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The laser at 50: Boom, bust, boomphysicsworld.com1 Lasting legacy1086diode42non-diode01998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008100%79%50%27%25%15% 10%1% 2% 7%0%–36% –24% –50%Laser sales peaked at the height of the bubble in 2000, then dropped dramatically as thetelecoms market collapsed (upper graph). These year-by-year totals show that sales of thediode lasers used in telecoms are still less than two-thirds of their bubble-era peaks, whencarriers paid premium prices to build what turned out to be excess capacity. By the timedemand recovered and construction resumed, prices had come down. The lower graph showsthe year-on-year percentage rises or falls in total sales. Despite the fluctuations, the cumulativeannual growth rate over the period shown was 7%.$ (billion)38ing to the companies developing them. As the bubblegrew, investors even chased half-baked ideas such as“all-optical networking”, which envisioned redirectingthe signals by changing their wavelength, but thoseschemes came to naught.Telecoms technologyThe telecoms bubble eventually collapsed because themarket had wildly overestimated the demand for telecomscapacity. Investors had funded too many companiesdoing similar things, while network operatorssuch as AT&T and Verizon had installed far more fibrethan they needed, and it took years for demand tocatch up. Yet the crucial technology – laser transmitters,fibre amplifiers and “closely packed” WDMoptics that could cram in dozens of different wavelengthsinto a single fibre – worked fine. Recent yearshave even seen data rates start to climb again. The firststep was to 40 Gbits s –1 per wavelength and in De cem -ber 2009 Verizon switched on a system transmitting at100 Gbit s –1 on a single wavelength between Paris andFrankfurt – enough capacity to send 2.5 fully packedsingle-sided DVDs in a single second.Tunable diode lasers have been another major winner.They have largely replaced fixed-wavelengthdiode lasers in WDM systems because they are onesize-fits-allcomponents. When a system is installed,software adjusts the lasers so that they emit at the de -sired wavelengths, then locks them in place. Althoughsome tunable lasers are used in subsystems where theirwavelengths may be changed when the network configurationis altered, most tunable lasers are set to onewavelength and left there. “It’s sort of a dull use of tunability,”says optoelectronics engin eer Larry Coldrenof the University of California, Santa Barbara. It doesthough fulfil system requirements.Laser Focus WorldColdren has personal experience of the telecomsbubble, having founded a company in 1998 calledAgility Com munications to produce tunable diodelasers in which the wavelength of the light emitted isselected by slightly expanding or contracting the multilayeredreflective structures that make up the diode.“Agility was worth a lot of money before it [even] hada product or a customer,” says Coldren, who had in -vented the diode-tuning technique a decade earlier.Investors poured more than $200m into the firm andColdren stayed with the company until it was boughtby JDS Uniphase in 2005 for $67m in stock and cash –a third of the money it had burned through.Although it took years to make tunable diode lasers asgood as fixed-wavelength diodes and to package themwith control electronics for system use, sales did, however,eventually begin to rise once the packaged tunablelasers beat the price and performance of fixed-wavelengthlasers. Indeed, Coldren is sure that such tu nablelasers will be around for years to come. “We even usethem here at the university because they’re robust, easyto make and very forgiving,” he says. Their design in -herently yields a single frequency – all it takes is tu ningand calibration to obtain a desired wavelength. Evenfirst-year graduate students can do it, Coldren explains.Pumped by successAnother bubble-era technology that has proved a hitbeyond the telecoms sector is the high-power diodelasers that were originally designed to pump fibre am -plifiers. These lasers have since been adapted to pumpa growing variety of solid-state lasers, which previouslyhad been pumped by bright lamps. This process wasinefficient because much of the lamp energy was emittedat wavelengths not ab sorbed by the laser material.Laser pumping is, in principle, much better because thelaser can be fabricated to emit only light matching theabsorption lines of chosen solid-state materials.Although early diode lasers could convert a largerfraction of input power into pump light than was poss -ible with lamps, they still only emitted milliwatts ofpower. Undeterred, military agencies took a keeninterest in developing more powerful pump diodes inthe 1980s and 1990s, realizing that such devices couldbe used to pump lasers that mark targets for smartbombs, or perhaps for laser weapons. But during thedot-com bubble, when building pump diodes for fibreamplifiers appeared more lucrative, companies likeSDL started switching their attention from the defenceto the telecoms market.That meant moving away from the 808 nm gallium–arsenide pump diodes sought for military applicationsand focusing instead on a new family of indium–gallium–arsenide (InGaAs) devices that emit in the980 nm erbium pump band, as well as indium– gallium–arsenide–phosphide (InGaAsP) devices emitting at the1480 nm erbium pump band. That new focus was onereason why SDL – the biggest manufacturer of pumpdiodes – was worth a staggering $41bn when JDS Uni -phase an nounced plans to buy it in July 2000.Market valuations have declined since those headydays, but pump-diode technology has boomed, andnot only in fibre amplifiers for telecoms. Pump diodesare displacing the pump lamps long used to powerPhysics World May 2010