DATE DUE

"S-QQ-^/OtU */

^1^1' XMd ; W-z >M*

f '

I

'

Demco, Inc. 38-?93

NITED STATES MTMENT OF MMERCE

ILICATION

*Ntmo» /

National Bureau of Standard

Library, E-01 Admin. Bldg.

OCT 6 1981

131102

/CO

NBS TECHNICAL NOTE 620

U.S.

PARTMENT

OF

OMMERCE

National

Bureau

QC > f loo ls 15753

o.4>10

973

Frequency and Phase Stabilization

of an HCN Laser by Locking to a

Synthesized Reference
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences 2 — Applied Radiation 2 — Quantum Electronics s — Electromagnetics 3 — Time

and Frequency * — Laboratory Astrophysics 3 — Cryogenics 3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety' — Building

Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Center consists of the following offices and divisions: Information Processing Standards — Computer Information — Computer Services —

Systems Development — Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Technical Information and Publications —

Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
ill a MAS. 'it ~ • .

6

jOO

13

a

Frequency and Phase Stabilization of

an HCN Laser by Locking to

a Synthesized Reference

Joseph S. Wells

Quantum Electronics Division

Donald Halford

Electromagnetics Division

Institute for Basic Standards

^UJ- National Bureau of Standards

Boulder, Colorado 80302

£ 7e d /> s> /c &< ^* ?A- j s?& && & -

NBS Technical notes are designed to supplement the

Bureau's regular publications program. They provide

a means for making available scientific data that are of transient or limited interest. Technical Notes may

be listed or referred to in the open literature.

c<^

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued May 1973
National Bureau of Standards Technical Note 620

Nat. Bur. Stand.(U.S-), Tech. !\ote 620. 56 pages (Ma) 1973)

CODEN: NBTNAE

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

(Order by SI) Catalog !\o. C 13.46:620). 60 cents
CONTENTS

Page

1. INTRODUCTION . . . 2

2. LASER PARAMETERS . 3

3. STABLE FREQUENCY REFERENCE 11

3.1. Short Term Stability Requirements 13

4. ACTIVE TECHNIQUES 24

4. 1. Frequency Lock Loop 24

4. 2. Phase Lock Loop 26

5. DISCUSSIONS 40

6. REFERENCES 48

in
LIST OF FIGURES

Page

Figure 1. Block diagram of experimental scheme for 4

an HCN laser frequency measurement.

Figure 2a. Schematic drawing of a frequency controllable " HCN laser.

Figure 2b. Ring anode and resulting stable plasma 6

configuration.

Figure 3. RF power spectrum of beat note between 10

HCN laser and 12th harmonic of 74- GHz

klystron.

Figure 4. Frequency synthesis scheme for stabilization 12

of HCN laser frequency.

Figure 5. Beat notes of a free running HCN laser versus 14

12th harmonic of 74- GHz klystron.

Figure 6. RF power spectrum of 0. 891 vs (12 x 0. 074 THz) 23

beat note when 74- GHz klystron is locked to a

phase locked klystron (a) and a cavity- stabilized

klystron (b).

Figure 7. Block diagram of laser stabilization scheme. 25

Figure 8. Response of the current controller- laser 29

combination to a fixed amplitude modulation

signal.

Figure 9. 74- GHz vs HCN laser beat notes. 32

Figure 10. Quality of phase lock of HCN laser. 33

Figure H. HCN laser stability as measured with a computing 35

frequency counter.

IV
LIST OF FIGURES (Continued)

Figure 12. Block diagram of scheme for measuring

linewidth and stability of a phase locked

HCN laser.

Figure 13. RF power spectrum on analyzer B of beat

between laser and multiplier chain B when

laser is (a) free running and (b) phase locked

to chain A.

Figure 14. Spectrum of phase lock correction signal

under various experimental conditions more

fully described in text.

Figure 15. Phase detector circuit used in HCN stabilization.

Figure 16. Filter and impedance matching circuit connecting

phase detector and series current controller.

Figure 17. Circuit for series current controller (SCC).

Page

37

39

41

45

46

47

Frequency and Phase Stabilization of an HCN

Laser by Locking to a Synthesized Reference

by

Joseph S. Wells and Donald Halford

Infrared frequencies as high as 88 THz have

recently been synthesized using diode harmonic

mixers with accuracies of parts in 1 . Stabilized

lasers are needed to make frequency measurements of higher accuracy. The HCN laser is the

lowest frequency basis laser used in these synthesis

schemes and its stabilization has been the subject of

recent interest. The laser is stabilized by locking

it to a phase locked microwave reference chain. Two

servo loops are utilized. The first loop is a relatively

slow frequency lock loop with the correction applied

to a P ZT driver. This loop not only accommodates

thermal expansion of the laser, but also serves as an acquisition aiding loop for the second servo. The latter is a phase locked system with the correction applied to the laser discharge current.

Details of the laser design and some noise

considerations relative to the microwave reference

chain are presented along with some experimental

data which indicate the results of the stabilization

techniques. Data regarding the system stability

and improved fast linewidth are included.

Key words: Fast linewidth; Frequency noise; HCN

laser; Infrared frequency synthesis; Laser frequency

measurements; Laser linewidth; Laser stabilization;

Phase locked laser.
1. INTRODUCTION

The HCN laser is currently the basis laser of lowest frequency that has been used in infrared frequency synthesis (IFS)

[1 2 3] schemes. The line of greatest interest (337 pm) can be made

to oscillate in a 0. 000 007 - THz bandwidth centered at 0. 890 76

1

THz. In order to improve the accuracy of subsequent laser frequency

measurements in general and to ultimately increase the accuracy of

the methane frequency measurement, it is necessary to stabilize the

frequency of the HCN laser to a high degree. To date, workers at

Martin Marietta have reported phase- locking the HCN laser to a microwave chain for short periods and obtaining beat note linewidths of

[4] less than 5 Hz. The accuracy of the reference was not measured

9 and could possibly be parts in 10 . A group at NPL in Teddington,

England, has recently stabilized the HCN laser to an absorption in

8 r 5

"1 difluoroethylene, and obtained a stability of 2.5 parts in 10 . This

note describes the HCN frequency stabilization work at NBS, Boulder.

In an effort to put all relevant information into a single document we

have included more detail than would be warranted for an external

publication.

