Sub-hertz fundamental linewidth photonic integrated Brillouin laser

Abstract

Photonic systems and technologies traditionally relegated to table-top experiments are poised to make the leap from the laboratory to real-world applications through integration, leading to a dramatic decrease in size, weight, power, and cost 1 . In particular, photonic integrated ultra-narrow linewidth lasers are a critical component for applications including coherent communications 2 , metrology 3-5 , microwave photonics 6 , spectroscopy 7 , and optical synthesizers 1 . Stimulated Brillouin scattering (SBS) lasers, through their unique linewidth narrowing properties 8 , are an ideal candidate to create highly-coherent waveguide integrated sources. In particular, cascaded-order Brillouin lasers show promise for multi-line emission 14 , low-noise microwave generation 6 and other optical comb applications. To date, compact, very-low linewidth SBS lasers have been demonstrated using discrete, tapered-fiber coupled chip-scale silica 9,10 or CaF2 11 microresonators. Photonic integration of these lasers can dramatically improve their stability to environmental and mechanical disturbances, simplify their packaging, and lower cost through wafer-scale photonics foundry processes. While single-order silicon 12 and cascade-order chalcogenide 13 waveguide SBS lasers have been demonstrated, these lasers produce modest emission linewidths of 10-100 kHz and are not compatible with waferscale photonics foundry processes. Here, we report the first demonstration of a sub-Hz (~0.7 Hz) fundamental linewidth photonic-integrated Brillouin cascaded-order laser, representing a significant advancement in the state-of-the-art in integrated waveguide SBS lasers. This laser is comprised of a bus-ring resonator fabricated using an ultra-low loss (< 0.5 dB/m) Si3N4 waveguide platform. To achieve a sub-Hz linewidth, we leverage a high-Q, large mode volume, single polarization mode resonator that produces photon generated acoustic waves without phonon guiding. This approach greatly relaxes phase matching conditions between polarization modes and optical and acoustic modes. By using a theory for cascaded-order Brillouin laser dynamics 14 , we determine the fundamental emission linewidth of the first Stokes order by measuring the beat-note linewidth between and the relative powers of the first and third Stokes orders. Extension of these high performance lasers to the visible and near-IR wavebands is possible due to the low optical loss of silicon nitride waveguides from 405 nm to 2350 nm 15 , paving the way to photonic-integrated sub-Hz lasers for visible-light applications including atomic clocks and precision spectroscopy.

Hybrid Integrated Semiconductor Lasers with Silicon Nitride Feedback Circuits

“…Stimulated Brillouin scattering (SBS) is another effect that can manifest as a nonlinear loss in the laser feedback circuit. This process is mediated by optoacoustic interactions in a medium, and has been observed in various waveguide platforms including silica [16], silicon [154], and silicon nitride [155]. The strength (or the intrinsic gain) of SBS is mainly dictated by the material properties, including refractive index, acoustic damping, and photoeleastic constant, as well as the optoacoustic overlap of the waveguiding structure [156].…”

“…With sufficient coherence, diode lasers can play a great role in precision metrology and timing, such as in portable atomic clocks [17,18,19], including satellite-based GPS systems [20,21]. When integrating narrow-linewidth semiconductor lasers into functional photonic circuits, they may serve as on-chip light engines, for instance, to drive Raman and Brillouin lasers [22,23,16]. A most recent development is driving Kerr frequency combs with narrowband diode lasers [24,25,26] which complements the frequency combs provided by mode-locked diode lasers [27].…”

Soliton and quasi-soliton frequency combs due to second harmonic generation in microresonators

“…Using nonlinear effects other than Kerr nonlinearity for comb generation is an active research area. One can mention here examples of the comb generation due to cascaded Raman and Brillouin effects [3][4][5][6], polariton [7,8] and graphene nonlinearities [9], and quadratic, χ (2) , nonlinear effects [10][11][12][13][14][15][16][17][18][19][20][21][22]. The latter hold a particular promise.…”

Unveiling delay-time-resolved phase noise statistics of narrow-linewidth laser via coherent optical time domain reflectometry

“…Considering the fact that slow phase fluctuations can be technically compensated 8,54 , noise spectra at low frequencies are not always of concern while the intrinsic linewidth characteristic associated with fundamental limits is non-trivially paid extensive attentions; for instance, the intrinsic linewidth governed by diverse phase noise dynamics were intensively studied in a plenty of lasers involving various gain mechanisms such as gas laser 55 , fibre laser 56 , semiconductor laser 10,39 , quantum cascade laser 9,32 , photonic integrated laser 13 and micro-resonator laser 57 . To demonstrate the different phase noise dynamics of lasers with various gain mechanism, we have experimentally measured the delay-time-resolved phase jitter variance of a fiber laser compared to that of an external cavity semiconductor laser, which are presented in Supplementary Figure S1.…”