Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 937–942 Part 1, No. 2B, February 2000 c 2000 Publication Board, Japanese Journal of Applied Physics Optical Disk Recording Using a GaN Blue-Violet Laser Diode Isao I CHIMURA ∗ , Fumisada M AEDA, Kiyoshi O SATO, Kenji YAMAMOTO and Yutaka K ASAMI Development Center, Home Network Company, Sony Corporation, 6-7-35 Kitashinagawa, Shinagawa-ku, Tokyo 141-0001, Japan (Received August 10, 1999; accepted for publication September 20, 1999) The combination of a GaN laser diode and a 0.85 numerical aperture objective has achieved an optical storage capacity of over 22 GB. Owing to sufﬁcient modulation ability and low-noise characteristics, GaN semiconductor lasers possess adequate quality to be light sources in optical recording systems. A bit size of 130 nm × 300 nm, corresponding to the areal recording density of 16.5 Gbit/in.2 , was realized on a six-layered thin-cover phase-change disk with a crystallization-promoting structure. Each active layer was inversely stacked and was thermally as well as optically optimized for the blue-violet light sources. A new small electromagnetic actuator with a lightweight φ3 mm two-element lens has extended its servo-bandwidth up to 8 kHz and enabled precise focusing control at the data transfer rate of 35 Mbps. KEYWORDS: GaN laser diode, two-element lens, high numerical aperture (NA), wide-band actuator, phase-change disk, high- density recording, high data transfer rate tive and a wavelength of 640 nm. It also gave extra fabrication 1. Introduction margins for assembling the lens unit, such as in the decenter Both a capacity of over 20 GB and a higher data transfer and the tilt of the two lenses. rate of more than 35 Mbps are required for optical data stor- In this paper, we examine the possibility of a GaN semicon- age in the era of digital broadcasting and of broadband net- ductor laser being employed as a light source for optical data works in order to record emerging commercial high-deﬁnition storage, especially from the viewpoint of modulation ability or multichannel digital video streams. Optical pickups with and noise characteristics. Our new two-element lens designed coherent light sources of shorter wavelength and also with for the wavelength of 400 nm is also discussed. As we al- higher numerical-aperture (NA) objectives are essential to ready reported, combined with a blue-violet light source, a improve resolution because the spot size is determined by high NA objective forces the defocus margin to become as diffraction to be approximately λ/NA, where λ is the free small as 600 nm.9) For making a high NA system practical, its space wavelength. To date, various papers have reported the residual focusing error must be kept within 1/10 of this range, advantage of utilizing blue light sources.1–3) Recent develop- i.e., 60 nm. Thickness ﬂuctuation of the disk cover must be ments in GaN semiconductor lasers enhance the probability kept sufﬁciently small to help this conﬁguration. In addition, for them to be practically used as light sources of optical disk the bandwidth of the two-axis focusing and tracking actuator systems in the near future.4, 5) needs to be extended since a high data transfer rate of over In addition, a solid immersion lens (SIL) as an add-on to 35 Mbps leads to increasing disk rotation. The combination a standard objective magniﬁes the effective NA by a factor of a lightweight two-element lens and a wide-band electro- of its refractive index n or n2 .6–8) The SIL is also one of the magnetic actuator, which has achieved the DC gain of 80 dB most promising technologies leading to a near-ﬁeld optical and the cutoff frequency of 8 kHz, meets the above require- disk storage whose NA exceeds 1. However, this type of high- ment. density storage would probably require an air-sealed system. In phase-change optical recording, crystallization-promot- Instead of employing the SIL described above, we pre- ing layers are known to improve overwrite cyclability at high viously reported that a 0.85 NA lens could double the areal data transfer rates.11) We have developed a six-layered phase- recording density of a digital versatile disk (DVD) in an un- change disk of the same structure for blue-violet light sources, sealed condition.9) In the case