RU2548940C1 - Integral fibre laser system and method for autogeneration of laser pulses - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: system employs two nested passive Q-switched fibre lasers with external pumping with radiation of an electrical current powered laser diode. The illuminated absorbing element used is the active fibre of one of the lasers. Pulse frequency is control is achieved owing to presence of optoelectronic feedback on the pulse frequency and the radiation power of the diode. Matching autogeneration of pulsed radiation is provided by synchronising modulation frequency and pumping current power with the frequency of the laser pulses of the system. Output radiation is amplified by an optical power amplifier.

EFFECT: stabilisation of pulsed radiation with two different optical frequencies, high accuracy and efficiency of converting energy.

20 cl, 5 dwg, 2 tbl

Description

Technical field

The invention relates to pulsed fiber laser systems generating passively Q-switched, namely, to dual-frequency optical systems laser systems with the conversion of short-wavelength laser pump radiation to long-wavelength, in the geometry embedded in each other by resonators in the presence of optoelectronic control feedback.

State of the art

Pulsed fiber lasers are widely used in various fields of science and technology. With a fully fiber implementation, such a laser is called solid fiber, with the combined use of fiber and other elements in the design of the laser, it is called fiber-discrete or hybrid. Fiber lasers are used in industry for cutting metals and marking products, in welding and processing of metals and composite materials, in fiber optic communication lines, locations, medicine and spectroscopy. Their main advantages are high optical radiation quality, ergonomics, compactness and the ability to integrate into technological equipment. There is a wide variety of designs of fiber lasers, due to the specifics of their application. In their manufacture, both ring resonators and linear Fabry-Perot-type resonators formed by pairs of fiber Bragg gratings (FBGs) are widely used. To obtain powerful nanosecond pulses with a repetition rate of units and tens of kilohertz, Q-switching circuits are often used.

The Q-switching mode is carried out using both active and passive optical media and elements in the cavity. Discrete electro- or acousto-optical modulator are examples of active elements inside a fiber resonator, controlled by an external electronic circuit. A discrete saturable optical element is an example of a passive element responsible for the self-generation of laser pulses. For example, crystals like U: CaF2, U: SrF2, U: BaF2 in the invention R.D. Stub et al., EP 0790684 A2, Glass fiber system using uranium-doped crystal Q-switch. However, discrete elements are not always easily mated with fiber, require the use of additional optical devices - focusers, mirrors, and therefore complicate, increase the cost of the product and make it not so compact and reliable in operation.

A known whole-fiber laser using doped fiber as a passive saturable element for implementing the Q-switching mode is Passive Q-switching all-fiber laser utilizing doped fiber as saturated absorption body, CN 102904153 A, SHI WEI et al. The authors propose an optical circuit - Fig. 1, of a fiber laser, in which passive Q-switching is performed using a saturable fiber absorber 103, in which there are levels of laser transitions. The fiber absorber is pumped by a fiber laser with an active fiber 105 inside the cavity, limited by FBG 102 and 106. Such an absorber replaces a discrete optical element (crystal) and provides a mode with passive Q switching, without converting the laser frequency characteristic of the type of laser active fiber 105 used, pumped by a diode 101. Due to the fact that the fiber absorber does not have its own resonator, radiation is generated at a single frequency. And other, longer wavelength radiation, characteristic of optical transitions in an absorber, is not used properly as laser radiation. But it is precisely this longer wavelength part of the spectrum that is often useful in practical applications, for example, in spectroscopy and surgery. In addition, the disadvantages of the proposed scheme include a low level of output power and insufficient stability of the pulse repetition rate.

