Mamychev laser oscillator for generating ultrashort pulses and device for starting the same

Description

TECHNICAL FIELD

[0001] The invention relates to the field of laser systems delivering ultra-short pulses, and more particularly to passive mode-locking fiber oscillators (MLs) of the "Mamyshev" type. These are particularly attractive because they offer remarkable performance (energies of several tens of nJ, extremely short pulse durations <50 fs) while being simple to produce (few components).

[0002] However, a drawback of these systems is that it is difficult or even impossible to start them. The proposed invention overcomes this problem by providing a simple and robust starting system for a passive mode-locked fiber oscillator of the "Mamyshev" type.

[0003] Such an oscillator is based on the concatenation of two Mamyshev regenerators.

[0004] The operating principle of a Mamyshev regenerator is as follows: a pulse propagating in an optical fiber is subject to the phenomenon of self-phase modulation (SPM), inducing spectral broadening. A spectral bandpass filter, with a central wavelength sufficiently shifted relative to the wavelength of the initial pulse, then allows only the part of the spectrum generated by SPM to pass through. Thus, the mechanism acts as a saturable absorber. Only pulses intense enough to produce the sufficient spectral shift can be transmitted. Insufficiently intense pulses are blocked.

[0005] The Mamyshev oscillator implements this principle. As illustrated in the Figure 1(based on the article by E. Poeydebat et al. entitled “All-fiber Mamyshev oscillator with high average power and harmonic mode-locking”, Vol. 45, No. 6 / 15 March 2020 / Optics Letters, p. 1395-1398), the laser cavity of a Mamyshev oscillator 10 in ring comprises two amplifiers 2, 4, an output coupler 8 and two bandpass filters 6, 12 between the amplifiers. By sufficiently offsetting the filters, only pulses intense enough to cross the gap between the two filters by SPM can propagate in the cavity. Designed to operate in a highly nonlinear regime, such a system thus provides access to extremely high intensity levels, as explained in particular in the article by E. Poeydebat et al. cited above. Acting as an ultra-fast saturable absorber, the mechanism can give rise to ultra-short pulses, less than 50 fs.

[0006] The modulation depth of the virtual saturable absorber, and consequently the operating regime and startability of a Mamyshev oscillator, strongly depend on the spectral separation between the filters (typically Gaussian or close to Gaussian shapes) relative to their spectral width. When the spectral separation between filters is equivalent to their half-maximum widths, a strong overlap of the transmitted spectra is obtained, which corresponds to a low modulation depth. The systems can then be self-starting, but they deliver strongly modulated spectra of small spectral widths (only a few nm or less, see in particular the article by N. Tarasov et al., "Mode-locking via dissipative Faraday instability, Nat. Commun. 7, 12441 (2016)).

[0007] When the spectral gap between the filters is slightly larger, roughly equal to their width at the base, typically twice the width at half-maximum, the spectral overlap at the base of the filters is still sufficient for CW lasing to be possible and for the system to start up on its own. The generation of fluctuations by modulation of the amplifier pump diodes (typically a few tens of kHz for a few tens of microseconds) helps to start the system, but the gap between the filters is then fixed by the CW lasing threshold, which limits the modulation depth and therefore the system performance (spectral width, mode-locking instabilities).

[0008] When the separation between the filters is much greater (typically several times the width of the filters), there is no longer any spectral overlap of the filters and very high modulation depths are obtained. It is in this case that extremely broad spectra (up to ~100 nm), corresponding to ultra-short pulses (< 50 fs) are obtained. On the other hand, the spectral gap is then such that CW lasing becomes strictly impossible and the system can no longer start from noise fluctuations alone or even by modulating the pump diodes. It is then essential to resort to additional means to start it.In this case, the most commonly used solutions for startup are either to inject a sufficiently intense initial pulse from an external source, or to add an auxiliary arm, specific to startup and which "bypasses" the first filter, including a saturable absorber or a mechanism such as non-linear rotation of the polarization. Once startup is complete and the ML regime is reached, the startup source can be stopped. The interest of these solutions is moderate because they require the use of a secondary laser source delivering short pulses (either an external laser or an auxiliary arm), considerably limiting the practical interest of such systems.

[0009] A more satisfactory solution that does not require an external source while offering the advantage of providing a significant modulation depth is for one of the two spectral filters to be a tunable spectral filter. Thus, the filter can first be placed close to the other filter, so as to initiate CW lasing, then the pump diodes are modulated to generate instabilities and, finally, the filters are moved apart once mode locking is achieved. The main disadvantage of this solution is that mode locking can only be maintained by gradually adjusting the pump power at the same time as the filters are moved apart, which requires either a complex and random manual procedure or the use of expensive motorized components driven by a path-finding algorithm.

