OPTICAL FREQUENCY COMB SETUP AND USE OF AN EXTERNAL CAVITY FOR DISPERSION COMPENSATION AND FREQUENCY TUNING

Abstract

An optical frequency comb setup including a semiconductor cascade laser drivable by a laser driver, emitting a laser beam through an end facet of the semiconductor cascade laser with a frequency comb with at least two given individual emission frequencies, repetition frequency, carrier envelope offset frequency shows improved comb stability and/or comb formation and/or comb bandwidth. This is achieved by an external cavity added outside of the cavity of the semiconductor cascade laser, having a reflective element with a mirror surface reflecting the at least two individual emission frequencies being arranged in a relative distance to the end facet allowing to adapt repetition frequency and/or carrier envelope offset frequency and/or the dispersion seen by the light in the optical frequency comb setup.

Claims

1-24. (canceled)

25. An optical frequency comb setup comprising a semiconductor cascade laser drivable by a laser driver, emitting a laser beam through an end facet of the semiconductor cascade laser with a frequency comb with at least two given individual emission frequencies, repetition frequency, carrier envelope offset frequency, wherein an external cavity is added outside of a cavity of the semiconductor cascade laser, comprising a reflective element with a mirror surface reflecting the given at least two individual emission frequencies being arranged in a relative distance (d) to the end facet (110) allowing to adapt repetition frequency (frep) and/or carrier envelope offset frequency (fceo) and/or the dispersion seen by the light in the optical frequency comb setup (1).

26. The optical frequency comb setup according to claim 25, wherein the semiconductor cascade laser and/or the reflective element are arranged in a linear translation mechanism such that the relative distance between the end facet and the mirror surface is adjustable by either movement of the reflective element fixed by holding means or movement of the semiconductor cascade laser and the end facet in the direction of the laser beam in such a way, that elongation of the external cavity respectively the relative linear position of the reflective element to the end facet leads to modification of the repetition frequency and/or the carrier envelope offset frequency and/or the dispersion.

27. The optical frequency comb setup according to claim 26, wherein the linear translation mechanism comprises a cascade laser mount and a mechanical actuator for coarse or fine adjustment of distance.

28. The optical frequency comb setup according to claim 27, wherein the mechanical actuator is able to move a sliding element, which movability is lockable by a blocking member, allowing to fix the position of the sliding element.

29. The optical frequency comb setup according to claim 28, wherein the mechanical actuator is formed by a micrometer screw.

30. The optical frequency comb setup according to claim 26, wherein the linear translation mechanism comprises a cascade laser mount, an electromechanical actuator and a steering electronics for fine adjustment of distance.

31. The optical frequency comb setup according to claim 30, wherein the electromechanical actuator comprises a piezo element.

32. The optical frequency comb setup according to claim 30, wherein the electromechanical actuator is a MEMS device operable by a steering electronics, usable to adjust the relative distance between the reflective element and the end facet in a coarse and fine adjustment.

33. The optical frequency comb setup according to claim 25, where the distance between reflective element and the end facet is such that the optical path outside the semiconductor chip is smaller than the optical path inside the semiconductor chip, at most half the length of the semiconductor chip.

34. The optical frequency comb setup according to claim 25, wherein the semiconductor cascade laser and/or the reflective element of the external cavity is mounted on a mounting plate, which is additionally used as a heatsink for the at least one semiconductor cascade laser, which is temperature controlled by a temperature controller.

35. The optical frequency comb setup claim 25, wherein the reflective element is partially reflective in the frequency range of the semiconductor cascade laser, allowing control of light fed back into the semiconductor cascade laser and/or exiting the external cavity.

36. The optical frequency comb setup according to claim 35, wherein the reflective element comprises at least one semiconducting material, in particular Gallium Arsenide, Silicon, indium phosphide or Germanium.

