WO1999053487A2

WO1999053487A2 - Laser driver with tracking servo

Info

Publication number: WO1999053487A2
Application number: PCT/US1999/007161
Authority: WO
Inventors: Stephen J. Hrinya
Original assignee: Seagate Technology, Inc.
Priority date: 1998-04-09
Filing date: 1999-03-31
Publication date: 1999-10-21
Also published as: WO1999053487A3

Abstract

A laser driver utilizes a difference amplifier circuit to produce a servo signal, and a tracking circuit for storing a digital approximation of the servo signal. During a write mode of operation, the tracking circuit produces a stored tracking signal which does not change. This tracking signal is used as the baseline current to which write pulses are added during the write mode. The digital approximation of the servo signal is input to a digital-to-analog converter to produce the tracking signal which does not drop over time. During a read mode of operation the tracking circuit incrementally updates the stored approximation of the servo signal. The laser driver also incorporates a correction mechanism in the difference amplifier circuit to prevent the servo signal from dropping too low during write mode operations, and eliminates large transients during the switch from write mode to read mode.

Description

LASER DRIVER WITH TRACKING SERVO

Inventor: Stephen J. Hrinya

CLAIM OF BENEFIT FROM PROVISIONAL APPLICATION This application hereby claims the benefit of commonly assigned provisional application with serial number 60/081,216, titled "LASER DRIVER WITH TRACKING SERVO", which was filed on April 9, 1998.

FIELD OF INVENTION

This invention pertains to the field of laser driver circuits for optical storage devices. More specifically, this invention pertains to a laser driver servo circuit which accommodates a laser changing between two modes of operation.

BACKGROUND OF THE INVENTION

The amount of power emitted from a diode laser is related to the laser drive current flowing into the laser. At low laser drive current levels, output power is almost non-existent until a threshold drive current, , is reached. As drive current is increased, the laser output power generally increases linearly (see Fig. 1). The rate at which laser output power increases with an increase in drive current is the slope efficiency of the laser. Although the slope efficiency of a laser is generally stable, the threshold current h varies strongly with temperature and time. Variation in is particularly strong for approximately the first 20 minutes of operation of the laser. As a laser ages, h gradually increases. In order to maintain a constant laser output power, a servo circuit is typically used to control the drive current. Such a servo circuit generally utilizes input from a photodetector to which a small portion of the laser beam output is diverted. The photodetector signal is used by the servo circuit to track changes due to variations in .

One area in which control of laser output power is critical is in magneto- optical drives. Magneto-optical drives operate in at least two modes: a read mode, and a write mode. The laser driver requirements for the two modes are different.

In the read mode of operation, constant laser output power is required. In the absence of modulation, however, diode laser output power tends to fluctuate due to laser "mode hopping," which is due to random perturbations in the cavity which cause the laser to select one of many available discrete longitudinal modes. The effects of mode hopping can be greatly reduced by modulating the laser drive current with a radio-frequency (RF) signal. The resulting laser drive current is generally an RF sinusoid on top of a direct current (DC) component near IT. The laser output power in read mode is often a clipped sinusoid, due to the fact that the amplitude of the RF signal is large enough that the laser drive current drops below on each cycle. A photodetector responds to the low frequency average of the laser output power, and this average is held constant by a laser power servo circuit which controls the DC component of the laser drive current. Typically the servo circuit output is amplified to produce the laser drive current.

For the write mode of a magneto-optical drive, the laser emits short pulses of relatively high energy. The duration of the pulses is typically on the order of a few nanoseconds. It is desirable to pulse the laser from a baseline current near . In some magneto-optical drive implementations it is possible to use the read mode DC component of the laser drive current as the baseline to which write pulses are added. For write mode operation, no RF signal is added to the laser drive current.

Magneto-optical drives which implement both read and write modes of operation in the laser driver circuitry must be able to switch quickly between these modes. When switching from read mode to write mode, the RF signal must be turned off, and the DC portion of the laser drive current must be held constant. Write pulses are then added to this baseline current. When switching from write mode to read mode, the write pulses are turned off, the RF signal is turned on, and the servo circuit must quickly stabilize to the correct DC component of the laser drive current.

