Thin-film lithium niobate circulator-free dispersion compensator in a time-lens system for femtosecond-pulse generation.

We demonstrate a circulator-free thin-film lithium niobate (TFLN) dispersion compensator based on the cascading 2 × 2 multimode interferometer (MMI) and two identical chirped Bragg gratings (CBGs). The cascaded MMI-CBG structure provides a dispersion value of 920 ps/nm/m over a 20 nm bandwidth covering 1537 to 1557 nm, featuring a compact footprint of 1 mm × 0.7 mm. Utilizing this device within a TFLN electro-optic time-lens system, we successfully generate 863-fs pulses at a 37 GHz repetition rate. Our compact, scalable, low-loss, and circulator-free dispersion compensator is the building block for the efficient generation of high-peak-power femtosecond laser pulses.

Bias-drift insensitive full-field frequency response characterization of a thin-film lithium niobate-based intensity modulator.

We propose a rapid and precise scheme for characterizing the full-field frequency response of a thin-film lithium niobate-based intensity modulator (TFLN-IM) via a specially designed multi-tone microwave signal. Our proposed scheme remains insensitive to the bias-drift of IM. Experimental verification is implemented with a self-packaged TFLN-IM with a 3 dB bandwidth of 30 GHz. In comparison with the vector network analyzer (VNA) characterization results, the deviation values of the amplitude-frequency response (AFR) and phase-frequency response (PFR) within the 50 GHz bandwidth are below 0.3 dB and 0.15 rad, respectively. When the bias is drifted within 90% of the Vπ range, the deviation fluctuation values of AFR and PFR are less than 0.3 dB and 0.05 rad, respectively. With the help of the full-field response results, we can pre-compensate the TFLN-IM for the 64 Gbaud PAM-4 signals under the back-to-back (B2B) transmission, achieving a received optical power (ROP) gain of 2.3 dB. The versatility of our proposed full-field response characterization scheme can extend to various optical transceivers, offering the advantage of low cost, robust operation, and flexible implementation.

Arbitrary-ratio 1 × 2 optical power splitter based on thin-film lithium niobate.

Optical power splitters (OPSs) have been widely used in photonic integrated circuits, but an OPS with a large fabrication tolerance and free choice of power splitting ratio (PSR) is still highly desired for thin-film lithium niobate (TFLN) platform. Here, we propose and experimentally demonstrate several 1 × 2 OPSs with PSRs from 50:50 to 5:95 using TFLN platform. The proposed devices are built by multimode interference structure to achieve a broad bandwidth and large fabrication tolerance. Various PSRs can be obtained by adjusting the geometry structure of the multimode interference region. All of our fabricated devices feature an insertion loss lower than 0.3 dB at the wavelength of 1550 nm, and a PSR variation less than 3% in the range of 1520 nm to 1590 nm.

Receive-diversity-aided power-fading compensation for C-band IM/DD OFDM systems.

A receive-diversity-aided power-fading compensation (RDA-PFC) scheme is proposed and demonstrated to eliminate the chromatic dispersion (CD)-induced power fading for C-band double-sideband (DSB) intensity modulation and direct detection (IM/DD) orthogonal frequency division multiplexing (OFDM) systems. By combining the responses before and after a dispersive element using a maximal-ratio combining (MRC) algorithm, the CD-induced power fading dips within the signal bandwidth of around 50 GHz can be effectively compensated for, which results in an up to 17.6-dB signal-to-noise ratio (SNR) improvement for the fading subcarriers after transmission over 10 km of standard single-mode fiber (SSMF). Using the 16 quadrature amplitude modulation (QAM) format, a diversity receiver with the proposed RDA-PFC scheme can support 170.6-Gbit/s OFDM signal transmission over a 10-km SSMF and reduces the bit error rate (BER) by more than an order of magnitude compared with a conventional receiver. Moreover, 208.1-Gbit/s adaptive bit and power loading OFDM signal transmission over a 10-km SSMF is realized by the proposed RDA-PFC scheme, which improves the capacity by 15.3% compared with the case without RDA-PFC at a BER of 3.8 × 10-3. The proposed RDA-PFC scheme shows great potential in CD-induced power-fading compensation for high-speed IM/DD OFDM systems.

