Abstract
A theoretical and experimental investigation of the effects of high-order
nonlinear distortion products produced by directly modulated semiconductor
lasers on the performance of high-loss analog optical communication links
requiring large RF dynamic range is reported. In order to provide sufficient
RF dynamic range to support radio services in links with high optical transmission
loss, for example in radio over free-space optics (RoFSO), while keeping costs
low, it is necessary to use directly modulated lasers. However, in these applications
the lasers must be driven to high modulation depths to maximize dynamic range.
Simulations show that under these unique conditions the first detectable nonlinear
distortion is often the result of dynamic distortion due to the laser being
driven near threshold. It is shown that this type of distortion is characterized
by a sharp increase in the contribution of high-order (fourth order or greater)
nonlinear terms resulting from the influence of laser relaxation oscillations.
As a consequence, the third-order spurious-free dynamic range (SFDR) metric
no longer accurately reflects the performance of such links as it assumes
that third order effects are dominant. An alternative measure of dynamic range
called dynamic-distortion-free dynamic range (DDFDR) is proposed. This differs
in that the upper limit is defined as the modulating power at which the peak
optical modulation index (OMI) reaches unity. At this point the error vector
magnitude (EVM) measured for a range of different wireless services starts
to increase rapidly due to high order distortion. This makes DDFDR a practical,
service-independent metric of dynamic range. For two different wireless services
it is observed experimentally that on average the DDFDR upper limit predicts
the EVM knee point to within 1.1dB, while the third-order
SFDR predicts it to within 6.2 dB. The DDFDR is thus shown to be a more accurate
indicator of real link performance when high-order distortion is dominant.
© 2011 IEEE
PDF Article
More Like This
Cited By
You do not have subscription access to this journal. Cited by links are available to subscribers only. You may subscribe either as an Optica member, or as an authorized user of your institution.
Contact your librarian or system administrator
or
Login to access Optica Member Subscription