WO2009063362A1

WO2009063362A1 - Universal optical key and lock system and method to initiate a security function

Info

Publication number: WO2009063362A1
Application number: PCT/IB2008/054631
Authority: WO
Inventors: Paraskevas Dunias
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-11-12
Filing date: 2008-11-06
Publication date: 2009-05-22
Also published as: TW200938712A

Abstract

Proposed is a security system(1), comprising a first member (10) and a second member (20). The first member comprises a coding surface (11) with embedded coding information. The second member (20) is capable to cooperate with the first member, and comprises a laser diode (30) having a laser cavity (31) for generating laser radiation to illuminate the coding surface (11). Furthermore, the second member comprises a detector 5 (40) arranged to initiate a security function. The security systemis characterised in that the detector (40) is arranged to initiate the security function in responds to a measurement of a variation (35) in an operational characteristic of the laser cavity (31), the variation comprising the coding information and being due to the self-mixing effect of an optical wave (36) in the laser cavity (31) and back-reflected radiation (38) re-entering the laser cavity (31) 10 after reflection from the coding surface (11). Such a security systemcan in particular be used as a lock and key combination to prevent theft of valuable items from anenclosure.

Description

Universal optical key and lock system and method to initiate a security function

FIELD OF THE INVENTION

The invention relates to a security system comprising a first member and a second member, the first member comprising a coding surface with embedded coding information, the second member, capable to cooperate with the first member, comprising a laser diode having a laser cavity for generating laser radiation to illuminate the coding surface and comprising a detector arranged to initiate a security function. Such a security system can in particular be used as a lock and key combination to prevent theft of valuable items from an enclosure. Furthermore the invention relates to a lock as well as a method to initiate a security function.

BACKGROUND OF THE INVENTION

An embodiment of a security system of the kind set forth is known from US4593185. That document discloses a lock and key combination. The key has a corrugated coding surface with a modulated depth profile corresponding to a security code. The lock, arranged to receive the key, comprises a laser for illuminating the coding surface and a photodetector for measuring reflected light. Comparing the measured reflected light - modulated by the depth profile - with information stored in a memory unit inside the lock, allows for the initiation of a security function, i.e. opening or closing of the lock, in accordance with the security code in the key. US4593185 discloses the necessity of a predefined key speed relative to the lock in order to correctly interpret the modulated light measured by the photodetector. According to a first method, the lock comprises a stationary sensing laser beam while the key moves into the sensing channel of the lock at a controlled motor driven speed. According to a second method, the sensing laser beam pivots at a controlled speed only after the key has been introduced completely into the sensing channel, i.e. the key has a fixed position enforced by a retention mechanism in the lock.

The disadvantage of this approach lies in the fact that the lock comprises complicated and expensive mechanically actuatable mechanisms. These not only make the prior art key and lock combination unattractive from an economical perspective, but also make the combination prone to failure due to amongst others wear and tear. Moreover, the presence of the mechanically actuatable mechanisms limits the miniaturisation of the system.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a security system of the kind set forth, solving at least one of the problems or disadvantages identified. The invention achieves this object according to a first aspect with the security system characterised in that the detector is arranged to initiate the security function in responds to a measurement of a variation in an operational characteristic of the laser cavity, the variation comprising the coding information and being due to the self-mixing effect of an optical wave in the laser cavity and back-reflected radiation re-entering the laser cavity after reflection from the coding surface.

The invention is based on the insight that the security system is able to let the coding information comprised by the first member (further identified as the key) be detected by the second member (further identified as the lock) through the application of the so-called self-mixing effect in a diode laser. (Note that although the first and second members are identified as the key and lock, in fact they may equally well be referred to as the lock and key, respectively.) In the self-mixing phenomenon, radiation emitted by the diode laser and re-entering the laser cavity after reflection from the coding surface induces a variation in an operational characteristic of the laser cavity due to the interference of radiation (i.e. light) taking place in the laser cavity. The front facet of the laser cavity and the coding surface constitute a so-called extended cavity. The self-mixing causes fundamental variations in the properties of the laser and the emitted radiation, such as for example the frequency and line width of the laser radiation, the optical power, and the threshold gain. Advantageously, the invention allows the coding information to be determined without the application of moveable parts in the system. Hence, the operation of the security system necessitates only the motion of the first and second member relative to each other.

