712 OPTICS LETTERS / Vol. 31, No. 6 / March 15, 2006

Optical smart card using semipassivecommunication

I. Glaser, Shlomo Green,* and Ilan Dimkov†

Holon Academic Institute of Technology, Department of Electrical and Electronic Engineering, 52 Golomb Street,Holon 58102, Israel

Received September 14, 2005; accepted October 31, 2005; posted December 12, 2005 (Doc. ID 64769)

An optical secure short-range communication system is presented. The mobile unit (optical smart card) ofthis system utilizes a retroreflector with an optical modulator, using light from the stationary unit; this mo-bile unit has very low power consumption and can be as small as a credit card. Such optical smart cards offerbetter security than RF-based solutions, yet do not require physical contact. Results from a feasibility studymodel are included. © 2006 Optical Society of America

OCIS codes: 350.3950, 060.4510.

Small devices such as magnetic cards, smart cards,and RF identification devices are routinely used forapplications ranging from simple identification atbank teller machines and parking lots to sophisti-cated two-way communication applications such aselectronic wallets and noncontact car keys. Increas-ingly, there is a demand to upgrade such devices sothey will be more difficult to counterfeit while, at thesame time, easier and faster to use legitimately. Mag-netic cards and smart cards usually need to be in-serted into a reader device, an inconvenience. Non-contact RF devices use nondirectionalcommunication, so unauthorized people can easilyeavesdrop.

An alternative to contact-based devices and to RFidentification is cards that have a pattern printedover a retroreflective background.1 Unfortunately,since the pattern is in plain view, such cards are veryeasy to counterfeit. We need a device combining thepassive directional capability of retroreflectors withtwo-way communication, thus facilitating authentifi-cation dialog and, possibly, secure data transmission.In the system described here, a combination of a re-flective light modulator and a lenslet array acts as aretroreflector that can be turned on and off at will,thus sending a temporal signal in the reflectedbeam.2

Figure 1 presents a schematic view of the opticaland electronic system for both the stationary unitand the mobile unit (the optical smart card), and Fig.2 shows a block diagram of the communication proto-col. While the stationary unit has its own opticalsource, the mobile unit utilizes a light modulator tomodulate and reflect light received from the station-ary unit. We shall begin our discussion with the mo-bile unit, depicted by the cross section in Fig. 1. Asshown in the figure, most of the area of the mobileunit is covered by a lenslet array, behind which thereis a reflective light modulator, such as a liquid crystal(LC) device. The reflective surface of the modulator islocated precisely at the common focal plane of thelenslets of the array. The combination of a lens with areflector in its focal plane acts as a retroreflector(known as a cat’s-eye retroreflector) and directs thereflected light back at its source3; the entire lenslet

array–reflective modulator assembly thus acts as a

0146-9592/06/060712-3/$15.00 ©

switchable retroreflector. When the assembly is illu-minated by light from the optical emitter at the de-tector, an electric signal fed to the modulator willmodulate the reflected light, which will be detectedby the optical detector of the stationary unit, locatednext to the emitter. This detector can then receive thesignal fed to the modulator of the mobile unit.

From Figs. 1 and 2 we can now see how this capa-bility of semipassive communication can be used. Atfixed time intervals the stationary unit transmitssome known temporal signal through its optical emit-ter. If the mobile unit is within range, its detector re-ceives this signal and the onboard microcontrollerrecognizes it. After sending this call signal, the sta-tionary unit transmits a CW signal of fixed intensityfor a predetermined amount of time. The mobile unitcan now use its modulator to modulate this light andsend it back to the mobile unit. This cycle can now berepeated as needed for the communication task. Forexample, the stationary unit could be an automaticteller machine (ATM), and the signals (which can beencrypted using a public key4) could contain identifi-cation codes for each of the units and possibly somedetails of the transaction or the status of the account.As shown in the figures, the mobile (card) unit con-tains, in addition to the lenslet array–modulator as-sembly, an optical detector and a microcontrollerchip. Additionally, it must have some power source,possibly a combination of some battery and a photo-voltaic power cell. The stationary unit contains, inaddition to the parts already described, an opticalinput–output (I/O) control subunit (LED power sup-

Fig. 1. Schematic view of the stationary and the mobile

units for the semipassive communication system.

2006 Optical Society of America

March 15, 2006 / Vol. 31, No. 6 / OPTICS LETTERS 713

ply and driver, detector preamplifier and read-channel electronics, etc.), a computer that controlsboth the dialog between the two units and the dialogwith the client machine (for example, the ATM ma-chinery), and some interface subunit that is con-nected to both the computer and the client machine.

Since the optical smart card scheme that we havejust described requires neither physical contact norprecise alignment, it can be less time consuming thanusing a conventional smart card5 that relies on physi-cal electrical contact. A common alternative commu-nication channel for smart cards, RF, is also noncon-tact and requires no alignment, but RFtransmissions can be easily intercepted by a thirdparty and thus are less secure. Table 1 compares themajor performance attributes of the three methods.

