S.I. Najafi

Bio: S.I. Najafi is an academic researcher from École Polytechnique. The author has contributed to research in topics: Phosphate glass & Ultraviolet light. The author has an hindex of 8, co-authored 10 publications receiving 300 citations.

Sol-gel integrated optical coupler by ultraviolet light imprinting

Abstract: The authors describe a simple low temperature method for fabricating a low-loss singlemode integrated optical directional coupler in a photosensitive, hybrid, organically modified sol-gel silica glass. The fabrication process is appealing for its simplicity, entailing few steps and using elementary photodefinition to give robust, mechanically rigid devices.

Cooperative upconversion effects on the performance of Er 3+ -doped phosphate glass waveguide amplifiers

Abstract: The cooperative upconversion processes of erbium ions in silver film ion-exchanged waveguides in a phosphate glass were studied from the pump intensity dependence of luminescence decay curves. Cooperative upconversion coefficients of the 4I13/2 level, 7.7±0.7×10-19 and 9.3±0.7×10-19 cm3/s, were obtained for an Er3+ concentration of 1×1020 cm-3 in bulk and waveguide samples, respectively. These values are 1 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. The effects of cooperative upconversion on the gain performance were analyzed for different Er3+ concentrations by a theoretical model based on experimentally measured gain with a 1.48-µm pump wavelength. Given the small cooperative upconversion coefficients in this glass, Er3+ concentrations potentially as high as 3.7×1020 cm-3 were shown to be feasible. Such high concentrations would result in a 12-dB gain at 150-mW pump power with a 4-cm-long waveguide.

Single-step UV recording of sinusoidal surface gratings in hybrid solgel glasses.

Abstract: Efficient surface gratings are UV imprinted in an acrylate-rich hybrid solgel glass. Structural changes occurring in the material upon exposure to light are investigated. Large amplitude modulation (700 nm) and low surface roughness (0.2 nm) were measured by atomic force microscopy. Diffraction efficiencies were found to be close to theoretical predictions. The method of fabrication does not involve any etching step, and an all-UV process is proposed. The gratings are resistant to temperature and to solvent effects. These devices could have useful applications in sensor and spectroscopic devices.

Potassium ion-exchanged Er-Yb doped phosphate glass amplifier

Abstract: A process is developed to make potassium ion-exchanged waveguides in erbium-ytterbium co-doped phosphate glasses. The effects of pump and signal wavelengths, and of the waveguide length on the gain in the waveguides are investigated. At a signal wavelength of 1.530 /spl mu/m, 19.5 dB internal gain is achieved in a 6 mm long waveguide; the measured noise factor is 4 dB.

UV-light imprinted Bragg grating in sol-gel ridge glass waveguide with almost 100% reflectivity

Abstract: The authors report the fabrication of sol-gel ridge waveguides with a grating. The singlemode ridge waveguide for 1.55 µm applications is made by ultraviolet light imprinting. The grating is written by the phase mask technique. 97 and 95% reflection is achieved for TE and TM polarisations, respectively, in a waveguide with a 1.5 mm long grating.

Erbium-doped integrated waveguide amplifiers and lasers

Abstract: Erbium-doped fiber devices have been extraordinarily successful due to their broad optical gain around 1.5–1.6 μm. Er-doped fiber amplifiers enable efficient, stable amplification of high-speed, wavelength-division-multiplexed signals, thus continue to dominate as part of the backbone of longhaul telecommunications networks. At the same time, Er-doped fiber lasers see many applications in telecommunications as well as in biomedical and sensing environments. Over the last 20 years significant efforts have been made to bring these advantages to the chip level. Device integration decreases the overall size and cost and potentially allows for the combination of many functions on a single tiny chip. Besides technological issues connected to the shorter device lengths and correspondingly higher Er concentrations required for high gain, the choice of appropriate host material as well as many design issues come into play in such devices. In this contribution the important developments in the field of Er-doped integrated waveguide amplifiers and lasers are reviewed and current and future potential applications are explored. The vision of integrating such Er-doped gain devices with other, passive materials platforms, such as silicon photonics, is discussed.

Er3+-doped phosphate glasses for fiber amplifiers with high gain per unit length

Abstract: The effect of the types of alkaline earth metal ions (Mg2+, Ca2+ and Ba2+) and concentration of Al2O3 and BaO on the refractive index and the effective emission linewidths of the 4I13/2−4I15/2 transition of Er3+ ions in phosphate glasses, 64P2O5 · 12Al2O3 · 3.5( Er 2 O 3 + La 2 O 3 )·20.5 MO ( M = Mg , Ca , Ba ) and 64P2O5 · 3.5(Er2O3+La2O3) · (21.5 − x)Al2O3 · (11+x) BaO (x=0,3.5,6.5 and 9.5) , were investigated. A single mode Er3+ doped phosphate glass fiber with a core diameter of 4 μm was fabricated by the rod-in-tube technique. A new Er3+ doped fiber amplifier is demonstrated pumping with a 980 nm fiber pigtailed-laser diode. A gain per unit length greater than 2 dB/cm is demonstrated, which is the largest gain per unit length for fiber amplifiers to our knowledge.

Cooperative upconversion and energy transfer of new high Er 3+ - and Yb 3+ –Er 3+ -doped phosphate glasses

Abstract: Systematic studies of cooperative upconversion and Yb3+–Er3+ energy transfer in newly developed phosphate glasses were performed by a rate-equation formalism. The cooperative-upconversion coefficients of the 4I13/2 level for different Er3+ concentrations were determined from the luminescence-decay curves for high pump intensities. A small cooperative-upconversion coefficient of 1.1×10-18 cm3/s was obtained for a high Er3+ concentration of 4×1020 ions/cm3.Yb3+–Er3+ energy-transfer coefficients for an Er3+ concentration of 2×1020 ions/cm3 codoped with different Yb3+ concentrations were calculated from the lifetime measurements of the 2F5/2 level of Yb3+ ions. For Er3+ codoped with an Yb3+ concentration of 6×1020 ions/cm3, an energy-transfer coefficient of 1.1×10-16 cm3/s and an energy-transfer efficiency higher than 95% were determined from our measurements under weak excitation. The cooperative-upconversion coefficients of Yb3+–Er3+-doped samples were found to be consistent with that of an Er3+-doped sample with the same Er3+ concentration. The weak cooperative-upconversion effect of high Er3+ concentrations and efficient Yb3+–Er3+ energy transfer indicate that these newly developed Er3+- and Yb3+–Er3+-doped phosphate glasses are excellent for active device applications.

Sol-gel glass waveguide and grating on silicon

Abstract: This paper reports on the fabrication and characterization of hybrid organic-inorganic glass sol-gel slab and channel waveguides by ultraviolet light imprinting in thin films deposited by a one-step dip-coating process. The adjustment of chemical composition of the materials provides precise selection of refractive index from 1.48 to 1.52 at the wavelength of 632.8 mn. The refractive index of the waveguides at 1.55 /spl mu/m is similar to that of optical fiber, thus reducing the reflection loss between the two to less than 0.01 dB. The effect of ultraviolet light exposure and heat treatment on waveguide refractive index is studied. Fabrication parameters to produce ridge waveguides are optimized to achieve very smooth side walls. Propagation losses in these waveguides are /spl sim/0.1 dB/cm. Single mode buried waveguides, at 1.55 /spl mu/m wavelength, with circular mode profile are demonstrated.