MAMYSHEV LASER OSCILLATOR FOR GENERATING ULTRA-SHORT PULSES AND DEVICE FOR STARTING SAME

Abstract

A laser device including a first cavity forming a Mamyshev oscillator, comprising a bandpass first filter at the first wavelength, and a second filter that also transmits the wavelength but that is reflective at a second wavelength, a second cavity, containing the first cavity, for forming a continuous-wave laser beam at the first wavelength and/or at a third wavelength neighbouring the first wavelength, the separation between the first and third wavelengths being smaller than the spectral width of the first filter and of the second filter, and means for allowing or interrupting a continuous-wave oscillation at the first wavelength or at said neighbouring wavelength, in the second cavity.

Claims

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

2. The laser device according to claim 1, the separation between the first and second wavelengths of the filters being between 5 nm and 25 nm.

3. The laser device according to claim 1, the second filter being disposed downstream of an optical circulator.

4. The laser device according to claim 1, the second cavity being delimited by a mirror, being a fiber mirror or by a fibre Bragg mirror.

5. The laser device according to claim 1, the means for allowing or interrupting the oscillation of the first wavelength, in the second cavity, comprising an optical switch or a variable optical attenuator.

6. The laser device according to claim 1, the second filter comprising a Bragg grating mirror.

7. The laser device according to claim 1, said first filter comprising a transmission filter or an optical circulator and a reflection filter.

8. The laser device according to claim 1, the first cavity being in a ring or being a linear cavity.

9. A method for starting the laser device according to claim 1, the method comprising: generating, in the second cavity, a continuous-wave and multimode laser beam, at the first wavelength; generating, in the first cavity, a pulsed laser beam at the second wavelength and at the first wavelength, and stopping the continuous-wave laser beam.

10. The method according to claim 9, wherein the losses or the gain at the first wavelength are modulated during the starting period.

11. The method according to claim 9, the first cavity comprising 2 fiber amplifiers each pumped by a laser diode, the method further comprising modulating the beam of the diodes.

12. The laser device of claim 2, the second cavity being delimited by a fiber mirror or by a fiber Bragg mirror.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows a known Mamyshev oscillator;

[0033] FIG. 2 shows an example of embodiment of a Mamyshev oscillator according to the invention;

[0034] FIG. 3 shows a reflectivity curve of the fibre mirror depending on the coupling ratio of the fibre coupler;

[0035] FIG. 4A

[0036] FIG. 4B illustrate a use of a fibre Bragg mirror instead of a fibre mirror;

[0037] FIG. 5 illustrates a use of a reflection filter 1 instead of the first transmission filter;

[0038] FIG. 6 is an example of device according to the invention using a linear cavity.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0039] FIG. 2 shows an example of embodiment of a Mamyshev oscillator according to the invention.

[0040] In this example, a 1.sup.st cavity 20, here in a ring, of length for example less than 30 metres, comprises two amplifiers 22, 24, which may in particular be fibre amplifiers, for example doped with ytterbium (with a small signal gain that may be between 20 dB and 30 dB). 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 Mar. 2020/Optics Letters, p. 1395-1398. Each amplifier comprises pumping means, for example a diode at 976 nm.

[0041] The cavity 20 further comprising a first spectral filter 26, for example a Gaussian filter, which allows radiation to pass through in a centred spectral bandwidth at a wavelength ?.sub.1, for example of width 1 nm. It may be centred in a band that corresponds to the emission of amplifier fibres, for example the ytterbium band.

[0042] It further comprises an output coupler 28, which makes it possible to extract the output beam of the cavity.

[0043] It may comprise an isolator 30, which ensures a unidirectional propagation.

[0044] The system also comprises a second spectral filter 32, for example a Gaussian filter, centred at a wavelength ?.sub.2, for example of width 1 nm. It may be centred in a band that corresponds to the emission of amplifier fibres, for example the ytterbium band. This filter 32 is preferably a reflection filter (for example of the Bragg grating mirror type): it reflects any radiation at the wavelength ?.sub.2 and sends it back into the 1.sup.st cavity 20. The first filter 26 does not allow any radiation, or very little radiation, to pass through at the wavelength ?.sub.2.

[0045] This second filter 32 is disposed, in relation to the ring, behind an optical circulator 34 and upstream of an optical switch 36 making it possible to block the passage of the beam. Finally, the cavity is completed by a mirror 38, for example a fibre mirror, for example again a 2 by 2 fibre coupler looped back on itself.

[0046] The separation between the central wavelengths ?.sub.1 and ?.sub.2 of the filters 26, 32 is preferably greater than n times (for example: n=4) the width of the individual filters, this separation 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.

[0047] As the transmission profile of a Bragg mirror is opposite to its reflection profile, the filter 32 allows the non-reflected wavelengths to pass through, and in particular any radiation at the wavelength ?.sub.1.