In an embodiment of the invention, the variation in an operational characteristic of the laser cavity is due to a modulation of the coding surface. Advantageously, measuring the variation allows the determination of the coding information embedded in the coding surface of the key. In an embodiment of the invention, the modulation is chosen from the group consisting of reflectivity modulation, corrugation modulation, and magnetic modulation. Advantageously, these coding surface modulations allow the coding information to be read applying an optical - i.e. a non-mechanical - interaction between the key and the lock, reducing the possibility of malfunctioning. Advantageously, reflectivity modulation allows for varying the amount of radiation to be coupled back into the laser cavity, thus inducing the variation in an operational characteristic. Advantageously, the corrugation modulation allows for varying the size of the extended laser cavity, thus inducing the variation in an operational characteristic. Advantageously, magnetic modulation allows for varying the polarization of the reflected radiation in dependence of the magnetic state of the coding surface (magneto-optic Kerr effect). In combination with the polarising properties of the laser cavity mirrors, this will again allow for varying the amount of radiation coupled back into the laser cavity. In an embodiment of the invention, the modulation of the coding surface comprises at least a first state and a second state characterizing at least one bit of the coding information. The logical "zero" value of the bit may be characterized by the first modulation state, while the logical "one" value may be characterized by the second modulation state, or vice versa. Advantageously, the invention provides a security system that allows for the key and lock to correctly cooperate - i.e. the initiation of the security function in accordance with the security code embedded in or applied on the coding surface - at any relative speed. Hence, the invention makes the complicated and expensive mechanically actuatable mechanisms present in prior art systems superfluous. Advantageously, the invention allows for a non-mechanical interaction between the first and second member, reducing the possibility of malfunctioning.

In an embodiment of the invention, the modulation of the coding surface comprises a third state, enabling two bits of the coding information to be distinguishable. Advantageously, the third modulation state functions as a reference state interposed between two bits encoded in the coding surface. This allows the determination of two bits with an equal value, independent of the speed with which the key and lock cooperate.

In an embodiment of the invention, due to a Doppler frequency shift of the back-reflected radiation re-entering the laser cavity, the variation in an operational characteristic of the laser cavity further comprises information on a speed with which the first member and the second member cooperate. Advantageously, arranging the movement of the key and lock relative to each other such that the direction of movement has a component in the direction of the laser radiation emitted by the laser cavity (i.e. the laser beam), the radiation reflected by the coding surface induces a Doppler shifted frequency proportional to the relative velocity. Focussing a part of the reflected radiation back on the diode laser by the same optics that focuses the laser beam on the coding surface, results in the interference / self-mixing effect inside the cavity. The operational characteristics of the laser cavity fluctuate with a frequency equal to the difference of the two radiation frequencies. Additionally modulating the laser light using an periodic sequence of an increasing/up and a decreasing/down laser current, the operational characteristics of the laser cavity fluctuate with a frequency equal to the difference between the increasing Doppler shifted frequency and the decreasing Doppler shifted frequency. In case of a corrugation profile on/in the coding surface, the profile now appears proportional to the sum of the increasing Doppler shifted frequency and the decreasing Doppler shifted frequency. Hence, measuring the value of one of operational characteristics using the detector allows determining the velocity and, by integration over time, the displacement of the key relative to the lock. Advantageously, measuring the speed with which the key and lock cooperate, allows an absolute determination of the lateral size (i.e. along the surface) of the modulation on the coding surface. This opens up the possibility to use this lateral modulation extension as a coding state. Advantageously, when applying a corrugation modulation of the coding surface, this embodiment allows for an absolute determination of the corrugation profile (both in depth and lateral size), opening up the possibility of encoding information in/on the surface with a very high density.

