Laser radiation source

Abstract

The present invention relates to a laser radiation source (10) comprising a pump laser (1) and an optical parametric oscillator (OPO (5)), further comprising a beam shaping optic (3) arranged between the pump laser (1) and the OPO (5), which is configured such that a pump laser beam (2) generated by the pump laser (1) is guided through the OPO (5) forming an intensifying lens in the OPO (5), such that the following condition is met: | α 2 | ∗ wp 2 > 35 m − 1 ; where w p the pump jet radius and α2 the square coefficient of the amplifying lens.

Description

[0001] The present invention relates to a laser radiation source, particularly for a spectroscope, a lidar system, or an optronic countermeasures system. In such laser applications, it is necessary to realize a compact and robust laser source capable of generating and selectively emitting a desired, time-varying average power and pulse energy (modulation) according to an electronic input signal. Infrared laser beams in the region of the atmospheric transmission windows around 2 µm and in the range of 3–5 µm are particularly required.

[0002] An optronic countermeasure is a defense mechanism designed to target sensors or guidance systems based on optical or infrared technology. This defense mechanism aims to reduce or neutralize the effectiveness of optical and infrared sensors used in guided missiles, reconnaissance, and surveillance equipment.

[0003] LiDAR (Light Detection and Ranging) is a remote sensing method that uses laser light to make precise measurements of distances and objects in the environment.

[0004] German patent DE 19745785 A1 describes a laser radiation source for a guided or directed infrared countermeasures system, in which pulsed laser sources are generated at a wavelength of 1 µm and converted to a wavelength range of 3–4.5 µm by means of a lithium niobate-based optical parametric oscillator (OPO). The laser radiation source according to DE 19745785 A1 has proven to have two disadvantages: firstly, it cannot cover the entire range from 3 µm to 5 µm; secondly, emission above 4.3 µm is only possible with a significant loss of efficiency due to the intrinsic absorption of the OPO material.

[0005] Furthermore, modulated laser sources for optronic countermeasures are known from the prior art. While a line spectrum in the 3-5 µm range can be generated, the adaptation to the infrared signature or the spectral sensitivity of the optronics is limited. The signature of the target to which the optronics react is generally thermal and therefore broadband. The atmosphere also exhibits narrowband absorption properties in its transmission windows. Consequently, the radiation received by the optronics decreases with an average extinction, where the extinction of a laser describes the attenuation of light intensity due to absorption and scattering in a medium. A laser with narrow lines has an extinction that is specific to its wavelength. It has been found that in previously known laser systems, the extinction of the laser is not sufficiently similar to that of thermal radiation.This has not yet been achieved sufficiently well and satisfactorily across the entire transmission window.

[0006] One object of the present invention is to overcome the disadvantages of the prior art, in particular to provide an efficient and / or simple laser radiation source that covers a high spectral bandwidth in the infrared range.

[0007] This problem is solved by the subject matter of independent claims.

[0008] A laser radiation source is then provided for generating laser radiation in the infrared range, for example in the range of the atmospheric transmission windows around 2 µm and / or in the range of 3 µm to 5 µm.

[0009] The laser radiation source comprises a pump laser and an optical parametric oscillator (OPO). A beam shaping optic is arranged between the pump laser and the OPO, configured such that a pump laser beam generated by the pump laser is guided through the OPO, forming a gain lens within the OPO, thus fulfilling the following condition: |α2|∗wp2>35 m−1, where w pThe pump beam radius and α2 are the square coefficient of the gaining lens. For pulsed pump lasers, this must be fulfilled at least once during a pulse. When the OPO is pumped in this way, the interaction of the gaining lens, pump beam caustic, temperature gradient in the OPO crystal, and OPO resonator creates an interaction geometry between the pump, signal, and idler. This geometry generates a spectrally broadband phase matching, resulting in broadband emission spectra in which no separate individual lines appear in the signal or idler.

