Monitoring and adjusting laser write power in an optical disk recorder using pulse-width modulated power level checking signals

A write-once optical disk data recorder automatically calibrates a laser during a write data operation and using write pulses focussed to a disk. First, the laser is calibrated using a non-focussed laser beam. Each sector of the disk has a laser checking or test area, such as an automatic laser power correction field (ALPC) of two byte lineal extent. During a first write operation after a power up or disk load, a pulse width modulated (PWM) laser test signal is recorded using a laser power level set using the non-focussed laser beam and an indicated desired recording power level on the disk. The recorded laser test signal is read back. The length of the read back laser test signal is measured. The measured length is then compared with a desired length of the PWM laser test signal that indicates a desired laser power level. That is, as laser recording power levels increase, a resultant recorded signal grows in size. This property is used to measure laser power for calibrating laser operation.

DOCUMENT INCORPORATED BY REFERENCE 
Call et al U.S. Pat. No. 5,185,734. 
FIELD OF THE INVENTION 
This invention relates to write-once read-many (WORM) optical disk storage 
devices, more particularly to controlling the laser used to write data on 
a WORM disk. 
BACKGROUND OF THE INVENTION 
Write-once read-many (WORM) optical media have stamped indications of 
manufacturer selected laser power level for recording or writing data unto 
the respective media. It is a current common practice to read and use the 
indicated laser power level for recording on such write once media. 
Usually, optical disk laser controls use feedback for accurately 
controlling laser emission levels. As such, laser calibration upon loading 
a disk medium is avoided. 
Because of variability between optical media, media aging and laser control 
circuit aging, media contamination, actual emitted laser beam power for 
optimum recording may be different from laser drive current settings based 
upon such media-indicated laser beam power level. The above-mentioned 
aging may vary the operation of the circuits and media response resulting 
in either a laser over-power level or laser under-power level situation. 
Such over-power laser beam levels can over ablate a track so as to 
obliterate adjacent recorded data, destroy a groove that interferes with 
tracking following and seeking, and the like. Such under-power levels may 
result in defective recording, such as recording over previously recorded 
data. Therefore, it is desired to calibrate laser power level in a 
write-once media recorder to avoid such under or over power laser beam 
levels. 
In prior-art laser-beam level calibration for write-once disks used 
defocussed beams for avoiding ablating usable data storage space of the 
disk. Such out-of-focus calibration may result in a laser beam power level 
that is different than the calibrated laser beam level. That is, the laser 
beam power level for a given laser drive current changes as the beam is 
focused. This phenomena is caused by a shift in the differential 
efficiency of a semiconductor laser used in optical recording as the laser 
beam spot on an optical disk becomes focussed. This shift occurs because 
of light reflected from the disk into the laser cavity creating a cavity 
external to the laser. This shift in laser differential efficiency not 
only varies from laser to laser but also is affected by the efficiency of 
the optical feedback path. The path variability is caused by media 
variations and by contamination of the optical path (objective lens). 
Therefore, it is desired to calibrate and control laser beam power level 
in a write-once recorder using an in-focus beam and in a manner that 
data-storage space is not used. Further, in write-once media systems, it 
is not effective to use the media data-storing areas for calibrating a 
laser, such as is reasonable in rewriteable optical disks (usually 
magnetooptical). It is therefore also desired to calibrate a laser beam 
power level in an in-focus condition without using data fields of disk 
sectors that would reduce the data-storage capacity of the disk. 
In many write-once optical disks, a two-byte automatic laser power 
correction (ALPC) field is provided in each write-once disk sector. This 
ALPC field enables correcting laser power for writing data at a correct or 
desired emitted laser power level. The ALPC field also enables the laser 
to be operated at write level outside of the data area. Such writing of a 
laser test signal in the ALPC field is monitored for ensuring that the 
laser is emitting a proper level laser beam to the disk. Such measurement 
is made with a photo detector receiving either the using so-called wasted 
light from a beam splitter or using light from an auxiliary port of the 
laser. This write testing merely turns the laser continuously on at write 
level for a period of time equal to scanning one or two bytes on the disk. 
