津田研究室 - 慶應義塾大学理工学部電子工学科

The structure of the AWG is shown in Fig. 3. The number of the output ports is doubled to retrieve N wavelengths in a single diffraction order when optical signals enter all possible input ports, and N 2 ? 1 couplers multiplex optical signals from output port 1~N/2 with signals from output port N/2+1~N

Fig. 3 Structure of AWG that combines an Nx2N AWG with 2x1 couplers

We designed a 10-ch, 6.4 THz spaced cyclic AWG. Fig. 4 shows the transmission spectrum for input port N. On the left side of the passbands resulted from output ports 1~N/2 are the passbands resulted from output ports N/2+1~N. Switching the input port from N to 1 will shift the passbands to the left side, which confirms that cyclic operation is possible. The largest deviation is 26% of the channel spacing in a frequency grid

Fig. 4 Transmission spectrum of the 10-ch, 6.4 THz spaced cyclic AWG. Dotted lines show 6.4 THz frequency spacing.

Sub-topic 1-2 : AWG with interleaved chirp waveguides

The structure of the AWG is shown in Fig. 5. We employ an interleaved chirp onto the arrayed waveguides, where the center wavelength of the even numbered waveguides is shifted N times of the channel spacing from the center wavelength of the odd numbered waveguides to produce an identical neighboring light pattern. By adjusting the center wavelength of the even numbered waveguides, we can match the diffraction order number of the even and odd numbered waveguides, making it possible to largely reduce the peak wavelength deviation.

Fig. 5 Structure of AWG with interleaved chirp waveguides

We designed a 10-ch, 30 nm spaced cyclic AWG. Fig. 6 shows the transmission spectrum for input port 1. On the right side of the passbands resulted from odd numbered waveguides are the passbands resulted from even numbered waveguides. Switching the input port from 1 to N will shift the passbands to the right side, which confirms that cyclic operation is possible. The largest deviation is 2% of the channel spacing in a wavelength grid.

Fig. 6 Transmission spectrum of the 10-ch, 30 nm spaced cyclic AWG. Dotted lines show 30 nm wavelength spacing

Sub-topic 2 : Using multi-step taper waveguides for high performance AWG

Sub-topic 2-1 : Low loss AWG

Light scattering at the gaps between arrayed waveguides near the boundary of arrayed waveguides and slab waveguide is the primary cause of loss.
⇒To reduce loss, we proposed employing multi-step taper structures on the boundary of the 1st slab waveguide and arrayed waveguides.

Fig. 7 (a) Multi-step taper structure. (b) Transmittance of each arrayed waveguides (normal and multi-step tapers).

Fig. 7(a) shows the multi-step taper structure. We achieved loss reduction by optimizing W and L. The results are shown in Fig. 7(b). Transmittance is improved approximately 1.5 dB for all arrayed waveguides compared to waveguides with normal tapers.

Sub-topic 2-2 : AWG with uniform peak transmittance

The structure of the star couplers in an AWG leads to lower transmittance the further the output port position is from the center.
•Produces loss imbalance between channels
⇒We proposed employing and optimizing multi-step taper structures on the boundary of the 2nd slab waveguides and arrayed waveguides to reduce loss imbalance between channels

Fig. 8 (a) Multi-step taper structure. (b) Transmittance of each output waveguides (normal and multi-step tapers).

Fig. 8(a) shows the multi-step taper structure. We achieved loss imbalance reduction by optimizing W and L. The results are shown in Fig. 8(b). Loss imbalance is reduced to as low as 0.36 dB, which is approximately 1.0 dB better compared to waveguides with normal tapers.

Sub-topic 3 : Reducing loss imbalance in a cyclic AWG using interleaved chirp waveguides

Doubling the free spectral range (FSR) is well-known method for reducing loss imbalance between channels of an AWG.
•However, doubling the FSR makes cyclic operation impossible
⇒We propose an AWG that functions cyclically even with a doubled FSR by employing interleaved chirp on the arrayed waveguides.
•? A cyclic AWG with low loss imbalance is realizable

Fig. 9 shows the operating principle of the AWG. Neighboring peaks are produced at half of the FSR by the interleaved chirp waveguides to enable cyclic operation.

Fig. 9 Principle of the cyclic AWG using interleaved chirp waveguides.

We designed a 32-ch, 50 GHz spaced AWG. The loss imbalance is reduced as much as 4.0 dB compared to normal AWG. The maximum and minimum loss of each output port of the AWG is shown in Fig. 10.

Fig. 10 Maximum and minimum loss for each output port of the AWG using interleaved chirp waveguides.

Sub-topic 4 : Wavelength tunable external cavity laser using AWG

A wavelength tunable laser is an essential component in a WDM network. In particular, it is an indispensable component for a router that uses AWGs to control the wavelength route.
•However, there is still few developments on a wavelength tunable laser in the T band
⇒We propose a wavelength tunable external cavity laser that utilizes the demultiplexing properties of the AWG.
•Wavelength tuning is performed by changing the input port of the AWG.
•The wavelength spacing of the AWG is 400 GHz.
•The gain chip is an InGaAs quantum dot gain chip with an oscillation wavelength of 1140 nm.

Fig. 11 Structure of the tunable external cavity laser with AWG.

Fig. 12 Oscillation spectrum control of the external cavity laser

(4)Achievements

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