Method of monitoring and controlling a laser diode

A method of controlling a laser diode measures an average light output power of the laser diode and compares the average light output power to a desired average light output power within a target range. If the average light output power is not within the target range, the slope efficiency is determined by measuring two light output powers at two different bias conditions. Each of the two light output powers is greater than a selected minimum light output power, which insures that each measurement occurs within the slope efficiency portion of the laser diode curve. A new bias current for the laser diode is calculated based on the measured slope efficiency so as to produce a new average light output power within the target range.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

Optical fiber communication systems (“OFCS”) use modulated light to transmit information over optical fibers. Unlike electronic transmissions, optical transmissions are not susceptible to electromagnetic noise and interference, and provide very broad bandwidth. Light emitting devices, such as laser diodes, are used to produce light pulses that are transmitted on the OFCS. Examples of suitable laser diodes include distributed feedback (“DFB”) lasers, Fabry-Perot (“FP”) lasers, and vertical-cavity surface-emitting lasers (“VCSELs”).

It is generally desirable that a light emitting device used in an OFCS produce pulses having a selected light power. Pulses that are too dim might not be reliably received by a photodetector at the opposite end of the optical fiber link, and pulses that are too bright might create an eye-hazard for a human operator. Light output (i.e. pulse light power) from a laser diode is a function of bias current, and increasing or decreasing the bias current increases or decreases the light output. However, different laser diodes will produce different light intensities for the same bias current. In other words, the light produced varies from part to part.

Binning is used to separate diode parts according to their operating characteristics, such as threshold current and slope efficiency. The laser diodes are used in circuits that provide external control for setting the biasing current so that it is suitable for a particular application. This approach requires extra pads to set the desired biasing current, and one-by-one testing to determine which bin each die is sorted to.

Aging and temperature can affect the bias current needed to achieve the desired light power. The control bits used to set the bias current in a particular application might not be sufficient to overcome changes in light power arising from aging or temperature effects. Closed-loop systems have been developed to compensate for changes in laser diode operating characteristics arising from aging and/or changes in temperature.

A closed-loop system is generally a feedback system that detects, evaluates, and compensates for changes in laser diode operating characteristics. This can ensure that a laser diode is able to operate at the desired bias point. Many different methodologies are used in the design and implementation of closed-loop monitoring system.

One closed-loop system computes the slope efficiency of a laser diode by reading the output light power at two different bias current levels of the power versus bias current curve. Both light power readings are taken at a power level above the minimum output light power, which occurs at a bias current above the threshold current. However, this assumes that the slope efficiency remains constant over time and temperature, which it does not.

Another closed-loop system computes the slope efficiency by measuring light power over a range of bias currents, and then sets the target bias current in a step-wise fashion based on the measured threshold current. However, this approach can take a long time, especially if the light power drifts outside the target range. Even if the light power is within the target range, determining the exact value of the threshold current is quite difficult and can take several measurements. Therefore, an improved technique for quickly and accurately determining the bias current for a particular light power from a laser diode is desirable.

BRIEF SUMMARY OF THE INVENTION

A method of controlling a laser diode measures an average light output power of the laser diode and compares the average light output power to a desired average light output power within a target range. If the average light output power is not within the target range, the slope efficiency is determined by measuring two light output powers at two different bias conditions. Each of the two light output powers is greater than a selected minimum light output power, which insures that each measurement occurs within the slope efficiency portion of the laser diode curve. A new bias current for the laser diode is calculated based on the measured slope efficiency so as to produce a new average light output power within the target range.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1shows a plot100of average light output power (“PAVE”) versus bias current for an exemplary laser diode. PAVEis the average output power (typically expressed in mW) when the laser diode is transmitting a 1-0-1-0-1 . . . data stream. A DATA0(LOW) condition is when the laser diode is biased at a low bias current I0, and a DATA1(HIGH) condition is when the laser is biased at a high bias current I0+IMOD. PAVEis the average of the output power level at DATA1and the output power level at DATA0. For a fixed IMOD, PAVEincreases slightly with increasing bias current until the threshold current (“ITH”)101is reached, after which PAVEmore rapidly increases with increasing bias current. The relationship between PAVEand bias current above PAVE—MINis called the slope efficiency (“SE”).

