Cylindrical Microlens Research Articles

Compact High Power Diode Laser MOPA System With 5.5 nm Wavelength Tunability

A hybrid master oscillator power amplifier (MOPA) diode laser with 6.2 W output power at 971.8 nm is presented. It consists of a distributed Bragg reflector (DBR) laser, which is collimated and focused into a tapered amplifier using cylindrical micro-lenses. The emitted light has a spectral width of about 20 pm and a propagation factor of $M^{2}_{\mathrm {1/e^{2}}} = 1.3$ in the slow axis. The emission is nearly diffraction limited with 79% of the total power within the central lobe (4.9 W diffraction limited). The MOPA system, including the micro-lenses, is realized on a compact conduction cooled package with a footprint of 25 mm $\times25$ mm. In addition, a micro-heater is embedded on top of the DBR-grating, which can provide up to 5.5 nm of electrical wavelength tuning. The output power is thoroughly constant (6.22 W ± 0.01 W) over the entire tuning range.

Extended 9.7 nm tuning range in a MOPA system with a tunable dual grating Y-branch laser.

In this Letter, we present a tunable Y-branch hybrid master oscillator power amplifier (MOPA) with 5.5W output power, emitting between 973.7 and 983.4nm. The MO is a monolithic Y-branch distributed Bragg reflector (DBR) diode laser, which is collimated and coupled into a tapered amplifier using cylindrical microlenses in a compact 25 mm×25 mm conduction cooled laser package. The wavelength spacing between the two laser branches is 2.2nm. Each branch can be electrically tuned by up to 7.5nm of quasi-continuous wavelength tuning, which in combination covers up to 9.7nm of spectral range. Over this range, an output power variation of 0.5% and 0.2% is observed for the left and right arms, respectively, while maintaining a spectral width of less than 17pm. In addition, the MOPA system can be operated as a dual-wavelength system by operating both branches simultaneously. The presented device is suitable for nonlinear frequency conversion applications.

Depth-of-focus extension in optical coherence tomography via multiple aperture synthesis

In this paper, we report what we believe is a novel technique to overcome the depth-of-focus (DOF) limitation in optical coherence tomography (OCT). Using confocal optics on a sample arm, we scanned the illumination beam across the under-filled objective lens pupil plane by steering the beam at the pinhole using a microcylindrical lens. The detected interferometric signals from multiple distinctive apertures were digitally refocused, which is analogous to synthetic aperture radar (SAR). Using numerical simulations and imaging experiments, we verified that this technique can maintain a diffraction-limited transverse resolution along a DOF that is ∼10 times larger than the confocal parameter. The ability to extend the DOF without signal loss and sidelobe artifacts may ultimately overcome the DOF limitation in high-resolution OCT.

Femtosecond laser one-step direct-writing cylindrical microlens array on fused silica.

We demonstrate an efficient method for fabricating high-quality cylindrical microlens arrays (CMLAs) on the surface of fused silica, fully based on spatially shaping of a femtosecond laser beam from Gaussian to Bessel distribution. As the envelope of shaped spatial intensity distribution matches the profile of cylindrical microlens perfectly, a CMLA with more than 50 uniform microlenses is fabricated by simple line scanning. The radius and height of these microlens units can be finely controlled by adjusting the power of laser pulses. Excellent optical imaging and high-speed fabrication performances are also demonstrated by our fabricated CLMA.

Simulation of hard x-ray focusing using an array of cylindrical micro-holes in a diamond film

The focusing of hard x-rays was numerically simulated using a beam propagation method implemented in the BeamPROP software package and a Rayleigh-Sommerfeld integral. It was shown that when focusing hard X-rays of wavelength 1.34 A with sequentially arranged circular cylindrical microlenses (shaped as 10-µm micro-holes) found in a polycrystalline diamond film, a nearly 10 times shorter focal length of the microlens array can be achieved, reducing from 14.4 cm to 15 mm, via increasing the number of microlenses from 1 to 20. Meanwhile, the x-ray focal spot size was found to decrease 7 times along the minor axis from 1.5 μm to 200 nm, while the maximum intensity in the focus experienced a 40-times increase.

