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02 Dec 2005
A look at the scientific innovations reported in journals in November including measurements of femtoforces.
Ceramic lasers
A Nd:YAG ceramic laser with a record-breaking slope efficiency of 62% has been reported by scientists in China. The team from the Shanghai Institute of Optics and Fine Mechanics surrounded a 5mm-diameter, 75 mm-long doped ceramic rod with nine QCW laser diode arrays emitting at 808 nm. The ceramic laser generated 236 W for a pump power of 45O W, corresponding to an optical-to-optical conversion efficiency of 52.5% and a slope efficiency of 62%.(Optics Express 13 8725)
Optical traps
An ultraweak force of 25 femtoNewtons, the smallest force ever directly measured, has been experimentally analysed by a scientist in Germany. Alexander Rohrbach from the European Molecular Biology Laboratory in Heidelberg used a 0.53 micron-sized latex bead held in an optical trap to make the measurement. The force was generated by the radiation pressure from a 11 mW blue (488 nm) laser beam that illuminated the bead. According to Rohrbach, such femtoforces are commonly found in nature. Examples include the force exerted by daily sunlight on a small dust particle in the atmosphere, or the friction that that a bacterium has to overcome when travelling through water. (Optics Express 13 9695)
Underwater imaging
An underwater optical radar that can generate 3D images of an object at a range of about 10 m in clear sea water has been built by Italian researchers. The device consists of an amplitude-modulated blue (405 nm) laser diode, an aspheric lens mounted on a pizeoactuator-based scanning system and a photomultiplier tube detection system. Luciano Bartolini and his colleagues from Ente Nazionale Energie Alternative (ENEA) in Rome say that in laboratory conditions the scheme provided a sub-millimeter accuracy at a laser modulation frequency of 36.7 MHz. The team is now exploring the use of lasers that can be driven at higher frequencies and output powers in order to improve the imaging performance. (Applied Optics 44 7130).
Diffractive optics
Thin diffractive optical elements known as “photon sieves” could potentially be the answer for creating ultralarge (>20 m) optics for space-based telescopes, according to a scientist from the US Air Force Academy in Colorado. Geoff Andersen from the Academy’s Laser and Optics Research Center has fabricated such a sieve and analysed its imaging performance. The sieve is made from a 2.5 mm-thick quartz plate that has a chrome coating perforated with over 10 million tiny (10-279 micron diameter) holes. The 10 cm diameter element has a focal length of 1m and offered diffracted-limited focusing of green light. Work is now looking at producing a broadband version. (Optics Letters 30 2976).
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