WO2015061411A1

WO2015061411A1 - Ultra high power single mode pulsed laser source with multiple oscillators operating to control population inversion level in amplifier

Info

Publication number: WO2015061411A1
Application number: PCT/US2014/061716
Authority: WO
Inventors: Sergey Guskov; Valentin Gapontsev; Alex Yusim; Igor Samartsev
Original assignee: Ipg Photonics Corporation
Priority date: 2013-10-25
Filing date: 2014-10-22
Publication date: 2015-04-30

Abstract

A light source includes a seed laser operative to output a train of substantially uniform signal pulses at a desired wavelength and polarization, and at least one additional laser configured to output radiation at a wavelength λ2 and/or different polarization. The output of the additional laser is controllable to maintain a level of population inversion in the booster between the signal pulses substantially the same as during the amplification of these signal pulses. The booster emits a substantially continuous of alternating pulses discriminated by a spectral or/and polarization filter.

Description

ULTRA HIGH POWER SINGLE MODE

PULSED LASER SOURCE WITH MULTIPLE OSCILLATORS OPERATING TO CONTROL POPULATION INVERSION LEVEL IN AMPLIFIER

BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

[001] The disclosure relates to an ultra-high power pulsed fiber laser system provided with a portable booster which extends over free space between a main console and laser head. Specifically, the disclosure relates to an ultra-high power pulsed laser source with an unconfmed booster and a multi-oscillator unit which is operative to control gain dynamics of the booster.

Prior Art

[002] Many laser machining applications require pulsed lasers. Main characteristics of pulsed lasers include, among others, high peak powers and controlled pulse energy delivered to the workpiece. This controlled laser pulse energy delivery is important to maintain good cut, weld, or mark quality. One of the most attractive configurations of the pulsed lasers is an efficient, compact, and rugged fiber laser. Often, the process involving a pulsed laser requires that the subsequent pulses be modified in real time.

[003] For example, many industrial operations require the uniform pulse energy from pulse to pulse. However, such unifoimity can be problematic during the on- and off- switching operations, typically performed by a shutter, due to the known gain dynamics of fiber amplifiers in general and, in particular, those of a master oscillator-power amplifier ("MOPA")

configuration. In other words, the build-up of the desired inversion population level and depletion of the accumulated population inversion at excited electronic levels are not

instantaneous, but gradual varying from pulse to pulse. FIG. 1 shows the energy variation from pulse to pulse at tum-on and turn-off conditions due to the typical gain dynamics which unevenly rises and slows down the turn-on and turn-off transient response of the fiber amplifier.

[004] In a typical deployment, pulsed fiber laser systems, i.e., light sources, deliver energy to the workpiece to be processed. The workpiece may have a complex geometry and/or an intricate pattern of formations and/or an amalgam of different materials which require a variable pulse-to- pulse energy. For example, the laser processed workpiece requires initially half the maximum energy for one of pulses within a pulse burst, 75% in the second pulse, and a 100% in subsequent pulses within the same burst. The pulse-to-pulse energy control is extremely difficult due the transient gain dynamics of a gain medium.

[005] The transient response of a gain medium is recognized in the field of fiber lasers. The U.S. patent application publication 21012/0320450 discloses a single laser seed source followed by a fiber amplifier. The seed source is controlled to output the seed pulses and radiation between adjacent seed pulses all at a uniform wavelength so as to maintain the collective average of the seed pulse and intermediate powers constant. As a result, the fiber-amplifier has substantially the same gain. However, the presence of radiation between the pulses may be a detriment in certain industrial applications.

[006] A need therefore exists for a compact, rugged ultra-high power single mode ("SM") fiber light source capable of controUably outputting a train of pulses with a desirable shape and energy in a nano- to sub-nanosecond frequency range and peak power in a MW - G peak power range which are free from the mentioned inconvenience of between-the-pulse radiation.

SUMMARY OF THE DISCLOSURE

[007] The present invention resolves the issue of controllable variation of pulse energy from pulse to pulse during the turn-on and turn-off of the laser. This is realized by adding at least one additional radiation generating channel which, in combination with the master oscillator, provides substantially constant inversion in the gain medium.

