1. Introduction

Banknotes, seeds, high priced industrial products, jewelry, and even pharmaceuticals have recently required authenticity verification processes by using various methods developed for preventing forgery [1–8]. Nowadays, because of the improvement and generalization of high-resolution equipment, such as color printers or scanners, image copy based counterfeiting poses serious threats. To combat counterfeiting based on image replication, more difficult and complex technologies have been developed and applied for anti-counterfeiting [9–11]. Since it is impossible to counterfeit by using simple copy devices, the most representative method is holograhy, which show changes in the images and colors depending on the viewing angles [12, 13]. Holograms are so-called “level 1” anti-counterfeiting method which everybody can check without any devices. To print the holographic images, precision mold fabrication is necessary, and thus counterfeiting can be difficult. However, as the cost of fabricating the mold is continually decreasing because of continuous advances in the molding technology, even the hologram technique has become vulnerable to the risk of counterfeiting. In addition, the need to fabricate new precision molds each time so as to create different hologram patterns is a disadvantage. Furthermore, most of the holograms are produced in sticker forms so as to attach it to products; thus, it could be at risk for being transferred to counterfeit goods. Although various optical techniques can be applied for anti-counterfeiting, the greatest limitation of the existing methods is that most of these methods try to prevent counterfeiting images. Therefore, in order to make it more difficult to replicate, as mentioned above, image anti-counterfeiting methods are becoming more complicated images. As a result, it induces more process steps for fabrication and becomes more expensive in terms of production costs. Therefore, in recent years, the barcode and quick response (QR) code are being applied to products [14,15]. These methods provide some information, such as the product price and direct linkage to a web page, including counterfeiting or not. In addition, radio frequency identification (RFID) tags [16,17] and electronic circuits [18,19] can be applied to authentic products. However, barcode and QR code patterns can be duplicated by simply copying; thus, it is meant to be used for information delivery rather than for anti-counterfeiting. The circuit installation process for anti-counterfeiting is complicated because circuit fabrication involves several process steps. Moreover, mass spectrometry is developed, but it is also slightly complicated [20, 21].

In this work, we propose a high-speed process in periodic patterning with Fresnel biprism(FB) interference and in analysis of azimuth angles of diffractive fringes. Micro-sized grating patterns were quickly and accurately fabricated on the surface of the material directly by using a nanosecond pulse laser. Diffraction fringes at various angles are generated when the fabricated grating patterns are illuminated using a continuous wave (CW) laser for measurement, and each of these angles is matched to a letter and a number. Depending on the angle combination of the diffraction fringes, a lot of information can be recorded. Alternatively, if the size of grating patterns is large enough to be detected by a smartphone’s camera and a simple portable lens, then the direction of gratings could be recognized quickly and easily without a measurement equipment using a CW laser.

2. Experiment

The simple 1-dimensional and 2-dimensional periodic microscale patterns can be engraved precisely by ultra-short laser interference using a FB, grating and beam-splitter [22–24]. The different ripples were generated on the surface by optimizing a pulse laser fluence and a beam scanning speed [25,26]. Figure 1(a) shows the experimental set up for interference fringe patterning on the surface of various materials by using a high energy pulsed laser. Since the laser’s power should be controlled based on the characteristics of the material, it is controlled using an attenuator. While rotating the FB, the interference fringe rotates according to the angle of rotation of the FB, and thus the direction of the manufactured grating is also rotated. However, in order to get a precise interference pattern, the polarization control is very important because the interference pattern can be affected depends on the relative polarization state of two beams [27, 28]. In order to avoid the polarization effect due to the rotation of the FB, the two quarter-wave plates were used. When linearly polarized light passes through the first quarter-wave plate, it is converted to circularly polarized light; when this circularly polarized light passes through the second quarter-wave plate, then it is converted to linearly polarized light. Therefore, the first quarter-wave plate should be fixed and the second quarter-wave plate should rotate in accordance with the FB. This configuration enables the two beams to be interfered by same polarization direction, by avoiding the polarization changes due to the rotation of the FB. By placing the mask of the desired shape in front of the specimen to be fabricated, the appearance of the micropattern can be determined. In this study, a circular shape was fabricated by placing a circular mask. In Figs. 1(b)–1(d), the grating pattern was fabricated by using a mask of 250 μm aperture size, on each NAK80(plastic mold steel), aluminum, and silicon. The energy level of the laser pulse used for processing can be adjusted using an attenuator having a wavelength of 1064 nm, pulse duration of 6 ns, and repetition rate of 10 Hz. Since most of the processing is possible with a pulse count of less than 10, the fabrication of the micropattern for Figs. 1(b)–1(d) took less than 1 s, and thus, it is capable of high-speed fabrication processing. The grating patterns can be fabricated by beam focusing and scanning. Since the lens based focusing method requires a precise control and longer time for laser beam scanning, it has drawbacks as compared to the inference method. In this paper, it increased the speed of micropatterning as well as its accuracy by using the interference method. The followings are the processing conditions in Figs. 1(b)–1(d): For NAK80, the laser pulse energy is 112 mJ, five pulses; for aluminum, the laser pulse energy is 163 mJ, five pulses; for the silicon wafer, the laser pulse energy is 182 mJ, 10 pulses.

