; UNITED STATES APARTMENT OF COMMERCE

oUBLICATION

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NBS TECHNICAL NOTE 603

U.S.

EPARTMENT

OF

COMMERCE

National Bureau

of Iandards

Construction of a

Flashlamp-Pumped Dye Laser

and an Acousto-Optic Modulator

for Mode-Locking
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6.

TECHNICAL NOTE 603

ISSUED JULY 1971

Nat. Bur. Stand. (U.S.), Tech. Note 603, 33 pages (July 1971) CODEN: NBTNA

Construction of a Flashlamp-Pumped Dye Laser

and an Acousto-Optic Modulator for Mode-Locking

D. A. Jennings and D. L. Baldwin

Quantum Electronics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide

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be listed or referred to in the open literature.

This research was partially supported by the Advanced Research Projects

Agency of the Department of Defense under ARPA Order No. 891.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.46:603), Price $1.00

Stock Number 0303-0867

Construction of a Flashlamp- Pumped Dye

Laser and an Acousto- Optic Modulator for Mode- Locking

D. A. Jennings and D. L. Baldwin

In this paper is presented the design of a flashlamp- pumped dye laser capable of emitting

light pulses 500 ns wide (FWHM) with a risetime

of 300 ns. The energy output in the visible region

of light is 1 to 1 mj with an energy conversion

efficiency of about 0. 01% at a repetition rate of

30 pps. The design of an acousto- optic modulator

used to mode-iock the dye laser by intracavity loss

modulation is presented. The laser output for a given cavity length depends on the frequency and

voltage applied to the inodulator; a 10% - 100%

modulated output can be obtained with 1 V rms - Z0 V rms, whereas a train of light pulses nar- rower than 0. 8 ns (FWHM) can be obtained with

80 V rms.

Key Words; Acousto- optic modulator; flashlamppumped dye laser; mode-locking; subnanosecond pulses.

1. Introduction

Flashlamp -pumped dye lasers were first reported in 1967,

and improvements too numerous to mention have been reported since

(2) that date. With the use of a grating or a few prisms and proper

choice of dyes, a tuning range from 400 nm to 65 nm is accessible.

(3) An excellent review article on mode-locking of lasers is

available. Mode-locking of a flashlamp- pumped dye laser at 460 nm
(4) has been reported. The theory of mode-locking has been discussed

(5) (6) by Di Domenico and Harris.

2. Dye Laser Construction

Several articles have described the construction of flashlamp-

/ "7 \ / O \ pumped dye lasers. However, these designs are for coaxial

systems which do not lend themselves to fast repetition rates due to

the inability for flashlamp cooling to be designed into the system.

Heating of the dye solution will cause thermally induced index of refraction gradients which lower the quality of the optical cavity and

hence inhibit the performance of the laser. What is needed is a system

in which the flashlamp and dye solutions may be cooled separately. The

use of elliptical pumping geometry is adaptable to this cooling requirement. In this part of the note we describe a flashlamp -pumped dye

laser with elliptical pumping and repetition rates of up to 30 pps.

Several papers have discussed the use of elliptical geometries

for laser pumping cavities. ' A block diagram of the dye

laser is shown in figure 1 complete with the ultrasonic modulator.

The detailed construction drawings are shown in the foldouts 1-5.

The laser pumping cavity was an aluminum cylinder of elliptical

cross section with a major axis of 5. 4 cm and eccentricity of 0. 45. The

flashlamp and dye cell were placed at the foci of the ellipise. The

interior surfaces of the pump cavity were highly polished. The output
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mirror was flat with a multilayer dielectric coating of 60 - 95% reflectivity. The other mirror was either a 2100 line/mm grating blazed at

5 00 nm in the first order with a reflectivity of 73% at 633 nm, or a

100% reflecting mirror. The mirror separation was 60 - 100 cm.

-3 -4

The dye laser uses 10 - 10 mole solution of Rhodamine 6G

in ethanol. The dye solution was circulated at a flow rate of about 10

liters per minute. The flashlamp has a 9 cm arc length and a 3 mm

bore and is water cooled. It is powered by a 0. 1 \j,£, 20 KV, thyratron

dumped capacitor. The flashlamp-pumpihg system is capable of 1 J

per pulse and 30 pps.

