An integrated magneto-optic (MO) Bragg cell modulator incorporating a pair of curved hybrid lenses in a tapered yttrium iron garnet—gadolinium gallium garnet (YIG-GGG) waveguide substrate was developed by Cheh Wang and Chen Tsai from the US Department of Electrical and Computer Engineering and Institute for Surface Science, University of California (Irvine, CA). The curved hybrid lens pair provides near-diffraction-limited focal spot size and relatively large angular field of view at greatly reduced coma.
The focused diffracted light beam was scanned by varying the carrier frequency of the magnetostatic forward volume wave (MSFVW). Radio frequency (RF) spectral analysis was performed by simultaneous application of multiple microwave frequencies. The tapered waveguide structure proved capable of satisfying the requirements for large MO Bragg bandwidth as well as good lens performance.
Magnetostatic wave modulators have several advantages over the more conventional acousto-optic Bragg cell modulators, says Tsai, who worked with acousto-optic Bragg cells for many years. Magnetostatic waves are slow electromagnetic waves that can propagate over limited bandwidths at microwave frequencies in material such as YIG. Garnets are of particular interest because of their ability to tune to specific electronic frequencies depending on the strength of an applied magnetic field. Guided-wave optical modulators using Bragg MO interaction in YIG-GGG waveguides can process wideband RF signals at carrier frequencies from 0.5 to 40 GHz. This is ten times higher than possible with acousto-optic modulators.
Another advantage, says Tsai, is that the carrier frequency is electronically tunable. When the magnetic field is changed, the frequency is altered. An acousto-optic modulator cannot be directly tuned. Also, a MO Bragg cell modulator easily produces a higher bandwidth.
Tsai and Wang constructed an MO Bragg cell modulator by incorporating a pair of identical microstrip line transducers in the central section of the taper waveguide. They inserted the finished modulator into a compact magnetic housing to provide the required bias magnetic field for saturation of the YIG layer and excitation of wideband MSFVWs.
A magneto-optic modulator is composed of a simple microstrip transducer, making it simpler to fabricate than an acousto-optic modulator. The MO Bragg cell modulator also has a high switching speed--the time required to switch from one terminal to another. "The velocity of propagating magnetostatic waves is 10, 100, 1000 times faster than the velocity of acoustic wavelengths," says Tsai. Therefore, the time required to switch from one pulse to another pulse is at least 10 times faster. A natural application for the MO modulator, therefore, would be as a high-speed multiple-port optical switch.
Future work will involve optimization of device parameters. A larger MO Bragg bandwidth, for example, could be achieved by using a microstrip transducer of smaller linewidth than 60 µm. A higher diffraction efficiency might be accomplished by using a longer interaction length and/or a nonuniform bias magnetic field.