This paper proposes an innovative optoantena design, result of the combination of a
laser and photodiode for  ber transmission and reception with a dual port full-duplex antenna.
This antenna can operate at di erent frequency bands simultaneously, while keeping it's two ports
highly isolated. Moreover the antenna combines  ltering, duplexing and radiating thus reducing
cost, volume and weight when compared to a more traditional approach. This properties make this
design a perfect candidate for the implementation of radio over  ber techniques. Both the design
of the antenna and the characterization of the optoelectronic elements of the device are thoroughly
discussed in this paper2020/202

Casas Casajuana, Damià

Cela París, José María

English

UPCommons. Portal del coneixement obert de la UPC

Bidirectional optoantenna for radio over fiber architecturesJosé Maŕıa Cela Paŕıs and Damià Casas CasajuanaUniversitat Politècnica de Catalunya, Barcelona, Spain(Dated: June 5, 2021)Abstract: This paper proposes an innovative optoantena design, result of the combination of alaser and photodiode for fiber transmission and reception with a dual port full-duplex antenna.This antenna can operate at different frequency bands simultaneously, while keeping it’s two portshighly isolated. Moreover the antenna combines filtering, duplexing and radiating thus reducingcost, volume and weight when compared to a more traditional approach. This properties make thisdesign a perfect candidate for the implementation of radio over fiber techniques. Both the designof the antenna and the characterization of the optoelectronic elements of the device are thoroughlydiscussed in this paper.I. INTRODUCTIONThe present evolution of the communications and re-mote sensing markets, allows to predict that only throughthe Smart combination of wireless and fiber optics tech-nologies can the demand for seamless coverage, low-losstransmission and wide bandwidth be satisfied [1]. Thedesign of compact, low cost volume and weight wirelessterminals that may connect directly to fiber is expectedto play a key role towards the next systems generation.That explains the recent popularity growth of Radio overfiber (RFoF) techniques in which the wireless signal istransmitted in native format through fiber so the wire-less nodes can be kept simple and transparently bridgethe radiofrequency envelope in between the optical andthe wireless domainsA device that encompasses such characteristics canplay a big role in the upcoming internet of things (IoT)technology. This technology has many promising applica-tions, but at the same time several technical challengesarise. Some of these obstacles are spectrum saturationand attenuation of signals at high frequencies. In thepresent paper, we present a solution to solve these prob-lems with the design of a dual-band optoantenna for forthe full-duplex fiber-radio connection. The LTE 22 bandwith a FDD duplex mode has been targeted for this de-sign.II. DEVICE STRUCTUREThe device can be divided in two parts: the optoelec-tronic circuitry and the antenna.The main electronic parts of the optoelectronic cir-cuit are the low-cost Transmitter/Receiver Optical Sub-Assembly (TOSA/ROSA) units. The TOSA consistson a distributed feedback (DFB) transmitter laser withemission band centered at l = 1.3µm. And the ROSA,which includes a PIN photodiode receiver connected to atransimpedance (TIA) amplifier. Their main function isto convert the received optical signal into an electric sig-nal and vice versa. The circuitry also accomplishes DCfeeding requirements and impedance matching with theFIG. 