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Abstract
We report a small-core UV-grade silica multimode photonic crystal fiber for nonlinear frequency conversion in the ultraviolet spectral region. The fiber has been fabricated using F110 UV-Grade glass from Heraeus, which features excellent transmission and low solarization in the UV window. Pumping the fiber core at 355 nm with picosecond laser pulses, we observe the appearance of parametric sidebands in several spatial modes up to 380 nm. We modelled this process using intermodal phase-matching conditions and obtained excellent agreement between calculations and the measured data. We further report frequency conversion pumping into the fiber microstructured cladding where broadband cascaded Raman scattering spanning up to 391 nm is observed. These results represent a significant step towards the efficient and stable generation of UV supercontinuum spectra in an all-silica fiber platform.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A major current challenge in supercontinuum (SC) generation is the generation of spectral components in the ultraviolet (UV) wavelength range below 400 nm [1]. There is particular need for UV broadband light sources in applications such as fluorescence microscopy for the simultaneous coherent excitation of multiple fluorophores, and in UV absorption spectroscopy [2]. However, UV generation in conventional silica-core fibers has proven to be extremely difficult because of factors such as high material electronic absorption, increased Rayleigh scattering, glass solarization (photo-darkening and optically-induced color centers), and large normal dispersion which limits the generated bandwidth from nonlinear frequency conversion processes [3]. These factors have stimulated much recent work on alternative approaches, primarily using gas-filled hollow-core photonic crystal fibers (PCFs) [4–10]. Although these efforts show great promise for deep UV generation down to 110 nm [8], compatibility with the ubiquitous silica platform remains a problem. There is therefore great interest in generating UV-light using modified UV-resistant glasses instead.
In this work, we report on the design and fabrication of a UV-resistant silica-based multimode photonic crystal fiber (PCF) for broadband UV generation below 400 nm exploiting intermodal nonlinear wave mixing. The fiber was fabricated using F110 UV-Grade glass from Heraeus, selected for its excellent transmission and low solarization in the UV-A range (260 nm-400 nm). Using a picosecond UV pump laser at 355 nm, we demonstrate a number of intermodal four-wave mixing (FWM) processes in the UV, which extend beyond the pump wavelength to 380 nm. The generated FWM signal and idler sidebands were carefully analyzed and imaged to determine their modal content. We determine the intermodal phase-matching conditions from the computed first and second-order dispersion parameters obtained from finite-element method (FEM) modelling, and obtain good agreement between experimental results and calculations. Significantly, we also observe a broadband Raman cascade from 355–391 nm when pumping the fiber directly in the microstructured cladding. This observation is interpreted using generalized nonlinear Schrödinger equation simulations of propagation in one bridge of the microstructure. We specifically show that the tight optical confinement allows for strong enhancement of the Raman effect with few intermodal FWM.
2. UV-Grade silica photonic crystal fiber
Figures 1(a-c) show, respectively, the UV-grade PCF design, a scanning electron microscope (SEM) image of the fabricated fiber cross-section, and a contrast-enhanced black and white image for improved visibility. The fiber was designed using a finite element method approach (COMSOL Software) to support multiple spatial modes with effective indices and dispersion profiles suitable to enable intermodal nonlinear FWM in the UV range. The fiber was then drawn at Photonics Bretagne in Lannion using the F110 UV-Grade silica glass from Heraeus. This glass has a high OH content up to 400 ppm and features good transmission in the UV-A range, from 260 nm to 400 nm. It also possesses a low solarization threshold around 300 nm and has a good transmission near the OH absorption band minima around 670, 800 and 1030 nm. The typical measured fiber attenuation provided by the manufacturer was 100 dB/km at 300 nm. This compares with an attenuation as large as 1000 dB/km at this wavelength for standard (low-OH) F300 silica glass used in conventional telecommunications fibers (SMF-28).
Fig. 1. (a) Design of the UV-grade solid-core microstructured fiber for nonlinear UV generation. (b) SEM image of the PCF manufactured by Photonics Bretagne using F110 UV-grade silica glass. (c) Contrast-enhanced black and white image from the SEM image (b).
