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Active and Passive Elec. Comp., 1995, Vol. 17, pp. 275-282Reprints available directly from the publisherPhotocopying permitted by license only
1995 OPA (Overseas Publishers Association)Amsterdam BV. Published under license byGordon and Breach Science Publishers SA.
Printed in Malaysia
OPTICALLY CONTROLLED MICROWAVEATTENUATORS
E BHUSHAN MITALFellow Member IETE, Department of Electronics & Communication Engg., C.R. State College of
Engineering, Murthal (Sonepat), India-131039
(Received August 2, 1994; in final form September 7, 1994)
The concept of photoconductivity is utilized in a number of microwave applications such as switchesand attenuators. Optically controlled microwave attenuators prove an alternative to the conventionalattenuators due to their simplicity, easy of operation, and very narrow ranges of continuously varyingattenuation levels available. This paper describes the design of such configurations of optically controlledattenuators on a microstrip line fabricated on a semiconductor substrate.
INTRODUCTION
Attenuators are important components in a microwave system. The conventionalattenuators have limitations of small bandwidth, low power handling capability, andless control. The optically controlled attenuators are expected to be free from theseshortcomings. These depend on the photoconductivity generated in a high resistivitysemiconductor substrate.When a semiconductor is illuminated by an optical source of an appropriate
wavelength, the optical energy is absorbed by the semiconductor, elevating theelectrons to the conduction band in mass and, hence, the conductivity of the sem-iconductor is increased dramatically. The absorption of optical energy is a strongfunction of wavelength of the illuminating source as shown in Fig. 1A. The ab-sorption coefficient c is given by as
(hf)2-trgx ao (hf)exp (-2-rrgx)’
hf Eg
where
gx [Rx,/(hf- Eg)]l/2
Rx 4.2 meV
ao(hf) (/3/hf)(hf Eg) 1/2
/3: a complex function of wavelength
h: Plank’s constant
f: frequency of the optical source
275
276 EB. MITAL
FIGURE AWAVELENGTH (pm)
Optical absorption coefficient wavelength for silicon germenium and gallium arsenide.
E: bandgap energy
Depending on the absorption coefficient c, the conducting plasma can be createdup to a certain depth in the semiconductor on illumination. The expression forplasma penetration depth d is given by2:
de[La(1 + cLa) + Vst] A
ce(L 4- Vt)
where
(aL.2 + vt)A=cL. (L. + vt)
x L/(1
/3 exp(-W/2L)W dimensions of the illuminated slot
c optical absorption coefficient
L. ambi-polar diffusion length
V. surface recombination velocity
excess carrier lifetime
MICROWAVE ATTENUATORS 277
If the penetration depth d is very small compared to the substrate thickness,then the plasma so created is called the surface plasma. In contrast, if d is large,then the plasma is called the bulk plasma.
The surface and the bulk plasma can be used to electrically bridge the gapbetween two conductors (Fig. 1B). This phenomenon is used in optically controlledmicrowave switches and attenuators.
PRINCIPLE OF THE OPTICALLY CONTROLLED MICROWAVEATTENUATOR
The semiconductor material between two conductors can be made photoconductingby optical illumination, creating an equivalent photoconductance. Fig. 2A shows amicrostrip line on a semiconducting substrate. If the semiconductor between thetwo conductors of the microstripline is made photoconducting, this will create shuntconductance between the two conductors. Due to this shunt conductance, some ofthe incoming microwave power is reflected back and only part of it is transmitted.We have
Pr g:ZPi
1/G Zog=I/G+ Zo
G: shunt photoconductance
Zo: characteristic impedance of the transmission line.
The attenuation is obtained, since the transmitted power is a function of theshunt photoconductance, which can be varied by optical illumination. The equiv-alent circiut is shown in Fig. 2B.
SEMICONDUCTOR LAYER
LASER SP01
M ICROWAVE ORC.CONTACT
FIGURE 1B
CW LASER BEAM
X=OTypical arrangement for the optical control of photoconductivity.
278 P.B. MITAL
LOAD
PINFIGURE 2A Microstripline on semiconducting substrate.
The effective photoconductivity re, produced by the optical illumination is givenby2:
[Arth (1 /3)]-]
o- Crm(W/4La)(1 /3) 1/2l_ /3)1/2
where
O’m (1 oLa)
ro (q/hc)(/Xn + /Zp)(1 R)aS tP/A
Pou./p(/PIN trREF
FIGURE 2B Equivalent circuit.