1: Diagram of the proposed full-duplex deviceantenna.The antenna consists of a two-port aperture-fed patchto radiate and receive the RF signal and a filter. Thefilter is made of various resonators. These resonatorsare responsible of separating the uplink and downlinkfrequencies, in order to deliver two different signals to thecircuitry. Furthermore, this antenna has the capabilityof operating at both frequencies while keeping both portsisolated.III. ELECTRONIC COMPONENTSThe chosen components are the FP-1310-4I laser andthe PIN-1310-10LR-LC photodiode. Those are off-the-shelf (OTS) cost-effective components easily integrablewith the proposed structure and the optical backend.In the laser side, DC power and signal are deliveredsuperposed in the same input pin. The current comingfrom the power supply is regulated using three 20Ω re-sistances. Three resistances were used instead of one, inorder to be within the power ratings, and to have a bettercontrol of the current flow. Additionally an inductanceof 100nH was used to not let RF signal into the powersupply.As mentioned before, the package of the photodiodeincludes an amplifier inside of it, which is fed with thesame DC input as the laser. Unlike the laser, powerand signal go through different pins, but we also used aninductance (100nH) to protect the power supply in caseit exists some coupling. Both photodiode and laser work2at the wavelength of 1310 nm and support bit rates upto 10Gb/s.A. CharacterizationThe characterization and modeling of the two com-ponents is necessary to isolate their real frequency re-sponse from the rest of testing circuit, like transmissionlines and welds (de-embedding). The first objective isto find a lumped element model of the components. Af-terwards, a matching network is proposed to adapt thecircuit and antenna impedances. The modeling of thecomponents is made using the Keysight Advanced De-sign System (ADS) software. The laser model followsthe circuit proposed by M. S. Ozyazici [2]. Although, thescheme was simplified because our measures did presentnegligible losses.With the purpose of making the initial measurementsa testing microstrip board is printed to fit our model. Weused FR4 as the substrate and copper coattings (σCU =5.96 · 107S/m) of 35µm. The laser, the photodiode andthe basic components are included in the prototype.The frequency response of the components is obtainedusing a vector network analyzer (VNA). The VNA re-turns S-parameter measurements at various frequencies.The measurements are performed between 3 and 4 GHzas this spectrum band was targeted. The equivalent in-put impedance is obtained with single-port independentmeasurements of the laser and the photodiode. The sig-nal transmission between both elements is also testedthrough an optical fiber. The S21 parameter shows a-20 dB gain from both ends, mainly due to the mismatchbetween the ports and the impedance of the laser andthe photodiode.MSubLRTermGMSUBMLINLLRR CCC RL5TermG2R10TL2L2L1R2R1C4C2C1MSub1R8R=L=4.12257 nH {t} {o}TanD=0.017Z=50 OhmNum=1R=16.9774 Ohm {t} {o}H=1.5 mmEr=4.4Mur=1Cond=5.96e7Hu=1e+33 mmSubst="MSub1"W=2.7 mmL=2.84639 cm {t} {-o}R=L=0.105 nHR=L=0.08 nHR=1.4 OhmR=3.62 OhmC=1.26286 pF {t} {o}C=8.9 pFC=851 pFT=35 umR=25369.2 Ohm {t} {o}FIG. 2: Photodiode modeling on ADSA flat zone with low loses was observed near 3.5GHz.Thereby, the LTE 22 band is proven a good choice forthis design.The S-parameters of the model are fitted to those mea-sured, by tunning the values of the lumped components.Then, when a good starting point is reached, one can usethe optimizing algorithms of the ADS software. Usingan error function with the difference between simulatedand real responses, the software provides the best valuesof the lumped elements of the model. In practice, sev-eral optimization algorithms were used. Most useful onesMSubMSUBsr_ims_RC-I_0805_J _19950814sr_ims_RC-I_0805_J _19950814sr_ims_RC-I_0805_J _19950814V_DCLsl_cft_0805CS_J _19950814RMLINTermL LR RC CCLMSub1R8R6R7SRC1R9L5L4C3TL1Term1L1 