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NBS TECHNICAL NOTE 680

U. S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

Application of Infrared Frequency

Synthesis Techniques With MetalInsulator-Metal Diodes to the

Spin Flip Raman Laser
NATIONAL BUREAU OF STANDARDS

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'L

Application of Infrared Frequency Synthesis

Techniques With Metal-lnsulator-Metal

Diodes to the Spin Flip Raman Laser

J. S. Wells

G. E. Streit

F. R. Petersen

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

r V"\

£

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

aS NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1976
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 680

Nat. Bur. Stand. (U.S.), Tech Note 680, 1 4 pages (Apr. 76)

CODEN: NBTNAE

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

Stock Number 003-003-01616-1 (Order by SD Catalog No. C13.46:680) $0.50
Application of Infrared Frequency Synthesis Techniques

with Metal-Insulator-Metal Diodes to the Spin Flip Raman Laser

J. S. Wells, G. E. Streit*, and F. R. Petersen

Infrared frequency synthesis techniques with a metal-insulator-metal (MIM) diode

have been extended to include the measurement of the frequency of a spin flip Raman

Laser (SFRL) . As a result of this extension, spectroscopy in the 5.3 ym region can

be put on a frequency rather than a wave-length metrology basis. Additional observations with the diode are in qualitative agreement with recent work relating to nonlinear tuning over axial SFRL modes.

Key words: Frequency measurements on tunable lasers; IFS with a tunable laser;

infrared frequency synthesis; SFRL frequency measurement; spin flip Raman laser.

I. Introduction

The metal-insulator-metal (MIM) diode has been used with success to measure the

frequency of several important gas lasers over the past few years. These measurements

2 have led to a new value for the velocity of light, and to new secondary frequency standards ' in the stabilized CO laser at. 9.4 and 10.6 ym ' and in the methane stabilized

helium neon laser at 3.39 ym. We report here an extension of frequency synthesis tech7 niques with the MIM diode to include a tunable laser: namely, the spin flip Raman laser

8 9 (SFRL) at 5.3 ym. ' This new development when coupled to recent developments of external

cavity operation of the SFRL, ' should help the SFRL achieve the potential for spectroscopy which has long been expected of it. Spectroscopy in this region can be put on a

12 frequency metrology rather than a wavelength metrology basis. The capabilities of

making frequency measurements at 5.3 ym have acquired added importance by virtue of a

recent class of two photon experiments at 10.6 ym with the CO laser.

II . CO Laser Frequency Measurement

The frequency of the SFRL is measured in two steps. In the first step, the frequency

of the CO pump laser is measured; the second step consists of a laser - SFRL difference

frequency measurement. Before proceeding, the essentials of infrared frequency synthesis

14

(IFS) will be briefly reviewed. To measure accurately the frequency of a laser, V , a

''National Research Council Post Doctoral Associate with

National Oceanic and Atmospheric Administration
frequency, v , which is close to the frequency to be measured, must be synthesized. The

difference between these two frequencies is an intermediate frequency, v , typically less

IF than one GHz. The unknown laser frequency is:

= V„ : t V M S IF

v = £,v. 1 mv„ + nv

S 1 2 yW

The quantities V and V are basis laser frequencies which have been determined by prior

synthesis measurement, and v is a microwave frequency. The harmonic numbers I, m, and n

y w are allowed both positive and negative values. The quantity (1 + | I \ + |m| + |n|) is

called the mixing order. " The harmonic generation, as well as the mixing which produces

the intermediate frequency to be measured, occurs in a suitable MIM diode, typically a

sharpened tungsten wire antenna on a nickel base. One particular consideration has been

crucial to these experiments. In prior IFS experiments the laser beams have been polarized

with the electric field vector in the horizontal plane which contained the antenna. The

angles between the laser beams and antenna were then dictated by long-wire antenna theory.

According to this theory, the angle between the antenna direction and the direction of the

first maximum in the radiation pattern is given by

= cos m

1 (i - <MP)

where A is the wavelength of the radiation being coupled to the antenna and L is the

distance between the tip of the tungsten wire and some discontinuity in the wire, either

in shape or direction. Although some contention exists as to the type of coupling that

prevails at 3.39 ym (perhaps a conical antenna) our experiments indicate that the long

wire antenna theory is applicable at 5.3 ym. Our measured value of 6 =11 ±1 is

commensurate with the antenna length at 100 ym. The unetched diameter of the antennas

used in this work is 25 ym.

