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Frequency conversion in nonlinear materials is an extremely useful solution to the generation of new optical frequencies. Often, it is the only viable solution to realize light sources highly relevant for applications in science and industry. In particular, supercontinuum generation in waveguides, defined as the extreme spectral broadening of an input pulsed laser light, is a powerful technique to bridge distant spectral regions based on single-pass geometry, without requiring additional seed lasers or temporal synchronization. Owing to the influence of dispersion on the nonlinear broadening physics, supercontinuum generation had its breakthrough with the advent of photonic crystal fibers, which permitted an advanced control of light confinement, thereby greatly improving our understanding of the underlying phenomena responsible for supercontinuum generation. More recently, maturing in fabrication of photonic integrated waveguides has resulted in access to supercontinuum generation platforms benefiting from precise lithographic control of dispersion, high yield, compact footprint, and improved power consumption. This Review aims to present a comprehensive overview of supercontinuum generation in chip-based platforms, from underlying physics mechanisms up to the most recent and significant demonstrations. The diversity of integrated material platforms, as well as specific features of waveguides, is opening new opportunities, as will be discussed here.

In this work, ultrashort pulse generation based on optical fiber coupled distributed feedback (DFB) laser diode and pulse regeneration technique is presented. Two-stage Mamyshev filtering, double-pass amplification in the first stage, and a tunable filter are used to achieve output pulses of 3.6 ps duration from the gain switched DFB diode with no additional pulse compression at the source output. Optimization of spectral broadening and isolation of initial pulse core components are achieved by a tunable spectral filter within a fiber. The resulting pulses are characterized by highly sensitive frequency resolved optical gating (FROG). The concept is compatible with today’s requirements for pulse on demand operation as it enable precise synchronization with external systems. Further, its architecture offers mitigation of stimulated Raman scattering (SRS), which usually represents one of the limiting factors for potential applications of such systems. The spectrally broadened output up to 2 nm can be used with well-established chirped pulse amplification systems.

We demonstrate a hybrid-structure fiber laser composed of single-mode fiber (SMF) and four-mode fiber (FMF), capable of delivering stable near-chirp-free soliton with the bandwidth of 75 pm. Ascribed to the spectral-domain intermodal interference, the SMF-FMF cavity serves not only as a ring resonator but also as an intrinsic spectral filter. The wavelength of the narrowband soliton is tunable within 10 nm by a polarization controller. Simulation results validate the experimental observations, revealing that the filtering effect simultaneously balances the self-phase modulation induced spectral broadening and saturable absorption induced temporal compression. The narrowband fiber laser is attractive for spectroscopy, quantum optics, and material processing, and offers a promising platform to study the evolution dynamics of few-mode solitons.

We theoretically investigate and optimize the performance of four-wave mixing (FWM) in microring resonators (MRRs) integrated with two-dimensional (2D) layered graphene oxide (GO) films. Owing to the interaction between the MRRs and the highly nonlinear GO films as well as to the resonant enhancement effect, the FWM efficiency in GO-coated MRRs can be significantly improved. Based on previous experiments, we perform detailed analysis for the influence of the GO film parameters and MRR coupling strength on the FWM conversion efficiency (CE) of the hybrid MRRs. By optimizing the device parameters to balance the trade-off between the Kerr nonlinearity and loss, we achieve a high CE enhancement of ∼18.6 dB relative to the uncoated MRR, which is ∼8.3 dB higher than previous experimental results. The influence of photo-thermal changes in the GO films as well as variations in the MRR parameters such as the ring radius and waveguide dispersion on the FWM performance is also discussed. These results highlight the significantly improved FWM performance that can be achieved in MRRs incorporating GO films and provide a guide for optimizing their FWM performance.

We demonstrate the damage-free delivery of nearly diffraction-limited picosecond laser pulses at 1064 nm with a maximum peak power of 3.5 MW in a 5 m air-filled anti-resonant hollow-core fiber (AR-HCF). In the air-filled AR-HCF, the transmission efficiency of picosecond pulses is degraded due to stimulated Raman scattering for the incident peak power above 3.59 MW. The temporal compression of pulses is also observed in the air-filled AR-HCF, where the self-phase modulation plays a key role in the anomalous dispersion region. By vacuuming the air in the core, a nearly constant coupling efficiency of 77% is achieved through the 5 m AR-HCF free of nonlinear effects, with M2 of the output beam less than 1.17.

A tapered Er-doped fiber amplifier for high peak power pulses amplification has been developed and tested. The core diameter changed from 15.8 µm (mode field diameter (MFD) 14.5 µm) to 93 µm (MFD 40 µm) along 3.7 m maintaining single-mode performance at 1555 nm (according to the S2-method, the part of the power of high-order modes does not exceed 1.5%). The amplification of 0.9 ns pulses with spectral width below 0.04 nm up to a peak power above 200 kW (limited by self-phase modulation) with a slope pump-to-signal conversion efficiency of 15.6% was demonstrated.

