45 results about "Self-phase modulation" patented technology

The invention discloses an ultrafast laser generating device. The device comprises an ytterbium-doped femtosecond laser light source, a first focusing lens, a first optical window, a first-stage energy transfer device, a first-stage hollow-core capillary optical fiber, a second optical window, a concave reflecting mirror, a half-wave plate, a wire grid polarizing film, a second focusing lens, a third optical window, a first air chamber, a second-stage hollow-core anti-resonance optical fiber, a second air chamber and a fourth optical window. The invention discloses a 90-degree off-axis parabolic mirror and a frequency resolution optical switch pulse measuring device. The incident pulse spectrum is broadened by utilizing a self-phase modulation effect by changing the type and the air pressure of inert gas in the first-stage hollow-core capillary optical fiber; and then the second-stage hollow-core anti-resonance optical fiber is filled with inert gas to generate an air pressure gradient, and sub-20fs-order ultrafast laser flexible transmission is finally realized by utilizing a soliton self-compression effect.

The invention provides a femtosecond pulse width precision adjustment compensation device and method, and relates to the technical field of laser processing, the device comprises a mode locking seed source, a stretcher, an amplifier, a first compressor and a second compressor; the second compressor comprises a movable reflecting mirror, a climbing mirror, a grating and a grating base material; the movable reflecting mirror is configured to adjust the diffraction times of the femtosecond laser on the grating by changing the distance between the movable reflecting mirror and the grating base material; the grating base material is made of a nonlinear material and is used for generating a self-phase modulation effect under the action of femtosecond laser to realize spectrum broadening and pulse compression; precise adjustment compensation of the femtosecond pulse width is achieved by adjusting the diffraction times. The invention provides a technical scheme which not only can bear high-power and high-energy laser, but also can realize on-line precise adjustment and compensation of pulse width.

The utility model discloses a femtosecond laser pulse compression device. The femtosecond laser pulse compression device comprises an amplifier, a first half-wave plate, a compressor, a second half-wave plate, a plano-convex lens and a solid sheet group which are sequentially arranged along a light path, laser pulses output by the amplifier enter the compressor after the polarization state of the laser pulses is adjusted by the first half-wave plate, the compressor is used for pre-compensating negative dispersion, the laser pulses output by the compressor enter the plano-convex lens after the polarization state of the laser pulses is adjusted by the second half-wave plate, and the laser pulses are focused by the plano-convex lens, enter the solid sheet group, are compressed and then are output; wherein the laser pulse generates a self-phase modulation effect in the solid sheet group, and the pre-compensation negative dispersion can counteract the positive dispersion generated by the self-phase modulation effect. According to the technical scheme, a chirp mirror does not need to be arranged, the cost can be reduced, and system operation is stabilized.

The invention relates to the technical field of super-continuum spectrum generation, and particularly provides a cross-octave super-continuum spectrum light source generation device and optical equipment, the cross-octave super-continuum spectrum light source generation device comprises a focusing lens and at least one group of solid structures, and ultrashort pulse fundamental frequency light enters the solid structures after being tightly focused; incident fundamental frequency light is firstly subjected to self-phase modulation in a first nonlinear action medium, a first super-continuum spectrum with a certain spectral width at the central wavelength of the fundamental frequency light is generated under the cross-phase modulation action, and except for the central wavelength, the first super-continuum spectrum is transmitted through a frequency doubling crystal; the spectrum at the central wavelength passes through a frequency doubling crystal to realize ultrashort pulse frequency doubling light, the frequency doubling light with high peak intensity generates a self-phase modulation effect in a second nonlinear action medium to generate a second super-continuum spectrum at the central wavelength of the frequency doubling light, and the two super-continuum spectrums form a cross-octave super-continuum spectrum light source; the super-continuum spectrum generation with high pulse energy is realized, the super-continuum spectrum generation range is further widened, and the multi-spectrum super-continuum spectrum generation is realized.

