Soft glass photonic crystal fibers (PCFs) have been fabricated for the first time with the stack and draw process. The same SF6-PCFs have been successfully tapered using a brush flame method. The transverse structure of the PCF does not collapse in the tapering process and core dimensions of the fabricated photonic nanowire has been measured to be 400 nm in diameter. Supercontinuum radiation in excess of one octave has been generated in both the untapered and tapered PCF and, in the latter case, pulse energy thresholds of 65 picojoules at a pump wavelength of 1550 nm were observed.
©2007 Optical Society of America
Photonic crystal fibers (PCF) have garnered widespread interest for their efficient way of enhancing the nonlinear optical interaction between pulses of light and their bulk constituents. These processes, particularly in relation to SC generation in PCFs, have been the subject of intense research since its inception [for a complete recent review on the subject see Ref. 1].
PCFs made of soft glasses are particularly interesting since their bulk nonlinearity is significantly higher than the more conventionally used fused silica and many efforts have been directed towards their fabrication [2–6]. Recently, Schott SF6 photonic crystal fiber has been shown to be a convenient source for very broad short-pulse supercontinuum (SC) covering a bandwidth approaching 3000 nm . Such soft-glass PCFs have been difficult to fabricate because of their low melting temperature and, to date, they have been manufactured by extrusion  or by a die-cast method . We present here results on the fabrication of SF6 PCF, SF6 PCF tapers and the performance of both these fibers when ultrafast pulses in the near infrared (λ=1550 nm) are coupled into them.
For these experiments, Schott SF6 photonic crystal fibers were manufactured by using the more traditional stack and draw process for the first time. This method has been repeatedly applied to silica glass with success however it is more challenging to carry out with soft glasses such as SF6 which has a softening temperature of 811 K  and where the microstructure can collapse during the draw. The fiber perform is made by stacking several capillaries and solid rods in a desired geometry after which it is carefully and slowly heated in order to reach the melting temperature without causing any stress fracture in the glass. As the preform softens, the PCF is drawn.
A cross section image of the fabricated fiber is shown in Fig.1 which shows a nicely formed regular microstructure. The fiber used in these experiments has a core diameter of 4.5 microns. These fibers typically have dispersion curves with zero dispersion crossing and anomalous dispersion in the near infrared at longer wavelengths (λ>1300 nm) than the similarly structured silica PCFs, which makes them suitable for pumping at (longer) fiber laser wavelengths. For the untapered PCF, the zero dispersion wavelength is estimated to be in the vicinity of 1300 nm at these dimensions.
Previous studies have shown a dominant Kerr nonlinearity that gives rise to spectrally smooth supercontinuum radiation in short pieces of this PCF . A short segment (Z=6 mm) of the stacked and drawn SF6-PCF is tested by coupling in it femtosecond pulses from a parametric oscillator operating at λ=1550 nm (80 MHz, Pavg=300 mW, t=100 fs). Generally, the maximum average power coupled in the fiber is around 100 mW, which corresponds to pulse energies slightly above 1 nJ.
As expected, the SC radiation generated (Fig. 2) from this short piece of PCF is lacking significant spectral structure from higher order nonlinear effects. The possibility of generating SC at low pump pulse energy is particularly interesting and the combination of the high bulk nonlinearity of soft glasses and the pump wavelength range makes these PCFs compelling for the realization of a compact fiber-based, low-coherence pulsed supercontinuum source.
While the physical parameters of the bulk material are certainly important, increased mode confinement significantly contributes to the enhancement of the nonlinearity and to the lowering of the SC generation threshold. In recent years there has been considerable interest in the tapering of regular [9, 10] and photonic crystal fibers [11, 12] giving rise to what is sometimes now referred to as a photonic nanowire.
Whereas a solid photonic nanowire poses mechanical challenges in terms of its support, a nanowire based on photonic crystal fiber affords more stability given the larger supporting structure around the sub-wavelength guiding core.
