Key enabling technologies for optical communications at 2000 nm

Low-loss silicon waveguide and an ultrahigh-Q silicon microring resonator in the 2 µm wave band

Silicon photonic-integrated circuits (PICs) operating in the 2 µm wave band are of great interest for spectroscopic sensing, nonlinear optics, and optical communication applications. However, the performance of silicon PICs in this wave band lags far behind the conventional optical communication band (1310/1550 nm). Here we report the realization of a low-loss waveguide and an ultrahigh-Q microring resonator in the 2 µm wave band on a standard 200 mm silicon photonic platform. The single-mode strip waveguide fabricated on a 220 nm-thick silicon device layer has a record-low propagation loss ∼0.2 dB/cm. Based on the low-loss waveguide, we demonstrate an ultrahigh-Q microring resonator with a measured loaded Q-factor as high as 1.1 × 106 and intrinsic Q-factor of 2 × 106, one order of magnitude higher than prior silicon resonators operating in the same wave band. The extinction ratio of the resonator is higher than 22 dB. These high-performance silicon photonic components pave the way for on-chip sensing applications and nonlinear optics in the 2 µm wave band.

Low-threshold 2 µm InAs/InP quantum dash lasers enabled by punctuated growth

2 µm photonics and optoelectronics is promising for potential applications such as optical communications, LiDAR, and chemical sensing. While the research on 2 µm detectors is on the rise, the development of InP-based 2 µm gain materials with 0D nanostructures is rather stalled. Here, we demonstrate low-threshold, continuous wave lasing at 2 µm wavelength from InAs quantum dash/InP lasers enabled by punctuated growth of the quantum structure. We demonstrate low threshold current densities from the 7.1 µm width ridge-waveguide lasers, with values of 657, 1183, and 1944 A/cm2 under short pulse wave (SPW), quasi-continuous wave (QCW), and continuous wave operation. The lasers also exhibited good thermal stability, with a characteristic temperature T0 of 43 K under SPW mode. The lasing spectra is centered at 1.97 µm, coinciding with the ground-state emission observed from photoluminescence studies. We believe that the InAs quantum dash/InP lasers emitting near 2 µm will be a key enabling technology for 2 µm communication and sensing.

Silicon-based compact eight-channel wavelength and mode division (de)multiplexer for on-chip optical interconnects

A compact wavelength and mode division (de)multiplexer is proposed for multiplexing a total of eight guided TE modes of a 220 nm thick silicon-on-insulator waveguide with input channels at two wavelengths of 1.55 and 2 µm for wavelength division multiplexing. The (de)multiplexer is composed of four sequentially arranged sections with bus waveguides of increasing widths. The first section uses an asymmetric directional coupler to couple one TE mode at 1.55 µm, while each of the next three sections consists of two collocated directional couplers to simultaneously couple two TE modes of the bus waveguide, one at each wavelength of 1.55 and 2 µm. Three linear adiabatic tapers are designed to connect the consecutive bus waveguides. The fundamental TE mode of the bus waveguide at 1.55 or 2 µm is coupled by using another adiabatic taper from a single-mode input waveguide. The simulation results show that over a broad bandwidth of >100n m the insertion loss and crosstalk for both wavelength bands is <1.15d B and <-27d B, respectively. In addition, a compact device footprint with a total coupling length of ∼61µm is achieved due to the use of collocated directional couplers in three sections.

High-Speed Carbon Nanotube Photodetectors for 2 μm Communications

In the era of big data, the growing demand for data transmission capacity requires the communication band to expand from the traditional optical communication windows (∼1.3-1.6 μm) to the 2 μm band (1.8-2.1 μm). However, the largest bandwidth (∼30 GHz) of the current high-speed photodetectors for the 2 μm window is considerably less than the developed 1.55 μm band photodetectors based on III-V materials or germanium (>100 GHz). Here, we demonstrate a high-performance carbon nanotube (CNT) photodetector that can operate in both the 2 and 1.55 μm wavelength bands based on high-density CNT arrays on a quartz substrate. The CNT photodetector exhibits a high responsivity of 0.62 A/W and a large 3 dB bandwidth of 40 GHz (setup-limited) at 2 μm. The bandwidth is larger than that of existing photodetectors working in this wavelength range. Moreover, the CNT photodetector operating at 1.55 μm exhibits a setup-limited 3 dB bandwidth over 67 GHz at zero bias. Our work indicates that CNT photodetectors with high performance and low cost have great potential for future high-speed optical communication at both the 2 and 1.55 μm bands.

