Method for controlling a swept source interferometer system based on semiconductor laser diodes

Abstract

The invention relates to a method for controlling an SS interferometer system (SS=swept source) based on a semiconductor laser diode, which interferometer system allows a wide range of applications and is suitable for applications in ophthalmology, in particular for imaging and determining biometric measurements of the eye. In the method according to the invention, the operation of individual semiconductor laser diodes is set by means of periodic current modulation in order to be able to tune a high-coherence spectral laser line in a wide wavelength range with the highest possible repetition rate. The parameters: center wavelength, scanning rate, scanning range, optical power at the eye and coherence length are adapted in this case so that the method is suitable for imaging and biometric applications by full-eye scanning. The proposed SS interferometer system based on a semiconductor laser diode is particularly designed for biometric measurements of the eye. Since the presentation is preferably based on optical coherence tomography recordings, the main applications are in ophthalmic diagnosis, preparation for therapeutic and surgical interventions and their subsequent examination.

Description

Technical Field

[0001] The invention relates to a method for controlling a SS interferometer system (SS=swept source) based on semiconductor laser diodes, which allows for a wide range of applications. The system should also be suitable for applications in ophthalmology, in particular for imaging and determining biometric measurements of the eye. Background Art

[0002] An interferometer is an optical device that uses interference for precision measurement. Any effect that changes the effective path length of the waves and thus changes the properties of the superimposed waves is measured.

[0003] Examples of this are: varying the length of one of two superimposed light paths to measure length, varying the refractive index to measure material properties or minimal changes in the spacing between test objects in a gravitational wave detector.

[0004] Correspondingly, a large number of solutions are known from the prior art.

[0005] A special interferometric application as an imaging method is optical coherence tomography (OCT). With the aid of OCT systems, 2- and 3-dimensional recordings of scattering materials with micrometer resolution can be obtained. The main application areas of OCT are medicine, especially ophthalmology.

[0006] In the OCT method, coherent light is used for distance measurement and imaging at reflecting and scattering samples with the aid of an interferometer. In the human eye, the OCT method provides measurable signals during depth scanning due to the refractive index changes and volume scattering that occur at optical boundary surfaces. Optical coherence tomography is a very sensitive and fast method.

[0007] In order to design the measured value detection even more efficiently, OCT systems based on the so-called "swept source" technology (SS-OCT) have been used in recent years. The frequency of the light source is tuned and a depth signal is generated from this. This technology allows a full eye scan to be performed at the human eye. Of course, this requires a corresponding selection and control of the illumination light source.

[0008] Current SS-OCT systems use complex microelectromechanical laser diode systems (MEMS) to tune spectral laser lines with high coherence lengths (in the cm to m range) in a wide wavelength range up to 150 nm at high repetition rates (in the kHz to MHz range). This is necessary to obtain highly resolved images very quickly with high axial resolution over large measurement depths, especially in transparent organic tissues (e.g. the human eye).

[0009] In optics, the coherence length is the maximum path length or travel time difference that two light beams from the same source can have in order to still generate a (spatially and temporally) stable interference pattern when they are superimposed.

[0010] Detection in an OCT system cannot of course be performed in a noise-free manner. Therefore, reflections in a sample can only be detected if the sample provides a signal that is greater than the background noise in the OCT system.

[0011] This minimum reflection that can still be detected is a very important characteristic variable of an OCT system and is called sensitivity and is usually stated in dB.

[0012] When considering the sensitivity, the so-called “Sensitivity Roll-Off” criterion must also be taken into account, which is understood as the decrease in the amplitude of the interference signal with increasing length difference between the reference arm and the sample arm.

[0013] For the OCT system described here, a sensitivity of -6 dB is defined (considering the "sensitivity roll-off" criterion).

[0014] However, according to the known prior art, there have also been efforts and attempts to use other laser diodes, such as VCSELs (Vertical Cavity Surface Emitting Lasers). An overview can be found in the article “Ultra-Widely Tunable VCSELs” by author Garrett D. Cole [1].

[0015] For this purpose, reference is made, for example, to the single-mode (also mono-mode) VCSEL laser diode from Philips, whose technical data are described in [3]. This VCSEL laser diode, which can be tuned by 2 nm, can be operated thermoelectrically in a temperature range of 10°C to 40°C with small differences in the central wavelength.

[0016] The authors Sucbei Moon and Eun Seo Choi describe in [4] a low-cost OCT system based on a VCSEL laser diode, which can be briefly tuned by current pulsation or temperature shock at a wavelength of approximately 1300 nm. However, in order to exclude motion artifacts when measuring the eye, very high laser scanning rates (repetition rates) in the range of 10-100 kHz are proposed. However, due to absorption in the vitreous body, the disclosed design for a low-cost OCT system with a wavelength of 1300 nm is not suitable for whole-eye measurements. In addition, when using a wavelength of 1300 nm, a scanning range (tuning range) of at least 25 nm (preferably 75 nm) must be achieved in order to achieve the required resolution of 30 μm (preferably 10 μm) in air. However, such a scanning range cannot be expected. Therefore, the parameter combinations mentioned here are not very suitable for building an optical biometer that meets the requirements of a competitive system.

[0017] DE 10 2008 028 312 A1 describes the use of VCSEL laser diodes for eye measurements. Here, the laser diode spectrum operates in a narrowband at a wavelength of approximately 850 nm with a typical coherence length of 100 mm and a spectral width of approximately 0.007 nm, thereby providing a scanning depth sufficient for measuring the entire length of the eye. However, in order to be able to achieve the required measurement accuracy with a maximum spectral tuning of 3 nm at a wavelength of 850 nm, a slow laser diode tuning in the range of less than 10 Hz can be used as a basis. This necessitates the additional use of a position recognition system, which is necessary in the case of a slowly tuned laser diode, in order to be able to measure moving objects, such as the human eye, at a relatively slow repetition rate of the tuned laser light.

[0018] A miniaturized, inexpensive OCT system for ophthalmic applications is described in WO 2018 / 119077 A1. In particular, the system is provided for measuring the thickness of the retina, wherein the system is suitable for enabling patients to carry out the measurements themselves at home due to its compactness and lightness. The SS-OCT system described here is based on a VCSEL laser diode, which is controlled by periodically varying the current. Due to the parameters used, a full eye scan is not possible with the described system. Instead, a system is described in which the optical device comprises an optical scanning element in order to enable the light source to be impressed onto different locations on the retina.

