Selin Dogru and Nadir Dagli of the U. of California at Santa Barbara have developed an electro-optic (EO) modulator with bandwidth >67 GHz and drive voltage of 0.77 V. The traveling-wave EO modulator contains a Mach–Zehnder modulator with 0.49-μm-thick, 2-μm-wide undoped multiple quantum well (MQW) waveguides with 33 x 10 nm indium aluminum gallium arsenide (In0.53Al0.08Ga0.39As) quantum wells and 32 × 5 nm In0.52Al0.48As barriers. The modulator has an operating wavelength of 1.55 μm, well away from the device’s photoluminescence peak at 1.37 μm, which thus does not contribute significantlyto propagation loss.
The MQW waveguide core is sandwiched between two indium phosphide (InP) cladding layers; the optical mode resides entirely within MQW and InP cladding. Ohmic losses are reduced by the substrate-removal fabrication method, in which the epilayer is sandwiched by metal electrodes. An experimental device with a 1-cm-long electrode had an extinction ratio of ~3 dB, arising from imperfect coupling of light into the waveguide. Also, a glue layer in the prototype device carried a portion of incoming light all the way to the output, where it was difficult to spatially filter out. Improved future versions will not have this spurious source of constant background light. Reference: Selin Dogru and Nadir Dagli, Opt. Lett. (2014); http://dx.doi.org/10.1364/OL.39.006074.