Maximizing the bandwidth of supercontinuum generation in As2Se3 chalcogenide fibers

Ultra-broadband frequency shifting of laser pulses in a square multicore chalcogenide fiber

The process of Raman frequency shifting of out-of-phase laser pulses in fibers with a square configuration of weakly coupled cores having two or more zero dispersion wavelengths has been studied. The use of out-of-phase distributions in multicore fibers makes it possible to increase pulse energies by orders of magnitude in comparison with the case of single-core fibers. Conditions for the stability of out-of-phase laser pulses are determined and confirmed by numerical simulations. A configuration of chalcogenide multicore fiber with three zero dispersion wavelengths is proposed, allowing ultra-broadband frequency shifting of laser pulses up to 6.2 μm with an energy efficiency of more than 25%.

Study of low-peak-power highly coherent broadband supercontinuum generation through a dispersion-engineered Si-rich silicon nitride waveguide

Since the first observation by Alfano and Shapiro in the 1970s [Phys. Rev. Lett.24, 584 (1970)PRLTAO0031-900710.1103/PhysRevLett.24.584], supercontinuum generation study has become an attractive research area in the field of broadband light source design, including its use in various applications associated with nonlinear optics in recent years. In this work, the numerical demonstration of ultrabroadband supercontinuum generation in the mid-infrared (MIR) region via the use of complementary metal-oxide semiconductor compatible Si-rich silicon nitride as the core in a planar waveguide design employing one of two materials, either LiNbO₃ or MgF₂ glass, as the top and bottom claddings is explored. A rigorous numerical investigation of broadband source design in the MIR using 2 mm long Si-rich silicon nitride waveguides is carried out in terms of waveguide structural parameter variations, input peak power variation, varying unexpected deformation of the waveguide along the core region during fabrication, and spectral coherence analysis. Among the several waveguide models studied, two promising designs are identified for wideband supercontinuum generation up to the MIR using a relatively low input peak power of 50 W. Simulation results reveal that spectral coverage spanning from 0.8 µm to 4.6 µm can be obtained by using a LiNbO₃-cladded waveguide, and similar spectral coverage is also predicted for the other design, a MgF₂-cladded waveguide. To the best of our knowledge, this is the widest spectral span in the MIR region employing a Si-rich silicon nitride waveguide so far. In dispersion tuning as well as in supercontinuum generation, the effect of possible unexpected waveguide deformation along the transverse directions during fabrication is also studied. No significant amount of spectral change is observed in the proposed model for a maximum of 10° inside/outside variation along the width. On the other hand, even 1° upward/downward variation along the thickness could cause substantial spectral change at the waveguide output. Finally, the obtained output spectra from the proposed waveguides are found to be highly coherent and can be applied in various MIR region applications such as optical coherence tomography, spectroscopic measurement, and frequency metrology.

Multi-octave mid-infrared supercontinuum and frequency comb generation in a suspended As2Se3 ridge waveguide

In this paper, we numerically investigate the mid-infrared supercontinuum (SC) generation in a suspended ${{\rm As}_2}{{\rm Se}_3}$As₂Se₃ ridge waveguide, which is designed with the two zero-dispersion wavelengths. Simulation results show that when the pump pulses at wavelength 3.3 µm with width of 100 fs and peak power of 900 W are launched into the anomalous dispersion region of the designed waveguide with a length of 0.87 mm, the SC can be generated in the wavelength range from 1.76 to 14.42 µm (more than three octaves), extending deep into the "fingerprint" region. The stability of the generated SC is confirmed by the first-order coherence. Moreover, we demonstrate the performance of the SC-based frequency comb by assuming a 50 pulse pump source at a repetition rate of 100 MHz.

Simple method for estimating the fractional Raman contribution

We propose a novel and simple method for estimating the fractional Raman contribution, f_R, based on an analysis of a full model of modulation instability (MI) in waveguides. An analytical expression relating f_R to the MI peak gain beyond the cutoff power is explicitly derived, allowing for an accurate estimation of f_R from a single measurement of the Raman gain spectrum.

