A VCSEL is a vertical cavity surface-emitting laser – a kind of laser diode which emits in a direction perpendicular to the wafer surface – in contrast to the more common edge-emitting semiconductor lasers. Such a laser can produce only quite moderate powers in the milliwatt region. However, it is possible to fabricate two-dimensional arrays containing many such VCSELs on a common semiconductor chip and to utilize the combined emission of those. One can then obtain several watts, dozens or even hundreds of watts, depending on the number of used emitters. Therefore, a 2-D VCSEL array containing many thousand emitters can compete with diode bars and (combining multiple arrays) even with diode stacks based on edge-emitting semiconductor lasers.
Compared with the fabrication of single emitters, the production process (typically with MOCVD and photolithography) is fairly similar. There are only few additional fabrication steps related to the electrical connections. The outer shape of the produced arrays is often rectangular, but other shapes (e.g. triangular ones) are equally possible. The emitters may be arranged in a square or hexagonal pattern, for example.
Typically, a single emitter has a diameter between 5 μm and 20 μm for an output power between a few milliwatts and some tens of milliwatts. The spacing (pitch) of emitters is sometimes as close as possible concerning fabrication, but in other cases significantly larger e.g. for limiting the density of heat generation. For example, one may use a 40-μm pitch for emitters with an active diameter of 8 μm. Such a chip with an area of e.g. 5 mm2 may produce an output power of the order of 10 W.
Note that edge emitters are suitable for one-dimensional arrays (= diode bars), but hardly for two-dimensional arrays, while VCSELs can be quite naturally made as 2D arrays with wafer level fabrication. Therefore, they have a substantial potential for low cost manufacturing.
Assemblies with Multiple VCSEL Arrays
For some applications, dozens of such VCSEL arrays are mounted side by side on a water-cooled holder which provides both the electrical connections and the cooling. Typically, the arrays are electrically connected in series in order to limit the required electrical current. Such a device with a total emitting area of several square centimeters may emit powers of hundreds of kilowatts, and larger devices can even generate multiple kilowatts. The optical intensity at some distance from the emitters can be fairly uniform over a large area.
Output Power and Beam Quality; Importance of Effective Cooling
The total output power of a VCSEL area is simply the sum of the output powers of all the emitters. However, the emission per VCSEL may have to be somewhat reduced if the limits of the cooling system are reached, as the combined dissipated power of many VCSELs and the thermal power density can be substantial (often hundreds of watts per cm2). The cooling can be made easier by increasing the spacing between the emitters – which however reduces the beam quality, as explained in the following.
The beam quality of a single VCSEL can be very high – close to diffraction-limited. When optimizing such devices for high output power, the beam quality may no longer be perfect. More importantly, however, when combining multiple VCSELs one effectively obtains a laser source with an accordingly increased emitting area; that area is substantially more than the number of emitters times the area of a single emitter, since some spacing between the emitters is required. At the same time, the beam divergence remains that of a single emitter, if the emission is uncoordinated (incoherent), as usual. As a consequence, the beam parameter product and M2 factor, which are indicators for the beam quality, are substantially increased.
Thus, the output power can in principle be scaled up (“power scaling”) simply by increasing the number of emitters, but at the same time the beam quality is reduced. Therefore, one should not regard that method as being true power scaling; the radiance (brightness) is not increased and typically stays at the order of 50 kW / (cm2 sr).
For optimizing the beam quality and radiance, one can use different approaches:
- One may maximize the emission of each single emitter, so that a small number of emitters is required, and the total emitting area is reduced accordingly. Some reduction in beam quality of a single emitter is relevant only as far as the beam divergence is increased.
- One will try to maximize the power conversion efficiency of each emitter, thus reducing the amount of generated heat.
- One can try to minimize the spacing of the emitters. That approach is essentially limited by how effective the cooling is, at least for continuous-wave operation. Methods for improving that aspect are explained in the following section.
- One can try to obtain a coherent emission by use of optical feedback, which is also explained further below.
Methods for Effective Cooling
Heat can be efficiently conducted away from a surface by using a material with particularly high thermal conductivity. In particular, one often uses synthetic diamond heater spreaders. As those are transparent, they can sometimes even be attached to the light-emitting surface.
A heat spreader can be combined with a microchannel cooler.
In the traditional type of design, where the emission is directed away from the epitaxial side and cooling usually must be done on the backside, the thermal impedance of the substrate is a problem. That can be reduced by using a substrate as thin as possible, or by thinning it after the epitaxial process.
At least for longer operation wavelengths, it is also possible to fabricate VCSELs which do not emit vertically away from the chip, but in the opposite direction, i.e., from the epitaxial side into the substrate. The output beam is then obtained on the bottom side, which is polished and obtains an anti-reflection coating for minimizing power losses. The top side can then be used for effective cooling, with a minimum thermal resistance between the heat source and the cooling medium.
A disadvantage, however, is the loss of optical power by light absorption in the semiconductor substrate, which needs to be doped (often n-doped) in order to carry the electrical pump current. Therefore, one will also need to reduce the substrate thickness as far as possible, e.g. to 150 μm.