[0010] The term "pump laser" refers to the laser source that provides the necessary energy to operate the OPO. The optical parametric oscillator (OPO) is a nonlinear optical converter that changes the wavelength of the incoming light and converts the incident pump light into two different wavelengths: signal and idler, whose frequencies sum to the pump frequency. The beam shaping optics are an optical component that shapes the beam so that it is optimally coupled into the OPO and creates the desired gain lens. The pump beam radius is the radius of the laser beam that pumps the OPO, and the quadratic coefficient of the gain lens describes the gain characteristics of the lens within the OPO.

[0011] According to the invention, a compact laser radiation source requiring few components is to be created that provides a broad emission spectrum with high efficiency over the entire atmospheric transmission range around 2 µm and in the 3-5 µm range, which is particularly important for applications in spectroscopy, LiDAR, and optronic countermeasures. In general terms, the idea of ​​the present invention is to optimize the beam shaping and / or pulse shaping between the laser and the OPO so that the OPO is able to generate a broad spectrum, particularly greater than 50 nm, in both signal and idler phases in the mid-infrared range.

[0012] According to an exemplary embodiment of the present invention, a transfer matrix of the intensifying lens is formed by (10−(n2±iλα22π)Δz1) Given, where λ is the wavelength, n2 the quadratic refractive index coefficient, i the imaginary unit, and Δz an infinitesimal beam propagation length in the OPO, specifically in the OPO crystal. The infinitesimal beam propagation length can refer to a nearly infinitesimally small length element used to analyze the propagation of the optical beam through the crystal, particularly to investigate the stepwise changes in light fields within the crystal. The transfer matrix plays a crucial role in the interaction between the pump laser beam and the optical parametric oscillator (OPO). To achieve this transfer matrix, focusing can be performed within the OPO, with different phase front curvatures further broadening the phase matching.Alternatively or additionally, pumping is performed with a correspondingly high pumping energy or medium power, so that the forming gaining lens and the thermal lens increase the phase matching range. The transfer matrix enables finely tuned phase matching, resulting in broader spectral coverage and better adaptation to atmospheric transmission windows. Implementing this specific transfer matrix allows the laser radiation source to generate a particularly broad spectrum in the mid-infrared range, which is highly advantageous for applications such as spectroscopy and optronic countermeasures. Precise control of the gaining lens and optimization of the optical parameters contribute to maximizing the efficiency and power of the laser radiation source while simultaneously minimizing the number of required optical components.This results in a more compact and robust design of the laser source, which is particularly suitable for use in platform-mounted laser systems.

[0013] According to a further aspect of the present invention, which can be combined with the preceding aspects and exemplary embodiments, a laser radiation source is provided for generating laser radiation in the infrared range, for example in the range of the atmospheric transmission windows around 2 µm and / or in the range of 3 µm to 5 µm.

[0014] The laser radiation source comprises a pump laser and an optical parametric oscillator (OPO). Beam shaping optics are positioned between the pump laser and the OPO, designed to focus or bundle the pump laser beam generated by the pump laser into the OPO. The pump laser serves as the primary light source, generating a laser beam which is then guided through the beam shaping optics. These optics modify the beam for optimal coupling into the OPO. An advantage of this arrangement is improved beam conversion efficiency, as the beam shaping optics precisely focus or bundle the pump laser beam onto the OPO. This enables optimal interaction between the pump beam and the nonlinear medium of the OPO, resulting in a broader spectral bandwidth of the generated radiation.

[0015] Another advantage is the compactness and robustness of the entire laser radiation source, as the number of required optical components is minimized.

[0016] According to an exemplary embodiment of the present invention, the beam shaping optics are further configured to broaden the phase matching of the pump laser beam by means of additional phase front curvatures. These phase front curvatures are specific modifications of the laser beam wavefront that serve to optimize the interaction between the pump laser beam and the optical parametric oscillator (OPO). By introducing additional phase front curvatures, the phase matching range is extended, which means that the OPO can operate efficiently over a broader spectrum. A broader spectrum enables better matching to the atmospheric transmission windows and the spectral sensitivity curves of the target systems.Furthermore, this extension of the phase matching helps to increase the stability and efficiency of the laser source by reducing its sensitivity to fluctuations in operating conditions.

