Pulse frequency control of laser pulse control technology

Pulse frequency control of laser pulse control technology

1. The concept of Pulse frequency, laser pulse Rate (Pulse Repetition Rate) refers to the number of laser pulses emitted per unit time, usually in Hertz (Hz). High frequency pulses are suitable for high repetition rate applications, while low frequency pulses are suitable for high energy single pulse tasks.

2. The relationship between power, pulse width and frequency Before laser frequency control, the relationship between power, pulse width and frequency must first be explained. There is a complex interaction between laser power, frequency and pulse width, and adjusting one of the parameters usually requires considering the other two parameters to optimize the application effect.

3. Common pulse frequency control methods

a. External control mode loads the frequency signal outside the power supply, and adjusts the laser pulse frequency by controlling the frequency and duty cycle of the loading signal. This allows the output pulse to be synchronized with the load signal, making it suitable for applications requiring precise control.

b. Internal control mode The frequency control signal is built into the drive power supply, without additional external signal input. Users can choose between a fixed built-in frequency or an adjustable internal control frequency for greater flexibility.

c. Adjusting the length of the resonator or electro-optical modulator The frequency characteristics of the laser can be changed by adjusting the length of the resonator or using an electro-optical modulator. This method of high-frequency regulation is often used in applications that require higher average power and shorter pulse widths, such as laser micromachining and medical imaging.

d. Acousto optic Modulator (AOM Modulator)  is an important tool for pulse frequency control of laser pulse control technology. AOM Modulator uses acousto optic effect (that is, the mechanical oscillation pressure of sound wave changes the refractive index) to modulate and control the laser beam.

4. Intracavity modulation technology, compared with external modulation, intracavity modulation can more efficiently generate high energy, peak power pulse laser. The following are four common intracavity modulation techniques:

a. Gain Switching by rapidly modulating the pump source, the gain medium particle number inversion and gain coefficient are rapidly established, exceeding the stimulated radiation rate, resulting in a sharp increase in photons in the cavity and the generation of short pulse laser. This method is particularly common in semiconductor lasers, which can produce pulses from nanoseconds to tens of picoseconds, with a repetition rate of several gigahertz, and is widely used in the field of optical communications with high data transmission rates.

Q switch (Q-switching) Q switches suppress optical feedback by introducing high losses in the laser cavity, allowing the pumping process to produce a particle population reversal far beyond the threshold, storing a large amount of energy. Subsequently, the loss in the cavity is rapidly reduced (that is, the Q value of the cavity is increased), and the optical feedback is turned on again, so that the stored energy is released in the form of ultra-short high-intensity pulses.

c. Mode Locking generates ultra-short pulses of picosecond or even femtosecond level by controlling the phase relationship between different longitudinal modes in the laser cavity. The mode-locking technology is divided into passive mode-locking and active mode-locking.

d. Cavity Dumping By storing energy in the photons in the resonator, using a low-loss cavity mirror to effectively bind the photons, maintaining a low loss state in the cavity for a period of time. After one round trip cycle, the strong pulse is “dumped” out of the cavity by quickly switching the internal cavity element, such as an acousto-optic modulator or an electro-optic shutter, and a short pulse laser is emitted. Compared to Q-switching, cavity emptying can maintain a pulse width of several nanoseconds at high repetition rates (such as several megahertz) and allow for higher pulse energies, especially for applications requiring high repetition rates and short pulses. Combined with other pulse generation techniques, the pulse energy can be further improved.

Pulse control of laser is a complicated and important process, which involves pulse width control, pulse frequency control and many modulation techniques. Through reasonable selection and application of these methods, the laser performance can be accurately adjusted to meet the needs of different application scenarios. In the future, with the continuous emergence of new materials and new technologies, the pulse control technology of lasers will usher in more breakthroughs, and promote the development of laser technology in the direction of higher precision and wider application.