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Abstract
An all-normal dispersion supercontinuum generation in the range of 950 ÷ 1100 nm in photonic crystal fiber with core infiltrated with toluene is reported. The regular lattice hollow core photonic crystal fiber with a large core of 12 μm was designed and developed. The photonic crystal fiber core was selectively filled with toluene. The investigated fiber has normal dispersion in the wavelength range of 0.5 ÷ 2μm, while the absolute value of dispersion varies from 150 to 5 ps/nm/km in the range of 1 ÷ 2µm wavelength. The fiber nonlinear coefficient is 130 W−1km−1 as a result of the trade-off of high nonlinearity of toluene and a large mode area of the fundamental mode. The large mode area is required for the efficient delivery of high power pulses from large mode area input fiber in an all fiber system. As a pump source, a standard subpicosecond fiber laser emitting at 1030 nm with 10nJ pulse energy was used.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The supercontinuum light source has had a transformative influence on important applications in imaging and spectroscopy [1]. With the development of glass chemistry and arrival of mid-infrared (MIR) nonlinear fibers, the fiber-based supercontinuum begins to enable applications covered so far almost exclusively by synchrotron sources [2]. The most common approach in supercontinuum generation in optical fibers is launching intense pulses from nanosecond lasers into photonic crystal fibers with engineered dispersion profile to match its zero dispersion wavelength to the pump laser wavelength. This results in generation of supercontinuum light with spectrum covering even multiple octaves, albeit at the cost of time coherence. An alternative approach assumes the use of engineered all-normal dispersion (ANDi) fibers and pumping with femtosecond lasers [3]. In such a case octave spanning spectrum can be obtained with light pulses of high time coherence and a preserved temporal profile. This opens up attractive application areas in synthesis of few-to-single cycle pulses by coherent seeding of optical-fiber based amplifiers [4,5]. Because of this, solutions are required at either the pump laser level or the nonlinear medium design level.
The current research in SG is revolves around the further increase of spectral coverage including mid-infrared and UV wavelengths, as well as on enhancement of time stability of the supercontinuum pulses [6].Very important are also practical issues related to decreasing of cost and improvement of robustness of supercontinuum sources in which the main cost driver remains the pump source.
The common method is development of supercontinuum sources based on photonic crystal fibers (PCFs) made of silica or highly nonlinear glasses. Silica fibers can be effectively used for coherent supercontinuum generation in the visible and near-infrared (NIR) wavelength range, while chalcogenide fibers have been reported to enable pulse preserving supercontinuum in the mid-infrared, but this usually requires complex pump systems, capable of delivering sub-100 fs pump pulses, in the latter case also at an exotic pump wavelength [7,8]. Optical fibers based on soft glasses can support SC generation in across visible and mid infrared range 0.4 μm ÷ 5 μm as Z. X. Jia et.al. reported for fluorotellurite fibers [9], or even in the range of 1.4 μm to 13.3 μm obtained in a PCF made of chalcogenide glass [10], however again complex pump schemes are required because hyperspectral supercontinuum generation in chalcogenide fiber with a zero dispersion wavelength in the 6-7 µm wavelength range, requires a matching pump laser wavelength – and only very sophisticated optical parametric oscillator or amplifier systems cover this part of spectrum with ultrashort laser pulses.
Another method for SG uses gas filled PCFs. In this case hollow-core PCFs are infiltrated with gas or plasma. Travers et.al. used kagomé PCFs infiltrated neon or argon at high pressure for SG in the visible range [11]. Other gas filled core fibers were used for SG in ultraviolet or NIR wavelength region with pump sources with pulse energies of microjoules and above [12,13]. The main advantage of this method is related to tuneability obtained by means of temperature and pressure change. On the other hand the spectral width of supercontinuum is limited by the range of the photonic bangap, since low-index core guiding mechanism is used in this case. Also, a practical issue related to high bending losses and gas core protection against moisture and dust are still a challenge [14].
