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17 Jun 2002
Accessing difficult wavelengths will be made easier with a new ultralow-threshold, micron-sized laser.
US researchers have developed a Raman laser with an ultralow threshold that could lead to compact sources for difficult-to-access wavelengths and non-linear optical studies. The laser, based on a silica microsphere, emits at 1670 nm and has a threshold of 86 µW - 1000 times lower than previous microsphere lasers (Nature 415 621).
Kerry Vahala and colleagues at the California Institute of Technology first made a 40 µm-diameter sphere by melting a standard telecoms fiber with a carbon dioxide laser. The sphere was then optically coupled to a tapered fiber, which the researchers say provides a convenient way to transport optical fields between the two.
"In our experiment we left a small air-gap between the fiber and microsphere to ensure the high quality of the resonator modes," said Vahala. "The microsphere can also be made to touch the taper but this increases the threshold power."
To transmit light to the sphere - the laser's cavity - the researchers pumped the fiber with a tunable 1550 nm diode laser. Typically the light orbits the sphere's perimeter in a so-called whispering gallery mode at least 100 000 times before leaving the cavity.
The researchers report an ultralow lasing threshold of only 86 µW at a red-shifted wavelength of 1670 nm. They also achieved a 36 % pump-signal conversion. Vahala says efficiently coupling light to and from the microspheres contributes to the low lasing threshold, but adds that the whispering gallery mode is also crucial to the lasing action.
"Whispering gallery modes have ultrahigh Q-values and very small volumes," he said. "This means that the light can orbit very tightly around the spheres many times which leads to a much higher efficiency in a much smaller device."
Vahala believes he can reduce the lasing threshold even further by decreasing the microsphere diameter to between 10 and 20 µm. Beyond these sizes however, temperature effects destabilize the whispering gallery mode.
The researchers plan to investigate making discs, rather than spheres, so they can produce true singlemode lasing. But while microdiscs are easier to fabricate, Vahala says that matching the quality factor of the microspheres will be a challenge.
The researchers also plan to couple many microsphere lasers to a single fiber for multiple wavelength applications, such as chemical sensing. "You could use this to create a comb of wavelengths which could probe the optical properties of a chemical at several wavelengths," said Vahala.
"And because the device is fiber compatible, it is easily adapted for excitation with low-power semiconductor lasers," he added. "This means the device could be used to extend the wavelength range of semiconductor sources used in telecommunications."
Author
Rebecca Pool is news editor on Optics.org and Opto & Laser Europe magazine.
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