The infrared frequency synthesis techniques described above are used in the system

shown in Fig. 1. The simpler CO pump laser frequency measurement is described first.

The CO laser power from the grating is focused onto a MIM diode thru an adjustable iris

(for power control) and 12.5 cm focal length lens. The output from the CO laser is also

focused onto the diode through an adjustable iris and a 25.4 cm lens. The microwave power
is coupled to the diode by aiming a sawed-off section of waveguide (which terminates the

microwave circuit) in the general direction of the diode. In the particular experiment

here, the frequency of P (17) of the CO laser is two harmonics of the P(18) line of the

CO laser frequency standard (2 X 28.359 774 THz) plus .046 607 THz from the klystron plus

an 0.000 030 THz IF beatnote, or 56.766 185 ± 0.000 001 THz. The klystron is phased

locked to a signal synthesized from the 4th harmonic of an X-band klystron plus the IF

locking frequency. The 46.63794 GHz frequency is thus determined by a frequency counter

operating at X-band frequencies. The resulting beat note (V - 2v -V ) which is co co ywave

shown in Fig. 2a not only completes the frequency measurement of the CO pump laser but is

used to stabilize the CO pump laser by the scheme in Figure 1. Over 100 CO lines lie

within 40 GHz of the second harmonic of some CO laser line and these CO frequencies may

be measured and stabilized in this manner. The CO laser would then have nearly the same

long term stability as the CO laser, and its absolute frequency would be known to

9 within a part in 10 . This procedure increases the utility of the CO laser as a pump for

18 a tunable Raman spin flip laser to be used for high resolution spectroscopy. Also, as a

monitor of the CO laser operation, one can observe the output from the diode and make

appropriate adjustments to insure that the CO laser is operating in a single mode.

III. SFRL - CO Pump Difference Frequency Measurement

The problem of measuring the difference frequency between the pump laser and the SFRL

output is complicated by three factors. First, the spin flip and pump signals have mutually

orthogonal polarizations, which originally presented difficulties in coupling to the diode

antenna. Second, the power output from the SFRL is low compared to levels generally used

for synthesis in MIM diodes. Third, the large collinearly-transmitted pump signal makes

it difficult to establish that the weaker SFRL output has been coupled to the diode.

Long wire antenna theory indicated that for best coupling the antenna should be

rotated in the plane of polarization by the angle with respect to the beam direction. m

An arrangement which tips the diode antenna down 11 in the vertical plane while maintaining an 11 projection in the horizontal plane with respect to the beam direction has

permitted 5 ym signals with either polarization to be coupled to the diode.

The research group at Heriot-Watt University has found maxima in the output power at

2 tesla for the I = Landau level and at 0.2 tesla for the I = 1 Landau level electrons

3
15 -3 involved in the spin flip process when using crystals with a 4.5 X 10 cm concentra19 14 -3

tion. Following this lead, the 8 X 10 cm concentration InSb crystal in our original

system was replaced with a crystal with a nominal concentration of 2.5 X 10 cm" . With

this 4 X 4 X 8 mm resonator, a broad maxima with an estimated 30 mw output power near 0.2

tesla was then obtained. This point of operation was selected since the frequency difference between the CO pump and the SFRL was predicted to be about 150 GHz corresponding

to the 2nd harmonic of an available klystron. Since the power available from the SFRL was

low, it was deemed desirable to keep the mixing order as low as possible in the initial

experiments. The SFRL power density is increased at the diode junction by focusing it

down with a 2.5 cm focal length lens. Sufficient transmitted pump power (although the

radii of curvature of the pump and SFRL wave fronts are not generally equal) was also

coupled to the diode through the same lens to produce a beat note between the laser and

the 75 GHz klystron. This beat note (shown in Figure 2b) has a frequency of 350 ± 5 MHz.

Our spin flip laser frequency (at a magnetic field of 0.2142 tesla) was the CO pump frequency (56.766 185 THz) minus two harmonics of the 0.074 057 THz microwave frequency plus

a 350 ± 10 MHz beat as determined by the spectrum analyzer calibration or 56.618 421 ± 0.000

010 THz. Again the 74.057 286 GHz is related to the 7th harmonic of an X band signal

which is measured with a frequency counter.

