Hello, everyone!
Today, I would like to share with you an
article about CML transmission performance.
The experimental setup is shown in Figure 1. A commercially available DFB laser at 1548.9 nm is directly modulated at 40-Gb/s using a PRBS with a word length of 27-1 or 231-1 generated from SHF 50 GHs pattern generator (SHF 12100B). The laser is biased at 94 mA and driven by 2.7 V (peak-to-peak) to produce 9 dBm average power and ~ 11 GHs of adiabatic chirp.
The bias and drive voltage are optimised for the best BER performance after transmission. After the DML, a tunable optical filter (TOF) with 3-dB bandwidth of 0.32 nm and 20-dB bandwidth of 0.76 nm is used as the OSR filter to generate the desired chirp- managed signals.
The optical eye diagrams before and after the filter are inserted in Figure 2. The extinction ratio of the DML output before the OSR filter is 1.3 dB and increased to 5 dB, after the OSR filter. The optical spectra before and after the OSR filter are shown in Figure 3. After the OSR filter, the CML signal is launched into different lengths of standard SMF-28 fiber.
Figure 1. Chirp-managed 40-Gb/s transmission experimental setup. (TOF: tunable optical filter.)
Figure 2. Received optical spectra with 0.01-nm resolution before and after OSR filter.
Figure 3. Measured BER curves of 40-Gb/s CML signals in different distance and PRBS length.
The dispersion and loss at 1548.9 nm of this fiber are 17 ps/nm/km and 0.2 dB/km, respectively. The receiver consists of an EDFA pre-amplifier and a 50 GHs PIN photodiode. Another TOF with 3-dB bandwidth of 1.4 nm is used to reduce amplified spontaneous emission (ASE) noise from the EDFA. A SHF 50 GHs error analyser (SHF11100A) is used to measure the BER performance.
The clock signals for the SHF error analyser are directly obtained from the pattern generator. Figure 3 shows the BER performance for the 40-Gb/s CML signals transmission over different fiber lengths with different patterns. When the PRBS pattern is 27-1, the receive sensitivity at a BER of 10-9 after transmission over 14.8 km and 15.8 km are -
17.6 and -16.5 dBm, respectively.
Increasing the pattern length to 231-1, the BER increases to 10-8 after transmission over 15.8 km at -16.5 dBm received power. The pattern dependence penalty is mainly due to the low frequency thermal chirp of the DFB, which is not compensated in this experiment.
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