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WO2010057290A1 - Oscillateur laser à fibre à impulsions sélectif de façon spectrale - Google Patents

Oscillateur laser à fibre à impulsions sélectif de façon spectrale Download PDF

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Publication number
WO2010057290A1
WO2010057290A1 PCT/CA2009/000365 CA2009000365W WO2010057290A1 WO 2010057290 A1 WO2010057290 A1 WO 2010057290A1 CA 2009000365 W CA2009000365 W CA 2009000365W WO 2010057290 A1 WO2010057290 A1 WO 2010057290A1
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Prior art keywords
pulse
fiber laser
phase
laser oscillator
optical pulses
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PCT/CA2009/000365
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English (en)
Inventor
Pascal Deladurantaye
Louis Desbiens
Marco Michele Sisto
Mathieu Drolet
Vincent Roy
Yves Taillon
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Institut National Optique
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Priority to CA2743648A priority Critical patent/CA2743648C/fr
Publication of WO2010057290A1 publication Critical patent/WO2010057290A1/fr

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    • 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
    • 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
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • 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
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • 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
    • 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
    • H01S3/10084—Frequency control by seeding
    • 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
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
    • H01S3/2308—Amplifier arrangements, e.g. MOPA

Definitions

  • the present invention relates to the field of laser devices and more particularly concerns methods for tailoring the spectrum of a narrow linewidth pulsed fiber laser.
  • High power pulsed fiber laser are currently in demand for a number of applications and uses.
  • numerous material processing applications such as memory repair, milling, micro-fabrication, drilling, etc. require pulsed laser systems which provide, among others, the four following characteristics all at the same time and with a great stability over the different conditions of operation and over time:
  • Scaling the output power without deteriorating other essential characteristics of the laser, such as beam quality or spectral purity, is a main challenge for high power pulsed fiber laser designers.
  • SBS Stimulated Brillouin Scattering
  • SPM Self-Phase Modulation
  • SRS Stimulated Raman Scattering
  • SBS threshold the first nonlinear effect that manifests when the pulse peak power exceeds a certain level, the so-called SBS threshold.
  • the impacts of SBS are mainly a degradation of the pulse amplitude stability, the appearance of counter-propagating satellite pulses, a roll-off in the laser output power vs pump power curve or even permanent damages to the laser's optical components.
  • SBS The process of SBS can be described classically as a parametric interaction among the pump wave (which is formed by the optical pulses), the Stokes wave (partially reflected optical pulses) and an acoustic wave.
  • the pump wave generates acoustic waves through electrostriction which in turn causes a periodic modulation of the refractive index in the fiber. This periodic index modulation creates a grating that partially scatters the pump wave through Bragg diffraction, causing the detrimental impacts just described.
  • SBS has been studied extensively since its discovery in 1964. For a general presentation of the SBS theory in the context of optical fibers see for example Govind P. Agrawal, "Nonlinear fiber optics", Academic Press, San Diego, 2001 , chapter 9.
  • the strain distribution solution is thought to be more adapted to passive single mode fibers in telecom applications but is not considered practical for high power fiber applications, since applying a controlled strain distribution on a LMA fiber while keeping stable beam characteristics is not really attractive from a practical point of view, due to the quite high modal sensitivity to mechanical constraints (torsion, curvatures, etc.) usually displayed by LMA fibers.
  • relatively high temperature gradients > 100 0 C
  • SBS threshold increase can reduce unacceptably the lifetime and reliability of an LMA fiber incorporating a high index polymer cladding to guide the pump light.
  • SBS mitigation schemes include designing a fiber with tailored acoustic properties.
  • PAPEN et al. U.S. patent no. 6,587,623
  • HASEGAWA European patent application no. EP 1 674 901
  • HASEGAWA European patent application no. EP 1 674 901
  • the fluorescence-based seed sources are not plagued by longitudinal mode beating noise. However, they are relatively inefficient since only a very small fraction of the produced fluorescence (about 0.1% for fiber gain media) is initially selected by the filter element. For polarized sources, the efficiency is even lower as half of the fluorescence power is lost after polarization filtering. Additional optical amplifier stages are therefore often required to boost the output power to usable levels, which increases the overall complexity, component count and cost of the device.
  • broad linewidth sources also suffer from a susceptibility to nonlinear effects other than SBS, especially SPM 1 which may quickly broaden the spectrum beyond the maximum acceptable width as the peak power increases in the fiber amplifier.
  • This effect is greater for broad linewidth than for narrow linewidth sources, due mainly to the low coherence and to the important phase noise of the former. This transfers the optical power from the spectral region of interest into large spectral "wings", thereby reducing the spectral power density of the source.
  • MURISON et al. disclose a seed source based on a frequency chirp induced by amplitude modulation [see International patent application published under no. WO 2008/086625].
  • the chirp is obtained using an amplitude modulator having a non-zero chirp parameter.
  • the injection current of a semiconductor laser diode is modulated in order to generate pulses with a frequency chirp along the pulse.
  • pulses having triangular shapes are generated and an amplitude modulator located downstream the laser diode further gates the pulse in the time domain.
  • the SBS threshold is increased as a result of the spectral broadening corresponding to the frequency chirping.
  • amplitude modulation approach for pulsed lasers
  • the SBS suppression therefore imposes variable limits or constraints on the pulse shape depending upon the conditions of operation (pulse repetition rate, output power, etc.), limiting the flexibility of the device.
