Single-sweep laser writing of 3D-waveguide devices - OSA Publishing

Single-sweep laser writing of 3D-waveguide devices Matthias Pospiech1∗ , Moritz Emons1 , Benjamin Väckenstedt1 , Guido Palmer1 , and Uwe Morgner1,2 1 Institute

of Quantum Optics, Leibniz Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany

2 Laser

Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany ∗ [email protected]

Abstract: We report on a method to create multiple waveguides simultaneously in 3D in fused silica. A combination of adaptive beam shaping with femtosecond laser writing is used to write two waveguides with changing separation and depth. The method is based on a programmable phase modulator and a dynamic variation of the phase-pattern during the writing process. The depth difference can be dynamically varied by changing a chirp parameter of the applied phase grating pattern. It can be employed in various photonic devices such as couplers, splitters and interferometers. Here we demonstrate splitters with both outputs ending in different depth. © 2010 Optical Society of America OCIS codes: (050.1950) Diffraction gratings; (070.6120) Spatial light modulators; (140.3300) Laser beam shaping; (230.7370) Waveguides; (250.5300) Photonic integrated circuits; (350.3390) Laser materials processing

References and links 1. K. Minoshima, A. M. Kowalevicz, E. P. Ippen, and J. G. Fujimoto, “Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing,” Opt. Express 10, 645–652 (2002). 2. A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett. 26, 42–43 (2001). 3. R. Osellame, V. Maselli, N. Chiodo, D. Polli, R. M. Vazquez, R. Ramponi, and G. Cerullo, “Fabrication of 3d photonic devices at 1.55 μ m wavelength by femtosecond ti:sapphire oscillator,” Electron. Lett 41, 315–317 (2005). 4. D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett. 24, 1311–1313 (1999). 5. S. Nolte, M. Will, J. Burghoff, and A. Tünnermann, “Femtosecond waveguide writing: a new avenue to threedimensional integrated optics,” Appl. Phys. A 77, 109–111 (2003). 6. J. Liu, Z. Zhang, S. Chang, C. Flueraru, and C. P. Grover, “Directly writing of 1-to-n optical waveguide power splitters in fused silica glass using a femtosecond laser,” Opt. Commun. 253, 315–319 (2005). 7. C. Florea and K. A. Winick, “Fabrication and characterization of photonicdevices directly written in glass using femtosecond laser pulses,” J. Lightwave Technol. 21, 246–253 (2003). 8. Y. Gu, J.-H. Chung, and J. G. Fujimoto, “Femtosecond laser fabrication of directional couplers and mach-zehnder interferometers,” in “Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies,” (Optical Society of America, 2007), p. CThS3. 9. M. Pospiech, M. Emons, A. Steinmann, G. Palmer, R. Osellame, N. Bellini, G. Cerullo, and U. Morgner, “Double waveguide couplers produced by simultaneous femtosecond writing,” Opt. Express 17, 3555–3563 (2009). 10. C. Mauclair, G. Cheng, N. Huot, E. Audouard, A. Rosenfeld, I. V. Hertel, and R. Stoian, “Dynamic ultrafast laser spatial tailoring for parallel micromachining of photonic devices in transparent materials,” Opt. Express 17, 3531–3542 (2009). 11. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat Photon 2, 219– 225 (2008).

#124199 - $15.00 USD Received 11 Feb 2010; revised 12 Mar 2010; accepted 18 Mar 2010; published 19 Mar 2010

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12. A. Steinmann, G. Palmer, M. Emons, M. Siegel, and U. Morgner, “Generation of 9-μ j 420-fs pulses by fiberbased amplification of a cavity-dumped yb:kyw laser oscillator,” Laser Phys. 18, 527–529 (2008). 13. L. P. Boivin, “Multiple imaging using various types of simple phase gratings,” Appl. Opt. 11, 1782–1792 (1972). 14. J. Liesener, M. Reicherter, T. Haist, and H. J. Tiziani, “Multi-functional optical tweezers using computergenerated holograms,” Optics Communications 185, 77 – 82 (2000). 15. J. Leach, G. Sinclair, P. Jordan, J. Courtial, M. Padgett, J. Cooper, and Z. Laczik, “3d manipulation of particles into crystal structures using holographic optical tweezers,” Opt. Express 12, 220–226 (2004).

1.

Introduction

Based on femtosecond laser written optical waveguides several integrated optical devices have been demonstrated, such as couplers [1, 2, 3], splitters [4, 5, 6], and interferometers [7, 8]. Recently, the use of a Spatial Light Modulator (SLM) has been adopted in waveguide writing for the creation of more complex planar photonic devices [9, 10]. Adaptive beam shaping of the femtosecond laser beam with an SLM allows for the generation of multiple foci with controlled power distribution and dynamically variable distance. Here, we extend the single sweep writing technique into the third dimension, which is one of the major advantages of direct imprinting compared to standard planar lithographic waveguide fabrication. In particular, we report on the writing of complex photonic devices based on two waveguides in a single sweep in different depths using the transversal waveguide writing geometry. The depth difference of the foci can be dynamically varied by changing a chirp parameter of the applied SLM grating mask. 2.

