Production of tuneable picosecond light impulses in a visible spectral range

The invention relates to a device for generating light pulses in the visible spectral range, comprising a laser system which emits femtosecond light pulses, ie light pulses with a duration of up to 1 ps, in the infrared spectral range, the wavelength of the light pulses emitted by the laser system being tunable , And with an optical frequency converter for converting the wavelength of the light pulses into the visible spectral range.

Laser systems that are capable of generating femtosecond light pulses are increasingly used in basic physical research as well as in other fields of research. With such laser systems, fast physical, chemical and biological processes can be observed virtually in "real-time". Commercial fields of application for femtosecond light-emitting laser systems are in the field of material testing and processing, in the field of medicine and in the so-called "life science" area. Multi-photon microscopy and optical coherence tomography are examples of specific applications.

In the recent past, so-called time-resolved fluorescence spectroscopy has developed into an important method of investigation in the field of chemistry, biochemistry, physical chemistry and semiconductor technology. In the case of time-resolved fluorescence spectroscopy, as a rule the lifetime of an excited state of a fluorophore used is determined. This is the average time that a single fluorophore molecule spends in the excited state before returning to the ground state by irradiation of a fluorescence photon. The lifetime is specific for the respective fluorophore and its local environment so that the fluorophore molecules are used as probes for investigating the microscopic processes of interest. It is also possible, for example, to distinguish two or more different fluorophores with overlapping optical absorption and emission spectra, by means of different lifetime periods, as a result of which the possible applications are further improved. Such investigations are, for example, used in the case of automated drug discovery (HTS) "high throughput screening".

In the case of time-resolved fluorescence spectroscopy, fluorophores are generally used as so-called markers, their lifetimes in the range between a few 100 ps and a few 100 ns. A short lifetime is generally advantageous, since a slight signal noise can be achieved by repeated measurement and averaging. At the same time, high demands are placed on the pulse length of light pulses emitted for excitation of the fluorophores from the short lifetime. The pulse length must in any case be short compared to the lifetime of the fluorophores. It should also be noted that the excitation spectra of the fluorophores in question are essentially in the visible spectral range. The number of fluorophores which can be used in certain investigated objects for time-resolved fluorescence spectroscopy is, as a rule, very limited by the experimental environment and by the specific interactions with the object to be investigated, which results in restrictions on the wavelength of the light pulses.

A device of the type mentioned at the outset is known from the state of the art (Herrmann et al., "Experimental evidence for super continuum generation by fission of higher order solitons in photonic fibers", Applied Physics Letters 88 (17), 173.901 / 1 to 173.901 / 4, 2002). In the known device, a broad-band, so-called supercontontuum is produced by means of a photonic crystal fiber from femtosecond light pulses of a titanium-sapphire laser. The titanium-sapphire laser system is tunable in the prior art device (narrowband). However, this tunability is not disadvantageously transferred to the visible spectral region of the supercontinuum. As a result, the previously known device is not suitable for producing light pulses which can be tuned in the visible spectral region. Accordingly, the application of the previously known laser system for time-resolved fluorescence spectroscopy is restricted.

It is an object of the present invention to provide a light source, in particular for time-resolved fluorescence spectroscopy, which supplies light pulses with a pulse duration in the picosecond range, wherein the wavelength of the light pulses is to be tunable substantially over the entire visible spectral range.

SUMMARY OF THE INVENTION The present invention solves this object from a device of the type mentioned in the introduction in that the conversion of the wavelength of the light pulses by means of the frequency converter takes place in such a way that the wavelength of the light pulses can be tuned in the visible spectral range Pulse duration of the frequency-converted light pulses can be increased to at least 1 ps.

According to the invention, femtosecond light pulses in the infrared spectral range are generated by means of a suitable laser system which have a sufficiently high pulse energy (order of magnitude nanojoule), so that the wavelength of the light pulses can be converted in a manner known per se using corresponding non-linear optical effects in the Desired visible spectral range. The frequency-converted light pulses are then adjusted according to the invention by means of an optical stretcher to the desired pulse duration of up to 1000 ps. For time-resolved fluorescence spectroscopy, light impulses with a pulse duration in the range from 1 ps up to 1000 ps are required. Light pulses with a pulse duration in the range between 10 and 100 ps are required for most applications. The repetition rate of the light pulses should be in the range of up to 250 MHz.