  • the chirp creates an additional pulse shape distortion factor for applications using the laser harmonic wavelengths. As the frequency varies more or less linearly along the pulse, the frequency conversion efficiency will also vary along the pulse, leading to pulse shape distortion. Maintaining stable pulse characteristics from laser to laser and over the laser lifetime becomes usually more difficult to achieve as the number of coupled operating parameters increases.
  • a pulsed fiber laser oscillator including a light generating module generating optical pulses at a repetition rate. Each optical pulse has a spectral profile, an amplitude profile and a pulse duration.
  • the oscillator further includes a spectrum tailoring module for tailoring the spectral profile of the optical pulses.
  • the spectrum tailoring module has at least one phase modulator for imposing a time-dependent phase variation on each of the pulses. Synchronizing means are provided for activating the phase modulator in synchronization with the optical pulses.
  • a method for providing high power optical pulses while avoiding the onset of non-linear effects comprising: a) generating seed optical pulses at a repetition rate, each seed optical pulse having a spectral profile, an amplitude profile and a pulse duration; b) broadening the spectral profile of the seed optical pulses, said broadening comprising propagating the seed optical pulses through at least one phase modulator imposing a time-dependent phase variation on each of said pulses, thereby obtaining spectrally broadened optical pulses, said broadening comprising activating the phase modulator in synchronization with the seed optical pulses; and c) amplifying said broadened optical pulses, thereby obtaining said high power optical pulses.
  • FIG. 1 is a schematized representation of a laser system according to an embodiment of the invention.
  • FIG. 2 is a schematic representation of an example of RF frequency spectrum corresponding to a time-dependent phase variation characterized by a plurality of frequencies.
  • FIGs. 3A to 3D are graphs showing the calculated tailored spectra obtained with a single frequency sinusoidal phase variation, for different values of the peak phase deviation.
  • FIGs. 4A and 4B are examples of optical spectral profiles obtained respectively with and without modulation of the frequency of the phase variation.
  • FIG. 5 is a timing diagram exemplifying the operation of components of an oscillator according to embodiments of the invention.
  • FIG. 6 is a graph comparing the peak phase deviation that can be obtained with a given average RF power budget for a synchronously gated sinusoidal phase variation with respect to the case where the phase variation is enabled at all times (CW phase modulation scheme).
  • FIG. 7 is a schematized representation of a laser oscillator according to an embodiment of the invention, using a pulsed seed source.
  • FIG. 8A is a schematized representation of a laser oscillator using a "MOPA” configuration
  • FIG. 8B is a timing diagram for the laser oscillator of FIG. 8A
  • FIG. 9A is a schematized representation of a laser oscillator using a "MOPA” configuration according to another embodiment
  • FIG. 9B is a timing diagram for the laser oscillator of FIG. 9A.
  • FIG. 1OA is a schematized representation of a laser oscillator using a "MOPA" configuration according to yet another embodiment
  • FIG. 1OB is a timing diagram for the laser oscillator of FIG. 1OA.
  • FIG. 11A is a schematized representation of a phase modulator driver according to one embodiment of the invention.
  • FIG. 11B shows the time variation of the various signals transmitted within the phase modulator driver of FIG. 11 A.
  • FIG. 12A is a schematized representation of a phase modulator driver according to another embodiment of the invention.
  • FIG. 12B shows the time variation of the various signals transmitted within the phase modulator driver of FIG. 12A.
  • FIG. 13A is a schematized representation of a phase modulator driver according to another embodiment of the invention.
  • FIG. 13B shows the time variation of the various signals transmitted within the phase modulator driver of FIG. 13A.
  • FIG. 14A is a schematized representation of a phase modulator driver according to another embodiment of the invention.
  • FIG. 14B shows the time variation of the various signals transmitted within the phase modulator driver of FIG. 14A.
  • FIG. 15A to 15C are schematized representations of phase modulator drivers using a plurality of source elements according to embodiments of the invention.
  • FIG. 16A is a schematic representation of a laser oscillator according to an embodiment of the invention, using a double-pass configuration.
  • FIG. 16B is a timing diagram for the embodiment of FIG. 16A.
  • FIG. 17 is a graph showing the experimental comparison of spectral broadening obtained using a single pass vs a double pass configuration.
  • FIG. 18 is a schematic representation of a laser oscillator according to an embodiment of the invention, using a plurality of phase modulators with associated drivers and synchronization means.
  • FIG. 19 is a schematic representation of a laser oscillator according to an embodiment of the invention, using a plurality of phase modulators with associated drivers and synchronization means, in a double-pass configuration.
  • FIG. 20 is a graph showing the SBS threshold dependence on the pulse duration.
  • FIG. 21 is a graph of experimental result showing SBS suppression for a pulsed fiber laser.
  • FIG. 22 is a graph of experimental result showing SBS suppression, where the output pulse energy can be increased by a factor of ten at least when using a synchronously gated phase variation scheme.
  • FIG. 23 is a graph of experimental spectra obtained at different output pulse energy levels.
  • FIG. 24 is a graph of experimental results showing that most of the energy can be maintained in a narrow spectral bandwidth while scaling the output power.
  • FIG. 25A is a graph of calculated spectra showing an example of the impact of SPM for a seed based on filtered ASE;
  • FIG. 25B is a graph of calculated spectra showing an example of the impact of SPM for a seed based on a narrow linewidth laser diode and a synchronously gated phase variation scheme;
  • FIG. 25C shows a superposition of the spectra of FIGs 25A and 25B to compare the impact of SPM for the two associated types of seed sources.