Such an extended-time continuous write signal can have excessive energy 
resulting in so-called over ablation, i.e. the area ablated (physical size 
of the recorded laser test signal) exceeds the track width and may exceed 
the length of the ALPC field. Remember that such laser power level 
verification is measured at the output of the laser and does not measure 
ablation on an optical disk. 
A reason for this over ablation is the duty cycle is different from a usual 
write pulse. That is, a usual write pulse has a duty factor of about 10% 
that ablates about one-half of the track width. In contrast, the 100% duty 
cycle used in the write qualification is extreme to often ablate radially 
outside of the track being written to. Such over ablation not only extends 
radially but also circumferentially (at the trailing edge of the DC 
pulse). While the duration of a recorded laser test signal need not fill 
the ALPC 2-byte field, many recorders do record such a laser test signal. 
In the latter instance, excessive laser power level results in a recorded 
laser test signal that crowds or extends to an ensuing write area, such as 
a sync area that precedes recorded user data. Recorded laser test signals 
having a shorter length may still radially over ablate in the ALPC area. 
Therefore, such extended continuous laser emissions may have undesired 
heating of the laser and its immediate environs. It is desired to avoid 
such over ablation. 
The so-called correct write power level is also dependent on the duration 
or width of a laser write pulse. Writing in write-once media often assumes 
that the recorded write power level on each disk is correct. Because of 
circuit variations, signal delay tolerances of .+-.5%, signal propagation 
asymmetries in various circuits, and the like results in variations of 
actual recorded write pulses of a same power level that is non-linear. It 
is desired to avoid such unintended variations of write-once recording. 
DISCUSSION OF THE PRIOR ART 
Romeas et al in U.S. Pat. No. 4,631,713 show recording binary test words on 
a write-once optical disk having a 10 repeated pattern. The durations of 
the respective "1" and "0" portions of the test pattern are measured. The 
laser write power that results in equality of the durations of the 1 and 0 
portions are equal is selected as the recording value. This calibration 
requires using valuable disk space that is desired to be avoided. 
McGee in U.S. Pat. No. 5,067,122 shows a system for measuring monitor 
sensitivity in an optical recording system. The test includes measuring a 
rate of change of the monitor response to a given test write signal for 
deriving a desired write laser power. Again, it is desired to avoid using 
write-once disk space for such calibration. 
Bletscher, Jr et al in U.S. Pat. No. 5,070,495 show an extensive 
calibration system based on symmetry parameters. This calibration requires 
excessive data storage space. While practical for rewriteable media, it is 
not economical for write-once media. 
Fennema et al in U.S. Pat. No. 5,136,569 show a write calibration based 
upon the type of medium is sensed. If a rewriteable medium is sensed, then 
extensive calibration of the laser, such as discussed above, is employed. 
If a write-once(WORM) medium is detected, then limited calibration is 
performed. This patent illustrates the need for conserving disk space in 
write-once media. It is desired to provide for an extended laser 
calibration technique that does not use data-storing areas. 
Finkelstein et al in U.S. Pat. No. 5,185,733 show another extensive laser 
calibration using randomly selected tracks. This method and apparatus are 
appropriate to rewriteable media only. The calibration results in 
selecting a write laser power level that results in a maximum read back 
signal amplitude. 
Call et al in U.S. Pat. No. 5,216,659 show a laser power calibration by 
measuring laser drive current in an out-of-focus beam condition at the 
surface of a WORM medium and in an in-focus condition of the laser beam. A 
slope is generated representing variations in laser beam power level 
versus laser current that enables calculations of laser power based on 
laser current. 
Bakx in U.S. Pat. No. 5,226,027 shows obtaining an optimum parameter for a 
disk that influences recording quality. Testing and auxiliary pattern 
areas are required. Also, a series of calibration areas are required. It 
is desired to reduce the disk area requirements for calibration from that 
required by Bakx. 
