1. Introduction

As a newly emerging integrated optics platform in recent years, lithium niobate (LN) thin film has attracted more interests because it still keeps the excellent characteristics of LN crystal in many aspects including electro-optic, acousto-optic, and nonlinear optical properties [1–5]. LN thin film can be bonded directly to the surface of SiO₂ [1, 2]. Due to the high refractive index contrast of the fabricated channel waveguides, light can be confined to a much smaller size in the LN thin film than in the conventional LN wafer, bent waveguide radius can therefore be reduced to a level smaller than 10 μm [2], which makes the ultra-compact LN photonic integration possible. With the continued improvement of the fabrication techniques of the LN thin film materials and the corresponding waveguides, the LN thin film has been considered as a very promising platform for the development of a wide range of ultra-compacted, active integrated devices, such as modulators, switches, tunable filters, wavelength converters and so on [1, 2]. Wherein fabricating low-loss bent channel waveguides with a small radius in the LN thin film is a key step toward this direction.

As far as the fabrication of LN waveguides is concerned, the annealed proton-exchange (APE) and the Ti-diffusion are two kinds of mature techniques, and commercial LN waveguides devices fabricated by these two techniques on the conventional LN wafer are available [6]. Moreover, by using the APE technique, a channel waveguide with a low propagation loss of 0.6 dB/cm was also realized in the LN thin film [7]. However, these waveguides formed with these two techniques have a small refractive-index change, thus it is difficult to keep the bending loss low with a small bend radius. To avoid an excessive bending loss, a large bend radius must be used, which greatly increases the footprint of the LN waveguide devices. To realize the ultra-compacted integrated photonic circuit on the LN film platform, the waveguides with high refractive index contrast are needed. For this purpose, two main methods used to fabricate high index-contrast waveguides in the LN thin film have been suggested, they are etching of the LN film by Ar milling [8–10] and hybrid-integration of LN with Si [11, 12] or SiN [13]. Among these two methods, the former is more popular because the fabricated waveguides have a lower propagation loss. Ar⁺ plasma etching allows propagation loss as low as 2.7 dB/m has been demonstrated [10]. But this method has poor etching selectivity and is well known to result in a trapezoid waveguide cross section with very shallow sidewalls and large slopes. In Ref. [9], it takes 60 minutes to etch LN layer to a depth of 460 nm in 760-nm thick LN thin film and the fabricated waveguide has a slope of ~27°. An alternative to Ar milling is to introduce chemical reaction between LN and F⁺ ions. The fact that LN can react with fluorine (F) ions make it a logical choice for achieving faster etching rates. Recently, a mix of fluorine (CHF₃) and Ar plasma allowing propagation loss as low as 0.4 dB/cm and a slope of 15° has been reported [14]. However, the re-deposition of LiF on the LN surface during the etching process results in a high scattering loss and impedes the realization of deep etching. Wet etching with HF and HNO₃ is a way to keep the LiF from depositing on the surface of the LN in the etching process [15]. However, this method is not a good choice for the fabrication of the waveguides in the LN thin film which is bonded to a buffer layer of silica. Because the layer of silica beneath the LN thin film is corroded faster than the LN film itself in the mixed solution of HF and HNO₃.

In this letter, we propose an efficient method combining the proton exchange with subsequent dry etching for the fabrication of low-loss bent channel waveguides in the LN thin film. Our proposed method introduces the chemical etching caused by F⁺ ion to increase the etching rate but with a key improvement to carry out a proton exchange process before etching. The proton exchange results in the replacement of Li⁺ ions with H⁺ protons, which helps to prevent the re-deposition of LiF during the subsequent ICP etching process [16]. As a consequence, the etching rate and depth can be increased on the premise of realizing a high etching quality. To evaluate the proposed method, we fabricated a straight reference waveguide and a set of 4 × 90° bent channel waveguides with different radii. The measurement shows that these fabricated waveguides have a trapezoid cross section with a top width of ~1.0 µm, a height of 900 nm and a slope of ~20° with respect to the vertical direction. To the best of our knowledge, this is the largest etching depth but with a small slope so far. In addition, at 1.55 µm wavelength our fabricated bent channel waveguide with a radius of 20 μm can achieve low bending losses of 0.455 dB/90° and 0.488 dB/90° for the fundamental quasi-TE (qTE) and quasi-TM (qTM) modes, respectively. Compared with the aforementioned methods, our method can realize a faster etching rate and a larger etching depth while maintaining a high etching quality.

