We report on a compensation characteristic of the phase shifts, originating from the cavity detuning and phase mismatching, in the process of squeezed state generation. In the experiment, the maximum level of squeezing was not generated under coresonance in a doubly resonant optical parametric oscillator (OPO), which was theoretically explained based on the motion equation of the OPO. Furthermore, the power dependence of the squeezing and antisqueezing was also measured at the coresonance and the optimum noise reduction status. The results revalidated the compensation mechanism. Finally, the amplitude quadrature (or vacuum) noise reduction was raised to 11.7 dB (or 12.3 dB) at 8 MHz by finely tuning the crystal temperature away from coresonance.
We report on a compensation characteristic of the phase shifts, originating from the cavity detuning and phase mismatching, in the process of squeezed state generation. In the experiment, the maximum level of squeezing was not generated under coresonance in a doubly resonant optical parametric oscillator (OPO), which was theoretically explained based on the motion equation of the OPO. Furthermore, the power dependence of the squeezing and antisqueezing was also measured at the coresonance and the optimum noise reduction status. The results revalidated the compensation mechanism. Finally, the amplitude quadrature (or vacuum) noise reduction was raised to 11.7 dB (or 12.3 dB) at 8 MHz by finely tuning the crystal temperature away from coresonance.
Optical parametric amplifiers (OPA's) and optical parametric oscillators (OPO's) can exhibit squeezed amplification when an injected signal beam is used to elicit small-signal gain saturation. This paper extends previous work on OPA/OPO noise behavior under signal injection. Doubly resonant and triply resonant, single-ended cavities are considered in the good-cavity limit. Owing to the nonlinear coupling between the signal and idler beams within the cavity, both are sensitive to the detuning of each beam from its respective resonance. Signal and idler output noise spectra are found by linearizing the quantum coupled-mode equations about the classical mean-field solutions. Cavity detunings lead to frequency dependent phase shifts between the low-noise and high-noise quadratures of both the signal and idler beams. Moreover, the phase of the low-noise quadrature need not coincide with that of the corresponding mean field. The resulting large-signal, signal-to-noise ratio improvement afforded by squeezed amplification is therefore sensitive to the cavity detunings. The effects of classical excess noise on the pump beam and the injected signal beam are also considered.
The field of squeezed state is an important quantum resource in the study of quantum optics. In the application of quantum information, the spectrum bandwidth of the squeezed light field is an important index to limit the information transmission capacity. Currently, the optical parametric oscillator (OPO) is one of the most efficient ways to generate high squeezed non-classical optical fields. In this paper, the degenerate singly-resonant and doubly-resonant OPO structures are introduced. Both OPOs are composed of concave mirrors and periodically poled potassium titanyl phosphate crystals (PPKTP). The length of PPKTP crystal is 10 mm. The curvature radius of the curved surface is 12 mm, and it has high reflectivity at 1550 nm and 775 nm. The plane surface is coated with anti-reflection coating. The air gap length is 21 mm. The concave mirror is an output coupling mirror, and its radius of curvature is 25 mm. In the singly-resonant OPO, only the signal light resonates in the cavity, and the pump light passes through the nonlinear crystal twice and then outputs out of the cavity. The reflectivity of OPO output coupling mirror to the wavelength of 1550 nm is 88%. The linewidth of the corresponding fundamental frequency wave is 77.4 MHz. For doubly-resonant OPO, both the signal light and the pump light resonate simultaneously in the cavity. The reflectivity of OPO output coupling mirror to 1550 nm and 775 nm is 85% and 97.5%, respectively. The linewidth of the corresponding fundamental frequency wave and harmonic is 97.1 MHz and 15.6 MHz, respectively. Then the threshold of OPO is calculated. The threshold pump power of OPO increases with signal light transmittance increasing, but the threshold value of doubly-resonant OPO is obviously smaller than that of singly-resonant OPO. After that, the variation of the squeezing bandwidth of the squeezed light field generated by OPO with the transmittance of the signal is analyzed. Finally, we complete the design of quantum squeezer with low threshold (18 mW), broadband (84.2 MHz) and high stability (the standard deviation of locking baseline is 0.32 MHz) experimentally. The results show that compared with the singly-resonant optical parametric oscillator, the doubly-resonant cavity has the characteristics of low threshold and high stability, which is more suitable for the preparation and practical application of broadband squeezed light field.
