Patent Publication Number: US-2007097837-A1

Title: Use of external optical feedback in a laser to generate a data signal

Description:
BACKGROUND  
      Lasers are employed to read data from and write data to optical data storage media such as compact discs and the like. Lasers may also be employed to write a label to a label surface of such optical media. In a typical optical media drive such as an optical disc drive, lasers generate an optical power output that is directed to the media. Such drives also typically employ various sensors to read data from the optical media. For example, assuming that an optical medium is an optical disc, there may be a series of “lands” and “pits” or other data marks that embody data stored on the optical disc. The lands reflect the laser light toward sensors with a reference amplitude and a reference phase, whereas the pits reflect the laser light toward these sensors with a modified amplitude and a modified phase. Data marks may also have the form of areas of altered phase in a phase change material. The phase of the recording material may be changed from amorphous to crystalline, or from crystalline to amorphous, during data recording. By angling the trajectory of the laser toward the optical disc, or by the use of beam splitting optics, laser light emitted by the laser may be reflected by the lands and pits of the optical disc, and captured by the sensors. In this respect, reflected laser light may represent a data signal having transitions between a state produced by lands and a state produced by pits. The timing of these data signal transitions relative to a reference signal produced by a data clock is used to generate a digital data stream corresponding to the encoded information stored on the disk. The use of sensors and beam splitting optics as described above present a relatively significant expense when optical storage devices are mass produced. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      The invention can be understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Also, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
       FIG. 1  is a block diagram of an optical disc drive according to various embodiments of the present invention;  
       FIG. 2  is a drawing of a laser employed in the optical disc drive of  FIG. 1  according to an embodiment of the present invention; and  
       FIG. 3  is a graph of a laser power output versus laser drive current input that illustrates a threshold current associated with the laser of  FIG. 2  according to various embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION  
      With reference to  FIG. 1 , shown is an optical disc drive  100  according to an embodiment of the present invention. The optical disc drive  100  is in data communication with a host  103 . The host  103  may be, for example, a computer system, server, or other similar device. The optical disc drive  100  may or may not be enclosed within the enclosure of the host  103 . For the purposes of the following discussion, first the structural aspects of the optical disc drive  100  are discussed. Thereafter, the operation of the optical disc drive  100  is discussed with particular focus on the operation of the laser in generating a data signal corresponding to data embodied on an optical medium according to the various embodiments of the present invention.  
      In one embodiment, the optical disc drive  100  includes a processor circuit  106 . The processor circuit comprises a processor  113  and a memory  116 , both of which are coupled to a local interface  119 . In this respect, the local interface  119  may be, for example, a data bus with an accompanying control/address bus as can be appreciated by those with ordinary skill in the art. The optical disc drive  100  further includes an optical pickup unit  123 , an actuator  126 , and a spindle  129 . According to various embodiments, the optical disc drive  100  also includes a rotational position laser sensor  133 . When in use, an optical disc  136  is placed on the spindle  129  as shown. The optical pickup unit  123 , actuator  126 , spindle  129 , and rotational position laser sensor  133  are all operatively or electrically coupled to the processor circuit  106 . In particular, these components are coupled to the processor circuit  106  by way of an electrical connection through which electrical signals may be received from or transmitted to the processor circuit  106  in orchestrating the operation of the optical disc drive  100  as will be described. In one implementation, the optical pickup unit  123 , actuator  126 , spindle  129 , and the rotational position laser sensor  133  are coupled to the local interface  119  through appropriate interface circuitry (not shown) as can be appreciated.  
      The actuator  126  may comprise, for example, a stepper motor or other such device. The actuator  126  is operatively coupled to the optical pickup unit  123 , for example, using a screw shaft  139 . In this respect, the actuator  126  may be manipulated by the processor circuit  106  in order to move the optical pickup unit  123  back and forth along the length of the screw shaft  139  during the normal operation of the optical disc drive  100  as will be described. In this respect, the actuator  126  positions the optical pickup unit  123  relative to the optical disc  136  during the normal course of operation of the optical disc drive  100 . Alternatively, other approaches may be employed to move the optical pickup unit  123  as desired beyond a screw shaft  139 .  
