Patent Abstract:
A method for a fiber optic device to conserve power includes turning off components in the fiber optic device and turning them back on when a detection signal is at a specified level. A method for a laser system to adjust a threshold level for signal detection includes generating a digital gain signal, amplifying at least one data signal with a gain based on the gain signal, comparing the at least one amplified data signal with a reference signal, and generating a signal based on the comparison. A method for a laser system to set error warnings includes receiving control bits that indicate if a host desires to be notified of certain error conditions and generating at least one signal based on the control bits to indicate at least one error condition.

Full Description:
FIELD OF INVENTION 
     This invention relates to error detection in laser transmitters, receivers, and transceivers. 
     DESCRIPTION OF RELATED ART 
     Laser transceivers are commonly used to transmit and receive data in optical networks. Error detection is vital to ensure that malfunctions in the laser transceivers are quickly identified and repaired so that communication is not interrupted. Thus, what are needed are methods and apparatuses that enhance error detection in the laser transceivers. 
     SUMMARY 
     In accordance with one aspect of the invention, a method for a laser system to conserve power includes turning off components in the laser system, detecting a data signal after a duration of time, and generating a detection signal in response to the detecting. The detection signal can be a loss of signal (LOS) signal or a signal detect (SD) signal. The method further includes repeat the turning off, the detecting, and the generating when the detection signal is at a first level, and turning on the plurality of components when the detection signal is at a second level. 
     In accordance with one aspect of the invention, a method for a laser system to adjust a threshold level for signal detection includes generating a digital gain signal, amplifying at least one data signal with a gain based on the gain signal, comparing the at least one amplified data signal with a reference signal, and generating a detection signal based on the comparison. The method further includes adjusting the gain signal based on the comparison of the at least one amplified data signal, comparing the value of the gain signal with at least a second reference signal, and generating the detection signal based on the comparison of the value of the gain signal. The method further includes setting a control bit to generating the signal as a loss of signal or a signal detect signal. 
     In accordance with one aspect of the invention, a method for a laser system to set error warnings includes receiving control bits that indicate if the host desires to be notified of certain error conditions, detecting occurrences of the error conditions, writing error bits based on the detected occurrences of the error conditions, performing a logical operation between the corresponding control bits and error bits, and generating at least one signal based on the result of the logical operation to the host to indicate at least one error condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a laser system in one embodiment of the invention. 
         FIG. 2  is a schematic of an LOS (loss of signal) circuit in the laser system of  FIG. 1  in one embodiment. 
         FIG. 3  is a schematic of an RX (receiver) circuit in the laser system of  FIG. 1  in one embodiment. 
         FIG. 4  is a flow chart of a method for the laser system of  FIG. 1  to conserve power. 
         FIG. 5  is a schematic of a programmable amplifier in the LOS circuit of  FIG. 2  in one embodiment. 
         FIG. 6  is a schematic of a peak detector in the LOS circuit of  FIG. 2  in one embodiment. 
         FIG. 7  is a schematic of a comparator with hysteresis in the LOS circuit of  FIG. 2  in one embodiment. 
         FIG. 8  is a flow chart of a method for the laser system of  FIG. 1  to adjust the LOS threshold level. 
         FIG. 9  is a flow chart of a method for the laser system of  FIG. 1  to generate a LOS signal using a closed feedback loop. 
         FIG. 10  illustrates an interrupt mask used to control error detection in one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a laser system  100  (e.g., a laser transceiver) in one embodiment of the invention. The anode of a laser  10  (e.g., a laser diode) is connected to a supply rail  11  that provides a power supply voltage Vcc_tx. The cathode of laser diode  10  is connected at a node  12  to an Ibias driver  14  that sinks a constant current Ibias from laser diode  10 . A controller  16  sets the magnitude of current Ibias by outputting a control signal IBIAS to driver  14 . Control signal IBIAS can be an analog signal or a digital signal having multiple bits. 
     The cathode of laser diode  10  is also connected at node  12  to an Imod driver  18  that sinks a modulation current Imod from laser diode  10 . Controller  16  sets the magnitude of current Imod by outputting a control signal IMOD to driver  18 . Control signal IMOD can be an analog signal or a digital signal having multiple bits. The drive current applied to laser diode  10  is either current Ibias or the sum of currents Ibias and Imod. 
