Octave-spanning Supercontinuum Research Articles

Octave Spanning Supercontinuum in Titanium Dioxide Waveguides

We report on the experimental generation of an octave-spanning supercontinuum in a 2.2 cm-long titanium dioxide optical waveguide with two zero dispersion wavelengths. The resulting on-chip supercontinuum reaches the visible wavelength range as well as the mid-infrared region by using a femtosecond fiber laser pump at 1.64 µm.

Asterisk-shaped microstructured fiber for an octave coherent supercontinuum in a sub-picosecond region.

We selected two thermally matched silicate glasses with fair refractive index contrast and developed an asterisk-shaped all-solid microstructured optical fiber. The fiber presents a low, ultra-flat, and all-normal dispersion in a wide wavelength range, allowing for the generation of an octave-spanning coherent supercontinuum (SC) in a 20dB dynamic range with 0.5ps pump pulses at 1.55μm. This result improves pump pulse duration that is only ∼100 fs, related to the broadband and highly coherent SC generation in fibers with all-normal dispersion. This enables broadband SC sources with all-fiber, high-power, and highly coherent properties.

Power and chirp effects on the frequency stability of resonant dispersive waves generated in photonic crystal fibres

Optimization of laser output parameters vs. f-to-2f beating signals can be mutually contradicting, when an octave-spanning supercontinuum is employed for f-to-2f measurements. We show that resonant dispersive waves will solve this issue, thanks to their frequency stability against changes in laser power and chirping.

Octave-spanning visible supercontinuum generation from an aluminum nitride single crystal pumped by a 355 nm nanosecond pulse

Supercontinuum generation (SC) of more than one octave spectrum spanning covering from 400 nm to 820 nm was achieved by pumping a piece of aluminum nitride (AIN) single crystal using a nanosecond 355 nm ultraviolet laser. The AlN with a thickness of ∼0.8 mm was grown by an optimized physical vapor transport technique and polished with solidification technology. Compared to previously reported ones, the achieved visible SC exhibited the broadest spectrum spanning from bulk materials pumped by a nanosecond pulse laser. The visible supercontinuum in AlN presents new opportunities for bulk material-based white light SC and may find more potential applications beyond typical applications in integrated semiconductive photoelectronic devices.

Coherent synthesis of ultrashort pulses based on balanced optical cross-correlator

Coherent synthesis of laser pulses is a major trend in the generation of ultrafast pulse field. There is no good way to compensate for the whole spectrum when the spectrum of ultrashort pulses is wide enough to reach an octave, so it is difficult to realize a sub-cycle pulse in a single-path laser system even if the spectrum range is wide enough. In this paper, 0.8 mJ, 30 fs laser pulses at 1 kHz repetition rate with 790 nm center wavelength from a Ti:sapphire chirped pulse amplifier (CPA) system are focused into hollow fiber with an inner diameter of 250 μm and a length of 1 m to produce an octave-spanning white-light supercontinuum (450-950 nm). Using this supercontinuum, we conduct two sets of comparative experiments. 1) We split the supercontinuum into two pulses with different spectrum ranges (450-750 nm and 650-1000 nm) by a dichroic mirror (HR, 500-700 nm; HT, 700-1000 nm), and we compress the two pulses by the double-chirped mirrors and wedge pairs to generate two few-cycle pulses:the long and short wavelength yielding pulses are 7.9 fs and 6.1 fs, respectively. Then we coherently synthesize two pulses by using another dichroic mirror, and controlling the relative time delay between the two pulses, and thus we synthesize a pulse of 4.1 fs. 2) We directly compress the supercontinuum by the double-chirped mirrors and wedge pairs, and obtain an optimization result of 5.3 fs, of which the pulse duration is wider than that in experiment 1. In these comparative experiments, the advantage of coherent synthesis for shorter pulse duration is preliminarily verified. Besides, the balanced optical cross-correlator technique is used to lock the relative time delay between two pulses. The root-mean-square value of relative time delay drift is less than 80 as in the case with feedback control loop, which ensures the stability of coherent synthesis system. This scheme can be adopted to accurately compensate for the dispersion and obtain the sub-cycle synthesized pulse, which is useful for generating the high harmonic and atto-second pulse.

