EP3987618A1 - Method of determining and/or setting an emission wavelength of a laser device and laser device - Google Patents

Method of determining and/or setting an emission wavelength of a laser device and laser device

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Publication number
EP3987618A1
EP3987618A1 EP19735497.0A EP19735497A EP3987618A1 EP 3987618 A1 EP3987618 A1 EP 3987618A1 EP 19735497 A EP19735497 A EP 19735497A EP 3987618 A1 EP3987618 A1 EP 3987618A1
Authority
EP
European Patent Office
Prior art keywords
laser
cavity
wavelength
section
voltage
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19735497.0A
Other languages
German (de)
French (fr)
Inventor
Magnus HAPPACH
David DE FELIPE MESQUIDA
Norbert Keil
Martin Schell
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Fraunhofer Gesellschaft zur Foerderung der Angewandten Forschung eV
Original Assignee
Fraunhofer Gesellschaft zur Foerderung der Angewandten Forschung eV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Fraunhofer Gesellschaft zur Foerderung der Angewandten Forschung eV filed Critical Fraunhofer Gesellschaft zur Foerderung der Angewandten Forschung eV
Publication of EP3987618A1 publication Critical patent/EP3987618A1/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
    • H01S5/0625—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in multi-section lasers
    • H01S5/06255—Controlling the frequency of the radiation
    • H01S5/06256—Controlling the frequency of the radiation with DBR-structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/068—Stabilisation of laser output parameters
    • H01S5/06808—Stabilisation of laser output parameters by monitoring the electrical laser parameters, e.g. voltage or current
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14—External cavity lasers
    • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
    • H01S5/142—External cavity lasers using a wavelength selective device, e.g. a grating or etalon which comprises an additional resonator
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14—External cavity lasers
    • H01S5/146—External cavity lasers using a fiber as external cavity
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/022—Mountings; Housings
    • H01S5/0225—Out-coupling of light
    • H01S5/02251—Out-coupling of light using optical fibres
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/026—Monolithically integrated components, e.g. waveguides, monitoring photo-detectors, drivers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/0607—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying physical parameters other than the potential of the electrodes, e.g. by an electric or magnetic field, mechanical deformation, pressure, light, temperature
    • H01S5/0612—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying physical parameters other than the potential of the electrodes, e.g. by an electric or magnetic field, mechanical deformation, pressure, light, temperature controlled by temperature
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/0617—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium using memorised or pre-programmed laser characteristics
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/065—Mode locking; Mode suppression; Mode selection ; Self pulsating
    • H01S5/0656—Seeding, i.e. an additional light input is provided for controlling the laser modes, for example by back-reflecting light from an external optical component
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/1003—Waveguide having a modified shape along the axis, e.g. branched, curved, tapered, voids
    • H01S5/101—Curved waveguide
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/1039—Details on the cavity length

Definitions

  • the invention relates to a method of determining and/or setting an emission wavelength of the laser device according to claim 1 and a laser device according to claim 9.
  • Laser devices comprising a feedback section, i.e. a cavity external to the laser cavity that reflects a portion of the light emitted by the laser back into the laser cavity can be configured to control the emission wavelength using a voltage measured at an active section of the laser.
  • a laser device is described e.g. in WO 2017/137 587 A1.
  • these laser devices are not capable of determining an absolute value of the emission wavelength.
  • external devices such as wavelength meters, optical spectrum ana lyzers or monochromators have to be used for determining the emission wavelength. More over, such external devices require that a portion of the emitted laser beam is separated and reflected towards the external devices.
  • the object of the invention is to provide a method and a device that permit to determine or set the emission wavelength in a more efficient manner.
  • a method of determining and/or setting an emission wavelength of the laser device comprising a laser having a laser cavity with a tunable reflection element and a feedback cavity external to the laser that reflects a portion of the light emitted by the laser back into the laser cavity, the method comprising the steps of:
  • the external feedback cavity of the laser device provides a feedback section which reflects a portion of the laser light back into the laser cavity (optical feedback). Due to the feedback cavity the effective reflectivity of the reflection element depends on the wavelength and the optical length of the feedback cavity.
  • the light amplification and attenuation changes the voltage at the active section (gain section) of the laser such that that voltage depends on the wavelength, too.
  • the course of the wavelength dependent voltage at the active section of the laser during the tuning of the reflection element is used for determining an absolute value of the emission wavelength of the laser device.
  • the tuning of the reflection element in particular has an effect on the mode excited in the laser cavity since a mode is only excited in the laser cavity as long as it has the lowest losses compared to all other possible modes or experiences the highest reflectivity at the reflection element. For exam ple, a mode k is only excited until there are better lasing conditions for a mode k+1.
  • the tuning of the reflection element may push its maximum of the reflectivity through the pos sible modes so that different modes are excited one after the other.
  • L the optical length of the laser cavity
  • the gain voltage measured during tuning of the reflection ele ment due to the subsequent excitation of different modes oscillates with a rather large pe riodicity. This allows to unambiguously assign a wavelength to a reference point of the gain voltage signal (e.g. a minimum or maximum of the gain voltage curve) and thus a determi nation of absolute emission wavelength values as will be explained below.
  • the reflection element may comprise or consist of a Bragg grating. Further, the reflection element may be tuned by thermal impact. For example, a heating section is arranged in proximity to the reflection element, wherein by actuating the heating section the reflective properties of the reflection element can be changed; and in particular the wavelength de pendency of the reflectivity of the reflection element may be changed under the impact of the activated heating section.
  • the“actuating variable” may be an electric current supplied to the heating section of a temperature of the reflection element. It is noted that it is possible that by heating the reflection element the length of the laser cavity might be changed as well. That effect might be compensated by activating (in particular by heating) a phase section (see below).
  • the wavelength assigned to the refer ence point is determined i) as a function of the optical length of the laser cavity and the optical length of the external cavity, or ii) by a measurement (using e.g. an external device such as a wavelength meter, an optical spectrum analyzers or a monochromator).
  • the laser may have a phase section which is tuned by changing at least a phase section actuating variable starting from the wavelength assigned to the reference point, wherein the voltage at the active section of the laser is measured as a function of at least the phase section actuating variable, and wherein the measured voltage is used to deter mine and/or set the emission wavelength.
