RU2606348C1 - Laser with resonator q-factor modulation and mode synchronization - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to laser equipment. Laser with modulation of q-factor and mode synchronization contains in first arm of optical resonator series-arranged first end mirror, acoustic-optical modulator, active element and first auxiliary mirror, and in other arm it contains second auxiliary and end mirrors, nonlinear element installed between them. Wherein modulator in different moments of time operates at two audio frequencies transmitted through additionally introduced electric signals adder from two additionally introduced generators of modulated oscillations synchronized by third generator, setting laser pulse repetition frequency selected in range of (0.1–100) kHz. Value of first audio frequency is determined by parameters of optical resonator to meet laser mode synchronization conditions, and second audio frequency, providing modulation of q-factor of resonator is selected in one and half times higher than first one. Light beam, coming from modulator after diffraction at second frequency, is overlapped by additional diaphragm.

EFFECT: technical result consists in opportunity to increase level of modulation.

1 cl, 5 dwg

Description

The invention relates to the field of optical quantum generators (lasers) using acousto-optical modulation of the resonator Q factor and simultaneously mode synchronization to obtain high peak radiation power.

In this area, a technical solution is known in accordance with RF patent No. 2478242, the authors of which are also part of the authors of the proposed invention. The known solution comprises a laser resonator consisting of four mirrors, of which two end and two auxiliary, between the first end mirror of the resonator and the active element is placed an acousto-optic traveling wave modulator, and a nonlinear crystal and a diaphragm are placed in front of the second end mirror, while the first end mirror is made in the form of a concave sphere of radius R ₁ , the center of the modulator is separated from the reflecting surface of the first end mirror by a distance equal to the radius R ₁ , the operating frequency f of the modulator is set equal to (or a multiple of) the half-mode interval of the 2f laser (2f = c / 2L, where c is the speed of light, L is the cavity length), and the switching frequency of the modulator determines the pulse repetition rate.

When conducting experiments using the laser according to patent No. 2478242, several shortcomings in its work were found out.

The first of them is caused by a small (not exceeding 50%) level of modulation of losses in the cavity during a double pass, which leads to a decrease in the total energy and power of the laser pulse in the case of an increased gain of the active medium. The reason for the low modulation of losses in the cavity is the return to the active medium of light beams reflected from the first end mirror and undergoing re-diffraction by a traveling acoustic wave in the modulator. As a result of this, the so-called pulse precursors (time-structured noise radiation) are observed in the laser before the main pulses (Q-switch pulses). The experimental results are shown in FIG. 1, which shows the waveform of three Q-switch pulses of a laser with an active medium Nd: YAG. The rightmost pulses in FIG. 1 are the main Q-switch pulses with a half-maximum duration of 120 ns. Their repetition period is 1 ms. To the left of the main Q-switch pulses, a pulse precursor radiation is observed with a total duration of about 400 μs. The appearance of precursors is due to the fact that with continuous continuous pumping of the active medium, the population inversion in the active medium and, correspondingly, the gain increase with time as it approaches the moment of generation of the Q-switch pulse. At some point in time, the amplification of radiation in the active medium begins to exceed losses, which manifests itself in the generation of pulse precursors. In a laser with a high gain, the total energy of the pulse precursors can be comparable with the energy of the main pulse, so the presence of precursors reduces the total energy and radiation power in the main Q-switch pulse by about half.

The second drawback of the known solution is caused by the impossibility of independently controlling the Q-switching mode and the mode synchronization mode, which limits the functionality of the laser. In particular, in the known solution, there is no possibility of prompt (without resonator tuning) laser switching from the mode of operation with Q switching and mode synchronization to the mode with Q switching without mode synchronization and vice versa. At the same time, such a switching of operating modes is often necessary when using a laser for scientific research, for example, when studying the features of laser welding and cutting of various materials.

The aim of this invention is to increase the modulation level of resonator losses and increase by two (or more) times the energy and power of the Q-switch pulse with mode synchronization in lasers with a high gain of the active medium, as well as expanding the functionality of the laser.

This goal is achieved by the fact that in a laser with Q-switched resonator and mode synchronization according to RF patent No. 2478242, the acousto-optic modulator used in it operates at different time points at two sound frequencies supplied through an additionally introduced adder of electrical signals from two additionally introduced modulated oscillation generators synchronized by a third generator that sets the pulse repetition rate of the laser pulses, selected in the range (0.1-100) kHz, and the magnitude of the first sound frequency op is determined by the parameters of the optical resonator to satisfy the laser mode synchronization condition, and the second sound frequency, which provides modulation of the Q-factor of the resonator, is selected one and a half times higher than the first, and the light beam emerging from the modulator after diffraction at this frequency is blocked by an additionally introduced diaphragm.

