阅读说明：本技术 激光二极管驱动方法及装置 (Laser diode driving method and device ) 是由 罗伯特·斯塔克 于 2019-08-01 设计创作，主要内容包括：公开了一种驱动激光二极管的方法和设备,包括将激光二极管的偏置电流增加到阈值水平,其中阈值水平低于激光二极管的启动水平,并且其中将电阻器与激光二极管并联设置,将电容充电到包括有激光二极管的电路的预充电电容,其中预充电电容低于激光二极管的电容启动水平；并启动激光二极管。(A method and apparatus for driving a laser diode is disclosed, including increasing a bias current of the laser diode to a threshold level, wherein the threshold level is below a start-up level of the laser diode, and wherein a resistor is arranged in parallel with the laser diode, charging a capacitance to a pre-charge capacitance of a circuit including the laser diode, wherein the pre-charge capacitance is below a capacitance start-up level of the laser diode; and the laser diode is activated.)

1. A method of driving a laser diode, comprising:

increasing a bias current of the laser diode to a threshold level, wherein the threshold level is lower than a start-up level of the laser diode, and wherein a resistor is arranged in parallel with the laser diode;

charging a capacitor to a pre-charge capacitor of a circuit including the laser diode, wherein the pre-charge capacitor is below a capacitor activation level of the laser diode; and

the laser diode is activated.

2. The method of claim 1, wherein increasing the bias current of the laser diode is performed by providing a single amplitude pulse of current to the laser diode.

3. The method of claim 1, wherein increasing the bias current of the laser diode is performed by providing a burst pulse sequence of amplitude pulses to laser pulses.

4. The method of claim 1, wherein the bias current of the laser diode is increased by an adjustable voltage source.

5. The method of claim 4, wherein the adjustable voltage source is controlled by an analog switch.

6. The method of claim 1, wherein the bias current of the laser diode is increased to the threshold level by sweeping current from a zero level to the threshold level.

7. A method of driving a laser diode, comprising:

increasing a bias current of the laser diode to a threshold level in a series of pulses, wherein the threshold level is below a start-up level of the laser diode, and wherein a resistor is disposed in parallel with the laser diode, and wherein a frequency of the series of pulses is greater than a laser diode current discharge rate;

charging a capacitor to a pre-charge capacitor of a circuit including the laser diode, wherein the pre-charge capacitor is below a capacitor activation level of the laser diode; and

the laser diode is activated.

8. The method of claim 7, wherein the bias current of the laser diode is increased by an adjustable voltage source.

9. The method of claim 8, wherein the adjustable voltage source is controlled by an analog switch.

10. The method of claim 7, wherein the bias current of the laser diode is increased to the threshold level by sweeping current from a zero level to the threshold level.

11. An apparatus for providing current to a device, comprising:

a diode;

a resistor disposed in parallel with the diode;

at least two transistors, wherein each transistor has a collector, an emitter, and a base, and each collector is connected to the diode;

at least one operational amplifier connected to each base of the at least two transistors;

a DC power supply connected to the at least one operational amplifier; and

at least one DC power supply connected to each of the emitters of the at least two transistors.

12. The apparatus of claim 11, further comprising:

at least one capacitor disposed in parallel with the at least one operational amplifier.

13. The apparatus of claim 11, wherein the at least two transistors are a first transistor and a second transistor.

14. The apparatus of claim 13, further comprising:

at least two resistors located between the emitter of the first transistor and the emitter of the second transistor, and the at least one DC power source is connected to each of the emitters.

15. The apparatus of claim 14, wherein the at least two resistors are a first resistor and a second resistor.

16. The apparatus of claim 15, wherein the first resistor has a higher resistance value than the second resistor.

17. The apparatus of claim 14, further comprising:

at least one resistor disposed between the emitter of the first transistor and the emitter of the second transistor and ground.

18. The apparatus of claim 17, wherein the diode is a laser diode.

19. The apparatus of claim 13, further comprising:

at least one capacitor disposed in parallel with one of at least two resistors, the at least two resistors located between the emitter of the first transistor and the emitter of the second transistor.

