Frequency comb calibration setup. Light from the frequency comb is split into two fibers—calibration and reference. The calibration fiber is fed to an integrating sphere for illumination of the spectrograph fiber. The reference fiber provides comblight spatially offset at the spectrograph detector array. IS: integrating sphere; ND: neutral density; GPSDO: global positioning system disciplined oscillator.
250 MHz spaced optical frequency comb source. is controlled by changes to the laser cavity length, while is controlled by changes to the laser pump power. Both and are locked to oscillators referenced to a GPSDO. is locked via the copy of at (134 MHz). For the lock, part of the laser output is amplified with EDF before the HNLF that broadens the spectrum to cover . A short section of single-mode fiber (SMF) after the HNLF sets the delay between the 1 and light for optimal detection. Light at is frequency doubled in periodically poled lithium niobate (PPLN). Light at passes through the PPLN and interferes on the photodetector (PD) with the frequency-doubled light. BPF: bandpass filter; SMF: single-mode fiber; WDM: wavelength division multiplexer; WP: waveplate.
12.5 GHz spaced comb generation. HPEDFA: high power erbium-doped fiber amplifier; SOA: semiconductor optical amplifier; PZT: piezoelectric transducer; DCF: dispersion compensating fiber; PD: photodetector. All fibers are single mode at 1550 nm.
Power per optical mode at different stages of comb filtering and amplification. The inset is a high resolution close-in view of the spectrum after the SOAs.
Power per optical mode after nonlinear broadening. The inset is a high resolution close-in view around 1600 nm. The comb signal to noise is limited by the grating-based optical spectrum analyzer employed for this measurement. The asymmetry of the modes in the inset is also due to the optical spectrum analyzer.
250 MHz offset mode suppression measurements. (a) Example heterodyne measurement where the higher and lower frequency sidemodes are suppressed 34 dB. (b) Summary of all sidemode suppression measurements. Circles: after the first filter cavity, showing higher and lower frequency sidemode suppression. Crosses: after the SOAs, showing higher and lower frequency sidemode suppression. Squares: after the second filter cavity, where no asymmetry in sidemodes was detected. Black diamonds: after HNLF where no asymmetry was detected.
(a) Microwave spectrum of the photodetected pulse train after the SOAs. Sidemodes offset by 250 MHz from the 12.5 GHz carrier are 57 dB below the carrier. (b) optical heterodyne measurement at 1560 nm where only 27 dB suppression is observed. This heterodyne measurement is also represented in Fig. 6.
Setup for measuring the frequency stability, accuracy, and linewidth of an optical mode. Light from the Ti:sapphire frequency comb is combined with the frequency-doubled light from the cw laser via a polarization beam splitter (PBS) and polarizer (Pol). Half-wave plates control the polarization into the PBS. For frequency accuracy and stability measurements, the 1 GHz Ti:sapphire reference comb is locked to the H maser and the beat frequency is sent to the frequency counter. For linewidth measurements, the reference comb is locked to an ultrastable optical reference and is sent to an rf spectrum analyzer (RFSA). EDFA: erbium-doped fiber amplifier.
Fractional frequency offset of (a) the GPSDO, (b) , (c) , and (d) an optical mode near 1550 nm.
Residual fractional frequency offset of the 1550 nm mode relative to the GPSDO.
Frequency and RV bias of an optical mode near 1550 nm measured along the lightpath. Zero offset is determined by the H maser. For the measurement after the HNLF, the mode number and frequency were determined to be 77 3587 and 193.9966 THz, respectively. For all measurements the accuracy and stability is dominated by the GSPDO.
Linewidth of an optical mode near 1550 nm. (a) Linear scale measurement of the linewidth along the lightpath. For each trace the spectrum analyzer sweep time is 5 ms and averaged 100 times. (b) Log scale measurement of the linewidth after the HPEDFA, after the second filter cavity, and after the HNLF. For each trace the spectrum analyzer sweep time is 113 ms and averaged 100 times. Note the difference in frequency scales for (a) and (b).
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