## 1 Introduction  
Continuous wave (CW) light beams that exhibit nonclassical statistics and quantum correlation are of interest for fundamental tests of quantum physics, and potential applications including high-precision measurements beyond the shot-noise level (SNL) and quantum information such as quantum key distribution and quantum teleportation [1; 2; 3; 4; 5; 6]. The common method to generate quantum correlated light beams is using an optical parametric oscillator (OPO) with a nonlinear crystal in the cavity [7; 8; 9; 10; 11; 12]. In 1987, the quantum correlated twin beams generated by OPO was firstly proposed [7], and the intensity difference fluctuations of the twin beams below the corresponding SNL was experimentally observed [8]. Furthermore, with the help of high-quality nonlinear crystals, high-level quantum correlations between down-conversion beams from OPO could be obtained [9; 10; 11; 12]. In these studies, the frequencies of quantum-correlated twin beams (the signal and idler beams) were degenerate, and polarizations of that were orthogonal. However, the quantum-correlated beams with different frequencies have significant advantages when they are used in the above-mentioned applications. As a matter of fact, the optical entangled state with different frequencies, which is an essential quantum source for constructing continuous variable (CV) quantum information networks and establishing connections between systems with different natures, consists of two sub-modes with quantum correlation at different frequencies between the quadrature amplitudes and between the quadrature phases [13; 14; 15]. In recent years, two-color, even three-color quantum correlations between signal-idler or pump-signal-idler beams were demonstrated using a non-degenerate optical parametric oscillator (NOPO) with doubly or triply resonant configurations [13; 14; 15; 16; 17]. The two-color quantum correlated beams at 0.8 and 1.5 \(\upmu\)m can be utilized directly to make a connection between a quantum memory device based on alkaline atoms and a quantum communication device based on telecommunication optical fibers [16; 17]. In addition, the quantum-correlated twin beams had been successfully employed to improve the sensitivity beyond the SNL for signal recovery, absorption measurement, two-photon absorption spectroscopy, and realize the sub-shot-noise imaging of weak absorbing objects [18; 19; 20; 21; 22; 23; 24]. When a light beam transmits an absorbing medium, the absorption spectroscopy can be directly recorded with light frequency scanning through resonance. The sensitivity of the measurement is limited by the intensity noise of the light beam, which is usually above or near the SNL. If one of the quantum correlated twin beams is used to transmit an absorbing medium, the quantum correlation between twin beams would be partially destroyed because the absorptions are regarded as kinds of losses. The absorption signal can be measured precisely according to the change of the quantum correlation between twin beams. Owing to the possibility of the intensity difference fluctuations of twin beams being reduced below the SNL, sub-shot-noise high-sensitivity absorption spectroscopy would be realized [18; 19]. It is well known that the light beam at the telecommunication wavelength of 1.5 \(\upmu\)m is particularly suitable for distribution over long-distance optical fibers because the attenuation of light beam at 1.5 \(\upmu\)m is as low as 0.17 dB/km in optical fibers [25], while various hydrocarbons, hydrochlorides, soil pollutants, components of human breath, and some explosive agents display strong absorption features in the range of 3.3 \(\upmu\)m [26; 27; 28]. So, the two-color quantum-correlated twin beams at 1.5 and 3.3 \(\upmu\)m are of interest for high-precision measurements beyond the SNL using a 3.3 \(\upmu\)m beam and for optical signal distribution over long-distance optical fiber using a 1.5 \(\upmu\)m beam.