Aluminum-free Technology Improves Diode Laser Performance
By: Dan Botez, University of Wisconsin-Madison
Contents
Compatible materials
Diode lasers for 0.73-0.81 µm
Incoherent, high-power devices at 0.98 µm
Coherent high-power device at 0.98 µm
Aluminum-free (Al-free) diode lasers have been the subject of intense research because of several advantages they offer compared to their aluminum-containing counterparts. Lasers with Al-free active regions (ALFAs) have significant advantages over diode lasers containing aluminum in the active region, such as aluminum gallium arsenide (AlGaAs)-based lasers for the wavelength range from 0.70 to 0.83 µm. Experimental data have demonstrated that for the wavelength range from 0.78 µm to 0.81 µm , ALFA lasers can provide reliable operation at output powers at least twice as high as those achievable from from AlGaAs lasers.1-3
Also , due to the high Pcomd value of indium gallium arsenide(InGaAs)-active devices , 0.98 µm- emitting lasers have recently demonstrated record-high continuous wave(CW) and quasi-CW powers for single-stripe diodes(see Figure 1) .
These results are a direct consequence of the differences in power density at catastrophic optical mirror damage (PCOMD) between devices with indium gallium arsenide phosphide (InGaAsP) and AlGaAs active regions.4 Furthermore, for the 0.70 to 0.78 µm wavelength range, (In)AlGaAs-active devices tend to be unreliable due to bulk degradation5-6. Al-free devices thus open up the 0.70 to 0.78 µm wavelength range for commercial use.
Compatible materials
A second key advantage of Al-free lasers is that they are based on indium gallium phosphide (InGaP). This material has a low reactivity to oxygen, making it compatible with regrowth processes for spatial-mode control (i.e. index-guided devices) 7,8 or frequency-mode control (i.e. distributed-feedback (DFB) lasers).9 This feature has allowed researchers to fabricate continuous wave (CW), watt-range devices capable of producing spatially coherent powers in stable beams, 10 as well as broad-stripe DFB lasers that generate watt-range CW powers with narrow spectral width (less than 1 Å).9 Finally, the use of the InGaP for the device cladding layers opens the wavelength range from 1.1 to 1.25 µm for commercial use. In the case of InGaAs devices with AlGaAs claddings, compressive strain in the structure imposes an upper emission wavelength limit of 1.1 µm. InGaP claddings, unlike AlGaAs claddings, are almost perfectly lattice-matched to the GaAs substrate, which allows designers to extend the wavelength range of InGaAs-active CW devices to values as large as 1.25 µm.11, 12
Diode lasers for 0.73-0.81 µm
Al-free devices operating at 0.73 µm have recently demonstrated substantial increases in maximum CW power as well as the maximum reliable power. Using broad waveguide (BW), 100-µm stripe devices with a GaAsP tensile-strained quantum well (QW), researchers from the Ferdinand Braun Institute (Berlin, Germany) have generated CW powers as high as 7 W, and demonstrated close to 3000 hours of reliable operation at 0.5 W CW. 13
At the University of Wisconsin (Madison, WI), our group has demonstrated CW output powers of 1 W from Al-free devices with operating lifetimes of 1000 hrs. The BW-type devices incorporate InGaAsP strain-compensated QW for the active layer. 14 The use of InGaAsP rather than GaAsP for the active layer presents two key advantages: the presence of In atoms eliminates sudden device failures; and the devices have higher potential reliable power since the PCOMD value for the InGaAsP is roughly 60% higher than that for GaAsP. 4, 13 Based on the PCOMD value for the InGaAsP active-layer material, 100-µm aperture ALFA devices operating at 0.73 µm can potentially achieve reliable CW output powers as high as 3 to 4 W.
At the important wavelength of 0.81 µm, researchers from Coherent Inc. (Santa Clara, CA) reported 40 W of reliable CW output for more than 6000 hrs. from thirteen 1-cm laser bars. 2, 15 More recently, at CLEO '99 (May 23-28; Baltimore), Coherent introduced the first commercially-available 60 W CW laser bars operating at 0.80 to 0.82 µm. The current performance is mostly limited by packaging and heat-management considerations; 1-cm bars could potentially produce from 80 to 100 W CW. In contrast, commercially-available 0.81µm AlGaAs-active bars only produce about 20 W CW, and only one manufacturer (Siemens) commercially advertises reliable 40 W CW from bars with InAlGaAs active regions.
Incoherent, high-power devices at 0.98 µm
Record CW and quasi-CW (QCW) operation has recently been reported for both 100-µm-stripe (see Figure 2) 16 and 200-µm-stripe InGaAs/InGaP(As)/GaAs lasers. 17 The 2-mm-long devices have 1.3-µm-wide waveguides and are mounted on copper heatsinks. Maximum output powers reported to date for 100-µm-aperture devices are 11 W CW and 14.3 W QCW. This is true not only for devices operating at 0.98 µm, but for any operating wavelength emitted from conventionally facet-passivated diode lasers. As a consequence of the low surface recombination velocity of In0.20Ga0.80As, the PCOMD for these devices is quite high: 18 MW/cm2 and 23 MW/cm2 for CW and QCW operation, respectively. In comparison, the CW PCOMD value for 100-µm-stripe Al0.1Ga0.9As devices is 7-8 MW/cm2.