IV. Further Diode Coupling Considerations and Observations

The first step in coupling the desired signals to the diode has been to monitor the

rectified signals on the diode when chopping the laser beams. On past occasions this

rectified signal has indicated that either the SFRL power or the transmitted pump power

was coupled to the diode, although the more general case is to have both coupled simultaneously to some degree. Since the transmitted CO pump is somewhat larger than the SFRL

signal, it is often difficult to ascertain that the latter is coupled to the diode.

In order to affect the most favorable situation, we carefully align the InSb resonator

normal to a red laser beam which is collinear with the CO pump. After manipulating the

SFRL output and transmitted beams with a three dimensional translator to obtain a maximum

rectified signal from the diode, a 1.25 GHz spectrum analyzer is then used to monitor the

beat notes. The essential features of the spectrum analyzer display are sketched in
Figure 3a. Three classes of beat notes are indicated. Beat notes labelled Class I are

intermode beats (possibly off axis modes) associated with a single longitudinal mode.

Class II beat notes occur between adjacent axial modes and the Class III beats result from

beating of signals from the CO pump laser, the SFRL, and the microwave source. The SFRL

coupling is indicated by intermode beat notes of the Class I designation. The focusing

lens is further manipulated and the diode impedence is varied to maximize these beat

notes. An additional periodic variable is the magnetic field since the SFRL gain reflects

the resonator modes to some degree. When the amplitude-bandwidth area for Class I beat

notes is minimized, beat notes designated as Class II appear, tune rapidly, and disappear

as a function of magnetic field. These Class II beats were initially mistaken for those

beats denoted by Class III, however they require only the spin flip laser for interpretation.

The notes we have designated Class II result from the non linear tuning over an axial

20 21 22 23 mode and two axial modes oscillating simultaneously near the mode hopping point.

These considerations are sketched in Fig. 3b. Because of the varying structure of the

aggregate of Class II beat notes (due to intermittent modes as the SFRL frequency is

swept) and the smallness of the interval, AH, it is difficult to ascertain the value

Av/AH. Our estimate of 0.1 to 0.2 GHz per millitesla is compatible with results obtained

20

elsewhere.

The beat note of prime interest is Class III which completed the SFRL frequency

measurement. The amplitude of this depends quite strongly on the microwave power level as

expected. In order to obtain the 30 db beat note shown in Figure 2b, we have directed all

the available microwave power (which is nominally 500 mW toward the diode. The transmitted

pump power was about 100 mW and the SFRL power estimated to be 30-50 mW. The fraction of

any of these levels coupled to the diode of course is not readily determined. Operable

diode impedence ranged from about 100 to 700 0..

The 300 MHz beat array of intermode beats at the origin might be some cause for

dismay without some elucidation. The intermode beating is a 2nd order process and the

modes indicated could thus be relatively weak. Another possibility is that some of the

structure is due to harmonics of two modes beating together. The observation of one beat

note of the Class 111 variety (4th order mixing) does not necessarily mean that the SFRL
is going single mode, but it does indicate that if other modes are present, they are

relatively weak compared to the principle mode and probably would not limit resolution of

molecular spectra.

We do not imply by the exhibition of the single beat note in Figure 2b that our

system did not exhibit the undesirable situation of more than one mode at some magnetic

fields in addition to the mode hopping region. The non-linear mode tuning is another

feature which is undesirable from a spectroscopic viewpoint. Clearly the SFRL requires

some improvement and recent work elsewhere indicates that these solutions are very close. '

In order to utilize fully the infrared frequency synthesis techniques described here,

it is desirable to improve the SFRL performance in three areas. These objectives, minimization of mode hopping, reduction of cavity pulling, and operation at higher power on a

single mode, can possibly be attained by an external cavity operation ' of the spin

flip laser. We plan to attempt some of these improvements in the near future. The technique of scanning an external mirror with the magnetic field drive signal seems particularly appealing at present.

We would like to acknowledge helpful discussions with Dr. D. A. Jennings, NBS , Boulder

and thank Dr. W. Schade and Dr. S. Miller of the Naval Electronics Lab in San Diego for

pointing out their "poor man's" polishing technique which we used to polish our InSb

samples. The pleasant association with Dr. J. J. Jimenez (a guest worker from LPTF,

Observatoire de Paris) also proved of worth to this experiment.
References

1. Daneau, V., Sokoloff, D., Sanchez, A., and Javan, A., "Extension of Laser Harmonicfrequency Mixing Techniques into the 9 y Region with an Infrared Metal-Metal Point

Contact Diode," Appl. Phys. Lett., Vol. 15, pp. 398-401, 1970.