[0019] literature:

[0020] [1] Cole et al.; “Ultra-Widely Tunable VCSELs”;

[0021] http: / / www.aomicro.com / tech / Cole_TUM_27_Sep_2012.pdf

[0022] [2] Nanoplus; “DFB laser diodes from 760nm to 830nm”;

[0023] https: / / nanoplus.com / fileadmin / user_upload / Data_sheets / nanoplus_DFB_760-830nm.pdf

[0024] [3] Philips, 760 / 763nm single-mode VCSEL;

[0025] http: / / www.photonics.philips.com / pdf / ULM76X-SingleMode_TO5.pdf

[0026] [4] Moon et al.; “VCSEL-based swept source for low-cost optical coherence tomography”, Biomedical Optics Express, Vol. 8, No. 2, Feb 1, 2017, p. 1110-1121.

[0027] [5] Flogan B.; “Operation of VCSELs Under Pulsed Conditions”, VIXAR Application Note, 21. January 2010;

[0028] [6] Bublitz et al, “SS-OCT-interferometry for measuring a sample”;

[0029] US 8,632,181 B2

[0030] [7] Choi W, et al, “Phase-sensitive swept-source optical coherence tomography imaging of the human retina with a vertical cavity surface-emitting laser light source”; Opt Lett. (Optical Communications), 2013; 38(3): 338-340. Summary of the invention

[0031] The object of the invention is to develop a method for controlling a simple SS interferometer system based on semiconductor laser diodes, which is suitable for imaging and biometric measurements at the eye. The parameters for controlling the laser beam source are optimized in such a way that a wide wavelength range can be tuned with a large coherence length and a relatively high repetition rate. In particular, biometric measurements of the eye should be possible with the aid of a full eye scan.

[0032] According to the invention, this object is achieved by the features of the independent claim. Preferred developments and embodiments are the subject matter of the dependent claims.

[0033] According to the invention, this object is achieved by setting up the operation of a simple semiconductor laser diode by means of periodic current modulation in such a way that a highly coherent spectral laser line can be tuned at the highest possible repetition rate and in a wide wavelength range. The parameters: center wavelength, scanning rate, scanning range, optical power at the eye and coherence length are adapted in this process so that the method is suitable for imaging and biometric applications by full eye scanning.

[0034] A first group of advantageous embodiments relates to the configuration or adaptation of the semiconductor laser diodes used, for example to the type of laser diodes and the surface emitters used, the configuration of their active regions and the optimization of the emission geometry.

[0035] A second group of advantageous embodiments relates to the control of the semiconductor laser diode, in particular the setting and stabilization of a periodic current modulation and / or a defined nm / K gradient, wherein Peltier elements can be used.

[0036] In a method for controlling a SS interferometer system based on a semiconductor laser diode, in which the operation of the semiconductor laser diode is provided according to the invention by means of periodic current modulation in order to be able to tune a highly coherent spectral laser line with the highest possible repetition rate and in a wide wavelength range, the following parameters are provided in particular:

[0037] - with a coherence length of at least 20 mm,

[0038] - a central wavelength in the range of 600nm to 1400nm,

[0039] - scanning rates in the range of 100 Hz to 100 kHz,

[0040] - a scanning range in the range of 3 nm to 75 nm, and

[0041] - Optical power at the eye in the range of 50 μW to 20000 μW.

[0042] Regarding the optical power of SS interferometer systems based on semiconductor laser diodes, it is proposed that the maximum permissible optical power be provided at the patient's eye at a given wavelength in order to achieve the maximum signal / noise ratio while complying with safety regulations. Since approximately 50% of the power of the beam source (semiconductor laser diode) is considered as loss in the optical system of the interferometer, the higher power of the semiconductor laser diode is planned accordingly:

[0043] Central wavelength λ Maximum optical power at the eye Semiconductor laser diode power 600nm 390μW >0.8mW 800nm 620μW >1.2mW 1050nm 1.95mW >4mW 1300nm 15.6mW >31mW

[0044] The optical power refers to the radiation power to be applied to the human eye, as defined, for example, in the laser standard DIN EN 60825-1. The optical power is related to the wavelength and the temporal shaping of the laser beam. For simplicity, the laser powers listed here refer to CW operation of the diode and laser class 1. For pulsed radiation and other laser classes, other pulse peak values ​​may apply. If necessary, country-specific standards must also be taken into account.

[0045] The proposed SS interferometer system based on semiconductor laser diodes is particularly suitable for biometric measurements of the eye. Since the demonstration is preferably based on optical coherence tomography recordings, the main applications are in ophthalmic diagnosis, preparation for therapeutic and surgical interventions and their follow-up. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The present invention is described in more detail below based on an embodiment. Here, English and German terms are used synonymously. For this purpose, it is shown:

[0047] Figure 1 shows a schematic diagram for controlling a semiconductor laser diode-based SS interferometer system according to the present invention by means of periodic current modulation, and

[0048] Figure 2 Some further optical parameters of the radiation emitted by the semiconductor laser diode of the SS interferometer system are shown. DETAILED DESCRIPTION

[0049] SS interferometer systems based on semiconductor laser diodes consist of a semiconductor laser diode and a control unit, wherein the control unit is designed to control the operation of the semiconductor laser diode by means of periodic current modulation in order to be able to tune a highly coherent spectral laser line in a wide wavelength range with the highest possible repetition rate.

[0050] For this purpose, the control unit is designed for periodic current modulation in order to vary the time and amplitude of the current pulsation.

[0051] As semiconductor laser diodes, for example VCSEL laser diodes are used which can be based on GaAs wafer surface emitters or on single-mode AlGaInP wafers with a multi-quantum well structure.

[0052] In particular, it is proposed according to the present invention that only single-mode VCSEL laser diodes are used, and no multi-mode VCSEL laser diodes are provided.

[0053] Single-mode VCSEL laser diodes are characterized by a very small instantaneous line width, a large coherence length and the measurement depth required for biometric measurements. In this case, single-mode VCSEL laser diodes should have a coherence length of at least 20 mm, but in particular 60 mm. This ensures that very long eyes, such as those in the case of high myopia, can be measured reliably.

[0054] The system is therefore also suitable for serial myopia checks, as are currently being carried out in Asia, for example, in order to combat myopia in the population.

[0055] Single-mode VCSEL laser diodes should be used with a typical laser line spectral width of 100 MHz. Corresponding lasers are available from Trumpf, for example.

