# Charge-Line Dual-FET High-Repetition-Rate Pulsed Laser Driver
## 2 Foundation of Charge-Line Pulsed Laser Driver  
### Dual-FET Output Stage Topology  
One of the most popular solutions dedicated to high-power drivers utilizes single switching devices [1, 2, 6, 7]. For this reason, the avalanche transistors for over 60 years [9] were the commonly used devices in laser pulsed generators [6, 7]. However, nowadays most of the modern systems utilize Field Effect Transistors (FET) [1, 2]. The basic example topologies, shown in Figure 1, are based oncharge storage passive elements. In order to briefly analyze their principle of operation, a steady state (transistors are switched-off and storage elements are charged) is assumed.  
If the trigger signal is applied, the C\({}_{\text{C}}\) capacitor in Figure 1a is discharged through a semiconductor switch and a laser diode, resulting in an optical pulse of Gaussian shape [1,2]. The amplitude of the laser pulse is determined both by the +HV voltage applied to the driver, and the C\({}_{\text{C}}\) capacitance. Therefore, it is impossible to adjust the pulse width and amplitude separately. Nevertheless, the main advantage of this simple circuit is its high laser current [1,6], limited only by the parasitic inductances and resistances of the C\({}_{\text{C}}\), Q\({}_{\text{CAP}}\), and laser diode itself.  
In the other topology, which is shown in Figure 1b, an open-end delay line was utilized as a charge storage element. When the trigger signal (denoted as Trg) is applied, the transistor Q\({}_{\text{LINE}}\) starts to discharge the line. The delay line acts then as a constant current sink (the peak current is determined by the characteristic impedance Z0 of the transmission line and initial line voltage) by the time needed by the falling edge signal to propagate back and forth through the passive element. The duration of the optical pulse resulting from the square wave applied to the laser is approximately 2 \(\cdot\) DL, thus a pulse width adjustment is possible only by replacing the physical element (i.e., the delay line). On the other hand, the peak power is independent of the pulse timing and might be adjusted by the +HV voltage setting.  
The novel topology of the pulsed laser driver described in this paper is shown in Figure 2. It combines two symmetrical structures from Figure 2b, although a single shared delay line is used [7]. The operation of the circuit can be divided into two adjacent phases. During the first phase, the transistor Q\({}_{\text{1}}\) is switched on at time t\({}_{\text{1}}\) and a pre-charged delay line supplies the laser diode (assures the current flow). If no trigger signal is applied to the Q\({}_{\text{2}}\), the resulting optical pulse width is 2 \(\cdot\) DL, as in the circuit shown in Figure 1b. However, if a second trigger signal is also supplied to the Q\({}_{\text{2}}\) in the succeeding second phase, i.e., t\({}_{\text{2}}\) = t\({}_{\text{1}}\), the transistors Q\({}_{\text{1}}\) and Q\({}_{\text{2}}\) start to discharge the delay line at the same time. This causes the premature termination of the lasing current at time t\({}_{\text{1}}\) + DL. By adjusting the mutual position t\({}_{\text{2}}\) and t\({}_{\text{1}}\) in the time domain, according to Equation (1), the available regulated laser pulse width ranges from 0 to 2 \(\cdot\) DL.  
\[\text{t}_{\text{FWHM}}=\text{t}_{\text{2}}-\text{t}_{\text{1}}+\text{DL} \tag{1}\]  
where t\({}_{\text{FWHM,MIN}}\)\(\rightarrow\) 0 and t\({}_{\text{FWHM,MAX}}\) = 2 \(\cdot\) DL  
Figure 1: Simplified schematics of two basic pulsed laser diode drivers utilizing a single switching device: (**a**) The driver with a pulse shaping capacitor C\({}_{\text{C}}\); (**b**) the driver with a pulse shaping delay line.  
The lasing peak current is adjusted via the +HV voltage, applied to the driver. According to Equation (2), the main current limiting factors in the dual-FET topology are laser forward voltage, Q\({}_{1}\)-on resistance and Z\({}_{0}\) of the delay line.  
\[\mathrm{I_{LAS}}\ =\ \frac{U_{+HV}-\ U_{F\ LAS}}{R_{on\ Q1}+Z_{0}} \tag{2}\]