Generating synchronized laser pulses at variable wavelengths

Abstract

The invention relates to an apparatus for generating laser pulses. It is an object of the invention to provide a method for generating synchronized laser pulse trains at variable wavelengths (e.g., for coherent Raman spectroscopy/microscopy), wherein the switching time for switching between different wavelengths should be in the sub-μs range. For this purpose the apparatus according to the invention comprises a pump laser (1), which emits pulsed laser radiation at a specified wavelength, an FDML laser (3), which emits continuous wave laser radiation at a cyclically variable wavelength, and a nonlinear conversion medium (4), in which the pulsed laser radiation of the pump laser (1) and the continuous wave laser radiation of the FDML laser (3) are superposed. In the nonlinear conversion medium (4) the pulsed laser radiation of the pump laser (1) and the continuous wave laser radiation of the FDML laser (3) are converted in an optical parametric process into pulsed laser radiation at a signal wavelength and an idler wavelength that differs therefrom. Furthermore the invention relates to a method for generating laser pulses.

Claims

1. An apparatus for generating laser pulses, comprising: a pump laser, which emits pulsed laser radiation at a specified wavelength, an FDML laser, which emits continuous wave laser radiation at a cyclically variable wavelength, and a nonlinear conversion medium, in which the pulsed laser radiation of the pump laser and the continuous wave laser radiation of the FDML laser are superposed, whereby signal and idler pulses at different wavelengths are formed in an optically parametric process within a gain and phase matching range.

2. The apparatus as claimed in claim 1, wherein an optical amplifier is arranged in the beam path between the pump laser and the nonlinear conversion medium, which optical amplifier amplifies the laser radiation of the pump laser.

3. The apparatus as claimed in claim 1, wherein the nonlinear conversion medium is a microstructured optical fiber, a fundamental-mode fiber, a multimode fiber, a periodically polarized birefringent crystal, a birefringent crystal, a hollow-core fiber filled with nobel gas, a kagome fiber filled with nobel gas or a “negative curvature” fiber filled with nobel gas.

4. The apparatus as claimed in claim 1, wherein the pump laser emits laser pulses with a repetition rate in the range of 1 kHz to 1 GHz and a pulse duration in the range of 1 μs to 10 fs.

5. A method for generating laser pulses, comprising least the following steps: generating pulsed laser radiation at a specified wavelength with a pump laser, generating continuous wave laser radiation at a cyclically variable wavelength with an FDML laser, and superposing the pulsed laser radiation of the pump laser and the laser radiation of the FDML laser in a nonlinear conversion medium, wherein signal and idler pulses at different wavelengths are formed in an optically parametric process within a gain and phase matching range.

6. The method as claimed in claim 5, wherein the optical parametric process is based on difference frequency generation or on four wave mixing.

7. The method as claimed in claim 5, wherein the pulsed laser radiation of the pump laser before the superposition with the continuous wave laser radiation of the FDML laser is amplified by an optical amplifier.

8. The method as claimed in claim 5, wherein the repetition rate of the laser pulses of the pump laser is in the range of 1 kHz to 1 GHz and the pulse duration is in the range of 1 μs to 10 fs.

9. The method as claimed in claim 5, wherein the frequency of the cyclical wavelength change of the FDML laser is equal to an integer multiple of the repetition rate of the laser pulses of the pump laser.

10. The method as claimed in claim 5, wherein the frequency of the cyclical wavelength change of the FDML laser is not equal to an integer multiple of the repetition rate of the laser pulses of the pump laser.

11. The method as claimed in claim 5, wherein the difference of the frequency of the cyclical wavelength change of the FDML laser and the repetition rate of the laser pulses of the pump laser is equal to an integer multiple of the frequency of the cyclical wavelength change of the FDML laser.

