RU206388U1 - Fiber thulium power amplifier for ultrashort pulses at a wavelength of 1.9 μm - Google Patents

Abstract

The utility model relates to laser technology, in particular to fiber laser systems that generate optical radiation at a wavelength of 1.9 μm in the form of ultrashort pulses with a peak power of up to 250 kW and an adjustable duration in the range from tens of femtoseconds to a few picoseconds. Laser systems of this type can be used for: creating broadband laser radiation (supercontinuum) in the mid-IR range, scientific applications, precision material processing, surgery, gas detectors. The technical result consists in obtaining pulses with an adjustable duration in the range from tens of femtoseconds to a few picoseconds with a peak power of up to 250 kW at a wavelength of 1900 nm. The specified technical result is achieved by the fact that the thulium all-fiber master oscillator power amplifier contains a mode-locked thulium all-fiber master oscillator connected to a fiber-optic isolator connected to a fiber-optic coupler of optical radiation, which is connected to a germanosilicate fiber with normal group dispersion. velocities at a wavelength of 1.9 μm, the output of which is connected to an optical radiation polarization controller connected in turn to a signal input operating at a wavelength of 1.9 μm, a spectrally selective fiber multiplexer, the pump input of which, operating at a wavelength of 1 , 55 μm, is connected to a fiber optic isolator, which is connected to a continuous laser source at a wavelength of 1.55 μm. The output of a spectrally selective fiber multiplexer containing radiation at wavelengths of 1.55 and 1.9 μm is connected to an active germanosilicate fiber doped with thulium ions with normal group velocity dispersion at a wavelength of 1.9 μm. The other end of the active fiber is connected to a fiber with an increased field diameter of the mode with anomalous group-velocity dispersion at a wavelength of 1.9 μm for pulse compression. A fiber with an increased mode field diameter makes it possible to compress a pulse in the time domain to a duration of less than 100 fs with a peak power of up to 250 kW. All fiber optic components have inputs and outputs made of quartz single-mode fiber that meets the ITU-TG652 standard. D. As a tunable element of the duration of ultrashort pulses, a radiation polarization controller is used, which allows you to vary the duration from tens of femtoseconds to a few picoseconds. Thanks to its all-fiber construction, the device is compact. 2 ill.

Description

Technology area

The utility model relates to laser technology, in particular to fiber laser systems that generate optical radiation at a wavelength of 1.9 μm in the form of ultrashort pulses with a peak power of up to 250 kW and an adjustable duration in the range from tens of femtoseconds to a few picoseconds. Lasers of this type can be used for: creating broadband laser radiation (supercontinuum) in the mid-IR range, scientific applications, precision material processing, surgery, gas detectors. The supercontinuum in the mid-IR range can be used for: precision spectroscopy, diagnostics of human diseases, detection of explosives in the air, optical coherence tomography.

State of the art

A lot of devices are known for obtaining radiation using thulium fiber sources (patent US 20130188660 A1, publ. 07.25.2013; patent EP 2521505 B1, publ. 06.09.2017; patent CN 105337146 A, publ. 17.02.2016; patent CN 105406329 A, publ. 03/16/2016; patent CN 205231454 U, publ. 05/11/2016; patent CN 110429459 A, publ. 11/08/2019; RF patent for invention No. 2682628 C1, publ. 03/19/2019; RF patent for invention No. 2693542 C1, publ. 03.07.2019; RF patent for invention No. 2690864 C2, publ. 06.06.2019).

For example, an all-fiber femtosecond laser with an amplifier in the form of a thulium active optical fiber is known (US patent US 8792158 B2, published on July 29, 2014). The amplification in this patent is implemented in an active fiber doped with thulium ions with anomalous group velocity dispersion, which requires the use of a longer branch for stretching pulses compared to active fibers with normal group velocity dispersion, which complicates the design.

The closest analogue (prototype) of the proposed device can be recognized as an amplifier of ultrashort pulses, described in the work: Chen Y. et al. "High Energy Ultrafast Laser at 2 μm Using Dispersion Engineered Thulium-Doped Fiber" // IEEE Photonics Journal. - 2019. - T. 11. - No. 6. - C. 1-12. In the prototype circuit, the all-fiber amplifier consists of a passively mode-locked master oscillator, the output of which is connected to an optical radiation isolator, which is connected to a spectrally selective optical radiation multiplexer. High-power erbium pumping is connected to the other input of the spectral-selective multiplexer, which, in turn, is connected to a thulium fiber with normal dispersion.

