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210   5 Near Field
                                           50
                                                                     3,000 nm
                                           40                        2,000
                                                                     1,000
                                         CNR (dB)  30  600  500
                                           20
                                                     400
                                                         300
                                           10
                                                                        l/4NA = 413 nm
                                                            200
                                           0
                                            1    2    3   4    5    6    7   8    9
                                                         Read power P r  (mW)
                            Fig. 5.56. Read power dependence on CNR of mark written at power of P w =
                            8.5 mW, with mark length as parameter
                                       (a)
                                          CNR (dB)  0 MHz  46.4 dB  0 MHz  44.1 dB  0 MHz 31.6 dB







                                             P  = 2.0 mW    P  = 4.0 mW    P  = 6.0 mW
                                              r
                                                             r
                                                                            r
                                                              1,000 nm
                                       (b)     0dB            11.6 dB        3.9 dB
                                          CNR (dB)
                                               = 2.0 mW    P  = 4.0 mW     P  = 6.0 mW
                                             P r
                                                             r              r
                                                              200 nm
                            Fig. 5.57. Read power dependence on signal amplitude and spectrum for mark
                            lengths of 1,000 nm with 10 dB div −1  and 1 MHz div −1  (a), and 200 nm with
                            10 dB div −1  and 10 kHz div −1  (b),withreadpower P r as parameter


                            Read power dependence

                            Figure 5.56 shows the read power P r dependence on the CNR of the mark
                            written at the power of P w =8.5 mW, where mark length is a parameter.
                            From the figure, it is evident that signals (400–200 nm) appear beyond the
                            diffraction limit and the highest CNR of each mark length can be obtained at
                            P r = 4 mW. On the other hand, CNRs are high and almost independent from
                            P r for longmarks (1,000–3,000 nm). All the signals disappear at read powers
                            greater than P r =6.5 mW because the amorphous mark and the bubble pit
                            are erased due to the continous large P r . We define the optimum super-RENS
                            read power as 4 mW.
                               Figure 5.57 shows the read power dependence on the signal spectrum and
                            the amplitude for mark lengths of 1,000 nm (a), and 200 nm (b), where read
                            power P r as a parameter. Both the signal spectrum and the signal amplitude
                            increase at P r = 4 mW. On the other hand, we find that noise increases at
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