Polarized Input Light Research Articles

Polarization-independent orbital angular momentum generator based on a chiral fiber grating.

A polarization-independent orbital angular momentum (OAM) generator, based on a chiral fiber grating (CFG), is proposed and demonstrated for the first time, to the best of our knowledge. Helical phases were successfully excited in this device for the arbitrary polarization states of the input light, and the resulting OAM modes could be produced with a desired polarization state. The employed CFG was fabricated by twisting a fused few-mode fiber during hydrogen-oxygen flame heating. The polarization characteristics, helical phase, and purity of the coupled modes in the CFG were experimentally investigated for varying input light polarization states. The results showed that the coupled higher-order modes had the same polarization as the input fundamental mode. The chirality of the excited OAM was polarization-independent and determined solely by the helicity of the CFG. The purity of the OAM mode was higher than 93% with a slight fluctuation of 2% which occurred during measurement processing.

Degree of optical polarization as a tool for detecting melanoma: proof of principle.

Determining the optical polarization properties of a skin lesion is a proposed method to differentiate melanoma from other skin lesions. We developed an in vivo Stokes polarimetry probe that fires a laser of known polarization at the skin and measures the Stokes parameters of the backscattered light in one shot. From these measured Stokes parameters, we can calculate the degree of polarization (DOP). Through testing on rough skin phantoms, a correlation between backscattered DOP and skin roughness was identified for both linear and circular input polarization, the latter of which was found to be more useful. In a pilot clinical trial of 69 skin lesions in vivo, it was found that the mean DOP for melanoma (linear input on melanoma: 0.46 ± 0.09) was greater than that of other lesions (linear input on all other lesions: 0.28 ± 0.01). This separation is greater for circular polarized input light, and it is likely that circular polarized light's greater sensitivity to surface roughness contributes to this result. In addition, all skin lesions demonstrated a stronger depolarizing effect on circular polarized light than linear polarized light. We have identified DOP as a potentially useful measurement to identify melanoma among other types of skin lesions.

Twisted tilted fiber Bragg gratings: new structures and polarization properties.

In this Letter, it has been shown that the twisting of TFBGs (tilted fiber Bragg gratings) decreases their sensitivity to the polarization of the input light. It has been proved by a theoretical simulation study that twisting the TFBG by 180° eliminates the sensitivity of the grating to input light polarization changes. We have experimentally proved that our structure produced has an 18 times lower coefficient of variation of transmission for polarization changes than the untwisted TFBG. This Letter also presents a new method for manufacturing the twisted tilted fiber Bragg grating (TTFBG). We have demonstrated the possibility of writing the TTFBG structure twisted by 90° on a fiber with a length of 10mm. The structure has been further twisted in the other direction by 90°, which is consistent with a structure twisted by 180°. The properties of both structures were determined. Due to the fiber's strength, the TTFBG twisted by 180° has to be 2cm long. An advantage of the proposed method of producing the TTFBG is that there is a twist of the structure itself after fabrication. This creates new possibilities of using the structure in many applications. This type of grating can be used to reduce the sensitivity to polarization in various sensing applications, in particular, to measure the refractive index. In addition, a 180° twisted TTFBG has properties of direction discrimination in twist sensing applications.

Highly Sensitive Twist Sensor Based on Partially Silver Coated Hollow Core Fiber Structure

Interferometer based on multiple beam interferences inside a hollow core fiber (HCF) structure (also known as an antiresonant reflecting optical waveguide) has been attracting interest of many researchers due to its periodic transmission spectrum containing high quality factor spectral dips. Functionalized HCF structures have been demonstrated for a wide range of applications in humidity, magnetic field, and biosensing. Here, we report a new application of the HCF-based structure with a partial silver coating layer for sensing of twist. It is configured by a fusion splicing a section of 4.5-mm long HCF between two standard single mode fibers (SMFs), followed by a sputter-coating of a very thin layer of silver on one side of the HCF surface. It is found that the spectral response of the partially silver coated HCF structure is very sensitive to the changes of input light polarization. An increase in sensitivity of the fiber structure to twist after deposition of the silver coating when the twist is applied to both the SMF and HCF sections is demonstrated by comparison with an uncoated HCF fiber structure. Experimental results show that twisting of the HCF section results in much greater changes in a selected dip's strength compared to that in the case of twisting the SMF section of the structure. The proposed HCF fiber sensors with 4.5-nm and 6.7 nm-thick silver layers show the highest sensitivities of 0.647 dB/°and 0.717 dB/° in the twist angles range of up to 10°. To the best of our knowledge, this is the highest twist sensitivity reported for intensity modulation-based fiber sensors. Moreover, the proposed sensor offers excellent measurement repeatability.

