Abstract:
A method for calibrating a laser transmitter includes (a) detecting an eye diagram of an output from the laser transmitter, (b) determining if the eye diagram is acceptable, (c) if the eye diagram is not acceptable, changing a value of a control signal in the laser transmitter, wherein the control signal sets an amplitude characteristic of a limiting amplifier coupled to a laser driver in the laser transmitter, and (d) repeating steps (a), (b), and (c) until the eye diagram is acceptable.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates to fiber optic laser transmitters.  
       DESCRIPTION OF RELATED ART  
       [0002]     As the speed of fiber optic transmitter exceeds 1 Gbps (gigabits per second), the existing VCSEL (vertical cavity surface emitting laser) based fiber optic transmitters suffer from degraded eye quality of the laser output signal from the interaction between the laser driver circuit and the laser diode. Since the laser output quality mainly depends on the impedance-matching between the laser driver circuit and the laser diode, the conventional way to improve the output eye quality is to use a matching network between the laser driver circuit and the laser. Such a matching network compensates the impedance mismatch and therefore improves the output eye quality.  
         [0003]     However, matching network is undesirable in fiber optic transmitters in parallel or multi-channel application due to the size of on-chip or off-chip matching network circuitry. Furthermore, the matching network cannot compensate the effect of random variation from channel to channel and from part to part.  
         [0004]     Thus, what is needed is a method and an apparatus that address the problems identified above.  
       SUMMARY  
       [0005]     In one embodiment of the invention, a method for calibrating a laser transmitter includes (a) detecting an eye diagram of an output from the laser transmitter, (b) determining if the eye diagram is acceptable, (c) if the eye diagram is not acceptable, changing a value of a control signal in the laser transmitter, wherein the control signal sets an amplitude characteristic of a limiting amplifier coupled to a laser driver in the laser transmitter, and (d) repeating steps (a), (b), and (c) until the eye diagram is acceptable.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a block diagram of a laser transmitter in one embodiment of the invention.  
         [0007]      FIG. 2  is a circuit diagram of a limiting amplifier in the laser transmitter of  FIG. 1  in one embodiment of the invention.  
         [0008]      FIGS. 3A, 3B , and  3 C are charts of the output voltage from the limiting amplifier of  FIG. 1  in one embodiment of the invention.  
         [0009]      FIG. 4  is a diagram of a system used to calibrate the laser transmitter of  FIG. 1  in one embodiment of the invention.  
         [0010]      FIG. 5  is a flowchart of a method for calibrating the laser transmitter of  FIG. 1  in one embodiment of the invention.  
         [0011]      FIG. 6  is a block diagram of a laser transmitter in another embodiment of the invention.  
         [0012]      FIG. 7  is a circuit diagram of a limiting amplifier in the laser transmitter of  FIG. 6  in one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  illustrates a laser transmitter  10  in one embodiment of the invention. Laser transmitter  10  includes a conventional input stage  12 , a limiting amplifier  14 , an amplitude/common-mode control circuit  16 , a conventional laser driver  18 , and a conventional laser diode  20 . Although illustrated as a single channel laser transmitter, one skilled in the art understands that a multi-channel laser transmitter can be constructed by repeating such a structure.  
         [0014]     Input stage  12  has its non-inverted and inverted input terminals coupled to receive input voltage signals IN +  and IN − , respectively. Input stage  12  provides output voltage signals Vin +  and Vin −  with steady voltage swings in response to input voltage signals IN +  and IN +  that may have variable voltage-swings.  
         [0015]     Limiting amplifier  14  has its non-inverted and inverted input terminals coupled to the non-inverted and inverted output terminals of input stage  12 , respectively. Furthermore, limiting amplifier  14  has one or more control terminals coupled to one or more output terminals of amplitude/common-mode control circuit  16 . Limiting amplifier  14  provides a gain over input stage  12 . Thus, limiting amplifier  14  generates output voltage signals Vout +  and Vout −  with improved rise and fall times over input voltage signals Vin +  and Vin − . Limiting amplifier  14  also holds output voltage signals Vout +  and Vout +  to levels prescribed by one or more digital control signals Di. received from control circuit  16 . Furthermore, limiting amplifier  14  may provide a level shift to change the common-mode voltage of output voltage signals Vout +  and Vout −  prescribed by digital control signals Di. In one embodiment, control circuit  16  includes a register  22  that stores and outputs control signals Di.  
         [0016]      FIG. 2  is a circuit diagram of limiting amplifier  14  in one embodiment of the invention. Limiting amplifier  14  includes a differential pair  32  digitally calibrated by variable resistors  37 ,  38 , and  40 , and a programmable current source  42 . Differential pair  32  consists of bipolar transistor  32 A and  32 B having their bases coupled to receive differential inputs and their emitters tied at a common node.  