SUMMARY OF THE INVENTION 
In a preferred form of the invention, a laser-test signal is recorded in 
the same sector as data but before data are recorded, such as in a ALPC 
field. If the recorded laser-test signal is detected as having a width 
different from an established pulse length, then the laser write pulse 
power level is adjusted in a direction to produce a previously established 
length laser-test signal. Data are recorded after the adjustment in a 
predetermined number of sectors. Then, the recorded laser-test signals in 
those sectors are measured to determine their recorded lengths. The 
measured lengths are averaged to produce an averaged measured length 
value. The averaged width is then compared with the established width. If 
the average measured length is not proper, the adjustment is repeated 
until an averaged measured length matches the desired length. Such 
adjustments are performed as a part of usual write operations. Data from 
the sectors written by a laser pulse creating an improper width write 
pulse-generated PWM signal may be reassigned to other sectors. 
The present invention also enables calibrating laser operations in a 
write-once optical disk player by measuring a width (duration) of a write 
pulse-generated pulse-width-modulated (PWM) laser-test signal. The write 
pulse-generated PWM signal is generated by a pulsed laser beam that 
provides for overlapping of recorded pulse signals along a track being 
scanned. Such write pulse-generated PWM signal is preferably recorded in 
an automatic laser pulse correction (ALPC) field of the sector. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following more particular description of 
preferred embodiments of the invention, as illustrated in the accompanying 
drawings.

DETAILED DESCRIPTION 
Referring now more particularly to the appended drawing, like numerals 
indicate like parts and structural features in the various figures. 
Referring first to FIG. 1, microprocessor 10 operates and controls the 
illustrated optical disk recorder. The usual internal memories (random 
access memory RAM and read only memory ROM) are included in microprocessor 
10. The usual data flow circuits 11 connect to a host processor (not 
shown) over attachment cable 12. Data detect circuits 13 respond to 
modulated reflected light from optical disk 20 via the usual optics to 
detect data and format-indicating signals. The detected data signals 
travel directly to data flow circuits 11 for error detection and 
correction processing. Data flow 11 supplies indications of the detected 
format signals to microprocessor 10 for enabling it to control the 
recorder. Laser 14 supplies a data-modulated laser beam through optics 15, 
thence over bi-directional optical path 16 to track 22 (represented as 
being in a groove) of optical disk 20. A usual motor (not shown) on drive 
spindle 21 rotates disk 20 for enabling the beam on path 16 to scan tracks 
on disk 20. Optics 15 are relatively radially movable with respect to disk 
20 for accessing a single spiral track at different radii. 
Microprocessor 10 controls laser 14 via digital to analog converter (DAC) 
25 and laser control 26. DAC 25 sets laser power levels for writing and 
reading. Data modulation of the laser 14 laser beam is controlled by data 
flow circuits 11, all under control of microprocessor 10. Verify circuit 
23 received read back signals from data detect 13 to compare with data 
received from the attaching host processor via data flow 11. Verify 
circuit 23 supplies the results of the comparison to microprocessor 10. If 
the comparison shows an identity (error correctable identity that is 
corrected by less correction power than is available by an error 
correction code being used for data correction) of read back data with 
data to be recorded, then verification of recording is given, otherwise 
verify circuit 23 supplies a verify failure signal to microprocessor 10. 
Power level detect circuits 28 receive so-called wasted light from a beam 
splitter (not separately shown) in optics 15 for indicating to 
microprocessor 10 emitted laser light power level (intensity). 
Microprocessor 10 includes a program that controls laser 14 to emit a 
laser beam having one of several predetermined power levels via digital 
inputs to DAC 25, as is known. The diverse power levels respectively are 
gated by data to be recorded on disk 20. 
As usual, optics 15 is mounted on a carriage, represented by dashed line 
28, that relatively moves optics 15 radially of disk 20 for accessing each 
one of the track 22 rotations on disk 20. Sector select and positioning 
servo 27 moves the carriage 28 radially of disk 20 and also selects 
(addresses) which of the addressable sectors in track 22 are to be 
accessed for either reading or writing. Since such arrangements are well 
known, further detail is dispensed with. 