Efficient and reproducible squeezed-light sources are essential for quantum information processing and precision metrology. Compact linear standing-wave optical parametric oscillators (OPOs) are attractive because they combine low optical loss, low pump-power requirements, and large longitudinal mode spacing. In doubly resonant cavities, however, the nonlinear interaction is not determined solely by bulk phase matching: forward- and backward-generated fields recombine coherently, making the effective gain sensitive to crystal-edge termination, wavelength-dependent coating phases, and the cavity resonance condition. Here, we show that these microscopic phase contributions can produce large threshold variations between nominally similar OPOs. We combine double-pass second-harmonic generation with OPO threshold measurements to extract the relevant crystal-cavity phases and analyse three linear OPO systems. The observed devices exhibit threshold variations of up to nearly six-fold, traced to the phase-dependent nonlinear-gain envelope at accessible doubly resonant operating points. Our results establish a phase-aware framework for compact linear OPOs and provide design guidelines for reproducible low-threshold squeezed-light sources in scalable photonic quantum systems.
We generate squeezed state of light at 860 nm with a monolithic optical parametric oscillator. The optical parametric oscillator consists of a periodically poled KTiOPO(4) crystal, both ends of which are spherically polished and mirror-coated. We achieve both phase matching and cavity resonance by controlling only the temperature of the crystal. We observe up to -8.0±0.2 dB of squeezing with the bandwidth of 142 MHz. Our technique makes it possible to drive many monolithic cavities simultaneously by a single laser. Hence our monolithic optical parametric oscillator is quite suitable to continuous-variable quantum information experiments where we need a large number of highly squeezed light beams.
A cw optical parametric oscillator (OPO) operated above threshold is of particular interest in current squeezing experiments. Quadrature squeezing in the signal and idler modes and in the pump mode is predicted. Intensity squeezing in the difference intensity of the signal and idler is predicted and has recently been observed.1 We have built a doubly resonant cw OPO with a magnesium oxide- doped lithium niobate crystal that is single-pass-pumped with the 530.9-nm line of a kryptonion laser. Mode hopping instability near degeneracy puts stringent requirements on the frequency stability of the pump laser and the mechanical stability of the OPO cavity. We have oberved such instability near degeneracy and determined the stability requirements for this OPO. Stable operation can be obtained as the OPO is operated away from degeneracy but at a higher threshold. Classically the single-pass pump output is predicted to be clamped at the threshold value so that the OPO in this configuration acts as a pump power limiter. Quantum mechanically, we have found that the pump intensity noise is enhanced. Measurements on the pump intensity noise spectrum in this particular configuration have been made and compared to our theoretical predictions.
Squeezed states are important sources in quantum physics, which have potential applications in fields such as quantum teleportation, quantum information networks, quantum memory, and quantum metrology and precise measurements. For our interest, the squeezed vacuum will be used in the quantum-enhanced optical atomic magnetometers, filling the vacuum port of the probe beam to improve measurement sensitivity. Based on the sub-threshold optical parametric oscillator (OPO) with PPKTP crystal, the squeezed vacuum at rubidium D1 line of 795 nm is obtained. In our work, we investigate the noise sources in an OPO system. By carefully controlling the classical noise source, the squeezing band extends to the analysis frequency of 2.6 kHz. The flat squeezing trace is 2.8 dB below the shot noise limit. In our work, we focus on the difference between the squeezing results at the analysis frequency of kilohertz regime at two different wavelengths, 1064 nm and 795 nm. The difference mainly comes from the absorption of 795 nm laser and its second harmonic at 397.5 nm in crystal (397.5 nm laser is at the edge of transparent window of PPKTP crystal that has an absorption index much higher than at other wavelength). The absorption induced nonlinear loss and thermal instability greatly affect the squeezing results, which is discussed in our work. Squeezing level at 795 nm is worse than at 1064 nm due to the above-mentioned factors. Noise coupling to the detection system limits the squeezing band. In the audio frequency band, squeezing is easily submerged in roll-up noises and the measured squeezing level is limited. Two factors limit the obtained squeezing:the technical noise induced in the detection and the squeezing degradation by the noise coupling of the control beams. In experiment, we carefully control the classical noise at analytical frequency of kilohertz by means of a vacuum-injected OPO, a counter-propagating cavity locking beam with orthogonal polarization, low noise homodyne detector, stable experimental system and quantum noise locking method for squeezing phase locking. Firstly, to preclude the classical noise from coupling the laser source, we use the vacuum injected OPO. A signal beam helps optimize the parametric gain and is blocked in the squeezing measurement process. In order to maintain the OPO, a counter-propagating beam with orthogonal polarization is used for locking the cavity. Then, a low noise balanced homodyne detector with a common-mode rejection ratio of 45 dB helps improve the audio frequency detection. Finally, the quantum noise locking provides a method to lock the relative phase between the coherent beam and the squeezed vacuum field. With the combination of these technical improvements, a squeezed vacuum of 2.8 dB is obtained at the analysis frequency of 2.6-100 kHz. The obtained squeezing level is mainly limited by the relatively large loss in OPO, which is induced by ultra-violet absorption in PPKTP crystal. The generated squeezed field is used to reduce the polarization noise of probe beam in an optical magnetometer, in order to increase detection sensitivity.