Patent Application
20140002893
The invention relates to a fiber amplifier system for amplifying and emitting pulsed radiation, having a master source, which emits pulsed output radiation, and at least one amplifier stage, which is arranged after the master source in the direction of radiation, and which amplifies the output radiation.
Fiber amplifier systems of this kind, so-called Master Oscillator Power Amplifier (MOPA) systems with flexible pulse forms within a range of nanoseconds, represent attractive resources both for industrial and scientific applications. Amongst others, interesting systems are those in which all parameters such as repetition rate, pulse form, pulse energy, and pulse duration can be influenced by means of a master source. It opens up an extremely broad field of application. In micro-material machining, laser parameters can be adapted to a given material to be machined or to the desired machining technique, for example drilling, welding, ablating or trepanning. Moreover, fiber-based systems become particularly interesting, because they can be set-up in an integrated-optical and adjustment-free manner, and are able to render high average performance. Even on generating EUV radiation, a fiber amplifier system of the initially described kind is of great interest.
Usually, a fiber-based oscillator power amplifier (MOPA) system is comprised of a master source and several successively arranged amplifier stages based on rare-earth doped fibers. Between the master source and the successive amplifier stage and/or between successive amplifier stages, it is possible to arrange a filtering element which, for example, is a spectral filter or a polarization filter.
To be able to generate arbitrary pulse forms, in most cases a fiber-coupled Fabry-Perot laser diode is applied as master source in prior art. Such a Fabry-Perot laser diode disposes of a nearly linear power output characteristic accompanied by sufficiently fast rise and decay times. By the aid of a function generator and driver electronics, any arbitrary pulse form can thus be generated.
A problematic issue with laser diodes of this type is the development of extremely narrow-band, isolated lines in the emission spectrum if the laser diode is controlled with a slowly rising pulse. These narrow-band lines are individual longitudinal modes which with a rising pump current reach in isolated form over to the laser threshold and dominate the spectrum on account of gain competition. By means of the successive amplifier stage, these longitudinal modes are amplified to pulses with a very high spectral intensity. From a certain pulse peak rate on, it leads to a stimulated Brillouin scattering (SBS). Since the stimulated Brillouin scattering runs contrary to the propagation direction of the laser pulse, the backscattered portion gets lost for the output pulse. The resulting output pulse is hereby distorted in its temporal shape. Likewise, the backscattered portion may reach such high rates that may entail damage to the amplifier system. Hence, the stimulated Brillouin scattering is the limiting factor on increasing the pulse energy in fiber-based MOPA systems.
A stimulated Brillouin scattering occurs as a non-linear effect if inelastic interactions occur between photons and acoustic phonons of the fiber material. The energy of the acoustic phonons reduces the energy of the pump photons which leads to a shifting of optical frequencies of the output radiation in the backscattered portion. An amplification of the Brillouin scattering rises exponentially with the length of the fiber, whereas the amplification coefficient depends on the material of the fiber, fiber geometry, and fiber temperature. The critical performance Pcr at which a Brillouin scattering starts in a glass fiber can be assessed by applying the following formula:
P
cr=˜21 Aeff(1+ΔνS/ΔνBr)/gBrLeff
where Aeff is the effective mode area, ΔνS is the bandwidth of the output radiation, ΔνBr is the bandwidth of the stimulated Brillouin scattering, gBr is the amplification coefficient, and Leff is the effective length of the fiber.
The equation shows that the critical performance at which stimulated Brillouin scattering starts to occur, is proportionate to the ratio of the bandwidth of the output radiation ΔνS and the bandwidth of the stimulated Brillouin scattering ΔνBr. The bandwidth ΔνBr depends on the service life of the acoustic phonons, and with quartz glass and a wavelength of roughly 1 μm it amounts to approx. 10-20 MHz. The spectral bandwidth ΔνS of a spectrally transformation-limited
Gauss pulse having a pulse duration of for example 50 ns amounts to approximately 1.8 MHz. This value ranges well below the bandwidth of the stimulated Brillouin scattering ΔνBr and thus it leads to a very low critical performance rate Pcr. In contrast therewith, a spectral bandwidth of the output radiation ΔνS in the amount of 1 THz—which roughly corresponds to 4 nm bandwidth with 1 μm wavelength—would comparably increase the critical performance rate Pcr by approximately 5 orders of magnitude. The advantage of utilizing a spectrally broad-band master source for fiber amplifiers with a high performance rate thus becomes evident.
