Abstract:
An apparatus and method for modifying the spread of a laser beam. The apparatus comprises a laser source operable to generate a laser beam having a flux that exceeds a predetermined value and an optical train operable to modify the beam such that the flux of the beam through a predetermined aperture does not exceed the predetermined value. The optical train may include a focusing lens, a diffractive focusing vortex lens, a beam splitting device, or a diffraction grating.

Description:
BACKGROUND OF THE INVENTION 
     Many of today&#39;s devices use lasers to implement various functions. For example, an optical mouse or laser pointer use lasers in their respective operations. Additionally, many fiber-optic communication devices use a laser as a source of signal generation. 
     These lasers generate optical beams that can sometimes generate light powerful enough to damage the human eye. For example, a laser-optic pointing device typically includes a beam-modifying apparatus in optical alignment with the laser designed to focus or collimate the laser beam for its intended use. Therefore, if a person looks into the laser beam, it may cause damage to the unaided eye. And even if the laser beam is not powerful enough to cause eye damage, one may further focus the laser beam with a magnifying lens or other optical instrument such that the further focused beam is capable of causing damage to the eye. 
     Because products containing laser devices are potentially hazardous to the eye, they are classified accordingly by their potential hazard level. One such classification scheme is the International Standards for The Safety of Laser Products (ISSLP). The least hazardous laser devices, as classified by the ISSLP, are classified as Class 1 laser devices. Laser devices in this category are defined as being safe under reasonably foreseeable—although perhaps not recommended—conditions of use, such as the use of optical instruments such as a magnifying lens for intra-beam viewing. The classification for the next-least-hazardous category of laser devices is Class 1M. This classification covers laser devices that are safe under reasonably foreseeable conditions of operation, but may be hazardous if the user employs an optical instrument such as a magnifying lens for intra-beam viewing. 
     According to the ISSLP, the maximum-allowed power, i.e., acceptable emission light (AEL) level, for a commonly used single-mode 850 nanometer (nm) wavelength Class 1 laser device is 0.78 milliwatts (mW) when measured according to the ISSLP-defined standard. The defined standard is the amount of flux (power per unit area) through a 7 millimeter (mm) aperture in a radial plane that is 14 mm from the point where the laser beam exits the device. Thus, if the flux of the laser beam that passes through the 7 mm aperture is less than 0.78 mW, then the laser device is considered Class 1 safe. 
     A speckle-based motion sensor, which could be used in an optical mouse, is a device that uses a laser for its functionality. In a speckle-based motion sensor, a laser beam is directed to a surface and the reflection of the laser beam from the surface creates a complex diffraction pattern, called a speckle pattern. If the laser beam moves relative to the surface, then the speckle pattern changes. Detectors for receiving the reflected speckle pattern can then determine the relative changes in the speckle pattern and translate these changes into the relative lateral motion of the laser-beam source. For acceptable performance, however, a laser in a speckle-based motion sensor is typically operated at power levels that exceed the maximum-allowable AEL for a Class 1 rating. That is, if the power is reduced to meet the Class 1 requirements, the speckle-based motion sensor may not perform at an acceptable level. 
     SUMMARY OF THE INVENTION 
     An embodiment of the invention comprises a laser source operable to generate a laser beam having a flux that exceeds a predetermined value and an optical train operable to modify the beam such that the flux of the beam through a predetermined aperture does not exceed the predetermined value. The optical train may include a focusing lens, a diffractive focusing vortex lens, a beam splitting device, or a diffraction grating. 
