CN113366712A - Multi-stage parametric module and picosecond pulse laser source comprising same - Google Patents

Abstract

A multi-stage Optical Parameter (OP) module is configured with an upstream Optical Parametric Amplification (OPA) stage, a plurality of intermediate OPA stages and an output OPA stage, the OPA stages being arranged along a single optical path and aligned at respective λ p, λ₃And λ_fThe pump, signal and IR beams at wavelength interact. Each of the OPA stages is provided with a Time Delay Compensation (TDC) component that alternates with the OPA stages along a single optical path. The TDC components are each configured to complementThe group velocity mismatch between the pump and signal beams is compensated for and the IR, pump and signal beams are guided along the optical path between the OPA stages while preventing the idler beam from propagating after each subsequent parametric interaction.

Description

Multi-stage parametric module and picosecond pulse laser source comprising same

Technical Field

The present disclosure relates to picosecond lasers. In particular, the present disclosure relates to a high power quasi-continuous (QCW) picosecond laser based on an optical parametric module configured with multiple nonlinear crystal based parametric amplifier (OPA) stages, and applications utilizing the OPA module.

Background

Many scientific and industrial applications require high average power pulsed lasers with tunable center wavelengths, variable spectral widths, and different pulse durations in the visible frequency range. Although high power Single Mode (SM) lasers in the infrared frequency range have been developed and widely used in various industries, high power blue lasers in general, and ultra-fast high power blue lasers in particular, have not been implemented in any desired power range, between several hundred watts to at least 1 kW.

There are few practical technical methods for generating blue laser light. A method includes fabricating a gallium nitride (GaN) based blue diode laser. GaN-based diode lasers with long lifetimes are unknown and suffer from limited optical efficiency.

For many years, the most efficient method of developing sources based on ultrafast blue lasers has been based on nonlinear conversion, i.e., frequency tripled and quadrupled, mode-locked lasers including Kerr lens mode-locked (KLM) Ti: sapphire laser, Nd: YAG lasers, mode-locked EDFAs, and, more recently, nanosecond (ns) Yb fiber lasers.

Another popular approach for developing ultrafast blue laser sources is based on parametric (parametric) nonlinear optical devices. In particular, green-pumped OPA, Optical Parametric Oscillator (OPO) and OP generator (OPG) are attractive laser sources due to the wide tuning range and average power scalability. For example, a synchronously pumped optical parametric apparatus is a useful ultrafast source of tunable coherent radiation that provides average power across a broad spectral region.

The law of conservation of energy in the parametric process is as follows:

second harmonic generation of signal/idler SHS/SHI:

sum frequency of the signal/idler and IR laser wavelengths generates SFS/SFI:

fig. 1 shows an exemplary schematic diagram of a blue laser source based on a stepped and frequency parametric amplifier disclosed in USP 7,106,498. This schematic diagram represents a fairly good representation of an existing blue laser source and comprises a beam splitter 1, which beam splitter 1 guides the light portions at the fundamental frequency and the pump frequency, respectively, along two different optical arms by means of a plurality of mirrors 9. Optically combined together in a sum frequency tunable generator (SFG)14, the output comprises four optical signals 17, 18, 19 and 20 at respective different frequencies generated in a manner well known to those of ordinary skill in the laser art. The shown two-arm schematic with beam splitter, beam combiner and multiple mirrors has a large footprint and a complex configuration, which is a great disadvantage of the known blue lasers based on parametric amplification.

Among the various NLOs used in parametric processes, lithium triborate LiB3O5(LBO) NL crystals have proven to be particularly damage resistant and non-absorbing, and thus, suitable for use in high power parametric devices that may otherwise be easily damaged. Other borate NL crystals (β -barium borate, BaB2O4 or BBO and bismuth borate, BiB3O6 or BiBO) and periodically poled NL crystals (PPTL, PPLN and PPKTP) may be equally effective and sometimes even superior to LBO crystals for parametric amplification, mainly due to their large non-linearity coefficient d_eff. Unfortunately, the expected minimum pulse peak power when using BBO and BIBO is relatively high due to the critical phase matching conditions employed. The most common periodically poled crystals are susceptible to photorefractive or photochromic damage, which limits the scaling of the average power. However, various applications may only benefit from high average power blue light. Parametric conversion is not limited to blue light generation. Other wavelengths generated by means of parametric processes can be advantageously used in various industries.

It is therefore desirable to provide a tunable ultrashort parametric module with a compact structure that does not feature optical splitters and optical combiners.

It is also desirable to provide a ps-pulse fiber laser source in combination with the disclosed parametric module.

Disclosure of Invention

According to one aspect of the present disclosure, a multi-stage Optical Parametric (OP) module is configured with an upstream Optical Parametric Amplification (OPA) stage, a plurality of intermediate optical parametric amplification stages, and an output optical parametric amplification stage, the OPA stages being arranged along a single optical path and associated with a respective λ p, λ₃And λ_fThe pump, signal and IR beams at wavelength interact. The OPA stages are each provided with a Time Delay Compensation (TDC) component which alternates with the OPA stages along a single optical path. The TDC components are each configured to compensate for a group velocity mismatch between the pump and signal beams, and to direct the IR, pump beams along the optical path between the OPA stagesAnd a signal beam while preventing propagation of the idler beam after each subsequent parametric interaction.

