Photorefractive Materials for Dynamic Optical Recording E-Book

Jaime Frejlich

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Seitenzahl: 530

Veröffentlichungsjahr: 2020

Beschreibung

A comprehensive and up-to-date reference on holographic recording Photorefractive Materials for Dynamic Optical Recording offers a comprehensive overview of the physics, technology, and characterization of photorefractive materials that are used for optical recording. The author, a noted expert on the topic, offers an exploration of both transient and permanent holographic information storage methods. The text is written in clear terms with coherent explanations of the different methods that allows for easy access to the most appropriate method for a specific need. The book provides an analysis of the fundamental properties of the materials and explores the dynamic recording of a spatial electric charge distribution and the associated spatial electric field distribution. The text also includes information on the characterization of photorefractive materials using holographic and nonholographic optical methods and electrical techniques, reporting a large number of actual experimental results on a variety of materials. This important resource: * Offers an in-depth source of information on the physics and technology of all relevant holographic recording methods * Contains text written by a pioneer in the field--Jaime Frejlich's research defined the field of dynamic holographic recording * Presents a one-stop resource that covers all phenomena and methods * Includes a review of the practical applications of the technology Written for materials scientists, solid state physicists, optical physicists, physicists in industry, and engineering scientists, Photorefractive Materials for Dynamic Optical Recording offers a comprehensive resource on the topic from the groundbreaking expert in the field.

Leseprobe

Photorefractive Materials for Dynamic Optical Recording

Fundamentals, Characterization, and Technology

Jaime Frejlich†

State University of CampinasGleb Wataghin Institute of Physics (IFGW)Campinas‐SP, Brazil

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This edition first published 2020

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Library of Congress Cataloging‐in‐Publication Data

Names: Frejlich, Jaime, 1946- author.

Title: Photorefractive materials for dynamic optical recording :

fundamentals, characterization, and technology / Jaime Frejlich

State University of Campinas, Gleb Wataghin Institute of Physics

(IFGW), Campinas-SP Brazil.

Description: First edition. | Hoboken, N.J. : John Wiley & Sons Inc., 2020.

| Includes index.

Identifiers: LCCN 2019032247 (print) | LCCN 2019032248 (ebook) | ISBN

9781119563778 (hardback) | ISBN 9781119563730 (adobe pdf) | ISBN

9781119563761 (epub)

Subjects: LCSH: Laser recording–Materials. | Photorefractive materials.

Classification: LCC TK7882.S3 S67 2020 (print) | LCC TK7882.S3 (ebook) |

DDC 621.382/34--dc23

LC record available at https://lccn.loc.gov/2019032247

LC ebook record available at https://lccn.loc.gov/2019032248

Cover Design: Wiley

Cover Image: © ArtLight Production/Shutterstock

List of Figures

Figure 1

Naturally birefringent uniaxial lithium niobate crystal view under converging white light between crossed polarizers with its

c

‐axis (optical axis) laying perpendicular to the plane (upper) and on the plane (lower).

Figure 1.1

Refractive index ellipsoid.

Figure 1.2

Refractive indices for a plane wave propagating in an anisotropic medium.

Figure 1.3

Crystallographic axes of a sillenite and an applied 3D electric field.

Figure 1.4

Structure of an undistorted cubic perovskite structure with general chemical formula ABX

3

. The differently shaded spheres represent X atoms (usually oxygens), B atoms (a smaller metal cation, such as

) and A atoms (a larger metal cation, such as

).

Figure 1.5

Three‐dimensional sillenite structure: darker spheres represent

ions and paler gray ones are

. Acknowledgments to Prof. Jesiel F. Carvalho, IF/UFG‐Goiânia‐GO, Brazil.

Figure 1.6

Schematic representation of a raw BTO crystal boule with its striations, indicating the way it will be sliced (top left); already sliced crystal with striations not perpendicular to the (011)‐face (top right) and ready‐to‐use crystal with renamed axes (bottom).

Figure 1.7

crystal boule as grown along its [001]‐axis.

Figure 1.8

Actual undoped sillenite crystals: raw

crystal boule grown along its [001]‐axis, showing striations on the lateral surfaces with both opposite (001)‐faces cut and polished (left);

crystal showing its (110)‐surface cut and polished (center) and

crystal with its larger (110)‐face cut and polished with its [001]‐axis direction along its longer dimension (right).

