NBSIR 74-360

A DYE MODE-LOCKED Nd'^ GLASS LASER FOR

GENERATING ELECTRICAL REFERENCE WAVEFORMS

Tatsutoku Honda

Norris S. Nahman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

September 1972

NBSIR 74-360

A DYE MODE-LOCKED Nd GLASS LASER FOR

GENERATING ELECTRICAL REFERENCE WAVEFORMS

Tatsutoko Honda

Norris S. Nahman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

September 1972

iT* W ^ ^4

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts, Director
FOREWORD

This report summarizes the research performed by Mr. T.

Honda during his stay at the National Bureau o£ Standards,

Boulder, Colorado, as a visiting guest scientist from

September 1971 to September 1972. Mr. Honda's home laboratory is the Electrotechnical Laboratory, Tokyo, Japan. This

report is typical of the high quality research performed in

the international scientific exchange programs that NBS participates in and strongly supports. The major publication from

this research was "The Optical Impulse Response of Biplanar

Vacuum Photodiodes , " coauthored by Mr. Honda and Dr. R. Smith

of NBS. The paper was published July, 19 73 in Applied Optics.

The second author Dr. Norris S. Nahman was the section

chief of the Pulse and Time Domain Section in which Mr. Honda

performed his research. Dr. Nahman provided scientific

counseling and direction of this research effort. He is

no longer with NBS. He is presently the chairman of the

Electrical Engineering Department of the University of Toledo,

Toledo, Ohio.

James R. Andrews

January 19 74
CONTENTS

Page

1. INTRODUCTION 1

2. CUTLER MODEL FOR SATURABLE DYE ABSORBER MODE -LOCKED

LASER 5

2.1 Linear Active Medium 5

2.2 Saturable Absorber 7

3. DYE MODE -LOCKED Nd"^^ GLASS LASER 10

3.1 Geometry 10

3.2 Pump Energy Charging and Discharging Circuit 11

4. . OPERATION OF THE Nd"*"^ GLASS LASER 12

4.1 Alignment of the Laser System 12

4.2 Concentration o£ Absorber Dye 12

4.3 Excitation Energy 13

4.4 Optical Damage 14

4.5 Time Interval of Operation 14

5. STABILITY OF DYE MODE -LOCKED LASER 15

5.1 Laser Rod 15

5.2 Excitation 16

5.3 Degradation of the Bleachable Dye 16

6. SINGLE PULSE EXTRACTION AND MEASUREMENT 20

6.1 Pulse Extraction 20

6.2 Two Photon Fluorescence Pulse Measurements 20

iii
CONTENTS (Continued)

Page

7. GENERATION OF ELECTRICAL PULSE WAVEFORM OF KNOWN

SHAPE 23

7.1 Introduction 23

7.2 Pyroelectric Detector - 23

7.3 Biplanar Vacuum Photodiode - 25

7.4 Optical Rectification 26

REFERENCES- 29

LIST OF FIGURES

Figure 1. A Model for Electronic and Optical Regenerative Pulse Generators — 34

Figure 2a. Optical Nonlinear Element for Mode Locked

Laser 35

Figure 2b. Pulse Sharpening Due to Repeated Traversals of

the Feedback Loop 35

Figure 3. Laser Model for Cutler Analysis 36

+ 3 Figure 4. Mode Locked Nd Glass Laser 37

Figure 5. Conventional Laser and Mode Locked Laser

Outputs 38

Figure 6. Laser Flash Lamp Electronics 39

Figure 7. Laser Burn Patterns 40

Figure 8. Shot to Shot Instability of Mode Locking 41

Figure 9. Excitation Energy Dependence of Mode Locking-- 42

Figure 10. Fatigue of Dye 43

Figure 11. Measurement of Intensity Dependence of Transmission of 9860 Dye 44

iv
LIST OF FIGURES CContinued)

Page

Figure 12. Optical Attenuation Versus Intensity of 9860

Dye (T=6%) 45

Figure 13. Transmission Property of 9860 Dye 46

Figure 14. Change of Transmission Property of 9860 Dye

Resulting from Mode Locked Laser Operation 47

Figure 15. Small Signal Transmission of 9860 Dye Versus

Exposure Time Under Blue Light 48

Figure 16. Single Pulse Extraction Technique 49

Figure 17. Single Pulse Generation Waveform 50

Figure 18. Two Photon Fluoresence Experimental Setup 51

Figure 19. Two Photon Fluoresence Results 52

Figure 20. Two Photon Fluoresence Results 53

Figure 21. Generation of Picosecond Pulsed Waveform of

Known Shape 54

Figure 22. Equivalent Circuit for a Pyroelectric Detector 55

Figure 23. Pyroelectric Detector Waveforms 56

Figure 24. Pyroelectric Detector 57

Figure 25. Biplanar Photodiode 58

Figure 26. Equivalent Circuit for Biplanar Photodiode 58

Figure 27. Impulse Response of Biplanar Photodiode 58

V

A DYE MODE-LOCKED Nd"^^ GLASS LASER FOR GENERATING

ELECTRICAL REFERENCE WAVEFORMS

T. Honda* and N.S. Nahman**

The theory, design, construction, operation and

stability o£ dye mode-locked Nd"*"^ glass laser are discussed. Optical properties of a saturable dye, single

pulse generation and two-photon fluorescence are experimentally studied. For the application of picosecond optical pulses to the field of the baseband

pulse measurements, three types of modelable detectors are described which have possible applications

for the generation of electrical reference waveforms.

+ 3 Key words: Glass laser; laser; mode-lock; Nd ; picosecond; reference waveform.

