Optical Materials 73 (2017) 45e49
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Low-loss optical waveguides in b-BBO crystal fabricated by femtosecond-laser writing Ziqi Li a, Chen Cheng a, Carolina Romero b, Qingming Lu c, zquez de Aldana b, Feng Chen a, * Javier Rodríguez Va a b c
School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Laser Microprocessing Group, Facultad Ciencias, Universidad de Salamanca, Salamanca 37008, Spain School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 June 2017 Received in revised form 25 July 2017 Accepted 26 July 2017
We report on the fabrication and characterization of b-BBO depressed cladding waveguides fabricated by femtosecond-laser writing with no signiﬁcant changes in the waveguide lattice microstructure. The waveguiding properties and the propagation losses of the cladding structures are investigated, showing good transmission properties at wavelengths of 400 and 800 nm along TM polarization. The minimum propagation losses are measured to be as low as 0.19 dB/cm at wavelength of 800 nm. The well-preserved waveguide lattice microstructure and good guiding performances with low propagation losses suggest the potential applications of the cladding waveguides in b-BBO crystal as novel integrated photonic devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: Optical waveguides b-BBO crystal Femtosecond-laser writing
1. Introduction As one of the most important nonlinear optical crystals, betabarium borate (b-BaB2O4 or b-BBO) combines many outstanding features such as its high nonlinear optical coefﬁcients, low groupvelocity dispersion, broad transparency range (189e3500 nm) and high damage threshold [1,2]. This unique combination ensures b-BBO crystal a promising candidate for a wide range of nonlinear optical applications such as frequency converters and optical parametric oscillators. In the realm of quantum optics, b-BBO crystal can be used to generate entangled photon pairs and tenphoton entanglement has been achieved by Wang et al . The transparency at 800 nm plays a prominent part since many optical photon sources operate at the wavelength of 800 nm. Optical waveguides with diverse conﬁgurations are the basic component of integrated photonic circuits due to the tight conﬁnement of light ﬁeld within microscale volumes of multifunctional optical materials [4,5]. Owing to the compact geometry of the guiding structure, much higher optical intensities can be achieved within the waveguides in comparison with the bulk, and some optical performances such as the nonlinear effects are expected to be enhanced
* Corresponding author. E-mail address: [email protected]
(F. Chen). http://dx.doi.org/10.1016/j.optmat.2017.07.049 0925-3467/© 2017 Elsevier B.V. All rights reserved.
to a certain extent . Beneﬁting from above advantages, optical waveguide in b-BBO crystal could possess a number of potential applications when integrated with other optical devices. Several techniques have been utilized to fabricate the waveguide such as ion/proton exchange , epitaxial layer deposition , ion-beam implantation/irradiation [9,10], and femtosecond (fs) laser writing [11,12]. Among those developed techniques, fs laser writing has been proven to be a powerful technique to fabricate various three-dimensional optical waveguiding structures in diverse transparent materials since the pioneering work by Davis et al. in 1996 [13e23]. During the inscription process, the fs laser interacts with the sample through nonlinear process such as twophoton or multiphoton absorption, and then followed by strongﬁeld ionization, i.e., avalanche ionization and tuning ionization, leading to the permanent modiﬁcation of refractive-index of transparent dielectric materials [11,24]. The fs-laser-induced refractive-index changes could be either positive (Dn > 0) or negative (Dn < 0), depending on the laser irradiation parameters (pulse energy, repetition rate, scanning speed, polarization, etc.) as well as on the physical properties of the materials [25e27]. According to the relative position between laser inscribed tracks and refractive-index modiﬁcation regions, the fs-laser micromachined waveguides can be classiﬁed into several groups, including directly written waveguides (so-called Type I) , stress-induced dual-line waveguides (so-called Type II) [16e18], depressed cladding
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waveguides [19e21], and ablated ridge waveguides [22,23]. The depressed cladding waveguides, which are constituted by numbers of low-index tracks around the waveguiding core, have superior performances over other conﬁgurations due to the tighter conﬁnement of light and the ease of integration with ﬁbers . So far, researchers have investigated the planer and ridge waveguides fabricated by ion beam irradiation as well as type II waveguides by fs laser writing in b-BBO crystal [29e31]. The fs-laser micromachined cladding waveguides in b-BBO have not been reported. In this work, we report on the fabrication of depressed cladding waveguides in b-BBO crystal produced by fs-laser writing with different fabrication parameters. Waveguiding performances of the fabricated cladding structures were investigated, showing good guidance at wavelengths of 800 and 400 nm. 2. Experimental The b-BBO crystal used in this work was cut into wafers along the direction (q ¼ 29.2 , j ¼ 0 ), which is the angle can be potentially used to realize the spontaneous parametric down conversion (SPDC) at 400 nm to generate entangled photon pairs, with dimensions of 10(x) 10(y) 2(z) mm3 and optically polished. The two polished 10 2 mm2 endfacet are coated with anti-reﬂection (AR) ﬁlms at wavelengths of 400 and 800 nm. Fig. 1 demonstrates the schematic process of the direct femtosecond laser writing. The cladding waveguide structures in b-BBO crystal were produced by utilizing the laser facilities at the Universidad de Salamanca, Spain, in which the Ti:sapphire regenerative ampliﬁer (Spitﬁre, Spectra Physics) was utilized to generate linearly polarized 120 fs pulses at a central wavelength of 800 nm with 1 kHz repetition rate. The maximum available pulse energy on the sample was 1 mJ, and it was controlled by employing a calibrated neutral density ﬁlter placed after a half-wave plate and a linear polarizer. The fs laser beam was focused through a 40 microscope objective (N.A. ¼ 0.65) at certain depth beneath the largest sample surface (with dimensions of 10 10 mm2) with a pulse energy of 0.21 mJ and 0.32 mJ. During the irradiation, the sample was placed in a computer-controlled XYZ translation stage and was scanned at a constant speed of 500 mm/s in the direction perpendicular to the laser polarization and the pulse propagation. The scanning direction was carefully aligned with the 10-mm-long edge of the sample
