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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
CHAPTER 1
INTRODUCTION
IN RECENT years, optical communication systems and optical information
processing systems have been more and more required for achievement of high-speed and
high-capacity performances. These optical systems attract much attention because they can
overcome the problems associated with high-speed electronic information and
communication systems such as electromagnetic inference noise, signal delay, clock skew,
and wire congestion. Especially, integrated optical devices are beneficial for optical
interconnects, photonic networks, optical waveguides and so on. In other words, works on
integrated optical devices have been actively researched. Observation and visualization of the
behavior of light pulses in these devices is useful to characterize and evaluate them and to
improve their performance. One of the ways for improvement of performance is to use a
femtosecond pulsed laser as an optical source.
A femtosecond pulsed laser has ultrahigh temporal resolution and is actively used not
only in optical communication but also in material processing, induction of ultra-fast
phenomena, and so on. Therefore, techniques capable of visualizing a femtosecond light
pulse are quite important. Examples of techniques for visualizing light propagation are
femtosecond time-resolved optical polarigraphy (FTOP), photon scanning tunneling
microscopy (PSTM), a streak camera, and light-in-flight recording by holography (LIF
holography). FTOP uses a nonlinear optical phenomenon and indirectly observes
femtosecond light pulses. Because FTOP requires repetitive femtosecond light pulses, it
cannot always correctly observe pulse propagation in the case when repetitive characteristics
of femtosecond light pulses are not uniform. PSTM can visualize femtosecond light pulses by
utilizing evanescent waves by near-field optical microscopy. Also PSTM indirectly visualizes
femtosecond light pulses because it requires numerical estimation by computer.
In addition, visualization by using a single pulse is impossible in principle for FTOP
and PSTM since these techniques require repetitive light pulses or sequential recording.
Dept. Of ECE, KIT, Tiptur 1
Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Indeed a streak camera can directly visualize femtosecond light pulses, but it can only obtain
1-D information of pulses. On the other hand, LIF holography can record a moving picture of
propagation of femtosecond light pulses by using a single pulse and can obtain 2- or 3-D
information of pulses. Additionally, this technique is capable of directly observing
femtosecond light pulse propagation as a form of a spatially and temporally continuous
moving picture.
These characteristics are also advantages of LIF holography because other techniques
for observing propagation of a femtosecond light pulse require sequential recording with a
delay line or only obtain a moving picture of a few frames. In the past, observation of
picosecond or femtosecond light pulse propagation in air and inside optical devices has been
reported by means of LIF holography. In LIF holography, high-resolution holographic plates
such as Agfa Holotest 8E75HD, which was barely sensitive to near-infrared light, were
mainly used as a holographic recording material. However, production of holographic plate
was discontinued. Although photoconductor plastic hologram (PPH) is one alternative to a
holographic recording material, photosensitivity of the PPH depends on wave-length of light
and requires high energy of light for recording
of a hologram. On the other hand, digital LIF (DLIF) holography which used an image
sensor to record holograms was reported. DLIF holography applies LIF holography to digital
holography that records holograms by an image sensor such as a charge-coupled device
(CCD) or a CMOS image sensor.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
CHAPTER 2
LIGHT-IN-FLIGHT RECORDING BY HOLOGRAPHY
2.1 RECORDING PROCESS:
Following figure 2.1 shows a basic recording arrangement of LIF holography. A light
pulse which is generated from an ultra-short pulsed laser is divided into two pulses by a beam
splitter. Two pulses are collimated by microscope objectives and collimator lenses. One
ultra-short light pulse is called as an illuminating light pulse and is introduced into a diffuser
plate with inclined angle.
Fig 2.1 Top view of a basic arrangement of LIF holography.
BS-beam splitter; M-mirror; OB- microscope; objective; CL-collimator lens
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Light pulse diffused or scattered by diffuser plate is called as an object light pulse.
