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Resonant-cavity LEDs fire up in-car networks

18 Apr 2006

The plastic optical fibre networks that feature in many of today's cars have data transmission rates that are limited by LED switching speeds. This barrier can be overcome, says Firecomms' John Lambkin, if these sources are replaced with resonant-cavity LEDs.

Today's car owners are demanding more from their vehicles than just good looks. They now want models offering greater levels of comfort, safety, fuel efficiency and reliability, along with features such as in-car navigation systems, GPS telephones, satellite radios, DVD players and Internet browsing facilities.

All of these features are implemented through the car's infotainment bus, which is a network of digital electronic devices operating at high data rates in an extremely harsh and electromagnetically noisy environment. Today these networks operate at 25 Mb/s, but car manufacturers are planning to migrate to 150 Mb/s, 200 Mb/s, 400 Mb/s and even 1 Gb/s speeds for real-time camera applications.

A copper-based wiring harness operating at these bit rates requires complex, heavy and expensive shielded cable assemblies, so the automotive industry has started to turn to cost-effective systems employing plastic optical fibre (POF). Largely immune from electromagnetic interference, POF offers several advantages over glass fibre, including easy termination and a high tolerance to mechanical vibration. These benefits have led to low-cost connectors and a rugged assembly process.

Many of today's cars already feature Media Oriented Systems Transport (MOST) POF-based multimedia buses (see "POF in-car network evolution"). The key components of these systems are transmitter (Tx) and receiver (Rx) fibre-optic transceivers (FOTs), which are currently supplied by companies such as Infineon and Hamamatsu. The plastic-encapsulated FOTs are fully integrated and contain both the light source and detector, together with the controlling LED driver and limiting amplifier integrated circuits. These components form a digital transceiver that can be driven by standard transistor-transistor logic signals and operate between -40 and +95 °C for the full 15-year lifetime of the vehicle. These FOTs, although operating at relatively low data rates, are highly sophisticated optoelectronic devices that offer the car industry an excellent price-to-performance ratio.

The MOST bus was specifically designed to support higher data rates, and work is already under way to implement next-generation MOST systems operating at 150 Mb/s. This development will face competition from the IDB-1394, a rival optical standard operating at 200 Mb/s, which is based on the consumer multimedia IEEE 1394.b (FireWire) protocol. One of the advantages of this standard is that it allows products such as Apple's iPod and Sony's PlayStation Portable (PSP) to be directly connected into the multimedia bus. This bus is still to be deployed, but several automotive manufacturers are actively supporting the development of this standard in Europe and Japan.

Red's the colour

POF requires 630-685 nm light sources, because these are the wavelengths where fibre attenuation is at its lowest. For the systems installed in today's cars, which operate below 50 Mb/s, conventional surface-emitting red LEDs are used for the transmitter. These devices are highly attractive as they have no threshold current and can consequently use simple drive circuits. In addition, they have good thermal stability. The LEDs are also intrinsically eye-safe, and being red it is trivial to literally see optical signals during maintenance operations.

However, as the MOST standard upgrades to 150 Mb/s and IDB-1394 progresses to 200 Mb/s, the challenge is to find POF-compatible light sources that deliver these data rates while also meeting the car's arduous environmental operating conditions. Conventional surface-emitting LEDs are limited to speeds of up to 50 Mb/s. The devices are also inefficient because most of the light generated in the active region is internally reflected, and less than 4% is extracted. High-bandwidth visible edge-emitting lasers can address these issues, but their threshold current characteristics and poor thermal behaviour means that these emitters are awkward and costly devices to control compared with the simpler, more reliable and more temperature-stable LEDs.

At Firecomms we believe that more suitable sources for higher data rates are resonant-cavity LEDs (RCLEDs), which combine high extraction efficiencies with good spectral and modulation characteristics. Since 1992, significant effort has been directed towards the development of these devices - visible RCLEDs in particular - and today these emitters can meet the specifications and lifetime performance demanded by the automotive industry.

