Why Can't High-power Fiber Laser Diodes Use Single-mode Fiber?

With the continuous breakthroughs in high-power continuous fiber laser technology, fiber laser technology has been widely used in industrial processing, medical treatment, aerospace and other fields.

In the pursuit of higher power output, high-power fiber laser diodes are incompatible with single-mode optical fibers. Because the core diameter of single-mode optical fibers is small and the numerical aperture is limited, it is difficult to efficiently couple and transmit the high-energy optical signals generated by high-power fiber laser diodes, resulting in loss and distortion of optical signals during transmission, which will seriously restrict the overall performance and application effect of fiber laser systems.

Characteristics of high-power fiber laser diodes:
Working principle: Laser is generated based on the stimulated radiation principle of semiconductor diodes, and light is transmitted and amplified through optical fiber structures. Under the action of the pump source, the particles in the working material are excited to a high energy level, forming a particle number inversion distribution, thereby realizing light amplification, and outputting lasers of specific wavelengths and powers through the mode selection of the resonant cavity.
Power characteristics: Usually have a relatively high output power, which can meet the needs of some scenarios that require high energy output, such as material processing, medical and other fields. For example, kilowatt-class fiber lasers are already relatively common and can be used for cutting, welding and other processes in large-scale industrial manufacturing.
Beam quality: Generally, it has good beam quality. Its output beam is mostly fundamental transverse mode or low-order mode, with high brightness and low divergence angle. This enables it to obtain a smaller spot size and higher power density after focusing, which is beneficial to improve processing accuracy and efficiency. However, there are still differences compared with the requirements of single-mode fiber for beam quality. Single-mode fiber requires higher beam quality and smaller spot size, and the laser generated by high-power fiber laser diode may have certain difficulties when coupled into single-mode fiber, requiring special optical system design and optimization.

Single-mode fiber characteristics:
Structure and principle: It consists of a core, cladding and coating. Light propagates axially in the core in a total reflection manner, and can only transmit one mode of light. Its maximum core diameter is generally 9μm or 10μm, and the diameter of the cladding is about 125μm. Because the core diameter is small and close to the wavelength of light, light can only propagate along the axial direction of the core when propagating in it, avoiding mode dispersion, thereby ensuring higher transmission quality.
Advantages:
Low loss: The transmission loss of single-mode optical fiber is relatively small, usually between 0.2dB/km and 0.4dB/km, which allows the signal to be transmitted over a longer distance in the optical fiber without too much relay amplification, and can support transmission distances of up to 80 kilometers or even longer.
Small dispersion: Because only one mode of light can be transmitted, there is no inter-mode dispersion, and the total dispersion is small. At a wavelength of 1.31μm, the material dispersion and waveguide dispersion of single-mode optical fiber are positive and negative, and the sizes are exactly equal, making the 1.31μm wavelength region an ideal working window for optical fiber communication, which can achieve high-speed data transmission.
High reliability: It has good durability and corrosion resistance, is not affected by chemical or other environmental factors, can maintain a certain transmission quality for a long time, and is suitable for long-distance data communication.
Strong anti-interference ability: Since the single-mode optical fiber design meets the optical single transmission mode, it can effectively reduce the scattering of light, so it is not easily affected by electromagnetic interference from the outside world, and is suitable for long-distance data communication.

There are mainly the following reasons why high-power fiber laser diodes cannot use single-mode optical fibers:
Mode mismatch:
The light output by high-power fiber laser diodes may contain multiple modes, while single-mode optical fibers can only transmit one mode of light. When multi-mode optical signals enter single-mode optical fibers, problems such as mode conversion and low coupling efficiency will occur, resulting in increased loss of optical signals during transmission, affecting transmission performance. For example, multi-mode laser diodes can be well coupled with multi-mode optical fibers because both support multiple modes of light transmission, but high-power fiber laser diodes and single-mode optical fibers are difficult to achieve efficient coupling.
Spot size difference:
The light-emitting surface size of high-power fiber laser diodes is usually large, while the core size of single-mode optical fibers is small, generally a few microns. This makes it difficult to effectively couple light into the core of single-mode optical fibers, resulting in large coupling losses. In some practical applications, due to the difference in spot size, complex optical systems may be required to adjust the spot size to accommodate single-mode optical fibers, but this will increase the cost and complexity of the system.
Aberration and beam quality:
Although high-power fiber laser diodes have a certain beam quality, there is still a gap with the high-precision, low-aberration beam quality required by single-mode optical fibers. Aberrations can cause problems such as phase distortion when light is transmitted in optical fibers, further affecting the transmission effect. In high-precision communication systems, aberrations may cause signal distortion and reduce communication quality.

Solutions and application suggestions
Use multimode optical fiber: For high-power fiber laser diodes, a matching multimode optical fiber can be selected for light transmission to improve coupling efficiency and transmission performance.
Application scenario selection: According to specific application requirements, weigh the necessity of using high-power fiber laser diodes and single-mode optical fibers. In some application scenarios where the power requirements are not particularly high but the transmission distance and bandwidth requirements are high, other types of light sources can be considered in combination with single-mode optical fibers.

High-power fiber laser diodes cannot use single-mode optical fibers, mainly due to problems such as mode mismatch, spot size differences, and inconsistent beam quality. In practical applications, in order to achieve the best transmission performance and application effect, the fiber type and light source combination should be reasonably selected according to specific needs.

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