2.2.1 InP-DFB

InP-based DFB lasers have been extensively developed with wavelengths at 1.3 and 1.55 μm for fibre optic communications over the last three decades. Similar processing techniques can be used for fabrication of single mode lasers operating in the 2 μm wavelength range. NTT-Japan demonstrated a DFB laser with an emission wavelength of 2.051 μm and output power of 10 mW [22]. A schematic of their buried heterostructure DFB grating is shown in Figure 5. The DFB grating was buried and required two regrowth stages after grating formation. There are no reported measurements on the spectral linewidths but they are expected to be in the Mid-Infrared InP-Based Discrete Mode Laser Diodes DOI: http://dx.doi.org/10.5772/intechopen.86458

Figure 3. Schematic diagram of the ridge waveguide metal grating DFB [20].

#### Figure 4.

Calculated bandgap wavelength for InGaAs on InP as a function of In composition. The upper horizontal axis shows the mismatch strain of InGaAs with respect to InP. Red dashed line indicates the lattice constant of InP 5.869A [13].

2 MHz range. The ex-facet power of 10 mW was improved on in a subsequent paper [23] with a value of 20 mW reported. The wavelength shift with current and temperature is reported to be 0.0025 nm/mA and 0.125 nm/<sup>o</sup> C [24].

### 2.2.2 Vertical-cavity surface-emitting laser

Light propagation and emission normal to the semiconductor layer structure is characteristic of a vertical-cavity surface-emitting laser (VCSEL) [25]. The main feature of the VCSEL design is the regrown buried tunnel junction (BTJ) (see Figure 6), which accomplishes current confinement and wave guiding. The active region contains five heavily compressively strained InGaAs-quantum wells

Figure 5. Schematic of BH-DFB laser diode [23].
