Laser system and method

Abstract

A laser system is provided that includes a pump laser source configured to generate pump laser pulses, a first nonlinear medium, and a second nonlinear medium. A cascade of nonlinear processes generates optical pulses at different wavelengths. A method for generating optical pulses is also provided.

Description

[Technical Field]

[0001] The present invention relates to laser systems that utilize nonlinear optical processes, and in particular to laser systems for generating mid-infrared light pulses. [Background technology]

[0002] Mid-infrared laser sources, i.e., laser sources operating in the wavelength range from about 2.5 μm to about 25 μm, are becoming increasingly important for a variety of applications across industry, research, and defense, because this region lies within the atmospheric transparency window and many organic molecules exhibit strong and characteristic rotational and vibrational absorption properties in this region.

[0003] One such application of interest is the precise ablation of biological tissue using pulsed laser systems operating at MIR wavelengths.

[0004] Ablation of biological tissue can be used in surgical procedures.

[0005] Ablation of biological tissue can also be used with high-resolution laser mass spectrometry bioimaging techniques, as described in a review article by KK Murray et al. (Methods 104 (2016) 118-126).

[0006] Briefly, this technique involves focusing a laser pulse to a fine spot size and using it to ablate biological tissue. The products of this ablation are directed into a mass spectrometer, allowing for chemical analysis of the tissue site. By raster scanning the laser spot across the tissue, mass analysis can be performed "pixel by pixel" across the tissue sample, providing rich data on the tissue composition within the sample at high resolution.

[0007] However, such applications require laser sources operating in the MIR wavelength range, specifically tuned to the absorption peak of water molecules around 3 or 6 μm. The laser source must also have pulse parameters (in terms of pulse duration and pulse power) suitable for ablation purposes and a high beam quality that allows for near-diffraction-limited focusing.

[0008] It is generally impossible to obtain optical pulses with suitable characteristics from the direct output of a laser oscillator. Prior art approaches rely on optical parametric amplifiers (OPAs), which use nonlinear processes to convert the output of a laser source from one wavelength to another. OPAs typically require the use of two lasers (a pump laser and a second seed laser), which must be synchronized in time and carefully overlapped in space within a nonlinear conversion medium. As a result, typical MIR OPA systems are complex, unreliable (due to alignment drift), and often have poor output beam quality. These deficiencies prevent the systems from being used outside of typical laser laboratories, meaning they cannot be installed in clinics, biological research environments, and operating rooms (for example).

[0009] It is an object of the present invention to provide a pulsed mid-infrared laser source that is less complex, more reliable, and has more idealized output characteristics than those available using prior art systems. Summary of the Invention [Means for solving the problem]

[0010] In a first aspect of the present invention, there is provided a laser system including a pump laser source configured to generate pump laser pulses, a first nonlinear medium, and a second nonlinear medium. The laser system is configured such that pump laser pulses from the pump laser are coupled into the first nonlinear medium to generate first generated optical pulses at a different wavelength from the pump laser pulses via a first nonlinear optical process in the first nonlinear medium. The pump laser pulses and the first generated optical pulses are coupled from the first nonlinear medium to a second nonlinear medium to generate second generated optical pulses at a different, longer wavelength from the pump laser pulses and the first generated optical pulses via a second nonlinear optical process in the second nonlinear medium.

[0011] In such a laser system, the cascade of nonlinear processes in the first and second nonlinear media allows access to an optical wavelength range (for the second generated optical pulse) that would otherwise be inaccessible using a single nonlinear stage. Furthermore, the laser pump pulse and the first generated optical pulse generated in the first nonlinear media are used to generate the second generated optical pulse in the second nonlinear media. Because the first generated optical pulse is temporally co-located with and travels together with the pump laser pulse, both the pump laser pulse and the first generated optical pulse are simultaneously introduced into the second nonlinear media. This eliminates the need to adjust the time delay between the pulses introduced into the second nonlinear media to ensure they overlap in time, as is typically the case when mixing optical pulses in a nonlinear media to generate a new optical pulse via a nonlinear process.

[0012] In some embodiments, the first nonlinear optical process is four-wave mixing. In particular, the first nonlinear optical process may be spontaneous emission four-wave mixing.

[0013] Advantageously, the use of spontaneous emission four-wave mixing (rather than a seed) eliminates the need for an additional seed source as a signal to the first nonlinear optical stage, allowing the laser system to operate without the additional complexity of introducing a seed source into the first nonlinear stage along with the pump laser pulses.

[0014] In some embodiments, the first nonlinear medium is an optical fiber, preferably a photonic crystal fiber (PCF).

[0015] Advantageously, PCFs can be designed to confine light to a relatively small core, increasing the intensity of the light within the PCF and enhancing the first nonlinear process. Furthermore, the dispersion characteristics of the PCF can generally be designed to provide the correct dispersion profile for efficient generation of the first generation light pulses in the first nonlinear process.

[0016] In some embodiments, the dispersion profile of the first nonlinear medium is such that the first nonlinear medium exhibits normal dispersion at the wavelength of the pump laser pulse. In particular, the dispersion of the first nonlinear medium at the wavelength of the pump laser pulse may be less than 0 to −20 ps / (nm·km), and preferably, the dispersion of the first nonlinear medium at the wavelength of the pump laser pulse may be 0 to −10 ps / (nm·km).

[0017] Advantageously, pumping in the low normal dispersion region suppresses optical effects such as soliton self-frequency shift and supercontinuum generation that can deplete optical power at the wavelength of the first generated optical pulse.

[0018] In some embodiments, the dispersion profile of the first nonlinear medium is such that the walk-off length between the first generated light pulse and the pump laser pulse is greater than the optical path length through the first nonlinear medium. Preferably, the walk-off length is at least twice the optical path length, and most preferably, the walk-off length is at least five times the optical path length.

[0019] Advantageously, a walk-off length longer than the optical path length can ensure minimal walk-off between the pump laser pulse and the first generated optical pulse as they propagate through the first nonlinear medium, which helps ensure that the first generated optical pulse overlaps in time with the pump laser pulse at the exit of the first nonlinear stage, thereby maximizing the overlap of these pulses in the second nonlinear stage and increasing the efficiency of the second nonlinear process and the generation of the second generated optical pulse.

[0020] In some embodiments, the second nonlinear optical process is three-wave mixing. In particular, the second nonlinear optical process may be difference frequency generation.

[0021] Advantageously, difference frequency generation in the second nonlinear medium can be used to access a longer wavelength range than can be accessed by the first nonlinear process alone. Furthermore, because the wavelength of the second generated optical pulse is determined by the difference between the optical frequency of the pump laser pulse and the optical frequency of the first generated optical pulse, the wavelength of the second generated optical pulse can be determined by the design of the pump laser and the first nonlinear stage.

[0022] In some embodiments, the difference in optical frequency between the pump laser pulse and the first generated optical pulse may be greater than 40 THz, preferably greater than 80 THz.

[0023] By configuring the laser system to have such an optical frequency difference between the pump laser pulse and the first generated optical pulse, the second generated optical pulse can be generated at an optical wavelength that is useful in a particular application.

[0024] In some embodiments, the wavelength of the first generated light pulse corresponds to a signal wavelength in the second nonlinear optical process, the wavelength of the second generated light pulse corresponds to an idler wavelength in the second nonlinear optical process, and the wavelength of the pump laser pulse corresponds to a pump wavelength in the second nonlinear optical process.

[0025] That is, the wavelength of the first product light pulse generated in the first nonlinear process can be designed to act as the signal in the second nonlinear process. When excited by the pump laser pulse wavelength, this ultimately results in the second product light pulse being generated at a specific target idler wavelength, thereby allowing the laser system to access a predetermined wavelength longer than could be achieved using a single nonlinear stage.

[0026] In some embodiments, the second nonlinear medium is a nonlinear crystal, preferably a periodically poled nonlinear crystal, and more preferably a periodically poled lithium niobate crystal.

[0027] The use of periodically poled nonlinear crystals is particularly advantageous as it can enable quasi-phase matching to target a specific wavelength or wavelength range for the second generated optical pulse.