Tapered tellurite step-index optical fiber for coherent near-to-mid-IR supercontinuum generation: experiment and modeling

We demonstrate broadband highly coherent near-to-mid-IR supercontinuum generation using a short length of tapered tellurite step-index fiber pumped with an ultrafast laser in normal dispersion regime. The tapered tellurite fiber possesses all-normal dispersion characteristics within the whole range of the generated supercontinuum spectrum. A highly coherent near-to-mid-IR supercontinuum spectrum spanning 1.28 to 3.31 μm at a -40  dB intensity level is obtained using a 3.2 cm long tapered tellurite fiber when it is pumped with a 200 fs laser pulse with a peak power of 19.8 kW at 2 μm. To obtain the supercontinuum spectrum, we also carried out numerical modeling for the tapered tellurite step-index fiber with the same geometrical parameters and pump conditions used in the experiment. The numerical observation supports the experimentally obtained result. The findings of this work show that the fabricated tapered tellurite step-index fiber is a promising nonlinear medium to obtain a coherent near-to-mid-IR supercontinuum spectrum in a short length of the fiber.

Highly coherent supercontinuum in the mid-infrared region with cascaded tellurite and chalcogenide fibers

We numerically investigate two-step supercontinuum (SC) generation using cascaded tellurite and chalcogenide fibers with all-normal group velocity dispersion pumped by a femtosecond laser at 2 μm. The optimized tellurite fiber is a hybrid microstructured optical fiber with a core surrounded by 12 rods. It has flat normal chromatic dispersion from 2 to 5 μm. The chalcogenide fiber is a double-core fiber with flat normal chromatic dispersion from 4 to 10 μm. The output SC spectrum from the best candidate fibers spans from 0.78 to 8.3 μm with coherence of unity all over the spectrum. Such high coherence pulse with broad spectrum will be valuable for many applications in tomography, ultrafast transient absorption spectroscopy, etc. The proposed fiber structures are all-solid and are feasible for fabrication with the common rod-in-tube method. This implies that two-step SC is a potential way to obtain broad, highly coherent SC in the mid-infrared.

Supercontinuum generation at 1.55 μm in As2S3 core photonic crystal fiber

This paper proposes a design and mathematical study of As₂S₃ chalcogenide photonic crystal fiber (PCF) for broadband supercontinuum generation. The proposed design offers a large nonlinearity coefficient and ultra-flattened dispersion. The proposed design was analyzed using the full-vectorial finite element method. Through this method, it is shown that an ultra-broad supercontinuum spectrum of 0.8-4.5 μm is attained using an As₂S₃ core PCF design with 20 fs pump pulse width and a length of 10 mm, having 3 kW power at a -40  dB spectral and temporal intensity. The proposed octagonal PCF has shown a low zero dispersion wavelength at the pump wavelength of 1.55 μm.

Mid-infrared supercontinuum generation in multimode As2 Se3 chalcogenide photonic crystal fiber

We rigorously investigated the mid-infrared supercontinuum generation in a multimode As₂Se₃ chalcogenide photonic crystal fiber (PCF). We studied the impact of the intermodal nonlinear effects on the nonlinear propagation of the fundamental and high-order modes. By solving the multimode generalized nonlinear Schrödinger equation, we have predicted the generation of a very broadband supercontinuum in both polarizations of the fundamental mode spanning from 2 to 11 μm at -20  dB in only 5 cm PCF length. The proposed study confirms that the energy transfer occurs only between the optical degenerate modes when propagating in the multimode chalcogenide PCF in the mid-infrared region.

Reconfigurable opto-thermal graded-index waveguiding in bulk chalcogenide glasses

In the absence of suitable deposition processes, the fabrication of graded-index chalcogenide waveguides or fibers remains an outstanding challenge. Here, by exploiting the strong thermo-optic effect present in chalcogenide glasses, we experimentally demonstrate non-permanent optically-induced waveguides in bulk As₂Se₃ rods using a 1.55 μm wavelength laser. This single-step process can be used not only to self-trap the writing beam, but also to guide another optical beam at a different wavelength in the opto-thermally inscribed waveguide channel. These results could pave the way towards harnessing nonlinear effects in graded-index chalcogenide guided settings.