[0056] To set and stabilize a defined nm / K gradient, the VCSEL laser diode can be embedded in an active semiconductor material, wherein the active semiconductor material is dimensioned accordingly and matched to the adjacent semiconductor material layer. To optimize the resulting heat sink, thermally conductive materials and / or Peltier elements can be used.

[0057] In particular, the construction of VCSEL laser diodes can be designed to be able to modify their active regions to achieve up to 20 mW of continuous optical power at the eye.

[0058] According to the invention, a VCSEL laser diode is provided with a wavelength in the range of 600 to 1400 nm, in particular approximately 1050 nm.

[0059] It should be noted here that for the same resolution, shorter wavelengths require a smaller tuning range (SweepRange), and the repetition rate is especially the biggest technical challenge in this technology. Therefore, short wavelengths in the red spectral range of, for example, 600 nm and in the IR (infrared) range of, for example, 700 nm are most suitable for this. For example, a single-mode AlGaInP laser diode with a multi-quantum well structure and a wavelength of 690 nm is suitable. In addition, VCSEL laser diodes with a wavelength of 840 nm are available and well suited.

[0060] Currently, VCSEL laser diodes with a wavelength of about 1050 nm are more suitable for cataract penetration or deeper penetration depths into tissue and are therefore preferred.

[0061] Furthermore, according to the invention, a VCSEL laser diode is provided with a repetition rate (sweep rate) of 100 Hz-100 kHz, in particular 1 kHz. In sharp contrast to [6], the SS interferometer system according to the invention does not require any additional movement signals of the eye in order to be able to evaluate the measurement results.

[0062] It should be taken into account that a repetition rate of 1 kHz is sufficient to compensate for motion artifacts and that, with the smallest possible repetition rate, a higher signal-to-noise ratio can be achieved at a given limited power of the laser diode. In particular, a repetition rate of 1-3 kHz is set as the optimal range for biometric eye measurements.

[0063] It is also important to consider that the instantaneous linewidth, coherence length and measurement depth are dependent on the tuning speed and repetition rate of the A-scan.

[0064] According to the invention, an optimization of this correlation is proposed. In the foreground, there is a sufficient measurement depth of 60 mm for full-eye biometry.

[0065] A frequency of at least 1 kHz is selected for the tuning speed / repetition rate of the A-scan in order to exclude eye movement artifacts in any case. If the measuring depth is thus not less than 60 mm, an increase of up to 100 kHz is provided for biometric measuring systems.

[0066] Another technical challenge in selecting and controlling a suitable VCSEL laser diode may be the existing temperature gradient of the wavelength change. For a tunability of 5 nm, a corresponding temperature change of 50 K may be required.

[0067] In order to achieve a large tuning range at a repetition rate of about 1 kHz, commercially available VCSEL laser diodes must be modified or they must be operated in a pulsed manner. For this purpose, existing control units are designed to vary the time and amplitude for the periodic current modulation.

[0068] To this end, it is proposed according to the invention that a periodic current modulation (electrical tuning) is performed independently of the direction of the wavelength change, so that up- and down-scanning is used.

[0069] In particular, it is proposed that both up-sampling and down-sampling of the wavelength are used in the simple swept source VCSEL biometer according to the invention when tuning the wavelength by means of current pulses. In this case, as the current increases, a longer wavelength is set. Therefore, in down-sampling, it must be noted that the wavelength is shortened in the current pulse interval.

[0070] It is generally known that a temperature gradient of approximately 0.1 nm / K can be achieved. Therefore, a temperature change of 50 K is required for a tunability of 5 nm.

[0071] According to the invention, it is proposed that the gradient be increased to more than 0.1 nm / K, preferably to approximately 0.3 to 0.5 nm / K.

[0072] This is achieved by selecting an adapted active semiconductor material. However, it is also possible to additionally embed the VCSEL laser diode in the active semiconductor material.

[0073] For example, VCSEL laser diodes with wavelengths between 600 nm and 1400 nm are based on GaAs wafers as surface emitters.

[0074] Furthermore, it is proposed to use a single-mode AlGaInP laser diode having a multiple quantum well structure at a wavelength of 690 nm and a large temperature gradient of at least 0.25 nm / K.

[0075] In order to optimize the heat sink to achieve a large temperature gradient within the possible operating temperatures of the VCSEL laser diode (eg −20 to +70° C.), it is proposed, for example, to couple highly thermally conductive materials such as copper, indium, etc. to the semiconductor material.

[0076] In addition to the purely electrically implemented wavelength tuning of the single-mode VCSEL laser diode according to the invention, there is also a combination of purely electrical wavelength tuning with additional thermal wavelength tuning.

[0077] For this purpose, the electrically controlled module is operated synchronously with a heat sink, which adjusts the ambient temperature of the single-mode VCSEL laser diode, in principle -80°C to +180°C, at the highest possible repetition rate of >100 Hz, in particular >1 kHz, within a certain range of, for example, 100 K. This allows the scanning range (wavelength tuning range in nm) to be extended and thus the resolution of the biometer to be increased.

[0078] Additionally, active cooling can be performed by means of Peltier elements.

[0079] However, since the additional cooling may cause condensed water to accumulate at the exit window of the VCSEL laser diode, it is proposed according to the present invention to provide a heating device for the exit window of the VCSEL laser diode.

[0080] As mentioned previously, VCSEL laser diodes with a wavelength of approximately 1050 nm are more suitable for penetrating cataracts or for deeper penetration depths into tissue.

[0081] According to the invention, a light power in the range of 200 μW to a maximum of 2000 μW, in particular greater than 1000 μW (at a wavelength of 1050 nm), is provided at the eye.

[0082] It must be taken into account that higher optical powers should only be achieved within the scope of laser safety regulations. To achieve higher optical powers, the design of the VCSEL laser diode needs to be changed. For this purpose, it is possible, for example, to change the active region or other parameters of the VCSEL laser diode in order to achieve optical powers of up to 2 mW.

[0083] According to the invention, in order to construct a VCSEL-based SS interferometer system, it is proposed that the laser radiation emitted by a semiconductor laser diode is coupled into the interferometer by means of free-radiation or optical fiber optics. For this purpose, it is necessary to design the structure of the VCSEL laser diode accordingly. In both cases, the beam quality is an important criterion at a given power, because it limits the laser power that can be used to perform the measurement.

