Novel photonic crystal fibre for low-noise coherent supercontinuum generation

A double-clad AsSe2-based photonic crystal fiber possessing ultra-flat near-zero dispersion has been introduced, here to achieve flat-top and coherent supercontinuum generation at Mid-IR range. Also, the required conditions to obtain flat-top, broadband, and coherent supercontinuum generation have been discussed based on the systematic study carried out here, by GNLSE regarding the input pump pulse characteristics and the dispersion regime. The proposed photonic crystal fiber in this study, presents nearly-zero all-normal dispersion of about D ∼ −3.4 ps(nm.km)−1 corresponding to minimum group velocity dispersion at 6.9 μm. For the pump pulse with λ = 6.9 μm, time duration of T = 50 fs, and low peak power of P = 1 kW, a coherent flat-top supercontinuum generation has been realized with the span of 4.14 μm and 4.97 μm at 8 dB and 20 dB levels, respectively. Moreover, a figure of merit covering the essential characteristics of supercontinuum generation spectra (bandwidth, coherency, and flatness) has been introduced to compare the performance of different structures. It has been shown that β 2 tailoring with near-zero and flat characteristic is essential to achieve higher figure of merit.

Dispersion-flattened dispersion-decreased all-normal dispersion (DFDD-ANDi) photonic crystal fibers have been identified as promising candidates for high-spectral-power coherent supercontinuum (SC) generation. However, the effects of the unintentional birefringence of the fibers on the SC generation have been ignored. This birefringence is widely present in nonlinear non-polarization maintaining fibers with a typical core size of 2 μm, presumably due to the structural symmetry breaks introduced in the fiber drawing process. We find that an intrinsic form-birefringence on the order of 10−5 profoundly affects the SC generation in a DFDD-ANDi photonic crystal fiber. Conventional simulations based on the scalar generalized nonlinear Schrödinger equation (GNLSE) fail to reproduce the prominent observed features of the SC generation in a short piece (9-cm) of this fiber. However, these features can be qualitatively or semi-quantitatively understood by the coupled GNLSE that takes into account the form-birefringence. The nonlinear polarization effects induced by the birefringence significantly distort the otherwise simple spectrotemporal field of the SC pulses. We therefore propose the fabrication of polarization-maintaining DFDD-ANDi fibers to avoid these adverse effects in pursuing a practical coherent fiber SC laser.

The degree of coherence of the supercontinuum(SC) directly determine the resolution and the precision in many optical apparatus, so how to achieve highly coherent SC is one of the focuses in nonlinear optics. It is shown that modulation instability (MI) is the key element that influences the coherence of SC. Therefore an effective way of achieving coherent SC is to avoid MI and to use other nonlinear effects such as self-phase modulation (SPM). We design a kind of photonic crystal fiber (PCF) which has an all-normal group velocity dispersion (GVD) profile. After numerical investigation on the generation of SC in the PCF we obtain the degrees of the SC in different lengths. Results indicate that for the generated SC by pumping 50 cm of this PCF with TFWHM=400 fs unchirped pulse Gaussian pulse can realize high coherence in the entire broadening area on condition that relative power is larger than -80 dB.