1. INTRODUCTION

The purpose of this work was to develop a pulsed laser

for generating optical picosecond pulses. The laser is to

be used as an optical pulse source in a study on the generation of electrical pulses by optical pulses. A mode-locked

+ 3 Nd glass laser was chosen for this work because a glass

laser is able to generate the shortest pulses and large

peak power [ 1]

.

*Mr. Honda was an NBS guest scientist during 1971-72. He is

with the Laser Research Section, Radio § Opto-Electronics

Division, Electrotechnical Laboratory, Tanashi Branch,

Tan as hi, Tokyo, Japan.

**Dr. Nahman is presently with the Electrical Engineering

Dept., University of Toledo, Toledo, Ohio.
The generation of large output pulses from laser devices

was first achieved by McClung and Hellworth [2] in 1962, This

technique is now called laser-Q switching; the shortest pulse

available from a Q-switched laser is about 10 nsec. By the

mode- locking technique it is now possible to produce 0.1 psec

pulses [3] at powers up to 100 Gigawatts.

The mode-locked laser pulse generator can be characterized as a regenerative pulse generator. It is well known

that a feedback loop consisting of an amplifier, a filter,

a delay line and a nonlinear element (which provides less

attenuation for a high-level signal than for a low- level

signal) behaves as a regenerative pulse generator, figure la.

In 1955, Cutler [4] discussed the regenerative pulse generator as a microwave pulse generator and was able to generate

microwave pulses having 2 ns pulse duration at 4 GHz.

Referring to figure la, when the loop-gain exceeds

unity, a pulse recirculates indefinitely around the loop

and each traversal gives rise to an output pulse at the

output terminal. The nonlinear element called an "expandor"

by Cutler has the three following characteristics:

1) Exhibits low loss for the peak region of the recirculating pulse while a high loss for lower amplitude

region

.

2) Discriminates against noise and reflections.

2
3) Acts to shorten the pulse duration until the pulse

duration is limited by the inherent bandwidth of the

filter.

The output of the regenerative oscillator has a pulse repetition rate equal to the reciprocal of the loop delay, a pulse duration equal to the reciprocal of the system bandwidth, and a carrier frequency determined by the filter

frequency.

A mode-locked laser pulse generator is an optical regenerative pulse generator, figure 1. The laser medium

serves as the amplifier, the combination of the Fabry-Perot

resonator and the line width of the laser transition serves

as the filter, and the time required for an optical pulse

to transverse twice the distance between the mirrors serves

as the loop time delay.

The optical analog of the "expandor" is a saturable

absorber, such as the reversible bleachable dye solution

commonly used in laser Q-switches . The three fundamental

requirements for a saturable absorber are as follows:

1) An absorption line at the laser wavelength.

2) A line width equal to or greater than the laser

line width.

3) A dye recovery time which is shorter than the loop

time delay of the laser.

3
Such a nonlinear element, figure 2a, sharpens pulses upon

repeated traversals of the feedback loop. The pulses become

successively peaked until a steady state is reached, figure

2b.

The steady state represents a balance between the narrowing caused by the nonlinear element and the spreading out

of the pulse due to the finite line width (frequency bandwidth) of the laser material. The study of short pulse formation in the laser has been approached from two theoretical

directions [5-8]. One method is based upon detailed studies

of the laser cavity modes [5,7,8]. Such an approach leads

to difficult analytical or numerical computation, because

of the complicated sets of coupled mode equations. However,

such studies have had notable success in explaining short

pulses in terms of the locking of the phases of the modes.

The other approach is simpler and is based upon the

method used by Cutler in studying microwave pulse generators

[4].

The Cutler method of analysis is in terms of functions

of time, and it has advantages both in terms of the physical

picture and of the mathematical operations involved. Also,

the Cutler method of analysis applied by Creighton, et al

.

[6] is described here because their analysis has rather

general forms for the functions representing operations on

signal in the laser, whereas most other analyses assume

these functions to be Gaussian.
2. CUTLER MODEL FOR SATURABLE DYE ABSORBER MODE-LOCKED LASER

2 . 1 Linear Active Medium

The laser is assumed to be operating on a particular line

o£ central frequency and approximate width fiw, figure 3.

A ring laser is imagined to simplify considerations. The

light transverses a complete cycle and returns to the input

side of the active medium.

Assuming that the active material is not saturated, thus

the electric field from it is a linear function of the incident field; also its response is assumed not to vary with time.

A linear operator represents both the linear operation of the

active medium on the signal, and the lumped losses in the

entire optical circuit. The linear operator is taken to have

a transfer function F((ja) . Referring to figure 3, if the input signals at A is I(t) and the output signal at B is 0(t)

,

then

00 0(t) = / rCco) • e"^'^''^ do) (1)

oo

where T(a3) is the Fourier transform of I(t) and 7(a)) is the

transfer function of the linear operator. From (1)

oo 0(t) = / F(t-t') 1(f) dt' (2)

5
The response function

- 00 which has a response time p on the order o£ magnitude o£ the

reciprocal o£ the bandwidth. 0 in a narrow range

about the optical frequency o) . Explicitly in terms of u) ,

F((jo) has the form

F(oo) = M'CciO-oo ) + fT*C-aj+co ) = MC^) + FT*(-J^) (3)

c c

M* results from the requirement that linear transfer functions

must satisfy the condition F((jo) = F*(-oo),

If the initial carrier pulse is

G(t) = G^^(t)cos oa^t (4)

and is such that G^^(t) varies slowly with respect to the

carrier frequency, then the output pulse envelope can be

approximated by _ /

' '

'

00 G ^(t) = / F(t-t')G. (f) dt' (5)

— 00

which varies slowly compared to optical carrier frequency.