Attosecond relative delay measurement using transient-grating frequency-resolved optical grating

The accurate and precise controlling of the attosecond time delay between the sub-pulses within a hundredth of an optical cycle is the key ingredient for the sophisticated custom-tailored coherent waveform synthesizer. The attosecond delay control technique commonly experiences the “complete” characterization of the ultrashort sub-cycle pulses, which includes the spatiotemporal pulse characterization of the synthesized waveform and the attosecond relative delay between the parent pulses. In this work, the relative time delay between spectrally separated ultrashort parent pulses is characterized in an interferometer scheme with a background-free transient-grating frequency-resolved optical grating (TG-FROG). The TG-FROG geometry accurately measures the full time-dependent intensity and phase of ultrashort laser pulses in a wide range of regime (from ultraviolet to infrared) and offers significant advantages over other nonlinear-optical processes geometries (i.e., the polarization-gate-FROG, the self-diffraction-FROG, the second-harmonic generation-FROG and the third-harmonic-generation-FROG). The attosecond measurement accuracy is achieved for the first time, to the best of our knowledge. In this experiment, the output of a carrier-envelope-phase-stable Ti:sapphire amplifier (sub-30-fs, over-1-mJ, 1 kHz) is spectrally broadened in a neon-filled hollow-core fiber with an inner diameter of 250μm. The transmission through the pressure-gradient hollow-core fiber results in an mJ-level octave-spanning whitelight supercontinuum, supporting a sub-3-fs Fourier transform-limited pulse. The supercontinuum is spectrally divided into two parent pulses by using a dichroic mirror. The sub-pulses are individually compressed by the custom-designed double-chirped mirrors and wedge pairs. The short and long wavelength pulses are separately compressed in few-cycle regime, yielding pulses with 6.7 fs and 9.8 fs, respectively. This technique overcomes the bottlenecks in the traditional delay measurement and should be applicable for many ultra-broadband pulse characterizations with extremely simple and alignment-free delay control device used. Furthermore, this new method will be easily adapted for the ultra-broadband two-dimensional electronic spectroscopy, the advanced temporal cloaking, and the field of sub-cycle arbitrary coherent waveform synthesizer for controlling strong-field interactions in atoms, molecules, solids, and nanostructures. We foresee that in the near future this novel technology will be very attractive for various applications in the next-generation light sources such as the Synergetic Extreme Condition User Facility in Beijing, China.

Carrier-envelope offset frequency stabilization of a gigahertz semiconductor disk laser

Optical frequency combs based on ultrafast lasers have enabled numerous scientific breakthroughs. However, their use for commercial applications is limited by the complexity and cost of femtosecond laser technology. Ultrafast semiconductor lasers might change this issue as they can be mass produced in a cost-efficient way while providing large spectral coverage from a single technology. However, it has not been proven to date if ultrafast semiconductor lasers are suitable for stabilization of their carrier-envelope offset (CEO) frequency. Here we present what we believe to be the first CEO frequency stabilization of an ultrafast semiconductor disk laser (SDL). The optically pumped SDL is passively modelocked by a semiconductor saturable absorber mirror. It operates at a repetition rate of 1.8 GHz and a center wavelength of 1034 nm. The 273 fs pulses of the oscillator are amplified to an average power level of 6 W and temporally compressed down to 120 fs. A coherent octave-spanning supercontinuum spectrum is generated in a photonic crystal fiber. The CEO frequency is detected in a standard f–to–2f interferometer and phase locked to an external reference by feedback applied to the current of the SDL pump diode. This proof-of-principle demonstrates that ultrafast SDLs are suitable for CEO stabilization and constitutes a key step for further developments of this comb technology expected in the coming years.

Coherent, directional supercontinuum generation.

