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Laser Direct Writing of Photonic Structures in X-cut Lithium Niobate using Femtosecond Pulses

S. Venugopal Raoa*, T. Shuvan Prashant,a K.L.N. Deepak,b Surya P. Tewari,a G. Manoj

Kumar,a and D. Narayana Raob

a Advanced Center for Research in High Energy Materials, bSchool of Physics University of Hyderabad, Hyderabad 500046, India

* Author for Correspondence: [email protected] OR [email protected]

ABSTRACT

We have fabricated straight line structures and Y-couplers in X-cut lithium niobate crystals using femtosecond laser pulses. A systematic characterization study was performed initially to determine the effects of pulse energy on feature size. The optimal parameters were determined from experiments and simulations obtained using a two dimensional split step beam propagation method. Later the waveguides and couplers were fabricated using these optimized parameters. We present our results on the physical and optical characterization of these structures. Keywords: lithium niobate, femtosecond, laser direct writing, photonic devices, beam propagation

2. INTRODUCTION Lithium Niobate (LNB) has emerged as promising material for fabricating integrated optoelectronic circuits because of its favorable physical properties such as large anisotropy, high electro-optic coefficients and large second order nonlinearity. LNB crystals are positively birefringent and, hence, the extraordinary refractive index ne for z-plane polarized light is higher than the ordinary refractive index no along the x and y direction. We refer to the crystal as X-cut or Z-cut depending on which crystal axis is perpendicular to the top surface. Many approaches have been implemented to fabricate waveguides on LNB crystals including ion implantation, liquid phase epitaxy and laser direct writing (LDW) [1-4]. However, the LDW technique stands apart from others because of the relative ease of realization of the microstructures and possibility of 3D photonic devices and integration. Incident intensity of the focused ultrashort pulses determines the type of modification in LNB: (a) an increase in refractive index ne (type I) (b) a decrease of both no and ne (type II) and (c) formation of micro voids [1, 2]. Thomas et al. have recently achieved a hybrid fs laser written chip in lithium niobate that comprised a rare-earth-doped laser section, a frequency doubling unit, Bragg reflectors, waveguide splitters, and an amplitude modulator [2]. However, detailed studies with different pulse durations and writing conditions are essential and enable us to identify the optimal conditions for achieving low loss (insertion, propagation etc.) photonic structures in lithium niobate [10-14].

3. EXPERIMENTAL DETAILS

The schematic and the experimental set up are illustrated in figure 1. The femtosecond oscillator amplifier system is capable of generating ultrashort pulses of ~100 fs duration at a repetition rate of 1 kHz and wavelength of 800 nm. A vertical microscope configuration is employed using a dry 40X (0.65 NA) microscopic objective (Olympus) to focus the laser beam on to the sample and 3D XYZ stages were employed to translate the sample [5]. A Brewster polarizer-half wave plate combination was used to control the intensity of the pulses. M1 to M5 are the mirrors. The crystals were X-cut and were polished and cleaned/sonicated in distilled water before laser direct writing. The modified regions of the crystal were examined using confocal and micro-Raman spectroscopy. Initially, the energy dependence of feature size was characterized. The parameters namely, the stage scan speed, the number of writes and the energy of the pulse were varied and optimized for realizing the photonic devices.

Photonics 2010: Tenth International Conference on Fiber Optics and Photonics, edited by Sunil K. Khijwania,Banshi D. Gupta, Bishnu P. Pal, Anurag Sharma, Proc. of SPIE Vol. 8173, 81730G © 2011 SPIE

CCC code: 0277-786X/11/$18 · doi: 10.1117/12.899559

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Figure 1 A

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NB can be desB = 0.11768, C 432 & D = -0.0

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set up (left) and SIMULATIO

dysh parameter×c×n×Eg× ε0/I

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ength from Sellm

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ides informatiohe laser frequeectively, and c is the permitti

n as the domin

+ B/(λ2+C) +fractive index n [6,7]

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right).

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Figure 2 illustrates the dependence of refractive index on wavelength. The typical He-Ne wavelength (λ=0.633 μm) was chosen as the input beam wavelength for the beam propagation simulations for which the indices no = 2.287 and ne = 2.204. Figure 3 illustrates the refractive index profile of a double line structure along with the parameters and also the input beam profile assumed for propagation studies. The dimensions for the waveguides and Y couplers are optimized using the 2D Fast Fourier transform beam propagation method (FFT-BPM) [8].

Figure 3 Above curve shows the refractive index profile of a double line structure and the various parameters line width (LW),

waveguide width (WW) and RIC (∆n). Below curve depicts the input Gaussian mode used in all the simulations.

For the type I waveguides, written structures or the adjacent regions act as the waveguiding regions. However, type I structures are susceptible to thermal degradation leading to reduction of light confinement. The following parameters have been varied to simulate type I structures: line width and the refractive index change (RIC=∆n) induced in the modification.

Figure 4 Double line y-branch used for simulations with three regions: linear waveguide (z1), taper region (z2) and split waveguides

(z3) and other parameters are waveguide width (x1) and branch angle (θ).



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hange in refractwritten lines. T

and RIC (∆nifications. Theacing between

5. RESULT

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ting speeds werere is monoton, we have obsedent pulses on t

was used for wn of the microfirmed damagemicrostructure

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arameters whicguides.