20. The apparatus of claim 11, wherein the diode is a laser diode.

Technical Field

Embodiments of the present disclosure generally relate to laser diodes. More particularly, embodiments of the present disclosure relate to a laser diode driving method and apparatus.

Background

Laser Phosphor Display (LPD) generates a video image by irradiating pixels with a plurality of focused Laser beams scanned on a screen. Each pixel embedded in the display screen contains a phosphor material that radiates light on the pixel in proportion to the laser beam power and time. Thus, the brightness of each pixel can be controlled by a combination of the laser diode peak drive current and the pulse width duration. High quality LPD displays require the achievement of high resolution (pixels close together) and a wide optical dynamic range of brightness levels. When multiple lasers are used, it is also important to have low drive circuit cost and simple construction.

Achieving perfect black is critical for seamless LPD display since each light engine image area is overlapping. Adding a black level in the overlap region causes the viewer to see a checkerboard visual effect unless the black is truly black. Due to the simplicity and low cost of the architecture and the non-linear nature of the laser diode, current driving methods require a trade-off between perfect black and pulse performance.

Conventional methods and devices have several limitations. A first limitation of current designs is that the bandwidth of the driver decreases as the peak current level decreases. The lower bandwidth results in slower rise and fall times and ultimately makes it difficult to achieve optical power control in the so-called "low gray scale region" of the display curve. One method of improving linearity in the "low gray scale region" involves increasing the dc bias current of the laser diode. Unfortunately, since the LD operates like an LED at low current, the screen phosphor still receives sufficient illumination, resulting in a black level that appears gray.

A second limitation of the existing driving methods is that the peak level of the driver pulse is affected by the previous pulse or pulses, since the pixels are close together. This makes it more difficult to control the linearity of the image and may even result in the pixel failing to illuminate if it was previously a black area of sufficient duration. Using higher level processing in software and FPGA hardware can minimize both of these limitations, but eliminating this interaction is highly desirable.

A third limitation arises from the non-linear nature of the laser diode. This provides a non-linear load to the driver circuit, making it difficult to optimize over the entire operating range.

The fourth limit is the calibration required to control the previous limit. Since all laser diodes have different output characteristics than the current, optical factory equipment must be employed to measure the black level and balance the driver currents.

It is desirable to provide a method of driving a laser diode that is superior to conventional driving methods.

It is desirable to provide a method in which the bandwidth of the driver is not reduced when the peak current level is reduced.

It is also desirable to provide a method in which the peak level of the driver pulse is not affected by the previous pulse.

It is also desirable to provide a method that optimizes the load of the driver circuit over the entire operating range.

Disclosure of Invention

In one embodiment, a method of driving a laser diode is disclosed, comprising increasing a bias current of the laser diode to a threshold level, wherein the threshold level is below a start-up level of the laser diode, and wherein a resistor is arranged in parallel with the laser diode, charging a capacitance to a pre-charge capacitance of a circuit comprising the laser diode, wherein the pre-charge capacitance is below a capacitance start-up level of the laser diode, and starting the laser diode.

In another embodiment, a method of driving a laser diode is disclosed that includes increasing a bias current of the laser diode to a threshold level in a series of pulses, wherein the threshold level is below a start-up level of the laser diode, and wherein a resistor is disposed in parallel with the laser diode, and wherein a frequency of the series of pulses is greater than a laser diode current discharge rate, charging a capacitance to a pre-charge capacitance of a circuit including the laser diode, wherein the pre-charge capacitance is below a capacitance start-up level of the laser diode, and starting the laser diode.

In another embodiment, an apparatus for providing electrical current to a device is disclosed, the apparatus comprising: a diode; a resistor arranged in parallel with the laser diode; at least two transistors, wherein each transistor has a collector, an emitter, and a base, and each collector is connected to a laser diode; at least one operational amplifier connected to each base of the at least two transistors; a DC power supply connected to the at least one operational amplifier; and at least one direct current power supply connected to each of the emitters of the at least two transistors.

Drawings

So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

Fig. 1 is a plot of optical output power versus forward current for a laser diode.

Fig. 2 is a typical modulation of the optical output power versus forward current curve for a laser diode.

Fig. 3 is a conventional apparatus for controlling laser diode current.

Fig. 4 is a graph of laser diode resistance versus current.

