Method for controlling semiconductor-laser-diode-based SS-interferometer system

Abstract

To provide a method for controlling a semiconductor-laser-diode-based SS-interferometer system (SS=swept source).SOLUTION: A method is suitable for use in ophthalmology, in particular for imaging and for determining biometric measurement values of an eye. In the method, by means of periodic current modulation, operation of single semiconductor laser diodes is designed such that a highly coherent spectral laser line can be adjusted with a highest possible repetition rate and over a wide wavelength range. A centre wavelength, sweep rate, sweep range, optical output in the eye and coherence length are adjusted such that the method is suitable for imaging and biometric applications via whole-eye scans. A semiconductor-laser-diode-based SS-interferometer system is provided, in particular, for biometric measuring of the eye. Given that the embodiments are based preferably on optical coherence tomographic scan images, the main application lies in ophthalmological diagnostics, treatment, and preparation of surgical procedures and follow-up thereof.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for controlling a semiconductor laser diode based SS interferometer system (SS = swept source) which allows for a wide range of applications. Furthermore, the system is intended to be suitable for use in ophthalmology, in particular for ocular imaging and biometric determination. [Background technology]

[0002] An interferometer is an optical technology device that uses interference to perform precise measurements: any effect that changes the effective path length of the waves and, therefore, the properties of the superimposed waves is measured. Examples include a change in the length of one of two superimposed optical paths when measuring length, a change in the refractive index when measuring material properties, or a small change in the distance between test masses in a gravitational wave detector.

[0003] Thus, many prior art solutions are known. One particular interferometric application is optical coherence tomography (OCT) as an imaging method. With the use of OCT systems it is possible to obtain two- and three-dimensional recordings of scattering materials with micrometer resolution. The main field of use of OCT is medicine, in particular ophthalmology.

[0004] OCT utilizes coherent light using an interferometer to perform distance measurements and imaging in reflective and scattering samples. In the human eye, OCT relies on the refractive index changes and volume scattering that occur at optical interfaces to generate a measurable signal during depth scanning. Optical coherence tomography is a very sensitive and fast method.

[0005] To make the acquisition of measurements even more effective, OCT systems using the so-called "swept source" technique (SS-OCT) have been used in recent years. This involves generating a depth signal by adjusting the frequency of the light source. This technique makes it possible to perform full-eye scans on the human eye. However, this requires a proper selection and control of the illumination source.

[0006] 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) and high repetition rates (in the kHz to MHz range) over a wide wavelength range up to 150 nm, which is necessary for very rapid acquisition of high-resolution images over large measurement depths with high axial resolution, especially in transparent organic tissues such as the human eye.

[0007] In optics, coherence length is the maximum allowable path length or propagation time difference between two light beams from the same source such that a stable (spatially and temporally) interference pattern results when the two beams are superimposed.

[0008] Of course, detection in an OCT system is not noise-free, so a sample reflection can only be detected if it produces a signal that is greater than the background noise of the OCT system.

[0009] This smallest still detectable reflection is a very important characteristic variable of an OCT system and is called its sensitivity, usually expressed in dB. When considering sensitivity, one must further take into account the so-called "sensitivity roll-off" criterion, which is understood to mean that as the difference in length between the reference arm and the sample arm increases, the amplitude of the interference signal decreases.

[0010] In the OCT system described herein, a sensitivity of -6 dB is defined (taking into account the "sensitivity roll-off" criterion). However, according to known prior art, there have already been efforts and attempts to use other laser diodes, such as VCSELs (Vertical Cavity Surface Emitting Lasers), as outlined in the article "Ultra-Widely Tunable VCSELs" by Garrett D. Cole in Non-Patent Document 1.

[0011] By way of example, reference is made in this respect to the single-mode (also called monomode) VCSEL laser diode from the company Philips, whose technical data are described in non-patent document 3. This VCSEL laser diode is tunable over 2 nm and can be thermoelectrically operated in the temperature range from 10 to 40 ° C with a small change in the central wavelength.

[0012] In Non-Patent Document 4, Sucbei Moon and Eun Seo Choi describe a low-cost OCT system using a VCSEL laser diode that is tunable by instantaneous current pulses or thermal shocks at a wavelength of about 1300 nm. However, to eliminate the effects of movement during ocular measurements, the sweep rate (repetition rate) of the laser is set very high, in the range of 10-100 kHz. The disclosed design of a low-cost OCT system with a wavelength of 1300 nm is not suitable for whole-eye measurements due to absorption in the vitreous. Furthermore, at the wavelength of 1300 nm used, a sweep range (tuning range) of at least 25 nm (preferably 75 nm) needs to be realized to enable the required resolution of 30 μm (preferably 10 μm) in air. No prospect of achieving such a sweep range has been shown. The combination of parameters described here is therefore not well suited to be able to build an optical biometer that meets the requirements for a competitive system.

[0013] The use of VCSEL laser diodes for eye measurements is described in US Pat. No. 5,399,433. In this case, the laser diodes operate in a spectrally narrowband manner with a wavelength of about 850 nm, a coherence length of typically 100 mm, and a spectral width of about 0.007 nm, thus providing a sufficient scanning depth for the measurement of the entire length of the eye. However, in order to achieve the required measurement accuracy with a wavelength of 850 nm and a spectral tuning of up to 3 nm, these laser diodes are supposed to be tuned at a slow speed, in the range of less than 10 Hz. This therefore creates the need for the additional use of a position recognition system, which is necessary for a laser diode tuned at a slow speed, in order to be able to measure moving objects such as the human eye with a relatively slow repetition rate of the tuned laser.

