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Title:
OPTICAL AMPLIFICATION USING THIN SINGLE CRYSTAL ROD
Document Type and Number:
WIPO Patent Application WO/2019/094714
Kind Code:
A1
Abstract:
Techniques, methods, systems, apparatus, and articles of manufacture that can be used for optical amplification may be particularly, though not exclusively, applicable to optical amplification using thin single crystal rods (TSCRs). Such TSCRs may be particularly useful for use in Master Oscillator - Power Amplifier systems. Use of TSCRs in such systems presents an attractive alternative to using glass fibers for ultra-short pulsing lasers with output power levels exceeding 100 W. Such systems are desirable because virtually any engineering material can be processed by ultra-short pulses and the achieved processing quality using such systems generally meets stringent quality requirements.

Inventors:
KATCHANOV ALEXANDRE (US)
MAXWELL GISELE (US)
Application Number:
US2018/060024
Publication Date:
May 16, 2019
Filing Date:
November 09, 2018
Export Citation:
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Assignee:
SHASTA CRYSTALS INC (US)
International Classes:
H01S3/06; H01S3/094; H01S3/10
Foreign References:
US20140371730A12014-12-18
US5177396A1993-01-05
US20060174658A12006-08-10
US20070242714A12007-10-18
US20050058165A12005-03-17
US20090073550A12009-03-19
Attorney, Agent or Firm:
AUSTIN, James E. et al. (WEAVER AUSTIN VILLENEUVE & SAMPSON LLP, P.O. Box 70250Oakland, California, 94612-0250, US)
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Claims:
CLAIMS

1. A system, comprising: a thin single crystal rod (TSCR) having an entrance surface and an exit surface, the TSCR comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength, the active substance is configured to emit amplified radiation having a second wavelength, and the active substance is distributed such that the absorption is distributed substantially uniformly throughout the rod; and a pump radiation source having an output surface, the pump radiation source being configured to provide the pump radiation to the TSCR such that an image of the output surface is formed at the entrance surface and the TSCR when the pump radiation source is providing the pump radiation to the TSCR.

2. The system of claim 1, further comprising a reflective optical element configured to cause a substantial fraction of a first portion of the pump radiation that reaches the exit surface of the TSCR to be reflected back into the TSCR via the exit surface, the reflective optical element being transparent to the amplified radiation. 3. The system of claim 2, wherein the reflective optical element is a dichroic coating applied to an outer side of the exit surface.

4. The system of claim 2, wherein the reflective optical element is a dichroic mirror positioned to face an outer side of the exit surface such that the dichroic mirror forms an image, at the exit surface, of the outer side of the exit surface. 5. The system of claim 1, wherein the TSCR has an inwardly tapering diameter along a lengthwise axis of the TSCR in a vicinity of the entrance surface.

6. The system of claim 5, wherein the TSCR has an outwardly tapering diameter along a transverse axis of the TSCR in a vicinity of the exit surface.

7. The system of claim 6, wherein: the TSCR has a first length along the lengthwise axis, the entrance surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

8. The system of claim 7, wherein: the TSCR has a first length along the transverse axis, the exit surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

9. The system of claim 1, wherein, the active substance is distributed non-uniformly throughout the TSCR.

10. The system of claim 9, wherein, the active substance is distributed such that a ratio of a first power absorption density in a vicinity of the exit surface to a second power absorption density in a vicinity of the entrance surface is between 0.5 and 1.

11. The system of claim 2, wherein the substantial fraction of the pump radiation is at least 25% of the pump radiation.

12. The system of claim 1, wherein: the pump radiation has a power of at least 140 Watts, and the TSCR has a transverse temperature variation of less than 15 degrees Celsius when the pump radiation source is providing the pump radiation to the TSCR.

13. The system of claim 1, wherein there is a substantially parabolic temperature distribution along a radial axis of the TSCR when the pump radiation source is providing the pump radiation to the TSCR.

14. The system of claim 1, further comprising imaging optics positioned in an optical path between the pump radiation source and the entrance surface, the imaging optics being configured to form the image of the output surface at the entrance surface with substantially no aberration. 15. The system of claim 14, wherein the imaging optics comprise a plurality of aspheric lenses.

16. The system of claim 15, wherein the imaging optics further comprise a dichroic mirror configured to combine the pump radiation with seed radiation, the dichroic mirror being transparent to the seed radiation and the dichroic mirror being reflective of the pump radiation.

17. The system of claim 1, wherein the entrance surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

18. The system of claim 17, wherein the exit surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

19. A thin single crystal rod (TSCR), comprising: an entrance surface and an exit surface, the TSCR further comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength; the active substance is configured to emit amplified radiation having a second wavelength; and the active substance is distributed such that the absorption is distributed substantially uniformly throughout the rod. 20. The TSCR of claim 19, further comprising a dichroic coating applied to an outer side of the exit surface.

21. The TSCR of claim 19, wherein the TSCR has an inwardly tapering diameter along a lengthwise axis of the TSCR in a vicinity of the entrance surface.

