Both square-shaped and circular-shaped flattop modes were experimentally demonstrated in extended-cavity broad-area VCSELs using aspheric feedback mirrors. These refractive aspheric mirrors were fabricated by electron-beam lithography on curved substrates. Excellent single-mode operation and improved power extraction efficiency were observed. The three-mirror structure of the VCSEL and the state-of-the-art fabrication of the aspheric mirror contribute to the superior VCSEL performance. The modal loss analysis using a rigid three-mirror-cavity simulation method is discussed.
© 2004 Optical Society of America
Vertical cavity surface emitting lasers (VCSELs) offer tremendous advantages over traditional edge-emitting semiconductor lasers because of their low fabrication cost, low threshold current, circular-mode profile, and high modulation speeds. In addition, two-dimensional VCSEL arrays can be easily fabricated due to their planar structure and orthogonal emission to the wafer surface. While VCSELs have become the ideal choice in many low-power applications, their utilization is limited in high-power applications such as high-speed optical communication and optical storage, where high-power single-spatial-mode emitters are preferred. Single-spatial-mode VCSELs are often limited by aperture size. The resulting small gain volume restricts the maximum single-mode power to just a few milliwatts . There have been numerous efforts aimed at designing large-aperture single-spatial-mode VCSELs during the past years. Among those efforts, the extended-cavity VCSELs consisting of epitaxially grown VCSELs and collinearly aligned external elements [2,3] have attained some of the highest single-mode powers. In this paper, we describe flattop mode shaping in an extended-cavity broad-area VCSEL with an external aspheric feedback mirror.
In many of the previous single-mode VCSEL designs, the primary goal was to improve the cavity modal discrimination (the squared ratio of the fundamental-mode eigenvalue and the next-higher-order mode eigenvalue [/]). In addition to improving the modal discrimination, our current design promotes single-mode oscillation through spatial hole burning. The higher-order modes are effectively prevented from lasing because the fundamental flattop VCSEL mode can deplete virtually all the VCSEL gain. Furthermore, the uniform flattop mode can interact with the uniformly pumped VCSEL gain medium more efficiently for more laser power. Excellent single-mode operation and improved power extraction efficiency were observed experimentally. The generated flattop VCSEL mode is expected to find applications ranging from laser pumping to optical illumination.
It has been shown that the spatial mode of a laser cavity can be shaped into a specific intensity profile by using an aspheric mirror as a fixed phase-conjugate surface in a mode-conjugate resonator [4,5]. So far this aspheric resonator technique has only been successfully applied in high-gain lasers due to the fabricated aspheric mirror loss. In this paper, for the first time, we demonstrate flattop mode shaping in a low-gain VCSEL by using an external-cavity aspheric mirror in combination with the existing top mirror. The combination of the reduced-reflectivity top DBR of the VCSEL and the external aspheric mirror can be viewed as an etalon . The equivalent reflectivity of this etalon is significantly greater than either of the reflecting surfaces alone. This three-mirror resonator configuration contributes to the cavity loss reduction and enables lasing in the low-gain VCSEL medium at the desired spatial mode.
Both square-shaped and circular-shaped flattop VCSEL modes are experimentally discussed and implemented in this paper. While the square-shaped flattop mode dramatically demonstrates the mode-shaping capabilities of the aspheric extended-cavity technique, the circular-shaped flattop mode better utilizes the geometry of the circular VCSEL aperture and offers higher conversion efficiency and power performance.
2. External-cavity VCSEL configuration and aspheric mirror fabrication
We have previously demonstrated mode shaping in a three-mirror solid-state laser consisting of an aspheric third mirror collinearly aligned with a two-mirror Fabry-Perot resonator . A novel numerical method based on the Fox-Li simulation was used for the modal analysis. In this simulation method, the spatially dependent optical-field exchange through the center mirror was considered for modeling the coupled-cavity resonator. Because of the small Fresnel number associated with the three-mirror solid-state laser, only mild mode shaping, namely Gaussian mode shaping, was implemented. Taking advantage of the large Fresnel number of a VCSEL cavity, we discuss the implementation of more dramatic mode shaping—flattop mode shaping—in this paper.
