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Frequency division using a soliton-injected semiconductor gain-switched frequency comb

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Science Advances  25 Sep 2020:
Vol. 6, no. 39, eaba2807
DOI: 10.1126/sciadv.aba2807
  • Fig. 1 Microcomb repetition rate division with a soliton-injected GSL.

    (A) The conceptual illustration of the scheme. The VCO is controlled by the SPL servo, outputting microwave frequency that is a subharmonic of the soliton repetition rate. (B) A photograph of the DFB laser used in this work. (C) Illustration of the gain-switched comb when the SPL is off. Multiple frequencies caused by the beating between sidebands of different microcomb teeth are generated. (D) The output of a soliton-injected GSL (with the SPL off) measured by an optical sampling oscilloscope. a.u., arbitrary units. (E) Illustration of the subharmonic phase–locked scenario in the frequency domain. (F) Output of the GSL in the time domain when it is subharmonic phase–locked to the soliton repetition rate (14.09 GHz). Photo credit: Wenle Weng, EPFL.

  • Fig. 2 Frequency division of a soliton microcomb generated by a crystalline microresonator.

    (A) Experimental setup. A fiber Bragg grating (FBG) and a circulator are used to separate the pumping external cavity diode laser (ECDL) from the solitons. A photodetector (PD1) and an electro-optic modulator (EOM) are used to produce PDH signals to lock the laser-resonance detuning. Half of the GSL power is registered by a photodetector (PD2) to produce subharmonic locking error signals. A servo locks the gain switching frequency to be an integral submultiple of frep via frequency modulation (FM) of the signal generator. Simultaneously, the dc output of the servo is used as an error signal to stabilize frep through amplitude modulation (AM) of the pump laser power with an acousto-optic modulator (AOM). (B) The optical spectra of the microcomb and the soliton-injected GSL gain-switched at frep/2 and frep/3, respectively. The inset shows an enlargement of a portion of the spectra. (C) The error signals generated by PD2. The dashed line indicates the locking point. (D) The power spectra of the soliton-injected GSL when the SPL is on (red) and off (blue). (E to G) The spectra of DKS frep and the phase-locked gain switching frequencies at frep/2 and frep/3, respectively.

  • Fig. 3 Phase noise spectra of synthesized microwave signals.

    The 14.09-GHz DKS frep is generated by photodetection of the soliton train. The synthesized signals at 7.05 (frep/2) and 4.70 GHz (frep/3) are output by the frequency-modulated signal generator when the SPL is activated. The servo has a control bandwidth of ∼10 kHz, causing the rising noise level around that frequency. Within the control bandwidth, the noise levels of the two synthesized microwaves are lower than that of frep by 6 and 9.5 dB (the dashed lines with a slope of f−2 are plotted for visual guidance). At frequencies above 100 kHz, the phase noise of the synthesized microwaves are identical to the levels intrinsic to the free running signal generator because of the servo’s limited bandwidth. The inset shows a photograph of the MgF2 resonator. Photo credit: Tobias Herr, EPFL.

  • Fig. 4 Frequency division of a 100-GHz repetition rate photonic chip Si3N4 microcomb.

    (A) From top to bottom: the optical spectra of the amplified soliton microcomb and the GSL with gain switching rates of frep/6, frep/10, and frep/15, respectively. (B) The enlargement of the comb spectra from 1546 to 1550 nm. (C) The phase noise spectra of the synthesized microwave signals. The blue dotted trace is the phase noise level of the 100-GHz frep inferred from the measured phase noise levels of the synthesized signals. The trace is not plotted above 30 kHz because of the limited control bandwidth of the SPL servo. The inset is a photograph of the Si3N4 microresonator.

Supplementary Materials

  • Supplementary Materials

    Frequency division using a soliton-injected semiconductor gain-switched frequency comb

    Wenle Weng, Aleksandra Kaszubowska-Anandarajah, Junqiu Liu, Prince M. Anandarajah, Tobias J. Kippenberg

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