The generation of high-order harmonics in a semi-infinite cell by femtosecond laser pulses is a common practice for reliable coherent and low divergence XUV source beams for applications. Despite the relative simplicity of the experimental method, several phenomena coexist that affect the generated spectral and divergence characteristics of the high harmonic XUV frequency comb. The ionisation degree of the medium and the consequent plasma formation length imposes a spatiotemporal evolution of the fundamental EM field and XUV absorption. Varying the laser pulse chirp and the focusing conditions, as well as the gas density, we measured intense harmonic spectral and divergence variations attributed mainly to self-phase modulations of the laser EM field in the partially ionised medium. Additionally, low-divergence high harmonics are observed for certain laser chirp values attributed to the strong phase matching of only the short electron quantum path. Thus, a tunable, low divergent, and coherent XUV source can be realised for spatiotemporal imaging applications in the nanoscale.

Controlling an ultrafast laser oscillator repetition rate is crucial for the emerging laser burst micromachining. The efficiency of a material surface treatment process as well as the quality of live tissue ablation depends, to a considerable extent, on the number of laser pulses in a burst window. In the following article, we present an all-PM fiber harmonically mode-locked Mamyshev ring oscillator capable of producing laser pulses with a tunable period. The oscillator can be operated at any frequency, being a multiple of the fundamental repetition rate up to 305 MHz at 19th harmonic. We found that the operation frequency is limited by the available pump power only. What is worth noting is that the energy of the emitted pulses is independent of the operating frequency. The essential laser parameters are excellent: an amplitude noise of 0.1% and fundamental frequency suppression levels of 70 dB at every repetition rate. Additionally, the mechanism of harmonic mode-locking is discussed in detail and is found to be caused by gain depletion and recovery effect.

We achieve a broadband near-zero dispersion in a thin silicon nitride waveguide array at near-infrared utilizing the multiple mode couplings. Different orders of modes are coupled at four different wavelengths, generating eight zero dispersion wavelengths (ZDWs). The broadband near-zero dispersion profile within 90 ps/nm/km is achieved, spanning over the spectral range of 1350 $-$ 1900 nm. Using the dispersion engineered waveguide array, an octave-spanning supercontinuum is numerically generated with a pump of 100 fs and 200 pJ. The concept of using multiple mode couplings to obtain broadband near-zero dispersion can be applied to other spectral regimes and material platforms as well, by engineering the structural parameters to shift the positions of the ZDWs.

Purpose: In this paper, optical solitons of higher-order nonlinear Schrödinger equation with Kudryashov's sextic power-law of nonlinear refractive index are investigated via the direct mapping method. The considered model identifies the optical soliton pulse propagation in the optical fibers. Deriving the optical solutions of investigated model such as sextic power is critical but difficult work. The primary aim of this paper is to graphically examine the impact of power-law nonlinearity (pLawNL) and chromatic dispersion (CD) parameters clarifying self-phase modulation (SPM) in the equation on soliton behavior as well as obtaining optical soliton solutions. Methodology: To according to the used technique, we first used the complex wave transform to generate the nonlinear ordinary differential equation (NLODE) form of the nonlinear Schrödinger equation (NLSE) with Kudryashov's sextic power-law the nonlinear refractive index (SPLawNRI). Then, we were able to produce a system of linear equations in polynomial form by using the approach. Different solution sets including the values of the parameters of the studied equation and the suggested approach were produced by solving the linear system of equations. Findings: We acquired the optical soliton solutions of the main equation after inserting the sets and wave transformation into the solution functions suggested by the approach. The constraint conditions for the related solutions were suggested. We proved that the gained solutions satisfied the NLSE with Kudryashov's SPLawNRI under the suggested constraint conditions. Originality: We present contour, 3D and 2D depictions in various simulations in figures to comment the obtained solution functions. Besides, we investigate the effects of the power-law nonlinearity parameter that expresses SPM in the main equation and the parameter that are the group velocity dispersion (GVD) or chromatic dispersion (CD) on soliton behavior. The results suggest that the utilized approach is efficient, reliable, and powerful to be readily applied to various NLSEs with higher-order or higher pLawNLs that characterize real-life problems.

We propose a practical scheme to demonstrate the combination and subsequent self-similar compression of two pulses with the same or different central wavelengths while propagating through a nonlinear fiber with exponentially decreasing dispersion. To initiate these processes, two raised cosine pulses with the same or different wavelengths is modulated using a phase modulator to acquire the same chirp at the input of the fiber. While propagating through the nonlinear fiber, these chirped pulses first coalesce into a single pulse and during further propagation get compressed into a single ultrashort high-power pulse. The output pulse possesses a large compression factor, high proportion of energy and peak power compared to a single input pulse. We also report the combination and compression of five raised cosine pulses with different wavelengths to achieve an appreciable compression effect, indicating that this system works well even with a small number of input pulses. The proposed scheme provides a simple way to generate high power ultrashort pulse with high energy and good quality in a short length of fiber.