[0028] In some embodiments, the laser system further comprises means for controlling the temperature of the nonlinear crystal, preferably the means for controlling the temperature of the nonlinear crystal is a crystal oven in which the nonlinear crystal is held.

[0029] By controlling the temperature of the nonlinear crystal, the phase matching of the second nonlinear optical process can be controlled to target a specific wavelength for the second generated optical pulse.

[0030] In some embodiments, the system is configured such that the peak wavelength of the second generated optical pulse is tunable over a range of peak idler wavelengths by adjusting the temperature of the nonlinear crystal using a means for controlling the temperature of the nonlinear crystal.

[0031] Advantageously, this allows the peak wavelength of the second generated light pulse to be tuned over a range of wavelengths, which may be useful in certain applications.

[0032] In some embodiments, the temperature of the crystal determines the phase matching between the wavelength of the pump laser pulse and the second generation laser pulse, and the optical spectrum of the first generation laser pulse is sufficiently broad to provide a phase matching signal over a range of peak idler wavelengths. The laser system may be configured such that the range of peak idler wavelengths exceeds 20 nm. Preferably, the range of peak idler wavelengths exceeds 50 nm, and more preferably, the range of peak idler wavelengths exceeds 100 nm.

[0033] Advantageously, if the optical spectrum of the first generated laser pulse is broad compared to the phase-matching acceptance bandwidth of the nonlinear crystal in the second nonlinear stage, tuning of the peak wavelength of the second generated optical pulse can be achieved simply by adjusting the temperature of the crystal in the second nonlinear stage, without the need to adjust the pump laser or the first nonlinear stage.

[0034] In some embodiments, the second generated light pulse has a peak wavelength in the mid-infrared range, and preferably the second generated light pulse has a peak wavelength in the range of 2.7 to 3.2 μm.

[0035] This wavelength range advantageously corresponds to the absorption peak wavelength of water (associated with O-H bonds), making it well suited for various biomedical and spectroscopic applications. For example, the second generated laser pulse emitted from the laser system can be used to rapidly heat tissue and ablate biological tissue. As a further example, the laser system can be used for optical spectroscopy applications.

[0036] In some embodiments, the first generated light pulse has a peak wavelength in the range of 1.5 to 1.8 μm.

[0037] In some embodiments, the first nonlinear process is vectorial four-wave mixing, wherein a pump laser pulse is coupled into a first polarization axis of the first nonlinear medium and a first generated optical pulse is generated on a second polarization axis of the first nonlinear medium.

[0038] Advantageously, this means that the laser pump pulse and the first generated light pulse will be co-located in time and space but polarized on essentially orthogonal axes, eliminating the need to adjust the polarization of one pulse relative to the other.

[0039] In some embodiments, the second nonlinear optical process is Type I collinear phase-matched three-wave mixing, and preferably the three-wave mixing is difference frequency generation.

[0040] Generally, such phase matching requires that the polarization of the pump field be orthogonal to the polarizations of the signal and idler fields. By using such a nonlinear crystal in combination with vectorial four-wave mixing in the first nonlinear stage, the pump laser pulse and the first product optical pulse are essentially phase matched to the second nonlinear optical process in the second nonlinear medium, enabling efficient generation of the second product optical pulse by the second nonlinear optical process.

[0041] In some embodiments, the second nonlinear medium may be a nonlinear crystal, and preferably, the second nonlinear medium may be a cadmium silicon diphosphide crystal.

[0042] Advantageously, cadmium silicon diphosphide (CdSiP2, sometimes also referred to as cadmium silicon phosphide or "CSP") provides a nonlinear optical material with a high nonlinear coefficient, has high thermal conductivity, and is suitable for generating optical pulses across the infrared wavelength range that are useful in a variety of applications.

[0043] In some embodiments, the first generated light pulse has a peak wavelength in the range of 1.2 to 1.3 μm. Furthermore, the second generated light pulse may have a peak wavelength in the range of 4 μm to 8 μm. Preferably, it may be in the range of 5 μm to 7 μm, and more preferably, it may be in the range of 6 μm to 6.5 μm.

[0044] This wavelength range advantageously corresponds to the absorption peak wavelength of water (associated with the HOH bending of water molecules), making it well suited for a variety of biomedical and spectroscopic applications. For example, the second generated laser pulse emitted from the laser system can be used to rapidly heat tissue and ablate biological tissue. As a further example, the laser system can be used for optical spectroscopy applications.

[0045] In some embodiments, the second generation light pulse may have a pulse duration in the range of 5 to 2000 picoseconds, preferably in the range of 20 to 1000 picoseconds, and most preferably in the range of 20 to 150 picoseconds.

[0046] Implementing systems with pulse durations on the order of tens to hundreds of picoseconds has proven advantageous for certain applications. On the one hand, pulses are long enough to ablate tissue without plasma formation, an effect associated with light pulses of femtosecond or picosecond duration. This plasma formation generates harmful free radicals that can affect neighboring cells and / or (important for mass spectrometry analysis) alter the chemical and biological profile of the ablated tissue plume. On the other hand, compared to nanosecond to microsecond light pulses, pulses are short enough to confine ablation to a relatively small area where the laser is focused, preventing damage to surrounding tissue regions and / or ablation of material from large areas of the biological sample.

[0047] In some embodiments, the pump laser pulses have a peak wavelength in the near infrared region, preferably around 1 μm.

[0048] Many laser sources operating in the near-infrared region, particularly near 1 μm (i.e., in the 1.0-1.1 μm range), are available, offering long-term stable operation and good energy efficiency. By implementing the cascade of nonlinear processes of the present invention, it is possible to realize the benefits of such laser systems while providing laser pulse output in otherwise inaccessible wavelength ranges.

[0049] In some embodiments, the pump laser pulses have a pulse duration in the range of 5 to 3000 picoseconds, preferably in the range of 20 to 1500 picoseconds, and most preferably in the range of 30 to 225 picoseconds.

[0050] Advantageously, by providing a laser pump pulse of a specified duration, the first and second generation optical pulses are generated with pulse durations of similar order of magnitude. In this manner, it is advantageously possible to provide a second generation optical pulse of a duration appropriate for a particular application without the need to temporally stretch or compress the optical pulse at the output of the second nonlinear stage or between the first and second nonlinear stages. That is, by adjusting the pulse duration of the pump laser pulse, an inherent temporal overlap of the optical pulse generated through cascading with the pump laser pulse occurs, resulting in the second generation laser pulse being generated with the appropriate duration.

[0051] In some embodiments, the pump laser source is an ytterbium-doped fiber laser source, which may include an ytterbium-doped fiber modelocked oscillator and / or one or more ytterbium-doped fiber amplifiers.

[0052] Ytterbium-doped fiber laser systems generally offer advantages such as being compact, energy-efficient, and reliable. Furthermore, in embodiments where optical fiber is used as the first nonlinear medium, the use of an ytterbium-doped fiber laser system means that the first nonlinear medium can be fiber-integrated with the pump laser system, further reducing the laser system's footprint and improving its reliability.

[0053] In some embodiments, the pump laser source comprises a semiconductor laser, and in other embodiments, the pump laser source comprises a microchip laser.

[0054] Semiconductor lasers or microchip lasers are available that produce pulses on the order of hundreds of picoseconds and are suitable for particular applications of the present invention.

[0055] In some embodiments, the pump laser source includes a pulse stretcher.

[0056] Advantageously, by using a pulse stretcher as part of the pump laser source, a laser oscillator that provides desirable (for example) thermal or output characteristics can be used, while achieving a pulse duration of the pump laser pulse that results in efficient conversion of optical energy through a cascade of nonlinear optical media.

[0057] In some embodiments, the laser system further includes an optical modulator between the pump laser source and the first nonlinear medium.

[0058] Advantageously, this allows the pulse repetition rate of the laser system to be varied and individual pulses to be selected, while the characteristics of the second generated light pulses remain constant.

