Octave-spanning Supercontinuum Generation Research Articles

Octave-spanning supercontinuum generation in a CMOS-compatible thin Si3N4 waveguide coated with highly nonlinear TeO2.

Supercontinuum generation (SCG) is an important nonlinear optical process enabling broadband light sources for many applications, for which silicon nitride (Si3N4) has emerged as a leading on-chip platform. To achieve suitable group velocity dispersion and high confinement for broadband SCG the Si3N4 waveguide layer used is typically thick (>∼700 nm), which can lead to high stress and cracks unless specialized processing steps are used. Here, we report on efficient octave-spanning SCG in a thinner moderate-confinement 400-nm Si3N4 platform using a highly nonlinear tellurium oxide (TeO2) coating. An octave supercontinuum spanning from 0.89 to 2.11 µm is achieved at a low peak power of 258 W using a 100-fs laser centered at 1565 nm. Our numerical simulations agree well with the experimental results giving a nonlinear parameter of 2.5 ± 0.5 W-1m-1, an increase by a factor of 2.5, when coating the Si3N4 waveguide with a TeO2 film. This work demonstrates highly efficient SCG via effective dispersion engineering and an enhanced nonlinearity in CMOS-compatible hybrid TeO2-Si3N4 waveguides and a promising route to monolithically integrated nonlinear, linear, and active functionalities on a single silicon photonic chip.

Exceeding two octave-spanning supercontinuum generation in integrated As2S3 waveguides pumped by a 2 μm fiber laser

In this work, we demonstrate over two octaves-spanning supercontinuum generation in integrated As2S3 chalcogenide glass waveguides pumped by a 2 μm femtosecond fiber laser. The pump laser provides 182 fs pulses at 50 MHz repetition rate. Through optimization of the waveguide dimensions, a broad supercontinuum spectrum spanning from 0.6 to 2.8 μm was achieved using a 4 μm wide, 5 mm long waveguide with 357 mW coupled pump power. The findings showcase the potential of dispersion engineered, integrated As2S3 photonics for realizing compact, efficient on-chip supercontinuum sources for mid-infrared applications.

Numerical exploration of multi-octave spanned supercontinuum generation with high coherence in single material photonic crystal fiber

We propose a single material microstructured optical fiber with more than five octave spanning supercontinuum generation. Due to using single material, compatibility checking between core and cladding material need not required. Moreover, the material is such chosen that optical transmission range is quite high in comparison to others. The fiber is designed in COMSOL platform and according to finite element method (FEM), numerical analysis is accomplished. Dispersion, confinement loss, nonlinearity etc. are investigated and exposed for a wavelength range of 2000–3000 nm. The proposed fiber reveals zero dispersion at 2200 nm, nonlinearity of 14,370 W−1km−1 and confinement loss around 10−9 order in dB/m. Finally, supercontinuum generation has been investigated inside the structure and obtained high coherence that span (from 1500 nm to 34,500 nm) over five octave. The generated spectrum fulfills the requirements for being applicable in frequency metrology, fiber laser and biophotonics. With the best of our knowing, such coherent and widened supercontinuua for single material fiber is yet to be proposed.

Circular lattice benzene-core PCFs with flat near-zero dispersion for low-power broad-spectrum supercontinuum generation

A circular photonic crystal fiber infiltrated with benzene with different air-hole diameters is proposed as a new supercontinuum light source. Optical properties related to dispersion, effective mode area, nonlinear coefficient, and attenuation of the fundamental mode are investigated numerically. Two optimized structures are selected and verified against supercontinuum generation (SCG) in detail. The first structure (#F1) possesses all-normal dispersion, while the second (#F2) has a zero-dispersion wavelength. The possibility of coherent, octave-spanning SCG is proved by a 40 fs pulse, 1.064 μm wavelength, and 0.45 kW of power in-coupled into the core of #F1. Otherwise, injecting a 90 fs duration, 1.5 μm wavelength, and 0.555 kW peak power pump pulse into #F2 generates a broad SC spanning 0.76–4.23 μm. With the advantages of flat near-zero dispersion, high nonlinearity, low attenuation, and low input power used for SCG, the proposed fibers may lead to new low-cost all-fiber optical systems.

