Some of the recent advances in the image engineering technology are three-dimensional and multi-spectral image acquisition. In order to obtain depth information of objects, many technologies are possible such as stereography, ToF, coded aperture and so on. Optical filters, diffraction gratings and variation of light source are available for multi-/hyperspectral imaging. A simple scheme for realizing those advanced technologies are discussed in this study. The proposed system includes only a monochromatic image sensor and a randomly coded aperture without lenses. The lensless imaging technologies are available not only for conventional image acquisition but also for refocusing method. As a statistical optics approach associating with the random coded aperture, simple imaging optics with diffusers are introduced. Those have been applied to acquire depth information and color images. However, none of the proposal to acquire depth and color information simultaneously has been discussed. The size of the point spread function (PSF) of a randomly coded aperture depends on the depth position of a point source. The inverse function of the PSF can retrieve the image on the corresponding depth position, because PSFs of different depths do not correlate to each other. The different wavelengths lead the different PSFs even for the same depth position. It is caused by the nature of diffraction of the aperture. Because of the nature, the selectivities of depth and wavelength are independent, then the proposed system realizes fourdimensional image acquisition with a deterministic approach by using a designed randomly coded aperture.
Proc. SPIE. 11125, ODS 2019: Industrial Optical Devices and Systems
KEYWORDS: Clocks, Modulation, Optical tracking, Modulation transfer functions, Analog electronics, Orthogonal frequency division multiplexing, Signal detection, Tolerancing, Optical discs, Systems modeling
The efficiency of a communication channel is basically determined by its signal to noise ratio. Digital data is reliable but its rigorous waveform has extra information. So, it should occasionally be converted to an analog modulated signal suitable for the channel. I have demonstrated an application and improvement of the Orthogonal Frequency Division Multiplexing (OFDM) method to optical disc systems. The frequency sub-band separation scheme not only has the advantage of the efficient use of the channel, but also seems to be valid for perturbations in practical use of optical disc systems. I will discuss about the tolerance of the OFDM readout to the perturbations such as defocus, disc tilt and so on. Some variations of sub-band separation and improvements such as the suppression of peak to average power ratio is another issue. In order to implement the OFDM for an optical disc system, it is necessary to perform analog signal recording. One of the method available is the Delta-Sigma conversion method. Using this method, an analog signal is converted to a binary digital waveform that can be used to modulate the light pulses for recording. However, the recording characteristics of media is not linear in general. An arrangement of the method in addition to the simulation with a typical optical recording model will be discussed.
Optical disc systems usually have several detection channels in order to obtain information and objective lens control signals. As well as used in the area of wireless communication known as Multiple-Input Multiple-Output (MIMO) or the diversity technique, these multiple-channel signals should bring a higher quality of the information for the optical disc systems. Partial Response (PR) signal processing is an effective method to increase the recording density for the current optical disc systems. In this report, dual-input channels, sum and tangential push-pull detections are considered. Different PR classes can be adapted to the dual-input channels. Three kinds of signal modulation methods, the 1-7 RLL, the Eight-Fourteen Modulation (EFM) and un-modulated signals are examined. The qualities of the dual-channel signals are expressed by drawing the constellation maps. Compare to the conventional single-channel detection case, dual- channel one tend to indicate smaller signal level deviations normalized by the detection window. The normalized signal level deviation in multiple-channel detection is defined as the deviation in the channel dimension space normalized by the minimum distance between the average levels of the states. In many cases, levels of states are separated in dual- channel detection case though those are degenerated in single-channel detection case. The signal levels of the additional channel increase the number of states. It should enhance the performance of the later signal processing part such as maximum likelihood (ML) method.
Proc. SPIE. 10384, Optical Data Storage 2017: From New Materials to New Systems
KEYWORDS: Quadrature amplitude modulation, Telecommunications, Signal processing, Signal generators, Optical recording, Analog electronics, Orthogonal frequency division multiplexing, Optical discs, Binary data, Systems modeling
An application of Orthogonal Frequency Division Multiplexing (OFDM) method to optical disc recording/readout is presented. OFDM has been widely used in the field of telecommunication owing to its highly efficient frequency usage. However OFDM has not been applied to optical disc recording because it is a multiple data transfer method and needs to record analog signals. Partial Response Maximum Likelihood (PRML) used in the current optical disc systems requires a certain kind of analog recording. Although optical recording usually creates binary marks, it is possible to obtain arbitrary analog readout signals by using PWM method. Another technique to generate analog signals using the oversampled binary recording is described and applied to multiple level recording. In addition, it is found that the level adjustment of multiple carriers for OFDM leads to the advantage when it is applied to the optical disc system. Using the simple transfer function model of the optical disc system, two types of readout signals using PRML and OFDM are calculated and then their qualities are compared. Since Quadrature Amplitude Modulation (QAM) method can be combined with OFDM, it is possible to increase the recoding density of optical disc systems. A method employing OFDM with 64-QAM and the pre-enhance method to the high frequency carrier shows an ability of 1.5 times recording density of the conventional Bru-ray Disc (BD).
