A novel terahertz wave reflective polarizer for THz communication

We design terahertz wave reflective polarizer that operates over a wide terahertz wavelength range and is based on a periodic bilayer structure. The structure is characterized by transfer matrix calculations. Results of simulations show that the mirror is highly reflecting for incidence angle θ ≤ 60°and TE as well as TM polarization in the wavelength range between 541.6 μm and 574.2 μm (i.e. frequency band between 522.5GHZ and 533.9 GHZ). As the incidence angle increases this reflection band blueshifts for both TM and TE polarizations.     

The effects of hydroxyapatite coatings on stress distribution near the dental implant-bone interface

The effects of different thickness of hydroxyapatite (HA) coatings on bone stress distribution near the dental implant-bone interface are very important factors for the HA-coated dental implant design and clinical application. By means of finite element analysis (FEA), the bone stress distributions near the dental implant coated with different thicknesses from 0 to 200 μm were calculated and analyzed under the 200 N chewing load. In all cases, the maximal von Mises stresses in the bone are at the positions near the neck of dental implant on the lingual side, and decrease with the increase of the HA coatings thickness. The HA coatings weaken the stress concentration and improve the biomechanical property in the bone, however, in HA coatings thickness range of 60-120 μm, the distinctions of that benefit are not obvious. In addition, considering the technical reason of HA coatings, we conclude that thickness of HA coatings range from 60 to 120 μm would be the better choice for clinical application.     

Enhancement of terahertz wave difference frequency generation based on a compact walk-off compensated KTP OPO

A compact, walk-off compensated dual-wavelength KTP OPO near the degenerate point of 2.128 μm pumped by a Nd:YAG pulsed laser is employed as the pump for terahertz (THz) source based on difference frequency generation (DFG) in a GaSe crystal. Coherent THz radiation that is continuously tunable in the range of 81-1617 μm (0.186-3.7 THz) is achieved. An enhancement of 76.7% in average for the THz energies at different wavelengths is realized using the walk-off compensated KTP OPO than the common one. Using a 8 mm-long GaSe crystal, the maximum output THz pulse energy is 48.9 nJ with the peak power of 11 W, corresponding to the energy conversion efficiency of 5.4 × 10^− 6 and the photon conversion efficiency of about 0.09%.     

A combined multiple-SLED broadband light source at 1300 nm for high resolution optical coherence tomography

We demonstrate a compact, inexpensive, and reliable fiber-coupled light source with broad bandwidth and sufficient power at 1300 nm for high resolution optical coherence tomography (OCT) imaging in real-time applications. By combining four superluminescent diodes (SLEDs) with different central wavelengths, the light source has a bandwidth of 145 nm centered at 1325 nm with over 10 mW of power. OCT images of an excised stage 30 embryonic chick heart acquired with our combined SLED light source (<5 μm axial resolution in tissue) are compared with images obtained with a single SLED source (∼10 μm axial resolution in tissue). The high resolution OCT system with the combined SLED light source provides better image quality (smaller speckle noise) and a greater ability to observe fine structures in the embryonic heart.     

High power room-temperature design of terahertz quantum cascade laser based on difference frequency generation

Terahertz (THz) quantum cascade lasers (QCLs) are key elements for high-power terahertz beam generation for integrated applications. In this study, we design a highly nonlinear THz-QCL active region in order to increase the output power of the device especially at lower THz frequencies based on difference frequency generation (DFG) process. It has been shown that the output power increases for a 3.2 THz structure up to 1.2 μW at room temperature in comparison with the reported power of P = 0.3 μW in [1]. The mid-IR wavelengths associated with this laser are λ₁ = 12.12 μm and λ₂ = 13.93 μm, which are mixed in a medium with high second-order nonlinearity. A similar approach has been used to design an active region with THz frequency of 1.8 THz. The output power of this structure reaches to 1 μW at room temperature where the mid-IR wavelengths are λ₁ = 12.05 μm, λ₂ = 12.99 μm.     