  • FIG. 26 is a graph comparing the effect of SPM on the spectral linewidth of pulsed fiber lasers as a function of the output energy per pulse for a seed source based on filtered ASE with respect to a seed source based on a narrow linewidth, phase modulated laser.
  • FIG. 27 is an example of spectral width control enhancement provided by a synchronously gated phase variation scheme.
  • FIG. 28 is an example of SPM-induced spectral broadening slope as a function of the source linewidth.
  • FIG. 29 is an example of a synchronously gated phase variation signal used to impose a linear frequency chirp along the optical pulse.
  • Embodiments of the present invention generally provide pulse generating methods and pulsed fiber laser oscillators for laser systems adapted for high power applications such as memory repair, milling, micro-fabrication, drilling and other material processing applications. It will be understood that embodiments of the present invention may also be used in other contexts such as remote sensing or any other application which may benefit from high power pulses having good optical characteristics.
  • oscillator is understood to refer to the portion of a laser system which generates light pulses.
  • the oscillator may include a laser cavity or alternatively be based on fluorescent emissions.
  • the oscillator may be part of a larger system including amplifying, beam shaping or any other optical component further defining the properties of the optical pulses generated by the oscillator.
  • the laser oscillators according to embodiments of the present invention are preferably fiber-based, which is understood to mean that light circulating in the oscillator is generally guided by optical fiber. It is however not excluded from the scope of the invention that the oscillator may include components external to optical fibers. In addition, the components of the laser oscillator may be embodied in more than one length of optical fiber, coupled together through known techniques such as fiber pigtails, fused coupling, mechanical couplers and the like.
  • the optical fiber or fibers embodying each components of the laser oscillator may have any appropriate structure. Depending on its function the optical fiber may be single mode or multimode, with a single or multiple cladding. It may be embodied by a standard fiber, a polarisation maintaining (PM) fiber, a microstructured (or “holey”) fiber or any other appropriate specialized type of fiber. It may be made of any suitable materials such as pure silica, doped silica, composite glasses or sapphire.
  • the oscillator 22 first includes a light generating module 24, which generates optical pulses 26.
  • the light generating module 24 determines the characteristics of the optical pulses 26 outputted thereby such as their repetition rate, pulse duration, spectral profile and amplitude profile.
  • the light generating module 24 includes a seed assembly 28 generating the optical pulses 26 and a pulse generator 30 collaborating with the seed assembly 28 to control the pulse characteristics.
  • the laser oscillator 22 further includes a spectrum tailoring module 42 for tailoring the spectral profile of the optical pulses 26 generated by the light generating module 24.
  • the spectrum tailoring module 42 includes a phase modulator 44 which imposes a time-dependent phase variation on each optical pulse 26 therethrough.
  • a phase modulator driver 48 drives the activation of the phase modulator 44 through a phase variation drive signal 50 providing the desired phase variation.
  • the spectrum tailoring module imposes a phase component on the electrical field of the optical pulses which is not constant over the duration of each pulse.
  • the time- dependent phase variation may be periodic, quasi-periodic, linear or have any other appropriate time-dependence.
  • the phase variation drive signal 50 is a RF signal characterised by one or more frequencies in the range of 100 MHz to 100 GHz.
  • FIG. 2 illustrates the general case of an RF spectrum corresponding to a phase variation drive signal having a plurality (in this case a continuum) of frequencies.
  • the phase modulator 44 may be embodied by an electro-optic component based modulator such as well known in the art.
  • the electro-optical material included in the phase modulator can be LiNbO 3 , LiTaO 3 , KNbO 3 or any other appropriate nonlinear material.
  • the phase modulator may be based on an acousto-optical component such as an acousto-optic modulator.
  • the spectrum tailoring module 42 may include more than one phase modulator 44 in a cascade, each applying a phase variation to the optical pulses therethrough so that their combined effect on the phase of the optical pulses results in the desired tailoring of their spectral profile.
  • the spectrum tailoring module 42 may be configured so that the light pulses make more than one pass through one of more phase modulator 44, as will be explained further below with respect to the embodiments of FIGs. 16A and 19.
  • each optical pulse 26 At the input of the phase modulator 44 each optical pulse 26 generally has a spectral profile centered at an optical frequency v with a linewidth ⁇ v.
  • additional spectral components can be added to the pulse spectral profile under certain conditions specific to the characteristics of the pulse and of the phase variation, thereby broadening the pulse spectral profile.
  • ⁇ (t) is the time-dependent phase term that varies when applying the phase variation.
  • this term has the profile:
  • ⁇ (t) ⁇ pe ⁇ k sin(2 ⁇ x t + ⁇ 0 ) (2)
  • ⁇ 0 is the initial phase
  • ⁇ peak the peak phase deviation.
  • the peak phase deviation obtained when applying a peak voltage V peak on the phase modulator 44 is given by:
  • V n is a characteristic of the phase modulator 44.
  • the Fourier decomposition of E(t) with ⁇ (t) given by equation (2) is a well known result of applied mathematics (see for example Bruce Carlson, "Communication systems - An introduction to Signals and Noise in Electrical Communication", McGraw-Hill, New York, 1986, chapter 7).
  • n is an integer
  • the spectral power density associated with a side band of index n is dependent upon the value of ⁇ peak . In general, for n > ⁇ peak , the spectral power density decreases rapidly as n increases.
