Abstract
The invention relates to a MOPA laser system having at least one laser oscillator (MO), which generates laser radiation at an emission wavelength (.sub.0), and having an optical amplifier (PA) downstream the laser oscillator (MO) in the propagation direction of the laser radiation, which optical amplifier amplifies the laser radiation and thereby spectrally broadens it to a useful bandwidth (). It is an object of the invention to provide an improved MOPA laser system which is designed for a high power of the amplified laser radiation and which is insensitive to back-reflection. Unavoidable back-reflections should neither affect the output power of the optical amplifier (PA), nor lead to the destruction of the laser oscillator (MO) or other components of the system. This object is achieved by the invention in that an optical bandpass filter (BPF) is arranged between laser oscillator (MO) and amplifier (PA), which optical bandpass filter is transparent to laser radiation at the emission wavelength (.sub.0), wherein those spectral components of the returning, that is, counter to the propagation direction, laser radiation impinging on the bandpass filter (BPF), which, in terms of wavelength, lie outside the passband (4), are reflected at the bandpass filter (BPF) in the propagation direction.
Claims
1. A laser system comprising at least one laser oscillator (MO), wherein said laser oscillator generates laser radiation at an emission wavelength (.sub.0), and at least one optical amplifier (PA) downstream of the laser oscillator (MO) in the propagation direction of the laser radiation, wherein said optical amplifier amplifies the laser radiation and thereby spectrally broadens it to a useful bandwidth (), wherein said laser system further comprises: an optical bandpass filter (BPF) arranged between said laser oscillator (MO) and said optical amplifier (PA), wherein said optical bandpass filter is transparent to laser radiation at the emission wavelength (.sub.0), and a spectral passband of the optical bandpass filter is smaller than the useful bandwidth (), wherein those spectral components of the returning, that is, counter to the propagation direction, laser radiation impinging on the bandpass filter (BPF), which, in terms of wavelength, lie outside the passband, are reflected at the bandpass filter (BPF) in the propagation direction.
2. The laser system according to claim 1, wherein the passband of the bandpass filter (BPF) is essentially equal to the spectral bandwidth of the laser radiation generated by the laser oscillator (MO).
3. The laser system according to claim 2, wherein the useful bandwidth () is at least twice as large, preferably at least five times as large, more preferably at least ten times as large as the spectral bandwidth of the laser radiation generated by the laser oscillator (MO).
4. The laser system according to claim 3, wherein the spectral bandwidth of the laser radiation generated by the laser oscillator (MO) is less than 0.01 nm to 1 nm, wherein the useful bandwidth () is at least 3 nm.
5. The laser system according to claim 1, wherein two or more laser oscillators (MO1, MO2) are provided, which generate laser radiation at a respectively different emission wavelength (.sub.1, .sub.2), wherein the bandpass filter (BPF) has two or more passbands, the spectral positions and widths of which are adapted to the emission spectra of the laser oscillators (MO1, MO2), wherein a combination element (WDM) is provided in the propagation direction in front of the amplifier (PA), the combination element combining the laser radiation of the two or more laser oscillators (MO1, MO2) into a laser beam.
6. The laser system according to claim 1, wherein the laser oscillator (MO) is coupled to an optical fiber and the amplifier (PA) is a fiber amplifier, wherein the bandpass filter (BPF) is integrated into the fiber path between the output of the laser oscillator (MO) and the input of the amplifier (PA).
7. The laser system according to claim 6, wherein the bandpass filter (BPF) is formed by two or more fiber Bragg gratings arranged one behind the other in the propagation direction, the Bragg wavelengths of which lie outside the emission wavelength (.sub.0) of the laser oscillator (MO) and within the useful bandwidth ().
8. The laser system according to claim 1, wherein the bandpass filter (BPF) is a dielectric multilayer filter.
9. The laser system according to claim 1, wherein the amplifier (PA) is power-modulated.
10. The laser system according to claim 3, wherein the spectral bandwidth of the laser radiation generated by the laser oscillator (MO) is less than 0.01 nm to 1 nm, wherein the useful bandwidth () is at least 10 nm.
Description
(1) The invention is explained in mere detail below with reference to the drawings. Embodiments of the invention are shown schematically in the drawings. Shown are:
(2) FIG. 1 MOPA laser system for material processing according to the prior art;
(3) FIG. 2 a laser system according to the invention in a first embodiment;
(4) FIG. 3 illustration of spectral properties of the optical bandpass filter according to the invention;
(5) FIG. 4 laser system according to the invention in a second embodiment.
(6) FIG. 1 schematically shows a MOPA laser system according to the prior art as a block diagram. The system comprises a laser oscillator MO which generates laser radiation at an emission wavelength. The generated laser radiation is supplied to an optical amplifier PA, which is, for example, an optically pumped, fiber amplifier doped with rare earth ions. The optical amplifier PA amplifies the laser radiation. At the same time, a nonlinear spectral broadening of the laser radiation to a useful bandwidth occurs in the optical amplifier PA, which broadening is typically significantly greater than the emission bandwidth of the laser oscillator. The amplified laser radiation travels in the propagation direction (indicated in FIG. 1 by the light arrows pointing to the right) through transmission optics (beam delivery optics) BDO, until it gets to the workpiece OBJ to be processed. For example, the BDO transmission optics can be an optical transport fiber. This can be adapted with respect to dispersion to the other components of the laser system in order to achieve, for example, optimal pulse quality on the workpiece OBJ in the transmission of laser pulses. During machining, a part of the incident laser radiation is reflected at the workpiece OBJ. This is indicated by the dark, left-pointing arrows in FIG. 1. This back-reflected laser radiation passes through the system counter to the propagation direction. First, the back-reflected laser radiation passes through the transmission optics BDO and then the optical amplifier PA. The back-reflected laser radiation is amplified in the optical amplifier PA. The amplified back-reflected laser radiation then gets into the laser oscillator MO. Fluctuations in the output power of the laser system occur due to the amplification of the back-reflected laser radiation in the optical amplifier PA, since energy is extracted from the medium of the optical amplifier PA by the amplification of the back-reflected laser radiation. The amplified back-reflected laser radiation getting into the laser oscillator MO can cause damage there, since the optical components of the laser oscillator MO and other optical components, which are arranged in front of the optical amplifier PA, are not designed for high powers.
