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In order to reduce the fluctuations of traffic flow and the impacts of traffic flow fluctuations, thereby increasing the level of service of roads and constructing the Internet of Things-based traffic system, based on the various causes of traffic flow fluctuations, the countermeasures were taken, such as the optimization of lanes, the management of intersections, the improvement and perfection of slow-vehicle lanes and bus stops, the timely renewal of traffic signs and marks, the rational arrangements of road operations, and the removal of visual obstacles, etc. The research results showed that these countermeasures could effectively reduce the hidden peril of accidents and improve the road capacity, thereby improving the level of service of roads, which provided a theoretical foundation and basis for road engineering design and traffic management. It can be seen that improving urban traffic through rational management and control measures can effectively improve the level of service of urban traffic.

Graphene and hBN as two-dimensional materials with excellent optical properties can be combined with other materials. In this paper, graphene and hBN is sequentially deposited on the surface of the silicon-core fiber to serve as transverse magnetic (TM) or transverse electric (TE) mode polarizer alternatively depending on the input wavelength and the diameter of the silicon-core. The Gr/hBN stack structure on the surface silicon-core microfiber is used not only for enlarging the wavelength bandwidth, such as from 350 nm (TM) and 670 nm (TE) for double Gr/hBN stack to 300 nm (TM) and 550 nm (TE) for single Gr/hBN stack, but also enhancing the light-graphene interaction to multiply the polarization extinction ratio, as compared with that of monolayer graphene. Small radius silicon core could provide large light-graphene interaction for flat extinction ratio with different thickness of the hBN.

A coupled equivalent circuit for high-speed Si photodiodes is proposed, which is composed of a traditional resistance-capacitance-limited equivalent circuit and a carrier transit-limited equivalent circuit connected by a voltage controlled current source. All the resistances and capacitances in the carrier transit-limited equivalent circuit have physical meaning according to Laplace Transform. The proposed coupled equivalent circuit was applied to the Si p-i-n photodiodes and agreed well with the measured reflection coefficients and frequency response curves. Again, the minority carrier lifetime, diffusivity and interface state is verified to be the main limiting factors to the frequency response. This improved modeling can provide an effective way to design high speed photodiodes.

Photodetection with high responsivity at the wavelength of 1060 nm is highly desirable for light detection and ranging (LiDAR) as well as the recent emergence of swept-source optical coherent tomography (SS-OCT) applications. However, the absorption coefficient $\alpha $ of Si material at 1060 nm under the bias of 0 V is very low due to its approach to the absorption bandgap edge of Si material. In this letter, the carrier-collection-enhanced structure is proposed, which functions as multiple carrier collection paths, which enhances the external quantum efficiency of photodiodes. An efficient Si photodiode with responsivity of 0.49 A/W at 1060 nm at 0 V is demonstrated, which is 2.5 times of that of commercial products. The cost-effective fabrication of Si photodiodes greatly enhances the system performance of LiDAR and OCT.

An absorption-adjustable graphene saturable absorber is used in a simple fiber laser system for an active control of the pulse-width of optical pulses. Such a saturable absorber is created by tightly attaching the graphene film onto the upper surface of the microfiber, thus an enhanced evanescence-field interaction between the propagating light along the microfiber length and graphene can be achieved. By tuning the polarization state of the light beam launched into the saturable absorber from TE to TM mode, the pulse-width of the pulses is varied between ~ 9.24 and ~ 2.32 ps.

Graphene has attracted a lot of research interest, especially as a saturable absorber (SA). However, improvement on thermal damage threshold is critical for the SA. Here, graphene covered on the microfiber is developed for this purpose by use of the light-graphene interaction via the evanescent field of the guided mode in the microfiber. Such interaction is numerically studied by using the theory of the electromagnetic field. Theoretical and experimental results indicate that graphene covered on the upper surface of the microfiber can be used as a polarization-dependent SA as well as an optical polarizer. When the radius size of the microfiber is down to 0.8 μm, its polarization extinction ratio is up to ~27 dB. When the radius of microfiber is up to ~3 μm, a polarization-dependent SA can be obtained with high thermal damage threshold of ~975.82 MWcm-2 for p -polarization and ~1233.2 MWcm-2 for s-polarization, and its polarization-dependent modulation depth varies from ~10.25% to ~12.85%.

