Solid-state laser modulator control unit

This repository contains the circuit and code of the control unit of the Q-factor modulator of a solid-state laser based on the Atmega 328p microcontroller embedded in Arduino.

What is a Q-factor modulator for?

The laser without a Q-factor modulator operates in continuous mode. But some lasers, such as ruby, cannot work in this mode due to heating, and as a result, damage to the material. Such lasers can only be used in pulsed mode. To create such a laser operation mode, Q-factor modulation is necessary. The essence of the process is that during pumping, the properties of the optical resonator are intentionally "degraded", thus preventing the laser from emitting. Due to this, the power is not spent on radiation and it is possible to obtain a high level of inverse population of the energy levels of the active medium. Further, the properties of the resonator are quickly "improved", and all the accumulated energy is realized in the form of a short, powerful pulse. To control these properties of the optical resonator, a Q-factor modulator control unit is needed, which creates control pulses.

Technical specifications

• Start delay time of the first control pulse t_d = 210 \mu s
• Duration of the control pulse t_p = 2.6\ \mu s;
• The duration of the leading edge of the pulse t_f \le 0.3\ \mu s
• The duration of the rear edge of the pulse t_b \le 0.4\ \mu s
• The amplitude of the control pulse on the load U_m = 34\ V
• The number of pulses in the series – 11
• Pulse repetition period T = 0.1\ \mu s
• Load resistance R_l = 5\ k\Omega
• Load capacity C_l = 50\ pF

The Arduino UNO hardware platform based on the Atmega 328p 8-bit microcontroller was chosen to perform this task, but basically, it could be any other Arduino or even bare Atmega chip. The control unit consists of only two components: a microcontroller (processes an external signal and generates pulses) and a power amplifier.

Description of the progam main.asm

• Configure the ports: PD7 for output, the rest for input
• Set 1 to PD7
• Check the PD5 for a signal (until a signal appears)
• If there is a signal, then pulse generation is started: set 0 to PD7, wait 2.6 microseconds, set 1, wait 6.5 microseconds
• Check the PD5 for the absence of a signal (until the signal disappears)
• Return to checking for the presence of a signal.

PD7 and PD5 are 7 and 5 pins on the Arduino board, respectively. The program generates inverted pulses, since there will be a transistor switch at the output that inverts the signal.

Calculation of the transistor key

A transistor switch is used as a power amplifier in this work. I used a soviet NPN transistor KT603V (КТ603В).

Initial data

• I_{c.sat} = 150\ mA - collector saturation current
• U_{c.-e.sat} = 0.3\ V - collector-emitter voltage in saturation mode
• E_s = 34\ V - external power supply voltage
• U_{in} = 5\ V - voltage at the transistor input
• \beta = 40 - transistor gain
• S = 1.2\ldots1.5 saturation coefficient. Let's take it equal to 1.5

Collector resistance

R_c = \frac{E_s - U_{c.-e.sat}}{I_{c.sat}} = \frac{34 - 0.3}{0.150} = 224.7\ \Omega

Base current

I_b = \frac{I_{c.sat}}{\beta}S = \frac{0.150}{40} \cdot 1.5 = 3.75\ mA

Base resistance

R_b = \frac{U_{in} - 0.7 V}{I_b} = \frac{5 - 0.7}{0.00375} = 1146.7\ \Omega

It is also necessary to choose the accelerating capacity of the C_a in order for the pulses to have a sharper front shape. Let's take C_a = 240\ pF.

The calculated resistors must be brought to the standard row E24. Thus we get the parameters of the power amplifier:

• R_c = 240\ \Omega
• R_b = 1.1\ k\Omega
• C_a = 240\ pF

Results

Using an oscilloscope, we can estimate the characteristics of the pulses.

Without amplifier

With amplifier

Accuracy of the characteristics of the received pulses

\Delta t_d = \frac{210 - 210}{210} = 0\% < 10\%

\Delta t_p = \frac{2.6 - 2.52}{2.6} = 3.07\% < 10\%

\Delta T = \frac{9.1 - 9.04}{9.1} = 0.67\% < 10\%

t_f = 0.244\ \mu s < 0.3\ \mu s

t_b = 0.034\ \mu s < 0.4\ \mu s

Arduino adjust

In order to program Arduino using assembly, the following article can be used.