[0084] In particular, fiber-based designs must ensure that coupling in is as lossless as possible. In single-mode fibers with a fiber diameter of 5 μm, almost lossless coupling in with losses of <10% can be achieved at a numerical aperture of NA=0.14. Coupling into single-mode fibers with a core diameter of 3 to 9 μm is usually proposed.

[0085] Another embodiment of the invention relates to the generation of current pulses that are modulated in wavelength. By evaluating the currents of two photodiodes with different spectral characteristics, a wavelength measurement signal can be generated in a simple manner by means of analog electronics and used as a feedback system.

[0086] The following description is exemplarily made for a wavelength range of about 1050 nm.

[0087] In this wavelength range, the spectral sensitivity of the InGaAs photodiode varies only by about +0.1 A / W per 100 nm, and in the case of the Si photodiode this variation is about -0.5 A / W per 100 nm. Of course, fluctuations in the power of the laser act identically on both photodiodes. Therefore, after appropriate normalization of the current of the Si photodiode with the current of the InGaAs photodiode, only the dependence of the resulting current on the wavelength remains, which is about 0.4 A per 100 nm (4 μA / nm). The measurement signal can now be used to form current pulses in order to achieve the desired variation curve of the wavelength variation.

[0088] Preferably, a wavelength variation curve over time is generated, which corresponds to an equidistant variation of the wave number over time. Thus, the spectral information in the interference signal (frequency space) can be directly transformed into a measurement result in the position space by Fourier transformation without further correction.

[0089] Suitable shaping of the current pulses can be performed by digitizing the wavelength-dependent signal and processing it algorithmically (eg improving it by filtering and averaging). The current pulses are then iteratively deformed until the desired result is achieved.

[0090] In addition to appropriate shaping of the current pulses until a linear or equidistant change in the wave number over time is achieved, it is also proposed according to the invention to use structured current pulses in which the current is interrupted multiple times, i.e. a rapidly clocked current fills the envelope of the current pulse. The clocked current pulses are optimized according to the invention until their structure leads to an equidistant change in the wave number over time of the single-mode VCSEL laser diode with respect to the spacing between the individual pulses and the amplitude of the individual pulses.

[0091] Another possibility is a closed analog or digital control loop which uses the desired wavelength profile as a preset value, the wavelength-dependent signal as a measured value and the current pulse as a control signal.

[0092] According to the invention, it is also proposed that the shaping of the current pulses is optimized by means of a continuous variation of the current within the pulse, so that the widest possible scanning range (tuning range of the wavelength in nm) of the single-mode VCSEL laser diode is generated. Thus, for example, a range of >3 nm or in particular >8 nm up to 20 nm is provided.

[0093] In addition to appropriately shaping the current pulses until a scanning range (tuning range of the wavelength in nm) that is as wide as possible is achieved, the invention also proposes to use structured current pulses in which the current is interrupted multiple times, i.e., a rapidly clocked current fills the envelope of the current pulse. The clocked current pulses are optimized according to the invention until their structure leads to a maximum scanning range (tuning range of the wavelength in nm) of up to approximately 20 nm over time for a single-mode VCSEL laser diode with regard to the spacing between the individual pulses and the amplitude of the individual pulses.

[0094] In the final design of the present invention, when the current is higher than the threshold current I th In the range of , short current pulses are selected, which have a duration of the current rising edge of <500 μs, in particular <50 μs, up to ≈1 μs. In addition to the described thermally induced spectral scanning range of, for example, 0.1 nm / K, a non-thermally induced extension of the spectral scanning range (thermal, spectral scanning range) is thereby achieved. The effect used according to the invention is based on a purely electrically induced shift of the electron energy levels / band gaps of the semiconductor laser material due to the high electric field strengths that are briefly loaded. This extension of the spectral scanning range is produced in particular by the type of current pulse and its rising edge. An advantage of this operating type is the small or negligible effect on the thermal stability of the SS-OCT system. In this operating type, cooling and thermal stabilization can be minimized or avoided.

[0095] This operating mode according to the invention is provided when the following conditions apply:

[0096] Δλ>ΔTΔλ therm

[0097] where Δλ corresponds to the spectral scan range,

[0098] Δλtherm corresponds to the thermal spectral scan range (in nm / K), and

[0099] ΔT corresponds to the temperature change (in K) during the current pulse.

[0100] In terms of measurement, this operating mode can be realized, for example, by means of an integrated temperature sensor for ΔT on the laser chip and measurement during a current pulse and determination of the spectral scanning range Δλ achieved with a known statically measured spectral drift travel Δλtherm of the laser material.

[0101] In the proposed method for controlling a SS interferometer system based on semiconductor laser diodes, the operation of the semiconductor laser diode is designed by means of periodic current modulation in order to be able to tune a highly coherent spectral laser line in a wide wavelength range with the highest possible repetition rate.

[0102] According to the invention, the following parameters are provided:

[0103] - with a coherence length of at least 20 mm,

[0104] - a central wavelength in the range of 600nm to 1400nm,

[0105] - scanning rates in the range of 100 Hz to 100 kHz,

[0106] - a scanning range in the range of 3 nm to 75 nm, and

[0107] - Optical power at the eye in the range of 50 μW to 20000 μW.

[0108] to this end, Figure 1 A schematic diagram showing the principle of controlling a semiconductor laser diode-based SS interferometer system according to the present invention by means of periodic current modulation.

[0109] The semiconductor laser diode-based SS interferometer system consists of a semiconductor laser diode 1 having a control unit 2, wherein the control unit 2 is designed to control the operation of the semiconductor laser diode 1 by means of periodic current modulation in order to be able to tune a highly coherent spectral laser line in a wide wavelength range with the highest possible repetition rate.

[0110] For this purpose, the control unit 2 is designed for periodic current modulation in order to vary the time and amplitude of the current pulses. In particular, the control unit 2 is designed to emit a short rise time of the current pulses in order to electrically pump the semiconductor diode 1 .

[0111] The semiconductor laser diode 1 is in particular a VCSEL laser diode which has a variation in the wavelength of the highly coherent spectral laser line that is dependent on the current operating temperature. For example, a variation Δλ of 0.07 nm / K is provided for a GaAs-based semiconductor laser diode. therm / ΔT, and a change of 0.25nm / K is set for the AlGaInP-based semiconductor laser diode.

