A multi-branch waveguide may be a solution to reduce the dimensions of optical power dividers and optical space switches. In this thesis an optical power divider and an optical switch based on a four-branch waveguide structure are proposed and fabricated for the first time.
The proposed four-branch optical power divider has one beam separator and two beam expanders in the branching region of a four-branch waveguide. The uniform output power distribution can be obtained by adjusting the sizes of the beam separator and the beam expander. To demonstrate the proposed power divider, the four-branch waveguide with a beam separator and two beam expanders is fabricated. Benzocyclobutene (BCB) and UV-curable epoxy are used as core and cladding layers, respectively. The measured powers of four output ports are uniform for both TE and TM modes within the difference of 0.3 dB.
A 1x4 thermo-optic switch based on the four-branch waveguide is proposed and fabricated. The proposed switch consists of a four-branch waveguide and four electrodes on the top of branches. The operation of this device is based on the adiabatic mode evolution in branch waveguides with a small angle of separation. If three among four electrodes are heated, the effective refractive indices of waveguides are decreased in the heated branches. Thus the input light is eventually coupled into the unheated branch that has an effective refractive index relatively higher than the other branches. The proposed four-branch thermo-optic switch is fabricated by using the standard photolithography and reactive ion etching. Polyimides with the refractive index difference of 1.2 % are used as materials of core and cladding layers. The fabricated device has the crosstalk of -14 dB or less, and its switching power is about 1200 mW.
However, this four-branch thermo-optic switch has a problem that driving powers in the switching states are different from each other; the power consumption for the inner output port is more than twice as large as that for the outer output port. To solve this problem unequal width heaters and the waveguide structure with a thin overcladding layer are proposed in a four- branch thermo-optic switch. This structure is fabricated with the polymer materials with high index difference, BCB and PFCB. Polarization-independent and wavelength-insensitive operation is demonstrated at the wavelength of 1.3 ㎛ and 1.5 ㎛. And the measured characteristics exhibit the smaller difference in the power consumption between the switching states and the driving power less than the previous four-branch thermo-optic switch with equal width heaters. As for the device performance, the crosstalk is better than -15 dB at about 170~190 mW, the insertion loss is 7.2 ~ 7.8 dB, and the switching time is less than 0.76 ms.
