Squeezed light from a singly resonant frequency doubler

Abstract

We demonstrate experimentally that singly resonant second-harmonic generation can be used for the production of squeezed light at high power levels. This new method has the advantage of a simple implementation and the potential for an excellent long-term stability. In earlier experiments doubly resonant frequency doublers (with resonant enhancement of both fundamental and second-harmonic wave) have been used for the production of amplitude squeezed light. A severe disadvantage of this concept is that it is very difficult to stabilize both resonances at the same time; in the best experiment so far the system could be stabilized for about 10 sec only. We have realized that a singly resonant frequency doubler, where only the fundamental wave is resonantly enhanced, can also be used for squeezing. We used a monolithic standing-wave resonator consisting of a MgO:LiNbO3 crystal with dielectric mirror coatings on the curved end faces. The crystal is pumped with a monolithic diode-pumped Nd:YAG laser. The produced green light is analyzed with a balanced homodyne detection scheme (see Fig. 1). For our first experiment the mirror reflectivities were chosen so that good squeezing is possible at relatively modest power levels; higher powers would lead to problems with the detection since the photodiodes could be saturated. 19% intensity noise reduction has been observed at 16 MHz in the second-harmonic wave with a power of 50 mW. A typical noise spectrum is shown in Fig. 2. Taking into account the limited quantum efficiency of our detection system we find 30% noise reduction in the light beam. Theory predicts 47% for the chosen operating point on the basis of measured classical parameters of the system. The deviation is due to a thermal instability which we hope to avoid with a new resonator made of KNbO3; this material exhibits a considerably higher nonlinearity compared to LiNbO3. It should be possible to generate light with 70% noise reduction at power levels in the order of 100 mW. The theoretical limit is at nearly 90% noise reduction. These prospects together with the simplicity and stability make us believe that our new approach is an important step towards possible applications of squeezed light in the near future, namely the realization of low noise optical amplifiers pumped with squeezed light. Show more