a cd system is taken for example . fig2 a is a schematic diagram of a pickup module according to an embodiment of the invention . the pickup module comprises a first laser diode 201 for dvd , a second laser diode 203 for cd , the first polarized beam splitter 205 with a reflective coating layer 204 , the second polarized beam splitter 207 with a reflective coating layer 206 , a folding mirror 209 with a reflective coating layer 208 , and a photo detector 211 . the first laser diode 201 and second laser diode 203 generate laser beams for dvd and cd . referring to fig2 a , after the laser diode 203 for cd emits a cd laser beam , orthogonal s - wave and p - wave components are generated , having initial intensities of respectively i s1 and i p1 . if an initial phase difference is δ s − p = 0 °, as shown in fig2 b , when the light beam enter the second polarized beam splitter 207 with the coating layer 206 , the reflectivity of the s - wave and the p - wave are respectively 0 % and 90 %, as shown - by solid arrows in fig2 c . when the light beam enters the folding mirror 209 with the coating layer 208 , reflectivity of the s - wave and the p - wave are respectively 70 % and 20 %, as shown by solid - arrows in fig2 d . in the embodiment of the invention , coating layers of 206 and 208 respectively generate phase differences of 70 ° and 200 °, as shown by dashed ellipses in fig2 c and 2d . optical characteristics of the second polarized beam splitter 207 and the folding mirror 209 are shown in table ii . table ii the second polarized beam folding splitter 206 mirror 208 with coating with coating reflectivity layer 207 layer 209 s - wave reflectivity 0 % 70 % p - wave reflectivity 90 % 20 % phase difference δ s − p 70 ° 200 ° thus , when cd laser beam enters the second polarized beam splitter 207 and the coating layer 206 , the coating layer 206 redistributes energy of the light beam and generates a phase difference of 700 between the s - wave and the p - wave . as a result , the cd laser beam is elliptically polarized and a long axis thereof modified to the proximity of the p - wave due to higher reflectivity of the p - wave for coating design . for example , if the phase difference δ s − p generated by coating layer 206 on the second polarized beam splitter 207 equals 70 °, intensities i s and i p respectively equal 0 . 15i s1 and 0 . 85i p1 after the cd light beam passes through the second polarized beam splitter 207 with the coating layer 206 , generating phase difference . if there is no coating layer with phase shift design , no s - wave component energy is retained after the light beam passes the optical component 207 . subsequently , the same principle can be utilized such that the folding mirror 209 with coating layer 208 redistributes and focuses most energy to the p - wave component . in the absence off coating layer 208 with phase - shift design , since no s - wave component energy is retained after the light beam passes through the second polarized beam splitter 207 , the energy in the original s - wave component is never used . for example , if the phase difference δ s − p of the second polarized beam splitter 207 with the coating layer 206 is 70 °, the intensities i s and i p respectively equal 0 . 15i s1 and 0 . 85i p1 after the cd light beam passes the second polarized beam splitter 207 with coating layer 206 for cd system , generating a phase difference . the coating layer 208 on the folding mirror 209 generates a phase difference δ s − p of 200 degrees , with most cd light beam energy focused on the p - wave component . as a result , the intensities i s and i p respectively equal 0 . 13i s1 and 0 . 10i p1 after the cd light beam passes the folding mirror 209 with the coating layer 208 , generating a phase difference . intensities i s1 and i p1 of the s - wave and p - wave components of the original light beam are equivalent . thus , the total energy of the light beam through the pickup module according to an embodiment of the invention is higher than in a conventional configuration . in addition , the optical components generates a total phase difference of δ s − p = 70 °+ 200 °= 270 ° or − 90 °, as shown in fig2 e . as a result , a circularly polarized state is obtained . the same principle is also used for dvd system . the folding mirror 209 reflects a dvd light beam and changes propagation direction thereof . the coating layers 204 , 206 , and 208 on the first polarized beam splitter 205 and second polarized beam splitter 207 and the folding mirror 209 change polarization of the dvd light beam and phase difference between the s - wave and p - wave . when the phase difference between the s - wave - and p - wave reaches 90 ° or 270 ° (− 90 °), the light beam is converted to a circularly polarized light . the photo detector 211 receives the dvd light beam reflected from a surface of a disc . the wavelength of a laser diode 201 for dvd is typically 660 nm . the coating layers 204 , 206 , and 208 on the first polarized beam splitter 205 the second polarized beam splitter 207 and the folding mirror 209 can provide phase difference with wavelength as shown in fig3 a , 3b and 3 c . thus , when the dvd laser beam passes through the first polarized beam splitter 205 with the coating layer 204 and the second polarized beam splitter 207 with the coating layer 206 , and the folding mirror 209 with the coating layer 208 , the total phase difference is as shown by the curve in fig3 d . when the wavelength of the dvd laser beam is 660 nm , the coating layers 204 , 206 , and 208 respectively generates phase differences of θ 1 , θ 2 , and θ 3 between the s - wave and p - wave such that θ 1 , θ 2 , and θ 3 are : accordingly , the pickup module converts light to a circularly polarized light beam . the effect thereof is the same as that of a quarter wavelength plate . as a result , no quarter wavelength plate is needed . those skilled in the art can add or remove optical components in the pickup module according to needs . optical components commonly used in the pickup module can be a laser diode , a beam splitter , a cubic , a grating , a folding mirror , a polarizer , and a collimator . the optical components respectively generate a phase difference of θ 1 , θ 2 , . . . , and collectively generate a total phase difference of ± 90 ° to obtain a circularly polarized light beam . in summary , the principle is used to take both the phase difference and efficiency into consideration by coating design to reach the better efficiency and phase shift . in addition , the invention converts a light beam to a state with higher transmissive or reflective efficiency by generating phase difference . in other words , selection of material , number of coating layers and thicknesses thereof are made according to the required phase difference and reflectivity / transmittance . during design of the optical components , as shown in fig4 , a formula as follows can be used . tan 2α = 2 i s0 i p0 cos θ sp / i s0 2 − i p0 2 ( 2 ) i s0 and i p0 respectively represent reflectivity or transmittance of the optical components with coating layers generating a phase difference . θ sp represents the phase difference between the s - wave and p - wave components , generated by the optical components with coating layers . the angle a represents an angle between a long axis of an elliptically polarized light and an s - wave axis . the optical component set according to the embodiment of the invention does not require a quarter wavelength plate or the other phase retarded plate for required purpose ( ½λ , ¼λ , ⅛λ . . . ), reducing cost thereof and avoiding problems resulting from poor quality or poor assembly thereof . in addition , phase difference is induced by combinations of different optical components with coating layers . as a result , coating layers on different optical components change polarization of a light beam such that transmittance or reflectivity is optimized , and signal intensity improved . while the invention has been described by way of example and in terms of preferred embodiment , it is to be understood that the invention is not limited thereto . to the contrary , it is intended to cover various modifications and the advantages would be apparent to those skilled in the art . therefore , the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications .