EP4305715A1 - Variable repetition rate multiplier based on polarization rotation - Google Patents

Variable repetition rate multiplier based on polarization rotation

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Publication number
EP4305715A1
EP4305715A1 EP21933413.3A EP21933413A EP4305715A1 EP 4305715 A1 EP4305715 A1 EP 4305715A1 EP 21933413 A EP21933413 A EP 21933413A EP 4305715 A1 EP4305715 A1 EP 4305715A1
Authority
EP
European Patent Office
Prior art keywords
repetition rate
multiplier
input
polarization
variable
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP21933413.3A
Other languages
German (de)
French (fr)
Other versions
EP4305715A4 (en
Inventor
Seydi YAVAS
Söhret Görkem KARAMUK
Hamit KALAYCIOGLU
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Lumos Lazer AS
Original Assignee
Lumos Lazer AS
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Lumos Lazer AS filed Critical Lumos Lazer AS
Publication of EP4305715A1 publication Critical patent/EP4305715A1/en
Publication of EP4305715A4 publication Critical patent/EP4305715A4/en
Pending legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2726—Optical coupling means with polarisation selective and adjusting means in or on light guides, e.g. polarisation means assembled in a light guide
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2861—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using fibre optic delay lines and optical elements associated with them, e.g. for use in signal processing, e.g. filtering
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2706—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
    • G02B6/2713—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations
    • G02B6/272—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations comprising polarisation means for beam splitting and combining

Definitions

  • the invention relates to a system and a method that relates to laser optics for generating variable pulse repetition rate multiplication based on polarization rotation.
  • the invention especially relates to the field of laser optics specifically developed for applications which utilize pulsed laser radiation.
  • the invention provides a user adjustable variable pulse repetition rate which will make possible a versatile laser source that can be used for various different applications or optimized for processing different materials.
  • Pulsed lasers find wide use in numerous fields such as material processing, spectroscopy, microscopy and medical applications with an increasing frequency every year. Specifically, material processing with ultrafast pulses has superb aspects as minimal collateral damage and high precision with the down-sides of traditionally being a slow process, requiring complex high- energy lasers [1]
  • ablation-cooled laser material regime [2] opened the door to transcending the limited ablation rates and the need for tens to hundreds of microjoules. In this regime, the repetition rate has to be high enough such that there is insufficient time for the targeted spot size to cool down substantially by heat conduction into the rest (bulk) of the target material by the time the next pulse arrives. Subsequent pulses ablate a target material that is already extremely hot, thereby reducing the individual pulse energy threshold and thermal effects to the bulk of the target. Furthermore, the simultaneous reduction of pulse energy and the pulse-to-pulse spacing reduces plasma shielding effects.
  • thermal relaxation times can be in the nanosecond range, which requires the repetition rates to be in the few-GHz range to fully exploit the ablation cooling effect.
  • signals either from mode-locked or modulated laser sources, at the desired repetition rates.
  • SESAM-mode-locked semiconductor disk lasers and vertical-external-cavity surface-emitting lasers have achieved few picosecond and sub-picosecond pulse durations at multi-GHz repetition rates [3], but they are specialized solutions.
  • femtosecond lasers with few hundred MHz to several GHz repetition rate have been developed [4-9], which generally are built of specialized components such as very high doped glass fibers, hybrid WDM-collimators.
  • the laser systems with modulated seed sources [10, 11] can provide repetition rates up to few hundred MHz which is limited mainly by the capacity and bandwidth of the driver electronics.
  • direct generation of femtosecond pulses at multi-GHz repetition rates faces practical challenges.
  • Figure 1 shows an example of a pulse repetition rate multiplier made of 3-dB couplers. Free space version of this technique is also viable where 50/50 beam splitters replace the 3-dB couplers [18]
  • this method at each stage light beam is divided into two beams equal in power and combined after a delay is imposed on one of them equal to one half of the input pulse repetition period. Hence, the repetition rate is doubled at output of each stage and cascading this process multiplies the repetition rate by two at each stage.
  • Figure 2 shows an example of a pulse train in burst mode.
  • continuous GHz-level pulse trains can be used for processing; however, this would require very high average powers from the lasers introducing thermal management problems and also very high scanning speeds from the scanners which direct the laser beam onto the material, where such rates are not available in the present technology.
