Passive high-energy Q-switch laser system with optical synchronization multi-stage / multi-pass amplification

Abstract

A sub-nanosecond laser system is disclosed. The sub-nanosecond laser system may include an excitation laser source operable to generate an excitation laser beam having an excitation wavelength, a first excitation beam splitter operable to receive the excitation laser beam and split the excitation laser beam into at least a first split excitation laser beam and a second split excitation laser beam, a passive Q-switch seed laser operable to receive the first split excitation laser beam and generate a seed laser beam, and an amplifier assembly operable to receive the second split excitation laser beam and the seed laser beam. The amplifier assembly may have one or more amplifiers arranged in series in a multi-stage configuration, one or more amplifiers arranged in a multi-pass configuration, or a combination thereof.

Description

Technical Field

[0001] This system relates to a passive high-energy Q-switch laser system, and more particularly, to a passive high-energy Q-switch laser system having optically synchronized multi-stage / multi-pass amplification.

Background Art

[0002] Lasers are used in many applications in industry, research, and medicine. Of particular interest are lasers that produce high-energy, short-duration pulses, such as nanosecond and sub-nanosecond pulses. Such lasers facilitate high-precision microfabrication of materials. Such lasers can also accommodate many scientific and technical applications, as well as certain medical procedures where the short interaction time with tissue can reduce heating effects, increase mechanical stress, and improve the quality of the interaction.

[0003] Q - switched solid - state lasers are currently used to generate short - time laser pulses. These pulses typically have a short duration of about a few nanoseconds. More generally, active optical components (acousto - optic or electro - optic) are used to generate Q - switched pulses. However, the pulse duration of active Q - switched lasers is limited to more than a few nanoseconds due to the limitations of the high - speed switching time of the active Q - switch and the larger physical size that results in a longer cavity length. To further shorten the pulse duration, either a mode - locked laser or a special type of Q - switched laser generally called a microchip laser is used. Mode - locking can generate ultrashort laser pulses, but its cavity contains a complex optical arrangement and is vulnerable to environmental perturbations. Furthermore, the typical pulse energy generated from a mode - locked resonator is on the order of nanojoules (nJ). To generate millijoule energy for meaningful applications, a regenerative amplifier that includes a complex optical arrangement and precise electronic synchronization makes the entire system large, expensive, and less reliable. Passive Q - switched short - cavity lasers can provide a compact and cost - effective alternative for generating pulses in the sub - nanosecond regime. A typical Q - switched short - cavity laser consists of a laser medium, a saturable absorber as a passive Q - switch, and two cavity mirrors (a high - reflector and an output coupler). The cavity is designed such that the crystal length is minimized, so the cavity length is on the order of millimeters. The short cavity length results in an extremely short cavity lifetime and potentially shorter Q - switched pulses. High - energy excitation (pump) sources combined with saturable absorbers with low initial transmittance enable the generation of mJ. Q - switched short - cavity lasers have demonstrated the ability to generate output pulses shorter than 150 ps, which is as short as those generated by large mode - locked lasers.

[0004] The energy from a Q-switch short cavity oscillator is high enough (at the millijoule level) so that there is no need to introduce complex amplification systems (such as regenerative amplifiers) to obtain a significantly large amplification rate in practical applications such as those performed by mode-locked lasers. Instead, master oscillator power amplifier (MOPA) systems are typically used to generate sub-nanosecond high-energy laser pulses. MOPA systems, especially those with high amplification gains, are sensitive to parasitic reflections and can cause problems of self-laser oscillation. Self-laser oscillation can significantly limit the amplification achievable in the output amplifier. A single-stage amplifier with single-pass amplification has a simple configuration. However, there are problems with extracting high energy (>1J). In a single-pass or single-stage amplifier, it is necessary to strongly pump (excite) the amplifier to accumulate a higher gain for amplification. On the other hand, a higher gain incorporated in the amplification medium tends to promote the occurrence of amplified spontaneous emission (ASE) or self-laser oscillation, which in turn depletes the gain and limits the amplified output energy.

Brief Description of the Drawings

[0005] To more fully understand the present disclosure and its features, reference is now made to the following description in conjunction with the accompanying drawings. Like reference numerals indicate like elements.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10A

Figure 10B

Figure 11A

Figure 11B

DETAILED DESCRIPTION OF THE INVENTION

[0006] To make the description concise and clear, it will be understood that reference numerals are repeated between different figures to indicate corresponding or similar elements where appropriate. Further, numerous specific details are set forth in order to provide a thorough understanding of the examples described herein. However, it will be understood by those of ordinary skill in the art that the examples described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related features being described. Also, this description should not be construed as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale, and the ratios of particular portions may be exaggerated to better illustrate details and features of the present disclosure.

[0007] Here, some definitions that apply to the entire above disclosure are presented.

[0008] As used herein, "about" refers to a numerical value including integers, fractions, percentages, etc., whether explicitly indicated or not. The term "about" generally refers to a range of numerical values that are considered equivalent to the recited value, e.g., ±0.5 to 1%, ±1 to 5%, or ±5 to 10% of the recited value, which is considered to have, for example, the same function or result.

[0009] The term "coupled" is defined as being directly or indirectly connected through intervening components and is not necessarily limited to physical connections. The connection can be such that the objects are permanently connected or removably connected.

[0010] The term "substantially" is defined such that a component need not be exact, as it is essentially adapted to a particular dimension, shape, or other word that substantially modifies it.

[0011] In the present disclosure, the terms "comprise", "include", and "have" are used interchangeably. The terms "comprise", "include", and "have" mean to include, but are not necessarily limited to, what is so described. The term "consisting essentially of" is more limiting than "comprise", but less limiting than "consisting of". Specifically, the term "consisting essentially of" is limited to those that do not materially affect the specified materials or steps and the essential characteristics of the claimed invention. The terms "a", "an", and "the" are understood to encompass not only the singular but also the plural.

[0012] As used herein, the term "high-energy sub-nanosecond laser" or "sub-nanosecond laser" may refer to a laser capable of providing energy of 0.1 to 10 J with a pulse duration of 10 to 900 psec. Generally, high-energy sub-nanosecond lasers can provide, for example, more effective microfabrication, scientific spectroscopy, and medical treatments than commonly used nanosecond lasers. Therefore, the availability of high-energy sub-nanosecond lasers can facilitate the improvement of existing applications and support the development of new laser applications.

