The paper reviews the state-of-the-art of miniaturized tunable lasers constructed by microelectromechanical systems (MEMS) technology, covering various topics of laser configurations, theoretical studies and some design issues, with primary focus on the uniqueness of MEMS tunable lasers in comparison to conventional opto-mechanical counterparts. Further studies have also been presented to investigate the tuning range and stability in order to provide a deep understanding of the specialities of MEMS lasers in the sense of physics. The introduction of MEMS has endowed two special features to tunable lasers. One is that MEMS facilitates external cavities at very short (<100 µm) and even extremely short length (<10 µm), leading to unusual tuning behaviors and different design concerns. The other is that the photolithography of the MEMS process makes it possible to fabricate gratings/mirrors in arbitrary profiles, which may inspire the innovation of new laser configurations that can only be realized by MEMS technology. With further work on integration and packaging, MEMS lasers would be able to deliver their merits of small size, fast tuning speed, wide tuning range and IC integration compatibility, and to broaden their applications to many fields. (Some figures in this article are in colour only in the electronic version)

https://nocweba.ntu.edu.sg/laq_mems/journal%20papers/2007/A%20review%20of%20MEMS%20external-cavity%20tunable%20lasers.pdf

A review of MEMS external-cavity tunable lasers

Estimation of rice leaf nitrogen contents based on hyperspectral LIDAR

Based on a triboelectric nanogenerator (TENG), the first active micro-actuator for optical modulation driven by mechanical energy without external power or mechanical joint is presented. This demonstrates the enormous potential of TENGs for independent and sustainable self-powered micro/nano electromechanical systems, and opens up new -applications of TENGs in triboelectric-voltage-controlled devices.

http://www.nanoscience.gatech.edu/paper/2015/15_AM_02.pdf

Active micro-actuators for optical modulation based on a planar sliding triboelectric nanogenerator.

This topical review discusses recent development and trends on scanning micromirrors for biomedical applications. This also includes a biomedical micro robot for precise manipulations in a limited volume. The characteristics of medical scanning micromirror are explained in general with the fundamental of microelectromechanical systems (MEMS) for fabrication processes. Along with the explanations of mechanism and design, the principle of actuation are provided for general readers. In this review, several testing methodology and examples are described based on many types of actuators, such as, electrothermal actuators, electrostatic actuators, electromagnetic actuators, pneumatic actuators, and shape memory alloy. Moreover, this review provides description of the key fabrication processes and common materials in order to be a basic guideline for selecting micro-actuators. With recent developments on scanning micromirrors, performances of biomedical application are enhanced for higher resolution, high accuracy, and high dexterity. With further developments on integrations and control schemes, MEMS-based scanning micromirrors would be able to achieve a better performance for medical applications due to small size, ease in microfabrication, mass production, high scanning speed, low power consumption, mechanical stable, and integration compatibility.

https://www.mdpi.com/2072-666X/7/2/24/pdf

Scanning Micromirror Platform Based on MEMS Technology for Medical Application.

Miniaturized optical benches process free-space light propagating in-plane with respect to the substrate and have a large variety of applications, including the coupling of light through an optical fiber. High coupling efficiency is usually obtained using assembled micro-optical parts, which considerably increase the system cost and integration effort. In this work, we report a high coupling efficiency, monolithically integrated silicon micromirror with controlled three-dimensional (3D) curvature that is capable of manipulating optical beams propagating in the plane of the silicon substrate. Based on our theoretical modeling, a spherical micromirror with a microscale radius of curvature as small as twice the Gaussian beam Rayleigh range provides a 100% coupling efficiency over a relatively long optical path range. Introducing dimensionless parameters facilitates the elucidation of the role of key design parameters, including the mirror's radii of curvature, independent of the wavelength. A micromachining method is presented for fabricating the 3D micromirror using fluorinated gas plasmas. The measured coupling efficiency was greater than 50% over a 200-μm optical path, compared to less than 10% afforded by a conventional flat micromirror, which was in good agreement with the model. Using the 3D micromirror, an optical cavity was formed with a round-trip diffraction loss of less than 0.4%, resulting in one order of magnitude enhancement in the measured quality factor. A nearly 100% coupling was also estimated when matching the sagittal and tangential radii of curvature of the presented micromirror’s surface. The reported class of 3D micromirrors may be an advantageous replacement for the optical lenses usually assembled in silicon photonics and optical benches by transforming them into real 3D monolithic systems while achieving wideband high coupling efficiency over submillimeter distances. Scientists from France and Egypt have developed spherical micromirrors that can couple optical fibres with almost 100% efficiency. Yasser Sabry and colleagues fabricated their monolithically-integrated mirrors in a silicon platform by a technique incorporating fluorinated gas plasmas and then coating the etched surface with a 100-nm-thick layer of aluminium. The resulting broadband mirrors had curvatures of 30–200 μm and exhibited reflectivities of >92% in the visible and near-infrared spectral regimes. The combination of high reflectivity and almost perfect coupling to optical fibres allows for the creation of miniature integrated optical cavities with a round trip transmission of better than 99.6% multiplied by the mirror reflectivity and an overall Q-factor of around 3000. This performance is an order of magnitude better than that achieved using flat mirrors.

