Patent Application
20230327392
The invention refers to methods for the autogeneration of ultrashort laser pulses and a laser device based upon, in particular to pulsed fiber lasers with passive mode locking in a polarization-maintaining ring resonator which contains a non-linear optical element (NOE).
Ultrashort pulse lasers in the pico- and femto-range are widely used in various branches of technology to solve a variety of problems, amongst others in material processing for industrial applications and for surgical applications in medicine. Fiber lasers occupy a special place among them due to their low cost, simplicity of use and high degree of reliability. Single-mode quartz fiber light guides, in which effects related to Kerr nonlinearity take place, are often used to design pulsed ring lasers with passive mode locking.
In the patent RU2605639C1 (which is a Russian counterpart of the European patent EP3317930B1 belonging to the same applicant as the present application) a scheme of a passive mode-locked ring fiber laser based on the non-linear polarization evolution effect contains a polarization sensitive isolator, an active fiber pumped by a laser diode, and two fiber polarization controllers regulated by micro-controllers. The polarization controllers are the weak point in this scheme since their regulation range is limited and generation stability may be disrupted by both mechanical and temperature factors.
In patent US9276372B2, discrete optical elements are used in the ring resonator scheme. The scheme which is not completely fiber-based results in unnecessary optical loss when matched with the fiber-optic sections. A Faraday rotator in a non-reciprocal element (which the inventors called a “non-reciprocal phase shifter”) is an additional component which Verdet constant is dependent on temperature and therefore as we know shall be thermostatically controlled. The Verdet constant depends on a wavelength, so problems with generating the broad spectra inherent in ultrashort pulses may arise.
In: Boivinet S. et al. All-Fibre 1-\(\mu\) m PM Mode-Lock Laser Delivering Picosecond Pulses at Sub-MHz Repetition Rate //IEEE Photonics Technology Letters. - 2014. - T. 26. - №. 22. - C. 2256-2259, to generate ultrashort pulses, a non-linear optical element was formed as a segment of a polarization-maintaining fiber guide (PM-fiber), into which radiation is introduced at an angle to the slow axis by a polarizer and a Faraday mirror. The imperfections of this scheme, as in the previous example, lie in the use of a Faraday rotator with all its disadvantages.
In: Wang Y. et al. Cross-splicing method for compensating fiber birefringence in polarization-maintaining fiber ring laser mode locked by nonlinear polarization evolution //Applied optics. - 2016. - T. 55. - №. 21. - C. 5766-5770, For the first time, an approach was used when the linear birefringence of one fiber-optic guide was compensated by another same PM-fiber spliced (welded) at 90 degrees to the polarization axes, without using a Faraday mirror. However, it is difficult to obtain stable generation and pulse-repetition stability in a scheme like this because of the temperature instability of the PM-fiber sections and, consequently, the phase instability of the polarization components which is also conditioned upon the inhomogeneity of the fibers both in length and in cross section.
In: Szczepanek J. et al. Ultrafast laser mode-locked using nonlinear polarization evolution in polarization maintaining fibers //Optics letters. - 2017. - T. 42. - №. 3. - C. 575-57, an approach similar to the previous one was used, but there was a third segment of PM-fiber with an offset along the splice angle. Mode locking was achieved by careful alignment of the fiber lengths; however, pulsing stability for generation with this kind of ring laser scheme was not at the level required and more work on the scheme is needed, there is no pulsed lasing stability or thermal stability.
In: I. Bychkov, A. Baranov. Simulation of ultrashort pulsed lasing in a laser based on the effect of the non-linear evolution of polarization in two sections of polarization-maintaining optical fibres //Quantum Electronics.-2019 - T.49. - №8. - C.762-767 - the closest technical solution is described and a theoretical numerical simulation of the previous case is run, but the third segment of PM-fibre selected was replaced by a model reconfigurable phase element. Areas of stable operation are established in a combined non-linear optical element consisting of two segments of polarization-maintaining fibres L1 and L2, equal in length and spliced to each other with the orthogonal orientation of the polarization axes and having a spliced joint with a fibre resonator at input and output mismatched according to the angle of orientation of the polarization axes at an acute angle of less than 40 degrees. However, in the study there is no impact of external medium/temperature parameters on the stability of the phase characteristics of the non-linear optical element which in practice result in the breakdown of pulsed lasing.
This invention puts forward a method and a device for the stable autogeneration of ultrashort laser pulses free from the above disadvantages, in particular because of temperature instability.
