Abstract:
Improvements to an optical recording apparatus comprising a laser light source are provided. The improvements include installing a photodiode in an optical path of at least a portion of the laser light beam from the laser light source, and connecting said photodiode to a detection circuit. The photodiode detects the laser light beam and generates a detection signal based on the detected beam. The detection circuit monitors the detection signal and generates an alarm signal if the detection signal indicates a dropout in the laser beam. The alarm signal can trigger an alarm indicator and/or cause a recording process to be halted.

Description:
TECHNICAL FIELD 
   This application relates to optical recording. In particular, the application relates to improvements for detecting laser dropout in optical recording. 
   DESCRIPTION OF RELATED ART 
   Use of CDs (compact discs) and DVDs (digital versatile discs or digital video discs) as optical storage media (“optical disc”) for storing and transporting content (such as audio, video, graphics, computer software, etc.) in an optically readable manner has been popular for a number of years. Several formats of optical discs are currently available, including (A) read-only formats such as CD-DA (digital audio compact disc), CD-ROM (CD-read-only memory), DVD-ROM, and other formats wherein content is prerecorded on the disc (such as by using an injection molding process), and (B) recordable formats in the form of (i) write-once read-many times formats such as CD-R (CD-recordable), and DVD±R (DVD-recordable), etc., or (ii) rewritable formats such as CD-RW (CD-rewriteable), DVD-RAM (DVD-Random Access Media), DVD−RW or DVD+RW (DVD-rewriteable), PD (Phase change Dual disk) and other phase change optical discs. Optical disc players for these optical discs use a red laser. Optical discs using a blue laser have also been introduced, such as HD DVD and BD (each of which includes read-only, recordable and rewritable formats). In addition, hybrid optical media which include a combination of formats are also available. 
   In conventional read-only type optical discs (for example, CD-ROM, DVD-ROM, etc.), data is generally stored as a series of “pits” embossed in a plane of “lands”. Microscopic pits formed in a surface of a plastic medium [for example, polycarbonate or polymethyl methacrylate (PMMA)] are arranged in tracks, conventionally spaced radially from the center hub in a spiral track originating at the medium center hub and ending toward the medium&#39;s outer rim. The light reflected from a read-only medium&#39;s surface by an optical disc player or reader varies according to the presence or absence of pits along the information track. A photodetector and other electronics inside the optical disc player translate the signal from the transition points between these pits and lands caused by this variation into the 0s and 1s of the digital code representing the stored information. 
   Read-only type optical discs generally are produced by an injection molding process. Initially, data representing the content to be recorded, encoded as a run length limited digital code (commonly known as an EFM signal in CD manufacturing) which contains its digital information in the timing between transitions, is used to control a laser beam recorder to form pits in a photoresist or a dye-polymer layer on an optical grade glass disc known as a glass master in a mastering process. A metallized glass master is used in an electroforming process to form (typically, metal) stampers. A stamper is used on one side of an injection molding cavity to emboss an information layer of pits and lands on a transparent polymer substrate formed by injection molding. The information bearing surface of the substrate is then covered with a reflective film (of metal or alloy) or the like. In the case of a CD, a plastic protective coating is applied over the film, and then art (for example, a picture, a design, text, etc.) is typically printed on the upper surface of the disc, to form an end product. In the case of DVDs, two half-thickness substrates are typically formed, metallization is applied to one (for example, DVD-5) or both (for example, DVD-10, DVD-9, DVD-18) half-thickness substrates, and the two half-thickness substrates are bonded by an adhesive (for example, hotmelt adhesive, ultraviolet light-cured adhesive, etc.). A second information layer can be formed for a DVD (for example, DVD-18) by applying a photo-polymer coating over a metallization layer applied to a substrate and the second information layer is embossed by a stamper into the photo-polymer layer which is then UV cured, metallized and protective coated. 