FIG. 2 diagrammatically shows, in a simplified form, the format used for 
each one of multiple addressable sectors in track 22, a single spiral 
track on disk 20. Begin portion 30 includes the usual clock synchronizing 
signals plus identification signals for the sector. In one embodiment, 
begin portion 30 starts with a usual sector mark (not shown--none of the 
internal details of begin portion 30 are shown in the drawing), then a 
first clock synchronization burst termed VFO1 (variable frequency 
oscillator burst type one) followed by a usual first address mark (AM). A 
first sector address identification ID1 follows the first AM. Then a 
second clock synchronization burst VF02, second address mark AM and second 
sector identification ID2 follow. A third repetition of the clock sync, AM 
and ID3 cluster follows. Other usual control fields were also used. A gap, 
represented by the vertical line between begin portion 30 and flag field 
31 separates portion 30 from flag field 31. A similar gap separates flag 
field 31 from two-byte ALPC field 32. 
The present invention uses ALPC field 32 in a new and novel manner. ALPC 
field 32 circumferentially abuts data field 33. Data field 33 starts with 
a fourth clock synchronization burst termed VFO3 followed by a usual 
data-bit synchronization (sync) signal then recorded data. The internal 
details of data field 33 are not shown in the drawing. End portion 34 
includes usual error detecting correcting redundancies, such as cyclic 
redundancy check signals and error correcting signals, as is known. A gap, 
not shown, separates adjacent sectors, i.e. between end portion 34 of one 
sector and begin portion 30 of a next adjacent sector. 
FIG. 3 illustrates the signal interference effects of prior art laser power 
recorded signal 40 in ALPC 32 with respect to the data clock 
synchronization burst VFO3, indicated by numeral 41. Laser 14 beam power 
is measured by power detect 28 while the laser is emitting a constant 
write intensity beam to the ALPC field 32. Note that the emitted beam 
power indicates a write pulse having a predetermined power for a 
predetermined period of time (empirically determined). Laser 14 emitted 
constant high intensity (DC) laser power test beam over ablates ALPC 32 
resulting in a trailing edge of recorded laser test signal 40 
circumferentially extending to data field 33. As a result, a negative 
effect on the read back circuits (not separately shown) in data detect 13 
can cause errors in delayed clock synchronization because of the high 
amplitude DC laser test signal 40. According to one aspect of this 
invention, a close string of write pulses are recorded in ALPC 32 
resulting in recorded write-pulse PWM laser test signal 42 that does not 
extend to data field 33 nor produces a high amplitude DC read back signal. 
The resultant reduced amplitude and less encroachment toward data field 33 
provides faster and more reliable clock synchronization in data field 33. 
Another negative effect of over-ablated test signal 41 is the erosion of 
the groove radially. This erosion can cause missing counting the track 
groove during a track seek. This count error results in the track seeking 
to an unintended spiral track circumvolution rather than to the target 
circumvolution. The present invention obviates this additional risk. 
It is also desired to measure laser beam power level while the laser 14 
emitted beam is focussed on the recording layer of disk 20 (groove 22). It 
is usual practice to measure laser beam power while the laser beam is 
defocussed so as not to ablate disk 20 write-once recording surface. 
According to this invention, the ALPC fields 32 in the various sectors are 
used for adjusting laser beam power as a result of performing a data 
recording or write operation, while the beam is focussed for writing and 
without using any data storing sectors nor accessing a sector separately 
from a write operation. FIG. 4 graphically illustrates a relationship of 
laser beam power at the recording surface with respect to laser current in 
focussed and defocussed states. Horizontal line 45 indicates minimum 
lasing power for laser 14. Line 46 indicates the change in beam power 
level with laser current in an out of focus condition. This line 
represents operation of the laser when focussing using an out-of-focus 
beam. Line 48 indicates in-focus beam using 1% feedback. Line 47 indicates 
in-focus beam using 2% feedback. Point 46F indicates a possible 
calibration of laser 14 emitted beam power level in an out-of-focus 
calibration. Point 47F indicates the resultant in-focus beam power level 
using 2% feedback. This graph shows that if a laser is power calibrated 
for writing using an out-of-focus beam, then the in-focus write laser 
power level is greater than if calibration used an in-focus laser beam. 
