WO2018166883A1 - Method for operating a surface-emitting semiconductor laser with variable wave number - Google Patents

Abstract

The invention relates to a method for operating a surface-emitting semiconductor laser (2) having an HCG mirror with variable wave number k. A trigger signal (17) is produced on the basis of a time-dependent control voltage U(t) or simultaneously with the control voltage in a time-controlled manner, the trigger signal comprising a plurality of successive trigger pulses and the wave number changing by a fixed value in a time interval between any two successive trigger pulses.

Description

 A method of operating a variable wavelength waveguide surface emitting semiconductor laser

Technical area

The invention relates to a method for operating a surface emitting semiconductor laser having a variable wave number k.

The evaluation and image generation algorithm in optical coherence tomography (OCT) based on frequency domain (OCT) requires inter alia the application of Fourier transforms to the signal obtained. A special case of the method is the Swept Source OCT (SSOCT), in which the necessary acquisition of the spectrum takes place sequentially and for this purpose a high-frequency tunable laser source ("swept source") is used It may be a tunable laser or a tunable LED, and for the required further processing and Fourier transformation of the data in the SSOCT method it is necessary that the spectrum with equidistant interpolation points with respect to the wave number or the frequency the light is present.

However, the swept-source dynamics usually follow a nonlinear function, so a signal recorded at a fixed sampling rate does not meet the requirements outlined above. For this reason, the swept source always generates a so-called k-clock signal in addition to the tunable light output signal and outputs it. The k-clock signal is a trigger signal, which indicates the detuning of the laser by a certain change of the wave number k during tuning of the laser (so-called sweep).

The SSOCT system can now use this signal as a trigger signal for data acquisition in order to directly record a spectrum with equidistant interpolation points in k-space. Alternatively, in a data acquisition with a fixed sampling rate, the k-clock signal can be used to convert the time equidistant signal by interpolation to a frequency equidistant basis.

State of the art

The necessary k-clock signal is optically generated in common OCT systems. For this purpose, a part of the generated laser power is passed through a frequency filter element and the transmitted or reflected power measured. As filter elements, for example, an etalon as Fabry-Perot filter or an interferometer - for example in Mach-Zehnder arrangement - are used. During the frequency sweep, the changing wavelength of the laser results in a periodic power signal at the output of the filter element. The periodicity can be adjusted by the parameters of the filter element to the requirements of the application, for example over the distance difference z of the two interferometer arms.

EP2884224A1 each describe a SSOCT with interferometrically generated clock signal. EP 3020105A1 recommends deriving an interferometric clock signal from the backside of the laser. The above-mentioned approach to generating the k-clock signal requires precisely designed optics as well as a measuring device for determining the power at the output of the spectral filter element. As a result, the assembly contains a significant proportion of the manufacturing costs of the swept-source laser. In addition, the k-clock characteristic is fixed by the execution of the optics. A change in the number of

Trigger pulses per wave number interval is thus not readily possible. Furthermore, a portion of the output power of the swept-source laser is always used up for the generation of the k-clock signal and thus the usable output power is reduced.

Object of the invention

 The object of the invention is a method for operating a light source for the SSOCT method, in which a trigger signal is generated in a simple manner.

Solution of the task:

 The object is achieved by a method according to claim 1. Advantages of the invention

It is advantageous to significantly reduce the manufacturing costs and the costs compared to arrangements that rely on the optical generation of a trigger signal. In addition, there is the possibility for a parameterizable trigger signal, for example by software, eg for a preselectable number of trigger pulses per wavelength interval, which can be easily adapted to the requirements of the respective application. description

 The SSOCT method requires a narrow band light source which emits light with a high coherence length. The light of a certain wavelength λ has a wave number k which corresponds to the reciprocal of the wavelength. The wave number k must be variable over a certain wavenumber range ki ... k2. From the multiplicity of available light sources, a surface-emitting semiconductor laser is selected according to the invention. A surface emitting semiconductor laser is also referred to as VCSEL (English vertical-cavity surface-emitting laser). Such a laser is particularly suitable because of the high coherence length of several 10 cm or more. Advantageous may be a single-mode operation. According to the invention, a surface emitting semiconductor laser (VCSEL) is provided. The surface emitting semiconductor laser comprises a resonator. The resonator comprises a first mirror and a second mirror. The resonator may be limited by the first and second mirrors. The first and second mirrors may be arranged parallel to each other. The first mirror and the second mirror are spaced from each other. The VCSEL may emit laser radiation in a direction z which may be perpendicular to the first mirror.