Tunable dual optical frequency comb at 2 μm for CO2 sensing

In this article, we demonstrate a dual frequency comb (DFC) based on the gain-switching of mutually injection-locked semiconductor lasers in the 2 μm wavelength region with a tunable free spectral range (FSR) between 500 MHz and 3 GHz. Through the down-conversion process enabled by DFCs, the beating spectra of the optical frequency combs were captured in a 15 MHz electrical bandwidth with high resolution and millisecond acquisition times. A first experimental demonstration of sensing CO₂ with this architecture is also presented.

1.8 µm band broadband hybrid light source employing a combination of a super luminescent diode and thulium-doped fiber amplifier

We have proposed a broadband hybrid light source that combines a super luminescent diode (SLD) and thulium-doped fiber amplifier (TDFA) and operates in the 1.8 µm band. This light source can improve the characteristics of the output spectrum by amplifying the output light of the SLD with TDFA. In this study, we investigate the dependence of the output spectral characteristics of the hybrid broadband light source on the thulium-doped fiber (TDF) length used in the TDFA as well as the output spectra of three newly developed SLDs of 1660, 1690, and 1730 nm bands. In the evaluation of the output bandwidth, there are various definitions of output bandwidth, but we adopted the bandwidth with power density of over -40dBm/0.1nm. This is because it is possible to evaluate in this band with a dynamic range of "30 dB/0.1 nm" by using a general optical spectrum analyzer. The hybrid light source achieves the bandwidth of 332 nm, from 1579 to 1911 nm, and a high total output power of over 15 dBm. The maximum ripple was less than ∼0.1dB, which is similar to the maximum value of that of the SLD, without any deterioration in the ripple characteristics owing to the hybrid configuration of the SLD and TDFA.

Recent Advances in High Speed Photodetectors for eSWIR/MWIR/LWIR Applications

High speed photodetectors operating at a telecommunication band (from 1260 to 1625 nm) have been well studied with the development of an optical fiber communication system. Recent innovations of photonic systems have raised new requirements on the bandwidth of photodetectors with cutoff wavelengths from extended short wavelength infrared (eSWIR) to long wavelength infrared (LWIR). However, the frequency response performance of photodetectors in these longer wavelength bands is less studied, and the performances of the current high-speed photodetectors in these bands are still not comparable with those in the telecommunication band. In this paper, technical routes to achieve high response speed performance of photodetectors in the extended short wavelength infrared/mid wavelength infrared/long wavelength infrared (eSWIR/MWIR/LWIR) band are discussed, and the state-of-the-art performances are reviewed.

Thulium-doped tellurium oxide waveguide amplifier with 7.6 dB net gain on a silicon nitride chip

We report on thulium-doped waveguide amplifiers integrated on a low-loss silicon nitride platform. The amplifier structure consists of a thulium-doped tellurium oxide thin film coated on a silicon nitride strip waveguide on silicon. We determine a waveguide background loss of 0.7 dB/cm at 1479 nm based on the quality factor measured in microring resonators. Gain measurements were carried out in straight and 6.7-cm-long s-bend waveguides realized on a 2.2-cm-long chip. We measure internal net gain over the wavelength range 1860-2000 nm under 1620 nm pumping and up to 7.6 dB total gain at 1870 nm, corresponding to 1.1 dB/cm. These results are promising for the realization of highly compact thulium-doped amplifiers in the emerging 2 μm band for silicon-based photonic microsystems.

Si-rich Si nitride waveguides for optical transmissions and toward wavelength conversion around 2 μm

We show that subwavelength Si-rich nitride waveguides efficiently sustain high-speed transmissions at 2 μm. We report the transmission of a 10 Gbit/s signal over 3.5 cm with negligible power penalty. Parametric conversion in the pulsed pump regime is also demonstrated using the same waveguide structure with an efficiency as high as -18  dB.

On acceleration sensitivity of 2 μm whispering gallery mode-based semiconductor self-injection locked laser

While whispering gallery mode resonators are well known for their low acceleration sensitivity, there has not been much published experimental research on the subject. We performed environmental sensitivity tests of a 2 μm semiconductor distributed feedback (DFB) laser, self-injection locked to a high-Q crystalline whispering gallery mode resonator. Measured acceleration sensitivity of the laser is below 5×10^-11  g^-1 in the 1-200 Hz frequency bandwidth and thermal sensitivity does not exceed 12  MHz/°C. The laser's frequency noise is below 50  Hz/Hz^1/2 at 10 Hz, reaching 0.4  Hz/Hz^1/2 at 400 kHz. The instantaneous linewidth of the laser is improved by nearly 4 orders of magnitude compared to the free-running DFB laser and is measured to be 50 Hz at 0.1 ms measurement time. The Allan deviation of the laser frequency is on the order of 10^-9 from 1 to 1000 s. All these features make the laser attractive for metrology applications involving low-noise 2 μm seed lasers.