Generation of ultrashort laser pulses

Abstract

Method for generating pulsed laser radiation, comprising the following steps: - Generating pulsed laser radiation at a fundamental wavelength; - Increasing the spectral bandwidth of the laser radiation by guiding the laser radiation through a first section of a periodically polarized nonlinear crystal material in which the polarization period is larger or smaller than the polarization period that satisfies the quasi-phase matching condition at the fundamental wavelength; and - Frequency conversion of the spectrally broadened laser radiation by passing the laser radiation through a second section of the periodically polarized nonlinear crystal material section, wherein the crystal material in the second section is quasi-phase-matched at the fundamental wavelength.

Description

TECHNICAL FIELD

[0001] The invention relates to a method for generating pulsed laser radiation. Furthermore, the invention relates to a laser system for generating pulsed laser radiation. BACKGROUND OF THE INVENTION

[0002] There is still growing interest in the technology of ultrashort pulsed laser systems. A pulse duration of only a single optical cycle and a peak power of up to terawatts or even petawatts are already feasible today. Numerous promising applications have emerged for which the high peak power and ultrashort pulse duration of ultrafast lasers are particularly desirable. Both give rise to nonlinear effects and open new avenues in engineering and scientific research. Users of the technology demand simple and robust devices that are comfortable to handle and can be easily integrated into complex experiments and instruments. Areas of application for such laser systems include life sciences, terahertz spectroscopy, two-photon polymerization, optical coherence tomography, excitation-interrogation or pump-probe spectroscopy, materials processing, and metrology.

[0003] Commercially available ultrafast laser systems are advantageously implemented as fiber laser systems. A summary of fiber laser technology for generating ultrashort laser pulses and their application was provided by Marion Lang, Thomas Hellerer, and Jürgen Stuhler ("Technology and applications of ultrafast fiber lasers," Proc. SPIE 8330, Photonics and Optoelectronics Meetings (POEM) 2011: Laser and Terahertz Science and Technology, 833007).

[0004] Known from US Pat. No. 6,650,466 B1 is an optical pulse compression technique based on quadratic nonlinear media. Negative nonlinear phase shifts are generated by non-phase-matched second harmonic generation, with the phase-modulated pulses subsequently compressed by propagation in materials with normal dispersion.

[0005] Shuanggen Zhang et al. (IEEE Journal of Quantum Electronics, 2015, Vol. 51, No. 5, pp. 1-5) analyzed the spectral shift and spectrum broadening properties of femtosecond laser pulses propagated in periodically poled lithium niobate under cascaded quadratic nonlinearity. This study revealed that increasing the input pulse power from an optically parametric amplifier-laser system resulted in the generation of multiple peak structures in the range from 1400 nm to 1700 nm. The maximum spectrum broadening was measured at 300 nm at a pump power of 67 mW. A spectral blue and red shift of up to 50 nm was observed under phase mismatch.

[0006] Fiber laser gain materials typically exhibit a limited bandwidth compared to solid-state laser systems (e.g., titanium-sapphire lasers). This effect can be limiting in laser systems that utilize frequency conversion (e.g., frequency doubling) to deliver pulsed laser radiation at a target wavelength that differs from the fundamental wavelength of the laser radiation generated in the fiber gain material. Users often specify laser pulses below 100 fs. However, without specific measures for spectral broadening, commercially available fiber laser-based systems typically deliver frequency-converted laser pulses with a duration significantly longer than 100 fs. The nonlinear frequency conversion shortens the laser pulses. However, this shortening is limited to a minimum factor of 1 / √2 compared to the original pulse length, which is often insufficient.Methods known to be suitable for achieving short pulse durations through spectral broadening are self-phase modulation and soliton compression in third-order nonlinear materials. Depending on the pulse energy, the nonlinearity of common bulk materials may be too small to achieve significant spectral broadening. Broadening in an optical fiber is possible. However, suitable photonic crystal fibers must be used, and the resulting fiber lengths are impractical. Furthermore, additional robust free-space optics are required for efficient coupling into the fiber. SUMMARY OF THE INVENTION

[0007] From the foregoing, it is already clear that there is a need for an improved technique for generating pulsed laser radiation. One object of the invention is to provide a method and a corresponding laser system that are simple and robust and capable of producing laser pulses below 100 fs using nonlinear frequency conversion.

