Abstract
1. Introduction
Applications of laser diodes (LDs) have been widely spreading into many areas, from the industrial manufacturing like welding and cutting to daily life such as data storage and telecommunication. Red-emitting laser sources play an important role in the medical field and digital appliances[
In this Letter, we adopt the structure of the mode expansion layer to narrow down the vertical beam divergence and analyze the mode behavior in the vertical direction. A low vertical divergence angle of 10.94° is realized in the experiment. The peak power of the broad area (BA) laser can reach 0.94 W, limited by the thermal rollover under continuous-wave (CW) operation. Furthermore, we fabricate the low coherence red LD device, which can achieve speckle contrast of 3.6% and peak power of 1.42 W under pulsed operation. This red LD can also realize the directional emission in both lateral and vertical directions.
2. Passive Mode Analysis and Structure Design
The refractive index of AlGaInP is critical to the calculation of the near-field amplitude, which further influences the far-field intensity profile. We adopt the modified single effective oscillator (MSEO) and related parameters to ensure the accuracy of refractive index of the layers[
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Figure 1.Schematic of the sectional refractive index distribution (left axis) and the optical NFP distribution (right axis). The inset shows the detail of the active region.
In the epitaxial direction, the following layers include 0.1 µm GaInP:Si -buffer layer, 2.5 µm -cladding layer , 1.2 µm mode expansion layer , and the 1 µm -cladding layer , followed the 0.05 µm highly -doped buffer layer GaInP:Mg and 0.2 µm GaAs:C cap layer. The active region consists of 0.05 µm undoped waveguide layers and compressively strained GaInP triple quantum wells (QWs) with a thickness of 5 nm, where the wells are separated by 10 nm barrier layers.
The maximum conduction band offset () of the AlGaInP material system is about 270 meV, while that of the AlGaAs is 350 meV[
Figure 2.Calculated conduction energy band diagram at different doping levels. The inset shows the ΔEc between the waveguide layer and the p-cladding layer at different doping levels.
The thicknesses of the cladding layer () and mode expansion layer () have critical influences on the near field and vertical divergence. We theoretically calculate the vertical divergence angle, which is based on the parameters mentioned above. At the first step, the mode characteristics with a constant of 1.8 µm and different are studied. Figure 3(a) shows the dependence of the calculated Γ, vertical divergence angle at FWHM, and on the , where the is defined as the ratio between Γ of the fundamental mode and the largest confinement factor among the high-order modes. Γ and increase as is increased, and the lowest vertical divergence is realized at a medium thickness of 1.2 µm. Second, the far-field characteristic with a constant of 1.2 µm is studied. Figure 3(b) depicts the vertical divergence decreasing with the increased . For the low vertical divergence structure, the is set to 2.5 µm. Based on the analysis above, the of 1.2 µm and the of 2.5 µm are chosen for the structure. Figure 4 shows the calculated vertical divergence angle, and a vertical divergence angle of 10.7° is obtained.
Figure 3.(a) Dependence of calculated Γ, vertical divergence, and RF/H on dME; (b) dependence of vertical divergence on dn−cladding.
Figure 4.Simulated far-field pattern (FFP) in the fast axis.
In order to realize the low coherence, we prefer to have a number of modes lasing simultaneously by designing the cavity structure. We directly fabricate the low coherence LD according to the good results in our previous work[
3. Device Fabrication and Results
The epitaxial layer is grown by metal organic chemical vapor deposition (MOCVD) on a Si-doped misoriented GaAs substrate tilted 15° off (100) toward [111]. The BA lasers are formed by photolithography and dry etching, and film is deposited as the electrical isolation layer. The injected window is opened by the wet chemical etching. Ti/Pt/Au is sputtered on the top of the highly doped -GaAs layer, which used the PVD method for ohmic contact. The GaAs substrate is thinned down to the 130 µm via mechanical polishing. Finally, the AuGeNi/Au is grown on the side by PVD and annealed at 420°C.
Uncoated BA lasers are fabricated to obtain parameters such as the internal quantum efficiency (), internal optical loss (), modal gain (), and transparent current density (), which can quantitatively assess the quality of the wafer. The BA lasers have a width of 100 µm and different lengths varying from 0.5 mm to 3 mm. The lasers are measured in the pulsed mode with a pulse width of 40 µs at a repetition rate of 100 Hz at 20°C. Figure 5(a) shows the typical cavity length dependence of the inverse external differential quantum efficiency (). We can fit the experimental results and obtain of 71% and of . Figure 5(b) is the representation of threshold current density versus the inverse cavity length 1/L for lasers of different cavity lengths. of and of are extracted from the fitting curve.
Figure 5.(a) Cavity length dependence of the inverse external differential quantum efficiency; (b) threshold current density versus the inverse cavity length.
We adopt as a facet mirror passivation coating. The front facet is coated with 10% anti-reflection (AR) layer, and the rear facet is coated with a 99% reflectivity layer for high reflection (HR). Figure 6 shows the light–current–voltage () and wall plug efficiency (WPE) curves for the device with a cavity length of 1500 µm and a width of 100 µm under 3 A CW at 20°C. The LD has a slope efficiency of 0.71 W/A, a threshold of 1.07 A, and the maximum WPE of 16.04% at 2.3 A. The maximum power of 0.94 W is achieved at 2.7 A, limited by the thermal rollover. The inset shows the emitting peak wavelength at 647.54 nm with an FWHM of 0.65 nm at 1.5 A CW.
Figure 6.Experimental L–I–V and WPE characteristics for 100 µm BA laser with a 1500 µm long cavity. The laser device is operated with coated AR of 10% and HR of 99% under 3 A CW at 20°C heatsink temperature. The inset shows the spectrum at 1.5 A CW.
The contrast of the vertical far-field profile is presented in Fig. 7. We obtain the vertical far-field divergence as low as 10.94° (FWHM) and 19.7° () measured at 1.5 A CW at 20°C, which agrees well with the theoretical result.
Figure 7.FFP of simulation and experiment. The dashed line indicates
To the best of our knowledge, the vertical divergence of 10.94° is the smallest for the red LD based on the mode expansion layer, other than the longitudinal photonic crystal structure.
Figure 8(a) shows the schematic of the low coherence red LD structure. The sizes of the R, , and L are 500 µm, 250 µm, and 1000 µm, respectively. Lasers are tested in pulsed mode with a 40 µs width and 100 Hz repetition rate. The corresponding curves are measured under the same condition at 20°C. The peak power of 1.42 W is obtained at 12 A, as shown in Fig. 8(b). Figure 8(c) shows the spectrum at 10 A at 20°C. The peak wavelength is 643.7 nm with the FWHM of 2.73 nm. Figure 8(d) exhibits the far-field pattern (FFP) with (FWHM) at the current of 10 A, which shows a better directional emission[
Figure 8.(a) Schematic of the low coherence red LD structure; (b) L–I–V curves; (c) the spectrum at 10 A; (d) the FFP at 10 A.
The vertical divergence angles between the low coherence LD and BA LD are different. We doubt that the possible reasons are the lateral mode oscillation and the different test conditions due to the test equipment.
We characterize the low coherence by the speckle contrast. Figure 9(a) shows the schematic of the experimental setup of the speckle measurement. The light emitted from laser is coupled into a fiber (, , ) and collected by the CCD camera (Lt545R, Lumenera) with the pixel size of . We adopt the to calculate the speckle contrast. Figure 9(b) shows the speckle pattern of the low coherence red LD structure, and the speckle contrast is reduced to 3.6%, compared with the 13.3% of the BA laser. We can estimate that the approximate 771 modes lase independently[
Figure 9.(a) Schematic of the experimental setup of the speckle measurement; (b) speckle pattern of low coherence red LD structure; (c) speckle pattern of BA laser.
4. Conclusion
We have demonstrated red LDs with a narrow vertical far field in the 645 nm range. An efficient mode expansion layer is designed to produce a strong penetration of the near-field distribution, which is beneficial to narrow down the FFP. A narrow vertical far-field angle of 10.94° is achieved experimentally. The maximum output power of 0.94 W at 2.7 A, limited by the thermal rollover, is achieved under the CW condition.
We have realized red LDs with a low coherence and directional emission with this epitaxy structure. The peak power of 1.42 W and the lateral divergence of 8.3° are achieved under pulsed operation. The speckle contrast is reduced to 3.6%, which means it has a bright prospect in laser display. With the optimization of epitaxial layers, the low coherence red LD can operate at the CW condition.
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