[0013] The terms used herein generally have their ordinary meanings in the relevant art, in the context of the disclosure, and in the particular context in which each term is used. For any one or more of the terms described herein, alternative terms and synonyms may be used, and no special significance should be placed on whether a term is elaborated or discussed in this specification. In some cases, synonyms for particular terms are provided. The listing of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification, including examples of any of the terms discussed herein, is merely illustrative and is not intended to further limit the scope or meaning of the disclosure or any of the illustrative terms. Similarly, the present disclosure is not limited to the various embodiments shown herein.

[0014] This specification discloses a sub-nanosecond laser system having a passive Q-switch seed laser and an amplifier assembly. The laser system described herein can overcome one or more of the problems described hereinabove with respect to conventional Q-switch lasers by using a multi-stage amplifier or a multi-pass amplifier within the amplifier assembly.

[0015] Multi-stage amplification includes two or more amplifiers arranged in series and separated by isolators. In such a MOPA system, instead of depositing the excitation energy in a single laser medium, a strong excitation energy is distributed among multiple amplifier media, so that the ASE or self-lasing oscillation process can be reduced. Thus, it can be a relatively simple and reliable solution for achieving high-energy amplification. In a multi-pass MOPA system, since the seed beam traverses the amplification medium multiple times, the gain can be efficiently extracted and used for amplification of the seed laser. Effective gain extraction will suppress the unnecessary ASE or self-lasing oscillation that depletes the gain. Thus, this can be a compact and cost-effective approach.

[0016] Typically, a MOPA having a multi-stage amplifier comprises at least three excitation sources, one excitation source exciting a master oscillator and the other excitation sources exciting output amplifiers. Excitation can include a flash lamp or other light source. In the case of a generally used multi-pass or multi-stage MOPA system, multiple separate excitation sources are used. For proper operation of a MOPA, it is necessary to strictly synchronize the excitation sources with the Q-switch operation. However, synchronization requires a complex electronic system. Any electronic noise or fluctuation can cause significant undesirable energy variations. Furthermore, using complex electronics for timing synchronization can increase the complexity and cost of the system. The laser system described herein provides optical synchronization between a single excitation laser for exciting a seed laser and multi-pass and multi-stage amplifiers, thus avoiding all of these problems. Additionally, a laser system having a single excitation laser can be less expensive than a laser system having separate excitation lasers configured to excite a seed laser and separate excitation laser(s) configured to excite an amplifier system.

[0017] The MOPA laser system and method disclosed herein perform optically synchronized multi-stage / multi-pass amplification seeded by a short cavity Q-switch laser to generate joule-level sub-nanosecond laser beams. The overall MOPA system comprises one passive Q-switch laser having a cavity length of a few millimeters acting as a seed laser, followed by two or more amplifiers arranged in series or multi-pass configuration, or a combination of multi-pass and multi-stage amplifiers. In the disclosed MOPA laser system, there are no active components involved in performing proper laser operation. Thus, this is a fully passive approach. The seed laser can generate millijoule-level laser pulses having nearly perfect beam quality and a short pulse duration of 50 - 150 ps. One or more isolators can be placed between the seed laser and the amplifier assembly. In one example, one isolator can be placed between the seed laser and the amplifier assembly. In a multi-stage system, additional isolators may be used between two adjacent stages. The isolator is operable to block any unwanted feedback from the current stage to the previous stage adjacent thereto to prevent optical damage or malfunction of the passive Q-switching timing. A single excitation source is used to excite both the seed laser and the amplifiers. Thus, synchronization of the multi-stage amplifier and the seed laser is automatically performed optically.

[0018] Among some of the advantages of the disclosed approach are all-passive approach, optical synchronization for proper functioning of multi-stage / multi-pass amplification, self-energy compensation, reliable performance, and / or energy scaling. For example, the disclosed laser system can have much higher energy stability compared to an electronic synchronization amplification system and can reduce performance dependency on environmental variations (temperature, electronic noise, etc.). Also, eliminating expensive optical devices for performing time synchronization and their expensive electronic drivers can also help reduce COGs (cost of goods sold).

[0019] The all-passive approach means that there are no active components used in the system. Q-switching is achieved with a passive attenuator, i.e., a saturable absorber. An electronic driver is not required for the timing synchronization of the amplifier and the seed laser pulse. Optical synchronization means that a single excitation source is used for both the seed laser and the amplifier, enabling optical execution of timing synchronization without introducing electronic components. This also enables proper functioning of multi-stage / multi-pass amplification. Self-energy compensation means that optical synchronization can help mitigate amplified output energy fluctuations due to excitation energy fluctuations. As shown in Figure 10A, when the excitation energy is high, the accumulated energy overcomes the Q-switch laser threshold, and the time required to generate a Q-switch laser pulse from the seed laser becomes shorter. In the case of an amplifier, a shorter Q-switching delay time helps balance the higher gain in the amplification medium due to higher excitation energy. As a result, the accumulated energy available for extraction may be the same, and it may not generate a larger amplification energy. On the other hand, as shown in Figure 10B, when the excitation energy is low, the longer the QS delay time, the more time is obtained to accumulate energy in the amplification medium to offset the gain reduction. Therefore, such optical synchronization with a passive Q-switching amplifier functions as an energy self-stabilization mechanism for reducing excitation laser energy fluctuations. High-reliability performance means that compared with an electronically synchronized MOPA system, an all-passive optical synchronization system is less affected by electronic noise and not vulnerable to environmental changes. Energy scaling is provided by a modular amplifier configuration for a multi-stage amplifier assembly. Since each stage can be considered an independent amplifier, it can be easier to perform energy scaling by adding more stages of the amplifier without complicating the system setup (especially synchronization) and the alignment of the previous stage.

