Abstract:
A dual-axis optical encoder device is disclosed. The optical encoder includes a substrate having a first surface and a second surface opposite the first surface and a first optical encoder on the first surface of a substrate and a second optical encoder on the second surface of the substrate. Each optical encoder includes an optical emitter and an optical detector. The dual-axis optical encoder device provides, within a single device, the ability to acquire movement information on two different axes.

Description:
The present invention relates to an optical encoder device for dual-axis encoding applications. 
     A dual-axis encoding application refers to an application where information of a motion on two different axes are acquired and processed. An example of such dual-axis encoding application can be found in a printer. The position of the printer head of the printer with respect to a print medium can be tracked by obtaining the position of the roller which feeds the print medium into the printer, and the position of the printer head in the printer. The position of the roller can be obtained by monitoring the amount of rotation of the roller about an axis, and the position of the printer head in the printer can be obtained by monitoring the movement of the printer head along another axis in the printer. By obtaining the information from these two axes, the position of the printer head with respect to the print medium can thus be obtained. A dual-axis optical encoder device used in this specification shall be used to refer to the optical encoder device for dual axis encoding application according to the invention. 
     An encoder is a device that provides feedback to a closed loop system. The encoder enables a signal interpretation such as to obtain information on a position, velocity, an acceleration and/or the like when the encoder works in pair with a codewheel or a codestrip. Codewheels are generally used for detecting the rotation motion, for example of the paper feeder drum in a printer or a copy machine, while codestrips are used for detecting the linear motion, for example of the print head of the printer. 
     Usually, the motion of the codewheel or the codestrip is detected optically by means of an optical emitter and an optical detector. Therefore, the encoder is usually an optical encoder. The optical emitter emits light in a light emission direction towards the codewheel/codestrip. The codewheel/codestrip comprises a regular pattern of slots and bars. According to the position of the slots and bars, relative to the light emission direction, the codewheel/codestrip permits, reflects or prevents light from passing through. The optical detector detects the light that is transmitted or reflected by the codewheel/codestrip and provide an unambiguous information on the motion of the codewheel/codestrip based on the detected light signal. 
     Optical encoders are generally classified into transmission-based optical encoders and reflection-based encoders. 
       FIG. 1  shows a typical transmission-based optical encoder  100 . The encoder  100  comprises an optical emitter  101  and an optical detector  102  in a housing material  104 . An optical lens  106  is provided in the housing material  104  directly below the optical emitter  101  to collimate light emitted by the optical emitter  101  into parallel light  105 . A free area  107  is provided between the optical emitter  101  and the optical detector  102  and a codewheel/codestrip  103  is free to rotate or move inside the free area  107 . The light emitted by the optical emitter  101  is collimated by the optical lens  106 , transmitted through the free area  107  and the codewheel/codestrip  103  and detected by the optical detector  102 . 
     A typical reflection-based optical encoder is shown in  FIG. 2 . The encoder  200  has an optical emitter  201  and an optical detector  202  mounted on a leadframe  207 , which are encapsulated in an expoxy optical element  204 . The optical element  204  has two dome-shaped surfaces, with the first dome-shaped surface  205  directly above the optical emitter  201  and the second dome-shaped surface  206  directly above the optical detector  202 . The codewheel/codestrip  203  is placed outside the optical element  204 , above the dome-shaped surfaces  205 ,  206 . The light emitted by the optical emitter  201  is transmitted through the dome-shaped surface  205  and is concentrated or collimated by the surface  205  into parallel light  208 , reflected by the codewheel/codestrip  203 , transmitted through the dome-shaped surface  206 , and is converged by the surface  206  to the optical detector  202 . 
     The described optical encoders are only able to provide information on a single axis, i.e. for single-axis encoding applications. For example, an optical encoder can be used with a codewheel connected to a roller of a printer to determine the position of the paper in the printer. To determine the position of the printer head of the printer with respect to the paper, a separate optical encoder to be used with a codestrip is needed. Therefore, for dual axis encoding applications like the printer or for tracking the position of a mouse of a computer, two encoders are required, resulting in a higher number of processes, piece parts and a larger operational space. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the invention to provide an optical encoder device that can simplify the manufacturing operations, reduce the number of processes, minimize the number of piece parts and minimize the operational space for dual-axis encoding applications. 