  • the phase section and the reflection element are tuned in such a way that the emission wavelength of the laser device is tuned continuously, i.e. the emission wavelength is tuned in such a way that no mode hops occur.
  • the emission wavelength is determined (i.e. an absolute value of the emission wavelength is determined) on the basis of the number of maxima and/or minima of the measured voltage occurring until a final value of the phase section actuating variable is reached or the emission wavelength is set by tuning the phase section actuating variable until the measured voltage has at least passed through a predetermined number of maxima and/or minima.
  • the meas ured voltage at the final value of the phase section actuating variable may be used to de- termine the emission wavelength.
  • the phase section actuating variable may be further changed until a predetermined voltage is reached.
  • both the phase section and the reflective element may be tuned at the same time.
  • the phase section and the reflective element are tuned in such a way that the emission wavelength is tuned continuously, i.e. without mode hops.
  • the phase section may be tuned by thermal impact.
  • a heating section may be arranged in proximity to the reflection element, wherein by actuating the heating section properties of the phase section can be changed; for example, a refraction index of a portion of the laser cavity (e.g. of a waveguide of the laser cavity) is changed under the impact of the activated heating section.
  • the“phase section actuating variable” may be an electric cur rent supplied to the heating section or a temperature of the heating section.
  • the invention is also related to a laser device, in particular for carrying out the method described above, comprising
  • a voltage measuring device for measuring a voltage at an active section of the laser
  • a measuring arrangement configured to tune the reflection element by changing an ac tuating variable during operation of the laser and to measure a voltage at an active sec tion of the laser during tuning of the reflection element;
  • an evaluation unit configured to identify a reference point in the voltage curve measured by the measuring arrangement and dependent on the actuating variable, to assign a wavelength to the actuating variable at the reference point, and to determine and/or set the emission wavelength of the laser device using the assigned wavelength.
  • the laser and the external cavity may be designed as de scribed PCT application PCT/EP2017/053051 .
  • the external cavity includes an optical waveguide coupled to the laser, wherein the optical waveguide of the external cavity, for example, is an integrated optical waveguide or a fiber waveguide (in particular a glass fiber, e.g. an APC glass fiber).
  • the laser of the laser arrangement according to the invention may be an external cavity laser, wherein the feedback cavity external to the laser is different from an external cavity of the external cavity laser.
  • the laser can also be an external-cavity laser, i.e. a laser which itself has an external cavity.
  • This external cavity is part of the "laser cavity” (e.g. together with a cavity formed by the active section of the laser).
  • the external cavity of the laser arrangement according to the invention is different from the external cavity of an external-cavity laser.
  • the external feedback cavity of the laser device may comprise an optical waveguide coupled to the laser, wherein the optical waveguide of the external cavity it is e.g. an integrated optical waveguide and/or an optical fibre.
  • the feedback cavity may also be provided by a free beam region, which for example is delimited by a first and a second lens (e.g. GRIN lenses).
  • the laser cavity may comprise an integrated optical waveguide coupled to the waveguide of the external feedback cavity.
  • the waveguide of the external feedback cavity has a first and a second partial section, the first partial section being formed at least in sections from a different material than the second partial section.
  • the first portion is formed by an integrated optical waveguide and the second portion is formed by an optical fiber.
  • the optical fibre may comprise a reflective structure with which the reflection of the light emitted by the laser back into the laser cavity is carried out.
  • the laser cavity and the feedback cavity may have (at least slightly) different optical lengths.
  • the length of the feedback cavity L 3 ⁇ 4 is between 50% and 99%, for example between 90% and 99%, of the length L cav of the laser cavity; in particular Lfb is 97% of Lcav.
  • the feedback cavity may be also located outside the optical path of the laser (in particular of the laser output).
  • Figure 1 a block diagram showing the principle setup of the laser device according to mention;
  • Figure 2 the gain voltage measured during tuning of a Bragg reflection element and a phase section, respectively;
  • Figure 3 the gain voltage measured using a different configuration of the laser device according
  • Figure 5 a laser device according to an embodiment of the invention
  • Figure 6 a laser device according to another embodiment of the invention.
  • FIG. 1 schematically shows a laser device 1 according to the invention.
  • the laser device 1 comprises a laser 2 and an external feedback cavity (feedback section) 30.
  • the laser 2 comprises a gain section (active section) 21 equipped with a high reflecting surface 221 and a reflection element in the form of a Bragg grating 23, the surface 221 and the Bragg grating 23 forming a laser cavity 20.
  • the external feedback cavity 30 is formed between the Bragg grating 23 and a reflective structure 31 .
  • laser 2 comprises a phase sec tion 25.
  • the external cavity 30 may comprise a first and a second waveguide section 341 , 342 that may have different thermo-optical coefficients T0i and TO2, respectively, wherein e.g. TO1 ⁇ 0 and T0 2 > 0.
  • the first section 341 comprises a polymer structure (e.g. a polymer waveguide)
  • the second section 342 comprises a semiconductor structure (e.g. semiconductor waveguide).
  • the gain section 21 and the remaining section 242 of laser cavity 20 may have different thermo-optical coefficients TO2 1 - and T0i L , wherein e.g. T0 2 L > 0 and T0i L ⁇ 0.
  • the section with a thermo-optical coefficient below zero i.e.
  • the second section 242 may comprise a polymer structure (e.g. a polymer waveguide), while the other section, the gain section 21 may comprise a semi conductor structure (e.g. semiconductor waveguide).
  • Providing sections with different thermo-optical coefficients allows to mitigate changes of the optical length L cav of the laser cavity 20 and of the optical length I_ 3 ⁇ 4 the external cavity 30, respectively, such that for example controlling the temperature of the laser device 1 may be omitted.
  • the lengths of the sections 21 , 242 of the laser cavity 20 are denominated“
  • heating devices 41 , 42 are associated with the Bragg grating 23 and the phase section 25 for tuning the Bragg grating 23 and the phase section 25, respectively. More particularly, the Bragg grating 23 is tuned by changing an electric current supplied to heat ing device (heating section) 41 , which may be a heating electrode, the electric current thus being an“actuating variable”. Similarly an electric current can be supplied to heating device 42, the electric current in this case being a“phase section actuating variable”.
