Surface-emitting Semiconductor Lasers
Semiconductor lasers can generally be grouped into two classes:
- Edge-emitting lasers, where the laser light propagates parallel to the wafer surface of the semiconductor chip and is reflected or coupled out at a cleaved edge.
- Surface-emitting lasers, where the light propagates in the direction perpendicular to the semiconductor wafer surface.
This article covers only surface-emitting lasers, providing an overview on different types (with the details of those being explained in other articles). Surface-emitting lasers can further be distinguished according to different criteria:
Monolithic vs. External-cavity Lasers
- Monolithic means that the laser is fabricated as a monolithic semiconductor piece, not containing additional elements such as separate mirrors. Such compact devices include VCSELs (vertical cavity surface-emitting lasers) and PCSELs (photonic crystal surface-emitting lasers), as explained further below.
- External-cavity devices, called vertical external cavity surface-emitting lasers (VECSELs), consist of a semiconductor device plus at least one additional external mirror for realizing the laser resonator. There can also be additional resonator mirrors and possibly other optical elements in the resonator. Overall, such a device looks similar do a solid-state bulk laser, except that the gain medium is a semiconductor rather than a doped insulator.
In any case, many monolithic lasers or gain chips for VECSELs can be fabricated together on a single wafer, which is sliced into many pieces.
Monolithic surface-emitting lasers may also be fabricated as two-dimensional laser arrays (e.g. VCSEL arrays) for generating much higher total output powers.
Vertical Cavity vs. In-plane Cavity
Vertical emission (i.e., an output laser beam perpendicular to the wafer surface) can fundamentally be achieved in two different ways:
In vertical-cavity lasers, the intracavity laser light also essentially propagates in directions perpendicular to the wafer surface. The output beam is obtained simply by partial transmission through an output coupler mirror.
Practically, that approach implies that the path length in the semiconductor gain medium is rather short – on the order of micrometers. The laser resonator may also be very short for a monolithic device (VCSEL), or much longer for an external-cavity laser (VCSEL). In any case, the available round-trip laser gain is quite limited, typically to at most a few percent. It is therefore necessary to realize a laser resonator with relatively low losses, i.e., using Bragg mirrors with high reflectivity.
There are (so far less common) lasers where the intracavity laser light essentially propagates in a plane along the wafer surface. That approach allows one to combine some of the advantages of edge-emitting and surface-emitting lasers: The active area and thus the output power can be much higher, also the round-trip gain due to the longer propagation length in the gain medium. This creates the potential of generating watt-level output power in combination with spatially single-mode emission, i.e., high beam quality.
There are different ways of implementing an in-plane laser, which may also be called horizontal cavity surface-emitting laser (HCSEL):
- One may use a structure similar to a edge-emitting laser, i.e. with a waveguide structure along the wafer surface, but complemented with an additional structure which reflects the output light upward in order to achieve vertical emission of the output.
- Another realization is the photonic crystal surface-emitting laser (PCSEL), where the in-plane laser resonator is realized with a photonic crystal structure. The same structure also reflects some of the light such as to form the output beam. Here, one may even achieve single-frequency operation, which is important, for example, for applications in 3D sensing and optical data transmission.
Electrical vs. Optical Pumping
One of the most important attractions of semiconductor lasers is that most of them can be electrically pumped. That is the most convenient and cheap way of providing the required power. This approach is also realized in most of the surface-emitting semiconductor lasers.
However, in some cases one uses optical pumping – usually with radiation from another semiconductor laser –, because that also has some advantages:
- It is substantially simpler to produce the semiconductor structure if one does not require electrical contacts. Also, optical pumping eliminates the requirement of sufficiently high electrical conductivity, so that undoped semiconductor media can be used, which are often also advantageous in terms of light absorption. One may thus produce devices with substantially simpler technology.
- Optical pumping allows one to ideally distribute the energy input in the spatial dimensions. In particular, it is easy to uniformly pump a larger area, whereas electrical pumping with a ring electrode around an active area properly works only for a limited ring diameter. Otherwise, one would need transparent electrodes, which are difficult to realize.
The advantages of optical pumping are particularly realized in high-power VECSELs. These can work with much larger active areas than electrically pumped VCSELs and thus produce much higher output powers, while still maintaining diffraction-limited beam quality due to the external cavity.
The highest electrical-to-optical efficiencies, partly well above 60%, are achieved by some electrically pumped lasers, and that kind of efficiency is hardly achievable with optical pumping. At least, the optical-to-optical efficiency of an optically pumped laser can be quite high, and despite the overall lower power conversion efficiency, the thermal handling can be simplified, since the pump laser does not need to have a very high beam quality and can thus be of a robust type.
The concept of surface-emitting semiconductor lasers offers various advantages, comparing with edge-emitting lasers:
- For a given optical output power, the emitting area is substantially larger, which implies correspondingly low optical intensities. Therefore, the challenge of catastrophic optical damage (COD), which is a limiting factor for many edge-emitting lasers, is effectively eliminated.
- The technical difficulties of carefully processing the cleaved outer sides of semiconductor gain chips are also eliminated, since the light does not go through those edges. Overall, the fabrication is tentatively simpler and thus potentially faster and cheaper.
- It can also be convenient that many gain chips, which are fabricated together on a large wafer, may be tested even before separating them.
- The often very short laser resonator makes it relatively easy to achieve single-mode emission, and this over a wide range of operation parameters like pump power and temperature.
- The emission wavelength is often not determined by the gain maximum of the semiconductor material, but by an optical resonance. That often leads to a substantially smaller temperature coefficient of the emission frequency.
The RP Photonics Buyer's Guide contains 11 suppliers for surface-emitting semiconductor lasers. Among them:
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See also: semiconductor lasers, vertical cavity surface-emitting lasers, vertical external-cavity surface-emitting lasers, photonic crystal surface-emitting lasers, edge-emitting semiconductor lasers
and other articles in the categories optoelectronics, lasers