Principles of Optical Devices

WHAT IS THE PRINCIPLES OF OPTICAL DEVICES,NAME AND EXPLAIN FEW ! ANS:Storage and retrieval of data on optical disks can be described in two simple steps. First, data marks are recorded on a surface. Data marks can be prerecorded, like on a music CD, or they can be recorded by users on blank disks, like with CD-recordable products. The second step is retrieval of information from the disk, where a light beam scans the surface. Modulation in the reflected light is used to detect the data-mark pattern under the scanning spot. The process for exposing data marks on a recordable optical disk is shown in Fig. 3, where an input stream of digital information is converted with an encoder and modulator into a drive signal for a laser source. The laser source emits an intense light beam that is directed and focused onto the surface with illumination optics. As the surface moves under the scanning spot, energy from the intense scan spot is absorbed, and a small, localized region heats up. The surface, under the influence of heat beyond a critical writing threshold, changes its reflective properties. Modulation of the intense light beam is synchronous with the drive signal, so a circular track of data marks is formed as the surface rotates. The scan spot is moved slightly as the surface rotates to allow another track to be written on new media during the next revolution. Data marks on prerecorded disks are fabricated by first making a master disk with the appropriate data-mark pattern. Masters for prerecorded CDs and DVDs are often exposed in a similar manner to exposing data marks on recordable optical disks, except that the light-sensitive layer is designed to produce pits in the master that serve as data marks in the replicas. Inexpensive replicas of the master are made with injection-molding equipment. Readout of data marks on the disk is illustrated in Fig. 4, where the laser is used at a constant output power level that does not heat the data surface beyond its thermal writing threshold. The laser beam is directed through a beam splitter into the illumination optics, where the beam is focused onto the surface. As the data marks to be read pass under the scan spot, the reflected light is modulated. Modulated light is collected by illumination optics and directed by the beam splitter to servo and data optics, which converge the light onto detectors. The detectors change light modulation into current modulation that is amplified and decoded to produce the output data stream.