The description of the technical work is divided into three

main areas. The first area involves determining the optimum laser

parameters, eliminating sources of frequency fluctuations from
the laser, preparing the diode to obtain best signal to noise beat note,

etc. The second and most difficult area is to obtain a microwave

frequency reference that has the requisite stability in both long and

short term. The third area involves active techniques, and includes

devising the necessary servos to impose upon the laser the long and

short term stability permitted by the reference. Two servo loops

have been used simultaneously. A phase lock loop was used to obtain

line narrowing, and a frequency lock loop served as an acquisition

aiding loop for the phase lock. The frequency lock loop applies a

mechanical correction to the laser cavity via a mirror on a piezoelectric translator (PZT). The phase lock loop applies a correction

to the laser plasma discharge current. The phase lock was achieved

with the aid of two different current controller combinations which we

denote as controllers A and B. Since controller A was difficult to

work with and required a third order loop to achieve phase lock, we

will limit our description to controller B which can be easily duplicated.

2. LASER PARAMETERS

Figure 1 is a block diagram of a representative scheme used

to measure the frequency of 337-pm line of the HCN laser. The master

oscillator source for the frequency measurement is a stable quartz

crystal oscillator. An X-band klystron is phase locked 30 MHz from
e

106.296 350 MHz

QUARTZ REF

OSCILLATOR

PHASE LOCKI

AND POWER

X-BAND

KLYSTRON

PHASE LOCK

AND POWER

E-BAND

KLYSTRON

30 MHz

(X100)

HARM.

GEN.

MIXER

FREQUENCY

COUNTER

T

J^ 10.599 635 GHz

30 MHz

(X7)

HARM.

GEN.

MIXER

X 7

'

SPECTRUM

ANALYZER

A

HORN

0.8907606 THz

5 GHz

30 MHz

IF AMP

(xl2)

HARM.

GEN.

MI VCD

i I

1*1 1 A t K MATCH.

>t 30 V Hz 1MET

.

Figure 1. Block diagram of experimental scheme for an HCN

laser frequency measurement.

|B- 1Q2751 |
the 100th harmonic of the quartz crystal oscillator. An E-band

klystron is in turn phase- locked 30 MHz from the 7th harmonic of the

X-band klystron. The output of the E-band klystron goes to a harmonic

generator/mixer where its 12th harmonic is compared with the 891-

GHz output of the HCN laser and the approximate 3 0- MHz frequency

difference is measured on the RF power spectrum analyzer. In addition to measuring the frequency of the laser, this beat note can be

utilized in frequency control. These various laser parameter adjustments are used to obtain the best signal to noise ratio for the beat note

as seen on the spectrum analyzer. These techniques are applied to the

components in the block diagram in Fig. 1. A schematic drawing of the 0. 1-m diameter, 4-m long folded

confocal HCN laser with Michelson coupling output is shown in

[6]

Fig. 2a. Invar spacers are used to minimize thermal expansion

and the high Young's Modulus reduces acoustically induced length

[7] fluctuations. Both the laser and the klystrons are supported on an

air piston isolation table to decouple the system from building vibrations

at 30, 60, and 120 Hz. Without the isolation, the resulting 30-Hz frequency modulation has produced frequency deviations of the 891 -GHz

output as large as 100 kHz for one particularly unfavorable mounting

arrangement. (This occurred when some normally inoperative
-DIELECTRIC BEAM-SPLITTER

COUPLER

POLYETHYLENE LENS RING ANODE

Figure 2 a. Schematic drawing of a frequency controllable HCN

laser. The translatable mirror A may be replaced

by a window to allow one output for frequency control

and the other for synthesis to higher frequencies.

Figure 2 b. Ring anode and resulting stable plasma configuration.

IJ-Httt
mechanical equipment was working in the room below our laboratory).

The beat note amplitude depends (within limits) on the laser power. The

output power from the laser is limited by two factors (discharge current

and beam splitter absorption) in addition to the design length. To avoid

deterioration of the water cooled cathode, the d. c. current is usually

limited to 0. 6A. This current is maintained by a regulated power

supply and is controlled by an external current controller which permits frequency modulation. Modulation experiments with the current

controller indicated a drum head resonance at about 25 Hz in the

original beam splitter. This unwanted response necessitated going

to a thicker beam splitter (0. 5-mm to 1-mm polyethylene was sufficiently thick) to avoid the problem. This thicker beam splitter

limited the laser power to 65% of the output power with a 0. 05-mm

beam splitter.

A stable gas discharge in the laser cavity is required because

the frequency of the laser output is a function of the resonant frequency

of the cavity which in turn depends on the refractive index of the plasma.

A turbulent discharge introduces fluctuations of the frequency of the

laser. The stability of the plasma is influenced by the d. c. discharge

current, pressure of gases, and electrode geometry. The optimum

pressures we have found for a current of 0. 6A are 0. 150 torr (1 torr=
2

13 3. 3 N/m ) of methane and 0. 275 torr of ammonia. These parameters

yield a stable plasma with stationary striations about 2. 5 cm apart.

Several anode geometries have been tried. The best plasma

stability was obtained by the one shown in Fig. 2b. It consists of an

elongated aluminum ring with 9. 9- cm O. D. and 7. 4- cm I. D. The

aluminum is anodized to prevent the plasma from terminating anywhere

except on some 15 conducting inserts. The 2. 5-cm long inserts are

made of stainless steel and the conducting ends are concave with an

11 -mm radius. Good electrical contact with the aluminum is obtained

by drilling undersized holes which will initially accommodate the inserts

only after they have been cooled to cryogenic temperatures.

The one disadvantage of this cathode is the difficulty of renovating it in the event that one of the contacts develops a hot spot.

Such a hot spot permanently alters the contact and can cause the discharge to terminate on a single contact. A more recent design permits

the anode to be disassembled and the stainless steel surface renovated

without damaging the insulating anodized layer. Details of this design

are available upon request.

The harmonic generator mixer used to mix the E-band signal

with the laser output consists of a silicon chip and tungsten catwhisker.

The catwhisker is pointed in a 10-molar solution of KOH while applying

a small voltage between the whisker and the solution. The chip and
whisker dimensions used in the commercial mount and a guide to the

proper etching voltage also are available on request. Best results have

been obtained after reducing the diameter of the whisker to a slight

tapered configuration, although reasons for this are not clear. Several

run-in contacts are usually made before one obtains a good junction.

