Optically Controlled Coupled Microstripline Microwave Power Attenuator

Transcription

1 IOSR Journal of Electrical and Electronics Engineering (IOSR-JEEE) e-issn: ,-ISSN: , Volume 9, Issue 5 Ver. IV (Se Oct. 2014), PP Otically Controlled Couled Microstriline Microwave Power Attenuator Raghbendra Singh Tomar,Enakshi K. Sharma and A K Verma (Deartment of Electronic Science, University of Delhi South Camus, India) Abstract: This aer resents an otically controlled couled microstriline microwave owerattenuator on a high resistivity silicon substrate in the 3GHz-7GHz frequency band. The structure is based on the10db microstri directional couler. Attenuation control is erformed by otical control of the oen end resistances. The oen end act as a variable resistance under sufficient otical illumination conditions. The attenuator has been designed for 10dB and 25dB attenuation control for one-ort and two ort otical illuminations resectively. The analysis has been carried out for one ort illumination using a 30mW, 850nm laser diode and two orts illumination using two 20mW, 650nm laser diode. The attenuator has been modeled by the Comuter Simulation Technique s Micro Wave Studio (CST MWS) 3D EM simulator. At 5GHz, the structure rovides a continuous variation of S41 between 0 and 25dB with both oen end illumination. Keywords: Attenuator, Couled Lines, CST MWS, Directional Couler, Otical Control I. Introduction In the last years, there has been a great interest in the develoment of laser controlled systems because of the fact that the otical control of microwave devices offers high isolation between the controlling otical beam and the controlled microwave signal, short resonse time, high-ower handling caacity, immunity to electromagnetic interference and low cost [1]. Otical control of microwave devices based on the hotoconductivity effect. When a semiconductor is illuminated with a hoton of the aroriate wavelength,an electron-hole air is generated in the semiconductor substrate creating a variable load that changes the roagation characteristics in the microwave device. This change in characteristics has alications such as in tunable filters [2], hase shifters [3], and microwave matching techniques [4]. One of the new romising alications of microstri technology is the otically controlled microwaveattenuators [5].Such otical control is based on the fact that when hotons of energy greater than the band ga are incident on the silicon substrate, electron-hole lasma created by light absortion [6]. The otically generated lasma at the end of an oen microstriforms an otically controlled load [7-8]. The main reason of using these devices is due to the demand of new and emerging alications, which leads to the develoment of circuits and subsystems in the otical range. In this aer, a modified directional couler with two wide oen-ended couled lines is resented. The even-odd mode technique [9-10] is emloyed to analyze and synthesize such a structure. After that, the laser tuning is achieved by modifying the electrical length of the oen-ended couled lines. An otically controlled microwave attenuator using a microstri directional couler on high resistivity silicon substrate has been exerimentally demonstrated by Haider et al. [11]. They used two orts illumination by a high ower argon laser (600mW, 514nm) to obtain 10dB attenuation control at 6GHz in the frequency band 3-8GHz. However, neither an analytical nor a simulated model has been resented by them. In this aer we resent investigations on oneort and two-ort illuminated couled microstri attenuators on the high resistivity silicon substrate, in the frequency band 3-7GHz. One ort illumination using a 30mW, 850nm laser diode and two orts illumination using two 20mW, 650nm laser diode has been done. The attenuator has been modeled by the Comuter Simulation Technique s Micro Wave Studio (CST MWS) 3D EM simulator [12], taking into account the SMA to microstri transition. II. Theory Of Otically Controlled Attenuator The otical control of microstriattenuators based on the henomenon of otical load discussed in [7-8]. When the oen end on the microstri line is illuminated by a laser sot, electron-hole airs are created by light absortion sread into the substrate due to carrier diffusion to give an inhomogeneous carrier distribution in the illuminated substrate. The absortion and enetration deth of the lasma deend on the otical wavelength and substrate arameters. Such electron-hole lasma created at the end of the oen microstri line due to illumination by laser sot changes the dielectric constant within the illuminated region in the semiconducting substrate. The otically illuminated region can be considered as a cylinder filled with a dielectric constant of comlex dielectric constant whose value varies with substrate deth and can be modeled as a caacitor with a lossy dielectric, which gives a comlex caacitance. This leads to an equivalent circuit model 19 Page