     The object is achieved by a dual-axis encoder device which comprises a first optical encoder on a first surface of a substrate and a second optical encoder on a second surface of the substrate, wherein an optical emitter-detector pair of the first optical encoder and an optical emitter-detector pair of the second optical encoder are mounted on the first surface and the second surface of the substrate, respectively. 
     In the dual-axis optical encoder device according to the invention, a codewheel/codestrip can work in pair with the first optical encoder to provide feedback information on one axis of the dual-axis encoding application, and another codewheel/codestrip can work in pair with the second optical encoder to provide feedback information on another axis of the dual-axis encoding application. The codewheel/codestrips of the first and second optical encoders of the dual-axis encoder device are coupled indirectly by external devices to the two axes of the dual-axis encoding application of the dual-axis encoding application that transform the direction of movement or rotation of the two axes into the direction and rotation of the codewheel/codestrips. 
     Therefore, in the dual-axis encoding device according to the invention, two optical encoders originally required for the dual-axis encoding application are integrated into a single encoder and therefore, the space required for encoder mounting and encoding operation is reduced. 
     A further advantage of integrating two separate optical encoders into a dual-axis optical encoder device for dual axis encoding applications is that fewer piece parts are required. The manufacturing operations, and hence the product cost for the device is also reduced since two separate encoders can be merged into one. 
     The substrate used may be a leadframe, an insert-molded leadframe, a double-sided printed circuit board (PCB), a ceramic substrate or a micro-interconnected device (MID) wherein the optical emitter and the optical detector can be mounted on both the first surface, for example a top surface, and the second surface, for example a bottom surface, of the substrate. A flat substrate is preferred as it gives a more compact design of the dual-axis optical encoder device according to the invention. Therefore, a leadframe is used as the substrate in the preferred embodiment of the invention, as it is slimmer compared to the other types of substrates, resulting in a smaller and more compact dual-axis optical encoder device. A leadframe substrate is also less expensive compared to the other types of substrate. 
     In the preferred embodiment of the invention, the optical emitter-detector pair of both the first optical encoder and the second optical encoder are arranged in a parallel direction on the first and second surface of the substrate, respectively. The direction of the optical emitter and optical detector pair is defined as the line intersecting both the optical emitter and the optical detector. Also, the optical emitter-detector pair of the first optical encoder on the first surface of the substrate is arranged such that it is directly above the optical emitter-detector pair of the second optical encoder on the second surface of the substrate. This arrangement allows a highest compact design of the dual-axis optical encoder device according to the invention. 
     The light emitted by the optical emitter follows an optical path from the optical emitter to the optical detector of the same optical emitter-detector pair. An optical element, which is a 3-dimensional epoxy encapsulation is provided for enclosing both the optical emitter-detector pair. The optical element has two internal reflecting surfaces arranged such that the light emitted by the optical emitter is reflected by the first surface to the second surface of the optical element, and is further reflected by the second surface of the optical element to the optical detector. Therefore, the optical element is used to control the optical path so that the optical path stays within the optical element, and is substantially U-shaped. In this way, the size of the dual-axis encoder device according to the invention is limited to the height of the optical element on the substrate, resulting in a more compact device. 
     In the preferred embodiment of the invention, a free area is provided in the optical element between the optical detector and the second internal reflecting surface for accommodating a codewheel/codestrip. This arrangement ensures that the encoder is maintained in its compact size and do not extend beyond and above the encoder device. Also in this embodiment, the codewheel/codestrip is nearer to the optical detector and therefore, optical diffractions and the response time to the movement of the codewheel/codestrip are reduced. 
     Alternative embodiments are possible for the free area for accommodating the codewheel/codestrip to be provided between the first surface and the second surface, resulting the orientation of the codewheel/codestrip to differ from the preferred embodiment. This alternative embodiment may be suitable if it is more convenient to arrange at least one of the codewheel/codestrip of the first and second optical encoders in a different orientation from the preferred embodiment when the size or compactness of the dual-axis optical encoder device is not of great importance. 