Known tunable VCSEL use a so-called MEMS mirror (English microoptomechanical system) as one of the two end mirrors of the laser cavity. By applying an electrical voltage, the position of this mirror can be influenced and thus the total length of the laser cavity can be changed. Such a MEMS mirror typically consists of a movably mounted DBR mirror (English disributed bragg reflector). Due to the high mass of a built-up of several layers known DBR mirror relatively high actuating forces are required to tune the resonator length. The reproducibility of the wave adjustment is therefore so bad, in particular with rapid adjustment or at high tuning frequencies, that an optically generated trigger signal is required. In contrast, according to the invention, the first mirror is designed as HCG (high-contrast grating). An HCG level may be about 200 times lower in mass compared to a DBR level. As a result, the setting accuracy can be significantly improved. High contrast gratings are also referred to as sub-wavelength grating. HCG are known, for example, from US200701 15553A1, their use for VCSEL

US20070153860A1, US20100316079A1 and US20100316083A1.

The method according to the invention for operating a light source with a variable wave number k for an SSOCT method comprises providing a surface-emitting the semiconductor laser (VCSEL). The surface-emitting semiconductor laser comprises a resonator and has a semiconductor surface lying in the resonator. Such a semiconductor surface lying in the resonator can be produced, for example, by undercutting the first mirror described below. The semiconductor surface may be parallel to the first mirror. The VCSEL may comprise a substrate. The VCSEL may have a semiconductor structure. The semiconductor structure may have at least one pn junction and at least one quantum well. The semiconductor structure may be intended to effect an optical amplification required for laser operation. The semiconductor surface may be arranged on an upper side of the semiconductor structure. The resonator comprises a first mirror and a second mirror. The first mirror is designed as HCG (high-contrast grid). The first mirror is arranged at a variable distance I to the semiconductor surface. The first mirror can thus be arranged with a parallel gap of thickness I to the semiconductor surface. The gap may preferably be filled with a gaseous medium. Also advantageously, the gap can also be operated without medium, ie under vacuum. The refractive index of the gap may be 1. The distance I can be less than 5μηι. The first mirror may be connected to a spring element. The spring element may be made of the same material as the first mirror. The first mirror may have a first electrical contact. The first mirror may represent a first electrode. The VCSEL may include a second electrode having a second electrical contact. The second electrode can advantageously be arranged on the semiconductor structure. By applying a control voltage between the first and the second electrode, the distance I can be reduced. The second contact may have an aperture through which the laser light can pass. However, the second electrode can also be arranged at a distance from the semiconductor structure which is greater than the distance I. By applying a control voltage between see the first and the second electrode, the distance I can be increased in this case.

According to the invention, a time-varying control voltage U (t) is generated.

According to the invention, a trigger signal is also generated by means of an electronic module.

The trigger signal can be generated exclusively on the basis of the time-varying control voltage U. Alternatively, the generation of an exclusively time-triggered trigger signal can be provided, wherein the trigger signal is generated simultaneously with the time-varying control voltage U.

The trigger signal comprises several pulse trains. Each pulse train comprises several temporally successive trigger pulses. In a time interval between any two successive trigger pulses of a pulse train, the wave number k changes by a fixed value. The change can be called Ak. The trigger pulses may, but need not, be equidistant in time.

The time-varying control voltage U is applied to the first mirror. This changes the distance I. This change in the distance I can be effected due to the electrostatic attraction between the mirror and a second electrode. The mirror serves as the first electrode. The control voltage may be applied between a first contact electrically connected to the mirror and the second contact electrically connected to or identical to the second electrode. This can lead to an electrostatic attraction between the first electrode and the second electrode, which depends on the applied control voltage.

The change in the distance I causes a change in the wave number k.

The temporal change of the control voltage can be continuous. Ideally, the change in wavenumber may also be continuous, i. without fashion changes of the laser.

A first pulse train can be generated while the wave number k is continuously changed from a first value ki of the wavenumber to a second value k2 of the wavenumber. Subsequently, a second pulse train can be generated while the wave number is continuously changed from the first value ki of the wave number to the second value k2 of the wave number. In the period between the first pulse train and the second pulse train, the distance I can be reset by resetting the control voltage U to an initial value.