L2R1 R2C1 C2L6Dpeaks=Bbase=Rough=0 mmTanD=0.017T=35 umHu=1e+33 mmCond=5.96e7Mur=1Er=4.4H=1.5 mmPART_NUM=RC-I-0805-20R0-J  20 OhmPART_NUM=RC-I-0805-20R0-J  20 OhmPART_NUM=RC-I-0805-20R0-J  20 OhmVdc=3.3 VPART_NUM=0805CS-101XJ BC 100 nHL=0.98403 nH {t} {o}R=R=2.97022 Ohm {t} {o}C=2.05492 pF {t} {o}W=2.7 mmL=2.84639 cm {t} {o}Z=50 OhmNum=1R=L=0.08 nHR=L=0.105 nHR=3.62 Ohm R=1.4 OhmC=851 pF C=8.9 pFSubst="MSub1"L=2.85403 nH {t} {o}R=FIG. 3: Laser diode modeling on ADSfor this task are the gradient and simulated annealing al-gorithms. The results of the fitting are presented on aSmith chart in Fig. 4 and 5.0.10.20.30.40.50.60.70.80.91.01.21.41.61.82.03.04.05.010 2020-2010-105.0-5.04.0-4.03.0-3.02.0-2.01.8-1.81.6-1.61.4-1.41.2-1.21.0-1.00.9-0.90.8-0.80.7-0.70.6-0.60.5-0.50.4-0.40.3-0.30.2-0.20.1-0.1freq (3.003GHz to 4.001GHz)S(1,1)measures..S(2,2)FIG. 4: Photodiode real measures in front ofcharacterization.0.10.20.30.40.50.60.70.80.91.01.21.41.61.82.03.04.05.010 2020-2010-105.0-5.04.0-4.03.0-3.02.0-2.01.8-1.81.6-1.61.4-1.41.2-1.21.0-1.00.9-0.90.8-0.80.7-0.70.6-0.60.5-0.50.4-0.40.3-0.30.2-0.20.1-0.1freq (3.003GHz to 3.902GHz)S(1,1)measures..S(1,1)FIG. 5: Laser real measures in front of characterization.IV. IMPEDANCE MATCHINGFinally, a circuit to match the impedance of the an-tenna is designed with the help of ADS software. We3FIG. 6: Vertical dissection of the antenna. Bottom layercontains the resonatorsdecided to match both the antenna and the RF circuitryto 50 Ohms. This way, the optoantenna design can beadapted to different requirements by changing only theantenna part. Thanks to the de-embedding, we were ableto match the response of the circuit without the extratransmission lines of the test board and DC-blocks. Theimpedance matching is carried out with a transmissionline and an open stub, since a narrowband operation isenough for the current solution.. The electrical length ofeach component is such that the impedance matching ismaximized at the center uplink/downlink frequency.V. ANTENNA DESIGN AND SIMULATIONThe antenna is designed and simulated using the soft-ware CST Studio Suite.The design of the antenna is based on the one proposedin [3]. This design makes use of open loop and hairpinresonators to implement the desired filters. The structureof the antenna is shown in Fig. 6 is made of two RogersRO4003C substrates represented by the orange pieces.Between the substrates a small permittivity foam (bluepiece in Fig6) is placed to separate them. A square patchis printed on the top layer of the upper substrate and aconductor plane with a slot is printed on the entirety thetop layer of the lower substrate. Finally, the feeding linesand the resonators (purple surface in Fig.6) are printedthe bottom layer of the lower substrate.The filtering of the two channels is accomplished bytwo open loop resonator pairs, each with a different res-onant frequency, f1 = 3.4GHz for the top one, actingas uplink, and f2 = 3.6GHz for the lower one, actingas downlink. The two pairs of resonators are coupled tothe hairpin resonator in between them, this hairpin res-onator is tuned to a central frequency f0, which is sharedby the patch. The conductor plane printed on top of thelower substrate acts as a common ground for both thepatch and the resonators, as well as for the feeding lines.FIG. 7: Resonator diagramThis conductor plane also helps to reduce the interfer-ence between the radiation of the patch and the resonatornetwork. However a small slot is left uncovered in thisplane, to allow the coupling of the hairpin resonator andthe patch.To design the resonators, we make use of the fact thatthis resonators select a wavelength approximately equalto their total length, in other words:LT ≈λ2(1)Where LT represents the total length of the resonator(both open-loop and hairpin). If