A novel polarization splitter based on dual-core elliptical-holes hybrid photonic crystal fiber

A novel kind of polarization splitter in dual-core elliptical holes hybrid photonic crystal fiber is proposed. Numerical results show that the splitter can reach small coupling length ratio of 0.5, for wavelength from 1.15 μm to 1.9 μm. At wavelength 1.55 μm, the extinction ratio can achieve ?64 dB and the 1.92-mm-long splitter is suggested to achieve extinction ratio better than ?10 dB, a bandwidth of 150 nm. The fiber has small coupling length ratio, small coupling length and high extinction ratio and it is more suitable for fabricating polarization splitter.     

A novel polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber

A kind of polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber has been proposed. The polarization splitter is based on the phenomenon of resonant tunneling. We use the finite element method and the full-vector beam propagating method to analyze the characteristics of three-core photonic crystal fiber. Compare with the silica glass three-core PCF, the ZnTe tellurite glass three-core PCF have higher extinction ratios and lower coupling loss, the extinction ratios ERA = ? 164.2681 dB and ERC = ? 37.1742 dB at the wavelength λ = 1.55 μm, and the coupling loss is lower than 0.02 dB. The 8.7983-mm-long splitter is proposed to achieve extinction ratio better than ? 20 dB and a bandwidthof 20 nm.     

Analysis of the characteristics of the polarization splitter based on tellurite glass dual-core photonic crystal fiber

A novel kind of polarization splitter based on tellurite glass dual-core photonic crystal fiber is proposed. The characteristics of the polarization splitter have been studied by the finite element method. By adjusting the ellipticity of the dual-core PCF, a very high value of birefringence can be reached. The high birefringence can increase the D-value of coupling length in the two orthogonal polarization directions, then the polarization-independent splitter can be achieved. Comparing with the SF57 glass dual-core PCF, the tellurite glass dual-core PCF has higher extinction ratio, the extinction ratio can reach ?31 dB at the wavelength $\lambda =1.55\mathrm{\mu }\mathrm{m}$. The 0.36-mm-long splitter is proposed to achieve extinction ratio better than ?10 dB and a bandwidth of 20 nm.     

A low transmission loss THz polarization splitter based on dual-core optical fiber

We propose a design of a low loss terahertz polarization splitter based on a dual-core terahertz fiber with crossed dielectric strips in the fiber cross section. Low transmission loss is realized by extending the mode field to the air holes adjacent to the solid material. An 11.4-cm-long terahertz polarization splitter is obtained with the extinction ratio better than −15 dB and a bandwidth of 16 μm.     

A compact polarization splitter utilizing directional coupler with left-handed material

A compact polarization splitter (PS) with left-handed material (LHM) sandwiched between two right-handed material (RHM) waveguides of a directional coupler (DC) is proposed. Its operation is by setting the coupling length of TM polarization be half that of TE polarization. Theoretic analysis exhibits that the PS with a very small size of 138.4 μm × 18.6 μm can be realized due to the inclusion of LHM even when the RHM with small birefringence is used.     

A surface-plasmon-polariton wavelength splitter based on a metal–insulator–metal waveguide

A surface-plasmon-polariton (SPP) wavelength splitter based on a metal–insulator–metal waveguide with multiple teeth is proposed. Using the transfer-matrix method, a plasmonic band gap is identified in the multiple-toothed structure, and the splitting wavelength of the SPP splitter can be easily adapted by adjusting the widths of the teeth and the gaps. The proposed wavelength splitter is further verified through finite-difference time-domain (FDTD) simulations, in which SPPs with incident wavelengths of 756 nm and 892 nm are successfully split and guided in opposite directions in the waveguide, with extinction ratios of 30 dB and 29 dB, respectively.     