Optical nonlinearities of excitons in monolayer MoS2

We calculate linear and nonlinear optical susceptibilities arising from the excitonic states of mono- layer MoS2 for in-plane light polarizations, using second-quantized bound and unbound exciton operators. Optical selection rules are critical for obtaining the susceptibilities. We derive the valley-chirality rule for the second harmonic generation in monolayer MoS2, and find that the third- harmonic process is efficient only for linearly polarized input light while the third-order two photon process (optical Kerr effect) is efficient for circularly polarized light using a higher order exciton state. The absence of linear absorption due to the band gap and the unusually strong two-photon third-order nonlinearity make the monolayer MoS2 excitonic structure a promising resource for coherent nonlinear photonics.

Novel twist measurement method based on TFBG and fully optical ratiometric interrogation

In this paper we present a novel twist measurement technique based on the influence of changes in the input light polarization state on the tilted Bragg grating. In the proposed sensor with the interrogation method, changes in the amplitudes of high-order cladding modes are measured by wavelength-matched fiber Bragg grating filters working in reflection mode. The key element of the method is the use of parameters of two selected parts of one cladding mode that are in close proximity to each other. Application of appropriate technological parameters to the manufactured photonic structures of the sensor and interrogation filter elements allows to obtain high-resolution measurement with simultaneous immunity to temperature fluctuations. The differential character of the proposed method also provides independence from changes in the optical power of the illuminating light source.

Tunable Fano-Like Lineshape in an Adiabatic Tapered Fiber Coupled to a Hollow Bottle Microresonator

Fano-like resonances and electromagnetic induced transparency (EIT)-like resonances in whispering gallery-mode (WGM) microcavities have important applications in high-sensitivity biosensing, optical switches, and slow light. Hollow microbottle resonators (HMBRs) provide special opportunities for optofluidic devices. Here, we theoretically and experimentally demonstrate a simple system to produce Fano resonance lineshapes in which a HMBR is coupled to the transition region of an adiabatic tapered optical fiber. The Fano resonance results from the interference between the fundamental mode and higher order modes of the taper that are coupled to the WGM of the HMBR. The degree of asymmetry can be tuned by scanning the coupling position along the taper and controlling the input light polarization. It is shown that by tuning polarization, the Fano resonance line converts from a Lorentzian shape to an EIT-like line. This system potentially can be used as an optical filter with the advantage of easy alignment.

Sensing Characteristics of Arc-Induced Long Period Gratings in Polarization-Maintaining Panda Fiber

We present and discuss an investigation of the sensing features of long period gratings fabricated in a polarization-maintaining Panda optical fiber, by means of the electric arc discharge technique. In particular, the investigation regards the resonant wavelength shift of first three cladding modes as function of three environmental parameters, such as strain, temperature, and surrounding medium refractive index. We also investigate and discuss the variations of the sensing behavior as a function of the polarization input state. The achieved results show that the sensitivity of each resonant wavelength to environmental parameters is modulated by the cladding mode order and by polarization input light, with some appealing characteristics. For instance, the lowest order cladding mode in slow polarization state is practically insensitive to temperature and surrounding refractive index, but sensitive to strain. Temperature and strain sensitivities show opposite signs with the input light polarization. Sensitivity to surrounding refractive index is higher in the case of fast polarization state. These results represent a concrete perspective of the possibility that this device can act as a 3-parameter sensor.

Polarimetry with azimuthally polarized light

Nonuniformly polarized light can be used for Mueller polarimetry of homogeneous linear samples. In this work, a set up based on using azimuthally polarized input light and a modified commercial light polarimeter is proposed and developed. With this set up, a Mueller submatrix of a sample can be obtained by measuring the Stokes parameters at only three different positions across the output beam section. Symmetry constraints for linear deterministic samples allow the complete Mueller matrix to be deduced for this kind of specimens. The experimental results obtained for phase plates and for a linear polarizer confirm the validity of the proposed method.