  • the laser device 1 further comprises a measuring and evaluation device 100 having a measuring unit 101 and an evaluation unit 102.
  • the measuring unit 101 is configured for supplying and e.g. controlling and the electric current supplied to the Bragg grating 23 and the phase section 25. Further, measuring unit 101 is capable of measuring a voltage V at the gain section 21 of the laser 2 during e.g. tuning of the Bragg grating 23 or to obtain such a voltage from a voltage measuring device 4 (in this case the measuring unit 101 and the voltage measuring device together form a measurement arrangement).
  • the evaluation unit 102 is configured to identify a reference point in the voltage curve de pendent on the actuating variable measured by the measuring arrangement 101 , to assign a wavelength to that reference point, and to determine the emission wavelength of the laser device 1 using the reference point and the assigned wavelength.
  • the voltage V at the gain section 21 is measured while a constant current is supplied to the gain section 21.
  • Figure 2 shows an exemplary measurement of the voltage V at the gain section 21 of laser 2 during tuning of i) the Bragg grating 23 (“Bragg sweep”) and ii) at least the phase section 25 (“wavelength sweep”).
  • Carrying out a“wavelength sweep” results in a voltage V measured at the gain section 21 that oscillates depending on the wavelength (as illustrated in Figure 2).
  • the continuously tuning of the emission wavelength in particular means that the heating current to at least the phase section 25 (and if required also to the Bragg grating 23) is changed in such a way that no mode hops occur.
  • the wavelength sweep voltage signal depends on the emission wavelength, wherein, however, due to the oscillation of that voltage signal with a rather short period it is not possible to unambiguously derive a wavelength from a value of the voltage signal.
  • Tuning of the reflection element (the Bragg grating 23) alone (without tuning the phase section 25 or with an additional compensation tuning of the phase section 25), i.e. carrying out a“Bragg sweep”, has the effect that the maximum of the reflectivity of Bragg grating 23 is moved through the possible modes of laser cavity 20 so that different modes are excited one after the other as already indicated above, wherein the wavelength difference between
  • a voltage at the gain section 21 can be measured for the excited modes (black dots in Fig. 2) such that the voltage during the Bragg sweep oscillates with a lower frequency than the gain voltage during the wavelength sweep. It could be said that the voltage signal obtained during the Bragg sweep is an“aliased” signal because the Bragg sweep“samples” the wavelength sweep signal with a sample frequency that is too low.
  • the Bragg sweep voltage signal different from the continuous wavelength signal obtained by carrying out the“wavelength sweep“ has such a large period (distance between two adjacent max ima) that a reference point can be identified in the voltage signal and an emission wave length can be unambiguously assigned to that reference point, which, in turn, can be used to determine and/or set the emission wavelength of laser device 1.
  • FIG. 3 again shows the gain voltage for a Bragg sweep (here denominated “Wavelength-Meter”) and a wavelength sweep (here denominated ’’Wavelength-Locker”), wherein, however, a different configuration of the laser device is used (in particular a different ration between the lengths of the laser cavity and the external cavity).
  • the Bragg sweep is carried out first, i.e. the gain voltage is determined depending on the heating current supplied to the heating device 41 assigned to the Bragg grating 23. After the recording of the voltage during the Bragg sweep, a reference point R is deter mined.
  • a minimum of the Bragg sweep curve (the“Wavelength-Meter” signal in Figure 2) is chosen as reference point R.
  • an emission wavelength is assigned to reference point R, e.g. by calculating the corresponding emission wavelength (which depends on the optical length L cav of the laser cavity 20 and of the optical length L fb the external cavity 30). It is also possible that the emission wavelength is measured (e.g. using an external device) at the reference point R. It is noted that in the example shown in Figure 3 the voltage curve has two minima. However, it is nevertheless possible to assign the correct wavelength to the reference point R, e.g. taking into account the gain characteristics of the active section or the heating parameters (e.g. the heating current supplied to the Bragg grating 23 and e.g. also to the phase section 25), wherein the heating parameters might be associated with a particular wavelength (using e.g. the results of a calibration measurement).
  • the emission wavelength of the laser device 1 is tuned by tuning the Bragg grating 23 and the phase section 25 in such a way that the emission wavelength changes continuously (i.e. without mode hops).
  • the continuous tuning of the emission wavelength results in a gain voltage (see the“Wave length-Locker” signal in Figure 3) oscillating with a higher frequency than the gain voltage during the Bragg sweep.
  • the wavelength space between two adjacent maxima or minima of the Wavelength-Locker signal depends on the length L 3 ⁇ 4 of the external cavity and can be calculated.
  • the wavelength at the target emission state can be derived from the gain voltage measured at the target emission state. For example, if five maxima occur between the reference point R and the target emission state, the period of the gain voltage following the fifth maximum of the calibrating Wavelength-Locker curve is to be used to assign the measured gain voltage for the target emission state to an emission wavelength.
  • the required number of maxima of the gain voltage signal and the required voltage value can be predetermined and used to set the heating currents towards the heating devices 41 , 42 accordingly.
  • the determination of the reference point R, of the number of maxima of the gain voltage between the reference point R and the target emis sion state and of the gain voltage at the target emission state can be carried out by evalu ation device 100, in particular by his evaluation unit 102.
  • Figure 4 illustrates wavelength measurements (carried out in particular by an external de vice) as a function of changes of the heating temperatures (i.e. heating currents) of the Bragg grating 23 (horizontal axis) and of the phase section 25 (vertical axis).
  • the above described Bragg sweep corresponds to the arrow BS in Figure 4.
  • the temperatures at the Bragg grating 23 changes, while the temperature of the phase section 25 remains constant.
  • the continuous wavelength sweep as indicated by an arrow WS wherein during the continuous wavelength the temperature at the Bragg grating 23 and the phase section 25 are altered in such a way that the emission wavelength changes continuously, i.e. without any mode hops (that is, without crossing the curved black lines in figure 4).
  • FIG. 5 shows an embodiment of the laser device 1 according to the invention.
  • a polymer section 210 is coupled to the active (e.g. semiconductor) section 21 of the laser 2.
