FR3099254A1 - Gain Switched Laser Diode Short Light Pulse Generation System and Method - Google Patents

Abstract

The invention relates to a system and method for generating a light pulse. According to the invention, the system comprises a gain-switched laser diode (1) adapted to emit a source light pulse (10), an optical device (20, 21, 22, 23, 41, 42, 43, 44, 45) phase self-modulation generation and Raman effect generation adapted to receive the source light pulse (10) and generate a spectrally divided light pulse (15) comprising a first pulse portion (11) spectrally widened by self-phase modulation and a second pulse portion (12) spectrally shifted by Raman effect and a spectral separation device (30, 44, 45) adapted to receive the spectrally divided light pulse and to extract the first pulse portion ( 11) spectrally widened by phase self-modulation Figure for the abstract: Fig. 1

Description

System and method for generating a short light pulse based on a gain-switched laser diode

The present invention relates to the technical field of short or ultra-short pulse lasers.

In the field above, mode-locked oscillators have been known for about twenty years, which make it possible to generate light pulses at a fixed repetition frequency. SESAM chips can be used for mode blocking but have a very limited lifetime.

More recently, gain-switched laser diodes (or gain-switched laser diode, denoted GSDL) have appeared. A gain-switched laser diode is an optical device that emits light pulses with a duration typically between 10 ps and 1 ns. The main advantage of such a gain-switched laser diode is to emit light pulses at an adjustable repetition rate from 1 Hz to 50 GHz. A gain-switched laser diode also offers longer life than a mode-locked oscillator using a SESAM chip. The major drawback of such a gain-switched laser diode is that the temporal quality of the emitted pulse is poor. Indeed, the temporal profile of the light pulse emitted by a gain-switched laser diode generally consists of a short intensity peak flanked by extended pedestals before and after the intensity peak, these pedestals being induced by a non-linear phase. The reproducibility of the temporal profile (and therefore of the duration) of the pulses emitted by a gain-switched laser diode is quite low to date. Moreover, the temporal pedestals (outside the main peak) are variable in shape from one pulse to another for the same system, and also variable from one system to another. These instabilities are due to both the switching electronics and the physics of the laser diode gain switching. The reproducibility is even lower from one diode to another. Another disadvantage of such a gain-switched laser diode is that the duration at the diode output is not or only slightly compressible and does not allow access to durations of less than 5 ps. Finally, this solution based on gain-switched laser diodes is not compatible with frequency-drift amplification (called “Chirped Pulse Amplification” in English, denoted CPA) because the optical spectra emitted are too fine and the drift in the initial frequency of the pulses is very strongly non-linear. The CPA amplification technique very commonly used in the laser industry to obtain ultra-short light pulses of very high energies.

Linear and non-linear optical techniques are known to temporally shorten the light pulse emitted by a gain-switched laser diode.

The publication Y. Kusama, Y. Tanushi, M. Yokoyama, R. Kawakami, T. Hibi, Y. Kozawa, T. Nemoto, S. Sato, and H. Yokoyama, "7-ps optical pulse generation from a 1064- nm gain-switched laser diode and its application for two-photon microscopy," Opt. Express 22, 5746-5753 (2014) describes a system based on a gain-switched laser diode and a narrow bandwidth (0.3 nm or 0.5 nm) spectral filter to generate a 7 ps light pulse.

US Patent 10,230,208 B2 describes a high intensity femtosecond light pulse generator system based on a gain-switched laser diode, a Mamyshev regenerator using an optical fiber generating non-linear optical effects, a tunable band-pass spectral filter and an optical amplifier for generating a pulse having a parabolic shaped temporal intensity.

However, these techniques eliminate most of the initial pulse to keep only a small part of the spectrum (a few percent in intensity) and require additional amplification of the pulse. Moreover, these systems present strong variations of intensity from one pulse to another pulse and an instability in pulse duration.

One of the aims of the invention is to propose a laser system generating short pulses, of stable intensity and duration over time, while being tunable in repetition frequency and of reduced manufacturing cost.

There is thus proposed, according to the invention, a light pulse generation system comprising a gain-switched laser diode suitable for emitting a source light pulse, an optical device for generating phase self-modulation and for generating Raman effect adapted to receive the source light pulse and generate a spectrally divided light pulse comprising a first pulse part spectrally widened by self-phase modulation and a second pulse part spectrally shifted by Raman effect and a spectral splitting device adapted to receiving the spectrally split light pulse and for extracting the spectrally broadened first pulse portion by self-phase modulation.

