Microwave Photonics - DARPA

8/9/2019 Microwave Photonics - DARPA

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3428 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 20, OCTOBER 15, 2014

Microwave Photonics Programs at DARPARichard W. Ridgway, Senior Member, IEEE, Carl L. Dohrman, Senior Member, IEEE,

and Joshua A. Conway, Senior Member, IEEE

(Invited Paper)

AbstractOver the past ten years, DARPA has made significantinvestments toward advancing the field of microwave photonics.This paper reviews DARPA-funded progress in this subject overthe past decade. DARPA-funded research has advanced the state-of-the-art formicrowave-photonic components, including low noiselaser diodes, electrooptic modulators and high power photodiodes,as well as microwave photonic link configurations, including pho-tonic downconversion, reconfigurable optical filters and opticalphase-locked loops. These investments have yielded dramatic im-provements in spurious-free dynamic range (SFDR). Measuredperformance includes SFDRs exceeding 115 dB Hz2/3 at 16 GHzusing broadband externally modulated links; exceeding 120 dB

Hz2/3 at 10 GHz using sub-octave electrooptic modulators; near135 dB Hz2/3 at 100 MHz using optical phased-locked loopsas linear phase demodulators; and exceeding 125 dB Hz2/3 at5 GHz using optical filtering, downconversion and predistortioncompensation.

Index TermsElectrooptic modulator, high power photodiode,microwave photonics, noise figure, optical fiber links, opticalphased-locked loops, photonic downconversion, spurious free dy-namic range (SFDR).

I. INTRODUCTION

MULTIFUNCTIONAL receivers, capable of performing

communications, radar, and electronic warfare func-

tions, are of considerable interest to the military. The next gen-eration of multifunctional receiver technology will require sub-

stantial gains in several key performance parameters, including

increased frequency of operation (f 18 GHz), increased in-

stantaneous bandwidth (B 1 GHz), increased spurious-free

dynamic range (SFDR > 120 dBHz2/3) and enhanced receiver

sensitivity (S 90 dBm). Furthermore, there is a continued

desire to reduce the system size, weight, power, and cost. The

application of microwave photonic components and links in

such receivers could yield significant improvements to these

performance parameters. Microwave photonic components op-

erate at very high frequencies with very wide bandwidths [1][4]

and can efficiently transfer signals from the RF to the optical

Manuscriptreceived January15, 2014; revised March 27,2014;accepted May11, 2014. Date of publication June 17, 2014; date of current version September1, 2014. The views opinions, and/or findings contained in this article are thoseof the authors and should not be interpreted as representing the official views

or policies, either expressed or implied, of the Defense Advanced ResearchProjects Agency or the Department of Defense.

R. W. Ridgway and J. A. Conway are with the U.S. Defense AdvancedResearch Projects Agency (DARPA), Arlington, VA 22203 USA (e-mail:[email protected]; [email protected]).

C. L. Dohrman is with the Booz Allen Hamilton, Inc., Arlington, VA 22203USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2014.2326395

domains and back. Furthermore, microwave photonic technolo-

gies are compatible with frequency channelization to simultane-

ously allow wide instantaneous bandwidths and good receiver

sensitivity. Finally, microwave photonic links use optical fiber

to transport the signals around the platform, which reduces the

RF loss and significantly reduces the cable size and weight. As

an example, the weight of a representative low-loss military-

grade RF cable (113 kg/km [5]) is more than three times higher

than the weight of a comparable military-grade fiber-optic ca-

ble (31 kg/km [6]), while the loss of the RF cable (0.72 dB/m

at 18 GHz) is orders of magnitude more than fiber-optic ca-ble (0.2 dB/km). While there are numerous components and

architectures that have been used to create microwave photonic

links, the three dominant components that enable the effective

use include:

1) Low-noise, high-power laser diodes.

2) Low-loss electrooptic modulators with low drive voltage,

and

3) High-power, highly-linear photodiodes.

Over the last ten years, the Defense Advanced Research

Projects Agency (DARPA) has invested in numerous programs

aimed specifically at the improvement and maturation of these

three components. The Ultra-Wideband Multifunction PhotonicTransmit/Receive Module (ULTRA-T/R) and Photonic Simul-

taneous Transmit and Receive (P-STAR) programs aimed to

improve the electrooptic modulator as a means of achieving

good RF isolation between transmit and receive signals sharing

a common aperture at X-band. The Transmit and Receive Op-

timized Photonics (TROPHY) program focused on improving

lithium niobate modulators and high power photodiodes to in-

crease the RF transmit power of microwave photonic links. The

Linear Photonic RF Front-End Technology (PHOR-FRONT)

program investigated the use of optical phase-locked loops as a

means of improving the linearity of microwave photonic links

and explored photonic downconversion. The Photonic Analog

Signal Processing Engines with Reconfigurability (PhASER)program considered RF signal processing in the optical domain,

using reconfigurable optical filters anddelay lines to improve the

SFDR of an RF photonic link. The Network Enabled by WDM-

Highly Integrated Photonics (NEW-HIP) program investigated

low-noise laser diodes and high-power photodetectors to create

a wavelength-division-multiplexed network capable of trans-

mitting analog and digital signals on a common single-mode

fiber-optic network. While these programs measured various

performance metrics, a common metric was SFDR. This pa-

per will summarize some of the results from these and other

DARPA-funded programs and describe how these performance

0733-8724 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications standards/publications/rights/index.html for more information.


