Editing Optical disc drive (section)

=== Laser and optics ===

==== Optical pickup system ====
[[File:CD Player focusing lens assembly.jpg|thumb|Pickup head]]
[[File:LG DVD GDR-8164B 07 laser read head.jpg|thumb|Optical pickup unit with two visible [[potentiometer]]s]]
[[File:Philips 1 beam tracking Laser Optical Assembly.jpg|thumb|Pickup head, side view]]
[[File:Detail of DVD-RW optics.png|thumb|Optical path]]
The most important part of an optical disc drive is an ''optical path'', which is inside a ''pickup head'' (''PUH''). The PUH is also known as a laser pickup, optical pickup, pickup, pickup assembly, laser assembly, laser optical assembly, optical pickup head/unit or optical assembly.<ref name=dvd>{{Cite book |first1=Jim H. |last1=Taylor |first2=Mark R. |last2=Johnson |first3=Charles G. |last3=Crawford |title=DVD Demystified |publisher=McGraw-Hill Professional | year=2006 |url=https://books.google.com/books?id=ikxuL2aX9cAC&pg=PT229 |pages=7–8 |isbn=0-07-142396-6}}</ref> It usually consists of a semiconductor [[laser diode]], a [[lens (optics)|lens]] for focusing the laser beam, and [[photodiode]]s for detecting the light reflected from the disc's surface.<ref name=cd>{{Cite book |first=Sorin G. |last=Stan |title=The CD-ROM Drive: A Brief System Description |publisher=Springer |year=1998 |url=https://books.google.com/books?id=Gs3pdRb83VcC&pg=PA14 |page=13 |isbn=0-7923-8167-X}}</ref>

Initially, CD-type lasers with a [[wavelength]] of 780&nbsp;nm (within the infrared) were used. For DVDs, the wavelength was reduced to 650&nbsp;nm (red color), and for Blu-ray Disc this was reduced even further to 405&nbsp;nm (violet color).

Two main [[servomechanism]]s are used, the first to maintain the proper distance between lens and disc, to ensure the laser beam is focused as a small ''laser spot'' on the disc. The second servo moves the pickup head along the disc's radius, keeping the beam on the ''track'', a continuous spiral data path. Optical disc media are 'read' beginning at the inner radius to the outer edge.

Near the laser lens, optical drives are usually equipped with one to three tiny [[potentiometer]]s (usually separate ones for [[compact disc|CD]]s, [[DVD]]s, and usually a third one for [[Blu-ray Disc]]s if supported by the drive<ref>{{cite web |title=Tearing Down A PS3 Blu Ray Drive |url=https://hackaday.com/2019/11/12/tearing-down-a-ps3-blu-ray-drive/ |website=Hackaday |access-date=31 July 2020 |date=12 November 2019}}</ref>) that can be turned using a fine screwdriver. The potentiometer is in a [[series circuit]] with the laser lens.

The laser diode used in DVD writers can have powers of up to 100 [[milliwatt]]s, such high powers are used during writing.<ref>{{cite web |title=100mW-Laserdioden aus 16x-DVD-Brennern |url=https://www.mikrocontroller.net/attachment/12346/Laserdiode8.pdf |website=mikrocontroller.net |access-date=11 August 2020 |language=de}}</ref> Some CD players have [[automatic gain control]] (AGC) to vary the power of the laser to ensure reliable playback of CD-RW discs.<ref name="auto"/><ref>{{Cite web|url=https://users.cs.cf.ac.uk/Dave.Marshall/Multimedia/node134.html|title=Compatibility of DVD}}</ref>

Readability (the ability to read physically damaged or soiled discs) may vary among optical drives due to differences in optical pickup systems, firmwares, and damage patterns.<ref>{{cite web |title=5. Conditions That Affect CDs and DVDs • CLIR |url=https://www.clir.org/pubs/reports/pub121/sec5/ |website=CLIR}}</ref>

