A ferrite modulator for the far infrared

Infrared Physics, 1970, Vol. 10, pp. 217-224. Pergamon Press. Printed in Great Britain

A FERRITE MODULATOR

FOR THE FAR INFRARED

J. R. BIRCH and R. G. JONES Division of Electrical Science, National Physical Laboratory,

Teddington,

Middlesex, England

(Received 14 August 1970) Abstract-The Faraday effect in two polycrystalline spine1 ferrites has been investigated at 29.7 cm-r using a HCN maser. From the results, large aperture room temperature modulators have been constructed, capable of use from 50 cm-r into the microwave region at modula-

tion frequenciesin excessof 2 MHz. INTRODUCTION

UNTIL recently, research in the far infrared spectral region from 200 to about 5 cm-l had

been made difficult by a lack of bright sources, sensitive detectors having short response times and efficient high frequency modulation techniques. The development of submillimetre maser sources (~2) and liquid helium cooled photoconductive and bolometric detectors (a-5) has considerably eased the problems of generation and detection of this radiation, but that of efficient, non-mechanical modulation techniques is largely unresolved. Most infrared modulation techniques using solids depend either on the modulation of free carrier absorption of a field effect such as the Faraday, Stark, Zeeman and Kerr effects. Free carrier absorption modulators are constructed from semiconducting material using a variety of techniques for modulating the number of free carriers in the conduction band. Minority carrier injection at a p.n. junction(s) has been used at 10*6pm, and free carrier injection due to different surface recombination velocities in single crystal germanium has been used for both near infrared(7) and centimetric (8)wavelengths. In the far infrared, modulation frequencies greater than 10 MHz have been obtained by the impact ionisation of impurity atoms in n- and p-type germanium at liquid helium temperatures.@) We have investigated the Faraday effect in certain polycrystalline spine1 ferrites with the aim of producing room temperature, large aperture, high frequency modulators which would be more suitable than present devices for many non-laboratory applications (e.g. aeroplane, balloon or satellite-borne spectroscopy) where prolonged or confined operation is essential. The saturation magnetisation, saturation Faraday rotation, refractive index and absorption coefficient of several rare earth iron garnets and spine1 ferrites at 297°K have been determined at 29.7 cm-l.(lO) These results indicate that two of the ferrites (5El and 8Cl*) may be used as the basis of far infrared modulators. In ferrites the Faraday effect may be represented by(U)

8=X

WIG

[(

If--ow+

8- 1+x ) (

wo -

t w )I

where : l9

is

the saturation rotation per unit length;

*Mullard Group commercial

type number for sintered aggregates of Ni, Zn, Cu, Mn, Fe, 0. 217

I.P.-c

(1)

218

J. R. BIRCH and R. G. JONES

WM is a constant for a given material equal to 4rry,M, ye being the effective gyromagnetic ratio and M the saturation magnetisation; we is the ferromagnetic resonance frequency, w is the radiation frequency; K is the dielectric constant and c the velocity of light. When W$,WOwhich will generally be the case for submillimetre and millimetre radiation, (1) reduces to

indicating that the saturation rotation is independent of the frequency of the radiation. The response of the Faraday rotation in these ferrites to a high frequency magnetic field will be limited by the relaxation time of the precessional motion of individual atomic magnetic moments about the applied field. This can be measured directly by flux reversal experimentsus) or may be inferred from measurements on the ferromagnetic resonance line width. Typical results(il) from line width measurements give lo-10 sec. for the precessional relaxation time, which indicates that theoretical modulation frequencies in excess of 1 GHz could be achieved with room temperature Faraday rotation devices. Practical considerations such as the prohibitively high powers needed to generate high frequency magnetic field@) will probably limit this to about 10 MHz. EXPERIMENTAL

In order to find the most suitable ferrite for a modulator material it was necessary to investigate the absorption and refractive index properties of both ferrites as well as the variation of Faraday rotation with magnetic flux density. The former measurements establish the ferrite transmission properties and the latter show which will cause the greatest rotation of plane of polarisation for a given change in applied field. Transmission and refractive index measurements on the ferrites were made from 10 to IO

Hz chopper

grid

FIG. 1. Schematic representation

of apparatus used to measue magnetic flux density.

the variation

of Faraday rotation

with

52 cm-r using a standard NPL Michelson Fourier Transform Interferometer with a Putley detector.@) Measurements were made on two different thicknesses of each ferrite. The variation of Faraday rotation with magnetic field was studied with the system shown in Fig. 1. Measurements were made at a single wavelength, 29.7 cm-l, using a hydrogencyanide maser. This had a cavity 2 m long, 50 mm diameter with 2.5 m radius of curvature