An alternative solution considered in recent years is use a hollow core PCFs and capillaries infiltrated with highly nonlinear liquids. Several considered organic liquids have nonlinear refractive index as high as soft-glasses [15]. Hence, nonlinear coefficient of liquid-core optical fiber can be comparable with the ones of solid-core fibers from highly nonlinear glass. The typical approach used for development of liquid-core fibers is based on silica capillary or hollow-core CPFs and further filling liquids into the air core. So the issues related to drawing fibers based on exotic, toxic and expensive soft glasses as well as thermal matching between core and cladding glasses are omitted. Highly nonlinear organic liquids usually have refractive index higher than silica, therefore index guiding mechanism can be exploited [16].
A number of experiments demonstrating SG in microcapilary fibers with a liquid core were reported. For example, step-index liquid-core optical fibers (LiCOF) filled carbon disulfide (CS2) enabling supercontinuum generation from NIR to MIR ware reported in [18-121]. Fanjoux et. al. reported supercontinuum generation in the visible range in a fiber with a toluene-filled core [22], however in this case a very high pulse energy of 200 nJ was used. Bozolan et. al. measured supercontinuum generation of the water-core PCF over wavelengths of 0.66 μm ÷ 1.14µm at normal dispersion [23]. Supercontinuum covering 0.5 ÷ 1.5 µm wavelengths in a water-filled core fiber with anomalous dispersion has also been reported by Kedenburg et. al. [24]. An innovative method of selective infiltration of a PCF microstructure of nonlinear liquids has been reported by Vieweg et al., which further enabled to record interesting NIR soliton dynamics and spectral broadening in carbon tetrachloride (CCl4) infiltrated solid-core PCF structureas and liquid-core PCF (LC PCF) [25] with high input power (330mW). Infiltration of fiber core with CS2 enabled to generate supercontinuum over the range of 1.2 ÷ 3 μm, where the dispersion profile had two zero dispersion points and soliton dynamics played an important role in the dynamics [21]. An overview of published experimental results on supercontinuum generation in fibers filled with different liquids is given in Table 1.
Table 1. State-of-the-art of experimental results on liquid core optical fiber supercontinuum generation.
A large number of experimental results of in liquid core fibers SG is focused on use of step-index liquid-core optical fibers, because it is straightforward and convenient to fill microcapillaries with liquids, as an equivalent of a step-index optical fiber. This approach suffers from a limitation in dispersion engineering, because of the lack of flexibility in designing of the fibers. Use of hollow core PCFs, in which only the core is selectively filled with a nonlinear liquid, allows enhanced dispersion modification [26]. It has been shown, that mode field manipulation at the nanoscale level can also yield normal dispersion characteristics, but that particular case involved technologically challenging fluoride glass fibers [27]. In the case of selective infiltration of the fiber air-core, it is possible to control and optimize the dispersion profile, the effective mode area and mode properties, by independently changing the photonic cladding parameters, i.e. the size of air holes and lattice pitch. Several methods have been proposed for selective introducing of liquids to specific air holes in a PCF [25,28–30]. The photopolymerization method based on the two-photon direct-laser writing technique was successfully used to close individual air-holes [25]. The drawback of this method is the requirement of high stability of the setup, precision of the air hole addressing and considerable work load, due to the sequential nature of the procedure. The UV-adhesive method, which uses the high pressure pump system to push the glue into all air-holes of the fiber, was reported in [28]. The drawback of this method is the need for precision control of length of infiltration of air-holes with the glue. Also milling of microchannels with focused ion beam was applied to fill liquids into desired air-holes of solid core PCFs which then is spliced with single mode fiber [29]. The thermal collapse of holes in the PCF cladding can be performed with a fusion splicer [30]. This method uses the electric current-driven discharge to increase locally the temperature of glass in the fiber and to collapse air-holes. This method does not allow to collapse individually selected holes. However, in the case of collapsing whole air-holes and only leaving the air core open, the splicer method is quite simple to implement with just controlling of the splicing arc parameters. For this reason, the splicer method was chosen to selective introduction toluene into core in this work.