Ultra broadband supercontinuum pulses are commonly used as a source of different wavelengths from a wide spectral bandwidth or as a source of very short pulses. However the processes responsible for wide spectral broadening are still under investigation. In this paper we examine the temporal and spectral characteristics of the pulses broadened upon propagation in the highly nonlinear photonics crystal fibers with different dispersion profiles. Generated supercontinuum pulses were experimentally characterized using cross-correlation frequency resolved optical gating technique. Full Text: PDF ReferencesM. Bradler, P. Baum, and E. Riedle, "Femtosecond continuum generation in bulk laser host materials with sub-?J pump pulses", Appl. Phys. B 97, 561 (2009). CrossRef T. M. Kardas, B. Ratajska-Gadomska, W. Gadomski, A. Lapini, and R. Righini, "The role of stimulated Raman scattering in supercontinuum generation in bulk diamond", Opt. Express 21, 24201 (2013). CrossRef A. Brodeur and S. L. Chin, "Band-Gap Dependence of the Ultrafast White-Light Continuum", Phys. Rev. Lett. 80, 4406 (1998). CrossRef R. R. Alfano, ed., The Supercontinuum Laser Source: Fundamentals with Updated References, 2nd ed (Springer, 2006). DirectLink A. L. Gaeta, Phys. "Catastrophic Collapse of Ultrashort Pulses", Rev. Lett. 84, 3582 (2000). CrossRef J. M. Dudley, G. Genty, and S. Coen, "Supercontinuum generation in photonic crystal fiber", Rev. Mod. Phys. 78, 1135 (2006). CrossRef M. Klimczak, B. Siwicki, P. Skibinski, D. Pysz, R. Stepien, A. Heidt, C. Radzewicz, and R. Buczynski, "Coherent supercontinuum generation up to 2.3 ?m in all-solid soft-glass photonic crystal fibers with flat all-normal dispersion", Opt. Express 22, 18824 (2014). CrossRef D. J. Kane and R. Trebino, "Characterization of arbitrary femtosecond pulses using frequency-resolved optical gating", IEEE J. Quantum Electron. 29, 571 (1993). CrossRef J. Dudley, X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O'Shea, R. Trebino, S. Coen, and R. Windeler, "Cross-correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments", Opt. Express 10, 1215 (2002). CrossRef N. Nishizawa and T. Goto, "Experimental analysis of ultrashort pulse propagation in optical fibers around zero-dispersion region using cross-correlation frequency resolved optical gating", Opt. Express 8, 328 (2001). CrossRef X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O'Shea, A. P. Shreenath, R. Trebino, and R. S. Windeler, "Frequency-resolved optical gating and single-shot spectral measurements reveal fine structure in microstructure-fiber continuum", Opt. Lett. 27, 1174 (2002). CrossRef S. Roy, S. K. Bhadra, and G. P. Agrawal, "Effects of higher-order dispersion on resonant dispersive waves emitted by solitons", Opt. Lett. 34, 2072?2074 (2009). CrossRef S. Bose, S. Roy, R. Chattopadhyay, M. Pal, and S. K. Bhadra, "Experimental and theoretical study of red-shifted solitonic resonant radiation in photonic crystal fibers and generation of radiation seeded Raman soliton", J. Opt. 17, 105506 (2015). CrossRef T. Roger, M. F. Saleh, S. Roy, F. Biancalana, C. Li, and D. Faccio, "High-energy, shock-front-assisted resonant radiation in the normal dispersion regime", Phys. Rev. A 88, (2013). CrossRef G. P. Agrawal, Nonlinear Fiber Optics, Fifth edition (Elsevier/Academic Press, 2013). DirectLink J. Szczepanek, T. Kardas, M. Nejbauer, C. Radzewicz, and Y. Stepanenko, "Simple all-PM-fiber laser system seeded by an all-normal-dispersion oscillator mode-locked with a nonlinear optical loop mirror", Proc. SPIE 9728, 972827 (2016). CrossRef C. Iaconis and I. A. Walmsley, "Self-referencing spectral interferometry for measuring ultrashort optical pulses", IEEE J. Quantum Electron. 35, 501 (1999). CrossRef L. E. Hooper, P. J. Mosley, A. C. Muir, W. J. Wadsworth, and J. C. Knight, "Coherent supercontinuum generation in photonic crystal fiber with all-normal group velocity dispersion", Opt. Express 19, 4902 (2011). CrossRef J. Szczepanek, T. M. Kardas, and Y. Stepanenko, "Sub-160-fs pulses dechriped to its Fourier transform limit generated from the all-normal dispersion fiber oscillator", Optical Society of America Frontiers in Optics conference, FTu3C?2 (2016). CrossRef G. Genty, M. Lehtonen, and H. Ludvigsen, "Effect of cross-phase modulation on supercontinuum generated in microstructured fibers with sub-30 fs pulses", Opt. Express 12, 4614 (2004). CrossRef S. Roy, S. K. Bhadra, K. Saitoh, M. Koshiba, and G. P. Agrawal, "Dynamics of Raman soliton during supercontinuum generation near the zero-dispersion wavelength of optical fibers", Opt. Express 19, 10443 (2011). CrossRef Y. Liu, Y. Zhao, J. Lyngso, S. You, W. L. Wilson, H. Tu, and S. A. Boppart, "Suppressing Short-Term Polarization Noise and Related Spectral Decoherence in All-Normal Dispersion Fiber Supercontinuum Generation", J. Light. Technol. 33, 1814 (2015). CrossRef

