DE102017204478A1 - A method of operating a variable wavelength waveguide surface emitting semiconductor laser - Google Patents

Abstract

The invention relates to methods for operating a surface emitting semiconductor laser with HCG mirror with variable wave number k. In this case, a trigger signal based on a time-dependent control voltage or simultaneously with the control voltage is generated in a time-controlled manner, wherein the trigger signal comprises a plurality of temporally successive trigger pulses and the wave number changes in a time interval between any two temporally successive trigger pulses by a fixed value.

Description

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. In German, one could call a swept source an optical wobble generator. This can be a tunable laser or a tunable LED. For the required further processing and the Fourier transformation of the data in the SSOCT method, it is necessary that the spectrum is present with equidistant interpolation points with respect to the wavenumber or the frequency of the light.

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 recorded signal by interpolation on 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 measured the transmitted or reflected power. 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 back 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 consumed for the generation of the k-clock signal, thus reducing the usable output power.

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 of a parameterizable trigger signal, for example by software, e.g. for a pre-selectable 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 wave number range k ₁ ... k ₂ . 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, especially in the case of 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. For example, HCG are off US20070115553A1 known to use their 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 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 p-n 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 l from 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 may also be without medium, i. be operated under vacuum. The refractive index of the gap may be 1. The distance l can be less than 5μm. 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 second electrodes, the distance ℓ 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 that is greater than the distance l. By applying a control voltage between the first and the second electrode, the distance l 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 one 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 denoted by Δk. 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 l. 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 of the distance l causes a change of 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 jumps of the laser.

A first pulse sequence can be generated while the wave number k is changed continuously from a first value k _{1 of} the wave number to a second value k _{2 of} the wave number. Subsequently, a second pulse train can be generated while the wave number is continuously changed from the first value k _{1 of} the wave number to the second value k _{2 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 k _{1 of} the wave number to a second value k _{2 of} the wave number, and subsequently a second pulse train can be generated while the wave number is continuously from the second value k ₂ the wave number is changed to the first value k _{1 of} the wave number. In this case, the resetting of the distance l 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 may have an inverse function g in an interval [U _min .. U _max] . 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)). For this purpose, discrete setpoint values of the control voltage U can be stored as function values g (k (t ₁ )), g (k (t ₂ )). G (k (t _n )) in a first look-up 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 t ₁ , t ₂ ,..., T _n be converted into the control voltage U by means of a digital-to-analog converter (DA converter).

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 setpoint values of the trigger signal can be continuously called up at the times t ₁ , t ₂ ,..., T _m . 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 is provided with a variable wavenumber k 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 formed as HCG (high contrast grating) and the first mirror is arranged at a variable distance l to a semiconductor surface and also comprising an electronic module. The light source is characterized in that it can be operated by the method according to claim 1.

• 1 zeigt einen oberflächenemittierenden Halbleiterlaser mit veränderlicher Wellenzahl.
• 2 zeigt eine erfindungsgemäße Lichtquelle.
• 3 zeigt eine Lichtquelle nach dem Stand der Technik.
• 4 zeigt ein erstes Ausführungsbeispiel des Verfahrens.
• 5 zeigt ein zweites Ausführungsbeispiel des Verfahrens.
• 6 zeigt ein drittes Ausführungsbeispiel des Verfahrens.
• 7 zeigt ein viertes Ausführungsbeispiel des Verfahrens.
• 8 zeigt ein fünftes Ausführungsbeispiel des Verfahrens
• 9 zeigt ein sechstes Ausführungsbeispiel des Verfahrens noch mehr.
• 10 zeigt ein siebentes Ausführungsbeispiel des Verfahrens

The figures show the following:

• 1 shows a waveguide surface emitting semiconductor laser.
• 2 shows a light source according to the invention.
• 3 shows a light source according to the prior art.
• 4 shows a first embodiment of the method.
• 5 shows a second embodiment of the method.
• 6 shows a third embodiment of the method.
• 7 shows a fourth embodiment of the method.
• 8th shows a fifth embodiment of the method
• 9 shows a sixth embodiment of the method even more.
• 10 shows a seventh embodiment of the method

EXAMPLES

1 shows a waveguide surface emitting semiconductor laser. The surface emitting semiconductor laser ( 2 ) (VCSEL) comprises a resonator. It has a semiconductor surface lying in the resonator ( 10 ) on. The resonator comprises a first mirror (3) and a second mirror ( 4 ) and is limited by these two mirrors. The first mirror is designed as HCG (high-contrast grid). The first mirror has one or more spring elements on the sides 12 that have spacers 13 connected to the VCSEL. The first mirror ( 3 ) is at a variable distance l to the semiconductor surface ( 10 ) arranged. The distance l can be changed by applying a control voltage U (t) to the first mirror. The control voltage is via the first contact 7 created on 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 8th , which is simultaneously formed as a second contact. This has an aperture 9 on, 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 leads to a deformation of the spring element and thus to a change in the distance l. Also shown is a semiconductor structure 5 , which provides the optical amplification and a substrate 6 , which serves as starting material for the production of the VCSEL. The laser radiation 11 is emitted here perpendicular to the semiconductor surface in the z-direction.