[0020] Figures 1-8 show various exemplary embodiments of a high-energy sub-nanosecond laser system. The disclosed high-energy sub-nanosecond laser system 100 includes an excitation laser source 102 operable to generate an excitation laser beam having an excitation wavelength, a first excitation beam splitter 104 operable to receive the excitation laser beam 103 and split the excitation laser beam into at least a first split excitation laser beam 105 and a second split excitation laser beam 107, a passive Q-switch seed laser 110 operable to receive the first split excitation laser beam 105 and generate a seed laser beam 109, and an amplifier assembly 120 operable to receive the second split excitation laser beam 107 and the seed laser beam 109, the amplifier assembly 120 having one or more amplifiers. The one or more amplifiers may be arranged in series in a multi-stage configuration, arranged in a multi-pass configuration, or a combination thereof. The first split excitation laser beam 105 and the second split excitation laser beam 107 are delivered simultaneously to both the seed laser 110 and the amplifier assembly 120, optically and automatically time-synchronizing the seed laser and the amplifier assembly.

[0021] Further, the system 100 may further include an excitation beam splitting optical system such as a beam splitter 104, one or more steering mirrors 106 at the excitation wavelength, and an isolator 108 inserted between the seed laser 110 and the amplifier assembly 120. FIG. 1 shows a typical all-passive high-energy laser system 100 having an amplifier assembly 120 optically synchronized with a single excitation laser source 102. For practical reasons, a two-stage or three-stage amplifier may be used for the multi-stage amplification assembly.

[0022] (Excitation Laser) In one embodiment, a common excitation laser source 102 is provided for exciting the disclosed high-energy (joule-level) sub-nanosecond laser system 100 with a high-energy pulsed laser using a multi-stage / multi-pass amplifier. More specifically, the excitation laser source 102 simultaneously excites the seed laser 110 and the individual amplifiers within the amplifier assembly 120, enabling the passively Q-switched seed laser 100 to be automatically synchronized with all the amplifiers.

[0023] The excitation laser source 102 may be operable to generate an excitation laser beam 103 having an excitation wavelength with excitation energy, and may be pulsed over an excitation pulse duration. At the excitation wavelength, the gain media 115 in the seed laser 110 and the amplifiers may absorb substantial excitation energy. In various embodiments, for a neodymium (Nd)-doped gain medium, the excitation wavelength may range from about 730 nm to about 900 nm. The choice of excitation wavelength depends on several factors, such as sufficiently high absorption by the gain medium, available excitation sources, and considerations of reducing quantum defects. In at least one example, the excitation wavelength may be about 750 nm. The excitation pulse duration may be selected to be equivalent to the fluorescence lifetime of the gain media in the seed and amplifiers for efficient excitation. In some embodiments where an Nd-doped gain medium is used, the excitation pulse duration may be from about 200 μs to about 500 μs. In at least one example, the excitation pulse duration may be about 250 μs. The excitation energy of the excitation laser source 102 may be high enough to enable passive Q-switching of the seed laser 110 and may also be sufficient to supply sufficient energy storage in the gain media at each amplification stage to achieve efficient amplification. In various embodiments, the excitation energy may range from about 50 mJ to about 10 J. For example, the excitation laser source emits an excitation laser beam pulse having an excitation energy of 1 J or more, 2 J or more, 3 J or more, 4 J or more, 5 J or more, 6 J or more, 7 J or more, 8 J or more, or 10 J or less. In at least one example, the excitation energy may be about 8 J.

[0024] In one example, the pump laser source 102 may be a flashlamp pumped Alexandrite laser that emits a pulsed laser of about 753 nm. In other examples, the gain medium of the seed laser or amplifier may include a neodymium-doped crystal or ceramic material. In the case of neodymium-doped crystals or ceramic materials (i.e., yttrium aluminum garnet (YAG), yttrium aluminum perovskite (YAP), or yttrium lithium fluoride (YLF)), their lifetimes are about 230 - 500 μs and the pump pulse duration is in approximately the same range. In a Nd:YAG-based joule-level MOPA laser system, about 8 J of pump energy may be required.

[0025] In some embodiments, the pump laser source 102 may be fiber-coupled or free-space propagated to the disclosed MOPA system.

[0026] (Passive Q-Switched Seed Laser) In one embodiment, the passive Q-switched seed laser 110 may include a cavity that includes a gain medium 115 and a passive attenuator (e.g., a saturable absorber 116).

[0027] In some embodiments, the passive Q - switched seed laser 110 may further comprise a pair of laser mirrors (e.g., a high reflector 113 and an output coupler 114) to form a laser resonator. The passive Q - switched seed laser 110 may further comprise an attenuator 111 for adjusting the excitation energy and an excitation beam shaping lens (s) 112 for generating an appropriate excitation spot size on the gain medium. The attenuator 111 is disposed within the split excitation beam path 105 before the excitation beam enters the passive Q - switched laser resonator. For example, FIG. 2 shows that the passive Q - switched seed laser 110 cavity is formed by a high reflector laser mirror 113 and an output coupler laser mirror 114 and has a gain medium 115 and a saturable absorber 116 therebetween. Unlike an active Q - switched laser, the passive Q - switched laser resonator does not have an active Q - switching mechanism (i.e., acousto - optic or electro - optic) and requires an external electronic driver. Instead, Q - switching is performed using a passive attenuator / optics (e.g., a saturable absorber 116) that functions as a fast - switching optical shutter. Passive Q - switching can offer the excellent advantage of generating shorter pulses (i.e., sub - nanosecond) compared to active Q - switching because the physical size of the saturable absorber is smaller and the switching time is faster. Further, a highly doped laser medium is typically used to reduce the thickness of the gain material and shorten the cavity length. In some embodiments, the exit face of the gain medium is coated with a high - reflection coating at the excitation wavelength to implement a double - pass excitation geometry, which can further shorten the length of the gain medium and the cavity length. The passive Q - switched short - cavity seed laser 110 is arranged to be a linear cavity to achieve compactness for generating sub - nanosecond laser pulses. In various embodiments, the passive Q - switched seed laser cavity may have a length of less than 10 mm, less than 8 mm, less than 6 mm, or less than 4 mm.

[0028] In one embodiment, the gain medium 115 and the passive attenuator 116 may be integrally coupled to form a monolithic structure. The front surface of the gain medium 115 may be coated with a high-reflection coating, and the back surface of the passive attenuator 116 may be partially coated with a reflective coating.