Tunable terahertz generation from one CO2 laser in a GaSe crystal

Coherent terahertz pulses have been generated at a range of 236.3-1104.5 μm (0.27-1.3 THz) by one CO₂ laser with dual-wavelength output based on collinearly phase-matched different frequency generation (DFG) in a GaSe crystal. This source has the advantages of compact and simplicity for tuning. The output power of the THz pulse and phase-matching conditions were investigated. The maximum single pulse energy of 11 nJ was generated at a frequency of 1.23 THz (243.6 μm), corresponding to a peak output power 182 mW.     

A tunable fiber parametric oscillator for the 2 μm wavelength range employing an intra-cavity thulium doped fiber active filter

We report an efficient fiber parametric oscillator operating in the wavelengths range of 1.97 μm to 2.14 μm. The oscillator is based on narrow band parametric amplification and employs a thulium doped fiber placed at the loops end as an intra-cavity active filter. The filter eliminates any signal generated by stimulated Raman scattering which inherently accompanies the parametric process. Short wavelength parametric oscillations and the pump signal are also absorbed. Only the long wavelength parametric oscillations can build up in the resonantly pumped system which emits 4 ns pulses at ~ 1 MHz with a maximum peak pulse power of 20 W.     

Long range surface plasmon polariton waveguides at 1.31 and 1.55 μm wavelengths

We investigate characteristics of gold metal strip waveguides based on long range surface plasmon polaritons (LRSPPs) along thin metal strips embedded in a polymer for practical applications at the telecommunication wavelengths of 1.31 and 1.55 μm. Guiding properties of the gold strip waveguides are theoretically and experimentally evaluated with the limited thickness and width up to ∼20 nm and ∼10 μm, respectively. The lowest propagation loss of ∼1.4 dB/cm is obtained with a 14.5-nm-thick and 2-μm-wide gold strip at 1.55 μm. With a single-mode fiber, the lowest coupling loss of ∼0.4 dB/facet is achieved with a 14.5-nm-thick and 5-μm-wide gold strip at 1.55 μm. The lowest insertion losses are obtained 8-9 dB with 1.5 cm-long gold strips of a limited thickness and width at both the wavelengths. We demonstrate a 10 Gbps optical signal transmission via the LRSPP waveguide with a 14 nm-thick, 2.5 μm-wide, and 4 cm-long gold strip. These LRSPP waveguides have potential applications for optical interconnects and communications.     

Hybrid photonic crystal 1.31/1.55 μm wavelength division multiplexer based on coupled line defect channels

The objective of this paper is to investigate the implementation of a hybrid photonic crystal (PhC) 1.31/1.55 μm wavelength division multiplexer (WDM) and wavelength channel interleaver with channel spacing of roughly 0.8 nm between the operating wavelengths of 1.54-1.56 μm. It is based on 1-D photonic crystal (PhC) structure connected with an output 2-D PhC structure. The power transfer efficiency of the hybrid PhC WDM at 1.31 μm and 1.55 μm were computed by eigen-mode expansion (EME) method to be about 88% at both the wavelengths. The extinction ratios obtained for the 1.31 μm and 1.55 μm wavelengths are − 25.8 dB and − 22.9 dB respectively.     

Multi-crystalline silicon as active medium for terahertz intracenter lasers

Stimulated emission at terahertz frequencies has been obtained from multi-crystalline silicon doped by phosphor under optical excitation by a mid-infrared laser. The silicon samples consist of grains with a characteristic size distribution in the range from 50 to 500 μm. The maximum operation temperature of the laser made from multi-crystalline silicon is 6 K less than that of monocrystalline lasers and the maximum output power is three times less while its laser threshold is only slightly higher and the emission frequency is the same. These effects are attributed to internal strain and enhanced phonon scattering induced by grain boundaries.     

High sensitivity long-period grating-based temperature monitoring using a wide wavelength range to 2.2 μm

Temperature effects on the various cladding modes of a long-period grating (LPG) fabricated in B-Ge co-doped fibre have been investigated to create a high sensitivity measurement device. The temperature sensitivities of the attenuation bands of the LPG over the wavelength region 1.2-2.2 μm, for a grating with a 330 μm period, were obtained by monitoring the wavelength shift of each attenuation band, with a temperature increment of 20 °C, over the range from 23 °C to 140 °C. The attenuation band appearing over the 1.8-2.0 μm wavelength range has shown a nearly five times higher temperature sensitivity than that of lower order modes, and thus it shows significant promise for fibre optic temperature sensor applications.     