22. The TSCR of claim 21, wherein the TSCR has an outwardly tapering diameter along a transverse axis of the TSCR in a vicinity of the exit surface.

23. The TSCR of claim 22, wherein: the TSCR has a first length along the lengthwise axis, the entrance surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

24. The TSCR of claim 23, wherein: the TSCR has a first length along the transverse axis, the exit surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

25. The TSCR of claim 24, wherein the active substance is distributed non-uniformly throughout the TSCR.

26. The TSCR of claim 25, wherein, the active substance is distributed such that a ratio of a first power absorption density in a vicinity of the exit surface to a second power absorption density in a vicinity of the entrance surface is between 0.5 and 1.

27. The TSCR of claim 19, wherein the entrance surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

28. The TSCR of claim 27, wherein the exit surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

29. A method comprising: providing pump radiation, from a pump radiation source, to an entrance surface of a thin single crystal rod (TSCR), the pump radiation source being configured to provide the pump radiation to the TSCR such that an image of an output surface of the pump radiation source is formed at the entrance surface and the TSCR; and causing the pump radiation to be distributed substantially uniformly throughout the rod.

30. The method of claim 29, wherein the thin single crystal rod (TSCR) has an entrance surface and an exit surface, the TSCR comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength, the active substance is configured to emit amplified radiation having a second wavelength, and the active substance is distributed such that the absorption is distributed substantially uniformly throughout the rod.

31. The method of claim 30, further comprising providing a reflective optical element configured to cause a substantial fraction of a first portion of the pump radiation that reaches the exit surface of the TSCR to be reflected back into the TSCR via the exit surface, the reflective optical element being transparent to the amplified radiation.

32. The method of claim 31, wherein the reflective optical element is a dichroic coating applied to an outer side of the exit surface.

33. The method of claim 31, wherein the reflective optical element is a dichroic mirror positioned to face an outer side of the exit surface such that the dichroic mirror forms an image, at the exit surface, of the outer side of the exit surface.

34. The method of claim 29, wherein the TSCR has an inwardly tapering diameter along a lengthwise axis of the TSCR in a vicinity of the entrance surface.

35. The method of claim 34, wherein the TSCR has an outwardly tapering diameter along a transverse axis of the TSCR in a vicinity of the exit surface.

36. The method of claim 35, wherein: the TSCR has a first length along the lengthwise axis, the entrance surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

37. The method of claim 36, wherein: the TSCR has a first length along the transverse axis, the exit surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

38. The method of claim 35 wherein, the active substance is distributed non- uniformly throughout the TSCR.

39. The method of claim 29, wherein, the active substance is distributed such that a ratio of a first power absorption density in a vicinity of the exit surface to a second power absorption density in a vicinity of the entrance surface is between 0.5 and 1.

40. The method of claim 30, wherein the substantial fraction of the pump radiation is at least 25% of the pump radiation.

41. The method of claim 29, wherein: the pump radiation has a power of at least 140 Watts, and the TSCR has a transverse temperature variation of less than 15 degrees Celsius when the pump radiation source is providing the pump radiation to the TSCR.

42. The method of claim 29, wherein there is a substantially parabolic temperature distribution along a radial axis of the TSCR when the pump radiation source is providing the pump radiation to the TSCR.

43. The method of claim 29, further comprising imaging optics positioned in an optical path between the pump radiation source and the entrance surface, the imaging optics being configured to form the image of the output surface at the entrance surface with substantially no aberration.

44. The method of claim 43, wherein the imaging optics comprise a plurality of aspheric lenses. 45. The method of claim 44, wherein the imaging optics further comprise a dichroic mirror configured to combine the pump radiation with seed radiation, the dichroic mirror being transparent to the seed radiation and the dichroic mirror being reflective of the pump radiation.

46. The method of claim 29, wherein the entrance surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

47. The method of claim 46, wherein the exit surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

Description:
Optical Amplification Using Thin Single Crystal Rod

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Patent Application No. 62/585,440, filed November 13, 2017, and titled, "APPARATUSES, SYSTEMS AND METHODS FOR OPTICAL AMPLIFICATION USING A THIN SINGLE CRYSTAL ROD," the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND

Existing optical amplification systems, particularly for high power radiation, have a number of shortfalls.

SUMMARY

Techniques, methods, systems, apparatus, and articles of manufacture that can be used for optical amplification, and may be particularly, though not exclusively, applicable to optical amplification using thin single crystal rods (TSCRs), e.g., single crystal rods with a diameter of 2 millimeters or less, e.g., 1 millimeter, are described. Such TSCRs may be particularly useful for use in Master Oscillator - Power Amplifier (MOPA) systems. Use of TSCRs in such systems presents an attractive alternative to using glass fibers for ultra-short (e.g., picosecond or femtosecond) pulsing lasers with output power levels exceeding 100 W. Such systems are desirable because virtually any engineering material can be processed by ultra-short pulses and the achieved processing quality using such systems generally meets stringent quality requirements.