Therefore, master sources are applied in state-of-the-art technology, whose bandwidth of the signal to be amplified is chosen to be so high that the critical performance rate becomes sufficiently high to suppress stimulated Brillouin scattering for the relevant performance parameters.
But problems arise for applications with a fiber-based MOPA system because in most cases a pulsed Fabry-Perot laser diode is utilized as master source, the spectral bandwidth of which depends on the pulse form, pulse duration, and repetition rate. As has been described hereinabove, extremely narrow-band, isolated lines occur in the emission spectrum with a rising pump current, thus causing stimulated Brillouin scattering.
Furthermore, approaches have been made in prior art to modify, for example, the geometry of the fiber, material composition, temperature or mechanical tension within the fiber in such a way that the resonance condition changes simultaneously over the overall length of the fiber. This prevents a development of dominating modes, because each partial section of the fiber amplifies another frequency and thus the amplification relative to a certain frequency remains low. The increase thus achieved concerning the threshold for stimulated Brillouin scattering is nevertheless insufficient for numerous applications.
Another prior art method provides for imprinting a temporally modulated phase onto the signal of the master source by means of a phase modulator, said modulated phase leading to side bands in the frequency domain and thus to an effective broadening of the spectrum. A phase modulator in combination with an existing pulsed master source, however, is very costly.
Consequently, prior art concepts have a disadvantage in that they are constructively costly and expensive and/or fail to achieve a sufficient increase in the threshold for Brillouin scattering.
Now, therefore, it is the object of the present invention to provide a fiber amplifier system for amplifying and emitting pulsed radiation that avoids stimulated Brillouin scattering as effectively as possible and at the same time can be produced simply and inexpensively.
To achieve this object, the present invention based on a fiber amplifier system of the initially described kind proposes that the output radiation emitted from the master source is broadband and is generated substantially by spontaneous emission.
Broadband within the sense of the present invention is an output radiation which emits not only a few wavelengths. With a central wavelength of 1045 nm, this applies, for example, to a spectral bandwidth of roughly 4 nm (=roughly 1 THz). Particularly optimal would be a spectral bandwidth in a range of 10 nm (=roughly 2.5 THz).
It is an essential feature of the present invention that the output radiation is generated by means of spontaneous emission. Those problems known from prior art due to individual longitudinal modes of high intensity which cause stimulated Brillouin scattering within the amplifier stages are thus avoided.
Advantageously the master source is an LED whose end facets have an antireflective coating. Components of this type are designated hereinafter as super-luminescence diodes (SLD). They represent low-cost and robust components which dispose of a nearly linear power output characteristic and generate a broadband spectrum. Moreover, super-luminescence diodes dispose of the coupling-in properties of a laser. In contrast with a laser, however, they are not comprised of a dedicated resonator and consequently they have a low coherence length. Thus, a development of extremely narrow-band longitudinal modes in the emission spectrum is effectively prevented.
A super-luminescence diode practically is constructively identical to a Fabry-Perot diode. However, whereas a Fabry-Perot diode utilizes the Fresnel reflection of the end facet of the active semiconductor layer system in order to form a resonator with a highly reflecting layer on the opposite facet, the end facets with a super-luminescence diode are provided with an antireflective coating or tilted versus the propagation direction of the laser pulse. This configuration leads to a suppression of optical feedback and thus it prevents the outset of laser emissions with the described formation of individual longitudinal modes. On account of the high small-signal amplification in the active semiconductor layer system of the super-luminescence diode, spontaneous emission is amplified and because of the waveguide properties of the semiconductor layer system it exits from the super-luminescence diode as a diffraction-limited beam, whose spectrum is only determined by the amplification bandwidth of the semiconductor layer system and the effect of gain narrowing. Because of the anisotropic structure of the semiconductor layer system, the emitted light is linearly polarized and can be coupled into a polarization-maintaining single mode fiber.
