1. Introduction

Supercontinuum (SC) generation has already found numerous technological applications so far. Typical applications involve the pulse compression for ultrashort femtosecond laser source [1], multi-wavelength optical source for dense wavelength division multiplexing telecommunications [2], and optical frequency metrology for the measurements of optical frequencies with unprecedented accuracy [3]. Rapid development of research on SC benefits greatly from the technological maturity of silica glass photonic crystal fiber, which is characterized by a small core, and a chromatic dispersion which can almost be engineered freely. However, for the silica glass photonic crystal fiber two barriers can not be broken through. Firstly, it is not transparent at the wavelengths longer than 3 µm, which makes SC beyond this wavelength difficult. Secondly, the nonlinear refractive index n₂ of silica glass is only 2.2×10^-20 m²/W. This restricts the further improvement of the nonlinear coefficient of the fiber. Highly nonlinear fiber is the prerequisite of a SC source composed of low-cost and compact devices. Nonsilica glasses such as tellurite glass and chalcogenide glass are transparent in the mid-infrared range, and have a higher n2 than silica glass by at least one order of magnitude. Investigations on SC from nonsilica glass microstructure fibers have already been reported in some papers recently [4–8]. However, most of them adopted the pump wavelength around 1.5 µm. There is no report about SC from these glass fibers by 1064 nm excitation except for lead silicate fiber, which, similar to the silica fiber, has a limited transparency range. One important reason why there are so few reports from 1064 nm excitation is that highly nonlinear glass always has high refractive index, so in order to shift the zero dispersion wavelength (ZDW), which is the reference for selection of pump wavelength, to around 1.0 µm, a microstructure cladding together with a core diameter around 1 µm is necessary. However, fabricating microstructure fiber with such a small core is a challenge, since low confinement loss for this ultra small core requires further reduction of cladding glass web sizes, which becomes geometrically difficult and impractical to fabricate [9]. For nonsilica glasses it is even more difficult because nonsilica glasses are usually soft glasses which have a viscosity very sensitive to the variation of temperature. For example, the operating temperature range of tellurite glass for fiber-drawing is less than ten percent that of the silica glass [10]. The latest report about air-cladding fiber in the core size around 1 µm is a lead silicate fiber in the shape of steering wheel, which has a core in the shape of triangularity. Meanwhile the diameter ratio of holey region to core (DRHC) is limited by the fabrication technology [11]. Nevertheless, a 1 µm hexagonal core surrounded by six holes has never been realized by soft glass before. A larger holey region can provide a tighter mode field, higher nonlinear coefficient, lower confinement loss, and different chromatic dispersion, which will result in different nonlinear phenomena.

Compared with a triangular core, a hexagonal core is more easily deformed in the fabrication process since the number of the holes doubles. In this research, we have for the first time succeeded in the fabrication of a hexagonally shaped tellurite microstructure fiber with a core diameter of 1 µm through controlling the temperature field exactly in the process of fiber-drawing. By controlling the pressure in the holes of fiber we fabricated fibers with DRHC varying from 3.5 to 20. The ZDW is shifted to around 1 µm so that the efficient, popular and commercially available laser can be the pump source [12]. One octave flattened SC was generated from the pump of 15 ps pulse from a 1064 nm fiber laser. To the best of our knowledge, this is also the first SC generation from soft glass fiber by a picosecond laser in this wavelength. SC sources based on highly nonlinear fiber pumped with temporally broad pump pulse is of significance because it avoids the complications and inconveniences of using a complex femtosecond oscillator [13]. Additionally, SC pumped by a femtosecond fiber laser at 1557 nm was also investigated.

2. Fiber fabrication and characterization

 

Fig. 1. Scanning electron microscope images of the fibers.