  • burst mode by controlling the burst repetition rate, the average power can be kept at reasonable levels.
  • burst-mode laser systems with high intra-burst repetition rates obtained via repetition rate multipliers.
  • the generation of burst mode laser beam where method of cascaded 3-dB fiberoptic couplers is used for dividing pulses and combining them with time offset, has been patented [19]
  • patents have been issued for a fiber-based laser system switchable between two repetition rates, one less than 0.5 GHz and the other at least GHz [20], and a high power high repetition rate laser system generating ultrashort pulses with GHz repetition rate [21], both of which use cascaded 3dB couplers.
  • Amplitude Systems owner of the patent in [21] and an active researcher of material processing with GHz repetition rates, has been offering burst-mode ultrafast lasers with GHz intra-burst repetition rates.
  • a method to obtain a flexible laser system that can generate a variable pulse repetition rate covering a wide range up to several GHz’s and down to MHz and even possibly to kHz levels.
  • Figure 3 gives an idea on the pulsed laser applications with different repetition rates.
  • the pulse repetition rate of ultrafast laser amplifier systems (pulse duration ⁇ few picoseconds) which use mode-locked laser resonators as seed sources, is either fixed or switchable at most between two levels, a low and high one as in [20]
  • the laser systems with modulated seed sources [10, 11] can provide repetition rates up to few hundred MHz which is limited mainly by the capacity and bandwidth of the driver electronics.
  • the present invention relates to laser systems specifically developed for applications which utilize pulsed laser radiation meeting the needs mentioned above, eliminating all disadvantages, and providing some additional advantages.
  • Main purpose of the invention is to propose a novel technique and a system for variable pulse repetition rate multiplication which will be user adjustable and will make possible a flexible laser system with flexible repetition rate parameters that can accommodate many different applications.
  • the technique in the invention converts a laser seed signal with pulse repetition rate of f 0 , to a signal with repetition rate choices of f 0 , 2fo, 4f 0 , ... ,2 n f 0 where n is the number of multiplier stages used in the system, limited by practical means.
  • this signal can cover from 100 MHz to several GHz range which would provide the means to choose optimal processing frequency for most of the materials.
  • variable repetition rate multiplier (1) for a laser system comprising: • A number n of cascaded controllable repetition rate multiplier units (2) for providing variable repetition rate with choices of f 0 , 2fo, 4f 0 , ... ,2 n f 0 where f 0 is the repetition rate of input beam and n is equal to at least 1,
  • a delay (6) equal to one half with a tolerance of ⁇ 5% of the input pulse repetition period in one of the two arms of each multiplier stage (A) which the split beams travel on,
  • Figure 1 Exemplary repetition rate multiplier made of 3-dB couplers.
  • Figure 2 Exemplary pulse train in the burst mode.
  • Figure 4 The variable repetition rate multiplier with cascaded stages.
  • Figure 6 The fiberoptic version of the single multiplier unit.
  • Figure 7 The free space version of the single multiplier unit with beam splitter cubes as the beam splitting and combining elements.
  • the invention relates to a variable repetition rate multiplier (1) of a laser system for applications which utilize pulsed laser radiation.
  • the variable repetition rate multiplier (1) is given in Figure 4.
  • the variable repetition rate multiplier (1) comprises several cascaded controllable repetition rate multiplier units (2).
  • Variable pulse repetition rate is enabled by cascaded controllable repetition rate multiplier units (2) which provide user controllable choices of f 0 , 2f 0 , 4f 0 , ... ,2 n f 0 at output, where f 0 is the input pulse repetition rate and n is the number of multiplier stages.
  • the upper limit of n is determined by practical limitations.
  • Each repetition rate multiplier unit (2) ( Figure 5) comprises at least one controllable polarization rotator (3) at input followed by at least one polarization dependent beam splitting element (5), at least one beam combining element (8) at the exit end.
  • the polarization rotator (3) controls the repetition rate multiplication factor by rotating the polarization of the input beam.
  • the beam splitting element (5) splits the incident beam to two beams with orthogonal polarizations.
  • the control on the repetition rate multiplier unit (2) is enabled by at least one user controllable electronic driver (4) or directly by at least one computer which controls the polarization rotator (3) by sending electrical signal to the driver of the polarization rotator (3).