The invention provides spectral shaping of pulses emitted by a laser diode, which, after passing through a compressor, makes it possible to generate short pulses, having a duration between 100 fs and 10 ps, of high energy and high peak power. Once the Raman peak has been exfiltrated, the pulse thus spectrally filtered and compressed is shorter than the source pulse emitted by the gain-switched laser diode which emits picosecond pulses, having a duration between 10 ps and 1 ns. In addition, the spectrally filtered and compressed pulse exhibits excellent temporal quality and greater symmetry with respect to its maximum intensity. The spectral separation of the Raman part of the pulse is stable and self-centered. The wavelength of the source light pulse can vary from 0.1 nm to a few nanometers without requiring readjustment of the spectral separation device and without any negative effect on the system of the invention. Since the gain-switched laser diode is frequency tunable, it is thus possible to generate a light pulse on demand or a series of pulses at an adjustable repetition rate between approximately 1 Hz and 50 GHz. Finally, the present disclosure implements optical components which are relatively inexpensive (low-pass filter, band-pass filter with wide spectral band, dichroic filter, optical fiber with photonic crystals inducing losses at the Raman wavelength or a highly absorbent material at the Raman wavelength) and not a notch filter with a narrow bandwidth and steep edges, such as a notch type filter, which is relatively more expensive. In addition, this technical solution does not necessarily require an additional amplification stage.

In a particular and advantageous embodiment, the optical device for generating phase self-modulation and generating the Raman effect comprises a passive optical fiber adapted to receive the source light pulse and generate a light pulse modulated spectrally by self -phase modulation.

In another particular and advantageous embodiment, the optical device for generating phase self-modulation and for generating the Raman effect comprises an optical amplifier suitable for amplification and for generating phase self-modulation.

Advantageously, the optical device for generating phase self-modulation and for generating the Raman effect comprises a second passive optical fiber or the same passive optical fiber adapted for the generation of the Raman effect.

According to a particular and advantageous embodiment, the optical device for generating phase self-modulation and for generating the Raman effect comprises an optical amplifier adapted for the amplification and for the generation of the Raman effect.

According to a particular example, the optical amplifier is a bandgap photonic fiber amplifier adapted to form the spectral separation device.

Advantageously, the spectral separation device comprises a wavelength low-pass filter, a band-pass filter, a dichroic filter, an optical fiber with photonic crystals, an optical amplifier having a gain band suitable for selectively amplifying the first pulse part or a diffraction grating compressor.

According to a particular and advantageous embodiment, the optical device for generating phase self-modulation and for generating the Raman effect and the spectral separation device consist of an optical fiber amplifier suitable for the amplification, the phase self-modulation generation, Raman effect generation and spectral separation.

Advantageously, the system comprises another optical amplifier arranged between the laser diode and the optical device for generating phase self-modulation and for generating the Raman effect.

Alternatively or additionally, the system comprises at least one optical amplifier and/or a compressor arranged downstream of the spectral separation device.

Preferably, the gain-switched laser diode emits source light pulses at an adjustable repetition rate in a frequency range between 1 Hz and 50 GHz.

The invention also relates to a method for generating a light pulse comprising the following steps: emission, by a gain-switched laser diode, of a source light pulse; generation, from the source light pulse, of a spectrally divided light pulse comprising a first part of the pulse spectrally widened by self-phase modulation and a second part of the pulse spectrally shifted by the Raman effect; and spectral separation adapted to extract the first spectrally widened pulse part by self-phase modulation.

Of course, the different characteristics, variants and embodiments of the invention can be associated with each other in various combinations insofar as they are not incompatible or exclusive of each other.

In addition, various other characteristics of the invention emerge from the appended description made with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

is a schematic view of a light pulse generation system according to the invention,

schematically represents a light pulse generation system according to a first embodiment,

schematically represents a light pulse generation system according to a second embodiment,

schematically represents a light pulse generation system according to a third embodiment,

schematically represents a light pulse generation system according to a fourth embodiment,

schematically represents a light pulse generation system according to a fifth embodiment,

schematically represents a light pulse generation system according to a sixth embodiment,

schematically represents a light pulse generation system according to a seventh embodiment,

schematically represents a light pulse generation system according to an eighth embodiment.

It should be noted that in these figures the structural and/or functional elements common to the different variants may have the same references.

detailed description

In the remainder of this document, the following terms are used equivalently: laser diode, gain-switched diode, gain-switched laser diode or gain-switched laser diode.

In the following description, the various embodiments are mere examples and do not limit the scope of the invention.

FIG. 1 schematically represents the general principle and operation of a light pulse generation system according to the present disclosure. This laser system comprises a laser diode 1 connected to an electric pulse generator 2. In the remainder of this document, the laser diode 1 is a gain-switched laser diode. More specifically, the electric pulse generator 2 applies an electric pulse of duration between 10 ps and 5 ns to the electrodes of the laser diode 1 so as to switch the gain of the laser diode. Thus, the gain-switched laser diode 1 emits a source light pulse 10 with a duration of between about 10 ps and 1 ns. The laser diode emits the source light pulse 10 at a wavelength of 1 μm, 1.03 μm, 1.064 μm, 1.55 μm, 1.56 μm or 2 μm for example. The source light pulse 10 has a spectral bandwidth between 1 µm and 1 nm.