A ferrite modulator for the far infrared

219

concave mirrors. The radiation was coupled out of the cavity through a 4.5 mm dia. hole in the fixed mirror, and by a Melinex beam splitter at the adjustable mirror. The beam splitter coupled radiation was used as a reference for the maser output level. A plane mirror system directed the hole coupled radiation to pass in the Faraday configuration through the magnet pole gap, and then through an analyser to be focused onto a Golay detector. The maser radiation was greater than 98 % plane polarised so a polariser grid before the magnet was not necessary. (The light pipe had no measurable depolarising effect.) Parallel faced samples of each ferrite wele placed in the centre of the pole gap and the Faraday rotation caused by each found as a function of magnetic flux density up to 0.6 tesla, measured with a Hall effect probe. A comparison of the measurements taken indicated that 5E1 would be a better modulator material than 8C1 due to its lower absorption coefficient giving it higher transmission at all wavenumbers for a given thickness. Figure 6 shows that it would also have a greater modulation efficiency for a given alternating field. A 5El modulator was constructed from an 8 mm dia. by 2 mm thick disc of the ferrite firmly held in the centre of a small water cooled solenoid wound on a Perspex former. The device is operated by transmitting plane polarised radiation through the ferrite and an analyser grid in the presence of a longitudinal alternating magnetic field. As the intensity of radiation transmitted through an analyser set at an angle + to the plane of polarisation of that radiation varies as COS‘$#~), the maximum modulated signal through the ferriteanalyser system will occur when the analyser is set at 45” to the unperturbed plane of polarisation of the radiation transmitted through the ferrite. Experimentally we define the modulation efficiency as the ratio of modulated signal intensity produced by the device to that produced by complete (mechanical) modulation of the radiation transmitted through the ferrite-analyser system. With the device operated in the mode described above, i.e. 13= 45”, the modulation efficiency will lie between 0 and 2. The modulation efficiency of the device was measured as a function of d.c. magnetic field using the apparatus of Fig. 1. The solenoid was placed with its axis coincident with the magnet axis and the modulation efficiency at 29.7 cm-l measured as a function of d.c. magnetic flux density up to 0.25 tesla for 10 Hz solenoid currents of 1 and 2A r.m.s. For the solenoid geometry used these corresponded to r.m.s. fields of 55 and 110G. respectively. The frequency response of the device was measured in zero external magnetic field at 29.7 cm-l with a Putley detector. The modulated signal was again measured for 1 and 2A r.m.s. solenoid cmrents from 10 to 3 x 106 Hz, the completely modulated reference being derived from a 10 Hz mechanical chopper. Up to 1.5 x 105 Hz measurements were made directly from the detector output signal displayed on a Tektronix lA7 unit, and on a frequency tuneable microvoltmeter at higher frequencies. The 5El device modulation efficiency was also measured at 650 Hz for 3 cm-l radiation generated by a Froome Harmonic Generator. (15) RESULTS Figure 2 shows the variation of 10 Hz modulation efficiency at 29.7 cm-1 with constant applied magnetic flux density. The data shows qualitative agreement with the Faraday rotation results of Fig. 6. For small alternating fields the modulation efficiency will be proportional to the gradient of that curve and hence the maximum efficiency should occur at low values of flux density and tend to zero as the flux density approaches 0.5 tesla.

220

J. R. BIRCH and R. G. JONES

The frequency response results are presented in Fig. 3 and both curves show a flat response out to 300 kHz. At higher frequencies measurements were only made for IA r.m.s. currents due to the high reactance of the solenoid. At the higher frequencies the modulation efficiency gradually falls off to 3 MHz, the highest frequency measured. As the limiting relaxation frequency in the ferrite is expected to be much greater than this, the cutoff is most probably

2A

o.ost

L -.+-

0

0.1

Maqnclic

FIG. 2. Variation

flux density

(TESLA)

of 10 Hz modulation efficiency with magnetic flux density for 5El device at 29.7 cm-l for 1 and 2A r.m.s. solenoid currents.

due to the time response of the Putley detector and the associated circuitry used being considerably greater than the expected 0.1 psec t3) of the detector alone. The measurements made with the Froome Harmonic Generator at 3 cm-l for an r.m.s. solenoid current of 2A at 650 Hz gave a modulation efficiency of 0.22 f 0.02 The corresponding value at 29.7 cm-l is 0.23 f 0.02. The refractive index results for 5E1 are presented in Fig. 4, together with the mean values of refractive index for both ferrites. These were found from the displacement of the zero path difference fringe of the interference pattern caused by introduction of the ferrite into one arm of the interferometer.(l@ It is expected that the refractive index of SC1 will behave with wavenumber in a similar manner to that of 5El The absorption measurements are shown in Fig. 5. These were computed from the transmission and refractive index spectra using T=-

