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InP lasers break spectral bandwidth record

Researchers have developed indium phosphide based long-wavelength VCSELs which have a bandwidth of 100nm
Working in collaboration with their partners under the EU’s “Subtune” project, scientists at the TU Darmstadt have developed semiconductor lasers that emit light over a wavelength range of 100 nm.

The researcher say this is a new world record for a single semiconductor laser.

Such lasers might allow more efficient, lower-cost operation of future fibre optic telecommunications networks and the development of high-responsivity gas sensors.

Currently, continuous tuning and mode-hop free tuning over 60 nm is typically achieved at wavelengths around 1.55 µm.

The goal of the Subtune project is to develop InP-based long-wavelength VCSELs ranging from 1.5 µm to 2.0 µm and GaAs-based VCSELs with wavelengths down to 800 nm, to introduce widely tuneable VCSELs in a broad range of the optical spectrum.

The concept is that the active component or so called half-VCSEL, is combined with a bulk-micromachined moveable mirror membrane in a hybrid two-chip assembly. The mirror can be electrothermally actuated to expand the air-gap and shift the cavity resonance towards longer wavelengths.

The eight European partners involved in the Subtune project are:     

The Technische Universität Darmstadt (Germany), The Technische Universität München (Germany), Vertilas GmbH (Germany), Chalmers University of Technology (Sweden), Tyndall National Institute (Ireland), IR Microsystems SA (Switzerland), Commissariat à l’Energie Atomique (France) and Consiglio Nazionale Delle Ricerche, IEIIT (Italy).

Design of TU Darmstadt's semiconductor laser. (Picture Credit: Institute for Microwave Technology and Photonics)

Surface-emitting semiconductor lasers emit light at a right angle to the plane of the semiconducting wafer on which they have been fabricated. They don't need much power to operate and are therefore used as light sources, for example in computer mice and laser printers.

Christian Gierl and Karolina Zogal of the TU Darmstadt’s Institute for Microwave Technology and Photonics have recently significantly extended the tuneabilities of such lasers.

Their approach involves taking advantage of yet another benefit of surface-emitting semiconductor lasers, namely, their very large resonator-length/emitting-area ratios, which greatly increases the spacings of their emitted wavelengths.

Their broad free spectral range allows tuning the wavelength of their strongest emission line over a broad range. It converts them into transmitters whose output may be set to any wavelength falling within a certain, broad range, just as radio transmitters may be tuned to various frequencies/wavelengths.

Gierl, a physicist, and Zogal, a materials scientist, managed to tune the wavelength of the output beam of a semiconductor laser provided by one of their Subtune partners, the Walter Schottky Institute at the TU München, over a range of more than 100 nm.

This, they claim, is the broadest tuning range so far achieved by any semiconductor laser and that these devices have excellent, emission characteristics, such as high output power and high spectral purity, similar to their conventional counterparts.

In order to obtain this broad tuneability, the scientists applied to its emitting surface a flexible membrane having a reflectance exceeding 99 percent at its lasing wavelengths that served as its output mirror, and whose flexing could be externally controlled.

Every laser is equipped with a pair of facing mirrors that reflect light back and forth in order to amplify the laser’s active medium on each pass. The spacing of its mirrors determines which wavelengths from the amplified range will be emitted. The TU Darmstadt duo’s new lasers allow accurately varying that spacing at will and smoothly tuning laser output wavelength over a broad range.

Such lasers might allow more efficient, lower-cost operation of future fibre optic telecommunications networks and the development of high-responsivity gas sensors. (Picture Credit: Institute for Microwave Technology and Photonics)

What's more, the TU Darmstadt duo have said that this fundamentally new technology could be easily transferred to practical applications without a hitch. Their lasers are tuneable over a range centred on 1.55 µm, the wavelength utilised by fibre optic telecommunications systems. They also claim to have developed the world’s first tuneable laser covering a range centred on 2.0 µm.

Gierl explains, “The telecommunications industry is extremely interested in this technology because in the future it will need to service households via fibre optic networks operating at various wavelengths.” If there were no tuneable lasers, a special type of semiconductor laser would have to be fabricated for each wavelength to be involved. He adds, “Tuneable lasers obviate that necessity, since only a single type of laser will have to be fabricated.”

The wavelength range centred on 2.0 µm is of particular interest to sensors for detecting the presence of gases, since it falls within the range where the vibrational modes of molecules, such as CO2, are excited. Gases may be identified and their concentrations determined by means of precision measurements of the wavelengths at which they absorb radiation and the absorption coefficients occurring at those wavelengths.

Gierl also says, “Since that absorption is very strong, gas sensors based on our technology have high responsivities, in addition to being extremely compact and highly energy-efficient.”

He also points out that thanks to their tuneability, a single laser would be sufficient for detecting various gases.

The next goal is to close the remaining gaps for utilisation in practical applications.

According to the two researchers, another benefit of its new lasers is that they are easy to fabricate. As Gierl puts it, “Although the method we employ for applying the membrane directly to the laser is new, we utilised methods that have become well-established in the semiconductor industry for that purpose.” The method involved is microphotolithography, a sort of engraving technique employed for fabricating microchips that allows generating structures having dimensions of the order of a few micrometres.

Gierl goes on to say that, they are able to fabricate chips having numerous, tuneable, surface emitters that meet all of the requirements for the particular applications involved.

A follow-on project is intended to close the remaining gaps in such chips’ readiness for utilisation in practical applications. Closing one of those gaps involves providing that their output may be modulated at high frequencies in order that data may be transmitted at high transfer rates. The researchers also plan to incorporate their chips into modules similar to USB sticks that may be readily integrated into telecommunications systems.

They are already collaborating with the Lawrence Livermore National Laboratory, Livermore, California, and Leister Technologies AG, Kaegiswil, Switzerland, on improving their gas sensors.

The new lasers have already been successfully tested on a communications network at Subtune partner Tyndall.

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