Measurement of impurity concentration in chalcogenide glasses using optical principle

Owing to impurity concentration, is important in chalcogenide glass to study various commercial applications, this paper presents a novel technique to measure the impurity concentration in chalcogenide glass at wavelength of 633 nm and 1500 nm using optical principle. Here both reflection and absorption losses are considered to estimate the same impurities. Reflectance is found using plane wave expansion method, where absorption factor is determined using Maxwell's curl equations. Simulation result reveals that reflectance, absorption factor and transmitted intensity vary linearly with respect to different impurity concentrations. The excellent linear variation of transmitted intensity gives an accurate measurement of impurity concentration in chalcogenide at aforementioned wavelength.     

Numerical study of sub-wavelength plasmonic waveguide

The authors present an analysis of a plasmonic waveguide, simulated using a two-dimensional finite-difference time-domain technique. With the surface structures located on the surface of the metal, the device is able to confine and guide light waves in a sub-wavelength scale. And two waveguides can be placed within 150 nm (∼6% of the incident wavelength) that will helpful for the optoelectronic integration. Within the 20 μm simulation region, it is found that the intensity of the guided light at the interface is roughly two to four times the peak intensity of the incident light, and the propagation length can reach approximately 40 μm at the wavelength of 2.44 μm.     

Tolerance and tuning properties of the optical parametric processes using periodically poled RbTiOAsO4

The maximal tolerance parameters of poling period and phase-matching, temperature in second harmonic generation (SHG) using periodically poled RbTiOAsO₄(PPRTA) as a function of the fundamental wavelength are investigated theoretically. The tolerance of the poling period ΔΛ of PPRTA is found larger than that of PPLN and PPKTP when the fundamental wavelength is beyond 2 μm, which reaches its maximum ΔΛ_max for PPRTA at a fundamental wavelength of 2.7433 μm. However, the tolerance for the phase-matching temperature ΔT of PPRTA is found smaller than that of PPLN and PPKTP with an exception that PPRTA has a larger tolerance of the temperature or a larger temperature phase-matching bandwidth at fundamental wavelength of 2.2474 μm, where the maximum of ΔT(ΔT_max) is obtained. Furthermore, the tuning characteristics of the optical parametric processes using PPRTA for collinear quasi-phase-matching (CQPM) is analyzed. The combination of temperature tuning and poling period tuning enables a quasi-continuous wavelength tuning range of 1493.2-1593.7 nm for the signal and 3201.8-3699.2 nm for the idler, where poling period of 39 μm, 39.5 μm and 40 μm and a temperature between 20 and 120° have been employed in the corresponding theoretical analysis.     

The photodetector of Ge nanocrystals/Si for 1.55 μm operation deposited by pulsed laser deposition

The photodetector properties of a Ge nanocrystals detector fabricated by pulsed laser deposition and in situ rapid thermal annealing treatment at 600 °C have been studied. Strong optical absorption and photocurrent response of the detector are measured in the wavelength range 1.3-1.55 μm. The detector possesses a low dark current of 61.4 nA and a photocurrent responsivity of 56 mA/W at the reverse bias 5 V. The external quantum efficiency at 1.55 μm is estimated to be 15%. The stop wavelength of absorption spectra extends to 1.65 μm. It indicates that these kind of Ge nanocrystals devices can be used as a 1.3-1.55 μm near infrared detector.     

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.     

Narrow line-width resonant cavity enhanced photodetectors operating at 1.55 μm

A method for fabrication of long-wavelength narrow line-width InGaAs resonant cavity enhanced (RCE) photodetectors in a silicon substrate operating at the wavelength range of 1.3-1.6 μm has been developed. A full width at half maximum (FWHM) of 0.7 nm and a peak responsivity of 0.16 A/W at the resonance wavelength of 1.55 μm have been accomplished by using a thick InP layer as part of the resonant cavity. The effects of roughness and tilt of the InP layer surface, and its free carrier absorption, as well as the thickness deviation of the mirror pair on the resonance wavelength shift and the peak quantum efficiency of the RCE photodetectors are analyzed in detail, and approaches for minimizing them toward superior performance are suggested.     