In Micro-Reflector optical disc systems, multi-layer index modulation distribution causes wavefront fluctuation in the
light beam passing through the layers. Generally, the Merecial's criterion 0.07λrms is widely used as a figure of merit
for the wavefront quality of a light beam for readout. However, due to the fine structure of the wavefront fluctuation by
the recorded marks, the decay of readout signal is less comparing to the general case.
This paper reviews the analyses and the experimental results of Micro-reflector optical disc system. In Micro-reflector
optical disc, data are recorded on multiple virtual planes in a monolithic holographic medium. We have demonstrated
the possibility of huge capacity from our analyses of readout characteristics of the Micro-reflector. In addition, we have
developed the five-axis servo control system in order to achieve precise control of two counter-propagating light spots in
recording media. Using this system, we succeeded in four-layer recording/readout.
The hemisphere type SIL of KTaO3 that has a high refractive index was developed. NA 1.84 was achieved by the hemisphere SIL of KTaO3 that has a wide thickness tolerance in combination with a high NA aspherical lens of NA 0.77. Huge capacities ROM of 50-100 GB were evaluated by means of KTaO3 hemisphere SIL. The same playback performance as that of LAH79 SIL was confirmed.
Progress is reported on cover-layer incident Near-Field Recording. Very good recording results are obtained on a CuSi disc demonstrating excellent signal quality with modulation corresponding to the full effective numerical aperture of 1.45. Comparing the Near-Field system with Blu-ray Disc further shows that an extension to at least 4 data layers should be possible. In addition, a method for dynamic tilt control is presented. Controlling the tilt between the solid immersion lens and the disc surface improves the system robustness and relaxes media manufacturing tolerances for a roadmap towards 500 GB capacity on a single-sided, 4-layer disc which is protected by a polymer cover-layer.
We have ever demonstrated an optical disc system with the capacity of 100 Gbyte over and with the user transfer rate of 100 Mbps over, using near-field optics with a solid immersion lens of a 1.8-NA or more. In this paper, we introduce and discuss some technologies toward the removability of a near-field optical disc drive system.
In a volumetric optical disc medium, two counter-propagating light beams focusing to the same point create a diffraction-limited size micro-reflector. We propose a single-sided optical layout for a three-dimensional optical disc recording system using the micro-reflectors. Calculation results of readout signal characteristics, estimation of interlayer crosstalk noise, and recording density of Micro-Reflector three-dimensional optical disc recording are described.
We demonstrate the capability of 100-GB density recording by electron beam mastering and readout by a near-field optical pickup with an effective NA of 2.05 and a blue LD of 405-nm wavelength. A silicon (Si) disk of 100-GB density is fabricated by an optimized Si etching process condition to form suitable pit pattern shapes for the near-field readout.
We improved the electron beam recorder with a differential pumping head for higher density discs and mass production. The beam diameters were improved by exchanging the aperture size of the objective lens and beam stability were also improved by adding a sound proof case. As for the performance of the improved electron beam recorder, we showed that a 104Gb/in2 (150GB capacity/layer) density disc with EFM plus modulation codes can be fabricated. We also improved the pit shape uniformity and a margin of the process by introducing the appropriate write strategy that is simulated by the Monte Carlo simulation to the recording pulses.
We have achieved high density near field readout of a 100 GB capacity (69.5 Gbit/in2) disc by using a solid immersion lens with numerical aperture of 2.05. In order to realize the solid immersion lens wtih numerical aperture of 2.05, the solid immersion lens was made from Bi4Ge3O12 mono-crystal. The refractive index of Bi4Ge3O12 is 2.23 at the wavelegnth of 405 nm. A conventional optical pick-up actuator with the solid immersion lens was used for the near field optical disc system. We confirmed that the near field readout system is promising method of realizing a high density optical disc system.
We have demonstrated the capability of 100GB density recording by the electron beam mastering and readout by a near-field optical pick-up with an effective NA of 2.05 and a blue LD of 405 nm wavelength. The Si disc of 100GB density was fabricated by the optimized Si etching process condition to form suitable pit pattern shape for the near-field readout.
