RU2723230C1 - Laser system with laser frequency stabilization - Google Patents

Abstract

FIELD: laser engineering.

SUBSTANCE: laser system with frequency stabilization of lasers has two tunable diode lasers with external resonators (DLER1 and DLER2) installed on the plate, which radiation beams pass through optical insulators 1 and 2, respectively, half-wave plates and adjustment rotary mirrors are directed as follows. Beam of the pumping laser DLER1 after reflection from the polarization cube (PC1) enters the frequency stabilization system by intra-Doppler resonance, is divided into two beams by means of a division plate (D.Pl1); after passing D.Pl1, beam of higher power passes around reference cell with cesium atoms (without buffer gas), which is located in magnetic field created by solenoid, and returns through cell opposite to test beam of lower power reflected from plate D.Pl1; trial beam of linear polarization passes a cell with cesium atoms, a quarter-wave plate, a polarization cube which separates orthogonal polarization fields, and beams of these orthogonal polarisations are directed to a balanced photodetector "Balancing". Second part of the DLER1 beam passing through the PC1 reflected from the dividing plate (D.Pl2) and passing through the polarization cube (PC3) is directed to the confocal scanning interferometer (IPR), and after passing PC4 is detected by photodetector PR2, which signal, which is due to harmonic modulation of the length of the confocal interferometer, after synchronous detection, generates an error signal which is transmitted to the PID controller which controls the length of the interferometer resonator. Third part of the DLER1 beam passing through the dividing plate D.Pl2 is directed to the fiber waveguide, which outputs radiation from the system. Part of the beam of the probing DLER2 laser after reflection by the D.Pl3 division plate and reflection in the PC3 and PC4 falls on the photodetector PR3, the signal of which after synchronous detection comes to the PID controller. Second part of the DLER2 beam after passing the dividing plate D.Pl3 is directed into the optical fiber waveguide.

EFFECT: technical result consists in stabilization of frequency of two lasers without expansion of their spectra due to modulation.

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Description

The claimed invention relates to magnetometry and gyroscopy.

The prior art scientific work relating to laser systems with stabilization of the laser frequency, in particular from the "Stabilization of the frequency of radiation of a semiconductor laser by the whispering gallery mode", A.N. Oraevsky, A.V. Yarovitsky, V.L. Velichansky “Quantum Electronics”, No. 10, 2001 and “Laser Frequency Standards at the Lebedev Physical Institute”, V.L. Velichansky, M.A. Gubin, Uspekhi Fizicheskikh Nauk, Volume 179, No. 11, 11.2009.

In addition, from the patent for invention RU 94014549 A1 05.27.1996, a method is known for stabilizing the frequency of a laser radiation, characterized in that the laser generates a peripheral multi-path mode and adjusts the cavity so that its length is close to the critical one for this multi-path mode, and its beam fluxes did not merge with each other in space. Then, radiation detectors are installed behind the output mirror, which record the transverse displacements of the beam fluxes of the multi-path mode, and adjusts the length of the resonator and the tilts of its mirrors, achieving minimal transverse displacements of the beam flux relative to the receivers.

Also from the prior art, from the patent for the invention RU 2431909 C2 04/20/2011, a laser radiation frequency stabilization system is known, comprising a regulator connected to its output with a stabilized laser input and a generator, as well as a first beam splitter located in the radiation beam of the laser to be stabilized. After the beam splitter, the following are installed in series: standard, first photodetector and first detector. The device comprises a second beam splitter having two channels and located in the radiation beam of a stabilized laser after the first beam splitter. The modulator is located between the first and second beam splitters, connected by its input to the output of the generator. A second photodetector and a second detector, as well as a differential amplifier, are installed in series. The output of the first photodetector and the output of the second photodetector are connected through a differential amplifier to the input of the regulator. The input of the second channel of the second beam splitter is connected to the output of the first beam splitter, and the output of each of the channels of the second beam splitter is optically connected to the input of one photodetector directly and to the input of another photodetector through a standard. The technical result consists in increasing the stabilization accuracy of the average radiation frequency without increasing the high-frequency deviations of this frequency.

However, in the known devices there is no possibility of stabilizing the laser without modulating their frequencies to generate an error signal in the extreme control method so that this detuning can be changed.

