Erbium-doped fiber amplifiers (EDFAs) are indispensable components in optical fiber communication systems attributed to their excellent advantages, which include a low noise figure [1], broad gain bandwidth [2], and high gain efficiency in telecommunication wavebands [3]. The energy level transitions of the erbium ion exhibit improved radiative lifetimes and more efficient energy conversion when compared to electron-hole interactions [4]. This improvement facilitates the photonic integration of erbium-doped waveguide amplifiers and lasers in conjunction with a variety of passive optical components on a monolithic chip.

Numerous material platforms have been employed to develop erbium-doped waveguide amplifiers through different techniques, including lithium niobate [5–9], high-energy ion-implanted silicon nitride [10], and aluminum oxide [11–13]. Among these materials, thin film lithium niobate (TFLN) stands out due to its impressive electro-optic, nonlinear, photorefractive, piezoelectric, and pyroelectric effects, as well as a wide transparent window ranging from 0.35 to 5 μm. TFLN has experienced significant progress across a range of applications, including high-speed electro-optical modulators [14], high-efficiency on-chip arrayed waveguide gratings [15], heterogeneous detectors [16], ultrafast tunable lasers [17], and integrated microwave photonic processing engines [18]. However, most of these components require additional external standalone EDFAs for power compensation, resulting in a low level of integration and diminishing the advantages of photonic integrated circuits (PICs) in large-scale on-chip systems. Consequently, many researchers have embarked on the investigation of erbium-doped optical waveguide amplifiers based on lithium niobate. Dedicated fabrication processes such as electron beam lithography-argon ion dry etching [19] and photolithography-assisted chemo-mechanical etching (PLACE) techniques [20,21] have been developed to achieve considerable optical gain. In addition, a coherent beam combination strategy has been utilized to improve the gain performance of the amplifier, exhibiting a high output power of 12.9 mW [22]. Furthermore, recent reports show that the output power can exceed 100 mW in a large mode field (LMF) waveguide configuration [20]. However, it is not trivial to obtain high-quality lithium-niobate-based waveguides due to their chemical inertness. In addition, the processing procedure is not compatible with the complementary metal-oxide-semiconductor (CMOS) technology due to lithium contamination issues. Moreover, the rib-like waveguide geometry with a slanted sidewall profile limits its applicability in scenarios that require strong coupling or sensitivity to sidewall effects. Therefore, it is instructive to develop high-efficiency lithium niobate waveguide amplifiers in an etchless way to overcome the above limitations.

In this study, we propose a heterogeneous waveguide amplifier by integrating a low-loss GeSbS waveguide with an etchless erbium-doped lithium niobate thin film. By optimizing the geometries of both the GeSbS strip waveguide and the lithium niobate thin film, we can achieve a significant overlap between the signal and pump waves. Under these conditions, we fabricate low-loss GeSbS-loaded erbium-doped lithium niobate waveguides using a standard waveguide processing flow, thereby avoiding the complex etching processes typically associated with lithium niobate. This research presents a prototype for heterogeneous integration of GeSbS and lithium niobate platforms aimed at developing efficient on-chip waveguide amplifiers. This advancement is anticipated to enhance functionalities within a monolithic integration scheme for future applications in optical signal processing, sensing, and computing.

Figures 1(a) and 1(b) show the cross-section and the schematic of the GeSbS-${\text{LiNbO}}_3$ heterogeneous optical waveguide amplifier, which is conceived with the below features and advantages. Firstly, it offers an effective way to realize high-gain waveguide amplifiers without the need to etch the lithium niobate film, which simplifies the fabrication procedure and provides more degrees of freedom for waveguide designs. Secondly, given the similar refractive indices between GeSbS (2.23 at 1.55 μm) and lithium niobate (2.2 at 1.55 μm), the combined waveguide can achieve good optical mode confinement for pump and signal waves simultaneously. By furthermore optimizing the dimensions of the GeSbS waveguide and the thickness of lithium niobate, we can control the spatial distribution of the optical mode field and signal enhancement (SE) to get optimized gain performance. Moreover, the heterogeneous waveguide configurations have been employed in various domains, including passive photonics [23], electro-optical modulators [24], acousto-optical modulatosr [25], and parametric frequency conversions [26], offering the potential to integrate more building blocks within the monolithic integration systems.

