OPTICAL SCANNING BEAM STEERING MIRROR

This patent application claims the benefit of U. S. Provisional Patent Application

No. 60/914,723 filed APRIL 28, 2007

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to optical scanning by rotating mirrors and, more particularly to high speed rotating mirrors. Rotating mirrors are used in a very large variety of optical scanning devices including confocal microscopes, structured illumination, bar code readers, laser displays and laser printers. Rotating mirrors are characterized by their angular range, the optical clear aperture and the dynamic characteristics including response time and accuracy. Technologies to rotate mirrors include rotating polygons, resonant actuators, galvo-acruators and Micro Electro Mechanical System (MEMS) techniques. Typical rotating polygons may have a six facets hexagon driven by a DC- motor typically at 35,000 RPM. A major limitation of rotating polygons is that they must be operated in a constant speed and in one direction only. The resonant actuators can drive a mirror to rotate in a sinusoidal motion and in one single resonant frequency. Typical resonant scanner can drive a 3mm clear aperture mirror at the rate of 10 KHz with angular range of 20 degrees. The resonant mirrors have the fastest scan rates, but they are limited to one single scan rate matched with the resonant frequency and to sinusoidal motion only. A full control of speed and direction can be achieved with a mirror attached galvo-actuator. A galvo-actuator can drive a mirror in typical angular range of 30-60 degrees and full step response of lms resulting in typical scanning rate of 1 KHz. Smaller steps can be even faster, going down 0.1 ms.

Optical Micro Electro Mechanical Systems (MEMS), also known as MOEMS provides an inexpensive method to produce large quantities of scanning mirrors in different size and thickness. Applications of MOEMS include laser displays, projectors displays and telecommunication devices including optical cross-connect. Optical MEMS may have a single or multiple mirrors capable to rotate in one or two axes.

US patent specifications US3,549,800 discloses a projection display system using a laser, light modulator and an optical scanner. The scanner of this device use a single rotating mirror actuated by either a galvanometer or resonance actuator. The actuator is synchronized by a driving circuitry to video signal, to produce a video image. US3, 186,115 disclose a visual display system based on array of multiple bi-stable rotating mirrors. Each mirror can take one of two stable positions (on-off) and an electro-magnetic actuator does the transition. In one position, the mirror reflects light towards the viewer, therefore it is optically "on" positipn, and the other position is optically "off. The development of MEMS technologies introduced improved methods to produce optical scanners of multiple rotating mirrors. US4,662,746 disclose a MEMS device for image display, consisting an array of multiple mirrors. Each mirror is hinged by a flexible cantilever and can be bent by electrostatic force. US5,696,619 disclose a further improvement of this device by replacing the bending cantilever by two torsion bars. When operated in bi-stable mode, the device of US5,696,619 is easy to control and can have a fast response of typically 16μs. A rotating mirror, driven by magnetic field rather than electrostatic force is provided by US6,285,485. According to a method disclosed by US6,285,485, a rotating mirror is driven to rotate using two current conducting coils. A first conducting coil is attached

to a rotating mirror and a magnetic field is applied. The second stationary coil induces alternating current in the first coil to drive the mirror.

A major issue of scanning mirrors in all the above technologies is improving the scanning speed while maintaining the optical quality. Improving the speed of mirrors is significant to all applications and especially for laser displays, where the scanning speed limits the resolution and the refresh rate. One way to improve the response time of a scanning mirror is to reduce the thickness of the moving parts, including the mirror. Care should be taken in this process, that the optical quality of the mirror will not be degraded. A common way to produce a thin mirror is to deposit a reflecting aluminum or silver layer on silicon substrate. A single crystal silicon can be polished below lmm thickness and yet maintain a very good flatness. Other substrate materials for mirrors include glass mirrors, beryllium and silicon carbide. See for example, Kevin F. Carr, "Silicon carbide mirrors benefit high-speed laser scanning", Laser Focus World April 2008.

It would be highly advantageous to have a high speed beam steering mirror, with thickness below 0.1mm without degradation of flatness and optical qualities. It would be further advantages to have a fast scanning mirror, in which the flatness of the mirror is not primarily dependent of its thickness.