Coherent supercontinuum (SC) generation in optical fibers has spurred extensive research interest and found many applications. Here, we report a hybrid optical crystal fiber with an yttrium aluminum garnet (YAG) core. The optimized fiber features broad and flat near-zero normal dispersion and three wavelengths where the dispersion slope is zero. Through numerical modeling, we in detail investigate the influence of the pump wavelengths near the three wavelengths on the spectral broadening. The spectral broadening shows asymmetric towards the high-frequency region. The longer the pump wavelength, the more is the short wavelength components of the generated SC. Thus, the high-frequency components of the SC spectra can be engineered in the fiber. Interestingly, the occurrence of optical wave breaking (OWB) in the low-frequency region is prior to that in the high-frequency region when the pump wavelength is at 1660 nm. The broadest spectrum could be obtained when the pump wavelength is near the middle wavelength where the dispersion slope is zero. The result provides a new strategy for fiber design and broadband SC generation, which is different from the viewpoint that the broadest spectrum can be obtained when the pump wavelength near the local maximum dispersion. Combined with high nonlinearity and flat dispersion, the hybrid fiber allows octave-spanning SC generation with the pump power low to 1 kW at 1100 nm. This provides the potentials for high-power, broadband, and coherent SC generation at 1064 nm where the dispersion slope is near zero.

An all-normal dispersion photonic crystal fiber with nearly zero flattened dispersion at 1550 nm is designed for generating high coherent broadband supercontinuum. It is found that an all normal dispersion photonic crystal fiber with nearly zero flattened dispersion at 1550 nm can be obtained by appropriately designing geometrical parameters and optimizing the index of the first ring of air-holes with filling different index liquid of the photonic crystal fiber. The results show that the optimized design photonic crystal fiber for pumping at 1550 nm is suitable for flat broadband and high coherent supercontinuum generation with only 4 kW input peak power in a 40 cm long of the photonic crystal fiber. It is also found the weaker the dispersion effect is, the more advantage to the high coherent broadband supercontinuum generation due to the self-phase modulation.

We design and simulate an all-normal dispersion arsenic trisulfide (As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) photonic crystal fiber (PCF) with high nonlinearity to enable a flat and coherent orbital angular momentum (OAM) supercontinuum (SC) generation. The photonic crystal fiber features a near-zero and flat negative dispersion with variation between -96.5 and -36.5 ps/(nm·km) over a 940-nm wavelength range from 1740 to 2680 nm. A 1946-nm supercontinuum forms from 959 to 2905 nm at -20 dB level which covers a 1.6-octave bandwidth, by launching a 100-fs 5-kW chirp-free hyperbolic secant pulse with wavelength at 2000 nm into a 1.0-cm designed fiber. The generated supercontinuum of the other two vortex modes (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> ) can cover more than two octaves by optimizing the proposed fiber structures. The coherence of the generated supercontinuum of the three modes all shows nearly perfect property over the whole bandwidth. In general, we found that the designed ring-core As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> PCF with all-normal dispersion could be used for broadband coherent supercontinuum generation of various vortex modes.

