# What is the Cut-off Wavelength?

**URL:** <https://forum.ofscn.net/t/what-is-the-cut-off-wavelength/406>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology, sensing-cable, specialty-fiber, high-temp-cable, wiki-component\
**Created:** [January 25, 2026, 2:36am UTC](https://forum.ofscn.net/t/what-is-the-cut-off-wavelength/406 "2026-01-25T02:36:44Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![frostweaver](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/f/da6949/32.png) [@frostweaver](https://forum.ofscn.net/u/frostweaver)\
**Post date:** [January 25, 2026, 2:36am UTC](https://forum.ofscn.net/t/what-is-the-cut-off-wavelength/406/1 "2026-01-25T02:36:44Z")

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Why does a single-mode fiber support multi-mode transmission below a specific wavelength?

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<div class="post-metadata">

**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\
**Post date:** [January 25, 2026, 2:36am UTC](https://forum.ofscn.net/t/what-is-the-cut-off-wavelength/406/2 "2026-01-25T02:36:49Z")

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When the operating wavelength in a single-mode fiber falls below its cut-off wavelength, the fiber will support the propagation of multiple modes, essentially behaving like a multi-mode fiber.

This phenomenon occurs because the V-number (or normalized frequency) of the fiber, which determines the number of modes a fiber can support, is inversely proportional to the wavelength. As the wavelength decreases, the V-number increases. A fiber designed as single-mode for wavelengths above its cut-off will start to guide higher-order modes when the V-number exceeds the threshold for single-mode operation (typically around 2.405 for a step-index fiber). This transition leads to multi-mode transmission, which can introduce modal dispersion and increase signal loss.

OFSCN provides various types of single-mode optical fibers, including standard and specialized options. For instance, our range includes:

- [OFSCN® G.652D Optical Fiber](https://www.ofscn.org/optical-fibers/g-652-d.html)
- [OFSCN® G.657 Optical Fiber](https://www.ofscn.org/optical-fibers/g-657-a.html)
- [OFSCN® 120℃ SM High-temperature Optical Fiber](https://www.ofscn.org/optical-fibers/high-temperature-120-sm.html)
- [OFSCN® 300℃ Small diameter optical fiber](https://www.ofscn.org/optical-fibers/small-diameter-pi-sm.html)
- [OFSCN® Gold-coated Optical Fiber](https://www.ofscn.org/optical-fibers/gold-coated.html)

Here is an image related to optical fibers:

 ![85-fiber](https://forum.ofscn.net/uploads/default/original/1X/6a319a4dd5ea0f7c0cc52e7310cea412ad0410b5.jpeg)