For application as a master source in a MOPA system, it is for example feasible to employ a super-luminescence diode with a spectral bandwidth of 10 nm with a central wavelength of 1045 nm. This corresponds to a bandwidth of approx. 2.5 THz. The relevant peak rate of 150 mW ranges within the same order of magnitude of comparable Fabry-Perot laser diodes. The shape of the spectrum is virtually independent of pulse duration, pulse form, and repetition rate. With the so-called spectral bandwidth of approx. 2.5 THz, an adequate increase in the Brillouin scattering threshold is in any case ensured.
Very simple fiber-based MOPA systems can be realized by applying the present invention. Because of the extremely small finesse of the super-luminescence diode, the photons service life is practically equal to the duration of a single pass through the active range of the super-luminescence diode so that there is no resonator dynamics due to the non-existing feedback. In combination with fast control electronics, arbitrary pulse forms ranging from sub-nanoseconds to continuous wave operation are thus possible, whereby a super-luminescence diode in a temporal range represents a highly dynamic synthesizer.
Eligible as diode within the scope of the present invention are all construction styles of light-emitting semiconductor structures such as edge emitters, surface emitters, trapezoidal structures, gain-guided and index-guided structures. Likewise, both direct and indirect semiconductors may be applied.
Advantageously the end facets of the super-luminescence diode, as outlined hereinabove, are provided with an antireflective coating. Alternatively, the surface normal of the end facets may have an angle versus the direction of radiation that deviates from 0°. Reflection is hereby suppressed so that an outset of laser emission is prevented. Thus merely a spontaneous emission will occur within the diode whereby coherence length is short.
In accordance with the invention, a filtrating element may be arranged upstream to and/or downstream of the at least one amplifier stage. It is preferably a spectral filter or a polarization filter. Particularly eligible for use are band pass filters which adapt the spectrum in the desired bandwidth and/or which filtrate non-desired amplified spontaneous emission. Moreover eligible for use are polarization filters and optical isolators. All filters can be designed to suit specific application requirements and be tunable, respectively.
Alternatively, the fiber amplifier system can be so designed that the amplifier stage is passed through by the radiation several times in order to increase the amplifying factor. This is advantageously achieved by arranging a circulator upstream to the amplifier stage, with a spectral grating being allocated to said circulator downstream of the amplifier stage in the direction of radiation. The spectral grating may in particular be a fiber Bragg grating, a long-periodic grating or a tunable grating. This version comprising a circulator and a spectral grating has an advantage in that the amplifier stage works in a double pass mode, i.e. it is passed through twice and thus it supplies substantially more amplification. Moreover, by the aid of the spectral grating, the optical spectrum can be influenced to suit specific application requirements.
It is of advantage that one amplifier stage, several amplifier stages or all amplifier stages are waveguides. Fiber-optical amplifier stages lend themselves suitable in particular because they can be optimally integrated into a fiber-based amplifier system, thus reducing the otherwise usually needed adjustments substantially.
Alternatively, the amplifier stage or several or all amplifier stages may be a volume-optical element. Volume-optical elements may work with media in solid, liquid or gaseous statuses which are pumped optically, electrically or chemically.
Apart from the inventive fiber amplifier system, the invention relates to a method for amplifying and emitting pulsed laser radiation in which the pulsed output radiation from a master source is amplified by means of at least one amplifier stage, with the master source emitting a broadband output radiation generated by means of spontaneous emission.
In particular, the fiber amplifier system can be so configured that it is optimized for non-linear frequency conversion.
Practical examples of the present invention are elucidated more precisely in the following by means of figures, where:
The fiber amplifier system according to
The setup according to
The arrangement in accordance with
The version of the fiber amplifier system according to
| Number | Date | Country | Kind |
|---|---|---|---|
| 102010052907.9 | Dec 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/005935 | 11/25/2011 | WO | 00 | 9/9/2013 |