Nonlinear characterization of GeSbS chalcogenide glass waveguides

GeSbS ridge waveguides have recently been demonstrated as a promising mid - infrared platform for integrated waveguide - based chemical sensing and photodetection. To date, their nonlinear optical properties remain relatively unexplored. In this paper, we characterize the nonlinear optical properties of GeSbS glasses, and show negligible nonlinear losses at 1.55 μm. Using self - phase modulation experiments, we characterize a waveguide nonlinear parameter of 7 W-1/m and nonlinear refractive index of 3.71 × 10-18 m2/W. GeSbS waveguides are used to generate supercontinuum from 1280 nm to 2120 nm at the -30 dB level. The spectrum expands along the red shifted side of the spectrum faster than on the blue shifted side, facilitated by cascaded stimulated Raman scattering arising from the large Raman gain of chalcogenides. Fourier transform infrared spectroscopic measurements show that these glasses are optically transparent up to 25 μm, making them useful for short - wave to long - wave infrared applications in both linear and nonlinear optics.

Ultra broadband mid-IR supercontinuum generation in Ge11.5As24Se64.5 based chalcogenide graded-index photonic crystal fiber: design and analysis

In this paper, we report design and numerical analysis of a Ge_11.5As₂₄Se_64.5 based chalcogenide glass graded-index photonic crystal fiber structure for mid-IR ultra broadband supercontinuum generation. The proposed dispersion engineered photonic crystal fiber offers a zero dispersion wavelength at a pump wavelength of 2.8 μm. To simulate the supercontinuum generation spectrum, the orders of dispersion coefficient up to the ninth order are considered. Simulated results indicate that an ultra broadband supercontinuum spectrum spanning 1-16 μm has been achieved using a 10 mm long photonic crystal fiber structure pumped with 50 fs secant hyperbolic pulses of 3 kW at a -30  dB spectral intensity level. To the best of our knowledge, this is the first time such broad supercontinuum spectrum has been reported. This ultra broadband mid-IR supercontinuum spectrum is applicable in many diverse fields, including medical, defense, metrology, and spectroscopy.

Multimode supercontinuum generation in chalcogenide glass fibres

Mid-infrared supercontinuum generation is considered in chalcogenide fibres when taking into account both polarisations and the necessary higher order modes. In particular we focus on high pulse energy supercontinuum generation with long pump pulses. The modeling indicates that when only a single polarisation in the fundamental mode is considered the obtainable supercontinuum bandwidth is substantially exaggerated compared to when both polarisations are taken into account. Our modeling shows that if the pump pulse is short enough (≤ 10 ps) then higher order modes are not important because of temporal walk-off. In contrast long pump pulses (≥ 40 ps) will efficiently excite higher order modes through Raman scattering, which will deplete the fundamental mode of energy and limit the possibility of obtaining a broadband supercontinuum.

Highly efficient cascaded amplification using Pr(3+)-doped mid-infrared chalcogenide fiber amplifiers

We computationally investigate cascaded amplification in a three-level mid-infrared (IR) Pr(3+)-doped chalcogenide fiber amplifier. The overlap of the cross-sections in the transitions (3)H(6)→(3)H(5) and (3)H(5)→(3)H(4) enable both transitions to simultaneously amplify a single wavelength in the range between 4.25 μm and 4.55 μm. High gain and low noise are achieved simultaneously if the signal is at 4.5 μm. We show that 45% of pump power that is injected at 2 μm can be shifted to 4.5 μm. The efficiency of using a mid-IR fiber amplifier is higher than what can be achieved by using mid-IR supercontinuum generation, which has been estimated at 25%. This mid-IR fiber amplifier can be used in conjunction with quantum cascade lasers to obtain a tunable, high-power mid-IR source.

Simulation of octave spanning mid-infrared supercontinuum generation in dispersion-varying planar waveguides

A dispersion-varying tapered planar waveguide is designed to generate supercontinuum efficiently in the mid-infrared region. The rib waveguide of lead-silicate glass on silica is 1.8 cm long, consisting of a segment with longitudinally increasing etch depth. The mechanism involves nonlinear soliton dynamics. The dispersion profile is shifted along the propagation distance, leading to continuous modification of the phase-matching condition for dispersive wave (DW) emission and enhancement of energy transfer efficiency between solitons and DWs. With low input pulse energy of 45 pJ, simulation demonstrates the generation of both broadband and flat near-octave spectrum spanning 1.3-2.5 μm at the -20  dB level.

