WO2008086996A1

WO2008086996A1 - Illuminating apparatus with nonlinear optical elements for producing laser light in a broad spectral range with homogeneous spectral power density

Info

Publication number: WO2008086996A1
Application number: PCT/EP2008/000232
Authority: WO
Inventors: Peter Westphal
Original assignee: Carl Zeiss Microimaging Gmbh
Priority date: 2007-01-16
Filing date: 2008-01-14
Publication date: 2008-07-24
Also published as: WO2008086996A8; DE102007002203A1

Abstract

Provided is an apparatus for illuminating an object, comprising illuminating optics (8), a module (3) which is arranged upstream of the illuminating optics (8) and has at least two nonlinear optical elements (4; 5; 6) which are arranged one behind the other, and a laser (2) which is arranged upstream of the module (3) and outputs laser radiation (L1) to the module (3), which laser radiation travels sequentially through the elements (4-6), which are arranged one behind the other, and is output to the illuminating optics (8) which can use the laser radiation (L4) coming from the module (3) to illuminate the object, wherein each nonlinear element (4; 5; 6) effects, due to its nonlinear action, that the spectral bandwidth of the laser radiation (L2; L3; L4) output by the nonlinear element (4; 5; 6) is greater than the spectral bandwidth of the laser radiation (L1; L2; L3) which is coupled into the nonlinear element (4; 5; 6), and wherein the apparatus has a control circuit which measures intensity fluctuations of the laser radiation (L4) coming from the module (3) and drives, as a function of the measured intensity fluctuations, a controllable component of the apparatus such that the intensity fluctuations are at least partially compensated for.

Description

Carl Zeiss Microlmaging GmbH January 14, 2008

Lawyer's file: PAT 4024/021 -PCT

LIGHTING DEVICE WITH NONLINEAR OPTICAL ELEMENTS FOR PRODUCING LASER LIGHT IN A WIDE SPECTRAL AREA WITH A HOMOGENEOUS SPECTRAL POWER DENSITY

The present invention relates to a lighting device that is particularly suitable for a microscope. Furthermore, the invention relates to a corresponding illumination method.

Especially with confocal microscopes, it is often desired that a spectral variable, spatially coherent lighting device is provided. The illumination device is intended to be able to deliver any desired radiation in the spectral range from UV (about 300 nm) to IR (about 2000 nm), the range from 350 to 1000 nm being preferred.

In confocal microscopy, it is still largely customary to combine lasers with different output wavelengths via color splitters or similar elements and to couple them into the microscope beam path. In order to cover the visible spectral range with sufficiently many laser wavelengths, at least three to five individual lasers must be used. This means that the technical complexity is high and correspondingly high costs.

In order to get away from the multiplicity of individual lasers, US Pat. No. 6,154,310 describes a laser system in which ultrashort pulses are coupled into an optical splitter. In each of the branches of the splitter, wavelength conversion occurs via harmonic or parametric generation. Subsequently, the branches are united again into a beam. The disadvantage here is that after conversion only a few discrete wavelengths are available.

US Pat. No. 6,888,674 B1 describes a scanning microscope which contains a primary laser and a single optical component which spectrally widens the primary laser light of the primary laser in such a way that it contains a substantial portion of the entire visible spectrum behind the optical component. In this arrangement, however, is disadvantageous that the visible spectrum, which is generated by the individual optical component, has a disadvantageous course for microscopy, since little light power is delivered in the green-yellow spectral range. However, it is precisely this spectral range that is of particular interest for microscopy, since many common fluorescent dyes are present here.

WO 2005/119328 A1 describes a light source for a scanning microscope, in which a photonic crystal fiber (PCF) is used to generate continuum. In Fig. 2 of WO 2005/119328 A1 a measured power spectrum is shown, wherein the PCF length was chosen so that the total continuum power in the visible spectral range was maximum. This spectrum shows extreme break-ins at different wavelength ranges, in particular around 505 nm and around 545 nm. Instead of the PCF length, it is also possible to change the length of the laser pulses in order to vary the spectrum. As a result, the gaps are usually only shifted spectrally.

DE 101 39 754 B4 describes a device in which both the primary laser light and the frequency-converted by a component laser light is used for microscopic purposes. Apart from the additional use of the primary laser light, this illumination device does not differ significantly from the illumination device of US Pat. No. 6,888,674 B1 and thus also has its disadvantages.

EP 1 558 967 B1 describes a device for generating pulsed light in the visible spectral range, which is similar to the illumination device of US Pat. No. 6,888,674 B1. In EP 1 558 967 B1, however, additional optical means are provided to influence the pulse duration. These optical means may also include movable components, thereby significantly increasing the complexity and manufacturing cost of the device.

