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GaAs Lasers Eye Multiple Targets

Infrared edge emitters and VCSELs are targeting a growing number of lucrative markets: Gaming, ultra-high density data storage, finger navigation and optical cables for USB and HDMI interconnects. Oclaro’s Robert Blum and Karlheinz Gulden discuss them all in detail.


The GaAs-based laser is famous for transforming the way we listen to music. This infrared emitter lies at the heart of every compact disc player, and its role in helping to read the noughts and ones off countless albums has ushered in the era of digitally recorded music that continues to this day.

Less well known are other applications for this laser, which continue to increase in number, enabling the manufacturers of these chips to grow their sales. These infrared emitters are used in a wide variety of optical links and they are seeing greater and greater deployment in computer-related technologies: They are widely used today in optical mice and optical trackpads (also known as finger navigation) as well as gesture recognition systems; and a promising variety of new consumer applications is emerging, such as active optical cables and high-density magnetic recording.

The first class of infrared lasers to hit the market was the edge-emitter. These are still widely used in compact disc players and, in a far higher power output format, as high-reliability sources for pumping of Erbium-doped fibres in communication links, where they help amplify and regenerate the light transmitted in the long-haul section of optical networks.

More recently, these infrared sources have been joined by VCSELs, which made their commercial debut in 1996. Thanks to the combination of high reliability, lower power dissipation, and lower cost –especially when integration and assembly costs are included –these devices quickly usurped their edge-emitting cousins in the short-reach optical network market.

And the GaAs-based VCSEL continued to evolve. In 2001, as data rates started to exceed 1 Gbit/s, conventional ion-implant manufacturing technology held back performance. But this bottleneck has been addressed by introducing a novel, non-lithographic VCSEL technology – selective oxidation –leading to products with improved lateral current distributions and superior optical guiding within the device. In turn, modulation speeds increased, while operating currents fell.

 

GaAs lasers could hold the key to increasing the density of data storage on hard disc drives

Selective oxidation is still used in today’s VCSELs, which play a key role in 10 Gbit/s Ethernet and 8/16 Gbit/s Fiber Channel links that transfer data over distances of up to several hundred metres. Only a few fabs worldwide have mastered the production of these devices which require state-of-the-art processing capabilities. VCSEL fabrication involves a non-lithographic lateral structuring step and the introduction of a highly strained current-blocking layer close to the active region; and implementing these features to yield a reliable product demands careful optimization of the design and manufacturing process.

The rewards of getting this right are strong sales to datacom markets, which order millions of units every year. That’s a significant level of orders for this industry, but it is dwarfed by shipments for consumer applications, which can require 100 million units per year.

 

An emerging market for VCSELs is associate with the Light Peak concept. This has evolved into the Thunderbolt interface which can be found on several Apple computers

 

Oclaro has produced a 10 Gbit/s VCSEL chip for use in non-hermetic, high-speed optical interconnects

One consumer market that the GaAs-based VCSEL has already tapped into is the optical mouse. Logitech lead the way in 2004, replacing the LED in the navigation engine with a VCSEL. This switch trimmed power consumption; improved tracking, which is a feature valued by gamers; and thanks to a move from a visible to an infrared source, made the mice more compatible with a glass surface. Thanks to all these attractive features, the VCSEL broke into its first high-volume consumer market, wracking up sales of many tens of millions within a few years.

Similar success could also occur in the finger navigation sector. In 2009, smartphone manufacturer Blackberry, which ships about 7 million handsets per quarter, replaced its mechanical trackball with an optical trackpad. This featured an LED at the time, but again a VCSEL offers advantages, such as superior temperature stability, narrower spectral line width, and a symmetrical, more directed beam shape that leads to cheaper collimating optics delivering great performance.

Short, superfast cables

Another growing consumer application are high-speed optical interconnects. Today, USB cables are in our homes, linking external disk drives and headphones to our PCs, while HDMI cables are connecting our DVD and Blu-ray players to high-resolution displays and flat screen TVs. Although most USB ports are currently based on the USB 2.0 standard, which corresponds to a data rate of 480 Mbit/s, newer standards have a much higher data rate, such as 5 Gbit/s for USB 3.0 and 10.2 Gbit/s for HDMI 1.3. These conventional, electrical higher-speed cables deliver good performance over short reaches, but this quickly deteriorates as links are lengthened beyond 3 metres.

A promising alternative is to move to optical cables. Interest in this technology has mushroomed following Intel’s 2009 announcement of Light Peak, a high-speed optical interconnect for consumer devices using 10 Gbit/s VCSELs. In 2011, the Light Peak concept was implemented in one of the USB ports of Sony’s VAIO Z Laptop series, with a fibre optic cable feeding data to and from the media dock. The release of Sony’s flagship laptop product thus marked the first commercial introduction of high-speed VCSELs in consumer applications. Since then, the original Light Peak concept has evolved into the Thunderbolt interface which can be found on several Apple computers and many peripheral devices today. The connection between devices can be electrical for short very distances or through active optical cables which boast a data rate of up to 20 Gbit/s for longer distances. What is common to these active optical cables is that VCSELs are at work, hidden away inside the connector, and enabling a cable that is thin, flexible, lightweight, and potentially significantly cheaper than its electrical counterpart.

