Patent Document

FIELD OF THE INVENTION 
       [0001]    The present invention relates to a light detector attached on an optical fiber for an imaging head and a light detector at a distal tip of the optical fiber to provide feedback to a light source controller. 
       BACKGROUND OF THE INVENTION 
       [0002]    Optical heads for imaging emit a plurality of light spots on a light sensitive medium. The optical imaging head may be configured from an array of pigtailed laser diodes. Each laser diode is optically coupled to a proximal tip of a multi-mode optical fiber. The distal tips of the optical fibers are supported in a linear array by opto-mechanical means and imaged onto a printing plate. 
         [0003]    The power calibration of the optical head is traditionally done as follows, the optical head is moved and adjusted in front of a light detector situated externally to the imaging head; and the power of each laser diode is then adjusted to emit the desired power intensity. This calibration is usually performed before each print. 
         [0004]    Prior art techniques currently monitor power from back reflected light at the proximal tip of the fiber. See, for example, U.S. Pat. No. 6,061,374 (Nightingale et al.). It would be desirable to measure the light along the distal tips of the fiber, which would detect different parameters, such as the loss of optical power along the fiber. 
       SUMMARY OF THE INVENTION 
       [0005]    Briefly, according to one aspect of the present invention a fiber optic imaging apparatus includes a light source; at least one optical fiber for transmitting light from the light source; a mechanical assembly for supporting at least one optical fiber; a detector which measures light transmitted by at least one optical fiber; and a controller for adjusting light intensity emitted from the light source according to a level of light detected by the light detector. 
         [0006]    The present invention provides a hybrid structure of a light detector and an optical fiber assembly. The optical fibers are densely assembled in a linear array. A light detector measures the light from this array and the measured results are used to adjust and monitor the optical power in real time by deploying a feedback mechanism. Additionally, improper measurement results can invoke an alarm to notify of hazardous safety situations. 
         [0007]    The present invention provides few unique features to the optical head. The combined structure of the optical head and the light detection means enable real time monitoring of the power and the shape of the pulse emitted from the distal tip of each fiber. 
         [0008]    Additionally, the light detector is placed within the same structure of the imaging head. This hybrid configuration enables instant alarm of hazardous situations. For example, a fault, such as a break along one of the fibers that can cause a fire in the machine, can be immediately identified. To avoid such situations, an interlock configured to sense the light detection measurements is automatically activated to shutdown the diode laser thus avoiding any damage or harm. This feature is important when it is used in conjunction with high power diode lasers. 
         [0009]    According to the present invention light is measured, along the distal tips of the fibers. The optical power measured along the distal tip of the fiber is proportional to the power emitted from the distal tip of the fiber. 
         [0010]    These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic illustrating measurement of light that is back reflected from the proximal tip of fiber in the prior art; 
           [0012]      FIG. 2  is a schematic illustrating light measurement along the distal tip of a fiber; 
           [0013]      FIG. 3  are graphs showing optical power emitted from the distal tips of a fiber versus the optical power measured along the distal tips of the fiber; 
           [0014]      FIG. 4A  is a plan view showing an end view of the optical fibers mechanical structure with a detector on top of the structure; 
           [0015]      FIG. 4B  is a side view illustrating an angled polished hybrid structure of the detection shown in  FIG. 4A ; 
           [0016]      FIG. 5  is a schematic illustrating a v-groove layout with detector on top of the v-groove; 
           [0017]      FIG. 6  is a schematic illustrating an imaging drum integrated with the detector along the distal tip of the fibers; and 
           [0018]      FIG. 7  is a schematic illustrating a hybrid structure with the detector and a light trap. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  and  FIG. 2  illustrates a rudimentary optical path. The optical path comprises a light source  12 , such as a laser diode. Micro-optics  13  couples the light generated by light source  12  into fiber  14 . The coupling of light can also be done by forming a micro-lens on the proximal tip of the fiber itself. Fiber  14  can be a single fiber or plurality of fibers arranged into a bundle of fibers. The light emitted from the distal tip of fiber  14  is propagated through imaging lens  18  and is imaged on the imaging plate  16 . 
         [0020]      FIG. 1  shows a prior art method, wherein light detector  11  is positioned at the beginning of the optical path, to measure the power that is back reflected from the micro-lens and the fiber proximal tip. An external light detector  15  is usually positioned in front of the imaging plate to measure laser emission  17 . This procedure is typically performed before each print for performing laser diode calibration. 
         [0021]      FIG. 2  illustrates one of the embodiments of the described invention, wherein the internal light detector  41  is positioned along the distal tips of the fibers. Internal light detector  41  measures the power of the light  45  emitted along the distal tip of fibers  47  as is depicted in  FIG. 4A . 
         [0022]    Reflective coating  46  may be applied on the internal surfaces of fibers mechanical housing structure  43  and/or fibers v-groove housing structure  53 , this is done in order to intensify the power of the light that will reach to internal light detector  41 . 
         [0023]    Measurements conducted in the lab showed a correlation between the power levels emitted from the distal tips of the fibers and the measured light  45  emitted along the fibers  47 . 
         [0024]    The hybrid structure of an internal light detector  41  and an optical fiber assembly  14  for the imaging head is described in  FIGS. 4A ,  4 B, and  5 . 
         [0025]    Referring to  FIG. 4A , fibers  47  are arranged in the mechanical housing structure  43 . The arrangement fibers  47  can also be arranged in a v-groove type structure, as is illustrated in  FIG. 5 . 
         [0026]    The fibers  47  are attached to a transparent fiber structure slab  42 . A transparent optical glue  50  with a suitable index of refraction may be used. An internal light detector  41  is attached to the top of transparent fiber structure slab  42  to measure the power of the light  45  formed along distal tips of the fibers  47 . 
         [0027]      FIG. 3  shows light powers as measured by detector  41  versus light powers emitted from the distal tips of fibers. The light power, plotted on the x-axis, was measured by internal light detector  41  along the distal tips of few fibers as a function of the light power, plotted on the y-axis, that was guided within the optical fibers and emitted from the distal tips of these fibers. In this specific case the bundle was constructed from  48  multimode optical fibers marked from channel  0  to channel  47 . The  48  optical fibers were aligned in a v-groove assembly and angled polished  492  in 8 degrees as is shown in  FIG. 4B . The pitch between the optical fibers was 250 microns. Regular silica fibers with stepped indexed profile of the index of refraction were used. The core diameter of the fibers was 60 microns and the cladding was 125 microns. A silicon detector in size of 10×10 millimeter 2 , was adjusted on top of the fiber array and used to measure the light. In this specific case a linear relationship can be seen between the light measured by internal light detector  41  along the distal tips and the light emitted from the distal tips. This is indicated by charts  31 ,  32 , and  33  for channels  0 ,  24 , and  44 , respectively. 
         [0028]    Internal light detector  41  measures one or more of the following light phenomena:
       1. light that is back reflected from the distal tip of the fiber;   2. light that is scattered along the distal tip of the fiber; and   3. leaky rays and evanescent waves emitted along the distal tip of the fiber.       
 