We have the approximate result that the filter acts on solely

the envelope function as if its transfer function were M(f2) ,

FCt) = ^

/ M(J^)e^"^ dt (6)

-00

6
The width of the response function F(t) is designated by the

symbol p where is the variance of F(t) (see (11)). The

time p is long compared to the carrier period, but short

compared to the cycle time of the laser.

Unfortunately, there is no simple way of writing the

output from a bleachable dye in terms of its input. The

expandor in Cutler's paper was assumed to act on the amplitude of the pulse envelope by a power of S, where S > 1. Although a bleachable dye does not behave in this simple way,

it does generally narrow pulses which pass through it as does

Cutler's expandor. A pulse going through a laser gives rise

to the next pulse; the signal at the output C is

where (t) is the j-th pulse at the input A. The input for

the (j+1) pulse is

2.2 Saturable Absorber

(7)

A

j+1 (t) = / F(t-T^^-t')G^(t') (8)

2 . 3 The Steady St ate

The steady state condition is given by

dt (9)

7
where

^' = T^C = f - (10)

Here T is the total time for a pulse to travel around the

loop; T^^ is the time for travel from A to C, while T^^ is

that for C to A.

For a criterion for the pulse duration, we use the

variance defined by

00/ Ct-<t>^)f(t) dt

= :^— (11)

/ £Ct) dt

-00

where

00

, . / tf(t) dt

<t> =

/ £Ct) dt

-00 This is a convenient criterion because variances add under

convolution, i.e., for

00 f(t) = / g(t-f)h(t') dt', (12)

- 00

we have /

From (9) we have

oo G(t-A')^/^ = / F(t-t')G(t') dt'

8
Furthermore, if G(t) is a Gaussian function, then the variance

1/S for G(t) ' is the variance of G(t) multiplied by S. Denoting

the variances of G(t) and F(t) by and p^, respectively, then

= — (14)

S - 1

where P

- ^ (approximately the lifetime of the gain medium

line). When S = 2 , we have a square law saturable absorber

and (14) reduces to

" = p = L' (1=)

which says that the pulse width p is approximately equal to

the reciprocal of the gain line bandwidth. Cutler's analysis

[4] yields a similar result.

In the case of a Gaussian filter function and pulse shape,

and a square law saturable absorber, Cutler finds for the pulse

duration

,

S/tt

T =

Ao)

1 Ml + )

(16)

18

where y is the coefficient for the nonlinear effect originating

from the dispersion, and Ago is the bandwidth for a Gaussian

filter defined by the response at the minus -one neper points.

If y is small enough,

which is the same form as (15) .

9
3. DYE MODE-LOCKED Nd"^^ GLASS LASER [9], [11]

3 . 1 Geometry

A schematic diagram of the dye mode- locked laser is shown

in figure 4. The laser produces an output at 1.06 ym which

is invisible and in the infrared range. The laser glass rod

is 1.6 cm in diameter and 19.3 cm in length; the rod is

+ 3 brewster cut at both ends; and made of Nd doped laser glass.

The laser rod is housed in the center of a double elliptical

pumping cavity having a gold-polished surface , containing two

straight Xenon lamps. 14.5 cm of the laser rod is in the

pumping cavity. The mirrors are wedged and anti-reflection

coated on the back surfaces at 1.06 ym. In some experiments,

we used an output mirror having a lower reflectivity of

60% in order to change the threshold of lasing. Also the

other mirror had a radius of curvature of 10 m to form a stable resonator. The saturable dye (9860) is housed in a

thin cell constructed with a 1.65 mm PTFE spacer; the dye

was dissolved in 1 , 2- dichloroethane and the transmission

adjusted to be 75 to 80 percent. The dye was put in close

proximity to the 100% mirror [12]. The laser emission shown

in figure 5 is observed with a photodetector (Biplanar Vacuum

Photodiode) and a traveling-wave oscilloscope (over all risetime 0.55 nS) . It has the form of a train of approximately

50 short pulses spaced by about 10 ns , the cycle time T of

the laser.

10
3 . 2 Pump Energy Charging and Discharging Circuit

The block diagram of the laser flashlamp pumping system

is shown in figure 6. The capacitor bank consists of ten

capacitors of 25 yf parallel connected so as to store

1000 joules when a charging voltage of 2.8 kV is applied.

The Xe lamp is triggered in a series mode [13] by using a

pulse transformer having a saturable inductance, which also

serves as a current limiter.

11
4. OPERATION OF THE Nd"*"^ GLASS LASER

4.1 Alignment of the Laser System

Alignment of the system initially is done by using the

visible beam of a cw He-Ne laser (0.6 ym) as a "guidingstring." It is rather hard to make a good alignment using

a simple serial alignment on the He-Ne beam. A better procedure is to employ a Fabry- Perot interference pattern from the

incident side of the resonator where the screen on which the

pattern images is kept as far as possible from the resonator.

The reflected beams from the mirrors should be completely

superimposed by adjusting mirrors while watching the interference pattern. However, it should not be expected that

the alignment will be perfect before lasing because the lasing

wavelength (1.06 ym) will be different from that of the HeNe beam (0.6 ym) . The final adjustments must be made under

actual lasing conditions. The actual far-field pattern

will show whether or not the alignment has been done

correctly and is obtained from burn patterns produced by

the laser beam, figure 7.

4.2 Concentration of Absorber Dye

The dye 9860 was chosen as a saturable absorber for modelocking because it has some advantages involving high saturation power density, fast relaxation time and high damage

threshold, table 1.

12
For determination of the dye concentration, it is necessary to consider the excitation energy capacity and optical

damage threshold of the rod and mirrors . Too high a concentration of dye makes the lasing threshold of a mode-locked

laser too large; this in turn could require optical energy

stored inside the laser to be equal to or larger than the

optical damage threshold.