A production smart card based on these principleswould probably use a ferroelectric LC modulator thatcombines a relatively fast response ��1 �s� with verylow power consumption.6 Since we do not need to dis-play an image, a simple, full-area modulator (ratherthan a pixelated image display) would be suitable.The lenslet array for the device would probably be in-tegrated with the modulator and would probably bemanufactured by plastic molding. Operation of sucha production system will, most likely, be in the nearIR, where inexpensive, low-cost emitters and detec-tors are readily available. While many LC modula-tors come packaged with a polarizer to affect inten-sity modulation, this system can use a modulatorwith no polarizer, so the reflected signal will be polar-ization modulated rather than intensity modulated—polarizers would be installed in the stationary unit.As we envision that for each stationary unit manymobile units will be manufactured and used, thiswould be more economical.

For feasibility demonstration of the optical smartcard we used an off-the-shelf reflective LC alphanu-meric display (see Fig. 3A). This unit was not optimalfor the application—the area of the display is pix-elated (part of the area is not modulated), and the LCis nematic instead of the preferred ferroelectric LCtype, with a rather slow response, limiting the datarate of the demonstration system to a few tens ofhertz. The lenslet array (Fig. 3B) was fabricated fromphotoresist over glass substrate, using the reflowprocess.7 While the reflow photoresist process giveshigh-quality lenslet surfaces, the resulting individuallenslets have circular apertures (and space between

Table 1. Basic Attributes of Smar

Attribute Conventional Smart Card

Range (m) ContactSignal beam width (deg)

Outgoing (fromstationary unit)

–

Return –Achievable data rate (Hz) 108

Card positioning Contact

neighboring lenslets), so the fill factor is inevitablylower than � /4. An alternative microlens fabricationprocess, continuous tone microlithography8 (possiblyfollowed by etching into the substrate or replication),can provide a fill factor near 100% but was not avail-able for these experiments. The optical power sourcewas a red �650 nm� LED; the use of visible light inthis experiment was useful for alignment and demon-stration, although for a production system invisibleIR light is more suitable. We found that the modula-tion ratio on the returned signal was around 5.4:1 (of-f:on), which is sufficient for low-error readout. Tohelp differentiate between legitimate optical signalsand background light the LED source was modulatedat 38 kHz, a frequency much higher than that of theactual signal as modulated by the LC device. The op-tical receiver treated this frequency as a carrier fre-quency and rejected signals not modulated by it.Computing power for the mobile unit was supplied bya single-chip microcontroller (Atmel AT89C52). Thestationary unit was essentially a personal computerwith some added input–output facilities, using astandard operating system and software comprisingLabVIEW for device control; access for the transac-tion database; and control, encryption, and decryp-tion code written specifically for the feasibility dem-onstration. The system employed RSA encryption fordemonstrating security and Manchester code9 for theoptical channel. Using these components, the station-ary unit was able to identify the user of the mobile

Fig. 2. Flow diagrams for the communication protocols ofboth units.

rd Communication Technologies

Method

RF Optical Retroreflecting

10−3 to 103 10−3 to 102

60° to omnidirectional 10° to 180°

Omnidirectional 2° or less108 106

Not critical Not critical

t Ca

714 OPTICS LETTERS / Vol. 31, No. 6 / March 15, 2006

unit, validate his access permissions, and securelyexchange data between the two units (see Fig. 4).

We have demonstrated the feasibility and viabilityof efficient semipassive optical communication andthe optical smart card concept. Systems based on

Fig. 3. Components used in the experimental feasibilitydemonstration: A, LC device; B, photomicrograph of thelenslet array.

Fig. 4. Example of an oscilloscope trace for a detected sig-nal transmitted from the mobile unit to the stationary unitin the feasibility demonstration system.

these ideas can provide more security because the re-

turn signal beam is essentially focused (see Table 1),with no need for precise card placement. While RF ordirect contact can deliver higher data rates than aLC-based optical smart card, there are numerous ap-plications where the data rate of such optical smartcards is more than needed.

The authors thank Naftali Eisenberg and MichaelManievich, who fabricated the lenslet array used inthe experiments, and Meir Zagon for his help in thelaboratory. I. Glaser’s e-mail address isiglaser@hait.ac.il.

*Present address, 22 Jerusalem Street, Bat-Yam,Israel.

†Present address, Cellvine, Ltd., 6 Yonni-Netanyahu Street, Or-Yehuda, Israel.

References

1. H. Takada, “Card having retroreflective bar codes anda magnetic stripe,” U.S. patent 5,237,164 (August 17,1993).

2. N. P. Eisenberg, I. Glaser, A. Drori, R. Karoubi, and Y.Arieli, “Directed reflectors and systems utilizing same,”U.S. patent 6,507,441 (January 14, 2003).

3. J. J. Snyder, Appl. Opt. 14, 1825 (1975).4. P. R. Zimmermann, Sci. Am. 268(10), 110 (1998).5. H. Shogase, IEEE Spectrum 25(10), 35 (1988).6. K. Yoshino and M. Ozaki, Jpn. J. Appl. Phys., Part 1

23, L385 (1984).7. Z. D. Popovic, R. A. Sprague, and G. A. N. Connel,

Appl. Opt. 27, 1281 (1988).8. H. P. Herzig, Micro-Optics Element, Systems and

Applications (Taylor & Francis, 1997).9. G. Fairhurst, “Manchester encoding,” (2001), http://

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