1. Introduction

In the past ten years, supercontinuum (SC) generation in microstructured fibers (MFs) have attracted great interest due to their remarkable dispersion and nonlinearity properties [1–4], finding numerous applications in optical frequency metrology [5], coherent spectroscopy [6], nonlinear microscopy [7], and biomedical optics [8]. MFs, designed with various types of core shape and core diameter, have been extensively fabricated and explored to generate supercontinuum of different features in previous work [9–13]. In these MFs, the silica nanoweb fibers, possessing much lower optical attenuation than the other planar waveguides, can be used in nanochemistry and sensing with an improved surface-to-volume ratio advantage especially useful for surface enhanced interaction. In terms of nonlinear optics, nanoweb fibers can be drawn to submicrometer thickness pursuing for larger birefringence and better polarization control of SC generation. Previous works have reported the nonlinear propagation and self-focusing in the 640 nm, 800-nm-thick silica nanoweb fiber [14,15]. The modes in nanoweb fibers can be divided into two groups according to their polarization properties, the TE modes in the width direction and TM modes along the thickness direction.

In our work a novel nanoweb fiber is reported, in which the cross section consists of web-like glass membranes with different thickness. This, in turn, allows tuning the dispersion of TE and TM polarized modes in your desirable pumping region in one fiber. The optical attenuation in the 0.55μm-thickness membrane is measured around 0.8 dB/m in the TE mode direction and 1.5 dB/m in the TM mode direction. Polarization dependent visible SC generation is obtained in the nanoweb fiber pumped in the anomalous dispersion region near the second ZDW [16,17]. The spectral evolution with polarization and input power are recorded and discussed in detail.

2 Fiber parameters and experimental setup

The fiber is manufactured by the Yangtze Optical Fibre and Cable Company Ltd using the stack-and-draw method. A scanning electron micrograph (SEM) of the fiber is shown in Fig. 1(a) . In the center of the fiber cross section is a large regular hexagon air hole surrounded by six annular air holes. The inner six narrow weblike glass membranes are labeled from No.1 to 6 here, all of them have a ~20 μm width, their measured thickness are 0.55 μm, 0.53 μm, 0.5 μm, 0.47 μm, 0.45 μm and 0.34 μm in sequence. A part of the No. 1 membrane is carefully measured as shown in Fig. 1(b). We can see its thickness is quite uniform except the connection point. The whole structure is embedded in a protective silica tube with an out diameter of 170 μm.

 

Fig. 1 (a) The SEM of nanoweb fiber, (b) The SEM of No.1 membrane.

Download Full Size | PDF

The attenuation of the No.1 membrane are measured around 0.8 dB/m along the TE mode direction as marked in Fig. 1(b) and 1.5 dB/m along the TM mode direction using the cut-back method. It is difficult to measure the dispersion in membrane of interest. The dispersion is calculated using the finite element module (Comsol Multiphysics, version 3.5a) in our work, the theoretical method is verified and simulation results agree well with that of the fiber structure in reference [14]. Figure 2 shows the calculated dispersion curve of the TM₀₀ and TE₀₀ in membranes with different thickness. For the TE₀₀ mode, the dispersion is normal over the entire concerned wavelengths. The dispersion curve is quite different for orthogonal polarization states. For the TM₀₀ mode, the dispersion of No.1 membrane is anomalous in wavelength range of ~0.57 μm to 0.83 μm, for the No.2 membrane it is 0.58 μm to 0.79 μm, and for No.3 membrane it is ~0.55 μm to 0.72 μm, while the dispersion is normal for both TM and TE modes when the thickness is smaller than 0.45μm. Thus, the membranes in nanoweb fiber allow SC generation in many different nonlinear regimes by tuning the pump wavelength.

 

Fig. 2 The dispersion curve of TM₀₀ and TE₀₀ modes in membrane with different thickness T.

Download Full Size | PDF

The nonlinear spectrum broaden is studied by pumping the fiber with a Ti:Sapphire laser (Mira900 Coherent) which delivers 200-fs linearly polarized pulses centered at 800 nm. The fiber used in experiment is 1.5 m long. A 40 X micro-objective lens with a NA of 0.65 is used to couple the pump into the membrane of interest. Here we select the No.1 membrane according to the dispersion curve. A similar method can determine the light coupling into which membrane as mentioned in [18]. A half-wave plate is used to vary the incident pulse polarization and a tunable attenuator to change the pump power. The output spectrum is recorded after collimating the output beam by an optical spectrum analyzer (Avaspec-3648, 200-1100nm).

