Patent Application
20210033945
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 or fluorescence-based microscopy due to a particularly fast response.
Over the last decades, improved analytical methods have enabled more and more extensive characterization of samples of all kinds and have contributed to a deep understanding of e.g. the structures and reaction mechanisms of biological systems. Among the most commonly used analytical methods are spectroscopic methods, which are, at least in principle, capable of resolving even larger structures and reaction processes in samples in real time with a spatial resolution in the sub-micrometer range. Laser-based methods such as multiphoton microscopy (MPM) and especially nonlinear coherent Raman spectroscopy (CRS) are particularly suitable as a subclass, since in the best case they can image the behavior of molecules in their “natural” chemical environment without additional staining. The basis of these modern methods is the further development in laser technology, which has led to the fact that, in addition to the basic advantages of laser light, flexible experiments can now be carried out with a single laser setup, for example at different wavelengths. However, due to the complexity of the devices available on the market and the associated costs for acquisition and maintenance, the majority of these applications are currently still limited to specialized laboratories. In order to make these techniques suitable for mass production and to enable a further flexibilization of the experiments to be carried out, a transformation into simple, low-maintenance and robust, for example fiber-based setups is necessary.
Well known are laser systems that achieve output wavelength detuning by changing the repetition rate (e.g. Shenping L. et al. in “Electrical wavelength-tunable actively mode-locked fiber ring laser with a linearly chirped fiber Bragg grating. IEEE Photonics Technology Letters” 10, 799-801 (1998)). In this paper, the repetition rate is only considered as a parameter for tuning the wavelength of the pump laser. The exact value of the repetition rate is of no further interest, and only a small wavelength change of the pump laser is obtained, which is limited to the gain ranges of existing laser materials. A use in connection with parametric amplifier systems is not disclosed.
A possibility to quickly change the wavelength of fiber-integrable parametric oscillators (FOPOs) is described for example in Brinkmann, M. et al. “Electronically and rapidly tunable fiber-integrable optical parametric oscillator for nonlinear microscopy” Optics Letters, 41, 2193, (2016)”. The wavelength change is obtained by using a dispersive filter in the FOPO and tuning the repetition rate of the pump laser. The disadvantage of this technical solution, however, is that the parametric gain spectrum must provide all wavelengths to be generated simultaneously at a fixed pump wavelength. This means that the pumping is done near the zero dispersion wavelength of the parametric amplifier. Accordingly, the maximum achievable tunability is limited (1100 nm to 1300 nm for the idler pulse). A further disadvantage of the used method is the high peak power of the pump pulse (approx. 5 kW) which is necessary to achieve a wide amplification range. Pump pulses with such a high peak power make the fiber integration of the pump pulses up to the parametric gain medium much more difficult in practice due to pulse-changing non-linear effects.
The tuning range of a FOPO can be extended by slightly detuning the wavelength of the pump pulses. In this case a small change of the pump pulse wavelength is translated by a steep phase matching curve of the parametric gain medium into a large wavelength change of the FOPO output pulses. This principle was used e.g. in T. Gottschall et al. “Four-wave mixing based light sources for real-world biomedical applications of coherent Raman microscopy” (Proc. of SPIE, Vol. 9712, p. 971202, 2016). Disadvantage of this technique is that in order to detune the FOPO wavelength, both the wavelength of the pump pulses and the repetition rate of the pump pulses must be detuned in order to capture the time overlap between pump and seed pulses in the gain medium. For this reason, known systems always require the independent control of two parameters (repetition rate and central wavelength of the pump pulses), which results in an increased material and control engineering effort. Since these required control loops are typically based on mechanical delay sections or temperature changes, known systems can only be detuned on relatively slow time scales (over 100 ms). Another disadvantage of known systems is that typical designs do not allow a complete integration into robust fiber optics, which makes a successful, cost-efficient and low-maintenance application outside of specialized laser laboratories considerably more difficult or even impossible.
Therefore, the object of the present invention is to open up a flexible laser system that can be implemented without mechanical delay lines or temperature tuning, in which the output wavelength can be tuned very fast and in a wide range. It is further the object of the present invention to disclose a method for the rapid tuning of laser output waves.