         [0017]     In one embodiment, variable resistors  37 ,  38 , and  40  are voltage controlled resistors (VCRs). VCR  37  has an input terminal coupled to rail and an output terminal coupled in parallel with VCRs  38  and  40 . VCRs  38  and  40  have output terminals coupled to the corresponding collectors of bipolar transistors  32 A and  32 B. Current sources  34  and  36  have input terminals coupled to the corresponding collectors of bipolar transistors  32 A and  32 B, and output terminals that provide the corresponding output voltage signals Vout +  and Vout − . Programmable current source  42  has an input-terminal coupled in parallel to the emitters of bipolar transistors  32 A and  32 B and an output terminal coupled to ground. The resistances of VCRs  37 ,  38 , and  40  are adjusted to vary the amplitude characteristics of limiting amplifier  14 , such as peak-to-peak amplitude, peak amplitude, and common-mode. Additionally, the current sank by programmable current source  42  can be adjusted to vary the output amplitude characteristics limiting amplifier  14 .  
         [0018]     In one embodiment, VCR  37  includes four resistors coupled in series, and four bypass transistors coupled in parallel with their corresponding resistors so the resistors can be bypassed by turning on their corresponding bypass transistors. The bypass transistors are controlled by digital control signals D 1 , D 2 , D 3 , and D 4  to set the resistance of VCR  37 .  
         [0019]     In one embodiment, VCRs  38  and  40  each includes four resistors coupled in series, and three bypass transistors coupled in parallel with their corresponding resistors so the resistors can be bypassed by turning on their corresponding bypass transistors. The bypass transistors are controlled by digital control signals D 5 , D 6 , and D 7  to set the resistance of VCRs  38  and  40 .  
         [0020]     In one embodiment, programmable current source  42  includes four transistors with their drains coupled in parallel the emitters of bipolar transistors  32 A and  32 B to sink a current from differential pair  32 . The transistors are controlled by digital control signals D 8 , D 9 , D 10 , and D 11  to set the amount of current to sink from differential pair  32 .  
         [0021]     Referring back to  FIG. 1 , register  22  stores the values of digital control signals D 1  to D 11 . Register  22  outputs digital control signals D 1  to D 11  to VCRs  37 ,  38 , and  40 , and programmable current source  42  to set the amplitude characteristics of limiting amplifier  14 .  
         [0022]     Laser driver  18  has its non-inverted and inverted input terminals coupled to the non-inverted and inverted output terminals of limiting amplifier  14 , respectively. Laser drive  18  converts input voltage signals Vout +  and Vout −  to a drive current for laser diode  20 . In one embodiment, laser diode  20  is a vertical cavity surface emitting laser (VCSEL).  
         [0023]     Simulations and tests have shown that the optimum output eye pattern can be achieved by selecting the proper output amplitude and the proper common-mode of limiting amplifier  14 , which is controlled by digital control signals D 1  to D 11 . When the output amplitude of limiting amplifier  14  is bigger than the optimum value, its residue portion contributes to the output eye pattern&#39;s overshoot and/or undershoot. When the output amplitude of limiting amplitude  14  is too small, limiting amplifier  14  cannot properly drive laser driver  18 . This causes extended rise and fall times of the laser output, which deteriorates the quality of the output eye pattern.  
         [0024]      FIGS. 3A, 3B , and  3 C are charts of a differential output voltage V from limiting amplifier  14  generated with digital control signals D 1  to D 7  listed in Table 1 below in one embodiment of the invention. Although not shown, one skilled in the art understands that digital signals D 8  to D 11  can also be varied to change the amplitude and the common-mode of limiting amplifier  14 .  
                                                   TABLE 1                                   Line   D1   D2   D3   D4   D5   D6   D7                           A   1   1   1   1   0   0   0           B   1   1   1   1   0   0   1           C   1   1   1   0   0   0   1           D   1   1   0   0   0   1   1                        
         [0025]     In  FIG. 3A , line A illustrates an output voltage V (Vout + −Vout − ) and a common-mode A&#39; when the digital control signals D 1  to D 7  are set at a first set of values. Line B illustrates output voltage V and a common-mode B&#39; when the digital control signals D 1  to D 7  are changed to a second set of values. By decreasing the resistance of VCRs  38  and  40 , the peak-to-peak amplitude of limiting amplifier  14  is decreased from four (4) units A to three (3) units A (where unit A is any arbitrary unit). As a result, common-mode B&#39; is greater than common-mode A&#39;.  
         [0026]     In  FIG. 3B , line C illustrates the output voltage V and a common-mode C&#39; when the digital control signals D 1  to D 7  are changed to a third set of values. By increasing the resistance of VCR  37 , the peak amplitude of limiting amplifier  14  is decreased by one (1) unit B (where unit B is any arbitrary unit). As a result, common-mode C&#39; is less than common-mode A&#39;.  