However, due to the absence of a compressor in the prototype, the pulse duration is 1.98 ps, which is significantly greater than the value used in this utility model.

Disclosure of a utility model

The technical result to be achieved by the proposed utility model consists in obtaining ultrashort pulses with adjustable duration in the range from tens of femtoseconds to units of picoseconds with a maximum peak power of up to 250 kW using an all-fiber circuit.

The specified technical result is achieved by the fact that the radiation of ultrashort pulses at a wavelength of 1.9 μm from an all-fiber laser with mode locking passes through a fiber-optic isolator, which is used to prevent the return of radiation from entering the laser, then the radiation passes through a fiber-optic coupler, with the help of which from 1 to 5 percent of the radiation is output to the radiation receiver to control the generation mode, the rest of the radiation passes through a germanosilicate fiber with normal group velocity dispersion at a wavelength of 1.9 μm for time stretching of the pulses before the amplification process, then the radiation passes through the polarization controller, with the help of which the polarization of the radiation entering the amplifier is adjusted, then the radiation enters the input of the fiber spectral selective multiplexer operating at a wavelength of 1.9 μm, continuous radiation from the laser pump source for not a wave of 1.55 μm passes through the second input of the fiber spectrum-selective multiplexer, operating at a wavelength of 1.55 μm, thus, at the output of the fiber spectral-selective multiplexer, continuous pump radiation at a wavelength of 1.55 μm is propagated along with ultrashort pulses to wavelength of 1.9 μm, the output of the fiber spectral-selective multiplexer is connected to an active thulium fiber with normal group velocity dispersion at a wavelength of 1.9 μm, in which ultrashort pulses are amplified and simultaneously increased in duration, then the radiation enters the fiber with increased mode field diameter and anomalous group-velocity dispersion at a wavelength of 1.9 μm for pulse compression to minimum durations and maximum peak power values.

All fiber optic components have inputs and outputs made of quartz single mode fiber that meets the ITU-TG652.D standard.

A radiation polarization controller is used as a tunable element of the duration of ultrashort pulses. Adjusting the polarization of the radiation before the amplification process makes it possible to vary the duration in the range from tens of femtoseconds to a few picoseconds.

Thanks to the all-fiber construction, the device is compact and structurally located in a single housing.

List of figures

In fig. 1 shows the structure of a series circuit of elements of the proposed thulium all-fiber master oscillator of ultrashort pulses.

In fig. 2 shows the structure of the series circuit of the elements of the proposed thulium all-fiber power amplifier of the master oscillator of ultrashort pulses.

Implementation of the utility model

In fig. 1 shows: 1 - laser source of continuous radiation at a wavelength of 1.55 μm with a maximum power of 3 W; 2 - spectral-selective fiber optical radiation multiplexer (1.55 / 1.9); 3 - an active optical fiber doped with thulium ions with anomalous group-velocity dispersion at a wavelength of 1.9 μm; 4 - germanosilicate fiber with normal group-velocity dispersion at a wavelength of 1.9 μm; 5 - controller for polarization of optical radiation; 6 - fiber optic isolator-polarizer of optical radiation; 7 - fiber optic coupler of optical radiation; 8 - laser output connector with a beveled fiber end face at an angle of 8-9 degrees to the optical axis to minimize back reflections; 9 - single-walled carbon nanotubes located in an optical socket between two FC / APC connectors.

