.png){kind=icon}
.png){kind=icon}
Return
Optical Synthetic Aperture Technique Based on Intensity Interferometry (Invited)
.png){kind=icon}
PRE
AI
L
L
H
X
DOI：10.3788/LOP252481.png)
.png){kind=icon}
Original request for help.png){kind=icon}
Share.png){kind=icon}
Save
Abstract
En 中文
Significance Since the dawn of the 21st century, the exploration of the universe has demanded increasingly higher angular resolution to resolve fine celestial details, including stellar surfaces, active galactic nuclei, and exoplanets. The resolution of traditional telescopes is constrained by the Rayleigh criterion (similar to lambda/D), necessitating larger apertures for improved resolution. However, constructing monolithic mirrors exceeding 30 meters, such as the European Extremely Large Telescope (E-ELT), approaches the limits of engineering capabilities and economic feasibility. Consequently, optical synthetic aperture (OSA) has emerged as a revolutionary solution, synthesizing a large effective aperture from multiple separated small telescopes to achieve resolution determined by the baseline length (B) rather than the individual aperture diameter (D). Among OSA techniques, amplitude interferometry (Michelson interferometry) measures the first-order correlation of the electric field. Although it offers high sensitivity, it requires optical path differences to be stabilized within a fraction of a wavelength, rendering it extremely susceptible to atmospheric turbulence and mechanical vibrations. In contrast, intensity interferometry, based on second-order coherence theory, measures correlations of light intensity fluctuations-the Hanbury Brown and Twiss (HBT) effect. By converting optical signals into electronic signals at the detector, intensity interferometry is insensitive to atmospheric turbulence and eliminates the need for complex phase-locking mechanisms. Recent advancements in single-photon detectors and high-speed electronics have revitalized intensity interferometry, offering a cost-effective pathway to construct kilometer-scale arrays capable of microarcsecond-resolution imaging. Progress This review systematically summarizes the principles, historical development, and recent advances in intensity interferometry. The fundamental distinction between amplitude and intensity interferometry lies in the physical quantity measured. As illustrated in the comparison of instrument components, amplitude interferometry requires precise optical beam combination to measure interference fringes, whereas intensity interferometry correlates electronic signals from independent detectors to measure the second-order coherence function g((2))(tau). The relationship between the measured intensity correlation and the source's spatial coherence is governed by the Siegert relation. This unique mechanism enables intensity interferometry to circumvent stringent phase stability requirements, as rapid atmospheric phase fluctuations are averaged out over the intensity measurement integration time. To reconstruct an image, spatial coherence information-specifically the visibility modulus-is collected in the Fourier plane (u-v plane). Earth's rotation is exploited to enhance u-v plane sampling coverage, tracing elliptical tracks that improve image fidelity. Because intensity interferometry discards Fourier phase information, image reconstruction relies on phase retrieval algorithms or higher-order correlations. Simulations demonstrate that combining bispectrum-based maximum-entropy methods with phase closure techniques can successfully reconstruct complex stellar objects from sparse interferometric data, whereas amplitude-only data fail to recover structural details. Historically, the feasibility of intensity interferometry was demonstrated by the Narrabri Stellar Intensity Interferometer (NSII) in the 1960s, which successfully measured the angular diameters of 32 stars. However, the technique remained dormant for decades due to low signal-to-noise ratios (SNR) and the limited sensitivity of early photomultiplier tubes. The modern revival of intensity interferometry has been driven by the advent of high-efficiency single-photon avalanche diodes (SPADs) and picosecond-precision time-to-digital converters (TDCs). These technologies have enabled the repurposing of existing imaging air Cherenkov telescopes (IACTs), which possess large light-collecting areas, into powerful intensity interferometers. Currently, major IACT arrays worldwide, including VERITAS, MAGIC, and H.E.S.S., have been employed for intensity interferometry observations. Recent experiments have successfully measured the spatial and temporal coherence of various stars. For instance, VERITAS and H.E.S.S. have reported precise angular diameter measurements and even dual-wavelength observations, confirming the capability of modern digital correlators to process high photon fluxes across long baselines. These large-scale experiments serve as critical testbeds for the upcoming Cherenkov telescope array (CTA), which promises significantly enhanced sensitivity and improved u-v coverage. Beyond traditional long-baseline astronomy, the paradigm of intensity interferometry is expanding into new domains. Spatial intensity interferometry analyzes the speckle patterns recorded by array detectors to enable imaging through scattering media. Innovations such as ptychographic intensity interferometry and lensless Wiener-Khinchin telescopes have demonstrated high-resolution imaging capabilities and three-dimensional tomographic reconstruction. Furthermore, novel experimental schemes have been proposed to overcome bandwidth and field-of-view limitations. These include color erasure intensity interferometry, which employs nonlinear frequency conversion to enable interference between different wavelengths, and active illumination schemes for long-range terrestrial imaging. Conclusions and Prospects intensity interferometry has evolved from a controversial theoretical concept into a robust practical technique for high-resolution imaging. The transition from analog to digital correlation, combined with the use of large-aperture Cherenkov telescopes, has addressed the historical limitation of low sensitivity. Today, intensity interferometry serves as a complementary technique to amplitude interferometry, particularly advantageous for observing bright, hot stars at short wavelengths over extremely long baselines where phase locking is infeasible. Looking forward, the construction of the CTA will open a new window for astronomy, potentially achieving sub-milliarcsecond resolution. This capability will enable direct imaging of stellar surface convection, mass transfer in binary systems, and the geometric structures of shock waves in Wolf-Rayet stars. Although challenges remain in theoretical error analysis under finite sampling and in improving signal-to-noise ratios for fainter targets, the integration of computational imaging techniques and quantum optical concepts ensures that intensity interferometry will play a pivotal role in the future of optical synthetic aperture imaging.
Keywords:
interferometry
intensity interferometry
optical synthetic aperture
angular resolution
computational imaging
phase retrieval
Journal
L
IF:
1
Papers:
505
Citations:
0