Breaking the optical efficiency limit of virtual reality with a nonreciprocal polarization rotator

A catadioptric lens structure,also known as pancake lens,has been widely used in virtual reality(VR)displays to reduce the formfactor.However,the utilization of a half mirror(HM)to fold the optical path thrice leads to a significant optical loss.The theoretical maximum optical efficiency is merely 25%.To transcend this optical efficiency constraint while retaining the foldable characteristic inherent to traditional pancake optics,in this paper,we propose a theoretically lossless folded optical system to replace the HM with a nonreciprocal polarization rotator.In our feasibility demonstration experiment,we used a commercial Faraday rotator(FR)and reflective polarizers to replace the lossy HM.The theoretically predicted 100%efficiency can be achieved approximately by using two high-extinction-ratio reflective polarizers.In addition,we evaluated the ghost images using a micro-OLED panel in our imaging system.Indeed,the ghost images can be suppressed to undetectable level if the optics are with antireflection coating.Our novel pancake optical system holds great potential for revolutionizing next-generation VR displays with lightweight,compact formfactor,and low power consumption.

A band-pass frequency selective surface with polarization rotation

A band-pass frequency selective surface(FSS) with polarization rotation property is proposed. The proposed polarization rotating FSS(PR-FSS) is a two-dimensional periodic structure, its unit cell is an antenna-filter-antenna(AFA) module,and the polarization directions of the upper and lower antennas in each AFA module are orthogonal to each other, so the PR-FSS can achieve frequency selection and 90 degrees polarization rotation at the same time. The numerical simulation demonstrate that the anticipated frequency selection and polarization rotation are realized by the PR-FSS in the frequency band from 8.84 GHz to 10.30 GHz with a relative bandwidth of 15.26%, and the maximum insertion loss in the pass band is only 0.17 d B. Finally, one effective experiment validation is carried out, a reasonable agreement is observed between the experimental and simulated results except for a slight deviation caused by fabrication error and measurement tolerance.

All-Optical Sampling Using Nonlinear Polarization Rotation in a Single Semiconductor Optical Amplifier

We propose a novel all-optical sampling method using nonlinear polarization rotation in a semiconductor optical amplifier. A rate-equation model capable of describing the all-optical sampling mechanism is presented in this paper. Based on this model, we investigate the optimized operating parameters of the proposed system by simulating the output intensity of the probe light as functions of the input polarization angle, the phase induced by the polarization controller, and the ori- entation of the polarization beam splitter. The simulated results show that we can obtain a good linear slope and a large linear dynamic range,which is suitable for all-optical sampling. The operating power of the pump light can be less than lmW. The presented all-optical sampling method can potentially operate at a sampling rate up to hundreds GS/s and needs low optical power.

Ellipsometric configurations using a phase retarder and a rotating polarizer and analyzer at any speed ratio

In this paper, we propose an ellipsometer using a phase retarder and rotating polarizer and analyzer at a speed ratio I：N. Different ellipsometric configurations are presented by assuming N = 1, 2, and 3. Moreover, two values of the offset angle of the retarder are considered for each ellipsometric configuration. The Mueller formalism is employed to extract the Stokes parameters, from which the intensity received by the detector is obtained. The optical properties of c-Si are calculated using all configurations. A comparison between different configurations is carried out considering the effect of the noise on the results and the uncertainties in the ellipsometric parameters as functions of the uncertainties of the Fourier coefficients. It is found that the alignment of the phase retarder has a crucial impact on the results and the ellipsometric configuration with speed ratio 1：1 is preferred over the other configurations.