[0010] There is currently no system for obtaining ultra-short pulses controlled by the spacing between spectral filters in a simple, robust, reproducible and inexpensive manner.

[0011] US 2019 / 305516 describes a nested cavity multi-filter Mamyshev oscillator.

[0012] The invention solves this problem by proposing a Mamyshev oscillator capable of having a large modulation depth (spectral gap between the filters of several times the width of the filters), which can be started without a secondary source of short pulses (external laser or auxiliary arm), simple to implement (without a complex start-up procedure), robust (fully fibered) and inexpensive (no tunable spectral filters). STATEMENT OF THE INVENTION

[0013] The invention firstly relates to a laser device comprising: a first cavity forming a Mamyshev oscillator comprising a first bandpass filter at a first wavelength (λ 1 ), and a second filter which also transmits the wavelength (λ 1 ), but which is in reflection at a second wavelength (λ 2 ), a second cavity, containing the first cavity, for forming a continuous laser beam at the first wavelength (λ 1 ) and / or at a third wavelength (λ 3 ) close to the first wavelength (λ 1 ), the difference between λ 3 and λ 1 being less than the spectral width of the first filter and of the second filter, means for allowing or interrupting a continuous oscillation at the wavelength (λ 1 ) or at said neighboring wavelength (λ 3 ), in the second cavity.

[0014] In other words, according to the invention, the laser device comprises a first cavity forming a Mamyshev oscillator comprising a first filter at wavelength (λ 1 ), a second filter, in reflection at wavelength (λ 2 ) but in transmission at wavelength (λ 1 ), and a second cavity, containing the first, to form a continuous laser beam at wavelength (λ 1 ) or at a wavelength (λ 3 ) close to λ 1 and means for allowing or interrupting an oscillation at wavelength (λ 1 ) or at the neighboring wavelength, in said second cavity.

[0015] According to the invention, a system is therefore produced comprising a first cavity and a second cavity, the first cavity being nested in the second cavity, the second cavity being for start-up and making it possible to obtain continuous lasing (CW), and the first cavity being for ML operation, the start-up of the ML laser being based on fluctuations in the lasing (CW); after start-up of the ML laser, continuous lasing (CW) is inhibited.

[0016] The invention therefore differs from the usual techniques used to start a Mamyshev oscillator with a large modulation depth (with no superposition of filters), since in general an external source of short pulses is used, an additional arm including a physical or virtual saturable absorber, or a motorized tunable filter.

[0017] The difference between the wavelengths λ 1 and λ 2 is, for example, between 5 nm and 25 nm. The second filter can be placed downstream of an optical circulator.

[0018] The second cavity can be delimited by a mirror, for example a fiber mirror or a fiber Bragg mirror.

[0019] The means for enabling or interrupting the oscillation of the wavelength (λ 1 ), in the second cavity, comprise for example an optical switch or a variable optical attenuator.

[0020] According to a particular embodiment, the second filter comprises a Bragg grating mirror.

[0021] The first filter may for example include a transmission filter or an optical circulator and a reflection filter.

[0022] In a laser device according to the invention, the first cavity may be a ring cavity or a linear cavity.

[0023] The invention also relates to a method of starting a laser device as described above or in the present description, comprising: the generation, in the second cavity, of a continuous laser beam at wavelength (λ 1 ); the generation, in the first cavity, of a pulsed laser beam at wavelength (λ 2 ) and at wavelength (λ 1 ), stopping the continuous laser beam.

[0024] According to a particular embodiment, the losses or the gain at the first wavelength (λ 1 ) can be modulated during the start-up period.

[0025] The first cavity comprises, for example, two fiber amplifiers, each pumped by a laser diode, the method further comprising modulation of the beam from these diodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] [ Fig. 1 ] represents a known Mamyshev oscillator; [ Fig. 2] represents an exemplary embodiment of a Mamyshev oscillator according to the invention; [ Fig. 3 ] represents a reflectivity curve of the fiber mirror as a function of the coupling rate of the fiber coupler; [ Fig. 4A ] [ Fig. 4B ] illustrate a use of a fiber Bragg mirror instead of a fiber mirror; [ Fig. 5 ] illustrates a use of a filter 1 in reflection instead of the first filter in transmission; [ Fig. 6 ] is an example of a device according to the invention using a linear cavity. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0027] There Figure 2 shows an example of an embodiment of a Mamyshev oscillator according to the invention.

[0028] In this example, a 1st cavity 20, here in a ring, of length for example less than 30 meters, comprises two amplifiers 22, 24, which can in particular be fiber amplifiers, for example doped with ytterbium (with a “small signal” gain which can be between 20 dB and 30 dB).

