High-power two-stage cascade GaSb-based type-I quantum well diode lasers emitting near 2 μm were designed and fabricated. Coated devices with cavity length of 3 mm generated about 2 W of continuous wave power from 100-μm-wide aperture at the current of 6 A. The power conversion efficiency peaked at 20%. Carrier recycling between quantum well gain stages was realized using band-to-band tunneling in GaSb/AlSb/InAs heterostructure complemented with optimized electron and hole injector regions. Design optimization eliminated parasitic optical absorption and thermionic emission, and included modification of the InAs quantum wells of electron and composition and doping profile of hole injectors. Utilization of the cascade pumping scheme yielded 2 μm lasers with improved output power and efficiency compared to existing state-of-the-art diodes.

High power semiconductor lasers emitting near 2 μm are in demand for various applications including resonant pumping of the Ho-doped hosts1 and seeding the LIDAR optical systems.2 Both InP-based and GaSb-based type-I quantum well (QW) diode lasers can operate in this spectral region. InP-based lasers can be fabricated within standardized telecommunication technology, while less widespread GaSb-based devices can offer superior efficiency and temperature stability. Cascade pumping scheme can significantly increase the laser efficiency, and it was applied to both InP- and GaSb-based type-I QW laser heterostructures, for instance, see Refs. 3–5. Broken gap band alignment at the heterointerface between InAs and GaSb can be utilized in design of GaSb-based cascade bipolar devices.6 Efficient band-to-band tunneling at GaSb/InAs heterointerface can be realized without excessive doping and, hence, without severe free carrier absorption penalty. This unique feature can be viewed as a critical advantage of the GaSb-based material system for development of high power lasers emitting in spectral region from below 2 to over 3 μm. We reported on design and development of the GaSb-based type-I QW high power cascade diode lasers emitting 960 mW of continuous wave output power near 3 μm.7 The “heart” of the device was tunnel junction/electron injector comprising GaSb/AlSb/InAs/AlSb heterostructure followed by InAs/AlSb chirped superlattice. The junction/injector recycled electrons between type-I QW gain stages in a manner similar to interband cascade lasers utilizing type-II QWs.8 The fundamental absorption edge in InAs QWs of an electron injector defines a minimum wavelength for an efficient laser operation. The injector design used in previous works was still adequate for lasers emitting near 2.4 μm (Ref. 5), but introduced excessive optical loss for shorter wavelength devices. In this work, we report on optimization of the design of the junction/injector region leading to the first efficient operation of the cascade λ ∼ 2 μm diode lasers. The devices with 3-mm-long cavities and anti-/high-reflection coated mirrors demonstrated CW output power level near 2 W from 100–μm-wide aperture at room temperature. This is a significant improvement compared to previous reports on non-cascade laser performance, namely, ∼1.3 W (Ref. 9) or ∼1.5 W (Ref. 10) per 100 μm of output aperture.

Figure 1 shows the schematic band diagram of the laser heterostructures studied in this work. The devices had 1.5-μm-thick Al0.8Ga0.2As0.07Sb0.93 cladding layers and 400-nm-wide Al0.3Ga0.7As0.03Sb0.97 waveguide core layers on both sides of the active region containing two 8-nm-wide Ga0.75In0.25Sb QWs. All aforementioned layers are typical for high power GaSb-based type-I QW non-cascade diode lasers emitting near 2 μm.10 The non-cascade diode laser structure would contain about 20 nm or so of waveguide core material between QWs.11 The devices considered in this work had tunnel junction and carrier injector layers connecting QW1 and QW2 in series rather than in parallel (Figure 1). For electron transport, these layers should (a) stop electrons from going from QW1 to QW2 through conduction band; and (b) provide for efficient electron delivery from valence band of GaSb to conduction band of QW2. The former must be accomplished by band offset between QW1 and AlGaAsSb graded barrier (Figure 1, left inset), while the latter is to be achieved by tunneling from valence band of 10-nm-wide GaSb layer through 2.5-nm-wide AlSb layer into conduction band state of the first InAs QW in chirped AlSb/InAs SL electron injector (Figure 1, right inset).