The invention relates to the technical field of ultrafast laser, and discloses a negative chirp management method and device of a laser and a medium. The method comprises the following steps: pre-processing a seed pulse, outputting the seed pulse through a mode-locked optical fiber oscillator, and primarily amplifying the seed pulse through a single-mode gain optical fiber; temperature tuning negative chirp introduction is carried out, a chirp fiber grating is packaged in a temperature tuning device, and the temperature of a semiconductor chilling plate is linearly increased within a preset range, so that the second-order dispersion amount of the chirp fiber grating is changed, and controllable negative chirp is introduced into a pulse; carrying out nonlinear amplification and spectrum broadening, inputting the pulse introduced with the negative chirp into a multimode gain fiber, injecting pumping energy into the multimode gain fiber, and broadening the pulse spectrum width to be greater than a preset width by utilizing the synergistic effect of self-phase modulation, dispersion and gain; pulse width compression is carried out, and the pulse after spectrum broadening is adjusted through a transmission-type grating pair compressor, so that the ultra-narrow pulse width is obtained.

A high-performance single-shot contrast measurement device with real-time self-verification of accuracy is characterized in that the sampling light of the single-shot contrast measurement device is obtained based on spectral broadening and spectral filtering in a self-phase modulation process, so that the sampling light has high contrast, high energy, and a large difference from the central wavelength of the incident light, and good stability; at the same time, the sampling light introduces a series of intensities through a pulse replication unit to be attenuated in sequence, and is offset at equal intervals in position. The sampling light is also evenly spaced in time for self-verification of the measurement accuracy of the measurement device. This invention realizes the characteristics of single-shot contrast measurement with high measurement dynamic range, wide time window, high time resolution, high measurement fidelity, simple and compact device, and real-time verification of the accuracy of measurement results, providing better support for improving the contrast of ultra-strong and ultra-short laser pulses and related laser-matter interaction experimental research.

This invention provides a semiconductor mode-locked laser and its fabrication method, relating to the field of optoelectronics. It addresses the problem in existing technologies where femtosecond pulses are difficult to output due to self-phase modulation and dispersion effects, thereby avoiding the use of external pulse compression technology and reducing production costs. The semiconductor mode-locked laser includes a mirror region, a gain region, and a saturable absorption region. The gain region is located between the mirror region and the saturable absorption region and is connected to both regions. The semiconductor mode-locked laser also includes a substrate, and a chirped grating layer and an active layer disposed on the same side of the substrate. The chirped grating layer is located in the mirror region and includes multiple gratings with a grating period linearly decreasing along the emission direction of the semiconductor mode-locked laser. The active layer is located in the gain region and the saturable absorption region, and the distance between the chirped grating layer and the substrate is greater than the distance between the active layer and the substrate.

The invention discloses a D-type concave mirror multi-pass hundred-femtosecond laser pulse compression system and method, and belongs to the technical field of ultrafast laser, and the system comprises a mode matching module which focuses a light beam into a cavity of a spectrum broadening module, the spectrum broadening module and a chirp compensation module. The spectrum broadening module comprises a cavity filled with inert gas, a D-shaped concave mirror is fixed at each of the two ends in the cavity, the two concave mirrors are oppositely arranged and are used for realizing multi-pass focusing of light beams, and reflectors for leading the light beams into or out of the D-shaped concave mirrors are arranged at the straight edges of the two concave mirrors; inert gas is used as a nonlinear broadening medium, femtosecond laser pulses are spread back and forth between the two D-shaped concave mirrors and are focused, a self-phase modulation effect is generated through the inert gas argon, and the spectrum is broadened. The pulse energy and quality can be improved while wide spectrum broadening is achieved, and shorter femtosecond pulses are obtained.

The application discloses a ytterbium-doped fiber mode-locked laser, which comprises a first pump source, a first regenerator, a second pump source and a second regenerator. The first regenerator comprises a first ytterbium-doped fiber and a pulse stretching device, and the second regenerator comprises an isolator, a pulse compression device, a spectrum stretching fiber and a band-pass filter. The design of the staggered band-pass filter is realized through the two regenerators, and the spectrum is widened due to the self-phase modulation effect; in addition, the energy input of the first pump source and the second pump source can realize the mode-locked pulse output. The application reduces the peak power by pulse stretching to reduce the nonlinear effect, realizes the generation of high-energy output pulses in the fiber mode-locked laser, and the interval between the pulse stretching device and the central wavelength of the band-pass filter can effectively prevent the noise and continuous component from oscillating, and improve the pulse quality.

