Optics and Lasers in Engineering ] (]]]]) ]]]–]]]

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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng

Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems Wen-Shing Sun a, Yan-Nan Lin a, Chuen-Lin Tien b,n, Chih-Hsuan Tsuei a, Chien-Cheng Kuo c, Jenq-Yang Chang a a

Department of Optics and Photonics, National Central University, Chung-Li 32001, Taiwan Department of Electrical Engineering, Feng Chia University, Taichung 40724, Taiwan c Thin Film Technology Center, National Central University, Chung-Li 32001, Taiwan b

a r t i c l e i n f o

abstract

Article history: Received 30 August 2011 Received in revised form 23 December 2011 Accepted 11 February 2012

We present a compact design for a unitary photo detector and single-path combination optical pickup head (OPH) which can be used for Blu-ray Disc, digital versatile disc and compact disc systems. The number of components in the OPH design can be reduced by focusing three discrete wavelength laser beams on the photo detector. The three laser diode chips and a photo detector can be encapsulated within an integrated optical unit (IOU), which is then combined with a compensator and rhomboid beam-splitter prism. The blazed holographic optical elements (HOEs) are designed for concentrating the diffracted light power to the 0th-order and the 1st-order to improve the round-trip efficiency. The simulation results, including the S-curve and optical efficiency, are compared for this combo-OPH with three IOUs. The optical efficiency of the proposed OPH for BD/DVD/CD systems as detected by the photo detector are 5.545%, 3.889%, and 3.517%, respectively. Our design results show that the combo-OPH with one IOU is better than the combo-OPH with three IOUs. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.

Keywords: Optical pickup head Integrated optical unit (IOU) Blazed holographic optical element Blu-ray disc

1. Introduction With the recent progress in information technology, the capacity and speed of optical storage systems have been continuously increasing, leading to a new generation of Blu-ray Disc (BD) devices [1]. Although BD technology has matured, CD-ROM devices must still offer multiple choices of disc reading function to be able to compete in the market. In other words, the optical pickup heads (OPHs) of new storage systems have to be capable of reading all previous types of disc systems. The development of combination OPHs with multiple laser wavelengths and numerical apertures (NAs) is necessary to ensure compatibility with BD, DVD and CD discs. Recently, there have been many studies regarding the development of OPH and objective lens designs for multiple wavelength applications [2–5]. In an OPH design several optical elements are combined inside a module that replaces the traditional optical components. For example, holographic optical elements (HOEs) can be used to replace both the beam-splitter (BS) prism and the cylindrical lens within an OPH system. Optical components like the laser diode, HOE, and photo detector integrated circuit (PDIC) can be packed

n

Corresponding author. E-mail address: [email protected] (C.-L. Tien).

inside a miniaturised package called the integrated optical unit (IOU) [6,7]. The utilisation of this type of module means that the volume and weight of the OPH can be minimised. The IOU, which has recently come to be universally used in the optical drives of DVD devices, is fabricated by integrating two lasers with two discrete wavelengths inside a single OPH [8–11]. This module can also be applied to make OPHs that are compatible with three different types of disc. There are two BS prisms and three IOUs in the combo-OPH with three wavelengths. Each IOU device has a laser diode, a photo detector and an HOE. However, this type of OPH has the drawback of the low diffraction efficiency of the HOE. The general groove profile of the HOE used in pickup heads has a 2-level binary structure leading to a 0th-order diffraction efficiency of about 70% and a 1st-order diffraction efficiency of 12% [12]. The round-trip efficiency is only about 8.4% when light passes through the HOE. However, the blazed type of HOE can be used to improve its round-trip efficiency. The blazed HOE is designed to concentrate the diffracted light power to the 0th-order and 1st-order. The round-trip efficiency of the blazed HOE reaches roughly 16% [13]. Our proposed design approach will improve the total optical efficiency of the combo-OPH. In order to reduce the number of components in an OPH system to improve convenience of fabrication and alignment, we suggest encapsulating three laser diode chips and photo detectors within an IOU. In 2009, Shih et al. utilised a single-path OPH with

0143-8166/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

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three wavelengths, using an IOU to reduce the number of photo detectors required [14]. They used two BS prisms, two laser diodes and one IOU device in the OPH. Their IOU device had one 780 nm laser diode and two photo detectors within it. They used the HOE to deflect three wavelength beams onto two photo detectors. Their OPH design can be reduced to use two photo detectors. Here, we want to further reduce the number of photo detectors in the unit for the OPH compact design. In general, the wavelength of the spectral response in the silicon photo detector series is specified to be in the range of 190– 1100 nm [15]; thus the silicon photo detector could detect three discrete wavelengths (405 nm, 650 nm, and 780 nm) of light. This allows the number of photo detectors in the combo-optical pickup head to be decreased to one. In this work, we propose a new design for a unitary photo detector and single-path combo-OPH to be used for BD, DVD and CD systems. Only one photo detector is needed in this OPH design. The main components, including three laser diode chips and a photo detector, can be encapsulated within an integrated optical unit (IOU), which simplifies the assembly process. After describing the OPH design, the simulation results, including the S-curve, beam shapes for different disc positions, and optical efficiency, are presented. Finally, we analyse and compare the proposed comboOPH design with the combo-OPH with three IOUs.

