# Design of a Laser Driver and Its Application in Gas Sensing
## 2 Structure of the Laser Driver
### Design of the Current Source  
#### 2.2.1 Schematic of the Circuit  
A float-ground type current source for laser driving was sketched in Figure 4. In order to make the simulation more realistic, the built macro-model of DFB-LD was introduced.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Symbols** & **Description** \\ \hline Rs & parasitic resistor in series \\ Cp & parasitic capacitor in parallel \\ Rd & parasitic leakage resistor \\ Rph & a constructed resistor with the resistance of \(kT/q^{2}\) \\  & (\(k\) the Boltzmann constant, \(T\) the temperature and \(q\) the electric charge) \\ Cd & diffusion capacitor \\ Csc & space-charge capacitor \\ KI & the ratio of the optical power emitted from the left facet of the optical \\  & resonator to the photon density \\ Kr & the ratio of the optical power emitted from the right facet of the optical \\  & resonator to the photon density \\ Beta & spontaneous emission coefficient \\ NA & positive end of laser electrical ports \\ NB & negative end of laser electrical ports \\ NL & virtual port on the left facet of the optical resonator \\ NR & virtual port on the right facet of the optical resonator \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the electric symbols in the equivalent circuit of DFB-LD.  
Figure 3: Test circuit of DFB-LD.  
Figure 2: The internal equivalent circuit (**left**) and sub-circuit symbol (**right**) of a DFB-LD.  
A 500 \(\Omega\) resistor (R1) was connected to the output of operational amplifier U1 to limit the maximum current flowing into the base of T1. The current supplied to the laser diode was sampled by a 1 \(\Omega\) Z-Foil resistor (Rs) and amplified by operational amplifier U2. The adding of resistor R4 was to balance the input resistance of the non-inverting and inverting terminals of U2. The output of U2 was fed back to U1 and compared with the preset voltage of DAC2 (\(V_{ref}\)). The result of the comparison causes the internal resistance of T1 to be automatically adjusted, thereby closing the negative feedback loop. The current outputted from the emitter of T1 was linearly regulated by \(V_{ref}\), leading to the expression as follows:  
\[I_{s}=\frac{V_{s}}{R_{s}}=\frac{V_{ref}R_{3}}{R_{s}(R_{2}+R_{3})}. \tag{1}\]  
Reverse surge, current spike and fast start-up current are harmful to the laser, so they should be avoided. While the laser power supply is turning off, a momentary reverse surge will be generated. The reverse biased high-speed switching diode (D1) is connected in parallel to the laser module to prevent the reverse surge from flowing into the cathode of the laser. Capacitor C1 can absorb a large amount of current spike which is caused mainly by the fluctuation of power supply [24; 25]. Two series switching diodes (D2 and D3) connected to the non-inverting input of U1 act as a current limiter. In this way, the lower and upper threshold of the preset voltage, which confines the range of desired current, is clamped between 0 V and the output of DAC1 (Vmax), respectively. The soft-start of the laser driver is implemented by charging a capacitor (C2) through a resistor (R5). Meanwhile, the Darlington pair (T2 and T3) is gradually turned on and its emitter voltage rises smoothly until a stable value is reached. Voltage regulator U3 (TPS73533) converts the emitter voltage of T3 to a 3.3 V DC output with the maximum current of 500 mA. Although increasing the feedback coefficient of the feedback loop can improve the stability of the circuit, increase the input impedance, reduce the output impedance and expand the pass-band, there is a hidden danger of self-oscillation. To reduce the possibility of self-oscillation, capacitor C5 is connected between the base and emitter of T1 for loop compensation. In Section 4.2, the stability of the circuit with and without compensation will be discussed separately.  
Figure 4: The float-ground type current source.