SUMMARY OF THE INVENTION

The present invention successfully addresses the shortcomings of the presently known configurations by providing a beam steering mirror including a static member, a thin mirror, a thin flexible hinge connecting the thin mirror to the static member allowing

the mirror to rotate with respect to the static member, a magnetic field applied to the mirror, a ferromagnetic layer associated with the mirror wherein the magnetic field attracts the ferromagnetic layer and stretches the thin mirror and a conducting layer associated with the thin mirror capable to conduct electrical current wherein the electrical current and the magnetic field introduce a force perpendicular to the magnetic field capable to accelerate the rotation of the mirror with respect to the substrate. In many embodiments the attraction force stretching the thin mirror improves the stiffness and flatness of the thin mirror. In some embodiments the conducting layer is patterned to form a conducting coil and in some embodiments the electrical current is induced in the conducting layer by a second conducting coil associated with the static member. In some embodiments the ferromagnetic layer is a permanent magnet and in some embodiments the static member includes a polymer. The invention also discloses an optical scanner including the disclosed beam steering mirror, a light source for providing a light beam in the direction of the beam steering mirror wherein the light beam is reflected from the beam steering mirror in a direction depending on the rotation angle of the thin mirror, a sensor for measuring the rotation angle of said thin mirror and a control device for controlling the rotation angle of the thin mirror. In many embodiments the light source includes at least one laser and in some embodiments the light beam has intensity that can be varied while scanning. In one embodiment of the invention the light beam reflected in a direction depending on the rotation angle of the beam steering mirror displays an image during an integration period shorter than the integration time of the human eye.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic view of a rotating mirror in one embodiment of the invention; FIG. 2 shows a top view of the rotating mirror of FIG 1 ;

FIG. 3 is a schematic view of a rotating mirror with a primary and a secondary coil to drive the mirror in one embodiment of the invention;

FIG. 4 shows a process to produce a rotating mirror in one embodiment of the invention; FIG. 5 is a Fabry Perot resonator (Fabry Perot Etalon) with a rotating mirror in another embodiment of the invention;

FIG. 6 is an optical scanner with a rotating mirror in one embodiment of the invention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention can be better understood from Figure 1, showing a schematic layout of an embodiment of the invention. Figure 1 shows a cross section of a mirror 100, made of a thin layer of reflective material such as aluminum or silver to minimize the mass of the rotating member, thus allowing high acceleration and low forces. A thin flexible layer 101 connects the mirror to a static member 104 and acts as a hinge, allowing rotation of the mirror 100 in one axis relative to the static member 104. The hinge 101 is made thin and flexible to avoid undesired stresses that may deform the thin mirror 100. A ferromagnetic layer made of ferromagnetic material 102 is attached to the mirror 100 as seen in Figure 1 (for example by plating NiFe) and a magnetic

field B applied to the mirror acts to pull the ferromagnetic layer 102 and stretch the mirror 100 and the hinge 101 by force Fs (stretching force). As the mirror and hinge are made very thin, the stretching force maintains the position and flatness of the mirror 100. A conducting layer 103 in Figure 1 is also attached to the mirror 100, (for example by deposition of Aluminum or plating Copper) wherein the current induced in the conducting layer 103 and the magnetic field B create a force F _R , perpendicular to the magnetic field. The force FR act to rotate the tilting mirror and to control the angular acceleration, velocity and position of the tilting mirror 100. The stiffness of the mirror 100 results from the stretching force FR and not only from the moment of inertia, therefore the mirror 100 of Figure 1 has a string model of stiffness (or stretched membrane stiffness in two dimensional model). A thin mirror of the invention is defined as a mirror whose thickness and resulting moment of inertia are insufficient to provide the required stiffness and flatness without an external stretching force. The mirror 100 can be thin and yet maintain optical quality because of the additional stiffness of the stretching force Fs-

Figure 2 shows a top view of the rotating mirror of Figure 1. The mirror 200 is attached to the static member 204 by thin layer 201 acting as a hinge allowing rotation in one axis. A conducting layer 203 is patterned to form a conducting coil. Electrical current i induced in the conducting layer 203 attached to the mirror 200 provides the driving force (F _R of Figure 1) to control the rotation of the mirror 200.

