阅读说明：本技术 光学通信中效率改善的激光二极管驱动器 (Laser diode driver with improved efficiency in optical communications ) 是由 C.巴扎尼 莫迟 于 2018-10-03 设计创作，主要内容包括：电路和方法为驱动激光二极管的激光驱动器提供了净空电压,使得激光二极管向光通信设备提供信号。该电路包含：从激光驱动器接收净空电压的净空控制电路,该净空控制电路基于净空电压生成受控电压,以及DC-DC转换器,其从净空控制电路接收受控电压、基于受控电压生成电压Vout,并将电压Vout作为输入施加至激光二极管。净空控制电路和DC-DC转换器在反馈回路中与激光二极管相连,以向激光二极管连续地提供电压Vout,并且DC-DC转换器调整电压Vout以补偿激光二极管随时间的老化特性或温度漂移,从而为激光驱动器保持最佳净空电压。(The circuit and method provide a headroom voltage for a laser driver that drives a laser diode such that the laser diode provides a signal to an optical communication device. The circuit comprises: a headroom control circuit receiving a headroom voltage from the laser driver, the headroom control circuit generating a controlled voltage based on the headroom voltage, and a DC-DC converter receiving the controlled voltage from the headroom control circuit, generating a voltage Vout based on the controlled voltage, and applying the voltage Vout as an input to the laser diode. The headroom control circuit and the DC-DC converter are connected in a feedback loop with the laser diode to continuously provide a voltage Vout to the laser diode, and the DC-DC converter adjusts the voltage Vout to compensate for aging characteristics or temperature drift of the laser diode over time to maintain an optimum headroom voltage for the laser driver.)

1. A circuit for providing a voltage Vout having an optimal headroom voltage to a laser diode driven by a laser driver, the laser diode configured to provide a signal to an optical communication device, the circuit comprising:

a headroom control circuit having an input connected to receive the headroom voltage from a connection between the laser diode and the laser driver, the headroom control circuit configured to generate a controlled voltage Vc based on the headroom voltage and a modulation current of the laser driver; and

a DC-DC converter connected to receive the controlled voltage Vc from the headroom control circuit and to generate the voltage Vout based on the controlled voltage Vc and to apply the voltage Vout as an input to the laser diode,

wherein the headroom control circuit and the DC-DC converter are connected in a feedback loop with the laser diode to continuously provide the voltage Vout to the laser diode, and the headroom control circuit controls the voltage Vc to compensate for high frequency reflections in the laser driver to maintain an optimal headroom voltage for the laser driver.

2. The circuit of claim 1 wherein the DC-DC converter is configured to generate a control voltage based on a controlled voltage from the headroom control circuit and to generate a driver signal based on a value of the control voltage to adjust the voltage Vout.

3. The circuit of claim 2, wherein the DC-DC converter compares the control voltage to a sawtooth waveform to generate the driver signal to adjust the voltage Vout based on a value of the control voltage.

4. The circuit of claim 1, wherein the DC-DC converter includes a plurality of switches controlled by the driver signal to adjust the voltage Vout as needed.

5. The circuit of claim 1 wherein the headroom control circuit comprises at least one programmable current source and/or at least one programmable resistor.

6. The circuit of claim 5, wherein the value of the at least one programmable current source and/or the at least one programmable resistor is set to control a headroom voltage of the laser driver.

7. The circuit of claim 6, wherein a change in the value of the at least one programmable current source and/or the at least one programmable resistor is arranged to control a headroom voltage of the laser driver and to control the voltage Vout input to the laser diode.

8. A circuit for providing a headroom voltage for a laser driver driving a laser diode configured to provide a signal to an optical communication device, the circuit comprising:

a feedback loop circuit connected between an output of the laser diode and an input of the laser driver, the feedback loop configured to generate a voltage Vout based on the headroom voltage and apply the voltage Vout to the input of the laser diode, wherein the feedback loop circuit is configured to generate Vout to compensate for high frequency reflections in the laser driver in order to maintain an optimal headroom voltage for the laser driver.

