Classifications

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  • 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
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  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/08018—Mode suppression
  • H01S3/0804—Transverse or lateral modes
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  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/1065—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using liquid crystals
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  • H01S5/00—Semiconductor lasers
  • H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping
  • H01S5/0078—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping for frequency filtering
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  • 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
  • H01S5/1092—Multi-wavelength lasing
  • H01S5/1096—Multi-wavelength lasing in a single cavity
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  • 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
  • H01S5/11—Comprising a photonic bandgap structure
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  • 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
  • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
  • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
• • 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
  • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
  • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
  • H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
• • H—ELECTRICITY
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  • 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
  • H01S2301/00—Functional characteristics
  • H01S2301/16—Semiconductor lasers with special structural design to influence the modes, e.g. specific multimode
• • H—ELECTRICITY
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  • 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
  • H01S2302/00—Amplification / lasing wavelength
  • H01S2302/02—THz - lasers, i.e. lasers with emission in the wavelength range of typically 0.1 mm to 1 mm
• • H—ELECTRICITY
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  • H01S5/00—Semiconductor lasers
  • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
  • H01S5/041—Optical pumping
• • 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
  • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
  • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
  • H01S5/18308—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement
  • H01S5/18319—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement comprising a periodical structure in lateral directions
• • 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
  • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
  • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
  • H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
  • H01S5/18369—Structure of the reflectors, e.g. hybrid mirrors based on dielectric materials

Definitions

• the present invention relates to surface-emitting vertical external cavity cavity (VeCSEL) laser devices, and more specifically to the generation of two transverse modes stabilized with such devices.
• VeCSEL vertical external cavity cavity
• the field of the invention is, but not limited to, semiconductor laser terahertz (THz) sources and their applications.
• THz waves cover the electromagnetic spectrum between microwaves and infrared.
• the field of application is very wide with some promising applications, for example in astronomy, in radar systems, in time-frequency metrology, in biomedical detection and imaging, in high-speed wireless communications or in Security.
• the bandwidth in the upper range of the THz spectrum, the sources are generally pulsed type and / or require low temperature operation; and in the lower range of the THz spectrum, the electronic sources usually operate easily at ambient temperature continuously and at a determined transmission frequency.
• multiple frequency sources provide increased tunability but do not simultaneously provide both sufficient output power and high bandwidth modulation, but also good compactness or reasonable cost.
• Different techniques are implemented to generate THz waves, distinguishable according to two categories:
• the first category consists of a photonic coupling of a configuration with two laser sources and through which the difference in frequency between the two sources is largely tunable. Moreover, since two different sources are used, this first technique will be sensitive to the drift effects of each of the sources and the beat frequency is not very stable;
• the second category consists of a two-mode laser: in this case, the frequency difference is intrinsically more stable because the two modes will be sensitive to common effects and the effects of drift will be homogeneous on each of the two modes.
• Another technique consists of an external cavity laser diode using spectral (spectral) filter (s) which can produce two stable frequencies, but to the detriment of output power, complex cavities, sensitivity / robustness of 'Degraded alignment, beam quality and consistency because the laser tends to operate two sets of longitudinal modes rather than two modes.
• spectral filter s
• Surface Emitting Vertical External Cavity (VeCSEL) laser devices are well known devices and are very promising solutions for dual frequency lasers because they are inherently compact, wavelength flexible, tunable over a broad spectrum, powerful and highly coherent (spectrally, spatially and also in terms of polarization).
• VeCSEL VeCSEL-Experimental Demonstration of a Dual-Frequency Semiconductor Laser Free of Relaxation Oscillation", Optics Letters, Vol. 34, Issue 21, 3421-3423 (2009). It discusses the use of a VeCSEL in the GHz range and is based on a dual polarization beam with intracavity beam splitting on the amplifying chip.
• the object of the present invention is to respond to the problems mentioned above and to lead otherwise to other advantages.
• An object of the invention is to generate a laser effect with two transverse modes, stable and robust / controlled within a single cavity.
• Another object of the present invention is to provide laser sources having a very high stability (i.e. high coherence or low phase noise) of the beat frequency.
• Another object of the present invention is to provide laser sources adapted for high power operations with good consistency and good stability.
• an optical wave generating laser device comprising at least two frequencies, said laser device comprising:
• a first element comprising a gain region situated between a first end defined by a first mirror and a second end defined by an output region
• a second mirror distinct from the first element, and arranged so as to form with the first mirror an optical cavity including the gain region and a free space between the output region and the second mirror,
• the laser device further comprising means for shaping the light intensity and / or the phase profile of the optical wave and arranged to select at least two transverse modes of the optical wave, in particular of the Laguerre-Gauss type.
• the means for shaping the light intensity comprise at least one mask, each mask being arranged with a surface having a non-uniform absorbance, said surface including at least one absorption zone and / or at least one transmission area; the means for shaping the phase profile of the optical wave are arranged to adjust the at least two transverse modes of the optical wave, and in particular comprise at least one photonic crystal and / or a diffraction grating of at least one end of the gain region and arranged to shape the transverse phase and / or the transverse intensity of the optical wave, to select at least two transverse modes.
• the gain region (any) may be a separate element of the second mirror. More generally, the number of stabilized frequencies emitted by the laser according to the invention corresponds to the number of transverse modes selected.
• the pumping means of the gain region may include optical and electrical means.
• the first element can be a half-VCSEL type semiconductor element, or any other type of laser in which the transverse modes can resonate inside the cavity.
• the laser device according to the present invention takes advantage of VeCSEL structures in terms of power output, stability and robustness.
• the use of a half VCSEL makes it possible to use the laser device according to the present invention at room temperature and in a continuous mode, which greatly simplifies its use and makes it more versatile.
• the intrinsic optical qualities of the cavity are very good and are particularly suitable for such applications: the optical losses are low and there is no amplified spontaneous emission, which leads to a highly coherent wave.
• the frequency can be continuously tuned over a very wide range of frequencies because there is no gain coupling and no mode coupling in the cavity.
• the at least two selected transverse modes of the optical wave according to the present invention can be chosen between (i) the stabilized Laguerre-Gauss modes, and / or (ii) the stabilized Hermite-Gauss modes.
• the spatial filtering means of the optical wave can be chosen between (i) light diffraction means, (ii) light scattering means and (iii) absorption means light.
• the Spatial filtering means of the optical wave may be located at a belly of the stationary longitudinal field within the cavity.
• the light absorbing means may comprise a single mask, said mask being arranged to select the at least two transverse modes of the optical wave.
• the above-mentioned light absorption or diffraction means may be integrated on the semiconductor element.
• the present invention makes it possible to stabilize resonant modes inside the cavity by selecting these isolated transverse modes and by preventing the coupling with certain undesired modes: the laser source according to the invention provides a very stable beat frequency.
• the thickness of each mask may be of the order of a few nanometers or less than the wavelength.
• the absorption zone provides an absorbance at least two times higher than the absorbance of the transmission zone.
• the transmission zones can transmit more than 99% of the light intensity of the wave, and the absorption zones can absorb between 5% and 100% of the light intensity of the wave. .
• each mask may be located at or near a belly of the optical wave within the cavity.
• the means for shaping the longitudinal phase profile has a non-uniform transverse distribution of the gain inside the optical cavity, in order to modulate transversely the intensity of the gain emission wavelength and to adjust the frequency difference between the at least two transverse modes of the optical wave.
• the means for shaping the longitudinal phase profile may further comprise at least one non-uniform loss mask transversely between the two modes in order to adjust the frequency difference between the two modes by locally adjusting the injected carriers in gain areas to change the gain emission length on these areas.
• the fine adjustment of oscillating transverse modes inside the cavity can be obtained by means of a phase profile forming means comprising a heat source arranged to generate inside the cavity.
• Optical nonuniform transverse temperature distribution to determine and control the frequency difference between the at least two transverse modes of the optical wave.
• the phase profile forming means comprises a spectral filter having a non-uniform optical length along its transverse section in order to adjust the frequency difference between the two transverse modes of said waveform. optical.
• the means for shaping the phase profile can be arranged to modify the refractive index "seen" by the optical wave and by at least one of the transverse modes oscillating within the cavity. Since the refractive index is, in the first order, sensitive to the temperature, the pressure and the hygrometry of the medium traversed by the optical wave, such a means of shaping the phase profile can consist of a means which varies the pressure and / or the hygrometry and / or the temperature of at least part of the path taken by the optical wave inside the cavity.
