This paper reports the theoretical analysis and experimental results of a far-field focus sensor. This system utilizes moving interference fringes at the far-field caused by the one-dimension uniform-pitch grating embedded inside a disc. The intensity of the moving fringes was sampled and processed by a digital signal processor and synchronization circuits including a phase lock loop. A well-shaped and low-offset focus error signal was observed all over the disc rotation, as well as, the focus serve operation being confirmed. Finally, how this sensor can be adopted to the collinear hologram data storage system is discussed.
This paper deals with the dynamic characteristics of a sliding, rotary type small lens actuator for high recording density disk system such as digital video disc. In particular the influence of the vibration modes of the lens holder on the frequency characteristics in the tracking direction is discussed. In order to understand the vibration mode phenomena, we performed numerical analysis by using the finite element method and observed actual vibration with a scanning type laser doppler velocimeter. As a result, we determined the rotational movement of the lens holder around the jitter axis at a few kHz, which turned out additional phase delay in the tracking servo loop and sometimes appears when the size of the actuator decreases. To suppress this movement, we considered the mechanism of this rotational movement and proposed a method of increasing the moment of inertia around the jitter axis. The effectiveness of this method was also confirmed experimentally.
The optical components in the detection train of a conventional magneto-optical (M-O) disk head include a half-wave plate and a polarization beamsplitter. These polarization components are bulky and require specialized mounting hardware. In order to realize a more compact head, we propose that these elements be replaced by an integrated device composed of cascaded volume and surface-relief gratings. In this paper, we describe the proposed system, detail designs for the individual elements, compare theoretical and prototype element performance, and discuss the operational tolerances of these elements.
Key factors in designing the optics, lens actuator, and head base for commercial magneto- optical heads are discussed in this paper. The design and performance of two types of magneto-optical disk heads developed at Mitsubishi are presented.
To cope with the trend of high-speed access, a high-performance optical head with a sliding and rotary-type of lens actuator has been developed. An advantage of this type of lens actuator is that the moving part of the actuator is dynamically well balanced so that the objective lens is not shaken by acceleration or deceleration during seeking. As a result, the stability of the velocity control during seeking has been improved and the waiting time when the operation mode changes from the seek mode to the tracking mode has been reduced. To cope with the trend of high-speed transfer, we have increased driving efficiency and set the frequency of the parasitic resonance sufficiently high. Also, the rotational latency, which is an important factor of the access time, has been shortened. As a result, a data transfer rate of 7.4 Mbits/s and average access time of 48 ms have been attained in a 130-mm rewritable optical disk drive.
A new composite tracking method suitable for a two-beam split-type optical head is suggested. Using twin spot and push-pull tracking method, and with ana auxiliary actuator combined, this method is designed so as to be applied, in particular, to optical disks having continuous grooves. In this method, a read-out spot is controlled by the twin-spot method, and moreover an auxiliary actuator for write spot positioning is driven by a differential signal of two push-pull tracking error signals derived from read-out and write beams. To examine tracking performance of this method, we constructed a two-beam optical head and successfully confirmed that the two spots could be easily positioned on the same track center independent of +/- 100 micrometers objective lens shift.