Photodiodes are typically used as the light detectors in optical feedback loops. Such photodiodes have leakage (“dark”) current. Measuring the SE above PAVE—MINensures that the photodiode is measuring light from the laser diode, and not just its own leakage current. In other words, power detected above PAVE—MINis valid for computing the SE. Measuring the average power level (between the DATA1and DATA0states) of the laser diode while it is being modulated is more desirable than measuring the laser diode in a static condition because the average power is higher than the output power at the DATA0state. Furthermore, the modulation does not have to be turned off to measure average power, which allows monitoring PAVEwhile an application is running. Modulation would be turned off to measure a DATA0power in a static condition, which would interrupt the application.

It is desirable that the HIGH data state be easily distinguished from the LOW data state. Generally, I0is above ITHto provide a rapid increase in output power with IMOD. It is desirable to maintain an average output power within a target range between PAVE—TARGET—MAXand PAVE—TARGET—MIN. A typical application will have a desired target average output power PAVE—TARGET. If SE drops, such as from aging or at elevated temperature, PAVEwill drop because both the power at I0(the LOW data state) and at I0+IMOD(the HIGH data state) will decrease, and might drop below PAVE—TARGET—MIN. Furthermore, the difference in output power between the HIGH and LOW data states decreases, which can make data reception more difficult.

Similarly, ITHtypically increases with age and temperature. ITHcan vary as much as 50% within a typical operating temperature range of an optical communication system. If ITHincreases past the low bias current I0, PAVEwill decrease because the power-current plot100below ITHhas a relatively shallow slope, and there will be less difference between the HIGH and LOW power outputs. It is desirable to adjust the manner in which the laser diode is driven to account for changes in both ITHand SE.

The targeted range for the average light power is dependent on the product application. In some applications, laser diodes are specified according to the average power and extinction ratio (“ER”). In a particular embodiment, the laser diode is specified according to the media-oriented systems transport (“MOST”) standards. The targeted range insures that the output power provided by the laser diode is suitable for the application, even as the output power varies due to age or temperature.

FIG. 2shows plots200,202of light output power versus bias current for an exemplary laser diode illustrating adjusting the bias current to account for temperature or aging drifts, or process variations according to an embodiment of the invention. During the fabrication of a run of laser diodes, some will have different power-current plots than others. In an application, the appropriate bias points (I0and I0+IMOD) for one laser diode will be different than for another.

The first plot200shows an initial power-current characteristic of a laser diode, and the second plot202shows a subsequent power-current characteristic of the laser diode after aging or at an elevated temperature. The threshold current has moved from a first current ITHto a higher current ITH′. The new threshold current ITH′is greater than the original low bias current I0. This condition is highly undesirable. In order to maintain DATA0at its original output power level, the low bias current I0is increased to I0′.

One way to determine the proper value for I0′is by computing the difference (I0) between I0and I0′that will provide a PAVEwithin the target range. This is done by first determining the present SE of the laser diode. SE is determined by measuring the light output power at two valid current levels. A valid current level is a current level that produces a PAVEgreater than PAVE—MIN(seeFIG. 1). Upon power-up, when the laser output power is zero, a valid laser output power is found by increasing the bias current in a pre-determined step until an average power above PAVE—MINis detected. Determining the exact value of ITH, as in done in a conventional technique and which can be difficult and time consuming, is not important, nor is taking multiple data points near ITHbecause PMINis well above the light output power that would occur at ITHin either condition (e.g. at either temperature), thus insuring that both points are within the SE portion of the laser diode output curve. The appropriate value of PMINdepends on the amount of leakage current specified of an associated photo-diode (detector) in an optical system. Generally, the chosen value of PMINproduces a detector current sufficient to provide reliable data transmission between the laser diode and the photo-diode.

Computing the desired change in low bias current according to the measured SE quickly sets the laser diode to maintain PAVEwithin the target range (seeFIG. 1). This approach increases the speed of adjustment of I0compared to a conventional monotonic step-wise adjustment, and avoids fluctuations in the output power that can arise as the step-wise adjusted laser diode settles into a steady state.

However, merely increasing the DATA0current to I0′still results in a decrease in the average light output power if the SE decreases with age/temperature. In a further embodiment, IMODis also adjusted to produce a PAVEat a selected value (e.g. PAVE—TARGET) or within a target range. In a particular embodiment, IMODis adjusted in situations where I0′would be at a maximum specified value I0—MAX(seeFIG. 3). The lifespan of the laser diode is reduced if I0is too high. Similarly, a high I0increases noise generation and reduces the extinction ratio of the laser diode. Therefore, it is desirable to keep I0below I0—MAX.