     By diverging or diffracting some of the flux in the laser beam, a more powerful laser may be used, and yet the device incorporating the laser can still receive a Class 1 rating. Such an apparatus and method may be used in devices that require a more powerful laser but still need a Class 1 rating, such as an optical mouse using speckle motion detection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a diagram of a laser-beam modifying apparatus that uses a focusing lens according to an embodiment of the invention; 
         FIG. 2  is a top view of a diffractive focusing vortex lens for use in the apparatus of  FIG. 1  according to an embodiment of the invention; 
         FIG. 3  is an orthogonal view of a ring-shaped irradiance distribution pattern that results from modifying a laser beam with the diffractive focusing vortex lens of  FIG. 2  according to an embodiment of the invention; 
         FIG. 4  is a graph of the percent of flux with respect to the displacement of the aperture that passes though the aperture when using the diffractive focusing vortex lens of  FIG. 2  according to a embodiment of the invention; 
         FIG. 5  is a diagram of a laser-beam modifying apparatus that uses a beam splitting device according to an embodiment of the invention; 
         FIG. 6A  is a top view of a two-dimensional diffractive grating for use in the apparatus of  FIG. 1  according to an embodiment of the invention; 
         FIG. 6B  is a plot of an array of divergent laser beams that result from a laser beam that passes through the two-dimensional diffractive grating of  FIG. 6B  according to an embodiment of the invention; and 
         FIG. 7  is a block diagram of an electronic system that incorporates the apparatus of  FIG. 1  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is presented to enable a person skilled in the art to make and use the invention. The general principles described herein may be applied to embodiments and applications other than those detailed below without departing from the spirit and scope of the present invention. The present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein. 
       FIG. 1  is a diagram of an apparatus for modifying a laser beam for use in an optical mouse according an embodiment of the invention. In this embodiment, a Vertical Cavity Surface Emitting Laser (VCSEL)  101  emits an 850 nm wavelength laser beam along an optical axis  102 . A typical VCSEL  101  will have an associated divergence angle  125  which is a measure of the widest angle at which individual rays of the laser beam emanate from the VCSEL  101 . Typically, the divergence angle  125  is defined as the angle at which the optical intensity (power per unit area) of the laser beam falls to one-half of the peak, on-axis value. A typical laser beam may have a divergence angle  125  of up to about 20. That is, individual rays of the laser beam emanate in a divergent, omni-directional manner at an angle of up to 10 from the optical axis  102 . For the purposes of laser eye safety, however, a worst case scenario is taken into account where the divergence angle  125  is 12. 
     The optical axis  102  of the VCSEL  101  is aligned with a collimating lens  105  located at a distance  120  of approximately 1.5 mm from the VCSEL  101 . The collimating lens  105  refracts the diverging laser beam  103  to produce a collimated laser beam  106  that is aligned with the optical axis  102 . Given a 12 divergence angle  125  before the diverging laser beam  103  enters the collimating lens  105 , the diameter of the collimated laser beam  106  is approximately 0.62 mm. Thus, without further conditioning, the entire collimated laser beam  106  would easily pass through a test aperture  115 . The power emitted by the VCSEL  101  is, therefore, limited to the 0.78 mW AEL, which may not produce an adequate signal level for a speckle-based motion sensing. Therefore, the collimated laser beam  106  is passed through another beam modifying medium  107 , such as a lens, prism, or grating according to various embodiments of the invention as discussed below. 
     In the embodiment shown in  FIG. 1 , the beam-modifying medium  107  is a refractive focusing lens  107 . The collimated laser beam  106  is passed through the focusing lens  107 , which is at a distance  121  of approximately 1 mm away from the collimating lens  105  along the optical axis  102 . This distance  121  is typically equal to thickness of an optical substrate (not shown) used to hold the lenses  105  and  107  in position. The focusing lens  107  focuses the collimated laser beam  106  to converge at a point  108 , which is at a distance  122  of approximately 1 mm away from the focusing lens  107  along the optical axis  102 . If the converging laser beam  109  were to strike an object, such as the target  110 , the reflection would produce a speckle pattern, and a detector  111  can detect motion of the beam  109  relative to the target  110  by detecting changes in the speckle pattern. Since an optical mouse (not shown in  FIG. 1 ) is typically placed on a mouse pad or other flat surface, the mouse pad or surface functions as a suitable target  110  and changes in the reflected speckle pattern, and thus movement of the mouse, may be detected by the detector  111  within the mouse. 
