High Q-factor distributed bragg reflector resonators with reflectors of arbitrary thickness

The Bragg reflection technique improves the Q-factor of a resonator by reducing conductor and dielectric losses. This is achieved by designing a low-loss inner resonant region (usually free space) surrounded by an outer anti-resonant region made of distributed Bragg reflector layers. In this paper we develop a simple non-Maxwellian model and apply it to design three distinct cylindrical Bragg resonators based on the same set of single-crystal sapphire plates and rings by changing only the dimension of the cavity that supports the structure. To accomplish this, the simple model allows an arbitrary thickness for either the horizontal or the cylindrical dielectric reflectors by relaxing the condition that they must be lambda/4 thick. The model also allows for higher-order field variations in both the resonant and the anti-resonant regions. The resonators were constructed and experimental results were compared with the simple model and the rigorous method of lines analysis. For the fundamental mode, an unloaded Q-factor of 234,000 at 9.7 GHz was obtained. This is larger than that for a whispering gallery mode resonator. The resonator also exhibited a greatly reduced spurious mode density when compared to an overmoded whispering gallery mode resonator.     

A study of distributed dielectric bragg reflectors for vertically emitting lasers of the near-IR range

Studies aimed at optimization of the design of a dielectric distributed Bragg reflector (DBR) produced by the reactive magnetron sputtering method for applications in near-IR vertical-cavity surface-emitting lasers with intracavity contacts (ICC-VCSELs) are carried out. It is shown that the reflectivity of the dielectric DBRs based on SiO₂/TiO₂ decreases due to the polycrystalline structure of the TiO₂ layers, which causes diffusive scattering of light. In contrast, amorphous Ta₂O₅ layers is characterized by a low surface roughness and low fluctuation in the refractive index. Single-mode ICC-VCSELs in the 980-nm spectral range with dielectric DBR based on SiO₂/Ta₂O₅ with a threshold current less than 0.27 mA, electric resistance of less than 200 Ω, and differential efficiency of more than 0.8 W/A are demonstrated.

     

Studying the characteristics of pulse-pumped semiconductor 1060-nm lasers based on asymmetric heterostructures with ultrathick waveguides

High-power semiconductor lasers based on asymmetric quantum-dimensional separate confinement InGaAs/GaAs heterostructures with ultrathick waveguides were fabricated by means of metalorganic hydride vapor phase epitaxy technology. The laser characteristics were studied in a pulsed pumping regime, in which the emission was excited by current pulses of 100 ns duration at a repetition frequency of 10 kHz and an amplitude of up to 200 A. The passage to a pulsed lasing regime allowed the active region heating to be reduced and the output power to be increased to 145 W for a laser diode with a 100-μm exit aperture. The results obtained for the pulsed lasing regime show that saturation of the output power-current characteristic observed in the continuous-wave regime is fully determined by overheating of the active region of a semiconductor laser.     

Output radiation focusing in curved-grating distributed Bragg reflector laser

Output radiation focusing in AlGaAs/GaAs quantum-well curved-grating distributed Bragg reflector (c-DBR) lasers has been theoretically and experimentally studied. It is established that the focal spot size for c-DBR lasers is determined to a considerable extent by the spectral perfection of laser radiation.     

Harmonic mode-locking in quantum dot lasers with tunnel-coupled waveguides

Passive mode-locking has been studied in the lasers with tunnel-coupled waveguides with the active region based on InAs/InGaAs quantum dots emitting in the range of 1.3–1.31 μm. Specific optical mode formation in this composite waveguide reduces the output beam divergence and raises the temperature stability of the lasing wavelength. The reduced optical confinement factor facilitates obtaining passive modelocking regimes. High response rate of the saturable absorber associated with the carrier escape time shorter than 20 ps allows generating a third harmonic with the frequency as high as 46.1 GHz.     