Among various embodiments described herein is a system, comprising: a thin single crystal rod (TSCR) having an entrance surface and an exit surface, the TSCR comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength, the active substance is configured to emit amplified radiation having a second wavelength, and the active substance is distributed such that the absorption is distributed substantially uniformly throughout the rod; and a pump radiation source having an output surface, the pump radiation source being configured to provide the pump radiation to the TSCR such that an image of the output surface is formed at the entrance surface and the TSCR when the pump radiation source is providing the pump radiation to the TSCR.

Also described is a thin single crystal rod (TSCR), comprising: an entrance surface and an exit surface, the TSCR further comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength; the active substance is configured to emit amplified radiation having a second wavelength; and the active substance is distributed such that the absorption is distributed substantially uniformly

throughout the rod.

Further disclosure is provided for a method comprising: providing pump radiation, from a pump radiation source, to an entrance surface of a thin single crystal rod (TSCR), the pump radiation source being configured to provide the pump radiation to the TSCR such that an image of an output surface of the pump radiation source is formed at the entrance surface and the TSCR; and causing the pump radiation to be distributed substantially uniformly throughout the rod. These and other features will be described in more detail below with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1A shows a simplified block diagram of an example of an optical amplification system, in accordance with some implementations. Fig. IB shows a simplified schematic diagram of an example of an optical amplification system, in accordance with some implementations.

Fig. 2 shows a simplified diagram of a thin single crystal rod (TSCR), in accordance with some implementations. Fig. 3A shows a close-up entrance surface end view view of an entrance surface of a TSCR, in accordance with some implementations.

Fig. 3B shows a close-up radial cross-sectional view of the entrance surface end portion of a TSCR in a cooling block configuration, in accordance with some implementations.

Fig. 4 shows a close-up radial cross-sectional view of a portion of a TSCR with a tapered entrance surface end, in accordance with some implementations.

Fig. 5 shows a close-up radial cross-sectional view of a portion of a TSCR with a tapered exit surface end, in accordance with some implementations. Figs. 6A and 6B shows comparative plots of temperature distributions at varying positions along a transverse axis (across the diameter) of a conventional TSCR and a TSCR in accordance with some implementations.

Fig. 7A shows a plot of an absorbed pump energy distribution along the length a TSCR, in accordance with some implementations. Fig. 7B shows a plot of an absorbed pump energy distribution along the length a TSCR with a non-uniform active substance concentration, in accordance with some implementations.

Figs. 8A and 8B show simplified block diagrams of an example of the last stage of an amplification system an optical amplification system, in accordance with some implementations.

DESCRIPTION

This disclosure describes techniques, methods, systems, apparatus, and articles of manufacture that can be used for optical amplification. The techniques may be particularly, though not exclusively, applicable to optical amplification using thin single crystal rods (TSCRs), e.g., single crystal rods with a diameter of 2 millimeters or less, e.g., 1 millimeter. Such TSCRs may be particularly useful for use in Master Oscillator - Power Amplifier (MOPA) systems. Use of TSCRs in such systems presents an attractive alternative to using glass fibers for ultra-short (e.g., picosecond or femtosecond) pulsing lasers with output power levels exceeding 100 W. Such systems are desirable because virtually any engineering material can be processed by ultra-short pulses and the achieved processing quality using such systems generally meets stringent quality requirements. By using ultra-short pulses, material can be removed by ablation in ultra-thin slices, because of minimal thermal penetration depth. Such ultra-short pulses may be especially beneficial for processing of very hard materials - e.g., cutting hardened cover glasses for cell phones. However, in many cases, the processing speeds of such ultra-fast systems are severely limited because the power output of most existing ultra-fast systems does not exceed ten watts. On the other hand, when high accuracy and/or stringent surface standards are to be met, picosecond or femtosecond pulses with energies from 1-100 micro joules at repetition rates of tens of kilohertz to tens of megahertz are desired. Such pulse parameters result in average output power the range from 100 watts to 1 kilowatt. Conventional glass fiber lasers have achieved multi-kW power levels for longer pulses. However such conventional glass fibers are generally unable to achieve such levels of output power for ultra-short pulses without complex systems, such as Chirped Pulse Amplification (CPA) that suffer from mode instabilities. As a result commercial systems based on glass fiber are now limited to below 100W of average power.

A few manufacturers of amplification systems have attempted to achieve such high-power ultra-short pulses using TSCRs rather than glass fibers; however, such existing systems have a number of shortfalls. By way of example, existing TSCR amplification systems generally suffer effects of high aberrations in thermal lensing, leading to beam quality deterioration. The TSCRs of such existing amplification systems are often susceptible to uneven temperature distributions with significant hotspots because pump radiation is often focused into a spot that is less than half the size of the TSCRs diameter. Furthermore, in such existing systems, a large fraction of pump radiation is often absorbed within the first third of the length TSCR. The maximum gain that can be achieved in an amplifier of a MOPA system is generally limited by a variety of factors. Such maximum gain in not only limited by parasitic laser action occurring on reflections of the amplified spontaneous emission (ASE) internally within an amplifying rod, but also by the fact that surface stress due to the thermal load in the amplifying rod may reach the fracture limit of the rod material. As such, avoidance of the thermal hotspots described in the preceding paragraph is desirable.

