Abstract:
An integrated circuit for providing burn-in current to a laser diode. The integrated circuit includes a laser driver having an output for connection to the laser diode. Burn-in circuitry is formed on the integrated circuit and generates a burn-in current. A switch is formed on the integrated circuit and couples the burn-in current to the laser diode in response to an enable signal.

Description:
BACKGROUND 
     Increasingly, data communications involve transmissions by optical sources that can deliver high volumes of digitized information as pulses of light. This is especially true for many communication companies that utilize laser diodes and optical fibers as their primary means for the transmission of voice, television and data signals for ground-based communications networks. 
     To achieve high bandwidth, laser diodes such as edge-emitting lasers and Vertical Cavity Surface Emitting Lasers (VCSELs) are commonly utilized as optical sources. These types of laser diodes are preferred due to their minute dimensions. For example, the typical VCSEL is measured in the order of micrometers. Consequently, an array of laser diodes can be integrated into a system to achieve high bandwidth transmissions. 
     In the manufacturing and production of VCSEL arrays, such as 1×12 or 1×4 parallel channel optical arrays, target optical and electrical characteristics are assigned to the arrays. To determine whether the VCSEL arrays will be operating at their target levels, each laser diode of the array is subjected to a burn-in process. That is, each VCSEL must be submitted to a quality control (QC) procedure that includes subjecting the VCSEL to a constant current at an elevated temperature for an extended time period. The burn-in current can be selected to be at a level that is higher than the standard operating current, since the QC procedure is a short-term test of whether the VCSELs will provide long-term performance during actual operating conditions. Similarly, the burn-in temperature is selected to be at a higher temperature than the anticipated operating temperature. Finally, the burn-in time period is selected on the basis of the type, specification and stringency of the devices. 
     Existing configurations use the laser driver to provide burn-in current to the laser diode.  FIG. 1  is a block diagram of a conventional burn-in arrangement. Integrated circuit  10  includes an input buffer  12 , limiting amplifier  14  and laser driver  16 . An ASIC or other device provides signals to the integrated circuit  10  for applications such as communications. To perform the burn-in process, commands from an external digital controller  24  are submitted to an on-chip digital controller  22 . The on-chip digital controller  22  sends commands to laser driver  16  that generates the high burn-in current for laser diode  20 . 
     The use of an on-chip burn-in controller has several drawbacks. First, the on-chip digital controller  22  requires additional bonding pads on the integrated circuit  10 . These extra pads add parasitics that degrade performance (e.g., eye quality), particularly for high transmission rates such as 2.5 Gbps or higher. Furthermore, bonding between the integrated circuit  10  and the digital controller  22  requires a delicate and difficult procedure. 
     Another drawback to the configuration of  FIG. 1  is that the laser driver  16  provides the burn-in current. This may require redundant calibration circuitry used to control the accuracy of the laser driver  16  during the burn-in phase. Furthermore, using laser driver  16  as the burn-in current source may cause degradation of laser driver  16 . The burn-in process is typically performed in a harsh environment (e.g., high temperature, high humidity), at a high DC current for a long time. This can result in degradation of laser driver  16 . 
     SUMMARY OF INVENTION 
     An embodiment of the invention is an integrated circuit for providing burn-in current to a laser diode. The integrated circuit includes a laser driver having an output for connection to the laser diode. Burn-in circuitry is formed on the integrated circuit and generates a burn-in current. A switch is formed on the integrated circuit and couples the burn-in current to the laser diode in response to an enable signal. 
     Another embodiment of the invention is a system for performing burn-in including a current source generating an input current and a laser diode. An integrated circuit includes a laser driver having an output for connection to the laser diode, burn-in circuitry formed on the integrated circuit generating a burn-in current and a switch formed on the integrated circuit for coupling the burn-in current to the laser diode in response to an enable signal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a conventional system for laser diode burn-in. 
         FIG. 2  is a block diagram of a system for laser diode burn-in in an embodiment of the invention. 
         FIG. 3  is a schematic diagram of an on-chip burn-in circuit in an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a block diagram of a system for laser diode burn-in in an embodiment of the invention. In  FIG. 2 , integrated circuit  100  is an N-channel laser drive chip. Each channel includes an input buffer  102 , a limiting amplifier  104 , and a laser driver  106 . Input buffer  102  stores signals from a communication device prior to transmission to the laser driver  106 . Limiting amplifier  104  amplifies the output of input buffer  102  and provides the amplified signals to laser driver  106 . Laser diver  106  drives laser diode  110  to generate optical signals for communications. An ASIC  112  provides signals to each channel for communications. The laser diodes  110  may be part of a VCSEL in a communications device, such as a fiber optic transmitter. 
     Integrated circuit  100  includes a burn-in circuit  120 . As described in further detail herein, the burn-in circuit  120  generates burn-in current for the laser diodes  110  during the burn-in process. The burn-in circuit  120  is formed in the integrated circuit  100  and may be implemented in BiCMOS circuits. The burn-in circuit  120  includes N-outputs connected to each of the N laser diodes through switches  122 . A current source  124  provides a precise current to the burn-in circuit  120 . In exemplary embodiments, the burn-in circuit amplifies the input current and generates a burn-in current that is fed to the laser diodes  110  through switches  122 . An enable signal is provided to the burn-in circuit to activate the amplifier and close switches  122 . In alternate embodiments, the burn-in circuit serves as a signal buffer that buffers the input current without amplification. 
       FIG. 3  is schematic diagram of an on-chip burn-in circuit in an embodiment of the invention. Portions of integrated circuit  100  are not shown for ease of illustration. Shown in  FIG. 3  is one section  130  of the burn-in circuit  120  generating burn-in current for one laser diode. Circuit section  130  includes an amplifier to amplify current from current source  124  and provide the amplified current to laser diode  110 . In the embodiment shown in  FIG. 3 , the amplifier is a current mirror configuration implemented through field effect transistors  132  and  133 . Input current Ii from current source  124  is amplified to produce burn-in current Ib. Alternatively, the burin-in circuit may buffer the input current without amplification. 
     An enable block  134  receives an enable signal from an external source. The external source may be a controller that generates an analog or digital enable signal that is applied to a pad on integrated circuit  100  or transmitted via an interface (e.g., a two wire serial interface). In response to the enable signal, the enable block  134  opens or closes switch  122  to provide the burn-in current to the laser diode  110 . In the embodiment shown in  FIG. 3 , switch  122  is implemented using a field effect transistor. It is understood that other switch elements may be used to couple the burn-in current to laser diode  110  and that multiple transistors may be used to provide switch  122  (e.g., a multiple transistor pass gate). 
     Enable block  134  also sends signals to switches  142  and  144 . When switch  142  is closed, the gates of transistors  132  are connected to ground disabling these FETs. Similarly, when switch  144  is closed, the gates of transistors  133  are connected to Vcc disabling these FETs. Since the burn-in circuit  120  is operated during manufacturing, it is preferable to disable the current mirror amplifier once the burn-in process is complete. This prevents the burn-in circuit  120  from generating stray currents that may effect operation of the integrated circuit  100 . 
     In operation, when burn-in is to be performed, the enable signal is provided to enable block  134 . This causes switch  122  to close and switches  142  and  144  to open. Current from current source  124  is amplified by the burn-in circuit  120  and provided to the laser diodes  110  through switches  122 . When the burn-in process is complete, the enable signal changes states, closing switches  142  and  144  and opening switch  122 . This deactivates the burn-in circuit  120  and isolates the burn-in circuit from the laser diode  20 . 
     While exemplary embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.