[0029] Typically, as seen in FIG. 1, in order to generate the first split excitation laser beam 105, a small amount of excitation energy can be split from the excitation laser beam 103 by the beam splitter 104. The second split excitation laser beam 107 having the majority of the energy can be sent to the amplifier assembly 120. For example, the excitation laser beam 103 may be split such that 10% or less of the excitation laser beam 103 energy is directed as the split excitation laser beam 105 to the seed laser 110, and 90% or more of the excitation laser beam 103 energy is directed as the split excitation laser beam 107 to the amplifier assembly 120. In some embodiments, the split excitation laser beam 105 is deflected by the mirror 106 and directed to the seed laser 110. The beam splitter 104 can be any suitable beam splitter such as, for example, a cube beam splitter, a partial reflector, a high refractive index plate, and a fiber splice. In one example embodiment, the beam splitter 104 may be an uncoated sapphire plate or window.

[0030] Returning to FIG. 2, the split excitation laser beam 105 is received by the seed laser 110, passes through the attenuator 111 to adjust the excitation energy to the seed cavity, and then passes through the excitation beam shaping lens 112 to provide an appropriate excitation spot size within the gain medium 115. The attenuator 111 may be a variable neutral density filter or a rotating polarization cube. This is used to adjust the time delay of the Q-switch pulse relative to the start of the excitation laser pulse.

[0031] The seed laser beam 109 emitted from the seed laser 110 may have an energy of from about several hundreds μJ to about 20 mJ. The seed laser beam may be a sub-nanosecond laser pulse having a short pulse duration of about 50 ps or 100 ps. The seed laser beam may have a seed wavelength in the range of from about 1 μm to about 1.1 μm.

[0032] In some embodiments, a high-speed photodetector 117 may be disposed near or within the seed laser 110 to monitor the Q-switch pulse and its time delay. When the Q-switch pulse is detected, the photodetector 117 may provide a feedback signal to the excitation laser source 102 to stop the excitation laser source 102 and prevent double or multiple pulses. In some examples, the photodetector 117 may communicate with the control system of the excitation laser source 102. The communication may be wireless.

[0033] Several factors can affect the Q-switch pulse energy. These include the excitation spot size in the gain medium 115, the initial transmittance of the saturable absorber 116, and the reflectivity of the output coupler laser mirror 114. In one example, the seed laser 110 may be an alexandrite laser pumped Nd:YAG laser having Cr 4+ :YAG as a passive Q-switch. With an excitation pulse of about 50 mJ and appropriate laser cavity parameters (i.e., cavity length, excitation spot size, initial transmittance of Cr4+:YAG, and reflectivity of the output coupler) of about 250 μs or less, the Q-switch Nd:YAG short cavity laser can generate a millijoule-level sub-nanosecond laser pulse having a short pulse duration of about 100 ps. In another example, the laser medium may be Nd:YAP or Nd:YLF.

[0034] (Multi-stage amplifier assembly) In one embodiment, one or more amplifiers of the amplifier assembly 120 may be arranged in a multi-stage configuration having a first stage and a second stage. The amplifier assembly 120 may further include a seed laser beam shaping lens 121 and a second excitation beam splitter 122 operable to split a portion of the second split excitation laser beam 107 to excite the first amplifier 126 of the first stage and direct the remaining portion of the second split excitation laser beam 107 to the second amplifier 128 of the second stage. In some embodiments, the amplifier assembly 120 may further include one or more dichroic mirrors 124 and 131 to couple the excitation beam and the seed beam / pre-amplified seed beam.

[0035] Returning to FIG. 1, the pulse of the sub-nanosecond seed laser beam 109 may propagate through an isolator 108 (e.g., a Faraday isolator) before being sent to the multi-stage amplifier assembly 120 for amplification. Using the isolator 108 can prevent feedback from the amplifier assembly 120 from being sent back to the seed laser 110. If feedback from the amplifier assembly 120 is sent back to the seed laser 110, it may cause optical damage and timing fluctuations in Q-switching. The multi-stage amplifier assembly 120 may also be operable to receive most of the excitation energy (e.g., the split excitation laser beam 107) from the same excitation laser source 102 used to excite the seed laser 110.

[0036] FIGS. 3-6 show an example of a fully passive sub-nanosecond MOPA system 100 having a dual-stage amplifier within the amplifier assembly 120. In some embodiments, there may be two inputs to the amplifier assembly 120, namely, the seed laser beam 109 after the isolator 108 and the input from the split excitation beam laser beam 107 after the beam splitter 104. The sub-nanosecond seed laser beam 109 generated from the seed laser 110 propagates through the optical isolator 108 and enters the first stage of the amplifier assembly 120.

[0037] In one embodiment, the seed laser beam shaping lens 121 is disposed in front of the amplification laser medium of the first stage amplifier 126 to generate a preferred spot size and divergence of the seed beam, and achieve a better spatial overlap with the second split excitation laser beam 107 within the amplification laser medium (i.e., the first stage amplifier 126). Most of the remaining excitation energy (i.e., the second split excitation laser beam 107) after being split to excite the seed laser 110 by the beam splitter 104 is utilized as a single excitation source for supplying energy to the amplifier assembly 120. The amplifier assembly 120 may include an amplifier beam splitter 122 that splits a part of the incoming second split excitation laser beam 107 energy to excite the first stage, and the remaining excitation energy is sent to the second stage amplifier 128. The excitation energy ratio of the two stages can be set for optimal final extraction efficiency. Three configuration examples are shown in FIGS. 3 to 5 with respect to the propagation direction of the excitation laser beam with respect to the input seed laser.

[0038] FIG. 3 shows a front-end excitation configuration in which an excitation laser beam propagates forward in a straight line with a seed laser for the first stage and an amplified seed laser from the first stage for the second stage, respectively, through an amplification medium (i.e., the first-stage amplifier 126 and the second-stage amplifier 128). The first excitation beam shaping lens 123 can be used to shape the beam to achieve an optimized spatial mode matching between the second split excitation laser beam 107 and the seed laser beam 109 within the amplification medium of the first-stage amplifier 126. The second excitation beam shaping lens 130 can be introduced to provide mode matching between the pre-amplified seed laser and the excitation beam for the second stage in the second-stage amplifier 128. The first dichroic mirror 124 and the second dichroic mirror 131 can be used to combine the excitation beam with the seed laser beam for the first stage and the pre-amplified seed laser for the second stage, respectively. These are coated to be highly reflective at the excitation wavelength and highly transmissive at the seed laser wavelength.