[0014] In patent document 2, a miniaturized and inexpensive OCT system for ophthalmic applications is described. In particular, the system is provided for measuring the thickness of the retina, which, due to its compactness and ease of use, is suitable for patients who can perform their own measurements at home. The SS-OCT system described there uses a VCSEL laser diode controlled by a periodic change in the electric current. In the described system, a scan of the whole eye is not possible due to the parameters used. Instead, a system is described in which the optical unit includes an optical scanning element in order to be able to move the light source to different positions on the retina. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] DE 102008028312 [Patent Document 2] International Publication No. 2018 / 119077 [Patent Document 3] U.S. Patent No. 8,632,181 [Non-patent literature]

[0016] [Non-Patent Document 1] Cole et.al., "Ultra-Widely Tunable VCSEL", http: / / www.aomicro.com / tech / Cole_TUM_27_Sep_2012.pdf [Non-Patent Document 2] Nanoplus, "760nm~830nm DFB laser diodes", https: / / nanoplus.com / fileadmin / user_upload / Data_sheets / nanoplus_DFB_760-830nm.pdf [Non-Patent Document 3] Philips, 760 / 763nm single mode VCSEL, http: / / www.photonics.philips.com / pdf / ULM76X-SingleMode_TO5.pdf [Non-Patent Document 4] Moon et.al., "VCSEL-based swept source for low-cost optical coherence tomography," Biomedical Optics Express, vol. 8, No. 2, February 1, 2017, pp. 1110-1121 [Non-Patent Document 5] Hogan B., "Operation of VCSELs Under Pulsed Conditions," VIXAR Application Note, January 21, 2010. [Non-Patent Document 6] 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", Optolet, 2013, 38(3), 338-340 Summary of the Invention

[0017] The present invention is based on the objective of developing a method for controlling a simple semiconductor laser diode-based SS interferometer system suitable for ophthalmic imaging and biometric measurements. In this case, the parameters for controlling the laser beam source should be optimized to allow tuning over a wide wavelength range with a high coherence length and a relatively high repetition rate. In this case, the biometric measurements of the eye are intended to be performed in particular by whole eye scanning.

[0018] According to the invention, the object is achieved by means of the features of the independent claims. Preferred developments and embodiments are the subject of the dependent claims. This object is achieved according to the invention by a method in which the operation of a simple semiconductor laser diode is configured such that, by periodic current modulation, a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate, with the parameters of central wavelength, sweep rate, sweep range, optical power at the eye and coherence length being adapted to make the method suitable for imaging and biometric applications with whole eye scans.

[0019] A first group of advantageous embodiments relates to the configuration or adaptation of the semiconductor laser diode used, for example the type of laser diode and surface emitter used, the configuration of its active region and optimization of the emission geometry.

[0020] A second group of advantageous embodiments relates to the control of semiconductor laser diodes, in particular the periodic current modulation and / or the setting and stabilization of a defined nm / K gradient, where Peltier elements can be used.

[0021] A method for controlling a SS interferometer system using a semiconductor laser diode, the operation of which is configured according to the invention in such a way that a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate by periodic current modulation, in particular in the case Central wavelength in the range of 600-1400 nm, Sweep rates in the range of 100Hz to 100kHz, A sweep range within the range of 3 to 75 nm, and The parameters of the light output at the eye are set within the range of 50 to 20,000 μW, A coherence length of at least 20 mm is provided.

[0022] Regarding the optical output of the SS interferometer system using semiconductor laser diodes, it is intended to provide the maximum possible permissible optical output at a given wavelength to the patient's eye so as to achieve the maximum signal / noise ratio while complying with safety regulations. Since about 50% of the output of the beam source (semiconductor laser diode) in the interferometer optical system must be considered as a loss, the semiconductor laser diode is predetermined to have a correspondingly higher output.

[0023] [Table 1] The optical power relates to the radiation power that is permitted to be irradiated to the human eye, as specified, for example, in the laser standard DIN EN60825-1. This power depends on the wavelength and the temporal shaping of the laser beam. For ease of explanation, the laser powers indicated here relate to cw operation of diodes and laser class 1. Other pulse peak values ​​can be used for pulsed radiation and other laser classes. Where necessary, national standards should also be taken into account.

[0024] The proposed SS interferometer system with semiconductor laser diodes is provided in particular for ocular biometry. The main applications are in ophthalmic diagnosis, treatment, preparation for surgical interventions and their follow-up examinations, since the proposal is based on scan recordings preferably performed by optical coherence tomography.

[0025] The invention is explained in more detail below on the basis of exemplary embodiments, in which English and German terms are used synonymously. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 is a basic diagram for controlling a SS interferometer system using a semiconductor laser diode according to the present invention by periodic current modulation. [Diagram 2] FIG. 13 illustrates some further optical parameters of the radiation emitted by the semiconductor laser diode of the SS interferometer system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The SS interferometer system using a semiconductor laser diode consists of a control unit and a semiconductor laser diode, and the control unit is designed to control the operation of the semiconductor laser diode by periodic current modulation so that a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate.

[0028] To that end, the control unit is designed to vary the time and amplitude of the current pulses for periodic current modulation. The semiconductor laser diodes used are, for example, VCSEL laser diodes, which can use surface emitters on GaAs wafers or single-mode AlGaInP wafers with a multi-quantum well structure.

[0029] In particular, the present invention comprises using only single mode VCSEL laser diodes. No multimode VCSEL laser diodes are provided. Single-mode VCSEL laser diodes are characterized by a very small instantaneous linewidth, a high coherence length and the measurement depth required for biomedical measurements. In this case, single-mode VCSEL laser diodes are intended to have a coherence length of at least 20 mm, in particular 60 mm, which allows reliable measurements even in very long eyes, for example in cases of severe myopia.

[0030] The system is therefore also suitable for mass screening for myopia, for example as is currently being carried out in Asia to stem the rise of myopia in the population. Single-mode VCSEL laser diodes should be used with a laser line spectral width of typically 100 MHz. Corresponding lasers are available, for example, from Trumpf.

[0031] For setting and stabilization of a defined nm / K gradient, the VCSEL laser diode can be embedded in an active semiconductor material that is appropriately dimensioned and matched to adjacent semiconductor material layers. Thermally conductive materials and / or Peltier elements can be used to optimize the fabricated heat sink.

