ISSN 1990-3413, Astrophysical Bulletin, 2018, Vol. 73, No. 2, pp. 249–256.
Pleiades Publishing, Ltd., 2018.
Original Russian Text
P.G. Kryukov, 2018, published in Astroﬁzicheskii Byulleten’, 2018, Vol. 73, No. 2, pp. 259–267.
Methods of Laser, Non-Linear, and Fiber Optics in Studying
Fundamental Problems of Astrophysics
P. G . K r y u k o v
Fiber Optics Research Center of RAS, Moscow, 119333 Russia
Lebedev Physical Institute of RAS, Moscow, 119991 Russia
Received May 19, 2017; in ﬁnal form, April 1, 2018
Abstract—Precise measurements of Doppler shifts of lines in stellar spectra allowing the radial velocity
to be measured are an important ﬁeld of astrophysical studies. A remarkable feature of the Doppler
spectroscopy is the possibility to reliably measure quite small variations of the radial velocities (its
acceleration, in fact) during long periods of time. Inﬂuence of a planet on a star is an example of such a
variation. Under the inﬂuence of a planet rotating around a star, the latter demonstrates periodic motion
manifested in the Doppler shift of the stellar spectrum. Precise measurements of this shift made it possible
to indirectly discover planets outside the Solar system (exoplanets). Along with this, searching for Earth-
type exoplanets within the habitable zone is an important challenge. For this purpose, accuracy of spectral
measurements has to allow one to determine radial velocity variations at the level of centimeters per second
during the timespans of about a year. Such measurements on the periods of 10–15 years also would serve as
a direct method for determination of assumed acceleration of the Universe expansion. However, the required
accuracy of spectroscopic measurements for this exceeds the possibilities of the traditional spectroscopy
(an iodine cell, spectral lamps). Methods of radical improvement of possibilities of astronomical Doppler
spectroscopy allowing one to attain the required measurement accuracy of Doppler shifts are considered.
The issue of precise calibration can be solved through creating a system of a laser optical frequency
generator of an exceptionally high accuracy and stability.
Key words: techniques: spectroscopic—instrumentation: spectrograph
Asigniﬁcant part of knowledge on the origin of
cosmic bodies in the Universe and on its structure
is based on spectral studies of astrophysical objects.
High-resolution-spectroscopy methods in the optical
domain are of importance in such studies. Besides
a possibility to determine chemical abundances and
characteristic of astronomical bodies, spectroscopic
measurements allow one to detect their radial velo-
cities (RV) with a high accuracy using the Doppler
eﬀect. The Doppler spectroscopy is based on this
method. The accuracy of RV measurements is de-
termined by the resolving power and calibration of an
astronomical spectrograph used in observations.
An outstanding achievement of the Doppler spec-
troscopy in astronomy was the discovery that a red-
shift of spectral lines of distant galaxies is proportional
to a distance to them (the Hubble law) which led to
development of the concept of the expansion of the
Universe as a result of the Big Bang. The Doppler
shifts are huge in this case, corresponding RVs reach
fractions of the speed of light.
However, analyzing small Doppler shifts makes it
possible to obtain extremely important results. Thus,
a high-accuracy determining of Doppler shifts in stel-
lar spectra can reveal  minor but strictly periodic
changes in the RV of a star. They were interpreted as
the inﬂuence of a planet.
The planet and the star rotate around the common
center of mass (see Fig. 1) which causes a periodic
change in the velocity of the star towards the ob-
server. Thus, for the ﬁrst time ever, the existence of
planets in stars outside the Solar system (exoplanets)
was proved in an indirect way. Thereby the Doppler
spectroscopy makes it possible to discover and study
The amplitude of the Doppler shift of spectral lines
and, correspondingly, RV variations depends on the
mass ratio of an exoplanet and a star. For exam-
ple, RV variations for a Jupiter-type exoplanet are
of about tens of meters per second, while for an
Earth-type exoplanet they are in the range of several