1. Introduction

As the basic components in integrated photonic system, optical waveguide structures could confine light propagation in very small volumes with dimensions of several micrometers, resulting in much high optical intensities with respect to the bulk materials [1]. The 1 × N waveguide beam splitter that divides an optical signal into N outputs plays key roles in the optical communication systems. Waveguide beam splitters with advantages of low propagation loss and high fiber matching efficiency are important elements in a variety of applications including power dividers, switches, couplers and modulators [1].

Several techniques, including ion/proton exchange, metal-ion indiffusion, ion implantation/irradiation and femtosecond laser writing, have already been utilized to produce waveguides in various optical materials [2–7]. Among these methods, the femtosecond laser writing has become a powerful and unique technique to implement direct 3D microstructuring of various optical materials owing to the advantages of wide applicability of materials, maskless 3D processing ability and negligible thermal-diffusion effect [8–12]. Both the laser parameters (pulse energy, pulse duration, repetition rate, focusing condition, scanning speed, etc.) and material properties affect the refractive index modification of the targets. According to the induced index changes (Δn) of the laser irradiated region, the femtosecond-laser written waveguides could be generally classified into three types: Type I (directly written waveguides, which locate inside the laser-write tracks with positive Δn), Type II (stress-induced waveguides locating in the vicinal regions of the laser written tracks with negative Δn), and depressed cladding waveguide (consisting of a core surrounded by a number of low-index tracks). An advantage of Type I modification of positive-index tracks is the capability of 3D waveguide structures, which can be relatively easily realized in optical materials. It is common to fabricate waveguides with Type I index modifications in amorphous materials, e.g., in most glasses, whilst Type I waveguides with positive index changes in the laser-induced tracks have only been fabricated in several crystals, including LiNbO₃ [13–16], Nd:YCOB [17], and BGO [18].

LiNbO₃ (lithium niobate or LN) is one of the most favorable materials used in integrated photonics owing to its excellent electro-optic, piezoelectric and nonlinear optical properties [19–21]. LiNbO₃-based electro-optic devices, including couplers, switches, modulators and splitters, play an important role in optical communications. The femtosecond-laser writing has become a very promising technique for waveguide fabrication in LiNbO₃. So far, the waveguide structures with Type I and II modifications have been realized in LiNbO₃ crystals [13–16,22,23]. In this work, we report on the fabrication of the waveguide beam splitters in 2D and 3D configurations by direct femtosecond laser writing in the z-cut LiNbO₃ crystal, as based on the Type I index modification of n_e. The guiding properties of the waveguides are numerically and experimentally investigated at the wavelengths of 632.8 nm and 1064 nm.

2. Experiments in details

The z-cut LiNbO₃ crystal used in this work was cut into dimensions of 10(x) × 10(y) × 1(z) mm³ and optically polished. The waveguide beam splitters were fabricated by femtosecond-laser writing. During the microstructuring process, an optical fiber laser system (origami-10 XP, OneFive) was utilized to generate linearly polarized laser beam (central wavelength of 1031 nm, repetition rate of 5 kHz, maximum pulse energy of 50 μJ and pulse duration of 420 fs). The LiNbO₃ sample was placed at a micro-positioning X-Y-Z motorized stage and the laser beam was focused by a 40 × microscope objective (N.A. = 0.6) at the point approximately 150 μm beneath the upper 10 × 10 mm² surface of the sample. A slit was inserted before the focusing objective to control the shape of the focused beam. The pulse energy incident on the sample was set to 4.9 μJ. The sample was scanned at a constant velocity as high as 4 mm/s along the y-axis. Figure 1 shows the schematic process of the laser-write of the LiNbO₃ waveguide beam splitters. In this work, a straight waveguide (WG1) and two waveguide beam splitters (WG2 and WG3 with configurations of 1 × 2 and 1 × 4 splitters) were fabricated, respectively. A metalloscope (Axio Imager, Carl Zeiss) was used to photograph the cross sections of the waveguides. For the 1 × 2 waveguide beam splitter (WG2), the input straight arm of 2 mm-long was connected to two splitted arms with the angular and lateral separation of ~0.229° and 32 μm, respectively. The output of the 1 × 4 beam splitter (WG3) was a squared 2 × 2 array. The angular separation and lateral separation (both in x-axis and z-axis) between every two output arms are ~0.229° and 32 μm, respectively.

 

Fig. 1 The schematic process of the laser-writing the LiNbO₃ waveguide beam splitters.

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We utilized an end-face coupling arrangement to characterize the waveguide mode profiles at 632.8 nm and 1064 nm excited from the He-Ne and Nd:YVO₄ solid-state lasers, respectively. A half-wave plate was employed to control the polarization of the incident laser beams. A pair of spherical convex lenses (N.A. = 0.4) were used to couple the light beam into and out of the waveguide. Afterwards, the modal profiles were recorded by a CCD camera and analyzed by the RayCi software. Figure 2 depicts the schematic plot of the end-face coupling system. Based on the above end-face coupling arrangement, we measured the light powers coupled in and out of the waveguide end-facets, and estimated the coupling loss and the Fresnel reflection loss of the waveguide. Finally, the propagation loss of the waveguide could be determined.