A first pulse train can be generated while the wave number k is continuously changed from a first value ki of the wavenumber to a second value k2 of the wavenumber, and Subsequently, a second pulse train can be generated while the wave number is continuously changed from the second value k2 of the wave number to the first value ki of the wave number. In this case, the resetting of the distance I between the first and the second pulse train can be omitted. The first and second pulse trains may each have a first and a last trigger pulse. Between the last trigger pulse of the first pulse train and the first trigger pulse of the second pulse train can pass a time period which is greater than the average time period between two temporally successive trigger pulses of the first trigger signal. This can have the advantage that the respective pulse sequences can be synchronized in a subsequent evaluation with an OCT signal.

The wave number k may be a function f of the control voltage U. The function f can have an inverse function g in an interval [ILm .. Umax]. The control voltage U may correspond to the function g (k (t)) in a time interval including a pulse train. The control voltage U can be selected in this time interval according to the function g (k (t)). By kön- NEN discrete set values of the control voltage U as a function of values g (k (ti)), g (k (t.2)) .. g (k (t _n)) to be stored in a first lookup table.

The control voltage can be generated in such a way that a series of desired values of the control voltage are stored in a first lookup table in a first memory area of the electronic module and the setpoint values of the control voltage are continuously called at times ti, t.2, t _n and Analog converter (DA converter) are converted into the control voltage U.

A sequence of desired values of the trigger signal can be stored in a second lookup table in a second memory area of the electronic module. The nominal values of the trigger signal can at times ti, t.2, t _m be accessed continuously. The retrieved values can be output as the trigger signal.

A sequence of voltage values can be stored in a third look-up table in a third memory area of the electronic module. Then, in each case a trigger pulse can be output from the electronic module continuously if the desired value of the control voltage corresponds to the respective next table value of the third lookup table. In addition, a light source according to the invention with a variable wave number k is specified, comprising a surface emitting semiconductor laser, wherein the surface emitting semiconductor laser comprises a resonator, wherein the resonator comprises a first mirror and a second mirror and the first mirror is designed as HCG (high contrast grid) and the first mirror is arranged at a variable distance I to a semiconductor surface and further comprising an electronic module. The light source is characterized in that it can be operated by the method according to claim 1.

The figures show the following:

 Fig. 1 shows a waveguide surface emitting semiconductor laser.

Fig. 2 shows a light source according to the invention.

 Fig. 3 shows a light source according to the prior art.

 Fig. 4 shows a first embodiment of the method.

 Fig. 5 shows a second embodiment of the method.

 Fig. 6 shows a third embodiment of the method.

Fig. 7 shows a fourth embodiment of the method.

 Fig. 8 shows a fifth embodiment of the method

 Fig. 9 shows a sixth embodiment of the method even more.

 Fig. 10 shows a seventh embodiment of the method

EXAMPLES

Fig. 1 shows a waveguide surface emitting semiconductor laser. The surface emitting semiconductor laser (2) (VCSEL) comprises a resonator. It has a semiconductor surface (10) lying in the resonator. The resonator comprises a first mirror (3) and a second mirror (4) and is bounded by these two mirrors. The first mirror is designed as HCG (high-contrast grid). The first mirror has on the sides one or more spring elements 12, which are connected via spacers 13 with the VCSEL. The first mirror (3) is arranged at a variable distance I from the semiconductor surface (10). The distance I can be changed by applying a control voltage U (t) to the first mirror. The control voltage is applied via the first contact 7 to the first mirror. The mirror has the additional function of a first electrode. Reference potential for the electrical control voltage U (t) is a second electrode 8, which is formed simultaneously as a second contact. This has an aperture 9, so that the laser light can pass. By applying the control voltage, an electric field is formed which causes an attraction force between the first and second electrodes. This results in a deformation of the spring element and thus a change in the distance I. Also shown is a semiconductor structure 5, which provides for the optical amplification and a substrate 6, which serves as a starting material for the production of the VCSEL. The laser radiation 1 1 is emitted here perpendicular to the semiconductor surface in the z direction. Fig. 2 shows a light source according to the invention. The variable wavelength light source 1 comprises a VCSEL 2 and an electronic module 14. The electronic module outputs a time-dependent control voltage U (t) and a trigger signal 17.

