978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 531 33rd Int. Spring Seminar on Electronics Technology

Optical Measurements upon Compact Discs in Education in Optoelectronics

Tamara Pencheva, Berkant Gyoch and Petko Mashkov

Physics Department, University of Rousse, Rousse, Bulgaria tgp@ru.acad.bg

Abstract: The aims of this article are connected with application of optical measurement methods in education in electronics. They are used for evaluation of some of peculiarities of different types of compact discs used as optical storage memories. A semiconductor laser diode is used as light beam source during measurements. The compact disc irradiated by laser beam at certain conditions operates as a diffraction grating. By measuring geometrical features of the diffraction patterns and using results from diffraction grating’s theory students can determine the average size of track pitch of different types compact discs (CD-ROM; CD-R: 650 MB, 700 MB, 790 MB, 870 MB, 1300 MB and DVD 4,7 GB). The length of the spiral track is calculated too. Different manners for recording data on compact discs are considered. Comparison between obtained data helps students to understand and clear up peculiarities of different types CDs and possibilities of their applications.

1. INTRODUCTION

Compact discs are wide used as optical storage memories. There are many types of discs which differ by capacity, physical properties of recording media and information’s recording methods [1]. This variety makes difficult understanding of peculiarities of discs for students. During education in Optoelectronics students are introduced in theory of diffraction and properties of different types of compact discs. Experiments and calculations presented in this paper allow better understanding and learning of:

• optical measuring method based on light diffraction on the disc’s track structure;

• data storage structure for different types of optical discs CD-DA (digital audio), CD-ROM (read only memory), CD-R (recordable), CD-RW (rewritable) and DVD;

• capacity (650 MB, 700 MB, 790 MB, 870 MB, 1 300 MB; 4 700 MB);

During measurements students can determine experimentally track pitch (step of adjacent rows of pits). The track is a continuous spiral line. For CD-ROM and DVD-ROM discs each physical track consists of a succession of depressions as seen by the

Double Density CD

Conventional

CD (Mode1)DVD

Data Capacity 1,3GB 650MB 4,7GB

Wavelength 780nm 650nm

Disc Size Diameter: 120mm Thickness: 1.2mm

Track Pitch 1,1 μm 1,6 μm 0,74μm

Table 1. Main parameters of CD, Double Density CD and DVD [2 – 4].

optical stylus, called pit, in the otherwise flat reflective layer. The encoded information is represented by variations of the pit length and by variation of the distance between pits. The depth of the pits should be such that the requirements for interference recognition of recorded information to be met [2]. For other types of optical storage discs the track can be formed in other ways. For instance, for CD-WORM (Write Once - Read Many) the arrangement of the data pits and lands is on preliminary prepared spiral groove [4].

The track pitch values for different types of CD are given in Table 1. Reducing of track pitch leads to

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 532 33rd Int. Spring Seminar on Electronics Technology

increasing of the length of the spiral track which increases CD’s capacity.

2. LIGHT DIFFRACTION ON CD TRACKS PATTERN

Spiral CD and DVD track pattern with constant step may be considered as diffraction grating with central (radial) symmetry.

In Fig. 1 some track pitch differences for CD, Double Density CD (DDCD) and DVD are shown. As it can be seen from the picture track pitches on CD are larger in comparison to these on DVD. Track pitch value for CD-ROM is 1,6 μm, for DVD-ROM is 0,74 μm. Moreover in the picture pit’s minimal length can be seen. It is 0,83 μm on CD-ROM and 0.4 μm on DVD-ROM. This decrease of the marker’s (pit’s) length is possible because the wavelength of lasers which are used for information recognition for these discs are different: for CD λ = 780 nm and for DVD λ = 650 nm. This shortening allows increasing recording density and leads to bigger disc’s capacity.

Light beam normally incident on diffraction grating diffracts in according to Bragg’s law (1):

d.sinθ = m λ, (m =0, ±1, ±2, …), (1)

where d is diffraction grating’s constant (step), θ is angle of diffracted beam deviation, m is order of diffraction maximum, λ is wavelength of incident light. Typical diffraction pattern consists on diffracted maxima and minima which are shown in Fig. 2 for transmissive diffraction grating.

