1. Introduction

The development of photonic crystal fibers (PCFs) has enabled a whole range of new or improved experiments in the fields of fundamental and applied laser science [1]. For example, the enhanced control of dispersion offered by highly nonlinear small-core PCF has made it possible, simply and massively, to broaden the spectrum of laser pulses [2,3]. When a mode-locked laser source is used, such supercontinua consist of a comb of frequencies spaced by the repetition rate of the laser. Octave-spanning versions of these frequency combs have important uses in frequency metrology [4,5].

In recent years lead-silicate glass PCFs have been used to generate octave-spanning supercontinua from pulsed laser sources at 1.5 μm (~200 pJ, 65 fs pulses) and 1 μm (100 pJ, 300 fs pulses) [6–8]. These results demonstrate the advantages of the roughly ten times higher glass nonlinearity compared to fused silica.

In addition, compact and reliable passively mode-locked Yb-fiber oscillators operating at 1 µm and delivering sub-100 fs laser pulses have appeared in recent years [9–11]. Although their use in generating supercontinua in pure silica PCF has already been investigated, octave-broad supercontinua could only be achieved by using an optical amplifier [12,13]. In this Letter we present the combination of a highly nonlinear SF6-glass PCF and an unamplified diode-pumped fs Yb-fiber oscillator (pulse duration 60 fs), producing a supercontinuum spanning the range 600 to 1450 nm from pulse energies less than 20 pJ. To our knowledge this is the lowest reported pulse-energy for octave-broad supercontinuum generation at 1 μm. Such simple octave-spanning light sources will have immediate applications in many fields where efficient spectral broadening is paramount.

Applications can range from optical coherence tomography (OCT) [14], where the short coherence length of “white light” is used, to frequency comb generation, where the full coherence of the light pulses is used. In certain key applications, such as the calibration of astronomical spectrographs [15,16] or space applications, the efficiency, stability and compactness of the supercontinuum sources becomes of critical importance.

2. SF6-glass fiber

The cross-sectional structure of the SF6-glass cane (a) and the final fiber (b) are shown in Fig. 1. The cane was fabricated by the now standard stack-and-draw technique, with a two-ring structure encircling a solid glass core, allowing guidance by modified total internal reflection. Pressure was applied during fiber drawing to inflate the holes in the cladding. The final outer diameter of the fiber was 190 μm, with a core diameter of ~1.7 μm. Despite uniform pressure being applied throughout the cane, the final hole sizes were slightly unequal due to the effects of surface tension and differing initial hole diameters and shapes. The result was a slightly asymmetric core geometry deviating from symmetry by ~0.1 μm.

 

Fig. 1. Optical micrograph of a 1.2 mm diameter cane (left) and scanning electron micrograph of the core region in the final fiber (right).

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The optical properties of the fiber are shown in Fig. 2. The loss was measured by the cutback method using a white-light source, and was ~ten-times higher (~4 dB/m) than in bulk material at the pump wavelength of 1 μm. This value is ~1 dB/m higher than previously reported in lead-silicate PCFs made by extrusion [7,8], which we attribute to the extra processing steps required in the stack-and-draw procedure.

The dispersion was measured using a Mach-Zehnder interferometer and a white-light source (Fig. 2, right-hand side). The fundamental core mode has a zero dispersion point at ~1.06 μm, compared to ~1.8 μm in the bulk glass. We observed a splitting of about 20 nm in the zero dispersion point for the two polarization eigen-states, which we attribute to the core asymmetry.

 

Fig. 2. (Left) Loss of the SF6-glass PCF measured by the cut-back method. It is about 10 times higher than in bulk glass (red dots). (Right) Group velocity dispersion of bulk SF6 glass (blue line) and measured group velocity dispersion of the fiber: Two zero dispersion wavelengths appear at 1060 and 1080 nm, caused either by slight geometrical asymmetry in the core structure or residual stresses and corresponding to orthogonal polarization eigen-states.

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For an estimate of the nonlinearity γ, we took the core diameter (1.7 μm) as the effective mode diameter. Based on a nonlinear refractive index n₂ of 2.2×10^-19 m²/W [3], the resulting nonlinearity is about 570 W^-1·km^-1 at 1.06 μm, which is more than two orders higher than in telecommunications fiber (~1 W^-1km^-1).

3. Supercontinuum generation

Figure 3 shows the set-up. The pump laser was a home-built, shoe-box sized passively mode-locked fs Yb-fiber oscillator based on nonlinear polarization rotation. In order to compensate the dispersion of the Yb-doped fiber, we used an internal grating compressor. The oscillator had a fundamental repetition rate of 166 MHz and maximum pulse energy of ~1 nJ. The mode-locking was reliably self-starting with excellent long-term stability. The extracted pulses were compressed, using an external transmission grating arrangement, to a duration of 60 fs. A Faraday isolator was used to suppress back-reflections. The maximum pulse energy after the compressor and isolator was about 500 pJ.