Design and analysis of a silicon waveguide with embedded pin diode are proposed here for the realization of second harmonic generation (SHG). The reverse-biased pin structure is employed to apply a DC field across an intrinsic silicon region which gives rise to electric-field induced (EFI) second-order nonlinearity χ (2)EFI in silicon. The resulted χ (2)EFI is then used to generate a second harmonic (SH) frequency of an input 2.29-μm signal in which second-order mode at 1.145 μm is used to satisfy modal phase matching with fundamental mode at 2.29 μm through suitably designing the silicon waveguide. It is shown that by engineering the geometry of pin diode, not only a high modal overlap integral is yielded by eliminating the counteractive parts of second-order mode at SH wavelength but also the required external voltage to induce the possible maximum achievable χ(2)EFI is significantly reduced compared to the previous SHG structure with the EFI nonlinearity. The simulation results in P 2ω / P ω = 6.5% for a pump power of 25 mW and only 7.6 V external voltage which can be improved up to 27% by increasing the input power to 260 mW.

We propose and demonstrate a high fidelity narrowband picosecond pulse two-stage fiber amplification scheme with inter-stage filtering technique using fiber Bragg grating. This technique can not only narrow the bandwidth of the pulses broadened by self phase modulation in the first-stage fiber amplifier, but also reshape the pulse temporal profile to near Gaussian shape, which reduces the spectral broadening slope of the pulses in the second-stage fiber amplifier and improves the narrowband fiber amplification ability. Using this technique, pulse energy of 10 nJ is achieved from a 0.2-nJ initial pulse by only using standard single mode fiber amplifiers while retaining 10 ps pulse width and 0.4 nm root mean square bandwidth. This technique may provide a simple and effective solution for high fidelity fiber amplification of low-energy ps seed pulse from fiber oscillators.

A multipass cell for nonlinear compression to few-cycle pulse duration is introduced composing dielectrically enhanced silver mirrors on silicon substrates. Spectral broadening with 388 W output average power and 776 µJ pulse energy is obtained at 82% cell transmission. A high output beam quality ( M 2 < 1.2 ) and a high spatio-spectral homogeneity (97.5%), as well as the compressibility of the output pulses to 6.9 fs duration, are demonstrated. A finite element analysis reveals scalability of this cell to 2 kW average output power.

We have developed an innovative extrusion with double peeling-off process to fabricate chalcogenide (GhG) preform, with an aim to remove the defects, such as bubbles, oxidation, and particles that always induce large optical losses in ChG glass fibers. As a result, none of the defects is found in the core-cladding interface of the fiber. The loss in a wavelength range of 5.5–8.2 μm is generally lower than 1 dB/m, with a minimum value of 0.2 dB/m at 6.3 μm. Moreover, a flat mid-infrared (MIR) supercontinuum (SC) spectrum spanning from 1.4 μm to 13.7 μm is generated in an 18 cm-long Ge-As-Se ChG fiber, accompanied with a highest spectrum flatten ratio of 72.4%.

We investigate a silicon nitride (Si3N4) horizontal slot waveguide with silicon dioxide (SiO2) cladding, which can achieve a flat and low normal dispersion of −15.56 ps/(nm•km) over a 3270-nm wavelength range from 1170 to 4440 nm, covering a 1.9 octave bandwidth. By launching a 100-fs 90-kW hyperbolic secant input pump pulse, a three-octave supercontinuum (SC) from 504 to 4229 nm can be generated using a 5-mm long waveguide. The generated SC spectrum is highly coherent, as it is mainly expanded by self-phase modulation (SPM) and optical wave breaking (OWB) in all-normal dispersion regime. The degree of coherence can achieve >0.99 from 1250 to 4300 nm. Therefore, the proposed waveguide could potentially serve as a key element in the on-chip broadband source for silicon photonic platform.

Two-stage compression of laser pulses with a power of 250 TW is experimentally realised by broadening their spectrum during self-phase modulation in fused silica and subsequent dispersion compensation upon reflection from chirping mirrors. A five-fold decrease in the duration is demonstrated, from 75 to 15 fs, with a B-integral value of about 5 at each stage. It is possible to avoid small-scale self-focusing due to self-filtering of the laser beam during free propagation in vacuum. With optimal parameters of the dispersive mirror, the pulse can be compressed to a duration of less than 5 fs.

An all-fiber mode-locked laser based on Mamyshev mechanism operating at 1550 nm is demonstrated. Self-phase modulation and offset spectral filtering provide high peak-power pulse selection in the laser cavity and induce mode locking. With selected operating parameters, the all-fiber mode-locked laser easily achieves mode locking without external seed pulses or auxiliary starting arms, making the laser convenient to operate. Pulses with maximum energy ∼18 nJ and width 230 fs are obtained. Pulse width is almost unchanged with increasing pump power, while pulse energy increases linearly. Starting of the mode-locked laser based on Mamyshev mechanism and output spectra with different pump powers are experimentally investigated.