  • the polarization rotator (3) rotates the polarization of the linearly polarized incident beam by a user set angle.
  • the beam splitting element (5) is a polarization selective element which splits the input beam to two beams with orthogonal polarizations.
  • the splitting ratio is determined by the orientation of the incident polarization with respect to the axes of the beam splitting element (5), incident polarization direction is in turn controlled by the polarization rotator (3).
  • the incident beam goes through with no splitting when its polarization is parallel to one of the two orthogonal axes of the beam splitting element (5), or it splits into two beams with orthogonal polarizations of equal power when its polarization makes 45° with the beam splitting element (5) axes.
  • At least one beam combining element (8) is responsible for combining the two split beams.
  • the repetition rate is unchanged, while in the second case it is doubled since one of the split beams go through a path with delay (6) with respect to the other one, equivalent to half the pulse repetition period.
  • Delay (6) which is a length difference between the paths of the split beams, facilitates repetition rate doubling by generating a delay equal to half the input pulse repetition period between the split beams.
  • one of the split beams is subjected to a polarization rotation (7) within a range of 80° to 100° by either a polarization rotator component or a polarization rotation scheme, to align its polarization with the other one before the recombination so that the combined output beam has a single linear polarization.
  • each one of the first m stages in the cascaded multiplier system is set to split the input beam to double the repetition rate as explained above, and the rest of the following stages are used to transmit the input signal without splitting.
  • the basic fiberoptic repetition rate multiplier unit (2) ( Figure 6) comprises at least one externally controllable fiber-coupled polarization rotator (3) at the input followed by at least one beam splitting element (5) which is polarization dependent and at least one beam combining element (8) at the exit end.
  • the fiber-coupled polarization rotator (3) can be made of two collimators aligned with each other and a half-wave plate in between them held in a motorized rotation stage or a liquid crystal polarization rotator, or a fiber squeezing system based on a piezo, motor or manual translation system.
  • the rotation stage (or the liquid crystal polarization rotator, orthe fiber squeezing system) is controlled by an electronically controlled driver (4).
  • the wave plate is rotated or the liquid crystal or fiber squeezing system is sent a signal, the linear polarization of the input beam is rotated and via this rotation the transmission through the polarization dependent beam splitting element (5) is controlled.
  • the linear polarization output from the polarization rotator (3) can be set such that either it is parallel to either one of the orthogonal axes of the polarization maintaining input fiber of the beam splitting element (5), or it makes 45° with them.
  • the beam is transmitted through beam splitting element (5) without splitting, while in the second case it is split into two orthogonally polarized beams of equal power.
  • the beam splitting element (5) has a single input and two output arms, each of which transmits a polarization along the slow or fast axis.
  • the output arms of the beam splitting element (5) are connected to two input arms of the beam combining element at the exit end of the unit.
  • the beam combining element can be a 3-dB coupler or a polarization dependent beam splitting element.
  • the 3-dB coupler is a fiberoptic 50/50 coupling element with two input, and one or two output arms.
  • the beam entering from one arm is split into two beams of equal power on the other side, when beams enter from both arms on one side of the coupler, an arm on the output side, either there is one or two of them, will have a signal equal to the sum of half of the beams from each input arm.
  • a polarization dependent beam splitting element may be used in reverse direction to combine the orthogonally polarized split beams.
  • the polarization rotation in one of the split fiber arms can be implemented in a few alternative ways.
  • First and simplest one is, to have the beam combining element (8) of the type with one of the polarization axes (called fast and slow axes) blocked so that the split beams in the two arms have to enter the coupler with their electrical fields aligned along the same axis.
  • Second one is in the form of a shifted splice, where the two orthogonal axes (called fast and slow axes) of the fibers to be connected are aligned so that the polarization after the splice shifts to the same axis as of the beam in the other arm.
  • Third alternative is the placement of a fiberoptic circulator and Faraday mirror which will rotate the polarization by 90°, where in this case the fiber length difference would also be embedded in the circulator for practical reasons.