Some operating characteristics of this dye laser without the

modulator are as follows: the threshold using a 95% and 100% reflectivity mirror is 1 - 2 J, depending on the dye concentration. With

an 80% reflectivity mirror and the grating, the threshold is around

5 J.

The peak emission of Rhodamine 6G is near 580 nm. However,

by using methanol as a solvent for shorter wavelengths and high dye

concentration in ethanol for longer wavelengths, we have been able to

tune the laser from 560 to 630 nm. The line width of the emission

using the grating is ~ 0. 1 nm.

The flashlamp has a risetime (10 - 90% power points) of 300 ns

and fall time of about the same with a 15% overshoot in the current
pulse. The current pulse width is 7 00 ns full width at half maximum

(FWHM). The laser output when pumped at 10 J is 200 - 5 00 ns (FWHM) in duration and 1-10 mJ, depending on mirror reflectivity,

dye concentration, and wavelength.

3. Mode-Locking of the Dye Laser

3. 1 Mode- Locking by Modulation Internal to the Cavity.

One can mode-lock a laser by modulating the losses of the

(1 2) cavity at a frequency near c/2L, ' where c is the speed of light in

a vacuum and L is the optical distance between the mirrors of the

cavity. Modulation depths of a few percent to 5 0% internal to the

cavity can give rise to 100% modulation of the laser output. In fact one

can produce a train of pulses with subnanosecond widths separated by

the cavity round trip transit time, in this case 10 ns. In this part of

the note we describe the theory of modulation of light by acoustic waves,

the construction of an acousto- optic modulator, and the results from

mode-locking a dye laser.

3. 2. Simplified Theory of Acousto- Optic Modulation.

In the acousto-optic method of mode-locking, the modulator is

a fused quartz block excited at a longitudinal acoustic resonance. In

our system the laser beam is incident parallel to the wave front of the

standing acoustic wave; the laser beam diameter is much greater than

the acoustic wavelength; the transit time of light through the block is
short compared to the period of the acoustic wave; and the frequency

of the acoustic wave is near one-half the frequency of the fundamental

longitudinal mode of the laser cavity.

(13) We use the Raman-Nath theory to describe the diffraction of

light by a sound wave. For a given width of the sound beam measured

along the path traversed by the light, the maximum change in index

of refraction, An, has an upper limit for the Raman-Nath theory to

(14) be applicable. Willard gives an expression for the value of An

above which the Raman-Nath theory yields the wrong quantitative

results for the intensity of light in the various diffraction orders.

Even so, from experiments we know that one may increase An past

the limit given by Willard to produce additional modulation depths in

the zero order diffracted beam of light. Thus, even for An so great

that the Raman-Nath theory breaks down quantitatively it still yields

the correct qualitative intensity for the undeviated beam of light.

A sound wave propagating through a material in the x- direction

causes variation in the index of refraction:

n(x, t) = n + A n sin (k x- w t)

where n(x, t) is the index of refraction with the confinement of the

sound beam,

n is the index of refraction of the modulator material with no sound

o beam,
A n is the maximum change in index of refraction,

k is the propagation constant of the sound wave, and

0) is the angular frequency of the sound wave.

In our case, the sound wave is a standing acoustic wave where:

n(x, t) = n + An (cos k x) (sin uu t) .

The block then appears as a phase grating to light incident perpendicular to the x-axis. The analysis of light passing through a phase

grating is very similar to the analysis of an ordinary diffraction

grating, but one replaces the sum over the apertures of a diffraction

grating with an integral over x. From the Raman- Nath theory we get

for the intensity I of the undeviated beam passing through a traveling

sound wave:

1=1 J

2 (v)

o o

2TTAnW

v =

X

where v is the Raman- Nath parameter,

X is the wavelength of incident light,

W = the width of sound beam measured along the path traversed by

the light,

I is the intensity of incident light, and

J (v) is the zero order Bessel function,

o
Under the conditions listed above, this intensity is independent of time.