Quadrature fringes wide-field optical coherence tomography and its applications to biological tissues

We propose and demonstrate quadrature fringes wide-field optical coherence tomography (QF WF OCT) to expand an optical Hilbert transformation to two-dimensions. This OCT simultaneously measures two quadrature interference images using a single InGaAs CCD camera to obtain en face OCT images. The axial and lateral resolutions are measured at 29 μm in air and 70 μm limited by a pixel size of camera using a superluminescent diode with a wavelength of 1.3 μm as the light source; the system sensitivity is determined to be −90 dB. The area of the en face OCT images is 4.0 mm × 4.0 mm (160 × 160  pixels). The OCT images are measured axially with steps of 10 μm. The en face OCT images of a in vivo human fingertip and a in situ rat brain are three-dimensionally measured up to the depth of about 3 mm with some degradations of a lateral resolution.     

Silicon donor and Stokes terahertz lasers

Two types of lasers based on hydrogen-like impurity-related transitions in bulk silicon operate at frequencies between 1 and 7 THz (wavelength range of 50-230 μm). These lasers operate under mid-infrared optical pumping of n-doped silicon crystals at low temperatures (<30 K). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized in the first type of terahertz silicon lasers. These lasers have a gain ∼1-3 cm⁻¹ above the laser thresholds (>1 kW cm⁻²) and provide 10 ps-1 μs pulses with a few mW output power on discrete lines. Raman-type Stokes stimulated emission in the range 4.6-5.8 THz has been observed from silicon crystals doped by antimony and phosphorus donors when optically excited by radiation from a tunable infrared free electron laser. The scattering occurs on the 1s(E)→1s(A₁) donor electronic transition accompanied by an emission of the intervalley transverse acoustic g-phonon. The Stokes lasing has a peak power of a few tenths of a mW and a pulse width of a few ns. The Raman optical gain is about 7.4 cm GW⁻¹ and the optical threshold intensity is ∼100 kW cm⁻².     

Surface temperature evolution in pulsed laser action of millisecond range

An originally developed multi-wavelength pyrometer (12 wavelengths in the range 1.001-1.573 μm, 50 μs acquisition time for each photodiode, 800 μm spatial resolution, 900-3500 °C brightness temperature range) is used to measure brightness temperature under the pulsed action of Nd:YAG laser (HAAS-HL62P) on stainless steel (INOX 304L) substrates. Specially developed “notch” filters (10⁻⁶ transparency at 1.06 μm wavelength) are applied to avoid the influence of laser radiation on temperature measurements. The true temperature is restored on the basis of method of multi-colour pyrometry. The accuracy of brightness temperature measurements is examined by comparing the temperature evolution for pulses with different durations but with the same value of energy density flux.The influence of the following parameters is studied keeping the remaining ones constant: pulse duration (6-20 ms, rectangular pulse shape), energy per pulse (10-33 J, rectangular pulse shape), pulse shape (three types of triangulars and one rectangular). Finally the evolution of surface temperature for pulses with more complex shapes but with the same pulse duration and energy per pulse is compared.     

Graphene nanoribbon based terahertz antenna on polyimide substrate

Graphene nano ribbon based terahertz patch antenna on polyimide substrate is designed and its radiation characteristics are investigated in the 725–775 GHz band. The terahertz communication system consists of higher data rate transmission, low transmit power with secured wireless communication. The proposed antenna consists of graphene nano ribbon as radiating patch and also the ground plane separated by a 20 μm thin polyimide substrate. The antenna has achieved the broad impedance bandwidth (>5%) in the band of operation. The design has yielded a peak gain of 5.71dB at 750 GHz. The antenna is simulated by using the finite element method based simulator Ansys - HFSS.     