we translate this re-sult to frequency, we get the following rule to design aresonator at frequency f :LT ≈c2f√εr(2)In the case of our material of choice, Rogers RO4003C,it’s relative permittivity is εr = 3.55. Expression 2 is agood starting point, but a finer tuning is required to re-ally select the desired frequency. Additionally the widthof the microstrip lines is calculated in order to get a char-acteristic impedance Z0 = 50Ω.The simulation of the antenna behavior and the use ofthe optimizing tools allows us to choose the set of geomet-rical parameters that result in the desired performance.The numeric values of this parameters are contained inTable I and represented in figures 6 and 7HHP BHP BR1 HR1 GR1 DR1 DR1L DR1HP9.74 9.20 9.20 7.42 0.60 0.50 0.40 0.40Lg Lp BR2 HR2 GR2 DR2 DR2L DR1HP90.0 28.0 9.20 5.60 0.60 0.51 0.40 0.4W H1 H2 H3 WS LS DS HL1 HL21.13 0.50 2.50 1.0 2.02 19.83 5.00 1.0 1.0TABLE I: Antenna Dimensions (mm)43.3 3.35 3.4 3.45 3.5 3.55 3.6 3.65 3.7Frequency (GHz)-50-45-40-35-30-25-20-15-10-50Magnitude (dB)S11S12S21S22FIG. 8: S-parameters of the antennaThe parameter DS needs some clarification, since it isthe deviation of the slot from the centre of the groundplane, along the slot’s long axis.With this values, the whole antenna was simulated.And the S-parameters obtained are those seen in Fig.8. It can be observed that both S(1, 1) and S(2, 2) arewell below −20dB at their respective frequencies. Thisensures a very low reflection coefficient in both ports 1and 2, and thus a maximum power transfer when oper-ating at their respective frequencies. Moreover, S(1, 2)and S(2, 1), are both quite low, at around −30dB for thewhole frequency range, and therefore a good isolation be-tween ports is guaranteed. It is clear that uplink channelhas a smaller bandwidth than the downlink one. Moreprecisely, the bandwidth computed at −10dB is:FBW1 =3.408− 3.3943.4= 0.4118% (3)FBW2 =3.628− 3.5763.6= 1.4444% (4)The radiating behaviour of the antenna is presented inFig. 9 and 10. In these figures the direction of the Z axisgoes from the patch towards the resonator. Thereforethe angle θ = 180º is aligned with the patch, and so,the main lobe is also aligned with it. Hence the antennaradiates mainly from the patch, and quite less from theresonators.FIG. 9: Farfield directivity at f1 = 3.4GHz, port 1 ex-citedFIG. 10: Farfield directivity at f2 = 3.6GHz, port 2 ex-citedVI. CONCLUSIONThis paper has demonstrated the use of an optoantenalike the one proposed as an effective solution for radioover fiber bidirectional communications. Their low cost,reduced size and good performance makes designs likethis one strong contenders for future applications spe-cially in the field of IoT.However this design has only been tested from a sim-ulation standpoint, and is still lacking experimental ev-idence to support it. In the near future the whole sys-tem should be printed and tested to see if measured dataagrees with the simulations. If so, the validity of thisdesign would be reaffirmed.[1] Francisco Aznar Concepción Aldea Diego F. Paredes-Páliz, Guillermo Royo and Santiago Celma. Radio overfiber: An alternative broadband network technology foriot. Electronics, 9:1785–1791, 2020.[2] M. S. Ozyazici. The complete electrical equivalent circuitof a double heterojunction laser diode using scatterringparameters. Journal of Optoelectronics and Advanced Ma-terials, 6(4):1243 – 1253, 2004.[3] Chun-Xu Mao, Steven Gao, and Yi Wang. Dual-bandfull-duplex tx/rx antennas for vehicular communications.IEEE Transactions on Vehicular Technology, 67(5):4059–4070, 2018.

Bidirectional optoantenna for radio over  ber architectures

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Bidirectional optoantenna for radio over ber architectures

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