This migration from electrical to fibre-optic cabling is being enabled not just by high volume VCSELs but also new types of fibre optic cables. Corning, for example, has been marketing its bend-insensitive multimode fibre, named ClearCurve, specifically for consumer applications. But cables can also be constructed from plastic optical fibres or other fibre, and in all cases they deliver a significantly longer reach than electrical links, while also being a lot less bulky. These sets of attributes make active optical cables even more attractive, for example to those customers who need to connect their slim HDTVs to surround sound systems without wanting to also showcase a thick electrical cable.

Meanwhile, edge-emitting GaAs-based lasers are starting to be deployed in systems for three-dimensional gesture recognition or skeletal tracking, with chip sales to this sector offering great potential for growth. The most popular product in this area to date is Microsoft’s Kinect system, which uses a near infrared laser to illuminate a scene that is then captured with a CMOS detector (this imaging system often features an optical bandpass filter matched to the laser wavelength).

Two different approaches are adopted in these systems: A structured light approach, which involves projecting a pattern onto the scene, usually from a continuous-wave, single-transverse-mode laser; or a time-of-flight technology, where a laser that is modulated in burst mode illuminates the scene. With the latter technology, reflected light is resolved in time, enabling calculations of the distances between the device and the objects in the scene. Both approaches favour lasers over LEDs, because their higher modulation speeds make them preferable for time-of-flight technologies, while narrow linewidth and high-optical output power are highly desirable characteristics for the structured light approach.

Gesture recognition and skeletal tracking offer a new dimension to gaming, and according to Microsoft and Primesense, 20 million devices have already been shipped for this application. This could be the tip of the iceberg, given that other applications are under discussion, such as the incorporation of similar systems into smart TVs and even handheld tablets. 250 million TVs are sold worldwide every year, and 125 million tablet computers should be sold in 2012, according to analysts DisplaySearch and iSuppli, respectively.

Boosting hard drive capability

Last, but by no means least in the list of emerging high-volume opportunities for the laser is the hard disk drive (HDD) industry. Here progress is measured in terms of storage density per square inch, and recently Seagate and then also TDK exceeded the 1 terabit mark by using a technology known as heat-assisted magnetic recording. These two firms, plus WD – collectively, they are the world’s three leading disk drive and head manufacturers – are working on this technology, which will employ a laser in each of the heads in a disc drive. Assuming four heads per disk drive, today’s market would consume a whopping 2 billion lasers per year.

Introducing lasers into disc drives is a radical step, but one that many insiders believe will have to happen. To increase storage density in disk drives, manufacturers can shrink the size of the magnetic bit cells (the “1"s and “0"s), but physics demands that the grain size – or the size of the magnetic domains in the magnetic layer – shrink accordingly if one wants to maintain the same signal-to-noise ratio. And to further guarantee the stability of the stored information and prevent self-erasing, these smaller grain sizes must be realised in magnetic materials with a high coercivity.



300 mW CW single mode laser diode for use in structured light applications

Coercivity is a term that describes how hard a magnetic material is, and materials with high coercivity will only change their magnetization (i.e., store information) under very high magnetic fields. Today’s magnetic heads, which generate fields of up to 2.4 Tesla and use a perpendicular magnetic recording technology, are now holding back data storage to well below 1 Tb/inch2.

Higher storage densities are possible by temporarily lowering the coercivity of the magnetic layer in a small area. The trick is to apply localized heat during the write process, thereby decreasing the magnetic field required to re-orient the magnetic domains. This local heat is provided by coupling light into a plasmonic antenna (also known as a near-field transducer) to create heated domains with dimensions of tens of nanometres.

If lasers are to make an impact here, they must combine affordability with high output power and great reliability. Sources emitting several tens of milliwatts are needed to ensure that enough optical power reaches the recording medium, after it has been coupled to an optical waveguide and focused into a spot of around 50 nm in diameter. And since such a small spot size can’t be achieved using classical optics and a laser wavelength of around 850nm, a near-field optical transducer has to be employed – making use of evanescent waves and near-field optics very similar to those utilized in a near-field scanning optical microscope (NSOM).

Other requirements for the laser include a very, very small chip size. This aids mechanical design, allowing the emitter to be fitted onto existing read/write heads, and it also trims the cost of production and increases the output from a fab.

This opportunity for chipmakers, plus those in gaming, cabling and finger navigation, highlights the great opportunities for high-volume sales of GaAs-based edge-emitters and VCSELs. Although the CD may be under threat due to the rapid take up of digital downloads, its key component, its infrared source, seems assured of a bright future.



An array of four 14 Gbit/s VCSELs for a 56 Gbit/s active optical fibre



Oclaro’s infrared laser pedigree

Oclaro has emerged as a leader in the design and high-volume manufacture of semiconductor laser diodes for consumer applications, thanks to its proven expertise and leadership in the design and production of highly reliable, single-mode and multi-mode VCSELs and edge emitting laser diodes. The company has been involved in VCSEL development and production for more than 15 years, and during that time it has increased the number of VCSEL die on a 3-inch wafer from 2,500 to 100,000, leading to increased chip production at a lower cost. To date, Oclaro has shipped more than 150 million VCSELs and several million edge emitting laser diodes into both telecom and consumer applications.



Oclaro has dramatically increased the number of VCSEL die that it can yield from a 3-inch wafer.



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