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Annual Meeting Optical Society of America

Frequency-tunable squeezed light for spectroscopy

Eugene Simon Polzik, J. Carri, Harry J. Kimble

Published Jan 1, 1991• 0 citations
Publisher

Abstract

A frequency-tunable source of squeezed light would allow investigations of the atom-field interaction in a regime where fundamental new phenomena have been predicted.1 There are also opportunities for spectroscopy with sensitivity beyond the shot-noise limit. Toward these objectives, in our experiment we employ a single-frequency Ti-sapphire laser with tuning optimized around 852 nm (cesium resonance transition). The infrared output of the laser is frequency doubled in an external buildup cavity with a KNbO3 crystal as the nonlinear element. Efficient doubling provides 0.65 W of tunable blue output for 1.35 W of infrared input. This blue output is then used to pump an optical parametric oscillator (OPO) containing a second KNbO3 crystal for downconversion back to the infrared. The high power of the blue pump that we have achieved and the large nonlinearity of KNbO3 allows us to employ an OPO that is resonant only for the subharmonic infrared light. This design facilitates tuning of the OPO compared with the case in which the blue pump and the infrared are both resonant. The doubling cavity, OPO, and laser itself are locked relative to a stable reference cavity by an rf sideband technique, thus providing the possibility for tunability of the system with a single knob.

