  We present a pluggable radiation-tolerant 4-level Pulse-Amplitude-Modulation
(PAM4) optical transmitter module called GBT20 (Giga-Bit Transmitter at 20
Gbps) for particle-physics experiments. GBT20 has an OSFP or firefly connector
to input 16-bit data each at 1.28 Gbps. The GBT20 drives a VCSEL die with an LC
lens or a VCSEL TOSA and interfaces an optical fiber with a standard LC
connector. The minimum module, including the host connector, occupies 41 mm x
13 mm x 6 mm. At 20.48 Gbps, the minimum Transmitter Dispersion Eye Closure
Quaternary (TDECQ) is around 0.7 dB. The power consumption is around 164 mW in
the low-power mode. The SEE cross-section is below 7.5x10^(-14) cm^2. No
significant performance degrades after a TID of 5.4 kGy.