MICROWAVE ATTENUATORS 279
1 cLa2 + VstB=ceL t -t- Vst
x aLa/(1 aLa)
/3 exp (-W/2La)
W dimensions of the illuminated slot
a optical absorption coefficient
L ambi-polar diffusion length
v surface recombination velocity
excess carrier lifetime
R surface reflectivity
S relative spectral response
The photoconductivity is dependent on the optical absorption coefficient and,hence, on the wavelength of the illuminating source. A program was made to obtainvalues of photoconductivity corresponding to different wavelengths. The results aregiven in Table 1 and Fig. 3A & B.
1.0
0.6
0.:2
SILICON
.I ,... ., ,.____I.-100 200 300 400 SO0
POWER/AREA( mwlsq, cm )FIGURE 3A Attenuation vs power/area (silicon).
280 EB. MITAL
ZO
Z
0.30
0.2
0.18
0.12
0.06
0.00
GALLIUM ARSENIDE
ATTENUATION AT WAVELENGTH 865 nm
100 200 300 400 500POWER I AREA (mwlsq. cm)
FIGURE 3B Attenuation vs power/area (GaAs).
Theoretically, attenuation can be evaluated by calculating S parameters for theequivalent circuits. The attenuation obtained is:
Attenuation (dB) 10 logo]$212
where
2 + GZoG: photoconductance
Zo: characteristics impedence of the microstripline
DESIGN AND FABRICATION
The design of various configurations in a coplanar waveguide/microstrip line usingsilicon/gallium arsenide substrate has been carried out using design equations givenin [3]. Various parameters are given below:
MICROWAVE ATTENUATORS 281
Parameter Si GaAs
Er 11.7 12.9Bandgap (Eg) 1.12 eV 1.43 eVhg hc/Eg 1109 nm 869 nmw/h 0.8083 0.7275
The penetration depth de, to which to photoconducting plasma is created, isgiven by
La(1 + ceLa) + Vst (aLa2 + Vst)aLa/(1 ceLa)d ,
a(L + Vst) (La(L + Vst)ceLa/(1 txLa)
where
a: absorption coefficient
La: ambi-polar diffusion length
v,," surface recombination velocity
t: excess carrier lifetime
The expression for d shows that it is dependent on a, which in turn is a strongfunction of wavelength. This dependence of penetration depth on wavelength isused to decide the wavelength of the optical source.4-5
EXPERIMENTAL RESULTS
A plot of attenuation vs optical power is shown in Fig. 3A & B and readingstabulated in Table 1 at wavelengths 632 nm & 830 nm.
TABLE 1
Power/Area (mW/sq cm) SILICON at 632 nm At 830 nmAttenuation (dB) Attenuation (dB)
0.00 -0.0000 -0.000050.00 0.1338 0.1712100.00 0.2655 0.3390150.00 0.3953 0.5037200.00 0.5231 0.6653250.00 0.6491 0.8240300.00 0.7733 0.9798350.00 0.8958 1.1328400.00 1.0165 1.2832450.00 1.1356 1.4311500.00 1.2531 1.5765
282 EB. MITAL
The following results are obtained:
(i) Source: Laser Diode with emission wavelength 830 nmLaser Diode bias current 55 mADetected microwave voltage 38 mV; before attenuation (withoutillumination)Detected microwave voltage 37 mV; after attenuation (with illumination)Attenuation (dB) 10 Log (37/38)
0.116 dB
(ii) Source: He-Ne Laser with emission wavelength of 632 nmLaser Beam intensity at the gap 400 mW/sq omDetected microwave voltage 30 mV; before attenuation (withoutillumination)Detected microwave voltage 29.3 mV; after attenuation (with illumination)Attenuation (dB) 20 Log (29.3 / 30)
0.1025 dB
CONCLUSIONS
The attenuation obtained experimentally is less than that of the theorectical cal-culations due to various theoretical and experimental errors. Due to discrete valuesof the strip conductor width and substrate thickness available, the characteristicimpedance of the microstripline is less than 50 ohms. The dimensions of the beamwidth of the laser and of the illuminated gap of the transmission line structure usedcould not be measured accurately. There is a possibility of better alignment of theoptical source beam and the gap to be illuminated. The results, although not ab-solutely perfect, are considered encouraging.
REFERENCES
[1] J.S. Blakemore, "Characteristics of GaAs," J. Appl. Phys., Vol. 53, No. 10, October 1982.[2] Walter Platte and Bernhard Sauerer, "Optically CW-Induced Losses in Semiconductor Coplaner
Waveguide," IEEE Transaction on Microwave Theory and Technique, Vol. 37, No. 1, January 1989.[3] K.C. Gupta, R. Garg and I.J. Bahl, Microstrip Lines and Slotlines. Dedham, MA: Artech House,
1979.[4] Gerd Keiser, Optical Fiber Communication. McGraw Hill Book Company, 1989.[5] Chi H. Lee, "Picosecond Optoelectronic Switching in GaAs," Applied Physics Letters, Vol. 30,
No. 2, 15 January 1977.
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