A waveguide lens composed of multistage polymer-filled thin grooves in a silica planar lightwave circuit (PLC) is proposed and the low-loss structure is designed. Both an imaging optical system and a Fourier-Transform optical system can be configured in a PLC by use of a waveguide lens. It makes a PLC functional and its design flexible. Moreover, a focal length of a lens is tunable with large thermo-optic effect of the polymer.
A concatenated lens is formed to attain a desirable focal length with low-loss. The thickness of each lens and the spacing are about 10-50 microns. The simulation showed that the radiation loss of the light propagate through 20-stage grooves filled with a polymer was only 0.868 dB when the refractive index of the polymer was 1.57, the groove width was 30 microns, and the spacing between adjacent grooves was 15 microns. For example, the single lens structure that the center thickness is 30 microns, the diameter is 300 microns, and the refractive index of the polymer was 1.57, have a focal length of 4600 microns. The focal length of 450 microns can be obtained with 20-stage concatenated lens structure. The larger numerical aperture can be realized with a polymer of higher refractive index.
We have applied the concatenated lens structure to various photonic circuits including optical couplers, a variable optical attenuator.
We propose a 1 x 2 optical prism deflector switch which consists of two parabolic waveguide mirrors, multiple-stage micro prisms filled with polymer, and input/output waveguides. This type of switch has high extinction ratio characteristics compared with the interference-type switch.
The number of prisms is 20 or 30, and the vertex angle of the prism is about 0.1 rad. The prisms are so thin that the coupling loss can be reduced. The waveguide mirror is formed by depositing Ag on the side wall of the deep trench in a slab waveguide. The light from the input waveguide reflects at the parabolic mirror to be the parallel light, propagates through the prisms and reflects at the other parabolic mirror to converge on the output waveguide.
The prisms have thermo-optic (TO) coefficient of -1.3 x 10-4/K while the slab waveguide has that of -6.7 x 10-6/K. Therefore, by raising the temperature around prisms, the light is deflected through the prisms and the switch is brought into the cross state. On the other hand, without rise in temperature, the light goes straight. The distance between adjacent outputs waveguides is 30mm, corresponding to the change in the temperature of 30 K. The size of devise is 4.4 mm x 8.0 mm with 20 prisms and 3.3mm x 6.5mm with 30 prisms, respectively. With 20 prisms at a wavelength of 1555 nm, a minimum insertion loss of 0.65 was measured in bar state.
The novel compact arrowhead arrayed-waveguide grating (AWG) using v-bend waveguides in each arrayed waveguide is proposed and fabricated. The v-bend structure is useful to reduce bending area of a silica waveguide with low Δ (0.3%) to high Δ (0.8%) design. It is composed of curved single mode waveguides and a slab waveguide with an integrated elliptic metal mirror. The mirror structure in the v-bend waveguide is fabricated by the reactive ion etching of the deep trench with smooth and vertical side wall, and by the deposition of silver metal with chemical reaction. The proposed AWG with arrowhead shape has a footprint of only 1/10 of the conventional one with the same performance. The small footprint leads to low cost and to high-resolution because the small circuit is less subject to the fluctuation of the effective index of the substrate.
In this paper, the 8-ch, 25 GHz spacing arrowhead AWG composed of the same v-bend structures in each arrayed waveguide in order to obtain the uniform propagation loss of the v-bend is fabricated with the effective index of 0.75 % silica and its property is reported. Its size of the slab and arrayed waveguide region is 1.3 mm × 15.5 mm and only 1/10 of the conventional type.
A beam steering type 1:4 optical switch with phase shifters in a silica arrayed-waveguide is proposed. It consists of collimating waveguide mirrors, an arrayed-waveguide which has deep trenches with polymer materials, and input/output waveguides. It can switch the output port of the incident light at high extinction ratio. The incident light is guided to the front mirror, collimated, and input to the arrayed-waveguide. Each waveguide in the arrayed-waveguide has the same length. The number of narrow trenches filled with polymer linearly increases in order. The refractive index of the polymer is set to the effective index of the single mode silica waveguide at certain temperature. The propagation direction of the output light from the arrayed-waveguide can be controlled by changing the temperature of the device because of the large thermo-optic coefficient of the polymer. The second mirror converges the light into one of the output waveguide.
We designed two types of the switch which had 9 or 15 waveguides in the arrayed waveguide and they are under fabrication. The chip sizes are about 2.5 mm x 8.0 mm and 2.5 mm x 9.0 mm, respectively. The required temperature shift for the switching from one output port to the adjacent output port is 20 [K] when we use the polymer with a TO coefficient of -1.8x10-4 [1/K].