2. Design and fabrication

Figure 1 shows schematically the configuration of the 4 × 90° bent channel waveguides and the straight reference waveguide. The straight waveguide has a length of 200 μm and a width 1.0 μm. These 4 × 90° bent channel waveguides, each of them consists of 4 sections of identical 90° circular bends, have the same width of 1.0 μm but different radius r. To reduce the coupling loss between the input (or output) fiber and the bent or the straight waveguides, the input and output waveguides have a width of 3.5 μm, and a pair of identical tapered waveguides, which have a length of 3.0 mm and a linearly changed width from 3.5 μm to 1.0 μm, are used to connect the bent (or straight) waveguide to the input waveguide at one end and the output waveguide at the other end. Here, to realize an adiabatic propagation of the fundamental mode, the taper is sufficiently long and change its width slowly. In addition, all the input, output, and tapered waveguides, as shown in Fig. 1, are totally identical so as to investigate the bending loss with different bend radii.

 

Fig. 1 Configuration of the straight tand the 4 × 90° bent channel waveguides.

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In our experiment, the LN thin film sample (NANOLN), which consists of a layer of 900-nm thick X-cut LN thin film direct bonded to the surface of a 2-μm thick silica layer formed on 500-μm thick X-cut LN substrate, was used to fabricate the channel waveguides. The fabrication process is shown schematically in Fig. 2. A layer of ~90-nm thick chromium (Cr) film was firstly deposited on the surface of the LN film by radio-frequency (RF) magnetron sputtering. Then the photoresist (PR) pattern of the designed bent and straight channel waveguides was formed by standard photolithography and transferred to the Cr film by chemical etching. Subsequently, the photoresist was removed and the patterned LN substrate was submerged in stearic acid (SA, the proton source) for 40 minutes at 250 °C. Here, the depth of the proton exchange should be equal to the required height of the waveguide at least. Although a longer proton exchange time will result in H⁺ protons diffusing into silica because the diffusion of H⁺ protons is more quickly in silica than in LN [17], the refractive index change in silica can be ignored in view of the large refractive index difference between LN and silica. After the proton exchange, the sample was etched using the mixture gases of N₂, Ar and CF₄ for 30 minutes with 85 W RF power inductively coupled into the plasma (ICP) and 80 W RF power coupled to the sample table. Here a N₂ flow was used to protect the surface of the LN thin film from carbonization in the etching process. Finally, the Cr mask was removed and the two ends of the fabricated waveguides were polished carefully.

 

Fig. 2 Fabrication process fora channel waveguide in the X-cut LN thin film: (a) Cr deposition;(b) photoresist spin-coating; (c) photolithography; (d) Cr corrosion; (e) photoresist removal;(f) proton exchange,(g) ICP etching; (h) Cr removal.

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The total length of the chip with different radii of 4 × 90° bent waveguides and straight reference waveguides was 7.0 mm. The scanning electron microscope (SEM) pictures of the fabricated 4 × 90° bent channel waveguide with a radius of 5 μm, the straight reference channel waveguide, and their respective input (or output) waveguides are shown in Fig. 3(a), 3(b), 3(c), and 3(d), respectively. The fabricated bent and straight channel waveguides have an identical top width of 1.0 μm and their respective input (or output) waveguides have a top width of 3.40 μm and 3.48 μm, which are in excellent agreement with the designed parameters. From Fig. 3(b), it can be deduced that the fabricated waveguides have a trapezoid cross section with a height of ~900 nm and a slope of ~20° with respect to the vertical direction. The etching rate is ~30 nm/min, which is faster than the direct etching by Ar milling with the same power. In addition, it can be seen from Fig. 3, there is a layer of residual LN on both sides of the waveguides. It is, however, so thin (see Fig. 3(b)) that does not influence the mode distribution but helps enforce the adhesion of the LN channel waveguides on the silica layer.