The power detected by the photodetector is very high during write mode, due to the magnitude of the write pulses. The servo circuit output is sampled at the entry to write mode, and this sampled value is used as the baseline for writing during the write mode. In conventional systems, however, this sampled value begins to droop over time. To compensate for this, conventional systems periodically switch into read mode long enough to reestablish the servo circuit output. This limits the duration of the write mode. The laser power servo responds to the higher level of power at the photodetector during write mode by attempting to reduce the laser power. Because the servo circuit feedback drives its output low in response to the writing pulses, conventional systems require a long time for the servo circuit to settle on the correct output value upon switching back to read mode.

What is needed is a laser driver circuit which allows a baseline current to be held constant for a write mode of indefinite duration. Also, it is desirable to provide for a shorter transition from write mode to read mode.

SUMMARY OF THE INVENTION

A laser driver utilizes a difference amplifier circuit to produce a servo signal, and a tracking circuit for temporary storage of an approximation of the servo signal. During the read mode of operation, the tracking circuit periodically updates the stored approximation of the servo signal. In the write mode, the tracking circuit is deactivated and holds its approximation of the servo output signal. The output of the tracking circuit is substituted for the servo during write mode. In one embodiment of the invention, the tracking circuit stores an approximation of the servo output in a digital register. This digital tracking value is provided to a digital to analog converter, producing a stable signal which does not droop over time.

In the writing mode, the servo loop is opened. The laser driver can incorporate a correction mechanism in the difference amplifier to prevent the servo output from deviating too far from the last value held in the tracking register. This mechanism allows the servo circuit to more quickly settle to the proper level after a switch from write mode to read mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an illustration of typical laser drive current to output power curves. Fig. 2 is an illustration of the difference amplifier circuit of one embodiment.

Fig. 3 is an illustration of the tracking circuit of one embodiment.

Fig. 4 is an illustration of one embodiment of the laser driver. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to Fig. 4, a laser driver circuit 400 is shown. Laser driver circuit 400 takes as inputs detector signal 290, set-point signal 280, mode gate signal 250, and digital input from I/O port 350. The output from laser driver circuit 400 passes through resistor 414 to the base of transistor 408. The emitter of transistor 408 is connected to ground through resistor 410, and the collector is connected to laser 402. Transistor 408 acts to convert the output voltage of laser driver 400 into laser drive current 418 to laser 402. During write mode, write pulse driver 404 adds write pulses to laser drive current 418. Write pulse driver 404 takes write data signal 460 and write gate signal 450 as inputs. During read mode, RF oscillator 406 adds an RF signal to the base of transistor 408, through capacitor 412. RF oscillator 406 takes as input RF gate signal 470.

A portion of the laser beam emitted from laser 402 is diverted to photodetector 202, which produces detector signal 290 based on the diverted light. Digital to analog converter (DAC) 210 is used to establish the servo set-point signal 280. Mode gate signal 250 is used to differentiate the different modes of operation. The read mode of operation corresponds to mode gate signal 250 being 1, and the write mode corresponds to mode gate signal 250 being 0. RF gate signal 470 and write gate signal 450 may be equivalent to the mode gate signal 250 in some embodiments.

Laser driver circuit 400 includes difference amplifier circuit 200, tracking circuit 300, and switch 416. Switch 416 responds to mode gate signal 250 to establish the source of the output signal from laser driver circuit 400. When mode gate signal 250 is 1, servo signal 270 is output. When mode gate signal 250 is 0, tracking signal 260 is output. During read mode, difference amplifier 200 compensates for changes in the power characteristics of laser 402 by changing servo signal 270, which determines laser drive current 418. Tracking circuit 300 produces tracking signal 260 which follows servo signal 270 during read mode, and which stays constant during write mode. Referring now to Fig. 2, difference amplifier circuit 200 is shown.

Difference amplifier circuit 200 takes as inputs detector signal 290, set-point signal 280, tracking signal 260, and mode gate signal 250. The output of difference amplifier circuit 200 is servo signal 270.

Variable resistor 206 is used to scale the gain of amplifier 204 such that the voltage at VPD 240 is VREF when laser 402 is emitting no light, and VREF/1 when laser 402 is emitting the maximum amount of light. The selection of the value for

V_REF is arbitrary, as is the scaling. In other embodiments other scalings can be used.

For example, scalings of VREF to VREF/ 3 and VREF to IVREF can be used.

