Multiwavelength HCG-VCSEL Array - Semantic Scholar
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Multiwavelength HCG-VCSEL Array Yi Rao, Christopher Chase, and Connie J. Chang-Hasnain Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, 94720, USA e-mail: [email protected]
Abstract: A multiwavelength HCG VCSEL array has been demonstrated experimentally operating at 1550 nm. The operating wavelength of single mode VCSELs spans from 1540 nm to 1591 nm. Multiwavelength vertical cavity surface emitting lasers (VCSEL) arrays are potentially a low cost, low power, monolithically-integrated and high speed solution for wavelength division multiplexing (WDM) and optical interconnects. Previously, multiwavelength VCSEL arrays have been achieved by varying the thickness of one layer inside of the VCSEL cavity during epitaxial growth [1], which changes the cavity roundtrip path length, and thus lasing wavelength. However, this approach lacks flexibility and controllability, and the range of the wavelength shift is limited by achievable thickness variation during growth. Here we show a new method to achieve multiwavelength arrays with pure post-growth fabrication, which does not involve any complicated growth techniques. The new approach is based on high contrast gratings (HCG), which have been shown as a light-weight top mirror in tunable VCSELs [2] with a tuning range of ~20 nm. For some applications such as coarse WDM (CWDM), wavelength tunability is not required and is an added complication and cost, so it is also desirable to have a fixed multiwavelength array. Recently, it was proposed that a multiwavelength HCG array could be achieved by varying the HCG design to obtain a phase response as a function of the design [3]; however, this approach has not been experimentally demonstrated to date. Additionally, the wide band (>20 nm) ultra high reflectivity (>99%) of the HCG has never been experimentally demonstrated before. Here, we use an alternative approach to achieve a multiwavelength VCSEL array – varying the air gap between the active region and HCG by design. We use this approach to experimentally demonstrate HCG VCSELs spanning 50 nm from the same epitaxial wafer with the wavelength determined by the fabrication process. Additionally, this approach demonstrates that the HCG is indeed highly reflective across over 50 nm of wavelengths.
Fig. 1 Schematic of a multiwavelength VCSEL array achieved by varying the air gap between the HCG and the rest of the cavity
The multiwavelength array structure is shown schematically in Fig. 1. Wavelength variation across the structure is achieved by varying the air gap size underneath the HCG. The full VCSEL cavity used here is described in [4]. The optical path length between the 2 mirrors, one of which is the bottom DBR, and other is the HCG, determines the lasing wavelength. Devices with shallower air gaps have more high index semiconductor in the cavity than those with a deeper air gap. Thus these devices have a longer optical path in the cavity and will then emit on a redder wavelength. Limiting the range of wavelengths possible is the reflectivity bandwidth of the mirrors. Fig. 2 a) shows the reflectivity of the HCG (blue) and bottom DBR (green), which consists of 45 pairs. The HCG (Fig. 2a inset) is a TE HCG where reflectivity is high for light polarized along the direction of the HCG. For the devices shown here, the HCG dimensions are Λ=~1070 nm, a=~700 nm, and tg=195 nm. In this case, the tuning range is limited by the DBR, which has enough reflectivity to support lasing from ~1530 nm to ~1595 nm. By adding more pairs of DBR or replacing the DBR with a dielectric mirror, the potential wavelength range could be further increased. The HCG bandwidth could also be further increased. The simulated reflectivity of the HCG as a function of period (Λ) and
978-1-4244-5684-0/10/$26.00 ©2010 IEEE
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grating air gap (a) is shown in Fig. 2 b). The HCG design here was optimized so that it is highly tolerant to errors in grating air gap, the most difficult to control value in HCG fabrication. This design shows a tolerance to >±100 nm in air gap variation. The tradeoff for the tolerance is bandwidth, but by adjusting the HCG dimensions, a bandwidth >100 nm can be obtained as shown previously [2].
a) b) Fig. 2 a) Simulation of HCG and bottom DBR reflectivity, 99% reflectivity band is from 1530 to 1595; b) Design tolerance of HCG is calculated at fixed wavelength of 1550 nm, and the grating air-gap variation could be >±100 nm.