of designing high NA objec- and propose an optical disk system based upon this concept. tives, two-element lenses are preferable because their refrac- The obtained results indicate that both the capacity of 25 GB tive power can be distributed over four refraction surfaces. On and the data transfer rate of 35 Mbps are feasible through fur- the other hand, high NA objectives generally reduce the sys- ther improvement of the disk materials. tem tolerances, particularly with regard to disk tilt and cover thickness, by the third and the fourth power of the NA, respec- 2. Optical Setup for Achieving a Capacity of Over 20 GB tively. The authors proposed a two-element aspherical lens to Figure 1 illustrates the schematic diagram of the optical increase the tolerances described above.10) In our former de- block. A GaN laser diode (a prototype sample from Nichia sign, each lens was mounted on an independent electromag- Chemical Industries, Ltd., maximum output power = 5 mW, netic actuator, and both focusing and tracking controls were λ ≈ 400 nm) was installed as a light source coupled with a implemented by moving the two lenses together as a singlet collimator lens and an anamorphic prism (β = 2.7), which unit. Moreover, the spherical aberrations due to cover vari- compensated the emitting beam proﬁle. Part of the laser out- ation were compensated by the mechanism of changing the put was monitored by a photodetector, and the driving current distance between the two lenses. We also demonstrated that a was controlled through an automatic-power-control (APC) 0.1 mm thin-cover disk effectively increases the tilt tolerances circuit to keep the illuminating power constant for each op- for the high NA optics. This lens design allowed the working erating mode such as recording and readout. A 0.85 NA ob- distance of 300 µm for the combination of a 0.85 NA objec- jective consisted of two aspherical glass lenses. The lens unit weighed 62 mg in total and the effective diameter was ∗ E-mail address: firstname.lastname@example.org 3.0 mm. Its working distance was 130 µm for the blue-violet 937 938 Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 2B I. I CHIMURA et al. Fig. 1. Schematic diagram of the optical pickup. Fig. 2. Spherical aberrations compensated by the expander. light sources. The two-element lens was rigidly connected in this setup in order to keep the total weight of the unit as low as possible for a precise and wide-band focusing servo, and the spherical aberration due to cover thickness variation was canceled by altering the distance between two additional expander lenses. Calculated spherical aberrations after thickness compensation are shown for various covers in Fig. 2, compared with no ex- pander setup. Although its compensation ability is slightly inferior to our former method, more than ±10 µm of de- viation can be adjusted. This ﬂexibility indicates that our two-element lens system is also capable of dual-layer phase- Fig. 3. Basic structure of the wide-band actuator. change recording. The reﬂected beam from a phase-change disk was split into two optical paths, one for knife-edge focusing error detection servo were 80 dB and 8 kHz, respectively. and the other for RF signals and differential push-pull (DPP) In the experiment, disk displacement along the optical axis tracking error detection. Each signal was detected by silicon due to disk rotation was set at 100 µmp-p . The measured resid- PIN photodetectors optimized for this particular wavelength. ual focusing error was within 35 nmp-p , as shown in Fig. 5, at the linear velocity of 5.72 m/s. Since this experimental 3. Electromagnetic Two-Axis Actuator for High Data condition corresponds to the 35 Mbps of data transfer rate Transfer Rate in our system, the results indicate that the combination of a Although both a blue-violet light source and a high NA lightweight two-element lens and a wide-band actuator meets objective reduce servo tolerances in various aspects, the de- the requirement even at this high data rate. focus tolerance may be the most vulnerable in this system. Other aberrations due to disk tilt or thickness deviation of a 4. GaN Semiconductor Laser as a Light Source for Op- disk cover can be effectively reduced by introducing a thin- tical Data Storage substrate disk