One criterion for a contact suitable for harmonic mixing is a fast risetime square wave of at least 2 mV resulting from the rectification (the

antenna is positive for a silicon diode) of the chopped laser beam, which

has been optimally directed into the input horn of the mixer. One can

sometimes improve the signal by additional pressure by further runningin the differential micrometer screw. The mount is designed to allow

one revolution of the screw after contact without deforming the whisker

proper. Normally the diode contact is quite susceptible to vibrations,

however when located on the vibration isolation table, the diode will

generally last three to four weeks without requiring recontacting.

The signal to noise ratio is optimized after a suitable beat note

is observed on the spectrum analyzer. The variables are the positions

of the 74 and 891-GHz waveguide sliding shorts, laser side-mirror

position (mirror A of Fig. 2a), 74- GHz power level, diode contact

pressure, laser power, compensation values in the klystron phase lock

loops, and laser beam positioning mirror orientation. The signal and

noise levels for a typical adjustment are shown in Fig. 3. In order
Figure 3. RF power spectrum of beat note between HCN laser

and 12th harmonic of 74- GHz klystron. Dispersion is

20 kHz/ cm, bandwidth is 1 kHz, and sweep rate is

5 ms/cm. Above, display is linear with 100 mV/cm

and input signal attenuated 50 dB. Below, display is

logarithmic with 10 dB/cm, Signal to noise ratio is

about 25 dB. The white square in the figures denotes

1 cm2

.

10
to eliminate noise and extraneous beats near 30 MHz (which are picked

up from various klystron lock boxes), the output from the IF amplifier

has been passed through a 30-MHz crystal filter with a 3-dB band pass

of 200 kHz.

3. STABLE FREQUENCY REFERENCE

An immediate objective is to obtain reference stabilities of parts

in 1 in both long and short term at one THz. The basic reference

chain is as follows: An X-band klystron is phase locked to some

harmonic (say the 100th) of a quartz crystal oscillator reference,

then an E-band klystron is locked to the 7th harmonic of the X-band

source. The HCN laser frequency is then compared with the 12th

harmonic of the E-band source. The basic frequency relations and

the signs of the offset frequencies in the reference chain are indicated

in Fig. 4. (An alternative scheme would go from 1 0. 6 GHz to 0. 891

[8] THz using a Josephson junction; however the disadvantages outweigh the advantages at this time. )

The absolute stability of the locked laser depends upon the

driving source of the reference chain, a quartz crystal oscillator. To

synthesize from a good quality quartz oscillator at 5. 06 MHz to the

HCN laser at 0. 891 THz requires a multiplication factor of 176, 400.

11
HCN LASER

E-BAND

KLYSTRON

X 12

0.8907600480 THz +30.000000 MHz T

X 11 —

r

I

QUARTZ

OSCILLATORS

X 90 X 100 X 100

X-BAND v i !

I CLYSTRON I 1-74.227

: 7 +30.150

T X 6

KLYSTRON " ' ' 1-74.227 50400 GHz

x 7 +30.150000 MHz

-30.000000 „ , ±J I 1-10.59962200 GHz + ft T

!

I

Li

l

i

i

I l

18.1106911 MHz

I I 106.2962200 MHz

X 21 *

5.061724762 MHz

Figure 4. Frequency synthesis scheme for stabilization of HCN

laser frequency.

12
This places some rather stringent requirements on the spectral purity

[9]

of the quartz crystal oscillator's output.

Our most convenient qualitative check on the short term

stability of the reference oscillator (5 MHz to 100 MHz) is obtained

by examining the beat note between the multiplied reference signal

and the HCN laser at 0. 891 THz. When multiplied to this frequency,

the short term stability is sometimes such that the multiplied reference linewidth is broader than the laser linewidth (which was to be

narrowed by locking to the reference!). Figure 5 shows an extreme

example of this, as well as of some sources of noise. These linewidths are to be compared with the 2 to 5 kHz linewidth of a current

state of the art HCN laser. The HCN laser is free running in these

tests. This sequence of beat notes suggests that a Gunn diode oscillator

when stabilized by a superconducting cavity might be the best reference for locking an HCN laser. Another moderately good reference

might be a Gunn diode oscillator locked to a high quality quartz

crystal oscillator.

3. 1. Short Term Stability Requirements

We let a superscript caret (/^) indicate quantities at the output

frequencies, for example, <j = nv (0.891 THz = 84 x 0. 0106 THz). A

13
14

vu
minimally acceptable reference signal should have a linewidth, W ,

at 891 GHz of about 100 Hz or less. If we were to assume white frequencynoise in the X-band source, then the requirements on the X-band source

[9] would be

W-3dB 100 Hz . nn . W rT ^hr

= 2~~ = T = °* ° 14 Hz

" • (D 3dB (n

2

) (84)

2

The 0. 014-Hz linewidth requirement on the X-band source can be

expressed alternatively in terms of its spectral density of frequency

fluctuations, S. (f). Since we assume only white frequency noise is

present, we may use the well-known relation [9]:

W-3dB

:5 TTS (f) , 6v

S

6v

(f) = B

a

For a 100- Hz noise analysis bandwidth, B , and white frequency noise

cl V

W-3dB

> _^/ (0.014Hz) (100 Hz)

tt ^~&' ' f rr

(2)

(3)

(6v) =\ ^^ ^ )=\ "

' V (4a)

rms f tt a rr

= 0. 668 Hz in a 100 Hz bandwidth 1000 Hz (4b)

(or any frequency) from carrier.

15
This is a factor of two larger than the 0. 3 Hz rms in a 100 Hz bandwidth at f of 1000 Hz as measured for a high quality two-cavity klystron.

However, the assumption of white frequency noise is a critical one, and

often a l/f model (flicker noise of frequency) is a better engineering

approximation for high quality sources.

Reference [9 ] is concerned with relevant noise aspects of

microwave to infrared frequency synthesis. It uses the concept of

the fast linewidth W . In its Fig. 7 it summarizes some results for

power law noise spectral densities, i. e. , spectral densities of the form

S

6v

(f) " V • < 5 >

The coefficient H in the f power law expression is (numerically)

a

the density evaluated at unity frequency. We now assume that the

0. 3 Hz rms frequency fluctuation of the X-band source is due to a

flicker of frequency noise, i.e., S c (f) = H ,f with a = -1. The

ov -

1

relevant equation for the fast linewidth in Fig. 7 of reference [ 9] is

Wf, -3dB

ffi)

/_8n_l H

-1

(6)

The fast linewidth is a measure of the coherence rate of a nearlyperiodic signal. For white frequency noise the fast linewidth is the

same as the classical linewidth. For detailed discussion, see [9] •

16
or Gf, -3dB mm

(7)

The operation of frequency multiplication increases the spectral density of frequency fluctuations by the factor (n ). Provided that B «f

H . = 4ivj >

B

(8)

or

H = n

2 <(6y)

2

>

B

(9)

Hence for flicker of frequency noise

Wf, -3dB w) B

n(6v)

rms

(10)

For n = 84, with (6v ) = 0. 3 Hz at X-band in a bandwidth B of

rms a

100 Hz at f = 1000 Hz, the fast linewidth is

W = 170 Hz.

f, -3dB

(ID

This is only slightly worse than the desired 100 Hz linewidth.