9. The circuit of claim 8, wherein the feedback loop circuit comprises:

a headroom control circuit having an input connected to receive the headroom voltage from an output of the laser driver, the headroom control circuit configured to generate a controlled voltage based on the headroom voltage; and

a DC-DC converter configured to convert the controlled voltage to the voltage Vout, the voltage Vout applied to an input of the laser diode.

10. The circuit of claim 9 wherein the DC-DC converter is configured to generate a control voltage based on the controlled voltage from the headroom control circuit and to generate a driver signal based on a value of the control voltage to adjust the voltage Vout.

11. The circuit of claim 10, wherein the DC-DC converter compares the control voltage to a sawtooth waveform to generate the driver signal to adjust the voltage Vout based on a value of the control voltage.

12. The circuit of claim 8, wherein the DC-DC converter includes a plurality of switches controlled by the driver signal to adjust the voltage Vout as needed.

13. The circuit of claim 8 wherein the headroom control circuit comprises at least one programmable current source and/or at least one programmable resistor and an error and error amplifier.

14. The circuit of claim 13, the value of the at least one programmable current source and/or the at least one programmable resistor being set to control a headroom voltage of the laser driver.

15. The circuit of claim 14, wherein a change in the value of the at least one programmable current source and/or the at least one programmable resistor is arranged to control a headroom voltage of the laser driver and to control the voltage Vout input to the laser diode.

16. A method of controlling a headroom voltage of a laser driver, the laser diode being driven by a laser driver, a headroom control circuit connected to receive the headroom voltage of the laser driver, a DC-DC converter connected to receive an output outward from the headroom control circuit and to input an output voltage Vout of the laser diode, the method comprising:

generating, with the headroom control circuit, a controlled voltage based on the headroom voltage; and

generating a voltage Vout based on the controlled voltage with the DC-DC converter and applying the voltage Vout as an input to the laser diode, wherein the headroom control circuit adjusts the voltage Vout to compensate for high frequency reflections in the laser driver in order to maintain an optimal headroom voltage for the laser driver.

17. The method of claim 16, further comprising generating a control voltage based on the controlled voltage from the headroom control circuit, and generating a driver signal based on a value of the control voltage to adjust the voltage Vout.

18. The method of claim 17, further comprising comparing the control voltage to a sawtooth waveform to generate the driver signal to adjust the voltage Vout.

19. The method of claim 18, further comprising controlling a plurality of switches in the DC-DC converter to regulate the voltage Vout.

20. The method of claim 16, further comprising setting a value of at least one programmable current source and/or at least one programmable resistor in the headroom control circuit to set the headroom voltage to a desired value.

Technical Field

The invention relates to a laser diode and a method of driving the laser diode.

Background

Laser diodes have found increasing use as optical transmitters in fiber optic communication systems. In such systems, the laser diode is typically driven by a constant current from an on-chip laser driver. In some optical transmission systems, an external DC-to-DC converter may be used to provide a fixed supply voltage for the laser diode from a main input voltage. However, this solution is not ideal. For example, a fixed DC converter output cannot compensate for aging characteristics or temperature drift of the laser diode over time. Furthermore, if the laser diode is operated at high frequencies, the modulation current in the laser driver may change at high speed, and when more or less modulation current flows into the laser and the voltage drop across it changes accordingly, the DC converter will not be able to adjust the supply voltage to provide sufficient headroom voltage (headroom) for the laser diode.

The power dissipated on or within the laser driver can be calculated as a constant current multiplied by the headroom across it. Unfortunately, headroom is not optimized in typical systems, wasting power. The efficiency of the overall system would be improved if the optimum and constantly updated headroom voltage could be determined and applied for the laser diode. In addition to the efficiency advantage, the optimum and constantly updated headroom voltage will automatically compensate for the I-V curve drift over the lifetime of the laser diode and changes due to temperature variations, thus keeping the bias current constant.

