Nhng tin b trong Quang hc, Quang ph và ng dng VI ISSN 1859 - 4271

Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271and low cost. In addition, the laser emission at this short wavelength is suitable for exci<s<strong>trong</strong>>tin</s<strong>trong</strong>>g awide range of dye molecules. To generate picosecond laser pulses, the driving circuit of the laserdiode needs to provide a DC bias current and short electrical pulses. Our pulser circuit is basedon the design of Uhring et al. (2004) as shown in Figure 1. The bipolar transistors Q2 and Q3are wide band NPN with a maximum collector current of 200 mA. Q1 is the PNP wide-bandtransistor. The TTL digital pulses with variable repetition rate up to 50 MHz from the generatorU2 can saturate the transistor Q3 and produces a voltage ramp at the collector as it quicklychanges its voltage from VCC level to the ground. The combination of capacitor C3 and resistorR7 forming a high pass filter produces a short (˜1ns) negative pulse. The transistor Q1 invert thisnegative pulse and produces a positive pulse at the base of Q2. The transistor Q2 plays the roleof a buffer, providing the short duration positive pulse to the laser diode.Fig. 1. Schematics of the picosecond diode laser. Transistor Q1 is the wideband PNP type.Transistors Q2 and Q3 are wide-band NPN type. The current source based on the regulatorLM317T provides the bias current to the laser diode. TTL pulses are generated by the pulsegenerator. The pulse forming circuit consis<s<strong>trong</strong>>tin</s<strong>trong</strong>>g of transistors Q1, Q2 and Q3, together wit<strong>hc</strong>apacitors C2, C3, resistor R7 produces electrical pulses of 1 ns in duration.III. SIMULATIONSThe equations describing the operation of a semiconductor under injection of an intense shortelectric pulse are as follows (Tatum et al. 1992, Paulus et al. 1988):dN 1= − [N + I(N − N0) − P(t)]dt τ2(1)dI 1= − [I − βN− I(N − N0)]dt τcWhere N is the population of the excited state, N o is the background population governed bythe doping and structure of the laser diode. Two time constants τ 2 and τ c are the spontaneouslifetime of the upper state and the cavity lifetime, respectively. The mode couple factor β isrelated to the geometry of the laser diode. We approximate this factor to be 10 -3 . We also use avalue of 4 ns for τ 2 and 4 ps for time constant τ c . The pumping electrical pulse P(t) isapproximated as a Gaussian with a full width at half maximum of 1 ns, similar for that generatedby the driving circuit described in the previous section. The rate equations are integrated using4th order Runge-Kutta method. The results of our calculations for the two cases when the114