FIG. 3shows plots of light output power versus bias current for an exemplary laser diode illustrating adjusting the modulation current to account for temperature, process, or aging drifts according to an embodiment of the invention. A first plot300shows an initial power-current characteristic of a laser diode, and the second plot302shows a subsequent power-current characteristic of the laser diode after aging or at an elevated temperature. PAVE—TARGETis maintained by adjusting both I0and IMOD.

As discussed above in relation toFIG. 2, I0is increased. However, increasing I0sufficiently to maintain the original DATA0output power would exceed I0—MAX. Thus, the output power at I—MAX(DATA0′) is less than the original DATA0output power. Furthermore, when the original IMODis added to I0—MAX, the output power304is only slightly higher than PAVE—TARGET. The average of the new DATA1output power level304and the DATA0′ output power, results in an average power much less than PAVE—TARGET.

Adjusting IMODallows I0to remain at or below I0—MAXwhile providing the desired PAVE—TARGET. A value IMODis calculated and added to the original IMODvalue to result in IMOD—NEW. IMOD—NEWproduces a new HIGH output power level (DATA1′) that, when added to the DATA0′ output power level, provides the desired PAVE—TARGET.

FIG. 4is a flow chart of an exemplary method400of controlling a laser diode according to an embodiment of the invention. PAVEis measured by a photodetector, such as a photodiode, converted into a voltage (e.g. using a transimpedance amplifier), and digitized (step402). If PAVEis less than or equal to PAVE—MIN(seeFIG. 1) (branch404), I0is increased by an amount (“X”) (step406) and the method returns to the starting point. The value of X depends on the specific application of the laser diode and is generally chosen to provide a reasonably optimum value that will not violate an eye safety or timing specification (e.g. by increasing the current in too big of an increment). In a particular embodiment, X is about 1.3 mA. When PAVEis not less than PAVE—MIN(branch408), PAVEis checked to see if it is within the target range. Variation from the target range will prompt a digital controller to carry out a compensation operation.

If PAVEis within the target range (branch410), the ER is optionally checked by determining whether the measured SE is greater than a desired SE that is computed (“SECOM”). If the measured SE is greater than SECOM(branch413), IMODis reduced by a selected value (step415), and the SE is measured again. The SECOMis an SE known to produce an ER within a specified range, based on the characterization between SE, ER, and IMOD. In a particular embodiment, the reduction in IMODis calculated according to the relationship between SE (which has been measured), ER (which is specified), and IMOD. Thus, IMODis calculated to produce the desired ER.

If the measured SE is not greater than the desired SECOM(branch419), IMODis checked to determine if it is greater than the initial IMOD. If IMODis greater than the initial IMOD(branch421) and10is at the minimum value (branch423), a fault condition occurs (step425) because the bias current cannot be reduced further. If I0is not at the minimum value (branch427), I0is reduced a selected amount (step429) and the process is returned to the start. An example of determining the desired selected reduction is described below in reference toFIG. 5A.

If IMODis not greater than the initial IMOD(branch431) and I0is at its maximum value (branch433), a fault condition (step435) occurs because the bias current cannot be further increased. If I0is not at its maximum value (branch437), I0is increased a selected amount (step439) and the process returns to the start. An example of determining the desired selected increase is described below in reference toFIG. 5B.

If PAVEis greater than the maximum target value (branch426), I0is checked to insure that it is not already at the minimum value. If I0is at the minimum value (branch418), an attempt is made to adjust IMOD. If IMODcannot be reduced (branch424), a fault (step428) occurs because both the bias current and the modulation currents cannot be further reduced to reduce PAVE. If IMODcan be reduced (branch422), IMODis reduced a selected amount (step432), and the process returns to the start. An example of determining the desired selected decrease in IMODis described below in reference toFIG. 6A.

If I0is not at the minimum value (branch446), I0is decreased to decrease PAVE(step448). In a particular embodiment, I0is decreased by a value (I0) calculated from the slope of diode characteristic. Knowing the slope, one can solve for the bias current that would produce a desired PAVEfor a given IMOD. If I0is greater than X mA (see step406), I0is decreased by X mA and the process returns to the start, otherwise I0is decreased by I0and the process returns to the start.

If PAVEis less than the minimum target value (branch450), I0is checked to insure that it is not already at the maximum value. If I0is at the maximum value (branch452), an attempt is made to adjust IMOD. If IMODcannot be increased (branch454), a fault (step456) occurs because both the bias current and the modulation currents cannot be further increased to increase PAVE. If IMODcan be increased (branch458), IMODis increased a selected amount (step460) and the process returns to the start. An example of determining the desired selected increase in IMODis described below in reference toFIG. 6B.