Such increased write beam power level can result in over-ablative writing 
as discussed above for the APLC test. Therefore, it is important to 
calibrate a laser write power level when the beam is in focus. 
FIG. 5 diagrammatically shows a recorded write pulse-generated PWM laser 
test signal 42. Again note that the laser beam power level is measured by 
power detect 28 using wasted light from optics 15 during the time ALPC 
field 32 test signal is being written. A portion of track 22 is indicated 
by the two horizontal dashed lines. Numeral 50 indicates a series of write 
intensity laser beam recorded "dots" that overlap longitudinal of track 22 
that is a write-pulse-generated PWM laser test signal. The recorded "dots" 
are about the radial width of track 22 (dots 50 can be smaller in diameter 
and still be used to practice the present invention). The resultant 
recording, because of thermal effects, creates a write-pulse-generated PWM 
laser test signal 42 indicated by wavy lines 51. Usually, the recorded 
write-pulse-generated PWM signal when sensed results in a read back signal 
envelope that has a minor modulation component. This write pulse-generated 
PWM signal results in signal 42 of FIG. 3. An advantage to the 
write-pulse-generated laser test signal is that the test signal replicates 
use of a data recording operation as opposed to the extended time DC 
maximum peak signal of the prior art. 
The FIG. 5 illustrated PWM signal is generated using the known write 
control by microprocessor 10 of laser 14 via DAC 25 and laser control 26. 
Data flow 11 supplies write-clock timed pulses to a laser control 26 to 
gate the DAC 25 laser level to laser 14. The timed pulses are generated by 
a usual write clock (oscillator, not shown) in data flow 11. As usual, the 
write clock emits a timed pulse once each bit period, i.e. defines a bit 
period. Either a binary one signal or a binary zero signal is "recorded" 
in each bit period. A binary zero signal is no ablation in track 22 while 
a binary one signal is an ablation (also termed a "pit"). The bit periods 
are selected to ensure that the recorded 1's do not overlap, either by 
duration of the bit period or by coding that avoids two one's from being 
recorded in adjacent bit periods. If a (2,7) code, for example, is used, 
then no adjacent bit periods each have a one signal. In this instance, to 
create the PWM signal 42, the (2,7) code is ignored by recording a binary 
one in each of a succession of bit periods. In such recording, the 
recorded binary ones result in overlapping ablations as seen in FIG. 5. It 
is to be noted, that the circumferential length of such ablated pits vary 
with radius when using a constant duration bit period. On the other hand, 
a recording system may employ timed write pulses that result in spaced 
apart ablated pits, such as represented in FIG. 5 by hash marks 55 that 
respectively indicate successive bit period centers. Successive recorded 
one signals then will not create a PWM signal. As an example, a (2,7) data 
pattern of 1001001001 (each 1 indicates a pulse to be recorded) can be 
used to produce recorded dots 50 that do not overlap, i.e. are separated 
along the length of track 22 as represented by numerals 55. Changing a 
(2,7) code data pattern to 101010101 results in the recorded dots 50 to 
overlap to produce the PWM signal 42. Such recording controls are 
preferably programmed controlled by microprocessor 10. Such program 
control may include a DAC set by microprocessor 10 to select a bit period 
to be used in recording. The DAC 25 (not shown) outputs an analog 
frequency (bit period) control signal to bias a VFO (not shown) in a known 
manner to oscillate for producing timing pulses having a selected period. 
In accordance with another aspect of the present invention, the actual 
laser 14 write-intensity beam power level at the surface of disk 20 is 
measured by measuring the length of a recorded write-pulse-generated PWM 
laser test signal 42 (FIG. 3). FIG. 6 illustrates the linear variation 
(within the operating range of write recording beam power levels 
(intensities) of pulse length with changing laser beam power level. Line 
60 illustrates the linear change in pulse duration (length) as beam power 
level changes. Line 61 indicates a 10% low power level. Line 63 indicates 
optimal write-beam power level for recording. Line 62 indicates a peak 
power level greater than optimal for desired recording. Because the higher 
peak values rise and fall in the same time duration as lower power pulses, 
the effective length of the recorded pulses are longer, as best understood 
with respect to FIG. 7. 
