High Polarization Extinction Ratio Research Articles

High-extinction-ratio directional-coupler-type polarization beam splitter with a bridged silicon wire waveguide.

We demonstrate an integrated polarization beam splitter (PBS) of a simple and compact device geometry with high polarization extinction ratios (PERs). A silicon PBS based on a three-waveguide directional coupler is numerically simulated for optimum device parameters, and fabricated experimentally. The measured PER values of the fabricated PBSs are 40.74dB and 39.01dB for transverse electric and transverse magnetic modes, respectively, and their corresponding insertion losses are 0.35dB on average for a coupler length of about 29.4μm.

Power Scaling of Linearly Polarized Random Fiber Laser

The power scaling of linearly polarized random fiber lasers (RFLs) is investigated in detail based on master oscillator power amplifier configuration. A 442-W linearly polarized output with a narrow 3-dB linewidth of 0.28 nm and a 621-W linearly polarized output with a relatively wider 3-dB linewidth of 2.7 nm are realized. Besides, near-diffraction-limited beam quality and high polarization extinction ratio are obtained in both situations, and further power scaling is limited by the onset of mode instability (MI). To the best of our knowledge, these results are highest output powers of linearly polarized RFLs for the time being, in the narrow-linewidth and wide-linewidth situations, respectively. Meanwhile, the differences in the spectral broadening factor and the MI threshold between the narrow-linewidth linearly polarized RFL and the wide-linewidth one are discovered and discussed for the first time. It is experimentally demonstrated that relatively wider linewidth of the RFL seed is in favor of reducing the spectral broadening and increasing the MI threshold, providing significant reference for power scaling of linearly polarized RFL.

All-fiber linearly polarized laser oscillator by fiber coiling loss control**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB1104500), the National Natural Science Foundation of China (Grant No. 61675114), and theTsinghua University (THU) Initiative Scientific Research Program, China (Grant No. 20151080709).

In this paper, we demonstrate an all-fiber linearly polarized fiber laser oscillator. The single polarization of the oscillator is achieved through the careful designing of the active fiber coiling. The relationship between fiber coiling diameter and polarization extinction ratio and optical efficiency is studied, whose results lead to an optimized system. The thermal management of the oscillator is also refined, which allows the oscillator to reach a maximum output power of 44.1 W with an optical-to-optical efficiency of 57.9%. A high average polarization extinction ratio of 21.6 dB is achieved during a 2-hour stability test. The oscillator also owns a narrow 3-dB bandwidth of 0.1 nm, as well as near-diffraction-limit beam quality of .

Design of a High Extinction Ratio Tunable Graphene on White Graphene Polarizer

A thermally/electrically tunable graphene on white graphene polarizer is proposed, in which the resonant coupling between the plasmonic surface modes of the structure and the transverse electric (TE) [or transverse magnetic (TM)] polarized incident wave is used to absorb this polarization, while the TM (or TE) polarized incident wave is totally reflected. It is then shown that the thermal and electrical tunability of surface conductivity of graphene can be used to control the optical properties of the proposed polarizer, including the selection of the desired polarization, and adjusting the amplitude of the reflected (desired) polarization. The application of the hexagonal boron-nitride (white graphene) as the substrate of graphene increases the propagation of the surface waves of the structure, which in turn results in the very high polarization extinction ratio of 75 dB. Moreover, the ultra-small insertion loss of 0.022 dB and relatively large bandwidth of ~ 60 nm are calculated for the proposed polarizer.

Ultracompact, high extinction ratio polarization beam splitter-rotator based on hybrid plasmonic-dielectric directional coupling.

A polarization beam splitter-rotator based on asymmetric directional coupling is proposed. An ultrashort cross-polarization coupling length of 7.7μm is achieved by manipulating the optical field distribution via the plasmonic effect, which is the shortest one reported so far, to the best of our knowledge. At the wavelength of 1.55μm, the extinction ratios are as high as 50.9dB and 28.2dB for fundamental transverse magnetic (TM0) and transverse electric (TE0) polarizations, respectively, and the corresponding insertion losses are 1.545dB and 0.037dB. In addition, the TM0-to-TE0 polarization conversion efficiency is higher than 95% within a bandwidth of 70nm.

