Research round-up

29 Nov 2004

A look back at some of the optical innovations unveiled this month.

VCSELs

Researchers in Sweden have fabricated long wavelength VCSELs emitting 1 mW of single mode power. The 1265 nm device is said to maintain a stable output power between 10 - 140°C. The team from the Royal Institute of Technology (KTH) and Zarlink Semiconductor believe the VCSEL will find applications such as 10 GBit Ethernet where single mode emission is required. The source comprises an InGaAs VCSEL which has a thin, patterned silicon layer on the top distributed Bragg reflector. The silicon layer acts as a mode filter by promoting the fundamental mode and suppressing higher-order modes. (Applied Physics Letters 85 4851)

CERAMIC LASERS

With the goal of producing high-average-power diode-pumped solid-state lasers, a pair of Italian scientists have published their initial studies into ceramic Nd:YAG slabs as the lasing media. Their initial device produces more than 160 W and has a slope efficiency of 20%. The 160 x 100 x 60 mm-sized setup uses a polycrystalline ceramic YAG slab which is side-pumped by two diode bars emitting at 802 nm. Using their experimental data together with finite-element simulations, the pair from Italy's Istituto di Ottica Applicata in Florence believe that the power extraction can be scaled up to 900 W using the same laser head geometry. (Applied Optics 43 6174)

CAVITY RING-DOWN SPECTROSCOPY

Single-cell, label-free detection of biological agents is possible using an optical fiber cavity ring-down spectroscopy (CRDS) resonator, according to a team from Princeton University's department of chemistry, US. The setup uses a fiber with a tapered section, produced by heating and mechanically drawing the fiber. According to the authors, this transforms the core mode into a cladding-dominated mode containing an evanescent portion that interacts with the external environment. The taper is coated with a specific polypeptide which binds to one type of target cell. Excited by laser light at 1520 nm, the optical sensor traps the target cells in the localized evanescent field. (Applied Physics Letters 85 4523)

LEDs

Three researchers have come up with a way to measure the temperature profile of the junction of visible LEDs using nematic liquid crystals (NLCs). A 540 nm LED is coated with an NLC and is examined under an optical microscope. Highly polarized light from a 780 nm laser diode is launched onto the sample through the microscope objective lens. "When the LED surface is heated above the transition temperature, the liquid crystal changes from anisotropic to isotropic," explain the authors. "The linearly polarized light that enters into and is reflected from the NLC will not change the polarization orientation, and the region appears dark under the optical microscope because of a cross-polarizer inserted into the optical path." The team can then monitor both the efficiency and electrical power versus temperature. (Optics Letters 29 2656)