[0112] It should also be noted that: according to the present invention, due to the short rise time of the current pulse, a wider tuning range (Δλ>ΔTΔλ) of the wavelength Δλ exceeding the value known by thermal variation can be achieved by the control unit 2 therm ).

[0113] Optionally, stable long-term operation of the SS laser source is provided with temperature stabilization or temperature management via an actively and passively operable heat sink 3. These additional features of the heat sink 3 can also be omitted if the thermal capacity of the semiconductor laser diode 1 and its existing heat sink is sufficient for stable operation.

[0114] Thermally conductive materials and / or Peltier elements can be used to optimize the heat sink 3 . Active cooling of the laser chip of the semiconductor laser diode 1 is synchronized with the periodic current modulation by the control unit 2 only at times outside the current rise time (rise time).

[0115] In addition to the function of cooling the laser chip for thermally stable operation due to heating by current pulses of the control unit 2, the heat sink 3 is optionally also provided for additionally heating the laser chip of the semiconductor laser diode 1 only within the current rise time in synchronization with the current pulses of the control unit 3. It is thereby possible to increase the thermally induced wavelength detuning by increasing the temperature change ΔT due to the electrically induced temperature increase and due to this optional direct active temperature increase during the current rise time.

[0116] The semiconductor laser diode 1 preferably emits an output power in the range of a few mW, which has the desired parameters for SS interferometer systems for OCT and biometric applications, which can compensate for losses in other optical systems and achieve a good signal-to-noise ratio taking into account the maximum ophthalmologically usable intensity in diagnostic applications. If the output power of the semiconductor laser diode 1 is too low, a semiconductor-based optical amplifier (Semiconductor Optical Amplifier, SOA for short) can optionally be integrated in the optical path as SOA 4.

[0117] exist Figure 2 Some further optical parameters of the radiation emitted by the semiconductor laser diode of the SS interferometer system are shown in FIG.

[0118] FIG. 5 schematically shows the tuning of individual highly coherent wavelengths in a wavelength range Δλ, i.e., a so-called scanning range with a power P in the mW range. The spectral resolution of the scanning determines the coherence length Lc or the measurement depth in air, as shown in the figure. This function is realized during the current rise (rise time) of the semiconductor laser diode 1 according to the invention. The figure shows a displacement (spectral jitter) of the tuning range Δλ, which can be produced in particular by heating the semiconductor laser diode 1 over a longer period of time. This effect can be suppressed by temperature management with the aid of the heat sink 3.

[0119] 6 shows the emission parameters with the time of the individual laser pulses by means of an example overview, wherein each laser pulse of 300 μs contains the spectral tuning properties of the high coherence wavelength within Δλ according to Graph 5. Here, these laser pulses are shown with a repetition rate of 1 kHz and a pulse spacing of 1 ms. In this case, a duty cycle of 30% is achieved. Thus, for example, after an active current pulse of 300 μs, the laser chip has another 700 μs for thermal and / or electromagnetic relaxation in order to achieve a long-term stable function.

[0120] The laser radiation of a semiconductor laser diode 1 having the characteristics according to Figures 5 and 6 is coupled into the eye 7 to be examined in an interferometric system (not shown in detail). The intensity of the light scattered back from the eye is detected by means of a beam splitter 8 in a biometer 9. The raw signal of the biometer 9 is processed by means of a post-processing unit 10 in order to output a depth scan (also called A-scan) over the entire eye 7, which in particular yields information about the position and distance of the cornea, lens and retina as a measurement result 11. In a manner not outlined here, this A-scan can be scanned laterally above the pupil opening of the eye in order to produce a cross-sectional image of the proposed distance in the eye 7 as a so-called B-scan.

[0121] In order to compensate for undesired drifts of the wavelength tuning range (scanning range) caused by thermal environmental influences and operating temperature variations, the so-called spectral jitter, the single-mode VCSEL laser diode can be built with a temperature-controlled heat sink having a temperature that is stable at a temperature constant of approximately + / -1 K in a temperature range of -80° C. to +180° C. in principle. In this case, high output powers of the single-mode VCSEL laser diode can also be achieved at low temperatures by means of high currents.

[0122] In order to achieve a temperature-dependent wavelength shift without an additional heat sink, the invention proposes using a fiber Bragg grating in the interferometer of a short-coherence biometric measuring system, which specifies a wavelength reference with which the measuring system can be continuously recalibrated.

[0123] Bragg reflectors / gratings (FBGs) (e.g. based on optical fibers) can be used for better spectral stabilization of the tuning range. The reflectors are introduced into the system so that light reflections are formed at precisely defined wave numbers during the tuning process. The light reflections can be recorded together with the interference signal. The light reflections serve as a reference for the precise and absolute spectral positioning of the spectral interference signal.

[0124] To this end, the authors W. Choi et al. describe a solution in [7], in which one of the detection arms is guided to a balanced detector via an FBG. The FBG creates a "notch" in the detection signal, which can be used for spectral normalization of the signal.

[0125] A first variant of the method relates to imaging based on an SS-OCT system. Here, too, it should be possible to tune a highly coherent spectral laser line in a wide wavelength range with the highest possible repetition rate.

[0126] In this variant of the proposed method, the following exemplary embodiments are possible.

[0127] According to the first design solution, one, more or all of the following preferred parameters are provided:

[0128] - In case of coherence length greater than 60 mm,

[0129] -1050nm central wavelength,

[0130] - a scan rate of at least 1kHz,

[0131] - A scanning range of at least 16nm, and

[0132] - Maximum 2mW optical power at the eye.

[0133] According to the second design solution, one, more or all of the following preferred parameters are provided:

[0134] - In case of coherence length greater than 60 mm,

[0135] -840nm central wavelength,

[0136] - a scan rate of at least 1kHz,

[0137] - A scanning range of at least 9nm, and

[0138] -Maximum 0.8mW optical power at the eye.

[0139] According to the third design solution, one, more or all of the following preferred parameters are provided:

[0140] - In case of coherence length greater than 60 mm,

[0141] -600nm central wavelength,

[0142] - a scan rate of at least 1kHz,

[0143] - A scanning range of at least 5 nm, and

[0144] - Maximum optical power at the eye of 0.4mW.

[0145] For example, a coherence length Lc or measurement depth in air of approximately 25 mm is sufficient for measurements from the anterior eye segment to the posterior side of the lens.