The device according to the invention is, of course, also suitable as a tunable light source which supplies quasi-continuous laser light for normal, ie not time-resolved, fluorescence spectroscopy.

The frequency converter used according to the invention may comprise one or more frequency doublers of the conventional type. As frequency doubler, for example, commercially available SHG crystals (second harmonic generation) or so-called periodically polarized crystals or suitable waveguide structures can be used.

In addition, an optical frequency filter, which is either connected upstream or downstream of the frequency converter, can be provided in the device according to the invention. The nonlinear optical element used for the frequency conversion of the light pulses into the visible spectral range, depending on the application, can also fulfill the function of the optical stretcher and the frequency filter. Because of the phase matching conditions to be met for the frequency doubling, the frequency conversion is generally narrow-band, which is equivalent to a spectral filtering of the frequency-converted light. Due to the narrow band-width, the temporal stretching of the light pulses also results in most cases.

Depending on the non-linear optical element used for frequency conversion, accordingly, it is not absolutely necessary to provide a separate optical element which fulfills the function of the optical stretcher.

Expediently, the wavelength of the light pulses emitted by the laser system should be tunable in a sufficiently large range, if possible at least between 1 μm and 2 μm, in order to achieve a sufficiently large tunability, if possible in the entire visible spectral range, for the frequency-converted light pulses. The laser system is usually tunable so far that the entire visible spectral range plus the adjacent ultraviolet (UV) and near infrared (NIR) spectral range can be covered by frequency conversion according to the invention. If, according to the invention, a simple frequency doubler is used as a frequency converter, a spectral coverage at the output of the device according to the invention between 400 nm and 1 μm results for a tunability of the wavelength of the light pulses emitted by the laser system between 800 nm and 2 μm, Complete visible spectral region including the near infrared spectral region.

The optical stretcher of the device according to the invention can expediently be formed by at least one dispersive optical element that is connected downstream of the frequency converter. The optical stretcher can also be designed in several stages and, for example, consist of a glass rod, followed by a dispersive glass fiber. In the first stage, the pulse duration of the frequency-converted light pulses can be increased from approximately 100 fs to approximately 1 ps, while in the second stage the pulse duration is further increased from 1 ps to the desired 10 to 100 ps. Overall, for the time-resolved frequency spectroscopy, the pulse duration of the frequency-converted light pulses should be adjustable in the range mentioned. It must be taken into account that, as a rule, an average spectral power density of more than 1 mW per nanometer, preferably more than 10 mW per nanometer, is required for time-resolved fluorescence spectroscopy.

According to the invention, a laser system is required which emits both high-power and spectrally variable light pulses in the infrared spectral region. This laser system should be inexpensive and easy to operate, especially for use in the field of time-resolved fluorescence spectroscopy.

To date, femtosecond light pulses of high power have typically been generated in the laboratory using titanium-sapphire laser systems. These systems are disadvantageously very expensive, can be expensive to use and can be handled with difficulty. Also, the tunability of the optical spectrum of the generated light pulses is not satisfactory in such laser systems.

Nowadays it is possible to produce femtosecond light pulses with impulse energies of one nanocouple and more by means of purely fiber-based laser systems. Such systems usually consist of a pulsed laser light source which emits femtosecond light pulses in the energy range of 100 pikojoule. These light pulses are then amplified by means of an optically pumped amplifier fiber so that the light pulses are available in the desired pulse energy range.