[0059] A second aspect of the present invention provides a laser system including a pump laser source configured to generate pump laser pulses, a first nonlinear medium, and a second nonlinear medium, wherein the pump laser pulses from the pump laser are coupled into the first nonlinear medium to generate first generated optical pulses at a different wavelength from the pump laser pulses via a first nonlinear optical process in the first nonlinear medium, the first nonlinear process being vectorial four-wave mixing, the pump laser pulses being coupled into a first polarization axis of the first nonlinear medium to generate the first generated optical pulses on a second polarization axis of the first nonlinear medium, and the pump laser pulses and the first generated optical pulses being coupled from the first nonlinear medium into a second nonlinear medium to generate second generated optical pulses at a different wavelength from the pump laser pulses and the first generated optical pulses via a second nonlinear optical process in the second nonlinear medium.

[0060] Similar to the first aspect of the present invention, in such a laser system, the cascade of nonlinear processes in the first and second nonlinear media allows access to an optical wavelength range (for the second generated optical pulse) that would be inaccessible using a single nonlinear stage. Furthermore, the laser pump pulse and the first generated optical pulse generated in the first nonlinear media are used to generate the second generated optical pulse in the second nonlinear media. Because the first generated optical pulse is temporally co-located with and travels with the pump laser pulse, both the pump laser pulse and the first generated optical pulse are simultaneously introduced into the second nonlinear media without the need to adjust the time delay between the pulses introduced into the second nonlinear media to ensure they overlap in time, as is typically the case when mixing optical pulses in a nonlinear media to generate new optical pulses via nonlinear processes. Furthermore, by using vectorial four-wave mixing in the first nonlinear media, the laser pump pulse and the first generated optical pulse are not only temporally and spatially co-located, but are also inherently polarized on orthogonal axes without the need to control the polarization of one pulse relative to the other.

[0061] A third aspect of the present invention provides a method for generating optical pulses, the method including: generating pump laser pulses using a pump laser source; coupling the pump laser pulses into a first nonlinear medium; generating first generated optical pulses at a different wavelength than the pump laser pulses via a first nonlinear optical process in the first nonlinear medium; coupling the pump laser pulses and the first generated optical pulses from the first nonlinear medium to a second nonlinear medium; and generating second generated optical pulses at a different wavelength than the pump laser pulses and the first generated optical pulses via a second nonlinear optical process in the second nonlinear medium, the second generated laser pulses being at a longer wavelength than the pump laser pulses and the first generated optical pulses.

[0062] A fourth aspect of the present invention provides a method for generating optical pulses, the method including generating pump laser pulses using a pump laser source, coupling the pump laser pulses into a first polarization axis of a first nonlinear medium, generating first generated optical pulses on a second polarization axis of the first nonlinear medium at a different wavelength from the pump laser pulses via vectorial four-wave mixing in the first nonlinear medium, coupling the pump laser pulses and the first generated optical pulses from the first nonlinear medium to a second nonlinear medium, and generating second generated optical pulses at a different wavelength from the pump laser pulses and the first generated optical pulses via a second nonlinear optical process in the second nonlinear medium. [Brief explanation of the drawings]

[0063] [Figure 1] 1 illustrates a laser system of the present invention. [Figure 2] 1 illustrates a laser system according to an embodiment of the present invention. [Figure 3a] 3 is a graph of the dispersion profile of the photonic crystal fiber used in the laser system of FIG. 2. [Figure 3b] 3b is a graph of a phase-matching curve for four-wave mixing based on the dispersion profile of FIG. 3a. [Figure 4a]3 is a graph of the temperature dependent phase matching curve of the nonlinear crystal used in the laser system of FIG. 2. [Figure 4b] 3 is a graph of a phase matching curve of the nonlinear crystal used in the laser system of FIG. 2. [Figure 5a] 3 is a graph showing the optical spectrum of the output from the first nonlinear stage in the laser system of FIG. 2. [Figure 5b] 3 is a graph showing the optical spectrum of the output from the laser system of FIG. 2. [Figure 6] 3 is a graph showing the optical spectrum of the output from the laser system of FIG. 2 over a range of nonlinear crystal temperatures. [Figure 7] 1 illustrates a laser system according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0064] FIG. 1 shows a laser system of the present invention.

[0065] Generally speaking, the laser system includes a pump laser source 100, a first nonlinear medium 200, and a second nonlinear medium 300. In the system of Figure 1, the first nonlinear medium takes the form of an optical fiber 200, and the second nonlinear medium takes the form of a nonlinear crystal 300. It will be appreciated that other nonlinear media may be used in place of one or other of the optical fiber 200 and the optical crystal 300, depending on the particular situation.

[0066] The pump laser source output 500 is comprised of a train of optical pulses, i.e., pump laser pulses. The output 500 from the pump laser source 100 is coupled into an optical fiber 200. Within the optical fiber, the pump laser pulses interact with the optical fiber 200 and, via a first nonlinear optical process, produce a first resulting optical pulse.

[0067] In the system shown in FIG. 1, the first nonlinear optical process is four-wave mixing in the optical fiber 200, specifically pump-degenerate spontaneous emission four-wave mixing.

[0068] In general, four-wave mixing is a process that involves the third-order nonlinearity of a material (i.e., the χ (3) 1. In the case of pump degenerate four-wave mixing, the two input optical frequency components are at the same optical frequency. In the case of Figure 1, the optical field of the pump laser pulse provides amplification of two new frequency components equally spaced in frequency around the pump optical frequency. The new component with a higher optical frequency (i.e., shorter wavelength) is commonly referred to as the "anti-Stokes" component. The new component with a lower optical frequency (i.e., longer wavelength) is commonly referred to as the "Stokes" component.

[0069] Four-wave mixing is a phase-sensitive process, meaning that the interaction depends on the relative phase of the input and output components. Therefore, its effect can be efficiently accumulated only if the phase-matching condition is met. In optical fibers, tuning the chromatic dispersion of the fiber can provide effective phase matching between the wavelength of the pump laser component and a specific target wavelength of the newly generated light component.

[0070] It will be appreciated, of course, that the four-wave mixing process can be enhanced by coupling in an optical component at either the Stokes or anti-Stokes wavelength, which will be amplified by the four-wave mixing process and thereby enhance the production of the other respective component. However, this involves the additional complexity of coupling an additional laser source to the PCF, and further requires that the component introduced at the Stokes or anti-Stokes wavelength be synchronized in time with the pump optical pulse to ensure effective nonlinear interaction. For these reasons, the use of spontaneous emission four-wave mixing (rather than seeded four-wave mixing) in the first stage is preferred, as it reduces system complexity and also ensures that the naturally occurring Stokes and anti-Stokes pulses are inherently synchronized with the pump laser pulse.

[0071] Because not all of the pump laser pulse is converted into the Stokes and anti-Stokes components, the output from optical fiber 200 consists of unconverted pump along with Stokes component 510 and anti-Stokes component 515. Effectively, a portion of the optical energy of each pump laser pulse is converted into a co-propagating optical pulse at the Stokes wavelength (i.e., a Stokes pulse) and a co-propagating optical pulse at the anti-Stokes wavelength (i.e., an anti-Stokes pulse).

[0072] In the laser system of Figure 1, a Stokes pulse provides the first generated optical pulse. Because the Stokes pulse is generated by a nonlinear optical process, it overlaps in time and space with the first generated optical pulse. Because the Stokes pulse is generated by a nonlinear optical process, it overlaps in time and space with the first generated optical pulse.

[0073] Although the present invention has been described with particular reference to four-wave mixing in optical fiber 200, it will be understood that the first nonlinear optical process may be a different nonlinear optical process known in the art and the first nonlinear medium may be a different type of nonlinear medium known in the art. Those skilled in the art will appreciate that by selecting a nonlinear medium and configuring the system to utilize a particular nonlinear optical process, it will be possible to configure the system to generate a first generated optical pulse at a particular target wavelength for the first generated optical pulse.

[0074] 1, the residual pump laser pulse and the Stokes pulse are coupled into nonlinear crystal 300, where a second nonlinear process occurs to provide a second generated optical pulse. In particular, the pump laser pulse and the Stokes pulse interact within nonlinear crystal 300 via the nonlinear optical process of difference frequency generation.