• the means for shaping the phase profile may comprise a means of heating at least a portion of the optical cavity, for example by using an electrical resistor placed in the vicinity in order to dissipate thermal energy by conduction and / or convection and / or radiation.
• the spatial filtering means comprises a two-dimensional array of micro-etchings (ie a photonic crystal)
• the characteristic dimensions such as the diameter and the periodicity of their repetition, can be adjusted to generate a predetermined transverse profile of optical length of said spatial filtering means and a phase shift of each transverse mode oscillating in the cavity, thereby setting the value of a frequency difference between said at least two transverse modes.
• This configuration advantageously makes it possible to define any phase profiles by varying one and / or the other characteristic dimension of the network.
• This variation can be monotonous or non-monotonic, and / or continuous or discontinuous: it is thus possible to precisely control the filtering performed using dimensional variations of a network of microcavities.
• the spectral filter may also comprise a dynamic means for controlling the transverse profile of the optical length of said spectral filter, such as for example a matrix of controllable liquid crystal pixels: it is thus possible to control during the operation of said device the frequency difference between the two transverse modes in order to vary it and / or to achieve servocontrol in order to stabilize it.
• a dynamic means for controlling the transverse profile of the optical length of said spectral filter such as for example a matrix of controllable liquid crystal pixels: it is thus possible to control during the operation of said device the frequency difference between the two transverse modes in order to vary it and / or to achieve servocontrol in order to stabilize it.
• the pumping means of the gain region may comprise (i) a pump laser emitting a pump laser beam and (ii) means arranged for spatially shaping the light intensity of the pump laser beam; and the means for selecting at least two transverse modes of the optical wave may comprise means for spatially shaping the light intensity of the pump laser beam and may be arranged to project a beam intensity pattern of pumping on the output region of the first element corresponding to the at least two selected transverse modes of the optical wave.
• the second mirror may be a concave mirror or a phase conjugate mirror.
• a spectral filter may be inserted within the cavity, on the surface of one of the mirrors, or in the free space.
• the laser device according to the invention may further comprise tuning means for moving the second mirror so as to change the length of the optical cavity.
• tuning means for moving the second mirror so as to change the length of the optical cavity.
• the laser device according to the invention may further comprise means for tuning the difference in frequency between the at least two modes of the optical wave.
• the means for tuning the frequency difference may be thermal tuning means arranged to establish a thermal gradient in the transverse direction of the semiconductor element, or preferably a nonhomogeneous transverse distribution of heat.
• Thermal tuning is a very fine means of changing the temperature of the optical cavity (i.e., the length of the optical path) and thus, the present embodiment of the invention provides frequency offsets. very fine: the laser device according to the invention is thus continuously tunable.
• the thermal tuning means may comprise one of the following components:
• At least one local thermal conducting element mounted transversely on the semiconductor element and arranged to conduct or dissipate heat, and / or means for modulating the power of the pumping laser in order to heat, and preferably to heat locally:
• the spatial filtering means and / or
• a shaped pumping beam which enhances / optimizes the thermal gradient or, or the inhomogeneous heat distribution transverse to the two beams, with a beam intensity profile of the pumping laser of a shape similar to a petal for example.
• a method for generating an optical wave comprising at least two frequencies and comprising at least one of the following steps: (i) generating an optical wave inside a gain region, (ii) pumping the optical wave with pumping means, (iii) shaping the light intensity and / or phase profile of the optical wave to select at least two transverse modes of the optical wave.
• the step of selecting the at least two transverse modes of the optical wave may consist of the spatial filtering of the intensity of the optical wave.
• the method according to the invention may further comprise a step of tuning the length of the optical cavity to shift the at least two frequencies of the optical wave.
• the method according to the invention may further comprise a step of tuning the frequency difference between the at least two modes.
• FIG. 1 describes a schematic view of the dual frequency laser according to the invention.
• FIGURE 2 is a diagram of the transverse temperature profile that could be measured inside the semiconductor element.
• FIG. 3 is a schematic representation of the thermal tuning of the semiconductor element.
• FIGURE 4 is a schematic view of the optical transmission spectra for a dual frequency VeCSEL device.