[0032] In particular, the VCSEL laser diode design can be configured such that its active area is tunable to achieve continuous optical output at the eye of up to 20 mW. According to the invention, a VCSEL laser diode is provided having a wavelength in the range of 600-1400 nm, in particular around 1050 nm.

[0033] What should be taken into account here is that shorter wavelengths require a narrower tuning range (called sweep range) for the same resolution and repetition rate, which is the biggest technical challenge in this technology in particular. Short wavelengths in the red spectral range, for example 600 nm, and in the IR range, for example 700 nm, are therefore best suited for this. As an example, single-mode AlGalnP laser diodes with a multi-quantum well structure and a wavelength of 690 nm are suitable. Furthermore, VCSEL laser diodes with a wavelength of 840 nm are available and well suited.

[0034] However, for cataract penetration or deeper tissue penetration, a VCSEL laser diode having a wavelength of about 1050 nm would be more suitable and should be preferred.

[0035] Furthermore, according to the invention, a VCSEL laser diode is provided having a repetition rate (called sweep rate) of 100 Hz to 100 kHz, in particular 1 kHz. In this case, in particular as a distinction with respect to patent document 3, the SS interferometer system according to the invention is intended not to require an additional movement signal of the eye in order to be able to evaluate the measurement result.

[0036] What should be taken into account here is that a repetition rate of 1 kHz is sufficient to compensate for motion effects, and that at the lowest possible repetition rate it is possible to achieve an all-around higher signal-to-noise ratio for a given limited power of the laser diode. In particular, a repetition rate of 1-3 kHz is set as the optimal range for in vivo ocular measurements.

[0037] Furthermore, it should be taken into account that the instantaneous linewidth, the coherence length and the measurement depth depend on the modulation rate and the repetition rate of the A-scan. An optimization of this dependency is provided by the present invention, where the first and foremost importance is the adherence to a sufficient measurement depth of 60 mm to perform full-eye biomeasurements.

[0038] In any case, the adjustment rate / repetition rate of the A-scan is chosen to have a frequency of at least 1 kHz so that the effects of eye movements can be eliminated, with an increase of up to 100 kHz being set for embodiments of the biometric system, provided that the result is not below a measurement depth of 60 mm.

[0039] A further technical challenge in the selection and control of suitable VCSEL laser diodes can be seen in the temperature gradient of wavelength change that exists: thus, a 5 nm tuning requires a temperature change of 50 K.

[0040] To achieve a wide tuning range at a repetition rate of about 1 kHz, commercially available VCSEL laser diodes need to be modified or operated in a pulsed manner, for which the control unit is designed to vary the time and amplitude of the current pulses for periodic current modulation.

[0041] For this purpose, the invention provides that a periodic current modulation (electrical adjustment) is performed, independent of the direction of wavelength change, so that both up-sweep and down-sweep are used.

[0042] In particular, when using current pulses to tune the wavelength, the present invention is adapted for simple swept-source VCSEL bioinstrumentation applications to use both up- and down-sweeps of the wavelength, where as the current increases, a longer wavelength is set, and during the down-sweep, it is necessary to take into account that the wavelength decreases between the current pulses.

[0043] It is generally known that a temperature gradient of about 0.1 nm / K can be achieved. Therefore, a temperature change of 50 K is required for an adjustment of 5 nm. The present invention provides for increasing this gradient to greater than 0.1 nm / K, preferably to about 0.3-0.5 nm / K.

[0044] This is achieved by selecting a suitable active semiconductor material, however it is also possible to additionally embed the VCSEL laser diode in the active semiconductor material. In a VCSEL laser diode for wavelengths of 600 nm to 1400 nm, for example, a GaAs wafer is used as a surface emitter.

[0045] Further, a single mode AlGaInP laser diode having a wavelength of 690 nm and a multi-quantum well structure with a large temperature gradient of at least 0.25 nm / K is used.

[0046] To optimize the heat sink to achieve a high temperature gradient within the operating temperature range of the VCSEL laser diode (e.g., -20 to +70°C), materials with high thermal conductivity, such as copper or indium, are bonded to the semiconductor material.

[0047] Besides the purely electrical tuning of single mode VCSEL laser diodes according to the invention, there is further provided a combination of purely electrical tuning with additional thermal tuning.

[0048] For that purpose, the electronic control module works in synchronism with the heat sink to regulate the ambient temperature of the single mode VCSEL laser diode mainly within the range of -80°C to +180°C, for example within a specific range of 100K, at the highest possible repetition rate, for example above 100Hz, in particular above 1kHz. Thus, the sweep range (tuning range of wavelength in nm) can be extended and the resolution of the biometer can be increased.

[0049] Active cooling using Peltier elements can additionally be carried out. However, additional cooling is provided in the present invention by heating the exit window of the VCSEL laser diode, as condensation of water may occur on the exit window of the VCSEL laser diode.

[0050] As already explained, for cataract penetration or deeper tissue penetration, VCSEL laser diodes with a wavelength of about 1050 nm are more suitable. According to the invention, in this case a light output at the eye in the range of 200 μW up to 2000 μW, in particular more than 1000 μW (for a wavelength of 1050 nm) is provided.

[0051] Here, it must be taken into account that a higher optical output can only be achieved within the limits of laser safety regulations. To achieve a higher optical output, the design of the VCSEL laser diode must be modified. For this purpose, it is possible to change the active area or other parameters of the VCSEL laser diode in order to achieve an optical output of up to 2 mW, for example.

[0052] For the construction of a SS interferometer system using a VCSEL, the present invention provides that the laser radiation emitted by a semiconductor laser diode is coupled into the interferometer by free space or optical fiber. For this purpose, the design of the VCSEL laser diode must be configured accordingly. In both cases, the beam quality for a particular power is an important criterion, since it determines the laser power that can be used to perform the measurement.

[0053] Especially in fiber-based designs, it is important to perform input coupling with as little loss as possible. For monomode fiber with a fiber diameter of 5 μm, it is possible to achieve practically lossless input coupling with less than 10% loss for a numerical aperture NA=0.14. Input coupling of monomode fibers with core diameters of 3-9 μm is typically provided.

