Vertical-cavity Surface-emitting Laser Beam Research Articles

Fourier Ptychography-Based Measurement of Beam Divergence Angle for Vertical Cavity Surface-Emitting Laser

The Vertical Cavity Surface-Emitting Laser (VCSEL) has led to the rapid development of advanced fields such as communication, optical sensing, smart cars, and more. The accurate testing of VCSEL beam quality is an important prerequisite for its effective application. In this paper, a method for measuring the divergence angle of the VCSEL far field spot based on transmissive Fourier ptychography is proposed. First, a single CCD multi-angle VCSEL far-field spot acquisition system is designed. Second, based on the proposed Fourier ptychographic algorithm with synchronous optimization of embedded optical transfer function, a resolution-enhanced phase image of the spot is reconstructed and the boundary extracted by the Sobel operator of the phase image is defined as the boundary position of the beam waist. In this way, the beam waist radius of the laser beam is calculated. Finally, the divergence angle of the laser beam is measured via the radius of the beam waist. Compared with the traditional Gaussian beam definition method, the method proposed in this paper has higher accuracy in divergence angle measurement. The experimental results show that this method can improve the divergence angle measurement accuracy by up to 9.7%.

Demonstration of real-time structured-light depth sensing based on a solid-state VCSEL beam scanner.

We demonstrated a real-time scanning structured-light depth sensing system based on a solid-state vertical cavity surface-emitting laser (VCSEL) beam scanner integrated with an electro-thermally tunable VCSEL. Through a swept voltage added to the tunable VCSEL, a field of view of 6°×12° could be scanned with a scanning speed of 100 kHz by the beam scanner. Adopting the beam scanner, the real-time depth image with a lateral resolution of 10,000 (20×500) was obtained by measuring a step target placed at 35cm. The frame rate could be >10Hz even if sunlight shot noise is artificially added to the experimental data. By using a higher-speed camera, a potential lateral resolution could be reached at 50,000 (100×500) with a frame rate of > 20Hz. By using flat optics, a compact scanning module offering line pattern with FoV of >40°×20° was also demonstrated. It could help to realize high-resolution and high-accuracy structured-light sensing with a compact module.

Advances in Polymer-Based Optical MEMS Fabrication for VCSEL Beam Shaping

In this paper, we discuss the design, fabrication, and characterization of electrothermally actuated polymer-based microoptical electromechanical microsystems (MOEMS) for active microoptics in vertical cavity surface emitting laser (VCSEL) devices. We describe in particular the principle of a SU8-based MOEMS designed for single-mode VCSEL beam active focusing. The ultimate objective is the realization of parallel compact optical scanners for sensing applications using collective and low-cost technologies. After discussing the advantages of the epoxy resist SU-8 for fabricating an integrated movable lens on active optical devices, we present our latest advances in technology for ensuring precise MOEMS fabrication on small III–V samples and for achieving accurate alignment of lenses on suspended circular membranes. Finally, we present our first results on the beam focusing of multimode VCSELs, which demonstrate the feasibility of our approach and could provide new insights in the MEMS-VCSEL field.

The Generation of a Compact Azimuthally Polarized Vertical-Cavity Surface Emitting Laser Beam with Radial Slits

We propose the simple and efficient generation of a compact azimuthally polarized laser beam. Radial slits with annular distribution are patterned onto the output facet of a standard vertical-cavity surface emitting laser. Due to the polarization modulation of the surface plasmon polariton mode excited by the radial slits, a compact azimuthally polarized laser beam at 850 nm is experimentally demonstrated. The finite-difference time-domain method is introduced to analyze the polarization characteristics of the transmitted light through the radial slit, which agrees well with our experimental results.

Modelling of vertical-cavity surface-emitting laser beam collimation using a nanostructured gradient index microlens

In this paper we show that the recently developed nanostructured gradient index (nGRIN) rod microlens can be utilised for the collimation of the beam generated by a vertical-cavity surface-emitting laser (VCSEL). The modelling of the nanostructured lens structure is performed using the finite difference time domain (FDTD) method with realistic nGRIN parameters and a Gaussian model of the light source. The large refractive index gradient of the nanostructured microlens allows the final microlens thickness to be only 70 μm with a diameter of 10 μm. Successful collimation of a single-mode VCSEL beam with a waist half-width of 1.53 μm is presented with a reduction in divergence half-angle from 10.1° to 3.3°. We show that the linear polarisation of the incident beam is preserved as well as presenting the tolerance of this type of lens to variations in overall thickness.

Analytical models for channel crosstalk in short-range free space optical interconnects

Analytical models for optical channel crosstalk in short-range free space optical interconnect systems are proposed. The diffraction crosstalk noise is modeled as a function of many parameters of the interconnect system, including the interconnect distance, the interconnect spacing, and the photo-detector size. The dependence of crosstalk noise on different systems' parameters is discussed and evaluated using the proposed models. It is shown that optimum parameters for the optical system and the detectors array can be easily determined to optimize and improve the interconnect performance. The effect of high modes of vertical cavity surface emitting lasers (VCSELs), VCSEL aperture, and the incident angle of the VCSEL beam on crosstalk are also discussed.