In contrast to existing systems, the disclosed techniques, articles, apparatus and systems allow pumping of an active substance within a TSCR amplifier in to provide high power extraction while also minimizing distortion due to thermal lensing as compared to existing systems. Additionally, the techniques, articles, apparatus and systems disclosed herein provide for increased gain in contrast to conventional optical amplifiers by more evenly distributing absorption of pump radiation throughout a TSCR and thereby avoiding hotspots that could threaten the integrity of the TSCR.

Fig. 1A shows a simplified block diagram of an example of an optical amplification system 100, in accordance with some implementations. In Fig. 1A, pump radiation 104 passes through imaging optics 108. As discussed further below, imaging optics 108 may be configured to form a sharp-edged image of an output surface of the pump radiation source 102 on the input. By way of example, as discussed further below in the context of Fig. IB, such imaging optics 108 may include optical components such as aspheric lenses, achromatic doublets, and best shape lens singlets.

Pump radiation 104 and seed radiation 112 are combined in combining optics 116 to pass into TSCR 120. As demonstrated below in the context of Fig. IB, such combining optics may include a dichroic mirror that reflects pump radiation and allows seed radiation to pass through. One having skill in the art may appreciate that a reverse scheme that includes a dichroic mirror that reflects seed radiation and allows pump radiation to pass through is a possibility; however, it may be advantageous to use an arrangement where pump radiation is be reflected by a dichroic mirror rather than passed through the dichroic mirror because transmission through a dichroic mirror may introduce aberration. In some implementations, as discussed below, the pump radiation 104 and the seed radiation 112 may enter an entrance surface of the TSCR 120. The TSCR 120 may include an active substance (e.g., any suitable dopant such as chromium, or a rare earth element such as ytterbium, neodymium, erbium, thulium, etc.) distributed within the TSCR 120. The active substance may be configured to absorb the pump radiation 104, which has a first wavelength. The active substance may be configured to emit amplified radiation 124 which has a second wavelength.

In some implementations, the active substance may be distributed such that absorption of the pump radiation 104 is distributed substantially uniformly throughout the TSCR 120. For example, the pump radiation 104 may distributed sufficiently uniformly throughout the TSCR 120 such that there are minimal transverse temperature variations across the diameter of the TSCR 120, as described further below in the context of Fig. 6. Such substantially uniform distribution of absorption may be accomplished in a variety of manners. By way of example, as discussed further below, the active substance may be distributed non-uniformly throughout the TSCR 120. Also or alternatively, as explained in further detail below, the active substance may be distributed sparsely such that a significant fraction (e.g., 25-30%) of the pump radiation 104 is not absorbed in a first pass through the TSCR 120. Such unabsorbed pump radiation 128 may then be re-transmitted back into the TSCR 120 by reflective optical element 132 for a second pass through the TSCR 120 during which most or all of the previously unabsorbed pump radiation 128 is absorbed by the active substance of the TSCR 120.

A benefit of using such a non-uniform or sparse concentration of active substance is that the distribution of absorbed pump power becomes more uniform than in existing TSCR amplifiers, and therefore the local variation in thermal load anywhere in the TSCR will minimal, avoiding the hot spots described above in the context of existing systems. Yet another benefit of this arrangement is that the transverse temperature distribution of the TSCR may be relatively uniform across any cross section of the TSCR, resulting in a parabolic or substantially parabolic distribution of temperature, and thus a nearly aberration-free thermal lens, as discussed further below in the context of Fig. 6. Fig. IB shows a simplified schematic diagram of an example of an optical amplification system in accordance with some implementations. The optical amplification system of Fig. IB is one particular example of the optical amplification system 100 of Fig. 1A. In Fig. IB, pump radiation is imaged onto an entrance surface of a TSCR (Single Crystal Fiber with cooling block) using a pair of aspheric coupling lenses and combined with seed radiation using a dichroic mirror that reflects the pump radiation and allows the seed radiation to pass through, as described above. The system also shows pump reflecting and return optics return optics beyond the exit surface end of the TSCR, as discussed, with alternatives, further below with reference to Fig. 2.