[0075] In general terms, difference frequency generation is a nonlinear optical process that arises from the second-order nonlinearity of a material (i.e., the χ of the material through which the optical field propagates).(2) (Governed by the π-term). In this process, two input beams interact to produce another beam with an optical frequency that is the difference between the optical frequencies of the input beams. Difference frequency generation is an example of the so-called "three-wave mixing" process, in which three beams (usually two input beams and an output beam) interact. Other examples of three-wave mixing include sum frequency generation, optical parametric amplification, and second harmonic generation.

[0076] In Figure 1, coupling the pump and Stokes laser pulses into the nonlinear crystal 300 amplifies the Stokes (signal) pulse, but, importantly, generates an idler component 520 via a difference frequency generation process. The idler component consists of a second generated optical pulse, the idler pulse, where the Stokes laser pulse is at the signal wavelength corresponding to the idler wavelength of the difference frequency generation process. The idler pulse is generated at an optical frequency equal to the frequency difference between the pump laser pulse and the Stokes signal pulse.

[0077] Therefore, the optical frequency (or equivalently, wavelength) of the idler pulse is determined by the wavelength of the pump laser pulse and the wavelength at which the Stokes pulse occurs in the first nonlinear medium. The wavelength of the pump laser pulse is determined by the characteristics of the pump laser source. The wavelength of the Stokes pulse is determined by phase matching of the nonlinear process in the optical fiber 200 and can be controlled by appropriately adjusting the chromatic dispersion of the optical fiber 200.

[0078] The cascade of nonlinear processes described above advantageously allows for the generation of optical pulses with longer wavelengths than can be achieved with a single nonlinear stage.

[0079] Some examples of this architecture are described below.

[0080] Example 1: Access to the 3 μm wavelength range using PCF and PPLN crystals In Example 1, we generate mid-infrared (MIR) pulses at approximately 3 μm using the cascaded nonlinearities of spontaneous four-wave mixing in a photonic crystal fiber (PCF) and quasi-phase-matched difference frequency generation in a periodically poled lithium niobate crystal (PPLN).

[0081] A schematic diagram of the laser system used in this example is shown in FIG.

[0082] In this case, the pump laser source 100 consists of an ytterbium fiber master oscillator power amplifier (MOPA) system. Specifically, the pump laser source 100 consists of a 50 MHz mode-locked ytterbium fiber oscillator 110, which generates optical pulses with a 5 ps duration and a wavelength of 1064 nm. These pulses are double-passed through a length of polarization-maintaining fiber 130 via an optical circulator 140 and reflected off a fiber loop mirror 135. The circulator 140, fiber 130, and fiber loop mirror 135 form a pulse stretcher that increases the pulse duration from 5 ps to 35 ps.

[0083] It has been found that the frequency conversion efficiency at each stage of a nonlinear cascade is sensitive to the peak power of the pump laser pulse, which in turn depends on its duration. This is because there are two nonlinear stages, and the overall efficiency of the system is sensitive to the duration of the pump laser pulse. In this regard, a pulse stretcher can be beneficial, as it can increase the overall efficiency of frequency conversion across the two stages by adjusting the duration of the pump laser pulse.

[0084] The next component in the optical chain of the pump laser system 100 is a pulse picker 145, in this case in the form of an acousto-optic modulator (AOM), which can be used to select pulses from the train of pump pulses, thereby reducing the effective repetition rate. For example, the repetition rate can be reduced to a few MHz. Of course, if a higher repetition rate is desired, the AOM may be omitted.

[0085] In another embodiment, a fiber-integrated semiconductor laser can be used instead of a mode-locked ytterbium fiber oscillator. The semiconductor laser can be operated to provide pulses in the picosecond to nanosecond range. In particular, semiconductor lasers providing pulses with durations between 100 ps and 2000 ps, ​​preferably in the hundreds of ps range (i.e., 100-400 ps), can be used. In this case, the pulse stretcher can be omitted. Furthermore, because semiconductor lasers can be operated at lower repetition rates than mode-locked lasers, it is possible to omit the AOM.

[0086] In further embodiments, a microchip laser may be used instead of the mode-locked ytterbium fiber oscillator. Such laser systems can be operated to deliver pulses with durations in the hundreds of picoseconds range (i.e., 100–400 picoseconds). Such lasers can also be operated at lower repetition rates or controlled to deliver “single-shot” pulses. The pulse energy from such lasers tends to be higher than that of mode-locked ytterbium fiber oscillators or pulsed semiconductor laser systems, making it easier for the amplified pump laser pulse to reach high pulse energies, and therefore for the second generated optical pulse (i.e., the MIR idler pulse) to reach high pulse energies. The pump laser pulse is then amplified in an optical amplifier chain including a first ytterbium-doped fiber amplifier (YDFA) 120 and a second YDFA 125. Alternatively, a single amplifier may provide sufficient power amplification, or more amplifiers may be included in the amplifier chain.

[0087] After amplification, the pump laser pulses are output from the pump laser system 100 .

[0088] In the system of FIG. 2, a series of free-space optics 600 is used to couple the output of a pump laser system into a PCF 205 that provides the first nonlinear medium 200 .

[0089] Specifically, the output is collimated by a lens and relayed through a series of mirrors via a free-space optical isolator (ISO), a half-wave plate (HWP), and a polarizing beam splitter (PBS). In this case, adjusting the HWP allows one to adjust the amount of pump laser power that passes through the PBS and is coupled into the PCF. A second HWP is used to align the polarization of the pump beam with the polarization axis of the PCF 205, and a second lens is used to couple the light into the facet of the PCF 205.

[0090] Of course, it will be appreciated that the free-space optics 600 used to couple the pump laser pulses into the PCF 205 can take many different forms. For example, a simple series of lenses may be used to couple the pump laser pulses into the input facet of the PCF 200. Furthermore, it will be appreciated that the output of the pump laser source 100 may be fiber integrated with the PCF 205, thereby eliminating the requirement for free-space optics 600 to couple the pump laser pulses into the first nonlinear medium 200.

[0091] Optionally, some embodiments can include a modulator between the output of the optical amplifier chain and the input of the PCF. For example, the collimated output of the pump laser system can be directed through a bulk acousto-optic modulator (AOM). This can be used to select pump pulses from single shots up to the repetition rate of the pump laser while maintaining the peak power (and energy) of each pulse. This is advantageous in many applications. For example, in mass spectrometry imaging (discussed above), it is desirable to deliver pulses synchronized with the scan rate of the mass spectrometer, which can be on the order of tens of hertz. It is desirable to trigger the AOM to deliver pulses at the desired time (e.g., the scan rate of the mass spectrometer in this example). Despite changes in the repetition rate due to the use of the AOM to select specific pulses, the pulse peak power (or equivalently, pulse energy) at the output of the pump laser remains constant. As a result, the nonlinear processes in the PCF and nonlinear crystal (which are inherently dependent on the peak power of the pulse) remain constant. As a result, the pulse characteristics of the MIR pulses remain constant, and the AOM can be used to vary the pulse repetition rate or select individual pulses.

[0092] This is advantageous compared to using an AOM at the output of a nonlinear stage (for example), as the nonlinear stage will be exposed to less overall power and have improved lifetime, while it is advantageous compared to relying on an AOM before the amplifier chain, as the pulse repetition rate can affect the degree of amplification within the amplifier chain, so that the peak power of the pulses output from the amplifier chain (and therefore the characteristics of the MIR pulses generated) may change as the pulse repetition rate is changed.

[0093] The dispersion profile of the PCF is shown in Figure 3a, where the dispersion profile is represented by the dispersion parameter D, which can be defined as:

number

[0094] As can be seen, at the pump wavelength of 1.064 μm, the PCF exhibits normal dispersion (i.e., the dispersion parameter D is negative), which is as low as −6 ps / (nm km).

[0095] The calculated phase-matching profile of the four-wave mixing process in the PCF is shown in Figure 3b. As can be seen, the pump wavelength of 1.064 μm (i.e., the pump laser wavelength) is phase-matched to the Stokes component near 1.65 μm and the anti-Stokes component near 0.785 μm. Therefore, the Stokes component (i.e., the first generated optical pulse) is generated at a wavelength of 1.65 μm. Strictly speaking, as should be understood, the phase-matching condition has a nonlinear term that is a function of the pump peak power. In Figure 3b, a pump peak power of 10 kW is assumed, but different pump peak powers have no substantial effect on the phase-matching mechanism.