   Recordable type optical media typically include a spiral wobble groove in the substrate. The groove defines recording channels on the disc for recording data, provides information for tracking of the disc while writing or reading data, and has its wobble frequency modulated to contain addressing and other information for the write and read processes. The substrate (including information layer bearing the spiral wobble groove) can be formed by injection molding, using a stamper electroformed with a glass master. In addition, recordable-type optical media generally include at least a recording layer, and in addition a reflective layer (of metal or alloy) and a protective layer. Information is recorded in the recordable-type optical medium by directing a laser light beam modulated by signals to selectively change optical characteristics (reflectivity or extinction coefficient) of the recording layer. The recording layer in write-once read-many times optical media typically includes a photosensitive organic dye which is heated during recording to irreversibly to form a pattern of marks or pits into the recording layer. 
   Each recording side of a rewritable disc also uses multiple layers beginning with a polycarbonate plastic substrate containing a shallow spiral groove extending from the inside to the outside diameter of the disc. A DVD-RW disc may additionally include pits and lands and a DVD-RAM disc also inside the groove itself. The substrates (including information layer bearing the spiral groove, land pre-pits and embossed areas) may be formed by injection molding, using a stamper electroformed with a glass master. Next in the multiple layers of a rewritable disc typically comes a dielectric layer, followed by a phase-change type recording layer having a polycrystalline structure, another dielectric layer and a reflective layer (of metal or alloy). Additional layers may also be incorporated above or below the dielectric layer, with a protective coating being applied as a last layer in single-sided optical media. During recording of the rewritable optical medium, the laser selectively heats tiny areas of the recording track to change the phase of each heated area from more crystalline into less crystalline (also known as “amorphous”) phase, in order to create marks that can be called “pits” (the term “pit” is used broadly herein to cover, for example, a pit in a read-only type optical disc, and a pit or mark in a recordable or rewritable optical disc). During erase, the laser (in a process called “annealing”) changes the amorphous areas back into more crystalline areas. 
   Various types of hybrid optical media are available. For example, one type of hybrid media has data pit patterns prerecorded on a surface of a substrate and has in addition a recording layer allowing a user to record data to the recording layer. Further, optical recording media having multiple recording layers are now also available. In addition, in another hybrid optical recording disc a read-only area and a recordable area are present on one side of the hybrid optical disc, with a wobble groove in the read-only area modulated by depressions in the substrate and extending into the substrate in the recordable area. 
   Accordingly, a laser source is used for pit formation for assorted types of optical discs. It is generally desired for the laser source to provide a laser beam having sufficiently consistent energy levels, and the beam is typically modulated by a modulator, according to the digital code encoding the data to be recorded (discussed above). 
   However, some laser sources occasionally experience dropouts. The term “dropout” as used herein refers to the undesirable occurrence in a laser beam in which the beam blinks (for example, fluctuated from an “ON” level to an “OFF” level and then back to an “ON” level). Laser dropouts can adversely affect pit formation in optical discs. 
   For example, laser dropouts may go through undetected in conventional apparatuses and processes for manufacturing read-only type optical discs. In conventional processes, discs are tested at the end of the process to cull out unreliable discs. In some instances, laser dropouts are so small that test equipment used to verify disc playability at the end of the manufacturing process may automatically correct for the missing data and play the program. When this happens, the person conducting the test has no way of knowing that there was a dropout. However, disc players purchased by consumers are not always able to make the same error correction, resulting in titles that do not play well in some players. 
   Conventional optical disc manufacturing processes and apparatuses do not include inexpensive, reliable means for monitoring the laser beam for dropouts during a mastering process. 
   A device which monitors a modulated beam, and then decodes the modulated beam and compares the decoded information to the actual data stream for any differences has been proposed. However, such a device is highly complex and expensive, and not necessarily reliable. 
   There is a need for a practical, inexpensive and reliable means for detecting dropouts during the mastering process, before discs are sold to consumers. 
   SUMMARY 
   This application describes improvements to an optical recording apparatus which adapt the apparatus to check for laser dropouts. Such an apparatus includes a laser light source. The improvement includes (1) a photodetector in an optical path of a light beam from the laser light source, and (2) a detection circuit. The photodetector detects the light beam and generates a detection signal based on the detected beam. The detection circuit monitors the detection signal and generates an alarm signal if the detection signal indicates a dropout in the laser beam. 