Other ultra-short light pulse is called as a reference light pulse and is introduced into a
holographic recording material with an inclined angle. Only when both illuminating light
pulse and reference light pulse arrive at recording material at the same time, interference
fringes are formed by both pulses and recorded on recording material. Reference light pulse
sweeps over recording material in recording process because pulse is introduced into plate
with an inclined angle. Therefore, behavior of the light pulse propagating on diffuser plate at
each instant is recorded in different parts of recording material.
2.2 RECONSTRUCTION PROCESS:
Fig.2.2(a) shows hologram obtained after chemical development of recording
material is set at position where recording material was prepared in recording process. A
continuous wave (CW) laser is used for reconstruction of the moving picture of light
propagation. Wavelength of CW laser is same as that of the ultra-short pulsed laser which
was used in recording process.
Light emitted from CW laser is collimated and illuminates hologram at the same
angle as the reference light pulse. Each different part of hologram reconstructs image of light
propagation at a different instance as shown in fig 2.2(b). Therefore, by moving gazed point
on the hologram along same direction in which reference light pulse swept, one can directly
observe an optical image of a spatially and temporally continuous moving picture of ultra-
short light pulse propagation without any camera.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Fig 2.2(a) Reconstruction.
BS-beam splitter; M-mirror; OB- microscope; objective; CL-collimator lens
Fig 2.2(b) Reconstruction of holograms for obtaining moving picture in DLIF holography.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
CHAPTER 3
Digital Light-in-Flight Recording by Holography
In DLIF holography, a hologram is recorded by an image sensor. A reconstructed
image is obtained from recorded hologram by computer. Therefore, instead of moving gazed-
point, images reconstructed from a set of pieces of digital holograms that were extracted
from digitally recorded hologram as shown in Fig. 2.2(a). In this case, relation between
temporal and spatial resolutions is trade-off. Here moving picture can be obtained by
sequentially reconstructing images from holograms.
Definition of Parameters
θ0 -Incident angle of illuminating light pulse against normal of diffuser plate
θR -Incident angle of reference light pulse against recording material
LA-R -Distance between A and R
LO-A0 - Distance between O and A0
LO-R - Distance between O and R
LR-R0 - Distance between R and R0
tA-R -Time required for the object light pulse to propagate from A to R
to-A0 - Time required for the illuminating light pulse to propagate from O to A0
to-R -Time required for the object light pulse to propagate from R to R0
tR-R0 - Time required for the reference light pulse to propagate from R to R0
T0 -Time when the illuminating light pulse arrives at O
TA0 - Time when the illuminating light pulse arrives at A0
TR0-Time when the object light pulse scattered at O arrives at R0
TR - Time when the object light pulse scattered at A arrives at R
τR0 -Time when the reference light pulse arrives at R0
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
CHAPTER 4
NUMERICAL SIMULATION
DLIF holography which numerically simulated that uses a femtosecond pulsed laser.
Simulation model was based on optical setup shown in Fig. 4(a). The plane on which diffuser
plate was set and axis that was perpendicular to the diffuser plate as xy-plane and z-axis,
respectively. Behavior of light was calculated by means of ray tracing. Coordinates of points
and parameters in Fig. 4(a) as shown in Table below. It is specified that distance between
diffuser plate and recording material as d.
In simulation, following conditions were applied for calculation simplicity.
1) An object light pulse scattered at O propagates in z-direction. This object light pulse and
reference light pulse simultaneously arrive at R0. Because optical path length of reference
light pulse is same as that of the object light pulse, interference fringes are generated at R0
and object light pulse is recorded at R0.
2) The cross sections are linear not only between illuminating light pulse and diffuser plate,
but also between reference light pulse and recording material.
3) The diffuser plate and recording material are placed parallel to each other.