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RIDGWAYet al.: MICROWAVE PHOTONICS PROGRAMS AT DARPA 3429

Fig. 1. SFDR and receiver sensitivity as a function of bandwidth for a typicalRF receiver, assuming an SNR requirement of 5 dB, noise figure of 5 dB, andSFDR of 110 dB in a 1 Hz band.

metrics could translate to system attributes of interest to military

platforms.

II. MICROWAVERECEIVERS

The three performance metrics of interest for a microwave

receiver include the instantaneous bandwidth, the receiver

sensitivity, and the dynamic range. The receiver sensitivity (S),

which is sometimes referred to as the minimum detectable sig-

nal, is determined by 1) the thermal noise power associated with

the bandwidth of the receiver, 2) the noise figure of the receiver,

and 3) the signal-to-noise ratio needed to detect or demodulate

the received signal:

S= 10log (kT B) + N F+ SNRre q (1)

wherekis Boltzmanns constant,Tis the apparent receiver tem-

perature in Kelvins, and B is the bandwidth in Hertz. As the

bandwidth increases, the thermal noise will increase, causing

degradation in the receiver sensitivity. The red line in Fig. 1

shows an example of this relationship for a receiver with 5 dB

noise figure and 5 dB signal-to-noise ratio requirement. The dy-

namic range of a receiver is also impacted by the instantaneous

bandwidth. The SFDR of a receiver is often normalized to a

1 Hz bandwidth. For an instantaneous bandwidth, B, the SFDR

for a receiver can be calculated as:

SFDR(B) = SFDR (1Hz) 2

3 10log (B) . (2)

The black dashed line in Fig. 1 shows an example relationship

between bandwidth and SFDR for a receiver with normalized

SFDR= 110 dBHz2/3.Clearly, the bandwidth and the noise figure influence the

receivers sensitivity as well as the SFDR. The dashed line at

110 dB Hz2/3 is representative of the SFDR of a state-of-the-art

electronic receiver. The solid line denotes the sensitivity as

a function of bandwidth for a typical X-Band electronic

receiver chain with a noise figure of 5 dB. To achieve the

Fig. 2. Block diagram of an external modulation microwave photonics link.

Fig. 3. Plot of measured laser relative intensity noise (RIN) versus opticaloutput power. Filled markers denote shot-noise-limited measurements and theopen markers denote estimates of the laser RIN, with the shot-noise removed.The red X depicts the desired performance of a low-RIN laser.

desired dynamic range of greater than 60 dB and sensitivity of

90 dBm requires a receiver bandwidth of about 10100 MHz.

III. MICROWAVEPHOTONICTECHNOLOGIES

A block diagram of an external modulation microwave pho-

tonic link is shown in Fig. 2. A low-noise, continuous-wave

laser diode provides light to an electrooptic modulator, such

as a lithium niobate modulator configured as a MachZehnder

interferometer (MZI). A microwave signal drives the electroop-

tic modulator, which modulates the light that is directed over

a single-mode fiber to a high-power photodiode. This section

summarizes the recent progress made on these three key mi-

crowave photonic components.

A. Low-Noise, High-Power Laser Diodes

Minimizing the relative intensity noise (RIN) of the laser

diode while attaining high output power is an essential step

in achieving the desired link noise figure and SFDR. DARPA

has made a number of investments in achieving low-RIN, high-

power lasers at = 1550 nm for microwave photonics, withseveral noteworthy successes, including demonstration of semi-

conductor lasers operating at or below the shot-noise floor [7] of

state-of-the-art measurement systems, as summarized in Fig. 3.

The DARPA RF Lightwave Integrated Circuits (RFLICS) pro-

gram, researchers demonstrated a tunable sampled-grating dis-

tributed Bragg grating laser with shot-noise-limited RIN below


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160 dB/Hz with 6 mW output power and 50 nm of wavelength

tunability [8]. On another DARPA-funded program, InP-based

slab-coupled optical waveguide external cavity lasers achieved

shot-noise-limited system RIN ( 163 dB/Hz) from 0.2 to

10 GHz with output powers of 370 mW at 1550 nm [9]. Addi-

tionally, underthe DARPA NEW-HIP program, low-noise, high-

power InP-based DFB lasers were demonstrated. Unpackaged

versions of these devices demonstrated output powers of over

200 mW with shot-noise-limited system RIN measurements of

158 dB/Hz, with estimated laser RIN of less than165 dB/Hz

[10]. Packaged versions of these devices also demonstrated

shot-noise-limited RIN measurements while also showing laser

operation over a temperature range from 10 to 85 C. These

packaged lasers demonstrated 50 mW output power with shot-

noise-limited RIN of 155 dB/Hz from 0.1 to 20 GHz, with

estimated laser RIN of 166 dB/Hz [11]. Fig. 3 shows the

summary of these RIN measurements.