2. System description Fig. 1(a) shows the layout of the unitary photo detector and single-path combo-OPH for BD, DVD and CD systems. Two rhomboid beam-splitter (RBS) prisms are used to produce three laser beams with three different wavelengths that follow one optical axis. Therefore, after passing through the two RBS prisms, the three laser beams emitted from individual laser diode chips can fall incident on a collimating lens. After each beam passes through a collimating lens it is reflected by a mirror. A quarterwave plate, with a bandwidth from 400 nm to 800 nm, is added behind the mirror [16]. The beam emitted from the laser diode is approximately 90% polarised, meaning that the directions of polarisation along the forward path and backward path will become orthogonal to each other after passing forward and backward through the quarter-wave plate. This may reduce the feedback-induced noise. The incidence of the beam through the wavelength selector (WS) is matched to the numerical apertures (NAs) for different types of discs. The different wavelength signals are focused on the disc by a common objective lens. Along the backward path of the OPH, the beams back-reflected from the disc are deflected by the blazed HOE, to be focused on the common photo detector. The number of components needed in the proposed combo-OPH can be decreased by integrating the three laser diode chips and one photo detector within a single IOU. The laser diode chips in this combo-OPH were adopted edge-emitting laser diode chips. Three 451 mirrors were placed in front of their own laser diode chips for redirecting the laser beam by 901 to vertically enter the system. The 405 nm and 650 nm laser diode chips are in back of their own 451 mirrors and emitting light from the direction out of the paper, as shown in Fig. 1(b). The 780 nm laser diode chip is in front of 451 mirror and emitting light from the direction into the paper. 2.1. Rhomboid beam-splitter (RBS) prism An RBS prism is used to adjust the position of the light source for the different wavelengths and positions of the laser diodes [17,18]. Due to the different positions of the three laser diode chips, two RBS prisms are needed to align signals with different

Fig. 1. (a) Layout of the unitary photo detector and single-path combo-OPH for BD, DVD and CD systems; (b) configuration of the IOU in the combo-OPH.

wavelengths on the same optical axis for BD, DVD and CD devices. The design is shown in Fig. 2. The dimensions of the RBS prisms are 5.0  2.5  2.5 mm3. The two optical axes are spaced 2.5 mm apart. The two RBS prisms are placed perpendicular to each other to avoid deflection of the three laser beams on the backward path of the common photo detector by the blazed HOEs. The 405 nm laser beam penetrates the two RBS prisms directly. The 650 nm laser beam is reflected by the mirror and interface A of the first RBS prism, and then penetrates the second RBS prism, as shown in Fig. 2(a). The 780 nm laser beam is reflected by the mirror and interface B of the second RBS prism, as shown in Fig. 2(b). The thin film coating on interface A in the first RBS prism reflects the 650 nm wavelength for DVD applications, and transmits the 405 nm wavelength for BD applications. The thin film of interface B on the other RBS prism reflects the 780 nm wavelength for CD applications, and transmits the 650 nm for DVD applications and the 405 nm wavelengths for BD applications. 2.2. Wavelength selector (WS) The different wavelengths of light and different NAs for the objective lens are necessary for BD, DVD and CD devices. A WS, composed of a glass plate with three regions, C, D, and E, is needed in front of the objective lens [17,18], as shown in Fig. 3. Since the CD, DVD and BD pickup heads use light sources of different wavelengths, the NAs of the corresponding objective lens will be different. Region C is transparent to the BD (405 nm),

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

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Fig. 2. Optical paths of the three-wavelength beams through two RBS prisms corresponding to the: (a) view on the xz plane; (b) view on the yz plane; and (c) the 3-D layout.

Fig. 3. Configuration of the WS for the BD, DVD and CD systems.

DVD (650 nm) and CD (780 nm) wavelength signals, region D transmits the BD and DVD wavelength signals, while the CD signal is reflected. In region E, the CD and DVD wavelength signals are reflected, while only the BD signal is transmitted. With this design, the NAs of the objective lens must correspond to the different CD, DVD and BD signals.

2.3. Blazed holographic optical element We adopt the commonly used astigmatic detection technique for the OPH. The blazed HOE has the function of splitting and focusing the returning laser beam to generate an astigmatic focusing error signal. The wavefront of the diffraction pattern can be represented by the following phase polynomial:

jðx,yÞ ¼

M X N 2p X

lo

m¼0n¼0

amn xm yn

ð1Þ

Usually, we take ten terms of the phase polynomial so that

jðx,yÞ ¼

2p

lo

ða00 þ a10 x þa01 yþ a20 x2 þ a11 xy þa02 y2 þa30 x3

þ a21 x2 y þa12 xy2 þa03 y3 Þ

ð2Þ

where a00 denotes a constant phase; a10x provides the diffraction angle in the x-direction; a01y provides the diffraction angle in the ydirection; a20x2, a11xy and a02y2 serve as the combination of a focusing lens and a cylindrical lens that converges the beam and generates the astigmatism; the remaining terms are used for correcting the coma, spherical and high-order aberrations. All the coefficients of the phase polynomial can be calculated and optimised by ray tracing. The coefficients of x and y form the off-axis term, but the period (L) of blazed HOE can be calculated by

lo L ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ð3Þ

ða10 Þ2 þ ða01 Þ2

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

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Fig. 4. Schematic cross-section of a blazed HOE.