or by utilizing a spherical aberration canceling 4.1 Frequency response of a GaN laser diode mechanism, as described in §2. In order to investigate the modulation capability of a GaN In order to achieve a residual focusing error of less than semiconductor laser, its frequency response was examined us- 60 nm, we developed a small two-axis electromagnetic actua- ing the setup illustrated in Fig. 6. The small sinusoidal out- tor for the two-element lens. The basic structure is illustrated put of a signal generator up to 350 MHz was AC-coupled to in Fig. 3. The movable section of the actuator, sustained by the laser diode to modulate the emitting output power. Mod- four lateral wire suspensions, weighed 250 mg including the ulation current (MI ) was measured using an FET probe and lens unit, and was capable of increasing both focusing and an oscilloscope. The modulated amplitude of the emitting tracking servo-bandwidths. Figures 4(a) and 4(b) show the output (Mopt ) was observed using a sampling optical oscillo- frequency responses along the focusing and the tracking axes scope (Hamamatsu OOS-01) through an optical ﬁber at each measured with a laser-Doppler vibrometor. The second res- measured frequency. DC current above threshold level was onance frequencies were nearly 80 kHz in the focusing di- constantly supplied to the diode throughout the measurement. rection and above 100 kHz in the tracking direction. The The frequency response of the laser diode is deﬁned here as achieved DC gain and the cutoff frequency for the focusing Mopt /MI . The experimental result, shown in Fig. 7, reveals Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 2B I. I CHIMURA et al. 939 Fig. 4. Frequency responses of the actuator: (a) focusing directions, (b) tracking directions. Fig. 5. Residual focusing error. Fig. 7. Frequency response of a GaN LD. Fig. 6. Experimental setup for examining the frequency response. that a GaN semiconductor laser has a virtually ﬂat frequency response up to at least 350 MHz and the capability of modu- lating the illuminating power at a very high data transfer rate. Fig. 8. Driver circuit for a GaN LD. 4.2 Driver circuitry and noise measurement Since not only the operating voltage but also the impedance of GaN semiconductor lasers were higher than previous red or the output of an APC circuit. In a readout operation, high- infrared semiconductor lasers, we employed a GaAs FET, as frequency (HF) oscillation of around 500 MHz was added to depicted in Fig. 8, to supply sufﬁcient voltage and to modulate the gate while its DC voltage was adjusted to keep the illumi- the laser current in the following experiments. In a recording nating power constant. operation, pulse-train signals were inputted to the gate of the Figure 9 shows the modulated light output at the data trans- FET, while the driving current was controlled by changing the fer rate of 35 Mbps. The peak power of 30 mW was squeezed amplitude and the DC levels of the pulses in accordance with in the measurement, whereas this GaN laser diode allowed 940 Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 2B I. I CHIMURA et al. Fig. 9. Modulation of the emitting output at 35 Mbps. only a low output of 5 mW. Owing to the high switching Fig. 10. RIN of a GaN LD. speed of the GaAs FET, the obtained rise-and-fall time of the emitting output was approximately 1 ns. The measured relative intensity noise (RIN) versus output power during the readout operation was plotted in Fig. 10. As long as the laser emitting power was kept above 2 mW, the RIN was well be- low −125 dB/Hz. This noise level is sufﬁciently low for a coherent light source to be used in an optical disk drive. 