The use of the self-mixing effect for measuring velocities of objects is known per se. Until now, however, the use of this effect in a security system of the kind set forth has not been suggested. Recognising that a measuring module, comprising a laser diode and detector, applying the self-mixing effect can be made so small and cheap, it opens up this new application. Moreover, due to the miniaturization possibilities the key may comprise the measuring module while the lock comprises the coding surface. Advantageously, such an embodiment allows recognising whether or not the key has been inserted in the correct lock. The security function in such a case might include sending (or blocking) information embedded in the key to the lock. Advantageously, implementing a measuring module and coding surface in both the key and the lock allows for handshaking functionality. As an example, such handshaking functionality between a bank card and an ATM prohibits the use of skimming equipment, and thus improves security.

In an embodiment, the detector is arranged to measure a variation of an impedance of the laser cavity. In addition to the properties identified above, the self-mixing effect also changes the impedance of the laser diode. Advantageously, measuring the voltage across the laser diode and dividing the measured voltage value by the known value of the electric current sent through the laser diode, determines the impedance. In an embodiment, the detector is a radiation detector arranged to measure the variation in the amount of light emitted by the laser cavity. Advantageously, measuring the intensity of the laser radiation forms a very simple way to determine the variation in the operational properties of the laser cavity, as it can be done using a simple and low cost photodiode.

In an embodiment, the radiation detector is arranged at a side of the laser cavity opposite from where the laser radiation to illuminate the coding surface is emitted. Generally, laser diodes are provided with a monitoring photodiode at their rear side. Usually, such a monitoring photodiode is used to stabilize the intensity of the laser beam emitted at the front side of the diode laser. The invention uses the monitoring photodiode to detect the variation in the laser cavity operational properties generated by the radiation reflected back from the coding surface and re-entering the cavity.

In an embodiment, the laser diode is of the VCSEL (vertical cavity surface emitting laser) type. Such lasers emit radiation in the vertical direction, making it highly suitable for application in the security system according to the invention. Alternatively, the laser diode is of the horizontal emitting type. This later embodiment makes use of a reflecting member (for example a mirror or an optical element capable of showing total internal reflection) reflecting the emitted laser beam to the coding surface. Advantageously, horizontal emitting laser diodes belong to the most commonly used laser, and thus have a considerably lower price tag than VCSELs. Note that the reflecting member adds hardly any additional costs.

According to a second aspect, the invention provides a lock capable of cooperating with a key comprising a coding surface, the lock comprising a laser diode having a laser cavity for generating laser radiation to illuminate the coding surface, the lock furthermore comprising a detector arranged to initiate a security function, arranged to initiate a security function, characterised in that the detector is arranged to initiate the security function in responds to a measurement of a variation in an operational characteristic of the laser cavity, the variation comprising the coding information and being due to the self-mixing effect of an optical wave in the laser cavity and back-reflected radiation re-entering the laser cavity after reflection from the coding surface.

According to a third aspect, the invention provides a method to initiate a security function comprising the steps providing a first member comprising a coding surface with embedded coding information, providing a second member, capable to cooperate with the first member, comprising a laser diode having a laser cavity for generating laser radiation to illuminate the coding surface, characterised in that the method further comprises the steps inducing a variation in an operational characteristic of the laser cavity through the self- mixing effect of an optical wave in the laser cavity and back-reflected radiation re-entering the laser cavity after reflection from the coding surface, and initiating the security function in responds to a measurement of the variation.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS Further details, features and advantages of the invention are disclosed in the following description of exemplary and preferred embodiments in connection with the drawings.

Fig. 1 schematically shows an embodiment of the invention Fig. 2 illustrates the principle of the method according to the invention Fig. 3 shows the variation of an operational characteristic of the laser cavity

Fig. 4 illustrates an embodiment for measuring the coding induced variation of the impedance of the laser cavity

Fig. 5 shows several embodiments for embedding the coding information Fig. 6 illustrates a variation of the principle of the method according to the invention

Fig. 7shows the optical frequency as a function of time of radiation measured by the security system according to the invention

