As an excellent optical crystal material, lithium niobate (${\mathrm{LiNbO}}_3$ or LN) is considered one of the optical integration platforms by virtue of small absorption coefficient ($0.02{\mathrm{cm}}^{-1}$ at 1064 nm), wide transparent window (0.35–5 µm), high nonlinear effect ($d_{33}=-41.7\mathrm{pm}/\mathrm{V}$ at 1.058 µm), strong electro-optic effect ($r_{33}=32.2\mathrm{pm}/\mathrm{V}$ at 1.547 µm), and acousto-optic effect. Traditionally, integrated optical devices were produced based on Ti-diffusion or proton-exchanged waveguides with weak refractive index contrast (∼0.1) and large waveguide width (∼10 µm), which hinders the development of integrated photonics on the LN platform. Fortunately, with the development of LN on insulator (LNOI) and the advance of corresponding micro-/nano-processing technologies, wire and ridge waveguides with high refractive index contrast, small footprint, and low loss were demonstrated^[1,2]. The propagating loss of the LNOI waveguide with micrometer scale has been reduced to as low as 0.027 dB/cm^[3,4]. LNOI microresonators with quality factors higher than ${10}^8$ were also reported very recently, whose loss approaches the material absorption limit^[5]. In recent years, passive integrated optical devices such as frequency converters^[6–13], electro-optic modulators^[14,15], optical frequency combs^[16–18], and spectrometers^[19] have been developed on the LNOI platform.

Another important part of integrated optical systems is the active LNOI devices, such as lasers and amplifiers. It is known that LN is an indirect band gap material emitting light inefficiently. The luminescence of LN can be achieved with the help of doping rare earth ions. Recently, some researchers first prepared waveguide and microring structures on LNOIs using electron beam lithography (EBL) and dry etching processes, and then doped them with erbium and ytterbium ions by ion implantation followed by thermal annealing at ∼500°C to repair the lattice damage. Based on these rare-earth-ions-doped LNOI devices, photoluminescence experiments were carried out^[20,21]. However, limited by the low doping concentration and the inhomogeneous distribution of rare earth ions, no laser radiation or light amplification is observed. Ingeniously, a good solution was conducted that first doped LN crystals during the growth process and then used ion-cut technology to prepare rare-earth-doped LN films with high doping concentration and uniform ion distribution. Currently, thulium- and erbium-doped LNOI platforms have been reported based on this method^[22–25], and 1550 nm band erbium-doped LNOI micro-disk lasers were achieved very recently^[23–25].

The on-chip LNOI amplifier, as a fundamental optical component of active LNOI devices, however, is rarely reported. Here, we present the fabrication of LNOI waveguide amplifiers using EBL and inductively coupled plasma reactive ion etching (ICP-RIE) processes on homemade erbium-doped LNOI wafers. Under the 980 nm band pump, a net internal gain of ∼15 dB at 1531.5 nm was realized on a ∼5-mm-long waveguide structure. The dynamic behavior of gain from linear to saturation was observed in the cases of both fixed signals with increasing pump power and fixed pump power with increasing signal power.

Fabrication of erbium-doped waveguide amplifiers starts from an erbium-doped X-cut LNOI wafer with a doping concentration of 0.1% (mole fraction). The wafer is composed of a 600-nm-thick erbium-doped LN film that was sliced from a homemade LN crystal wafer, a 2-µm-thick silicon dioxide (${\mathrm{SiO}}_2$) buffer layer, and a 500-µm-thick silicon substrate. The preparation process is schematically illustrated in Fig. 1(a), which is mainly divided into four steps. Firstly, a layer of hydrogen silsesquioxane (HSQ) resist was spin-coated on the erbium-doped LN film. Subsequently, the patterns of waveguide amplifiers were defined by EBL. Then, ${\mathrm{Ar}}^{+}$ plasma etching was carried out to transfer the mask patterns into the erbium-doped LN film, resulting in ridge waveguides with a 280 nm etching depth and a 60° wedge angle. Finally, the chip was immersed in HF solution for 5 min to remove residual resist mask. Lastly, the facets of the optical waveguides were mechanically processed for efficient fibers to realize facet coupling. Figure 1(b) shows the optical micrograph of an array of fabricated erbium-doped LNOI waveguides. The final length of the straight waveguide is about 5 mm. Ridge LNOI waveguides in the micrometer scale have a high refractive index contrast leading to strong light field confinements in both the 980 nm band and 1550 nm band. Based on the structure parameters of the fabricated waveguides with a top width of 1.4 µm, the field distributions and effective refractive indices of waveguide eigenmodes can be numerically calculated. The mode distributions of fundamental transverse electric modes at pump (∼974.3 nm) and signal (∼1531.5 nm) wavelengths were calculated and are shown in Fig. 1(c) as examples.

Respecting the higher absorption coefficient of erbium ions in the 980 nm band compared with that in the 1480 nm band^[26], a continuous laser working at 974.3 nm was selected as the pump to investigate the optical amplification performance of the erbium-doped LNOI waveguides. A tunable narrow-band laser operating in the 1550 nm band served as the signal laser. Figure 2 schematically illustrates the experimental setup for the characterization of the LNOI amplifier. After passing through a variable optical attenuator (VOA), an optical coupler (OC), and a polarization controller (PC), respectively, the pump and signal lasers were combined using a wavelength division multiplexer (WDM) and then launched into the LNOI waveguide via a lensed fiber. At the same time, the pump/signal power in the optical path was monitored by sending the light from the second port of the corresponding OC to a power meter (PM). While propagating in the on-chip waveguide, the pump is absorbed by erbium ions and converted into gain to realize signal amplification under the signal light induction (stimulated emission). Similarly, both the amplified signal and residual pump light output from the chip were collected by a second lensed fiber and sent to an optical spectrum analyzer to detect the 1550 nm band signal. It is worth noting that under higher pump power levels, strong green up-conversion fluorescence along the waveguide was generated, as shown in the device photography in Fig. 2.

