Photonic integration holds the promise for realizing high-performance, low-cost, scalable solutions for communication, sensing, and quantum computation applications. Lithium niobate on insulator (LNOI) is one of the most suitable material platforms for integrated photonic circuits (PICs) due to its unique material properties including the large electro-optic coefficient, the large second-order nonlinear susceptibility, and a wide optical transparency window ranging from the visible to the infrared.
Chip-based LNOI devices, such as wide bandwidth electro-optic modulators have recently become available and work fully compatibly with complementary metal-oxide semiconductor (CMOS) voltages. Lately, lasers were proven to have the compatibility to be integrated on the LNOI platform with low loss. Until recently, all building blocks for a complete PIC have been realized on the LNOI platform with outstanding performance except one—photodetectors.
Photodetectors that absorb light and can convert high-speed modulated optical signals into a photocurrent are essential components in optical communication networks and microwave photonics systems. Conventionally, and since LNOI is transparent, the optical signal from a PIC is fiber-coupled into a discrete, stand-alone photodetector. This, however, introduces additional loss and is not compatible with the need for fully integrated chip-scale photonic solutions.
To address the problems, a joint research group led by Prof. Andreas Beling and Xiangwen Guo at the University of Virginia in collaboration with Dr. Mian Zhang at Hyperlight and Prof. Marko Lon?ar at Harvard University has demonstrated and experimentally verified the first-ever integrated high-bandwidth photodetector (PD) on LNOI waveguide.
The reported approach is based on an InP-based modified uni-traveling carrier PD that was optimized for high speed and high efficiency, and heterogeneous material integration using adhesive wafer bonding to join the two dissimilar materials, InP and LNOI. The relevant research results were published in Photonics Research, Volume 10, No. 6, 2022 (Xiangwen Guo, Linbo Shao, Lingyan He, Kevin Luke, Jesse Morgan, Keye Sun, Junyi Gao, Ta-Ching Tzu, Yang Shen, Dekang Chen, Bingtian Guo, Fengxin Yu, Qianhuan Yu, Masoud Jafari, Marko Lon?ar, Mian Zhang, Andreas Beling, High-performance modified uni-traveling carrier photodiode integrated on a thin-film lithium niobate platform[J]. Photonics Research, 2022, 10(6): 1338).
As shown in fig.1, the PD is integrated on top of the LNOI waveguide from which light couples into the absorber and converts into photocurrent. In the experiments a lensed fiber was used to input-couple light at 1550 nm wavelength and the photocurrent was extracted by a high-frequency probe.
Fig.1 (a) Illustration of integrated photodetector on LNOI waveguide. (b) Microscope image of a PD with lensed fiber (left) and a high-frequency probe (right) for testing. (c) Microscope image of complete chip with integrated PDs and modulators.
The results show that the photodiode reaches a record-high 3-dB bandwidth of 80 GHz (fig. 2a) and a high responsivity of 0.6 A/W. Digital data detection performance of the PD was characterized by an eye diagram measurement (fig. 2b). The non-return-to-zero on-off keyed data pattern was generated at 40 Gbit/s and launched into the LNOI waveguide. Figure 2(b) shows the clearly open eye diagram from the integrated PD and the eye diagram of a commercial PD as a reference.
It is worth mentioning that the integrated PD can handle sufficient photocurrent and provides high signal-to-noise ratio to the oscilloscope so no amplifier to boost up the electrical signal was necessary in the measurements. The clear open eye diagram proves the high-bandwidth performance of the integrated PD and also demonstrates its capability for digital communication applications.
Fig.2 (a) Measured frequency responses showing 3-dB bandwidths of up to 80 GHz for PDs with small absorber area. (b) Setup for eye diagram measurements (top) and measured eye diagrams for a commercial PD (left) and the integrated PD on LNOI (right).
These results show that the heterogeneously integrated InP-based modified uni-traveling carrier PD is a promising candidate for LNOI PICs that require wide-bandwidth and efficient light detection on chip.
