METHOD AND DEVICE FOR CONTROLLING AN ACOUSTOPTIC COMPONENT

Description

State of the art

[0001] A laser system in a laser scanning microscope, as described, among others, in EP 1 795 938 A2, DE 19702753C2, has an acousto-optic element (AOTF / AOM) for wavelength selection and influencing the intensity of the laser light, the spectral properties of which are adapted to the spectral characteristics of the laser light to be transmitted.

[0002] Fig. 1 shows an example of an ideal transfer function H(λ) of an AOTF with a rectangular transducer when excited by a single sine wave.

[0003] The optical transmission properties of the acousto-optic element (AOTF / AOM) are typically determined by the crystal's cut and the geometry of the transducer mounted on it. The choice of geometry can influence both the width of the main lobe and the decay behavior of the side lobes of the transfer function H0(λ). A rectangular transducer is frequently used, whose normalized transfer function H0(λ) describes the sinc2 function. H 0 λ = sinc 2 λ λ 0

[0004] The frequency fn of the exciting RF signal for a given laser wavelength λn is determined, among other things, by the material properties and the geometry of the crystal. It is inversely proportional to the laser wavelength λn, which can be influenced by the excited diffraction grating. λ n ∼ 1 f n

[0005] The transfer function H 0 (λ) is characterized, among other things, by the following parameters: B 3dB - bandwidth at which the main lobe of the transfer function has decayed to half its maximum (also FWHM - full width at half maximum ) BN - Distance between the zeros that limit the main lobe of the transfer function

[0006] For the simultaneous control of two or more laser lines (λ₁, λ₂, ... λₙ), the RF signal used to control the acousto-optic element contains a sinusoidal carrier for each laser line. Each sinusoidal carrier of frequency f₁, f₂, ... fₙ excites a diffraction grating in the acousto-optic element for the corresponding laser line.

[0007] Based on the relationship λn ~ 1 / fn, the properties of the transfer function can also be characterized as a function of the frequency of the driving RF signal. In particular, a ΔfBN can be specified to describe the width of the main lobe as a function of the tuning frequency of the drive signal. The following holds true: Δ f B N ~ 1 / B N

[0008] For the rectangular For the transducer, B 3dB = 0.89 and BN = 2. The values ​​BN and B 3dB are therefore related to each other as follows: B 3 dB = 0 , 445 B N bzw . FWHM = 0 , 445 B N Conventional use of the AOTF

[0009] The distance Δλ between two adjacent laser lines is chosen to be at least large enough that the main lobes of the transfer function H₀(λ) of the acousto-optic element do not overlap. This is the case if the distance Δλ between two laser lines is not less than the zero-intercept BN of the main lobe of the transfer function H₀(λ) of the acousto-optic element. The following applies: Δλ ≥ B N

[0010] For the rectangular transducer, this results in: Δλ ≥ 2 , 247 FWHM

[0011] Modifying the transducer geometry to adjust the transmission characteristics typically involves a significant increase in the required power of the AOTF's high-frequency drive signal to achieve the required diffraction efficiency of >90%. Compared to a standard AOTF with a rectangular transducer, an increase in RF drive power of up to 7 dB may be necessary, depending on the laser wavelength. Typical values ​​are >+27 dBm (~500 mW) for the modified AOTF, as opposed to +20 dBm (~100 mW) for a standard AOTF.

[0012] Increasing the RF control power causes additional heating of the acousto-optic element and, due to the dependence of the speed of sound on the temperature in the crystal (e.g. TeO2), results in a change in its optical properties.

[0013] The heating process causes a significant spectral shift, particularly in the transfer function H 0 (λ).

[0014] This undesirable temperature drift can be counteracted by heating the AOTF crystal to a temperature significantly above room temperature (e.g. 50°C) using a controlled electric heater (DE-19827140-A1).

[0015] Alternatively, it is possible to stabilize the temperature using a Peltier element or to implement temperature-controlled frequency tracking of the RF drive signal, as described for example in DE202007015506 U1 and in DE19827140 A1.

[0016] Local changes in crystal temperature due to short-term variations in RF drive power – for example, due to intensity modulation of the laser light during image acquisition – cannot be detected and compensated for by such methods and, with respect to the first diffraction order of the AOTF, result in undesirable changes in the angle and intensity of the laser light due to the spectral shift of the transfer function H 0 (λ).