Two-dimensional layered materials (2DLM) have become the subject of intensive research in various applications such as electronics, photonics and optoelectronics due to their unique physical properties. As a new class of 2DLM, MXenes have attracted great interest due to their superior performance in a wide variety of applications such as batteries, supercapacitors, catalysts, electronics and optics. Here, we have investigated the broadband spatial self-phase modulation (SSPM) and ultrafast response of the MXene Ti3C2Tx (T=O, OH or F) experimentally. The MXene Ti3C2Tx exhibited the broadband nonlinear optical response via SSPM from 400 nm to ~1 μm under the ultrafast laser excitation, and ultrafast carrier characteristics with an ultrafast recovery time with femtosecond transient absorption spectroscopy. The experimental results have shown that the MXenes have the broadband nonlinear optical response, which can lay a foundation for the application prospect for the MXene-based ultrafast optoelectronic devices.

We propose an integrated photonic circuit that acts as an optical coherent Ising machine and simulates its performance on the basis of some example problems. In contrast to previous all-optical approaches, the proposed integrated Ising machine does not require an optical parametric oscillator and can, hence, operate at a single wavelength, reducing the overall design complexity. We present a symmetric nonlinear photonic device as the fundamental building block that can use self-phase modulation in two microring resonators to emulate a continuously tunable, symmetrically bistable Ising node. We derive and verify using numerical simulation the key properties of the single Ising node device and the full Ising machine circuit. We estimate the full Ising machine's tolerance to realistic fabrication errors on the basis of randomly sampled example problems, and we discuss which technologies are required to obtain large-scale systems.

Bismuth sulfide (Bi2S3) is a binary chalcogenide semiconductor compound that has received much attention in optoelectronic devices because of its stratified structure. In this work we showed that the two-dimensional (2D) Bi2S3 shows strong nonlinearity using spatial self-phase modulation and that the all-optical photonic devices, e.g. the all-optical switches and all-optical diodes, have been demonstrated experimentally by observing the nonlinear behavior of the diffraction rings. In addition, an all-optical diode is designed in this work using combined structure with 2D Bi2S3/SnS2 nanosheet by taking advantage of the reverse saturated absorption of 2D SnS2 and saturated absorption of 2D Bi2S3. Nonreciprocal light propagation has been achieved with different incident wavelength and a variety of incident intensities. Those characteristics make 2D Bi2S3 a potential candidate for the next generation nonreciprocal all-optical device.

We propose a wideband chaos generation scheme that shows significant bandwidth enhancement and excellent time delay signature (TDS) suppression, by introducing self-phase modulation in the feedback loop of external-cavity semiconductor laser (ECSL) and passing the ECSL-generated chaos through a dispersive component. Proof-of-concept experiment and thorough complementary numerical simulations are carried out to study the efficient bandwidth and TDS characteristics of the generated chaos. The results demonstrate that with the joint contributions of self-phase modulation and dispersion, the efficient bandwidth of chaos can be enhanced by several times and the spectrum gets much flatter. Simultaneously, the TDS can be completely suppressed, in a wide operation parameter range. The proposed wideband chaos generation scheme shows great potential for the applications of fast random bit generation, high-resolution chaotic radar, and high-speed chaos communication.

We evaluate the threshold power for self-focusing in gold nanorod colloids of varying concentration by a power limiting method in the femtosecond filamentation regime. The pulses are tuned near the longitudinal plasmon peak of the nanorods, leading to saturation of linear absorption and reshaping of the particles. We evaluated the last two effects by optical transmission measurements and spectroscopic analysis and estimated that considerable particle deformation does not occur before the collapse of the beam. We performed numerical simulations based on the experimental results, and evaluated only a subtle, monotonically increasing enhancement of the nonlinear refractive index of the host material (water) as the nanoparticles concentration increases. The role of higher-order contributions is discussed. Our work provides an alternative characterization approach of ultrafast nonlinearities in absorbing media. It further emphasizes that self-focusing of intense femtosecond pulses in gold nanocomposites is hampered by the ultrafast modulation of the susceptibility of the metal.

SPM-enabled spectral selection (SESS) constitutes a powerful fiber-optic technique to generate wavelength broadly tunable femtosecond pulses. In the current demonstration, the maximum tuning range is 400 nm and the energy conversion efficiency from the pump source to the outmost spectral lobes is ∼25%. In this submission, we apply the particle swarm optimization method to the generalized nonlinear Schrödinger equation to identify the optimal parameters that maximize both the tuning range and the conversion efficiency. We show that SESS in an optical fiber with the optimized dispersion can deliver SESS pulses tunable in one octave wavelength range and the conversion efficiency can be as high as 80%. We further show the feasibility of experimental implementation based on specially designed fibers or on-chip waveguides.