      The optical pickup unit  123  includes a laser assembly  140  that may be employed to read data from and, in some embodiments, to write data to a track  141  of the optical disc  136 . The laser assembly  140  and the rotational position laser sensor  133  may each comprise, for example, a laser diode or other type of laser. While the use of the laser assembly  140  is described in the context of the optical disc drive  100 , it is understood that the laser assembly  140  may be employed in optical data storage devices other than an optical disc drive as can be appreciated. In this respect, the optical disc  136  is one example of the various types of optical media that may be employed according to the various embodiments of the present invention.  
      In the context of the optical disc drive  100 , the laser assembly  140  is controlled to generate laser light  146  that is directed to the optical disc  136 . The laser assembly  140  may operate at any one of a number of optical wavelengths as can be appreciated by those with ordinary skill in the art. At least a portion of the laser light  146  may reflect off the optical disc  136  as reflected laser light  149 . Data structures are embodied in the optical disc  136  that reflect the laser light  146  as can be appreciated by those with ordinary skill in the art.  
      The optical pickup unit  123  further comprises a lens focus actuator  153  that controls the focus of a lens. In this respect, the lens focus actuator  153  adjusts the position of the lens in relation to the optical disc  136  in response to a focus error signal, value, or other input setting. The lens focus actuator  153  is operatively coupled to the processor circuit  106  that provides the focus error signal or data to the lens focus actuator  153 .  
      The laser assembly  140  also detects reflected laser light  149  during a read operation and generates a voltage signal that is applied to the processor circuit  106 . The magnitude of the voltage signal generated by the laser assembly  140  is generally proportional to the magnitude of the incident reflected laser light  149  that enters a gain medium of a laser within the laser assembly  140  as optical feedback, as will be described. Alternatively, a current signal may be generated by the laser assembly  140 .  
      The optical pickup unit  123  may be caused to write data to the optical disc  136  by controlling the laser assembly  140  in the optical pickup unit  123  so as to form the data structures in the optical disc  136 . The writing capabilities of the optical disc drive  100  may also be employed to write a label on a label surface of the optical disc  136 . Specifically, the label surface of the optical disc  136  may be chemically treated so as to change an optical property such as optical density, reflectivity, or color upon being irradiated with laser light from the laser assembly  140 . Such treatment includes, for example, a coating of thermo-chromic material that has been screen-printed or otherwise disposed on the label surface such that this material changes from light to dark color when activated by laser light  146  from the laser assembly  140 . The thermo-chromic material may comprise, for example, a mixture of color-forming dye, activator, and infrared antenna contained in a polymer matrix. The infrared antenna absorbs laser energy of a particular wavelength or range of wavelengths and converts it to heat. The heat causes the activator, dye, and the polymer matrix to melt, thereby allowing the activator to interact with the dye. The interaction results in a chemical change to the dye that causes a change in color. The label material may vary slightly from manufacturer to manufacturer, from one disc to another disc from the same manufacturer, or even from one region on a disc to another region on the same disc. As a consequence, the appearance of the generated label may vary accordingly.  
      The spindle  129  comprises a motor or other such device that spins the optical disc  136 . This motor may be, for example, a brushless DC motor or other type of motor. In this respect, the optical disc  136  is mounted onto the spindle  129 . Thereafter, the optical disc  136  may be spun relative to the optical pickup unit  123  and the rotational position laser sensor  133 . The rotational position laser sensor  133  obtains positional data from a position encoder  159  on the optical disc  136  as it rotates on the spindle  129 . By virtue of the positional data obtained from the position encoder  159 , the precise azimuth location of the optical pickup unit  123  relative to the optical disc  136  can be tracked during the operation of the optical disc drive  100 .  
      The optical disc drive  100  further comprises a number of components stored in the memory  116  and executable by the processor  113  in order to control the operation of the various components of the optical disc drive  100 . These components comprise, for example, an operating system  163  and a disc drive controller  166 . The disc drive controller  166  is executed by the processor  113  to control the various operations of the optical disc drive  100 . In this respect, the disc drive controller  166  orchestrates the general operation of the optical disc drive  100  in writing data to and reading data from optical discs  136 . The disc drive controller  166  also orchestrates the operation of the optical disc drive  100  in writing a label on a surface of an optical disc  136 .  
      Where embodied in the form of software or firmware, the disc drive controller  166  and the laser focus actuator control  169  may be implemented using any one of a number of programming languages such as, for example, C, C++, Assembly, or other programming languages. The disc drive controller  166  may be implemented, for example, in an object oriented design or in some other programming architecture. Where any portion of the disc drive controller  166  and/or the laser focus actuator control  169  is represented in a flow chart herein, assuming that the functionality depicted is implemented in an object oriented design, for example, then each block of such flow charts may represent functionality that is implemented in one or more methods that are encapsulated in one or more objects, etc.  