HCG VCSELs were fabricated with different air gaps but otherwise similar structures. Fig. 3 a) shows the light-current characteristics of these devices and b) shows the optical spectrum at a fixed current of 10 mA. Some variation in output power and threshold current is seen in the devices. This is likely due to a lack of uniformity in the HCG and other fabrication-related variations as well as the wavelength dependence of gain. With further fabrication optimization, we hope to achieve more uniform device characteristics regardless of the lasing wavelength. Devices were obtained with output wavelengths ranging from 1540 nm to 1591 nm. This is the first experimental verification that the HCG is reflective enough for a VCSEL to lase over such a broad wavelength range (>50 nm).
a) b) Fig. 3 a) LI curve and b) spectrum of multi-wavelength HCG VCSELs under continuous wave operation at room temperature. All the devices in b) are biased at 10mA, and wavelength range is from 1540nm to 1591nm.
In summary, we present a novel multiwavelength HCG-VCSEL array. VCSELs were fabricated spanning a 50 nm wavelength range with a simple post-growth, process-determined wavelength. In addition, ultra high reflectivity (>99%) of the HCG across >50 nm is experimentally demonstrated for the first time. We gratefully acknowledge support from the CIAN NSF ERC under grant #EEC-0812072. [1] [2] [3] [4]
C. Chang-Hasnain, M. Maeda, N. Stoffel, J. Harbison, L. Florez, and J. Jewell, Electron. Lett. 26(13), 940–941 (1990). M.C.Y. Huang, Y. Zhou, and C.J. Chang-Hasnain, Nature Photonics, 2, 180 (2008). V. Karagodsky, B. Pesala, C. Chase, W. Hofmann, F. Koyama, C. J. Chang-Hasnain, Optics Express, 18, 694-699 (2010). C. Chase, Y. Rao, C. J. Chang-Hasnain, “Single-Mode, Long-Wavelength HCG-VCSEL Using Proton-Implant-Defined Aperture”, submitted to International Semiconductor Laser Conference 2010, Kyoto, Japan.
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Multiwavelength HCG-VCSEL Array Yi Rao, Christopher Chase, and Connie J. Chang-Hasnain Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, 94720, USA e-mail: [email protected]
Abstract: A multiwavelength HCG VCSEL array has been demonstrated experimentally operating at 1550 nm. The operating wavelength of single mode VCSELs spans from 1540 nm to 1591 nm. Multiwavelength vertical cavity surface emitting lasers (VCSEL) arrays are potentially a low cost, low power, monolithically-integrated and high speed solution for wavelength division multiplexing (WDM) and optical interconnects. Previously, multiwavelength VCSEL arrays have been achieved by varying the thickness of one layer inside of the VCSEL cavity during epitaxial growth [1], which changes the cavity roundtrip path length, and thus lasing wavelength. However, this approach lacks flexibility and controllability, and the range of the wavelength shift is limited by achievable thickness variation during growth. Here we show a new method to achieve multiwavelength arrays with pure post-growth fabrication, which does not involve any complicated growth techniques. The new approach is based on high contrast gratings (HCG), which have been shown as a light-weight top mirror in tunable VCSELs [2] with a tuning range of ~20 nm. For some applications such as coarse WDM (CWDM), wavelength tunability is not required and is an added complication and cost, so it is also desirable to have a fixed multiwavelength array. Recently, it was proposed that a multiwavelength HCG array could be achieved by varying the HCG design to obtain a phase response as a function of the design [3]; however, this approach has not been experimentally demonstrated to date. Additionally, the wide band (>20 nm) ultra high reflectivity (>99%) of the HCG has never been experimentally demonstrated before. Here, we use an alternative approach to achieve a multiwavelength VCSEL array – varying the air gap between the active region and HCG by design. We use this approach to experimentally demonstrate HCG VCSELs spanning 50 nm from the same epitaxial wafer with the wavelength determined by the fabrication process. Additionally, this approach demonstrates that the HCG is indeed highly reflective across over 50 nm of wavelengths.