The step-index heavily germanium-doped silica fibers with all normal dispersion (ANDi) are promising candidates for highly coherent supercontinuum (SC). These ANDi step-index silica fibers are easier to fabricate, splice and handle than silica photonic crystal fibers (PCFs). 40 % GeO2 doped step-index silica fiber of 3 &#x03BC;m core diameter has flat and near-zero dispersion which is between -5.5 and 0 ps/nm/km from 1.4 to 2.4 &#x03BC;m. Highly coherent SC spanning from 1050 nm to 2100 nm generated from the ANDi step-index heavily germanium-doped silica fiber pumped at 1560 nm. All-fiber coherent SC source from 1200 nm to 2200 nm is achieved by splicing the ANDi silica fiber with a 1560 nm femtosecond fiber laser of 90 % couple efficiency. Moreover, the step-index ANDi germania-core silica fiber of 2.2 &#x03BC;m core diameter is proposed to generate coherent mid-infrared SC from 1.7 to 3 &#x03BC;m. The ANDi step-index silica fibers not only can generate highly coherent broadband SC at near infrared or mid-infrared region but also can easily achieve all-fiber structure coherent SC source. And the experiment of supercontinuum generation in UNNA4 and UHNA7 fibers with different pump wavelengths indicates that the pump of 1064 nm is not suitable for coherent supercontinuum generation in the fiber of UNNA4 and UHNA7.

A microstructured optical fiber (MOF) structure with longitudinal dispersion variation is theoretically investigated for broad, coherent supercontinuum (SC) generation in the infrared (IR) region. The MOF possesses anomalous dispersion at 1550 nm but changes its dispersion to normal dispersion by a tapering process. The inline dispersion controlled structure provides a linear chirp and the subsequent pulse compression without additional components. The compressed pulse then seamlessly enters a short normal dispersion section for the coherent SC generation. Our simulation suggests that the inline structure can be as short as around 11.6 cm to realize the pulse compression and the SC generation. In particular, only 0.5-cm-long final normal dispersion section is required for the coherent SC generation, hence avoiding high background loss particularly in the spectral region above 2 $\mu\text{m}$ , where silica glass absorption becomes considerably high. In the structure, the peak power of a pulse is enhanced by 14 times in the compression process, and the corresponding generated SC spectrum has a bandwidth of 1260 nm, extending to wavelengths above 2 $\mu\text{m}$ . The simulation also shows that the generated spectrum is highly coherent. We experimentally demonstrated the dispersion shift from anomalous to normal dispersion with the microstructure fiber fabricated in-house.

In this paper, the supercontinuum (SC) generation in a carbon disulfide (CS2)-filled photonic crystal fiber (PCF) with strong slow nonlinearity is investigated. When the PCF is pumped at 1.55 μm in the anomalous dispersion region, we obtain highly coherent SC spanning from 0.99 to 2.32 μm, at -40 dB level. Moreover, the influences of the slow nonlinearity, the input pulse width, the pulse peak power, the fiber length, and the temperature on the supercontinuum generation (SCG) are studied. The role of the slow nonlinearity in enhancing the coherence of SC is proved. To our best knowledge, this is the first demonstration on generating the octave-spanning SC with high coherence using the slow nonlinearity of CS2. CS2 is a material that has high nonlinearity coefficient and well transparency in infrared. What’s more, the slow nonlinearity is very strong in this material.