2 Otically Controlled Couled Microstri line Microwave Power Attenuator of a caacitance in arallel to a resistance. The resistance decreases from several kilo-ohms in dark state to a few ohms with increase in otical intensity. Considering a uniform distribution of the hoto induced charges, the relative comlex dielectric constant in the microwave range can be written as r j (1) The real and imaginary art of the comlex dielectric constant given by e m L l l m h h n e (2) e e e m e 0 e 2 l h 2 m 2 l m h ne e e (3) 01 me 01 e Whereε L is the relative dielectric constant in the dark state, e is electronic charge, τ and τ e are the collision time for electron and hole, l and h are the densities of light and heavy holes, m l and m h are the effective mass of the light and heavy holes, resectively, v is the microwave ulsation.the comlex refractive indexis exressed as follows [8] The corresonding comlex refractive index n of illuminated silicon can be defined as n j (4) where, and are the real and imaginary art of the refractive index, resectively, and are related to by the relation 2 (5) n r comlex roagation constant defined as r j k 0 - j k ( 0 j ) (6) The real art is hase constant and imaginary art, the attenuation constant of the comlex roagation constant of a wave. The quantity / k 0 referred to as the slowing factor, defines the ratio of hase velocity in free sace to hase velocity in the semiconductor referred to skin deth and defined as 1 (7) / k 0 and increases as carrier concentration or frequency increases while, the skin deth decreases with either carrier concentration or frequency. [10]. If the conductivity becomes too large, the deth of the conducting region will be defined by the skin deth and not by the diffusion length. From the value of, it can be seen that the resence of the lasma region alters the wave velocity. Another issue is that α increases together with.this will obviously reduce the transmission coefficient.if the diffusion length is small comared to the absortion deth 1 / the conductivity and resistance becomes 20 Page

3 Otically Controlled Couled Microstri line Microwave Power Attenuator S P ( y) (1 R) e( n ) e hc A y (8) hc R ( e (1 R) e( ) S P n d 1) (9) In this case the otically induced resistance is controlled by radiation absortion characteristics of semiconducting substrate and radiation wavelength. The relation between otical ower and otically induced resistance is given in table 1. 6mm 0.095mm 5.52m 0.234mm 3mm 0.203mm Figure:1Otically Controlled Couled Microstri line Microwave Power Attenuator TABLE: 1the relation between otical ower and otically induced resistance Power(mW) Resistance () III. Design and Simulation Fig. 1 shows the layout of the roosed attenuator. The rimary structure is quarter wavelength 10dB directional couler at the center frequency of 5GHz. The couler consists of two conductor layers interleaved by one substrate between the conductor layers. The couled microstri lines designed on the high resistivity silicon substrate having dielectric constant 11.8 and resistivity 3000Ω-cm and thickness 285μm. The dimensions of the couled lines are calculated from CAD oriented software called microwave office TCAD [13] using the even and odd mode imedances. In fig. 1, for the illumination the ort 2 and ort 3 left oen. The ort-1 is the inut ort and the isolated ort-4 is the outut ort of the attenuator. In this configuration the four ort device becomes a new two ort device. Due to mismatch at the couled ort-2, the microwave signal gets reflected and aears at the outut ort-4. This reflection is a maximum under dark condition. With otical illumination at ort-2, the load resistance reduces at ort-2, which in turn reduces the ower at the outut ort-4. The minimum available outut ower at the ort-4 is determined by the directivity of the mictrostri couler. A large reflection occurs on the main line resulting into oor return loss because ort-3 is oen. In the fig. 2 above described henomena can be seen. In fig. 2a when the device is four orts couler maximum surface current density is on the through ort3 and a couling of 10 db ower is showing on ort 2. In fig.2b when ort 2 and 3 kee oen the 10dB down current density is on outut ort 4. S11 can be imroved by otical illumination of ort-3, as the illumination creates a better otically controlled matched load. At the center frequency of the couler, we can estimate S11 and S14 for a couling coefficient C by the following exressions: 21 Page