We demonstrate a novel approach to producing coherent, directional supercontinuum and cascaded dispersive waves using dispersion engineering in waveguides. By pumping in the normal group-velocity dispersion (GVD) regime, with two zero-GVD points to one side of the pump, pulse compression of the first dispersive wave generated in the anomalous GVD region results in the generation of a second dispersive wave beyond the second zero-GVD point in the normal GVD regime. As a result, we achieve an octave-spanning supercontinuum generated primarily to one side of the pump spectrum. We theoretically investigate the dynamics and show that the generated spectrum is highly coherent. We experimentally confirm this dynamical behavior and the coherence properties in silicon nitride waveguides by performing direct detection of the carrier-envelope-offset frequency of our femtosecond pump source using an f-2f interferometer. Our technique offers a path towards a stabilized, high-power, integrated supercontinuum source with low noise and high coherence, with applications including direct comb spectroscopy.

Toward all-fiber supercontinuum spanning the mid-infrared

Broadband laser light sources in the mid-infrared region attract enormous interest due to the plethora of applications they are enabling, including multispecies trace gas detection, free-space communications, and infrared countermeasures. Key to the progress in supercontinuum generation has been the wide availability of fiber-based near-infrared and bulk-optic mid-infrared pump sources and suitably engineered nonlinear media capable of supporting high-brightness supercontinua. A large proportion of the system complexity relates to the pump source itself with free-space systems based on parametric conversion being the most common for the generation of long-wavelength supercontinua. In an effort to realize all fiber and all mid-infrared supercontinuum sources, we combine a recently developed 2.9 µm ultrafast fiber laser based on holmium with an environmentally stable, polymer-protected all-chalcogenide tapered fiber. By launching 230 fs, 4.2 kW peak power pulses into the As2Se3/As2S3 tapered fiber, we demonstrate a spectrum spanning from 1.8 to 9.5 µm at the −20 dB points with an average power of more than 30 mW. This >2 octave-spanning supercontinuum is a crucial step toward robust mid-infrared broadband sources required for future field-deployable instruments.

Improvement of signal-to-noise ratio of a beat note by cascading an Yb-doped fiber amplifier in an Er-fiber comb

Femtosecond optical frequency combs (FOFCs) make it possible to measure the absolute frequency of a laser, which greatly simplifies quantity traceability and comparison of the absolute frequency. In order to ensure the accuracy and reliability of the measurement, the signal-to-noise ratio (SNR) of the beat note (fb) between an FOFC and a laser should be above 30 dB. In this paper, an Er-doped FOFC (Er-FOFC) is employed, at a repetition rate of 305 MHz. The spectral intensity near the 1 µm wavelength in an octave-spanning supercontinuum generated from the Er-FOFC is enhanced effectively by cascading an Yb-doped fiber amplifier after spectral broadening, which helps improve the SNR of the beat note between the comb light and an iodine-stabilized 532 nm laser. This method can effectively reduce the intensity requirements on the 1 µm wavelength when the spectrum is directly broadened in an Er-FOFC.

Octave-spanning coherent supercontinuum generation in silicon on insulator from 1.06\u2009\u03bcm to beyond 2.4\u2009\u03bcm

Efficient complementary metal-oxide semiconductor-based nonlinear optical devices in the near-infrared are in strong demand. Due to two-photon absorption in silicon, however, much nonlinear research is shifting towards unconventional photonics platforms. In this work, we demonstrate the generation of an octave-spanning coherent supercontinuum in a silicon waveguide covering the spectral region from the near- to shortwave-infrared. With input pulses of 18 pJ in energy, the generated signal spans the wavelength range from the edge of the silicon transmission window, approximately 1.06 to beyond 2.4 μm, with a −20 dB bandwidth covering 1.124–2.4 μm. An octave-spanning supercontinuum was also observed at the energy levels as low as 4 pJ (−35 dB bandwidth). We also measured the coherence over an octave, obtaining , in good agreement with the simulations. In addition, we demonstrate optimization of the third-order dispersion of the waveguide to strengthen the dispersive wave and discuss the advantage of having a soliton at the long wavelength edge of an octave-spanning signal for nonlinear applications. This research paves the way for applications, such as chip-scale precision spectroscopy, optical coherence tomography, optical frequency metrology, frequency synthesis and wide-band wavelength division multiplexing in the telecom window.