If I0is not at the maximum value (branch462), I0is inecreased to increase PAVE(step464). In a particular embodiment, I0is increased by a value (I0) calculated from the slope of diode characteristic. Knowing the slope, one can solve for the bias current that would produce the desired PAVEfor a given IMOD. If I0is greater than X mA (see step406), I0is increased by X mA and the process returns to the start, otherwise I0is increased by I0and the process returns to the start.

FIG. 5Ais a plot of a portion of a diode characteristic500illustrating how to adjust the bias and modulation currents to improve ER. A new bias current I0—newis calculated according to the slope of the curve and the desired change in ER (which is determined according to a known relationship between power and ER). The laser diode is operating at an initial bias current I0and at an initial modulation current IMOD, which is added to the bias current during the modulated portion of the output. The initial average power PAVE—initialoccurs at a current of I0+½IMOD. The bias current is reduced to I0—new, and the modulation current is increased to IMOD—newin order to maintain the average output power at an essentially constant level while increasing ER. The new average output power, occurs at I0—new+½IMOD—new, might not be exactly the same as PAVE—initialdue to digitization errors. For example, in a particular embodiment, the modulation current is stepped from a minimum value (e.g. zero mA) to a maximum value (e.g. about 12 mA) in eight steps (digital values 0-7). The bias current is stepped from a minimum value (e.g. a current above the expected threshold current) to a maximum value (e.g. the bias current just below which a safety issue might arise when the total of the bias current and the modulation current produces an unsafe light power output) in 256 steps (digital values 0-255). These values are merely exemplary. Other laser diodes might have different minimum and maximum modulation current values, and other control systems might have finer or coarser adjustments.

FIG. 5Bis a plot of a portion of a diode characteristic510illustrating how to adjust the bias and modulation currents to reduce ER. A new bias current I0—newis calculated according to the slope of the curve and the desired change in ER (which is determined according to a known relationship between power and ER). The laser diode is operating at an initial bias current I0and at an initial modulation current IMOD, which is added to the bias current during the modulated portion of the output. The initial average power PAVE—initialoccurs at a current of I0+½IMOD. The bias current is increases to I0—new, and the modulation current is decreased to IMOD—newin order to maintain the average output power at an essentially constant level while reducing ER. The new average output power, occurs at I0—new+½IMOD—new, might not be exactly the same as PAVE—initialdue to digitization errors. For example, in a particular embodiment, the modulation current is stepped from a minimum value (e.g. zero mA) to a maximum value (e.g. about 12 mA) in eight steps (digital values 0-7). The bias current is stepped from a minimum value (e.g. a current above the expected threshold current) to a maximum value (e.g. the bias current just below which a safety issue might arise when the total of the bias current and the modulation current produces an unsafe light power output) in 256 steps (digital values 0-255). These values are merely exemplary. Other laser diodes might have different minimum and maximum modulation current values, and other control systems might have finer or coarser adjustments.

FIG. 6Ais a plot of a portion of a diode characteristic600illustrating an embodiment of adjusting the modulation current to achieve a target average power when the bias current is at a maximum value. In a particular embodiment, modulation current is adjusted in a step-wise fashion. Each step increases or decreases the modulation current by a pre-selected amount. In some embodiments, each step is the same amount, alternatively, some steps are greater or less than others.

The bias current I0is at the maximum allowable value in a particular diode and application. To achieve a target average power level PAVE—targetwhen the current power PAVEis too low, the average power is increased by increasing the modulation current. The initial modulation current is at an initial selected level. For example, in a system providing an 8-step adjustment of modulation current, the modulation current is set at a level according to the fourth adjustment step (i.e. n=3, where n is a value between 0-7). The modulation current is increased by increasing n to 4 or higher, and is decreased by decreasing n to 2 or less. In some embodiments, the adjustment of modulation current is relatively coarse. In such cases, it is desirable to set n to the value that will produce an average output power closest to the PAVE—target.

In the linear portion of the diode characteristic, the average output power occurs at the bias current plus one-half the modulation current. Knowing the slope of the diode characteristic in this region allows calculation of the modulation current that will produce PAVE—targetat a given bias current I0, which is this example is at the maximum value. However, because modulation current is adjusted in relatively coarse steps (compared to the bias current), increasing the modulation current (i.e. increasing the value of n) usually does not result exactly in PAVE—target. It is desirable to select the value of n that results in an average power level closest to PAVE—target.