C-Band optical 90-degree hybrid using thin film lithium niobate.

The integrated optical 90-degree hybrid is a crucial component for coherent receivers. Here, we simulate and fabricate a 4 × 4 multimode interference coupler as a 90-degree hybrid using thin film lithium niobate (TFLN). The device features low loss (0.37 dB), high common mode rejection ratio (over 22 dB), compact footprint, and small phase error (below 2°) within the whole C-band experimentally, which is promising for integration with coherent modulators and photodetectors for TFLN-based high-bandwidth optical coherent transceivers.

Widely tunable O-band lithium niobite/III-V transmitter.

The ever-increasing traffic has been driving the demand for compact, high-speed, and low-power-consumption optical transmitters. Thin-film lithium niobite (TFLN) platforms have emerged as promising photonic integrated solutions for next-generation optical transmitters. In this study, we demonstrated the first widely tunable optical transmitter based on a butt-coupling a TFLN modulator with an electrically pumped tunable laser. The tunable laser exhibited a side-mode suppression ratio of > 60 dB, linewidth of 475 kHz, and wavelength-tuning range of over 40 nm. The TFLN modulator presented a voltage-length product of 2.9 V·cm and an electro-optic response of 1.5 dB roll-off at 50 GHz. The optical transmitter support data rate was as high as 160 Gb/s.

Comparison of low-complexity sparse and weight-sharing nonlinear equalizers for C-band 100-Gbit/s DSB PAM-4 transmission over 60-km SSMF.

To cope with the nonlinear distortions and the chromatic dispersion (CD) induced power fading in double-side band (DSB) intensity modulation and direct detection (IM/DD) transmission systems, high-performance Volterra nonlinear equalizers (VNLEs) including Volterra feed-forward equalizer (VFFE) and Volterra decision-feedback equalizer (VDFE) are widely applied. However, the conventional VNLEs have high computational complexity, especially for longer memory lengths. In this paper, based on sparse and weight-sharing strategies for significant kernel reduction, we propose four low-complexity NLEs including a sparse diagonally pruned VDFE (S-DP-VDFE), a sparse diagonally pruned absolute-term DFE (S-DP-ATDFE), a weight-sharing DP-VDFE (WS-DP-VDFE), and a weight-sharing DP-ATDFE (WS-DP-ATDFE), and present a comprehensive comparison among them in terms of computational complexity and bit error ratio (BER) performance in a C-band 100-Gbit/s PAM-4 transmission system over 60-km standard single-mode fiber (SSMF). The experimental results show that the proposed S-DP-VDFE and WS-DP-VDFE not only exhibit comparable performance with the conventional DP-VDFE but also reduce the complexity by 54.5% and 45.9%, respectively. While the proposed S-DP-ATDFE and WS-DP-ATDFE yield lower complexity at the expense of a slight performance degradation. Compared with the proposed S-DP-VDFE, S-DP-ATDFE, and WS-DP-VDFE, the proposed WS-DP-ATDFE with the lowest number of real-valued multiplications of 45 achieves up to 90.9%, 81.6%, and 95.8% complexity reduction, respectively, at the 7% hard-decision forward error correction (HD-FEC) BER limit of 3.8 × 10-3. The proposed low-complexity WS-DP-ATDFE shows great potential in low-cost and high-performance IM/DD optical transmission systems.

Nonlinearity-aware PS-PAM-16 transmission for C-band net-300-Gbit/s/λ short-reach optical interconnects with a single DAC.

