• Acta Photonica Sinica
  • Vol. 51, Issue 1, 0151106 (2022)
Bin ZHANG, Lei WANG, Yuechen JIA, and Feng CHEN*
Author Affiliations
  • School of Physics,State Key Laboratory of Crystal Materials,Shandong University,Jinan 250100,China
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    DOI: 10.3788/gzxb20225101.0151106 Cite this Article
    Bin ZHANG, Lei WANG, Yuechen JIA, Feng CHEN. Research Advances of Optical Waveguides by Light-manipulation Based Femtosecond Laser Writing(Invited)[J]. Acta Photonica Sinica, 2022, 51(1): 0151106 Copy Citation Text show less

    Abstract

    Integrated optical circuits play an essential role in the field of optical communication, by which the high-speed processing and transmitting of optical signals can be realized. Optical waveguide, in which the light will be confined into a micron or submicron volume for non-diffraction propagation, is one of the most importantly basic components in integrated optical circuits. The low-loss optical waveguides can be applied to fabricate high-performance photonic devices, e.g., beam splitters, frequency converters, and waveguide lasers. Hence, the fabrication of low-loss optical waveguides is of great significance to many applications in integrated optics and quantum photonics. The optical waveguides in transparent materials can be produced by ion exchange, ion implantation, and Ti-indiffusion. Nevertheless, these waveguides are limited to a 2D planar geometry. The 3D optical waveguides could be fabricated by femtosecond laser direct writing. Femtosecond laser direct writing is a maskless, efficient, and flexible 3D fabrication technique, which has become one of the most widely used techniques for precision machining of materials. The femtosecond laser possesses ultrashort pulse width and extremely high peak intensity, which could lead to the suppression of heat-affected zones and the appearance of nonlinear interactions (e.g., multiphoton absorption, tunneling ionization, and avalanche ionization), respectively. The microscopic objective is often utilized to focus NIR femtosecond laser into transparent materials, resulting in material modifications in focal regions. The material modifications can be classified into two types: Type-I modification and Type-II modification. The refractive index change is positive in the areas of Type-I modification, and the refractive index change is negative in the areas of Type-II modification. By using these two types of modifications, the single-line waveguide, dual-line waveguide, vertical-dual-line waveguide, multi-line waveguide, and depressed-cladding waveguide have been fabricated in transparent materials (e.g., glasses and crystals). In the past 20 years, a variety of photonic devices have been produced with femtosecond-laser-written optical waveguides, such as waveguide arrays, electro-optic modulators, and directional couplers. It can be anticipated that the novel, multi-functional, and high-efficient waveguide-based photonic devices will be created in succession with the in-depth study on laser-matter interactions. Although femtosecond laser direct writing has made a series of achievements in waveguide fabrication, there are still some challenges to rapidly produce low-loss optical waveguide with circular cross-section, due to spherical aberration at the interface caused by refractive index mismatch. In order to improve the waveguide quality and fabrication efficiency, the researchers are dedicated to develop the femtosecond laser writing technique based on light-manipulation. First, slit beam shaping. In this technique, a slit is inserted before the microscopic objective (slit orientation is parallel to laser-scanning direction), by which the aspect ratio of femtosecond-laser-induced track can be greatly reduced. It has been reported that the propagation loss of waveguide written by this processing technique can be reduced to less than 0.5 dB/cm, which is suitable to construct high-performance photonic devices. The slit beam shaping is an effective technique to improve the performance of femtosecond-laser-written waveguides. However, the existence of slit will inevitably result in a lot of loss of femtosecond laser energy, which is a disadvantage of slit beam shaping. Second, astigmatic beam shaping. As for this technique, an astigmatic cylindrical telescope is placed before the microscopic objective to reshape femtosecond laser, by which the waveguide with circular cross-section could be obtained as well. The minimum propagation loss of waveguide fabricated with this processing technique is less than 0.5 dB/cm, which is also applicable to constitute low-loss 3D waveguide configurations. It should be noted that, when fabricating 2D and 3D optical waveguides, the slit beam shaping and astigmatic beam shaping need to adjust slit orientation and cylindrical lens direction, respectively. It is this additional complexity that restricts the further applications of these two beam shaping techniques in integrated photonics. Third, deformable mirror beam shaping. In this technique, a 2D deformable mirror is utilized to reshape the spatial profile of femtosecond laser, by which the propagation loss of waveguide can also be reduced to some extent (~1.5 dB/cm). Fourth, simultaneous spatiotemporal focusing. This technique can strongly reduce nonlinear side effects, and have many potential applications for fabricating low-loss waveguides. However, the waveguide written by this processing technique has not been reported yet. Fifth, spatial light modulator beam shaping. It is a versatile and energy-efficient technique to control energy distribution of laser focus, which is promising to fabricate low-loss and high-quality optical waveguides. This paper, starting from the introduction of five beam shaping techniques, summarizes the latest research advances of waveguides fabricated by shaped femtosecond laser. An outlook is presented including several potential spotlights.
    Bin ZHANG, Lei WANG, Yuechen JIA, Feng CHEN. Research Advances of Optical Waveguides by Light-manipulation Based Femtosecond Laser Writing(Invited)[J]. Acta Photonica Sinica, 2022, 51(1): 0151106
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