Fig. 5 is a graph of driver transistor Beta versus collector current.

Fig. 6 is a graph of drive open loop gain versus laser current.

Fig. 7 is a graph of a conventional driving method of a non-bias resistor of a laser diode.

FIG. 8 is a graph of an improved driving method in one embodiment of the present disclosure.

Fig. 9 is a graph of a conventional driving method of a non-bias resistor of a laser diode.

Fig. 10 is a graph of a conventional driving method of a non-bias resistor of a laser diode.

Fig. 11 is a graph of a device for driving a laser diode current.

Fig. 12 is a graph of driver load resistance, R-bias, and current versus driver current.

Fig. 13 is a graph of voltage versus time for a laser diode without pre-charging.

Fig. 14 is a graph of voltage versus time for a single low precharge of a laser diode.

Fig. 15 is a graph of the voltage of two precharge pulses of a laser diode as a function of time.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

Detailed Description

Hereinafter, reference is made to embodiments of the present disclosure. It should be understood, however, that the present disclosure is not limited to the specifically described embodiments. Rather, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the present disclosure. Moreover, although implementations of the disclosure may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given implementation is not a limitation of the disclosure. Thus, the following aspects, features, embodiments and advantages are merely illustrative and should not be considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, references to "the present disclosure" should not be construed as a generalization of any inventive subject matter disclosed herein and should not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).

In one embodiment, a driving method for a laser diode is disclosed. The driving method provides a way to realize the full potential of LDP technology by allowing the laser diode "off" current to reach zero while still maintaining high bandwidth pulse control of the laser diode peak current waveform. The method also reduces "history effects" by preventing the pulsed current from one pixel from affecting other pixels. The driving method also simplifies product calibration by eliminating optical factory equipment to fine tune the bias current for each laser channel (typically 20 channels) compared to conventional methods. In an embodiment, a board tester is used to set all channels to a predetermined offset value.

Background of laser diode driving

Fig. 1 shows a laser diode curve of output power in mW versus current in mA for a conventional application for driving a laser diode. There are two distinct regions of operation: below threshold (LED area) and above threshold (laser area). In all laser diode applications, the goal is to operate a device above threshold in the lasing region, where the light output rises linearly with increasing current.

In typical pulsed applications, small residual "off" LED output levels can be tolerated, so no circuitry is required to dynamically reduce the laser diode current to zero. In contrast, in conventional applications, most laser diode devices operate at the dc-biased "off" level shown in fig. 2, just below or just above the turn-off, to keep the driver in a linear operating range and support fast entry into the lasing region.

However, in LDP display applications, it is desirable to set the "off" dc level as close to zero as possible to minimize any LED light output. In LDP displays, the system operates in the laser region above the threshold, and a wide luminance range from several hundred nits to a few tenths of nit must be achieved. The bright end of the range is achieved by generating a fast rise time 1000mW light pulse, close to a 50% on-to-off duty cycle, to fully illuminate each phosphor pixel area. The low end of the range is achieved by generating very narrow current pulses with amplitudes exceeding the laser threshold. As mentioned above, the low end of the dynamic range requires that the "off" or black level be less than 1mnit, which can only be achieved with laser diode currents of less than 1 mA. Since each laser diode will output a different light output level for the same drive current, the circuitry must be very accurate or require careful calibration to achieve good background uniformity across the screen.

Conventional method of controlling laser diode current

Fig. 3 shows a typical drive circuit for a conventional manner of controlling the laser diode current through a laser diode identified as LD 1. The circuit includes an operational amplifier U1, the operational amplifier U1 drives the bases of two high speed transistors Q1 and Q2 connected in parallel by conductive lines. Two transistors are used to provide peak currents of up to one ampere. The anode of LD1 is connected to LSR _ PWR, which in one embodiment is set to +8 volts. The cathode of LD1 is connected to the collectors of Q1 and Q2. The two emitters are connected to a current sense resistor R1 which converts the current flowing through the transistor to a value labeled V_SensingIs connected to the negative input of U1 through a feedback resistor R2. This completes the feedback loop that converts the pulse _ input voltage to a current pulse that flows from the LSR _ PWR voltage source, through the laser diode LD1, and into Q1 and Q2. The input signal labeled pulse _ in is typically the output from an analog switch whose input is an adjustable voltage source. When the switch is on, the voltage source is connected to the pulse _ in and generates a laser diode current pulse until the switch is off. The second input of the circuit is an adjustable voltage source labeled vbias, which is set to correct any offset error in amplifier U1 and minimize the LD current when the pulse _ input is off. Note that the pulse _ in typically comprises a voltage pulse with a rise and fall time of less than 5ns, and the pulse on time ranges from 5 to 100ns and can be considered close to ideal.

