Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-11-06
2004-03-16
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S222100, C250S224000, C345S157000, C345S166000, C345S173000, C345S174000, C345S175000, C345S158000, C345S163000, C356S028500, C356S496000
Reexamination Certificate
active
06707027
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of measuring the movement of an input device and an object relative to each other along at least one measuring axis, the method comprising the steps of:
illuminating an object surface with a measuring laser beam for each measuring axis, and
converting a selected portion of the measuring beam radiation reflected by the object surface into an electric signal representative of the movement along said measuring axis.
The invention also relates to an input device provided with an optical module for carrying out the method, and to an apparatus comprising such an input device.
2. Description of the Related Art
Such a method and input device known from European Patent Application No. EP-A 0 942 285, corresponding to U.S. Pat. Nos. 6,246,482, 6,330,057, 6,424,407 and 6,452,683. The input device may be an optical mouse used in a computer configuration to move a cursor across the computer display or monitor, for example, to select a function of a displayed menu. Such an optical mouse is moved across a mouse pad by hand, like a conventional mechanical mouse. As described in EP-A 0 942 285, the input device may also be an “inverted” optical mouse. The input device is then stationary and, for example, built in the keyboard of a desktop, notebook or palm computer and a human finger is moved over, for example, a transparent window in the housing of the input device. In the latter case, the input device may be small, because the optical module for measuring the finger movement can be made very small. In fact, the input device is reduced to the optical measuring module. This opens the way to new applications for the input device. For example, an input function can be built in a mobile phone for selecting functions on a menu and for accessing Internet pages, or in a remote control device for a TV set for the same purposes, or in a virtual pen.
EP-A 0 942 285 discloses several embodiments of the optical measuring module, in all of which homodyne or heterodyne detection is used. All embodiments comprise a diffraction grating arranged close to the module window. The grating reflects a portion of the illumination beam radiation, preferably radiation diffracted in one of the first orders, to a detector which also receives a portion of the radiation reflected and scattered by the object surface. The laser radiation diffracted in the first order by the grating is denoted a local oscillator beam, and the detector coherently detects the radiation from the object surface using this local oscillator beam. The interference of the local oscillator beam and the radiation reflected by the object surface reaching the detector gives rise to a beat signal from the detector, this beat signal being determined by the relative motion of the object surface parallel to this surface. The optical measuring module of EP-A 0 942 285 comprises, besides the grating, a collimator lens, a focusing lens and a pinhole diaphragm, preceding the detector, these elements having to be aligned very accurately. This complicates the manufacture and increases the cost of the module, which is intended to be a mass-produced consumer product.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method as described in the opening paragraph, which is based on another detection principle and allows the use of an optical configuration with fewer components, and is easier to manufacture. This method is characterized in that measuring beam radiation reflected back along the measuring beam and re-entering the laser cavity emitting the measuring beam, is selected, and in that changes in operation of the laser cavity, which are due to interference of the re-entering radiation and the optical wave in the laser cavity and are representative of the movement, are measured.
This new method of measuring the relative movement of an input device and an object, for example, a human finger or another object, uses the so-called self-mixing effect in a diode laser. This is the phenomenon that radiation emitted by a diode laser and re-entering the cavity of the diode laser induces a variation in the gain of the laser and thus in the radiation emitted by the laser. The object and the input device are moved relative to each other such that the direction of movement has a component in the direction of the laser beam. Upon movement of the object and the input device, the radiation scattered by the object gets a frequency different from the frequency of the radiation illuminating the object, because of the Doppler effect. Part of the scattered light is focused on the diode laser by the same lens that focuses the illumination beam on the object. Because some of the scattered radiation enters the laser cavity through the laser mirror, interference of light takes place in the laser. This gives rise to fundamental changes in the properties of the laser and the emitted radiation. Parameters which change due to the self-coupling effect, are the power, the frequency and the line width of the laser radiation and the laser threshold gain. The result of the interference in the laser cavity is a fluctuation of the values of these parameters with a frequency that is equal to the difference of the two radiation frequencies. This difference is proportional to the velocity of the object. Thus the velocity of the object and, by integrating over time, the displacement of the object can be determined by measuring the value of one of these parameters. This method can be carried out with only a few and simple components and does not require accurate alignment of these components.
The use of the self-mixing effect for measuring velocities of objects, or, in general, solids and fluids, is known per se. By way of example, reference is made to the article: “Small laser Doppler velocimeter based on the self-mixing effect in a diode laser” in Applied Optics, Vol. 27, No. 2, Jan. 15, 1988, pages 379-385, and the article. “Laser Doppler velocimeter based on the self-mixing effect in a fiber-coupled semiconductor laser: theory” in Applied Optics, Vol. 31, No.8, Jun. 20, 1992, pages 3401-3408. However, up to now, use of the self-mixing effect in an input device as defined above has not been suggested. This new application is based on the recognition that a measuring module using the self-coupling effect can be made so small and cheap that it can be installed easily and without much additional cost in existing devices and apparatus.
In order to detect the direction of movement, i.e., to detect whether the object moves forward or backward along the measuring axis, the method may be characterized in that the shape of the signal representing the variation in operation of the laser cavity is determined. This signal is an asymmetric signal and the asymmetry for a forward movement is different from the asymmetry for a backward movement.
Under circumstances where it is difficult to determine the asymmetry of the self-mixing signal, preferably another method is used. This method is characterized in that the direction of movement along said at least one measuring axis is determined by supplying the laser cavity with a periodically varying electric current and comparing first and second measuring signals with each other, these first and second measuring signals being generated during alternating first half-periods and second half-periods, respectively.
The wavelength of the radiation emitted by a diode laser increases, and thus the frequency of this radiation decreases, with increasing temperature, thus with increasing current through the diode laser. A periodically varying current through the diode laser in combination with radiation from the object re-entering the laser cavity results in a number of radiation pulses per half-period and thus in a corresponding number of pulses in the measured signal. If there is no relative movement of the input device and the object, the number of signal pulses is the same in each half-period. If the device and the object move relative to each other, the number of pu
Liess Martin Dieter
Vermeulen Olaf Thomas Johan Antonie
Weijers Aldegonda Lucia
Meyer David C
Porta David
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