This technical article describes an application example of FiberLabs PDFA for enhancing the link budget of 100GBASE-LR4 transmission. We show that, by adding a FiberLabs PDFA (AMP-FL5601-OP15) as a preamplifier, the link budget can be increased by 7 dB.

Experiment and result

The benefits of adding a preamplifier PDFA was evaluated by conducting a series of BER measurements in the following two conditions:

  • Back-to-back
  • With preamp PDFA (FiberLabs AMP-FL5601-OP15)

The experimental setup and results are summarized in Figure 1. The BER measurements were conducted using a 100GBASE-LR4 transciever (Gigalight GQS-SPO101-LR4C) and a QSFP optics checker (Gigalight 40G QSFP+ & 100G QSFP28 Optics Checker). A variable optical attenuator was used to change the signal power launched to the receiver or preamplifier. BERs were measured as a function of signal power before receiver (back-to-back), and as a function of power before preamplifier (with PDFA). The signal pattern was PRBS31.

Figure 1: Setup of experiment and results.

With the addition of a preamplifier PDFA, the minimum power for meeting the IEEE specification (BER<10-12) was decreased from -12 dBm/lane to -19 dBm/lane, providing 7-dB improvement. The maximum attenuation for meeting the IEEE spec were 13 dB (back-to-back) and 20 dB (with preamp), showing that the link budget was increased by 7 dB.

Comparison to booster amplifier configuration

The link-budget enhancement achieved by this preamplifier configuration is somewhat lower than the booster amplifier configuration. This is mainly because the BERs were limited by noise figure (i.e. ASE noise) of PDFA in the preamplifier configuration. We justify this assumption by our experimental observation that further increasing the preamplifier gain (>15 dB) did not result in BER improvement.

It is therefore preferred to use a PDFA in the booster configuration for best performance, while the preamplifier configuration may be preferred if a high signal power from a booster PDFA (~100 mW) is a concern.