When trying to minimize the laser diode off current and maximize pulse performance, there are several limitations to this circuit that affect performance. These limitations are mainly due to the low cost and simple architecture of the circuit. Laser diodes are very sensitive to transient negative voltages and over-currents, so this design provides reliable operation using only positive power supplies. The circuit employs an open collector topology and controls the laser current by sinking current from the laser into the collector under closed loop control. Thus, the circuit can increase the laser current very quickly by driving more base current into the transistor, but reduce the current by virtue of the laser impedance. Furthermore, when the laser is located far from the driver, there will be additional capacitance from the collector to ground and from the collector to LSR _ PWR, which will slow down the turn-off.

As shown in fig. 4, the turn-off is complicated because the voltage drop of the laser diode and the resistance varies non-linearly with the current. When the current increases above the threshold (about 120mA), the resistance approaches a few ohms, but at 1mA or less, the resistance can easily reach 10K or higher. The laser diode also has a forward voltage drop of a few volts.

When a high peak pulse current is applied to the laser diode, the turn-off time is very fast because the low resistance of the laser provides a fast discharge path. When the nonlinear device is driven using the circuit of fig. 3, the impulse response will start to drop as we reduce the bias or "off current close to 0 mA. The laser diode resistance increases and the current decreases, resulting in a slower decay of the laser diode current. This makes the circuit sensitive to increased capacitance, which limits the laser diode from being located far from the driver.

The second limitation of this circuit is due to the non-ideal nature of transistors Q1 and Q2. These transistors are chosen because of their high frequency and high current drive characteristics, so their gain or Beta drops at lower currents. Referring to fig. 5, a graph of transistor Beta IC/IB versus current for a typical component in a prior art driver is shown. When the current drops from about 15mA to.8 mA (first point), it can be seen that the combined transistor gain drops by about 50%. Since the circuit uses closed loop control to precisely control the current level, closed loop control can result in a reduction in drive bandwidth as the current decreases.

One way to measure the limitations of the conventional approach is to plot the open loop gain and dc bias current in the feedback loop. According to control theory, the bandwidth of the driver is proportional to the frequency in Hz, with an open loop gain equal to 1. This is plotted in fig. 6 for the existing and new driving method. As shown, the open loop gain for both methods is the same for laser currents above 150mA, but the existing method drops to 1MHz at 1 mA. The new approach increases it to 17MHz, which is sufficient to produce very good control of the laser drive current. A second way of demonstrating the limitations of the existing methods and the improvements of the new driving method is to measure and compare the optical pulse response.

Fig. 7, 8, 9 and 10 show the improvement of the described embodiment over conventional driving methods when working in the most difficult region of narrow pulse width and low amplitude pulses. The optical pulse response of the laser diode is measured by focusing the output of the laser onto a high-speed optical detector. For each of these figures, a burst of 5 identical pulses is generated and the pulse width is adjusted for the same steady state peak detector output. Fig. 7 and 8 compare the existing (conventional) and new methods and apparatus for a 100mV peak pulse. As shown in fig. 7 and 8, the first pulse in the burst pulse sequence has an amplitude that is about 50% lower than the subsequent pulses of the prior method. Fig. 8 shows that for the described aspect, all 5 pulses are close to the same amplitude. The second observation is that the pulse width of the conventional method is 9.7ns and has been reduced to 9.2ns for the new aspect described. Both improvements are consistent with the bandwidth increase shown in fig. 6. Fig. 9 and 10 compare the prior (conventional) and new methods for a 50mV peak pulse. In this case, the conventional method cannot generate the first pulse, but all five (5) pulses are present in the new method unlike in the conventional graph. Note that both methods result in all five pulses reaching the same amplitude because the pulse width is increased and the peak level is increased.