[0017] According to an exemplary embodiment of the present invention, the laser radiation source is equipped with beam-shaping optics configured either as a focusing lens or as a telescope. This specific design of the beam-shaping optics enables precise focusing or bundling of the pump laser beam generated by the pump laser into the optical parametric oscillator (OPO). A focusing lens offers the advantage of focusing the pump laser beam to a narrow point within the OPO, resulting in increased beam intensity at that point and thus improving the efficiency of the nonlinear conversion within the OPO. A telescope, on the other hand, can collimate or expand the pump laser beam over a greater distance before it enters the OPO, which can improve beam quality and the homogeneity of the beam profile.This can be particularly advantageous when a uniform energy distribution across the beam cross-section is required to reach the OPO threshold over a larger area, thereby increasing the efficiency and stability of the system. By offering the choice between a focusing lens and a telescope, the invention provides flexible adaptation of the beam shaping to the specific requirements of each application. This flexibility contributes to optimizing the performance and reliability of the laser radiation source.

[0018] According to an exemplary embodiment of the present invention, the beam shaping optics are further configured to modify the transverse pump beam profile such that the oscillation threshold of the OPO can be achieved over a larger beam cross-section, for example, by means of a super-Gaussian or a rectangular profile. This modification makes it possible to achieve the oscillation threshold of the optical parametric oscillator (OPO) over a larger beam cross-section. The term "transverse pump beam profile" refers to the intensity distribution of the laser beam perpendicular to the direction of propagation. A super-Gaussian profile is characterized by a flatter intensity distribution in the center and steeper edges compared to a normal Gaussian profile, while a rectangular profile has a uniform intensity distribution over the beam cross-section.These profiles are advantageous because they distribute the energy of the pump laser more efficiently across the OPO crystal, thus lowering the threshold for optical parametric oscillation.

[0019] According to an exemplary embodiment of the present invention, the pump laser comprises a laser diode and at least one fiber amplifier. The pump laser is configured to modify the oscillation threshold of the optical parametric oscillator (OPO), for example, by generating a rectangular pump pulse waveform. The laser diode serves as the primary source of the pump radiation, while the fiber amplifier amplifies the radiation generated by the laser diode to provide the necessary energy for the operation of the OPO. The oscillation threshold is the minimum pump power or energy required to efficiently initiate the OPO process and achieve stable generation of signal and idler waves. At this threshold, the OPO begins to continuously generate light at two new frequencies (signal and idler) derived from the incident pump light during the pumping process.Modifying this threshold allows for influencing the efficiency and spectral bandwidth of the generated radiation. A rectangular pump pulse profile means that the intensity of the pump radiation varies in a rectangular shape, resulting in a uniform and constant energy input during the pulse. This has the advantage that the phase matching conditions in the OPO are met over a longer period of the pulse, leading to broader spectral emission.

[0020] According to an exemplary embodiment of the present invention, the pump laser can be operated by means of an operating current such that varying the operating current results in a temporal shift of the central wavelength of the laser diode. Varying the operating current leads to a change in the temperature of the laser diode, which in turn causes a shift in the emitted wavelength. This temporal shift of the central wavelength is particularly advantageous because it results in a spectral broadening of the pump pulse. This broadening of the pump laser's spectrum leads to more efficient and wider spectral coverage in the downstream optical parametric oscillator (OPO). The OPO converts the pump radiation to a different wavelength, and the broader spectral distribution of the pump beam also results in a wider and more uniform distribution of the OPO's output radiation.