A.S. offer a more advanced solution in their work. Kurkov et al, All fiber Yb-Ho pulsed laser, Laser Phys. Lett. 6, No.2, 135-138 (2009), in which the authors perform passive Q-switching with a saturable absorber. This saturable absorber is placed in its own resonator with blind mirrors — fiber Bragg gratings with a characteristic wavelength corresponding to the laser transition of the absorber, so that this radiation does not go beyond the limits of the blind resonator. For example, such an absorbing element may be a segment of a fiber doped with ions of the rare-earth metal holmium. The fiber is placed in the resonator of a fiber iterbium laser. However, due to the use of deaf mirrors, this scheme does not allow longer-wavelength radiation at a holmium wavelength of 2.0 μm to be pulled out of the cavity, and the fiber laser operates at a single optical frequency, namely, a frequency of an iterbium laser of 1.125 μm. In addition, the proposed scheme does not provide the ability to control and fine-tune the frequency of pulse generation.

Thus, the task of creating an inexpensive all-fiber double-frequency laser with passive Q switching and a controlled pulse repetition rate at the resonant wavelengths of the optical absorber and the pump laser is relevant and significant.

SUMMARY OF THE INVENTION

The invention proposes a scheme of a controlled pulsed whole-fiber laser system, Figure 2, which allows generation at two different wavelengths without resorting to complex optical discrete elements (modulators), which achieves higher technical and economic indicators - reliability, stability, controllability and economy (cheapness). It uses a fiber laser 202, in which passive Q-switching is performed using a saturable absorber in the form of active fiber 201, in which there are levels of laser transitions due to the presence of rare-earth metal ions, for example Tm (and / or Ho). Unlike the traditional Q-switching scheme, this saturable fiber absorber is placed in its own resonator, inside the pump cavity (between the FBG mirrors 206 and 209), limited by its own FBG mirrors - 207 and 208, and one of the mirrors is not blind, which allows laser generation at the wavelength of the absorber and the output of radiation 210 or 211 outside the cavity. In the example of FIG. 2, such a absorbing element is a thulium laser 201 with thulium doped active fiber (Tm), which is placed in a fiber erbium (Er) fiber resonator by fiber optic welding. Such an optical scheme allows pulsed generation of laser radiation simultaneously at two different optical frequencies with wavelengths characteristic of the erbium - 1.5 microns and thulium - 1.9 microns laser transition. The short-wavelength cavity with an active erbium fiber is pumped by a semiconductor laser or an LD laser diode, powered by a current driver 204, controlled by a programmable microchip or microcontroller (MS) 205. In this example, a laser diode with a wavelength of 962 nm is used to pump the erbium active fiber.

Thus, in the laser system, the generated pulses of generation of an erbium pump laser 202 at a wavelength of 1567 nm pump an embedded cavity in which the saturable absorber is a thulium laser 201. This leads to pulsed generation at a longer wavelength of 1908 nm, in comparison with pump wavelength of an erbium laser. Depending on the location of non-deaf (translucent) FBG mirrors in each pair of 206, 209 and 207, 208, transmitting radiation from both resonators to the outside, one can obtain various options for removing one or another radiation from the system. Namely, the radiation of the long wavelength Tm laser and the short wavelength Er laser pump are output:

- in different directions or in the same direction,

- only long-wave radiation is output,

- only the short-wavelength radiation of the pump laser is output.

Some possible combinations follow from table. 1 in the configuration of FIG. 2.

Table 1 Pairs of FBG, (Figure 2) Deaf FBG Unswerving FBG Wavelength, microns Direction (Figure 2) 106 x - 1567 110 109 - x 1567 106 - x 1567 104 109 x - 1567 106 x - 1567 - 109 x - 1567 107 x - 1908 110 108 - x 1908 107 x - 1908 - 108 x - 1908

Pulse generation occurs at a constant level of the pump current LD. A mode of coordinated self-oscillation is proposed: for a modulated pump current LD with a modulation frequency fo, conditions arise for synchronizing the frequencies of the laser pulses of the system and the modulation current at this frequency.

