Po Sheun Chung

Extraction of intrinsic response from S-parameters of laser diodes

We describe a new method for extracting the intrinsic response of a laser diode from S-parameters measured using a calibrated vector network analyzer. The experimental results obtained using the new method are compared with those obtained using the optical modulation method and the frequency response subtraction method. Good agreement has been obtained, confirming the new method validity and accuracy. The new method has the advantages of obtaining the intrinsic characteristics of a laser diode with conventional measurements using a network analyzer.

Characterization and modeling of Bragg gratings written in polymer fiber for use as filters in the THz region

We demonstrate fiber Bragg gratings written in polymer fiber for use in the THz window for the first time. A KrF excimer laser operating at 248 nm was used to inscribe notch-type gratings in single component Topas subwavelength fiber. A transmission loss at the centre wavelength of the grating of 60 dB is observed in short gratings containing only 192 notches. Experimental results and modeling are presented. The gratings are expected to find use in THz signal filtering and chemical or biosensing applications.

Polymer planar waveguide device using inverted channel structure with upper liquid crystal cladding

We propose a composite waveguide configuration based on an inverted polymer channel structure with upper nematic liquid crystal cladding. This configuration can achieve a more homogenous liquid crystal molecular alignment between the core and the liquid crystal material by minimizing the rubbing damage during preparation of the alignment layer. We demonstrated our idea with a variable optical attenuator which exhibited a 24 dB of attenuation range over a tuning peak voltage of 10 V at 1550 nm.

Proton-exchanged LiNbO/sub 3/ optical waveguides using stearic acid

Proton-exchanged optical waveguides have been fabricated in z-cut LiNbO/sub 3/ using a new proton source: stearic acid. These waveguides were characterized optically and were found to exhibit a step index profile with Delta n=0.118 measured at 0.633 mu m. The propagation losses were typically around 1.5 dB/cm, and the diffusion constant and the activation energy for the proton-exchange process were measured to be 5*10/sup 6/ mu m/sup 2//h and 69 kJ/mol, respectively.<<ETX>>

Extended IWKB method for determination of the refractive-index profile in optical waveguides

A simple method is outlined for the calculation of the refractive-index profiles of both multimode and monomode planar optical waveguides. The method is an extension of the well-established IWKB method. The parameters required are the dispersion relation of the surface index changes and the effective indices at different wavelengths. As an example, the refractive-index profile of a monomode z-cut KTiOPO(4) planar waveguide fabricated by Rb(+):K(+) exchange is computed. Close agreement is obtained, indicating the practicality of this method. The method is also applicable to optical waveguides in other materials, such as LiNbO(3), LiTaO(3), and glass.

Analysis of velocity-matched Ti:LiNbO3 optical intensity modulators with an extended point-matching method

The point-matching method has been extended to analyse a travelling-wave electrode structure for shielded velocity-matched Ti:LiNbO3 Mach-Zehnder optical modulators. This method provides a simple and fast approach to analysing the coplanar waveguide with buffer layer, cover shield, and electrodes of finite thickness. The calculated characteristic impedances and microwave effective indices agree well with those obtained using the spectral domain technique and the finite-element method. It is found that in the coplanar waveguide without cover shield it is difficult to achieve velocity matching. The coplanar waveguide with cover shield can be used to achieve velocity matching, but the corresponding characteristic impedance is lower than 50Ω. Both velocity matching and impedance matching can be obtained using a narrow centre electrode. The calculated electric field has been used to estimate the modulation efficiency of the device. Velocity matching can be achieved by adjusting the height of the cover shield, but a compromise must be made between high interaction efficiency and low microwave reflection when choosing the buffer layer thickness and the electrode thickness.

Ion‐exchanged Fe doped KTiOPO4 optical waveguides

Planar and channel waveguides were fabricated in Fe doped KTiOPO4 (KTP) using the Rb+:K+ ion‐exchange process. These waveguides were characterized optically. It was found that the index change is smaller than that obtained using undoped KTP substrates. However, the diffusion depth is larger. The Fe ions appear to be an effective means to modify the properties of ion‐exchanged KTP waveguides. The photorefractive effect (PRE) in Fe doped KTP channel waveguides was also investigated at wavelengths of 0.543, 0.6328, and 0.83 μm. PRE was not observed, indicating that unlike that in LiNbO3 or KNbO3, Fe ion is not a major contributing factor in KTP.

Erbium Doping in Silicon by Thermal Indiffusion

A rare-earth ion, namely erbium, was doped into silicon (Si) using thermal indiffusion. Codoping using ytterbium oxide (Yb2O3) and germanium (Ge) to increase photoluminescence (PL) intensity was also demonstrated, and the effect of different Yb/Er ratios on Si was studied. Room temperature PL intensity relative to Er-diffused sample was found to increase by ~30 times using Yb/Er (27/1) codopant. When a SiGe/Yb/Er layer was used, room temperature PL intensity was further increased to more than 60 times.

High‐index proton‐exchanged MgO:LiNbO3 optical waveguides using adipic acid

We report the fabrication and characterization of proton‐exchanged optical waveguides in z‐cut MgO:LiNbO3 using adipic acid as a new proton source. These waveguides exhibit propagation losses of less than 1 dB/cm, a linear‐step index profile, and a surface index increase of 0.149 measured at 0.633 μm wavelength. This is the largest surface index change reported for proton‐exchanged MgO:LiNbO3 waveguides. The diffusion parameters were characterized optically, and the diffusion constant D0 and the activation energy Q were found to be 5.40×1010 μm2/h and 103.27 kJ/mol, respectively. The diffusion rate is slower than that of the popular phosphoric acid.

Experimental studies of proton-exchanged waveguides in lithium niobate using toluic acid

Low-loss proton-exchanged planar waveguides have been fabricated in z-cut LiNb03 using a new organic proton source: toluic acid. These waveguides exhibit a step refractive index profile with a surface index increase of 0.124 measured at 0.633/mm. The propagation loss was typically around 1 dB/cm. The diffusion constant D0 and the activation energy Q for the proton exchange process were found to be 7.02 x107 mm2/hr and 75.58 kJ/mol, respectively.