[0096] Returning to FIG. 2, the output from PCF 205 (including residual pump 500 , Stokes component 510 and anti-Stokes component 515 ) is collimated with a lens and directed into second nonlinear medium 300 .

[0097] Using a lens 330 , the output from the PCF 205 is coupled into a periodically poled lithium niobate (PPLN) crystal 310 .

[0098] As is known in the art, periodically poled nonlinear crystal materials provide quasi-phase matching of nonlinear interactions within the nonlinear crystal. In such crystals, the domain orientation within the nonlinear crystal periodically reverses, such that the sign of the nonlinear coefficient changes periodically along the length of the crystal. The periodic distance over which the domain orientation reverses is called the "poling period." This poling period determines the phase matching within the crystal. Due to thermal expansion, the length of the poling period changes slightly with the crystal temperature. Therefore, the exact phase matching within a periodically poled crystal depends on its temperature.

[0099] To this end, the PPLN crystal 310 is held in a crystal oven (not shown) controlled by a temperature controller 320. This allows for control of the crystal temperature. A temperature-dependent phase-matching curve for the PPLN crystal is shown in Figure 4a, and an exemplary plot of the phase-matching curve for a crystal temperature of 60°C is shown in Figure 4b. As can be seen, the pump wavelength of 1.064 μm (i.e., the wavelength of the pump laser pulse) is phase-matched to the signal wavelength of 1.65 μm (i.e., the wavelength of the first generated laser pulse generated in the first nonlinear medium), near the MIR idler wavelength of 3.0 μm. This means that the energy from the pump laser component amplifies the Stokes signal component at 1.65 μm, resulting in the generation of a new component at the MIR idler wavelength. It can be seen that the synchronous coupling of the pump and signal pulses leads to the amplification of the signal pulse and the generation and amplification of the idler pulse. Therefore, the efficiency of energy transfer from the pump component to the signal and idler components is significantly improved compared to when only the pump component is coupled to the PPLN crystal 210.

[0100] Therefore, coupling the pump laser pulse into the PPLN crystal 310 along with the first generated optical pulse (i.e., the Stokes pulse) from the output of the PCF 205 will generate a second generated optical pulse at the MIR idler wavelength, thereby having a wavelength around 2.9 μm.

[0101] The output from the PPLN crystal 310 is recollimated using lens 335 and passed through a long-pass dichroic filter 340 which filters out the short wavelength components (i.e., the laser pump component, the Stokes component from the first nonlinear stage, and the anti-Stokes component), leaving only the MIR idler component 520.

[0102] It will be appreciated that other nonlinear crystals can be used in place of PPLN, including other periodically poled and patterned nonlinear crystals for temperature-controlled quasi-phase matching, such as, for example, orientation-patterned gallium phosphide (OP-GaP) or orientation-patterned gallium arsenide (OP-GaAs).

[0103] The results of this system are shown in Figure 5. Figure 5a shows the optical spectrum of the output from PCF 205. This spectrum contains a peak 705 near 1.064 μm, corresponding to the unconverted pump laser power 500. It also contains a peak 710 near 0.785 μm, corresponding to the anti-Stokes component 515 generated by four-wave mixing in PCF 205. It also contains a peak 715 near 1.65 μm, corresponding to the Stokes component 510 generated by four-wave mixing in PCF 205. A detailed view of peak 715 is shown on a linear scale in the inset of the graph in Figure 5a.

[0104] Figure 5b shows the optical spectrum at the output from the second nonlinear stage after filtering with a long-pass dichroic filter at a crystal temperature of 100 °C. This shows that a strong spectral component is generated around 2.9 μm. We found that optical powers exceeding 100 mW can be obtained at a repetition rate of 8 MHz (pulse energy exceeding 12.5 nJ).

[0105] It will be appreciated that frequency conversion in the second nonlinear medium 300 is limited by the so-called acceptance bandwidth of the medium. In practical systems, phase matching of the nonlinear process does not occur at a single defined wavelength, but rather over a finite wavelength range. Therefore, for efficient nonlinear conversion in difference frequency generation in the second nonlinear medium, a significant amount of the optical power of the Stokes signal (i.e., the first generated) optical pulse must fall within the phase-matched acceptance bandwidth for effective generation of the MIR idler (i.e., the second generated optical pulse).

[0106] As can be seen from Figure 5a, the optical spectrum around the 1.65 μm peak at the output of PCF 205 is relatively broad, which can be partly attributed to the use of spontaneous emission four-wave mixing (rather than seeding) in the first nonlinear stage, as there is a wavelength range over which effective phase matching is achieved in the PCF.

[0107] However, a relatively broad spectrum does not prevent effective difference frequency generation in the PPLN crystal 310 because it has been found that nonlinear processes that convert optical power to longer wavelength components (as compared to three-wave mixing processes for conversion to shorter wavelengths, such as sum frequency generation or second harmonic generation) generally have relatively broad acceptance bandwidths. Therefore, it has been found that MIR pulses can be effectively generated with sufficient optical power even when the spectrum of the Stokes signal pulse is relatively broad. Therefore, it is possible to generate sufficient power in the MIR region while achieving the benefits associated with spontaneous four-wave mixing (discussed above).

[0108] In the system of FIG. 2, the wavelength of the second generated output pulse can also be controlled by controlling the temperature of the PPLN crystal 310 using the temperature controller 320.

[0109] This can be seen by referring to Figure 4a, which shows temperature-dependent phase matching. Even though the pump wavelength is constant at 1.064 μm, phase matching over a wide range of MIR wavelengths is possible, depending on the crystal temperature. Conversion efficiency to the MIR idler wavelength is, of course, contingent on the presence of an appreciable optical signal at the corresponding signal wavelength from the Stokes signal pulse (generated in the first nonlinear stage), as provided in the present laser system. Due to spontaneous four-wave mixing in the first nonlinear stage, the spectrum of the Stokes signal components is sufficiently broad to provide a phase-matched signal over a range of phase-matching wavelengths, allowing the PPLN crystal 310 to be tuned by changing its temperature using the temperature controller 320 to control the temperature of the oven in which the crystal 310 is held.

[0110] The results of tuning the temperature of the spectrum of the MIR idler component (i.e., the second generated optical pulse) are shown in Figure 6. As can be seen, the peak wavelength of the MIR idler can be tuned over a range of more than 200 nm, with the peak wavelength being less than 2.9 μm for a crystal temperature of 100 °C and exceeding 3.0 μm for a crystal temperature of 40 °C.

[0111] Example 2: Access to the 5-6 μm wavelength range using vectorial four-wave mixing in birefringent fiber and type I phase-matched DFG in CSP crystals In Example 2, the cascaded nonlinearities of spontaneous vector four-wave mixing in a birefringent PCF and type I phase-matched difference frequency generation in a cadmium silicon diphosphide (CSP) crystal are used to generate MIR pulses around 6 μm.

[0112] The system of Example 2 is shown schematically in Figure 7. This system is similar in many respects to the system of Example 1 shown in Figure 2 and the general schematic diagram shown in Figure 1.

[0113] Similar to Example 1, the laser system includes a pump laser source 1100 , a first nonlinear medium in the form of a PCF 1200 , and a second nonlinear medium in the form of a nonlinear crystal 1300 .

[0114] In this embodiment, PCF 1200 is a birefringent PCF and is configured to have a chromatic dispersion profile across its two polarization axes to provide phase matching for the vectorial four-wave mixing process.

[0115] Vectorial four-wave mixing is a specific case of four-wave mixing. The general description of four-wave mixing has been given above and will not be repeated here. Specifically, vectorial four-wave mixing involves the generation of light waves with polarizations different from that of the input light wave due to nonlinear interactions. This typically requires a birefringent medium to phase-match the different frequency components along different polarization axes. This can be achieved (as in this example) by using a birefringent PCF that exhibits different dispersion characteristics for each polarization axis. The dispersion profile of such a fiber can be designed so that wavelengths along one polarization axis are phase-matched to wavelengths along the orthogonal polarization axis. In particular, in vectorial four-wave mixing, the dispersion characteristics along the different polarization axes are such that the pump wave is phase-matched to the Stokes and anti-Stokes waves along the polarization axis orthogonal to the pump wave.