An RCLED consists of a thin quantum-well active region in a Fabry-Pérot cavity formed from metallic mirrors or distributed Bragg reflectors (DBRs). The RCLED cavity promotes emission into resonant modes within the LED's "escape cone", while suppressing emission into off-resonance modes. This means that the emission profile, which is significantly different from the isotropic nature of a standard LED, can substantially improve the device's extraction efficiency.

Firecomms' red RCLEDs contain an InGaP/AlGaInP quantum-well active region and AlGaAs-based DBR mirrors, and are grown by conventional MOVPE. The device structure is similar to that of a VCSEL, but the number of mirror pairs in the top DBR is significantly smaller, normally less than 10. However, because the RCLED is a spontaneous emission device, the tolerances needed during growth and fabrication procedures are significantly relaxed in comparison with a VCSEL, leading to yields and production costs similar to those of conventional LEDs.

RCLEDs also have a higher bandwidth than conventional LEDs. This improvement comes from the use of thin quantum-well active regions and current-confining oxide apertures. These apertures, which are larger than those routinely used in VCSEL production, reduce the optical loss from below the wire-bond contacts. They also help to maintain higher carrier densities for a given current, which reduces the photon's spontaneous lifetime and increases the device's bandwidth.

With small diameter apertures of less than 50 μm, RCLEDs can produce remarkably high bit rates of 1.25 Gb/s, although internal heating effects limit the useful output power. Arguably more important, though, bit rates of 150 and 200 Mb/s, which are required for next-generation MOST and IDB-1394 standards, can be easily achieved while still maintaining high output powers.

The RCLEDs' thermal behaviour differs substantially from that of a conventional LED, but a highly stable output power can be produced by optimizing the RCLEDs' active region. Reliability is the other critical characteristic for automotive applications. At Firecomms' headquarters in Ireland we have collected over 500,000 device hours of reliability data for RCLEDs in plastic-encapsulated FOT packages and on TO style headers, run in both DC and AC, at temperatures of -40, 85, 95, 110 and 140 °C. We recorded no device failures, which shows that RCLEDs can deliver the same high levels of reliability as their more conventional LED cousins.

The MOST 1.1 standard currently deployed in car manufacturing requires a maximum data rate of 50 Mb/s (including signalling overhead), so it may seem unnecessary to consider using RCLEDs in MOST FOTs. However, RCLEDs designed for automotive applications can deliver high fibre-coupled power, good temperature stability and a significant increase in the launched optical power, compared with the current MOST optical budget. In addition, this technology offers an evolutionary roadmap for MOST to deliver 150 Mb/s data transfer over POF, and that allows car manufacturers to continue using the well-established POF harnesses.

First to market

At the MOST Interconnectivity Conference held in Japan late last year, we launched the world's first MOST 1.1 compliant FOT using RCLED technology. Through optimized design of the RCLED and associated driver IC, our MOST FOT can deliver a minimum fibre-coupled output power over operating temperature and life of -7 dBm, which is a 3 dB improvement over this standard's minimum power specification. The change in output power of our RCLEDs over temperature is as low as 2 dB, and the devices can maintain a full-width half-maximum of their emission below 25 nm across the entire temperature range, which reduces absorption loss within the fibre. Our corresponding receiver FOT has a -28 dBm sensitivity over operating temperature and life, which is a 5 dB improvement over the MOST standard requirement. This means that our system can deliver an additional 8 dB of margin over the MOST specification, which allows the POF harness to incorporate either an increased number of in-line connectors or tolerate higher bend losses in the fibre.

Today MOST FOTs in high volume have a $2-3 target price per FOT pair, and this year fully digital FOTs meeting the 200 Mb/s IDB 1394 and IEEE 1394.b standards will be launched by several suppliers. Consequently, we expect that this automotive-driven technology will not only continue to impact the car industry, but migrate to non-automotive applications such as public audio/visual address systems, home entertainment networks and security-camera systems. The aerospace industry is also beginning to look at adopting POF-based technology for cabin-based multimedia systems. In all of these cases we believe that the combination of the POF fibre physical layer and the multimedia-oriented bus standards of MOST and IEEE 1394 will make for a reliable, cost-effective and high-performance optical system.

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