     If the target  110  is removed, i.e., the optical mouse pointing device is lifted away from the reflecting surface, then the convergent laser beam  109  will propagate past the focal point  108  and then proceed to diverge. If one were to measure the flux of the now diverging laser beam  113  through any 7 mm aperture at a radial distance  123  of 14 mm from the focusing lens  107  (which in one embodiment is the closest point of human access as defined by the ISSPL for determining the AEL level for the laser eye-safety standard), one would find that only a portion of the diverging laser beam  113  would pass through the aperture  115 . As discussed below, the maximum flux passes through the aperture  115 , which has its center aligned with the axis  102 , and this maximum flux is low enough to meet the requirements for a Class 1 rating. Furthermore, the ISSPL requires that the radial distance  123  be measured from the point where the beam  109  exits the device that generates the beam; thus, the distance  123  may be measured from other than the lens  107  in other embodiments. 
     In the embodiment shown in  FIG. 1 , only about 25% of the diverging laser beam  113  passes through the aperture  115 . As discussed above, even if the aperture  115  is moved to different angles with respect to the optical axis  102  (which is akin to looking at the focusing lens  107  from different angles), 25% of the divergent laser beam  113  will typically be the maximum amount to pass through the aperture  115 . Thus, in this particular example, the AEL level of the VCSEL  101  could be as high as 3.0 mW yet still only produce a flux of 0.75 mW (which is below the maximum flux allowed for a Class 1 rating) through the 7 mm aperture  115 . 
     Other embodiments are contemplated wherein the modifying medium may be different. For example, instead of using a focusing lens  107 , as is the case in the embodiment of  FIG. 1 , the modifying medium  107  may be a diffractive focusing vortex lens (not shown in  FIG. 1 ). 
       FIG. 2  shows a top view of a diffractive focusing vortex lens  200  according to an embodiment of the invention. There are several well-known ways of creating diffractive light using a diffractive focusing vortex lens  200 . The diffractive focusing vortex lens  200  is created by superimposing a concentric-edge microstructure lens  201  with a radial-edge microstructure lens  202 . The resulting effect to light, i.e., the collimated laser beam  106 , passed through the superimposed lenses  201  and  202  causes light to “twist” away from diffractive focusing vortex lens  200  along the optical axis  102 . 
     Referring to  FIG. 3 , the twisting nature of light passed through the diffractive focusing vortex lens  200  results in the formation of a donut- or ring-shaped irradiance distribution pattern  300  in the plane  315  orthogonal to the optical axis  102 . That is, this phenomenon causes the pattern  300  to appear as a hollow “cone” when looking into the axis  102  toward the modifying medium  107  (which here is the lens  200 ). If designed properly, i.e., designed with the Class 1 rating in mind, the diameter of the “cone” of the ring-shaped irradiance distribution pattern  300  at the 14 mm point that corresponds to the aperture  115  position will be slightly larger than the aperture  115  diameter of 7 mm. Thus, if the aperture  115  is centered on the optical axis  102 , little or no laser light passes through it. 
     A person, however, may not always choose to look directly into the optical axis  102 . As such, portions of the ring-shaped irradiance distribution pattern  300  may enter the eye. Depending on the angle (measured as displacement from the optical axis  102 ) one is looking, the amount of flux will also vary. 
       FIG. 4  is a graph that plots the percent  401  of flux passing through the aperture  115  as a function of the displacement  402  in millimeters radial from the optical axis  102 . A worst case scenario point  410  occurs when the aperture  115  is displaced 5 mm in a direction perpendicular to the optical axis  102 . However, the maximum flux passing through the aperture at any given point is 20%. Thus, to receive a Class 1 rating, a VCSEL  101  may have an AEL level of up to 4 mW. 