   The application also describes a method for adapting an optical recording apparatus to detect laser dropouts. The optical recording apparatus includes (A) a laser light source, (B) a mirror in an optical path of a laser light beam from the laser light source, and (C) a modulator for modulating a first light beam which reflects from the mirror. The method, according to an exemplary embodiment, comprises (a) installing a photodetector in an optical path of a light beam which is functionally related to the laser light beam which passes through the mirror, and (b) connecting the photodetector to a detection circuit. The photodetector detects the light beam and generates a detection signal based on the detected beam. The detection circuit monitors the detection signal from the photodetector and generates an alarm signal if the detection signal indicates a dropout in the laser beam. 
   The application also provides an apparatus which can detect laser dropout in a laser beam recorder configured for an optical recording format. In one embodiment, the apparatus includes a laser light source, a mirror, a modulator, a photodetector and a detection circuit. The mirror is in an optical path of a laser light beam from the laser light source. A portion of the laser light beam which is reflected by the mirror forms a first light beam. A portion of the laser light beam which passed through the mirror forms a second light beam. The photodetector is in an optical path of a second light beam, and the photodetector detects the second light beam and generates a detection signal based on the detected beam. The detection circuit monitors the detection signal and generates an alarm signal if the detection signal indicates a dropout in the laser beam. 
   In addition, this disclosure also provides a method for handling laser dropouts in an optical recording process. In one embodiment, the method includes (i) using a photodetector in an optical path of at least a portion of a laser light beam to convert the at least a portion of the laser light beam into a detection signal, (ii) using a detection circuit to monitor the detection signal and based on the monitoring detect a dropout in the laser light beam, and (iii) generating an alarm signal if the dropout is detected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present application can be more readily understood from the following detailed description with reference to the accompanying drawings wherein: 
       FIG. 1  shows a schematic diagram of a conventional apparatus for recording data on optical discs; 
       FIG. 2  shows a schematic diagram of an improved apparatus for recording data on optical discs, according to an exemplary embodiment of the present application; 
       FIG. 3  shows a block diagram of a detection circuit, according to an exemplary embodiment; 
       FIG. 4  shows a schematic diagram of an apparatus for recording data on optical discs, according to another exemplary embodiment; 
       FIG. 5  shows a schematic diagram of an apparatus for recording data on optical discs, in accordance with yet another exemplary embodiment; 
       FIG. 6  shows a flow chart of a method, according to an exemplary embodiment, for adapting an apparatus for recording data on optical discs to enable the apparatus to detect laser dropouts; and 
       FIG. 7  shows a flow chart of a method for handling laser dropouts in an optical recording process, according to an exemplary embodiment. 
   

   Similar reference numerals designate identical or corresponding parts throughout the several drawings. 
   DETAILED DESCRIPTION 
   This application describes an improved optical recording apparatus and a method for adapting an optical recording apparatus to detect laser dropouts. The apparatus may be integrated in an optical recording device for recording data on recordable or rewritable optical discs. Alternatively, the apparatus may be integrated in a laser beam recorder (LBR) apparatus used for cutting a glass master based on a formatted signal, and (if no laser dropout during the cutting process is detected) the glass master is used for manufacturing prerecorded optical discs. The improved laser beam recorder apparatus can be adapted to monitor a laser for dropouts during, for example, a photo resist mastering process, and interfaced with the control electronics to monitor the beam only when needed. 
   Any suitable laser may be used as the laser light source, depending on the optical disc format. However, a UV laser source which radiates laser light of a wavelength of 351 nm is preferred if the format is HD-DVD. HD-DVD and Blu-ray are two proposed formats which are competing to be the standard for the next generation optical recording disc. Each of HD-DVD and Blu-ray provides a pre-recorded format and a recordable format. Each format is designed for use with blue laser technology, in order to achieve higher data density, such as would be required for recording high definition video. Laser dropouts can affect recordation of data in any of the optical disc formats, regardless of whether a red laser, a blue laser or another suitable laser is used, and the techniques of this application can be utilized to detect the dropouts. 
