Modes classification

[Aadil Raza]

Dear Friends,

In Spatial CW laser we have two options of modes which is Hermite-Gaussian modes and Laguerre-Gaussian modes. but in most of the mode division multiplexing papers I have seen they are only talking about LP modes for transmission of data. What is the reason of this?

[Steve Dods]

LP modes (Linear Polarized modes) are popular for use as an approximation to propagation on weakly guiding optical fibres. The real modes are vector modes that have complicated field patterns, but the refractive index of an optical fibre core is usually only a fraction of a percent higher than the cladding. Because of this small contrast, the vector modes are almost linearly polarized, and so the LP approximation is pretty good for most optical fibres.

Propagation in free space, such as in the cavity of a gas laser, is a different thing. The Gaussian modes are appropriate for that case.

[Aadil Raza]

Thanks for reply. So it means we have to generate LP modes for data modulation and then transmit them in optical fiber for MDM? or we can also use Hermite-Gaussian modes and Laguerre-Gaussian modes for this purpose as well? please clear my confusion. secondly if fiber support only lp modes then why we have Hermite-Gaussian modes and Laguerre-Gaussian modes in spatial CW laser? waiting for your reply please..

[Steve Dods]

The mode is the form the wave assumes when it follows a waveguide. Different waveguide, different mode. Typical optical fibres have LP modes. Free space, such as in gas laser cavity, the Gaussian modes are better for the model. Integrated optical devices will have different modes. Light in one mode can pass into another, but usually with some coupling loss.

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