ISCUSSION

fs pulses are istructure widt

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ation of ‘pearl t higher writingses. We strongve. A careful estigations are ng application

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illustrated in fth in LNB is dre width on enachieved with f

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0.8, 1, 2, 4, 6, 8width with incr

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inspection of pending to ide

ns in fabricatio

realized by encstigate the effeee figure 4), wed are the angl

figures 5a anddirectly propornergy for speedfew tens of μJ

e are correspondnd 1 mm/s.

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entify the exacton of photonic

closing a ect of the we have le of the

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ding

s and the g speeds. bute this

ures were res could after the t reason. crystals



Figure 6a Lin

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volume, b) Strucsible for the x(zzRaman peaks ob

gy of 40 μJ. b De

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ependence of stru

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modes due to mng across z-axcrease in Ramae 8 shows the entified in theer irradiation.

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Figure 8 (a) and 635 nJ r

Figure 8 presIn the FFT-Bothers constaand LW = 1transmitted oindex contrasthe plots. Ou

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Figure 9 F

and (b) are confespectively. The

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Figure on the leftcontrast o

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f typical 2-mmns, the differenepicts a typicalex contrast of le line wavegu

2, FFT-BPM mshow that for s

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6. CONCLUSIONS

In conclusion, we have fabricated and simulated simple photonic structures in LNB wafers. Confocal and micro-Raman characterization has been done to understand the structural and lattice deformations. Varying the different writing parameters allowed us to fabricate structures of desired dimensions. Optimization of photonic device dimensions is achieved by undertaking simulation studies using FFT-BPM technique. Our future studies will be to understand the differences in structures written with ~100 fs and ~40 fs.

Figure 10 Dependence of transmittance on waveguide width (a) for different line widths, for different index contrasts (b) Dependence of transmittance on line width w0 stands for input Gaussian mode’s spot size (c) (LW = 10 μm and w0 = 4 μm) (d) (LW = 10 μm and

w0 = waveguide width)

7. REFERENCES

[1] Gattass, R. R. & Mazur, E., "Femtosecond laser micromachining in transparent materials," Nat Photonics 2, 219-225, (2008).

[2] Thomas, J., Heinrich, M., Zeil, P., Hilbert, V., Rademaker, K., Riedel, R., Ringleb, S., Dubs, C., Ruske, J.-P., Nolte, S., and Tu¨nnermann, T., "Laser direct writing: Enabling monolithic and hybrid integrated solutions on the lithium niobate platform," Phys Status Solidi A 208, 276-283, (2011).



[3] Thomson, R. R., Campbell, S., Blewett, I. J., Kar, A. K. & Reid, D. T., "Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime," Appl Phys Lett 88, 111109 (2006).

[4] Nejadmalayeri, A.H., Herman, P.R., “Ultrafast laser waveguide writing: lithium niobate and the role of circular polarization and picosecond pulse width,” Opt. Lett. 31, 2987-2989 (2006).

[5] Kallepalli, D. L. N., Desai, N. R. & Soma, V. R., "Fabrication and optical characterization of microstructures in poly(methylmethacrylate) and poly(dimethylsiloxane) using femtosecond pulses for photonic and microfluidic applications," Appl. Opt. 49, 2475-2489 (2010).

[6] Smith, D. S., Riccius, H. D. & Edwin, R. P. "Errata" Opt. Commun. 20, 188-188, (1977). [7] Smith, D. S., Riccius, H. D. & Edwin, R. P., "Refractive-Indexes of Lithium-Niobate," Opt. Commun. 17, 332-

335 (1976). [8] Banerjee, P. P. & Jarem, J. M. [Computational Methods for Electromagnetic and Optical Systems] Optical

Science and Engineering, CRC Press, 2000. [9] Rodenas, A., Nejadmalayeri, A. H., Jaque, D. & Herman, P., "Confocal Raman imaging of optical waveguides in

LiNbO3 fabricated by ultrafast high-repetition rate laser-writing," Opt Express 16, 13979-13989 (2008). [10] Gui, L., Xu, B., Chong, T.C., "Microstructure in Lithium Niobate by Use of Focused Femtosecond Laser Pulses,"

IEEE Photonics Technol. Lett. 16, 1337-1339 (2004). [11] Burghoff J, Hartung H, Nolte S, A. Tunnermann, “Structural properties of femtosecond laser-induced

modifications in LiNbO3,” Appl. Phys. A. 86, 165-167 (2007). [12] Amir H. Nejadmalayeri and Peter R. Herman, "Rapid thermal annealing in high repetition rate ultrafast laser

waveguide writing in lithium niobate," Opt. Express 15, 10842-10854 (2007) [13] Burghoff, J., Grebing, C., Nolte, S., and Tunnermann, A., "Efficient frequency doubling in femtosecond laser

written waveguides in lithium niobate," Appl. Phys. Lett. 89, 081108 (2006). [14] Nejadmalayeri, A.H., Herman, P.R., "Rapid thermal annealing in high repetition rate ultrafast laser waveguide

writing in lithium niobate," Opt. Express 15, 10842-10854 (2007)



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