Tunable mid-IR laser absorption sensor for time-resolved hydrocarbon fuel measurements

A wavelength-tunable mid-infrared (mid-IR) laser is used to make time-resolved absorption measurements of methyl-cyclohexane (MCH) and n-dodecane vapor concentration, demonstrating the use of this novel laser source for sensing hydrocarbon fuels. Two sensitive and species-specific diagnostic strategies are investigated: (1) direct absorption at a fixed wavelength, and (2) dual-wavelength differential absorption with two rapidly-alternating wavelengths. The tunable laser light is produced using difference frequency generation by combining two near-infrared diode lasers in a periodically poled lithium niobate crystal, providing a continuous-wave (cw), room temperature mid-IR source with the low intensity noise, and rapid wavelength tunability typical of telecommunications diode lasers. Direct absorption measurements of MCH with a wavelength of 3413.7 nm demonstrate fast time response (1 μs) and low noise in cell (300-675 K) and shock tube (650-1450 K) experiments. The detection limits of MCH range from 0.5 ppm-m at 300 K to 11 ppm-m at 1440 K (pressure = 101 kPa). Next, time-division multiplexing is used to alternately generate two mid-IR wavelengths at 20 kHz, enabling the use of dual-wavelength differential absorption to eliminate interference absorption. Measurements of MCH concentration are first made in a cell, with varying amounts of n-heptane interference absorption. Accurate values of MCH concentration are obtained for n-heptane/MCH ratios as high as 15, demonstrating the utility of this sensor for species-specific hydrocarbon detection in systems with interfering absorption. Finally, time-resolved n-dodecane vapor concentration measurements are made in a shock-heated evaporating aerosol. The dual-wavelength differential absorption diagnostic is sensitive only to the vapor concentration, rejecting droplet extinction. These measurements illustrate the power of the differential absorption strategy for sensitive vapor-phase detection in the presence of particle scattering. The tunability of this new source will allow these concepts to be extended to other hydrocarbon fuels.     

Widely phase-matched tunable difference-frequency generation in periodically poled LiNbO3 crystal

We report on the development of a laser source in the mid-infrared spectral region based on difference-frequency generation (DFG) in a periodically poled LiNbO₃ (PPLN) crystal. Continuously tunable coherent radiation from 2.75 to 4.78 μm was produced by optical parametric interaction between a diode-pumped monolithic continuous-wave (CW) Nd:YAG laser operating at 1.064 μm and a CW Ti:Sapphire laser tunable from 767 to 871 nm. Temperature-dependent quasi-phase-matched DFG wavelength acceptance bandwidth was studied and characterized. An empiric formula is given to estimate the phase-matched wavelength acceptance bandwidth as a function of the crystal temperature at Λ = 22.5 μm. A large frequency scan of 128 cm⁻¹ (about 78 cm⁻¹ above 1 μW) near 4.2 μm was achieved. The whole absorption spectrum of the P and R branches of the ν₃ band of atmospheric carbon dioxide has been recorded with a single phase-matched frequency scan.     

Low-pump-threshold tunable optical parametric oscillator using periodically poled MgO:LiNbO3

We fabricated and characterized periodically poled MgO:LiNbO₃ device with five gratings in 0.5 μm increments from 29 μm to 31 μm for optical parametric oscillator (OPO). The OPO operation threshold is 30 μJ using this device with a 50 mm effective length. At 560 mW input pump power, we have achieved 300 mW of the total output power, and the conversion efficiency is 54%. Multi-periods and temperatures tuning of the OPO yields a signal wavelength range from 1.45 to 1.72 μm and an idler wavelength range from 2.8 to 4.05 μm in the mid infrared.     

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.     

A complete study of the line intensities of four bands of CO2 around 1.6 and 2.0 μm: A comparison between Fourier transform and diode laser measurements

Atmospheric carbon dioxide is a key specie for the Earth climate. Two spectral windows at 1.6 μm and 2.0 μm are of particular interest for the in situ and remote monitoring of carbon dioxide from satellite, balloon or airborne platforms using infrared absorption spectroscopy. A precise knowledge of the line strengths is a prerequisite for an accurate concentration retrieval. In this paper, we have revisited in the laboratory the (30⁰1)_III←(00⁰0) and (30⁰1)_II ← (00⁰0) bands of CO₂ near 1.6 μm and the (20⁰1)_III ← (00⁰0) and (20⁰1)_II ← (00⁰0) bands near 2.0 μm by implementing both a high-resolution Connes-type Fourier-transform spectrometer and a tunable diode laser spectrometer equipped with several telecommunication-type semiconductor laser devices. Approximately 200 (respectively 18) transitions of CO₂ have been carefully investigated in spectra recorded with the FT spectrometer (respectively with the tunable diode laser spectrometer). The intensity measurements achieved with both instruments are thoroughly compared to previous instrumental determinations, ab-initio calculations and available atmospheric molecular database.     

