WO2015080832A1

WO2015080832A1 - System and method for separation of signal and input beams utilizing walk-off phenomenon

Info

Publication number: WO2015080832A1
Application number: PCT/US2014/063411
Authority: WO
Inventors: Alexey Avdokhin; Oleksiy Andrusyak
Original assignee: Ipg Photonics Corporation
Priority date: 2013-11-26
Filing date: 2014-10-31
Publication date: 2015-06-04

Abstract

A frequency converter is configured from a phase-matched non-linear crystal portion which frequency-converts radiation of one or more input beams to the desired frequency of the signal beam. Due to birefringence of the phase-matched crystal portion, the input and signal beams propagate at a walk-off angle relative to one another along the entire length of the phase-matched portion. The frequency converter further includes a non-phase-matched non-linear crystal portion receiving the input and signal beams which continue to propagate at the walk-off angle so as to be spatially separated from one another at the desired distance after exiting the output of the non- phased portion. The desired distance is selected to provide unobstructed propagation of the signal beam to the target to be irradiated while the input beam(s) is incident on an optical component preventing the input beam from impinging upon the target.

Description

SYSTEM AND METHOD FOR SEPARATION OF SIGNAL AND INPUT BEAMS

UTILIZING WALK-OFF PHENOMENON

BACKGROUND OF THE DISCLOSURE

Field of the disclosure

[001] This disclosure generally relates to laser systems and more particularly to laser systems that feature non-linear frequency conversion. Specifically, the disclosed method and device involve spatial separation of at least one input and generated beams within a non-linear frequency converter by utilizing the walk-off phenomenon.

Prior art

[002] The non-linear frequency conversion is a process of converting the frequency of one or more input beams at a fundamental frequency to the desired frequency of a signal beam utilizing non-linear properties of a frequency converting media. The frequency conversion is often realized by using non-linear crystals. In practice, the input beam at the fundamental frequency is not fully converted to the signal beam at the desired frequency which means that multiple wavelengths exit a non-linear crystal and need to be separated if the output in a single wavelength range is desired, which is the goal of the disclosed method and laser system.

[003] Typically, bulk beam splitters, such as dichroic mirrors, are used to separate beams at fundamental and desired frequencies after these beams exit the crystal and before the signal beam impinges on the target to be irradiated. Unfortunately, high intensity radiation at converted frequencies, particularly those in a UV range, quickly degrades the coating of the dichroic mirror. As a result, the operational lifetime of the entire laser system is limited by rapid degradation of dichroic mirrors.

[004] Efficient frequency conversion is realized in non-linear crystals if the phase matching condition is met, as Icnown to artisan. Because of the birefrmgent properties of nonlinear crystals, the critical phase matching, i.e., technique for obtaining phase matching, leads to an angular deviation of the signal beam from one or more input beams at the fundamental frequency. The deviation angle is Icnown to artisan as a walk-off angle and the deviation itself is a phenomenon known to artisan as spatial walk-off. [005] The walk-off phenomenon is undesirable since it reduces the interaction length within the non-linear crystal in which the frequency conversion takes place. The reduced interaction length leads to the reduced efficiency of conversion. In addition, the shape of the signal beam can be modified compared to the shape of the input beam at the fundamental frequency. For example, a circular input beam at the fundamental frequency can yield an elliptical signal beam.

[006] FIG. 1A illustrates an exemplary architecture of a frequency converter configured to generate a particular harmonic, such as second harmonic. In particular, frequency converter 10 is a phase-matched nonlinear crystal configured to convert a single input beam 12 at the fundamental frequency to a frequency-doubled signal beam 14. The frequency conversion process is observed along the entire crystal length between input and output facets 18 and 20, respectively. As a result, despite walk off of signal beam 14 from input beam 12 within the crystal, these input beams are partially overlapped at output facet 20, as shown in FIG. IB. After exiting converter 10 through output facet 20 both beams further propagate parallel to one another and remain partially overlapped. The input beam 12 at the fundamental frequency is then reflected by a dichroic mirror 24 which is transparent to signal beam 14 propagating further to eventually irradiate a target 20'. Commonly, radiation at shorter wavelengths, particularly those in a UV range, has higher photon energies which can quickly degrade the dichroic coating of mirror 24.