The same expression (1) describes diffraction of reflected light beams. The disposition of maximums depends on wavelength of light λ (except the central maximum m = 0), so diffraction grating produces a separation of the colors of white light that is why optical discs looked colorized like rainbow in reflected light.

Fig. 1. Track pitches comparison of CD-R, Double Density CD -R and DVD [3].

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 533 33rd Int. Spring Seminar on Electronics Technology

Fig. 2. Operating of transmissive diffraction grating [5].

From expression (1), for m = 1 (first order spectrum) diffraction grating’s constant d is:

.sinθλ

=d (2)

3. DETERMINATION OF TRACK PITCH VALUE FOR DIFFERENT TYPES OF DISCS

The standard track pitch on CD-ROM is 1,6 μm which corresponds to about 625 tracks per millimeter [2]. Pit width is marked as a (usually 0,5 μm) and distance between pits b is usually about 1,1 μm, Fig. 3. Diffraction grating’s constant (track pitch) can be considered as d = a + b.

Fig. 3. Structure of CD-ROM reflecting surface

operating as diffraction grating.

A scheme of experimental equipment for track pitch measurements is shown in Fig. 4, and a photo of realized experiment – in Fig. 5.

Laser beam normally incident on the disc plane diffracts, Fig. 4. The order m = 0 corresponds to a direct reflection of light from the grating plane to the light source, order m = 1 corresponds to the first maximum which is deflected to point A.

Laser

Screen

L

x

2x

CD

А B

С

θ'

θ

Fig. 4. Scheme of experimental equipment for CD track

pitch measurements.

Fig. 5. Photo of experiment for track pitch measurements with diffraction maximum m = 0 in the center and m = ± 1

symmetrically on both sides.

The deviation of first maximum may be determined from triangle ΔАBС which is rectangular and angle θ' = θ, Fig. 4. A substitution BC = x can be made:

ACx

ACBC

=='sinθ and x

ACBCAC

=='sin

1θ

(3)

Using Pythagoras' theorem the follow expression becomes in:

(AC)2=(BC)2 + (AB)2 = x2 + L2. (4)

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 534 33rd Int. Spring Seminar on Electronics Technology

Then substitution of (4) in (3) leads to:

2

2

2

2222

1xL

xLx

xLx

xAC

+=+

=+

= . (5)

As a result diffraction grating’s constant for CD can be expressed as:

2

1 ⎟⎠⎞

⎜⎝⎛+=

xLd λ , (6)

where x is the distance from the central maximum to the first maximum in the diffraction pattern, L is the distance from disc to the scale on the screen, Fig. 4 and Fig. 5. Distances x and L are measured directly.

4. MEASURMENT OF TRACK PITCH

Measurement procedure:

1. Put CD under investigation with outer diameter 120 mm into the arrangement, shown in Fig. 5. CD-ROM, CD-R 650 MB, 700 MB, 790 MB, 870 MB, 1 300 MB, DVD – 4 700 MB are used.

2. Distance 2x between the first maxima in the diffraction pattern, Fig. 4, is measured directly. Precision about 1 mm may be accepted as adequate. The results for different discs are written into the Table 2.

3. According to relationship (6) diffraction grating’s constant d are calculated.

4. Distance L from disc to the scale on the screen is fixed (L = const = 100 mm) and is acceptable for direct measurement, if need.

During measurements a semiconductor laser diode with wavelength λ= 655 nm is used as light beam source.

Calculations of track pitch. As an example of calculations described above, the data from the first row on Table 2 (for CD-R 74 min) are obtained as follows:

=⎟⎠⎞

⎜⎝⎛+=

2

461001655

mmmmnmd

= 655* 2,392885=1567nm ≈ 1,58μm.

Results for upper mentioned discs obtained by described method are presented in Table 2.

Type of optical disc

2x, mm

d, μm

CD-R 650 MB 92 1,567

CD-R 700 MB 96 1,514

CD-R 790 MB 114 1,323

CD-R 870 MB 122 1,258

CD-R 1300 MB 148 1,101

DVD SS, SL 376 0,742

CD-ROM 90 1,596

Table 2. Measured track pitch values for different types of optical discs.

In Table 2 abbreviation for DVD - SS means single sided, SL means single layer.