Extrusion printing for fabrication of spherical and cylindrical microlens arrays.

In this paper, we present an extrusion printing technique for producing spherical and cylindrical plano-convex microlens arrays with controllable feature dimensions. This technique employs a robotic adhesive dispenser for robotically controlled microextrusion of ultraviolet (UV) curable polymer onto a glass substrate surface to directly deposit the microlens arrays. It provides a simple and flexible alternative to fabricate both spherical and cylindrical microlens arrays.

Rapid fabrication of cylindrical microlens array by shaped femtosecond laser direct writing

In this study, a remarkable spatial shaping approach is proposed to transform Gaussian femtosecond laser into quasi-Bessel optical field with compressed central lobe and amplified side lobes of the spatial intensity profile. Based on this technique, inward bulge trench (IBT) structures are fabricated with high efficiency on the surface of PMMA by a single illumination step, whose cross-sectional profile is opposite to the results fabricated by Gaussian beam. And plano-convex cylindrical microlens array, which is consistent in size and shape throughout a large sample area, is formed through simply piecing together the IBT structures during fabricating process. Furthermore, numerical simulations of optical field in radial direction and on-axial direction are exploited to rationalize the dependence of the patterned microstructures on the spatial intensity distribution of femtosecond laser.

Assessment of replication fidelity of optical microstructures by hot embossing

This work reports the performance evaluation of low-cost hot embossing processes of fine three-dimensional microstructures, namely (i) cylindrical microlens array, (ii) plano-convex aspheric Fresnel lens, and (iii) pyramidal array structures. All those optical elements were replicated by employing a low-cost hot embossing tool in optical quality acrylic material. Scanning electron microscopy and optical interference microscopy techniques were used to quantitatively measure the obtained structures. In terms of the replication fidelity with respect to the mold counterpart, the resulting optical elements presented at least 96 % of dimensional fidelity at micro- and nanometer scale, including the structures with sharp edges present in the Fresnel lens. In terms of the surface optical quality, resulting root mean square surface roughness of at least one tenth of wavelength was obtained, considering applications in the visible range of spectrum. The results showed that even nanostructures generated by the material removal mechanisms during mold fabrication, such as crystal grain elastic recovery, were well replicated with differences in the range of few of nanometers which is within the vertical resolution of the employed optical interference technique.

Formation of cylindrical micro-lens array on fused silica glass surface using CO2 laser assisted reshaping technique

Abstract This paper demonstrates the laser assisted formation of plano-convex cylindrical and flat-top curved micro-lens array on fused silica glass surface. Initially, femtosecond laser pulses are irradiated on the sample glass to fabricate periodic linear micro-gratings on the glass surface. Afterwards, we reshape the micro-gratings by several times irradiation of a CO 2 laser beam by focusing the laser beam on top of the micro-gratings. As a consequence, plano-convex cylindrical micro-lens array with a period varying from 20 to 40 µm are formed on fused silica glass surface. However, flat-top curved gratings’ array is observed on the glass surface for a gratings’ period of 50 µm. The fabricated micro-lenses show great consistency in size and shape throughout the sample area. Furthermore, we analyze the formation mechanism of micro-lens array on glass surface using the CO 2 laser assisted reshaping technique. The proposed reshaping technique exhibits great potential for forming a large variety of micro-lens arrays on the surface of various transparent materials.

Cylindrical Liquid Crystal Microlens Array With Rotary Optical Power and Tunable Focal Length

A novel cylindrical liquid crystal microlens array with rotary power and tunable capability is proposed and experimentally demonstrated. This structure is capable of generating tunable cylindrical microlenses in two different axis by low voltage control. For this, complex electrical signals are required (electrical phase shift of 180°). A control over the maximum phase shift from $4\pi $ to $34\pi $ radians has been demonstrated. This effect modifies the effective focal length (from 0.2 to 1.2 mm). Moreover, an OFF-state is also possible (behaving like a transparent medium). The structure is simple and does not require a complex fabrication process. The proposed device has several advantages over the existing microlens arrays and is simple and low cost. The device could contribute to developing new applications and to reducing the fabrication costs of current devices. For example, this device can be used in the next generation of mobile displays with autostereoscopic capability.