Towards 640 Gbit/s wavelength conversion based on nonlinear polarization rotation in a semiconductor optical amplifier

Taking into account ultra-fast carrier dynamics, this paper models 640 Gbit/s wavelength conversion scheme based on nonlinear polarization rotation （NPR） in a single semiconductor optical amplifier （SOA） and investigates the performance of this kind of wavelength conversion scheme in detail. In this model, two carrier temperature equations are introduced to substitute two energy density equations, which reduce the complexity of calculation in comparison with the previous model. The temporary gain and phase shift dynamics induced by ultra-short optical pulses are numerically simulated and the simulated results are qualitatively in good agreement with reported experimental results. Simulated results show that non-inverted and inverted 640 Gbit/s wavelength conversions based on NPR are achieved with clear open eye diagrams. To further investigate the performance of the non-inverted wavelength conversion scheme, the dependence of output extinction ratio （ER） on some key parameters used in simulation is illustrated. Furthermore, simulated analyses show that high performance non-inverted wavelength conversion based on NPR can be achieved by using a red-shifted filtering scheme.

Generation of diffraction-free vectorial elliptic hollow beams with space-varying inhomogeneous polarizations

Diffraction-free vectorial elliptic hollow beams(vEHBs)are generated by an optical system composed of a short elliptic hollow fiber(EHF)and an axicon.Each beam has a closed elliptic annular intensity profile and space-varying polarization states in its diffraction-free distance of more than 1 m.The generated beams have a counter-clockwise or clockwise periodically-rotated inhomogeneous polarization.And the spin angular momentum(SAM)of the vEHBs is 1ħor-1ħwhich is consistent with the type of dual-mode in the EHF and the periodic polarization rotations of the vEHBs.The vEHBs have potential applications in optically trapping and micromanipulating the micro-or nano-particles,quantum information transmission,and Bose-Einstein condensates,etc.

Frequency-selective microwave polarization rotator using substrate-integrated waveguide cavities

A frequency selective polarization rotator that can rotate the polarization angle of an incident electromagnetic wave at the microwave frequency by 45 is presented. The polarization rotator is based on a two-dimensional periodic array of substrate integrated waveguide cavities, realizing the polarization rotation by coupling the input signal to the output wave through three metallic slots. Two layers of frequency selective surfaces are cascaded by substrate and form the polarization rotator. A vertical slot on the top layer is used to select the horizontal polarization from the incident wave, the vertical and the horizontal slots on the bottom layer are, respectively, used to obtain horizontally and vertically polarized outgoing waves. The two orthogonal outgoing waves are combined to result in the 45~ polarized wave. Both full wave simulation and experimental measurement are carried out, together validating the proposed method.

Passive polarization rotator based on silica photonic crystal fiber for 1.31-μm and 1.55-μm bands via adjusting the fiber length

A new polarization rotator based on the silica photonic crystal fiber is proposed. The proposed polarization rotator photonic crystal fiber （PR-PCF） possesses a triangle jigsaw-shape core region. The full-vector finite-element method is used to analyze the phenomenon of polarization conversion between the quasi-TE and quasi-TM modes. Numerical simulations show that the wavelengths of 1.31 μm and 1.55 μm are converted with a nearly 100% polarization conversion ratio with their matched coupling length and has a relatively strong realistic fabrication tolerance - 100 nm on the y axis and 50 nm on the x axis. The full vectorial finite difference beam propagation method is used to confirm the performance of the proposed PR-PCF.

An all-optical buffer based on polarization rotation in an EAM

A theoretical model of the refractive index changes of the TE and TM modes in an electro-absorption modulator （EAM） is deduced. The photon absorption and refractive index changes are analyzed numerically. The influence of pump intensity on the phase difference between the TE and TM modes is studied. The polarization rotation effect is obtained in the EAM, and a novel all-optical fiber loop buffer is designed.

Dynamo saturation in rapidly rotating solar-type stars

The magnetic activity of solar-type stars generally increases with stellar rotation rate. The increase, however, saturates for fast rotation. The Babcock-Leighton mechanism of stellar dynamos saturates as well when the mean tilt angle of active regions approaches ninety degrees. Saturation of magnetic activity may be a consequence of this property of the Babcock-Leighton mechanism. Stellar dynamo models with a tilt angle proportional to the rotation rate are constructed to probe this idea.Two versions of the model- treating the tilt angles globally and using Joy＇s law for its latitude dependence- are considered. Both models show a saturation of dynamogenerated magnetic flux at high rotation rates. The model with latitude-dependent tilt angles also shows a change in dynamo regime in the saturation region. The new regime combines a cyclic dynamo at low latitudes with an（almost） steady polar dynamo.