where a10 and a01 are the coefficients of x and y, respectively; and lo is the wavelength of the incident light. The angular separation of the diffraction orders is controlled by the period (L) of blazing in the HOE. However, the diffraction efficiency for different orders is determined by the depth (d) of the HOE blazing, as schematically illustrated in Fig. 4. The diffraction efficiency is calculated based on the scalar diffraction theory [19]. When the beam is normally incident on the blazed HOE grating, the mth-order diffraction efficiency (Zm) is (  )2 sin pðkmÞ  Zm ¼  ð4Þ pðkmÞ The depth of blazed HOE groove profile can be expressed as k¼

dðn1Þ

ð5Þ

lo

where n is the refractive index of the blazed HOE. The blazed HOE is utilised in the OPH. All beams emitted from the laser diodes are diffracted by the blazed HOEs twice (the 0thorder on the forward path and the 1st-order on the backward path). The blazed HOE is designed to concentrate as much of the diffracted light power as possible in the 0th-order and the 1st-order. The round-trip diffraction efficiency of the blazed HOE (Z) is  2 sinðpkÞ sin½pðk1Þ Z ¼ Z0 Z1 ¼ ð6Þ ðpkÞ ½pðk1Þ Using Eq. (6), the maximum round-trip diffraction efficiency of the blazed HOE is calculated to be 16% when k is set 0.5. Furthermore, the diffraction efficiency is about 40% for the 0th-order and 40% for the 1st-order.

3. Optical design for a unitary photo detector and single-path combo-OPH The optical path design of the proposed combo-OPH is based on the specifications of the BD system. In the objective lens design, the numerical aperture of the objective lens (NAo) is 0.85 [4,5]. The numerical aperture is a characteristic parameter of an optical system defined by NAo ¼ no sin yo ¼ no

Do 2f o

ð7Þ

where no is the refractive index of the disc; yo is half the focus angle; Do is the clear aperture of the objective lens; and fo is the effective focal length (EFL) of the objective lens. In our objective

lens design, the EFL and diameter are 3.12 mm and 3.27 mm, respectively. However, the required specifications for the NAs of the objective lens will differ for the BD, DVD, and CD. The NAs of the objective lens for the BD, DVD, and CD are 0.85, 0.6, and 0.45, respectively. For the WS design, Eq. (7) can be used to calculate the clear apertures for the BD, DVD and CD system, due to the NA matching in these systems, which are 3.27 mm, 2.31 mm and 1.73 mm, respectively. In the collimating lens design, the numerical aperture of the collimating lens (NAc) should match the half emitting angle of the laser diode. We found the normalised intensity to be at 1/e2 when the angle in the horizontal direction is 7.1611. In order to ensure that the beam shape on the disc is circular, the numerical aperture of the collimating lens is defined as NAc ¼ sin7:1611 ¼

Dc ¼ 0:125 2f c

ð8Þ

where Dc is the aperture of the collimating lens; fc is the EFL of the collimating lens. The tracking error of the objective lens should be kept within70.3 mm, meaning that the aperture size of the collimating lens will be Dc ¼ Do þ0.6 mm. The OPH for the BD system is established by defining NAc and NAo. In the OPH design for DVD and CD systems, there will be additional spherical aberrations on the optical paths, because the same collimating and objective lenses are used. In addition, the objective lens cannot focus the 650 nm and 780 nm laser beams on the DVD and CD discs lens, because the thicknesses of their cover layers are different from that of the BD disc. The thicknesses of the cover layers of the BD, DVD, and CD discs are 0.1 mm, 0.6 mm, and 1.2 mm, respectively [4]. This problem can be solved by changing the spacing between the objective lens and the disc. When this is done the 650 nm and 780 nm laser beams can be focused on the DVD and CD discs by the objective lens. In addition, the 650 nm and 780 nm laser diode chips are not at the focal point of the collimating lens. The length of the optical path from the laser diode chip to the collimating lens for the 650 nm and 780 nm laser beams are different from that for the 405 nm laser beams. It is thus impossible for these beams to be collimated by passing through the collimating lens. These extra aberrations in the DVD and CD systems are resulted that their overall optical qualities are worse than that of the BD system. The compensators in the DVD and CD systems are adopted to compensate for these aberrations, including additional spherical aberrations on the optical paths and aberrations caused by defocusing of the 650 nm and 780 nm light. These compensators made of plastic lens with an aspherical surface have specified optical powers to correct the aberrations. The optical qualities of

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

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the DVD and CD systems will thus be as good as those of the BD system. Another advantage is that the compensator and the RBS prism can be combined together, and fabricated from plastic, as shown in Fig. 5. In recent years, plastic prisms and beam-splitters have been produced for the commercial market [20,21]. Rhomboid prisms with aspherical surfaces can be fabricated by plastic injection methods. This technology makes it possible to replace the beam-splitter prism and compensator by an RBS prism, thereby reducing the number of optical components in the combo-OPH.

Fig. 5. Combination of RBS prisms and compensators for DVD and CD systems.

