Dense WDM transmission at 2 μm enabled by an arrayed waveguide grating

Silicon MMI-based power splitter for multi-band operation at the 1.55 and 2 µm wave bands

Multimode interference (MMI)-based power splitters are fundamental building blocks for integrated photonic devices consisting of an interferometer structure. In order to forestall the 'capacity crunch' in optical communications, integrated devices capable of operating in multiple spectral bands (e.g., the conventional telecom window and the emerging 2 µm wave band) have been proposed and are attracting increasing interest. Here, we demonstrate for the first time, to the best of our knowledge, the realization of a dual-band MMI-based 3 dB power splitter operating at the 1.55 and 2 µm wave bands. The fabricated power splitter exhibits low excess losses of 0.21 dB and 0.32 dB with 1 dB bandwidths for 1500-1600 nm and 1979-2050 nm, respectively.

Silicon photonic flat-top WDM (de)multiplexer based on cascaded Mach-Zehnder interferometers for the 2 µm wavelength band

The 2 µm wavelength band has proven to be a promising candidate for the next communication window. Wavelength-division multiplexing (WDM) transmission at 2 µm can greatly increase the capacity of optical communication systems. Here, we experimentally demonstrate a high-performance silicon photonic flat-top 8-channel WDM (de)multiplexer based on cascaded Mach-Zehnder interferometers for the 2 µm wavelength band. A three-stage-coupler scheme is utilized to provide passbands and reduce channel crosstalk, and 11 thermo-optic phase shifters have allowed active compensation of waveguide phase errors. The fabricated device shows low insertion loss (< 0.9 dB), channel crosstalk (< 20.6 dB) and 1-dB bandwidth of 2.3 nm for operating wavelength ranging from 1955nm to 1985nm. The demonstrated (de)multiplexer could potentially be used for WDM optical data communication in the 2 µm spectral band.

Silicon photonic arrayed waveguide grating with 64 channels for the 2 µm spectral range

Driven by the demand to extend optical fiber communications wavelengths beyond the C + L band, the 2 µm wave band has proven to be a promising candidate. Extensive efforts have been directed into developing high-performance and functional photonic devices. Here we report an integrated silicon photonic arrayed waveguide grating (AWG) fabricated in a commercial foundry. The device has 64 channels with a spacing of approximately 50 GHz (0.7 nm), covering the bandwidth from 1967 nm to 2012 nm. The on-chip insertion loss of the AWG is measured to be approximately 5 dB. By implementing a TiN metal layer, the AWG spectrum can be thermally tuned with an efficiency of 0.27 GHz/mW. The device has a very compact configuration with a footprint of 2.3 mm × 2 mm. The demonstrated AWG can potentially be used for dense wavelength division multiplexing in the 2 µm spectral band.

Tri-layer gradient and polarization-selective vertical couplers for interlayer transition

We demonstrate and optimize a tri-layer vertical coupler for a silicon nitride (Si₃N₄) multilayer platform operating at a 2 µm band. The large spacing between the topmost and bottommost layers of a gradient structure enables ultra-low crossing loss and interlayer crosstalk without affecting the efficiency interlayer transition. We achieve a 0.31 dB transition loss, ultra-low multi-layer crosstalk of -59.3 dB at a crossing angle of 90° with an interlayer gap of 2300 nm at 1950nm. With width optimization of this structure, the fabrication tolerances toward lateral misalignment of two stages in this coupler have increased 61% and 56%, respectively. We also propose a vertical coupler, based on this design, with mode selectivity and achieve an extinction ratio of < 15 dB for wavelengths in the 1910-1990 range. Meanwhile, a multi-layer interlaced AWGs centered at 1950nm and based on vertical coupler has been demonstrated. The proposed vertical couplers exhibit potential for application in large-scale photonic-integrated circuits and broadly in photonic devices.