     An amplifier  28  receives data signal IN_P and its complement IN_N to be transmitted by laser diode  10 . In response, amplifier  28  outputs amplified signals IN_P′ and IN_N′ to an LOS (loss of signal detection) circuit  30 .  FIG. 2  illustrates on embodiment of LOS circuit  30 . LOS circuit  30  outputs signals IN_P′ and IN_N′ as control signals IN_P″ and IN_N″ to driver  18 . LOS circuit  30  also outputs a signal TX_LOS_INT when signals IN_P′ and IN_N′ are not detected because their amplitudes are below a threshold. The purpose and operation of LOS circuit  30  are described later in one aspect of the invention. 
     A mirror  32  reflects a part of the light signal from laser diode  10  to a light detector  34  (e.g., a photodiode) and transmits a part of the light signal to a fiber  36  that carries the light signal to another component. Photodiode  34  is connected between supply rail  11  and an Imon ADC (analog-to-digital converter)  38 . Photodiode  34  outputs an analog signal Imon to Imon ADC  38 . Analog signal Imon is proportional to the reflected power received by photodiode  34 . The reflected power is proportional to the transmitted power received by fiber  36  and the total output power of laser diode  10 . Imon ADC  38  outputs a digital signal IMON to controller  16 . 
     An Iref source  40  outputs a reference signal IREF to controller  16 . Reference signal IREF can be either digital or analog. Controller  16  typically compares signals IREF and IMON to control laser diode  10  in a closed feedback loop. 
     A temperature sensor  42  outputs a signal TEMP to controller  16 . Signal TEMP can be either digital or analog. Signal TEMP is proportional to the temperature of laser system  100 . 
     A Vcc ADC  44  is connected to supply rail  11  and outputs a digital signal VCC_TX to controller  16 . Signal VCC_TX is proportional to supply voltage Vcc_tx supplied to laser diode  10 . 
     A memory  46  outputs parameters for calibrating laser system  100  to controller  16 . Controller  16  communicates with a host on a bus DIG_IO. The host can be an external processor, computer, or a test equipment. Memory  46  may be a programmable nonvolatile memory such as an EEPROM. 
     Laser system  100  may include an RX circuit  50  that decodes a light signal received from a fiber  52 . A light detector  54  (e.g., a photodiode) converts the light signal to an analog current signal IN 1 . RX circuit  50  converts signal IN 1  to a voltage data signal OUT_P and its complement OUT_N. RX circuit  50  also outputs a signal RX_LOSS_INT when the data signals are not detected because their amplitudes are below a threshold. 
       FIG. 3  illustrates one embodiment of RX circuit  50 . A resistor  56  and a capacitor  57  form an RC filter that provides a filtered voltage Vbias to photodiode  54 . Photodiode  54  outputs signal IN 1  to a non-inverted terminal of a transimpedance amplifier (TIA)  58 . TIA  58  has a inverted terminal coupled to the ground as a reference. 
     TIA  58  converts current signal IN 1  to a voltage signal OUT_P′ and its complement OUT_N′. TIA  58  outputs signals OUT_P′ and OUT_N′ to an amplifier  60  and a LOS circuit  61 . Amplifier  60  outputs amplified signal OUT_P and its complement OUT_N to the host. Additional gain stages  62  may be used to further amplify signals OUT_P and OUT_N. 
     LOS circuit  61  outputs a signal RX_LOS_INT when signals OUT_P′ and OUT_N′ are not detected because their amplitudes are below a threshold. LOS circuit  61  can be similarly constructed as LOS circuit  30  as shown in  FIG. 2  where the signals to and from circuit  61  are shown in parenthesis. The purpose and operation of LOS circuit  61  are described later in one aspect of the invention. 
     TIA  58  includes a DC cancellation circuit  63  that feeds back a DC cancellation current into the non-inverted terminal of amplifier  58 . The DC cancellation current is proportional to the average optical power of the light signal received by photodiode  54 . 
     In accordance with one aspect of the invention, a host can set a control bit to determine whether laser system  100  generates a LOS or a SD (signal detect) signal. As described above, the LOS signal indicates that the data signals are not detected because their amplitudes are below a threshold. The SD signal is the complement of the LOS signal. The SD signal indicates that the data signals are detected because their amplitudes are above a threshold. Some applications of laser system  100  may prefer the LOS signal while others may prefer the SD signal. 