The application discloses a kind of ultrafast laser generation device.The device includes Yb-doped femtosecond laser light source, first focusing lens, first optical window, first energy transfer device, first level hollow capillary fiber, second optical window, concave mirror, half-wave plate, wire grid polarizer, second focusing lens, third optical window, first gas chamber, second level hollow anti-resonant fiber, second gas chamber, fourth optical window, 90° off-axis parabolic mirror, frequency-resolved optical gating pulse measurement device.By changing the inert gas type and air pressure in the first level hollow capillary fiber, the incident pulse spectrum is broadened using self-phase modulation effect;Then, the second level hollow anti-resonant fiber is filled with inert gas to generate air pressure gradient, and finally realizes flexible transmission of sub-20 fs ultrafast laser using soliton self-compression effect.

A device for the generation of ultrashort pulses, wherein an oscillator is formed by: a first and a second non-overlapping transmission band-pass filter, which can serve as reflecting end element of the oscillator; optically transparent means with non-linear Kerr coefficient χ(3) different from zero configured to achieve a spectral broadening by self-phase modulation of the signal transiting through these means; an optical waveguide that produces a positive gain; a node configured to receive a trigger signal designed to activate the operation of the oscillator; a trigger signal generating device comprising: a laser source, for example a microchip, configured to generate a laser pulse, preferably with a minimum bandwidth, having a duration of hundreds of ps, up to the ns; a coupling system designed to introduce the pulse of the trigger laser into a waveguide made of an optically transparent material characterised by a non-linear Kerr coefficient χ(3) different from zero, which is configured to produce two distinct effects in order to spectrally broaden the pulse of the trigger laser, and precisely: a) self-phase modulation four-wave mixing; the output of the waveguide supplies the trigger signal to the node. The pulses produced by the oscillator typically have a duration of the order of the picosecond and are easily reduced to the Fourier limit of circa 100 femtoseconds by means of a dispersive device.

The purpose of the present invention is to provide a reflective mask blank having an absorber film that has a low refractive index n for EUV light, a high extinction coefficient k for EUV light, and excellent SPM resistance. An absorber film (18) of a reflective mask blank (10) contains palladium and at least one element X selected from the group consisting of ruthenium, iridium, and platinum. The total content of palladium and the element X in the absorber film (18) is 80 at% or more with respect to the total metal atoms contained in the absorber film (18). When the element X is only ruthenium, the content of the element X in the absorber film (18) is 20-75 at% with respect to the total atoms in the absorber film (18). When the element X contains at least one of iridium or platinum, the content of the element X in the absorber film (18) is 30 at% or more with respect to the total atoms in the absorber film (18).

The invention provides a twin beam generation method based on self-phase modulation and an application method thereof. The twin beam generation method comprises the following steps: acquiring an ultrafast laser pulse; the ultrafast laser pulse is controlled to be propagated in the optical Kerr medium, the ultrafast laser pulse interacts with the optical Kerr medium during propagation to generate self-phase modulation, two symmetric sidebands are generated, and the symmetric center of the two sidebands is a center frequency region of the ultrafast laser pulse; the two sidebands are taken out from the ultrafast laser pulse at the same time, and two twin beams with ideal quantum correlation characteristics are obtained. The ultrafast laser pulse generates self-phase modulation in the optical fiber or interacts with the optical Kerr material in a free space to generate self-phase modulation, and the method for taking out the twin beams with the quantum correlation characteristic also belongs to the category of the invention. The method can generate high-power and high-repetition-frequency ultrafast twin beams, simplifies the generation process of the twin beams, and is high in robustness, low in cost and easy to implement.

It is provided a method for generating a supercontinuum, the method comprising the following steps: a) radiating a carrier laser pulse having a first temporal width onto a first non-linear material; b) at the same time, radiating a second shorter laser pulse having a second temporal width onto the first non-linear material, thereby changing the non-linear properties of the first non-linear material and imprinting a ghost pulse having a third temporal width into the carrier pulse; the second temporal width being at least two times shorter than the first temporal width, and c) radiating the carrier pulse with imprinted ghost pulse onto the first non-linear material or a second non-linear material and generating, by self-phase modulating, a supercontinuum around the center frequency of the carrier pulse.