In our modulator the light passes through a standing sound wave so one

cannot neglect the time dependence of the index of refraction. In

this case we have for the intensity of the undeviated beam:

2 *

I(t) = I J (v sin id t) . o o

(15) A fourier analysis of I(t) will show that the fundamental frequency

(JO ID of modulation of the light is f = 2 —— = . Modulation at this & m 2tt tt frequency dominates up to modulation depths on the order of 30% where

the second harmonic begins to give appreciable distortion. Second

harmonic distortion as produced here might even improve mode-locking.

3. 3. Acousto- Optic Modulator and Other Apparatus for

Mode-Locking a Dye Laser

The apparatus we used to mode-lock the dye laser consisted of

a transducer bonded to a fused quartz block and a RF voltage source

with variable frequency and voltage. The fused quartz block was a

cube 1. 4 cm on a side with two pair of opposite sides optically polished.

It is desired that the two sides be parallel to within X /100 (X = wavelength of sound used) and flat to within X /100. The two Brewster

angle faces through which the light passes should be parallel to 1 min.

of arc and flat to 1/5 of the optical wavelength.

The x-cut quartz transducer had a fundamental frequency of
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20 MHz and was driven at its third harmonic. It was circular in shape,

1. 3 cm in diameter, and it had evaporated gold electrodes. The top

electrode was 1 cm in diameter. The bottom electrode covered the

whole bottom of the transducer and wrapped around the edge of the

transducer for electrical connection from the top. The transducer

was bonded to the block with a clear epoxy spread thin with a razor

blade. It is desired that the bond thickness be less than X /100 and be

uniform. A copper sheet with a 1. 1 cm diameter hole was used to make

contact to the lower electrode as shown in figure 2, since the thin gold

plating wrapped around the transducer was troublesome when soldered

to a small wire lead. It seems probable that the trouble could also

have been avoided by evaporating gold right onto the fused quartz block

for the lower electrode.

A signal generator providing up to 1. 74 V rms into 50 Q was

used as the RF voltage source. It had an output bandwidth of less

than 1 KHz and was easily tuned from 58 - 62 MHz with nearly constant

output. The voltage was amplified to 20 - 150 V rms by a tuned pushpull tube amplifier. The quartz transducer with a 20 MHz fundamental frequency and a 1 cm diameter top electrode had a capacitance

of 22. 5 pf. The effective shunt resistance of the acousto- optic

modulator was more than 5 KQ when the drive frequency was a

resonant frequency of the transducer and the fused quartz block.

10
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11
Thus, undesirable operation would result from putting the modulator

at the end of a coaxial cable. The best results were obtained by including the transducer as part of the resonant circuit of a tuned

amplifier and mounting the modulator near this amplifier using tinned

copper wires less than two inches long. Since one must match three

frequencies, it is desirable that the applied frequency be easily adjustable at least over ± 2 MHz.

A cavity of optical length of 1. 25 m was used with a modulator

drive frequency of 60 MHz. To minimize reflection losses and to

avoid resonances in sub-cavities of the system, the modulator was

placed at Brewster's angle as shown in figure 3. For the same reason,

the AR coated windows of the dye cavity were tilted a few degrees from

normal incidence. The modulator was as close to the totally reflecting

mirror as possible. The multi- layer, dielectric coated rear mirror

reflected 99. 9% of light at 580 nm wavelength and had a 2. 15 m

radius of curvature. Dielectric coated flat mirrors which transmitted

for 5% - 20% of the light at 580 nm were used for the output mirror.

The detection equipment consisted of a vacuum biplaner photodiode driving an oscilloscope. The risetime (10% - 90% max. signal)

of the photodiode depends on the DC voltage and is calculated to be

0. 4 ns with 1000 V DC applied. The oscilloscope has a risetime of

0. 28 ns giving a risetime of the system as f 0.4 + 0. 28 - 0. 48 ns.