[0017] Interference occurring when using multiple wavelengths is described by Tian & Herrmann (Journal of Lightwave Technology 13: 1146–1154; 1995). Adjusting the bandpass of an AOTF by excitation with multiple frequencies is also known from US3729251. Furthermore, DE102006053187 A1 discloses a microscope with an AOTF whose bandpass is adjustable. Description of the invention and its effects and advantages

[0018] The invention, in the case of a device or a method according to the preambles of the independent claims, is characterized by the respective characterizing features.

[0019] The invention defined by a device according to claim 1 or a method according to claim 5, and its effects and advantages, are explained in more detail below. Novel use of the AOTF

[0020] According to the invention, the acousto-optic element is driven by two or more sinusoidal RF signals per laser line. The spectral spacing Δf of the sinusoidal RF signals is deliberately chosen such that the main lobes of the excited transfer functions H0(λ) overlap. This overlap of the main lobes of the excited transfer functions HN(λ) is intentional, as the superposition of the individual transfer functions leads to a broadening of the main lobe in the resulting transfer function HΣ(λ). Example:

[0021] AO Tunable Filter AOTF. nc-VIS from AA Opto-Electronic (datasheet values): • Optical Wavelength λ 450 - 700 nm • Spectral Resolution (FWHM) 1.5 nm • Drive Frequency f 153 - 80 MHz • Tuning slope k Tune 0.292 MHz / nm

[0022] Using the relationships given above and assuming a rectangular transducer, the following values ​​can be calculated: • Distance between zeros BN 3.37 nm (BN = FWHM / 0.445) • Distance between zeros Δf BN 0.98 MHz (Δf BN = k Tune · BN ) In conventional use, this means: • Minimum distance between laser lines Δλ ≥ 3.37 nm (Δλ ≥ 2.247 FWHM) • Spacing of the sinusoidal supports Δf when used in accordance with the invention: ≥ 0.98 MHz • Spacing of the sinusoidal supports Δf < 0.98 MHz

[0023] This datasheet specifies a value of 30kHz / °C for the drift of the AOTF frequency as a function of temperature.

[0024] By cleverly choosing the initial phase and amplitude of the individual carriers of the generated signal, it is possible to achieve a (nearly) constant envelope for the signal I.

[0025] The invention is described below with reference to the schematic drawings in Fig. 2-5 explained in more detail.

[0026] In Fig. 2 A light source is schematically represented, for example by a laser L.

[0027] The light source can also consist of several lasers combined via beam splitters, as shown in the prior art. The light from the light source L is directed via an AOTF (Automatic Optical Fiber) for wavelength selection and intensity control towards a microscope, preferably a laser scanning microscope.

[0028] The light source L and AOTF can also be arranged in an illumination module BM (shown in dashed lines) which is optically connected to the microscopic beam path by direct coupling or by one or more optical fibers.

[0029] A control unit AS, usually combined with other microscope functions, also serves to control the AOTF.

[0030] Between the control unit and the AOTF, a multi-frequency mixer unit MIX is advantageously provided as a frequency generator according to the invention, the operation of which will be explained in more detail below: The invention relates to the control of an acousto-optical element (AOTF / AOM) with a high-frequency signal. Such a signal contains several high-frequency carriers which – not necessarily – have a uniform distance Δf from each other and exhibit identical or different signal amplitudes.

[0031] This type of control causes several transfer functions H 0 (λ) specified by the crystal and transducer geometry to be superimposed, which are shifted relative to each other by a multiple of Δf and are evaluated by the amplitude of the respective exciting individual carrier.

[0032] The resulting transfer function HN (λ) is obtained from the superposition of the spectrally shifted transfer function H 0 (λ) of the individual carriers exciting the acousto-optic element.

[0033] Regarding the decay behavior of the side lobes of the transfer function, it is useful to choose the carrier spacing Δf of the driving signal such that the main lobes of neighboring transfer functions complement the minima of their respective direct neighbors.

[0034] The transfer function HN(λ) of the acousto-optic element undergoes spectral spreading due to this control and the resulting superposition of several transfer functions H0(λ), which in particular results in a widening of its main lobe. The decay behavior of the side lobes, which is determined by the sinc 2< function in rectangular transducers, does not change or does not change significantly.