In the case of too low concentration of dye, the laser

would be poorly mode-locked.

4 . 3 Excitation Energy

Usually, excitation level is determined by a "cut and

try" method. The threshold of a mode-locked laser depends

upon the gain and losses of the whole laser system involving

the reflectivity of mirrors and the absorption of the dye.

It is necessary to avoid pumping near the threshold,

because too low pumping causes instability of lasing and

mode- locking, and is strongly influenced by the fluctuation

of the excitation power from Xenon lamps which can be 10 to

20 percent. Too high a pumping level can cause damage to

the laser components. Consequently, care must be taken in

increasing the pumping level to achieve stable operation.

13
4.4 Optical Damage

Optical damage on the surfaces of optical devices such

as mirrors and in bulk materials such as rods can be produced by high power mode-locked laser pulses. The surface

damage results from optical power absorbed at dirty spots

on surfaces of optical devices. Surfaces should always be

kept clean.

In bulk materials an excessively high power laser beam

can lead to self - focusing of the optical beam. To avoid the

production of "hot-spots" in the beam uniform pumping and

homogeneous dye solution are required.

4.5 Time Interval of Operation

The shot to shot time interval allowed depends on the

cooling time constant of an overall system and pumping energy

level. Generally speaking, regular [periodic) operation results in a good reproducibility of lasing; however, the

repetition rate must be slow enough so that the system cools

down to the same initial state. Typical repetition rates

would be of the order one shot/10 minutes for highest power

operation.

14
5. STABILITY OF DYE MODE -LOCKED LASER

Instability o£ a dye mode - locked laser is caused by

various conditions o£ the components in the laser system.

The main causes which impede stability of mode-locked laser

and the appropriate counter-measures are shown in table 2.

The detail about some of these causes will be discussed

below. A typical example of shot to shot instability of

dye mode-locked laser is shown in figure 8.

5 . 1 Laser Rod

The pumping energy heats the laser rod and produces a temperature gradient throughout the rod. This gives rise

to a change of refractive index and mechanical distortion in

the rod if the distribution of pumping energy and absorption

is not uniform.

It is well known that glass is a poor thermal conductor.

It will take a considerable amount of time until the heated

rod will cool. For a simple estimation of the thermal diffusivity, we have to take into account a thermal time constant

of about 30 to 50 sec for the glass material. If an energy

of 100 joules (equivalent to electrical input energy of 1000

joules applied to Xenon lamp of 101 efficiency) is absorbed

by the glass rod, the temperature would correspondingly rise

by 1.6 to 2.6°C.

15
5.2 Excitation

An example of excitation energy dependence of mode- locking

is shown in figure 9. From these pictures, the mode- locking

threshold can be observed.

By observing the intensity of 0.53 ym emitted from a Xenon lamp, a shot to shot intensity fluctuation of 10 to 201

was found. The fluctuation is influenced by the change of

charging voltage, triggering, and physical situation of the

lamp. The electrically induced fluctuation troubles may

be minimized by utilizing an electronic automatic and internal

(series) triggering method for driving the Xenon lamps.

The physical situation of the lamp is complicated. The

emission around 0.5 8 y is most effective for lasing; however,

the spectrum emitted from a Xenon lamp depends upon temperature,

pressure, input energy and use -de gradation of the lamp. Overall, better shot to shot energy stability can be achieved by

regular operation at not too low a discharging voltage.

5 . 3 Degradation of the Bleachable Dye

It has been noted that the dye solution dominates the

stability of a saturable absorber mode-locked laser. Unfortunately, only experience can provide the information for

establishing the generation of good mode-locked pulses.

16
The dye manufacturer suggests that the life of dye is

about 500 shots at optical power level of 1 to 5 MW. This

power level seems to be quite low for the laser system

discussed here. Experimentally poor mode-locking has been

observed after 30 to 60 shots, figure 10. The degradation of

the dye solution results in the change of its transmission

property, which destroys mode - locking . If the dye solution

loses its essential optical nonlinearity , no mode-locking can

be achieved.

The transmission property of a dye can be measured by

transmission experiments. The intensity dependence of the

optical absorption for a dye solution was measured by using

a train of short pulses from a mode-locked laser. A block

diagram of the measurement system is shown in figure 11.

To obtain an appropriate optical power density for

bleaching the dye, a lens having a focal length of 50 cm

was used. A dye cell 1.65 mm thick was placed away from the

lens by about 30 cm in order to give approximately the same

optical power density as that stored in the mode-locked laser.

The laser beam is split into two beams; one is used as a reference and the other as the probe. The test cell has the

same size as one being used for mode - locking . Both beams are

detected by a vacuum photodiode (0.5 ns risetime) and the output pulses are displayed by a traveling wave oscilloscope

CO. 35 ns risetime).

17
The intensity dependence of the optical absorption of the

9860 dye solution is demonstrated in figure 12; less absorption at high power can be seen. The results of the transmission measurements on the 9 860 dye solution are shown in figure

13. The solid-line curves are theoretical ones computed by

using the two-level model developed by Kercher [14].

For a thin cell mode (propagation through the cell < opti

cal pulse duration), Hercher gives for the transmission, T,

T = 1 - —

where a is the small signal absorption, and I^, is the optical

intensity at which absorption is Sq/Z.

The small signal transmission was measured separately

by using a combination of a white spectrum lamp, a spike

(transmission) filter for 1.06 ym and a photomultiplier . The

small signal transmission was adjusted to 37.51, 62.5%, and

78%. The curves of the thin cell model account well for the

experimental points for the weak absorbing dye solution. The

two-level model has been supported experimentally [15]. As

noted by the ref. [15], we have also observed a decrease of

the transmission for optical pulse intensities larger than

a certain value.