According to another aspect of the present disclosure, an optical pump is configured with a Linearly Polarized (LP) ps fiber laser source pumping an optical parametric module. Advantageously, but not exclusively, the disclosed source may be used to output red, green and blue light simultaneously. The ps laser source is configured with a mode-locked ps fiber laser using Chirped Pulse Amplification (CPA) technology to generate an output at a fundamental wavelength in the 1 μm spectral range. By appropriate selection of the desired fundamental wavelengths, the source can simultaneously output red and blue light with average pulse powers in excess of 100W. For example, the fundamental wavelength may be selected from the group consisting of 1030nm, 1048nm, 1060nm, and 1071 nm.

Each of the above aspects is characterized by a plethora of features that can be combined with each other in all configurations, as detailed in the following detailed description and claims. The features of each aspect may be combined in a manner readily discernible by one of ordinary skill in the laser art.

Drawings

The above and other aspects and features of the present disclosure will become more apparent in conjunction with the following drawings, in which:

FIG. 1 is an optical schematic of a blue laser according to known prior art;

FIG. 2 is a schematic view of a fiber optic light source of the present invention;

FIG. 3A is a schematic diagram of a red light generating OPA module incorporated in the fiber laser source of the present disclosure of FIG. 2;

FIG. 3B is an example of a parametric blue light generation module combined with the red light generation module of FIG. 3A;

FIG. 3C is another example of a blue light generating module combined with the red light generating module of FIG. 3A;

FIG. 3D is yet another example of an OPA module;

FIG. 3E is yet another example of an OPA module of the present invention;

FIG. 4A is a detailed optical schematic of a blue laser of the present invention according to one specific example;

FIG. 4B is a detailed optical schematic of a blue laser of the present invention according to another example;

FIG. 5 shows the blue-red wavelength relationship of the disclosed RG light source;

FIG. 6 is an optical schematic of the disclosed ps-fiber laser based pump module;

FIG. 7 is a schematic representation of one example of a pulse replicator module in accordance with aspects of the present technique; and

fig. 8 is a schematic representation of another example of a pulse replicator module in accordance with aspects of the present technique.

DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION

Reference will now be made in detail to the disclosed system. Wherever possible, the same or similar reference numbers are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. For convenience and clarity only, the terms "connected," "coupled," "combined," and similar terms and their lexical variations do not necessarily mean directly and directly connected, but also include connections through intermediate elements or devices.

Fig. 2 schematically shows a light source 10 according to the invention, the light source 10 comprising several modules optically coupled to each other. The fiber laser source module 12 is operative to generate a sequence of ps IR pulses propagating along the optical path at a fundamental wavelength λ f selected from, for example, 1030nm, 1048nm, 1060nm, or 1071 nm. The IR beam interacts with the NLO of pump 14 to generate SH by converting a portion of the IR light to pump light at a pump wavelength λ p ═ λ f/2. The pump 14 may be packaged in a pump module 16 that also includes the fiber laser source 12 and outputs the pump light and the remaining IR light at the respective λ p and λ f wavelengths. The parametric NLO module 18 processes the output received from the pumping module 16 and is configured to output red, green, and blue light at respective signal, pump, and blue wavelengths. As described below, various structural examples illustrate the concepts of the present disclosure. Common to all disclosed examples is the optical schematic of NLO module 18, including several OPA stages disposed along a single optical path, as better shown in the following figures.

FIGS. 3A-3C show N of FIG. 2The operating principle and structure of LO 16, in particular, fig. 3A shows four OPA stages, each stage comprising an LBO crystal. The upstream OPA1 is configured to generate a signal beam, i.e., red light in the 777-854 nm wavelength range, that is directed along the optical path through the subsequent intermediate OPA2-OPA3 stage and gradually amplified. The schematic shown comprising at least three or more upstream and intermediate LBO OPA stages is necessary to provide sufficient amplification of the light components propagating and generated therein, since the peak power of the fiber laser source 12 is limited by a few MW. A laser source not shown here but disclosed below outputs ps IR pulses that are coupled into a Second Harmonic Generator (SHG) (not shown here), after which the IR light is filtered out, while the green light generated at the pump wavelength λ p is coupled with quantum noise to the upstream LBO OPA stage 1. As a result, the latter outputs a green pump beam at the corresponding signal wavelength and idler wavelength λ₃、λ₄A red signal beam and an idler beam generated.

Ultra-fast OPA requires Time Delay Compensation (TDC) of the group velocity mismatch between the pump beam and the signal pulse beam at the output of the LBO crystal. Thus, prior to additional parametric amplification of the red signal beam in LBO OPA stage 2, the propagating green/pump and red signal beams are incident on TDCs located in-line with all OPA stages between adjacent LBO OPA stages₁The above. In summary, the OPA module 18 is configured with a plurality of OPA stages and TDCs, which alternate with each other along the optical path.

A TDC is an optical system whose components may include a chirped mirror, a dichroic mirror (a drop idler), and a birefringent window made of various materials and placed at various temperatures. As a result, spectral tuning can be achieved by merely changing the temperature of the LBO using, for example, a combination of chirped mirrors and material dispersion.