37. The optical frequency comb setup according to claim 25, wherein an additional coating layer or multilayer in form of a dielectric and/or metallic dispersive coating is provided on the mirror surface of the reflective element and/or the outside surface of the end facet and/or surfaces of beam shaping or dispersive elements placed between end facet and the reflective element, for dispersion compensation and frequency stabilization.

38. The optical frequency comb setup according to claim 25, wherein the outside surface of the end facet and therewith the reflectivity of the laser facet is modified by optical facet coating, comprising at least one electrically non-conducting layer directly coated on the surface of the end facet.

39. The optical frequency comb setup according to claim 25, where the distance between the reflective element and the end facet is most preferred between 5 microns and 100 microns.

40. The optical frequency comb setup according to claim 25, wherein beam shaping elements are placed in the external cavity, between the end facet and the reflective element in the optical path, comprising in particular at least one lens or a curved mirror.

41. The optical frequency comb setup according to claim 25, wherein in the external cavity at least one dispersive element is placed between the end facet and the mirror surface, in particular in form of a prism, a reflective grating, a phase grating or a multilayer element.

42. The optical frequency comb setup according to claim 25, wherein the laser beam with the frequency comb generated by the semiconductor cascade laser used for measurements is exited in direction to the reflective element out of the external cavity and/or in direction of the end facet of the semiconductor cascade laser at the side opposite to the side with the external cavity.

43. The optical frequency comb setup according to claim 25, wherein beside the external cavity at one side of the semiconductor cascade laser a second external cavity between a second end facet and a second reflective element on the opposite side of the external cavity is attached, wherein the laser beam for measurements with the frequency comb generated and modified by both external cavities exits the first reflective element and/or the second reflective element.

44. An external cavity added to an end facet of at least one semiconductor cascade laser, comprising a reflective element with a mirror surface being arranged spaced apart in a distance to the end facet, wherein the external cavity is arranged in the optical frequency comb setup of claim 25.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] A preferred exemplary embodiment of the subject matter of the invention is described below in conjunction with the attached drawings.

[0023] FIG. 1a shows a schematic view of a dual optical frequency comb setup, comprising two optical frequency comb setups according to prior art, while

[0024] FIG. 1b shows an amplitude vs. frequency diagram of a dual optical frequency comb setup according to prior art.

[0025] FIG. 2 shows an optical frequency comb setup with external cavity in a schematical view with fixed distance between end facet and mirror surface, while

[0026] FIG. 3 shows a schematic view of an optical frequency comb setup affixed in a linear translation mechanism for adjustment of distance d.

[0027] FIG. 4 shows a schematic view of a cascade laser structure affixed in a linear translation mechanism comprising an electromechanical actuator.

[0028] FIG. 5 a) shows a schematic view of emission of a modified laser beam using one external cavity, while

b) shows a schematic view of emission of two modified laser beams using two external cavities attached to one semiconductor cascade laser.

DESCRIPTION

[0029] Examples of optical frequency setups 1 will be described starting with FIG. 2 in the following. Such optical frequency setups 1 comprise a semiconductor cascade laser structure 11 and a laser driver 10. If a dual frequency comb setup is build, it further comprises a second frequency comb setup 1 with a semiconductor cascade laser 11 and a laser driver 10, not depicted in FIG. 2, which can be identical in construction as the cascade laser structure 1.

[0030] Here the used semiconductor cascade laser structures 11, 11 are especially quantum cascade lasers (QCL) or an interband cascade lasers (ICL), which are based on semiconducting gain media, where optical gain is achieved by stimulated emission of intersubband transitions respectively interband transitions. Such semiconductor cascade laser 11, 11 are operating in mid- to far-infrared spectral region, with central wavelength above 3 microns and a person skilled in the art knows different QCLs and ICLs and how to provide such semiconductor cascade laser 11. The cavity of the QCL or ICL is formed like a chip, which can be manufactured by cheap mass production.