The Influence of Fibre Bending on Polarization-Dependent Twist Sensor Based on Tilted Bragg Grating

AbstractThis paper presents an experimental study on influence of input light polarization on the spectral characteristics of a fibre twist sensor based onTilted Fiber Bragg Grating(TFBG) with simultaneous application of bending to an optical fibre. The application of proposed measurement stand could provide the ability of transforming the bending to a displacement. The twist measurement was performed by tuning of the sensor illuminating light polarization angle. The spectral parameters of selected cladding mode which are sensitive to the rotation of input light polarization angle have been shown. This paper shows the characteristics of transmittivity and wavelength shift for an incident high-order cladding mode measured with different curvatures of fibre. The dependency of selected cladding mode spectral parameters related with the twist measurement on the influence of temporary bending has been shown. The measurements were performed for two positions of sensing structure refractive index perturbations in relation to the bending direction plane. The experimental results show that the direction of TFBG structure bending has a small influence on the stability of spectral parameters characteristic for twist measurement, assuming that the bending direction is fixed while measurement.

Single-Tone Optical Frequency Shifting and Nonmagnetic Optical Isolation by Electro-Optical Emulation of a Rotating Half-Wave Plate in a Traveling-Wave Lithium Niobate Waveguide

In this paper, we demonstrate a frequency shifter at 780 nm based on an X -cut Z -propagating lithium niobate waveguide platform with an isotropic orientation. The electro-optic effect modulates the signal with traveling-wave configuration. The frequency-shifting device emulates a rotating half-wave plate when it converts left circular polarized input light to right circular polarized output light and changes the original optical frequency by its modulation frequency. We characterize the device by conducting the polarization conversion experiment and achieve the frequency shift at up to 2 GHz and more than 20 dB extinction ratio. The device needs 30 dBm RF power to perform a frequency shift with high extinction ratio. With the same device, we also demonstrate optical isolation with 18.5 dB extinction ratio.

Arc-Induced Long Period Gratings in Polarization-Maintaining Panda Fiber

In this letter, we report about the fabrication of long period gratings (LPGs) in a polarization-maintaining Panda optical fiber, by means of the electric arc discharge technique. This is the first time that LPGs are arc-induced in a polarization maintaining fiber. The dependence of the LPG’s spectral features on the input light polarization alignment is illustrated, by considering the two principal polarization states, i.e., along the fast and slow axes of the Panda fiber. Moreover, an investigation about the surrounding refractive index sensitivity, as a function of the two principal polarization states, is also reported with interesting results.

Experimental realization of a 2 × 2 polarization-independent split-ratio-tunable optical beam splitter.

We realized a polarization-independent split-ratio-tunable optical beam splitter supporting two input and output ports through a stable interferometer. By adjusting the angle of a half-wave plate in the interferometer, we can tune the beam splitter reflectivities for both input ports from 0 to 1, regardless of the input light polarization. High-fidelity polarization-preserving transmission from input to output ports was verified by complete quantum process tomography. Nearly optimal interference effects at the beam splitter with various split ratios were observed by two-photon Hong-Ou-Mandel interference for different input polarization states. Such a beam splitter could find a variety of applications in classical and quantum optical technologies.

Silicon-nanowire-based optical hybrid with insensitive operation for TE/TM states of polarization

An optical 90° hybrid consisting of a polarization splitter–rotator (PSR) and a multimode interferometer (MMI) is proposed to realize insensitive optical frequency mixing for TE/TM states of polarization. The PSR is based on the mode hybridization and mode coupling between waveguides; input light at a fundamental transverse electric (TE) mode and a transverse magnetic (TM) mode are output at separate ports, but both output signals are in the fundamental TE mode. These two output fields are then guided to the MMI, where the optical frequency mixing with the light of a local oscillator takes place. This mixing is then independent of the state of polarization of the hybrid input light. We designed such an optical hybrid based on silicon-nanowire waveguides. At a wavelength of 1540nm, simulation results for the hybrid show a transmission efficiency of 98.6% or 95.8% for TE- or TM-polarized input light, respectively, while the transmission efficiencies for both polarizations are above 90% in a 30nm wavelength range, namely [1525nm, 1555nm]. Further, in the wavelength range [1510nm, 1565nm], the transmission imbalance between the output fields is below 0.85dB for TE- or TM-polarized input light, respectively, while the phase errors in both cases are less than 4°.

In-line polarization rotator based on the quantum-optical analogy.