Figure 3 presents a diagram of a laser system covered by feedback on the pulse repetition rate to increase the generation stability. Part of the output radiation branches off with a 1% coupler 310 to the PD photodiode of the optoelectronic converter 311, the output signal of which (shown by a dotted line) is fed to the input of the analog-to-digital converter of the MC 305 microcontroller. Under the control of the MC, negative feedback is implemented by counting the number of pulses for a fixed period and if their number deviates during the next calculation, the pump current LD changes (the radiation power LD increases or decreases) at the command of the MS in the current driver LD 304, so that compensate for the frequency deviation from the previous value.

To increase the power of pulsed radiation 313, 315 of any frequency component at the output of the resonators, a fiber optical amplifier 312 and / or 314 is used. In particular, an amplifier based on stimulated recombination scattering (Raman amplifier) is used as such an amplifier.

Thus, the invention proposes two systems:

1. An all-fiber pulsed laser system for generating laser pulses in the passive Q-switching mode (FIG. 2), containing two embedded fiber resonators - short-wave 201 and long-wave 202, with external pumping of the short-wave resonator by radiation from a semiconductor laser 204 (LD laser diode) excited by electric current, and generating at the optical frequency of the short-wave resonator 202, characterized in that the long-wave resonator 201 is a long-wave fiber resonator about a laser limited by its own pair of fiber Bragg gratings 207, 208 - one of which transmits radiation outside the resonators, and the short-wave resonator 202 is pumped by the radiation of a laser diode LD, powered by an electric current from an electronic device - current driver 204, by command of a programmable microcircuit MS 205 so that pulsed generation of a long-wavelength fiber laser pumped by the radiation of a short-wavelength laser is carried out;

and includes:

- a resonator of a short-wave fiber laser 202, limited by its own pair of fiber Bragg gratings 206, 209, containing a portion of the active fiber, for example erbium (Er), and

- LD laser diode, supplied with electric current from an electronic device

- current driver 204, at the command of programmable chip 205, and

- a resonator of a long-wave fiber laser 201 containing a portion of an active fiber, for example thulium (Tm), and placed inside the resonator of a short-wave laser 202 by connecting using fiber optic welding, and limited to its pair of fiber Bragg gratings 207, 208,

- one of which passes radiation 210 or 211 beyond the limits of the resonators; wherein:

- apply active fibers of long-wave 201 and short-wave 202 fiber lasers doped with the same or different rare-earth metal ions from the series: Yb, Er, Nd, Ho, Sm, Tu;

- one of the mirrors 206 or 209 of the short-wave resonator is not deaf, and the other is deaf.

- bring out only long-wave radiation;

- bring out in opposite directions along the resonator fiber short-wave and long-wave radiation;

- bring out in the same direction on the same fiber and short-wave and long-wave radiation;

- control of the amplitude and frequency of the electric current of the laser diode LD is carried out by the current driver 204 at the command of a programmable chip 205;

- the current of the laser diode LD is maintained constant or pulsed by the signal of the programmable chip 205;

- the current of the laser diode is modulated in accordance with the signal of the programmable chip 205, so that the modulation frequency is synchronous with the pulse repetition rate of the fiber lasers 201, 202.

2. A whole-fiber pulsed laser system for generating laser pulses in the passive Q-switching mode (Fig. 3), containing two embedded fiber resonators - short-wave 302 and long-wave 301, with external pumping of the short-wave resonator by radiation from a semiconductor laser 304 (LD laser diode) excited by electric current, and generating at the optical frequency of the short-wave resonator 302, characterized in that the long-wave resonator 301 is a long-wave fiber resonator about a laser limited by its own pair of fiber Bragg gratings 307, 308 - one of which passes radiation outside the resonators, and the short-wave resonator 302 is pumped by the radiation of a laser diode LD, powered by an electric current from an electronic device - current driver 304, by command of the MC 305 microcontroller so so that pulsed generation of a long-wavelength fiber laser pumped by the radiation of a short-wavelength laser is carried out, and the pulse repetition rate is stabilized by splendens negative feedback optoelectronic communication under the control of microcontroller 305, with the proviso that:

