# Long-Wavelength VCSELs: Status and Prospects
## 2 Distributed Bragg Reflectors for the 1300-1550 nm Spectral Range
For wavelengths at or below 1300 nm, the number of CaF\({}_{2}\)/ZnS pairs is 3.5 [30], due to an \(\Delta\)n of 0.85 [24]. For 1300 nm VCSELs DBRs based on AlF\({}_{3}\)/ZnS were also used [31], showing a larger difference in the refraction index (\(\Delta\)n = 0.95). A completely novel approach to largely increases heat dissipation based on circumventing the mirrors for current injection and heat dissipation is reported by [PERSON] et al. [32; 33]. High-contrast gratings (HCG) might well be used in the future for the top DBRs [34; 35; 36; 37; 38; 39; 40; 41]. Buried tunnel junction (BT)) and/or proton implantation are used to provide current confinement for HCG at 1300-1550 nm VCSELs. The principal advantages of HCGs in a VCSEL include transverse mode control, fixed polarization, and reduced manufacturing cost (reduction of the epitaxial growth time) [42]. Lasing via fundamental mode allows one to realize high SM output power for HCG devices with large apertures. HCGs are promising to realize widely tunable VCSELs. A review by [PERSON] in this volume reports in depth about HCG-based VCSELs. HCG provides an extraordinarily wide stopband [43]. More than 98.5% reflectivity in the 1120-1620 nm spectral range was demonstrated [44]. The additional advantage is wavelength scalability. By multiplying the dimensions of HCG (by 6.5), the reflection band center shifts from 1.55 \(\upmu\)m to 10 \(\upmu\)m [45]. HCG-VCSEL lasing wavelength can be tuned by the HCG period and duty cycle [43]. As a result, monolithic multi-wavelength VCSELs were realized [46]. The use of HCG as a movable mirror allows one to realize the tunable VCSELs with a wide tuning range as well as rapid tuning speed [47; 48; 35; 49]. The detailed study of a systematic review of experimental and modeling results as well as fabrication tolerance can be found in ref. [43]. In both uniform and random-size HCGs, a very large fabrication tolerance in HCG-VCSELs is demonstrated [43]. The main drawback of near-infrared subwavelength HCG VCSELs is their modest dynamic performance in relation to VCSELs with semiconductor mirrors as well as the challenging processing related to electron-beam lithography usage [50]. On the other hand, the dynamic performance of SWIR HCG VCSELs is comparable (about 8 GHz modulation bandwidth for 1500 nm VCSELs [3; 34; 39]) with the same value for wafer-fused VCSELs but inferior to the superior modulation performance of hybrid LW VCSELs. MEMS-tunable 1550 nm VCSELs demonstrate the reliability of Telcordia GR468 [51]. A different type of DBR is based on MEMS. Using 11.5 SiO\({}_{\text{x}}\)/SiN pairs [6] allows one to realize 7 GHz [5] and 9 GHz [7] modulation bandwidth for 1500 nm VCSELs. The \(\Delta\)n value for SiO\({}_{\text{x}}\)/SiN is about 0.47. Using SiO\({}_{\text{x}}\)/SiC\({}_{\text{z}}\) DBRs [52] allows one to increase the \(\Delta\)n value to 1 [53]. As a result, the pair number can be decreased to 6.5 instead of 11.5 for reflectivity of \(>\)99.5%. Finally, the stop band width is 243 nm (at a reflectivity of \(>\)99.5%) and is about twice that of the SiO\({}_{\text{x}}\)/SiN pair. As a result, the small IR absorption of SiC\({}_{\text{z}}\) can be diminished due to the wider tuning range compared to SiO\({}_{\text{x}}\)/SiN-based MEMS DBRs.