Abstract:
A transmitter uses an on-chip pattern generator to provide an input signal, and a built-in monitor to detect the power of the light emitted by the light source. The transmitter determines the correlation between the output power measured by an external power meter, and the output power detected by the built-in monitor. After the correlation is determined, the external power meter is no longer needed. Instead, further characterizations, such as over-temperature characterizations, are performed by determining the power detected by the built-in monitor, and then using the known correlation to calculate the actual power output and other transmitter characteristics.

Description:
FIELD OF THE INVENTION 
     The invention is directed towards optical fiber transmitters and more specifically, towards characterizing and adjusting the response of the transmitters. 
     BACKGROUND OF THE INVENTION 
     Optical fiber transmitters use a variety of different light sources, including Vertical Cavity Surface Emitting Lasers (VCSEL); Light Emitting Diodes (LED); lasers, and other light-emitting devices. However, due to variations in the manufacturing process, the individual produced transmitters have dissimilar behaviors. Furthermore, the output properties of these optical sources change with temperature as well. Therefore, each transmitter is first characterized to determine its output response, and then is subsequently adjusted to make its response comply with the required specifications. For example, bias current and modulation current of the transmitter may need to be adjusted so that its minimum and maximum power output levels fall within acceptable limits. This process of adjusting the transmitter to change its output response is referred to as “programming” the transmitter. 
     The prior art conventional programming method is to connect a pattern generator to the transmitter input, connect an optical power meter at the transmitter output, and then change the temperature while characterizing and programming the transmitter&#39;s output response to different inputs and at different temperatures to bring its performance in line with specifications. 
     Unfortunately, the pattern generators and optical power meters used for measuring the output characteristics are expensive, and the number of devices that can be simultaneously tested is constrained by the number of test pattern generators and power meters available. These constraints are especially limiting during the temperature characterization because it is such a time-consuming process. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment of the present invention, an optical transmitter performs self-characterization and self-programming by using an on-chip pattern generator for an input signal, and a built-in optical monitor to detect the optical power of the light emitted by the light source. Since the pattern generator and optical monitor are already incorporated into the transmitter design, less test equipment is needed during the characterization and programming process. This increases the number of devices that can be simultaneously characterized given the same capital investment. 
     The optical transmitter first determines, at room temperature, the correlation between the output power as measured by an external optical power meter, and the output power detected by the built-in optical monitor on the transmitter. Once the correlation is determined, the external optical power meter is no longer needed. Further measurements of the optical transmitter, such as over-temperature characterizations, are performed by determining the power detected by the built-in optical monitor, and then using the known correlation to calculate what the actual power output would be if measured using the external power meter. 
     Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a high-level block diagram of a preferred embodiment of an optical transmitter made in accordance with the teachings of the present invention. 
         FIG. 2  is a flowchart that illustrates the correlation process in the optical transmitter, according an embodiment of the present invention. 
         FIG. 3  is a flowchart that illustrates how to characterize and program the optical transmitter after the correlation of  FIG. 2  between P out  and A mon  has been determined, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a high-level block diagram of a preferred embodiment of an optical transmitter  101  made in accordance with the teachings of the present invention. A light source  103 , which may be a VCSEL, LED, or other light-emitting device, is driven by a transmission driver  105 . The transmission driver  105  controls the light output of the light source  103  as well as its extinction ratio. A controller  107  communicates with the transmission driver  105  to control and adjust parameters such as the highest and lowest emitted power from the light source. The controller  107  also communicates with a memory  108  for storing and retrieving data. The memory  108  may be an EEPROM local to the optical transmitter  101 , cache memory on the same chip as the controller  107  itself, or any other storage mechanism. 
     A pattern generator  109  generates a pattern that may be used as input to the transmission driver  105  during characterization. Possible patterns that may be generated include a continuous one, a continuous zero, a Pseudo Random Bit Sequence (PRBS), a 1010 pattern, and many others. The pattern generator  109  is incorporated into the optical transmitter  101 , and can be combined on-chip with other circuitry of the optical transmitter  101 . 
     The controller  107  controls the pattern generator  109  to determine which pattern is selected. The controller  107  also controls a mutiplexer  113  to select between a data stream  119  (in normal operation), and pattern from the pattern generator  109  (during characterization and programming) for input to the transmission driver  105 . The controller  107  can be implemented in hardware circuitry, software programming algorithms, or firmware. It can be a microprocessor or a specialized Application Specific Integrated Circuit (ASIC). 
     An optical monitor  121  in the optical transmitter  101  responds to and monitors the light emitted by the light source  103 . The optical monitor  121  generates a signal A mon  that is indicative of the optical power emitted by the light source  103 . A mon  can be any characteristic of the optical monitor that is responsive to the power of light, such as its current, resistance, capacitance, etc. For example, if a photodiode is used as the optical monitor  121 , A mon  may be derived from the current through the photodiode. Or, if a photoresistor is used as an optical monitor  121 , then A mon  may be derived from the resistance of the photoresistor. The present invention is readily adaptable for use with other photosensitive devices, such as phototransistors and photocapacitors. 
     An optical power meter (OPM)  123 , external to the optical transmitter  101 , also measures the light emitted from the light source  103  and generates a power indicator P out . However, the OPM  123  is only needed temporarily for characterization at room temperature, as will become apparent in the paragraphs to come. 
