Strained Chirped Fiber Bragg Gratings Based all-fiber Variable Optical Delay Line for Optical Coherence Tomography

We present the novel all-fiber optical delay line that can be realized by exploiting the fundamental distributive reflection characteristics of a chirped fiber Bragg gratings (CFBG). With the assist of a strained CFBG and a designed fiber stretcher, the proposed delay line could perform the function (such as attainment of variable large enough optical delay without dispersion mismatch) of the conventional optical delay line within the optical fiber. While the strained CFBG produced 3.1 mm optical delay, the pair of CFBGs in the opposite direction can manage dispersion imbalance of the delay line. The use of the PZT-based fiber stretcher can enable depth scanning at a high repetition rate for real-time imaging. The performance of the delay element is demonstrated with measured experimental results. OCT system embedded with the all-fiber variable optical delay line showed the axial resolution of 100 μm and the dynamic range of 50 dB. The implemented system was used for the imaging of a biomedical sample, which proves the utility of the proposed delay element as a promising alternative of optical delay line.     

Photonic true-time delay unit for broadband adaptive nulling in antenna arrays

An Opto-VLSI-based (VLSI: very-large-scale-integration) tunable true-time delay (TTD) generation unit used for adaptive null steering in phased array antennas is presented. The system is based on continuous tunable optical TTD technique. Arbitrary multiple true-time delays to generate multiple broadband nulls can simultaneously be synthesised for each antenna element. The proposed structured are characterised both theoretically and experimentally. Experimental results demonstrating the principle of the true-time delay for null generations are presented. The maximum time delay measured for a wavelength tuning from 1530 to 1560 nm is 9.5 ns.     

Flattop beam illumination for 3D imaging ladar with simple optical devices in the wide distance range

We have developed an illumination optical system for 3D imaging ladar (laser detection and ranging) which forms flattop beam shape by transformation of the Gaussian beam in the wide distance range. The illumination is achieved by beam division and recombination using a prism and a negative powered lens. The optimum condition of the transformation by the optical system is derived. It is confirmed that the flattop distribution can be formed in the wide range of the propagation distance from 1 to 1000 m. The experimental result with the prototype is in good agreement with the calculation result.     

Chirped-pulse four-wave Raman mixing in molecular hydrogen

Four-wave Raman mixing (FWRM) in molecular hydrogen was studied using chirped pump and Stokes pulses emitting at 802 and 1,203 nm, respectively. The group delay dispersion (GDD) of the anti-Stokes pulse was examined employing a frequency-resolved optical gating system at different GDDs of the pump and Stokes pulses (0 or ±1,000 fs²). As a result, the energy and the sign of GDD for the anti-Stokes pulse remained unchanged, when the pump and Stokes pulses had the GDD with the same sign. When the sign was not the same, the energy decreased and only the portion useful for resonant FWRM was converted into a Raman emission. This technique has a potential for use in compensation of dispersion by passing the negatively chirped high-order Raman sidebands through the optics with positive chirps in the spectral region from the deep-ultraviolet to the near-infrared, to generate multiple transform-limited Raman pulses and then to produce an ultrashort optical pulse by a Fourier synthesis of these Raman emissions.     

Crystal growth and characterization of a novel inorganic–organic hybrid NLO crystal: (NH4)[Cd(NCS)3]·C12H24O6

It is reported here, for the first time, that high-quality bulk size (18 × 5 × 4 mm³) single crystals of a new nonlinear optical crystal, [(NH₄)[Cd(NCS)₃]·C₁₂H₂₄O₆] [Ammonium (18-crown-6-ether) Cadmium(II) tri-thiocyanate; ACCTC], have been grown from aqueous solution via slow evaporation technique. Solubility of ACCTC has been determined for various temperatures. The grown crystals were characterized by single crystal X-ray diffraction, FT-IR, FT-Raman, and UV–Vis–NIR studies. ACCTC crystallizes in orthorhombic system with cell parameters a = 14.7568 ?, b = 15.4378 ?, and c = 10.6383 ? with space group Cmc2₁. The optical second-harmonic generation effect has been measured by using the Kurtz powder technique and is found to be 2 times higher than that of KDP (KH₂PO₄). The sample possesses wide optical transparency range from 200 to 2,500 nm. The TG-DSC thermal analysis revealed that the sample is thermally stable up to 237.92 °C, which is comparatively far better than the thermal stability of [(18C6)Li][Cd(SCN)₃]; CLTC (170 °C).     