[0054] A further embodiment of the invention relates to the generation of wavelength-controlled current pulses: by evaluating the currents of two photodiodes with different spectral behavior, a wavelength measurement signal is generated in a simple manner using analog electronics and used as a feedback system.

[0055] This is explained below using an example for the wavelength range around 1050 nm. In this wavelength range, the change in spectral sensitivity is about +0.1 A / W per 100 nm for the InGaAs photodiode and about -0.5 A / W per 100 nm for the Si photodiode. However, the effect of variations in the power output from the laser is the same for both photodiodes. Thus, after properly normalizing the Si photodiode current with the InGaAs photodiode current, all that remains is the resulting current's dependence on wavelength, which is about 0.4 A per 100 nm (4 μA / nm). This measurement signal can then be used to shape the current pulse to achieve the desired profile of wavelength change.

[0056] A profile of wavelength over time, corresponding to an equidistant change in wavenumber over time, is preferably generated. In this way, the spectral information of the interference signal (frequency domain) can be directly converted to a spatial domain measurement by Fourier transformation without further equalization.

[0057] Appropriate shaping of the current pulse can be achieved by digitizing the wavelength-dependent signal and processing it algorithmically (improving it by filtering and averaging, etc.). The current pulse is then repeatedly modified until the desired result is achieved.

[0058] In addition to the suitable shaping of the current pulses until linearity or an equidistant change in wave number over time is reached, the invention further comprises using structured current pulses in which the current is interrupted many times, i.e. a rapidly clocked current fills the envelope of the current pulse, which is optimized according to the invention in terms of the interval between the individual pulses and the amplitude of the individual pulses, until its structure results in an equidistant change in wave number over time of a single-mode VCSEL laser diode.

[0059] Additionally, a closed analog or digital control loop is contemplated, with the desired wavelength profile as the setpoint, the wavelength dependent signal as the measurement, and the current pulse as the actuation signal. The invention further comprises optimizing the shaping of the current pulse by continuous variation of the current within the pulse in order to obtain the widest possible sweep range (tuning range of wavelength in nm) of the single mode VCSEL laser diode, in this respect a range of more than 3 nm, in particular more than 8 nm up to 20 nm is set.

[0060] In addition to the appropriate shaping of the current pulses until the widest possible sweep range (tuning range of the wavelength in nm) is reached, the invention further comprises using structured current pulses in which the current is interrupted many times, i.e. a rapidly clocked current fills the envelope of the current pulse, which is optimized according to the invention in terms of the spacing between the individual pulses and the amplitude of the individual pulses, until the structure provides over time a maximum sweep range of up to about 20 nm (tuning range of the wavelength in nm) of a single-mode VCSEL laser diode.

[0061] In a final embodiment of the present invention, the short current pulse is th, and the duration of the rising edge of the current is less than 500 μs, in particular less than 50 μs, up to about 1 μs. Thus, in addition to the thermally induced spectral sweep range of, for example, 0.1 nm / K, an extension of the non-thermally induced spectral sweep range (non-thermal spectral sweep range) is achieved. The effect used in the present invention is based on a purely electrically induced shift of the electronic level / band gap of the semiconductor laser material due to the instantaneously applied high electric field strength. This extended part of the spectral sweep range is brought about in particular by the type of current pulse and its rising edge. The advantage of this mode of operation is the low or negligible cost in terms of thermal stability of the SS-OCT system. In this mode of operation, cooling and thermal stabilization can be minimized or avoided.

[0062] This mode of operation according to the invention satisfies the condition Δλ>ΔTΔλ therm is set if where Δλ corresponds to the spectral sweep range, Δλ therm corresponds to the thermally induced spectral sweep range (unit: nm / K), ΔT corresponds to the temperature change (unit: K) during the current pulse.

[0063] This mode of operation can be achieved, for example, by using an integrated temperature sensor for ΔT in the laser chip and the known spectral drift behavior Δλ of the laser material measured in steady state. therm This can be done quantitatively using measurements during the current pulse combined with measurements of the spectral sweep range Δλ achieved with

[0064] In the proposed method for controlling a SS interferometer system using a semiconductor laser diode, the operation of the semiconductor laser diode is configured such that, by periodic current modulation, a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate. According to the invention, in this case: Central wavelength in the range of 600-1400 nm, Sweep rates in the range of 100Hz to 100kHz, A sweep range within the range of 3 to 75 nm, and The parameters of the light output at the eye are set within the range of 50 to 20,000 μW, A coherence length of at least 20 mm is provided.

[0065] In this regard, FIG. 1 shows a basic diagram of the control of a semiconductor laser diode-based SS interferometer system according to the invention by periodic current modulation. The SS interferometer system using a semiconductor laser diode consists of a control unit 2 and a semiconductor laser diode 1, where the control unit 2 is designed to control the operation of the semiconductor laser diode 1 by periodic current modulation such that a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate.

[0066] To that end, the control unit 2 is designed to vary the time and amplitude of the current pulses for periodic current modulation, in particular to output current pulses with short rise times for electrically pumping the semiconductor diode 1.

[0067] In this case, the semiconductor laser diode 1 is in particular a VCSEL laser diode, which changes the wavelength of the highly coherent spectral laser line depending on the current operating temperature. In this regard, for example, a semiconductor laser diode based on GaAs has a variation Δλ of 0.07 nm / K. therm / ΔT is set, and the variation Δλ of 0.25 nm / K is set for a semiconductor laser diode using AlGaInP. therm / ΔT is set.

[0068] Furthermore, according to the present invention, the rise time of the current pulse from the control unit 2 is short, so that the rise time exceeds the values ​​known as a result of thermal fluctuations (Δλ>ΔTΔλ therm), it should be noted that it is possible to achieve a wider tuning range of the wavelength Δλ.

[0069] Optionally, mainly for stable long-term operation of the SS laser source, there is provided temperature stabilization or temperature management by a heat sink 3 which can operate actively and passively. If the thermal capacity of the semiconductor laser diode 1 and its already existing heat sink is sufficient for stable operation, these additional functions of the heat sink 3 can be omitted.