Development of fibers containing highly nonlinear liquid in the photonic lattice, including fibers with selectively filled air core, cannot neglect toxicity of certain nonlinear liquids. In particular, organic solvents with high nonlinear refractive index are usually toxic. Their level of toxicity is typically described by the toxic index which was discussed in [31,32]. Only toluene has the unique set of properties including relatively low toxicity and high nonlinear refractive index. The nonlinear refractive index of toluene is n2 = 16.10−19 m2/W at the wavelength of 1064 nm [15]. It is 60 times higher compared to silica (n2 = 2.74·10−20 m2/W) [33], similar to nonlinearity of benzene and nitrobenzene and only above 3 time lower than carbon disulphide (CS2). However, CS2 and other high nonlinear liquids are more toxic than toluene [31,32]. Moreover linear refractive index of toluene is higher than silica, therefore a silica PCF with toluene core can support index guiding. Toluene offers relatively high transmission in the visible and NIR range. Therefore short length fibers with toluene can be considered for SG with femtosecond and subpicosecond pump lasers [22,26].
In this work we report for the first time an all-normal dispersion supercontinuum generation in a PCF with hollow core infiltrated with toluene. The core of the fiber was selectively infiltrated with solvent using a pump system. Air holes in the cladding were collapsed using a fiber fusion splicer. Dispersion properties of the tested fiber were simulated and measured in a Mach-Zehnder interferometer setup combined with a pump system to ensure solvent presence in the core. Next, using generalized nonlinear Schrödinger equation (GNLSE), SG was investigated theoretically and verified experimentally with fiber subpicosecond pulses for a range of pump pulse energies. Since toluene evaporates rapidly in an open core fiber, continuous toluene pumping was applied to maintain solvent presence in the core throughout the experiments.
2. Design of a toluene core photonic crystal fiber
We consider a silica PCF with regular hexagonal lattice and large, hollow core filled with toluene. We aim to use large core to ensure high coupling efficiency for butt coupling an all-fiber pump laser. Short pulse all-fiber lasers use large mode area fibers to deliver the pulse without nonlinear distortion. We modelled dispersion properties of PCFs with a liquid core diameter in the range of 2 – 16 µm and for different parameters of the photonic cladding. In the simulations we took into account the measured transmission of toluene and material dispersion of toluene and silica, as presented in Fig. 1.
Fig. 1 (a) real part of refractive index of silica and toluene, (b) transmission of toluene in 10mm sample.
Chromatic dispersion characteristics, calculated for different considered fiber structures are shown in Fig. 2. The dispersion profiles rise with increase of the relative air hole size of the photonic lattice (f = d/Λ). In the case of Λ smaller than 3 µm, the shifting of dispersion is clearly noticeable for varying relative air hole size. However, in the case of lager Λ, relative air hole size influences the dispersion characteristics only moderately. Normal dispersion in the whole wavelength range under consideration from 0.8 to 2 µm can be obtained for fibers with a large core, as shown in Fig. 2(f). However, in this case the dispersion is not flat and is highly sloped from −300 to around 0 ps/nm/km over 0.8 µm - 2 µm wavelengths. Reducing the core diameter narrows the range of normal dispersion, but also reduces the absolute value of dispersion.
Fig. 2 Dispersion properties of PCF with toluene core for various lattice constants Λ = 1µm (a), Λ = 2µm (b), Λ = 3µm (c), Λ = 4µm (d), Λ = 5µm (e), Λ = 6µm (f), Λ = 7µm (g), Λ = 8µm (h) and various filling factor f (f = d/Λ). Size of toluene core is equal to 2Λ.
3. Development of photonic crystal fiber with a core infiltrated with toluene
A hollow core PCF was developed using the standard stack-and-draw method [33]. The fiber with outer diameter of 103 μm had structure comprised of five rings of air holes arranged in a hexagonal lattice with lattice constant Λ = 6.2 μm, relative air-hole size d/Λ = 0.88 and diameter of the core Dcore = 12 μm (Fig. 3(a)).