• FIG. 5 is an example of a masking file used to manufacture masks according to the invention by electron beam lithography.
• FIG. 6 illustrates absorbent metal masks made of chromium according to the invention.
• FIGURE 7 is a schematic view of a laser device according to the invention with an absorbing metal mask located on the semiconductor element.
• FIGURE 8 is a schematic view of a laser device according to a second embodiment of the invention with two absorbent metal masks separated by a spacer located on the semiconductor element.
• FIGURE 9 is a view of the far-field intensity of the laser device according to the invention.
• FIG. 10 is the measured optical spectrum of the laser device according to the invention.
• FIGURE 11 is a graph illustrating the output power of the laser as a function of optical pumping power.
• FIGURE 12 is a laser device according to the invention with a diffractive mask on the semiconductor element.
• FIGURE 13 illustrates another diffractive mask design.
• FIG. 14 illustrates the THz spectrum of the Laguerre-Gauss double-frequency VeCSEL by photonic coupling and the time evolution of its mode intensity.
• FIG. 15 describes a diagram of a spectral filter making it possible to shape the phase profile of the oscillating optical wave in the optical cavity.
• FIG. 16 schematically illustrates an embodiment of the device according to the invention, in which a non-uniform gain medium makes it possible to adjust the frequency difference between the two selected transverse modes.
• the adjective “longitudinal” refers to the direction corresponding to that of the propagation of the optical wave 107 or the axis of the cavity.
• the adjective “transversal” refers to a direction orthogonal to the longitudinal direction.
• the first element 111 is considered a semiconductor element, without limiting the present invention.
• FIGURE 1 a general embodiment of the present invention is described. This mode makes it possible to obtain stable and compact laser modules operating at ambient temperature with robust, stabilized and tunable bi-frequency transverse modes.
• the present invention is based on a well-known VeCSEL technology and incorporates features using III-V technologies as well as a concave-type external optical cavity.
• VeCSEL technology comprises a semiconductor element with a gain region for generating optical radiation and a first mirror 103.
• the gain region may for example comprise quantum wells and may be electrically or optically pumped.
• the first mirror 103 may comprise a succession of layers constituting a Bragg grating which reflects the optical waves 107.
• the semiconductor element is mounted on a Pelletier module heat sink in order to stabilize and regulate the temperature.
• the Bragg mirror has a high reflectivity, 99% or more higher than that of the exit mirror; for example, it may consist of 27.5 pairs of AlAs / GaAs quarter-wave layers.
• the gain region 104 is, for example, consisting of six compensated-constrained InGaAs / GaAs (P) quantum wells located at the belly of the intra-cavity laser field.
• the total optical thickness of the active region between the Bragg grating and the air is 13/2 half-wave layers.
• Such a semiconductor element is manufactured using well-known semiconductor manufacturing technologies, such as molecular beam epitaxy (MBE), electronic lithography, dry etching and chemical etching. .
• a second concave-shaped external mirror is provided and arranged to form an outer optical cavity with the first mirror 103 and to stabilize certain transverse modes. It should be noted that the present invention may be able to stabilize Laguerre-Gauss modes and / or Hermite-Gauss modes and / or Bessel-Gauss modes.
• the external optical cavity is termed "external" because it includes a portion that is distinct from the semiconductor element. It does not require any extra components on the inside for nominal laser operation.
• the optical cavity of the laser device 100 according to the invention may include a solid spacer between the output region and the second mirror 106 in order to form a cavity monolithic.
• a monolithic cavity could be made of a monolith of sapphire or glass.
• the outer cavity has axial symmetry.
• the second mirror 106 may be a dielectric glass-based mirror or may include photonic crystals on a semiconductor or etched surface. Its transmittance is a few percent so as to allow the laser beam to exit the device.
• the second external mirror is located 1 mm from the semiconductor element, and has a radius of curvature of about 10 mm.
• a free space 105 separates the concave outer mirror from the semiconductor element.
• optical pumping is used to excite the low-energy electrons to some higher levels, thanks to a single-mode laser diode.
• its wavelength can be 785 nm and the output power 300 mW.
• the pump laser beam 110 is focused on the half-VeCSEL surface with a diameter of 50 to 100 ⁇ m.
