Computers, cell phones and XBoxes are powered by the ability of electrons to control electrons. In the world of photonics, the challenge is to develop the ability of photons to control photons. Researchers at Tampere University of Technology in Finland have demonstrated the efficient modulation of a signal beam with a saturable absorber in a monolithic semiconductor structure.Their goal was to develop a novel intensity modulator that exploits saturable absorption -- which can be actively controlled -- in multiple quantum wells. When used with a mode-locked fiber laser, such a device would provide both passive pulse shaping and active pulse synchronization to an external radio frequency signal.In the monolithic optically controlled intensity modulator, the input beam is absorbed in the quantum-well region with the control laser off. With the control laser on, the saturable-absorption region is bleached, and the input beam reflects out. Essentially, the semiconductor structure is a conventional edge-emitting laser diode with a window in the top. The input beam enters the window perpendicular to the intracavity control beam. If the intracavity laser is off, the input is absorbed by the quantum wells in the active region of the laser. If the control laser is on, the intracavity photons saturate the absorption, and the input beam passes through the quantum-well region, reflects off a distributed Bragg reflector fabricated below the edge-emitting laser and exits.The researchers used the device to synchronize a passively mode-locked pulse train to an external signal and discovered that the modulation index was sufficient to actively mode-lock a fiber laser. With some improvements, the modulator, which to date has displayed a maximum modulation index of about 20 percent, could be used for applications such as optical processing.Team member Mircea Guina said that optical saturation of absorption could be exploited to build efficient vertical optical modulators. He believes that the concept behind the modulator lays the foundation for a new class of optoelectronic integrated circuits that would generate and control light beams. The next step is to improve the design to offer higher modulation index and speed, broader bandwidth and smaller threshold.