Optical arrangement

The present invention relates to an optical arrangement, particularly to an optical arrangement including an optical element for use in scanning optical record carriers of different types, or to an optical element in an optical system such as a camera.

Progress in the field of optical reading and recording has resulted in the introduction of increasingly complex systems having extended capabilities, such as higher information capacity and density. Unfortunately, with these extended capabilities the margins of tolerance of the systems decrease.

An example of a new optical system is the Blu-ray Disc system (BD) which is a new optical recording protocol. The BD system uses a radiation beam with a wavelength of approximately 405 nm, a numerical aperture (NA) of 0.85 and a spherical aberration compensation for a record carrier of substrate thickness of 0.1 mm. When this system is compared with the Digital Versatile Disk system (DVD) which uses a radiation beam of wavelength 650 nm, a numerical aperture of 0.6 and a record carrier substrate thickness of 0.6 mm, the increase in NA and the decrease in wavelength makes the margins of tolerance for BD much smaller than for DVD. The BD system, capable of significantly increase data density (capable of recording 25 GB discs) is much more sensitive to wavelength variations that may arise when the temperature of the laser varies, or the wavelength of the laser varies from batch to batch by a few nm.

In another example in the field of optical recording, information is stored on an information layer of an optical record carrier such as a compact disc (CD) or a digital versatile disc (DVD). An increase in the density of information which can be stored on such an optical disc can be achieved by decreasing the size of a focal spot of a radiation beam which is used to scan the information layer of the optical disc. Such a decrease in spot size can be achieved by using a shorter wavelength and a higher numerical aperture (NA) of the radiation beam. However, increased resolution tends to reduce tolerances which apply to optical elements within an optical system. These reduced tolerances cause the focal spot of the radiation beam for scanning the optical disc to be more susceptible to degradation in quality.

Optical systems may be designed to be compatible with different types of optical disc, for example both a CD and a DVD. In such a system a separate radiation beam with an appropriate and different wavelength for scanning each type of disc is used. Each radiation beam is generally directed along a common portion of an optical path within the system along which lies optical elements for focusing the radiation beam to a focal spot on the optical disc. A problem arises during the design of these optical elements, for example an objective lens, as it is needed to ensure that the radiation beam being used to scan the type of optical disc is focused to a spot of sufficient quality on the optical disc.

This problem is caused in part by the different wavelength and numerical aperture of the radiation beam for scanning each type of disc, but also to a difference between an information layer depth of a transparent cover layer of a first and a second type of optical disc through which the radiation beam passes. This cover layer modifies the radiation beam passing through the cover layer. This modification is considered when designing the precise specifications of the objective lens so that a wavefront deviation is introduced into the radiation beam which compensates the modification by the cover layer and ensures that the focal spot achieved is of the highest quality. In the case of a CD and a DVD, the thickness of this cover layer is approximately 1.2 mm and 0.6 mm respectively. As a result, when an objective lens designed to focus a radiation beam for scanning a DVD to a focal spot is used to scan a CD, a wavefront deviation comprising a spherical aberration is introduced into the radiation beam for compensating the wavefront deviation introduced by the cover layer of the DVD. As the cover layer of the CD is of a different depth than that of the DVD the focal spot is of a reduced quality.

As described above, parameters of the optical system affecting the quality of the focal spot include environmental influences, for example a change in temperature, on the optical system. An optical system of this type generally comprises a collimator lens for modifying the vergence of the radiation beam scanning the optical disc and an objective lens for focusing the radiation beam to the focal spot on the optical disc. The optical system is designed for use at a standard operating temperature and the precise specifications of the optical elements including the collimator and the objective lens are determined based upon this standard temperature.

With a variation from this standard temperature the properties of the optical elements are affected, leading to a decrease in the quality of the focal spot of the radiation beam. In the case of the objective lens a change in temperature causes a refractive index of a material from which the lens is formed, a shape of the lens and a dimension of the lens to vary. Additionally this variation in temperature causes a slight change in a wavelength of the radiation beam being used to scan the disc to occur. The consequent decrease in the quality of the focal spot is typically in the form of a wavefront deviation comprising a spherical aberration of the radiation beam.

The unwanted aberrations arising when the wavelength varies in a certain range can be reduced using a periodic notched lens structure, as described in the article “An Objective with a Phase Plate”, Optics and Spectroscopy, volume 6 (1959) pp. 126-133 by A. Tudorovskii. The notched lens described in this article can be viewed as a combination of a normal lens and a diffractive structure. Each step in the lens introduces a phase step in the wavefront of the transmitted beam equal to a multiple of the wavelength. The precise method by which the unwanted aberrations can be reduced is discussed in full in the article. However, there is a significant drawback with using this method in the optical systems described above, namely that although the notched lens structures can make an optical system achromatic, they generally lead to a rather large number of small zones. This can make the structures difficult to manufacture, as a high degree of accuracy is required to maintain the fine periodic zones of the structure.

