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.     

Design and experiment of spectrometer based on scanning micro-grating integrating with angle sensor

A compact, low cost, high speed, non-destructive testing NIR (near infrared) spectrometer optical system based on MOEMS grating device is developed. The MOEMS grating works as the prismatic element and wavelength scanning element in our optical system. The MOEMS grating enables the design of compact grating spectrometers capable of acquiring full spectra using a single detector element. This MOEMS grating is driven by electromagnetic force and integrated with angle sensor which used to monitored deflection angle while the grating working. Comparing with the traditional spectral system, there is a new structure with a single detector and worked at high frequency. With the characteristics of MOEMS grating, the structure of the spectrometer system is proposed. After calculating the parameters of the optical path, ZEMAX optical software is used to simulate the system. According the ZEMAX output file of the 3D model, the prototype is designed by SolidWorks rapidly, fabricated. Designed for a wavelength range between 800 nm and 1500 nm, the spectrometer optical system features a spectral resolution of 16 nm with the volume of 97 mm × 81.7 mm × 81 mm. For the purpose of reduce modulated effect of sinusoidal rotation, spectral intensity of the different wavelength should be compensated by software method in the further. The system satisfies the demand of NIR micro-spectrometer with a single detector.     

High-power tunable Er,Yb ribbon fiber laser

A high-power Er,Yb double-clad ribbon fiber laser pumped by a 9-diode-bar pump module is reported. The laser yielded 102 W of continuous-wave output at 1566 nm for a launched pump power of 244 W, corresponding to a slope efficiency of ~ 44% with respect to launched pump power. Tunable operation was achieved using a simple external feedback cavity with a diffraction grating and the operating wavelength could be tuned from 1533 nm to 1567 nm. Temperature distribution in the ribbon fiber geometry and prospects of power scaling will be discussed.     

High-power single-longitudinal-mode operation of Tm:YAG laser using Fabry–Perot etalons and volume Bragg grating

A diode-pumped high-power single-longitudinal-mode (SLM) Tm:YAG laser was investigated. To obtain a single-frequency 2 μm laser output, Fabry–Perot (F–P) etalons combined with a volume Bragg grating (VBG) were used as frequency selection devices. The transmission losses of the VBG and etalons were analyzed and the angles of F–P etalons were optimized theoretically. Considering the gains and the insertion losses, the output wavelength of the Tm:YAG laser was estimated to be 2012.47 nm. Using this method, as much as 574 mW SLM laser was obtained experimentally, corresponding to a slope-efficiency of 18.6% and an optical-to-optical efficiency of 8.2%. The output wavelength was measured to be 2012.47 nm, which was in excellent agreement with the theoretical result. The power instability was less than 1% in 30 minutes test, and the degree of the linear polarization was over 20 dB.     

Polarization-independent rapidly tunable optical add-drop multiplexer utilizing non-polarizing beam splitters in Ti:LiNbO3

A polarization-independent four-port wavelength-tunable optical add drop multiplexer (OADM) that utilizes non-polarizing relaxed beam splitters has been analyzed and demonstrated in Ti:LiNbO₃ at the 1530 nm wavelength regime. The design utilizes an asymmetric interferometer configuration with strain induced index grating for polarization coupling along its arms that are shifted in position relative to each other. Experimental results of the filter response agree with theoretical predictions. Electrooptic tuning over a range of 15.7 nm at a rate of 0.08 nm/V has been measured. A temporal response < 46 ns to a 20 V step change in tuning voltage has been demonstrated. Fiber-to-fiber insertion loss is ~ 6.5 dB.     

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.     

Polarization insensitive hollow optical waveguide

A polarization insensitive hollow optical waveguide is proposed. The propagation characteristics of orthogonal polarizations in the hollow waveguide are effectively controlled in simulation to provide polarization insensitivity by tailoring the parameters associated with the two mirrors—a high-index contrast grating (HCG) mirror and a distributed Bragg reflecting (DBR) mirror, on either side of an air-core. The polarization insensitivity is evidenced by a low polarization dependence loss of 1.36 dB/cm and a low modal birefringence of 1.01 × 10^? 4.     

