RU2343611C1 - STABILISED DOUBLE-MODE He-Ne/CH4 LASER - Google Patents

Abstract

FIELD: electronics.

SUBSTANCE: invention is attributed to the field of optical maser frequency standards. It has been offered to make stabilised double-mode He-Ne/CH₄ laser as single glass ceramic monoblock containing linear Fabty-Perot resonator of broken structure. Channels filled with absorbing (methane) and gain (He-Ne) mediums are made along optical axis. Gas-discharge channel with gain medium contains three electrodes. All optical components are installed on optical contact. Automatic frequency control system is made capable to independently regulate currents between electrode pairs of gain medium.

EFFECT: resonator passive stability improvement and automatic stabilisation of resonance asymmetry factor.

5 cl, 1 dwg

Description

The invention relates to the field of optical frequency standards, in particular laser frequency standards with an internal absorption cell, using narrow nonlinear absorption or dispersion resonances to stabilize the frequency. Frequency-stabilized lasers of this type are widely used in precision metrology and ultra-high resolution spectroscopy as length standards, as well as narrow-spectrum oscillators and secondary standards with a stability level of 10 ^-13 -10 ^-14 [1-3].

Frequency stability of laser standards depends on three main components:

- characteristics of a quantum reference - a spectral line along the top of which the optical frequency is stabilized by an electronic self-tuning system (AFC),

- frequency bands and gain of the AFC system,

- magnitude of external disturbances.

For a given level of external disturbances, an extremely important characteristic of a frequency-stabilized laser is the passive stability of its optical cavity, i.e. stability of sizes and angular settings to vibrations, changes in ambient temperature, pressure, etc.

In portable stabilized lasers based on internal absorbing cells (He-Ne / CH ₄ , He-Ne / I ₂ [2-5], etc.), a Fabry-Perot linear resonator made in the form of a massive structure is used to increase passive stability from Invar pipes or rods (Invar - metal with a coefficient of thermal expansion α≈1 * 10 ^-6 ), on which mirror alignment units with rigid spring elements are installed (usually welded). Structural rigidity and a small coefficient α make it possible to reduce slow frequency drifts and increase the time of continuous operation of devices under given temperature conditions without rearrangements within the limits determined by the properties of the materials used and the capabilities of manufacturing technology.

Known [5] is a stabilized single-mode He-Ne / CH ₄ laser, consisting of a linear Fabry-Perot resonator made in the form of a parallelepiped made of Invar rods, in which: a gas discharge glass tube with an amplifying (He-Ne) and a quartz tube with an absorbing (CH ₄ ) medium, and mirrors adjusted by springs are fixed at the ends of the parallelepiped.

The disadvantage of this design, recruited from individual elements, is the inability to further increase the stability of the mutual arrangement of its constituent elements. When the invar coefficient α is ≈1 * 10 ^-6 and the temperature gradient is 1 degree on a characteristic transverse linear dimension of 100 mm, angular misalignment of mirrors of the order of 5 * 10 ^-6 occurs. A similar misalignment can occur due to the tilt of the optical windows that seal the glass tube with a reinforcing medium, since the coefficient α for the glass from which the gas-discharge tube is made with a reinforcing (He-Ne) medium is 20-30 times larger than that of the Invar. In addition, the gas discharge tube is a source of heat, and the elongated structure (length / diameter ratio of the order of (10-20): 1) increases the possibility of temperature gradients, which leads to a change in the angular and spatial arrangement of the optical windows. Such changes, together with the misalignment of the mirrors, lead to a drift and periodic fluctuations in the average lasing power caused by changes in the spectrally selective losses associated with spurious back reflections and radiation diffraction at the internal diaphragms of the resonator. Changing the laser generation regimes ultimately distorts the shape of the resonances by which the laser frequency is stabilized. The resulting shape distortions are characterized by the so-called "asymmetry coefficient", which is not stable over time, which leads to slow drifts of the stabilized frequency of portable He-Ne / CH ₄ lasers at the level of 10 ^-11 -10 ^-12 [6].

Known [7,8] is a two-mode stabilized (“reference”) He-Ne / CH ₄ laser with a narrow emission spectrum, selected as a prototype consisting of a Fabry-Perot linear resonator having a broken “P” -shaped configuration, in which two arms sequentially arranged:. a gas discharge tube with glass reinforcement (He-Ne) and medium quartz tube with absorbent (CH ₄₎ medium is excited in the discharge tube with an amplifying medium voltage is applied to two electrodes located at opposite ends of the discharge tube on each half of y. transverse magnetic fields superimposed orthogonally to each other along the "S" and "P" polarizations of the radiation. Intermediate cavity mirrors, onto which the beam is incident non-orthogonally, introduce a phase difference for radiation with "S" and "P" polarizations, which Together with magnetic fields superimposed on the amplifying medium, it provides stable generation of two modes with linear, mutually orthogonal ("S" and "P") polarizations and a frequency difference determined by the cavity length and phase difference on the mirrors. The application of orthogonal magnetic fields also leads to the fact that each part of the He-Ne medium enhances predominantly one of the orthogonal “S” or “P” polarizations, i.e. one of the mods. Stable two-mode generation makes it possible, in comparison with a single-mode laser, to increase the sensitivity of the resonance emission in methane, by which the radiation frequency is stabilized, and thereby increase the stability of the radiation frequency with small laser dimensions.

The Fabry-Perot resonator of the laser prototype is made in the form of a rigid metal structure welded from invar tubes with four mirrors mounted on mechanical quotation heads, and a glass gas discharge tube with an amplifying medium and a cell with methane made of quartz are placed in the invar tubes. Fine (within the wavelength) adjustment of the cavity length is provided by piezoelectric elements through which the mirrors are mounted on mechanical alignment devices. The radiation from the prototype laser is detected by a photodetector and a signal proportional to the intensity of one of the modes and / or the frequency of intermode beats is fed to an electronic frequency-locked loop (AFC). The AFC system extracts derivatives with respect to the frequency of resonances of saturated absorption or saturated dispersion in methane: for this, auxiliary modulation of the cavity length is introduced and harmonics (1st, 2nd, ...) of the modulation frequency are extracted. Next, the signal of one of the resonance harmonics (frequency derivatives of the resonance) is applied to the piezoelectric element controlling the cavity length, and the radiation frequency stabilizes at the point where the signal of the selected derivative is zero, i.e. near the center of resonance.

The disadvantages of the laser prototype are the same as for the analogue, since it is also technologically impossible to significantly increase the transverse and longitudinal rigidity of the structure (despite the use of welded joints, Invar), which leads (under the influence of weight, vibration, temperature, temperature gradients) to slow alignment of the resonator and drift of the generation parameters — total power, mode power difference, etc. As a result, even in laboratory conditions, the stability of the radiation frequency begins to deteriorate, starting from the averaging time of 10-20 seconds, which does not allow to obtain high long-term (up to a day) frequency stability in such a laser.

In addition, it also manifests the physical mechanism of stabilized frequency drifts mentioned above, associated with a change in the shape of the reference resonance in the process of stabilizing the frequency at the center of the resonance in methane. This change in shape is described by the appearance in the signal used to stabilize the harmonic of a small harmonic additive of a different parity. The relative value of this additive, called the asymmetry coefficient, can reach (0.1-1.0)% of the fundamental value.

The problem solved by the invention is to drastically reduce the drifts of the stabilized frequency and thereby increase by one or two orders of long-term frequency stability, in particular, due to a sharp improvement in the passive stability of the resonator and automatic stabilization of the resonance asymmetry coefficient.

This is achieved by the fact that:

- the resonator is made in the form of a single monoblock made of a material with an ultralow coefficient of thermal expansion, for example, a sitall (coefficient α of the sitall is ≈1 × 10 ^-8 , instead of 1 × 10 ^-6 from the invar);

- the size ratio of the monoblock lies in the range (3: 3: 1) - (4: 4: 1), which helps to increase the rigidity of the structure and a more uniform temperature distribution;

- mirrors are mounted on the optical contact, without mechanical (spring) adjustment nodes;

- to align the axis of the resonator one of the mirrors of the resonator is made spherical and mounted on a prism located on one of the kinks of the axis of the resonator or on one of the ends of the resonator;

- amplifying and absorbing media are filled into channels made directly in the body of the monoblock;

- for each part of the amplifying medium in the monoblock, pair slots are made for magnets creating transverse, mutually orthogonal fields;

- the automatic frequency control system is configured to independently adjust the currents between the pairs of electrodes of the amplifying medium;

- an additional electrode is introduced into the channel with the amplifying medium, which allows independent control of currents between pairs of electrodes.

The signal for the automatic stabilization of the resonance asymmetry coefficient is another harmonic of the auxiliary modulation of the reference resonance in frequency, which is different from that used in the main AFC system. If the value of this other harmonic differs from the initial value, the mismatch signal is supplied to the device acting on the laser generation mode.