[0116] Returning to FIG. 7 , pump laser source 1100 is configured to provide a pump beam 1500 having a particular polarization, indicated by the double-headed arrow. This is coupled into birefringent PCF 1200. It will be appreciated that the system may include means for adjusting the polarization of pump laser beam 1500 to match the appropriate polarization axis of fiber 1200. This may take the form of a half-wave plate or the like. It will also be appreciated that the output of the pump laser source is unpolarized, but that a polarizing element (such as a polarizing beam splitter) is used to select a particular polarization component for coupling into the PCF. The pump laser source 1100 itself may be substantially similar to pump laser source 100 of Example 1.

[0117] Each pump laser pulse propagates through the PCF and generates an anti-Stokes component 1515 and a Stokes component 1510. As in Example 1, the anti-Stokes component 1510 provides the first generated optical pulse. These new components are generated by vectorial four-wave mixing and therefore have orthogonal polarization to the pump laser pulse, as indicated by the double-headed arrow.

[0118] As in Example 1, not all of the pump laser pulse is converted into the Stokes and anti-Stokes components, so the output from optical fiber 1200 consists of unconverted pump along with Stokes component 1510 and anti-Stokes component 1515. Effectively, a portion of the optical energy of each pump laser pulse is converted into a co-propagating optical pulse at the Stokes wavelength (i.e., a Stokes pulse) and a co-propagating optical pulse at the anti-Stokes wavelength (i.e., an anti-Stokes pulse).

[0119] As mentioned above, in this laser system, the Stokes pulse provides the first generated optical pulse, and because the Stokes pulse is generated by a nonlinear optical process, it overlaps with the first generated optical pulse in time and space.

[0120] Importantly, the Stokes pulses are generated by vectorial four-wave mixing, and therefore are essentially generated with polarization orthogonal to the pump laser pulse. In this example, it is preferable to use spontaneous vectorial four-wave mixing (rather than a seeded process) for the same reasons as explained in Example 1 above.

[0121] 7, the residual pump laser pulse and the Stokes pulse are coupled into nonlinear crystal 1300, where a second nonlinear process occurs to provide a second generated optical pulse. In particular, the pump laser pulse and the Stokes pulse interact within nonlinear crystal 1300 via the nonlinear optical process of difference frequency generation.

[0122] However, in contrast to Example 1 above, phase matching in a nonlinear crystal does not have to be achieved by periodic poling to achieve quasi-phase matching.

[0123] Instead, the second nonlinear optical process is Type I collinear phase-matched difference-frequency generation. In the laser system of Figure 7, the birefringence inherent in the nonlinear crystal provides phase matching between the different optical components in the nonlinear optical process. In Type I phase matching, the pump and signal waves have orthogonal polarizations, and an idler wave is generated with a polarization that matches the signal wave.

[0124] Typically, this type of phase matching requires careful control of the relative polarizations of the various optical beams input to the nonlinear crystal. However, as explained above, by using vectorial four-wave mixing in the first nonlinear stage, the pump laser component 1500 and the Stokes (signal) component 1510 are essentially orthogonally polarized relative to each other, allowing Type I phase matching to be used in the nonlinear crystal without (for example) having to adjust the polarization of one beam relative to the other.

[0125] It will be appreciated that the inherent orthogonal polarization between the laser component 1500 and the Stokes (signal) component can be utilized in a Type II phase-matched nonlinear crystal, where the pump and signal components are orthogonal, but the idler component is generated with a polarization that matches the pump wave. The type of phase-matching employed in a nonlinear crystal depends on the birefringence properties of the crystal, which determine the phase-matching conditions for a given nonlinear process.

[0126] In the example system of FIG. 7, the nonlinear crystal 1300 is a cadmium silicon diphosphide crystal and Type I phase matching is used.

[0127] In particular, in the laser system of FIG. 7, the dispersion profile of the birefringent PCF 1200 is controlled to provide phase matching between the Stokes components near 1.064 μm and 1.26 μm.

[0128] With this frequency separation between the pump laser beam 1500 and the Stokes component 1510, the nonlinear crystal 1300 produces an idler component at 6 μm.

[0129] This wavelength range is of particular interest because it corresponds to the absorption peak wavelength of water (associated with the HOH bending of the water molecule). For example, the second generated laser pulse emitted from the laser system can be used to rapidly heat tissue and ablate biological tissue. As a further example, the laser system can be used for optical spectroscopy applications.

[0130] It will be appreciated that by properly engineering the dispersion characteristics of the birefringent fiber and phase matching to the properly separated Stokes components, other wavelength ranges can be targeted for other applications. Furthermore, other nonlinear crystals, such as zinc germanium phosphide (ZGP, ZnGeP2), can be used to generate the MIR idler, and these can also be used in place of the CSP.

[0131] It will be understood that the above-described embodiments are exemplary and that the invention is defined by the appended claims.

[0132] [Embodiment] (1) a pump laser source configured to generate pump laser pulses; a first nonlinear medium; a second nonlinear medium, The laser system includes: a pump laser pulse from the pump laser is coupled into the first nonlinear medium to generate a first generated optical pulse at a wavelength different from the pump laser pulse via a first nonlinear optical process in the first nonlinear medium, the first nonlinear process being vectorial four-wave mixing, the pump laser pulse being coupled into a first polarization axis of the first nonlinear medium, and the first generated optical pulse being generated on a second polarization axis of the first nonlinear medium; the pump laser pulse and the first generated optical pulse are coupled from the first nonlinear medium to the second nonlinear medium, and a second generated optical pulse is generated at a different wavelength from the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium. (2) The laser system of embodiment 1, wherein the second nonlinear optical process is type I collinear phase-matched three-wave mixing, and preferably the three-wave mixing is difference frequency generation. (3) The laser system of embodiment 2, wherein the second nonlinear medium is a nonlinear crystal, and preferably, the second nonlinear medium is a cadmium silicon diphosphide crystal. (4) The laser system according to any one of the first to third embodiments, wherein the first nonlinear medium is a birefringent photonic crystal fiber. (5) A laser system according to any one of embodiments 1 to 4, wherein the second generated light pulse has a wavelength longer than the wavelength of the first generated light pulse and the wavelength of the pump laser pulse.

[0133] (6) The laser system according to any one of the first to fifth embodiments, wherein the first generated light pulse has a peak wavelength in the range of 1.2 to 1.3 μm. (7) A laser system according to any one of embodiments 1 to 6, wherein the second generated light pulse has a peak wavelength in the range of 4 μm to 8 μm, preferably in the range of 5 μm to 7 μm, and more preferably in the range of 6 μm to 6.5 μm. (8) a pump laser source configured to generate pump laser pulses; a first nonlinear medium; a second nonlinear medium, The laser system includes: a pump laser pulse from the pump laser is coupled to the first nonlinear medium to generate a first generated optical pulse at a wavelength different from that of the pump laser pulse via a first nonlinear optical process in the first nonlinear medium; the pump laser pulse and the first generated optical pulse are coupled from the first nonlinear medium to the second nonlinear medium, and a second generated optical pulse is generated at a different and longer wavelength than the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium. (9) A laser system as described in embodiment 8, wherein the first nonlinear optical process is four-wave mixing. (10) A laser system as described in embodiment 9, wherein the first nonlinear optical process is spontaneous four-wave mixing.

[0134] (11) The laser system of any one of claims 9 to 10, wherein the first generated light pulse has a longer wavelength than the pump laser pulse. (12) The laser system according to any one of embodiments 8 to 11, wherein the first nonlinear medium is an optical fiber, preferably a photonic crystal fiber. (13) A laser system according to any one of embodiments 8 to 12, wherein the second nonlinear optical process is three-wave mixing. (14) The laser system of embodiment 13, wherein the second nonlinear optical process is difference frequency generation. (15) A laser system according to any one of embodiments 8 to 14, wherein the second nonlinear medium is a nonlinear crystal, preferably a periodically poled nonlinear crystal, more preferably a periodically poled lithium niobate crystal.