 

Fig. 1 Rotatable interference fringe patterning on different materials. (a) Schematic illustration of the laser interference fringe patterning with FB. Laser beam diameter has been magnified for understanding. The power of laser is appropriately controlled by attenuator based on the properties of the target material. FB and quarter-wave plate are mounted on rotation stage perpendicular to the optical axis in order to precisely rotate the interference fringe. Mask of circular aperture is placed in front of the target material. (b~d) Optical microscopic images of the fabricated grating patterns of NAK80, aluminum, and silicon plates, in alphabetical order.

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3. Results and discussion

When beams pass through FB, according to Snell’s law, as in the Fig. 2, the beam will refract toward the center of FB. Using Eq. (1) with conditions of FB used in the experiment, we obtain $\theta$ = 2.3°; where refractive index of FB $\mu$ = 1.5, side angle of FB $\alpha$ = 4.6°

 

Fig. 2 Beam refraction direction and angle at the surface of Fresnel biprism.

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Fig. 3 Analysis of interference characteristics. (a) Conceptual illustration for observing the interference intensity pattern with the magnification optical system combined with lens and CCD according to interference position. R denotes the rotation angle of combination FB and λ/4 plate. Z denotes the distance from mask to interference position, which is observed by a combination of lens and CCD. (b) Laser interference intensity images acquired by moving CCD and lens along the optical axis. The scale bar indicates 200 μm. (c) Line profiles of interference intensity along the dashed lines in Fig. 2(b). The inset shows the FFT analysis of line profiles. Optical microscope images show the patterning results with respect to Z positions. (d) Interference images according to rotating angle of FB and λ/4 plate. The scale bar indicates 100 μm.

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To find the best grating fabrication result of the specimen with the best diffraction fringe visibility, the laser’s processing condition was changed and examined. Depending on the laser illumination time, the degree of processing could change. Figure 4 shows the results obtained by setting the pulse energy at 112 mJ and varying the laser illumination time on the specimen. NAK80 was used as a material; the laser beam was irradiated for 1 s in Figs. 4(a) and (b), and for 5 s in Figs. 4(c) and (d). As shown in the figures, at 5 s, the inscription was deeper, but the surface was not uniform. Although both conditions have the same grating period, as shown in inset of Figs. 4(b) and (d), the reason why the diffraction fringe is sharper at the 1 s illumination may be related to the roughness of the processing surface. To generate a sharp diffraction fringe, the information of the constructive and destructive phase must be accurate from the grating to the detector. However, as in Fig. 4(c), where the surface is very rugged and irregular, it seems that it cannot produce a sharp diffraction fringe because the phase information is distorted. Therefore, to generate a highly visible diffraction fringe, it is important not only to maintain a constant period of the grating pattern but also to enhance the surface roughness to reduce the phase error, rather than trying to carve the grating deeper. The insets of Figs. 4(b) and 4(d) illustrate the diffraction fringe generated by illuminating a 532 nm wavelength laser on the grating. When the roughness of the processing surface is smoother and regular, as in Fig. 4(b), the diffraction fringe is sharp, but using a relatively coarser surface roughness as in Fig. 4(d), the high-order diffraction fringes get weaker.

 

Fig. 4 Comparing the fabricated patterns on the NAK80. The images were captured by scanning electron microscope. (a) A circle-shaped grating pattern was fabricated using a 112-mJ pulse for 1 second. (b) Magnified image of Fig. 3(a). The inset shows the diffraction fringe generated by the continuous laser. (c) Grating pattern was fabricated using a 112-mJ pulse for 5 seconds. (d) Magnified image of Fig. 3(c). It shows the rougher fabricated surface compared to Fig. 3(b). Inset shows the diffraction image.