To calibrate the net internal gain of erbium-doped LNOI waveguide amplifiers, we first characterized the optical propagation losses for both the pump and signal of chip waveguides using whispering-gallery-resonator-loss measurements based on an erbium-doped microring resonator coupling with the waveguide structure on the same chip. As shown in Fig. 3, through fitting the resonance spectrum by a Lorentz function, the load $Q_L$ at 1531.5 nm and 974.5 nm resonance modes was derived as $9.78\times {10}^4$ and $2.42\times {10}^5$, respectively. According to the coupling state (1550 nm band over-coupling and 980 nm band under-coupling) inferred by the gap between the microring and the waveguide, the intrinsic quality $Q_i$ was obtained as $5.0\times {10}^5$ (1531.5 nm) and $2.6\times {10}^5$ (974.5 nm), respectively. Finally, the propagation loss coefficient $\alpha$ was estimated based on$$\alpha =\frac{2\pi n_{\mathrm{eff}}}{\lambda Q_i},$$(1)where $n_{\mathrm{eff}}={\lambda }^2/(2\pi R·\mathrm{FSR})$ is the effective refractive at the relevant wavelength, $R$ and $\mathrm{FSR}$ denote the microring radius (100 µm) and free spectral range, respectively. The calculated propagation losses for the signal and pump are 0.83 dB/cm and 2.5 dB/cm, respectively. The propagation loss is mainly composed of the scattering loss of the waveguide side wall and the absorption loss from erbium ions. Assuming that both ends are identical, we estimated the single-end-facet coupling losses as 6.76 dB at 1531.5 nm and 6.35 dB at 974.5 nm by considering the fiber-to-fiber insertion loss and chip propagation loss. Based on the above calibration results, the pump and signal powers are referred to as on-chip power in this paper.

The net internal gain of the erbium-doped LNOI waveguide amplifier was defined as$$g=10{\log }_{10}\frac{P_{\mathrm{on}}}{P_{\mathrm{off}}}-\alpha L,$$(2)where $P_{\mathrm{on}}$ and $P_{\mathrm{off}}$ are the output signal power in pump-on and pump-off cases, $\alpha$ and $L$ represent the propagation loss coeffcient at the signal wavelength and waveguide length, respectively. Here, $\alpha L$ is estimated as 0.42 dB. The net internal gain under the pump power from 0 to 64 mW is shown in Fig. 4(a) with fixed signal power ∼5 nW at 1531.5 nm. At low pump power, the net internal gain increases rapidly with the increase of the pump power, which belongs to the small signal amplification stage. Obviously, the internal loss is enough to compensate with the pump power at ∼1 mW. When the pump power increases to 60 mW, the net internal gain tends to be saturated. The evolution of the measured signal spectrum with the increase of pump power (0, 0.10 mW, 7.35 mW, 16.19 mW, 32.31 mW, and 64.02 mW) is shown in Fig. 4(b). The maximum net internal gain of ∼5.5 dB was obtained in our device within a waveguide length of ∼5 mm at higher pump power of ∼64 mW, which corresponds to a net internal gain of 11 dB/cm. Such a net internal gain could be further improved by increasing the doping concentration of erbium ions. Compared with a traditional $\mathrm{Er}:\mathrm{Ti}:{\mathrm{LiNbO}}_3$ waveguide amplifier with 2–3 dB/cm gain, the net internal gain has been significantly improved^[27–30]. This is mainly due to the increase of optical power density for both pump and signal modes and the spatial overlap with erbium ions.

Moreover, we characterized the gain dependence of an erbium-doped LNOI waveguide amplifier on the signal power. As shown in Fig. 4(c), at low signal power, the net internal gain decreases linearly with the increasing signal power for a fixed pump power of ∼23 mW, corresponding to the small signal gain state (−65 dBm to −50 dBm). As the signal power continues to increase, gain saturation is observed. The maximum gain is ∼15 dB (∼30 dB/cm) with the signal power of ∼65 dBm, and there is still ∼0.13 dB amplification at ∼34 dBm signal power. As shown in the amplified spontaneous emission (ASE) spectrum of the erbium-doped LNOI waveguide with pump power of 0.57 mW [Fig. 4(d)], although the optimal working wavelength is ∼1531.5 nm, the amplifier works in a wide range of the communication band.

In summary, we fabricated on-chip erbium-doped LNOI waveguide amplifiers using EBL and ICP-RIE processes. Under the 980 nm band pump, the communication band amplifiers were demonstrated with maximum net internal gain of ∼15 dB achieved in a 5-mm-long chip. Compared with the previous $\mathrm{Er}:\mathrm{Ti}:{\mathrm{LiNbO}}_3$ bulk waveguide amplifier, the amplification performance has been greatly improved. The amplifier could be compatible with passive LNOI devices by selectively doping LN wafers via erbium diffusion during the fabrication of LNOI wafers, which may significantly promote the development of LNOI on-chip integrated LNOI optics.

During the preparation of this article, we noticed that two researches on erbium-doped waveguide amplifiers were posted on arXiv^[31,32], in which the authors had reported similar amplification performance. The differences of our researches are as follows. (1) Our preparation process is more simplified, without Cr film deposition and chemical mechanical polishing (CMP) steps. (2) Our amplifiers have relatively high net gain per unit length for a weak signal (∼30 dB at a signal power of ∼−65 dBm), so it may have more advantages in weak signal amplification.