     It should be noted that alternative embodiments for the dual-axis according to the invention are also possible by using different design configurations for the first and/or second optical encoders on the first and second surface of the substrate, respectively. For example, the dual-axis optical encoder device allows the flexibility of using the reflection-based encoder as described in  FIG. 2  as the first and/or second optical encoder. However, using the reflection-based encoder described in  FIG. 2  for the first and/or second optical encoder in the dual-axis encoder device will result in the device to be less compact, and the size of the device will be extended beyond the optical element to the codewheel/codestrip which is positioned outside and above the optical element. Furthermore, the reflection-based encoder does not perform as well because part of the optical path of the light in the reflection-based encoder is outside the optical element, and hence more susceptible to external environment factors, like vibrations, that may cause the distance or the alignment between the codewheel/codestrip and the optical element to fluctuate. 
     The dual-axis optical encoder device according to the invention also provides the flexibility of allowing the optical emitter-detector pair of the first optical encoder and the second encoder to be arranged on the substrate in different directions with respect to each other in further alternative embodiments. Such arrangement, although compromising on the compactness of the device, is necessary for example when the circuitries present on the substrate are arranged such that the first and second optical encoders are not able to be arranged in the same direction. 
     The other features and advantages of the invention will become apparent from the following descriptions of the preferred and alternative embodiments and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference numbers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-section through a typical transmission-based optical encoder. 
         FIG. 2  shows a cross-section through a typical reflection-based optical encoder. 
         FIG. 3  shows a cross-section through a dual-axis optical encoder device according to the preferred embodiment of the invention. 
         FIG. 4  shows a cross-section through a dual-axis optical encoder device according to the first alternative embodiment of the invention. 
         FIG. 5  shows a cross-section through a dual-axis optical encoder device according to the second alternative embodiment of the invention. 
         FIG. 6  shows a cross-section through a dual-axis optical encoder device according to the third alternative embodiment of the invention. 
         FIG. 7  shows a cross-section through a dual-axis optical encoder device according to the fourth alternative embodiment of the invention. 
         FIG. 8  shows a cross-section through an optical encoder mounted on an insert-molded leadframe with the free area accommodated between the second flat surface and the optical detector. 
         FIG. 9  shows a cross-section through an optical encoder mounted on an insert-molded leadframe with the free area accommodated between the first and the second flat surface. 
         FIG. 10  shows a cross-section through a dual-axis optical encoder device according to the invention, with the first optical encoder arranged orthogonal to the second optical encoder. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment and other alternative embodiments of the invention will now be described in details with the accompanying drawings. 
     According to the invention, a dual-axis optical encoder device is proposed, wherein two optical encoders are integrated on a single substrate. A first optical emitter and detector pair belonging to a first optical encoder are mounted on a first surface of a substrate, and a second optical emitter and detector pair belonging to a second optical encoder are mounted on a second surface of the substrate. 
     The substrate is preferably flat, has two surfaces for the optical emitter and detector pair of the first and second optical encoder to be mounted on. The substrate may be a leadframe, an insert-molded leadframe, a double-side PCB, a ceramic substrate or a micro-interconnecting device, wherein an optical encoder can be mounted on each side. In the preferred embodiment of the invention, a leadframe which is essential flat, is used as the substrate. 
     The dual-axis optical encoder device according to the preferred embodiment is shown in  FIG. 3 . 
     In the optical encoder device  300  according to the preferred embodiment of the invention, a first optical emitter  306  and a first optical detector  307  are mounted directly on the first surface of the leadframe  301 . A second optical emitter  308  and a second optical detector  309  are mounted directly on the second surface of the leadframe  301  and the optical emitter  308  and the optical detector  309  of the second encoder are arranged such that they are in parallel direction with the first optical emitter  306  and detector  307  of the first optical encoder. Also, the first optical emitter  306  and optical detector  307  on the first surface of the leadframe  301  are directly above the direction of the second optical emitter  308  and optical detector  309  which are on the second surface of the leadframe  301 . The direction of the optical emitter and optical detector pair is defined as the line intersecting both the optical emitter and the optical detector. 
