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Journal of the Optical Society of America B

Influence of birefringence induced at low temperature on balanced detection of polarization-dependent photon-number squeezing and its optical compensation

Jing Zhang, Tiancai Zhang, Ruifang Dong, Junxiang Zhang, Junmin Wang, Changde Xie, Kunchi Peng

Published Jul 1, 2001• 1 citation
Publisher

Abstract

The polarization-dependent photon-number squeezing of 1.5 dB from a grating feedback locking quantum-well laser in a cryostat has been experimentally detected with a balanced homodyne detector including an imperfect nonpolarized-beam splitter. The great influence of the birefringence induced at low temperature on the determination of the shot-noise-limit and on the detection of photon-number squeezing is discussed in detail. Birefringence is compensated for by insertion of a half-wave plate of adjustable inclination into the output beam before the polarization components of the optical system.

DOI
10.1364/josab.18.001014

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Annual Meeting Optical Society of America•Jan 1, 1987•0 citations
The polarization-dependent photon-number squeezing of 1.5 dB from a grating feedback locking quantum-well laser in a cryostat has been experimentally detected with a balanced homodyne detector including an imperfect nonpolarized-beam splitter. The great influence of the birefringence induced at low temperature on the determination of the shot-noise-limit and on the detection of photon-number squeezing is discussed in detail. Birefringence is compensated for by insertion of a half-wave plate of adjustable inclination into the output beam before the polarization components of the optical system.
Amplitude noise spectra from the nonclassical output of an injection-locked low-temperature quantum-well laser analyzed along orthogonal polarization axes reveals correlated polarization-dependent noise. Using polarization-preserving balanced homodyne detection to suppress these effects, we demonstrate photon-number fluctuations 4.5 dB below the semiclassical shot-noise limit (within 95% of the expected squeezing based on the device efficiency). Polarization mixing through birefringent components and, more significantly, correlated fluctuations originating in the laser gain medium are shown to account for the observations.
In this dissertation measurements are presented that extend our understanding of the important noise processes involved in the semiconductor laser generation of photon-number squeezed light. Strong sub-shot noise correlations between photon-number fluctuations on orthogonal polarization axes are observed in the output of a low temperature injection-locked quantum-well laser. Such polarization dependent effects are accounted for by intrinsic correlations generated in the laser gain medium and polarization mixing due to birefringence. Using polarization preserving balanced homodyne detection, up to 4.5 dB of photon-number squeezing is reported, which is within 95% of the expected maximum squeezing based on the device quantum efficiency. The work in this dissertation demonstrates the application of such nonclassical fields to practical measurements that achieve sub-shot noise sensitivity. Coherent nonlinear optical spectroscopy with a photon-number squeezed probe field is used to measure the Urbach tail and exciton resonances in a GaAs/AlGaAs multiple quantum well with sensitivity below the semiclassical shot-noise limit. Direct drive current modulation of an injection-locked room temperature laser results in a nonclassical amplitude modulated field with a signal to noise ratio that exceeds the maximum ratio permitted for a classical source under equivalent modulation conditions.
Due to the birefringence effect of the quantum well laser and the collimating lens in a low temperature cryostat, the polarization of the output light was changed, therefore the photon number fluctuation of the output individual linearly polarized mode was increased. By compensating the birefringence with a quarter wave plate, and utilizing an inphase external weak feedback to laser diode laser by means of residuary reflection from the two surfaces of a half wave plate, we obtained the output light of 24mW with the photon number fluctuation 2.8dB below the Standard Quantum Limit at the pump current of 34.65mA.
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.
Since the first proposal by Caves1 that squeezed light could be used to enhance the sensitivity of a gravitational wave interferometric antenna, squeezed states have been demonstrated in the laboratory but not yet used in an actual measurement system. This paper describes a polarization interferometer experiment, operated with a squeezed light source, designed to improve the interferometer sensitivity beyond the standard quantum limit. The interferometer consists of two polarization beam splitters, whose axes are oriented 45° with respect to each other. Linearly polarized incident laser light is transmitted by the first polarizer and split by the second one, which also acts as the splitter for a balanced homodyne detector. A rotation of the polarization between the two polarizers can be detected with quantum-limited sensitivity (e.g., ±1 μrad). The squeezed light entering the other port of the interferometer is produced using a χ2 crystal (KTP) in an optical cavity, pumped by a frequency-doubled cw ring Nd:YAG laser. The interferometer sensitivity can thus be increased by a factor depending on the degree of squeezing achieved and on the detector quantum efficiency. Sensitivity increases by factors between 2 and 3 are predicted, and current performance is described.