 

Fig. 2 The schematic plot of the end-face coupling system using a laser source at 632.8 nm/1064 nm.

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3. Results and discussion

Figure 3 depicts the microscopic images of the cross section of the straight waveguide WG1, 1 × 2 WG2 and 1 × 4 WG3 waveguide beam splitters. The measured near-field intensity distributions of the straight waveguide WG1, 1 × 2 WG2 and 1 × 4 WG3 waveguide beam splitters along the TM polarization (e.g., the E vector of light is parallel to z-axis of crystal) at 632.8 nm and 1064 nm by the end-face coupling system are also shown in Fig. 3. As one can see, for the WGs 1-3, the modal profiles exhibit fundamental modes, i.e., TM₀₀. For the 1 × 2 waveguide beam splitter WG2, the measured output splitting ratio for the two arms is 50:50 both at 632.8 nm and 1064 nm. This result indicates that the light energy is divided equally into two parts and guided out, which shows excellent performance for the Y-branch splitter function. For the 1 × 4 waveguide beam splitter WG3, the measured output intensity splitting ratios for the four arms are 50:48:50:49 at 632.8 nm and 50:48:49:48 at 1064 nm, respectively. This indicates that our fabricated 1 × 4 waveguide beam splitter possesses good performance of nearly equal output power. The splitting ratio could be further improved by optimizing the laser-writing design and processing parameters.

 

Fig. 3 The microscopic images of the cross section (left) and measured near-field intensity distributions at 632.8 nm (middle) and 1064 nm (right) of the straight waveguide WG1, 1 × 2 WG2 and 1 × 4 WG3 waveguide beam splitters along TM polarization. The dashed lines indicate the spatial locations of laser-induced tracks.

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Assuming a step-like refractive index profile and measuring the numerical aperture of the waveguide, we can estimate the maximum refractive index change at the waveguide core with respect to the unmodified bulk using the formula [24]

Afterwards, we simulated the light propagation at 632.8 nm by using BeamPROP (Rsoft, Inc) software, which is based on the finite-difference beam propagation method (FD-BPM) [26], to test the splitting efficiency of the beam splitters. Figure 4(a) shows the top-view of simulated light intensity profile after light propagating on the XY-plane at 632.8 nm, TM polarization, for the 1 × 4 waveguide beam splitter WG3. The simulated beam profile evolution process of light propagating along the beam splitter at 632.8 nm is also depicted in Fig. 4(b). By comparing the shapes of the profiles in Fig. 3, Figs. 4(a) and 4(b), one can conclude that the simulated results of 1 × 4 waveguide beam splitter WG3 match quite well with the experimental data.

 

Fig. 4 (a) The top-view of simulated light intensity profile after light propagating on the XY-plane and (b) the simulated beam profile evolution process of light propagating along the photonic structure at 632.8 nm on TM polarization for the 1 × 4 waveguide beam splitter WG3 by using FD-BPM code.

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Table 1 shows the propagation losses (α) of the laser-written straight waveguide (WG1), 1 × 2 (WG2) and 1 × 4 (WG3) waveguide beam splitters on TM polarization at 632.8 nm. It can be seen that as the number of output arms increases, the propagation loss of the waveguide splitter increase. Comparing the propagation losses of the straight waveguides and beam splitters with the same fabrication parameters, the additional losses were determined to be less than ~0.3 dB, which may be partly attributed to the imperfections of the multi-arm structures. Our obtained values are comparable to those reported for laser-written waveguide splitters in LiNbO₃ [27]. Figure 5 depicts, for the WGs 1-3, the angular dependence of the output light power to investigate the thorough information of the polarization effects of the guidance with the input light power of 1.7 mW. As one can see, for the same straight waveguide/waveguide splitter, as the light polarization angle changes the output light power varies, which show the polarization-sensitive guidance. At the same polarization angle, as the number of output arms increases, the output power of the waveguides decreases. Particularly, when the polarization angles are 90° and 270°, the output power is zero, which means that there is no guidance along the TE polarization due to the negative ordinary index change (∆n_o < 0) inside the tracks. When polarization angles are 0° and 180°, which is along the TM polarization, the output power reaches maximum. This behavior is in good agreement of the reported Type I waveguides in LiNbO₃ [13].

Table 1. The propagation losses (α) of the laser-written straight waveguide (WG1), 1 × 2 (WG2) and 1 × 4 (WG3) waveguide beam splitters on TM polarization at 632.8 nm

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Fig. 5 The Polar images of the output light power of WG1 (red squares), WG2 (blue circles) and WG3 (black triangles) and the corresponding fits (lines) at 632.8 nm with the input light power of 1.7 mW.

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4. Summary

We have successfully fabricated the 2D 1 × 2 and 3D 1 × 4 waveguide beam splitters in z-cut LiNbO₃ crystal by direct femtosecond laser writing based on the Type I positive refractive index changes of n_e. The experimentally measured modal profiles of waveguide beam splitters are in good agreement with the results based on the simulation. The propagation loss of the waveguide splitter is less than 4 dB/cm and the splitting ratio is approximately equal, which implies that our fabrication approach to 2D and 3D waveguide beam splitters has a potential application in integrated optics and optoelectronics devices.