1. Introduction

Femtosecond lasers have been extensively used for microfabrication and micromachining of various three-dimensional structures in dielectrics, such as waveguides [1,2], couplers [3,4], gratings [5,6] and three-dimensional channels [7–9]. Optical data storage in nonphotosensitive medium has also been demonstrated [10–14]. There are mainly two approaches to record and retrieve information. When ultrashort laser pulses are tightly focused inside a transparent material, permanent submicrometer-sized voids can be created by microexplosion. This process is used to record digital information in three dimensions by writing multiple bit planes [10–12]. This bit-oriented storage has high density and huge capacity but the bit-by-bit readout is relatively slow. On the other hand, holographic storage by surface ablation needs only a single pulse to record and the whole data in the hologram can be reconstructed with a reference beam [13,14]. Both writing and reconstruction are extremely fast. Nevertheless, it is hard to write the hologram inside transparent materials because the two recording beams are loosely focused to obtain enough overlapping area. The surface is usually ablated before internal permanent modification could occur for the surface ablation threshold is much lower.

In this paper we proposed and realized a novel technique for optical storage. An object image was sampled and Fourier transformed by a computer. Then the complex amplitude distribution was encoded by detour phase method to form a binary computer-generated hologram (CGH). The resulted CGH was recorded inside silica glass in a single step by microexplosion induced by tightly focused femtosecond laser pulses. The fabricated CGH was permanent because it was written inside silica glass without surface damage and was not annealed out at 600°C. The whole image was reconstructed with a collimated He-Ne laser beam. The recording is like that in bit-oriented storage and the reconstruction is similar to that in holographic storage. In addition to optical storage, this technique can be also applied to microfabrication of diffractive optical elements for microoptics.

 

Fig. 1. (a) Object image with 64×64 pixels - logo of Peking University; (b) Simulation of reconstruction from the CGH encoded by detour phase method.

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CGH’s can produce wavefronts with any prescribed amplitude and phase distribution and have, therefore, found many applications to matched filtering [15], interferometric test of aspheric surfaces [16], generation of three-dimensional image [17,18], and holographic optical manipulation [19]. There are two steps for production of a Fourier hologram. The first step is to calculate the complex amplitude of the virtual or physical object wave at the hologram plane. The second step involves encoding and production of a transparency. Traditionally a large scale version of the hologram is produced by a plotter or a printer and then photographically reduced to the required size. The photoreduction can be eliminated by using electronic beam writing that produces only surface relief patterns and usually needs multiple procedures. By using femtosecond laser pulses, we are able to directly fabricate the expected CGH inside a bulk of silica glass.

2. Experimental and results

The logo of Peking University was selected as the object image. It consisted of 64×64 pixels as shown in Fig. 1(a). The desired CGH was the discrete Fourier transform of the input object field. The complex amplitudes (modulus and phase) were encoded by detour phase method. The simulation of the reconstruction from the CGH is given in Fig. 1(b).

In our experiment, the final binary hologram was divided into 64×64 equally spaced cells. The top left of the CGH is shown in Fig. 2(a). As demonstrated in Fig. 2(b), each cell comprised 8×8 dots. Rectangular apertures were drawn inside each cell. Each aperture was determined by its height, h_mn, its width, w_mn, and its center with respect to the center of the cell, c_mn. In our experiment, the width was equal to the half width of the cell. The height and the center were proportional to the modulus, A_mn, and the phase, φ_mn, of the sampled values of the wavefront $A_{mn}(x,y)e^{-i{\phi }_{mn}(x,y)}$ taken at x=md and y=nd, where d was the sampling distance along x and y coordinates. A_mn was normalized in the calculation. Therefore, h_mn=A_mnd, w_mn=w d/2, and c_mn=φ_mn d/2π. This coding procedure was carried out for all the cells in the hologram.

 

Fig. 2. (a) Top left of the CGH encoded by the detour phase method. The whole CGH is divided into 64×64 equally spaced cells; (b) One of the cells. Each cell consists of 8×8 dots and a rectangular aperture is drawn inside it. The aperture’s height, h_mn, and the center with respect to the center of the cell, c_mn, are determined by the wavefront $A_{mn}(x,y)e^{-i{\phi }_{mn}(x,y)}$ taken at x=md and y=nd, where d is the sampling distance along x and y coordinates. The width, w_mn=w=d/2, is constant for all apertures.

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It is obvious that only a finite combination of aperture heights and positions can be drawn when an incremented plotter or printer is used. The number depends on the resolution and the incremental step size. If the number is too small, reconstruction quality suffers. In our experiments, the cell was made up of 8×8 dots, the number of effective phase values was 8 while the number of effective modulus value was 5 because the aperture height could be changed only by an even number of increments due to the symmetrical fabrication. The last row was set to be opaque due to the need of symmetry.