In fig. 2 shows: 10 - thulium all-fiber master generator of ultrashort pulses at a wavelength of 1.9 μm; 11 - fiber optic isolator at a wavelength of 1.9 microns; 12 - fiber optic coupler; 13 - control output of the laser; 4 - germanosilicate fiber with normal group-velocity dispersion at a wavelength of 1.9 μm; 5 - controller for polarization of optical radiation; 14 - laser source of continuous radiation at a wavelength of 1.55 μm with a maximum power of 6 W; 15 - fiber optic isolator at a wavelength of 1.55 microns; 2 - spectral-selective fiber optical radiation multiplexer (1.55 / 1.9); 16 - active optical fiber doped with thulium ions, with normal group-velocity dispersion at a wavelength of 1.9 μm; 17 - optical fiber with an increased diameter of the mode field and anomalous dispersion of group velocities at a wavelength of 1.9 μm; 18 - amplifier fiber output connector with beveled fiber end at an angle of 8-9 degrees to the optical axis to minimize back reflections. The proposed device works as follows.

An all-fiber ultrashort pulse thulium laser with hybrid mode locking with a pulse duration of 330 fs and a repetition rate of 24 MHz at a wavelength of 1.9 μm is used as a master oscillator of ultrashort pulses. (1, Fig. 1) at a wavelength of 1.55 µm with a maximum power of 3 W. Pump radiation is injected into the cavity using a spectrally selective fiber optical radiation multiplexer (2, Fig. 1) and enters an active aluminosilicate fiber (3, Fig. 1) doped with thulium ions, (0.8 wt% Tm, 3.6% (mass) Al) with a stepped refractive index profile (the difference between the refractive indices of the core and the cladding is 0.012) with a core diameter of 10 μm, a cladding diameter of 125 μm, group velocity dispersion at a wavelength of 1.9 μm - 70.56 ps ² / km length 1.25 m, 98% of the pump radiation is absorbed in the active fiber. On the other hand, the active fiber contains a germanosilicate fiber (4, Fig. 1) with a germanium oxide content in the core of ~ 30 mol. % with a core diameter of 2.2 µm with a refractive index difference of 0.0324 with the group velocity dispersion at a wavelength of 1.9 µm equal to 108 ps ² / km and a length of 3.22 m to adjust the group delay dispersion of the laser resonator. 81% of the radiation is extracted from the cavity using a fiber-optic alloy coupler (7, Fig. 1). For unidirectional lasing, an insulator-polarizer is used (6, Fig. 1), which is also an important component of the mode locking method based on nonlinear polarization evolution. Single-wall carbon nanotubes located in an optical socket between two FC / APC connectors (9, Fig. 1) are used as a slow saturable absorber for self-triggering of mode locking. Polarization controllers 5 are used to adjust the mode synchronization mode. All fiber-optic components have inputs and outputs made of a quartz single-mode fiber that meets the ITU-TG652.D standard, the total length of this type of fiber in the cavity is 4.02 m.

At a pump power of 800 mW, the radiation power of the master oscillator at the output of the fiber connector (8, Fig. 1) in the mode locking mode is 8 mW. The power spectral density at half maximum is 22 nm, the pulse duration at half maximum is 330 fs. The signal-to-noise ratio is 60 dB. The laser operates in the extended pulse generation mode.