Towards ten-watt-level 3-5 µm Raman lasers using tellurite fiber

Raman lasers based on mid-infrared fibers operating at 3-5 µm atmospheric transparency window are attractive sources for several applications. Compared to fluoride and chalcogenide fibers, tellurite fibers are more advantageous for high power Raman fiber laser sources at 3-5 µm because of their broader Raman gain bandwidth, much larger Raman shift and better physical and chemical properties. Here we report on our simulations for the development of 10-watt-level 3-5 µm Raman lasers using tellurite fibers as the nonlinear gain medium and readily available continuous-wave (cw) and Q-switched erbium-doped fluoride fiber lasers at 2.8 µm as the pump sources. Our results show that a watt-level or even ten-watt-level fiber laser source in the 3-5 µm atmospheric transparency window can be achieved by utilizing the 1st- and 2nd-order Raman scattering in the tellurite fiber. The presented numerical study provides valuable guidance for future 3-5 um Raman fiber laser development.

Midinfrared supercontinuum generation via As2Se3 chalcogenide photonic crystal fibers

Using numerical analysis, we compare the results of optofluidic and rod filling techniques for the broadening of supercontinuum spectra generated by As2Se3 chalcogenide photonic crystal fibers (PCFs). The numerical results show that when air-holes constituting the innermost ring in a PCF made of As2Se3-based chalcogenide glass are filled with rods of As2Se3-based chalcogenide glass, over a wide range of mid-IR wavelengths, an ultra-flattened near-zero dispersion can be obtained, while the total loss is negligible and the PCF nonlinearity is very high. The simulations also show that when a 50 fs input optical pulse of 10 kW peak power and center wavelength of 4.6 μm is launched into a 50 mm long rod-filled chalcogenide PCF, a ripple-free spectral broadening as wide as 3.86 μm can be obtained.

Mid-infrared supercontinuum generation using dispersion-engineered Ge(11.5)As(24)Se(64.5) chalcogenide channel waveguide

We numerically investigate mid-infrared supercontinuum (SC) generation in dispersion-engineered, air-clad, Ge(11.5)As(24)Se(64.5) chalcogenide-glass channel waveguides employing two different materials, Ge(11.5)As(24)Se(64.5) or MgF(2) glass for their lower cladding. We study the effect of waveguide parameters on the bandwidth of the SC at the output of 1-cm-long waveguide. Our results show that output can vary over a wide range depending on its design and the pump wavelength employed. At the pump wavelength of 2 μm the SC never extended beyond 4.5 μm for any of our designs. However, supercontinuum could be extended to beyond 5 μm for a pump wavelength of 3.1 μm. A broadband SC spanning from 2 μm to 6 μm and extending over 1.5 octave could be generated with a moderate peak power of 500 W at a pump wavelength of 3.1 μm using an air-clad, all-chalcogenide, channel waveguide. We show that SC can be extended even further when MgF(2) glass is used for the lower cladding of chalcogenide waveguide. Our numerical simulations produced SC spectra covering the wavelength range 1.8-7.7 μm (> two octaves) by using this geometry. Both ranges exceed the broadest SC bandwidths reported so far. Moreover, we realize it using 3.1 μm pump source and relatively low peak power pulses. By employing the same pump source, we show that SC spectra can cover a wavelength range of 1.8-11 μm (> 2.5 octaves) in a channel waveguide employing MgF(2) glass for its lower cladding with a moderate peak power of 3000 W.

Dispersion engineered Ge₁₁.₅As₂₄ Se₆₄.₅ nanowire for supercontinuum generation: a parametric study

A promising design of Ge₁₁.₅As₂₄ Se₆₄.₅ nanowires for supercontinuum generation is proposed through numerical simulations. It can be used for generating a supercontinuum with 1300-nm bandwidth. The dispersion parameters upto eighth-order are obtained by calculating the effective mode index with the finite-element method. We have investigated dispersion curves for a number of nanowire geometries. Through dispersion engineering and by varying dimensions of the nanowires we have identified a promising structure that shows possibility of realizing a wideband supercontinuum. We have found significant variations in its bandwidth with the inclusion of higher-order dispersion coefficients and indicated the possibility of obtaining spurious results if the adequate number of dispersion coefficients is not considered. To confirm the accuracy of dispersion coefficients obtained through numerical computations, we have shown that a data-fitting procedure based on the Taylor series expansion provides good agreement with the actual group velocity dispersion curve obtained by using a full-vectorial finite-element mode-solver.