5. Thin-Cover Phase-Change Disk for a Blue-Violet Light Source A cross-sectional view of a phase-change disk is shown in Fig. 11. Cross-sectional view of a six-layered phase-change disk. Fig. 11. For the purpose of achieving a high data transfer rate of over 25 Mbps, the concept of crystallization promoting was applied to our system. The phase-change disk, compris- Table I. Experimental conditions for capacity of 22 GB. ing six layers and a land-and-groove disk structure, was ther- User capacity 22 GB mally as well as optically optimized both for the wavelength Format efﬁciency 80% of 400 nm and for the 0.85 NA objective. SiC layers, func- Disk size (φ) 120 mm tioning as crystallization-promoting materials, sandwiched a Modulation 1–7 code GeSbTe recording layer and improved the rewritability at high Bit density 16.5 Gbit/in.2 data transfer rates. Bit length 130 nm After pits and grooves were embossed on the surface of Minimum mark length 173 nm a 1.1-mm-thick poly-carbonate (PC) substrate (φ120 mm), Track pitch 300 nm each layer was inversely stacked. A thin-cover PC sheet of Channel clock 50 MHz (27 Mbps) 0.1 mm was placed on top after all the active layers had been Linear velocity 4.33 m/s sputtered. Because the combination of short wavelength and Peak power 4.0 mW high NA leads to a narrow cover tolerance, the thickness- Bias power 1.5 mW ﬂuctuation of the disk cover, i.e., a PC sheet plus adhesive, Cooling power 0.5 mW must be kept small enough to help this conﬁguration. Our Readout power 0.25 mW measurement results have proven that the thickness varies slightly from the lead-in (innermost area of a disk) through to the lead-out (outermost area), and that the typical peak-to- capacity was estimated at 22 GB when the recording area was peak difference is 3 µm. Fluctuation within one revolution is 24 mm through 58 mm in radius, and the format efﬁciency of the order of 1 µm. was 80%. Although having introduced the crystallization- promoting layers, we have not yet achieved a data transfer 6. Phase-Change Recording up to 25 GB Capacity rate of 35 Mbps after numbers of direct overwrite (DOW) cy- 6.1 Evaluation of 22 GB capacity at 27 Mbps of data trans- cles because of its poor rewritability at this target data rate. fer rate The following experiment was performed at 27 Mbps, the In order to certify the performance of GaN semiconduc- maximum data transfer rate attained by this disk structure. tor lasers and to investigate the achievable bit density brought At ﬁrst, a bit size of 130 nm × 300 nm, corresponding to by this high NA optics, 1–7 modulated mark length recording the areal recording density of 16.5 Gbit/in.2 , was investigated, was carried out on a land-and-groove phase-change disk. Ex- since good results were obtained without any cross-write phe- perimental conditions are summarized in Table I. The total nomena. In the experiment, the illuminating beam was pulse- Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 2B I. I CHIMURA et al. 941 modulated at the three different power levels described in Disk tilt tolerances were also examined at this recording Table I. Figure 12 shows eye-patterns after linear equaliza- density. Figure 14 shows the results for tangential (a) and ra- tion at 1000 DOW cycles with the existence of cross talk for dial tilt margin (b) at 1000 DOW cycles. If the tolerance is the land (a) and the groove (b), respectively. An adequate jit- deﬁned by the jitter value of 15% (dotted line in both charts), ter of 9.4% on the land and that of 9.0% in the groove were this system has skew tolerances of ±0.5 degrees in the tangen- obtained. tial directions and of ±0.7 degrees in the radial directions. Additionally, in order to evaluate this areal recording den- sity, both overwrite cyclability and cross-write phenomena 6.2 Investigation into 25 GB capacity from adjacent tracks were examined by the following pro- Since the capacity for a φ120-mm-size disk, predicted cedures. The results were plotted in Figs. 13(a) and 13(b), from the wavelength of 400 nm and the numerical aperture as the data-to-clock jitter versus DOW cycles. First, random of 0.85, is expected to be 25 GB, we also examined the 1–7 data were recorded repeatedly (1 to 5 × 104 times) on land-and-groove recording at this density. In the experiment, the same track (e.g., in the groove). Second, random 1–7 data track pitch was narrowed down to 270 nm while bit length were recorded on both adjacent tracks (on the lands for this was set to the same as the previous condition, i.e., 130 nm. case), and then the jitter of the recorded signals was checked Its areal recording density corresponds to 18.4 Gbit/in.2 . Fig- on the track of concern. In Fig. 13(a), circles and triangles ure 15 shows the eye-pattern on the land (a) and that in the show the DOW-cycle characteristics measured on the lands groove (b). The amplitudes of RF signals were not well bal- and those in the