Titanium:sapphire-on-insulator integrated lasers and amplifiers.

Titanium:sapphire (Ti:sapphire) lasers have been essential for advancing fundamental research and technological applications, including the development of the optical frequency comb1, two-photon microscopy2 and experimental quantum optics3,4. Ti:sapphire lasers are unmatched in bandwidth and tuning range, yet their use is restricted because of their large size, cost and need for high optical pump powers5. Here we demonstrate a monocrystalline titanium:sapphire-on-insulator (Ti:SaOI) photonics platform that enables dramatic miniaturization, cost reduction and scalability of Ti:sapphire technology. First, through the fabrication of low-loss whispering-gallery-mode resonators, we realize a Ti:sapphire laser operating with an ultralow, sub-milliwatt lasing threshold. Then, through orders-of-magnitude improvement in mode confinement in Ti:SaOI waveguides, we realize an integrated solid-state (that is, non-semiconductor) optical amplifier operating below 1 µm. We demonstrate unprecedented distortion-free amplification of picosecond pulses to peak powers reaching 1.0 kW. Finally, we demonstrate a tunable integrated Ti:sapphire laser, which can be pumped with low-cost, miniature, off-the-shelf green laser diodes. This opens the doors to new modalities of Ti:sapphire lasers, such as massively scalable Ti:sapphire laser-array systems for several applications. As a proof-of-concept demonstration, we use a Ti:SaOI laser array as the sole optical control for a cavity quantum electrodynamics experiment with artificial atoms in silicon carbide6. This work is a key step towards the democratization of Ti:sapphire technology through a three-orders-of-magnitude reduction in cost and footprint and introduces solid-state broadband amplification of sub-micron wavelength light.

Multi-dimensional data transmission using inverse-designed silicon photonics and microcombs.

The use of optical interconnects has burgeoned as a promising technology that can address the limits of data transfer for future high-performance silicon chips. Recent pushes to enhance optical communication have focused on developing wavelength-division multiplexing technology, and new dimensions of data transfer will be paramount to fulfill the ever-growing need for speed. Here we demonstrate an integrated multi-dimensional communication scheme that combines wavelength- and mode- multiplexing on a silicon photonic circuit. Using foundry-compatible photonic inverse design and spectrally flattened microcombs, we demonstrate a 1.12-Tb/s natively error-free data transmission throughout a silicon nanophotonic waveguide. Furthermore, we implement inverse-designed surface-normal couplers to enable multimode optical transmission between separate silicon chips throughout a multimode-matched fibre. All the inverse-designed devices comply with the process design rules for standard silicon photonic foundries. Our approach is inherently scalable to a multiplicative enhancement over the state of the art silicon photonic transmitters.

Hybrid integrated mode-locked laser using a GaAs-based 1064 nm gain chip and a SiN external cavity.

External cavity mode-locked lasers could be used as comb sources for high volume application such as LIDAR and dual comb spectroscopy. Currently demonstrated chip scale integrated mode-locked lasers all operate in the C-band. In this paper, a hybrid-integrated external cavity mode-locked laser working at 1064 nm is demonstrated, a wavelength beneficial for optical coherence tomography or Raman spectroscopy applications. Additionally, optical injection locking is demonstrated, showing an improvement in the optical linewidth, and an increased stability of the comb spectrum.

Heterogeneous integration of Si photodiodes on silicon nitride for near-visible light detection.

Silicon nitride (SiN) is used extensively to complement the standard silicon photonics portfolio. However, thus far demonstrated light sources and detectors on SiN have predominantly focused on telecommunication wavelengths. Yet, to unlock the full potential of SiN, integrated photodetectors for wavelengths below 850 nm are essential to serve applications such as biosensing, imaging, and quantum photonics. Here, we report the first, to the best of our knowledge, microtransfer printed Si p-i-n photodiodes on a commercially available SiN platform to target wavelengths <850 nm. A novel heterogeneous integration process flow was developed to offer a high microtransfer printing yield. Moreover, these devices are fabricated with CMOS compatible and wafer-scale technology.

Hybrid modeling approach for mode-locked laser diodes with cavity dispersion and nonlinearity.

Semiconductor-based mode-locked lasers, integrated sources enabling the generation of coherent ultra-short optical pulses, are important for a wide range of applications, including datacom, optical ranging and spectroscopy. As their performance remains largely unpredictable due to the lack of commercial design tools and the poorly understood mode-locking dynamics, significant research has focused on their modeling. In recent years, traveling-wave models have been favored because they can efficiently incorporate the rich semiconductor physics of the laser. However, thus far such models struggle to include nonlinear and dispersive effects of an extended passive laser cavity, which can play an important role for the temporal and spectral pulse evolution and stability. To overcome these challenges, we developed a hybrid modeling strategy by unifying the traveling-wave modeling technique for the semiconductor laser sections with a split-step Fourier method for the extended passive laser cavity. This paper presents the hybrid modeling concept and exemplifies for the first time the significance of the third order nonlinearity and dispersion of the extended cavity for a 2.6 GHz III-V-on-Silicon mode-locked laser. This modeling approach allows to include a wide range of physical phenomena with low computational complexity, enabling the exploration of novel operating regimes such as chip-scale soliton mode-locking.

Hybrid integrated mode-locked laser diodes with a silicon nitride extended cavity.