Incident angle and temperature dependence of WSi wire-grid polarizer

The dependences of the incident angle and thermal durability of a tungsten silicide (WSi) wire-grid polarizer were examined. A WSi grating with a 0.5 fill factor, 260 nm depth, and 400 nm period was formed on a Si surface using two-beam interference and dry etching. The TM transmission spectrum of the fabricated element was greater than 60% at the incident angle of θ = 40° (the angle between the incident direction and the perpendicular axis to the grating direction) in the 4–10 μm wavelength range. An extinction ratio of 22.2 dB was achieved at 2.5 μm wavelength. Additionally, results show that this polarizer has higher thermal resistance than that of commercial infrared polarizers. Therefore, this polarizer is effective for taking a polarized thermal image of high temperatures.     

Improvement of polarization extinction in silicon waveguide devices

The SOI based waveguide devices are found to be highly polarization sensitive. Unwanted polarization excitations can be attenuated by integrating a TE- or TM-pass polarizer. A large attenuation of TM-polarized light has been observed when a thin film of metal is coated on the top of silicon rib waveguide, while TE-polarized light remains almost unaffected. The attenuation of TM-polarized light is attributed to the plasmonic absorption of the evanescent field in the metal cladding. Typically, with an Al cladding of thickness ～ 100 nm and a length of 1 mm on top of a single-mode (λ ～ 1550 nm) SOI rib waveguide structure, TE vs TM extinction ratio of ～ 15 dB has been obtained. Integrating such waveguide polarizers in a directional coupler and MZI based DWDM channel interleaver, we have also achieved an improvement in polarization extinction by ～ 15 dB.     

Fabrication and characterization on an organic/inorganic 2 × 2 Mach–Zehnder interferometer thermo-optic switch

An organic/inorganic hybrid 2 × 2 directional coupler (DC) Mach–Zehnder interferometer (MZI) thermo-optic (TO) switch was successfully designed and fabricated using simple direct ultraviolet photolithography process. The hybrid organic/inorganic waveguide structure includes poly-methyl-methacrylate-glycidyl-methacrylate (P(MMA-GMA)), SU-8 2005 and silica as core, upper cladding and under cladding, respectively. Device optimization and simulation were performed to decrease radiation loss and leakage loss, quicken response time and cut down power consumption. Measurements of the fabricated devices at 1550 nm wavelength result in a switching power of 7.2 mW, a response time of ∼100 μs, and crosstalk of −22.8 and −26.5 dB under cross state and bar state, respectively. Besides, the driving-noise-tolerance characteristics of this device were experimentally investigated by directly imposing a generated tunable noise on the pure driving signal (4 V_pp) and the minimum extinction ratio is larger than 18 dB under a noise level of 2.5 V_pp. The effect of noise on extinction ratio was found decreased with the increase of noise frequency.     

Low-threshold all-fiber 1000 nm supercontinuum source based on highly non-linear fiber

We present an highly efficient all-fiber compact supercontinuum source that exhibits a nearly flat spectrum from 1.1 μm to 2.1 μm. This broadband infrared optical source is made-up of a highly non-linear fiber pumped by a 1.55 μm self-Q-switched Er-Brillouin nanosecond pulsed fiber laser, which in turn is pumped by a low-power 1480 nm laser diode. In this work we highlight the great potential of highly non-linear fiber for supercontinuum generation with respect to conventional dispersion-shifted fiber by demonstrating a significant 10 dB power enhancement in the short wavelength side of the supercontinuum.     

Novel optical add-drop multiplexer based on dual racetrack resonators

In this paper, we designed and fabricated a four-channel optical add-drop multiplexer (OADM) based on dual racetrack resonators. The size of the fabricated device is only 2400 μm × 500 μm. The fabricated device can effectively and perfectly realize the signals upload and download. The free spectral range (FSR) of OADM is about 15.2 nm. We take the spectral responses near 1555 nm as an example. When the device acts as an optical drop multiplexer, the minimum insertion loss is 4.481 dB and the maximum extinction ratio is 31.931 dB. The maximum adjacent channels crosstalk is -9.845 dB. When the device acts as an optical add multiplexer, the minimum insertion loss is 0.944 dB and all of the extinction ratios are bigger than 25 dB. The maximum crosstalk is -16.531 dB which indicates the crosstalk can be neglected.     