  • the cavity 20 of the laser 2 comprises an integrated optical waveguide 24 that extends from the highly reflecting coating 221 of the active section 21 up to the Bragg grating 23.
  • the wave guide 24 forms a first sub-section 241 that extends in the region of the active section 21 and correspondingly is formed by layers of a semiconductor material.
  • the second sub section 242 of the waveguide 24 extends as a polymer waveguide in the polymer section 210.
  • the two sub-sections 241 , 242 of the waveguide 24 are coupled to each other, wherein at least one coupling facet of the waveguide portions 241 , 242 can have an anti reflection coating.
  • the external cavity 30 extends from the Bragg grating 23 up to an outlet 31 of the external cavity 33.
  • the Bragg grating 23 at the same time also forms a first reflecting resonator element of the external cavity 30, wherein a second resonator element delimiting the ex ternal cavity 30 for example is formed by a correspondingly reflecting coating of the outlet 31 or by another reflecting element.
  • the laser 2 may be an external-cavity laser, i.e. the laser itself has an external cavity that forms one part of the laser cavity. This external cavity of the laser is of course different from the external cavity 30.
  • the external cavity 30 likewise includes an integrated optical waveguide 34.
  • This integrated optical waveguide 34 consists of a first sub-section 341 , which is formed by a continuation of the second sub-section 242 of the waveguide 24 of the laser cavity 20, i.e. the first sub section 341 of the waveguide 34 of the external cavity 30 is integrally connected with the second sub-section 242 of the waveguide 24.
  • a second sub-section 342 of the waveguide 34 of the external cavity 30 on the other hand is not formed as a polymer waveguide, but is part of a compensation section 2100 of the external cavity 30, which is coupled to the substrate on which the polymer section 210 of the laser 2 is formed.
  • the compensation section 2100 is formed from another material than the waveguide 34, e.g. on the basis of indium phosphide or another semiconductor material.
  • the second sub section 342 of the waveguide 34 is formed by layers of a semiconductor material.
  • heating devices 41 , 42 are assigned to the Bragg section 23 and the phase section 25, respectively.
  • Current sources 401 , 402 are provided for supplying electrical power (current) RB G and Pp h to the Bragg grating 23 and the phase section 25, respectively.
  • the current sources 401 , 402 are controlled by the evaluation device 100.
  • a constant power source supplying electrical power P g to the active region is used to operate laser 2, wherein the voltage drop V g (corresponding to voltage drop“V” in Fig. 1 and 5) is measured.
  • Figure 6 shows a variation of the embodiment of Figure 5, wherein instead of the compen sation section 2100 an optical fiber 2200 (in particular having a thermo-optical coefficient larger than zero) is used.
  • a front face 2201 of the optical fiber 2200 may be antireflection coated, wherein, however, the fiber 2200 in a distance from the front face 2201 comprises a reflective structure 2002 that together with the Bragg grating 23 defines the external cav ity 30.
  • a portion of the optical fiber 2200 (including the front face 2201 ), e.g. a glass or polymer fibre, may be arranged on the same substrate as laser 2, while another portion (e.g. includ ing the reflective structure 2202) is not arranged on that substrate.
  • the laser device shown in Figure 7 corresponds to a device shown in Figures 1 and 5, wherein in optical fiber 2300 is coupled to a compensation section 2100 of the external cavity 30, wherein an interface between an output surface 2101 of the compensation sec tion 2100 and a front face 2301 of fiber 2300 forms a reflecting structure that together with the Bragg grating 23 defines the external cavity 30.
  • Figure 8 is related to another variation of Figure 5, wherein an portion of an optical fiber 2400 is arranged on the substrate of the compensation section 2100.
  • a front face 2401 his antireflection coated, wherein a reflective structure 31 is formed in the waveguide 341 of compensation section 2100, the reflective structure 31 together with the Bragg grating 23 defining the external cavity 30.

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  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Optics & Photonics (AREA)
  • Semiconductor Lasers (AREA)

Abstract

The invention relates to a method of determining and/or setting an emission wavelength of a laser device (1) comprising a laser (2) having a laser cavity (20) with a tunable reflection element (23) and an external feedback cavity (30) external to the laser (2) that reflects a portion of the light emitted by the laser (2) back into the laser cavity (20), the method comprising the steps of tuning of the reflection element (23) by changing an actuating variable during operation of the laser (2); measuring a voltage (V) at an active section (21) of the laser (2) during tuning of the reflection element (23); identifying a reference point (R) in the voltage curve dependent on the actuating variable and assigning a wavelength to the reference point (R); and determining and/or setting the emission wavelength of the laser device (1) using the reference point and the assigned wavelength. The invention also relates to a laser device.

Description

Method of determining and/or setting an emission wavelength of a laser device and laser device
Description
The invention relates to a method of determining and/or setting an emission wavelength of the laser device according to claim 1 and a laser device according to claim 9.
Laser devices comprising a feedback section, i.e. a cavity external to the laser cavity that reflects a portion of the light emitted by the laser back into the laser cavity can be configured to control the emission wavelength using a voltage measured at an active section of the laser. Such a laser device is described e.g. in WO 2017/137 587 A1. However, these laser devices, for example, are not capable of determining an absolute value of the emission wavelength. Rather, external devices such as wavelength meters, optical spectrum ana lyzers or monochromators have to be used for determining the emission wavelength. More over, such external devices require that a portion of the emitted laser beam is separated and reflected towards the external devices.
The object of the invention is to provide a method and a device that permit to determine or set the emission wavelength in a more efficient manner. According to the invention, a method of determining and/or setting an emission wavelength of the laser device is provided, the laser device comprising a laser having a laser cavity with a tunable reflection element and a feedback cavity external to the laser that reflects a portion of the light emitted by the laser back into the laser cavity, the method comprising the steps of:
tuning of the reflection element by changing an actuating variable during operation of the laser;
measuring a voltage at an active section of the laser during tuning of the reflection element;
identifying a reference point in the voltage curve dependent on the actuating variable and assigning a wavelength to the reference point; and
determining and/or setting the emission wavelength of the laser device using the ref erence point and the assigned wavelength.