The laser system further comprises an optical device 20 for generating self-phase modulation (also denoted SPM for the acronym of the English expression Self-Phase Modulation) and for generating the Raman effect and a spectral separation device 30. On the one hand, the optical device 20 for generating self-phase modulation and generating the Raman effect receives the source light pulse 10 and generates a light pulse 14 broadened spectrally by self-phase modulation (SPM), the spectral bandwidth of the spectrum broadened by SPM is between 1 nm and 150 nm. The spectrum of light pulse 14 spectrally broadened by SPM generally has a main peak 141 and side lobes 142 on either side of the main peak. The optical device 20 for generating phase self-modulation and for generating the Raman effect also induces a transfer of energy from the main peak 141 of the spectrum of the light pulse 14 broadened by SPM towards a spectral component offset with respect to to the main peak, for example approximately 13.2 THz in optical frequency in the case where a silica glass optical fiber is used to generate the Raman effect, i.e. several tens of nanometers in wavelength, in the above example of about 80 nm. In other words, the optical device 20 for generating self-phase modulation and for generating the Raman effect generates at the output a light pulse 15 consisting of two spectral components: a first pulse part 11 and a second pulse part 12, the second pulse part 12 being shifted spectrally by Raman effect. The terms “first” and “second” part of the pulse here do not imply any notion of order and are only intended to differentiate the spectral components of the light pulse 15 at the output of the optical device 20 for generating self-modulation of phase and Raman effect generation. This Raman effect is non-linear: it depends on the square of the peak power and selectively affects the main peak 141 of the spectrum of the light pulse 14 without modifying the side lobes 142 of the spectrum. Indeed, the main peak 141 of the spectrum of the light pulse 14 has a high peak power in the time domain compared to the peak power of the side lobes 142 of the spectrum. The first pulse part 11 is thus spectrally broadened by self-phase modulation and has a central peak 111 strongly attenuated by the Raman effect. In other words, the first pulse part 11 is purified or cleaned by the Raman effect, because energy is transferred from this central peak to a Raman peak shifted in wavelength. The side lobes 112 of the first pulse part 11 are in practice not affected by the Raman effect because they have a low temporal peak power. In this way, the sidelobes 112 of the first part of pulse 11 have the same intensity and the same spectral bandwidth as the sidelobes 142 of the light pulse 14 widened by SPM.

The spectral separation device 30 receives the spectrally divided light pulse 15 comprising the first pulse part 11 spectrally broadened by self-phase modulation and the second pulse part 12 spectrally shifted by the Raman effect. The second pulse part 12 is spaced from the first pulse part 11, for example by several tens of nanometers.

By way of non-limiting example, the spectral separation device 30 comprises a wavelength low-pass filter having a cut-off wavelength λ _c greater than the wavelength of the source light pulse 10 and less than the wavelength of the second pulse part 12 shifted spectrally by Raman effect. For example, consider a laser diode 1 emitting a source light pulse 10 at the wavelength of 1064 nm, the light pulse 14 broadened by SPM having a main peak 141 at 1064 nm and a spectral bandwidth between 1 nm and 50 nm and the second pulse part 12 shifted spectrally by Raman effect being centered at 1116 nm. A band-pass filter is used, with a wide spectral band, having a high cut-off wavelength at about 1090 nm and a low cut-off wavelength less than about 1050 nm or even 1030 nm so as to avoid cutting part of the spectrum expanded by SPM. The bandpass filter is for example a bandpass mirror. Alternatively, a low-pass filter is used having a cut-off wavelength at approximately 1090 nm, i.e. between the highest wavelength of the first pulse part 11 and the length wave of the second pulse part 12 shifted spectrally by Raman effect.

The spectral separation device 30 extracts the first part of the pulse 11 spectrally broadened by SPM and cleaned by the Raman effect from the main peak 141 of the spectrum of the light pulse 14. Now, advantageously, the major part of the noise and the incompressible phase (inducing the temporal pedestals) of the light pulse is contained in the main peak 141 of the light pulse 14 spectrally broadened by SPM. The second part of the pulse 12 shifted spectrally by Raman effect comes from the main peak 141 of the light pulse 14 broadened by SPM, this main peak 141 having an incompressible and noisy phase. On the contrary, the first part of the pulse 11 spectrally broadened by SPM and cleaned by Raman effect of the main peak comprises side lobes 112 corresponding to the side lobes 142 of the light pulse 14 broadened by SPM, these side lobes 112, 142 having a stable and compressible linear temporal phase (by means of a compressor) and therefore a temporally specific content. By proper pulse is meant here a pulse having at least 70% of its energy in the main temporal peak. The proportion of energy in the main temporal peak of the pulse is measured by integrating either an autocorrelator measurement or an optical spectrum.

The first pulse part 11 can then be amplified by a system of optical amplifier(s).

Particularly advantageously, the first pulse part 11 can be temporally compressed to obtain a light pulse of shorter duration than the source light pulse 10. To this end, the light pulse generation system further comprises a compressor arranged downstream of the spectral separation device 30. The compressor can be chosen from among a diffraction grating compressor, a prism compressor, a GRISM compressor, a Treacy compressor, a 4f compressor, a compressor optical fiber and a volume Bragg grating (or VBG) compressor.