Fabrication of polymer-based optical microsystem arrays suited for the active focusing of vertical laser diodes

We present a low-cost fabrication technique of a polymer-based micro-optical-electrical-mechanical systems (MOEMS) suited for the dynamic focusing of VCSELs (vertical-cavity surface-emitting lasers). A simple method based on an SU(8) double exposure is proposed for the collective integration of small footprint transparent suspended membrane arrays on vertical laser diodes. We demonstrate that this kind of MOEMS can provide under thermal actuation a vertical displacement of around 0.2 µm W−1 over a maximal range of 8 µm. As a wide range of initial gaps between the membrane and the laser source can be chosen, this approach opens new insights into the dynamic control of the VCSEL beam waist position as well as for tunable VCSEL fabrication.

A microtip self-written on a vertical-cavity surface-emitting laser by photopolymerization

We present the integration of a self-aligned microtip on a vertical-cavity surface-emitting laser (VCSEL) by near infrared photopolymerization. This one-step fabrication process is triggered by the laser source itself. It is based on the use of photopolymers sensitive at the lasing wavelength and can be applied to VCSEL devices after their process fabrication. We have characterized the fabricated microtips and shown that they focus laser light at few micrometers from the device. The applications of this simple method may concern VCSEL beam shaping as well as the fabrication of microprobes for near-field optical microscopy.

Analysis of hexagonal array geometry for free-space optical interconnects with improved signal-to-noise ratio

The effect of transmitter and receiver array configurations on the performance of free-space optical interconnects (FSOIs) was investigated. Experimentally measured, spectrally resolved, near-field images of vertical-cavity surface-emitting laser (VCSEL) transverse modes were used as extended sources in our simulation model and combined with laser relative intensity noise and the receiver noise to determine the optimal array geometry. Our results demonstrate the importance of stray-light cross talk in both square and hexagonal configurations. By changing the array lattice geometry from square to hexagonal, we obtained an overall optical signal-to-noise ratio improvement of 3 dB. We demonstrated that the optical signal-to-noise ratio is optimal for the hexagonal channel arrangement regardless of the transverse mode structure of the VCSEL beam. We also determined the VCSEL drive current required for the best performance of the FSOI system.

MEMS-based VCSEL beam steering using replicated polymer diffractive lens

This paper describes a fully integrated micro-optical system, in which dynamic angular control of the beam from a vertical cavity surface emitting laser (VCSEL) is realized by laterally moving a collimating diffractive lens in the light path. The lens is mounted on a translatable silicon stage, which consists of a frame with an opening for the light to traverse the lens and electro-statically driven comb actuators, by which the lateral movement is achieved. Devices implementing both one- (1D) and two-dimensional (2D) scanning have been fabricated and evaluated. Integration of the lens onto the translatable silicon stage is done using a newly developed fabrication process based on hot embossing of an amorphous fluorocarbon polymer. This fabrication process relies on a reversed-order protocol, where the structuring of the optical element precedes the silicon microstructuring. Assembly and packaging of the VCSEL-MOEMS system, using low temperature cofired ceramic (LTCC) technique, is also demonstrated. Optical evaluation of the system and beam steering function shows significant beam deflection for a relatively low driving voltage (∼70 V).

Application of 3D glycerol-compensated inclined-exposure technology to an integrated optical pick-up head* *This paper is based on the two papers presented at the TRANSDUCERS'03 conference, Boston MA, USA.

This paper proposes a novel method for integrating inclined mirrors with precise angles and smooth surfaces in on-chip Fresnel lens optical pick-up-head applications. A glycerol-compensated inclined-exposure technology was adopted for 45° mirror pair fabrication. This technology can fabricate 19–90° inclined structures onto SU-8 negative tone resist with thickness from 100 to 1000 µm. Glycerol is employed as an index matching material during exposure to compensate for the refractive index difference between the air and SU8, thus extending the possible inclined angles from 54° (in air) down to 19° (in glycerol). To eliminate reflected induced patterns, CK-6020L resist is employed as an antireflection layer. The surface roughness, measured by AFM, is below 7 nm for various inclined angles. Device fabrication has been successfully completed and test results demonstrated a functional VCSEL (vertical cavity surface emitting laser) beam focusing on a 5 µm spot. Further beam size reduction into the sub-micrometer range requires an objective lens with high numerical aperture, which is not within the scope of this paper. The fabricated optical components are robust and accurately aligned. This new integrated approach provides an easy and low-cost fabrication method for optical pick-up heads eliminating the need for manual assembly.

Design of microchannel free-space optical interconnects based on vertical-cavity surface-emitting laser arrays.