Proceeding from this, it is an object of the invention to provide a lighting device that can generate laser radiation having approximately the same spectral power density in the entire visible spectral range. Furthermore, a corresponding lighting method is to be provided.

The object is achieved by a device for illuminating an object, according to claim 1.

By providing a cascade of at least two nonlinear optical elements arranged one behind the other, it is advantageously achieved that the spectral power density in the entire visible spectral range is relatively constant. In particular, extreme dips in the spectrum are avoided.

By means of the control loop, which measures intensity fluctuations of the laser radiation coming from the module and controls a controllable component of the device depending on the measured intensity fluctuations such that the intensity fluctuations are at least partially compensated, a very constant spectral power density in the entire visible spectral range can be achieved.

The more nonlinear optical elements the module has, the more constant the resulting spectrum can be designed. Preferably, therefore, three, four, five or even more non-linear elements are provided.

The device for illuminating may be part of a microscope in particular, so that a microscope is provided with a lighting device that can deliver laser radiation with nearly constant spectral power at least in the green-yellow spectral range and preferably in the entire visible spectral range.

In particular, the microscope can be a point or line scanning microscope which operates confocally or partially confocal. Furthermore, it may also be a microscope that operates on the principle SPIM (Selective Plane Illumination Microscopy). The microscope may also be an optically operating cytometer or an optically operating Biochiop reader.

The illumination device can be used both for detection and manipulation of microscopic objects.

The microscope can be used in particular in the field of fluorescence microscopy. It is particularly suitable for simultaneous or quasi-simultaneous (i.e., with fast wavelength switching) excitation of multiple fluorescent dyes. Because the illumination device of the microscope provides visible light and infrared radiation, it is suitable for both single-photon and multi-photon excitation.

The microscope may also be an OCT microscope (OCT = Optical Coherence Topography) or a CARS (Coherent Anti-Stokes Raman Spectroscopy) microscope. In the embodiment as an OCT microscope, the wide spectrum of the illumination device can be used advantageously, since in this way a low temporal coherence and thus a high spatial resolution is possible. In the variant as a CARS microscope can be used to advantage that the at least two different, for the CARS necessary wavelengths can be varied continuously with the lighting device.

The microscope can also be embodied as a material microscope, as a cytometer, as a laser-based biochiop reader or as a fundus camera.

In particular, in the illumination device according to the invention, the laser can emit laser radiation in the infrared spectrum and the spectrum of the laser radiation emitted by the module can extend into the visible region.

Furthermore, it is possible for the spectrum of the laser radiation emitted by the first nonlinear element to lie exclusively in the infrared spectral range.

The non-linear optical elements can be designed so that complement the spectral broadening of the individual nonlinear optical elements as complementary as possible, so that a total of a very wide range can be achieved. Of course, the spectral ranges of the individual nonlinear optical elements may overlap.

At least one of the optical elements may be formed as a tapered optical fiber. The dispersion zero point of the group velocity of the optical fiber is preferably chosen so that it lies below the wavelength of the coupled-in laser radiation. This can be achieved, for example, by varying the length of the tapered portion of the fiber.

Furthermore, at least one of the non-linear optical elements may be formed as a photonic crystal fiber, which are often also called microstructured glass fibers.

At least one of the elements may also be formed as a nonlinear crystal.

A spectral filter unit can be arranged between the module and the illumination optics. This filter unit may be controllable, e.g. advantageous in the use of the illumination optics in a CARS microscope. With the filter unit, the wavelength or the wavelengths can be filtered out of the spectrum of the laser radiation emitted by the module, which are required for the respective application.

The controllable component can be designed as a controllable intensity modulator, which is arranged between the module and the illumination optics. , ,

However, the component may be e.g. also the laser itself and / or a component in the illumination optics.

Furthermore, a device for illuminating an object is provided, having an illumination optical system, a module arranged upstream of the illumination optical unit, which has at least two non-linear optical elements arranged one behind the other, and a window

Module upstream laser that emits laser radiation to the module, which passes in sequence through the elements arranged one behind the other and is delivered to the illumination optics, which can illuminate the object with the laser radiation coming from the module, each non-linear element causes due to its non-linear effect, that the spectral

Bandwidth of the laser radiation emitted by the nonlinear element is greater than the spectral bandwidth of the laser radiation coupled into the nonlinear element.

The object is further achieved by a method according to claim 10.

With this illumination method, laser radiation with approximately constant spectral power density can be provided.

In the method, the primary laser radiation may be in the infrared spectrum and the spectrum of the laser radiation emitted by the last element may extend into the visible region.

