Plastic Objective Lenses for Blue LD Optical Pick-up
Mori, Nobuyoshi ＊Hashimura, Junji ＊＊Kimura, Tohru ＊＊Ori, Yuichiro ＊＊Abstract

Takada, Kyu ＊Mushiake, Nobuo ＊２ Technical issues for application of Hi-NA
plastic objective lenses

Plastic objective lenses are also promising in the BlueLD optical pick-up, since temperature dependency can becompensated by distribution of power or using movable

Table 1 Specifications of plastic singlets of NA0.85

element.

１ Introduction
Plastic objective lenses have played an important partin spreading optical disc systems since the CD was invented. Take molds, for instance; molds for glass lenses
are no longer usable after 10 thousand uses. On the otherhand, molds for plastic lenses are able to hold out over 1
million uses. So, plastic lenses are essential to mass proPlastic singlets for Blu-ray can be designed as shown

duction of optical disc drives.

in Fig.１. The specifications of these singlets are listed in

Recently, new high density optical disc systems em-Table １.

ploying Blue-LD have been proposed. One of them is theHD-DVD system that adopts an objective lens, of which

It’s a well-known fact that the refractive index of plas-

the NA and the cover layer thickness are the same as ontic changes with temperature, and the refractive index

the DVD. Another is the Blu-ray system adopting anchange causes spherical aberration in a plastic singlet. If

objective lens, of which the NA is 0.85 and the cover

the NA is not so high, however, the spherical aberration

layer thickness is 0.1mm. Is it still possible to use plastic

caused by temperature change（hereafter, called “tem-objective lenses, especially for such a high NA system asperature aberration” for short）is negligible.
The spherical aberration increases in proportion to theBlu-ray?
In this paper, we introduce some examples of plastic

4th power of NA. So in the case of Blu-ray, temperature

objective lenses and discuss technical issues concerning

aberration is a critical issue except with small size lenses.
We proposed two ways to correct the temperature

their application.

ABC

Fig.1 Plastic singlets for Blu-ray＊コニカミノルタオプト㈱＊＊コニカミノルタオプト㈱開発グループ

光学研究部 光学開発センター
オプティカルコンポーネント事業部aberration１）２）. The first solution is a doublet that combines two positive lenses. Fig.２ is a schematic diagram（本論文 SPIE の許可を得て、ODS2004 Technical Digest より転載しました。）

showing how to correct temperature aberration. Spheri-

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cal aberration changes the opposite way when the tem-plasticobjective singlet. The temperature aberration isperature of only one of the two lenses increases. And

corrected by the collimator’s shift along the optical axis.

when the magnification of the 2nd lens is chosen appro-

Additionally, if the diffractive structure applies to one

priately, temperature aberration can be corrected wellof the surfaces of the collimator, the chromatic aberra-by itself.

tion of the objective lens can be compensated for.

The second solution is the optical system that consists

Fig.４shows residual wavefront error after compensa-

of a plastic objective and a movable collimator or a mov-tion by the collimator shift. This will be the simplest ofable beam expander with an actuator.

all the optical systems for Blu-ray if the spherical aberra-Fig.３shows one of the configurations.

tion can be detected.

Movable plastic collimator works together aFig.2 Schematic diagram for correcting temperature aberration

Fig.3 Plastic singlet and movable collimator

Fig.4 Residual wavefront error

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３ Compatibility
Compatibility is desired and various approaches have
Blu-ray/ DVD compatible objective
been proposed.３−５）
lens assembly using a singlet made of glass and a plastic
phase plate has been developed recently.６）The plastic
phase plate has a wave selective diffractive structure（WSE）
that compensates for spherical aberration due tothe different thickness of the cover layer.
The cross section view of this lens is shown in Fig.５

Fig.5 Schematic view of the objective lens assembly

and its specifications are listed in Table ２.

Table 2 Specifications of a compatible objective lensFig.6 Cross sectional view of the WSE

The wavelength selective diffractive structure is drawn
in Fig.６. It has steps, height（h）of which is as follows;

for mass production of the first solution of plastic objective lens for Blu-ray shown in Fig.１. The diameter ofentrance pupil is 3a and the working distance is 0.24a.

h＝2λ1/（n1−1）

It is assembled automatically combining their flanges di-where λ1 is wavelength of 405å, and n1 is refractive

rectly. As shown in Fig.７, it can be handled like a sin-index of the phase plate for 405å. Therefore, theglet. Fig.８shows its interferogram. The wave front er-wavefront of 405å gets through without phase change.

rors are stably below 0.04 λrv in preproduction.

On the other hand, in the case of 655å, certain amountsof phase shift remain and then this periodic structure
affects the wavefront as diffraction gratings. The diver-５Conclusion

gent power of the diffractive lens corrects the spherical

We proposed a few ways to compensate the tempera-aberration due to the difference of cover layer thickness.

ture aberration of Hi-NA plastic objectives and have dem-

Needless to say, this means can be adopted also for a

onstrated a compatible objective lens using a plastic WSE.

plastic objective lens.

Plastic objective lenses have a disadvantage regarding
temperature dependency but at the same time, they pos-４

sess huge advantages regarding mass production andFabrication results

flexibility to achieve compatibility. It would be neces-

As an example of fabrication, we are now getting readysary to make good use of plastic objectives in order toKONICA MINOLTA TECHNOLOGY REPORT VOL.2（2005）

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achieve a low-cost and high-density optical disk system.
Plastic objective lenses will play an important part in the
optical pick-up systems using Blue-LD including Hi-NA
objective lenses and compatible objective lenses.

Fig.7 Plastic doublet

Fig.8 Interferogram of plastic doublet● Reference１）N. Mori, T. Kimura, Proc. of the 26th Optics Symposium, Japan
5（2001）（In Japanese）
２）T. Kimura, N. Mori, Proc. of ODF2002, 83（2002）
３）R. Katayama, Proc. of ISOM2003, 216（2003）
４）K. Koike, M. Ogasawara, I. Kikuchi, T. Matsuda, T. Ezuka andS. Ohtaki, Proc. of ODS2003, 271（2003）
５）Y. Komma, Y. Tanaka and S. Mizuno, Proc. of ISOM2003, 108（2003）
６）K. Takada, J. Hashimura, Y. Ori, N. Mushiake, Proc. of ISOM2003,230（2003）

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