According to the invention, a system for the generation of laser pulses of variable wavelength, which comprises at least
a pump pulse generator for generating pump pulses and
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, wherein the wavelength of the pump pulses is adjustable either as such or also via the repetition rate of the pump pulse generator, and in the event of a change in either the repetition rate or the wavelength, the respective other parameter changes simultaneously in such a way that pump and seed pulses overlap in time in the optically parametric amplification medium. Surprisingly, it has been shown that by means of the laser system constructed according to the invention, laser pulses can be generated which can be tuned over a wide range of wavelengths and where, in particular, the change between different wavelengths occurs within a very short time scale. For example, parametric laser systems are available which can be easily built up in fiber-integrated design and are electronically tunable quickly (for example <100 ms). The tunability of the system can be more than 2000 cm−1 or 70 THz, and efficient systems with low peak pump pulse power, for example less than 5 kW or even less than 1 kW, can be realized. This flexibility is achieved in particular by the inventive design of the pump pulse generator with additional optical parametric amplification medium, whereby the variability in the pump wavelengths in particular can contribute to a significantly more efficient and higher spreading of the output wavelengths. Furthermore, the design is flexible enough to allow the use of additional optical components or systems between the pump pulse generator and the parametric amplification medium to adapt the power, duration, spectral width and shape of the pump pulses. For this purpose it is used that laser pulses with a central wavelength outside the amplification range of classical laser materials can be generated by parametric amplifier systems, divided into optical parametric amplifiers and optical parametric oscillators. The parametric amplifier systems convert pump pulses previously generated by a pump laser, stimulated by further seed pulses, into wavelength shifted signal and idler pulses. An important prerequisite for this is an overlap of the seed pulses with the pump pulses in the pa-rametric amplification medium.
This is in contrast to the state-of-the-art systems that either allow only very small wavelength changes or where the change between different wavelengths occurs on a time scale (>100 ms to a few seconds) that is not suitable for modern high performance spectroscopy, e.g. in modern nonlinear or fluorescence based microscopy. In addition, these well-known systems always require the independent control of two parameters (repetition rate and central wavelength), which results in an increased material and control engineering effort. Another disadvantage of the known systems is that typical designs do not allow a complete integration into robust fiber optics technology, which makes a successful, cost-efficient and low-maintenance application outside of specialized laser laboratories considerably more difficult or even impossible.
The system according to the invention generates laser pulses of variable wavelength. This means that the system can not only provide laser pulses of a fixed or, in the case of output of a wavelength range, of a single central wavelength, but that laser pulses of different wavelengths or central wavelengths can be provided. In the following the term wavelength is used synonymously for the central wavelength if the laser system in question outputs a range of wavelengths. In particular, the tunable system consisting of a pump pulse generator and an optically parametric amplification medium can provide different output laser pulses with wavelength differences of up to several hundred nanometers.
The pump pulse generator generates the pump pulses, which are fed into the optically parametric gain medium. The pump pulse generator can provide laser pulses of different wavelengths and repetition rates, either with wavelength or repetition rate control. The pump pulse generator can have or include one or more optical amplifier chains and one or more spectral filters. For example, a pump pulse generator usable in accordance with the invention may be a passive mode-locked pump pulse generator, such as a mode-locked fiber laser. In addition, it is also possible to decouple part of the power from the pump pulse generator to generate the seed pulses.
The optical parametric amplification medium can be used in optical parametric oscillators (OPOs) or optical parametric amplifiers (OPAs). These systems can be fiber based (FOPO, FOPA). The optical parametric amplification medium is characterized by the fact that the incident pump pulses, stimulated by further seed pulses, are converted into wavelength shifted signal and idler pulses. The wavelengths of the output waves and thus the conversion in the optical parametric amplification medium is performed according to the known relationships (energy conservation and phase matching) of nonlinear optics.
The wavelength of the pump pulses leaving the pump pulse generator can be adjusted both as such and via the repetition rate of the pump pulse generator. The pump pulse generators which can be used according to the invention are thus capable of providing pump pulses with a wavelength within a certain range. The wavelength of the outgoing laser pulses can be adjusted independently either by the repetition rate of the pump pulses, e.g. by electro-optical modulation at the desired repetition rate, or by the wavelength as such, e.g. by an electrically adjustable wavelength filter.