[0031] The semiconductor cascade laser 11 is operated by the laser driver 10 emitting a laser beam with an optical frequency comb (12) with given central wavelength and individual emission frequencies (fn), repetition frequency (frep), and carrier envelope offset frequency (fceo). The dispersion is an intrinsic property of the optical frequency comb setup (1), but it does not show in the optical frequency comb (12). It helps the formation of the optical frequency comb (12) if the dispersion of the optical frequency comb setup (1) is low.

[0032] The first laser beam with frequency comb 12 exits through an end facet 110 of a cavity formed between the two end facets 110 of the semiconductor cascade laser 11. The end facet 110 can be partially reflective in the frequency range of the used semiconductor cascade laser 11, reflecting a part of the emitted light back into the waveguide of the semiconductor cascade laser 11.

[0033] The end facet 110 is a flat surface of semiconducting solid, which is at least partly reflective, while the reflectivity of the end facet 110 is adjustable by an antireflection coating or a high-reflection coating placed on the outside surface of the end facet. The end facet 110 can also be coated with a dispersion compensation coating.

[0034] For compensation of dispersion and independent control of f.sub.ceo and f.sub.rep an external cavity 5, comprising a reflective element 50 affixed with holding means 51 in a distance d, is defined between a mirror surface 500 and outside surface of the end facet 110.

[0035] The external cavity is coupled outside of the cavity of the semiconductor cascade laser 11. Values of the distance d can be from the order of a micrometer to the order of a meter.

[0036] A more complex part of the optical frequency comb setup 1 is depicted in FIG. 3, based on the optical frequency comb setup 1 of FIG. 2, showing a linear translation mechanism 6. The linear translation mechanism 6 comprises a cascade laser mount 60, an optional guiding device 61 and a mechanical actuator 62. The cascade laser mount 60 and the holding means 51 of the reflective element 50 are connected to the guiding device 61, in order to make the distance d between surface of the end facet 110 and the mirror surface 500 adjustable. In another embodiment holding means 51 and cascade laser mount 60 are separated and not connected to the guiding device 61. The relative linear position of the reflecting element 50 can be changed by linear translation of the holding means 51 relative to the semiconductor cascade laser structure 11 respectively the end face 110 by adjusting the mechanical actuator 62. The mechanical actuator 62 can be a micrometer screw, which is schematically indicated by broken lines.

[0037] The external cavity 5 is added outside of the cavity of the semiconductor cascade laser 11. With the linear translation mechanism 6 the distance d in direction of the exiting laser beam comb 12 between reflective element 50 with mirror surface 500 and the end facet 110 can be adjusted by either movement of the reflective element 50 fixed by the holding means 51 or movement of the semiconductor cascade laser 11 respectively its end facet 110 in direction of the laser beam in such a way, that the elongation of the external cavity 5 respectively the linear position of the reflective element 50 relative to the end facet 110 leads to modification of repetition frequency frep and/or carrier envelope offset frequency f.sub.ceo and/or dispersion.

[0038] While the semiconductor cascade laser structure 11 is emitting the optical frequency comb 12, the dispersion and at the same time frequencies f.sub.ceo and f.sub.rep can be controlled by adjusting distance d. The quality of measurements, if such an optical frequency comb setup 1 is used in a spectroscopy setup can therewith be improved.

[0039] FIG. 4 shows a schematic setup of an optical frequency comb setup 1 with the semiconductor cascade laser 11, without the laser driver 10 and with the linear translation mechanism 6 in another embodiment. The optical frequency comb setup 1 is mounted on the cascade laser mount 60. The distance d between reflective element 50 and the end facet 110 and therewith the external cavity 5 are depicted. The possible elongation of the external cavity 5 is indicated with the double arrow.

[0040] The holding means 51 of the here wedge-like formed reflective element 50 is composed of a tip 51, which is connected to the mechanical actuator 62 and an electromechanical actuator 63.

[0041] The mechanical actuator 62 comprises a micrometer screw 620 and a sliding element 621, which are connected to the holding means 51 for reaching the linear movement of the mechanical actuator 62 and the holding means 51 with reflective element 50 relative to the semiconductor cascade laser 11 along a mounting plate 61, which is defined here as guiding device 61. The micrometer screw 620 allows to move the sliding element 621. A blocking screw 622 as possible blocking means allows to fix the position of the first sliding element 621. With the mechanical actuator 62 a coarse linear movement can be done with an accuracy of the order of a micron.

