Femtosecond pulsed laser ablation of thin gold film

Laser micromachining on 1000 nm-thick gold film using femtosecond laser has been studied. The laser pulses that are used for this study are 400 nm in central wavelength, 150 fs in pulse duration, and the repetition rate is 1 kHz. Plano-concave lens with a focal length of 19 mm focuses the laser beam into a spot of 3 μm (1/e² diameter). The sample was translated at a linear speed of 400 μm/s during machining. Grooves were cut on gold thin film with laser pulses of various energies. The ablation depths were measured and plotted. There are two ablation regimes. In the first regime, the cutting is very shallow and the edges are free of molten material. While in the second regime, molten material appears and the cutting edges are contaminated. The results suggest that clean and precise microstructuring can be achieved with femtosecond pulsed laser by controlling the pulse energy in the first ablation regime.     

Design of Ultra Compact Polarization Splitter Based on the Complete Photonic Band Gap

A novel design of polarization splitter based on the complete photonic band gap has been proposed in this paper. The proposed Photonic Band Gap (PBG) polarization splitter is formed by two photonic crystal waveguides composed of dielectric rods in air in honeycomb structure for which complete photonic band gap is obtained using the plane wave expansion (PWE) method. The splitting properties (i.e. coupling length, extinction ratio and insertion loss) of PBG polarization splitter have numerically been investigated using the finite difference time domain (FDTD) method. It has been shown that polarization splitter of length as small as 32 μm can be designed at λ=1.55 μm. The proposed polarization splitter offers a large bandwidth of 120 nm.     

Refractive index of standard oils as a function of wavelength and temperature

The refractive indices (n) of eight standard oils from Physikalisch Technische Bundesanstalt, Germany were determined with an accuracy of ±1×10⁻⁴ by using Abbe Refractometer. The measurements were performed at temperature 20°C in the spectral range 0.4–0.7 μm. The experimental data were fitted to the simple Cauchy dispersion formula and the results were found to be consistent within the limits of experimental error. In all cases, the refractive index decreased monotonically with increasing wavelength. The refractive indices (n) of these oils have been measured as a function of the temperature t (20°C up to 50°C) at λ=0.589 μm and were found to have linear temperature dependencies. The refractive indices of the studied oils and the uncertainty in their values are calculated at λ=0.589. The Lorentz–Lorenz (L–L) formula has been tested and it was found to be valid with a maximum deviation of 0.4% and was used to calculate the molecular polarizability θ.     

Compact eye-safe laser sources based on OPOs with KTP or PPKTP crystals

We report on compact eye-safe nanosecond laser sources emitting in the 1.5 μm wavelength range based on non-critically phase-matched parametric interaction in optical parametric oscillators (OPOs) with KTP and periodically poled KTP (PPKTP) crystals, pumped by the fundamental frequency of Nd:YAG lasers. As much as 250 μJ signal pulse energy at 1.5 μm wavelength, 6.5 ns FWHM pulse-width, has been obtained in a PPKTP-OPO, extracavity pumped by a Nd:YAG microlaser oscillator–amplifier at 650 μJ pump pulse energy, 8 ns pulse-width. A single signal pulse of 2.7-mJ output energy at 1.57 μm wavelength, less than 5 ns pulse-width, was generated in a KTP-OPO, intracavity pumped by a passively Q-switched Nd:YAG laser.     

Short wave pass and long wave pass dichroic coating at 45° on zinc selenide substrate for dual band thermal imager

In dual band thermal imager dichroic coating plays a vital role in separating 3–5 μm and 7.5–10.5 μm wavelength region for observing better image quality from two different channels. In this work a study has been carried out on the design and fabrication of short and long wave pass dichroic coating at 45° on zinc selenide flat substrate. These dichroic coated optics can be used to separate 3–5 μm (in reflection or transmission channel) and 7.5–10.5 μm (in transmission or reflection channel) wavelength region. An inhomogeneous refractive index profile which is a polynomial of 5th order was considered to design the high and low wave pass dichroic coating on zinc selenide substrate. The inhomogeneous profile was then approximated with five steps from substrate to air medium. These steps were then converted in terms of durable coating materials of six and seven layer stack for short and long wave pass dichroic coating respectively. The coating material combination used was germanium as high index material and IR-F625 as low index material. Result achieved for short wave pass dichroic filter was 94% average transmission in 3–5 μm region and 95% average reflection in 7.5–10.5 μm region. Similarly, result achieved for long wave pass dichroic filter was 95% average reflection in 3–5 μm region and 94% average transmission in 7.5–10.5 μm.     