The above disclosed operation is repeated each time the red signal is amplified in the four sequentially positioned stages OPA1-OPA4 shown, while the idler generated in each OPA stage is discarded by the designated TDC1-TDC3 located before the subsequent OPA stage. As recognized by one of ordinary skill, meaningful parameter interaction does not occur if the idlers are coupled to subsequent OPA stages. At the output of the illustrated schematic, the source light strikes a spectral filter, such as a dichroic mirror, which is transparent to the amplified red signal and reflects the green pump light to a specified output.

Fig. 3B, which shows an OPA block having the structure of fig. 3A, represents one example of an output blue light generating OPA stage of the inventive module 18 of fig. 2. In particular, the red signal beam at signal wavelength λ 3 amplified according to the schematic of fig. 3A is further coupled into an output LBO SHG that generates the second harmonic (i.e., the blue beam). The additional dichroic mirror transmits the red signal beam but reflects the generated blue beam.

Fig. 3C shows another example of an OPA stage generating output blue light using two polarizations of output blue and red light. In particular, the output OPA stage is configured with two SHGs spaced apart from each other along the optical path and on either side of a fourth TDC4 comprising a double half-wave plate. At the output of module 18, the cross-polarized blue light and the remaining red light are incident on a subsequent dichroic mirror that separates the light at the output of light source 10 of FIG. 2. Note that the schematic diagrams of the NLO module 18 of fig. 3B and 3C provide separate outputs for the respective green, red and blue light, which are advantageously used in RGB engines, as described below.

FIGS. 3D and 3E show the use at the fundamental wavelength λ_fA slightly different configuration of the NLO module 18 for IR light below, where a portion of the IR light remains after an upstream SHG (not shown here) configured to generate a green beam at the pump wavelength. In particular, fig. 3D illustrates a multi-stage NLO module 18 configured similar to the configuration of the respective fig. 3A-3C. The IR light is incident on the OPA block, which has the same structure as the OPA block of fig. 3A, and propagates through all OPA stages unaffected by parametric operation. Thus, as shown in this figure, the output of the OPA block includes an IR beam, a signal red beam, and a pump green beam, all of which are incident on an output upstream dichroic mirror that is transparent to both red and IR light, but reflects green light. The IR and red beams are coupled into the SFG stage, producing blue light due to the mixing of the coupled IR and red light. An output downstream dichroic mirror spaced downstream from the SFG along the optical path reflects and transmits the blue lightThe remaining portions of the IR and red beams are radiated. Thus, the light source 10 includes three output ports through which the red, green and blue beams pass, respectively.

Similar to fig. 3C, fig. 3E advantageously provides the possibility of utilizing orthogonal polarization directions of the red, blue and IR beams. This is achieved by combining TDC4 with spaced upstream and downstream outputs LBO SFG1 and LBO SFG 2. TDC4 contains triple half-wave plates and other mirrors and is located between SFG1 and SFG 2. The upstream output SFG mixes the IR and red beams to produce a blue beam output. All three lights are incident on a triple half-wave plate that shifts the polarization direction of linearly polarized light. The downstream SFG provides sum frequency generation for blue light whose polarization direction is orthogonal to the polarization of the blue light at the input of the SFG. At the output of the downstream SFG, the cross-polarized light is incident on an output dichroic mirror that reflects the blue light beam and transmits the red/IR light beam.

With reference to fig. 3A-3E above, it is readily seen that the disclosed schematic is characterized by being disposed in line and alternating OPAs and TDCs, which together define a single optical path through the entire illustrated arrangement. The experimental phase of the structure disclosed above gives very promising results. For example, for a conversion efficiency of 50%, the ultrafast OPA of fig. 3A-3C can be pumped with 50-500nJ pulse energy, which is easily achieved by conventional fiber lasers. Using LBO blocks NLO of low absorption and high damage threshold instead of the periodically poled materials commonly used, the above disclosed schematic of light source 10 (fig. 2) may in the near future produce a blue light pulse output of greater than 1kW with a spectral linewidth in the 1.5 to 3nm spectral range.

FIG. 4A shows an exemplary detailed optical schematic of the light source 10 generating a source blue light output in the 443-467nm wavelength range. However, one of ordinary skill in the laser art can readily reconfigure the schematic shown so that it will have a similar straight-line architecture to those shown in fig. 3A-3E, and be constructed identically to the schematic of fig. 3D-3E.