Recently, the generation of coherent, octave-spanning, and recompressible supercontinuum (SC) light has been demonstrated in optical fibers with all-normal group velocity dispersion (GVD) behavior by femtosecond pumping. In the normal dispersion regime, soliton dynamics are suppressed and the SC generation process is mainly due to self-phase modulation and optical wave breaking. This makes such white light sources suitable for time-resolved applications. The broadest spectra can be obtained when the pump wavelength equals the wavelength of maximum all-normal GVD. Therefore each available pump wavelength requires a specifically designed optical fiber with suitable GVD to unfold its full power. We investigate the possibilities to shift the all-normal maximum dispersion wavelength in microstructured optical fibers from the near infra red (NIR) to the ultra violet (UV). In general, a submicron guiding fiber core surrounded by a holey region is required to overcome the material dispersion of silica. Photonic crystal fibers (PCFs) with a hexagonal array of holes as well as suspended core fibers are simulated for this purpose over a wide field of parameters. The PCFs are varied concerning their air hole diameter and pitch and the suspended core fibers are varied concerning the number of supporting walls and the wall width. We show that these two fiber types complement each other well in their possible wavelength regions for allnormal GVD. While the PCFs are suitable for obtaining a maximum all-normal GVD in the NIR, suspended core fibers are well applicable in the visible wavelength range.

Recently, the generation of coherent, octave-spanning, and recompressible supercontinuum (SC) light has been demonstrated in optical fibers with all-normal group velocity dispersion (GVD) behavior by femtosecond pumping. In the normal dispersion regime, soliton dynamics are suppressed and the SC generation process is mainly due to self-phase modulation and optical wave breaking. This makes such white light sources suitable for time-resolved applications. The broadest spectra can be obtained when the pump wavelength equals the wavelength of maximum all-normal GVD. Therefore each available pump wavelength requires a specifically designed optical fiber with suitable GVD to unfold its full power. We investigate the possibilities to shift the all-normal maximum dispersion wavelength in microstructured optical fibers from the near infra red (NIR) to the ultra violet (UV). In general, a submicron guiding fiber core surrounded by a holey region is required to overcome the material dispersion of silica. Photonic crystal fibers (PCFs) with a hexagonal array of holes as well as suspended core fibers are simulated for this purpose over a wide field of parameters. The PCFs are varied concerning their air hole diameter and pitch and the suspended core fibers are varied concerning the number of supporting walls and the wall width. We show that these two fiber types complement each other well in their possible wavelength regions for all-normal GVD. While the PCFs are suitable for obtaining a maximum all-normal GVD in the NIR, suspended core fibers are well applicable in the visible wavelength range.

A photonic crystal fibre with hollow core filled with toluene is considered as a new system for coherent supercontinuum generation. The dispersion characteristics are studied for various geometrical parameters of photonic crystal fibres. Two structures with lattice constant 2 μm, filling factors d/Λ 0.3 and 0.35 and toluene core of diameters of 3.34 and 3.23 μm have flat dispersion in the near infrared range. The structure with d/Λ = 0.3 has all-normal dispersion characteristics in whole near-infrared wavelength range, while the second structure (d/Λ = 0.35) has anomalous dispersion for wavelengths longer than 1.5 μm. Although confinement losses in the considered structures are as high as 0.4 dB cm−1, we show that the generation of coherent supercontinuum in the range 1.0–1.7 μm with the pulse energy conversion of 16% is feasible in 4 cm long fibre samples with standard fibre femtosecond lasers.

A rib waveguide structure in As2Se3 chalcogenide glass has been designed and numerically analyzed for on-chip coherent supercontinuum generation in the midinfrared region. The waveguide structure possesses an all-normal dispersion profile with dispersion value of -13.22 ps/nm·km at the pump wavelength. Coherent midinfrared supercontinuum spectrum spanning 1.2 to 7.2μm has been obtained using a 2.5mm long rib waveguide when pumped with 200fs laser pulses of a peak power of 2.5kW and a repetition rate of 1kHz at 2.8μm. Such highly nonlinear subwavelength size rib waveguide structures are highly applicable for the power efficient on-chip midinfrared coherent supercontinuum sources. Coherent midinfrared supercontinuum sources are very important in frequency metrology, nonlinear microscopy, nondestructive testing, molecular spectroscopy, and optical coherence tomography.

Numerical investigation of a broadband coherent supercontinuum generation in Ga8Sb32S60 chalcogenide photonic crystal fiber with all-normal dispersion