Read back laser test signal (pulse) 68 of FIG. 7 illustrates a PWM pulse 
generated at the optimal power level. The shape of read back PWM pulse 67 
is determined by measuring amplitudes at a sequence of points, such as 
indicated by numerals 69-76. Points 72 and 73 together measure the 
amplitude of PWM read-back pulse 67. Measurement points 70 and 71 combine 
to define the slope of the leading edge while measurement points 74 and 75 
combine to define the trailing pulse slope. Interpolating between the 
slope indicating points (70,71 or 74,75) locates the time at which the 50% 
threshold 66 amplitude is exceeded. The 50% threshold 66 value is 
calculated by halving the sum of the values indicated as the baseline 
level at 69 or 76 plus the PWM amplitude value indicated by points 72 or 
73. The elapsed time between the leading and trailing edges 70-71 and 
74-75 at threshold line 66 indicates pulse length. Increasing the peak 
amplitude of the pulse as shown by pulse 68 results in a greater length 
pulse measured by the respective times that PWM threshold 79 is crossed by 
read-back PWM signal 67, as can be easily seen in FIG. 7. At the trailing 
edge, numerals 77 and 78 indicate the increase in pulse duration or length 
as caused by increased pulse peak amplitude. 
Before using the recorder, such as during manufacture or before, these PWM 
pulse durations are calibrated to an empirically determined optimal laser 
power level for enabling calculating relative laser power level used to 
create the recorded pulse. For example, a first WORM disk may require 
eleven milliwatts for optimal recording. A PWM pulse 68 recorded at ten 
milliwatts on this first disk results in a recorded PWM signal that is too 
short. If the pulse width to relative power is calibrated, it than can be 
calculated by the time measure that such PWM pulse was in recorded at 
about 10% below optimal power level. Conversely, if the recorded PWM 
signal is too long, that indicates the optimal power level is lower. This 
measurement is used for each individual disk for obtaining optical 
recording on each and every disk. This principle is used in the present 
invention. Measuring the duration or length of the read back 
write-pulse-generated PWM laser test signal yields the relative laser beam 
power level that recorded the pulse (same in-focus condition). 
Accordingly, during a first data write operation (write command execution) 
during any recording session on a write-once disk or upon detecting a 
write error, the ALPC field 32 recorded laser test signal 42 is measured, 
as seen in FIG. 7. Then microprocessor 10, using a table lookup generated 
as set forth above and stored in microprocessor 10, calculates a recording 
power level used to record laser test signal. To practice the present 
invention, quantitative analysis of the write-pulse-generated PWM signal 
is not required. A desired length for the write-pulse-generated PWM signal 
is predetermined and stored in an internal memory of microprocessor 10. If 
a measured length of the write-pulse-generated PWM signal is greater than 
the desired length, then laser power is decreased. If a measured length of 
the write-pulse-generated PWM signal is less than the desired length, then 
laser power is increased. The latter process is preferably iterative as 
will become apparent. 
FIG. 8 illustrates one embodiment of the present invention. This embodiment 
includes a program embedded in microprocessor 10. Therefore, the machine 
operations chart represent the embedded program means that may be 
implement using any known programming technique. Steps 90-95 effect 
initialization of laser 14 operation. Call et al U.S. patent illustrates 
initialization of a laser in an optical recorder after an optical disk has 
been received by the illustrated optical disk recorder. In the illustrated 
embodiment, step 90 calibrates laser 14 write beam power level using an 
out-of-focus laser beam (see line 46 of the graph shown in FIG. 4). The 
laser 14 is preferably calibrated without using feedback via power detect 
28 (FIG. 1) as indicated in step 91. Step 92 measures the average power of 
calibrating laser write pulse for setting laser 14 drive current level, as 
is known. In the illustrated embodiment a 3T test pattern was used. This 
pattern is a series of pulses (1's) separated a number of bit recording 
positions indicated by a number of vacant (no-pulse) bit positions (0's), 
such as 100010001 . . . . The usual recorded desired write beam power 
level on each optical disk 22 is used in this calibration. As pointed out 
above, such a calibration may not be optimum nor suitable for a given 
recording operation. In accordance with this invention, the laser 14 power 
level is automatically adjusted by measuring the duration or length of the 
write-pulse-generated PWM signal. 