DETAILED DESCRIPTION OF THE EMBODIMENTS Figure 1 shows a cross section of a security system 1. The system comprises a first member 10 (the key) and a second member 20 (the lock). The first member 10 comprising the coding surface 11 with embedded coding information, for instance on an elongated spine. The second member 20 may comprise a sensing channel or slit 21 for receiving the first member 10, enabling it to cooperate with that member. Furthermore, the second member 20 comprises a laser diode 30 and a detector 40. The laser diode 30 has a laser cavity 31 for generating laser radiation to illuminate the coding surface 11 when operating security system by inserting the key into the lock. The detector 40 is arranged to initiate a security function (i.e. opening or closing the lock) in accordance with the encoded information. Figure 2 illustrates the physical principle behind the invention, while figure 3 shows a variation 35 of an operational characteristic of the laser cavity 31. The security system 1 according to the invention is characterized in that the detector 40 is arranged to initiate the security function in responds to a measurement of a variation 35 in an operational characteristic of the laser cavity 31, the variation comprising the coding information and being due to the self-mixing effect of an optical wave 36 in the laser cavity and back- reflected radiation 38 re-entering the laser cavity 31 after reflection from the coding surface 11. The invention is based on the insight that the security system 1 is able to detect the coding information through the application of the so-called self-mixing effect in the diode laser 30. Figure 2 schematically represents the diode laser 30 with its cavity 31 and its front 33 and rear 32 facets (or mirrors), respectively. The cavity 31 has a length L. The space between the coding surface 11 and the front facet 33 forms an external cavity 34 with length L_e. The laser radiation (or laser beam) 37 emitted through the front facet 33 will be - at least partly - reflected from the coding surface 11 back to the cavity 31. A part of the back- reflected radiation 38 re-enters the cavity 31, causing interference between the optical wave 36 and the back-reflected radiation 38 inside the cavity. Through the interference, a change in the external cavity 34 causes a variation 35 in an operational characteristic of the cavity 31. A modulation of the coding surface will induce the change in the external cavity 34. The variation 35 in an operational characteristic of the cavity 31 may be observable as a variation in the frequency of the laser radiation 37 (Fig. 3). The line width of the laser radiation 37, the intensity or optical power of the laser radiation 37, the threshold gain, and impedance of the laser diode 30 form alternative examples of operational characteristics in which the variation 35 may be observable.

In an embodiment of the invention the detector 40 measures the variation 35 in the intensity of the laser radiation 37. Elegantly, the detector 40 is arranged in an embodiment at a side of the laser cavity 31 opposite from where the laser radiation 37 to illuminate the coding surface 11 is emitted. Generally, laser diodes 30 are provided with a monitoring photodiode at their rear side detecting a small amount of light 39 leaking out of the rear facet 32. Usually, such a monitoring photodiode is used to stabilize the intensity of the laser beam 37 emitted at the front side of the diode laser 30. The invention uses the monitoring photodiode to detect the variation 35 in the laser cavity operational properties generated by the radiation reflected back from the coding surface 11 and re-entering the cavity 31. In another embodiment, the detector 40 is arranged to measure a variation 35 of the impedance of the laser cavity 31. This embodiment makes use of the fact that a variation 35 in the gain of the laser cavity 31 is proportional to the number of electrons in the conduction band in the junction of the laser diode 30. This number in turn is inversely proportional to the resistance of the junction. By measuring the resistance the coding information can be determined. Fig. 4 schematically illustrates such an embodiment. The current source 70 supplies electrical current to the laser diode 30. The voltage across the laser diode 30 is supplied to an electronic circuit 75 functioning as the detector 40 via the capacitor 72. This voltage, which is normalized with the current through the laser, is proportional to the resistance or impedance of the laser cavity 31. The inductance 71 in series with the laser diode 30 forms a high impedance for the signal across the laser diode.

Figure 5 shows several embodiments for embedding the coding information in/on the coding surface 11 of the first member 10. The information may be embedded in the coding surface 11 using a modulated corrugation profile (Fig. 5A). The depth of the corrugation profile may indicate the logical "zero" and "one" bit values of digital embedded information. More specific, a first modulation state 101 may represent "zero" and a second modulation state 102 may represent "one". In order to identify two adjacent bits having the same logical value, it may be beneficial - especially if no information is available with respect to the (relative) speed with which the first 10 and second 20 members cooperate, for instance if the coding surface 11 moves perpendicular to the laser beam 37 - to use a third modulation state 103. In that case, delimiting each bit by a corrugation depth having the third modulation state 103 beneficial makes such identification possible. Finally, the availability of a forth (and so on) modulation state 104 enables a higher coding density of the information. Alternatively, the information may be embedded in the coding surface 11 using a modulated reflectivity profile (Fig. 5B). The logical "zero" and "one" bit values of digital embedded information may be indicated by a first modulation state 201 and a second modulation state 202 having a high and a low reflectivity, respectively. Again, a third modulation state 203, having for instance an intermediate (or at least distinguishable) reflectivity, enables the identification of two adjacent bits having the same logical value, without the need for information on the cooperation speed of the first 10 and second 20 members.

