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Network reliability is very important for the next generation of optical networks. In this paper, a highly reliable protection mechanism, which integrates the approaches of optical protection and microwave protection, is proposed for metro-access optical networks based on the wavelength division multiplexing (WDM) technique. By designing a scheme for the convergence of optical and wireless access and cooperating with designed remote nodes (RNs), not only can access for both wired and wireless service be provided simultaneously in each optical network unit (ONU) but also microwave protection can be realized when a failure occurs in a distribution segment. By developing a novel optical switch module (OSM) to cooperate with the dual-fiber ring structure, flexible switching can be achieved to protect against multiple types of failures in the feeder segment, including single-fiber faults, dual-fiber faults and multifiber faults. Based on the above structure, the influence of Rayleigh backscattering (RB) noise can be mitigated when the network is operating under all operating scenarios except for an inner-fiber fault. To further mitigate the influence of noise, optimization for upstream transmission performance is implemented by configuring a reflective semiconductor optical amplifier (RSOA). Furthermore, the feasibility of the overall scheme is verified through experiments and simulations under different operating conditions. The results show that the proposed mechanisms can achieve acceptable bit error rate performance and quality of service (QoS) metrics while also offering various advantages, including high reliability, large scale, and a relatively low cost.

In this paper, we verify the feasibility of our proposed design scheme for customizable optical pressure sensor based on the photo-elastic effect of SiO2 waveguide in asymmetric Mach-Zehnder interferometer (a-MZI) structure. The curve lengths and minimum radii of curvature of several common types of a-MZI arm curves are calculated theoretically. By analyzing the relationship between the radius of curvature of the long arm curve, the length of the long arm curve and the arm length difference of the a-MZI sensor under different types of arm curves, the design scheme for an a-MZI pressure sensor with a specific arm length difference is obtained. By selecting a rationalized arm design scheme, customization of the sensing range can be achieved. No special processing (such as etching) is required for the waveguide, which improves the pressure tolerance of the sensor. When using the SiO2 waveguide with a refractive index difference of 2%, the a-MZI pressure sensor based on sine type arm with arm length difference of 5 mm has a sensing range of 4.83 MPa and a sensitivity of 1.24 mW/MPa. The a-MZI pressure sensor with the ease of fabrication can be used to monitor various scenarios, such as marine and building monitoring.

A tunable microwave photonic filter (MPF) based on a polarization beam splitter (PBS) and waveguide grating true-time delay line (TTDL) is proposed and demonstrated experimentally. Taking advantage of the high birefringence property of lithium niobate (LN), the PBS is achieved by a simple structure in a short length. The TTDL consists of an LNOI waveguide grating and flexible electrodes covering along the grating. Owing to the electro-optic effect of LN, a wide tuning range and high tuning accuracy of delay time can be achieved simply by controlling the charging position of the electrodes. With the proposed PBS and TTDL, the MPF exhibits low drive voltage, low power consumption, and high response rate in a simple construction.

An optical sensor based on dual asymmetric Mach–Zehnder interferometers (AMZIs) cascade with temperature-independent is proposed. The optical sensor based on AMZI is very sensitive to the change of ambient temperature. The temperature shift will lead to the change of the refractive index of the optical waveguide, which will affect the accuracy of the measurement of target-parameters. The designed optical sensor is composed of two AMZIs cascaded, one acting as a sensing AMZI and the other as a compensating AMZI. When the temperature changes, the sensing curve of the sensing AMZI shifts. The compensating AMZI can make the sensing curve goes into reverse to realize the compensation. The error of compensation results is reduced by 2–3 orders of magnitude, the compensation temperature range is from 5 to 45 °C, and the error is reduced to

In this paper, an optical pressure sensor based on pressure-induced birefringence in Lithium Niobate (LN) crystal is designed to achieve large sensing dynamic range. By using the photo-elastic effect of LN crystal, the optical phase change caused by the external pressure can be converted into the change of the output light intensity. Considerable sensor range can be obtained without special processing of the LN crystal, which makes it have high pressure tolerance. The sensing range of 24.85 MPa is demonstrated to be achievable on x-cut LN crystal with a length of 10 mm in the z-axis direction. The sensing sensitivity can reach 0.281 mW/MPa. By customizing the sensing range, it is expected to use the LN crystal based pressure sensor with the ease of fabrication to monitor high pressure scenarios, such as marine and large buildings monitoring.

A precisely tunable true time delay line (TTDL) based on lithium niobate on insulator waveguide grating is proposed. Utilizing the electro-optic effect of lithium niobate, active tuning of reflected wavelength and time delay can be realized. The numerical simulation results show that, with the optimized grating structure and flexible electrode distribution, wide tuning range and high tuning accuracy of time delay can be achieved by simply controlling the charging position of the electrodes. This TTDL is applied to a tunable bandpass microwave photonic filter (TBMPF), which using PM-IM conversion to suppressed dc signal to achieve bandpass. In the TBMPF, the TTDL plays the role of introducing the dispersion for PM-IM conversion, time delay between the taps and tuning of central frequency. This TTDL can reduce the system complexity and cost of microwave photonic filter.