[0008] According to the invention, a method for generating pulsed laser radiation is disclosed. The method comprises the steps of: - generating pulsed laser radiation at a fundamental wavelength; - increasing the spectral bandwidth of the laser radiation by passing the laser radiation through a first section of a periodically poled nonlinear crystal material in which the poling period is greater or smaller than the poling period that satisfies the quasi-phase matching condition at the fundamental wavelength; and - frequency converting the spectrally broadened laser radiation by passing the laser radiation through a second section of the periodically poled nonlinear crystal material, wherein the crystal material in the second section is quasi-phase matched at the fundamental wavelength.

[0009] According to the invention, the pulsed laser radiation at the fundamental wavelength can be generated using a conventional and commercially available diode-pumped mode-locked fiber laser (such as an ytterbium, erbium, or thulium fiber laser system) with a pulse duration of less than 1 ps, preferably 100 fs to 800 fs, and a repetition rate of 10-100 MHz. The initially generated pulsed laser radiation is typically amplified, for example, using a conventional rare-earth-doped fiber amplifier to an average power of 100 mW or more to enable efficient nonlinear frequency conversion.

[0010] It is known that nonlinear processes (frequency doubling, sum and difference frequency generation, etc.) are phase-sensitive and require efficient phase matching. It must be ensured that an appropriate phase relationship between the waves of the fundamental radiation and the frequency-converted radiation is established and maintained along the propagation direction in the nonlinear crystal used. Only when the phase-matching condition is met are amplitude contributions from different locations to the resulting radiation at the end of the nonlinear crystal all in phase. Quasi-phase matching is a well-known technique for achieving similar results. Instead of a homogeneous nonlinear crystal material, a material with spatially modulated nonlinear properties is used.The concept essentially consists in allowing a phase mismatch over a certain propagation distance, but reversing the nonlinear interaction at positions where the interaction would otherwise occur with the wrong conversion direction. In this way, the total amplitude of the frequency-converted radiation can continuously increase along the nonlinear crystal, and sufficient overall conversion efficiency is achieved. Periodic poling of a nonlinear crystal is a well-known technique for achieving quasi-phase matching. This involves creating a periodic reversal of the domain orientation (domain inversion) in the nonlinear crystal, so that the sign of the nonlinear coefficient also changes.The most common technique for periodic poling is the application of a strong electric field to a ferroelectric crystal using patterned electrodes on the crystal surface, which typically have a period of between a few micrometers and several tens of micrometers. The poling period (i.e., the period of the electrode structure) determines the wavelengths for which the nonlinear process is quasi-phase-matched.

[0011] The invention takes advantage of the fact that effective spectral broadening can be induced by self-phase modulation on the laser radiation at the fundamental frequency through the cascading interaction with the frequency-converted laser radiation in the nonlinear crystal under phase-mismatch conditions. Thus, the invention proposes spectrally broadening the pulsed laser radiation by passing it through a periodically poled nonlinear crystal that is phase-mismatched, i.e., the poling period is greater or lesser than the poling period that satisfies the quasi-phase-matching condition at the fundamental wavelength. The phase mismatch depends on the propagation constants of the fundamental and frequency-converted laser radiation in the nonlinear crystal and the poling period. In a commonly used third-order material (e.g., lithium niobate), the nonlinearity has a positive value.Therefore, a larger or smaller poling period than the poling period satisfying the quasi-phase matching condition can be used to obtain a negative or positive contribution of the self-phase modulation, respectively.

[0012] It should be noted that the term "periodically poled nonlinear crystal," used throughout the description and claims, refers to any crystal material with spatially modulated nonlinear properties. The modulation need not be strictly periodic.