  • the free space repetition rate multiplier unit (2) ( Figure 7) is made up of at least one externally controllable polarization rotator (3) at the input, followed by at least one polarization dependent beam splitting element (5), polarization rotation (7) in one of the arms after the polarization beam splitting element (5), at least two flat mirrors (9) for steering the beam in one of the arms towards the beam combining element and at least one beam combining element (8) that recombines the beams.
  • the elements that make up the free space version of the multiplier unit are shown in the Figure 7. In exceptional case of one-stage multiplier, the polarization rotation (7) may be omitted.
  • the controllable polarization rotator (3) can either be a half-wave plate manually adjustable or held by a motorized rotational stage, or a liquid crystal polarization rotator both of which can be controlled via electronically controlled drivers (4).
  • the polarization dependent beam splitting element (5) may be a polarization selective cube or plate that splits the incoming beam into two beams with orthogonal polarizations travelling in directions perpendicular to each other, where a path difference equivalent to half the input pulse repetition period exists between the paths of the split beams which meet at the beam combining element at the exit end of the multiplier stage.
  • the polarization rotation (7) can be realized by a half-wave plate which is set to impose a rotation on the transmitted beam with respect to the incident beam in order to align the polarization of the two split beams before combining.
  • the beam combining element may be a beam splitter cube (10) or plate either of which is polarization independent and splits the incoming beam to two beams of equal power travelling in perpendicular directions.
  • each beam will be split into two equal components on perpendicular directions.
  • the incoming beams can be aligned such that in each of the two exit directions one obtains the sum of halves of the incident two beams. Hence each exiting beam will be a combination of the incident beams on the beam splitter and equal in power.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Plasma & Fusion (AREA)
  • Signal Processing (AREA)
  • Theoretical Computer Science (AREA)
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Abstract

The invention relates to a method and a system for generating variable pulse repetition rate for a laser system. The invention comprises n number of cascaded controllable variable repetition rate multiplier (1) stages, where n is equal to at least one. In each repetition rate multiplier unit (2), the linearly polarized incident laser beam is transmitted through an externally controllable polarization rotator (3) followed by a polarization dependent beam splitting element (5) which either splits the beam into two orthogonally polarized beams or transmits it without splitting where either case can be selected using the polarization rotator (3). In one of the two optical paths following the polarization selective element a delay (6) equal to half the input pulse repetition period is present. A beam combining element (8) at the output of each repetition rate multiplier unit (2) combines the split beams or transmits the single beam. Thus at each multiplier stage, either the input pulse repetition rate is doubled or preserved, so that with n stages, selectable repetition rates of f0, 2f0, 4f0,…,2nf0 are generated where f0 is the repetition rate of the input beam.

Description

VARIABLE REPETITION RATE MULTIPLIER BASED ON POLARIZATION ROTATION
Technical Field
The invention relates to a system and a method that relates to laser optics for generating variable pulse repetition rate multiplication based on polarization rotation.
The invention especially relates to the field of laser optics specifically developed for applications which utilize pulsed laser radiation. The invention provides a user adjustable variable pulse repetition rate which will make possible a versatile laser source that can be used for various different applications or optimized for processing different materials.
The State of the Art
Pulsed lasers find wide use in numerous fields such as material processing, spectroscopy, microscopy and medical applications with an increasing frequency every year. Specifically, material processing with ultrafast pulses has superb aspects as minimal collateral damage and high precision with the down-sides of traditionally being a slow process, requiring complex high- energy lasers [1] The recently demonstrated ablation-cooled laser material regime [2] opened the door to transcending the limited ablation rates and the need for tens to hundreds of microjoules. In this regime, the repetition rate has to be high enough such that there is insufficient time for the targeted spot size to cool down substantially by heat conduction into the rest (bulk) of the target material by the time the next pulse arrives. Subsequent pulses ablate a target material that is already extremely hot, thereby reducing the individual pulse energy threshold and thermal effects to the bulk of the target. Furthermore, the simultaneous reduction of pulse energy and the pulse-to-pulse spacing reduces plasma shielding effects.