[0146] For example, for the purpose of IOL calculation, with the aid of a measuring device for determining biometric values, measured values ​​such as eye length AL, lens thickness LD, anterior chamber depth VKT and corneal thickness HHD are determined essentially from so-called A-scans or B-scans. For this purpose, an axial resolution of approximately 100 μm in air is sufficient.

[0147] By way of example, reference is made here to the device IOLMaster 500 from Zeiss Meditec AG, in which a multimode laser diode with a half-value width of Δλ≈2.7 nm at a central wavelength of 785 nm achieves a resolution of 100 μm in air.

[0148] The axial resolution Δz in the Gaussian spectral power density is defined as:

[0149]

[0150] Where: Δλ defines the half-value width, and

[0151] λ defines the central wavelength.

[0152] Other typical spectral power densities can also be well approximated by the above formula. In the case of a semiconductor laser diode with a central wavelength of 1050 nm, Δλ≈5 nm is required in order to achieve a resolution of 100 μm in air.

[0153] If a semiconductor laser diode with a central wavelength of 840 nm and an electrical tuning range of 5 nm is used, an improvement in the resolution in air of about 65 μm is achieved. At a wavelength of 840 nm, the axial resolution of 100 μm required for a biometer in air is achieved in a tuning range of about 3 nm.

[0154] Therefore, the following values ​​are obtained from it:

[0155]

[0156]

[0157] According to the invention, at a wavelength of 1050 nm, a half-value width in the range of 10-100 nm, in particular 16 nm, is provided (this corresponds to the scanning range of the semiconductor laser diode).

[0158] Furthermore, according to the invention, a VCSEL laser diode is provided with a repetition rate (scanning plastic) of 100 Hz-100 kHz, in particular >1 kHz.

[0159] In this scenario, reference is again made to the already mentioned relationship between motion artifacts, repetition rate and signal-to-noise ratio.

[0160] The sensitivity of SS-OCT systems scales with the energy (number of photons) deposited in the tissue and not with the peak power. Therefore, according to the present invention, it is sought to select a scan rate that is fast enough, but also long enough to exclude eye motion artifacts, so that a sufficiently large pulse energy can be provided for a given power of the laser diode.

[0161] According to the invention, a light power in the range of 200 μW to a maximum of 20,000 μW, in particular a maximum of 2,000 μW (at a wavelength of 1,050 nm) is provided.

[0162] Here again, it is important to comply with the already mentioned laser safety regulations.

[0163] However, it is also possible to vary other parameters, so that, for example, the line width of a VCSEL laser diode can be widened.

[0164] According to the invention, a coherence length Lc of at least 60 mm is also provided, since the resulting measurement depth is necessary to determine the entire human eye.

[0165] However, it is also possible to realize SS-OCT systems with a low axial resolution of >30 μm, in particular >50 μm. This is of interest, for example, for the display of tomography and volumetric measurements of the entire eye (anterior chamber up to the fundus). Likewise, the loss of axial resolution can be compensated by "machine learning" algorithms. To this end, images must first be recorded at a standard resolution and used to train the algorithm. Subsequently, the algorithm learns based on machine learning and also improves the resolution in images with low axial resolution.

[0166] In order to expand the scope of use of the technology according to the invention, it is proposed in particular to improve the evaluation of full-eye scans as A-scans and / or as B-scans by means of algorithms based on machine learning / deep learning / artificial intelligence. The advantage of the biometer according to the invention is the large measurement depth, which allows full-eye scans at high repetition rates, such as an A-scan rate of 28 kHz. However, it is disadvantageous that it has a low resolution due to the low scanning range of about 5 nm compared to modern high-resolution SS-OCT systems with a scanning range of up to 100 nm. It is therefore proposed to measure at least about 20 patient eyes completely and / or partially by means of a biometer according to the method and at the same time by means of other commercially available ultra-high-resolution OCT systems. With the help of algorithms based on machine learning / deep learning / artificial intelligence, the known high-resolution measurements should now be compared and evaluated with the measurements of the biometer according to the invention. As a result, these algorithms should improve the resolution and accuracy of the subsequent individual measurements of the biometer in the A and / or B scans.

[0167] A second variant of the method for controlling a SS interferometer system based on semiconductor laser diodes relates in particular to biometric applications by full-eye scanning.

[0168] In the proposed method for controlling a SS interferometer system based on a semiconductor laser diode, the operation of the semiconductor laser diode is set by means of periodic current modulation in order to be able to tune a high-coherence spectral laser line in a wide wavelength range with the highest possible repetition rate. In this case, according to the invention, the following parameters are set for biometric applications:

[0169] - with a coherence length of at least 20 mm,

[0170] - a central wavelength in the range of 600nm to 1400nm,

[0171] - scanning rates in the range of 100 Hz to 100 kHz,

[0172] - a scanning range in the range of 3 nm to 20 nm, and

[0173] - Optical power at the eye in the range of 50 μW to 20000 μW.

[0174] According to a preferred design solution, one, more or all of the following priority parameters are set for full eye scanning:

[0175] - In the case of coherence lengths greater than 60 mm,

[0176] -1050nm central wavelength,

[0177] - a scan rate of at least 1kHz,

[0178] - A scanning range of at least 5 nm, and

[0179] - Maximum 2mW optical power at the eye.

[0180] Semiconductor laser diodes are operated with the aid of periodic current modulation by varying the duration, amplitude and shape (rising edge) of the current pulses.

[0181] For the maximum repetition rate, a maximum temperature gradient must be achieved by the current pulses in the active VCSEL semiconductor layer. However, in order to be able to achieve high repetition rates with constant laser parameters in almost continuous operation, it is necessary to ensure that the average temperature of the active material is constant.

[0182] According to the invention, this is achieved with only passive cooling of the VCSEL laser diode by:

[0183] - Short current pulses at the threshold current I thabove, where the duration of the current rise edge is in particular <500 μs and up to ≈50 ns, and

[0184] - A duty cycle that is sufficient to cool the active VCSEL semiconductor layer to a stable operating temperature in the range of + / -2 K. In particular, according to the invention, a pulse duty cycle of <50% and up to 1%, preferably 30%, is provided for this purpose. For example, the next current pulse is carried out after a current pulse of 100 ns or 10 μs after 10000 ns, and a repetition rate of 100 kHz can be set.

[0185] According to an advantageous design, the VCSEL-based SS interferometer system according to the present invention is characterized by the following technical parameters:

[0186] - Average laser wavelength at 840nm,

[0187] -5nm scanning rate,

[0188] -2mW VCSEL laser diode output power,

[0189] -28kHz scan rate,

[0190] -30% duty cycle,

[0191] - This corresponds to a power of 0.2 mW at the eye and thus to a measurement sensitivity of 100 dB.