The invention provides a coherent anti-Stokes Raman microscope system, a driving method thereof and a microscopic device. The coherent anti-Stokes Raman microscope system comprises a femtosecond laser front-end module, a dual-wavelength module, a pulse broadening amplification module, a time delay module, a spectrum broadening filtering module, a double frequency generation module, a dispersion compensation module and a microscope module. According to the coherent anti-Stokes Raman microscope system driven by self-phase modulation spectrum selection, a self-phase modulation spectrum selection (SESS) method is adopted to convert the pulse wavelength of 1550nm to 1600nm, a second harmonic generation process is realized through a frequency doubling crystal, the pulse wavelength of 1600nm is converted to 800nm, coherent anti-Stokes Raman microscopic imaging is realized, and the coherent anti-Stokes Raman microscope system driven by self-phase modulation spectrum selection has the advantages that the coherent anti-Stokes Raman microscope system is simple in structure and convenient to operate. Therefore, selective imaging of various biological tissue components is realized. Compared with other wavelength conversion methods, the device is simple, the conversion efficiency is high, the wavelength is tunable, and the expansibility is high.

This invention discloses a tunable saddle-shaped spectrum generation device and method, belonging to the field of precision machining. It includes: a pump source, a resonant cavity with overall positive dispersion, a mode-locking device, a dispersion management device, and a spectral adjustment component. The pump source excites a gain fiber to generate laser light. The mode-locking device outputs a mode-locked pulse, and the dispersion management device provides positive dispersion. Working in conjunction with the nonlinear effect of self-phase modulation within the cavity, the mode-locked pulse spectrum is directly shaped into a saddle-shaped spectrum with a central depression. The spectral adjustment component achieves flexible tuning of the center wavelength and bandwidth of the output spectrum by adjusting the relative positions of the diffraction grating and the fiber collimator. This invention actively generates a tunable saddle-shaped spectrum from the source, eliminating the need for complex modulation elements in the amplification link. It allows for precise pre-compensation of the gain characteristics of subsequent amplifiers, effectively suppressing gain narrowing effects. It has the advantages of compact structure, low cost, high stability, and high energy utilization.

The application provides a high-power narrow-pulse-width laser and a regulation method thereof, the laser comprising a seed source, a first pulse-width regulation grating, a gain fiber, a spectrum shaping system and an amplifier; the first pulse-width regulation grating is arranged behind the seed source and is used for introducing dispersion in initial signal light; the gain fiber is used for amplifying the optical signal to realize spectrum broadening by using the self-phase modulation effect of the optical signal; the spectrum shaping system is arranged behind the gain fiber and is used for performing amplitude modulation and phase modulation on the broadened optical signal; and the amplifier is arranged behind the spectrum shaping system and is used for performing energy amplification on the shaped optical signal to obtain high-power narrow-pulse-width laser. By introducing dispersion in the initial signal light, the self-phase modulation effect of the gain fiber is used to realize spectrum broadening, and the amplitude and phase modulation of the spectrum shaping system on the broadened optical signal effectively compensates the gain narrowing and nonlinear phase accumulation in the amplification process.

The invention relates to the technical field of ultrafast laser, and discloses a pumping intensity optimization method of a laser, equipment and a medium. The method comprises the following steps: carrying out graded pumping design, and pre-amplifying signal light output by a mode-locked fiber oscillator by adopting a single-mode laser diode; the method comprises the following steps: establishing a mathematical model of pumping power and gain fiber length, and when the pumping power is increased from low power to preset power, adjusting the chirp amount of a chirp fiber grating through a temperature tuning device, and performing pre-chirp management on signal light input into a multimode gain fiber; under high pumping intensity, the initial chirp amount of signal light is optimized, so that the signal light entering the multimode gain optical fiber is in a negative chirped pulse state, the self-phase modulation and dispersion effect of the signal light when the signal light is amplified in the multimode gain optical fiber is balanced, and the stimulated Raman scattering and self-focusing phenomena are avoided; and the compressed pulse width is used as a feedback signal to realize the optimal matching of the signal light energy and the pulse width.

This invention discloses a multi-channel broadband chaotic source and random number generation method based on self-phase modulation feedback. Specifically, the chaotic signal undergoes self-phase modulation to generate a large number of new frequency components, which interact with the dispersion of a chirped fiber grating (FBG) and are then fed back into a semiconductor laser. Detuning the chirped fiber grating's central wavelength shifts the reflection spectrum, redistributing the chaotic signal's energy. This invention can generate a multi-channel chaotic signal source and, if the experimental equipment bandwidth is sufficiently large, produce ultra-wideband chaotic signals. It can also hide delay characteristics within background noise, effectively increasing the complexity of the chaotic signal.

