Switchable Single- to Multiwavelength Conventional Soliton and Bound-State Soliton Generated from a NbTe2 Saturable Absorber-Based Passive Mode-Locked Erbium-Doped Fiber Laser.

As a member of transition metal dichalcogenides (TMDs), NbTe2 has a work function of 5.32 eV and a band gap of 0 eV at the Fermi level, which enables it to possess broadband absorption characteristics and has huge potential in optoelectronic devices. In this work, a combination of liquid phase exfoliation (LPE) and optical deposition methods (ODMs) were used to fabricate a NbTe2 saturable absorber (SA). Based on the NbTe2 SA, a ring passive mode-locked erbium-doped fiber laser (PML-EDFL) was constructed by adding NbTe2 SA into the laser cavity. A switchable single- to multiwavelength (dual/triple/quadruple) conventional soliton (CS) and a bound-state soliton (BS) were observed for the first time. The results reveal that NbTe2 SA has excellent saturable absorption characteristics (modulation depth of 2.6%, saturation intensity of 177.4 MW/cm2, and unsaturated loss of 63.8%) and can suppress mode competition and stabilize multiwavelength oscillation. This study expands the applications of NbTe2 nanosheets in ultrafast optoelectronics. The proposed switchable PML-EDFL has extensive applications in high-capacity all-optical communication, high-sensitivity optical fiber sensing, high-precision spectral measurements, and high-energy-efficiency photon neural networks.

Neuromorphic Photonics Based on Phase Change Materials.

Neuromorphic photonics devices based on phase change materials (PCMs) and silicon photonics technology have emerged as promising solutions for addressing the limitations of traditional spiking neural networks in terms of scalability, response delay, and energy consumption. In this review, we provide a comprehensive analysis of various PCMs used in neuromorphic devices, comparing their optical properties and discussing their applications. We explore materials such as GST (Ge2Sb2Te5), GeTe-Sb2Te3, GSST (Ge2Sb2Se4Te1), Sb2S3/Sb2Se3, Sc0.2Sb2Te3 (SST), and In2Se3, highlighting their advantages and challenges in terms of erasure power consumption, response rate, material lifetime, and on-chip insertion loss. By investigating the integration of different PCMs with silicon-based optoelectronics, this review aims to identify potential breakthroughs in computational performance and scalability of photonic spiking neural networks. Further research and development are essential to optimize these materials and overcome their limitations, paving the way for more efficient and high-performance photonic neuromorphic devices in artificial intelligence and high-performance computing applications.

Chromium oxide film for Q-switched and mode-locked pulse generation.

Chromium oxide (Cr2O3) is a promising material used in the applications such as photoelectrochemical devices, photocatalysis, magnetic random access memory, and gas sensors. But, its nonlinear optical characteristics and applications in ultrafast optics have not been studied yet. This study prepares a microfiber decorated with a Cr2O3 film via magnetron sputtering deposition and examines its nonlinear optical characteristics. The modulation depth and saturation intensity of this device are determined as 12.52% and 0.0176 MW/cm2. Meanwhile, the Cr2O3-microfiber is applied as a saturable absorber in an Er-doped fiber laser, and stable Q-switching and mode-locking laser pulses are successfully generated. In the Q-switched working state, the highest output power and shortest pulse width are measured as 12.8 mW and 1.385 µs, respectively. The pulse duration of this mode-locked fiber laser is as short as 334 fs, and its signal-to-noise ratio is 65 dB. As far as we know, this is the first illustration of using Cr2O3 in ultrafast photonics. The results confirm that Cr2O3 is a promising saturable absorber material and significantly extend the scope of saturable absorber materials for innovative fiber laser technologies.

Flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a fiber Lyot filter.

In this paper, we report a flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a novel fiber optic Lyot filter. The wavelength, pulse duration and spectral bandwidth of passive mode-locked lasers can be tuned by controlling the polarization controller. The proposed Lyot filter was constructed by a single-mode fiber insertion between two polarization-maintaining fibers. The filter bandwidth and laser output tunability were based on the birefringence characteristics of the polarization-maintaining fibers. This all-fiber laser is simple and stable and can be used for various applications where width-tunable or wavelength-tunable pulses are necessary.

On-chip all-optical quantizer based on the cross polarization modulation effect in a double-ridge a-Si:H-Si3N4 waveguide.

The on-chip all-optical quantizer is a key functional module to realize all-optical analog-to-digital conversion. It has important applications in future optical communication and ultrafast all-optical signal processing systems. To the best of our knowledge, a novel all-optical quantization scheme based on the cross-polarization modulation effect in a double-ridge a-Si:H-Si3N4 waveguide is proposed. For the optimized waveguide, the maximum nonlinear coefficient and transmission loss at the probe wavelength are 2680.5m-1W-1 and 2.246 dB/cm, respectively. The simulation results of the proposed all-optical quantizer show that a nonlinear phase shift of 8 can be generated in the 8.79 mm long waveguide when the peak pump light power is 0.8 W. A 16-level quantization and a 4-bit quantization resolution are obtained. The effective number of bits is 3.79-bit, which is only 0.21-bit away from the ideal resolution.

