VCSEL Industry. Babu Dayal Padullaparthi
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Figure 1.13 Stages of VCSEL development. The inset figure shows the sketch of VCSEL drawn by Kenichi Iga on March 22, 1977.
Source: Figure by K. Iga and B. D. Padullaparthi [copyright reserved by authors].
Figure 1.14 The first demonstration of a surface‐emitting laser.
Source: Figure by K. Iga [29] [copyright reserved by author].
The first device (prototype) was realized in 1979 using a GalnAsP‐InP material for the active region. The VCSEL operated at a 1300 nm wavelength [29]; a schematic cross section of the device is shown in Figure 1.14. This VCSEL used a double heterostructure with GaInAsP as an active layer, which was grown on an InP substrate. Light is emitted by injecting current from circular electrodes, and metal reflectors are formed above and below the substrate to form a resonator. This laser was driven by a pulsed current and was cooled to 77 K using liquid nitrogen. At 800 mA the device lased. When we looked at the light coming out of the device, it flashed rapidly at a certain current. It was possible to finally measure the spectrum, and it was much narrower than LEDs, which indicated laser oscillation. As mentioned above, the device was named surface‐emitting laser. The threshold was very high, more than 20 times that of a normal laser, and as such, the device was out of order immediately!
1.3.1.2 Stage Ib: First Room‐Temperature Continuous‐Wave Operation
In 1982, Iga and co‐workers made a VCSEL with 10 μm length cavity and confirmed the clear VCSEL oscillation [30]. In 1982, Iga’s group made a buried confinement VCSEL with a 6 mA threshold GaAs device using liquid phase epitaxy (LPE) [31]. A major breakthrough was the achievement of continuous‐wave (CW) operation at room temperature (RT) at 820 nm wavelength on GaAs substrate by Iga and Koyama (also from Tokyo Institute of Technology) in 1988 [32, 33]. The device structure is shown in Figure 1.15(a). The device was grown by metal organic chemical vapor deposition (MOCVD). With this achievement, global R&D of VCSELs has outperformed ordinary semiconductor lasers in the area of expertise. The concept of semiconductor DBR demonstration in 1988 [34] and the introduction of multi‐quantum wells into VCSEL [35] contributed to the improvement of VCSEL development in later years.
After this breakthrough from Tokyo Institute of Technology, continuous room‐temperature operation of the VCSEL, as shown in Figure 1.15(b), was also achieved by Jack Jewell and co‐workers at Bell Laboratories in 1989 [36, 37]. The concept of periodic gain or matched gain in quantum wells contributed to reduce the threshold by Larry Coldren and co‐workers [38, 39].
Figure 1.15 Initial VCSELs achieving room‐temperature continuous operation. (a) The VCSEL device that exhibited the first room‐temperature continuous‐wave operation by Koyama and Iga in 1988 [32].
Source: Copyright reserved by Fumio Koyama and Kenichi Iga
(b) A 2D micro‐post array by Jewell and Lee in 1989 [36, 37].
Source: Adapted from IEEE.
1.3.2 Stage‐II: Spread of Worldwide R&D
The second stage (1991–2000) covers the expansion of VCSEL research, the advancement in growth technology, and the emerging application needs in data communications. The first DARPA funding was driven by the Joint Strike Fighter (JSF) program. Three centers for optoelectronics were started in universities. Honeywell, Motorola, and HP were the primary companies working on the programs. More details can be found in the wiki page:
https://en.wikipedia.org/wiki/Vertical‐cavity_surface‐emitting_laser.
Areas of emphasis in Stage II include mass production technology [40], threshold current reduction[39–42], transverse mode control, oxidation [43, 44], polarization control, initial tunable VCSELs [45], MEMS elements [46], 2D arrays [47], high‐speed and high‐power VCSELs, InP‐based device with continuous operation [48] and quantum wells VCSELs, and so on. This was a golden period in the VCSEL journey to mass production, and many technical and manufacturing advances contributed to the foundation of VCSEL technology.
1.3.3 Stage III: Extension of Applications and Initial Commercialization
Stage‐III of VCSEL development started in 1999, shown in Figure 1.12, as we entered a new information and technology era in 2000. The third stage (1999–2010) brought on new development of wavelengths, single mode, VCSEL arrays, volume manufacturing driven by Internet traffic demand, autofocus, and so forth, and the focus has shifted to commercial efforts. Why did 1310–1550 nm VCSELs not become widely adopted? That was primarily due to the technical difficulty of making mirrors and overcoming optical loss in materials.
In 2000 one of the authors (Iga) wrote a VCSEL review paper [49] and in the same special issue, DARPA managers Elias Towe, Robert F. Leheny, and Andrew Yang wrote the following about VCSEL in their review paper [50]: “Its size, manufacturability, and potential ease of heterogeneous integration of electronics promise a range of applications that have yet to be explored.” This was the time when DARPA invested considerable human and monetary resources in the R&D of VCSEL, in particular the massive integration of VCSELs, detectors, micro‐optics and driving electronics for free‐space optical interconnects and all optical switching. However, practical and commercial free‐space interconnect and all optical switching did not really pick up during the subsequent years. This investment nonetheless continued to drive VCSEL innovations such as high‐power VCSEL arrays, high‐contrast gratings, athermal VCSELs, coupled cavity VCSELs, VCSELs‐based slow light waveguide devices, multi‐wavelength VCSELs/WDM [51], quantum dot VCSELs, high‐bandwidth VCSELs (>20 GHz), and so forth.
VCSELs are currently applied in various optical systems, such as optical networks, parallel optical interconnects, laser printers, computer mice, and so on. The three critical application areas that provided the commercial impetus for continued VCSEL expansion were high‐speed data connectivity, computer mice, and laser printing.
1.3.3.1 LAN for Internet
A first large market for VCSELs with large‐scale production had begun in 1995 [40]. Around 1999, the Internet spread rapidly worldwide. The dramatic growth of data centers created communication networks that support the Internet, including long‐distance optical fiber networks and local area networks (LANs). Metaphorically, the artery of the blood vessel is the long‐distance line, and the LAN is a capillary. VCSEL was adopted as a light source for LANs operating at 1 Gbit/s and running Fiber Channel and Ethernet protocols.
The protocols were further standardized (10G; IEEE802.3ae in 2002, 100G; IEEE802.3ba in 2011) for the optical fiber communication that constitutes the LAN of the Internet. In 2020, high speed VCSELs and the pulse amplitude modulation (PAM) scheme have been developed for 400 Gbit/s high‐speed Ethernet. Information flows through the capillaries of companies and universities. Details