Waveguide writing setup

When a femtosecond laser pulse is focused tightly inside the bulk transparent material, the intensity in the focal volume becomes high enough to cause nonlinear absorption, leading to a permanent refractive index modification after relaxation of the plasma. Under suitable irradiation conditions the refractive index change is positive, allowing for direct fabrication of guiding structures [11]. The laser system employed for our experiments is a novel home-built femtosecond Yb-fiber based amplifier. The experimental setup is shown in Fig. 1. This system operates at 1030 nm and provides up to 9 µJ at 420 fs at a repetition rate of 1 MHz [12]. The substrate material is fused silica, where nonlinear absorption is an eight photon process at 1030 nm. The femtosecond laser pulses are tightly focused inside the fused silica sample at a depth of approximately 150 µm using either a 25× microscope objective with a numerical aperture of NA = 0.5 or a 40× microscope objective with a numerical aperture of NA = 0.65 while moving with a translation speed of 100 µm/s perpendicular to the laser beam axis. The SLM is a liquid-crystal phase modulator (Hamamatsu PPM, model X8267-15) consisting of an array of 768 × 768 pixels with a pixel size of ≈ 26 µm and is addressed via computer control. The incident power can be varied by a λ /2-plate and a polarizing beam splitter; the beam is expanded to a Gaussian radius of 8 mm so that the 20 mm × 20 mm aperture of the

Fig. 1. Waveguide writing setup. Half-wave plate (motorized) and polarizer (PBS) for power adjustment. #124199 - $15.00 USD Received 11 Feb 2010; revised 12 Mar 2010; accepted 18 Mar 2010; published 19 Mar 2010

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SLM is almost filled. The SLM is inserted under a small angle, and the shaped laser beam is demagnified (2.6×) by a second telescope in order to match the aperture of the final microscope objective (6 mm). After this telescope the beam is collimated, so that the phase mask from the SLM is reimaged on the input aperture of the microscope objective with a smaller size. 3.

Writing of 3D multiple waveguides

Multiple foci are created by applying a phase mask to the SLM, which distributes the incident radiation in distinct diffraction orders. The focusing lens transforms the diffraction angles into well separated foci in the focal plane. Periodic rectangular phase masks can be used to distribute the light into two symmetric displaced foci [13]. The dependency of the separation of the diffraction orders on the SLM pixel period is following a hyperbolic curve, with a period p between 20. . . 500 px. In our setup a focal spacing of 100 µm corresponds to a period of 20 px and 6 µm to a period of 500 px [9]. We extended the writing of parallel waveguides into the third dimension. This is achieved by a combination of blazed phase gratings and Fresnel phase masks. In general, foci can be moved in the lateral plane (x-direction) by the utilization of a blazed grating with period Γ and in the axial direction (z-direction) by adding a Fresnel lens with positive or negative sign of the focal length f [14, 15]. The corresponding phase functions in one dimension are 2π 2π x 2 x mod 2π φFresnel (x) = − and φBlazed (x) = (1) mod 2π λ 2f Γ

Fig. 2. Phase mask of blazed gratings and Frensel lenses. (a) φ1 : focusing Fresnel lens (shifting upwards), φ2 : blazed grating (shifting to the left), φ3 = φ1 + φ2 . (b) φ4 : defocusing Fresnel lens (shifting downwards), φ5 : blazed grating (shifting to the right), φ6 = φ4 + φ5 . left and upwards , φ6 : phase mask shifting right and downwards, (c) φ3 : phase mask shifting φ7 = arg ei φ3 +ei φ6 .

The sum of both phases φfocus (x) = φFresnel (x) + φBlazed (x) allows to control the position in x and z-direction of a single focus, see Fig. 2(a) and (b). Multiple foci can be realized by summing up the complex functions ei φfocusi and applying the argument of the complex sum to the SLM. Equation (2) shows the corresponding function for the phase masks in Fig. 2(c). φ7 (x) = arg ei(φ3 (x)) + ei(φ6 (x)) (2) In the case of an equal shift of two foci in the horizontal and vertical plane, realized by sign changes of focal length and blazed grating, the resulting phase function corresponds to a binary rectangular grating with chirped periodicity, varying across the width of the aperture, as displayed in Fig. 2(c).