[0192] In particular, it is proposed in this optimization that a greater peak power can be achieved if the driver current pulses are shorter, so as to enable a pulse duty cycle of <10%. However, the energy (number of photons) deposited in the tissue is then reduced by a factor of more than ten.

[0193] According to the invention, for biometric applications, at 1050 nm a half-value width in the range of 3-20 nm, in particular of approximately 5 nm, is provided (this corresponds to the scanning range of the semiconductor laser diode).

[0194] According to the invention, in order to achieve a stable, static operating temperature of a VCSEL-based SS interferometer system, a duty cycle adapted in terms of measuring sensitivity and thermal stability is implemented in the range of 1% to 50%, in particular 30%.

[0195] Since a minimum repetition rate of 1 kHz is provided according to the invention, a pulse duty cycle of 0.01% is also feasible in the case of current pulses of 100 ns duration, which means a cooling time in the active laser material that is 10,000 times longer than the current pulse time. In order to keep the expenditure for the required control within limits, a duty cycle of 0.1% is realistic. A stable static operating temperature of the SS-OCT system can thereby be set. Within the limits mentioned above for the pulse energy required for the measurement, a duty cycle of 10% was sought, and the first practical solution was found with a duty cycle of 30%. Since the spectral tuning range remains permanently stable even at a duty cycle of 30% and does not undergo spectral drift, good heat dissipation of the active volume of the laser diode is ensured in the case of the configuration selected according to the invention.

[0196] In the above advantageous design, an average power of 2 mW of diode emission can be ensured. When using such a tunable VCSEL laser diode, a VCSEL-based SS interferometer system can provide a measurement power of 200 μW at the eye and enable biometric measurement of the eye with a sensitivity of 100 dB.

[0197] According to the invention, it is also proposed that, for pumping the laser source, the amplitude of the variation curve of the current pulsation is controlled in time so as to obtain a linear variation curve of the emission wavelength in time of the current pulse within the spectral tuning range of the laser emission. This is advantageous for minimizing error sources in the evaluation of the measurement signal.

[0198] As already mentioned, the sensitivity of VCSEL-based SS interferometer systems scales with the energy (number of photons) deposited in the tissue and not with the peak power. Therefore, according to the invention, an advantageous optimization is proposed in which the laser diode allows >10 times the power in pulsed operation, which is still considered practical in principle.

[0199] Ultimately, the thermal load limits the output power of the laser diode. Therefore, an optimum tuning range is provided, especially at low operating temperatures.

[0200] For the operation according to the invention of a VCSEL-based SS interferometer system, it is therefore proposed to define and set a stable nm / K gradient for the VCSEL laser diode.

[0201] As already mentioned, a temperature gradient of approximately 0.1 nm / K can be achieved, thus requiring a temperature change of 50 K for a tuning range of 5 nm. According to the invention, it is provided that the gradient is set to >0.1 nm / K, preferably approximately 0.3 to 0.5 nm / K. This can also be achieved by the above-mentioned measures.

[0202] According to the invention, a light power in the range of 200 μW to a maximum of 20,000 μW, in particular a maximum of 2,000 μW (at a wavelength of 1,050 nm) is provided.

[0203] Therefore, according to the present invention, it is proposed that the driving current of the laser diode is higher than the threshold current I th The current can be tuned within a range of , wherein the minimum and maximum current values ​​of the range result in an output power of the laser diode of at least 50 μW for measurements in the eye in the corneal plane.

[0204] According to another advantageous design, for the periodic current modulation, a clock fluctuation of less than 1 ns, a current pulse repetition of less than 10 ns, and an amplitude stability of less than + / -5% need to be set and ensured.

[0205] Therefore, in order to optimize the already mentioned repeatability of the tuning range it is proposed that:

[0206] - very small clock fluctuations in the control pulses set in the range of <1ns,

[0207] - Maintains a repeatability of <10ns for the sequence of control pulses, and

[0208] - Set its amplitude stability to <+ / -5% in time variation.

[0209] This ensures that the tuned wavelength range remains constant. Changes in the wavelength range of >0.1 nm have an influence not only on the axial resolution but also on the axial scaling and thus on the accuracy of the biometric measurement.

[0210] Furthermore, in particular when the necessary stability requirements for the spectral tuning range cannot be fully met, it is proposed to correct the changes in the trigger and driver signals by means of a mechanically stable reference signal of the interferometer. For example, the correction can be carried out by correlating and recording interference patterns and reference signals.

[0211] The solution according to the invention for constructing a simple tunable diode laser is based on the fact that a wavelength change can be achieved when the temperature changes in the active laser material. Here, the temperature change can be carried out indirectly by current pulses and / or by additional heating or cooling.

[0212] In addition to this effect desired according to the invention, however, there is also a change in the output power at a given pump current due to the temperature change of the active material of the laser diode. Therefore, according to the invention, firstly a selection of a laser diode design is proposed in which the change in the output power as a function of the operating temperature is small or reproducible. Furthermore, the above-mentioned optimization of the operating conditions is provided with respect to this dependence.

[0213] However, in addition to the technical challenges already mentioned, other dependencies must be considered that influence the selection and control of suitable VCSEL laser diodes.

[0214] The application of the measurement technique according to the invention at a moving eye requires fast measurement times so that motion artifacts in the measurement signal can be excluded. For individual measurements, the measurement time should be approximately less than or equal to 1 ms. Therefore, a combination of duty cycle and scan rate of, for example, 10% and 100 Hz can be selected. In the case of this temporal tuning rate, a spectral tuning width in the range of 3-20 nm is then also required to ensure the required resolution. However, the known behavior of laser diodes is that large tuning widths can be achieved, especially at low tuning rates, which is another challenge for realizing the measurement system according to the invention. For this combined requirement, the selection of the best version based on the laser diode design is proposed.

[0215] Furthermore, an absolutely constant spectral tuning range is required for reproducible evaluation of the measurement signal.On the other hand, the repeated electrical / thermal tuning of the laser diode required in the proposed measurement operation causes temperature drifts and thus wavelength drifts of the tuning range.

[0216] For this requirement, the selection of an optimal version of the laser diode is provided, which has, for example, a large thermal conductivity or a large heat capacity and thus a small undesired temperature drift.