and perpendicular to these polished endfacet, thus producing low refractive-index parallel damage ﬁlaments with the separations of 3 mm at different depths of the sample, forming the desired circular geometry of the cladding waveguide. The overlayed text in Fig. 1 shows the measured pulse energy on sample. Two waveguides with diameters 20 and 30 mm (No. 1 and No. 2 respectively) were fabricated with pulse energy of 0.21 mJ, while another two claddings with the same diameters (No. 3 and No. 4) were produced with pulse energy of 0.32 mJ. The crosssectional microscope images of Nos. 1 to 4. are shown in Fig. 1(b)(e), respectively. It can be found that the damage track becomes more intense as the pulse energy increases. In order to obtain the waveguiding properties of the fabricated waveguides in b-BBO crystal, the end-face coupling system was carried out to make the optical characterization as depicted in Fig. 2. In this conﬁguration, linearly polarized light beam at 800 nm was generated from a tunable Ti:sapphire laser (Coherent MBR-PE) and laser beam at 400 nm was emitted from a solid-state laser. The generated laser was focused through microscope objective lens (Obj 1) and coupled into the waveguides, then the other microscope objective (Obj 2) was utilized to collect the output lasers transmitted through the waveguides. The modal proﬁles at the output of waveguide were imaged by CCD (CinCam CMOS, Cinogy), and the input and output powers were measured through a power meter. A half-wave plate was inserted into the optical path to investigate the polarization dependent features of the output beams. 3. Results and discussion Fig. 3 demonstrates the measured room-temperature m-Raman spectrum of the track, bulk and the core of waveguide in the same measuring conditions. The m-Raman spectrum was measured along the direction perpendicular to the input facet of the waveguide under the excitation of 532 nm laser and the spatial resolution of our Raman measurement is around 1 mm, which is ﬁne enough to estimate the features of the microstructures. It can be seen that there is no obvious changes of the m-Raman spectra according to the peak positions and intensity in the waveguide core and bulk two regions. The intensity of m-Raman spectra from the track area is about 8.7% lower in comparison with the bulk region, which may be attributed to the lattice modiﬁcation induced by fs laser writing.
Fig. 1. (a) The schematic plot of the fabrication procedure of fs-laser micromachined waveguides in b-BBO crystal. Translating the sample with respect to the laser focus allows the fabrication of various cladding waveguiding structures. (b)e(e) The cross sectional microscope image of the end face of b-BBO waveguiding structures.
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Fig. 2. The schematic plot of experimental setup used to investigate the guiding properties of b-BBO waveguides.
assumption of a step-index proﬁle. The maximum refractive index increase (Dn) was calculated using the following formula
Fig. 3. Confocal micro-Raman spectrum obtained from the track (blue), bulk (red) and the core (green) of b-BBO waveguide No. 3 at the excitation wavelength of 532 nm. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
The results indicate the lattice structure as well as the physical properties of the waveguide region are expected to be well preserved. Fig. 4 illustrates the measured near-ﬁeld proﬁle of the output light, with TM polarized 400 and 800 nm input light launched into waveguide Nos. 1 to 4, respectively. For the wavelength of 800 nm, fundamental mode was achieved for the cladding waveguides of Nos. 1 to 4 depicted in Fig. 4 (a)-(d). The propagation mode was found to be multimode at the input wavelength of 400 nm. The allangle light transmission experiments of waveguide No. 1 along the transverse plane at 800 nm and 400 nm were carried out to ﬁgure out the polarization dependent effects of the guidance. As shown in Fig. 5, it can be found that the output power reaches maximum along TM polarization (90 and 270 ), while there is no guidance along the TE polarization (0 and 180 ), which may be attributed to the anisotropic changes of refractive index along different directions in the process of fabrication. In addition, we calculate the variation of the refractive index between the irradiated tracks and the guiding area, which can be estimated by measuring the N.A. of the waveguide with an
sin2 Qm 2n
where Qm is the maximum incident angular deﬂection at which no transmitted power change occurs, and n is the refractive index of the unirradiated bulk area at the corresponding wavelength of 800 nm. In this work, the obtained value of refractive index changes Dn for waveguides Nos. 1 to 4 is 5.3 103, 5.9 103, 6.5 103, and 6.8 103, respectively. Based on the measured Dn and the ﬁnite difference beam propagation method, the near-ﬁeld modal distributions at 800 nm wavelength were also simulated by using the software Rsoft®. Fig. 6 (a) illustrates the reconstructed 2D refractive index proﬁle of the cladding waveguide No. 2 at 800 nm. In according to the modelling result shown in Fig. 6 (b), the fullwidth-half-maximum (FWHM) of calculated modal distribution along X and Y directions were 16.9 and 15.45 mm and exhibited fundamental mode, which is in good agreement with the experimental results with FWHM of 16.01 and 15.16 mm, respectively. By applying the end-face coupling arrangement, the propagation losses of the waveguides in b-BBO crystal were estimated and calculated by direct measurement of the laser from the input and output end-facets, in which the mismatch coefﬁcient and Fresnel reﬂection losses were considered. The correlation function between launched Gaussian ﬁeld proﬁles and the propagation modal proﬁles can be expressed as the respective overlap integrals .