[0021] According to an exemplary embodiment of the present invention, the pump laser comprises at least one enhanced spontaneous emission (ASE) filter, wherein this filter is arranged, in particular, upstream of a main fiber amplifier in the beam propagation direction. The term "enhanced spontaneous emission (ASE)" refers to unwanted light emissions that can occur in an amplifying medium and can impair the quality of the laser beam. The ASE filter serves to suppress these unwanted emissions before the laser beam enters the main fiber amplifier. This is particularly important because ASE can degrade the signal-to-noise ratio and reduce the efficiency of the amplification process. By placing the filter upstream of the main fiber amplifier, it is ensured that only the desired laser beam is amplified, resulting in higher beam quality and efficiency.One advantage of this configuration is the improvement in the overall power and stability of the laser radiation source. Another advantage is the increase in pump pulse energy, as the filter removes unwanted emissions, thus enabling higher efficiency in the amplification process. This also facilitates achieving optimal operating conditions in the optical parametric oscillator (OPO), resulting in broader spectral emission in the mid-infrared range. The application of such a filter can be realized through various technologies, such as acousto-optical modulators (AOMs) or wavelength-selective filters operating in either time or spectral space.

[0022] According to a further aspect of the present invention, which can be combined with the preceding aspects and exemplary embodiments, a method for generating laser radiation in the infrared range, in particular in the range of the atmospheric transmission windows around 2 µm and / or in the range of 3 µm to 5 µm, is provided by means of a laser radiation source designed according to one of the previously described aspects and / or exemplary embodiments.

[0023] Preferred embodiments are specified in the dependent claims.

[0024] Further properties, features and advantages of the invention will be clarified below by describing preferred embodiments of the invention with reference to the accompanying exemplary drawings, which show: Fig. 1 a schematic view of an embodiment of a laser radiation source comprising a pump laser, beam shaping optics and an optical parametric oscillator; Fig. 2. A further schematic view of an embodiment of a laser radiation source comprising a laser diode, a fiber amplifier, beam shaping optics, and an optical parametric oscillator; and Fig. 3 Another schematic view of an embodiment of a laser radiation source comprising a laser diode, a filter, a fiber amplifier, a beam shaping optics and an optical parametric oscillator.

[0025] The Fig. Figures 1 to 3 show schematic representations of exemplary embodiments of laser radiation sources according to the invention 10.

[0026] According to Fig. The laser radiation source comprises a pump laser 1, which generates a pump laser beam 2. The pump laser beam 2 is guided through a beam-shaping optic 3, which is arranged between the pump laser 1 and an optical parametric oscillator (OPO) 5. The beam-shaping optic 3 shapes the pump laser beam 2 into an OPO pump laser beam 4, which enters the OPO 5. The OPO 5 then generates the OPO output beam 6. This arrangement enables the focusing or bundling of the pump laser beam 2 in the OPO 5 to create an intensifying lens within the OPO 5. The following condition applies at least once during a pulse: |α2|∗wp2>35 m−1, where w p The pump jet radius and α2 are the square coefficient of the intensifying lens. Furthermore, the transfer matrix of the intensifying lens is defined by (10−(n2±iλα22π)1) given where λ is the wavelength, n2 is the quadratic refractive index coefficient, i is the imaginary unit and Δz is an infinitesimal beam propagation length in the OPO crystal.

[0027] Fig. Figure 2 shows a further embodiment of the laser radiation source 10, in which the pump laser 1 comprises a laser diode 1', optionally with fiber amplifiers, and a main fiber amplifier downstream of the laser diode 1'. The pump radiation 7 generated by the laser diode is amplified by the main fiber amplifier 10. The amplified pump radiation 2 is then guided through the beam shaping optics 3, which transforms the beam into an OPO pump laser beam 4, which enters the OPO 5 and is output as output radiation 6.The pulse shaping achieved in this way within a fiber amplifier system as a pump laser, in which pulses are generated by a laser diode and amplified to the required pulse energies in at least one fiber amplifier, allows the oscillation threshold of the OPO to be varied during a pump pulse so that it occurs as early as possible in the pump pulse's temporal profile. This can be achieved, for example, by generating a rectangular pump pulse profile or a largely rectangular pump pulse with an initial overshoot. This not only improves the conversion efficiency but also significantly broadens the emission spectrum, since the improved phase matching conditions and the gain lens effect occur over a larger temporal fraction of the pump pulse compared to, for example, a Q-switched pump laser with a largely Gaussian temporal pump pulse profile.

