S.I. Najafi

Publications (40)70.27 Total impact

• Article: Self‐Processing of Surface‐Relief Gratings in Photosensitive Hybrid Sol‐Gel Glasses

  D. Blanc, S. Pelissier, K. Saravanamuttu, S. I. Najafi, M. P. Andrews
  Advanced Materials 12/1999; 11(18):1508 - 1511. · 13.88 Impact Factor
• Article: Single-step UV recording of sinusoidal surface gratings in hybrid solgel glasses.

  S Pelissier, D Blanc, M P Andrews, S I Najafi, A V Tishchenko, O Parriaux
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  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.
  Applied Optics 12/1999; 38(32):6744-8. · 1.41 Impact Factor
• Article: Ion-exchanged glass waveguide Mach-Zehnder interferometers for wavelength multi/demultiplexing and the effect of thermal post-annealing on spectral transmission

  Guangwen Zhang, S Honkanen, S I Najafi, A Tervonen, P Katila
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  ABSTRACT: A non-symmetric three-port integrated optical Mach-Zehnder interferometer configuration is employed to make 1.3/1.55 mu m and 1.48/1.55 mu m wavelength multiplexers/demultiplexers. The devices are produced by a potassium-silver double-ion exchange process. The effect of thermal post-annealing on the spectral behaviour of the fabricated devices is investigated. It is observed that the spectral transmission of the 1.3/1.55 mu m device is not sensitive to post-annealing while the transmission spectrum of the 1.48/1.55 mu m device changes after post-annealing. The losses and extinction ratios of the fabricated devices are also determined.
  Pure and Applied Optics Journal of the European Optical Society Part A 12/1998; 3(5):907.
• Article: Sol-gel glass waveguide and grating on silicon

  S.I. Najafi, T. Touam, R. Sara, M.P. Andrews, M.A. Fardad
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  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 μm 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 ~0.1 dB/cm. Single mode buried waveguides, at 1.55 μm wavelength, with circular mode profile are demonstrated
  Journal of Lightwave Technology 10/1998; 16(9):1640-1646. · 2.78 Impact Factor
• Article: Yb3+sensitized Er3+-doped waveguide amplifiers: a theoretical approach

  A. Shooshtari, T. Touam, S. I. Najafi, S. Safavi-Naeini, H. Hatami-Hanza
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  ABSTRACT: Silicate and phosphate glass waveguide amplifiers doped with Er3+, and co-doped with Er3+/Yb3+ are theoretically studied. Configurations for core and core–cladding doped waveguide amplifiers are considered. It is shown that gain in the core–cladding doped amplifiers is considerably higher than core doped amplifiers. It is also shown that with input signal power up to 1 and 200mW pump power, a 12.5dB gain can be achieved in a 3cm long waveguide amplifier, with a noise figure of 3dB.
  Optical and Quantum Electronics 01/1998; 30(4):249-264. · 0.82 Impact Factor
• Article: UV-light imprinted Bragg grating in sol-gel ridge glass waveguide with almost 100% reflectivity

  M.A. Fardad, T. Touam, P. Meshkinfam, R. Sara, X.M. Du, M.P. Andrews, S.I. Najafi
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  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
  Electronics Letters 07/1997; · 0.96 Impact Factor
• Conference Proceeding: Potassium ion-exchanged Er-doped phosphate glass integrated optics amplifier

  P. Fournier, P. Meshkinfam, M.A. Fardad, M.P. Andrews, S.I. Najafi
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  ABSTRACT: Not Available
  Lasers and Electro-Optics, 1997. CLEO '97., Summaries of Papers Presented at the Conference on; 06/1997
• Article: Symmetric profile beams from waveguides with asymmetric grating couplers.

  T Touam, S I Najafi
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  ABSTRACT: An accurate method to analyze integrated optics waveguides with a grating is used to design diffractive gratings. A novel approach to produce symmetric diffracted beams is proposed. In this approach, we vary the grating duty cycle along the propagation direction. We demonstrate that this method results in symmetrical diffracted beam profiles.
  Applied Optics 05/1997; 36(12):2554-8. · 1.41 Impact Factor
• Article: Potassium ion-exchanged Er-Yb doped phosphate glass amplifier

  P. Fournier, P. Meshkinfam, M.A. Fardad, M.P. Andrews, S.I. Najafi
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  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 μm, 19.5 dB internal gain is achieved in a 6 mm long waveguide; the measured noise factor is 4 dB
  Electronics Letters 03/1997; · 0.96 Impact Factor
• Article: Gain characteristics of a high concentration Er3+‐doped phosphate glass waveguide

  T. Ohtsuki, N. Peyghambarian, S. Honkanen, S. I. Najafi
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  ABSTRACT: A channel waveguide on an erbium‐doped phosphate laser glass 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 0.63 dB/cm at 1.3 μm. Er³⁺ concentration of 1×10²⁰ ions/cm³ was chosen so that no concentration quenching occurred. This was confirmed by measuring a fluorescence lifetime of 8.4 ms at 1.54 μm. Gain of the fabricated waveguide was measured by using a Ti:sapphire laser at a wavelength of 980 nm and a laser diode of 1.532 μm 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 showed that lasing is expected at the emission peak with a 3.6 cm long waveguide fabricated in a similar manner. Calculation results showed that the potential gain of 8.8 dB can be realized with 250 mW pump power, and a 7.2 cm long waveguide provided that mode overlap can be increased by 20% and propagation loss can be reduced by 50%. No significant upconversion effect was observed up to 1.1×10⁶ W/cm² pump intensity. © 1995 American Institute of Physics.
  Journal of Applied Physics 10/1995; · 2.17 Impact Factor
• Article: Length optimization of single-mode rare-earth doped waveguides using saturated absorption