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We employ these binary rectangular chirped gratings for the creation of two foci with different separations and depths. These phase gratings are described by the formula in equation (3) with chirp parameter c in the order of c = 10−3 px−1 . xSLM is the spatial coordinate on the SLM in units of pixels. 2 xSLM + cxSLM 1 φchirped (xSLM ) = π · rect sin 2π +1 (3) 2 p The calculated focal distributions of the chirped phase grating with different chirp parameters are shown in Fig. 3 together with the resulting cross section profiles of the actually written waveguides.

Fig. 3. Focusing properties of a laser beam with uniform (left (a,c,d): c = 0, p = 40 px) and chirped phase profile (right (b,e,f): c = 0.39 × 10−3 px−1 , p = 53 px). (c+e) Calculated focal intensity distribution of the laser beam. (d+f) Cross-sectional microscope views of the end faces of the parallel waveguides simultaneously written with the upper phase mask at the SLM. These waveguides were written with NA 0.5 at 100 µm/s with 500 nJ (d) and 900 nJ (f).

The two parameters chirp and period are not orthogonal, changing the relative x and z positions of the foci in the focus volume at the same time. It is nevertheless easily possible to calculate a path in the c-p space which results in a well defined change of depth while keeping the horizontal separation constant and vice versa. Figure 4 (a-b) demonstrates the variation of chirp and period that was used for a linear change in depth from 0 to 13.8 µm while keeping the x-separation constant at 100 µm. The separation in z-direction depends linearly on the chirp factor and hyperbolically on the period factor, as shown in Fig. 4 (a)). The increase in chirp factor c causes as well a linear increase in xseparation, which has to be counteracted by a concurrent modification of the period according to the curve in Fig. 4 (b). The color scales in figures 3(c) and (e) are normalized to the same maximum intensity. Obviously, the maximum focal intensity decreases substantially with increasing chirp parameter c (here to about 64 %), which has to be considered in the experiment to keep the waveguide quality high. The peak intensity of both foci in terms of depth difference is following an exponential decay, as shown in Fig. 4(c). The peak intensity difference between the two foci is quite independent of the depth difference (here in the example: 3 %).


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Fig. 4. Dependence of separation in z-direction and intensity of the foci as a function of the chirp and period parameters c and p. (a) z-separation in the c-p space. The white dotted line indicates the path from z = 0 to 13.8 µwith a constant separation in x as was used in the experiment. (b) chirp c and period p for the white line from (a) as a function of the separation in z. (c) Corresponding intensity decay.

4.

Writing of 2D Y-splitters

To create an Y-splitter in a single sweep, we extended the multiple waveguide writing from a static phase pattern to a moving focus scheme, where the distance between the two foci was dynamically changed during the substrate movement. Due to the finite response times of the pixels going from zero to π phase level with rise and fall times between 190 ms (rise) and 330 ms (fall), undefined and undesired diffraction can occur during the pixel adaption. To minimize this problem, we optimized towards a smooth change of the phase grating structure. This ensures a gap-less constant intensity in both foci over the whole time of phase shape variation. This requirement of a smooth variation is the reason for our self-limitation on binary phase gratings. The temporal phase grating evolution at the SLM is shown in Fig. 5 (a). The corresponding calculated focal distribution in the Fourier plane is pictured in Fig. 5 (b).

Fig. 5. Splitter structures from an evolving phase grating. (a) Temporal phase profile applied to the SLM. (b) Calculated focal intensity distribution.

With this grating sequence we wrote splitter structures in fused silica with an NA 0.65 focusing objective in a single sweep at a translation speed of 100 µm/s. At 20 Hz frame rate we applied 200 different phase masks per millimeter substrate movement. At the splitting point, the laser intensity had to be increased (factor 2.5) to keep the writing intensity for each waveguide constant. The intensity was controlled using a motorized rotation stage (Standa 8MR150) holding the half-wave plate. At the splitting point the intensity was changed by a rotation of the rotation stage. The total time for the intensity change was 300 ms which corresponds to approx. #124199 - $15.00 USD Received 11 Feb 2010; revised 12 Mar 2010; accepted 18 Mar 2010; published 19 Mar 2010

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30 µm movement of the substrate.

Fig. 6. Images of three different splitters written in fused silica with an end separation of 98 µm and separating lengths of 4 mm, 6 mm, and 8 mm. The image is composed by stitching together several microscope images, each scaled down in width by a factor of 20.

Figure 6 shows some examples of different splitters with a total length of 20 mm each. The pulse energy for the single waveguide was 280 nJ and for the simultaneous written double waveguides 290 nJ per focus. The zoom in Fig. 6 reveals that the waveguides do not really overlap at the slitting point. The closest possible separation is 6.2 µm, simply limited by the largest possible phase grating period on the SLM. This is sufficient for splitting anyhow, since the mode field diameter of the resulting waveguide modes is of the same size. The guiding properties were proven by butt-coupling the light from an infrared fiber laser at 974 nm (JDSU 2600) into the input port and observing the guided modes with a camera (Sumix, SMX-10Mx) at the output. Figure 7 proves the splitting and the single mode guiding.

