HYBRID MICROWAVE FIBER OPTIC LINKS

John A MacDonald and Allen KatzJohn A. MacDonald and Allen Katz

Linear Photonics, LLC3 Nami Lane, Suite 7C, Hamilton, NJ 08619

609-584-5747

macdonald@linphotonics.com

LINEAR PHOTONICS, LLC

June, 20081

Bringing Performance to Light!

Hybrid Photonics• LPL has developed Hybrid Microwave Photonic Link

Components– Directly-modulated Microwave-over-fiber extended to Ku Band– Very small Size and Weight– Lower Cost & Complexity than external modulationLower Cost & Complexity than external modulation– High Dynamic Range: equivalent to external modulation

Microwave Photoreceiver

June, 20082Microwave Laser Transmitter

Directly Modulated Links• Directly-Modulated (DM) Fiber Optic Links

– Transmitter: RF pre-amplification, biasing, and matching to low-impedance laser diode G1

F

RF IN

G1 F

RF IN

matching to low impedance laser diode• Output is Intensity Modulated• Modulation Efficiency = ratio of peak output modulation

envelope to peak microwave input current – Receiver: PIN photodiode matching from high-

F1

Laser Diode

F1

Laser Diode

Receiver: PIN photodiode, matching from highimpedance diode, RF post-amplification

• Responsivity = ratio of generated current to incident light intensity

• PIN diode performs direct envelope detection

OPTICAL OUTPout

Link Length

OPTICAL OUTPout

Link LengthPIN diode performs direct envelope detection

OPTICAL INPinOPTICAL INPin

OLGdBG refL ⋅−= 2)(Link Gain, GL OLGdBG refL ⋅−= 2)(Link Gain, GL

)(174)/()( dBGHzdBmNdBF Lout −+=Link Noise Figure F )(174)/()( dBGHzdBmNdBF Lout −+=Link Noise Figure F

G2 F2

Photodiode

G2 F2

Photodiode)()()( Lout

⎥⎥⎦

⎤

⎢⎢⎣

⎡++=

−−10

21010 101010log10

OLcOLcc

out

rinshotth

N

Link Noise Figure, Fand

Output Noise Power Density, Nout

)()()( Lout

⎥⎥⎦

⎤

⎢⎢⎣

⎡++=

−−10

21010 101010log10

OLcOLcc

out

rinshotth

N

Link Noise Figure, Fand

Output Noise Power Density, Nout

⎤⎡ + eqeq IIPOIP ⎤⎡ + eqeq IIPOIP

June, 20083

RF OUTRF OUT⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

+

= +

1010

10

3

1010

10log10)(Leqeq

eqeq

GIIPOIPdBmIIPLink Input Intercept, IIP3⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

+

= +

1010

10

3

1010

10log10)(Leqeq

eqeq

GIIPOIPdBmIIPLink Input Intercept, IIP3

Packaged Laser Limitations• Low Cost Commercial Laser Transmitters utilize

packaged laser diodes– Widespread use in CATV, RF-over-Fiber

• Thermal and Frequency Limitations– Frequency Response Limited by package parasiticsFrequency Response Limited by package parasitics– Thermal Operation limited by changing gain slope

– TEC-cooled “butterfly” packages solve thermal problem– At expense of even lower bandwidth (more package effects)At expense of even lower bandwidth (more package effects)

– Uncooled “TO-Can” packages can operate to ~ 4 GHz– Slope change limits thermal range for stable link gain to ~ 0 to 50 C

Cooled “butterfly” laserFrequency Response < 3 GHzHigh Current (TEC)Low Reliability (TEC)

Uncooled “TO” laserFrequency Response < 4 GHzLimited Thermal Range

June, 20084

y ( )

Laser Diode Thermal OperationCyoptics 293BT - S/N 448

3.500

4.000

Laser L-I curve over Temperature

-20C

Cyoptics 293BT S/N 449 - S21 Gain - LO2

1

0

1

8 GHz Link Gain over Temperature

1 000

1.500

2.000

2.500

3.000

al O

utpu

t Pow

er (m

W)

70C

-5

-4

-3

-2

-1

Gai

n (d

B)

-0.500

0.000

0.500

1.000

0 10 20 30 40 50 60 70 80 90

Laser Current (mA)

Opt

ica

-9

-8

-7

-6

8.000 8.050 8.100 8.150 8.200 8.250 8.300 8.350 8.400 8.450 8.500

Frequency (GHz)

-20 de g C 0 deg C +25 deg C +50 deg C +75 deg CSlope efficiency and Threshold change with temperature

-20 deg C, 50 mA 0 deg C, 50 mA +25 deg C, 50 mA +50 deg C, 75 mA +75 deg C, 100 mAResults in large variation in link gain over temperature

• Link gain = f(Slope Efficiency) Link Gain = f(Optical Power)g ( p y) ( p )• Bias adjust can maintain optical power, but not slope efficiency• Use of TEC maintains laser temperature

• Requires ~ 2 W DC power• Not suitable for high reliability

June, 20085

• Not suitable for high-reliability• TEC MTBF may be less than Laser Diode MTBF

• Hi-Rel, Low Power solution is RF Variable Attenuation

Hybrid Laser TransmitterOPTICAL SECTION

– Uncooled Laser Diode• High Reliability, Low Noise 1310 nm

RF in

• Low DC Power (no TEC)– Optical Alignment to Single Mode fiber

• Focusing Optics and Optical Isolator

MICROWAVE INPUT SECTION MICROWAVECONTROL

+5

+12

MICROWAVE INPUT SECTION– Temperature Compensation Network– Optimal Microwave Matching

