Klaus-Jochen Boller

We present experimental and theoretical evidence of dispersive self-Q switching in hybrid III–V/SiN extended cavity integrated lasers with a frequency-selective narrow-band mirror. Although similar regimes have been previously predicted in photonic crystal lasers, our results demonstrate their occurrence in a considerably simpler and more accessible platform based on silicon photonics technology and without the need for a saturable absorber section. The measurements are accurately reproduced by a theoretical model, whose excellent agreement with experiments establishes a robust and versatile framework for analyzing and optimizing future device designs.

Integrated coherent sources of ultra-violet (UV) light are essential for a wide range of applications, from ion-based quantum computing and optical clocks to gas sensing and microscopy. Recently, approaches that use frequency upconversion have received considerable attention. Among these, the integrated thin-film lithium niobate (TFLN) photonic platform shows particular promise. However, to date, the high propagation losses and lack of reliable techniques for consistent poling of cm-long waveguides with small poling periods have impeded progress. Here, we present a sidewall poled lithium niobate (SPLN) waveguide approach that overcomes these obstacles and results in a two-orders-of-magnitude increase in generated UV power. We demonstrate SPLN waveguides featuring record-low propagation losses of 2.3 dB/cm, complete domain inversion across the waveguide cross-section, and an optimum 50% duty cycle, resulting in a record-high normalized conversion efficiency of 5050%W ⁻¹ cm ⁻² , and 4.2 mW of generated on-chip power at 390 nm wavelength. This advancement makes the TFLN platform a viable option for high-quality on-chip UV generation, benefiting emerging applications.

We show that using strongly coupled dual-core waveguides for supercontinuum generation shifts the wavelength of the high-frequency dispersive waves towards shorter wavelengths, as compared to generation in a single-core waveguide having the same core dimensions. In a demonstration experiment, we launch ultrashort infrared pump pulses at 1- μ m wavelength (285-THz frequency) into silicon nitride waveguides, where soliton formation and fission leads to generation of dispersive waves in the visible range. Efficient input coupling and controlled excitation of the two lowest order supermodes of the dual-core waveguide is provided with adiabatic tapers and a dual-prong input structure. For the dual-core waveguide, the short-wavelength dispersive wave is located at 540 nm (green, 555 THz), which is blue-shifted by 80 nm (70 THz) compared to that of the single-core waveguide. Simultaneously, the dual-core waveguide generates broadband radiation spanning from the blue into the UV range, reaching to below 350 nm (above 855 THz), with typically a spectral density 25 dB below that of the dispersive wave. The broadband component can be attributed to third harmonic generation and is not observed in single-core supercontinuum generation. Numerical modeling shows good agreement with experimental measurements. The demonstrated dual-core approach and dedicated input coupling appear to hold promise also for other waveguide structures, independent of specific materials or core dimensions, by providing shorter wavelengths than with the respective single-core waveguide.

Embedding multi-wavelength lasers in photonic waveguide circuits is of interest for next-generation ion traps, such as for miniaturizing optical clocks or upscaling ion-based quantum computing. Critically, this path involves photonic integration of highly coherent lasers in the ultraviolet (UV) range, which is presently obstructed by the transparency limit of materials used in established integrated waveguides. Here, we demonstrate the first integrated, extended cavity diode laser based solely on UV-transparent materials. We integrate aluminum oxide waveguide circuits with gallium nitride amplifiers to generate milliwatt-level on-chip output power near the ultraviolet range. The extended cavity approach allows for wide wavelength coverage and precise frequency control, which is demonstrated by tuning mode-hop-free to a Sr-transition frequency. Due to the inherent stability of photonic circuits and UV-compatible integration, the intrinsic laser linewidth reaches a record-low value around 300 kHz with better than 43-dB side-mode suppression. These results announce the viability of a novel class of integrated lasers that opens access to the UV.

We experimentally investigate on-chip control and analysis of spatially multimode nonlinear interactions in silicon nitride waveguide circuits. Using widely different dispersion of transverse supermodes in a strongly-coupled dual-core waveguide section, and using integrated pairs of input and output single-mode waveguides, we enable controlled excitation of nonlinear processes in multiple supermodes, while a basic physical mode decomposition aids the identification of parallel and cascaded processes. Pumping with ultrashort pulses at 1.5-μm wavelength (around 195-THz light frequency), we observe simultaneous dual-supermode, near-infrared supercontinuum generation having different spectral widths, in parallel with third-harmonic generation at around 515 nm (582 THz). Cascaded four-wave mixing with supercontinuum components upconverts the third-harmonic radiation toward a set of four shorter blue wavelengths emitted in the range between 485 and 450 nm (617 to 661 THz). The approach taken here, i.e., using chip-integrated spatial multiplexing and demultiplexing for excitation and analysis of broadband transverse nonlinear conversion, can be an advanced tool for better understanding and control in multimode nonlinear optics, such as for extending frequency conversion to wider spectral ranges via extra phase matching paths.

