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Abstract
High-power high-brightness super large optical cavity laser diodes with an optimized epitaxial structure are investigated at the wavelength of 980 nm range. The thicknesses of P- and N-waveguides are prudently chosen based on a systematic consideration about mode characteristics and vertical far-field divergences. Broad area laser diodes show a high internal quantum efficiency of 98% and a low internal optical loss of 0.58 cm−1. The ridge-waveguide laser with 7 μm ridge and 3 mm cavity yields 1.9 W single spatial mode output with far-field divergence angles of 6.8° in lateral and 11.5° in vertical at full width at half maximum under 2 A CW operating current. The corresponding M2 values are 1.77 and 1.47 for lateral and vertical, respectively, and the corresponding brightness is 76.8 MW‧cm−2‧sr−1. The far-field divergence angles with 95% power content are in the range of 24.7° to 26.1° across the whole measured range.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power high-brightness edge-emitting laser diodes are of interest for a wide range of applications including materials processing [1], pump sources [2], as well medicine and biology [3]. Lasers emitting at 980 nm range are key components for Erbium- and Ytterbium-doped fiber lasers [4]. However, the conventional thin waveguide design usually leads to edge-emitting diode lasers suffering from a high facet optical load and a very large divergence angle in the fast axis (vertical to the epitaxial layers), and thus restricts their applications in many fields. A higher coupling efficiency and a larger alignment tolerance can be achieved by a lower vertical divergence. The results have been demonstrated both simulation and experiment [5].
Several approaches have been introduced in laser designs to reduce the vertical divergence angle, such as the asymmetric waveguide [6], the large optical cavity [7], the super large optical cavity (SLOC) [8–11], the photonic band crystal waveguide [12–15], the coupled large optical cavity [16], the modified Bragg-like waveguide (MBW) [17], and the high-brightness vertical broad-area edge-emitting laser [18]. These methods have reduced the vertical divergence angle to less than 11° at full width at half maximum (FWHM) [10–15,17]. Especially for the designs of SLOC combining with low-index quantum barriers [10] and MBW [17], they achieved vertical beam divergence of 15.6° and 9.8° enclosing 95% power content (θ95%), that is very meaningful for the laser applications. In 2002, J. N. Walpole et al. proposed the concept of slab-coupled optical waveguide laser (SCOWL) [19]. Recently, SCOWL has record-high diffraction-limited CW output power of 3 W and a maximum wall-plug efficiency (WPE) of 45% [20]. By proper mis-orientation substrates, an extraordinarily high WPE of 59% was realized [21]. However, except for SCOWL, most of the above epitaxial structures are very thick and the WPE of lasers are no more than 40% by typical packaging. Although there is some reported work on high power ridge waveguide lasers with high WPE [5,22], the vertical θ95% needs to be improved further
In this paper, we report on 980 nm lasers based on 5.2 μm thick asymmetric waveguide SLOC structure with reduced internal optical loss (αi) and improved WPE. At first we designed and calculated the structure parameters for the SLOC by the commercial tool LaserMod [23]. The epiwafer was grown by metal organic chemical vapor deposition (MOCVD) and processed into both multimode broad area (BA) lasers and single-spatial-mode ridge-waveguide (RW) lasers. Then the electro-optical properties of the fabricated lasers were studied.
2. Passive mode analysis and epitaxial design
In order to achieve a low vertical divergence beam, we designed and optimized the super large optical cavity structure with an active region located in the waveguide asymmetrically. In our epitaxial structure, the waveguide was designed to be Al0.2GaAs. For quantum-well lasers, αi caused by free-carrier absorption depends on their doping densities in light doping region linearly and in heavy doping region quadratically, and most of αi for semiconductor lasers occur in active region and in heavily doped P-cladding [24]. In order to prevent the fundamental mode from penetrating into the P-cladding layer, the aluminum content of the P-cladding layer is chosen as 0.5, producing a large refractive index difference of 0.17 compared to the waveguide. In lightly doped semiconductors, the free-hole absorption coefficient for photon is almost twice larger than that of free-electron at 980 nm [25,26]. According to some direct measurements, it actually could be as large as 10 times greater at 1550 nm [27]. Therefore, the designed structure is composed of a thick N-waveguide and a thin P-waveguide to realize a low vertical divergence and a low αi. Al0.3GaAs is constructed as N-cladding, and the thicknesses of P- and N-cladding are fixed to 800 nm and 600 nm, respectively.