     Both the optical power indicator A mon  as measured by the optical monitor  121 , and the optical power P out  as measured by the OPM  123 , are inputs to the controller  107 . The controller  107  calculates the correlation between A mon  and P out  and stores the correlation in the memory  108 . The correlation between A mon  and P out  is the relationship between the values of A mon  and P out  such that the optical transmitter  101  can calculate and predict what the value of P out  would be for a given value of A mon  based on this relationship. 
     The format in which the correlation is stored may vary. For example, the correlation can be recorded locally in the memory  108  as a look-up table of data points, using the values of A mon  as an index to the corresponding values of P out . Interpolation between or extrapolation of the stored values may be used to calculate values of P out  for any values of A mon  that are not stored in such a table. Or, the correlation may be able to be stored as an equation or algorithm, where P out  is a function of A mon . Other methods of storing the correlation may also be used. 
       FIG. 2  is a flowchart that illustrates the correlation process in the optical transmitter  101 , according to an embodiment of the present invention. In step  201 , the OPM  123  is coupled to the output of the optical transmitter  101  at room temperature. Next, in step  203 , the controller  107  varies the output power P source  of the light source  103  while the OPM  123  and the optical monitor  121  simultaneously measure the output power of the light source  103  and generate indicators P out  and A mon , respectively. Multiple values of P out  and the corresponding values of A mon  should be measured, to ensure accuracy in the final stored correlation. 
     In step  205 , the controller  107  determines the correlation between P out  and A mon  and stores it in the memory  108  (step  207 ) for later retrieval by the optical transmitter  101 . Once the correlation between P out  and A mon  has been determined, the external OPM  123  can be decoupled from the optical transmitter  101  and removed from the system (step  209 ), since it is no longer needed. Instead, the correlation between P out  and A mon  is used to determine other parameters that may be needed to program the transmitter. 
     For example, the power level of the optical transmitter that represents a digital one (P 1 ), and the power level that represents a digital zero (P 0 ), are typically measured in order to fully characterize the light source. P 1  and P 0  can be easily determined without the external OPM  123  after the correlation between P out  and A mon  has been calculated and stored, by reading A mon  from the optical monitor  121  and translating this value to the corresponding P out  using the correlation data previously collected. 
       FIG. 3  is a flowchart that illustrates how to characterize and program the optical transmitter  101  after the correlation between P out  and A mon  has been determined, according to an embodiment of the present invention. In step  301 , the controller  107  signals the pattern generator  109  to send a sequence of continuous digital ones to the transmission driver  105 . The light source  103  will emit light at its power level that represents a digital one. Since the pattern generator  109  is incorporated into the optical transmitter  101 , there is no need to hook up an external pattern generator. 
     Next in step  303 , the optical monitor  121  generates a power indicator A mon  that is representative of the power level it measures from the light source  103 . Then, in step  305 , the controller  107  uses the stored correlation to find or calculate the corresponding value P out  for the given A mon . This corresponding P out  also happens to be P 1  for the optical transmitter  101 , which is the power emitted by the light source  103  when the transmission driver  105  drives a digital one. Due to the initial correlation process described in  FIG. 2 , this calculated value is virtually identical to the value that would have been measured by an OPM  123  if one had actually been hooked up to the optical transmitter  101 . If P 1  is not within prescribed limits, the controller  107  programs the transmission driver  105  to bring P 1  within acceptable levels (step  307 ). This may mean adjusting the bias current of the light source  103 , its modulation current, or both. 
     A similar process is followed to determine P 0 , with the difference that the controller  107  signals the pattern generator  109  to send a sequence of continuous digital zeroes rather than digital ones. After calculating P 1  and P 0 , other parameters can easily be determined, such as the average power (P avg =(P 0 +P 1 )/2), the extinction ratio (ER=P 1 /P 0 ), etc. The pattern generator  109  may also generate other patterns, such as a 1010 pattern or a PRBS, to enable the calculation of other transmitter parameters. 
     The same processes can also be performed as the temperature is changed, to characterize the optical transmitter  101  over a wide range of temperatures. Using the correlation is especially useful during these over-temperature characterizations. No external test equipment is needed, since the pattern generator  109  is integrated into the optical transmitter  101  and no external optical power meter is required. Therefore, the number of devices that can be tested is limited only by the capacity of the temperature-controlled test chamber. 
     Depending on the design of the optical transmitter, it is possible that the correlation between A mon  and P out  may not be strictly independent of temperature. The output response of the optical transmitter when an integrated pattern generator  109  drives its input may also be different from the output response when the optical transmitter is connected to an external pattern generator. However, the controller  107  can be programmed to account and compensate for any such variations. 
     These techniques may also be used to characterize any changes in the response of the optical transmitter over time, or other environmental changes. 
     In an alternate embodiment of the invention, the controller  107 , or the memory  108 , or both the controller  107  and the memory  108 , are located outside of the optical transmitter  101 . For example, the controller  107  and/or memory  108  may be located in an external computer that is connected to the optical transmitter  101  only during characterization and programming. Since the controller  108  and/or memory  108  are not strictly needed during normal operation, it may not be necessary for these components to be a permanent part of the optical transmitter  101 . 
     While primarily described in terms of transmitters of optical light, the present invention could also apply to transmitters of infrared light, ultraviolet light, and other segments of the electromagnetic spectrum. 
     Although the present invention has been described in detail with reference to particular preferred embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.