S. Iraj Najafi

Gain characteristics of a high-concentration Er3+ doped phosphate glass waveguide

A channel waveguide on an erbium doped phosphate laser glass (Kigre Q89) was fabricated by a dry silver-film ion exchange technique, and its gain properties were studied experimentally. The propagation loss of the fabricated waveguide was 0.6 dB/cm at 1.3 micrometers . Er3+ concentration of 1 X 1020 ions/cm3 was chosen so that no concentration quenching occurred. This was confirmed by measuring a fluorescence lifetime of 1.54 micrometers (8.4 ms). Gain of the fabricated waveguide was measured by using a Ti:sapphire laser at a wavelength of 977 nm and a laser diode of 1.530 micrometers producing pump and signal beams respectively. The signal wavelength used for the experiments was shorter than the emission peak, and the measured gain of the 1.8 cm waveguide was comparable to the total loss. However, the model which adopted experimental conditions showed that lasing is expected at the emission peak with a 3.6 cm long waveguide fabricated by current method. Calculations results showed that the 6.5 dB gain can be realized at 300 mW pump power with a 5.4 cm long waveguide, provided that mode overlap can be increased by 25% and propagation loss can be reduced by 50%. No significant upconversion effect was observed up to 1.1 X 106 W/cm2 pump intensity.

Double-ion-exchange process in glass for the fabrication of computer-generated waveguide holograms.

We experimentally optimize double-ion-exchange process parameters to achieve a designed phase modulation for a wave front passing through a computer-generated waveguide hologram and numerically analyze the effects of fabrication errors. We also demonstrate a gradient-thickness waveguide hologram for ? beam splitting.

High Er concentration phosphate glasses for planar waveguide amplifiers

Cooperative upconversion processes and Yb-Er energy transfer efficiencies in high Er concentration phosphate glasses were studied. The cooperative upconversion coefficients were deduced from the pump intensity dependence of luminescence decay curves. Cooperative upconversion coefficients of 4I13/2 level, for Er3+ concentrations higher than 1 X 1020 cm-3, are one order of magnitude smaller than the ones reported for silica glass. The increase in the cooperative upconversion coefficient with the increase in Er3+ concentration was found to be small and Er3+ concentrations as high as 3.7 X 1020 cm3+ in this glass are feasible. Yb-Er energy transfer efficiency in Yb/Er co-doped phosphate glasses, with Er concentrations as high as 1.9 X 1020 cm-3 and 2.4 X 1020 cm-3, were measured with a pump and probe technique and also estimated from lifetime measurements. The energy transfer efficiencies exceed 95 percent, although the ratio of the concentrations, Yb/Er, is only about 1.2 and 2 in the samples studied.This confirms that efficient pumping of high Er3+ concentration phosphate glass, required in waveguide amplifiers, can be achieved utilizing Yb/Er co-doping.

Organoaluminophosphate sol-gel silica glass thin films for integrated optics

We report on the fabrication and characterization of waveguide thin films on silicon by ultraviolet light imprinting in a new photosensitive, organically modified aluminophosphate sol-gel silica glass prepared by a one-step dip-coating process. Appropriate combinations of ultraviolet light exposure time, sol-gel film thickness and postbake parameters produce waveguides suitable for optical telecommunication applications.

Gratings fabrication by ultraviolet light imprinting and embossing in a sol-gel silica glass

We present simple, low temperature methods for inexpensive fabrication of gratings in a photosensitive, organically modified silicate (ORMOSIL) system for integrated optical applications. The material is prepared by the sol-gel method. Gratings are made by UV imprinting through a mask and by an embossing technique. In the first case, UV exposure induces a refractive index change of (Delta) n equals 0.005 in the film and results in an index modulation type grating. Relief type gratings are made by mechanical embossing. Gratings are compared and characterized in terms of diffraction efficiency and grating period.