      The memory  116  may comprise, for example, random access memory (RAM), such as, for example, static random access memory (SRAM), dynamic random access memory (DRAM), or magnetic random access memory (MRAM) and other such devices. In addition, the memory  116  may also include, for example, read-only memory (ROM) such as a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or other like memory device.  
      In addition, the processor  113  may represent multiple processors and the memory  116  may represent multiple memories that operate in parallel. In such a case, the local interface  119  may be an appropriate network that facilitates communication between any two of the multiple processors, between any processor and any one of the memories, or between any two of the memories etc. The processor  113  may be of electrical, optical, or molecular construction, or of some other construction as can be appreciated by those with ordinary skill in the art.  
      The operating system  163  is executed to control the allocation and usage of hardware resources such as the memory, processing time and peripheral devices in the optical disc drive  100 . In this manner, the operating system  163  serves as the foundation on which applications depend as is generally known by those with ordinary skill in the art.  
      Next, the general operation of the optical disc drive  100  in reading from or writing to the optical disc  136  is described according to an embodiment of the present invention. The disc drive controller  166  controls the operation of the various components of the optical disc drive  100  in order to read from or write to the optical disc  136 . The disc drive controller  166  also controls the operation of the various components of the optical disc drive  100  when writing data to and reading data from the optical disc  136 .  
      To the extent that the disc drive controller  166  orchestrates the operation of the various components of the optical disc drive  100  in order to read data from or write data to the optical disc  136 , it controls the movement of the optical pickup unit  123  by directing the actuator  126  to cause the optical pickup unit  123  to move along the screw shaft  139  as needed. In addition, the disc drive controller  166  controls the rotation of the optical disc  136  by controlling the speed of the spindle  129 .  
      The laser assembly  140  performs the dual operations of both a light source and a sensor. Specifically, the laser assembly  140  is configured to write data to the optical disc  136  by acting as a light source, and to read data from the optical disc  136  by acting as a sensor. The disc drive controller  166  controls the write functions of the optical disc drive  100  by directing the laser assembly  140  to generate the laser light  146  of sufficient power to cause pits or other data marks to be formed at appropriate locations in the optical disc  136  to represent data as can be appreciated. The disc drive controller  166  controls the read functions by directing the laser assembly  140  to generate relatively lower power laser light  146  that is insufficient to cause the formation of data marks. The corresponding reflected laser light  149  that reenters the gain media of a laser within the laser assembly  140  acts as optical feedback that varies in amplitude or phase, or both, as the laser light  146  falls incident onto the data marks, and results in the generation of a data signal that represents the data embodied in the optical disc  136 .  
      Also, the rotational position laser sensor  133  comprises a laser that generates laser light  161  that is directed to the position encoder  159 . When the laser light  161  falls incident to a land portion of the position encoder  159  (as opposed to pits that cause the laser light  161  to be absorbed, diffracted, or otherwise attenuated), at least a portion of the laser light  161  is reflected in the form of reflected laser light  162  that is directed back to the rotational position laser sensor  133 . In a manner similar to the laser assembly  140 , the reflected laser light  162  reenters the gain media of the rotational position laser sensor  133  and acts as optical feedback resulting in the generation of a position signal that represents position encoder  159  data embodied in the optical disc  136 .  
      The disc drive controller  166  tracks the position of the optical disc  136  based upon the position signal from the rotational position laser sensor  133 . In particular, the rotational position laser sensor  133  senses the passing of position encoder  159  in the form of, for example, encoder bars disposed on the optical disc  136  near, for example, the inside diameter, although the encoder bars may be located at some other position on the optical disc  136 . To track the actual location of the optical disc  136 , the disc drive controller  166  may include a counter that counts pulses of the position signal up to a total number of pulses in a single rotation to determine the actual position of the optical disc  136  at a given time.  
      Thus, the azimuthal location of the laser beam  146  generated by the optical pickup unit  123  relative to the optical disc  136  may be determined at any given time by virtue of the positional data tracked by the disc drive controller  166  based upon the data generated by the rotational position laser sensor  133 . In particular, the azimuthal location of the optical pickup unit  123  relative to a predefined position on the optical disc  136  of each pixel or segment of a label that is to be written to the optical disc  136  may be calculated based upon the relative positions of each of the encoder bars of position encoder  159  sensed by the rotational position laser sensor  133 .  