     A first optical element  314  is provided for the first optical encoder on the first surface of the leadframe  301 . The first optical element  314  is a three-dimensional epoxy-filled encapsulation which encapsulates the first optical emitter  306  and the optical detector  307  on the first surface of the leadframe  301 . The first optical element  314  has a first surface  302  and a second surface  303  facing each other. The first surface  302  has a three-dimensional parabolic form and the second surface  303  is flat and arranged at an angle of 45° with respect to the direction of the light emitted from the first optical emitter  306  to the first surface  302 . The first surface  302  is arranged above the first optical emitter  306 , and the second surface  303  is arranged above the first optical detector  307 . 
     Light is emitted by the first optical emitter  306  in the direction towards the first surface  302  of the first optical element  314  and travels along an optical path towards the first optical detector  307 . The light emitted by the first optical emitter  306  travels along the optical path to the first surface  302 , and is reflected and collimated by the first surface  302  to the second surface  303 . The second surface  303  reflects the light to the first optical detector  307 . Therefore, the optical path of the light from the first emitter  306  to the first optical detector  307  is substantially U-shaped, with the first and second surfaces  302 ,  303  of the first optical element  314  acting as internal reflecting surfaces. 
     A free area  312  is arranged inside the first optical element  314  between the second surface  303  and the first optical detector  307  and a codewheel/codestrip  310  is accommodated within the free area  312 . The codewheel/codestrip  310  comprises a plurality of alternating transparent and opaque encoding elements in a form of slots and bars (not shown), which encoding elements are arranged such that they are able to affect the optical path of the light emitted by the first optical emitter  306  passing through the free area  312  in the first optical element  314 . The codewheel/codestrip  310  moves or rotates in the free area  312  in a manner such that the encoding elements on the codewheel/codestrip  310  advances in a direction substantially orthogonal to the direction of the first optical emitter  306  and detector  307  pair of the first optical encoder. 
     The light emitted by the first optical emitter  306  therefore travels along the optical path in the optical element  314  towards the first surface  302 , and is reflected and collimated by the first surface  302  into a parallel light beam towards the second surface  303 . The second surface  303  reflects the parallel light beam into the free area  312  and onto the encoding elements of the codewheel/codestrip  310 . Part of the parallel light beam passes through the transparent portion of the encoding elements on the codewheel/codestrip  310  and travels toward the first optical detector  307 , and is subsequently detected by the first optical detector  307 . 
     To ensure a complete reflection at the first and second surfaces  302 ,  303  of the first optical element  314 , the first and second surfaces  302 ,  303  may be coated with reflective material to make the first optical element  314  less susceptible to manufacturing inaccuracies or fluctuations of the light emission direction during the operation of the first optical emitter  306 , and therefore, prevents an undesired light loss at these two surfaces  302 ,  303 . 
     A second optical element  315  is provided for the second optical encoder on the second surface of the leadframe  301 . The second optical element  315  is a three-dimensional epoxy-filled encapsulation which encapsulates the second optical emitter  308  and the second optical detector  309  on the second surface of the leadframe  301 . The second optical element  315  has a first surface  304  and a second surface  305  facing each other. The first surface  304  has a three-dimensional parabolic form and the second surface  305  is flat and arranged at an angle of −45° with respect to the direction of the light emitted from the second optical emitter  308  to the first surface  304  of the second optical element  315 . The first surface  304  of the second optical element  315  is arranged below the second optical emitter  308 , and the second surface  305  is arranged below the second optical detector  309 . 
     Light is emitted by the second optical emitter  308  in the direction towards the first surface  304  of the second optical element  315  and travels along an optical path towards the second optical detector  309 . The light emitted by the second optical emitter  308  travels along the optical path to the first surface  304  of the second optical element  315 , and is reflected and collimated by the first surface  304  to the second surface  305 . The second surface  305  reflects the light to the second optical detector  309 . Therefore, the optical path of the light from the second optical emitter  308  to the second optical detector  309  is also substantially U-shaped, with the first and second surfaces  304 ,  305  of the second optical element  315  acting as internal reflecting surfaces. 