B. The Electrooptic Modulator

The MachZehnder modulator (MZM) has become the

predominant electrooptic modulator for high-performance

microwave-photonic applications. One of the attractive features

of the MZM is its well-defined transfer function, which allows

for accurate link performance analysis as well as the potential

for linearization techniques using pre- or post-distortion. The

optical transfer function of a MachZehnder electrooptic mod-

ulator can be written as [12]:

Pou t (t) = TffP1

2

1 + cos

(VB + vmsin(m t))

V

(3)

where Tff is the fiber-to-fiber transmission of the modulator

when biased for maximum transmission,P1 is the optical input

power in milliwatts,VB is the dc bias voltage, V is the voltage

needed to induce a phase shift, m is the input modulation

voltage, and m is the modulation frequency in radians per

second. The performance of a MZI is often characterized by its

slope efficiency [12], given by

sm zi(, f) = Tf fP1Rs

2V (f) cos (4)

whereRs is the source impedance and is angle relative to the

quadrature bias point (where = 0). Although there has beena great deal of interest and exploration of the role of the bias

condition of the modulator [13], the most common bias point isat quadrature, where the second-order distortion is minimized.

When the modulator is biased at quadrature, the third-order

distortion limits the SFDR, as will be discussed later.

Critical metrics for the electrooptic modulator include Tf f,

V , maximum optical power handling, and the 3 dB cutoff

frequency. DARPA has explored these parameters in a num-

ber of programs, including ULTRA-T/R [13], P-STAR, and

TROPHY. Each of these programs aimed to increase the opera-

tional frequency of the modulators by reducing the RF-optical

velocity mismatch and by reducing the attenuation coefficient of

the traveling-wave electrodes [14]. Achieving RF-optical veloc-

ity matching requires increasing the velocity of the RF signal by

Fig. 4. MeasuredV as a function of frequency [13].

reducing the effective dielectric constant of the RF signal. Meth-

ods of reducing the effective dielectric constant have included

reducing the thickness of the lithium niobate [15], increasing thethickness of the microwave electrodes [14], and etching away

unneeded portions of the lithium niobate [16].

An innovative lithium niobate modulator developed under

ULTRA-T/R focused on achieving velocity matching between

the traveling wave RF signal and optical signal and maximiz-

ing the length over which this velocity matching occurred. The

novel design resulted in an interaction length between the RF

and optical signals of nearly 14 cm [17]. The dual-drive z-cut

design yielded one of the lowest measured V for a broadband

lithium niobate modulator: 1.4 V at 12 GHz. Fig. 4 summarizes

the measuredV as a function of frequency [13]. The fiber-to-

fiber optical insertion loss (

10 log(Tff)) was measured to be8 dB [13]. While this lowV modulator has a great deal of util-

ity for microwave-photonic links, the focus of the ULTRA T/R

program was to allow simultaneous transmit and receive of mi-

crowave signals from a common antenna. Experiments carried

out under the ULTRA T/R program achieved 40 dB of isolation

between transmitted and received signals above 10 GHz [18].

The TROPHY program focused on improving the linearity

of microwave-photonic links as well as moving to higher oper-

ational frequencies. TROPHY included significant electrooptic

modulator development, and the focus was again on achiev-

ing velocity matching between the RF and optical signals. The

TROPHY program explored both thick electrodes and etching

of unneeded lithium niobate as a means of increasing the veloc-ity of the microwave signal. As part of the TROPHY program, a

lithium niobate modulator waspackagedwith 1 mm coaxial con-

nectors and demonstrated to have operational performance out

to 110 GHz. Fig. 5 shows the measured RF-optical frequency

response of the modulator. The fiber-to-fiber optical insertion

loss was measured to be 3 dB at a wavelength of 1550 nm [19].

On a DARPA-funded Small Business Innovation Research

program, velocity matching between the RF and optical signals

was achieved by transferring a very thin layer (110 m) of

lithium niobate onto a quartz substrate using crystal ion slic-

ing [20]. The dc V of the device was shown to be 2.5 V at

1550 nm wavelength with a VL product of 4.75 V

cm [21].


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Fig. 5. Measured RF-optical frequency response of a velocity-matched

lithium niobate modulator [19].

While these devices have not been packaged and fully character-

ized at high frequencies, preliminary results show preservation

of the electrooptic coefficient and low optical loss in the optical

waveguides formed in the thin films of lithium niobate. Fur-

thermore, electromagnetic models show that the low dielectric

constant of the quartz substrate enables simultaneous velocitymatching and low RF loss out to millimeter-wave frequencies

[20].

In addition to lithium niobate modulators, DARPA has also

made significant investments in electrooptic modulators based

on alternative materials. While lithium niobate has a relatively

high electrooptic coefficient (r33 ) of 30.8 pm/V, novel materi-

als engineering approaches have been investigated to produce

electrooptic polymers with potentially much higher r33 . The

DARPA Supermolecular Photonics Engineering (MORPH) pro-

gram developed electrooptic polymers and investigated their

use in electrooptic modulators. Under this program, electroop-

tic polymers based on poly(methyl methacrylate)-anthrylmethylmethacylate (PMMA-AMA) were synthesized and shown to

have r33 of over 300 pm/V. Additionally, MZMs were fabri-

cated using these polymers, and dc V as low as 0.75 V was

demonstrated, while insertion losses of 17.1 dB were measured

[22]. Subsequent work with electrooptic polymer-based MZMs

has resulted in commercial products; as an example, an elec-

trooptic polymer-based MZM with dc V of 2.5 V, insertion

loss of 6.5 dB, and speeds of up to 45 Gbit/s for differential

phase shift keying is commercially available [23].

