Optical analyser supports advanced laser gas detection

London – Laser manufacturer nanoplus, based in Gerbrunn, Germany, is using an optical spectrum analyser to verify the high spectral quality of its distributed feedback (DFB) lasers using a spectral characterisation technique based on molecular excitation.

The nanoplus DFB lasers are used, for example, to detect gas leaks, monitor concentrations of explosive gases or to analyse industrial processes. Other application areas include space exploration: a nanoplus laser will soon be used to detect traces of water, methane and carbon dioxide on Mars, on board the NASA Curiosity.

The highly accurate method for the detection of gases in low concentrations is based on the use of laser light. This technique relies on the fact that each type of gas molecule has distinctive vibrational and rotational states, which can be excited by a specific quantity of energy.

With a laser that emits photons of this exact energy, specific molecules inside a gas mixture can be stimulated to “dance”. This process can be observed indirectly by measuring the amount of laser light being absorbed by the gas molecules, depending on their concentration.

To achieve high accuracy in the absorption measurement, a laser is required that emits light only at the energy (and hence frequency) that is relevant for the gas type being examined. The frequency of such a single-mode laser can then be tuned extremely precisely around the molecule’s absorption frequency by a variation of the operating current.

nanoplus develops single-mode lasers which make use of a patented technology for distributed feedback (DFB). For this technique, metal gratings with dimensions in the 100 nm range are patterned next to ridge structures in semiconductor material using electron-beam lithography.

The grating structure acts as a filter for the DFB laser’s resonator modes, allowing only one frequency/wavelength to pass with low loss.

To verify the high spectral quality of the lasers, nanoplus uses an AQ6375 optical spectrum analyser from Yokogawa. This instrument’s accessible spectral range extends well beyond typical wavelengths for telecommunications (around 1700 nm).

As a result, the instrument can be used to characterise DFB laser products, for example, for an application in the sensing of water vapour (around 1877 nm), carbon dioxide (2004 nm) and carbon monoxide (2332 nm).

With the AQ6375, the individual Fabry-Pérot laser modes can be clearly resolved. In addition, the high dynamic range enables the demonstration of single-mode laser operation with a side-mode suppression ratio of more than 50 dB.

The detection of trace gases with concentrations of less than one part per billion in a gas mixture can be achieved with these DFB lasers. To achieve a high throughput of  characterised lasers, nanoplus uses a fully automated test setup using the GPIB interface of the AQ6375.

Despite the relatively complex measurement process, the test setup allows for a quick determination of the lasers’ tuning properties at different chip temperatures and operating currents.

The lasers are tested in the range from 1200 to 2400 nm before they are supplied to the customers, according to their applications.