Each time the FIG. 1 recorder is powered on, steps 90-95 are performed. 
Such powering on starts a recording session. No further action is taken 
until data are received for recording on disk 20, such as at step 96. The 
description assumes that the optics 15 are relatively positioned to disk 
15 for accessing a target sector. In a first pass over the sector, step 97 
records a write-pulse-generated PWM laser test signal in ALPC field 32 
using the steps 90-95 calibrated beam power level. This recorded laser 
test signal is preferably not more than one byte in duration, i.e. is less 
than the 2-byte length of ALPC field 32. In a second pass over the sector, 
step 98 measures the recorded pulse duration as best seen in FIG. 7. Since 
automatic linear interpolation to find a threshold crossing is known, it 
is not further detailed herein. 
After the measurement, microprocessor 10 in step 100 compares the measured 
length of the read back write-pulse-generated PWM laser test signal with a 
desired length. If the comparison shows that the measured length is within 
an empirically determined length tolerance of the desired length (step 10 
OK exit YES), then data are written in the target and other sectors as 
indicated by numeral 101. 
If the step 100 comparison shows the measured length to be outside the 
predetermined tolerance (step 100 "NO" exit), then step 102 determines 
whether the measured test signal length is longer or shorter than the 
desired laser test signal length. Step 104 reduces laser drive current 
amplitude for a too-long measured write-pulse-generated PWM signal while 
step 105 increases laser drive current amplitude for a too-short measured 
write-pulse-generated PWM signal. The reduction or increase in laser drive 
current amplitude can be a constant amount fixed in the recorder design, 
or since the slope indicated by line 60 (FIG. 6) is known, that slope 
determines the estimate for amount of change in drive current amplitude, 
i.e. the drive current amplitude change should change the laser beam power 
by an amount not greater than, preferably equal to, the difference between 
measured power level (measured pulse length) and desired power level. 
Further, small adjustments in laser drive current can include 
compensations for radial position of the beam on disk 20. The laser drive 
current amplitude adjustment is completed by microprocessor 10 changing 
the digital input to laser drive DAC 25. 
From step 104 or 105, machine operations proceed to step 106 for writing 
"N" sectors of data, including the original target sector, all in a second 
pass. N is a positive integer, as 3, 5, 8, 10, etc. This writing is done 
at the adjusted drive current level. That is step 104 or 105 are completed 
for laser drive write current amplitude change between the trailing edge 
of the ALPC field 32 write-pulse-generated PWM signal and the onset of 
recording. An alternative is to write the target sector at the original 
laser drive current value, then perform step 106 on an additional N 
sectors. If optics 15 include read-while-write or read-after-write 
capabilities, then the above described measurement operations occur in but 
one pass over the target sector. 
Step 110 during a next pass over the sectors during a verify operation, 
measures the lengths of all the ALPC field 32 read back and just-recorded 
write-pulse-generated PWM laser test signals. Alternately, step 110 can 
measure, in addition to or in replacement of, the lengths of PWM pulse 
marks or signals recorded in data area 33 to the PWM signal recorded in 
the ALPC areas 32. The measured PWM signal lengths are averaged to produce 
an average measured PWM signal length. The averaged measured PWM signal 
length is then compared with the desired or optimal PWM signal length. If 
the averaged measured length is within a predetermined tolerance, then 
recording continues at arrow 101 using the just adjusted write drive 
current amplitudes. If step 110 finds the averaged measured length to be 
out of tolerance with the desired length, then step 112 reassigns data 
recorded in all of the N sectors for recording in alternate sectors. Such 
reassignment and recording is well known. After the reassignment but 
before the recording in the alternate sectors, steps 97-105 are repeated 
in step 113. Then the N sectors of data are written to the alternate 
sectors. The number of cycles of steps 106-113 may be limited to a fixed 
number of iterations, such as three. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that various changes in form and details may be made 
therein without departing from the spirit and scope of the invention.