Considering that metals and many other technologically important materials, particularly, silicon have high thermal diffusivity, thermal relaxation times can be in the nanosecond range, which requires the repetition rates to be in the few-GHz range to fully exploit the ablation cooling effect. In order to obtain such high repetition rates with ultrafast pulses, one needs signals, either from mode-locked or modulated laser sources, at the desired repetition rates. SESAM-mode-locked semiconductor disk lasers and vertical-external-cavity surface-emitting lasers have achieved few picosecond and sub-picosecond pulse durations at multi-GHz repetition rates [3], but they are specialized solutions. On the fiber side, femtosecond lasers with few hundred MHz to several GHz repetition rate have been developed [4-9], which generally are built of specialized components such as very high doped glass fibers, hybrid WDM-collimators. On the other hand, the laser systems with modulated seed sources [10, 11] can provide repetition rates up to few hundred MHz which is limited mainly by the capacity and bandwidth of the driver electronics. Thus, direct generation of femtosecond pulses at multi-GHz repetition rates faces practical challenges. It is possible to use Fabry-Perot and other passive cavities to scale up the repetition rate with excellent fidelity and low noise, but this approach is quite complex [12, 13] Free space methods have also been proposed for repetition rate multiplication [14] A simple method appears to be the use of pulse repetition rate multiplication (PRRM) via cascaded couplers [15-17] Among such methods, multiplication with cascaded 3-dB fiberoptic couplers offers a simple and effective solution by an integer multiplication factor of 2(W_1), where N is the number of couplers [15, 16,
18] Figure 1 shows an example of a pulse repetition rate multiplier made of 3-dB couplers. Free space version of this technique is also viable where 50/50 beam splitters replace the 3-dB couplers [18] In this method, at each stage light beam is divided into two beams equal in power and combined after a delay is imposed on one of them equal to one half of the input pulse repetition period. Hence, the repetition rate is doubled at output of each stage and cascading this process multiplies the repetition rate by two at each stage.
Despite the reduction of the required pulse energy (typically fractions of a micro- joule), the need for high repetition rates (typically in the GHz range) can only be met through burst-mode operation (Figure 2) for practical reasons with the current state of technology, where groups of high- repetition pulses are repeated at a much lower frequency. Figure 2 shows an example of a pulse train in burst mode. In principle, continuous GHz-level pulse trains can be used for processing; however, this would require very high average powers from the lasers introducing thermal management problems and also very high scanning speeds from the scanners which direct the laser beam onto the material, where such rates are not available in the present technology. On the other hand in the burst mode, by controlling the burst repetition rate, the average power can be kept at reasonable levels. Following the discovery of the ablation-cooled regime of laser material processing [2], research and development efforts have intensified in burst-mode laser systems with high intra-burst repetition rates obtained via repetition rate multipliers. As such, the generation of burst mode laser beam, where method of cascaded 3-dB fiberoptic couplers is used for dividing pulses and combining them with time offset, has been patented [19] Further, patents have been issued for a fiber-based laser system switchable between two repetition rates, one less than 0.5 GHz and the other at least GHz [20], and a high power high repetition rate laser system generating ultrashort pulses with GHz repetition rate [21], both of which use cascaded 3dB couplers. And in the laser market for material processing, Amplitude Systems, owner of the patent in [21] and an active researcher of material processing with GHz repetition rates, has been offering burst-mode ultrafast lasers with GHz intra-burst repetition rates.
A method to obtain a flexible laser system that can generate a variable pulse repetition rate covering a wide range up to several GHz’s and down to MHz and even possibly to kHz levels. Figure 3 gives an idea on the pulsed laser applications with different repetition rates.
The current state of the pulsed laser technology is summarized below.
1- The pulse repetition rate of ultrafast laser amplifier systems (pulse duration < few picoseconds) which use mode-locked laser resonators as seed sources, is either fixed or switchable at most between two levels, a low and high one as in [20]
2- On the other hand, the laser systems with modulated seed sources [10, 11] can provide repetition rates up to few hundred MHz which is limited mainly by the capacity and bandwidth of the driver electronics.
Purpose of the Invention
The present invention relates to laser systems specifically developed for applications which utilize pulsed laser radiation meeting the needs mentioned above, eliminating all disadvantages, and providing some additional advantages.
