30 Apr 2007
An electrically pumped photonic-crystal laser with improved tuning sensitivity could be used for free-space communications.
Researchers at the California Institute of Technology have made the first demonstration of an electrically pumped, large-area, edge-emitting photonic crystal laser that operates at room temperature. The laser, which allows the lasing wavelength to be tuned with much greater sensitivity than other photonic-crystal designs, could be used as a high-quality pump laser or as a source for free-space communications.(Optics Letters 32 1256)
Until now it has been difficult to obtain this type of laser with narrow spectral line widths and narrow beam divergences in a large area. "It is the first demonstration of an electrically pumped, large-area, edge-emitting photonic crystal laser. We show that the lasing mode is truly defined by both the longitudinal and transverse Bragg conditions by systematic lasing wavelength tuning," Lin Zhu from the Department of Electrical Engineering told optics.org.
In addition, the team claims a tuning sensitivity 30 times smaller than that of a regular distributed feedback (DFB) laser or a photonic-crystal laser with large index contrasts. The laser also has narrow spectrum line width of 0.7 nm, while a 1.7x diffraction-limited far-field divergence angle of 1.8 degrees is obtained at a 3.5x current threshold for devices measuring 100x480µm. "These lasers are very good candidates for high-power (large-area) lasers with good spectral and spatial beam quality (PC control)," said Lin.
The team achieved a tuning sensitivity of 0.08 by using lithography to change the photonic-crystal lattice constants in the transverse direction. The transverse wave vector is much smaller than the longitudinal wave vector, which means that relatively large changes in the transverse constant enables fine tuning of the laser wavelength.
"Tunability is important for multiple-wavelength communications and sensing. The transverse lasing-wavelength tuning sensitivity is much smaller compared to a DFB laser. This allows much better and easier control of the lasing wavelength," added Lin.
Conventional photonic-crystal research focuses on small cavities with large differences in refractive index. "In comparison, our research uses a weak-index perturbed, polymer-planarized, surface photonic-crystal structure to control the lasing mode in a large area."
The photonic-crystal structure is first written onto the wafer surface using electron-beam lithography, and is then etched into the InGaAsP semiconductor for around 360 nm to obtain the desired index contrast. An etch-back process is then used to create a polyimide "post" inside each etched hole, which separates the etched holes from the subsequently evaporated metal contact to reduce optical losses and help obtain a good contact quality.
The team now plans to develop a single-mode continuous wave laser that operates at room temperature and has a single-lobed, diffraction-limited far field. Key difficulties to be overcome include reducing the current threshold while maintaining a single-lobed, diffraction-limited far field as well as thermal control and confinement of the electrical current.