Laser driver oscillation requirement: Reading data - Blu-ray Forum

smlogan · 06-19-2005, 03:22 AM

Hi,

I am trying to understand the requirements of a blue-violet laser driver when reading a Blu-ray disc. Based on my reading of documents on this site, I believe I understand the timing requirements for the write operation. However, when reading a Blu-ray disc, it seems the laser driver must operate at a much higher oscillation frequency than for the "write" operation as the photodiode may miss recorded marks if the laser is turning on and off at the "write speed". For example, for the 1X rate, teh laser driver must write data at 36 Mb/sec. However, it seems the oscillation frequency of the driver must be much greater than 36 MHz to read the same data.

If I may ask, does anyone have any insight into the requirement laser driver oscillation frequency when reading a 1X or 2X Blu-ray disc?

Thank you very much for any information anyone may have.

Shawn

georgir · 06-25-2005, 10:16 AM

i dont imagine the reading laser to be switching on/off at all, it'd probably be constant while the reflection varies according to the data on the disk.
but meh, this is too technical so i'm really clueless... forgive my ignorance

smlogan · 06-26-2005, 12:47 AM

No problem!

I've continued studying the issue and I think I've got a much better understanding now.

The laser driver is modulated at a relatively high frequency (200 MHz to 500 MHz) when it places the laser in its read mode of operation. This is a mower power mode than its "write" mode of operation. The motivation for modulating the laser driver relates to the fact that a laser can actually operate at many modes. These modes are close together in frequency but show up as "noise" during the read mode. Unfortunately, as the laser disc rotates past the laser, a portion of its laser light is reflected back into the laser and can excite these other modes of the laser - causing a source of unwanted noise in the read operation. By modulating the laser driver at 200 MHz to 500 MHz, it "moves" these unwanted modes in frequency (similar to the modulation used in broadcasting). This allows the data recovery algorithm to filter out these additional modes and effectively increase the signal to noise ratio.

The modulation frequency, contrary to my understanding, is not set by the minimum frequency required to read the marks on a disc.

I'm not sure if this makes sense to you - but I do appreciate your response! Thanks!

06-19-2005, 03:22 AM	#1
smlogan New Member Member since:Jun 2005	Laser driver oscillation requirement: Reading data Hi, I am trying to understand the requirements of a blue-violet laser driver when reading a Blu-ray disc. Based on my reading of documents on this site, I believe I understand the timing requirements for the write operation. However, when reading a Blu-ray disc, it seems the laser driver must operate at a much higher oscillation frequency than for the "write" operation as the photodiode may miss recorded marks if the laser is turning on and off at the "write speed". For example, for the 1X rate, teh laser driver must write data at 36 Mb/sec. However, it seems the oscillation frequency of the driver must be much greater than 36 MHz to read the same data. If I may ask, does anyone have any insight into the requirement laser driver oscillation frequency when reading a 1X or 2X Blu-ray disc? Thank you very much for any information anyone may have. Shawn

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06-25-2005, 10:16 AM	#2
georgir Active Member Member since:Mar 2005	i dont imagine the reading laser to be switching on/off at all, it'd probably be constant while the reflection varies according to the data on the disk. but meh, this is too technical so i'm really clueless... forgive my ignorance

06-26-2005, 12:47 AM	#3
smlogan New Member Member since:Jun 2005	No problem! I've continued studying the issue and I think I've got a much better understanding now. The laser driver is modulated at a relatively high frequency (200 MHz to 500 MHz) when it places the laser in its read mode of operation. This is a mower power mode than its "write" mode of operation. The motivation for modulating the laser driver relates to the fact that a laser can actually operate at many modes. These modes are close together in frequency but show up as "noise" during the read mode. Unfortunately, as the laser disc rotates past the laser, a portion of its laser light is reflected back into the laser and can excite these other modes of the laser - causing a source of unwanted noise in the read operation. By modulating the laser driver at 200 MHz to 500 MHz, it "moves" these unwanted modes in frequency (similar to the modulation used in broadcasting). This allows the data recovery algorithm to filter out these additional modes and effectively increase the signal to noise ratio. The modulation frequency, contrary to my understanding, is not set by the minimum frequency required to read the marks on a disc. I'm not sure if this makes sense to you - but I do appreciate your response! Thanks!

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