[008] For pulsed lasers adding an additional channel is a degree of freedom that can be used to keep the pulse to pulse energy constant in order to maintain fast turn-on and turn-off times. The additional channel can be filtered at the output using a spectral discriminator, such as a dichroic coated optic or a volume Bragg grating that works well with chirped pulse amplification systems.

[009] The disclosed laser configuration includes a shutter before the amplification stage, at least one or more additional pulse generating channel coupled into the amplifier after the shutter; and a spectral filter that discriminates between the additional channel(s) and the seed signal. The shutter and the additional channel work so as to maintain the input power level to the subsequent amplifying stages such that inversion level is controlled from pulses to pulse. In one

embodiment the inversion level in the booster is substantially constant maintaining the same pulse to pulse energy extraction during the transient on/off condition. In another embodiment the level of inversion may be controllably changed to meet the objectives of any given process.

BRIEF DESCRIPTION OF THE DRAWINGS

[010] The above and other features of the disclosure will become more readily apparent from the following specific description in conjunction with the drawings, in which:

[Oi l] FIG. 1 illustrates a train of pulses output by the known prior art fiber laser configurations which utilize the shutter positioned before the booster stage; the pulses each have respective gradual leading and trailing edges and are characterized by non-uniform energy;

[012] FIG. 2 shows a conceptual schematic of the disclosed laser source;

[013] FIG. 3 shows an exemplary fiber laser source configured in accordance with one aspect of the inventive concept;

[014] FIG. 4 illustrates another exemplary fiber laser source configured in accordance with a further aspect of the inventive concept.

[015] FIGS. 5A - 5C show a sequence of bursts of signal light pulses at the input of a booster of FIGS. 2 and 4, additional radiation at the input of the booster, and the output of the booster, respectively;

[016] FIG. 6 shows the output of the booster of FIGS. 2 and 3 with a modified formation of bursts of signal light pulses; and

[017] FIGS. 7 A - 7B are the screen shot showing alternating bursts of pulses which are emitted by respective seed and additional light sources of the disclosed system and have non-matching energy levels.

SPECIFIC DESCRIPTION

[018] Reference will now be made in detail to embodiments of the invention. Wherever possible, same or similar numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. Unless specifically noted, it is intended that the words and phrases in the specification and claims be given the ordinary and accustomed meaning to those of ordinary skill in the fiber laser arts. The word "couple" and similar terms do not necessarily denote direct and immediate connections, but also include mechanical and optical connections through free space or intermediate elements.

[019] The disclosure teaches controlling characteristics of subsequent signal pulses, which are emitted by a seed, by modulating the power of secondary pulses emitted by another laser so that the signal and secondary pulses are coupled into a booster in an alternating fashion.

[020] In one aspect, the disclosure teaches a laser source configured to emit bursts of pulses characterized by a substantially uniform energy. The inventive concept includes maintaining a substantially constant level of population inversion in the gain medium, such as fiber amplifier.

[021] FIG. 2 shows one of numerous possible laser configurations implementing the inventive concept. A laser source 20 is configured as a pulsed laser system operating in a nanosecond pulse duration range or chirped pulse laser system capable of emitting pulses in a pico- to femtosecond range.

[022] The laser source 20 includes a seed laser 22 operative to emit short and ultra-short signal light pulses 24 at the desired wavelength λΐ . The seed laser 22 may be selected from any type of actively mode-locked, passively mode-locked, continuous wave ("CW") or externally modulated CW lasers. In schematics of FIG. 2, seed laser 22 is used with acousto-optical modulators (AOM) the output of the seed laser 22.