In particular, the laser module 12 generates ps pulses of IR light at a base wavelength λ f of 1030nm and a green pump signal at a pump wavelength λ p of 515 nm. During second harmonic generation, a portion of the IR light is passed to the pump signal in the SHG 14 of fig. 2. The sequence of ps pulse sequences at the fundamental and pump wavelengths is incident on a focusing lens L2, which focusing lens L2 targets each pulse to the central region of a first NLO 26, which serves as both an OPG/a parametric device of the upstream stage that constitutes the NLO module 18. The first nonlinear crystal 26 interacts with the green pump beam to produce a red signal beam and an idler beam at respective third and fourth wavelengths λ 3 and λ 4. The combination of the fundamental wavelength, a portion of the pump beam and the generated red signal beam is incident on the first dichroic mirror 28, the portion of the fundamental wavelength remaining after interaction with the upstream SHG (not shown) propagates freely between the output of the OPG 26 and the input of the SFS 46, the portion of the pump beam at wavelength λ p follows the interaction between the NLO 26 of the upstream OPA stage and the pump signal beam, and the generated red signal beam has a wavelength λ 3. The dichroic concave mirror 28 is transparent to the idler, but reflects the pump and signal light beams in a direction opposite to and nonparallel to the one direction so that the reflected light is incident on the same NLO 26 operating as an OPA. The dichroic mirror 28 is therefore part of the TDC component, compensating for the group velocity mismatch between the reflected pump beam and the red beam. The reflected light propagates through the NLO 26 of the upstream OPA stage in the opposite direction, repeating a process similar to that in the NLO 26 in the other direction.

The red signal beam at the signal wavelength λ 3 is further amplified when the reflected light interacts with the first NLO 26. The idler beam at wavelength 4 is regenerated and propagates coaxially with the further attenuated green pump beam and the amplified red beam. Upon incidence on the dichroic mirror 30, the light beams at all wavelengths, except for the newly generated idler, which propagates through the mirror 30, are reflected to at least one intermediate OPA stage of the module 18.

The reflected beam is further incident on and reflected from a flat mirror 32 which directs the light to a concave mirror 34, which concave mirror 34 reflects the light in one direction through the second nonlinear crystal of the intermediate OPA stage. The interaction between the second NLO36 and the pump signal further amplifies the red signal at λ 3, which is then incident on the dichroic curved mirror 38 with the remaining pump signal, red signal, and idler, where the idler is transmitted through the mirror. The reflected light propagates through the second NLO36 in the opposite direction, repeating a process similar to that in the upstream OPA stage. As a result, the amplified red, third, idler and IR signals at respective λ 3, λ p, λ 4 and λ f are incident on the mirror 40, and the mirror 40 directs all light except the idler to the downstream stage.

Similar to the previous OPA stage, the downstream stage is configured with a flat mirror 42 and a curved mirror 44 that sequentially direct the received light in one direction through an SFG (denoted as SHS) that includes an LBO NLO 46. The latter mixes the previously amplified red light signal at wavelength λ 3 with the remaining IR light at wavelength λ f to produce light at wavelength λ_SSO＝(1/λf+1/λ3)^-1The source signal output of (1). Therefore, the output ps blue light is transmitted through the plane mirror 52 at a wavelength in the wavelength range of 443-.

As shown, the corresponding pump wavelength λ 3 and signal wavelength λ may further be used_fThe remaining red and infrared light. This light is reflected from the curved mirror 48 in the opposite direction through the NLO 46, again generating blue light that is incident on the flat mirror 50 and further reflected toward the flat mirror 54 of the output stage. The half-wave plate 56 is in optical communication with the mirror 54 and is configured to change the polarization of the blue light to be orthogonal to the polarization of the blue light transmitted through the mirror 48. Downstream of the half-wave plate 56, a Thin Film Polarizer (TFP)58 combines the blue light output at the desired 443 or 467nm center wavelength. In general, the output OPA stage may have various parametric devices alone or in combination with devices that produce the desired output wavelength. Accordingly, various types of parametric generation including sum and difference frequencies and various types of frequency conversion including sum, difference and second harmonic generation may be used by appropriately configuring the output stage that can be realized by those skilled in the art.

As described above, each of all OPA stages may be configured to provide a one-way LBO NLO. With this modification, the schematic of fig. 4A can be reconfigured to provide the same straight-line architecture as those shown in fig. 3A-3E, but with twice as many LBO NLOs.

Fig. 4B shows another example of an OPA module 18 of the present invention of a light source 10, which outputs a 343nm blue light beam. The source used in fig. 4B includes a ps UV fiber laser source (not shown) operating at a 343nm pump wavelength. Configured substantially similarly to the schematic of fig. 4A, the module 18 has a downstream OPA stage provided with an optical parametric amplifier instead of the SFG NLO of the previously disclosed module using a 515nm wavelength pump beam.

Various types of NLO crystals may be used in the context of the present disclosure. The crystal may or may not be critically phase matched because any existing spatial walk-off can be compensated by the WOC plate. Thus, crystal types of BBO, BIBO, KTP, KTA, periodically poled LiNbO3(PPLN), periodically poled LiTaO3(PPT), and the like can be implemented in the schematic diagrams of fig. 2 and 3. However, the only crystal that can provide an output power of 100W or higher is LBO (and possibly BIBO) because it has the lowest bulk absorption and highest damage threshold among all other alternative NLO crystals. The NLO may or may not be a non-critical phase matched crystal. If there is any spatial deviation, it can be compensated for, as known to those skilled in the art. Each NLO crystal is thermally controlled individually to provide the best phase matching conditions.