Design, fabrication, characterisation of optical imaging cylindrical micro-lens array based on liquid crystal

The electrical adjustment of the optical performance of liquid crystal cylindrical micro-lens array of 128×128, which is based on the field controlling effect of liquid crystal dielectric behaviour, is realised through constructing smart liquid crystal electro-optic structure, and orderly modulating the refractive index of liquid crystal material and its spatial distribution. According to the elastic continuum theory of liquid crystal, it is firstly described on the formation of specific refractive index spatial distribution of liquid crystal molecules, which is driven by applied electric-field. Furthermore, the white-light focusing, the far-field white-light interference and the monochromatic laser interference are all measured extensively and discussed in details.

Liquid crystal microlens arrays recorded by polarization holography.

We report the characterization of diffractive microlens arrays (MAs) using a polarization holographic approach assisted by a spatial light modulator (SLM), in a nematic liquid crystal (NLC) cell. The MAs were recorded in the photoaligning substrates of the cell and then replicated in the NLC bulk, through the surface interactions. The transparency of the NLC on a wide range of wavelengths and the ability to tune its optical birefringence, through an external voltage, allowed us to create MAs with high efficiency. We have presented the results obtained for diverse MAs configurations, composed by spherical and cylindrical microlenses and characterized by different focal lengths. The efficiency reaches a value of 90%, at a wavelength of 633nm.

Beating Shapes Relying on Moiré Level Lines

We consider the problem of obtaining a recognizable shape as superposition moiré of two line gratings. The method we propose generates moiré lines located between the shape foreground and background centers and the shape boundaries. Upon relative displacement of the superposed base- and revealing-layer gratings, the moiré lines move and give the impression of beating shapes. Original bilevel shapes are converted into elevation profiles that are embedded by small local shifts within the base-layer grating layout. The elevation profile's level lines are revealed as moiré by superposing the base and revealing layers. Similar level line moirés can be obtained by applying an identical geometric transformation to both the base and the revealing layers. In order to create grayscale or color images embedding several distinct moiré shapes, we create as base layer a dither array made of several ditherband gratings, each one embedding its specific elevation profile. Further variants include the possibility of incorporating elevation profiles within both the base and the revealing layers, or having two elevation profiles sharing a same base layer by tiling the available space. The moiré-level line techniques can be advantageously used for decorative or for anti-counterfeiting applications. Their base- and revealing-layer gratings may be produced by printing techniques, by imaging on film, or by arrays of cylindrical microlenses.

High-integrated spectral splitting solar concentrator with double-light guide layers

Individual secondary optical components in a spectral splitting solar concentrator utilizing a microlens array require multiple photovoltaic (PV) cells, which leads to the complexity of system alignment and a high cost. In order to improve the integration of the PV cells and thermal management, a spectral splitting concentrator coupled to double-light guide layers has been proposed. Using one-axis tracking, we further investigate the optical performance of the concentrator combined with a cylindrical microlens array with double vertically staggered light guide layers in detail. The results show that this solar concentrator maintains a good acceptance angle of ±2 deg in the east-west direction and an acceptable angle of ±14 deg in the perpendicular direction on both low and high spectrums, achieving a concentration ratio of 10×. Finally, the capability of lateral displacement tracking has been explored for an aperture angle of ±24 deg in this concentrator.

Design and evaluation of a large-scale autostereoscopic multi-view laser display for outdoor applications.

State-of-the-art autostereoscopic displays often do not comply with mandatory requirements for outdoor use, because of their limitations in size, luminance, number of 3D viewing zones, and maximum 3D viewing distances. In this paper we propose a concept for a modular autostereoscopic multi-view laser display with sunlight readable luminance, theoretically up to several thousand 3D viewing zones, and maximum 3D viewing distances of up to 70 meters. Each picture element contains three laser diodes, a cylindrical microlens, as well as a MEMS mirror, which deflects the collimated light beams to the left and right eyes of multiple viewers in a time-multiplexed manner. To demonstrate the principle, we have developed a prototype display with 5 x 3 picture elements.