If a wavelength change of the pump pulse is either controlled by the repetition rate or directly, a change of the respective other parameter occurs simultaneously. The adjustment of one parameter is therefore immediate and simultaneous to the change of the other parameter. This ensures that after a wavelength change the system adapts very quickly to the new wavelength and the full pulse power can be made available. This is in contrast to state-of-the-art systems in which adaptation is not simultaneous but consecutive, for example by adjusting a filter and subsequently changing the length of the resonator.
By simultaneously changing the wavelength and the repetition rate it is achieved that pump and seed pulses overlap in time in the optically parametric amplification medium. The overlapping of the two pulses leads to an optimal amplification in the parametric amplification medium, and high output powers can be provided again within very short times after the wavelength change. A temporal overlap of the pump and seed pulses occurs when, within the same time interval, both pump and seed pulses make a joint contribution to the power of the laser system's output pulses.
In a preferred configuration of the system, the simultaneous change of the repetition rate when changing the wavelength or the simultaneous change of the wavelength when changing the repetition rate of the pump pulse generator can be achieved by an additional optically dispersive element. The additional optically dispersive element allows the system to be designed in such a way that a simultaneous and simultaneous adjustment of repetition rate and wavelength is achieved. Optically dispersive elements are those elements which are in principle able to give light waves a different propagation speed (phase velocity) as a function of their wavelength. Furthermore, it can be advantageous to be able to fine-tune the amount of dispersion afterwards in order to achieve a particularly good adjustment of the arrival time of pump and seed pulse in the parametric amplification medium for all wavelengths to be amplified.
The amount of dispersion of the dispersive element can be derived from the different group velocities of pulses of different wavelengths. The dispersion has the consequence that when tuning, i.e. changing the pump wavelength and the corresponding seed wavelength, an overlap that is set once for a pair of pump and seed wavelengths is usually not preserved:
The period of revolution τ(ω), resp. the repetition rate R=1/τ in a pump pulse generator can be calculated from the group velocity vg(ω) resp. the group delay per length β1(ω)=1/vg and the lengths Li of the used components for each frequency:
For example, for a change of the wavelength from 1020 nm to 1050 nm in a typical pump pulse generator based on an ytterbium fiber laser with a repetition rate of about 10 MHz, the period of revolution is shortened by about 3 ps, which corresponds to a change of the repetition rate of about 3 kHz. If, for example, the seed radiation is generated and amplified in a typical optical parametric oscillator (OPO) also based on glass fibers (parametric amplification medium e.g. NKT Photonics LMA-PM-5), the period of revolution between the seed wavelengths of 749 nm and 930 nm belonging to the respective pump wavelengths is shortened by about 400 ps due to the larger wavelength change. An existing temporal overlap of pump and seed pulses cannot be maintained for pump pulses of a duration t<<400 ps without changing the period of revolution in the pump pulse generator or OPO (e.g. by changing the length L) when tuning the wavelength. In the example given above, the length of the pump pulse generator would have to be shortened by about 12 cm, e.g. by using mechanical delay rails.
In general, a frequency-dependent mismatch Δτ of the time interval between the corresponding seed and pump pulses can be defined:
Δτ(ωp)=τSeed(ωs(ωp))−τPumpe(ωp).
This mismatch is compensated by an additional element with a suitable dispersion, preferably in the pump pulse generator. A mathematical calculation of the required amount of additional dispersion can be done by a Taylor expansion of the mismatch Δτ by the pump wavelength ω0:
The dispersion of the additional dispersive element of length L can be chosen according to this estimation with β2*L=τ1, β3*L=τ2 etc. Typically, a development of the mismatch Δτ up to the second order and a control of the dispersion up to the third order β3 based on this is sufficient to ensure sufficient compensation. However, it can also be advantageous to compensate for higher orders. In this way a fast detuning of the laser system over a particularly wide range can be made possible.
Within a preferred embodiment of the system, the additional optically dis-persive element can be selected from the group consisting of chirped Bragg structures, gratings, prisms, or combinations thereof. Especially this group of optically dispersive elements has proven to be very efficient in achieving a simultaneous adaptation between repetition rate and wavelength. These elements can be easily integrated into fiber optic systems and require a minimum of maintenance. In this respect, low-cost and robust systems can also be provided for applications in “industrial” environments.
