One wafer yields varied diode lasers
Researchers at the Opto-Electronic Device Research Laboratories of nec Corp., Tsukuba, Japan, have developed a novel technology to manufacture diode lasers with different emission wavelengths on a single wafer. The resulting lower fabrication costs, nec believes, will speed the development of high-capacity multimedia networks, including those based on wavelength-division multiplexing (wdm).
The current conventional process requires that all the lasers on a wafer have the same emission wavelength and lasing characteristics (see figure on page 31). Masayuki Yamaguchi, assistant manager at the laboratories, says that several diode lasers in a 75-nm-wide wavelength range and a frequency band to 10 THz can be produced via the new process on the same indium phosphide (InP) wafer with uniform lasing characteristics. Moreover, the threshold current can be stabilized at 9 mA, regardless of the output wavelength of the diode laser. nec estimates that use of the new process will lead to a one-hundredfold reduction in fabrication costs and a significant reduction in production times for laser diodes used in wdm lightwave systems.
The Tsukuba laboratory specifically targeted the development of made-to-order, simultaneous fabrication of different-wavelength diode lasers on a single wafer to match the pre-assigned wavelength grids of future wdm systems. Ideally, each diode laser with a different wavelength should have uniform performance characteristics over a wide range of the erbium-doped fiber amplifier gain bandwidth of 1530 to 1560 nm. nec confirms that fine pitch control of gratings by electron beam lithography combined with bandgap energy control by selective metal-organic vapor phase epitaxy (movpe) will achieve the uniform performance goal.
Researchers achieved arbitrary wavelength control of a distributed- feedback (dfb) diode laser by controlling the grating pitch to increments of less than 0.01 nm. Yamaguchi notes that the current minimum drawing step of a conventional electron beam machine is fixed at 2.5 nm, which means that laser diodes currently made on the same InP wafer have the same lasing characteristics. To reduce the pitch value, nec developed a "field-size variation technique" that continuously varies the electric field and the positioning of the electron beam before it hits the wafer. The field size can be changed by 0.05%, allowing fine pitch control in 0.0012-nm steps. In one experiment, the researchers formed a 40-channel device with different pitch gratings from 235.98 to 247.68 nm in 0.3-nm steps.
The researchers also developed a selective movpe growth technology for active layer bandgap energy control. Yamaguchi notes that the lasing characteristics of dfb diode lasers deteriorate if the emission wavelength deviates greatly from the active layer gain peak wavelength. Selective movpe growth adjusts the gain peak wavelength to the emission wavelengths for uniform laser performance for all channels.
These technologies, nec predicts, are capable of producing laser diodes in a 100-nm-wide wavelength range and in a frequency band to 10 THz on one wafer, with stabilized threshold direct current. q
Paul Mortensen writes from Japan.