Fig. 11 illustrates one aspect of the disclosure of a driving method that includes adding a single 200 ohm resistor disposed in parallel with the laser diode. This resistor is referred to as the "R-bias" resistor. The use of an R-bias resistor in combination with V-bias and the laser diode nonlinear transfer curve provides a substantially improved and cost-effective method for controlling pulsed laser diode current. The curve of fig. 4 shows that the voltage drop across the laser diode has a sharp knee that occurs at low current levels, approaching 3V. It is also known that below about 15mA, the gain of transistors Q1 and Q2 will begin to decrease, resulting in a loss of open loop gain. Increasing the appropriate resistance value in parallel with the laser diode, in combination with setting the dc bias current through the resistor, will allow the driver to operate at a minimum current and at the same time allow the laser diode current to approach zero. In the case of driving a laser diode, setting the bias current to 15mA would result in a 3V drop across Rbias and result in a laser diode current of less than.1 mA. It is highly desirable to set this voltage to match the laser diode forward threshold because this minimizes the voltage swing from "off" to "on" required to drive the laser, resulting in faster optical rise and fall times.

The above is also shown in fig. 12. In this figure, the driver current is swept from 0mA to 36mA on the horizontal axis and is plotted by the bias resistor, the current through the laser diode, and the combined parallel resistance of the bias resistor and the laser diode. The figure shows that the bias current can be increased in the driver to 15mA and still result in the laser diode current remaining at zero. As the driver current increases, the laser diode resistance decreases, causing most of the driver current to flow into the laser diode. This becomes more efficient when the laser diode is driven at higher currents. As expected, the combined driver load resistance is 200 ohms at zero current and then transitions to a few ohms as the current increases.

The 200 ohm resistor plays an important role in reducing the capacitive effect between the collector and ground and between the collector and the LSR _ PWR source. When the driver current is turned off, the current decay is dominated by the RC time constant, which is now controlled by the resistor at a lower current. This allows for a significant improvement in preventing one pulse from affecting the next as the time between pulses becomes shorter.

Finally, the introduction of the R-bias provides a method of calibrating the "off" light output level. Instead of relying on the optical measurement equipment required by conventional methods, each circuit is calibrated by simply opening the switch that applies the pulse _ in to the driver and adjusting the vbias voltage to produce three (3) volts across the laser diode terminals. This is effective because at this voltage all laser diodes will result in a very low current due to their high off-resistance. This calibration may be performed with or without connection of the laser diode, allowing each circuit to be calibrated when testing the pcb.

Pre-charging method

A pulse command method called "precharge" is now described in a non-limiting embodiment, which provides additional margin to ensure that the first pulse in a burst pulse sequence will always occur and prevent missing pixels on the display screen. As provided in fig. 9, the first pulse in a burst pulse sequence occurring after a sustained off-period may not be able to illuminate the pixel. However, this potential failure mechanism is greatly improved with the driving method described herein. However, it is important to note that light failure can still occur when the laser diode deviates substantially from normal. To address these situations, a "pre-charge" method that includes commanding a burst pulse sequence of either a single low-amplitude pulse or a normal-amplitude pulse may be used to further increase the bias current and charge any capacitance in the laser diode electrical path.

As provided in fig. 3, the "pulse _ in" is typically from an adjustable voltage source connected through an analog switch. Fig. 13 shows an example case where precharging can improve what is defined as "first pixel up" after a long period of no pulse. In fig. 13, trace 1300 is the "pulse _ in" signal input to the driver. Trace 1302 shows the laser diode current pulses for the first two pixels in the burst pulse sequence. The peak current of the first pulse is slightly lower than the second pulse due to the charging of the capacitor. The resulting laser output power is shown at 1304 and it can be seen that very good current control is required to produce a consistent peak output power.

FIG. 14 shows a graph using "pre-charging" of the laser and the electrical connection capacitance to produce a consistent output optical power level. In this case a low amplitude pulse of voltage is applied just before the larger pulse used to illuminate the pixel. Typically the low amplitude pulses are adjusted to produce low current pulses well below a threshold (about 5 to 10mA) just before the high amplitude pulses used to illuminate the pixels. In fig. 14, the low precharge pulse causes the first and second pulses to match in the light output.

