Researchers close in on easy-to-make lasers for optical fibre communications

Researchers close in on easy-to-make lasers for optical fibre communicationsSteve Bush For the first time, a long wavelength vertical-cavity surface-emitting laser has been grown directly on a simple GaAs substrate. Previous successful attempts have involved complex processes like substrate back-etching and wafer to wafer bonding. In this case, the emissive quantum well is GaInNAs within a GaAs cavity. The laser’s distributed Bragg reflectors are both made from alternating layers of GaAs and AlAs. The top has 21 periods, the bottom 25.5 The whole thing sits on a N-type GaAs substrate. Emission is at 1.18?m and its designers now see a route to a version for use in the 1.3?m optical fibre transmission window.
Hitachi’s Central Research Laboratory made the laser and the work was performed under the management of Real World Computing as part of the MITI program ‘Interconnection for the high-density data bus’. Why go for a vertical structure? Conventional semiconductor lasers are made horizontally in a wafer and are long (0.5mm) rectangular structures. Once they are cut from the wafer, two edges of each chip are polished to form the end mirrors for the laser cavity. The chips then edge-emit. Ignoring technical performance, this method of construction has several practical disadvantages. The structures cannot be packed very closely together on the wafer, limiting the number that can be made at a time; edge polishing is a fiddly process, outside mainstream semiconductor processing, and edge-emission means that non-standard chip mounting is required if an optical connector is to be mounted on top of a package. On the other hand, edge emission is an advantage if a laser is to inject into surface optical waveguides, like those in Bookham Technology’s optical Asics.
Vertical cavity lasers pack tightly on to wafers (<0.1mm centres), do not require edge polishing and emit at right angles to the wafer surface. How they work Avertical-cavity surface-emitting laser (VCSEL) consists of a cavity and, occupying the bulk of the structure, a pair of distributed Bragg reflectors (DBRs). The cavity is a sandwich. Its filing is a very thin quantum well. Above is a P-doped semiconductor, below is equal thickness of N-type. The cavity is one wavelength thick and forms a P-I-N diode where photons are emitted during the recombination of carriers. Above and below the cavity are the Bragg reflectors. Making a reflector using semiconductors is not easy. Bragg reflectors use the reflecting effect caused when materials of two different refractive indices are in contact. A large difference in reflective index makes a good reflector, but semiconductors have small differences. The answer is to have a stack of interfaces, each one wavelength apart – a DBR.
An electrode is plated onto the top of the upper reflector. The substrate is the other contact and emission is through the bulk of wafer and out of its back.


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