RU2801363C1 - Generation of ultrashort pulses in submicron region of the spectrum on neodymium fibre in all-fibre circuit - Google Patents

Abstract

FIELD: computer technology.

SUBSTANCE: method for generating ultrashort pulses with neodymium fibre in the submicron range in an all-fibre scheme consists in diode pumping a 1.3-meter polarization-maintaining single-mode fibre doped with neodymium with core absorption through a spectral compressor with a wavelength separation of 800 nm/920 nm. On one side, the resonator is terminated with a chirped fibre Bragg grating (CFBG) with group delay dispersion used to compensate for high normal dispersion of the fibres inside the resonator. The CFBG serves as a bandpass optical filter with a centre wavelength of 920 nm and a Gaussian profile to detect the laser wavelength. To obtain mode locking at the connector tip, a saturable semiconductor mirror absorber (SESAM) is installed at the other end of the linear resonator. Linear polarization is provided by a polarizer inside the circuit.

EFFECT: generation of ultrashort pulses with neodymium fibre in the submicron range in an all-fibre scheme.

2 cl, 11 dwg

Description

FIELD OF TECHNOLOGY

The present technical solution relates to the field of computer technology, in particular, to methods for generating ultrashort pulses with neodymium fiber in the submicron range in an all-fiber circuit.

BACKGROUND OF THE INVENTION

The prior art solution, selected as the closest analogue, WO2009076967(A1), publ. 06/25/2009 This solution relates to a mode-locked fiber laser system made with a generation passband and having a linear resonator, moreover, said resonator contains an amplifying medium, a saturable absorber having a saturation power, and a filter having a spectral response, while the specified mode-locked fiber laser. The system is designed in such a way that it is possible to obtain mode-locked operation together with continuous wave (CW) at an energy density less than three times the saturation energy density of the saturable absorber.

The proposed technical solution is aimed at eliminating the shortcomings of the state of the art and differs from the known solutions in that the proposed method provides the generation of ultrashort pulses with neodymium fiber in the submicron range in an all-fiber circuit.

SUMMARY OF THE INVENTION

The technical problem to be solved by the claimed solution is the creation of a method for generating ultrashort pulses with neodymium fiber in the submicron range in an all-fiber scheme.

EFFECT: generation of ultrashort pulses with neodymium fiber in the submicron range in an all-fiber circuit.

The claimed result is achieved through the implementation of a method for generating ultrashort pulses with neodymium fiber in the submicron range in an all-fiber scheme, containing the steps in which:

a 1.3-meter single-mode PM (polarization-maintaining single-mode fiber) neodymium-doped fiber with core absorption is pumped by a diode through an 800 nm / 920 nm wavelength division multiplexer (WDM), with the resonator terminated on one side with a chirped Bragg fiber grating (CFBG) with a group delay dispersion used to compensate for the large normal dispersion of the fibers inside the resonator, while the CFBG serves as a band-pass optical filter with a center wavelength of 920 nm and a Gaussian profile, determining the wavelength of laser radiation, while obtaining mode locking on A saturable semiconductor mirror absorber (SESAM) is mounted on the connector tip at the other end of the linear resonator, with linear polarization provided by a polarizer inside the circuit.

In a particular embodiment of the described method, an additional spectral filter is implemented to filter the radiation at 1064 nm and transmit in the region below a micron.

DESCRIPTION OF THE DRAWINGS

The implementation of the invention will be described hereinafter in accordance with the accompanying drawing, which is presented to explain the essence of the invention and does not limit the scope of the invention in any way. The following drawings are attached to the application:

Fig. 1 - Linear scheme of a mode-locked fiber laser.

Fig. 2 - Mode locking threshold and laser efficiency.

Fig. 3 - Spectrum of the laser output pulse at 920 nm, no signs of amplified spontaneous emission (ASE) at 1064 nm.

Fig. 4 - Map of the obtained lasing regimes for various total dispersions and output powers.

Fig. 5 - Harmonic mode locking of the 1st, 6th and 12th order with almost zero total dispersion.

Fig. 6 - Spectra of output pulses for different total dispersion of the resonator.

Fig. 7 - Pulse energy (brown) and pulse width (green) at different total dispersion. For guidance, lines connecting points are presented.

Fig. 8 - Experimental spectrum of pulses for different pump powers with anomalous total dispersion.

Fig. 9 - Calculated spectrum of pulses for different pump powers with anomalous total dispersion.

