Strong Normal Dispersion Research Articles

Widely-tunable, multi-band Raman laser based on dispersion-managed thin-film lithium niobate microring resonators

Stimulated Raman scattering is an attractive way to extend the operation spectral range of optical sources. However, the spectral extension range of a tunable Raman laser is limited by the Raman frequency shift and pump tuning bandwidth. This makes it challenging to realize chip-scale, widely tunable Raman lasers, as on-chip lasers only provide limited pump power and tuning bandwidth. Here, we tackle this by dispersion engineering of a thin-film lithium niobate microring resonator, where its high-quality factor ( ~ 2.5 million) ensures a sub-milli-watt (0.8 mW) threshold for Raman lasing while its strong normal dispersion with suppressed avoided mode crossing restrains the competing Kerr comb generation process. Combining the multi-wavelength Raman gain response of lithium niobate and cascaded Raman lasing, we demonstrate a widely tunable Raman laser covering 1592–1955 nm, showing a 335-nm spectral extension range from a 94-nm-tuning-bandwidth pump laser. Our demonstration paves the way to realize chip-scale, widely-tunable Raman lasers.

Temporal characteristics of stationary switching waves in a normal dispersion pulsed-pump fiber cavity.

Kerr cavities driven in the normal dispersion regime are known to host switching waves. These consist of a traveling wavefront that connects separate regions associated with high- and low-intensity steady states of the cavity. In this Letter, we drive a 230-m custom built fiber ring cavity with strong normal dispersion using nanosecond pulses, allowing us to directly resolve the fine structure of individual switching waves, including resonant oscillations occurring over periods of the order of ∼10 ps. We demonstrate the intimate connection between the temporal and spectral features of the dispersive waves associated with switching waves, while also investigating how these dispersive waves evolve with cavity parameters, namely the frequency detuning and pump desynchronization. Furthermore, by applying a localized and temporary perturbation to our driving field in the presence of a phase modulation trapping potential, we are able to generate a stable and persistent dark pulse, allowing us to directly observe and model the interlocking of two stationary switching waves under quasi-CW pumping conditions. These results further verify the accuracy of the dispersive wave formalism used, and show that their temporal modulation frequency and decay rate in a pulsed-pumped cavity are accurately captured from theory previously applied to CW-pumped systems.

Microfiber-enabled dissipative soliton fiber laser at 2 μm.

Optical microfibers can show very strong normal dispersion (as high as 2700 ps2/km) around 2μm, a highly desired feature for dispersion management in ultrafast lasers. Here we report a dissipative soliton fiber laser at 2μm enabled by an optical microfiber. The full width at half-maximum of the optical spectrum was about 50nm, and its flatness could be attributed to the third-order dispersion compensation provided by the optical microfiber. The pulse duration was measured to be 195fs after a dechirping single-mode fiber. Our results suggest a new route to dissipative soliton fiber lasers at 2μm, with a wide and flat spectrum and thus a short dechirped pulse.

Broadband biphoton generation and statistics of quantum light in the UV-visible range in an AlGaN microring resonator

We present a physical investigation on the generation of correlated photon pairs that are broadly spaced in the ultraviolet (UV) and visible spectrum on a AlGaN/AlN integrated photonic platform which is optically transparent at these wavelengths. Using spontaneous four wave mixing (SFWM) in an AlGaN microring resonator, we show design techniques to satisfy the phase matching condition between the optical pump, the signal, and idler photon pairs, a condition which is essential and is a key hurdle when operating at short wavelength due to the strong normal dispersion of the material. Such UV-visible photon pairs are quite beneficial for interaction with qubit ions that are mostly in this wavelength range, and will enable heralding the photon-ion interaction. As a target application example, we present the systematic AlGaN microresonator design for generating signal and idler photon pairs using a blue wavelength pump, while the signal appears at the transition of ytterbium ion (171Yb+, 369.5 nm) and the idler appears in the far blue or green range. The photon pairs have minimal crosstalk to the pump power due to their broad spacing in spectral wavelength, thereby relaxing the design of on-chip integrated filters for separating pump, signal and idler.

Extending chip-based Kerr-comb to visible spectrum by dispersive wave engineering.