      By virtue of the above-mentioned components, the disc label controller  163  orchestrates writing to and reading from the optical disc  136 . In addition, the disc drive controller  166  causes the optical pickup unit  123  to position the lens  156  to properly condition the laser light  146  as it is directed at the optical disc  136 . In this respect, the positioning of the lens  156  may be performed continuously while the optical disc  136  spins and the laser  143  is directed thereto during the performance of label writing functions, or other operations. Conditioning the laser light  146 , in some embodiments, includes focusing the laser light  146  on the desired location of the optical disc  136 .  
      Turning then to  FIG. 2 , shown is an example of the laser assembly  140  as it is employed to generate a data signal when reading data from the optical disc  136  according to an embodiment of the present invention. In the example illustrated, the laser assembly  140  includes a laser  173  that comprises, for example, a laser diode of several layers. Alternatively, the laser  173  may comprise some type of laser other than a laser diode.  
      The layers of the laser  173  include a ground electrode  176  and a substrate  179 . Disposed on the substrate  179  is a guiding layer  183 . The laser  173  also includes a second guiding layer  176 . Between the guiding layers  183  and  186  is a gain medium  189 . The gain medium  189  includes a forward facet  193  and a rear facet  196 . Both the forward and rear facets  193  and  196  are at least partially reflective, allowing the laser  173  to generate laser light  146 . In particular, the forward and rear facets  193  provide internal feedback of optical power that facilitates the generation of laser light as can be appreciated.  
      In this respect, the forward facet  193  is partially reflective thereby enabling laser light  146  to be emitted when the laser drive input current reaches a predefined level generally referred to as a “threshold current” as will be described. The laser assembly  140  also includes a photodetector  203 . A predefined percentage of the output power generated by the laser is transmitted as laser light  199  that exits the rear facet  196  of the laser  173  and is directed to the photodetector  203 . The photodetector  203  generates an output signal that may be, for example, a data signal or other signal based upon the information obtained from the optical disc  136 . This signal is transmitted to the processor circuit  106  ( FIG. 1 ) as described above.  
      The laser  173  also includes a cladding layer  206  and a positive electrode  209 . Generally, the example structure of the laser  173  is typical of the structures of the laser diodes in this respect and is not described herein in detail.  
      When power is applied to the laser  173 , the laser  173  emits optical power in the form of the laser light  146  toward the optical disc  136 . Also, the laser  173  emits optical power in the form of the laser light  199  through the rear facet  196  to the photodetector  203 . In one embodiment, the magnitude of the optical output power emitted by the laser  173  is a function of the external optical feedback coupled into the gain medium of the laser  173 .  
      The “power efficiency” of the laser  173  is defined herein as the optical output power divided by the electrical input current, and the “gain” of the gain medium of the laser  173  is defined as optical output power divided by optical input power. When the optical power is emitted from the front facet  193  in the form of the laser light  146 , the laser light  146  may strike a reflective surface associated with the optical disc  136 . Consequently, at least a portion of the laser light  146  is reflected as the reflected laser light  149  back toward the gain medium  189  of the laser  173 . According to the various embodiments of the present invention, the reflected laser light  149  is coupled back into the gain medium  189 . In this respect, the reflected laser light  149  that enters the gain medium  189  acts as external optical feedback into the laser  173 . While an optical disc  136  is shown as reflecting the laser light  146 , it is understood that any optical medium that allows reflection of the laser light  146  back into the gain medium  189  may be employed.  
      In this respect, the laser  173  may include two types of feedback. First, is the internal feedback inherent by the use of the forward and rear facets  193  and  196 . In this respect, laser light moves back and forth between the forward and rear facets  193  and  196  gaining strength by causing the stimulated emission of radiation as can be appreciated. The laser light moves back and forth between the forward and rear facets  193  and  196  until sufficient power is reached.  
      The forward facet  193  and rear facet  196  may be dielectrically coated or may be constructed in some other manner so as to provide the desired amount of reflectivity for the internal feedback, while at the same time allowing the emission of the desired amount of optical power therethrough as can be appreciated. The external optical feedback comprises the amount of reflected laser light  149  that re-enters the gain medium  189  of the laser  173 .  
