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A wide angle microscanning architecture is presented in which the angular deflection is achieved by displacing the principle axis of a curved silicon micromirror of acylindrical shape, with respect to the incident beam optical axis. The micromirror curvature is designed to overcome the possible deformation of the scanned beam spot size during scanning. In the presented architecture, the optical axis of the beam lays in-plane with respect to the substrate opening the door for a completely integrated and self-aligned miniaturized scanner. A micro-optical bench scanning device, based on translating a 200 μm focal length micromirror by an electrostatic comb-drive actuator, is implemented on a silicon chip. The microelectromechanical system has a resonance frequency of 329 Hz and a quality factor of 22. A single-mode optical fiber is used as the optical source and inserted into a micromachined groove fabricated and lithographically aligned with the microbench. Optical deflection angles up to 110 degrees are demonstrated.
©2013 Optical Society of America
1. Introduction
Optical beam scanners are key components in a wide range of applications including displays, optical communications, imaging, printing and surgery [1–5]. Miniaturization of the scanners reduces their cost and size enabling new applications that were not possible using their original bulky form; these include handheld barcode readers, pico projectors, confocal microscopy, Optical Coherence Tomography (OCT) and endoscopy [6–12]. The integrability of the scanner with other optical components on the same substrate is an important asset that allows great cost reduction and opens the door for more applications. A good example of this application is the OCT in which the optical scanner could be integrated with a beam splitter and either a moving mirror or a tunable laser to form an integrated OCT head working in either time domain or frequency domain, respectively.
In the conventional microscanning architecture, incident beam from a laser source is first collimated using a lens in order to reduce its divergence angle, and then reflected into different directions using a flat mirror rotating around its central axis as shown in Fig. 1(a). To this end, several actuation mechanisms were reported including electrostatic [10], piezoelectric [13], thermal [14] or magnetic [15] actuation with the aim of increasing the achievable scan angle and speed, which is a challenging task. Another challenge in the rotating micromirrors architecture arises due to the static or the dynamic bending of the mirror surface during the micromachining process or during the scanner operation, respectively [16]. MEMS mirrors are usually coated with a thin metal film to improve the reflectivity. This coating, in addition to the method of fabrication, can cause stress gradient across the mirror thickness leading to a permanent curvature of the reflecting surface, which may be compensated by the accompanying optics. More seriously, dynamic bending during the operation cannot be compensated. Indeed, a radius of curvature equal to or smaller than the scanned beam Rayleigh range can lead to a severe increase of the far-field beam radius and renders the beam spot size vary significantly during the scanning [17]. Assembly cost of the scanner shown in Fig. 1(a), bringing up the laser source, the collimating optics as well as the rotating mirror all aligned together, is a third challenging issue in this architecture. Monolithic integration of a diffractive Fresnel zone plate on a scanning micromirror was reported as a step towards simplifying the assembly [18], yet source integration and the possibility of monolithic integration of other micro-optical components were not possible.
Fig. 1 Conventional scanning architecture based on a rotating flat mirror is shown in (a). The presented scheme based on a translating curved mirror is shown in (b). In the presented scheme, wide angle scanning can be easily obtained, the mirror dimensions can be controlled independent of the mechanical components of the system and the source, as well as other micro optical components, can be integrated on chip.
Microscanners based on refraction of light from a translated microlens were also reported [19–21]. A micro stage attached to an electrostatic or piezoelectric actuator is usually micromachined, and then the lens is inserted and fixed on that stage. Again the assembly and fixation of the microlens on the stage and in conjunction with the source is not a trivial task. The weight of the lens harms the agility of the system while the scanning angle is limited by the optical properties of refraction. Typical maximum scanning angle of ± 5-10 degrees and speed in the order of 200 Hz were reported [20]. Therefore, there is a need for a wide angle scanner that can be monolithically integrated with other components on the same chip.
In this work, we present an optical scanning architecture based on translating a curved mirror as shown in Fig. 1(b) [22, 23]. The incident beam from the source is simultaneously collimated and reflected by the micromirror while the latter is attached to a translational motion mechanism. At one position, the incident beam is reflected in a given direction. By translating the curved mirror to another position, a new deflection angle is achieved. In this way, ultra-wide optical scanning angles, in principle up to 180 degrees, can be obtained. More importantly, the mirror surface and its translation path are designed carefully to maintain the uniformity of the beam spot size across the scanning window [24]. The optical beam propagates parallel to the wafer substrate and, thus, the optical source, the curved mirror and a microelectromechanical actuator can be all integrated on chip. This significantly reduces the alignment and assembly issue usually encountered in the conventional scanners. The source can be also replaced by an optical fiber inserted into a fiber groove where the latter can be lithographically aligned with the mirror. The presented architecture opens the door for a completely integrated laser scanner with beam splitters, filters and laser sources self-aligned on the same chip that allows low cost and mass production of this important product family.