Handheld probe integrating laser diode and ultrasound transducer array for ultrasound/photoacoustic dual modality imaging

Ultrasound and photoacoustics can be utilized as complementary imaging techniques to improve clinical diagnoses. Photoacoustics provides optical contrast and functional information while ultrasound provides structural and anatomical information. As of yet, photoacoustic imaging uses large and expensive systems, which limits their clinical application and makes the combination costly and impracticable. In this work we present and evaluate a compact and ergonomically designed handheld probe, connected to a portable ultrasound system for inexpensive, real-time dual-modality ultrasound/photoacoustic imaging. The probe integrates an ultrasound transducer array and a highly efficient diode stack laser emitting 130 ns pulses at 805 nm wavelength and a pulse energy of 0.56 mJ, with a high pulse repetition frequency of up to 10 kHz. The diodes are driven by a customized laser driver, which can be triggered externally with a high temporal stability necessary to synchronize the ultrasound detection and laser pulsing. The emitted beam is collimated with cylindrical micro-lenses and shaped using a diffractive optical element, delivering a homogenized rectangular light intensity distribution. The system performance was tested in vitro and in vivo by imaging a human finger joint.

Stable-wavelength and narrow-bandwidth high-power external-cavity laser diode stack

We demonstrated a simple external-cavity configuration to effectively suppress the spectral bandwidth of a high-power laser diode (LD) stack. The external cavities consisted of a set of microcylindrical lenses for fast-axis collimation and a single volume Bragg grating. On the basis of this configuration, we reduced the bandwidth from 2.34 to 0.36 nm (6.5-fold reduction) and achieved a peak power output of 335 W. The external-cavity LD stack showed very good spectrum stability in the entire current and wide temperature ranges considered.

Laser Light Module with Integrated MEMS Mirror for Autostereoscopic Outdoor Displays

We have developed highly compact light modules to be used as pixels in multi-view autostereoscopic outdoor displays. Each light module consists of three laser diodes, associated monitor photodiodes, a cylindrical microlens, and an electromagnetic 1D MEMS mirror. The mirror deflects the collimated light beams to the left and right eyes of multiple viewers in a time- multiplexed manner. By synchronizing the MEMS mirror actuation with the three-dimensional image information, i.e. the laser diode driving signals, theoretically up to several thousand autostereoscopic views can be displayed.

Multiple lateral photo-Dember terahertz emitters illuminated by a cylindrical micro-lens array

We demonstrate a terahertz multiple emitter design based on the lateral photo-Dember effect and illuminated with a cylindrical micro-lens array. The multiple emitter produces an average of 5.2 times the output power of a single lateral photo-Dember emitter and is capable of reaching bandwidth comparable to that of a single commercial photoconductive antenna.

Measurement and statistical analysis toward reproducibility validation of AZ4562 cylindrical microlenses obtained by reflow

This paper presents the statistical analysis applied into the shape of microlenses (MLs) for validating the high-reproducibility feature of their fabrication process. The MLs were fabricated with the AZ4562 photoresist, using photolithography and thermal reflow processes. Two types of MLs arrays were produced for statistical analysis purposes: the first with a cross-sectional diameter of 24μm and the second with a cross-sectional diameter of 30μm, and both with 5μm spacing between MLs. In the case of 24μm diameter arrays, the measurements showed a mean difference in diameter of 2.78μm with a standard deviation(SD) of 0.22μm (e.g., 2.78±0.22μmofSD) before the reflow, and 2.34±0.35μm ofSD after the reflow. For the same arrays, the mean difference in height obtained was, comparatively to the 5.06μm expected, 0.76±0.10μm ofSD before the reflow and 1.91±0.15μm of SD after the reflow, respectively. A mean difference in diameter of 2.64±0.41μm ofSD before the reflow, and 1.87±0.34μm of SD after the reflow was obtained for 30μm diameter MLs arrays. For these MLs, a mean difference in height of 0.71±0.12μm of SD before the reflow and 2.24±0.24μm of SD after the thermal reflow was obtained, in comparison to the 5.06μm of height expected to obtain. These results validate the requirement for reproducibility and opens good perspectives for applying this fabrication process on high-volume production of MLs arrays.