The external feedback cavity of the laser device provides a feedback section which reflects a portion of the laser light back into the laser cavity (optical feedback). Due to the feedback cavity the effective reflectivity of the reflection element depends on the wavelength and the optical length of the feedback cavity. The light amplification and attenuation changes the voltage at the active section (gain section) of the laser such that that voltage depends on the wavelength, too. Embodiments of such a laser device will be described below and are disclosed in the PCT application PCT/EP2017/053051 filed February 10, 2017 which in this respect is incorporated herewith.
According to the invention, the course of the wavelength dependent voltage at the active section of the laser during the tuning of the reflection element is used for determining an absolute value of the emission wavelength of the laser device. The tuning of the reflection element in particular has an effect on the mode excited in the laser cavity since a mode is only excited in the laser cavity as long as it has the lowest losses compared to all other possible modes or experiences the highest reflectivity at the reflection element. For exam ple, a mode k is only excited until there are better lasing conditions for a mode k+1. The tuning of the reflection element may push its maximum of the reflectivity through the pos sible modes so that different modes are excited one after the other. The wavelength differ ence between two adjacent modes k and k+1 is represented by the free spectral range wherein L as the optical length of the laser cavity and l0 is the central wavelength (for example, l0 = 1,55mth). The gain voltage measured during tuning of the reflection ele ment due to the subsequent excitation of different modes oscillates with a rather large pe riodicity. This allows to unambiguously assign a wavelength to a reference point of the gain voltage signal (e.g. a minimum or maximum of the gain voltage curve) and thus a determi nation of absolute emission wavelength values as will be explained below.
The reflection element may comprise or consist of a Bragg grating. Further, the reflection element may be tuned by thermal impact. For example, a heating section is arranged in proximity to the reflection element, wherein by actuating the heating section the reflective properties of the reflection element can be changed; and in particular the wavelength de pendency of the reflectivity of the reflection element may be changed under the impact of the activated heating section. In this case the“actuating variable” may be an electric current supplied to the heating section of a temperature of the reflection element. It is noted that it is possible that by heating the reflection element the length of the laser cavity might be changed as well. That effect might be compensated by activating (in particular by heating) a phase section (see below).
According to another embodiment of the invention, the wavelength assigned to the refer ence point is determined i) as a function of the optical length of the laser cavity and the optical length of the external cavity, or ii) by a measurement (using e.g. an external device such as a wavelength meter, an optical spectrum analyzers or a monochromator).
Moreover, the laser may have a phase section which is tuned by changing at least a phase section actuating variable starting from the wavelength assigned to the reference point, wherein the voltage at the active section of the laser is measured as a function of at least the phase section actuating variable, and wherein the measured voltage is used to deter mine and/or set the emission wavelength.
For example, the phase section and the reflection element are tuned in such a way that the emission wavelength of the laser device is tuned continuously, i.e. the emission wavelength is tuned in such a way that no mode hops occur.
For example, the emission wavelength is determined (i.e. an absolute value of the emission wavelength is determined) on the basis of the number of maxima and/or minima of the measured voltage occurring until a final value of the phase section actuating variable is reached or the emission wavelength is set by tuning the phase section actuating variable until the measured voltage has at least passed through a predetermined number of maxima and/or minima. As to the first alternative, in order to enhance the accuracy of the wave length determination in addition to the number of voltage maxima and/or minima, the meas ured voltage at the final value of the phase section actuating variable may be used to de- termine the emission wavelength. Regarding the second alternative, when the predeter mined number of maxima and/or minima of the measured voltage has been registered, in order to fine tune the emission wavelength, the phase section actuating variable may be further changed until a predetermined voltage is reached. Moreover, in order to tune the emission wavelength, both the phase section and the reflective element may be tuned at the same time. In particular, the phase section and the reflective element are tuned in such a way that the emission wavelength is tuned continuously, i.e. without mode hops.
The phase section may be tuned by thermal impact. A heating section may be arranged in proximity to the reflection element, wherein by actuating the heating section properties of the phase section can be changed; for example, a refraction index of a portion of the laser cavity (e.g. of a waveguide of the laser cavity) is changed under the impact of the activated heating section. In this case the“phase section actuating variable” may be an electric cur rent supplied to the heating section or a temperature of the heating section.
The invention is also related to a laser device, in particular for carrying out the method described above, comprising
- a laser having a laser cavity comprising a tunable reflection element;
- at least one feedback cavity external to the laser which reflects a portion of the light emitted by the laser back into the laser cavity;
- a voltage measuring device for measuring a voltage at an active section of the laser;
- a measuring arrangement configured to tune the reflection element by changing an ac tuating variable during operation of the laser and to measure a voltage at an active sec tion of the laser during tuning of the reflection element; and
- an evaluation unit configured to identify a reference point in the voltage curve measured by the measuring arrangement and dependent on the actuating variable, to assign a wavelength to the actuating variable at the reference point, and to determine and/or set the emission wavelength of the laser device using the assigned wavelength.
As already mentioned above, the laser and the external cavity may be designed as de scribed PCT application PCT/EP2017/053051 . For example, the external cavity includes an optical waveguide coupled to the laser, wherein the optical waveguide of the external cavity, for example, is an integrated optical waveguide or a fiber waveguide (in particular a glass fiber, e.g. an APC glass fiber).
Further, the laser of the laser arrangement according to the invention may be an external cavity laser, wherein the feedback cavity external to the laser is different from an external cavity of the external cavity laser. In other words, the laser can also be an external-cavity laser, i.e. a laser which itself has an external cavity. This external cavity, however, is part of the "laser cavity" (e.g. together with a cavity formed by the active section of the laser). Correspondingly, the external cavity of the laser arrangement according to the invention is different from the external cavity of an external-cavity laser.
According to another embodiment of the invention, the external feedback cavity of the laser device may comprise an optical waveguide coupled to the laser, wherein the optical waveguide of the external cavity it is e.g. an integrated optical waveguide and/or an optical fibre. The feedback cavity may also be provided by a free beam region, which for example is delimited by a first and a second lens (e.g. GRIN lenses).
