Abstract
The present invention relates to a system and method for generating wavelength-tunable laser output pulses using parametric processes, wherein a simultaneous and tuned tuning of the pump pulse wavelength and repetition rate ensures a temporal overlap between pump and seed pulses in a parametric gain medium. Based on this parameter coupling, lasers with a wide tunable wavelength range can be obtained, which can be fully fiber-based and which are also suitable for modern nonlinear microscopy or fluorescence microscopy due to a particularly fast response.
Claims
1. A system for generating laser pulses of variable wavelength, comprising at least: a pump pulse generator for generating pump pulses; an optically parametric amplification medium pumped through the pump pulse generator, which converts incoming pump pulses, stimulated by further seed pulses, into wavelength shifted signal and idler pulses; and an optically dispersive element wherein the wavelength of the pump pulses can be adjusted either as such or via the repetition rate of the pump pulse generator, and wherein, when either the repetition rate or the wavelength is changed, the other parameter changes by means of the optically dispersive element simultaneously in such a way that pump and seed pulses in the optically parametric amplification medium overlap in time.
2. (canceled)
3. The system according to claim 2, wherein the optically dispersive element is selected from the group consisting of chirped Bragg structures, gratings, prisms or combinations thereof.
4. The system according to claim 2, wherein the optically dispersive element is a chirped Bragg grating.
5. The system according to claim 1, where the change of the wavelength of the pump pulse generator is done by an adjustable spectral filter.
6. The system according to claim 1, wherein at least a part of the optically dispersive element is arranged outside the pump pulse generator.
7. The system according to claim 1, wherein the repetition rate of the pump pulse generator is an integer multiple of the repetition rate of the seed pulses.
8. The system according to claim 1, wherein both the parametric amplification and the seed pulse generation are performed in an optically parametric oscillator (OPO).
9. The system according to claim 1, wherein the parametric amplification is carried out in an optically parametric amplifier (OPA), the seed pulses are generated from a portion of the pump pulses and these seed pulses are delayed in time by the inverse of the repetition rate of the pump pulses before being superimposed with the pump pulses and are thus superimposed with a pump pulse following from the pump pulse train.
10. The system according to claim 2, wherein the repetition rate of the pump pulse generator is greater than or equal to 5 MHz and less than or equal to 80 MHz and the group velocity dispersion of the optically dispersive element is less than or equal to 15 ps2 and greater than or equal to 0.5 ps2.
11. A method for changing the wavelength of laser pulses using an optically parametric gain medium comprising at least the steps of a) Generation of pump pulses with defined wavelength and defined repetition rate, b) Generation of seed pulses, c) Superposition of the seed and pump pulses in the optically parametric amplification medium, wherein when the wavelength or the repetition rate of the pump pulse changes, the respective other parameter changes by means of an optical dispersion simultaneously in such a way that an overlap in time of the pump and seed pulses in the optically parametric amplification medium is ensured.
12. The method according to claim 11, wherein the simultaneous change of wavelength and repetition rate of the pump pulses is affected by means of a dispersive element.
13. The method according to claim 11, wherein the change in wavelength of the laser pulses is affected by a simultaneous variation of pump pulse wavelength and repetition rate within less than or equal to 100 ms.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description and the accompanying drawings, in which:
[0050] FIG. 1 a system for generating output laser pulses with a tunable wavelength based on parametric amplification according to a preferred embodiment of the invention in the form of a pump pulse generator (9), an amplifier (10) and a fiber-based OPO;
[0051] FIG. 2 a further system for generating output laser pulses with a tunable wavelength based on a parametric gain according to a preferred embodiment of the invention in the form of a pump pulse generator (9), an amplifier (10) and a fiber-based OPO;
[0052] FIG. 3 a pump pulse generator with suitable coupling of repetition rate to wavelength according to a preferred embodiment of the invention;
[0053] FIG. 4 another pump pulse generator with suitable coupling of repetition rate to wavelength according to a preferred embodiment of the invention;
[0054] FIG. 5 another pump pulse generator with suitable coupling of repetition rate to wavelength according to a preferred embodiment of the invention;
[0055] FIG. 6 an example of the change of the repetition rate of an ytterbium master oscillator (Yb-MO, dotted line) at the change of wavelength compared to the repetition rate required to pump a FOPO based on a commercial photonic crystal barrel (dotted line). Also shown is the resultant repetition rate change in dispersion matching of a Yb-MO with a chirped fiber bragg grating C-FBG (solid line);