     Referring back to  FIG. 1 , the host can set a control bit in a register or in memory  46  to indicate its preference for the LOS or the SD signal from LOS circuit  30 . Controller  16  can read the control bit and generate a control signal TX_LOS_INVERT that controls if LOS circuit  30  generates the LOS or the SD signal. Alternatively, the register can directly output control signal TX_LOS_INVERT. A logic gate  70  (e.g., an exclusive OR gate) receives signals TX_LOS_INT and TX_LOS_INVERT. If control signal TX_LOS_INVERT is low, gate  70  generates the LOS signal (e.g., a signal TX_LOS). If control signal TX_LOS_INVERT is high, gate  70  generates the SD signal (e.g., a signal TX_SD). 
     Similarly, the host can set a control bit that indicates its preference of the LOS or the SD signal from LOS circuit  61  ( FIG. 3 ) in RX circuit  50 . A logic gate  72  (e.g., an exclusive OR gate) receives signals RX_LOS_INT and RX_LOS_INVERT. If control signal RX_LOS_INVERT is low, gate  72  generates the LOS signal (e.g., a signal RX_LOS). If control signal RX_LOS_INVERT is high, gate  72  generates the SD signal (e.g., a signal RX_SD). 
     In accordance with one aspect of the invention, laser system  100  can be brought out of a sleep mode by periodically checking for an incoming data signal and bringing laser system  100  out of the sleep mode when the incoming data signal is detected.  FIG. 4  is a flow chart of a method  90  for bringing laser system  100  out of the sleep mode in one embodiment. 
     In step  92 , controller  16  puts laser system  100  in an AWARE (Awake on Remote Event) mode and clears a count. Controller  16  does so in response to a host command or when data signals have not been received by either LOS circuit  30  or  61  over a period of time. In the AWARE mode, laser system  100  conserves power by turning off the current sources in laser system  100 . Controller  16  can turn off the various current sources by blocking the reference voltages (e.g., collectively shown as reference voltage Vref in  FIG. 1 ) to the current sources. For example, current sources in Ibias driver  14 , Imod drive  18 , and LOS circuits  30  and  61  can be turned off. 
     In step  94 , controller  16  increments the count. Controller  16  can perform the actual counting or use a counter. 
     In step  96 , controller  16  determines if a specific time T 1  has passed. If so, step  96  is followed by step  98 . If time T 1  has not passed, then step  96  loops back to step  94  where the count is continued. 
     In step  98 , controller  16  turns on LOS circuit  30  ( FIG. 1 ) and/or LOS circuit  61  ( FIG. 3 ) for a specific time T 2  to check for any incoming data signals. 
     In step  100 , controller  16  determines if either LOS circuit  30  or  61  detects an incoming data signal at time T 2 . If so, step  100  is followed by  104 . If neither LOS circuit  30  or  61  detects an incoming data signal at time T 2 , step  100  is followed by step  102 . 
     In step  102 , controller  16  resets the count. Step  102  is followed by step  94  and method  90  repeats until an incoming data signal has been detected. 
     In step  104 , controller  16  turns on the current sources in laser system  100  by providing them with their reference voltages. 
     In step  106 , controller  16  exits the AWARE mode. 
     In accordance with one aspect of the invention, LOS threshold levels can be adjusted digitally. Referring back to  FIG. 2 , LOS circuit  30  includes a programmable amplifier  130  that has a control terminal receiving a control signal GAIN from controller  16 . Programmable amplifier  130  also has input terminals that receive data signals IN_P and IN_N. Programmable amplifier  130  amplifies signals IN_P and IN_N with a gain set by control signal GAIN and outputs the data signals as signals IN_P′ and IN_N′. 
     Peak detectors  132  and  134  are each coupled to receive signals OUT_P′ and OUT_N′. Peak detector  132  outputs an analog signal Vpeak′ that is proportional to the peak level of signals OUT_P′ and OUT_N′. Peak detector  132  also outputs a reference signal Vpeakref′ derived from signals OUT_P′ and OUT_N′. Peak detector  132  shifts signal Vpeak′ down by an offset such that without any input signals, signal Vpeak′ is offset below signal Vpeakref′. Similarly peak detector  134  outputs signals Vpeak and Vpeakref but without the offset. 
     A comparator  136  with hysteresis circuitry compares signals Vpeak′ and Vpeakref′. When Vpeak′ is less than signal Vpeakref′, comparator  136  brings a signal TX_LOS_INT high. 
