Vertical laser diodes for modern high-performance data links
The usual laser diodes, such as those found in CD players, emit their light laterally. They are far too large for future telecommunications and computer network requirements. Laser diodes with vertical cavities are a promising alternative due to their small size and higher power. The demands on data transfer are constantly increasing. In modern computer networks, such as the Internet, transmission is required using 32 parallel channels with data rates of more than one gigabit (one billion binary characters) per channel and second. These orders of magnitude can only be realized efficiently with the help of optical glass fibers. Tiny laser diodes, the dimensions of which are smaller than those of a grain of s alt, serve as the transmission source. Classic laser diodes, such as those found in every CD player, have edge dimensions of 200 micrometers (two tenths of a millimeter). They emit their light horizontally from the sandwich-like chip, which is broken out of a semiconductor crystal.
Innovative laser diodes with vertical resonators, on the other hand, emit light perpendicular to the surface. They have only been intensively researched since the early 1990s. With their small dimensions of only ten micrometers (one hundredth of a millimeter) edge length, they are the smallest microlasers in the world. The active zone of the laser consists of nanostructured quantum films that are tailored to the exact atomic layer and are located between highly reflective optical resonator mirrors. They can be manufactured and installed individually or in parallel arrays (rows) without splitting or breaking, easily and therefore inexpensively. In addition, they achieve higher performance than the horizontally emitting diodes. Because of their numerous advantages, vertical laser diodes are key components for today's high-performance data links.
Prof. dr. Karl Joachim Ebeling, Head of the Department of Optoelectronics at the University of Ulm. As early as 1991, he and his working group presented the first continuously emitting vertical laser diode in Europe. In 1992 they achieved what was then the world's smallest threshold current of 650 microamperes, in 1993 the largest tuning range (8.2 nanometers) in the emission wavelength and the smallest emission line widths (30 megahertz). In 1995, the working group achieved the highest transmission rates of ten gigabits per second.
The Ulm optoelectronics specialists have now developed laser sources that do not require a bias current, i.e. without a continuously applied voltage to start the laser, and at the same time achieve data transmission rates of 2.5 gigabits per second on single-mode light (a radiation that only occurs in one single wavelength is emitted) allow. The highest output power is 4.8 milliwatts. A technological achievement that deserves special mention is the epitaxial growth of highly reflective, but electrically low-impedance AlGaAs Bragg mirrors and electro-optical quantum films with an internal efficiency of over 95%. The vertical laser diodes that can be realized in this way require threshold currents of less than 500 microamperes in the technically relevant temperature range of -40° to +80° C. 2.5 gigabits per second data transmission can be increased to ten gigabits with a fiber length of more than 500 meters through high-frequency control. The bit error rates are less than 10-11 and are therefore extremely minimal. These values, which are achieved not only with individual elements but also with densely packed laser arrays with 14 elements for parallel transmission, are unmatched in the world to date. They characterize the rank of the Ulm working group, which is significantly involved in the development of vertical laser diodes worldwide and is the undisputed leader in Europe. On December 12, Professor Karl Joachim Ebeling was awarded the 1997 Karl Heinz Bekurts Prize for his work on vertical laser diodes in optical connection technology.
Vertical lasers could also play a role in optical backbones for data networks and in interchip connections in the future. Furthermore, their use in optical storage technology for CD players is conceivable. If they were one day to be used instead of infrared light-emitting diodes in control and sensor technology, which is also within the realm of possibility, they would soon be taken over by mass markets thanks to their high efficiency and cheap manufacturability.