Sub-100 fs Kerr-lens mode-locked femtosecond Yb:CaYAlO<sub>4</sub> laser with GHz repetition rate

Femtosecond lasers with GHz repetition rate play an important role in scientific and industrial applications such as spectroscopy, optical frequency combs and GHz-Burst pulse trains for micro-machining in the ablation-cooled regime. Kerr-lens mode-locked (KLM) technique and passively mode-locking based on semiconductor saturable absorber mirror (SESAM) are the primary methods to generate GHz femtosecond all-solid-state lasers (ASSLs). Kerr-lens mode-locked Ti:Sapphire lasers have made significant progress benefited from the high-power green pump lasers, and repetition rate up to 10 GHz has been obtained with the average power of 1.2 W. In the early 21st century, ytterbium ion (Yb<sup>3+</sup>) doped laser crystals and ceramics with emission wavelengths near 1 μm received attention due to their high conversion efficiency and broad gain-bandwidth. Combining the customized SESAM and high-power multimode fiber-coupled laser diodes (LDs), GHz Yb-doped ASSLs with watt-level average power may be easily attained and have made rapid progress. However, GHz KLM lasers have strict requirements for the cavity design and pump sources. For satisfying mode matching and enhancing the soft aperture effect within the gain medium, a high-brightness pump source with excellent beam quality (<i>M</i><sup> 2</sup> ～ 1) is desired, such as the single-mode fiber coupled LD, however, the maximum pump power of which is only ～1 W. As a result, the average power of GHz KLM femtosecond laser is typically limited to few tens of milliwatts, which limits the further applications. In this work, we report the first GHz high-power KLM Yb:CaYAlO<sub>4</sub> laser by using a high-power single-mode fiber laser instead of the low-power single-mode fiber coupled LDs as the pump source. On the basis of <i>ABCD</i> matrix, a simple four-mirror bow-tie ring cavity is built so that the laser mode can match well with the focused pump spot in the crystal. At the pump power of 8 W, stable unidirectional KLM is achieved, the laser has the average power of 2.1 W with a pulse duration of 88 fs and a repetition rate of 1.8 GHz, corresponding to the peak power of 11.57 kW. The high peak power and extremely short pulse duration are crucial for coherent octave-spanning supercontinuum generation. The powerful GHz KLM laser with sub-100 fs pulse duration provides an attractive source for realizing the optical frequency combs and micro-machining applications.

Supercontinuum generation in highly birefringent fiber infiltrated with carbon disulfide

Supercontinuum generation in highly birefringent fiber infiltrated with carbon disulfide is numerically studied with a linearly polarized femtosecond laser (1560 nm wavelength, 90 fs pulse duration) as a pump source. The fiber has two polarized modes (x- and y-polarized LP01) with different dispersion characteristics, meaning all-normal dispersion for the y-polarized mode and anomalous dispersion for the x-polarized mode. We build a numerical modeling for nonlinear propagation including Kerr effects and Raman effects with positive and negative nonlinear refractive. Our results point out that spectral bandwidth, flatness, and coherence of supercontinuum generation depend on the input polarized angle (an angle with respect to the x-axis) of the laser pulses. In particular, the input pulses polarized near slow-axis (x-axis) provide 1.5 octave-spanning soliton-induced SC generation with bandwidth from 1000 nm to 3300 nm via 10 kW input peak power, while input pulses polarized near the fast-axis (y-axis) create octave-spanning all-normal dispersion supercontinuum generation (from 1200 nm to 2500 nm). Modification of the input angle enables controlling spectro-temporal properties of supercontinuum generation, such as polarization state, coherence, spectral bandwidth, and flatness. In addition, the use of highly birefringent fiber also enables suppression of polarization modulation instability; therefore coherence solely depends on effects of modulation instability and it is possible to tailor via changing the input angle.

Engineering the high-frequency components of coherent supercontinuum generation in hybrid optical fibers with yttrium aluminum garnet core

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.