The design in Fig. 1(a) was based on a triangular photonic lattice with core diameter of 4.26 µm, hole diameter d = 3.5 µm, pitch $\Gamma ~=~3.88$ µm, and air-fill fraction $d/\Gamma =0.902$. The UV fiber theoretically supports the first eight LP modes at 355 nm including the fundamental mode LP01 and 7 higher-order modes from LP11 up to LP22. They are plotted in Fig. 2(a) in colormap. Figures 2(b) and (c) show their inverse group velocity $\beta _{1}$ and their second-order dispersion $\beta _{2}$ in the UV range, respectively. As can be seen, the second-order dispersion is highly normal for all modes at 355 nm (up to $12 \times 10^{-26} \, \mathrm {s}^{2} \, \mathrm {m}^{-1}$ for the LP22 mode) because it is mainly dominated by material dispersion. This strong normal dispersion in turn precludes standard soliton and dispersive wave dynamics usually involved in anomalous-dispersion supercontinuum generation [11–13], and it this limitation which motivates the design of a multimode fiber for intermodal FWM in the UV [14–20].
Fig. 2. (a) Computed mode profiles of the 8 main guided modes from LP01 to LP22. (b) Computed inverse group velocity $\beta _{1}$ for 8 spatial modes versus wavelength. (c) Computed group velocity dispersion parameter $\beta _{2}$ for 8 spatial modes versus wavelength.
3. Experimental setup
Figure 3 shows the experimental setup used to investigate intermodal FWM and Raman scattering in the UV. As a pump source, we used a passively Q-switched Nd:YAG microchip laser (Teemphotonics PowerchipTM series) at 355 nm with a repetition rate of 1 kHz, a pulse duration (FWHM) of 350 ps, and an output mean power of 18 mW (peak power of $\sim$50 kW). The laser power was further controlled by a variable density filter and the laser beam was injected into the fiber using a UV-coated aspherical injection lens controlled by a 3-axis translation stage. We used several different injection lenses with focal lengths f of 7.5 mm, 8 mm, 11 mm and 15.3 mm and numerical aperture (N.A.) of 0.3, 0.5, 0.3 and 0.16. This allowed a change of beam waist diameter from 3.4 µm, 3.6 µm, 4.9 µm to 6.8 µm, modifying the fiber injection conditions and modal properties, allowing us to explore the various intermodal FWM processes. A coupling efficiency of 20 % was obtained using aspherical lenses with focal length f = 7.5 mm, f = 8 mm and f = 11 mm, respectively, and of 28 % for an aspherical lens with f = 15.3 mm. These measurements of optimal injection efficiency were measured using a short fibre length of 75 cm, but we note that in the experiments described below to excite particular mode combinations, the coupling efficiency could be significantly less than the optimal. The fiber output beam was then imaged using a CCD camera after a diffraction grating to record the modal profiles of the generated parametric sidebands. Another part of the fiber output beam was deflected using a beam splitter and recorded with an UV-visible optical spectral analyzer (OSA) operating in the wavelength range 350-1200 nm (Yokogawa AQ6373).
4. Experimental results
4.1 Intermodal four-wave mixing
Initial experiments were carried out using a 4 m long fiber sample and a coupling lens with focal distance f = 7.5 mm. Figure 4 shows two typical optical spectra recorded out of the fiber for two different coupling powers and injection conditions. We can clearly see a series of intermodal FWM sidebands around the pump wavelength at 355 nm. In Fig. 4(a), a first Stokes sideband is generated at 359 nm with a weaker anti-Stokes idler at 350.5 nm. The modal content of the Stokes signal (shown in inset) reveals that it propagates in the LP31 mode. We can further notice the onset of the first-order stimulated Raman scattering sideband at 360.8 nm, which matches with the Raman frequency shift of fused silica (13.2 THz). Figure 4(b) shows, for different transverse coupling conditions, two new FWM processes involving a Stokes signal at 357.3 nm in the LP41 mode, and an anti-Stokes sideband below the pump at 352 nm, respectively. The second FWM involves a Stokes signal at 363.1 nm, still in the LP41, due to the coupling with the Raman Stokes at 360.8 nm. The image in the middle shown in Fig. 4(b) is a mix between the first Raman order at 360.8 nm and an intermodal FWM process at 361 nm [21].
