Performance of Ar/sup +/-milled Ti:sapphire rib waveguides as single transverse-mode broadband fluorescence sources

Rib waveguides have been fabricated in pulsed-laser-deposited Ti:sapphire layers using photolithographic patterning and subsequent Ar/sup +/-beam milling. Fluorescence output powers up to 300 /spl mu/W have been observed from the ribs following excitation by a 3-W multiline argon laser. Mode intensity profiles show high optical confinement and the measured beam propagation factors M/sub x//sup 2/ and M/sub y//sup 2/ of 1.12 and 1.16, respectively, indicate single transverse-mode fluorescence emission. Loss measurements using the self-pumped phase conjugation technique have yielded comparable values (1.7 dB/cm) for the ribs and the unstructured planar waveguide counterparts. The combination of optimum modal properties and strong optical confinement, together with sufficient levels of fluorescence output, make the single-moded Ti:sapphire rib waveguides a very interesting candidate as a fluorescence source for optical coherence tomography applications.     

GaAs／GaAlAs截止型光波导调制器的设计

利用近似解析方法，详细地讨论了金属覆盖层介质波导在截止点附近的模式传播特种和截止条件，并结合有效折射率方法讨论了脊高和脊宽对脊形波导截止条件的影响，设计了一种双层质结截止型强度调制器。     

Variable optical attenuator based on a cutoff modulator withtapered waveguides in polymers

A variable optical attenuator was demonstrated by using a thermooptic cutoff modulator in polymers. It combined horizontally and vertically tapered waveguide structures to improve both the attenuation efficiency and the fiber coupling. The rib height of the waveguides in the input and output region was chosen to achieve minimum fiber coupling loss. For the waveguide in the active region with the electrodes, the rib height was reduced and the width was tapered in order to enhance the attenuation performance by weakening the mode confinement. The two waveguides with different rib heights were connected smoothly by employing a vertical taper, which was realized by utilizing two steps of reactive ion etching with a shadow mask. Then a fiber coupler built in a silicon block was attached directly to the output end of the device. A fraction of the main attenuator output was tapped and fed back to the electrical driver to achieve constant output regardless of variations in input light power and polarization. The measured insertion loss of the attenuator was 2.5 dB at 1550 nm. The dynamic range was more than 20 dB with an electrical power consumption of 160 mW. And the optical response time was faster than 1.5 ms. The effect of polarization on the attenuation was reduced to 0.1 dB by employing a continuous electronic feedback control. The wavelength uniformity was as small as 0.3 dB over the range from 1530 to 1560 nm. Finally, the attenuator was successfully used to regulate channel powers within 0.4 dB in a wavelength division multiplexed transmission system     

Large single-mode rib waveguides in GeSi-Si and Si-on-SiO2

Mode-matching and beam-propagation methods are used to analyze single-mode operation of optical GeSi-Si and Si-SiO₂ semiconductor rib waveguides. The waveguide dimensions that allow only the fundamental HE₀₀ or EH₀₀ mode to propagate have been determined. For both material systems, it is found that the rib can be several microns wide and several microns high, thus allowing efficient coupling to single-mode fibers. Numerical examples are given for monomode guiding group-IV materials, but the results apply to III-V rib waveguides. It is shown that single-mode rib guides with large cross sections are feasible as long as the waveguide is at least several millimeters long     

Photobleached refractive index tapers in electrooptic polymer rib waveguides

Photobleached refractive index tapers in electrooptic polymer rib waveguides, which act as two-dimensional optical mode transformers, are investigated. One taper method involves a discrete step mask-shifting scheme with a fixed intensity UV light source. A second method utilizes a gray-scale mask to precisely control the intensity of UV light reaching each portion of the taper. Using a gray-scale mask, adiabatic refractive index tapers can be inscribed into electrooptic polymer waveguides in a single fabrication step with no scanning parts. Optimized taper profiles are derived and applications for their incorporation into passive-to-active waveguide transitions are described.     

Glass waveguides on silicon for hybrid optical packaging

Work on deposited glass waveguides on silicon to form waveguides and filters is reviewed. The choice of these particular waveguides makes sense only as part of a consistent approach to optoelectronic packaging. Hybrid optical packaging on silicon (HOPS) is described and briefly compared with other techniques. For these packages, two waveguides were developed: a tight mode waveguide with a silicon nitride rib core for matching a semiconductor laser and a loose mode waveguide with a phosphosilicate glass core for matching an optical fiber     

Finite element beam propagation method for three-dimensionaloptical waveguide structures

A beam propagation method (BPM) based on the finite element method (FEM) is described for longitudinally varying three-dimensional (3-D) optical waveguides. In order to avoid nonphysical reflections from the computational window edges, the transparent boundary condition is introduced. The present algorithm using the Pade approximation is, to our knowledge, the first wide-angle finite element beam propagation method for 3-D waveguide structures. To show the validity and usefulness of this approach, numerical results are shown for Gaussian-beam excitation of a straight rib waveguide and guided-mode propagation in a Y-branching rib waveguide     