[0039] In some embodiments, an additional optical isolator 125 can be inserted between the first-stage amplifier 126 and the second-stage amplifier 128 to avoid crosstalk between the two amplifiers that causes unwanted optical damage and low execution efficiency. To block the residual excitation energy leaking out through the amplification medium before exiting the system, the amplifier assembly can include another dichroic mirror 133 together with a beam dump 132, so that only the amplified sub-nanosecond laser beam can be the single output.

[0040] In another embodiment, another excitation laser configuration is shown in FIG. 4. In this embodiment, the excitation beam enters the amplifier from the rear end face of the amplification medium, and as a result, the excitation beam propagates in the reverse direction through the amplification medium (i.e., the first-stage amplifier 126 or the second-stage amplifier 128) together with the seed laser beam or the pre-amplified seed laser.

[0041] In some embodiments, the amplification medium may be excited from both the front end and the rear end to provide a more uniform excitation energy distribution, and thus, as shown in FIG. 5, a uniform gain distribution is provided across the length of the crystal / amplification medium. For each amplification stage, the incident split excitation laser beam 107 split by the amplification beam splitter 122 is further split into two parts with equal or different energy ratios by the second beam splitter 134 or the third beam splitter 135. The excitation beam shaping lenses 123 and 130 can be configured to generate an appropriate spot size and beam divergence to achieve optimized mode matching in the forward and backward directions with the incident seed beam of the first stage. The same function is realized by the excitation beam shaping lenses 138 and 139 of the second stage. The dichroic mirrors 124 and 133 are each operable to couple the excitation beam with the seed laser beam for the first stage and the amplified seed laser beam for the second stage. Another set of dichroic mirrors 131 and 140 can be used to send another part of the excitation beam to the back of the amplification medium of the first stage amplifier 126 and the second stage amplifier 128, and to transmit the pre-amplified seed laser beam and the amplified seed laser beam, respectively.

[0042] In one embodiment, the multi-stage amplifier assembly 120 may be composed of amplifiers with more than two stages. In various embodiments, the amplifier assembly 120 may include one, two, three, or four amplifiers. In at least one example, the amplifier assembly 120 may have three stages including three amplifiers. As an example, FIG. 6 shows a three-stage amplifier assembly 120 having a forward pumping configuration. The pumping beam splitters for amplifiers 122 and 134 split the available pumping beam energy in the amplifier assembly into three parts and can be used to pump three amplifiers, namely the first-stage amplifier 126, the second-stage amplifier 128, and the third-stage amplifier 142. The pumping beam shaping lenses 123, 130, 138 are arranged in the pumping beam path to perform mode matching between the pumping beam and the corresponding seed laser beam in three steps. The same function may be performed by the seed laser beam shaping lenses 121, 127, and 136 for the seed laser beam. Similar to the dual-stage system, other pumping configurations (i.e., backward pumping and double-end pumping) may also be associated with this configuration.

[0043] (Multi-pass amplification) Figures 7 and 8 are examples of an optical synchronous MOPA system having a multi-pass amplifier assembly and a backward excitation configuration in a single stage (Figure 7) or a dual stage (Figure 8). In some embodiments, the high energy laser system 200 includes a passive Q-switch short cavity seed laser 210, a multi-pass amplification assembly 220, and an excitation laser 202. Further, it may further include an excitation beam splitting optical system 204, a steering mirror 206, and an isolator 208 inserted between the seed laser 210 and the amplifier assembly 220. The amplifier assembly 120 may include one or more amplifiers 226, 236, one or more seed laser beam shaping lenses 221, 227, one or more excitation beam shaping lenses 223, 235, a polarization optical system 222, one or more dichroic mirrors 224, 231, one or more steering mirrors 229, 230, 232, and a quarter-wave plate 228. In one embodiment, the amplifier assembly 120 includes one amplifier and is arranged in a multi-pass configuration. Figure 7 shows a typical all-passive high energy laser system 200 having a multi-pass amplifier assembly 220 optically synchronized with a single excitation laser 202.

[0044] Similar to the multi-stage amplifier assembly, the multi-pass amplifier assembly 220 receives the first split excitation laser beam 207 and the seed laser beam 209 after the isolator 208. In this configuration, only one laser amplification medium (e.g., amplifier 226) is used. The seed laser beam 209 is arranged to pass through the amplifier 226 multiple times in order to obtain efficient amplification before being emitted from the amplifier 226. The seed laser beam 209 travels back and forth within the amplification medium either on the same straight line or spatially separated. In a multi-pass system on the same straight line, the control of the output beam depends on the polarization difference. One example is the double-pass amplifier on the same straight line shown in FIG. 7. The seed laser 210 and the isolator 208 are set such that the seed laser beam 209 after the isolator 208 is P-polarized. The polarization optics 222 is operable to transmit the P-polarized seed laser and propagate through the amplifier 226 as the first pass before being reflected by the dichroic mirror 224. The reflected amplified seed laser beam further passes through a quarter-wave plate 228 and is accurately reflected in the opposite propagation direction by a planar or curved mirror 232. When the amplified seed laser beam passes through the quarter-wave plate 228 for the second time, its polarization rotates by 90 degrees to become S-polarized and is directed towards the amplifier 226 again by the dichroic mirror 224 as the second pass to obtain further amplification. The S-polarized beam is reflected by the polarization optics 222 after passing through the amplification medium for the second time. Note that the polarization optics 222 may be a thin-film polarizer or a polarization cube. The dichroic mirror 224 can act as a beam combiner to transmit the excitation beam, combine it with the reflected seed beam, and cause the two beams to propagate on the same straight line towards the amplification medium. The multi-pass amplifier assembly also includes one or more excitation beam steering mirrors 229, 230 and an excitation beam shaping lens 223.