In this paper, we report that the conversion efficiency and spectrum of femtosecond optical parametric amplification (fs-OPA) can be significantly enhanced by employing a compact cascaded femtosecond OPA (CF-OPA) scheme with the self-compensation of the temporal walk-off between two nonlinear gain media. Correspondingly, the gain related temporal contrast can also be improved. The feasibility of the CF-OPA method using three cascaded BBO crystals is numerically and experimentally analyzed. Moreover, by replacing the conventional fs-OPA with the CF-OPA and optimizing the design, the performance of a nonlinear temporal filter combining cross-polarized wave generation and fs-OPA is comprehensively improved. The experimental results demonstrate the superiority of the CF-OPA scheme, which can generate high-performance cleaned pulses at 1 kHz repetition rate with energy of 340μJ, energy fluctuation below 0.9% (RMS), spectral width of 97 nm (FWHM), Fourier-transform-limited pulse width of 12 fs and temporal contrast better than 10−12. To the best of our knowledge, this is the first reported temporal walk-off self-compensated quasi-collinear CF-OPA geometry adopting three cascaded BBO crystals, which can be easily generalized to other wavelengths or nonlinear crystals. The above nonlinear temporal filter with a CF-OPA scheme has the rarest comprehensive parameters, which can provide excellent seed pulses for PW and 10 PW class femtosecond laser systems.

We showed that the nonlinear Mach–Zehnder interferometer may be used not only for enhancing temporal contrast, as proposed earlier, but also for increasing pulse power due to efficient pulse compression. The interferometer introduces into the output pulse a nonlinear phase equal to π/2. This allows increasing laser power by a factor of 1.5 only by means of a chirped mirror. Use of an additional nonlinear plate leads to a multi-fold power increase retaining the contrast enhancement.

The analytical propagation formulae of twisted Gaussian Schell-model (TGSM) beams through nonlinear Kerr media are derived. It is found that a TGSM beam is less sensitive to Kerr nonlinearity than a Gaussian Schell-model (GSM) beam. Furthermore, the propagation characteristics of TGSM beams with stronger twist and worse spatial coherence are less affected by Kerr nonlinearity. The self-focusing effect enhances the beam twist, but degrades the beam spatial coherence. In the atmosphere (one kind of self-focusing media), a TGSM beam has greater resistance to self-focusing effects and atmospheric turbulence effects than a GSM beam or an ideal Gaussian beam.

In this paper, we study the limitations of decreasing the repetition rate for the narrowband dissipative soliton picosecond (ps) pulsed Figure-9 fiber laser with periodically saturable absorber (SA), and demonstrate how to decrease the repetition rate of this kind of fiber laser. By asymmetrically increasing the passive fiber length of nonlinear amplifying loop mirror (NALM) to lower SA saturation power, Q-switching instability can be avoided, thus effectively reducing the repetition rate of ps pulses. To combat noise-like pulse caused by excessive reduction of SA saturation power, we invoke the non-reciprocal output characteristics of periodic SA, and combined with increasing the intracavity fiber length outside the SA, we further reduce the laser repetition rate. Repetition rates for ∼10 and ∼20 ps pulses are reduced to 1.7 MHz and 848 kHz, respectively, which are, to the best of our knowledge, the lowest repetition rates of Figure-9 lasers reported thus far.

We have experimentally demonstrated the post-compression of a long-wave infrared (9.2 μm) 150 GW peak power pulse from 2 ps to less than 500 fs using a sequence of two bulk materials with negative group velocity dispersion (GVD). The compression resulted in up to 1.6-fold increase of the peak power and up to 2.8-fold increase of the intensity in the center of a quasi-Gaussian beam. The partial decoupling of the self-phase modulation and chirp compensation stages by using two materials with significantly different ratios of nonlinear refractive index to GVD provides accurate optimization of the compression mechanism and promises a viable path to scaling peak powers to supra-terawatt levels. During the preparatory study, we measured, for the first time to our knowledge, the nonlinear refractive indices of NaCl, KCl, and BaF2 for picosecond pulses in the long-wave infrared region.

We present a novel optical signal processing scheme for enhancing the minimum detectable environmental perturbation of intensity-based fiber sensors. The light intensity is first stabilized by inducing a sinusoidal intensity modulation and extracting the first-order sideband generated by self-phase modulation (SPM) in a nonlinear medium. The light with stabilized intensity is then sent through a sensor and the sensor induced power variation is magnified by first inducing a sinusoidal intensity modulation, then undergoing SPM, and finally extracting a higher-order sideband. The advantage of the proposed stabilization-magnification (SM) sensing scheme is experimentally demonstrated by applying a damped vibration on an intensity-based fiber sensor and comparing the minimum detectable strain value of the proposed scheme with that of a conventional sensing scheme. Experimental results demonstrate minimum detectable strain improvement by a factor of 3.93. This new SM sensing scheme allows for the detection of perturbations originally too weak to be detected by a given intensity-based fiber sensor, which will be beneficial for a variety of applications such as high frequency ultra-sound detection.

