Properties of a Thin-Film Optical Waveguide Using Fluorinated Silicon Oxide and Organic Spin-on-Glass Films

A thin-film optical waveguide using a fluorinated silicon oxide (SiOF) as a core layer was investigated. An organic spin-on-glass (SOG) film was used for a cladding layer. The SiOF films were formed at 23°C by a liquid-phase deposition (LPD) technique using a supersaturated hydrofluosilicic acid (H₂SiF₆) aqueous solution. A thin-film optical waveguide structure for single mode was designed and fabricated, based on the dispersion properties of refractive indices for the LPD-SiOF and organic SOG films. The refractive indices at a wavelength of 632.8 nm were 1.430 and around 1.400 for the LPD-SiOF and organic SOG films, respectively. The thickness of LPD-SiOF films deposited was 1.18 μm. Thicknesses of cladding organic SOG films cured at 300 and 400°C were 1.28 and 1.31μm, respectively. The effective refractive indices for single mode were 1.4169 and 1.4158 at a wavelength of 632.8 nm for the cladding organic SOG films cured at 300 and 400°C, respectively, and differences between the measured and calculated incident angles were 0.84° and 1.29° for the cladding organic SOG films cured at these respective temperatures. A streak of guided-light was observed for the LPD-SiOF/SOG structure optical waveguide. The transmission loss was 7.6-7.9 dB/cm.     

IR-induced nonlinear optics in Ge-doped Bi12TiO20 large-sized nanocrystallites

Photoinduced non-linear optical effects in large-sized (up to 25 nm) nanocrystallites (NC) of Ge-doped Bi₁₂TiO₂₀ (BTO:Ge) incorporated within olygoether photopolymer matrix have been studied. Photoinduced second harmonic generation (PISHG) was measured. Nd:YAG pulsed laser (λ=1.06 μm) was used as a source of photoinducing light. As a fundamental light source for the SHG and two-photon absorption, Er:LiYF₄ laser (λ=2.065 μm) was used. We have found that with increasing IR pump power density, the output doubled frequency SHG signal (λ=1.03 μm) increases and achieves its maximum value at the pump power density about 0.45 GW/cm² and NC size about 12 nm.The values of second-order optical susceptibilities were almost 20% larger than for the pure BTO NC single crystals. With decreasing temperature below 60 K, the SHG signal increases achieving maximal value at LHeT.     

Angular distribution of intensity of reflected radiation investigations of the influence of CO2 laser treatment on optical properties of hydrogenated amorphous silicon

Thin films of hydrogenated amorphous silicon (a-Si:H) were annealed using CO₂ laser radiation (λ=10.6 μm). Changes of optical properties of the treated a-Si:H were investigated using optical transmittance spectroscopy and the angular distribution of intensity of reflected radiation (ADIRR). The CO₂ laser annealing influences the spectral characteristics of the real part of refractive index n and absorption coefficient α of light in a-Si:H. This treatment increases the n and α values as well as the Urbach energy of a-Si:H. Simultaneously it decreases the optical energy gap of this material. The changes of optical parameters at the interfaces of a-Si:H–glass substrate and a-Si:H–air were established.     

Efficient diode-pumped Nd:YVO4 continuous-wave laser at 1.34 μm

We report a high-power continuous-wave(cw) diode-pumped efficient 1.34 μm Nd:YVO₄ laser. The laser properties of a low Nd³⁺-doped concentration of the Nd:YVO₄ crystal operating at 1.34 μm formed with a simple plane-concave cavity have been demonstrated. With the incident pump power of 22 W, an output power of 8.24 W was obtained, giving an optical conversion efficiency of 37.5% and slope efficiency of 40%. The thermal effects of cw end-pumped solid-state lasers were studied.     

Characteristics of 980/1550 nm WDM coupler based on planar curved waveguides

A wavelength division multiplexer (WDM) for 980/1550 nm based on planar curved waveguide coupler (CWC) is proposed. Compared with conventional parallel straight waveguide coupler (SWC), this structure has more flexibility with two variable parameters of bending radius R and minimum edge-to-edge spacing d₀, which are the two main parameters for the splitting ratio of coupler and decrease the complexity of device design and fabrication. Based on coupled mode theory (CMT) and waveguide theory, R and d₀ of the WDM CWC are designed to be R=13.28 m and d₀=4.39 μm. The contrast ratio (CR) and insertion loss (IL) for 980 and 1550 nm are CR₁=24.62 dB, CR₂=24.56 dB and IL₁=0.014 dB, IL₂=0.015 dB, respectively. The 3D beam propagation method (BPM) is used to verify the validity of the design result. The influence of R and d₀ variations on the device performance is analyzed. For CR>20 dB, the variation ranges of R and d₀ should be within −0.10 to +0.44 m and −0.05 to +0.02 μm, respectively.     

SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3–1.55 μm

The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–1.55 μm. The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by 60 nm Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BV_ceo, was about 6 V. Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at 1.3 μm. When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and 5 mA/W at 1.3 and 1.55 μm, respectively.     