       FIG. 5  illustrates one embodiment of programmable amplifier  130 . Amplifier  130  includes a differential gain stage  150 . Stage  150  includes bipolar transistors  152  and  154  having their collector terminals coupled by resistors  156  and  158  to supply rail  11 , respectively. The output voltages of stage  150  are set by the number of current sources  160 - 0  to  160 - 2  that are coupled in parallel to sink a current from the emitter terminals of transistors  152  and  154 . Current sources  160 - 0  to  160 - 2  can be individual turned on by corresponding control signals GAIN 0  to GAIN 2  (collectively referred to as “control signal GAIN”). The output voltages of stage  150  are level-shifted by bipolar transistors  162  and  164 . Transistors  162  and  164  have their collector terminals connected to supply rail  11  and their emitter terminals connected to current sources  166  and  168 , respectively. 
       FIG. 6  illustrates one embodiment of peak detector  132 . Peak detector  132  includes a differential pair  190  that selectively passes signals OUT_P′ and OUT_N′ to a holding capacitor  192 . Holding capacitor  192  captures the highest voltage output from differential pair  190 , which is the peak voltage of signals OUT_P′ and OUT_N′ level-shifted one base emitter junction voltage drop. The peak voltage, minus a voltage drop across a resistor  191 , is provided as signal Vpeak′. The voltage drop across resistor  191  provides an offset by which signal Vpeak′ will be less than reference signal Vpeakref′ to generate a LOS condition unless sufficient input signals are present such that Vpeak′ is greater Vpeakref′. 
     A voltage divider  194  is coupled between the lines that carry signals OUT_P′ and OUT_N′. Voltage divider  194  outputs the average of their voltages to a holding capacitor  196 . Holding capacitor  196  captures the highest voltage output from voltage divider  194  level-shifted one base emitter junction voltage drop, which is provided as reference signal Vpeakref′. Signal Vpeakref′ represents the DC level of the input signals. 
     Peak detector  134  is similarly constructed as peak detector  132  but may not include resistor  191  that provides the voltage offset. This is because peak detector  134  is used with an ADC  230  (described later) to determine a digital representation of the optical modulation amplitude (OMA) of signals OUT_P′ and OUT_N′. 
       FIG. 7  illustrates one embodiment of hysteresis comparator  136 . When voltage signal Vpeak′ is larger than voltage signal Vpeakref′ by a hysteresis amount, NMOS transistor  210  is turned on to pull low the gate terminals of PMOS transistors  212 ,  214 , and  216 . This causes transistor  216  to turn on and couple supply rail  11  to the input terminal of an inverter  218 . Thus, inverter  218  generates a low signal TX_LOS_INT when voltage signal Vpeak′ is larger than Vpeakref′. 
     When voltage signal Vpeak′ is less than Vpeakref′ by a hysteresis amount, NMOS transistor  230  is turned onto pull low the gate terminals of PMOS transistors  232 ,  234 , and  236 . This causes transistor  236  to turn on and couple supply rail  11  to the gate terminals NMOS transistors  238  and  239 . This causes transistor  239  to turn on and ground the input terminal of inverter  218 . Thus, inverter  218  generates a high signal TX_LOS_INT when voltage signal Vpeak′ is less than Vpeakref′. Transistors  212  and  232  provide hysteresis to prevent oscillation of signal TX_LOX_INT. 
     Instead of comparator  136 , ADC  230  ( FIG. 2 ) can be used to determine if signal TX_LOS_INT should be generated. ADC  230  has an input terminal receiving analog voltage signal Vpeak and a reference terminal receiving reference signal Vpeakref. ADC  230  converts the analog input signal to a digital signal TX_OMA, which corresponds to the peak level of data signals IN_P and IN_N. Controller  16  then compares the value of digital signal TX_OMA with two reference signals for hysteresis purposes. If signal TX_OMA is greater than a first reference signal, then controller  16  generates a low signal TX_LOS_INT′. If signal TX_OMA is less than a second reference signal, then controller  16  generates a high signal TX_LOS_INT′. Controller  16  then uses a switch  231  ( FIG. 1 ) to pass signal TX_LOS_INT′ instead of signal TX_LOS_INT as an input to gate  70 . 
     Controller  16  can use an ADC in LOS circuit  61  in RX circuit  50  as described above. Controller  16  generates a signal RX_LOS_INT′ after comparing the value of digital signal RX_OMA with reference signals. Controller  16  uses a switch  233  ( FIG. 1 ) to pass signal RX_LOS_INT′ instead of signal RX_LOS_INT as an input to gate  72 . 
     In accordance with one aspect of the invention, the LOS threshold level can be adjusted according to the temperature of laser system  100 .  FIG. 8  is a flow chart of a method  240  for controller  16  to adjust the LOS threshold level in one embodiment. Method  240  is explained with reference to LOS circuit  30  but it is also applicable to LOS circuit  61 . 