O-band reconfigurable silicon polarization rotator

Silicon waveguides typically exhibit optical anisotropy,which leads to polarization correlation and single-polarization operations.This consequently creates a demand for polarization-control devices.This paper introduces a CMOS-compatible O-band reconfigurable TE/TM polarization rotator comprising two symmetrical polarization rotator-splitters and phase shifters.This configuration enables dynamic conversion of any linear polarization to its quadratic equivalent.Experimental results indicate that the reconfigurable polarization rotator exhibits an insertion loss of less than 1.5 dB.Furthermore,the bandwidth for a polarization extinction ratio beyond 15 dB exceeds 60 nm.

Generation of cavity-birefringence-dependent multi-wavelength bright-dark pulse pair in a figure-eight thulium-doped fiber laser

We experimentally demonstrated a stable multi-wavelength bright-dark pulse pair in a mode-locked thulium-doped fiber laser(TDFL).The nonlinear polarization rotation(NPR)and nonlinear optical loop mirror(NOLM)were employed in a figure-eight cavity to allow for multi-wavelength mode-locking operation.By incorporating different lengths of high birefringence polarization-maintaining fiber(PMF),the fiber laser could operate stably in a multi-wavelength emission state.Compared with the absence of the PMF,the birefringence effect caused by PMF resulted in rich multi-wavelength optical spectra and better intensity symmetry and stability of the bright-dark pulse pair.

A theoretical and experimental study on all-normal-dispersion Yb-doped mode-locked fiber lasers

We report on a theoretical and experimental study of an all-normal-dispersion （ANDi） Yb-doped mode-locked fiber laser, in which nonlinear polarization rotation （NPR） is used to realize mode-locking without any dispersion compensation. Based on the coupled nonlinear Schr6dinger （CNLS） equation, a model simulating the mode-locked process of an all-normal-dispersion ring fiber laser is developed, which shows that the achievement of stable mode-locking depends on the alignment of the polarization controller （PC） along the fast-polarization axis of the fiber, the birefringence intensity, and the net cavity dispersion. According to the theoretical analysis, stable mode-locked pulses with pulse duration 300 ps and average output power 33.9 mW at repetition rate 36 MHz are obtained.

Transition state to mode locking in a passively mode-locked erbium-doped fibre ring laser

The transition state between the continuous wave region and the mode-locked region in a passively mode-locked erbium-doped fibre ring laser has been experimentally observed by utilizing the nonlinear polarization rotation technique. When the pump power reaches the mode-locked threshold, the metastable pulse train with a tunable repetition rate is obtained in the transition from the continuous wave state to the passive mode-locked state via proper adjustment of the polarization controller. A simpie model has been established to explain the experimental observation.

Energy and rotation-dependent stereodynamics of H(~2S) + NH(a^1?) → H_2(X^1Σ_g^+) + N(~2D) reaction

Quasi-classical trajectory calculations are performed to study the stereodynamics of the H（~2S） ＋ NH（a^1？） →H_2（X^1Σ_g~＋） ＋ N（~2D） reaction based on the first excited state NH_2（1~2A＇） potential energy surface reported by Li et al.[Li Y Q and Varandas A J C 2010 J. Phys. Chem. A 114 9644] for the first time. We observe the changes of differential cross-sections at different collision energies and different initial reagent rotational excitations. The influence of collision energy on the k-k＇ distribution can be attributed to a purely impulsive effect. Initial reagent rotational excitation transforms the reaction mechanism from insertion to abstraction. The effect of initial reagent rotational excitations on k-k＇ distribution can be explained by the rotational excitation enlarging the rotational rate of reagent NH in the entrance channel to reduce the probability of collision between incidence H atom and H atom of target molecular. We also investigate the changes of vector correlations and find that the rotational angular momentum vector j＇ of the product H_2 is not only aligned, but also oriented along the y axis. The alignment parameter, the disposal of total angular momentum and the reaction mechanism are all analyzed carefully to explain the polarization behavior of the product rotational angular moment.