The spectrum of the laser radiation emitted by the first non-linear element, to which primary laser radiation is incident, can lie exclusively in the infrared spectral range.

Further, at least one of the optical elements may be formed as a tapered optical fiber. It is also possible to form at least one of the non-linear optical elements as a photonic crystal fiber or as a nonlinear crystal.

Furthermore, in the method, the laser radiation emitted by the last element can be spectrally filtered and the object can be illuminated with the spectrally filtered laser radiation.

In particular, for controlling the method, the laser radiation coming from the last element can be guided through a controllable intensity modulator, which is controlled accordingly and emits the laser radiation with reduced intensity fluctuations for illumination of the object. The illumination of the object can be carried out in particular for a microscopy method, so that a microscopy method is provided which has an illumination which has an approximately constant spectral power density in the visible spectral range.

There is further provided a method for illuminating an object in which a primary laser radiation passes through at least two nonlinear optical elements arranged one behind the other and the laser radiation emitted by the last optical element is used to illuminate the optical object, each nonlinear element due to its nonlinear effect causes the spectral bandwidth of the laser radiation emitted by the non-linear element is greater than the spectral bandwidth of the laser radiation coupled into the non-linear element.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the specified combinations but also in other combinations or alone, without departing from the scope of the present invention.

The invention will be explained in more detail for example with reference to the accompanying drawings, which also disclose characteristics essential to the invention. Show it:

Fig. 1 is a schematic representation of a device for illuminating an object for explaining an embodiment of the invention

Lighting apparatus;

FIGS. 2a-2e each show the spectral power of the laser radiation emitted by the individual elements;

3 shows an embodiment of the lighting device according to the invention;

Fig. 4 shows a further embodiment of the lighting device according to the invention, and

Fig. 5 shows a further embodiment of the lighting device according to the invention.

In the embodiment shown schematically in Fig. 1, the device 1 for illuminating an object comprises a laser 2, a module 3 with three nonlinear optical elements 4, 5, 6, a spectral filter 7 and an illumination optical system 8, for example, be part of a microscope can. The laser 2 emits pulsed laser radiation having a wavelength of 1064 nm and a pulse length of 5 fs at a pulse rate of 100 MHz and a mean optical power of 25 watts. In Fig. 2a, the spectral power (y-axis) of the laser radiation of the laser 2 as a function of wavelength in nm (x-axis) is plotted.

The laser radiation of the laser 2 strikes the first non-linear optical element 4, which may be formed for example as a highly tapered glass fiber. At the output of the first nonlinear optical element 4, a broader spectrum of the laser radiation is present in comparison to the spectrum of the laser radiation L1 impinging on the first nonlinear optical element 4.

The spectral power of the laser radiation L2 emitted by the first non-linear optical element is shown schematically in FIG. 2b, wherein the wavelength of the coupled-in laser radiation L1 is denoted by L1. The scale of the y-axis may differ in FIGS. 2a-2e, since FIGS. 2a-2e are only intended to illustrate the qualitative course of the spectral power.

The first non-linear optical element 4 is designed so that a significant light output is present only for wavelengths greater than 800 nm and thus exclusively in the infrared spectral range.

The laser radiation L2 then impinges on the second non-linear optical element 5, which due to non-linear optical effects leads to a broadening of the spectral bandwidth and emits laser radiation L3. The spectral power of the laser beams L3 is shown schematically in FIG. 2c.

The laser radiation L3 passes through the third non-linear optical element 6, which leads to a further spectral broadening of the bandwidth. The emitted laser radiation L4 has an approximately equal spectral power density in the visible range, as can be seen from FIG. 2d. The laser radiation L4 strikes a spectral filter 7, which filters out the desired wavelengths from the spectrum of the laser radiation L4 and supplies them to the illumination optics 8 as laser radiation L5. The spectral power of the laser radiation L5 is shown in FIG. 2e. The spectral filter 7 may also be omitted, e.g. if a broadband laser radiation L4 is desired or a spectral filtering in the illumination optical system 8 takes place.

The illumination device 1 thus comprises a module with three non-linear optical elements, which lead to a spectral broadening of the laser radiation L1 of the laser 2, , ,

wherein the broadening due to the non-linear optical elements 4 to 6 connected in series causes the spectral power density in the visible spectral range to be approximately the same or at least to have no pronounced dips in the visible wavelength range.

FIG. 3 shows a development of the lighting device 1 according to the invention of FIG. 1, wherein identical elements are designated by the same reference numerals and reference is made to the above explanations for their description.