We describe a FMM (Fourier Modal Method) based rigorous vector diffraction method that can be applied to trapezoidal groove structures. Applying this method to the calculation of the electric field on the surface of the recording layer for DVR groove-only recording media, we found a significant difference of the electric field distributions between two recording schemes, one when the light spot is focused on-groove, the other when it is focused in-groove. These calculation results clearly explained the measured differences of the recording power sensitivities and the cross-writing characteristics between the two schemes.
We developed a near field readout system for an optical ROM disc with a GaN laser diode and a 1.4-effective-NA objective lens. We read out a 25 Gbyte optical ROM disc using a gap servo and a tracking servo in the near field read system, and experimentally confirmed an effective NA of 1.4.
We propose a method to record a multilevel signal onto an optical read only memory disc. In this method, we use the signal processing to generate the multilevel recording signal, which satisfies zero-intersymbol interference condition and zero-direct current condition. The resultant multilevel signal is emboss recorded as position displacement of groove walls. To playback a disc, the push- pull detection and the adaptive equalizer is used. Experimental disc of 0.6 micron trackpitch and a 0.28 micron/bit of density is made. When a DVD equivalent optical pickup is used to playback this disc, we confirm the error rate of 3 X 10-4 and a clear 2D eye pattern of 16 levels.
The contributions to noises in optical disk readout caused by the surface roughness on the disk are calculated using a full vector diffraction theory. The sources of noise are assumed to be small periodic bumps existing on land or groove, as well as periodic jaggedness on the groove walls. Under these conditions, the polarization dependence of noise, the difference of noise when the bumps are on land or groove, and the depolarization noise in magneto-optical readout are calculated.
We developed a 1.5-Numerical-Aperture optical setup using a GaN blue-violet laser diode. We used a 1.0 mm-diameter super-hemispherical solid immersion lens, and optimized a phase-change disk structure including the cover layer by the method of MTF simulation. The disk surface was polished by tape burnishing technique. An eye-pattern of (1-7)-coded data at the linear density of 80 nm/bit was demonstrated on the phase-change disk below a 50 nm gap height, which was realized through our air-gap servo mechanism.
Recently, solid immersion lenses (SILs) have yielded a great interest in near-field optical data storage. However, convincing data on areal recording density have yet to be reported. The difficulty is attributed to the fact that an air-bearing slider needs to be kept optically contacted and also that a tiny SIL is vulnerable to aberrations caused by an air gap between a lens and a disk. The air-bearing system cannot easily achieve the condition above because its linear velocity changes the air gap. We have developed a new SIL device mounted on an actuator, which allows the air gap to be varied arbitrarily and enables optical contact independent of disk rotation. The obtained eyepattern of (l,7)-coded data on a phase-change disk demonstrates near-field recording over 1.2- numerical-aperture (NA).
troduction The scalar diffraction theory represented by the Hopkins’ method has been widely used for simulating the readout signals from optical pickups. This method is based on the partially coherence theory, and was well improved by previous works, for example the classification into the two types of scanning microscopes,2< extension for MO pickups, and so on. Furthermore, various super-resolution techniques and the crosstalk reduction technique were analyzed theoretically by using this scalar diffraction theory. On the use of this theory, the light amplitude distribution on the exit pupil of the objective lens or the modulation transfer function (MTF) is usually employed for the explanation of the functions of optics. Meanwhile, the method we present here is based on the work by Velzel. His method is basically an approximation of the scalar theory and concentrates on the point spread function (PSF) of the illumination optics in the pickup. And it provides a simple and practical way for computations and understanding of the readout process. For example, Kubota presented in his study the influences of wave front aberrations on the readout characteristics by using jitter calculations, in which the readout signal was calculated as a convolution of the pits on the disk and the PSF. In this paper, we present a method using an extended point spread function (EPSF) that can be applied for the case when the detector has an arbitrary shape and information pits are recorded on a groove. After that, two examples, differential phase detection (DPD) method and jitter analysis of the readout signal from pits on a groove, will be shown. Although our method is also an approximation, physical and intuitive considerations will be acquired even under such complex cases.
We have developed a brand-new method using a gyrator for magneto-optical pickup. This method provides a simple optical pickup with some unique characteristics. It achieves both higher optical coupling efficiency, higher readout signal level, and improved modulation transfer function called ultra-resolution. In this paper, we describe the principle of MO signal readout, theoretical analysis, some results of simulation and experiments.