Theoretical study of stereodynamics for reaction O（3p）＋HCl

The vector correlation between products and reagents for reaction O（3P）＋HCl→OH＋Cl is studied using a quasi- classical trajectory （QCT） method on the benchmark potential energy surface of the ground 3A″ state [Ramachandran and Peterson, J. Chem. Phys. 119 （2003） 9550]. The generalised differential cross section （2π/σ）（dσ00/dwt） is presented in the centre of mass frame. The distribution of dihedral angles, P（Фr）, and the distribution of angles between k and j′, P（θr）, are calculated. The influence of the collision energy and the influence of the reagent rotation and vibration on the product polarization are studied in the present work. The calculated results indicate that the rotational polarization of product molecule is almost independent of collision energy but sensitive to the reagent rotation and vibration. product rotational polarization, vector correlations, differential cross sections, quasi- classical trajectory

Dynamic polarization rotation and vector field steering based on phase change metasurface

Polarization rotation and vector field steering of electromagnetic wave are of great significance in modern optical applications.However,conventional polarization devices are bulky,monofunctional and lack of tunability,which pose great challenges to the miniaturized and multifunctional applications.Herein,we propose a meta-device that is capable of multi-state polarization rotation and vector field steering based on phase change metasurface.The supercell of the meta-device consists of four Ge2Sb2Te5(GST)elliptic cylinders located on a SiO2 substrate.By independently controlling the phase state(amorphous or crystalline)of each GST elliptic cylinder,the meta-device can rotate the polarization plane of the linearly polarized incident light to different angles that cover from 19.8°to 154.9°at a wavelength of 1550 nm.Furthermore,by merely altering the phase transition state of GST elliptic cylinders,we successfully demonstrated a vector field steering by generating optical vortices carrying orbital angular momentums(OAMs)with topological charges of 0,1 and–1,respectively.The proposed method provides a new platform for investigating dynamically tunable optical devices and has potential applications in many fields such as optical communications and information processing.

Noise-like pulse generation from a thulium-doped fiber laser using nonlinear polarization rotation with different net anomalous dispersion

A mode-locked thulium-doped fiber laser(TDFL) based on nonlinear polarization rotation(NPR) with different net anomalous dispersion is demonstrated. When the cavity dispersion is-1.425 ps^2, the noise-like(NL) pulse with coherence spike width of 406 fs and pulse energy of 12.342 nJ is generated at a center wavelength of 2003.2 nm with 3 dB spectral bandwidth of 23.20 nm. In the experimental period of 400 min, the 3 dB spectral bandwidth variation, the output power fluctuation, and the central wavelength shift are less than 0.06 nm, 0.04 d B, and0.4 nm, respectively, indicating that the NPR-based TDFL operating in the NL regime holds good long-term stability.

Design of a C-band polarization rotator-splitter based on a mode-evolution structure and an asymmetric directional coupler

A C-band polarization rotator-splitter based on a mode-evolution structure and an asymmetric directional coupler is proposed. The mode-evolution structure is designed in a bi-level taper through which the TM;mode can evolve into the TE;mode. Then the TE;mode is coupled to the TE;mode at the cross port using the asymmetric directional coupler. The input TE;mode propagates along the waveguide without mode conversion and output at the through port. From the experimental results, the extinction ratio is lower than 30 dB and the excess loss is less than 1 dB for input TE;mode at the whole C-band. For input TM;mode, the ER and the EL are, respectively,lower than-10 and 1.5 dB.

Temporal contrast enhancement by nonlinear elliptical polarization rotation in a multi-pass cell

We demonstrate the simultaneous temporal contrast improvement and pulse compression of a Yb-doped femtosecond laser via nonlinear elliptical polarization rotation in a solid state multi-pass cell.The temporal contrast is improved to 109,while the pulse is shortened from 181 to 36 fs,corresponding to a compression factor of 5.The output beam features excellent beam quality with M^(2) values of 1.18×1.16.The total efficiency of the contrast enhancement system exceeds 50%.This technique will have wide applications in high temporal contrast ultra-intense femtosecond lasers.