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
  • H01S5/10—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
• • 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
  • H01S5/005—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
  • H01S5/0064—Anti-reflection components, e.g. optical isolators
• • 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
  • H01S5/0651—Mode control
  • H01S5/0653—Mode suppression, e.g. specific multimode
  • H01S5/0654—Single longitudinal mode emission
• • 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
• • 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
  • H01S5/1071—Ring-lasers

Definitions

• the field of the invention is that of high-dynamic lasers, used in particular in telecommunications systems with digital signals, in radar systems with analog signals, etc.
• the increase in the dynamics of a laser is obtained by increasing its power and / or by reducing its intrinsic intensity noise.
• lasers with very low noise are considered. These lasers are also used in new applications such as optical manipulation of atoms, atomic and molecular spectroscopy, quantum memories, quantum cryptography, large interferometers, gravitational wave detection, etc.
• An important object of the invention is therefore to provide a laser with very low noise on spectral bands greater than 20 GHz.
• the invention proposes a laser comprising a semiconductor active medium with a population inversion lifetime ⁇ c , and a resonant cavity with a lifetime of photons in the cavity r p , mainly characterized in that the cavity comprises means for being monomode longitudinally and means for that ⁇ p > ⁇ c .
• Such a laser then has a quasi-white noise spectrum, on a potentially infinite frequency band, an ideal condition for the transport of broadband analog signals for example.
• the cavity being able to produce several modes, the means for obtaining a monomode cavity comprise filtering means of these modes.
• the semiconductor having a length I the cavity is external and has a length L> 100 I in order to obtain ⁇ p > ⁇ c .
• the filtering means of these modes comprise for example a Bragg grating and / or a Fabry-Perot interferometer; the cavity optionally comprises an insulator and / or an optical fiber.
• the filtering means when the external cavity comprises filtering means and at least one mirror external to the semiconductor, the filtering means comprise this external mirror and this mirror is photorefractive.
• the external cavity comprises an external exit mirror, and the latter is a concave mirror or a plane mirror associated with a collimating lens or comprises at least one photo-refractive crystal.
• the cavity comprises mirrors having a reflection coefficient R> 80%.
• the laser may be monolithic and have two faces having a reflection coefficient R> 80%.
• the semiconductor is a half-VCSEL or a quantum-island semiconductor or a quantum cascade semiconductor.
• the semiconductor is a quantum cascade semiconductor, the cavity is external and comprises a waveguide external to the semiconductor.
• the laser may further comprise a servo device.
• FIG. 1 schematically represents an example of a laser according to FIG. invention whose outer cavity is in a ring
• FIG. 2 schematically represents transmission curves T of the signal as a function of the wavelength ⁇ in the presence of a spectral filtering obtained by the insertion of a Bragg grating and a Fabry-Perot interferometer into the cavity.
• FIGS. 1 schematically represents an example of a laser according to FIG. invention whose outer cavity is in a ring
• FIG. 2 schematically represents transmission curves T of the signal as a function of the wavelength ⁇ in the presence of a spectral filtering obtained by the insertion of a Bragg grating and a Fabry-Perot interferometer into the cavity.
• FIGS. 1 schematically represents an example of a laser according to FIG. invention whose outer cavity is in a ring
• FIG. 2 schematically represents transmission curves T of the signal as a function of the wavelength ⁇ in the presence of a spectral filtering obtained by
• FIG. 3 schematically represent different examples of linear cavity lasers according to the invention: with an external cavity with concave mirror (3a), with plane mirror and collimation lens (3b), with photo-refractive crystal and collimation lens ( 3c), with mirror and waveguide (3d), and a monolithic laser without external cavity (3e). From one figure to another, the same elements are identified by the same references.
• these lasers exhibit relaxation oscillations at the frequency ⁇ r whose value is proportional to the pumping rate ⁇ of the laser and, conversely, to the lifetimes ⁇ p and ⁇ c :
• the resonance frequency ⁇ r disappears when the lifetime of the photons in the laser cavity becomes greater than the characteristic recombination time of the carriers, characteristic property of so-called "class A" lasers.
• Such a laser then has a quasi-white noise spectrum, on a potentially infinite frequency band, an ideal condition for the transport of broadband analog signals for example.
• the principle of the invention consists in acting on the dynamics of interaction between the photons and the amplifying medium of the laser in order to be placed in a particular operating regime which makes it possible to significantly extend the lifetime of the photons in the laser cavity. compared to that of the population inversion in the amplifying medium or carriers in the case of a semiconductor laser.
• a mode of operation equivalent to that of class A lasers is obtained from a class B standard laser such as a semiconductor laser, by significantly increasing the service life of the class B lasers. photons in the laser cavity and / or decreasing the life of the population inversion in the amplifying medium.
• the laser source must remain monomode longitudinally in order to overcome the noise of beat between modes.
• the laser 1 according to the invention has as active medium 2 a semiconductor of length I, and a external cavity length L> 100 I.
• the original cavity which is that of the semiconductor is enlarged by means of an optical fiber 3 which loops back on the semiconductor.
