Please note this is a comparison between Version 3 by Beatrix Zheng and Version 2 by Beatrix Zheng.

External cavity semiconductor lasers (ECSLs) usually refer to the gain chip based on the introduction of external optical components (such as waveguides, gratings, prisms, etc.) to provide optical feedback. By designing the type, position and structure of external optical components, the optical properties of SLs (such as center wavelength, linewidth, tuning range, side-mode suppression ratio (SMSR), etc.) can be changed. Bragg grating external cavity semiconductor laser (BG-ECSL) is a device with a specific optical element (Bragg grating) in the external cavity. BG-ECSLs have excellent performances, such as narrow linewidth, tunability and high SMSR. They are widely used in WDM systems, coherent optical communication, gas detection, Lidar, atomic physics and other fields.

- tunable
- narrow linewidth
- Bragg grating
- BG-ECSL

Grating, also known as diffraction grating, is an optical element composed of a large number (tens of thousands) of equally wide and equally spaced slits, which can make the amplitude or phase of the incident light, or both, produce periodic spatial modulation.

Bragg gratings (period less than 1 μm, also known as reflection gratings) are transparent devices with a periodically varying refractive index. Their structure is shown in **Figure 1**. The reflectance is large in the wavelength region (bandwidth) near a particular wavelength, which satisfies the Bragg condition:

mλ_{B} = 2n_{eff}Λcosθ

In Formula (1), m is the diffraction order, λ_{B} is the Bragg wavelength, n_{eff} is the effective refractive index of the medium, Λ is the grating period and θ is the propagation angle in the medium relative to normal incidence.

If the above conditions are met, the difference between the wave number of the grating and the wave number of the incident and reflected waves is matched. Other wavelengths of light are hardly affected by the Bragg grating but still produce some sidelobes in the reflection spectrum. Similarly, there is almost no reflection of the beam at other angles of incidence. When the grating is long enough, even a very weak refractive index modulation can achieve almost total reflection of the beam near Bragg wavelength. The principle is shown in **Figure 2**.

The linewidth (∆ν) of BG-ECSLs (linewidth is usually used to quantitatively characterize the spectral purity of its temporal coherence) can be expressed as follows:

$$\Delta v=(1+{\alpha}^{2})\frac{{{\upsilon}_{g}{2}^{}\mathrm{h}\nu \mathrm{g}{n}_{sp{}_{}{\alpha}_{m}\frac{8\pi {P}_{0}\frac{}{}}{}}}^{}}{}$$

(2)

In Equation (2), α is the specific linewidth increase factor of the semiconductor laser, υ_{g} is the group velocity, h is the Planck constant, ν is the frequency, g is the gain factor, n_{sp} is the spontaneous emission factor (reflecting incomplete particle number inversion), α_{m} is the output loss of the cavity and P_{0} is the output power.

Where α can be expressed as:
vector n and g represent the real and imaginary parts of the complex refractive index of the active medium, respectively, and dg/dN is the differential gain. Due to the different materials, it is generally between 2 and 5, and from Equation (2), it can be seen that the output power of the laser is inversely proportional to the linewidth.

$$\alpha =\frac{\mathrm{d}\overrightarrow{\mathrm{n}}/\mathrm{dN}\frac{\mathrm{dg}/\mathrm{dN}\frac{}{}}{}}{}$$

(3)

The SMSR of BG-ECSLs (SMSR is usually a replay index used to characterize its single-longitudinal modeling) can be expressed as:
where P_{1} is the optical power of longitudinal mode, and P_{2} is the maximum optical power of edge mode. The larger the SMSR of the laser, the better its single-mode characteristics, and the more stable the single-mode output of the laser. When the edge mode rejection ratio is greater than 20 dB—that is, the optical power of the main longitudinal mode is more than 100 times of the maximum optical power of the edge mode—the laser in this working state can be considered as a single longitudinal mode laser.

$$SMSR=10\mathrm{lg}\frac{{P}_{1}\frac{{P}_{2}\frac{}{}}{}}{}$$

(4)

Type | VBG | FBG | WBG |
---|---|---|---|

Main Materials | Photo-Thermo-Refractive, Polymer | Glass, Crystal, Plastomer | Si, SiO_{2}, Si_{3}N_{4}, LiNbO_{3}, Polymer |

Linewidth Range | 2 kHz~1.5 THz | 125 Hz~312 GHz | 320 Hz~85.3 GHz |

Min linewidth | 2 kHz | 125 Hz | 320 Hz |

SMSR Range | 16 dB~57 dB | 15 dB~82 dB | 15 dB~60 dB |

Max SMSR | 57 dB | 82 dB | 60 dB |

Tuning Range | 0.011 nm~2 nm | 0.1 nm~85 nm | 0.16 nm~100 nm |

Max Tuning Range | 2 nm | 85 nm | 100 nm |

Output Power | 10 mW~106.4 W | 0.05 mW~670 mW | 0.5 mW~312 mW |

Maximum Output Power | 106.4 W | 670 mW | 312 mW |

- Saktioto, T.; Fadilla, F.D.; Soerbakti, Y.; Irawan, D.; Okfalisa. Application of Fiber Bragg Grating Sensor System for Simulation Detection of the Heart Rate. J. Phys. Conf. Ser. 2021, 2049, 012002.
- Butt, M.A.; Kazanskiy, N.L.; Khonina, S.N. Advances in Waveguide Bragg Grating Structures, Platforms, and Applications: An Up-to-Date Appraisal. Biosensors 2022, 12, 497.
- Jinachandran, S.; Rajan, G. Fibre Bragg grating based acoustic emission measurement system for structural health monitoring applications. Materials 2021, 14, 897.
- Park, T.H.; Kim, S.M.; Oh, M.C. Polymer-waveguide Bragg-grating devices fabricated using phase-mask lithography. Curr. Opt. Photonics 2019, 3, 401–407.
- Nishijima, Y.; Ueno, K.; Juodkazis, S.; Mizeikis, V.; Fujiwara, H.; Sasaki, K.; Misawa, H. Lasing with well-defined cavity modes in dye-infiltrated silica inverse opals. Opt. Express 2009, 17, 2976–2983.
- Zheng, Y.; Yue, J.; Zhang, P. Analysis of parameter influence law of waveguide Bragg grating. Opt. Laser Technol. 2022, 146, 107576.
- Mikutis, M.; Kudrius, T.; Šlekys, G.; Paipulas, D.; Juodkazis, S. High 90% efficiency Bragg gratings formed in fused silica by femtosecond Gauss-Bessel laser beams. Opt. Mater. Express 2013, 3, 1862–1871.
- Roth, G.L.; Kefer, S.; Hessler, S.; Esen, C.; Hellmann, R. Polymer photonic crystal waveguides generated by femtosecond laser. Laser Photonics Rev. 2021, 15, 2100215.

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