Figure 1.9

Index‐of‐refraction of BTO that is formulated by

[6].

Figure 1.10

‐type cubic crystal and its crystallographic axes

,

and

with an externally electric field

applied along the “

x

”‐direction.

Figure 1.11

Principal coordinate axes system

arising by the effect of an electric field

applied along the “

x

”‐axis, as shown in Fig. 1.10.

Figure 1.12

Sillenite crystal cut along its principal crystallographic axes, with an electric field along the [001]‐axis.

Figure 1.13

Lithium niobate crystal with an applied electric field along the photovoltaic

c

‐axis.

Figure 1.14

Lithium niobate crystal ellipsoid (black) and its modified (gray) size by the action of an applied field in opposite directions (left and right pictures) along the

c

‐axis.

Figure 2.1

Energy diagram for a typical CdTe crystal doped with vanadium, with the Te in the Cd anti‐sites at 0.23 eV below the CB and the Cd vacancies 0.4 eV above the VB [19].

Figure 2.2

Dark conductivity measured at various temperatures for a CdTe:V crystal (labeled CdTeBR16B3) produced and measured by Dr. J.C. Launay, ICMCB‐Bordeaux, France. From the Arrhenius plot, the energy of the Fermi level

eV is computed.

Figure 2.3

Representation of the sillenite octahedra unit with the lone‐electron pair in one corner.

Figure 2.4

Octahedra sharing corners.

Figure 2.5

Sillenite structure showing (dashed lines) the empty tetrahedra formed by four double‐octahedra units.

Figure 2.6

Localized states in the Band Gap of nominally undoped

crystal, from [29]. Filled electron‐donors are in gray and empty ones in white; the DOS (density of states) is qualitatively represented by the width of the full‐line limited levels whereas the dashed‐line ones are not. The succession of states close to the VB represents the almost continuous states except the few discrete ones at 2.4 and 2.5 eV. Reproduced from [29].

Figure 2.7

Schematic representation of luminescence effect on a sillenite crystal.

Figure 2.8

Photoluminescence in BTO‐008. The dashed line is the spectrum of the light of an LED illuminating the BTO crystal sample. The continuous curve is the spectrum of the light measured at the crystal output, very closed to it. A luminescent peak appears at 570 nm (

2.2 eV).

Figure 2.9

Intrinsic semiconductor: Fermi level for an intrinsic semiconductor and its “energy vs. occupation‐of‐states diagram” (right side).

Figure 2.10

Doped semiconductor: Fermi level pinned at the position of the dopant in the BG. On the right‐hand side is the “energy vs. occupation‐of‐states” diagram.

Figure 2.11

Doped semiconductor: Fermi

and quasi‐stationary Fermi levels upon illumination. The “energy vs. occupation‐of‐states” graphics is shown on the right‐hand side.

Figure 2.12

Recombination centers.

Figure 2.13

Traps.

Figure 2.14

Schematic representation of a material with one center (one single species) with two valence states (electron donors and electron acceptors) on two correspondingly slightly different localized states in the Band Gap. Electron acceptors are here represented as positively charged so that a nonphotoactive negative ion should be close to it in order to produce electrical neutrality at equilibrium for the as‐grown crystal.

Figure 2.15

Under the action of light (of adequate wavelength) electrons are excited to the CB, thus increasing the electron density in the CB and therefore increasing the n‐type (photo)conductivity. In the CB they diffuse (or are drifted if there is an externally applied electric field) and are retrapped (on the available acceptors) again and re‐excited and so on.

Figure 2.16

In this example, under the action of light, electrons and holes are excited to the CB and VB, respectively, so that the photoconductivity is due to electrons and holes. In this case, electrons do predominate but it could also be the opposite, or even be only holes being excited and the photoconductivity being of the p‐type.

Figure 2.17

Under nonuniform light, negative charges (in this case, we assume to be electrons only) accumulate in the darker (less illuminated) regions leaving behind, in the more illuminated regions, opposite (positive here) charges.

Figure 2.18

Photochromic effect and the band‐transport model. On the left side, we represent deep photoactive centers (acceptors and donors) and shallower centers close to the CB, with empty donors (acceptors) only, labeled

. In this figure electron acceptors, both for deep and for shallow centers, are represented as positively charged so that a nonphotoactive negative ion should be close to these charged acceptors to ensure local electric neutrality. On the right side, we see that under the action of light (represented by the arrows), the electrons are excited into the conduction band. Some of the electrons are retrapped to the

and some others to the

centers. The latter ones, that slowly relax to the deeper

centers in the dark, have a higher light absorption coefficient and are therefore responsible for the photochromic darkening effect.