Direct intensity modulation of a rectangular ring laser with bidirectional lasing characteristics

The direct intensity modulation of a three-guide coupled rectangular ring laser having bidirectional lasing characteristics is reported for the first time. The rectangular laser cavity consists of four low loss total internal reflection mirrors and an output coupler made out of three passive coupled waveguides. The laser is fabricated using a total cavity length of 580 μm with active section lengths of 250 μm. For both the clockwise and counterclockwise circulating directions, a lasing threshold of around 38 mA is obtained at room temperature under continuous wave operation. A maximum 3-dB modulation bandwidth of approximately 3.2 GHz is observed in both circulating directions.     

The nonlinear refraction and absorption dependence on the thermal effect for 4 ns pulse duration in binuclear Zn(II) phthalocyanine solution

The influence of thermal effect on the third-order nonlinear optical properties of binuclear Zn(II) phthalocyanine in chloroform solution was studied. The nonlinear refraction and absorption of the sample was measured by using Z-scan technique with 4 ns laser pulses at 532 nm wavelength. The opposite signs of the effective nonlinear refraction index were observed by changing the focal length of focusing lens from 10 cm to 20 cm in the experimental setup. Changing the focusing lens increased the beam waist radius from 7 μm to 20 μm. The nonlinear absorption coefficient was reduced about 200 times based on changing the fluence or beam waist radius. The drastic changes in the third-order nonlinear optical parameters were attributed to thermal effect. To investigate the role of thermal effects even further the effective nonlinear refraction and absorption coefficients were studied by using different repetition rates, input powers and concentrations.     

The influence of light intensity on surface recombination in GaS single crystals

Gallium sulphide (GaS) is a layer structure semiconductor with relatively wide energy gap (E_g (295 K) = 2.5 eV and E_g (80 K) = 2.62 eV). It has potential applications in some areas of optoelectronics. This paper presents the investigations of the influence of light intensity on surface recombination velocity of charge carriers in GaS single crystals. To attain this purpose spectral dependences (between 420 and 550 nm) of absorption coefficients, reflectivity coefficients and photoconductivity were measured in vacuum. The investigations were performed for various light intensities in several temperatures from 80 to 333 K. The least square method was applied to fit the theoretical dependences of photoconductivity on wavelength and intensity of illumination at these temperatures. From the fittings the temperature and light intensity dependences of surface recombination velocity and bulk lifetime of charge carriers were obtained.     

Fabrication of Si-based waveguide splitters using photosensitive sol-gel method

In this paper, a photosensitive sol-gel method for 1 × 8 Y-branch optical splitter is reported. The poly (methyl methacrylate) (PMMA) cladding films with different thicknesses (above 10 μm) on Si substrates are prepared on Si substrates by dip-coating, the root-mean-square roughness (Rrms) of which is less than 0.7 nm. The UV photosensitive organic-inorganic composite SiO₂/ZrO₂/H core films are then coated on these PMMA cladding films using photosensitive sol-gel method. The refractive indexes of PMMA cladding films examined by spectroscopic ellipsometer are 1.4854, 1.4815 and 1.4806 at 0.85, 1.31 and 1.55 μm, while that of the SiO₂/ZrO₂/H gel films are 1.5569, 1.5489 and 1.5472 μm, respectively, which is larger than that of PMMA cladding films. Therefore, the composite structure of SiO₂/ZrO₂/H gel films on PMMA-on-Si substrates could be used to fabricate waveguide splitter. Based on the inherent photosensitivity of the SiO₂/ZrO₂/H gel film, 1 × 8 Y-branch optical power splitters with a thickness of 5 μm are fabricated by irradiating the gel film with UV light through a mask followed by dissolving the non-irradiated area in a suitable solvent. The line-width and output spacing of this splitter are 25 and 110 μm, respectively. The observed near-field pattern indicates that the light with a wavelength of 1.53-1.56 μm can be transmitted and split into eight branches in the optical splitter, which is fabricated by using the above technique.     