grooves, respectively. Cross-write was inves- anced between the land and the groove. Obtained data-to- tigated in the same manner and the measurement results were clock jitters on the land and in the groove were 9.6% and plotted in Fig. 13(b). In this case, after the random data were 12.0%, respectively, at 1000 DOW cycles with the existence recorded 1000 times on the same track, both adjacent tracks of cross talk. At this track density, cross-write phenomena were overwritten up to 5 × 104 cycles. On the basis of the were observed. In particular, recorded marks in the groove results of Figs. 13(a) and 13(b), (if the jitter value limitation were affected by the DOW on the adjacent lands. is considered to be 10%) we conclude that the cyclability of this phase-change disk is limited around 1 × 104 times, and 6.3 Possibility of 35 Mbps of data transfer rate that the degradation of adjacent tracks affects the cross-write Although a six-layered phase-change disk still limits the measurement in the region over 5 × 104 DOW cycles. data transfer rate of our high-density system after numbers of DOW cycles, the possibility of realizing 35 Mbps was ex- amined. From the results described in §3, our high NA lens unit and two-axis actuator are both capable of 35 Mbps. As demonstrated in §4, GaN semiconductor lasers possess the ability of high-frequency modulation and their noise level (RIN) is sufﬁciently low. A GaAs FET laser driver has achieved a rise-and-fall time of 1 ns. For further investigation, a series of 2T marks (173 nm length) were recorded on the land only once at a data transfer rate of 35 Mbps (channel clock = 66 MHz, linear velocity = 5.72 m/s). In the experiment, the track pitch was selected to be 270 nm, the condition for achieving 25 GB capacity. Mea- Fig. 12. Eye-patterns of 22 GB land-and-groove recording at 1000 DOW sured carrier-to-noise (C/N ) ratio of the 2T marks along with cycles: (a) on the land, (b) in the groove. ampliﬁer noise is shown in Fig. 16. Obtained C/N ratio of Fig. 13. (a) Overwrite cyclability at 27 Mbps of data transfer rate, (b) cross-write from adjacent tracks at 22 GB of capacity. 942 Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 2B I. I CHIMURA et al. Fig. 14. Tilt tolerances: (a) tangential directions, (b) radial directions. for a φ120 mm phase-change disk. Realized areal record- ing density was 16.5 Gbit/in.2 . A small new actuator with a lightweight φ3 mm two-element lens has extended its focus- ing and tracking servo-bandwidths to be applicable to emerg- ing high data transfer rate applications such as high-deﬁnition digital video streams. The current data transfer rate is still limited by the disk structure of our six-layered phase-change disk because of its insufﬁcient rewritability over 27 Mbps. Cross-write phenom- Fig. 15. Eye-patterns of 25 GB land-and-groove recording at 1000 DOW ena from adjacent tracks at a capacity of 25 GB are also at- cycles: (a) on the land, (b) in the groove. tributed to this crystallization-promoting layer design. Im- proving the erasability at high data transfer rates and optimiz- ing the layer structure for the blue-violet light sources and for the high NA objectives could lead to a much higher capac- ity of over 25 GB and also a higher data transfer rate of over 35 Mbps. C/N = 48 dB Acknowledgments The authors are grateful to all the members of Optical Total noise Disc Laboratory and Media Laboratory for supporting this research. We would like to express our sincere thanks to K. Nishitani, H. Ogawa and M. Yamamoto for encouraging us throughout this work. Amplifier noise 1) W. J. Kozlovsky, A. G. Dewey, A. Juliana, J. E. Hurst, M. R. Latta, D. A. Page, R. N. Payne and H. Werlich: Proc. SPIE 1663 (1992) 410. 2) F. Yokogawa, S. Ohsawa, T. Iida, Y. Araki, K. Yamamoto and Y. Moriyama: Jpn. J. Appl. Phys. 37 (1998) 2176. 3) H. A. Wierenga: Proc. SPIE 3401 (1998) 64. Fig. 16. C/N ratio of 2T marks and ampliﬁer noise at 35 Mbps. 4) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto and H. Kiyoku: Jpn. J. Appl. Phys. 36 (1997) L1059. 5) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. 48 dB and the low ampliﬁer noise level, compared to the total Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Uemoto, M. 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