Moreover, the laser cavity may comprise an integrated optical waveguide coupled to the waveguide of the external feedback cavity. For example, the waveguide of the external feedback cavity has a first and a second partial section, the first partial section being formed at least in sections from a different material than the second partial section. According to an embodiment, the first portion is formed by an integrated optical waveguide and the second portion is formed by an optical fiber. The optical fibre, in turn, may comprise a reflective structure with which the reflection of the light emitted by the laser back into the laser cavity is carried out.
Furthermore, the laser cavity and the feedback cavity may have (at least slightly) different optical lengths. For example, the length of the feedback cavity L¾ is between 50% and 99%, for example between 90% and 99%, of the length Lcav of the laser cavity; in particular Lfb is 97% of Lcav.
It is noted that the feedback cavity may be also located outside the optical path of the laser (in particular of the laser output).
The embodiments described above with respect to the method according to the invention can of course analogously be used in conjunction with the laser device according to the invention.
Embodiments of the invention will be described in more detail hereinafter with reference to the drawings, which show: Figure 1 a block diagram showing the principle setup of the laser device according to mention;
Figure 2 the gain voltage measured during tuning of a Bragg reflection element and a phase section, respectively;
Figure 3 the gain voltage measured using a different configuration of the laser device according
Figure 4 emission wavelengths depending on the heating temperature of the phase section and the Bragg reflection element, respectively;
Figure 5 a laser device according to an embodiment of the invention;
Figure 6 a laser device according to another embodiment of the invention;
Figure 7 a variation of Figure 6; and
Figure 8 another variation of Figure 6.
Figure 1 schematically shows a laser device 1 according to the invention. The laser device 1 comprises a laser 2 and an external feedback cavity (feedback section) 30. The laser 2 comprises a gain section (active section) 21 equipped with a high reflecting surface 221 and a reflection element in the form of a Bragg grating 23, the surface 221 and the Bragg grating 23 forming a laser cavity 20. The external feedback cavity 30 is formed between the Bragg grating 23 and a reflective structure 31 . Further, laser 2 comprises a phase sec tion 25.
The external cavity 30 may comprise a first and a second waveguide section 341 , 342 that may have different thermo-optical coefficients T0i and TO2, respectively, wherein e.g. TO1 < 0 and T02 > 0. For example, the first section 341 comprises a polymer structure (e.g. a polymer waveguide), while the second section 342 comprises a semiconductor structure (e.g. semiconductor waveguide). Similarly, the gain section 21 and the remaining section 242 of laser cavity 20 may have different thermo-optical coefficients TO21- and T0iL, wherein e.g. T02 L > 0 and T0iL < 0. Similarly to the external cavity, the section with a thermo-optical coefficient below zero, i.e. the second section 242 may comprise a polymer structure (e.g. a polymer waveguide), while the other section, the gain section 21 may comprise a semi conductor structure (e.g. semiconductor waveguide). Providing sections with different thermo-optical coefficients allows to mitigate changes of the optical length Lcav of the laser cavity 20 and of the optical length I_¾ the external cavity 30, respectively, such that for example controlling the temperature of the laser device 1 may be omitted. The lengths of the sections 21 , 242 of the laser cavity 20 are denominated“|_icav” and“|_2cav”, wherein the length of sections 341 , 342 of the external cavity 30 are denominated“l_ifb” and“l_2fb”, re spectively.
Moreover, heating devices 41 , 42 are associated with the Bragg grating 23 and the phase section 25 for tuning the Bragg grating 23 and the phase section 25, respectively. More particularly, the Bragg grating 23 is tuned by changing an electric current supplied to heat ing device (heating section) 41 , which may be a heating electrode, the electric current thus being an“actuating variable”. Similarly an electric current can be supplied to heating device 42, the electric current in this case being a“phase section actuating variable”.
The laser device 1 further comprises a measuring and evaluation device 100 having a measuring unit 101 and an evaluation unit 102. The measuring unit 101 is configured for supplying and e.g. controlling and the electric current supplied to the Bragg grating 23 and the phase section 25. Further, measuring unit 101 is capable of measuring a voltage V at the gain section 21 of the laser 2 during e.g. tuning of the Bragg grating 23 or to obtain such a voltage from a voltage measuring device 4 (in this case the measuring unit 101 and the voltage measuring device together form a measurement arrangement).
The evaluation unit 102 is configured to identify a reference point in the voltage curve de pendent on the actuating variable measured by the measuring arrangement 101 , to assign a wavelength to that reference point, and to determine the emission wavelength of the laser device 1 using the reference point and the assigned wavelength. The voltage V at the gain section 21 is measured while a constant current is supplied to the gain section 21.
Figure 2 shows an exemplary measurement of the voltage V at the gain section 21 of laser 2 during tuning of i) the Bragg grating 23 (“Bragg sweep”) and ii) at least the phase section 25 (“wavelength sweep”). Tuning the phase section 25 (and possibly also the Bragg grating 23) in such a way that the emission wavelength of the laser to is continuously tuned, wherein. Carrying out a“wavelength sweep” results in a voltage V measured at the gain section 21 that oscillates depending on the wavelength (as illustrated in Figure 2). The continuously tuning of the emission wavelength in particular means that the heating current to at least the phase section 25 (and if required also to the Bragg grating 23) is changed in such a way that no mode hops occur. Thus, the wavelength sweep voltage signal depends on the emission wavelength, wherein, however, due to the oscillation of that voltage signal with a rather short period it is not possible to unambiguously derive a wavelength from a value of the voltage signal.
Tuning of the reflection element (the Bragg grating 23) alone (without tuning the phase section 25 or with an additional compensation tuning of the phase section 25), i.e. carrying out a“Bragg sweep”, has the effect that the maximum of the reflectivity of Bragg grating 23 is moved through the possible modes of laser cavity 20 so that different modes are excited one after the other as already indicated above, wherein the wavelength difference between
A
two adjacent modes is represented by the free spectral range AA= 0 . Accordingly, a voltage at the gain section 21 can be measured for the excited modes (black dots in Fig. 2) such that the voltage during the Bragg sweep oscillates with a lower frequency than the gain voltage during the wavelength sweep. It could be said that the voltage signal obtained during the Bragg sweep is an“aliased” signal because the Bragg sweep“samples” the wavelength sweep signal with a sample frequency that is too low. However, the Bragg sweep voltage signal different from the continuous wavelength signal obtained by carrying out the“wavelength sweep“ has such a large period (distance between two adjacent max ima) that a reference point can be identified in the voltage signal and an emission wave length can be unambiguously assigned to that reference point, which, in turn, can be used to determine and/or set the emission wavelength of laser device 1.
