In-line Polarization Research Articles

Ghost spintronic THz-emitter-array microscope

Terahertz (THz) waves show great potential in nondestructive testing, biodetection and cancer imaging. Despite recent progress in THz wave near-field probes/apertures enabling raster scanning of an object’s surface, an efficient, nonscanning, noninvasive, deep subdiffraction imaging technique remains challenging. Here, we demonstrate THz near-field microscopy using a reconfigurable spintronic THz emitter array (STEA) based on the computational ghost imaging principle. By illuminating an object with the reconfigurable STEA followed by computing the correlation, we can reconstruct an image of the object with deep subdiffraction resolution. By applying an external magnetic field, in-line polarization rotation of the THz wave is realized, making the fused image contrast polarization-free. Time-of-flight (TOF) measurements of coherent THz pulses further enable objects at different distances or depths to be resolved. The demonstrated ghost spintronic THz-emitter-array microscope (GHOSTEAM) is a radically novel imaging tool for THz near-field imaging, opening paradigm-shifting opportunities for nonintrusive label-free bioimaging in a broadband frequency range from 0.1 to 30 THz (namely, 3.3–1000 cm−1).

In-fibre LP11–LP21 mode conversion using force-induced long-period fibre gratings with polarization controllers

ABSTRACTMode conversion between higher-order modes is attained in a step-index four mode fibre using an in-line polarization controller followed by a force-induced long period fibre grating. By adjusting the field orientation of the LP11 mode to the antisymmetric fibre grating, the LP11 mode is converted to the LP21 mode with a spatially periodic force of 50 N along the fibre length of 50 mm. The mode conversion is evaluated by observing the far-field and near-field patterns of the fibre output and by comparing the transmitted optical power with and without the additional two-mode fibre. The phase-matching grating period is required of the accuracy of roughly ±5 μm and the radiation loss during the mode conversion is as low as 0.01 dB.

Multi-Shuttle Behavior Between Dissipative Solitons and Noise-Like Pulses in an All-Fiber Laser

A multi-switchable generation between dissipative solitons (DSs) and noise-like pulses (NLPs) in a fiber laser is demonstrated. A 45° tilted fiber grating is applied as an effective in-line all-fiber polarizer to realize the nonlinear polarization rotation (NPR) mechanism. With the fixed orientations of polarization controllers, the operation regimes of the fiber laser can be alternatively switched between DS and NLP by monotonically increasing pump power only. We observed five switching stages between DS/NLP regimes with the pump power elevating from 200 to 660 mW. Remarkably, the whole procedure could be precisely controlled forwardly and backwardly. This multi-shuttle behavior between the two distinct types of output pulse can be attributed to the transmission of NPR mechanism together with the gain filtering effect. To the best of our knowledge, this is the first report on the multi-shuttle behavior between DS and NLP in a fiber laser under monotonic change of pump power. Our findings may contribute to in-depth understanding of nonlinear dynamics and laser physics.

All-fiber passively mode-locked ultrafast laser based on a femtosecond-laser-inscribed in-fiber Brewster device.

We report on an in-fiber Brewster device with a 45° tilted fiber grating (TFG) directly written by a plane-by-plane femtosecond laser inscription method. Up to 10dB polarization-dependent loss was achieved, proving effective polarizing functionality. Furthermore, we employ it as an in-line polarizer to successfully mode lock a fiber laser through the nonlinear polarization rotation technique. A stable soliton pulse train has been generated at 1563.64nm with a pulse width of 624fs and pulse energy of 0.42nJ. With proper polarization adjustment, the laser also can operate in a noise-like regime. The parameters of this kind of 45°-TFG can be flexibly customized owing to the high flexibility and controllability of the femtosecond laser-inscription approach. In particular, such in-fiber polarizing devices inscribed by femtosecond laser inscription without removing the fiber coating are extremely robust for fiber lasers working at a broad wavelength region including the mid-infrared.

