Research round-up

09 Mar 2005

A look at some of the innovations in optics unveiled this month.

POLLUTION SENSOR
An optical waveguide chip that senses multiple organic pollutants could soon find use in environmental, clinical or security applications. Developed by a team from Germany and the UK, the fiber-pigtailed chip consists of a channel waveguide circuit that distributes evanescent light to 32 separate patches each sensitive to a specific water pollutant. The result is rapid, simultaneous and highly-sensitive fluorescence detection from the semiconductor laser powered device. The team has demonstrated its sensor for the key pollutant estrone, achieving a detection limit of less than 1 ng/L. (OPTICS EXPRESS 13 1124)

AVALANCHE PHOTODIODE
Researchers in the US have made a silicon carbide (SiC) based avalanche photodiode (APD) that is sensitive over the UV range 250 - 370 nm. The team used the APD to perform single photon counting at room temperature. Dubbed SPAD and based on a 4H-SiC wafer design, the device measures 160 x 160 µm. Scientists from Rutgers University, NASA-Goddard Space Flight Centre and United Silicon Carbide measured a photon count rate of around 4.5 MHz when evaluating their device with a commercial UV LED source. The photodiode's quantum efficiency was observed to peak at 270 nm. (ELECTRONICS LETTERS 41 212)

DFB LASER
Scientists in Japan have created a tunable distributed feedback (DFB) laser from an organic polymer waveguide. The team from Kyoto Institute of Technology tunes its laser between 484 and 508 nm by rotating an assembly formed by the waveguide and an aluminium mirror. The waveguide is made from a polymer-based solution that is spin-coated onto a glass substrate and left to dry under ambient conditions for 6 hours. Measuring 0.6 µm, the film supports TE0 mode lasing when pumped with 30 ps pulses from a frequency tripled Nd:YAG laser (355 nm) at a repetition rate of 10 Hz. (Applied Physics Letters 86 061101-1)

YELLOW-ORANGE LIGHT SOURCE
An all solid-state yellow-orange light source developed by the Technical University of Denmark and the Swedish technology institute KTH could suit a range of biomedical applications. Such yellow-orange sources are attractive as their emission matches the absorption widow of haemoglobin in blood. The team's diode laser pumped all-solid-state system generates more than 750 mW of yellow-orange (593.5 nm) continuous-wave output by intra-cavity sum-frequency mixing the 1064 and 1342 nm lines of two Nd:YVO4 lasers. Conventional sources such as copper vapour lasers or various dye lasers are said to be inconvenient due to their large size and requirement for water cooling. (OPTICS EXPRESS 13 1188)

DATA TRANSMISSION
Researchers at the University of Paderborn, Germany claim to have achieved a data-rate of 5.94 Tbit/s in the C band over 324 km of optical fiber. According to the team, this is the largest data rate transmitted in a single optical band over such a distance. The transmission consists of four fiber spans and features Raman amplification and polarization multiplexing. (IEEE Photonics Technology Letters, April issue)