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Quantum Electronics

Excess broadband noise at equal intensities in the interferometer arms

Valentin M. Gelikonov, Vladimir N. Romashov, Grigory V. Gelikonov

Published Apr 27, 2021• 2 citations
Publisher

Abstract

Abstract We report a theoretical and experimental study of the excess photocurrent noise in the detection of low-coherence radiation caused by the beats of the random components of the optical spectrum at equal intensities in the interferometer arms. It is shown that, in this case, the spectral density of photocurrent fluctuations is times higher than when detecting radiation without interference. This result makes it possible to clarify the value of the limiting level of the total fundamental noise of low-coherence radiation during interference. Excess noise cannot be completely subtracted in balanced detection.

DOI
10.1070/qel17536

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Abstract We report a theoretical and experimental study of the excess photocurrent noise in the detection of low-coherence radiation caused by the beats of the random components of the optical spectrum at equal intensities in the interferometer arms. It is shown that, in this case, the spectral density of photocurrent fluctuations is times higher than when detecting radiation without interference. This result makes it possible to clarify the value of the limiting level of the total fundamental noise of low-coherence radiation during interference. Excess noise cannot be completely subtracted in balanced detection.
Optical interference usually requires two interfering paths to be balanced within the coherence length. For the case of unbalanced paths, interference effects can be recovered in the frequency-domain analysis of the superposed resultant optical field because optical frequency filtering in the spectral analysis of the optical field lengthens the coherence length to overcome the path imbalance. However, since optical detection usually is intensity measurement that loses the phase information of the detected optical field, the output photocurrent of intensity measurement cannot recover the lost interference effect in the postprocessing of the data such as electronic spectral analysis. On the other hand, if amplitude measurement can be used to measure the resultant optical field from the path-unbalanced superposition of two fields, the output photocurrent carries complete information of two fields and can restore interference in postdetection processing, even though no interference shows up in direct intensity measurement. In this paper, we use homodyne detection method for amplitude measurement to analyze the output of a path-unbalanced interferometer in the spectral domain. Since complete information about the superposed optical field is contained in the photocurrent, spectral analysis shows interference fringes in frequency that reveal information about the path imbalance. It is found that shot noise is the limiting factor in the visibility of interference but can be reduced with a squeezed state input in the unused port of the unbalanced interferometer. This approach is a different paradigm from the traditional one for studying the coherence between optical fields and should find wide application in remote sensing and precision measurement.
The phase noise affecting the beat photocurrent produced by coherent detection of a single-frequency laser field with a time-delayed and frequency-shifted image of itself in a heterodyne two-wave interferometric system using a single-mode optical fiber is investigated as a function of source spectrum and phase-decorrelation time under the assumptions of quantum laser phase noise and high-local-oscillation level. The influence on the photocurrent spectrum of partial correlation between detected fields is pointed out. These calculations are an extension of a previously reported high-resolution spectroscopic method and account for the phase-noise limitation in unbalanced heterodyne fiber sensors and heterodyne-transmission receivers with an optical reference wave generated from the received signal. An experimental study is reported consisting of a Mach–Zender-type interferometer employing a single-frequency amplitude-stabilized laser, an acousto-optical single-side-band modulator for frequency shifting one of the arms, and several lengths of single-mode optical fiber to decorrelate the detected fields partially. Results are found to be in good agreement with theory.
We calculate the intensity fluctuations of the light transmitted by a Fabry-P\'erot interferometer when a broadband laser is incident upon it. The laser field is assumed to have a Lorentzian spectrum and to be broadened by random amplitude fluctuations (real or complex) or random frequency fluctuations. In each case we obtain closed-form expressions for the intensity autocorrelation function, from which we can obtain the magnitude of the intensity fluctuations, as well as the power spectrum. These results indicate a strong dependence of the transmitted intensity fluctuations on the higher-order statistical properties of the incident field. Agreement with similar results concerning atomic fluorescence fluctuations is very good.
The quantum-limited operation of balanced mixer optical homodyne and heterodyne receiver using a 50-50 percent beam splitter and two identical photodetectors is experimentally confirmed. The absolute quantum noise levels are calibrated with an incoherent GaAs light-emitting diode output. The photoelectron statistics, obtained through the analog photon counting experiment, are reduced to the quantum limit. The photocurrent fluctuation spectral density measured with a spectrum analyzer also agrees with the quantum noise spectral density. Furthermore, the deviations from the absolute quantum noise level are within ±0.02 dB. Finally, complete suppression of local oscillator excess noise is demonstrated in the 0-1.5 GHz frequency region.
Quantum-mechanical calculations of the mean-square fluctuation spectra in optical homodyning and heterodyning are made for arbitrary input and local-oscillator quantum states. In addition to the unavoidable quantum fluctuations, it is shown that excess noise from the local oscillator always affects homodyning and, when it is broadband, also heterodyning. Both the quantum and the excess noise of the local oscillator can be eliminated by coherent subtraction of the two outputs of a 50-50 beam splitter. This result also demonstrates the fact that the basic quantum noise in homodyning and heterodyning is signal quantum fluctuation, not local-oscillator shot noise.