Читать книгу VCSEL Industry - Babu Dayal Padullaparthi - Страница 32

Now, what is this new surface‐emitting laser (SEL) or the vertical‐cavity surface‐emitting laser (VCSEL)? The structure is substantially different from conventional edge‐emitting lasers (EELs), i.e., the vertical cavity is formed by the surfaces of epitaxial layers, and light output is from one of the mirror surfaces orthogonal to the substrate as has been shown in Figure 1.13. It is recognized that one of the authors (Iga, from Tokyo Institute of Technology) invented VCSEL in 1977 [25–28] as shown in the inset of Figure 1.13. This new invention was coined VCSEL (vertical‐cavity surface‐emitting laser), following the naming of a “pixel,” which means any of the small discrete elements that together constitute an image (as on a television or digital screen). In the first stage, Ia, there were many technical challenges to overcome to realize this new device. The main challenges were the relatively low optical gain, overall mirror quality, and efficient current injection.

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!