Multifunctional Metamirror: Polarization Splitting and Focusing

Metasurfaces are paving the way to improve traditional optical components by integrating multiple functionalities into one optically flat metasurface design. We demonstrate the implementation of a multifunctional gap surface plasmon-based metasurface which, in reflection mode, splits orthogonal linear light polarizations and focuses into different focal spots. The fabricated configuration consists of 50 nm thick gold nanobricks with different lateral dimensions, organized in an array of 240 nm x 240 nm unit cells on the top of a 50 nm thick silicon dioxide layer, which is deposited on an optically thick reflecting gold substrate. Our device features high efficiency (up to ~65%) and polarization extinction ratio (up to ~30 dB), exhibiting broadband response in the near-infrared band (750-950 nm wavelength) with the focal length dependent on the wavelength of incident light. The proposed optical component can be forthrightly integrated into photonic circuits or fiber optic devices.

Design of a tunable graphene plasmonic-on-white graphene switch at infrared range

A tunable Y-branch graphene plasmonic switch operating at the wavelength of 1.55 μm is proposed in which graphene is placed on white graphene. The switch structure is investigated analytically and numerically by the finite difference time domain method. The graphene plasmonic switch considered here supports both transverse magnetic and transverse electric graphene plasmons whose propagation characteristics can be controlled by modulating the external electric field and the temperature of graphene. Our calculations show that by strong coupling between the incident waves and the graphene plasmons of the structure, a high polarization extinction ratio of 45 dB and relatively large bandwidth of 150 nm around the central wavelength of 1.55 μm are achievable. Furthermore, the application of white graphene as the substrate of graphene decreases the propagation loss of the graphene plasmons and the required applied electric field. It is also shown that the propagation mode of the graphene plasmons can be tuned by changing the temperature and the calculated threshold temperature is 650 K.

Polarization splitting phenomenon of photonic crystals constructed by two-fold rotationally symmetric unit-cells

We present an intrinsic polarization splitting characteristic of low-symmetric photonic crystals (PCs) formed by unit-cells with C2 rotational symmetry. This behavior emerges from the polarization sensitive self-collimation effect for both transverse-magnetic (TM) and transverse-electric (TE) modes depending on the rotational orientations of the unit-cell elements. Numerical analyzes are performed in both frequency and time domains for different types of square lattice two-fold rotational symmetric PC structures. At incident wavelength of λ = 1550 nm, high polarization extinction ratios with ∼26 dB (for TE polarization) and ∼22 dB (for TM polarization) are obtained with an operating bandwidth of 59 nm. Moreover, fabrication feasibilities of the designed structure are analyzed to evaluate their robustness in terms of the unit-cell orientation: for the selected PC unit-cell composition, corresponding extinction ratios for both polarizations still remain to be over 18 dB for the unit-cell rotation interval of θ = [40°–55°]. Taking all these advantages, two-fold rotationally symmetric PCs could be considered as an essential component in photonic integrated circuits for polarization control of light.

Submicron-scale broadband polarization beam splitter using CMOS-compatible materials

We propose a polarization beam splitter (PBS) with a footprint of only 600 × 790 nm2 operating at a wavelength of λ = 1550 nm, which is the smallest PBS ever demonstrated. This device uses CMOS-compatible materials, namely, silicon and silica. The present PBS comprises two Si waveguides with different geometrical aspect ratios adjoined side-by-side, which separates the transverse-electric (TE) and transverse-magnetic (TM) modes without relying on an additional coupling region. The designed PBS achieves a polarization extinction ratio of approximately 25 dB for both modes and insertion losses of approximately 0.87 and 1.09 dB for the TE and TM polarizations, respectively. Over a wide bandwidth of 150 nm (from λ = 1475–1625 nm), a high polarization extinction ratio (greater than 20 dB) and a low inversion loss (lower than 1.3 dB) can be obtained. The proposed PBS allows for geometrical errors of ±15 nm while maintaining a polarization extinction ratio of >20 dB and inversion losses of >1.1 and 1.3 dB for the TE and TM modes, respectively. With the submicron footprint, the reported PBS may be able to be used in high-density photonic integrated circuits and nanophotonic devices.

High extinction ratio D-shaped fiber polarizers coated by a double graphene/PMMA stack.