- PD photodetector with optoelectronic converter 311 receives the optical signal as part of the generated pulse radiation and transmits an electronic signal to the input of the digital-to-analog converter of microcontroller 305, which provides stabilization of the pulse mode by controlling the current of the laser diode LD by means of a current driver 304; and includes:

- a resonator short-wave fiber laser 302, limited to its own pair of fiber Bragg gratings 306 and 309, containing a portion of the active fiber, and

- LD laser diode, supplied with electric current from an electronic device

- current driver 304 controlled by microcontroller 305, and

a resonator of a long-wave fiber laser 301, containing a portion of the active fiber, and placed inside the cavity of the short-wave laser 302 by connecting using fiber optic welding, and limited by its pair of fiber Bragg gratings 307 and 308 - one of which passes radiation outside the resonators, and

- PD photodetector with an optoelectronic converter 311, which receives an optical signal as part of the generated pulsed radiation, and transmits an electronic signal to the input of the digital-analog converter of the microcontroller 305, which provides stabilization of the pulse mode due to the feedback between the optical signal and the electronic signal of the current driver of the laser diode 304 laser diode 304 LD

wherein:

- radiation is removed through a fiber coupler 310 or through the outer sheath of the fiber;

- carry out negative feedback on the pulse repetition rate, so as to prevent the frequency from changing by controlling the driver of the pump current 304, changing the direct current of the laser diode LD, i.e. pump power;

- carry out the adjustment of the pulse repetition rate in the mode of coordinated self-generation by adjusting the modulation frequency and the amplitude of the pump current of the laser diode LD to achieve synchronization with the pulse repetition rate of the laser pulses under the control of the microcontroller 305.

In any of the laser systems proposed above, a fiber power amplifier 312 and / or 314 is used to amplify the output radiation 313/315 for any optical component, in particular, the fiber power amplifier is a Raman amplifier.

The invention also proposes a new method for laser pulse auto-generation by a system of two fiber lasers with resonators embedded in one another in the passive Q-switching mode, when externally pumped by a semiconductor laser (laser diode) excited by electric current, characterized in that they achieve the mode of coordinated laser pulse auto-generation radiation at two different optical frequencies, provided that the modulation frequency of the pump current is synchronized with the frequency of the laser pulses in the laser second system; wherein:

- the mode of self-consistent auto-generation is achieved by adjusting the modulation frequency and the pump current power of the laser diode until synchronization with the laser pulse repetition rate occurs;

- carry out the pumping of one fiber resonator by the radiation of a laser diode;

- carry out the pumping of two fiber resonators by the radiation of a laser diode;

- carry out stabilization of the mode of coordinated self-generation of pulses using optoelectronic feedback, matching the pulse repetition rate with the current modulation frequency and the pump diode pump power, so as to prevent the pulse repetition rate from changing by correcting the power and / or modulation frequency of the pump diode pump current, for example , the diode is powered by pulse-width modulated current and the duty cycle of the pulses is adjusted by the command of the microcontroller included in the c Feedback loop with an optoelectronic device - the microcontroller analyzes the frequency of the laser pulses and, when changed, generates a control signal to compensate for the deviation by changing the duty cycle of the current pulses, i.e. pump power.

Brief Description of the Drawings

Figure 1. Optical design of a fiber laser with a passive fiber absorber without its own resonator (CN 102904153 A).

102, 106 - a pair of fiber Bragg gratings,

103 - fiber absorber,

104 - fiber capler,

105 - active laser fiber,

107 - radiation output.

Figure 2. Schematic of an all-fiber pulsed laser system.

201 - long wavelength fiber laser,

202 short-wave fiber laser,

203 - optical input of laser diode radiation into an active fiber,

204 — LD laser diode current control device (LD current driver),

205 - programmable logic integrated circuit (or microcontroller MS),

206, 209 - a pair of fiber Bragg gratings of the laser resonator 202,

210, 211 - radiation output,

207, 208 - a pair of fiber Bragg gratings of the laser cavity 201.

Figure 3. Scheme of a controlled all-fiber pulsed laser system with optical power amplification.