A new design of low-loss and ultra-flat zero dispersion photonic crystal fiber using hollow ring defect

New hollow ring defect structure is introduced in photonic crystal fiber design for ultra- flat zero dispersion with very low waveguide losses. The hollow ring defect consisted of a central hole surrounded by a doped silica ring provides highly flexible defect engineering capabilities in photonic crystal fibers to achieve precise control of dispersion value and dispersion slope while independently maintaining low waveguide losses, which was not attainable in previous designs. A nearly flat zero dispersion of D=0±0.51 ps/nm km was obtained in the wavelength range of 1.44–1.61 μm with the maximum slope of ?2.7×10^?2 ps/nm² km. The confinement loss was less than 5.75×10^?8 dB/m along with the bending loss of 2.8×10^?6 dB/m for the radius of 10 mm, and splice loss of less than 1.57 dB to conventional single mode fiber at 1.55 μm.     

100-mW linear polarization single-frequency all-fiber seed laser for coherent Doppler lidar application

A compact short-cavity fiber laser configured with Er³⁺/Yb³⁺ highly co-doped phosphate glass fiber with stable linear polarization and single frequency output is proposed and investigated experimentally. The fiber laser is composed of a high-reflectivity fiber Bragg grating (HRFBG) and a polarization-maintaining fiber Bragg grating (PMFBG) with the matched wavelengths at 1540.3 nm, which aims at one of the center wavelengths of the atmospheric transmission windows and may be used as the local oscillator (LO) of the coherent Doppler lidar (CDL). The output power of the laser reaches more than 114-mW, the signal-to-noise ratio is larger than 70 dB and the laser linewidth is about 4.1-kHz. Moreover, the linear polarization with 40.5 dB extinction ratio, the power fluctuation of less than ± 0.25% and the frequency fluctuation of less than ± 80 MHz are also obtained. Compared with the DFB fiber laser, the proposed fiber laser is more suitable for the CDL applications.     

Compact polarization rotator on silicon for polarization-diversified circuits

We demonstrate a polarization rotator based on adiabatic mode evolution on silicon for polarization-diversified circuits. The rotator has a device length of 420?μm, a polarization-conversion efficiency of more than 90%, and an insertion loss less than 1?dB for a wavelength range of 80?nm. Combining the rotator with a compact, broadband polarization beam splitter based on cascaded directional couplers enhances the polarization conversion extinction ratio to over 30?dB with less than 1.5?dB total insertion loss over a 60?nm spectral range.     

TE/TM-mode pass polarizers and splitter based on an asymmetric twin waveguide and resonant coupling

A new passive TE/TM-mode polarization filter for an InP system based on an asymmetric twin waveguide and resonant coupling is investigated. Linear taper sections with different taper angles are introduced to couple between the two vertically separated waveguides. The underlying waveguide is designed to enable direct edge coupling from an optical fiber. At a wavelength of 1.55?μm power extinction ratios of 20 dB for the TE- and more than 10 dB for the TM-polarization are reported for devices shorter than 400?μm. An increased extinction ratio can be obtained by concatenating structures. Furthermore, we show this concept can be expanded to a polarization splitter.     

Low loss propagation in slow light photonic crystal waveguides at group indices up to 60

We have designed slow light photonic crystal waveguides operating in a low loss and constant dispersion window of Δλ = 2 nm around λ = 1565 nm with a group index of n_g = 60. We experimentally demonstrate a relatively low propagation loss, of 130 dB/cm, for waveguides up to 800 μm in length. This result is particularly remarkable given that the waveguides were written on an electron-beam lithography tool with a writefield of 100 μm that exhibits stitching errors of typically 10–50 nm. We reduced the impact of these stitching errors by introducing “slow–fast–slow” mode conversion interfaces and show that these interfaces reduce the loss from 320 dB/cm to 130 dB/cm at n_g = 60. This significant improvement highlights the importance of the slow–fast–slow method and shows that high performance slow light waveguides can be realised with lengths much longer than the writing field of a given e-beam lithography tool.     

