The use of silicon nitride in integrated photonics has rapidly progressed in recent decades. Ultra-low-loss waveguides based on silicon nitride are a favorable platform for the research of nonlinear and microwave photonics and their application to a wide variety of fields, including precision metrology, communications, sensing, imaging, navigation, computation, and quantum physics. In recent years, the integration of Si and III-V materials has enabled new large-scale, advanced silicon nitride-based photonic integrated circuits with versatile functionality. In this perspective article, we review current trends and the state-of-the-art in silicon nitride-based photonic devices and circuits. We highlight the hybrid and heterogeneous integration of III-V with silicon nitride for electrically pumped soliton microcomb generation and ultra-low-noise lasers with fundamental linewidths in the tens of mHz range. We also discuss several ultimate limits and challenges of silicon nitride-based photonic device performance and provide routes and prospects for future development.

Silicon nitride passive and active photonic integrated circuits: trends and prospects

The development of integrated semiconductor lasers has miniaturized traditional bulky laser systems, enabling a wide range of photonic applications. A progression from pure III-V based lasers to III-V/external cavity structures has harnessed low-loss waveguides in different material systems, leading to significant improvements in laser coherence and stability. Despite these successes, however, key functions remain absent. In this work, we address a critical missing function by integrating the Pockels effect into a semiconductor laser. Using a hybrid integrated III-V/Lithium Niobate structure, we demonstrate several essential capabilities that have not existed in previous integrated lasers. These include a record-high frequency modulation speed of 2 exahertz/s (2.0 × 1018 Hz/s) and fast switching at 50 MHz, both of which are made possible by integration of the electro-optic effect. Moreover, the device co-lases at infrared and visible frequencies via the second-harmonic frequency conversion process, the first such integrated multi-color laser. Combined with its narrow linewidth and wide tunability, this new type of integrated laser holds promise for many applications including LiDAR, microwave photonics, atomic physics, and AR/VR. 

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Integrated Pockels laser

The field of silicon photonics has experienced widespread adoption in the datacoms industry over the past decade, with a plethora of other applications emerging more recently such as light detection and ranging (LIDAR), sensing, quantum photonics, programmable photonics and artificial intelligence. As a result of this, many commercial complementary metal oxide semiconductor (CMOS) foundries have developed open access silicon photonics process lines, enabling the mass production of silicon photonics systems. On the other side of the spectrum, several research labs, typically within universities, have opened up their facilities for small scale prototyping, commonly exploiting e-beam lithography for wafer patterning. Within this ecosystem, there remains a challenge for early stage researchers to progress their novel and innovate designs from the research lab to the commercial foundries because of the lack of compatibility of the processing technologies (e-beam lithography is not an industry tool). The CORNERSTONE rapid-prototyping capability bridges this gap between research and industry by providing a rapid prototyping fabrication line based on deep-UV lithography to enable seamless scaling up of production volumes, whilst also retaining the ability for device level innovation, crucial for researchers, by offering flexibility in its process flows. This review article presents a summary of the current CORNERSTONE capabilities and an outlook for the future.

https://www.mdpi.com/2076-3417/10/22/8201/pdf

CORNERSTONE’s silicon photonics rapid prototyping platforms: current status and future outlook

A hybrid integrated low-noise linear chirp frequency-modulated continuous-wave (FMCW) laser source with a wide frequency bandwidth is demonstrated. By employing two-dimensional thermal tuning, the laser source shows frequency modulation bandwidth of 10.3 GHz at 100 Hz chirped frequency and 5.6 GHz at 1 kHz chirped frequency. The intrinsic linewidth of 49.9 Hz with 42 GHz continuous frequency tuning bandwidth is measured under static operation. Furthermore, by pre-distortion linearization of the laser source, it can distinguish 3 m length difference at 45 km distance in the fiber length measurement experiment, demonstrating its application potential in ultra-long fiber sensing and FMCW light detection and ranging.

Hybrid integrated low-noise linear chirp frequency-modulated continuous-wave laser source based on self-injection to an external cavity

A low noise linearly frequency modulated continuous wave laser source with a wide frequency bandwidth is demonstrated. By two-dimensional thermal tuning, the laser source shows 42 GHz continuous frequency tuning with 49.86 Hz intrinsic linewidth under static operation. For dynamical FMCW, the laser source has 10.25 GHz frequency bandwidth at 100 Hz chirped frequency and 5.56 GHz at 1 kHz chirped frequency. With pre-distortion linearization, it can distinguish 3 m length difference at 45 km distance in the fibre length measured experiment, which demonstrate a potential for application in ultra-long fibre sensing and FMCW LiDAR.

Hybrid integrated low noise linearly chirped Frequency Modulated Continuous Wave laser source based on self-injection to external cavity

Wavelength shift, caused by temperature fluctuation, critically limits the application of photonic systems. Here, the waveguide geometry is optimized to minimize the wavelength shift due to temperature change and fabrication error. A temperature-insensitive Mach–Zehnder interferometer filter is proposed for a wavelength locker, based on a silicon nitride waveguide. The proposed device achieves a 0.6 pm/K spectral shift over the C-band, which meets the requirements of a wavelength locker for application in dense wavelength division multiplex systems.

Temperature-insensitive Mach-Zehnder interferometer based on a silicon nitride waveguide platform.

In multi-chip integrated photonic micro-systems, it is important to protect the laser source from reflection because the semiconductor laser is sensitive to the back reflection induced by the roughness of the waveguide sidewall or other on-chip optical structures. In order to maintain the stable operation of the light source, an additional micro-ring resonator coupled with the laser source is design before the optical circuit which enables the laser diode to enter a self-injection locking state and improves the reflection tolerance. It is experimentally demonstrated in the hybrid system that by tuning the phase shifter and the micro-ring resonate wavelength, the distributed feedback (DFB) laser enters a self-injection locking state, resulting significant improvement of the critical reflection ratio from −18.69 dB to −4.31 dB. The method can isolate the external reflection of laser diode without using heterogeneous integration, which provides a new idea for on-chip source isolation. The parameters design of the micro-ring and the coupling method with the laser are also introduced in the letter.

A Method for Improving Reflection Tolerance of Laser Source in Hybrid Photonic Packaged Micro-System

Narrow linewidth and fast-chirped frequency are essential in frequency-modulated continuous-wave lasers. We introduce a laser that meets these requirements by coupling a distributed feedback laser with an external high-Q microring resonator, where a bulky stacked piezoelectric chip is attached to the resonator for fast tuning. The laser demonstrates an ultranarrow intrinsic linewidth of 22 Hz in the self-injection-locked state. Actuated by the bulky piezoelectric chip, the maximum triangular actuation bandwidth can reach 100 kHz. The driving voltage is filtered to avoid a resonant mechanical mode, obtaining the minimum residual linearity error at 10 kHz with a 4.2 GHz tuning range. A light detection and ranging system was set up for a proof-of-concept experiment, demonstrating a high detection precision with standard deviations of 2.7 and 4.0 cm for targets at 15 and 30 m, respectively.

Liwei Tang

Papers

Hybrid integrated low-noise linear chirp frequency-modulated continuous-wave laser source based on self-injection to an external cavity

Hybrid integrated low noise linearly chirped Frequency Modulated Continuous Wave laser source based on self-injection to external cavity

Temperature-insensitive Mach-Zehnder interferometer based on a silicon nitride waveguide platform.

A Method for Improving Reflection Tolerance of Laser Source in Hybrid Photonic Packaged Micro-System

Hybrid integrated ultralow-linewidth and fast-chirped laser for FMCW LiDAR.