         [0032]    For this specific measurement, regular stepped indexed multimode silica fibers were used, but other types of optical fibers can be used as well, and the intensity of the light can be controlled by constructing fibers in various ways. For example, by adjusting the roughness  493  of the core  47   a  and clad  47   b  interface, the intensity of the scattered rays  491  can be controlled. The distal tips of the fibers can be angled, cleaved, or polished in order to control the light that is back reflected from these tips. The distal tips of the fibers can be coated using optical filters of various types in order to control the power of the transmitted and back reflected light. Scattering particles  490  may be formed within core  47   a  in order to control the amount of the scattered light. Grating formed within the core can be used to reflect part of the guided radiation toward internal light detector  41 . 
         [0033]    In the case where more then one wavelength is guided within the optical fiber, several detectors, each sensitive to a specific wavelength, can be aligned along the fiber in order to monitor each light source. 
         [0034]    This hybrid structure configuration provides few advantages:
       a. The detection of the light power levels, measured by internal light detector  41  along the distal tip, helps to calibrate the optical power needed to be generated by the light source  12  in order to form a good print.   b. The measurement of the light is performed along the distal tip of the fiber. This helps to detect malfunctioning light sources or cuts or breaks on fibers  47  along the entire fiber.   c. The hybrid structure enables performing light measurements simultaneously during a print or a print test procedure. On the contrary when using an external detector, adjusted aside to the printing plate, simultaneous measurements are not possible.   d. Properly and individually activating the light sources and performing simultaneous light measurements with the print enables fast alert of possible hazardous situations. In the case that such a hazardous state is detected, the laser sources will be automatically shut down by usage of interlocking means for example. A fast automatic shut down of the light source is vital for eye safety application and to prevent burns that may be caused by laser radiation.   e. Properly and individually activating the light source and performing simultaneous light measurements with the print enables real time monitoring of parameters such as optical powers, rise and fall times, and power stabilities.       
 