• the ring cavity thus formed has a length L of a few meters, for example 5 m.
• Such a cavity length corresponds to a free spectral interval of a few tens of MHz, which allows the simultaneous oscillation of several thousand longitudinal modes (spectral width of the gain 40 nm).
• the insertion of a Bragg grating 4 in the cavity makes it possible to reduce the oscillation range to 0.05 nm; curve a illustrates this filtering.
• the light passes through, in order, the insulator then the Fabry-Perot.
• a circulator 7 directs the light to the Bragg grating which serves as an output coupler and spectral filter.
• the light reflected by the Bragg grating is finally redirected to the semiconductor 2.
• the Fabry-Perot resonance frequency is locked in this longitudinal mode.
• a servo device 8 such as a synchronous detection device.
• Such a servocontrol also makes it possible to compensate for the mode drifts induced by a temperature change or mechanical stress variations.
• Such a laser oscillates at 1549 nm and remains monomode longitudinally.
• the modulation response of the laser shows that the resonance has disappeared and that we are in the presence of a class A laser, that is to say such that ⁇ p > ⁇ c .
• the results obtained on the noise measurements confirm that the laser obtained is a laser with very low noise: the noise spectrum of this laser is far below that of a standard DFB laser. Indeed, the intensity noise Relative Intensity Noise (“RIN”) of the laser is limited by shot noise over the entire spectral range experimentally accessible by the test bench (100 MHz-21 GHz). The output power of the laser under experimental conditions being 1.8 mW, its relative shot noise is at -156 dB / Hz.
• RIN intensity noise Relative Intensity Noise
• a linear external cavity of a few centimeters but strongly overstretched is used. Indeed, in a highly extended cavity, photons perform several hundreds of round trip before exiting the cavity. The result is therefore identical to that which one would obtain with a very long cavity.
• Using a cavity a few centimeters has a certain advantage compared to a long cavity since it eliminates complex spectral filtering.
• a highly stretched cavity is a cavity whose mirrors have a reflection coefficient greater than 80%. In the following examples, the cavity is linear.
• An example of an extended cavity low noise laser will be described.
• the semiconductor used is a half-VCSEL.
• a VCSEL Very Cavity Surface Emittivity Laser
• a half-VCSEL is a VCSEL whose exit face has no Bragg grating.
• the laser oscillation is then obtained by placing an exit mirror in an external cavity.
• the output mirror may be a concave mirror or a planar mirror associated with a collimating lens.
• a cavity length of a few centimeters is then sufficient to obtain a class A laser and consequently an intrinsically low-noise laser over a large spectral width.
• the half-VCSEL which acts here as the amplifying medium can be pumped either optically or electrically.
• a spectral filtering device such as a Bragg grating and / or a Fabry-Perot interferometer may further be included in the cavity.
• the cavity loops on itself with a photo-refractive crystal.
• the photo-refractive crystal simultaneously makes it possible to increase the lifetime of the photons and to perform a spectral filtering.
• using a semiconductor whose life inversion of the population in the active medium is very short The use of such an active medium makes it possible to reduce the laser cavity length to a few centimeters, or even a few millimeters.
• Active media that meet this criterion are quantum-island semiconductors or quantum cascade semiconductors. These active media allow, in addition, to cover wavelengths ranging from near infrared (quantum islands) to THZ (quantum cascade).
• the approach based on a decrease in the life of population inversion in the active medium can of course be combined with the approach based on the increase of the lifetime of the photons in the laser cavity.
• the laser comprises an external cavity, that is to say one that extends beyond the semiconductor 2.
• the first face 21 of the semiconductor 2 plays the role of the first mirror of the laser cavity 14.
• the second face 22 is, in turn, anti-reflective treatment.
• a mirror 9 placed a few centimeters from the active medium 2 closes the laser cavity 14.
• the exit mirror 9 may be a concave mirror (FIG. 3a) or a plane mirror associated with a collimation lens 11 (FIG. 3b) or a photo crystal -refractive 12 associated with a collimation lens 11 ( Figure 3c).
• a face 13 of the photo-refractive crystal plays the role of the second mirror of the cavity.
• the extended cavity further comprises the mirror 9, a waveguide THz 10 as diagrammatically shown in FIG. 3d.
• the laser is monolithic and the means for obtaining ⁇ p > ⁇ c are based on the overvoltage factor of the cavity and on the choice of the active medium 2 which is for example a quantum cascade laser.
• the active medium 2 which is for example a quantum cascade laser.
• a reflective treatment is deposited on both faces 21, 22 of the active medium 2.
• the length of the active medium (of the order of mm) can be optimized so as to reduce the linewidth of the laser.
• This last architecture has the advantage of being monolithic so easy to implement and less sensitive to external disturbances.
• the semiconductor is for example a quantum-island or quantum cascade laser or a half-VCSEL.
• the reflection coefficients of the mirrors of the cavity are preferably greater than 80%.

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

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• 2005
  • 2005-06-10 FR FR0505937A patent/FR2887082B1/fr active Active
• 2006
  • 2006-06-07 JP JP2008515215A patent/JP2008543101A/ja active Pending
  • 2006-06-07 EP EP06777276A patent/EP1889340A1/fr not_active Withdrawn
  • 2006-06-07 US US11/917,148 patent/US20090225800A1/en not_active Abandoned
  • 2006-06-07 WO PCT/EP2006/062975 patent/WO2006131536A1/fr active Application Filing
  • 2006-06-07 AU AU2006256749A patent/AU2006256749A1/en not_active Abandoned

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