Strong reduction of laser power noise by means of a Kerr nonlinear cavity

We demonstrated the power noise reduction of a continuous-wave laser field by means of an effective third-order Kerr nonlinear cavity. In contrast to conventional noise reduction schemes relying on linear cavities, a strong noise suppression at Fourier frequencies below the linewidth of the nonlinear cavity was possible. The laser light was reflected off a Kerr nonlinear cavity that had a half width half maximum linewidth of 4.5 MHz. The cavity was operated slightly off-resonance at approximately half of the maximum power buildup, close to its so-called critical state; a power noise reduction of up to 32 dB at Fourier frequencies below 1 MHz was observed after reflection. The effective third-order nonlinearity was a so-called cascaded second-order nonlinearity of $\text{MgO}:{\text{LiNbO}}_3$. The laser had a power of 0.75 W at the wavelength of 1064 nm.

Figure 1

(Color online) Phase space description of a light field (a) before and (b) after the interaction with an optical Kerr medium. $X_{1,2}$ denote the amplitude and the phase quadratures. Due to the intensity-dependent phase shift, the noise distribution is transformed into an elliptic shape.

Figure 2

(Color online) Schematic of the experimental layout. FI: Faraday isolator, EOM: electro-optical modulator, MC: mode cleaner cavity, PD: photodetector, RM: mode-matched reference mirror, DBS: dichroic beam splitter, AT: variable attenuator, and Analyzer: spectrum analyzer. Details are described in the text.

Figure 3

(Color online) Measured second-harmonic generation (SHG) conversion efficiency vs crystal temperature (input laser power $P_{{\text{TEM}}_{00}}=95\text{mW}$). The temperature was measured via a thermoresistor. The absolute values could be calculated with an accuracy of $\pm 1°\text{C}$. The Kerr effect can be observed in conversion minima slightly offset from the optimum SHG temperature: $\left(T_{\text{Kerr},\text{lower}}-T_{\text{SHG}}\right)=-1.41°\text{C}$ and $\left(T_{\text{Kerr},\text{upper}}-T_{\text{SHG}}\right)=+1.77°\text{C}$.

Figure 4

(Color online) Line shapes of the resonator mode recorded in transmission. The cavity was operated at the upper temperature Kerr point (see Fig. 3). The hysteresis clearly demonstrates the presence of an intensity-dependent phase shift and hence a strong cascaded nonlinearity. An equivalent hysteresis, but with “flipped” asymmetry, was also observed in the other Kerr point (not displayed), as expected for opposite signs of the nonlinear phase shift. The incident fundamental power was approximately 780 mW.

Figure 5

(Color online) Reflected fundamental power noise vs Fourier frequency for laser beams of identical dc power. The top peaked curve shows the laser’s initial power noise after two transmissions through the mode cleaner MC, measured using the mode-matched reference mirror as shown in Fig. 3. The other curves show the noise on the beam reflected at the Kerr nonlinear cavity locked on several states on the steep resonance slope. The maximum amplitude noise reduction (at about 0.9 MHz) was achieved at a detuning where the internal cavity field reaches approximately 3/4 of the maximum buildup obtained at resonance.