Fig. 2 shows a simplified diagram of a TSCR 200, in accordance with some implementations. TSCR 200 is one example of a TSCR, which may be used in the optical amplification systems disclosed herein such as optical amplification system 100 of Fig. 1A and the optical amplification system of Fig. IB. As discussed above, an output surface of a pump radiation source (e.g., the exit surface of a pump fiber) is imaged directly on an entrance surface 2 of TSCR 200 of Fig. 2. The output surface may be imaged in in such a way that a sharp edged image of the output surface is formed on the entrance surface 2. In other words, in some implementations an image of the output surface may be formed with minimum aberrations such that the there is virtually no pump power beyond the edges of the image. As discussed above this can be accomplished using optical components such as aspheric lenses, achromatic doublets, and best shape lens singlets, as will be readily understood by those skilled in the art. For instance, as depicted in Fig. IB, a combination of simple singlet aspheric lenses can be used to provide such sharp imaging. Fig. 3A shows the image 303 of the pump output fiber on the input face of the TSCR amplification medium 301. The diameter of the image of the output surface of the pump radiation source may be made as close to the diameter of the entrance surface of the TSCR as possible, e.g., in some implementations, 90-95% of the diameter of the entrance surface of the TSCR (so not on only 0.05 to 0.1 at perimeter 305 of rod diameter). The diameter of the image of the output surface of the pump radiation source 303 is limited to being less than 100% of the entrance surface to avoid exposure of the material surrounding the TSCR to intense pump radiation. While some residual pump intensity may spill out beyond the sharp edge of the image due to imperfections of the aberration compensation system (e.g., aspheric coupling lenses of Fig. IB), the inventors have determined, for example, that by forming an 0800 μιη image on the entrance surface of a 01000 μιη TSCR, surrounding material damage can be avoided. This result was verified experimentally with a pump power in excess of 400 W. Since the diameter of the image of the output surface of the pump radiation source may be made as close to the diameter of the entrance surface of the TSCR as possible, the pump radiation may have negligible free propagation, and the entire TSCR may become a guided pump propagation region.

The formation of the image 303 of the pump output fiber on the input face of the TSCR amplification medium and the corresponding pumped volume 304 is shown in the view from the side in radial cross-section in Fig. 3B. Fig. 3B also illustrates the role of aberrations that will lead to pump radiation 302 interacting with the thermally conductive wrap 307 (e.g., a soft, heat conductive material such as Indium wrap) around the crystal amplification medium 301 and with the material of the heat sink 309. Such interaction may lead to the ablation of both materials and contamination of the crystal input face followed by its consequent damage. In some implementations, such potential ablation may be mitigated using an arrangement depicted in radial cross-section in Fig. 4. In Fig. 4, there is an inward taper 402 in a portion of the TSCR 401 near the entrance surface 403. Such a TSCR may be fabricated using a technique known as Laser Heated Pedestal Growth (LHPG), such as described in US Patent Application No. 15/554,703, filed August 30, 2017, and titled APPARATUSES AND METHODS FOR PRODUCING THIN CRYSTAL FIBERS USING LASER HEATING PEDESTAL GROWTH, incorporated by reference here for the description of the LHPG technique, including creation of tapering profiles of TSCRs, which allows creation of a TSCR with a variable diameter. As depicted in Fig. 4, a short section 405 at the entrance surface of the TSCR has a larger diameter (D2) than the diameter of main cylindrical portion 407 of the TSCR (Dl). This section 405 may be formed by tapering down the TSCR to the diameter of the main cylindrical part of the rod Dl, on a length (Lt) that may be determined from the relationship (Dl-D2)/Lt > tan(0), where Θ is the divergence angle of the pump radiation cone. The diameter of the sharp image of the output surface of the pump radiation source, as described above, may be is made equal to the diameter Dl of the main cylindrical part of the TSCR and may be positioned in the plane of the TSCR where the tapered section becomes equal to the diameter Dl of the main cylindrical part of the TSCR.

Returning to Fig. 2, in some implementations, the number of active ions in the active substance of the TSCR 200 of Fig. 2 may be limited such that a significant fraction of pump radiation 5, e.g., 20% to 35% of the pump radiation 5, will reach exit surface 7 of the TSCR 200. In some implementations, the non-absorbed fraction of the pump radiation 5 may be returned into the TSCR 200, as indicated by an arrow 8, so that non-absorbed pump radiation will propagate back to the entrance surface 2 thereby causing the majority or all of the pump radiation 5 to be absorbed within the TSCR 200. The return of the non-absorbed fraction of the pump radiation 5 into the

TSCR 200 may be accomplished in a variety of manners. By way of example, in some implementations, a dichroic coating, which is highly reflective to the pump and highly transparent for the amplified radiation, may be applied to the exit surface 7 of the TSCR 200. Alternatively, an aspherical mirror 510 can be placed behind the thin rod 501 at a distance that provides a perfect imaging of the exit crystal face into itself, as depicted in Fig. 5. The dashed lines in Fig. 5 depict the edge of the unabsorbed pump radiation which is reflected by the aspherical mirror 510. This reflected unabsorbed pump radiation 504 could have hit the thermally conducting soft wrap 507 around the TSCR, damaging the wrap and the crystal. This could have occurred with a slight misalignment of the spherical mirror or because of edge effects on the connection between the barrel surface of the TSCR and its back surface. As shown in Fig. 5, such effects may be mitigated by using LHPG techniques to form a short section 505 of the TSCR with an outward taper 502 to a slightly increased diameter close to the exit surface 7/503 of the TSCR such that the pump radiation reimaged onto the back face 7 by the pump re-entry optics will not spill beyond the exit surface 7 of the TSCR, as described above with reference to the input face of the TSCR in Fig. 4. This solution is shown schematically in Fig. 5. The principles of forming the tapered section at the crystal fiber rod end are parallel to the explanations above described in the context of Fig. 4.