The technical problem consists in solving these drawbacks with the achievement of a technical result, which consists in stabilizing the frequency of two lasers without expanding their spectra due to modulation, the frequency of one of the lasers being stabilized directly along the atomic line, and the frequency of the second along the same atomic line, but with a large widely tuned by the detuning of its frequency relative to the atomic line. The indicated technical result is ensured in a laser system with stabilization of the laser frequency, comprising two tunable diode lasers with external resonators (DLVR1 and DLVR2) mounted on the plate, the radiation beams of which pass through optical isolators 1 and 2, respectively, half-wave plates and alignment rotary mirrors are guided along to such channels:

- the DLVR1 pump laser beam after reflection from the polarization cube (PC1) enters the frequency stabilization system by the intra-Doppler resonance, is divided into two beams by means of a dividing plate (D.Pl1); a beam of higher power, after passing through D.Pl1, goes around a support cell with cesium atoms (without buffer gas), which is in the magnetic field created by the solenoid, and returns through the cell towards a test beam of lower power reflected from the plate of D.Pl1; a linear polarization probe beam passes through a cesium atom cell, a quarter-wave plate, a polarizing cube that separates the fields of orthogonal polarizations and the beams of these orthogonal polarizations are sent to the Balance balanced photodetector;

- the second part of the DLVR1 pump laser beam transmitted through PC1, reflected from the dividing plate (D.pl2) and passed through the polarization cube (PC3), is sent to the confocal scanning interferometer (IFP), and after passing through PC4, the FP2 photodetector is recorded, whose signal arising due to harmonic modulation of the length of the confocal interferometer, after synchronous detection generates an error signal, which is fed to the PID controller that controls the length of the resonator of the interferometer;

- the third part of the pump laser beam DLVR1, passed through the dividing plate D. pl2 is sent to the optical fiber, which removes radiation from the system;

- a part of the DLVR2 probe laser beam after reflection by the dividing plate D.pl3 and reflected in PC3 and PC4 is incident on the FP3 photodetector, the signal of which after synchronous detection is fed to the PID controller, which stabilizes the DLVR2 frequency according to the transmission resonance of the IFP, which differs in longitudinal index from the transmission resonance which the laser frequency DLVR1 is tuned to;

- the second part of the DLVR2 probe laser beam after passing through the dividing plate D.pl3 is sent to the optical fiber.

The claimed invention is illustrated using the drawings.

Figure 1. Laser frequency stabilization scheme, where:

Optical isolator - optical isolator,

λ / 2 and λ / 4 are half-wave and quarter-wave phase plates, respectively,

PC - polarization cube,

Z - 100% of the mirror,

D.pl - dividing plates,

FP - photodetector,

L - lenses

IFP - Fabry-Perot confocal interferometer.

FIG. 2a, b - the appearance of the device, where

a) In the foreground, a solenoid with a support cell. Behind him on the opposite side is an interferometer. Vertical parallelepipeds that stand out in height are photodetectors.

b) The pump laser (in the white case) in the lower right corner. A probe laser in the same housing in the upper right corner. In the upper left corner there is an optics for introducing radiation into optical fibers.

FIG. 3a — all components of the D ₁ absorption line of cesium atoms (λ = 895 nm) with intra-Doppler resonances.

FIG. 3b pump laser frequency

FIG. 4. Intra-Doppler resonances for orthogonal circular polarizations converted to orthogonal linear polarizations by a quarter-wave plate and separated by PC2: green curve for a beam passing through a polarization separation cube, blue curve for a beam reflected from it.

FIG. 5. The difference of the two signals shown in the previous figure. The dispersion curve with a large slope in the center of the dependence is due to the Doppler transmission resonance. It is observed against the background of a flatter curve of the same dispersion form and arises due to the wider Doppler contour of the absorption line. She has the opposite sign.

FIG. 6. A system for introducing laser radiation into optical fibers that remove radiation from the system for further use in a magnetometer or other applications

A diagram of the laser system is shown in FIG. 1, and its appearance in FIG. 2. Radiation of the required frequency and power is generated by tunable diode lasers with an external resonator. These lasers maintain the single-frequency generation regime with a cw power of up to 200 mW and provide smooth tuning of the generation frequency within the 30 GHz interval, covering all ultrathin components of the D ₁ cesium line. To ensure the stability of the frequency and the generation regime with respect to spurious reflections from many elements of the system, the radiation of both lasers passes first through optical isolators (Opt. Isolator 1, Opt. Isolator 2). Since these insulators rotate the linear polarization direction by 45 °, half-wave plates are installed in front of the turning mirrors. They ensure that the plane of polarization of radiation coincides with the plane of incidence on the mirror, which ensures the preservation of linear polarization during reflection. After the rotary mirror, the pump laser radiation is divided into 2 parts. The beam reflected from the polarization cube PK1 goes to the frequency stabilization system by the intra-Doppler resonance at the transition F _g = 3 → F _e = 3 D _{1 of} the cesium line [4,11,12,14]. In this system, the laser radiation is again divided into two parts by a dividing plate D.Pl2. A beam of greater power passes through this plate, bypasses around a support cell with a diameter of 25.4 mm and a length of 70 mm with cesium atoms (without buffer gas), and returns through the cell towards the test beam of lower power reflected from the plate D.Pl1. In such a scheme, depending on the transmittance of the probe beam radiation by the cell on the laser frequency, intra-Doppler resonances are recorded. All components of the D ₁ absorption line of cesium atoms (λ = 895 nm) with intra-Doppler resonances are shown in Fig. 3a. The width of the resonances is ~ 25 MHz. This spectrum is recorded by a pump laser. The two digits near each line correspond to the quantum numbers of the total angular momentum of the lower (F _g ) and upper (F _e ) hyperfine sublevels. A part of this spectrum with two high-frequency components on a different scale is shown in Figure 3b. The laser frequency DLVR1 is stabilized by one of these intra-Doppler resonances.

