Fig. 1. Schematic of the full setup. A 9μm wavelength CW QCL emits around 100 mW. The beam goes to an external modulator connected to DC and RF sources. The resulting signal passes through a 31-m Herriott cell before being collected on a high-speed detector and recorded with a fast oscilloscope.

Fig. 2. (a) Picture of the connectorized external modulator on its mount with a coplanar waveguide. (b) Modulation depth estimated while measuring the signal voltage on the QCD when voltage pulses are applied on the modulator (blue dots). The measurements are well fitted with a Beer–Lambert law (solid curve). (c) QWIP biased at 4 V and showing a bandwidth >25GHz with a heterodyne beating signal⁴² (in red). We also display the electrical rectification of the QCD, showing a 3 dB bandwidth of 4.5 GHz (in blue).

Fig. 3. Bandwidth measurements. Optical response of the full system (modulator, QWIP, and amplifiers) measured using a VNA and injecting a 5 dBm signal in the modulator (green curve). In blue, the rectified current from the modulator when injecting a 0 dBm input signal. The gray curve represents the noise obtained in the same configuration while the midinfrared beam is blocked.

Fig. 4. Eye diagrams of transmission without processing for two different data rates using a QWIP. The figure shows the normalized voltage as a function of time: (a) 16Gbits−1 for a B2B transmission for a BER < 0.38% and (b) 16Gbits−1 through a 31-m Herriott cell for a BER < 4%.

Fig. 5. Comparison of the two eye diagrams (a) without and (b) with FFE equalization (ntap=391 and μ=1×10−5) at 30Gbits−1 with a 215-bit long PRBS in B2B configuration. FFE corrects systematic channel defects and intersymbol interference, hence opening the eye. (c) Eye diagrams for back-to-back transmission after equalization for a 27-bit long PRBS at 24Gbits−1.

Fig. 6. Eye diagrams of the transmission through a 31-m Herriott cell for three different data rates and a sequence length of 27 for the QCD and 215 for the QWIP. (a) QCD: 14Gbits−1 for a BER=0.21%. (b) QWIP: 30Gbits−1 for a BER=0.33%. (c) QWIP: 40Gbits−1 for a BER=2.5%. (d) Evolution of the BER (equalized and nonequalized) with respect to the baudrate for the QWIP transmission. The dashed lines recall BER limits for 7% HD-FEC (green) and 27% HD-FEC (cyan).

Fig. 7. (a) Eye diagram of a B2B equalized 27 PRBS transmission for the QCD with parameters (ntap=391, μ=1×10−4 with no RRC shaping) at 10Gbits−1. (b) Eye diagram of a 31-m equalized 27 PRBS transmission for the QWIP with parameters (ntap=391, μ=1×10−5 with no RRC shaping) at 24Gbits−1. (c) Eye diagram of a 31-m equalized 27 PRBS transmission for the QWIP with parameters (ntap=391, μ=1×10−5 with an RRC shaping with ρ=0.5) at 40Gbits−1.