Tunable transmitters nearing the market

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At the 1998 National Fiber Optic Engineers Conference, employees of a pair of wavelength-division multiplexing (WDM) companies were discussing preliminary announcements of tunable-laser sources for carrier applications. "If they were available, I'd buy them tomorrow," said one staffer. WDM vendors and users will be reaching for their wallets in the near future, as vendors have announced availability of tunable-laser transmitters, while others expect to have sample units available in the first quarter of 2000. Tunable-laser sources available as stand alone units also have been announced.Th Acf1a1

Lucent Microelectronics' C488 should be the first tunable-laser transmitter module available to vendors and carriers. Technology choices and cost issues, however, promise to keep the market competitive for some time as several vendors ready competing products.

Lucent Technologies' Microelectronics Group (Breinigsville, PA) recently unveiled its C488 transmitter, a unit that can be tuned across four channels spaced 50 GHz apart across the wavelength range between 1528.77 and 1563.86 nm (see photo). The unit, samples of which are expected to be available as this quarter ends, uses the company's electroabsorptive modulated laser technology to provide 2.5-Gbit/sec operation for applications ranging from 200 to 640 km. The module provides multiple-channel selection, wavelength locking, output power adjustment, and remote monitoring and programming.

According to Stefan Rochus, European marketing manager for optoelectronics and Lucent Microelectronics, and Conrad Burke, marketing director of optoelectronics components and solutions, the tunable laser will cost 20% less than the combined cost of four single-channel lasers. The company is already looking into expanding the channel count as well as boosting the operational capacity to 10 Gbits/sec. Production on the units should begin in the first quarter of next year.

The Lucent transmitter joins that of Altitun AB (Kista, Sweden), which has been showcasing tunable-laser technology since the end of 1998. Altitun recently unveiled its NYW-50 integrated semiconductor laser module, which follows its work with the Anywave NYW-30 tunable-laser source. The module generally will be supplied with 40 preset wavelengths corresponding to the International Telecommunications Union's (ITU's) 100-GHz wavelength grid, although custom channel plans will be accommodated. The module includes all drive, control, and temperature-control electronics in a 40-pin DIL package.

Meanwhile, JDS Uniphase (San Jose, CA) has demonstrated a 10-Gbit/sec tunable-transmitter module that combines a laser, lithium niobate modulator, and driver electronics. The module provides a tuning range of 8 nm, which translates to 10 channels on the ITU grid. The unit displayed at trade shows in late September featured a source developed at JDS Uniphase's facility in Eindhoven, the Netherlands, and modulators from the company's plant in Bloomfield, CT. An Altitun microprocessor control board provided the wavelength control, while the driver electronics came from Veritech Microwave (Plainfield, NJ). According to the company's chief technology officer, Fred Leonberger, samples of a commercial product should be available next quarter.

Severa companies have jumped into the market with sources only. For example, Alcatel Optronics (Nozay, France) has announced its 1905 TLM broadband continuous-wave tunable laser, which comes in a standard 14-pin butterfly package. It covers 32 channels at 50-GHz spacing. Meanwhile, Marconi Communications (Nottingham, UK) has announced an electrically tunable laser for its SmartPhotonix family of products. The semiconductor-based laser was part of the "Application and Control of Widely Tunable Lasers in HD-WDM Networks" (actual) program, under the umbrella of the European Advanced Communications Technologies and Services (ACTS) effort. As such, it benefited from a tuning mechanism based on a lookup table. The mechanism allows remote tuning of gain and phase, along with course and fine wavelength tuning. (Altitun and NTT Corp. of Japan also contributed tunable-laser technology to the program.) The laser itself tunes over 40 nm, providing 50 channels over the ITU grid.

The wide variety of suppliers is nearly matched by the variety of laser technologies that underpin their offerings. Three technologies currently dominate the market. The first, distributed-feedback (DFB) lasers, are common sources for today's single-channel applications. They are well understood and supply adequate output power for long-haul applications. Tuning generally is accomplished via thermal techniques. Grating coupler sampled reflector (GCSR) lasers generally provide very wide tuning; however, this tunability can come at the expense of output power. Finally, distributed Bragg reflector (DBR) lasers take a position on the middle ground, where they generally provide more power than a GCSR laser and slightly better tuning than DFB lasers.

All three technologies have their champions. For example, Nortel Networks (Paignton, UK), which has demonstrated tunable technology for over a year, favors DFB lasers. The company plans to link several DFB lasers together to broaden its offering's tuning range. Altitun uses GCSR, while Alcatel and JDS Uniphase use DBR technology. The current Lucent offering uses DFB sources, although Lucent's Rochus and Burke say that the company will switch to DBR in the future.

The number of companies operating in the field is a testament to the expected market potential of tunable lasers.

"We're being pushed pretty hard by a number of customers to provide tunable lasers just for regular deployments," reports Paul Morkel, senior manager, photonic product planning, at transmission equipment vendor Fujitsu Networks Corp. (Richardson, TX) The higher channel counts of WDM systems mean higher complexity and inventory problems for carriers. The use of a 32-channel dense WDM system currently means that carriers must stock 32 laser transmitters as spares. To help solve this inventory problem, Fujitsu plans to offer a standard tunable transmitter for its products in the first quarter of 2000 that likely will cover four to eight channels. "The marketplace is looking for tunable lasers to be used for standard deployments," says Morkel. "They have a smaller number of units [in inventory] and they just provision it to the wavelength they need in the field and then it just sits there."

The key to opening this market, however, will be cost. "One of the key requirements of that application is that the cost has to be comparable to that of a single-channel laser, because they're not, from a performance standpoint, gaining anything else; they're just minimizing the amount of inventory that they have to have and the number of units they have to manage," Morkel explains. "The feedback we've got is that the marketplace will accept some small premium for tunable lasers. It's difficult to tie people down to exactly what that might be, but it's certainly less than 50%. We might want to consider that between a 15% and 20% increment might be acceptable."

Of course, WDM vendors also have inventory considerations, and tunable-laser developers envision a day when tunable lasers will be a standard component of new systems. However, cost is an issue here as well. "I think the day when this becomes a clear replacement for all of the standard lasers is really going to be driven by the price--no doubt about it. And I think that is some years away," offers JDS Uniphase's Leonberger. "But I do think that some of the other markets, such as a strategy for a sparing laser or a standby laser, there are people who will pay more of a premium for that than they will pay for inventory issues. So that kind of market will occur first."

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