FIG. 1.

Schematic band diagrams of the laser heterostructure under flat band condition. Insets show enlarged parts of laser heterostructure containing AlGaAsSb linearly and compositionally graded layer grown as a random alloy and tellurium doped (nominal 5e17 cm−3 in InAs layers only) AlSb/InAs/…/InAs/AlSb (2.5/3.0/1.2/2.2/1.2/1.8/1.2/1.5/1.2/1.2/1.2/0.9/1.2/0.9/2.5 nm) chirped superlattice electron injector.

FIG. 1.

Schematic band diagrams of the laser heterostructure under flat band condition. Insets show enlarged parts of laser heterostructure containing AlGaAsSb linearly and compositionally graded layer grown as a random alloy and tellurium doped (nominal 5e17 cm−3 in InAs layers only) AlSb/InAs/…/InAs/AlSb (2.5/3.0/1.2/2.2/1.2/1.8/1.2/1.5/1.2/1.2/1.2/0.9/1.2/0.9/2.5 nm) chirped superlattice electron injector.

Close modal

Devices having four different tunnel junction and carrier injector layers were considered. Figure 2 plots the pulsed (200 ns/100 kHz) light-current characteristics of the 100-μm-wide ridge, 1-mm-long uncoated lasers mounted epi-side up. The devices using AlGaAsSb layer compositionally graded from 50% to 5% of aluminum and AlSb/InAs electron injector with 4.2-nm-wide first InAs QW were hardly able to start lasing at room temperature. These types of devices were designed with junction/injector structure similar to the one previously utilized for the demonstration of the cascade diode lasers emitting in spectral region from ∼2.4 to ∼3.3 μm (Ref. 5). The experiment confirmed the anticipated loss increase for wavelength near 2 μm caused by strong fundamental absorption in 4.2-nm-wide InAs QW (calculations predict cut-off wavelength about 2.3 μm). In order to move the cut-off wavelength below 2 μm, the AlSb/InAs electron injector (Figure 1, right inset) was redesigned and had the first InAs QW thickness decreased down to 3 nm (the rest of the chirped superlattice was modified appropriately). The devices based on new injector design (structure A, Figure 2) demonstrated reduced threshold and increased efficiency, and became comparable to reference double-QW diode lasers at room temperature.

FIG. 2.

Light–current–voltage characteristics measured for 100-μm-wide, 1 mm-long uncoated lasers in low duty cycle regime (200 ns/100 kHz) at 20 °C. Insets show laser spectra near thresholds.

FIG. 2.

Light–current–voltage characteristics measured for 100-μm-wide, 1 mm-long uncoated lasers in low duty cycle regime (200 ns/100 kHz) at 20 °C. Insets show laser spectra near thresholds.

Close modal

Strong improvement of the laser performance was achieved, when initial composition of the AlGaAsSb graded layer was increased to 80% aluminum and its doping level was set to nominal 1017 cm−3 using beryllium acceptors (structure B, Figure 2). Addition of the 30-nm-thick Al0.8Ga0.2As0.07Sb0.93 constant composition layer just before 100-nm graded layer led to further slight increase of the device efficiency (structure C, Figure 2). Figure 3 plots modal gain spectra measured by Hakki–Paoli method12 for lasers based on structures A and B. Long wavelength part of the gain spectra saturates to total cavity loss and, thus, allows estimating an internal optical loss to be 4–5 and 6–7 cm−1 for devices with structures A and B, respectively. Thus, the difference in optical loss level cannot explain improved efficiency in devices with increased aluminum content and doping level in graded layer. However, the transparency current and differential gain with respect to current do improve in devices based on structure B. Specifically, transparency current decreases nearly twofold, while differential gain increases nearly fourfold in lasers based on structure B compared to devices based on structure A.

FIG. 3.

