## 3 Polymer AWG-Based OLTs  
Optical line terminals (OLTs) and optical network units (ONU) are basic building blocks in the WDM-PON infrastructure. The hybrid integration approach allows high-performance and cost-effective OLTs to be assembled using polymer arrayed waveguide gratings (AWGs) for wavelength multiplexing (MUX) and de-multiplexing (DMX). InP LD arrays can be direct modulated as transmitters (Tx) and planar PD arrays function as high speed receivers (Rx). On the transmitter side, the OLT-Tx is shown in Figure 5a,b. This device consists of four InP-based 10-chanel DFB LD arrays first placed on a Si submount and then butt-joint coupled to a polymer MUX AWG. The complete device, combining the AWG and the LD arrays, is 29 mm long and 30 mm wide. The 40-channel MUX AWG is based on a polymer waveguide with square cross-section of 3.5 \(\upmu\)m \(\times\) 3.5 \(\upmu\)m and a refractive index contrast of 1.47/1.45. It is designed to cover the L-band (1574.4 nm-1606.4 nm) with 100 GHz channel spacing. The spectral characteristic of the AWG channels is of Gaussian type with a 1-dB bandwidth of 0.25 nm. The temperature dependence is \(\sim\)0.04 nm/\(\,\)C. A heater electrode placed on top of the arrayed waveguide region can be biased to thermally adjust the AWG central wavelengths to match the ITU-grids [11]. Figure 4: (**a**) SEM photo of a SiN\({}_{x}\) waveguide Bragg grating and (**b**) SEM photo of a SiN\({}_{x}\)/Polymer grating assisted coupler. Both photos are taken before the polymer top cladding is spin-coated. Figure 6a shows the overlaid spectra of the AWG at different heater powers. Figure 6b exacts and plots the central wavelength during the thermal tuning. At \(\sim\)900 mW, the tuning range exceeds the channel spacing of \(\sim\)0.8 nm. The schematic layout of the InP LD array is shown in Figure 6c. Each single element comprises a 150-\(\upmu\)m-long DFB section and a 250-\(\upmu\)m-long extra amplification section to increase the front side optical output power. The emission wavelength for each individual LD is defined by the e-beam lithography to set the 100-GHz channel spacing. A heater electrode is included in the DFB section and up to 3-nm tuning can be realized thermally, in order to pin-point the wavelengths at the ITU grid. The emitting wavelengths of different LDs on the array are listed in Figure 6d. The \(4\times 10\)-channel LD arrays construct the 40 defined channels in the L-band. The LD arrays are first mounted on the Si-submounts with pre-defined RF lines and then butt-joint coupled to the polymer-based AWG. The arrays have been high-reflection (HR) coated on the back side against air, whereas the front facet has an anti-reflection (AR) coating against polymer to avoid back reflections. Index-matching epoxy is first applied to secure the optical coupling, followed by a mechanical fixing epoxy to support the large array. Figure 5: 40-channel OLT-Tx based on 4 \(\times 10\) direct modulated LD arrays butt-joint coupled to a thermally tunable polymer AWG. (**a**) The schematic layout and (**b**) photograph of the assembly. Figure 6: (**a**) Overlaid AWG spectra at different heater powers; (**b**) AWG central wavelength during thermal tuning; (**c**) schematic layout of the multi-wavelength LD array; (**d**) the measured emission wavelengths. The assembled OLT-Tx is placed on the thermo-electric cooler (TEC), which stabilizes the assembly temperature at 25 \(\,\mathrm{\SIUnitSymbolCelsius}\). The performance is evaluated and displayed in Figure 7, where the spectra of the 40 channels are measured at the outputs of the MUX AWG. An optical power larger than 0 dBm has been obtained in the fiber at a drive current \(\sim\)120 mA. The spectrum of the AWG is firstly adjusted to match the central wavelengths defined by the ITU-Grid. The emitting wavelengths of the individual LDs are then aligned to the grid. The coinciding spectra from the AWG and the LDs lead to a stunningly sharp wavelength response and the achieved side mode suppression ratio (SMSR) exceeds 70 dB for all the 40 channels.