# What is the "side-mode suppression ratio (SMSR)"? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [October 8, 2026, 9:30am UTC](https://forum.ofscn.net/t/what-is-the-side-mode-suppression-ratio-smsr/2954 "2026-10-08T09:30:54Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [October 8, 2026, 9:30am UTC](https://forum.ofscn.net/t/what-is-the-side-mode-suppression-ratio-smsr/2954/1 "2026-10-08T09:30:54Z")
Why can’t there be messy small peaks next to the main peak? Do small peaks interfere with measurements? ---
**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:** [October 8, 2026, 9:32am UTC](https://forum.ofscn.net/t/what-is-the-side-mode-suppression-ratio-smsr/2954/2 "2026-10-08T09:32:36Z")
In Fiber Bragg Grating (FBG) sensing technology, the “small peaks” appearing next to the main peak are typically referred to in optical physics as **Side Lobes** or **Side Modes**. These small peaks **significantly interfere with measurements**. A key metric measuring the relative difference in optical intensity between the main peak and the largest side lobe is the **Side Mode Suppression Ratio (SMSR)**, defined as: \text{SMSR} = 10 \log\_{10}\left(\frac{P\_{\text{main}}}{P\_{\text{side}}}\right) = P\_{\text{main}}(\text{dBm}) - P\_{\text{side}}(\text{dBm}) If prominent secondary peaks exist around the main peak (i.e., if the SMSR is too low), they can cause severe interference in the sensing demodulation system in several ways: * * * ### 1. Causing Measurement Errors and “Mode Hopping” in Demodulator Peak-Tracking Algorithms When measuring strain or temperature, the core task of a Fiber Grating Interrogator is to precisely lock onto the center position of the Bragg wavelength (\lambda\_B). Common peak-tracking algorithms include Gaussian fitting, polynomial fitting, and the centroid method: - **Centroid Offset Error** : The centroid algorithm calculates the integral center of the reflected peak’s energy envelope. If asymmetric small peaks exist on one or both sides of the main peak, the algorithm can be dragged by the energy of the side lobes, causing the calculated center wavelength to deviate from the true Bragg wavelength, thus introducing a systematic calculation error. - **Extreme Value Misjudgment and Mode Hopping (Peak Hopping)**: In practical engineering scenarios, optical fibers may experience power attenuation due to bending or long-distance transmission. If the intensity of the main peak decreases, or if small peaks abnormally rise due to non-uniform stress, threshold-based peak-tracking algorithms might mistakenly identify a small peak as the main peak. This leads to abrupt jumps in the demodulated wavelength, resulting in severe outliers or erroneous data. * * * ### 2. Inducing Channel Crosstalk in Wavelength Division Multiplexing (WDM) Systems In quasi-distributed measurements, multiple FBGs with different center wavelengths are typically written in series on a single fiber (e.g., dozens of sensors connected in the same channel). - To fully utilize the demodulation bandwidth, the wavelength spacing between adjacent sensors is usually designed to be very small (e.g., 2 \ \text{nm} \sim 5 \ \text{nm}). - If the side modes of the gratings are not sufficiently suppressed, their spectral width will spread laterally, directly encroaching into the working wavelength window of adjacent FBGs. - This spectral overlap can cause spectral aliasing and optical crosstalk between adjacent measurement points, making it impossible for the demodulation system to independently resolve the reflection peaks of adjacent sensors, and even causing adjacent measurement points to fail directly. * * * ### 3. Physical Causes and Suppression Methods for Side Lobes - **Causes** : In uniform fiber gratings, the abrupt truncation of the refractive index modulation at the ends of the grating region (equivalent to applying a rectangular window function in the spatial domain) inevitably leads to side lobe distribution resembling a \text{sinc}^2(x) function on both sides of the main lobe, according to the principles of Fourier transform. Additionally, multi-mode effects, cladding mode coupling, or non-uniform refractive index modulation can also generate spurious side modes. - **Engineering Suppression Techniques** : During the writing process, **Apodization techniques** are employed to ensure a smooth transition in the amplitude of the grating refractive index modulation along the fiber axis (e.g., Gaussian apodization, raised cosine apodization). This eliminates boundary discontinuities and suppresses side lobes below the noise floor. * * * ### 4. Industry Specification Indicators and Product Standards In industrial-grade high-precision sensing systems, fiber gratings are typically required to have a sufficiently high Side Mode Suppression Ratio to ensure the purity of the main peak and the stability of the demodulation algorithm. For example, the standard factory specifications for bare grating products such as [OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)](https://www.ofscn.net/fbg-products/femtosecond-fbg.html) and [OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare)](https://www.ofscn.net/fbg-products/high-strength-fbg.html) produced by Beijing Dacheng Yongsheng Technology Co., Ltd. strictly adhere to a default SMSR of \ge 15 \ \text{dB} (meaning the main peak power is more than 30 times higher than the maximum side lobe). This ensures no mode hopping or crosstalk occurs when used with the [OFSCN® Fiber Bragg Grating Interrogator](https://www.ofscn.net/fbg-products/analyzer.html) for high-frequency and high-precision demodulation. ![OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)](https://forum.ofscn.net/uploads/default/original/1X/7cf0a1575059014c779931d799099c9ade047022.jpeg) ![OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)](https://forum.ofscn.net/uploads/default/original/1X/4a94dfba62e5a2cb3f7f62109fdd11f2dc7659a7.jpeg) ![OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)](https://forum.ofscn.net/uploads/default/original/1X/9f5e7be032063f6d8d776ff92e37de0db24c3be2.jpeg) ![OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)](https://forum.ofscn.net/uploads/default/original/1X/67c06a72909c28554efb33535409db562f9f970d.jpeg) ![OFSCN® Fiber Bragg Grating Interrogator](https://forum.ofscn.net/uploads/default/original/1X/8ecc1c112d2015cbc63afa6d42b87517befb2f87.jpeg) ![OFSCN® Fiber Bragg Grating Interrogator](https://forum.ofscn.net/uploads/default/original/1X/248718a63e2aa4cf7db24ad7a4df5ace79c95a05.jpeg)