Abstract
We demonstrate a new approach for pulse formation in mode-locked lasers, based on exciting intracavity solitons in a two-dimensionally patterned quasi-phase-matching (QPM) grating. Through an adiabatic following process enabled by an apodized QPM crystal, we transiently excite multicolor nonlinear states within the crystal, utilize their advantageous properties for pulse formation and stabilization, and then convert the energy back to the resonating laser pulse before the end of the crystal in order to suppress losses. This idea gives access to large nonlinearities that would otherwise be too lossy for use intracavity. In our case, the states accessed are self-defocusing Kerr-like nonlinearities based on phase-mismatched second-harmonic generation. The QPM device has an additional transverse gradient, for tuning the nonlinearity and to aid in laser self-starting. We demonstrate the technique in a semiconductor saturable absorber mirror mode-locked laser with Yb:CALGO as the gain medium, producing 100 fs pulses at 540 MHz repetition rate, with 760 mW of average output power. We present comprehensive theoretical and numerical modeling of the laser to understand the new mode-locking regime. Our approach offers a flexible and compact route to managing nonlinearities inside laser cavities while suppressing the losses that could otherwise prevent or deteriorate mode-locked operation, and is particularly interesting for highly compact bulk, fiber, and waveguide lasers with gigahertz repetition rates and operating wavelengths from the near- to mid-infrared spectral regions.
Highlights
Soliton dynamics play a critical role in many optical systems and are of fundamental interest in several areas of physics
We experimentally demonstrate a new type of two-dimensionally patterned quasi-phase-matching (2D-QPM) device designed to adiabatically excite intracavity solitons based on the second-order nonlinearity χ 2
Both cases correspond to a net positive cavity group delay dispersion (GDD), and the main source of self-phase modulation (SPM) is from the negative contribution of the aperiodically poled lithium niobate (APPLN) crystal
Summary
Soliton dynamics play a critical role in many optical systems and are of fundamental interest in several areas of physics. The concept has immediate practical importance for lasers with gigahertz repetition rates, because the features offered by adiabatic cascading simultaneously address many of the challenges faced when scaling femtosecond diode-pumped solid-state lasers to high repetition rates These features include (1) low-loss soliton formation at low intracavity fluences in a highly compact device, enabled by the large self-defocusing n2, of order −1.5 × 10−5 cm2∕GW in our case; (2) straightforward dispersion management, even in restrictive cavity geometries, since the dispersion provided by bulk materials leads to soliton formation; and (3) suppression of damage to optics from Q-switching instabilities through the variable and nonabsorptive losses from SHG, which allow the laser to be operated at the rollover point of the nonlinear reflectivity to suppress Q-switched mode locking [34]. We first present the theory of adiabatic SH excitation (Section 2), our experimental results (Sections 3 and 5), numerical simulation of the laser (Section 4), and concluding remarks (Section 6)
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