Another materials system currently under development is the

GaAs/AlGaAs system. GaAs-based MZMs have been demon-

strated in the past with V as low as 0.3 V in a pushpull config-

uration [24], but their overall performance is limited by the r33of GaAs. Additionally, the small mode sizes of these devices re-

sult in increased coupling losses, which limits their utility in mi-

crowave photonic systems. Recently, DARPA has funded work

to increase the electrooptic efficiency of GaAs-based modula-

tors through the use of InAlGaAs/InAlAs multi-quantum well

structures in the waveguide core [25]. These structures were

used to demonstrate MZMs with V of 2 V for single-arm-drive

and device length of 1.8 mm, resulting in a V L product of

0.36 Vcm. The device has potential for redesign in a pushpull

configuration to reduce V by a factor of two. The capacitance

of the devices was measured as 2 pF/cm, which is expected to

enable high-speed operation.

Fig. 6. Plot ofV versus 3 dB bandwidth for a number of MZMs in the recentliterature. Data markers are color-coded to indicate DARPA funded work, andshape-coded to indicate materials system. Data points which did not include3 dB frequency measurements are shown at 1 GHz. Based upon data compiled

by [37].

A summary of the state-of-the-art in MZMs is shown in

Fig. 6, which plots V versus bandwidth. It should be noted

that insertion loss and power handling also play an impor-

tant role in modulator performance but are not captured in this

figure.

C. Photodetectors

When operating in the linear region of the photodiode re-

sponse, the output current of the photodiode is given by:

ide t =rd Pou t (5)

where rdis the detector responsivity in A/W and Pou tis the light

from the electrooptic modulator. Neglecting the frequency re-

sponse of the photodiode and electrooptic modulator, the output

RF signal is given by:

PRF out=i2de t RL (6)

where RL is the load resistor. To address the high-power,

high-speed, high-linearity requirements of microwave photonic

receivers, the unitraveling carrier (UTC) photodiode has estab-

lished itself as the leading photodiode architecture for high-performance receivers. Under the DARPA TROPHY program,

significant advances in high-power, high-linearity UTC photo-

diodes were demonstrated [38]. Specific developments include

the demonstration of UTC photodiodes with RF output power

up to 750 mW at 15 GHz with an output third-order inter-

cept point (OIP3) of >55 dBm. The maximum power level

was achieved using InP/InGaAs-based UTC photodiode struc-

tures bonded to a high thermal conductivity substrate. Using a

novel charge-compensated modified unitraveling-carrier (CC-

MUTC) photodiode design, high RF output powers were at-

tained in the V-band, including devices with 3 dB bandwidths

of 50 and 65 GHz demonstrated with saturation currents of 95


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Fig. 7. Plot of saturation current versus frequency for high-power, high-speedphotodiodes. Based upon data compiled by J. Campbell and A. Beling [48].

and 55 mA, respectively [39]. A plot of the state of the art in

saturation current versus 3 dB bandwidth is shown in Fig. 7.Another area of DARPA-funded photodiode research is

the topic of balanced photodetectors. Balanced photodetectors

have potential to reduce the impact of RIN on the NF in a mi-

crowave photonic link, by enabling a differential measurement

of the RF signal relative to the original optical carrier. Balanced

photodetectors also enable increased optical power handling.

While the prospect of noise cancellation is appealing, the im-

plementation of microwave photonic links using balanced pho-

todetectors is limited by the ability to achieve high-precision

optical power splitting and equal optical delay in each path.

Under the DARPA TROPHY program, an InP/InGaAs modi-

fied UTC photodiode was demonstrated with a 3 dB bandwidthof 8 GHz, saturation current of 320 mA, maximum RF output

power of 1.5 W, and OIP3 of up to 47 dBm [40].

IV. MICROWAVEPHOTONIC LINKS

The inherent wide-bandwidth capabilities of microwave pho-

tonic components as well as the attractive size and weight ben-

efits of optical fibers make microwave photonic links of signifi-

cant interest to military systems for microwave antenna remoting

and signal distribution. Key figures of merit for a microwave-

photonic link are:

1) Noise figure (NF)The measure of degradation of the

signal-to-noise ratio from the input microwave signal tothe output microwave signal.

2) SFDRThe ratio of the input signal and the induced

spurious signals.

3) Microwave frequency of operation.

4) Instantaneous bandwidth.

These key figures of merit are dependent, in part, on the mi-

crowave photonic components described in the previous section.

However, they can also be influenced by other system design

considerations, such as the use of photonic down-conversion,

optical phase-locked loops, and reconfigurable optical filtering,

as explored in the DARPA programs TROPHY, PHOR-FRONT,

and PhASER, respectively.

Fig. 8. Calculated link noise figure as a function of laser RIN.

This section will review the key figures of merit of microwave

photonic links and review results from several DARPA-funded

programs.