5

Fig. 6 shows the position of the laser diode chips and the photo detector in the IOU of the combo-optical pickup head corresponding to the: (a) view on the xz plane; and (b) view on the xy plane. The two laser diode chips are spaced 2.5 mm apart. The photo detector is located at the centre of the three laser diode chips but separated from them by 1.25 mm in the x- and y-directions. The enlargement of Fig. 6(b) shows the six detection areas on the photo detector. Regions F to I comprise the quadrant photo detector for astigmatic focusing detection. A focusing error signal (FES) is output based on the distribution of the beam spot between F, G, H and I. Region J to region K are used for detection of the three-beam tracking error signal in the CD system. For BD and DVD systems, it is necessary to use the differential phase detection (DPD) method for tracking error signal (TES) detection. However, the coma aberration caused by the shifting of the objective lens is an issue in DVD and CD systems. The coma aberration arises from the movement of the objective lens along the radial direction of the disc and it affects the accuracy of both FES and TES. This problem could be solved by using the method of dynamic coma aberration correction reported by Katayama and Komatsu [4,5]. Since the photo detector is not positioned on the optical axis, blazed HOEs are used to deflect light in the direction of the photo detector. Fig. 7 shows the configuration of the blazed HOEs and the IOU: (a) view on the xz plane; and (b) view on the yz plane. The solid coloured lines represent the forward paths of the three

Fig. 6. Positions of laser diode chips and photo detector corresponding to the: (a) view on the xz plane; (b) view on the xy plane.

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

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3 are around 3.43 mm and 4.11 mm, respectively. The periods of the three blazed HOEs are acceptable for standard fabrication methods but they cannot be encapsulated within a single IOU because of the 1.25 mm spacing in the x- and y-directions between the photo detector and three laser diode chips. This spacing restricts the dimensions of the RBS prisms, as shown in Fig. 2. If the three blazed HOEs are encapsulated within the IOU, the smallest period of blazing on the HOE will be less than 1 mm, which may not be possible with standard fabrication methods. Based on the periods of the blazing on the HOEs, the maximum round-trip efficiencies are calculated using the Gsolver software. When the depth of the blazing for HOE 1 is set to 386 nm, the round-trip efficiency is 15.656% (Z0 ¼42.675% and Z1 ¼36.687%). When the depth for blazed HOE 2 is set to 644 nm, the round-trip efficiency is 15.726% (Z0 ¼42.631% and Z1 ¼36.888%). When the depth for blazed HOE 3 is set to 775 nm, the round-trip efficiency is 15.733% (Z0 ¼42.958% and Z1 ¼36.623%). Regarding the optical design of the combo-OPH systems, the specifications for the BD, DVD and CD systems are shown in Tables 2–4, respectively. There are some aspheric surfaces to be used such as the collimating lens, objective lens and compensating surfaces on RBS prisms. The equation for an aspheric surface can be expressed as s¼

cv y2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ Ay4 þ By6 þ Cy8 þ Dy10 1 þ 1ð1 þ KÞc2v y2

ð9Þ

where s represents the surface sag; cv is the vertex curvature of the lens surface; R¼1/cv; R is the curvature radius; y is the vertical distance from any point on the surface to the optical axis of revolution; K is the conic constant; A, B, C and D are the high order coefficients of the aspheric surface. The design method has been reported in a previous publication [22].

4. Simulations and comparative analysis

Fig. 7. Configuration of blazed HOEs and the IOU corresponding to the: (a) view on the xz plane; (b) view on the yz plane.

After designing the unitary photo detector and single-path combo-OPH, a simulation is carried out. The results, including optical properties and optical efficiency are then compared to those obtained for a comparable OPH with three IOUs. 4.1. Combo-OPH with three IOUs

Table 1 Coefficients of the blazed HOE phase profile obtained using the astigmatic focusing detection method.

Construction wavelength (nm) Coefficient on x1y0 (a10) Coefficient on x0y1 (a01) Coefficient on x2y0 (a20) Coefficient on x1y1 (a11) Coefficient on x0y2 (a02) Coefficient on x3y0 (a30) Coefficient on x2y1 (a21) Coefficient on x1y2 (a12) Coefficient on x0y3 (a03)

blazed HOE 1

blazed HOE 2

blazed HOE 3

405 0.134247 0.134247  0.001339  0.002363  0.001339  0.000723  0.000699  0.000699  0.000723

650  0.134134 0.134134  0.001306  0.007748  0.001306 0.000726  0.000701 0.000701  0.000726

780 0.134113  0.134113  0.001311  0.007761  0.001311  0.000727 0.000702  0.000702 0.000727

laser beams and the dotted lines the backward paths. HOE 1 is used in the BD system; HOE 2 is used in the DVD system; and HOE 3 is used in the CD system. The three laser beams are deflected along the backward path and focused on the photo detector. Table 1 gives the optimised coefficients for the blazed HOE phase profiles obtained using the astigmatic focusing detection method. The period for the blazed HOE 1 plate is calculated with Eq. (3) to be around 2.13 mm. The periods for HOE 2 and HOE