[0029] Examples of such amplifiers are given in the article by E. Poeydebat et al. entitled “All-fiber Mamyshev oscillator with high average power and harmonic mode-locking”, Vol. 45, No. 6 / 15 March 2020 / Optics Letters, pp. 1395-1398. Each amplifier includes pumping means, for example, a 976 nm diode.

[0030] The cavity 20 further comprises a first spectral filter 26, for example a Gaussian filter, which allows radiation to pass in a spectral band centered at a wavelength λ 1 , for example of width 1 nm. It can be centered in a band which corresponds to the emission of the amplifying fibers, for example the ytterbium band. It further comprises an output coupler 28, which makes it possible to extract the output beam from the cavity.

[0031] It may include an isolator 30, which ensures unidirectional propagation.

[0032] The system also comprises a second spectral filter 32, for example a Gaussian filter, centered at a wavelength λ 2 , for example of width 1 nm. It can be centered in a band which corresponds to the emission of the amplifying fibers, for example the ytterbium band. This filter 32 is preferably a reflection filter (for example of the Bragg grating mirror type): it reflects all radiation at the wavelength λ 2 and returns it to the 1st cavity 20. The first filter 26 does not allow any radiation, or very little radiation, to pass at the wavelength λ 2 .

[0033] This second filter 32 is arranged, relative to the ring, behind an optical circulator 34 and upstream of an optical switch 36 (such as for example an optical switch) making it possible to block the passage of the beam.

[0034] Finally, the cavity is completed by a mirror 38, for example a fiber mirror, for example again a 2 by 2 fiber coupler looped back on itself.

[0035] The difference between the central wavelengths λ 1 and λ 2 of the filters26, 32 is preferably greater than n times (for example: n = 4) the width of the individual filters, this difference is for example between 5 nm and 15 nm (or even 25 nm), which makes it possible to avoid a superposition of the optical spectra defined by these filters.

[0036] The transmission profile of a Bragg mirror being inverse to its reflection profile, the filter 32 lets through the non-reflected wavelengths, and in particular any radiation at the wavelength λ 1 .

[0037] Thus the device of the Figure 2 has two nested cavities: the first cavity is defined by the reflection on filter 32; it is equivalent to a Mamyshev type oscillator as illustrated in the Figure 1 ; the second cavity, which includes the first cavity but also a linear part, is defined by the reflection on the mirror 38, which reflects all the wavelengths reaching it, in particular the radiation at the wavelength λ 1 .

[0038] The operation of this system is as follows.

[0039] When the amplifiers 22, 24 are switched on and the optical switch 36 is closed (turned on), and the gain of the amplifiers is greater than the total losses in the second cavity (this latter condition is obtained by the oscillations in the cavity), the laser starts on the fluctuations of the noise in free multimode mode.

[0040] A continuous multimode beam, exhibiting intensity fluctuations, is emitted at a central wavelength λ 1 defined by filter 26. The high intensity peaks, sufficiently intense to cross, by self-phase modulation (SPM), the gap between filters 26 and 32, are then reflected by filter 32 and can oscillate in the first cavity.

[0041] A train of short pulses is output.

[0042] We therefore find the operation of a Mamyshev oscillator with a virtual saturable absorber of great modulation depth. Once the mode-locking operation ("ML" for Mode-Locking in English) is obtained, the optical switch 36 can then be opened (made non-conducting) in order to inhibit the CW operation of the second cavity. The laser then emits radiation which includes the wavelengths λ 1 and λ 2 .

[0043] In order to prevent the continuous wave (CW) lasing from being too dominant and inhibiting the possibility of obtaining ML lasing, the reflectivity of the fiber mirror 38 is preferably adjusted so that the two operations have average powers of the same order of magnitude (typically a few hundred mW in this example). This reflectivity of the fiber mirror 38 can be adjusted between 0% and 100% depending on the coupling rate of the fiber coupler used, as illustrated in Figure 3 The coupling ratio can be changed by changing the coupler.

[0044] It is also possible to replace the optical switch 36 with a variable optical attenuator, which not only allows fine adjustment of the power reflected by the fiber mirror 38 in continuous operation (CW) but also to inhibit CW lasing in ML operation (by attenuating in such a way that the total losses of the second cavity are greater than the gain of the amplifiers).

[0045] If the natural fluctuations of the CW laser are insufficient to trigger the ML operation, it is possible to modulate the pump diodes of the amplifiers 22, 24 to generate larger fluctuations. In one embodiment, a function generator is used to control the power supply of the pump diodes. For example, this is a generator from Keysight, for example the Keysight 33210A Waveform / Function Generator.