2 shows a light source according to the invention. The light source with variable wave number 1 includes a VCSEL 2 and an electronics module 14 , The electronics module outputs a time-dependent control voltage U (t) and a trigger signal 17 out.

3 shows a light source according to the prior art. To operate an optical interferometer 15 is required, which is an interferometric measurement signal 16 generated. This is an optical k-clock signal, which is used as a trigger signal. To generate the k-clock signal must be part of the power of the laser beam 11 be removed.

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 waveforms are shown only as an example. 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 several trigger pulses 20 , It will be a first pulse train 18 generated while the wave number k is continuously from a first value k _{1 of} the wave number is changed to a second value k _{2 of} the wave number. The following is a second pulse train 19 generated while the wave number is continuously changed from the second value k _{2 of} the wave number to the first value k _{1 of} the wave number. The first and second pulse trains each have a first 19 and a last trigger pulse 20 on. 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, this can hide the locations from the OCT signal which are close to the reversal points of the wavenumber, ie at the reversal points of the direction of movement of the first mirror.

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 several pulse trains ( 18 . 19 ). Each pulse train comprises several trigger pulses 20 , It will be a first pulse train 18 generated while the wave number k is continuously changed from a first value k _{1 of} the wave number to a second value k _{2 of} the wave number. The following is a second pulse train 19 generated while the wave number is continuously changed in turn from the first value k _{1 of} the wave number to the second value k _{2 of} the wave number. In the period between the first pulse train and the second pulse train, the distance l is reset by resetting the control voltage U to an initial value.

6 shows a third embodiment of the method. The wave number k is a function f of the control voltage U, which has an inverse function g in an interval [U _min .. U _max ]. 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 (t ₁ )), g (k (t ₂ )). G (k (t _n )) in a first lookup table 23 deposited. The setpoint values of the control voltage are continuously called up at times t ₁ , t ₂ ,..., T _n and converted into the control voltage by means of a digital-to-analog converter 26. The times are thereby by a first clock 30 specified. From the first bar is by means of a divider 27 , which is followed by a gate 28, which generates only timed trigger signal.

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 t ₁ , t ₂ , ..., t _n continuously retrieved and by means of a digital-to-analog converter 26 converted into the control voltage. The times are thereby by a first clock 30 specified. 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 updated via a comparator 11 in each case a trigger pulse output from the electronic module, when the target value of the control voltage corresponds to the respective next table value of the third lookup table.

8th 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 t ₁ , t ₂ , ..., t _n continuously retrieved and by means of a digital-to-analog converter 26 converted into the control voltage. The times are thereby by a first clock 30 specified. A sequence of desired values of the trigger signal U ¹ _clock. U ⁿ _clock 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 t ₁ , t ₂ ,..., T _m and the retrieved values are output as the trigger signal.

9 shows a sixth embodiment of the method even more. Here, the trigger signal by means of a second clock 11 generated, which has a higher clock frequency than the first clock, and is synchronized with this.

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 the generated control voltage U via a stage comparator 32 is compared with predetermined voltages U _comp and in each case a trigger pulse is output when exceeding the following predetermined voltage value.

LIST OF REFERENCE NUMBERS

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

8th

Second contact, second electrode

9

aperture

10

Semiconductor surface

11

laser radiation

12

spring element

13

spacer

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 converter

27

divider

28

gate

29

comparator

30

First bar

31

Second bar

32

Stufenkomparator

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2884224 A1 [0005]
• EP 3020105 A1 [0005]
• US 20070115553 A1 [0011]
• US 20070153860 A1 [0011]
• US 20100316079 A1 [0011]
• US 20100316083 A1 [0011]

Claims (9)

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 grating) is formed and the first mirror (3) is arranged at a variable distance l from 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 l is changed and the change in the distance l a change causes a change in the wave number k, Method according to Claim 1 characterized in that a first pulse train (18) is generated while the wave number k is changed continuously from a first value k _{1 of} the wavenumber to a second value k _{2 of} the wavenumber and subsequently a second pulse train (19) is generated while the Wavelength is continuously changed from the first value k _{1 of} the wave number to the second value k _{2 of} the wave number Method according to Claim 1 characterized in that a first pulse train (18) is generated while the wave number k is changed continuously from a first value k _{1 of} the wavenumber to a second value k _{2 of} the wavenumber and subsequently a second pulse train (19) is generated while the Wavelength is continuously changed from the second value k _{2 of} the wave number to the first value k _{1 of} the wave number 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 t ₁ , t ₂ , ..., t _n continuously retrieved and converted into the control voltage by means of a digital-to-analog converter (26). 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 t ₁ , t ₂ , ..., t _m continuously retrieved and the retrieved values are output as the trigger signal. 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 of the third lookup table equivalent. Light source (1) with a variable wave number k comprising 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 l is arranged to a semiconductor surface (10) and b. an electronic module, characterized in that the light source with the method according to Claim 1 can be operated.

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