Fig. 8 shows a schematic representation of the combo-OPH with three IOUs, for the BD, DVD, and CD systems. The IOUs used for 405 nm and 650 nm light both include a laser diode, a HOE plate, and a photo detector. However, the component for the 780 nm IOU has an additional three-beam grating because of the three-beam tracking error signal detection needed for the CD system. This configuration is simple and is often used in combo optical pickup heads. Fig. 9(a) shows the FES, the so-called S-curve, for the comboOPH with three IOUs. The S-curves for the three OPH operations are symmetrical, with linear ranges of 3.5 mm (BD system), 7.5 mm (DVD system) and 9.5 mm (CD system). This meets the requirements of linear range for all systems. Because the optical path of the OPH is designed to meet the specifications of the BD system, it is better that the maximum and minimum intensity of the normalised FES for the BD system be close to þ1 and 1, respectively, rather than the normalised FES corresponding to the DVD and CD systems. Figs. 9(b)–(d) show the beam shapes on the photo detector with varied disc positions obtained with the combo-OPH with three IOUs corresponding to the BD, DVD, and CD systems, respectively. Table 5 shows the optical efficiency of the combo-OPH with three IOUs. The three HOEs in this kind of OPH still use a 2-level

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

W.-S. Sun et al. / Optics and Lasers in Engineering ] (]]]]) ]]]–]]]

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Table 2 Lens design data for the BD system. Number

Surface

0

Object

1

Plane

2

Plane

3

Plane

4

Plane

5

Plane

6

Plane

7

Plane

8

Plane

9

Plane

10

Plane

11

Asphere

12

Plane

13

Plane

14

Plane

15

Plane

16

Plane

17 (stop)

Asphere

18

Asphere

19

Plane

20

Image

Collimating lens Surface #11 K=  0.2506 A=  5.9088  10  5 B=  5.1621  10  7 C=  2.0097  10  8 D=1.2538  10  9

Medium

Air

Radius (mm) N

Length (mm)

Optical material

0.1400

N Air Glass Air Plastic Air Plastic Air Plastic Air Plastic Air Air Glass Air Glass Air Glass Air Plastic

N N N N N N N N N 8.3946 N N N N N 1.7622  8.2223 N

 4.8600

(451 mirror)

 1.0000

B270_Schott (cap glass)

 3.4181  1.0000

Z-480R (blazed HOE 1)

 0.2000  2.5000

Z-480R (RBS prism 1)

 0.2000  2.5000

Z-480R (RBS prism 2)

 1.0000  1.5000

Z-E48R (collimating lens)

 3.0000 3.0000

(451 mirror)

0.7400

Crystalline quartz (quarter-wave plate)

0.2000 0.5000

B270_Schott (WS)

0.1500 2.0000

MNBFD130 (objective lens)

0.8718 0.1000

P-CARBO (cover layer)

N Objective lens Surface #17 K=  0.2857 A=  9.3624  10  3 B=  3.3183  10  3 C=  7.6986  10  4 D=  1.9344  10  4

Surface #18 K=5.4551 A=  2.1602  10  3 B=  6.9523  10  4 C=  2.0555  10  4 D=9.1492  10  5

Centre wavelength: 405 nm NA=0.85

Table 3 Lens design data for the DVD system. Number

Surface

0

Object

1

Plane

2

Plane

3

Plane

4

Plane

5

Plane

6

Asphere

7

Plane

8

Plane

9

Plane

10

Plane

11

Plane

12

Plane

13

Plane

14

Asphere

15

Plane

16

Plane

17

Plane

18

Plane

19

Plane

20 (stop)

Asphere

21

Asphere

22

Plane

23

Image

RBS prism 1 Surface #6 K=0 A=  2.7995  10  2 B=7.8111  10  3 C=0 D=2.2560  10  4

Medium

Air Air Glass Air Plastic Air Plastic Plastic Plastic Plastic Air Plastic Air Plastic Air Air Glass Air Glass Air Glass Air Plastic

Radius (mm) N N N N N N N N N N N N N N 8.3946 N N N N N 1.7622  8.2223 N

Length (mm)

Optical material

0.1400  4.8600

(451 mirror)

 1.0000

B270_Schott (cap glass)

 3.4181  1.0000

Z-480R (blazed HOE 2)

 0.2000  0.1000

Z-480R (RBS prism 1)

 1.2400

Z-480R (RBS prism 1)

2.5000

Z-480R (RBS prism 1)

 1.2500

Z-480R (RBS prism 1)

 0.2000  2.5000

Z-480R (RBS prism 2)

 1.0000  1.5000

Z-E48R (collimating lens)

 3.0000 3.0000

(451 mirror)

0.7400

Crystalline quartz (quarter-wave plate)

0.2000 0.5000

B270_Schott (WS)

0.1500 2.0000

MNBFD130 (objective lens)

0.5921 0.6000

P-CARBO (cover layer)

N Collimating lens Surface #14 K=  0.2506 A=  5.9088  10  5 B=  5.1621  10  7 C=  2.0097  10  8 D=1.2538  10  9

Objective lens Surface #20 K=  0.2857 A=  9.3624  10  3 B=  3.3183  10  3 C=  7.6986  10  4 D=  1.9344  10  4

Surface #21 K=5.4551 A=  2.1602  10  3 B=  6.9523  10  4 C=  2.0555  10  4 D=9.1492  10  5

8

W.-S. Sun et al. / Optics and Lasers in Engineering ] (]]]]) ]]]–]]]