The invention puts forward a method and a device (laser) for generating ultrashort laser pulses with passive mode locking based on the well-known effect of non-linear birefringence in the polarization-maintaining fibre-optic guide - in the PM-fibre - of a ring resonator in which they form unidirectional polarized radiation. It is amplified on the active fibre section of the optical amplifier, filtered in a spectral band of several nanometres after removing part of the amplified radiation from the resonator. The presence of a non-linear optical element (NOE) at the polarizer input leads to excitation of pulsed lasing in the ring resonator in dissipative soliton mode, at which non-linear birefringence is achieved and therefore the non-linear evolution of polarization also. The NOE consists of two segments of optical fibre L1 and L2, equal in length and joined to each other by a splice with the orthogonal orientation of the polarization axes and having spliced joints with the external fibre/loop of the ring resonator at input and output mismatched according to the angle of orientation of the polarization axes at an acute angle of less than 40 degrees, while at the input of the non-linear optical element band-pass optical filtering is done and, at its output, the polarization of the output radiation along its slow polarization axis.
In contrast to well-known schematics for the construction of pulsed lasers like this, the proposed method and device for exciting autogeneration of ultrashort laser pulses differs in its increased generation stability, thermal stability, the possibility of self-excitation and transition to pulsed autogeneration mode at start-up. This is the result of the use of two thermal compensation loops by means of special placing together of the asymmetric parts of fibre segments L1 and L2, which form the NOE. Each loop is thermostatically controlled and their thermal stabilization is done by an electronic controller at two different temperatures T1 and T2, respectively. The choice of temperature for these two loops ensures the optimal ratio of the linear and non-linear parts of the phase difference between the polarization components of the optical wave at the NOE output, at which there is the required thermal stability and stability of the ultrashort pulsing autogeneration mode with self-excitation at start-up each time the laser is switched on, which is the technical result.
Other advantages and distinctive features of the proposed invention will become obvious from the following detailed description of its embodiment, quoted with references to the accompanying drawing and a brief description.
The
1 - optical fibre ring resonator,
2 - polarization-maintaining fibre-optic guide (PM-fibre),
3 - optical fibre polarizer (OP),
4 - optical amplifier (OA),
5 - optical isolator (ISO),
6 - PM-fibre optical coupler (PM CP),
7 - band-pass optical filter (BOF),
8 - segment AB of NOE PM-fibre (segment L1),
9 - segment BC of NOE PM-fibre (segment L2),
10 - regulation controller (CONT) based on a microcontroller,
11 - first loop of thermostat with temperature T1,
12 - second loop of thermostat with temperature T2,
13 - optical spectrum analyser (OSA) for analysing output radiation.
A - NOE input connection with resonator’s PM-fibre (the angle mismatch of the polarization axes is indicated on the extended dotted line: 30°),
B - internal orthogonal connection by PM-fibre in the NOE,
C - NOE output connection with resonator’s PM-fibre (the angle mismatch of the polarization axes is indicated on the extended dotted line: -30°).
The
In known previous similar ring resonator schemes insufficient attention was paid to the method of placing the NOE fibre. But this factor is of decisive importance and noticeably affects the stability of the pulsed lasing mode during self-excitation of the resonator.
In optical amplifier 4, an optical fibre doped with ytterbium ions is usually used as an active medium if obtaining radiation amplification near the 1.0 µm wavelength is required. Ions of other rare-earth elements are used for amplification in other optical ranges - Er, Ho, Tm, etc.
To ensure unidirectional generation in optical scheme 1, an optical isolator 5 is used. A fibre coupler 6 is used to deliver the useful part of the power from the resonator to the consumer, as well as for analysis of spectral characteristics or to further compress the pulse and obtain ultrashort femtosecond pulses. In the invention, the NOE is formed by two pieces of PM-fibre with lengths L1 and L2, the polarizer 3 is connected to its output at point C. At the same time, optical fibre segments L1 and L2 are aligned precisely to within the beat length (about 2 mm) of the polarization-maintaining fibre-optic guide. It is known that PM-fibres have a beat length dependent on the temperature. For example, when one metre of Nufern PM980-XP fibre is heated by one degree Celsius, the phase difference change will be approximately 3.5 rad. Therefore, by varying the temperatures of the fibres, it is possible to change the phase shift ΔφL between the radiation propagating in the fast and slow axes of the PM-fibre. For example, for a 10 cm long segment, the change in the linear component of the phase difference ΔφL = π will already occur when heated by 10 degrees, and correspondingly with temperature instability within 0.1 degrees- the instability of the linear part of the phase difference will be 0.01π radians, or 1.8 angular degrees.