The present invention discloses a method for generating a tunable laser in the visible to deep ultraviolet band, comprising: providing a laser with a center frequency of ω p , wavelength is λ p The near-infrared ultrafast laser is used as the pump light source, and the pump light intensity exceeds the self-phase modulation threshold; the pump light is focused and incident on the nonlinear optical crystal to stimulate the self-phase modulation effect in the crystal, generating a center frequency of ω p The method involves adjusting the position or orientation of the nonlinear optical crystal so that the pump light and the broadened spectrum generated by self-phase modulation meet the phase matching conditions of four-wave mixing (FWM) in the NLO crystal, resulting in laser output from the visible to deep ultraviolet range. By continuously changing the phase matching conditions of the NLO crystal, the output laser frequency can be continuously tuned from the visible to deep ultraviolet range. This method overcomes the current problem of four-wave mixing technology that prevents wide-range frequency tuning of the output light and has potential applications in ultra-fine machining, UV lithography, air pollution monitoring, microsurgery, and other fields.

The invention relates to a laser communication transmitter system, a communication method, a lunar laser communication ground terminal, a communication system, a computer readable storage medium and a computer program product. The laser communication transmitter system comprises a transmitting unit and a free space light transmitting mirror; a hollow-core optical fiber jumper wire is adopted between the transmitting unit and the free space light transmitting mirror; and an optical signal generated by the transmitting unit is transmitted to the lunar laser communication satellite-borne terminal through a signal uplink via the hollow-core optical fiber patch cord and the free space light transmitting mirror. According to the mode, the laser communication transmitter system based on the hollow-core optical fiber patch cord is adopted, the influence of the nonlinear effect of the optical fiber on optical signals is remarkably reduced, and the quality and the stability of signal transmission are improved. By using the hollow-core optical fiber, spectral distortion caused by self-phase modulation and a four-wave mixing effect is effectively reduced, and the risk that beacon light is buried by noise is reduced, so that the signal detection and demodulation performance of a receiving end is improved.

The invention relates to a method and an arrangement for the nonlinear spectral broadening of temporally incoherent or partially coherent optical pulses of a pulsed laser beam, which have a pulse duration not limited by bandwidth. In one embodiment of the method, the pulsed laser beam is split into two partial beams, which are spectrally shifted relative to each other by a frequency difference Δf. The spectrally shifted partial beams are then at least partially superimposed such that they interfere in the time domain during or after superposition and form at least one modulated pulsed laser beam, the pulses of which exhibit a temporal modulation of intensity with a period Δt = 1 / Δf.The pulses of the modulated pulsed laser beam are then spectrally broadened by self- or cross-phase modulation in an optically nonlinear medium, achieving a large spectral broadening through this modulation. This method and setup are particularly advantageous for IFE driver lasers.

The invention relates to a device (10) for spectral broadening of a pulsed laser beam (12) by means of self-phase modulation with features of claim 1 and a laser arrangement (32) for spectral broadening of a pulsed laser beam (12) by means of self-phase modulation with features of the dependent claim.

The invention discloses an optical isolator based on nonlinear absorption and self-phase modulation, which sequentially comprises a first aperture diaphragm, a first lens, a nonlinear medium, a second lens and a second aperture diaphragm along the forward propagation direction of an optical path, and is characterized in that the focal length of the first lens is greater than that of the second lens; the focus of the first lens coincides with the focus of the second lens, and the nonlinear medium is arranged between the coincided focus and the second lens. The embodiment of the invention can reduce the insertion loss, improves the isolation of the reverse optical signal, is simple in structure, and can be widely applied to the technical field of optical devices.

The invention discloses a method for generating ultraviolet band tunable laser, which comprises the following steps of: providing near-infrared ultrafast laser with the central wavelength of lambda p as a pump light source, and enabling the pump light intensity of the pump light source to exceed a self-phase modulation threshold value; the pump light is focused and then enters a nonlinear optical crystal, and the pump light generates self-phase modulation light lambda s through a self-phase modulation effect; the nonlinear optical crystal is non-centrosymmetric; the self-phase modulation light lambda s and the pump light lambda p are subjected to five-wave mixing according to phase matching conditions, and ultraviolet laser with the output frequency being omega o and the corresponding wavelength being lambda o is generated; the space direction of the nonlinear optical crystal is adjusted to change the phase matching condition of five-wave mixing, and then continuous tuning of the output light frequency omega o is achieved. The method has the remarkable advantages of being short in wavelength, high in intensity and narrow in line width, the overall performance of deep ultraviolet laser output is improved, the special application requirements of multiple short-wavelength ultraviolet lasers can be met, and the defects of the prior art are overcome.