Referring again to Fig. 2, in some implementations the amplified beam 6 may be configured to enter the TSCR 200 with a diameter 1.2 times smaller (at l/e2 levell) than the diameter the TSCR 200. Such configuration of the diameter of the amplified beam 6 provides unobstructed free propagation within the TSCR 200, leading to high extraction efficiency.

Returning to Fig. 2, in a particular example, TSCR 200 may be a 30 mm long ytterbium-doped yttrium aluminum garnet (Yb:YAG) TSCR with a 1000 μιη diameter that is pumped by a 140 W 940 nm laser diode. In this example, the output surface of the pump fiber may be imaged into a sharp image with a 800 μιη diameter onto the entrance surface of the TSCR. The TSCR may have Yb concentration of 0.5%, resulting in the absorption of approximately 75% of the pump radiation on the first pass through the TSCR. The unabsorbed pump radiation (with a power of approximately 35 W) is re-injected into the thin rod so that only 8.8 W was reaching the entrance face 2 of the crystal. This corresponds to absorption of 94% of the pump power in the TSCR.

The calculated temperature distributions for positons along the length of the TSCR in the example described in the preceding paragraph are shown in Fig. 6A. The letters in the corners of the graphs indicate positions of the transverse temperature distributions in accordance with Fig. 2. A comparison with existing TSCR systems, as depicted in the graphs of Fig. 6B, highlights significant differences. First of all, as depicted in Fig. 6A, the temperature distributions in all four sections of the TSCR are so close to parabolic that both curves are practically indistinguishable on all graphs. This is in contrast with the graphs depicted in Fig. 6B for a representative existing TSCR system in which the temperature distributions in different sections of the TSCR diverge substantially from the parabolic ideal. The second difference is that, unlike existing TSCR systems, the temperature distributions along the TSCR length is close to uniform, only varying between about 10°C to about 20 ° C from towards the exit end of the TSCR to towards the entry end of the TSCR where more pump radiation has been absorbed; whereas, the temperature variation along the length of the conventional TSCR is far greater, ranging from greater than 50 ° C to less than 5 ° C.

Fig. 7A shows a graph of an absorbed pump energy distribution along the TSCR discussed in the context of the example described in the preceding paragraphs. As illustrated in Fig. 7A, the absorbed pump power density is distributed relatively uniformly along the length of the TSCR. The density of the power as a function of the position along the length of the TSCR changes only by a factor of about two, as represented by the dashed line c) in Fig. 7A. For reference, the absorbed part of the forward propagating (first pass) pump radiation is shown by the solid line a) and for the back propagating (second pass) pump radiation - by solid line b). By contrast, as described above, in existing configurations using TSCRs most absorbed pump power is concentrated in the portions of the rod nearest the entrance surface.

As discussed above, in some implementations, uniform absorption of pump radiation may be achieved by distributing an active substance non-uniformly throughout a TSCR. By way of example, as discussed above the TSCR may be fabricated using a technique known as Laser Heated Pedestal Growth (LHPG). This technique allows TSCRs to be grown such that concentration of active substance is variable along the length of the TSCR. The concentration may gradually increase from the entrance surface of the TSCR to the exit surface of the TSCR. The final active substance concentration may be determined using an iterative process of calculating the pump radiation propagation and absorption within the TSCR with the target of the pump power absorption density at the exit end of the TSCR being in the range of 0.5-1 of the value of the pump power absorption density at the front end of the rod. An example of such an increasing active substance concentration towards the end of the TSCR is depicted by dashed line a) in Fig. 7B. The uniform pump power absorption density corresponding to such a gradually increasing active substance concentration increase is depicted in Fig. 7B by solid line b).

In another embodiment, the present disclosure relates to avoidance of damage to anti-reflective coatings (AR-coatings) that may be deposited on one or both end faces of a TSCR used as an optical amplifier to improve optical performance. Such AR-coatings may be damaged by a high power beam at the last stage of amplification. This damage can limit the average output power of the amplifier to a value corresponding to the damage threshold of the AR-coating deposited onto the face(s) of the TSCR. In accordance with this embodiment, several times increase in the output power of the amplifier system may be obtained, as compared to the AR-coating optical damage limitation referenced above. This embodiment leverages the very high value of the radiative damage threshold of optical crystals, which is several times higher than the damage threshold of AR- coatings deposited onto the same crystal.