Fig. 3 SEM image of cross-section of the fabricated silica PCF (a), end faced (b) and cross-section (c) of the PCF after thermal collapse of air holes in the photonic cladding.
To collapse air holes in the cladding of developed PCF we applied thermal method with use of a 3SAE fusion fiber splicer [30]. The particular model used (LDS) is equipped with three electrodes spaced around the fiber, to ensure uniform distribution of the heat in the processed sample. During the heating the fiber was fixed at both ends in order to immobilise it in the heating zone. The fiber is heated in the middle using single short electrical pulse, in order to collapse selectively only holes in the photonic cladding and leave the hollow core open. The desired result – collapsed air holes and open core – is achieved when fusion time is set to 1.5 seconds while applied voltage of electric arc is similar to the one typically used for fusion splicing of standard single mode fibers, Fig. 3(b). The thermal collapse process reduces also the diameter of the air core. The diameter of the core after the heating process was reduced more than 3 times down to 3.5 µm, and the length of the collapsed section is 140 µm, as shown in Fig. 3(c). A 12 cm long sample of the fiber was processed in this way at both ends. Next, an ultrasonic knife fiber cleaver was used to cleave the fiber with collapsed cladding at both ends.
The toluene infiltration process of the developed fiber with collapsed cladding air holes was based on capillary forces [34]. We placed horizontally the end of the fiber into a droplet of toluene. For 120 mm length of the sample, the infiltration time of toluene was 5 minutes. A setup shown in Fig. 4 was used to observe the process. A commercial supercontinuum source was used to illuminate the fiber from the side, through a 20 × microscope objective L1. The output face of the fiber was imaged on a CCD camera with the 60 × microscope objective L2. Changes in brightness of fiber core indicated degree of the core infiltration, Fig. 5.
Fig. 5 Image of output of the test fiber illuminated with scattered light when hollow core is filled by air (a), when hollow core is fully filled with toluene (b).
We concluded that toluene fully infiltrated the core of the tested fiber, when the core was brighter than photonic cladding, because part of the light focused on the sample was guided in the high index core, as shown in Fig. 5(b).
4. Characterization of the toluene core photonic crystal fiber
Chromatic dispersion D can be expressed as the second order derivative of the effective refractive index neff
where Re(neff) is the real part of the complex effective refractive index of the guided mode.Dispersion of the developed fiber has been calculated with the use of the finite difference method (FDM) and then verified experimentally in standard Mach-Zehnder interferometer setup [35] as shown in Fig. 6. Because of liquid evaporation and leakage at the open end of the fiber, toluene was constantly pumped into the core of the fiber with use of the reservoir of toluene, connected to a pump system in which the collapsed end was immersed. The pump system enabled maintaining a pressure of ΔP = 50 mPa to push toluene though the fiber core to ensure its full infiltration during the measurement. As the light source we used a supercontinuum laser (Koheras SuperK), emitting in the range of 450–2400 nm with an average output power of 105 mW. Input beam is collimated with lens MO1 and divided with the beam splitter BS1 into signal and reference beams. Two lenses, MO2 and MO3 are used in signal arm to couple light into the fiber and collimate the output beam, respectively. Input face of the fiber is placed in a custom made toluene reservoir equipped with a glass window which allows coupling of light into the fiber with lens MO2. The reference arm contains variable neutral density filter to adapt beam intensity, a mirror M2 to redirect light beam and a set of two mirrors M3, M3 placed on a translation stage to balance length of the reference arm with respect to signal arm of the interferometer. A set of two spectrometers with spectral ranges of 500–1100 nm and 900–1700 nm were used as detectors. All the components such as lenses and variable intensity filters were characterized and their influence on the measured dispersion characteristics of the fiber has been accounted for.
Fig. 6 Mach-Zehnder interferometer setup for dispersion measurement of toluene core PCF. The pump system is applied to keep to ensure fiber core filling with toluene during measurements.
Experimental results and calculated dispersion characteristics are well matched as shown in Fig. 7. The tested fiber has an normal dispersion profile in the considered spectral range of λ = 0.5 ÷ 2 µm. The absolute value of dispersion varies between −150 and −5 ps/nm/km in wavelength range of 1 ÷ 2µm.