Fig 4(a). Simulation model.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Definitions Of co-ordinates of points
In order to numerically obtain reconstructed image recorded at set of R whose y-
coordinate was 0 on recording material, we calculated set of coordinates of points at which
object light pulses, which were recorded at R, were scattered. We considered case in which
object light pulse scattered at O at TO when illuminating light pulse arrived at O. to-R0 was
expressed as
to-Ro = dc………………………………………………………..(1)
Then τR0 was given as,
τR0 = TO + tO − R0……………………………………………(2)
By considering (1), TRo expressed as
TR0 = τR0…………………………………………………(3)
Because the reference light pulse was incident to recording material at angle of θR , the cross
section propagated in x-direction. Then, vR , propagation speed of cross section between
reference light pulse and recording material, was given as
vR =c
sin θR…………………………………..(4)
Then TR0-R and TR were respetively expresssed as
tR0 − R =LR0−RvR
………………………………..(5)
τR = τR0 + tR0 – R……………………………….(6)
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
On the other hand, illuminating light pulse arrived at A0 at TA0 . Here, case in which the
object light pulse scattered at A. Because illuminating light pulse was incident to diffuser
plate at the angle of θO , vO , propagation speed of cross section between illuminating light
pulse and diffuser plate, was given as
vR =c
sin θ0………………………………..(7)
Then tO-A0 and TA0 were respectively expressed as
tO − A0 =LO−A 0vO
……………………………(8)
TA0 = TO + tO − A0……………………………….(9)
Object light pulse were scattered at A at TA0 and arrived at R at TR . Then, tA− R and TR
were, respectively, given as
tA− R=LA−Rc
…………………………(10)
TR = TA0 + tA− R……………………………….(11)
Here, if the object light pulse scattered at A is recorded at R, object light pulse and reference
light pulse simultaneously arrive at R. That is to say,
τR = TR………………………………..(12)
By applying (1)–(11) to (12), obtained following equation is:
d + LR0 − R sin θR = LO − A0 sin θO + LA− R……. (13)
If (13) is satisfied, object light pulse containing information on A is recorded at R. L R0
− R , LO − A0 and LA− R are given if the coordinates of A, R0 , and R are determined.And can
calculate a set of the coordinates of the points on diffuser plate that scatters object light
pulses recorded at R by examining (13) at a fixed R and a varying A and finding all
coordinates of A satisfying (13).Then image can be reconstructed from R. By changing
coordinate of R and conducting same calculations mentioned above, image reconstructed
from each point whose y-coordinate is 0 on recording material calculated.
Dept. Of ECE, KIT, Tiptur 9
Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
According to the condition discussed: d =30 cm, θO = 0.5◦ , and θR , = 0.5◦ . The
sampling interval of A in both x- and y-directions was set to be 500 μm. Setup is to
difference between optical path length of object light pulse and that of the reference light
pulse to be shorter than coherence length of a femtosecond light pulse. Coherence length was
set to be same as the length of a 96 fs light pulse. Also, we assumed that size of the diffuser
plate was 40 mm (H) × 20 mm (V).
Fig. 4 shows simulation results of femtosecond light pulse propagation. From (a) to
(e), xR changed as follows: (a) – 2.0 mm, (b) –1.0 mm, (c) 0.0 mm, (d) 1.0 mm, and (e) 2.0
mm. X- and y-axes correspond to those of reconstructed images of femtosecond light pulse
propagation. Small dots composing each circle in Fig. 4 represent sampled points of a
femtosecond light pulse calculated, that is, set of points represent reconstructed image of
femtosecond light pulse propagating on diffuser plate. The reconstructed images move when
coordinates of R change along same direction in which reference light pulse swept and
curvature caused by principle of LIF holography.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Fig. 4(b) The time interval between adjacent pictures is 175 fs.
CHAPTER 5
OPTICAL SETUP
Mode-locked Ti:sapphire laser (Mira 900-D, Coherent, Inc.) to generate femtosecond
light pulses. Center wavelength and duration of the femtosecond light pulses were 800 nm
and 96 fs, respectively. Femtosecond light pulses were introduced into recording system of
LIF holography. Magnification of OB1 and OB2 were 20 and 10, respectively. The focal
lengths of CL1 and CL2 were 300 and 150 mm, respectively. We set the distance between
the diffuser plate and the recording material to be 30 cm.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Fig.5 Optical setup of experiment. (a) Schematic diagram of top view. M: mirror, HM: Half mirror, OB:
microscope objective, CL: collimator lens. (b) Photograph of the diffuser plate. A 1951 USAF resolution test
chart was patterned on the plate.