[007] A need, thereby, exists for a frequency converter with a high signal to input ratio and long operational lifetime.

[008] Another need exists for a laser system incorporating a frequency converter with a high signal to input ratio and long operational lifetime.

[009] A further need exists for a method that utilizes a walk-off phenomenon in order to efficiently separate signal and input beams within a short distance from the output facet of a frequency converter.

SUMMARY OF THE DISCLOSURE

[010] The inventive frequency converter, structured in accordance with one aspect of the disclosure, meets these needs by including a phase-matched non-linear crystal portion optically coupled to a non-phase-matched non-linear crystal portion. The input phase- matched portion thus converts a fundamental input frequency of at least one input beam to the desired frequency of the signal beam, which propagates at a walk-off angle to the input beam(s) within this portion. The output non-linear crystal portion is not phase- matched which terminates the frequency conversion process. Consequently, the distance between at least one input and signal beams gradually increases towards the exit facet of the output portion as the beams continue to propagate through the output portion at the walk-off angle. The second portion has a length required to completely separate the signal and input beams such that these beams are completely spaced from each other without even a partial overlap.

[Oi l] Upon exiting the frequency converter, the signal and input beams propagate parallel to one another and are spaced apart at a distance that allows a splitter to reflect one of the beams while not clipping or distorting the shape of the other beam. For example, the signal beam can bypass the splitter, such as mirror, on a way to the target to be irradiated, while the mirror reflects the input beam(s).

[012] According to one structural embodiment of the disclosed frequency converter, the phase-matched and non-phase matched non-linear crystal portions are two separate crystals. The coupling of these portions is established by diffusion bonding two crystal portions together so as to provide a single-piece slab having a seam. The rest of the components are similar to those of the previous embodiment.

[013] Another embodiment of the disclosed frequency converter also includes two separate non-linear crystal portions. The portions are in optical contact with one another held by intermolecular forces.

[014] In accordance with still another embodiment of the disclosed frequency converter, the separate portions, like those in previously disclosed embodiments, are spaced apart at a distance from one another but still remain optically coupled. The mechanism of separating input and signal beams is identical to that disclosed above.

[015] According to another embodiment, the frequency converter is configured with a monolithic seamless non-linear crystal. To provide the termination of the frequency conversion, which takes place along one portion of the crystal, the other portion is acted upon by an external actuator so as to modify one or more physical parameters of the crystal. In one variation of this embodiment, the actuator is operable to bend the other portion until the phase-matching condition in this portion is no longer satisfied. The length of the crystal's non-phase-matched portion is selected to provide the desired separation between the beams as they exit the crystal parallel to each other.

[016] In another variation of this embodiment, the force generating actuator is replaced by a heat generating device. The latter is configured to maintain such a temperature of the second output portion that the phase-matching condition is not satisfied and the frequency conversion occurs only in the phase-matched input portion.

[017] Another aspect of the disclosure relates to a laser system incorporating the inventive frequency converter. The laser system may have one or more single-mode light sources emitting a input beam(s) and at least one frequency converter. The separation between the signal and input beams that occurs within the non-phase-matched portion allows the signal beam to bypass a splitter which prevents the input beam(s) from impinging on the target.

[018] A further aspect of the disclosure includes a method of spatially separating signal and input beams by utilizing the walk-off phenomenon.

BRIEF DESCRIPTION OF THE DRAWINGS

[019] The above and other features of the disclosed frequency converter will become more readily apparent from the following drawings, in which:

[020] FIG. 1 A shows a frequency converter of the known prior art.

[021] FIG. IB is a diagrammatic representation of input and signal beams, exiting the frequency converter of FIG. 1 , as the beams would appear on the screen placed directly after the crystal.

[022] FIG. 2A is a view of the inventive frequency converter shown in one plane and configured in accordance with one embodiment of the disclosure in;

[023] FIG. 2B shows the beams as they would appear on the screen placed directly after the crystal of FIG. 2 A.

[024] FIG. 2C is a view of the inventive frequency converter of FIG. 2A shown in a plane perpendicular to that of FIG. 2A;

[025] FIG. 3 is a schematic illustration of technique used in preparation of the frequency converter of FIGs. 2A-2C; [026] FIG. 4 illustrates the inventive frequency converter configured in accordance with another embodiment.