Octave-spanning supercontinuum generation at telecommunications wavelengths in a precisely dispersion- and length-controlled silicon-wire waveguide with a double taper structure

We demonstrate on-chip octave-spanning supercontinuum (SC) generation with a Si-wire waveguide (SWG). We precisely controlled the SWG width so that the group velocity becomes flat over a wide wavelength range. By adjusting the SWG length, we could reduce the optical losses due to two-photon absorption and pulse propagation. In addition, for efficient coupling between the laser pulse and waveguide, we fabricated a two-step inverse taper at both ends of the SWG. Using a 600-nm-wide SWG, we were able to generate a broadband SC spectrum at wavelengths from 1060 to 2200 nm at a −40 dB level with only 50-pJ laser energy from an Er-doped fiber laser oscillator. We found that we can generate an on-chip broadband SC spectrum with an SWG with a length even as small as 1.7 mm.

High-energy continuum generation in an array of thin plates pumped by tunable femtosecond IR pulses

We report the generation of octave-spanning supercontinuum pulses in an arrangement of thin transparent plates pumped by 60 fs infrared pulses tunable from 1.2 to 1.75 $$\upmu \mathrm{m}$$ . We demonstrate that the multiple plate technique allows scaling up the energy of the continuum to the level of hundreds of $$\upmu \mathrm{J}$$ while avoiding optical damage and beam breakup due to multiple filamentation. We investigate the spatial and temporal structures of the continuum by performing XFROG and frequency-resolved angular spectra measurements and confirm that the continuum wave packet is well-behaved in time and space and possesses an estimated spectral energy density in excess of 10 nJ/nm throughout the range, which exceeds that of continua generated in bulk solids by more than two orders of magnitude. Finally, we demonstrate for the first time that the carrier-envelope phase stability of the multi-plate continuum is comparable to that of a continuum generated in a bulk medium.

Ultrafast middle-IR lasers and amplifiers based on polycrystalline Cr:ZnS and Cr:ZnSe

Transition-metal-doped II-VI semiconductors possess a unique blend of physical, spectroscopic, optical, and technological parameters. These materials enable high power lasers in important middle-infrared range. Furthermore, they combine superb ultra-fast laser capabilities with high nonlinearity and polycrystalline microstructure, which provides random quasi-phase matching. We developed flexible design of femtosecond polycrystalline Cr:ZnS and Cr:ZnSe lasers and amplifiers in the spectral range 2–3 µm. We obtained few-optical-cycle pulses with a multi-Watt average power in a very broad range of repetition rates 0.07–1.2 GHz. We also report on efficient nonlinear frequency conversion directly in the polycrystalline gain elements of ultra-fast lasers and amplifiers including second harmonic generation with sub-Watt power and generation of an octave-spanning middle-infrared supercontinuum.

Mid-Infrared Octave-Spanning Supercontinuum and Frequency Comb Generation in a Suspended Germanium-Membrane Ridge Waveguide

Stable octave-spanning supercontinuum (SC) in the mid-infrared (MIR) region finds extensive applications in spectroscopy, metrology, biochemistry, etc. The absorption of conventional silicon- or silicon oxide-dominated nonlinear media makes SC generation in MIR region technically challenging. In this paper, we propose ultrabroadband MIR-SC generation using a suspended germanium-membrane ridge waveguide. We theoretically showed that when pump pulses centered at 4.8 μm with pulse width at 180 fs and peak power at 800 W are injected into a 4-mm-long proposed ridge waveguide, the SC generated ranges from 1.96–12 μm (about 2.6 octaves), extending deep into the “fingerprint” region. The first-order coherence is calculated to confirm the stability of the generated SC. The performance of the SC-based frequency comb is also investigated by assuming a 100-pulses pump source at a repetition rate of 100 kHz.

Room-temperature-deposited dielectrics and superconductors for integrated photonics.