This is further illustrated in Figure 3 which again shows the gain voltage for a Bragg sweep (here denominated “Wavelength-Meter”) and a wavelength sweep (here denominated ’’Wavelength-Locker”), wherein, however, a different configuration of the laser device is used (in particular a different ration between the lengths of the laser cavity and the external cavity). The Bragg sweep is carried out first, i.e. the gain voltage is determined depending on the heating current supplied to the heating device 41 assigned to the Bragg grating 23. After the recording of the voltage during the Bragg sweep, a reference point R is deter mined. In this case, a minimum of the Bragg sweep curve (the“Wavelength-Meter” signal in Figure 2) is chosen as reference point R. Further, an emission wavelength is assigned to reference point R, e.g. by calculating the corresponding emission wavelength (which depends on the optical length Lcav of the laser cavity 20 and of the optical length Lfb the external cavity 30). It is also possible that the emission wavelength is measured (e.g. using an external device) at the reference point R. It is noted that in the example shown in Figure 3 the voltage curve has two minima. However, it is nevertheless possible to assign the correct wavelength to the reference point R, e.g. taking into account the gain characteristics of the active section or the heating parameters (e.g. the heating current supplied to the Bragg grating 23 and e.g. also to the phase section 25), wherein the heating parameters might be associated with a particular wavelength (using e.g. the results of a calibration measurement).
Subsequently, for example, starting from the reference point R, the emission wavelength of the laser device 1 is tuned by tuning the Bragg grating 23 and the phase section 25 in such a way that the emission wavelength changes continuously (i.e. without mode hops). The continuous tuning of the emission wavelength results in a gain voltage (see the“Wave length-Locker” signal in Figure 3) oscillating with a higher frequency than the gain voltage during the Bragg sweep. The wavelength space between two adjacent maxima or minima of the Wavelength-Locker signal depends on the length L¾ of the external cavity and can be calculated. Thus, if the laser device 1 is tuned to a target emission state (having a target wavelength), i.e. by tuning the heating current supplied to the Bragg grating 23 and the phase section 25 to target values, the value of the target emission wavelength at the target emission state can be inferred from the gain voltage at the target emission state:
For example, if a calibrating Wavelength-Locker curve is registered such as the one shown in Figure 3 and the wavelength at the reference point R and the number of maxima of the gain voltage signal between the reference point R and the target emission state is taken into account, the wavelength at the target emission state can be derived from the gain voltage measured at the target emission state. For example, if five maxima occur between the reference point R and the target emission state, the period of the gain voltage following the fifth maximum of the calibrating Wavelength-Locker curve is to be used to assign the measured gain voltage for the target emission state to an emission wavelength.
The other way around, if the emission wavelength of the laser device 1 is to be set to a predetermined value, the required number of maxima of the gain voltage signal and the required voltage value can be predetermined and used to set the heating currents towards the heating devices 41 , 42 accordingly. The determination of the reference point R, of the number of maxima of the gain voltage between the reference point R and the target emis sion state and of the gain voltage at the target emission state can be carried out by evalu ation device 100, in particular by his evaluation unit 102. Figure 4 illustrates wavelength measurements (carried out in particular by an external de vice) as a function of changes of the heating temperatures (i.e. heating currents) of the Bragg grating 23 (horizontal axis) and of the phase section 25 (vertical axis). The above described Bragg sweep corresponds to the arrow BS in Figure 4. In the case of the Bragg sweep BS the temperatures at the Bragg grating 23 changes, while the temperature of the phase section 25 remains constant. The continuous wavelength sweep as indicated by an arrow WS, wherein during the continuous wavelength the temperature at the Bragg grating 23 and the phase section 25 are altered in such a way that the emission wavelength changes continuously, i.e. without any mode hops (that is, without crossing the curved black lines in figure 4). The measured device had a feedback cavity with an optical length Lfb = 0,97*Lcav (optical length of the laser cavity).
Figure 5 shows an embodiment of the laser device 1 according to the invention. A polymer section 210 is coupled to the active (e.g. semiconductor) section 21 of the laser 2. The cavity 20 of the laser 2 comprises an integrated optical waveguide 24 that extends from the highly reflecting coating 221 of the active section 21 up to the Bragg grating 23. The wave guide 24 forms a first sub-section 241 that extends in the region of the active section 21 and correspondingly is formed by layers of a semiconductor material. The second sub section 242 of the waveguide 24 extends as a polymer waveguide in the polymer section 210. The two sub-sections 241 , 242 of the waveguide 24 are coupled to each other, wherein at least one coupling facet of the waveguide portions 241 , 242 can have an anti reflection coating.
The external cavity 30 extends from the Bragg grating 23 up to an outlet 31 of the external cavity 33. Thus, the Bragg grating 23 at the same time also forms a first reflecting resonator element of the external cavity 30, wherein a second resonator element delimiting the ex ternal cavity 30 for example is formed by a correspondingly reflecting coating of the outlet 31 or by another reflecting element. It is noted that the laser 2 may be an external-cavity laser, i.e. the laser itself has an external cavity that forms one part of the laser cavity. This external cavity of the laser is of course different from the external cavity 30.
The external cavity 30 likewise includes an integrated optical waveguide 34. This integrated optical waveguide 34 consists of a first sub-section 341 , which is formed by a continuation of the second sub-section 242 of the waveguide 24 of the laser cavity 20, i.e. the first sub section 341 of the waveguide 34 of the external cavity 30 is integrally connected with the second sub-section 242 of the waveguide 24. A second sub-section 342 of the waveguide 34 of the external cavity 30 on the other hand is not formed as a polymer waveguide, but is part of a compensation section 2100 of the external cavity 30, which is coupled to the substrate on which the polymer section 210 of the laser 2 is formed.
It is conceivable that analogous to the active section 21 of the laser 2 the compensation section 2100 is formed from another material than the waveguide 34, e.g. on the basis of indium phosphide or another semiconductor material. Correspondingly, the second sub section 342 of the waveguide 34 is formed by layers of a semiconductor material.
