Visible-near-middle infrared spanning supercontinuum generation in a silicon nitride (Si3N4) waveguide

We demonstrate the generation of a supercontinuum spanning more than 1.5 octaves over the 1.2–3.7 μm range in a silicon nitride waveguide using sub-40-fs pulses at 2.35 μm generated by a 75MHz Kerr-lens mode-locked Cr:ZnS laser. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

absorption and emission bands, Cr:ZnS and Cr:ZnSe are often referred to as the "Ti:sapphire of the middle-infrared". All recent developments in ultrafast Cr-based laser technology are related to Kerr-lens mode-locking in polycrystalline Cr:ZnS and Cr:ZnSe, which was first achieved in 2014 [2]. The most current achievements include 2-3 optical-cycle oscillators with Watt-level average power and MW-level peak power [3,4,5]. Efficient power scaling of ultra-fast MIR oscillators in simple and robust single-pass polycrystalline Cr:ZnS and Cr:ZnSe amplifiers has been demonstrated, resulting in few optical-cycle MIR sources with multi-Watt (multi-MW) average (peak) powers which operate in very broad range of pulse repetition rates [6,7]. Supercontinuum (SC) generation using photonic waveguides is a promising approach for spectral broadening of pulsed coherent sources at low pulse energies and small footprint. SC generation using femtosecond Cr:ZnS laser in As 2 S 3 -silica double-nanospike waveguide was reported previously [8]. Many materials like stoichiometric Si 3 N 4 (SiN) [9,10], AlGaAS [11], silicon on insulators [12], and chalcogenide glasses [13] are appropriate for integrated nonlinear optics. Among them Si 3 N 4 holds a unique place due to its high χ (3) nonlinearity, absence of two photon absorption in the 1550 nm band, ultralow propagation losses [14], space compatibility [15], CMOS compatible fabrication process, and spectral coverage over the visible-middle-infrared range. For this reason Si 3 N 4 has been successfully used in microresonator frequency comb generation (micro-combs) [16] as well as low pulse energy supercontinuum generation [9], as well as mid infrared frequency comb generation [10].
The goal of this work is to demonstrate SC generation in Si 3 N 4 waveguides pumped by a radiation of Cr:ZnS femtosecond (fs) oscillator to realize a coherent expansion of the pump laser spectrum enabling self-referenced f-2f stabilization of the Cr:ZnS pump source.

