Fig. 1. 3D schematic of the ultra-silicon-rich nitride (USRN) device showing the CMBG and USRN waveguide parameters, where Lapod is the apodization length, Λ is the grating pitch, and G1 and Gapod are the distances between the pillars and the USRN waveguide. The insets show the principle of our thermo-optic tuning method.

Fig. 2. (a) Group index (ng) and second-order dispersion (β2) coefficient of the CMBG at different temperatures located at 1536 nm. (b) Third-order (β3) and fourth-order dispersion (β4) coefficients of the CMBG at different temperatures located at 1536 nm.

Fig. 3. (a) Simulated spectral broadening at different temperatures at a fixed pulse wavelength of 1536 nm. Simulated and measured (b) −30  dB and (c) −20  dB bandwidth of the output spectrum as a function of temperature.

Fig. 4. (a) Simulated spectral profiles at the end of the CMBG. (b) Simulated temporal profiles as a function of temperature at the end of the CMBG. (c) Simulated temporal FWHM at the end of the CMBG stage for each temperature point.

Fig. 5. (a) Schematic of the experimental setup, where blue lines denote the polarization maintaining (PM) fibers, with DUT, device under test; OSA, optical spectrum analyzer; TLF, tapered lensed fiber; TC, temperature controller; OA, optical attenuator; FFL, femtosecond fiber laser. (b) Measured spectral broadening at different temperatures and the input pulse at a fixed input wavelength of 1536 nm. (c) Measured spectral bandwidth at −30  dB level as a function of temperature.

Fig. 6. (a) Measured temperature-dependent stopband of a characteristic cladding modulated Bragg grating with a stopband centered at 1557 nm at 25°C. (b) Measured transmission spectra of USRN grating used in the spectral broadening measurement showing the stopband of 1553 nm at 25°C. (c) The measured Bragg wavelength at different temperatures. (d) The measured refractive index of USRN at different temperatures.

Table 1. Comparison of Various Approaches toward Tuning of Laser Pulse Bandwidth