## 6 Wavelength Tunable 100G Transmitter Module
Improper RF arrangement can throttle the overall system bandwidth to a level much below the capacity of the individual components. The RF links cannot solely rely on the bond-wires, as the dangling wires with varying structural and contact properties often lead to severe signal deterioration, interference and unbalanced transmission. An interposer is often needed to connect the electronic signals among the components and also to the cable connectors on the housing. The interposer can be fabricated by structuring metal transmission lines on a ceramic film. The design must be compatible with the housing as well as the overall placement of the individual components to avoid interference, back-reflection and other possible signal impairment. The package shown in Figure 16 is based on the gold-plated submounts of CuW that provide mechanical support for the optical and electrical components. Also included in the package are DC and RF coupling substrates, active thermal control with a thermo-electric cooler (TEC), and a lensed fiber for the coupling of the modulated signals out of the box. The optical loss of the transmitter module is estimated to be \(\sim\)12.5 dB. This includes the propagation loss on the passive polymer and the EO polymer chip, the insertion loss of the half-wave plate and the MZM, the coupling loss between the passive and the EO polymer chip and the coupling loss between the EO polymer chip and the lensed fiber. The power consumption of the device is around 7.2 W, taking into account the consumption of the tunable laser (0.25 W for the GC and max. 0.06 W for the thermal tuning), the MUX-DRV circuit (3.8 W), the phase-shifter for the biasing of the MZM (0.04 W), and the TEC (around 3 W). The DC characteristics of the transmitter module, plotted in Figure 17a, are obtained by adjusting the bias current of the thermal phase shifter on one of the MZM arms. Three wavelengths are selected within the tuning range from 1544 nm to 1566 nm. The static extinction ratio is around 16 dB and the maximal output power reaches \(-9.5\) dBm. The overlaid wavelength tuning spectra are summarized in Figure 17b, which are collected directly at the output of the transmitter when modulated with on-off keying (OOK) signals at 100 Gb/s. Continuous wavelength tuning from 1544 nm to 1566 nm has been demonstrated. The typical tones of the OOK modulation can be seen on both sides of the carriers with 100 GHz (0.8 nm) spacing. The transmission performances at 100 Gb/s data rate are evaluated at the three selected wavelengths and the eye-diagrams are recorded for the back-to-back (B2B) transmission and propagation through a SSMF up to 1625 m. The results are listed in Figure 17c. The characteristics at the three wavelengths are very similar and the eyes remain open after 1625-m SSMF transmission. However, when the transmission distance goes beyond 1000 m, the chromatic dispersion in the SSMF starts to degrade the signal quality, indicated by the immerging hills on the top of the eye diagram. Further experiments show that error free transmission with BER lower than \(10^{-10}\) is achievable at 100 Gb/s for a distance up to 500 m, beyond which the BER increases to \(10^{-9}\) for the 1000-m and \(10^{-7}\) for the 1625-m transmission, respectively. Forward error correction (FEC) coding or dispersion management in the fibers will then be needed. The all-in-one packaged transmitter module allows direct plug-n-play deployment in the optical network systems. The single-lane 100 Gb/s connectivity can significantly reduce the number of  
Figure 17: **(a)** The DC characteristics of the transmitter module at 1544 nm, 1533 nm and 1565 nm. **(b)** overlaid spectra of the 100 Gb/s on-off keying (OOK) signals at the output of the transmitter module covering the tuning range from 1544 nm to 1566 nm. **(c)** Eye-diagrams of the 100 Gb/s OOK signals, evaluated at three wavelengths from B2B to 1625 nm SSMF transmission. components, shrink down device footprint and improve the network flexibility.