Two octaves mid-infrared supercontinuum generation in As₂Se₃ microwires

We report the first demonstration of mid-infrared supercontinuum generation in As₂Se₃ chalcogenide microwires with the added advantage of using low energy pulses. The generated SC covers two octaves of bandwidth from 1.1 μm to 4.4 μm at -30 dB. This exceeds the broadest reported SC bandwidth in As₂Se₃ microwires by a factor of 3.5. The microwire geometry and pumping conditions are the key parameters in generating the 3.3 μm bandwidth while using a low pump pulse energy of 500 pJ.

Calculation of the expected output spectrum for a mid-infrared supercontinuum source based on As ₂ S₃ chalcogenide photonic crystal fibers

We computationally investigate supercontinuum generation in an As ₂ S₃ solid core photonic crystal fiber (PCF) with a hexagonal cladding of air holes. With a goal of obtaining a supercontinuum output spectrum that can predict what might be seen in an experiment, we investigate the spectral and statistical behavior of a mid-infrared supercontinuum source using a large ensemble average of 10⁶ realizations, in which the input pulse duration and energy vary. The output spectrum is sensitive to small changes (0.1%) in these pulse parameters. We show that the spectrum can be divided into three regions with distinct characteristics: a short-wavelength region with high correlation, a middle-wavelength region with minimal correlation, and a long-wavelength region where the behavior is dominated by a few rare large-bandwidth events. We show that statistically significant fluctuations exist in the experimentally expected output spectrum and that we can reproduce an excellent match to that spectrum with a converged shape and bandwidth using 5000 realizations.

Mid-infrared supercontinuum generation to 12.5μm in large NA chalcogenide step-index fibres pumped at 4.5μm

We present numerical modeling of mid-infrared (MIR) supercontinuum generation (SCG) in dispersion-optimized chalcogenide (CHALC) step-index fibres (SIFs) with exceptionally high numerical aperture (NA) around one, pumped with mode-locked praseodymium-doped (Pr(3+)) chalcogenide fibre lasers. The 4.5um laser is assumed to have a repetition rate of 4MHz with 50ps long pulses having a peak power of 4.7kW. A thorough fibre design optimisation was conducted using measured material dispersion (As-Se/Ge-As-Se) and measured fibre loss obtained in fabricated fibre of the same materials. The loss was below 2.5dB/m in the 3.3-9.4μm region. Fibres with 8 and 10μm core diameters generated an SC out to 12.5 and 10.7μm in less than 2m of fibre when pumped with 0.75 and 1kW, respectively. Larger core fibres with 20μm core diameters for potential higher power handling generated an SC out to 10.6μm for the highest NA considered but required pumping at 4.7kW as well as up to 3m of fibre to compensate for the lower nonlinearities. The amount of power converted into the 8-10μm band was 7.5 and 8.8mW for the 8 and 10μm fibres, respectively. For the 20μm core fibres up to 46mW was converted.

Thulium pumped mid-infrared 0.9-9μm supercontinuum generation in concatenated fluoride and chalcogenide glass fibers

We theoretically demonstrate a novel approach for generating Mid-InfraRed SuperContinuum (MIR SC) by using concatenated fluoride and chalcogenide glass fibers pumped with a standard pulsed Thulium (Tm) laser (T(FWHM)=3.5ps, P0=20kW, ν(R)=30MHz, and P(avg)=2W). The fluoride fiber SC is generated in 10m of ZBLAN spanning the 0.9-4.1μm SC at the -30dB level. The ZBLAN fiber SC is then coupled into 10cm of As2Se3 chalcogenide Microstructured Optical Fiber (MOF) designed to have a zero-dispersion wavelength (λ(ZDW)) significantly below the 4.1μm InfraRed (IR) edge of the ZBLAN fiber SC, here 3.55μm. This allows the MIR solitons in the ZBLAN fiber SC to couple into anomalous dispersion in the chalcogenide fiber and further redshift out to the fiber loss edge at around 9μm. The final 0.9-9μm SC covers over 3 octaves in the MIR with around 15mW of power converted into the 6-9μm range.