Resonant-cavity-enhanced photodetectors and LEDs in the mid-infrared

In this paper we outline the use of resonant-cavity enhancement for increasing the exterior coupling efficiency of photodetectors and light-emitting diodes (LEDs) in the mid-infrared (MIR) spectral region. This method is potentially very important in the MIR because encapsulation is not presently feasible due to the lack of suitable materials. Among other potential applications, resonant-cavity-enhanced (RCE) photodetectors and LEDs could be particularly suitable for greenhouse gas detection because of their ‘pre-tunable’ spectrally narrowed resonantly enhanced peaks. We also present the optical characterization of an InAs RCE photodetector aimed at the detection of methane gas (λ≈3.3 μm), and an InAs/InAs_0.91Sb_0.09 resonant-cavity LED (RCLED) aimed at carbon dioxide gas (λ≈4.2 μm). The high peak responsivity of the RCE photodetector was 34.7 A/W at λ=3.14 μm, and the RCLED peaked at λ=3.96 μm. These are among the longest operating wavelengths for III–V RCE photodetectors and RCLEDs reported in the literature.     

Observation of all-optical wavelength conversion based on cascaded effect in periodically poled lithium niobate waveguide

We experimentally demonstrate multi-channel wavelength conversion through the cascaded second-order nonlinear processes in periodically poled lithium niobate waveguide in the optical communication wavelength band of 1.5 μm. The normalized conversion efficiency as high as 90% W⁻¹ cm⁻² is obtained. Wavelength conversion over a broad wavelength range of 66 nm has been confirmed.     

Actively Q-switched intracavity second-harmonic generation of 1.06 μm in BiB3O6 crystal

We have investigated the acousto-optically Q-switched intracavity second-harmonic generation of 1.06 μm in a 1.9-mm-long BiB₃O₆ crystal, cut for type-I phase-matching direction of (θ,)=(168.9°,90°), performed in a diode-end-pumped Nd:YVO₄ laser. When the incident pump power was 4.3 W at 30 kHz of pulse repetition frequency, a maximum average green output power of 480 mW, the shortest pulse with FWHM width of 72 ns, the highest single pulse energy of 16 μJ and the maximum peak power of 222 W were obtained, giving the corresponding optical conversion efficiency of 11.2%. The effect of varying temperature in BIBO crystal on the average green output power was also investigated.     

A high-precision optical metrology system for determining the thickness of birefringent wave plates

This study develops a non-destructive measurement system for determining the thickness and refractive indices of birefringent optical wave plates. Compared to previous methods presented in the literature, the proposed metrology system provides the ability to measure the thickness of the birefringent optical plate in high-precision. The results show that for a commercially available birefringent optical wave plate with refractive indices of n_e=1.5518, n₀=1.5427 and a thickness of 452.1428 μm, the experimentally determined value for the error in the wave plate thickness measurement is just 0.046 μm. The measurement resolution of the proposed system exceeds that of the interferometer hardware itself. The proposed method provides a simple yet highly accurate means of measuring the principal optical parameters of birefringent glass wave plates.     

Ultra-fast nonlinear response around 1.5 μm in 2D AlGaAs/AlOx photonic crystal

We report the fast switching capabilities of a two-dimensional Al_0.3Ga_0.7As photonic crystal slab around 1.5 μm. The slab is supported by an AlOx low-index thick layer that plays the role of an efficient heat sink. By pumping at 0.8 μm in the absorption of the Al_0.3Ga_0.7As quantum wells, the optical response is modified in the transparency region: a 200% change in the reflectivity is obtained with a total response time of 8ps.     

Loss reduction of dielectric-coated metallic hollow waveguides for CO2 laser light transmission

A condition for reducing transmission loss of a dielectric-coated metallic hollow waveguide is shown theoretically when the inner metallic surface is not perfect. It is also shown that the transmission loss for CO₂ laser light is 10.6 μm wavelength is able to be estimated from an attenuation spectrum of a short waveguide. By fabricating a ZnSe coated Ag hollow waveguide of short length, we have shown that it has a lower loss than a Ge coated waveguide for CO₂ laser light.     

Second harmonic generation in In2O3–SiON films doped by Al and Sn

Photoinduced optical and second-order non-linear optical effects in the interfaces separating In₂O₃–SiON (O/N ratio equals 1) films doped with A1, Sn and glass substrates were investigated using the photoinduced optical second harmonic generation. The photoinduced effective second-order optical susceptibility d_eff (at λ=1.76 μm) shows a good correlation with the linear optical susceptibility, particularly with the shift of the absorption edge. The maximal response of the photoinduced optical response signal was observed for the pump–probe delaying time of 34 ps. The performed experimental measurements indicate that the observed effects are mainly caused by the interface potential gradients on the border glass–In₂O₃–SiON film and by additional polarization due to insertion of the Al and Sn atoms. The observed phenomenon may be proposed as a sensitive tool for investigation of thin semiconducting interfacial layers and simultaneously such films may be used as materials for non-linear optical devices.     