   Modification of a conventional laser beam recorder apparatus will be discussed below as an exemplary embodiment of the techniques of this application. However, it should be understood to one skilled in the art that implementation of the techniques of this application is not limited to manufacturing of read-only optical discs, and that the techniques can be readily integrated in any optical disc recording process in which a laser light source is used. 
   A conventional laser beam recorder apparatus which is configured for an optical recording format (for example, CD or DVD) is shown schematically in  FIG. 1 . Apparatus  10  includes a light source  11 , a modulator (for example, acoustic-optical or electro-optical modulator)  13  and modulation interface  15 . In addition, the apparatus  10  includes assorted optical components such as mirrors  12   a - 12   c,  etc. 
   The light source  11  generates a light beam for optical media mastering. The light beam is redirected via the mirror  12   a  to the modulator  13 . The modulation interface  15  converts a source signal into a pulse-width formatted data signal which is supplied to the modulator  13 . The modulator  13  modulates an intensity of the light beam according to the formatted data signal from the modulation interface  15 . The modulated beam output by the modulator  13  switches amongst multiple intensity levels, including a high-level and a low-level. 
   The modulated beam is redirected via the mirrors  12   b  and  12   c  onto a rotating glass substrate coated with photoresist (or dye polymer), to form a pattern on the photoresist (or dye polymer) for forming a glass master (or, in the case of an optical disc recording device for recording onto recordable or rewritable optical discs, onto a recordable or rewritable optical disc, to form pits or marks in the recording layer). 
   As discussed above, some laser light sources sporadically experience dropouts. 
   The improvements disclosed by this patent specification involve supplementing the apparatus  10 , in order to detect laser dropouts, as further discussed below. A relatively simple design can be implemented using relatively inexpensive parts to monitor the integrity of the laser beam when it is actually being used. This improvement can be used on any type of laser beam when it is necessary to know that the beam blinked (for example, fluctuated from an “ON” level to an “OFF” level and then back to an “ON” level). 
   An exemplary embodiment of such an improved apparatus is shown in  FIG. 2 . The differences between apparatus  20  shown in  FIG. 2  and the apparatus  10  shown in  FIG. 1  include a photodetector  27  and a detection circuit  28 . 
   The photodetector  27  is preferably a photodiode, and can be inserted after the mirror  12   a . While the larger part of the laser light beam LB is reflected by the mirror  12   a  towards the modulator  13 , a small percentage of the laser light beam LB passes through the mirror  12   a  and is detected by the photodetector  27 . The photodetector  27  is placed in an optical path of the (preferably, unmodulated) light beam L 2  which passes through the mirror  12   a.  The photodetector  27  generates a detection signal based on the detected beam. 
   The detection circuit  28  is connected to the photodetector  27  and monitors the detection signal. The detection circuit generates an alarm signal if the detection signal indicates a dropout in the laser beam. The detection circuit may take on any of various configurations which can detect a variation in the detection signal indicating a laser dropout. 
   In one exemplary embodiment (detection circuit  38  in  FIG. 3 ), the photodetector may be connected to an analog-to-digital (A/D) converter  38   a  to digitize the detection signal into an “on” or “off” state, and a latching circuit  38   b  is coupled to the output of the A/D converter to receive the digitized detection signal. In another exemplary embodiment, the detection signal supplied by the photodetector device is a digitized signal, and the digitized detection signal from the photodetector device is supplied to the latching circuit. In either event, the latching circuit is configured to output the alarm signal when the beam blinks (for example, experiences a rapid ON to OFF to ON fluctuation). 
   According to another embodiment ( FIG. 4 ), an improved apparatus  40  is similar to apparatus  20 , and includes in addition an indicator  49  (for example, a LED, speaker, etc.). An audible or visual alarm may be triggered on the indicator  49  by the alarm signal, to inform the user that a laser dropout has been encountered. The alarm signal may be supplied to a control circuit, and the control circuit may halt a recording process of the optical recording apparatus when the alarm signal output by the detection circuit indicates that a laser dropout occurred. In addition, a reset may be provided with the indicator to allow it to be reset for the next run. In another embodiment, the alarm signal may be recorded or logged with appropriate annotation of time information and/or other information related to occurrence of the laser dropout in the recording process. 