Single-mode, single-polarization and dispersion-flattened waveguides based on silicon carbide and diamond

Fundamental properties of an optical waveguide are mainly related to its guided modes and associated loss, dispersion, polarization and nonlinearity. Single-mode and single-polarization waveguiding has long been of interest, i.e., so-called truly single mode. We propose SiC- and diamond-based waveguides, which are truly single-mode and simultaneously dispersion-flattened. This is preferred for broadband nonlinear applications, such as octave-spanning supercontinuum and frequency comb generation. An ultra-low and flat dispersion profile with four zero-dispersion wavelengths (ZDWs) can be obtained over an octave-spanning bandwidth from 1.8 to 3.6 μm. Meanwhile, only TE00 mode is supported in the proposed waveguide over the low-dispersion bandwidth. Scattering loss of the waveguide is greatly reduced by 8.6 times, compared to a standard strip waveguide. Fabrication of the proposed waveguide is easier than previous dispersion-tailored waveguides, without dry etching SiC and diamond. It is expected that the proposed waveguides can find applications in nonlinear photonics, optomechanics and quantum optics.

On-chip octave-spanning supercontinuum generation in hybrid slot waveguide by power-efficient dual pump

We propose an on-chip cascaded structure consisting of a microresonator and a slot waveguide to generate the high-performance supercontinuum harnessing the dual pump scenario. Utilizing a single CW pump with the power of only 0.1 W, the dissipative Kerr soliton with the full width at half maximum of 69.2 fs and the peak power of 103.3 W is generated and couples out the microresonator together with the CW pump to serve as a dual pump seed for the supercontinuum generation in the followed slot waveguide. The slot waveguide built upon a silicon-organic hybrid structure is carefully designed to possess remarkable nonlinearity and appropriate dispersion profile for smooth and broadband supercontinuum generation. We have numerically achieved the 1.56 octave-spanning supercontinuum spanning 1 ~ 3 μm with the proposed dual pump scenario. The supercontinuum generated by the dual pump scenario shows flattened, broadband, and quasi-right-angled-trapezoid shape, along with good coherence. Compared with the single pump scenario of CW or soliton pulse only, the supercontinuum generated by the dual pump scenario possesses the much stronger spectrum, broader spanning, and medium coherence. Our work opens up a new path for on-chip and cost-effective supercontinuum generation with the high performance of intensity, bandwidth, and coherence.

Octave-spanning coherent supercontinuum generation in a step-index tellurite fiber and towards few-cycle pulse compression at 2 [formula omitted

We experimentally demonstrate 140-THz bandwidth (at −20 dB) supercontinuum generation in a 10 cm-long all-normal dispersion step-index tellurite fiber pumped by a turn-key femtosecond fiber laser emitting at 2.11μm at a repetition rate of 19 MHz. The soliton self-frequency shifted thulium-doped fiber mode-locked laser emits initial transform-limited pulses, with 85-fs pulse duration, that are subsequently quasi-linearly chirped (over more than 50 THz) during the above nJ-level nonlinear propagation. Moreover, we numerically demonstrate the possible pulse compression down to 12 fs by means of additional linear propagation in a standard step-index fluoride fiber with anomalous dispersion.

Octave-spanning supercontinuum generation in infrared by MoS2-filled hollow core fiber

We report the generation of an infrared (IR) supercontinuum spanning over an octave using a centimeter (6 cm) length of photonic crystal fiber (PCF) filled with MoS2, which has a considerably large nonlinear refractive index. The fiber is pumped with a femto-second laser pulse with an infrared wavelength of 2080 nm, pumping intensity of 336.76 GW/cm2, peak power of 0.75 GW, and pulse width of 150 fs. With the assistance of highly nonlinear MoS2, a supercontinuum bandwidth that remarkably extends from 1400 to 2700 nm below 20 dB is achieved. This supercontinuum bandwidth significantly exceeds the previously reported spectra for other nonlinear glass-based optical fibers, even with a fairly short fiber length. Furthermore, this short fiber length allows PCF to be more efficiently utilized, and is expected to be suitable for various applications, such as broadband light source through liquid-based fibers and waveguide-based optical communications, especially in the IR region.

