1. Introduction

Low cost single mode semiconductor laser diodes emitting at wavelengths, λ, in the 1.6–2.1 μm wavelength range are highly desirable as light sources for trace gas spectroscopy due to the strong absorption bands [1] in this spectral region. Compared to conventionally used systems based on electrochemical point sensors, tuneable diode laser absorption spectroscopy (TDLAS) offers a number of benefits for the detection of gases such as; rapid response time, long term stability, high selectivity and sensitivity, rugged systems and measurement capability over long distances using open path systems [2]. TDLAS is based on the molecular rotationalvibrational absorption of gases that produce distinct peaks in the near to midinfrared (IR) spectral range. These molecule resonances cause characteristic 'fingerprints' by selective absorption of laser light as the wavelength is tuned by changing the laser bias current or heat sink temperature. Great interest is due to the strong absorption lines of various important gases, such as methane (1.665 μm), hydrogen chloride (1.743 μm), and nitrous oxide (1.795 μm), in this wavelength range. Two pertinent greenhouse gases, water vapour (H2O) and carbon dioxide (CO2), have strong absorption bands at wavelengths centred around 1.877 and

Figure 1.

Absorption spectra of two important greenhouse gases CO2 and H2O, with strong absorption lines in the nearto-mid-IR extracted from the HITRAN database [1].

2.004 μm respectively and are shown in Figure 1. There is also an increasing number of other applications, such as high data-rate communications over hollow core photonic crystal fibre [3–6] and noninvasive optical blood glucose monitoring [7] which also require compact and robust low-cost laser sources emitting in the 1.6–2.1 μm wavelength range.

Semiconductor laser diode materials with light emission in the 1.6–2.1 μm wavelength range include indium phosphide (InP) and the gallium antimonide (GaSb) material systems. It must be noted that cost sensitivity is a significant issue for many of these applications and in order to keep the laser chip cost down the InP material is often preferred over the GaSb material system for emission in the mid-IR region. In comparison with the GaSb material platform, the processing technologies for InP-based materials are more mature as they were developed for telecommunications lasers. In addition, superior substrate quality, lower substrate cost, better thermal performance and mature growth methods make InP-based lasers attractive candidates for light sources in this wavelength region [8–15]. The III-V compound semiconductor material system (AlGaIn)-(AsP) constitutes an ideal basis for the realization of diode lasers in this wavelength region [16]. InGaAs, either lattice matched to InP or deliberately strained, can be used for the active layer with a direct band gap between 1.1 and 2.3 μm. Recently InP-based type-II QWs have extended the wavelength up to 3 μm [17] which opens up another important wavelength region for the fabrication of lower cost lasers for sensing applications.

This chapter begins with an overview of mid-IR single mode laser diodes and then outlines the state of the art in InP based mid-IR discrete mode laser diodes.