[0042] The electromechanical actuator 63 comprises a piezo element 630. Optionally the piezo element 630 is arranged in a piezo element housing 631, a second sliding element 632 and a non-depicted steering electronics, which generates a high voltage.

[0043] The piezo element housing 631 is mounted on the same mounting plate 61 as the semiconductor cascade laser 11. Here the mounting plate 61 is used as well as a heatsink for the semiconductor cascade laser 11 and is temperature controlled. Suitable temperature control means are known, but not depicted here. In the piezo element housing 631, two sliding elements are placed. The first one is attached to the back of the housing with springs in order to have a restoring force toward the back of the housing. The two ends of the piezo element 630 are glued on each sliding element 621, 632. By applying a voltage on the piezo element 630, the second sliding element 632 can be displaced with a precision of a few nanometers. Therewith a fine linear movement can be reached. The stepwise resolution of the linear movement due to the electromechanical actuator 63 should be in a range below one micron. The relative position of the reflective element 50 is controlled by the piezoelectric actuator 63 in a closed loop operation. Depending on the distance d between reflective element 50 and the end facet 110, the inherent dispersion of the semiconductor cascade laser 11 can be decreased to values considerably smaller than 1000 fs.sup.2.

[0044] In another embodiment of the optical frequency comb setup 1, the linear translation mechanism 6 can be provided by a Microelectromechanical systems or MEMS device, which can provide the linear coarse and fine adjustment in one device. In such a MEMS device the cascade laser structure 11, the linear translation mechanism 6 with cascade laser mount 60, the electromechanical actuator 63 and the parts of the external cavity 5 can be integrated or connected to the same mounting plate 61 or can be separated on different mounts. Such a MEMS device also needs a steering electronics for controlling the distance d by either changing the linear position of the semiconductor cascade laser 11 or the reflective element 50.

[0045] In the following part, some possible specialities usable in all embodiments of the Optical frequency comb setup 1 are described. For example, the reflective element 50 can be formed of a single or multilayer of sufficient reflective material respectively materials. Beside a reflective element 50 of gold, a preferred choice is a block of a semiconducting material with direct band gap, in particular Gallium Arsenide, Silicon, indium phosphide or Germanium. For example reflective element 50 in form of a mirror of a cleaved piece of GaAs is placed in distance d from the end facet 110 of the semiconductor cascade laser 11. The reflective element 50 should be partly reflective in the wavelength region of QCLs and ICLs 11.

[0046] The outside surface of the end facet 110 and therewith the reflectivity of the laser facet 110 can be modified by an optical facet coating, comprising at least one layer directly coated on the outside surface of the end facet 110. The natural reflectivity of the laser facet 110 is around 30%. This value can be adjusted by either a high-reflection coating to increase this value or an anti-reflection coating to decrease it. In practice the coating of the outside surface of the end facet 110 can be done with single or multiple layers of Al2O3, SiO2, Si, Ge, YF3, BaF2, Au, Ti or other material proven to work for facet coatings of cascade lasers.

[0047] Not depicted in the Figures, but due to the divergence of the laser beam of semiconductor cascade lasers 11, beam shaping elements can be placed in the basic external cavity 5. Possible beam shaping elements are lenses or curved mirrors, which are placed between end facet 110 and the reflective element 50 in the optical path.

[0048] In other embodiments, at least one dispersive element can be placed inside the external cavity 5, between end facet 110 and mirror surface 500. Possible dispersive elements are a prism, a reflective grating, a phase grating or a multilayer element.

[0049] An additional dispersive coating layer can be provided on the mirror surface 500 of the reflective element 50 and/or the outside surface of the end facet 110 and/or the surfaces of beam shaping or dispersive elements placed between end facet 110 and the reflective element 50, for dispersion compensation and frequency stabilization of QCL/ICL frequency combs 12.

