Abstract:
An optical transceiver system includes circuitry including a light-emitting device, a driver for the light-emitting device being coupled to the light-emitting device for providing activating power to said light-emitting device and an optical detector disposed to receive light emitted from the light-emitting device, said circuitry being substantially constructed using CMOS technology. A calibration interface is coupled to the circuitry for automatically executing an iterative cycle for a calibration of optical link parameters. And, an optical loop-back is included for optically coupling the light-emitting device and the optical detector, the optical detector receiving light emitted by the light-emitting device, the light emission being selectively stimulated by the calibration interface, the received light emission being communicated to the calibration interface for comparison with a known light signal. A method of automatic calibration and control of optical link parameters in a VCSEL-based optical transceiver is included.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 60/073,481, filed Feb. 3, 1998, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to fiber optic communication, and more particularly, to a method and system for the calibration and control of primary optical link parameters for use within fiber optic communication systems, as well as in other applications. 
     2. Description of Related Art 
     Optical transmission systems have three general components: the light source, the transmission medium, and the detector. The light source for an optical transmission system typically includes one or more Light Emitting Diodes (LEDs) or lasers. A pulse of light from the light source commonly indicates a one bit and the absence of light indicates a zero bit. As a light source, the semiconductor laser has distinct advantages over the LED, including higher data rates and longer distance transmission capabilities. The transmission medium is commonly ultra-thin glass fiber. The detector generates an electrical pulse when light falls upon it. With current technology, transceiver modules containing both the light source and the light detector are increasingly preferred. 
     Low-cost, high-performance, highly integrated fiber optic interface circuits are becoming increasingly necessary to meet the demands of high-speed digital data communication. With the advent of gigabit Ethernet systems, for example, fiber optic technology has become increasingly preferred. A fiber optic transmission line typically uses one or more newly developed and relatively inexpensive vertical-cavity surface-emitting laser (VCSEL) diodes as the light source to transmit optical data. 
     The high-speed nature of fiber optic communication necessitates that the VCSEL-based optical transceivers operate quickly, accurately and efficiently, for best results. 
     Calibration of prior art VCSELs is performed manually. An operator typically manually modifies resistance or other parameters while watching the laser output wave form on an oscilloscope or other device. This modification is typically performed by laser trimming or by the use of potentiometers. This method of iterative active manual calibration must typically be performed before assembly of any module containing the laser, which adds undesirable time and expense to the calibration process and the optical transmission systems. Further, recalibrating the transceiver to new or different optical link parameters requires disassembly of the transceiver module, followed by another round of iterative active manual calibration and then reassembly. This adds considerable time and expense to the calibration process. 
     To enhance the operation of the one or more VCSEL diodes, and to make more efficient and cost-effective the methods and systems for calibrating VCSELs that are compatible with the modularity of present laser transceiver systems, new methods and systems for calibrating VCSEL-based transceivers are needed. Particularly, methods and systems are needed for intelligent, active and automatic calibration and control of primary optical link parameters. 
     SUMMARY OF THE INVENTION 
     Accordingly, methods and systems for control and calibration of VCSEL-based optical transceivers are provided that meet many, if not all, of the above-described needs. A control scheme for a VCSEL-diode-based optical transceiver preferably replaces present control methods of manual calibration. The new control scheme preferably utilizes an automated system incorporating an optical feedback loop. Further, the control scheme preferably reduces cost and improves optical transceiver performance. To accomplish these goals, systems and methods for control and calibration of VCSEL-diode-based optical transceivers according to embodiments of the invention are provided. 
     Accordingly, an object of the present invention is to provide an optical loop-back for sampling the laser output of the transceiver. 
     Another object of the invention is to provide an integrated A/D converter for dark, mark and space input readings. 
     Yet another object of the invention is to provide an integrated digitally programmable interface for programming laser bias and modulation currents. 
     Still another object of the invention is to provide a serial EEPROM interface for non-volatile target parameters and calibration data storage. 
     Still another object of the invention is to provide algorithmic control of the above-described features, to provide intelligent and automatic control and calibration of the primary optical link parameters. 
     The present invention is an optical transceiver system that includes circuitry including a light emitting device, a driver for the light emitting device being coupled to the light emitting device for providing activating power to said light emitting device and an optical detector disposed to receive light emitted from the light emitting device, said circuitry being substantially constructed using CMOS technology. A calibration interface is coupled to the circuitry for automatically executing an iterative cycle for a calibration of optical link parameters. And, an optical loop-back is included for optically coupling the light emitting device and the optical detector, the optical detector receiving light emitted by the light emitting device, the light emission being selectively stimulated by the calibration interface, the received light emission being communicated to the calibration interface for comparison with a known light signal. The present invention is further a method of automatic calibration and control of optical link parameters in a VCSEL-based optical transceiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a VCSEL-based optical transceiver according to an embodiment of the present invention. 
     FIG. 2 is a flow chart showing the states of the operational mode of the optical transceiver. 
     FIG. 3 is a flow chart showing the operation of an embodiment of the invention in calibration mode. 