     A free area  313  is arranged inside the second optical element  315  between the second surface  305  and the second optical detector  309  and a codewheel/codestrip  311  is accommodated within the free area  313 . The codewheel/codestrip  311  comprises a plurality of alternating transparent and opaque encoding elements in a form of slots and bars (not shown), which encoding elements are arranged such that they are able to affect the optical path of the light emitted by the second optical emitter  308  passing through the free area  313  in the second optical element  315 . The codewheel/codestrip  311  moves or rotates in the free area  313  in a manner such that the encoding elements on the codewheel/codestrip  311  advances in a direction substantially orthogonal to the direction of the second optical emitter  308  and detector  309  pair of the second optical encoder. 
     The light emitted by the second optical emitter  308  therefore travels along the optical path in the second optical element  315  towards the first surface  304 , and is reflected and collimated by the first surface  304  into a parallel light beam towards the second surface  305 . The second surface  305  reflects the parallel light beam into the free area  313  and onto the encoding elements of the codewheel/codestrip  311 . Part of the parallel light beam passes through the transparent portion of the encoding elements on the codewheel/codestrip  311  and travels toward the second optical detector  309 , and is subsequently detected by the second optical detector  309 . 
     To ensure a complete reflection at the first and second surfaces  304 ,  305  of the second optical element  315 , the first and second surfaces  304 ,  305  of the second optical element  315  may also be coated with reflective material to make the second optical element  315  less susceptible to manufacturing inaccuracies or fluctuations of the light emission direction during the operation of the second optical emitter  308 , and therefore, prevents an undesired light loss at these two surfaces  304 ,  305 . 
       FIG. 4  shows a first alternative embodiment of the invention. The optical encoder device  320  in the first alternative embodiment is identical to the optical encoder  300  according to the preferred embodiment described in  FIG. 3 , except that the free area  313  of the second optical encoder in this alternative embodiment is provided between the first surface  304  and the second surface  305 . In this case, the light emitted by the second optical emitter  308  therefore travels along the optical path in the second optical element  315  towards the first surface  304 , and is reflected and collimated by the first surface  304  into a parallel light beam into the free area  313  and onto the encoding elements of the codewheel/codestrip  311 . Part of the parallel light beam passes through the transparent portion of the encoding elements on the codewheel/codestrip  311  and travels towards the second surface  305 , and is reflected by the second surface  305  to the second optical detector  309  to be detected. 
     The first alternative embodiment described in  FIG. 4  above is suitable when it is more convenient to arrange at least one of the codewheel/codestrip of the first and second optical encoders in a different orientation from the preferred embodiment in  FIG. 3 . 
     It should be pointed out again that the dual-axis optical encoder device according to the invention allows the flexibility of using the reflection-based encoder described in  FIG. 2  as at least one of the first optical encoder and second optical encoder as alternative embodiments. 
       FIG. 5  shows a cross section through a dual-axis optical encoder device according to a second alternative embodiment of the invention, wherein the reflection-based optical encoder described in  FIG. 2  is used as the first optical encoder. 
     In the optical encoder device  330  according to the second alternative embodiment of the invention, the first optical emitter  306  and the first optical detector  307  are mounted on the first surface of the leadframe  301 , and the second optical emitter  308  and a second optical detector  309  are mounted on the second surface of the leadframe  301 . The arrangement of the first and second optical emitters  306 ,  308  and detectors  307 ,  309  on the leadframe  301  are the same as the arrangement in the preferred embodiment of the invention as described in  FIG. 3 . 
     The first optical element  314  provided for the first optical encoder in this embodiment is also a three-dimensional epoxy-filled encapsulation over the first optical emitter  306  and the optical detector  307  on the first surface of the leadframe  301 . However, the first optical element  314  in this embodiment has a first three-dimensional dome-shaped surface  302  and a second dome-shaped surface  303  arranged adjacent to each other, and directly over the first optical emitter  306  and the first optical detector  307  respectively. The first dome-shaped surface  302  acts as a light concentrator or collimator for light emitted by the first optical emitter  306  and the second dome-shaped surface  303  serves to converge a reflected light beam onto the first optical detector  307 . The codewheel/codestrip  310  is arranged outside the first optical element  314 , such that the two dome-shaped surfaces  302 ,  303  are between the codewheel/codestrip  310  and the leadframe  301 . 