All fiber mode-locked thulium-doped fiber laser using a novel femtosecond-laser-inscribed 45°-plane-by-plane-tilted fiber grating

Using the novel 45°-plane-by-plane-tilted fiber grating inscribed by a femtosecond laser as an in-line fiber polarizer, we demonstrate an all-fiber passively mode-locked thulium-doped fiber laser. We characterize the grating for polarization-dependent loss, indicating high contrast and broad operating wavelength. The 45°-tilted fiber grating together with polarization controller act as an artificial saturable absorber suitable for mode-locking through nonlinear polarization rotation. By properly pre-adjusting the polarization controller, stable mode-locking is achieved for a pump power in the range of 300 mW–400 mW at 1565 nm. Owing to the net anomalous dispersion, a stable mode-locked soliton pulse centered at 1870 nm with a pulse width of 862 fs is reported at a fundamental repetition rate of 22.34 MHz. A long-term stability test reveals that the laser is quite stable for 24 h under laboratory conditions, confirming the capabilities of the 45°-tilted fiber grating for stable laser operation.

Highly sensitive temperature sensor based on a long period grating inscribed metal clad ridge waveguide with PDMS surrounding

A highly sensitive temperature sensor based on a long period grating (LPG) written metal clad ridge waveguide (MCRW) with poly-dimethylsiloxane (PDMS) surrounding is proposed and theoretically analyzed. We have exploited the coupling between the fundamental and a higher order quasi-TE mode via the long period grating written in the core of the MCRW. It is shown that owing to the differential enhancement of the two participating modes' evanescent fields in the PDMS surroundings due to the metal under cladding and the high thermo-optic coefficient of PDMS, the thermal dependence of the higher order mode is significantly enhanced as compared to the fundamental mode. In addition, a dispersion turn around behavior in the phase matching graph of the LPG is observed due to the metal under cladding. As a result, a temperature sensitivity as high as ∼100 nm/°C can be achieved by using the dual resonance near the dispersion turning point. Further, due to the highly lossy nature of the quasi-TM modes, no inline polarizer is required to be used with the proposed structure.

Fiber nanogratings induced by femtosecond pulse laser direct writing for in-line polarizer.

Self-organized fiber nanogratings (FNGs) induced by femtosecond pulse laser direct writing are demonstrated in optical silica fibers for the first time. Ge doping in the silica fiber core plays an important role in the formation of nanogratings. Negative changes in refractive index with sawtooth fluctuation and strong birefringence of FNGs were investigated by an optical fiber refractive index analyzer and a crossed polarization microscope, respectively. The polarization features of FNGs were characterized by the Mueller matrix analytical method and in-line polarizers based on FNGs were fabricated. FNGs are promising integrated components because of their prominent properties of polarization and salient compatibility with most fiber-optic systems.

Wavelength tunable soliton rains in a nanotube-mode locked Tm-doped fiber laser

We report soliton rains in a tunable Tm-doped fiber laser mode locked by carbon nanotubes. We also detect their second- and third-harmonics. We achieve a tunability of over 56 nm, from 1877 to 1933 nm, by introducing a polarization-maintaining isolator and two in-line polarization controllers. This makes our system promising as a tunable filter for ultrafast spectroscopy.

Realizing multi-functional all-optical data processing on nanoscale SiC waveguides

All-optical logics are realized on nanoscale SiC waveguides with add-drop micro-ring functionality, including the TE/TM polarized data decoding, the dual-port Kerr switching and gating beyond 12 Gbit/s. With employing the C-C bond enriched SiC thin film upon thermal oxide, the nonlinear refractive index of up to 2.44 × 10−12 cm2/W enables the asymmetric waveguide with polarization distinguishable transmission, which provides a polarization-selectivity to discreminate the TE/TM polarized data decoding with an nearly 9-dB extinction ratio. The TE/TM polarized decoding performance is comparable with a state-of-the-art fiberized in-line polarizer. The complementary transmission in the bus waveguide port facilitates the dual-port Kerr switching for data format conversion/inversion in both add/drop channels. Owing to the TE/TM polarization discriminated throughput, the asymmetric add-drop waveguide micro-ring also permits all-optical AND logic gating functions, where the ON-state outputs only if the pump bit is set at ON state and the probe bit with matched polarization. These results reveal the multi-functionality of the nanoscale SiC add-drop micro-ring waveguide for future photonic logics on chip.

Phase-shifted Solc-type filter based on thin periodically poled lithium niobate in a reflective geometry.