Theoretical reflective performance of a sandwiched two-layer grating polarizer

The reflective polarizer is described by a sandwiched two-layer grating with a metal slab. Such a new polarizer is aimed to improve the performance of a reflective grating-based polarizer. The grating is optimized with the usual duty cycle of 0.5, where TE and TM polarizations are reflected in the –1st and the 0th diffraction orders, respectively. With optimized grating parameters, the extinction ratio can reach 45.5 dB and 41.9 dB in two diffraction orders, which are greatly improved compared with the conventional reported surface grating polarizer with the simple structure. Attractive merits of the new design are high efficiency, high extinction ratio, wide incident wavelength bandwidth for TE polarization, and wide angular range for TM polarization. Numerical results are expected to open new opportunities for the design of a grating-based polarizer with the enhanced performance by the complicated grating configuration.     

Compact WDM demultiplexer for seven channels in photonic crystal

We demonstrate a new device concept for wavelength division demultiplexing based on planar photonic crystal waveguides. The filtering of wavelength channels is realized by shifting the cutoff frequency of the fundamental photonic bandgap mode in consecutive sections of the waveguide. The shift is realized by modifying the size of the border holes.The proposed demultiplexer has an area equal to (16.5 μm × 6.5 μm) and thus it is verified that this structure is very small and can be integrated easily into optical integrated circuits with nanophotonic technologies. The output wavelengths of designed structure can be tuned for communication applications, around 1550 nm. The wavelengths of demultiplexer channels are λ₁ = 1.590 μm, λ₂ = 1.566 μm, λ₃ = 1.525 μm, λ₄ = 1.510 μm, λ₅ = 1.484 μm, λ₆ = 1.450 μm, λ₇ = 1.400 μm respectively. Designs offering improvement of number of the separate wavelengths (seven), miniaturization of the structure (107.25 μm²) is our aim in this work.In our structure, we consider that the 2D triangular lattice photonic crystal is composed of air holes surrounded by dielectric. Its parameters are: radius of holes (r = 0.130 μm), lattice constant (a = 0.380 μm), and index of membrane (n = 3.181:InP). The numerical model used to simulate the structure of the demultiplexer is based on the finite difference time domain (FDTD).     

Highly birefringent photonic crystal fiber polarization splitter made of soft glass

A soft glass dual core polarization splitter based on highly birefringent photonic crystal fiber (PCF) is proposed and the full vector finite element method (FEM) is employed to analyze the impacts of structural parameters on birefringence and the coupling length, and simulation results show that high birefringence on the order of 10⁻² can be obtained at 1.55 μm, moreover, hole size, hole pitch and elliptic ratio all affect birefringence and the coupling length. Based on these results, the PCF's structure is optimized to realize a polarization splitter of 282 μm whose largest extinction ratio is around −45.42 dB at 1.55 μm. Meanwhile, the bandwidth at the extinction ratio of −10 dB is about 90 nm, and around 32 nm at −20 dB.     

Broad band polarizing beam splitter with a dielectric layer embedded in the groove of metal grating

We described the use of a metal grating embedded with a dielectric layer in the grating groove as a highly efficient polarizing beam splitter (PBS), to reflect s-polarized light and transmit p-polarized light in a wide spectrum band. This novel structure is designed at a wavelength of 1550 nm to be used in the optical communication wavelength band. High polarization efficiency and extinction ratio for both reflected and transmitted light are obtained by optimization of the designed structure. Moreover, it is shown that this structure can exhibit extremely high extinction ratio in the optical communication band with less dependence on the grating parameters and on the incidence angle, which greatly ease the fabrication of such kind of gratings.