         [0040]    In order to better understand the disclosed invention, reference is made to  FIG. 6 , which illustrates an imaging drum  61  rotating in the direction of rotation axis  63 . An imaging substrate such as a printing plate  16  is mounted on imaging drum  61 . The disclosed optical emitting light with the light detector mechanism is shown in conjunction with the imaging drum  61 . 
         [0041]    Light is emitted by light source  12  and is coupled utilizing micro-optics  13  into optical fiber  14 . Further, along the distal tip of the optical fiber, light values are detected and measured by internal light detector  41 . The measured results are communicated via the measurement results line  65  into the light source intensity control device  64 . Light source intensity control device  64  will set the intensity of light source  12  via intensity control line  66  to conform with to the measured results in order to form a well balanced imaged spot  67  on printing plate  16 . 
         [0042]    The use of an internal light detector  41  as well as an external detector  15  to calibrate and monitor the optical head carries few advantages. Using both light detectors  15  and  41 , may lead to a more reliable and precise laser calibration and laser monitoring procedure. For example, reading different results from the detectors may indicate a malfunction in one of them, thus alerting detectors service event. 
         [0043]    For laser safety applications more than one light detector such as internal light detector  41  can be used. For example, a second light detector  48  can be placed along the proximal tip of the fiber and or at some other place along the fiber. Sensing emitted light from additional internal light detector  48  without any light sensed from internal light detector  41  may indicate a cut or a break somewhere along the fiber between the two adjacent detectors. 
         [0044]    Additionally, the readings from internal light detectors  41  and  48  can also be compared to the readings of light detector  11 , that measures the back reflected light, or to electrical signals such as the current and voltage of the light source. The reading of external light detector  15  can be also used in comparison to the current and voltage of the light source or to the reading of internal light detectors  41 . Reading more than one light detector and using an adequate algorithm to analyze the results will help identifying malfunction and will improve the optical head reliability in respect with laser safety aspects. 
         [0045]      FIG. 7  describes another embodiment of the invention wherein a light trap  71  is used. A light trap may be for example of a half sphere form or a cone that has an internal reflecting coating. 
         [0046]    It will be appreciated that the examples shown in  FIGS. 2-7 , are for the purpose of example only and are not limiting. The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
       PARTS LIST 
       [0000]    
       
           11  light detector positioned at proximal tip 
           12  light source (e.g. laser diode) 
           13  coupling micro-optics 
           14  fiber 
           15  external light detector 
           16  printing plate 
           17  laser emission 
           18  imaging lens 
           31  graph describing the power measured by detector  41  versus the power ted from the distal tip of the fiber of channel  0   
           32  graph describing the power measured by detector  41  versus the power ted from the distal tip of the fiber of channel  24   
           33  graph describing the power measured by detector  41  versus the power ted from the distal tip of the fiber of channel  44   
           41  internal light detector 
           42  transparent fiber structure slab 
           43  fibers mechanical housing structure 
           45  light emitted along the distal tips of optical fibers 
           46  internal reflective coating 
           47  fibers 
           47   a  core 
           47   b  clad 
           48  additional internal light detector 
           50  transparent optical glue 
           53  fibers v-groove housing structure 
           61  imaging drum 
           63  imaging drum rotation axis 
           64  light source intensity control device 
           65  measurement results line 
           66  intensity control line 
           67  imaged spot 
           71  light trap 
           490  scattering particle 
           491  light reflection due to a scattering particle 
           492  angled polish 
           493  core clad interface adjusted roughness

Technology Category: 3