18
In figure 14, the change in the transmission property for

a 9860 dye solution resulting from mode-locked laser operation

is shown. Apparently the nonlinearity of the dye was weakened.

The dye solution was degraded in the mode-locked laser after

35 shots; note the increase in the small signal transmission.

The degraded dye could no longer provide the necessary modelocking nonlinearity.

In this case, the number of shots of mode-locking operation is not so important, but rather the change of the transmission property of the degraded dye solution when it happened.

It should be also noted that the excitation power for modelocked lasing with fatigued dye is lower than with new dye.

Also the transmission property of dye solution is strongly

influenced by absorption of blue light and ultra-violet radiation. The change of the small signal transmission of the dye

solution versus exposure time under a fluorescent lamp is

shown in figure 15. The completely bleached out state of

the dye is shown. It may be concluded that the degradation

of dye could be considerably influenced by blue and ultraviolet light from Xe lamps.

19
6. SINGLE PULSE EXTRACTION AND MEASUREMENT

6.1 Pulse Extraction

A single pulse can be extracted from a train of picosecond pulses by using an optical gate, figure 16. The gate

consists of an electroopt ical polarization switch placed between two polarizers. When the gating pulse is applied to

the polarization switch, the polarization of the optical

signal is changed; consequently, if the signal is then passed

through a polarization discriminator, the signal will be

spatially separated into its two distinct polarization

components. The gating pulse of 4 to 5 kV, 8 to 10 ns is

generated by a spark gap pulse generator which is triggered

by the laser beam. A typical picture of single pulse generation is shown in figure 17.

A single picosecond pulse is considered to be very useful

for electrical time domain studies. Furthermore, this technique makes it possible to obtain the single pulse having the

shortest duration in the train of optical pulses.

6 . 2 Two Photon Fluorescence Pulse Measurements

Optical coincidence techniques involving intensity autocorrelation functions have been very useful in the study of

ultrashort optical pulses of a few picoseconds in duration.

20
A commonly used technique is that of two photon fluorescence

(TPF) [16] ; the block diagram for a TPF measurement of pulse

width is shown in figure 18. The laser beam is divided into

two beams of equal intensity; these interfere in a dye which

exhibits two-photon induced fluorescence. The pulse duration

is proportional to the width of the auto- correlation function.

The time - integrated fluorescence intensity F(t) excited

by the two identical wave packets V(t) propagating in the

opposite direction is given in terms of the correlation

function (x)

:

F(t) = 2F^[1+2G2(t)/G^(0)]

in which

G^Ct) = <VCt)VCt+T)> <V*(t)V*(t+T)>

where x = 2Z/V (V = group velicty of the wave packets and Z is the

position) and F^ the single pass fluorescence. The notation

<V(t)V(t+x)> denotes the integration

,. , T/2

T ! V(t)V(t-x) dt

-T/2

The contrast ratio K of the fluorescence pattern is defined as the ratio of the peak fluorescence at x = 0 to the

background fluorescence at large values of x;

K = F(0) /F(x->°=)

It is well known that K assumes the value of 3 in the absence

of background intensity and reduces to 1.5 for Gaussian noise

which is caused by the thermal radiation generated by a laser

beam.

21
Rhodamine 6G was utlized as the dye medium. In ethanol

solution, this dye has its primary absorption peak at a wave

length that is quite close to the 0.5 3 um, the second har-

+ 3 monic of the Nd glass laser. In figures 19 and 20, some

typical pictures of TPF by mode- locked pulses are shown.

For TPF experiments, it is always necessary to adjust

carefully the optical beams to be completely superimposed in

optical power density. Too strong optical power increases

noise and reduces the contrast ratio.

The two-photon fluorescence method has considerable

ambiguity for determinating pulse duration because the pulse

waveform is unknown. Many workers have been trying to find

the waveform of mode- locked pulses and measure these pulse

duration. It is said that the front part of a mode-locked

pulse is expressed with Gaussian and the tail exponential.

In spite of the ambiguity of TPF, the method is very

useful for the estimation of picosecond pulse duration because of its simplicity and one shot measurement capability.

22
7. GENERATION OF ELECTRICAL PULSE WAVEFORM OF KNOWN SHAPE

7.1 Introduction

In the NBS Section 2 72.20, research has been done on

reference waveform generators and standards [17] to be used

in pulse measurements and other applications. The method

employs the band- limiting properties of a lossy uniform transmission line to produce a known waveform and generator

impedance. An example of generator design employing planar

skin effect metal loss and debye dielectric loss with a unit

ramp generator is shown in figure 21a.

The ultimate application of the laser system described

in this report is to replace the convolution problem of the

combination of a unit ramp generator and a modeled line by the

combination of a picosecond optical pulse generator and a modeled detector, figure 21b.

Three kinds of optical detectors have been considered for

the modeled detector having a predictable impulse response:

1) fast thermal photodetector ; pyroelectric effect, 2) biplanar

vacuum photodiode , and 3) nonlinear optical effect: optical

rectification.

7 . 2 Pyroelectric Detector

The pyroelectric effect [18, 21] concerns the change in

the polarization of a polar crystal when it undergoes a variation in its temperature. Optical radiation is absorbed
by means o£ a surface block of an optically absorbing material

attached to a piece of spontaneously polarized pyroelectric

material. If the pyroelectric material serves as the dielectric in a capacitor, then the resultant change of polarization due to heating of the pyroelectric gives rise to a

pyroelectric voltage across the capacitor electrodes.