As already indicated in conjunction with Figure 1 heating devices 41 , 42 are assigned to the Bragg section 23 and the phase section 25, respectively. Current sources 401 , 402 are provided for supplying electrical power (current) RBG and Pph to the Bragg grating 23 and the phase section 25, respectively. The current sources 401 , 402 are controlled by the evaluation device 100. A constant power source supplying electrical power Pg to the active region is used to operate laser 2, wherein the voltage drop Vg (corresponding to voltage drop“V” in Fig. 1 and 5) is measured.
Figure 6 shows a variation of the embodiment of Figure 5, wherein instead of the compen sation section 2100 an optical fiber 2200 (in particular having a thermo-optical coefficient larger than zero) is used. A front face 2201 of the optical fiber 2200 may be antireflection coated, wherein, however, the fiber 2200 in a distance from the front face 2201 comprises a reflective structure 2002 that together with the Bragg grating 23 defines the external cav ity 30.
A portion of the optical fiber 2200 (including the front face 2201 ), e.g. a glass or polymer fibre, may be arranged on the same substrate as laser 2, while another portion (e.g. includ ing the reflective structure 2202) is not arranged on that substrate.
The laser device shown in Figure 7 corresponds to a device shown in Figures 1 and 5, wherein in optical fiber 2300 is coupled to a compensation section 2100 of the external cavity 30, wherein an interface between an output surface 2101 of the compensation sec tion 2100 and a front face 2301 of fiber 2300 forms a reflecting structure that together with the Bragg grating 23 defines the external cavity 30.
Figure 8 is related to another variation of Figure 5, wherein an portion of an optical fiber 2400 is arranged on the substrate of the compensation section 2100. A front face 2401 his antireflection coated, wherein a reflective structure 31 is formed in the waveguide 341 of compensation section 2100, the reflective structure 31 together with the Bragg grating 23 defining the external cavity 30.

Claims

Claims
1. Method of determining and/or setting an emission wavelength of a laser device (1 ) comprising a laser (2) having a laser cavity (20) with a tunable reflection element (23) and an external feedback cavity (30) external to the laser (2) that reflects a portion of the light emitted by the laser (2) back into the laser cavity (20), the method comprising the steps of:
- tuning of the reflection element (23) by changing an actuating variable during oper ation of the laser (2);
- measuring a voltage (V) at an active section (21 ) of the laser (2) during tuning of the reflection element (23);
- identifying a reference point (R) in the voltage curve dependent on the actuating variable and assigning a wavelength to the reference point (R); and
- determining and/or setting the emission wavelength of the laser device (1 ) using the the reference point and the assigned wavelength.
2. The method according to claim 1 , wherein the reflection element (23) comprises a Bragg grating.
3. The method according to claim 1 or 2, wherein the reflection element (23) is tuned by thermal impact.
4. The method according to any of the preceding claims, wherein the wavelength as signed to the reference point (R) is determined i) as a function of the length (Lcav) of the laser cavity (20) and the length of the external cavity (!_¾,), or ii) by a measurement.
5. The method according to any of the preceding claims, wherein the laser (2) has a phase section (25) which is tuned by changing at least a phase section actuating var iable, and wherein the voltage (V) at the active section (21 ) of the laser (2) is measured as a function at least of the phase section actuating variable, wherein the measured voltage (V) is used to determine and/or set the emission wavelength.
6. Method according to claim 5, wherein the phase section (25) and the reflection element (23) are tuned in such a way that the emission wavelength of the laser device (1 ) is tuned continuously.
7. Method according to claim 5 or 6, wherein the emission wavelength is determined on the basis of the number of maxima and/or minima of the measured voltage (V) occur ring until a final value of the phase section actuating variable is reached or the emission wavelength is set by tuning the phase section actuating variable until the measured voltage (V) has at least passed through a predetermined number of maxima and/or minima.
8. Method according to any of claims 5 to 7, wherein the phase section is tuned by ther mal impact.
9. Laser device, in particular for carrying out the method according to any of the preceding claims, comprising
a laser (2) having a laser cavity (21 ) comprising a tunable reflection element (23); at least one external feedback cavity (30) external to the laser (2) which reflects a portion of the light emitted by the laser (2) back into the laser cavity (20);
a measuring arrangement (4, 101 ) configured to tune the reflection element (23) by changing an actuating variable during operation of the laser (2) and to measure a voltage (V) at an active section (21 ) of the laser (2) during tuning of the reflection element (23); and
an evaluation unit (102) configured to identify a reference point (R) in the voltage curve measured by the measuring arrangement (4, 101 ), to assign a wavelength to the reference point (R), and to determine the emission wavelength of the laser de vice (1 ) using the reference point and the assigned wavelength.
10. The laser device according to claim 9, wherein the external feedback cavity (30) com prises an optical waveguide (34) coupled to the laser (2).
1 1. The laser device according to claim 10, wherein the optical waveguide (34) of the ex ternal feedback cavity (30) is an integrated optical waveguide, an optical fibre and/or a free beam region.
12. The laser device according to claim 10 or 1 1 , wherein the laser cavity (20) comprises an integrated optical waveguide (24) coupled to the waveguide (34) of the external cavity (30).
13. The laser device according to any of claims 10 to 12, wherein the waveguide (34) of the external feedback cavity (30) has a first and a second partial section (341 , 342), the first partial section (341 ) being formed at least in sections from a different material than the second partial section (342).
14. The laser device according to claim 13, wherein the first portion (341 ) is formed by an integrated optical waveguide and the second portion is formed by an optical fiber
(2200, 2400).
15. The laser arrangement according to claim 14, wherein the optical fibre comprises (2200) a reflective structure (2202) with which the reflection of the light emitted by the laser (2) back into the laser cavity (20) is carried out.
16. The laser device according to any of claims 10 to 15, wherein the laser cavity (21 ) and the feedback cavity (30) have different optical lengths (Lf , Lcav)·
EP19735497.0A 2019-06-19 2019-06-19 Method of determining and/or setting an emission wavelength of a laser device and laser device Withdrawn EP3987618A1 (en)

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