NTT Laser Senses Carbon Monoxide
Researchers from NTT Photonics Laboratories have made an InP-based continuous-wave laser that they say emits at the longest ever wavelength reported for such a device.
The laser, described in Electronics Letters on October 11, emits 2.33 µm infra-red light from three 5 nm InAs quantum wells sandwiched between four 5 nm-thick In0.53Ga0.47As barriers.
“This laser will be useful as a light source for CO sensing because its wavelength agrees well with the CO absorption line," say the authors.
The Japanese company is now looking to deploy the laser in commercial detectors that sense CO before the gas reaches lethal concentrations, according to a report on Nikkei.net.
Nikkei.net says that NTT s sensors should be able to detect CO concentrations down to 10 parts per million, whereas the permissable exposure limit for CO is 50 parts per million.
For InAs multi-quantum well (MQW) lasers fabricated on InP substrates, attaining wavelengths longer than 2 µm is difficult because the two materials are lattice mismatched by about 3.2 percent. This makes it difficult to grow the thicker InAs layers that allow longer wavelength lasing.
To make the new laser, the NTT group initially grew a 100 nm-thick InGaAsP guide layer on top of the substrate. This helps to contain the emitted light in a separate confinement heterostructure (SCH) laser design.
This layer and the first InGaAs barrier "“ immediately above "“ were both grown by MOVPE at 500°C. After this the researchers grew the InAs quantum wells at 620°C.
When the MQW structure was finished, a second 100 nm InGaAsP layer completed the SCH design, and finally a p-InP layer was deposited to make the laser essentially symmetrical. The reported device operates with threshold current and threshold current density of 560 mA and 1.56 kA/cm2 respectively.
NTT Electronics has already started shipping sample lasers to equipment manufacturers but prototypes cost thousands of dollars, Nikkei.net says. This report also says that NTT is confident that volume manufacturing will ultimately bring prices down enough to see its lasers used in residential alarms.