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Ti2AlC is a kind of ternary layered structured ceramic metal compound, combining the advantages of both ceramic and metal. Herein, the saturable absorption performance of Ti2AlC at the 1-µm wave band is investigated. The Ti2AlC behaves with excellent saturable absorption, which has a modulation depth of 14.53% and a saturable intensity of 13.27 MW/cm2. An all-normal dispersion fiber laser based on the Ti2AlC saturable absorber (SA) is constructed. The repetition frequency of the Q switched pulses increased from 44 to 49 kHz as the pump power rose from 276 to 365 mW, and the corresponding pulse width decreased from 3.64 to 2.42 µs. The maximum output single Q switched pulse energy is as high as 169.8 nJ. Our experiments prove that the MAX phase Ti2AlC has potential as a low-cost, simple preparation, and broadband SA material. To the best of our knowledge, this is the first demonstration of Ti2AlC serving as a SA material achieving Q switched operation at the 1-µm wave band.

In this Letter, we propose and experimentally validate a sparse deep learning method (SDLM) for terahertz indoor wireless-over-fiber by transmitting a 16-quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM) signal over a 15-km single-mode fiber (SMF) and a wireless link distance of 60 cm at 135 GHz through a cost-effective intensity-modulated direct detection (IM-DD) communications system. The proposed SDLM imposes the L1-regularized mechanism on the cost function, which not only improves performance but also reduces complexity when compared with traditional Volterra nonlinear equalizer (VNLE), sparse VNLE, and conventional DLM. Our experimental findings show that the proposed SDLM provides viable options for successfully mitigating nonlinear distortions and outperforms conventional VNLE, conventional DLM, and SVNLE with a 76%, 72%, and 61% complexity reduction, respectively, for 8-QAM without losing signal integrity.

The traditional angular spectrum method has an inherent problem that the region of diffraction propagation should be homogeneous. However, in some cases, the medium of the diffraction propagation region is inhomogeneous. In this Letter, based on iteration we proposed the non-uniform angular spectrum method for diffraction propagation calculation in a complex medium. By phase pre-processing in the spatial domain and diffraction calculation in the spatial frequency domain, the diffraction propagation problem of the light field in a complex medium is solved. Theoretical formulation and numerical examples as well as experimental investigation are presented to confirm the validity of the proposed method. The advantages of this method include faster computation, smaller memory requirement, and the ability to compute a larger area compared with the finite element method as well as the ability to compute the non-paraxial case compared with the standard fast Fourier transform beam propagation method.

This study investigates coherent detection that is free from the Doppler frequency shift effect for orthogonal frequency division multiplexing radio-over-fiber (OFDM-RoF) links using period-one (P1) nonlinear dynamics of semiconductor lasers. Even under a dynamically time-varying Doppler frequency shift of up to 100 kHz, corresponding to a relative motion between a transmitter and a receiver with a velocity of more than 3850 km/h at 28 GHz, the microwave carrier of a received OFDM-RoF signal can be successfully regenerated instantaneously and uninterruptedly with its phase highly preserved through the P1 dynamics. No carrier frequency offset (CFO) due to the Doppler frequency shift effect happens if the regenerated microwave carrier is used as a microwave local oscillator for coherent detection of the received OFDM-RoF signal. As a result, a bit-error ratio of around 10−9 is achieved for coherent detection of a 28 GHz OFDM-RoF signal carrying 4 Gb/s 16-quadrature amplitude modulation data. Thus, no digital signal processing, either photonic or electronic, is required to compensate for such a CFO. This all-optical system is capable of operation up to at least 100 GHz.

In this Letter, we propose and experimentally demonstrate a novel, to the best of our knowledge, sparse deep neural network-based nonlinear equalizer (SDNN-NLE). By identifying only the significant weight coefficients, our approach remarkably reduces the computational complexity, while still upholding the desired transmission accuracy. The insignificant weights are pruned in two phases: identifying the significance of each weight by pre-training the fully connected DNN-NLE with an adaptive L2-regularization and then pruning those insignificant ones away with a pre-defined sparsity. An experimental demonstration is conducted on a 112 Gbps PAM4 link over 40 km standard single-mode fiber with a 25 GHz externally modulated laser in O-band. Our experimental results illustrate that, for the 112 Gbps PAM4 signal at a received optical power of − 5 d B m over 40 km, the proposed SDNN-NLE exhibits promising solutions to effectively mitigate nonlinear distortions and outperforms a conventional fully connected Volterra equalizer (VE), conventional fully connected DNN-NLE, and sparse VE by providing 71%, 63%, and 41% complexity reduction, respectively, without degrading the system performance.

This study investigates an all-optical scheme based on period-one (P1) nonlinear dynamics of semiconductor lasers, which regenerates the microwave carrier of an orthogonal frequency division multiplexing radio-over-fiber (OFDM-RoF) signal and uses it as a microwave local oscillator for coherent detection. Through the injection locking established between the OFDM-RoF signal and the P1 dynamics, frequency synchronization with highly preserved phase quality is inherently achieved between the recovered microwave carrier and the microwave carrier of the OFDM-RoF signal. A bit-error ratio down to 1.9×10sup-9/supis achieved accordingly using the proposed scheme for coherent detection of a 32-GHz OFDM-RoF signal carrying 4 Gb/s 16-quadrature amplitude modulation data. No electronic microwave generators or electronic phase-locked loops are thus required. The proposed system can be operated up to at least 100 GHz and can be self-adapted to certain changes in the operating microwave frequency.

