Research ArticlePHYSICS

On-chip dual-comb source for spectroscopy

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Science Advances  02 Mar 2018:
Vol. 4, no. 3, e1701858
DOI: 10.1126/sciadv.1701858
  • Fig. 1 Cascaded microring resonators for dual comb generation.

    (A) Schematic of the device. A cw laser pumps the silicon nitride waveguide, which is coupled to two silicon nitride rings: R1 and R2. Through parametric oscillation and cascaded FWM, rings R1 and R2 generate frequency combs with repetition rates frep1 and frep2, respectively. The insets show schematic optical spectra after the first and second rings. The spectrum detected at the output of the chip using a fast photodiode shows a series of beat notes in the rf domain. Note that although the optical spectrum insets span several tens of terahertz, the rf spectrum spans only a few gigahertz, enabling ease of detection in the electronic domain. (B) Optical microscopy image of the fabricated device showing the silicon nitride rings with integrated platinum microheaters.

  • Fig. 2 Optical spectra.

    (A to C) Generated spectrum when the heaters on (A) R1, (B) R2, and (C) both rings are tuned into resonance with the pump laser at 1561.4 nm. OSA resolution, 0.01 nm. The insets are top-view IR camera images showing which of the two rings are on resonance. The combs are in the single-soliton mode-locked state. Solid red and green lines are fits to a sech2 spectral envelope.(D) Dual-comb state with a single soliton in R1 and two solitons in R2. The zoomed-in inset in (C) and (D) reveals dual combs with pairs of lines closely spaced in wavelength. By changing the magnitude of the final upward voltage ramp shown in (E), one can choose the number of solitons in the final state. As an example, the state shown here has a single soliton in R1 and two solitons half a roundtrip apart in R2 (harmonic mode-locking). All spectra span 400 nm (51 THz). (E) Time domain traces of the voltage ramp used to access the mode-locked states of both the rings. The blue (green) line represents the voltage applied to the heater on R1 (R2). The red and purple lines represent the transmitted pump power and the converted comb power (excluding the pump and vertically offset for clarity), respectively. Discrete steps are seen in the pump power, as well as the comb power, characteristic of multisoliton states in the ring. The inset shows the stages of comb formation in R1, including the high-noise state (I), the multisoliton state (II and III), and the single-soliton state (IV). a.u., arbitrary units.

  • Fig. 3 Time-domain interferogram and rf heterodyne beat notes.

    (A) Twenty microsecond time-domain interferogram of the mode-locked dual comb. (B) The same time-domain interferogram expanded 2000 times shows periodic pulses at the inverse of the beat-note frequency (1/Δfrep). (C) The dual comb is amplified by an L-band EDFA and detected on a fast photodiode to highlight features of the time domain interferogram. This trace is zoomed in by a factor of 8000 compared to (A). Note that the interferogram in (C) represents a filtered version of the real time-domain trace because of the ~40-nm gain bandwidth of the L-band EDFA. Comb lines close to the pump (1561 to 1570 nm) and those beyond 1610 nm are attenuated by the EDFA. (D) rf multiheterodyne beat notes obtained by a fast Fourier transform (FFT) of the interferogram in (A). The data were taken with a longpass filter and represent the dual comb lines on the red side of the pump. The beat notes correspond to the dual comb shown in Fig. 2C, where both combs are in the single-soliton mode-locked state. A beat-note spacing of 1.12 GHz is observed. The inset shows the linewidth of the first beat note, measured using an rf spectrum analyzer with a resolution bandwidth of 2 kHz. We measure a high SNR (60 dB) with a linewidth of 10 kHz, which corresponds to a relative coherence time of 100 μs. (E) rf beat notes of the comb shown in Fig. 2D, where the comb generated in R1 is in the single-soliton mode-locked state and the comb in R2 is in the two-soliton harmonic mode-locked state. The beat notes are spaced by 2.24 GHz because the R2 comb is missing a line in every alternate mode of the ring. In both (D) and (E), the beat notes close to the pump have a high SNR in excess of 40 dB. The spurious peaks at 5, 10, 15, 20, and 25 GHz are artifacts of the FFT analyzer.

  • Fig. 4 Dual-comb heterodyne technique to study beat-note evolution.

    The device used in this measurement had a beat-note spacing of 8.6 GHz. Keeping the comb generated in R1 in the mode-locked state, the resonance of R2 is tuned with respect to the pump laser and the rf spectrum is recorded. The left panel shows the spectrum of the first beat note as the heater power on R2 is varied starting from a far–blue-detuned state to the mode-locked state on the red-detuned side. The spectra on the right show the formation of beat notes (stages I and II), which broaden to form a wide beat note in the high-noise state (stage III and IV). On tuning further to the red-detuned side, we observe an abrupt transition to a narrow and high-SNR beat note as shown in Fig. 3. (see also the Supplementary Materials and movie S1).

  • Fig. 5 DCS of dichloromethane.

    (A) Schematic of the setup. The output of the chip generating a dual comb is sent to a 10-mm cuvette containing dichloromethane, and the transmitted light is sent to a 45-GHz photodiode and analyzed with an rf spectrum analyzer or with a fast oscilloscope. (B) Spectrum of dichloromethane acquired using DCS and corroborated with the same spectrum measured with a broadband supercontinuum source and an OSA. The solid line represents the absorption spectrum measured with a supercontinuum source (Fianium SC-450-4) and an OSA. The red (blue) circles represent the absorption spectrum acquired using the rf beat notes and a shortpass (longpass) filter. All spectra are normalized by the corresponding spectrum without dichloromethane.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/e1701858/DC1

    Details of soliton mode-locking using heaters

    Details of rf multiheterodyne spectroscopy

    Filtering

    Beat-note spacing

    Evolution of combs in the high-noise state

    Details of the experimental setup

    fig. S1. Detailed experimental setup.

    fig. S2. Heater response.

    fig. S3. Frequency comb filtered with a longpass filter.

    fig. S4. Quality factor of microrings.

    fig. S5. Optical spectra of combs in the high-noise state.

    fig. S6. Evolution of dual-comb beat note when the R1 comb is in the high-noise state.

    movie S1. Beat-note evolution.

    References (5964)

  • Supplementary Materials

    This PDF file includes:

    • Details of soliton mode-locking using heaters
    • Details of rf multiheterodyne spectroscopy
    • Filtering
    • Beat-note spacing
    • Evolution of combs in the high-noise state
    • Details of the experimental setup
    • fig. S1. Detailed experimental setup.
    • fig. S2. Heater response.
    • fig. S3. Frequency comb filtered with a longpass filter.
    • fig. S4. Quality factor of microrings.
    • fig. S5. Optical spectra of combs in the high-noise state.
    • fig. S6. Evolution of dual-comb beat note when the R1 comb is in the high-noise state.
    • Legend for movie S1
    • References (59–64)

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mp4 format). Beat-note evolution.

    Files in this Data Supplement: