1. Introduction

Since the first report [1] of an optical fiber based on a microstructured “photonic crystal” material, most reported research on photonic crystal fibers (PCF’s) has concerned fibers made of silica using a stack-and-draw process, although fibers drawn from other glasses and even polymers [2] have also been reported. Extrusion is an alternative fabrication technology for preforms for these fibers [3], and this has recently been extended to the use of soft glasses [4]. In this paper we report the fabrication and characterisation of PCF produced by extrusion of commercially available SF6 glass. Light in these fibers is guided in a solid glass core that is surrounded by air holes. The “holey” material acts as a reduced-index cladding from which guided light in the core is completely reflected [5,6]. We describe the waveguiding properties of such a fiber, and first results of supercontinuum generation using an ultrashort pump source at a wavelength of 1550 nm.

Nonlinear applications of photonic crystal fibers require a high nonlinearity (by forming a small core and by using a material with a large value of nonlinear refractive index n ₂) and good control of the waveguiding characteristics [7]. Previous work [4] has concentrated on optimizing the nonlinearity in PCF formed from soft glass. However, the group-velocity dispersion (GVD) characteristic plays a central role in determining the nonlinear response of fibers to short pulses. For example, nonlinear optical effects observed with silica photonic crystal fibers when using ultrashort pump pulses in the 800 nm band [7] have differed substantially from those found using similar pulses in the 1550 nm band [8]. This results directly from the differences in the material properties of silica at the two wavelengths, which make it difficult to reproduce the form of the GVD curve found around 800 nm at the longer wavelengths. For many applications, it is thus essential to simultaneously optimize both the nonlinearity and the GVD characteristic of the fiber. In this paper, we report the fabrication and characterization of an SF6 - PCF with high nonlinearity and anomalous or zero GVD in the 1550 nm band, and large higher-order dispersion.

2. Fabrication

SF6 is a commercial glass produced by Schott, and has a softening temperature of 811 K [9]. We adapted a direct or forward extrusion process (where product and punch move in same direction) to produce both preforms and jacketing tubes. The extrusion rig was mounted vertically on an existing fiber-drawing tower, and the drawing furnace was used as the heat source. We used a pneumatic actuator attached to the punch to force an SF6 billet of 20 mm diameter through various dies. The extruded glass was then drawn directly from the die to create preforms of 1 mm outer dimension. We also extruded jacketing tubes of 1 mm internal diameter and 4 mm outer diameter in a similar fashion. We used the preforms to draw fibers of tens of meter lengths with core diameters in the range 1 – 10 μm. Results presented in this paper were obtained using fibers with two rings of air holes surrounding a solid core, with four holes in the inner ring and eight in the outer ring. Figure 1 shows micrographs of a preform (Fig. 1, left) and a fiber (Fig. 1, right). During fiber drawing, collapse of the jacketing tube onto the preform caused the slight enlargement of air holes in the outer ring.

 

Fig. 1. Preform (left, optical micrograph) and fiber (right, electron micrograph) fabricated from SF6 glass by extrusion. The preform (left) is 1mm in outer diameter, and was jacketed prior to drawing the fiber shown on the right. The fiber has a nominal 2.6μm core dimension, and the glass strands in the second ring of air holes are 150nm across and 6μm in length..

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3. Basic waveguiding properties

SF6 has a refractive index of 1.76 at 1550 nm [9], and has a nonlinear index n₂ = 2.2 10^-19 m²/W [10]. It is transparent over the spectral range 500 – 2000 nm, with a minimum bulk loss of about 1 dB/m [9]. Our methods could also be applied to other commercial glasses, including some with some what higher nonlinearity and slightly lower intrinsic loss. We have performed cut-back measurements of loss in the fiber shown in Fig. 1 using a broadband light source: Fig. 2 shows the measured loss curve in the wavelength range up to 1750 nm. The minimum observed loss is 2 dB/m at a wavelength of 1200 nm. Loss at the long wavelength end is dominated by confinement loss (which has a dramatic dependence on wavelength), while water peaks are the most prominent spectral feature in the low-loss window. A similar level of water absorption is also present in the bulk material. At wavelengths shorter than 1400 nm, confinement loss no longer appears significant, and yet losses in the fiber remain slightly higher than quoted figures for the bulk material. The excess loss is attributed to enhanced Rayleigh scattering at the optically rough glass-air interfaces. The fibers studied have core dimensions in the range 1.8 μm to 5 μm and the cores of our fibers were rectangular with a length ratio of about 1:2, leading to significant birefringence. An observed near-field pattern for the fiber is shown in Fig. 3. These fibers are not intrinsically single-mode, but the small core size and differential loss of high order modes make it possible to employ just the fundamental mode of propagation. Our main intention in this work is to optimize the GVD while maintaining a relatively high nonlinearity.