2. Evenson, K. M., Wells, J. S., Petersen, F. R. , Danielson, B. L. , Day, G. W. , Barger

,

R. L. , and Hall, J. L. , "Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser," Phys. Rev. Lett., Vol 29, pp. 1346-1349, 1972,

3. Evenson, K. M. , Wells, J. S., Petersen, F. R. , Danielson, B. L. , and Day, G. W.

,

"Accurate Frequencies of Molecular Transitions Used in Laser Stabilization: the 3.39

ym Transitions in CH and the 9.33 and 10.18 urn Transitions in CO , " Appl. Phys.

Lett., Vol 22, No. 3, pp. 192-195, Feb. 1973.

4. Barger, R. L., and Hall, J. L., "Pressure Shift and Broadening of Methane Line at

3.39 y Studied by Laser-saturated Molecular Absorption," Phys. Rev. Lett., Vol. 22,

pp. 4-8, 1969.

5. Petersen, F. R. , McDonald, D. G. , Cupp, J. D., and Danielson, B. L. , "Rotational

Constants for C from beats Between Lamb-dip Stabilized Lasers," Phys. Rev.

Lett. Vol. 31, pp. 573-576, 1973.

6. Freed, C, and Javan, A., "Standing-wave Saturation Resonances in the CO 10.6 y

Transitions Observed in a Low-pressure Room Temperature Absorber Gas," Appl. Phys.

Lett., Vol. 17, pp. 53-56, 1970.

7. Patel, C. K. N. , and Shaw, E. D., "Tunable Stimulated Raman Scattering from Conduction

Electronics in InSb," Phys. Rev. Lett., Vol. 24 pp. 451-455, 1970.

8. Mooradian, A., Brueck, S. R. J., and Blum, F. A., "Continuous Stimulated Spin-Flip

Raman Scattering in InSb," Appl. Phys. Lett., Vol. 17, pp. 481-483, 1970.

9. Patel, C. K. N., "Tunable Spin-flip Raman Laser at Magnetic Field as Low as 400 G,"

Appl. Phys. Lett., Vol. 19, pp 400-403, 1971.

10. Mooradian, A., lecture notes at Scottish University Summer School in Physics, Edinburg,

Scotland, Aug. 1975.

11. Scragg, T., and Smith, S. D., "External Cavity Operation of the Spin-flip Raman

Laser," Opt. Commun., Vol. 15, pp. 166-168, Oct. 1975.
12. Wells, J. S., and Petersen, F. R. , Streit, G. E. , Goldan, P. D. , and Sadowski, C. M.

"An Infrared Spectrometer Utilizing a Spin Flip Raman Laser, IR Frequency Synthesis

Techniques, and CO Laser Frequency Standards," NBS Tech Note #670, January 1976.

13. Bischel, W. K., "Application of Nonlinear Optical Techniques for the Investigation of

Molecular Properties and Collisional Processes," Ph. D. Thesis, University of California,

Livermore, Report UCRL-51889, Sept. 1975.

14. Wells, J. S., Evenson, K. M. , Day, G. W. , and Halford, Donald, "Role of Infrared

Frequency Synthesis in Metrology," Proc. IEEE, Vol. 60, No. 5, pp. 621-623, May 1972.

(This paper includes references to the early development of the MIM diode by both the

MIT and NBS groups.)

15. Sakuma, E., and Evenson, K. M., "Characteristics of Tungsten-nickel Point Contact

Diodes Used as Laser Harmonic-Generator Mixers," IEEE J. Quant. Electron., Vol. QE10, No. 8, pp. 599-603, Aug. 1974.

16. Matarrese, L. M. , and Evenson, K. M. , "Improved Coupling to Infrared Whisker Diodes

by Use of Antenna Theory," Appl. Phys. Lett., Vol. 17, No. 1, pp. 8-10, July 1970.

17. Kildal, H., R. S. Eng . , Mikkelsen, J. C. , and Spears, D. L. , "Determination of Absolute

Frequencies of C and C Laser Lines," Appl. Phys. Lett., Vol. 24, pp. 231-

233, 1974.

18. Patel, C. K. N., "Saturation Spectroscopy with a Tunable Spin-flip Raman Laser,"

Appl. Phys. Lett., Vol. 25, pp. 112-114, 1974.