Figure 2.19

Schema for the crystal samples: undoped

(labeled BTO‐J40), lead‐doped

(labeled BTO‐Pb), undoped

(labeled BSO) and photovoltaic iron‐doped

(labeled LNb) with the photovoltaic “

c

” axis parallel to the [

10] crystal axis. The light is always incident on the (110) crystal plane. Dimensions for all samples are reported in Fig. 2.20.

Figure 2.20

Crystal samples.

Figure 2.21

(left) and

:Fe (right) crystal samples showing the [010] and

c

‐axis that are their photovoltaic axes, respectively.

Figure 2.22

Average photovoltaic current density measured along axes [010] and “

c

”, respectively, on the BTeO and LNbO:Fe crystal samples (depicted on the left side) illuminated with spatially uniform

nm laser light normally incident on their (100) faces, as a function of the intensity I(0) as computed at the input plane inside the material. Reproduced from [12]. Fitting data to Eq. (2.67) with

for BTeO [50] and

for LNbO:Fe [12] it is possible to compute their corresponding

, which are reported in Tables 2.1 and 2.2. Reproduced from [12].

Figure 2.23

Polarization‐dependent photovoltaic photocurrent for both BTeO and LNbO:Fe crystal samples, as a function of the polarization direction of the

nm laser light, with the angular position referred to the axes [010] and “

c

”, respectively, for the incident (onto the (100) crystal faces) intensity (outside the material)

. Reproduced from [12].

Figure 2.24

Photocurrent (

)

, for undoped

as a function of the angle

. The photocurrent was measured along the [

10]‐crystal axis using

 = 532 nm and incident light intensity

 = 102 mW/

measured outside the crystal. The initial point,

 = 

, corresponds to the polarization parallel to the [

10]‐axis (see Fig. 2.19).

Figure 2.25

Photovoltaic current versus light intensity

(uniform

nm laser incident on the (110) crystal plane with light polarization direction along

) for undoped

(BTO‐J40) sample. The

and

represent the photovoltaic current measured along the [001] and [

]‐axis, respectively. The continuous line is the best fitting with Eq. (2.78) and the parameters computed from fitting are reported in Table 2.3.

Figure 2.26

Photovoltaic current versus light intensity (uniform

nm laser incident on the (110) crystal plane with light polarization direction along [001]) for an undoped

(BSO) sample. The continuous line is the best fitting with Eq. (2.80) and the parameters computed from fitting are reported in Table 2.3.

Figure 2.27

Photovoltaic current versus light intensity (uniform

nm laser incident on the (110) crystal plane with light polarization direction along [001]) for a lead‐doped

(BTO‐Pb) sample. The dashed line is only a guide for the eyes.

Figure 2.28

Average photovoltaic current density data, measured along the

c

‐axis, versus light (

nm) intensity (light polarization direction along crystal

c

‐axis) for an iron‐doped

(LNb) sample show a strict linear behavior with the continuous line being the best fitting with Eq. (2.67).

Figure 2.29

Light‐induced absorption spots produced in the center of an undoped

crystal by the action of a thin

 = 532 nm laser line beam; the second spot is due to the beam reflected from the rear crystal face.

Figure 2.30

Photochromic relaxation time for

as a function of inverse absolute temperature. Arrhenius data fitting leads to an activation energy of 0.42

0.02 eV.

Figure 2.31

Transmitted versus incident power (both measured in the air) for a 8.1 mm thick photorefractive

crystal slab labeled BTO‐010 using a

nm Gaussian cross‐section intensity laser beam (1.3 mm radius, P = 800

W corresponding to

mW/

). Data in the graphics are fitted by a linear equation for the limits

(black line) and

(gray line) as shown in the graphics.

Figure 2.32

Light‐induced Schottky barrier at the illuminated transparent conductive ITO electrode‐photorefractive crystal interface.

Figure 2.33

Schema of a photorefractive BTO crystal plate between two conductive transparent ITO electrodes including crystal axes and the illuminated front (001) plane.

Figure 2.34

Cross‐section schema of the ITO‐sandwiched BTO plate indicating the photocurrent flow under illumination.