   In another embodiment ( FIG. 5 ), an improved apparatus  50  is similar to apparatus  20 , and includes in addition a control circuit  59  (for example, LBR control electronics). The control circuit  59  enables the detection circuit  28  when a recording process of the optical recording apparatus commences, and disables the detection circuit when the recording process ends. The apparatus  50  may optionally include the detection circuit  38 , and the latching circuit  38   b  can be interfaced with the control circuit  59  such that the latching circuit turns “on” during the actual recording (for example, glass master cutting) process and “off” after the process is completed. The turning “on” and “off” of the latching circuit can be performed by the control circuit or by the latching circuit itself while monitoring outputs of the control circuit. 
   The control circuit  59  may include a controller which causes a mechanical shutter of the laser light source to be opened when a recording process of the optical recording apparatus commences, and closed when the recording process ends. A time delay preferably follows after the shutter opens, before the controller causes the detection circuit to be enabled, and a time delay follows after the detection circuit is disabled, before the controller causes the shutter to be closed. For example, time delay circuitry may be added to allow time between opening of the mechanical shutter and commencement of actual monitoring. 
   A method for adapting an apparatus for recording data on optical discs to enable the apparatus to detect laser dropouts, according to an exemplary embodiment, will now be described with reference to  FIGS. 1 ,  2  and  6 . A method for adapting the optical recording apparatus  10  to detect laser dropouts includes installing photodiode  27  in an optical path of light beam L 2  which passed through the mirror  12   a  (step S 61 ) and connecting the photodiode  27  to detection circuit  28  configured to monitor a detection signal from the photodiode  27  and generate an alarm signal if the detection signal indicates a dropout in the laser beam (step S 63 ). The method may additionally include connecting the detection circuit  28  to an alarm indicator (not shown) which is triggered by the alarm signal from the detection circuit. 
   The method may further comprise connecting the detection circuit  28  to a control circuit (not shown). The control circuit may be configured to halt a recording process of the optical recording apparatus when the alarm signal is output by the detection circuit  28 . In another embodiment, the control circuit may be configured to enable the detection circuit  28  when a recording process of the optical recording apparatus commences, and disable the detection circuit  28  when the recording process ends. 
   The method may further include connecting a controller to a mechanical shutter of the laser light source, and the controller causes the shutter to be open when a recording process of the optical recording apparatus commences, and the controller causes the shutter to be closed when the recording process ends. The controller preferably allows a time delay after the controller causes the shutter to be open, before the controller causes the detection circuit to be enabled, and allows a time delay after the controller causes the detection circuit to be disabled, before the controller causes the shutter to be closed. 
   The adapted apparatus can be used, for example, in an optical recording device for recording a formatted signal onto a recordable or rewritable optical disc. Alternatively, the adapted apparatus can be integrated in a laser beam recorder for cutting a glass master, and the glass master is then used for manufacturing prerecorded optical discs in a format using multiple modulation levels. 
   A method for handling laser dropouts in an optical recording process, according to an exemplary embodiment, will now be described with reference to  FIGS. 2 and 7 . 
   The photodetector  27  in an optical path of light beam L 2  is used to convert the light beam L 2  into a detection signal (step S 71 ). The detection circuit  28  is used to monitor the detection signal and based on the monitoring detect a dropout in the laser light beam LB (step S 73 ). An alarm signal is generated if the dropout is detected (step S 75 ). 
   The method may further comprise one or more of the following: (a) enabling the detection circuit when a recording process commences, and disabling the detection circuit when the recording process ends; (b) halting a recording process when the alarm signal is output; (c) converting the detection signal from the photodetector to a digital signal, and detecting a change in state of the digital signal, and when the change in state is detected, generating the alarm signal. 
   In describing exemplary embodiments, specific terminology is employed for the sake of clarity in this disclosure. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. In addition, the above specific embodiments are illustrative, and many variations can be introduced on these embodiments without departing from the spirit of the disclosure or from the scope of the appended claims. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. As another example, the steps in the methods described herein need not be performed in exactly the order described (for example, methods of  FIG. 6  and  FIG. 7 ).