VCSEL Industry. Babu Dayal Padullaparthi

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      Now, in a laser cavity with a resonator length L longer than the wavelength, waves of many wavelengths with slightly different lengths can resonate. These modes are called longitudinal modes. On the other hand, the modes in the perpendicular direction are called the transverse modes.

      Considering a normal semiconductor laser, if λ is 1.3 μm, n = 3.5, and L = 3 μm, then q = 16.

      Therefore, even if q differs by 1, the resonance wavelength changes only slightly as Δλ. With |Δ λ | ≪̸ λ in mind, if λ → λ ₀ + Δλ , q → q + 1, then we obtain,

      (1.8)

      This |∆ λ | is called free spectral range (FSR) and is inversely proportional to cavity length, L.

      Here, n_eff is the effective index considering the dispersion of the medium and is given by the following expression:

      (1.9)

      Since ∂n/∂ λ < 0 in ordinary semiconductors, n_eff is usually larger than n. In the above example, n_eff = 4.0 and |∆ λ | = 70 nm.

      1.1.5.3 Cavity Formation

To achieve laser oscillation, a resonator that provides optical feedback to the gain medium is required. The laser resonator is formed by a pair of mirrors; a so‐called Fabry‐Pérot (FP) resonator is shown in Figure 1.8(a). In an edge‐emitting laser, the gain width is w, the cavity length is equal to L, and the mirror is usually made by simply cleaving the semiconductor crystal. In this case the refractive index of about 3.5 is higher than outside air, and the resonator edges look like open termination. (Here φ = 2 π . φ is defined in Eq. (1.1)).

      In Table 1.1 we have touched on DFB and DBR structures for single‐mode operation of edge‐emitting lasers [20–23]. In both cases, we utilize a pair of Bragg mirrors having an electric field reflectivity expressed by that sandwich some space or active region. These wavelength‐selective cavities can provide single longitudinal‐mode operation. For using those lasers in optical pulse code modulation (PCM) for optical fiber communications, they should maintain single mode under high‐speed modulation (~100 Gb/s). Moreover, in the case of coherent digital communications, the laser should operate with narrow spectrum (~kHz). This kind of lasers is called a dynamic single‐mode laser [24].

      In the case of VCSELs the mirrors are formed by semiconductor Bragg reflectors or dielectric mirrors, and therefore, we can design the resonator as open or short terminations. We can use its large free spectral range (FSR) for pure single longitudinal‐mode operation and wide‐range wavelength tuning. The details will be described in Chapters 2 and 8.

1.2 Semiconductor Lasers and Manufacturing

      1.2.1 Manufacturing Process of Edge‐Emitting Lasers

A schematic of EEL manufacturing and testing processes is shown in Figure 1.10. The edge‐emitting lasers (such as FP and DFB lasers) require facet coatings after cleaving and often need regrowth of specific steps.

      The FP‐EELs with cleaved laser mirrors have only 33% reflectivity. The reflectivity of the cleaved surface can be modified to be either higher or lower reflectivity by coating multi‐layer films that can be used to optimize the performance characteristics and to protect the surfaces of the EEL. The facet coatings are applied after the lasers are cleaved from the substrate and require extensive handling, which makes them more difficult to manufacture.

      Another approach is to form the distributed reflectors through multiple epitaxial steps with intermediate fabrication steps. The regrowth and the intermediate processing result in a nonmonolithic epitaxial growth making this process very complex and not manufacturing‐friendly compared to a fully monolithic VCSEL fabrication.

Figure 1.10 The manufacturing processes of edge‐emitting lasers.

      Source: Figure by K. Iga and J. A. Tatum [copyright reserved by authors].

      1.2.2 Vertical‐Cavity Surface‐Emitting Laser

In vertical‐cavity surface‐emitting lasers, the optical cavity is designed to be normal to the wafer surface, and the light emits vertically from the surface, as shown in Figure 1.2(b). High reflectivity mirrors (>99%) can be obtained with the growth of multiple epitaxial layers just above and below the active cavity without regrowth, resulting in an optical resonator cavity on the order of the emission wavelength, which is often referred to as a microcavity resonator. The lateral optical and electrical confinement is achieved by an oxidation process and may have sizes from one to several tens of microns. This lateral size controls the mode profile of the laser and will be further described in Chapter 2.

Figure 1.11 The manufacturing and testing processes of VCSELs.

      Source: Figure by K. Iga and J. A. Tatum [copyright reserved by authors].

As shown in Figure 1.11, the epi‐growth process is fully monolithic and device fabrication is manufacturing‐friendly and scalable. Mass production of VCSELs thus appears more like modern LED and IC manufacturing. In contrast to FP‐edge‐emitting lasers, the handling of full wafers only (no bar handling or facet coatings), the ability of fully testing on wafer (see Appendix СКАЧАТЬ