[0135] (16) The laser system of embodiment 15, further comprising a means for controlling the temperature of the nonlinear crystal, preferably the means for controlling the temperature of the nonlinear crystal being a crystal oven in which the nonlinear crystal is held. (17) A laser system as described in embodiment 16, configured such that the peak wavelength of the second generated optical pulse is tunable over a range of peak idler wavelengths by adjusting the temperature of the nonlinear crystal using the means for controlling the temperature of the nonlinear crystal. (18) The laser system of embodiment 17, wherein the temperature of the crystal determines phase matching between the wavelength of the pump laser pulse and the second generation laser pulse, and the optical spectrum of the first generation laser pulse is sufficiently broad to provide a phase-matched signal over the range of the peak idler wavelength. (19) The laser system of embodiment 18, wherein the peak idler wavelength range is greater than 20 nm, preferably the peak idler wavelength range is greater than 50 nm, and more preferably the peak idler wavelength range is greater than 100 nm. (20) A laser system according to any one of embodiments 8 to 19, wherein the second generated light pulse has a peak wavelength in the mid-infrared range, preferably, the second generated light pulse has a peak wavelength in the range of 2.7 to 3.2 μm.

[0136] (21) The laser system according to any one of embodiments 8 to 20, wherein the first generated light pulse has a peak wavelength in the range of 1.5 to 1.8 μm. (22) a wavelength of the first generated optical pulse corresponds to a signal wavelength in the second nonlinear optical process; the wavelength of the second generated optical pulse corresponds to an idler wavelength in the second nonlinear optical process; A laser system described in any one of embodiments 1 to 21, wherein the wavelength of the pump laser pulse corresponds to a pump wavelength in the second nonlinear optical process. (23) A laser system according to any one of embodiments 1 to 22, wherein the dispersion profile of the first nonlinear medium is such that the first nonlinear medium exhibits normal dispersion at the wavelength of the pump laser pulse. (24) A laser system as described in embodiment 23, wherein the dispersion of the first nonlinear medium at the wavelength of the pump laser pulse is between 0 and -20 ps / (nm km), and preferably, the dispersion of the first nonlinear medium at the wavelength of the pump laser pulse is between 0 and -10 ps / (nm km). (25) The dispersion profile of the first nonlinear medium is such that a walk-off length between the first generated optical pulse and the pump laser pulse is greater than an optical path length through the first nonlinear medium; More preferably, the walk-off length is at least twice the optical path length; Most preferably, the laser system according to any one of embodiments 1 to 24, wherein the walk-off length is at least five times the optical path length.

[0137] (26) A laser system according to any one of embodiments 1 to 25, wherein the difference in optical frequency between the pump laser pulse and the first generated optical pulse is greater than 40 THz, preferably greater than 80 THz. (27) The laser system according to any one of embodiments 1 to 26, wherein the second generated light pulse has a pulse duration in the range of 5 to 2000 picoseconds, preferably in the range of 20 to 1000 picoseconds, and most preferably in the range of 20 to 150 picoseconds. (28) A laser system according to any one of embodiments 1 to 27, wherein the pump laser pulse has a peak wavelength in the near-infrared region, preferably, the pump laser pulse has a peak wavelength around 1 μm. (29) The laser system of embodiment 28, wherein the pump laser pulse has a pulse duration in the range of 5 to 3000 picoseconds, preferably in the range of 20 to 1500 picoseconds, and most preferably in the range of 30 to 225 picoseconds. (30) The laser system of any one of embodiments 28 and 29, wherein the pump laser source is an ytterbium-doped fiber laser source.

[0138] (31) The laser system of embodiment 30, wherein the pump laser source includes an ytterbium-doped fiber mode-locked oscillator and / or one or more ytterbium-doped fiber amplifiers. (32) A laser system according to any one of embodiments 28 to 30, wherein the pump laser light source includes a semiconductor laser, or the pump laser light source includes a microchip laser. (33) A laser system according to any one of embodiments 1 to 32, wherein the pump laser light source includes a pulse stretcher. (34) A laser system according to any one of embodiments 1 to 33, further comprising an optical modulator between the pump laser light source and the first nonlinear medium. (35) A method for generating an optical pulse, comprising: generating pump laser pulses using a pump laser source; coupling the pump laser pulse into a first polarization axis of a first nonlinear medium; generating a first generated optical pulse at a different wavelength than the pump laser pulse and on a second polarization axis of the first nonlinear medium via vector four-wave mixing in the first nonlinear medium; coupling the pump laser pulse and the first generated optical pulse from the first nonlinear medium to a second nonlinear medium; generating a second generated optical pulse at a different wavelength than the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium.

[0139] (36) The method described in embodiment 35, wherein the second nonlinear optical process is type I collinear phase-matched three-wave mixing, and preferably the three-wave mixing is difference frequency generation. (37) The method of embodiment 36, wherein the second nonlinear medium is a nonlinear crystal, preferably, the second nonlinear medium is a cadmium silicon diphosphide crystal. (38) The method according to any one of embodiments 35 to 37, wherein the first nonlinear medium is a birefringent photonic crystal fiber. (39) A method for generating an optical pulse, comprising: generating pump laser pulses using a pump laser source; coupling the pump laser pulse into a first nonlinear medium; generating a first generated optical pulse at a wavelength different from the pump laser pulse via a first nonlinear optical process in the first nonlinear medium; coupling the pump laser pulse and the first generated optical pulse from the first nonlinear medium to a second nonlinear medium; generating a second generated optical pulse at a different wavelength than the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium; The method, wherein the second generated laser pulse is at a longer wavelength relative to the pump laser pulse and the first generated optical pulse. (40) The method of embodiment 39, wherein the first nonlinear optical process is four-wave mixing, preferably spontaneous emission four-wave mixing.

[0140] (41) The method of embodiment 39 or 40, wherein the first nonlinear medium is an optical fiber, preferably a photonic crystal fiber. (42) The method according to any one of embodiments 39 to 41, wherein the second nonlinear optical process is three-wave mixing, and preferably, the second nonlinear optical process is difference frequency generation. (43) The wavelength of the first generated optical pulse corresponds to a signal wavelength in the second nonlinear optical process; the wavelength of the second generated optical pulse corresponds to an idler wavelength in the second nonlinear optical process; A method according to any one of embodiments 39 to 42, wherein the wavelength of the pump laser pulse corresponds to the pump wavelength in the second nonlinear optical process. (44) The method according to any one of embodiments 39 to 43, wherein the second nonlinear medium is a periodically poled nonlinear crystal. (45) controlling the temperature of the periodically poled nonlinear crystal; 45. The method of embodiment 44, further comprising changing the temperature of the periodically poled nonlinear crystal to change the phase matching between the wavelength of the pump laser pulse and the second generated laser pulse, thereby changing the wavelength of the second generated light pulse.

Claims

1. a pump laser source configured to generate pump laser pulses; a first nonlinear medium; and a second nonlinear medium, The laser system includes: a pump laser pulse from the pump laser is coupled into the first nonlinear medium to generate a first generated optical pulse at a wavelength different from the pump laser pulse via a first nonlinear optical process in the first nonlinear medium, the first nonlinear process being vectorial four-wave mixing, the pump laser pulse being coupled into a first polarization axis of the first nonlinear medium, and the first generated optical pulse being generated on a second polarization axis of the first nonlinear medium; the pump laser pulse and the first generated optical pulse are coupled from the first nonlinear medium to the second nonlinear medium, and a second generated optical pulse is generated at a different wavelength from the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium.

2. 2. The laser system of claim 1, wherein the second nonlinear optical process is Type I collinear phase-matched three-wave mixing, and preferably the three-wave mixing is difference frequency generation.

3. 3. The laser system of claim 2, wherein the second nonlinear medium is a nonlinear crystal, preferably a cadmium silicon diphosphide crystal.

4. 4. The laser system according to claim 1, wherein the first nonlinear medium is a birefringent photonic crystal fiber.

5. 10. The laser system of claim 1, wherein the second generated light pulse has a wavelength longer than the wavelength of the first generated light pulse and the wavelength of the pump laser pulse.

