FIBER BRAGG GRATING

WHAT IS FIBER BRAGG GRATING

Fiber Bragg grating (FBG) is passive optical components that reflect specific wavelengths of light from the fiber core. The schematic diagram of FBG is illustrated in Figure 1, where Λ represents the grating period, which is typically less than 1μm. FBG is created by exposing the fiber core to ultraviolet (UV) light and a phase mask, which creates a periodic modulation of the refractive index. This periodic refractive index change acts as a mirror within the fiber, reflecting the specific wavelength of light. The specific wavelength reflected by FBG is related to the grating period by the equation λc=2neff Λ, where λc is the reflected wavelength, and neff is the effective refractive index. The reflectivity of FBG depends on the amount of refractive index modulation and the grating length. Higher refractive index modulation and grating length will result in higher reflectivity.



THE APPLICATIONS OF FBG IN FIBER LASER ENGINE

FBG is widely used in a variety of applications, including optical communications, sensors, fiber laser engines, optical filters, and optical amplifiers. As shown in figure 2, FBG are used in fiber laser engines (shown in red color) as high reflection fiber Bragg grating (HR FBG) and output coupler fiber Bragg grating (OC FBG). The resonant cavity, also known as the optical resonant cavity, is a cavity in which light of a specific wavelength resonates. It is one of the three elements of a laser. The resonant cavity is used to allow the gain medium to act as a light gain amplifier after it has achieved population inversion. The amplified signal is collected through the resonant cavity to form an oscillator.



HIGH REFLECTION FIBER BRAGG GRATING

High reflection fiber Bragg grating (HR FBG) is a type of fiber Bragg grating (FBG) that have a higher reflectivity than output coupler FBG (OC FBG). As shown in Figure 3, HR FBG can reflect up to 99.5% of light at a specific wavelength.



OUTPUT COUPLER FIBER BRAGG GRATING

Output coupler fiber Bragg gratings (OC FBG) is a type of fiber Bragg grating that have a lower reflection than HR FBG. OC FBG only reflect a small amount of light back to the input end and allow the rest of the light to pass through. The reflection of OC FBG can be customized to meet specific needs. As shown in Figure 4, the input, reflection, and transmission spectra of OC FBG show that it has a lower reflection for a specific wavelength of light.



MEASUREMENT AND PARAMETERS OF FBG

When fabricating fiber Bragg grating on few-mode fibers, modes other than the fundamental mode are unable to form normal angle reflection when incident on the fiber Bragg grating. HR FBG and OC FBG in the laser cavity model accumulate energy by targeting modes with stronger reflectivity. In order to clearly understand the measurement conditions of FBG in the system, FBG measurements are divided into three categories: (1) Reflection spectrum measurement of FBG; (2) Transmission spectrum measurement of FBG considering all modes in the core; and (3) Transmission spectrum measurement of FBG considering only the fundamental mode Pn01. The main parameters of FBG can be obtained through spectral measurement of FBG, including (A) Center wavelength; (B) 3dB bandwidth; (C) side-lobe; and (D) reflectivity.

(1) Reflection spectrum measurement of FBG: Figure 5 shows a photo of the measurement of FBG reflection spectrum. In figure 5, the red dashed line intersects the green line at the points where the green line drops by 3dB from its peak, forming two points denoted as λ1 and λ2. These parameters are obtained and used to calculate the FBG crucial parameters.

The reflection spectrum provides crucial parameters such as:

  • Center wavelength (λc): Calculated using the formula λc = (λ1 + λ2) / 2;
  • 3dB bandwidth (Δλ): Calculated using the formula Δλ = λ2 - λ1
  • Side-lobe height (ΔL): Calculated by finding the difference between the peak and the adjacent trough of the side-lobe.

(2) Transmission spectrum measurement of FBG considering all modes in the core: In figure 6, the yellow curve represents the transmission spectrum of the FBG as measured by the OSA, and the green curve represents the normalized transmission spectrum of the FBG (Obtained by subtracting the light source spectrum from the actual measured spectrum to eliminate the influence of the light source itself). We indicate the difference between the peak and trough values of the green curve (usually taking the second trough) with a red mark in Figure 6. This difference is used to calculate the reflectivity of the FBG when considering all modes in the core. The equation of Reflectivity:

(3)Transmission spectrum measurement of FBG considering only the fundamental mode Pn01 :

In figure 7, the yellow curve is the measured FBG transmission spectrum by OSA, and the green curve represents the normalized FBG transmission spectrum (the actual measured spectrum minus the light source spectrum to remove the influence of the light source). We mark the difference between the peak and valley values of the green curve (usually the second valley) in this figure, as marked in P.P. red in figure 7, to calculate the reflectivity of the FBG at the fundamental mode Pn01.
The equation of reflectivity is