The first part of the pulse 11 spectrally broadened by SPM and cleaned by the Raman effect retains all of the side lobes 142 contained in the optical spectrum of the light pulse 14 broadened by SPM. In addition, the first pulse part 11 spectrally broadened by SPM and cleaned by Raman effect has an energy corresponding to a relatively large part of the light pulse 14 broadened by SPM, of the order of more than 20% of the energy is conserved after Raman filtering. Consequently, at the output of the light pulse generation system, a high energy pulse is obtained. The signal is generally amplified after Raman filtering, using a limited number of optical amplifiers. On the contrary, in a prior system based on narrow-band spectro-temporal filtering of a pulse emitted by a gain-switched laser diode, only a small percentage (about 2%) of the initial energy is retained after temporal filtering, which requires a greater number of optical amplifiers.

The spectral separation is easy to achieve by means of, for example, a standard low-pass filter, because the first part of the pulse 11 is clearly and clearly separated spectrally from the second part of the pulse 12 by several tens of nanometers. Moreover, this spectral separation is repeatable from an industrial point of view. A low-pass (or wide-band band-pass) spectral filter having a predetermined cut-off wavelength λ _c is much less expensive than a narrow-band band-pass filter and easily reproducible industrially.

Alternatively, the spectral separation device 30 comprises a dichroic filter to spatially separate the first pulse part 11 from the second pulse part 12. According to another variation, the spectral separation device 30 comprises a photonic crystal optical fiber , for example a bandgap photonic fiber, adapted to extract the first pulse part 11 and to induce high losses on the second part of the pulse 12, for example via a coupling between the core modes and the sheath at the wavelength of the second part of the pulse 12. According to yet another variant, the spectral separation device 30 comprises an optical amplifier having a gain band adapted to selectively amplify the first part of the pulse 11 without amplify the second part of pulse 12, the wavelength of the first part of pulse 11 being inside the gain band of this optical amplifier and the wavelength of the second part of the pulse 12 being outside the gain band of this optical amplifier.

Advantageously, the system comprises a compressor with diffraction grating(s) to compress the pulses at the output of the system. The compressor can also be used in an alternative or complementary way to spatially separate the first pulse part 11 from the second pulse part 12. For example, the diffraction grating compressor is configured so that the second part of the pulse 12 is diffracted in a separate direction from the first pulse part 11 in the compressor.

The Raman effect is automatically and selectively applied to the main peak 141 of the SPM broadened light pulse 14. There is therefore no fine adjustment of the filter to be made. The cut-off wavelength of the spectral separation device 30 mainly depends on the wavelength of the source pulse emitted by the gain-switched laser diode 1. More precisely, the cut-off wavelength of the spectral separation device 30 is selected according to the wavelength of the main peak 141 and the value of the Raman shift, which can vary slightly depending on the optical fiber used to generate the Raman shift.

By retaining a large part of the optical spectrum, this technical solution is not very sensitive to the spectral drifts which occur naturally in the short and long term with a laser diode 1 switched in gain. Indeed, the spectral gap between the wavelength of the main peak 141 and the wavelength of the Raman line 12 being of the order of several tens of nanometers, the solution of the invention is both robust and wavelength tolerant.

This technical solution has various advantages. It makes it possible to clean the temporal profile of a laser pulse emitted by the gain-switched laser diode 1, which then makes it possible to amplify the first part of the extracted pulse 11 then to recompress the amplified pulse to a duration between 100 fs and 5 ps.

In addition, this technical solution makes it possible to retain a significant part of the energy of the pulse after cleaning by the Raman effect, which allows effective amplification in one or more subsequent amplification stages.

Another advantage of the system of the present disclosure, compared to a narrow band spectro-temporal filtering system, is to make it possible to recompress the light pulse 11 at the output, with better stability in terms of duration. Indeed, in a system of the prior art with spectro-temporal filtering, the slightest fluctuation in the peak power of the source light pulse emitted by the gain-switched laser diode translates into a change in the parameters (phase, duration, spectrum …) of the temporally filtered pulse after SPM. On the contrary, according to the present disclosure, a peak power fluctuation of the pulse emitted by the gain-switched laser diode 1 mainly results in a variation of the energy transferred by Raman effect in the second part of the pulse 12 which is spectrally filtered. Thus, the first pulse part 11 spectrally broadened by SPM and cleaned by the Raman effect has a stable energy independently of the peak power fluctuations of the laser diode 1.

Starting from the general principle described in connection with FIG. 1, we describe various particular embodiments in connection with FIGS. 2 to 9. In all the embodiments, the light pulse generation system comprises a generator 2 of electrical pulse connected to a gain-switched laser diode 1 to generate a source light pulse 10 with a duration of between approximately 10 ps and 1 ns. The laser diode 1 can be a laser diode of the Fabry Perot type or, preferably, a distributed feedback laser diode (DFB for distributed feedback) which offers better pulse stability. Many laser diodes 1 are commercially available today. The source light pulse 10 has a wavelength between 380 nm and 5 micrometers (µm) and a spectral bandwidth between 1 pm and 1 nm. Advantageously, the laser diode 1 emits source 10 light pulses from a demand pulse up to a repetition frequency of about 50 GHz. The repetition frequency is easily tunable via the electrical pulse generator 2.

