We present operating and lifetest data on 795 and 808 nm bars with aluminum-free active regions. Conductively cooled bars operate reliably at CW power outputs of 40 W, and have high efficiency, low beam divergence, and narrow spectra. Record CW powers of 115 W CW are demonstrated at 795 nm for 30% fill-factor bars mounted on microchannel coolers. We also review QCW performance and lifetime for higher fill-factor bars processed on identical epitaxial material.
© Optical Society of America
High power diode lasers and bars emitting at 780–810 nm are widely used as pump sources for Ho-, Ho/Tm- and Nd-based solid state lasers, for rubidium pumping in inert-gas magnetic-resonance-imaging (795 nm), and for material processing. For wavelengths shorter than 810 nm, AlGaAs/GaAs lasers using Al in the active region have experienced long-term reliability problems.
Al-free lasers based on InGaAsP active regions with InGaP claddings lattice matched to the GaAs substrate offer a promising alternative to conventional AlGaAs laser diodes. The InGaAsP/GaAs Al-free material system has strong advantages over the conventional AlGaAs/GaAs system due to resistance to the motion of dark-line defects , a lower surface recombination rate , a lower device series resistance, and a higher threshold for catastrophic optical mirror damage (>17 MW/cm2) . However, due to the small bandgap offsets, Al-free lasers suffer from carrier leakage resulting in low characteristic temperatures T0 and T1. The addition of Al in the cladding layers has been shown to improve carrier confinement in 830-nm lasers . Because the mode overlap with the InAlGaP cladding layers is negligible, this design maintains the advantages of an Al-free active region; but it offers higher efficiencies and improved thermal performance over Al-free structures, due to suppressed carrier leakage in the higher bandgap InAlGaP claddings. Because Al is absent in the active regions, we use the term “AAA” to refer to this structure. The present paper reviews our results for 795 and 808 nm continuous wave (CW) and 808 nm quasi-CW (QCW) bars fabricated on AAA material.
The wafers are grown by solid-source molecular beam epitaxy (SSMBE) . The device structure consists of a single tensile-strained Al-free quantum well (QW) surrounded by Al-free waveguide layers and InAlGaP claddings. Additional tensile strain in the QW allows continued high performance characteristics while moving to shorter wavelengths. Due to the tensile strain in the QW, the first light-hole subband (lh1) has higher energy than the heavy-hole subbands. As a result, optical gain for TM-polarized light is significantly higher than that for the TE mode, which explains the TM-polarization of the laser emission. A schematic diagram of the AAA structure is shown in Figure 1.
Figure 2 illustrates the CW bar geometry and conduction-cooled package schematics. For CW applications, bar dimensions are 1 mm by 10 mm, with a 30% fill factor (i.e. nineteen broad-area emitters, each 150-μm wide, spaced on 500-μm centers).
The QCW bars fall into 50% and 90% fill factor categories. The 50% fill-factor bars are nominally 1 mm by 10 mm and have forty-nine 100-μm-wide emitters on 200-μm centers. The 90% fill-factor bars are nominally 0.6 mm by 10 mm, and have sixty 150-μm wide emitters on 160-μm centers.
3.1 808 nm 40 W CW Bars
Forty-watt CW bars are typically mounted junctions down on conductively cooled heatsinks, and are operated at room temperature (25 °C). The typical thermal resistance of the conductively cooled package is ~ 0.7°C/W. This was measured using a variation of the spectral calibration technique in reference . Figure 3 illustrates the typical power vs. current (P-I) characteristics of the 808-nm 40 W bars. Slope efficiencies of ~ 1.2 W/A and power conversion efficiencies of > 52% are typical at 25 °C[7,8]. Typical pulsed threshold current densities and characteristic temperatures T0 are 250–280 A/cm2 and 160–180 °C, respectively. At 40 W the devices operate with a nominal peak wavelength of 808 nm. The spectral widths are typically < 1.5 nm full width at half maximum (FWHM). The nominal beam divergence is 10 degrees by 30 degrees FWHM in the slow and fast axes respectively.
Figures 4(a) and (b) show the thermal performance of the 40 W bars. Shown are the power vs. current and power conversion efficiency vs. current for bars operating over the range of 20 to 60 °C. Due to the low carrier leakage in the AAA structure, the power conversion efficiency exceeds 42% even at 60 °C.
