3.1.2 Mid-IR FP laser characterization

To evaluate the material quality before fabricating the DM lasers, broad area and FP lasers with varying cavity length were fabricated and characterised on the four wafers, only the results from the 2 μm wafer are presented in this section. Ascleaved broad area lasers with 50 μm wide ridge widths and varying cavity lengths were analysed under pulsed conditions (1 μs pulses with 0.1% duty cycle) to evaluate the material quality. The threshold current density (Jth) for the 1000 μm cavity length was 358 A/cm<sup>2</sup> (119 A/cm<sup>2</sup> /QW) indicating good material quality. The slope efficiency variation with cavity length allowed the internal quantum efficiency (ni) and the internal optical loss (ni) to be estimated to be 80% and 8 cm<sup>1</sup> respectively.

#### Figure 11.

(a) SEM image of a ridge waveguide FP laser diode. (b) Picture of a processed 3-inch InP wafer.

### Optical Fiber Applications

A 600 μm long FP ridge waveguide laser was packaged in a fiberised 14-pin butterfly module which contained a thermoelectric cooler and thermistor and the optical characteristics were measured under CW conditions. Figure 12 shows the overlapped CW measurement of light-current (LI) characteristics measured at chip temperatures 10, 25, 45, 50, 60 and 70°C. The power was measured with a large area (Ø3 mm) extended wavelength InGaAs detector (GPD 3000). The light coupling efficiency into the fibre was measured to be 60% and the chip ex-facet power was >20 mW at 200 mA, 25°C. The extracted threshold currents were 18, 30 and 58 mA at 25, 50 and 70°C respectively. The characteristic temperature of threshold current between 25–50 and 25–70°C was calculated to be 49 and 38 K. The measured slope efficiencies in the fibre at 25 and 50°C were 0.08 and 0.06 W/A respectively [3].

In Figure 13 the emission spectrum is plotted at a heat sink temperature of 25°C and bias current of 80 mA using a Yokogawa (AQ6375) long wavelength optical spectrum analyser. In Figure 14 we overlap the measured optical spectra over a

Figure 12. Overlapped light-current curves in the temperature range 10–70°C for the 2 μm FP laser.

Figure 13. Emission spectrum at a bias current of 80 mA and heat sink temperature of 25°C.

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

#### Figure 14.

Overlapped emission spectra at a bias current of 80 mA in the temperature range 25–50°C.

#### Figure 15.

Overlapped FP emission spectra at bias currents of 80 mA and heat sink temperature of 25°C for the four InP wafers.

temperature range from 15 to 50°C, the centre lasing wavelength shows a linear dependence with temperature with a tuning rate Δλ/ΔT of 0.83 nm/o C, which is consistent with that expected due to the temperature-induced change in the refractive index.

To demonstrate the wide wavelength coverage from 1.65 to 2.15 μm 400 nm we overlap the FP emission spectrum for the four wafers as shown in Figure 15.

#### 3.2 Design of DM laser diodes

Single wavelength operation in DM laser diodes is achieved by introducing index perturbations in the form of shallow-etched features, or slots, positioned at a number of sites distributed along the ridge waveguide as shown in Figure 16a [27–32]. The slots are realized using ICP dry etching, with a typical depth in the region of 1.5–2 μm and a width of 1 μm. The slots are relatively shallow and are not etched into the active (wave guiding) region, however, they will still interact with the

Figure 16.

(a) SEM image of 2 μm wide ridge waveguide with etched grating. The slot width is 1 μm and the spacing between the slots L is 4 μm in this example. (b) Illustration of slot reflection and transmission for N slots.

mode's electric field as the mode profile is not fully confined to the active region and will expand into the surrounding cladding regions. This interaction results in a proportion of the propagating light being reflected at the boundaries between the perturbed and the unperturbed sections. In effect the slots act as reflection centres and through suitable positioning the slots manipulate the mirror loss spectrum of an FP laser so that the mirror loss of a specified mode is reduced below that of the other cavity modes [27–32]. Using a simplified model, developed in [33, 34], a slot can be described as a one dimensional discontinuity inserted into the cavity; as most of the reflection comes from the front of the slot interface. Figure 16b shows a schematic of a laser cavity with slots introduced into the cavity; where rs is the slot reflectivity, ts is the slot transmission, N is the number of slots, L is the distance between the slots, and ϒ<sup>i</sup> is the reflectivity in a section of the cavity where i is the slot number. The reflectivity from the first slot, ϒ<sup>1</sup> is given by rs. The introduction of a slot into the waveguide changes its effective refractive index, so that it differs slightly from the segments of the waveguide without slots. The reflectivity from the waveguide to slot interface can be approximated using Eq. (1):

$$r\_s \approx abs \left(\frac{n\_2 - n\_1}{n\_2 + n\_1}\right) \tag{1}$$

where n<sup>1</sup> is the effective refractive index of the waveguide and n<sup>2</sup> is the effective refractive index of the waveguide with a slot.