To implement such a device, which maintains a constant value of the asymmetry coefficient, an additional electrode located approximately in the middle of the channel is introduced into the gas-discharge channel with an amplifying medium, and two independent outputs for excitation of the discharge in each half of the active medium are introduced into the power source. Since mutually orthogonal magnetic fields are superimposed on each half of the active medium, each half of the gas discharge tube amplifies predominantly one of the orthogonally polarized modes, and current regulation, for example, in one part changes the mode gain difference and the asymmetry coefficient depending on the mode gain.

In addition to automatic stabilization of the asymmetry coefficient, the presence of an additional electrode makes it possible to compensate for the initial difference between the excesses of the S and P polarization modes, thereby reducing the initial resonance asymmetry and the scale of the stabilized frequency shifts.

Since there is a technological limitation on the maximum channel length in the body of a monoblock, the compact design while maintaining the required (~ 1 m) optical length of the resonator is ensured by the “broken” geometry of the optical path due to the introduction of additional highly reflective mirrors also mounted on the optical contact. The number of kinks is chosen to be greater than or equal to (N-2), where N is the total number of resonator mirrors, based on the required limit stability of the radiation frequency (σ), defined by the relation:

σ = Г * {Ш / С} ≈Г · T / (L ² · S · n _Ф )

where Γ is the width of the reference resonance in methane, C is the value of the spectral resonance signal of the saturated dispersion at the intermode frequency, and - is the magnitude of the noise of the intermode frequency determined by spontaneous emission, i.e. natural fluctuations of the mode frequencies, L is the cavity length, S is the cross-sectional area of the resonator mode, n _f is the bulk density of photons in the cavity.

In the above formula, the Shavlov – Townes relation was used for natural radiation noise [9], which includes the cavity bandwidth (~ 1 / L) and the total number of photons (for a fixed density of the saturating field it is ~ L). As a result, at fixed losses on the mirrors, the ultimate stability quadratically depends on the cavity length. In the present invention, an increase in the length L is associated with kinks in the beam path and the number of such kinks (N-2), where N is the total number of mirrors, is selected taking into account the losses introduced by each additional mirror and technological problems in the manufacture of a monoblock.

The drawing shows a block diagram of the proposed stabilized two-mode laser, containing:

1 - monoblock ceramic base of the resonator; 2 - flat mirrors; 3 - spherical mirror; 4 - sealing windows; 5 - a glass prism for mounting a spherical mirror (3); 6 - anodes; 7 - cathode; 8 - registration system; 9 - electronic resonance frequency tuning system in methane; 10a - channels with an amplifying medium, 10b - channels with an absorbing medium, 12 - a piezoelectric element controlling the length of the resonator; 14 - block independent power supply of the discharge in two channels with an amplifying medium.

The drawing also shows: 11 - the path of the beam in the cavity and to the registration system (8); 13 - the path of the auto-tuning signal to the piezoelectric element (12) of the laser; 15 - the path of the auto-tuning signal to the power supply unit of gas-discharge channels (14).

The device operates as follows.

Flat mirrors (2) are mounted on optical contact on the polished faces of the monoblock base (1), the angular inaccuracy of which does not exceed 2-5 arc seconds, and separation vacuum-tight windows for each medium are installed. After filling the channels with amplifying and absorbing media, the laser is finally aligned with a spherical mirror (3) mounted on the optical contact on an additional prism (5), which, in turn, is mounted on the optical contact on the monoblock (1). A spherical mirror (3) can also be mounted on one of the ends of the resonator, but in this case an additional vacuum-tight window (4) is introduced in front of it, so that the resonator can finally be adjusted to the “atmosphere”.

To excite two parts of the gas discharge between the pairs of electrodes (7.6), two independent outputs of a high-voltage power source (14) are used and currents are set that provide not only the radiation intensity of two modes necessary for saturation of methane absorption with orthogonal linear polarizations, but also an imbalance of mode amplifications necessary to compensate for the inevitable small difference in mode loss. To obtain a stable two-mode generation mode, magnets with mutually orthogonal field direction are installed in the slot along the discharge channels. Laser radiation emerging through one of the mirrors is directed to a registration system (8), after which the electrical signal contains, in particular, a beat signal at the intermode frequency. The beat signal is fed to a frequency-locked loop (9), which generates at the output harmonic signals (frequency derivatives) of the resonance of the saturated dispersion of the methane spectral line (or saturated absorption when the intensity of one of the modes is recorded). Further, the signal of one of the harmonics of the resonances enters the piezoelectric element (12), which closes the main feedback loop and stabilizes the laser frequency at the center of the resonance of the saturated dispersion (saturated absorption). The signal for stabilizing the asymmetry of the shape of the reference resonance, proportional to a different derivative of the reference resonance in frequency, which is different from the first stabilization, is taken from the second output of the automatic frequency control system (9) and fed to the independent power supply unit for discharges in two channels with an amplifying medium so that its effect on one of the currents or the current difference in the channels stabilizes the asymmetry coefficient of the shape of the reference resonance.

