# What is a "transmission spectrum"? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [October 8, 2026, 9:31am UTC](https://forum.ofscn.net/t/what-is-a-transmission-spectrum/2957 "2026-10-08T09:31:54Z")\ **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, 9:31am UTC](https://forum.ofscn.net/t/what-is-a-transmission-spectrum/2957/1 "2026-10-08T09:31:54Z")
Why is there a “dip” in the spectrum when viewed at the other end of a grating? ---
**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, 9:33am UTC](https://forum.ofscn.net/t/what-is-a-transmission-spectrum/2957/2 "2026-10-08T09:33:53Z")
In the technical analysis of Fiber Bragg Gratings (FBGs), the spectrum measured at the other end of the grating is known as the **Transmission Spectrum**. The “dips” appearing on the spectrum are commonly referred to as **Transmission Dips / Notches** in optical engineering. This phenomenon is a physical effect jointly determined by **coherent Bragg reflection** and the **law of conservation of energy** : * * * ### 1. Physical Principle: Bragg Reflection and Coherent Superposition A Fiber Bragg Grating is a micro-structure with periodic refractive index modulation fabricated within the core of a single-mode fiber. When broadband light is injected into the grating region along the fiber, Fresnel reflection occurs at each weak interface of refractive index change. When the wavelength of the incident light satisfies the Bragg resonance condition: \lambda\_B = 2 n\_{\text{eff}} \Lambda _(Where \lambda\_B is the Bragg center wavelength, n\_{\text{eff}} is the effective refractive index of the guided mode in the fiber core, and \Lambda is the grating modulation period)_ The reflected lights from each interface undergo completely in-phase constructive interference and superposition, strongly reflecting the light of this specific narrow band wavelength \lambda\_B back at the input end (appearing as a narrow band “peak” at the reflection end). ### 2. Conservation of Energy: Energy Deficit at the Transmission End Ignoring the extremely small intrinsic scattering and absorption losses in the fiber material, the optical power input into the system follows the law of conservation of energy: T(\lambda) + R(\lambda) \approx 1 _(Where T(\lambda) is transmittance and R(\lambda) is reflectance)_ - **Non-resonant wavelength region** : The light wave does not satisfy the Bragg condition, its reflectance is close to zero (R \approx 0), and the light wave passes through the grating region with almost no loss (T \approx 1), appearing as a flat baseline. - **Resonant wavelength region (\lambda\_B)** : The light energy is coherently reflected back in the incident direction (R \to 1), leading to a significant reduction in the energy propagating forward and reaching the other end of the grating (T \to 0). Therefore, on the logarithmic scale (dB scale) of an Optical Spectrum Analyzer (OSA), the transmitted power at this wavelength position drops sharply, forming a steep “dip”. * * * ### 3. Engineering Characterization of Transmission Dips The shape of the transmission dip directly characterizes the core performance parameters of the fiber grating: - **Depth (Transmission Loss / Suppression Depth)**: The attenuation at the bottom of the transmission dip is directly determined by the **Reflectivity** of the grating. - For example, if the grating reflectivity is 90\%, the transmitted optical power drops to 10\% of the initial value, and the dip depth is approximately 10\ \text{dB}; - If the reflectivity is 99\% (high-reflection grating), the transmission dip depth can reach 20\ \text{dB}; - If the reflectivity is 99.9\%, the transmission dip depth can reach 30\ \text{dB}. - **Width (Filter Bandwidth)**: The Full Width at Half Maximum (FWHM) of the transmission dip corresponds to the **3dB bandwidth** of the grating, which is mainly determined by the length of the grating region (e.g., 2\ \text{mm} to 10\ \text{mm}) and the depth of the modulation refractive index (\Delta n). * * * ### Related Product Series Beijing Dacheng Yongsheng Technology Co., Ltd.'s multi-category Fiber Bragg Grating products are manufactured with strictly controlled reflection/transmission spectral quality, covering single-point FBGs and multi-point Fiber Grating Strings (FBG Strings): - **[OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)](https://www.ofscn.net/fbg-products/femtosecond-fbg.html)**: Femtosecond point-by-point written gratings, with customizable reflectivity and bandwidth. Side-mode suppression ratio is defaulted to \ge 15\ \text{dB}. - **[OFSCN® Polyimide Fiber Bragg Gratings / FBG Strings (Bare)](https://www.ofscn.net/fbg-products/polyimide-fbg.html)**: Polyimide temperature-resistant fiber gratings, with a standard wavelength range of 1525\ \text{nm} \sim 1565\ \text{nm} (customizable to 1510\ \text{nm} \sim 1590\ \text{nm}). ![OFSCN® Polyimide Fiber Bragg Gratings](https://forum.ofscn.net/uploads/default/original/1X/63e9a4173f5f2c03733dc13affabe75e789e155a.jpeg)