• VeCSEL has been designed to generate a stable laser state consisting of at least two transverse modes, each transverse mode operating in a single longitudinal mode with a linear polarization state, and therefore at a well-defined frequency.
• FIG. 2 illustrates the transverse temperature profile 201 that could be measured inside the semiconductor element 111 and the intensity of certain transverse modes of Laguerre-Gauss (LG) 202, 203 and 204.
• the transverse profile The temperature is Gaussian and the modes LG 0 o 202, LG02 203 and LG 0 3 204 are shown.
• the beat frequency between the two transverse LG modes is based on a thermal effect, and more precisely on the radial thermal gradient 201 generated by optical pumping (or preferably a nonhomogeneous transverse distribution of heat): as the pump beam is focused on the half-VCSEL with a quantum efficiency defect, a radial thermal gradient is then generated.
• FIG. 3 is a schematic representation of the thermal tuning on the semiconductor element 111.
• the pump laser beam 110 focused on the top surface 112 of the semiconductor element 111 induces a thermal gradient 301.
• the index refraction varies within the cavity, and may be different between two axial optical paths (corresponding for example to two modes).
• Thermal tuning can be accomplished by any means and methods.
• the thermal tuning is provided by the optical pump laser beam 110.
• the temperature within the semiconductor element can be regulated.
• thermal pumping can be achieved by a local metal heater deposited on the surface of the semiconductor element. Thin metal layers are placed transversely to generate a thermal gradient during heating, or preferably a nonhomogeneous transverse distribution of heat.
• FIGURE 4 illustrates a schematic view of the optical transmission spectra for a dual frequency VeCSEL device. Due to their intrinsic characteristics, the transverse modes have different resonance frequencies which are situated in different frequency ranges 401, 402 and 403. Thus, as illustrated in FIGURE 4, the mode N 'th order 401 is located on the left side of the spectra, whereas the orders of N + 1 th and N + M th 402, 403 are located on the right side. Thus, for each order, the Laguerre-Gauss transverse modes exist in a frequency comb 411, 412, 413 which is separated from the next order by a distance corresponding to approximately the free spectral interval 402 plus the difference in phase difference. from Gouy. The passage between these frequency combs can be accomplished by tuning the cavity length. Nevertheless, this type of tuning does not affect the beat frequency.
• the present invention proposes an original and innovative solution that consists of means for shaping the light intensity and / or the phase of the optical wave 107 which resonates inside the optical cavity, in order to select two transverse modes and avoid the appearance of other damaging transverse modes.
• the intensity shaping means may be spatial filtering means of the optical wave 107 of the resonator. It may be for example at least one mask located on or inside the semiconductor element 111 - preferably at a belly of the laser wave field - and with a specific shape so providing first areas through which the optical wave 107 may be transmitted, and other areas through which the propagation of the optical wave 107 may be predominantly interrupted.
• the transversal spatial filter according to the invention induces high losses for undesired cavity modes and low losses for the desired modes.
• these devices can be fabricated by organometallic chemical vapor deposition (MOCVD) and electron beam lithography, without the present invention being limited to these manufacturing processes.
• MOCVD organometallic chemical vapor deposition
• electron beam lithography electron beam lithography
• the means for shaping the intensity of the optical wave 107 within the resonator may be a laser beam pumping device whose intensity is contoured.
• the laser device 100 according to the invention is stable and highly tunable: as the tuning of the beat frequency is based on thermal effects, the tuning capacity can be adjusted to a certain extent by a regulation of the temperature gradient. the interior of the semiconductor element, for example by regulating the power of the pump laser beam 110 and / or some additional heating on the semiconductor element.
• the devices according to the embodiment of the present invention are suitable for filtering the Laguerre-Gauss transverse modes.
• FIGURE 5 With reference to FIGURE 5, FIGURE 6, and FIGURE 7, a subwavelength absorbent filter is described.
• the overall dimensions of said filter are compatible with the semiconductor element, since it can be deposited on the top surface of said semiconductor element.
• the structured absorbent layer may be deposited at any location within the cavity, between the first mirror 103 and the exit region.
• the intensity of undesired transverse modes of the optical wave 107 that resonate within the cavity can be absorbed between 5 % and 100% by the absorption areas 530, while the transmission areas 520 tend to reduce the losses as much as possible, and typically less than 1%.