We theoretically investigate the wavelength-dependent pulse self-compression dynamics of intense femtosecond laser pulses in gas-filled capillaries. Simulations with λ = 1, 2, 3 and 4 µm using the multimode carrier-resolved unidirectional pulse propagation equation reveal pulse self-compression or pulse broadening depending on plasma and modal dispersion. Our study shows that the pulse at 1 µm exhibits better pulse self-compression compared with longer wavelengths due to smaller group velocity mismatch between fundamental and higher-order capillary modes.

A few-cycle mid-infrared (MIR) laser is demonstrated via nonlinear self-compression in solid thin plates. In this novel solution, the anomalous material dispersion in the MIR band and the chirp induced by self-phase modulation are mutually compensated, which can achieve self-compression. Finally, with the 4 µm laser injection with 4.8 mJ/155 fs and few-cycle pulses with 3.44 mJ, 29.4 fs are generated with a high efficiency of 71.7%, and the system maintains very good spectral stability in 10 days. Compared with other post-compression methods, this self-compression technique has the advantages of high efficiency and robust and large energy expansion scale, which can be further extended to MIR lasers with other wavelengths and higher peak power.

A proposal for additional temporal compression and peak power enhancement of intense ( > T W / c m 2 ) femtosecond laser pulses using two thin plane-parallel plates is presented. The first ultrathin plate (order of mm) induces spectral broadening due to self-phase modulation, and the second ultrathin plate (order of micron) corrects the spectral phase. The elimination of the negative dispersive multilayer coating from the scheme offers an improved laser-induced damage threshold for the post-compression process.

The dynamics and stability of continuous-wave and multi-pulse structures are studied theoretically, for a generalized model of passively mode-locked fiber laser with an arbitrary nonlinearity. The model is characterized by a complex Ginzburg-Landau equation with saturable nonlinearity of a general form ($I^m/(1+\Gamma I)^n$), where $I$ is the field intensity, $m$ and $n$ are two positive real numbers and $\Gamma$ is the optical field saturation power. The analysis of fixed-point solutions of the governing equations, reveals an interesting loci of singular points in the amplitude-frequency plane consisting of zero, one or two fixed points depending upon the values of $m$ and $n$. The stability of continuous waves is analyzed within the framework of the modulational-instability theory, results demonstrate a bifurcation in the continuous-wave amplitude growth rate and propagation constant characteristic of multi-periodic wave structures. In the full nonlinear regime these multi-periodic wave structures turn out to be multi-pulse trains, unveiled via numerical simulations of the model nonlinear equation the rich variety of which is highlighted by considering different combinations of values for the pair ($m$,$n$). Results are consistent with previous analyses of the dynamics of multi-pulse structures in several contexts of passively mode-locked lasers with saturable absorber, as well as with predictions about the existence of multi-pulse structures and bound-state solitons in optical fibers with strong optical nonlinearity such as cubic-quintic and saturable nonlinearities., Comment: 13 pages, 7 figures, Feature issue on "Optics in Africa"

Nonlinear propagation of light pulses can excite dispersive waves anchored at frequencies determined by the chromatic dispersion curve. In this work, conditions enabling dispersive-wave self-frequency shift over the propagation distance are analytically derived in the normal dispersion regime. Importantly, this novel, to the best of our knowledge, scenario is not found by solving the complex dynamics of the pulse, but by studying the evolution of the nonlinear and dispersive length scales. This approach allows a simpler, yet consistent and insightful, analysis that may also be very useful in other nonlinear regimes. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We experimentally investigate the tunable Doppler shift in an 80 nm thick indium-tin-oxide (ITO) film at its epsilon-near-zero (ENZ) region. Under strong and pulsed excitation, ITO exhibits a time-varying change in the refractive index. A maximum frequency redshift of 1.8 THz is observed in the reflected light when the pump light has a peak intensity of ∼ 140 G W / c m 2 and a pulse duration of ∼ 580 f s , at an incident angle of 40°. The frequency shift increases with the increase in pump intensity and saturates at the intensity of ∼ 140 G W / c m 2 . When the pump pulse duration increases from ∼ 580 f s to ∼ 1380 f s , the maximum attainable frequency shift decreases from 1.8 THz to 0.7 THz. In addition, the pump energy required to saturate the frequency shift decreases with the increase in pump pulse duration for ∼ x < 1 p s and remains unchanged for ∼ x > 1 p s durations. Tunability exists among the pump pulse energy, duration, and incident angle for the Doppler shift of the ITO-ENZ material, which can be employed to design efficient frequency shifters for telecom applications.