We demonstrate theoretically and experimentally a high extinction ratio and compact size TE-pass polarizer made by a D-shaped fiber coated with a double graphene/PMMA stack. The light propagating in the core of the fiber can be efficiently coupled into the graphene sheet thanks to the giant enhancement of the modal evanescent field associated with the high refractive index graphene/PMMA cladding. The strong interaction between the light and graphene produces a large attenuation difference between modes with orthogonal polarizations, resulting in an improved extinction ratio and a reduced insertion loss due to the device compactness. A double graphene/PMMA stack coated polarizer with an extinction ratio of up to 36 dB and an insertion loss of 5 dB has been achieved when the device length is only 2.5 mm. The double graphene/PMMA stack has proved to be significantly better than single graphene/PMMA stack and bilayer graphene/PMMA structures, providing a polarizer with maximum extinction ratio of 44 dB for a length of 4 mm. The achieved results indicate that the proposed high extinction ratio polarizer is a promising candidate for novel in-fiber graphene-based devices.

Wavelength tunable, linearly polarized mode-locked fiber laser using chirped fiber Bragg grating

We demonstrate a wavelength-tunable, linearly polarized mode-locked fiber laser using carbon nanotube and chirped fiber Bragg grating. The chirped fiber Bragg grating is mounted on a plastic cantilever and serves as a tunable spectral filter. Linearly polarized mode-locked pulses are obtained with typical pulse duration of ~7.89 ps and repetition rate of 8 MHz, respectively. The high polarization extinction ratio is maintained over 30 dB for a long period of time. The central wavelength of the mode-locked laser is tuned continuously by adjusting the cantilever, while maintaining the polarization state.

Generation of 180 W average green power from a frequency-doubled picosecond rod fiber amplifier.

We report on the generation of 180 W average green power from a frequency-doubled picosecond rod fiber amplifier. In an Yb-doped fiber master-oscillator-power-amplifier system, 2.3-ps 704 MHz pulses are first amplified in small-core fibers and then in large-mode-area rod fibers to produce 270 W average infrared power with a high polarization extinction ratio and diffraction-limited beam quality. By carrying out frequency doubling in a lithium triborate (LBO) crystal, 180 W average green power is generated. To the best of our knowledge, this is the highest average green power achieved in fiber-based laser systems.

High-extinction ratio and short-length polarization splitter based on microstructured optical fiber with tellurite glass

A new type of tellurite glass based on dual-core microstructured optical fiber with an Au wire has been numerically evaluated. This novel substance is a suitable optical fiber material for nonlinear applications due to its high nonlinear coefficient. A full-vector finite element method is employed to analyze the characteristics of the polarization splitter. A splitter with an ultrashort length of 1.079 mm and a high extinction ratio of 174.92 dB at the wavelength of 1.55 μm has been obtained. Moreover, the splitter exhibits a bandwidth with an extinction ratio as high as 20 dB of about 70 nm. An effective mode area Aeff of the optical fiber of 21.53 μm2 for one of the supermodes of the x even mode has been calculated.

High Extinction Ratio Polarization Beam Splitter Design by Low-Symmetric Photonic Crystals

A novel concept of polarization beam splitting with high polarization extinction ratio is proposed based on the polarization sensitive self-collimation feature of symmetry reduced photonic crystals. The idea utilizes self-collimation mechanism and rotated isofrequency contours arising due to the deliberately implemented symmetry reduction in the photonic structure. Square lattice of low-symmetric rectangular air holes in dielectric background are numerically analyzed in both frequency and time domains. The operating bandwidth of the device is Δ λ = 53 nm. At λ = 1.55 μm, polarization extinction ratios are ∼23 and ∼18 dB for transverse electric and transverse magnetic ports, respectively. The investigated device is 46.4 μm × 12.4 μm in size and based on a uniform, planar, and homogenous structure. Therefore, there is no need for an additional splitting assistance to separate the two orthogonal polarizations. Possible fabrication imperfections are also analyzed and it is observed that polarization extinction ratios stay above 17 dB when error percentage is around 5.5%. With having these advantages, symmetry reduced photonic crystal-based polarization beam splitters can be a good candidate for optical communication applications.

Environmentally Stable Pulse Energy-Tunable Picosecond Fiber Laser

We have reported a mode-locked pulse energy tunable ytterbium-doped fiber laser constructed by the polarization-maintaining (PM) fibers with a position-tunable focuser and a semiconductor-saturable absorber mirror (SESAM). The narrow bandwidth fiber Bragg grating in the cavity produces self-starting picosecond (ps) mode-locked operation. The linear laser cavity achieved tunable single pulse energy varied from 0.591 to 0.988 nJ, with a 24.7-MHz repetition rate and a 6.03-ps pulse duration at 1064.18 nm. High environmental stability is also confirmed by a high polarization extinction ratio over 17.8 dB.

Broadband high birefringence and polarizing hollow core antiresonant fibers.