Stable, low-loss optical waveguides and micromirrors fabricated in acrylate polymers

Acrylate-based optical waveguides have been fabricated with optical loss of 0.5 dB/cm at 1300 nm by means of a new material system that ensures stable optical and mechanical properties over a wide temperature range. No increase in loss was measured after 500 h at temperatures up to 150 degrees C, and there was no significant increase in loss during short (<5 min) temperature excursions to 300 degrees C for bonding. Single-mode waveguides were fabricated with refractive indices for core and clad of 1.505 and 1.500, respectively, so that the mode field is very similar to that of single-mode silica fiber. Guides were fabricated on both planar and structured substrates of Si and GaAs as well as on substrates coated with metals and dielectrics. Fabrication involved spin coating and UV exposure to cross-link the polymer, but the substrate temperature did not exceed 180 degrees C. With this method guides could be fabricated on a range of substrates up to 125 cm in diameter, including those with multilayer metallization for multichip modules, providing optical interconnect capability. Microprism reflecting surfaces were fabricated in the waveguides to couple light out normal to the substrate. All the processing was compatible with normal semiconductor fabrication.     

Ferroelectric nanostructures fabricated by focused-ion-beam milling in epitaxial BiFeO3 thin films

We studied a suitable route to fabricate ferroelectric islands by focused-ion-beam milling in bismuth ferrite epitaxial thin films. Piezoresponse force microscopy shows that the damage induced by the milling process is extended to 1 μm away from the edge of the focused-ion-beam patterned islands. After a combined vacuum and oxygen atmosphere annealing procedure, ferroelectricity is fully recovered in structures with sizes down to 500 nm, while for 250 nm islands the defects at the interfaces induce polarization direction pinning.     

Organo-lanthanide complexes as luminescent dopants in polymer waveguides fabricated by hot embossing

Lanthanide complexes, Eu(dbm)₃(Phen) and Er(dbm)₃(Phen), are employed as luminescent dopants within polymer channel waveguide devices fabricated by hot embossing. Spectroscopic properties of the complexes as dopants in the waveguide core polymer are investigated in detail. Judd–Ofelt parameters are calculated for the europium chelate and radiative properties are determined viz. potential for optical amplification. Channel waveguides fabricated by single level embossing are shown to be capable of guiding visible and infrared light emitted following optical excitation of the dopants. Multi-level polymer micro-optical benches incorporating doped channel waveguides and passive locational features for self-alignment and integration of optical fibres are fabricated in a multi-level single-step embossing process and are shown to successfully out-couple the waveguided dopant emission.     

Optical properties of waveguides fabricated in fused silica by femtosecond laser pulses

With tightly focused femtosecond laser pulses, waveguides are fabricated in fused silica. The guiding and attenuation properties of these waveguides at wavelengths of 514 nm and 1.5 microm are studied. We demonstrate that by changing only the writing speed, waveguides with a controllable mode number can be produced.     

分布反馈量子级联激光器的光栅制备

利用全息曝光方法制备了分布反馈量子级联激光器的光栅掩模,选择和发展了恰当的用于InGaAs/InP材料的光栅腐蚀优化工艺,得到腐蚀规律,讨论了腐蚀机制。在量子级联激光器的InGaAs/InP层上制备光栅得到分布反馈量子级联激光器,其单模特性较好,信噪比大于30dB。     

Polarization-independent wavelength filter based on dual asymmetric long-period gratings in channel waveguides

Channel waveguides with asymmetric long period gratings exhibiting coupling from the fundamental (symmetric) core mode to the first-order (antisymmetric) cladding mode were analyzed. Using this geometry, we show that it is possible to realize polarization-dependent and polarization-independent operations at desired wavelengths. We have chosen the device parameters to demonstrate its operation in the C+L band. Sensitivity of the coupling behavior to random variation in various parameters such as core and cladding dimensions, grating period, etc., has been analyzed. The proposed structure should be interesting from the point of view of realizing waveguide devices such as filters, polarizers, etc., based on waveguide gratings.     