[0046] The example of Gaussian spectral profiles of filters 26, 32 was given above, but it is quite possible to use profiles of different shapes.

[0047] The fiber mirror 38 can be replaced by the Fresnel reflection of the cleavage of the output fiber of the optical switch 36: the cleavage angle determines the reflection coefficient but this is then limited to a maximum of 5%.

[0048] As illustrated in Figure 4A, the fibered mirror 38 can be replaced by a fibered Bragg mirror 38' (filter 3) centered at a wavelength λ 3 ( Figure 4B ): λ 3 is taken close to λ 1 , the difference between λ 3 and λ 1 being less than the spectral widths of each of the filters 26, 38', so that there is spectral superposition of the filter 26 and the filter 38', as illustrated in Figure 4B . CW lasing occurs at a wavelength defined by this spectral overlap, with the CW lasing power adjusted by the spacing between the 26 and 38' filters.

[0049] As illustrated in Figure 5 , the transmission filter 26 can be replaced by a 26' optical circulator and a 26" reflection filter.

[0050] According to another embodiment, illustrated in Figure 6, the cavity is only linear. In this case, a single amplifier 40, operating in both directions, is used. The filter 26 (for example a fiber Bragg mirror with reflectivity less than 100%) serves both as a spectral filter and as an output coupler.

[0051] In the examples presented, the system operates in the ytterbium spectral band (around 1 µm) but it can be adapted to other wavelength ranges (in particular the 1.5 µm telecom or 2-3 µm mid-infrared bands). The invention makes it possible to produce a simpler system than existing solutions, such as those implementing the injection of a short pulse from an external laser, or an auxiliary arm including a saturable absorber. It makes it possible to obtain a start of mode locking with a large modulation depth (even if there is no spectral superposition between the filters) without complex and expensive components such as a motorized tunable filter or a saturable absorber. In addition, there are no adjustable parameters, which gives the system great robustness.

[0052] The invention therefore provides a simple solution to the problem of starting Mamyshev oscillators with high modulation depth (with non-overlapping filters). The invention can be applied as a source of ultrashort pulses for a wide variety of applications such as spectroscopy, laser ablation of thin layers, micromachining / microcutting / microdrilling / micromarking, ophthalmic surgery, multiphoton imaging, laser structuring and texturing, etc. The proposed scheme, simple, compact, robust and inexpensive, is perfectly suited to implementation by a laser manufacturer.

Claims

1. Laser device comprising: - a first cavity (20) forming a Mamyshev oscillator, comprising a bandpass first filter (26) at a first wavelength (λ1), and a second filter (32) that also transmits the wavelength (λ1) but that is reflective at a second wavelength (λ2), - a second cavity, containing the first cavity, for forming a continuous-wave laser beam at the first wavelength (λ1) and / or at a third wavelength (λ3) neighbouring the first wavelength (λ1), the separation between λ3 and λ1 being smaller than the spectral width of the first filter (26) and of the second filter (32), - means (36) for allowing or interrupting a continuous-wave oscillation at the wavelength (λ1) or at said neighbouring wavelength, in the second cavity.

2. Laser device according to claim 1, the separation between the wavelengths λ1 and λ2 of the filters (24, 32) being between 5 nm and 25 nm.

3. Laser device according to one of claims 1 or 2, the second filter (36) being disposed downstream of an optical circulator (34).

4. Laser device according to one of claims 1 to 3, the second cavity being delimited by a mirror (38), for example a fibre mirror or by a fibre Bragg mirror (38').

5. Laser device according to one of claims 1 to 4, the means for allowing or interrupting the oscillation of the wavelength (λ1), in the second cavity, comprising an optical switch (36) or a variable optical attenuator.

6. Laser device according to one of claims 1 to 5, the second filter (36) comprising a Bragg grating mirror.

7. Laser device according to one of claims 1 to 6, said 1st filter comprising a transmission filter or an optical circulator (26') and a reflection filter (26").

8. Laser device according to one of claims 1 to 7, the first cavity (20) being in a ring or being a linear cavity.

9. Method for starting a laser device according to one of claims 1 to 8, comprising: - generating, in the second cavity, a continuous-wave and multimode laser beam, at the wavelength (λ1); - generating, in the first cavity, a pulsed laser beam at the wavelength (λ2) and at the wavelength (λ1), - stopping the continuous-wave laser beam.

10. Method according to claim 9, wherein the losses or the gain at the first wavelength (λ1) are modulated during the starting period.

11. Method according to one of claims 9 or 10, the first cavity comprising 2 fibre amplifiers each pumped by a laser diode, the method further comprising modulating the beam of these diodes.