[0056] FIG. 7 an experimental implementation of the appropriate coupling of repetition rate to wavelength. Shown are the theoretical curves from FIG. 6 (YbMo-CFBG solid line, YbMo dashed line) and the experimentally measured values of a YbMo with a C-FBG in the resonator (crosses);
[0057] FIG. 8 is an example of the tuning curve of a FOPO (signal and idler wavelength in dependence of the central pump wavelength) realized with a commercial photonic crystal fiber (NKT-Photonics LMAS). The solid lines show the theoretical curve and the crosses the experimentally achieved values. The tuning was done purely by electronically detuning a filter in the pump pulse generator; with a normal FOPO, the resonator length would have had to be mechanically altered by 10.8 cm by a delay bar;
[0058] FIG. 9 the dependence of the temporal overlap in a FOPA of pump pulse and a chirped seed pulse on the repetition rate fr1 in a) and fr2 in b), if the seed pulse is generated by a preceding pump pulse and delayed by a fixed time i to this.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0059] FIG. 1 shows a preferred design of the laser system. The pump pulse generator (9) is brought to the required peak power (e.g. 1 kW) by an optical amplifier chain (10) before its output beam is coupled into the FOPO. The pump pulses are superimposed with seed pulses via a WDM coupler (11) in a parametric amplifier fiber (12). The amplified pulses (signal, idler or preferably both, as well as the remaining pump pulses) are coupled out (e.g. 80%) via a fiber-based coupler (13) and the rest is fed back via a feedback fiber (14).
[0060] FIG. 2 shows another preferred version of the laser system based on an OPA. The pulses generated in the pump pulse generator (9) in a dispersion-matched manner are divided by a coupler (15) into an arm for pump pulse amplification (with fiber amplifier chain (10)) and an arm for seed pulse generation. The seed pulse is preferably generated by supercontinuum generation (16), if necessary by use of optical amplifiers. Other methods of seed pulse generation, e.g. stimulated Raman scattering, are also possible. The seed pulse is superimposed by a delay fiber (17) and a WDM coupler (11) with a pump pulse following in the pump pulse train and then amplified in a parametric amplifier fiber (12). The coupling of the repetition rate of the pump pulse to the wavelength of the pump pulse is chosen to match the dispersion in the seed gut (see FIG. 8), taking into account the dispersion acting on the pump pulses in the amplifier arm (10). In comparison to the FOPO in FIG. 1, the entire seed generating arm takes the role of the FOPO resonator after the splitting of the pump pulse, and the dispersion of the entire pump pulse amplifying arm must be added to the dispersion within the pump pulse generator.
[0061] FIG. 3 schematically shows a passive mode-locked fiber laser as pump pulse generator for use in the system of the invention. The fiber laser can be constructed entirely in fiber optics technology, in particular polarization-maintaining glass fibers, and is pumped by a fiber-coupled pump diode (1), which provides the amplification in the actively doped amplifier fiber (2) shown as thick dashed lines. The amplifier fiber can be an ytterbium-doped polarization-maintaining glass fiber. Inside the resonator an electronically tunable bandpass filter (3) can be used to control the emitted wavelength. The saturable absorber mirror (4) used is preferably applied directly to a glass fiber. The second end mirror in the linear resonator is formed by the chirped fiber Bragg grating (5), which also serves as an output element. The dispersive element (5) impresses a large part of the total dispersion in the resonator on the circulating laser light and thus ensures a strongly wavelength-length-dependent period of revolution. In this way, the coupling of the repetition rate to the given wavelength can be controlled.
[0062] FIG. 4 shows another version of a laser system which uses active mode coupling. The ring resonator can consist of polarization-maintaining glass fiber. An electro-optical modulator (6) is used to start the mode coupling and control the repetition rate. The dispersion of the fiber resonator is adjusted by a C-FBG (5), which can also serve as an output mirror. Alternatively, an additional coupler can be built into the resonator. To use the reflective C-FBG in a ring resonator, a circulator (7) is used.
[0063] FIG. 5 shows a further design of a possible pump pulse generator, which uses artificial passive mode coupling to shorten the emitted pulse duration (e.g. below 50 ps or below 10 ps at repetition rates of e.g. 50 MHz or 10 MHz). The nonlinear amplifying loop mirror (NALM) used for this purpose consists of an amplifier fiber (2) made of passive polarization-maintaining glass fiber pumped through the pump diode (1). It is connected to the actual resonator ring by a fiber-based coupler (8). The coupler can preferably provide an asymmetrical splitting ratio (e.g. 45/55%). The wavelength can be controlled by an electronically tunable bandpass filter. The circulator (7) used here additionally ensures a clockwise direction of rotation in the left fiber ring and blocks light components reflected from the NALM. The dispersion of the fiber resonator is again adjusted by a C-FBG (5). It can be advantageous to use an amplifier fiber with its own pump diode in the main ring of the resonator as well, e.g. to adjust the power in the resonator independently of the gain in the NALM.