In our preliminary idea, the thickness of N-waveguide (TN-w) is designed in the range of 3 μm to 6 μm. Thus, at the first step, we fixed the TN-w to be center of preconception namely 4.5 μm, and calculated the dependences of mode characteristics and vertical divergences on the thickness of P-waveguide (TP-w) for both single quantum well (SQW) and double quantum wells (DQW). The active region consisted of undoped 7 nm In0.16GaAs quantum well(s) sandwiched between 10 nm GaAs barriers. Figures 1(a) and 1(b) presents the calculated optical confinement factors (OCFs) in the active region (ΓQW(s)) for the first ten order modes as function of TP-w for both SQW and DQW. They show that only the ΓQW(s) of the fundamental mode increases with P-waveguide getting thicker, and the ΓQW(s) of high order modes vary within a certain range. For SQW structure, the ΓQW(s) of the fundamental mode becomes larger than ΓQW(s) of all other high order modes until the TP-w increases to 700 nm. For DQW structure, the critical thickness is 500 nm. In addition, with the increasing of TP-w, the increase rate of ΓQW(s) for the fundamental mode is getting smaller. The TP-w for this phenomenon is 900 nm and 700 nm for SQW and DQW structure, respectively. The right axis of Figs. 1(a) and (1)b show the ratios between ΓQW(s) of the fundamental mode and the biggest ΓQW(s) among other high order modes (RF/H). The larger RF/H for thicker P-type waveguide indicates an improved mode discrimination. It is obvious that the RF/H values of SQW structure are smaller than that of DQW structure.
Fig. 1 The dependence of the calculated ΓQW(s) (left axes) and the RF/H (right axes) on the TP-w (a) SQW, (b) DQW.
Figure 2 shows the dependences of the calculated vertical divergences at FWHM and OCFs of the fundamental mode in P-waveguide and P-cladding layer (ΓP-side) on the TP-w. The omitted data in the figure imply that the calculated far-field patterns are multi-lobes under the conditions. For both SQW and DQW structures, the vertical divergence reduces rapidly with the increase of TP-w (when the thickness less than 700 nm), and the ΓP-side increases nearly linearly with the increment of TP-w. In fact, the fraction of the fundamental mode penetrating into P-cladding is negligible. It is because the main intensity distribution locates in N-waveguide, and the refractive index step between P-waveguide and P-cladding is large enough. For the same TP-w, SQW structure achieve a smaller vertical divergence and a lower ΓP-side than that of DQW structure. However, as shown in Figs. 1(a) and 1(b), the RF/H values of SQW structure are smaller than that of DQW structure. In view of reducing vertical divergence, decreasing αi, and preventing from lasing high order modes, we choose the active region as DQW and 700 nm P-waveguide.
Fig. 2 The dependence of calculated vertical divergences angles (left axis) and ΓP-side values (right axis) on the TP-w.
After we fixed the active region as DQW and TP-w to be 700 nm, we calculated the mode characteristics and vertical divergences with various TN-w in the range of 3 μm to 6 μm in a step of 250 nm. Figure 3(a) presents the dependence of the calculated ΓQWs and RF/H on TN-w. Figure 3(b) shows the vertical far-field angles and ΓP-side in dependence of TN-w. As presented in Figs. 3(a) and 3(b), ΓQWs, ΓP-side, and vertical divergence are decreasing with the increase of TN-w. However, the thicker the N-waveguide becomes, the lower the rate of decrease is, especially for ΓP-side and vertical divergence. This phenomenon can be contributed to the influence of QWs on the waveguide properties in such waveguide thicknesses [26]. Above all, we choose the TN-w as 4.5 μm. The total thickness of waveguide is 5.2 μm.
Fig. 3 The dependence of (a) calculated ΓQWs (left axis) and RF/H (right axis) and (b) calculated vertical angles (left axis) and ΓP-side (right axis) on the TN-w.
