## 4 Colorless ONU  
Within the WDM-PON technology, a colorless optical network unit (ONU) supporting all wavelength channels is needed to simplify network operation, reduce installation cost, and keep the maintenance efforts under control [22,23]. Such a device can be assembled on the polymer platform and functions including bi-directional connectivity, real-time data rate monitoring and crosstalk suppression can be enabled simultaneously [14]. The schematic layout and the photo of the assembled ONU are shown in Figure 10a,b. The entire assembly measures 6.5 mm (L) by 2.5 mm (W). The InP gain chip (GC) is similar to an LD except that the internal optical feedback loop is intentionally removed. At least one facet of the gain chip is AR coated to elimin  
Figure 10: ONU assembled on the polymer-based hybrid integration platform. (**a**) The schematic layout and (**b**) photograph of the assembly. Figure 9: Measured responsivities of the PD arrays on the OLT-Rx. enhance light emission. It usually employs a multi-quantum well design to enhance the output power and to broaden the gain bandwidth. The dual GC array is first placed and wire-bonded onto a RF-submount and then butt-joint coupled to the dual-channel polymer Bragg gratings. Together they form a tunable transmitter (Tx) when the GC is direct modulated. This dual-channel design helps relieve the thermal strain from the single polymer Bragg grating and the wavelength tuning across the C-band is split between the two paths. In this way, the local temperature at maximal tuning position still stays far below the certified degradation temperature of 300 \(\,\mathrm{\SIUnitSymbolCelsius}\) and long-term stable performance is verified over more than 6000 h at stressed conditions [24]. Five percent of the Tx power is tapped out via a pair of asymmetric Y-couplers to be received by the monitor PD. The TFE-based C/L band filter reflects the Tx signal, which further propagates through the polymer waveguide into the single mode fiber that is attached passively in the U-groove [25]. On the receiver (Rx) side, the incoming optical signal at any wavelength within the L-band goes through the same fiber into the polymer waveguide. The TFE passes the signal through to the receiver side of the ONU, where the 45\(\,\mathrm{\SIUnitSymbolCelsius}\) mirror deflects the light and feeds it to the high-speed planar PDs mounted on top. A series of air trenches are etched on the polymer chip together with the U-Groove and the TFE slot. These air trenches provide many optical interfaces and deliberately scatter the stray light in the polymer slab in the direction away from the location where the Rx PD is mounted to avoid cross-talks. The TFE is a key element performing the C/L-band wavelength (de)multiplexing. It consists of alternating layers of sputtered SiO\({}_{2}\) and TiO\({}_{2}\) films on a thin polymer substrate. The total thickness of the filter adds up to 24 \(\,\mathrm{\SIUnitSymbolMicro m}\). The fabrication is done on a standard 3-inch silicon wafer. After sputtering, the TFE is cut by an excimer laser into desired dimensions. A photo of the TFE after cutting is shown in Figure 11a. The TFE has been characterized by single mode fibers, prior to the integration into the polyboard. The reflection/transmission spectra are displayed in Figure 11b. The TFE introduces a minimal insertion loss \(<\)2 dB. Due to the band-edge effect, however, the transition slope between 1562 nm to 1572 nm causes signals at these wavelengths to suffer from extra attenuation. Filters with steeper band-edges at reasonable thickness are currently being investigated. Figure 11: (**a**) Microscope image of the C/L band filter after laser cutting. (**b**) Reflection and transmission spectra of the TFE-based filter. After the assembly, the ONU is first characterized on the Tx side. The drive current for the GC is set to 50 mA. The lower arm of the tunable laser path covers a tuning range from 1525 nm to 1548 nm, as indicated in Figure 12a, while the upper arm from 1545 nm to 1568 nm, as shown in Figure 12b.