17
Another significant source of noise in the reference chain is

the flicker of phase noise associated with the quartz crystal oscillator

to which the X-band klystron is phase- locked. The spectral density of

this noise is

S

6v

(f) = H

l f

1 (12)

Unfortunately, the analysis of this flicker of phase noise situation

(a equal to unity) is more complicated than for the power law spectral

densities where a is less than unity. From equation (38) of

reference [ 9 ], the fast linewidth for a less than unity is

a

W = 2 (13)

f, -3dB * W H

a

IT sin

2 -

1 - a

We can see that the behavior of the linewidth is becoming pathological

as a approaches unity.

Due to the f behavior of the spectral density, in order to

achieve high-order frequency synthesis it is necessary to have some

filter with a bandwidth B which furnishes a sharp cutoff at some

frequency f « B/2. (This could be a filter or filters say just after

the quartz crystal oscillator, at some intermediate stage in the

multiplier chain, and/or in the klystron phase lock circuitry. )

18
If the bandpass filtering of the reference signal is done at a

low (carrier)frequency stage, such that at that frequency

W£,-3dB <<f

u' < 14>

and if subsequently this signal is frequency multiplied such that at the

output frequency

V3dB > V < 15 >

then we can use a graphical construction similar to Fig. 6 of

r I /**

reference |_9J to obtain an estimate of W, , . The behavior of

f, -3dB

the filtered flicker of phase noise is as f for f less than f , and

u

_ 3

as f (for a single pole bandpass filter) for f greater than f .

The corresponding algebraic analysis is as follows:

S

6v

(f) = "l

£

' (16)

and therefore

hi (f

> = V

-1

(17)

for the unfiltered flicker of phase noise. After filtering by a single

pole bandpass filter of bandwidth B = 2 f , for f greater than f

the resultant signal is

u u

19
s ** ( f>f > 0$ u

= H

filtered

1 \f/ '

or

S *Jf>f > ov u

filtered

f 2 H f

1 u 1

After frequency multiplication by the factor n we have

(18)

(19)

/\ 2 2 -1

's. (£>f ) = (n f*H.) f 6v u u 1 (20)

A 2 2

that is, a = -1 and H = n f H, . u 1

Substituting into the equation for a = -1 for the fast linewidth (a procedure which is approximately valid provided W ~ f )

we obtain

Wf, -3dB (#)

AH, (21)

To multiply from 5 MHz to 0. 891 THz, we have n = 176, 400. If the

flicker of phase noise at 5 MHz has the commonly encountered

intensity [ ll] of

-11? 2 -

1

S..(f) = (10 '^rad ) f , o$

(22)

then

20
S

6v

(f)=(10

_11 - 2

) f

1

, ( 2 3)

H = 10" 11 * 2

. (24)

For an assumed bandwidth of 1 kHz, f = 5 kHz, ana" u

Wf, -3dB 2. 20 n f Ju _ , (25a)

u^f -1

4. 8 kHz. (25b)

This contribution from the flicker of phase noise is severe. At v = r o

0. 891 THz for our assumed conditions it is a greater contribution to

/\

the W than is the contribution from the flicker of frequency noise

of the X-band klystron. Several methods are available to avoid

deterioration of the laser spectrum by the multiplied phase noise.

One is to decrease the bandwidth B of the klystron lock loop for

example. However, the klystron loses lock if the bandwidth is

narrowed by too great a factor.

Recently, we have investigated a scheme similar to that

[12 ] developed by Risley to obtain the best spectral purity for direct

multiplication from X-band to frequencies in the 10- THz region.

This scheme involves using the quartz referenced klystron as a

source to injection lock a second cavity-stabilized klystron. The

21
amount of phase noise on the reference oscillator that is imposed

upon the injection locked klystron is dependent upon the level of the

injected signal, i. e. , the control bandwidth depends on the injected

power. The attenuator between the two klystrons is analogous to a

variable upper frequency cutoff on a filter.

Risley' s source is intended to be multiplied directly to the

laser frequency in a Josephson junction. In our case we are using

room temperature diode harmonic generators and the 7th harmonic of

X-band is used to phase lock a 74- GHz klystron.

Both a cavity- stabilized X-band klystron and a phase locked

X-band klystron are connected to a waveguide switch. In position one,

the 74- GHz klystron is locked to the phase locked X-band klystron.

In position two, the 74- GHz klystron is locked to the cavity-stabilized

klystron and the phase locked X-band signal simultaneously fed

through an attenuator to injection lock the cavity -stabilized klystron.

Sufficient attenuation is available to avoid injection locking. Figures

6a, b show the resulting improvement in the signal to noise ratio of

the beat note when using the cavity-stabilized system. At this level of

examination, an injecting signal caused no deterioration of the signal

to noise ratio. The difference is even more pronounced in Fig. 6c, d

where the Lissajous figures are shown when the laser is phaB-e locked.

22
° s

o a

tuD 23
(It is pointed out here that the controller A was used in this experiment. ) The resulting absolute linewidth of the laser when phase

locked to the cavity- stabilized klystron is discussed in the last

section of this report.

4. ACTIVE TECHNIQUES

Figure 7 shows a block diagram of the scheme for stabilizing

the laser. It consists of two servo loops, one for frequency locking,

the other for phase locking. The frequency lock loop consists of a

frequency discriminator, preamp and filter, and a translator driver

with a slow (up to 100-Hz frequency response) correction applied to

the piezoelectric translator (PZT). This loop serves as the acquisition aiding loop for the phase lock system.

The phase lock portion of the scheme is shown on the right. It

consists of a filter, a 29. 75-MHz quartz oscillator, a balanced mixer,

a 250-kHz reference oscillator, a phase detector and filter, and a

current controller.