[0013] In particular, the period of the spatial modulation can vary along the propagation path of the laser radiation. In other words, the meaning of the term "periodically poled nonlinear crystal" according to the invention also includes a "non-uniformly poled nonlinear crystal."

[0014] After guiding the fundamental laser radiation through the phase-mismatched, periodically poled nonlinear crystal, the resulting spectrally broadened laser radiation is frequency-converted according to the invention by guiding the laser radiation through a second section of the periodically poled nonlinear crystal material section, which is arranged downstream of the first section in the laser radiation path, with the crystal material in the second section being quasi-phase-matched at the fundamental wavelength. In this way, spectrally broad, frequency-converted laser pulses are obtained, which support particularly short pulse durations.

[0015] The invention only requires that the phase-mismatched periodically poled nonlinear crystal increase the spectral width and enable the generation of laser pulses below 100 fs. Therefore, the invention provides a very simple, robust, and cost-effective solution to the above-mentioned problem of the invention.

[0016] In a preferred embodiment, the pulse duration of the frequency-converted laser radiation is shortened by passing the frequency-converted laser radiation through a dispersive element. By passing the frequency-converted laser radiation through an element of suitable dispersion, the pulse duration can be reduced in a straightforward manner to the minimum value, which is supported by the spectral width of the frequency-converted laser radiation. Advantageously, as a result of the spectral broadening step of the invention, the thus shortened pulse duration can be less than 1 / 2 times the pulse duration of the originally generated laser radiation at the central wavelength. In other words, the invention enables the shortening of the laser pulses significantly beyond the shortening effected by the nonlinear frequency conversion.

[0017] In a further preferred embodiment, the originally generated pulsed laser radiation is chirped. In many applications, e.g., in microscopy, a defined pre-chirp of the laser pulses is required to compensate for the dispersion of the downstream optical components in the beam path, e.g., the microscope objective. Typically, these optical components exhibit normal dispersion. Thus, pre-chirped laser pulses with anomalous dispersion, i.e., a negative pre-chirp, are required. This is conventionally achieved using Gires-Tournois interferometer (GTI) mirrors, microstructured fibers, prism pairs, or grating pairs. All of these solutions have disadvantages such as high cost and alignment sensitivity.It has been found that the method of the invention enables the transmission of the chirp of the fundamental laser pulses through the frequency conversion step such that the frequency-converted laser radiation is negatively pre-chirped, as required to compensate for the dispersion of the downstream optical components in the beam path. Appropriate (pre-)chirping of the fundamental laser pulses can be achieved, for example, by applying chirped pulse amplification (CPA) in the step of generating the laser radiation at the fundamental wavelength with a suitable adjustment of the pulse stretcher and pulse compressor. No additional components are required.

[0018] According to the invention, a laser system for generating pulsed laser radiation is also disclosed. The laser system comprises: - a laser source which generates pulsed laser radiation at a fundamental wavelength; - a first periodically poled nonlinear crystal section arranged in the beam path and having a poling period that is greater or smaller than the poling period that satisfies the quasi-phase matching condition at the fundamental wavelength in order to spectrally broaden the laser radiation; and - a second periodically poled nonlinear crystal section arranged in the beam path downstream of the first section, the second section being quasi-phase matched at the fundamental wavelength to frequency convert the laser radiation.

[0019] The laser system implements the above-described method of the invention by employing the first periodically poled nonlinear crystal section for spectral broadening of the laser radiation and the second periodically poled nonlinear crystal section for frequency conversion. Therefore, both spectral broadening and frequency conversion occur in one periodically poled nonlinear crystal.

[0020] In a particularly advantageous embodiment, the two sections are formed in a single monolithic periodically poled nonlinear crystal.

[0021] In one possible embodiment, the single periodically poled nonlinear crystal has two regions of different poling periods, one region (forming the first periodically poled nonlinear crystal section) which is phase-mismatched according to the invention to spectrally broaden the fundamental radiation, and one region (forming the second periodically poled nonlinear crystal section) which is quasi-phase-matched at the fundamental wavelength to frequency-convert the laser radiation.

