Conventionally, enhancement of storage capacities of various kinds of optical disks have been achieved by making smaller an area on disk tracks where information is written, and by making smaller the size of a light collecting spot on a focus face by adopting a shorter-wavelength laser beam for recording and reproducing and an objective lens with a high numerical aperture.
For example, a CD (compact disk) has a capacity of 650 MB, in which the thickness of the disk substrate serving an optically transparent layer (a transparent cover layer and a spacer layer that are provided on an information recording layer and may be referred to as a transparent substrate) is about 1.2 mm, the wavelength of its laser beam is about 780 nm, and a numerical aperture (NA) of its objective lens is 0.45, whereas a DVD (digital versatile disc) has a capacity of 4.7 GB, in which the thickness of the disk substrate serving as an optically transparent layer is about 0.6 mm, the wavelength of its laser beam is about 650 nm, and the NA is 0.6.
In DVD, two substrates whose thickness is, for example, 0.6 mm, are bonded together to be used as a 1.2 mm-thickness disk.
In BD (Blu-ray disc), which has a higher recording density, a large capacity of 25 GB per recording layer is realized by using an optical disk with its optically transparent layer thinned to 0.1 mm, and by applying a laser beam with its wavelength about 405 nm and the NA of 0.85.
Besides, there is HDDVD (high definition digital versatile disc), for example, in which a large capacity more than 18 GB is realized by using an optical disk whose disk substrate serving as an optically transparent layer has the same thickness of 0.6 mm as that of DVD, and by applying a laser beam with its wavelength being about 405 nm and the NA of 0.65.
Recently, in the field of optical recording, a high-density recording method has been studied that uses a super-resolution optical disk in which an optically nonlinearly mask layer whose refractive index varies according to light intensity is formed.
In this method, by causing a refractive index variation at a portion in a light collecting spot on the optical disk where light is locally intensified or temperature is locally raised, it is possible to reproduce information (for example, Non patent document 1) from a mark whose size is smaller than a diffraction limit λ/4NA that is determined from the numerical aperture NA of a condensing lens—a component of the optical head device—and the wavelength λ of the light.
In the super-resolution optical disk as described above, to the extent that the mask layer absorbs light, more readout power is required than that in a conventional optical disk. As a result, low frequency noises (or disk noises) outstandingly appear in a reproduction signal (for example, Non patent document 2).
The low frequency noises mainly appear in a frequency band lower than the diffraction-limited spatial frequency.
By the way, a basic principle of reproducing data on an optical disk is that a mark having concave and convex portions or a mark having differences in refractive index diffracts light, which is then transmitted through an objective lens having a numerical aperture NA so as to be detected as a return light by a receiver, which gives a reproduction signal. That is, in a peripheral circumferential portion of the objective lens with the numerical aperture NA, there is much diffracted light that is produced by a mark sequence with a high spatial frequency. On the contrary, in the central portion of the objective lens, there is much diffracted light that is produced by a mark sequence with a low spatial frequency, so that there are also lots of low frequency noises in the central portion of the objective lens. In addition, there is a tendency that, as the spatial frequency of the shortest-mark sequence becomes higher due to densification, the diffraction angle of the diffracted light by the shortest mark becomes larger, reducing the quantity of light that is taken into the objective lens.
Therefore, the signal-noise ratio becomes lower along with densification. Especially, when the shortest mark becomes shorter than the diffraction limit, the ratio is remarkably lowered. In the super-resolution optical disc, because its readout power is being enhanced, low frequency noises themselves become large, making the signal-noise ratio even lower.
In the super-resolution optical disk, it is considered to be an effective method that peripheral portions of a return light beam, where the ratio of a reproduction signal component to low-frequency-band noises is observed to be high, are intensified to produce a reproduction signal, and thus it is desired that the peripheral portions of the return light beam, where the quantity of light is small, be detected as much as possible without losses.
In a typical device that records or reproduces data to/from an optical disk, a light beam radiated from a light source is focused by an objective lens onto the optical disk, then reflected or diffracted by the optical disk, again taken into the objective lens as a return light beam, and led to a light receiving device. Then, a reproduction signal is produced based on an electric signal outputted from the light receiving device.
In order to form a light collecting spot in good quality on the optical disk through the objective lens, it is necessary to establish a predetermined-aperture limitation so that a light beam having an entrance pupil diameter (effective entrance pupil diameter) which is determined by the design of the objective lens to be effective to focus the light beam, enters the objective lens.
In many cases of the aperture limitation, a light-beam portion outside the effective entrance pupil diameter is light-blocked, and this is realized, for example, by providing an aperture having a diameter equal to the effective entrance pupil in an objective lens holder.
On the other hand, a return light beam from the optical disk is light-blocked except for a light beam portion that passes through the aperture having the effective entrance pupil diameter, and only the limited light beam portion corresponding to within the effective entrance pupil diameter out of the return light beam reflected or diffracted by the optical disk as aforementioned, is led to a light receiving device.
Here, if light beams are considered to be ideal light rays, the return light beam is collimated by the objective lens and then naturally becomes a light beam having a diameter equal to the effective entrance pupil diameter. That is, the diameter of the return light beam becomes equal to the effective entrance pupil diameter. However, actually, the return light beam spreads due to the diffraction of the optical disk and the diffraction action of the light beam under propagation, so that the return light beam again enters into the objective lens with its diameter larger than the effective entrance pupil diameter, meaning that there is an aperture limitation for the return light beam when the effective entrance pupil diameter is equal to the exit pupil diameter of the objective lens, like the above aperture.
As a method for taking in much light from peripheral portions of the return light beam, not only the exit pupil diameter but also the effective entrance pupil diameter may be enlarged by using an objective lens with an increased numerical aperture so as to enhance the reproduction resolution itself. However, this requires using a high-manufacturing-cost objective lens whose light-condensing performance is ensured also for an increased portion in its numerical aperture so as not to degrade the quality of the light collecting spot, leading to a situation where inexpensive optical head devices cannot be provided.
[Non patent document 1] “Observation of Eye Pattern on Super-Resolution Near-Field Structure Disk with Write-Strategy Technique”, Jpn. J. Appl. Phys., Vol. 43, No. 7A, 2004, pp. 4212-4215
[Non patent document 2] “Low Frequency Noise Reduction of Super-Resolution Near-Field Structure Disc with Platinum-Oxide Layer”, ODS Technical Digest, ThC3 (2005)