2. Optical beam transformation
Before studying the performance of the presented scanner, this section briefly describes the transformation of a Gaussian beam (GB) after reflection on a surface, which is curved along one dimension. This allows to evaluate both cylindrical and acylindrical reflectors. For this purpose, consider an incident GB on a curved reflector with an angle of incidence θinc as shown in Fig. 2. In this case, the incidence plane is referred to as the tangential plane (t-plane) and the plan perpendicular to the tangential plan while containing the beam optical axis is referred to as the sagittal plane (s-plane). Let the reflector surface has a radius of curvature R in the t-plane while the surface cross section is flat in the s-plane. The incident GB undergoes a transformation in the t-plane described by the ABCD matrix given by Eq. (1) [31]:
The focal length of the transformation is given by Eq. (2):The output GB waist radius wout and the distance dout between the location of the output GB waist and the point of incidence are related to the input GB counterparts by Eqs. (3) and (4) [32]: where win is the waist radius of the input GB, din is the distance between the location of the input GB waist and the point of incidence and zo is the Rayleigh range of the input GB. Using the matrix parameters in Eq. (1) the normalized output beam parameters are given by Eqs. (5) and (6): where zR is the Rayleigh range of the output GB. Equations (2), (5) and (6) will be used to study the deformation in the output GB Rayleigh range and waist location with scanning.Fig. 2 A Gaussian beam is incident on a curved reflector with an angle of incidence θinc where the reflector radius of curvature is R. The tangential plane contains the incident as well as the reflected beam Optical Axis (OA) while the sagittal plane is normal to the tangential one and contains the beam OA.
3. Scanning based on cylindrical mirrors
In its simplest form, the optical scanning architecture can be built using a cylindrical mirror. The scanning shown in Fig. 1(b) can be achieved by shifting the mirror principle axis in a normal direction with respect to the incident beam optical axis while holding the input GB waist location fixed at a distance fo from the mirror vertex, where fo = −0.5R. The focal length of the curved reflector varies with the incidence angle as given in Eq. (2). As a result, the reflected beam parameters given by Eqs. (5) and (6) vary significantly with scanning as shown in Fig. 3, when a cylindrical mirror is used. The study is carried out for fo / zo ratios of 1, 5 and 10. The variation amplifies with fo / zo both for zR / zo and dout / fo. For example, a 10% reduction in zR / zo is found at 10, 13 and 25 degrees for fo / zo of 10, 5 and 1 respectively. The negative peak in dout / fo is reduced and shifted to larger angles for smaller fo / zo. The variation of the output beam waist location may not be critical as long as we are interested in the output beam at the far field. The significant roll off in the Rayleigh range of the output beam is, however, a critical drawback in the scanner performance based on a cylindrical reflector. This deteriorated output limits the overall scan angle of the curved reflector and the Rayleigh range of the output beam that are predominate factors in defining the scanner performance.
Fig. 3 The variations of the output beam normalized parameters with the incidence angle. The study is carried out for a mirror focal length to input beam Rayleigh range (fo / zo) ratio of 1, 5 and 10. The variations of the output beam Rayleigh range is shown in (a) while the variation of the output beam waist location is shown in (b). The variations are significant limiting the wide-angle scanning performance.
4. Scanning based on a specially-designed acylindrical mirror
The aforementioned drawback of cylindrical reflector based scanning is encountered due to the reduction in focal length by the inclined incidence factor cos(θinc). In addition, a mismatch arises between the focal length and the distance between the input GB waist and mirror surface, which varies according to Eq. (7):
The focal length is maximized at zero degree angle of incidence where din is matched to the focal length only at this specific angle. Therefore, a special design for the mirror curved profile coupled to its translation path is needed.The proposed solution is to tilt the translation path of the curved mirror with the appropriate angle α with respect to the incident beam optical axis as shown in Fig. 4. This tilting can play an important role in minimizing the input distance variation. The mirror surface has to be designed carefully, in accordance. The differential equation describing the mirror surface is formulated by equating the effective focal length of the mirror surface to the input distance in conjunction with keeping the tilted motion path mentioned previously:
where R = R (θinc) is now a function of the incidence angle, leading to an acylindrical shape for the mirror. By substituting for the radius of curvature of the mirror surface as well as the cosine of the angle of incidence in terms of the surface first and second derivatives, Eq. (8) can be put in the form:where x and y are the Cartesian coordinate system. The right hand side in Eq. (9) represents the input distance and the factor m = 1/tan (α). The solution of Eq. (9) could be found numerically as a family of curves resulting from different values of m. For example, Fig. 5 depicts the solution for m = 1, 2, 10 and infinity where the latter value describes the case of translating the mirror in a normal direction with respect to the incident beam optical axis. The cylindrical cross sectional profile is close to the solution of Eq. (9) when m = 10. Indeed, the different values of m don’t change the starting behavior of the curve as governed by the derivative boundary value at the vertex, which was set to infinity in our case. For smaller values of m, the curve tends to be more linear at larger y-coordinate values. This will, consequently, limit the maximum obtainable scanning angle for a given translation.Fig. 4 A linear tilted motion is applied on the mirror with respect to the incident beam axis from position a to position b. The tilted motion minimizes the variation in input distance din while the mirror profile is designed thereof to have its effective focal length (f) matching the input distance.