After the epitaxial structure was finalized, we calculated the dependence of series resistance on doping profile. Based on our designs, the doping of N- and P-cladding were fixed to be 1 × 1018 cm−3. We calculated the series resistances of devices with 100 μm stripe and 2 mm cavity. The results are shown in Table 1. It shows that the series resistance induced by doping profile increases from 9.1 mΩ to 40.9 mΩ when the waveguide doping are changed from 1 × 1018 cm−3 to undoped. Unfortunately, the improvement of series resistance will lead to deterioration of internal loss. Finally, the doping was chosen as 1 × 1017 in N-waveguide and 5 × 1016 in P-waveguide.
Table 1. The dependence of series resistance on doping profile
3. Device fabrication
The epitaxial structure was grown on (1 0 0) GaAs substrate in an flip top CCS 6x2” Aixtron MOCVD system. The source gases were TMIn, TEGa, TMAl, DEZn, SiH4, AsH3, and PH3. The growth temperature and the chamber pressure were 600°C and 100 mbar, respectively. The V/III ratios of all epitaxial layers were over 100. The epiwafer was processed into both BA lasers with stripe width of 100 μm and RW lasers. For BA lasers, the heavy doped contact layer was removed by wet etching. For RW lasers, ridge waveguides were formed by pairs of 30 μm wide and deep-etched trenches. According to our calculation results, the thickness of residual layer above the active region was chosen to be 300 nm, introducing an effective index step of 2.71 × 10−3 between the trench and the ridge. In the experiment, the 1437 nm deep trenches were defined by inductively coupled plasma dry etching, corresponding to an effective index step of 2.84 × 10−3. The width of ridge was chosen to be 7 μm in order to achieve a single spatial mode output according to our previous experimental work [22]. Part of BA devices with uncoated facets and different cavity lengths were used to analyze the general characteristics of the epitaxial structure. The other BA lasers and RW lasers were cleaved into 3 mm-long bars and coated with 99% high-reflection and 4% antireflection coatings. The devices were mounted P-side down on Cu mounts which has a thermal resistance of 7.3 W/K.
4. Results
Uncoated BA lasers were measured in pulsed mode with a duration of 30 μs and a repetition of 100 Hz. We use the Eq. (1)
to fit the αi and the internal differential quantum efficiency (ηi) of the epitaxial structure by analyzing the function relation between external quantum differential efficiency (ηex) and device length [28]. The reflectivity of cleaved facet is R ≈30%. Figure 4 shows the dependence of reciprocal ηex (left axis) on the cavity length at heat sink temperature T = 20°C. The devices show a high ηi of 98% and a low αi of 0.58 cm−1. The ηex of 1 mm long device is as high as 93%. Owing to the low αi, 4 mm long devices have a relatively high ηex of 82%.Figure 5 (a) shows the light-current-voltage (L-I-V) and the WPE characteristics of BA lasers measured at 20°C under continuous wave (CW) mode. The laser diodes have a slope efficiency of 1.16 W/A. After the driving current being larger than 7 A, the slope efficiency has an obvious drop due to the thermal limitation. The output power only increases 0.64 W when the current increases from 9 A to 10A. The laser has a threshold of 0.6 A, a maximum WPE of 56.7% at 4.9 A, and a maximum power of 9.9 W in CW mode. The output power reaches saturation due to the thermal roll-over.
Fig. 5 Measured L-I-V and WPE characteristics for (a)100 μm BA laser with 3 mm long cavity and (b) 7 μm wide ridge RW lasers with 3 mm long cavity under CW mode at 20°C. The inset of (b) shows the measured optical spectrum at 1.5 A.
The L-I-V and the WPE characteristics of 3 mm long RW lasers with 7 μm ridge are shown in Fig. 5(b) at heat sink temperature of 20°C in CW mode. The lasers have a threshold of 75 mA, a slope efficiency of 1 W/A, and a maximum WPE of 54.8% at 400 mA. A maximum optical power of 2.4 W is achieved at 2.7 A limited by COMD. As shown in the inset, the peak wavelength is emitting at 980.2 nm measured at CW current of 1.5 A.