A DFG spectrometer at 3 μm for high resolution molecular spectroscopy and trace gas detection

We report the realization and characterization of a spectrometer based on difference frequency generation using a periodically poled lithium niobate crystal. As signal and pump lasers we used a diode-pumped Nd–YAG laser (λ=1.064 μm) and an extended cavity semiconductor diode laser (λ=0.785 μm), respectively. The mid-infrared coherent radiation was produced at 3 μm with a maximum power of about 160 nW obtained with 340 mW of signal and only 3.4 mW of pump. That corresponds to an efficiency of 0.01%/Wcm, which is in good agreement with other data available in literature. The generated radiation around 3 μm has allowed us to study fundamental absorption bands of molecules of great atmospheric and physical interest such as water vapor and the hydroxyl free radical. In this work we report preliminary spectroscopic results concerning the ν₁ 5₄₁→6₅₂ H₂O line at 2.968 μm. In particular, for this line we provide the first experimental estimation of self-, N₂- and O₂-broadening coefficients.     

Si1−xGex/Si multi-quantum well phototransistor for near-infrared operation

For the first time in the literature, we report the monolithic integration of SiGe near-infrared phototransistor and planar hetero-junction bipolar transistor (HBT). The phototransistor is made with SiGe/Si multi-quantum well structure (MQW_PHT). At room temperature, the MQW_PHT reveals an optical responsivity of 1904 mW/A at 0.85 μm and 1.25 mW/A at 1.3 μm under the reversed bias of V_CE=1.5 V. For electrical DC and microwave performance, the SiGe HBT has a current gain of 160 and a cut-off frequency (f_T) of 25 GHz, respectively.     

Line Parameters for Ozone Hot Bands in the 3.3-μm Spectral Region

Line positions, intensities, and lower state energies have been calculated for eight hot bands of ¹⁶O₃ in the 3.3-μm spectral region. The results are based on spectroscopic parameters deduced in recent high-resolution laboratory studies and improved rotational energy levels of the (103), (004), and (310) vibrational states derived by refitting earlier data and experimental (004) energy levels from measurements of the 4ν₃ - ν₃ hot band. The good quality of the new parameters has been verified through comparisons of line-by-line simulations with high-resolution laboratory spectra. The present work and the results of our analyses of the main bands at 3.6 μm [Smith et al., J. Mol. Spectrosc.139, 171-181 (1990)] and 3.3 μm [Camy-Peyret et al., J. Mol. Spectrosc.141, 134-144 (1990)] provide a complete set of ozone spectroscopic line parameters covering the 3-μm region.     

Optical particle sizers for on-line applications in industrial plants

We present three optical particle sizers properly conceived for different on-line applications in industrial plants. The first one is a diffraction-based particle sizer utilising an innovative optical scheme that allows the instrument to operate at very low particle concentration regimes (i.e. at extinction values as small as 10⁻⁵) in the size range 0.9–90 μm. The second one is based on the same principle of operation, but has been designed with a different optical configuration that makes it suitable for the characterisation of high concentration particle laden flows (like for e.g. pulverised coal downstream of the grinding mills) in the size range 3–300 μm. The third instrument is based on a multi-wavelength extinction technique and provides measurements over long optical paths (up to 10 m) in the 0.1–3 μm size range.     

Excitation dependent losses and temperature increase in various hollow waveguides at 10.6 μm

Theoretical and experimental studies have been made on excitation dependent transmission properties of various hollow waveguides at 10.6 μm. A heat problem of the waveguide has been experimentally treated when high powered CO₂ laser light is launched.     

Effect of buffer gas on the generation of eye-safe 1.54 μm radiation using the stimulated Raman scattering technique in CH4

Experimental results on the generation of 1.54 μm eye-safe radiation in pure CH₄, CH₄:He and CH₄:Ar mixtures pumped by the fundamental of an Nd:YAG laser, using the stimulated Raman scattering (SRS) technique, are described. A decrease in the energy conversion efficiency and degradation in the beam quality of S₁ was observed in pure CH₄ at high pump energies. This problem was overcome in CH₄:He and CH₄:Ar mixtures. Compared with the first Stokes (1.54 μm) energy conversion efficiency in pure CH₄, at a pump energy of 126 mJ, an enhancement of 50% in energy conversion efficiency was observed in the CH₄:Ar mixture (60% argon concentration) and as much as 100% in the CH₄:He mixture (60% helium concentration). The use of these buffer gas mixtures improved the spatial beam quality of the Stokes radiation considerably and also resulted in raising the pump threshold for optical breakdown of the Raman gain medium.     

High-power end-pumped CW Nd:GdVO4 laser formed with a flat–flat cavity

Formed with a flat–flat resonator, a diode-laser-array end-pumped CW Nd:GdVO₄ laser at 1.06 μm, capable of generating 8.6 W of TEM₀₀ output power with optical conversion efficiency of 43% and slope efficiency of 48%, has been developed. The laser beam was nearly diffraction limited, with the beam quality factor measured to be M²=1.22. Under the conditions of multi-mode operation, the laser was able to produce 11.2 W of low-order transverse mode radiation (M²<2) at the incident pump power of 22 W, giving an optical conversion efficiency of 51%, and a slope efficiency of 55%.