Experimental results
The fully oxide cladded waveguide core is made of stoichiometric Si 3 N 4 deposited via low-pressure chemical vapor deposition (LPCVD) following the photonic Damascene process described in [14,17,18]. The chips were ranged from 5 to 15 mm long, with double inverse taper mode converters at both ends [19]. In particular, MIR operations require large-cross-section waveguides not only for MIR wave confinement but also for the dispersion engineering in the waveguide. We applied conformal deposition of Si 3 N 4 , which enables a record-large cross section. The waveguide height is up to 2.3 µm, while the width is selected from 1.0-1.6 µm, as shown in Fig. 2. The waveguide length is from 5-15 mm. On both the input and output side of the waveguide, inverse taper section is introduced for increasing the coupling efficiency. The cross section at the taper end is 0.65 µm in the width and ∼1.7 µm in the height. Figure 1 shows a simulation of a scenario where a 50 fs pulse centered at 2.4 µm with 6 kW peak power is launched into a Si 3 N 4 waveguide with 1.3 µm width and 2.3 µm height. The shown output spectrum is obtained after 4.8 mm propagation distance in the waveguide. The dispersive wave around 4.4 µm is generated by its phase matching to the pump pulse (i.e. the zero-valued relative phase constant in the dispersion landscape), and with the group velocity exceeding the pump wave, it behaves analogous to the Cherenkov radiation.
Numerical simulations are based on the nonlinear Schrodinger equation (NLSE), with full dispersion included which corresponds to the waveguide geometry and is calculated via a finite element method. The dispersion is expressed as a relative difference of wave's phase constant compared with the pump wave (assumed as solitons), i.e.: indicates the wave's phase constant (also called propagation constant), ω s is the angular frequency of the pump wave, and v g is the group velocity of the pump wave. The Raman effect is ignored and not included in the simulation, since it is weak in Si 3 N 4 . For ultra-short femtosecond pulses, the self-steepening effect is included in our simulation, which in the frequency domain is termed as a frequency-dependent Kerr nonlinear coefficient, i.e. γ = ωn 2 /cA eff , where ω is the angular frequency of the optical wave, n 2 is the  nonlinear refractive index, A eff indicates the effective mode area, and c is the speed of light in vacuum. We select the waveguide that produces anomalous group velocity dispersion (GVD) at the pumping wavelength, such that the supercontinuum generation is in the soliton regime and is generated by means of the soliton compression over the wave propagation. This process requires short waveguide length (much shorter than the dispersion length L D = τ 2 /| β (2) |, where τ indicates the pumping pulse duration and β (2) is the GVD at the pumping wavelength). In the presented Si 3 N 4 waveguides and in the presence of the Cr:ZnS laser pulses (pulse duration 45 fs, peak power >10 kW), the first occurrence of the supercontinuum (also referred to as the first compression point) is typically within few millimeters of the propagation distance. This makes a 5-mm-long waveguide sufficient for the spectral broadening of the pumping laser. Moreover, the soliton regime can ensure a high level of coherence underlying the supercontinuum, since solitons are immune to perturbations, such as power fluctuation and timing jitter, which are typical origins of coherent degradation and can be introduced when operating around the zero GVD.
The pump laser is a CLPF (CLPF-2400-15-80-1-SHG, IPG Photonics) laser system, which is a Kerr-lens mode-locked, middle-infrared laser with the output power up to 1.2 W in fundamental, and 0.3 W in of second harmonic, generated simultaneously in the Cr:ZnS gain element via random quasi-phase-matching process (RQPM) [1]. It operated at 2350 nm with 36 fs pulse duration at 75 MHz repetition rate. Typically, two orthogonal polarizations are generated simultaneously. Figure 3 summarizes the output characteristics of the pump source for horizontal polarization.  Figure 4 shows the experimental setup for SC generation in Si 3 N 4 waveguides. The setup consists of an optical isolator, half wave plate, AR coated molded IR aspheric lens (Black diamond, Thorlabs) placed on a precise XYZ translation stage, gold coated parabolic reflector, monochromator (SpectraPro 300i, Acton Research) and a set of detectors sensitive in VIS, near-IR and MIR spectral regions. We used PMT (P2, Acton Research), TE cooled InGaAs (ID 441, Acton Research), and LN cooled InSb (J10D-M204-R04M-60, EG&G Optoelectronics) for detection of SC spectra. Due to negative GDD of the optical isolator the Cr:ZnS pump laser pulses were stretched to 79 fs and later re-compressed to 45 fs due to the positive GDD of the black diamond lens. The laser beam was focused to a spot of size approximately 5 µm and coupled to the Si 3 N 4 waveguide with coupling efficiency of 16-20%. The average power of laser radiation was about 260-300 mW after the optical isolator.
During our experiments, we found that efficiency and SC spectrum strongly depends on geometry of the waveguides as well as polarization of the incident light. The results shown here in Fig. 5 correspond to the best performing waveguide "E7 5 mm W1 #8". The longer waveguides (e.g 10 and 15 mm) showed worse performance presumably due to losses in SiO 2 clsding. The E7 substrate contains 13 waveguides with width sweeping from 1000 nm to 1600 nm. The 8th waveguide has 1200 nm width with group velocity dispersion minimum at 2.5 µm according to numerical simulations. The incident pump power was 260 mW and had horizontal polarization. The coupling efficiency was about 16%, which corresponds to 0.56 nJ of pulse energy and 12.4 kW peak power. We also have observed that threshold for SC generation was about 50 mW of incident pump power corresponding to 2.4 kW peak pump power. The spectrum of second harmonic, shown in Fig. 4, generated simultaneously in the Cr:ZnS gain element via random quasi-phase-matching process and can be directly used for f-2f stabilization of the Cr:ZnS pump source.  (a) (i) Supercontinuum spectrum generated in "E7 5 mm W1 #8" waveguide at 260 mW incident average power, 45 fs pulse duration, 75 MHz rate and horizontal polarization (Black); (ii) spectrum of CLPF laser system (Green); (iii) spectrum of near IR second harmonic output of CLPF laser system shown for assessment of possibility of using 2 nd harmonic generated in laser gain element and SC for f-2f stabilization of the Cr:ZnS pump source (Red); (vi) Visible emission -the third harmonic generation in Si 3 N 4 waveguide (Blue). (b) Picture of the set-up during operation, showing on the left the black diamond lens, at the center Si 3 N 4 waveguide shining red, and at the right -gold parabolic reflector.

Conclusions
We experimentally demonstrated SC generation spanning over 1.5 octaves over 1.2-3.7 µm spectral range from Si 3 N 4 waveguides using 75 MHz Cr:ZnS fs oscillator as a pump. The demonstrated presumably coherent 1.5 octaves spanning bandwidth is ideal for self-referenced f-2f detection of the f ceo . In addition, this represents a promising broadband coherent source for dual comb spectroscopy.