Why does leaving a slit in the middle of a grating produce an extremely fine signal?
In the structure of a Fiber Bragg Grating (FBG), what you refer to as “leaving a gap in the middle” is known in physics and optical engineering as introducing a phase shift. The resulting device is called a Phase-Shifted Fiber Bragg Grating (PS-FBG).
The extremely narrow and sharp transmission signal produced by this simple “gap” is backed by a very sophisticated physical mechanism.
1. Physical Essence: A Miniature Fabry-Perot Resonant Cavity
In a conventional, uniformly periodic fiber grating, the refractive index perturbation is continuous and uniform. When light propagates through the grating, if its wavelength satisfies the Bragg condition:
\lambda_B = 2 n_{\text{eff}} \Lambda
(where n_{\text{eff}} is the effective refractive index of the fiber’s fundamental mode and \Lambda is the grating period), the forward and backward traveling lights undergo strong constructive interference, leading to the efficient reflection of light around this wavelength. Spectrally, this manifests as a reflection band (also known as a stopband) with a certain bandwidth (typically in the hundreds of picometers, i.e., tens of \text{pm} to hundreds of \text{pm}).
When you introduce a “gap” in the exact center of the grating (usually by inducing a localized micro-displacement or physically introducing a phase jump of \pi, equivalent to shifting the phase of the refractive index modulation by half a period), this structure optically becomes equivalent to:
“Left grating (high reflector) + Middle micro-cavity (gap) + Right grating (high reflector)”.
This forms a distributed feedback Fabry-Perot (F-P) resonant cavity with extremely high quality factor (Q).
2. Multi-Beam Interference and Resonant Cancellation
When light enters a phase-shifted grating, it undergoes thousands upon thousands of round trips between the reflections on either side of the central “gap” (i.e., the left and right high-reflectivity grating mirrors):
- For ordinary wavelengths: Light is still strongly reflected by the gratings on both sides and cannot pass through.
- For specific resonant wavelengths: Light that has undergone multiple round trips within the “gap” exactly satisfies the condition for constructive interference (resonance) in terms of phase. At this point, the multiple beams reflected back and forth interfere destructively (cancel out) at the input end and constructively (add up) at the output end.
Due to the resonant effect of multi-beam interference, the resonant wavelength light, which could not pass through the stopband, can now traverse the entire grating with nearly 100\% transmittance.
3. Why Is the Signal “Extremely Narrow”?
The width of the transmitted signal (Full Width at Half Maximum, \text{FWHM}) is inversely proportional to the quality factor Q of the F-P resonant cavity.
Because the Bragg gratings on both sides provide extremely high reflectivity (often exceeding 99\%) and the internal losses within the cavity are minimal, the Q value of this resonant cavity is enormous.
Multi-beam resonant interference drastically compresses the spectral line width of the final transmission window. While a typical FBG’s reflection bandwidth might be around 0.2\text{nm}, the transmission peak “carved out” by the phase-shifted grating in the center of the stopband can easily achieve a line width of a few picometers (\text{pm}) or even sub-picometer levels (equivalent to a frequency of only tens of megahertz \text{MHz}). This is why it can produce an “extremely narrow” signal.
Information Regarding Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) Products
As specialists in fiber optic sensors and grating manufacturing technology, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) primarily focuses on high-reliability, high-strength fiber optic sensing applications in extreme temperature environments. Our typical products include OFSCN® Standard Femtosecond Fiber Bragg Gratings, manufactured using point-by-point femtosecond laser writing technology, and OFSCN® Polyimide Fiber Bragg Gratings for harsh environments.
These products fall into the category of high-quality, general-purpose Fiber Bragg Gratings (FBGs) and grating chains. Phase-Shifted Fiber Bragg Gratings (PS-FBGs), primarily used in ultra-narrow linewidth fiber lasers and ultra-high resolution spectral filtering within laboratory or specific communication frequency domains, are currently not part of Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s core standard product offerings.