DAC 210 generates set-point signal 280, which serves as a reference for the desired voltage at VPD 240. In the illustrative embodiment, DAC 210 is scaled such that it produces VREF at minimum code and VREF/1 at maximum code. These endpoints correspond to the voltages at V_PD 240 for maximum and minimum laser output power. Servo set-point signal 280 feeds into the inverting input of amplifier 220 through resistor 212. A feedback loop formed by resistors 214, 216, and capacitor 218, regulates the gain of amplifier 220 for set-point signal 280. The non-inverting input of amplifier 220 is connected to V_PD 240 through resistor 222, and to tracking signal 260 through resistor 224. Resistors 212 and 222 are equal to R ohms, and resistors 214 and 224 are equal to A_SR ohms. At low frequencies, the feedback path for amplifier 220 reduces to R ohms. Assuming a very large gain for amplifier 220, and low frequency changes to DAC 210, the voltage of servo signal 270 connected to the output of amplifier 220 is approximated by Equation 1:

' Servo ⁼ *Y PD ~ "setPoint ) ⁺ Tracking ^Q' ^ where Vsewo is the voltage of servo signal 270, VsetPoint is the voltage of servo set-point signal 280, and Vτrackin_S is the voltage of tracking signal 260. Because photodetector 202 reacts to the intensity of light produced by laser 402, laser 402 is driven by laser drive current 418, and laser drive current 418 responds to servo signal 270 in read mode, amplifier 220 closes this loop, changing servo signal 270 so as to minimize the difference between VPD 240 and set-point signal 280. The laser output power for read mode can effectively be set through DAC 210, which determines set-point signal 280.

Referring now to Fig. 3, tracking circuit 300 is described. Tracking circuit 300 accepts as inputs servo signal 270, mode gate signal 250, and digital information through I/O port 350. Tracking circuit 300 produces as outputs tracking signal 260 and digital information through I/O port 350.

When mode gate signal 250 is 1, indicating read mode, clock 304 produces periodic clock pulses. Comparator 302 produces a signal which indicates whether servo signal 270 is higher than tracking signal 260. D flip-flop 306 receives clock pulses from clock 304, and the output from comparator 302. D flip-flop 306 provides synchronous samples of the output of comparator 302 to be used as the up/ down control input to up/ down counter 308 and prescaler 312. Prescaler 312 is an up/ down counter with fewer bits than counter 308. In the illustrative embodiment prescaler 312 is a 3-bit counter. Ordinarily, prescaler 312 decreases when tracking signal 260 is higher than servo signal 270, and prescaler 312 increases when servo signal 270 is higher than tracking signal 260. Overflow signal 314 is active when prescaler 312 is at maximum code and the up/ down input indicates the up direction, or when prescaler 312 is at minimum code and the up/ down input indicates the down direction. Counter 308 changes only when indicated by overflow signal 314. The direction of change is indicated by the comparator signal from gate 306. The effect of prescaler 312 is to introduce a lag into the movement of counter 308, which responds to comparator 302. Although this lag is not necessary, in some embodiments it helps keep tracking signal 260 from large excursions around servo signal 270.

During read mode, when mode gate signal 250 is 1, counter 308 increases if tracking signal 260 is lower than servo signal 270, and decreases if tracking signal 260 is higher than servo signal 270. The value in counter 308 is provided to DAC 310, which produces tracking signal 260. During read mode, the changes to counter 308 generally cause tracking signal 260 to move closer to servo signal 270. When the difference between tracking signal 260 and servo signal 270 is less than one bit of DAC 310, tracking signal 260 will alternate between less than a bit over and less than a bit under servo signal 270. The accuracy of tracking signal 260 can generally be increased by using more bits in DAC 310. In the illustrative embodiment, eight bits are used in DAC 310. DAC 310 is scaled to produce voltages which result in minimum current at zero code, and the maximum current at maximum code.

During write mode, when mode gate signal 250 is 0, counter 308 and prescaler 312 do not change, because there is no signal from clock 304. Because the value in counter 308 is not changed, DAC 310 produces an unchanging tracking signal 260, which is ordinarily approximately equal to servo signal 270 at the time write mode is entered. Because tracking signal 260 is generated by a digital value stored in counter 308, and does not droop over time, it is not necessary to periodically engage read mode in order to refresh tracking signal 260. This allows for write mode operation of indefinitely long duration.