We theoretically investigate methods of controlling pulse generation in normal-dispersion fiber optical parametric chirped-pulse amplifiers. We focus on high-energy, ultrashort pulses at wavelengths widely separated from those of the pump, and find that within this regime, a number of simple properties describe the essential phase and gain dynamics. Of primary importance are the relationships between the chirps of the pump, seed, and parametric gain, which we theoretically predict and then experimentally validate. By properly arranging these parameters, the signal and idler waves can be widely customized to fulfill a remarkable range of application requirements, spanning from narrowband to few-cycle.

Mode-locked mid-infrared (MIR) fiber laser research has been dominated by the generation of pulses in the picosecond regime using saturable absorbers (SAs) and more recently frequency shifted feedback (FSF). Despite the significant emphasis placed on the development of materials to serve as the SAs for the MIR, published pulse durations have been substantially longer than what has been reported in the near-infrared (NIR). In this report we present experimental data supporting the view that the majority of demonstrations involving SAs and FSF have been limited by the presence of molecular gas absorption in the free-space sections of their cavities. We show that the pulse duration is directly linked to the width of an absorption-free region of the gaseous absorption profile and that the resulting optical spectrum is nearly always bounded by strong absorption features.

This work outlined the integrated role between vertical cavity surface emitting laser diodes with Gaussian pulse generators and Ideal erbium doped fiber amplifier (EDFA) for the self-phase modulation instability management, as well as the signal/NP level (Power level [PL]) with the spectral frequency is simulated after wavelength division multiplexing (WDM) fiber at length of 10 km based third order Gaussian pulse generators. The light power variations against temperature after WDM fiber is also studied at length of 10 km based various order Gaussian pulse generators. Electrical received power (ERP) variations against temperature is clarified after avalanche photo diode (APD) photodetectors based various order Gaussian pulse generators. The study emphasis the bad effects of the increase of the temperature, and higher Gaussian pulse generators order on the signal power levels.

In this paper, we proposed a highly sensitive temperature sensor based on self-phase modulation (SPM) in an in-house fabricated microstructured optical fiber (MOF) which had three rings of air holes. The temperature sensing performance was evaluated by detecting the 3 dB bandwidth of SPM spectrum with the variation of temperature at different pump wavelengths and average pump power. At the pump wavelength of 1400 nm with the average pump power of 600 mW, the temperature sensitivity was obtained to be as high as 1.296 nm/°C. Moreover, the theoretical simulation was carried out, the results of which corresponded well with the experiment. To the best of our knowledge, this is the first experimental study concerning SPM-based temperature sensing. This work proves experimentally and theoretically a new temperature sensing mechanism drawing on the SPM effect in optical fibers, which is expected to develop temperature sensors of low cost, simple structure and high sensitivity.

Optical frequency comb (OFC) and picosecond pulse generation are demonstrated experimentally based on a directly modulated AlGaInAs/InP square microcavity laser. With the merit of a high electro-optics modulation response of the microcavity laser, power-efficient OFCs with good flatness are produced. Ten 8-GHz-spaced optical tones with power fluctuation less than 3 dB are obtained based on the laser modulated by a sinusoidal signal. Moreover, the comb line number is enhanced to 20 by eliminating the nonlinear dynamics through optical injection locking. Owing to the high coherence of the OFC originating from the directly modulated microcavity laser, a 6.8 ps transform-limited pulse is obtained through dispersion compensation. The optical pulse is further compressed to 1.3 ps through the self-phase modulation effect in high nonlinear fiber.

Period doubling (PD) and the merging of multiple dissipative-soliton-resonance (DSR) pulses are investigated in a fiber laser. Depending on the initial conditions and operation settings, various PDs of multiple DSR pulses are achieved. For two coexisting DSR pulses, we observe PD on both pulses or PD on a single pulse while the other pulse maintains period one. For three coexisting DSR pulses, PD on all pulses or PD on one/two pulse(s) are achieved when the other pulses maintain period one. It is observed that excessively increasing the pump power could destroy the PD behavior. Within certain parameters, PD behavior can be maintained by increasing the pump power. Apart from the appearance of PD, it is found that two DSR pulses may merge into a single DSR pulse if the pulse interval is small enough during pump power increase.

The impact of pump pulse fluctuations on ultrafast nonlinear frequency conversion processes in optical fibers is investigated. A simple measure of the sensitivity of a particular process to pump technical noise is introduced and tested against numerical ensemble simulations for normal-dispersion continuum generation and dispersive-wave generation. The proposed measure is found to adequately describe the impact of technical noise at the per mill or percent level.

