GaN PIC for Fceo Detection at Low Pulse Energy

Gallium Nitride (GaN) has high nonlinear coefficients <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\chi^{2}=20 p m / V, \chi^{3}=2.2 \cdot 10^{-20} p m^{2} / V^{2})$</tex> that, in combination with its mature fabrication process and its space graded qualification, make it extremely interesting for nonlinear optical devices based on Photonics Integrated Circuits (PICs). Here we present the design, fabrication and testing of a GaN waveguides for detecting the carrier envelope offset frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(f_{ceo})$</tex> of a mode-locked laser within a single PIC. Dispersion engineering is crucial for the efficiency of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> generation process. The final goal is to ensure the frequency overlap between the dispersive wave (DW) generated through the Supercontinuum and the second harmonic (SH) of the signal. We selected by numerical Finite Element Method (FEM) simulations an optimized waveguide cross-section height of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$725nm$</tex>, etch depth of 435nm and length of 1<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cm</inf>. We then fabricated and tested 29 waveguides with different widths to fine tune the DW position and compensate for fabrication tolerances. The waveguides are fabricated by deep UV lithography and reactive ion etching. The setup used for the experiment is shown in Fig. 1a). A mode locked laser (repetition rate 100MHz, pulse length 120 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$fs$</tex>) is injected into the system from right hand side. A half-wave plate in combination with a polarizer controls the input pulse energy. Light is coupled into the quasi-TM00 mode of the waveguide with 5.3dB loss. The chip output is directed either to an OSA, to measure the optical spectrum, or to a fast Photodiode where the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> signal is recorded, filtered, amplified and then measured with an electrical spectrum analyzer (ESA). The spectra from all the waveguides are reported (interpolated) in the spectrogram of Fig. 1 b). We clearly see how the DW location changes with waveguide width (left side of the plot). Also, we see the broadening of the pump centered around 1560nm due to self-phase modulation. The SH signal at 780nm is not distinguishable and burried in DW signal. We benchmarked the recorded data by simulating the Supercontinuum dynamics with a generalized nonlinear Schrödinger equation (GNLSE) code [1]. Fig. 1c reports a good agreement between the experimental and theoretical location of the DW position showing the capability to effectively engineer DW location and increase <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> efficiency. Octave-spanning Supercontinuum generation with good <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> signal has been detected for most of waveguides while the waveguide featuring a width of 1.2um showed <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> detection with the highest SNR and the lowest pulse energy. The position of the DW at 710nm, suggests that SH is generated from a blue-shifted part of the broadened input pulse. In Fig. 1 d) we plot three spectra for such waveguide corresponding to pulse energies of 30 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$pJ$</tex>, 62 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$pJ, 118pJ$</tex>. The 62 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$pJ$</tex> pulse, equivalent to a pulse power of 517W generates an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> shown in Fig. 1e), featuring an SNR of 30dB measured at a resolution bandwidth of 100KHz and at video bandwidth of 100H z. Such values are sufficient to use the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> signal for laser stabilization [2]. In conclusion, dispersion engineered waveguides on GaN PICs for low-energy octave spanning supercontinuum generation was shown. Pulse energy is on pair with state-of-the art f-2f scheme based on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\chi^{3}$</tex> non linearity in one single waveguide [3] and are eight times lower than results reported in previous demonstration in GaN [4]. Building on the feedback of this experiment, further improvements in the design and fabrication are under study. The aim is to obtain a lower power <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> measurements with higher SNR by enhancing SH generation and optimizing the spectral overlap with the DW. This work was supported by European Space Agency (contract no. 4000144309/24/NL/GLC/ov).