[0045] If further amplification is required, an amplifier assembly 220 that combines a complex multipass amplifier and a multistage amplifier is needed. For example, the amplifier assembly 220 may include a plurality of amplifiers arranged in a configuration that combines multipass and multistage. In some embodiments, the first amplifier and / or the second amplifier is excited from the front end or the rear end of the amplification medium. In some embodiments, the first amplifier and / or the second amplifier is excited from both the front end and the rear end to provide a uniform excitation energy distribution. In other embodiments, the amplifier assembly 120 includes a double-pass amplifier in the first stage, followed by a single-pass amplifier in the second stage. FIG. 8 shows an example of an amplifier assembly 220 that includes a double-pass amplifier 226 as the first stage, followed by an amplifier 236 in the second stage. Similar to a typical double-stage amplification system, this hybrid amplification system uses a double-pass amplifier instead of a typical single-pass amplifier as the first stage. One feature common to all the disclosed configurations is that the excitation beam and the seed laser for each stage or each pass are obtained from the same excitation laser, which is necessary to perform optical synchronization and all-passive operation of the laser system.

[0046] The above configurations are not exclusive and are provided by way of example. Other configurations for a multistage / multipass MOPA system with optical synchronization are considered to be part of this disclosure.

[0047] (Optical synchronization) The advantage of the sub-nanosecond laser system is that a common excitation laser optically synchronizes the passive Q-switch laser and the multi-stage / multi-pass amplifier assembly. As an example, FIG. 9 shows the principle of how the synchronization of the dual-stage amplifier assembly functions. With the start of the excitation laser pulse, time zero is set for the clock of the entire system. When excitation occurs, the gain of the seed laser gain medium and the two amplifiers starts to increase simultaneously. If Q-switching is not occurring, all gain curves reach their maximum values simultaneously and then decrease to zero. The magnitude of the gain value of a given gain medium depends on its excitation energy and doping concentration. The time required to reach the peak of the gain curve is determined by the fluorescence lifetime of the gain medium and other depletion mechanisms such as amplified spontaneous emission (ASE) or self-laser oscillation.

[0048] If there is a passive Q-switching mechanism in the seed laser, short Q-switching pulses may be generated when the accumulated gain value exceeds the cavity losses. The Q-switching delay is the time required for the gain of the seed gain medium to exceed the losses. In some embodiments, the control of the amplified laser energy can be performed by adjusting the Q-switching time delay of the seed laser while maintaining a constant excitation energy for each amplification stage in order to achieve a stable thermal profile within the amplification medium. The Q-switch delay time can be varied by adjusting the decay of the excitation energy to the seed laser.

[0049] Sub-nanosecond Q-switch pulses function as the end point of gain accumulation for the amplification gain medium. And this end point can be considered to be approximately the same for all amplification media. Although the optical path lengths of each stage can be different in meters, when compared on the scale of hundreds of microseconds set by the fluorescence time of the gain medium, the time it takes for light to pass through each amplification stage is approximately the same. The gain accumulated in each amplification medium within the Q-switching delay time can be available for extraction. Therefore, synchronization can be achieved by an excitation laser pulse having a rising edge that defines time zero and a passive Q-switch pulse that sets the end time. In some embodiments, the same principle may also be applied to multi-pass amplification and to amplification systems that combine multi-pass amplification and multi-stage amplification.

[0050] (Energy control by adjustment of Q-switch time delay) Most commonly, the output energy adjustment of a MOPA system is mainly performed by adjusting the pump power / energy to the amplifier. However, such an energy control method may cause several problems. First, in the case of a MOPA system that requires a wide range of energy adjustment, it is necessary to greatly change the pump power / energy to the amplifier from low to high. As a result, the thermal lens effect and polarization extinction change significantly, which becomes a problem in some applications that require stable beam polarization. Some examples include harmonic generation, excitation anisotropic laser media, polarization sensitivity measurement, etc. The second problem is due to the instability of the laser energy amplified at a very low energy level. Low output energy is accompanied by low pump energy. However, the most commonly used pump lasers may become more unstable, and their energies vary much more greatly at very low energy levels compared to higher energy levels. Therefore, a large variation in pump energy will be directly converted into unstable energy of the laser amplified at the lower limit of the energy level. Furthermore, the unstable pump energy causes a variation in the delay of the Q-switch seed laser pulse with respect to the start of the pump pulse, which further results in unstable output energy.

[0051] To overcome these problems, energy control can be implemented while maintaining a constant excitation energy for the amplifier assembly by controlling the excitation energy to the seed rather than the excitation energy to the amplifier. In this case, the excitation energy to the amplifier is set to a level at which stable operation can be achieved. As an example, it is to operate the excitation at the highest constant energy level for the pump amplifier.

[0052] In one embodiment, the optical synchronization of the disclosed laser system can perform energy control by adjusting the Q-switching time delay while maintaining the same excitation energy for each amplification stage. As shown in FIG. 2, an attenuator for adjusting the excitation beam energy is disposed in the excitation beam path of the seed laser. When the excitation beam energy to the seed laser is set to a lower value, more time is required to generate the Q-switching pulse, and thus the Q-switching delay becomes longer. For optical synchronization, a longer Q-switching delay will result in a larger gain accumulation available for extraction in the amplification medium, which will result in a higher amplified output energy, as shown in FIG. 11B. In contrast, as shown in FIG. 11A, by increasing the excitation energy to the seed and adjusting the Q-switching delay time to a shorter time, it is possible to obtain a lower output energy. FIGS. 11A and 11B show controlling the Q-switch seed laser pulse delay to adjust the amplified energy by adjusting the excitation energy to the seed of the dual-stage amplifier assembly. It should be noted that the gain profile in the amplification medium remains the same for a constant excitation energy to the amplifier assembly, but the gain in the seed laser changes as a result of the change in the excitation energy to the seed laser. The same principle can be applied to a single-pass single-stage amplifier, or a multi-pass amplifier, as well as an amplifier assembly consisting of a combination of multi-pass amplifiers.

[0053] It will be understood by those skilled in the art that the present disclosure is not limited to what has been particularly shown and described herein. Rather, the scope of the laser system includes both the various combinations and sub-combinations of the features described above, as well as those modifications and variations thereof that would occur to one of ordinary skill in the art upon reading the above description and that are not in the prior art.

