As a method of increasing a capacity of an optical disc, there are a method of increasing a recording density and a multilayering method. Further, as a method of increasing the recording density, there are a method of decreasing a channel bit length, that is, a mark length, and increasing the density in a linear-density direction and a method of narrowing a track pitch. However, if the density is increased in the linear-density direction, a problem in which inter-symbol interference is increased occurs. Further, if the track pitch is narrowed, leak-in (adjacent track crosstalk) of information from an adjacent track is increased, and quality required for a tracking error signal and a characteristic required for a tracking servo are degraded.
Methods for reducing the adjacent track crosstalk (hereinafter simply referred to as “crosstalk”) have been proposed. Techniques of reducing influence of the crosstalk by spatially splitting reflected light from an optical recording medium into three in a track width direction, detecting each of the three into which the light is split, multiplying (weighting) the detection signal by a constant, and performing an addition operation are disclosed in Patent Literatures 1 and 2. Further, a technique capable of emphasizing and reproducing a reproduction signal of a small recording mark by further performing weighting in a beam traveling direction has been suggested in Patent Literature 1 as an idea.
Further, in the case of multilayering, in order to secure a sufficient recording light amount and a reproducing light amount for each layer, it is effective to keep coupling efficiency of an optical system high using only one beam in an outward path. A push-pull technique (abbreviated as a “PP technique”) has been known since the past as a method of detecting a tracking error using only one beam in an optical disc device which performs recording and/or reproducing on an optical disc having a groove structure on an optical disc recording surface, and an advanced push-pull technique (abbreviated as “APP technique”) disclosed in Patent Literature 3 and the like have been known as a method of suppressing an offset of a tracking error signal obtained by the push-pull technique which is caused by scrolling of an objective lens. In the case of a one-beam PP technique, the offset of the tracking error signal occurs at a boundary portion between an already recorded region (referred to as a “recorded region”) and a region in which recording is not performed yet (referred to as a “non-recorded region”). The offset in the recording boundary occurs when a reflectance distribution of the recording boundary is asymmetric to a track, and it becomes an amplitude grating with a period coarser than that of a track and diffracts a light beam collected on an optical disc.
Due to the occurrence of the offset, a recording characteristic and a reproducing characteristic deteriorate, and in the least favorable case, a tracking servo is not performed. In order to suppress such an offset, a technique of performing focus control such that an offset amount of a tracking error signal is adjusted so that a level of the tracking error signal in a track jump state becomes an intermediate level between a peak level and a bottom level is disclosed in Patent Literature 4.
Further, techniques of detecting a tilt of a recording medium on the basis of light intensity of an interference region between 0th-order light and 1st-order diffracted light are disclosed in Patent Literatures 5 and 6.