We present the generation of nonclassical state using an amplified diode laser as a light source. The intensity noise of an amplified diode laser was significantly suppressed and reached the shot noise limit at 15 MHz using both a filter cavity and resonant optical feedback. Frequency doubling efficiency of 66% and up to 120 mW output power of green has been achieved in cw second-harmonic generation from 1080 nm to 540 nm. Bright two-mode amplitude-squeezed state was generated from a type-II nondegenerate optical parametric amplifier pumped by generated green light. The measured noise reduction is 2.1±0.2 dB below the shot-noise level.
© 2006 Optical Society of America
Among many methods of generation of squeezed state, the optical parametric oscillator (OPO) and amplifier (OPA) appear to be relatively simple and efficient method. Type-II continuous–wave nondegenerate optical parametric oscillators (cw-NOPOs) have been utilized to generate quantum-correlated twin beams and two-mode squeezed vacuum when they are operating above and below their threshold, respectively . In most of these experiments, type-II KTP crystals were usually used with Nd:YAG lasers as pump source since stable diode-pumped Nd:YAG lasers are commercially available [2, 3]. Unfortunately, these experimental setups have a severe problem of non-zero walk-off, which leads to a limited nonlinear interaction length and polarization crosstalk. In addition, it is impossible to operate under frequency degenerate condition without walk-off compensation when one uses the KTP crystal with the Nd:YAG laser at 1064 nm. The fundamental wavelength of 1080 nm can solve this problem since it can realize type II noncritical phase matching (NCPM) in an a-cut KTP crystal. Kimble’s group and group of Shanxi University conducted lots of interesting experiments using an a-cut KTP crystal pumped by their homemade Nd:YAP laser at 1080 nm, such as the landmark experiment of realization of EPR paradox , quantum dense coding  and entanglement swapping . In their successful experiments, not only the experimental setup is simplified but also intracavity losses are reduced and the efficiency of downconversion is improved by exploiting the type-II NCPM condition in an a-cut KTP crystal.
On the other hand, recent laser-diode technology is commercially supplying laser diodes (LDs) operating at 1080 nm. Considering future applications of nonclassical light in quantum information science, the combination of a LD at 1080 nm and an a-cut KTP crystal to generation of nonclassical light is a straightforward view. The compactness, portability, and low cost of LDs are advantageous to developing the nonclassical light sources using two or more OPOs in which all the pumping LDs may be phase-locked through optical injection. In our previous experiment, we efficiently generated twin beams from a NOPO pumped by second harmonic of a LD at 1080nm using an a-cut KTP crystal . In the case of twin-beam generation, the intensity-difference detection system can cancel the classical excess noise of the pumping laser despite the large excess noise of the LD. For the other types of nonclassical light generation, such as two-mode squeezed states, we need to reduce the large excess noise of the LD source. Recently we have achieved efficient noise suppression of an amplified diode laser by optical filtering and resonant optical feedback .
In this paper, we report the first experiment on generation of a two-mode bright squeezed state by NOPA using the above-mentioned noise-suppressed amplified diode laser. To our best knowledge, the setup is the first cw NOPA pumped by a frequency-doubled diode laser. The measured noise reduction is 2.1±0.2 dB below the shot noise level.
2. Experimental setup
The experimental setup is shown schematically in Fig. 1. A grating-stabilized single-mode extended-cavity diode laser (ECDL) severs as the primary light source. The wavelength of diode laser is selected at 1080 nm in our experiment. The master laser provides 50 mW of power after the grating, and more than 30 mW is available for injection into the tapered amplifier chip. The output power from the amplifier chip is about 450 mW. To reduce the excess noise of the diode laser, we sent the beam through a filter cavity and used the resonant optical feedback. A small fraction of the transmitted light was picked off by the polarizing beamsplitter (PBS) with a half wave plate, which can also control the optical feedback level, and sent into ECDL as a resonant optical feedback. When the cavity is locked, we measured more than 200 mW transmitted light power for an incident power of 380mW, representing a transmission efficiency of 53%. Most of the noise suppressed laser power is introduced into the frequency doubler to generate second harmonic at 540 nm as pump beam for the NOPA. About 5 mW of power is injected to the NOPA as a seed wave. The polarization of seed wave is 45° relative to the b axis of the KTP crystal, and it is decomposed to signal and idler seed waves with identical intensity and the orthogonal polarizations along the b and c axes, respectively, which correspond to the vertical and horizontal polarization. The generated bright two-mode squeezed state is detected by a homodyne detector.