[0070] Thermally conductive materials and / or Peltier elements can be used to optimize this heat sink 3. Active cooling of the laser chip of the semiconductor laser diode 1 is set in synchronism with the periodic current modulation by the control unit 2, only at times other than the rising edge (rise time) of the current.

[0071] In addition to the cooling function of the laser chip by heating by the current pulses of the control unit 2 for a thermally stable operation, the heat sink 3 optionally comprises additional heating of the laser chip of the semiconductor laser diode 1 only within the rising edge of the current in synchronism with the current pulses of the control unit 2. As a result, it is possible to increase the thermally induced wavelength detuning by increasing the temperature change ΔT not only based on an electrically induced temperature increase but also based on an optional direct active temperature increase within the time of the rising edge of the current.

[0072] The semiconductor laser diode 1 preferably outputs an output power in the range of several mW with the desired parameters of the SS interferometer system for OCT and biometric applications, which can compensate for losses in further optical systems and achieve a good signal / noise ratio while considering 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 (referred to as semiconductor optical amplifier, SOA for short) SOA4 is optionally incorporated into the optical beam path.

[0073] FIG. 2 shows some further optical parameters of the radiation emitted by the semiconductor laser diode of the SS interferometer system. Diagram 5 shows diagrammatically the tuning of individual highly coherent wavelengths over a wavelength range Δλ, the so-called sweep range, with a power P in the mW range. As shown in the graph, the spectral resolution of the sweep determines the coherence length Lc or the measurement depth in air. This function is realized within the current rising edge (rise time) of the semiconductor laser diode 1 according to the invention. The diagram shows in particular a shift of the tuning range Δλ (spectral jitter) that can occur as a result of heating of the semiconductor laser diode 1 after a relatively long operation. This effect can be suppressed by temperature management by a heat sink 3.

[0074] Diagram 6 shows, by way of example, the schematic temporal emission parameters of individual laser pulses, each laser pulse of 300 μs containing a spectral tuning characteristic of a highly coherent wavelength within Δλ according to diagram 5. These laser pulses are shown here with a repetition rate of 1 kHz, i.e. a pulse interval of 1 ms. In this case, a duty cycle of 30% is realized. In this respect, for example, after an active current pulse of 300 μs, the laser chip again has 700 μs time for thermal and / or electromagnetic relaxation for long-term stable functionality.

[0075] The laser radiation of a semiconductor laser diode 1 having characteristics according to diagrams 5 and 6 is coupled into the eye 7 to be examined in an interferometer system (not shown in detail). The light intensity backscattered from the eye is detected by a beam splitter 8 in a biometer 9. The broad signal of the biometer 9 is processed with a post-processing unit 10 in order to output a depth scan (also called A-scan) over the entire eye 7, in particular information about the positions and distances of the cornea, lens and retina as a measurement result 11. In a procedure not shown diagrammatically here, said A-scan can be scanned laterally over the pupil opening of the eye in order to generate a cross-sectional image of said distances in the eye 7, as a so-called B-scan.

[0076] To compensate for the undesired drift of the wavelength tuning range (sweep range) resulting from the effects of the thermal environment and changes in the operating temperature, the so-called spectral jitter, the single-mode VCSEL laser diode can be installed on a temperature-stabilized heat sink with a temperature stability of about + / - 1 K in the temperature range from -80 ° to + 180 ° C. In this case, it is also possible to achieve a high output power of the monomode VCSEL laser diode as a result of the high current at low temperatures.

[0077] The present invention comprises the use of optical fiber Bragg gratings in the interferometer of a short coherent biometric system that predefine a wavelength reference that allows the measurement system to be continuously recalibrated to achieve temperature dependent wavelength drift without an additional heat sink.

[0078] For better spectral stabilization of the tuning range, a (e.g. fiber-based) Bragg reflector / grating (FBG) can be used. The said reflector is introduced into the system in such a way that during the tuning process, a light reflection occurs when a precisely defined wave number is given. The said light reflection can be recorded together with the interference signal. This serves as a reference for an accurate and absolute spectral localization of the spectral interference signal.

[0079] In this regard, W. Choi et al. in Non-Patent Document 6 describe a solution in which one of the detection arms is guided to a balanced detector via an FBG. In the detected signal, the FBG generates a "notch" that can be used for spectral referencing of the signal.

[0080] A first variant of the method relates to imaging based on an SS-OCT system, in which too it is intended that a highly coherent spectral laser line is tunable over a wide wavelength range with as high a repetition rate as possible.

[0081] In this variant of the proposed method, for example the following embodiments are possible: According to the first embodiment, Central wavelength of 1050 nm, 1kHz sweep rate, 16nm sweep range, and One, more, or all of the preferred parameters of light output at the eye of up to 2 mW are set; Coherence lengths of over 60 mm are provided.

[0082] According to the second embodiment, Central wavelength of 840 nm, A sweep rate of at least 1 kHz, A sweep range of at least 9 nm, and One, more, or all of the preferred parameters of light output at eye of up to 0.8 mW are set; Coherence lengths of over 60 mm are provided.

[0083] According to a third embodiment, Central wavelength of 600 nm, A sweep rate of at least 1 kHz, 5nm sweep range, and One, more, or all of the preferred parameters of light output at eye of up to 0.4 mW are set; Coherence lengths of over 60 mm are provided.

[0084] As an example, to measure from the front of the eye to the back of the lens, a measurement depth or coherence length Lc in air of about 25 mm is sufficient. In measuring devices for determining biometric measurements, e.g. for the purpose of IOL calculation, measurements such as the eye length AL, the lens thickness LD, the anterior chamber depth VKT, the corneal thickness HHD etc. are essentially determined from so-called A-scans or B-scans. An axial resolution of about 100 μm in air is sufficient for this purpose.

[0085] As an example, reference is made for this purpose to the IOL Master 500 from Zeiss Meditec AG, in which a multimode laser diode with a central wavelength of 785 nm and a full width at half maximum of Δλ ≈ 2.7 nm achieves a resolution of 100 µm in air.