A. Noise FigureThe noise figure is a critically important metric of the link.

While low noise figure can often be achieved through the use of

an electronic low-noise amplifier at the front of an RF receiver

chain, this diminishes the considerable bandwidth advantage

of using a microwave-photonic receiver and is impractical for

many antenna-remoting applications. The noise figure of a link

is given by [12]:

NF = 10log

1 +

nad d

gi nin

(7)

where nin is the input noise (nin =kT B), nad d is the noise

added by the link, andgi is the intrinsic gain of the analog link.For a MZI,gi is given by

gi =

Tf fP1Rs

2V

2r2d . (8)

Recognizing that there are two resistors that contribute to the

system noise, the noise figure can be rewritten as [1]

N F= 10log

1 + constant +

RL

gi kT

i2RI N + i

2SN + i

2t

(9)

where i2RI N , i2SN , and i

2t are the noise-like currents associated

with laser RIN, shot noise, and thermal noise, respectively, and

constant term refers to added thermal noise from the modu-lation device circuit. This term has a marked frequency depen-

dence,whichcan play a significant role in links using modulators

with traveling-wave electrodes (this term was first recognized

in [49] and is analyzed in greater detail in [1]).

Laser RIN tends to be the dominant noise current for mi-

crowave photonic links operating with high average photode-

tector current [12]. Fig. 8 shows the calculated noise figure

as a function of laser RIN for a link with high optical power

(P1 = 30 dBm), and low optical loss through the electroop-tic modulator (Tf f = 0.5). The three curves denote modula-tors withV = 4 V,V = 2 V, andV = 1 V, respectively. It

should be noted that it is challenging to achieve a laser RIN of


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Fig. 9. Calculated RF Transfer curve for a microwave photonic link.

TABLE IPARAMETERSUSED FORLINKMODEL

Parameter Value

Laser Power,P1 1 W

Laser RIN 170 dB/Hz

Modulator transmission,Tf f 5 dB

V 1.7 V

Detector responsivity,r d 0.9 A/W

Modulator load resistance,R m 50

Detector load resistance, R l 50

Noise figure 10 dB

Bandwidth 10 MHz

less than 170 dB/Hz. For this reason, it is difficult to achieve

a noise figure of less than 10 dB without using a noise-figure-

reducing technique, such as balanced detection [13] or using a

low noise amplifier at the input of the MZI to set the link noise

figure.

B. SFDR

For external modulation, amplitude-modulated links of the

type depicted in Fig. 2, the component that fundamentally lim-

its the dynamic range performance of the photonic link is the

electrooptic modulator. When biased at quadrature, the cosine

response shown in the modulator transfer function (see Eq. (3))

will generate third-order nonlinearities as the input signals be-

come large. Equations (3), (5), and (6) can be used to calculate

the microwave photonic links transfer function. Fig. 9 shows a

calculated transfer function for a link with component metrics

shown in Table I.

The SFDR of a microwave photonic link without predistortion

is limited by the third-order intermodulation distortion as is

given by [50]

SFDR =2

3[IIP3 NF (kT B)] (10)

where IIP3 is the input third-order distortion intercept point. For

an electrooptic modulator in a MZI configuration, IIP3 increases

with increasingV [50]. For the example shown in Fig. 9, IIP3

was calculated to be 13.7 dBm and the SFDR was calculated to

be 71 dB for a 10 MHz instantaneous bandwidth.

Fig. 10. Photonic mixer architecture [19].

C. The Role of Modulator Bias on SFDR

The modulator bias conditions role in SFDR has been the

subject of several DARPA-funded programs. One DARPA-

funded seedling program recognized that shifting the bias angle

( in Eq. (4)) away from quadrature has no effect on third-order

input power intercept but increases the even-order distortions

significantly. It was experimentally shown that the normalized

SFDR at an operational frequency of 2 GHz could be increased

by 9 dB to 122 dB

Hz2/3 simply by adjusting the bias an-

gle 168 from the maximum transmission point to a value of

= 78and increasing the laser power to provide an equivalentamount of photocurrent at the detector [50]. Additional exper-

iments demonstrated operation of the link at 18 GHz with an

SFDR of 118 dB Hz2/3. However, it is important to recognize

that these SFDR values assume that the bandwidth is limited to

less than an octave, since the second-order distortion terms have

not been minimized.

D. Photonic Downconversion

As described previously, photonic links can achieve largeSFDR and gain while reducing noise figure. For links spanning

tens of meters in length and with high microwave frequencies,

photonic links can meet or exceed the performance of conven-

tional coaxial cable while reducing size and weight. In these

systems, the dynamic range of the receiver chain tends to be

limited by the microwave mixer.

To push system performance, the TROPHY program de-

signed and demonstrated photonic mixers for wideband down-

conversion [19], [51]. Fig. 10 illustrates the architecture used

to implement photonic mixing. The system separately modu-

lates the RF signal and local oscillator onto the split optical

source. The optical carrier and lower sidebands are removed in

the optical regime by using fiber Bragg grating filters. This stepis critical to the performance. By removing sidebands and har-

monicsbefore mixing on the photodiode, the spurs are greatly

reduced. The SFDR of the microwave photonic link with pho-

tonic downconversion was compared with the SFDR from a

conventional microwave mixer. In every spur measurement, the

photonic mixer outperformed the conventionalmicrowave mixer

by at least 30 dB [51]. As empirically demonstrated, mixing in

the optical regime provided significantly reduced spurious sig-

nals over state-of-the-art electronic mixers while maintaining

operation over wider instantaneous bandwidths. The balanced

detection also rejected common mode noise such as RIN and

ASE beat noise. Using commercially available components, the


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Fig. 11. Microwave photonic link employing an optical phase-locked loop[53].

TROPHY team was able to demonstrate photonic downconver-

sion at X-Band with an SFDR near 120 dBHz2/3 [52].