To test the operation of the proposed device, a monoblock He-Ne / CH ₄ laser was implemented with the number of mirrors N = 6, which ensured the optical cavity length L≈1 m and a low (≈0.3 Hz / Hz ^1/2 ) level of natural intermode frequency noise, which is also important for obtaining high short-term frequency stability. In addition to the fundamentally new design, which dramatically increased the passive stability of the resonator compared to the prototype laser, active stabilization of the asymmetry coefficient of the reference resonance was used: for the second feedback loop, the 4th harmonic of the saturated dispersion resonance was used, which regulated the current difference in the discharge channels. Comparison with a stationary (“reference”) stabilized He-Ne / CH ₄ laser with an intracavity telescope having high long-term stability due to stabilization by a much narrower reference resonance (the central component of the allowed magnetic hyperfine structure of the methane line ~ 3 kHz wide) showed that the long-term stability of a monoblock laser increased by 50-100 times compared with the prototype: an increase in the Alan parameter, characteristic of the prototype, depending on the averaging time instead of 10 sec about for 1000 sec. Measurements at longer averaging times were not available due to the slow drifts of the “reference” laser itself, arising from the insufficient passive stability of its cavity.

LITERATURE

1. "Handbook of Lasers", ed. A.M. Prokhorov, publishing house "Soviet Radio", 1978, volume 1, p. 225.

2. B.C. Letokhov, V.P. Chebotaev, Non-linear super-high resolution laser spectroscopy. Science, 1990, p. 411.

3. M. Gubin, E. Kovalchuk, et al, "Absolute frequency measurements with a set of transportable He-Ne / CH ₄ OFS and prospects for future design and applications", in Proceedings of the 6 ^th International Symposium on Frequency Standards and Metrology, 9-14 September 2001, St. Andrews, Scotland, World Scientific Publishing Ltd, editor P. Gill., Pp. 453-460.

4. W. G. Schweitzer, Jr., E. G. Kessler, R. D. Deslattes, et al, "Description, performance and Wavelength of Iodine stabilized lasers", Applied Optics, Vol. 12, # 12, pp. 2927-2938. December 1973.

5. Y. Akimoto, "A transportable methane stabilized He-Ne laser", IEEE Trans. on Instrumentation and Measurement, vol IM-36, # 2, pp. 633-635, June 1987.

6. Krylova D.D., Shelkovnikov A.S., Petrukhin E.A., Gubin M.A., Quantum Electronics, 34, 554 (2004).

7. Gubin MA, Protsenko ED, Quantum Electronics, 24, 1080 (1997).

8. M. Gubin, D. Tyurikov, A-Shelkovnikov, E. Kovalchuk, G. Kramer and B. Lipphardt, "Transportable He-Ne / CH ₄ Optical frequency Standard and Absolute Measurements ...", IEEE, J. of Quantum Electronics , v.31, No.12, p.2177 (1995).

9. A. Meytland, M. Dan, “Introduction to Laser Physics”, Moscow “Nauka”, FML, 1978, p. 113.

Claims (5)

1. A stabilized two-mode He-Ne / CH ₄ laser containing a Fabry-Perot linear resonator with a broken structure, in which an absorbing methane cell and a He-Ne gas discharge tube with two electrodes at the ends are arranged along the optical axis, along which two pairs of constant magnets with field directions orthogonal to each other and the optical axis, as well as a self-tuning system, characterized in that the resonator is made as a single monoblock made of a material with a thermal expansion coefficient Ia is not greater than 1 × 10 ^{-8 -1} degrees, and amplifying the absorbent medium placed in the channels disposed along the optical axis of the resonator and formed by broken lines, are installed on the resonator mirror optical contact at the edges of breaks, the total number of breaks is greater than or equal to (N-2 ), where N is the number of resonator mirrors. 2. The laser according to claim 1, characterized in that the material is a monoblock ceramic. 3. The laser according to claim 1, characterized in that an additional electrode is introduced into the channel with an amplifying medium for independent adjustment of currents between the pairs of electrodes. 4. The laser according to claim 1, characterized in that for aligning the axis of the resonator one of the mirrors of the resonator is made spherical and mounted on a prism located on one of the kinks of the axis of the resonator. 5. The laser according to claim 1, characterized in that for aligning the axis of the resonator one of the mirrors of the resonator is made spherical and mounted on a prism located at one of the ends of the resonator.