A second alternative approach for cost-effective design is to take advantage of the fact that the laser diode current rise and fall times are slower than the "pulse _ in" waveform. In this case, the "precharge" is performed using a pulse control method that quickly turns the "pulse _ in" signal on and off, leaving the driver to low pass filter the pulse train input. By appropriately modulating the on-off precharge switch timing we can achieve an average bias current level that is matched in performance to the first approach. This is shown in fig. 14. The benefit of this embodiment is that it can be implemented using existing circuitry that already exists.

In one non-limiting embodiment, a method of driving a laser diode is disclosed that includes increasing a bias current of the laser diode to a threshold level, wherein the threshold level is below a start-up level of the laser diode, and wherein a resistor is disposed in parallel with the laser diode, charging a capacitance to a pre-charge capacitance of a circuit including the laser diode, wherein the pre-charge capacitance is below a capacitance start-up level of the laser diode, and starting the laser diode.

In another non-limiting embodiment, the method may be performed wherein increasing the bias current of the laser diode is performed by providing a single amplitude pulse of current to the laser diode.

In another non-limiting embodiment, the method may be performed wherein increasing the bias current of the laser diode is performed by providing a burst pulse sequence of amplitude pulses to the laser pulses.

In another non-limiting embodiment, the method may be performed wherein the bias current of the laser diode is increased by an adjustable voltage source.

In another non-limiting embodiment, the method may be performed wherein the adjustable voltage source is controlled by an analog switch.

In another non-limiting embodiment, the method may be performed wherein the bias current of the laser diode is increased to a threshold level by sweeping the current from a zero level to the threshold level.

In another non-limiting embodiment, a method of driving a laser diode is disclosed, comprising increasing a bias current of the laser diode to a threshold level in a series of pulses, wherein the threshold level is below a start-up level of the laser diode, and wherein a resistor is disposed in parallel with the laser diode, and wherein a frequency of the series of pulses is greater than a laser diode current discharge rate; charging the capacitor to a pre-charge capacitor of a circuit including the laser diode, wherein the pre-charge capacitor is below a capacitor activation level of the laser diode, and activating the laser diode.

In yet another non-limiting embodiment, the method may be performed wherein the bias current of the laser diode is increased by an adjustable voltage source.

In yet another non-limiting embodiment, the method may be performed wherein the adjustable voltage source is controlled by an analog switch.

In yet another non-limiting embodiment, the method may be performed wherein the bias current of the laser diode is increased to a threshold level by sweeping the current from a zero level to the threshold level.

In yet another non-limiting embodiment, an apparatus for providing electrical current to a device is disclosed, the apparatus comprising: a diode; a resistor arranged in parallel with the laser diode; at least two transistors, wherein each transistor has a collector, an emitter, and a base, and each collector is connected to a laser diode; at least one operational amplifier connected to each base of the at least two transistors; a DC power supply connected to the at least one operational amplifier; and at least one direct current power supply connected to each of the emitters of the at least two transistors.

In yet another non-limiting embodiment, the apparatus further comprises at least one capacitor disposed in parallel with the at least one operational amplifier.

In yet another non-limiting embodiment, the apparatus may be implemented wherein the at least two transistors are a first transistor and a second transistor.

In yet another non-limiting embodiment, the apparatus may further include at least two resistors located between the emitters of the first and second transistors, and at least one direct current power supply connected to each of the emitters.

In yet another non-limiting embodiment, the apparatus may be configured wherein the at least two resistors are a first resistor and a second resistor.

In yet another non-limiting embodiment, the apparatus may be configured wherein the first resistor has a higher resistance value than the second resistor.

In yet another non-limiting embodiment, the apparatus further comprises at least one capacitor disposed in parallel with one of at least two resistors located between the emitters of the first and second transistors.

In yet another non-limiting embodiment, the apparatus further comprises at least one resistor disposed between the emitters of the first and second transistors and ground.

In yet another non-limiting embodiment, the apparatus can be configured wherein the diode is a laser diode.

While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.