The VCSEL used in this study was from NOVALUX Inc. The multiple InGaAs quantum wells provide relatively high gain at λ~970 nm, and the reduced top DBR of this VCSEL gives an approximate reflectivity of 80%. These characteristics make this laser particularly suitable for external-cavity operation. The circular aperture of this VCSEL is 100 µm in diameter. The VCSEL was mounted on an electrical-thermal cooling stage with a stabilized temperature of 25°C for the optimal gain at the DBR wavelength. To maximize the modal discrimination, the external cavity was formed by placing an aspheric mirror 6~7 mm from the VCSEL aperture. A pellicle beamsplitter was inserted in between the aspheric mirror and the VCSEL aperture to directly sample the lasing mode for measurement.
The external aspheric mirror has a surface profile exactly matching the wavefront of a diffracted flattop mode whose beam waist is at the VCSEL aperture, as shown in Fig. 1. A 20th-order square SuperGaussian with a beam width of 80 µm (or a beam waist of 40 µm) was chosen as the desired mode, and the aspheric mirror was to be placed 7 mm away from the VCSEL aperture. (A nth-order SuperGaussian is defined as φ(x)=exp[-x2n/ω0]) The same setting was used for the square-flattop mode shaping discussed elsewhere in this paper.
The aspheric mirrors for mode shaping have been fabricated as diffractive elements using wet and dry etching, and as continuous reflective elements using diamond turning. The losses associated with these two techniques at near-visible wavelengths make them unsuitable for low-gain lasers. Fortunately, the surface figure of an aspheric mirror for flattop mode shaping has an underlying envelope quite close to a sphere (Fig. 1). We therefore chose to fabricate the aspheric mirror on a spherical substrate. The small phase departure from the spherical surface was fabricated by electron-beam lithography. The electron beam only needs to expose a thin resist layer, reducing the defocus through the resist layer and the proximity effect. When the commercially available concave substrate of high surface precision is combined with a somewhat less accurate micro-fabricated surface relief structure, the overall aspheric mirror quality can be significantly improved. We fabricated our aspheric mirror on a BK7 blank concave mirror with a radius of curvature of 9.3 mm. A 2-µm-thick layer of PMGI electron-beam resist was spun on top of the substrate for electron-beam exposure. Fig. 2 shows the surface of a fabricated aspheric mirror viewed with an interference microscope (Zygo New View 100) where the underlying spherical envelope has been removed. For a 512 µm×512 µm mirror size, the maximum surface relief depth was ~600 nm, while the maximum relief depth would have needed to be ~4 µm if the aspheric structure were fabricated on a flat substrate. The subsequently developed aspheric mirror was then coated with 20-nm-thick titanium followed by 100-nm-thick gold for optical reflection.
Unlike the two-mirror Fabry-Perot cavities used in many other reports of beam shaping, the external-cavity VCSEL has three mirrors aligned collinearly. To accurately model the cavity modes and modal discrimination, a three-mirror cavity simulation was adopted to include coupled-cavity effects .
Figure 3 shows the simulated modal loss and modal discrimination as a function of the external cavity length d 2 of an extended-cavity VCSEL. The resonator parameters used in this computation were chosen to simulate the experimental parameters: the effective length of the VCSEL cavity d 1 is 3 wavelengths, the VCSEL aperture is 100 µm in diameter, the top DBR reflectivity is 80%, the effective reflectivity of the aspheric mirror is 95% (taking into consideration the combined loss on the aspheric mirror and the pellicle beamsplitter). We set the diameter of the external aspheric mirror (also an effective aperture) to be 0.2d 2—large enough to eliminate the modal discrimination induced by this aperture. In this simulation, a 20th-order circular SuperGaussian with a beam diameter of 90 µm (or a beam waist of 45 µm) was chosen as the fundamental mode. The fundamental-mode diffraction loss changes very little with d 2, and both the high-order-mode diffraction loss and the modal discrimination reach their maximums with d 2 around 5 mm, roughly a Rayleigh range of a fundamental Gaussian with the same beam waist. Similar curves were also obtained when a square-shaped SuperGaussian was chosen as the fundamental mode. We therefore chose the external cavity lengths to be around 5 mm in our extended-cavity VCSEL design for maximizing the modal discrimination.