Overview of Nd- and Er-Doped Glass Integrated Optics Amplifiers and Lasers

There has been significant progress since 1990 on development of rare-earth-doped glass integrated optics amplifiers and lasers. Several fabrication processes were utilized to make rare-earth-doped waveguides. Neodymium and erbium doped waveguides were successfully produced, and amplifiers and lasers were demonstrated. Recently, high performance erbium-doped amplifiers in phosphate glasses were achieved. In this paper, we review the progress in development of neodymium and erbium doped lasers and amplifiers.

UV-light-imprinted surface, ridge, and buried sol-gel glass waveguides and devices on silicon

New processes are developed to produce surface, ridge and buried sol-gel glass waveguides by photo-inscription using a ultraviolet light source. The fabricated waveguides demonstrate very low propagation losses (approximately 0.1 dB/cm). Several integrated optical devices such as cascaded Y-branch power splitters, directional couplers, Mach-Zehnder interferometer wavelength division multiplexers have also been produced.

Novel sol-gel fabrication of integrated optical waveguides

The sol-gel method is employed to implement a new, simple and highly versatile fabrication process for integrated optical waveguides on silicon. Photosensitive glass films of several microns thickness are dip-coated and optical components of various configurations through suitable masks are directly inscribed in them by ultraviolet light. Undefined regions are removed by soaking the films in an organic solvent for a few minutes. The developed ridges are then treated and planarized with a cladding layer. Altogether, the procedure is reproducible, and leads to waveguides with low propagation losses, on the order of 0.13 dB/cm.

Improving CdS quantum-dot materials by the sol-gel method

In this paper we discuss our approaches and results for the preparation of improved semiconductor quantum dot materials by the sol-gel method. By taking advantage of the low- temperature sol-gel route, the nanostructure and optical properties of CdS quantum dots were greatly improved for device applications. Pore-free sodium borosilicate (NBS) glasses and organically modified silicates (ORMOSILs) were used as matrices for the CdS nanocrystallites. Results from both degenerate four-wave mixing and pump-probe techniques indicated large third-order nonlinear responses of the order of 10-6 esu from CdS-doped NBS glasses. By using potassium ion exchange, the first CdS-doped channel waveguides were fabricated in NBS glasses. Propagation of femtosecond laser pulses through the waveguide was investigated. Pulse breakup effect that may be the result of soliton formation was observed in these waveguides. Polydimethylsiloxane (PDMS) was uses as the organic component to modify the silica gel. CdS-doped ORMOSILs derived from the PDMS- TEOS system exhibits improved mechanical property and film-forming ability compared to purely inorganic gels. Both CdS-doped PDMS-TEOS ORMOSILs and NBS glasses were much more resistant to photodarkening than the glasses made by melting. The use of the bifunctional ligand 3-aminopropyltriethoxysilane (APTES) significantly reduced the average particle size and narrowed the size distribution of CdS quantum dots in silica gels and densified NBS glasses.

Passive and active sol gel materials and devices

This paper examines sol-gel materials for photonics in terms of partnerships with other material contenders for processing optical devices. The discussion in four sections identifies semiconductors, amorphous and crystalline inorganic dielectrics, and amorphous and crystalline organic dielectrics as strategic agents in the rapidly evolving area of materials and devices for data communications and telecommunications. With Zyss, we trace the hierarchical lineage that connects molecular hybridization (chemical functionality), through supramolecular hybridization (collective properties and responses), to functional hybridization (device and system level constructs). These three concepts thread their way through discussions of the roles sol-gel glasses might be anticipated to assume in a photonics marketplace. We assign a special place to glass integrated optics and show how high temperature consolidated sol-gel derived glasses fit into competitive glass fabrication technologies. Low temperature hybrid sol-gel glasses that combine attractive features of organic polymers and inorganic glasses are considered by drawing on examples of our own new processes for fabricating couplers, power splitters, waveguides and gratings by combining chemical synthesis and sol-gel processing with simple photomask techniques.