Efficiency of VCSEL Arrays
The power conversion efficiency of VCSEL arrays is often of the order of 50%, which is roughly comparable with that of many edge-emitting high-power laser diodes. However, some of those reach efficiencies of well over 70% , while VECSEL arrays are currently limited to around 63% at 976 nm .
Operation at higher temperatures (e.g. 50 °C at the case) is also possible, but with somewhat lower efficiency.
The efficiency is also often lower if operation with a particularly high beam quality is enforced.
Quite high peak powers are possible in pulsed operation with nanosecond to microsecond pulse durations. Even in pulsed operation, the device reliability can be high: catastrophic optical damage (COD) can usually not occur due to the moderate optical intensities – they are normally far lower than those in edge-emitting lasers, where the achievable peak power is limited by COD to a value not that much higher than the continuous-wave output power.
Pulsed operation is particularly useful for pumping Q-switched solid-state lasers with low pulse repetition rates. Here, the pumping duration will typically be a few hundred microseconds. However, one can also consider the use with much shorter pulse durations (e.g. a few nanoseconds) for time-of-flight measurements, where a high peak power is often of vital importance.
Emission Linewidth and Temperature Coefficient
The emission linewidth of a single VCSEL can be very small – far below 1 nm. In principle, the linewidth of a whole VCSEL array can be similarly small, provided that all emitters work at quite precisely the same wavelength. That requires a high uniformity achieved in the production process and in addition a sufficiently homogeneous temperature distribution.
The emission wavelength of a VCSEL typically has a several times lower temperature dependence than that of an edge-emitting laser diode – normally around 0.065 nm/K for GaAs-based devices. This is because the emission wavelength is essentially determined by a cavity resonance and not by the gain maximum, and cavity resonances have several times smaller temperature coefficients. They are influenced only by the temperature dependence of the refractive index and not by carrier densities involving Fermi distributions.
Note that the emission wavelength may drift during pulse generation, where waste heat is accumulated in a relatively small volume and needs some time to be removed.
Beam Collimation with Microlens Arrays
Some applications require the reduction of beam divergence by the use of a microlens array, where each emitter obtains its own lens (See Figure 1). That can facilitate the further processing of the beam, for example the focusing to a small spot with an additional large lens.
One may either use a separately fabricated lens array, which needs to be precisely aligned to the VCSEL array, or fabricate microlenses directly on the semiconductor chip.
Coherent Emission with External Resonator
Normally, the different VCSELs are emitting in an independent and thus uncoordinated manner, i.e., without mutual coherence. Even when considering only rather short averaging times, one can then simply add up the intensity distribution of the different emitters, ignoring any interference effects. As a result, the beam parameter product is large, since the emitting area is increased by combining many emitters, while the divergence angle remains unchanged.
This can be profoundly changed if the emission of all the VCSELs can be made mutually coherent based on optical feedback. For that, one will usually employ the combination of two measures:
- One uses a common output coupler mirror, which may be placed a few millimeters away from the emitting surface. It can be flat or curved, depending on the amount of thermal lensing and the priorities of beam quality vs. power conversion efficiency.
- The top mirror reflectivity of the VCSELs is substantially reduced, so that lasing is possible only with the optical feedback from the external mirror.
Ideally, one also uses adapted VCSEL designs with minimized current crowding and spatial hole burning effects, which are more easily forced to operate with low divergence. With such external-cavity VCSELs (EC-VECSELs), one can obtain emission with a substantially reduced beam divergence, which e.g. enables one to launch the light into a multimode fiber. In some cases, the M2 factor is even close to 1, approximating a TEM00 profile. However, the power conversion efficiency is often significantly compromised – particularly when optimizing a device for highest beam quality Nevertheless, the achieved radiance can be orders of magnitude higher (hundreds of MW / (cm2 sr)) than in cases where uncoordinated emission takes place.
At the same time, the emission linewidth may be reduced, since the different emitters are tentatively pulled towards common emission frequencies.
Applications for VCSEL Arrays
VCSEL arrays are suitable for pumping solid-state lasers. Due to their relatively low beam divergence, they may sometimes be used even without any pump optics, which of course substantially simplifies the laser setup. In other cases, one may use a fiber-coupled laser with simple pump optics. Further, the low temperature dependence of the emission wavelength may make a temperature stabilization system obsolete. Even pulsed pumping is easily possible, e.g. for Q-switched lasers, where pump pulse durations will typically be a few hundred microseconds.
Arrays which do not reach a particularly high radiance, but only a high output power, can be applied for thermal material processing – for example, for the forming of thermoplastic parts, for the production of carbon fiber parts, for offset printing, for sintering processes, and the local heat treatment of steel parts. For such applications, one does not need to strongly focus the generated infrared radiation. One can often profit from the flat and smooth intensity profile, achieved without an additional beam homogenizer. In some cases, one may activate the VCSEL arrays only in certain spatial zones for obtaining modified illumination patterns.
VCSELs may be used for illumination e.g. in inspection systems, where again the limited beam quality is usually not relevant, but only the homogeneous illumination. The option to quickly modulate the illumination intensity, possibly even use pulsed illumination, can be very helpful in some situations.
Arrays with a small number of emitters, which can often be independently addressed, are used for optical fiber communications.
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This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!