[0054] The above-described and recited disclosure is merely an example. Although many features and advantages of the present technology are described in the foregoing explanation together with details of the structure and function of the present disclosure, the present disclosure is merely illustrative and can be modified within the scope of the principles of the present disclosure in the full scope indicated by the broad general meaning of the terms used in the appended claims, particularly with respect to details such as the shape, size, and arrangement of components. Accordingly, it will be understood that the above examples can be varied within the scope of the appended claims.

[0055] To enhance the understanding of the present disclosure, a number of examples are provided herein. A specific set of statements is provided as follows.

[0056] Statement 1. A sub-nanosecond laser system includes an excitation laser source operable to generate an excitation laser beam having an excitation wavelength, a first excitation beam splitter operable to receive the excitation laser beam and split the excitation laser beam into at least a first split excitation laser beam and a second split excitation laser beam, a passive Q-switch seed laser operable to receive the first split excitation laser beam and include a cavity having a gain medium and a passive attenuator and be operable to generate a seed laser beam, and an amplifier assembly operable to receive the second split excitation laser beam and the seed laser beam and generate an amplified laser beam, the one or more amplifiers of the amplifier assembly being arranged in series in a multi-stage configuration, arranged in a multi-pass configuration, or a combination thereof, wherein the first split excitation laser beam and the second split excitation laser beam are simultaneously delivered to both the seed laser and the amplifier assembly to optically and automatically time-synchronize the seed laser and the amplifier assembly.

[0057] Statement 2. In the system of Statement 1, the one or more amplifiers are arranged in a multi-stage configuration having a first stage and a second stage, and the amplifier assembly further comprises a seed laser beam shaping lens and a second excitation beam splitter operable to split a portion of the second split excitation laser beam to excite a first amplifier of the first stage and direct the remaining portion of the second split excitation laser beam to a second amplifier of the second stage.

[0058] Statement 3. In the system of Statement 2, the amplifier assembly further comprises one or more dichroic mirrors.

[0059] Statement 4. In the system of Statement 2, the excitation laser beam is incident on the first amplifier and / or the second amplifier from a front end face of the amplifier, and the excitation laser beam propagates collinearly through the first amplifier or the second amplifier together with the seed laser beam or a pre-amplified seed laser beam.

[0060] Statement 5. In the system of Statement 2, the excitation laser beam is incident on the first amplifier and / or the second amplifier from a rear end face of the amplifier, and the excitation laser beam propagates in a reverse direction through the first amplifier or the second amplifier together with the seed laser beam or a pre-amplified seed laser beam.

[0061] Statement 6. In the system of Statement 5, the first amplifier and / or the second amplifier are excited from both the front end face and the rear end face to provide a uniform excitation energy distribution over the length of the amplification medium.

[0062] Statement 7. In the system of Statement 2, the amplifier assembly further comprises a third stage having a third amplifier.

[0063] Statement 8. In the system of Statement 1, the amplifier assembly comprises one amplifier and is arranged in a multipass configuration.

[0064] Statement 9. In the system of Statement 8, the seed laser beam is arranged to pass through the amplifier multiple times in order to obtain efficient amplification before being emitted from the amplifier.

[0065] Statement 10. In the system of Statement 9, the seed laser beam travels back and forth through the amplifier either linearly or spatially separated within the amplifier.

[0066] Statement 11. In the system of Statement 1, the amplifier assembly comprises two or more amplifiers arranged in a combination of multipass and multistage configurations.

[0067] Statement 12. In the system of Statement 11, the amplifier assembly comprises a first-stage double-pass amplifier followed by a second-stage amplifier.

[0068] Statement 13. In the system of Statement 1, the length of the cavity of the passive Q-switch seed laser is less than 10 mm.

[0069] Statement 14. In the system of Statement 1, the seed laser is operable to generate laser pulses in the millijoule level and with a pulse width of 100 ps or less.

[0070] Statement 15. In the system of Statement 1, the system further comprises one or more isolators arranged between the seed laser and the amplifier assembly.

[0071] Statement 16. In the system of Statement 1, the amplifier assembly further comprises additional isolators between two adjacent stages.

[0072] Statement 17. In the system of Statement 1, the passive attenuator is a saturable absorber.

[0073] Statement 18. In the system of Statement 1, the passive Q-switch seed laser further comprises a pair of laser mirrors for forming a laser resonator.

[0074] Statement 19. In the system of Statement 18, the pair of laser mirrors comprises a high reflector and an output coupler.

[0075] Statement 20. In the system of Statement 1, the passive Q-switch seed laser comprises a separate gain medium and a passive attenuator, the system according to claim 1.

[0076] Statement 21. In the system of Statement 1, the passive Q-switch seed laser comprises a gain medium and a passive attenuator coupled to each other so as to form a monolithic structure.

[0077] Statement 22. In the system of Statement 21, the front surface of the gain medium is coated with a high reflection coating, and the back surface of the passive attenuator is coated with a partially reflective coating.

[0078] Statement 23. In the system of Statement 1, the passive Q-switch seed laser further comprises an attenuator for adjusting the excitation energy and an excitation beam shaping lens.

[0079] Statement 24. In the system of Statement 23, the attenuator comprises a variable neutral density filter or a rotating polarization cube.

[0080] Statement 25. In the system of Statement 1, the passive Q-switch seed laser further comprises a photodetector capable of monitoring the Q-switch pulse from the seed laser and its time delay, providing a feedback signal to the excitation laser source, and triggering the excitation laser source to shut down to prevent double pulses or multi-pulses.

[0081] Statement 26. In the system of Statement 1, the excitation laser source emits an excitation laser beam pulse having an excitation energy exceeding 1 J.

[0082] Statement 27. In the system of Statement 26, the excitation energy is high enough not only to enable passive Q-switching of the seed laser but also to supply sufficient energy storage to the gain medium of each amplifier to achieve efficient amplification.

[0083] Statement 28. In the system of Statement 26, the pulse width of the excitation laser beam pulse is about 100 - 500 μs.

[0084] Statement 29. In the system of Statement 1, the gain medium of the seed laser or amplifier includes a neodymium-doped crystal or a ceramic material.

[0085] Statement 30. In the system of Statement 29, the gain medium is selected from the group consisting of yttrium aluminum garnet (YAG), yttrium aluminum perovskite (YAP), or yttrium lithium fluoride (YLF).