As detailed in the following, in certain embodiments, the self-phase modulation generation, Raman effect generation and spectral separation steps are implemented in separate optical devices arranged in series. In other embodiments, two or more of these steps are implemented within the same optical component.

FIG. 2 represents a light pulse generation system according to a first embodiment. The light pulse generation system comprises a first passive optical fiber 21 favoring the generation of the phase self-modulation phenomenon and a second passive optical fiber 22 adapted for the generation of the Raman effect. The first passive optical fiber 21 is arranged between the laser diode 1 and the second passive optical fiber 22. Optionally, the system comprises an optical amplifier 40 arranged between the laser diode 1 and the first optical fiber 21 passive. The light pulse generation system comprises a spectral separation device 30 arranged downstream of the first passive optical fiber 21 and of the second optical fiber 22.

In this first embodiment, the self-phase modulation (SPM) is generated in the first passive optical fiber 21 and the Raman effect is generated in the second passive optical fiber 22.

The first passive optical fiber 21 is, for example, a step-index fiber, a gradient-index fiber, a photonic crystal fiber (PCF) or a photonic bandgap fiber. The first passive optical fiber 21 has a core size of between 1 μm and 50 μm and a length of between 1 cm and 100 m. The generation of SPM being a non-linear optical effect, it is preferable for the peak power at the input of the first passive optical fiber 21 to be between 1 −W and 1 MW. If the peak power delivered by laser diode 1 is insufficient, an optical amplifier 40 is used. Preferably, the first optical fiber 21 is a fiber lightly or undoped with germanium (or any other element having a strong Raman way to favor the generation of SPM. The first passive optical fiber 21 receives the source light pulse 10 or, where applicable, the amplified source light pulse 13 by the optical amplifier 40. The first optical fiber 21 generates a spectrally broadened light pulse 14 by phase self-modulation ( MPS). The light pulse 14 has a main peak 141 and side lobes 142.

The second passive optical fiber 22 is, for example, a step-index fiber, a gradient-index fiber, a microstructured fiber, a silica glass fiber or a fluorinated glass fiber. The second passive optical fiber 22 has a core size of between 2 μm and 50 μm and a length of between 1 m and 100 km. The second passive optical fiber 22 is preferably a fiber with a core doped with germanium or any other element having a strong non-linear Raman response, such as a chalcogenide element, for example fluorine. Core doping produces an index difference between the refractive index of the core, denoted n _core , and the refractive index of the sheath, denoted n _sheath , is such that (n _core -n _sheath is between 10 ^-5 to a few 10 ^-1 The second passive optical fiber 22 preferably comprises a high-density vitreous matrix.Advantageously, to promote the generation of non-linear Raman effect, the second optical fiber 22 has a group speed almost identical to the wavelength of the source light pulse 10 and the wavelength of the Raman peak 12. Preferably, the group velocity difference between these two wavelengths is less than or equal to 6 ps/ m. Since the generation of the non-linear Raman effect is proportional to the square of the peak power of the light pulse received, it is preferable that the peak power at the input of the second passive optical fiber 22 be between 1 kW and 1 MW The second passive optical fiber 22 receives the light pulse 14 spectrally broadened by self-phase modulation (SPM) and generates the light pulse 15 spectrally divided by the Raman effect. In other words, by non-linear Raman effect, the second passive optical fiber 22 spectrally divides the light pulse into the first pulse part 11 and the second pulse part 12, which is spectrally shifted with respect to the first part d pulse 11. More precisely, the second pulse part 12 comes from the transfer of energy by Raman shift of the main peak 141 of the light pulse 14 widened by SPM. The first pulse part 11 comprises side lobes 112 corresponding to the side lobes 142 of the light pulse 14 widened by SPM and a central peak 111 resulting from the Raman effect attenuation of the central peak 141 of the light pulse 14 widened by SPM.

Downstream of the second passive optical fiber 22, the spectral separation device 30 separates the first pulse part 11 and the second pulse part 12, for example by eliminating the Raman part of the light pulse 15, this is that is to say the second pulse part 12. In the first embodiment, the spectral separation device 30 allows the first pulse part 11 to pass. The spectral separation device 30 comprises for example a low-pass filter in wavelength, the low-pass filter having a cut-off wavelength between the wavelength of the laser diode and the wavelength of the second pulse portion 12, a centered wide band-pass filter on the wavelength of the diode and having a high cut-off wavelength as defined for the low-pass filter or a dichroic filter also having a cut-off wavelength as defined for the low-pass filter down.

The spectral separation device 30 has an extended spectral band, for example of at least 10 nm. On the contrary, in a prior system based on narrowband spectro-temporal filtering of a pulse emitted by a gain-switched laser diode, the spectro-temporal filter has a spectral bandwidth of less than a few nanometers at most.