[023] The signal light pulses 24 each, when coupled into booster 28, excite rare earth ions which leave the ground level for higher excited electronic levels - the process known as population inversion. The build-up of ions on excited electronic levels is not instantaneous. Similarly, the depletion of the ion population on the excited levels is also gradual, not instantaneous. Therefore, the time necessary to complete the build-up and the desired population inversion vary from one burst of pulses to another. Accordingly, sequential pulses are not uniform which means that the energy level carried in pulses also varies which, in turn, leads to unsatisfactory performance of laser systems in applications requiring the pulse uniformity.

[024] In accordance with one embodiment of the invention, the desired population inversion level in booster 28 is kept substantially constant both during bursts of seed pulses and between the bursts. This is realized by employing at least one additional channel including at least one additional light oscillator or laser 32 which emits a light beam at a wavelength λ2 different from wavelength λΐ . The additional laser 32 which may include pulsed, continuous wave, single frequency, broad band, etc., laser configurations. The source may be configured as a single mode or multimode fiber laser, semiconductor laser or any other appropriate laser type.

Furthermore, with a polarization maintaining ("PM") active fiber, the sources may be configured to output respective differently polarized bursts of pulses, as discussed in reference to FIG. 4. Importantly, the output of additional light oscillator 32 is controlled to keep the population inversion level in booster 28 at a desired level. For example, the level of inversion may be substantially the same as that obtained in response to seed pulses 24. As a result, the output of booster 28 is practically continuous and includes pulses at the desired wavelength λΐ and other wavelength %2, respectively, having substantially a constant energy level. In the end a spectral filter 34 is operative to discriminate between these two wavelengths. Alternatively, the inversion level may be controllably modified.

[025] The population inversion level depends, among others, on the output power and wavelength of the seed signal. The adjustment of the output of additional light oscillator 32, if a need arises to stabilize the population inversion level in booster 28, can be easily carried out in semiconductor diode lasers by modulating the input current. Accordingly, second light oscillator 32 is preferably, but not necessarily, a semiconductor diode laser.

[026] FIG. 3 illustrates an exemplary schematic of chirp pulse laser source 20 operating to maintain, for example, a substantially constant level of population inversion in booster 28.

Specifically for this example, light source 20 may be configured with a main console and a laser head. As typical for pulsed lasers, the console houses a fiber laser configured as a master oscillator/power amplifier ("MOP A") in which mode-locked seed 22 generates signal light pulses 24 further initially amplified in a pre-amplifier 26.

[027] The selection of the format of each burst 24 of signal light pulses is realized, for example, by a shutter or pulse sheer 36 also housed in the console which allows seed 22 to operate in a CW regime. The format of the burst is defined by a pulse repetition rate, individual pulse duration and the number of pulses within burst 24. Any format modification of the burst may lead to changes in the integrated energy of each burst 24. In the shown laser source, pulse slicer 36 is configured as an acousto-optic modulator (AOM) located between seed 22 and preamplifier 26, which thus provides . The AOM 36 may operate in a kHz - MHz range and perform a double function. First, as an easily adjustable slicer, it may modify the format of bursts 24. Second, it operates as a gate shutting down the propagation of bursts of signal light pulses and therefore operative to control pulse bursts. Other configurations of slicer 36, such as an electro-optic modulator (EOM) which may perform even at a GHz level and be used in disclosed chirped pulse source 20 instead of AOM.

[028] The individual pulses within burst 24, which are emitted by seed laser 22, are

subsequently temporally stretched in a pulse stretcher 38 which may be positioned before or, as shown, after pulse slicer 36 in the console. Pulse stretching reduces the peak power of the pulses by the temporal stretching factor. The energy of the pulse can then be increased by at least the same factor while remaining always below the limit of various destructive nonlinear effects. Properly designed element having high dispersion can be used to function as a stretcher. In the shown configuration, stretcher 38 is configured as a chirped fiber Bragg grating in combination with pigtailed circulator. Alternatively to the position of stretcher 38 in FIG. 3, it may be positioned before pulse slicer 36.

