1-GHz Offset-Free Frequency Comb in the Mid-Infrared

A. S. Mayer1, C. R. Phillips1, C. Langrock2 A. Klenner1,3, A. R. Johnson3, K. Luke4, Y. Okawachi3, M. Lipson5, A. L. Gaeta3, M. M. Fejer2, and U. Keller1

1Department of Physics, Institute of Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland 2Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305,USA

3Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA 4School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USA

5Department of Electrical Engineering, Columbia University, New York, NY 10027, USA mayeral@phys.ethz.ch

Abstract: We generate offset-free gigahertz frequency combs tunable from 2.75 to 3.8 µm by using PPLN waveguides for 30-dB-amplification of a supercontinuum generated in a Si3N4 waveguide by the same Yb:CALGO-laser that pumps the OPA process. OCIS codes: (320.7090) Ultrafast lasers; (320.7360) Waveguides

1. Introduction

Optical frequency combs have become an indispensable tool to enable fundamental advances in fields including biochemical sensing, astronomical spectroscopy and frequency metrology. Due to the commercial availability of comb sources based on Ti:sapphire or fiber laser systems containing Erbium or Ytterbium as a gain material, most frequency combs operate in the near-infrared (NIR) region from 800 nm-1.5 µm and with comb lines spaced by a few 100 MHz. Especially for sensing applications involving the characterization of vibrational transitions in molecular species, however, combs extending to wavelengths longer than 2 µm are required. Moreover, a high power per comb line and a free spectral range in the gigahertz regime is desired to facilitate rapid measurements with high signal-to-noise ratio. Quantum cascade lasers and frequency combs based on microresonator technology feature among the candidates to provide combs in the mid-infrared and show great potential for low-cost semiconductor integrability, but they are typically more suitable for line spacings >10 GHz [1,2] and have not yet reached the capabilities of modelocked laser-based combs in terms of noise performance.

Here, we demonstrate a unique combination of three compact platforms that enables us to achieve tunable and high-spectral-density frequency combs in the range of 2.75 to 3.8 µm: We start with a low-noise 1-GHz diode-pumped solid state laser oscillator (DPSSL) [3] that directly provides two output beams. One beam is used for low-energy coherent supercontinuum generation (SCG) in a silicon nitride (Si3N4) waveguide [4], producing a spectrum extending from 650 nm to 1.8 µm. The part of SC around 1.5 µm is coupled into a periodically poled lithium niobate (PPLN) reverse-proton-exchanged (RPE) waveguide together with the second laser output beam centered around 1.05 µm. The latter acts as a pump for the single-pass optical parametric amplification (OPA) process. The idler output of this OPA is an offset-free mid-IR comb.

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Fig. 1. (a) Experimental setup (b) Microwave frequency spectrum showing the repetition rate frequency at 1.025 GHz. (c) Measured optical spectrum of the laser output, used for both the input for supercontinuum generation (SCG) in the Si3N4 waveguide and the pump for the OPA process (d) Coherent octave spanning supercontinuum measured with an optical spectrum analyzer (OSA) and a Fourier transform infrared spectrometer (FTIR)

