Fig. 1. (a) Schematic showing the principle of HCF-based PT phase modulator. (b) Calculated absorption coefficient of the P(9) line of pure C₂H₂ at 1 atm as a function of T by use of HITRAN database²². (c) Transverse distribution of T and p calculated by using COMSOL Multiphysics with P_ctrl=500 mW and f=100 kHz. (d) Variation of local phase modulation with a step length of 1 mm and accumulated phase modulation along the length of an acetylene-filled HCF.

Fig. 2. (a) Schematic diagram and (b) photo of the fabricated gas-filled hollow-core fiber phase modulator. (c) Measured loss spectrum of the phase modulator. (d) Experimental setup for characterizing the phase modulator. TECF: thermal-expanded core fiber, DFB: distributed feedback laser, FG: function generator, AOM: acoustic optical modulator, EDFA: erbium-doped fiber amplifier, WDM: wavelength-division multiplexer, PZT: piezoelectric transducer, PC: polarization controller, PD: photodetector, OSC: oscilloscope, LPF: low-pass filter.

Fig. 3. Characteristics of the HCF-based PT phase modulator with a modulation frequency of 100 kHz. (a) MZI output waveforms with different P_ctrl in the AR-HCF. (b) Phase modulation amplitude as a function of P_ctrl. (c) Wavelength dependence of phase modulation amplitude for P_ctrl=502 mW.

Fig. 4. (a) Frequency response of phase modulation with P_ctrl =502 mW. (b) Transient response detected by use of the MZI for control light beam pulse width of 50 μs.

Table 0. Temperature-dependent thermodynamic parameters of materials at 1 atm^25,26.

Table 0. Summary of state-of-the-art all-fiber optical phase modulators.