[029] The pre-amplified pulses 24 are then amplified in booster 28 housed in the laser head. The booster 28 is preferably a fiber laser amplifier capable of amplifying light to a MW level. The booster 28 is configured with an active fiber extending, in this embodiment, between the main console and laser head over free space. However, other configurations of laser source 20 are within the scope of this disclosure and may, for example, include a typical fiber block located within the laser head and including an active fiber with opposite ends thereof fused to respective ends of input and output active fiber. The input passive fiber receives the pre-amplified signal and secondary pulses which are further processed in the manner common to different

configurations of disclosed source 20, as disclosed hereinbelow in detail.

[030] The laser head may further house pump 44 which preferably, but not necessarily is configured to emit pump light coupled into the output end of booster 28 in a direction which is counter to the propagation direction of the pulses. The end counter-pumping scheme includes a mirror 46, as disclosed in co-pending provisional US patent applications 61/773,370;

61/773,377; and 61/773370, respectively which are fully incorporated herein by reference.

Obviously, a side pumping scheme or end pumping in the propagation direction can be used as well. To prevent detrimental consequences of backreflection, preferably a pigtailed isolator or any other optical component, which functions similarly to the isolator, is introduced between the amplifying cascades.

[031] The additional light generating channel includes oscillator 32 and a stretcher 40 configured to control duration of pulses 48. The oscillator 32 is preferably configured as a diode laser. The stretching can be performed by any element with high dispersion, such as a chirped volume Bragg grating ("CVBG") or chirped fiber Bragg grating ("CFBG"). The chirped light pulses are coupled out by the circulator 38 before being pre-amplified in preamplifier 26. The outputs of respective seed and additional lasers 22 and 32, respectively, are finally amplified in booster 28. The gain medium of all light fiber generating and amplifying devices can be selected from any of rare earth elements and their combination. The additional oscillator 32 may be driven by the same driver as seed laser 22 or by a separate one and can emit a continuous wave or pulsed radiation.

[032] The output of booster 28 thus may be viewed as a continuous train of pulses in which signal light pulses 24 at the desired wavelength λΐ alternate with radiation at wavelength X2. The spectral separation of pulses at different wavelengths is realized by spectral filter 34 which can be of any known configuration, such as FBG, VBG, dichroic mirror, and others.

Importantly, spectral filter 34, such as the VBG shown in FIG. 3, also operates as a compressor which at least reduces but preferably eliminates the chirp.

[033] A controller 42 is operative to generate a control turn-on/turn-off signal coupled into pulse sheer 36 which either prevents further propagation of burst of signal pulses 24 or allows the propagation. Simultaneously, controller 42 outputs a tum-on/turn-off signal coupled into second diode oscillator 32.

[034] FIG. 4 illustrates a slightly different approach for the controlling the inversion level of booster 28 and therefore the characteristics of signal pulses at the booster's output. Specifically, this embodiment may have the same structure of source 20 as shown in FIG. 3, but in a PM format, i.e., all components of source 20 shown in FIG. 3 are polarization maintaining. The elements of the laser head of FIG. 3, i.e., booster 28, pump and fibers are also manufactured in a PM format.

[035] Specifically, seed 22 and laser 32 are configured to output respective bursts of pulses 60 and 62 at the same or different wavelengths but having different polarizations. Coupled in to a polarization discriminator 58, pulses of respective bursts 60 and 62 interact with one another. The pumping scheme may be configured analogously to that of FIG. 3, i.e., pump 44 and mirror 46 coupling the pump light into the output end of booster 28 in a counter propagating direction. The disclosed pumping scheme, as mentioned above, is not limited to the illustrated structure and may have other configurations easily realized by one of ordinary skill in the laser arts. [036] The light output from booster 28 is further coupled into an optional polarization discriminator 58 which is configured to process the first output at the desired polarization. The respective wavelengths λΐ and λ2 may be uniform or different. If a need arises, the

configurations shown in FIGS. 2-4 may operate with the outputs of respective lasers 22 and 32 having different wavelengths and different polarizations.