In vivo non-mechanical scanning grating-generated optical coherence tomography using an InGaAs digital camera

We demonstrated in vivo cross-sectional imaging of human fingers by non-mechanical scanning optical coherence tomography (OCT), using a diffracted light as the reference beam and a linear illumination beam at a center wavelength of 1.3 μm for deeper penetration into biological tissues. By applying the three-step phase-shifting method, our system can measure OCT images at 10 frames/s with a sensitivity of 90 dB for a 2.45 × 4.80 mm (axial × lateral) measurement range using an InGaAs digital camera (320 × 256 pixels).     

Nanolayered multilayer coatings of CrN/CrAlN prepared by reactive DC magnetron sputtering

Single-phase CrN and CrAlN coatings were deposited on silicon and mild steel substrates using a reactive DC magnetron sputtering system. The structural characterization of the coatings was done using X-ray diffraction (XRD). The XRD data showed that both the CrN and CrAlN coatings exhibited B1 NaCl structure with a prominent reflection along (2 0 0) plane. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy and the surface morphology of the coatings was studied using atomic force microscopy. Subsequently, nanolayered CrN/CrAlN multilayer coatings with a total thickness of approximately 1 μm were deposited on silicon substrates at different modulation wavelengths (Λ). The XRD data showed that all the multilayer coatings were textured along {2 0 0}. The CrN/CrAlN multilayer coatings exhibited a maximum nanoindentation hardness of 3125 kg/mm² at a modulation wavelength of 72 Å, whereas single layer CrN and CrAlN deposited under similar conditions exhibited hardness values of 2375 and 2800 kg/mm², respectively. Structural changes as a result of heating of the multilayer coatings in air (400-800 °C) were characterized using XRD and micro-Raman spectroscopy. The XRD data showed that the multilayer coatings were stable up to a temperature of 650 °C and peaks pertaining to Cr₂O₃ started appearing at 700 °C. These results were confirmed by micro-Raman spectroscopy. Nanoindentation measurements performed on the heat-treated coatings revealed that the multilayer coatings retained hardness as high as 2250 kg/mm² after annealing up to a temperature of 600 °C.     

A new solution of reducing polymer optical fiber losses

Polymethyl methacrylate (PMMA) is one of the most commonly used optical materials. However, the application of it in the area of optical communication is strongly limited by the intrinsic absorption loss of carbon-hydrogen stretching vibration. In this paper, we present a method to solve the problem by adopting the hollow-core fibers with cobweb cladding structure. The fibers use a single dielectric material and may solve the problem of structural support. Thus the feasibility of the “OmniGuide” fibers is improved, while a series of advantages of the “OmniGuide” hollow-core fiber are retained. It is promising that a fiber with low transmission loss, high bandwidth, large-core, and low costs can be designed and fabricated using PMMA. At the same time, a very broad range of the wavelengths (from visible to near infrared region, for instance, wavelengths at 0.65-1.12 μm, and even 1.30 μm, 1.54 μm and their neighbors) may be adopted for signal wavelength.     

Micro-spectrometry in the visible range with full-field optical coherence tomography for single absorbing layers

A time-domain full-field OCT adapted to the visible range and with an original configuration using an interferometric objective, that minimizes mechanical vibrations and some settings and that performs imaging without moving the sample, is presented. This setup achieves micrometer scale imaging, 1.5 μm in the axial direction and 1.2 μm in the lateral one. The principle of micro-spectrometry from OCT data by Fourier transform is described and the influence of some key data processing parameters is simulated and discussed. The experimental spectra reconstruction from tomographic data is validated by comparison with transmittance spectra. Imaging and spectra of dyes at a micrometer scale are obtained from the same data volume.     

A multiband absorber based on multipolar plasmon excitation

We report a multiband absorber with a top-layer grating structure based on the multipolar plasmon excitation. The simulation results show that the absorber has three distinctive absorption peaks originated from multipolar plasmon excitation at wavelengths λ = 0.576 μm, λ = 0.760 μm and λ = 5.630 μm with the absorption magnitudes more than 0.86, 0.96 and 0.99, respectively. The multipolar plasmon excitation can be described by surface plasmon standing waves.