[0050] This dispersive coating can for example form an external Gires-Tournois Cavity. Possible coating layer are dielectric and metallic coatings, which are designed to change the dispersion of the external cavity 5.

[0051] Such coatings can comprise in particular Al2O3/SiO2, YF3/Ge, . . . layers and can as well be only partly reflective.

[0052] The dispersion of the whole optical frequency comb setup 1 is then the sum of the dispersion of the semiconductor cascade laser 11 chip dispersion and the dispersion of every element and coatings placed between the laser end facet 110 and the mirror surface 500. It includes any beam shaping element, any dispersive element and any coatings on the laser end facet 110, on the mirror surface 500 and on the beam shaping and dispersive element. By implementation with locally fixed reflective element 50 and a coating layer, the reflective element can be used to control f.sub.ceo and f.sub.rep.

[0053] Different beam exiting modes are possible in an optical frequency comb setup 1 in different directions after passing at least one external cavity. For example as presented in the schematic drawing of FIG. 5a). A frequency comb laser beam A can exit the semiconductor cascade laser 11 after passing the external cavity 5 in a first direction, while a frequency comb laser beam A exits the semiconductor cascade laser 11 through a second end facet 110 at the opposite side of the semiconductor cascade laser 11 in an opposite direction after passing the internal cavity of the semiconductor cascade laser 11.

[0054] In FIG. 5b) another optical frequency comb setup (1) is depicted, comprising two external cavities 5, 5. Beside the first external cavity 5 at one side of the semiconductor cascade laser 11, a second external cavity 5 is attached, comprising a second end facet 110 and a second reflective element 50 on the opposite side of the first external cavity 5. While a first frequency comb laser beam A exits through the first reflective element 50 in a first direction, a second frequency comb laser beam A passes through the second reflective element 50 in a second direction, opposite to the first. Both frequency comb laser beams A, A were modified by both external cavities 5, 5.

[0055] Best results can be achieved if the external reflective element 50 is in very close proximity to the cascade laser end facet 110, such that the optical path of the laser cavity that is outside the semiconductor chip is smaller than the path within the laser chip. Improved suitable distance d between the external reflective element 50 and the cascade laser end facet 110 is preferred less than 0.1 mm, most preferred between 5 microns and 100 microns. In such a setup, the reflective element 50 is in such a close proximity to the laser end facet 110, such that the optical path outside the semiconductor chip 11 is much smaller than the optical path inside the semiconductor chip 11. Also in that configuration, the external cavity can be used to adjust f.sub.rep and/or f.sub.ceo and/or the dispersion of the frequency comb setup. This is either achieved by a dispersive coating on the mirror surface, or just by choosing the mirror position in a way that the external cavity positively influences f.sub.rep and/or f.sub.ceo and/or the dispersion of the overall frequency comb setup.

LIST OF REFERENCE NUMERALS

[0056] 0 Dual optical frequency comb setup

[0057] 1 Optical frequency comb setup [0058] 10 laser driver [0059] 11 semiconductor cascade laser (QCL, ICL) [0060] 110 end facet [0061] 12 optical frequency comb

[0062] 2 beam combiner/combining and deflecting means

[0063] 3 sample

[0064] 4 detector/signal processing unit

[0065] 5 External Cavity (outside the laser cavity) [0066] 50 Reflective element (mirror, single or multi layer) [0067] 500 mirror surface [0068] 51 holding means of reflective element (tip, glued) [0069] d distance between mirror surface 500 and surface of end facet 110

[0070] 6 linear translation mechanism [0071] 60 cascade laser mount [0072] 61 guiding device/mounting plate [0073] 62 mechanical actuator [0074] 620 micrometer screw [0075] 621 sliding element [0076] 622 blocking screw [0077] 63 electromechanical actuator [0078] 630 piezo element [0079] 631 piezo element housing [0080] 632 sliding element