We investigate the design of free-space optical interconnects (FSOIs) based on arrays of vertical-cavity surface-emitting lasers (VCSELs), microlenses, and photodetectors. We explain the effect of the modal structure of a multimode VCSEL beam on the performance of a FSOI with microchannel architecture. A Gaussian-beam diffraction model is used in combination with the experimentally obtained spectrally resolved VCSEL beam profiles to determine the optical channel crosstalk and the signal-to-noise ratio (SNR) in the system. The dependence of the SNR on the feature parameters of a FSOI is investigated. We found that the presence of higher-order modes reduces the SNR and the maximum feasible interconnect distance. We also found that the positioning of a VCSEL array relative to the transmitter microlens has a significant impact on the SNR and the maximum feasible interconnect distance. Our analysis shows that the departure from the traditional confocal system yields several advantages including the extended interconnect distance and/or improved SNR. The results show that FSOIs based on multimode VCSELs can be efficiently utilized in both chip-level and board-level interconnects.

Wireless infrared data links: ray-trace simulations of diffuse channels and demonstration of diffractive element for multibeam transmitters

Free-space IR transmission provides high bandwidth and good security with small-sized and low-cost links, for instance, for high- bit-rate wireless LANs. Some robustness against shadowing is achieved using diffuse channels, but disadvantages are high path loss and multipath propagation. We use ray-trace simulation software to analyze IR channels in realistic office rooms. Simulations are performed to test the methods and to specify the bandwidth and power budget requirements of diffuse links. Both diffuse and specular reflections as well as shadowing effects due to furniture are considered. Based on the study, novel Monte Carlo ray-tracing software, such as ASAP (Advanced System Analyses Program), are suitable for analyzing the multipath dispersion and the optical power budget of infrared links in realistic indoor environments. The simulations also reveal the benefits of quasidiffuse link configurations, which are composed of multibeam transmitters with restricted beam divergences as well as of detectors with narrow fields of view. For implementation of multibeam transmitters we design an array- type diffractive element that modifies the beams of a 2-D vertical-cavity surface-emitting laser (VCSEL) array. A single element providing the largest 50-deg illumination angle is fabricated for demonstration. The measured diffraction efficiency is in fair agreement with the calculated one after considering the properties of the real VCSEL beam.

Diode-laser absorption sensor for line-of-sight gas temperature distributions

Line-of-sight diode-laser absorption techniques have been extended to enable temperature measurements in nonuniform-property flows. The sensing strategy for such flows exploits the broad wavelength-scanning abilities (>1.7 nm approximately 30 cm(-1)) of a vertical cavity surface-emitting laser (VCSEL) to interrogate multiple absorption transitions along a single line of sight. To demonstrate the strategy, a VCSEL-based sensor for oxygen gas temperature distributions was developed. A VCSEL beam was directed through paths containing atmospheric-pressure air with known (and relatively simple) temperature distributions in the 200-700 K range. The VCSEL was scanned over ten transitions in the R branch of the oxygen A band near 760 nm and optionally over six transitions in the P branch. Temperature distribution information can be inferred from these scans because the line strength of each probed transition has a unique temperature dependence; the measurement accuracy and resolution depend on the details of this temperature dependence and on the total number of lines scanned. The performance of the sensing strategy can be optimized and predicted theoretically. Because the sensor exhibits a fast time response (~30 ms) and can be adapted to probe a variety of species over a range of temperatures and pressures, it shows promise for industrial application.

Optical beam steering using MEMS-controllable microlens array

Novel, two-dimensional MEMS-controllable microlens array has been integrated with a vertical cavity surface emitting laser (VCSEL) array using flip-chip assembly. The MEMS/VCSEL hybrid system is built as an efficient and reliable smart pixel array for board-to-board or chip-to-chip optical interconnects in digital systems. Theory including geometrical and Fourier transform analysis and beam steering experimental results are presented. By translating polymer microlens, fabricated on a MEMS X– Y movable plate, using electro-thermal actuators, a beam steering angle of 70 mrad is achieved. VCSEL beam steering was successfully demonstrated in our MEMS/VCSEL hybrid system to collimate and steer laser beam for a precision alignment in a two-dimensional free-space optical interconnect.

VCSEL beam waists from optical spectra

We relate the optical beam waist of a vertical-cavity surface-emitting laser (VCSEL) to a measurement of the transverse mode splitting found in the device's optical output spectra and to a measurement of beam divergence. We show experimentally that the results are in good agreement. Accurate knowledge of beam waists is useful for tailoring devices for single-mode or for high-speed operation and to better understand and interpret the mode spectrum.

Analysis of a vertical-cavity surface-emitting laser-based bidirectional free-space optical interconnect

The design of a bidirectional free-space optical interconnect system is presented. Vertical-cavity surface-emitting laser (VCSEL) arrays as a coherent light source and VCSEL beam collimation are described. Hologram array design and a way to improve the diffraction efficiency by use of a copying technique utilizing Dupont photopolymers are presented. Scattering from the hologram as a noise source is measured. An optical model for the design of system parameters such as the VCSEL beam diameter, size and apodization of the hologram, and size of the detector is given on the basis of cross-talk analysis of the system. The effect of VCSEL wavelength variation on system design is considered. Aberrations caused by the Fourier lens in the system are calculated, and ways for correction of the aberrations are discussed.