Octave spanning supercontinuum generation with a few-mode fiber

We propose and demonstrate the generation of a supercontinuum with a few-mode fiber. Due to the accommodation of a few-mode fiber in LP01 and LP11 modes, the system was engineered to detect the octave spanning supercontinuum in both modes. A mode-locked laser with 5.2 nm spectral bandwidth at a central wavelength of 1557 nm, 0.92 ps pulse width, 13.36 MHz repetition rate, and 61.3 dB peak-to-pedestal extinction ratio was implemented with a carbon nanotube saturable absorber. This mode-locked laser was employed as a seed laser for the consecutive generation of supercontinuum with a 500 m few-mode fiber. The laser modes were adjusted with a mode filter to propagate in either LP01 or LP11 modes, which were accessed with a charge-coupled device camera. A fair comparison between a few-mode fiber and a single-mode fiber was made for supercontinuum generation in the LP01 mode. Despite the fact that the few-mode fiber has lower optical power than the single-mode fiber due to higher attenuation, this fiber provides flexible design of laser modes. This work demonstrates the first attempt to generate a supercontinuum with a few-mode fiber, whereby the broad tunable soliton and associated dispersive wave sources for both laser modes were comprehensively studied.

Performance scaling of a 10-GHz solid-state laser enabling self-referenced CEO frequency detection without amplification.

A simple and compact straight-cavity laser oscillator incorporating a cascaded quadratic nonlinear crystal and a semiconductor saturable absorber mirror (SESAM) can deliver stable femtosecond modelocking at high pulse repetition rates >10 GHz. In this paper, we experimentally investigate the influence of intracavity dispersion, pump brightness, and cavity design on modelocking with high repetition rates, and use the resulting insights to demonstrate a 10.4-GHz straight-cavity SESAM-modelocked Yb:CALGO laser delivering 108-fs pulses with 812 mW of average output power. This result represents a record-level performance for diode-pumped femtosecond oscillators with repetition rates above 10 GHz. Using the oscillator output without any optical amplification, we demonstrate coherent octave-spanning supercontinuum generation (SCG) in a silicon nitride waveguide. Subsequent f-to-2f interferometry with a periodically poled lithium niobate waveguide enables the detection of a strong carrier-envelope offset (CEO) beat note with a 33-dB signal-to-noise ratio.

Efficient supercontinuum generation enabled by dispersion engineering in a dual-core waveguide

A compact dual-core waveguide is proposed for efficient supercontinuum generation, in which the use of Si-rich nitride and titanium dioxide provides a large enough index contrast for dispersion engineering. Strong anomalous group velocity dispersion is obtained over a wide bandwidth of 1860 nm which greatly reduces the dispersion length. The nonlinear length can also be significantly decreased due to high nonlinear Kerr index of Si-rich nitride. As a consequence, it could facilitate octave-spanning supercontinuum generation from 1160 to 2385 nm in a greatly reduced propagation distance, as short as 165μm. It is more than one order of magnitude shorter than those previously reported and can be further reduced to 70μm. This compact broadband source is promising not only for on-chip applications but also for applications in tiny environments such as nano-scale spectroscopic probing and study of biological molecules, cells and tissues.

Highly coherent supercontinuum generation in CS2-infiltrated single-core optical fiber

As is generally known, the broadband near-infrared supercontinuum (SC) is significant for applications such as pump-probe spectroscopy, nonlinear spectroscopy, frequency combs and nonlinear optical parametric amplification. We present the first detailed demonstration of approximate near-infrared, octave-spanning SC generation in an all-normal dispersion CS2 single-hole liquid core optical fiber (LCOF). By solving the generalized nonlinear Schrödinger equation, the SC can be calculated at different parameters. The influence of important parameters—including but not limited to the fiber core diameter, pump wavelength, pump power and pulse width—on the supercontinuum generation (SCG) of a CS2-infiltrated LCOF can be analyzed for the optimization of near-infrared SCG. The results show that at low pump power (such as P0 = 4 kW), the SC generated by using two pump wavelengths (1.55 μm and 1.95 μm) has spectral flatness, good coherence and is octave-spanning. However, the bandwidth is severely affected by the large pulse duration T0, and the coherence is seriously influenced by the pulse duration T0 > 1200 fs at the pump wavelength of 1.55 μm. The effect of pulse duration on the first-order coherence is greater at short pump wavelengths (1.55 μm) than long wavelengths (1.95 μm). Under high power conditions (such as P0 = 20 kW), the coherence of the SC generated remains good at the two-pump wavelength. This study explores an optimized scheme for obtaining a broadband SC with high coherence and fine spectral flatness suitable for specific applications.

