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Technical Insight

Surface emitters revolutionize photonics (Forum - Fiber-Optic Components)

New approaches to the design and manufacture of surface emitting lasers will help fiber to further penetrate all areas of the network. Malcolm Thompson and Aram Mooradian of Novalux explain how.
Recent years have seen a massive proliferation in fiber infrastructure, starting with a few high bandwidth, long distance, submarine-based systems, and eventually reaching down to enterprise and even residential fiber applications which represent a potentially huge fiber market. For each one of these applications, the available channel bandwidth is growing at a phenomenal rate, as illustrated in , with all applications seeing growth of 12 orders of magnitude in the last decade. This growth rate is expected to continue for the next decade. Lasers lift the barriers It is often assumed that it is the core of fiber infrastructure the physical fiber lines that are the limiting factor to future growth. To examine this hypothesis, the fiber environment in the vicinity of the Novalux headquarters building in Sunnyvale, California was investigated. Nine different fibers had been laid within 100 yards of the building. That so many fiber lines lie so closely together demonstrates that fiber is not the rate limiter to further fiber penetration. The true limiter lies in the component cost of connecting to the fiber. A typical optical add/drop multiplexer (OADM) might cost $100 000. The need for low-cost lasers, a basic building block of all optical systems including the OADM, is critical to further penetration. Lasers are used in several components in the system, including erbium-doped fiber amplifiers (EDFAs), which need to be replaced, especially in the dense metro environment, by lower cost amplifiers. It would also be best to incorporate lasers that are directly modulated, chirpless and tunable into today s transmission systems. Tunability not only reduces the laser count in DWDM systems (where currently up to 160 different wavelengths are required), but would also radically reduce the number of spare transmitters needed to maintain system reliability. A single tunable laser can act as a back-up for a multitude of different wavelengths. The opportunities in this area are huge. In the period 20002004, total networking expenditures are expected to grow from $10.5 to $47.5 billion, with the components segment growing from $4.5 to $24 billion, according to RHK, a market researcher in telecom components. In addition, the infrastructure needed to support and utilize fiber networks is growing at an explosive rate, but the continued growth of all of these sectors relies on a decrease in the cost of components. This cost reduction will be lead by those components that are designed for automation and high-volume manufacturing. Currently the photonics manufacturing industry is in its infancy. This typically means an industry focussed on product research, brute force approaches to soling problems, homegrown equipment, a lack of sophisticated high-volume infrastructure, and low revenue per employee. In many ways the photonics industry is where the silicon industry was 2030 years ago. The lessons learned by the silicon industry, and even more importantly the attitude of silicon manufacturers, must be applied to the photonics industry. Part of this attitude is the establishment of highly manufacturable, baseline products. These would be based on standard design rules and standard process modules, and would utilize the vast amounts of data generated in the manufacturing process to control and maintain product specifications. In this environment manufacturing times for a given product will be radically shortened, and this will act as a vehicle for increasing yields and outputs. NECSELs At the core of Novalux s approach to manufacturing a wide range of optical components is the Novalux extended cavity surface emitting laser (NECSEL) platform. The characteristics of NECSELs make these devices ideal for all laser functions in the optical network, including very high power pumps, fixed wavelength and tunable transmitters, and fast switching architectures. In essence, this is the first high-power laser platform amenable to high-volume manufacturing because of its surface-emitting characteristic and thus lower costs for telecom components. NECSEL technology Key to high performance optical communication networks are high power EDFA pumps. These are typically edge-emitting 980 nm lasers (as opposed to surface emitting lasers). The limitations of these lasers are the damage to the optical facet that can occur at high power levels, as well as the need for external gratings for wavelength stabilization. Other laser sources include surface emitters that are capable of very high powers from large apertures, but which experience current crowding and operate in a multimodal fashion, which is not appropriate for a conventional EDFA. NECSEL technology solves these problems with an electrically pumped GaInAs surface-emitting laser in which both the optical modes and the wavelength are controlled by an extended cavity. These lasers are capable of producing extremely high power levels of more than 1 W cw in multimode operation and more than 500 mW cw in a single TEM00 mode. NECSELs use a three-mirror coupled cavity design (see Compound Semiconductor May 2001, p51). DBR layers are used to form the p-type bottom mirror and an intermediate n-type mirror, while the third output mirror is located at the end of the extended laser cavity. The active region of the laser consists of GaInAs strain-compensated 8 nm-wide MQWs grown by MOCVD on n-type GaAs substrates. Current is injected through a circular p-aperture defined by a Si3N4 layer, while the n-contact has an anti-reflection coated circular aperture to allow the beam through to the external mirror. The n-aperture is larger than the p-aperture to avoid clipping of the beam inside the cavity. High performance was achieved by greatly reducing carrier crowding effects in the gain region for efficient power extraction in a fundamental or low-order spatial mode. At the same time, both the substrate optical absorption and the electrical resistance are kept low. The GaAs substrate thickness ranged from 50 to 350 m, depending on the device design. High-power performance Large area devices are used to reduce laser power densities and therefore eliminate potential catastrophic damage effects. Devices with 150 m diameters operating in a multi-mode regime delivered cw output power of 850 mW at a current of 2000 mA, therefore operating at a current density of less than 104 A/cm2 (see ). The large areas of these devices result in a low output beam divergence of about 3 mrads at full angle half maximum. As a result, fiber coupling efficiencies of about 90% with powers of 420 mW cw in single-mode fiber have been achieved. When operating in the TEM00 mode at up to 500 mW, the laser wavelength is stabilized near 980 nm by the short cavity FabryPerot, with a linewidth of less than 0.1 nm and shifted with temperature at a rate of only 0.07 nm/C. Accelerated lifetime testing has shown that with over 3000 hours of continuous operation at 110C junction temperature and 50% of threshold to rollover, these lasers exhibit reliability characteristics similar to those demonstrated for low power VCSELs. Manufacturing for high yields At Novalux, modeling and simulation are an integral part of the manufacturing process. A device modeling cycle that gives information on optical, thermal, mechanical and electrical properties is used to test any given design (see ). A design is modeled in various configurations until the desired properties are realized. Devices are fabricated in a high volume, cassette-to-cassette wafer processing facility running both 4 inch GaAs and 3 inch InP wafers. Continuous in-line monitoring and testing is critical to maintaining high yields. Laser testing is done at the wafer level and as a function of variable burn-in, allowing the identification of known-good-die that are then picked and passed along to a fully automated packaging and assembly step. Relatively loose alignment tolerances are possible in the fiber to laser mounting due to the use of circular TEM00 beams. The manufacturing sequence is illustrated in . With this highly automated and monitored process, cycle times are greatly reduced, with laser testing occurring only six days after the GaAs substrates enter the processing line. This rapid cycle time allows very fast device optimization times. The future for fiber Using NECSELs as the underlying platform technology opens the way for fiber infrastructure to penetrate every part of the network, not just the long haul and short reach areas. Initial products that Novalux will implement based on this platform will broadly be divided into two families: pumps and transmitters. On the pump side, the 980 nm single and multimode pumps described above, will be followed by 14xx nm pumps. On the transmitter side, a fixed-frequency transmitter will be introduced, followed by a demonstration of very fast tunability, enabling some very advanced switching architectures.
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