Compact silicon photonic hybrid ring external cavity (SHREC)/InGaSb-AlGaAsSb wavelength-tunable laser diode operating from 1881-1947 nm

In recent years, the 2 µm waveband has been gaining significant attention due to its potential in the realization of several key technologies, specifically, future long-haul optical communications near the 1.9 µm wavelength region. In this work, we present a hybrid silicon photonic wavelength-tunable diode laser with an operating range of 1881-1947 nm (66 nm) for the first time, providing good compatibility with the hollow-core photonic bandgap fiber and thulium-doped fiber amplifier. Room-temperature continuous-wave operation was achieved with a favorable on-chip output power of 28 mW. Stable single-mode lasing was observed with side-mode suppression ratio up to 35 dB. Besides the abovementioned potential applications, the demonstrated wavelength region will find critical purpose in H₂O spectroscopic sensing, optical logic, signal processing as well as enabling the strong optical Kerr effect on Si.

Key enabling technologies for optical communications at 2000 nm

This paper discusses the potential for opening a new wavelength window at the 2 μm waveband for optical communications, showing current limitations of the system's performance. It focuses on novel results for key enabling technologies, including the analysis of laser injection locking at this waveband, an improved responsivity for bulk and strained InGaAs edge-couple detectors, and also an increased gain profile for thulium-doped fiber amplifiers.

Silicon arrayed waveguide gratings at 2.0-μm wavelength characterized with an on-chip resonator

Low-loss arrayed waveguide gratings (AWGs) are demonstrated at a 2.0-μm wavelength. These devices promote rapidly developing photonic applications, supported by the recent development of mid-infrared lasers integrated on silicon (Si). Multi-spectral photonic integrated circuits at 2.0-μm are envisioned since the AWGs are fabricated with the 500-nm-thick Si-on-insulator platform compatible with recently demonstrated lasers and semiconductor optical amplifiers on Si. Characterization with the AWG-ring method improves the on-chip transmission uncertainty to ∼6% compared to the conventional method with an uncertainty of ∼53%. Channel losses of ∼2.4  dB are found, with -31  dB crosstalk per channel. Fully integrated multi-spectral sources at 2.0 μm with pump lasers, low-loss multiplexers, and an output amplifier are now feasible.

Demonstration of High-Speed Optical Transmission at 2 µm in Titanium Dioxide Waveguides

We demonstrate the transmission of a 10-Gbit/s optical data signal in the 2 µm waveband into titanium dioxide waveguides. Error-free transmissions have been experimentally achieved taking advantage of a 23-dB insertion loss fiber-to-fiber grating-based injection test-bed platform.

High speed single-wavelength modulation and transmission at 2 μm under bandwidth-constrained condition

The 2-μm optical band has gained much attention recently due to its potential applications in optical fiber communication systems. One constraint in this wavelength region is that the electrical bandwidth of components like modulators and photodetectors is limited by the immature manufacturing technologies. Here we experimentally demonstrated the high-speed signal generation and transmission under bandwidth-constrained scenario at 2-μm. It is enabled by the direct-detection optical filter bank multicarrier (FBMC) modulation technique with constant amplitude zero autocorrelation (CAZAC) equalization. We achieved a single wavelength 80 Gbit/s data rate using the 16-QAM FBMC modulation format which is the highest single channel bit rate at 2-μm according to our best knowledge. The signal is transmitted through a 100m-long solid-core fiber designed for single-mode transmission at 2-μm. The measured bit error rates of the signals are below the forward error correction limit of 3.8 × 10^-3, and the 100m-fiber transmission brings negligible penalty.

High gain holmium-doped fibre amplifiers

We investigate the operation of holmium-doped fibre amplifiers (HDFAs) in the 2.1 µm spectral region. For the first time we demonstrate a diode-pumped HDFA. This amplifier provides a peak gain of 25 dB at 2040 nm with a 15 dB gain window spanning the wavelength range 2030 - 2100 nm with an external noise figure (NF) of 4-6 dB. We also compare the operation of HDFAs when pumped at 1950 nm and 2008 nm. The 1950 nm pumped HDFA provides 41 dB peak gain at 2060 nm with 15 dB of gain spanning the wavelength range 2050 - 2120 nm and an external NF of 7-10 dB. By pumping at the longer wavelength of 2008 nm the gain bandwidth of the amplifier is shifted to longer wavelengths and using this architecture a HDFA was demonstrated with a peak gain of 39 dB at 2090 nm and 15 dB of gain spanning the wavelength range 2050 - 2150 nm. The external NF over this wavelength range was 8-14 dB.