Nested loop feedforward linearization of directly modulated laser diode

A nested loop feedforward linearization scheme is applied to an uncooled laser diode. This scheme in effect performs linearization twice, thus leading to further reduction of third-order intermodulation products when compared to conventional feedforward linearization. Results are presented for a DFB laser transmitter operating within the 2.4 GHz ISM band. For a two-tone spacing of 20 MHz and 10 dBm input power, conventional feedforward linearization results in third-order intermodulation products at -32.5 dBc as compared to -46.5 dBc for the nested scheme. However, the latter topology requires four cancellation loops as opposed to two for conventional feedforward. As a compromise solution, one of the loops is removed from the nested structure in order to reduce the component count, and in this case third-order intermodulation products are at -43.5 dBc, which is an acceptable trade-off between complexity and distortion reduction.