The spectral separation device 30 retains at least about 50% of the energy of the first pulse part 11 and rejects at least about 50% of the energy of the second pulse part 12.

As a variant, the spectral separation device 30 comprises a microstructured fiber, for example a photonic bandgap optical fiber, adapted to extract the first part of the pulse 11 and to induce high losses on the second part of the pulse 12, for example via a coupling between the core modes and the cladding modes at the wavelength of the second part of the pulse 12. This variant has all the advantages of an optical fiber known to those skilled in the art at namely compactness, robustness and flexibility.

FIG. 3 represents a light pulse generation system according to a second embodiment. The light pulse generation system comprises a passive optical fiber 21 and a linear optical amplifier 42. Optionally, the system comprises an optical amplifier 40 arranged between the laser diode 1 and the passive optical fiber 21. The passive optical fiber 21 is structurally and functionally similar to the first passive optical fiber 21 described in connection with FIG. 2. The first passive optical fiber 21 receives the source light pulse 10 or, where appropriate, the amplified source light pulse 13 by the optical amplifier 40. The first optical fiber 21 generates a spectrally broadened light pulse 14 by self-phase modulation (SPM). In order to more particularly favor the generation of the self-phase modulation (SPM), in this embodiment, the peak power of the source pulse 10 or of the amplified source pulse 13 is less than or equal to 1 kW. In addition, the passive optical fiber 21 is preferably lightly or not doped with a chemical element having a strong non-linear Raman response (such as germanium or a chalcogenide element, for example fluorine) and has a length of between 1 cm and 100m. Preferably, the difference in index between the refractive index of the core, n _core , and the refractive index of the sheath, n _sheath , is such that n _core -n _sheath is less than 10 ^{- 3} The device for spectral separation 30 is structurally and functionally similar to that described in connection with the first embodiment.

In this second embodiment, the linear optical amplifier 42 receives the light pulse 14 spectrally broadened by SPM, amplifies it while generating a Raman effect from the main peak 141 of the light pulse 14 spectrally broadened by SPM. The linear optical amplifier 42 thus generates the light pulse 15 consisting of two spectral components: a first pulse part 11 and a second pulse part 12, the second pulse part 12 being spectrally shifted by the Raman effect. Advantageously, the linear optical amplifier 42 comprises an active optical fiber, for example a core index fiber doped with rare earth ions, a core index gradient fiber doped with rare earth ions or a microstructured fiber core doped with rare earth ions.

Since the generation of the non-linear Raman effect is proportional to the square of the peak power of the light pulse received, it is preferable for the peak power at the input of the linear optical amplifier 42 to be between 1 kW and 1 MW. This second embodiment advantageously makes it possible to make the system more compact because it does not require a second optical fiber 22. In addition, this second embodiment generally makes it possible to obtain more energetic pulses at the output of the spectral separation 30 because the first pulse part 11 is amplified. In addition, costs are reduced due to the reduction in the number of optical components.

In an example of the second embodiment, the light pulse generation system produces light pulses at a repetition rate of 250 kHz. Such a spectral separation device produces light pulses having a temporal duration (width at half maximum) after compression of 3.6 ps, a peak power of 3 MW, an energy per pulse of 10.46 μJ and an average power of 2.614 W. This light pulse generation system makes it possible to generate light pulses of duration and pulse temporal quality comparable to that obtained in the prior art with a narrow band spectro-temporal filtering, but here allows to get 10 times more energy. However, it is known to those skilled in the art that the more the peak power of a pulse increases, the more difficult it is to obtain pulses of good temporal quality.

FIG. 4 represents a light pulse generation system according to a third embodiment. The light pulse generation system comprises an optical amplifier 41 and a passive optical fiber 22. The optical amplifier 41 is arranged between the laser diode 1 and the passive optical fiber 22. The passive optical fiber 22 is structurally and functionally similar to the second passive optical fiber 22 described in connection with FIG. 2. The spectral separation device 30 is structurally and functionally similar to that described in connection with the first embodiment.

In this third embodiment, the self-phase modulation (SPM) is generated directly in the optical amplifier 41 intervening after the laser diode 1 switched in gain. The Raman effect is generated in the passive optical fiber 22. More precisely, the optical amplifier 41 receives the source light pulse 10. The optical amplifier 41 amplifies the source light pulse 10 while broadening it spectrally by self-modulation of phase to generate a spectrally broadened light pulse 14 by phase self-modulation. In one example, the optical amplifier 41 comprises an active optical fiber doped with rare earth ions, for example an active optical fiber with a step index, an active optical fiber with an index gradient or even an active photonic crystal fiber (PCF ). The active optical fiber has a core size between 2 µm and 30 µm in and a length between 1 cm and 100 m. The generation of SPM being a non-linear optical effect, it is preferable that the peak power at the input of the optical amplifier 41 be between 1 mW and 1 MW.