E. Phase Modulation and Optical Phased-Locked Loops

The DARPA PHOR-FRONT program explored the use of

phase modulators and optical phase-locked loops (OPLL) as a

means of increasing the SFDR of a microwave-photonic link. A

phase modulator formed in lithium niobate is intrinsically linear

[53], [54]. PHOR-FRONT explored optical phase-locked loopsas linear demodulators. In one implementation of the PHOR-

FRONT concept, an attenuation-counterpropagation (ACP) in-

loop phase modulator was used to modulate the phase of the

LO laser and a directional coupler to serve as a phase detector,

as shown schematically in Fig. 11. Two photodetectors monitor

the phase difference and provide a feedback signal to the ACP

in-loop modulator to essentially match the phase modulation

placed on the optical signal by the incoming RF signal [53].

Initial measurements of the SFDR of the microwave-photonic

link were carried out with a commercial-grade lithium nio-

bate phase modulator (V = 1.95 V) as the transmitter phase

modulator and an ACP-OPLL. At a modulation frequency of100 MHz, the link was shown to have an SFDR of 135 dB

Hz2/3 [53]. This paper projects even higher SFDR if the opti-

cal power is increased, detector responsivity is increased and

ACP phase modulator is improved. Calculations suggest that an

SFDR of 147 dB Hz2/3 could be achieved with a 3 dB band-

width of 500 MHz. These calculations further predict a link

noise figure of 2.2 dB [53].

A key challenge with optical phase locked loops is that the

latency around the loop limits the RF operational frequency.

Integrating the detectors and phase modulators into a common

substrate can reduce the latency and increase the operational fre-

quency. In a second implementation of the PHOR-FRONT opti-

cal phased-lock loop, a coherent receiver comprised a balancedphotodetector, a two-by-two waveguide multimode interference

coupler, and the integrated tracking phase modulators [55]. The

photonic chip used an adjacent electronic chip comprising a

transconductance amplifier in its feedback path to drive the in-

tegrated phase modulator. The receiver was shown to have a

bandwidth of 1.45 GHz. The SFDR was measured to be 125 dB

Hz2/3 at a frequency of 300 MHz, 121 dB Hz2/3 at 500 MHz

and 113 dB Hz2/3 at 1 GHz.

PHOR-FRONT also explored a self-homodyne coherent re-

ceiver that relies on linear optical phase encoding of the RF

signal and uses a 90 optical hybrid for in-phase/quadrature

phase demodulation [56], as shown schematically in Fig. 12.

Fig. 12. Self-homodyne coherent receiver [56].

The system used a fiber stretcher in one arm of the receiver to

provide interferometric bias control. The SFDR was measured

to be 126.8 dBHz2/3 at test frequencies near 1 GHz. The noise

figure was measured at 18.6 dB. Calculations suggest that this

architecture could achieve an SFDR of 135.3 dB Hz2/3 with a

shot-noise-limited optical source [56].

This same in-phase/quadrature (I/Q) phase demodulation was

also used to demonstrate photonic downconversion. Here, an

additional phase modulator was added to one arm to provide the

LO. With the two testtones set at10.0195 GHz and 10.0225 GHz

and the LO phase modulator driven at 5 GHz, the photonic

downconversion and I/Q demodulation demonstrated an SFDR

exceeding 105 dB Hz2/3 [36]. With the two test tones set at

3.0195GHz and3.0225 GHzand theLO phase modulator driven

at 1.5 GHz, the photonic downconversion and I/Q demodulation

demonstrated an SFDR exceeding 105 dB Hz2/3 [36].

F. Microwave-Photonic Filters

Microwave photonic filters are of substantial interest as an

essential component to a wide variety of practical microwavephotonic system configurations. Moreover, the high Q values at-

tainable with photonics could enable filtering performance that

is not practical in the electronic domain [57]. Additionally, re-

configurability of microwave photonic filters is desired because

of its potential to enable more dynamic microwave photonic sys-

tems and to serve as a universal filtering component in photonic

systems. To meet this need, theDARPA PhASER program lever-

aged emerging photonic integrated circuit (PIC) technologies

to develop reconfigurable microwave photonic filters with high

dynamic range and wide bandwidths. The PhASER program ad-

vocated an integrated photonic unit cell concept, which would

enable scaling to very high-order filters. PhASER investigated

PIC-based unit cells both with and without integrated opticalgain. Optical gain enables implementation of more complex fil-

ters with a greater number of unit cells, but this can result in

added noise and distortion, and it requires the use of a fabrica-

tion process supporting optical gain (typically InP-based). As

part of the PhASER program, reconfigurable microwave filters

were demonstrated on a number of platforms, with and without

optical gain. In the InP-InGaAsP platform, a unit cell with re-

configurable poles and zeroes, based on an asymmetrical MZI

with feedback, was demonstrated [58]. This unit cell was cas-

caded to produce a fourth-order filter with passband tunability

of 1.95.4 GHz with stopband rejection greater than 32 dB. The

linearity of these filters was investigated, and optimization of


8/12


Fig. 13. Filtered coherent microwave photonic link [63].

the demonstrated structure is expected to enable SFDR of up to

117 dB Hz2/3 for filters in the 12 GHz range [59], Also of

interest was the demonstration of a widely tunable filter unit cell

on a hybrid silicon platform with integrated InP-based materials

for optical gain [60]. These unit cells were shown to demon-

strate poles and zeroes simultaneously with unit cell transfer

characteristics that closely matched theory.