Fig. 4. Experimental output spectrum showing intermodal FWM sidebands for (a) an output power of 23 µW and (b) a fiber output power of 31 µW. The spectral resolution is 0.5 nm and the images in the inset show the modal content of each component of interest (f = 7.5 mm for both).
Next, we investigated in the same fiber sample the intermodal FWM with a focal length f = 8 mm. In Fig. 5, we show an output optical spectrum for a mean output power of 14 µW. It features different intermodal FWM and modal content. A first FWM signal appears at 358.1 nm in the LP11 mode, a second signal at 359.8 nm in the LP21, a third signal at 363.4 nm in the LP21, and a fourth signal is at 369 nm in the LP11 mode, respectively, while the pump beam at 354.8 nm has a complex mixed-mode profile.
Fig. 5. Intermodal FWM spectrum for a fiber output power of 14 µW with the modal images of each FWM component of interest. The spectral resolution is 0.1 nm (f = 8 mm).
Other intermodal FWM sidebands have been observed using the coupling lens with f = 15.3 mm with a beam waist of 6.8 µm that over matches the core diameter of around 4.14 µm. One can first see in Fig. 6(a), in addition to the Raman line at 360.8 nm, a new FWM sideband generated at 364.9 nm in the LP02 mode. Figures 6(b-d) also shows more FWM spectra. We retrieve the FWM signals at 358.1 nm and 359.9 nm as in Fig. 5. However, new FWM Stokes signals are now generated at 358.9 nm and 363.7 nm with a modal image of LP11. The fifth peak at 366 nm with a large modal image of LP01 is similar to a Raman second order. In Fig. 6(c), there is only one FWM case with an anti-Stokes sideband at 351.1 nm in the LP01 and a Stokes sideband at 358.3 nm in the LP11 mode. In Fig. 6(d), we again show four FWM. A first signal wavelength is at 358.6 nm with a modal image of LP31, a second signal wavelength is at 359.2 nm, a third signal wavelength is at 361.1 nm without any clear modal image, a fourth signal wavelength is at 364.2 nm with a modal image of LP31. The first modal image is the one of the pump at 355 nm. The other peak at 360 nm with a modal image of LP21 corresponds to the first Raman line involved with the FWM process.
Fig. 6. Experimental results of intermodal FWM spectra in the UV-grade photonic crystal fiber pumped at 355 nm with the aspherical lens of f = 15.3 mm and for different fiber output power and fiber length (a) L = 1 m, P$_{out}$ = 187 µW, (b) L = 4 m, P$_{out}$ = 21 µW, (c) L = 1 m, P$_{out}$ = 145 µW, (d) L = 4 m, P$_{out}$ = 23.5 µW.
Finally, using the coupling lens with f = 11 mm with a beam waist of 4.9 µm we observed the broadened intermodal FWM spectrum generated with a pump at 355 nm (see Fig. 7). Fig. 7 shows three strong intermodal FWM bands at 359.5 nm in LP21, at 363.6 nm in LP02 and at 364.9 nm in LP02, respectively. It also shows the broadened peak generated via intermodal FWM at 379.7 nm in the LP22 mode. These results have been further compared to the theoretical calculations of intermodal phase-matching conditions (see Table 1 in the next section).
Fig. 7. Intermodal FWM spectrum for a fiber output power of 34 µW, with a resolution of 0.5 nm and with the modal images of each component of interest (f = 11 mm).
Table 1. Comparison between theory and experiment for a number of intermodal FWM processes. The experimental values listed correspond to results shown in different figures as follows: Wavelengths shown in black correspond to the results in Fig. 6(a) or (b) or (c) or $\underline{\underline{(\textrm{d})}}$. Wavelengths shown in $\color{blue}{\textrm{Blue}}$ correspond to the results in Fig. 5; Wavelengths in $\color{green}{\textrm{Green}}$ correspond to the results in Fig. 9(b); Wavelengths in $\color{red}{\textrm{Red}}$ correspond to the results in Fig. 7; Wavelengths in $\color{purple}{\textrm{Purple}}$ correspond to the results in Fig. 4(a) or (b). Those idler wavelengths marked (*) have been calculated from the experimental signal wavelengths.