Modal gain spectra measured by Hakki–Paoli method at several pulsed (200 ns/1 MHz) currents under threshold at 20 °C for 100-μm-wide, 1-mm-long uncoated two-stage cascade diode lasers based on structures A and B.

FIG. 3.

Modal gain spectra measured by Hakki–Paoli method at several pulsed (200 ns/1 MHz) currents under threshold at 20 °C for 100-μm-wide, 1-mm-long uncoated two-stage cascade diode lasers based on structures A and B.

Close modal

Reduction of the transparency current accompanied by enhancement of the differential gain and after threshold efficiency can be explained by suppression of the thermionic leakage of electrons from QW1 in lasers based on structure B. Indeed, increase of the initial Al composition from 50% to 80% is expected to increase the band offset between Γ minimum in QW1 and X minimum in AlGaAsSb graded barrier by about 50 meV. Introduction of the doping in AlGaAsSb graded layer, arguably, minimizes voltage drop required for hole transport from GaSb to QW1, hence further improving the barrier limiting the electron thermionic emission and, possibly, tunneling. This scenario is supported by measurement of the slope efficiency and threshold current density of lasers A and B in wide temperature range (Figure 4). In the low temperature limit, structure A has adequate barrier to stop electron leakage from QW1, and the corresponding devices demonstrate similar threshold current densities and slope efficiencies compared to lasers based on structure B. However, at elevated temperatures, the devices based on structure B have better efficiency and lower threshold. Further reinforcement of the barrier (aimed at stopping possible field emission) with 30 nm of Al0.8Ga0.2As0.07Sb0.93 in devices with structure C led to only marginal improvement (Figure 2), i.e., within the range of typical device-to-device variation. Thus, both structures B and C yielded efficient lasers. These devices were characterized by parameters T0 ≈ 65 K and T1 ≈ 300 K as measured for 1-mm-long uncoated lasers in pulsed regime (200 ns/100 kHz) in temperature range from 20 to 60 °C. The laser threshold current density at 20 °C was just below 100 A/cm2 for uncoated 1-mm-long devices, i.e., similar to the best previously reported values for high power non-cascade λ ∼ 2 μm lasers.10,11 Laser injection efficiency near room temperature was estimated as ∼180% confirming efficient carrier recycling between cascaded gain stages.

FIG. 4.

Temperature dependences of the threshold current densities and slope efficiencies measured in pulsed regime (200 ns/30 kHz) for 100-μm-wide, 1 mm-long uncoated lasers based on structures A and B, respectively.

FIG. 4.

Temperature dependences of the threshold current densities and slope efficiencies measured in pulsed regime (200 ns/30 kHz) for 100-μm-wide, 1 mm-long uncoated lasers based on structures A and B, respectively.

Close modal

Temperature dependence of the power–current–voltage characteristics of lasers based on structure A (Figure 5) shows peculiarities that can be invoked to explain the strong dependence of the laser threshold, efficiency, and differential gain on AlGaAsSb graded barrier properties. At temperatures below ∼250 K, the devices demonstrate classic laser behavior with linear continuous increase of the output power after well-defined threshold current. Starting from ∼280 K, the transition to lasing occurs abruptly at threshold currents that are higher than “virtual thresholds,” i.e., the one which could be estimated by linear extrapolation of the power–current curve to zero power levels. This peculiarity becomes more apparent at higher temperatures, implying the underlying phenomenon is thermally activated (Figure 5). Moreover, the actual device threshold depends on how power–current curve is measured, i.e., by increasing or by decreasing current (Figure 6). The actual threshold determined when decreasing current is lower while “virtual threshold” remains the same for both ways of changing current during measurement. The abrupt jump to lasing is accompanied by abrupt voltage change (Figures 5 and 6). The voltage–current dependence demonstrates apparent hysteresis as well (Figure 6).

FIG. 5.

Light–current–voltage characteristics measured in pulse regime (200 ns/30 kHz) in temperature range from 79 to 340 K for 100-μm-wide, 1 mm-long uncoated lasers based on structure A.

FIG. 5.