Fig. 7. Near field image of the splitter output (middle in figure 6).

The losses of the splitters were calculated from PT 1 LdB = − CLdB − FLdB , 10 · log 2 cm PI · ηSplit

(4)

with PT : transmitted power from one output, PI : incident power, ηSplit : splitting ratio, CLdB : mode matching loss at the entrance, FLdB : Fresnel losses at both surfaces. From the splitter structures shown in Fig. 6 we produced and analyzed 30 samples and observed an average loss of LdB = −6.8 ± 1.5 dB/cm. Compared to single or parallel waveguides, the splitter imposes an additional loss of 20 %. The average splitting ratio is ηSplit = 50 % ± 8.3 %. #124199 - $15.00 USD Received 11 Feb 2010; revised 12 Mar 2010; accepted 18 Mar 2010; published 19 Mar 2010

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5.

Writing of 3D Y-splitters

For the creation of single sweep 3D-splitters, we added the chirp parameter from equation (3) to the grating evolution. Figure 8 demonstrates the scheme. The left splitter shows the 2D-splitter from the last section, whereas in the 3D version on the right side, behind the splitting point the two waveguides leave the 2D plane in different directions and both outputs end at different depths. A linear slope for the change in depth was chosen, but any curved variation of the depth difference could be realized as well.

Fig. 8. 3D sketch of 2D and 3D splitters.

The 3D splitter devices have been created with an NA 0.5 objective to emphasize the depth difference which is proportional to the focal length of the focusing lens. The lower NA results in more elliptical mode profiles and the losses in the splitters increase to an average loss of LdB = −10.2 ± 0.7 dB/cm (taken from 34 samples).

Fig. 9. 3D Y-Splitter with linearly increasing depth difference on the right half. The resulting depth difference at the output is 13.8 µm. (a) Full microscope top image of the splitter. (b) Cross-sectional view at the output. (c) Near field profile of the output beams.

A 3D-Splitter is displayed in Fig. 9. On the left half of Fig. 9(a) the structure is 2D, then on the right, the chirp parameter increases from 0 to 0.39 × 10−3 px−1 , and the pulse energy increases #124199 - $15.00 USD Received 11 Feb 2010; revised 12 Mar 2010; accepted 18 Mar 2010; published 19 Mar 2010

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linearly from 0.5 µJ to 0.9 µJ per focus. The corresponding values of chirp and period variation are demonstrated in Fig. 4. The pulse energy was changed using a half-wave plate mounted in a motorized rotation stage. Coupling light into the input port of the splitter, we observe an imbalance between the two output ports of about ηSplit = 50 % ± 14 %. This imbalance vanishes for depth differences below 10 µm. This difference originates either from the splitting point or from unexpected imabalance between the two waveguides. Further investigations will identify the major contributing factor. The losses in these splitters are comparable to the 2D versions. Larger depth differences are not accessible due to the maximum power capability of the beam shaper according to Fig. 4(c). Additionally, higher chirp factors required for larger depth differences are limited by the smallest realizable periodicity of the phase grating, which is limited by the resolution of the SLM. For periods lower than 10 pixels the diffraction efficiency decreases dramatically. 6.

Conclusion

We demonstrated 2D and 3D waveguide splitters written in single sweeps in fused silica. The concept of single sweep creation of complex photonic structures allows for a fine control of the waveguide spacing independent from the reproducibility of the micro positioning system. The splitting of the laser beam into two foci was realized by a phase grating from a computer programmable beam shaper. By dynamic variation of the phase grating period during the substrate translation we demonstrated splitting of single-mode waveguides. We introduced a novel concept for phase gratings for 3D shaping by including a chirp parameter. With this concept, the depth difference of the two foci can be tuned. To our knowledge this is the first demonstration of simultaneous 3D writing of multiple waveguides. This concept is directly and universally applicable to much more complex structures. With today’s high power femtosecond laser systems the number of parallel written waveguides is only restricted by technical limitations of the available light modulators (pixel resolution, phase switching rates and power capability). With the demonstrated beam shaping method various photonic devices are easily achievable just by software control; examples are directional couplers, splitters, and interferometers for complex but flexible 3D photonic networks, e.g. for optical sensing in lab-on-a-chip devices. Acknowledgments This work was supported by the European Union within contract no. IST-2005-034562 (Hybrid integrated bio-photonic sensors created by ultrafast laser systems - HIBISCUS).


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