[0035] The excitation of an acousto-optic element (AOTF / AOM) according to the invention thus allows the spectral width of the main lobe of the transfer function to be arbitrarily increased without significantly altering the decay behavior of the side lobes. This makes it possible to spread the main lobe of the transfer function so that both the pulse of a laser system, as described in EP 1 795 938 A2, is transmitted optimally, and the temperature drift of the crystal no longer influences the transmission of laser light of a specific wavelength. This is achieved by spreading the width of the main lobe by the number of carriers of the radio frequency signal and their spacing to such an extent that the pulse to be transmitted, with its spectral width, or the wavelength to be transmitted of a gas or DPSS laser system, remains within the main lobe of the transfer function even at maximum temperature drift of the crystal.The temperature range should encompass ambient temperatures between 15 °C and 35 °C, whereby the operating temperature of the crystal can be about 20 °C higher than the ambient temperature.

[0036] The inventive solution for controlling acousto-optical elements (AOTF / AOM) makes it possible to adapt the spectral properties of the crystal to the spectral properties of the coupled laser light without modifying the transducer geometry, i.e., using standard components. An additional benefit is that heating the crystals can be completely eliminated. This reduces the detrimental heat input into the system. The elimination of the heating element and the associated electronic and mechanical components enables the design of more compact assemblies, allowing for a higher degree of system integration. Furthermore, adverse effects from local heating of the crystals no longer occur, as the resulting drift of the transfer function is also compensated for by the widening of its main lobe.

[0037] Another advantage is the almost rectangular characteristic of the main lobe of the transfer function HN (λ), which ensures that the angle and intensity of the laser light in the first diffraction order of the AOTF remain stable over the entire specified temperature range of, for example, 15°C to 45°C.

[0038] The principle of compensating for temperature drift when controlling the acousto-optic element with a high-frequency signal is illustrated. Figure 3 .

[0039] It demonstrates the inventive principle of compensating the temperature drift of an AOTF / AOM by controlling the crystal with five carriers of essentially the same power.

[0040] The transfer function HN (λ) at room temperature (2) is spectrally shifted by heating the crystal (3); the width of its main lobe and the decay behavior of the side lobes are not affected. By a suitable choice of the number and spacing of the carriers, the main lobe of the transfer function is spectrally spread by superimposing several individual main lobes, such that the laser line (1) with a fixed wavelength is transmitted without distortion over the entire temperature range.

[0041] In Fig. 4 and 5 The generation of the sum transfer function when controlling an AOTF with an OFDM signal (Orthogonal Frequency-Division Multiplexing) should be further clarified.

[0042] In both figures, an OFDM signal with three carriers is used – the resulting transfer function HN(λ) is thus obtained from the superposition of three curves Ho(λ), H+1(λ) and H-1(λ), ​​respectively, which are spectrally shifted by λ or 0.7 n λ relative to each other. Fig. 4 and Ho(λ) H+0.7(λ) H -0.7(λ).

[0043] The images differ in the magnitude of the spectral shift. In the drawing Fig. 4 The shift is chosen such that the main lobe of a transfer function always falls into a minimum of the adjacent function. In Figure 5, the shift is slightly smaller (0.7 instead of 1).

[0044] The high-frequency signal is preferably generated using an I / Q mixer, which can be either analog or digital. The complex baseband signals for driving the in-phase and quadrature paths of the I / Q mixer can be advantageously calculated using the inverse discrete Fourier transform (IDFT) when the signal carrier spacing is equidistant.

[0045] The I / Q mixer also has the advantageous property of suppressing the sidebands of the set main signal, so that an optimal main lobe of a signal can be generated.

[0046] To adjust the frequency generator and the I / Q mixer, a measured distribution of the temperature-dependent frequency drift of the AOTF can advantageously be used, wherein, for example, a monitor diode is advantageously arranged in the beam path, which detects the intensity change of the signal transmission of the AOTF as a function of the ambient temperature.

[0047] The control of the described signal generator to generate the signal can then advantageously be carried out in such a way that the resulting broadened transfer function has essentially the same spectral characteristic for a desired temperature range.

