Visible light modulator by sputter-deposited lithium niobate

Recently, thin-film lithium niobate (LN) optical modulators based on the electro-optic (EO) effect have been studied for future ultra-high-speed optical communications.^1–13 However, these studies have been limited to infrared light (typical wavelengths λ = 1550 nm); few studies have been conducted for light with shorter wavelengths, such as visible light. By decreasing the wavelength λ of the laser beam from infrared to visible light, the product of the half-wave voltage V_π and interaction electrode length L (V_π · L) required for modulation decreases proportionally. A low V_π · L can result in a significant reduction in the power consumption and device size. Moreover, it can open up new windows for emerging markets using a thin-film LN modulator.

Concerning thin-film LN optical modulators using visible light, research on modulation using red (R) light (λ = 637 nm) has been conducted;¹⁴ however, shorter wavelengths, such as green (G) light (λ = 520 nm) or blue (B) light (λ = 473 nm), have not yet been studied. By using G or B colors, a reduction in V_π · L might occur.

Furthermore, as an emerging technology, visible light communications (VLCs) have widely been studied.^15–21 All previous studies have used direct laser light modulation; however, by using external modulators, the speed limit associated with this type of modulation can be overcome, and ultra-high-speed VLC can be realized. The use of modulators represents the only alternative to the current use of direct laser modulation for increasing the communication speed of VLC.

Because VLC is mainly intended for consumer-use applications, a significant cost reduction of the modulator is necessary. However, in all existing studies, thin-film LN modulators were fabricated using the adhesion process of bulk LN to a substrate.^1–14 Therefore, it is difficult to decrease the fabrication cost. The use of sputter-deposited thin-film LN on a substrate can be expected to realize a much lower fabrication cost than the existing bulk LN material adhesion process. Although studies have been conducted on sputter-deposited thin-film LN,^22–27 none of these studies have researched thin-film LN modulation for visible or conventional infrared light.

In this study, we fabricated an LN modulator using sputter-depositing thin-film LN directly on a sapphire substrate to realize the significant fabrication cost reduction required for much wider applications. By using visible light of R, G, and B colors, we experimentally demonstrated the modulation function of sputter-deposited thin-film LN modulators and realized small V_π · L values for these colors.

To reduce the fabrication cost, we introduced a sputtering deposition method for thin-film LN directly on a 4 in. Al₂O₃ sapphire (001) substrate. Radio frequency (RF) sputter deposition of the LN target was performed with the substrate heated to 650 °C. Oxygen gas was introduced during the deposition with Ar gas to compensate for oxygen reduction during the LN sputtering. The oxygen partial pressure and other sputtering conditions were carefully controlled to yield high-quality crystalline LN.

Figure 1(a) shows a typical x-ray diffraction (XRD) pattern of an LN film with a thickness of 1.6 µm. The sputter-deposited thin-film LN depicts a good (006) crystal orientation on a sapphire (001) substrate. The full width at half maximum (FWHM) of the rocking curve of the LN (006) peak was 0.36°.

After the LN deposition, the wafer fabrication process was started. Figure 1(b) shows a processed wafer. A ridge-shaped waveguide of the LN was fabricated using Ar ion physical etching. On the LN waveguide, a buffer insulator of SiInO_x was deposited, which was in contact with the Au electrodes to apply voltage to the LN. Our LN modulator using a Mach–Zehnder interferometer (MZI) structure is shown in Fig. 2. An electrode to apply voltage to LN is fabricated just above the LN waveguide so that the resultant electric field can be parallel to the LN (006) crystal orientation. This electrode geometry is similar to the one which uses Z-cut LiNbO₃.¹³ 

The EO measurement setup for the sputter-deposited thin-film LN modulator is shown in Fig. 3. The visible laser light for the R, G, and B color wavelengths λ was 638, 520, and 473 nm, respectively. The laser powers of R, G, and B were 40, 50, and 15 mW, respectively. The laser light was guided into the LN modulator via an objective lens whose numerical aperture is 0.70. Dichroic mirrors that reflect 98% of the laser beam of each wavelength were used at the beam incident part. A modulation voltage was applied to the LN modulator. The modulated output light from the LN modulator was detected via a photodetector, with a frequency bandwidth of 3 kHz by Thorlabs Model PDA36A2. The electrical signal from a photodetector was monitored using a digital oscilloscope.