  • Typical spectral profiles of the optical pulses after tailoring are shown in FIGs. 3A to 3D, for various values of ⁇ peak and a same value of ⁇ . It can be easily understood that by simply choosing appropriate values for both parameters of the phase variation, ⁇ and ⁇ peak , tailoring of the spectral profile of the optical pulses is readily achieved.
  • the resulting spectral profile of the optical pulses may be uniform along the pulse. This may be achieved by ensuring that the spectrum tailoring takes place within a duration significantly shorter than the pulse duration. This will be the case if the condition ⁇ » 1/ ⁇ is satisfied, where ⁇ is the pulse duration. For example, for 10 ns pulses this condition corresponds to ⁇ » 100 MHz, and ideally for such pulses ⁇ is of the order of at least 1 GHz.
  • phase variation frequency may further be advantageous in embodiments where the initial phase of the phase variation with respect to the optical pulse leading edge is not kept fixed or not controlled (parameter ⁇ Q in equation (2)).
  • the optical frequency sweep provided by the phase variation will in general vary differently along the pulse from pulse to pulse, which could lead to a situation where the spectral characteristics of the pulse can change significantly from pulse to pulse in an uncontrolled manner.
  • the phase variation may differ from the simple oscillatory case described above, providing an even greater versatility in the spectrum tailoring capacities of the device.
  • the profile of the phase variation may be adapted in view of the requirements of the application to which the laser oscillator is destined.
  • the phase variation signal ⁇ (t) spectrum can be considered to include a spectrum of n discrete frequencies Q 1t ⁇ 2 , ⁇ 3, ... , Cl n or a continuum of frequencies.
  • the phase variation preferably has a spectrum of frequencies having a lower cut-off frequency ⁇ c significantly larger than 1/ ⁇ .
  • An example of such an RF spectrum is illustrated in FIG. 2.
  • f(t) can be a sinusoid, leading to
  • ⁇ it) ⁇ peak sin(2;r( ⁇ 0 + ⁇ peak sin(2 ⁇ t)) x t + ⁇ 0 ) (4)
  • ⁇ o the central phase variation frequency
  • Cl peak is the maximum deviation of ⁇ with respect to ⁇ o
  • ⁇ the frequency at which the phase modulation frequency is varied.
  • FIG. 4B shows an example of spectral profile which can be obtained with this scheme, compared to the corresponding case where the phase variation frequency is fixed as shown in FIG. 4A.
  • the modulation of the phase variation frequency allows for the distribution of the optical power among many additional spectral components with respect to the fixed phase variation frequency case, which reduces the maximum spectral power density, shown on the vertical axis, for a fixed spectral band.
  • Such a reduction can be advantageously used for mitigating SBS in high power fiber amplifiers.
  • the pulsed laser oscillator 22 further includes means for synchronizing the activation of the phase modulator 44 with the arrival of the optical pulses 26 thereat.
  • This synchronization is an advantageous aspect of the present invention as it allows for using low average power RF amplifiers to drive the phase modulators and limits the thermal stress on the phase modulators, which is beneficial in terms of system cost, complexity and reliability, as will be explained below.
  • the synchronizing means are conceptually represented by the arrow 46 extending between the pulse generator 30 and the phase modulator driver 48 and one skilled in the art will understand that any combination of components and signals allowing the control of the timing of the activation of the phase modulator 44 in relation to the propagation of the optical pulses 26 therethrough could be used without departing from the scope of the present invention.
  • phase modulation 44 is activated to the "on” state while an optical pulse 26 propagates therethrough and is kept in the "off state during inter-pulse period.
  • the expression “synchronously gated phase variation” is used herein to refer to this regime of operation.
  • a conceptual timing diagram is represented in FIG. 5 for a laser operating at a pulse repetition rate PRR.
  • the phase modulation drive signal is enabled synchronously with the optical pulse for a duration approximately equal to the pulse duration ⁇ .
  • the delay ⁇ is in general dependent upon the optical pulse propagation delay between the seed assembly 28 and the phase modulator 44 and also depends upon the electrical propagation delays between the pulse generator 30, the seed assembly 28 and the phase modulator driver 48.
  • the delay ⁇ is adjusted in order to make sure that the phase variation is active while an optical pulse is transmitted through the phase modulator and not active otherwise.
  • the duty cycle is only 0.1 %, which mean that the phase modulator 44 can be advantageously kept in the "off' state for up to 99.9% of the time using this method.
  • the phase modulator is activated synchronously with the optical pulses for a delay that is slightly longer than the pulse duration so as to provide a security margin with respect to the possible variations associated to jitter or to other time-related tolerances.
  • the phase modulator can be driven synchronously with the optical pulses for a duration approximately equal to ⁇ maX) this duration being kept constant for any chosen pulse duration.
  • the duration of the phase variation can be set to be approximately equal to the optical pulse duration over the whole range ⁇ min to ⁇ max , which means that it is adjusted accordingly every time the optical pulse duration is modified.
  • the initial phase of the phase modulation signal with respect to the optical pulse leading edge can be controlled or not. All of these variants and equivalents thereof are considered activations of the phase modulator "in synchronization" with the optical pulses.
  • the synchronously gated phase variation scheme considerably reduces the RF average power level required for efficiently tailoring the pulse spectral profile, as the phase modulation is active for only a small fraction of time with respect to a continuous wave phase modulation scheme (phase modulation active at all times).
  • This much lower RF power consumption is advantageous in terms of cost and reliability.
  • the power dissipated in the phase modulator can be considerably reduced (three orders of magnitude reduction in the example mentioned above), which significantly mitigates the risk of experiencing thermal issues with the device.