The mode of operation of the acousto-optical modulator proposed in the claimed solution, when the laser beam at different times experiences diffraction at two different sound frequencies, is new. Declared as an invention, the solution is dependent, since it uses all the essential features of the invention according to patent No. 2478242.

Description of the proposed technical solution is illustrated by drawings.

FIG. 1 Oscillogram of three Q-switch pulses with precursors of pulses (obtained in an experiment with a laser according to US Pat. RF No. 4278242). The division value along the abscissa is 200 μs. The pulse repetition rate is 1 kHz.

FIG. 2 Scheme of the proposed laser. 1, 4, 5, 8 — resonator mirrors, 2 — acousto-optic traveling wave modulator, 3 — active element, 6 — non-linear element, 7 — diaphragm, 9 — additional diaphragm, 10 — signal amplitude adder, 11 — carrier signal generator frequency f ₁ , 12 is a control signal generator with a carrier frequency f ₂ , 13 is a pulse generator that sets the laser pulse repetition rate (0.1-100) kHz.

FIG. 3 Timing diagrams of the control electric signals and the intensity of the laser radiation, a) - output signals of the pulse generator 13 at the first output (dashed line) and generator 11 with a frequency f ₁ (solid line, b) - output signals of the pulse generator 13 at the second output (dashed line) and generator 12 with a frequency f ₂ (solid line), c) is the signal supplied to the acousto-optical modulator, d) is the intensity of the laser radiation.

FIG. 4 Oscillogram of a pulse of generation of an Nd: YAG laser, made in accordance with the proposed technical solution, in the Q-switch mode with mode synchronization. The division value along the abscissa is 50 ns. Average laser power 2 W, repetition rate 1 kHz.

FIG. 5 Oscillogram of three Q-switch lasing pulses in the proposed Nd: YAG laser. The division value along the abscissa is 200 μs. The pulse repetition rate is 1 kHz, the energy per pulse is 2 mJ.

The proposed laser is shown in FIG. 2, where in addition to the laser elements of the patent No. 4278242, generators 11-13, an adder 10, and an aperture 9 are additionally introduced.

The acousto-optic modulator 2 is connected to the output of the adder 10 of the signal amplitudes, and the inputs of the adder 10 are connected to the outputs of the generators 11 and 12. Generators 11, 12 contain frequency synthesizers that generate continuous sinusoidal signals at frequencies f ₁ and f ₂ , as well as modulation schemes (keys) performing modulation (switching) of these signals. The pulse generator 13 has two outputs connected to the inputs of the modulation circuits of the generators 11, 12. It sets the frequency of the laser pulses and synchronizes the operation of the generators 11 and 12.

The control signal representing the sum of the amplitudes of the two signals is supplied to the piezoelectric transducer of modulator 2. The first of these signals, generated by the generator 11, has a carrier frequency f ₁ that is equal to (or a multiple of) half of the inter-mode interval (f ₁ = c / 4L). This signal is used to synchronize the laser modes. The second of the signals generated by the generator 12 has a carrier frequency f ₂ = 1,5⋅f ₁ . This signal is designed to modulate the quality factor of the resonator (Q-switch mode) of the laser.

In FIG. 3 a) the dashed line shows the dependence of the voltage U _{1 of the} pulse signal on time t supplied from the first output of the generator 13 to the modulating input of the generator 11. The solid line shows the output signal of the generator 11, which is a periodic sequence of radio pulses with a carrier frequency f ₁ having a duration τ ₁ and the repetition period T. Laser mode synchronization occurs over a time τ _{1 of the} duration of the radio pulse. Typically, a value of τ ₁ equal to several microseconds is selected.

In FIG. 3 b) the dashed line shows the time dependence of the voltage U _{2 of the} pulse signal supplied from the second output of the generator 13 to the modulating input of the generator 12. The solid line shows the output signal of the generator 12, which is a periodic sequence of radio pulses of duration τ ₂ following period T. The duration of the pause between the radio pulses is equal to τ ₁ . The period T is equal to the period following the laser Q-switch pulses. In practice, T values can range from 10 microseconds to 100 milliseconds.

The shape of the total (U ₁ + U ₂ ) signal supplied to the input of modulator 2, as a function of time, is shown in FIG. 3 s).

The laser radiation intensity J versus time t is conventionally shown in FIG. 3 d), where, using the vertical arrows, the moments of Q-switch pulse generation are shown. The repetition rate of the Q-switch pulses F = 1 / T is in the range of 0.1-100 kHz.

Let us explain the operation of the proposed laser.