[027] FIG. 5 illustrates the inventive frequency converter configured in accordance with another embodiment.

[028] FIGs. 6A shows the inventive frequency converter configured in accordance with a further embodiment and shown in the same plane as that of FIG. 2C.

[029] FIG. 6B shows the image of input and signal beams as they would appear on the screen placed directly after the nonlinear crystals in FIG. 6A.

[030] FIG. 6C shows the inventive frequency converter of FIG. 6 A shown in the same plane as that of FIG. 2 A.

[031] FIG. 7 illustrates the inventive frequency converter configured in accordance with another embodiment.

[032] FIG. 8 is a schematic of the exemplary laser system utilizing the disclosed frequency converter.

SPECIFIC DESCRIPTION

[033] 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 portions 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 optical connections. The term "input beam" is predominantly used in a singular form, but should be construed as including more than one input beam for the reasons explained below.

[034] FIG. 2A is a view of one of the embodiment of the disclosed frequency converter 50. The frequency converter 50 includes an input portion 52 made from a non-linear crystal. The input portion 52 is phase-matched to convert a frequency of input beam 56 to a frequency of signal beam 58 along its entire length. The input and signal beams 56 and 58, respectively still overlap after these beams exit input portion 52. The overlap between the beams is eliminated by an output portion 54 of frequency converter 50, as disclosed below. [035] The output portion 54, like input portion 52, is made from a non-linear crystal and coupled to the input portion forming a seam 64. In contrast to input portion 52, output portion 54 is not phase-matched. With proper configuration of output non-linear crystal portion 54, the walk-off phenomenon still exists in that portion. However, no frequency conversion occurs as input and signal beams propagate through output portion 54 towards its exit facet at the walk-off angle relative to each other. After propagating a certain distance in portion 54, two beams become spatially separated with the distance between them gradually increasing towards the end of portion 54. As input and signal beams 56 and 58, respectively, exit output portion 54, the beams are completely spatially separated for all practical purposes of this disclosure, as shown in FIG. 2B. The distance d between the beams is sufficient for signal beam 58 to bypass a beam guiding component, such as beam splitter 60, and impinge on a target 62, whereas input beam 56 can be reflected by splitter 60 or collected by other means.

[036] FIG. 2C shows frequency converter of FIG. 2A in the plane perpendicular to that of FIG. 2A. In this view, beams 56 and 58 appear to propagate co-linearly in both crystal portions.

[037] Various frequency conversion methods are considered to be within the scope of the disclosure. For example, the technique known as second harmonic generation which includes converting frequency of a long wavelength input radiation to the second harmonic thereof. Another frequency conversion technique includes mixing two input beams at different frequencies to produce the desired frequency of the signal light through nonlinear interaction. Numerous other frequency conversion techniques can be realized within the scope of the present invention. The possibility of having multiple input beams readily explains why the scope of the present disclosure covers, in addition to a single input beam, numerous input beams.

[038] FIG. 3 illustrates one of possible practical implementations of frequency converter 50 of FIG. 2 A. In accordance with the shown implementation, input and output portions 52 and 54, respectively, can be cut from the same or different boules so that their respective optical axes Oin and Oout are not parallel to one another. The portions 52 and 54can be then diffusion bonded at seam 64. Alternatively, portions 52 and 54 can be spaced apart at a short distance. Still another configuration of the disclosed frequency converter includes input and output portions 52 and 54, respectively, which are in optical contact.

[039] Still another implementation of frequency converter 50 includes forming both phase-matched and non-phase-matched portions from a single (and thus seamless) monolithic non-linear crystal. This can be realized in accordance with techniques illustrated in FIGs. 4 -5.

[040] Refening to FIG. 4, a pressure-generating device 66 is used to bend output portion 54 of seamless monolithic frequency converter 50 until phase-matching condition is no longer satisfied in this portion. The required applied pressure can be easily determined by one of ordinary skill.

[041] FIG. 5 illustrates a heater 68 which provides a temperature difference between input and output portions 52 and 54. The required temperature difference is determined such that phase matching condition is satisfied only in input portion 52 while output portion 54 is not phase-matched. One or both portions 52 and 54 can be thermo- regulated to achieve the desired temperature difference. FIG. 5 shows the embodiment in which both portions are thermally controlled by respective heaters Tl and T2 that may be located along a single, for example, bottom side or both top and bottom sides.