  V. Francois, T. Ohtsuki, N. Peyghambarian, S.I. Najafi
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  ABSTRACT: Absorption saturation was demonstrated to provide a high, uniform population inversion over a length of waveguide longer than the material absorption length. The small signal gain was measured experimentally in single-mode waveguides, made by silver ion exchange, on a commercially available neodymium doped glass. The ratio of the excited states was calculated as a function of waveguide length and was shown to be a convenient criteria to determine the optimum device length.< >
  IEEE Photonics Technology Letters 09/1995; · 2.19 Impact Factor
• Article: Sol-gel integrated optical coupler by ultraviolet light imprinting

  C.-Y. Li, J. Chisham, M. Andrews, S.I. Najafi, J.D. Mackenzie, N. Peyghambarian
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  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
  Electronics Letters 03/1995; · 0.96 Impact Factor
• Article: Threshold gain and threshold current analysis of circular grating DFB and DBR lasers

  Chunmeng Wu, T. Makino, S.I. Najafi, R. Maciejko, M. Svilans, J. Glinski, M. Fallahi
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  ABSTRACT: A TE-cylindrical wave threshold gain and threshold current analysis is presented for circular grating distributed feedback (CG-DFB) and circular grating distributed-Bragg-reflector (CG-DBR) lasers. Mode properties of CG-DFB/DBR lasers are discussed. It is found that the threshold gain, threshold current density and threshold current of CG-DBR lasers can be much lower than those of CG-DFB lasers
  IEEE Journal of Quantum Electronics 11/1993; · 1.88 Impact Factor
• Article: Integrated optical circular grating tap power divider

  S.I. Najafi, M. Fallahi, P. Lefebvre, C. Wu, I. Templeton
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  ABSTRACT: A new compact integrated optical tap and multiport power divider is provided. A four-port device is made using a glass substrate and is tested at 514 nm wavelength.
  Electronics Letters 09/1993; · 0.96 Impact Factor
• Article: Loss characteristics of potassium and silver double‐ion‐exchanged glass waveguides

  W. J. Wang, S. Honkanen, S. I. Najafi, A. Tervonen
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  ABSTRACT: Loss characteristics of potassium and silver double‐ion‐exchanged glass waveguides are studied. Low propagation losses (less than 0.25 dB/cm) at 1.296 μm wavelength are measured, although aluminum as a mask and inexpensive multipurpose glass as a substrate are used in the fabrication process. The losses are compared to the losses of waveguides made by one‐step aluminum masked silver ion exchange and by silver ion exchange with ionic masking (dielectric mask). The reasons for the loss reduction in double‐ion‐exchanged waveguides compared to one‐step silver ion‐exchanged waveguides are discussed.
  Journal of Applied Physics 09/1993; · 2.17 Impact Factor
• Article: Integrated 1.3 mu m/1.55 mu m wavelength multiplexer and 1.8 splitter by ion exchange in glass

  G. Zhang, S. Honkanen, S.I. Najafi, A. Tervonen
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  ABSTRACT: Integration of a 1.3 mu m/1.55 mu m wavelength multiplexer and a 1/8 splitter in a single glass substrate is demonstrated by potassium and silver double-ion exchange. The device is composed of a nonsymmetric Mach-Zehnder interferometer for wavelength multiplexing and symmetrical Y-junctions for achromatic splitting. The facet-to-facet excess loss is less than 2.5 dB.
  Electronics Letters 07/1993; · 0.96 Impact Factor
• Article: Waveguide coupler for potassium- and silver-ion-exchanged waveguides in glass.

  S Honkanen, P Pöyhönen, A Tervonen, S I Najafi
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  ABSTRACT: A waveguide coupler for integrating potassium- and silver-ion-exchanged glass waveguides in a single device is proposed and demonstrated. The measured insertion loss is 1.2 dB.
  Applied Optics 04/1993; 32(12):2109-11. · 1.41 Impact Factor
• Article: Reply to Comment on erbium-doped composite glass waveguide a

  S. Honkanen, S.I. Najafi, W.J. Wang, Q He
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  ABSTRACT: Not Available
  Electronics Letters 02/1993; 29(1):132-. · 0.96 Impact Factor
• Article: Simplified coupled-wave equations for cylindrical waves in circular grating planar waveguides

  C. Wu, T. Makino, R. Maciejko, S.I. Najafi, M. Svilans
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  ABSTRACT: A simplified form of the coupled-wave equations for cylindrical waves in circular grating distributed feedback (DFB) planar waveguides is presented. The coupling coefficients can be expressed as the product of a cylindrical-wave factor and the corresponding coupling coefficient between planar waveguide modes. Under the large radius approximation (β r &Gt;1), which covers most practical cases, the interpolarization coupling between TE and TM cylindrical waves approaches zero. The coupled-wave equations can then be reduced to two independent families, one for TE- and the other for TM-cylindrical waves
  Journal of Lightwave Technology 12/1992; · 2.78 Impact Factor
• Article: New integrated optical ring resonator in glass

  W.J. Wang, S. Honkanen, S.I. Najafi, A. Tervonen
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  ABSTRACT: A new integrated optical ring resonator using a four-port nonsymmetric Mach-Zehnder interferometer as an input/output coupler is demonstrated. The device is fabricated in a glass substrate by the double-ion-exchange process. A finesse of 5 at lambda =1.523 mu m is measured. Propagation losses of 0.17 dB/cm in the ring waveguide are estimated.
  Electronics Letters 11/1992; · 0.96 Impact Factor