The formation of a discrimination curve for frequency stabilization is performed by the DAVLL method. To do this: 1) a magnetic field is created in the cell using a solenoid parallel to its axis and the direction of propagation of the probe beam radiation; 2) the radiation of the probe beam after passing through the cell is separated by a quarter-wave plate and a polarizing cube PK3 into two beams with orthogonal linear polarizations. The dependences of the cell transmission on the laser frequency for these two beams are shown in FIG. 4, and the dependence of the balanced photodetector signal detecting the signal difference for the aforementioned beams on the laser frequency has the form of a discriminating curve of FIG. 5. This error signal is fed to the PID controller that controls the frequency of DLVR2. The advantage of this frequency stabilization scheme is the generation of an error signal without forced modulation of the laser frequency. The stabilization of the frequency of the pump laser by the intra-Doppler resonance provides additional advantages: firstly, the width of the reference resonance is one and a half orders of magnitude less than the width of the Doppler contour for a cell without a buffer gas and about 2 less than in a cell with a buffer gas; secondly, the magnetic field required for the DAVLL circuit is reduced by the same amount.

The radiation of the pump laser transmitted through PC1, reflected from D.pl2 and transmitted through PC3, is directed to a confocal scanning interferometer (IFP), and after PC4 it is detected by an FP2 photodetector. By changing the length of the interferometer using piezoceramics, the transmission resonance of the interferometer is tuned to the frequency of the pump laser. An additional harmonic voltage applied to the piezoceramics allows modulating the resonance frequency with an amplitude that is a small fraction of the transmission resonance width. After synchronously detecting the signal from FP2, an error signal is generated, which is fed to the PID controller, which stabilizes the frequency of the transmission resonance of the interferometer at the frequency of the pump laser. In this case, the frequencies of all other transmission resonances of this interferometer are also stabilized.

The radiation of the second DLVR2 probing laser after reflection by the dividing plate D.pl3 and reflection in PC3 and PK4 is incident on the FP3 photodetector. The frequency of the probe laser is tuned to a different transmission resonance of the interferometer and stabilized by a feedback loop similar to that described above. Only the first loop stabilizes the resonance frequencies of the interferometer by the frequency of the first laser attached to the atomic line, and the second loop stabilizes the frequency of the probe laser by another resonance shifted by an integer number of free dispersion regions of the interferometer. Thus, the interferometer allows you to transfer the stability of atomic resonance to a second laser. To suppress the mutual influence of two lasers and separate the signals, their radiation entering the interferometer has orthogonal linear polarizations.

Claims (6)

A laser system with stabilization of laser frequency, containing two tunable diode lasers mounted on a plate with external resonators (DLVR1 and DLVR2), the radiation beams of which pass through optical isolators 1 and 2, respectively, half-wave plates and alignment rotary mirrors are directed through such channels: - the DLVR1 pump laser beam after reflection from the polarization cube (PC1) enters the frequency stabilization system by the intra-Doppler resonance, is divided into two beams by means of a dividing plate (D.Pl1); a beam of higher power, after passing through D.Pl1, goes around a support cell with cesium atoms (without buffer gas), which is in the magnetic field created by the solenoid, and returns through the cell towards a test beam of lower power reflected from the plate of D.Pl1; a linear polarization probe beam passes through a cesium atom cell, a quarter-wave plate, a polarization cube that separates the fields of orthogonal polarizations, and the beams of these orthogonal polarizations are sent to the Balance balanced photodetector; - the second part of the DLVR1 pump laser beam transmitted through PC1, reflected from the dividing plate (D.Pl2) and passed through the polarization cube (PC3), is sent to the confocal scanning interferometer (IFP), and after passing through PC4, the FP2 photodetector is detected, the signal of which arising due to harmonic modulation of the length of the confocal interferometer, after synchronous detection generates an error signal, which is fed to the PID controller that controls the length of the resonator of the interferometer; - the third part of the laser beam DLVR1, passed through the dividing plate D.Pl2, is sent to the fiber optic fiber, which removes radiation from the system; - a part of the DLVR2 probe laser beam after reflection by D.Pl3 dividing plate and reflected in PC3 and PC4 gets to the FP3 photodetector, the signal of which after synchronous detection comes to the PID controller, which stabilizes the DLVR2 frequency according to the IFP transmission resonance, which differs from the transmission resonance in longitudinal index which the laser frequency DLVR1 is tuned to; - the second part of the DLVR2 probe laser beam after passing through the dividing plate D.Pl3 is sent to the optical fiber.

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