[042] FIGs. 6A - 6C illustrate another embodiment of converter 50 formed by two separate nonlinear phase-matched and non-phase-matched portions 52 and 54, respectively. In contrast to the embodiments of FIGs. 2-5, the portions are coupled in such a way that their respective optical axes are orthogonal to each other. This represents a specific example of a general case in which optical axes of the portions lie in different planes (not necessarily orthogonal to each other).

[043] Refening specifically to FIG. 6A, converter 50 is shown in the plane identical to that of FIG. 2C. However in contrast to the embodiment of FIGs. 2A - 2C where the signal beam deviates from the input beam due to walk-off, in the embodiment of FIGs. 6A - 6C input beam 56 deviates from signal beam 58 due to the same phenomenon. This difference between the present and previously disclosed embodiments is the result of the optical axis of output portion 54 being in a different plane from the optical axis of input portion 52. In the example of this embodiment of FIG. 6, the axes are orthogonal to each other. FIG. 6C illustrates propagation of the same beams 56, 58 of FIG. 6A but in the plane orthogonal to the plane of FIG. 6A (and in the same plane as that of FIG. 2A). The goal achieved by the converter in this embodiment is the same as that of previously

disclosed embodiments - spatial separation of the signal and input beams at the output which is shown in FIG. 6B.

[044] FIG. 7 illustrates a further embodiment of converter 50 which is configured with input portion 52 that is similar to that of previously disclosed embodiments. The output portion 54 is configured as a birefringent prism. However, not any birefringent prism can be incorporated within the scope of the disclosure. Technologically, an upstream portion 82 of the prism should be made from the same material as that of input portion 52 of converter 50. This condition is necessary since if materials of portions 52 and 82 are different, respective coefficients of thermal expansion are different as well which would lead to a poor bonding between portions 52 and 82. The other condition necessary to successfully use a birefringent prism is the angle between input portion 82 and output portion 84. Since the beams incidents on input portion 82 are already polarized, what is necessary for their successfully separation is the same angular relationship as the one on FIG. 6A-6C. Of course, in contrast to previous embodiments, input and signal beams 56, 58 do not leave output portion 84 in parallel planes, but rather are emitted at an angle relative to one another. The technique of separation remains the same as in the previously disclosed embodiments and includes deflection of input beam 56 by element 60.

[045] FIG. 8 illustrates an exemplary single mode laser system 70 incorporating the disclosed frequency converter 50. The source 72 may have a variety of configurations emitting a single mode beam 74 at about 1064 nm. For example, the source can be

configured as a master oscillator-power amplifier ("MOP A") single mode, single

frequency source including a fiber amplifier.

[046] The IR beam 74 is incident on a frequency converter 76 which converts the fundamental frequency of source radiation to a second harmonic producing an output beam 78 at about 532 nm. The converter 76, as known to artisan, is configured with a non-linear crystal. While not necessary, converter 76 may have the same configuration as inventive frequency converter 50. The frequency converter 76 is preferably made from Lithium triborate (LiB₃0₅), but may include any known non-linear crystals appropriate for the given task. [047] The output beam 78 of the first frequency converter 76 is finally coupled into the disclosed frequency converter 50 which is operative to further frequency convert the radiation to the desired output wavelength. In this particular system, the last frequency converter 50 is a second harmonic generator producing output at about 266 nm (fourth harmonic of fundamental radiation 74). The last frequency converter 50, when configured in accordance with the disclosed invention, allows one to efficiently separate the input radiation 78 from a system output beam 90 by one of numerous conventional methods. Numerous types of non-linear crystals may be used for manufacturing the disclosed frequency converter. The illustrated architecture of system 70 incorporates the disclosed frequency converter which is made from Barium borate ("BBO").

[048] A variety of changes of the disclosed structure may be made without departing from the spirit and essential characteristics thereof. Thus, it is intended that all matter contained in the above description should be interpreted as illustrative only and in a limiting sense, the scope of the disclosure being defined by the appended claims.