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The composition of the tellurite glass was 76.5TeO₂-6Bi₂O₃-11.5Li₂O-6ZnO (mol%). The raw materials were analytic grade. A tellurite glass rod in the shape of hexagon was prepared by casting the glass melt in an alloy mold and then annealing it at the transition temperature. The tellurite glass tubes were prepared by rotational casting method. The rod was inserted into a tube and then elongated into a cane with an outside diameter of 2 mm. The cane was inserted into another jacket tube of tellurite glass, and then was drawn into the fiber. The jacket tellurite tube was used to decrease the ratio of the core to cladding size. The profiles of furnace, jacket tube and cane were kept strictly concentric to ensure the core had a good shape. In the fiber-drawing process a positive pressure of nitrogen gas was pumped into the hole of the cane. As shown in Fig. 1 the fiber has a core diameter of 1 µm, and an outside diameter of about 120 µm. By increasing the pump pressure, the diameter of the whole holey region composed of six holes can be increased. Three types of fibers which have the same core size and different DRCH were fabricated for detailed investigation. The DRHCs were 3.5, 10 and 20. They were named as SHF (small hole fiber), MHF (moderate hole fiber), and LHF (large hole fiber) respectively. The pump pressures were 1.6 kPa, 4.3 kPa and 7.8 kPa respectively. Because the holes of the cane were not sealed, they collapsed totally without pump pressure. When the pump pressure was higher than 7.8 kPa, the holey region became obviously asymmetric. On the whole, the diameter of the holey region can be controlled in the range of 1–20 µm (The minimum 1 µm is just the diameter of core). To the best of our knowledge, the fiber LHF is a fiber with the largest DRHC for the small core air-cladding fiber so far.

 

Fig. 2. Chromatic dispersions of the fundamental mode of the fibers.

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Table 1. Zero dispersion wavelength ZDW1 and ZDW2, the group velocity dispersion β2 at 1064 nm and nonlinear coefficient γ for the fiber SHF, MHF and LHF.

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The fully vectorial finite difference method (FV-FDM) was used to calculate the wavelength dependent propagation constants from which the chromatic dispersion was calculated. The simulations were based on scanning electron microscope images. As shown in Fig. 2 and Table 1, the two ZDWs can be varied greatly by the variation of DRHC. Because the wavelength of propagated light is close to, or even larger than the size of the core for these microstructure fibers, a high proportion of power is propagated in the cladding. Consequently, the size of the holey region has an important influence on the chromatic dispersion properties. The nonlinear coefficient γ was calculated by:

where F(x,y) is the profile of the field at 1064 nm. n₂(x,y) is the distribution of nonlinear refractive index. It is 5.9×10^-19 m²/W for this tellurite glass, and is 2.9×10^-23 m²/W for air. γ for each fiber is shown in Table 1. It increases greatly with the initial increase of holey region, and then keeps constant with the further increase of holey region.

 

Fig. 3. Spectra of confinement loss of the fibers.

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The optical loss at 1557 nm for each fiber was measured using the standard cutback measurement technique. A homemade femtosecond fiber laser with the peak wavelength of 1557 nm was connected with a single mode fiber (SMF) by a connector. The beam from the SMF was collimated into parallel by a lens of 20×0.25 NA. The parallel beam was focused and coupled into the tellurite microstructure fiber by a lens of 40×0.47 NA. The output end of the fiber was mechanically spliced with a silica fiber cable with a large mode field by using a butt-joint method. The other end of the fiber cable was connected with the optical spectrum analyzer (OSA). After the SC generation measurement, the femtosecond fiber laser was replaced by a white light source. The optical loss for each fiber is around 5 dB/m at 1557 nm. Because the raw materials are analytic grade, the loss can be decreased greatly by improving the purity of raw materials. The spectra of confinement loss were calculated by the FV-FDM. The results are shown in Fig. 3. It indicates that there is a cutoff wavelength due to the confinement loss for each fiber. As the holey region increases in size, the cutoff wavelength shifts to longer wavelength.

3. Supercontinuum generation

 

Fig. 4. Power-dependent supercontinuum spectra under the pump of 1557 nm femtosecond fiber laser. The curve is displaced by 20 dB.

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Figure 4 shows the power-dependent SC spectra measured by using a homemade femtosecond fiber laser at 1557 nm. The pulse width is 400 fs and the repetition rate is 16.75 MHz. The lengths of the microstructure fibers were around 6 cm. The small peaks at 1178 nm, 1245 nm and 1318 nm are parasitic multi-wavelength Raman lasers from high power 1480 nm fiber Raman laser which is the pump source of the femtosecond fiber laser. The launched efficiency, defined as the launched power divided by the power incident on the lens of 40×0.47 NA, was about 10%. The difference in noise before and after 1000 nm was due to the change of detector in OSA. The maximal breadth of the SC was limited by the maximal power of the pump laser. Under the maximal pump power the energy of the launched pulse is 573 pJ. The peak power is 1433 W. An interesting phenomenon is the intense visible emission. Their wavelengths are almost not changed by the increase of pump power. They mainly ascribes to third harmonic generation (THG) [14]. For SC spectra of three fibers, the shortest wavelength is around 519 nm, which is corresponding to one-third of the pump wavelength. Visible emissions longer than 519 nm originate from the THG by Raman-shifted solitons [15,16]. The wavelength of THG depends on the phase-matching conditions. For the fiber MHF the emission covers all the visible range longer than 519 nm. The intensity of visible emission of SHF is much smaller than those of other two. It is ascribed to that a smaller holey region provides a poorer confinement of mode at infrared band. The most intense emission comes from the fiber LHF at 660 nm.