These record high powers reflect the fact that BW devices possess both a large equivalent (tranverse) spot size, and a temperature-insensitive external differential quantum efficiency hd—the value for hd decreases by only 3% between 20° C and 70° C, corresponding to a high value for the characteristic temperature T1 of 1800 K. For a given active-layer material, a high T1value is critical to achieving both high CW power and maximum possible PCOMD 16 For instance, temperature-sensitive devices (i.e., T1 ~200 K), reflect severe carrier leakage out of the quantum well, which causes them to experience either thermal rollover, which limits the maximum power, or reach COMD at power values roughly 70% of those for temperature-insensitive devices (T1 >1000 K). Further improvements can be achieved by decreasing the thermal resistance of the heatsink. 16
By using an identical BW laser structure to that discussed above, researchers from Sarnoff Corp. (Princeton, NJ) and the University of Wisconsin-Madison fabricated and tested 200-µm stripe devices. 17 The experimental devices demonstrated the highest powers achieved for any type of single stripe diode laser: 16.8 W CW and 23.5 W QCW (see Figure 1).
In addition, a preliminary lifetime test performed at 6 W CW demonstrated extremely robust performance (see Figure 3). Over the course of a 1200-hr. test at an operating temperature of 45° C, the device output decreased by only 6%. An extrapolation of this data predicts room-temperature operation of about 104 hr at the same output power level. 17
It should be noted that 6 W CW is at least a factor of three higher than previously reported reliable CW output powers for any type of single-stripe diode lasers. The performance is made possible by the use of InGaAs active layers and the attendant PCOMD, a large transverse spot size (0.66 µm), and a high T1 value (1800 K).
Coherent high-power device at 0.98 µm
Over the last decade, researchers have extended a significant amount of effort in achieving watt-range coherent power from large-aperture (greater than 100 µm) diode lasers. Devices such as the fanout-type master-oscillator power-amplifiers (MOPAs) and the a-DFB laser have displayed high diffraction-limited, single-frequency powers. Lacking lateral-mode confinement, however, such devices possess inherent instabilities stemming from refractive-index variations induced by thermal gradients and/or injected carriers; these issues raise serious questions of long-term stability and reliability. 18-21 There is thus a need for coherent, large-aperture devices that not only select fundamental-mode operation but also maintain a stable mode to high drive levels.
To achieve lateral-mode stability from large-aperture (100 to 200 µm) emitters, one has to introduce a periodic structure with strong built-in index guiding (Dn > 0.01). 22 The fabrication of such structures, called photonic lattices, involves regrowth processes. Al-free materials are well-suited to such fabrication techniques, because they offer oxide-free surfaces.
Researchers recently demonstrated a 40-element resonant optical waveguide (ROW) phase-locked laser arrays with an index step Dn=0.1 (see Figure 4). 10 The optical mode for the array is supported in low-index, high-gain regions called antiguides.22 The high effective index regions are created by embedding high-index GaAs stripes in the p-type InGaP upper cladding layer. The optimal element and inter-element widths, d and s, are 4 µm and 0.8 µm, respectively, yielding a 191-µm-wide aperture for the 40-element array.
The beam patterns for CW operation are shown in Figure 5. At the 1.6 W CW power level, the coherent power in the main lobe is 1.0 W. At 1.6 W and drive currents up to nine times greater than threshold, the lateral beam divergence is only twice the diffraction-limited value.
The price for coherency is not too high. At output powers of 0.8 W and 1.6 W CW, the devices operate at wallplug efficiencies hp of 24% and 23%, respectively. These values are roughly half those achieved by devices without spatial-mode control, which produce unstable beams with diameters 40 to 50 times as wide as the diffraction-limited value.
Thus, high Dn ROW arrays have produced watt-range, coherent CW powers with stable beams. With further device optimization, the potential exists for stable, near-diffraction-limited beam operation to output powers as high as 3 W CW.
Aluminum-free diode lasers have opened up two wavelength windows for reliable high-power CW operation: 0.70 to 0.78 µm, and 1.1 to 1.25 µm. At 0.98 µm, wide-stripe devices have generated record CW and QCW output powers. In addition, Al-free technology allows for the fabrication of stable-beam, watt-range coherent devices. Once strictly the province of the research lab, Al-free diode lasers have achieved commercial viability. Current and future developments should see the adoption of these devices for a host of applications.
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About the author…Dan Botez is the Philip D. Reed Professor of Electrical Engineering and director of the Reed Center for Photonics at the University of Wisconsin, 1415 Engineering Dr., Madison, WI 53706. Tel: 608-265-4643; Fax: 608-265-4623; e-mail: botez@engr.wisc.edu.