Use of Fringe-Resolved Autocorrelation for the Diagnosis ... · PDF fileUse of Fringe-Resolved...

Click here to load reader

  • date post

    21-Apr-2018
  • Category

    Documents

  • view

    221
  • download

    4

Embed Size (px)

Transcript of Use of Fringe-Resolved Autocorrelation for the Diagnosis ... · PDF fileUse of Fringe-Resolved...

  • Use of Fringe-Resolved Autocorrelation for

    the Diagnosis of the Wavelength Stability

    of KU-FEL

    and single-shot measurement of mid-IR laser spectra

    Institute of Advanced Energy

    Kyoto University

    Takashi Nakajima, Yu Qin, Xiaolong Wang,

    Heishun Zen, Toshiteru Kii,

    and Hideaki Ohgaki

  • Outline

    Introduction

    Autocorrelation

    intensity autocorrelation (IAC) fringe-resolved autocorrelation (FRAC)

    Idea to estimate the shot-to-shot change of

    laser wavelength

    spectrally unresolved method using IAC and FRAC

    Single-shot measurement of mid-IR FEL spectra

    sum-frequency mixing

    Summary

  • Introduction

    KU-FEL: Oscillator-type FEL for the mid-IR (5-14 m)

    time

    macro pulse

    micro pulse

    ~ s

    ~ 350 ps

    pulse structure of KU-FEL

    ~ ps

    0.2 m

    Shot-to-shot change of laser wavelength

    If 1 ps pulse and

    1 % change of 0

    scan

    12 11.5 13 (m)

    FEL wavelength

    1.0

    0.5

    0

    Inte

    ns

    ity

    0

    different macropulse

    different micropulse

    scanning-type spectrometer

    array-type mid-IR photodetector

    For the detection of single-micropulse spectra

  • Autocorrelation methods

    Nonlinear crystal

    filter

    Photo

    detector

    dttEtE

    4

    21 )()(

    FEL pulse

    Movable

    stage

    FRAC signal

    2

    Fringe-resolved autocorrelation (FRAC)

    dttItI )()(

    2

    Photo

    detector

    Inter-

    ferometer

    Intensity autocorrelation (IAC)

    FEL pulse

    Movable

    stage

    Nonlinear crystal

    IAC signal

    Delay time

    p 2

    IAC

    sig

    nal

    Techniques to measure the ps~fs pulse duration

    Delay time

    FRA

    C s

    ign

    al

  • Idea to estimate the shot-to-shot change of laser wavelength

    How does it work? (k) (k+1)

    train of

    micropulses time

    interference smeared out

    for larger delay if (k)(k+1)

    c/ period noscillatio

    (k) (k+1) (k)+ (k+1)

    + = narrowing

    replica

    4)1()1(

    4)()(

    )()(

    )()()(

    tEtE

    tEtEI

    kk

    kk

    FRAC

    (k)

    (k+1)

  • )(exp1

    1)1(2ln2exp)( )()(0

    )(

    3

    2

    0

    )(

    2

    )(

    0

    )(

    1

    )( ttiaia

    tktIatE kkk

    k

    kkk

    M

    k

    k tEtI1

    2)( )()(

    IAC and FRAC signals with various fluctuations

    autocorrelation

    signals

    dttItII IAC )()()(

    dttEtEIN

    k

    kk

    FRAC

    1

    4)()( )()()(

    Intensity of the macropulse

    Field amplitude of the kth micropulse

    micropulse k the of

    frequency central and duration, intensity, of nfluctuatio :, ,

    micropulse kth the offrequency central :

    duration micropulse

    th

    (k)

    0

    )(

    3

    )(

    2

    )(

    1

    :

    kkk

    p

    aaa

    021 p

  • 0% 0.5%

    1% 1.5%

    Delay time (ps) Delay time (ps)

    1.53ps 1.36ps

    1.09ps 0.89ps

    Narrowing of FRAC signals by wavelength fluctuation

    After averaging over thousands of shots,

    IAC signals : no change

    FRAC signals: narrowing by shot-to-shot wavelength change

    FRAC signal narrowing

    Wavelength fluctuation

    For 1 ps pulse,

    Qin et al., Jpn.J.Appl.Phys. (in press)

  • 2

    2

    0

    22

    0

    2

    2

    22

    02

    2222

    0

    2

    2

    22

    4

    2ln2exp2

    2cos2)1(2ln2

    exp

    cos2

    2lncos

    22

    )3(2lnexp4

    1)()(

    p

    p

    pp

    dttEtE

    Retrieval of pulse duration from FRAC signals

    If we can retrieve the pulse duration from the FRAC signals, we may only perform the FRAC measurement !