Fig. 3 shows a light source according to the prior art. To operate, an optical interferometer 15 is required, which generates an interferometric measurement signal 16. This is an optical k-clock signal, which is used as a trigger signal. In order to generate the k-clock signal, a part of the power of the laser beam 1 1 must be removed.

Fig. 4 shows a first embodiment of the method. Shown are the wave number k, the control voltage U and the trigger signal 17 over time. It should be noted that for the sake of clarity here, as in the following figures, the signal curves are only shown as examples. In practice, one will advantageously choose substantially more than the fifteen trigger pulses per pulse sequence shown here. This can be, for example, a few thousand pulses per pulse train. The trigger signal comprises a plurality of pulse sequences (18, 19). Each pulse train comprises a plurality of trigger pulses 20. It becomes a first pulse train

18, while the wave number k is continuously changed from a first value ki of the wave number to a second value k2 of the wave number. The following is a second pulse train

19 is generated while the wave number is continuously changed from the second value k2 of the wave number to the first value ki of the wave number. The first and the second pulse sequence each have a first 19 and a last trigger pulse 20. Between the last trigger pulse of the first pulse train and the first trigger pulse of the second pulse train elapses a time period which is greater than the time interval between two temporally successive trigger pulses of the first trigger signal. This can have the advantage that the pulse sequences can be synchronized in a subsequent evaluation with an OCT signal. In addition, the locations from the OCT signal which are close to the reversal points of the wave number, ie at the reversal points of the direction of movement of the first mirror, can thereby be masked out. Fig. 5 shows a second embodiment of the method. Shown are the wave number k, the control voltage U and the trigger signal 17 over time. The trigger signal comprises a plurality of pulse sequences (18, 19). Each pulse train comprises a plurality of trigger pulses 20. A first pulse train 18 is generated while the wave number k is changed continuously from a first value ki of the wavenumber to a second value k2 of the wavenumber. Subsequently, a second pulse train 19 is generated, while the wave number is continuously changed in turn from the first value ki of the wavenumber to the second value k2 of the wavenumber. In the period between the first pulse train and the second pulse train, the distance I is reset by resetting the control voltage U to an initial value. Fig. 6 shows a third embodiment of the method. The wave number k is a function f of the control voltage U to be, which has an inverse function g [LLin .. U _ma x] at an interval. The control voltage U corresponds to the function g (k (t)) in a time interval including a pulse train. The control voltage U is selected in this time interval according to the function g (k (t)). For this purpose, discrete setpoint values U ¹ MEMS... U ⁿ MEMs of the control voltage U are present as function values g (k (ti)), g (k (t.2)). G (k (t _n )) in a first lookup - Table 23 deposited. The target values of the control voltage are at times ti, t.2, t _n consecutively accessed and implemented in the control voltage by means of a digi tal-to-analog converter 26th The times are given by a first clock 30. From the first clock is by means of a divider 27, which is followed by a gate 28, the only time-triggered trigger signal generated. Fig. 7 shows a fourth embodiment of the method. A sequence of desired values of the control voltage is stored in a first lookup table 23 in a first memory area of the electronic module. The setpoints of the control voltage at times ti, t.2, t _n continuously retrieved and converted by means of a digital-to-analog converter 26 in the control voltage. The times are given by a first clock 30. A sequence of voltage values is stored in a third look-up table 25 in a third memory area of the electronic module. It is continuously output via a comparator 1 1 each trigger pulse from the electronics module when the setpoint of the control voltage corresponds to the next table value of the third lookup table.

8 shows a sequence of desired values of the control voltage is stored in a first look-up table 23 in a first memory area of the electronic module. The setpoints of the control voltage at times ti, t.2, t _n continuously retrieved and by means of a digital-to-analog converter 26 converted into the control voltage. The times are given by a first clock 30. A sequence of desired values of the trigger signal U ¹ _C iock .. U ⁿ ci _0C k is _stored in a second lookup table 24 in a second memory area of the electronic module. The setpoint values of the trigger signal are continuously called up at the times ti, t.2, t _m and the retrieved values are output as the trigger signal.

Fig. 9 shows a sixth embodiment of the method even more. Here, the trigger signal is generated by means of a second clock 1 1, which has a higher clock frequency than the first clock, and is synchronized with this.

Fig. 10 shows a seventh embodiment of the method. Here, the trigger signal is generated exclusively on the basis of the time-varying control voltage U, by comparing the generated control voltage U via a step comparator 32 with predetermined voltages Ucom and a trigger pulse is output in each case when exceeding the following predetermined voltage value.