[0048] Thus, the device of FIG. 2 comprises two interlocked cavities: [0049] the first cavity is defined by the reflection on the filter 32; it is equivalent to a Mamyshev oscillator such as illustrated in FIG. 1; [0050] the second cavity, which comprises the first cavity but also a linear portion, is defined by the reflection on the mirror 38, which reflects all of the wavelengths that reach it, in particular the radiation at the wavelength ?.sub.1.

[0051] The operation of this system is as follows.

[0052] When the amplifiers 22, 24 are switched on and the optical switch 36 closed (rendered conducting), 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.

[0053] A multimode continuous-wave beam, having intensity fluctuations, is emitted at a central wavelength ?.sub.1 defined by the filter 26. The high intensity spikes, sufficiently intense to cross, by Self-Phase Modulation (SPM), the separation between the filters 26 and 32, are then reflected by the filter 32 and can oscillate in the first cavity.

[0054] A train of short pulses is emitted as output.

[0055] Therefore, this gives an operation of a Mamyshev oscillator with a virtual saturable absorber of large modulation depth. Once the Mode-Locking (ML) operation has been obtained, the optical switch 36 may then be opened (rendered non-conducting) in order to inhibit the CW operation of the second cavity. The laser then emits a radiation that includes the wavelengths ?.sub.1 and ?.sub.2.

[0056] In order to avoid the continuous-wave (CW) lasing from being too dominant and inhibiting the possibility of obtaining a ML lasing, the reflectivity of the fibre mirror 38 is preferably adjusted so that the two operations have average powers of the same magnitude (typically a few hundreds of mW in this example). This reflectivity of the fibre mirror 38 may be adjusted between 0% and 100% according to the coupling ratio of the fibre coupler used, as illustrated in FIG. 3. The coupling ratio may be modified by changing coupler.

[0057] It is also possible to replace the optical switch 36 with a variable optical attenuator, which makes it possible not only to accurately adjust the power reflected by the fibre mirror 38 in continuous-wave (CW) operation but also to inhibit the CW lasing in ML operation (by attenuating such that the total losses of the second cavity are greater than the gain of the amplifiers).

[0058] 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 greater fluctuations. According to one embodiment, a function generator is used that controls the supply of the pump diodes. For example, it concerns a generator from Keysight, for example the Keysight 33210A Waveform/Function Generator.

[0059] The example of Gaussian spectral profiles of the filters 26, 32 has been given above, but it is entirely possible to use profiles of different shapes.

[0060] The fibre mirror 38 may be replaced with the Fresnel reflection of the cleaving of the output fibre of the optical switch 36: the cleaving angle determines the reflection coefficient but the latter is then limited to a maximum of 5%.

[0061] As illustrated in FIG. 4A, the fibre mirror 38 may be replaced with a fibre Bragg mirror 38 (Filter 3) centred at a wavelength ?.sub.3 (FIG. 4B): ?.sub.3 is taken close to ?.sub.1, the separation between ?.sub.3 and ?.sub.1 being smaller than the spectral widths of each of the filters 26, 38, such that there is spectral superposition of the filter 26 and of the filter 38, as illustrated in FIG. 4B. The CW lasing occurs at a wavelength defined by this spectral superposition, the power of the CW lasing being adjusted by the separation between the filters 26 and 38.

[0062] As illustrated in FIG. 5, the transmission filter 26 may be replaced with an optical circulator 26 and a reflection filter 26.

[0063] According to another embodiment, illustrated in FIG. 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 fibre Bragg mirror of reflectivity less than 100%) is used both as a spectral filter and an output coupler.

[0064] In the examples presented, the system operates in the ytterbium spectral band (around 1 ?m) but it may be adapted to other wavelength ranges (in particular the telecom 1.5 ?m or mid-infrared 2-3 ?m bands).

[0065] The invention makes it possible to produce a system that is simpler than the existing solutions, such as those implementing the injection of a short pulse from an external laser, or an additional arm including a saturable absorber. It makes it possible to obtain starting of the mode-locking with a large modulation depth (even if there is no spectral superposition between the filters) without a complex or expensive component such as a motorised tunable filter or a saturable absorber. In addition, there are no adjustable parameters, which gives the system a significant robustness.

[0066] Therefore, the invention proposes a simple solution to the problem of starting Mamyshev oscillators with large modulation depth (with non superposition of the filters).

[0067] The invention may be applied as a source of ultra-short pulses for a wide variety of applications such as spectroscopy, ablation of thin layers by laser, micro-machining/micro-cutting/micro-drilling/micro-marking, ophthalmic surgery, multiphotonic imaging, laser structuring and texturing, etc. The simple, compact, robust and inexpensive scheme proposed is entirely adapted to an implementation by a laser industrialist.