Main purpose of the invention is to propose a novel technique and a system for variable pulse repetition rate multiplication which will be user adjustable and will make possible a flexible laser system with flexible repetition rate parameters that can accommodate many different applications. The technique in the invention converts a laser seed signal with pulse repetition rate of f0, to a signal with repetition rate choices of f0, 2fo, 4f0, ... ,2nf0 where n is the number of multiplier stages used in the system, limited by practical means. Depending on the input source, this signal can cover from 100 MHz to several GHz range which would provide the means to choose optimal processing frequency for most of the materials. Also, it is possible to obtain kHz and GHz pulse repetition rates from the same laser system with the use of a pulse picker after the repetition rate multiplier, hence this technique will make possible a versatile laser source which can be used for very different applications.
In order to achieve above mentioned purposes, the invention is related to a variable repetition rate multiplier (1) for a laser system comprising: • A number n of cascaded controllable repetition rate multiplier units (2) for providing variable repetition rate with choices of f0, 2fo, 4f0, ... ,2nf0 where f0 is the repetition rate of input beam and n is equal to at least 1,
• At least one externally controllable polarization rotator (3) for controlling the repetition rate multiplication factor by rotating the polarization of the input beam,
• At least one polarization dependent beam splitting element (5) for splitting the incident beam to two beams with orthogonal polarizations,
• A delay (6) equal to one half with a tolerance of ± 5% of the input pulse repetition period in one of the two arms of each multiplier stage (A) which the split beams travel on,
• At least one beam combining element (8) for combining the two beams generated by the beam splitting element.
The structural and characteristics features of the invention and all advantages will be understood better in detailed descriptions below, and therefore, the assessment should be made taking into account the said detailed explanations.
Brief Description of the Drawings
Figure 1 - Exemplary repetition rate multiplier made of 3-dB couplers.
Figure 2 - Exemplary pulse train in the burst mode.
Figure 3 - The pulsed laser applications with different repetition rate ranges.
Figure 4 - The variable repetition rate multiplier with cascaded stages.
Figure 5 - The single unit of the variable repetition rate multiplier in detail.
Figure 6 - The fiberoptic version of the single multiplier unit.
Figure 7 - The free space version of the single multiplier unit with beam splitter cubes as the beam splitting and combining elements.
Description of Parts References
1 Variable repetition rate multiplier
2 Repetition rate multiplier unit
3 Polarization rotator
4 Driver
5 Beam splitting element
6 Delay
7 Polarization rotation
8 Beam combining element 9 Mirror
10 Beam splitter cube
Detailed Description of the Invention
In this detailed description, the invention has been described in a manner not forming any restrictive effect and only for purpose of better understanding of the matter.
The invention relates to a variable repetition rate multiplier (1) of a laser system for applications which utilize pulsed laser radiation. The variable repetition rate multiplier (1) is given in Figure 4. The variable repetition rate multiplier (1) comprises several cascaded controllable repetition rate multiplier units (2). Variable pulse repetition rate is enabled by cascaded controllable repetition rate multiplier units (2) which provide user controllable choices of f0, 2f0, 4f0, ... ,2nf0 at output, where f0 is the input pulse repetition rate and n is the number of multiplier stages. The upper limit of n is determined by practical limitations. Each repetition rate multiplier unit (2) (Figure 5) comprises at least one controllable polarization rotator (3) at input followed by at least one polarization dependent beam splitting element (5), at least one beam combining element (8) at the exit end. The polarization rotator (3) controls the repetition rate multiplication factor by rotating the polarization of the input beam. The beam splitting element (5) splits the incident beam to two beams with orthogonal polarizations.
The control on the repetition rate multiplier unit (2) is enabled by at least one user controllable electronic driver (4) or directly by at least one computer which controls the polarization rotator (3) by sending electrical signal to the driver of the polarization rotator (3). The polarization rotator (3) rotates the polarization of the linearly polarized incident beam by a user set angle.