     The light emitted by the first optical emitter  306  travels in the optical path in the first optical element  314  towards the first dome-shaped surface  302  and is concentrated or collimated by the first dome-shaped surface  302  into an at least substantially parallel light beam. The at least substantially parallel light beam travels toward the codewheel/codestrip  310  and depending on the encoding elements on the codewheel/codestrip  310 , a part of the at least substantially parallel light beam is reflected towards the second dome-shaped surface  303  of the first optical element  314 . The reflected parallel light beam enters the second optical element  314  through the second dome-shaped surface  303  and is converged by the second dome-shaped surface  303  onto the optical detector  307 . As can be seen, the optical path of the light for the first optical encoder is substantially V-shaped. 
     The second optical encoder in this alternative embodiment is identical to the second optical encoder in the preferred embodiment as described in  FIG. 3 , and will not be described again. 
       FIG. 6  shows a cross-section of a dual-axis optical encoder device  340  according to a third alternative embodiment of the invention. 
     The arrangement of the optical encoder device  340  is similar to the optical encoder device  330  described in  FIG. 5 , except that the free area  313  of the second optical encoder of the optical encoder device  340  is provided between the first surface  304  and the second surface  305 . 
     The other parts of the optical encoder device  340  are identical to the optical encoder device  330  in the second alternative embodiment described in  FIG. 5 , and will not be described again. 
       FIG. 7  shows a cross-section of a dual-axis optical encoder device  350  according to a fourth alternative embodiment of the invention. 
     The dual-axis optical encoder device  350  uses the reflection-based optical encoder described in  FIG. 2  for the first and the second optical encoders on both the first surface and the second surface of the leadframe  301 . 
     The second optical element  315  provided for the second optical encoder in this embodiment is also provided as a three-dimensional epoxy-filled encapsulation over the second optical emitter  308  and the second optical detector  309  on the second surface of the leadframe  301 . The second optical element  315  in this embodiment has a first three-dimensional dome-shaped surface  304  and a second dome-shaped surface  305  arranged adjacent to each other, and directly below the second optical emitter  308  and the second optical detector  309  respectively. The first dome-shaped surface  304  acts as a light concentrator or collimator for light emitted by the second optical emitter  308  and the second dome-shaped surface  305  serves to converge a reflected light beam onto the second optical detector  309 . The codewheel/codestrip  311  is arranged outside the second optical element  315 , such that the two dome-shaped surfaces  304 ,  305  are between the codewheel/codestrip  311  and the leadframe  301 . 
     The light emitted by the second optical emitter  308  travels in the optical path in the second optical element  315  towards the first dome-shaped surface  304  and is concentrated or collimated by the first dome-shaped surface  304  into an at least substantially parallel light beam. The at least substantially parallel light beam travels toward the codewheel/codestrip  311  and depending on the encoding elements on the codewheel/codestrip  311 , a part of the at least substantially parallel light beam is reflected towards the second dome-shaped surface  305  of the second optical element  315 . The reflected parallel light beam enters the second optical element  315  through the second dome-shaped surface  305  and is converged by the second dome-shaped surface  305  onto the optical detector  309 . The optical path of the light for the first optical encoder is substantially V-shaped. 
     Further alternative embodiments can be derived by replacing at least one of the first and second optical encoders of the described embodiments with the optical encoders shown in  FIG. 8  or  FIG. 9 . 
       FIG. 8  shows an optical encoder for use in further alternative embodiments of the invention. 
     The optical encoder  400  comprises an insert-molded leadframe  407 , an optical emitter  401 , an optical detector  402 , an optical element  404  and an optical lens  412 . The optical element  404  in this case is arranged directly on the insert-molded leadframe  407  and encloses an air gap  411 . Hence it does not encapsulate the optical emitter  401  and the optical detector  402 . The optical emitter  401  and the optical detector  402  are each enclosed in an encapsulation capsule  410  and are arranged on the insert-molded leadframe  407  in the air gap  411 . The optical element  404  has a first flat surface  405  and a second flat surface  406 , wherein the first flat surface  405  is arranged above the optical emitter  401 , and the second flat surface is arranged above the optical detector  402 . The optical emitter  401  emits light in the direction towards the first flat surface  405 , and the optical detector  402  detects light from the direction of the second flat surface  406 . The optical lens  412  is arranged in the air gap  411 , in the path of the light emitted by the optical emitter  401  to collimate the emitted light into a parallel light beam. The light, which is emitted by the optical emitter  401 , travels along optical path  409  inside the optical element  404  towards the optical detector  402 . 
