This technical article describes an application of FiberLabs PDFA for enhancing the link budget of 100GBASE-LR4 transmission. We show that adding a FiberLabs PDFA (AMP-FL5601-OB16) as a booster amplifier

  • increases the link budget by more than 10 dB, and
  • does not add power penalty.

 

Experiment and result

The benefits of a booster amplifier was evaluated by conducting a series of BER measurements in the following two conditions:

  • Back-to-back
  • With booster PDFA (FiberLabs AMP-FL5601-OB16)

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 to the receivers, and BERs were measured as a function of received signal power (each lane, not total power).

 

Figure 1: Setup of experiment and results.

Two graphs (receiver power vs BER) are almost identical even with the addition of booster PDFA, showing that the PDFA did not add power penalty. Of course, the link with PDFA accommodates higher attenuation as a result of amplification. The maximum attenuation for meeting the IEEE specification (BER<10-12) was increased from 13 dB to 24 dB, showing that the link budget was increased by 11 dB.

Other applications

The high output power and low noise figure of a PDFA allowed the enhancement of link budget without power penalty. This scheme can also be applied to extended-reach transceivers (e.g. 100GBASE-ER4, 100G ER4-lite, and 100G 4WDM-20/40), as they are based on high-sensitivity receivers, and not high-output transmitters. The maximum reach may be extended to >60 km with the simple addition of a booster PDFA.

Other than link budget enhancement, the use of booster PDFA may hugely benefit transceiver testing (e.g. receiver stress test), or branching-loss compensation for network monitoring, etc.

So far, PDFA is the only practical choice if you need to simultaneously amplify the LR4 signal to a high power (>+15 dBm). SOA may be used as a preamplifier but not as a booster amplifier, as the saturation output power of SOA is typically below 10 dBm. Another critical reason an SOA cannot be used for this application is nonlinear effects, which result in significant BER degradation.