The design of triple rib waveguide couplers by the discretespectral index method

The operations of three-guide couplers are usually controlled by the width and refractive index of the guides. However, when rib waveguides are used in three-guide couplers and power dividers, we propose to use the dimensions of the ribs as the governing parameters instead. Formulating a new transcendental equation using the discrete spectral index (DSI) method, triple rib waveguide couplers are accurately analyzed for the first time. The new accurate and fast analysis of triple rib waveguides shows that the propagation constants of the various modes are dominantly controlled by the height of the middle rib. A systematic and simple procedure is consequently introduced to design quasi-transverse electric (TE) polarized triple rib waveguide couplers, which ensures maximum power transfer from one outermost guide to the other outermost guide. For triple rib power dividers, the height of the middle rib is found to control the amount of power that can be transferred from the excited middle rib waveguide to the two outer rib waveguides. Maximum power division is achieved when the three ribs of the waveguide are identical     

GaAs/GaAlAs curved rib waveguides

Curved dielectric optical waveguides suffer from radiation loss due to bending. To minimize the bending loss and reduce the radius of curvature, it is necessary to fabricate guides which provide strong optical confinement. This paper gives a brief review of curved waveguide analysis and presents some experimentally measured loss values for GaAs/GaAlAs curved rib waveguides. The rib waveguides, fabricated using ion beam milling, have a large rib height and are tightly guided structures. When corrected for reflection losses and input coupling efficiency, a minimum loss of approximately 3 dB has been achieved for a multimode 90° curved guide with a radius of curvature of 300 μm, and 8.5 dB for a single-mode curved guide with a radius of curvature of 400 μm. It is believed that most of this residual loss is not radiation loss due to bending, but rather scattering loss due to rib wall imperfections.     

Analysis of the guided modes in slab-coupled waveguides using a variational method

The waveguiding properties of the slab-coupled waveguides (i.e., the optical stripline and the rib waveguide) are investigated by the vector variational method. The slab-coupled waveguides are low-loss optical waveguides which are easy to fabricate and design. In this paper, the propagation constants and the field intensity distributions of the slab-coupled waveguides are presented, and the effects on the wave-guiding properties caused by changing their geometrical parameters are discussed. The results obtained by employing the variational method are compared with those obtained by using other methods.     

PMMA脊形光波导的设计与场分析

利用有效折射率法（EIM）理论设计了有机聚合物脊形光波导结构,并分析光波导中传输的模式场,确定出光波导结构的适宜尺寸范围。光波导结构中分别选用PMMA（聚甲基丙烯酸甲酯）与PDMS（聚二甲基硅氧烷）作为波导层与包层的材料,用光漂白法来实现脊形波导结构。首先利用EIM法分析脊形波导区的宽度与高度之间的关系对归一化色散曲线的影响,以及波导层与包层的折射率匹配的影响,然后又通过分析波导区传输的基模场来讨论传输的损耗问题。研究发现波导结构的脊宽与脊高之比w／d越大,其有效折射率neff越大,而此结构尺寸又需约束在一定范围内,否则将造成很大的传输损耗。     

Fabrication of optical inverted-rib waveguides using UV-imprinting

In this paper we introduce a fabrication process for polymer rib waveguides that uses UV-imprint lithography. In the structure of an inverted-rib waveguide, the lower cladding of the waveguide is patterned by UV-imprinting and the waveguiding layer is subsequently spin-coated. That makes the thickness of the formed slab layer on the rib waveguide controllable by tuning the spin-coating parameters. The fabrication process utilizes two steps of UV-imprinting. The first one is to form a rigid polymer mold from positive tone photoresist. The second one is to pattern the waveguide lower cladding with the formed polymer mold. Through the two steps of UV-imprinting, rib waveguides can be fabricated without an etching procedure. We demonstrated the proposed fabrication process by fabricating 2-μm-wide waveguides operating in single mode at 1310 nm. With TE-polarized light, the fabricated waveguides show an average transmission of 58.6% in a 30 mm long waveguide, corresponding to a loss of 2.3 dB.     

Comparison among various Si/sub 3/N/sub 4/ waveguide geometries grown within a CMOS fabrication pilot line

Low-pressure chemical-vapor deposition (LPCVD) thin-film Si/sub 3/N/sub 4/ waveguides have been fabricated on Si substrate within a complementary metal-oxide-semiconductor (CMOS) fabrication pilot line. Three kinds of geometries (channel, rib, and strip-loaded) have been simulated, fabricated, and optically characterized in order to optimize waveguide performances. The number and optical confinement factors of guided optical modes have been simulated, taking into account sidewall effects caused by the etching processes, which have been studied by scanning electron microscopy. Optical guided modes have been observed with a mode analyzer and compared with simulation expectations to confirm the process parameters. Propagation loss measurements at 780 and 632.8 nm have been performed by both using the cutback technique and measuring the drop of intensity of the top scattered light along the length of the waveguide. Loss coefficients of approximately 0.1 dB/cm have been obtained for channel waveguides. These data are very promising in view of the development of Si-integrated photonics.     