Fig. 1 Schematic of a multiwavelength VCSEL array achieved by varying the air gap between the HCG and the rest of the cavity
The multiwavelength array structure is shown schematically in Fig. 1. Wavelength variation across the structure is achieved by varying the air gap size underneath the HCG. The full VCSEL cavity used here is described in [4]. The optical path length between the 2 mirrors, one of which is the bottom DBR, and other is the HCG, determines the lasing wavelength. Devices with shallower air gaps have more high index semiconductor in the cavity than those with a deeper air gap. Thus these devices have a longer optical path in the cavity and will then emit on a redder wavelength. Limiting the range of wavelengths possible is the reflectivity bandwidth of the mirrors. Fig. 2 a) shows the reflectivity of the HCG (blue) and bottom DBR (green), which consists of 45 pairs. The HCG (Fig. 2a inset) is a TE HCG where reflectivity is high for light polarized along the direction of the HCG. For the devices shown here, the HCG dimensions are Λ=~1070 nm, a=~700 nm, and tg=195 nm. In this case, the tuning range is limited by the DBR, which has enough reflectivity to support lasing from ~1530 nm to ~1595 nm. By adding more pairs of DBR or replacing the DBR with a dielectric mirror, the potential wavelength range could be further increased. The HCG bandwidth could also be further increased. The simulated reflectivity of the HCG as a function of period (Λ) and
978-1-4244-5684-0/10/$26.00 ©2010 IEEE
11
grating air gap (a) is shown in Fig. 2 b). The HCG design here was optimized so that it is highly tolerant to errors in grating air gap, the most difficult to control value in HCG fabrication. This design shows a tolerance to >±100 nm in air gap variation. The tradeoff for the tolerance is bandwidth, but by adjusting the HCG dimensions, a bandwidth >100 nm can be obtained as shown previously [2].
a) b) Fig. 2 a) Simulation of HCG and bottom DBR reflectivity, 99% reflectivity band is from 1530 to 1595; b) Design tolerance of HCG is calculated at fixed wavelength of 1550 nm, and the grating air-gap variation could be >±100 nm.
HCG VCSELs were fabricated with different air gaps but otherwise similar structures. Fig. 3 a) shows the light-current characteristics of these devices and b) shows the optical spectrum at a fixed current of 10 mA. Some variation in output power and threshold current is seen in the devices. This is likely due to a lack of uniformity in the HCG and other fabrication-related variations as well as the wavelength dependence of gain. With further fabrication optimization, we hope to achieve more uniform device characteristics regardless of the lasing wavelength. Devices were obtained with output wavelengths ranging from 1540 nm to 1591 nm. This is the first experimental verification that the HCG is reflective enough for a VCSEL to lase over such a broad wavelength range (>50 nm).
a) b) Fig. 3 a) LI curve and b) spectrum of multi-wavelength HCG VCSELs under continuous wave operation at room temperature. All the devices in b) are biased at 10mA, and wavelength range is from 1540nm to 1591nm.
In summary, we present a novel multiwavelength HCG-VCSEL array. VCSELs were fabricated spanning a 50 nm wavelength range with a simple post-growth, process-determined wavelength. In addition, ultra high reflectivity (>99%) of the HCG across >50 nm is experimentally demonstrated for the first time. We gratefully acknowledge support from the CIAN NSF ERC under grant #EEC-0812072. [1] [2] [3] [4]
C. Chang-Hasnain, M. Maeda, N. Stoffel, J. Harbison, L. Florez, and J. Jewell, Electron. Lett. 26(13), 940–941 (1990). M.C.Y. Huang, Y. Zhou, and C.J. Chang-Hasnain, Nature Photonics, 2, 180 (2008). V. Karagodsky, B. Pesala, C. Chase, W. Hofmann, F. Koyama, C. J. Chang-Hasnain, Optics Express, 18, 694-699 (2010). C. Chase, Y. Rao, C. J. Chang-Hasnain, “Single-Mode, Long-Wavelength HCG-VCSEL Using Proton-Implant-Defined Aperture”, submitted to International Semiconductor Laser Conference 2010, Kyoto, Japan.
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