The opportunity of tapering a high nonlinearity glass to further confine the guided mode is clearly auspicious but riddled with the complication of dealing with the significantly lower melting temperatures (compared to >2000 K for silica) which would also potentially compromise the core’s supporting microstructure.
In an attempt to deliver the minimal thermal shock possible to the SF6-PCF, 15 cm segments of SF6-PCF were mounted vertically securing one end of the fiber by means of a fiber chuck while an adjustable constant pulling force was exerted at the other end. A “distance” brush flame approach was used by employing a butane torch where the flame is kept in close proximity but never directly applied onto the PCF.
When gradual heating of the central portion of the fiber is achieved, the constant pull tapers the fiber. Depending on the separation distance between the flame and the PCF, tapers as long as 2 cm were obtained. Certainly, only a few millimeters of taper are needed for SC generation . Scanning electron microscope images of the tapered cross section used in these measurements is shown in Fig. 3.
The core is now found to have a transverse dimension around 400 nanometers. It is remarkable to see that the support structure around the core is still present in spite of some collapse noticeable in some of the air holes (the holes themselves have dimensions between 120 and 200 nm).
Pulses are now coupled into this fiber taper to observe the nonlinear transformation in it. Expectedly, the 1550 nm pump pulses are significantly broadened and generate supercontinuum. The fiber used is a 3.5 cm segment of untapered PCF with a 0.5 cm tapered tip. It is important to decouple the nonlinear effects due to the fiber as a whole and the ones due to the tip. To this end, a comparison was carried out between a 4 cm untapered SF6-PCF and the equal length PCF with the 0.5 cm tapered tip. An illustration of the results is shown in Fig. 4 where it is observable that at low pump powers (10 mW) there is no considerable broadening in the SF6-PCF whereas the tapered PCF gives broad supercontinuum. The SC mode from the photonic nanowire is observed in the far-field to be quite smooth and seemingly the fundamental mode of the fiber. Modal calculations are being performed to verify these findings and will be the subject of an upcoming analysis.
Losses in the taper are estimated by taking measurements of the output power under identical coupling conditions both for the tapered and untapered fibers. The detected power in the two cases is found to be comparable (difference <2%) provided that the tapering has been executed appropriately.
The spectral data for the SC exhibits some spectral structure. This is most likely due the combination of the shortened nonlinear lengths caused by the smaller mode and the mechanical difficulties of holding the taper fixed and consistently pointing towards the optical spectrum analyzer. Under these conditions, however, it is established that broad supercontinuum (from 650 nm to >1750 nm) is generated for pump pulse energies of 125 pJ at 1550nm. Significant broadening (Δλ between 900nm and >1750 nm) is still found to occur for in-fiber average powers of 5 mW, which correspond to pulse energies of 65 picoJoules. The result is very encouraging for generation of broadband sources pumped by fiber-based femtosecond oscillators and could provide a very practical route for a convenient broadband source.
Summarizing, a soft-glass, high nonlinearity SF6-PCF was realized by stack-and-draw fabrication. This implies that added versatility and control in PCF design are made possible and realization of tailored dispersion curves, polarization control and other designs can be possible in this material. The nonlinear quality of this fiber has been verified by measuring the transformation of 1550 nm femtosecond pulses which has proven consistent with the previously studied extruded SF6 fiber and has generated smoother and broad SC radiation. The soft glass fiber was successfully tapered and a photonic nanowire having a 400 nm core diameter was realized. SC generation in this structure has been shown to have remarkably low threshold, down to below 10 mW or sub 100 pJ. While issues still need to be overcome to make the nanowires reliably mechanically stable, the low threshold SC generation, coupled with 1550 nm pumping, promises to introduce considerable opportunity in ultrabroadband, portable sources dramatically reducing the requirements placed on fiber-based ultrafast pump sources.
The authors gratefully acknowledge the help of Brian Lawrence for the SEM imaging and Hannah Perry with the manufacturing. This work has been supported under the NSF STTR Program.
References and links
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