[0086] The axial resolution Δz in terms of Gaussian power spectral density is given by

[0087]

number

[0088] where Δλ defines the full width at half maximum, λ defines the central wavelength. Other typical power spectral densities can also be reasonably well approximated by the above formula. For a semiconductor laser diode with a central wavelength of 1050 nm, Δλ ≈ 5 nm is required to achieve 100 μm resolution in air.

[0089] Even better values ​​of about 65 μm resolution in air are obtained when using semiconductor laser diodes with a central wavelength of 840 nm and an electrical tuning range of 5 nm. At this wavelength of 840 nm, the axial resolution of 100 μm in air required for biomeasurements is already achieved with a tuning range of about 3 nm.

[0090] This therefore gives the following values:

[0091] [Table 2] The present invention provides a full width at half maximum in the range of 10-100 nm, and in particular at least 16 nm at 1050 nm (corresponding to the sweep range of semiconductor laser diodes).

[0092] Furthermore, the present invention provides a VCSEL laser diode having a repetition rate (called a sweep rate) of 100 Hz to 100 kHz, and in particular 1 kHz or higher. In this connection, reference is again made to the relationship between motion effects, repetition rate and signal / noise ratio already mentioned.

[0093] The sensitivity of a SS-OCT system is proportional to the energy (number of photons) deposited in the tissue, not the peak power. Therefore, the present invention seeks to select a sweep rate that is fast enough to eliminate the effects of eye movement, but also long enough to make available a sufficiently high pulse energy for a given output of the laser diode.

[0094] In this case, the invention provides an optical output in the range of 200 μW up to 20000 μW, in particular up to 2000 μW (at a wavelength of 1050 nm). In this case too, the aforementioned laser safety regulations must be observed again.

[0095] However, other parameters can also be varied, in this regard, for example the linewidth of the VCSEL laser diode can be increased. Furthermore, the present invention provides a coherence length Lc of at least 60 mm, since the measurement depth resulting from the coherence length is necessary to measure the entire human eye.

[0096] However, SS-OCT systems with lower axial resolutions of more than 30 μm, especially more than 50 μm, can also be realized. This is of interest, for example, for tomography and volumetric representations of the entire eye (from the anterior chamber to the fundus). The loss of axial resolution can be compensated for by "machine learning" algorithms. For this purpose, images must first be recorded at a standard resolution and these images are used to train the algorithm. The machine learning-based algorithm then learns to improve the resolution even in images with low axial resolution.

[0097] In order to expand the field of application of the technology according to the invention, in particular algorithms based on machine learning / deep learning / artificial intelligence are intended to provide an improved evaluation of the whole eye scan as A-scan and / or B-scan. In this case, the advantage of the biometer according to the invention is the high measurement depth, which allows a whole eye scan at a high repetition rate, for example an A-scan rate of 28 kHz. However, compared to modern high-resolution ss-OCT systems with a sweep range of up to 100 nm, it is disadvantageous in terms of a low resolution due to a small sweep range of about 5 nm. Therefore, it is intended to use the biometer according to the invention and at the same time to measure about 20 eyes of a patient completely and / or partially using other commercially available ultra-high resolution OCT systems. By means of algorithms based on machine learning / deep learning / artificial intelligence, the known high-resolution measurements are intended to be compared and evaluated with the measurements of the biometer according to the invention. As a result, these algorithms are intended to improve the subsequent individual measurements of the biometer in A-scan and / or B-scan in terms of resolution and precision.

[0098] A second variant of the method for controlling a semiconductor laser diode based SS interferometer system is particularly relevant for full eye scanning biometric applications. In the proposed method for controlling a SS interferometer system using a semiconductor laser diode, by periodic current modulation, the operation of the semiconductor laser diode is configured in such a way that a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate. Central wavelength in the range of 600-1400 nm, Sweep rates in the range of 100Hz to 100kHz, A sweep range of 3 to 20 nm, and The parameters of the optical power at the eye in the range of 50 to 20,000 μW are set for biometric applications; A coherence length of at least 20 mm is provided.

[0099] According to one preferred embodiment, Central wavelength of 1050 nm, 1kHz sweep rate, 5nm sweep range, and one, more, or all of the preferred parameters of light output at the eye of up to 2 mW are set for the full eye scan; Coherence lengths of over 60 mm are provided.

[0100] Semiconductor laser diodes operate by periodic current modulation whereby the duration, amplitude and shape (rising edge) of a current pulse are varied. To obtain the highest repetition rate, the highest temperature gradient must be achieved by the current pulses in the active VCSEL semiconductor layers. However, for a substantially continuous mode of operation, the same average temperature of the active material must be ensured to be able to achieve a high repetition rate with constant laser parameters.

[0101] This is accomplished by simply passively cooling the VCSEL laser diode according to the present invention by the following means. - Threshold current I th short current pulses in the range of less than 500 μs to about 50 ns in duration, - a duty cycle sufficient to cool the active VCSEL semiconductor layer to a stable operating temperature within a range of + / - 2 K. In particular, for this purpose a duty cycle of less than 50% to 1%, preferably 30%, is provided according to the invention. In this regard, for example, if a 100 ns current pulse is applied, the next current pulse can be generated after 10000 ns or 10 μs to set a repetition rate of 100 kHz.

[0102] According to one advantageous embodiment, a SS interferometer system using a VCSEL according to the present invention comprises: 28kHz sweep rate, 30% duty cycle 840nm mean laser wavelength, 5nm sweep range, Characterized by the technical parameters of the VCSEL laser diode output power of 2mW, This output power corresponds to a power at the eye of 0.2 mW, and therefore the measurement sensitivity is 100 dB.

[0103] In particular, with this optimization, higher peak powers can be achieved using shorter driver current pulses, resulting in a duty cycle of less than 10%, although the energy (number of photons) deposited in the tissue can be reduced by more than 10 times.