I/O port 350 allows counter 308 to be monitored during operation of laser driver circuit 400. The value in counter 308, after tracking signal 260 has converged on servo signal 270, is proportional to the laser drive current 418 necessary to produce the level of laser output power specified in DAC 210. The laser output power to drive current curve can be measured by monitoring counter 308 through I/O port 350 with different set-points specified in DAC 210. By disabling RF gate signal 470, waiting for tracking signal 260 to settle, and monitoring counter 308, servo signal 270 can be determined for a particular value in DAC 210. The slope efficiency for laser 402 is calculated by dividing the change in DAC 210 values by the corresponding change in servo signal 270 values. The value of can similarly be estimated by extrapolating from a pair of DAC 210 values and a corresponding pair of servo signal 270 values to determine the laser drive current 418 that coincides with zero laser output power. As a laser ages, the value of for that laser increases. The expected end of life for a laser can often be predicted based on the history of values. In one embodiment, calculated IT values are periodically recorded. These values are used by the system to determine an expected end of life for laser 402. A warning message is issued to the system in time for laser 402 to be replaced, preventing unexpected failure and possible loss of data.

I/O port 350 also allows open loop operation of laser 402. By setting mode gate signal 250 to 0, laser drive current 418 is based on tracking signal 260, which is specified by the value in counter 308. Through I/O port 350, the value in counter 308 can be set to a desired level. This functionality makes it convenient to use laser 402 while aligning optics before photodetector 202 is installed.

Because laser 402 operates open loop during write mode, and the laser output power detected by photodetector 202 is high during write mode, amplifier 220 would ordinarily cause servo signal 270 to drop low in response. This slows down the transition from write mode to read mode, because it takes a long time for difference amplifier circuit 200 to converge on the proper servo signal 270. In the illustrated embodiment of Fig. 2, amplifier 228 is used to prevent servo signal 270 from dropping too low during write mode. When mode gate signal 250 is set to 1, indicating read mode, the output of amplifier 228 is switched away from amplifier 204, and VPD 240 is influenced only by photodetector 202. When mode gate signal 250 is set to 0, indicating write mode, however, the output of amplifier 228 is added into the non-inverting input of amplifier 204 through resistor 208. The inverting input of amplifier 228 receives tracking signal 260 through resistor 226, and the non-inverting input receives servo signal 270. Capacitor 230 integrates the difference between servo signal 270 and tracking signal 260. The output of amplifier 228 drives a current through resistor 208 to force servo signal 270 at amplifier 220 to remain at a level equal to tracking signal 260. When the mode is switched from write to read, servo signal 270 is the same as tracking signal 260 and the switching transient is thereby greatly reduced.

The above description is included to illustrate the operation of exemplary embodiments and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above description, many variations will be apparent to one skilled in the art that would be encompassed by the spirit and scope of the present invention. What is claimed is:

Claims

1. A laser driver for producing a laser drive signal which determines a component of a drive current for a laser, the laser driver comprising: a tracking circuit for producing a tracking signal, which tracking signal is an approximation of a servo signal during a first mode of operation and is constant during a second mode of operation; a difference amplifier circuit coupled to the tracking circuit, for producing the servo signal during the first mode of operation; and a switching device coupled to the tracking circuit and the difference amplifier circuit, for transmitting the servo signal as the laser drive signal during the first mode of operation, and transmitting the tracking signal as the laser drive signal during the second mode of operation.

2. A laser driver with at least a first mode and a second mode of operation for producing a laser drive signal which determines a component of a drive current for a laser, the laser driver comprising: a tracking circuit for producing a tracking signal based on a stored digital tracking value, receiving a servo signal during the first mode of operation, and changing the stored digital tracking value so as to cause the tracking signal to be an approximation of the servo signal during the first mode of operation; a difference amplifier circuit coupled to the tracking circuit, for receiving a set- point signal and a detector signal, the detector signal being proportional to a level of output power produced by the laser in response to the drive current, and causing the servo signal to converge such that the detector signal approximates the set-point signal during the first mode of operation; and a switching device coupled to the tracking circuit and the difference amplifier circuit, for transmitting the servo signal as the laser drive signal during the first mode of operation, and transmitting the tracking signal as the laser drive signal during the second mode of operation.

3. The laser driver of claim 2, wherein the difference amplifier is adapted to receive the tracking signal, and is adapted to cause the servo signal to be proportional to the sum of the tracking signal plus a value which is proportional to the difference between the set-point signal and the detector signal during the first mode of operation.