[0028] Fig.Figure 3 shows another embodiment of the laser radiation source 10, in which the pump radiation 7 generated by the laser diode 1' is passed through an enhanced spontaneous emission filter ASE 8, which produces filtered radiation 9. This filtered radiation 9 is then finally passed to the main amplifier 10. The ASE 8 enables an increase in the pump pulse energy and facilitates the achievement of the condition |α2|∗wp2>35 m−1. In all illustrated embodiments, the beam shaping optics 3 are used to focus or bundle the pump laser beam 2 and to broaden the phase matching of the pump laser beam. This results in a spectrally broadband phase matching and broadband emission in the mid-infrared range, particularly in the region of the atmospheric transmission windows around 2 µm and in the range from 3 µm to 5 µm. The laser radiation source thus created is compact and robust, making it particularly suitable for applications in the field of spectroscopy or optronic countermeasures.

[0029] The features disclosed in the foregoing description, figures and claims can be important for the realization of the invention in its various embodiments, both individually and in any combination. Reference symbol list 1 pump laser 1' Laser diode 2 pump laser beam 3 Beam shaping optics 4 OPO pump jet 5 OPO 6 OPO output radiation 7 pump radiation to be amplified 8 filters 9 filtered radiation 10 main amplifiers QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 19745785 A1

[0004]

Claims

[1] Laser radiation source (10) comprising a pump laser (1) and an optical parametric oscillator (OPO (5)), characterized by a beam shaping optic (3) arranged between the pump laser (1) and the OPO (5), which is designed such that a pump laser beam (2) generated by the pump laser (1) is guided through the OPO (5) forming an intensifying lens in the OPO (5) such that the following condition is met: |α2|∗wp2>35 m−1; where w p the pump jet radius and α2 the square coefficient of the amplifying lens. [2] Laser radiation source (10) according to claim 1, wherein a transfer matrix of the intensifying lens is formed by (10−(n2±iλα22π)Δz1) is given where λ is the wavelength, n2 is the quadratic refractive index coefficient, i is the imaginary unit and Δz is an infinitesimal ray propagation length in the OPO. [3] Laser radiation source (10), in particular according to claim 1 or 2, comprising a pump laser (1) and an optical parametric oscillator (OPO (5)), characterized by a beam shaping optic (3) arranged between the pump laser (1) and the OPO (5), which is configured to focus or bundle a pump laser beam (2) generated by the pump laser (1) into the OPO (5). [4] Laser radiation source (10) according to one of the preceding claims, wherein the beam shaping optics (3) is further configured to generate at least one phase front curvature in order to broaden the phase matching of the pump laser beam. [5] Laser radiation source (10) according to one of the preceding claims, wherein the beam shaping optics (3) is designed as a focusing lens or as a telescope. [6] Laser radiation source (10) according to one of the preceding claims, wherein the beam shaping optics (3) is further configured to modify the transverse pump beam profile in such a way that the oscillation threshold of the OPO (5) can be achieved over a larger beam cross-section, for example by means of a super-Gaussian or rectangular profile. [7] Laser radiation source (10) according to one of the preceding claims, wherein the pump laser (1) comprises a laser diode (1') and at least one fiber amplifier (10) and is configured to modify the oscillation threshold of the OPO (5), for example by generating a rectangular pump pulse profile. [8] Laser radiation source (10) according to claim 7, wherein the pump laser (1) can be operated by means of an operating current such that varying the operating current results in a temporal shift of the central wavelength of the laser diode (1'). [9] Laser radiation source (10) according to claim 7 or 8, wherein the pump laser (1) further comprises at least one filter for enhanced spontaneous emission (ASE (8)), wherein in particular the ASE (8) is arranged in the beam propagation direction in front of a main fiber amplifier (10). [10] Method for generating laser radiation (6) in the infrared range, in particular in the range of the atmospheric transmission windows around 2 µm and / or in the range of 3 µm to 5 µm, using a laser radiation source (10) designed according to one of the preceding claims.

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