/pdf/silicon-micromirrors-with-three-dimensional-curvature-56dofbmm59.pdf

Silicon micromirrors with three-dimensional curvature enabling lensless efficient coupling of free-space light

We propose an improved design of variable optical attenuator (VOA) using a micromachined elliptical mirror, which offers the advantages of both the shutter-type VOAs and the flat-mirror reflection-type VOAs while overcoming their problems. This design positions the input and output fibers at the two focal centers of the reflective elliptical mirror. Since the ellipse can focus the light from one center to the other, the VOA enjoys low insertion loss while using the normally cleaved single-mode fibers. As the mirror is displaced, the input beam is rapidly defocused, producing a large attenuation range without requiring a large mirror displacement. This VOA also has low polarization dependence loss (PDL), low wavelength dependence loss (WDL), and low back reflection. It is fabricated by the deep reactive ion etching process and achieves 44-dB attenuation at 10.7-V driving voltage. The PDL is 0.8 dB at the 40-dB attenuation level and the WDL is 1.2 dB at the 20-dB level for 100-nm wavelength change. More importantly, the measurement shows that attenuation has a nearly linear relationship with the mirror displacement over a 30-dB attenuation range.

Linear MEMS variable optical attenuator using reflective elliptical mirror

This letter presents a design of a microelectromechanical systems variable optical attenuator (VOA) that employs a pair of parabolic mirrors as the retro-reflector, which has obtained a linear relationship over a 62-dB range between the attenuation (in decibels) and the mirror rotation angle (in degrees). The insertion loss measures 0.6 dB thanks to the three-dimentional optical coupling design. The linearity comes from the simultaneous shift and defocus of the laser beam. Compared with the conventional coaxial and cross-axial VOAs, such retro-axial design has the two fibers arranged on the same side and thus facilitates the use of standard packaging formats

Retro-Axial VOA Using Parabolic Mirror Pair

This paper presents a 3-dimensional micro-optical system for improving the optical coupling efficiency in the external cavity tunable lasers. It is constructed by an optical fiber and a curved mirror; the former acts as a rod lens to transform the light in one direction while the latter converges the light in the other direction. Compared with the previous MEMS lasers that have no light focusing or only in one direction, this 3D micro-optical system allows for long external cavity length and high coupling efficiency, which are critical for wide tuning range. A prototype is fabricated on SOI wafer by DRIE with an etching depth of 75 µ m. It obtains a coupling efficiency as high as 46.5% with a total external cavity length of 220 µ m. In contrast, the coupling efficiency is only 13% when the fiber rod lens is absent, and is 1% when no light transformation is involved. Compared with conventional laser optical systems that employ external ball lenses and other optical components, this 3D micro-optical system possesses the advantages of simple packaging, large alignment tolerance and easy mass production.

https://nocweba.ntu.edu.sg/laq_mems/conference/IEEE%20MEMS/2006%20MEMS_MP45%20Design%20and%20experiment%20of%203-dimensional%20micro-optical%20system%20for%20MEMS%20tunable%20lasers.pdf

Design and Experiment of 3-Dimensional Micro-Optical System for MEMS Tunable Lasers

This paper reports a new design of MEMS variable optical attenuator (VOA) that makes use of a pair of parabolic mirrors as the retro-reflector in the horizontal plane and a normal fiber as the rod lens to focus in the vertical plane. It is advantageous over the conventional co-axial VOAs since it allows the fibers to be arranged in parallel and is thus convenient for the packaging. The parabolic mirrors improve the optical coupling and yield an insertion loss of only 1.4 dB even when only the normally-cleaved single mode fibers are used. The attenuation can go up to 63 dB when the mirror pair is rotated by 2.6 degrees. More importantly, a linear relationship is well maintained over the whole range, unlike the nonlinear relationship in many conventional VOAs. At the 20-dB level, the VOA measures a PDL of 0.2 dB and a WDL of 0.5 dB, which are superior to the typically values of 1-2 dB in the conventional VOAs.

https://nocweba.ntu.edu.sg/laq_mems/conference/IEEE%20MEMS/2007%20MEMS_Retro-reflection%20VOA%20using%20parabolic%20mirror%20for%20low%20insertion%20loss%20and%20linear%20attenuation%20relationship.pdf

Retro-reflection VOA using parabolic mirror for low insertion loss and linear attenuation relationship

This paper presents a tuning mechanism to provide an adjustable virtual pivot for MEMS tunable lasers, which eliminates the mode hopping problem associated with conventional Littrow lasers. The key idea is to attach the two ends of the blazed grating to two separate translational actuators. The relative displacements of the actuators can be used to translate and/or rotate the blazed grating in a controllable manner. As a result, the wavelength can be continuously tuned ideally over an unlimited range through a simultaneous sweep of the grating angle and cavity length. In the experiments, a MEMS Littrow laser is fabricated by the DRIE process on the SOI wafer and is integrated onto a single chip with the size of 3 mm times 3 mm. It achieves a wide tuning range of 53.2 nm thanks to the introduction of the new tuning mechanism and a 3D optical coupling system.

https://nocweba.ntu.edu.sg/laq_mems/conference/IEEE%20MEMS/2007%20MEMS_MEMS%20tuning%20mechanism%20for%20eliminating%20mode%20hopping%20problem%20in%20external-cavity%20lasers.pdf

H. Cai

Papers

Linear MEMS variable optical attenuator using reflective elliptical mirror

Retro-Axial VOA Using Parabolic Mirror Pair

Design and Experiment of 3-Dimensional Micro-Optical System for MEMS Tunable Lasers

Retro-reflection VOA using parabolic mirror for low insertion loss and linear attenuation relationship

MEMS tuning mechanism for eliminating mode hopping problem in external-cavity lasers