The present invention relates to an optical apparatus in which optical members such as lenses are fixed with respect to a holding frame and other lenses using laser light, and the manufacturing method thereof. 
Recently, digital cameras have become widespread such as to be mounted on mobile phones, and there have been growing demands for smaller sizes and higher accuracies thereof.  Moreover, as prices of digital cameras have gone down, it has become important to reduce the manufacturing cost by such as reducing the number of parts or improving the assembly process. 
On the other hand, although parts with higher accuracies are required as the number of parts of a digital camera is reduced, there is a limit on the finishing accuracy of parts and the man-hour for adjustment in the assembly process tends to be increased.  Particularly, since many adjustment processes are required in the assembly process of lenses which have a most significant effect on the camera performance, there is need for a assembly method of lenses which allows to perform adjustment with more ease and higher accuracies. 
Accordingly, there has been proposed a technique for adjusting the spacing of lenses in which the lenses are fixed at an optimal position by varying the position of a lens in the optical axis thereof by rotating a ring-like adjustment component provided with a plurality of lens receiving surfaces in the assembly process of lenses (see Japanese Patent Laid-Open No.  2005-49599). 
There has also been proposed a technique for adjusting the lens spacing in which a sheet-like spacing adjustment plate to be interposed between lenses is provided with a plurality of projection-and-depression shapes to follow the curved surface shape of a lens (see, for example, Japanese Patent Laid-Open No.  07-113936). 
Then, after adjusting the lens spacing, it is necessary to fix the lenses while keeping the positional accuracy of the lenses, but in a conventional fixing method by use of an adhesive, it would take time for the adhesive to cure.  For example, in the case of a typically used ultraviolet-curing adhesive, it would take not less than 10 seconds from coating until curing of the adhesive, and may take not less than one minute until the curing is completed. 
For this reason, there has been a problem in that the lens position is changed in the period from coating of an adhesive till curing thereof or the adhesive shrinks during curing, thereby causing a deviation of the fixing position of the lens. 
Then, in order to reduce the fixing time of lenses, there has been proposed a method in which a resin lens is provided with an infrared absorptivity and resin lenses or a resin lens and a resin lens frame are fixed in a few seconds using an infrared laser without using an adhesive (see Japanese Patent Laid-Open No.  2005-292441). 
However, in the above described Japanese Patent Laid-Open No.  2005-292441, it is necessary that resin lenses are provided with an infrared absorptivity, and the technique can not be applied to lenses which are not provided with an infrared absorptivity. 
Under such circumstances, the applicants of the present invention have proposed a method in which a hot-melt agent is disposed between a lens and a lens frame, and the lens frame and the hot-melt agent are subjected to laser irradiation to be instantly heated up so that the hot-melt agent is turned into an adhesive, thereby fixing the lens to the lens frame. 
According to this lens fixing method, it is possible to reduce the fixing time of lenses even when the lenses are not provided with an infrared absorptivity, thereby improving the positional accuracy of the lenses.  In the above described lens fixing method, when adjusting the spacing between the lens and the lens frame, it may be possible to adjust the spacing between the lens and the lens frame by changing the thickness of the hot-melt agent. 
However, when a hot-melt agent of a certain thickness is used to adjust the spacing between the lens and lens frame, it is necessary to change the laser irradiation condition to make the hot-melt agent exert an adhesive property appropriately. 
That is, since changing the thickness of hot-melt agent will change the proportion of laser absorption/scattering of the hot-melt agent itself, the intensity of laser which is transmitted through the hot-melt agent will be significantly varied, thereby making it necessary to change the condition of laser to be irradiated. 
In this case, if the adjustment spacing between the lens and the lens frame is in a certain range, it may be possible to cope with the above described situation, but if the spacing is changed for example from 20 μm to 40 μm, the irradiation energy needs to be increased by about two times, thereby leading to an upsizing of the laser apparatus.  Also, an attempt to cope with that situation by increasing the irradiation time of laser by about two times will also lead to the increase in the time needed for the assembly process of lens. 
Further, in order to produce a hot-melt agent for which laser irradiation condition needs not be changed even when the thickness thereof is varied, it is necessary to change the property of each hot-melt agent with a different thickness, which will lead to an increase in manufacturing cost. 
Moreover, since hot-melt agents have a lower rigidity compared with materials used for lens frames (for example, polycarbonate), and also have a large coefficient of thermal expansion, they are likely to undergo thermal deformation.  Therefore, as the thickness of the hot-melt agent increases, it is more likely to undergo dimensional changes, thereby increasing the risk of degrading the positional accuracy of the lens.