A tunable microwave photonic notch filter (MPNF) based on lithium niobate on insulator (LNOI) polarization beam splitter (PBS) and waveguide grating true time delay line (TTDL) is proposed and demonstrated experimentally. Based on LNOI, the PBS is achieved by simple structure in a short length utilizing the high birefringence property of lithium niobate (LN), and the TTDL is consisted of waveguide grating and flexible electrodes covering the grating. Utilizing the electro-optic effect of LN, wide tuning range and high tuning accuracy of time delay can be achieved by simply controlling the charging position of the electrodes. With the PBS and TTDL, the MPNF has a simple construction with low power consumption.

The integrated polarization multiplexing in-phase and quadrature (IQ) modulator based on lithium niobate (LN) thin film and all waveguide structure is designed. It consists of polarization demultiplexer (PDM), polarization multiplexer (PM) and two IQ modualtors (IQM). Each of IQM contains two different couplers and two Mach-Zehnder modulators (MZM). The theory of multimode interference (MMI) is used to design the PDM, PM and IQM. For PDM and PM, the extinction ratios of TE and TM polarized lights reach 23 dB and 21 dB. Quadrature Phase Shift Keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) are achieved well with the integrated modulator.

Multimode interference (MMI) structure is introduced in semiconductor laser used in optical communication system to realize higher power and better temperature tolerance. Using beam propagation method (BPM), Multimode interference laser diode (MMI-LD) is designed and fabricated in InGaAsP/InP based material. As a comparison, conventional semiconductor laser using straight single-mode waveguide is also fabricated in the same wafer. With a low injection current (about 230 mA), the output power of the implemented MMI-LD is up to 2.296 mW which is about four times higher than the output power of the conventional semiconductor laser. The implemented MMI-LD exhibits stable output operating at the wavelength of 1.52 µm and better temperature tolerance when the temperature varies from 283.15 K to 293.15 K.

In this letter, we design and experimentally demonstrate a four-channel full-band wavelength division multiplexing (WDM) device based on multimode interference in the silicon dioxide material platform. The simulation and performance analysis of WDM devices are carried out by the beam propagation method and finite-difference time-domain method. With a simple structure, the device is in small size, and easy to implement in process. The experimental results show that the device achieves a wavelength separation of 1.28, 1.34, 1.52, and $1.58~\mu \text{m}$ . The insertion losses and a contrast between two channels are also displayed.

We propose a DC-based polarization beam splitter (PBS) in lithium niobate on insulator (LNOI). Utilizing the high birefringence property of Lithium Niobate (LiNbO3, LN), the device is achieved by simple structure in a short length. With the use of beam propagation method (BPM), the simulation results show that the device has a good performance for the separation of TE and TM polarizations with a high extinction ratio (about 35 dB). The simulated fabrication tolerance for the variation of the waveguide width is about 100 nm and the bandwidth is about 65 nm when the extinction ratio is higher than 10 dB.

In this paper, an Arrayed-Waveguide Grating (AWG) filter based on Lithium Niobate on Insulator (LNOI) is designed to achieve tunable optical filtering by adjusting the refractive index of the waveguide area. By using the electro-optic effect of Lithium Niobate (LN), two triangular model electrode areas are designed to tune the central wavelength and bandwidth of the filter respectively. The adjustment in refractive index of two areas are relatively independent. By optimizing the structural parameters of the AWG, better tuning capability of bandwidth is achieved while maintaining the tuning range of central wavelength. Simulated by beam propagation method, excellent tuning capability of central wavelength and bandwidth can be obtained by applying a low voltage to the two electrode areas we designed.

In metro-access optical network, reconfigurable optical add/drop multiplexers (ROADMs) are required. A ROADM based on coarse wavelength division multiplexer (CWDM) and microring resonators (MRRs) has been proposed and demonstrated. The simulation and performance analysis of ROADM system are carried out by using beam propagation method (BPM) and finite-difference time-domain method (FDTD). CWDM based on multimode interference (MMI) principle is designed. A series of MRRs can make up optical switch array with over 28dB switching ratio.

In this paper, a reconfigurable Radio over Fiber system based on optical switches, optical multimode interference power splitter and tunable optical filters is designed, which can achieve quadrature phase shift keying wireless modulation in optical domain and switch carrier frequency between 30 and 60 GHz. The scheme can make good use of bandwidth resource and realize signal transmission in fiber within 10 km.

We demonstrate a bidirectional radio-over-fiber system based on optical coupling sing-sideband modulation, which can effectively reduce the impact of nonlinear distortion in transmission. With sidebands heterodyne detection, the system achieves 60-GHz wireless coverage. By optical carrier reusing, the base station can omit the laser source to make the system colorless. Through simulation analysis, the system has the credible transmission property both in downlink and uplink transmissions.

A polarization beam splitter based on lithium niobate thin film is proposed, which consists of a directional coupler. The device is easy to be achieved by simple structure. Numerical simulations show that the device has a good performance for the separation of TE and TM polarizations with a high extinction ratio (about 30 dB). The simulated fabrication tolerance for the variation of the waveguide width is about 110 nm and the bandwidth is about 70 nm for the extinction ratio higher than 10 dB.

A flexible optical filter based on tunable arrayed waveguide grating (AWG) with triangular-like model electrodes is designed on lithium niobate-on-insulator. Using the electro-optic effect of lithium niobate, the 10 − 3 increase of the waveguides refractive index could be achieved with a 4.348-V effective external voltage. The tunable range of the central wavelength of AWG can reach 3.8 nm and the 3-dB bandwidth is about 0.43 nm.