The fast thermal photodetector is expected to generate an

electrical pulse waveform having a risetime of 100 picoseconds.

The pyroelectric current is given by,

pcF

dP

where P = ^tjt- , P^ : spontaneous polarization, T: temperature,

C: specific heat, b: thickness of pyroelectric material,

A: area of detector, AW: change of radiation power, p: density

of pyroelectric material. An equivalent circuit is shown in

figure 22. The output voltage is given by

V = kl

0 0

1_

~ U

^e

t t

1 , -t; t;^

e - e

where Trr is thermal time constant, T circuit time constant

1 e

(Rj^C) , and peak intensity.

In figure 23, the output signals to be expected are shown

for the pyroelectric detection process in cases of both impulse

and step incident optical beams. Generally, this detector has a

24
disadvantage of small signal output. An example of pyroelectric photodete ctor design is shown in figure 24. Ni film

o electrodes of 200 A thick were evaporated on both surfaces

of a PVF2 pyroelectric film of - 100 ym thick.

7 . 3 Biplanar Vacuum Photodiode

The impulse response of a biplanar vacuum photodiodes

[22, 24] was experimentally investigated [27] by the use of

a single ultrashort laser pulse from a train of mode-locked

pulses. The impulse response of a biplanar vacuum photodiode

for the observation of ultrashort optical pulses is determined

by the transit time of the photoelectrons from the photocathode

to the anode, figure 25.

Transit time of a photoelectron traveling across the

distance between electrodes is given by

The photocurrent is written as

where v = velocity of charge, Q = total photoelectron charge,

and

25
The equivalent circuit for the biplanar diode is shown in

figure 26. The output voltage is

V = 2^

O ^2 V

= 2^

-t

t - T(l-e , 0 < t < T

-T (t-T)

T(l-e , T < t

where x = RC^. A graph of is shown in figure 2 7. By the

use of the planar vacuum photodiode, it is possible to generate

electrical pulses having large peak voltages with risetime of

100 ps.

7.4 Optical Rectification

The nonlinear optical effect induced by a strong optical

beam in a crystal has a component of DC electrical polarization; the production of the electrical polarization is called

optical rectification [25, 26]. The DC polarization induced by

light is given by

Pi(0,lY) = X2i.^C0,a)-to) EjCa) ,lY)E*C^,lT) ,

that is, proportional to optical power. The induced current

is represented by

. _ dP _ Y dl

^or " 3t " -^2

where P is the integrated polarization over the volume of a crystal and I is optical power. Typical materials available

26
for the optical rectification are KDP, ADP , LINbO^ and LiNbO^,

etc. each of which lack the inversion symmetry in the crystal

structure. The broadband response of the optical rectification

effects coupled with an appropriate transmission line and intense picosecond laser pulses suggests the use of the effect

for generating picosecond electrical pulses.

27
ACKNOWLEDGMENT

by T. Honda

I would like to extend thanks to Dr. R. Sangster and

Dr. H. Altschuler for the administrative actions which enabled

this work. I have been greatly impressed and stimulated by

the research activities in the NBS Pulse and Time Domain Section, 2 72.20. Also, I would like to thank Dr. Nahman for many

stimulating discussions and lectures and for his interest and

encouragement during this work. It is also a pleasure to

acknowledge the support of Dr. W. McCaa and the technical

assistance of Mr. T. Whittemore in the electronics work.

I would like to acknowledge and thank Dr. R. Lawton for

the helpful discussions on pyroelectric devices. I am indebted

to Mr. K. Wilson who has constructed many devices for this

work.

Finally, I am indebted to Dr. D. Jennings, NBS 271.00 , who lent me various optical devices. I would like to gratefully thank Dr. R. Smith, NBS 2 71.00, for the many helpful

discussions and suggestions, and furthermore, for the collaboration in the experiment on the vacuum photodiode responses.

28
REFERENCES

[I] A.J. DeMaria, W.H. Glenn, Jr., M.J. Brienza, and M.E.

Mack, "Picosecond Laser Pulses," Proc. IEEE, Vol. 57,

No. 1. pp. 2-25, January 1969.

[2] F.T. McClung and R.W. Hellwarth, "Gaint Optical Pulses

from Ruby," J. Appl. Phys . , Vol. 33, pp. 82 8-829, March

1962.

[3] R.W. Smith, "Mode-Locking of Lasers," Proc. IEEE, Vol. 58,

No. 9, pp. 1342-1357, September 1970.

[4] C.C. Cutler, "The Regenerative Pulse Generator," Proc.

IRE, pp. 140-148, February 1955.

[5] S.E. Schwarz, "Theory of an Optical Pulse Generator,"

IEEE J. Quantum Electronics, Vol. QE-4, No. 9, pp. 503-

514, September 1968.

[6] J.R. Creighton and J.J. Jackson, "Simplified Theory of

Picosecond Pulses in Lasers," J. Appl. Phys., Vol. 42,

No. 9, pp. 3409-3414, August 1971.

[7] N.G. Bosov, P.G. Kriukov, V.S. Letokhov, and Y.V. Senatskii,

"H-12-Generation and Amplification of Ultrashort Optical

Pulses," IEEE J. Quantum Electronics, Vol. QE-4, No. 10,

pp. 606-609, October 1968.

[8] D.J. Kuizenga and A.E. Siegman, "FM and AM Mode-Locking

of the Homogeneous Laser -- Part I: Theory," pp. 694-

708, "Part II: Experimental Results in a Nd: YAG Laser

with Internal FM Modulation," pp. 709-715, IEEE J. Quantum

Electronics, Vol. QE-6, No. 11, November 1970.

[9] K. Patek, "Glass Laser," London Iliffe Books.

