[attachment=14357]
1. INTRODUCTION
FUTURE optical networks will extend wavelength division multiplexing (WDM) technology to take full advantage of the wide bandwidth of the fiber to meet with the rapidly increasing demand for communication capacity. Large WDM networks will require wavelength converters for signal path routing, and this will enable us to use wavelength resources efficiently and realize flexible network construction.
Several wavelength-conversion methods have already been proposed , including cross-gain or cross-phase modulation in semiconductor amplifiers, four-wave mixing in highly nonlinear optical fibers, and quasi-phase-matched (QPM) difference-frequency generation (DFG) in periodically poled LiNbO (PPLN) devices. Of the several available types of wavelength converter, PPLN waveguide devices are especially promising. This is because these converters provide unique characteristics such as the simultaneous conversion of WDMchannels, a large signal bandwidth, transparency as regards modulation format, and high conversion efficiency.
1.1 EARLIER FABRICATION METHODS
Ti diffusion or annealed proton exchange (APE) has typically been employed to fabricate LiNbO3 waveguides. With a Ti-diffused waveguide, although low propagation losses are achievable, the waveguide exhibits low resistance to photorefractive damage and a relatively low conversion efficiency due to weak light confinement. On the other hand, waveguides fabricated with the APE method exhibit higher resistance to photorefractive damage and stronger light confinement than Ti diffused waveguides.
Although the APE method is well established and produces efficient wavelength converters, it has several drawbacks. First, even with APE waveguides, slight photorefractive damage causes a QPM wavelength shift. Secondly, it is difficult to ensure the long-term reliability of APE waveguides because of the presence of mobile protons in these proton-
exchanged waveguides. Third, proton exchange only increases the refractive index for an extraordinary wave. Therefore, waveguides made with this method support only a single polarization.A polarization diversity approach, either with two waveguides or with counterpropagating waves in a single waveguide, is needed for APE waveguides to achieve polarization independent operation, which is essential for telecommunication applications.
1.2 DIRECT BONDED WAVEGUIDE DEVICES
Direct-bonded or adhered ridge-waveguide devices have been studied with a view to overcoming these limitations. The ridge-shaped structure was fabricated by using ultra-precision machining with a diamond blade. Since no ion-exchange process is employed in the fabrication, the ridge waveguide exhibits strong resistance to photorefractive damage and there is no degradation of the nonlinear coefficient. The ridgewaveguide confines both the TM and TE modes. This feature enables us to realize polarization diversity using a single waveguide incorporating a polarization rotator. In addition, direct bonding provides strong light confinement owing to the step index profile. Therefore, a high conversion efficiency can be obtained by using ridge waveguide structures in PPLN waveguide devices.
2. FABRICATION OF RIDGE WAVEGUIDE
2.1 FABRICATION STEPS
In the fabrication process a 3-inch z-cut 7-mol% ZnO-doped LiNbO3 wafer is prepared for the core layer, which is highly resistant to photorefractive damage, and a 3-inch z-cut LiTaO3 wafer for the cladding. A periodically poled structure was formed in advance on the ZnO doped LiNbO3by the conventional electrical poling method. The method involves conventional photolithography to form a comb-shaped pattern in photo-resist on a LiNbO3 substrate.
We apply a liquid electrode consisting of a lithium chloride solution to both sides of the LiNbO3 while keeping them insulated from each other. Then, we apply a high-voltage electric field to the LiNbO3.This reverses the spontaneous polarization under the electrode. After that, the two wafers are brought into contact in a clean atmosphere and annealed at 500 C to achieve complete bonding. The waveguide layer thickness was reduced to 4 m by successive lapping and polishing.
After the polishing process, we fabricated the ridge shape structure using a dry etching system. Until now, the reactive dry etching technique has mainly been applied to the fabrication of LiNbO3 based optical modulators.
Chemically reactive gases, such as CF4 , C2F6 and CHF3 , were used for dry etching LiNbO3. However, LiF composite can be formed on the LiNbO3 substrate surface as a result of the reaction of Li ions with F ions during the chemical etching process. The LiF composite causes the roughness of the LiNbO3 surface morphology.
We utilized an Ar+C4F8gas mixture to avoid any deterioration of the surface morphology. We set the Ar/C4F8 gas flow ratio at 95/5. The LiNbO3 was mainly etched with a physical etching process rather than a chemical etching process.