• the shapes of these areas are designed to correspond with the transverse profile of the transverse modes that one seeks to select.
• FIG. 5 illustrates a masking file used for the lithography of a wafer 500 intended to design the laser device 100 according to the present invention and integrating a spatial filter on the surface of the top of the semiconductor element 111.
• Many types of patterns are designed on the wafer 500 with different dimensional characteristics. Some of them are larger than others to optimize the selection of transverse modes, depending on the size of the beam and the pump laser 110.
• these drawings are almost equivalent. They allow to filter different orders of transverse modes. For example, FIG.
• FIG. 6 illustrates a photograph by optical microscope of chromium metal masks 610, 620, 630, 640 according to the invention as well as the corresponding Laguerre-Gauss transverse modes obtained by simulation with such filters 615, 625, 635. , 645 respectively.
• Plates 6. a and 6.b illustrate metal masks 610, 620, 630, 640 for two different filtering orders of the transverse Laguerre-Gauss modes.
• the cross pattern 610, 630 and the flower-shaped pattern 620, 640 are equivalent, as can be seen in the photos 6.c and 6.d respectively.
• the images 6.c and 6d illustrate the corresponding LG transverse modes that can be obtained with these absorbent masks: the mask 610 and 620 makes it possible to filter the LG00 + LG02 modes (615 and 625) while the mask 630 and 640 is used to filter modes LG00 + LG03 (635 and 645).
• the masks are designed according to certain physical parameters.
• the corresponding Laguerre-Gauss modes can be calculated. Many of these modes can not achieve sufficient operation for laser emission because the Gaussian pump beam is focused in the center of the structure, thus allowing only (and thus selecting) the existence of low order transverse modes.
• the losses can be calculated for each Laguerre-Gauss mode and the lowest losses should be obtained for the selected modes LG 0 o and LG02 for example. Due to their almost complementary cross-sectional distributions, these two modes should coexist due to the burning of transverse spatial holes ("transverse spatial hole burning").
• a mask may be able to select one, two or more than two transverse modes. As described below in FIG. 8, two masks 820, 830 can be combined so that each one selects at least one different transverse mode.
• the mask is formed by the deposition of a layer of 10 nm of chromium and a width of a few microns on the half-VeCSEL.
• the transverse dimensional characteristics are about a few microns, depending on the beam diameter, and, as far as the longitudinal dimensions are concerned, the masks must be thin enough not to absorb transverse modes that one seeks to propagate to the other. inside the cavity. They must be fine enough to achieve light diffraction.
• a thickness of 10 nm is optimal for chromium for example, but it depends on the absorption coefficient of the material chosen. More generally, the thickness is less than the wavelength of the corresponding optical wave 107.
• the mask is deposited at a belly of the optical wave 107 which oscillates within the optical cavity.
• the deposition material is not limiting in the present invention. Depending on the applications, the desired wavelength, the technologies and / or the desired performances, it will be possible to deposit a metallic material, a doped semiconductor material, or a chemically etched material and / or a dielectric.
• FIG. 7 illustrates the implementation of such an absorbent mask 710 according to the invention, on the top surface 701 of a half-VeCSEL 111. Thanks to the absorption areas and the transmission areas, two transverse modes 720 , 730 are selected and are - finally - stabilized within the laser optical cavity by the so-called transverse spatial hole burning phenomenon in the gain region 104. These modes oscillate between the mirror of Bragg 103 located inside the semiconductor element 111 and the second external concave mirror 106. To the right of FIGURE 7, a diagram illustrates the propagation of the intensity of the field 740 inside the cavity: it shows the propagating wave inside the Bragg mirror and resonating through the active zone and the gain region 104 during optical pumping.
• FIG. 8 illustrates a schematic view of a laser device 800 according to another embodiment of the invention with two metal masks 820, 830.
• Each mask is deposited, one after the other. All first, the Bragg mirror 103 is deposited on the substrate 102. Next, the active area with the gain region 104 is deposited and lithographed on the Bragg mirror. Next, a first mask 810 with a first pattern is deposited on the surface; in the example shown it is a chrome mask whose thickness is 10 nm. Then, a spacer 820 is deposited with a thickness that depends on the wavelength. In the example shown, its thickness is equal to half the wavelength. Then, a second absorbent metal mask 830 with a second pattern that may be different from the first is deposited on the spacer. In a preferred embodiment, each mask is located on a belly of the optical wave 107 that is generated by the present laser device.
