Design and fabrication of Fourier plane diffractive optical elements for high-power fibre-coupling applications

In industrial processes using fibre beam delivery of high peak power laser light, diffractive optical elements are a very useful and flexible tool in maximising the amount of light reaching the work surface. This is due to the ability of diffractive optical elements to accurately couple light into multiple fibres, while conditioning the light in order to maximise the throughput for each fibre. We discuss the design techniques for diffractive optical elements and the application of these techniques to fibre-coupling problems. The flexibility of diffractive optical elements is demonstrated by their application to several fibre-coupling geometries, including a fibre bundle, a linear array of fibres, and a rotationally symmetric fibre connector. The diffraction efficiencies for the elements approach 90% with uniformity errors of less than 5%.     

采用分步迭代算法设计制作衍射光学元件

 结合均匀照明相位器件的设计实例，介绍最新提出的基于迭代框架的优化方法，并应用该方法设计制作了16台阶的衍射光学元件并对其进行了测试，验证了设计原理及方法的正确性，且对加工中可能出现的误差进行了分析和讨论。测试结果验证了采用多台阶相位结构的设计方案能有效提高工艺宽容度。     

Simplified mesh techniques for design of beam-shaping diffractive optical elements

This paper describes a simplified mesh generation technique that is based on the finite element method of calculation of beam-shaping diffractive optical elements (DOEs). The mesh generation technique uses the inherent symmetry of the incident beam to generate a mesh. Using the meshes so generated, DOEs that convert a Gaussian intensity beam to one of a specified shape, are calculated. Simulations of the results of such beam-shaping elements will be presented. Such elements have uses in industrial and medical applications where both the shape as well as the intensity distribution on the material that is to be processed is very important. For example, in industrial applications the beam may be used to uniformly heat up a specific area in which case the intensity has to be uniform across the beam. The Gaussian intensity variation of a laser has to be converted to a flat-top beam in order to achieve this. To reiterate, beam shaping refers to changing both the intensity distribution and the shape of the beam. Experimental results of the fabricated gratings will also be presented. These results will include experimental data on the method of additive lithography which can be used to improve the efficiency of DOEs.     

Vector fuzzy control iterative algorithm for the design of sub-wavelength diffractive optical elements for beam shaping

The vector fuzzy control iterative algorithm (VFCIA) is proposed for the design of phase-only sub-wavelength diffractive optical elements (SWDOEs) for beam shaping. The vector diffraction model put forward by Mansuripur is applied to relate the field distributions between the SWDOE plane and the output plane. Fuzzy control theory is used to decide the constraint method for each iterative process of the algorithm. We have designed a SWDOE that transforms a circular flat-top beam to a square irradiance pattern. Computer design results show that the SWDOE designed by the VFCIA can produce better results than the vector iterative algorithm (VIA). And the finite difference time-domain method (FDTD), a rigorous electromagnetic analysis technique, is used to analyze the designed SWDOE for further confirming the validity of the proposed method.     

A comparison of the iterative Fourier transform method and evolutionary algorithms for the design of diffractive optical elements

Three methods of designing diffractive optical elements (DOE) are compared. The iterative Fourier transform algorithm (IFTA) is compared to an evolutionary strategy (ES) approach and a combination of both methods. It is shown that the combination of both methods produces a better solution than the iterative method and is faster than using the evolutionary strategy only.     

Beam delivery system with a non-digitized diffractive beam splitter for laser-drilling of silicon

We report a beam-delivery system consisting of a non-digitized diffractive beam splitter and a Fourier transform lens. The system is applied to the deep-drilling of silicon using a nanosecond pulse laser in the manufacture of inkjet printer heads. In this process, a circularly polarized pulse beam is divided into an array of uniform beams, which are then delivered precisely to the process points. To meet these requirements, the splitter was designed to be polarization-independent with an efficiency>95%. The optical elements were assembled so as to allow the fine tuning of the effective overall focal length by adjusting the wavefront curvature of the beam. Using the system, a beam alignment accuracy of<5 μm was achieved for a 12-mm-wide beam array and the throughput was substantially improved (10,000 points on a silicon wafer drilled in ~1 min). This beam-delivery scheme works for a variety of laser applications that require parallel processing.     