(7) FIG. 2 schematically shows a MOPA laser system according to the invention. In this case, the laser oscillator MO is designed as a fiber oscillator. This comprises an optical fiber as a laser medium. A rear highly reflective grid HR and a front low reflective grid LR form the laser resonator. The laser radiation generated by the laser oscillator MO at the emission wavelength arrives at a fiber-integrated bandpass filter BPF over a fiber path. The bandpass filter BPF is transparent to the laser radiation at the emission wavelength both in and counter to the propagation direction, wherein the spectral bandpass of the bandpass filter smaller than the useful bandwidth on which the laser radiation in the subsequent optical amplifier PA is broadened compared to the emission bandwidth of the laser oscillator MO. Back-reflected laser radiation, that is, laser radiation propagating to the left from the workpiece OBJ in FIG. 2 through the transmission optics BDO and the fiber amplifier PA outside the passband of the bandpass filter, is reflected at the bandpass filter and then travels again in the propagation direction (to the right in FIG. 2) through the optical amplifier PA, is amplified in this and then gets through the transmission optics BDO back to the workpiece OBJ. In this way, the bandpass filter BPF effects an isolation against back-reflection. Back-reflections only get back into the laser oscillator MO in the passband of the bandpass filter which is narrow compared to the useful bandwidth. The power of this small proportion of the total back-reflected laser radiation is correspondingly low, so that no damage occurs in the laser oscillator. The laser radiation passing through the bandpass filter BPF in the back direction in the region of the emission wavelength is reflected at the rear reflector HR of the laser resonator of the laser oscillator MO and then likewise passes through the laser system in turn in the propagation direction. Optionally, an optical isolator of known design (not shown) designed for correspondingly small powers can be arranged between laser oscillator MO and BPF if it should be necessary to protect the laser oscillator as completely as possible against back-reflected radiation.
(8) FIG. 3 illustrates the functional principle of the bandpass filter BPF according to the invention. The diagram shows the emission line 1 of the laser oscillator MO at the wavelength .sub.0. Reference numeral 2 denotes the spectrum of the laser radiation amplified in the optical amplifier PA and spectrally broadened to the useful bandwidth . Reference numeral 3 denotes the reflection spectrum of the bandpass filter BPF. It can be seen that the bandpass filter BPF has a passband 4 adapted to the emission line 1 of the laser oscillator MO. The laser radiation is reflected at the bandpass filter BPF outside the passband 4. On the basis of the overlap of the spectrum 2, which corresponds to the spectrum of the back-reflected laser radiation, with the reflection spectrum 3, it becomes clear that the majority of the back-reflected laser radiation is reflected at the bandpass filter BPF and thus can not arrive at the laser oscillator MO.
(9) FIG. 3 schematically shows the spectral properties of the radiation and the bandpass filter. It should be noted that the spectral characteristics of the bandpass filter BPF can also be asymmetric in adaptation to the occurring spectra of the laser radiation, for example, when the non-linear spectral broadening mechanism of the optical amplifier PA is asymmetric to the emission wavelength .sub.0 (for example, in Raman processes). In any case, the bandpass filter should be adapted as far as possible to the bandwidth and spectral position of the back-reflected laser radiation, insofar as these differ from the spectrum of the laser radiation of the laser oscillator.
(10) A fiber laser can be used as a laser oscillator MO, as illustrated in FIG. 2. In principle, however, any CW or pulsed laser or a light-emitting diode (LED) of low bandwidth can be considered. The laser oscillator can be amplitude and/or phase modulated. The master oscillator MO can also be an ASE source which amplifies electromagnetic radiation resulting from spontaneous emission and optionally spectrally truncates it.
(11) In the exemplary embodiment shown in FIG. 4, two laser oscillators MO1, MO2 are provided which generate laser radiation at a different emission wavelength. The laser oscillator MO1 has the emission wavelength .sub.1, the laser oscillator MO2 has the emission wavelength .sub.2. The laser radiation of the two laser oscillators MO1, MO2 is combined by means of a combination element WDM into a single laser beam, which then, as in FIG. 2, propagates through an optical fiber to the bandpass filter BPF according to the invention.
(12) FIG. 4 schematically shows the reflection spectrum of the bandpass filter BPF in this exemplary embodiment. It can be seen that the bandpass filter BPF has two narrow passbands 4 at the emission wavelengths .sub.1, .sub.2. The laser radiation is reflected at the bandpass filter BPF in the remaining regions 3. The emission at two different wavelengths .sub.1, .sub.2 can be utilized in order to broaden the bandwidth of the laser radiation to the useful bandwidth in the optical amplifier PA via four-wave mixing in a particularly efficient manner. The two laser oscillators MO1, MO2 can preferably be phase modulated to suppress further undesired effects, such as stimulated Brillouin scattering. Suitable narrowband sources as laser oscillators MO1, MO2 can be easily realized as diode lasers with external resonator (ECDL) or as spectrally truncated ASE sources.