[10] M.A. Dugvay, et al. , "Study of the Nd: Glass Laser Radiation," IEEE J. Quantum Electronics, Vol. QE-6, pp. 725-

743, November 19 70 .

[II] D. Von Der Linde , "Experimental Study of Single Picosecond

Light Pulses," IEEE J. Quantum Electronics, Vol. QE-8,

No. 3, pp. 328-338, March 1972.

[12] E.M. Garmier and A. Yariv, "Laser Mode-Locking Saturable

Absorbers," IEEE J. Quantum Electronics, Vol. QE-3, No.

6, pp. 222-226, June 1967.

29
W.R. Hook, et al., "Xenon Flashlamp Triggering for Laser

Application," IEEE Transaction on Electron Devices,

Vol. ED-19, No. 3, pp. 308-314, March 1972.

M. Hercher, "An Analysis of Saturable Absorbers," Applied

Optics, Vol. 6, No. 5, pp. 347-954, May 1967.

A. Penzkofer, et al., "The Intensity of Short Light

Pulses Determined with Saturable Absorbers," Optics

Communication, Vol. 4, No. 5, pp. 377-379, January 19 72.

J. A. Giordmaine, et al., "Two-Photon excitation of

fluorescence by picosecond light pulses," Appl. Phy.

Lett., Vol. 11, pp. 216-218, October 1967.

W.D. McCaa, Jr., and N.S. Nahman, "Generation of Reference

Waveforms by Uniform Lossy Transmission," IEEE Transaction on Instrumentation and Measurement, pp. 382-390,

Vol. IM-19, No. 4, November 19 70.

E. Fatuzzo and W.J. Merz , Ferroelectricity , John Wiley

§ Sons, 1967, pp. 63-76.

J.R. Alday, "Millimeter Wave Detectors Using the Pyroelectric Effect," IEEE Trans, on Electron Devices, Vol. ED-16,

No. 6, pp. 598-601, June 1969.

R.A. Cowley, et al., "Dielectric Response in Piezoelectric

Crystals," J. Physics-C, Vol. 4, No. 10, pp. L-203,

July 19 71.

M. Simhony and A. Shavlor, "Pyroelectric Voltage Response

to Step Signals of Infrared Radiation in Triglycine

Sulphate and Strontium Barium Niobate," J. Appl. Phys .

,

Vol. 42, No. 10, pp. 3741-3744, September 1971.

L.K. Angers on and B.J. McMurtry, "High Speed Photodetectors," Proc. IEEE, Vol. 54, No. 10, pp. 1335-1349,

October 1966.

H. Melchior, M.B. Fisher, and F.R. Arams, "Photode tectors

for Optical Communications Systems," Proc. IEEE, Vol. 58,

No. 10, pp. 1466-1486, October 1970.

P. J.R. Laybourn, "Photodetectors for Laser Application,"

Optics and Laser Technology, pp. 76-82, 1971.

30
[25] M. Bass, P. A. Franken , J.F. Ward and G. Weinreich, "Optical Rectification," Phys . Rev. Letters, Vol. 9, No. 11,

pp. 446-448, December 1962.

[26] M.J. Brienza, A.J. DeMaria, and W.H. Glenn, "Optical

Rectification of Mode-Locked Laser Pulses," Physics

Letters, Vol. 26A, No. 8, pp. 390-391, March 1968.

[27] R.L. Smith and T. Honda, "The Optical Impulse Response of

Biplanar Vacuum Photodiodes , " to be published.

31
Table 1 PROPERTIES OF MODE -LOCKING DYES

DYE

ABSORPTION PEAK

SATURATION POWER

DENSITY

RELAXATION TIME*

DAMAGE THRESHOLD

(Relative Unit)

9740

1.045 vim

40 MW/cm^

25 'V. 35 X lO"-'-^ Sec

15

9860

1.051 ym

50 MW/cm^

6 9 X lO"-^^ Sec

43

32
Piwwo

C/2 mou < S

C3

COmHo

<;<

I—(

OhO

I—I o

PQ 2

I-H

O > S 2owo

Pi<

hHOO

CJ <

E-" I—I

CJH

CO

iz: ouIowH

u

Pi

1—1 u

CJ

CJ 2

HH Pi H W

CO

u

D CJ

•-3 h-l Q Pi < H

H

CO

>HCOwQ

CJ

HHH<U

Pi

(—I

CJ

C3

I—I O

CO<C5

£3 o

PQ<H

CO

Pi

H

W

CJ CO 5 <

PiCO

O

w

w

Q

CJ

PC o

l-< I o

CO S w

CO<CJ

H22CJ

I—<

CO

PiO<o

CO

PimH

rsi 2O

I-H E-OH

CO

HHQ§

Piw

CO< <

CJ

I—I Ho 0 o

Pi tJU o

Piw

CO

•Jo 2O

I—I <HUP

PL, <

cypio

Ph -o

CO

Pioo

a,

CJ

u

I—I

Pi

E-CJm

>-i m

CJ

Piw

CJ

CJ

I—I

PiH

I—t

CJ

Pi

m < ^ u

CO

PQO

PiH <

CJ

I—I m

>-QoOH

CJmQ

CO

PimO

COmu

I-H >mQo

COm

CJ<PL, PiCO2OH

COQ

33
EXPANDOR

DELAY

FILTER AMPLIFIER

OUTPUT

REFLECTOR

(a) System Model.

DELAY =

2 L

(b) Laser Pulse Generator.

REFLECTOR

OUTPUT

Figure 1 A MODEL FOR ELECTRONIC AND OPTICAL

REGENERATIVE PULSE GENERATORS.

