optical microwave converter ims2010

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February 2011 85

The Optical-to-Microwave Converter Student Design

Competition was sponsored by MTT-3 MicrowavePhotonics subcommittee. Fiber-optic links are at-

tractive for remoting microwave signals. However, formany applications, the optical link still has limited outputpower and undesirable power efciency. These can be ad-dressed by improving the optical-to-microwave conver-sion process in the link receiver. A cost-effective solutionis to power-combine photodetectors inside the receiver. Inthis design competition, each student team was requiredto design a 1 , 3 GHz optical-to-microwave converterwith greater than 10% bandwidth by power-combiningfour low-cost photodetectors supplied by Linear Photon-

ics. The design goal was to achieve more than 20 dBm RF

output power, at least 35 dBm third-order intercept point(OIP3), and greater than 5% power efciency.

The goals of the MTT-3 subcommittee are: to fa-cilitate the exchange of information about lightwavetechnology among members of the MTT-S, and to as-sist in educating the MTT-S community on the tech-niques and applicability of lightwave technology. Thecommittee emphasizes important applications in bothcivilian and military fields, including guided-wavetechniques, high-speed circuits, microwave tech-niques, and their combination and/or integration withoptoelectronic devices.

MTT-3 Optical-to-Microwave Converter ■ Yifei Li

An Optical-to-Microwave Converter ■ Renyuan Wang, Mohammad Ali Shirazi-H.D., and Yifei Li

RF/photonic links are ber-opticlinks used to convey RF signals(Figure 1). At the link transmit-

ter, the RF signal is encoded onto anoptical carrier and transmitted throughoptic bers. At the receiver, the opticalsignal is converted back to the micro-wave domain by a photodiode. The RF/photonic links are attractive for manyapplications, including cable televi-sion, radio-over-ber systems, antennaremoting, local oscillator signal distri- bution, and optical domain true-time-

delay beam forming [1]–[3]. For RF/photonic links, an efcient optical-to-

microwave converter capable of gen-erating sufcient RF power with highdelity is critical. This calls for a photo-detector with high frequency responseand high saturation optical power. Inorder to increase the saturation opticalpower of a conventional photodetector,a large photodetection area is necessary.But this increases the detector parasiticcapacitance. A traveling wave photode-tector, in principle [4]–[5], can overcomethis limit. However, it suffers from lowcharacteristic impedance, velocity mis-

match, and poor coupling efciency be-tween the optical ber and the detectorwaveguide. All these limit its achievableperformance. In light of these challeng-es, an alternative low-cost approach of

power combining an array of relativelylower power but high-frequency pho-todetectors to enhance the photodetec-tor output power seems very attractive[6]. The power combining might pose bandwidth limitations. However, theRF/photonic links mostly only requirehigh-carrier frequencies but not verylarge instantaneous bandwidth.

This article addresses the designof an optical-to-microwave converterfor the student design competitionat the IMS 2010. The design is based

on the power combining approach.The objective of the competition wasto design and realize an optical-to-microwave converter with a 10% rela-tive bandwidth (the center frequencyis selected by the competitors from 1GHz , 3 GHz), a peak output RF pow-er of . 20 dBm, and an output OIP3 of . 35 dBm. In our design the center fre-quency of 1 GHz was selected, and theoutputs of four low-cost photodetec-tors were combined to meet the power

requirement. In addition, to improvethe output power and efficiency, anDigital Object Identifier 10.1109/MMM.2010.939315

Renyuan Wang (rw364@cornell.edu) iswith the School of Electrical and Computer

Engineering at Cornell University. Yifei Li iswith the Department of Electrical and Computer

Engineering, University of Massachusetts,Dartmouth. Mohammad Ali Shirazi-H.D. (mali.shirazi@ieee.org) is a graduate (M.Sc.) student

with the Department of Electrical and ComputerEngineering, University of Massachusetts,

Dartmouth.

CWLaser

ExternalModulator

RFOutput

RF Input

Figure 1. RF/photonic link.Date of publication: 14 January 2011

1527-3342/11/$26.00©2011 IEEE

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86 February 2011

optimized photodetector load imped-ance was employed instead of the stan-dard 50- V termination. The converterdemonstrated 100 mW RF outputpower and OIP3 of 35 dBm, thus ful-filling the technical requirements spec-

ified by the competition. Compared toother approaches [6]–[8], our designprovides a simple, cost-effective wayof achieving optical-to-microwaveconversion with sufficient RF outputpower and high fidelity.

Converter DesignThe essence of designing the optical-to-microwave converter is to developappropriate power combining andmatching circuits for the selected pho-todiodes at the targeted operation fre-quency [6], [9]. Here, the photodiodes(Emcore 2651E) were supplied by thecompetition host, and the operationfrequency of the converter is targetedat 1 GHz. The bandwidth, responsiv-ity, and input saturation power of thephotodiodes are 3 GHz, 0.98 A/W at1,550 nm, and 10 mW, respectively.To achieve 20 dBm RF output, powercombining of multiple photodiodes is

necessary. The power combining alsoreduces the necessary RF current swingon each photodiode, thereby improv-ing the converter linearity. However,the trade-off is reduced bandwidth,increased system complexity, andcost. After considering all the factors,four photodiodes were finally used toachieve the output power requirementand to optimize the system complexityand linearity.