Figure 2.35

ITO sandwiched 0.81 mm thick BTO crystal plate with electrodes wired to a lock‐in amplifier.

Figure 2.36

Measured photocurrent data referred to Fig. 2.35 with

,

and

indicating the front illuminated sample, whereas

,

and

refer to rear plane illumination, with

and

data refer to the left‐side ordinate axis.

Figure 2.37

Photovoltaic‐based current data (

,

and

) computed from curves in Fig. 2.36 are plotted on the left‐side ordinate axis, whereas computed Dember‐based currents (

,

and

) are plotted on the right‐side ordinate axis. Because of logarithmic scales, all current are plotted as positive, although Dember and photovoltaic based ones have opposite signs. Data for

mW/

are represented by

and

whereas

and

are for

mW/

. Data for

mW/

are represented by

and

.

Figure 3.1

Photoactive centers inside the Band Gap. There are filled traps

(electron‐donors), empty traps

(electron‐acceptors) and nonphotoactive ions (+) to provide local charge neutrality.

Figure 3.2

Under the action of light the electrons are excited from the traps into the conduction band where they diffuse and are retrapped in the darker regions. A space modulation of electric charge results, with overall positive charge in the illuminated and negative charge in the less illuminated regions.

Figure 3.3

The charge distribution produces a space‐charge electric field modulation.

Figure 3.4

The electric field modulation may produce deformations in the crystal lattice.

Figure 3.5

If the photoconductive material is also electro‐optic, that is to say it is photorefractive, the space‐charge field may produce an index‐of‐refraction modulation in the crystal volume that is in‐phase (or counterphase) with the space‐charge field modulation and is

‐shifted to the recording pattern of light.

Figure 3.6

Holographic setup: A laser beam is divided by the beamsplitter

BS

, reflected by mirrors

M1

and

M2

and interfering with an angle 2

. A sinusoidal pattern of light, as described in the text, is produced in the volume where these two beams interfere. A photorefractive crystal

C

is placed in the place where this pattern of light is produced. The irradiance of the two interfering beams is measured behind the crystal using photodetectors

D1

and

D2

. Shutters

Sh1

,

Sh2

and

Sh3

are used to cut off the main beam and each one of interfering beams, if necessary.

Figure 3.7

Generation of an interference pattern of fringes.

Figure 3.8

Light excitation of electrons to the CB in the crystal.

Figure 3.9

Generation of an electric charge spatial modulation in the material.

Figure 3.10

Generation of a space‐charge electric field modulation.

Figure 3.11

The electric field modulation produces a index‐of‐refraction modulation (volume grating) via electro‐optic effect.

Figure 3.12

The recorded grating can be read using one of the recording beams that is transmitted and diffracted.

Figure 3.13

The grating is erased during reading.

Figure 3.14

Until all recording is erased.

Figure 3.15

Space‐charge electric field grating being recorded by the

‐shifted sinusoidal pattern of fringes.

Figure 3.16

Space‐charge electric field without an externally applied field for a pattern of fringes with modulation

(left), 0.60 (center) and 0.30 (right).

Figure 3.17

Simulated recording (from 0 to 20 au) and erasure (from 20 to 50 au) of a space‐charge field with

and

= 10 au.

Figure 3.18

Index‐of‐refraction modulation arising in the crystal volume. The upper figure shows the pattern of light fringes projected onto the crystal, the middle figure shows the resulting charge density and the lower figure shows the spatial‐charge field and index‐of‐refraction modulation in the absence of any externally applied electric field (

). All vertical coordinates are in “arbitrary units”.

Figure 3.19

Schematic description of running hologram generation in photorefractives. A moving pattern‐of‐fringes onto the sample produces a synchronously moving volume hologram that reaches a maximum amplitude at a resonance speed.

Figure 3.20

Plot of

for the assumed parameters:

m,

m,

,

rad/s,

and

for

nm; with the experimental conditions being

K

 = 10

,

V/m, and an intensity inside the front crystal plane

W/

. From Eq. (3.21) and

we compute

V/m at

T

 = 300 K, from

and

in Eq. (3.49) we get

V/m, from

and

in Eq. (3.50) we compute

, from

and

in Eq. (3.87) we get

rad/s.

Figure 3.21

Plot of

from Eq. (3.85) for the same parameters referred to in Fig. 3.20.