In this third embodiment, as in the first embodiment, the passive optical fiber 22 receives the light pulse 14 broadened spectrally by phase self-modulation and generates the pulse 15 spectrally divided by the Raman effect into two spectral components. The spectral separation device 30 extracts the first pulse part 11 and discards the second pulse part 12 shifted spectrally by Raman effect. This embodiment advantageously makes it possible to make the system more compact by replacing an optical amplifier 40 and an optical fiber 21 with a single optical amplifier 41. This embodiment also makes it possible to reduce manufacturing costs by reducing the number of components optics.

FIG. 5 represents a light pulse generation system according to a fourth embodiment. The light pulse generation system here comprises an optical fiber 23 generating phase self-modulation and promoting the generation of non-linear Raman effect. Optionally, the system may include an optical amplifier 40 placed between the laser diode 1 and the optical fiber 23. The spectral separation device 30 is structurally and functionally similar to that described in connection with the previous embodiments.

In this fourth embodiment, a single optical fiber 23 is used for the direct and combined generation of the Raman effect and the self-phase modulation. As in the two previous embodiments, a low-pass filter for example makes it possible to retain the spectral component of the first part 11 of the pulse while suppressing the spectral component of the second part 12 of the pulse, shifted by the Raman effect. This embodiment advantageously makes it possible to make the system more compact by replacing an optical fiber 21 and an optical fiber 22 with an optical fiber 23. This embodiment also makes it possible to reduce manufacturing costs by reducing the number of optical components .

FIG. 6 represents a light pulse generation system according to a fifth embodiment. The light pulse generation system here comprises an optical amplifier 43 arranged between the laser diode 1 and the spectral separation device 30. Advantageously, the optical amplifier 43 comprises an active optical fiber having a core diameter of between 1 and 50 µm. The optical amplifier 43 comprises an active optical fiber doped with rare earth ions, for example an active optical fiber with step index, active optical fiber with an index gradient or even an active photonic crystal fiber (PCF).

In this fifth embodiment, the non-linear optical effects of phase and Raman self-modulation are directly generated in the optical amplifier 43. This embodiment advantageously makes it possible to make the system more compact by replacing an optical fiber 21 , an optical fiber 22 and/or an optical amplifier 40, 41 or 42 by a single optical amplifier 43. This embodiment also makes it possible to reduce manufacturing costs by reducing the number of optical components.

FIG. 7 represents a light pulse generation system according to a sixth embodiment. The light pulse generation system here comprises a passive optical fiber 21 and an optical amplifier 44. Optionally, the system comprises an optical amplifier 40 disposed between the laser diode 1 and the passive optical fiber 21. The passive optical fiber 21 is structurally and functionally similar to the first passive optical fiber 21 described in connection with Figure 2.

In this sixth embodiment, the optical amplifier 44 combines several functions. The optical amplifier 44 receives the light pulse 14 spectrally broadened by self-phase modulation (SPM) from the passive optical fiber 21. In addition, the optical amplifier 44 produces a Raman effect on the peak 141 of the light pulse 14. Finally the optical amplifier 44 spectrally separates the amplified light pulse to cut the spectral component or second part 12 of the light pulse shifted by Raman effect and extract the first pulse part 11 which is spectrally broadened by self-phase modulation. This embodiment advantageously makes it possible to make the system more compact by replacing the spectral separation device 30 combined with an optical fiber 22 or an optical amplifier 42 by a single optical amplifier 44. This embodiment also makes it possible to reduce the costs manufacturing by reducing the number of optical components.

By way of example, the optical amplifier 44 comprises a special optical fiber of the photonic band gap fiber type doped with rare-earth ions. Rare-earth ions include, for example, ytterbium, erbium, thulium and/or holmium ions. A person skilled in the art uses digital simulation tools to determine the other characteristics of the optical fiber amplifier 44, such as its length, the core diameter, the dopant(s).

FIG. 8 represents a light pulse generation system according to a seventh embodiment. The light pulse generation system here comprises a first optical amplifier 41 arranged at the output of the laser diode 1 and a second optical amplifier 44 downstream. The first optical amplifier 41 is structurally and functionally analogous to the optical amplifier 41 described in connection with the third embodiment (illustrated in FIG. 4), for generating phase self-modulation directly at the output of the laser diode 1 switched in gain. The second optical amplifier 44 is structurally and functionally analogous to the optical amplifier 44 described in connection with the sixth embodiment (illustrated in FIG. 7), to amplify the pulse, generate a Raman shift and directly filter the spectral component shifted by Raman effect. This embodiment advantageously makes it possible to make the system more compact by replacing several optical components with two optical components. Thus the optical amplifier 40 and the first optical fiber 21 are combined in the optical amplifier 41, while the second optical fiber 22 and the optical amplifier 42 are combined in the optical amplifier 44. This embodiment also makes it possible to reduce manufacturing costs by reducing the number of elements.

FIG. 9 represents a light pulse generation system according to an eighth embodiment. The light pulse generation system here comprises a single optical amplifier 45 arranged at the output of the laser diode 1.