Optical quantum transmitter with finesse of 30 at 800-nm central wavelength using microring resonators

A microring resonator (MRR) system incorporating an add/drop system is presented. The finesse of the proposed system can be determined using the full width at half maximum (FWHM) and free spectrum range (FSR) of the generated multiple soliton pulses. The central wavelength of the bright input soliton pulse has been selected as 800 nm, at which a ring system with better sensitivity shows high finesse that is suitable for applications to many optical communication systems such as optical transmitters and sensors. Simulation results show that FSR of 0.3 nm and 1.1 ns and FWHM of 10 pm and 36.6 ps could be obtained. Therefore, a system with finesse of 30 can be obtained; in such a system, the MRR system shows high performance. This system can be used in optical communication networks as a transmitter system for optical soliton pulses with finesse of 30, and theses pulses can be detected via an optical receiver.     

Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N _DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ～150 fs, central wavelength ～800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from ?40 ps to +40 ps with a resolution of ～0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ～790 nm towards ～550 nm is demonstrated for delay changes of less than ～2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (～760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.     

Generation of an arbitrary chirped microwave waveform with high time-bandwidth product for increasing range resolution of RADAR by using photonic technique

An approach for photonic generation of an arbitrary chirped microwave waveform with an increased time-bandwidth product (TBWP) is proposed and experimentally demonstrated. In the proposed model, light from the mode locked laser is splitted into two parts by using 1 × 2 power splitter: one is sent to linearly chirped fiber Bragg grating (LCFBG) through circulator and the other is time delayed by fiber delay line. The optical pulse in upper arm is time stretched by the LCFBG. Meanwhile, the optical pulse in lower arm experiences a time delay and then stretched by the dispersive single mode fiber. Temporal interference pattern is generated with an increasing or decreasing free spectral range by combination of two time-stretched optical pulses. Finally, the temporal interference pattern which is obtained at the output of optical coupler is transformed into an arbitrary chirped microwave waveform by using a photo-detector. The main advantage of this proposed model is high TBWP in the range of 750–1000 which ultimately results in an increased range resolution of radio detection and ranging.     

A simple humidity sensor utilizing air-gap as sensing part of the Mach–Zehnder interferometer

A simple high-resolution refractive index (RI) and phase sensor has been demonstrated and the results numerically verified. A free space gap is employed in one arm of a Mach–Zehnder interferometer (MZI) to serve as the sensing mechanism with a physical spacing of 1.4 mm. The propagation constant of transmitted light in the MZI’s gap changes due to the small variation in the ambient RI that will further shift the optical phase of the signal. A free space optical delay line is embedded within the MZI’s other arm to set the phase reference point and compensate for variations in the optical phase difference. The ambient RI is computed by measuring the phase shift in the transmission spectrum A high-resolution sensing of 0.8 pm/%RH corresponds to phase change of 0.012°/%RH has been achieved in 1520 nm.     

A cost effective 100 Gbps hybrid MDM–OCDMA–FSO transmission system under atmospheric turbulences

FSO or free space optics is a familiar name used in a wide array of applications in the area of telecommunications. Due to its features of low maintenance cost and deployment time, most of the applications consider FSO as the alternative solution for appropriately replacing fiber optics. In this work, we have designed 100 Gbps FSO system by combining mode division multiplexing (MDM) and optical code multiple access scheme (OCDMA). Ten channels, each carrying 10 Gbps data, are transported over 8 km FSO link by using MDM of two Laguerre Gaussian modes and random diagonal codes. Moreover, the performance of proposed MDM–OCDMA–FSO system is also investigated under atmospheric turbulences.     

