astrophotonics (original) (raw)

Definition: the application of advanced photonics for astronomical instrumentation

Alternative term: photonics in astronomy

Categories: photonic devices, optoelectronics

• photonics
  • silicon photonics
  • quantum photonics
  • quantum electronics
  • space photonics
  • astrophotonics

Related: space photonicsphotonicsadaptive opticsphotonic lanternsfrequency combs

DOI: 10.61835/z6x Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Astrophotonics?

Astrophotonics is an interdisciplinary field at the intersection of astronomy and photonics that applies advanced optical and photonic technologies to astronomical instrumentation. It encompasses the design, development, and application of devices that manipulate light (across broad wavelength ranges, far beyond the visible) to improve the efficiency, sensitivity, stability, and functionality of astronomical instruments such as telescopes and spectrographs. The field well predates the year 2000 (even if it wasn’t yet called “astrophotonics”), and accelerated as photonic integrated circuits and nanophotonics matured — enabling miniaturized, highly stable and customizable devices to replace or augment bulk optics in astronomy [11, 29].

As optics has always been central to astronomy, one might think “astrophotonics” started centuries ago. However, the term specifically refers to developments that go beyond classical free-space optics to include guided-wave technologies and modal engineering (often exploiting telecom photonics), photonic integrated circuits (PICs), and advanced methods such as specialized interferometry, frequency combs and adaptive optics. These approaches not only improve performance but can dramatically reduce size and mass (a win for stability and cost) and enable highly multiplexed instruments — for example, compact, high-resolution hyperspectral imaging or multi-object spectrographs [11, 27].

There is overlap with space photonics where astronomical instruments operate in space, and when various enabling components are used in both fields. However, much of space photonics does not deal with astronomy.

Probed Aspects of Light

Astrophotonics is not only about optimizing telescope optics for imaging; it exploits several complementary properties of light:

• Spatial information is used for imaging as in traditional optics, but with improved technology.
• Spectral information is widely used for analyzing composition, temperatures, velocities and redshifts.
• Temporal intensity fluctuations (variability and photon statistics) play important roles: Nanosecond-scale correlation measurements (intensity interferometry) probe stellar diameters and surface structure [22]. Pulsar studies are another application.
• Polarization of light traces magnetic fields, scattering geometries and dust alignment [37].
• Orbital angular momentum (OAM) and mode structure are explored for spatial mode sorting and starlight suppression (e.g., vortex coronagraphs) [28, 23].
• Quantum properties such as second-order photon correlation functions ($g^{(2)}(\tau)$) are analyzed, with emerging quantum-enhanced metrology prospects [22].

Applications of Photonics in Astronomy

Adaptive Optics and Wavefront Control

Atmospheric turbulence scrambles wavefronts and is only partly mitigated by selecting superb sites with clear sky. Adaptive optics (AO), often realized with laser guide stars, restores diffraction-limited performance on large telescopes and is a foundational partner for many astrophotonic devices (e.g., for single-mode fiber injection and PIC beam combiners) [18].

See the article on adaptive optics for details.

Phase Mask Coronagraphy

Historically, the term coronagraphy comes from solar corona studies, where it is necessary to somehow remove the light from the far brighter photosphere. Similar techniques are now used to investigate other phenomena around stars, such as circumstellar disks and faint exoplanets.

In phase-mask coronagraphy, a focal-plane mask imposes a spatial phase pattern so that subsequent pupil-plane filtering with a Lyot stop removes the star’s diffracted light. Important families include the Roddier disk phase mask, the four-quadrant phase mask (FQPM) and vortex/AGPM masks that employ subwavelength gratings.

Interferometry

Modern photonics has substantially transformed astronomical interferometry:

• Intensity interferometry: Hanbury Brown–Twiss (HBT) type interferometers measure second-order correlations ($g^{(2)}(\tau)$) of star light collected with two or more separate telescopes, the spacing of which (baseline length) is varied. For thermal radiation, Siegert relation connects second- and first-order coherence: g^{(2)}(\tau) = 1 + |g^{(1)}(\tau)|^2$$

• Thus, intensity interferometry indirectly probes the spatial coherence (related to angular structure) without requiring a stabilized relative optical phase between the telescopes. This enabled classical stellar-diameter measurements, pioneered at the Narrabri Stellar Intensity Interferometer, and underpins current kilometer-baseline concepts with Cherenkov-telescope arrays [22].

