A novel multi-level read-only recording using signal waveform modulation (SWM) is presented. The SWM is realized by inserting a sub-pit/sub-land to the original land/pit. Numerical simulation provides a helpful tool for the write strategy optimization. This method is experimentally validated on the DVD platform. Comparing with 2-level recording, an increase of 50% in capacity can be expected. It shows superiority over signal amplitude modulation (SAM) multi-level in replication processing, capacity increase and servo performance.
©2008 Optical Society of America
A recognizable advantage of optical read-only memory (ROM) is its low-cost and fast mass-production. Multi-level (ML) technology can increase the storage capacity without changing the readout optics and mechanism [1–2].
The studies of our team on ML ROM recording have been reported [3–5]. In the previous work, different recording powers are employed to form pits with varied width and depth, and the RF signal amplitude is used to differentiate the levels. Hence, the previous ML method can be called signal amplitude modulation (SAM), which only makes use of the pits to obtain ML signals. To avoid replication difficulty caused by consecutive pits, an M-ray even nonzero symbol run-length limited (ENSRLL) coding is employed , which limits the increase of storage capacity. It also encounters a problem in tracking error detection, which deteriorates the quality of readout signal .
In this paper, a novel multi-level read-only recording is presented. A sub-land/pit is inserted to the original pit/land, leading to variations in waveform of readout signal. A signal waveform modulation (SWM) ML recording is realized by using the waveform to differentiate the levels, and is implemented on the DVD platform. Simulation-assisted write strategy optimization and experiment results are presented. SWM ML recording can overcome the shortcomings of SAM discussed above. Comparing to 2-level recording, an increase of 50% in capacity can be expected.
2. Basic principle
Readout of read-only optical disk is based on phase diffraction. Reflective lights from the pit and adjacent lands have a phase difference, which leads to a decrease of light intensity on the photon-electronic detector and a corresponding amplitude decrease in readout signal. The decrease amount is determined by the amount of phase difference and its distribution. This can be mathematically described by Hopkin’s classic optical disk diffractive model . Due to the diffraction limit, the size of focused readout light spot is definite and determined by the laser wave-length and numeric aperture of objective lens. When a mark (pit or land) is too short, the signal amplitude change will be too small for detection. Therefore, every format specifies its shortest mark length. For example, DVD’s specified shortest mark length is 400nm.
In our ML recording, we utilize marks shorter than the specified shortest mark length. They are called sub-marks (including sub-pits and sub-lands). A sub-pit/sub-land is inserted to the original land/pit by design. When scanned, the sub-mark will change the phase difference amount and distribution, resulting in local amplitude change of readout signal. The comparison of SWM and SAM ML is in Fig. 1 and Fig. 2. Figure 1 shows how the marks are recorded. Figure 2 shows how the signals are obtained. In Fig. 2, the circles indicate the focused reading light spot. The difference of these two ML methods can be obviously seen.
It can be seen in Fig. 2 that sub-marks have changed the waveform of readout signal. Using the waveform as the symbol of level differentiation, a signal waveform modulation (SWM) ML recording is realized. Furthermore, changing the length or/and position of the sub-marks, more levels can be realized. A long run-length has more spaces for sub-marks length/position variation on the disk, and it also has more time duration and amplitude margin for the partial amplitude change in the readout signal. So, for long run-lengths, more levels are realized; for short run-lengths, fewer levels are realized. This ML method is different from the previously reported limited multi-level (LML) . In LML, the sub-pit/sub-land is only positioned in the middle and its length is fixed for a certain run-length. Therefore, a run-length can only realize at most 4 levels.
3. Write strategy optimization
Write strategy (WS) optimization is a challenging part of the SWM ML recording. It is done by adjusting the values of T1, T2 and T3 in Fig. 1 for every run-length and level. Here, we define [T1 T2 T3] as the writing vector, which can determine the length and position of sub-marks.
There are two basic tasks for WS: (1) making sure levels can be easily differentiated, meaning that the difference of waveform between every two levels should be as big as possible; (2) making sure the run-length is not influenced by the inserting of sub-marks, meaning that run-length compensation will be implemented when needed.
Numerical calculation is carried out for the WS optimization. A lithography model for optical disk mastering presented in  is employed to calculate the pit profile on the disk. A diffractive model based on angle spectrum decomposition presented in  is employed to calculate the readout signal. All the parameters are the same as the actual DVD production and can be found in ref. . All the readout parameters are selected according to the DVD system.
After simulation trial, it is determined that 3T (T means the mark length corresponding to one channel bit), 4T and 5T remain the 2-level state, 6T and 7T realize 6 levels, 8T and 9T realize 10 levels, 10T and 11T realize 14 levels. Simulation results of typical run-lengths, 6T and 11T, will be discussed here.
