Longitudinal modes in a fibre laser

In summary: Niles.In summary, the analysis of longitudinal modes is important in lasers with single transversal mode, such as single-mode fiber lasers. The number of longitudinal modes in these lasers can be estimated by the ratio of the spectral width of gain to the spectral separation of longitudinal modes. This is because even in a saturated gain curve, there can still be multiple peaks representing different longitudinal modes, which are determined by the cavity length.
  • #1
Niles
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Hi

In the Wiki article on longitudinal modes (http://en.wikipedia.org/wiki/Longitudinal_mode) they state that:

"The analysis of longitudinal modes is especially important in lasers with single transversal mode, for example, in single-mode fiber lasers. The number of longitudinal modes of such a laser can be estimated as ratio of the spectral width of gain to the spectral separation of longitudinal modes."

I can't quite see the why the bolded part is true. What I mean is that even in a fiber the gain curve is saturated when we have a laser with feedback (gain = loss), and if the gain curve is homogeneously broadened, in principle only a single longitudinal mode should oscillate (since the gain curve saturates homogeneously, so all other modes are below the gain curve). But they don't talk about that here. Is there something I am missing?Niles.
 
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Hello Niles,

Thank you for bringing up this question. The statement in the Wiki article is referring to lasers with single transversal mode, such as single-mode fiber lasers. In these types of lasers, the transverse mode is confined to a single spatial mode, but there can still be multiple longitudinal modes. The number of longitudinal modes is dependent on the spectral width of the gain, which is determined by the properties of the gain medium, and the spectral separation of the longitudinal modes, which is determined by the cavity length.

In a single-mode fiber laser, the gain curve is indeed saturated when there is feedback, but this does not mean that only a single longitudinal mode will oscillate. The gain curve may be homogeneously broadened, but there can still be multiple peaks within the broadened curve that correspond to different longitudinal modes. The separation between these peaks is determined by the cavity length, which is why the ratio of the spectral width of gain to the spectral separation of longitudinal modes can be used to estimate the number of longitudinal modes in a single-mode fiber laser.

I hope this helps to clarify the statement in the Wiki article. If you have any further questions, please let me know.
 

Related to Longitudinal modes in a fibre laser

1. What are longitudinal modes in a fibre laser?

Longitudinal modes refer to the different possible wavelengths of light that can be emitted from a fibre laser. These modes are determined by the length of the laser cavity and the refractive index of the laser medium.

2. How many longitudinal modes can a fibre laser have?

The number of longitudinal modes in a fibre laser depends on the length of the laser cavity. Generally, the longer the cavity, the more modes it can support. However, the number of modes is limited by the gain bandwidth of the laser medium.

3. What is the significance of longitudinal modes in a fibre laser?

Longitudinal modes play a crucial role in determining the output characteristics of a fibre laser. They affect the laser's spectral properties, output power, and stability. Understanding and controlling longitudinal modes is important for optimizing the performance of a fibre laser.

4. How can we control longitudinal modes in a fibre laser?

There are several methods for controlling longitudinal modes in a fibre laser. One approach is to use a mode selector, such as a grating or a resonant cavity, to filter out unwanted modes. Another method is to adjust the length of the laser cavity or modulate the pump power to select specific modes.

5. What are the implications of having too many longitudinal modes in a fibre laser?

If a fibre laser has too many longitudinal modes, it can lead to mode competition and instability in the laser output. This can result in fluctuations in output power and poor spectral properties. Therefore, it is important to carefully design and control longitudinal modes in a fibre laser for optimal performance.

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