• Package parasitics removed

MICROWAVEINPUT

CONTROLCIRCUITPWR MON

• Operation to inherent laser diode frequency range (12 GHz+)– Broadband or narrowband tuning (application specific)– Provisions for Preamp / Predistortion OPTICAL SECTION

Optical outANALOG CONTROL SECTION

– Analog hybrid on ceramic• Power supply conditioning• Constant Current bias loop

Optical out

June, 20086

• Thermal Control• Backfacet telemetry

Transmitter Layout

RF Launch (GPO)Control Electronics

DC Interface

Optical Platform RF Matching& Attenuator

Fiber Pigtail(9/125 SM)

June, 20087

Link PerformanceTypical Performance for Microwave Links

Examples: C-Band and BroadbandLinks available tuned any bandwidth to 12 GHzLinks available tuned any bandwidth to 12 GHz

C-Band BroadbandBandwidth 3.2 to 4.0 GHz 4 to 12 GHzBandwidth 3.2 to 4.0 GHz 4 to 12 GHzOptical Power 6 dBmo 6 dBmoRF Link Gain -12 dB -15 dBGain Variation with Frequency 0.5 dBp-p 2 dBp-pGain Variation with Temperature 2 dBp-p 2dBp-pRF Input/Output Return Loss 10 dB 10 dBInput IP3 32 dBm 28 dBmNoise Figure 29 dB 32 dBSFDR 118 dB (1 Hz) 113 dB (1 Hz)DC PDC Power

@ -10 C@ 25 C@ 70 C

Operational Temperature Range

375 mW450 mW600 mW

-20 to +70 C

June, 20088

Operational Temperature RangeTransmitter Weight (less pigtail)Receiver Weight (less pigtail)

19 grams5 grams

20 to 70 C

Link Performance

C-Band IF Link

June, 20089

Design Flexibility• Link optimization

– Microwave matching design for any bandwidthMicrowave matching design for any bandwidth requirement

– Integrated preamplification for lower noise • Receive side antenna remoting

• Integrated Predistortion Linearization• Integrated Predistortion Linearization– Improves Intermodulation Distortion

• Bandwidth Extension to 20 GHz– Units available 4Q08

June, 200810

Q

Linearity Improvement• Linearization: Methods of improving the linearity of a

nonlinear network– Predistortion Linearization is one technique

• Employs a nonlinear element in the microwave signal path• Operates at instantaneous microwave rate

N t li it d b d l i f db k f df d h– Not limited by delay as in feedback or feedforward approach– Not limited by overly complicated component-count– Limited primarily by microwave matching, preamplifiers, etc.

• Linearizer Technology Inc (Linear Photonics’ sisterLinearizer Technology, Inc. (Linear Photonics sister company) has been manufacturing linearizers and linearized networks for > 15 yrs

T h l i dil li d t fib ti t k– Technology is readily applied to fiber optic networks

June, 200811

Linearizer Technology, Inc.

Predistortion Linearization

-2

-1

0

1

2

3

Output Power

-5

-4

-3

2

-5 -4 -3 -2 -1 0 1

4

Input Power

-1

0

1

2

3 Phase

-2

-5 -4 -3 -2 -1 0 1Input Power

• Nonlinear Device exhibits Gain and Phase Compression

June, 200812

Predistortion Linearization

-2

-1

0

1

2

3

-1

0

1

2

3

4

5

Output Power Output Power

-5

-4

-3

2

-5 -4 -3 -2 -1 0 1

4

-5

-4

-3

-2

-5 -4 -3 -2 -1 0 1

4

Input Power Input Power

-1

0

1

2

3

-1

0

1

2

3 Phase Phase

-2

-5 -4 -3 -2 -1 0 1

-2

-5 -4 -3 -2 -1 0 1Input Power Input Power

• Nonlinear Device exhibits Gain and Phase Compression

• Precede it with another nonlinear device that exhibits gain and phase expansion, in conjugate with the device to be linearized (the linearizer)

June, 200813

p , j g ( )

Predistortion Linearization

-2

-1

0

1

2

3

-1

0

1

2

3

4

5

-2

-1

0

1

2

Output Power Output Power Output Power

-5

-4

-3

2

-5 -4 -3 -2 -1 0 1

4

-5

-4

-3

-2

-5 -4 -3 -2 -1 0 1

4

-5

-4

-3

-5 -4 -3 -2 -1 0 1

3

Input Power Input Power Input Power

-1

0

1

2

3

-1

0

1

2

3

-4

-3

-2

-1

0

1

2Phase Phase Phase

-2

-5 -4 -3 -2 -1 0 1

-2

-5 -4 -3 -2 -1 0 1-5

-5 -4 -3 -2 -1 0 1Input Power Input Power Input Power

• Nonlinear Device exhibits Gain and Phase CompressionNonlinear Device exhibits Gain and Phase Compression

• Precede it with another nonlinear device that exhibits gain and phase expansion, in conjugate with the device to be linearized (the linearizer)

Th d i d t i id l li it

June, 200814

• The desired outcome is an ideal limiter– The linearity of an ideal limiter cannot be improved

Predistortion Linearization

• Result is reduction in IMD:

SFDR i i t d 1 3 (dB) ith IMD• SFDR is impacted 1:3 (dB) with IMD

June, 200815

Results: Gain and Phase Transfer

5.7 dB1.5 dB

Non linearized @ 8 GHz Linearized @ 8 GHzNon linearized @ 8 GHzP1 dB is 5.7 dB from saturationPhase compression rapidly above sat

Linearized @ 8 GHzP1 dB is 1.5 dB from saturationPhase nonlinearity held to < 1° past sat

June, 200816