    Expectation values of

    0

    0

    1.0

    ps p

    delay (ps)

    Pulse duration is retrieved by the

    time-integration of FRAC signals p

    1)( FRACI

    d

    d

    d

    pFRACId

    2ln

    21)(

    1)( FRACI

  • Experimental results

    step size = 2 m (= 1/150 ps)

    FRAC measurement

    exp.envelope

    fitted envelope

    delay (ps)

    exp.envelope

    fitted envelope

    delay (ps)

    step size = 10 m (= 1/30 ps)

    IAC measurement

    delay (ps)

    exp.data

    Gaussian fitting

    %4.1

    0

    66.0

    ps p

    %0

    52.1

    66.0

    ps p

    ps 64.02

    9.0 p

    0.9 ps

    pulse duration 0.64~0.66 ps by

    both IAC and FRAC methods

    wavelength stability is < 1.4 %

    Qin et al. (in preparation)

  • Toward single-shot measurement of Mid-IR FEL pulse spectra

    Mid-IR : MCT-based array photodetector (very expensive)

    NearIR: Si-based array photodetetor (cheap!)

    Convert the MIR laser pulse into the NIR pulse by sum-frequency mixing !

    1 (11 m) + 2 (1064 nm) 3970 nm

    KU-FEL Nd:YAG or

    microchip lasers

    SFM signal

    (near-IR)

    Signal intensity

    ISFM IMIR x I1064nm

    < 1 ps 20 ns or

    0.8 ns

    < 1 ps

    Wang et al. (in preparation)

  • spectrometer

    shortpass

    filter

    nonlinear

    crystal

    (AgGaS2)

    SFM signal

    ~970 nm

    Schematics of sum-frequency mixing

    SFM signal detectable?

  • Experimental setup for sum-frequency mixing

    2 mm

    5 mJ/pulse for YAG

    (20 ns)

    100J/pulse

    for microchip laser

    (0.8 ns)

    KU-FEL

    @11 m

    5mJ/macropulse at 1Hz

    ~5000 micropulses

    YA

    G / m

    icroch

    ip

    spectro -meter

    SP filter

    Spectral resolution

    ~0.8 nm

    KU-FEL

    macropulse

    control

    delay 1064 nm

    pulse

    /2

    AgGaS2

  • Shot-to-shot change of the SFM spectra by 20 ns Nd:YAG laser

    Sum of the micropulse spectra in the same macropulse

    =

    SF

    M s

    ign

    al

    966 968

    Wavelength (nm)

    972 974 970

    1064nm pulse

    macro pulse

    micropulse separation

    20 ns

    350ps

    SFM

    Wang et al. (in preparation)

  • = (

    )

    retrieved

    FEL spectrum

    Retrieval of the FEL spectra

    Wavelength (m)

    raw spectrum

    SF

    M s

    ignal (a

    .u.)

    Wavelength (nm)

    deconvolved

    spectrum

    FEL spectrum by

    monochrometer

    FE

    L s

    ignal (a

    .u.)

    Wang et al. (in preparation)

  • Time (s) wavelength (m)

    Inte

    nsi

    ty (

    a.u

    .)

    Inte

    nsi

    ty (

    a.u

    .) raw

    deconvolved

    Macropulse shape FEL spectrum

  • Time (s) wavelength (m)

    Inte

    nsi

    ty (

    a.u

    .)

    Inte

    nsi

    ty (

    a.u

    .) raw

    deconvolved

    Macropulse shape FEL spectrum

  • Time (s) wavelength (m)

    Inte

    nsi

    ty (

    a.u

    .)

    Inte

    nsi

    ty (

    a.u

    .) raw

    deconvolved

    Macropulse shape FEL spectrum

  • Summary

    Measurement of the pulse duration and wavelength stability

    of KU-FEL using fringe-resolved autocorrelation

    Spectrally unresolved detection !

    Periodicity of the fringe wavelength information

    micropulse duration = 0.64~0.66 ps

    wavelength instability