Options for EDFA pumps change to meet market needs

Qing 'Ken' Wang, Toby Strite, Stephen Eglash, and Kao-Yang Huang

EDFA designers can now specify efficient 500-mW 980- and 960-nm pumps for the booster stage or L-band, replacing 1480-nm pumps at comparable cost. With lower power consumption and coolerless modules also available, market segments will benefit from increased performance and reduced cost.

Dense WDM fiberoptic systems and their markets are rapidly changing. System architects are demanding greater amplifier performance while fully expecting a rapidly declining cost of ownership for the amplification function of the network. As a result, designers of erbium-doped fiber-amplifiers (EDFAs) are under increasing pressure to produce amplifiers with high performance across a range of performance criteria such as response time, power consumption, gain-bandwidth and flatness, noise figure, and dBm output power, while simultaneously reducing the cost of their amplifier products.To meet these criteria, new 980-nm pump-module designs are being developed. These products are designed to provide cost-effective solutions to these amplifier design challenges. Underlying this development is unprecedented progress in 980-nm laser-chip technology.

A typical aluminum gallium indium arsenide (AlGaInAs) 980-nm laser-chip platform features InGaAs strained-layer quantum-well technology and a separate confinement heterostructure. These single-mode laser diodes have real refractive index guiding for stable optical behavior and a simple and robust facet technology that greatly enhances facet durability. Technologies such as these result in 980-nm lasers capable of 750 mW of kink-free, ex-facet optical power, wide dynamic range, superior reliability, and outstanding electrical and thermal efficiencies. The reliability of AlGaInAs lasers is now well understood after several years of submarine deployment. For example, a 980-nm pump laser chip designed for submarine applications is able to provide >300 mW of fiber-coupled power, roughly double that of the previous generation, at sub-100 FIT (1 FIT = 1 failure in 10⁹ device-hours) reliability (see Fig. 1, top).

FIGURE 1. Long use in submarine applications has established the reliability of 980-nm pump laser chips such as JDS Uniphase's previous (6520 and 6530), current (6540), and next (6560) generations (top). Based on the operating power of past and prototype fiber-coupled pump lasers, the physical properties of AlGaInAs have permitted 980-nm laser power to outstrip 1480-nm pumps (bottom). (Data based on pump laser module power available in commercial volumes.)

Alternative pump lasers based on indium gallium arsenide phosphide (InGaAsP) operate in the 1480-nm wavelength range. Compared to AlGaInAs, InGaAsP lasers suffer higher losses, lower gain, and weaker confinement, which result in much lower wallplug efficiency and greater thermal handling problems. The heat management capability of standard pump laser packages, and the poor reliability of high-power 1480-nm laser diodes limit the development of higher-power 1480-nm pump lasers to be slower than that of 980-nm (see Fig. 1, bottom).

Great improvements in chip output power and reliability enable the 980-nm pump laser to be optimized for any EDFA application. Underlying this flexibility is a calculated design tradeoff of rated optical output power against operational temperature. Power, reliability, and innovative package technology are the key elements to the realization of a new generation of these lasers.

NEW FRONTIERSIncreased channel counts at higher bit rates drive the demand for pump photons in optical networks. System complexity increases this demand as wavelength management introduces localized losses that must be offset by added gain. EDFA designers have had little choice but to specify more pump lasers per EDFA, along with the additional circuitry and passive components those require, while struggling with increasingly challenging heat dissipation budgets.

Recently introduced 980-nm pump modules featuring up to 360 mW of fiber-Bragg-grating (FBG)-stabilized power, and soon 500 mW, offer large EDFA cost and complexity savings (see Fig. 2). Because the cost of any pump module is primarily constrained by package material cost, high-power 980-nm pumps sell at greatly reduced cost per milliwatt of power. EDFAs replacing multiple pumps with a high-power 980-nm laser realize additional savings in size, complexity, and electronic circuitry. An additional bonus arises when a 1480-nm pump is designed out, because noise performance is optimized while slashing electrical power consumption and thermal dissipation. Finally, the application of 980-nm pumping throughout amplifier architecture gives amplifier designers new freedom in choosing the location and magnitude of inter-amplifier loss components.

Table 1 illustrates this point with a typical example in which two 140-mW, 1480-nm pumps, in a co-/counter-propagating pumping scheme, are replaced by a single 980-nm co-propagating pump for the power stage of a multistage EDFA. The example considers the concurrent reduction of losses associated with the 1480/1550-nm thin-film coupler and splices, which are eliminated at the output of erbium-doped fiber in the design using 980-nm technology. The required 295-mW, 980-nm pump dissipates only 6 W of power, 60% less than the sum of the two 1480-nm (7 W x 2 = 14 W) pumps, in addition to improving the amplifier noise figure. In total, the design eliminates one DWDM coupler, two splices, and one pump laser (with associated TEC driver, laser driver, and control circuitry).

The simulation results of erbium-doped fiber pumped by 140-mW/140-mW, 1480-nm/1480-nm co-/counter-propagating lasers is shown in Design 1. Insertion losses of 0.25 dB for a fused-fiber 980/1550-nm coupler and 0.6 dB for a thin-film 1480/1550-nm coupler are assumed. Splice losses are modeled at 0.2 dB throughout. Launched 1480-nm power inserted in the EDF is 113 mW. In the dual-pump design, amplified power is hit by 1480/1550-nm coupler (0.6 dB) and splice (0.2 dB) losses totaling 0.8 dB. The single high-power 980-nm co-propagating pump in Design 2 does not suffer the final 0.8-dB loss, and can achieve the same output power with only 260-mW, 980-nm power into the erbium-doped fiber, corresponding to a 980-nm pump laser operating at 295 mW, well within present technological capabilities. Design 2 also has noise figure reduction advantage of 0.17 dB.