Fig. 10 - Comparison of the experimental and calculated spectral and temporal pulse widths.

Fig. 11 - Dynamics of pulse energy, spectral and time width inside the laser cavity.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the implementation of the invention, numerous implementation details are provided to provide a clear understanding of the present invention. However, one skilled in the art will appreciate how the present invention can be used, both with and without these implementation details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to unnecessarily obscure the features of the present invention.

Furthermore, it will be clear from the foregoing that the invention is not limited to the present implementation. Numerous possible modifications, changes, variations and substitutions that retain the spirit and form of the present invention will be apparent to those skilled in the subject area.

The present technical solution is based on a comprehensive study of the generation of ultrashort pulses in a polarization-maintaining, all-fiber, neodymium-doped, dispersion-compensated laser at a wavelength of 920 nm. A chirped fiber Bereg grating was used in the line laser circuit to compensate for the large normal dispersion of the fibers. It is also used as a spectral filter for laser wavelength selection and as an output translucent mirror with 25% reflectance. A semiconductor saturable absorption mirror (SESAM) is used to obtain mode locking and as a second blind mirror. Complete filtering of 1064 nm radiation is provided by a 920/1064 wavelength splitter. Self-triggered generation of ultrashort pulses in the range of total dispersion from positive 0.24 ps ² to negative 0.05 ps ² was studied numerically and experimentally by changing the cavity length. In addition, dispersion control makes it possible to generate harmonic mode locking up to the 12th order at a sufficiently close to zero total dispersion and for intracavity powers limited by the destruction threshold of SESAM. Numerical simulation was used to optimize the laser resonator circuit and study the intracavity pulse dynamics.

The present technical solution for the first time discloses the generation of ultrashort dispersion-controlled pulses in a neodymium-doped all-fiber laser at a wavelength of 920 nm, containing a chirped fiber Coast grating (FBG) for dispersion compensation and SESAM in a linear resonator circuit. In addition, dispersion control achieved harmonic mode locking up to the 12th order with a pulse repetition rate of 0.43 GHz with a fairly close to zero total dispersion for intracavity powers below the SESAM degradation threshold.

Fig. 1 - Linear scheme of a mode-locked fiber laser: FBG - fiber Bragg grating, WDM - spectral multiplexer, LD - laser diode, SESAM - semiconductor mirror with saturable absorber.

Fig. 2 - Mode locking threshold and laser efficiency.

Fig. 3 - Spectrum of the laser output pulse at 920 nm, no signs of amplified spontaneous emission (ASE) at 1064 nm (Experimental and numerical spectra of pulses with Gaussian approximation with anomalous total dispersion).

The line laser circuitry used to achieve mode locking is shown in Figure 1. Here, an 808 nm diode (LU0808M250 Lumics) pumps a 1.3 m neodymium-doped single-mode PM fiber (CorActive ND 103-PM) with a core absorption of ≈40 dB/m at 808 nm through a 800 nm/920 nm wavelength division multiplexer (WDM). On one side, the resonator ends with a chirped fiber Bragg grating (CFBG, TeraXion) with a group delay dispersion of 0.19 ps ² used to compensate for the large normal dispersion of the fibers inside the resonator. It provides 25% reflection, and sends 75% to the laser output. The CFBG also serves as a bandpass optical filter with a center wavelength of 920 nm and a Gaussian profile with a bandwidth of 10 nm at 3 dB (see FIG. 1), thus determining the wavelength of the laser light. To obtain mode locking, SESAM (BATOP GmbH) was used, mounted on the connector tip at the other end of the linear resonator. The SESAM is manufactured at 920 nm with a modulation depth of 15%, a relaxation time of 0.5 ps, and a saturation energy density of 30 µJ/cm2. more efficient amplified spontaneous emission of Nd fiber at 1064 nm. The total length of the resonator with a full bypass in the case of the shortest laser scheme is 4.0 m, which, in combination with CFBG, gives -0.05 ps ² of the total anomalous dispersion of the resonator. Further dispersion adjustment was carried out by changing the length of the normal dispersion passive fiber β 2 = +35 ps ² /km between SESAM and 800/920 WDM. Stable self-triggering pulse generation is achieved starting from a pump power of 100 mW, with an efficiency of 9%, while at lower pump powers the laser produces continuous wave (CW) radiation with somewhat lower efficiency, which can be explained by the higher losses of SESAM in CW than in mode locking mode (see Fig. 2). The pulse spectrum is shown on a large scale in Fig. 2 without any evidence of neodymium fiber ASE at 1064 nm. The spectrum of the emitted pulse with anomalous total dispersion has a Gaussian profile (see Fig. 3), which is confirmed by numerical simulation, which will be discussed later).