Laser ablation of absorbing liquids under transparent cover: acoustical and optical monitoring

Phase transition induced with infrared (λ = 2920 nm and λ = 2940 nm) nanosecond laser pulses in strongly absorbing liquids (water, ethanol) under transparent solid cover is investigated with the help of acoustical and optical monitoring. LiNbO₃ transducer is used for registration of pressure pulses generated in irradiated liquids. Optical signals due to scattering and specular reflection of probing optical beams are explored with the schemes involving total internal reflection and interference effects. Combination of these two optical diagnostic methods permits for the first time to show that irradiation of covered liquids leads to vapor cavity formation which is divided from the cover with thin (submicron) liquid film despite the fact that radiation intensity maximum is located just at the liquid–plate boundary. The cavity formation is due to explosive boiling which occurs when the superheated liquid reaches its superheating limit in near critical region. After the first acoustical signal, the second signal is observed with several hundreds microseconds time delay which is caused by the vapor cavity collapse. Some results of optical and acoustical diagnostics in the case of free liquid surface are also presented.     

Performance enhancement of the power penalty in DWDM FSO communication using DPPM and OOK modulation

This paper presents the design and performance enhancement of the power penalty (PP) in a dense wavelength division multiplexing based on free space optical communication (FSOC) link using digital pulse position modulation (DPPM) and on–off keying (OOK) modulation. Such a system has a high performance, low cost, robust and power efficient, reliable, excessive flexibility, and higher data rate for access networks. The system performance is evaluated for an 8-channel wavelength-division-multiplexing for hybrid fiber FSOC system at 2.5 Gbps on widely accepted modulation schemes under various atmospheric turbulence (AT) regimes conditions. The performance of system is introduced in terms of PP, bit-error rate (BER), transmission distance and the average received optical power. The numerical results shows that the improvement of the PP using DPPM modulation of 0.2–3.0 dB for weak turbulence (WT) regimes for BER of 10^?6 and above 20, 25 dB for strong turbulence (ST) regimes are reported for BER of 10^?6 and 10^?9, as respectively (depending on the AT level). Further, we develop of improvement the PP caused by multiple-access interference about 6.686 dB which is predicted for target BER of 10^?9 in WT and 1 dB at target BER of 10^?6 in ST when the 8 user are active on the system of optical network units. Additionally, the optical power budget and margin losses of a system are calculated with different link length. The proposed approach of DPPM merges superiority with higher enhancement of PP about 0.8 dB for BER equal 10^?9 at FSO link length l_fso?=?2000 m compared to OOK at 1 dB for WT. An improvement of 2 dB is observed using the DPPM scheme over an OOK due to capability of detect pulses under background noise conditions with increased receiver sensitivity.     

Optical characterization of tin containing novel chalcogen rich glassy semiconductors

Amorphous thin film materials with different compositions of Se_80?xTe₂₀Sn_x (0 ≤ x ≤ 10) system have been deposited on glass substrates by a well known thermal evaporation technique. Structural characterization of different compositions of aforementioned system has been done by Raman spectroscopy. The optical properties of thin films have been studied in the wavelength range 200–1100 nm by the utilization of the optical absorbance spectra of deposited thin films. To calculate the optical band gap from the optical absorption spectra, we have used Tauc model that follows the mechanism of allowed ‘non-direct electronic transition’. Subsequently, we have determined the energy band gap, metallization criterion and refractive index of thin films of aforesaid system. The variation in optical properties with composition has been interpreted in terms of density of defect states.     

A tunable multi-wavelength Brillouin–erbium fiber laser with fourfold Brillouin frequency spacing

We have experimentally demonstrated a tunable multi-wavelength Brillouin–erbium fiber laser with over 40 GHz spacing utilizing two cascaded double Brillouin-frequency-spacing cavities. In this laser configuration, two segments of 25 km-long single-mode fibers are used as Brillouin gain medium in each ring cavity, and a segment of 8 m-long erbium-doped fiber with 980 nm pump is employed to amplify Brillouin pump (BP). At BP wavelength of 1550 nm, BP power of 8.3 dBm (6.8 mW) and the maximum 980 nm pump power of 27.78 dBm (600 mW), seven output channels with fourfold Brillouin-frequency spacing, and the tuning range of 15 nm from 1545 to 1560 nm are achieved. The proposed multi-wavelength Brillouin–erbium fiber laser has wide applications, such as in microwave signal generation and optical communications.     