4. 1. Frequency Lock Loop

The frequency lock loop is adequate for purposes other than

high accuracy frequency synthesis, and can easily be improved with the

incorporation of a crystal discriminator (sensitivities like 60 mV/kHz)

24
<L> O

enCo

.1-1 (->

cti N

COOuo

CQ

25
or a frequency to voltage converter. The present discriminator

(0. 11 mV/kHz) requires a 100 to 120 dB IF amplifier to get proper

limiting in the discriminator. The resulting amplified Johnson

noise, the unwanted ground loops, and the spurious instabilities in

the plasma necessitate a high degree of filtering to avoid broadening

the laser lines. A time constant of 0. 2 s permits servoing out resonant frequencies of the isolation table on which the laser is mounted

and at the same time avoiding deterioration of the laser signal due to

the loop noise. The frequency control system can serve as an acquisition aiding loop for a phase lock system.

4. 2. Phase Lock Loop

To narrow the laser linewidth below the 2 to 5-kHz free- running

linewidth requires a narrow reference and a more sensitive discriminato:

or a phase lock circuit. The phase lock loop shown in Fig. 7 has been

used for this purpose. Two different current controllers were successfully used in the experiments.

An initial complication in attempting to phase lock the laser to

a microwave chain was the presence of noise primarily arising in the

chain. In order to avoid the noise and to test the loop, the laser was

[13]

initially locked to a second HCN laser. This proved to be a very

helpful step in empirically determining the initial gain and filter para26
meters. It also established that the beat note width was primarilydue to the laser rather than the microwave chain used next.

After determining the optimum filter parameters, the second

HCN laser was replaced by the microwave chain. The 30- MHz output

was filtered by a 200-kHz band pass filter and beat down to a nominal

250-kHz to facilitate use of the 250-kHz phase detector. A 30-MHz

phase detector could be used; however, the advantage of being able

to monitor the Lissajous on the scope (the scopes available in our lab

did not have X-axis deflection at 30 MHz) makes 250 kHz a desirable

working frequency. The maximum output of the phase detector, k , d

is 1 volt/rad and the gain of the current controller A -laser combina5 5

tion, k , is 10 Hz/volt. Hence k k = 10 Hz/rad. The transfer

o

' o d

function for an ideal controller- laser combination is

L(s) = k

okd

F(s)

s+ k kF(s) * o d

(26)

Phase lock was achieved and after empirically adjusting the filter

parameters for best lock, it was found that the filter

F(s) = (ST

2 + 1) (ST 4

+ 1)

(ST

1 + 1) (ST

3 + 1)

(27)

had zeros at 1. 28 kHz and 2. 55 kHz and poles at 25 Hz and 130 kHz.

27
The loop transfer function is then

T , , K(s)K

d

(s Vl)(s Vl)

s V3 + S K(S) K

d

T 2

T

4

+ SL(S) K

d

(T

2 + V + k(s) K

d

Useful design parameters such as the natural frequency U) and the

damping factor are not easily extracted from this third order loop

expression. Since current controller B is less complicated, more

amenable to analysis, and (more importantly) much easier to duplicate

elsewhere, we restrict our further description to current controller B.

5

Controller B has a gain constant k = 7. 5 x 10 and the frequency

dependence of amplitude and phase are as shown in Fig. 8.

Unlike controller A (which required a unique filter to achieve

phase lock) controller B can be locked with a simple filter whose values

are not critical, but can be adjusted for optimum damping, etc. A

typical filter for controller B is

(st + 1)

F( S } 1 (29)

S( V T2 + 1}

' where the filter parameters are:

R T = R C = 0. 002s

WWV } >

2 kQ

r 2 ^> 200 n

luF

T2

= R

2

C = °* °° 02S

28
r^^l T T I T T

0.5

10 20 50 100 200 500 1000 2000 5000 10000 20000

FREQUENCY, Hz

o

-60

UJ

UJ -120

UJ

UJo

. -180 Uto

UJ -240

a:

-300

10 20 50 100 200 500 1000 2000 5000 10000 20000

FREQUENCY, Hz

Figure 8. Response of the current controller - laser combination to a fixed

amplitude modulation signal.

29

Eaaaaz
[ 14] Then the transfer function is:

uu s (2uu - n + uu ) n iTTT n

o d

L(s) = 2 „ 2

s + 2 uu s + uu n n

(30)

where

uu = n

k

oKd

T

l 2

= 1.8x10 rad/s (31)

and the damping factor is

o d

This would lead to a pull in frequency of [14]

(32)

Auu = 2 (2 uu k-uu

2

) = 1.3x10 rad/s ( 33 ) p n n '

which is within the range of the acquisition loop. The phase lock loop

will hold lock for times of the order of a minute. The acquisition loop

increases this time about an order of magnitude, and, while it does not

prevent the laser from skipping cycles, it does return it to the locked

condition. The 10-minute lock time does permit a frequency measurement which is the objective of the work.

30
Figure 9 shows the beat note between the HCN laser and the

reference when the laser is frequency locked, a, and when the laser

is phase locked, b. Controller B was used for this test.

Figure 10a shows the beat note at 30 MHz on an expanded

scale for the phase locked condition. The post detection bandwidth

is greater than 10 kHz. The probability of seeing the beat note at the

center of this 200-Hz window is better than 97% during a 10-minute

period and thus the system could be utilized for IFS as is. If it is in

the center of the window we know its frequency to within about one

part in 1 . In general, one can make about 5 to 10 consecutive

frequency measurements with 10 s counting time for each and determine

the difference frequency when the beat note is as shown. (In the event

that the signal is too weak to count, one can count the 29. 75-MHz and

the 250-kHz references, the sum of which is the phase locked beat

frequency). From the data displayed we infer that the laser is following the reference but we still do not know how wide the laser is.

Figure 10b shows a Lissajous figure where the Y-axis is

derived from the beat between the laser and microwave reference

chain (down converted 25 kHz) while the X-axis is driven by the phase

detector reference oscillator. We reiterate here that there is an

advantage to operating the phase lock loop at 25 kHz, as say compared

to 30 MHz due to the fact that one can monitor the Lissajous figure

31
Figure 9. 74- GHz vs HCN laser beat notes.

500 Hz/ cm., 30 Hz bandwidth.

Sweep rate 1 s/cm. Linear display.

Top: frequency lock, lOmV/cm.

Below: phase lock, 20 mV/cm. The

white square denotes 1 cm .

32
Figure 10. Quality of phase lock of HCN laser.

Tope RF power spectrum of beat between HCN laser

and 12th harmonic of 74- GHz klystron with laser

phase locked to 12th harmonic. Vertical scale is

linear, dispersion is 20 Hz/ cm, bandwidth 10 Hz,

and sweep rate 1 s/cm (30 MHz is center frequency).