In yet another alternative, the information may be embedded in the coding surface 11 using a modulated magnetization profile (Fig. 5C). Again, the logical "zero" and "one" bit values of digital embedded information may be indicated by a first modulation state 301 and a second modulation state 302 having a "north" and a "south" magnetization, respectively. Similar to the previous embodiments, a third modulation state 303, for instance having no net magnetization, may function as the marker for distinguishing two adjacent bits. Note that also in the later two embodiments the availability of a forth, fifth, and so on modulation state (not shown) enables an increase in the density of the information embedded in / on the coding surface 11.

In an embodiment the lock 20 and key 10 cooperate with a (relative) speed 15 (Fig. 6). In general this speed 15 has a s-component 16 perpendicular to the laser beam 37 and a p-component 17 parallel to it. Elegantly, arranging the laser beam 37 eccentrically with respect to lens focussing it on the coding surface 11, induces a skewed illumination of the key 10 and consequently generates a p-component 17. This component induces a Doppler shift in the frequency of the reflected radiation 38 proportional to its value. The Doppler shifted frequency causes the operational characteristics of the laser cavity to fluctuate with a frequency equal to the difference of the radiation frequencies of the laser beam 37 and the reflected radiation 38. Hence, the embodiment enables the determination of the (parallel component of the) speed 15 with which the key 10 and lock 20 cooperate.

In an embodiment an increasing/up and a decreasing/down laser current frequency modulates the laser beam 37 emitted by the laser diode 30. Elegantly a triangular wave pulse current superimposed on a dc current introduces such a modulation. When a p- component 17 of the speed 15 is present, the output of the detector 40 will show a beat signal consisting of two parts. The first part comprises a Doppler signal observed during a non- modulation period, while the second part comprises a pseudo-Doppler signal superimposed on the triangular wave. While the first part represents a pure Doppler effect and depends on only the cooperation speed 15 (i.e. its p-component 17), the second part depends on both the speed 15 as well as the distance between the laser diode 30 and the key 10 (i.e. the length L_e of the external cavity 34.) Advantageously, this allows the (absolute) determination of the depth as well as the lateral extension of the corrugation profile in/on the coding surface 11. This opens up the possibility to use both the depth and the later extension for encoding information, increasing the information density considerably. Figure 7 shows the optical frequency/as a function of time t of radiation measured by the detector 40. The real line 81 represents the optical frequency of the emitted laser beam 37, modulated by the triangular wave pulse current. The dashed line 82 represents the optical frequency of the reflected radiation 38 in case the p-component 17 equals zero. Compared to the emitted radiation it has a time lag 2LJc, the round trip time of the external cavity 34. The dash-dotted line 83 represents the optical frequency of the reflected radiation 38 in case the p-component 17 is positive (i.e. the external cavity gets longer.) Compared to the emitted radiation it has both a time lag 2Le/c as well as a Doppler shift fo. As the Doppler beat frequency fo can be directly observed during the non-modulation period, the value V of λ the p-component 17 can be obtained directly from V = — f_D , with λ the wavelength of the

laser diode 30 radiation.

When V=O, the external cavity length L_e is given by L = , with T the modulation

period of the triangular wave current, i_m the peak-to-peak amplitude of the current, η the FM modulation efficiency , and β, the observed beat frequency. At positive F values, the beat frequency yL_/, during the rising period of the triangular wave is always greater than fd_own during the falling period. Therefore, the length L_e of the external cavity 34 is given by ). Analogously, for negative lvalues L_e is given by

L - f_D ) . Note that positive and negative F values can readably be

determined from the measured values off_up and /down-

Hence, as both V and L_e can be determined from the variation 35 of the operational characteristic of the laser cavity 31, the invention allows the determination of both the depth as well as the lateral extension of the corrugation profile. It has been found that the invention allows the corrugation profile to be determined with an accuracy of about 50μm in both the depth and lateral dimension.