Analysis of Effect of Chromatic Dispersion for Wavelength Tunable Ultrashort Soliton Pulse Generation

The performance characteristics of generated soliton and anti-Stokes pulses for the wavelength tunable ultrashort soliton pulse generation system are discussed for different fiber dispersion parameters of β₂ and β₃ at the pump wavelength. Numerical analysis showed that the broadband wavelength tunability is improved as both the absolute value of negative β₂ and the value of β₃ are decreased. When β₃ is the usual value, the best performance of broadband continuous wavelength tunability which broadens symmetrically to the pump wavelength is obtained for β₃ of around 蜢2 psβ²/km. The reason for this is that the behaviors of both the generated soliton and anti-Stokes pulses are due to the β₂. Through simulations for different fiber dispersion parameters of β₂ or β₃ we obtained interesting results. When β₃ is negative and the zero dispersion wavelength is set at the longer wavelength side than the pump wavelength, such as around 2 μm, it is possible to convert the generated soliton into new components at the longer wavelength side over 2 μm.     

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.     

Collimation Characteristics of Planar Microlens for Parallel Optical Interconnect

We evaluated the collimated beam quality of a planar microlens (PML) for parallel optical interconnect systems. It has been confirmed that the divergence beam from a single mode fiber was collimated by single PML and propagated with good beam quality by about 10 mm, which is the nominal length of microoptics components considered. The divergence beam from a vertical cavity surface emitting laser was collimated using stacked PML and the collimated beam width of 100 μm was obtained. The collimated beam profile was good enough for low loss beam interconnect in a micro-optical bench scheme.     

An alternative approach to determine the fractional heat load in solid state laser materials: application to diode-pumped Nd:YVO4 laser

A simple approach is described and used for on-line measurement of the fractional heat-load parameter ξ in an operating diode-pumped Nd:YVO₄ laser at 1.06 μm wavelength for different doping concentrations. The method is based on the fact that if the thermo-optical properties of the sample are known then the ξ-parameter can be estimated from the measured effective focal length induced by the pump beam and any other lensing effect due to mechanical mounting of the sample for a given pumping configuration. The value of the fractional heat-load parameter estimated by our technique was in excellent agreement with the earlier reported values.     

Reflectivity Characteristics of Resonant Four Wave Mixing Generated in Erbium-Doped Optical Fibers

Resonant four-wave mixing (RFWM) based on the photo-induced refractive index change in erbium-doped optical fibers is demonstrated experimentally. The erbium-doped fibers act as a saturable absorber at 1.536 μm pump wavelength. Dependence of efficiency of RFWM on the pump power, the dopant concentration, the fiber length, and the signal modulation frequency are analyzed using three types of erbium-doped fibers differing in dopant concentration. It is found that a signal modulation frequency higher than 10 kHz is required for the correct measurement. The RFWM efficiency is improved with an increase in the input pump power in a range up to 1.5 mW and the highest reflectivity of 5.1% is attained. The highest RFWM efficiency is achieved at αgsL ∼ 5 dB, which is in good agreement with the theory.     

Design and fabrication of a negative microlens array

An operation model of a negative microlens array is demonstrated. The array consists of two kinds of materials with different refractive indices. First of all, a positive microlens array with 256×256 elements serving as a pattern is fabricated by argon ion beam etching on the quartz. The diameter and average corona height of the element are 28 and 0.638 μm, respectively. The spacing between two neighboring elements is 2 μm. In the second phase, after being coated by epoxy, the positive microlens array pattern is spun and baked, leading to a complex negative microlens array. Surface stylus measurement shows that the surface of the positive quartz microlens array is smooth and uniform. Focal length measurement of the negative microlens array indicates that the focal length region with −731±3 μm is in good agreement with the theoretical calculation value of −729 μm.     

Four-channel coarse WDM for inter- and intra-satellite optical communications

A polymer volume grating-based four-channel coarse wavelength division multiplexer (WDM) for inter- and intra-satellite optical communication application is reported for the first time. This compact four-channel WDM device working at 0.83, 1.06, 1.34 and 1.55 μm is designed to build a complete optical link between two satellites, where wavelengths of 0.83 and 1.55 μm are used for data stream channels, 1.06 and 1.34 μm are used for inter- and intra-satellite connection. It is for the first time reported that a WDM device can cover such a large wavelength range in a single substrate. For transverse electric (TE) wave, the channel efficiencies at 0.83, 1.06, 1.34 and 1.55 μm are 55%, 40%, 35% and 45%, respectively. Channel efficiencies for transverse magnetic (TM) waves are 20% lower than those of TE waves on average. Wavelength shifts due to Doppler effect, temperature variations and radiation effects in space can be adequately accommodated.     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

Design, analysis and optimization of silicon-on-insulator photonic crystal dual band wavelength demultiplexer

A highly efficient photonic crystal dual band wavelength demultiplexer (DBWD) using silicon-on-insulator (SOI) substrates is proposed for demultiplexing two optical communication wavelengths, 1.31 μm and 1.55 μm. Demultiplexing of two wavelength channels is obtained by modifying the propagation properties of guided modes in two arms of Y type photonic crystal structure. Propagation characteristics of proposed DBWD are analyzed utilizing 3D finite difference time domain (FDTD) method. Enhancement in spectral response is further obtained by optimizing the Y junction of demultiplexer giving rise to high transmission and extinction ratio for the wavelengths, 1.31 μm and 1.55 μm. Hence it validates the efficiency of proposed optimized DBWD design for separating two optical communication wavelengths, 1.31 μm and 1.55 μm. Tolerance analysis was also performed to check the effect of variation of air hole radius, etch depth and refractive index on the transmission characteristics of the proposed design of SOI based photonic crystal DBWD.     

