| 摘要 |
A wavelength plate including an organic thin film adhered to a glass substrate through an adhesive is built in an optical head optical system used in a predetermined using temperature range. The adhesive has a glass transition temperature higher than the predetermined using temperature range, and the linear expansion coefficient (K 1 ) of the adhesive, the linear expansion coefficient (K 2 ) of the organic thin film, and the linear expansion coefficient (K 3 ) of the glass substrate have the relation of K 3 <=K 1 and K 1 <=K 2 under the predetermined using temperature range. The linear expansion coefficient (K 1 ) of the adhesive, the linear expansion coefficient (K 2 ) of the organic thin film and the linear expansion coefficient (K 3 ) of the glass substrate can satisfy the relation of K 3 <K 1 <K 2 when the temperature within the optical head optical system is an ordinary temperature, and satisfy the relation of K 3 <K 2 <K 1 when the temperature within the optical head optical system is higher than the glass transition temperature of the adhesive. In the event of fluctuation of laser wavelength by temperature change, the phase difference changes so as to match with this wavelength fluctuation. In an optical head device, the above-mentioned phase difference plate is provided so that its arrangement position is changeable.
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