Low-loss waveguides fabricated in BK7 glass by high repetition rate femtosecond fiber laser

For the first time femtosecond-laser writing has inscribed low-loss optical waveguides in Schott BK7 glass, a commercially important type of borosilicate widely used in optical applications. The use of a variable repetition rate laser enabled the identification of a narrow processing window at 1 MHz repetition rate with optimal waveguides exhibiting propagation losses of 0.3 dB/cm and efficient mode matching to standard optical fibers at a 1550 nm wavelength. The waveguides were characterized by complementary phase contrast and optical transmission microscopy, identifying a micrometer-sized guiding region within a larger complex structure of both positive and negative refractive index variations.     

Spatial-mode control of vertical-cavity lasers with micromirrors fabricated and replicated in semiconductor materials

Micromirrors were fabricated in gallium phosphide by mass transport to provide spatial-mode control of vertical-cavity surface-emitting lasers (VCSEL's). The concave mirrors were used in an external-cavity configuration to provide spatial filtering in the far field. Single-mode cw lasing was demonstrated in 15-mum-diameter VCSEL's with currents as high as 6 times threshold. The fabrication process was extended to micromirrors in gallium arsenide by use of a replication and dry-etch transfer process.     

Planar optical waveguides in Nd:BSO crystals fabricated by He and C ion implantation

Planar optical waveguides in Nd:BSO crystals were fabricated by the implantation of 500 keV He ions and 6.0 MeV C ions at two different substrate temperatures. The guiding modes were measured by the prism-coupling method with a He-Ne beam at 633 nm. The intensity calculation method (ICM) and reflectivity calculation method (RCM) were used for reconstructing refractive index profiles. The near-field intensity distribution of the waveguide, formed by He and C ions implanted after annealing at 300 °C, was measured by the end-face coupling setup. It was in reasonable agreement with the intensity of the waveguide mode simulated by the finite-difference beam propagation method (FD-BPM). The absorption spectra of the sample with He ions implanted at fluences of 3 × 10¹⁶ ions/cm² were measured using a spectrophotometer.     

Planar optical waveguides in Bi4Ge3O12 crystal fabricated by swift heavy-ion irradiation

We report on the fabrication of the planar waveguides in Bi4Ge3O12 crystal by using 17 MeV C5+ or O5+ ions at a fluence of 2×10(14) ions/cm2. The reconstructed refractive index profiles of the waveguides produced by either C5+ or O5+ irradiation are the "well" + "barrier" pattern distribution. The two-dimensional modal profiles of the planar waveguides, measured by using the end-coupling arrangement, are in good agreement with the simulated modal distributions. After thermal annealing treatment at 260 °C for 30 min, the propagation loss for C5+ and O5+ irradiated waveguides could be reduced down to ~1.1 and ~4.8 dB/cm, respectively, which exhibit acceptable guiding qualities for potential applications in integrated optics.     

Optimization of III–V compound semiconductor heterostructures for distributed Bragg reflector applications in VCSELs

A selectively oxidized vertical cavity surface emitting laser (VCSEL) has been designed and fabricated for operation at a wavelength of 1.546 μm. The device structure was grown on an InP substrate using III–V quaternary semiconductor alloys for the two mirror stacks and unstrained multi-quantum wells for the active layer. A threshold current as low as 2.2 mA has been achieved. The influence of the intentional and growth-related compositional grading at the heterointerfaces on the mirror reflectivity and laser characteristics has been investigated, and key sensitivities to laser performance have been determined.     

Switching of output coupling in a grating coupler by diffraction transition to the distributed Bragg reflector regime

In a proposed switching grating coupler (SGC), the grating period Lambda is chosen so that the SGC converts from a first-order grating coupler to a third-order distributed Bragg reflector by means of a small change in the guided mode index. The principle for switching the radiated wave power and the effective aperture of the SGC were experimentally confirmed by use of the thermo-optic effect of a polymer waveguide. The extinction of the peak intensity of the wave radiated by the SGC and collected by an external lens was measured to be 5 dB with a temperature change of 10 degrees.     