Analysis of materials selected for multilayer diffractive optical elements

An analysis model to optimize the materials selected for multilayer diffractive elements (MLDOEs) is presented with approximate Cauchy dispersion formula of refractive index and the maximum polychromatic integral diffraction efficiency (PIDE). The analysis model presents that the maximum PIDE of MLDOEs consisting of two materials with large Abbe number difference and small partial dispersion difference can be generated. The scope of application and the relationship between diffraction efficiencies of MLDOEs with different material pairs and different design wavelength pairs are presented and simulated with the analysis model of MLDOEs.     

Design of novel laser cavity with diffractive phase elements

A new method based on the general theory of amplitude-phase retrieval is proposed for designing the diffractive phase elements (DPEs) used in the laser cavity that can produce the desired mode with the specified shape of beam. Simulation results show that the laser cavity with the designed DPEs can successfully generate uniform square and triangle ring patterns. The loss of these fundamental modes is negligibly small and the separation of adjacent transverse modes is large enough to discriminate the fundamental mode from the higher-order modes.     

Directly laser-written integrated photonics devices including diffractive optical elements

Femtosecond laser-written integrated devices involving Fresnel Zone Plates (FZPs) and waveguide arrays are demonstrated as built-in optical couplers. These structures were fabricated in borosilicate glass using a direct laser writing technique. The optical properties of these integrated photonic structures were investigated using CW lasers and high-resolution CCDs. For a single FZP coupled to a single waveguide, the overall coupling efficiency was 9%. A multiplexed optical coupler composed of three FZP layers was demonstrated to couple three waveguides simultaneously in a waveguide array. Structures of this type can be used as platforms for multichannel waveguide coupling elements or as microfluidic sensors that require higher light collecting efficiency.     

Tailored TEMp0 beams for large size 3-D laser prototyping

The Laser-Lithography technique allows the fabrication of complex objects having microsizes by selectively solidifying polymeric materials layer by layer upon exposure to a focused Gaussian laser beam having a beam propagation factor M²=1. We can expect that extension of this technique to large sizes 3-D prototyping comes up against a large increase in the design time. A possible solution is the increase in the focused spot size, but unfortunately at the price of a great reduction in the longitudinal resolution due to the resulting increase in the depth of focus. To overcome these difficulties, we propose the use of a rectified TEM_p0 beam allowing the obtaining of a Gaussian beam intensity profile in the focus plane of a lens. The reshaped TEM_p0 beam has a beam propagation factor M²≈(2p+1), and this yields to a relative improvement of the longitudinal resolution although the spot size is increased for reducing the processing time.     

菲涅耳区衍射光学束匀滑器件的设计

 采用精细化设计方法，进行了菲涅耳区衍射光学束匀滑器件的设计，利用爬山-模拟退火混合优化算法，获得了真实的束匀滑分布，不仅控制了算法采样点上的光强分布，还控制了其他非采样点上的光强分布。优化得到的束匀滑器件的位相深度小于π，易于后续加工。     

用于光束整形的衍射光学元件设计的混合算法

提出了一种用于衍射光学元件优化设计的混合遗传迭代爬山算法,该算法将迭代量化傅里叶变换算法融入到遗传算法中,然后在整体遗传算法结束后,对找到的当前最优解再用爬山法进行局部寻优,从而得到最优的衍射光学元件表面相位分布.用该混合方法设计了衍射光学元件,可以将入射的高斯光束整形成方形的均匀光斑.模拟结果表明:该混合算法具有收敛速度快、设计准确度高等优点.相比于其它设计方法,本文提出的方法能较好地改善整形效果,特别适用于光束整形的衍射元件设计.     

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

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.     

用于光束整形的衍射光学元件设计的混合算法

提出了一种用于衍射光学元件优化设计的混合遗传迭代爬山算法,该算法将迭代量化傅里叶变换算法融入到遗传算法中,然后在整体遗传算法结束后,对找到的当前最优解再用爬山法进行局部寻优,从而得到最优的衍射光学元件表面相位分布.用该混合方法设计了衍射光学元件,可以将入射的高斯光束整形成方形的均匀光斑.模拟结果表明:该混合算法具有收敛速度快、设计准确度高等优点.相比于其它设计方法,本文提出的方法能较好地改善整形效果,特别适用于光束整形的衍射元件设计.     