The above NLO module 18 is particularly advantageous when used in an RGB engine in a visual display, which requires the generation of all three primary colors from one laser source. The efficiency of the RGB engine is based on the following major considerations. First, the primary colors red, green and blue should be chosen so that the human eye can detect more than 90% of the color gamut. Second, the speckle phenomenon that causes image distortion should be minimized and ideally eliminated. Furthermore, the RGB light source should be characterized in that the converted visible light has a high wall plug efficiency and a high luminous efficiency at the minimum output power of the fiber pump source disclosed in the present application. However, the display industry is not the only industry that benefits from the light source 10 disclosed in FIG. 2. The parametric module 18, such as shown in fig. 3A-3E, has been demonstrated to provide an output in the 1.7-2.5 μm wavelength range by using an IR beam as the pump beam and a BIBO crystal instead of an LBO crystal. Structurally, the output OPA stage may be configured with two SHGs while the remaining OPA stages are configured with parametric amplifiers similar to fig. 4A and 4B.

Returning to fig. 2, the pump module 16 includes an ytterbium (Yb) doped mode-locked ps fiber laser 12. Most industrial Yb fiber lasers operate at a fundamental wavelength λ f of about 1030nm or 1064nm (emission). The selection of any of these wavelengths determines the configuration of the NLO module 18, as described below.

Fig. 5 shows a blue-red wavelength relationship based on equations 1, 2 and 3, where the 515nm pump wavelength is generated by the interaction of the SHG 14 with the IR signal from the Yb fiber laser 12 at the 1030nm fundamental wavelength (fig. 2). The curve 120 is determined by using a combination of two second harmonic generators (SHGs and SHGi) that interact with light at the respective pump wavelengths. Alternatively, the curve 120 may be modeled by using a combination of sum frequency generators (SFG and SFGi). According to yet another alternative, the curve 120 is modeled by utilizing a combination of SFGs and SHG parametric devices. The curve 120 indicates that the desired red and blue wavelengths can be obtained by using two SHG devices or two SFG devices. If the blue wavelength is changed to, for example, a 445nm wavelength but still requires a 610nm idler wavelength, the combination of SFG stages corresponds to curve 122. The curve 124 corresponding to the combination of SHG and SFG stages shows that if a 610nm idler wavelength is required, the combination cannot be used for valid RGB 10. Various combinations of parametric devices may produce the desired wavelength. The efficiency of the light source 10 ultimately determines one or another combination of parametric mechanisms that can be readily obtained to accommodate any desired wavelength by using the structural flexibility of the NLO module 18 of fig. 3A-3E and 4A-4B.

Table 1 provides the parametric operation types and necessary data used by the high efficiency RGB source 10 at the 1030nm fundamental wavelength to generate blue and signal (red) outputs at the respective 478nm and 610nm wavelengths. As can be seen, the RGB source 10 is configured with an NLO 18 that includes an SHG of 1220nm wavelength to produce the desired source idler output at 610nm wavelength. However, the SHG mechanism does not work efficiently in order to generate a source output signal of 478nm wavelength. However, the use of the SFG device to interact with 892nm signals allows the RGB source 10 to operate efficiently at 478nm source signal output. The table also provides line widths for the source signal output, pump signal output, and source idler signal output, which have line widths of 3nm, 4nm, and 5nm, respectively, which help avoid speckle. At the minimum power of the laser source of the module 12, the conversion efficiency of IR light at the fundamental wavelength to the corresponding red, green and blue signals is between 8% and 20%, which is considered very high, while the average power of the output red and blue signals is 158W, 106W and 114W, respectively, which is also extraordinary for medium-sized 2D cinema. It is believed that for this type of projector, there is no comparable conversion efficiency and power in the corresponding red and especially blue spectral regions.

TABLE 1

G，λc(nm)	478	515	610
				Δλc(nm)	3	4	5
Signal/idler for SHG (nm)		1030	1220
				SHS/SHI efficiency to IR (%)		20	8
Signal/idler for SFG (nm)	892	1030
				SFS/SFI efficiency to IR (%)	11	20
Required power (W/50klm)	158	106	114

The relationship of the different wavelengths shown in table 1 is based on the pump module 16 of fig. 2, which pump module 16 generates pump light at a wavelength of 515 nm. Other pump wavelengths can obviously be used with parametric NLO module 18 based on the law of conservation of energy in the optical parametric process. Table 2 shows the signal red and blue wavelengths generated from OPA/OPO pumped by green light of wavelength 532nm, which is SH of IR light of wavelength 1064 nm. Table 2 provides the data and parametric mechanism for rendering RGB light sources with the 1064nm source that is most efficient for 2D cinema.

TABLE 2

Fig. 6 shows a fiber laser pump module 12 based on a ps fiber source 100 with Chirped Pulse Amplification (CPA) architecture in combination with pulse replication. The source 100 may have various configurations, including an all-fiber or YAG-based structure. Preferably, the source is characterized by a MOPA configuration, and thus, the MOPA configuration includes a master oscillator and fiber or other type of amplifier or booster 150 that outputs a sequence of ps pulses. The source 100 is also configured with: a pulse stretcher 130, a pulse copier block 140, an isolator 160, all located between the seed and the booster; and a pulse compressor 170 after the fiber booster 150. The input laser pulses 112 are temporally stretched using a pulse stretcher 130, amplified in an amplification stage comprising a booster 150 and an optional preamplifier 154, and decompressed using a pulse compressor 170. The stretched pulses 132 are replicated using a pulse replicator module 140 prior to amplification.