In order to facilitate the design and

simulation of the converter, an equiva-lent circuit model of the photodiodesFigure 3. (a) Experiment setup for measuring the peak output RF power,(b) peak output RF power at 1 GHz, and (c) measured converter bandwidth.

RF Input

MZ Modulator RF Output

Optical-to-MicrowaveConverter

1,550 nm

0.90 0.95 1.00 1.05 1.10

–40–30–20–10

01020

–50

30

Frequency (GHz)

O u

t p u

t R F P o w e r

( d B m

)

(a)

(b)

0.75 1.00 1.25 1.500.50

–40

–35–30

–25–20

–45

–15

Frequency (GHz)(c)

L i n k G a

i n ( S

2 1

) ( d B )

CW Laser

150 MHz

(d)

0.75 1.00 1.25 1.500.05

05

101520

–5

25

Frequency (GHz)

O u

t p u

t

S i g n a

l L e v e

l ( d B m

)

3 dB Bandwidth

(a)

C j = 1.29 pF

l = 1.29 nH

r = 2.2 Ω

D x = 23°

I

Measured S 22 of the PhotodiodeSimulated S 22 of the EquivalentCircuit Model

V Bias

V Bias

4 × 1

(c)

TransmissionLine

Transformer

Photodiode

ImpedanceMatching and PowerCombining Circuits

(b)

1 GHz

R x = 6.8 M Ω

0 . 2

0 . 5

1 . 0

2 . 0

5 . 0

Figure 2. (a) Photodiode equivalent circuit model, (b) S22 of the photodiode and the equivalent circuit model (frequency: 45 MHz to1.5 GHz), (c) schematic and photo of the optical-to-microwave converter, and (d) simulated output RF power versus frequency.

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February 2011 87

was first determined by curve-fittingthe measured S22 of the photodiode(obtained from an Agilent 8510C vec-tor network analyzer) [10]. Figure 2(a)shows the equivalent circuit model,where Rx and C j are, respectively, thedynamic impedance and junction ca-

pacitance of the reverse-biased photo-diode, l and r are the parasitic induc-tance and resistance of the bondingwires, respectively, and Dx accountsfor the electric delay caused by a shortsection of a 50- V transmission line inthe photodetector mount. The S22 of the equivalent circuit model fits themeasured data well from 45 MHzto 1.5 GHz [Figure 2(b)], the error iswithin 0.4%. Figure 2(c) shows theschematic of the converter. As the

photodiodes exhibit large internal im-pedances, a large load impedance is

required to extract high RF power. Weuse a 1:4 transmission line transformer(Minicircuits TC4–25G21) to boost theload impedance from the standard50 V to 200 V. Also, at 1 GHz, the im-pedance of the photodiode is capaci-tive. Therefore, shunt inductors are

used to resonate the capacitance [Fig-ure 2(c)]. The layout of the converteris on a 60-mil thick Duroid substrate(RO4350), where it is easier to imple-ment high- impedance transmissionlines. Figure 2(c) shows the fabricatedconverter. The simulation result [seeFigure 2(d)] shows a peak output pow-er of 22 dBm at 1 GHz and a 3-dB band-width of 120 MHz when the total inputoptical power to the four detectors is40 mW, assuming a 100% modulation

index. This meets the requirements of the competition.

MeasurementsThe peak output RF power of the con-verter was measured using the setupshown in Figure 3(a). A 1,550-nm con-tinuous wave (CW) optical carrier ismodulated using a Mach-Zehnder (MZ)intensity modulator. The modulator is a

traveling wave device with a 50V

ter-mination. It has a V p of 6 V and a band-width of 10 GHz. The output optical sig-nal is split into four paths and detectedwith the optical-to-microwave convert-er. At 1 GHz, the peak output RF poweris 20.7 dBm [Figure 3(b)] when the RFpower applied to the modulator is 21dBm and the combined photocurrentfrom the four photodiodes is 35 mA.The measurement is in good agreementwith the simulation result. The con-

verter bandwidth was also determinedthrough the small-signal link gain (S 21)

(a)

CW Laser 1(1,546 nm)

CW Laser 2(1,550 nm)

EDFA

EDFA

PolarizationController

PolarizationController

MZ Modulator 1

MZ Modulator 2

f 1

f 2

Optical-to-MicrowaveConverter

Input Voltage O p

t i c a

l I n t e n s

i t yMZ Modulator Input/Output

ResponseDistortion Causedby the Nonlinearity

of the Converter

10 15 20 25 30 35 40 45–80

–60

–40

–20

0

20

40

Applied RF Power (dBm)

O u

t p u

t S i g n a

l L e v e

l ( d B m

)

–60

–50

–40

–30

–20

–10

0

–70

10

Frequency (MHz)

O u

t p u

t S i g n a

l L e v e

l ( d B m

)

(b) (c)

998.5 998.7 998.9 999.1 999.3

Fundamental ToneThird-Order Distortion

Output IP3 = 35.1 dBmThird Order

Nonlinear DistortionFundamental

Response

Figure 4. (a) Experiment setup for linearity measurement, (b) output spectrum of the two-tone test, (c) output IP3 of theoptical-to-microwave converter.

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