknowledge the financial support of the Engineering and Physical Science Research Council (EPSRC), UK, through research grant GR/555217/01. J. R. Taylor is a Royal SocietyWolfson Research Merit Award holder. S. V. Popov is an EPSRC Advanced Research Fellow. J. C. Knight is a Research Fellow of the Leverhulme Trust. J. C. Travers and A. B. Rulkov are supported by EPSRC quota studentships.
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