[0086] Statement 31. In the system of Statement 1, the excitation laser source is a flashlamp-pumped alexandrite laser that emits a pulsed laser beam of about 753 nm.

[0087] Statement 32. In the system of Statement 1, the control of the amplified laser energy is carried out by adjusting the delay time of the seed laser while maintaining a constant excitation energy at each amplification stage and achieving a stable thermal profile in the amplification medium.

[0088] Statement 33. In the system of Statement 32, the delay time is varied by adjusting the decay of the excitation energy with respect to the seed laser.

Claims

1. An excitation laser source operable to generate an excitation laser beam having an excitation wavelength, A first excitation beam splitter operable to receive the excitation laser beam and split the excitation laser beam into at least a first split excitation laser beam and a second split excitation laser beam, A passive Q-switch seed laser operable to receive the first split excitation laser beam and generate a seed laser beam, and including a cavity containing a gain medium and a passive attenuator, An amplifier assembly operable to receive the second split excitation laser beam and the seed laser beam and generate an amplified laser beam, having one or more amplifiers, wherein the one or more amplifiers are arranged in series in a multi-stage configuration, arranged in a multi-pass configuration, or a combination thereof, A sub-nanosecond laser system in which the first split excitation laser beam and the second split excitation laser beam are simultaneously delivered to both the seed laser and the amplifier assembly, and optically and automatically time-synchronize the seed laser and the amplifier assembly.

2. The one or more amplifiers are arranged in a multi-stage configuration having a first stage and a second stage, The amplifier assembly further comprises A seed laser beam shaping lens, A second excitation beam splitter operable to split a portion of the second split excitation laser beam to excite a first amplifier of the first stage and direct the remaining portion of the second split excitation laser beam to a second amplifier of the second stage, The system according to claim 1.

3. The amplifier assembly further comprises one or more dichroic mirrors, The system according to claim 2.

4. The excitation laser beam is incident on the first amplifier and / or the second amplifier from the front end face of the first amplifier or the second amplifier, and the excitation laser beam passes through the first amplifier or the second amplifier together with the seed laser beam or a pre-amplified seed laser beam. Propagate in a straight line, The system according to claim 2.

5. The excitation laser beam is incident on the first amplifier and / or the second amplifier from the rear end face of the first amplifier or the second amplifier, and the excitation laser beam propagates in the reverse direction through the first amplifier or the second amplifier together with the seed laser beam or the pre-amplified seed laser beam. The system according to claim 2.

6. The first amplifier and / or the second amplifier is excited from both the front end face and the rear end face in order to provide a uniform excitation energy distribution over the length of the amplification medium. The system according to claim 5.

7. The amplifier assembly further comprises a third stage having a third amplifier. The system according to claim 2.

8. The amplifier assembly comprises one amplifier and is arranged in a multi-pass configuration. The system according to claim 1.

9. Before the seed laser beam is emitted from the amplifier, it is arranged to pass back and forth through the amplifier a plurality of times in order to obtain efficient amplification. The system according to claim 8.

10. The seed laser beam reciprocates linearly or spatially separated within the amplifier. The system according to claim 9.

11. The amplifier assembly comprises two or more amplifiers arranged in a combination of multi-pass and multi-stage configurations. The system according to claim 1.

12. The amplifier assembly comprises a double-pass amplifier in the first stage and an amplifier in the second stage following it. The system according to claim 11.

13. The length of the cavity of the passive Q-switch seed laser is less than 10 mm. The system according to claim 1.

14. The seed laser is operable to generate laser pulses in the millijoule level and a pulse width of 100 ps or less. The system according to claim 1.

15. The system according to claim 1 further comprises one or more isolators arranged between the seed laser and the amplifier assembly.

16. The amplifier assembly further comprises an additional isolator between two adjacent stages. The system according to claim 1.

17. The passive attenuator is a saturable absorber. The system according to claim 1.

18. The system according to claim 1, wherein the passive Q-switch seed laser further comprises a pair of laser mirrors for forming a laser resonator.

19. The system according to claim 18, wherein the pair of laser mirrors comprises a high reflector and an output coupler.

20. The system according to claim 1, wherein the passive Q-switch seed laser comprises a separate gain medium and a separate passive attenuator.

21. The system according to claim 1, wherein the gain medium and the passive attenuator are combined to form a monolithic structure.

22. The system according to claim 21, wherein a front surface of the gain medium is coated with a high reflection coating and a back surface of the passive attenuator is coated with a partially reflective coating.

23. The system according to claim 1, wherein the passive Q-switch seed laser further comprises an attenuator disposed in an excitation beam path before entering the laser resonator to adjust excitation energy, and an excitation beam shaping lens.

24. The system according to claim 23, wherein the attenuator comprises a variable neutral density filter or a rotating polarization cube.

25. The system according to claim 1, wherein the passive Q-switch seed laser further comprises a photodetector capable of monitoring a Q-switch pulse from the seed laser and its time delay, providing a feedback signal to the excitation laser source, and triggering to shut down the excitation laser source to prevent double pulses or multi-pulses.

26. The system according to claim 1, wherein the excitation laser source emits an excitation laser beam pulse having an excitation energy exceeding 1 J.

27. The system according to claim 26, wherein the excitation energy is high enough not only to enable passive Q-switching of the seed laser, but also to supply sufficient energy storage to the gain medium of each amplifier to achieve efficient amplification.

28. The system according to claim 26, wherein a pulse width of the excitation laser beam pulse is about 100 - 500 μs.

29. The system according to claim 1, wherein a gain medium of the seed laser or the amplifier comprises a neodymium-doped crystal or a ceramic material.

30. The gain medium of the system according to claim 29 is selected from the group consisting of yttrium aluminum garnet (YAG), yttrium aluminum perovskite (YAP), or yttrium lithium fluoride (YLF).

31. The system according to claim 1, wherein the excitation laser source is a flashlamp-pumped Alexandrite laser that emits a pulsed laser beam of about 753 nm.

32. The control of the amplified laser energy is performed by adjusting the delay time of the seed laser while maintaining a constant excitation energy at each amplification stage and achieving a stable thermal profile in the amplification medium, according to the system of claim 1.

33. The system according to claim 32, wherein the delay time is varied by adjusting the attenuation of the excitation energy with respect to the seed laser.

Images