In the embodiment of FIG. 3, a fast photodiode 9 (which can be a PIN or an avalanche photodiode, for example, a PIN or an avalanche photodiode) is additionally arranged between the spectral filter 7 and the illumination optics 8 in conjunction with a partial beam extraction for measuring the intensity of the laser radiation L 4, wherein only a small portion of the radiation L5 is coupled out for the photodiode 9. The temporal intensity fluctuation measured with the photodiode 9 is supplied to a control system 10 (arrow P1) which influences the laser 2 (as indicated by the arrow P2) to counteract the intensity fluctuation.

By means of the photodiode 9 and the control system 10, e.g. the fluctuation is measured in terms of time and extrapolated from the intensity changes of the future intensity course. The laser 2 is then influenced (as indicated by the arrow P2) so that the intensity fluctuations of the extrapolated intensity curve are compensated.

But it may also be sufficient to determine a moving average as a rule input.

The control system 10 can influence not only the laser 2 but, for example, also the spectral filter 7 (arrow P3). Furthermore, it is possible for the control system to influence an optionally provided intensity modulator 11 (shown in dashed lines in FIG. 3) (arrow P3), which is arranged between the photodiode 9 and the illumination optical unit 8. The intensity modulator 11 can be designed, for example, as an acousto-optic modulator (AOM) or as an electro-optical modulator (EOM).

Further, the control system 10 may drive a controllable component (not shown) of the illumination optics 8, as shown schematically by the arrow P5. The controllable component of the illumination optical unit 8 may be, for example, an AOM, an EOM, a tilting mirror matrix, an LCD element, a mechanically changeable diaphragm or an attenuator. The control system 10 can use one of the control circuits P2-P5, several of the control circuits P2-P5 or even all of the control circuits P2-P5. If the control loop P4 is not used, the intensity modulator 11 is omitted.

4, a further embodiment of the lighting device 1 is shown schematically, wherein the same elements as in the embodiment of Fig. 1 are designated by the same reference numerals and reference is made to the description thereof to the above statements. The embodiment of FIG. 4 may comprise a control system 10 in the same way as in FIG.

In the embodiment of FIG. 4, an optical diode 12 is disposed between the laser 2 and the first non-linear optical element 4. The optical diode 12 serves to avoid interference of the laser 2 by back reflections from the further beam path (to the right of the diode 12 in FIG. 4).

The first non-linear optical element 4 is designed here as a tapered fiber whose dispersion zero point of the group velocity is below the wavelength of the laser radiation L1 of 1064 nm. This ensures that the laser radiation L1 lies in the abnormal dispersion region of the tapered fiber 4, which is a prerequisite for effective continuum generation.

The tapered portion of the first nonlinear optical element 4 has a length of 50-200 mm and can be used as a design parameter. Thus, the length of the tapered portion can be selected so that the spectral range of the laser radiation L2 emitted from the first nonlinear optical element 4 extends from about 800 to 1300 nm. The diameter of the tapered area is about 2.5 microns.

The laser radiation L2 is coupled by means of a coupling optics 13, which is embodied here as lens optics, into the second non-linear optical element 5, which is designed as a photonic crystal fiber (PCF = Photonic Crystal Fiber). Instead of the coupling optics 13, the fibers 4 and 5 can also be optically connected by means of direct splicing (fusing or gluing).

The PCF 5 is designed so that its dispersion zero point is below 800 nm, so that it is ensured here that the wavelengths of the spectral range of the laser radiation L2 are in the abnormal dispersion range of the PCF 5. The length of the PCF 5 is a design parameter. The length is chosen so that the visible continuum of the wavelength L3 behind the PCF 5 has the highest possible optical power and / or spectrally possible is homogeneous. The core diameter and the diameter of the PCF 5 air holes are each about 2 μm. The length of the PCF 5 is in the range of 0.5 - 10 m.

In the embodiment of Fig. 4, the module 3 thus exactly two nonlinear optical elements 4, 5, which are preferably selected so that the continuums generated by them are as complementary as possible, so that in the sum of a substantially homogeneous spectrum. Of course, in the embodiment of Fig. 4, the module 3 may have more than two nonlinear optical elements. This can be used, for example, to further improve the spectral homogeneity.

FIG. 5 shows a modification of the lighting device 1 of FIG. 4. The embodiment of FIG. 5 can also have a control system 10 in the same way as in FIG. 3. In the illumination device 1 of Fig. 5, the laser 2 emits laser radiation having a wavelength of 1064 nm with a pulse length of 300 fs and a pulse rate of 50 MHz at an average optical power of 5 watts. Because of the shorter laser pulses of the laser 2 of the illumination device 1 of Fig. 5 compared to the laser pulses of the laser 2 of the illumination device 1 of Fig. 4, the tapered portion of the fiber 4 in Fig. 5 has a diameter of 3 microns and thus a slightly larger Diameter as the tapered portion of the fiber 4 of the lighting device 1 of Fig. 4 on.