Below: Lissajou figure at 250 kHz where Y-axis is

driven by down converted beat note and X-axis is

driven by phase detector reference.

33
while adjusting the filter parameters, gain, etc. for good operating

values. The Lissajous figure also tells when the loop becomes unlocked and skips cycles. Of course, with a scope with a 3 0-MHz

X-axis drive capability one could do the same.

Figure 11 shows some preliminary indications of beat frequency

fluctuations (square root of an Allan variance) as determined with a

commercial computing frequency counter. Each point represents 100

contiguous measurements except for the two points indicated by an

asterisk where M = 1 0. Shown are frequency fluctuations versus

averaging time for the HCN laser under three different modes: free

running, frequency locked, and phase locked. The rise at 1 s in

the phase locked mode is due to the laser dropping out of lock

momentarily. The frequency lock loop does cause it to return immediately to the phase locked condition. We are working towards

keeping it locked for longer periods, although it is not absolutely

essential for our present purposes of IFS. Again, this is a measure

of how closely the laser is following the reference, but gives no

measure of the laser linewidth.

We have an experiment planned to measure directly the linewidth of the reference (and consequently of the laser) at 1 THz. One

would like to beat the 84th harmonic of one X-band klystron with the

84th harmonic from another tuned to a slightly different frequency, but

34
i—QZ3

o

1/1

- <3 O o

ino

4J

p. cm oy

!>> u

s

CU O ua* rt ' a) -jh H

m rt CM >

bo ii p. q ^

p .-i a <

2 *

Id "g ~ a o

CU

f-i

CO

CD

u O

u p

i—i

1> <+-i u

rrl >^ p o

rn 013

<u a

1

+» Jh

CU

,0

CTJ CO

CO

cu

CO

cti i—

<

O

X

PQ N

o

- ii Epq

Z H 'SNOUVniOmj ADNjinOS'HJI IV3H cuM•i-i

35
there are ambiguities in such an experiment. However, by phase

locking an HCN laser to the 84th harmonic of klystron A, all the

other harmonics are discarded, and the 84th harmonic of klystron B

can be compared with the phase locked HCN laser with a Josephson

junction multiplier mixer.

The scheme for such an experiment is indicated in Fig. 12.

The HCN laser described earlier has been modified to obtain outputs

from both sides of the beam splitter. One output is used in the control

loop and the other is used for examination of the spectrum with an

independent multiplier chain. Figure 9 showed the gross features of

the beat note between multiplier chain A and the laser when the laser

is either frequency locked or phase locked as monitored on spectrum

analyzer A. Figure 10 gave a better indication of the degree to which

the phase of the HCN laser can be made to follow the phase of the

reference. Since the laser is tightly locked to the output of the

multiplier chain A, the display on spectrum analyzer B really compares the output of the two multiplier chains. This technique provides

an experimental means for checking theoretical predictions of the

linewidth based upon examination of noise spectra at 5 MHz and 10

GHz and helps to indicate what is needed to achieve the desired linewidth for the HCN laser.

36
Gi n) ^£

C£ OC o LO UJ LU z Cxi • h-—

1

3: l>N

PUTI UNTE G-+AA/V- RIES RREN TROL R

PO

PPLY > III 3

1' « s: o LU UJ DZ UJ ID o <_> *» -^ CD CO 00 CO CO CO o O

ZC

c_> _j

i l ^-^ s Jh

.-1£ ^ ""

PI 1—

t

" ""* ^

CO .»•—~^ <c CD >- •rH 2: 2: O C£ 3

id o: id q: —1 I— in OH LU *" -V DC UJ l-H

rt 1— M 1— r>j LU ID <D (J >- C_) >- CO CD

UJ _1 **"~^»» UJ _! «=c cc a

CL. <£ q- «=c DT i—

1

?H CO ^ UJ CO z Q- O O

<< ** * Q <E <+H O 0) .

1 ^ 1 1 1 § u

<•" " *w <=c 4

<L> CO 3 j co

4

00 Na to-/S

O N

ro D-- r V 2 n1 CD O

8 J

1 CO O

1 1—1 Td --I cc on CD

UJ UJ ^2 1—i CQ 5 ^_L •—1 <c

1 nn _i

CQ Ch D- Z j

.

1 O- z —( l-H /Mllll\V% l-H 1—

1

1— <c 1— < • _l DZ _l DT ID (_) ID

1—

I

2: 2: LU

2: CD CO

in «=c GO h

37
Ideally, one would like the multiplier chains to be identical,

however, present economic considerations preclude this. We have

nearly identical X-band cavity-stabilized klystrons. These have good

short term stability at 1 GHz, however, they need to be locked to a

quartz crystal oscillator for good long term stability. The output of

multiplier chain A is the 12th harmonic of a 74. 2- GHz klystron which

is phase locked to the 7th harmonic of a cavity-stabilized X-band

klystron (CSKA) at 10.6 GHz. The CSKA oscillator is injection locked

(at a low level) to an X-band klystron which in turn is phase locked to

the 90th harmonic of a 118-MHz quartz crystal oscillator reference.

The output of interest from multiplier chain B is the 92nd

harmonic of a cavity -stabilized klystron (CSKB) at 9. 6 GHz. The

92nd harmonic is generated in a Josephson junction which also

serves as the mixer to combine the 92nd harmonic with the laser

radiation. Oscillator CSKB is injection locked to the output of a

multiplier which is referenced to a 5-MHz quartz crystal oscillator.

The multiplier chain design is the same as that used in cesium

* A a v L15] standards work.

A preliminary experiment has been performed on a more

modest scale. In this experiment chain A consisted of a cavitystabilized X-band klystron and an E-band klystron which was phase

locked to the 7th harmonic of the X-band signal. Chain B consisted

38
Figure 13. RF power spectrum on analyzer B of beat between laser

and multiplier chain B when laser is (a) free running and

(b) phase locked to chain A. Dispersion is 5 00 Hz/ cm,

bandwidth is 100 Hz, and sweep rate is 0. 1 s/cm.

39
of a 118-MHz quartz crystal oscillator, a phase locked X-band ,

klystron, and a phase locked E-band klystron. The results (shown

in Fig. 13) while not spectacular, do indicate that the laser linewidth

has been narrowed. Further, problems formerly resulting from slow

drifts in the laser frequency are now controlled by the reference, and

cycle counting techniques can be used to monitor the frequency of the

HCN laser. The computing counter has been programmed to display

the laser frequency based upon its measured values for frequencies of

the X-band signal and the beat frequency signal (0. 891 vs 12x0. 074

THz).