Z CGP ¼
f*in ðxÞfðx; zÞdx
where fin ðxÞ ¼
cm fm ðxÞ is the incident ﬁeld and assumed as the P cm fm ðxÞeibz refers to the propagating Gaussian ﬁeld, fðx; zÞ ¼ m
modal proﬁles of the waveguide and b is the propagation constant. The values of mismatch coefﬁcient CGP can be calculated through FD-BPM algorithm (Rsoft® Beam PROP) and the coupling losses can be therefore obtained. Meanwhile, the losses caused by Fresnel reﬂections in the interfaces of the two waveguide end facets and the air can be obtained by the following equation:
n2 n1 n1 þ n2
Where n1 and n2 represent the refractive indices of the sample
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Fig. 4. Experimental near-ﬁeld intensity distributions of depressed cladding waveguides in b-BBO from No. 1 to 4 at the wavelength of (a)e(d) 800 nm and (e)e(h) 400 nm. The dashed circles are the boundaries of the waveguiding areas.
and air. With these considerations, the propagation losses a (as well as the total losses atotal) of the fs laser fabricated waveguides in bBBO crystal were determined and listed in Table 1. It is worth noting that practical propagation losses could be slightly less than the calculated value in the case of multi-mode waveguides. By comparing the data of the propagation losses a, we can conclude that a4
Table 1 The losses atotal (dB) and propagation losses a (dB/cm) of the cladding waveguides in b-BBO crystal. Waveguide
Fig. 5. Output power as a function of all-angle light transmission with the same launched power of b-BBO waveguide No. 4 at different wavelengths.
No. No. No. No.
1 2 3 4
5.13 3.54 4.79 2.75
2.06 0.98 1.71 0.19
6.31 5.79 6.26 5.41
3.2 3.18 3.14 2.80
Fig. 6. (a) The reconstructed 2D refractive index proﬁle and (b) calculated modal proﬁle of the cladding waveguide No. 2 at the wavelength of 800 nm.
Z. Li et al. / Optical Materials 73 (2017) 45e49
diameter of 20 mm, which can be attributed to the larger acceptance angle. Compared with the previous ridge waveguides produced by ion beam irradiation, the fs laser fabricated waveguides in b-BBO crystal possess lower propagation losses as well as easier and more ﬂexible fabrication procedure, showing the that the fs laser writing is an effective way for low-loss waveguide fabrication. 4. Conclusions In summary, we have fabricated the cladding waveguides in bBBO crystal by fs-laser writing with different fabrication parameters. The guiding properties and the propagation losses at 800 and 400 nm were investigated, exhibiting good guidance along TM polarization. The propagation losses are measured to be as low as 0.19 dB/cm at wavelength of 800 nm along TM polarization. The well-preserved waveguide lattice microstructure and good guiding performances with low propagation losses indicate the fs laser direct-written depressed cladding waveguide in b-BBO crystal as potential candidate in the photonic quantum circuits. Acknowledgements The work is supported by the 111 Project of China (No. B13029), n (Project SA116U13), and Ministerio de Junta de Castilla y Leo Economía y Competitividad (MINECO) (FIS2013-44174-P). References r, Opt. Mater. 22 (2003)  R.S. Klein, G.E. Kugel, A. Maillard, A. Siﬁ, K. Polga 163e169.  C. Lu, S.S. Dimov, R.H. Lipson, Chem. Mater. 19 (2007) 5018e5022.  X.L. Wang, L.K. Chen, W. Li, H.L. Huang, C. Liu, C. Chen, Y.H. Luo, Z.E. Su, D. Wu, Z.D. Li, H. Lu, Y. Hu, X. Jiang, C.Z. Peng, L. Li, N.L. Liu, Y.A. Chen, C.Y. Lu, J.W. Pan, Phys. Rev. Lett. 117 (2016) 210502.  E.J. Murphy, Integrated Optical Circuits and Components: Design and Applications, Marcel Dekker, New York, 1999.  G. Lifante, Integrated Photonics: Fundamentals, Wiley, Atrium, 2008.  C. Grivas, Prog. Quant. Electron 35 (2011) 159e239.
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