Fig. 5 Solution of Eq. (9) for different values of m compared to the cylindrical cross sectional profile.
A study was carried out seeking the appropriate tilt angle that minimizes the output beam parameters variations on a wide scanning angle range. The tilt angle was found to be α ≈26.5 degrees corresponding to m ≈2. For this case, the reflected beam parameters are depicted in Fig. 6 versus the incidence angle for fo / zo of 10, 5 and 1. Nearly uniform spot size and waist location are obtained versus incidence angle up to 55 degrees; corresponding to 110 degrees optical deflection angle. Indeed, there is a significant improvement as compared with the scanning performance shown in Fig. 3.
Fig. 6 Nearly uniform output beam parameters can be obtained using the surface described by Eq. (9) for m = 2 up to 110 degrees optical deflection angle. The study is carried out for fo / zo ratios of 1, 5 and 10. For the different ratios, the same normalized output distance (dout / fo) can be obtained as shown in (b).
5. Demonstration on an SOI optical-bench microscanner
An SOI optical bench was designed and fabricated using deep reactive ion etching of silicon [25,26]. The device layer height of the SOI wafer is 150 μm. The overall layout of the design is shown in Fig. 7(a). An electrostatic comb-drive was used in order to produce the desired translational motion. In principle, other actuation mechanisms are also possible. The comb-drive is double-sided and works in push-pull operation in order to produce large displacement [27,28]. At the center of the device, a double-folded flexure is used to avoid non-linearity at large displacement [29]. The electrical routing to the electrodes is made using the silicon layer while the electrodes as well as the mirrors are aluminum metalized using the shadow mask technique [26]. A scanning electron microscope image of the fabricated device is shown Fig. 7(b). Acylindrical mirror with fo = 200 μm is attached to the comb-drive and a fiber groove is etched facing the mirror where the optical fiber axis is tilted with respect to the comb-drive motion direction. The metalized area around the mirror can be observed by its higher brightness in Fig. 7(b). The optical fiber, inserted into the fiber groove, is working as an optical source with a GB output. The beam is incident on the mirror in an off-axis manner and the output beam deflection angle can be scanned by applying an electrical voltage on the comb-drive electrodes. Aside from the mirror attached to the actuator, acylindrical mirrors with focal length fo of 100, 200 and 400 μm were also fabricated. These mirrors enable the static testing of the optical performance in terms of the reflected beam spot size and intensity profile. In the latter case, the relative motion between the fiber and the mirror was achieved using external mechanical motion stage while the optical fiber is left free to move with respect to the mirror.
Fig. 7 Overall structure of the microscanner device in (a) and a Scanning Electron Microscope (SEM) image of the fabricated device in (b) with a zoom-in on the comb-drive fingers, attached curved mirror and an optical fiber inserted into the etched groove self-aligned with the mirror. A double-sided push-pull comb-drive is used to have a relatively large displacement while a double-folded spring is used to avoid non-linearity at large displacement.
6. Device characterization
Electrical measurements were carried out to characterize the comb-drive resonance frequency and quality factor. The equivalent parallel conductance and capacitance of the comb-drive actuator were measured using an HP 4149A impedance analyzer instrument [30]. The low voltage terminal of the instrument was connected to the electrode of the movable fingers while the high voltage terminal was connected to the fixed fingers. In order to prevent sticking between the movable fingers and the handle layer (substrate), the electrodes of both of them have to be at the same electrical potential. This could be achieved by connecting the movable fingers electrode to the instrument virtual ground while the substrate was connected to the real ground. The comb-drive was biased by a DC voltage and a small superimposed AC signal. The AC signal frequency was swept from 270 to 390 Hz around the expected comb-drive resonance frequency. The measurement results for the conductance and the capacitance are depicted using markers in Figs. 8(a) and 8(b), respectively. The motional parameters of the equivalent circuit were extracted [30] and fitting of the data was applied (see lines in Fig. 8). The estimated quality factor is close to 22 with a resonance frequency of about 329 Hz. These measurements were carried out at atmospheric pressure.