  • the required RF power is much lower than it is in the CW regime, much higher peak phase deviations can be obtained with a given amount of average RF power, thereby allowing broader spectra and more efficient SBS suppression.
  • the synchronously gated phase variation scheme is compared with the CW phase modulation scheme in terms of the peak phase deviation (which is proportional to the spectral broadening) that can be achieved as a function of the available RF average power budget.
  • the synchronously gated time-dependent phase variation authorizes much broader spectra when compared with the CW scheme for a given RF power budget. For example, a modest RF power budget of 10 dBm is sufficient to enable a peak phase deviation as large as 11/7 with the synchronously gated regime, whereas it is limited to about 0.4 ⁇ in the CW regime.
  • the tailoring of the spectral profile of the optical pulses may be accomplished by controlling the phase variation parameters from pulse to pulse using high speed electronics embodying the pulse generator, phase modulator driver and other related components.
  • a suitable platform is for example disclosed in U.S. provisional patent application no. 61/076337 by Deladurantaye P. et al., filed on 27 June 2008 and entitled "Embedded digital laser pulse shaping platform and method".
  • the phase variation characteristics as well as the pulse amplitude profile are both dynamically controlled from pulse to pulse, thereby allowing the production of extremely flexible and agile pulsed fiber lasers in both the time domain and the optical frequency domain. Examples of such embodiments are presented below.
  • the pulse seed source 32 may be a semiconductor laser diode of any appropriate configuration such as a Fabry-Perot cavity, a distributed- feedback diode, an external-cavity diode laser (ECDL), etc.
  • the semiconductor diode may be fiber-based and guide light in a single mode in a transverse or longitudinal regime.
  • the pulse generator 30 may for example be embodied by a device or platform apt to generate a pulse drive signal 36 of appropriate characteristics, and is preferably based on high speed electronics. In the embodiment of FIG.
  • the drive signal 36 should be tailored to vary the drive current of the pulsed seed source 32 in order for the optical pulses 26 generated thereby to have the desired shape.
  • the optical characteristics of the optical pulses 26 will depend on a number of factors such as the complex impedance of the pulsed seed source 32, which is itself dependent on the physics of the cavity of this source and on the diode package.
  • FIGs. 8A and 9A show alternative embodiments where the seed assembly 28 includes a continuous light source 36 generating a continuous light beam 38, followed by first and second amplitude modulators 40a, 40b modulating the continuous light beam 38 to provide the optical pulses 26.
  • the pulse generator 30 sends a first drive signal 34a to a first amplitude modulator 40a which is external to the continuous seed light source 36, and the first drive signal 34a controls the opening of the first amplitude modulator 40 to allow light therethrough according to the desired output shape of the optical pulses 26 formed in this manner.
  • a second drive signal 34b is provided to a second amplitude modulator 40b to open the same in complete or partial synchronization with the arrival of the optical pulses 26 thereat, providing a greater resolution and versatility in the characteristics of the resulting pulses.
  • a second amplitude modulator 40b is positioned upstream the spectrum tailoring module 42, and an optical amplifier 52 is placed in between.
  • the amplifier may for example be embodied by a pumped gain medium such as a rare-earth doped length of optical fiber.
  • FIG. 9A the second amplitude modulator 40b is positioned downstream the spectrum tailoring module 42, so that the final temporal shaping of the optical pulses is performed after the spectral tailoring thereof.
  • FIG.. 10A shows an other embodiment, where the spectrum tailoring module 42 is positioned upstream both modulators 40a and 40b.
  • the synchronously gated regime is made possible by driving the spectrum tailoring module 42 first and then synchronizing the two amplitude modulators 40a and 40b in order to generate and shape optical pulses 26 in synchronicity with the time windows for which the continuous light beam phase has been varied by the spectrum tailoring module 42.
  • An optical amplifier 52 may also be provided in those embodiments.
  • FIGs. 8B, 9B and 10B illustrate examples of timing diagrams corresponding to the embodiments of figures 8A, 9A and 10A respectively.
  • either type of seed source whether pulsed 32 or continuous 36 may be tunable in wavelength, according to techniques known in the art.
  • external spectral tuning components such as filters, gratings or the like (not shown) may be provided externally to the seed source.
  • the phase modulator driver preferably includes a source module which generates a source signal and a high speed switching module which selectively transmits this source signal from the source module to the phase modulator.
  • the high speed switching module is activated by the pulse synchronization signal.
  • a variable gain amplifying module is also preferably provided and disposed between the high speed switching module and the phase modulator.
  • the three modules of the phase modulator driver 48 may be embodied by an oscillator 64, a variable gain amplifier 68 and a high-speed switch 66.
  • the source signal provided by the oscillator 64 is an oscillating signal 70 at a fixed frequency which defines the frequency ⁇ of the phase variation drive signal 50.
  • the phase variation peak amplitude can be adjusted by controlling the gain of the amplifier 68, which may advantageously also be activated synchronously with the optical pulses as shown in the timing diagram of FIG. 11 B.
  • this case corresponds to the case of a single peak of frequency ⁇ with variable intensity from pulse to pulse.
  • the high speed switch 66 is preferably activated by the pulse synchronization signal 46 so as to selectively transmit the oscillating signal 70 through the amplifier 68 to the phase modulator 44 only when an optical pulse is present at its input, in order to minimize the RF power consumption of the device as explained above.
  • the pulse to pulse spectrum tailoring capability is exemplified for two consecutive pulses 26A and 26B.