     In another embodiment shown in  FIG. 5 , the modifying medium is a beam-splitting element  500 . The beam-splitting element  500 , which may include one or more prisms, is employed to reduce the amount of flux that passes through the 7 mm aperture  115  for any given viewing angle by refracting portions of the laser beam in different directions. When the collimated laser beam  106  enters the beam splitting element  500 , the collimated laser beam  106  is split into a first beam  501  and a second beam  502 . With an appropriate design in this embodiment, the two beam  501  and  502  diverge at an angle  510  large enough so that at most only one beam  501  or  502  passes through the 7 mm aperture 14 mm away at any given viewing angle. Thus, the minimum angle  510  between any two beams  501  and  502  is approximately 24.2 degrees. This effectively doubles the AEL level allowed for the VCSEL  101  beam  106  to still be within the Class 1 rating. 
     In another embodiment, the beam-splitting element  500  may split the collimated laser beam  106  into more than two beams. If the collimated laser beam  106  is split into n separate beams, the maximum allowable AEL level of the beam  106  will be 0.78 mW×n. This assumes that no more than one of the n beams passes through any 7 mm aperture at a radial distance of 14 mm. In the example shown in  FIG. 5 , the maximum AEL level of the beam  106  is calculated to be equal to 2×0.78 mW=1.56 mW maximum VCSEL power. Other possible beam splitting devices  500  include conventional diffractive and holographic elements or multiple refractive lenses. 
     In yet another embodiment, the modifying medium  107  may be a diffraction grating  600  as shown in  FIG. 6A . The diffraction grating  600  is a two-dimensional diffraction element that includes a double exposure of its photo-resist layer coating on a glass substrate, an x-direction exposure  601  and a y-direction exposure  602 . A well-known holographic exposure method in which two collimated UV laser beams impinge on the photo-resist surface at a known angle is used to create the two-dimensional diffraction grating  600 . In this method, the interference of the two equal-power collimated ultraviolet (UV) laser beams creates a sinusoidal intensity pattern whose period depends on the angle between the beams. The greater the angle, the smaller the period. The first exposure  601  creates a latent image of a sinusoidal phase grating along the x-direction, wherein the sine wave is with respect to the depth of the cut. After rotating the substrate by 90 degrees, a second exposure  602  creates a grating along the y-direction (also sinusoidal with respect to the depth of the cut). The final result is the two-dimensional diffraction grating  600  wherein the photo-resist layer pattern is a surface relief grating with a nearly sinusoidal groove shape in both x and y directions. [MSOffice1] 
     Referring to  FIG. 6B , when the collimated laser beam  106  passes through such a two-dimensional diffraction grating  600 , the collimated laser beam  106  is diffracted into an array of divergent laser beams  615  which emanate from the two-dimensional diffraction grating  600  at various angles. If the divergent laser beams  615  were to be displayed on a distant wall for viewing, an array of light spots  616  would be seen. For the purposes of this discussion, the spots  616  are labeled in a simple x-y axis beginning with the center spot  620  at (0, 0). The spot from the first divergent laser beam  621  to the right is (1, 0), and so on. As the distance between the display wall and the laser source becomes larger, the distance between spots also becomes larger. 
     The angles between the divergent laser beams  615  are determined by the period of the two-dimensional diffraction grating  600 . The angular separation is approximately λ/Λ x  for the x-direction, where Λ x  is the period of the x-grating  601  and λ is the laser wavelength. There is an identical expression for the diffraction angle for the y-grating  602 . 
