A transmitter optical sub assembly includes a laser source having associated therewith a laser driver and a package for housing the laser source (10) and the laser driver. The package includes respective separate compartments for the laser source and the laser driver, wherein a package wall, which is opaque to IR emission, separates the laser source from the laser driver. The laser source and the laser driver are arranged in those separate compartments whereby the laser source is exempt from being directly irradiated by the laser driver.