4 Otically Controlled Couled Microstri line Microwave Power Attenuator S 11 = 2C 2 1 (10) S 14 = 2C (1 C 2 ) (11) The roagation characteristics are calculated by the three-dimensional (3D) simulator CST Microwave Studio [12], based on the finite integration technique. Themicrostri to coaxial transition roduces discontinuities which in turn creates oen end losses and non smoothness in resonse of S41 due to mismatch. The fig. 3 is the modeling of transition from SMA to microstri. The SMA connector has been roerly designedand modeled by us with LC network. This modeling rovides smoother resonse for S14. Figure:2Equivalent Circuit ofotically Controlled Couled Microstri line Microwave Power Attenuator with the SMA to microstri transition. We can obtain otical control of the attenuator either by one-ort or by two-ort illumination.in the case of one-ort illumination either of the two orts, ort-2 or ort-3, can be illuminatedwhile the other is oen or terminated in a 50 Ω matched load Fig. 3 shows, modeling of the SMA to microstri line transition at both the inut ort-1 and atthe outut ort-4 by the LC-network.The values of C1, C2 and L are 0.1F, 0.3F and 1.8nHresectively. Both oen ends have been simulated by a arallel RCload with R changing according to illumination level at ort-2. The value of C has beenestimated by the oen end discontinuity. The CST MWS simulated resonse of the attenuator with these terminations (Fig. 4) show that under the dark condition maximum S41is 0dB at 4.5GHz and comes down to -8dB, i.e., 8dB attenuation control for the load change from5000ω to 50Ω. The attenuation control is 10dB when load comes down to 30Ω. Moreover,modeling of transition by LC network disturbs the smoothness of resonse for S41. Onillumination of both the through and couled orts, attenuation control more than 20dB couldbe obtained (a) (b) Figure:3 Current density distribution in (a) rimary directional couler and in (b) otically controlled attenuator when both ort oen (maximum ower at ort four) 22 Page

5 Otically Controlled Couled Microstri line Microwave Power Attenuator IV. Results And Discussion Fig. 4 and fig. 5 show the CST MWS simulatedtransmission resonsesfor attenuator with illumination at ort-2 and ort-3 oen. A changing otical resistance has been used at the lace of otical illumination. The S21 of the transmitted wave deends strongly on the injected otical ower. The magnitude of S21 not remains ractically constant at one given frequency.the variable otical control has been obtained by changingresistances 30Ω, 50Ω, 100Ω, 500Ω, and 5000Ω, where 30Ω resistancesmeans 20mW ower and 5000Ωresistance corresonds to the dark condition. The nature of S41 and S11 showing the correctness of the attenuatormodel. The simulated results for S21 show two maxima, at 4.5GHz and 8GHz, with a di at 7GHz. The S11 for ort 2 illuminations gives di at 4.6GH and at 8GHz. Fig. 6 and fig. 7 show theort 3 illumination simulated results.s21 show the same attern that is maxima at 4.5GHz and 8GHz, with a di at 7GHz. But for the S11 it changes and gives vary low reflection for 500Ω resistance. For one ort illumination whether it is at ort 2 or at ort 3, the simulation results show attenuation control of 10dB from the dark condition load resistance 5000Ω to the illuminated condition load resistance 30Ω. Fig. 8 and fig. 9 show results for both ort-2 and ort-3 are illuminated simultaneously by two indeendent 20mW, 650nm laser diodes. The Fig. 8 shows attenuation control of 25dB and fig. 9 shows that S11 degrades with increase in illumination. The nonsmoothness in resonse of S41 is due to the transition from SMA to microstri which has been modeled, by us with LC network. However, a roerly designed transition can rovide smoother resonse for S41. An attenuator designed for 6 db couling coefficient will rovide better return loss.thus, for a 10dB couler S11 is -1.94dB and S41 is -4.4 db, whereas, for a 6-dB couler S11 imroves to db and S14 is dB. Haider et al. [6] used a 6 db couling co-efficient for their design. However, for ease in fabrication we used a 10dB couling co-efficient in our design of the otically controlled attenuator. With otical illumination at ort-2, the load resistance reduces at ort-2, which in turn reduces the ower at the outut ort-4. The minimum available outut ower at the ort-4 is determined by the directivity of the mictrostri couler. A large reflection occurs on the main line resulting into oor return loss because ort-3 is oen. S11 In fig. 5 the increasing otical illumination hasbeen simulated by a decrease in load variation from 5000Ω (dark condition) to 50Ω. However,the return loss is not satisfactory. It can be imroved uto 20dB for the case of illumination atort-2 by terminating ort-3 in a 50Ω load with simultaneous imrovement in attenuation control (10dB). The simulated results for the case of illuminated ort-2 with ort-3 oen, showthat the maximum couling frequency shifts from 4.5GHz to 4.6GHz. Figure:4 Variation of S21 when ort 2 terminated with otical load and ort 3 remain oen 23 Page