用于光束整形的多功能衍射相位板的设计

提出一种设计多功能衍射相位板的随机搜索和模拟退火混合设计方法利用这种方法,通过采用多个评价函数,可设计出在不同观察面上同时产生所需光强分布的衍射相位板设计用于高斯光束整形的多功能衍射相位板可以在焦面上产生高能量利用率、小旁瓣的光强分布,同时在离焦面上获得具有良好顶部均匀性的光强分布该方法提高了衍射器件的设计灵活性,对于设计同一块器件在光路的不同位置产生所需的光强分布提供了新的思路,在激光武器、激光加工和激光手术等领域有广阔的应用潜力.     

Optical fibres for material processing lasers

Fibre transmission of high-power radiation has opened the laser a new dimension of material processing applications. The three-dimensional treatment revolutionised a far field of the production technology. One impressed example is the laser welding process in the automotive industry. The Nd–YAG solid state laser in combination with the optical fibre increase the automation and the flexibility of a whole industry. The next generation of Audis aluminium alloy car body is mainly welded by fibre-guided Nd–YAG laser beams. Since the past 10 years the maximum average laser power transmitted through optical fibres increase from some 100 W up to 10 kW. For industrial application only Nd–YAG lasers and recently the diode lasers are economically suited for fibre transmission.     

Off-axis collimation of diode laser beams by means of single-element holographic diffractive optics

Principles of single-element holographic diffractive optics for collimation of diode laser beams with a large divergence, an elliptic cross-section, and astigmatism are presented. Holographic off-axis transformation enables collimation of the beam in two variants: one with a perpendicular input beam and an oblique output beam, and the other with the beams arranged vice-versa. Diffraction due to an elliptic aperture is analyzed. Inspection of experimental samples demonstrates an agreement with theory in the case of diffraction limited focal pattern and shows increase of astigmatism with the departure from diffraction only limitation.     

Optimization of quantized diffractive elements with symmetry constraints

We employ the simulated annealing algorithm to optimize totally quantized Fourier diffractive elements, whose phase modulation is constrained by a symmetry similar to that of the desired diffraction pattern. Elements optimized under this restriction, show better performance than the conventionally optimized ones, for which the object phase function is randomly modified. In addition, the elements constrained by spatial symmetry are optimized with relatively low computational efforts.     

Excimer laser patterned dielectric masks for the fabrication of diffractive optical elements by laser ablation

Masks for laser processing are generated by laser ablation patterning of dielectric layer systems. The application of these masks for the rapid fabrication of diffractive optical elements (DOEs) is presented. The diffractive optical elements are designed as phase-only elements, assuming an illumination with a plane wave. A continuous phase function is calculated using an iterative Fourier transform algorithm (IFTA). This continuous phase function is reduced to two or four levels by an iterative Fourier quantisation algorithm (IFQA) that is able to include focal power. The fabrication of the DOE is performed in a two-step process. First, a binary amplitude mask (or a set of masks for multi-level DOEs) is made by structured ablation of a highly reflective dielectric coating (HR 248 nm) from a fused silica substrate. This is accomplished by using an ArF excimer laser emitting at 193 nm, a wavelength that is sufficiently absorbed in the HfO₂/SiO₂-dielectric layer system, leading to precisely ablated mask structures. In the second step, this mask is used in a 4:1 projection configuration to generate a surface profile in a polymer substrate by ablation at 248 nm. The depth modulation can be defined by adjusting laser fluence and pulse number. Examples of DOEs ablated in polycarbonate are shown and their performance is characterised.     

光谱色散匀滑技术对衍射光学器件束匀滑性能的影响

 模拟分析了光谱色散匀滑技术对衍射光学器件束匀滑性能的影响。光谱色散匀滑技术的使用提高了衍射光学器件的抗波前畸变能力，降低了对加工精度的高要求。光谱色散匀滑技术与衍射光学器件的联用有望满足惯性约束聚变对束匀滑性能的高要求。