301 - long wavelength fiber laser

302 short-wave fiber laser,

303 - optical input of the laser diode radiation into the active fiber,

304 — LD laser diode current control device (LD current driver),

305 - programmable logic integrated circuit (or microcontroller MS),

306, 309 - a pair of fiber Bragg gratings of the laser resonator 202,

307, 308 - a pair of fiber Bragg gratings of the laser cavity 201,

310 - fiber coupler,

311 - optoelectronic Converter with a PD photodetector,

312, 314 - optical power amplifiers (in particular Raman),

313, 315 - radiation output.

Figure 4. Oscillogram of fiber laser pulses (not to scale).

401 - laser pulse of long-wave radiation 1905 nm with a duration of 108 ns,

402 - laser pulse of short-wave radiation 1567 nm with a duration of 185 ns,

Figure 5. Oscillogram of synchronous laser pulses in the mode of coordinated self-generation.

The implementation of the invention

The laser system assembled in the configuration of Figure 2, allowed pulsed laser generation at wavelengths of 1567 and 1908 nm. The pulse length of a thulium laser with a wavelength of 1908 nm was 105 ns, when pumped with an erbium fiber laser with a wavelength of 1567 nm with a pulse duration of 185 ns and a repetition rate of 30 kHz (Figure 4), the energy in the pulse of a thulium laser was 40 μJ, with an average energy 1 watts.

When testing the stabilization circuit (Figure 3), it was found that the deviation of the pulse repetition rate was less than 1% with working feedback.

In the mode of coordinated self-generation, when the modulation frequency fo of the pump current LD is synchronized with the laser pulse repetition rate in the laser system, a frequency deviation of no more than 1% is achieved. In Fig. 5, the lower row of rectangular pulses corresponds to a modulated current at a frequency fo = 20 kHz, and the upper row corresponds to laser pulses of the system.

Table 2 shows the range of amplitudes of the pump currents Imin and Imax for LD type PLD-34-962 (IPG Photonics) and the corresponding duty cycle of rectangular current pulses with respect to the “unit” width in the optical scheme of FIG. 2.

table 2 Frequency fo, kHz Goodness Imin, a Imax, A twenty 1: 3.3 7.2 7.43 thirty 1: 2.2 6.2 6.31 40 1: 1.67 5.67 5.69

Industrial applicability

The proposed two-wave laser systems can be effectively applied in various fields of science and technology, in particular, in spectroscopy, laser location (in the region of 2 μm there is an atmospheric transparency window), processing of materials, medicine, especially as radiation sources, cutting and simultaneously coagulating blood vessels . The use of these laser systems and the method of coordinated auto-generation will allow achieving a new technical level and providing the best technical and economic indicators in the direction of economy (low cost), reliability, stability and controllability of pulsed radiation.

Claims (20)