Claims

1. A frequency converter based on a non-linear crystalline medium, comprising:

an input phase-matched portion frequency-converting radiation of at least one input beam to a desired frequency of a signal beam, the one input and signal beams propagating through the input phase-matched portion at a walk-off angle relative to one another; and

at least one output non-phase-matched portion receiving the one input and signal beams and configured so that the beams continue to propagate at the walk-off angle through the output portion and are spatially separated from one another at a desired distance at an output of the non- phased-matched portion, wherein the desired distance is sufficient to prevent the beams from overlapping one another.

2. The frequency converter of claim 1, wherein the input and output portions are diffusion bonded.

3. The frequency converter of claim 1, wherein the input and output portions are optical contact bonded.

4. The frequency converter of claim 1, wherein the input and output portions are spaced apart.

5. The frequency converter of claim 1, wherein the input and output portions are parts of a monolithic seamless slab of the non-linear crystalline medium.

6. The frequency converter of claim 5 further comprising an actuator coupled to the output portion of the monolithic seamless slab and operative to deform the downstream part to modify at least one of parameters defining the phase matching condition so as terminate the latter.

7. The frequency converter of claim 6, wherein the actuator includes a pressure generating device or a thermo-electrical cooler.

8. The frequency converter of claim 1 further comprising a second input beam at a second input frequency, the input frequencies of respective one and second input beams being mixed to convert to the desired frequency of the radiation of the signal beam.

9. The frequency converter of claim 1, wherein the output portion is configured as a birefringent prism which includes an upstream part bonded together with the input portion and made from material similar to that of the input portion, and a downstream part.

10. A single mode, single frequency laser system comprising:

the frequency converter of claims 1 through 8; and

a beam guiding component spaced downstream from the frequency converter, the input and signal beams being spatially separated at the desired distance selected so that while the input beam is incident on the component, the signal beam bypasses the beam guiding component to irradiate a target.

11. The laser system of claim 10 further comprising at least one laser system emitting a single mode laser beam at a fundamental frequency, a first input source receiving the laser beam and configured as a second harmonic generator which is operative to convert the fundamental frequency to the frequency of the input signal.

12. The laser system of claim 10 further comprising a second input source emitting radiation of a second input beam at a frequency different from the frequency of the input beam, the second input beam being incident on the phase-matching input portion of the frequency converter in which the frequencies of respective input beams are mixed to convert to the desired frequency of the signal beam.

13. A method for spatial separation of at least one input and signal frequencies of respective radiations of at least one input and signal beams, respectively, comprising:

irradiating an input phase-matched non-linear crystal portion with the one input beam, thereby converting input frequency to the signal frequency while input and signal beams propagate along the input portion at a walk-off angle relative to one another; and terminating frequency conversion in a non-phase-matched output non-linear crystal portion which is optically coupled to the input portion, wherein the input and signal beams continue to propagate through the output crystal portion at the walk-off angle and exit therefrom at a desired distance.

14. The method of claim 13 further comprising reflecting the one input beam by a beam guiding component before the signal beam hits a target, the desired distance between the input and signal beams being sufficient for the signal beam to bypass the beam guiding component.

15. The method of claim 13 further comprising iiTadiating the input phase-matched portion with a second input beam, thereby mixing up frequencies of respective input beams together to convert to the desired frequency of the signal beam.

16. The method of claim 13 further comprising diffusion bonding the input and output crystal portions to one another before irradiating the input portion.

17. The method of claim 13 further comprising optical contact bonding the input and output crystal portions to one another before iiTadiating the input portion.

18. The method of claim 13 further comprising:

providing a single seamless non-linear phased-matched crystal including the input and output crystal portions, and

externally acting upon the output crystal portion to terminate a phase matching condition therein.

19. The method of claim 18, wherein the externally acting on the output crystal portion includes generating a bending force or creating a temperature differential between the input and output crystal portions.

20. A frequency converter based on a non-linear crystalline medium, comprising: an input phase-matched portion frequency-converting radiation of at least one input beam to a desired frequency of a signal beam, the one input and signal beams propagating through the input portion at a walk-off angle relative to one another, the input and output portions being configured with a substantially uniform coefficient of thermal expansion; and

at least one output non-phase-matched portion and spaced from the input portion at most at a distance smaller than a wavelength of the input beam, the output portion receiving the one input and signal beams which continue to propagate at the walk-off angle through the output portion and are spatially separated from one another at a desired distance at an output of the output non- phased-matched portion, wherein the desired distance is sufficient to prevent the beams from overlapping one another.