Illuminating light pulse and reference light pulse were incident on diffuser plate and
image sensor at 0.5◦ against normal of each plate, respectively. Plate is patterned with 1951
USAF resolution test chart as shown in Fig. 5(b) so as to easily recognize motion of
femtosecond light pulse propagation. We used a CCD camera (ORCA-HR, Hamamatsu
Photonics K. K.) to record a hologram. The number of pixels and pixel pitch of camera were
4000 (H) × 2624 (V) pixels and 5.9 × 5.9 μm, respectively.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Fig.5(c) Optical setup results
Seven digital holograms were extracted from
recorded whole hologram. The time interval between adjacent pictures is 96 fs.
Each image reconstructed from 512 × 512 pixels extracted from recorded whole
hologram consisting of 4000 × 2624 pixels. Brilliant circle is reconstructed images of
collimated femtosecond light pulse. By sequentially displaying Reconstructed images on a
monitor of a computer, and obtained moving picture of propagation of the collimated
femtosecond light pulse.
Square image in center of brilliant circle is zeroth-order diffraction image. The
reconstructed light pulse propagates from left to right when we move observation position
from left to right, which is same direction as that of the reference light pulse on the image
sensor on camera.
Actual time of phenomenon and time interval between adjacent pictures in Fig. 5(c)
were 576 and 96 fs, respectively. Thus, succeeded in DLIF holography using a femtosecond
pulsed laser.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
CHAPTER 6
ADVANTAGES AND DISADVANTAGES
ADVANTAGES
By femtosecond pulse shaping, in which Fourier synthesis methods are used to generate
nearly arbitrarily shaped ultrafast optical wave forms according to user specification.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Applications of pulse shaping to optical communications, biomedical optical imaging,
high power laser amplifiers, quantum control, and laser-electron beam.
Reduce the spectral bandwidth of the diffracted signal. However, both theoretically and
experimentally that it is much easier in the frequency domain than in the space domain to
achieve a large enough diffraction bandwidth of volume holograms for the bandwidth of
100-fs pulses to be used for frequency-domain femtosecond pulse shaping.
The hologram is read out using forward-scattering phase conjugation to remove phase
distortion to all orders, including drift in the time of flight.
Dispersion of all orders is compensated by forming a dynamic spectral-domain hologram
of a signal pulse (that has a time-varying dispersion) referenced to a stable clock pulse.
DISADVANTAGES
It is a serial process and laser positioning is difficult.
There may be risk of cyto- and photo-toxicity.
CHAPTER 7
APPLICATIONS
Digital measurement of Three Dimentional Shapes.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
The interferogram of the object under study is only registered for the object parts for
which the difference in path length between the reference and the object beam is less than the
coherence length.
Digital recording.
Digital audio and digital video is directly recorded to a storage device as a stream of
discrete numbers, representing the changes in air pressure (sound) for audio and chroma and
luminance values for video through time, thus making an abstract template for the original
sound or moving image.
Moving picture recording applications.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Moving pictures of collimated and converging light pulses and some images
extracted from them. Inherent feature appearing in the images essential to determine the
actual shape of the propagating light pulse.
CHAPTER 8
CONCLUSION
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
Recording and observation of infrared femtosecond light pulse propagation as a form of a
digital motion picture.
Center wavelength and duration of the light pulse generated from laser were 800 nm and
96 fs, respectively.
DLIF holography will contribute not only to the evaluation and improvement of
integrated optical devices, but also the observation, visualization and elucidation of
ultrafast phenomena in frontiers of natural science, optimum control of a laser beam in
laser processing, and so on.
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Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12
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