For example, a device for generating tunable light pulses is known from EP 1 118 904 A1. The known device operates with a special non-linear optical fiber by means of which the optical spectrum of femtosecond light pulses delivered by a suitable pulsed laser light source can be selectively modified by utilizing solitonic effects and the Raman effect. In order to vary the spectrum of the generated light pulses, the intensity of the light coupled into the non-linear optical fiber is varied in the system described in the aforementioned printed document. This directly results in the disadvantage that, in the known system, the desired optical spectrum of the generated light pulses depends on the pulse energy. An independent variation of the pulse energy and the wavelength of the light pulses is therefore not possible with the prior art system. A further disadvantage is that, in the known system, the nonlinear optical fiber used must have a length of several tens of meters, so that the desired Raman effect becomes effective. Due to the long running distance, an undesirable loss of coherence of the generated light pulses can occur.

In order to avoid the disadvantages outlined, a laser system can be used which has a non-linear optical fiber for generating the tunable light pulses, by means of which the optical spectrum of femtosecond light pulses can be modified by utilizing purely solitonic effects, the nonlinear optical fiber being modified An optical compressor is connected.

As can be seen, nonlinear processes in the fiber into which the light pulses are coupled in the laser system of the device according to the invention lead to two separate light pulses forming in the fiber, the spectrum of which is opposite that of the coupled light pulse to the longwave or shortwave spectral range Is shifted. The spectral separation of the light pulses is adjustable by means of the optical compressor which is connected upstream of the nonlinear optical fiber in the device according to the invention. The temporal frequency response ( "chirp") of the coupled light pulses is deliberately influenced by the optical compressor. The optical spectrum modified by means of the non-linear optical fiber is then sensitive to the predetermined "chirp", so that the desired tunability of the light pulses is given. It is particularly advantageous that the optical spectrum of the light pulses generated by means of the device according to the invention can be varied independently of the pulse energy.

In experiments, it has been found that the laser system of the device according to the invention advantageously uses a very short nonlinear optical fiber with a length of only a few centimeters to achieve the desired modification of the optical spectrum of the light pulses. As a result, coherence losses of the generated light pulses are effectively avoided.

The light pulses coupled into the non-linear optical fiber of the laser system should have an impulse energy of at least one nanjoule. Such high pulse energies are desirable so that the solitonic optical effects occur to produce the tunable light pulses within the non-linear optical fiber to the extent required.

The optical compressor of the laser system of the device according to the invention should be designed such that the temporal frequency conversion of the light pulses coupled into the nonlinear optical fiber can be varied. This makes it possible, in a comfortable and simple manner, to match the generated light pulses to the desired wavelengths by suitably adjusting the adjustable elements of the optical compressor, such as, for example, prisms or optical gratings.

According to an advantageous embodiment of the device according to the invention, the non-linear optical fiber is designed to be polarization-maintaining and dispersed-displacement. Such a fiber is described, for example, in the article by T. Okuno et al. In the journal IEEE Joumal of Selected Topics of Quantum Electronics, vol. 5, p. 1385, 1999. *** " The mentioned solitonic optical effects which lead to the desired modification of the spectrum of the light pulses according to the invention occur in the non-linear optical fiber when the wavelength of the light pulses coupled into the fiber is in the range of the zero-dispersion wavelength of the fiber. In experiments, a nonlinear optical fiber whose zero-dispersion wavelength is in the range of 1.52 μm was used to generate the tunable light pulses.

Light pulses with a particularly broad optical spectrum can be generated with the laser system of the device according to the invention if the non-linear optical fiber has a particularly small core diameter of ≤ 5 μm. In experiments, a fiber with a core diameter of 3.7 μm was successfully used, with a fiber length of only 7 cm being sufficient. This results in a usable wavelength range for tuning the light pulses, which extends from about 1.1 μm to 2.0 μm.

In addition to conventional optical glass fibers, according to the invention, microstructured photonic fibers can also be used as non-linear optical fibers for producing the tunable light pulses. Such fibers have a transverse microstructure in the region of the core. By suitable adaptation of the zero-dispersion wavelength as well as by low core diameters and thus high non-linearity of such crystal fibers, the generation of widely tunable light pulses according to the invention is possible.