Table 4 Lens design data for the CD system. Number

Surface

0

Object

1

Plane

2

Plane

3

Plane

4

Plane

5

Plane

6

Plane

7

Plane

8

Asphere

9

Plane

10

Plane

11

Plane

12

Plane

13

Plane

14

Asphere

15

Plane

16

Plane

17

Plane

18

Plane

19

Plane

20 (stop)

Asphere

21

Asphere

22

Plane

23

Image

RBS prism 2 Surface #8 K=0 A=  2.5697  10–2 B=8.9004  10–3 C=0 D=  1.8548  10–3

Medium

Air Air Plastic Air Glass Air Plastic Air Plastic Plastic Plastic Plastic Air Plastic Air Air Glass Air Glass Air Glass Air Plastic

Radius (mm) N N N N N N N N N N N N N N 8.3946 N N N N N 1.7622  8.2223 N

Length (mm)

Optical material

0.1400  0.2600

(451 mirror)

 1.0000

Z-480R (grating)

 3.6000  1.0000

B270_Schott (cap glass)

 3.4181  1.0000

Z-480R (blazed HOE 3)

 2.8000  0.1000

Z-480R (RBS prism 2)

 1.2400

Z-480R (RBS prism 2)

2.5000

Z-480R (RBS prism 2)

 1.2500

Z-480R (RBS prism 2)

 1.0000  1.5000

Z-E48R (collimating lens)

 3.0000 3.0000

(451 mirror)

0.7400

Crystalline quartz (quarter-wave plate)

0.2000 0.5000

B270_Schott (WS)

0.1500 2.0000

MNBFD130 (objective lens)

0.2156 1.2000

P-CARBO (cover layer)

N Collimating lens Surface #14 K=  0.2506 A=  5.9088  10–5 B=  5.1621  10–7 C=  2.0097  10–8 D=1.2538  10–9

Objective lens Surface #20 K=  0.2857 A=  9.3624  10–3 B=  3.3183  10–3 C=  7.6986  10–4 D=  1.9344  10–4

Surface #21 K=5.4551 A=  2.1602  10–3 B=  6.9523  10–4 C=  2.0555  10–4 D=9.1492  10–5

Fig. 8. Schematic representation of the combo optical pickup head with three IOUs.

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

W.-S. Sun et al. / Optics and Lasers in Engineering ] (]]]]) ]]]–]]]

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Table 5 Optical efficiency of the combo-OPH with three IOUs. Item

BD system (%)

DVD system (%)

CD system (%)

Laser diode chip The second BS Collimating lens Objective lens Disc Photo detector

100 38.132 25.842 17.784 17.784 3.321

100 33.667 25.970 17.046 17.046 2.533

100 28.686 23.395 14.490 14.490 2.090

efficiency of the three-beam grating for the CD system is 90%. These results are taken into consideration in the optical efficiency analysis of the combo-OPH with three IOUs, as shown in Table 2. The reflectivity for the BD, DVD, and CD discs is assumed to be 100%. In the BD system, the 405 nm laser diode emits an optical efficiency of 100%. After the laser beam passes through the HOE and the BS prisms, the optical efficiency decreases to 38.132%, and then to 25.842% after passing through the collimating lens. As mentioned in Section 3, the numerical aperture of the collimating lens (NAc) is set to the emitting angle (7.1611) of the laser diode. Therefore, the optical power in the spatial region within the numerical aperture of the collimating lens (NAc ¼0.125) is 56.464% of the total emitted power. If the 0th-order diffraction efficiency of the HOE is taken into consideration, it is close to the optical efficiency across the collimating lens. When the laser beam passes through the collimating lens, it forms a parallel beam. However, the optical efficiency across the objective lens is 17.784%. This loss of energy by the laser beam as it moves from the collimating lens to the objective lens occurs, because of the track seeking of the pickup head and the quarterwave plate. The aperture diameter of the collimating lens is 0.6 mm larger than that of the objective lens. The transmittance of the quarter-wave plate is 99% [16]. Finally, the simulated optical efficiency on the BD disc is 17.784% and the total round-trip operation efficiency for the BD system is 3.321%. The simulated optical efficiency for the DVD system is 17.046% and the total roundtrip operation efficiency is 2.533%. The simulated optical operation efficiency for the CD system is 14.490% and the total round-trip efficiency is 2.090%. The total round-trip efficiencies for all systems are less than 4% owing to passing through the HOE twice. The round-trip efficiency of the 2-level binary HOE is only 9.414%. Therefore, if we want to improve the optical efficiency of this type OPH, the key is to increase both the 0th-order and 1st-order diffracted efficiency of the HOE. 4.2. Unitary photo detector and single-path combo-OPH

Fig. 9. (a) Simulated FESs for the combo optical pickup head with three IOUs, and beam profiles with varied disc position corresponding to the: (b) BD, (c) DVD, and (d) CD system.

binary structure. The optimal maximum round-trip efficiencies of the three HOEs are as follows: for the 405 nm light, the maximum round-trip efficiency is 9.414% and the 0th-order and the 1st-order diffracted efficiencies are 49.030% and 19.201%, respectively. The maximum round-trip efficiencies for the HOEs used in the 650 nm and 780 nm light IOUs are both 9.414%. Furthermore, the three-beam grating for the CD system allows 65% of the incident light to remain in the 0th-order and diffracts 12.6% of the incident light into each þ1 and  1 diffracted order. The total optical