Reference numerals:

1 light source with variable wave number

2 surface emitting semiconductor laser

3 first mirror

4 second mirror

 5 semiconductor structure

 6 semiconductor substrate

 7 First contact

 8 second contact, second electrode 9 aperture

 10 semiconductor surface

 1 1 laser radiation

 12 spring element

 13 spacers

14 electronic module

 15 Optical interferometer

 16 Interferometric measurement signal

 17 trigger signal

 18 First pulse train

19 Second pulse train

 20 trigger pulse

 21 First trigger pulse

 22 Last trigger pulse

 23 First Lookup Table

24 Second lookup table

 25 Third Lookup Table

 26 DA converters

 27 dividers

 28 gate switching

29 comparator

 30 First bar

 31 second bar

 32 stage comparator

Claims

claims: 1 . A method of operating a light source (1) having a variable wavenumber k for a SSOCT method  a. Providing a surface-emitting semiconductor laser (2), the surface-emitting semiconductor laser comprising a resonator and having a semiconductor surface (10) in the resonator, the resonator comprising a first mirror (3) and a second mirror (4) and the first mirror as HCG ( High contrast grid) is formed and the first mirror (3) is arranged at a variable distance I to the semiconductor surface (10), b. Generating a time-variable control voltage U (t),  c. Generating a trigger signal (17) exclusively on the basis of the time-varying control voltage U or generating an exclusively timed trigger signal (17) simultaneously with the time-varying control voltage U by means of an electronic module (14),  wherein the trigger signal (17) comprises a plurality of pulse trains (18, 19) and each pulse train comprises a plurality of temporally successive trigger pulses (20) and the wave number k changes by a fixed value in a time interval between any two temporally successive trigger pulses (20) of a pulse train.  d. Applying the time-varying control voltage U to the first mirror so that the distance I is changed and the change in the distance I causes a change in a change of the wave number k, 2. The method according to claim 1, characterized in that a first pulse train (18) is generated while the wave number k is continuously changed from a first value ki of the wavenumber to a second value k2 of the wavenumber and subsequently generates a second pulse train (19) while the wave number is continuously changed from the first value ki of the wave number to the second value k2 of the wave number 3. The method according to claim 1, characterized in that a first pulse train (18) is generated while the wave number k is continuously changed from a first value ki of the wavenumber to a second value k2 of the wavenumber and subsequently generates a second pulse train (19) while the wavenumber is continuously from the second value k2 of the wavenumber is changed to the first value ki of the wavenumber Method according to one of the preceding claims, characterized in that the first (18) and the second pulse train (19) each have a first (21) and a last trigger pulse (22) and that between the last trigger pulse of the first pulse train and the first trigger pulse the second pulse train passes a time period which is greater than the time interval between two temporally successive trigger pulses of the first trigger signal. Method according to one of the preceding claims, characterized in that the wave number is a function f of the control voltage U and that the function f in an interval [Umin .. Umax] has an inverse function g and that the control voltage U in a time interval which a Pulse train (18) includes the function g (k (t)) and the control voltage U is selected in this time interval according to the function g (k (t)). Method according to one of the preceding claims, characterized in that the control voltage is generated such that a sequence of desired values of the control voltage in a first lookup table (23) are stored in a memory area of the electronic module and that the setpoint values of the control voltage at times ti, t .2, t _n continuously retrieved and converted by means of a digital-to-analog converter (26) in the control voltage. A method according to claim 6, characterized in that a sequence of desired values of the trigger signal in a second lookup table (24) are stored in a second memory area of the electronic module and that the setpoint values of the trigger signal at the times ti, t.2, t _m continuously retrieved and the retrieved values are output as the trigger signal. A method according to claim 6, characterized in that a sequence of voltage values in a third look-up table (25) are stored in a third memory area of the electronic module and that each time a trigger pulse is output from the electronic module, if the target value of the control voltage to the respective next table value third lookup table matches. Comprising light source (1) with a variable wave number k  a. a surface emitting semiconductor laser (2), the surface emitting semiconductor laser comprising a resonator, the resonator comprising a first mirror (3) and a second mirror (4) and the first mirror is formed as HCG (high contrast grating) and the first mirror is in a variable Distance I to a semiconductor surface (10) is arranged and b. an electronic module, characterized in that the light source can be operated by the method according to claim 1.