| Femtosecond laser direct writing（Fs LDW）technology plays an important role in the preparation of optical waveguide integrated chips because of its advantages of true three-dimensional direct writing,high machining accuracy and maskless.Up to now,glass waveguides based on Fs LDW have been fully demonstrated in the fields of quantum photonics,astronomical photonics,topological photonics and Non-Hermite photonics.Although great progress has been made,in the process of the development of optical waveguide devices towards high integration,high performance and versatility,the performance of these waveguide integrated systems is difficult to be further improved due to the limitation of insufficient waveguide performance.Specifically,in high-performance polarization coding photonic chips,ideal polarization devices require that the birefringence intensity and optical axis angle of waveguides can be controlled arbitrarily as needed.However,the current waveguide processing technology has not met this requirement.In large-scale integrated photonic chips,each unit device is required to work in an ideal state to improve the system performance.However,the coupling process of devices such as directional couplers changes due to the existence of processing errors,which eventually leads to the decline of system performance with the increase of device cascade degree.In Non-Hermite photonic chips,the waveguide loss is required to be arbitrarily controlled in a large range in order to introduce the Non-Hermite degree required by the waveguide system,which puts forward new requirements for the waveguide loss control technology.In conclusion,in the future development of optical waveguide integrated chips,high performance and multi-function are an important trend in the development of waveguide integrated devices,and the functionality of devices is often related to various basic properties of waveguides.Therefore,the realization of arbitrary control of various basic properties of optical waveguides to improve the performance of waveguides can further promote the development of waveguide integrated chips.To solve the above problems,we have developed a variety of femtosecond laser processing technologies that can realize the arbitrary control of waveguide birefringence,coupling coefficient and transmission loss,and demonstrated the related applications of high-performance waveguides.Specifically,we proposed a beam shaping technology to realize the preparation of ultra-low birefringence waveguides,and realize the preparation of elliptical waveguides with birefringence intensity and optical axis angle that can be arbitrarily controlled,which provides a solution for the realization of ideal polarization encoder devices on-chip.We proposed a technology of reconstructing waveguide coupling coefficient by femtosecond laser secondary direct writing（Fs LSDW）,which realized the full cycle reset of directional coupler beam splitting ratio,and provides a solution for the implementation of high fidelity and high-performance chip.We controlled the coupling coefficient between waveguides by controlling the coupling spacing,and realized the on-chip photonic devices and high-fidelity optical quantum logic gates based on Non-Abelian braiding.We realized the arbitrary control of waveguide loss intensity and distribution,shown its application in dynamic non Hermite system around exceptional points,and verified the chiral transmission characteristics in multi waveguide system for the first time.These waveguide processing technologies greatly improve performances of waveguides and are of great significance to improve the integration and functionality of waveguide integration systems.The main work and innovations of this paper are as follows:1.In order to meet the demand that the waveguide properties can be finely controlled to realize the preparation of high-performance waveguides,an optical waveguide integrated chip processing system which can arbitrarily control the waveguide loss,birefringence and coupling coefficient between waveguides is built;Several test systems that can be used to test various basic properties of waveguides and the performance of waveguide devices are built.It provides an experimental basis for the preparation of high-performance waveguides and devices.2.In the aspect of waveguide loss control:in order to meet the demand for low loss waveguide in Hermite system,the preparation of low transmission loss（0.2 d B/cm）waveguides in borosilicate glass were realized by optimizing the parameters such as laser repetition rate,scanning speed and pulse energy.In order to meet the demand of arbitrarily controllable waveguide loss in a wide range in Non-Hermite systems,waveguides with arbitrarily controllable transmission loss from 0.2 d B/cm to 400d B/cm were prepared by implanting overexposed scattering points with controllable loss intensity and spacing in the waveguide.In addition,Non-Hermite devices which can dynamically surround multiple exceptional points were fabricated by using waveguides with arbitrarily controllable loss.The chiral transmission phenomenon in the system of dynamically surrounding multiple exceptional points was verified for the first time.3.In the aspect of waveguide coupling coefficient control:in view of the processing error of devices,on the one hand,we proposed a technology to reconstruct the transverse dimension of waveguides in the coupling region by Fs LSDW,and then reset the coupling coefficient between waveguides,so as to realize the arbitrary control of the coupling coefficient between waveguides from 0.47 rad/mm to 2.1 rad/mm.Using this technology,the directional coupler with machining error was repaired,and the arbitrary reset of the beam splitting ratio of the directional coupler from 0:100 to100:0 was further realized.This technology provides a solution for the implementation of high-fidelity and high-performance optical waveguide chips.On the other hand,the coupling coefficient between waveguides was controlled by controlling the coupling spacing,and the preparation of on-chip photonic devices and optical quantum logic gates based on Non-Abelian weaving was realized.This topologically protected photonic device has high fidelity,which provides another solution for the implementation of high-performance optical waveguide chips.4.In the aspect of waveguide birefringence control:Aiming at the problem that it is difficult to prepare ideal polarization devices on chip,a cylindrical lens-slit shaping technology was proposed to realize the preparation of ultra-low birefringence circular section optical waveguides,in which the birefringence is as low as 1.49×10-6.Based on two circular section waveguides,the preparation of elliptical waveguides with arbitrarily controllable waveguide birefringence intensity and optical axis angle in the range of 0 to 1.7×10-5 and 0 toπwere realized.Based on this arbitrary birefringence control method,various on-chip polarization devices such as high-performance quarter wave plate,half wave plate,polarization independent directional couplers,polarization sensitive directional couplers and polarization beam splitters were prepared respectively.The above waveguide birefringence control technology and the display of basic polarization devices provide a solution for the on-chip ideal polarization coding integrated chips.These processing technologies and application demonstrations of arbitrary control of waveguide transmission loss,coupling coefficient and birefringence provide the possibility for the implementation of high-performance integrated chips.In addition to being used alone,various control technologies can also be used in combination to realize the simultaneous control of various properties of glass waveguides,which is of great significance to increase the overall performance of chips in the future. |
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