     The first flat surface  405  encloses a first angle of −45° with respect to the direction of the light emitted by the optical emitter  401 , and faces both the optical emitter  401  and the second flat surface  406 . The second flat surface  406  encloses a second angle of +45° with respect to the direction of the light emitted by the optical emitter  401 , and faces both the optical detector  402  and the first flat surface  405 . 
     A free area  408  is arranged inside the optical element  404  between the second flat surface  406  and the optical detector  402 , and a codewheel/codestrip  403  is accommodated within the free area  408 . 
     The first flat surface  405  and the second flat surface  406  act as internal reflecting surfaces, so that light incident on the surfaces is reflected. The light emitted by the optical emitter  401  is collimated into parallel light by the optical lens  412  and travels along the optical path  409  in the optical element  404 . The parallel light is reflected by the first flat surface  405  towards the second surface  406 , and is again reflected by the second surface  406  into the free area  408 . Part of the parallel light which is transmitted through the transparent encoding elements of the codewheel/codestrip  403  travels towards the optical detector  402  and is subsequently detected by the optical detector  402 . The optical path of the light emitted by the optical emitter  401  is substantially U-shaped. 
       FIG. 9  shows a cross-section of another optical encoder which can be used in at least one alternative embodiment of the invention. The optical encoder  420  in  FIG. 9  is similar to the optical encoder  400  shown in  FIG. 8 , except that the free area  408  is arranged in the optical element  404  between the first flat surface  405  and the second flat surface  406 . 
     When any of the optical encoders shown in  FIG. 8  or  FIG. 9  are used in the dual-axis encoder device according to the invention, an inserted-mold leadframe  407  as shown in  FIG. 8  or  FIG. 9  is used instead of the leadframe  301  as used in the preferred embodiment shown in  FIG. 3 . 
     The dual-axis optical encoder device according to the invention also provides the flexibility of allowing the optical emitter-detector pair of the first optical encoder and the second encoder to be arranged on the substrate in different directions with respect to each other. Such arrangement, although compromising on the compactness of the device, is necessary for example when the circuitries on the substrate are arranged such that the first and second optical encoders are not able to be arranged in the same direction. 
       FIG. 10  shows a cross-section of a dual-axis optical encoder  500  when the first optical emitter  306  and the first optical detector  307  of the first optical encoder belonging to the dual-axis optical encoder  330  in  FIG. 5  are arranged in a direction orthogonal to the direction of the second optical emitter  308  and optical detector  309  of the second optical encoder. The parts and operation of the dual-axis optical encoder device  500  are identical to the dual-axis optical encoder device  330  described in  FIG. 5 . It should be noted that the orientation of the codewheel/codestrip  310  for the first optical encoder need not be changed even though the orientation of the first optical encoder is changed. This is because the encoding elements on the codewheel/codestrip  310  are still able to affect the optical path. However, the orientation of the photodiodes used in the optical detector  307  needs to be changed (not shown) correspondingly in order to detect the pattern of the light reflected by the codewheel/codestrip  310 . 
     The dual-axis optical encoder device according to the above described embodiments can thus be used to provide feedback information of a dual-axis encoding application, for example the position of the cursor controlled by a mouse of a computer. In this example, the codewheel/codestrip  310  of the first optical encoder can be used to provide feedback information on the movement of the mouse along one axis, for example the X-axis, and the codewheel/codestrip  311  of the second optical encoder can be used to provide feedback information on the movement of the mouse along another axis, for example the Y-axis. In this way, the movement and hence the new position of the cursor on the X-Y axis can be determined, without having to use two separate optical encoders. 
     The optical emitter used according to the invention is a light emitting diode, and the optical detector is an array of photodiodes strips. The first and/or second optical encoders further comprises a signal processor for processing the signals which are generated by the optical detector on the basis of movement of the codewheel/codestrip. 
     While the described embodiments of the invention have been described, they are merely illustrative of the principles of the invention. Other embodiments and configurations, including the various combinations and orientations of the optical encoders, may be devised without departing from the spirit of the invention and the scope of the appended claims.