Short-pulse, high-peak-power Q-switched Tm–silica fibre laser at 1.9 m

High-performance Tm–silica fibre lasers operating at 1.9 μm when pumped at 1319 or 1064 nm have been Q-switched using a rotating mirror mounted at an asymmetric angle. Pumping by 1319 nm gives better performance compared with 1064 nm pumping due to greater excited state absorption (ESA) at 1064 nm. A short Q-switched pulse duration of 25 ns and a peak pulse power of about 2.7 kW has been obtained from a 1.8 m fibre for 3.5 W launched pump power at 1.3 μm. The Q-switch performance has been studied for variation of fibre length and shown that the optimum length under these pumping conditions is around 2 m.     

Novel mid-IR laser based on hybrid AlGaAsSb/InAs/CdMgSe heterostructure

We report on the development of a novel design of a mid-IR laser combining III–V and II–VI compounds in a “hybrid” double heterostructure. It possesses large (1.5 eV) potential barriers both for injected electrons and holes, suppressing their leakage from the active region, and provides strong optical confinement. An AlGaAsSb/InAs/CdMgSe laser diode with a III–V/II–VI heterovalent interface at the 0.6 μm-InAs active region has been grown by molecular beam epitaxy on an InAs substrate. Despite a far from optimal defect density at the CdMgSe/InAs interface and high losses inherent for bulk active region of the laser, the structure demonstrates lasing at 2.8 μm (up to 100 K) in the pulsed regime with a threshold current density of 3–4 kA/cm². Type II InSb monolayer insertions into an InAs layer show bright photoluminescence at 3.8 μm (77 K), confirming the great potential of the InAs-based nanostructure active region for longer wavelength applications.     

Characteristics of InAlGaAs/InAlAs Superlattice Avalanche Photodiodes for Ultra-low Optical Power Detection in the Near Infrared

Static characteristics of two different structured InAlGaAs/InAlAs superlattice avalanche photodiodes (SLAPDs) cooled by liquid nitrogen were evaluated at a wavelength of 1.5 μm. The dark current of the SLAPD having a thick superlattice layer of 0.504 μm was 5 × 10⁻¹³ A. This was successively reduced by four orders of magnitude compared to that of the thin layer SLAPD of 0.231 μm at a breakdown voltage of around 20 V. The thickened layer was effective in suppressing tunneling dark current. An output current of 1.7×l0⁻¹² A at a bias voltage of 15 V was measured for an optical input with a wavelength of 1.5 μm and a signal power of 1 × 10⁻¹² W. This showed a sharp distinction from the dark current.     

Ordered tellurium nanowire arrays and their optical properties

High-density tellurium (Te) nanowire arrays were prepared in the nanochannels of an anodic aluminum membrane (AAM) template using the electrochemical deposition method. The as-synthesized Te nanowires, typically 60 nm in diameter and up to 40 μm in length, possess a hexagonal single crystalline structure following [001] growth direction. The optical polarization properties of Te nanowire arrays embedded in AAM were investigated using an optic parameter oscillator in the wavelength range from 0.7 to 1 μm. The high optical polarization of the Te nanowire arrays embedded in the AAM assembly system was observed. PACS 81.05.Cy; 78.67.-n; 82.80.Fk.     

One-dimensional single (In,Ga)As quantum dot arrays formed by self-organized anisotropic strain engineering

Well-defined one-dimensional single (In,Ga)As quantum dot (QD) arrays have been successfully formed on planar singular GaAs (1 0 0) in molecular beam epitaxy by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs quantum wire (QWR) superlattice (SL) template. The distinct stages of template formation, which govern the uniformity of the QD arrays, are directly imaged by atomic force microscopy (AFM). The AFM results reveal that excess strain accumulation causes fluctuations of the QWR template and the QD arrays. By reducing the amount of (In,Ga)As and increasing the GaAs separation layer thickness in each SL period, the uniformity of the QD arrays dramatically improves. The single QD arrays are straight over more than 1 μm and extended to 10 μm length. Capped QD arrays show clear photoluminescence emission up to room temperature.     

1.3-μm GaInAsP/InP Multi-Quantum-Well Surface-Emitting Lasers

A multi-quantum-well (MQW) active layer has been introduced to 1.3-μm GaInAsP/InP surface-emitting (SE) lasers. Room temperature pulsed operation of a 1.3-μm MQW SE laser was obtained for the first time and its threshold current was 15 mA. CW (continuous wave) operation up to 7°C (threshold current 1_th=7.6 mA at 7°C) was achieved.