  • both pulses 26A and 26B Before entering the spectrum tailoring module 42, both pulses 26A and 26B have an identical spectral profile (or spectrum) X.
  • the gain of the amplifier 68 may be controlled by a gain signal 72.
  • the gain signal 72 sets the gain of the amplifier 68 to two different values for the time windows corresponding to pulses 26A and 26B, respectively, the gain being set to a lower value for pulse 26B with respect to pulse 26A. Consequently, the phase modulator 44 is driven with a higher phase variation peak amplitude for pulse 26A than for pulse 26B, resulting in a more important peak phase deviation for pulse 26A than for pulse 26B.
  • the spectrum Y obtained for pulse 26A is broader than the spectrum Z of pulse 26B, as the broadening is approximately proportional to the peak phase deviation, as can be seen in FIGs. 3A o 3D.
  • both pulses also have different spectral profiles in general, as also shown in FIGs. 3A through 3D.
  • FIG.12A shows an alternative embodiment of FIG. 11A where the synchronized gain control signal of the RF amplifier has tailored characteristics in the time domain.
  • the amplitude profile of this signal is tailored in a different manner for two consecutive pulses. Consequently, the amplitude of the phase variation can be varied differently along each pulse using this scheme, which corresponds to different pulse spectral profiles having variable spectral linewidths along each pulse.
  • the optical pulse A is characterized by a broader linewidth at the beginning of the pulse and a narrower linewidth at the end of the pulse.
  • optical pulse B has a narrower linewidth at the beginning of the pulse and a broader linewidth at the end of the pulse.
  • various time-dependent spectral profiles can be produced along the optical pulses, and from pulse to pulse.
  • very flexible pulsed fiber oscillators offering both tailored pulse shapes and tailored pulse spectra can be produced.
  • phase modulation driver 48 where the source module is embodied by a noise source 74 generating a noise signal 76 having a variable frequency and defining a more complex phase variation frequency profile, as for example illustrated in FIG. 2.
  • the corresponding timing diagram is shown at FIG. 13B.
  • This embodiment could be useful for SBS suppression or for applications where a broad (or "white") optical spectrum is desirable, for example in some spectroscopic remote sensing applications where one or several chemical species are to be detected in a given spectral bandwidth, each specie having its own set of spectral absorption bands.
  • phase modulator driver architectures can be employed in order to generate different RF spectra, for example by combining several oscillators and/or several RF amplifiers, without departing from the scope of the present invention. Examples of such architectures are presented in FIGs. 15A to 15C.
  • FIG. 14A illustrates an alternative embodiment that can also provide optical pulses with spectral characteristics that vary along the pulses.
  • source module is embodied by a voltage-controlled oscillator (VCO) 80.
  • VCO 80 is a single frequency oscillator whose frequency can be controlled with a voltage over a certain range.
  • An example of VCO is the model ZX95-1200W+ (trademark) from Mini-Circuits of Brooklyn, NY.
  • the VCO frequency is controlled with a tailored control signal 82, in synchronization with the optical pulses.
  • the RF amplifier gain is controlled as in FIG. 12A. Using this configuration, both the frequency and the amplitude of the phase variation can be tailored along each pulse.
  • FIGs. 15A to 15C show alternative embodiments where the source module includes a plurality of source elements such as oscillators 64, having one or possibly different frequencies, VCOs 80 and noise sources 74. Each source element generates a corresponding source signal component. Of course, any appropriate number of source elements may be provided, in any appropriate combination depending on the desired end result.
  • a combiner 84 is preferably provided to combine the source signal components from each source element into the source signal. In the embodiment of FIG. 15A, the combiner positioned directly downstream the source elements, therefore combining their output before the switching module.
  • the switching module includes a plurality of high speed switches, each associated with a corresponding source element, and the combiner is provided between the switching module and the gain amplifying module.
  • FIG. 15C shows a variant where the gain amplifying module also includes individual gain amplifiers in series with a corresponding source element and high speed switch chain, the combiner being provided downstream the gain amplifying module.
  • the gain amplifying module also includes individual gain amplifiers in series with a corresponding source element and high speed switch chain, the combiner being provided downstream the gain amplifying module.
  • the spectrum tailoring module 42 includes a recirculation assembly recirculating the optical pulses 26 through the phase modulator 44 for a plurality of passes.
  • a circulator 54 and a reflective element 56 are used for this purpose.
  • the optical pulses 26 impinge on a first port 58 of the circulator and exit the same through a second port 60 in communication with the phase modulator 44.
  • a first pass of the optical pulses 26 through the phase modulator 44 tailors their spectral profile a first time.
  • the tailored optical pulses are then reflected by the reflective element 56, for example a fiber Bragg grating, located downstream the phase modulator 44 and connected to its output.
  • the reflected pulses 26 then traverse the phase modulator 44 for a second time where their spectral profile is further tailored, re-enter the second port 60 of the circulator 54 and are finally outputted at a third port 62 of the circulator 54.
  • the phase variation drive signal 50 is synchronized with the optical pulses 26 so that the phase modulator 44 is active only while an optical pulse is transmitted through the phase modulator 44 in either direction.
  • FIG. 16B shows an example of corresponding timing diagram.
  • the embodiment of FIG. 16A provides in principle a twofold increase of the spectral broadening factor with respect to single-pass schemes for the same phase variation condition.
  • a 3dB reduction in the RF average power required to drive the phase modulator is possible in principle.
  • the achievable improvement is dependent upon the phase modulator design, the optical path length and other practical limitations. For example, phase shifts occurring during the pulse propagation must be compensated. This can be done by a fine tuning of the phase modulation frequency.