PhASER also included development of microwave photonic

filters on silicon-based platforms without optical gain. This

study included the development of fully reconfigurable mi-

crowave photonic filters demonstrating both poles and zeroes

and comprising high-Q silicon microdisk resonators [61]. These

fourth-order filters were formed on a silicon-on-insulator plat-

form with tunable 3 dB bandwidths from 900 MHz to 5 GHz

and out-of-band rejection of 38 dB.

In order to minimize the need for gain in optical filters, de-

velopment of low-loss waveguides is of interest for improving

this technology. Optical loss in waveguides was the focus of

the DARPA Integrated Photonic Delay (iPhoD) program, which

made significant progress in reducing the loss of on-chip optical

waveguides, demonstrating losses of 0.08 dB/m through a 27 m

integrated waveguide, with losses of 0.037 dB/m achieved in

waveguide resonators [62].

These filters can be used to increase the SFDR of microwavephotonic coherent links as shown schematically in Fig. 13 [63].

The optical filtering allows only a single sideband of the mod-

ulated optical signal to reach the optical detector. The two elec-

trooptic MZI modulators were biased for carrier suppression.

An optical filter with 350 MHz bandwidth centered at the first

LO sideband ensures that the only signals that reach the detec-

tor are the first sidebands and the LO. An SFDR of 116 dB

Hz2/3 was measured when the optical power to each electrooptic

MZM was set to 20 dBm [63].

The SFDR was further improved using predistortion compen-

sation of the nonlinearities of the microwave photonic link by

inserting a compensating modulator in the LO path [63]. Thethird-order distortion was suppressed by 20 dB, resulting in a

13 dB reduction in SFDR [64]. The intermodulation distortion

as a function of input RF power was shown to have a slope of

five rather than three, indicating that the SFDR was now limited

by fifth-order distortion [63]. Like many other linearization ap-

proaches, these SFDR improvements are only valid over limited

bandwidths, as discussed in [65].

V. IMPACT OFMICROWAVEPHOTONICS ONSFDR

The state-of-the-art in microwave photonic link SFDR as

a function of frequency has been summarized in previous re-

views, in particular Coxet al. [65]. This paper showed that links

Fig. 14. SFDR in 1 Hz noise bandwidth as a function of measured perfor-

mance frequency. IMDD is an abbreviation for intensity modulation, directdetection.

employing directly modulated laser diodes had high dynamic

range (up to 125 dBHz2/3[66]) at frequencies less than 1 GHz

but suffered significantly as the frequencies increased. This pa-

per further showed that links employing laser diodes and stan-

dard lithium niobate modulators could achieve SFDRs of 112dB

Hz2/3 at frequencies between 2 to 18 GHz [67][69]. Links em-

ploying electrooptic modulators biased near the off-state were

shown to have an even higher SFDR of 115 dB Hz2/3 at fre-

quencies up to 20 GHz [70], although operating the modulatornear the off-state emphasizes the second-order distortion, thus

limiting the links operation to bandwidths of less than one oc-

tave.

Fig. 14 shows the published SFDR in 1 Hz noise bandwidths

for the representative values from [65] along with the values

summarized in this paper. In this chart, the black markers de-

note SFDR values from [65], and the red markers denote the

SFDR values from recent DARPA-funded efforts reviewed in

this paper. The filled triangles denote external modulators biased

at quadrature and the open triangles denote external modulators

biased near the modulator off-state and operating over less than

one octave. In both cases, recent DARPA investments have re-

sulted in a 3 to 5 dB improvement in SFDR.The filled squares show the SFDR of links that use electroop-

tic phase modulators and optical phase-locked loops for de-

modulation. These links have shown exceptionally high SFDR

(>130 dB Hz2/3) at low frequencies (


9/12


TABLE I IDESIREDSYSTEMPERFORMANCEMETRICS FOR A

MICROWAVE-PHOTONIC-BASEDCHANNELIZEDRECEIVER

Parameter Value

Frequency band > 50 GHz

Instantaneous bandwidth > 5 GHz

Dynamic range > 60 dBReceiver sensitivity 10 cm to achieve low drive voltages, making them bulky

and expensive. Lowering the size and cost of microwave pho-

tonic systems will require integration of photonic and electronic

circuits.

Continued development of silicon photonics offers significant

potential for leveraging highly advanced CMOS fabrication in-

frastructure to achieve levels of photonics integration which

cannot be achieved by other means. The DARPA Electronic-

Photonic Integrated Circuit (EPIC) program combined elec-tronic and photonic integration on a common silicon substrate

[72]. EPIC explored modulation, filtering, and downconversion

in a silicon photonic circuit [73]. While the SFDR of 94.3 dB

Hz2/3 measured at 11 GHz did not meet the performance of the

microwave photonic links reviewed in this paper, it is impressive

for a reconfigurable silicon photonic circuit.

Another DARPA effort, the Electronic-Photonic Heteroge-

neous Integration (E-PHI) program, is currently underway to

develop the design and manufacturing tools for electronic-

photonic heterogeneously integrated systems. The goal of this

effort is to deliver circuits employing cutting edge CMOS cir-

cuits co-integrated with active and passive photonic devices.