Figure 3.22

Plotting of

Q

as a function of

(

‐axis) and

(

‐axis) for

V/m,

,

nm with

,

and an intensity inside the front crystal plane

W/

.

Figure 3.23

Plotting of

Q

as a function of

K

, from Eq. (3.91), for typical values

m,

m and different applied electric fields from 5

, 7

, 10

to 15

V/m, represented by the progressively increasing size of the dashed lines, respectively.

Figure 3.24

Plotting of

(continuous curve),

(long dashing curve) and

(short dashing curve) versus

, for the same parameters referred to in Fig. 3.20.

Figure 3.25

One‐species/two‐valence/two‐charge carrier model contributing to charge transport: one single spatial trap modulation structure is produced.

Figure 3.26

Two‐species/two‐valence/two‐charge carrier model contributing to charge transport: two distinct spatial trap modulation structures are produced.

Figure 3.27

Hole‐electron competition on different photoactive centers under the action of low energetic photon recording light: only charge carriers close to the CB (electrons) and to the VB (holes) can be excited, but electrons cannot be excited from the hole‐donor level or holes from the electron‐donor level, because of energy considerations. In this case, an electron‐based hologram is recorded in the level closer to CB, and the same for holes in the level close to VB. However, electrons progressively accumulate in the (deeper) level closer to the VB and holes accumulate in the level close to the CB, where they cannot be re‐excited again because the recording light is not energetic enough. The recording is progressively decreasing, because of the decrease in the corresponding charge carriers, until a steady state is achieved because of the exhaustion of any one of the two levels.

Figure 3.28

Short circuit schema using conductive silver glue to electrically connect the opposite faces along the photovoltaic axis

(left) and open circuit schema, without any electrical connection (right).

Figure 4.1

Reading the recorded hologram with one of the recording beams.

Figure 4.2

Recording a fixed volume index‐of‐refraction hologram that is phase‐shifted by

referred to the recording pattern of fringes with

being the angle inside the material.

Figure 4.3

Bragg condition where

and

are the incident beam and the diffracted beam wavevectors, respectively (or vice versa), and

is the grating wavevector.

Figure 4.4

Amplitude coupling in two‐wave mixing: in this example, the weaker beam receives energy from the stronger, but could also be the other way round.

Figure 4.5

Phase coupling in two‐wave mixing: the pattern of fringes and associated grating are progressively shifted by the same amount. The picture shows some degree of amplitude coupling too.

Figure 4.6

Numerical plotting of

versus the normalized time

, from Eq. (4.80) for

,

and

with

= −0.5, −0.25, 0.25 and 0.5 from the smaller to the larger dashed lines, respectively.

Figure 4.7

Numerical plotting of

versus the normalized time

, from Eq. (4.80) for

,

and

with

= −0.5, −0.25, 0.25 and 0.5 from the smaller to the larger dashed lines, respectively.

Figure 4.8

Numerical plotting of

versus the normalized time

, from Eq. (4.80) for

,

and

with

= −0.5, −0.25, 0.25 and 0.5 from the smaller to the larger dashed lines, respectively.

Figure 4.9

Numerical plotting of

versus the normalized time

, from Eq. (4.80) for

,

and

with

= −0.5, −0.25, 0.25 and 0.5 from the smaller to the larger dashed lines, respectively.

Figure 4.10

Transient effect of a perturbation, in the form of a ramp voltage (thick curve) applied to the

PZT

‐supported mirror in the holographic setup, on the diffraction efficiency (thin curve) of a running hologram recorded in a photorefractive BTO‐crystal using the 514.4‐nm wavelength. The diffraction efficiency evolution to equilibrium is faster for the negative‐gain (lower graphics, with

) than for the positive‐gain (upper graphics with

) experiment. In both cases, the applied external field is

kV/cm, the total incident irradiance is

22.5 mW/

and the beam ratio is

. Reproduced from [94].

Figure 4.11

Computed running hologram

as a function of

(rad/s) for

and different material parameters.

Figure 4.12

Computed running hologram

as a function of

(rad/s) for

and different material parameters.

Figure 4.13

Computed running hologram

as a function of

(rad/s) for

and different material parameters.

Figure 4.14

Computed running hologram

as a function of

(rad/s) for

and different material parameters.

Figure 4.15

versus

(rad/s), computed for

and different material parameters, for a typical BTO crystal 2.05 mm thick and

= 1165

.