[0104] The present invention provides a full width at half maximum in the range of 3-20 nm for biometric applications, in particular about 5 nm at 1050 nm (which corresponds to the sweep range of semiconductor laser diodes).

[0105] According to the present invention, a duty cycle within the range of 1% to 50%, in particular 30%, adapted with respect to measurement sensitivity and thermal stability is further realized to achieve a stable steady-state operating temperature of the VCSEL-based tuned SS interferometer system.

[0106] According to the invention, a minimum repetition rate of 1 kHz is provided, so that even a duty factor of 0.01% is possible for current pulses with a duration of 100 ns, which means a cooling time 10000 times longer than the current pulse time in the active laser material. A duty cycle of 0.1% is realistic in order to keep the required control complexity within limits. This makes it possible to establish a stable steady-state operating temperature of the SS-OCT system. Within the limits of the pulse energy required for the measurements explained above, we aimed for a duty cycle of 10% and found a first practical solution with a duty cycle of 30%. Even with a duty cycle of 30%, the spectral tuning range is permanently stable and no spectral drift occurs, so that the configuration selected according to the invention ensures good heat dissipation for the active volume of the laser diode.

[0107] In the above advantageous embodiment, it was possible to ensure an average power of 2 mW of the diode emission. In the application of this tunable VCSEL laser diode, a VCSEL-based SS interferometer system can provide a measurement power at the eye of 200 μW, realizing eye biometric measurements with a sensitivity of 100 dB.

[0108] The invention further comprises controlling the current pulse for pumping the laser source in time with respect to its profile amplitude such that a profile of the emitted wavelength is obtained within the spectral tuning range of the laser radiation that is linear over the time of the current pulse, which is advantageous for minimizing error sources in the evaluation of the measurement signal.

[0109] As already mentioned, the sensitivity of a VCSEL-based SS interferometer system is proportional to the energy (number of photons) deposited in the tissue, not the peak power. Thus, the present invention provides an advantageous optimization that allows for over ten times the power of a laser diode during pulsed operation, which in principle may still be feasible in practice.

[0110] Ultimately, the thermal load limits the output power of the laser diode, which provides an optimal tuning range, especially at low operating temperatures. Therefore, for operation of the SS interferometer system using the VCSEL according to the present invention, a stable nm / K slope of the VCSEL laser diode is defined and set.

[0111] As already mentioned, a temperature gradient of about 0.1 nm / K can be realized, so that for a tuning range of 5 nm a temperature change of 50 K is necessary. The invention provides for setting said gradient above 0.1 nm / K, preferably to about 0.3-0.5 nm / K. This can be achieved in particular by the means described above.

[0112] In this case, the invention provides an optical output in the range of 200 μW up to 2000 μW, in particular up to 2000 μW (at a wavelength of 1050 nm). Therefore, the present invention provides a threshold current I th and adjusting the driver current of the laser diode in a range exceeding 100 μW, with minimum and maximum current values ​​in said range providing an output power of the laser diode for measurements at the eye in the corneal plane of at least 50 μW.

[0113] According to a further advantageous embodiment, for the periodic current modulation 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 set and ensured.

[0114] Therefore, in order to optimize the repeatability of the adjustment range already described, the following is done: - setting very small clock variations of the control pulses, within the range of less than 1 ns; - maintaining the repetition rate of the control pulses at less than 10 ns; and - Set the amplitude stability of the time profile to less than + / - 5%.

[0115] This keeps the tuning wavelength range constant. Changes in the wavelength range of more than 0.1 nm will affect both the axial resolution and the axial scaling, and therefore the accuracy of the biometric measurements.

[0116] Furthermore, the mechanically stable reference signal of the interferometer is provided to correct the fluctuations of the trigger and driver signals, especially when the necessary stability requirements mentioned above are not fully met with respect to the spectral tuning range, for example by correlation and registration of the spectral interference pattern of the reference signal.

[0117] The solution according to the invention for constructing a simple tunable diode laser is based on the fact that a wavelength change is achievable if there is a temperature change in the active laser material, where said temperature change is brought about indirectly by a current pulse or / and by additional heating or cooling.

[0118] However, according to the invention, in addition to such desired effects, temperature changes in the active material of the laser diode are also accompanied by a change in the output power for a given pump current. Thus, firstly, the invention comprises selecting a laser diode design in which the change in output power as a function of the operating temperature is small or reproducible. Furthermore, with regard to this dependency, an optimization of the above-mentioned operating conditions is provided.

[0119] However, in addition to the technical challenges already mentioned, there are many more dependencies that must be considered that affect the selection and control of a suitable VCSEL laser diode. The application of the measurement technique according to the invention to a moving eye requires fast measurement times in order to be able to exclude the effects of movement on the measurement signal. For an individual measurement, the measurement time needs to be about 1 ms or less. Thus, for example, a combination of 10% duty cycle and 100 Hz sweep rate should be selected. Given this temporal tuning rate, a spectral tuning width in the range of 3-20 nm is likewise required to ensure the required resolution. However, the known behavior of laser diodes is that, especially for slow tuning rates, high tuning widths are obtained, thus presenting an additional challenge for the realization of the measurement system according to the invention. For this combined requirement, a selection of the optimal version is provided based on the design of the laser diode.

[0120] Furthermore, an absolutely constant spectral tuning range is necessary to be able to evaluate the measurement signal in a reproducible manner. On the other hand, due to the repeated electrical / thermal tuning of the laser diode required during the envisaged measurement operation, a temperature drift and therefore a wavelength drift of the tuning range occurs as a result.

[0121] For example, selection of an optimal version of a laser diode with high thermal conductivity or high heat capacity can reduce undesired temperature drift relative to this requirement. A wide sweep range requires the VCSEL laser diode to be set at a high temperature, but at high temperatures the light output of the laser diode decreases.

[0122] Furthermore, the above dynamic thermal behavior of VCSEL laser diodes is itself dependent on the operating temperature. In this regard, the following obtains at higher operating temperatures: - higher laser thresholds, and - Lower maximum power P max and a lower maximum current I max .