A light beam propagating in the laser cavity from right to left (see Fig. 2), when passing through the modulator 2 at different points in time, experiences diffraction by a traveling acoustic wave with two different frequencies f ₁ and f ₂ . In this case, two diffracted beams are formed (indicated in Fig. 2 by the letters “b” and “c”), which propagate at angles θ ₁ and θ ₂ with respect to the transmitted (non-diffracted) beam (indicated in Fig. 2 by the letter “a”) .

The first of the diffracted rays "b" corresponds to diffraction by an acoustic wave with a frequency f ₁ . This beam has a Doppler frequency shift f ₁ . After reflection from the mirror 1, the transmitted beam "a" and the first diffracted beam "b" propagate back along the same path and fall back to modulator 2, where they experience re-diffraction. As a result of repeated diffraction, part of the “b” beam is returned back to the resonator with a doubled Doppler frequency shift 2f ₁ , and its other part passes without diffraction and is removed from the resonator. The transmitted beam "a" after repeated passage of the modulator partially returns to the resonator without changing the frequency, and partially experiences diffraction in the modulator with a frequency shift f ₁ and leaves the resonator. As a result of this, after a double passage of radiation through a modulator with an acoustic wave at a frequency f ₁ , the beam indicated in FIG. 2 with the letter “a”. Due to the interference of light waves with different frequency shifts, the intensity of the beam “d” extracted from the resonator is modulated in time with a frequency of 2f ₁ .

The second of the diffracted rays ("c") corresponds to diffraction by an acoustic wave with a frequency f ₂ . Its propagation angle θ is equal to _2. An additional off-axis diaphragm 9, which is located near the end mirror 1, which closes the diffracted beam “c” and prevents its return to the laser cavity, is installed in the laser resonator (compared to the known solution of the RF patent No. 4278242). The transmitted light beam "a" after reflection from the mirror 1 experiences repeated diffraction in the modulator 2 on a wave with a frequency f ₂ and is partially removed from the resonator. The output beam is indicated in FIG. 2 letter "e".

The loss coefficient T introduced into the resonator by an AO modulator is determined by the ratio of the intensity of the radiation beams emitted from the resonator in the forward and backward pass of the modulator (after reflection from the mirror) to the intensity of the light beam incident on the right of the modulator. The diffraction efficiency of the modulator in one pass of radiation through the modulator is denoted by η. If an acoustic wave with a frequency f ₁ propagates in the modulator, then the losses are determined by the average intensity of the beam “d” extracted from the resonator (see Fig. 2). Calculating the beam intensity “d”, for the loss coefficient T, depending on the modulator efficiency η, we obtain the expression T = 2⋅η⋅ (1-η). Note that a similar expression for losses will also be true for the well-known decision on the patent of the Russian Federation No. 2478242. An analysis of this expression shows that with an increase in η, the loss coefficient T reaches a maximum value of T = 0.5 at a value of η = 0.5 and then decreases to zero at η = 1. In addition, the maximum value of T = 0.5 must be considered as the average value over time. As mentioned above, the “d” beam extracted from the resonator has a modulation at a frequency of 2⋅f ₁ . Therefore, the loss coefficient T associated with this beam is also modulated at a frequency of 2%. At the same time, at the minimum points, the loss coefficient is T <0.5. Mode-locked laser pulses occur at these minimum loss factor points.

In the case when an acoustic wave with a frequency f ₂ propagates in the modulator, the intensity of the beam "c" delayed by the diaphragm 9 contributes to the loss of the resonator. Calculating the total intensity of the beam "c" and the beam "e" output from the resonator, for the loss coefficient we obtain the expression T = η (2-η). From this expression it follows that the maximum value of the losses T = 1 at η = 1. For a typical AOM with diffraction efficiency η = 0.7, we have a loss coefficient T = 0.91. Such a loss value is sufficient for Q-switching in most lasers with a high radiation gain.

To improve the spatial separation of the diffracted light beams "b" and "c" in the plane of the diaphragm 9 (see Fig. 2), it is necessary to increase the difference between the operating frequencies of the modulator f ₂ -f ₁ . It is advisable to select a frequency value f ₂ = 1,5⋅f _1, since in this case the Doppler frequency shift of the scattered light, the diaphragm 9 is not delayed during the frequency shift lies in an interval between the laser modes (antiresonance), which leads to further suppression of the light radiation in the cavity.

To ensure the operation of the modulator at two different frequencies f ₁ and f ₂ it is advisable to use an operating mode that is intermediate between the Raman-Nath and Bragg diffraction modes. A theoretical review of this regime is set forth in the book [1] by V.I. Balakshiy, V.N. Parygin, L.E. Chirkov. Physical foundations of acousto-optics. - M .: Radio and communications, 1985 .-- 280 p. In this work, it is shown that, when choosing the optimal value of the length of the modulator piezoelectric transducer in the direction of light propagation, it is possible to ensure both high diffraction efficiency of the modulator (η≈0.8) and a sufficiently wide band of operating frequencies reaching 60% of the average operating frequency.