Disclosure of Invention

Aspects of embodiments of the present invention include circuits and methods for providing a headroom voltage for a laser diode driver that drives a laser diode that provides a signal to an optical communication device. The circuit includes: a headroom control circuit receiving a headroom voltage from the laser driver; the clearance control circuit generates a controlled voltage Vc based on the clearance voltage and the modulation current of the laser driver; and a direct current-direct current (DC-DC) converter receiving the controlled voltage from the headroom control circuit and generating a voltage Vout based on the controlled voltage, and applying the voltage Vout as a power source to the laser diode. The headroom control circuit and the DC-DC converter are connected in a feedback loop with the laser diode to continuously provide a voltage Vout to the laser diode, and the headroom control circuit controls the voltage Vc to compensate for high frequency reflections in the laser driver due to the high frequency modulation current to maintain an optimum headroom voltage for the laser driver.

In other aspects of embodiments of the invention, the DC-DC converter is configured to generate the control voltage based on an input of the headroom control circuit and to generate the driver signal to adjust the voltage Vout based on a value of the input. The DC-DC converter may compare the control voltage with a sawtooth waveform to generate a driver signal based on the input value to adjust the voltage Vout. Other DC-DC converter implementations and/or control schemes may be implemented. In one embodiment, the DC-DC converter includes a plurality of switches controlled by the driver signal to adjust the voltage Vout as needed.

It is also contemplated that the headroom control circuit includes at least one programmable current source and/or at least one programmable resistor. In other aspects of embodiments of the invention, the value of the at least one programmable current source and/or the at least one programmable resistor is set to control the headroom voltage of the laser driver. In a variant, the change in the value of the at least one programmable current source and/or the at least one programmable resistor is arranged to control the headroom voltage of the laser driver and to control the voltage Vout supplied to the laser diode.

In other aspects of embodiments of the present invention, a circuit provides a headroom voltage for a laser driver that drives a laser diode configured to provide a signal to an optical communication device. The circuit includes a feedback loop circuit connected between the output of the laser diode and the input of the laser driver, such that the feedback loop is configured to generate a voltage Vout based on the headroom voltage and apply the voltage Vout to the input of the laser. The feedback loop circuit is configured to generate Vout to compensate for aging characteristics and/or temperature drift of the laser diode over time to maintain an optimal headroom voltage for the laser driver.

The circuit is also contemplated such that the feedback loop circuit includes a headroom control circuit having a headroom voltage connected to receive the headroom voltage from the output of the laser driver. The headroom control circuit is configured to generate a controlled voltage based on the headroom voltage. And, the DC-DC converter is configured to convert the controlled voltage into a voltage Vout such that the voltage Vout is applied to an input of the laser diode. In one embodiment, the DC-DC converter is configured to generate a control voltage based on the controlled voltage from the headroom control circuit, and to generate a driver signal to adjust the voltage Vout based on a value of the control voltage. The DC-DC converter may compare the control voltage to the sawtooth waveform to generate a driver signal to adjust the voltage Vout based on the value of the control voltage.

In one embodiment, the DC-DC converter includes a plurality of switches controlled by the driver signal to adjust the voltage Vout as needed. The headroom control circuit may comprise at least one programmable current source and/or at least one programmable resistor. In one variation, the value of the at least one programmable current source and/or the at least one programmable resistor is set to control the headroom voltage of the laser driver. The change in the value of the at least one programmable current source and/or the at least one programmable resistor is set to control the headroom voltage of the laser driver and to control the voltage Vout input to the laser diode.

Also disclosed is a method of controlling the headroom voltage of a laser driver, wherein the laser diode is driven by the laser driver, and a headroom control circuit is connected to receive the headroom voltage of the laser driver, while a DC-DC converter is connected to receive an output from the headroom control circuit and output a voltage Vout to an input of the laser diode. The method comprises the following steps: generating a controlled voltage based on the headroom voltage with a headroom control circuit, and generating a voltage Vout based on the controlled voltage with a DC-DC converter, then applying the voltage Vout as an input to the laser diode, such that the DC-DC converter adjusts the voltage Vout to compensate for aging characteristics or temperature drift of the laser diode over time, thereby maintaining an optimal headroom voltage for the laser driver.

In one embodiment, the method further comprises: the control voltage is generated based on the controlled voltage from the headroom control circuit, and the driver signal is generated based on the value of the control voltage to adjust the voltage Vout. The method may further include comparing the control voltage to a sawtooth waveform to generate a driver signal to adjust the voltage Vout. In one configuration, the method further includes controlling a plurality of switches in the DC-DC converter to regulate the voltage Vout. The method may also set a value of at least one programmable current source and/or at least one programmable resistor in the headroom control circuit to set the headroom voltage to a desired value.

Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.

Drawings

The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the drawings, like reference numerals designate corresponding parts throughout the different views.

FIG. 1 shows a block diagram of a circuit for providing a voltage to a laser diode that provides a signal to an optical fiber according to an embodiment of the invention.

Fig. 2 shows a DC-DC converter for providing a voltage to a laser diode according to an embodiment of the invention.

Fig. 3 shows a schematic diagram of a laser diode driver and associated circuitry for providing a high-speed current signal with optimal headroom to drive a laser diode, according to an embodiment of the invention.

Fig. 4 shows a schematic diagram of a headroom control circuit (greff) and an associated circuit of fig. 3 (in which the associated circuit is shown in detail) according to an embodiment of the present invention.

Fig. 5 shows a flow diagram according to an embodiment of the invention.

Fig. 6 shows test results for different values of Vout, vhheadroom and deltavhheadroom obtained when varying the programmable current source I2.

FIG. 7 shows test results for different values of Vout, Vheadroom, and DeltaVheadroom obtained when varying the programmable current source I1.

Detailed Description

In the following description, numerous specific details are set forth in order to provide a more thorough description of embodiments of the invention. It will be apparent, however, to one skilled in the art that embodiments of the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail so as not to obscure the embodiments of the invention.

Fig. 1 shows a block diagram of a circuit 100 for providing optimal headroom to a laser diode 102. The laser diode 102 of fig. 1 may be used to provide signals in optical communications (such as to the optical fiber 104), for example, for free space communications. However, other uses of the laser diode 102 may also be used with the preferred embodiment of the present invention.

Headroom is defined herein as the difference between the supply voltage and the sum of the individual voltage drops along a single circuit path. As shown in fig. 1, the headroom for the laser driver will be the voltage applied to the laser diode (Vout) minus the voltage drop across the laser diode and any circuitry associated with the driver disposed between the laser diode 102 and Vout. Laser drivers such as npn transistors 304 and 305 shown in fig. 3 may typically require a headroom of about 0.7V.

If the voltage at the laser diode input (Vout) is 3.3 volts (as defined for VccT by SFP + high speed electrical interface standard SFF-8431), the laser driver headroom may be insufficient due to the above-mentioned voltage drop increase. Thus, embodiments of the present invention continuously regulate the voltage Vout to provide optimal headroom for the laser driver.

In addition, the laser driver 212 operates at high frequency when turning the laser diode 102 on and off by sending current pulses into the laser diode 102, 202 (typically using a square wave signal with very sharp edges). Upon switching the laser on and off, current in the circuit is sent through the laser diode 202 (when the laser diode is switched on) or through the circuit branch of fig. 3 (when the laser diode is switched off), which contains the bipolar transistor 304.

However, due to high frequency effects, when a square wave pulse is applied, only the characteristic impedance of the load (e.g. 25ohm in the case of a DML laser) is initially applied, since the signal has not yet propagated along the transmission path, but when the signal propagates along the transmission line and is reflected back, an additional voltage peak occurs, which is generally equal to the modulation current Imod multiplied by the equivalent resistance Req1 of the laser. The Req1 of the laser depends on the physical specifications and manufacturing characteristics of the laser as well as the optical subassembly structure (bond wires to the laser, flex cables connecting the laser driver board to the laser, etc.). The peak voltage is typically about 1V. After the edges of the square wave pass, a continuous mode type condition now occurs and current flows into the laser diode and the voltage drop across the laser diode 202 is equal to the diode voltage of the laser plus the direct current resistance of the laser times the current. The direct current resistance of the laser diode is smaller than Reql (high frequency resistance of the laser diode). In order for the driver not to be compressed during the transition between off and on of the laser diode, a higher voltage than pure DC operation needs to be applied due to the extra peak voltage.

