PROCEEDINGS OF THE IRE

Combined Optical-Microwave SystemConsiderations*

S. H. CUSHNERt

I. INTRODUCTION

HIS PAPER presents a discussion of locating sys-

tems employing both infrared and microwaveenergy as intelligence carriers. The actual wave-

length regions which are of interest here are 3 X 10-5 cmto 3X10-3 cm and 0.3 cm to 30 cm. Emphasis is on a

particular class of such systems, called "dual systems,"in which the functions of the several components are

combined insofar as possible. This first section will pre-

sent the dual concept and will consider some generalapplications. Section II discusses various componentswhich prove useful in system design.

Speaking broadly, there are two approaches to thesystem problem of employing infrared and microwaveenergy. As previously mentioned, we will concentrate on

the dual approach. The alternative is the parallel ap-

proach, which simply means two separate detectionsystems-one for each wavelength region. This approachis a good choice in surface-based systems. Without toomuch anticipation of the results of Section II, we com-

pare the elements and functions which characterize air-borne infrared and radar detection and direction findingsystems.

Each system has a "dome" or entrance window, whichmust be transparent to the radiation and must give pro-

tection to the rest of the system. Each type of systemthen has what is termed an antenna or energy-collectinigsystem, the function of which may be stated in various

ways. In a microwave system, it can be considered tomatch the impedance of free space to the remainder ofthe system. It performs this by deforming the wave-

fronts. In the infrared, we may speak meaningfully ofrays; and in this case, parallel rays are transformed intoconverging rays.

After a scanning scheme for determining the angularcoordinates of the source being viewed, the next elementin both systems is the detector. At this point, it is clearto workers in either field that there must be somethingwhich directs the two- spectral regions to appropriatedetectors. We assume, for the time being, that the detec-tors are necessarily different.The last step is the information-processing system.

The data derived from infrared homing or tracking de-

vices are the instantaneous target angular coordinatesand the corresponding angular rates. Radars get thisdata and, in addition, can measure range and range rate.

*Original manuscript received by the IRE, June 30, 1959.t Ramo-Wooldridge Corp., a division of Thompson Ramo-

WVooldridge, Inc., Los Angeles, Calif.

Determining the best method for weighting the datawhich are common to both systems would require studyfor a given tactical situation, since some or all informa-tion could be spurious.Each of the foregoing items will be discussed in more

detail in later sections.

II. DOME

Since all infrared transparent materials are low-lossdielectrics, they are, in principle, transparent to micro-

waves. However, the problem is the large dielectricconstants which materials such as quartz, sapphire, andmagnesium oxide have in the microwave region. Thesematerials have dielectric constants relative to vacuum

in the range of five to ten. For normal incidence, thepower losses due to reflection are in the range of 44-67per cent. This difficulty may be avoided by making theelectrical pathlength through the material such that itbehaves as its own low-reflectance layer.

This condition is fulfilled for normal incidence on a

loss-free non magnetic homogeneous isotropic dielectricof refractive index n when the thickness d is given by

xod = N

2N

N = 1, 2, 3 ...

Xo=free space wavelength.

In transmisson- line parlance, this is the condition foran impedance match. The spectral bandwidth over

which the reflection loss for two surfaces is less than 20per cent is typically ± 5 per cent of the center frequencyfor first-order interference (N= 1).

Separating Elements

There are two general approaches to the problem ofseparatinig infrared radiation from microwave radiation.One can reflect the microwave and transmit the infraredor one can reverse the functions. The methods of realiz-ing these alternatives are as follows:

Transmit infrared, reflect microwaves1) wire grating or mesh2) dielectric structure.

Transmit microwaves, reflect infrared1) metallic array

2) dielectric structure.

Paper 4.2.6

1959 1553

PROCEEDINGS OF THE IRE

A wire grating will reflect incident microwaves whenthe electric vector is parallel to the grating elemenits,aiid have negligible effect if the wires are perpendicularto the electric vector of the incident waves. Such astructure is almiost perfectly transparenit to infraredwaves. The electromagnetic complement of the grid,which is obtailned by replacing the grid wires by spacesand the grid spaces by metal, can be made to be tralis-parent to microwaves and to reflect 90 per cent or mnoreof incident infrared energy.

Another type of beamsplitter is the all-dielectricstructure. This is simply a slab of dielectric which hasa high-reflectance coating in the infrared. These coatin-gsare also nonconducting and contribute a negligibleamount to the thickness of the slab. The entire struc-ture can therefore be made a low-reflectance thicknessfor the microwaves; or the alternative, if desired, canhave a low-reflectan-ce infrared coatinig and be made athickness which reflects all the miciowave radiation.

Energy- Collecting SystemIt has been mentioned that the operatioii of the

en-ergy-collecting elemen-ts of the system may be statedin several ways. In the microwave case, where the di-ameter of the main collector elema-ent is usually a simall(r-20) number of waveleni.gths, the full machinery ofvector diffractioln theory must be used to predict theefficienicy and characteristics of anitennas. In the imfraed, where the size of the collectinig elemen-ts is a few

times 105 wavelengths, the geometrical-optics approxi-mation of straight-line propagation may be used. In thecase of very high resolutioni systems such as are used foi

Paper 4.3.1

reconnaissance or precision tracking, there mnay be astop which is small enough to spread the waves by dif-fraction. In this case, additional optical elemrients calledfield lenses, which cani collect e.nergy distributed overlarge anigles, are used to gather the energy and bri-ngit to focus oni the detector. Field. lenses are ofteln usedin optical systems to produce high-quality images in theabsence of diffraction phenomena. There is a wide vanriation in the quality of the surfaces required to collectenergy in the two wavelength regions. It is only asseitedhere that the surface quality of an optical reflector ismore than adequate for use in the microwave regioni.The typical vacuum-evaporated aluminum film usedto make the reflecting surface on optical elements has acon-ductivity great enough to be a satisfactory reflectorof microwaves.

Detectors and Scanning

In the previous discussion, refereiice has beer m-Dade toa separating element which directs the microwave radi-ation and the infrared radiation to their respective de-tector. Present-day technology dictates the siliconI ci-ystal and catwhiskers for a microwave detector aind aphotoconductive cell or bolometer for the infrared system.A mnechanical scanning system for the microwave por-

tioIn of a dual system is less desirable than. for a conven-tional radar. The reason for this is the comisiderablysmaller latitude of vibration- which cani be tolerated byoptical-type systems. For dual systens, an1 electronicscanning system such as the feirorod type would be momedesirable.

Methods of Background Description andTheir Utility*DAVID Z. ROBINSONt

INTRODI CTION

j N orcder to design- or evaluate a given infrared sys-tem, it-is necessary to know the characteristics ofthe potential targets aiid backgrounds with which

it is expected to deal. The information desired can. beput in the form of questions.

1) What is the probability of detectinig a target in thepresence of a given background with a given IRsystem?

* Original maniuscript received by the IRE, Jun-ie 30, 1959.f Office of Naval Res., Lonidone, England. Formerly with Baird-

Atomiic, Inic., Cambr-idge, Mass.

2) What is the probability that a given backgroundwiTill n-ot give a false signal when nio target ispresen t?

3) How can one design the system to iicrease theseprobabilities.?

Note that it is not only the properties of the backgroundwhich are involved here. Of vital importaance also arethe properties of both the target and the IR systemnwhich is being used.

It is the hope of those working in the field to find aconcise description of backgrounds which will wxork withany system and with any target If the method of de

1554 SeptembDer