The Optical Pickup

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The optical pickup is largely governed by the principles of photonics. Here is a brief description of how an optical pickup works.

As the amount of current supplied to the laser diode reaches its threshold limit, it emits laser beam directed towards the grating lens. The grating lens then diffracts received beam into three major components: the 0th, +1th and -1th order beams. The 0th order beam or the main beam is used for focusing the beam into the desired pit of the disc while the +1th and -1th order beams or the side beams are used to track adjacent pits and reduce errors in tracking due to disc wobbling. The diffracted beam then goes to primary beam splitter which splits the received beam into two paths: one directed towards the quarterwave plate and another back to the grating lens. Ferrite beads are employed to attenuate the returning beam since this beam is unnecessary. The quarterwave plate then circularly polarizes the desired beam while passing through it. The polarized beam is then received by the collimator lens. This lens then collimates, i.e. concentrates the beam in a unidirectional pattern for more efficient usage of the beam. The mirror is now ready to receive this concentrated beam. The mirror then reflects the beam towards the objective lens which is a convergent type of lens so that the beam is concentrated at one focal point which in this case is the surface of the disc, specifically to the targeted pit. Depending on the reflectivity of the surface, it reflects the beam all the way back to the primary beam splitter which then again splits the reflected beam one towards the sensor lens and another towards the grating lens. Part of the beam is reflected by the front photodiode. This component converts the beam into an electrical signal. This signal is then fed to the automatic power controller to control the amount of current supplied to the laser diode driver. The optical pickup functions as reading or writing depending on the amount of current supplied to it. Commonly the higher rated power is used as the writing power while the lower rated power the reading power. The laser diode driver directly controls the amount of current supplied to the laser diode. The ferrite beads again attenuate the unwanted beam while the sensor lens which is also of convergent type concentrates the desired beam at the photoelectric sensors of the photodiode integrated circuit. These tiny sensors convert the beam into an electrical signal and op amps in cascade digitize the signal and this signal serves as the output of the optical pickup.



The Optical Pickup Diagram

An optical pickup is a part of an optical disc drive responsible for reading and/or writing data out of optical media discs such as compact discs (CD), digital versatile discs (DVD), etc. The heart of the optical pickup is its laser diode. This emits laser beams that serve as the reader/writer of the optical pickup. Depending on the power emitted by the laser diode can the optical disc drive function. More commonly, the higher amplitude power is used for writing data because more power is necessary to burn the surface of the disc than for reading data out of it.