12. A method, comprising: using an apparatus as claimed in claim 1 as a light source for generating synchronized laser pulse trains of variable wavelength in coherent Raman spectroscopy or microscopy.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Exemplary embodiments of the invention will be explained in more detail below with reference to the drawings. In the figures:

(2) FIG. 1 shows an apparatus according to the invention as a block diagram;

(3) FIG. 2 shows an illustration of the method according to the invention in a first operating mode;

(4) FIG. 3 shows an illustration of the method according to the invention in a second operating mode;

(5) FIG. 4 shows an illustration of the method according to the invention in a variation of the second operating mode;

(6) FIG. 5 shows an illustration of the method according to the invention in a third operating mode.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

(7) FIG. 1 schematically shows an apparatus according to the invention as a block diagram. It comprises a pump laser 1, e.g. a mode-coupled fiber laser, which generates laser pulses with a duration of approx. 30 ps at a repetition rate of approx. 20 MHz. The wavelength of the pulsed laser radiation is fixedly specified. The pulsed laser radiation of the pump laser 1 is amplified by an optical amplifier 2, e.g. a diode-pumped amplifier fiber. Furthermore the apparatus comprises an FDML laser 3, which generates continuous wave laser radiation at a cyclically variable wavelength. The output of the optical amplifier 2 and the output of the FDML laser 3 are connected via optical fibers to a nonlinear conversion medium 4, e.g. a microstructured optical fiber, so that the amplified laser pulses of the pump laser 1 and the laser radiation of the FDML laser 3 are superposed in the nonlinear conversion medium 4. In these the pulsed laser radiation of the pump laser 1 and the continuous wave laser radiation of the FDML laser 3 are converted in an optical parametric process, e.g. by four wave mixing, into pulsed laser radiation at a signal wavelength and an idler wavelength that differs therefrom, which exits the apparatus at the output of the nonlinear medium 4 (in FIG. 1 to the right) and can be used for e.g. CRS microscopy. On the basis of the cyclical change of the wavelength of the radiation of the FDML laser 3, the central wavelength of the signal or idler pulses is varied from pulse to pulse within the amplification bandwidth of the parametric process and according to the tuning range of the FDML laser 3.

(8) In the diagrams of FIG. 2-5 the cyclical variation of the continuous wave laser radiation of the FDML laser 3 in the wavelength range between λmin and λmax is illustrated as a function of time (in units of the inverse frequency f.sub.FDML of the cyclical wavelength change of the FDML laser 3). The vertical, dashed lines show the sampling of the radiation of the FDML laser 3 with the radiation of the pump laser 1 in the nonlinear conversion medium 4 at the frequency f.sub.pump at the corresponding discrete time.

(9) In the example shown in FIG. 2 the frequency of the cyclical wavelength change of the FDML laser is equal to the repetition rate of the laser pulses of the pump laser. So f.sub.FDML−f.sub.pump=0 is true. Then the same spectral part is always selected by the pump pulses from the radiation of the FDML laser 3, which is cyclically variable with respect to the wavelength. Thus a signal pulse train and an idler pulse train with a time-constant central wavelength in each case are formed. The phase difference between the tuning cycle of the FDML laser 3 and the pulse repetition of the pump laser 1 thereby determines the central wavelengths of the signal and idler pulses.

(10) In FIG. 3 the frequency of the cyclical wavelength change of the FDML laser 3 is not equal to an integer multiple of the repetition rate of the laser pulses of the pump laser 1. So f.sub.FDML−n.Math.f.sub.pump≠0 is true (where n is a natural number). Thereby the continuous wave laser radiation of the FDML laser 3, which varies cyclically with respect to the wavelength, is sampled with the pump repetition rate f.sub.pump, wherein every emerging signal and idler pulse forms at a different central wavelength.

(11) FIG. 4 shows a variation of the operating mode presented in FIG. 3. In FIG. 4, n.Math.f.sub.FDML−m.Math.f.sub.pump=0 is true (where n and m are natural numbers). Specifically, n=2 and m=3 in the presented case. In this case three different signal and idler wavelengths result, consecutively in time.

(12) In the operating mode shown at the end in FIG. 5, the difference of the frequency of the cyclical wavelength change of the FDML laser and the repetition rate of the laser pulses of the pump laser is equal to an integer multiple of the frequency of the cyclical wavelength change of the FDML laser. So f.sub.FDML−f.sub.pump=n.Math.f.sub.FDML is true. In this operating mode n different central wavelengths of the FDML radiation are chosen and successively converted into signal and idler pulses. In this way, for the presented case with n=2, two wavelengths are selected alternately.

(13) In the diagrams of FIG. 2-5 the temporal profile of the wavelength of the radiation of the FDML laser has a sawtooth-shaped characteristic. Another curve profile is readily conceivable, without thereby changing something about the functional principle of the invention. In FIG. 3 the sawtooth-shaped characteristic ensures that the successively selected wavelengths are equidistant.