[0022] In a further embodiment, the poling period varies linearly along the beam path in at least one of the regions. A linearly chirped, periodically poled nonlinear crystal is particularly well suited for efficient frequency conversion of chirped laser pulses. Thus, in the case of (pre-)chirped laser pulses at the fundamental wavelength, the frequency conversion efficiency can be improved by providing a linearly chirped region (i.e., a region with a linearly varying poling period) that forms the second periodically poled nonlinear crystal section.

[0023] A region of the nonlinear crystal with a fixed (longer or shorter) poling period for spectral broadening through self-phase modulation and a region of chirped periodic poling for frequency conversion can be replaced by a fully linearly chirped crystal, meaning that the poling period varies linearly along the optical path over the entire length of the nonlinear crystal. In this embodiment, longer poling periods in the first section of the nonlinear crystal effect self-phase modulation, and the shorter poling periods in the second section, i.e., toward the end of the crystal, establish quasi-phase matching to enable frequency conversion.This embodiment has the advantage of "relaxed" design requirements, since the region within the nonlinear crystal in which the laser beam is focused, and thus the poling period that provides the most efficient frequency conversion, can be adjusted.

[0024] In a further preferred embodiment, the laser system further comprises an adjustable dispersive element arranged in the beam path upstream of the first periodically poled nonlinear crystal section to generate a negative pre-chirp of the laser radiation. The requirement of applying a pre-chirp to the laser pulses has been explained in detail above.

[0025] In yet another preferred embodiment, the laser system comprises a dispersive element arranged in the beam path downstream of the second periodically poled crystal section to temporarily compress the frequency-converted laser pulses. The laser system of the invention requires only the specific periodically poled nonlinear crystal with two regions and the dispersive element to achieve spectral broadening at the fundamental wavelength in combination with temporary compression in the dispersive element to ultimately obtain frequency-converted laser pulses with minimal pulse duration (below 100 fs). BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are for illustrative purposes only and are not intended to define the limitations of the invention. In the drawings: Fig. 1 schematically shows a laser system according to the invention; Fig. 2 diagrams illustrating the properties (duration, spectrum) of laser pulses generated according to the invention; Fig. 3 schematically shows a first construction of a periodically poled nonlinear crystal to be used in the laser system of the invention; Fig. 4 schematically shows a second construction of a periodically poled nonlinear crystal to be used in the laser system of the invention; Fig. 5 schematically shows a third construction of a periodically poled nonlinear crystal to be used in the laser system of the invention; Fig. 6 schematically shows a laser system according to another embodiment of the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] Fig. 1 schematically shows an embodiment of a laser system according to the invention.

[0028] The laser system comprises a laser source 1. The laser source 1 can comprise a rare earth ion-doped fiber laser oscillator (not shown) which emits a laser pulse train at a fixed repetition rate of, for example, 100 MHz, a section of dispersive optical fiber (not shown) to temporarily stretch the laser pulses, and a diode-pumped fiber amplifier (not shown) which receives the chirped laser pulses and increases their energy such that the average power of the amplified pulse train is on the order of 100 mW or more. The thus amplified laser pulses are coupled out of the optical fiber and temporarily compressed in a grating compressor 2. In the embodiment shown, the resulting laser pulses have a duration of 175 fs at a fundamental wavelength of 1560 nm. The spectral width Δλ of the laser radiation is 25 nm.

[0029] The laser pulses are then passed through a non-uniformly poled nonlinear lithium niobate crystal (NUPLN) 3. The NUPLN 3 has a length of 2 mm with a linear chirp of the poling period of -2.00 µm / mm. With this design, no frequency conversion occurs, but rather spectral broadening through self-phase modulation occurs in a first section of the NUPLN, where the poling period is larger than the poling period that satisfies the quasi-phase matching condition. Frequency doubling occurs in a second section, where the poling period becomes small enough to establish quasi-phase matching. In this way, laser pulses are generated at 780 nm with a pulse duration of 340 fs and a spectral width Δλ of 15 nm. This spectral bandwidth supports a pulse duration of 80 fs. The frequency-doubled laser pulses exhibit a negative chirp because the phase mismatch in the first section of the NUPLN 3 is negative.This negative chirp can be easily compensated for by using a block of dense flint glass 4 as a dispersive element a few centimeters long. As can be seen, a pulse duration of 80 fs in the second harmonic is achieved with a fundamental pulse duration of 175 fs. The final pulse duration is thus considerably less than 1 / 2 times the pulse duration of the laser radiation at the fundamental wavelength. This is the result of the spectral broadening in the first section of the NUPLN 3.