The proposed laser was implemented with a cavity length L = 1.5 m. An Nd: YAG crystal with diode pumping (laser transition wavelength λ = 1.064 μm) was used as an active medium. The intermode frequency of the laser cavity was 100 MHz. Mirror 1 had a radius of curvature of the reflecting surface R ₁ = 400 mm. Modulator 2 is made on a quartz crystal with an acoustic wave velocity of V = 5.75 km / s. The first operating frequency of the modulator is f ₁ = 50 MHz, which is half of the intermode frequency of the resonator. The second operating frequency of the modulator is f ₂ = 1.5 f ₁ = 75 MHz. Based on these values of the operating frequencies, the length of the piezoelectric transducer of the modulator in the direction of light propagation is chosen equal to 25 mm The thickness of the piezoelectric transducer corresponded to its resonant frequency equal to the average value between the frequencies f ₁ and f ₂ . To expand the frequency band of the piezoelectric transducer, a matching electrical circuit was used. In this case, a range of operating frequencies of the modulator from 44 MHz to 82 MHz was obtained with a drop level equal to 0.8 of the maximum value of diffraction efficiency, about 80%.

To synchronize the laser modes, we used a control signal in the form of radio pulses with a carrier frequency f ₁ = 50 MHz, a duration of 4 μs, and a repetition rate of 1 kHz (see Fig. 3a). A signal with a frequency of f ₂ = 75 MHz was used to modulate the quality factor of the laser. For this purpose, the signal amplitude level of 75 MHz was periodically modulated to a zero level for a pause time of 4 μs at a repetition rate of 1 kHz. The modulated signal of 75 MHz was summed in amplitude with the signal of 50 MHz, amplified and fed to modulator 2. At the input of modulator 2, the pulse power of the 75 MHz signal was 35 W, which ensured the diffraction efficiency of the modulator η = 0.7. As a result, the coefficient of radiation loss in the laser cavity for a double pass in the Q-switching mode was T = 0.91. The pulse power of the signal at a frequency of 50 MHz was about 5 W, as a result of which a modulator efficiency of η≈0.15 was achieved, which is sufficient for efficient synchronization of laser modes.

In FIG. 4 shows an oscillogram of one Q-switch radiation pulse obtained in the proposed laser. The pulse has a half-width of the envelope equal to 130 ns. Inside the envelope, the pulse contains a series of short pulses with mode synchronization, followed by a period of 10 ns. The repetition frequency of the Q-switch pulses is 1 kHz, the average laser power is 2 watts. When the control signal with the frequency f ₁ = 50 MHz was turned off, the oscillogram had no short pulses with mode synchronization, and the time profile of the pulse coincided with the envelope drawn through the vertices of the short pulses in FIG. 4. Thus, the switching of the operating mode is realized, which confirms the advanced functionality of the laser.

In FIG. 5 shows an oscillogram of three Q-switch radiation pulses obtained in the proposed laser. The repetition frequency of the Q-switch pulses is 1 kHz. On the waveform there are no precursors of pulses observed in the laser according to the patent of the Russian Federation No. 2478242 (see Fig. 1). This is due to the increased modulation of the resonator losses (91%) in the proposed laser. In the proposed laser, an average radiation power of 2 W was obtained at a pulse repetition frequency of Q-switch pulses of 1 kHz, while in the laser according to RF patent No. 2478242 the same radiation power (in the absence of pulse precursors) is obtained at a pulse repetition frequency of Q-switch of 2 kHz . It follows that in the proposed laser the energy of one Q-switch pulse is approximately twice as high as in the well-known solution according to the RF patent No. 2478242.

The results obtained confirm the effectiveness of the proposed technical solution.

Claims (1)

A Q-switched laser with mode synchronization, comprising in the first arm of the optical cavity a first end mirror, an acousto-optic modulator, an active element and a first auxiliary mirror, and second auxiliary and end mirrors in the other arm, between which a nonlinear element is installed, characterized in that the said modulator at different points in time operates at two sound frequencies supplied through an additionally entered adder of electrical signals from two additional of modulators of oscillations introduced by us, synchronized by a third generator that sets the laser pulse repetition rate, selected in the range from (0.1-100) kHz, the first sound frequency being determined by the parameters of the optical resonator to satisfy the laser mode synchronization condition, and the second sound frequency, providing modulation of the Q-factor of the resonator, it is selected one and a half times higher than the first, and the complementary light beam exiting from the modulator after diffraction at this frequency is blocked but entered aperture.

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