[0064] FIG. 6 shows as an example a theoretical calculation of the adjustment of the resonator dispersion of an active-mode-coupled pump pulse generator (Yb-MO) according to the invention to ensure simultaneous gain and time overlap in a FOPO resonator consisting of a 50 cm long parametric amplifier fiber (LMA-PM-5) and feedback fiber (PM-980). The X-axis shows the pump wavelength versus the repetition rate. A typical active-mode coupled ytterbium fiber laser with a repetition rate of about 10 MHz changes its repetition rate by only <5 kHz when the pump pulse wavelength changes from 1020 nm to 1050 nm due to the dispersion of commonly used optical fibers. However, the signal wavelengths corresponding to the pump pulse wavelengths at the maximum parametric gain (in the range from about 750 nm to about 950 nm) show different repetition rates by about 40 kHz in a FOPO based on a parametric amplifier fiber (LMA-PM-5) and a feedback fiber (PM-980) (curve labeled FOPO). The use of an additional dispersive element in the form of a C-FBG with 2=+6.91 ps.sup.2 and 3=0.018 ps.sup.3 in the pump pulse generator causes an adjustment of the repetition rate in the pump pulse generator, shown by the solid line, thus ensuring a temporal overlap of seed and pump pulses in the parametric amplifier fiber when the pump wavelength changes.
[0065] FIG. 7 shows an experimental implementation of the scenario shown in FIG. 6. On the X-axis the pump wavelength is plotted against the theoretically calculated repetition rate for a Yb-MO and a Yb-MO adjusted with a C-FBG with 2=+6.91 ps.sup.2, 3=0.018 ps.sup.3. The crosses show the measured repetition rate of a version of such an adapted pump pulse generator as a function of the electronic tuning of the wavelength.
[0066] FIG. 8 shows an example of the tuning curve of a FOPO based on a commercial photonic crystal fiber (LMA-PM-5) and a pump pulse generator adapted according to the invention (adaptation result is described in FIG. 7). For tuning, only the wavelength of the pump pulses was electronically detuned. No mechanical (re-)adjustment or length adjustment of the system took place. The electronically adjusted wavelength of the pump pulse generator on the X-axis and the wavelengths of the signal and idler radiation on the Y-axis are plotted. The solid lines show the theoretically expected course and the crosses the experimentally achieved results. With the used repetition rate of the pump laser, either the length of the FOPO or that of the pump laser resonator would have had to be changed by approx. 10.8 cm during the tuning process in an ordinary FOPO system, e.g. by the mechanical method of a delay rail by 5.4 cm.
[0067] FIG. 9 shows exemplarily and schematically in a delayed-FOPA the dependence of the time overlap of the pump (20) and seed (21) pulses on the repetition rate. In this scheme, broadband and strongly chirped seed pulses (21) were generated by a preceding pump pulse (20) and experienced an additional, but repetition rate-independent delay i (e.g. by the seed generation arm in FIG. 2). The seed pulses (21) stretched by the Dispersi-on are now superimposed by the following pump pulses (20). If the repetition rate of the pump pulses (20) changes from fr1 to fr2, the time point (relative to the preceding pump pulses (20)) of the arrival of the following pulses in the amplifying medium changes, but not the time point of the arrival of the seed pulses (21). The repetition rate thus determines the time overlap between chirped seed pulses (21) and the pump pulses (20).
REFERENCE CHARACTER LIST
[0068] 1 Pump diode [0069] 2 Amplifier fiber [0070] 3 Electronically tunable spectral filter [0071] 4 Saturable absorber mirror [0072] 5 Chirped Bragg grating [0073] 6 Modulator [0074] 7 Circulator [0075] 8 Coupler [0076] 9 Pump pulse generator (dispersion adapted) [0077] 10 Optical fiber amplifier [0078] 11 WDM coupler [0079] 12 Parametric amplifier fiber [0080] 13 decoupler [0081] 14 Feedback fiber [0082] 15 Coupler [0083] 16 Seed pulse generation [0084] 17 Retardation fiber [0085] 20 Pump pulse [0086] 21 Seed pulse