Fig. 10(a) shows the FES for the unitary photo detector and single-path combo-OPH. These S-curves for the three OPH operations are symmetrical with linear ranges of 3 mm (BD), 7 mm (DVD) and 9 mm (CD). This meets the linear range requirements for all systems. Since the optical path of the OPH is designed to meet the specifications of a BD system, it is better that the maximum and minimum intensity of the normalised FES corresponding to the BD system be close to þ1 and  1, respectively, than that of the normalised FES corresponding to the DVD and CD systems. The beam profiles on the photo detector for the unitary photo detector and single-path combo-OPH with varied disc positions corresponding to BD, DVD, and CD systems are shown in Figs. 10(b)–(d), respectively. The variation in the wavelength of the laser diode remains an issue. The drawback of the IOU design for astigmatic focusing detection is wavelength variation, which comes from the temperature change. The variation of wavelength during diffraction will result in decentring of the beam spot on the photo detector. This shift may cause deterioration in the FES and increase in the

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

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W.-S. Sun et al. / Optics and Lasers in Engineering ] (]]]]) ]]]–]]]

Fig. 10. (a) Simulated FESs for the unitary photo detector and single-path combo optical pickup head, and beam profiles with varied disc position corresponding to the: (b) BD, (c) DVD, and (d) CD system.

tracking error signal. In order to avoid a wavelength shift of several nanometres due to temperature effect, the spot-size detection and knife-edge methods are often used in two different optical paths of the OPH design [23,24]. However, these methods are not suitable for our proposed design with the common path. It is well known that laser diode central wavelength stability is directly related to temperature stability. To solve this problem, one approach is to integrate an auto-power compensation circuit

Fig. 11. Optical efficiency of the unitary photo detector and single-path comboOPH corresponding to the: (a) BD, (b) DVD, and (c) CD system.

for the OPH [25]. Another approach is to use a temperatureinsensitive laser diode in the proposed combo-OPH, and to obtain the desired stability by a thermoelectric cooling and temperature control unit with sufficient heat conduction from laser diode chip location to the ambient temperature. Fig. 11 shows the results for the operation and optical efficiency of the unitary photo detector and single-path combo-

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007

W.-S. Sun et al. / Optics and Lasers in Engineering ] (]]]]) ]]]–]]]

OPH for the: (a) BD, (b) DVD and (c) CD systems. The simulated efficiencies for various components in the whole system are similar to the description in Section 4.1. For the 405 nm light, the simulated optical efficiency is 15.652% on the BD disc and 5.545% on the photo detector. For the 650 nm light, the simulated optical efficiency is 13.939% on the DVD disc, and 3.889% on the photo detector. For the 780 nm light, the simulated optical efficiency is 12.070% on the CD disc and 3.517% on the photo detector. The simulation results show that the optical efficiencies on the disc for all systems are different, because of the wavelength selector (WS). The diameter of the WS for the BD system is larger than that for the DVD system, and the WS diameter for the DVD system is larger than that for the CD system. This is the reason that the optical efficiency is higher for the BD disc system than for the DVD/CD disc systems. Furthermore, the optical efficiency is also higher for the BD system than that for the DVD and CD systems. According to the optimised results, the roundtrip efficiencies for the three blazed HOEs are almost the same. This is because the optical path of the proposed combo-OPH is based on the specifications for a BD system. The 650 nm and 780 nm signals still lose energy in the transfer process of the backward path, although the compensators in the DVD and CD systems ensure that their optical qualities are as good as those of the BD system. The simulation results are compared with for the combo-OPH with three IOUs. The linear regions of the S-curves for the whole system of the unitary photo detector and single-path combo-OPH are similar to those for the combo-OPH that with three IOUs. The linear range requirements are met for all systems, and the combo OPH has the potential to be practical. On the other hand, the optical efficiency on the photo detector of the unitary photo detector and single-path combo-OPH is overall higher than that of the combo-OPH with three IOUs. The round-trip diffraction efficiency of the blazed HOE is nearly two times that of the 2-level binary type HOE. The three blazed HOEs provide an improvement in optical efficiency over the unitary photo detector and single-path combo-OPH. The design with the blazed HOE is helpful for concentrating the energy of the specific order of diffracted light. A comparison of the optical elements of the two OPH systems shows there to be fewer optical elements in the unitary photo detector and single-path combo-OPH than in the combo-OPH with three IOUs. The combination of the compensator with the RBS prism, and the three laser diode chips and one photo detector in an IOU device can help to reduce the number of optical elements. This design has the potential to simplify the assembly process, and the tolerance of alignment can also be expected to be reduced. The physical dimensions of the final product can also be downsized.

5. Conclusions This study presents a compact design for a unitary photo detector and single-path combo-OPH for BD, DVD and CD systems. Our design takes advantage of already existing semiconductor techniques, from which the whole setup, including the laser diode chips, blazed HOEs, and corrected aspherical surfaces on RBS prisms can be assembled. The photo detector of the compact combo-OPH design can be reduced to one unit. Furthermore, three laser diode chips and one photo detector can be encapsulated within a single IOU, which helps to reduce the number of components. The blazed HOEs are designed for concentrating the power of the diffracted light to the 0th-order and 1st-order, to improve the round-trip efficiency. The optimised round-trip efficiencies of the three blazed HOEs are 15.656%,

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15.726% and 15.733%, respectively. The simulation results for the unitary photo detector and single-path combo-OPH are higher overall than those obtained with the combo-OPH with three IOUs. In addition, this design has the potential to simplify the assembly process, to downsize the physical dimensions of the final product and to reduce the cost for BD, DVD and CD pickup applications. We feel that the design has the potential to be put into practical use in future.

