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Researchers say GaSb suits soft tissue surgery

A bar comprising 20 individual GaSb emitters provides a high-power source that could offer better control over incisions made using laser surgery.

by Marie Freebody
A GaSb-based laser diode bar emitting at 2 µm is ideal for use in soft tissue surgery, say researchers from Fraunhofer Institute for Laser Technology who are developing the technology.

"We obtained a maximum output power of 18-20 W at an operating current of 70 A and a wavelength of 1.92 µm for a bar containing 20 emitters," Konstantin Boucke, head of the laser components department at Fraunhofer ILT, told optics.org.

"Compared with results obtained by other groups and other material systems such as InP, this to our knowledge is an improvement of more than a factor of two."

For surgical applications, the advantage of using 2 µm is that the light is absorbed easily but does not penetrate far. This means that it is easier to control surgical incisions.

"Most research to date has concentrated on GaAs and InP-based diode lasers for the 1.0 to 1.5 µm range," explained Marcel Rattunde from the Fraunhofer Institute for Applied Solid-State Physics (IAF). "The GaSb material family is best suited for development of high power diode lasers emitting at 2 µm. GaSb has a smaller and appropriate band gap energy and band offsets for efficient 2 µm emission."

The individual laser chips were fabricated using MBE at IAF s semiconductor processing equipment. The high-power modules also feature red or green "pilot" lasers to act as a guide for the infrared emission.

Each 10 mm-wide laser bar contains 20 emitters with an emitter width of 150 µm and a pitch of 500 µm. The laser bars were mounted on water-cooled micro-channel heatsinks with a cooling water temperature of 20 °C. "From a fiber-coupled diode laser module - based on two polarization-coupled laser bars - we have obtained an output power of 15 W via a 600 µm fiber core with an NA of 0.22," said Boucke.

According to Boucke, the overall optical efficiency of the two-bar module is less than 40 percent. The team is aiming to increase this to greater than 65 percent but is being hindered by the lack of detectors sensitive to 2 µm. "Around 2 µm there are only few and expensive camera systems available based on Mercury Cadmium Telluride (MCT) or indium antimonide (InSb) detectors, and the resolution of these cameras is not as good as for standard CCD cameras," said Boucke.

The device has not yet been used in surgery. "We are currently planning the first experiments on soft tissue cutting and coagulation properties together with the university hospital in Aachen," commented Boucke.

The next steps are to evaluate the long-term stability and reliability of the laser bars under operating conditions. "We intend to optimize the manufacturing process of the GaSb laser diodes and to further optimize the fiber coupling in the diode laser module," said Boucke.

The team is also considering commercializing the module, with the price of individual modules expected to drop significantly once small series production is established.

About the author
Marie Freebody is a reporter for Optics and Lasers Europe magazine and optics.org.

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