An in-line polarization rotator (PR) is proposed based on the quantum-optical analogy (QOA). The proposed PR possesses an auxiliary E7 liquid crystal (LC) waveguide in the vicinity of the single-mode fiber (SMF) core. Because of the matched core size, the PR demonstrates good compatibility with the established backbone networks which are composed of conventional SMFs. With optimized parameters for the auxiliary waveguide, the PR offers a near 100% polarization conversion efficiency at the 1550 nm band with a bandwidth of ∼30 nm, a length of ∼4625.9 μm with a large tolerance of ∼550 μm, and a tolerance of the input light polarization angle and rotation angle of the E7 LC of ∼π/30 and ∼π/36 rad, respectively. The performance was verified by the full-vector finite-element method. The proposed PR can be easily fabricated based on the existing photonics crystal fiber manufacturing process, making it a potentially inexpensive device for applications in modern communication systems. Moreover, the QOA, compared with the previous supermode-theory design method, allows a designer to consider several waveguides separately. Therefore, various unique characteristics can be met simultaneously which is consistent with the trend of modern fiber design.

Semi-reciprocal polarization maintaining fibre coupler with distinctive transmission characteristics.

Optical couplers are very important devices in optical communication systems and optical sensor systems. Several types of optical couplers with different materials or different transmission characteristics have been reported. Here we propose a semi-reciprocal polarization maintaining fibre coupler with unique transmission characteristics, which is distinct from conventional polarization maintaining fibre couplers and polarization beam splitters, and investigate the characteristics of the coupler theoretically and experimentally. The experimental results show that for circularly and elliptically polarized input light, the proposed coupler will act both as an in-line polariser and a conventional polarization maintaining fibre coupler. The output polarization extinction ratio of the transmission arm is 31.79 dB at a centre wavelength of 841 nm. For linearly polarized input light, the coupler will merely act as a conventional 3 dB polarization maintaining fibre coupler. The unique features of the proposed coupler enables the removal of polarisers from optical sensor systems and coherent optical communication systems, and reduces the insertion loss and production cost of the optical path. Therefore there is wide application for this device in optical sensor systems and optical communication systems.

Ultra high sensitive integrated optical waveguide refractive index sensor based on multimode interference

We propose an ultra high sensitive integrated optical waveguide refractive index sensor based on multimode interference which can be integrated with other sensing elements on a single chip. The proposed sensor has ultra high sensitivity around communication wavelength (1.55μm) varying from ∼1370 (1160) to 33,850 (26,050)nm/RIU for TM (TE) polarized input light for the refractive index changes from 1.33 to 1.44. To the best of our knowledge, this sensitivity is highest among multimode interference based sensors reported till date. A physical explanation for the ultra high sensitivity is also presented.

On the dual-cone nature of the conical refraction phenomenon.

In conical refraction (CR), a focused Gaussian input beam passing through a biaxial crystal and parallel to one of the optic axes is transformed into a pair of concentric bright rings split by a dark (Poggendorff) ring at the focal plane. Here, we show the generation of a CR transverse pattern that does not present the Poggendorff fine splitting at the focal plane, i.e., it forms a single light ring. This light ring is generated from a nonhomogeneously polarized input light beam obtained by using a spatially inhomogeneous polarizer that mimics the characteristic CR polarization distribution. This polarizer allows modulating the relative intensity between the two CR light cones in accordance with the recently proposed dual-cone model of the CR phenomenon. We show that the absence of interfering rings at the focal plane is caused by the selection of one of the two CR cones.

Equivalent-nanocircuit-theory-based design to infrared broad band-stop filters.

We theoretically introduced a design paradigm and tool by extending the circuit functionalities from radio frequency to near infrared domain, and a broad band-stop filter, is successfully demonstrated by cascading triple layers of nano-square arrays. The feasibility is confirmed by its consistency with the rigorous FDTD calculation. Moreover, such a third-order Butterworth filter is not only insensitive to the incident angle and but also to input light's polarization. The new paradigm forms a theoretical foundation for designing optical devices and also enriches the classic circuit operations at the optical frequency region.

Liquid Crystal Lens Array with Thermally Controllable Focal Length and Electrically Convertible Lens Type

This paper reports the fabrication of a lenticular liquid crystal (LC) lens array with thermally tunable focus and with the function of a convertible lens type, using the surface structure of a UV-curable polymer and a twisted-nematic (TN) LC cell. The TN LC cell makes the LC lenticular lens function as a converging or diverging lens by controlling electrically the polarization of input light. Therefore, the focal lengths for both the converging and diverging lenses, which can be switched from the TN cell, can be tuned by changing the effective refractive index of the LC by Joule heating of the transparent electrode. As a result, the focal length of the lens with the E7 LC was changed continuously from 8.7 to 31.2 mm for the converging lens type and from -9.8 to -14.2 mm for the diverging lens when the temperature was increased from 25 to <TEX>$56^{\circ}C$</TEX>.