超短脉冲强测量.pptx
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1、In order to measure an event in time,you need a shorter one.To study this event,you need a strobe light pulse thats shorter.But then,to measure the strobe light pulse,you need a detector whose response time is even shorter.And so onSo,now,how do you measure the shortest event?Photograph taken by Har
2、old Edgerton,MITThe Dilemma第1页/共44页Ultrashort laser pulses are the shortest technological events ever created by humans.Its routine to generate pulses shorter than 10-13 seconds in duration,and researchers have generated pulses only a few fs(10-15 s)long.Such pulses have many applications in physics
3、,chemistry,biology,and engineering.You can measure any eventas long as youve got a pulse thats shorter.So how do you measure the pulse itself?You must use the pulse to measure itself.But that isnt good enough.Its only as short as the pulse.Its not shorter.Techniques based on using the pulse to measu
4、re itself have not sufficed.第2页/共44页To determine the temporal resolution of an experiment using it.To determine whether it can be made even shorter.To better understand the lasers that emit them and to verify models of ultrashort pulse generation.To better study media:the better we know the light in
5、 and light out,the better we know the medium we study with them.To use pulses of specific intensity and phase vs.time to control chemical reactions:“Coherent control.”To understand pulse-shaping efforts for telecommunications,etc.Because its there.Why measure an ultrashort laser pulse?As a molecule
6、dissociates,its emission changes color(i.e.,the phase changes),revealing much about themolecular dynamics,not avail-able from the mere spectrum,or even the intensity vs.time.Excitation to excited stateEmissionGround stateExcited state第3页/共44页ExptTheoryLinearLinear or nonlinearmediumMeasuring the int
7、ensity and phase of the pulses into and out of a medium tells us as much as possible about the linear and nonlinear effects in the medium.Studying Media by Measuring the Intensity and Phase of Light PulsesWith a linear medium,we learn the mediums absorption coefficient and refractive index vs.With a
8、 nonlinear-optical medium,we can learn about self-phase modulation,for example,for which the theory is much more complex.Indeed,theoretical models can be tested.Time(fs)IntensityPhaseNonlinearEaton,et al.,JQE 35,451(1999).Time(fs)IntensityPhase第4页/共44页A laser pulse has the time-domain electric field
9、:EI(t)1/2 exp i0t i(t)IntensityPhase(t)=Re Equivalently,vs.frequency:exp -ij j(0)Spectral Phase(neglecting thenegative-frequencycomponent)E()=Re S()1/2We must measure an ultrashort laser pulsesintensity and phase vs.time or frequency.SpectrumKnowledge of the intensity and phase or the spectrum and s
10、pectral phaseis sufficient to determine the pulse.第5页/共44页t d dtThe instantaneous frequency:Example:“Linear chirp”Phase,(t)timetimeFrequency,(t)timeWed like to be able to measure,not only linearly chirped pulses,but also pulses with arbitrarily complex phases and frequencies vs.time.The phase determ
11、ines the pulses frequency(i.e.,color)vs.time.第6页/共44页The spectrometer measures the spectrum,of course.Wavelength variesacross the camera,and the spectrum can be measured for a single pulse.Pulse Measurement in the Frequency Domain:The SpectrometerCollimating Mirror“Czerny-Turner”arrangementEntrance
12、SlitCamera orLinear Detector ArrayFocusingMirrorGrating“Imaging spectrometers”allow many spectra to be measured simultaneously.Broad-bandpulse第7页/共44页Pulse Measurement in the Time Domain:DetectorsExamples:Photo-diodes,Photo-multipliersDetectors are devices that emit electrons in response to photons.
13、Detectors have very slow rise and fall times:1 nanosecond.As far as were concerned,detectors have infinitely slow responses.They measure the time integral of the pulse intensity from to+:The detector output voltage is proportional to the pulse energy.By themselves,detectors tell us little about a pu
14、lse.Another symbolfor a detector:DetectorDetector第8页/共44页Pulse Measurement in the Time Domain:Varying the pulse delaySince detectors are essentially infinitely slow,how do we make time-domain measurements on or using ultrashort laser pulses?Well delay a pulse in time.And how will we do that?By simpl
15、y moving a mirror!Since light travels 300 m per ps,300 m of mirror displacement yields a delay of 2 ps.This is very convenient.Moving a mirror backward by a distance L yields a delay of:Do not forget the factor of 2!Light must travel the extra distance to the mirrorand back!Translation stageInput pu
16、lse E(t)E(tt)MirrorOutput pulse第9页/共44页We can also vary the delay using a mirror pair or corner cube.Mirror pairs involve tworeflections and displace the return beam in space:But out-of-plane tilt yieldsa nonparallel return beam.Corner cubes involve three reflections and also displace the return bea
17、m in space.Even better,they always yield a parallel return beam:“Hollow corner cubes”avoid propagation through glass.Translation stageInputpulseE(t)E(tt)MirrorsOutput pulseEdmundScientific第10页/共44页Measuring the interferogram is equivalent to measuring the spectrum.Pulse Measurement in the Time Domai
18、n:The Michelson Interferometer Pulse energy (boring)Field autocorrelation(maybe interesting,but)The FT of the field autocorrelation is just the spectrum!Beam-splitterInputpulseDelaySlow detectorMirrorMirrorE(t)E(tt)第11页/共44页Okay,so how do we measure a pulse?V.Wong&I.A.Walmsley,Opt.Lett.19,287-289(19
19、94)I.A.Walmsley&V.Wong,J.Opt.Soc.Am B,13,2453-2463(1996)Result:Using only time-independent,linear filters,complete characterization of a pulse is NOT possible with a slow detector.Translation:If you dont have a detector or modulator that is fast compared to the pulse width,you CANNOT measure the pul
20、se intensity and phase.with only linear measurements,such as a detector,interferometer,or a spectrometer.We need a shorter event,and we dont have one.But we do have the pulse itself,which is a start.And we can devise methods for the pulse to gate itself using optical nonlinearities.第12页/共44页Pulse Me
21、asurement in the Time Domain:The Intensity AutocorrelatorCrossing beams in an SHG crystal,varying the delay between them,and measuring the second-harmonic(SH)pulse energy vs.delay yields the Intensity Autocorrelation:The Intensity Autocorrelation:DelayBeam-splitterInputpulseAperture eliminates input
22、 pulsesand also any SH created by the individual input beams.Slow detectorMirrorE(t)E(tt)MirrorsSHGcrystalLens第13页/共44页Single-Shot AutocorrelationWhile this effect introduces a range of delays on any given pulse andcould cause a broadening of the trace in multi-shot measurements,it allows us to meas
23、ure a pulse on a single laser shot if we use a largebeam and a large beam angle to achieve the desired range of delays.第14页/共44页Single-Shot Autocorrelationc2第15页/共44页Single-Shot AutocorrelationInput pulse(expanded in space to 1 cm)Beam-splitterSHGcrystalCameraE(t)E(tt)Cylindrical lens focuses the be
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