Configurable narrow bandwidth filters are indispensable components in optical communication networks. Here, we present an easily-integrated compact tunable filtering based on polarization-coupling process in a thin periodically poled lithium niobate (PPLN) in a reflective geometry via the transverse electro-optic (EO) effect. The structure, composed of an in-line polarizer and a thinned PPLN chip, forms a phase-shift Solc-type filter with similar mechanism to defected Bragg gratings. The filtering effect can be dynamically switched on and off by a transverse electric filed. Analogy of electromagnetically induced transparency (EIT) transmission spectrum and electrically controllable group delay is experimentally observed. The mechanism features tunable center wavelength in a wide range with respect to temperature and tunable optical delay to the applied voltage, which may offer another way for optical tunable filters or delay lines.

Toward 100 Micrometer per Refractive Index Unit Sensitive Sensor Using a Compact Long-Period Grating Inscribed Metal Clad Ridge Waveguide

A novel structure with long-period grating in a metal clad ridge waveguide (MCRW) as a highly sensitive ambient refractive index (ARI) sensor is proposed suitable for biosensing applications. We have exploited the coupling between the fundamental mode and a higher order mode via the long period grating written in the photosensitive core of the MCRW. Utilizing quasi-transverse electric (TE) modes of the MCRW structure, it is shown that ARI sensitivity of ∼ ${{\text{100}}}\,{\mu} {\text{m/}}$ refractive index unit (RIU) can be achieved by using dual resonance near dispersion turn around point. This ultrahigh sensitivity can be attributed to the metal under-cladding which enhances the evanescent field in the ambient medium more prominently for the higher order modes as compared to the fundamental mode. It is shown that device compactness is also increased significantly due to the metal under-cladding. Further, the propagation lengths of quasi-TE modes in the proposed structure are about 35 to 185 times higher than that of quasi-transverse magnetic modes due to which, no in-line polarizer is required in the proposed device.

Magnetic field modulating in-line fiber polarization modulator based on microfiber and magnetic fluid

We demonstrate an in-line fiber polarization modulator (FPM) which is constructed by immersing a microfiber into magnetic fluid (MF) and modulated with the magnetic field. The polarization modulation principle is based on the magneto-optical dichroism of the MF and the non-circular symmetric distribution of magnetic nanoparticles around the microfiber. Modulations of both the light polarization degree and the polarization direction are realized. The modulation capability, time response, insertion loss, and magnetic field direction dependence of the proposed FPM are studied.

Enhancement of refractive index sensitivity of Bragg-gratings based optical waveguide sensors using a metal under-cladding

We theoretically analyze, a compact Bragg grating inscribed metal clad leaky ridge waveguide (MCLRW) as a refractive index sensor which can be integrated with other elements on a single chip. The grating is considered to be written in the photosensitive core. Using the quasi-TE mode of the structure, it is shown that a metal layer underneath the core enhances the evanescent field in the ambient region resulting in a significant increase in the ambient refractive index sensitivity. The obtained sensitivity and the figure of merit of the considered sensor structure are found to be almost 13–14 times higher than that of a ridge waveguide without any metal layer. Further, due to the higher modal loss of the quasi-TM mode, no in-line polarizer is required to assemble with the device. Effect of the loss due to metal layer on the peak reflectivity as well as bandwidth of the spectrum is also discussed.

A terahertz in-line polarization converter based on through-via connected double layer slot structures

A terahertz (THz) in-line polarization converter that yields a polarization conversion ratio as high as 99.9% is demonstrated at 1 THz. It has double-layer slot structures oriented in orthogonal directions that are electrically connected by 1/8-wavelngth-long through-via holes beside the slot structures. The slots on the front metal-plane respond to the incident THz wave with polarization orthogonal to the slots and generates a circulating surface current around the slots. The surface current propagates along a pair of through-via holes that function as a two-wire transmission line. The propagating current generates a surface current around the backside slot structures oriented orthogonal to the slot structures on the front metal layer. The circulating current generates a terahertz wave polarized orthogonal to the backside slot structures and the 90° polarization conversion is completed. The re-radiating THz wave with 90° converted polarization propagates in the same direction as the incident THz wave.