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In order to shorten the analysis time for measuring a large number of fringe angles, as in Fig. 5(a), two lasers and polarizers were configured to produce four diffracted fringes simultaneously. Lasers with wavelengths of 532 nm and 632 nm are passed through beam splitters and proceed along the same beam path, passing through vertical and horizontal polarizer and the red and blue color filter. By doing so, four gratings are illuminated by four different combinations of laser colors and polarization directions. Due to the diffraction phenomenon, the distance between each order of the diffracted fringe, which is generated from a single grating, grows apart as it progresses. However, distance between the each order is not far until it is reflected by a beam splitter. Therefore, diffracted fringe up to the ± 5th order can pass through and be reflected by the beam splitter. Then, it will be directed toward the detector. However, when considering the distance between beam splitter and the detector, all of the ± 5th order diffracted fringes cannot be observed by the detection area of CCD. To enable the detection of diffraction fringes below the ± 5th order, we use a convex lens in front of the detector to bring the diverged fringes closer. Two linear polarizers, whose transmission axes are rotated by 90° relative to the axis of the each polarizer, are attached side-by-side, and the contact surface is placed at the center of beam axis. This polarizer is located in front of the target, and the other polarizer is placed in front of the detector. Both polarizers are oriented at 90° relative to each other. Half of the polarization direction of diffracted fringes generated by the target is not compatible with the transmission axis of polarizer located in front of detector, resulting in half of the diffracted fringe being blocked by the polarizer. Therefore, only other half is detected by the detector. When the grating lens, polarizer, and detector are aligned along the beam path, as shown in Fig. 5(b), it is possible to clearly observe that four diffracted fringes are rotated based on the center of detector at the same time. The angles are defined such that the 26 alphabets and 10 numbers could be expressed within 180°. For instance, alphabets were in five degree intervals from 10° to 120° and the numbers were implied with meanings to 4 degree intervals from 130° to 166° in sequence; these may be combined and adjusted in a variety of ways.. As a this rule, four diffracted fringes of Fig. 5(b) denote the ‘I’, ‘L’, ‘W’ for alphabets and ‘0’ for number respectively. In addition, based on the precision of the diffracted fringe pattern recognition, the measurement error range can be determined; for example, about ± 1 degree.. In this way, when meanings are assigned to the alphabet and numbers in an arbitrary angle, a variety of information, such as serial code, can be stored in each angle of the diffraction fringes. Figure 5(c) shows optical microscope images of fabricated patterns. If we fabricate the grating patterns that are relatively larger than the size and period of a grating which is for laser based detection system, or improve the resolution of smartphone camera by adding simple external magnifying lens, we can examine whether it is a phony with not the angle of diffraction fringe but the pattern directions. Although the experiment results of image processing to recognize the direction of gratings by smartphone is not shown in this work, it is anticipated that when a smartphone application is developed for anti-counterfeit serial number recognition, the user can quickly and easily find the products’ authenticity along with the product information. When the direct micro-patterning on the authentic product is difficult owing to the material’s rough surface, fabricating micro-pattern on thin film, such as a sticker, can be used instead of direct patterning on the authentic surface. For example, in the case of a banknote, grating is fabricated on the thin film of aluminum and then attached to the Korean banknotes, as shown in Fig. 5(d). Therefore, by using the proposed method in this paper, the interference fringe may be directly inscribed on the surface or made into a thin film sticker form in order to attach to authentic materials, in accordance with the characteristics of the material.

 

Fig. 5 Diffraction fringe detecting apparatus and grating patterning on samples. (a) Conceptual image of the system for measuring four different diffraction fringes simultaneously by using two lasers, two polarizers, and two color filters. The direction of the arrow indicates the transmitting axis of polarizer. Two linear polarizers are combined together such that their axes are perpendicular to each other. In addition, the red and blue color filters are placed adjacently. The bonding lines of the polarizer and the color filter are perpendicular to each other’s optical axis. The figure also illustrates the positioning of the Beam expander (BE), Beam splitter (BS), Polarizer (P), and Color filter (CF). The red laser beam path is illustrated, but the green laser is omitted for the sake of the reader’s understanding. (b) Images of the four diffracted fringes at CCD. The left half plane of the detector collects the horizontally linear polarized signal and right half plane of detector views the vertically linear polarized signal. The center dot line indicates that the CCD detecting plane is divided virtually into two areas, and the arrows in the image denote the detected linear polarization direction of beam at each side. (c) Fabricated interference pattern on a metal block for analysis with smartphone. (d) Attachment of the gratings patterned on aluminum film on a Korean banknote. See Visualization 1.

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4. Conclusions

In conclusion, in this work, we proposed a scheme for detecting counterfeiting by interference fringe patterning directly on the surface and detecting the rotation angle of diffraction containing the meaning of the serial code. Given that the present set-up would use the FB, a simple and efficient high-speed optical system with a rotated micro-sized grating can be produced while rotating the FB. Consequently, a high-speed and non-contact method can be realized. Moreover, the method is robust against counterfeiting with copy devices, such as printers, and one can record information, such as serial code, on the original product surface. In terms of detection, it requires the special optical equipment unlike hologram, the non-contact method can produce results a high speed without damaging the product. If a higher level of complexity of counterfeit prevention is required, then a laser based high precision detector is necessary. On the other hand, for low level of anti-counterfeiting, we anticipate that the counterfeits and product information can be found out using a personal smartphone and a simple external lens. Therefore, the anti-counterfeiting technology proposed in this study can find applications in anti-counterfeiting measures for high-end manufactured goods, currency, pharmaceuticals, and important documents.