[0123] The invention therefore comprises choosing an operating temperature below room temperature of about 20° C., in particular below 10° C., or cooling the laser diode to its minimum allowable operating temperature. The solution according to the invention uses VCSEL laser diodes to provide a SS interferometer system suitable for use in ophthalmology, in particular for determining ocular biometric measurements.

[0124] The SS-OCT system is tunable over a wide wavelength range and has a high coherence length and a relatively high repetition rate, making it particularly suitable for ocular biometry with whole-eye scanning.

[0125] The solution according to the invention for constructing a simple tunable diode laser is based on the fact that a wavelength change is achievable if there is a temperature change in the active laser material, where said temperature change is brought about indirectly by a current pulse or / and by additional heating or cooling.

[0126] As mentioned above, a temperature gradient of about 0.1 nm / K can be achieved, thus requiring a temperature change of 50 K for a tuning range of 5 nm. The present invention provides for setting said gradient above 0.1 nm / K, preferably about 0.3-0.5 nm / K. This can be optimized, inter alia, by the described choice of active semiconductor laser material, the selection of appropriate operating parameters, and thermal management of the system.

Claims

1. 1. A method for controlling a simple, semiconductor laser diode based swept-source (SS) interferometer system suitable for full-eye scanning, comprising: applying a periodic current modulation to configure the operation of said semiconductor laser diode such that a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate; Multiple parameters below A central wavelength in the range of 600 to 1300 nm; A sweep rate in the range of 100 Hz to 100 kHz; A sweep range in the range of 3 to 75 nm, and Light output at the eye in the range of 50-20,000 μW and using A coherence length of at least 20 mm is provided; and A method, characterized in that current pulses having a duration of 1 μs at a repetition rate of 100 kHz or a duration of 10 μs at a repetition rate of 10 kHz are realized.

2. Regarding imaging using OCT systems, Multiple parameters below Central wavelength of 1050 nm, A sweep rate of at least 1 kHz; A sweep range of at least 16 nm; and Maximum light output at the eye is 2mW using at least one of 2. The method of claim 1, wherein a coherence length of more than 60 mm is provided.

3. Regarding imaging using OCT systems, Multiple parameters below Central wavelength of 840 nm, A sweep rate of at least 1 kHz; A sweep range of at least 9 nm; and Maximum light output at the eye is 0.8mW using at least one of 2. The method of claim 1, wherein a coherence length of more than 60 mm is provided.

4. Regarding imaging with OCT systems, Multiple parameters below Central wavelength of 600 nm, A sweep rate of at least 1 kHz; A sweep range of at least 5 nm, and Maximum light output at the eye is 0.4 mW using at least one of 2. The method of claim 1, wherein a coherence length of more than 60 mm is provided.

5. The method of claim 1 , further comprising using a low axial resolution of greater than 30 μm for imaging by application of an OCT system.

6. The method of claim 5 , further comprising the step of compensating for the loss of resolution by application of a machine learning based image processing algorithm.

7. The method of claim 1 , further comprising making the periodic current modulation independent of a direction of wavelength change such that both up-sweeps and down-sweeps are used.

8. 1. A method for controlling a simple, semiconductor laser diode based swept-source (SS) interferometer system suitable for full-eye scanning, comprising: applying a periodic current modulation to configure the operation of said semiconductor laser diode such that a highly coherent spectral laser line is tunable over a wide wavelength range at the highest possible repetition rate; Multiple parameters below A central wavelength in the range of 600 to 1300 nm; A sweep rate in the range of 100 Hz to 100 kHz; A sweep range of 3 to 20 nm; and Light output at the eye in the range of 50-20,000 μW and using the same for ophthalmic biometry applications, A coherence length of at least 20 mm is provided; A method, characterized in that current pulses having a duration of 1 μs at a repetition rate of 100 kHz or a duration of 10 μs at a repetition rate of 10 kHz are realized.

9. Regarding whole eye scans, Multiple parameters below Central wavelength of 1050 nm, A sweep rate of at least 1 kHz; A sweep range of at least 5 nm, and Maximum light output at eye of 2mW [0023] 6. The method of claim 5, wherein a coherence length of more than 60 mm is provided.

10. The method of claim 8 further comprising making the periodic current modulation independent of a direction of wavelength change such that both up-sweeps and down-sweeps are used.

11. The amplitude of the current pulse is defined as the threshold current I ｔｈ The method of claim 8 , further comprising the step of:

12. 9. The method of claim 8, further comprising the step of: making the duration of the rising edge of the current pulse less than 500 μs to 50 ns.

13. 9. The method of claim 8, further comprising the step of providing for said periodic current modulation a clock variation of less than 1 ns, a current pulse repeatability of less than 10 ns, and an amplitude stability of less than + / - 5%.

14. The additional modulation of the (electronic level) electronic band gap of a semiconductor laser material by the application of a momentarily applied high electric field strength is defined as Δλ>ΔTΔλ ｔｈｅｒｍ The method further includes the step of: where Δλ corresponds to the spectral sweep range, Δλ ｔｈｅｒｍ corresponds to the thermally induced spectral sweep range (unit: nm / K), 2. The method of claim 1, wherein ΔT corresponds to the temperature change (in K) during the current pulse.

15. 15. The method of claim 14, further comprising the step of controlling the current pulses based on knowledge of the instantaneous wavelength by application of a feedback system, thereby achieving a known change in wave number over time.

16. The method of claim 14 further comprising the step of performing a linear variation of wavenumber over time.

17. The method of claim 15 further comprising the step of performing a linear variation of wavenumber over time.

18. The method of claim 14 further comprising the step of including two different photosensitive materials in the feedback system.

19. 10. The method of claim 1, further comprising achieving a continuous output power of the semiconductor laser diode of 2 mW with a sweep rate of 28 kHz, a duty cycle of 30%, an average laser wavelength of 840 nm, a sweep range of 5 nm, and a continuous output power of 0.2 mW at the eye, which corresponds to a measured sensitivity of 100 dB.

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