As shown in fig. 3, which shows the laser driver 212 in more detail, in a typical DML application, Vout must be about 4V or higher to provide sufficient headroom for the laser driver's n-p-n transistors 304, 305, taking into account the associated voltage drop across the laser diode 202 and the circuitry associated with the laser driver 212, when high frequency effects are considered. For one extreme example, the safety switch 306 may have a voltage drop of about 0.15V, the circuit board and ferrite voltage drop may be about 0.1V, the voltage drop across the laser diode 202 may be about 2.0V, and the peak voltage reflected back from the transmission line may be about 1.0V, as described above. Therefore, in order to provide the required headroom voltage of about 0.7V, Vout should be about 0.15V +0.1V +2.0V +0.7V +1.0V, or about 4.0V. This would prove problematic to use a module with a 3.3V power supply. Thus, embodiments of the present invention can provide Vout of approximately 4.0V or higher for the worst case.

The circuit 100 of fig. 1 for providing optimal headroom to the laser diode 102 includes a grerf 106 and a DC-DC converter 108. The greff 106 is a headroom control circuit that provides a controlled voltage to the DC-DC converter 108. The DC-DC converter 108 is configured to convert the voltage provided by grerf 106 to a voltage Vout that is used as an input to the laser diode 102. The digital signal processing 110 will monitor the performance of the laser diode and process it to send an appropriate signal to the DC-DC converter 108 for modulating the output voltage Vout so that data can be provided to the optical fiber 104 with a low error rate.

Greff 106 receives digital commands from digital signal processing 110 and receives an input voltage vheadrom from the output of laser driver 112, which can be used in a feedback loop to regulate the voltage Vout applied as an input to laser diode 102. The headroom of the laser driver can be adjusted by the grerf 106 to provide the desired headroom and to automatically compensate for changes in the modulation current.

The laser drivers 112, 212 modulate the current in the laser diode 102 to transmit optical signals at 28Gbps (gigabits per second), although other speeds may be used. The average current required by the laser diodes 102, 202 varies as a function of temperature and aging effects. The voltage drop across the laser diode 102 will change accordingly. The DC-DC converter 108 deals with or responds to very high frequency reflections caused by the mismatched impedance of the laser driver 112 with respect to the laser diode 102. This reflection depends on the impedance (particularly the laser bonding inductance) acting in the system and is fairly constant for a given system. Referred to herein as the equivalent laser resistance or Req 1. These reflections are also proportional to the modulation current (Imod).

The laser driver 112 may be a DML (direct modulated laser) driver. To guarantee the performance of the DML driver, the headroom should be:

Vheadroom＝Vld_min+Imod*Reql (1)

where Vld _ min is the minimum dc voltage at the output of the laser driver to guarantee performance (which is typically determined by design, on the order of 0.7V), Reql is the equivalent impedance of the laser diode 102, 202, which is proportional to the reflection from the transmission line, and Imod is the modulation current in the laser diode 102. As mentioned above, Req1 × Imod is typically about 1.0V.

The power consumption (power displacement) in the laser driver 112 can be calculated as the following equation (2). Equation (2) shows that the power consumption (Pdiss) is equal to the headroom voltage (vheadrom) multiplied by the average modulation current (Iave). If the headroom voltage is too low, the laser driver performance will suffer and the error rate will increase. If the headroom voltage is too high, not only will power dissipation be undesirable, but the high frequency bipolar transistor 304 used in the laser driver 112 risks breakdown.

Pdiss＝Vheadroom*Iave (2)

More details of the DC-DC converter 108 and its connections to the greff 206, the laser diode 202, and the laser driver 212 are shown in fig. 2. The DC-DC converter 108 outputs a voltage Vout to the laser diode 202 driven by the laser driver 212. Greff 206 adjusts the voltage received at its input to output a voltage to error amplifier EA1213, where a voltage of 0.4V is applied at the positive input of EA1213 to produce the control voltage Vc.

The sawtooth generator 218 generates sawtooth waveforms vsaw.bck and vsaw.bst, which are selected under the control of the mode selector 214 to output the sawtooth waveforms Vsaw. The sawtooth waveform may be a 2.5MHz sawtooth waveform, although other waveforms may be used. The sawtooth waveform is compared in PWM generator 216 with Vc in PWM generator 216 to generate PWM modulated signal 217 for use by driver 220 to generate PWM modulated signals A, B, C and D for controlling transistors 222, 224, 226 and 228 to produce the desired Vout. Appropriate control of transistors 222, 224, 226 and 228 serves to raise or lower Vout as needed so that Vout can be provided between approximately 2.0V to 4.5V.