3. Results and discussion

Figure 3 shows the measured average output power versus polarization angle θ, where θ is the angle between the polarization direction of the input pulse and the TE mode direction. By rotating the half-wave plate and real-time measuring the output power at the fiber end, we can identify the θ = 0° angle with a maximum output power of 44 mW. Correspondingly, a minimum output power is measured of 16 mW near the θ = 90° angle. The total shape of output power curve is quite similar with that of the cos²(θ) curve. The inset in Fig. 3 is the recorded field pattern for TM mode excitation at the 1.5m-long nanoweb fiber output end. It is observed that the main part of the spectral power still centers near the excitated point, also a little part of power leaks into the adjacent membranes through the interconnected points.

 

Fig. 3 the average output power vs. polarization angle θ.

Download Full Size | PDF

Figure 4 shows the recorded spectrum from θ = 0° to 90° in steps of 10° at the maximum pump power of 460 mW (the output power of our Ti:Sapphire laser). About 13% of pump power is measured incident into the membrane at the θ = 0° angle in the experiment. One can see that the output spectra exhibit highly polarization dependent feature, demonstrating quite different nonlinear effects involved during the SC generation at their corresponding angles. For easy of discussion, here we divide the incident power P _i into two parts according to θ, P _TE = P _icos² θ for TE mode excitation and P _TM = P _isin² θ for TM mode excitation, and interpret the SC generation is a combined contribution of P _TE and P _TM. At θ = 0°, only TE modes are excited in the normal dispersion regime as shown in Fig. 2(a), hence the spectral broadening of the initial laser pulse, mainly caused by the self-phase modulation, is narrow and symmetrical. At θ = 20°, the spectrum becomes asymmetrical. It indicates that the P _TM has caused the nonlinear spectral broadening of the input pulse in the anomalous dispersion region of TM modes. The SC becomes broader and broader with the increasing of θ (or P_TM). At θ = 50°, we observed new spectral components emerge around 400 nm. At θ = 90° only TM modes are excited, as a result the SC is broadest covering from ~350 nm to ~950 nm, we can notice the spectral components at ~400 nm grows up.

 

Fig. 4 The recorded spectrum for θ = 0° to 90° in steps of 10° at the pump power of 460 mW.

Download Full Size | PDF

We measured the clear mode photograph at the different θ as shown in Fig. 5 . It should be pointed out that the membrane is along with the white line direction as shown in θ = 0°. We can see that the mode photograph contains not only the fundamental mode, also high order modes, e.g. at least TE₀₀ mode and TE₀₁ mode can be distinguished out at θ = 0°. Therefore, the spectral broadening is a complicated process involving of high order modes in fact, which is proved by slightly changing the pumping condition and real-time measuring the field patterns in our experiment. Also, we noticed the fundamental mode is dominant in the mode photograph as shown in the figure from θ = 40°~90°, hence the spectral broadening is mainly affected by the dispersion properties of the TM₀₀ mode.

 

Fig. 5 The mode photograph at the different angle θ.

Download Full Size | PDF

Figure 6 is the recorded nonlinear spectral broadening for TM modes excitation with the increasing of pump power. The spectral evolution can be divided into three stages distinctly: In the first stage, the broadening is mainly determined by self-phase modulation, we find the splitting of red-shifted spectral peak at pump power of 9.2 mW, a common phenomenon of the self-frequency shift of Raman soliton [2,19] that plays an important role for SC generation in the long wavelength. In the second stage, the split peak red-shifts across the second ZDW (the inset solid line) and enters into the normal dispersion region at 855 nm, simultaneously the spectrum blue extends to 770 nm in the anomalous dispersion region at the pump power of 50 mW. Increased the pump power to 250 mW, we notice the spectral red-extension is limited near 950 nm, indicating that the soliton self-frequency shift is cancelled due to the large normal dispersion and high transmission loss, but the spectrum continues broadening to shorter wavelength with the increasing of pump. In the third stage, when the blue spectral trail touches the first ZDW (the inset dotted line), an additional spectral band appears near 400 nm and grows up rapidly beyond the pump power of 350 mW. Further increasing pump power leads to the generation of new spectral components in the range of 400 nm to 600nm via four-wave-mixing effect as shown in the maximum pump power of 460 mW.

 

Fig. 6 Spectral evolution for TM mode excitation with the increasing of pump power.

Download Full Size | PDF

4. Conclusion

In conclusion, we fabricated a novel nanoweb fiber with web-like glass membranes of different thickness, and obtained the polarization dependent visible SC in the 0.55μm-thickness membrane. The broadband spectrum range from ~350nm to 950nm is achieved for TM mode excitation pumped near the second ZDW by fs pulse at a wavelength of 800nm. The spectral evolution with polarization and pump power are recorded and discussed in detail. We think that the tunable visible SC in the nanoweb fiber may be used in the substrate integrated waveguide for sensing.