The device design of the MZI configuration was computationally simulated using the finite-element method. We used an electromagnetic simulation solver, which uses the finite difference method of Photon Design Ltd. Because the range of wavelengths used in this experiment was not very wide, from 473 to 638 nm, a common device dimension was sufficient to obtain a simple V_π · L dependence on the wavelength λ. In principle, V_π · L is proportional to λ. Since V_π · L is proportional to λ, a smaller V_π · L can be obtained using shorter wavelengths, such as B laser light, than with R and conventional infrared laser light. To realize a lower power consumption and smaller footprint of the modulator, the application of B light modulation is an extremely powerful tool.

We determined the modulator design such that all the R, G, and B colors could propagate through the LN waveguide. By this design, we could compare the V_π · L dependence on λ. Moreover, as this sputter-deposited LN modulator is geared toward wider consumer applications with lower cost, unlike current long-haul applications with higher cost, we analyzed the adoption for all colors of visible light using simple designs.

The MZI designs for R, G, and B are listed in Table I, where the electro-optic coefficient and the optical index used in the simulation are listed in Tables II and III, respectively. Here, n₀ is the refractive index of the ordinary ray and n_e is the refractive index of the extraordinary ray.

The simulated result for V_π · L is listed in Table IV. Figure 4 shows the simulated R, G, and B laser light propagation through the LN waveguide for transverse mode 0 (TM0). The R, G, and B colors can propagate through the LN waveguide using the same MZI design.

Using the MZI design shown in Table I, we fabricated a sputter-deposited thin-film LN modulator. The resultant LN waveguide is shown in Fig. 5. Using this device, we measured the R, G, and B visible light propagation through a sputter-deposited LN waveguide. Figure 6 shows the output port of the sputter-deposited LN waveguide. All the visible light colors (R, G, and B) were successfully propagated using the same waveguide.

Figure 7 shows the EO modulation results. The modulated output signal detected by the photodetector current as a function of the applied voltage is shown, where the modulation frequency is 100 Hz. Although our target frequency is higher than it, we intended to measure basic performance without taking care of electronics of high frequency. The other concern might be the affection by photorefractive effect at a low frequency. However, the photorefractive effect for LN is observed only when the continuous beam irradiation takes several minutes to several hours.^28–31 Therefore, the shot time scale of 10 ms for 100 Hz does not affect the result by photorefractive effect, and our experimental result was repeatable.

As shown in Fig. 7(a), for R, the half-wave voltage V_π is 3.8 V, and the half-wave voltage V_π and interaction electrode length L (0.5 cm) product of V_π · L is 1.9 V cm. By using shorter wavelengths λ than the general infrared laser light of 1550 nm for long-haul optical communications, an extremely small V_π · L of 1.9 V cm can be obtained even by using a sputter-deposited thin-film LN modulator. As shown in Fig. 7(b), for G, V_π decreases to 2.8 V, and V_π · L is 1.4 V cm. This is a relatively small number, which is extremely difficult to achieve as long as infrared light is used. Furthermore, as shown in Fig. 7(c), for B, V_π further decreases to 2.4 V, and V_π · L is 1.2 V cm. Notably, these significantly small values for V_π · L were obtained using sputter-deposited thin-film LN and not by bulk LN. In addition, the device fabrication cost can be significantly reduced compared to the existing bulk LN adhesion process. This means that utilizing visible light for optical communications can reduce the power consumption, device size, and fabrication cost simultaneously.

For the extinction ratio (ER), all three colors of light showed similar values of 8 dB. Although this might be insufficient for long-haul communications, it is sufficient for both data center applications and consumer applications. Rather than achieving an excellent value of ER, a lower fabrication cost using sputter deposition is much more beneficial for consumer applications from an industrial perspective, as long as it is within an acceptable range.

Figure 8 summarizes the V_π · L values for R, G, and B. As shown in Fig. 8(b), V_π · L is directly proportional to λ, as expected. By using the same devices, a clear V_π · L dependency on λ was successfully obtained. Moreover, as previously shown in Table I, these V_π · L values are consistent with the computationally simulated number.