• FIGURE 9 shows a view of the far-field intensity of the laser according to the invention with an integrated metallic absorbent mask.
• FIGURE 9 also illustrates the coexistence of the two Laguerre-Gauss modes.
• FIG. 10 illustrates the optical spectrum of the laser according to the invention. It shows that the laser according to the present invention provides a dual-frequency operation, illustrated by the two peaks plotted and having a frequency difference between the two modes of 162 GHz. By changing the patterns of the absorbing masks, it is possible to change the beat frequency up to 450 GHz.
• FIGURE 11 illustrates the output power of the laser as a function of optical pumping power.
• the output power of the laser is linear with a first slope up to a threshold value corresponding to the second transverse mode and from which the slope of the power of laser output increases: the efficiency of the laser according to the invention increases when the second mode appears.
• the threshold value of the pumping power is, for the first mode, around 150 mW, and the threshold value for the second mode is around 250 mW.
• Second embodiment diffraction masks
• FIGURE 12 is a laser device 100 according to the invention with a diffractive mask on the semiconductor element 111 according to the previous description. It consists of a single layer of Si 3 N 4 1210 completed by a 2D network of holes 1220 of diameters arranged along a square grid of period a. The diameter of the holes can be constant or variable, depending on the desired effects.
• the single layer of Si 3 N 4 is a photonic crystal mirror and deposited on the top surface of the half-VeCSEL 111, as for the absorbent masks. The principle is close to that of absorption masks but it is based on diffraction effects instead of absorption effects. Thus, the light is diffracted in order to diffract the Laguerre-Gauss modes which are not desired.
• FIG. 13 illustrates another diffractive mask pattern that introduces losses on the optical wave 107 that oscillates within the laser cavity.
• the diffractive mask 1310 according to this embodiment is composed of four diffraction gratings 1320, 1340, 1360, 1380 etched on the two axes of symmetry of the spatial filter 1310.
• the pitch is 1 ⁇ m and the size each brand is 200 nm.
• Two modes are selected thanks to this diffractive photonic crystal mask: the main mode TEMoo 1305 represented by a circular line centered on the mask and the LG02 mode represented by a concave and closed curve (1301 to 1304) at each corner of the mask.
• the typical thickness of the dielectric deposit on which the diffractive structures are etched is between about half the wavelength and one-eighth of the wavelength.
• Diffractive masks can be made by some microelectronic techniques, such as for absorbent masks.
• the optical pumping is entirely homogeneous on the zone corresponding to the desired beam - either Gaussian or top-of-shape profile - on the active surface of the semiconductor element on which pumping is done.
• the spatial transversal filter is mounted on the optical components that focus the pump laser beam 110 on the active surface of the semiconductor element, in order to reduce its intensity in certain zones, called absorption zones, and to enable transmission. light in some other areas, called transmission zones.
• the shape and location of these zones on the pump laser beam 110 are arranged to be projected onto the active surface on the semiconductor element, and so that they are superimposed on the transverse Laguerre-Gauss modes that the we try to intercept. Only those corresponding to the modes of Laguerre-Gauss transversal that one seeks to select will actually be pumped by this pumping laser of inhomogeneous intensity.
• another technique for shaping optical pumping can be achieved by focusing two elliptical pumping beams (the two ellipses being transverse to each other) on the chip. of semiconductor to produce a pumping area of similar shape to that of a petal.
• FIG. 14 illustrates the THz spectrum of the two-frequency Laguerre-Gauss VeCSEL by photo-coupling and the temporal evolution of its mode intensity. It shows the simultaneous laser generation of two mutually coherent optical waves (i.e. displaying a relative phase noise low) inside a single VeCSEL cavity, in 300K and moderate power (35mW) CW operation.
• FIG. 14 thus illustrates the coherent THz wave generation by photocoupling of the Laguerre-Gauss dual-frequency VeCSEL in a UTC photodiode, and confirms the theoretical study of the laser dynamics, showing the simultaneous emission of the two LG modes, in steady state. permanent, VeCSEL dual frequency.