• Synthesized apertures with PIC beam combiners: Photonic integrated circuits can coherently combine light from multiple telescopes in an interferometric array, allowing astronomers to synthesize apertures much larger than individual telescopes. Stabilizing optical phase is the core challenge, addressed with two methods: Adaptive optics (see above) mitigates atmospheric distortion, while carefully shielded cryogenic optical delay lines for connecting telescopes are monitored and phase-compensated with laser interferometry and automatic feedback techniques, also compensating for the effects of Earth's rotation. In fact, both methods are used not only simultaneously but in cooperation: A fringe tracker, based on a secondary interferometer, feeds additional corrections to the delay lines, keeping fringes stable during exposure times.

• Nulling interferometry: Photonic combiners can destructively interfere starlight to cancel the on-axis source while transmitting off-axis planetary light [1, 5].

Fiber Optics for Light Collection and Transport

Fiber optics underpin many modern astronomical instruments [36]:

• Single-mode and multimode fibers relay light from telescope foci to remote spectrographs, enabling stability and high multiplexing (e.g., early MEDUSA multi-object spectroscopy [3]). Spatial reformatting with fibers can adapt telescope PSFs to spectrograph inputs.
• Photonic lanterns adiabatically convert a multimode input into multiple single-mode outputs (and back), allowing multimode telescope light (often seeing-limited) to feed single-mode photonic devices (e.g., compact spectrographs or nullers) [21].

Spectroscopy

For many years, spectroscopy has been an essential method in astronomy. Examples of properties that can be analyzed with spectroscopy:

• properties of small objects in the solar system (asteroids)
• temperatures and chemical compositions of stars
• gas kinematics in circumstellar environments
• tiny oscillations of radial velocity of stars in the search for exoplanets, causing corresponding changes in spectroscopic lines
• composition and radial velocities of galaxies, partly within large galaxy surveys

In many cases, extreme performance is required — for example, efficient light processing in a photon-starved regime, extreme spectral resolution, the detection of faint signals in the presence of much stronger disturbing influences, or the combination with high spatial resolution.

Modern photonics devices enable improved performance, more compact devices, and partly even entirely new kinds of observations. Some examples:

• Compact integrated spectrographs: Photonic integrated circuits deliver stable, miniaturized, single-mode spectrographs. A common type is based on arrayed-waveguide gratings (AWGs) [25], but there are other on-chip concepts, including stationary-wave integral field Fourier-transform devices.
• Handling multimode inputs: Telescopes are inefficient at direct single-mode fiber coupling unless adaptive optics well compensates for atmospheric turbulence effects. Therefore, one often collects the light with few-mode or multimode fibers and uses photonic lanterns to split a multimode feed into many single-mode channels that are connected to compact spectrographs [21].
• Integral Field Spectroscopy (IFS) is spectroscopy applied to extended objects, i.e., with many spatial channels for 2D resolution — similar to hyperspectral imaging as used in remote sensing, but with differences concerning spectral resolution, field of view, snapshot acquisition and used components. An example is the ARGUS integral field unit within FLAMES, feeding the GIRAFFE spectrograph in ESO's Very Large Telescope (VLT) [4]. Fiber optics and microlens arrays are extensively used.
• Astrocombs provide ultra-precise, repeatable wavelength calibration, far surpassing spectral lamps or telluric standards. This is critical for ultra-precise measurement of radial velocities [9, 17, 26].

Spectral Filtering

Astronomical observations often require special kinds of spectral filtering. For example, airglow from atmospheric OH radicals, which can be relatively intense, must be removed in order not to overwhelm faint astronomical signals.

Using tailored masks for covering those spectral lines in the focal plane of a high-resolution spectrograph has been only partially successful. A better solution is to apply spectral filtering before the spectrograph:

• For spectrographs coupled with single-mode fibers, fiber Bragg gratings (FBGs) are well suited as simple notch filters.
• Most telescopes, however, are inefficient in collecting light into a single mode, and therefore use few-mode or multimode fibers. Unfortunately, FBGs in multimode fibers do not work as well due to mode-dependent reflection peaks. For solving that problem, photonic lanterns have been developed, which distribute multimode light without substantial power losses over multiple single-mode fibers, for which performant notch filter gratings are available.

With highly performant adaptive optics, single-mode collection with telescopes becomes feasible, so that simpler single-mode filtering can be applied — besides reducing the complexity of spectrographs.