      When the laser  173  is operated at near threshold levels, the external optical feedback provided by the reflected laser light  149  that enters the gain medium  189  may cause an appreciable change in the gain of the gain medium causing the laser to make a transition from operation below threshold to operation above threshold and significantly increasing the optical power emitted by the laser  173  as both the laser light  146  and the laser light  199  emitted through the rear facet  196  and directed to the photodetector  203 . Thus, when the external optical feedback exists such that the reflected laser light  149  re-enters the gain medium  189 , then a greater amount of optical power is detected at the photodetector  203 .  
      Conversely, when no reflected laser light  149  is directed back into the gain medium  189  then the optical power detected by the photodetector  203  is correspondingly diminished. Thus, the laser assembly  140  may be employed as a sensor that generates a data signal based upon data structures embodied in the optical disc  136 . In particular, when the laser light  146  falls onto a “land” portion of the optical disc  136  that represents, for example, a logical “1”, the resulting reflected laser light  149  enters the gain medium  189 , thereby resulting in a greater amount of emitted optical power from the laser  173 . Also, a greater amount of optical power is emitted through the rear facet  196  toward the photodetector  203 . The photodetector  203  can thus output a signal indicative of the logical “1” based on the detection of a greater amount of laser light  199  incident on the photodetector  203 .  
      Conversely, when the laser light  146  strikes a “pit” on the optical disc  136 , then there is little or no reflected laser light  149  that enters the gain medium  189 . Consequently, the amount of laser light  199  that is emitted through the rear facet  196  is correspondingly diminished due to the absence of external optical feedback. In response, the photodetector  203  generates a diminished signal. Thus, the pits of the optical disc  136  may represent, for example, a logical “0” for which a corresponding signal may be generated by the photodetector  203  and transmitted to the processor circuit  106  ( FIG. 1 ). In addition, much higher efficiency data coding schemes than the above described example may be used. For instance, the passage of the edges of pits through a laser beam causes transitions in a data signal, the locations of the transitions in time determining the digital data according to a data encoding scheme.  
      The use of the laser assembly  140  provides a distinct advantage in that the laser assembly  140  may be employed to write data or a label to the optical disc  136 , and, the same laser assembly  140  may be employed as a sensor to generate a data signal from a data structure embodied within the optical disc  136 . Consequently, there is no need for a separate sensor within the optical disc drive  100  beyond the photodetector  203  within the laser assembly  140  to perform the function of reading data from the optical disc  136 . Among other functions, the photodetector  203  may typically be employed to generate a laser power monitor signal that is used to control the output of the laser  173 . Consequently, laser assemblies  140  are commonplace and may be obtained at relatively low cost.  
      This presents a significant advantage in that the cost associated with a separate sensor is avoided. Thus, the photodetector  203  comprises, for example, a circuit that generates a data signal that embodies the data within the optical disc  136  based upon the variation in the power efficiency of the laser assembly  140  due to the external optical feedback of the reflected laser light  149  that is directed into the gain medium  189  of the laser  173 .  
      Referring next to  FIG. 3 , shown is a graph of the emitted output power of the laser assembly  140  in response to the laser drive input current I of the laser assembly  140 . In this respect, the laser drive input current I is the drive current applied to the laser  173  ( FIG. 2 ) of the laser assembly  140  by external control circuitry as can be appreciated. As shown, emitted output power of the laser  173  is relatively low until at least a threshold laser drive current T IN  is applied. This threshold laser drive current T IN  marks a substantial and dramatic change in the slope of the emitted output power relative to the laser drive input current. This slope determines the power efficiency of the laser.  
      Note that there may be several different approaches to determining the threshold laser drive current as can be appreciated, where that depicted in  FIG. 3  is provided as an example. In this respect, depending on the method used for defining and determining the threshold, the threshold laser drive current may be located at several different locations near the elbow of the output power curve as can be appreciated.  
      According to one embodiment of the present invention, the laser drive input current I applied to the laser  173  in order to read data from the optical disc  136  is set near the threshold laser drive current T IN . In this respect, the external optical feedback presented by the reflected laser light  149  that is coupled back into the gain medium  189  of the laser  173  results in a relatively significant change in the gain of the laser  173  causing it to transition from operating below threshold to operating above threshold. Note that once the laser is operating above threshold its gain stabilizes, and the gain is no longer influenced by additional optical feedback. Consequently, when drive current T IN  is applied to the laser  173 , laser output power generally increases rapidly and nonlinearly as a function of any optical feedback until the threshold is reached, and the laser output power increases substantially in proportion to additional optical feedback thereafter. Specifically, according to one embodiment, the emitted output power of the laser  173  falls within an operating range R for an applied drive current T IN , external optical feedback from pits and lands, and the resultant variation in the gain of the laser  173 . In another embodiment, the operating range R of the emitted output power of the laser  173  straddles the emitted output power T O  at the threshold laser drive current T IN . In this respect, the lowest and highest values of the emitted output power of the laser  173  due to the selective application of the external feedback straddles or fall on either side of the output power of the laser  173  at the threshold laser drive current T IN .  