The pulse stretcher 130 is configured to stretch pulse durations of the input pulse sequence 112 to produce a stretched pulse sequence 132, the stretched pulse sequence 132 having a reduced peak power. According to some embodiments, the pulse stretcher 130 stretches the pulses of the initial pulse train 112 to a pulse duration of about a few nanoseconds, and may be 10ns in some examples. The repetition rate of the stretched laser pulses 132 can be increased by a pulse replicator module 140 that replicates the optical waveform of the stretched laser pulses 132 in time to generate a modified pulse train 148. The time plot of the stretched laser pulse train 132 output by the pulse stretcher 130 has a pulse period t and a pulse repetition rate 1/t. According to some embodiments, the pulse replicator 140 may be for replicating the stretched laser pulses such that the t is reduced to an extent that the laser energy of the modified pulse train 148 appears continuous. The almost continuous wave characteristics of the modified pulse train 148 are a function of both the stretching performed by the pulse stretcher 130 and the copying performed by the pulse copying module 140. Examples of systems that utilize such lasers are discussed in further detail below. The pulse replicator 140 may be configured to increase the repetition rate of the stretched laser pulses 132 to tens of MHz and several GHz levels. The pulse stretcher 130 and/or the pulse replicator module 140 may be configured to generate modified pulses 148 having a desired peak-to-average power ratio. An example is discussed below.

Pulse replicator module 140 is an all-fiber device that includes at least two fiber couplers including an input fused fiber coupler, an output fiber coupler, and at least one fiber delay line disposed between the input fused fiber coupler and the output fused fiber coupler. All fiber couplers maintain polarization. The fiber coupler may also be configured as a single mode non-Polarization Maintaining (PM) fused fiber coupler. The components of the pulse stretcher 130 and the pulse replicator module 140 may be configured to output pulses at a trimmed (high) repetition rate. The pulse compressor 170 compresses the pulse width of the chirped amplified pulse 153. Non-limiting examples of pulse compressors include grating compressors, such as CVBGs and Treacy compressors, and Martinez and Prism compressors.

The amplified and compressed laser pulses 174 output from the pulse compressor 170 may be characterized as ultra-short pulsed laser with high repetition rate and high average power. Specific applications for such output include generating high average power UV laser radiation, and are discussed below.

Fig. 5 is a schematic representation of a first example of a pulse replicator module 440. According to this configuration, the input fused fiber coupler 442 is configured as a fiber optic splitter. One of the two fibers exiting the coupling region of the output fiber coupler 443 forms an output 436 containing the modified pulse train 148. The beam splitter 442 has an input 434, in which case the input 434 is coupled to the optical pulse stretcher 130 (fig. 4), and an output 436 of the output coupler 443 is connected or otherwise coupled to the fiber power amplifier 150. The pulse replicator module 440 also includes at least one fiber coupler 444 disposed between the input coupler 442 and the output coupler 443.

A delay tau is added to one of the outputs of input splitter 442 using a single mode fiber of appropriate length (i.e., fiber delay line 445) such that one leg or output segment (445) of the pair has a different (longer) optical path length than the other leg 446. This is at the output fiber 445 of splitter 442₁And 446₁Two pulses are generated which are separated by tau. Then, when the two outputs are combined in coupler 444, a delay of 2 τ is placed into one of the paths to produce two sets of four pulses. By doubling the differential delay between the two paths, the process can be repeated until the desired number of copies is obtained. The two paths are then combined using combiner 443. The length of the delay τ may be chosen to be slightly longer than the length of the laser pulse to avoid pulse overlap and interference.

Pulse replicator 440 includes a plurality of stages 449, each stage including a fiber delay line 445 such that each successive stage introduces a time delay to stretched laser pulse 132. For the example shown in FIG. 5, pulse replicator 440 includes four stages 449₁、449₂、449₃And 449₄Where at each stage the signal power is divided and recombined with a fixed time delay. Since the number of copies is doubled at each stage 449 (i.e., a 50: 50 coupler), the two outputs 445 propagate into the combiner 443₄And 446₄Each containing 2^xA copy, where x is the number of stages used (in this example, x-4). Thus, pulse replicator 440 is configured as a multi-stage passive pulse replicator, where each successive stage introduces a fixed time delay to stretched laser pulses 132. The time delay may increase or decrease by a predetermined amount at each successive stage.

Last stage 445 of the duplicator 440₄And 446₄Are combined in the combiner 443 to generate a time-delayed replica pulse train as the modified pulse train 148 at the output 436. As shown in fig. 6, the optical fiber branchWay 445₄And 446₄Each of the 8 replica pulses of (a) are combined in the combiner 443 to generate 16 pulses. These 16 pulses are configured as bursts of pulses, and thus, modified pulse sequence 148 will include a series of bursts of pulses, each burst of pulses including 16 pulses. Delay line 445₁-445₄The length of (c) specifies the burst repetition rate (i.e., the time interval between bursts).

