1.  Field of the Invention
The present invention relates to an optical signal pick-up device, especially to picking up data stored in compact disks with an optical pick-up head, transferring optical signals to a photo detector with dual light sources refracted by designed optical lens and the light path formed by it. 
2.  Background Description
Because of the rapid increase of multimedia content, there are more and more storage media needed and which meets this need are low-cost disc.  Both read-writable and single-read-writable disc are popular in enterprise, home users and personal users.  Therefore, there are a lot of types of disc are available in the market, such as CD, CD-R, CD-RW, DVD ROM, DVD-R/W, DVD+R/W, and so on.  However, there are different optical requirement in different disc types.  For example, the wavelength for CD (780 nm) and DVD (650 nm) is different, and as a result, there are usually two sets of laser diodes, lens, and photo detector in drives that can pick up signals in both CD and DVD.  Although the design is simple, the cost and assembly complexity are increased.  Moreover, although some drive manufactures have developed different designs of laser diode, lens, and photo detectors, there are always some problems remained.  To solve this problem, the invention presents a better-integrated solution, which can meet the criteria of cost, preciseness, and assembly. 
Firstly, referring to the FIG.  1, the conventional separate design provided different disc types separate optical devices.  As shown in the FIG.  1, the first optical pick-up device comprises the first objective lens 12, 780 nm laser diode 13, the first beam splitter 14 and the first photo detector, while the second optical pick-up device comprises the second objective lens 17, collimator 18, 650 nm laser diode 16, the second beam splitter 19 and the second photo detector 20.  The light path of the first optical pick-up device is similar to the one of the second optical pick-up device: the polarized light emitted by the laser diode is reflected by the beam splitter, passes through the objective lens, carries the signals in the disc 11, goes back through the objective lens and the beam splitter, and then is detected the optical signals by the photo detector. 
As we can see in the FIG.  1, although the idea of the separate design is simple, some components of this design, one of the photo detectors, and the rear circuit are redundant, which can be surmounted.  Therefore, one mechanism is added in another design to switch the optical lens in order to reach the optical requirement. 
As shown in the FIG.  2, the polarized lights emitted by two laser diodes 24, 25 with different wavelength (780 nm and 650 nm) is reflected by different beam splitters 26, 27, pass through the suitable objective lens 22, 23 and projects on the disc 21, carry the optical signals in disc 21 and pass through suitable objective lens 22, 23 and beam splitter 26, 27, and then detected by the single photo detector.  The adequate adjustment of the objective lens 22, 23 is due to the unique mechanism (not shown in the figure), which is used to switch the objective lens; however, due to the abrasion of the mechanism, the error of the design, and the different operation environment, the stability of this design is not good enough, also, the extra lens-adjustment mechanism increase extra cost, assembly time, and complexity. 
Another design can surmount the disadvantage of the FIG.  2.  That is, put different objective lens in the same light path and take the adjustment mechanism out, as shown in the FIG.  3.  This optical device comprises the first laser diode 33, the second laser diode 34, the first beam splitter 35, the second beam splitter 36, the collimator 32, the objective lens 31, and the single photo detector 37.  When data is picked up from different types of disc, different polarized lights are emitted by corresponding laser diode, pass through beam splitter, collimator, and objective lens, focus on the disc 30, and then feed back to the single photo detector 37.  The uniqueness of this design is that the objective lens 31 can reach different requirement based on different wavelength.  However, because the objective lens 31 needs special surface design and coating according to the optical characters, the price of the specific objective lens is high and the yield rate of it is low.  As a result, this design is not ideal for consumer products in terms of price.