The current to drive the mirror can be induced in the conducting coil 203 by an alternating magnetic field, as disclosed in US 6,285,485, or the coil 203 can be connected to a source of current by flexible connectors. A rotating mirror with driving current induced by alternating magnetic field is demonstrated in Figure 3, showing a

schematic layout of one embodiment of the invention. A primary coil 305 in Figure 3 is attached to the static member 304 and an alternating current conducted in the primary coil 305 induces current in the secondary coil 303 of Figure 3. The production of the rotating mirror of the invention involves common knowledge to deposit and pattern a reflecting layer, a conducting layer, a ferromagnetic layer and a sacrificial layer. Figures 4A, 4B and 4C demonstrate a method to produce the preferred embodiment of the invention. A thin reflecting layer 400 is deposited and patterned on substrate 404 to form a mirror. More layers including a thin flexible layer 401, a ferromagnetic layer 402 and a conducting layer 403 are deposited and patterned to form a hinge 401 and a conducting coil 403. If the reflecting layer is flexible and mechanically stable, it can also serve for the hinge. A permanent magnet 406 is attached to the substrate 404 to provide a magnetic field to attract the ferromagnetic layer 402. It is essential to apply a magnetic field prior to releasing the rotating mirror from the substrate else the mirror may roll and get damaged. To enhance the attraction, the ferromagnetic layer 402 can be magnetized to become a permanent magnet. In the preferred embodiment, the substrate 404 is made of polymer, for example polymethylmetacrylate (PMMA) and it also serves as a sacrificial layer and a static member. After attaching the permanent magnet, part of the substrate is dissolved, for example with CH ₃ COCH ₃ (acetone) solvent, to release the rotating mirror 400. Figure 4B shows the rotating mirror 400 after releasing, the rotating mirror 400 is attached by the hinge 401 to the remaining part of the substrate 404, which acts as a static member and the mirror 400 is free to rotate. Figure 4C shows a printed circuit board 417, with a primary conducting coil 419 attached to the substrate to induce current in the secondary coil of the conducting layer 403 associated with the

mirror. A ferromagnetic core 418 improves the coupling of the primary coil 419 to the secondary coil 403.

Figure 6 shows an optical scanner in one embodiment of the invention including a light source 61O ₅ a rotating mirror 600 as described in Figure 1, a rotation sensor 630 and a control device 650. The light source 610 directs a light beam A to the rotating mirror 600. The light source 610 may include one or several lasers, for example red, green and blue lasers for a laser display. For a laser display, the intensity of the light source is controllable and can be varied. The light beam AR is reflected from the rotating mirror 600 at the angle β according to the rotation angle of the rotating mirror 600. A beam splitter 650 splits the beam AR into a primary scanning beam B and a secondary beam C directed to a rotation sensor 630, which measures the rotation of the beam steering mirror 600. A control device 650 receives the information of the required angle as a first input R and the information of measured angle as a second input M. The output of the control device 650 is the current I of the primary coil 620 which controls the angle β of light beam B. The rotation sensor can be a Position Sensitive Detector (PSD) ₅ see for example Hamamatsu one dimensional PSD S8673 datasheet. Other sensors such as a capacitive sensor or a grating and counter are also possible. The optical scanner shown in Figure 6 can scan in one axis only, a second rotating mirror can be added to scan in the perpendicular axis and thus create a two axes scanner. A two axes scanner is required by some applications including laser display. In the application of a laser display, the reflected beam B displays an image within an integration period shorter than the integration time of the human eye.

A beam steering mirror is useful not only for scanning, but also as an optical shutter. Figure 5 shows an optical layout an optical shutter in another embodiment of the invention comprising the steering mirror 500 as described by Figure 1, coupled with a secondary mirror 510 to form a Fabry Perot resonator (Fabry Perot Etalon). When the steering mirror 500 is parallel to the secondary mirror 510, and the optical distance between the mirrors L match the resonance conditions for the wavelength of the incident light, the light would be transmitted through. Even small tilt of the steering mirror would break the resonant conditions causing the incident light to be reflected backwards. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Therefore, the claimed invention as recited in the claims that follow is not limited to the embodiments described herein.