We report the first demonstration of a regime of operation in optical parametric oscillators (OPOs), in which the formation of temporal simultons produces stable femtosecond half-harmonic pulses. Simultons are simultaneous bright-dark solitons of a signal field at frequency ω and the pump field at 2ω, which form in a quadratic nonlinear medium. The formation of simultons in an OPO is due to the interplay of nonlinear pulse acceleration with the timing mismatch between the pump repetition period and the cold-cavity roundtrip time and is evidenced by sech 2 spectra with broad instantaneous bandwidths when the resonator is detuned to a slightly longer round-trip time than the pump repetition period. We provide a theoretical description of an OPO operating in a regime dominated by these dynamics, observe the distinct features of simulton formation in an experiment, and verify our results with numerical simulations. These results represent a new regime of operation in nonlinear resonators, which can lead to efficient and scalable sources of few-cycle frequency combs at arbitrary wavelengths. DOI: 10.1103/PhysRevLett.120.053904 Introduction.-The temporal modes that arise from cavity nonlinear dynamics have attracted intense interest due to both their diversity of operating regimes and their applications. Recent work has focused on resonators with cubic nonlinearities that form dissipative solitons from a balance between intracavity dispersion and self-phase modulation [1][2][3]. These systems have successfully generated few-cycle pulses and phase-stabilized frequency combs in the range from 400 nm to 3.5 μm [4-6] and have been employed in optical clocks [7], spectroscopy [8], telecommunications [9], and attoscience [10]. While considerable effort is being invested to extend these sources to other wavelength ranges [11], such operation requires overcoming the challenges associated with developing broadband laser gain media and high finesse resonators at new wavelengths.Nonlinear resonators based on quadratic nonlinearities offer a compelling new direction for the field. In contrast with cubic nonlinearities, the χ ð2Þ associated with quadratic nonlinearities may be patterned to quasiphasematch a rich variety of multiwave interactions. χ ð2Þ materials are well developed and frequently used to produce pulses at otherwise inaccessible wavelengths, but many of the regimes of operation in these systems remain relatively unexplored. One promising system is the synchronously pumped degenerate optical parametric oscillator (OPO), in which a χ ð2Þ resonator pumped at 2ω generates a resonant halfharmonic at ω. While many pulse formation mechanisms have been proposed in continuous-wave-pumped degenerate OPOs [12][13][14][15], to date these systems have not yet achieved mode-locked femtosecond pulses by using such

For the growing demand of frequency combs in mid-infrared (mid-IR), known as the "molecular fingerprint" region of the spectrum [1], down conversion of near-IR frequency combs through halfharmonic generation offers numerous benefits including high conversion efficiency and intrinsic phase and frequency locking to the near-IR pump [2]. Hence cascaded half-harmonic generation promises a simple path towards extending the wavelength coverage of stable frequency combs. Here, we report a two-octave down-conversion of a frequency comb around 1 µm through cascaded half-harmonic generation with ∼64% efficiency in the first stage, and ∼18% in the second stage. We obtain broadband intrinsically-frequency-locked frequency combs with ∼50-fs pulses at ∼2 µm and ∼110-fs pulses at ∼4 µm. These results indicate the effectiveness of half-harmonic generation as a universal tool for efficient phase-and frequency-locked down-conversion, which can be beneficial for numerous applications requiring long-wavelength coherent sources.Femtosecond frequency combs in mid-IR region of the optical spectrum [1] are beneficial for numerous applications ranging from molecular spectroscopy in medicine [3] to higher harmonic generation [4], and dielectric laser accelerators [5]. The most effective path for generating femtosecond mid-IR frequency combs so far has been based on nonlinear methods to down-convert or extend the well-developed near-IR frequency combs of the mode-locked lasers. Parametric down conversion in optical parametric oscillators (OPOs) [6], difference frequency generation [7] and supercontinuum generation (SCG) [8] are among the widely used methods in this category.Recently, progress has been made towards more direct generation techniques without using near-IR frequency combs. Quantum cascade lasers (QCLs) are shown to be effective in generation of mid-IR frequency combs with outputs similar to frequency modulated lasers [9]. High-Q monolithic resonators pumped by c.w. mid-IR QCLs have also produced frequency combs [10,11]. These techniques promise a path for on-chip implementation, however, they currently have limited conversion efficiencies and spectral coverage in the mid-IR, and their frequency stabilization and temporal behavior are yet to be fully explored. Substantial research is also focused on extending the operation of femtosecond mode-locked lasers to longer wavelengths in the mid-IR, with demonstrations in free-space cavities at around 2.4 µm [12], and in fiber cavities at around 2.8 µm [13].Half-harmonic generation -the reverse of second-harmonic generation (SHG)-is the down-conversion of an optical input by one octave. It has been achieved in OPOs operating at degeneracy in c.w.[14] and arXiv:1510.03744v1 [physics.optics]

Abstract:We report generation of 48 fs pulses at a center wavelength of 2070 nm using a degenerate optical parametric oscillator (OPO) synchronously-pumped with a commercially available 36-MHz, femtosecond, mode-locked, Yb-doped fiber laser. The spectral bandwidth of the output is ~137 nm, corresponding to a theoretical, transform-limited pulse width of 33 fs. The threshold of the OPO is less than 10 mW of average pump power. By tuning the cavity length, the output spectrum covers a spectral width of more than 400 nm, limited only by the bandwidth of the cavity mirrors. Griebner, "175 fs Tm:Lu 2 O 3 laser at 2.07 µm mode-locked using single-walled carbon nanotubes," Opt. Express 20(5), 5313-5318 (2012

We demonstrate a mid-IR frequency comb centered at 3120 nm with 650-nm (20-THz) bandwidth at a comb-teeth spacing of 500 MHz. The generated comb is based on a compact ring-type synchronously pumped optical parametric oscillator (SPOPO) operating at degeneracy and pumped by a mode-locked Er-doped 1560 nm fiber laser at a repetition rate of 100 MHz. We achieve high-repetition rate by using a fractional-length cavity with a roundtrip length of 60 cm, which is one-fifth of the length dictated by conventional synchronous pumping.

Powerful and efficient optical frequency combs in the mid-infrared (MIR) spectral region are highly desirable for a broad range of applications. Despite extensive efforts utilizing various techniques, MIR frequency comb sources are still lacking power, efficiency, or bandwidth for many applications. Here, we report the generation of an intrinsically locked frequency comb source centered at 4.18 µm from an optical parametric oscillator (OPO) operating in the simulton regime, in which formation of purely quadratic solitons lead to enhanced performance. We show advantages of operation in the simulton regime in direct experimental comparisons to the conventional regime, which are also supported by simulation and theory.We achieve 565 mW of average power, 900 nm of instantaneous 3-dB bandwidth, 350% of slope efficiency, and 44% of conversion efficiency; a performance that is superior to previous OPO demonstrations and other sources in this wavelength range. This work opens a new avenue toward MIR frequency comb generation with high power and efficiency and suggests the great potential of soliton generation based on quadratic nonlinearity in the MIR spectral region.