A nonlinearity-aware signal transmission scheme based on a low-complexity 3rd-order diagonally pruned absolute-term nonlinear equalizer (NLE) with weight sharing (DP-AT-NLE-WS) and rate-adaptable probabilistically shaped 16-level pulse amplitude modulation (PS-PAM-16) signal is proposed and experimentally demonstrated for C-band net-300-Gbit/s/λ short-reach optical interconnects. By replacing the multiplication operation with the absolute operation and applying weight sharing to reduce the kernel redundancy, the computational complexity of the proposed 3rd-order DP-AT-NLE-WS is reduced by >40% compared with the 3rd-order DP-Volterra NLE (DP-VNLE), DP-AT-NLE, and DP-VNLE-WS, with the achieved normalized general mutual information (NGMI) above a threshold of 0.857. Employing a commercial 32-GHz Mach-Zehnder modulator (MZM) and a single digital-to-analog converter (DAC), we demonstrate the single-lane transmission of 100-GBaud PS-PAM-16 signal using DP-AT-NLE-WS in the C band at record 370-Gbit/s line rate and 300.4-Gbit/s net rate over 1-km standard single-mode fiber (SSMF), achieving 21.2% (15.5%) capacity improvement over 100 (105)-GBaud PAM-8 transmission. To the best of our knowledge, this is the first net-300-Gbit/s intensity modulation and direct detection (IM/DD) short-reach transmission in the C band using commercially available components.

High-performance polarization management devices based on thin-film lithium niobate.

High-speed polarization management is highly desirable for many applications, such as remote sensing, telecommunication, and medical diagnosis. However, most of the approaches for polarization management rely on bulky optical components that are slow to respond, cumbersome to use, and sometimes with high drive voltages. Here, we overcome these limitations by harnessing photonic integrated circuits based on thin-film lithium niobate platform. We successfully realize a portfolio of thin-film lithium niobate devices for essential polarization management functionalities, including arbitrary polarization generation, fast polarization measurement, polarization scrambling, and automatic polarization control. The present devices feature ultra-fast control speeds, low drive voltages, low optical losses and compact footprints. Using these devices, we achieve high fidelity polarization generation with a polarization extinction ratio up to 41.9 dB and fast polarization scrambling with a scrambling rate up to 65 Mrad s-1, both of which are best results in integrated optics. We also demonstrate the endless polarization state tracking operation in our devices. The demonstrated devices unlock a drastically new level of performance and scales in polarization management devices, leading to a paradigm shift in polarization management.

Low-loss edge-coupling thin-film lithium niobate modulator with an efficient phase shifter: publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 1478 (2021)OPLEDP0146-959210.1364/OL.418996.

Low-loss edge-coupling thin-film lithium niobate modulator with an efficient phase shifter.

Thin-film lithium-niobate-on-insulator (LNOI) is a very attractive platform for optical interconnect and nonlinear optics. It is essential to enable lithium niobate photonic integrated circuits with low power consumption. Here we present an edge-coupling Mach-Zehnder modulator on the platform with low fiber-chip coupling loss of 0.5 dB/facet, half-wave voltage Vπ of 2.36 V, electro-optic (EO) bandwidth of 60 GHz and an efficient thermal-optic phase shifter with half-wave power of 6.24 mW. In addition, we experimentally demonstrate single-lane 200 Gbit/s data transmission utilizing a discrete multi-tone signal. The LNOI modulator demonstrated here shows great potential in energy-efficient large-capacity optical interconnects.

Compact high-efficiency four-mode vortex beam generator within the telecom C-band.

Vortex beams carrying orbital angular momentum have attracted a great deal of attention over the past few years. An integrated vortex beam generator with high efficiency is desirable for wide-ranging applications. Here we demonstrate a highly efficient silicon photonic integrated vortex beam generator based on superposed holographic fork gratings. A metal mirror is used to enhance emission efficiency by reflecting the power leaking down to the substrate back to air. Experimental characterization confirms that the emission efficiency of the generator increases by ${\sim} 5\,{\rm dB}$∼5dB. Moreover, the present device shows preferable features of broadband, polarization diversity, and compact footprint.

Compact high-efficiency four-mode vortex beam generator within the telecom C-band: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 1607 (2020)OPLEDP0146-959210.1364/OL.385878.

Compact high-efficiency vortex beam emitter based on a silicon photonics micro-ring.

Photonic integrated devices that emit vortex beam carrying orbital angular momentum are becoming key components for multiple applications. Here we propose and demonstrate a high-efficiency vortex beam emitter based on a silicon micro-ring resonator integrated with a metal mirror. Such a compact emitter is capable of generating vortex beams with a high efficiency and small divergence angle. Vector vortex beams of various topological charges are selectively generated by the emitter at different wavelengths with an emission efficiency of up to 37%.