If I0is at its maximum value (e.g. 255) and IMODis not at its maximum value (e.g. n is less than seven), average power output can be increased by increasing IMOD, that is, by increasing n. If n=6, then n may only be increased to seven. However, if n is less than 6, it may be increased at least two steps. The modulation current that would produce PAVE—targetis calculated (PAVE—targetoccurs at I0+½IMODtarget), and the value of n that produces the closest average power output is determined. In other words, the value of n that produces a modulation current closest to IMOD—targetis calculated. In the event where n is increased to a higher value x, ½IMOD—xis compared to ½IMOD—(x−1)to see which value produces a modulation current closest to ½IMOD—target. In this example, x−1 produces the more desirable modulation current, and PAVE—(x−1)is slightly less than PAVE—target. In an alternative example (not illustrated), IMOD—xis the more desirable modulation current.

FIG. 6Bis a plot of a portion of a diode characteristic610illustrating an embodiment of adjusting the modulation current to achieve a target average power when the bias current is at a minimum value. In a particular embodiment, modulation current is adjusted in a step-wise fashion. Each step increases or decreases the modulation current by a pre-selected amount. In some embodiments, each step is the same amount, alternatively, some steps are greater or less than others.

The bias current I0is at the minimum allowable value in a particular diode and application. To achieve a target average power level PAVE—targetwhen the current power is too high, the average power is decreased by decreasing the modulation current. The initial modulation current is at an initial selected level. For example, in a system providing an 8-step adjustment of modulation current, the modulation current is set at a level according to the fourth adjustment step (i.e. n=3, where n is a value between 0-7). The modulation current is increased by increasing n to 4 or higher, and is decreased by decreasing n to 2 or less. In some embodiments, the adjustment of modulation current is relatively coarse. In such cases, it is desirable to set n to the value that will produce an average output power closest to the PAVE—target.

In the linear portion of the diode characteristic, the average output power occurs at the bias current plus one-half the modulation current. Knowing the slope of the diode characteristic in this region allows calculation of the modulation current that will produce PAVE—targetat a given bias current I0, which is this example is at the minimum value. However, because modulation current is adjusted in relatively coarse steps (compared to the bias current), decreasing the modulation current (i.e. decreasing the value of n) usually does not result exactly in PAVE—target. It is desirable to select the value of n that results in an average power level closest to PAVE—target.

If I0is at its minimum value (e.g. 0) and IMODis not at its minimum value (e.g. n is greater than zero), average power output can be decreased by decreasing IMOD, that is, by decreasing n. If n=1, then n may only be decreased to zero. However, if n is greater than 1, it may be decreased at least two steps. The modulation current that would produce PAVE—targetis calculated (PAVE—targetoccurs at I0+½IMOD—target), and the value of n that produces the closest average power output is determined. In other words, the value of n that produces a modulation current closest to IMOD—targetis calculated. In the event where n is decreased to a lower value x, ½IMOD—xis compared to ½IMOD—(x+1)to see which value produces a modulation current closest to ½IMOD—target. In this example, x+1 produces the more desirable modulation current, and PAVE—(x+1)is slightly greater than PAVE—target. In an alternative example (not illustrated), IMOD—xis the more desirable modulation current.

FIG. 7is a plan view of a laser diode system700according to an embodiment of the invention. A laser diode702emits light from a front facet703according to the current provided to the laser diode702from a controllable current source704that is part of a laser driver chip706. The laser driver chip706also includes a digital controller708. A photodetector710, such as a photodiode, produces a detector signal712that is proportional to the light power level emitted by the laser diode. In this embodiment, the laser diode702“leaks” light from a back facet705. The amount of light from the back facet705is proportional to the light emitted from the front facet703, but much less intense. In a particular embodiment, the photodetector produces a current that is converted to a voltage by passing the current through a sense resistor714. The sense resistor is not integrated in the laser driver chip in another embodiment. Alternatively, a trans-impedance amplifier is used to convert the photo-detector current to voltage instead of a resistor.

The detector signal is digitized by an analog-to-digital converter (“ADC”)715. The digital controller is a digital logic circuit that performs a method according to an embodiment of the invention. Alternatively, methods according to embodiments of the invention are performed by a microcontroller that is not integrated in the laser driver chip. The laser driver chip receives power on a bias line716that provides the power to operate both the laser diode and the laser driver chip, and receives a control signal on a control line718that indicates whether the laser diode should be in a DATA1condition or a DATA0condition.

While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to these embodiments might occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.