     FIG. 4 is a more detailed block diagram of the bias and modulation current control digital to analog converter shown in FIG.  1 . 
     FIG. 5 is a more detailed block diagram of the transimpedance amp measurement block shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows one preferred embodiment of a VCSEL-based optical transceiver that can be controlled and calibrated automatically. The system preferably comprises CMOS transceiver integrated circuit  10 , electrically erasable programmable read only memory (EEPROM)  15 , optical detector  20 , VCSEL diode  25 , and optical loop-back  30 . CMOS transceiver integrated circuit  10  preferably includes block  35  comprising the interface, control and state machine. 
     Block  35  is connected to data output line  40 , calibration initiate line  45 , and calibration out line  50  and data input line  55 . Circuit  10  also includes transimpedance amplifier measurement block  60 , post amplifier  65 , transimpedance amplifier  70 , bias and modulation current control digital-to-analog converter  75  and VCSEL laser driver  80 . Block  35  is connected to EEPROM  15 , transimpedance amplifier measurement block  60 , post amplifier  65 , and bias and modulation current control digital-to-analog converter  75 . Transimpedance amplifier measurement block  60  is directly connected to a first side of optical detector  20  for measuring light emission output of the VCSEL diode  25  sensed by optical detector  20 . Block  60  is further connected to transimpedance amplifier  70  and post amplifier  65 . Transimpedance amplifier  70  is connected to post amplifier  65  and further connected to optical detector  20 . Bias modulation current control digital-to-analog converter block  75  is connected to VCSEL laser driver  80 , which in turn is connected to VCSEL diode  25 . Optical loop-back  30  extends between VCSEL diode  25  and optical detector  20 . Optical loop-back  30  is an external component, preferably inserted into the duplex ports of transceiver integrated circuit  10  while the device is operating in calibration mode. 
     FIG. 4 is a more detailed view of block  75  showing two digital to analog converters, Imod DAC 85 and Ibias DAC 90, each converter having output lines  87 ,  92 , respectively. Block  75  is further comprised of Imod Register  95  and Ibias Register  100 , each register having respective data lines  105 ,  107  and corresponding write lines, Imod write  110  and Ibias write  115 . Step  120 , then, initiates registers  95  and  100  to zero values. 
     A more detailed view of block  60  is shown in FIG.  5 . As can be appreciated, block  60  contains analog to digital (A/D) converter  125 , which is connected to Tia analog output line  130  and Tia_out data line  135 . Based upon the signal received from optical detector  20  at Tia analog output  130 , A/D converter  125  generates a new Tia out signal on line  135 . 
     Full integration of VCSEL laser driver  80 , transimpedance amplifier  70  and post amplifier  65 , along with additional control measurement circuitry, as can be found in CMOS transceiver integrated circuit  10 , allows circuit  10  to measure key laser optical parameters and to optimize bias (Ibias) and modulation (Imod) currents for driving VCSEL diode  25 . Inserting optical loop-back  30  into the duplex ports permits the light output of VCSEL diode  25  to be received by optical detector  20 . 
     In operation, calibration initiate input  45  into block  35  is toggled to initiate the calibration cycle. An on-chip ring oscillator (not shown) preferably is used to generate a local clock source for the calibration cycle. Serial data input  55  and output  40  are disabled during the calibration cycle, and calibration output  50  toggles high to indicate successful completion of the calibration cycle. Data input  55  and data output  40  give programmable access to EEPROM  15  so that optical link parameters may be stored and charged. 
     These high (Imod) and low (Ibias) reference values are set and stored on internal registers of the DAC 75. VCSEL laser driver  80  is then activated and the Tia output is digitized and compared to the stored reference level, while bias and modulation currents within laser driver  80  are stepped by control of the current setting digital-to-analog converter  75 . The bias current setting controls the VCSEL diode  25  threshold current. The modulation current setting controls the maximum light output from VCSEL diode  25 . 
     The setting of the bias and modulation currents optimizes the VCSEL diode  25  operation, compensating for variations in laser transfer characteristics (slope efficiency) and ensuring correct light level and extinction ratio (the ratio of data mark to data space levels). The bias and modulation current values are stored in EEPROM  15 , ensuring non-volatile storage of the operating parameters after calibration. 
     Embodiments of the invention enable completely integrated measurement and control of the bias and modulation current requirements for VCSEL diode  25 . Further, no complex packaging or calibration is required for monitoring the light output of VCSEL diode  25 . 
     FIG. 3 is a flow chart of a calibration cycle according to an embodiment of the invention. The calibration cycle is initiated by transmitting the appropriate signal into calibration initiate  45  of block  35  (see FIG.  1 ). 
     First step  120  of the calibration cycle sets the values of Ibias and Imod to zero. These values are stored in Imod Register  95  (FIG. 4) and Ibias Register  100  located in the bias and modulation current control DAC  75  (FIG.  1 ). 