         [0027]     In  FIG. 3C , line D illustrates the output voltage V and a common-mode D&#39; when the digital control signals D 1  to D 7  are changed to a fourth set of values. By further increasing the resistance of VCR  37 , the peak amplitude of limiting amplifier  14  is decreased by two (2) units B. By further decreasing the resistance of VCRs  38  and  39 , the peak-to-peak amplitude swing of-limiting amplifier  14  is decreased from three (3) units A to two (2) units A. As a result, common-mode D&#39;. is less than common-mode A&#39;. By varying digital control signals D 1  to D 11 , a range of peak-to-peak amplitude, peak amplitude, and common-mode can be achieved.  
         [0028]      FIG. 4  is a diagram of a system  50  used to calibrate the optimum value of the output amplitude of limiting amplifier  14  in one embodiment of the invention. System  50  includes laser transmitter  10  having an output fiber connected to an oscilloscope  54 . Oscilloscope  54  includes a program that outputs the eye mask margin to a computer  56 . Computer  56  includes a program that varies the amplitude characteristics of limiting amplifier  14  until the eye mask margin reaches an acceptable value.  
         [0029]      FIG. 5  is a flowchart of a method  60  to use system  50  to calibrate laser transmitter  10  in one embodiment of the invention. In step  62 , the output fiber from laser transmitter  10  is connected to oscilloscope  54  with the eye mask margin program. Laser transmitter  10  then starts to transmit random data. Default values of digital control signals D 1  to D 11  in register  22  are used to control the output amplitude of limiting amplifier  14 .  
         [0030]     In step  64 , oscilloscope  54  downloads the eye mask margin to computer  56 .  
         [0031]     In step  66 , computer  56  determines if the eye mask margin value has reached an acceptable value. If the eye mask margin has not reached an acceptable value, then step  66  is followed by step  68 . If the eye mask margin has reached an acceptable value, then step  66  is followed by step  70 , which ends method  60 .  
         [0032]     In step  68 , computer  56  writes new values of digital control signals D 1  to D 11  in register  22 . Computer  56  can increment or decrement the values of digital control signal D 1  to D 11 . Step  68  is followed by step  64  and method  60  repeats until computer  56  determines optimum values of digital control signals D 1  to D 11  that produce an acceptable eye mask margin.  
         [0033]      FIG. 6  illustrates a laser transmitter  10 &#39; in one embodiment of the invention. Laser transmitter  10 &#39; is similar to laser transmitter  10  and their common elements share the same reference number. Laser transmitter  10 &#39; includes conventional input stage  12 , a limiting amplifier  14 &#39;, an amplitude/common-mode control circuit  16 &#39;, conventional laser driver  18 , and conventional laser diode  20 .  
         [0034]     Amplitude/common-mode control circuit  16 &#39; includes a register  22 &#39; that stores and outputs one or more digital control signals Di to a digital-to-analog converter (DAC)  24 . DAC  24  converts digital control signals Di into one or more analog control signals Ai. DAC  24  outputs analog control signals Ai to limiting amplifier  14 &#39; to control its amplitude characteristics such as peak-to-peak amplitude, peak amplitude, and common-mode.  
         [0035]      FIG. 7  is a circuit diagram of limiting amplifier  14  in one embodiment of the invention. Limiting amplifier  14 &#39; includes differential pair  32  calibrated by VCRs  37 &#39;,  38 &#39;, and  40 &#39;, and a programmable current source  42 &#39;. Limiting amplifier  14 &#39; is configured like limiting amplifier  14  of  FIG. 2  described above but for the implementation of VCRs  37 &#39;,  38 &#39;, and  40 &#39;, and programmable current source  42 &#39;.  
         [0036]     In one embodiment, VCR  37 &#39;includes one resistor and one bypass transistor coupled in parallel with the resistor so a variable amount of current can bypass the resistor by controlling the gate voltage of the bypass transistor. The gate of the bypass transistor is coupled to analog voltage signal A 1  to set the resistance of VCR  37 &#39;.  
         [0037]     In one embodiment, VCRs  38 &#39; and  40 &#39; each includes two resistors coupled in series, and one bypass transistor coupled in parallel with its corresponding resistor so a variable amount of current can bypass the resistor by controlling the gate voltage of the bypass transistor. The gates of the bypass transistors are coupled to an analog voltage signal A 5  to set the resistance of VCRs  38 &#39; and  40 &#39;.  
         [0038]     In one embodiment, programmable current source  42 &#39; includes one transistor with its drain coupled to the emitters of bipolar transistors  32 A and  32 B to sink a current from differential pair  32 . The gate of the transistor is coupled to an analog voltage signal A 8  to set the amount of current to sink from differential pair  32 .  
         [0039]     By changing digital control signals Di in register  22 &#39;, the amplitudes and the common-mode of limiting amplifier  14 &#39; can be modified to achieve the optimum output eye pattern. System  50  of  FIG. 4  and method  60  of  FIG. 5  can be used to calibrate laser transmitter  10 &#39; as described above.  
         [0040]     Thus, the present invention does not require external or internal matching circuitry for improving the output eye quality. In addition, the digital control can program the optimum value for each channel in an initial programming stage during production. This individual programming can compensate the part-to-part and channel-to-channel random variations.  
         [0041]     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.