Figure 2 demonstrates the impact of temperature on the P-I curves with temperature ranging from 286.15 - 333.15 K. As is shown in Figure 2, with the increase of temperature, the threshold current gradually increases, the decrease of the output light power gradually.

Figure 3 presents the threshold current as a function of temperature. It shows that the threshold current increases with the increasing temperature. Specifically, the threshold current increases from 0.28 A to 0.41 A at a rate of 0.0027 A/K. Previous studies found that the threshold current density at a given temperature J_th(T) can be calculated using the following equation [12]-[14]:

where J_th(T₁) is the threshold current density at temperature of T₁, in this experiment the T₁ values set to 295.15 K, T₀ represents the characteristic temperature [15]. The main underlying mechanisms for the changes in the threshold current density caused by the temperature [14] are as following: 1) Temperature changes the statistical distribution of the carrier, thereby affecting the light gain coefficient; 2) Temperature influences the internal loss existing in the semiconductor laser (auger recombination, surface composite, free carrier absorption; 3) Temperature affects heterojunction between the potential barrier heights, which in turn affects the hot carrier leakage. With the increase in the temperature, the internal wear of the semiconductor laser diodes increase, meanwhile both the internal and external quantum efficiency, external quantum efficiency drops. The potential barrier height increases with temperature, leading to the leakage of hot carriers, which results in a higher threshold current.

The characteristic temperature T₀ is calculated by linear fitting the data points in Figure 3, which is 120 K. The characteristic temperature T₀ is an important measure of the temperature sensitivity of semiconductor laser diodes. A bigger T₀ indicates that the laser diode is less sensitive to temperature. The calculation shows that the semiconductor laser diode are sensitive to temperature, which implies a strong fluctuation in the performance of this particular laser diode with temperature ranging from 286.15 K to 333.15 K.

Figure 4 depicts the change in output power with temperature given an input current of 2 A. it shows that the output power decreases gradually with the increasing temperature, particularly, the output power decreases from 1.47 W to 1.27 W at a rate of 0.00425 W/K. Given a current higher than the threshold current, the output power can be expressed with the function below [12]-[14]:

where, η_i is the internal quantum efficiency, η_ex is the external quantum efficiency, ai and am are internal loss and loss of end face. The internal loss increases, meanwhile the efficiency of internal quantum and external

quantum decreases as the temperature rises. In addition, the leakage of hot carriers increases, which increases the threshold current, thereby resulting in a reduced output power.

Plotted in Figure 5 is the V-I curves with temperature varying from 286.15 K to 333.15 K. At a given voltage, when temperature changes from 286.15 K to 295.15 K, current increases considerably. However, the change in the current is negligible with the temperature ranging from 295.15 K to 333.15 K as the rest curves overlap with each other. This can be explained by the relationship between voltage and current.

Analysis the reason, by the relationship between electric current and voltage and can know [12]:

where V stands for the forward voltage imposed on the laser diode, is the internal voltage, is the series resistance, E_g (T) is the forbidden bandwidth at a given temperature, The forbidden bandwidth narrows with the increasing temperature, which reduces, compared to the change in V and, the change in with

temperature can be ignored. Hence, current rises with temperature within a certain range. However, as the temperature continues to increase, the change in the conduction band and the valence band of Fermi level range reduces, leading to a smaller change in E_g, therefore there is minimum change in the current with temperature above 295.15 K.

The V-I function for other diodes is as following:

We obtain from Equation (3), derivation Equation (5) gives us the value of, If I > I_th,, the value of V_d is a constant. when I₀ << I, calculate the expression:

When the change in current and voltage is small, at a temperature of 295.15 K and a current blow

the threshold current, applying linear fitting to V-I curves with Equation (6), show in Figure 6 which gives an ideal factor of n = 1.076. Normally, the ideal factor for diodes ranges from 1 to 2, hence the calculated ideal factor in the present study is within the range. The series resistance is calculated to be 0.609 Ω.

Figure 7 shows the centre wavelength at different temperature while keeping the input current constant. It re-

veals that the centre wavelength increases as temperature rises. It is known that the material used in the semiconductor laser is direct band gap materials, and the centre wavelength is determined by E_g, which is a function of the temperature coefficient. The relationship between the output wavelength and forbidden bandwidth can be expressed as [12]-[14]:

Figure 8 shows the centre wavelength and the forbidden bandwidth as a function of temperature. It demonstrates that the centre wavelength increases as the temperature rises. The increase rate of the centre wavelength is calculated to be 0.275288 nm/K. Compared to an increase rate of 0.37 nm/K for temperature characteristic of 1.06 μm InGaAs/InGaAsP quantum-well laser diode [7] in a previous study, this change is relatively small. At a given temperature, the forbidden bandwidth E_g is calculated from Equation (8) by substituting the centre wavelength at that particular temperature. Figure 8 reveals that the forbidden bandwidth is reduced when temperature is higher. This result agrees with the inversely proportional relationship between temperature and the forbidden bandwidth as shown in Equation (9). The decrease in the forbidden bandwidth is responsible for the increasing centre wavelength at a higher temperature.