21 Sep 2007
Vertical-cavity surface-emitting lasers have long been associated with the datacom market but as Karlheinz Gulden of Bookham explains, the laser's unique emission properties are now proving useful in applications such as optical mice and atomic clocks.
Vertical-cavity surface-emitting lasers (VCSELs) were developed for commercial use in the 1990s and at that time offered the ideal alternative to edge-emitting lasers for short-range fibre-optic data communications. The rapid rise in VCSEL manufacturing paralleled the explosive growth of the internet and, by the late 1990s, VCSELs had all but replaced edge-emitting lasers in the datacom market.
In 2001, datacom modules were being produced in their millions and accounted for over 90% of the VCSELs market. But today there is a different story: datacom demand has been eclipsed by other, even larger markets, which are attracted by the low cost, low power consumption and high reliability offered by VCSELs.
A VCSEL consists of an active layer surrounded by additional cladding layers of semiconductor material. But, unlike edge-emitting lasers where the laser emission is parallel to the wafer surface, VCSELs emit light vertically.
The advantages of this different emission geometry are numerous. For example, emitting a beam perpendicular to the active region means that more devices can be fitted onto a wafer thanks to their smaller surface area.
VCSELs can also be tested and characterized on the wafer prior to cleaving. In comparison, an edge-emitting laser has to be cleaved from the wafer, sub-mounted and bonded before its quality can be established. This means that a VCSEL manufacturer can test tens of thousands of devices every hour using a fully automated characterization process. In turn, the cost to both the manufacturer and the customer is reduced substantially because there is no need for an expensive pre-testing process. It is this ability to produce millions of VCSELs, both reliably and cost-effectively, that made the technology viable to support the increasing data requirements of the new internet.
Although the essential principles of the laser technology are the same (requiring both a gain region and mirrors to achieve lasing), a VCSEL's construction is very different to its edge-emitting counterpart. Edge-emitting lasers have a comparatively long gain distance on the order of millimetres, whereas the gain region of a VCSEL is restricted by the size and number of quantum wells and is typically only 20–30 nm.
This drastically shorter gain section means that the laser mirrors need to have extremely high reflectivity to achieve lasing. Instead of the 30% reflectivity needed by a typical gallium arsenide edge emitter, VCSELs require 99.8% reflectivity. At the same time, because of the vertical geometry, the mirrors need to be highly conductive to avoid ohmic heating.
Creating highly conductive mirrors capable of achieving this degree of reflectivity was one of the main barriers to mass VCSEL production. The answer was to integrate distributed Bragg mirrors into the device structure. Such mirrors consist of layers of semiconductor material, such as AlGaAs, each with different elemental compositions to create alternating high and low refractive indices.
The electrical conductivity can then be kept high by changing the composition between the layers gradually and avoiding abrupt interfaces that act as barriers for electrical carriers. These highly reflective mirrors combined with the small volume of the microcavity reduce the VCSEL's threshold current and substantially lower the power consumption compared with edge-emitting lasers.
While creating manufacturing hurdles, a VCSEL's short cavity length also offers advantages in the form of single longitudinal mode emission. With a longer optical cavity, edge-emitting lasers can experience longitudinal mode instabilities whereas a VCSEL will always emit a single longitudinal mode. By modifying the lateral laser dimensions or geometry of a VCSEL, single lateral mode, and therefore a much more stable emission, can be achieved.
This singlemode emission leads to a further advantage. Because of the circular shape of the emission aperture of a singlemode VCSEL (typically 3–6 µm), very little beam distortion is created and a near ideal gaussian beam is emitted. In contrast, edge-emitting lasers often require special optics to correct, for example, a non-symmetric beam profile. The optics design for a singlemode VCSEL is much simpler.
The VCSEL's low cost and high reliability made it the natural choice for datacom applications. However, the combination of factors, such as the lower power consumption and stable singlemode emission has also made it increasingly attractive to other markets.
Mice and clocks
One such market is optical computer mice, which require extremely low power consumption to ensure maximum battery lifetime. Because of their unique geometry, VCSELs are replacing LEDs for this application. For example, LEDs typically require more than 20 mW of electrical power to produce 1 mW of useable optical output, but VCSELs can achieve the same optical output for less than 10 mW of electrical input.
The high coherence of the light allows improved sensor technologies to be used, leading to improved tracking performance. As a result, there are now more VCSELs supplied to the optical mouse market than are used in datacom applications.
Atomic clocks are another application in which VCSELs are enabling new concepts for smaller, lower-power and lower-cost solutions. The high-frequency modulation characteristics, combined with the singlemode properties, have been used in chip-scale atomic clocks with volumes of less than 1 cm3. Ultimately, these clocks may be implemented in consumer applications such as battery-driven GPS systems.
Singlemode VCSELs have also been used for several years in industrial oxygen measurement sensors. The advantages over existing electro-chemical and paramagnetic solutions are the fast response times, high sensitivity, high reliability and insensitivity to other gases. For these reasons, VCSEL-based optical oxygen sensors continue to gain an increasing share of this market.
Pushing the boundaries
As with all lasers, modulation speed, power, wavelength range and stability are critical parameters for anyone considering the use of VCSELs. In 2000, speed was addressed with the development of a new manufacturing technique called selective oxidation, which allowed efficiency improvements in output and modulation, and led to the development of high-speed VCSELs. As a result, 10 Gbit/s VCSELs are available today and research is pushing this up to 16 Gbit/s and even 25 Gbit/s, making VCSELs ideal for storage area networks.
The boundaries of a VCSEL's emission wavelength range are also being pushed. Within the AlGaAs material system, operating wavelengths between 750 and 980 nm are now commercially available. Using different material systems, such as diluted nitrides or phosphides, the upper wavelength boundary is being pushed up to 2000 nm and beyond, which opens up uses such as transceivers for telecom metro networks and high-sensitivity gas sensing. The development of sub-700 nm VCSELs also paves the way towards possibilities for plastic optical fibre and other applications where visible laser light is required.
While 850 nm is currently the most common singlemode wavelength, it is technically possible to make a VCSEL at any wavelength within the above range. This can be done by changing the thickness of the semiconductor layers in the distributed Bragg mirrors and active region. Many customers require VCSELs of various wavelengths. Manufacturers can accommodate this if the variant will be required in enough quantity to make it a commercially viable and fully qualified product.
The tolerance is an important consideration. Singlemode VCSELs can be tuned over a small range of nanometres by varying the current and/or operating temperature but, if a very specific wavelength is required, with a narrow tolerance, then the cost will increase substantially.
Power output is also being addressed and high-power VCSELs are being developed for use in illumination. With higher temperature stability and a narrower emission spectrum than competing technologies, VCSELs may offer a useful alternative to high-brightness LEDs in machine-vision systems. At the other end of the scale, even lower power VCSELs aim to use just 1 mW of electrical power to generate several hundred microwatts of optical power, increasing efficiency and opening VCSELs up to a new range of battery-powered applications.
Traditional datacom continues to provide a stable and growing market for VCSELs. However, the advantages that made VCSELs so attractive to this market are increasing their use in a range of applications without the need to change the underlying technology.
Optical mice have already proved to be a massive application for VCSELs. Today, many other markets offer huge potential for their future growth, particularly emerging consumer markets for datacom VCSELs, as well as industrial markets, optical encoders, atomic clocks, illumination and gas sensing.
• This article originally appeared in the September 2007 issue of Optics & Laser Europe magazine.