Towards optical-frequency-comb generation in continuous-wave-pumped titanium-indiffused lithium-niobate waveguide resonators

Much progress, both experimentally and theoretically, has recently been made towards optical-frequency-comb generation from continuously pumped second-order nonlinear systems. Here, we present observations towards finding an integrated solution for such a system, using a titanium-indiffused lithium-niobate waveguide resonator. These results are compared to the recently developed theory for equivalent systems. The system is seen to exhibit strong instabilities, which require further investigation in order to fully determine the suitability of this platform for stable optical-frequency-comb generation.

Figure 1

General scheme for cw-pumped integrated OFC generation utilizing the second-order nonlinearity. A titanium-indiffused periodically poled lithium-niobate waveguide (Ti:PPLN) with coatings deposited to the end faces, labeled front and rear, is pumped by a strong cw laser. Within the device, an optical frequency comb (with a spectral spacing equal to the free spectral range of the system) is generated. Representative optical spectra for the input and output fields are shown.

Figure 2

Spectral dynamics of a waveguide sample as a function of the pump (fundamental) field detuning from cavity resonance with a 330 mW pump field at 1550 nm. The spectrum is normalized to the maximum power in the field for each detuning value.

Figure 3

Experimental layout for the optical-frequency-comb generation measurements. ECDL: Newport Velocity tunable (1520–1570 nm) ECDL laser source; EOM: electro-optic phase modulator; EDFA: erbium-doped fiber amplifier; FC: fiber coupler; HP: half-wave plate; BD: beam dump; FI: Faraday isolator; WG: waveguide sample; DM: dichroic mirror; PD: photodetector; mFalc: servo electronics for laser locking.

Figure 4

Example of a single SH power measurement run. The SH power exiting the rear of the waveguide device is recorded as the pump field power is varied. The trace shows the theoretically predicted mean SH power with the nonlinear coupling coefficient fitted to the measured SH powers below an input pump power of 100 mW.

Figure 5

Measured SH power exiting the rear of the waveguide when the PDH lock is engaged. The coupled fundamental field power is approximately 100 mW and the ECDL wavelength is set to 1550.2 nm.

Figure 6

Transmitted optical pump field power measured as the wavelength of the pump is scanned over the cavity resonance for (a) 40 mW and (b) 150 mW input pump powers. The scan voltage ramp is shown in purple, where a larger scan voltage corresponds to a redshift of the pump field. The pump field wavelength is scanned with a 50 mHz ramp with a peak-to-peak amplitude of 0.3 nm. SH powers are normalized to the maximum output power for each trace.

Figure 7

Measured optical spectra for a 330 mW pump field power as the pump laser wavelength is scanned from red to blue. The traces are in chronological order, such that (f) is measured when the pump laser is the most blueshifted with respect to the cavity resonance. The pump field wavelength is 1549.8437 nm.

Figure 8

Predicted pump field spectra with a pump field power of 330 mW and decreasing cavity detunings (corresponding to a laser frequency scan from red to blue) of (a) $\delta =0$, (b) $-0.05$, and (c) $-0.15$.

Figure 9

Spectra of the SH field with a 330 mW pump field at 1549.78 nm. The shown spectra show sample spectra when the system is (a) far from resonance, (b) near to resonance and producing large amounts of SH power, and (c) very near to resonance.

Figure 10

Analogous cavity for the monolithic waveguide resonator. The cavity is unfolded using two perfect mirrors (in gray) such that the dynamics from $z=0$ to $z=L/2$ and the dynamics from $z=L/2$ to $z=L$ can be considered. Note that the system described in such a way is now a traveling-wave geometry, in contrast to the original system.