[0030] The diagram from Fig. Figure 2a shows the time course (power as a function of time) of a single pulse at the output of the flint glass block 4. A pulse 5 is depicted, which is generated using a conventional (constant poling period and fully quasi-phase-matched) periodically poled lithium niobate crystal (PPLN), compared to a pulse 6, which is generated according to the invention as explained above using the NUPLN 3. As can be seen, the pulse duration is reduced from approximately 150 fs to 80 fs by the concept of the invention.

[0031] The diagram from Fig. Figure 2b shows the corresponding spectra of the laser pulses. As can be seen, the width of spectrum 7 obtained using the conventional PPLN is approximately 6 nm, whereas the width of spectrum 8 obtained using the NUPLN 3 is approximately 15 nm.

[0032] The Fig. Figures 3-5 show possible designs of the NUPLN 3, which is used according to the invention for spectral broadening and frequency conversion. The beam path and direction of the laser radiation are indicated by dashed arrows.

[0033] Fig. 3 shows a construction of the crystal with two different directly adjacent poling regions, one for self-phase modulation (poling period Λ=Λ SPM ), without any frequency conversion, and one for efficient frequency doubling (poling period Λ=Λ QPM ), in which the quasi-phase matching condition is met. However, the efficiency of frequency conversion may be reduced in this embodiment because the laser pulse incident on the second region will generally be chirped, and the fixed poling period provides quasi-phase matching only for the central wavelength, but not for the chirped parts of the pulse.

[0034] A linear chirp of the poling period in the second range (Λ=Λ QPM +Λ'x, where x is the position along the beam path) has been calculated as in Fig. 4 has proven to be more efficient for frequency doubling of chirped laser pulses.

[0035] A region with a longer poling period for self-phase modulation and a region with chirped poling, which provides quasi-phase matching, in a single monolithic periodically poled nonlinear crystal 3 can also be realized by linearly varying the poling period (linear chirp) along the beam path over the entire length of the nonlinear crystal 3. This is shown in Fig. 5, in which the polarization period Λ=Λ SPMQPM +L' XThe longer poling periods in the first section of the crystal cause self-phase modulation, and the shorter poling periods in the second section toward the end of the crystal establish quasi-phase matching and thus cause frequency conversion. This embodiment allows for relaxed design rules, since the region within crystal 3 in which the laser beam is focused, and therefore the poling period, can be adjusted for the most efficient frequency conversion.

[0036] In other words, the invention proposes the use of a monolithic, non-uniformly poled, partially quasi-phase-matched material to broaden the spectrum at the fundamental wavelength of the pulsed laser radiation and to frequency convert the spectrally broadened laser radiation.

[0037] As mentioned above, many applications, e.g., confocal microscopy systems, require a defined pre-chirp of the laser pulses to compensate for the dispersion of the downstream optical components in the beam path. In the embodiment of Fig. 6, the grating compressor 2 is used to provide the laser radiation with a negative pre-chirp at the fundamental wavelength of 1840 nm. The resulting laser pulses have a pulse duration of 500 fs with a spectral width of Δλ=50 nm. Again, the frequency doubling is performed using the NUPLN 3 with the construction of Fig.5. The conversion efficiency has been proven to be as good as with an unchirped input pulse in a conventional PPLN. At the frequency-doubled wavelength of 920 nm, the pulse duration is still 500 fs with a spectral width Δλ of 30 nm. The pre-chirp of the laser pulses is maintained during the frequency-doubling process such that the laser pulses have their shortest duration of 70 fs after passing through the microscope objective 9 with normal dispersion. Fine adjustment of the negative pre-chirp can be achieved either by inserting glass wedges after the NUPLN 3 in front of the objective 9 or by adjusting the grating separation in the compressor 2. According to the invention, the NUPLN 3 is used for efficient frequency doubling with an increased spectral bandwidth. A walk-off in the NUPLN 3 may produce some additional pre-chirp, but this is small compared to the pre-chirp provided by the grating pair 2.