     In one example, if Λ is 15 microns and the wavelength of the collimated laser beam  106  is 850 nm, then the angle between grating orders is about 56.7 milliradians or about 3.25 degrees.  FIG. 6B  shows a diffraction pattern for equal x-grating  601  and y-grating  602  periods ( FIG. 6A ). The flux in the higher orders  650  decreases gradually with grating order and the higher orders  650  extend to nearly 90 degrees from the normal of the two-dimensional diffraction grating  600 . The fractional power diffracted into the (p,q) order can be shown ( Introduction to Fourier Optics,  J. W. Goodman, McGraw Hill, 1968) to be: 
                 J   q   2     ⁡     (     m   2     )       ⁢       J   p   2     ⁡     (     m   2     )             
where J is the Bessel function of the first kind, the order is p or q, and m is the peak-to-peak phase delay of the two-dimensional diffraction grating  600 , which is proportional to groove depth. In this embodiment, the x-grating  601  and the y grating  602  groove depths are the same, but in other embodiments, they may be different. If m=8 radians, then the maximum flux in the zero order laser beam  620  is about 2.5% of the flux of the collimated laser beam  106 . The flux in orders (0,1), (1,0), (0, −1), and (−1,0)  621  is about 0.07% of the flux of the collimated laser beam  106 . In orders (1,1), (1, −1), (−1,1), and (−1, −1) the flux is about 0.0019% of the flux of the collimated laser beam  106 . The deep grooves spread the flux into many higher order  650  laser beams. If the two-dimensional diffraction grating  600  is made shallower, i.e., reducing m, then the flux in the lower orders will rise. For example, reducing m to 2 increases the flux in the zero-order laser beam  620  to 34% of the collimated laser beam  106  beam.
 
     An advantage of using a two-dimensional diffraction grating  600  as the modifying medium  107  in the system of  FIG. 1  is that by designing the spatial frequency, wherein the angle between diverging laser beams  615  is high enough, typically only one of the diffraction orders can pass through the 7 mm aperture  115 . Therefore, for a 7 mm aperture  115  which is 14 mm away from two-dimensional diffraction grating  600 , the angle between divergent laser beams  615  is typically greater than about 24.2 degrees. Even if the angle between the divergent laser beams  615  is less than 24.2 degrees, so that multiple divergent laser beams  615  may enter the 7 mm aperture  115 , the divergent laser beams  615  will still not cause damage to the human eye. The reason for this is that the divergent laser beams  615  will not focus to a single spot on the retina, but rather an array of spots  616 . Since eye damage is typically caused by localized heating of the retina, spreading the light into an array of spots  616  reduces the possibility of damage to the eye, thus increases the maximum allowable AEL level for a VCSEL  101 . 
     Because only one divergent laser beam  615  will pass through the aperture  115  at any one angle of incidence, only the strongest divergent laser beam  615  (the zero-order laser beam  620  (0,0)) needs to be taken into consideration for the eye-safety standard since all other divergent laser beams  615  have a lower magnitude. As such, for a grating modulation of m=8, the flux of the undiffracted laser beam  620  is about 2.5% of the flux of the collimated laser beam  106  as discussed above. Therefore, an acceptable AEL level for a VCSEL  101  in this embodiment may be 40 times the Class 1 rating. Even if m=2, the AEL level for a VCSEL  101  may be about 3 times the Class 1 rating. 
       FIG. 7  is a block diagram of a general-purpose computer system  720  that includes an optical mouse  742  that incorporates the apparatus of  FIG. 1  according to an embodiment of the invention. The computer system  720  (e.g., personal or server) includes one or more processing units  721 , system memory  722 , and a system bus  723 . The system bus  723  couples the various system components including the system memory  722  to the processing unit  721 . The system bus  723  may be any of several types of busses including a memory bus, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory  722  typically includes read-only memory (ROM)  724  and random-access memory (RAM)  725 . Firmware  726  containing the basic routines that help to transfer information between elements within the computer system  720  is also contained within the system memory  722 . The computer system  720  may further include a hard disk-drive system  727  that is also connected to the system bus  723 . Additionally, optical drives (not shown), CD-ROM drives (not shown), floppy drives (not shown) may be connected to the system bus  723  through respective drive controllers (not shown) as well. 
     A user may enter commands and information into the computer system  720  through input devices such as a keyboard  740  and an optical mouse  742 . These input devices as well as others not shown are typically connected to the system bus  723  through a serial port interface  746 . Other interfaces (not shown) include Universal Serial Bus (USB) and parallel ports  740 . A monitor  747  or other type of display device may also be connected to the system bus  723  via an interface such as the graphics card  789 .