Nano-patterned visible wavelength filter integrated with an image sensor exploiting a 90-nm CMOS process

A highly efficient visible wavelength filter enabling a homogeneous integration with an image sensor was proposed and manufactured by employing a standard 90-nm CMOS process. A one dimensional subwavelength Al grating overlaid with an oxide film was built on top of an image sensor to serve as a low-pass wavelength filter; a microlens was then formed atop the filter to achieve beam focusing. The structural parameters for the filter were: a grating pitch of 300 nm, a grating height of 170 nm, and a 150-nm thick oxide overlay. The overall transmission was observed to reach up to 80% in the visible band with a decent roll-off near ～700 nm. Finally, the discrepancy between the observed and calculated result was accounted for by appropriately modeling the implemented metallic grating structure, accompanying an undercut sidewall.     

Multichannel wavelength division multiplexing system based on silicon rods of periodic lattice constant of hetero photonic crystal units

Characteristics of two different multichannel wavelength division multiplexing (WDM) systems composed of two-dimensional (2D) hetero photonic crystals (HPCs) are introduced. One utilizes five photonic crystal (PC) units, each fabricated with triangular and rectangular lattice. The other consists of five PC units in rectangular lattice. Both systems have a lattice constant difference of 4 nm between adjacent PC units, and both systems apply silicon rods with a radius of 120 nm. Finite-difference time-domain (FDTD) method and plan wave expansion (PWE) method reveal the ability of wavelength spacing ～8 nm with high quality factor (Q) in a system based on triangular and rectangular lattice; and ～8 nm with almost constant transmission efficiency based on rectangular lattice.     

An anti-reflective 1D rectangle grating on GaAs solar cell using one-step femtosecond laser fabrication

We report the fabrication of the anti-reflective micro/nano-structure on absorbing layer of GaAs solar cell surface using an efficient approach based on one-step femtosecond laser irradiation. Morphology of the microstructures and reflectance of the cell irradiated are characterized with SEM and spectrometer to analyze the influence of laser processing parameters on the change of microstructures induced and the reflectance. It has been found that the rectangle grating micro/nano-structure with a period of 700 nm and width of 600 nm is obtained neatly with laser pulse energy of 30.5 μJ(pulse duration is 130 fs, center wavelength is 800 nm, scanning speed is 2.2 mm/s and spot diameter is 22 µm). Reflectance has been suppressed to 23.6% with rectangle structure from 33% of planar cell. In addition, simulation using a finite-difference-time domain(FDTD) method results show that the rectangle grating micro/nano-structure can effectively suppress the reflection within large wavelength ranges.     

Metal-mirror-based grating for high-efficiency element in dense wavelength division multiplexing

It is desirable to obtain high efficiency with polarization-independence and wideband properties for incident wavelength. A metal-mirror-based grating is presented to diffract the incident wave into reflection orders with high efficiency for TE and TM polarization. The modal method and rigorous coupled-wave analysis (RCWA) are used together to optimize a metal-mirror-based grating effectively. From the analysis of the modal method, it is feasible to realize such a grating with the prescribed grating duty cycle and period. Accurate parameters of the grating depth and thickness of the connecting layer are optimized using RCWA. Compared with the reported binary simple grating, high efficiency can be improved greatly for the incident wavelength of 1550 nm in dense wavelength division multiplexing. The diffraction investigation indicates that a wideband property for incident wavelength can be obtained for such a novel metal-mirror-based grating.     

A linearly-polarized tunable Yb-doped fiber laser using a polarization dependent fiber loop mirror

A Ytterbium-doped linearly-polarized fiber laser is constructed with a polarization maintaining fiber Sagnac loop mirror. The fiber loop mirror made of polarization maintaining fiber coupler has a polarization dependent reflectivity, which provides the necessary polarization discrimination between the slow and fast axes. With a fiber Bragg grating written in normal polarization maintaining fiber as an output coupler, laser output of up to 5.6 W at 1070 nm is generated with a polarization extinction ratio of > 20 dB and an overall efficiency of 55%. The broadband polarization dependent reflection of the fiber loop mirror offers advantages of easy spectral tuning and simple linearly-polarized laser generation.     