[0048] For this purpose, an optimization procedure can also be advantageously carried out with the signal generator in order to generate an optimal number and distribution of the spectrally offset transfer functions depending on the given temperature difference in which the AOTF has the same transfer characteristics.

[0049] The high-frequency signals for controlling the signal generator and the I / Q mixer can advantageously be generated using a freely programmable component such as an FPGA.

[0050] The high-frequency signals for controlling the signal generator and the I / Q mixer can advantageously be generated using a freely programmable component such as an FPGA.

[0051] FPGAs have already been described in a completely different context in connection with laser scanning microscopes: DE102006034905 A1, DE19858456 A1.

[0052] The invention solves, in an unexpectedly simple yet highly versatile manner, the complex attempts described in the aforementioned prior art to overcome the problem of temperature dependence in acousto-optic components. The invention is applicable to acousto-optic components in microscopes that influence both the illumination and detection beam paths, for example, to adjust the bandwidth of transmitted or blocked wavelength ranges, even in the case of a broadband light source ("white light laser").

Claims

1. Apparatus for controlling an acousto-optic component for a microscope, wherein the acousto-optic component is configured to influence the laser light which is in the form of a pulse of a laser system with a spectral width or a laser line (1) at a specific wavelength and which passes through said acousto-optic component as illumination and / or detection light of a microscope, comprising - the acousto-optic component, wherein the acousto-optic component has a transfer function (H0(λ)) which describes a relationship between the spectral properties of the acousto-optic component and a spectral characteristic of the laser light to be transmitted, - and at least one frequency generator for setting an illumination wavelength of the illumination light and / or a detection wavelength of the detection light, wherein the frequency generator, for controlling the influence, is connected to the acousto-optic component, characterized in that the frequency generator is designed such that it creates a signal which, as a result of the superposition of a plurality of individual main lobes of transfer functions, leads to a resultant main lobe of a resultant transfer function being spectrally dispersed such that the spectral width of the pulse or the laser line (1) at a fixed wavelength is still located within the main lobe of the transfer function, even in the event of a maximum temperature drift of the AOTF crystal, wherein the acousto-optic component is controlled by two or more control signals in such a way that two or more overlapping and / or superimposing main lobes of the transfer function of the acousto-optic component or main maxima are created.

2. Apparatus according to Claim 1, wherein the acousto-optic component is an AOTF or an AOM.

3. Apparatus according to either of preceding Claims 1 and 2, wherein the frequency generator is an I / Q mixer.

4. Method for controlling an acousto-optic component for a microscope, wherein the acousto-optic component influences the laser light which is in the form of a pulse of a laser system with a spectral width or a laser line (1) at a specific wavelength and which passes through said acousto-optic component as illumination and / or detection light of a microscope, wherein the illumination and / or detection wavelength is set using at least one frequency generator that is connected to the acousto-optic component and controls the influence, characterized in that the frequency generator is used to create a signal which, as a result of the superposition of a plurality of individual main lobes of transfer functions, spectrally disperses a resultant main lobe of a resultant transfer function such that temperaturedependent shifts of the transfer function do not modify the transfer behaviour of the crystal of the acousto-optic component, and the spectral width of the pulse or the laser line (1) at a fixed wavelength is still located within the main lobe of the transfer function, even in the event of a maximum temperature drift of the AOTF crystal, wherein the acousto-optic component is controlled by two or more control signals in such a way that two or more overlapping and / or superimposing main lobes of the transfer function of the acousto-optic component or main maxima are created.

5. Method according to Claim 4, wherein the main lobes are each delimited by two minima.

6. Method according to either of Claims 4 and 5, wherein the main lobe intensity / side lobe intensity ratio is at least 10:1.

7. Method according to any of Claims 4-6, wherein spectrally shifted transfer frequencies are provided on both sides of the illumination and / or detection wavelength.

8. Method according to any of Claims 4-7, wherein the signal distribution of the frequency generator is set on the basis of a measured temperaturedrift curve of the acousto-optic component.

9. Method according to any of Claims 4-8, wherein, in an optimization method, the signal distribution is modified, and the arisen distribution is captured and corrected in respect of its temperature behaviour.

10. Microscope containing an apparatus according to any of Claims 1-3.