Si Lu

Broadband polarizing beam splitter based on the form birefringence of a subwavelength grating in the quasi-static domain

We propose a novel broadband polarizing beam splitter with a compact sandwich structure that has a subwavelength grating in the quasi-static domain as the filling. The design is based on effective-medium theory an anisotropic thin-film theory, and the performance is investigated with rigorous coupled-wave theory. The design results show that the structure can provide a high polarization extinction ratio in a broad spectral range.

Design and fabrication of a polymeric flat focal field arrayed waveguide grating

For the first time, a new-type flat focal field arrayed waveguide grating (AWG) demultiplexer, with the focal signals of all wavelengths of operation focusing along a straight line, is designed based on the aberration theory and fabricated based on a newly developed negative tone epoxy Novolak resin (ENR) polymer using electron-beam direct writing. A polymeric four-channel 400GHz spacing flat focal field AWG demultiplexer is fabricated and tested. Four modal images from the output waveguides are observed and the measured transmission spectra is presented. And we make error analysis.

Semiconductor laser diode to single-mode fiber coupling using diffractive optical elements

Abstract An integrated mode converter consisting of two diffractive optical elements (DOEs) and a Silicon slab is presented for low-loss coupling between a semiconductor laser diode (LD) and a single-mode fiber (SMF). The phase structures of the DOEs are designed using iterative phase retrieval algorithm. We introduce a new far-field amplitude constraint into the iteration to provide very high mode conversion quality. Compared with previously published mode converters, the scheme is more universal because it’s applicable for any semiconductor LD. In simulation, coupling losses lower than 0.02 dB are predicted for all the discussed LDs with aspect ratios of the elliptical fields from 1 to 9. The requirements on axial displacement and rotation angle have been removed. The tolerance for 1-dB loss increment for lateral misalignment is 0.9 μm. And the coupling loss is insensitive to tilt angle.

Integrated diffractive optical mode converter for fiber-to-waveguide coupling

Integrated diffractive optical mode converter, consisting of a diffractive optical element (DOE) and a slab waveguide, is used for fiber-to-waveguide coupling. The phase of the DOE is generally designed by optimization algorithm. In this paper, the precise design, a new method with one more restrictive way, is adopted to design the diffractive optical mode converter for fiber-to-waveguide coupling. Through this method, the intensity of any point on the output plane is fully filled with the required demand. Compared with what previously published, the coupling loss of the precise designed converter is lower.

Design and simulation of very low insertion loss arrayed waveguide grating using full-vectorial beam propagation method with circular cylindrical coordinates

A radial finite-difference beam propagation method is proposed systematically with circular cylindrical coordinates, which is more accurate for simulation of some radiated waveguide structures. Theoretical design and simulation of low insertion loss arrayed waveguide gratings with couplers of tapered arrayed waveguides are presented using this full-vectorial beam propagation method.

Beam-shaping application in laser heat processing

Beam-shaping plays a very important role in the field of laser processing. Laser quench needs a rectangular speckle, which has a flat top, steep sides, high efficiency and low sidelobes. Diffractive phase elements (DPEs) have many superexcellent characters, which conventional optical elements have difficulties to achieve. Designing of the phase plane comes down to phase retrieval problem. Geometrical transformation and multiform iterative algorithms, such as G-S algorithm, Input-Output algorithm, ST algorithm are adopted. Through comparison of the results from different methods, some evaluations about algorithms are made. ST algorithm is the most feasible method for the problem, the result of which can meet the requirements of practical process. Some simulation experiments and discussions about algorithms have been done. To be fabricated as a binary optical element (BOE), the result of design has been quantified to 16 steps.

Diffractive optical element for coupling semiconductor laser diode to single-mode fiber

An integrated scheme is proposed for low-loss coupling between a semiconductor laser diode (LD) and a single-mode fiber (SMF) by using a diffractive optical element (DOE). The DOE is designed based on diffractive optics principle, which phase distribution is optimization result of iterative phase retrieval algorithm. A new far-field amplitude constraint is introduced into the iteration to provide very high mode conversion quality. The scheme is applicable to realize high efficient coupling to SMF for any semiconductor LD. Coupling losses lower than 0.02dB have been reached for all the discussed LDs with aspect ratios of the elliptical fields from 1 to 9. The requirements on axial displacement and rotation angle have been removed. The tolerance for 1-dB loss increment for lateral misalignment is 0.9μm. And the coupling loss is insensitive to tilt angle.

Very low-loss fiber-to-waveguide coupling using diffraction optical element mode converter

Very low-loss coupling between standard single-mode fibers (SMF) and high refractive-index waveguides is achieved using diffraction optical element (DOE) as mode converter. The DOE is a beam-shaping element designed based on diffraction optics theory. DOE shapes the mode field of SMF to that of the waveguide, or vice versa. The mode mismatch between them is eliminated. DOE is a pure phase element, which phase distribution optimized by G-S algorithm. The most attractive point of this mode converter is that the DOE can be designed to achieve any mode field distribution of any waveguide structure. This advantage makes DOE a strong and flexible mode converter. Though being a phase element, DOE has certain wavelength tolerance covering a sufficiently wide band, which makes it applicable in multi-wavelength integrated planar lightwave circuits (PLCs). DOE can be approximated by binary optical element (BOE), so that it can be fabricated using existing PLC fabrication step

Full-vectorial beam propagation method for waveguide modeling

The beam propagation method (BPM) is one of the most popular approaches in modeling electromagnetic field propagation. The conventional BPM is established on scalar Helmholtz equation using paraxial approximation. There are limits in the analysis of the vector properties of electromagnetic field and wide-angle propagation. This paper presents the techniques of removing these limits. A full-vectorial BPM bases on finite-difference technique is described from vector wave equations, called finite-difference vectorial beam propagation method (FD-VBPM). The main disadvantage of FD-VBPM is the calculation inefficiency, especially in three dimensional modeling. To obtain two dimensional equivalent structures, a precise optimization approach is adopted. Propagation constant and field of fundamental mode best match those of the original waveguide. The method has very high accuracy. The decoupled semi-vectorial BPM is also derived. The full-vectorial method is extended to wide-angle BPM removing paraxial limit. The method bases on Pade approximant. Simulations are made on rib waveguide.

Design of flat-field arrayed waveguide grating based on aberration theory

A new type flat-field arrayed waveguide grating (AWG) has been designed. The focal points of all wavelengths of operation distribute on a straight line. With parallel output waveguides, it¡¯s convenient to connect with fiber-array directly. Flat-field AWG could function as a spectrometer when the output waveguides are thrown off. The work based on the developed aberration theory of AWG. In this theory, the restrains imposing on the conventional Rowland-type AWG have been removed. Various restrains will generate new type structures. In the flat-field AWG design, the restrains come from the concurrent imaging theory. Three aberration-free points restrain three dominant geometry parameters of AWG, geometry of star couplers, phased array ports distribution, and length increment between adjacent paths. A 16-channel flat-field AWG is designed. As stigmatic points introduced, the aberration of the device is much lower than that of the conventional Rowland-type AWG.