Acknowledgements This study was sponsored by the National Science Council of Republic of China, Taiwan, under contract number NSC 98-2221E-008-021-MY3. References [1] Maeda T, Terao M, Shimano T. A review of optical disk systems with blueviolet laser pickups. Jpn J Appl Phys 2003;42:1044–1051. [2] Komma Y, Tanaka Y, Mizuno S. Compatible objective lens for Blu-ray disc and digital versatile disc using diffractive optical element and phase-step element which corrects both chromatic and spherical aberrations. Jpn J Appl Phys 2004;43:4768–4771. [3] Aiba M, Kawakita S, Sato J, Kanno K, Yamada T. BD/DVD/CD compatible objective lens. Proc SPIE 2006;6282:62821J. [4] Katayama R, Komatsu Y. Blue/DVD/CD compatible optical head. Appl Opt 2008;47:4045–4054. [5] Katayama R, Komatsu Y. Optical head using single objective lens for four types of optical disks. Jpn J Appl Phys 2008;47:5784–5793. [6] Manoh K, Yoshida H, Kobayashi T, Takase M, Yamauchi K, Fujiwara S, et al. Small integrated optical head device using a blue-violet laser diode for Bluray disc system. Jpn J Appl Phys 2003;42:880–884. [7] Horiyama M, Kanazawa Y, Ogata N, Nishioka S, Miyake T, Nakata Y, et al. Optical pickup using integrated optical unit for Blu-ray disc. Proc SPIE 2006;6282:62820A. [8] Shih HF. Integrated optical unit design for the collinear holographic storage system. IEEE Trans Magn 2007;43:948–950. [9] Uchiyama M, Ebihara T, Omi K, Kitano H, Hoshino I, Mori K. Development of optical pickup for digital versatile disc using two-wavelength-integrated laser diode. Jpn J Appl Phys 2000;39:1549–1553. [10] Mori E, Komma Y, Yasuda K, Hotta N, Imafuji O, Kikuchi A, et al. Digital versatile disc read-only disc, rewritable disc and compact disc compatible optical pickup with a two-wavelength laser diode unit. Jpn J Appl Phys 2002;41:4845–4849. [11] Shih HF. Optical head with two wavelengths in single path using holographic optical element. Jpn J Appl Phys 2005;44:1797–1802. [12] Lee WH. Holographic optical head for compact disk applications. Opt Eng 1989;28:650–653. [13] Freeman MO, Shih HF, Chang HR, Wang JK, Chen CL, Chuang RN, et al. High efficiency HOEs for holographic DVD pickup heads. IEEE Trans Magn 1998;34:456–458. [14] Shih HF, Lu WC, Chang JY. Design of single-path optical pickup head with three wavelengths using integrated optical unit. IEEE Trans Magn 2009;45:2202–2205. [15] Held G. Introduction to light emitting diode technology and applications. New York: CRC Press; 2008 p. 114–7. [16] Thorlabs: AQWP05M-600 Datasheet, /http://www.thorlabs.hk/NewGroup Page9.cfm?ObjectGroup_ID=854S. [17] Sun WS, Sun CC, Chang JT, Tien CL, Ma SH, Chang SH. Triple-wavelength optical pickup head designs for compact disk, digital versatile disk and highdensity digital versatile disk devices. J Mod Opt 2005;52:2523–2547. [18] Sun WH, Tsai MC, Chang JT. US Patent 0126458, 2006. [19] O’Shea DC, Suleski TJ, Kathman AD, Prather DW. Diffractive optics: design, fabrication, and test. Washington: SPIE Press; 2003 p. 17–34. [20] Thornton CL. US Patent 0109404, 2009. [21] Plastic prisms, /http://www.purakou.co.jp/product/product.htmlS. [22] Sun WS, Chang H, Sun CC, Chang MW, Lin CH, Tien CL. Design of high-power aspherical ophthalmic lenses with a reduced error budget. Opt Eng 2002;41:460–470. [23] Koma Y, Kadowaki S, Hori Y, Kato M. Holographic optical element for an optical disk head with spot-size detection servo optics. Appl Opt 1990;29:5127–5130. [24] Freeman MO, Shih HF, Wang JK, Yang TP, Hsu L. Robust focus and tracking detection for holographic digital versatile disc optical pickup-head modules. Jpn J Appl Phys 1999;38:1755–1760. [25] Shimomura T, Izawa C, Matsui T. Development of a compact optical system for microarray scanning using a DVD pickup head. Rev Sci Instrum 2008;79:035101.

Please cite this article as: Sun W-S, et al. Compact design for a unitary photo detector and single-path combo optical pickup head for Blu-ray disc, digital versatile disc and compact disc systems. Opt Laser Eng (2012), http://dx.doi.org/10.1016/j.optlaseng.2012.02.007