A new publication from Opto-Electronic Sciences; DOI 10.29026/oes.2024.240003 , discusses a three-dimensional multichannel filter based on femtosecond laser direct writing technology.
Fiber Bragg gratings (FBGs) are optical wavelength-sensitive devices formed by periodic refractive index modulation within the core of an optical fiber. The strength of the counter-propagating coupling between incident and reflected waves in the device is closely related to the structure of the fiber grating. Only light waves that satisfy the Bragg reflection condition can be reflected, giving the device excellent wavelength-selective properties. Furthermore, the device exhibits excellent response characteristics to environmental changes in terms of refractive index and period. The center wavelength of the device's reflected spectrum can vary with the environment. By testing its reflected spectrum, information about environmental factors like temperature, stress, and refractive index can be extracted. Therefore, based on the wavelength-selective and environmentally responsive properties of FBGs, fiber gratings were widely applied in many fields such as filtering, communication, and sensing.
To enhance the communication filtering capability and achieve multi-parameter decoupling in sensing, it is necessary to utilize distributed FBGs composed of multiple sub-gratings with different periods cascaded longitudinally. However, as photons pass through the distributed fiber Bragg grating's various sub-gratings with different periods, multiple reflections can lead to significant signal crosstalk. Additionally, integrating multiple gratings at different longitudinal positions on the same optical fiber reduces the longitudinal integration density of the device. To address these issues, three-dimensional integration of multiple sub-gratings with different periods in the lateral direction of light transmission is crucial for achieving parallel filtering and sensing in multiple channels, thereby enhancing the performance of the Bragg gratings. In recent years, femtosecond laser technology has played a vital role in the field of three-dimensional photonics chip fabrication, owing to its unique three-dimensional modification capabilities. Devices manufactured using this technique find widespread applications in both classical and quantum information processing. Femtosecond laser direct writing is a vital technical solution for achieving the lateral 3D integration of multiple sub-gratings.
Professor Zhen-Nan Tian's team at Jilin University successfully achieved one-step fabrication of a multi-channel three-dimensional filter based on Bragg gratings with different periods using femtosecond laser direct-writing technology. The filter consists of a three-dimensional waveguide optical beam splitter and parallelly arranged Bragg gratings with different periods cascaded on the same chip, as shown in Figure 1. After passing through the 3D optical beam splitter, the light is evenly divided into four spatially parallel beams and is individually transmitted to BG structures with different periods that are in-situ integrated using femtosecond laser technology. Parallel filtering and reflection within the wavelength range from 1450 to 1600 nm were achieved on a 15.5 mm × 1 mm × 1 mm chip, with four center wavelengths spaced at 50 nm intervals and a 3-dB bandwidth of approximately 0.37 nm. All waveguide gratings (WGs) in the filter had a length of 6 mm and were arranged at the vertices of a square with a side length of 127 µm, demonstrating excellent compact integration characteristics and obvious powerful scalability.
The article presents experimental and simulation spectra of four-channel three-dimensional filter, as shown in Figure 2. From left to right, the four enlarged views correspond to the reflection spectra at 1450, 1500, 1550, and 1600 nm, whose 3-dB bandwidths are 0.42, 0.34, 0.33, and 0.37 nm, respectively. A good agreement between experiment and simulation can be observed. By controlling laser and grating structural parameters, the amplitude, central wavelength, and bandwidth of the reflection spectra can be manipulated, thus enabling the creation of arbitrary reflection and transmission curves.
The proposed multi-channel parallel waveguide Bragg grating structure in the paper eliminates crosstalk effectively as there is no interference between channels and no multiple reflections among the gratings. This enables simultaneous multi-channel network information transmission, holding application value in fields such as fiber-to-the-home construction and other optical network transmissions, and has promising prospects for on-chip all-optical information processing.
Keywords: fiber Bragg grating / waveguide grating filter / three-dimensional / multichannel / femtosecond laser direct-writing