Pulse generation characterization is performed using the following instruments: Yokogawa AQ6373 OSA, ThorLabs InGaAs DET08CFC photodetector and Tektronix MDO3102 1 GHz oscilloscope, Femtochrome FR-103WS autocorrelator, ThorLabs S132C photodiode power meter.

Fig. 4 - Map of the obtained lasing regimes for various total dispersions and output powers.

Fig. 5 - Harmonic mode locking of the 1st, 6th and 12th order with almost zero total dispersion.

When the total dispersion approaches zero, a relatively small change in it can lead to a significant change in the pulse generation mode. In this experiment, the total dispersion of the resonator was varied within -0.05 ps ² ÷ 0.24 ps ² and the modes of pulse generation were analyzed on the map depending on the intracavity power and the corresponding output power, which is shown in Fig. 4. Intracavity power is limited by the SESAM degradation threshold of 10mW average power, so the output is limited to 7mW. Approaching zero total dispersion from the negative side, the pulse generation threshold and the soliton splitting threshold decrease in accordance with the area theorem. This leads to a decrease in the maximum achievable pulse energy. At zero total dispersion, no pulse generation is observed. Typically, substantially more modulation depth is required to achieve stable mode locking. According to our numerical simulations, it is necessary to have 30% of the modulation depth, but in the experiment it is 15%. With a slightly positive dispersion of about +0.006 ps ^{2 ,} the system tends to generate in harmonic mode locking with minimum pulse energy. We were able to achieve the 12th harmonic with a repetition rate of 0.43 GHz, limited by the SESAM damage threshold, see FIG. 5. Further increase towards positive total dispersion expands the intracavity power range supporting single pulse generation. At the same time, the transition from single pulses to the second harmonic occurs through the Q-switched (QML) mode. For long cavities, starting from a total dispersion of 0.15 ps ² , the pulses exhibit a behavior more typical of dissipative solitons, with a flat top of the spectrum, highly chirped pulses, and a large single pulse power range. With a dispersion of more than 0.24 ^ps2, it was not possible to observe mode locking at the admissible maximum intracavity laser power.

Spectral measurements show sharp changes in the pulse spectra for various total cavity dispersions (see Fig. 6). For long cavities, the total normal dispersion of the fiber is several times the anomalous dispersion of the FBG (CFBG), so the resulting total dispersion remains highly positive. The generated pulse leaves the resonator, propagating through the CFBG without dispersion compensation at the end of a complete round trip, and its spectrum acquires a shape characteristic of a dissipative soliton. Increasing the resonator length narrows the pulse spectrum. For short resonators, with anomalous total dispersion, the shape of the output pulse spectrum is closer to Gaussian (see Fig. 2), the intracavity dynamics of which will be discussed below). All of the listed characteristics of the pulse are in good agreement with the behavior of a dispersion-controlled soliton, except for the largest case of normal dispersion 0.15 ÷ 0.24 ps ² , where dissipative solitons are observed.

The pulse energy and pulse width as a function of the total resonator dispersion are presented in Figure 7. As previously described, the pulse energy of 12 pJ is at its lowest near zero dispersion and increases with distance from this point. For the largest normal and anomalous dispersion, the pulse energy is limited by the SESAM destruction threshold (see Fig. 7, black dotted line). On the contrary, the pulse width shows a monotonic increase from negative to positive dispersion (see Fig. 7, green curve). Propagating through optical fibers with normal dispersion, the pulse accumulates a positive chirp. At the output of the laser, CFBG does not affect the pulse chirp when passing through it, and upon reflection, CFBG compensates for dispersion and can even change the sign of the pulse chirp. Thus, the output pulse always has a positive chirp, which is proportional to the pulse duration and gets larger as the resonator length increases. The resulting pulses reach a width of 1.8 ps with the shortest resonator circuit and exceed 50 ps with a bypass length of 11.6 m.