Nonlinear Saturable and Polarization-induced Absorption of Rhenium Disulfide

Monolayer of transition metal dichalcogenides (TMDs), with lamellar structure as that of graphene, has attracted significant attentions in optoelectronics and photonics. Here, we focus on the optical absorption response of a new member TMDs, rhenium disulphide (ReS2) whose monolayer and bulk forms have the nearly identical band structures. The nonlinear saturable and polarization-induced absorption of ReS2 are investigated at near-infrared communication band beyond its bandgap. It is found that the ReS2-covered D-shaped fiber (RDF) displays the remarkable polarization-induced absorption, which indicates the different responses for transverse electric (TE) and transverse magnetic (TM) polarizations relative to ReS2 plane. Nonlinear saturable absorption of RDF exhibits the similar saturable fluence of several tens of μJ/cm2 and modulation depth of about 1% for ultrafast pulses with two orthogonal polarizations. RDF is utilized as a saturable absorber to achieve self-started mode-locking operation in an Er-doped fiber laser. The results broaden the operation wavelength of ReS2 from visible light to around 1550 nm, and numerous applications may benefit from the anisotropic and nonlinear absorption characteristics of ReS2, such as in-line optical polarizers, high-power pulsed lasers, and optical communication system.

Novel in-line effective fiber polarizer based on chromium and gold nano metal films: design and experiment

The performance of the magnesium fluoride/chromium/gold in-line fiber optic polarizer on the thickness of magnesium fluoride buffer layer and chromium metal thin film was theoretically investigated. To prove the theoretical study, the magnesium fluoride/chromium/gold polarizer was made based on the simulation results. The obtained polarizer showed an extinction ratio of about 40 dB which is in a good agreement with the theoretical result at the corresponding thicknesses. Both theoretical and experimental results showed that by covering the side polished single mode fiber with the optimum layer thicknesses, a novel and an efficient fiber polarizer with the high extinction ratio could be obtained. This is the first time that this type of polarizer has been made.

Quasi-distributed twist/torsion sensor.

This paper presents a fiber-optic quasi-distributed in-line twist/rotation sensor. The presented sensor consists of a polarization maintaining lead-in fiber and series of in-line fiber polarizers with integrated semi-reflective mirrors that are interconnected by a standard single mode fiber. Sections of interconnecting single mode define multiple twist/rotation sensitive sensor segments, which can be interrogated individually by a simple optical time domain reflectometer. The presented system exploits the inability of the standard single mode fiber to change the polarization state, i.e. E-field vector orientation of a wave propagating down twisted/rotated fiber. Thus, twisting of the fiber segments between consecutively located polarizers causes modulation of the transmitted optical power along each sensor segment, which is further determined by observation of back-reflected signals from semi-reflective mirrors integrated together with in-line polarizers. A quasi-distributed sensing system with nine sensing segments is demonstrated with twist/rotational resolution better than 0.3 degrees and low cross-talk. The entire system is built out of a small number of simple and efficient optoelectronic components.

Laser scanning by rotating polarization gratings.

Laser beam scanning can be realized using two independently rotating, inline polarization gratings, termed Risley gratings, in a fashion similar to Risley prisms. The analytical formulas of pointing position as well as their inverse solutions are described. On this basis, the beam scanning is investigated and the performance of scanning imaging is evaluated. It is shown that the scanning function in 1D scanning evolves from a sinusoidal to triangular scan and the duty cycle increases rapidly as the ratio of grating period to wavelength is reduced toward 2. The scan pattern in 2D scanning is determined by the ratio k of the gratings' rotatory frequency. In imaging applications, when k tends toward 1 or -1, the scan pattern becomes dense and is inclined to be spiral or rose-like, respectively, which is desirable for the purpose of enhancing spatial resolution. There is a direct trade-off between spatial resolution and frame rate. The spiral and rose scanning enable multiresolution imaging, providing a preview of the scanned area in a fraction of the overall scan time, which is extremely useful for fast, real-time imaging applications.

Versatile mode-locked fiber laser with switchable operation states of bound solitons.

Bound states of two solitons are among the typical forms of bound states and can be observed in various operation states of mode-locked fiber lasers. We experimentally investigated bound solitons (BSs) in a passively mode-locked erbium-doped fiber laser based on a semiconductor saturable absorber mirror, whose operation states can be switched among multiple pulses, passively harmonic mode-locking, and "giant pulses" by simply adjusting the in-line polarization controller with the pump power fixed. Up to four pulses, fourth-order harmonic mode-locking (HML), and a "giant pulse" with four BSs were obtained with increasing pump power. Experimental results showed a correlative relationship among those operation states (N pulses/Nth-order HML/"giant pulses" of N bound solitons) at different pump power levels. The birefringence induced by the erbium-doped fiber inside the laser cavity played a vital role in the transitions of those operation states.

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.