• FIG. 15 describes a diagram of a spectral filter making it possible to shape the phase profile of the oscillating optical wave in the optical cavity, in a manner complementary to the selection means of the at least two transverse modes.
• Diagram (a) illustrates an optical cavity in which two transverse modes have been selected (means not shown) and integrating a spectral filter in order to adjust the frequency difference between the two modes
• the diagram (b) illustrates a front view. said spectral filter
• the diagram (c) illustrates on the one hand the bandwidth of such a filter, for the two transverse modes oscillating in the optical cavity and on the other hand the gain of the amplifying medium.
• the device according to the invention can comprise between the two mirrors M 1 and M 2 forming the optical cavity - complementary to the selection means of at least two transverse modes not shown - a filter absorption spectral and / or an amplifying spectral filter 1503 arranged to generate a phase shift between the two transverse modes and thus grant the frequency difference between the two oscillating transverse modes ( ⁇ , ⁇ 2) within the cavity.
• the intensity of oscillating transverse modes inside the cavity is illustrated in 1504.
• This filtering can for example be obtained using a Lyot or Farby-Perot filter comprising a gain medium and / or a loss medium.
• the adjustment of the cavity can thus be achieved by adjusting said filter so that each oscillating transverse mode ⁇ , ⁇ 2 passes through a different optical length through the filter.
• the spectral filter is arranged to have a refractive index, or even a nonuniform transverse optical length so as to generate a phase shift between the two oscillating transverse modes.
• such a filter can be obtained using a Bragg mirror (1-10 pairs) with low reflectivity and located on the structure of the semiconductor element (ie the VCSEL by example).
• a Bragg mirror (1-10 pairs) with low reflectivity and located on the structure of the semiconductor element (ie the VCSEL by example).
• Such a mirror can be made by well-known methods of microelectronic deposition.
• Such a spectral filter advantageously provides a "factory setting" of the frequency difference between the two oscillating transverse modes.
• such a spectral filter may also be dynamic in order to provide a control means on the frequency difference between the two oscillating transverse modes inside the cavity. It may be for example a matrix of controllable liquid crystal pixels.
• the adjustment of the characteristic dimensions of such a mirror makes it possible to control the refractive index "perceived" by each transverse mode oscillating in the cavity; and it is possible for example by increasing the optical thickness of the material traversed by a first mode, relative to that traversed by the second mode, to control very precisely the desired frequency deviation for the device according to the invention. More particularly, it will be sought to modify the values of refractive index (and therefore of thickness) of the central zone of said filter relative to the peripheral zone. For example, it is possible to engrave on the upper surface of the Bragg mirror a phase shift overlayer of a thickness typically less than ⁇ / 2 in one of the transverse modes relative to the other transverse mode.
• phase-shifting structure for example a photonic crystal or a meta-material whose refractive index is controlled spatially by the characteristic dimensions of a network.
• a phase-shifting structure for example a photonic crystal or a meta-material whose refractive index is controlled spatially by the characteristic dimensions of a network.
• it may be a two-dimensional array of holes whose diameters and periodicity evolve according to a particular profile that determines the phase profile of the transverse modes. It is already possible to achieve "on demand" particular phase profiles, including continuous, to obtain very fine settings between modes.
• transverse non-uniform heating so as to locally modify the value of the refractive index of the material and to control the phase difference between the two. transverse modes and therefore the frequency difference.
• FIG. 16 schematically illustrates an embodiment of the device according to the invention, in which a non-uniform gain medium makes it possible to adjust the frequency difference between the two transverse modes selected by said device, the means making it possible to select the two modes transverse not being represented.
• Figure (a) illustrates an optical cavity incorporating a non-uniform gain medium
• Figure (b) illustrates a front view of said gain medium
• Figure (c) illustrates the bandwidth of the gain medium.
• the device according to the invention can comprise between the two mirrors M 1 and M 2 forming the optical cavity at least one gain medium 1601, 1602 arranged to tune the frequency difference between the two oscillating transverse modes ( ⁇ , ⁇ 2 ) inside the cavity.
• the intensity of oscillating transverse modes inside the cavity is illustrated in 1603.
• the device according to the invention may comprise at least one non-uniform loss mask transversely between the two modes in order to adjust the frequency difference between the two modes.

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• 2014
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