What is Mode Mixing?

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- GoPhotonics

May 16, 2023

Mode Mixing in Lasers

Mode Mixing in Lasers refers to a phenomenon in which the laser output does not have a single well-defined spatial mode, but rather exhibits a mixture of different spatial modes. It leads to decreased performance and causes potentially damaging effects in lasers. Therefore, it is essential to prevent mode mixing for optimizing laser performance and ensuring safe operation.

In a laser, the light is typically confined to a single transverse mode, i.e., the light wave oscillates in a single plane perpendicular to the direction of propagation. This is important for achieving the high levels of coherence and intensity that make lasers so useful in a variety of applications. In some cases, the laser can experience mode mixing, where different transverse modes are excited and interfere with each other. The different transverse modes in a laser is depicted in figure 1.

Figure 1: Transverse modes in a laser

The main cause of mode mixing is stress-induced birefringence. This changes the refractive index of the material in the laser cavity due to mechanical stress. Stress is introduced into the laser during the manufacturing process or external processes, such as thermal expansion or vibration. Hence, the laser cavity can support multiple transverse modes which lead to mode mixing.

Some negative impacts are caused on laser performance due to this effect. One of the most important problems is reduced power output. The energy that would be concentrated in a single transverse mode is spread out over multiple modes. This also results in a less stable output beam with fluctuations in intensity and direction. Also, mode mixing increases the noise level and decreases coherence, thereby reducing the quality of the laser's output.

There are some solutions to reduce these problems. One approach is to use a mode filter, which selectively blocks certain transverse modes from the laser cavity and allows only the desired mode to propagate. Another approach is to use active control systems, such as adaptive optics or feedback control, to compensate for mode mixing and maintain the desired output. Also, careful design and selection of materials can help minimize stress-induced birefringence and reduce the probability of mode mixing.

Mode Mixing in Optical Fibers

Mode mixing in fibers is a phenomenon that occurs when different optical modes become coupled and interfere with each other as they propagate through an optical fiber. This can result in a degradation of the quality of the optical signal being transmitted through the fiber.

Figure 2: Illustration of mode mixing in fiber

In the case of polarization-maintaining fiber (PMF), the polarization states of different modes in the fiber get mixed due to imperfections or perturbations in the fiber structure. The loss of polarization coherence, which refers to the degree to which the polarizations of multiple waves are synchronized with each other, is caused by this phenomenon in the fiber, resulting in reduced performance. It can be problematic in applications such as fiber optic gyroscopes and telecommunications. The mode mixing in fiber is illustrated in figure 2.

Polarization-maintaining fibers are designed to maintain the polarization state of light traveling through them. They achieve this by introducing birefringence into the fiber, which creates two orthogonally polarized modes that can be selectively excited and propagated through the fiber. But imperfections or perturbations in the fiber structure can cause these modes to mix, leading to a degradation of the polarization coherence.

Stress-induced birefringence is one of the common cause of mode mixing in fibers and this can be introduced into the fiber during the manufacturing process or due to external stress. The stress is generated by heating and stretching the fiber, by applying pressure to the fiber, or by inserting a stress rod into the fiber during the drawing process. This stress causes the two modes in the fiber to mix if the stress is not uniform or if there are imperfections in the fiber. 

Fiber bending is another reason for mode mixing. When a PMF is bent, the birefringent structure is distorted which causes the two modes to mix. This effect is more noticeable in fibers that are bent tightly or have a small bending radius. To minimize mode mixing due to fiber bending and to avoid excessive fiber bending or twisting, fibers with a high degree of birefringence are used.

The other factors that can contribute to mode mixing in PMFs include temperature variations, mechanical strain, and fiber imperfections such as micro-bends or twists. Careful handling and installation of the fiber can also help to minimize external stress and prevent fiber imperfections that can lead to mode mixing.