1.55 μm distributed feedback (DFB) lasers subject to strong 1.4 μm optical injection

An experimental analysis of the influence of optical injection at 1.4 μm wavelength into two different commercial 1.55 μm DFB lasers is reported. The results demonstrate the strong dependence of the DFB behaviour on the injection parameters. Complete mode suppression or signal amplification can be obtained by varying the excitation wavelength and/or intensity, suggesting that these devices could be operated as logic ports or signal amplifiers, according to the injected signal.     

Optimal design of coupling waveguide structure for adiabatic optical directional full couplers weighted by sin-square and raised-cosine functions

The optimal design of coupling waveguide structure for adiabatic optical directional full couplers (AODFC) based on weighted sin-square function (SSF) and raised-cosine function (RCF) are investigated in this paper. The coupling lengths with a crosstalk smaller than −30 dB at the operating wavelength of 1.57 μm resulted in a value of 3.0 and 2.4 mm for the SSF and RCF cases, respectively. Under the same level of crosstalk, the wavelength ranges obtained by weighted SSF and RCF could be varied about 1.33-1.70 μm and 1.27-1.70 μm with coupler lengths of 6.0 and 5.5 mm, respectively. Clearly, the AODFC weighted by RCF has the superior performances of both short coupling length and broad wavelength ranges.     

High-power 3.8 μm tunable optical parametric oscillator based on PPMgO:CLN

The experimental results of a high-power 3.8 μm tunable laser are presented on a quasi-phase-matched single-resonated optical parametric oscillator in PPMgO:CLN pumped by a 1064 nm laser of an elliptical beam. Theoretical analyses of the PPMgO:CLN wavelength tuning are presented. The pump source was an acousto-optical Q-switched cw-diode-side-pumped Nd:YAG laser. The beam polarization matched the e-ee interaction in PPMgO:CLN. When the crystal was operated at 90 °C and the pump power was 150 W with a repetition rate of 10 kHz, average output power of 22.6 W at 3.86 μm and 63 W at 1.47 μm was obtained. The slope efficiency of the 3.86 μm laser with respect to the pump laser was 17.8%. The M² factors of the 3.86 μm laser were 1.74 and 4.86 in the parallel and perpendicular directions, respectively. The mid-IR wavelength tunability of 3.7-3.9 μm can be achieved by adjusting the temperature of a 29.2 μm period PPMgO:CLN crystal from 200 °C to 30 °C, which basically is accorded with the theoretic calculation.     

Temperature sensitivity of long period fiber grating in SMF-28 fiber

In this paper we have presented long period fiber grating (LPFG) as temperature sensor. Temperature based sensors have found a number of applications in commercial and industrial fields. In LPFG based temperature sensors, they respond to shift in various peak resonant wavelengths corresponding to various attenuation bands of the transmission spectrum. Temperature effect on the various attenuation bands of a LPFG have been investigated to create a highly sensitive measurement device. The temperature sensitivities of various attenuation bands of a LPFG over the wavelength region of 1.1–1.7 μm, for a grating period of 280 μm period, are obtained by monitoring the wavelength shift of each peak resonant wavelength with temperature increment of 20 °C, ranging from 0 °C to 100 °C.     

Modeling of femtosecond ablation of aluminum film with single laser pulses

Detailed investigation of pulsed laser ablation dynamics is performed for aluminum target under action of 100 fs pulses with peak intensity 3.95 × 10¹² W/cm² and wavelength 0.8 μm.Non-equilibrium two-temperature model with hydrodynamic Stephan problem was used for modeling. Explicit tracking of moving interphase boundaries permits exact determination of their velocity and amount of removed and evaporated material. Detailed ablation process is analyzed using the study of temperature, pressure and density evolution in the target. High phase front velocities (melting up to 5 km/s and evaporation up to 350 m/s) are caused by strong overheating of solid and liquid phases.