The overlapping of rectangles from adjacent cells was eliminated by using a slightly modified cell structure called circular overflow. Briefly put, when the rectangle flowed out of its own resolution cell, it was truncated at the edge. The overflowed portion was then placed at the opposite edge of the cell. For example, as shown in Fig. 2(a), quarter of the rectangle in the eighth cell in the first row was placed at the left edge.

After the CGH was encoded, we directly fabricated it inside a bulk of silica glass with femtosecond laser pulse. A regeneratively amplified Ti:sapphire laser system (Spitfire, Spectra Physics) was used, which delivered pulses with a duration of 120fs (FWHM), a center wavelength at 800nm and a repetition rate of 1kHz. A long working distance objective lens with a numerical aperture of 0.50 (LMPLFL50X, Olympus) was applied to focus the laser pulses. The sample was mounted on a computer-controlled XYZ translation microstage with 0.1µm resolution. The schematic setup is shown in Fig. 3(a). To record the desired CGH, the sample was moved step by step and irradiated by the focused pulses according to the hologram pattern. The irradiated dots became “black” by microexplosion while the unirradiated dots remained “white”.

 

Fig. 3. (a) Schematic setup for direct writing of a CGH inside silica glass with femtosecond laser pulses at 800 nm. The CGH is 300µm beneath the front surface. The incident energy is 0.7µJ per pulse at 1kHz; (b) Top left of the fabricated CGH. The designed counterpart is shown in Fig. 2 (a).

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When the pulses were focused into the silica glass, either refractive index change or damage could be induced, which was dependent on the pulse duration and the incident energy [20]. In our case, internal damage via microexplosion was needed to make the irradiated area opaque. Therefore, we stretched the pulse duration to ~300fs. The incident energy was 0.7µJ per pulse. Then the chirped pulses were focused 300µm beneath the front surface of the sample. The high fluence at focus induced microexplosion to create a void. The void damage scattered light strongly, resulting in an opaque spot inside the transparent glass.

 

Fig. 4. (a) Schematic setup for reconstruction from the fabricated CGH; (b) Reconstructed object image and high order images as well as their conjugate images; (c) An enlarged version of the upper part in (b).

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There were 512 or 64×8 dots in each row and 512 rows in total. The CGH was first scanned in x direction step by step and then moved to the next row by moving in +y direction. The odd rows were scanned in +x direction while the even ones in -x direction. The translation speed was 40µm/s. The lateral size of the void was about 3µm so that the translation step was set to be 3µm. The cell width was 24µm and the final CGH was ~1.54×1.54mm². The top left part of the fabricated hologram is presented in Fig. 3(b). Its designed counterpart is shown in Fig. 2(a).

Cell sizes and locations were almost the same as expected, but edges were not well defined and the surrounding of a void seemed gray rather than black because an opaque area was actually an array of separate ellipsoid-like voids. By reducing translation step to make adjacent voids partly overlap, uniform areas with smooth edges can be achieved at the expense of writing time. A feasible approach might be to shape cross-sections of foci rectangular. Then, the resulted rectangular voids would easily form a more accurate rectangular opaque area.

To reconstruct the object image from the fabricated CGH, a collimated He-Ne laser beam was normally incident on the CGH and the diffraction pattern was collected by a lens onto a screen. Then the output image was taken by a digital camera. The setup for reconstruction is shown in Fig. 4(a). In addition to the original object, there were high order images as well as their conjugate images as demonstrated in Fig. 4(b). The image could be selected by spatial filtering. The zero-order was attenuated for a better contrast. An enlarged version of the upper part is presented in Fig. 4(c). Though the diffraction efficiency was 3%, it was shown that the original object image was reconstructed with high fidelity.

Besides a plane image shown here, a 3D object can be stored and reconstructed using this direct writing method if the encoded CGH is binary. Because a CGH of an image or of a diffractive optical element is written inside glass in a single step, it can be integrated with others for a complex display or a functional device in one bulk of glass. In addition, femtosecond laser pulses have induced microexplosion inside many transparent materials, so CGH’s can be directly fabricated in these media like inexpensive and widely-used soda-lime glass.

3. Conclusion

We have stored an object image in a permanent computer-generated hologram directly written inside silica glass with femtosecond laser pulses and reconstructed with a collimated He-Ne laser beam. The recording is like that in bit-oriented storage and the reconstruction is similar to that in holographic storage. This technique can be applied to not only optical storage of virtual objects that can’t be realized with traditional holography, but also microfabrication of diffractive optical elements that are extensively used in microoptics.