Compact all-fiber source of coherent linearly polarized octave-spanning supercontinuum based on normal dispersion silica fiber

We report the generation of coherent octave-spanning supercontinuum in an all-fiber system, without any free-space optical components. The setup uses the femtosecond fiber laser as a pump and an all-normal dispersion microstructured fiber as a medium for supercontinuum generation. The generated spectrum is characterized both experimentally and numerically and shows a broad bandwidth (1.1−2.2 μm), a high signal to noise ratio reaching 100 at maximum, a high coherence (closing to 1), linear polarization and average output power up to 57 mW. The source is characterized by exceptional simplicity and does not require any alignment (the nonlinear fiber is spliced to the pump) which finally opens the path to outside-lab applications of supercontinuum radiation.

Visible to near-infrared octave spanning supercontinuum generation in tantalum pentoxide (Ta2O5) air-cladding waveguide.

In this work, for the first time, to the best of our knowledge, an anomalous dispersion CMOS-compatible Ta2O5 waveguide was realized, and broadband on-chip supercontinuum generation (SCG) was accordingly demonstrated. When pumped at a center wavelength of 1056nm with pulses of 100fs duration and peak power of 396W, a supercontinuum ranging from 585nm to 1697nm was generated, comprising a bandwidth of more than 1.5 octaves and leading to an efficient SCG source. The excellent performance for Ta2O5 to generate SCG benefits mainly from its high nonlinear refractive index, which enhances the efficiency of the nonlinear conversion process.

Octave-spanning supercontinuum generation in nanoscale lithium niobate waveguides.

We demonstrate octave-spanning supercontinuum generation in unpoled lithium niobate waveguides, which are engineered to possess anomalous dispersion and pumped by a turn-key femtosecond laser centered at 1560nm. Tunable dispersive waves and strong phase-matched second-harmonic generation are both observed by controlling the widths of the waveguides. The major features of the experimental spectra are reproduced by numerical modeling of the generalized nonlinear Schrödinger equation, which can be used to guide waveguide designs for tailoring the supercontinuum spectrum. Our results identify a path to a simple and integrable supercontinuum source in lithium niobate nanophotonic platform and will enable new capabilities in precision frequency metrology.

High-power short-wavelength infrared supercontinuum generation in multimode fluoride fiber

We demonstrate the generation of octave-spanning supercontinuum generation from 1200 nm to over 2500 nm with 600 mW average power in a short length of multimode fluoride fiber with 100 μm core diameter. We perform a detailed study of the supercontinuum generation process as a function of the pump wavelength and for two different fiber lengths. Beam profile characterization at the fiber output in different wavelength bands is also carried out. Our results open up new possibilities for the generation of high-power supercontinuum sources from the near- to mid-infrared.

High-power, octave-spanning supercontinuum generation in highly nonlinear fibers using noise-like and well-defined pump optical pulses

High-power, octave-spanning supercontinuum generation through pumping a highly nonlinear fiber with noise-like and well-defined optical pulses is investigated. A two-stage fiber amplifier, consisting of a pre-amplifier and a booster, is used to greatly enhance the average power of noise-like pump pulses from 14 mW to 13.1 W. Owing to the limited coupling efficiency, only a maximum optical power of 4.63 W is launched into the nonlinear fiber. As a result, a supercontinuum spectrum spanning from 940 to 2300 nm with an average power of 3.82 W is achieved with noise-like pump pulses. This is, to the best of our knowledge, the highest average power obtained over such an octave-spanning supercontinuum spectrum using noise-like pump pulses. Supercontinuum generation with a broader spectral range and a higher average power is feasible if the coupling efficiency of the pump pulse power launched into the nonlinear fiber is increased. For the purpose of comparison, well-defined pump pulses with a similar average power at a similar repetition rate are also investigated for similar octave-spanning supercontinuum generation. A supercontinuum spectrum spanning from 950 to 2500 nm with an average power of 3.62 W is obtained with well-defined pump pulses. Even though the spectrum of the resulting supercontinuum using the well-defined pump pulses is broader, a large portion of its optical power is concentrated around the center wavelength, leading to a lower spectral power density at other wavelengths; furthermore, a higher extent of spectral structure appears in its optical spectrum, resulting in a higher power variation among different wavelengths.