Fig 1. Schematic of the fiber laser frequency comb. The CEO frequency is detected by a photodetector (PD), and is used to control the pump diode current. A piezoelectric transducer (PZT) fiber stretcher in the cavity allowed for adjustment of the repetition rate. The thick solid lines represent free-space paths, the thin solid lines represent fiber paths and the dotted lines represent electrical paths. BPF: Bandpass filter; SHG: second-harmonic generation.

Fig 2. (a) The spectrum from the output of the fiber laser. The spectral full-width at half maximum (FWHM) is 20 nm. (b) The measured intensity autocorrelation of the amplifier output. The FWHM of the autocorrelation trace is ~90 fs, which sets a reasonable upper limit to the pulse width as in Ref. [12]. The original laser output duration is ~210 fs FWHM.

Fig 3. (a) The octave-spanning supercontinuum generated in the UV-exposed highly nonlinear fiber. (b) RF power spectrum from mixing the 1030 nm portion of the supercontinuum with the frequency-doubled 2060 nm portion. The repetition rate signal (f _r) at 49.8 MHz and CEO frequency (f ₀) are clearly seen. The CEO beat frequency has a SNR of 20–25 dB.

Fig. 4. The electronics used to phase-lock the CEO frequency. The f ₀ signal from the f-to-2f interferometer was filtered at 120 MHz (with a 6.7 MHz bandwidth), mixed with a 1 GHz signal, divided, and compared to a 4.375 MHz signal. All synthesizers were referenced to a common time base. The CEO frequency change was ~15 MHz/mW of pump power. The total pump power was ~60 mW.

Fig. 5. Scanning the laser’s repetition rate (f _r) while the up-shifted CEO frequency (2f _r+f ₀) is phase-locked. The repetition rate and divided-down CEO frequency were counted with a gate time of 1 s. (a) The divided-down CEO frequency experiences no phase slip as the repetition rate is scanned over a 40 kHz span in 200 s. (b) The divided-down CEO frequency remains locked over an 800 kHz span with a scan velocity of 2.48 kHz/s. Occasional phase slips occur during the scan. (c) The scan velocity is increased to 9.91 kHz/s and small oscillations in the locked f ₀ signal occur due to mechanical resonances of the delay line. (d) The scan velocity was increased to 19.8 kHz/s and the phase-lock was improved to prevent oscillations.

Fig. 6. Scanning a CW laser locked to the variable repetition rate frequency comb. (a) The initial frequency comb (blue) with an offset frequency f ₀ and an initial repetition rate f _r where a CW laser (red) is locked to the n ^th tooth of the comb. (The dotted lines indicate the extension of the frequency comb to zero frequency). (b) The final frequency comb (purple) with the same offset frequency f ₀ and a repetition rate f _r that has been increased from f _r to f _r+δf _r. At low frequencies (100 MHz) the shift of the comb lines is imperceptible on this scale. The CW laser (red) is assumed to be still locked to the n ^th tooth of the comb. As a result, its frequency has been increased by nδf _r.