SnS nanosheets for harmonic pulses generation in near infrared region.

Two-dimensional materials have attracted increasing attention because of their excellent mechanical, thermodynamic, magnetic, electrical and optical properties. Here, a new two-dimensional material of tin sulfide (SnS) is experimentally prepared. It is layered like black phosphorus and owns distinct optoelectronic properties, but eliminates the disadvantage of instability. The nonlinear saturable absorption characteristics of the SnS nanosheets is investigated at 1563.3 nm by the double-balanced detection method. The obtained modulation depth and saturation intensity are 5.4% and 66.3 MW/cm2, respectively. A passively harmonic mode-locked erbium-doped fiber laser based on the SnS saturable absorber (SA) has been demonstrated. The results show that mode-locking with fundamental frequency of 5.47 MHz is realized at pump power of 28.38 mW. With the increase of pump power, the laser can operate from fundamental frequency to high-order harmonic mode-locking. The maximum repetition rate of 412.73 MHz has been obtained, which is equivalent to the 76th harmonic mode-locking. This work reveals that SnS nanosheets is a novel and efficient SA with high damage threshold, which will find potential applications in optical communication, photoelectric detection, laser medicine, etc.

Sheet-structured bismuthene for near-infrared dual-wavelength harmonic mode-locking.

Bismuthene with a similar layered lattice structure belonging to group VA is regarded as a kind of novel two-dimensional material and has excellent properties such as small indirect bandgap (less than 1 eV) and unique electronic properties, etc. Based on the large magnitude of third-order nonlinear susceptibility and high carrier motility, bismuthene can be considered as a promising material for various optoelectronics, electronics, and nonlinear optics. Compared with the mass research about the few-layer bismuthene, we focus on the characteristics and nonlinear optical properties of bismuthene nanosheets in this work. Bismuthene nanosheets present high modulation depth over 7.7%. The sheet-structured bismuthene as saturable absorbers (SAs) is a technically important issue in laser technology. Here, for the first time, it is demonstrated that bismuthene nanosheets can be served as an SA to readily generate a harmonic dual-wavelength mode-locked picosecond pulse in a highly nonlinear fiber laser. A harmonic mode-locked pulse order from 1st to 20th is obtained at the pump power from 43.2 to 201.5 mW. When the pump power is greater than 408 mW, a 52th harmonic dual-wavelength pulse (corresponding to the repetition of 208 MHz) has been obtained. This study demonstrates the bismuthene saturable absorption is an intrinsic property independent from the structural dimension. Our work attests the promise of bismuthene in optical communication, optical detecting, sensor systems, and material processing, etc.

Slot-slot waveguide with negative large and flat dispersion covering C+L+U waveband for on-chip photonic networks.

A novel, to the best of our knowledge, dual-core slot-slot waveguide with extreme high dispersion is proposed. The high dispersion value at the desired wavelength is obtained based on strong resonance coupling between two slot-waveguide modes. The properties of dispersion magnitude and bandwidth are numerically analyzed by using the finite-difference time-domain method with a perfectly matched layer boundary. All numerical simulation results reveal that for the optimized geometrical parameters of H1=350 nm, L1=569 nm, S1=31.3 nm, La=1062.39 nm, H2=427 nm, L2=137.4 nm, and S2=63.5 nm, the maximum dispersion of negative 3.645×105 ps·nm-1·km-1 and dispersion full width at half-maximum of 6.3 nm at 1550 nm have been obtained. By cascading the slot-slot waveguides with varying width and height, a large and flattened dispersion of -3.5×105 ps·nm-1·km-1 covering the C+L+U waveband is obtained. Dispersion compensation of a 100 Gbit/s return-to-zero on-off-keying optical time-division multiplexing signal after 50 km full spectrum single-mode optical fiber transmission with five different central wavelengths is demonstrated through simulation for the first time. In addition, fabrication tolerance of the proposed slot-slot waveguide is analyzed. Such a waveguide is compatible with complementary metal-oxide-semiconductor technology and has potential applications in next-generation large-scale photonic integrated circuits.

Cu2S nanosheets for ultrashort pulse generation in the near-infrared region.

2D metal chalcogenide materials have received enormous attention due to their extraordinary bio-chemical, electronic, magnetic, thermal and optical properties. Compared with the typical two-dimensional transition metal dichalcogenides (TMDs) and topological insulators, cuprous sulfide (Cu2S) has very different two-dimensional lattice structures, along with excellent electro-catalysis and high conductivity. However, the nonlinear optical properties of Cu2S have never been studied until now. Here, the nonlinear photonics characteristics of Cu2S and its application in ultrafast lasers have been systematically studied for the first time. Through optical deposition of Cu2S nanosheets on a tapered fiber, the nonlinear optical properties of Cu2S nanosheets are measured through the interaction with the evanescent field. The results indicate that superior nonlinear saturable absorption properties with a modulation depth of 0.51% are achieved. An erbium-doped fiber (EDF) laser is constructed to verify the performance of the Cu2S saturable absorber (SA). The results show that an output pulse with 8.06 MHz repetition rate, 1.04 ps pulse duration, 1530.4 nm central wavelength and 3.1 nm spectral width without an obvious Kelly sideband is obtained. Considering the diversity of the metal chalcogenide family, various engineering applications may be developed from the nonlinear saturable absorption characteristics of Cu2S, including optical fiber communication/sensing, precision optical metrology, material processing and nonlinear optics.

