Advances make 980-nm pump lasers more reliable

Recent improvements in packaging technology and proprietary chip structures have produced 980-nm pump lasers that are far superior to earlier models.

Norman Kwong, Jeffrey Ungar, and David Huff Ortel Corp.

In response to demands for increased bandwidth, companies continue to turn to erbium-doped fiber amplifiers (edfas) because of their ability to amplify multiple signals at several wavelengths. High-power, highly reliable 980-nm pump lasers have emerged as the transmission engine of choice for pumping edfas used in long-haul fiber-optic telecommunications systems. Recent advances in packaging technology and enhancements in proprietary chip structures have yielded improved and cost-effective 980-nm pump lasers that are far superior to earlier models.

Because edfas are the only active devices in wavelength-division multiplexing (wdm) systems that can affect all channels simultaneously, reliability has received considerable attention. Semiconductor lasers emitting at the 980-nm wavelength were demonstrated more than 10 years ago, but only in the last few years have they attained viability for telecommunications applications.

The substrate for a 980-nm pump laser is GaAs, while the active area is composed of InGaAs quantum wells surrounded by AlGaAs confinement layers (see Fig. 1). Since this structure is patterned laterally on the substrate, the material layers provide only vertical optical confinement. Then the devices are metallized and cleaved, leaving a mirror-smooth surface at each end of the active area. These end facets are typically coated with high-reflection and antireflection coatings to direct most of the light in the output direction.

However, because of the high density of surface electronic states at the cleaved mirror facets, electrons and holes present during operation in the light-emitting layers of the laser near the facet recombine nonradiatively. This depletes the population inversion and makes the active layer near the facet absorptive to the laser output radiation. This absorption, and the high power density at the facet (>107W/cm2), can quickly lead to localized heating, which increases the absorption further. This thermal runaway condition eventually leads to melting of the facet and is called catastrophic optical damage (cod). This degradation can take place instantaneously if optical power exceeds the device specifications. At lower powers, localized heating of the facet will occur over a period of months or years, but still will result in premature facet failure. Degradation of the facet can also be directly caused by electrical breakdown at the facet surface.

Recognition of this phenomenon has stimulated development of devices with degradation-resistant facets. A few vendors use proprietary facet-coating techniques to reduce susceptibility to cod. Others have developed aluminum-free structures in an effort to reduce facet oxidation. Although these devices have demonstrated good reliability, they are still not free of cod at high powers.

Figure 2 shows an alternative, proprietary nonabsorbing facet technology. High-quality, AlGaAs "window" structures are epitaxially grown at the ends of the laser active area, separating the facet from the quantum wells. These AlGaAs windows are highly transparent at the 980-nm emission wavelength, and so are not heated by absorption of the output light. Further, the AlGaAs band-gap is designed to be higher than that of the active area, acting as a barrier to the diffusion of carriers to the surface states at the facet. Therefore, they cannot recombine and start the process leading to cod. The facet windows are also designed to prevent electrical conduction at the facet. These devices have remained cod-free even when pumped to the thermal limit for the device. Devices have functioned to 500-mW facet power without degradation.

Index-guided buried mesa

The type of waveguide used for the pump laser chip strongly influences the performance of the device. A commonly adopted approach, shown in Figure 3, uses a surface-ridge waveguide. Light propagating in the active layer below the surface has an evanescent "tail" extending to the surface of the semiconductor. The closer proximity of the semiconductor-air interface to the active layer on the sides of the chip compared to the center results in weak waveguiding. This design does not provide strong confinement of electrical current to the waveguide region, because current can spread out laterally in the cladding layer above the active region.

An index-guided buried mesa structure solves these problems (see Fig. 4). Here, the beam inside the laser cavity is entirely contained within the bulk of the device. Strong refractive index differences between the active layers and the AlGaAs on the top, bottom, and sides of the laser cavity provide strong optical waveguiding in both horizontal and vertical directions. Use of this structure results in a near-circular, nonastigmatic beam and high pumping efficiency. This strongly confining structure can be made highly resistant to beam shifts that cause light-current nonlinearities or "kinks" in the laser output. Further, the reverse-biased junctions of the cladding AlGaAs layers confine the current to the active area of the device.

Double-enclosure hermetic package

The packaging of the 980-nm chip represents a critical factor in device reliability. All organic materials can lead to long-term facet degradation. Therefore, the chip must be hermetically sealed in a flux-free, epoxy-free environment.

Two layers of hermetic protection can improve the reliability of chips even further. To achieve this enhanced level of protection, carefully handled die are fluxlessly soldered to TO-style headers. Caps with transparent windows are electrically welded to the headers. This sealed unit then provides environmentally secure, convenient handling during manufacturing processes. The coupled devices are then mounted into packages containing the thermo-electric cooler and hermetically sealed a second time. This double hermetic enclosure provides enough protection to ensure the more than 25-year intended lifetime of these devices.

Fiber-grating stabilization

Telecommunications applications require that the wavelength and power remain stable over the device`s lifetime. Furthermore, new amplifier architectures require that the spectrum of the pump remain stable over a variety of power levels and the wavelength of the pump be known or specified precisely.

The addition of a fiber Bragg grating (fbg) in the fiber pigtail of the device also can ensure wavelength precision and spectral stability. The fbg is chosen to provide a narrowband optical reflection at a specific wavelength. The low thermal and aging coefficients of the fbg wavelength assure that the device spectrum is stable with respect to output power, device age, and operating temperature. Because the wavelengths can be precisely and independently defined, pumps can be combined in bidirectional pumping schemes, or with pump-combining wdms. Further, the fbg enhances the stability in the presence of undesired reflections inside the amplifier.

Rigorous burn-in and qualification programs performed in iso 9001-certified facilities can ensure power stability and reliability. Module tests of temperature cycling, high- and low-temperature storage, shock and vibration, esd, etc., should be conducted in accordance with Bellcore`s (Bell Communications Research--Morristown, NJ) tr-nwt-00468 standard. Changes in packaging should be requalified where appropriate.

A system of wafer lot verification and device burn-in can aid screening. In one such program, device burn-in consists of 500 hours at 70C with 350 mA of applied current. Devices pass if they show less than 3 mA of change in threshold current and less than 10 mW of optical power change at 300 mA and 25C. Wafer lots are validated by taking successfully burned-in devices and subjecting them to an additional 1000 hours at burn-in conditions. If one failure is observed, an additional 200 hours of burn-in are required with no additional failures. If there are two failures, the lot is rejected. u

Norman Kwong is vice president and general manager, telecommunications products; Jeffrey Ungar is director, device materials and structures; and David Huff is marketing manager, telecommunications products, at Ortel Corp. (Alhambra, CA).