Anomalous group velocity dispersion is a key parameter for generating bright solitons, and thus wideband Kerr frequency combs. Extension of the frequency combs spectrum to visible wavelengths has been a major challenge because of the strong normal dispersion of conventional photonic materials at these wavelengths. In this paper, we numerically demonstrate a wideband frequency comb extending from near-infrared to visible wavelengths (∼1200 nm to 650 nm). The proposed frequency comb micro-resonator takes advantage of a wideband blue-shifted anomalous dispersion, achieved in an optimized over-etched silicon nitride waveguide and strong power transfer to shorter wavelengths through radiative dispersive waves, achieved by modulating the dispersion in a coupled resonator architecture. We show the possibility of obtaining a close to visible dispersive Cherenkov radiation peak that is only 10 dB below the overall comb peak and can be tuned by adjusting the coupling structure in the coupled resonator architecture.

Supercontinuum generation by stimulated Raman–Kerr scattering in a liquid-core optical fiber

We investigate nonlinear picosecond pulse propagation at 532 nm in an optical fiber capillary filled with toluene liquid and demonstrate supercontinuum generation from 550 nm up to 870 nm. Although it operates in the strong normal dispersion regime, we show that supercontinuum light is generated through the combined effects of multiple cascaded Raman scattering and stimulated Raman–Kerr scattering, which enables a large spectral broadening of the Raman lines owing to the strongly anisotropic orientational relaxation of liquid molecules. We further present numerical simulations of the nonlinear Schrodinger equation, including the Raman scattering, to show the spectro-temporal dynamics.

Wideband slow light based on plasmon-induced transparency at telecom frequency

We propose and demonstrate a metal–insulator–metal (MIM) waveguide side coupled with a series of stubs to realize broadband slow surface plasmon polaritons (SPPs) around the telecom wavelength 193.5THz. The obviously slow light effect results from the strong normal dispersion around the frequency of the plasmon-induced transparency. Theoretical calculations indicate that the plasmonic waveguide system of the length 11μm works on a broad bandwidth of 20THz. The group velocity of SPPs predicted by the improved transmission line theory is about 0.2c (c is light speed in vacuum), which is confirmed by the finite-difference time-domain (FDTD) numerical simulation. The waveguide system for slow light effect has important potential application in optical delay lines.

Ultranarrow linewidth and high gain of an optical cavity with enhanced self-Kerr nonlinearity in quantum dot molecules

The enhanced self-Kerr nonlinearity of quantum dot molecules may be used to realize optical cavities with an ultranarrow linewidth and high gain. The resonant tunneling induces constructive interference for the self-Kerr nonlinearity, and then a narrow gain window with large normal dispersion appears with frequency detuning. The competition between linear and nonlinear dispersion leads to strong normal dispersion of the total susceptibility, which significantly narrows the cavity linewidth; the nonlinear gain introduces the total gain effects contributing to high transmission. Simulation results show that the cavity linewidth could be narrowed by nearly 30 times and the transmission peak enhanced about 40 times compared with a linear case.

Coexistence and evolution of bright pulses and dark solitons in a fiber laser

Bright pulses and dark solitons coexisting and evolving in a fiber laser with strong normal dispersion have been observed. Experimental results show that there exists a transitional pulse regime with the pulse duty cycle of ∼0.5 that connects the bright pulse and dark soliton regimes. On adjusting polarization states and pump power, the duty cycle of pulses varies so that the laser emits bright, transitional, and dark pulses alternately. It is found that dark solitons are composed of two antiphase square pulses with very large durations and duty cycles. The experimental observations reveal that the bright and dark solitons are not completely isolated from each other. This work could provide a new perspective to the understanding of bright and dark soliton formations as well as their transformations.

Erbium-Doped Fiber Lasers Operated in a Strong Normal Dispersion Regime at Low Repetition Rate

We report on a passively mode-locked erbium-doped fiber laser with a repetition rate as low as 1.2 MHz in a strong normal dispersion regime. The laser was mode locked via spectral filtering. It generated a pulse with large and linear chirp that can be dechirped to 430 fs outside the cavity.