The radiation power of the master oscillator is increased in an all-fiber amplifier, the schematic of which is shown in Fig. 2. At the output of the laser (10, Fig. 2), there is a fiber-optic isolator (11, Fig. 2), which is used to prevent the return of radiation from entering the laser. After the insulator, there is a fiber-optic coupler (12, Fig. 2), which diverts a part of the radiation (1-5%) to the receiver using the output (13, Fig. 2) to control the laser operating modes, since under external influences, the laser can switch to the mode of generation of continuous radiation or the mode of Q-switching. The rest of the radiation (90-95%, there is also a few percent losses in the coupler) passes through a germanosilicate fiber (4, Fig. 2), which was described above and is used in a laser with normal group-velocity dispersion at a wavelength of 1, 9 microns to increase the duration of ultrashort pulses. Pulse stretching is necessary to prevent nonlinear pulse decay during amplification. The length of the germanosilicate fiber is 3.085 m.The total length of the optical fibers of the optical components up to the germanosilicate is 2.015 m.Further, the radiation propagates in the 0.756 m long fiber, which is an input operating at a wavelength of 1.9 μm, into the spectral selective multiplexer (2, Fig. 2), this fiber is installed in a mechanical polarization controller (5, Fig. 2), which works by changing the birefringence in the fiber by squeezing and twisting it. The other input of the spectral-selective multiplexer, operating at a wavelength of 1.55 μm, receives continuous pump radiation from a laser source at a wavelength of 1.55 μm (14, Fig. 2), protected from backward radiation using an insulator (15, Fig. . 2). A spectral selective multiplexer combines continuous pump radiation and ultrashort pulse radiation and directs it into an active fiber (16, Fig. 2). To enhance the used active germanosilicate fiber doped with thulium (0.9 mol.% Tm ^3+, 36 mol.% GeO _2), with a stepped profile with a refractive index difference between core and cladding indices of refraction of 0.045, the core diameter of which is 2, 2 microns, shell diameter - 125 microns. The dispersion of the group velocities of the active fiber at a wavelength of 1.9 μm is 130.55 ps ² / km. The active fiber length is 2.123 m. The active fiber absorbs ~ 98% of the pump radiation. Ultrashort pulses, passing through an active fiber, are amplified and stretched in time. Then, to compress the pulses, the radiation enters a silica fiber with an increased mode field diameter (17, Fig. 2) with a stepped refractive index profile with a difference between the refractive indices of the core and the cladding 0.112, length 2.366 m, the core diameter of which is 20 μm, the cladding diameter is 125 μm, the dispersion of the group velocities of this fiber is anomalous at a wavelength of 1.9 μm and close to the value of silica glass. The increased mode field diameter makes it possible to increase the peak pulse power compared to standard telecommunication fibers. All fiber optic components have inputs and outputs made of quartz single mode fiber that meets the ITU-TG652.D standard.

The operating mode of the laser system is achieved when the pump radiation power of the laser source (14, Fig. 2) is equal to 4 W, while at the output of the amplifier (18, Fig. 2), which is made in the form of an angled FC / APC connector, the radiation power is 600 mW ... In the process of pulse propagation in all elements of the amplifier, the pulse acquires a nonlinear phase incursion, which leads to the fact that at the amplifier output the pulse turns into a group of pulses, inside which there is a pulse containing 60% of the energy, and a group of several pulses with the rest of the energy, while the total pulse energy is 25 nJ, and the energy contained in the main pulse is 15 nJ. The amplification and compression process strongly depends on the polarization of the radiation; this is due to the dependence of the nonlinear coefficient of optical fibers on the state of polarization of the radiation. Thus, by adjusting the polarization, it is possible to change the duration of the pulses at the amplifier output in the range from 60 to 1270 fs. In this case, the peak power reaches 250 kW at a pulse duration of 60 fs.

Lasers of this type can be used for: creating broadband laser radiation (supercontinuum) in the mid-IR range, scientific applications, precision material processing, surgery, gas detectors. The supercontinuum in the mid-IR range can be used for: precision spectroscopy, diagnostics of human diseases, detection of explosives in the air, optical coherence tomography.

Claims (1)

Thulium all-fiber power amplifier of an ultrashort pulse master oscillator, characterized by the presence of a fiber compressor with an increased mode field diameter, which makes it possible to reduce the pulse duration to tens of femtoseconds and increase the peak power to 250 kW; containing thulium all-fiber mode-locked master oscillator connected to a fiber optic isolator connected to a fiber optic coupler connected to a germanosilicate fiber with normal group velocity dispersion at a wavelength of 1.9 μm, the output of which is connected to the controller polarization of optical radiation, connected in turn with the signal input operating at a wavelength of 1.9 μm, a spectrally selective fiber multiplexer, the pumping input of which, operating at a wavelength of 1.55 μm, is connected to a fiber-optic isolator, which is connected to laser source of continuous radiation at a wavelength of 1.55 microns; the output of the spectrally selective fiber multiplexer containing radiation at wavelengths of 1.55 and 1.9 μm is connected to an active germanosilicate fiber doped with thulium ions with normal group velocity dispersion at a wavelength of 1.9 μm; the other end of the active fiber is connected to a fiber with an increased field diameter of a mode with anomalous group velocity dispersion at a wavelength of 1.9 μm for pulse compression; As a tunable element of the duration of ultrashort pulses, a radiation polarization controller is used, which makes it possible to vary the duration in the range from tens of femtoseconds to a few picoseconds.

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