Use of a non-periodic phase structure (NPS) can reduce some of the problems associated with the notched lens device described above. An NPS has a phase structure comprising annular areas forming a non-periodic pattern of optical paths of different lengths. An NPS commonly introduces a wavefront deviation to a radiation beam passing through the NPS and may be used to modify or correct a wavefront deviation of a radiation beam by introducing a further wavefront deviation.

International patent application WO 01/48745 describes an optical head for scanning one type of optical record carrier. At a design temperature an objective lens is arranged to focus a radiation beam to a spot on the optical record carrier. At a temperature other than the design temperature the objective lens introduces a wavefront deviation into the radiation beam. An NPS is arranged to introduce a further wavefront deviation into the radiation beam such that the wavefront deviation introduced by the objective lens is reduced.

International patent application WO 02/082437 describes an optical scanning device for scanning optical record carriers of a first, second and a third different type with a radiation beam of a first, second or third different wavelength respectively. An objective system is provided for focusing the radiation beam upon the type of optical record carrier being scanned. Additionally an NPS is provided in a path of the radiation beam. The NPS approximates a flat wavefront for the first radiation beam, a spherical aberration wavefront at the second radiation beam and a flat or spherical aberration wavefront at the third radiation beam.

International patent application WO 02/29798 describes an optical device for scanning optical record carriers of a first and a second type with a first and a second radiation beam respectively. Each radiation beam has a different numerical aperture. Both devices include an NPS which does not affect the first radiation beam but introduces a spherical aberration into the second radiation beam. This introduced spherical aberration is for compensating a spherical aberration resulting when scanning through a difference in a cover layer thickness of the first and second optical record carriers.

International patent application WO 01/48746 describes an optical device for scanning optical record carriers where slight variations in wavelength emitted by a laser diode, usually caused by slight difference between laser diodes from different batches, are compensated for using a single NPS. Using radiation with a wavelength different from the wavelength for which the optical device was optimised will, in general, give rise to a certain amount of spherochromatism, hence a certain amount of spherical aberration proportional to the difference in wavelengths which will be corrected by the NPS.

It is a drawback of NPS devices, that although the NPS can correct for one variation in a given parameter, the lens and NPS together become sensitive to variations in other parameters. For example, if an NPS is used to compensate for thermal variations in the objective system, this can cause the optical device to become sensitive to wavelength variations. These wavelength variations may arise from the fact that the wavelength of the laser used in the optical system varies by a few nm from sample to sample.

In an optical pickup for optical recording, making the objective lens athermal with an NPS may cause unwanted wavefront aberrations depending on the wavelength of the laser used. This wavelength dependence is undesirable and can limit the application of the NPS for thermal correction.

Additionally, it is a further drawback of NPS devices that although an NPS can make an objective lens achromatic for zero field angles, at non-zero field angles the compensation becomes dependent on the field angle. As a result, the compensation at larger field angles is no longer optimal and a camera lens or a zoom lens made achromatic using an NPS is not optimal for large field of view systems.

It is one object of the present invention to provide improvements in performance of objective lenses in optical systems in order to overcome the above-mentioned limitations.

According to the invention there is provided an optical arrangement for interacting with a radiation beam, the optical arrangement comprising an optical system and a compensator, the compensator including a first optical element, the first optical element having a phase structure comprising stepped annular areas forming a non-periodic pattern of optical paths of different lengths, the compensator being arranged to generate:

a first wavefront deviation introduced by the variation of a first parameter during interaction of the radiation beam with the compensator, the first wavefront deviation being arranged to counteract a wavefront deviation introduced by the variation of the first parameter during interaction of the radiation beam with the optical system; and

a second wavefront deviation introduced by the variation of a second, different, parameter during interaction of the radiation beam with the compensator, characterised in that the compensator further includes a second optical element having a phase structure comprising stepped annular areas forming a non-periodic pattern of optical paths of different lengths, the second optical element being arranged to reduce said second wavefront deviation.

The effect of the first and second wavefront deviations is, in each case, quantifiable in terms of a root mean square optical path difference (RMS OPD) of the radiation beam. By reducing, in terms of the RMS OPD, the second deviation, the compensator has the effect of increasing the resolution of the beam when focused to a spot, under varying conditions, thus increasing tolerances of the system.

By use of the present invention, it is possible to compensate for the effect of variation of the first parameter, which may be a parameter such as temperature, angle of incidence, polarisation and wavelength of radiation, with the compensator without causing a substantial second wavefront deviation (which would otherwise be caused by the first optical element) when the second parameter, which may be another one of those listed above, is varied.