6. 10. The laser system of claim 1, wherein the first generated light pulse has a peak wavelength in the range of 1.2 to 1.3 μm.

7. 2. The laser system of claim 1, wherein the second generated light pulse has a peak wavelength in the range of 4 μm to 8 μm, preferably in the range of 5 μm to 7 μm, more preferably in the range of 6 μm to 6.5 μm.

8. a pump laser source configured to generate pump laser pulses; a first nonlinear medium; and a second nonlinear medium, The laser system includes: a pump laser pulse from the pump laser is coupled to the first nonlinear medium to generate a first generated optical pulse at a wavelength different from that of the pump laser pulse via a first nonlinear optical process in the first nonlinear medium; the pump laser pulse and the first generated optical pulse are coupled from the first nonlinear medium to the second nonlinear medium, and a second generated optical pulse is generated at a different and longer wavelength than the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium.

9. 9. The laser system of claim 8, wherein the first nonlinear optical process is four-wave mixing.

10. 10. The laser system of claim 9, wherein the first nonlinear optical process is spontaneous four-wave mixing.

11. 11. The laser system of claim 9 or 10, wherein the first generated light pulse is at a longer wavelength than the pump laser pulse.

12. 9. The laser system of claim 8, wherein the first nonlinear medium is an optical fiber, preferably a photonic crystal fiber.

13. 9. The laser system of claim 8, wherein the second nonlinear optical process is three-wave mixing.

14. 14. The laser system of claim 13, wherein the second nonlinear optical process is difference frequency generation.

15. 9. The laser system of claim 8, wherein the second nonlinear medium is a nonlinear crystal, preferably a periodically poled nonlinear crystal, more preferably a periodically poled lithium niobate crystal.

16. 16. The laser system of claim 15, further comprising means for controlling the temperature of the nonlinear crystal, preferably wherein the means for controlling the temperature of the nonlinear crystal is a crystal oven in which the nonlinear crystal is maintained.

17. 17. The laser system of claim 16, wherein the peak wavelength of the second generated optical pulse is tunable over a range of peak idler wavelengths by adjusting the temperature of the nonlinear crystal using the means for controlling the temperature of the nonlinear crystal.

18. 18. The laser system of claim 17, wherein a temperature of the crystal determines phase matching between a wavelength of the pump laser pulse and the second generation laser pulse, and wherein the optical spectrum of the first generation laser pulse is sufficiently broad to provide a phase-matched signal over the range of the peak idler wavelength.

19. 20. The laser system of claim 18, wherein the peak idler wavelength range is greater than 20 nm, preferably the peak idler wavelength range is greater than 50 nm, and more preferably the peak idler wavelength range is greater than 100 nm.

20. 9. The laser system of claim 8, wherein the second generated light pulse has a peak wavelength in the mid-infrared range, preferably in the range of 2.7 to 3.2 μm.

21. 9. The laser system of claim 8, wherein the first generated light pulse has a peak wavelength in the range of 1.5 to 1.8 μm.

22. a wavelength of the first generated optical pulse corresponds to a signal wavelength in the second nonlinear optical process; the wavelength of the second generated optical pulse corresponds to an idler wavelength in the second nonlinear optical process; 10. The laser system of claim 1, wherein a wavelength of the pump laser pulse corresponds to a pump wavelength in the second nonlinear optical process.

23. 10. The laser system of claim 1, wherein the dispersion profile of the first nonlinear medium is such that the first nonlinear medium exhibits normal dispersion at the wavelength of the pump laser pulse.

24. 24. The laser system of claim 23, wherein the dispersion of the first nonlinear medium at the wavelength of the pump laser pulse is between 0 and −20 ps / (nm km), and preferably the dispersion of the first nonlinear medium at the wavelength of the pump laser pulse is between 0 and −10 ps / (nm km).

25. the dispersion profile of the first nonlinear medium is such that a walk-off length between the first generated optical pulse and the pump laser pulse is greater than an optical path length through the first nonlinear medium; More preferably, the walk-off length is at least twice the optical path length; 10. The laser system of claim 1, wherein the walk-off length is most preferably at least five times the optical path length.

26. 2. The laser system of claim 1, wherein the difference in optical frequency between the pump laser pulse and the first generated optical pulse is greater than 40 THz, preferably greater than 80 THz.

27. 10. The laser system of claim 1, wherein the second generated light pulse has a pulse duration in the range of 5 to 2000 picoseconds, preferably in the range of 20 to 1000 picoseconds, and most preferably in the range of 20 to 150 picoseconds.

28. 10. The laser system of claim 1, wherein the pump laser pulses have a peak wavelength in the near infrared region, preferably around 1 μm.

29. 29. The laser system of claim 28, wherein the pump laser pulses have a pulse duration in the range of 5 to 3000 picoseconds, preferably in the range of 20 to 1500 picoseconds, most preferably in the range of 30 to 225 picoseconds.

30. 30. The laser system of claim 28 or 29, wherein the pump laser source is an ytterbium-doped fiber laser source.

31. 31. The laser system of claim 30, wherein the pump laser source comprises an ytterbium-doped fiber modelocked oscillator and / or one or more ytterbium-doped fiber amplifiers.

32. 30. The laser system of claim 28, wherein the pump laser source comprises a semiconductor laser, or wherein the pump laser source comprises a microchip laser.

33. 10. The laser system of claim 1, wherein the pump laser source includes a pulse stretcher.

34. 10. The laser system of claim 1, further comprising an optical modulator between the pump laser source and the first nonlinear medium.

35. 1. A method for generating light pulses, comprising: generating pump laser pulses using a pump laser source; coupling the pump laser pulse into a first polarization axis of a first nonlinear medium; generating a first generated optical pulse at a different wavelength than the pump laser pulse and on a second polarization axis of the first nonlinear medium via vector four-wave mixing in the first nonlinear medium; coupling the pump laser pulse and the first generated optical pulse from the first nonlinear medium to a second nonlinear medium; generating a second generated optical pulse at a different wavelength than the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium.

36. 36. The method of claim 35, wherein the second nonlinear optical process is Type I collinear phase-matched three-wave mixing, preferably wherein the three-wave mixing is difference frequency generation.

37. 37. The method of claim 36, wherein the second nonlinear medium is a nonlinear crystal, preferably, the second nonlinear medium is a cadmium silicon diphosphide crystal.

38. The method of any one of claims 35 to 37, wherein the first nonlinear medium is a birefringent photonic crystal fiber.

39. 1. A method for generating light pulses, comprising: generating pump laser pulses using a pump laser source; coupling the pump laser pulse into a first nonlinear medium; generating a first generated optical pulse at a wavelength different from the pump laser pulse via a first nonlinear optical process in the first nonlinear medium; coupling the pump laser pulse and the first generated optical pulse from the first nonlinear medium to a second nonlinear medium; generating a second generated optical pulse at a different wavelength than the pump laser pulse and the first generated optical pulse via a second nonlinear optical process in the second nonlinear medium; The method, wherein the second generated laser pulse is at a longer wavelength relative to the pump laser pulse and the first generated optical pulse.

40. 40. The method of claim 39, wherein the first nonlinear optical process is four-wave mixing, preferably spontaneous emission four-wave mixing.

41. 41. The method of claim 39 or 40, wherein the first nonlinear medium is an optical fiber, preferably a photonic crystal fiber.

42. 40. The method of claim 39, wherein the second nonlinear optical process is three-wave mixing, preferably wherein the second nonlinear optical process is difference frequency generation.

43. a wavelength of the first generated optical pulse corresponds to a signal wavelength in the second nonlinear optical process; the wavelength of the second generated optical pulse corresponds to an idler wavelength in the second nonlinear optical process; 40. The method of claim 39, wherein the wavelength of the pump laser pulse corresponds to a pump wavelength in the second nonlinear optical process.

44. 40. The method of claim 39, wherein the second nonlinear medium is a periodically poled nonlinear crystal.

45. controlling the temperature of the periodically poled nonlinear crystal; 45. The method of claim 44, further comprising: varying the temperature of the periodically poled nonlinear crystal to change phase matching between a wavelength of the pump laser pulse and the second generated laser pulse, thereby changing the wavelength of the second generated optical pulse.

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