We systematically study different approaches to introduce high birefringence and high polarization extinction ratio in hollow core antiresonant fibers. Having shown the ineffectiveness of elliptical cores to induce large birefringence in hollow core fibers, we focus on designing and optimizing polarization maintaining Hollow Core Nested Antiresonant Nodeless Fibers (HC-NANF). In a first approach, we create and exploit anti-crossings with glass modes at different wavelengths for the two polarizations. We show that suitable low loss high birefringence regions can be obtained by appropriately modifying the thickness of tubes along one direction while leaving the tubes in the orthogonal direction unchanged and in antiresonance. Using this concept, we propose a new birefringent NANF design providing low loss (~40dB/km) and high birefringence (>10-4) over a record bandwidth of ~550nm, and discuss how bandwidth can be traded off to further reduce the loss to a few dB/km. Finally, we propose a polarization mode-stripping technique in the birefringent NANF. As a demonstration, we propose a polarizing birefringent NANF design that can achieve orthogonal polarization loss ratios as large as 30dB over the C-band while eliminating any undesirable polarization coupling effect thereby resulting in a single polarization output in a hollow core fiber regardless of the input polarization state.

Ultracompact transverse magnetic mode-pass filter based on one-dimensional photonic crystals with subwavelength structures.

We propose and experimentally demonstrate an ultracompact transverse magnetic (TM) mode pass filter based on a rectangularly-shaped one-dimensional (1-D) photonic crystal silicon waveguide with an extremely high polarization extinction ratio (PER) of >30 dB and a low insertion loss (IL) of ~1 dB. The device structure of the TM mode-pass filter is numerically simulated using a three-dimensional (3-D) finite difference time domain (FDTD) method. The proposed device supports its fundamental TM mode only, whereas the transverse electric (TE) mode is reflected by the 1-D photonic crystals (PhCs). The measured PER of the fabricated TM mode-pass filter is ~34 dB, and the IL is about 1 dB. The entire device length is about 4 μm. Our simulation results predict that the device bandwidth of 30 dB PER is about 200 nm.

An Ultra-broadband Single Polarization Filter Based on Plasmonic Photonic Crystal Fiber with a Liquid Crystal Core

An ultra-broadband single polarization filter based on plasmonic photonic crystal fiber with a liquid crystal core is investigated using the full-vectorial finite element method. Numerical simulations show that the loss of x-polarized core mode is better than 248.95 dB/cm in a 1.25–2.1 μm wavelength range, while the corresponding loss of y-polarized core mode is lower than 0.21 dB/cm. The high polarization extinction ratio is obtained at the communication wavelength of 1.3 μm, where the losses of x- and y-polarized core modes are 433.25 and 0.0064 dB/cm, respectively. When the fiber length is 1 mm, a broad bandwidth of 850 nm with an extinction ratio lower than −20 dB is obtained. Moreover, the proposed single polarization filter exhibits a better tolerance of realistic fabrication.

Polarization splitter based on dual-core photonic crystal fiber with octagonal lattice

The polarization characteristics of a photonic crystal fiber with octagonal lattice are evaluated by using the finite element method. The coupled-mode theory between two cores is introduced. A novel high extinction ratio and short length polarization splitter based on dual-core octagonal photonic crystal fiber is designed in this paper. Numerical results show that the extinction ratio at the wavelength of 1.55 µm can reach to −175.01 dB, and the bandwidth as the extinction ratio as low as −15 dB is about 82 nm. Besides, a 0.8518-mm-long photonic crystal fiber is better applied to the polarization beam splitter at the communication band of λ = 1.55 µm.

Fabrication and application of a graphene polarizer with strong saturable absorption

By transferring 100 nm gold-coated CVD monolayer graphene onto the well-polished surface of D-shaped fiber, we achieve a graphene in-line polarizer with a high polarization extinction ratio of ∼27 dB and low insertion loss of 5 dB at 1550 nm, meanwhile achieving a strong saturable absorption effect of 14%. The manufacture of this graphene in-line polarizer also simplifies the graphene transfer process. To explore the potential applications of the new device, we also demonstrate noise-like pulse generation and supercontinuum spectrum generation. By launching the designed graphene device into a fiber ring laser cavity, 51 nm bandwidth noise-like pulse is obtained. Then, launching the high-power noise-like pulse into high nonlinear fiber, a 1000 nm wide supercontinuum spectrum is obtained, which is favorable for sensing and nonlinearities scientific fields.