Assuming no loss from the slot Eq. (2),

$$t\_t = \mathbf{1} - r\_t \tag{2}$$

ϒ<sup>2</sup> is the reflectivity from the second slot and is given by Eq. (3)

$$\Upsilon\_2 = r\_s t\_s^2 \exp\left(-2i\beta L\right) \tag{3}$$

where β is the complex propagation constant, and the term ts is squared to take account of forward and backward travelling waves. The exponential term describes the medium in which the light travels, and a factor of two is used again to take account of forward and backward travelling waves. The complex propagation constant takes account of the gain and loss in the transmission medium and is defined in terms of Eq. (4)

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

$$
\beta = \beta\_{re} + i\beta\_i = \frac{2\pi n}{\lambda} + i\frac{\mathbf{g} - a\_i}{2} \tag{4}
$$

where n is the refractive index, λ is the wavelength, g is the optical gain and α<sup>i</sup> is the internal cavity loss. The reflectivities of the third and fourth slots are given by Eq. (5):

$$\Upsilon\_3 = r\_s t\_s^4 \exp\left(-4i\beta L\right) \tag{5}$$

and Eq. (6),

$$\Upsilon\_4 = r\_s t\_s^6 \exp\left(-6i\beta L\right) \tag{6}$$

respectively; therefore, the reflectivity obtained from four slots is given by Eq. (7):

$$\begin{split} \Upsilon\_{\text{total}} &= \Upsilon\_1 + \Upsilon\_2 + \Upsilon\_3 + \Upsilon\_4 \\ &= \mathbf{r}\_s + \mathbf{r}\_s \mathbf{t}\_s^2 \exp\left(-2\mathbf{i}\mathfrak{J}\mathbf{L}\right) + \mathbf{r}\_s \mathbf{t}\_s^4 \exp\left(-4\mathbf{i}\mathfrak{J}\mathbf{L}\right) + \mathbf{r}\_s \mathbf{t}\_s^6 \exp\left(-6\mathbf{i}\mathfrak{J}\mathbf{L}\right) \end{split} \tag{7}$$

By letting Eq. (8)

$$X = t\_\circ^2 \exp\left(-2i\beta L\right) \tag{8}$$

the total reflectivity from N slots can be expressed by Eq. (9) the following series.

$$\mathbf{Y}\_{\text{total}} = \mathbf{r}\_s \left( \mathbf{1} + \mathbf{X} + \mathbf{X}^2 + \mathbf{X}^3 + \dots + \mathbf{X}^{N-1} \right) \tag{9}$$

Which in terms of known variables can be described as Eq. (10)

$$\Upsilon\_{\text{total}} = r\_s \left[ \frac{\mathbf{1} - \left( t\_s^2 \exp\left(-2i\beta L\right) \right)^N}{\mathbf{1} - t\_s^2 \exp\left(-2i\beta L\right)} \right] \tag{10}$$

The power reflection is related to the reflection amplitude by Eq. (11),

Figure 17. Simulated power reflection spectrum as a function of slot number.

$$R = abs\left(\mathbf{Y}\_{total}^2\right) \tag{11}$$

Using this model the power reflection versus etched feature number at a wavelength of 1887 nm was simulated and shown in Figure 17.

For a 600 μm long laser cavity at about 60 slots the peak reflectivity begins to saturate and the FWHM is about 0.9 nm which is equivalent to the FP mode spacing. So this is the optimum number of slots for this cavity length.

### 3.2.1 DM laser fabrication

The fabrication of the DM laser is exactly the same as the FP laser outlined in Section 3.1.2 width the addition of one extra dry etching step to etch the grating, the dry etch chemistry used again was Cl/N2 and was followed by a short wet-etch to remove surface roughness from the grating. A schematic of the etched features is shown in Figure 18.