High Fundamental Repetition Rate Fiber Lasers Operated in Strong Normal Dispersion Regime

We demonstrate a high fundamental repetition rate erbium-doped fiber ring laser operated in strong normal dispersion regime passively mode-locked by the nonlinear polarization rotation technique. A novel scheme of using a polarization beam splitter as both the pump input coupler and the fiber laser output coupler is employed to increase the repetition rate of the fiber laser. Gain-guided solitons with a fundamental repetition rate of 234 MHz and 1.11 ps pulsewidth are achieved from the ring laser cavity.

Dissipative solitons in a passively mode-locked Er-doped fiber with strong normal dispersion

We report on ultrashort pulse generation from a passively mode-locked erbium fiber laser operating in the highly positive dispersion regime. Highly-chirped pulses with 5.3 ps duration and spectral bandwidth of 8.3 nm are generated. They are extra-cavity compressed down to 757 fs. Numerical simulations confirm the experimental results and show that these pulses could be interpreted as dissipative solitons.

Propagation of electromagnetic waves in periodic and Fibonacci photonic loop structures

We investigate theoretically and experimentally the propagation of electromagnetic waves in one-dimensional periodic and quasi-periodic photonic band gap structures made of serial loop structures separated by segments. The quasi-periodic structures are ordered according to a Fibonacci sequence constituted of two blocks A and B where each block is composed of a loop attached to a segment. It is demonstrated that the general trend of the transmission spectrum of the Fibonacci structure may be considered as an intermediate spectrum between those corresponding to the periodic structures composed of only blocks A or only blocks B . In particular, besides the existence of extended and forbidden modes, some narrow frequency bands appear in the transmission spectra inside the gaps as defect modes. These modes are shown to be localized only within one of the two blocks constituting the structure. A numerical analysis of the transmission phase time enables us to derive the group velocity as well as the density of states in these structures. In particular, the localized modes may give rise to a strong normal dispersion in the gap, giving rise to a slow group velocity below the normal propagation speed in the coaxial cables. The dependence of the band gap structure on the lengths of the finite segment and the loop diameter is presented. The experimental results are obtained using coaxial cables in the frequency range of few hundreds of MHz. These results are in very good agreement with theoretical prediction using Green's function method. The scaling property has been checked theoretically in the case where the absorption in the cables is neglected.

Slow electromagnetic pulse propagation through a narrow transmission band in a coaxial photonic crystal

We demonstrate the slow group-velocity propagation of electromagnetic pulses through a narrow transmission band of a simple coaxial photonic crystal. The narrow transmission band was formed by creating a defect in a periodic coaxial cable filter which resulted in a narrow-frequency passband within an otherwise forbidden band stop region. Strong normal dispersion within this region causes the group velocity of the pulse to be slowed well below the speed of light and below the expected propagation speed in a coaxial cable. This phenomenon is essentially similar to the much-studied slow light propagation observed in atomic vapors through regions of strong dispersion. The simplicity with which this phenomenon can be observed in a one-dimensional coaxial photonic crystal makes this a versatile experiment for such studies. Group velocities of 0.30c, where c is the speed of light in a vacuum, were observed.

Group velocity manipulation in simple acoustic band gap filters

A simple experimental configuration is presented in which the group velocity of acoustic pulses is adjusted from much slower to much faster than the speed of sound. The experiment is an acoustic analog of two much studied optical phenomena: the superluminal group velocity achieved when optical pulses are tunneled through regions of high absorption or attenuation, and slow (or even stopped) light propagation in the vicinity of strong material dispersion, typically realized using electromagnetically induced transparency in atomic vapors. For the acoustic experiments described here, attenuation and dispersion were created using periodically structured waveguides. The periodicity results in one-dimensional acoustic band gaps, frequency intervals in which propagation of sound is strongly suppressed. The introduction of defects in the perfect periodicity leads to narrow transmission bands within the band gaps. There is strong normal dispersion in the vicinity of these defect modes leading to very slow group velocities. By tuning the carrier frequency of acoustic pulses through the forbidden and defect regions the group velocity can be adjusted from 0.22 to 1.8 times the speed of sound in the straight unstructured waveguide. These results are shown to be consistent with a straightforward theoretical model.