An optical communications subsystem is proposed to permit the multiplexing of multiple, parallel electronic data streams onto a serial, very high speed optical data channel. The subsystem may also be used to generate programmable ultrafast optical data words for the testing of optical components, and system performance testing of very high speed data transmission systems. The key device component, based on a modified arrayed waveguide grating structure, is directly integratable with a high-speed optoelectronic modulator array in a simple, cost effect, and manufacturable configuration. Pulse spacings as small as 1 picosecond have been demonstrated corresponding to an effective data rate of up to one terahertz. An integrated optical pulse generator is configured to receive a laser light input and output an optical pulse train. Direct space-to-time pulse shaping and optical pulse train generation is achieved by use of an arrayed waveguide (AWG) that is double-passed. A mask is utilized for time domain pulse shaping that is employed after a single pass through the arrayed waveguide. In the case of an optical data/word generator, a spatially patterned mask translates spatial data, for example representing binary data or a binary word, of the mask to the output optical pulse train. The arrayed waveguide (AWG) system has waveguide ports that double as inputs and outputs, and provides direct space-to-time pulse shaping of a single, short pulse laser/optic signal. Direct optical access to individual guides in the waveguide array allows one to control the light intensity in each guide and therefore control the output pulse intensities with a one-guide one-pulse effect.