Aperiodic fiber Bragg gratings (FBGs) are well suited for filtering out multiple spectral lines [7].

Infrared Detectors

Photodetectors for longer-wavelength infrared light are hard to make with high quantum efficiency for operation in photon-starved regimes. A possible solution is nonlinear upconversion by sum frequency generation in a ($\chi^{(2)}$) medium. Particularly suitable are waveguides made in periodically poled LiNbO3 (PPLN), pumped with a continuous-wave laser. Upconversion efficiencies on the order of 90% are feasible. For subsequent detection, silicon-based single-photon avalanche diodes (SPADs) can be used, for example. Upconversion stages can sit after adaptive-optics-fed single-mode injection or after a photonic lantern, feeding compact, visible-band detectors or spectrographs.

Note that the limited phase matching bandwidth can even be useful for spectral filtering of the signals, improving the signal-to-noise ratio.

It is also possible to do image-preserving upconversion in bulk crystals, even in the form of hyperspectral imaging.

Modern photonics also has other types of sensitive infrared detectors, for example quantum well infrared photodetectors (QWIPs), a kind of photoconductive detectors.

Space Telescopes

To avoid detrimental effects of Earth's atmosphere (aberrations, light pollution, natural atmospheric emission), space telescopes are increasingly used. The compactness and robustness of photonic devices make them particularly attractive for space missions, where mass, volume, and stability are critical constraints. Photonic integrated circuits can replace bulky optical benches on satellites and probes.

See the article on space photonics for more details.

General Challenges of Astrophotonics

To a substantial extent, photonic components originally developed for optical fiber communications are utilized. These often need to be developed further in various respects:

• Usable bandwidth: For astronomy, the usable wavelength range needs to be greatly expanded. This is a challenge particularly for photonic integrated circuits, where different material platforms are required for different spectral regions [29, 31].
• Losses of light: Collected light generally needs to be processed very efficiently, as collecting more light to compensate for some losses would be extremely difficult and expensive. (In contrast, optical fiber communications can easily tolerate substantial losses, e.g. by using fiber amplifiers.)
• Ultra-sensitive detection: While optical fiber communications generally avoids signal levels getting too low, astronomy requires ultra-sensitive photodetectors and related low-noise electronics.
• Dynamic range: Astronomy applications frequently require a far larger dynamic range than telecom components, sometimes even extreme contrast (e.g. in exoplanet search). Therefore, various components need to be optimized further. For example, optical modulators are required with high modulation contrasts, polarization states have to be processed with precision, and stray light has to be strongly suppressed.

From a supplier perspective, astrophotonics is a highly specialized niche market, with only a few players (e.g., large telescopes) utilizing certain developments. While the generated value can be high, challenges arise not only from extreme performance requirements, but also from long deployment cycles and small component counts.

Frequently Asked Questions

What is astrophotonics?

Astrophotonics is a field at the intersection of astronomy and photonics that applies advanced optical technologies, such as fiber optics and photonic integrated circuits, to improve astronomical instruments like telescopes and spectrographs.

What is the role of adaptive optics in astrophotonics?

Adaptive optics (AO) corrects wavefront distortions caused by atmospheric turbulence. This is crucial for astrophotonics as it enables the diffraction-limited performance required to efficiently couple starlight into single-mode devices like optical fibers and integrated circuits.

What are photonic lanterns and how are they used in astronomy?

Photonic lanterns are fiber-optic devices that convert light from a multimode fiber into several single-mode fibers with low loss. They allow light from seeing-limited telescopes to be fed into single-mode photonic instruments like compact spectrographs.

How are laser frequency combs used in astronomy?

Laser frequency combs (astrocombs) serve as ultra-precise rulers for wavelength calibration in spectrographs. They are essential for applications requiring extreme precision, such as measuring the subtle radial velocity shifts of stars to detect exoplanets.

How has photonics improved astronomical interferometry?

Photonics enables compact and stable integrated circuits to coherently combine light from multiple telescopes, which is essential for interferometry. This allows astronomers to synthesize a much larger virtual telescope, achieving exceptionally high angular resolution.

Why are photonic devices well-suited for space telescopes?

The compactness, low mass, and robustness of photonic devices, particularly integrated circuits, make them ideal for space missions. They can replace bulky conventional optics, which is critical given the strict constraints on mass and volume for space payloads.

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Bibliography

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