5. DISCUSSIONS

In addition to being a useful method of stabilizing the HCN

laser, the phase locking technique also facilitates a diagnostic

technique for determining some of the sources of noise modulation

of the laser frequency. As an example, see Fig. 14 which shows

photos of traces of the Fourier components of the phase lock correction signal for a series of different changes in the overall system.

At the start of the experiment the laser was locked to a phase-locked

reference, using controller A for the phase lock correction, and

the isolation table was inoperative. The PZT driver was connected

to the end mirror. Figure 14a shows 0- and 360- Hz calibration signals

40
Figure 14. Spectrum of phase lock correction signal under various experimental

conditions more fully described in text, a) Calibration marks at 0, 360 Hz.

b) Combination ps- sec- A (power supply and series current controller - A)

X-band is phase locked, c) ps-scc-A, X-band is cavity stabilized,

d) (same as c) except PZT driver is turned off. e) (same as d) except

ps-scc-A has been replaced by combination B.

41
for reference, and Fig. 14b shows correction components at start of

experiment. In Fig. 14c, a cavity stabilized klystron was substituted

for the phase locked reference; the signal has not changed appreciably.

However in Fig. 14d, the PZT driver has been turned off. Gone are

the components at 120 and 240 Hz. These were due to 2nd and 4th

harmonics of a 60-Hz extraneous signal in the PZT driver. The 3rd

harmonic was possibly of such phase that it partially cancelled the

180- Hz component which now appears enhanced. Note the encircled

region below 60 Hz which has disappeared in Fig. 14e when isolation

table is operative. In Fig. 14f, a new power supply and current

controller are put into use and the large 60- and 180-Hz components

are no longer present. The correction signal was examined up to

20 kHz, however no appreciable components were found at higher

frequencies. Figures 15-17 give details of some of the circuits used

in the frequency control equipment.

The techniques described have been recently extended to an

8-m HCN laser, and we are currently operating with only a phase

lock correction. Lock times appear sufficient for most of our

current experiments.

The stability of our present reference is a part in 1 for 1 s

averaging time. Further, since the laser is phase locked to the refer42
ence, we can use cycle counting techniques (which are especiallyconvenient with the aid of a programable computing counter) to displayits frequency on the counter.

We have demonstrated that the phase of a laser signal can be

made to follow quite closely the phase of a reference signal synthesized

from the microwave region. The microwave reference could be a

superconducting cavity stabilized oscillator for best stability in shortterm (narrowest linewidth) coupled with a cesium beam primary

frequency standard for highest accuracy and for best long-term stability.

We point out that the stabilized laser system which we have described

can facilitate greater accuracy in infrared frequency synthesis and

measurements.

43
We are indebted to L. B. Elwell of the electronics group for

his invaluable support, numerous lengthy discussions, and other

indispensable assistance to the project, and to J. Skudler for the

design of current controller B. We are grateful to A. S. Risley

and J. H. Shoaf of the Time and Frequency Division for their

contributions regarding the development of a suitable reference.

The support and encouragement by D. A. Jennings is greatly

appreciated. We wish to acknowledge useful discussions with

B. Danielson and D. G. McDonald. We also extend thanks to K. M.

Evenson for helpful suggestions and to L. Wood of NOAA for his

phase detector and useful advice.

44
cmOi

c:o

CM -VW-

^1*

T-*W 1

co

CT» fcflcfr

LO -VWHH*

CO

CM 7i\

cjo

CM >

-VW—O^

en on. oo

CNJ Q. Z

CM HH'

-iHKr*

HH"

00

<»-—j<}—)<] k VW-4 VA OTM

o

CM

o

.r-l

-f->

+-> CO u

cu03

•

O

u JM

CJ o

CD "£

rt o & id

ID

CD

bfl h

45
a

uo

o

Oi

+-><U -d

<dw

CO

Oh&0

• rHo

§

oo

O

Oh

13

a,

Q>

•rH (!) PR W

VO

u

•1-1
COMMERCIAL POWER SUPPLY SPEC I F ICA TIONS

0-12KV, 0- 1 AMP VOLTAGE REGULATED

0-02% OR .4V FOR 10% LINE OR 25% LOAD VARIATION

RIPPLE AND NOISE - SUM LESS THAN

30MV PEAK

" ro PEAK

z

rt

7"CN LASER

- I WWW

1 .5K BALLAST

PHASE LOCK SIGNAL

POWER SUPPLY SCC

POSITIVE

BOTTOM VIEW (j)

5.1

(o o 2

(?) NEGATIVE

A)50uA

ILOWER FOR 3-1000Z

Q

1 Q

2

= 2N4240

MODULAR POWER SUPPLIES

ZELTEX ZM1550

Figure 17. Circuit for series current controller (SCC).

Skudler-Elwell 9-72.

47

IB-105386
REFERENCES

[1.] L. O. Hocker, A. Javan, D. Ramachandra Rao, L. Frenkel,

and T. Sullivan, "Absolute Frequency Measurement and

Spectroscopy of Gas Laser Transitions in the Far Infrared,"

Appl. Phys. Lett., vol. 10, No. 5, March 1967, pp. 147-149.

[ 2.] J. S. Wells, K. M. Evenson, G. W. Day, and Donald Halford,

"Role of Infrared Frequency Synthesis in Metrology, " Proc.

IEEE, vol. 60, No. 5, May 1972, pp. 621-623.

[3.] K. M. Evenson, J. S. Wells, F. R. Petersen, B. L. Danielson,

and G. W. Day, "Accurate Frequencies of Molecular Transitions

Used in Laser Stabilization: The 3. 39 |im Transitions in CH and

the 9. 33 and 10. 18 am Transitions in CO " Appl. Phys. Lett. , vol. 22, No. 3, Feb. 1973, pp. 192-195.

[4.] R. E. Cupp, V. J. Corcoran, and J. J. Gallagher, "Line

Narrowing in a Phase- Locked Laser, " IEEE J. Quantum

Electronics, vol. QE-6, pp. 241-243, April 197 0.

[5.] D. J. E. Knight, "Accurate Infrared Frequency Measurement for

a New Determination of the Speed of Light, " Proc. of the Frequency Standards and Metrology Seminar, Pavilion de la Foret

Montmorency, Laval Universite, Quantum Electronics Laboratory,

Quebec, Canada, pp. 296-324, Aug. 1971.