Figure 5 illustrates the lifetime of 40 W 808 nm bars at 25 °C. Thirteen devices were operated in constant power mode. The largest degradation was ~7 %, and four devices show no degradation at all. The cumulative lifetime for the 13 devices approaches 75,000 device hours.
3.2 795 nm 40 W CW Bars
795 nm 40 w bars  have slope efficiencies as high as 1.3 W/A, and power conversion efficiencies as high as 60%. A typical P-I curve is shown in figure 6(a). Figure 6(b) shows a plot of CW threshold current and slope efficiency as functions of temperature. Even at 75 °C, slope efficiency of 795 nm bars is as high as 1.05 W/A. The CW values for T0 and T1 are calculated from the standard expressions:
The CW values for the bars are To=123°C, and T1=300°C.
Figure 7 illustrates the thermal performance of the 795 nm 40 W bars over the 25 °C to 75 °C ambient temperature range. The bars have excellent efficiencies even at 75 °C ambient.
As shown in Figure 8 between 25 °C and 75 °C, the center wavelength shifts from 794 nm to 813 nm with temperature, while the full width at half maximum (FWHM) of the spectra remains below 1.5 nm. Typical spectral widths for 795 nm 40 W bars are < 1.6 nm FWHM. The vertical beam divergence is typically < 32° FWHM.
Ten 795 nm bars have been operating in constant power mode at 40 W and 25 °C for a cumulative time of 46,400 device hours. One device has degraded ~ 13%, and three devices show no degradation at all. This is illustrated in Figure 9.
The biggest limitation to bar performance is the ability to remove heat. The data presented so far was obtained on conduction cooled packages with a measured thermal impedance of ~0.7 °C/W. We have also mounted 795 nm bars on microchannel heat exchangers, which have a thermal impedance of 0.36 °C/W. This has resulted in record powers from 30% fill-factor 795 nm bars, as shown in Figure 10. We were able to extract 115 W of CW power from a bar using a water temperature of 20 °C. It is our expectation that 30% fill-factor devices mounted on microchannel heat exchangers would operate reliably under CW operating conditions at powers in excess of 60W, based on thermal criteria.
3.3 808 nm QCW Bars
QCW bars made from 808-nm AAA material operate efficiently, and have shown good lifetimes. The devices are mounted on conductively cooled heatsinks and are operated at 25 °C. Figure 11 illustrates power versus current for 808 nm 50% fill-factor bars. The bars are operated at 10% duty cycle (1 ms pulses at 100Hz). Figure 11 (b) shows 808 nm 90% fill-factor bars operated at 1% duty cycle (200 μsec pulses at 50 Hz). Both types of bars are capable of reaching > 100 W peak powers, even at higher duty cycles. When mounted on conduction-cooled packages, the 50% fill-factor bars can operate at higher duty cycles (up to CW) at 60 W average powers, while the 90% fill-factor bars are better-suited for high peak power applications (100 W peak at < 5% duty cycle). A typical spectrum in the 200–250 μsec pulse-width, 10% duty cycle regime is centered at 808 nm, and has a width of ~ 2.5 nm FWHM at 100 W for either 50% or 90% fill-factor bar geometry.
Figure 12 illustrates the reliability of 90% fill-factor bars. The bars are operated at 80 A in the constant current mode, at 25 °C. The duty cycle is 25% (200 μsec pulses). We have demonstrated > 1.5×1010 shots with < 10% power degradation under these operating conditions, in spite of water-cooling interruptions.
In conclusion, we have demonstrated reliable 40 W CW operation of 795-, and 808-nm bars with Al-free active regions grown by solid source molecular beam epitaxy. We have also demonstrated record powers of 115 W CW for 30% fill factor 795 nm bars from this material mounted on low thermal resistance microchannel coolers. 808 nm QCW bars are capable of peak powers well in excess of 100 W at low duty cycles, and show good reliability (> 1.5×1010 shots) at 70 W peak power at 25% duty cycle. The excellent thermal performance of the bars, combined with their narrow spectral emission and low beam divergence, makes them ideally suited for pumping, graphics, illumination, medical and industrial applications.
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