As will be appreciated, the components of the pulse stretcher 130 and the pulse replicator module 140 may be configured to output pulses at a trimmed repetition rate. The repetition rate may be chosen such that the peak power is low enough to avoid undesired damage and yet high enough for efficient frequency conversion. The fiber couplers and fiber delay lines of the pulse replicator module may be used to create various pulse formats, and fig. 8 is a schematic representation of another example of a pulse replicator module 540. According to some embodiments, the pulse replicator module may include a series of sub-modules, where each sub-module may be individually configured. For example, the pulse replicator module 540 shown in fig. 5 uses the output from the pulse replicator module 440 of fig. 7 as an input, but it should be understood that the replicator module 540 may also be used alone or in combination with sub-modules having other configurations.

In a similar manner to the replicator module 440 of fig. 6, the pulse replicator module 540 further includes an input fused fiber coupler 542 and an output fiber coupler 543. Between the input coupler 542 and the output coupler 543 are intermediate fiber couplers (544), (547a), and (547 b). Instead of directing the delay line from each stage 549 to an adjacent (downstream) stage, at least one delay line bypasses one or more downstream stages as shown in the configuration of fig. 6. According to this example, delay line 545₄Is driven from the first stage 549₁Is directed to the output combiner 543 and thereby bypasses the second stage 549₂And tertiary 549₃And form the fourth stage 549₄The delay line of (1). Therefore, the time delay increases introduced at each successive stage are not all equal to each other. As shown in fig. 5, this configuration allows an initial 16-pulse burst with a total duration (envelope) of 9ns to pass throughFour stages 549₁-549₄Converted to 160-pulse bursts with an envelope of 90ns, with each pulse duration being 0.45ns and the pulse spacing being 0.56 ns. Table 1 below summarizes each stage.

TABLE 1-stages of the pulse copier Module 540 of FIG. 6

The examples of pulse replicators shown in fig. 7 and 8 are not meant to be limiting, and other configurations are within the scope of the present disclosure.

Having thus described several aspects of at least one example, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. For example, examples disclosed herein may also be used in other contexts. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the scope of the examples discussed herein. Accordingly, the foregoing description and drawings are by way of example only.

Claims (22)

1. A multi-stage optical parametric OP module comprising:

an upstream Optical Parametric Amplification (OPA) stage receiving a linearly polarized pump beam at a pump wavelength λ p and configured to generate a linearly polarized pump beam at a respective wavelength λ₃And λ₄A lower signal beam and an idler beam, wherein a portion of the pump beam, the signal beam, and the idler beam propagate along a single optical path;

a plurality of intermediate OPA stages optically coupled to each other along an optical path, and each subsequent OPA stage receiving the pump beam and a progressively weaker portion of the amplified signal beam from a preceding OPA stage;

a plurality of time delay compensated TDC components alternating with the OPA stages along a path and configured to compensate for group velocity mismatch between the pump and signal beams and to direct the pump and signal beams along an optical path while preventing the idler beam from propagating after each subsequent parametric interaction; and

an output OPA stage receiving the pump beam and a portion of the amplified signal beam and configured to generate an output beam at a desired output wavelength λ o that is different from a wavelength λ o of the signal beam₃。

2. The OP module of claim 1 wherein the signal beam is progressively amplified in at least two or more intermediate OPA stages, each intermediate OPA stage configured with an optical parametric amplifier, each optical parametric amplifier comprising a nonlinear crystal selected from LBO, BBO, BiBO, KTP, KTA, periodically poled lithium niobate PPLN, or periodically poled lithium tantalate PPLT.

3. The OP module of claim 2 wherein the output OPA stage that generates the output beam comprises at least one or more sum frequency generating SFG parametric devices, at least one or more second harmonic generating SHG parametric devices, or a combination of SFG parametric devices and SHG parametric devices, each parametric device comprising the nonlinear crystal.

4. The OP module of claim 1, wherein the output OPA stage is configured with a half-wave plate to modify a polarization of the output beam and a beam combiner configured to combine two polarizations of the output beam such that the output beams are cross-polarized.

5. The OP module of claim 1 wherein the output OPA stage is configured with two spaced apart upstream and downstream SFG devices and a half-wave plate between the SFG devices.

6. The OP module of claim 1, wherein the TDC components are each configured with a chirped mirror, a dichroic mirror or a birefringent window, or a combination of a mirror and a birefringent window.

7. The OP module of claim 1, wherein the pump wavelength λ p is in the green light range centered around the 515nm wavelength and the output wavelength λ o is in the blue light 443-467nm wavelength range, or the pump wavelength λ p is in the IR light range centered around the 1030nm wavelength and the output wavelength λ o is in the 1700-2500nm range.

8. The OP module of claim 1 wherein the upstream OPA stage, the intermediate OPA stage and the output OPA stage and the TDC assembly are all positioned along a straight single optical line, or along an angled single optical path.

9. The OP module of claim 1, further comprising at least one output wavelength filter providing output separation for the pump beam, the signal beam and the output beam.