Light–current–voltage characteristics measured in pulse regime (200 ns/30 kHz) in temperature range from 79 to 340 K for 100-μm-wide, 1 mm-long uncoated lasers based on structure A.

Close modal
FIG. 6.

Light–current–voltage characteristics of 100-μm-wide, 1-mm-long uncoated lasers based on structure A measured in CW regime at 20 °C by increasing and decreasing current.

FIG. 6.

Light–current–voltage characteristics of 100-μm-wide, 1-mm-long uncoated lasers based on structure A measured in CW regime at 20 °C by increasing and decreasing current.

Close modal

The plausible explanation can be based on assumption that at elevated temperatures, the electron transport in conduction band of structure A cannot be stopped by nominally undoped AlGaAsSb barrier with insufficient band offset and bended bands (due to possible residual deep donor charge and/or voltage drop needed for hole transport through nominally undoped layer). As a result, electrons populate both QW1 and QW2 somewhat similarly to the case of standard multiple-QW diode lasers, where QWs are connected in parallel. Unsatisfactory connection of QWs in series obliterates an advantage of the cascade pumping scheme. This factor and severe charge misbalance in QWs lead to increase of the transparency current and reduction of the differential gain. Eventually, enough electrons accumulate in QW1 and holes in QW2 to build an electric field for efficient band-to-band tunneling through GaSb/AlSb/InAs heterostructure. At this moment, net available gain increases and device jumps to lasing. Redistribution of the voltage drop between QWs under thus established favorable pumping conditions leads to reduction of the net voltage drop across laser heterostructure. Hysteresis can be explained by somewhat smeared difference in pumping levels of two QWs interacting through a large number of intracavity photons after threshold. In structures B and C, the proper carrier recycling between QWs is restored due to increased band offset and p-doping of AlGaAsSb barrier. In this case, the current through laser heterostructure starts flowing only when band-to-band tunneling is established and both QWs contribute to the gain. The corresponding laser efficiency is doubled and transparency current is nearly halved. The differential gain is increased nearly fourfold, presumably, due to mutual action of carrier recycling and improved charge balance in both QWs.

Figure 7 shows light–current–power conversion characteristics measured in CW regime for 100-μm-wide 3-mm-long coated lasers based on structure C. Insets show the laser emission spectra and far field patters of these two-stage cascade lasers. Cascade design improves the slope efficiency leading to record ∼2 W output power level in near 2 μm spectral region. The devices demonstrated the power conversion efficiency near 20% at the output power of about 700 mW and above 12% at maximum power level. These parameters were achieved using lasers soldered epi-down with indium directly onto gold plated copper blocks.

FIG. 7.

CW light–current–power conversion characteristics measured at 17 °C for 100-μm-wide, 3-mm-long AR/HR coated cascade lasers. Insets show the device's emission spectra and measured fast axis far field pattern.

FIG. 7.

CW light–current–power conversion characteristics measured at 17 °C for 100-μm-wide, 3-mm-long AR/HR coated cascade lasers. Insets show the device's emission spectra and measured fast axis far field pattern.

Close modal

In summary, we report on the demonstration of the high power cascade diode lasers emitting near and below 2 μm. To develop cascade heterostructure compatible with operating wavelength near 2 μm, the carrier injectors had to be redesigned compared to those used in 3 μm lasers. It was required to reduce thickness of the InAs QWs in InAs/AlSb electron injector to increase a transition energy between quantized states and eliminate fundamental absorption near 2 μm. In addition, the composition and doping level of the AlGaAsSb graded composition layer serving simultaneously as a hole injector and electron stopper were modified to suppress parasitic thermionic leakage of electrons. Optimization of the carrier injector regions yielded devices emitting near 2 μm with room temperature threshold current densities ∼80 A/cm2 and producing nearly 2 W of CW power from 100-μm-wide stripe at room temperature.

The work was supported by U.S. Army Research Office, Grant No. W911NF1110109, and National Science Foundation, Grant No. ECCS-1408126.

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