FURTHER REDUCING POWER CONSUMPTIONAlGaInAs 980-nm chip technology has dramatically outstripped designers' core requirement of 100- to 220-mW pump power in the preamplification EDFA stage. This situation opens the possibility of trading off rated optical output power against additional savings in electrical power consumption and dissipated power, while maintaining superior reliability. Low power consumption (LPC) 980-nm modules are now available that operate the laser chip at 45°C, as opposed to the standard 25°C.

EDFA electrical and thermal dissipation budgets consider worst-case pump laser performance, which occurs at the maximum rated case temperature (typically 75°C). In conventional pump modules, the TEC consumes >80% of the power budget. Table 2 illustrates the huge advantage of the LPC module. At 75°C case temperature the LPC pump consumes only 1.7 W, less than 40% the consumption of a conventional 980-nm pump and four times better than a comparable 1480-nm pump laser. LPC pumps are drop-in replacements for conventional 980-nm modules, allowing increased component packaging density with lower total heat dissipation for next-generation amplifiers.

Superior chip reliability is required to withstand the elevated 45°C operating temperature. The reliability model of the 6540 980-nm pump laser chip indicates the LPC pump module will perform below 250 FIT while producing 250 mW of fiber-coupled power. Further improvements in 980-nm chip technology will permit the advantages of LPC pump technology to be extended to powers appropriate to the EDFA booster stage and L-band requirements in the near future.

COOLERLESS 980-NM PUMPS
Another major development is the introduction of coolerless 980-nm pump modules at 120-mW FBG-stabilized output power. These are particularly suited to single-channel and narrowband EDFAs, and amplifiers deployed in environments that severely limit the electrical power and thermal dissipation budgets. Coolerless pumps do away with the TEC and thermistor, further reducing power consumption, heat dissipation, and electrical complexity in the EDFA. Eliminating the bulky TEC opens the door to modules of reduced size. However, to achieve the desired performance characteristics of the modern pump laser, coolerless operation places stringent demands on chip and package technologies.

A number of challenges were overcome to develop reliable and stable coolerless pumps. A typical coolerless 980-nm module is fiber-Bragg-grating (FBG)-stabilized, and maintains stable coupling efficiency over the full 0°C to 75°C case-temperature range, making coolerless operation transparent to the EDFA designer. For example, at 75°C case temperature, 220-mW fiber-coupled optical power results from only 1W electrical power input. The laser module efficiency changes by less than 30% over the full temperature range (see Fig. 3, left).

Spectral quality and dynamic range are evidenced by >30 dB side-mode suppression ratio (SMSR) over 0°C to 75°C, and >15 dB at 10 mW output power, respectively (see Fig. 3, right). Maintaining FBG operation is crucial for coolerless 980-nm products because the narrow erbium ion absorption spectrum around 980 nm means pumping efficiency and noise figure degrade rapidly if the pump wavelength varies. Like the LPC, the 120-mW power currently available in the coolerless platform will scale as the development of higher-power 980-nm chips continues. Finally, the elimination of the TEC will allow for future significant size reduction of the coolerless pump laser.

THE L-BAND
L-band EDFAs are even more power hungry than their C-band counterparts because of a reduction in the amplification efficiency of erbium ions at longer signal wavelengths. The longer length of the erbium-doped fiber used to offset the poor efficiency introduces other issues such as increased amplified spontaneous emission (ASE) noise. As a result of these issues, L-band EDFAs are even more ripe to capitalize on improved pump technologies tailored specifically to their needs.

A recent innovation for the L-band is a 960-nm pump laser. Because the erbium absorption cross section of EDF at 960-nm is smaller, the backward ASE is significantly less than for 980-nm pumping. Consequently, 960-nm pumping actually generates more gain than pumping at 980 nm. The 960-nm wavelength offers dramatic improvements over 980 nm, and comparable dBm output power to the 1480-nm pump, but at half the electrical power consumption and thermal dissipation levels (see Fig. 4).

FIGURE 4. Simulation models a single pump amplifier using Lucent L-band EDF at varied pump wavelengths and fiber lengths for a single 1607-nm wavelength input at 0 dBm. Pump powers are scaled to a photon count identical to 300 mW at 980 nm. Pumping at 960-nm produces ~2 dB more gain than at 980 nm, and comparable output as the 1480-nm pump at much lower electrical power consumption and thermal dissipation levels.

Important from a technical standpoint, 960-nm is equivalent to 980-nm chip and package technology, so 500 mW, LPC and coolerless 960-nm L-band pumps are just around the corner. These will follow the same upward power and downward cost trajectories as their 980-nm counterparts.

EDFA designers can now begin specifying efficient 500mW 980-nm pumps in the booster stage or L-band amps, replacing power hungry 1480-nm pumps at comparable cost. Within the conventional 100- to 220-mW output power band commonly used in the pre-amps, operating the chip at 45°C reduces pump module power consumption by a dramatic 60%. Narrowband or metro applications requiring

ACKNOWLEDGMENTS
The authors thank their colleagues Vincent V. Wong, Victor Rossin, Clem Burton, Alexander Schoenfelder, and Jo Major for their contributions to this article.

The authors are all from JDS Uniphase. Qing "Ken" Wang is marketing manager for pump modules based at 80 Rose Orchard Way, San Jose, CA 95134; Toby Strite is regional marketing director for pump modules, based at Bush Park, Estover Industrial Estate, Plymouth, PL6 7RG, England. Stephen J. Eglash is marketing manager for 980-nm coolerless pump laser modules at 80 Rose Orchard Way, San Jose, CA 95134. Kao Yang Huang is the program manager of the Fiber Optic Products Group at 625 Industrial Way West, Eatontown, NJ 07724. For more information, contact Wang at [email protected].