The invention relates to a device (10) for spectral broadening of a pulsed laser beam (12) by means of self-phase modulation, having the features of claim 1, and to a laser arrangement (32) for spectral broadening of a pulsed laser beam (12) by means of self-phase modulation, having the features of the additional independent claim.

The present invention proposes a method for optimizing the design of doped optical fibers, including the steps of core size, NA size design, output power threshold estimation, refractive index distribution refinement design, and optical fiber comprehensive performance evaluation. The present invention can simultaneously consider the influence of basic optical fiber parameters and optical fiber refractive index distribution on the performance of doped optical fibers in the optical fiber design link. By introducing the finite element simulation method, the effective mode field area, high-order mode loss and other characteristic indicators of the optical fiber can be accurately analyzed, and finally a good doped optical fiber design is obtained. The present invention has versatility, and the present invention can simultaneously take into account the threshold analysis of other nonlinear effects that limit the increase in optical fiber laser power, such as stimulated Raman scattering effect, four-wave mixing effect, self-phase modulation effect, thermal lens effect, etc. The present invention can provide a direct technical path for the efficient design and performance evaluation of high-performance doped optical fibers, and can serve the performance optimization design and efficient research and development of high-power doped optical fibers.

The application provides a multi-pass cell structure light pulse spectrum shape consistency regulation compression method and system. The method makes the structure light pulse multiple round trip propagation in the multi-pass cell and realizes the spectrum broadening dominated by self-phase modulation through the nonlinear medium, carries out spatial-spectrum interference measurement on the measured light after the spectrum broadening of the multi-pass cell and the dispersion compensation, restores the wavelength resolution spatial complex light field data, and establishes the evaluation index for representing the spectrum shape spatial consistency with the spectrum overlap index V. According to the evaluation index, the input structure light parameters, the multi-pass cell propagation parameters and the dispersion compensation parameters are feedback adjusted, so that the quantitative evaluation and optimization control of the structure light pulse spectrum broadening process and the compression output quality are realized, and the output pulse with the target spectrum width, the higher spectrum shape spatial consistency, the good compressibility and the higher peak power is obtained.

The invention belongs to the technical field of optical time domain reflectometer, and particularly relates to a phase-sensitive optical time domain reflectometer performance optimization method based on self-phase modulation. The method comprises the following steps: when a self-phase modulation effect occurs due to the fact that a high-power pulse reaches a nonlinear threshold value, or the pulse power of an input optical fiber is adjusted to exceed the nonlinear threshold value, the spectral range of a detected scattered signal can be expanded; based on short-time Fourier transform, extracting the characteristic that the frequency spectrum of the signal changes along with the transmission distance, designing a short-time segmentation filter adaptive to the change of the transmission distance, and carrying out phase shift transformation processing on the signal; processing and synthesizing a multi-frequency component caused by the self-phase modulation effect by using an analysis and synthesis algorithm to generate a complex analysis signal matrix; through IQ demodulation, phase difference calculation and unwrapping processes, high-precision demodulation phase information is obtained. According to the invention, the sensing distance can be obviously prolonged, the coherent fading effect is effectively inhibited, and the detection sensitivity and stability of the system are improved.

To provide a fiber amplifier that provides frequency modulation and amplitude modulation of a seed beam to increase beam power and reduce a beam width.SOLUTION: A fiber amplifier system includes: an optical source providing an optical seed beam; and a FM electro-optic modulator (EOM) that frequency-modulates the seed beam to broaden its spectral linewidth. The system also includes an AM EOM that amplitude-modulates the seed beam to provide an amplitude-modulated seed beam that is synchronized with the frequency-modulated seed beam. The system also includes a non-linear fiber amplifier receiving the AM and FM modulated seed beam, where the amplitude-modulated seed beam causes self-phase modulation in the fiber amplifier that phase-modulates the seed beam as it is being amplified by the fiber amplifier that acts to cancel the spectral linewidth broadening caused by the frequency modulation.SELECTED DRAWING: Figure 1