A half-wave plate and a polarizing beam-splitter were used before the focusing lens for variable adjustment of the pulse energy launched into a ~4 cm long PCF. A second half-wave plate was used to optimize the polarization state of the laser pulses in the fiber. The transmitted power was measured with a thermoelectric detector. Using uncoated lenses, we achieved a maximum transmission of about 13%, corresponding to a maximum transmitted pulse energy of 66 pJ. An increase in transmission up to 20% could be expected if anti-reflection coated lenses are used. The spectrum after the PCF was measured using an optical spectrum analyzer (OSA), the light being delivered with a multimode fiber.

 

Fig. 3. Set-up for supercontinuum generation: TGC = transmission grating compressor; I = Faraday isolator; PBS = polarization beam splitter. The output spectrum was launched into a multimode fiber and coupled to an optical spectrum analyzer.

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To explore the evolution of the spectrum with power, we increased the launched pulse energy in steps of ~12 pJ from 6 pJ to 66 pJ. The resulting spectra are shown in Fig. 4, plotted on a logarithmic scale for different launched pulse energies. It can be seen that some spectral broadening already appears at pulse energies of 6 pJ. First, a Raman peak appears at 1250 nm in the long-wavelength regime and second, symmetric broadening around the laser line was observed, indicating the presence of self-phase modulation. After increasing the pulse energy to 18 pJ, the Raman peak shifted from 1250 nm to 1350 nm. Simultaneously, a strong peak appeared at 650 nm, in the normal dispersion regime of the fiber. We also observed further broadening on the short-wavelength side of the pump for a launched energy of 18 pJ. At this energy level, the broadened spectrum already extended from 600 nm to 1450 nm. This is the lowest pulse energy yet reported for production of an octave-spanning supercontinuum from an Yb-based fiber laser at 1 μm.

On further increase in pulse energy (from 18 to 66 pJ), some additional spectral broadening from 1450 to 1550 nm was observed, together with a general increase in the spectral intensity between 600 nm and 1550 nm.

Based on previous experiments on supercontinuum generation in SF6-glass PCF pumped at 1550 nm [17], it is likely that the octave-spanning supercontinuum will have a clearly defined comb structure. Furthermore, since the pulse duration is similar and the fiber length four-times shorter than in [17], the comb coherence is likely to be higher. Detailed measurements of the frequency comb parameter are the subject of current investigation and will be reported in a future publication.

The visible spectral range is of particular interest in astro-spectroscopy, since most of the relevant atomic transitions occur in this region and the transparency of the earth atmosphere is better compared to the UV and IR spectral ranges. As a result, we investigated in more detail the visible spectral components of the supercontinuum. We used a reflection grating for spatial separation of the visible output spectrum. In Fig. 4 (above) one can see the separated visible spectrum for maximum launched pulse energy. It contains orange and red spectral components.

 

Fig. 4. Optical spectra of the Yb-oscillator (laser) before the PCF and for different launched pulse energies after 4 cm of SF6-glass PCF. The left-hand vertical scale refers to the 66 pJ plot; the other plots have been displaced downwards for clarity. An octave is reached at pulse energies below 20 pJ. The features on the extreme left-hand side (outside the shaded region) are an artefact of the spectrum analyzer. Above, the visible part of the spectrum at maximum launched pulse energy is shown after spatial separation with a grating.

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In summary, the spectrum at the maximum launched pulse energy covers the range 600 to 1550 nm, which is wider than an octave and offers the possibility of measuring the carrier-envelope-offset frequency of the Yb-oscillator without any need for additional pulse amplification. In contrast with fiber-laser based frequency combs generated from 1550 nm pump pulses, the current source has significant visible spectral components.

4. Conclusions and outlook

Bright, stable and octave-spanning supercontinuum spectra can be produced by pumping an SF6-glass photonic crystal fiber with 60 fs pulses from a passively mode-locked Yb-fiber laser operating at a wavelength of 1 μm. A suitable PCF has a core diameter of 1.7 μm and a zero dispersion wavelength of ~1060 nm, and can be made using the standard stack-and-draw technique. A simple unamplified fs Yb-fiber oscillator and only 20 pJ of launched pulse energy is needed, in a short 4 cm length of fiber, to generate a spectrum spanning from 600 nm to 1450 nm. This is the lowest pump pulse energy so far reported for generation of an octave-spanning supercontinuum at 1 μm.

The results demonstrate that SF6-glass photonic crystal fiber is a promising vehicle for applications where spectral broadening is required at very high repetition frequencies (several GHz), when the pulse energies are very low. Such a situation arises in the precise calibration of astronomical spectrographs with frequency combs. In this case a repetition frequency or mode spacing of above 10 GHz is needed for the best performance [15,16], which in turn requires very efficient spectral broadening to have any hope of covering most or a large part of the visible spectrum.