Integrated semiconductor mode-locked lasers have shown promise in many applications and are readily fabricated using generic InP photonic integration platforms. However, the passive waveguides offered in such platforms have relatively high linear and nonlinear losses that limit the performance of these lasers. By extending such lasers with, for example, an external cavity, the performance can be increased considerably. In this paper, we demonstrate for the first time that a high-performance mode-locked laser can be achieved with a butt-coupling integration technique using chip scale silicon nitride waveguides. A platform-independent SiN/SU8 coupler design is used to couple between the silicon nitride external cavity and the III/V active chip. Mode-locked lasers at 2.18 GHz and 15.5 GHz repetition rates are demonstrated with Lorentzian RF linewidths several orders of magnitude smaller than what has been demonstrated on monolithic InP platforms. The RF linewidth was 31 Hz for the 2.18 GHz laser.

50 GBd PAM4 transmitter with a 55nm SiGe BiCMOS driver and silicon photonic segmented MZM.

We demonstrate an optical transmitter consisting of a limiting SiGe BiCMOS driver co-designed and co-packaged with a silicon photonic segmented traveling-wave Mach-Zehnder modulator (MZM). The MZM is split into two traveling-wave segments to increase the bandwidth and to allow a 2-bit DAC functionality. Two limiting driver channels are used to drive these segments, allowing both NRZ and PAM4 signal generation in the optical domain. The voltage swing as well as the peaking of the driver output are tunable, hence the PAM4 signal levels can be tuned and possible bandwidth limitations of the MZM segments can be partially alleviated. Generation of 50 Gbaud and 53 Gbaud PAM4 yields a TDECQ of 2.8 and 3.8 dB with a power efficiency of 3.9 and 3.6 pJ/bit, respectively; this is the best reported efficiency for co-packaged silicon transmitters for short-reach datacenter interconnects at these data rates. With this work, we show the potential of limiting drivers and segmented traveling-wave modulators in 400G capable short-reach optical interconnects.

27 dB gain III-V-on-silicon semiconductor optical amplifier with > 17 dBm output power.

Hybrid III-V-on-silicon semiconductor optical amplifiers with high-gain and high-output-power are important in many applications such as transceivers, integrated microwave photonics and photonic beamforming. In this work we present the design, fabrication and characterization of high-gain, high-output-power III-V-on-silicon semiconductor optical amplifiers. The amplifiers support a hybrid III-V/Si optical mode to reduce confinement in the active region and increase the saturation power. A small-signal gain of 27 dB, a saturation power of 17.24 dBm and an on-chip output power of 17.5 dBm is measured for a current density of 4.9 kA/cm2 (power consumption of 540 mW) at room temperature for an amplifier with a total length of 1.45 mm. The amplifiers were realized using a 6 quantum well InGaAsP active region, which was previously used to fabricate high-speed directly modulated DFB lasers, enabling their co-integration.

Silicon photonics traveling wave photodiode with integrated star coupler for high-linearity mm-wave applications.

Next-generation wireless communication will require increasingly faster data links. To achieve those higher data rates, the shift towards mmWave frequencies and smaller cell sizes will play a major role. Radio-over-Fiber has been proposed as a possible architecture to allow for this shift but is nowadays typically implemented digitally, as CPRI (Common Public Radio Interface). Centralization will be important to keep next-generation architectures cost-effective and therefore shared optical amplification at the central office could be preferable. Unfortunately, limited power handling capabilities of photodetectors still hinder the shift towards centralized optical amplification. Traveling wave photodetectors (TWPDs) have been devised to allow for high-linearity, high-speed opto-electronic conversion. In this paper, an architecture is discussed consisting of such a TWPD implemented on the iSiPP25G silicon photonics platform. A monolithically integrated star coupler is added in the design to provide compact power distribution while preserving the high linearity of the TWPD. The traveling wave structure (using 16 photodetectors) has a measured 3 dB bandwidth of 27.5 GHz and a fairly flat S21 up to 50 GHz (less than 1 dB extra loss). Furthermore, the output referred third-order intercept point at 28 GHz, is improved from -1.79 dBm for a single Ge photodiode to 20.98 dBm by adopting the traveling wave design.

A III-V-on-Si ultra-dense comb laser.

Optical frequency combs emerge as a promising technology that enables highly sensitive, near-real-time spectroscopy with a high resolution. The currently available comb generators are mostly based on bulky and high-cost femtosecond lasers for dense comb generation (line spacing in the range of 100 MHz to 1 GHz). However, their integrated and low-cost counterparts, which are integrated semiconductor mode-locked lasers, are limited by their large comb spacing, small number of lines and broad optical linewidth. In this study, we report a demonstration of a III-V-on-Si comb laser that can function as a compact, low-cost frequency comb generator after frequency stabilization. The use of low-loss passive silicon waveguides enables the integration of a long laser cavity, which enables the laser to be locked in the passive mode at a record-low 1 GHz repetition rate. The 12-nm 10-dB output optical spectrum and the notably small optical mode spacing results in a dense optical comb that consists of over 1400 equally spaced optical lines. The sub-kHz 10-dB radio frequency linewidth and the narrow longitudinal mode linewidth (<400 kHz) indicate notably stable mode-locking. Such integrated dense comb lasers are very promising, for example, for high-resolution and real-time spectroscopy applications.