DOI
10.1364/oam.1991.mbb3

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A frequency-tunable source of squeezed light would allow investigations of the atom-field interaction in a regime where fundamental new phenomena have been predicted.1 There are also opportunities for spectroscopy with sensitivity beyond the shot-noise limit. Toward these objectives, in our experiment we employ a single-frequency Ti-sapphire laser with tuning optimized around 852 nm (cesium resonance transition). The infrared output of the laser is frequency doubled in an external buildup cavity with a KNbO3 crystal as the nonlinear element. Efficient doubling provides 0.65 W of tunable blue output for 1.35 W of infrared input. This blue output is then used to pump an optical parametric oscillator (OPO) containing a second KNbO3 crystal for downconversion back to the infrared. The high power of the blue pump that we have achieved and the large nonlinearity of KNbO3 allows us to employ an OPO that is resonant only for the subharmonic infrared light. This design facilitates tuning of the OPO compared with the case in which the blue pump and the infrared are both resonant. The doubling cavity, OPO, and laser itself are locked relative to a stable reference cavity by an rf sideband technique, thus providing the possibility for tunability of the system with a single knob.
A frequency tunable source of squeezed light has been developed for application to a variety of problems in atomic physics. The basic experimental arrangement consists of a subthreshold optical parametric oscillator (OPO) that is pumped by the frequency-doubled output of a titanium-sapphire laser. Squeezed light with continuous tunability of 2 GHz has been generated with a directly observed nonclassical noise reduction of 5.0 dB relative to the vacuumstate limit in a balanced homodyne detector (5.2 dB when corrected for detector noise). Although our source is potentially broadly tunable over the noncritical phase-matching bandwidth of potassium niobate, current research has concentrated on operation at 852 nm for excitation of the D2 line of atomic cesium. Relative to the usual quantum limit associated with vacuumstate fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in FM saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB after correction for detector noise.
The non-classical light resonance on the cesium D1 (894.6 nm) line has important applications in solid-state quantum information networks due to its unique advantages. The cesium D1 line has a simplified hyperfine structure and can be used to realize a light-atom interface. In our previous work, we demonstrated 2.8-dB quadrature squeezed vacuum light at cesium D1 line in an optical parametric oscillator(OPO) with a periodically poled KTP(PPKTP) crystal. However, the squeezing level is relatively low, and the tunability that has practical significance for squeezed light has not been further investigated. Theoretically, the increase of the transmittance of output mirror and the decrease of the intra-cavity loss of the OPO can improve the squeezing level. Here, we use super-polished and optimal coating cavity mirrors to improve the nonlinear process in OPO. We prepare 447.3 nm blue light from 894.6 nm fundamental light by a second harmonic generation cavity (SHG). The SHG is a two-mirror standing-wave cavity with a PPKTP crystal as the nonlinear medium. The power of generated blue laser is 32 mW when the incident infrared power is 120 mW. Using the blue light to pump an OPO, we achieve quadrature squeezed vacuum light at cesium D1 line. The OPO is a two-mirror standing-wave cavity with a PPKTP crystal. The threshold of OPO is reduced to 28 mW. The squeezing level of generated quadrature squeezed vacuum light is increased to 3.3 dB when the pump power is 15 mW. Taking into account the overall detection efficiency, the actual squeezing reaches 5.5 dB. We inject a weak signal beam into the OPO cavity to act as an optical parametric amplifier (OPA), and test the tunability of squeezzed light. The blue light and the squeezed light are tuned by using a low-frequency triangular wave signal to scan the Ti: sapphire laser. Gradually increasing the amplitude of the scanning triangle wave signal, the generated bright squeezed light can be continuously tuned over a range around 80 MHz without losing the stability of the whole system. The generated squeezed light offers the possibility for the efficient coupling between the non-classical source and solid medium in the process of quantum interface.
A single-frequency optical parametric oscillator (OPO) operating in the wavelength range around 2 µm is reported. The OPO comprises a periodically poled LiNbO 3 (PPLN) crystal for quasi-phase matching and a four-mirror ring-cavity resonant to the signal. The OPO signal and idler are tuned from 1.89 to 2.05 µm and from 2.21 to 2.42 µm, respectively, by changing the crystal temperature. Output powers in excess of 2 W are demonstrated at a pump power of 9 W for both the signal and idler over all of the respective tuning ranges. A detailed characterization of the intensity and frequency noise is presented. The OPO exhibits an integrated intensity noise below 0.1% and a linewidth of ∼58 kHz for an observation time of 1 ms. The reported performance makes this source an ideal candidate for the generation of squeezed light around 2 µm via cascaded second-harmonic generation (SHG) and parametric downconversion (PDC).
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.
Squeezed state of light is an important resource of optical measuerments below the shot noise limit and has been used to improve measurement sensitivity in many areas such as gravitational wave detection, especially in audio frequency region. Compared with the high-frequency squeezed states, the generation of the low-frequency squeezed states is more difficult, because it is limited by several technical noise sources. In this paper we report the observation of more than 2 dB of vacuum squeezing at 1064 nm in the gravitational-wave detection band down to 3 kHz with a double-resonant optical parametric oscillator (OPO). The OPO has a configuration of linear cavity consisting of an input coupling mirror with a transmission of 11% at 532 nm and an output coupling mirror with the transmission of 12% at 1064 nm. The nonlinear materials in the OPO is type-I periodically poled potassium titanyl phosphate (PPKTP) crystal which is chosen for this experiment due to its higher nonlinearity, broader phase matching temperature, and smaller photo-thermal effect. The OPO is pumped by the light of 532 nm from Nd: YVO4/KTP solid-state laser of maximum optical power 3 W. To avoid various noise coupled from the seed beam, the OPO is seeded by vacuum fluctuations instead of coherent field at the fundamental wavelength (1064 nm). A Pound-Drever-Hall (PDH) locking scheme is used to lock the OPO cavity length with the signal derived from the reflected pump beam, so as to lock the pump field and also lock the fundamental field. To make both the pump and seed beams resonant simultaneously, the temperature of the PPKTP is carefully adjusted. The squeezed state can be detected on a homodyne detection by interfering it with the local oscillator (LO) and detected by a balanced detector with two photodiodes (EXT500 T) but having the same quantum efficiency of 86% at 1064 nm. The subsequent electronic noise is analyzed with a low-frequency spectrum analyzer, which shows that the audio noise sources from lab enviroment, locking quality, escape efficiency, propagation loss, homodyne efficiency and detection efficiency have effect on the squeezing pruced by an OPO.