We present an approach to fabrication and packaging of integrated photonic devices that utilizes waveguide and detector layers deposited at near-ambient temperature. All lithography is performed with a 365 nm i-line stepper, facilitating low cost and high scalability. We have shown low-loss SiN waveguides, high-Q ring resonators, critically coupled ring resonators, 50/50 beam splitters, Mach-Zehnder interferometers (MZIs) and a process-agnostic fiber packaging scheme. We have further explored the utility of this process for applications in nonlinear optics and quantum photonics. We demonstrate spectral tailoring and octave-spanning supercontinuum generation as well as the integration of superconducting nanowire single photon detectors with MZIs and channel-dropping filters. The packaging approach is suitable for operation up to 160 °C as well as below 1 K. The process is well suited for augmentation of existing foundry capabilities or as a stand-alone process.

Carrier envelope offset frequency detection and stabilization of a diode-pumped mode-locked Ti:sapphire laser.

We demonstrate the first diode-pumped Ti:sapphire laser frequency comb. It is pumped by two green laser diodes with a total pump power of 3 W. The Ti:sapphire laser generates 250 mW of average output power in 61-fs pulses at a repetition rate of 216 MHz. We generated an octave-spanning supercontinuum spectrum in a photonic-crystal fiber and detected the carrier envelope offset (CEO) frequency in a standard f-to-2f interferometer setup. We stabilized the CEO-frequency through direct current modulation of one of the green pump diodes with a feedback bandwidth of 55 kHz limited by the pump diode driver used in this experiment. We achieved a reduction of the CEO phase noise power spectral density by 140 dB at 1 Hz offset frequency. An advantage of diode pumping is the ability for high-bandwidth modulation of the pump power via direct current modulation. After this experiment, we studied the modulation capabilities and noise properties of green pump laser diodes with improved driver electronics. The current-to-output-power modulation transfer function shows a bandwidth larger than 1 MHz, which should be sufficient to fully exploit the modulation bandwidth of the Ti:sapphire gain for CEO stabilization in future experiments.

Towards an analytical framework for tailoring supercontinuum generation.

A fully analytical toolbox for supercontinuum generation relying on scenarios without pulse splitting is presented. Furthermore, starting from the new insights provided by this formalism about the physical nature of direct and cascaded dispersive wave emission, a unified description of this radiation in both normal and anomalous dispersion regimes is derived. Previously unidentified physics of broadband spectra reported in earlier works is successfully explained on this basis. Finally, a foundry-compatible few-millimeters-long silicon waveguide allowing octave-spanning supercontinuum generation pumped at telecom wavelengths in the normal dispersion regime is designed, hence showcasing the potential of this new analytical approach.

Octave-spanning supercontinuum generation in highly nonlinear silica fibres based on cost-effective fibre amplifiers

We report a simple method for supercontinuum generation. The set-up consists of an Er-doped mode-locked fibre laser, used as seed, and a highly nonlinear fibre with zero dispersion at 1550 nm. Thus, all the components are easily attainable. With this novel system the requirements in terms of control of dispersion are reduced. In addition, the spectral width is optimized using fibres with positive and negative dispersion. The supercontinuum emission is characterized by means of an optical spectrum analyser and a PbS photodetector, showing an octave-spanning spectral width, with a flat profile from 1100 nm up to 2100 nm. Compared to other supercontinuum sources, this new proposal results in a very competitive and attainable system, particularly in the 1500–2100 nm region.

Octave-spanning supercontinuum generation in hybrid silver metaphosphate/silica step-index fibers

We reveal the potential of step-index fibers consisting of a metaphosphate glass core and a silica cladding as an ultrafast octave-spanning supercontinuum source. The hybrid waveguide was fabricated by pressure-assisted melt filling and possesses a sophisticated dispersion behavior with two zero-dispersion points in the proximity of the Erbium laser bands. The fiber generates an octave-spanning supercontinuum from 0.7 to 2.4 μm if pumped at 1.56 μm with 30 fs pulses and energies as low as 300 pJ. Numerical simulations reveal soliton fission and double dispersive wave generation as the dominant broadening effect. This study highlights phosphate glasses as a promising new candidate for the next generation of broadband photonic devices, as they allow for high rare earth-doping levels and dispersion posttuning via plasmonic nanoparticle growth.