Microresonator-based platforms with $\chi^{(2)}$ nonlinearities have the potential to perform frequency conversion at high efficiencies and ultralow powers with small footprints. The standard doctrine for achieving high conversion efficiency in cavity-based devices requires "perfect matching", that is, zero phase mismatch while all relevant frequencies are precisely at a cavity resonance, which is difficult to achieve in integrated platforms due to fabrication errors and limited tunabilities. In this Letter, we show that the violation of perfect matching does not necessitate a reduction in conversion efficiency. On the contrary, in many cases, mismatches should be intentionally introduced to improve the efficiency or tunability of conversion. We identify the universal conditions for maximizing the efficiency of cavity-based frequency conversion and show a straightforward approach to fully compensate for parasitic processes such as thermorefractive and photorefractive effects that, typically, can limit the conversion efficiency. We also show rigorously that these high-efficiency states are stable., Comment: 16 pages, 6 figures

Pure quartic soliton (PQS) is a new class of solitons demonstrated in recent years and provides innovations in nonlinear optics and its applications. Generating PQSs in micro-cavities offers a novel way to achieve coherent microcombs, presenting a promising application potential. Here we numerically investigate the PQS generation in a dispersion-engineered aluminum nitride (AlN) micro-cavity. To support PQS, a well-designed shallow-trench waveguide structure is adopted, which is feasible to be fabricated. The structure exhibits a dominant fourth-order dispersion reaching up to -5.35×10−3 ps4/km. PQSs can be generated in this AlN micro-cavity in the presence of all-order-dispersion and stimulated Raman scattering. Spectral recoil and soliton self-frequency shift are observed in the PQS spectrum. Furthermore, we find that due to the narrow Raman gain spectrum of crystalline AlN, the PQS evolves directly to chaos rather than turning into a breather. The threshold pump power with which the PQS turns into chaos is also theoretically calculated, which squares with the simulation results.

We propose here a new approach for compression and frequency up-conversion of short optical pulses in the regime of extreme nonlinear optics in optically dense absorbing media, providing an alternative route to attosecond-scale pulses at high frequencies. This method is based on dynamics of self-induced transparency (SIT) pulses of nearly single cycle duration, leading to single-cycle-scale Rabi oscillations in the medium. The sub-cycle components of an incident pulse behave as separate SIT-pulses, approaching each other and self-compressing, resulting in the threefold compression in time and frequency up-conversion by the same factor. As we show, the scheme can be cascaded, staying at the subsequent stage with nearly the same compression and up-conversion ratio. In this way, as our simulations show, after only few micrometers of propagation, a 700 nm wavelength single cycle pulse can be compressed to a pulse of 200 attoseconds duration located in XUV frequency range.

Long range phase-sensitive optical time domain reflectometer (ϕ-OTDR) sensing mainly employs distributed amplification in the sensing fiber. It requires the light to be injected into both ends of the sensing fiber, which reduces the degree of freedom in embedding the fiber into structures. To overcome this problem, the key factors that affects the signal-to-noise ratio (SNR) in ϕ-OTDR system based on matched filter is analyzed thoroughly, and a single-ended long-range ϕ-OTDR that does not require distributed amplification is proposed. In this system, two key techniques are adopted for SNR improvement. To boost the pulse energy and suppress the self-phase modulation, the distortion of the amplified pulse is rectified by using iterative predistortion method; to mitigate the influence of the interference fading and the stimulated Brillouin backscattering, a three-carrier pulse is employed. In combination with the non-linear frequency modulation technique which yields a 42.7 dB side lobe suppression ratio, these approaches guarantee an achievable ϕ-OTDR of 80 km sensing range, 2.7 m spatial resolution, 49.6 dB dynamic range in the experiment. To the best of authors’ knowledge, this is the first time that a ϕ-OTDR without optical amplification in the sensing fiber is realized over such a long sensing range.

We report novel functionalities of a very simple, robust, and low-cost optical metrology method for the reconstruction of an intensity profile and phase of pulses in the telecommunication range based on measuring the fundamental spectrum and two self-phase modulated spectra after a nonlinear element with Kerr nonlinearity. We demonstrate, for the first time, to the best of our knowledge, the possibility of implementing this method using a microstructured tellurite fiber (with a zero dispersion wavelength of 1.55 μ m and nonlinear coefficient >500 (W km)−1) as a nonlinear element for characterizing optical pulses with a wide range of parameters: wavelengths from O-band to L-band, durations from

Nonlinear optical properties of soda-lime glasses irradiated by 100 fs laser pulses with intensities ranging from 80 to 182 GW/cm2 and wavelengths from 780 to 870 nm were investigated by the Z-scan technique. Saturated three photon absorption (3PA) was observed and the dependence of the 3PA coefficient on the intensity and the irradiation wavelengths. The measured value of the nonlinear refractive index was found to be intensity dependent and wavelength independent. The estimated nonlinear optical properties were in good agreement with the experimental results. The self-compression of femtosecond laser pulses in a soda-lime glass was evaluated numerically taken into account the interplay between group velocity dispersion (GVD) and self-phase modulation (SPM) in the soda-lime. A numerical simulation of the transmission of a 200 fs laser pulse through soda-lime glass of 1.2 cm thickness showed that the interplay between anomalous GVD and SPM lead to a significant compression of the pulse to 8.5 fs i.e. by a factor of 23.