Using a compact waveguide technology for both stages provides several key advantages: The Si3N4 waveguide allows for octave-spanning SCG using just 36 pJ of coupled pulse energy due to the high nonlinearity and mode confinement, while the PPLN waveguide maximizes the interaction length compared to bulk devices and thus enables much higher signal amplification (up to 30 dB in this first demonstration). This approach allows us to omit any pre-amplification of the pulses, thus keeping the system based on one active element only. 2. Experimental Setup Our gigahertz diode-pumped SESAM-modelocked Yb:CALGO laser [3] (Fig. 1a) is used as a single source for pump and signal: One of the two output beams was free-space coupled into a 7.5-mm-long Si3N4 spiral waveguide, which is oxide clad with a rectangular cross section of 690 nm by 900 nm. The portion of the SC ranging from 1.2 to 1.8 µm is filtered with dichroic mirrors and used as the signal. We amplify these signal pulses in PPLN RPE waveguides [5], where lithium ions were exchanged with protons via diffusion, leading to an increase in refractive index for light polarized along the crystal c-axis. The buried channel waveguides obtained by removing protons near the surface with a subsequent reverse proton exchange (RPE) process can exhibit a near-Gaussian shape, and have only a small variation of the depth of the fundamental mode with respect to wavelength, thus increasing the efficiency of nonlinear mixing processes. The RPE waveguides use an exchange depth of 2.3 µm. The width of the proton-exchanged region is tapered to optimize coupling. This width is ≈2 µm wide at the input, designed to yield a single-mode waveguide with a round mode of comparable size to standard single-mode fiber. The width is adiabatically tapered to a final width of 12 µm [5], used for the main part of the waveguide where quasi-phase-matched (QPM) nonlinear mixing occurs. The QPM gratings are 18.5 mm long and the chip contains 90 waveguides with periods ranging form 17-30 µm. We use the second laser output beam as a pump, with ≈200 mW (200 pJ) being coupled into the PPLN waveguide. A Faraday isolator was used to block potential back reflections from the facet of the waveguides in both arms. Grating pairs are used to induce a negative group delay dispersion to stretch the pulses to ≈500 fs in order to maximize pump-signal interaction. Figure 2a shows the amplification of the signal while laterally scanning the chip through the waveguides with periods from 26.35 to 25.18 µm, while Fig. 2b shows idler spectra ranging from 2.75 to 3.8 µm. A high gain of up to 30 dB is achieved in this first proof-of principle experiment, and we already observe a corresponding drop in transmitted pump due to pump depletion. The mid-IR combs are expected to be offset-free, since the signal and pump used in the difference-frequency process originate from the same laser.

3. Conclusion and Outlook We have demonstrated a compact setup to extend low-noise DPSSL-based frequency combs at gigahertz repetition rates to the spectral region of interest for molecular spectroscopy and sensing. A single oscillator without any amplification stage acts as a source for both the pump and, via highly energy-efficient SCG in a Si3N4 waveguide, also as a signal to drive an OPA process in a PPLN RPE waveguide. We show a signal gain up to 30 dB and offset-free idler combs tunable from 2.75 to 3.8 µm. 4. References [1] A.Hugi, G.Villares, St.Blaser, H. C. Liu and J.Faist, "Mid-infrared frequency comb based on a quantum cascade laser", Nature 492, 229–233 (2012) [2] A.G. Griffith, R.K.W. Lau, J. Cardenas, Y.Okawachi, A.Mohanty, R.Fain, Y.H. Daniel Lee, M. Yu, C.T. Phare, C.B. Poitras, A. L. Gaeta, and M.Lipson, "Silicon-chip mid-infrared frequency comb", Nat.Commun. 6, 6299 (2015) [3] A. Klenner, S. Schilt, T. Südmeyer, U. Keller "Gigahertz frequency comb from a diode-pumped solid-state laser," Opt. Express 22, 31008-31019 (2014) [4] A. S. Mayer, A. Klenner, A. R. Johnson, K. Luke, M. R. E. Lamont, Y. Okawachi, M. Lipson, A. L. Gaeta, and U. Keller, "Frequency comb offset detection using supercontinuum generation in silicon nitride waveguides," Opt. Express 23, 15440-15451 (2015) [5] C. Langrock, M. M. Fejer, I. Hartl, and M. E. Fermann, "Generation of octave-spanning spectra inside reverse-proton-exchanged periodically poled lithium niobate waveguides," Opt. Lett. 32, 2478-2480 (2007)

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Fig. 2 (a): Signal spectrum recorded with the OSA ANDO AQ-6315 (limited to 1750 nm), showing up to 30 dB amplification. (b) Idler spectra ranging from 2.75 to 3.8 µm recorded with an FTIR (Thorlabs OSA205).