The beam splitting element (5) is a polarization selective element which splits the input beam to two beams with orthogonal polarizations. The splitting ratio is determined by the orientation of the incident polarization with respect to the axes of the beam splitting element (5), incident polarization direction is in turn controlled by the polarization rotator (3). In the two limiting cases, either the incident beam goes through with no splitting when its polarization is parallel to one of the two orthogonal axes of the beam splitting element (5), or it splits into two beams with orthogonal polarizations of equal power when its polarization makes 45° with the beam splitting element (5) axes. At least one beam combining element (8) is responsible for combining the two split beams. In the first limiting case, the repetition rate is unchanged, while in the second case it is doubled since one of the split beams go through a path with delay (6) with respect to the other one, equivalent to half the pulse repetition period. Delay (6), which is a length difference between the paths of the split beams, facilitates repetition rate doubling by generating a delay equal to half the input pulse repetition period between the split beams. Also, one of the split beams is subjected to a polarization rotation (7) within a range of 80° to 100° by either a polarization rotator component or a polarization rotation scheme, to align its polarization with the other one before the recombination so that the combined output beam has a single linear polarization. To obtain a certain repetition rate 2mf0 for arbitrary m>0 from a variable repetition rate multiplier (1) with n stages where n>m, each one of the first m stages in the cascaded multiplier system is set to split the input beam to double the repetition rate as explained above, and the rest of the following stages are used to transmit the input signal without splitting. Forthe case, m=0, the input repetition rate f0 is preserved such that all stages in the variable repetition rate multiplier (1) are adjusted to transmit the beam without splitting.
In the fiber version, all elements of the invention are made of polarization maintaining fibers, with the exception that in case of n equals one, that is a multiplier containing one stage, the beam combining element may be built of non-polarization maintaining fibers. The orthogonal axes of the polarization maintaining fibers are called slow and fast axes. The basic fiberoptic repetition rate multiplier unit (2) (Figure 6) comprises at least one externally controllable fiber-coupled polarization rotator (3) at the input followed by at least one beam splitting element (5) which is polarization dependent and at least one beam combining element (8) at the exit end. There is a length difference between the two fiber paths connecting the beam splitting element (5) to the beam combining element (8) providing a delay (6) equal to half of the input pulse repetition period. Also, in one of these fiber paths a polarization rotation is implemented so that polarization of the split beams are aligned (made parallel) before combining, with the exception that in case of n equals one, that is a multiplier containing one stage, polarization rotation may not be included. The fiberoptic elements that make up the repetition rate multiplier unit (2) are shown in the Figure 6. The fiber-coupled polarization rotator (3) can be made of two collimators aligned with each other and a half-wave plate in between them held in a motorized rotation stage or a liquid crystal polarization rotator, or a fiber squeezing system based on a piezo, motor or manual translation system. The rotation stage (or the liquid crystal polarization rotator, orthe fiber squeezing system) is controlled by an electronically controlled driver (4). When the wave plate is rotated or the liquid crystal or fiber squeezing system is sent a signal, the linear polarization of the input beam is rotated and via this rotation the transmission through the polarization dependent beam splitting element (5) is controlled. Hence, the linear polarization output from the polarization rotator (3) can be set such that either it is parallel to either one of the orthogonal axes of the polarization maintaining input fiber of the beam splitting element (5), or it makes 45° with them. In the first case, the beam is transmitted through beam splitting element (5) without splitting, while in the second case it is split into two orthogonally polarized beams of equal power. The beam splitting element (5) has a single input and two output arms, each of which transmits a polarization along the slow or fast axis. The output arms of the beam splitting element (5) are connected to two input arms of the beam combining element at the exit end of the unit. The beam combining element can be a 3-dB coupler or a polarization dependent beam splitting element. The 3-dB coupler is a fiberoptic 50/50 coupling element with two input, and one or two output arms. The beam entering from one arm is split into two beams of equal power on the other side, when beams enter from both arms on one side of the coupler, an arm on the output side, either there is one or two of them, will have a signal equal to the sum of half of the beams from each input arm. In the exceptional case of a one-stage multiplier, a polarization dependent beam splitting element may be used in reverse direction to combine the orthogonally polarized split beams. The polarization rotation in one of the split fiber arms can be implemented in a few alternative ways. First and simplest one is, to have the beam combining element (8) of the type with one of the polarization axes (called fast and slow axes) blocked so that the split beams in the two arms have to enter the coupler with their electrical fields aligned along the same axis. Second one is in the form of a shifted splice, where the two orthogonal axes (called fast and slow axes) of the fibers to be connected are aligned so that the polarization after the splice shifts to the same axis as of the beam in the other arm. Third alternative is the placement of a fiberoptic circulator and Faraday mirror which will rotate the polarization by 90°, where in this case the fiber length difference would also be embedded in the circulator for practical reasons.
