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Transverse mode confinement in lithographic VCSELs

2017; Institution of Engineering and Technology; Volume: 53; Issue: 24 Linguagem: Inglês

10.1049/el.2017.2780

1350-911X

D.G. Deppe, J. Leshin, J. Leshin, Latika Eifert, Frank M. Tucker, T. Hillyer,

Molecular Junctions and Nanostructures

Electronics LettersVolume 53, Issue 24 p. 1598-1600 PhotonicsFree Access Transverse mode confinement in lithographic VCSELs D.G. Deppe, Corresponding Author ddeppe@sdphotonics.com sdPhotonics LLC, 4304 Scorpius Street, Orlando, FL, 32816 USA Also with D.G. Deppe: College of Optics and Photonics, Building 53, University of Central Florida, Orlando, FL, USASearch for more papers by this authorJ. Leshin, sdPhotonics LLC, 4304 Scorpius Street, Orlando, FL, 32816 USASearch for more papers by this authorJ. Leshin, sdPhotonics LLC, 4304 Scorpius Street, Orlando, FL, 32816 USASearch for more papers by this authorL. Eifert, Army Research Laboratory, Simulation and Training Technology Center, 12423 Research Parkway, Orlando, FL, 32826 USASearch for more papers by this authorF. Tucker, Army Research Laboratory, Simulation and Training Technology Center, 12423 Research Parkway, Orlando, FL, 32826 USASearch for more papers by this authorT. Hillyer, Army Research Laboratory, Simulation and Training Technology Center, 12423 Research Parkway, Orlando, FL, 32826 USASearch for more papers by this author D.G. Deppe, Corresponding Author ddeppe@sdphotonics.com sdPhotonics LLC, 4304 Scorpius Street, Orlando, FL, 32816 USA Also with D.G. Deppe: College of Optics and Photonics, Building 53, University of Central Florida, Orlando, FL, USASearch for more papers by this authorJ. Leshin, sdPhotonics LLC, 4304 Scorpius Street, Orlando, FL, 32816 USASearch for more papers by this authorJ. Leshin, sdPhotonics LLC, 4304 Scorpius Street, Orlando, FL, 32816 USASearch for more papers by this authorL. Eifert, Army Research Laboratory, Simulation and Training Technology Center, 12423 Research Parkway, Orlando, FL, 32826 USASearch for more papers by this authorF. Tucker, Army Research Laboratory, Simulation and Training Technology Center, 12423 Research Parkway, Orlando, FL, 32826 USASearch for more papers by this authorT. Hillyer, Army Research Laboratory, Simulation and Training Technology Center, 12423 Research Parkway, Orlando, FL, 32826 USASearch for more papers by this author First published: 01 November 2017 https://doi.org/10.1049/el.2017.2780Citations: 2 AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract Index confinement is studied experimentally and through modelling for lithographic vertical-cavity surface-emitting lasers (VCSELs) and contrasted with other types of VCSELs. Modelling shows that the index confinement is set by the height of a shallow internal mesa that produces the optical mode confinement. Overgrowth with a semiconductor mirror enables a range of index confinement from zero (no index guiding) similar to proton-implanted VCSELs to much higher confinement than viable for oxide VCSELs. Lasing spectra are studied for side-mode-suppression ratio and beam patterns are compared for high and low indexes confined VCSELs of different sizes. Introduction Optical mode confinement has become an important design parameter for commercially available vertical-cavity surface-emitting lasers (VCSELs). Proton-implanted VCSELs first demonstrated in 1989 generally operate without a sizable built-in lateral index guide [1], and generally also have low efficiency. Oxide VCSELs, demonstrated in 1993 [2], in contrast have higher efficiency and can be designed over a range of transverse index values that depend on oxide thickness and placement in the cavity. However, the transverse index value of the oxide VCSEL cannot easily be made to be zero. In this Letter, data and analysis are presented that show that lithographic VCSELs can be made to operate with greater range of mode confinement than proton-implanted or oxide VCSELs. This increases the range of designs possible for single-mode operation [3], and very small cavity VCSELs. Owing to their internal mesa structure, transverse index values of lithographic VCSELs can in fact be zero such as for proton-implanted VCSELs. This low-index confinement can then be used to increase laser cavity size for single-mode operation [4], and thus increase the VCSEL power over single-mode oxide VCSELs. The opposite regime of much higher-index confinement can improve efficiency of the VCSEL that can yield higher optical power up to a few hundred milliwatts needed for chip-scale atomic clocks, for example [5]. The high-index confinement is also needed to continue the progression of higher-speed VCSELs for use in data centres and optical interconnects. Device structure The internal confinement is analysed in Fig. 1. Proprietary design and growth enable the intracavity mesa that confines the VCSEL's optical mode and shown in Fig. 1 to be inserted into a high-quality VCSEL cavity. The range of heights of interest for confinement range from 0 to ∼250 Å. The mesas can be fabricated with a precision of a few angstroms in height. The change in cavity length that occurs between where the mesa exists and where it is absent establish the transverse effective index of the VCSEL [6]. The effective index values can be calculated directly from changes in longitudinal cavity resonance due to changes in cavity length. The effective index change values versus ΔL, the mesa height, are shown in the plot of Fig. 1. Although in principle, quite thick oxide layers could be used to produce large index change for oxide