Figure 4 shows the experimental square-flattop mode in the VCSEL by using an external aspheric mirror with a surface figure depicted in Fig. 2. Rectangularly pulsed current with a repetition rate of 1 kHz and pulse durations of 1 µs was used to drive the VCSEL. The same pulsed current was used in other pulse-driven-VCSEL measurements; the performance of CW-driven VCSELs will be discussed at the end. Figure 4(a) is the near-field intensity with a CCD camera imaging the VCSEL plane. Figure 4(b) is the far-field intensity with the CCD camera directly placed 10 cm away from the VCSEL aperture. Distinct side lobes at the predicted spatial locations (the expected Fourier transform of the near-field pattern) in the far field are clearly visible.
A circular-flattop mode with a diameter of 90 µm was also implemented to better utilize the geometry of the circular VCSEL aperture. For both the square-flattop mode shaping and circular-flattop mode shaping, excellent modal discrimination was obtained. Figure 5 shows the measured spectrum of the extended-cavity VCSELs with different configurations under pulsed driving currents at ~3I th. The blue dotted curve reveals a broad spectrum of closely spaced spatial modes in a concave extended-cavity VCSEL, constructed by placing a concave mirror (radius of curvature of 9.3 mm) at roughly two times the focal length away from the VCSEL aperture. Since the external concave mirror just imaged the VCSEL aperture back into itself, this extended-cavity VCSEL was equivalent to a monolithic broad-area VCSEL with a full-grown top DBR. The red solid curve is the measured spectrum of an extended-cavity VCSEL with an external aspheric mirror for the circular-flattop mode shaping. This sharp spectrum indicates single-spatial-mode oscillation.
Fig. 6 shows the measured single-mode power as a function of pump currents for the extended-cavity VCSELs with different configurations under pulsed driving currents. The blue dashed curve is for the VCSEL with a concave feedback mirror, and the red solid curve is for the VCSEL with an aspheric feedback mirror for the circular-flattop mode shaping. As mentioned previously, placing an external concave mirror at 2×f (focal length) away from the VCSEL aperture only results in a multi-spatial-mode oscillation. One way to obtain single-mode concave extended-cavity VCSEL is to tune the external mirror away from the 2f position. In our experiment, the external concave mirror was moved about 4 cm closer to the VCSEL aperture, resulting in an external cavity length of 5.3 cm. Single-spatial-mode operation was observed at all current levels for this concave extended-cavity VCSEL design. The L-I curves indicate that the aspheric extended-cavity VCSEL had a slightly higher threshold current due to the mirror fabrication imperfection and misalignment, but had 20% higher slope efficiency compared with that of the concave extended-cavity VCSEL.
The laser slope efficiency is defined as:
where ħω is the photon energy, q is the electron charge, ηd is the differential efficiency, ηint is the internal quantum efficiency, αoc is the output coupling loss, and αint is the total of other losses. Note that we used the pellicle beamsplitter as the output coupling element with a coupling loss of αoc , and any power loss on the external mirror is considered to be part of αint . Since the concave and the aspheric extended-cavity VCSELs have the same beamsplitter setting for the power output, they have same αoc . The fact that the concave extended-cavity VCSEL has a lower threshold implies smaller αint . Thus an improved internal quantum efficiency ηint for the aspheric extended-cavity VCSEL is deduced from the slope efficiency measurement.
The power performance of the extended-cavity VCSELs under CW driving currents was also investigated. The aspheric extended-cavity VCSEL had about the same slope efficiency as that of the concave extended-cavity VCSEL at low pumping currents, but experienced an earlier thermal rollover (at 6I th for the aspheric extended-cavity VCSEL versus 8I th for the concave extended-cavity VCSEL). This was because thermal lensing was not modeled in the aspheric extended-cavity VCSEL design, but could not be ignored in the case of CW operation. Power performance of the aspheric extended-cavity VCSEL can be improved by better heat removal or incorporation of the thermal lensing effect into the external aspheric mirror design.
In conclusion, we have demonstrated flattop mode shaping in extended-cavity broad-area VCSELs with aspheric feedback mirrors. The effective etalon consisting of the reduced-reflectivity top DBR mirror and the high-quality aspheric mirror fabricated by electron-beam lithography on a curved substrate significantly lowered the fundamental mode loss, and made the mode shaping in a low gain laser possible for the first time. This extended-cavity VCSEL design possesses the advantages of high modal discrimination and improved power extraction efficiency.
The authors would like to thank Aram Mooradian of Novalux Inc for the assistance and material support.
References and links
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