[0038] In other words, the concept of the invention enables the transmission of the pre-chirp, which can be provided at the fundamental wavelength by the frequency conversion step, so easily that the frequency-converted laser radiation is negatively pre-chirped, as required to compensate for the dispersion of the downstream optical components in the beam path. At the same time, a minimum pulse duration in the range below 100 fs can be achieved according to the invention after guiding the optical components of the respective application by spectrally broadening the fundamental laser radiation in the phase-mismatched nonlinear crystal section.

Claims

[1] A method for generating pulsed laser radiation, comprising the following steps: - generating pulsed laser radiation at a fundamental wavelength; - increasing the spectral bandwidth of the laser radiation by passing the laser radiation through a first section of a periodically poled nonlinear crystal material in which the poling period is greater or smaller than the poling period that satisfies the quasi-phase matching condition at the fundamental wavelength; and - frequency converting the spectrally broadened laser radiation by passing the laser radiation through a second section of the periodically poled nonlinear crystal material section, wherein the crystal material in the second section is quasi-phase matched at the fundamental wavelength. [2] The method of claim 1, wherein the pulse duration of the frequency-converted laser radiation is shortened by passing the laser radiation through a dispersive element. [3] The method of claim 2, wherein the shortened pulse duration is less than 1 / √2 times the pulse duration of the originally generated laser radiation at the central wavelength. [4] Method according to claims 1-3, wherein the originally generated pulsed laser radiation is chirped. [5] A method according to any one of claims 1-4, wherein the frequency-converted laser radiation is negatively pre-chirped to compensate for the dispersion of downstream optical components in the beam path. [6] Method according to one of claims 1-5, wherein the frequency conversion of the spectrally broadened laser radiation is a frequency doubling. [7] Laser system for generating pulsed laser radiation, comprising: - a laser source which generates pulsed laser radiation at a fundamental wavelength; - a first periodically poled nonlinear crystal section arranged in the beam path and having a poling period that is greater or smaller than the poling period that satisfies the quasi-phase matching condition at the fundamental wavelength in order to spectrally broaden the laser radiation; and - a second periodically poled nonlinear crystal section arranged in the beam path downstream of the first section, the second section being quasi-phase matched at the fundamental wavelength to frequency convert the laser radiation. [8] A laser system according to claim 7, wherein the two sections are formed in a single monolithic periodically poled nonlinear crystal having two regions of different poling periods. [9] A laser system according to claim 8, wherein the poling period varies linearly along the beam path in at least one of the regions. [10] A laser system according to claim 8, wherein the two sections are formed in a single monolithic periodically poled nonlinear crystal, the poling period varying linearly along the beam path over the entire length of the nonlinear crystal. [11] Laser system according to any one of claims 7-10, further comprising an adjustable dispersive element arranged in the beam path upstream of the first periodically poled non-linear crystal section to generate a negative pre-chirp of the laser radiation. [12] A laser system according to any one of claims 7-10, wherein a dispersive element is arranged in the beam path downstream of the second periodically poled crystal section to temporarily compress the frequency-converted laser radiation. [13] A laser system according to any one of claims 7-12, wherein the laser source comprises a mode-locked rare earth doped fiber laser. [14] A laser system according to claim 13, wherein the laser source is arranged to emit laser pulses having a duration of less than 1 ps at a repetition rate of 10-100 MHz. [15] A laser system according to claim 13 or 14, wherein the laser source further comprises a rare earth doped fiber amplifier arranged to amplify the pulsed laser radiation to an average power of at least 100 mW.

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