Optical amplification devices are crucial components in optical communication systems. Commercially available amplification devices have their own pros and cons. For instance, erbium-doped fiber amplifiers (EDFAs) can provide high gain with low noise in a broad band, but typically requiring a fiber length of one to tens of meters. Semiconductor optical amplifiers (SOAs) feature high gain but polarization sensitive and relatively high noise figure. The lithium niobate on insulator (LNOI) platform combines the superior optical properties with strong mode confinement, making it a potential solution for next-generation photonic integrated circuits (PICs). Erbium ions exhibit stable and low-noise gain in a broad band covering the telecom bands with high output power, making Erbium doped media highly suitable for optical amplifiers and lasers. Doping erbium ions in LNOI platform is a promising solution to achieve low loss, high gain, and integrated erbium-doped waveguide amplifiers (EDWAs). Here, we demonstrate an efficient and practical integrated Er:LNOI micro-waveguide amplifier and validate the exceptional amplification capabilities and practicality of the Er:LNOI micro-waveguide amplifier.

We fabricate the array of the integrated Er:LNOI waveguide amplifiers (Fig. 1c) based on micrometer thick Er:LNOI wafer (Fig. 1b) through UV lithography and deep etching process. Figure 1d is the photograph of the Er:LNOI wafer containing the fabricated waveguide array, with a length of 5.6 cm. Figure 1f displays the scanning electron microscope (SEM) image of the end facet of a Er:LNOI micro-waveguide.

Fig. 1. (a) Bonding of the Er:LNOI wafer. (b) Thinning and polishing. (c)(d) Image of the micro-waveguide array. (e) Microscopy of the strong fluorescence under 980-nm pumping. (f) SEM image of the micro-waveguide end facet. (g) Simulated fundamental TE mode profiles at 980, 1460, and 1531 nm, respectively.

We assess the performance of the micro-waveguide amplifier under bidirectional pumping at the wavelength of 1460 nm. As the on-chip signal power increases, the internal net gain gradually decreases due to the depletion of erbium ion populations in the excited state. Gain saturation is observed when the on-chip signal power increases to 0.25 dBm, as depicted in Fig. 2a. It is noteworthy that our micro-waveguide amplifier, due to low coupling loss, has the potential to be packaged as a device with a fiber-to-fiber gain of 13.7 dB for a signal power of −4.5 dBm, which is superior to Er:TFLN nano-waveguide amplifiers. The dependence of internal net gain on on-chip pump power is investigated for three different on-chip signal powers, as presented in Fig. 2b. The maximum internal net gain is approximately 18.8 dB, corresponding to a normalized net gain of 3.36 dB/cm. The output signal power is as high as 20.7 mW at the gain point of 18.8 dB (Fig. 2c). The spectra with and without pump at the maximum gain point are shown as Fig. 2d, which is relatively high at the output fiber benefiting from low coupling loss, as compared to the majority of research based on Er:TFLN nano-waveguide amplifiers.

Fig. 2. (a) Internal net gain varies with signal power. (b) Internal net gain varies with on-chip pump power for different on-chip signal powers. (c) Measured output signal power varies with on-chip pump power. (d) Spectra of the output signal with and without pump at the maximum gain point.

This work is published in“Xiaotian Xue, Jing Qiu, Tingting Ding, Wenjun Ding, Jiayu Wang, Yongzhi Tang, Yuting Zhang, Hao Li, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, Integrated erbium-doped waveguide amplifier on lithium niobate on insulator, Optics Materials Express, 14(8), 1985-1994 (2024)”。

Link：https://opg.optica.org/ome/fulltext.cfm?uri=ome-14-8-1985&id=553382