What is the difference between single mode and multi-mode in pulsed laser?

In pulsed lasers, the terms single-mode and multi-mode describe the spatial characteristics of the laser beam. They refer to how the light propagates within the laser cavity, influencing the beam quality and performance for various applications. Below are the main differences between the two:

1. Beam Quality (M² Factor)

Single-Mode:

Exhibits a low M² value (close to 1), indicating a nearly perfect Gaussian beam profile.

Results in a fine, highly focused, and precise beam.

Multi-Mode:

Has a higher M² value (>1), resulting in a less homogeneous beam with multiple lateral modes.

The beam is wider and less focused.

2. Beam Focusability

Single-Mode:

Can be tightly focused to a small spot size, making it ideal for high-precision applications.

Multi-Mode:

Produces a wider beam that is less focused but delivers more power over a larger area.

3. Power Output

Single-Mode:

Typically supports lower power levels (e.g., less than 500W) due to the requirement for high beam quality.

Multi-Mode:

Capable of supporting higher power levels (up to several kilowatts), making it suitable for high-energy applications.

4. Applications

Single-Mode:

Precision marking and engraving

Micro welding and thin material cutting

High-precision cleaning (e.g., precision surfaces or microelectronics)

Multi-Mode:

Heavy-duty cleaning (e.g., rust and paint removal)

Cutting and welding thicker materials

Surface texturing and large-area processing

5. Heat Affected Zone (HAZ)

Single-Mode:

Generates a smaller HAZ, minimizing thermal damage to surrounding materials.

Multi-Mode:

Produces more heat, leading to a larger HAZ, which may affect material properties.

6. Laser Structure Differences

Single-Mode:

Resonant Cavity: Short cavity length + small aperture diaphragm

Gain Medium: Single-mode fiber or narrow-aperture crystal

Pump Source: Single-mode laser diode

Output: Gaussian beam, M² ≈ 1

Multi-Mode:

Resonant Cavity: Long cavity length + large aperture or no diaphragm

Gain Medium: Large mode field fiber or wide-aperture crystal

Pump Source: Multi-mode laser diode or flash lamp

Output: Multi-mode beam, M² > 1