It starts from an erbium-doped x-cut lithium niobate film with a doping concentration of 0.5% (mole fraction). The erbium-doped LN film is 600 nm thick with a 2-μm-thick ${\mathrm{SiO}}_2$ buffer layer. The upper low-loss GeSbS strip waveguide is then manufactured and the details about the fabrication process can be found in our previous work [27]. Figures 1(c) and (d) illustrate the simulated transverse electric (TE) mode field distributions of 1480 nm and 1531.4 nm, respectively. In addition, to achieve a higher gain coefficient, it is essential to confine as much of the optical mode field as possible within the erbium-doped LN layer to ensure sufficient population inversion, while also ensuring low propagation loss in the waveguide. To determine the waveguide dimensions to support high gain performance, we sweep the geometrical parameters of the GeSbS waveguides. Figure 1(e) illustrates the effective indices of fundamental TE and transverse magnetic (TM) modes at 1480 and 1531.4 nm varying with the waveguide width when the thickness of GeSbS film is 250 nm. It is noted that the waveguide could support high-order modes when the width exceeds 1.5 μm. The implementation of wider waveguides contributes to enhanced gain and reduced waveguide propagation losses. In our current configuration, no additional mode-selective structures are utilized. The measured insertion losses of the 1.2-cm-long waveguide with a width of 2.5 μm are $11.8\pm 0.35\mathrm{dB}$, $9.9\pm 0.17\mathrm{dB}$, and $9.0\pm 0.22\mathrm{dB}$, respectively at wavelengths of 1531.4 nm, 1550 nm, and 1640 nm. To prevent the excitation of higher-order modes and the occurrence of mode hybridization, the bending radius of the waveguide is chosen to be 200 μm. This approach ensures optimal performance by balancing between mode confinement and transmission efficiency, leveraging the combined merits of lithium niobate and chalcogenide materials for integrated photonic applications.

Here, we implement a bidirectional pumping method to test the performance of the optical waveguide amplifiers. The experimental setup is shown in Fig. 2(a). Considering the different mode profiles of the pump (980 nm or 1480 nm) and signal waves (1531.4 nm), we select 1480 nm light as the pumping source to maximize the mode overlap efficiency. The inset image shows the microscope image of the lensed fiber coupled to the waveguide facet when the pumping light is off and on. Figure 2(b) illustrates the microscope image of the bending region of the waveguide with a radius of 200 μm. Experimental observations confirm that no mode hybridization or excitation of higher-order modes has occurred. Figure 2(c) presents the SEM image of the waveguide cross-section with PMMA top cladding. The vertical and smooth sidewall of the GeSbS waveguide lays a solid foundation for low waveguide loss in our work. The strong green photoluminescence resulting from cooperative up-conversion is observed as shown in Fig. 2(d), which indicates that the erbium ions could be excited efficiently.

A series of waveguides with varying lengths was fabricated to characterize the background loss of the waveguide at a wavelength of 1640 nm, which is out of the absorption band of erbium ions to avoid the impact of absorption loss. As shown in Fig. 3(a), it can be observed that the insertion loss curve exhibits an excellent linear relationship with the waveguide length. It indicates that the waveguide insertion loss is related to the propagation loss and the coupling loss between the waveguide and fiber. The impact of bending loss is negligible. The fitted results reveal that the background propagation loss of the waveguide is approximately 0.51 dB/cm and the coupling loss is about 4.25 dB/facet. Furthermore, the total waveguide loss is characterized in the full spectral wavelength range from 1460 to 1640 nm with an on-chip signal power of $-14.4\mathrm{dBm}$ as shown in Fig. 3(b), which exhibits a typical absorption curve of erbium-doped waveguide amplifiers. The absorption loss is evaluated to be $2.68\pm 0.272\mathrm{dB}/\mathrm{cm}$ at 1531.4 nm, which is consistent with results reported in other studies [19,28]. The same characterization and fitting procedure are repeated for different launched signal powers to determine the saturated absorption effect during small-signal excitation where the ground-state transition reaches its maximum. It can be seen in Fig. 3(c) that when the launched signal power is lower than $-9.7\mathrm{dBm}$, the variation in absorption loss is less than 0.15 dB/cm at wavelengths of 1531.4 and 1550 nm.

We have characterized the gain performance of the waveguides with different lengths, specifically 4.8, 6.6, and 10.3 cm. Figure 4(a) illustrates the internal net gain of the EDWAs under various on-chip pump powers with a constant injected signal power of $-14.4\mathrm{dBm}$. All these three waveguides exhibit gain saturation as the injected pump powers increase. The 4.8 cm waveguide reaches saturation condition with an internal net gain of about 12.3 dB when the pump power is approximately 30 mW. In contrast, the 6.6-cm- and 10.3-cm-long waveguides exhibit saturation at higher pump powers of 35 and 45 mW due to larger waveguide lengths and higher losses. At the same time, they show higher peak internal net gains of 17.9 dB and 22.3 dB, respectively. The 10.3-cm-long waveguide demonstrates an on-chip output power of up to 7 mW, indicating its potential to offer efficient power compensation for on-chip optical interconnection systems without the need for discrete bulk EDFAs in practical applications.

Concurrently, we calculate the energy conversion efficiency during the operation of the gain waveguides, as illustrated in Fig. 4(b). It shows a maximum achievable energy conversion efficiency of approximately 15%, which surpasses the results of previous studies by bidirectional pumping [29,30]. This improvement is beneficial to minimize power consumption in future devices. Figure 4(c) illustrates the gain characteristics of the waveguides under various input signal powers. With the variation of input signal optical power, waveguides with different lengths exhibit the same gain trend. At low input signal power, higher internal net gain is achieved. Specifically, the 10.3-cm-long waveguide can reach an internal net gain of approximately 28 dB when the input signal power is $-25\mathrm{dBm}$. This is attributed to a sufficient long waveguide containing more erbium ions and achieving more adequate population inversion. Shorter waveguides, such as 6.6 cm and 4.8 cm, can also obtain internal net gains of about 23 dB and 16 dB, respectively. However, the internal net gain shows a declining trend with relatively high input signal powers due to the saturation limitation. We evaluate the saturation output power of the gain waveguides under different input signal powers as shown in Fig. 4(d). The green region represents the operational conditions under which off-chip net gain can be obtained. The yellow region indicates where an on-chip net gain can be achieved and the red region denotes the conditions under which no gain can be acquired.