Calculation of spiral track length. While we know that CDs are recorded in a single continuous spiral [2, 7, 9] determination of its length may be considered as a helpful problem. The physical length of the CD, DVD spiral track can be determined by converting the polar representation of the Spiral of Archimedes equation to rectangular coordinates. For simplification it is possible to approximate the physical track by a succession of concentric rings which radius changed with the step of spiral track. In this case the track length for optical disc may be calculated as:

TPRinRoutL ).( 22 −

=π

, (7)

where L - is length of spiral; Rout – is outer radius; Rin– is inner radius; TP- tracks pitch.

The Lead-in area of an audio CD contains the Table of Contents, listing up to 99 digital audio tracks and is also where the player synchronizes itself to the disc being played. The Lead-out area contains simple codes which allow the audio CD player to recognize the end of the disc. The Program area is where all the digital audio goes. Note that the Lead-out area and the outer buffer zones are both defined as being at least 0,5 mm wide (measured radially).

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 535 33rd Int. Spring Seminar on Electronics Technology

Fig. 6. CD dimensions [9]: radius of lead-in area; radius of

program area; radius of lead-out area.

In Fig. 6 radius of lead-in area Rin= 23mm, Rout = 25 mm; radius of program area Rin= 25mm, Rout = 58 mm; radius of lead-out area Rin= 58mm, Rout = 58,5 mm. Let us calculate lengths of track for separate disc areas and total length of spiral for standard CD-ROM:

• For lead in area, from (7):

6

622

10*6,110*)2325.(

−

−−=πL =189 m.

• For program area, from (7):

6

622

10*6,110*)2558.(

−

−−=πL = 5 378 m.

• For lead out area, from (7):

6

622

10*6,110*)585,58.(

−

−−=πL = 114 m.

• Total length of spiral on standard CD-ROM is 5 681 m.

For comparison, length of spiral track for other optical discs is:

CD-R 700 MB length of spiral is 6 003,575 m. CD-R 790 MB length of spiral is 6 870,305 m. CD-R 870 MB length of spiral is 7 225,289 m. CD-R 1 300 MB length of spiral is 8 255,597 m. DVD 4 700 MB length of spiral is 12 249,883 m.

From obtained result it can be seen that reducing of track pitch leads to increasing of the length of the spiral track which increases CD’s capacity. Comparison between measured with standard values of track pitch [2 – 4] shows quite good correlation. Measurement error is less than 0,004 μm. Standard ECMA-130 allow deviation about ± 0,1 µm. The spiral track should have a pitch of 1,6 µm ± 0,1 µm.

6. CONCLUSIONS

The problem under consideration allows to students to be introduced in main characteristics and peculiarities of different types of optical disc memories (CD-ROM; CD-R: 650 MB, 700 MB, 790 MB, 870 MB, 1300 MB and DVD 4,7 GB), to measure some latent parameters of data storage: track step and length of disc track using relatively simple and accessible optical measurements.

ACKNOWLEDGEMENT

The National Science Fund, Ministry of Education and Science of Bulgaria, is gratefully acknowledged for the financial support of research project VU-EES-301/2007 and of research project 10-FEEA-06.

REFERENCES

[1] J. D. Lenk, Lenk’s laser handbook-featuring CD, CDV, and CD-ROM technology, New York, McGraw-Hill, Inc., 1992.

[2] Data interchange on read-only 120 mm optical data discs, Standard ECMA-130, 2nd Edition - June 1996.

[3] Sony Establishes Double Density CD-ROM/-R/-RW Formats, http://www.sony.net/SonyInfo/ News/Press_Archive/200007/00-0705/.

[4] Basic Specifications of Double Density CD-ROM/-R/-RW formats, September 2000, http://www.licensing.philips.com/.

[5] Five Slit Diffraction http://www.mwit.ac.th/Physicslab/ hbase/phyopt/mulslid.html.

[6] http://www.media.utah.edu/dvdworkshop [7] Sony CRX200E DDCD-RW, February 19, 2002,

http://www.cdrinfo.com/Sections/Reviews/Specific.aspx?ArticleId=6135.

[8] What are CD-ROM Mode-1, Mode-2 and XA? http://sony.storagesupport.com/node/6405.

[9] Introduction to CD and CD-ROM, Graham Sharpless, Deluxe Global Media Services Ltd., July 2003.

[10] http://www.disctronics.co.uk/cdref/cdbasic5.htm.