Previously, we reported using a diffusing probe in conjunction with a two-layer diffusion model for accurately recovering the optical properties of superficial volumes. However, the two-layer diffusion equation is computationally less efficient than a standard diffusion equation (SDE) by 3 orders of magnitude. In this Letter, we present a new approach extended from the diffusing probe geometry that enables the use of a diffusion model that has efficiency comparable to an SDE. Our Monte Carlo simulation results indicate that this new approach is not only very accurate but also sensitive to the presence of 0.5 mm(3) inhomogeneities and could be used for monitoring the early progression of skin melanoma.

We report what we believe is the first experimental observation of bright-soliton-induced two-dimensional waveguides. The waveguides are induced by two-dimensional photorefractive screening solitons, and their guiding properties, i.e., whether the waveguides are single mode or multimode, are controlled by adjustment of the soliton parameters.

We demonstrate self-trapping of one-dimensional and two-dimensional optical beams in a photorefractive KNbO(3) crystal. We study the waveguides induced by the self-trapped beams for prospective applications of tunable nonlinear frequency conversion in soliton-induced waveguides.

We demonstrate theoretically and experimentally that induced spatiotemporal modulation instability can exist in a self-defocusing medium if the nonlinearity is noninstantaneous. We predict the growth rate as a function of the spatial and temporal frequencies of the modulation and the response time of the nonlinearity and confirm it by our experiments.

We demonstrate that the phase shift of a reference injection-locked optoelectronic oscillator (OEO) varies as the change of its fiber delay over a long period of time. The dynamic variation of the fiber delay is monitored using an injected probe signal and is compared with the phase shift. With actively stabilized fiber delays according to the monitored data, the long-term frequency stability of the reference injection-locked OEO is evaluated.

By employing the technique of asynchronous mode locking, we have successfully demonstrated direct generation of stable 10 GHz 816 fs pulse trains with a supermode-suppression ratio >70 dB from a hybrid mode-locked Er-fiber laser. When the modulation frequency deviates from the cavity harmonic frequency by 15-40 kHz, stable femtosecond soliton pulses are formed. Our results demonstrate that asynchronous mode locking can act as an effective mechanism for achieving a shorter pulse width and for stabilizing high-repetition-rate pulse trains in soliton fiber lasers.

We study, both theoretically and experimentally, the transverse modulational instability of spatial stripe solitons in anisotropic nonlocal photorefractive media. We demonstrate that the instability scenarios depend strongly on the stripe orientation, but the anisotropy-induced features are largely suppressed for spatial solitons created by self-trapping of partially incoherent light., Comment: 3 pages including 4 figures (2 theoretical and 2 experimental). submitted to Optics Letters

Thin films of Bi were grown by pulsed laser deposition on glass substrates at room temperature. The thickness and roughness of the films were characterized by grazing-incidence x-ray reflectivity, and the complex refractive indices were measured in the range from 1.5 to 4 eV by spectroscopic ellipsometry. We performed Z-scan measurements to study the third-order optical nonlinearity of the films. It was found that the Bi films exhibited an unusually large nonlinear refractive coefficient, n(I)~1.24x10(-1) cm(2)/kW and nonlinear absorption coefficient, alpha(I)~-3.97 cm/W , at low laser intensity, ~60 kW/cm(2) . This anomaly is believed to have an origin related to melting of the Bi films at the focus spot by the laser beam.

We predict the formation of optical spatial solitons in photorefractive polymers. The orientational enhancement from the doped chromophores and the dependency of the quantum efficiency of generating mobile holes on the electric field make the polymeric solitons behave differently from other photorefractive solitons.

We present an experimental study of coherent collisions between one-dimensional bright photorefractive screening solitons in a bulk strontium barium niobate crystal.

We report the observation of incoherent collisions between two-dimensional bright photorefractive screening solitons. The solitons remain intact and do not exchange energy whenever the collision angle exceeds the critical angle for guidance in the waveguide that each soliton induces, which is, in turn, fully controlled by the soliton parameters. When the collision angle is much smaller than the critical angle the solitons fuse to form a single beam.

We present an experimental study of steady-state dark photorefractive screening solitons trapped in a bulk strontium barium niobate crystal. We compare experimental measurements with theoretical calculations of the soliton properties and find good agreement between theory and experiments. We confirm the shapepreserving behavior of the dark soliton by measuring its beam profile as it propagates throughout a specially cut crystal and by guiding a beam of a different wavelength.

We study experimentally steady-state photorefractive screening solitons trapped in both transverse dimensions and measure their beam profiles as they propagate throughout the crystal. The solitons are observed to be axially symmetric, and they self-bend. We characterize the soliton dependence on the optical intensity, applied electric-field strength, and beam diameter.