An effective mode area Aeff of the tested fiber was calculated based on a SEM image. Since the core of the fiber is large, 12 μm of the diameter, the effective mode area is high and equals 73.2 μm2 at the pump wavelength of 1030 nm. Light is well confined in the toluene core, therefore variation of effective mode area is limited, as shown in Fig. 8(a). Based on the effective mode area, we have calculated the nonlinear coefficient defined as:
where Aeff denotes effective mode area of the fundamental mode and n2 – nonlinear refractive index of toluene.Fig. 8 Calculated effective mode area, nonlinear refractive index for developed toluene core PCF (a) and measured losses of tested fiber (b).
The nonlinear coefficient of the PCF is relatively small if we compare it with previously reported various highly nonlinear liquid core or soft glass fibers [9,10,26]. However if we take into account rather large mode area of the fiber, we can consider it as relatively high. The obtained value is a result of a trade-off between large mode area and high nonlinearity of toluene. Large mode area of the consider nonlinear fiber is highly demanded if we consider butt coupling or fusion splicing with pump fiber to deliver femtosecond pulses. Only matching of effective mode area between fibers ensures effective energy coupling into the liquid core fiber. Use of highly nonlinear fibers with high nonlinear coefficient and small effective mode area is not effective, because of high coupling losses in the considered all-fiber system.
Attenuation of the considered toluene core fiber is determined by the material loss of toluene and the waveguide confinement loss. Since the core is large and mode is well confined in the core, attenuation of the fiber mainly determined by the material loss of toluene. Figure 1(b) and Fig. 8(b) clearly present a correlation between transmittance of toluene and losses of the fiber. The low-transmittance around 1.14 µm and 1.4 µm wavelengths in toluene result in peak loss in the fiber. The obtained characteristics show that losses in the visible and NIR are relatively high up to 5.8 dB/cm and transmission is limited to 1.6 μm, due to high absorption further into the infrared range. Therefore for supercontinuum generation only few centimeter long samples of the fiber can be considered.
5. Supercontinuum generation in toluene core photonic crystal fiber
Supercontinuum generation of tested fiber was first simulated by numerically solving of the generalized nonlinear Schrödinger equation (GNLSE), based on the split-step Fourier method [36].
Here Ã(ω) is the frequency domain envelope of the input pulse A(t). The influence of dispersion characteristics β(ω) on the nonlinear properties are described by multiplication of the complex spectral envelope Ã(ω) and. Frequency domain of attenuation coefficient ᾶ(ω) is the sum of confinement loss and material loss, and Aeff(ω) is the frequency-dependent effective mode area. These parameters were obtained in calculations of the linear properties of the fiber and are shown in Fig. 7 and Fig. 8.
The response function R(t’) depends on the contribution mechanism e.g. molecular reorientation, molecular interactions, collision-induced polarizability, electronic response, etc. It can be expressed as [37]:
Here, the subscripts el, d, l, c indicate the bound-electronic, molecular reorientation, molecular interaction and collision-induced, respectively, while the coefficients C2l,2d,2c are normalized constant and N = (n2el + n2c + n2d + n2l). The term g(ω) stands for the distribution function of vibrational motion expressed as [37]:
where ω0 and σ are are the center frequency and bandwidth, respectively.In the simulations we assumed pulse parameters of fiber laser used further in the experiments with generation of the supercontinuum. The initial pulse parameters are as follows: 400 fs pulse duration,1030 nm central pumped wavelength, a Gaussian shape of the pulse and varying input energy pulse in the range 1 nJ ÷ 10 nJ, this corresponds to peak power of 2.5 kW ÷ 25 kW. In the response function R(t’) we assumed that n2el = 0.6∙10−19 (m2/W), n2d = 3∙10−19 (m2/W), τr,d = 0.25(ps), τf,d = 2.1(ps), n2l = 1.2∙10−19 (m2/W), n2c = 0.12∙10−19 (m2/W), τr,c = 0.25(ps), τf,d = 0.2(ps), ω0 = 11 (ps−1), σ = 8 (ps−1) [38,39].