The vertical and lateral far-field distributions are measured in gradual current to characterize the far-field divergence and investigate the stability of vertical divergence depending on driving current. Figures 6(a) and 6(b) show the measured far-field profiles for 3 mm long RW lasers with 7 μm wide ridge using a rotating photodiode under CW mode at heat sink temperature of 20°C. Single lobe far-field angles are obtained in both vertical and lateral directions over the whole measurement range. As presented in Fig. 6(a), the SLOC structure yields low vertical divergences below 13° at FWHM. Figure 6(b) shows that the lateral divergences are smaller than 8°. The θ95% are increased to about 26° in vertical and 14° in lateral, respectively. Compared to lateral divergences, vertical θ95% suffer from more remarkable increases. This is due to more intensity distribution in the wings of the vertical divergence distribution than that of lateral due to the impact of QWs [26].
Fig. 6 Measured far-field profiles for a 3 mm long RW lasers with 7 μm ridge using a rotating photodiode under CW mode at heat sink temperature of 20°C. (a) vertical, (b) lateral. (c) M2 values for both lateral and vertical direction (left axis) and brightness (right axis) versus driving current.
Figure 6(c) shows the beam quality factors M2 which were measured at heat sink temperature of 20°C by a commercial beam propagation analyzer system of M2-200s-FW (Ophir-Spiricon, LLC), which based on the second moment method recommended by ISO 11146. For 3 mm long RW lasers with 7 μm wide ridge, M2 values in vertical direction are in the small range of 1.40 to 1.61 indicating the stable single mode emission. M2 values in lateral direction suffer from a sharp increment from 1.28 to 1.84 with the driving current increasing from 400 mA to 800 mA and then vary within a range of 1.69 to 1.88. Due to the lateral divergence do nearly not change on power, the sharp increment results from the extending for the wings of the beam waist intensity distribution in lateral direction. Using the Eq. (2)
where B is brightness, P is optical power, and are lateral and vertical M2 values respectively, and λ is the peak wavelength, a maximum brightness of 76.8 MW‧cm−2‧sr−1 is obtained at 2 A injection current.5. Discussion
The value of θ95% must be minimized in real commercial system [10]. Some designs have been introduced and reported on the achievements of θ95% [9–11,14,15,17]. References [10] and [11] using the design of SLOC combined with low-index quantum barriers to reduce the θ95% to 15.6°and achieved a high brightness of 90 MW‧cm−2‧sr−1 at 1060 nm range. However, the maximum WPEs for BA lasers and RW lasers are only 34% and 26.4%, respectively. In [17], BA laser demonstrated a very low θ95% of 9.8°, but the WPE is no more than 30% according to estimation. In our work, we demonstrated 3 mm long RW lasers with 7 μm wide ridge achieved 1.9 W single spatial mode operation at 2A, and the corresponding brightness is 76.8 MW‧cm−2‧sr−1. The lasers obtained a maximum WPE of 54.8% and a maximum output power of 2.4 W, limited by COMD. The results of θ95% are in the range of 24.7° to 26.1° across the whole measuring range. Compared with [10,11,17], although the WPEs of both BA lasers and RW lasers of our design have a significant improvement, the θ95% is still large. In fact, 5% power content distributes in the range of more than 6°, based on our analysis of all far-field distributions. The work for the future is to eliminate the wings of the vertical far-field distribution, and it would reduce the θ95% significantly. Besides, we suppose that a higher output power and brightness can be obtained by introducing facet passivation and better package condition [29].
6. Conclusion
In summary, we presented 980 nm high-power high-brightness lasers based on systematic optimized asymmetric super large optical cavity (SLOC). Uncoated BA laser diodes show a high ηi of 98% and a low αi of 0.58 cm−1. 3 mm long BA laser achieved a slope efficiency of 1.16 W/A, a maximum output power of 9.9 W at 10 A, and a maximum WPE of 56.7% at 4.9 A. 3 mm long RW laser with 7 μm wide ridge achieved a maximum output power of 2.4 W and a maximum WPE of 54.8%. The values of θ95% are in the range of 24.7° to 26.1° across the whole measurement range. Under 2 A CW driving current, RW laser provided a 1.9 W single spatial mode output with the M2 values of 1.77 in lateral and 1.47 in vertical, and the corresponding brightness is 76.8 MW‧cm−2‧sr−1.
Funding
National Key R&D Program of China (2016YFA0301102, 2016YFB0402203); National Natural Science Foundation of China (Grant Nos. 61535013, 61234004, and 61675139).
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