Appl. Phys. Express 8, 022701 (2015)

Introducing an active external cavity has been shown to dramatically increase the bandwidth of a distributed feedback (DFB) laser diode. By adding a 220-μm-long feedback section that ends with a high reflectivity mirror to a DFB laser, researchers in Japan experimentally realized a light source with a 3 dB bandwidth of 40 GHz. Furthermore, theoretical simulations predict that this figure could be pushed to as high as 70 GHz following optimization. The increase in bandwidth is due to cross-gain modulation and feedback from the external cavity. The feedback section contains a waveguide, a part of which uses electro-absorption to control the intensity and phase of light travelling through it. The InGaAlAs device emits light in the telecoms window at 1,545 nm. The team at Tohoku University and NTT Photonics Laboratories says that the device could be well-suited as an all-optical high-speed wavelength converter or an optical NOT logic gate for optical signal processing circuitry.