Experimental Study Of Femtosecond Laser Direct Writing Glass Waveguide Devices With Controlled Performance

Femtosecond laser micromachining technology is gradually being widely applied in the preparation of waveguide and integrated photon circuits,due to its maskless,low-cost,and high-precision three-dimensional rapid processing capabilities,as well as demonstrating research value in areas such as quantum optics,astrophotonics,topological photonics,and non-Hermitian photonics.However,femtosecond laser micromachining technology is still in the developmental stage,and we urgently need to explore appropriate processing methods to achieve controllable waveguide performance and functional devices.For instance,in the face of large-scale integration,we need single-mode waveguides with low transmission loss as routing devices to connect various functional devices on the chip.In the process of fabricating polarization-encoded photon devices,we need precise control of the birefringence intensity and optical axis direction of the waveguide.However,there is currently no unified processing method that can simultaneously solve the problems of birefringence intensity and optical axis control.During on-chip energy transfer,signal exchange,and power distribution processes,we need precise modulation of the coupling function between waveguides.In the process of fabricating large-scale integrated photonics chips using Femtosecond laser micromachining technology,we need to address the performance degradation and functional failure caused by processing errors.In the process of photon transportation,the optical phase is a key factor affecting quantum interference and entanglement,optical quantum computing,and quantum logic gate devices.Thus,we need to maintain stable and precise optical phase.In summary,there are still many unresolved issues in the field of femtosecond laser direct writing integrated photonics circuits,such as transmission loss control,waveguide birefringence intensity and optical axis control,coupling control,and phase control.To address the above-mentioned issues,we have developed a complete set of femtosecond laser direct writing and testing platforms,and explored the methods of processing and testing integrated photonics circuits throughout the entire process.We have successfully fabricated optical waveguides with transmission losses as low as 0.24d B/cm,and achieved precise control over waveguide transmission losses,birefringence intensity and optical axis,coupling,and phase.The main contributions and innovations of this study are as follows:1.According to the experimental requirements for fabricating high-performance and finely controllable waveguides,we have built a reconfigurable processing platform with many beam-shaping modules.We have also set up many testing platforms to characterize the basic waveguide performance（loss,birefringence,optical axis,and coupling coefficient）and device performance,providing an experimental foundation for subsequent research.2.In terms of waveguide loss control:Comparing and optimizing processing parameters such as laser repetition rate,pulse width,focusing conditions,single-pulse energy,and scanning speed,we have successfully fabricated single-mode waveguides with low transmission losses（0.24 d B/cm）in Corning glass.Meanwhile,we have achieved arbitrary controllable waveguides with transmission losses ranging from 0.24to 20 d B/cm by controlling implantation scattering point positions,density,and exposure parameters,which has important application value for the study of Non-Hermitian devices.3.In the control of waveguide birefringence intensity and optical axis:To address the issue of polarization rotation during light transmission,we propose a"shape-stress"dual adjustment scheme and successfully fabricate waveguides with ultra-low birefringence(1×10^-9).Specifically,we use column lens-assisted slit shaping technology to fabricate circular optical waveguides,solving the birefringence influence caused by waveguide cross section.We also use auxiliary stress fields to control the internal stress of the waveguide,addressing the birefringence influence caused by internal stress.We further demonstrate the application of ultra-low birefringence waveguides in polarization-independent directional couplers and waveguide arrays.In addition,to meet the requirements of on-chip polarization devices,we induce circular waveguides with a tunable optical axis（0-360°）using auxiliary stress fields,demonstrating waveguide wave plates with high-performance 45°rotating optical axes.At the same time,we use an external stress field to tune the waveguide birefringence,achieving controllable birefringence intensity of the optical waveguide,and demonstrating various on-chip polarization devices such as polarization beam splitters and single-qubit quantum logic gates.The birefringence and optical axis control technologies mentioned in this paper provide noval solutions for on-chip polarization encoder devices and polarization-preserving transmission.4.In terms of waveguide coupling coefficient control:we propose an auxiliary field detuning coupling scheme to reconstruct the coupling coefficient between waveguides,allowing for arbitrary control within the range of 0.98-1.76 rad/mm.With the coupling coefficient reconstruction scheme,we achieve arbitrary splitting ratio operation in a single directional coupler.Moreover,to address the issue of fabrication errors in devices,we use second-order alignment technology and coupling coefficient reconstruction technology to restore directional couplers and their networks with processing errors.This waveguide coupling coefficient modulation technology provides a new approach to solving waveguide coupling tuning problems and achieving conversion of high-fidelity quantum logic gates.5.In terms of phase control:we proposed a waveguide effective refractive index reconstruction scheme to achieve static tuning of the optical phase.We use classical and quantum light to characterize the phase control capability of this noval scheme.Specifically,we achieve full-cycle 2πphase modulation within a 5 mm modulation length in an MZI,with a modulation accuracy ofλ/70.Importantly,this noval static phase modulation scheme has the advantages of simple process,low loss,and ease of integration into complex three-dimensional photonic devices,and is expected to solve phase error correction and phase control in the manufacturing of photonic quantum devices.The application demonstrations of the aforementioned control technologies for waveguide loss,birefringence,optical axis,coupling coefficient and phase,provide guarantees for the fabrication of high-performance integrated photonics cicuits using Femtosecond laser micromachining technology.