VCSELs, these cause reliability and fabrication problems. Lithographic VCSELs of various sizes were studied with two different intracavity mesa heights estimated to be ∼15 Å in a low-index VCSEL, and ∼150 Å in a high-index VCSEL. The effective index values of the two cavities were determined by analysing the resonance shifts in the cavity regions that either included or did not include the intracavity mesa. The effective index change for the low-index cavity is estimated to be Δn∼2.5 × 10−3, while the effective index for the high-index cavity is estimated to be Δn∼2.5 × 10−2. Modal properties of both VCSELs for similar laser cavity sizes but the contrasting mode confinement have been studied for their spectral and beam properties that include their side-mode-suppression ratio (SMSR) and beam divergence properties for various drive levels. The spectral separation between the lowest and first-order transverse cavity modes are used to estimate the laser mode sizes for each of the different-sized VCSELs. The estimated mode sizes agree with the design values based on fabrication masks for both high- and low-index VCSELs. Fig 1Open in figure viewerPowerPoint Change in effective index inside and outside cavity a Plot of effective transverse index change versus change in cavity lengths between the regions that either include or do not include the intracavity mesa b Schematic of the mesa structure Experimental results Fig. 2 shows the impact of mode confinement on the SMSR of 4 µm diameter, 5 µm diameter, and 6 µm diameter VCSELs of low- and high-index designs. For each size the low-index confinement extends the range of SMSR > 30 dB over current drive significantly. The 4 µm low-index VCSEL remains strictly single transverse mode (SMSR > 30 dB) throughout its full operating range. The 5 and 6 µm low-index VCSELs each have much higher SMSRs than for the 5 and 6 µm high-index VCSELs, respectively, for similar drive currents. Fig 2Open in figure viewerPowerPoint Plots of SMSRs versus drive current above threshold for Δn = 2.5 × 10−2 and 2.5 × 10−3 VCSELs of 4, 5 and 6 µm diameter sizes Fig. 3 compares the beam quality patterns measured for different currents for the 4 µm high- and low-index VCSELs. The effects of thermal lensing are generally observed in either implanted or oxide VCSELs. The thermal lensing produces a reduction of the VCSEL's actual mode size, and a broadening of the far-field radiation pattern with increasing drive current. Fig. 3 shows that the 4 µm low-index VCSEL remains stable in its far-field pattern over its full range of operating currents. This mode stability has been observed consistently for the 4 µm low-index VCSELs relative to the 5 and 6 µm low-index VCSELs, and the 4, 5, and 6 µm high index VCSELs. The measurements for the 4 µm high-index VCSEL are also shown. The increasing spot sizes for given distances for the high index 4 µm VCSEL indicate both reduction in lasing spot size and self-focusing to a different position just outside the VCSEL cavity. These changes in beam pattern are consistent with thermal lensing inside the VCSEL cavity. Fig 3Open in figure viewerPowerPoint Plots of beam size versus distance for 4 µm diameter VCSELs a Δn = 2.5 × 10−3 b Δn = 2.5 × 10−2 Fig. 4 shows the advantage of the high-index confinement for overcoming this problem and reaching high efficiency at small VCSEL size. The 2 µm diameter VCSEL based on high-index confinement also produces an SMSR ≥ 30 dB for transverse modes over its full operating range, and delivers high power and high efficiency at power levels of 1∼2 mW. This small aperture size for high-index confinement can improve the efficiency over existing oxide VCSELs for low-power sensors such as chip-scale atomic clocks. Fig 4Open in figure viewerPowerPoint Measured data plotted for optical output power versus current and efficiency versus current for the 2 µm Δn = 2.5 × 10−2 VCSEL. The SMSR for transverse modes remains > 34 dB through the full operating range of the VCSEL and power conversion efficiency is recorded high for this size VCSEL at 42% In addition, aperture size variations that are inherent in the oxide VCSEL manufacturing can also be minimised and possibly eliminated from impacting manufacturing yield. The lithographic VCSEL, therefore, solves both problems, and promises continued improved scaling properties for both low- and high-power sensor applications in a commercially viable VCSEL technology. Conclusion In summary, data have been presented showing that lithographic VCSELs can achieve both higher and lower effective index values than either commercial oxide or proton-implanted VCSELs. In addition, the low-index lithographic VCSELs can still operate at high efficiency, and smaller aperture size than proton-implanted VCSELs. Further development of low index lithographic VCSELs could lead to higher power laser sources needed for sensors such as magnetometers, gyrometers, and accelerometers needing 5–20 mW. In addition, refinement of the technology using smaller apertures appears capable of improving efficiency and uniformity over that which can be achieved by oxide VCSELs. Improvements in laser yield can also be expected through improved aperture control to more precisely set lasing wavelength and other operating characteristics. Acknowledgments This material was based on work supported by, or in part by, the U.S. Army Research Laboratory under contract W911NF14C0088 and the U.S. Army Research Office under grant W911NF1510579. The authors thank Dr. Mike Gerhold for useful conversations related to this submission. 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