Although the invention has been elucidated with reference to the embodiments described above, it will be evident that alternative embodiments may be used to achieve the same objective. The scope of the invention is therefore not limited to the embodiments described above, but can also be applied to any other application device where reading coding information may initiate a security function. For instance, the key will not necessarily have to have an elongated spine forming the coding surface. In stead the coding surface may be formed on a plane card, a ball, or any other appropriate object. Also the lock will not necessarily have to have a receiving slit or sensing channel for receiving the key. The lock may simply have a sensing window in front of which the key will have to be moved to enable the security function.

Claims

CLAIMS:

1. A security system (1) comprising a first member (10) and a second member (20), the first member (10) comprising a coding surface (11) with embedded coding information, - the second member (20), capable to cooperate with the first member, comprising a laser diode (30) having a laser cavity (31) for generating laser radiation to illuminate the coding surface (11) and comprising a detector (40) arranged to initiate a security function, characterised in that - the detector (40) is arranged to initiate the security function in responds to a measurement of a variation (35) in an operational characteristic of the laser cavity (31), the variation comprising the coding information and being due to the self-mixing effect of an optical wave (36) in the laser cavity (31) and back-reflected radiation (38) re-entering the laser cavity (31) after reflection from the coding surface (11).

2. A security system (1) according to claim 1, wherein the variation (35) is due to a modulation of the coding surface (11).

3. A security system (1) according to claim 2, wherein the modulation is chosen from the group consisting of reflectivity modulation, corrugation modulation, and magnetic modulation.

4. A security system (1) according to claim 2 or 3, wherein the modulation of the coding surface (11) comprises at least a first state and a second state characterizing at least one bit of the coding information.

5. A security system (1) according to claim 4, wherein the modulation of the coding surface (11) comprises a third state, enabling two bits of the coding information to be distinguishable.

6. A security system (1) according to claim 4, wherein the variation (35) further comprises information on a speed with which the first member (10) and the second member (20) cooperate due to a Doppler frequency shift of the back-reflected radiation (38) re- entering the laser cavity (31).

7. A security system (1) according to any of the claims 1 to 6, wherein the detector (40) is arranged to measure a variation (35) of an impedance of the laser cavity (31).

8. A security system (1) according to any of the claims 1 to 6, wherein the detector (40) is a radiation detector arranged to measure the variation (35) in the amount of light emitted by the laser cavity (31).

9. A security system (1) according to claim 8, wherein the radiation detector is arranged at a side of the laser cavity (31) opposite from where the laser radiation (37) to illuminate the coding surface (11) is emitted.

10. A security system (1) according to any of the claims 1 to 9, wherein the laser diode (30) is of the VCSEL type.

11. A lock (20) capable of cooperating with a key (10) comprising a coding surface (11), the lock comprising a laser diode (30) having a laser cavity (31) for generating laser radiation to illuminate the coding surface (11), - the lock furthermore comprising a detector (40) arranged to initiate a security function, arranged to initiate a security function , characterised in that the detector (40) is arranged to initiate the security function in responds to a measurement of a variation (35) in an operational characteristic of the laser cavity (31), the variation comprising the coding information and being due to the self-mixing effect of an optical wave (36) in the laser cavity (31) and back-reflected radiation (38) re-entering the laser cavity (31) after reflection from the coding surface (11).

12. A method to initiate a security function comprising the steps providing a first member (10) comprising a coding surface (11) with embedded coding information, providing a second member (20), capable to cooperate with the first member, comprising a laser diode (30) having a laser cavity (31) for generating laser radiation to illuminate the coding surface (11), characterised in that the method further comprises the steps inducing a variation (35) in an operational characteristic of the laser cavity (31) through the self-mixing effect of an optical wave (36) in the laser cavity and back- reflected radiation (38) re-entering the laser cavity (31) after reflection from the coding surface (11), and - initiating the security function in responds to a measurement of the variation

(35).