Lasers
Semiconductor lasers can be categorized into several types depending on:
1 Emission wavelength and materials;
2 Resonant cavity configuration;
3 Single mode or multi‐mode;
4 Direction of emitted light;
5 Direct modulation speed;
6 Power output;
7 Footprint of device;
8 Beam form and connectivity to optics such as optical fibers;
9 Price of device;
10 Manufacture volume;
11 1D or 2D array; and so on.
A simple category is outlined in Table 1.1. In this case, we categorized into two types on the basis of cavity configuration, i.e., edge‐ and surface‐emitting cavities. This book is focused on the research, development, and industrial sectors to demonstrate the impact of VCSEL.
A schematic of an edge‐emitting laser (EEL) is shown in Figure 1.2a. In edge‐emitting lasers, the light emits through the edges of the wafer that is in‐plane with the substrate and the optical cavity is horizontal for laser resonance. On the other hand, the structure of VCSEL is illustrated in Figure 1.2b. The resonance of light is vertical to the substrate and taken from the surface. The laser structure is formed on the substrate in just one manufacturing process.
Table 1.1 Categorization of semiconductor lasers.
Source: Table by K. Iga [copyright reserved by author].
| Type of cavity | Edge‐emitting laser | Surface‐emitting laser |
|---|---|---|
| Single Mode | ||
| Transverse | narrow stripe | narrow aperture |
| Wavelength | DFB, DBR | Fabry‐Pérot |
| Multi‐Mode | ||
| Transverse | broad area | wide aperture |
| Wavelength | Fabry‐Pérot | Fabry‐Pérot |
| Array | one‐dimensional | two‐dimensional |
Figure 1.2 Schematic of semiconductor lasers (a) Edge‐emitting laser (EEL). (b) Vertical‐cavity surface‐emitting laser (VCSEL).
Source: Figure by B. D. Padullaparthi [copyright reserved by authors].
In both cases, the emission and amplification of light in semiconductor lasers is due to the recombination of electrons and holes that exist in the active region. This is what most of the standard textbooks on optical properties of semiconductors teach. Later, we will show the readers a different explanation.
1.1.2 Light Emission and Absorption in Semiconductors
The characteristics of the semiconductor diode current (I) as a function of the applied voltage (V) are used to create several different types of optical devices. The voltage–current (V‐I) characteristic of a diode is shown in Figure 1.3. The first quadrant Q1 is known as forward bias, and the current increases exponentially with the applied voltage. When an electron and a hole recombine near the p‐n junction, the energy is released as a photon. This regime is where light‐emitting diodes (LEDs) and lasers operate.
The third quadrant Q3 is the reverse bias region, which is used as photodetectors. When a photon is absorbed near the p‐n junction, the light energy creates an electron and hole pair. The electrons drift to the positive electrode and the holes move to the negative electrode under reverse bias.
The fourth quadrant Q4 is where photoconduction occurs and is the operating regime for solar cells. When a photon is absorbed near the p‐n region, an electron hole pair is created. The resultant current times voltage (power) generates an electrical energy in the solar cell. The second quadrant is not used for practical optical devices.
The first and third quadrants play critical roles as key optoelectronics components for optical communication and sensing applications. To understand how light (photons) interacts with semiconductors, a deeper understanding of light emissions and absorption is needed.
Figure 1.3 Voltage–current (V–I) characteristic of a p‐n junction with no incident light (solid curve) and with incident light (dashed curves).
Source: Figure by K. Iga and J. A. Tatum [copyright reserved by authors].
1.1.3 Birth of Semiconductor Lasers
1.1.3.1 Homostructure and Double Heterostructure Lasers
Based on the principle of light amplification in semiconductors, the first semiconductor laser was realized by four groups almost simultaneously in 1962. This was two years later after Therdore Maiman demonstrated the first laser [1]. The optical gain layer was located near the p‐n homo‐junction parallel to the substrate [2–5]. The light resonance occurs between the mirrors formed at the edge of the substrate by cleaving or polishing the semiconductor crystal. These lasers are known as edge‐emitting lasers (EELs).
In 1970, eight years later, two groups reported a double heterostructure (DH)‐based laser that enabled room‐temperature continuous operation [6, 7]. The device using an AlGaAs‐GaAs DH structure, as shown in Figure 1.4, introduced by Hayashi and Panish reduced the threshold current density to about 1 kA/cm2 at 300 K, a major breakthrough over the initial homo‐junction devices.
Figure 1.4 Double heterostructure laser. (a) The carrier densities of electron and holes in p, i, and n regions. (b) The optical field intensity near the double heterostructure.
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