Planar LiTaO(3) waveguides fabricated by proton exchange in concentrated and diluted pyrophosphoric acid with annealing

Annealed proton-exchanged z-cut LiTaO(3) planar waveguides fabricated with pyrophosphoric acid have been characterized. For proton exchange, the extraordinary index increase Δ n(e) ranged from 0.0119 to 0.0141, depending on the exchange temperature. The effective diffusion coefficient D(e) ranged from 0.1325 μm(2)/h at 240°C to 0.545 μm(2)/h at 280 °C. Single-mode propagation losses were α = 0.7 dB/cm. Compared with benzoic acid, pyrophosphoric acid produces waveguides with a higher Δ n(e) and a lower propagation loss. For proton exchange in lithium phosphate-diluted pyrophosphoric acid, a lower Δ n(e) was obtained, but D(e) and propagation losses were not reduced. After proton exchange, the waveguides were annealed. The surface index initially increased, peaked, and then decreased. The waveguide depth d and the surface index n(s) were measured at regular intervals. Figures and empirical formulas relating the waveguide depth d and the surface index increase Δ n(s) to the anneal time, anneal temperatures, and the waveguide depth after proton exchange are given. An example is given in which, the desired waveguide parameters d and Δ n(s), the fabrication conditions could be calculated with the previously derived formulas. Propagation losses decreased to 0.4 dB/cm after prolonged annealing.     

Spectral characteristics of asymmetric directional couplers in graded index channel waveguides analyzed by coupled-mode and normal-mode techniques

We present a detailed analysis of the spectral characteristics of asymmetric directional couplers (ADC's) formed by K(+) -Na(+) ion exchange in BK7 glass and compare the results obtained by the use of normal-mode theory with those obtained by the use of the coupled-mode approach. Maximum power transfer is observed to occur at the wavelength at which the propagation constants of the perturbed component arms are equal. This is attributed to the strong coupling inherent in these devices. Strong coupling and asymmetry are observed to result in unequal confinement of the normal modes, leading to reduce power transfer, even when both arms of the coupler are synchronized. Additionally, significant polarization dependence is observed because of birefringence induced by both the K(+) -Na(+) exchange process and the chosen device structure. Polarization extinction ratios of 11.9 and 26.1 dB are obtained for the arms of the directional couplers. ADC's are observed to operate as bandpass filters, and by adjustment of structural parameters, the peak transmission wavelength can be tuned, with spectral bandwidths of 25-55 nm and coupling lengths of 6-12 mm. Over 90% of the input power in the TE polarization at 1.07 μm can be transferred to the second arm of the coupler, whereas power either in the TM polarization or at wavelengths outside the passband is relatively unaffected. We also show that within the bandpass, where the two arms are phase matched, exact normal-mode analysis yields the same results as the quasi-normal-mode approach, in which the normal modes are expressed in terms of the individual modes.     

Improving delivery performance for asymmetric Laplace distributed delivery time in a two-stage supply chain

This paper investigates strategies for improving supply chain delivery timeliness when the delivery time follows an asymmetric Laplace distribution. Delivery performance is measured using a cost-based analytical model which evaluates the expected cost for early and late delivery. This paper presents a set of propositions that define the effect of changes to the parameters of the delivery time distribution on the expected penalty cost for untimely delivery when a supplier uses an optimally positioned delivery window to minimise the expected cost of untimely delivery. The scale parameter increases the expected penalty cost, skewness decreases the cost and the location parameter has no effect on the expected penalty cost. The effects are illustrated in a numerical example with real-world supplier data. The results can be used in developing strategies for improving delivery performance from a supplier’s perspective and define how the delivery time distribution parameters can be modified to decrease the expected penalty cost of untimely delivery. The paper proposes a general approach to modelling delivery performance improvement and can be applied to other delivery time distribution forms. The approach can serve as guidance for practitioners undertaking a programme to improve delivery performance.