Figs. 8A-B show top views of beams propagating in the last stage of an amplification system. As depicted in Fig. 8A, according to this embodiment, input beam 1001 coming from a seed laser source or from a previous amplification stage is polarized vertically (perpendicular to the drawing plane), which is shown symbolically in the drawing by encircled crosses. The input beam 1001 goes through a polarizing beam splitter 1002 and propagates until it hits the entrance face 1003 of a TSCR 1015 of the amplification system. According to the present embodiment, the entrance face 1003 is oriented at a Brewster's angle relative to lengthwise axis of the TSCR 1015. As the input (not yet amplified, low power) beam 1001 is polarized orthogonally to the plane of incidence, some small fraction of it is lost to reflection 1004 from the input face 1003, but this small loss is insignificant in the whole amplifying system power budget. The input beam is amplified within the TSCR, reaches its intermediate power, and exits the TSCR 1015 through its AR-coated face 1005. The first pass amplified beam 1006 remains vertically polarized until it hits a quarter-wave plate 1007, and becomes circularly polarized.

As depicted in Fig. 8B, after reflection by a retro-reflecting optical system 1010 the second amplification pass beam 1011 still remains circularly polarized until it hits the quarter-wave plate 1007, whereupon it converts into a horizontally polarized beam 1012. The beam 1012 enters the TSCR through the AR-coated face 1005, and is then amplified to its final high power on the second pass through the TSCR 1015. The beam now has horizontal polarization and is able to pass through the Brewster oriented face 1003 without attenuation or reflection. The second pass amplified beam 1013 propagates in free space back towards the source until it hits the polarizing beam splitter 1002 from which it is now being reflected to form the output beam 1014. Now the average power of the output beam 1014 is limited not by the damage threshold of the AR-coating, but by the several times larger damage threshold of the crystal itself.

In yet another embodiment, the first pass output face of the TSCR is also oriented at Brewster's angle to the lengthwise axis of the TSCR. In this embodiment the AR-coating of the face is designed to have minimum reflection for both vertically and horizontally polarized radiation. An advantage of this embodiment is that it avoids chromatic dispersion due to the prism action of the TSCR having one tilted face and one straight face.

As will be apparent to a person skilled in the art, both described embodiments can be used in combination with all previously described embodiments, such as non-tapered or tapered TSCR ends.

As additional disclosure, following are listed some particular system, device and method embodiments in accordance with this description:

System Embodiments: 1. A system, comprising: a thin single crystal rod (TSCR) having an entrance surface and an exit surface, the TSCR comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength, the active substance is configured to emit amplified radiation having a second wavelength, and the active substance is distributed such that the absorption is distributed substantially uniformly throughout the rod; and a pump radiation source having an output surface, the pump radiation source being configured to provide the pump radiation to the TSCR such that an image of the output surface is formed at the entrance surface and the TSCR when the pump radiation source is providing the pump radiation to the TSCR. 2. System embodiment 1, further comprising a reflective optical element configured to cause a substantial fraction of a first portion of the pump radiation that reaches the exit surface of the TSCR to be reflected back into the TSCR via the exit surface, the reflective optical element being transparent to the amplified radiation. 3. System embodiment 2, wherein the reflective optical element is a dichroic coating applied to an outer side of the exit surface.

4. System embodiment 2, wherein the reflective optical element is a dichroic mirror positioned to face an outer side of the exit surface such that the dichroic mirror forms an image, at the exit surface, of the outer side of the exit surface. 5. System embodiment 1, wherein the TSCR has an inwardly tapering diameter along a lengthwise axis of the TSCR in a vicinity of the entrance surface.

6. System embodiment 1 or 5, wherein the TSCR has an outwardly tapering diameter along a transverse axis of the TSCR in a vicinity of the exit surface.

7. System embodiment 5 or 6, wherein: the TSCR has a first length along the lengthwise axis, the entrance surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

8. System embodiment 5, 6 or 7, wherein: the TSCR has a first length along the transverse axis, the exit surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

9. System embodiment 1, 5 or 6, wherein, the active substance is distributed non- uniformly throughout the TSCR. 10. System embodiment 9, wherein, the active substance is distributed such that a ratio of a first power absorption density in a vicinity of the exit surface to a second power absorption density in a vicinity of the entrance surface is between 0.5 and 1.

11. System embodiment 2, wherein the substantial fraction of the pump radiation is at least 25% of the pump radiation.

12. System embodiment 1, wherein: the pump radiation has a power of at least 140 Watts, and the TSCR has a transverse temperature variation of less than 15 degrees Celsius when the pump radiation source is providing the pump radiation to the TSCR.

13. System embodiment 1, wherein there is a substantially parabolic temperature distribution along a radial axis of the TSCR when the pump radiation source is providing the pump radiation to the TSCR.

14. System embodiment 1, further comprising imaging optics positioned in an optical path between the pump radiation source and the entrance surface, the imaging optics being configured to form the image of the output surface at the entrance surface with substantially no aberration.

15. System embodiment 14, wherein the imaging optics comprise a plurality of aspheric lenses.

16. System embodiment 15, wherein the imaging optics further comprise a dichroic mirror configured to combine the pump radiation with seed radiation, the dichroic mirror being transparent to the seed radiation and the dichroic mirror being reflective of the pump radiation.