==== Read-only media ====
{{Unreferenced section|date=December 2024}}

[[File:Optical Disk Drive Photodetector.jpg|alt=The optical sensor out of a CD/DVD drive|thumb|The optical sensor out of a CD/DVD drive. The two larger rectangles are the photodiodes for pits, the inner one for land. This one also includes amplification and minor processing.]]
On factory-pressed ''read only media'' (ROM), during the manufacturing process the tracks are formed by pressing a thermoplastic resin into a nickel stamper that was made by plating a glass 'master' with raised 'bumps' on a flat surface, thus creating ''pits'' and ''lands'' in the plastic disk. Because the depth of the pits is approximately one-quarter to one-sixth of the laser's wavelength, the reflected beam's phase is shifted in relation to the incoming beam, causing mutual destructive [[Interference (wave propagation)|interference]] and reducing the reflected beam's intensity. This is detected by photodiodes that create corresponding electrical signals.

==== Recordable media ====
{{missing information|section|laser wattages for reading and writing of individual media types|date=August 2020}}
An optical disk recorder encodes (also known as burning, since the dye layer is permanently burned) data onto a recordable [[CD-R]], [[DVD-R]], [[DVD+R]], or [[BD-R]] disc (called a ''blank'') by selectively heating (burning) parts of an organic [[dye]] layer with a laser.{{Citation needed|date=February 2007}}

This changes the reflectivity of the dye, thereby creating marks that can be read like the pits and lands on pressed discs. For recordable discs, the process is permanent and the media can be written to only once. While the reading laser is usually not stronger than 5 [[Watt|mW]], the writing laser is considerably more powerful.<ref>{{cite web |title=Laser diodes from CD-RW drives can cut and burn! |url=http://danyk.cz/laser_en.html |website=danyk.cz |access-date=11 August 2020}}</ref> DVD lasers operate at voltages of around 2.5 volts.<ref>{{cite web |title=Powerful laser diodes from DVD-RW drive |url=http://danyk.cz/laser3_en.html |website=danyk.cz |language=en}}</ref>

The higher the writing speed, the less time a laser has to heat a point on the media, thus its power has to increase proportionally. DVD burners' lasers often peak at about 200&nbsp;mW, either in continuous wave and pulses, although some have been driven up to 400&nbsp;mW before the diode fails.

==== Rewriteable media ====
For rewritable [[CD-RW]], [[DVD-RW]], [[DVD+RW]], [[DVD-RAM]], or [[BD-RE]] media, the laser is used to melt a [[crystal]]line metal [[alloy]] in the recording layer of the disc. Depending on the amount of power applied, the substance may be allowed to melt back (change the phase back) into crystalline form or left in an [[amorphous solid|amorphous]] form, enabling marks of varying reflectivity to be created.

==== Double-sided media ====
''Double-sided'' media may be used, but they are not easily accessed with a standard drive, as they must be physically turned over to access the data on the other side.

==== Dual layer media ====
''Double layer'' or ''dual layer'' (DL) media have two independent data layers separated by a semi-reflective layer. Both layers are accessible from the same side, but require the optics to change the laser's focus. Traditional ''single layer'' (SL) writable media are produced with a spiral groove molded in the protective [[polycarbonate]] layer (not in the data recording layer), to lead and synchronize the speed of recording head. Double-layered writable media have: a first polycarbonate layer with a (shallow) groove, a first data layer, a semi-reflective layer, a second (spacer) polycarbonate layer with another (deep) groove, and a second data layer. The first groove spiral usually starts on the inner edge and extends outwards, while the second groove start on the outer edge and extends inwards.<ref>{{Cite web|url=https://i.publiclab.org/system/images/photos/000/008/589/original/doublelayer1.gif|title=Cross Section of DVD+R DL Optical Media}}</ref><ref>{{Cite web|url=http://www.windowsusers.org/images/Dvd/R_dl.jpg|title=Cross Section of DVD+R DL Optical Media}}</ref>

==== Photothermal printing ====
Some drives support [[HP Inc.|Hewlett-Packard]]'s [[LightScribe]], or the alternative [[Labelflash|LabelFlash]] [[photothermal printing]] technology for labeling specially coated discs.