The second non-linear optical element 5 is also in the embodiment of Fig. 5 formed as a tapered fiber (and not as PCF), with their dispersion zero point is below 800 nm. The length of the tapered portion of the fiber 5 is again a design parameter and is in the range of 50-200 mm; the diameter of the tapered section is about 2 microns.

The features of the individual embodiments described can, as far as appropriate, be combined with each other.

In particular, the described embodiments can be trained or further developed as a microscope.

Claims

Carl Zeiss Microlmaging GmbH January 14, 2008Note: PAT 4024/021 -PCTPatentansprüche

1. A device for illuminating an object, comprising an illumination optical unit (8), a module (3) arranged upstream of the illumination optical system (8), which has at least two non-linear optical elements (4; 5; 6) arranged one behind the other, and one the laser (2) upstream of the module (3) which emits laser radiation (L1) to the module (3), which passes in sequence through the elements (4-6) arranged one behind the other and is emitted to the illumination optics (8) with the laser radiation (L4) coming from the module (3) being able to illuminate the object, each nonlinear element (4; 5; 6), due to its nonlinear effect, causing the spectral bandwidth of the non-linear element (4; 5; 6) to be emitted Laser radiation (L2; L3; L4) is greater than the spectral bandwidth of the laser radiation (L1; L2; L3) coupled into the non-linear element (4; 5; 6), and wherein the device comprises a control circuit which can compensate for intensity fluctuations of the module (L2; 3) coming lase radiation (L4) and depending on the measured intensity fluctuations a controllable component of the device so controls that the intensity fluctuations are at least partially compensated.

2. Apparatus according to claim 1, wherein the laser (2) emits laser radiation (L1) in the infrared spectrum and extends the spectrum of the module (3) emitted laser radiation (L4) in the visible range.

3. Device according to one of the above claims, wherein the spectrum of the first nonlinear element (4) emitted laser radiation (L2) is located exclusively in the infrared spectral range.

4. Device according to one of the above claims, wherein at least one of the optical elements (3 - 5) is formed as a tapered optical fiber.

5. Device according to one of the above claims, wherein at least one of the non-linear elements (3-5) is formed as a photonic crystal fiber.

6. Device according to one of the above claims, wherein at least one of the elements (3 - 5) is formed as a nonlinear crystal.

7. Device according to one of the above claims, wherein between the module (3) and the illumination optics (8) a spectral filter unit (7) is arranged.

8. Device according to one of the above claims, wherein the component is designed as a controllable intensity modulator (11) between the module (3) and the illumination optics (8).

9. Device according to one of the above claims, wherein the device is designed as a microscope.

10. A method of illuminating an object in which a primary laser radiation passes through at least two non-linear optical elements arranged one behind the other and the laser radiation emitted by the last optical element is used to illuminate the object, each nonlinear element causing, due to its nonlinear effect, that the Spectral bandwidth of the laser radiation emitted by the nonlinear element is greater than the spectral bandwidth of the laser radiation coupled into the nonlinear element, and whereby intensity fluctuations of the laser radiation coming from the last element are measured and depending on the measured intensity fluctuations, the method is controlled so that the intensity fluctuations at least partially be compensated.

11. The method of claim 10, wherein the primary laser radiation is in the infrared spectrum and extends the spectrum of the emitted from the last element laser radiation in the visible range.

12. The method of claim 10 or 11, wherein the spectrum of the first nonlinear element, to which the primary laser radiation impinges emitted laser radiation (L2) is located exclusively in the infrared spectral range.

13. The method of claim 10, wherein at least one of the optical elements is formed as a tapered optical fiber.

14. The method according to any one of claims 10 to 13, wherein at least one of the non-linear elements is formed as a photonic crystal fiber.

15. The method according to any one of claims 10 to 14, wherein at least one of the elements is formed as a nonlinear crystal.

16. The method according to any one of claims 10 to 15, wherein the laser radiation emitted by the last element is spectrally filtered and the object is illuminated with the spectrally filtered laser radiation.

17. The method according to any one of claims 10 to 16, wherein for controlling the method, the coming of the last element laser radiation by a controllable

Intensity modulator is performed, which is driven accordingly and emits the laser radiation with reduced intensity fluctuations to illuminate the object.

18. The method according to any one of claims 10 to 17, wherein the illumination of the object is performed for a microscopy method.