In theory, the amplified spontaneous emission (ASE) spectrum of a waveguide under saturation excitation can reflect the signal enhancement characteristics of the waveguide over a broad spectral range because it is related to the emission cross-section of the gain material due to sufficient ion inversions [29,31]. The tested ASE spectra of the waveguides with different lengths are shown in Fig. 4(e). To make a better comparison, the injected pump power is set to a constant high value of 80 mW to ensure the saturation excitation of the longest waveguide. The 10.3-cm-long waveguide exhibits an enhanced ASE spectrum. However, a slight decrease trend occurs at the 13-cm-long waveguide. This can be attributed to the poor waveguide quality and reabsorption effect in the longer waveguide. Figure 4(f) illustrates the signal enhancement at different signal wavelengths for the 10.3-cm-long waveguide with an on-chip signal power of $-14.4\mathrm{dBm}$. It shows a strong correlation with the ASE spectra presented in Fig. 4(e), exhibiting an SE peak at 1531.4 nm with 57.5 dB.

In optical communication systems, the noise figure is a key parameter for evaluating the performance of optical amplifiers. We can derive the noise figure using the optical source-subtraction method after power calibration. Here, the noise figure of the optical waveguide amplifier can be calculated using the following formula: $$\mathrm{NF}=\frac{P_{\mathrm{ASE}}-G{P}_{\mathrm{SSE}}}{GhvB}+\frac{1}{G},$$(1)where $G$ is the linear optical gain at the optical frequency $v$. $P_{\mathrm{ASE}}$ is the power of the amplified spontaneous emission and $P_{\mathrm{SSE}}$ is measured from the calibrated optical spectra. $B$, denoting the measured bandwidth, is set to be 0.02 nm here. Figure 5 shows the calculated noise figures concerning different signal input powers at 1531.4 nm in a 6.6-cm-long waveguide. It can be seen that the minimum noise figure is about 5.64 dB at an input signal power of $-29.7\mathrm{dBm}$. In addition, the noise figures of the EDWA reach 7.29 dB and 9.18 dB when the input signal power is set to $-14.4\mathrm{dBm}$ and $-8.8\mathrm{dBm}$, respectively. It indicates that as the input signal power increases, the noise figure also tends to increase. On one hand, this can be attributed to the insufficient population of erbium ions participating in stimulated emission. This limitation can be addressed by several approaches such as increasing the waveguide length, expanding the waveguide mode field area, or raising the effective doping concentration of erbium ions. On the other hand, the 1531 nm signal is more likely to participate in the excitation dynamics of erbium ions when the input signal is at an elevated power level. This process contributes to the enhancement of the ASE power.

In this study, we conduct a performance comparison of our heterogeneous waveguide amplifiers with other EDWAs reported in recent literature, as presented in Table 1. The finding reveals that the gain performance of our proof-of-concept EDWA is on par with the state-of-the-art erbium-doped lithium niobate devices [8,28]. Meanwhile, our waveguide amplifier outperforms other EDWAs employing heterogeneous integration schemes [11,27,30]. A notable feature of our study is the combination of etchless erbium-doped lithium niobate thin films and low-loss chalcogenide waveguides on a monolithic chip. This strategy enables the demonstration of lithium niobate etch-free waveguides. Nevertheless, there remains a performance gap between our device and the best-reported EDWAs to date. A critical aspect of our waveguide design involves the implementation of an undoped GeSbS strip waveguide to confine the optical mode. This results in approximately 30% of the optical field being confined in the passive region, leading to a relatively low gain per unit length compared to the etched waveguide schemes [28]. It is reasonable to conclude that gain enhancement could be achieved by optimizing the waveguide geometry, reducing the propagation loss, and increasing the doping concentration of erbium ions.Table 1.

Gain Performance Comparison of the Reported Er-Doped Waveguide Amplifiers

In summary, we have successfully manufactured low-loss GeSbS-loaded erbium-doped lithium niobate waveguide amplifiers. Approximately 28.35 dB internal net gain is achieved under pumping in the 1480 nm band. The maximum on-chip output power can reach about 8.2 dBm. Due to the high overlap ratio of signal and pump waves, the demonstrated amplifier can achieve signal amplification at a lower pump power of 45 mW with a high conversion efficiency of 15%. Upon low input signal power, the noise figure of the amplifier can be well controlled below 6 dB. The prototype proposed in this work has achieved exceptional gain performance by a heterogeneous integration design, avoiding the challenging etching process associated with lithium niobate. It paves the way for integrating diverse material properties while simultaneously addressing their intrinsic limitations, thereby promoting the development of versatile devices that can be utilized across various fields, including nonlinear integrated photonics, microwave photonics, and coherent optical communications.