A typical range of V_π · L for the bulk LN-based thin-film LN modulator varies from 2.8 to 2.1 V cm^3,4,6,9,11 for the general infrared laser light. Even for the smallest value, V_π · L is 1.75 V cm.¹² Compared to these values, the V_π · L values within the interval of 1.2–1.9 V cm obtained in this study are even smaller; these were achieved using sputter-deposited thin-film LN with much lower fabrication cost than the bulk LN adhesion process. From an industrial perspective, reducing the cost and power consumption is critical for realizing wider circulations with large market sizes. V_π · L of 1.2 V cm for B light is approximately half that of the general bulk LN-based thin-film LN modulator, even with much lower fabrication cost. The realization of B laser light modulation can create more openings for an emerging market.

An experiment on R (λ = 637 nm) laser light modulation using a bulk LN adhesion process was reported, and a small V_π · L of 1.6 V cm was successfully confirmed.¹⁴ This is similar to that obtained in our study for R light, although it is slightly larger. The difference in the electric field confinement may be partially attributed to the difference in crystal orientation: the z axis of our sputter-deposited LN was perpendicular to the film plane, whereas that of Ref. 14 is in the film plane. In addition, there must be a difference in the crystalline quality between bulk LN and sputter-deposited LN. However, considering the cost efficiency of sputter-deposited thin-film LN, this performance difference is surprisingly small. Furthermore, there have not been any reports on external modulation for G and B laser light. The small V_π · L of 1.2 V cm for B light enables us to use a Si complementary metal-oxide-semiconductor (CMOS) controller, which can reduce the total system cost and fabrication cost, using sputter-deposited thin-film LN. This could lead to the realization of VLC with fast communications at a much lower cost.

It is also meaningful that V_π · L is experimentally proportional to λ, as shown in Fig. 8(b). As long as the laser light can propagate through the LN waveguide, using shorter wavelengths λ is extremely beneficial for realizing smaller values for V_π · L. As expected, a good linear relationship between V_π · L and λ is a good indicator of the reasonably functioned sputter-deposited LN modulator.

This successful demonstration of light modulation results from the good crystalline structure of the sputter-deposited LN. For example, the orientation of the LN (006) crystalline structure with an FWHM of 0.36° is much better than that of sputter-deposited LN films with an FWHM of 5°.²⁷ Carefully controlling the sputtering conditions is critical for realizing such a good crystalline LN.

In addition to the good crystalline structure, the etching process for the LN waveguide is the key to realizing a smooth surface of the LN waveguide with shorter wavelengths than that of infrared laser light. Visible light applications require a milder etching process with greater care than infrared applications. This process development could further improve the performance of the modulator.

We have also measured propagation loss for the R, G, and B laser light by changing the waveguide length from 10 to 16 mm and 22 mm. As a result, propagation loss was 8, 10, and 11 dB/cm, respectively. These numbers are still large numbers than those we have typically measured for infrared light of a wavelength of 1550 nm. Due to the small wavelengths for visible light, the smoothness of waveguide effects needs to be cared more than that for infrared light.

Finally, we have also measured higher frequency by using different photodetectors, with a frequency bandwidth of 1 GHz by Menlo systems Model APD210. We have confirmed higher frequency modulation for all the R, G, and B laser light up to 300 MHz. However, this frequency limit is due to the limit of the bandwidth of the photodetector, and further high frequency modulation can be expected by using the high-end photodetector. This will be investigated in the following study.

In conclusion, we fabricated a thin-film LN modulator via direct sputtering deposition on a sapphire substrate. By using visible laser light with shorter wavelengths than those of infrared light, we experimentally achieved V_π · L values, exhibiting 1.9, 1.4, and 1.2 V cm for R (λ = 638 nm), G (λ = 520 nm), and B (λ = 473 nm) visible light, respectively. We also demonstrated that V_π · L decreased with decreasing wavelengths.

Our results show that the LN modulator can be adopted for emerging applications of VLC and that wafer-level fabrication using the sputtered film could present new opportunities for future mass production at a much lower cost.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.