For 6T, because it is short, the sub-mark is positioned in the middle, and only the length of the sub-mark is varied. Figure 3 shows the trial simulation results. For 6T pit, the calculated write vectors, from top to bottom, are respectively [2 2 2], [2.125 1.75 2.125], [2.25 1.5 2.25], …, [3 0 3]. The unit is the duration of one channel clock. For 6T land, the calculated writing vectors, from top to bottom, are respectively [3 0 3], [2.5 1 2.5], [2.375 1.25 2.375], …, [2 2 2]. Following can be seen from these results:
- The readout signal shows amplitude variation through the entire run-length, instead of waveform variation. This is because 6T is so short that the reading light spot irradiates the whole length. However, readout signal of different levels can also be differentiated.
- For the same T2 value in writing vector, the influence of a sub-land is more significant than that of a sub-pit. It means that different writing vectors should be selected for pits and lands to obtain symmetrical readout signals.
- The run-length is obviously shortened due to the inserting of sub-marks. So, run-length compensation is needed.
For other run-lengths, similar simulation trial is carried out, and then writing vectors are selected. Table 2 is the selected writing vectors for 11T, and Fig. 5 is the corresponding simulation results. 11T realize 14 levels. The sub-marks are positioned in the middle, middle-left and middle-right. The length of sub-marks is also varied. It can be seen that the waveform of readout signal is obviously modulated. The Waveform can be used for the level differentiation.
Furthermore, influence of adjacent marks on run-length deviation should also be considered. More complex WS optimization is implemented. Numerical simulation is also a helpful tool for corresponding parameter selection and adjustment.
The proposed SWM ML recording is implemented on the DVD platform. Commercially available mastering and injection molding equipment are used. All the processing parameters and conditions are the same as conventional 2-level DVD. Key mastering processing parameters are listed in Table 3. The only change is the formatter controlling the recording laser power. An ESP-7000 formatter by ECLIPSE corp. is employed to generate the required writing pulse.
Figure 6 shows the profile images of SAM ML and SWM ML disks scanned by an atom force microscope (AFM). Cross-section of sub-land/sub-pit is also shown in sub-figure (b) and (c). The sub-land is not as high as normal land. That is because the reading spot has a certain size, and the fringe part of spot will bleach the sub-land area. Also, the sub-pit is not as deep as normal pit. That is because the exposure of photo-resist exhibits integral effect, and irradiation time of sub-pit area is not so long due to its short writing pulse.
A commercial DVD pick-up and a commercial DVD servo circuit are used to readout the disk. The readout signal of random data is shown in Fig. 7. Actual signals of 6T and 11T are also shown in Figs. 8 and 9. Comparing Figs. 8 and 9 with Figs. 4 and 5, it can be seen that the simulated and experimental signals agree well.
A digital circuit based on field programmable gate array (FPGA) is developed to detect run-length and level data. A raw error bit error rate of 10-4 is obtained. The feasibility of SWM ML recording on DVD is validated.
5. Conclusion and discussion
A novel signal waveform modulation multi-level read-only recording is presented. Numerical simulation provides a helpful tool for the write strategy optimization. The feasibility of this method is experimentally demonstrated on the DVD platform.
A 2-level-to-multi-level mapping modulation coding with 2 steps is employed in our method. Firstly, a 2-level run-length limited (RLL) coding is carried out, where run-lengths smaller than 3T are allowed. Secondly, run-lengths smaller than 3T will be eliminated. These too short run-lengths will be combined with neighboring run-lengths to form a long run-length, and then mapped to a multi-level run-length. For example, 2T-2T-2T can be mapped to 6T of level 2, 2T-4T can be mapped to 6T of level 3, 3T-2T-3T can be mapped to 8T of level 2, 2T-2T-4T can be mapped to 8T of level 3.
Without using all the levels of every run-length, we realizes the same transfer rate and capacity as 2-level (1, 7) RLL coding, whose density ratio (DR) is 2 bits/(min symbol). Experiments show that it is feasible for 5T to realize 4 levels, which means that (0, 6) RLL coding can be mapped to our ML coding and the DR will be 2.25 bits/(min symbol). As the DR for 2-level DVD is 1.5 bits/(min symbol), an increase of 50% in capacity can be expected if the min symbol length and track pitch are kept the same as DVD. As a comparison, the latest reported realizable DR of 4-level SAM ML is only 2 bits/(min symbol) , whose corresponding increase in capacity is 33%. Therefore, SWM ML shows superiority over SAM ML in capacity increase.
This new SWM ML also has other merits. (1) In SWM ML, both land and pit can realize multi-level. It is naturally land-pit-spacing, which can facilitate the injection molding and RF signal DC-free control. (2) Experiments show that our SWM ML has the same track-following servo performance as 2-level DVD, while the SAM ML encounters the tracking error detection problem .
This work is supported by the National Natural Science Foundation of China grant 60677036 and 60707003.
References and links
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