Grerf 106, 206 will adjust the headroom voltage to automatically compensate for the I-V curve drift of the laser diode 202 over the life cycle and compensate for temperature changes to keep the bias current constant. For example, if the voltage drop across the laser diode 202 changes due to temperature changes or due to drift over time, the voltage at the output of the laser driver (Vheadroom) will change and may become too low or too high to provide headroom with optimal performance. For example, if the voltage Vheadroom decreases due to temperature changes and a large voltage drop across the laser diode 202, the voltages at the output of REGREF206 and the input of error amplifier EA1213 will decrease, resulting in a change in the control voltage Vc. The changed control voltage Vc will result in a changed Vout being applied to the laser diode 202.

The grerf 206 is shown in more detail in fig. 4, and its connections to the laser diode 202, the laser driver 212, and the DC-DC converter. Greff 206 includes an error amplifier EA2 and a set of programmable resistors R1410 and R2414 and programmable current sources I1406 and I2412. The headroom voltage vheadrom at the laser driver 212 is input to the positive input of the error amplifier EA2, and the voltage generated at the output of I1406 (I1 × R1) is input to the negative input of the error amplifier EA 1213. The voltage at the output of the error amplifier EA 2408 is Vref + I2 × R2. When the feedback loop is closed by the DC-DC converter, the voltage Vout will self-regulate in a way that satisfies equation (3):

Vheadroom＝Vref+R2*I2+R1*I1 (3)

i1 is used to generate a voltage (I1R 1) that when added to Vref at R2I 2 and EA1 will produce the minimum voltage Vld _ min required for the laser driver to operate, as shown in equation (4), where I1 and R1 are held constant. This minimum voltage for laser driver operation is typically about 0.7V.

Vld_min＝R2*I2+Vref (4)

Imod*Reql+I1*R1 (5)

Importantly, I1 is configured to track changes in the modulation current of laser driver 212. This may be accomplished in a number of ways, such as monitoring changes in the modulation current of the laser driver 212 using monitoring circuitry configured to make corresponding or proportional changes in the programmable current source I1406.

In closed loop operation, grerf 106, 206 will adjust Vc and hence Vout to ensure that equation (3) is valid, regardless of the laser diode operating voltage (which may change with temperature/aging) and other IR drops due to circuit board/ferrite resistance and the voltage drop of the safety switch (see fig. 3).

Fig. 5 shows a flow diagram according to an embodiment of the invention. In step 502, the headroom control circuit reref generates the controlled voltage based on the headroom voltage vheadrom. In step 504, the DC-DC converter generates the voltage Vout based on the controlled voltage received from the headroom control circuit.

In step 506, a voltage Vout is applied as an input to the laser diode, wherein the DC-DC converter adjusts the voltage Vout to compensate for aging characteristics or temperature drift of the laser diode over time to maintain an optimal headroom voltage for the laser driver.

Fig. 6 shows the test results of the headroom control circuit grerf 206, with a change in the control bits I2_ ctrl [2:0] that change the current injected at the programmable current source I2412 with R1, I1 and R2 held constant. As shown in FIG. 6, each step changes Vout and Vheadroom by about 100 mV. This allows a user of an embodiment of the present invention to set the desired headroom vheadrom by adjusting the programmable current source I2412. Different currents may be used as needed to produce higher or lower headroom vhheadroom.

Fig. 7 shows the test result of the headroom control circuit grerf 206, in which the injection current applied at the programmable current source I1406 is changed. The test set R1 to 11K ohms and varied the value of I1 with I2 and R2 held constant. The headroom voltage Vheadroom changes by about 38mV for every 3.5A change in I1. This allows a user of an embodiment of the present invention to set the desired headroom vheadrom by adjusting the programmable current source I1406. Different currents may be used as needed to produce higher or lower headroom vhheadroom.

While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the invention. In addition, the various features, elements, and embodiments described herein may be claimed or combined in any combination or arrangement.