Optionally, in the laser system of the device according to the invention of the non-linear optical fiber, an additional optical compressor can be connected downstream in order to obtain light pulses with a minimum pulse duration at the output of the laser system. In experiments, the use of a prism compressor using prisms made from SF10 glass has proven itself. Thus pulse durations of ≤ 25 fs could be achieved.

Exemplary embodiments of the invention are explained in the following with reference to the figures. Show it:<dl id="dl0001"><dt>FIG</dt><dd>1 shows the device according to the invention as a block diagram;</dd><dt>FIG</dt><dd>Laser system of the device according to the invention.</dd></dl>

The device shown in FIG. 1 consists of a laser system LS which emits femtosecond light pulses in the infrared spectral range. These light pulses are fed to an optical frequency converter FC, which can be, for example, a frequency doubler crystal of the known type. The frequency-converted light pulses are fed to an optical stretcher OS, by means of which the pulse duration of the frequency-converted light pulses can be increased to at least 1 ps. Depending on the requirements, an optical frequency filter, which is not shown in detail in FIG. 1, can be connected upstream or downstream of the frequency converter FC in order to be able to eliminate unwanted spectral components in the optical spectrum of the light pulses, depending on the application. According to the invention, the wavelength of the light pulses emitted by the laser system LS is tunable. A tunability between 1 μm and 2 μm should be ensured, if possible even between 800 nm and 2 μm, so that light pulses which can be tuned substantially over the entire visible spectral range are available at the output of the device shown in FIG . The optical stretcher OS can be any desired dispersive optical element, such as, for example, a glass fiber or an optical fiber with a suitable dispersion or a multi-stage structure of such elements.

FIG. 2 shows schematically the structure of the laser system LS of the device shown in FIG. In this case, a pulsed laser light source 1 is provided, which emits femtosecond light pulses with an impulse energy of more than one nanjoule. The laser light source 1 can advantageously be a completely fiber-based system which is composed of a commercially available pulsed fiber laser and an optical pumped amplifier fiber connected downstream thereof. However, the use of conventional free-beam lasers as a laser light source 1 is also possible. The temporal frequency profile of the light pulses emitted by the laser light source 1 is specified in a targeted manner by means of a prism compressor 2. In the exemplary embodiment shown, the prism arrangement is run through twice by the light pulses for this purpose. By means of the double arrow, it is indicated that one of the prisms of the compressor can be adjusted so as to be able to match the generated light pulses according to the invention. A nonlinear dispersion-shifted and polarization-maintaining optical fiber 3, into which the light is coupled by means of a lens 4, is connected downstream of the prism compressor 2. The light pulses coupled into the fiber 3 have a wavelength that essentially corresponds to the zero-dispersion wavelength of the optical fiber 3. Because of non-linear solitonic effects occurring in the fiber 3, the optical spectrum of the light pulses is greatly modified. The light pulses emerging from the optical fiber 3, which are coupled out by means of a further lens 5, have an optical spectrum which is sensitive to the "chirp" provided by the compressor 2. By adjusting the corresponding prism in the compressor 2, the light pulses emerging from the fiber 3 in the infrared wavelength range can be adjusted between 1.1 μm and 2.0 μm. As described above, the optical spectrum of the light pulses at the output of the fiber 3 has two separate components, which are shifted toward the long-wave or short-wave spectral range, in contrast to the wavelength of the coupled light pulse. An adjustable spectral separation of the two components of more than 100 THz can be realized with the illustrated construction. Even if a short non-linear optical fiber 3, which can have a length of ≤ 10 cm, is sufficient according to the invention, the light pulses within the fiber 3 are dispersed.

This can be compensated by an additional prism compressor 6. When SF10 glass prisms were used, tunable light pulses with a pulse duration of ≤ 25 fs were realized with the structure shown in the drawing. An FROG structure or a spectrometer 7 is provided for characterizing the light pulses.

It is to be pointed out that other dispersive optical components, such as, for example, grating compressors, so-called "chirped" mirrors, fiber Bragg gratings, can be used for the purposeful adjustment of the "chirp" of the light pulses coupled into the fiber 3, Additional dispersive optical paths, etc.