  • FIG. 17 presents experimental results comparing the double pass scheme with respect to the single pass configuration. Although not as important as the theoretically predicted value because of the experimental setup limitations, some improvement in the RF power budget was obtained.
  • FIG. 18 shows an embodiment where a plurality of phase modulators 44 are provided in series, each having an associated phase modulator driver 48. A pulse synchronization signal is transmitted to each phase modulator driver, taking into account the appropriate propagation delays. Each phase modulator may impose a same time-dependent phase variation on the light therethrough, or different phase variation components may be combined to obtain the desired phase at the output of the spectrum tailoring module 42.
  • FIG. 19 shows an alternative embodiment combining some of the features of the embodiments of FIGs. 16A and 18, providing a reflective element 56 and the end of a cascade of phase modulators 44 so that the optical pulses 26 are modulated by each phase modulator twice.
  • high power optical pulses is understood to refer to pulses having a peak power which is sufficient for typical material processing and sensing applications as explained above.
  • a pulse energy of at least 50 ⁇ J is considered “high power”, although this value is given as a general indication and is not considered limitative to the scope of the invention.
  • the methods taught can also be applied to limit the onset and the impact of SBS and SPM in low average power fiber oscillators comprising single mode fibers with mode field diameters of a few microns. As a matter of fact, the methods can be applied as soon as the intensity of the pulses in the fiber core reaches the non linear effects intensity threshold, independent of the fiber mode field diameter.
  • the method first includes generating seed optical pulses, for example with a light generating assembly according to one of the embodiments described above or equivalents thereof.
  • the spectral profile of these seed pulses is then broadened by propagation through at least one phase modulator imposing a time-dependent phase variation on each of these seed pulses.
  • the phase modulator is activated in synchronization with the seed optical pulses therethrough.
  • the optical pulses are amplified, thereby obtaining the high power optical pulses. It will be understood that any of the above described embodiments of the oscillator or equivalents thereof may be used to realize the method generally described herein.
  • SBS Stimulated BriHouin Scattering
  • Embodiments of the method described above may provide a powerful tool for overcoming the SBS limitations affecting other narrow linewidth, high power fiber lasers while not sacrificing the beam quality, the flexibility, the stability and the reliability of the lasers.
  • the SBS suppression efficiency provided by phase modulation in the field of high power pulsed fiber lasers is dependent upon two main factors. The first one is the maximum achievable spectral broadening, which is governed by the frequency spectrum of the phase variation and by the peak phase deviation. The second one is the phase modulation dynamics with respect to the SBS dynamics in the context of pulses having durations of the same order of magnitude than the lifetime of the SBS phonon.
  • a v w et t0 tne Brillouin-gain bandwidth Av B with respect to the maximum BriHouin gain obtained for a very narrow linewidth signal see for example Cotter, D. "Stimulated BriHouin Scattering in Monomode Optical Fiber", J. Opt. Commun. 4 (1983) 1 , 10-19).
  • Av B is of the order of 50-100 MHz in optical fibers at a wavelength of 1 ⁇ m.
  • the SBS threshold is generally dependent upon the relative values of the pulse duration and of the phonon lifetime, as predicted by transient SBS models (see Boyd, R.W., "Nonlinear Optics", Academic Press, 2003, pp.427-428)
  • FIG. 20 shows experimental results demonstrating that the SBS threshold varies with the pulse duration.
  • SBS can be suppressed using a synchronously gated, time-dependent phase variation through the broadening of the spectral profile of the optical pulses.
  • a duration 1/ ⁇ is required to sweep the complete optical frequency span.
  • 21 shows an example of SBS suppression obtained experimentally for a pulse duration of 10 ns.
  • FIG. 22 presents another experimental example of SBS suppression.
  • the LMA fiber amplifier maximum output pulse energy is clearly limited at 12 ⁇ J without a phase variation, as the backward signal intensity increases exponentially beyond this threshold.
  • time-dependent phase variation By applying a synchronously gated, time-dependent phase variation to the seed signal, no such exponential behavior is observed for pulse energy values about ten times higher.
  • the amplified pulse spectral profile remains quite narrow, as can be see in FIG. 23, an important characteristic for applications involving frequency conversion. This is also shown in FIG. 24, where the energy per pulse emitted in a spectral bandwidth of 0.5 nm is plotted against the injected pump power in the LMA fiber pump core.
  • the slight broadening of the spectrum is the result of the onset of SPM, which also means that SBS no longer represents a concern.
  • SPM mitigation is the subject of the next section.
  • SPM Self-Phase Modulation
  • FIG. 25B presents the calculated broadening for a phase- modulated narrow linewidth single longitudinal-mode seed.
  • SPM broadens the spectrum considerably, as the final spectral width is about ten times larger than the initial linewidth.
  • each of the individual spectral lines is slightly broadened by SPM and the energy remains in the initial spectral envelope.
  • FIG. 25C compares the spectra of FIGs 25A and 25B using the same horizontal scale, to emphasize the different magnitudes of the spectral broadening induced by SPM for both types of seeds.
  • FIG. 26 shows a typical example of how SPM affects the spectral width of the output pulses as a function of their energy. In this particular case the pulses are amplified in 2.5m of LMA fiber.
  • a phase modulated source is characterized by a spectral width, which corresponds to the width of the total envelope, and a single peak linewidth, which is the linewidth of the source before the time-dependent phase variation is applied.