The evolution of the broadened spectra as a function of input pulse energy is shown in Fig. 9a. Up to around 8 nJ of input energy, self-phase modulation is mainly contributing to the development of spectrum. Optical wave breaking (OWB) begins at input pulse energy higher than 8 nJ, this can be observed at the blue-shifted (trailing) edge of the pulse and at the redshifted (leading) edge of the pulse [41].
Fig. 9 Modeling of spectrum evolution along pulse energy with 10cm length of the fiber (a), supercontinuum spectrum estimated for 10 cm length of the fiber when pumped with pulses of 400 fs duration, 1030 nm pump wavelength and various pulse energy (b) and coherence degree obtained from 20 individual pairs of pulses with random initial noise seed.
The normal dispersion characteristics of the fiber across all wavelengths covered by the supercontinuum, together with pumping with sub-picosecond pulses are the prerequisites for a highly coherent SC [42]. The coherence property of the investigated supercontinuum has been evaluated numerically with the use of degree of first order coherence expressed as [43]:
The coherence degree was calculated with 20 individual pairs with different random noise seed (single photon per mode noise model was used in the simulations).The obtained value of first degree of coherence was equal to 1 across the spectrum, as shown in Fig. 9(b).
Figure 10 shows the evolution of spectrum along the fiber sample – and in the time domain for three selected points along the fiber length (numerically generated group delay traces) – for the case of 10 nJ input pulse energy, which corresponded to 25kW of peak power. At propagation lengths shorter than 6 cm, the spectral broadening is due to self-phase modulation. Optical wave breaking occurs first on the trailing pulse edge at 6 cm of propagation and generates a new wavelength band around 950 nm. On the leading edge, OWB occurs only after 7 cm with a new wavelength band generated at around 1100 nm. The spectral and temporal intensity profiles are presented in Fig. 9(b) and Fig. 10. Strong slope of normal dispersion limits the spectrum at the short wavelength edge. However, with increasing input energy, spectral broadening can continue at the red-shifted wavelengths (leading edge) due to favourably flat dispersion.
Fig. 10 Modeling of spectrum evolution along length of the fiber, supercontinuum spectrum estimated for 10 nJ input pulse energy, 400 fs duration, 1030 nm pump wavelength.
To verify experimentally SG in developed fiber we used a simple setup consisting of a laser source, toluene pump system and spectrometer, as shown in Fig. 11. The 1030 nm sub-picosecond fiber laser delivering 400 fs pulses, was used as a source (Menlo, BlueCut). The pump beam was coupled with MO1 lens into core of tested fiber. Input face of the fiber was placed in custom made toluene reservoir. The output beam is collected with lens MO2 and directed into spectrometer. The length of the fiber structure used in the experiment was 10 cm. Measured numerical aperture of the fiber was 0.08. The coupling efficiency into the fiber was estimated to be 16%. This relatively low figure is due to the fact, that to avoid pumping the fiber at the open end, where there would be some toluene missing, the fiber was pumped from the end immersed in the toluene reservoir. The relatively long distance between the in-coupling point and the fiber front facet, imposed by the toluene reservoir, necessitated use of a 10 × lens. This in turn resulted in the beam diameter of around 16 µm, thus much larger, than the slightly collapsed core, which further limited the coupling efficiency. We note, that the use of the lens enabled selective excitation of the fundamental mode in the fiber structure, which was confirmed by monitoring of the fiber output intensity profile. Because of this, in the next section we compared the experimental results with numerical simulations of nonlinear propagation using a scalar (single mode) model.
The experimental results of SG are presented in Fig. 12. The broadening occurs almost symmetrically around the pump wavelength and covers a wavelength range of 950nm ÷ 1100nm for the 10 nJ incident pulse energy. Experimental and numerical results are in very good agreement both in spectral width and shape of the spectrum. Symmetrical shape of spectrum suggests, that the broadening is contained within wavelengths of relatively flat part of the dispersion profile of the fiber structure. Further broadening was pump power-limited, in relation to the thermal properties of the proposed fiber setup. 10 nJ was the maximum incident pulse energy, and higher energy used in the experiment resulted in burning of toluene.