NOE transmission Tr with a polarizer 3 at the output, which blocks radiation along the fast axis of the fibre, is described by the claim (on condition that at input most of the linearly polarized radiation propagates along the slow axis):
where Θ is the angle of radiation input into fibre L1; ΔφL, ΔφNL respectively are the linear and non-linear phase shifts between the two polarization modes. ΔφL is caused by the different lengths L1 and L2 and different linear birefringence of these fibres (for example, as a result of their different temperatures). ΔφNL - is caused by the effects of self-phase modulation and phase cross-modulation between the two polarization modes. It is assumed that the angle between the axis of the L2 fibre and the PM-fibre to the polarizer is negative, opposite the angle in use in the NOE and is equal to: -Θ. At equal Θ angles to the NOE input and output, the linear component of the phase difference will shift by π radians. Thus, by choosing thermostat temperatures T1 and T2, it is possible to create a big loss for continuous radiation in the resonator when there is no pulse (i.e. create low transmission Tr right down to zero, since for continuous low power radiation ΔφNL = 0). It will be recalled that, for self-excitation of the pulsed lasing mode, the transmission function Tr of the non-linear element (after the polarizer) must have a non-zero value and a non-zero positive derivative at low input powers. In this case, a small increase in the input radiation power will lead to an increase in the transmission Tr by means of an additional non-linear phase incursion ΔφNL. Both the mathematical modelling and the experiments show that for the excitation of stable pulsed lasing, it is optimal to have an initial phase difference of about ΔφL = 0.1π. In the case of the optical scheme proposed, this can easily be achieved by selecting and stabilizing the temperatures T1 and T2 of the two NOE fibre loops using two thermostats.
Thus, the invention puts forward a method and device for generating ultrashort laser pulses with passive mode locking, based on the well-known effect of non-linear birefringence in the polarization-maintaining fibre-optic guide (in the PM-fibre) of a ring resonator 1 (the
The proposed method and device differ from known solutions in that self-excitation and stable autogeneration of pulsed radiation in the PM-fibre 2 ring resonator 1 are provided by optimal temperature matching and stabilization of the phase shift of the polarization components of the optical wave, its linear and non-linear phase components at the NOE output, creating in particular:
Notably, the NOE fibre input and output are spliced (welded) into the ring resonator at equal values, regardless of the sign, of the angular displacements of the polarization axes with respect to the resonator fibre in the range from 10 to 40 angular degrees, optimally at an angle of 30 angular degrees.
Notably, the phase difference of the polarization components of optical waves along the two axes of polarization at the NOE output, where there is no non-linear evolution of polarization, ΔφL is fixed by the temperature selection of the two NOE loops and lies within 15-25 angular degrees, being optimally equal to 20 angular degrees.
The proposed method for autogeneration of ultrashort pulses can be done in a device such as an ultrashort pulse laser, in the housing (in the
The laser device proposed in the invention goes into self-excitation and stable autogeneration in accordance with the method described above when switched on. The device differs from known ones in that self-excitation and stable autogeneration of pulsed radiation in the ring resonator 1 of the PM-fibre 2 laser are provided by optimal temperature matching and stabilization of the phase shift of the polarization components of the optical wave, its linear and non-linear phase components at the NOE output, achieved by means of two thermostats 11 and 12, respectively, regulated by an electronic controller 10, ensuring in particular:
Notably, as an option, the optical fibre polarizer is located in a thermostatically controlled loop together with the NOE output connection, or in a separate third thermostat at a fixed temperature T3.
Notably, the electronic controller 10 ensures the regulation and stabilization of thermostat temperatures T1 and T2 by means of electrical instrumentation circuitry accurate to no less than 0.5° C.
Notably, after partial removal of the radiation to outside the resonator via the fibre coupler 6, the duration of the laser picosecond pulse can be about 5-15 ps with an energy of up to 5 nJ at a repetition rate of 5-25 MHz.
Notably, after removal of the radiation to outside the resonator via the fibre coupler 6, the duration of the laser picosecond pulse with a radiation spectrum broadened by 15-30 nm at a central wavelength of about 1030 nm and a normal (positive) linear chirp, may be further reduced to the femtosecond range by a device with anomalous dispersion of group velocities (for example, on a pair of diffraction gratings).
For the embodiment of the invention, a device was assembled according to the
Passive mode locking at start up and transition to a stable autogeneration mode was done by selecting the level of optical amplification and the temperatures: T1 and T2 of the two NOE loops, in which the asymmetry along the length of the configured fibre segments L1 and L2 was about 10 cm with a total length of 5.5 m. Temperatures T1 and T2 can differ by 15° C. from one manufactured device to another.
The device ensured stable operation after tuning regulated by a microcontroller at the selected loop temperatures: T1 = 40° C., T2 = 45° C. The repetition rate of ultrashort pulses with a duration of about 10 ps - 13.5 MHz, with a 3.5 nJ energy per pulse at 2 W pump power in an optical amplifier, the spectral width at a central wavelength of 1030 nm was about 21.6 nm. In this case, the pulse had a normal (positive) linear chirp and was additionally compressed on a pair of diffraction gratings to a value of 150 fs.
The invention can be used in the laser industry, medicine, spectroscopy, optical communication lines, as well as for solving various fundamental and applied problems. In all areas where increased requirements are imposed on stable generation of ultrashort pulses in the femtosecond range with a broad spectrum of laser radiation at a given width and power.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020125448 | Jul 2020 | RU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/056554 | 7/20/2021 | WO |