 

Fig. 2. Observed loss in SF6 PCF with a 2.6 μm core, as shown in Fig. 1. The data points for the bulk glass are from [6]. The curve is the result of a cutback measurement using a white light source.

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Fig. 3. Near-field pattern observed in a fiber similar to that shown in Fig. 1. The excitation source is a halogen lamp and the core diameter is 2.6μm.

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4. Group Velocity Dispersion

The GVD of bulk SF6 is shown as the broken line in Fig. 4. The dispersion at 1550 nm is around -30 ps/nm.km, becoming rapidly more negative towards shorter wavelengths. The effect of waveguide dispersion in small-core SF6 fibers is even more pronounced than in silica fibers [16] because of the higher refractive index. Group velocity dispersion curves of several fibers were measured using low-coherence interferometry and the results are shown in figure 4 for four different values of the core size. In the experimental setup, we used a polarizer to isolate a single polarization state. Data for the orthogonal polarization state were similar, although slightly offset from the curves given in Fig.4. As in silica fibers, decreasing the core size results in a strong anomalous waveguide contribution to the GVD, resulting in the shifting of the zero-GVD wavelength towards the visible. Considering our fibers in sequence (in order of reducing core size) the zero-GVD wavelength is shifted to around 1600 nm for the 5 μm core size, moving to well below 1300 nm for the 2.6 μm core fiber. We can explain this trend by regarding the small core PCF as a strand of SF6 surrounded by air [11]. It is worth noting that the zero GVD wavelengths of our fibers lie around 1550 nm, in contrast to other PCF’s [7] that produced supercontinuum using 800 nm pump sources, which had zero-GVD wavelengths around 800 nm [6].

 

Fig. 4. Measured group-velocity dispersion curves for bulk SF6 (broken curve) and for the fundamental mode in fibers with four different core sizes

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5. Supercontinuum generation

Nonlinear experiments were performed using an ultrashort optical parametric oscillator emitting pulses of 100 fs duration at a wavelength of 1550 nm. The pulse repetition rate was 80 MHz and the average output power of the laser was 200 mW. The spectral characteristics of the fiber output were obtained using an optical spectrum analyzer (for the wavelength 350 nm – 1750 nm) and an auxiliary spectrometer with an InGaAs detector to extend the wavelength range to 2200 nm. Results reported here were obtained using a 75 cm length of fiber with a 2.6 μm core diameter. This core diameter was chosen as a compromise between the nonlinearity and the ease of coupling in the pump beam. As shown in Fig. 4, this fiber has a zero-GVD wavelength around 1.3 μm, so that the pump pulses propagate in the anomalous dispersion regime.

Supercontinuum formation is observed to occur for average incident powers as low as 30 mW (corresponding to less than 1 kW of peak power). By optimizing the coupling so as to achieve around 30 mW average output power, we observe a very broad supercontinuum at the fiber output. The generated radiation is in the fundamental fiber mode, and the spectrum extends from at least 350 nm to 2200 nm, as shown in Fig. 5. A color photograph (Fig. 6) taken with a digital camera shows only the very small portion of the whole spectrum that falls in the visible wavelength band. The actual spectrum observed at the output is a strong function of the fiber length as well as the coupled power, because of the significant fiber loss on both the long and the short wavelength ends of the spectrum. The material losses are quoted [9] as 43 dB/m at a wavelength of 420 nm and 22 dB/m at a wavelength of 2325 nm, so that losses will be significant in even the few centimeters of fiber needed to generate this very broad spectrum. The physical mechanisms responsible for this very dramatic spectral broadening remain to be investigated: however, we expect that a number of known processes such as soliton breakup, self-phase modulation and four-wave mixing are involved. The generation of a supercontinuum spanning more than a factor of six in frequency with an unamplified pump source is an impressive demonstration of the strength of the nonlinear interactions in this fiber.

 

Fig. 5. Optical spectrum of the continuum observed at the output of a 75 cm length of 2.6 μm core fiber.

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Fig. 6. Visible supercontinuum generation in an SF6-PCF of several meters length. The ultrashort pulse train at 1550nm is incident into the fiber from the top left, and the color variation is partly due to the strong wavelength dependent loss in the visible. For 200 mW incident power, the generated spectrum extends from below 350 nm to beyond 2.2 μm.

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6. Conclusions

We have fabricated and characterised photonic crystal fibers using SF6 glass. The fibers have high nonlinearity and have zero or anomalous group velocity dispersion around 1550 nm. We have used these fibers to demonstrate spectacular supercontinuum generation using an ultrashort pump source at 1550 nm wavelength. The generated spectrum extends at least from 350 nm to 2200 nm. Such fibers will enable the generation of octave-spanning supercontinua using a mode locked fiber laser source at 1550 nm, making a compact and stable high-brightness broadband light source.