In the free space version, the free space repetition rate multiplier unit (2) (Figure 7) is made up of at least one externally controllable polarization rotator (3) at the input, followed by at least one polarization dependent beam splitting element (5), polarization rotation (7) in one of the arms after the polarization beam splitting element (5), at least two flat mirrors (9) for steering the beam in one of the arms towards the beam combining element and at least one beam combining element (8) that recombines the beams. The elements that make up the free space version of the multiplier unit are shown in the Figure 7. In exceptional case of one-stage multiplier, the polarization rotation (7) may be omitted. The controllable polarization rotator (3) can either be a half-wave plate manually adjustable or held by a motorized rotational stage, or a liquid crystal polarization rotator both of which can be controlled via electronically controlled drivers (4). The polarization dependent beam splitting element (5) may be a polarization selective cube or plate that splits the incoming beam into two beams with orthogonal polarizations travelling in directions perpendicular to each other, where a path difference equivalent to half the input pulse repetition period exists between the paths of the split beams which meet at the beam combining element at the exit end of the multiplier stage. The polarization rotation (7) can be realized by a half-wave plate which is set to impose a rotation on the transmitted beam with respect to the incident beam in order to align the polarization of the two split beams before combining. The beam combining element may be a beam splitter cube (10) or plate either of which is polarization independent and splits the incoming beam to two beams of equal power travelling in perpendicular directions. When two beams are incident on the beam splitter from perpendicular directions, each beam will be split into two equal components on perpendicular directions. The incoming beams can be aligned such that in each of the two exit directions one obtains the sum of halves of the incident two beams. Hence each exiting beam will be a combination of the incident beams on the beam splitter and equal in power.
REFERENCES
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Claims

1. A variable repetition rate multiplier (1) for a laser system comprising:
• A number n of cascaded controllable repetition rate multiplier units (2) for providing variable repetition rate with choices of f0, 2fo, 4f0, ... ,2nf0 where f0 is the repetition rate of the input beam and n is equal to at least 1 ,
• At least one externally controllable polarization rotator (3) for controlling the repetition rate multiplication factor by rotating the polarization of the input beam,
• At least one polarization dependent beam splitting element (5) for splitting the incident beam to two beams with orthogonal polarizations,
• A delay (6) equal to one half with a tolerance of ± 5% of the input pulse repetition period in one of the two arms of each repetition multiplier unit (2) which the split beams travel on,
• At least one beam combining element (8) for combining the two beams generated by the beam splitting element.
2. A variable repletion rate multiplier (1) according to Claim 1 , wherein all elements are made of fibers.
3. A variable repletion rate multiplier (1) according to Claim 1, which is the free space implementation and includes at least two steering mirrors (9).
4. A method of generating variable pulse repetition rate for a laser system via a variable repetition rate multiplier (1) built of cascaded repetition rate multiplier units according to Claim 1 , comprising:
• wherein at the input of each repetition rate multiplier unit (2) a linearly polarized laser beam with input repetition rate is incident on an externally controllable polarization rotator (3),
• wherein the controllable polarization rotator (3) is followed by a polarization dependent beam splitting element (5) that either splits the beam into two orthogonally polarized beams or transmits it without splitting which is determined by the polarization rotator,
• wherein the case that the beam is split into two, the two beams travel in paths with an optical path difference equivalent to a delay (6) of half the input pulse repetition period with a tolerance of ± 5%,
• wherein the two split beams or the single beam is incident on a beam combining element (8), which combines the split beams or transmits the single beam as it is, • wherein the case that the input beam is split into two, the pulse repetition rate is doubled to twice the input repetition rate while in the case with no splitting it remains the same,
• wherein each multiplier stage either input repetition rate is doubled or it is kept fixed, which generates the user selectable repetition rate choices of f0, 2f0, 4f0, ... ,2nf0 at the final output of n multiplier units in the variable repetition rate multiplier (1), where n is the total number of stages which equals at least 1 and f0 is the input pulse repetition rate for the first multiplier unit at the input of the variable repetition rate multiplier (1).
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WO2000011766A1 (en) * 1998-08-20 2000-03-02 Orbotech Ltd. Laser repetition rate multiplier
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