Planar optical waveguide device and DP-QPSK modulator

1. A planar optical waveguide device, comprising:

two input portions that are waveguides that have the same width, are parallel to each other, and have rectangular cross-sections;

a wide portion that is a linear waveguide and is connected after the two input portions;

a tapered portion that is connected after the wide portion and that is a multi-mode waveguide which has a tapered shape having a width decreasing gradually and through which at least TE1 propagates; and

an output portion that is connected after the tapered portion and that is a multi-mode waveguide which has a rectangular cross-section and through which at least TE1 propagates,

wherein the planar optical waveguide device forms a high-order mode conversion combining element that outputs the TE0, which is input to the two input portions, as the TE1 from the output portion, and

assuming that a distance between the two input portions in a width direction of the two input portions is gap, a width of each of the two input portions is Wa, and a waveguide width of the wide portion is Wb, a relationship of Wb>Wa×2+gap is satisfied, and a center of the two input portions in the width direction matches a center of the wide portion in the width direction.

2. The planar optical waveguide device according to claim 1 , further comprising:

a bent waveguide that is connected before one or both of the two input portions.

3. The planar optical waveguide device according to claim 1 , further comprising:

a tapered waveguide that is connected before the two input portions and changes a width of a waveguide.

4. The planar optical waveguide device according to claim 1 , further comprising:

a tapered waveguide that is connected after the output portion and changes a width of a waveguide.

5. The planar optical waveguide device according to claim 1 , further comprising:

a high-order mode splitter that is connected after the output portion and extracts TE1.

6. The planar optical waveguide device according to claim 5 , further comprising:

a high-order polarization conversion element that is connected after the high-order mode splitter and converts TE1 to TM0.

7. The planar optical waveguide device according to claim 1 , further comprising:

a high-order polarization conversion element that is connected after the output portion and converts TE1 to TM0.

8. The planar optical waveguide device according to claim 7 , further comprising:

a polarization beam splitter that is connected after the high-order polarization conversion element and extracts TM0.

9. The planar optical waveguide device according to claim 1 ,

wherein a waveguide that forms the high-order mode conversion combining element is formed by a core and claddings surrounding the core, and

the claddings are formed of the same material.

10. The planar optical waveguide device according to claim 9 ,

wherein, in a waveguide that forms the high-order mode conversion combining element, a height of the core is 220 nm, the core is formed of Si, the cladding is formed of SiO 2 , and the Wa satisfies 400 nm≦Wa≦800 nm and the gap satisfies 100 nm≦gap≦500 nm, and

assuming that Wc=(Wb−(Wa×2+gap))/2 and Wd=(−0.0000000008350862×gap 2 −0.000000436986×gap+0.0008631515625)×Wa 2 +(0.0000016652002737×gap 2 +0.000208004578×gap−1.2672957688375)×Wa+(−0.00069425946204×gap 2 +0.670233759496504×gap+462.994458236249), 0<Wc<Wd is satisfied.

11. A DP-QPSK modulator comprising the planar optical waveguide device according to claim 6 .

12. A DP-QPSK modulator comprising the planar optical waveguide device according to claim 7 .

13. A DP-QPSK modulator comprising the planar optical waveguide device according to claim 9 .

14. A DP-QPSK modulator comprising the planar optical waveguide device according to claim 10 .