[037] The disclosed source of FIGS. 2-4 is a flexible structure which is operative to adjust the format of pulses. The pulse format is defined as the repetition rate of pulses, the pulse duration, and the number of pulses in a burst. Changing any of or a combination of the pulse

characteristics alters the overall energy in the burst of signal light pulses. To provide a substantially constant level of population inversion between the signal pulses, if the energy of signal light pulses is changed, controller 42, which can be utilized in each of the embodiments of respective FIGS. 3 and 4, outputs a control signal coupled into the driver of additional oscillator 32. As a result, the modulation of the current at the input of additional oscillator 32 adjusts the energy of additional radiation 48. Typically, the output of additional oscillator 32 is controlled to adjust the population inversion level between the signal pulses so that it remains substantially the same over the entire process which provides the desired shape (and energy) of signal light pulses 24 at the output of the disclosed source as exemplified in FIGS. 5A - 5C and 6.

[038] FIG. 5 A illustrates the format of the bursts 24 of signal light pulses at the input of booster 28 of FISs. 2 - 4. The bursts 24 are characterized by a uniform pulse repetition rate and the number of pulses sufficient to fill out the entire burst. FIG. 5B shows secondary burst of pulses at the input of the booster wliich are substantially uniform with bursts 24. FIG. 5C illustrates the output of the booster.

[039] FIG. 6 illustrates the output of booster 28 of FIGS. 2-4 with a population inversion level being substantially the same during each burst 24 of signal pulse light and between the bursts thereof. The format of burst 24 is different from that of FIGs. 5 A - 5C. Here each burst 24 has individual signal light pulses grouped in sub-bursts 24 'by means of the pulse slicer. Obviously, sub-bursts have a repetition rate different from that of individual pulses within the sub-burst.

[040] The output of additional oscillator 32 of FIGS. 2-4 can be easily controlled to adjust a population inversion level between bursts 24 of signal light pulses in booster 28 to a desired level. The output of oscillator 32 may be adjusted by modifying current at the input of oscillator 32. Alternatively or in addition, the wavelength of radiation emitted by additional oscillator 32 may be controllably changed. Also, if oscillator 32 has a pulse generating configuration, the pulse frequency can be controlled. All of the output adjusting techniques can be utilized to either maintain the energy of signal pulse bursts 24 constant, or to controllably adjust the energy of individual bursts 24.

[041] The operation of the disclosed light source of FIGS. 2-4 is not limited to alternating the pulses at different wavelengths. The complimentary source may be turned to output an additional pulse which overlaps burst 24 of light signal pulses. In general, depending on the operation of additional oscillator 32, each individual or desired combination of individual light pulses within burst 24 can be shaped to have the desired energy at the desired time.

[042] FIGs. 7A - 7B illustrate the operation of both light oscillators 22 and 32 of FIGS. 2-4 which emit respective bursts at least partially overlapping each other in time. Assume that burst 24 of light signal pulses is required to have a dip in the middle of the burst. By controllably switching on additional oscillator 32 to output pulse 48 at the right time and right amplitude while seed source 22 is still on, individual light signal pulse 50 located in the middle of the burst 24 can be reduced as shown in FIG. 6C. Obviously, any individual or selected combination of light signal pulses within burst 24 may be modified as well to meet any required shape of burst 24. For example, a dip 52 has sharp edges because pulse 48 of FIG. 6B and pulse 50 have the same duration and perfectly synchronized. However, if additional pulse 48 were time shifted relative to signal light pulse 50, it would affect neighboring individual signal light pulses. As a result, dip 50 would have gradually falling and rising flanks.

[043] Having described at least one of the preferred embodiments of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments. For example, the disclosed light source may have more than one second laser operating at a wavelength λ3 different from λΐ and λ2. Again adjusting the output of still another additional laser so as to obtain the population inversion level in the booster matching that of the seed is fully within the scope of this invention. The disclosed light source may operate in single or multiple transverse modes depending on the customer's requirements. Various changes, modifications, and adaptations including different wavelengths, fiber parameters and rare-earth dopants may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as disclosed above.

