A predistortion-type equi-path linearizer designed for radio-on-fiber system

A predistortion-type equi-path linearizer is created for a radio-on-fiber system based on wide-band code division multiple access; its main benefit is its simple optical circuit configuration. Experiments show that the phase difference between the carrier and third-order intermodulation (IM3) component of the laser diodes (LDs) is shifted by around 90° from that of the RF amplifier. To our knowledge, this is the first report to describe this phase shift, which is observed with both Fabry-Perot and distributed-feedback LDs. To counter this shift, we use additional phase shifters. The proposed design places a pre-amplifier between the linearizer and LD for lower power consumption and better insertion gain at the linearizer. Experimental and calculated results agree well, and more than 20-dB improvement in the IM3 components is obtained over the bandwidth of 60 MHz. The linearizer works well in the temperature range of 10°C to 40°C with a simple control circuit.