Effect of impurities on the surface morphology of Fe(111)

The surface composition and morphology of Fe(111) have been examined through a combined analysis that includes low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The preferential segregation of sulfur has been clearly identified by AES upon annealing. The STM images exhibit numerous triangular pits of various sizes, and the LEED patterns show diffused n × 1 spots. The triangular pits reveal a Sierpinski gasket fractal. For sulfur-free Fe(111), nitrogen segregates to the surface upon annealing, forming a 4√3 × 4√3 superstructure that is identified by LEED patterns and STM images. The STM images show nanoscale triangular clusters regularly aligned in a hexagonal 4√3 × 4√3 configuration. Ultra-thin chromium film deposited on a nitrogen-segregated Fe(111) surface with post-annealing induces further nitrogen segregation, resulting in the formation of triangular pyramid-shaped CrN nanoclusters.     

Development of an efficient coherent optical source at 6.04 μm

Efficiency as high as 26% is obtained for generation of mid-infrared radiation at 6.04 μm by frequency doubling of ammonia laser emission at 12.08 μm in a 15 mm long type-I cut AgGaSe₂ crystal. The NH₃ laser used for this work is optically pumped by a commercial TEA CO₂ laser operating on 9.22 μm and produces pulsed output of ∼210 mJ with a duration of ∼200 ns at 12.08 μm. The generated radiation at 6.04 μm is separated out from the residual radiation at 12.08 μm by exploiting the principle of polarization dependent diffraction of reflection grating.     

Two dimensional exciton polaritons in microcavities with embedded quantum wires

Optical anisotropy of the periodical array of quantum wires embedded in a semiconductor microcavity is shown to result in polarization-dependent vacuum-field Rabi-splitting and a triple-anticrossing shape of the exciton–polariton dispersion curves. Both effects originate from the resonant diffraction of light at the grating of quantum wires. The calculation has been done within the nonlocal dielectric response theory and using the 4 × 4 transfer matrix technique.     

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.     

Tunable multi-wavelength fiber laser based on a polarization-maintaining erbium-doped fiber and a polarization controller

A stable and tunable multi-wavelength fiber laser with a polarization-maintaining erbium-doped fiber (PM-EDF) and a polarization controller (PC) is proposed and demonstrated. A homemade PM-EDF incorporated in the ring cavity is used as the gain medium. Simultaneous multi-wavelength oscillation is achieved at room temperature. The theory of the PM-EDF and PC to suppress the wavelength competition is described in detail. The 3 dB bandwidth is less than 0.01 nm. The power fluctuation and wavelength shift are measured to be less than 0.5 dB and 0.05 nm over 32 min. The wavelength tuning between single-, double-, triple-, and four-wavelength is realized.     

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.     

Er/Yb co-doped fiber amplifier with wavelength-tuned Yb-band ring resonator

Erbium-ytterbium co-doped fiber amplifier with wavelength-tuned Yb-band loop resonator is presented. The amplified spontaneous emission (ASE) from Yb ions is utilized to stimulate a laser emission at several wavelengths from the 1 μm band in the 1550 nm amplifier. The wavelength of this lasing is tuned by introducing a fiber Bragg grating (FBG). The results show, that the overall efficiency of the amplifier at nominal 1550 nm wavelength can be increased by introducing a feedback loop with 1040 nm and 1050 nm FBG. This loop also protects the Er/Yb amplifier from parasitic lasing at 1 μm and allows significant output power scaling without risk of self-pulsing.     

Realization of periodic transmission filter based on a pair of cascaded long-period fibre gratings of different strength/wavelength position

Using high-intensity (560–650 GW/cm²) 264 nm 220 femtosecond laser pulses, we inscribed a periodic (comb) transmission filter in a photosensitive Ge/B-codoped fibre, based on a pair of long-period gratings of different strength/wavelength position. The irradiation conditions and grating parameters for the successful realization of the 24–28-nm-wide transmission filter in the region 1480–1580 nm with the fringe period of 1.7–3.1 nm and the fringe bandwidth of 0.8–1.3 nm were established.