[0217] A large scanning range depends on setting a high temperature of the VCSEL laser diode. However, at high temperatures, the optical power of the laser diode decreases.

[0218] Furthermore, the proposed dynamic thermal behavior of the VCSEL laser diode is related to the operating temperature itself. Thus, at higher operating temperatures:

[0219] - a higher laser threshold, and

[0220] - Smaller maximum power P max and a smaller maximum current I max .

[0221] According to the invention, it is therefore proposed to select an operating temperature less than or equal to room temperature of approximately 20° C., in particular <10° C., or to cool the laser diode to its minimum permissible operating temperature.

[0222] With the solution according to the invention, a SS interferometer system can be provided which is based on a VCSEL laser diode and is suitable for use in ophthalmology, in particular for determining biometric measured values ​​of the eye.

[0223] The present SS-OCT system can be tuned over a wide wavelength range with a high coherence length and a relatively high repetition rate and is therefore suitable for measuring eye biometry, in particular by means of a full-eye scan.

[0224] The solution according to the invention for constructing a simple tunable diode laser is based on the fact that a wavelength change can be achieved when a temperature change occurs in the active laser material. This temperature change can be achieved indirectly by current pulses and / or by additional heating or cooling.

[0225] As mentioned above, a temperature gradient of about 0.1 nm / K can be achieved, so that a temperature change of 50 K is required for a tunability of 5 nm. According to the invention, it is proposed that this gradient be set to >0.1 nm / K, preferably about 0.3 to 0.5 nm / K. This can also be optimized by selecting the active semiconductor laser material in the manner described, selecting suitable operating parameters and thermal management of the system.

Claims

1. A method for controlling a simple swept source interferometer system based on a semiconductor laser diode suitable for whole eye scanning, the method comprising: Applying periodic current modulation to set up the operation of semiconductor laser diodes to be able to tune highly coherent spectral laser lines at the highest possible repetition rate and over a wide wavelength range; And use the following parameters: - with a coherence length of at least 20 mm, - a central wavelength in the range of 600nm to 1300nm, - scanning rates in the range of 100 Hz to 100 kHz, - a scanning range in the range of 3 nm to 75 nm, and - optical power at the eye in the range of 50 μW to 20000 μW, and At a repetition rate of 100 kHz, current pulses having a duration of 1 μs or at a repetition rate of 10 kHz, current pulses having a duration of 10 μs can be realized.

2. The method according to claim 1, characterized in that At least one of the following parameters is set for imaging with the OCT system: - In case of coherence length greater than 60 mm, -1050nm central wavelength, - a scan rate of at least 1kHz, - A scanning range of at least 16nm, and - Maximum 2mW optical power at the eye.

3. The method according to claim 1, characterized in that: At least one of the following parameters is set for imaging with the OCT system: - In case of coherence length greater than 60 mm, -840nm central wavelength, - a scan rate of at least 1kHz, - A scanning range of at least 9nm, and -Maximum 0.8mW optical power at the eye.

4. The method according to claim 1, characterized in that At least one of the following parameters is set for imaging with the OCT system: - In case of coherence length greater than 60 mm, -600nm central wavelength, - a scan rate of at least 1kHz, - A scanning range of at least 5 nm, and -Maximum 0.4mW optical power at the eye.

5. The method according to claim 1, characterized in that A low axial resolution of more than 30 μm was set for imaging with the OCT system.

6. The method according to claim 5, characterized in that The loss of resolution is compensated by an image processing algorithm based on machine learning.

7. The method according to claim 5, characterized in that Set at least one of the following parameters for full eye scanning: - In the case of coherence lengths greater than 60 mm, -1050nm central wavelength, - a scan rate of at least 1kHz, - A scanning range of at least 5 nm, and - Maximum 2mW optical power at the eye.

8. The method according to claim 1, characterized in that: The periodic current modulation is performed independently of the direction of the wavelength change, so that both up-frequency scanning and down-frequency scanning are used.

9. The method according to claim 1, characterized in that: An additional modulation of the electronic band gap of the electronic energy levels of the semiconductor laser material is achieved by the brief application of high electric field strengths, for which the following applies: Δλ>ΔTΔλ therm Where Δλ corresponds to the spectral scanning range, Δλ therm corresponds to the thermal spectral scan range in nm / K, and ΔT corresponds to the temperature change during the current pulse in K.

10. The method according to claim 1, characterized in that Based on the knowledge of the instantaneous wavelength, the current pulses are regulated by means of a feedback system, so that a known change in the wave number over time is achieved.

11. The method according to claim 10, characterized in that The wave number varies linearly with time.

12. The method according to claim 10, characterized in that The feedback system consists of two different photosensitive materials.

13. The method according to claim 1, characterized in that With an average laser wavelength of 840 nm and a scanning range of 5 nm, a continuous output power of 2 mW of the semiconductor laser diode and a continuous power at the eye of 0.2 mW corresponding to a measurement sensitivity of 100 dB were achieved using a scanning rate of 28 kHz, a duty cycle of 30%.

14. A method for controlling a simple swept source interferometer system based on a semiconductor laser diode suitable for whole eye scanning, the method comprising: Applying periodic current modulation to set up the operation of semiconductor laser diodes to be able to tune highly coherent spectral laser lines at the highest possible repetition rate and over a wide wavelength range; And the following parameters are set for biometric applications at the eye: - with a coherence length of at least 20 mm, - a central wavelength in the range of 600nm to 1300nm, - scanning rates in the range of 100 Hz to 100 kHz, - a scanning range in the range of 3 nm to 20 nm, and - optical power at the eye in the range of 50 μW to 20000 μW, and At a repetition rate of 100 kHz, current pulses having a duration of 1 μs or at a repetition rate of 10 kHz, current pulses having a duration of 10 μs can be realized.

15. The method according to claim 14, characterized in that The periodic current modulation is performed independently of the direction of the wavelength change, so that both up-frequency scanning and down-frequency scanning are used.

16. The method according to claim 14, characterized in that Make the current pulsation amplitude within the threshold current I th Within the above range.

17. The method according to claim 14, characterized in that The duration of the rising edge of the current pulsation is set in the range of less than 500 μs and up to 50 ns.

18. The method according to claim 14, characterized in that A clock fluctuation of less than 1 ns, a current pulse repeatability of less than 10 ns and an amplitude stability of less than + / -5% are ensured for the periodic current modulation.

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