      Because the magnitude of the laser light  199  directed toward the photodetector  203  varies based upon the gain of the laser  173  as influenced by the external optical feedback from pits and lands, the photodetector  203  generates a data signal in response. The optical power emitted through the rear facet  196  varies in response to both the gain of the laser  173  and the optical feedback power coupled into the laser. Thus laser output increases rapidly as a function of optical feedback near threshold. In this respect, the rear facet  196  is conditioned to provide for the emission of a predefined fraction of the total optical power emitted by the laser  173 .  
      Because the operating range R of the output power results in emitted laser outputs that straddle the threshold laser drive current I of the laser  173 , a dramatic difference in the output of the laser  173  is seen in response to the application of the external optical feedback in the form of the reflected laser light  149  that is coupled to the gain medium  189  of the laser  173 .  
      Referring back to  FIG. 2 , according to the various embodiments of the present invention, the data signal generated by the laser assembly  140  based upon the data embodied in the optical disc  136  may be generated using one of a number of approaches. In one approach, a power source is coupled to the laser  173  that applies a substantially constant laser drive current to the laser  173 . In one embodiment, this substantially constant laser drive input current is the threshold laser drive current T IN  ( FIG. 3 ). The output of the photodetector  203  is thus proportional to the gain of the laser  173 . Because the gain changes based upon the external feedback provided by the reflected laser light  149  reflected from the optical disc  136 , the data signal may be generated from such gain changes. In this respect, the external optical feedback represented by the reflected laser light  149  is modulated by the data embodied in the optical disc  136 . Consequently, the data embodied in the optical disc  136  in turn modulates the output signal of the photodetector  203 .  
      In another approach, additional circuitry may be used to maintain the portion of the optical power  199  directed to the photodetector  203  at a predefined constant value. To do so, the laser drive input current is varied so as to maintain a substantially constant emitted output power  199  directed to the photodetector  203 . An appropriate feedback loop may be used to accomplish the substantially constant emitted output power  199  from the rear facet  196  as can be appreciated. A data signal may then be generated based upon the changes in magnitude of the laser drive current applied to the laser  173 . In addition, the laser assembly  140  may be operated in other modes that provide for the generation of the data signal that embodies the data of the optical disc  136  as can be appreciated.  
      In another embodiment, a laser drive input current may be applied to the laser that embodies an AC current. The AC current may be in the form, for example, of a sine wave. The AC current may be specified so that the threshold current of the laser  173  is between the highest and lowest magnitudes of the AC current. In this respect, the AC current would straddle the threshold current of the laser. This would provide an approach by which the location of the threshold current of the laser  173  may be detected and by which the range of input current applied to the laser may be controlled to straddle the threshold current of the laser. Controlling the operating range of the laser in this manner enables the sensitivity of the sensor to remain large and relatively constant as the laser threshold current changes due to temperature variations and other causes. Specifically, if the AC current straddles the threshold current in this manner, then the optical output of the laser  173  will vary significantly with the positive and negative peaks of the AC current due to the significant change in the optical output power of the laser  173  when passing through the threshold current. To detect the threshold current of the laser  173 , a DC offset of the AC current applied to the laser drive input current can be varied until the appropriate change in laser output power is detected. Alternatively, a duty cycle of the output waveform may be examined.  
      Additionally, the laser light  149  may not be in phase relative to the laser light  146  when it is reflected back into the gain medium  189  of the laser  173 . This is because the distance between the optical disc  136  and the front facet  193  may vary due to, for example, warpage as the optical disc  136  spins relative to the laser  173 . Consequently, the data signal generated by the laser assembly  140  as described above may experience fluctuations such as a sinusoidal or other fluctuation from constructive and destructive interference caused by the change in the phase of the reflected laser light  149 .  
      Although the invention is shown and described with respect to certain embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the claims.