In this eighth embodiment, the optical amplifier 45 combines several functions. The optical amplifier 45 is used for the generation of self-phase modulation, the generation of the Raman effect and the filtering of the Raman bump. Advantageously, the optical amplifier 45 comprises a special optical fiber of the microstructured fiber type doped with rare earth ions. Thus, the optical amplifier 45 receives the source light pulse 10, amplifies it and directly generates the first pulse part 11 broadened by SPM and spectrally separated from the second pulse part 12 spectrally shifted by Raman effect, the second pulse part 12 being cut off.

In a particularly advantageous manner, the light pulse generation system according to any one of the embodiments described above is combined with a compressor arranged downstream of the spectral separation device. The compressor can be a diffraction grating compressor, a prism compressor, a GRISM compressor, a Treacy compressor, a 4f compressor, a fiber optic compressor, a volume Bragg grating (or VBG) compressor.

The light pulse generation system of the present disclosure makes it possible to generate light pulses having a shorter time duration after compression than the source light pulse emitted by the gain-switched laser diode 1. The light pulses thus generated have a wavelength of between 380 nm and 5 μm, for example 1 μm, 1.55 μm or 2 μm. The light pulses are generated on demand or at an adjustable repetition rate, between 1 Hz and 5 GHz. The light pulses at the output of the spectral separation device have a peak power comprised between 30 kW and 150 kW, an energy per pulse comprised between 4 nJ and 100 μJ and an average power comprised between 1 W and 300 W. of a system illustrated in Figure 3, the output light pulses have a peak power of 3 MW after compression.

The light pulse generation system according to any one of the embodiments described above can serve as a source for a power amplifier or even for a frequency drift amplification chain (CPA) comprising at least one amplifier optics and a compressor.

The light pulse generation system of the present disclosure finds many applications, for example in LIDAR or remote sensing systems or laser machining.

Of course, various other modifications may be made to the invention within the scope of the appended claims.

Claims (10)

Light pulse generation system comprising a gain-switched laser diode (1) suitable for emitting a source light pulse (10), an optical device (20, 21, 22, 23, 41, 42, 43, 44, 45) phase self-modulation generation and Raman effect generation adapted to receive the source light pulse (10) and generate a spectrally divided light pulse (15) comprising a first pulse part (11) spectrally broadened by phase self-modulation and a second pulse part (12) spectrally shifted by Raman effect and a spectral separation device (30, 44, 45) adapted to receive the spectrally divided light pulse and to extract the first pulse part ( 11) spectrally broadened by self-phase modulation. System according to Claim 1, in which the optical device (20, 21, 22, 23, 41, 42, 43, 44, 45) for generating self-phase modulation and for generating the Raman effect comprises an optical fiber ( 21, 23) passive adapted to receive the source light pulse (10) and generate a light pulse (14) spectrally modulated by self-phase modulation. System according to Claim 1, in which the optical device (20, 21, 22, 23, 41, 42, 43, 44, 45) for generating self-phase modulation and for generating the Raman effect comprises an optical amplifier ( 41, 43, 45) suitable for amplification and for the generation of self-phase modulation. System according to Claim 2 or 3, in which the optical device (20, 21, 22, 23, 41, 42, 43, 44, 45) for generating self-phase modulation and for generating the Raman effect comprises a second passive optical fiber (22) or the same passive optical fiber (23) adapted for the generation of the Raman effect. System according to one of Claims 1 to 3, in which the optical device (20, 21, 22, 23, 41, 42, 43, 44, 45) for generating phase self-modulation and for generating the Raman effect includes an optical amplifier (42, 43, 44, 45) suitable for amplification and for generating the Raman effect. A system according to claim 5 wherein the optical amplifier (44, 45) is a bandgap photonic fiber amplifier adapted to form the spectral separation device (30, 44, 45). System according to one of Claims 1 to 5, in which the spectral separation device (30) comprises a wavelength low-pass filter, a band-pass filter, a dichroic filter, an optical fiber with photonic crystals, a optical amplifier having a gain band adapted to selectively amplify the first pulse part (11) or a diffraction grating compressor. System according to Claims 3 and 5, in which the optical device (20, 21, 22, 23, 41, 42, 43, 44, 45) for generating self-phase modulation and for generating the Raman effect and the device separation units (30, 44, 45) consist of an optical fiber amplifier (45) suitable for amplification, self-phase modulation generation, Raman effect generation and spectral separation. System according to one of Claims 1 to 8, in which the gain-switched laser diode (1) emits source light pulses at an adjustable repetition frequency in a frequency range between 1 Hz and 50 GHz. Method for generating a light pulse comprising the following steps: emission, by a gain-switched laser diode (1), of a source light pulse (10), generation, from the source light pulse (10), d 'a spectrally divided light pulse (15) comprising a first pulse part (11) spectrally widened by self-phase modulation and a second pulse part (12) spectrally shifted by Raman effect, and spectral separation (30, 44, 45 ) adapted to extract the first pulse part (11) broadened spectrally by self-phase modulation.