Numerical investigation on high power mid-infrared supercontinuum fiber lasers pumped at 3 µm

High power mid-infrared (mid-IR) supercontinuum (SC) laser sources in the 3-12 µm region are of great interest for a variety of applications in many fields. Although various mid-IR SC laser sources have been proposed and investigated experimentally and theoretically in the past several years, power scaling of mid-IR SC lasers beyond 3 μm with infrared edges extending beyond 7 μm are still challenges because the wavelengths of most previously used pump sources are below 2 μm. These problems can be solved with the recent development of mode-locked fiber lasers at 3 μm. In this paper, high power mid-IR SC laser sources based on dispersion engineered tellurite and chalcogenide fibers and pumped by ultrafast lasers at 3 µm are proposed and investigated. Our simulation results show that, when a W-type tellurite fiber with a zero dispersion wavelength (ZDW) of 2.7 µm is pumped at 2.78 μm, the power proportion of the SC laser beyond 3 µm can exceed 40% and the attainable SC output power of the proposed solid-cladding tellurite fiber is one order of magnitude higher than that of existing microstructured tellurite fibers. Our calculation also predicts that a very promising super-broadband mid-IR SC fiber laser source covering two atmospheric windows and molecules' "fingerprint" region can be obtained with a microstructured As₂Se₃ chalcogenide fiber pumped at 2.78 μm.

Mid-infrared supercontinuum generation in fluoroindate fiber

We report the generation of mid-infrared supercontinua in a step-index fluoroindate-based fiber. The large core of the fluoroindate fiber allows the guiding of multiwatt laser power over a broad spectral range. These fibers exhibit zero dispersion at 1.83 μm, minimal loss of 0.1 dB/m at 3.2 μm up to only 0.8 dB/m at 5 μm. These specifications enable mid-infrared supercontinuum generation and propagation with low loss. By using mid-infrared ultrashort laser pulses from an optical parametric amplifier, we demonstrate generation of a 20 dB spectral flatness supercontinua from 2.7 to 4.7 μm in the fluoroindate fiber, which is twice the spectral broadening compared to a ZBLAN fiber under similar conditions.

Supercontinuum generation in dispersion-managed tapered-rib waveguide

We have designed a tapered-rib waveguide and numerically studied the generation of supercontinuum using such waveguides. The Air-SF57 glass-SiO(2) waveguide is 3 cm long, with a varying etched depth to manage the total dispersion. Numerical simulations are conducted for input pulses at a wavelength of 1.55 μm with a width of 150 fs and peak power of 5 kW. The proposed waveguide geometry greatly broadens the output spectrum, extending from ∼1 to ∼6  μm, caused by the continuous modification of the phase-matching condition for the generated waves.

Octave-spanning supercontinuum generation in in situ tapered As₂S₃ fiber pumped by a thulium-doped fiber laser

We report a supercontinuum spanning well over an octave of measurable bandwidth from about 1 to 3.7 μm in a 2.1 mm long As₂S₃ fiber taper using the in situ tapering method. A sub-100-fs mode-locked thulium-doped fiber laser system with ~300 pJ of pulse energy was used as the pump source. Third-harmonic generation was observed and currently limits the pump pulse energy and achievable spectral bandwidth.

In-situ tapering of chalcogenide fiber for mid-infrared supercontinuum generation

Supercontinuum generation (SCG) in a tapered chalcogenide fiber is desirable for broadening mid-infrared (or mid-IR, roughly the 2-20 μm wavelength range) frequency combs(1, 2) for applications such as molecular fingerprinting, (3) trace gas detection, (4) laser-driven particle acceleration, (5) and x-ray production via high harmonic generation. (6) Achieving efficient SCG in a tapered optical fiber requires precise control of the group velocity dispersion (GVD) and the temporal properties of the optical pulses at the beginning of the fiber, (7) which depend strongly on the geometry of the taper. (8) Due to variations in the tapering setup and procedure for successive SCG experiments-such as fiber length, tapering environment temperature, or power coupled into the fiber, in-situ spectral monitoring of the SCG is necessary to optimize the output spectrum for a single experiment. In-situ fiber tapering for SCG consists of coupling the pump source through the fiber to be tapered to a spectral measurement device. The fiber is then tapered while the spectral measurement signal is observed in real-time. When the signal reaches its peak, the tapering is stopped. The in-situ tapering procedure allows for generation of a stable, octave-spanning, mid-IR frequency comb from the sub harmonic of a commercially available near-IR frequency comb. (9) This method lowers cost due to the reduction in time and materials required to fabricate an optimal taper with a waist length of only 2 mm. The in-situ tapering technique can be extended to optimizing microstructured optical fiber (MOF) for SCG(10) or tuning of the passband of MOFs, (11) optimizing tapered fiber pairs for fused fiber couplers(12) and wavelength division multiplexers (WDMs), (13) or modifying dispersion compensation for compression or stretching of optical pulses.(14-16.)