     The next sequential step, step  140 , reads the target ‘0’ information from EEPROM  15 . The target ‘0’ parameters are used to calibrate the laser to output a ‘0’ bit. Advantageously, EEPROM  15  can be pre-programmed with information to accommodate a wide range of optical transmission parameters. 
     Step  145  then increments the Ibias value by a predetermined amount, writing the new value to Ibias register  100 . A corresponding bias current is then generated by Ibias DAC  90  and is used by laser driver  80  to produce a light emission from diode  25 . This light emission is then looped back through optical loop-back  30  to optical detector  20  and input into the transimpedance amplifier measurement and calibration block  60 . 
     In step  150 , the output generated by the A/D converter  125  (FIG. 5) is read. In step  155 , the output from the A/D converter  125  is compared with the target ‘0’ values previously retrieved from EEPROM  15 . If the output from A/D converter  125  is greater than or equal to the target value, the calibration of the target ‘0’ parameter is complete, ‘yes’  160  is generated, and the cycle proceeds to step  165 . Otherwise, ‘no’  170  is generated and an iteration begins by jumping back to step  145 . The Ibias value is again incremented, and the new Ibias value is again used by laser driver circuit  80  to produce a new light emission from VCSEL  25 . Detector  20  receives the light emission and again supplies the A/D converter  125  with an analog signal which is converted to new digital signal Tia_out on line  135 . This new value of Tia_out is read in step  150 , and in step  155 , is again compared to the target parameter. This iteration continues until the generated Tia_out signal on line  135  is greater than or equal to the target ‘0’ parameter retrieved from EEPROM  15  in step  140 . If this condition is satisfied, the ‘yes’ value  160  is generated. 
     After ‘yes’  160  is generated, processing proceeds to step  165 . There, the last value generated by A/D converter  125  is written to Ibias register  100 . This value represents the calibrated value corresponding to the target ‘0’ parameter. In the following step, step  175 , the same Ibias value is stored in non-volatile EEPROM  15  for future use. Particularly, when the transceiver is turned off after calibration, the values written to the registers  95 ,  100  will be lost. When the unit is again powered up, the calibrated Ibias and Imod values are retrieved from EEPROM  15 , or other comparable memory storage device. 
     At step  180 , the calibrating of the target ‘1’ parameter commences. As with the calibrating of the target ‘0’ parameter, the target ‘1’ parameter is first read from EEPROM  15 . Step  185  increments the value of Imod by a predetermined amount from its initial zero value, as set in step  120 . The current value is then used to generate a light emission from VCSEL  25  that is received by optical detector  20  through optical loop-back  30 . A/D converter  125  generates a new value of Tia_out signal on line  135 , which is read at step  190  and compared with the retrieved target parameter at step  195 . If the newly generated value of the Tia_out signal is not greater than or equal to the target ‘1’ parameter, ‘no’  200  is generated and the cycle jumps back to step  185 . There, Imod is again incremented and a new value of Tia_out signal on line  135  is generated as described above. This iteration continues until the Tia_out signal is greater than or equal to the target ‘1’ parameter. At this point, ‘yes’  205  is generated and the cycle proceeds to step  210 . 
     At step  210 , the value of Imod that meets the target parameter is stored in Imod register  95 . As with the value of Ibias, the Imod value is also stored in EEPROM  15 , at step  215 , so that the Imod register can be restored to the correct value after the circuit has been powered down. The calibration cycle is then completed  217 , as shown. A signal is generated on calibration output line  50  to indicate a successful calibration. 
     FIG. 2 is a flow chart representing transceiver  10  running in its operational mode. When transceiver  10  is first powered up after calibration, the Ibias and Imod values stored in Ibias register  100  and Imod register  95  will no longer be present. The first step  220  in the operational mode is to read the Imod value from EEPROM  15 . In step  225 , the value is stored into Imod register  95  where it will be used by laser driver  80  to produce the calibrated logical ‘1’ output light emission. The algorithm then proceeds to step  230 , where the value stored for Ibias is retrieved from EEPROM  15  and written in step  235  to Ibias register  100  used by laser driver  80  to produce a logical ‘ 0 ’ light emission from light emitting device  25 . The algorithm is then completed  240 , as shown. 
     It is anticipated that the cycles and algorithms described herein can be implemented in many ways. For example, the logic of the algorithm can be hard-wired into block  35 . Further, block  35  can contain a microprocessor and minimal RAM and ROM that could store and execute programmed instructions in a manner consistent with the above described algorithm. It is anticipated that the invention can be implemented virtually completely using CMOS technology. 
     The specification is intended to be illustrative of the many variations and equivalents possible according to the invention. Various modifications in and changes to the above-described devices and methods will be apparent to those of ordinary skill. Though these systems and methods of control and calibration of an optical transceiver were described with particularity for uses directed at VCSEL diodes and high-speed fiber optic communications, other light emitting devices and other uses for such a system are contemplated. For example, LEDs may be used in some applications as a light source. Other applications that might benefit from this invention include bar code scanners, encoders, proximity sensors, laser printers, and laser range finders, among others.