     In step  242 , values of control signal GAIN for a range of temperatures are stored in a table in memory  46 . The values are experimentally determined to generate a constant LOS threshold level over different temperatures. Instead of the table, a function correlating the values of control signal GAIN to different temperatures can be extrapolated from experimental data and stored in memory  46 . 
     In step  244 , controller  16  determines the temperature of laser diode  10  by reading signal TEMP from temperature sensor  42 . 
     In step  246 , controller  16  determines a value of control signal GAIN at the present temperature. Controller  16  can look up the value of control signal GAIN in the table in memory  46 . Alternatively, controller  16  can calculate the value of control signal GAIN at the present temperature using the function extrapolated from experimental data. 
     In step  248 , controller  16  generates control signal GAIN to amplifier  130 . 
     In accordance with one aspect of the invention, LOS detection can be performed using a closed feedback loop where signal GAIN is adjusted to maintain a constant output of LOS ADC  230  and the value of signal GAIN is compared with one or more reference signals to determine if any data signals are detected.  FIG. 9  is a flow chart of a method  270  for controller  16  to generate a LOS signal using a closed feedback loop in one embodiment. Method  270  is explained with reference to LOS circuit  30  but it is also applicable to LOS circuit  61 . 
     In step  272 , controller  16  reads signal TX_OMA from LOS circuit  30 . 
     In step  274 , controller  16  determines if signal TX_OMA is approximately equal to a threshold OMAth. If not, step  274  is followed by step  276 . If signal TX_OMA is approximately equal to threshold OMAth, then step  274  is followed by step  278 . 
     In step  276 , controller  16  adjusts the value of signal GAIN so ADC  230  generates a constant output. If signal TX_OMA is less than threshold OMAth, then controller  16  increases signal GAIN, and vice versa. Step  276  loops back to step  272 . 
     In step  278 , controller  16  determines if the value of signal GAIN is less than a threshold GAINth 1 . If so, step  278  is followed by step  280 . If signal GAIN is not less than threshold GAINth 1 , step  278  is followed by step  282 . 
     In step  280 , controller  16  sets signal TX_LOS_INT low if it had been set high because the data signals are detected. Step  280  is followed by step  272  and method  270  repeats. 
     In step  282 , controller  16  determines if the value of signal GAIN is greater than a threshold GAINth 2 . If so, step  282  is followed by step  284 . If signal GAIN is not greater than threshold GAINth 2 , step  282  is followed by step  272  and method  270  repeats. By comparing signal GAIN against different thresholds in steps  278  and  282 , hysteresis is provided in method  270 . 
     In step  284 , controller  16  sets signal TX_LOS_INT high if it had been set low because the data signals are not detected. Step  284  is followed by step  272  and method  270  repeats. 
     Referring back to  FIG. 2 , a comparator  290  and a counter  292  are used in a closed feedback loop. Comparator  290  compares signal TX_OMA with a reference signal REF and provides an output signal to counter  292 . Counter  292  periodically increments or decrements a count depending on the output signal. A switch  293  provides the count as a signal GAIN′ to programmable amplifier  130 . Controller  16  again compares signal GAIN′ to one or more reference signals to determine whether or not to set signal TX_LOS_INT′ high. 
     In accordance with one aspect of the invention, a programmable interrupt mask is used to allow the host to determine which alarms and what warning levels are to be used by laser system  100 .  FIG. 10  illustrates an interrupt mask  310  that the host can write control bits through bus DIG_IO. Each control bit can set an alarm for a specific type of error. For example, a control bit  312  controls whether or not the host will receive an alarm when the supply voltage level is too low or too high. Other error conditions include laser diode temperature, power levels, and laser current. Furthermore, one or more control bits can set the threshold level for a specific type of error. For example, control bits  314  to  316  set the maximum value of the supply voltage while control bits  317  to  319  set the minimum value of the supply voltage. 
     When errors are detected, controller  16  writes error bits. For example error bit  322  corresponds to an alarm for the supply voltage. Controller  16  performs a logic operation (e.g., an OR or an AND operation) between the corresponding control bits and error bits to determine whether or not the host should be informed of the error. If the host desires to be informed, controller  16  can output an interrupt signal on bus DIG_IO. Control bits and error bits can be written in physical registers in laser system  100  or specific memory locations in memory  46 . 
     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.

Technology Classification (CPC): 7