L 6.] J. S. Wells, K. M. Evenson, L. M. Matarrese, D. A. Jennings,

and G. L. Wichman, "Design for a Variable- Output Coupling FarInfrared Michelson Laser, " NBS Tech. Note 395, Jan. 1971.

[7.] C. Freed, "Designs and Experiments Relating to Stable Gas Lasers,

Proc. of the Frequency Standards and Metrology Seminar,

Pavilion de la Foret Montmorency, Laval Universite, Quantum

Electronics Laboratory, Quebec, Canada, pp. 226-261, Aug. 1971.

[8.] D. G. McDonald, A. S. Risley, J. D. Cupp, and K. M. Evenson,

"Harmonic Mixing of Microwave and Far- Infrared Laser Radiation

Using a Josephson Junction," Appl. Phys. Lett., vol. 18, pp.

162-164, Feb. 1971.

48
[9. ] Donald Halford, "Infrared- Microwave Frequency Synthesis

Design: Some Relevant Conceptual Noise Aspects", Proc. of the

Frequency Standards and Metrology Seminar, Pavilion de la

Foret Montmorency, Laval Universite, Quantum Electronics

Laboratory, Quebec, Canada, pp. 431-466, Aug. 1971. Copies

are available upon request from the author.

[10. ] S. R. Stein and J. P. Turneaure, "A Superconducting Cavity

Stabilised Oscillator of High Stability", Electronics Letters,

Vol, 8, No. 13, pp. 321-323, June 1972.

[11.] Donald Halford, A. E. Wainwright, and James A. Barnes,

"Flicker Noise of Phase in RF Amplifiers and Frequency

Multipliers: Characterization, Cause, and Cure", Proc. 22nd

Annual Symposium on Frequency Control, Fort Monmouth, N. J. , April 1968, pp. 340-341. See also H. Brandenberger et al. , "High Quality Quartz Crystal Oscillators: Frequency Domain

and Time Domain Stability", Proc. 25th Annual Symposium on

Frequency Control, Fort Monmouth, N. J. , April 1971, pp. 226-230.

[12. ] A. S. Risley, John H. Shoaf, J. Robert Ashley, and Frank M.

Palka, "Frequency Stabilization of X-band Sources for Use in

Frequency Synthesis into the Infrared" (Submitted to IEEE Trans.

Instrum. Meas. ).

[13. J R. Strauch suggested locking the two lasers as a diagnostic.

[14. ] Floyd M. Gardner, "Phase Lock Techniques," John Wiley and

Sons, 1967, pp. 7-13, 44-46.

[15. j D. J. Glaze, "Improvements in Atomic Cesium Beam Frequency

Standards at the National Bureau of Standards", IEEE Trans, on

Instrumentation and Measurement, Vol. IM-19, No. 13, pp. 156-

160, August 197 0.

49
FORM NBS-114A (1-71) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN-620

2. Gov't Accession No.

4. TITLE AND SUBTITLE

Frequency and Phase Stabilization of an HCN Laser by

Locking to a Synthesized Reference

3. Recipient's Accession No.

5. Publication Date May 1973

6. Performing Organization Code

7. AUTHOR(S)

Joseph S. Wells and Donald Halford

8. Performing Organization

10. Project/Task/Work Unit No. 2710151

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

Boulder, Colorado 60302

11. Contract/Grant No. N/A

12. Sponsoring Organization Name and Address

Same as Item 9.

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.) Infrared frequencies as high as 88 THz have recently been synthesized

using diode harmonic mixers with accuracies of parts in 10 . Stabilized lasers

are needed to make frequency measurements of higher accuracy. The HCN

laser is the lowest frequency basis laser used in these synthesis schemes and

its stabilization has been the subject of recent interest. The laser is stabilized

by locking it to a phase locked microwave reference chain. Two servo loops

are utilized. The first loop is a relatively slow frequency lock loop with the

correction applied to a PZT driver. This loop not only accommodates thermal

expansion of the laser, but also serves as an acquisition aiding loop for the

second servo. The latter is a phase locked system with the correction applied

to the laser discharge current.

Details of the laser design and some noise considerations relative to the microwave reference chain are presented along with some experimental

data which indicate the results of the stabilization techniques. Data regarding

the system stability and impro ved fast linewidth are included.

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

Fast linewidth; Frequency noise; HCN laser; Infrared frequency synthesis; Laser

frequency measurements; Laser linewidth; Laser stabilization; Phase locked laser.

18. AVAILABILITY STATEMENT

X UNLIMITED.

J FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCI ASSIFIED

21. NO. OF PAGES

56

22. Price

$ . 60

USCOMM-DC 66244-P71
NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau of Standards research and development in physics, mathematics, and chemistry. Comprehensive

scientific papers give complete details of the work,

including laboratory data, experimental procedures,

and theoretical and mathematical analyses. Illustrated with photographs-, drawings, and charts. Includes

listings of other NBS papers as issued.

Published in two sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on standards of physical measurement, fundamental

constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $17.00; Foreign, $21.25.

• Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design,

numerical analysis, theoretical physics and chemistry,

logical design and programming of computers and

computer systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign,

$11.25.

TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's measurement, research, developmental, cooperative, and publication activities, this monthly

publication is designed for the industry-oriented

individual whose daily work involves intimate contact with science and technology

for engineers, chemists,

physicists, research managers, product-development

managers, and company executives. Includes listing of

all NBS papers as issued. Annual subscription: Domestic, $6.50; Foreign, $8.25.

NONPERIODICALS 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 co- operatively 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 background knowledge for shopping in today's tech- nological marketplace.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau

Cryogenic Data Center Current Awareness Service (Publications and Reports of Interest in Cryogenics).

A literature survey issued weekly. Annual subscription : Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00. Send subscription orders and remittances for the preceding bibliographic services to the U.S. Department

of Commerce, National Technical Information Service, Springfield, Va. 22151. Electromagnetic Metrology Current Awareness Service (Abstracts of Selected Articles on Measurement

Techniques and Standards of Electromagnetic Quantities from D-C to Millimeter-Wave Frequencies). Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and

remittance to the Electromagnetic Metrology Information Center, Electromagnetics Division, National Bureau

of Standards, Boulder, Colo. 80302. Order NBS publications (except Bibliographic Subscription Services)

from: Superintendent of Documents, Government Printing Office, Washington, D.C. 20402.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use, $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215