3.2.2 DM laser diodes emitting at 1.87 μm

Low cost single mode semiconductor laser diodes emitting at 1.87 μm are highly desirable as light sources for trace gas sensing of H2O. The measurement of H2O is important in many industrial applications, for example, continuous emission monitoring in combustion processes where the vapour concentration can be related to performance parameters, such as, efficiency of combustion and heat release. In this section we present data on DM laser diodes operating around λ = 1.877 μm. Fabricated DM lasers exhibit continuous wave (CW) mode hop free operation in the temperature range from 15 to 55°C with emission wavelengths centred at 1.87 μm at 25°C, and ex-facet optical output power >3 mW at 25°C.

A ridge waveguide 600 μm in length DM laser diode was die bonded to an aluminium nitride submount and the optical characteristics were measured under CW conditions. Figure 19 plots the overlapped CW measurement of ex-facet LI characteristics measured at chip temperatures of 15, 25, 35, 45 and 55°C, with the same power and wavelength measurement setup as mentioned in the previous section. The extracted threshold currents were 12, 15, 18, 22 and 29 mA at 15, 25, 35, 45 and 55°C, respectively. The measured slope efficiencies at 15 and 55°C were 0.038 and 0.022 W/A, respectively.

Figure 18. Schematic view of the InP DM laser.

Mid-Infrared InP-Based Discrete Mode Laser Diodes DOI: http://dx.doi.org/10.5772/intechopen.86458

#### Figure 19.

Overlapped CW LI curves as a function of heat sink temperature.

Figure 20.

In Figure 20 the emission spectrum of the DM laser diode is measured at a heat sink temperature of 25°C and bias current of 100 mA. A peak wavelength of 1877 nm is demonstrated, with a side mode suppression ratio (SMSR) of 45 dB achieved, in excellent agreement with simulated values. In Figure 21a we plot the peak wavelength versus bias current as a function of laser submount temperature. The peak wavelength tunes linearly with bias current at a tuning rate of 0.017 nm/mA. In Figure 21b the optical emission spectrum over a wide temperature range, from 5 to 55°C, is plotted. The single mode peak lasing wavelength shows a linear dependence with current and temperature, with a tuning rate of Δλ/Δ<sup>I</sup> 0.017 nm/mA and Δλ/ΔT of 0.113 nm/<sup>o</sup> C, consistent with that expected due to the temperature-induced change in the refractive index [29].

Wide gain bandwidth is demonstrated in Figure 22 where the DM peak wavelength is varied by changing the grating period wavelength tunability of 120 nm is achieved. This makes this material promising for widely-tunable mid-infrared single-mode devices such as DM arrays, external cavity lasers and sampled gratings. Eblana's DM technology has been used to demonstrate single frequency lasers with

Figure 21.

(a) Peak wavelength versus bias current over temperature. (b) Overlapped optical emission spectra versus heat sink temperature at a fixed laser bias current of 100 mA.

Figure 22.

Overlapped single frequency spectra of 10 different DM lasers demonstrating 120 nm wide gain for the 1.8 μm wafer.

#### Figure 23.

Overlapped single frequency spectra of different DM lasers based on four InGaAs quantum wells wafers spanning the 1.7–2.15 μm wavelength region.

high side mode suppression ratios spanning a wide wavelength range from 1.75 to 2.1 μm by using the appropriate InGaAs quantum well composition and thicknesses as shown in Figure 23.

The wavelength and tuning results demonstrated by DM lasers in this chapter show that the devices operate at a single wavelength with SMSR >40 dB robustly over current and temperature variations and, furthermore, that these devices are highly suitable as low cost sources for TDLAS and other sensor applications. A further important feature of the DM laser diode is that its fabrication is far less complex than that of a distributed feedback laser diode resulting in a significant cost advantage.