4.2 Intermodal FWM theory
Here we briefly review the basic theory of degenerate intermodal FWM and the related phase matching conditions [22–28]. First, we consider energy conservation,
with $\lambda$ the wavelength in nm, p, i and s denote the pump, idler and signal, respectively. Similarly, momentum conservation yields a phase-matching condition expressed as:Table 1 summarizes the theoretical FWM signal and idler wavelengths for different modal combinations and compares them with results obtained from experiment. The agreement between theory and experiments is particularly good for all FWM signal sidebands, confirming the intermodal FWM processes occurring in the UV-grade multimode PCF. Note however that most of the calculated idler sidebands are absent in the experimental spectra shown in Figs. (4–7) due to the wavelength limitation of our spectrometer.
4.3 Cascaded Raman scattering
We investigated the stimulated Raman scattering (SRS) response of the fiber using an aspherical lens with f = 8 mm, with a beam waist diameter of 3.6 µm and a N.A. of 0.5. Figure 8 shows two different coupling conditions that allow for the generation of a Raman cascade until the sixth-order. The fiber output image shown in Fig. 8(a) shows, surprisingly, a coupling of the pump in several bridges of the fiber with no propagation in the core. In Fig. 8(b) an example is shown where coupling occurs only over a few bridges. The bridges, which have a diameter around 1.6 µm, allow a tight confinement of the light. Despite the strong attenuation of this 4 m long fibre sample, a cascaded Raman response of six orders is generated through cladding mediated modal coupling.
Fig. 8. (a) Experimental output spectrum for a fiber length of 4 m showing wideband cascaded Raman scattering from 355 nm to 391 nm. The fiber output image shows pump coupling in almost all the bridges. (b) Experimental output spectrum for a fiber length 4 m showing similar results but with pump coupling in only a few bridges (f = 8 mm for both).
Then the aspherical lens with f = 15.3 mm and N.A. = 0.15 was used in order to broaden the Raman cascade since the coupling efficiency with this lens is better than with the two other aspherical lenses. However, the experiments revealed a decreasing Raman cascade, which is mixed with intermodal FWM processes to generate a continuum (see Fig. 9).
Fig. 9. (a) Experimental output spectrum for a 4 m long fiber showing wideband cascaded Raman scattering from 355 nm to 385 nm. The fiber output image shows where the pump is coupled in the fiber, here in all bridges and the core. (b) Experimental output spectra for a fiber length 4 m showing wideband cascaded Raman scattering from 355 nm to 377.7 nm and intermodal FWM effects at 358.1 nm and 370.5 nm. Output power level varies from 0.1 to 0.2 mW from bottom to top (f = 15.3 mm for both).
Figure 9(a) shows the fiber output spectrum with a cascaded Raman until the fourth order mixed with an intermodal FWM with an idler wavelength at 351.3 nm (see Figs. 9(a) and (b)) and a signal wavelength at 358.1 nm (not visible in Fig. 9(a) but visible in Fig. 9(b)). Perhaps the most surprising result is shown in the inset of Fig. 9(a) where light is seen to couple both into the whole photonic cladding as well as into the solid core.
Figure 9(b) shows the spectral evolution as a function of the fiber output power for the generation of the Raman cascade mixed with two FWM processes. At low power only the Raman cascade is generated. Then at high power one can see two FWM peaks, one with the signal wavelength at 358.1 nm and the idler wavelength at 351.3 nm and a second one with the signal wavelength at 370.5 nm. These combined effects generate a continuum from 350 nm to 377.7 nm. We performed additional FEM-based numerical simulations that confirm that the light can be indeed guided in the silica bridges (cladding modes) [5,29,30] in the UV-Grade PCF. Typical computed mode profiles are shown in Figs. 10(a-c) where we plotted three cases with their effective indices and losses, respectively.