Figure 4.16

versus

(rad/s), computed for

and different material parameters, for a typical BTO crystal 2.05‐mm thick and

= 1165

.

Figure 4.17

versus

(rad/s), computed for

and different material parameters, for a typical BTO crystal 2.05‐mm thick and

= 1165

.

Figure 4.18

versus

(rad/s), computed for

and different material parameters, for a typical BTO crystal 2.05 mm thick and a hypothetically low

.

Figure 4.19

Phase modulation setup:

BS

: beamsplitter,

PZT

piezoelectric‐supported mirror,

D

: photodetector,

LA‐

and

LA‐

: lock‐in amplifiers tuned to

and 2

respectively,

HV

high voltage source for the

PZT

,

OSC

oscillator to produce the dithering signal.

Figure 4.20

Wave‐mixing schema showing the hologram phase shift

and the phase shift

between the transmitted and diffracted beams at the crystal output.

Figure 4.21

Degenerate four‐wave mixing showing the signal

S

and reference

R

beams interfering to produce a real‐time hologram in the nonlinear material (left); then a pump beam

P

, identical to

R

but much stronger and counter propagating, is diffracted by the already recorded hologram and the diffracted beam is the conjugate

S

*

of the signal

S

beam, reflecting back along the same incidence direction.

Figure 5.1

Input and output light polarization.

Figure 5.2

Input and output polarization referred to actual principal axes coordinates.

Figure 5.3

General illustration of the polarization direction of the transmitted and diffracted beams through a crystal with optical activity and anisotropic diffraction. At mid‐crystal thickness, the polarization directions of the transmitted and diffracted beams are

shifted from the [110] and [001] axes, respectively.

Figure 5.4

Transmitted and diffracted beams orthogonally polarized at the output through a crystal with optical activity and anisotropic diffraction. Assuming

, the incident beam's polarization direction at the input plane should be

with reference to the [110]‐axis.

Figure 5.5

Transmitted and diffracted beams parallel‐polarized at the output through a crystal with optical activity and anisotropic diffraction. Assuming

, the incident beam's polarization direction at the input plane should be

with reference to the [110]‐axis.

Figure 6.1

Scanning electronic microscopy image of a 1D hollow sleeve structure first recorded on photoresist film, then metallic vacuum deposited and finally washed away from all remaining photoresist to produce hollow metallic structures. Produced and photographed by Lucila Cescato, Laboratório de Óptica, Instituto de Física, Universidade Estadual de Campinas, Brazil.

Figure 6.2

Scanning electronic microscopy image of a 2D‐array holographically recorded and chemically developed on photoresist film. Produced and photographed by Lucila Cescato, Laboratório e Óptica, Instituto de Física, Universidade Estadual de Campinas, Brazil.

Figure 6.3

Scanning electronic microscopy image of a blazed grating made by the holographic recording of the first and the second spatial harmonic components of a sawtooth‐shape profile on photoresist film. Produced and photographed at Laboratório de Óptica, Instituto de Física, Universidade Estadual de Campinas, Brazil. Reproduced from [105].

Figure 6.4

Block‐diagram of a self‐stabilized setup:

D

photodetector,

LA‐

phase sensitive lock‐in amplifiers tuned to

,

HV

voltage source for the phase modulation device

PM

,

OSC

oscillator at frequency

. The output phase shift, feedback and noise phases are

,

and

, respectively.

Figure 6.5

Schematic description of the actual self‐stabilized holographic recording setup:

C

photorefractive crystal,

D

photodetector,

LA

and

LA

phase sensitive lock‐in amplifiers tuned to

and

, respectively,

HV

high voltage source for the piezo‐electric supported mirror

PZT

acting as phase modulator,

OSC

oscillator at frequency

.

Figure 6.6

Schematic description of the effect of noise on the two‐wave mixing in the holographic setup.

Figure 6.7

Block‐diagram of fringe‐locked running hologram setup: same as for Fig. 6.4 with the addition of an integrator

INT

at the output of the lock‐in amplifier.

Figure 6.8

Schematic actual setup for self‐stabilized running hologram recording: same as for Fig. 6.4 with the addition of an integrator

INT

at the output of the lock‐in amplifier.

Figure 6.9

Fringe‐locked running hologram speed:

Kv

(rad/s) versus feedback amplification