We report on a beam of squeezed light with considerable power (3.2 mW). 52% noise reduction in the fundamental mode is observed by second-harmonic generation in a monolithic resonator with active frequency stabilization.
The most efficient sources of squeezed light have employed χ(2) processes in parametric downconversion or upconversion. In second harmonic generation both the fundamental and second harmonic modes are in an amplitude squeezed state. The upconverted mode is the squeezed state with the shortest wavelength involved in the process; therefore, frequency doubling is a promising scheme with which squeezing can be pushed toward the blue end of the spectrum. We measured the amplitude and phase noise properties of the second harmonic light generated in a frequency doubling monolithic cavity.
We present a new technique for the efficient and simple production of amplitude-squeezed light with high coherent excitation, using a frequency doubler, in which only the fundamental wave is resonantly enhanced. Our theory predicts up to nearly 90% squeezing in the second-harmonic wave under ideal conditions. In our first experiment we measured up to 30% squeezing [having taken into account the limited quantum efficiency (63%) of our detection system] in a stable 50 mW beam. Compared to doubly resonant frequency doubling our new approach has considerable technical advantages.
We have generated squeezed states of light by the process of second harmonic conversion within an optical cavity resonant at both fundamental and harmonic frequencies. The experiment consists of a standing-wave cavity containing a crystal of lithium niobate doped with magnesium oxide and driven by the field from a frequency-stabilized Nd:YAG laser (incident power, ~10 mW). The total field reflected from the cavity illuminates a pair of high quantum efficiency photodiodes, whose individual photocurrents are summed. Observations of squeezing are made by analyzing the spectral density of fluctuations for this sum photo-current while the cavity detuning is scanned through resonance. An intensity regulator holds the reflected light intensity constant during the scan by control of the incident light level. Reductions in photocurrent fluctuations of 12% relative to the vacuum-state or shot-noise level are achieved for frequency offsets near 4 MHz. Since significantly higher degrees of squeezing are predicted by our theoretical analysis, we discuss those processes that currently limit the observed noise reduction, including phase noise produced by the intracavity crystal in the absence of nonlinear conversion.