In a further advantageous design of the system, the additional optically dispersive element can be a chirped fiber bragg grating (C-FBG). In particular, the use of chirped fiber Bragg gratings as an additional dispersive element can help to obtain the widest possible tunable wavelength range. In addition, these structures can be fine-tuned so that the maximum possible overlap of pump and seed pulses is obtained. A further advantage is that the chirped fiber Bragg grating that can be used, for example, can also be used as a decoupling element for the radiation to be used from the resonator. However, it is also possible that a pump pulse generator according to the invention has an additional decoupling unit. If a particularly high level of control is required, a possible design can also use commercially available thermally fine-tunable C-FBGs. However, more complex temperature profiles can also be applied to the C-FBG.
In another aspect of the system, the wavelength of the pump pulse generator can be changed via an adjustable spectral filter. In addition to the possibility of a simple and direct selection of the wavelength, a further advantage is that the filter band width determines the pulse properties of the pump pulse generator (spectral width and temporal duration as well as the temporal chirp) and thus a greater control over the generated pump pulses is gained. The spectral width and temporal duration of the pump pulses provide more control over the spectral width and temporal duration of the pulses generated in the parametric amplification medium.
In a version of a pump pulse generator with spectral filter, saturable absorber mirrors (SAM), e.g. in a linear resonator geometry, can be used as passive mode coupling elements, as well as fast (<100 ms) detunable optical filters for adjusting the wavelength and, coupled to it, the repetition rate. For this purpose, e.g. Fabry-Perot filters tunable via piezoelectric effects (e.g. fiber-coupled), acousto-optical filters, but also optical gratings (e.g. in a reflective Littrow arrangement) in combination with fast beam deflectors can be considered.
A further embodiment can use active mode coupling. To control the repetition rate and the emission wavelength, an electro-optical modulator, e.g. based on lithium niobate, can be used. However, other modulation methods, e.g. acousto-optical modulation, can also be used.
In a preferred embodiment of the system, at least a part of the additional optically dispersive element can be located outside the pump pulse generator. This design allows a particularly flexible setup and can contribute to the use of standard wavelength tunable pump pulse lasers. In this case, the coupling between the repetition rate and wavelength, as required by the invention, takes place entirely in or before the optically parametric amplification medium. This can reduce the cost of the laser system and simplify maintenance operations.
In a preferred configuration of the system, the repetition rate of the pump pulses is an integer multiple of the repetition rate of the seed pulses. In this configuration, the additional optically dispersive element is selected so that the adjustment between repetition rate and wavelength is designed for different repetition rates of pump pulse generator and seed pulses. The special feature of this design is that the repetition rate of a power component of the pump pulse radiation is only reduced to the repetition rate of the seed pulses after generation in the generator, e.g. with an electro-optical modulator as a so-called “pulse picker”. This has the advantage that the remaining part of the pump pulses is available at a higher repetition rate, which makes it possible to use the generated light directly in lock-in detection methods, e.g. in the detection of coherent Raman scattering.
Furthermore, the parametric amplification as well as the seed pulse generation can preferably be done in an optical parametric oscillator (OPO). When using an OPO to generate the wavelength shifted signal and idler radiation, the seed radiation is generated by the pump pulses in the parametric amplification medium in a spontaneous process and superimposed by a feedback with the next pump pulses. This design offers the advantage of a cost-effective realization, since no external seed pulse generation is required. Furthermore, the system can be operated very efficiently due to the additional superelevation of the seed pulse energy.
In a further embodiment of the system, the seed pulses can be generated from a fraction of the pump pulses and these seed pulses can be delayed by the inverse of the repetition rate of the pump pulses in the parametric amplification medium before they are superimposed with the pump pulses. This setup allows the separate generation of the seed pulses before they are amplified in the parametric amplification medium. This use of an OPA, i.e. single-pass amplification, reduces the complexity of the overall system. A further advantage of this design is the possibility to use particularly efficient seed pulse generation mechanisms known from other areas of nonlinear photonics, such as spectral broadening by self-phase modulation, supercontinuum generation or soliton-self-frequency shift by stimulated spatial scattering.