10. The OP module of claim 1, wherein the OPA stages are spaced apart from each other along an angled single optical path,

the upstream and intermediate OPA stages each comprise an optical parametric amplifier OPA traversed by the pump and signal beams along an angled single optical path in one direction and in an opposite, non-parallel direction;

the output OPA stage comprises: at least one or more sum frequency generating SFG parametric devices, at least one or more second harmonic generating SHG parametric devices, or a combination of an SFG parametric device and an SHG parametric device, a portion of the pump beam, the signal beam, and the output beam crossing the output OPA stage in one direction and in an opposite direction such that the OPA stage generates two output beams at an output wavelength λ o; and a polarizing component configured to shift the linear polarization of one of the output beams and further combine the output beams having different polarizations to provide a combined cross-polarized output beam.

11. A picosecond ps single mode SM pulsed laser source comprising:

an optical pump based on a ps-fiber laser operated to generate a linearly polarized pulsed pump beam at a pump wavelength λ ρ along a single optical path;

a wavelength-conversion parametric module that receives a pump beam downstream from the optical pump and is configured with:

an upstream Optical Parametric Amplification (OPA) stage receiving a linearly polarized pump beam at a pump wavelength λ p and configured to generate a linearly polarized pump beam at a respective wavelength λ₃And λ₄A signal beam and an idler beam below, the signal beam and the idler beam propagating coaxially along a single optical path;

a plurality of intermediate OPA stages optically coupled to each other along an optical path, and each subsequent OPA stage receiving the pump beam and a progressively weaker portion of the amplified signal beam from a preceding OPA stage;

a plurality of time delay compensated TDC components alternating with OPA stages along a path and configured to compensate for group velocity mismatch between a pump beam and a signal beam and to direct the pump beam and the signal beam along the optical path while preventing the idler beam from propagating after each subsequent parametric interaction; and

an output OPA stage receiving the pump beam and a portion of the amplified signal beam and configured to generate an output beam at a desired output wavelength λ o.

12. The ps SM pulsed laser source of claim 11, wherein the signal beam is progressively amplified in at least two or more intermediate OPA stages, each configured with an optical parametric amplifier, the optical parametric amplifiers each comprising a nonlinear crystal selected from LBO or BBO or BiBO.

13. The ps SM pulsed laser source of claim 11, wherein the output OPA stage that generates the output beam comprises at least one or more sum frequency generating SFG parametric devices, at least one or more second harmonic generating SHG parametric devices, or a combination of SFG parametric devices and SHG parametric devices, each parametric device comprising the nonlinear crystal.

14. The ps SM pulsed laser source of claim 11, wherein the output OPA stage is configured with a half-wave plate to modify the polarization of the output beam and a beam combiner configured to combine the two polarizations of the output beam such that the output beams are cross-polarized.

15. The ps SM pulsed laser source of claim 11, wherein the output OPA stage is configured with two spaced apart upstream and downstream SFG devices and a half-wave plate between the SFG devices.

16. The ps SM pulsed laser source of claim 11, wherein the TDC components are each configured with a chirped mirror, a dichroic mirror or a birefringent window, or a combination of a mirror and a birefringent window.

17. The ps SM pulsed laser source according to claim 11, wherein the pump wavelength λ p is in the green light range centered at the 515nm wavelength and the output wavelength λ o is in the blue 443-467nm wavelength range, or the pump wavelength λ p is in the IR light range centered at the 1030nm wavelength and the output wavelength λ o is in the 1700-2500nm wavelength range, the blue light ps pulses at said output wavelength λ o each having an average power of up to 1000W and a spectral linewidth range of 1.5 to 3 nm.

18. The ps SM pulsed laser source of claim 11, wherein the upstream, intermediate and output OPA stages and the TDC assembly are in line with each other, or along a single optical path that is angled.

19. The ps SM pulsed laser source of claim 11, further comprising at least one output wavelength filter providing output separation for the pump beam, the signal beam, and the output beam.

20. The ps SM pulsed laser source of claim 11, wherein the fiber-laser based optical pumping comprises:

a Yb mode-locked fiber laser configured to operate under a pulse or burst directive providing an input sequence of ps pulses at a fundamental wavelength λ f;

an optical pulse stretcher configured to stretch pulse durations to produce a sequence of stretched pulses;

a pulse replicator module optically coupled to the optical pulse stretcher and configured to segment each pulse into a plurality of replicas;

a fiber power amplifier optically coupled to the pulse replicator module and configured to amplify each replica;

a pulse compressor optically coupled to the fiber power amplifier and configured to temporally compress the amplified replica to a respective ps pulse at a fundamental wavelength; and

a second harmonic generator SHG that outputs a pump output beam at the pump wavelength λ p.

21. The ps SM pulsed laser source of claim 20, wherein the pulse replicator module comprises one or more time delay stages, each time delay stage comprising an input splitter, an output fused fiber coupler, and a fiber delay line disposed therebetween, the fiber delay line providing a time delay between adjacent replicas, wherein the time delay increases or decreases by a predetermined amount in each successive stage, the predetermined amount being the same for all stages or different in at least one stage.

22. The ps SM pulsed laser source of claim 20, wherein the optical pump has a fiber master oscillator and a power amplifier MOPA architecture, the fiber master oscillator being a Yb mode locked fiber laser with a ring resonator, the power amplifier being either a fiber amplifier or a Yb: YAG.

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