19. Colles, M. J., Dennis, R. B., Smith, J. W. , and Webb, J. S., "A Study of Factors

Affecting the InSb cw Spin-flip Laser," Opt. Commun., Vol. 10, No. 2, pp. 145-148,

Feb. 1974.

20. MacKensie, H. A., Smith, S. D., and Dennis, R. B., "Tuning and Mode Characteristics

of the C. W. InSb Spin-Flip Raman Laser Determined by Use of a Fabry-Perot Interferometer," Opt. Commun., Vol. 15, pp. 151-156, Oct. 1975.

21. Firth, W. J., Wherrett, B. J., and Weaire, D., "Theory of Tuning Behavior of the Spin

Flip Laser," Opt. Commun., Vol. 15, pp. 157-160, Oct. 1975.

22. Brueck, S. R. J., and Mooradian, A., "Frequency Stabilization and Fine Tuning Characteristics of a CW InSb Spin-Flip Laser," IEEE J. Quant. Elect., Vol. QE-10, pp.

634-642, Sept. 1974.

23. Brueck, S. R. J., and Mooradian, A., "Efficient Single-Mode, CW, Tunable Spin-Flip

Raman Laser," Appl. Phys. Lett., Vol. 18, pp. 229-2 30, Mar. 1971.
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Figure 2. a) Beat note from metal-insulator-metal (MIM) diode when irradiated with outputs from CO laser, a CO laser and a 46.6 GHz microwave source. The center

frequency is 30 MHz with 1 MHz per division dispersion, 1 ms per division sweep

rate and 30 khz bandwidth. Display is logarithmic with 10 db per division. The

signal to noise ratio is slightly over 30 db.

b) Beat note from MIM diode when irradiated with outputs from a spin flip Raman

laser, a CO laser, and a 74.06 GHz microwave source. Center frequency is 370

MHz with 20 MHz per division dispersion, 5 msec per division sweep rate, single

sweep, and 300 khz bandwidth. Signal to noise ratio is slightly less than 30 db

on this 10 db per divison scale. This beat note jittered around over a 20 MHz

range at this particular point of operation for the spin flip Raman laser.

10
II

III

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(a)

250 500 750

FREQUENCY, MHz

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V -»-H

1000 1250

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0.01 0.02 0.03 0.04

MAGNETIC FIELD INCREMENT, TESLA

J0.05

Figure 3 a) Sketch of spectrum analyzer display of diode when irradiated with CO laser,

spin flip Raman laser and 74.06 GHz klystron. Three different classes of beat

notes are displayed. Class I consists of intermode beats from the spin flip

laser and are generally in the region 0-400 MHz. Class II beat notes tune with

the magnetic field, but are independent of the microwave frequency. Class III beat notes which are the beat notes of interest for spectroscopic purposes

(shown in Figure 2b) tune with the magnetic field and at twice the rate of the

microwave frequency.

b) Non linear axial modes of the spin flip Raman laser as a function of magnetic field. AH corresponds to the magnetic field interval for mode overlap.

Av and Av , correspond to the tuning rate of the beginning and end of the m , , m-1 e axial mode respectively.

gives Av/AH.

A measurement of the Class II beat notes tuning rate

(The frequency axis is not scaled since the non-linearity has been exaggerated

to illustrate the field dependence.)

11
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN 680

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

Application of Infrared Frequency Snythesis Techniques

with Metal-Insulator-Metal Diodes to the Spin Flip Raman

Laser

3. Recipient's Accession No.

5. Publication Date April 1976

6. Performing Organization Code

277.03

7. AUTHOR(S) J. S. Wells, G. E. Streit and F. R. Petersen 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2773153

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as Item #9

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

Infrared frequency synthesis techniques with a metal-insulator-metal (MIM)

diode have been extended to include the measurement of the frequency of a spin

flip Raman Laser (SFRL) . As a result of this extension, spectroscopy in the 5.3 ym

region can be put on a frequency rather than a wave-length metrology basis. Additional observations with the diode are in qualitative agreement with recent work

relating to non-linear tuning over axial SFRL modes.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Frequency measurements on tunable lasers; IFS with a tunable laser; infrared

frequency synthesis; SFRL frequency measurement; spin flip Raman laser.

18. AVAILABILITY QT Unlimited

2 For Official Distribution. Do Not Release to NTIS

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21. NO. OF PAGES

14

22. Price

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