Single-mode and polarization-independent silicon-on-insulator waveguides with small cross section

The fabrication restrictions that must be imposed on the geometry of optical waveguides to make them behave as single-mode devices are well known for relatively large waveguides, with shallow etch depth. However, the restrictions for small waveguides (/spl sim/1 /spl mu/m or less in cross section) are not well understood. Furthermore, it is usually a requirement that these waveguides are polarization independent, which further complicates the issues. This paper reports on the simulations of the conditions for both single-mode behavior and polarization independence, for small and deeply etched silicon-on-insulator (SOI) waveguides. The aim is to satisfy both conditions simultaneously. The results show that at larger waveguide widths, waveguide etch depth has little effect on the mode birefringence because the transverse-electric (TE) mode (horizontal-polarized mode) is well confined under the rib region. However, at smaller rib widths, the etch depth has a large influence on birefringence. An approximate equation relating the rib-waveguide width and etch depth to obtain polarization-independent operation is derived. It is possible to achieve single-mode operation at both polarizations while maintaining polarization independence for each of the waveguide heights used in this paper but may be difficult for other dimensions. For example, a 1-/spl mu/m SOI rib waveguide with an etch depth of 0.64 /spl mu/m and rib width of 0.52 /spl mu/m is predicted to exhibit such characteristics.     

n-p-n silicon lateral phototransistors for hybrid integrated optical circuits

An etched mesa silicon lateral phototransistor (EMS-LPT) suitable for detecting the light signal from optical channel waveguides has been designed and fabricated. In this paper both n⁺-p-n⁺uniform base and n⁺-p-p^--n⁺double-diffused EMS-LPT's are reported. The photoactive region of the EMS-LPT is highly localized and can be easily coupled either via an evanescent field or to a grating coupler on a channel waveguide. Light coupling, gain, speed, and signal-to-noise ratio of the device are thereby greatly improved. The fabrication techniques of the EMS-LPT's are compatible with those of MOSFET's, permitting integration of multiple EMS-LPT's and MOSFET load transistors to form optically addressed inverters on the same silicon chip. By flip-chip bonding LiNbO3and silicon substrates and coupling LiNbO3channel waveguides to EMS-LPT's via grating couplers, we produce electrooptic switches with optical input and output.     

The unitarity of split-operator finite difference andfinite-element methods: Applications to longitudinally varyingsemiconductor rib waveguides

Split-operator finite-difference and finite-element techniques are applied to the calculation of losses in semiconductor rib waveguide Y junctions. It is shown that, unlike the finite-difference procedure, which is unitary for both uniform and nonuniform grid-point spacings, considerable care must be exercised in formulating the split-operator finite-element method in order to preserve the power in the propagating electric field. The calculations are performed in the context of the Fresnel approximation to the scalar Helmholtz equation, which yields accurate solutions for transverse electric (TE)-polarized electric fields in rib waveguides far from cutoff     

Cutoff wavenumbers in truncated waveguides

Truncated waveguides are used in some microwave components. The cutoff wavenumbers in truncated circular waveguide and truncated square waveguide are desired but usually difficult to be computed. In this paper, they are efficiently determined using the unified method that we proposed earlier. Both TE-polarized and TM-polarized modes are studied. To demonstrate the applicability and correctness of this method, two practical examples are considered, one is a truncated circular waveguide and the other is a truncated square waveguide. Results obtained for the two cases are compared with existing data in literature and a good agreement is observed. A further extension of the work is made to compute cutoff wavenumbers of a truncated elliptical waveguide. It is found from the analysis that the cutoff wavenumbers in these irregular waveguides can be computed easily, rapidly, and accurately using the unified method     

Coupling light from incoherent sources to optical waveguides

Spatially coherent laser light can be coupled into an optical waveguide with nearly 100 percent efficiency. However, coupling light from a spatially incoherent source to a waveguide can be frustrating. Here the author describes how to efficiently couple blackbody radiation to optical fiber waveguides. The same basic principles hold for other types of waveguides, such as integrated optic waveguides. Radiation emitted from other types of extended sources, such as surface emitting LEDs or ionized gases, can be efficiently coupled to waveguides using the same techniques     

White light spectral analysis of lithium niobate waveguides

In this paper, the usefulness of evaluating optical channel waveguides in lithium niobate with white light illumination is shown. Spectral data can be interpreted to show various regimes of waveguide operation from multi-mode to cutoff, and input coupling coefficients and attenuation factors are derived for these different modes. This technique is shown to be useful as a monitor both for process development and process consistency. Finally, both a Mach-Zehnder and a phase modulator are swept under bias and analyzed     

A pixellated grating array using photoelectrochemical etching on aGaAs waveguide

We have demonstrated and evaluated a grating array outcoupler fabricated by photoelectrochemical (PEC) etching, a manufacturable and practical approach for fabrication of grating-based III-V semiconductor waveguide devices. An array of submicron period gratings was etched into photolithographically delineated areas in a single PEC step. The fabricated devices are: 10-μm wide rib waveguides with 0.35-μm first-order outcoupling gratings; and 10-μm wide rib waveguides with 10 μm×10 μm pixellated areas of gratings. Device characterization demonstrates the effectiveness of outcoupling grating fabrication using PEC and that the pixellated grating outcoupler is an effective and simple means of generating an optical beam array