Fig. 8 Measured electrostatic actuator equivalent parallel conductance is given in (a) and parallel capacitance in (b) versus frequency. The measured data is depicted in markers while equivalent circuit data fitting is depicted in lines. The resonance frequency is 329 Hz and the quality factor is 22.
Long travel range by the comb-drive actuator was achieved by applying a half-wave rectified sinusoidal signal on the push electrode simultaneously with a 180 degree phase shifted signal on the pull electrode. More than one combination of the applied signal offset voltage and amplitude can be used to achieve the required displacement of 400 μm, which corresponds to about 100 degrees optical deflection angle using the acylindrical mirror with fo = 200 μm. Microscope images for the comb-drive in static case as well as in resonance are given in Figs. 9(a) and 9(b), respectively where the scale etched in the silicon has a pitch of 25 μm. The oscillation envelope at resonance can be observed in Fig. 9(b) with an overall displacement slightly larger than 400 μm obtained using applied signal offset voltage and amplitude of 35 V.
Fig. 9 Microscope image for the comb at rest position is given in (a) and at resonance in (b). The scale etched in the silicon has a pitch of 25 μm. A peak-to-peak displacement slightly larger than 400 μm can be observed.
The SOI optical bench microscanner was attached and wire bonded to a PCB daughter board as shown in Fig. 10(a). A 4/125 OZ Optics optical fiber was used to ensure single mode operation at the visible spectrum. The optical fiber was inserted into the micromachined fiber groove, which is lithographically aligned with the acylindrical micromirror and the comb-drive actuator. The fiber was fed from Thorlabs S1FC675 Fabry-Perot laser source working at a wavelength of 675 nm. The comb-drive was operated at the resonance condition resulting in the scanning window shown in Fig. 10(b). A window width of about 21.5 cm could be projected on a screen 10 cm away from the scanning MEMS.
Fig. 10 An optical fiber is inserted into the fiber groove of the MEMS scanner working as a source for visible laser in (a). The MEMS scanner is operated at resonance resulting optical scanning window in (b). A window width of 20 cm could be projected on a screen 10 cm away from the scanning MEMS.
An experiment was carried out in order to characterize the scanned beam spot size and intensity profile. For this purpose, fabricated acylindrical mirrors aside from those attached to the actuator, were used in conjunction with an external mechanical motion stage enabling the characterization of the beam in a static manner. A typical scanned beam spot shape after 10 cm propagation is shown in the inset of Fig. 11(a) for an optical deflection angle of 100 degrees. An elliptical spot is obtained offering a flying-line scanning type instead of the dot scanning. Indeed, the scanned beam spot shape can be changed by controlling the incident or the reflected GB wavefront radius of curvature. This can be achieved using a lensed fiber or additional curved micromirrors integrated within the SOI optical bench [33,34].The scanned beam spot size and intensity profile were captured using DataRay Inc. BeamScopeTM-P8 scanning slit beam profiling system [35]. The measured normalized intensity profile of the beam along the x-direction is shown in Fig. 11(a) in markers where w is the spot radius of the beam. Gaussian fitting of the data is also presented in solid line with a root mean square error less than 0.03. The spot radius was recorded for various optical scanning angles in the range of 20 to 110 degrees and the results are shown in Fig. 11(b). In order to validate the scalability the design of the acylindrical mirror, the experiment was repeated for mirror focal length fo of 100, 200 and 400 μm. There is a good agreement between the measured data (markers) and the theoretical prediction (lines). There is a larger deviation for smaller focal length, which can be referred to axial misalignment in the process of fiber insertion. However, the deviation is still relatively small for a 100 μm focal length.
Fig. 11 The intensity profile of the scanned beam (measured in markers and Gaussian fitting in line) at an optical deflection angle of 100 degree is given in (a) versus the transverse direction as shown by the inset. The inset also shows the spot shape of the beam, which tends to be linear. The spot radius of the scanned beam is given in (b) versus the optical deflection angle using acylindrical mirrors with focal length (fo ) of 100, 200 and 400 μm.
7. Conclusion
A microscanner based on an electrostatic comb-drive actuator displacing a curved micromirror was presented. The mirror profile was designed to minimize the variation in the scanned beam parameters due to inclined incidence with different angles. The profile is coupled to a titled motion of the mirror and the comb-drive with respect to the incident beam axis. The optical beam axis is parallel to the wafer substrate enabling the composition of the microscanner in the form of a micro-optical bench. The microscanner was experimentally validated and optical deflection angle up to 110 degrees was obtained; positioning the presented scanner ahead of the wide-angle performing scanners specification. Moreover, the possible integration of laser sources, filters and beam splitters within the microscanner is a major advantage in the presented scanner architecture opening the door for a wide range of applications with a special potential in biomedical imaging.
Acknowledgment
The authors would like to acknowledge the Information Technology Industry Development Agency (ITIDA) for the financial support of this work through the ITAC program
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