Fig. 10. Typical computed cladding mode profiles with (a) an effective index ($n_{eff}$) of 1.460546 with 0.1897 dB/m losses, (b) $n_{eff}$ = 1.460524 and 1.3501 dB/m losses. (c) $n_{eff}$ = 1.460539 with 0.2276 dB/m losses.
Figures 10(a) and (b) show the first two computed cladding modes with their effective indices that match with the modal images from Fig. 8(a) and (b), respectively. The mode losses are 0.1897 dB/m and 1.3501 dB/m, respectively. Figure 10(c) shows the computed cladding mode involved in the mixing process between cascaded Raman and intermodal FWM seen in the experimental modal image in Fig. 9(a). The discrete cascaded Raman of six orders from 350 nm to 391 nm is generated through cladding modal coupling in a few bridges (see Fig. 8(b)). When the number of bridges involved in the cladding mode increases, the Raman cascade of six orders from 350 nm to 391 nm evolves towards a continuum, as shown in Fig. 8(a). Then the cladding modes are mixed with core modes. This is shown in Fig. 9(b) where we can see both cascaded Raman scattering and intermodal FWM from 350 nm to 380 nm. To interpret the cascaded Raman spectra shown in Fig. 8, we simulated the nonlinear pulse propagation and Raman cascade in one silica bridge of the microstructured cladding, using the computed first and second-order dispersion parameters for the fundamental mode (see Figs. 11(a) and (b)) from a finite-element method (FEM) computation. The generalized nonlinear Schrödinger equation (GNLSE) [31,32] was used to perform this simulation:
Fig. 11. (a) To calculate the dispersion properties of the PCF bridges, we performed FEM modelling of the structure as shown with each large air hole having diameter of 3.5 µm. The figure shows the computed fundamental mode in the bridge region between the air holes. (b) Computed group velocity dispersion parameter $\beta _{2}$ for the fundamental mode versus wavelength.
Fig. 12. Simulation with a peak power of 100 W and a loss of 1 dB/m with under a colormap of the simulation of the continuum generation over 4 m of the fiber length.
The colormap in Fig. 12 shows the evolution of the continuum generation versus distance in the silica bridge function of the length of the fiber (4 m in total). We see a typical Raman cascade generation over the propagation in one bridge until the sixth order. The result of the simulation in Fig. 12 supports well the experimental spectra in Fig. 8 as a Raman cascade is generated through the bridge in the simulation and the spectral broadening is also reached. However, as it is a simulation in only one bridge, we cannot go further in the interpretation of losses, peak power or number of bridges involved in the experimental results.
5. Conclusion
We have designed and manufactured a new photonic crystal fiber made of UV-grade silica glass for broadband light generation in the UV range. The fiber has been drawn from F110 UV-Grade silica glass for its low loss and solarization in the UV-A band. We have observed intermodal FWM and cascaded Raman scattering in the UV-Grade silica multimode PCF, which is enabled by the modal properties of the fiber core and the tight confinement in the fiber bridges, respectively. These results are highly promising in the search of new optical fibres, which feature both a high nonlinearity and UV-resistance thus opening the way toward supercontinuum generation in the strong normal dispersion regime, which does not rely on soliton dynamics and dispersive wave generation. However, further research is needed to improve the fabrication methods with this material to reduce absorption in the UV and to improve design parameters such as a smaller core size, which would lead to improved optical confinement.
Funding
European Union’s H2020 Research and Innovation Programme (722380); Agence Nationale de la Recherche (ANR-17EURE-0002, ANR-15-IDEX-0003); Conseil régional de Bourgogne-Franche-Comté.
Acknowledgments
The authors thank L Gauthier-Manuel, G. Soro, A. Mosset, F. Devaux, E. Dordor, M. Raschetti, J. Chretien and C. Billet for technical support.
Disclosures
Disclosures: L.P., T.T., A.M., D.L. (Photonics Bretagne, E). All other authors (S.P., C.P., A.N.G., G.F., C.F.K., J.M.D., T.S.) declare no conflict of interest.
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