 

Fig. 3 Scanning electron microscope pictures of the fabricated4 × 90° bent channel waveguide with a radius of 5 μm (a) and corresponding input (output) waveguide (b), as well as the straight reference waveguide (c) and corresponding input (output) waveguide (d)

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3. Experiments and results

To inspect the intensity distributions of the fundamental mode in the fabricated LN waveguides, the linear polarized light at 1550 nm wavelength from a distributed feedback laser (DFB, OPEAK OptoElectronics) with polarized light output function was first made to pass through a polarization controller so as to achieve the TE or TM polarization and then launched into the LN waveguide via a lensed single-mode fiber (SMF). The near-field spot output from the LN waveguide was amplified with a 20 × /0.4 objective lens and then captured with an infrared camera (MicronViewer 7290A). The measured intensity distributions of the fundamental qTE and qTM modes in the straight waveguide are shown in Fig. 4(a) and 4(b) respectively, while in the 4 × 90° bent channel waveguide with 5 μm radius are shown in Fig. 4(c) and 4(d), respectively. According to the results simulated with a commercial mode solver (COMSOL) at 1.55 µm wavelength for the qTE and qTM polarizations, although the fabricated ~1.0-µm width channel waveguide supports the fundamental and the second order modes as shown in Fig. 5(a), 5(b), 5(c), and 5(d), respectively, the captured four near-field images exhibit the distributions of quite pure fundamental mode. This is because we adjusted the lensed SMF carefully so as to launch the light exactly at the center of the input waveguide. Thus almost only the fundamental mode was selectively excited in the input waveguide.

 

Fig. 4 Measured intensity distributions of the fundamental mode at 1.55μm wavelength in the fabricated LN straight waveguide,(a) qTE mode, (b) qTM mode, and in the fabricated4 × 90° bent channel waveguide with 5 μm radius, (c) qTE mode, (d) qTM mode, with a pair of tapered waveguides as the input and output waveguides.

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Fig. 5 Simulated electric field intensity distributions of the modes using the parameters of the fabricated ~1.0-μm width channel waveguide at 1.55μm wavelength for the qTE polarization: (a) the fundamental mode, (b) the second order mode, and for the qTM polarization:(c) the fundamental mode, (d) the second order mode.

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To measure the insertion losses of the fabricated channel waveguides, the output light from the bent waveguides and the straight waveguide were collected by an SMF and measured with an optical spectrum analyzer (OSA) (Anristu MS97740A). The measured insertion losses of the fabricated straight waveguide at 1550 nm wavelength are 10.42 dB for the qTE mode and 10.15 dB for the qTM mode. As for the bent waveguides, although we designed different bend radii from 5 μm to 45 μm with an increase of 5 μm and from 50 μm to 80 μm with an increase of 10 μm, only these bent waveguides with radius of 5 μm, 15 μm, 20 μm, 45 μm, 70 μm, and 80 μm are in good condition. Fortunately, it is enough to evaluate the bending losses and hence the fabricated waveguide quality. In view of that all the input, output, and tapered waveguides are totally identical (see the configuration in Fig. 1), the bending losses of our fabricated 4 × 90° bent channel waveguides can be deduced by subtracting the insertion loss (in dB) of the straight reference waveguide from the insertion losses (in dB) of the bent waveguides with different radii, respectively. Then the loss per 90° bend, which is a quarter of the loss of the 4 × 90° bent channel waveguide, can be obtained and the results for different radii are shown in Fig. 6. For the qTE and qTM modes, the losses are, with a radius of 5 μm, 1.215 dB and 1.092 dB, respectively, while with a radius of 20 μm, only 0.455 dB and 0.488 dB, respectively. From Fig. 6, for the cases of the radius greater than 45 μm, the bending loss increases slightly with the increase of the radius. The possible reasons include the measurement errors, the differences of the fabricated waveguides, and the differences of the propagation loss due to different waveguides lengths.

 

Fig. 6 Bend losses of the fabricated 90° bent LN channel waveguides with different radii for the fundamental qTE and qTM modes at 1550 nm wavelength.

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4. Conclusions

In conclusion, we demonstrated an efficient method combining the proton exchange with dry etching to fabricate low-loss bent channel waveguides in the LN thin film. Due to the proton exchange process before etching, in which Li⁺ ions in LN is exchanged with H⁺ protons, our proposed method makes use of the chemical etching caused by F⁺ ion to increase the etching rate but avoid the re-deposition of LiF during the ICP etching process. Our experiment achieved an etching rate of 30 nm/min. Our fabricated bent channel waveguides have a trapezoid cross section with a top width of ~1.0 µm, a height of 900 nm and a slope of ~20° with respect to the vertical direction. And a 90° bent channel waveguide with a radius of 20 μm can achieve a low bending loss of 0.455 dB and 0.488 dB for the fundamental qTE and qTM modes, respectively. Our proposed method can be used to fabricate low-loss and ultra-compacted integrated waveguide devices in the LN thin film.