If successful, E-PHI could have several critical impacts to mi-crowave photonics, including reduced modulatorV , very-low-

linewidth semiconductor-based photonic sources with world-

class noise levels for intensity-modulated and coherent links,

integrated microwave signal sources with phase noise levels

surpassing all other compact sources, and major advances in

photonics integration, leading to smaller size and lower system

costs as well as improved functionality for more sophisticated

microwave photonic architectures. For instance, the tight co-

integration of electronics and photonics could also reduce the

latency in optical phase-locked loop systems, pushing the op-

erational frequency into the gigahertz levels while maintaining

exceptionally high SFDR.


10/12


VII. CONCLUSION

This paper has reviewed many of the DARPA-funded mi-

crowave photonic link developments carried out over the last

ten years. The key takeaways from these investments include

the following:

1) Reducing laser RIN and theV of electrooptic modula-

tors as well as increasing the linearity and power handlingcapability of optical detectors has improved link perfor-

mance, including SFDR and NF.

2) Microwave photonic links with photonic downconversion

can yield SFDRshigher than similar microwave-only links

employing a microwave mixer for downconversion.

3) Optical phase modulation with optical phase-locked loops

can yield exceptionally high SFDRs.

4) Photonic-based predistortion compensation can effec-

tively increase SFDR over limited bandwidths.

DARPAs Microsystems Technology Office has been a leader

in developing microwave photonic components, architectures

and links. Using these technical advances for microwave an-

tenna remoting, photonic downconversion, and channelized re-

ceivers could significantly improve the performance metrics for

the militarys advanced receivers, including extending the fre-

quency range, increasing the instantaneous bandwidth and dy-

namic range as well as improving receiver sensitivity. The next

step is to move microwave photonic technologies from these

experimental efforts and demonstrations to deployed military

systems.

ACKNOWLEDGMENT

The authors would like to acknowledge the past DARPA

program managers within the Microsystems Technology Office

who have developed and supported the innovative programs in

microwave photonics, including Dr. R. Leheny, Dr. S. Pappert,

Dr. M. Haney, Dr. R. Esman, and Dr. J. S. Rodgers. Without

their insights and dedication to microwave photonics, many of

these programs would not have been completed. The authors

also acknowledge Dr. J. S. Rodgers for discussions on elec-

trooptic modulators and Prof. J. Campbell and Prof. A. Beling

of the University of Virginia for discussions on high power

photodetectors.

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Richard W. Ridgway (M80SM05) receivedthe B.S. and the M.S. degrees inelectrical engineering from the University of Michigan, Ann Arbor, MI, USA,in 1978 and 1979, respectively, and the Ph.D. degree in electrical engineeringfrom The Ohio State University, Columbus, OH, USA, in 1985.

In July 2011, he joinedthe U.S. Defense Advanced Research ProjectsAgency(DARPA), Arlington, VA, USA, where he is currently a Program Manager forthe Strategic Technology Office. His interests include millimeter-wave commu-nications and microwave photonics. Prior to joining DARPA, he was a Senior

Research Leader at Battelle, Columbus, where for more than 25 years he wasinvolved in the development of integrated optical components for optical, mi-crowave, and millimeter-wave communication systems. From 2001 to 2007, heserved as Chief Technology Officer for the Battelle-spinout company, Optimer

Photonics, Inc., focused on bringing electrooptic waveguide technology to thetelecommunications industry. He holds 26 US patents.

Carl L. Dohrman(M08SM13) received the B.S. degree from the Universityof Illinois, Urbana, IL, USA, in 2002 and the Ph.D. degree from the Mas-

sachusetts Institute of Technology, Cambridge, MA, USA, in 2008, both inmaterials science and engineering.

Since 2008, he has been a Consultant with Booz Allen Hamilton, Inc., wherehe has provided subject matter expertise in microelectronic and photonic mate-rials, devices, and circuits to the Microsystems Technology Office and StrategicTechnology Office of theU.S. Defense Advanced Research Projects Agency. Hehas advised on a range of projects covering microwave photonics, photonic inte-

grated circuits, device-level heterogeneous integration technologies, RF/mixedsignal electronics, and nitride optoelectronics. He has more than 30 refereed

journal and conference pub lications, and one patent.

Joshua A. Conway(M13SM13) received the B.S. degree in physics and theM.S. degree in electrical engineering from the University of Illinois, Urbana,IL, USA, in 1999 and 2001, respectively, and the Ph.D. degree in electricalengineering from the University of California, Los Angeles, CA, USA, in 2006.

SinceAugust2012,he hasservedas a Program Manager forthe MicrosystemsTechnology Office at the U.S. Defense Advanced Research Projects Agency

(DARPA), Arlington, VA, USA. His interests include active integrated photonicdevices, RF photonics and advanced imaging systems. Prior to joining DARPA,he was with Kinsey Technical Services (KTSi), where he was Senior PrincipalEngineer of special programs at the Los Angeles Air Force Base. Prior to

joining KTSi, he served on the technical staff of The Aerospace Corporation,starting in 2003. From 2001 to 2003, he worked at Boeing Satellite Systems,where he designed, built, and tested fiber-optic subsystems for inter-satellitelaser communication systems.

Dr. Conway has received numerous awards, has authored more than 30technical papers and conference proceedings and holds eleven patents.

Microwave Photonics - DARPA

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