12
(a)

(b)

(c)

Figure 4. Oscilloscope traces of the dye laser output modulated

by two passes through the acousto- optic modulator

placed outside the cavity for various voltages applied

to the modulator. (5 ns/cm sweep speed)

(a) 60 V rms; (b) 100 V rms; (c) 134 V rms

13
(a)

(b)

(c)

(d)

Figure 5. Oscilloscope traces of the dye laser output with the

acousto- optic modulator inside the laser cavity for

various voltages applied to the modulator,

(a) 10 V rms, 20 ns/cm; (b) 15 V rms, 20 ns/cm;

(c) 30 V rms, 2 ns/cm; (d) 30 V rms, 20 ns/cm

14
mm

(a)

(b)

(c)

(d)

Figure 6. Oscilloscope traces of the dye laser output with the

acousto- optic modulator inside the laser cavity for

various voltages applied to the modulator,

(a) 80 V rms, 2 ns/cm; (b) 80 V rms, 5 ns/ cm

(c) 120 V rms, 2 ns/cm; (d) 120 V rms, 5 ns/cm

15
The photodiode is mounted in a housing with a 5 Q output impedance.

An impedance matching transformer was used to step up the impedance to the 125 Q impedance of the oscilloscope.

3. 4. Results of Using the Acousto- Optic

Modulator to Mode- Lock the Dye Laser

The width of the pulses observed on the oscilloscope depended

on the frequency of modulation and on the voltage applied to the

modulator Pulse widths of around 1. 5 ns (FWHM) were independent

of a frequency change of 200 KHz or 0. 3%. The depth of modulation

of light passing through the modulator increases with the RF voltage

applied to the modulator as shown in figure 4. A modulation depth of

a few percent per pass through the modulator is sufficient to produce

a 5 0% modulation depth of the light leaving the laser. Modulation

depths of 5% per pass produced pulses with widths less than 2 ns (FWHM). Modulation depths of 20% per pass produced pulses with

widths less than 0. 8 ns (FWHM). Results of the laser output with the

modulator inside the cavity are shown in figures 5 and b.

Pulses as narrow as 0. 75 ns (FWHM) have been observed on

the oscilloscope as shown in figure 7. The pulse shown has a risetime

(10% - 90% max. signal) of 0. 54 ns. Since the detection system has

a risetime of about 0. 5 ns, we believe that the actual light pulses are

even narrower than indicated by the oscilloscope. Note that in figure 7

16
(a)

(b)

Figure 7. Oscilloscope traces of the dye laser output with the

acousto- optic modulator inside the laser cavity after

critical adjustment of frequency. 100 V rms is applied

to the modulator,

(a) sweep speed of 5 ns/cm; (b) sweep speed of 2 ns/cm

(a)

(b)

Figure 8. Oscilloscope traces of the dye laser output with electrooptic modulator inside the laser cavity one meter long.

Approximately 600 V rms on the modulator,

(a) 10 ns/cm; (b) 2 ns/cm

17
there is a second pulse whose amplitude is about 10% the maximum

amplitude of the first pulse. This second pulse is about 1. 8 ns after

the main pulse. Independent measurements verify that the second

pulse is a reflection of the first pulse from an impedance mismatch

between the detector housing and the step up transformer.

The optical bandwidth of the dye laser depends on pump energy,

dye concentration and mirror reflectivity. Measurements with a

spectrometer indicate that the bandwidth for a given pump energy

near threshold was about 5 nm with the modulator turned off and also

with the modulator turned on producing a train of 1. 5 ns pulses.

3. 5. Other Methods of Mode- Locking

We have also tried to mode-lock the dye laser with an electro-

* , (17) optic modulator.

Pulses as narrow as 1. 8 ns (FWHM) as shown in figure 8 were

obtained using this modulator with about 600 V rms applied at 75 MHz

with a 1 meter cavity. The half wave voltage for the KD*P crystal

we used was about 6 KV.

Pulses less than 1 ns wide were also obtained by plaeing a no]

bleachable dye inside the cavity near one mirror, but to get these

pulses the concentration of the dyes had to be just right. This was

very difficult to accomplish. A photograph of the shortest pulses

obtained by this technique is shown in figure 9.

18
Figure 9. The dye laser output using saturable dye mode-locking,

the sweep speed is 2 ns/cm.

19
References

1. P. P. Sorokin and J. R. Lankard, IBM J. Res. Develop. 1A, 148

(1967).

2. B. B. Snavely, Proc. IEEE 57, 1374(1969).

3. P. W. Smith, Proc. IEEE 58, 1342(1970).

4. C. M. Ferrar, IEEE J. Quantum Electronics (Corresp. ) QE5,

550 (1969).