Thin-film lithium niobate (TFLN) has emerged as a promising platform for the realization of high-performance chip-scale optical systems, spanning a range of applications from optical communications to microwave photonics. Such applications rely on the integration of multiple components onto a single platform. However, while many of these components have already been demonstrated on the TFLN platform, to date, a major bottleneck of the platform is the existence of a tunable, high-power, and narrow-linewidth on-chip laser. Here, we address this problem using photonic wire bonding to integrate optical amplifiers with a TFLN feedback circuit. We demonstrate an extended cavity diode laser with an excellent side mode suppression ratio exceeding 60 dB and a wide wavelength tunability over 43 nm. At higher currents, the laser produces a high maximum on-chip power of 76.2 mW while maintaining 51 dB side mode suppression. The laser frequency stability over short timescales shows an ultra-narrow intrinsic linewidth of 550 Hz. Long-term recordings indicate a high passive stability of the photonic wire bonded laser with 58 hours of mode-hop-free operation, with a trend in the frequency drift of only 4.4 MHz/h. This work verifies photonic wire bonding as a viable integration solution for high performance on-chip lasers, opening the path to system level upscaling and Watt-level output powers.

Mode-locked lasers are of interest for applications such as biological imaging, nonlinear frequency conversion, and single-photon generation. In the infrared, chip-integrated mode-locked lasers have been demonstrated through integration of laser diodes with low-loss photonic circuits. However, additional challenges, such as a higher propagation loss and smaller alignment tolerances, have prevented the realization of such lasers in the visible range. Here, we demonstrate the first, to the best of our knowledge, chip-integrated mode-locked diode laser in the visible using an integrated photonic circuit for cavity extension. Based on a gallium arsenide gain chip and a low-loss silicon nitride feedback circuit, the laser is passively mode-locked using a saturable absorber (SA) implemented by focused ion beam (FIB) milling. At a center wavelength of 642 nm, the laser shows an average output power of 3.4 mW, with a spectral bandwidth of 1.5 nm at a repetition rate of 7.84 GHz.

We demonstrate absorber-free passive and hybrid mode-locking at sub-GHz repetition rates, using a hybrid integrated extended cavity diode laser operating near 1550 nm. The laser is based on InP as a gain medium and a Si₃N₄ waveguide feedback circuit. Absorber-free Fourier domain mode-locking with ≈15 comb lines at around 0.2 mW total power is achieved with repetition rates around 500 MHz, using three highly frequency-selective micro-ring resonators that extend the on-chip cavity length to 0.6 m. To stabilize the repetition rate, hybrid mode-locking is demonstrated by weak RF modulation of the diode current. The RF injection reduces the Lorentzian linewidth component from 8.9 kHz to a detection-limited value of around 300 mHz. To measure the locking range of the repetition rate, the injected RF frequency is tuned with regard to the passive mode-locking frequency and the injected RF power is varied. The locking range increases approximately as a square-root function of the injected RF power. At 1 mW injection, a wide locking range of about 80 MHz is obtained. We also observe the laser maintaining stable mode-locking when the DC diode pump current is increased from 40 mA to 190 mA, provided that the cavity length is maintained constant with thermo-refractive tuning.

We present hybrid-integrated extended cavity diode lasers tunable around 637 nm, with a gain-wide spectral coverage of 8 nm. This tuning range addresses the zero-phonon line of nitrogen-vacancy centers and includes the wavelength of HeNe lasers (633 nm). Best performance shows wide mode-hop free tuning up to 97 GHz and a narrow intrinsic linewidth down to 10 kHz. Also, the maximum output power is 2.5 mW in a single-mode fiber, corresponding to an on-chip power of 4.0 mW. Full integration and packaging in standard housing with fiber pigtails provide high intrinsic stability and enable integration into complex optical systems.

Brillouin enhanced four wave mixing in the form of a Brillouin dynamic grating (BDG) enables a uniquely tunable filter whose properties can be tuned by purely optical means. This makes the BDG a valuable tool in microwave photonics (MWP). BDGs have been studied extensively in fibers, but the only observation in an integrated platform required exotic materials. Unlocking BDG in a standard and mature platform will enable its integration into large-scale circuits. Here, we demonstrate the first observation of a BDG in a silicon nitride (Si3N4) waveguide. We also present a new and optimized design, which will enhance the BDG response of the waveguide, unlocking the path to large-scale integration into MWP circuits.