6 Otically Controlled Couled Microstri line Microwave Power Attenuator Figure:5 Variation of S11 when ort 2 terminated with otical load and ort 3 remain oen Figure:6 Variation of S21 when ort 3 terminated with otical load and ort 2 remain oen Figure:7 Variation of S21 when ort 3 terminated with otical load and ort 2 remain oen 24 Page

7 Otically Controlled Couled Microstri line Microwave Power Attenuator Figure:8 Variation of S21 when both ort 2 and ort 3 terminated with otical load Figure:9 Variation of S21 when both ort 2 and ort 3 terminated with otical load V. Conclusion An otical control of 10dB and 20dB attenuation could be obtained by the one and two orts illumination resectively. In case of the one ort illumination, the return loss could be imroved by terminating the ort-2 in the 50Ω matched load. In case of two orts illumination, return loss could be imroved by designing the attenuator for 6dB couling co-efficient. At 650nm illumination, otical load is modeled by a resistance. The transition from SMA to microsti is modeled by LC network and CST MWS simulated results correctly redict behavior of the otically controlled attenuator. Acknowledgements For the financial suort the CSIR-India is acknowledged. References [1] Herczfeld, P.R., Daryoush, A.S., Contarino, V.M., Rosen, A., Turski, Z., and Khana, A.P.S.: Otically controlled microwave devices and circuits. IEEE MTT-S Int. Microw. Sym. Dig., 1985, [2] Platte, W.: Periodic-structure hotoexcitation of a silicon colanar wave guide for selective otoelectronic microwave control, IEEE Trans. Microw. Theory Tech., 1990, 38, (5), [3] Cheung, P., Neikirl, D., and Itoh, T.: Otically controlled colanar waveguide hase-shifter, IEEE Trans. Microw. Theory Tech., 1990, 38, [4] Safwat, A., Khalil, D., Elhennawy, H., and Ragaie, H.: Quasi-static analysis of an otically illuminated directional couler, IEEE Trans. Microw. Theory Tech., 1997, 45, Page

8 Otically Controlled Couled Microstri line Microwave Power Attenuator [5] Lee, S., Kuga, Y., and Mullen, R.A.: Otically tunable, millimeter-wave attenuator based on layered structures, Microw. Ot. Technol. Lett., 2000, 27, (1), [6] J. Haidar, A. Vilcot, M. Bouthinon and E. Pic., "Otically controlled assive microwavestructures", Sino-French Worksho on Fibre& Integrated Otics, Set , Shanghai. [7] B. Boyer, J. Haidar,A. Vilcot and M. Bouthinon," Tunable microwave load based on biasedhotoinduced lasma in silicon', IEEE Trans. Microwave Theory Tech., Vol. 45, Aug, 1997.s [8] A.K. Verma, Enakshi K. Sharma, Nasimuddin, A. Bhaduria and B. R. Sigh, "Oticallycontrolledmicrostri load", National Sym. On Advances in Microwaves and Light waves, N.Delhi, India, Mar., 2000, [9] D. M. Pozar, Microwave Engineering, 3rd ed., New York: Wiley,2005; ISBN [10] R. Mongia, I. Bahl, P Bharia, RF and Microwave Couled-LineCircuits, Norwood MA, Artech House, 1999, ISBN [11] Jihad Haidar, Anne Vilcot and Michel Bouthinon, Otically tunable microwave attenuator using a quarter-wave microstri couler Microwave and Otical Technology Letters, Volume 10, Issue 6, ages , 20 December 1995 [12] User manual CST Microwave Studio, 2013 [13] User manual AWR Microwave office, Page