4. The laser driver of claim 2, wherein the tracking circuit is adapted to hold the stored digital tracking value unchanged during the second mode of operation.

5. The laser driver of claim 2, wherein the tracking circuit is adapted to incrementally change the stored digital tracking value so as to cause the tracking signal to converge towards an approximation of the servo signal during the first mode of operation.

6. The laser driver of claim 2, wherein the difference amplifier is adapted to receive the tracking signal during the second mode of operation, and to use feedback to urge the servo signal towards the tracking signal during the second mode of operation.

7. The laser driver of claim 6, wherein the difference amplifier circuit is adapted to use negative feedback on the servo signal.

8. The laser driver of claim 2, wherein the tracking circuit includes an up/ down counter, and the stored digital tracking value is stored in the up/ down counter.

9. The laser driver of claim 2, wherein the tracking circuit changes the stored digital tracking value in the direction necessary to reduce any difference between the tracking signal and the servo signal during the first mode of operation.

10. The laser driver of claim 2, wherein the tracking circuit includes a digital output port, and the stored digital tracking value is retrievable via the digital output port.

11. The laser driver of claim 2, wherein the tracking circuit includes a digital input port, and the stored digital tracking value can be set via an input signal received at the digital input port.

12. The laser driver of claim 2, wherein the first mode corresponds to a read mode of an optical storage device, and the drive current includes a radio-frequency signal during the read mode.

13. The laser driver of claim 2, wherein the second mode corresponds to a write mode of an optical storage device, and the drive current includes write signal pulses during the write mode.

14. A laser driver with at least a first mode and a second mode of operation for producing a laser drive signal which determines a component of a drive current for a laser, the laser driver comprising: a tracking circuit for producing a tracking signal based on a tracking value, receiving a servo signal during the first mode of operation, and changing the tracking value so as to cause the tracking signal to be an approximation of the servo signal during the first mode of operation; a difference amplifier circuit coupled to the tracking circuit, for receiving a set- point signal and a detector signal, the detector signal being proportional to a level of output power produced by the laser in response to the drive current, causing the servo signal to converge such that the detector signal approximates the set-point signal during the first mode of operation, receiving the tracking signal during the second mode of operation, and using feedback to urge the servo signal towards the tracking signal during the second mode of operation; and a switching device coupled to the tracking circuit and the difference amplifier circuit, for transmitting the servo signal as the laser drive signal during the first mode of operation, and transmitting the tracking signal as the laser drive signal during the second mode of operation.

15. The laser driver of claim 14, wherein the difference amplifier circuit is adapted to use negative feedback on the servo signal.

16. A method for estimating the threshold drive current of a laser in an optical storage device, the laser being driven by a laser drive signal produced by a laser driver, the laser driver including a tracking circuit which determines a digital tracking value corresponding to a level of current necessary to cause a level of laser output power corresponding to a laser power set-point value, the method comprising the steps of: accessing the digital tracking value more than once; changing the laser power set-point value between accesses of the digital tracking value; and calculating the threshold current from the accessed digital tracking values and the laser power set-point values corresponding to the accessed digital tracking values.

17. A method of estimating the remaining life expectancy of a laser in an optical storage device, the laser being driven by a laser drive signal produced by a laser driver, the laser driver including a tracking circuit which determines a digital tracking value corresponding to a level of current necessary to cause a level of laser output power corresponding to a laser power set-point value, the method comprising the steps of: accessing the digital tracking value more than once; determining from the plurality of accessed digital tracking values a characteristic laser current value; and correlating the characteristic laser current value to laser life expectancy through predetermined correlations.

18. The method of claim 17, wherein the step of accessing the digital tracking value more than once comprises the sub-steps of: setting the laser power set-point value to a first value; after the tracking circuit has determined the digital tracking value corresponding to the level of current necessary to cause the laser to produce the level of output power corresponding to the first value, accessing the digital tracking value; setting the laser power set-point value to a second value; and after the tracking circuit has determined the digital tracking value corresponding to the level of current necessary to cause the laser to produce the level of output power corresponding to the second value, accessing the digital tracking value.

19. The method of claim 17, wherein determining a characteristic laser current value includes using one of interpolation and extrapolation to determine the level of current corresponding to a characteristic output power level.

20. The method of claim 19, wherein the characteristic output power level is zero output power.