David B. Kay

Write noise from optical heads with nonachromatic beam expansion prisms

The purposes for achromatizing the optics in an optical recording head are: (a) to reduce focal shifts of the spot at the disk and in the focus sensing system, and (b) to eliminate lateral shifts of the spot at the disk when the laser wavelength shifts. Reference 1 discusses the use of a very low dispersion glass in the fabrication of molded singlet lenses for use in optical data storage. This paper discusses an effect of a lateral shift of the focal spot that occurs, due to the dispersion in the beam expansion prism, when the laser mode hops.

Problem of track offset in optical disk systems

In an optical disk drive, it is well known that a tilt of the disk causes an offset in the tracking-error signal (TES). One effect of disk tilt is the introduction of a dc component to the TES, which can be largely corrected by operation of the tracking system at the midpoint between the maximum and the minimum values of the open-loop TES. However, this method of correcting for the dc shift in the TES does not correct for the effect of coma in the focused spot, which leads to track offset. The track offset of a system is defined as the distance between the peak irradiance in the focused spot and the center of the groove when the tracking system is operating at the midpoint between the maximum and the minimum values of the open-loop TES in the presence of disk tilt. Calculations are performed that show the dependence of track offset on various system parameters, including track pitch, wavelength, and numerical aperture and rim intensity of the objective lens, and on the regions of the beam used to generate the TES. The track offsets for several beam-segmentation schemes are calculated for a digital versatile disk that uses push-pull and differential phase tracking. It is shown that for differential phase tracking the value of track offset depends on the mark length.

Heads and Lasers

Publisher Summary The Magneto-Optical (MO) optical head is the transducer in a MO optical disk drive. It must provide a small laser spot that is of <1.0 μm full width half maximum diameter at the recording media surface and maintain that spot at focus to <±0.5 μm and on the data track to <±0.1 μm. It must sense the very small Kerr rotations of the linear polarized light reflected from the magnetized domains, within the data track with a high signal-to-noise ratio. It must also record domains with alternating magnetization into the data track. The challenge is great, but it is met by a combination of a high-power single spatial mode laser diode, a high-performance voice-coil actuator, high quality optics, and detectors packaged together and supported with preamplifiers, laser driver, servo, and channel electronics.

Low-noise high-reliability 680-nm optical head enables robust 14.8-gigabyte/disk product

A high reliability and low noise 680 nm optical head is described. This head enables a high data integrity, 14.8 GByte/disk product (KODAK optical disk system 2000). The laser intensity noise is measured to be -133 to -137 dB/Hz at the data frequencies. This allows phase margin for the system to exceed 50%. The laser with drive electronics is shown to have a pulse lifetime of greater than 40,000 hours at exaggerated drive conditions.