17. Any preceding listed system embodiment, wherein the entrance surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

18. System embodiment 17, wherein the exit surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR. Device (TSCR) Embodiments:

1. A thin single crystal rod (TSCR), comprising: an entrance surface and an exit surface, the TSCR further comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength; the active substance is configured to emit amplified radiation having a second wavelength; and the active substance is distributed such that the absorption is distributed substantially uniformly throughout the rod.

2. Device embodiment 1, further comprising a dichroic coating applied to an outer side of the exit surface. 3. Device embodiment 1, wherein the TSCR has an inwardly tapering diameter along a lengthwise axis of the TSCR in a vicinity of the entrance surface.

4. Device embodiment 1 or 3, wherein the TSCR has an outwardly tapering diameter along a transverse axis of the TSCR in a vicinity of the exit surface.

5. Device embodiment 3 or 4, wherein: the TSCR has a first length along the lengthwise axis, the entrance surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

6. Device embodiment 3, 4 or 5, wherein: the TSCR has a first length along the transverse axis, the exit surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

7. Device embodiment 1, 2, 3 or 4, wherein the active substance is distributed non- uniformly throughout the TSCR. 8. Device embodiment 7, wherein, the active substance is distributed such that a ratio of a first power absorption density in a vicinity of the exit surface to a second power absorption density in a vicinity of the entrance surface is between 0.5 and 1.

9. Any proceeding listed device embodiment, wherein the entrance surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

10. Device embodiment 9, wherein the exit surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

Method Embodiments:

1. A method comprising: providing pump radiation, from a pump radiation source, to an entrance surface of a thin single crystal rod (TSCR), the pump radiation source being configured to provide the pump radiation to the TSCR such that an image of an output surface of the pump radiation source is formed at the entrance surface and the TSCR; and causing the pump radiation to be distributed substantially uniformly throughout the rod. 2. Method embodiment 1, wherein the thin single crystal rod (TSCR) has an entrance surface and an exit surface, the TSCR comprising an active substance distributed within the TSCR, wherein: the active substance is configured to absorb pump radiation having a first wavelength, the active substance is configured to emit amplified radiation having a second wavelength, and the active substance is distributed such that the absorption is distributed substantially uniformly throughout the rod.

3. Method embodiment 2, further comprising providing a reflective optical element configured to cause a substantial fraction of a first portion of the pump radiation that reaches the exit surface of the TSCR to be reflected back into the TSCR via the exit surface, the reflective optical element being transparent to the amplified radiation.

4. Method embodiment 3, wherein the reflective optical element is a dichroic coating applied to an outer side of the exit surface. 5. Method embodiment 3, wherein the reflective optical element is a dichroic mirror positioned to face an outer side of the exit surface such that the dichroic mirror forms an image, at the exit surface, of the outer side of the exit surface.

6. Method embodiment 1, wherein the TSCR has an inwardly tapering diameter along a lengthwise axis of the TSCR in a vicinity of the entrance surface.

7. Method embodiment 1 or 6, wherein the TSCR has an outwardly tapering diameter along a transverse axis of the TSCR in a vicinity of the exit surface.

8. Method embodiment 6 or 7, wherein: the TSCR has a first length along the lengthwise axis, the entrance surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle.

9. Method embodiment 6, 7 or 8, wherein: the TSCR has a first length along the transverse axis, the exit surface has a first diameter, a middle portion of the TSCR has a second diameter, the pump radiation has a divergence angle, first diameter and the second dimeter have a difference, and the difference divided by the first length is greater than a tangent of the divergence angle. 10. Method embodiment 2, 6 or 7 wherein, the active substance is distributed non- uniformly throughout the TSCR.

11. Method embodiment 10, wherein, the active substance is distributed such that a ratio of a first power absorption density in a vicinity of the exit surface to a second power absorption density in a vicinity of the entrance surface is between 0.5 and 1.

12. Method embodiment 2, wherein the substantial fraction of the pump radiation is at least 25% of the pump radiation.

13. Method embodiment 1, wherein: the pump radiation has a power of at least 140 Watts, and the TSCR has a transverse temperature variation of less than 15 degrees Celsius when the pump radiation source is providing the pump radiation to the TSCR.

14. Method embodiment 1, wherein there is a substantially parabolic temperature distribution along a radial axis of the TSCR when the pump radiation source is providing the pump radiation to the TSCR.

15. Method embodiment 1, further comprising imaging optics positioned in an optical path between the pump radiation source and the entrance surface, the imaging optics being configured to form the image of the output surface at the entrance surface with substantially no aberration. 16. Method embodiment 15, wherein the imaging optics comprise a plurality of aspheric lenses.

17. Method embodiment 16, wherein the imaging optics further comprise a dichroic mirror configured to combine the pump radiation with seed radiation, the dichroic mirror being transparent to the seed radiation and the dichroic mirror being reflective of the pump radiation. 18. Any preceding listed method embodiment, wherein the entrance surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.

19. Method embodiment 18, wherein the exit surface of the TSCR is oriented at a Brewster's angle to the lengthwise axis of the TSCR.