(1 - R)2 expad - Rzexp -

ad

(3)

A ferrite modulator for the far infrared

221

which gives the transmission T, through a parallel sided absorbing layer of thickness din terms of its absorption coefficient a and allows for multiple reflections within the 1ayer.o’) R is the reflection coefficient and is given by (4) where nl is the refractive index of the surrounding medium and 1t2 that of the absorbing layer. Equations (3) and (4) are only valid if the inequality

holds, which is the case for these ferrit~s.(lO) The curve for WI was calculated using the mean

0.24

x

:

c

“‘O-0

k 1% 020-

2A

5 c 0.16.z

f

^

b 0

I 102

IO’

ITrequency(Hz) 10s 106

I IO”

I

I

I

4

Modulation

FIG. 3. Frequency response of modulation

x

efficiency ar 29.7 cm-1 for 1 and 2A r.m.s. solenoid cmnts 5El device.

8CI I

____“_____________________________~____~~~~~~~~~~~~

3.90

3~85 :: c $380 --_-l"_____-_'Z I c 4 3.75 0 j ooo

3.70 -

3.65

0

0

I 5

I IO

I IS

I 20

1 25

I 30

I 35

I 40

I 45

I 50

Wave-number@n-~) FIG. 4. Refrative index spectra of ferrites. Dashed lines show the mean vafue. 12.

D

for

222

3.

R.

BIRCH

and R. 6. Jams

value of refractive index at all wa~~nnmbers, This wifl not introduce substantial errors due to the smalf deviation of refractive index from the mean value. In absorption measurements using NPL cube interferometers it has been shown that the use of convergent radiation introduces errors in measured absorption coefficient of as much as +14% for materials with refractive index about 1.5. (18)For these ferrites, which have IZ about 3.9, the measured coefficients may therefore be considerably in error. The main cause of error is defocusing by the sample, and this will be constant for a11wavenumbets if the refractive index is constant. As this approximately applies to the ferrites, we have measured their ‘true’ absorption coefficient at 29.7 cm-1 using parallel radiation, beamsplitter

FIG. 5. Absorption spectra of ferrifes.

coupled from an HCN maser, and have corrected for defocusing in the broadband results by normalising them to agree with the maser results. The values of absorption coefficient measured at 29.7 cm-l are 1.25 & 0.12 cm-l (5El) and 1.92 f O-12 cm-1 @Cl), which agree with previous values of 1.36 and 2.05 crn-J(lO) within experimental error. Figure 6 shows the variation of Faraday rotation with magnetic fiux density for both ferrites. The measurements were taken on thin samples and normalised to one centimetre thickness, giving errors of f 20” per cm. for 5E1 and rf 8” per cm. for 8Cf. The measured saturation rotations are 329” per cm in 5El and 300” per cm in 8Cf.

A ferrite modulator

for the far infrared

223

FIG. 6. Variation of Faraday rotation with magnetic flux density. CONCLUSIONS

A comparison of the optical properties of the two ferrite5 has shown 5El to be a better material for a Faraday rotation modulator than 8C1. The characteristics of such a 5E1 modulator have been measured and it has been found to be a useful device for broad band, high frequency modulation, although its transmission characteristics limit it to operate in the spectral region below 50 cm-l. The device operates most efficiently in a zero biasing magnetic field so that a permanent magnet system is not necessary. Much higher modulation frequencies than those reported here will be achieved when the solenoid size and frequency of operation are well matched to experimental requirements and available oscillator/ power amplifier systems. The flatness of the frequency response out to at least 300 kHz means that the device could form the basis of a multi-channel, room temperature, far infrared audio communications system. Tf the response is in fact flat beyond this frequency then video communications systems may be possible. With such high frequencies far infrared sideband spectroscopy may also be possible. The effectiveness of the application of the Faraday effect to the far infrared has been shown by the production of this cheap, solid state modulator which is capable of high frequency operation at room temperature and requires little space and unsophisticated driving electronics. Subsequent application of the effect in this ferrite means that many of the devices which are used in the microwave region, such as isolators, switches and rotators, would become available in the far infrared. Acknowledge~nenfs-We wish to thank Dr. R. F. Pearson of Mullards for supplying the ferrite samples used in these experiments and Dr. V. Sells of Queen Mary College, London for operating, and allowing us to use the Froome Harmonic Generator.

224

J. R. BIRCH and R. G. JONES REFERENCES

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1.

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