In an additional characteristic of the system, the repetition rate of the pump pulse generator can be greater than or equal to 5 MHz and less than or equal to 80 MHz and the group velocity dispersion of the additional optically dispersive element can be less than or equal to 15 ps2 and greater than or equal to 0.5 ps2. Within these parameter ranges, a particularly efficient coupling between repetition rate and wavelength can be achieved by very stable and commercially available C-FBGs. This results in a highly tunable laser, which returns to its original power very quickly after wavelength change. This allows the use in modern spectroscopic methods, especially for the investigation of biological systems. Here, laser systems with repetition rates between 5 and 80 MHz are of particular advantage because they offer an efficient compromise between high signal yield and low probability of damage to biological cells. Larger dispersions or group velocity dispersio-nes can be disadvantageous, since the dispersive elements required for this are very complex and can only be manufactured at high costs. In addition, these elements are usually very susceptible to temperature fluctuations, for example. Smaller dispersions or group velocity dispersions can be disadvantageous, because the relative error in the production of chirped Bragg structures with low dispersion becomes larger and larger. This can lead to the fact that the temporal overlap achieved by the coupling of wavelength and repetition rate becomes smaller and smaller and thus the desired generation of detunable wavelengths becomes more inefficient or is no longer possible.
In a further aspect fiber-based optical amplifiers can be used between the pump pulse generator and the parametric amplification medium. These amplifiers can consist of several stages and be designed, for example, as single-mode preamplifiers and main amplifiers with a large fiber core, i.e. a so-called “Large-Mode Area” fiber, whereby the amplifier fiber can be designed, for example, as an ytterbium-doped polarization-maintaining glass fiber.
In a further design, the “chirp” of the pump pulses can be specifically adjusted to achieve a wider or respectively narrower instantaneous gain bandwidth centered around the gain maximum. Furthermore, especially when using spectrally broadband and thus bandwidth limited very short pump pulses (e.g. <10 ps), it can be advantageous to stretch (“chirp”) them before feeding them into the parametric gain medium, e.g. to reduce disturbing nonlinear effects like self-phase modulation.
In an additional embodiment, the fiber laser can be constructed entirely in fiber optics technology, in particular from polarization-maintaining glass fibers.
Furthermore, a method for changing the wavelength of laser pulses using an optically parametric amplification medium, which comprises at least the steps
In step a) of the method pump pulses with defined wavelength and defined repetition rate are generated. This implies that the pump pulses are defined by a specific pair of values, namely wavelength and repetition rate. This does not mean that these two parameters are fixed in the sense that wavelength and repetition rate are not changeable. The generation of the defined pump pulses can be done by the above described pump pulse generators.
In step b) seed pulses are generated. This step can be done by a sepa-rate seed pulse generator or by decoupler and, if necessary, by processing a part of the pump pulses.
In step c) the seed and pump pulses are superimposed in the optical parametric amplification medium. This can be done, for example, by feeding the two pulses simultaneously into the amplification medium. According to the invention, a superposition occurs when both pump and seed pulses contribute to the power of the laser system output pulses within the same time interval.
The method is designed in such a way that if the wavelength or the repetition rate of the pump pulses changes, the other parameter changes simultaneously. Thus, within the method according to the invention, the two parameters wavelength and repetition rate are not considered to be largely independent of each other, but rather one of them is specifically adjusted when the other parameter is changed. This is in contrast to methods of the state of the art, which do without a specific coupling of both parameters.
In a preferred design of the method the simultaneous change of wavelength and repetition rate of the pump pulses can be achieved by means of a dispersive element. The dispersive elements described above are able to guarantee a fast and efficient coupling between repetition rate and wavelength of the pump pulse laser. Thus, widely tunable and quickly switchable methods can be realized.
Within another aspect of the method, the wavelength of the laser pulses can be changed by a simultaneous variation of the pump pulse wavelength and repetition rate within less than or equal to 100 ms. The method enables a fast achievement of the desired output power after a wavelength change by simultaneous coupling between repetition rate and wavelength without the use of mechanical components. In this respect, switching times can be realized which were not possible with the methods known so far.
The aforementioned components as well as the components to be used in accordance with the invention and described in the design examples are not subject to any particular exceptional conditions with regard to their size, shape, design, choice of material and technical concepts, so that the selection criteria known in the field of application can be applied without restriction.
Further details, features and advantages of the subject matter of the invention result from the sub-claims as well as from the following description of the associated drawings, in which—as an example—several preferred design examples of the concept according to the invention are presented.
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:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 129 637.9 | Dec 2017 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/083632 | 12/5/2018 | WO | 00 |