  • FIG. 28 A numerical comparison of the spectral broadening slope predicted for a filtered ASE source and for a phase modulated filtered ASE source is presented in FIG. 28 as a function of the source linewidth, considering the same length of the same LMA fiber employed to generate the graph of FIG. 26.
  • the phase modulated ASE source it is important to notice that the linewidth appearing on the horizontal axis of the graph of FIG. 28 corresponds to the spectral width of the source itself, not the spectral width of the envelope generated by the phase modulation. It is clear from those results that the broader the source is, the more it will be affected by SPM through spectral broadening.
  • a phase modulated source can have a significantly broader initial spectral width (see FIG.
  • the spectral bandwidth of the amplified optical pulses remains of the order of 500 pm for output energy levels as high as 150 ⁇ J, as shown in FIG. 26.
  • the second option is superior to the first one as it yields optical pulses with high energy levels and narrow spectral linewidths simultaneously.
  • some embodiments of the present invention can efficiently scale the peak power of high power pulsed fiber lasers and mitigate both SBS and SPM.
  • a seed source having a linewidth that is as narrow as possible such as a very coherent single longitudinal mode semiconductor laser diode. It may also be advantageous to use the phase modulation techniques disclosed with respect to embodiments of the present invention to add optical frequencies so as to broaden the spectral envelope of the seed by a factor that is just sufficient to suppress SBS.
  • the impact of SPM is minimized once SBS is suppressed by the phase modulation because the spectral broadening slope due to SPM, which increases with the initial source linewidth as shown in FIG. 28, is minimized.
  • the pulse to pulse spectral agility of some embodiments of the present invention is particularly well adapted to remote sensing applications such as Differential Absorption LIDAR (DIAL) and range-resolved Tunable Diode Laser Spectroscopy (TDLS). Both techniques are used to measure the concentration of gas or air- suspended particles, and require complex laser sources for illuminating the target, whose absorption or scattering is measured as a function of laser wavelength. DIAL requires a source with two emission wavelengths and measures the differential absorption loss at the two wavelengths due to the gas or particles. The differential measurement allows for higher sensitivity than standard LIDAR. TDLS instead uses a tuneable laser source and measures a spectrum of absorption or scattering by illuminating the target and sweeping the laser wavelength.
  • DIAL Differential Absorption LIDAR
  • TDLS range-resolved Tunable Diode Laser Spectroscopy
  • Embodiments of the present invention can provide high peak power laser sources with very narrow linewidths by choosing a suitable seed. Moreover, the laser output power or pulse energy can be significantly increased with minimum alteration of the spectrum over previously disclosed fibre based laser sources because the SBS threshold is increased and the SPM is controlled very well.
  • Both DIAL and TDLS can be implemented using embodiments of the present invention. For example, in the case of TDLS, a linear frequency chirp along the optical pulse can be imposed and controlled from pulse to pulse, which corresponds to a quadratic increase of the optical phase along the pulse for the phase variation signal, followed by a phase reset between two pulses, as illustrated in FIG.

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Abstract

L'invention porte sur des procédés de génération d'impulsions optiques haute puissance et sur des oscillateurs lasers. Un module de génération de lumière génère des impulsions optiques d'amorçage ayant des caractéristiques optiques prédéterminées. Un module de sélection spectrale est ensuite utilisé pour sélectionner le profil spectral des impulsions optiques. Le module de sélection spectrale comprend un modulateur de phase qui impose une variation de phase dépendant du temps sur les impulsions optiques. L'activation du modulateur de phase est synchronisée avec le passage de l'impulsion optique à travers celui-ci, réduisant ainsi efficacement l'énergie RF nécessaire pour faire fonctionner le dispositif.
PCT/CA2009/000365 2008-11-21 2009-03-20 Oscillateur laser à fibre à impulsions sélectif de façon spectrale WO2010057290A1 (fr)

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US8263903B2 (en) 2010-05-18 2012-09-11 Institut National D'optique Method for stablizing an output of a pulsed laser system using pulse shaping
CN101950913A (zh) * 2010-08-12 2011-01-19 上海拜安实业有限公司 基于脉冲种子源放大的光纤激光光源及光纤传感系统
CN104201550A (zh) * 2014-08-27 2014-12-10 广东高聚激光有限公司 一种皮秒脉冲光纤激光器及其脉冲生成方法
US9871339B1 (en) 2016-12-15 2018-01-16 Raytheon Company Laser transmitter for generating a coherent laser output signal with reduced self-phase modulation and method
WO2018111351A1 (fr) * 2016-12-15 2018-06-21 Raytheon Company Émetteur laser pour générer un signal de sortie laser cohérent à automodulation de phase réduite et procédé
US10186828B2 (en) 2016-12-15 2019-01-22 Raytheon Company Laser transmitter for generating a coherent laser output signal with reduced self-phase modulation and method
IL265828B (en) * 2016-12-15 2022-10-01 Raytheon Co A laser transmitter for generating a coherent laser output signal with reduced self-phase modulation and a method
IL265828B2 (en) * 2016-12-15 2023-02-01 Raytheon Co A laser transmitter for generating a coherent laser output signal with reduced self-phase modulation and a method

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CA2743648A1 (fr) 2010-05-27
US20100128744A1 (en) 2010-05-27
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US8798107B2 (en) 2014-08-05
CA2743648C (fr) 2014-11-04
CA2743373C (fr) 2016-05-03
US7974319B2 (en) 2011-07-05
US20100135347A1 (en) 2010-06-03

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