5. M. DiDomenico, Jr., J. Appl. Phys. 35, 2870(1964).

6. S. E. Harris, Proc. IEEE 54, 1401 (1966).

7. P. P. Sorokin, J. R. Lankard, V. L. Moruzzi and E. C. Hammond,

J. Chem. Phys. 48, 4726(1968).

8. A. Goldstein and F. H. Dacol, "A Reliable Flashlamp Pump

Tunable Organic Dye Laser," Rev. Sci. Inst. _40, pp. 1597-1598

(1969).

9. M. Ciftan, C. F. Luck, C. G. Schafer, and H. Statz, Proc. IRE

49, 960 (1961).

1 0. G. D. Boyd, R. J. Collins, S. P. S. Porto, A. Yariv and W. A.

Hargroves, Phys. Rev. Letters 8, 269(1962).

11. C. Bowness, D. Missio and T. Ragola, Proc. IRE 50, 1704(1962).

12. L. E. Hargrove, R. L. Fork, and M. A. Pollack, Appl. Phys.

Letters 5, 4 (1964).

20
13. C. V. Raman and N. S.N. Nath, Proc. Indian Academy of Science

2, 406 (1935).

14. G. W. Willard, J. Acoustical Soc. Amer. _21, 101 (1949).

15. L. E. Hargrove, IEEE Trans, on Sonics and Ultrasonics, SU14,

33 (1967).

16. A. T. Starr, "Radio and Radar Techniques" p. 450 (Pitman

Publishing Corporation, 1953).

17. E. Li. Steele, "Optical Lasers in Electronics" p. 58, (John Wiley

and Sons, Inc., 1968).

18. D. J. Bradley, A. J. F. Durrant, F. O'Neill and B. Sutherland,

Phys. Letters 30A, 535 (1969).

21





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form NBS-114A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-TN-603

2. Gov t Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Construction of a Flashlamp-Pumped Dye Laser and an

Acousto- Optic Modulator for Mode- Locking

5. Publication Date July 1971

6. Performing Organization Code

7. AUTHOR(S) D. A. Jennings and D. L. Baldwin

8. Performing Organization

10. Project/Task/Work Unit No. 2710153/2710456

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No ARPA Order No. 891

12. Sponsoring Organization Name and Address Advanced Research Projects Agency

1400 Wilson Blvd.

Arlington, Virginia 22209

13. Type of Report & Period Covered Final

14. Sponsoring 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.)

In this paper is presented the design of a flashlamp- pumped dye laser

capable of emitting light pulses 500 ns wide (FWHM) with a risetime of 300 ns. The energy output in the visible region of light is 1 to lOmJ with an energy

conversion efficiency of about 0. 01% at a repetition rate of 30 pps. The design

of an acousto- optic modulator used to mode-lock the dye laser by intracavity

loss modulation is presented. The laser output for a given cavity length depends

on the frequency and voltage applied to the modulator; a 1 0% - 100% modulated

output can be obtained with 1 V rms - 20 V rms, whereas a train of light pulses narrower than 0. 8 ns (FWHM) can be obtained with 80 V rms.

17. KEY WORDS (Alphabetical order, separated by semicolons) Acousto-optic modulator; flashlamp-pumped dye laser; mode- locking; sub- nanosecond pulses.

18. AVAILABILITY STATEMENT

1X1 UNLIMITED.

Z) FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

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UNCLASSIFIED

21. NO. OF PAGES

33

22. Price

JSCOMM-DC 66244-P71 GPO 836 - 131

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$3.00; foreign, $4.00*.

• Difference in price is due to extra coflt of foreign mailing:.

Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts, pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series. NSRDS provides quantitative data on the physical and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality and methods for testing various industrial products. These standards are developed cooperatively with interested Government and industry groups and provide the basis for common understanding of product characteristics for both buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards Publications. This series is the official publication within

the Federal Government for information on standards adopted and promulgated under the Public Law

89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

Consumer Information Series. Practical information, based on NBS research and experience, covering

areas of interest to the consumer. Easily understandable language and illustrations provide useful back- ground knowledge for shopping in today's technological marketplace.

NBS Special Publication 305, Supplement 1, Publications of the NBS, 1968-1969. When ordering, include Catalog No. C13. 10:305. Price $4.50;

foreign, $5.75.

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