Mid-infrared supercontinuum generation in a suspended-core As2S3 chalcogenide microstructured optical fiber

We demonstrate the supercontinuum (SC) generation in a suspended-core As(2)S(3) chalcogenide microstructured optical fiber (MOF). The variation of SC is investigated by changing the fiber length, pump peak power and pump wavelength. In the case of long fibers (20 and 40 cm), the SC ranges are discontinuous and stop at the wavelengths shorter than 3500 nm, due to the absorption of fiber. In the case of short fibers (1.3 and 2.4 cm), the SC ranges are continuous and can extend to the wavelengths longer than 4 μm. The SC broadening is observed when the pump peak power increases from 0.24 to 1.32 kW at 2500 nm. The SC range increases with the pump wavelength changing from 2200 to 2600 nm, corresponding to the dispersion of As(2)S(3) MOF from the normal to anomalous region. The SC generation is simulated by the generalized nonlinear Schrödinger equation. The simulation includes the SC difference between 1.3 and 2.4 cm long fiber by 2500 nm pumping, the variation of SC with pump peak power in 2.4 cm long fiber, and the variation of SC with pump wavelength in 1.3 cm long fiber. The simulation agrees well with the experiment.

Self-stabilized quantum optical Fredkin gate

The quantum optical Fredkin gate is an indispensable resource for networkable quantum applications. Its performance in practical implementations, however, is limited fundamentally by the inherent quantum fluctuations of the pump waves. We demonstrate a method to overcome this drawback by exploiting stimulated Raman scattering in fiber-based implementations. Using a Sagnac fiber-loop switch as a specific example, we show that high switching contrast can be maintained even in the presence of significant pump fluctuations. This unique feature of self-stabilization, together with high-speed and low-loss performance of such devices, point to a viable technology for practical quantum communications.

Octave-spanning infrared supercontinuum generation in robust chalcogenide nanotapers using picosecond pulses

We report on infrared supercontinuum generation extending over more than one octave of bandwidth, from 850 nm to 2.35 μm, produced in a single spatial mode from a robust, compact, composite chalcogenide glass nanotaper. A picosecond laser at 1.55 μm pumps a high-index-contrast, all-solid nanotaper that strongly confines the field to a 480 nm diameter core, while a thermally compatible built-in polymer jacket lends the nanotaper mechanical stability.

Mid-infrared supercontinuum generation in tapered chalcogenide fiber for producing octave-spanning frequency comb around 3 μm

We demonstrate mid-infrared (mid-IR) supercontinuum generation (SCG) with instantaneous bandwidth from 2.2 to 5 μm at 40 dB below the peak, covering the wavelength range desirable for molecular spectroscopy and numerous other applications. The SCG occurs in a tapered As(2)S(3) fiber prepared by in-situ tapering and is pumped by femtosecond pulses from the subharmonic of a mode-locked Er-doped fiber laser. Interference with a narrow linewidth c.w. laser verifies that the coherence properties of the near-IR frequency comb have been preserved through these cascaded nonlinear processes. With this approach stable broad mid-IR frequency combs can be derived from commercially available near-IR frequency combs without an extra stabilization mechanism.

High-spectral-flatness mid-infrared supercontinuum generated from a Tm-doped fiber amplifier

Broadband mid-infrared supercontinuum pulses were generated directly from a short piece of active fiber in a single-mode Tm-doped fiber amplifier. The broadband mid-infrared pulses have an extremely high spectral flatness with ~600 nm FWHM bandwidth (from 1.9 μm to 2.5 μm), >15 kW peak power, and >20 GW/cm(2) laser peak intensity. This new approach exhibits a significantly different physical mechanism from other supercontinuum generation demonstrations in the literature, in which usually a piece of passive fiber was used for nonlinear spectral broadening. The physical mechanism for the broadband mid-infrared supercontinuum generation in this approach has been attributed to a combined effect of two superradiative processes of Tm(3+) ions (i.e., the (3)F(4)-(3)H(6) transition covering the 1.8~2.1  μm spectral region and the (3)H(4)-(3)H(5) transition covering the 2.2~2.5  μm spectral region), and also nonlinear optical processes as well in the Tm-doped gain fiber. The spectra of the mid-infrared supercontinuum pulses were further broadened in a 2 m chalcogenide fiber with 20 dB bandwidth ~1100 nm and a 3 m fluoride fiber with 20 dB bandwidth ~2600 nm.