1. An all-fiber pulsed laser system for generating laser pulses in the passive Q-factor mode, containing two fiber cavities embedded in one another - a short-wave and a long-wave, with external pumping of a short-wave resonator by radiation from a semiconductor laser (laser diode), excited by an electric current, and generating on an optical current the frequency of the short-wave resonator, characterized in that the long-wave resonator is a resonator of a long-wave fiber laser, a pair of fiber Bragg gratings, one of which transmits radiation outside the resonators, and the short-wave resonator is pumped by the radiation of a laser diode, which is supplied with electric current from an electronic device - a current driver, by command of a programmable microcircuit so that pulsed generation of a long-wave fiber laser is pumped shortwave laser radiation. 2. The whole-fiber pulsed laser system according to claim 1, in which active fibers of long-wave and short-wave fiber lasers doped with the same or different rare-earth ions from the series Yb, Er, Nd, Ho, Sm, Tu are used. 3. The whole-fiber pulsed laser system according to claim 1, in which one of the mirrors of the shortwave resonator is deaf and the other is deaf. 4. An all-fiber pulsed laser system according to claim 1, in which only long-wave radiation is output. 5. The whole-fiber pulsed laser system according to claim 3, in which the short-wave and long-wave radiation are output outward in opposite directions along the resonator fiber. 6. The whole-fiber pulsed laser system according to claim 3, in which both the short-wave and the long-wave radiation are output outward in the same direction along the same fiber. 7. The whole-fiber laser system according to claim 1, in which the amplitude and frequency modulation of the electric current of the laser diode are controlled by a current driver by command of a programmable chip. 8. An all-fiber pulsed laser system according to claim 1, in which the current of the laser diode is kept constant or pulsed by a signal of a programmable chip. 9. The whole-fiber pulsed laser system according to claim 7, in which the current of the laser diode is modulated in accordance with the signal of the programmable microcircuit, so that the modulation frequency is synchronous with the pulse repetition rate of the fiber laser radiation. 10. A whole-fiber pulsed laser system for generating laser pulses in the passive Q-factor mode, containing two fiber cavities inserted one into the other - a short-wave and a long-wave, with external pumping of a short-wave resonator by radiation from a semiconductor laser (laser diode), excited by an electric current, and generating on an optical current the frequency of the short-wave resonator, characterized in that the long-wave resonator is a resonator of a long-wave fiber laser, faceted a pair of fiber Bragg gratings - one of which transmits radiation outside the resonators, and the short-wave resonator is pumped by the radiation of a laser diode, which is supplied with electric current from an electronic device - a current driver, by command of the microcontroller so that pulsed generation of a long-wave fiber laser pumped by radiation a short-wave laser, and the pulse repetition rate was stabilized due to the presence of negative feedback opt electronic communication under the control of the microcontroller, provided that:
- a photodetector with an optoelectronic converter receives an optical signal as part of the generated pulse radiation, and transmits an electronic signal to the input of the digital-to-analog converter of the microcontroller, which provides stabilization of the pulse mode by controlling the current of the laser diode by means of a current driver. 11. The whole-fiber pulsed laser system of claim 10, in which the radiation to the photodetector is removed through a fiber coupler or through the outer sheath of the fiber. 12. The whole-fiber pulsed laser system according to claim 11, in which negative feedback is made on the pulse repetition rate, so as to prevent a frequency change by controlling the pump current driver by changing the direct current through the laser diode. 13. The whole-fiber pulsed laser system according to claim 11, in which the pulse repetition rate is adjusted in the mode of coordinated self-oscillation by adjusting the modulation frequency and the pump diode pump power to achieve synchronization with the repetition rate of the laser pulses under the control of the microcontroller. 14. An all-fiber pulsed laser system according to claim 10, comprising a fiber power amplifier for amplifying output radiation from any optical component. 15. The whole-fiber pulsed laser system according to 14, in which the fiber power amplifier is an amplifier based on stimulated Raman scattering - a Raman amplifier. 16. The method of laser pulse auto-generation by a system of two fiber resonators inserted one into the other in the passive Q-switching mode, with external pumping by a semiconductor laser (laser diode) excited by an electric current, characterized in that they achieve the mode of coordinated auto-generation of laser radiation pulses on two different optical frequencies, provided that the modulation frequency of the pump current, at a given power, is synchronized with the repetition rate of the laser pulses in the laser system. 17. The method for the self-generation of laser pulses according to clause 16, in which the mode of coordinated auto-generation is achieved at different pump currents of the laser diode. 18. The method of auto-generation of laser pulses according to clause 16, in which carry out the pumping of one fiber resonator radiation of a laser diode. 19. The method of auto-generation of laser pulses according to clause 16, in which two fiber cavities are pumped by radiation of a laser diode. 20. The method of laser pulse auto-generation according to clause 16, in which stabilization of the mode of coordinated auto-generation of pulses is carried out using feedback optoelectronic coupling the pulse repetition rate with the current modulation frequency and the laser diode pump power, so as to prevent the pulse repetition rate from being changed by correction power and / or frequency modulation of the pump current of the laser diode.

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