====Multi beam drives====
Zen Technology and Sony have developed drives that use several laser beams simultaneously to read discs and write to them at higher speeds than what would be possible with a single laser beam. The limitation with a single laser beam comes from wobbling of the disc that may occur at high rotational speeds; at 25,000 RPMs CDs become unreadable<ref name="auto">{{Cite web|url=https://cdrfaq.org/faq05.html|title=CD-Recordable FAQ - section 5|website=cdrfaq.org}}</ref> while Blu-rays cannot be written to beyond 5,000 RPMs.<ref>{{Cite web |date=January 2018 |title=White Paper Blu-ray Disc™ Format General |url=http://www.blu-raydisc.com/Assets/Downloadablefile/White_Paper_General_5th_20180216.pdf |archive-url=https://web.archive.org/web/20211028160330/http://www.blu-raydisc.com/Assets/Downloadablefile/White_Paper_General_5th_20180216.pdf |archive-date=Oct 28, 2021 |website=blu-raydisc.com |edition=5th}}</ref> With a single laser beam, the only way to increase read and write speeds without reducing the pit length of the disc (which would allow for more pits and thus bits of data per revolution, but may require smaller wavelength light) is by increasing the rotational speed of the disc which reads more pits in less time, increasing data rate; hence why faster drives spin the disc at higher speeds. In addition, CDs at 27,500 RPMs (such as to read the inside of a CD at 52x) may explode causing extensive damage to the disc's surroundings, and poor quality or damaged discs may explode at lower speeds.<ref>{{Cite web|url=https://www.smh.com.au/technology/life-in-the-fast-lane-can-be-a-disc-shattering-experience-20021209-gdfxwp.html|title=Life in the fast lane can be a disc-shattering experience|date=December 9, 2002|website=The Sydney Morning Herald}}</ref><ref name="auto"/>

In Zen's system (developed in conjunction with Sanyo and licensed by Kenwood), a diffraction grating is used to split a laser beam into 7 beams, which are then focused into the disc; a central beam is used for focusing and tracking the groove of the disc leaving 6 remaining beams (3 on either side) that are spaced evenly to read 6 separate portions of the groove of the disc in parallel, effectively increasing read speeds at lower RPMs, reducing drive noise and 
stress on the disc. The beams then reflect back from the disc, and are collimated and projected into a special photodiode array to be read. The first drives using the technology could read at 40x, later increasing to 52x and finally 72x. It uses a single optical pickup.<ref name="Anandtech-Multibeam"/><ref>{{Cite web|url=https://hothardware.com/reviews/kenwoods-72x-true-x-cdrom-drive|title=Kenwood's 72X True X CDROM Drive |date=December 15, 2001|website=HotHardware}}</ref><ref name="CDRinfo-MultiBeam"/><ref>{{Cite web|url=https://gcn.com/articles/2000/08/07/kenwoods-72x-cd-cant-keep-pace-with-24x-cdrw.aspx|title=Kenwood's 72X CD can't keep pace with 24X CD-RW -|date=August 7, 2000|website=GCN}}</ref><ref>{{Cite web|url=https://www.targetpc.com/hardware/cd-rom/kenwood72x/|title=Target PC :: Kenwood 72X IDE CD-ROM|website=www.targetpc.com}}</ref><ref>{{Cite web|url=https://www.theregister.com/1999/11/08/kenwood_launches_72x_cd_unit/|title=Kenwood launches 72x CD unit|website=www.theregister.com}}</ref>

In Sony's system (used on their proprietary Optical Disc Archive system which is based on [[Archival Disc]], itself based on Blu-ray) the drive has 4 optical pickups, two on each side of the disc, with each pickup having two lenses for a total of 8 lenses and laser beams. This allows for both sides of the disc to be read and written to at the same time, and for the contents of the disc to be verified during writing.<ref>{{Cite web |title=Optical Disc Archive Generation 2 White Paper |url=https://pro.sony/s3/cms-static-content/file/49/1237494482649.pdf |access-date=2024-11-13 |website=Sony Professional}}</ref>