Design and characterization of arrayed waveguide gratings using ultra-low loss Si3N4 waveguides

Planar waveguides with ultra-low propagation loss are necessary for integrating optoelectronic systems that require long optical time delay or narrowband optical filters. In this paper, we review an ultra-low loss planar waveguide platform that uses thin (<150 nm) Si₃N₄ cores and thick (>8 μm) SiO₂ cladding layers. In particular, we discuss the performance of arrayed waveguide gratings (AWGs) fabricated with the platform. We propose the use of a practical design method that takes the statistical nature of worst-case crosstalk into account. We also demonstrate the measurement of amplitude and phase error distributions in an AWG using an optical backscatter reflectometer. We show that the waveguides have phase errors small enough to achieve AWG crosstalk below ?30 dB, while crosstalk below ?40 dB should also be possible with optimization of the component design.     

Structure and optical properties of Cd-substituted ZnO (Zn1−x Cd x O) thin films synthesized by the dc reactive magnetron sputtering

The ternary Zn_1?x Cd _x O (x = 0, 0.2) thin films with wurtzite structure and highly (002)-preferred orientations were deposited on glass substrates by the direct current (dc) reactive magnetron sputtering method. The X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), optical absorption spectra and photoluminescence (PL) were employed to investigate the structural and the optical properties in detail. The results indicated that as x varied from x = 0–0.2, the diffraction angle of the (002) peaks decreased from ~34.36° to ~33.38° and the lattice spacing increased from 0.260 to 0.268 nm. Moreover, the optical band-gap of the Zn_1?x Cd _x O thin films with the wurtzite structure decreased from 3.20 eV at x = 0–2.70 eV at x = 0.2. Correspondingly, the near-band-edge PL was tuned in a wide visible region from ~393 to 467 nm. The chemical bonding states of Cd in Zn_1?x Cd _x O alloy thin films were examined by XPS analysis.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.     

一种大视场反射光学系统的研究

从Schwarzschild系统出发,基于设计大视场光学系统的同心原则,设计了一种大视场的三反射光学系统.该系统有两个球面反射镜,一个二次非球面镜,系统在频率50 lp/mm处22°×11°的条形大视场范围内具有良好的成像性能,可以用于紫外探测光学系统中.     

Enhancing the performance of thin film CdS/PbS photovoltaic solar cells

This work investigates dependence of the short-circuit current density, open-circuit voltage, fill factor and efficiency of a thin film CdS/PbS solar cell on thickness of transparent conductive oxide (TCO) layer, thickness of window layer (CdS), concentration of uncompensated acceptors (width of space-charge region), carrier lifetime in PbS and the reflectivity from metallic back contact. The effect of optical losses, front and rear recombination losses as well as the recombination losses on space-charge region are also considered in this study. As a result, by thinning the front contact layer indium tin oxide from 400 to 100 nm and window layer (CdS) from 200 to 100 nm it is possible to reduce the optical losses from 32 to 20%. The effect of electron lifetime on the internal and external quantum efficiency can be neglected at high width of the space-charge region. The maximum current density of 18.4 mA/cm² is achieved at wide space-charge region (concentration of uncompensated acceptors = 10¹⁵ cm^?3) and the longest lifetime (τ_n = 10^?6 s) where the optical and recombination losses are about 55%. The maximum efficiency of 5.17%, maximum open-circuit voltage of 417 mV and approximately fixed fill factor of 74% are yielded at optimum conditions such as: electron lifetime = 10^?6 s; concentration of uncompensated acceptors = 10¹⁶ cm^?3; thickness of TCO = 100 nm; thickness of CdS = 100 nm; velocity of surface and rear recombination = 10⁷ cm/s and thickness of absorber layer = 3 μm. When the reflectance from the back contact is 100%, the cell parameters improve and the cell efficiency records a value of 6.1% under the above conditions.     

Design and analysis of all-optical 4–2 binary encoder based on photonic crystal

In this paper we theoretically propose combining optical waveguides with nonlinear photonic crystal based ring resonators for realizing an all optical 4–2 encoder. Nonlinear resonant rings are constructed from GaAs as the material used for dielectric rods. By application of optical beam to input ports of the structure the on/off states are realized and the resulting truth table confirms functionality of proposed device. The switching threshold and time delay of the device are about 1 kW/µm² and 1 ps, respectively. The normalized power at the output ports for all the ports in ON states is the same and equal to 60%.