Claims

1. A laser source for sequentially delivering controllable bursts of signal pulses, comprising: a seed laser outputting a first output including the bursts of signal pulses;

an additional laser lasing a second output different from the first output;

a fiber booster receiving the first and second outputs of respective seed and additional lasers and emitting the amplified first and second outputs in a propagating direction, the additional laser being adjustable to controllably output the second output so as provide a desired level of population inversion in the fiber booster, wherein the signal pulses of the amplified first output have a desired shape and energy; and

a filter located downstream from the fiber booster and configured to discriminate the amplified first output and second outputs, which are incident thereon, from one another.

2. The laser source of claim 1, wherein the first and second outputs from respective seed and additional lasers are emitted at different wavelengths λΐ and λ2, respectively, the filter being configured to discriminate different wavelength from one another.

3. The laser source of claim 1, wherein the first and second output from respective seed and additional lasers are emitted at the same wavelength, the first and second outputs from respective seed and additional lasers being emitted having respective different polarizations, the seed laser, additional laser, fiber booster being configured in a polarization maintaining (PM) format, the filter being configured to discriminate different polarizations from one another.

4. The laser source of claim 1, wherein the first and second outputs from respective seed and additional lasers are emitted at different wavelengths λΐ and X2, respectively and with different respective polarizations.

5. The laser source of claim 1, wherein the seed laser operates so that signal pulses in each burst of the first output having a controllable amplitude and duration.

6. The laser source of claim 5, wherein the second output emitted by the additional laser is coupled into the fiber booster between adjacent first outputs.

7. The laser source of claim 1, wherein the additional laser is controlled to have the second output providing the desired level of population inversion in the fiber booster substantially the same as that during amplification of the first output so that the fiber booster emits a continuous and uniform output.

8. The laser source of claim 1, wherein the seed and additional lasers each are configured to operate in a continuous wave ("CW") or pulsed regime, or quasi CW.

9. The laser source of claim 1 further comprising a main console housing the seed and additional lasers and further at least one preamplifier, wherein the seed laser and preamplifier are arranged in a master oscillator - power amplifier configuration.

10. The laser source of claim 9 further comprising a laser head spaced from the console which houses a beam guiding optic and the filter spaced downstream from the beam guiding optics.

11. The laser source of claim 10, wherein the fiber booster includes

a fiber doped with ions of rare-earth elements and extending over free space between the console and laser head, the doped fiber having

an input end region, which receives the first and second outputs of respective seed and laser in the console, and

an output end region which terminates in the laser head,

the beam guiding optic including a reflector which is provided with a central opening traversed by an output of the fiber booster and configured to reflect pump light in a counter- propagating direction so that the pump light is coupled into the output end region of the doped fiber, and

a protective cable surrounding the doped fiber between the main console and laser head.

12. The laser source of claim 1, wherein the additional laser is selected from diode lasers or fiber lasers.

13. The laser source of claim 2, wherein the filter being selected from the groups consisting of fiber Bragg grating, volume Bragg grating and bulk optics.

14. The laser source of claim 1 further comprising:

a first pulse stretcher located between the seed laser and fiber booster, the first stretcher being configured to chirp the signal pulses,

a second pulse stretcher receiving the second output, which includes a train of supplementary pulses, from the additional laser, and operative to chirp each of the supplemental pulses, the supplemental pulses being coupled into the fiber booster so that the fiber booster emits the continuous output including alternating signal and supplemental pulses.

15. The laser source of claim 1 further comprising a pulse sheer located between the seed laser and fiber booster and configured as an acousto-optic modulator (AOM) or electro-optic modulator (EOM) or semiconductor optical amplifier.

16. The laser source of claim 1, wherein the spatial filter is configured to compress the signal pulses.

17. The laser source of claim 1 further comprising a controller operative to emit a control signal governing a sequential switching operation of the seed and additional lasers.

18. The laser source of claim 17 wherein the controller is operative to output a control signal coupled into a driver of the additional laser to modify an output of the additional laser so as to maintain the level of the population inversion in the fiber booster constant.

19. The laser source of claim 1, wherein the inversion level is controlled to be constant or variable.