Investigation of the pulse dynamics and its intracavity behavior for the most important case of total dispersion -0.05 ps ² , when the shortest pulse and the highest peak power are observed. For this, a numerical calculation was carried out using the Ginzburg-Landau equation for the complex amplitude based on the Fourier splitting method for physical factors (Split Step Fourier Method). The dispersion and Kerr nonlinearity of all fibers are taken to be +35 ps 2 /km and 0.01 W 1 m -1, respectively. The SESAM saturable absorber is modeled using standard rate equations. Propagation through the CFBG is calculated by multiplying the complex electric field by the reflectance spectrum and the dispersion coefficient in frequency space. The radiation amplification in neodymium-doped fiber is modeled by the Ginzburg-Landau equation with saturable gain with a 50 nm parabolic gain band peaking at 905 nm and a saturation power of 20 mW, which is estimated from the ASE of the Nd fiber in the 900 nm range.

In FIG. Figures 8 and 9 show the experimental pulse spectra and the corresponding calculations for various pump powers, which show good agreement. The spectral and temporal width of the pulses increases with increasing power, as shown in FIG. 10.

Fig. 11. Dynamics of pulse energy, spectral and time width inside the laser cavity.

Calculations show that the pulse duration along the resonator is constantly changing, see Fig. 11. After reflection from the CFBG with anomalous dispersion, the pulse changes the sign of the chirp from positive to negative, and then, propagating through the fiber with positive dispersion, changes sign back again. At this moment, the pulse reaches the minimum width (lower panel) inside the resonator ~600 fs, as well as the minimum width of the spectrum (upper panel), which also breathes in the resonator. This behavior is typical for a dispersion-controlled soliton that changes sign twice. The corresponding pulse energy is shown in the top panel as a black line. It can be seen that the pulse exits the laser near the maximum energy. A slight increase in output energy is possible if the 920/1060 WDM filter is moved to a different position, but this will result in significant ASE emission at 1060 nm. There are limits to the SESAM damage threshold for maximum output power, and this can be significantly increased by further amplification with an external amplifier or by implementing a saturable absorber that does not undergo thermal degradation.

Thus, the proposed technical solution provides the generation of dispersion-controlled ultrashort pulses at a wavelength of 920 nm in an all-fiber laser doped with neodymium and maintaining polarization, due to the developed circuit of a linear laser with a fiber chirped Bragg grating as a semitransparent output mirror and SESAM as a second deaf mirror. Dispersion compensation with a chirped fiber Bragg grating made it possible to demonstrate different pulse modes depending on the total dispersion. The solution allows you to receive pulses with an energy of 132 pJ and a duration of 2 ps at a repetition rate of 51 MHz with anomalous total dispersion, as well as pulses with an energy of 14 pJ and with a duration of 5.5 ps at a repetition rate of 430 MHz in the mode of harmonic mode locking with total dispersion close to zero. The pulse energy and maximum repetition rate in harmonic mode locking are limited by the SESAM damage threshold and can be significantly increased by integrating a thermally stable saturable absorber.

This technical solution contains the following significant differences from the prior art.

This technical solution uses a filter element that allows you to remove generation at a wavelength of 1064, thereby increasing the gain efficiency by 920 (as well as an unnecessary glow from the output radiation by 1064).

The pumping of the proposed laser, unlike the known ones, is directed not towards SESAM, but towards CFBG, which is transparent to the remaining pumping, allowing it to go outside (in the proposed technical solution, all elements (including fiber) retain polarization).

All elements in this technical solution are fiber.

Dispersion compensation in the present technical solution is carried out by a chirped fiber grating.

In these application materials, a preferred disclosure of the implementation of the claimed technical solution was presented, which should not be used as limiting other, private embodiments of its implementation, which do not go beyond the scope of the requested legal protection and are obvious to specialists in the relevant field of technology.

Claims (2)

1. A method for generating ultrashort pulses with neodymium fiber in the submicron range in an all-fiber scheme, comprising the steps of: pumping a 1.3-meter single-mode PM fiber doped with neodymium with core absorption through a spectral compressor with a wavelength division of 800 nm by means of a diode /920 nm (WDM), and on one side the resonator ends with a chirped fiber Bragg grating (CFBG) with a group delay dispersion used to compensate for the large normal dispersion of the fibers inside the resonator, while the CFBG serves as a bandpass optical filter with a center wavelength of 920 nm and a profile Gauss, by determining the wavelength of laser radiation, while in order to obtain mode locking at the tip of the connector, a saturable absorber based on a semiconductor mirror (SESAM) is installed at the other end of the linear resonator, with linear polarization provided by a polarizer inside the circuit. 2. The method according to claim 1, in which an additional spectral filter is implemented to filter radiation at 1064 nm and transmit in the region below a micron.