Numerical investigation of mid-infrared supercontinuum generation up to 5 μm in single mode fluoride fiber

We numerically investigate mid-infrared supercontinuum generation in single mode fluoride fiber pumped by 1.56 μm picosecond fiber lasers. To get high energy conversion efficiency in mid-infrared region, the ratio of power generated in 2.5 ~5 μm range to the total input power for supercontinuum generation is optimized by varying the pulse width, peak power and fiber length. The long wavelength edge of the supercontinuum spectrum can be extended to 4.8 μm in a 100 cm long fluoride fiber pumped by a 1.56 μm fiber laser with a pulse width of 4 ps and a peak power of 100 kW, and the corresponding ratio of power generated in 2.5 ~5 μm range to the total input power is about 44.6%. The spectral broadening is mainly caused by self-phase modulation, stimulated Raman scattering and four-wave mixing. The simulated results show that high average power supercontinuum light source in 2.5 ~5 μm range could be obtained in fluoride fibers pumped by 1.56 μm picosecond fiber lasers.

Crystalline Cr²⁺:ZnSe/chalcogenide glass composites as active mid-IR materials

We propose new transition metal (TM)-doped ZnSe/As₂S₃:As₂Se₃ composite materials for mid-IR fiber lasers. The composites are the suspension of crystalline micro- and nanosized TM²⁺:ZnSe or TM²⁺:ZnS powders in chalcogenide glasses with the refraction index matching. Mid-IR room-temperature lasing of Cr²⁺:ZnSe/As₂S₃:As₂Se₃ microcomposite material is demonstrated at the 2.4 μm wavelength.

Progress in optical waveguides fabricated from chalcogenide glasses

We review the fabrication processes and properties of waveguides that have been made from chalcogenide glasses including highly nonlinear waveguides developed for all-optical processing.

Calculation of the expected bandwidth for a mid-infrared supercontinuum source based on As(2)S(3) chalcogenide photonic crystal fibers

We computationally investigate supercontinuum generation in an As(2)S(3) solid core photonic crystal fiber (PCF) with a hexagonal cladding of air holes. We study the effect of varying the system (fiber and input pulse) parameters on the output bandwidth. We find that there is significant variation of the measured bandwidth with small changes in the system parameters due to the complex structure of the supercontinuum spectral output. This variation implies that one cannot accurately calculate the experimentally-expected bandwidth from a single numerical simulation. We propose the use of a smoothed and ensemble-averaged bandwidth that is expected to be a better predictor of the bandwidth of the supercontinuum spectra that would be produced in experimental systems. We show that the fluctuations are considerably reduced, allowing us to calculate the bandwidth more accurately. Using this smoothed and ensemble averaged bandwidth, we maximize the output bandwidth with a pump wavelength of 2.8 μm and obtain a supercontinuum spectrum that extends from 2.5 μm to 6.2 μm with an uncertainty of ± 0.5 μm. The optimized bandwidth is consistent with prior work, but with a significantly increased accuracy..

Computational study of 3-5 microm source created by using supercontinuum generation in As2S3 chalcogenide fibers with a pump at 2 microm

We present simulation results for supercontinuum generation using As(2)S(3) chalcogenide photonic crystal fibers. We found that more than 25% of input power can be shifted into the region between 3 microm and 5 microm using a pump wavelength of 2 microm with a peak power of 1 kW and an FWHM of 500 fs. The broad dispersion profile and high nonlinearity in As(2)S(3) chalcogenide glass are essential for this application.

Soliton self-frequency shift performance in As(2)S(3) waveguides

The soliton self-frequency shift in As(2)S(3) is investigated theoretically. Differences in the shapes of the Raman spectra in silica and As(2)S(3) are predicted to lead to variations of less than 25% in the frequency shift rate of a fundamental soliton. Detailed simulation of the propagation of a low peak power pulse in a chalcogenide ridge waveguide shows the concepts of Raman soliton behaviour in silica to be transferrable to As(2)S(3). Thus we predict the effectiveness of the soliton self-frequency shift in contributing to wide bandwidth generation in low-power supercontinua at mid-infrared wavelengths in this highly nonlinear chalcogenide, as well as other nonlinear processing applications such as digital quantization for optical analogue to digital conversion.