3. Results and discussion
The intensity noise of the noise suppressed laser light is measured and shown in Fig. 2. To avoid the saturation of the detector, only particular power of laser light is measured. We see that for this particular power level (approx. 20 mW) the laser light is shot noise level at 15 MHz and hence transfers no classical noise into the second harmonic and seed at frequencies higher than 15 MHz. The detail of noise suppression of amplified diode laser will be reported else where . The frequency doubler has an a-cut KTP crystal with a length of 10 mm as a nonlinear optical medium in the enhanced cavity with a bow-tie type ring configuration. The crystal is mounted in an oven and its temperature is stabilized by a controller with a specified stability of ±0.005°C. The cavity was the same one based on our previous design . Different from previous design, the transmission of the input mirror Tin=2.1%, which is close to the calculated optimum input transmission when the input fundamental is about 180mW, is selected. Figure 3 is the functions of the second harmonic output power and the conversion efficiency versus the input fundamental power. We derived the theoretical curves by solving equation
where η=P 2ω/Pω is the SHG efficiency and LSHG=0.4% is a total round-trip loss of SHG cavity determined by the measured finesse of 250. The single-pass conversion efficiency ENL of the crystal, which is set to the value ENL=1.1×10-3 W -1, is the only adjustable parameter. The experimental results are in reasonable agreement with the expected curves. The maximum green output power of 120 mW is obtained when the input fundamental power is 180 mW. The directly measured doubling efficiency is 66%.
The NOPA has a semimonolithic configuration consisting of a concave mirror of 20-mm radius of curvature was coated with a Tout=2% transmission for 1080 nm and high reflection for 540 nm. It serves as an output coupler for our NOPA. A facet of KTP inside the cavity was coated for antireflection at both 1080 and 540 nm. The other facet was coated for antireflection at 540 nm and high reflection at 1080 nm. It acts as the input mirror of the pump field at 540 nm. The measured finesse of the resonator is 300, the free spectral range is 6 GHz, and the cavity bandwidth is γc=20 MHz. We calculated a total round-trip loss of LNOPA=0.3% by the measured finesse. The escape efficiency of ξ=Tout/(Tout+LNOPA)=0.87 is obtained. Due to the large transmission of input coupler at 540 nm, the pump field only passes the cavity twice without resonating. The crystal nonlinear efficiency of ENL=1.1×10-3 W -1 was estimated from the SHG process. From this we deduce an expected threshold pump power for parametric oscillation, Pth=(Tout+LNOPA)2/4ENL=120 mW.
The principal difficulty of the NOPA resides in frequency-degenerate operation. There are several ways to solve this problem, such as seed injection , inserting a half-wave plate in cavity , and active adding phase-locking between the signal and idler . For this purpose, the seed wave is injected in our work. The NOPA cavity must be simultaneously resonant at the signal and idler of seed beam frequency. By fine tuning of the crystal temperature the birefringence between the signal and idler waves in KTP is compensated and the simultaneous resonance is obtained. Once the double resonance is completed, phase-sensitive parametric amplification/deamplification was realized. Operating below threshold and scanning the pump phase with the PZT one observes maximum amplification factors up to 20. Stable operation of the squeezer can be achieved by locking the cavity on the frequency of the seed wave via a dither-locking technique.
It has been demonstrated that the orthogonally polarized modes, which are produced from the projection of the output signal and idler mode along direction at ±45° relative to the signal beam polarization, have squeezed fluctuations . Different from previous work , in which the squeezed vacuum is measured by introducing local oscillator light, we directly measured the squeezing of the bright two-mode from the NOPA. The signal and idler modes superposed by rotating half-wave plate at an angle of 22.5° relative to the signal beam polarization, then the coupled modes were separated by the polarizing beam splitter. The noise of the bright mode is measured by the self-homodyne detection system. The ac photocurrents of the detector are combined in a hybrid junction to generate the difference and sum currents i - and i+, which indicated the shot noise reference and the intensity noise levels, respectively.
Figure 4 shows the measured variances at ƒ=16 MHz when the pump power was about 100 mW. Trace (b) and (c) refer to the amplitude noise and the shot noise limit, respectively, when the pump phase is fixed on deamplification operating. A measurement of V det(i +) and V det(i -) is also given in traces (f) and (e) when the phase is scanned. The amplitude squeezing up to 2.1±0.2 dB is measured under the total detection efficiency of η=86% (detector quantum efficiency 90% and propagation efficiency 96%). Trace (d) gives the measured shot noise limit without pump and trace (a) gives the electronic noise of our detector. We note that the noise powers for the available light power is close to that the electronic noise floor, so the electronic noise floor should be subtracted. The inferred value after taking into account the electronics floor is 2.5±0.2 dB. In a simplified mode the measured amplitude spectrum is expressed by 
Using the parameters of our experiment indicated in the preceding paragraphs, a theoretical squeezing of 2.5 dB is predicted. The experiment results are very well in agreement with this prediction.
Although the squeezing of just 2.1dB is observed, it is the first experiment on generation squeezed state using LD as a light source directly. The frequency at which the noise of LD was suppressed to shot noise level can be further decreased by exploiting a high-finesse cavity. Therefore, a large magnitude of squeezing may be generated at low frequency. Furthermore, we can separate the two-mode squeezing state into signal and idler beams. The quantum correlation between amplitude and phase quadratures of intense signal and idler beams can be directly used as the quantum entanglement source for quantum information and communications experiments.
In conclusion, two-mode bright amplitude squeezing of 2.1±0.2 dB at an output power of about 1 mW is obtained from the type-II NCPM NOPA pumped by the 540 nm green light. The pump light, as great as 120 mW, is obtained by frequency doubling of a noise-suppressed diode laser operating at 1080 nm. We have demonstrated good agreement between our experimental results and the theoretical predictions.
We are grateful to T. Hirano for helpful discussions.
References and links
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