Photonic Crystal Lattice Controls Laser Polarization
By applying a photonic crystal lattice and changing the shape of the unit cell structure, researchers in Japan have developed a surface-emitting laser that promises higher output powers and better control of the lasing modes than either vertical-cavity or circular grating-coupled surface-emitting lasers.
Researchers have demonstrated control of the polarization modes from a semiconductor laser through the design of a photonic crystal incorporated into the structure. Courtesy of Susumu Noda.
"The vertical-cavity surface-emitting laser is now recognized as a very important light source for local area networks," said Susumu Noda, a professor in the department of electronic science and engineering at Kyoto University in Japan, and a researcher on the project. "However, there is a limitation of available power, typically less than a few milliwatts for complete single longitudinal and lateral modes."
To overcome this problem, Noda and researchers from the university, Minolta Ltd. of Takatsuki and Japan Science and Technology Corp. of Kyoto incorporated a square photonic crystal lattice in the laser structure. Using a wafer-fusion technique, they formed the lattice on an N-type InP substrate, to which they bonded an InGaAsP/InP multiple-quantum-well active layer sandwiched between N- and P-type InP cladding layers.
The photonic crystal enabled them to form a stable laser cavity in which the crystal controlled the propagation of light in the X- and Y-directions, yielding a two-dimensional standing wave, Noda explained. Moreover, the lattice points determine both the lasing wavelength and the beam pattern.
"Thus, even though the surface area of the device becomes very large, single longitudinal- and lateral-mode oscillation becomes possible," Noda said.
The polarization modes created by the square photonic-crystal lattice are complicated when the structure of the unit cells is circular, Noda said, and controlling polarization is one of the remaining hurdles to creating high-power semiconductor lasers that display perfect single-mode output over a large area and surface emission with a very narrow divergence angle. By changing the shape of the cells in the crystal lattice from a circle to an ellipse, the scientists have generated linearly polarized emission.
The team hopes to improve the design of the new photonic crystal laser for telecommunications applications.
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