34
a = a

1 +

s INTENSITY I Figure 2a OPTICAL NONLINEAR ELEMENT FOR

MODE LOCKED LASER.

NUMBER OF PASSES

0

1. 4

= 0.7

Pulse

Figure 2b PULSE SHARPENING DUE TO REPEATED

TRAVERSALS OF THE FEEDBACK LOOP.

35
CO.

r(t) or F(co)

©

Liinear Operator

Laser Linewidth

Linear Gain- Losses

CA

Propagation

Delay

Nonlinear

Element

Power Law

Figure 3 LASER MODEL FOR CUTLER ANALYSIS.

36
ouoo

vO

CO o

ino

CO

Q

U

ft; H

O

S: P i w ^ ^ ^ 5 Q 3 S o w oo w

<;

COQW

14uowQO

CD

•H

37
200 M s/ div

Conventional

50 ns/div

dye mode-locked

Figure 5 CONVENTIONAL LASER AND MODE

LOCKED LASER OUTPUTS.

38
COUo

a:Huw

:[: COS

OJW

COi

0)

39
Conventional Mode-locked

Figure 7 LASER BURN PATTERNS.

40
Shot #1

Shot #3

Figure 8 SHOT TO SHOT INSTABILITY OF MODE

LOCKING .

9860 dye(T = 80%), 100% & 70% mirrors,

800 J constant, time scale = 10 ns/div.

41
800 J

r

-"1

-4

760 J

610 J

Figure 9 EXCITATION ENERGY DEPENDENCE OF

MODE LOCKING.

9860 dye (T = 80%), 100% & 70% mirrors.
Shot #15

Shot #29

Shot #40

Figure 10 FATIGUE OF DYE.

9860 dye (T = 80%), constant 800 J input.

43
44
Strong Focusing '. (high power density)

less absorption

Weak focusing:

(low power density)

more absorption

Figure 12 OPTICAL ATTENUATION VERSUS INTENSITY

OF 9860 DYE (T = 6%).

45
(•/. ) NOISSIWSNVdi

46
s

<

Q _i

lU

I—<

»—o

I

o

J

Itop

?

ta\o

°\

\

t—o

X o

5

I/)

\ o

AV

\ °

3

I—

z8

i

lU

>-o

o

o o o

00 o o oin

(•/.) NOISSIWSNVai

47
(0

0)

in

CO

(M

OO

o o o o

in

00 O

O

z

oI—

I

to

z

o

u

OX)

•i-i

3

cqW

Q

Z

CO CO

<

CO>

W

Q

(%) NOISSIP^SMVHX

48
49
(a) A train of pulses missing a single pulse. 20 ns/div.

(b) Single pulse extracted from a train of pulses. 5 ns/div.

Figure 17 SINGLE PULSE GENERATION WAVEFORM

50
51
^ I 185 ps

^

TPF Pattern

Pulse Wavetrain

20 ns / div.

Figure 19 TWO PHOTON FLUORESENCE RESULTS.

52
2 ps

Figure 20 TWO PHOTON FLUORESENCE RESULTS.

53
Unit ramp

generator

y(s) z^(8)

, Tfanstnisslon bandwidth- limited I

I by planar skin effect and Debye

j

1 dielectric losses, i

(a) Present NBS reference waveform generator.

Mode Locked

Nd"*"^ Glass

Laser

Measurement

on Pulse

Duration

Modeled

Detector

Impulse response

(Transfer function)

to be predictable

(b) Proposed future NBS reference waveform generator.

Figure 21 GENERATION OF PICOSECOND PULSED

WAVEFORM OF KNOWN SHAPE.

54
Figure 22 EQUIVALENT CIRCUIT FOR A PYROELECTRIC

DETECTOR

.

55
I

step

impulse

^

Figure 23 PYROELECTRIC DETECTOR WAVEFORMS.

56
Figure 24 PYROELECTRIC DETECTOR.

57
U d

Q

anode photocathode

Figure 25 BIPLANAR PHOTODIODE.

Figure 26 EQUIVALENT CIRCUIT FOR BIPLANAR

PHOTODIODE.

Figure 2 7 IMPULSE RESPONSE OF BIPLANAR PHOTODIODE.

58
.FORM NBS-IKA (1.71) U.S. DEPT. OF COMM. 1 . PUBLIC ATION OR RE PORT NO. 2. Gov't Accctsloo BIBLIOGRAPHIC DATA NBSIR 74-360

SH E E T

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

^ 3

A Dye Mode-Locked Nd Glass Laser for Generating

Electrical Reference Waveforms

5. Publication Date September 1972

7. AUTHOR(S)

T. Honda and N.S. Nahman

1. |*et£ot(mag OtganintioB

9. PERFORMING ORGANIZATICffJ NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, BoulderLabs.

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

l6. Project/Taslc/Work Unit No. :722159 & Z/Zo±by

11. Contract/Grant No.

12, Sponsoring Otganization Name and Address National Bureau of Standards

Department of Commerce

Boulder, Colorado 80302

13. Type of Report & Period Covered NBSIR FY 7 2

14. Sponsonag Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The theory, design, construction, operation and stability of dye

saturable dye, single pulse generation and two-photon fluorescence are

experimentally studied. For the application of picosecond optical pulses

to the field of the baseband pulse measurements, three types of modelable

detectors are described which have possible applications for the

generation of electrical reference waveforms.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Glass laser; laser; mode-lock; Nd"*"^; picosecond; reference waveform

18. AVAILABILITY STATEMENT

r 1 UNLIMITED.

IXl FOR OFFIQAL MSTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

(TfflS PAGE)

UNCLASSIFIED

22. Price

USCOMM-OC ••244.P7I

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