Fig. 12 Experimental results of supercontinuum generation obtained for 10 cm length of the fiber under pumping with pulses of 400 fs duration, and 1030 nm pump wavelength and 4nJ, 10 nJ pulse energy, compared with numerically obtained results assuming similar pump conditions.
The observed pump power dependent development of supercontinuum spectra clearly indicates self-phase modulation and optical wave breaking as the dominant contributors. There is also reasonable agreement between the nonlinear propagation simulations and the experimental data, as shown in Fig. 12. This – in relation to the experimentally measured dispersion profile – enables to note, that the investigated liquid-filled fiber structure has the potential for coherent, pulse preserving supercontinuum generation, when femtosecond laser pumping is applied. The recent reports by two independent teams on coherence degradation in ANDi fibers point out, that Raman scattering and polarization induced beating can have a detrimental impact on the noise properties in these systems [40,41]. In particular it has been shown that, lowering absolute value of dispersion would contribute to limitation of Raman gain. This, along with any residual birefringence (not investigated in this work) could be improved for the fiber structure discussed herein. Therefore, a detailed analysis of coherence properties, i.e. using Michelson interferometry, e.g. for a range of different liquid-filled structures to investigate the relation between coherence and the dispersion, as well as birefringence induced effects, should be considered the next steps for characterization of the type of fiber structures demonstrated in this report – before final conclusions of coherence and noise performance are drawn.
6. Conclusion
We reported for the first time on a normal dispersion supercontinuum generation in a PCF with hollow core infiltrated with toluene The broadening spectrum from 950 nm to 1100 nm was achieved with 400 fs long input pulse with energy of 10 nJ at 1030 nm central wavelength. It is to be noted that this is a pump pulse duration well above the record, octave spanning all-normal dispersion supercontinuum generation reported for 100 fs or shorter pump pulses [3]. We used an advanced fusion fiber splicer to selectively collapse air holes in the cladding of photonic crystal fiber, while leaving out the open air core intact. Capillary forces were used to fill the hollow core with toluene. The pump system was combined with a typical Mach-Zehnder interferometer for dispersion measurement of toluene core PCF. The tested fiber has an all-normal dispersion profile with small absolute value of −150 ÷ −5 ps/nm/km in wavelength range 1 ÷ 2µm. Main reason for moderate spectral width of supercontinuum we have obtained is relatively low nonlinear coefficient related to assumed large more area and high attenuation of toluene. High nonlinear refractive index of toluene cannot compensate obstacles related to its attenuation and requirements of an all-fiber systems. Increase of nonlinear coefficient or peak power is obligatory for broadband supercontinuum generation in fibers with toluene core fibers.
The presented results show, that liquid core fibers can be an interesting approach for robust compact, all fiber systems for SG with all-fiber low power femtosecond pump systems. In case of toluene filled core, they can offer reasonable, but not record-high broadening, when pumped with low energy sub-picosecond pulses, typical for fiber femtosecond oscillators with simple 1 or 2 stage amplification systems. Use of low attenuation liquids as carbon tetrachloride are more promising for broadband supercontinuum all-fiber sources [17,28], albeit care has to be taken to assure enough propagation length (liquid infiltrated fiber length) to compensate for its lower nonlinearity, compared to toluene. Silica glass PCFs allow robust integration of liquid core fibers with large core area, step index pump and output fibers and bring the concept of low-cost all-fiber liquid core coherent supercontinuum to practical use.
Funding
Foundation for Polish Science Team Programme from the funds of European Regional Development Fund under Smart Growth Operational Programme (TEAM TECH/2016-1/1); National Science Centre (NCN) in Poland (OPUS UMO-2016/21/B/ST7/02249); National Foundation for Science and Technology Development (NAFOSTED) (103.03-2014.62).
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