Graphene with zero-band gap can be easily saturated under strong excitation due to Pauli blocking. In this work, the graphene oxide membrane is reduced on the surface of microfiber by use of high temperature heating. Such reduced graphene oxide can interact with the strong evanescent field of the microfiber and enable saturable absorption in passively mode-locked fiber laser. The system can be effectively utilized to directly generate ultra-wide-band doublet pulses for high-capacity wireless communication applications.

A theoretical study of nonlinear effects in the external cavity semiconductor laser (ECSL) with sampled fiber grating is presented; such effects are responsible for asymmetric side-mode suppression ratio or power characteristics, and hysteresis phenomena. Impacts of material and geometrical parameters on nonlinear effects of such lasers were analyzed and discussed. Apodization and interleaved technologies were demonstrated to be helpful with expanding tuning range and suppressing nonlinear effects. Based on this, a new ECSL with interleaved sampled grating has been proposed.

In this paper, an analytical method is proposed for calculating reflection-spectrum envelopes (RSEs) of various multipeak gratings. Consequently, impacts of parameters on the envelope of the sampled grating are analyzed in detail both with and without refractive index variation. Accuracy of those proposed methods has been verified by the good agreement with the simulated reflectivity spectrum of the transfer matrix method. A technology of multiple reflection spectrum envelopes concatenation (MRSEC) is proposed to design multipeak gratings with broad flat-top spectrum envelopes: one is the digital concatenated grating; the other is the consecutive interleaved sampled grating. The proposed technology can densify spectral channels of sampled gratings with fixed reflectivity peak space, as well as spatially physical corrugation of structure.

A polarization-dependent saturable absorber is developed by the use of graphene-polymer film covered microfiber. The evanescent field of the guided mode propagating along the microfiber can effectively interact with graphene. Polarization analysis shows that the single-layer graphene-based saturable absorber is more sensitive to polarization and has larger modulation depth than that of multi-layer graphene. Such a saturable absorber exhibits large saturable absorption power of >56.03 mW, and is easy to fabricate and compatible with optical fiber system. It has high potential in wide range of photonics applications.

A simple, compact, and robust refractive index sensor is developed, by twisting a pair of silica microfibers to form a coupling device. The transmission spectrum of the device is highly sensitive to the surrounding refractive index and the highest sensitivity obtained is -2377 nm/refractive index unit (RIU), at the refractive index value of - 1.3680, for the fibers with diameter of ~4.2 μm, and the sensitivity can be further increased to - 2735 nm/RIU for a smaller fiber diameter of ~2.8 μm. Such a device is easy to construct, of low cost, and compatible with the optical fiber system.

A digital concatenated grating is presented to tackle that problem of creating a flat ldquotop-hatrdquo comb reflector for tunable lasers. A novel, monolithic, widely tunable semiconductor laser with these gratings is proposed and demonstrated theoretically.

Two types of erbium-doped fiber lasers based on Sagnac interferometer incorporated with fiber grating are proposed and experimentally demonstrated, which effectively support tunable and switchable dual-wavelength single-longitudinal-mode laser operation. The mode selection is implemented by use of the Sagnac interferometer with fiber grating, an umpumped erbium-doped fiber, and a tunable fiber Bragg grating. The Sagnac interferometer with fiber grating plays the role as a narrowband comb filter, while the unpumped erbium-doped fiber together with the tunable fiber Bragg grating essentially forms a passive self-tracking filter to ensure a stable single-longitudinal-mode fiber laser operation. By use of the Sagnac interferometer with chirped fiber Bragg grating, tunable microwave signals can be generated by beating the dual wavelengths, within the frequency range between and GHz.