# InP-Based Foundry PICs for Optical Interconnects
## 2 InP-Based Generic-Foundry Photonic Integrated Circuits
### Fully-Integrated Dual-Polarization Electro-Absorption Modulated Laser  
Figure 3a shows a photograph of an InP PIC that was realized with the generic-foundry service described in the previous section, namely a dual-polarization electro-absorption-modulation laser (DPEML). The core concept of the DPEML PIC is its serial configuration of all individual devices and BBs. The integrated DFB laser on the left side of the PIC generates a transverse-electric-polarized (TE-polarized) optical signal, which then passes through a 45\({}^{\circ}\) polarization rotator (PR) that converts half of the TE-polarized signal into a transverse-magnetic-polarized (TM-polarized) signal. Then, both signals pass through the first electro-absorption modulator (EAM). The strong polarization selectivity of the EAM, which is based on compressively strained multi quantum-wells (MQW), means that only the TE-polarized optical signal is modulated, leaving the TM-polarized part of the optical signal un-modulated. Afterwards, both signals pass through a 90\({}^{\circ}\) PR that converts the TE signal to TM, and the TM signal to TE. The second EAM again only modulates the newly converted TE-signal (previously un-modulated TM), leaving the newly converted TM signal (previously modulated by the first EAM) intact, thereby avoiding cross modulation between both polarizations. Both PRs use a design with large fabrication tolerances reported in a previous study [17]. The generic-foundry platform's standard complex-coupled DFB laser uses the same MQWs as the EAM BBs. In this design, all active BBs are electrically isolated to eliminate any electrical crosstalk. The lengths of the 45\({}^{\circ}\) and 90\({}^{\circ}\) PRs are 0.5 and 1 mm, respectively. The EAMs and the DFB have active lengths of 200 \(\upmu\)m, respectively, resulting in a PIC size of 3.2 mm \(\times\) 0.2 mm. Figure 3b shows the response of the EAM to both polarizations as a function of the applied bias voltage. This plot shows that the EAM only responds to a TE-polarized optical signal, while leaving TM-polarized unchanged. Figure 3c shows the frequency response of the EAM for various applied voltages, illustrating a modulation bandwidth up to 12 GHz at present. Significantly higher bandwidths will be available in the future following dedicated design and processing optimization steps. The DC characterization of the fabricated DPEML PIC is done on a Stokes-calibrated measurement station. We use it to measure the Stokes vector of light exiting the device and its dependence on the two bias voltages. The measurement is done at a DFB bias of 130 mA, and the chip is placed on a TEC that is set to 20 \({}^{\circ}\)C. The fiber-coupled output power with unbiased EAMs is \(-\)23 dBm. This can be broken down to 3 dBm coming from the DFB, 2 dB loss per PR, 9 dB per EAM (including the epitaxial butt-joint  
Figure 3: Photograph of the fabricated dual-polarization electro-absorption-modulated laser (**a**), along with a plot showing the insertion loss of the EAM for hot polarizations (**b**), and a plot of the bandwidth of the EAM for various applied voltages (**c**). interface), and 4 dB coupling loss to a tapered single mode fiber. A schematic of the measurement setup is shown in Figure 4a, and eye diagrams of both polarizations at 20 Gb/s are shown in Figure 4b. To verify the device's data transmission capability, two electrical 20 Gbit/s non-return-to-zero (NRZ) signals are generated with a pseudo-random bit-sequence (PRBS) source. The signals are amplified with linear amplifiers to about 2 Vpp and launched onto the EAMs via high speed probes with a ground-signal-ground (GSG) configuration. To avoid back reflections, the probe has an AC-coupled 50 \(\Omega\) resistor integrated. DC bias voltages are applied via bias-tees. The optical signal is amplified with an L-band Erbium-doped fiber amplifier (EDFA) and subsequently spectrally filtered.