 

Fig. 5. Power-dependent supercontinuum spectra under the pump of 1064 nm picosecond fiber laser. The curve is displaced by 10 dB.

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Figure 5 shows the power-dependent SC spectra pumped by a picosecond fiber laser at 1064 nm. The pulse width is 15 ps and the repetition rate is 80 MHz. The laser was launched into the tellurite fiber by a lens of 40×0.47 NA. The launched efficiency, defined as the launched power divided by the power incident on the lens, was about 10%. The other end of the tellurite fiber was connected with a large mode field fiber which was connected with OSA. The length of the tellurite fiber was around 30 cm. The initial evolution of SC from the fiber SHF is very similar to the previous report where the spectra of stimulated Raman scattering (SRS) have been obtained by pumping a 7 µm core tellurite air-cladding fiber with a 1064 nm picosecond laser [17]. SRS plays an important role in the initial evolution of SC for the fiber SHF and MHF. Further evolution involves four wave mixing (FWM), and the soliton propagation dynamics. The intense anti-Stokes peak for the SC spectra of fiber MHF ascribes to the non-phase-matched coupling between SRS and FWM [18].

Fiber LHF was pumped far from the ZDW in the anomalous dispersion range. FWM-MI (modulation instability) and solitons broaden the SC. For the pulse shorter than 100 ps, self phase modulation (SPM) introduces new frequency components which acts as a probe pulse for MI [19]. The frequency shift of MI gain peaks is given by [20]:

where Ω_max is the angular frequency shift of the maximum gain. P₀ is the peak power, and β₂ is the group velocity dispersion. In Table 1 the absolute value of β₂ of LHF is much larger than that of MHF, so the Ω_max of LHF is much smaller. Additionally, for both fibers, because the nonlinear coefficients are the same, the frequency shifts by SPM are the same under the same pump conditions. As a result MI occurred more easily for the fiber LHF than for the fiber MHF, because Ω_max of LHF is much smaller and can be reached by frequency shift of SPM more easily. That is the reason why MI appears only for the fiber LHF. In Fig. 4 under the average pump power of 58 mW, it can be found that the spectrum of the fiber LHF exhibits symmetric sidebands if the amplified spontaneous emission was subtracted. With the increase of the pump power the symmetric sidebands became separate. These are the features of MI [21]. Here SRS does not appear obviously. This is because the threshold of SRS for this fiber is higher than that of MI. This can be found by comparing the initial pump powers of the fibers.

When the average pump power is 400 mW, the peak power of the launched pulse is 33 W and the energy is 500 pJ. The nonlinear lengths are 7.8, 5.3 and 5.3 mm for the fiber SHF, MHF, and LHF respectively. Here we used the picosecond pulse, but the energy of pulse is still lower than those of many researches where femtosecond pulse with energy of several nJ was used. For the fiber LHF, a much flattened section of SC covers the O, E, S, and C band of the fiber communication. The nonlinear lengths are much shorter than their effective lengths even when the peak power of pulse is only several tens of watt. It means that the high nonlinear coefficient counteracts the high loss to a large extent. It is of significance, because on the one hand it will make the device more compact, on the other hand the decrease of requirement for the purity of raw materials will decrease the cost of fiber to a large extent. In this experiment we used the microstructure fibers with the length of 30 cm only because this length was convenient for the measurement. The length can be decreased greatly if required.

4. Summary

In summary, the tellurite microstructure fiber with a core diameter of 1 µm and in a core shape of hexagon has been fabricated for the first time. By controlling the positive pressure in the holes in the fiber-drawing process the DRHC can be varied from 1 to 20. It provides much flexibility in engineering chromatic dispersion. The confinement loss and nonlinear coefficient show great dependence on the size of holey region. Intense visible emission was found for the fiber under the pump of 1557 nm femtosecond laser, which ascribed to the third harmonic generation. One octave flattened SC spectrum was generated under the pump of 1064 nm picosecond laser for the fibers. Because of the high nonlinear coefficient, controllable chromatic dispersion and low requirement for the purity of raw materials, such fibers have promising applications for the compact and low-cost supercontinuum source.