M2 factor of controllable dark-hollow beams through a multi-apertured ABCD optical system.

Under an optical system with multiple hard-edged apertures in a cylindrical coordinate system, the recurrence propagation expression is derived for the controllable dark-hollow beams (CDHBs) by expanding the hard-aperture function into a finite sum of complex Gaussian functions. Given the recurrence propagation expression, we deduce the approximate analytical expressions of the beam propagation factor M2 in terms of the generalized truncated second-order moments. This provides a fast algorithm for the evaluation of the beam propagation quality for CDHBs through complicated optical trains with a series of apertures. The propagation of CDHBs through a two-aperture-lens ABCD optical system serves as the special case of multi-apertured ABCD optical systems. Our numerical results suggest that a one-aperture-lens optical system reduces the beam propagation quality of CDHBs, and a two-aperture-lens optical system improves the beam propagation quality of CDHBs by selecting appropriate beam parameters and aperture parameters.

Interactions between self-accelerating beams in photorefractive media.

We investigate interactions between in-phase or out-of-phase Airy beams and nonlinear accelerating beams in photorefractive crystals by means of direct numerical simulations. Since self-accelerating beams possess infinite energy, we evaluated the propagation of two shifted counter beams by truncating the oscillating tails. The numerical results show that the external bias electric field has a profound effect on the propagation dynamics and the solitons shed from the Airy beam. We also found that during interactions between two nonlinear accelerating beams, single solitons, soliton pairs, and bound breathing solitons were produced. When the nonlinear accelerating beam collides with a soliton beam, the main lobe continues accelerating while the soliton intensity fluctuates with the lobes.

All-optical 1-to-8 wavelength multicasting at 20 Gbit/s exploiting self-phase modulation in dispersion flattened highly nonlinear photonic crystal fiber.

All-optical multicasting of performing data routing from single node to multiple destinations in the optical domain is promising for next generation ultrahigh-peed photonic networks. Based on the self-phase modulation in dispersion flattened highly nonlinear photonic crystal fiber and followed spectral filtering, simultaneous 1-to-8 all-optical wavelength multicasting return-to-zero (RZ) signal at 20 Gbit/s with 100 GHz channel spaced is achieved. Wavelength tunable range and dynamic characteristic of proposed wavelength multicasting scheme is further investigated. The results show our designed scheme achieve operation wavelength range of 25 nm, OSNR of 32.01 dB and Q factor of 12.8. Moreover, the scheme has simple structure as well as high tolerance to signal power fluctuation.

[Study on spectral gain characterization of FWM processes with multi-frequency pumps in photonic crystal fiber].

Spectral gain induced by four-wave-mixing with multi-frequency pump was investigated by exploiting the data signal and continue lights co-propagation in dispersion flattened high nonlinear photonic crystal fiber (PCF). The effects of wavelength drift of pump lights, polarization state of orthogonal or parallel of pump lights, polarization mismatch of signal light versus orthogonal pump lights, total power of signal and probe light on the spectrum gain were analyzed. The results show that good FWM gain effects with multi-frequency pump can be obtained in 36.4 nm wavelength range when power ratio of pump to probe light is appropriate and with identical polarization. Furthermore, the gain of FWM with multi-frequency pump is very sensitive to polarization fluctuation and the different idle waves obtain different gain with the variation in signal polarization state. Moreover, the impact of pump numbers was investigated. The obtained results would be helpful for further research on ultrahigh-speed all optical signal processing devices exploiting the FWM with multi-frequency pump in PCF for future photonics network.

[Study on spectral broadened characterization of cross phase modulation in photonic crystal fiber].

Spectrum broadening induced by cross phase modulation (XPM) was investigated by exploiting the optical time-division multiplexing (OTDM) data signal and continue wave probe light co-propagation in dispersion flattened high nonlinear photonic crystal fiber (PCF). The effects of wavelength drift of probe lights, polarization mismatch, total power and power ration of pump and probe light on the spectrum broadening were analyzed. The results show that good XPM effects can be obtained in 36 nm wavelength range when the total power is higher than 23 dB, power ration of pump and probe light is appropriate and with identical polarization. Furthermore, polarization independent XPM effect can be achieved by using the remainder birefringence of the PCF with the pump state of polarization (SOP) aligned at 45 degrees to the PCF principal axes. The obtained results in this paper would be helpful for research on ultrahigh-speed all optical signal processing devices exploiting the XPM in PCF for future photonics network.