OpticalFiberCommunicationsAnd

Cr-dopedfiberamplifier

ASSOC. PROF. CHUNNIEN LIU

NATIONAL CHUNG-HSING UNIV.1

2020.06.04

HomeworkReports (A4, total 2 pages, submit as MS-word file, not pdf.)Question 1 (A4, 1 page)Please explain the optical fiber communication system.

Question 2 (A4, 1 page)Please tell me the several advantages of optical fiber communications. And how to add the communicated speed.

Deadline: 2020.6.15TTI-students to Prof. YoshimuraNCHU-students to Prof. Lin

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IntroductionVisiblelight:

Electromagneticwave(wavelength:0.38-0.76μm)

Color Wavelength Frequency Photonenergy

Violet 380–450 nm 680–790 THz 2.95–3.10 eV

Blue 450–485 nm 620–680 THz 2.64–2.75 eV

Cyan 485–500 nm 600–620 THz 2.48–2.52 eV

Green 500–565 nm 530–600 THz 2.25–2.34 eV

Yellow 565–590 nm 510–530 THz 2.10–2.17 eV

Orange 590–625 nm 480–510 THz 2.00–2.10 eV

Red 625–740 nm 405–480 THz 1.65–2.00 eV

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IntroductionA communication system is a collection of individual communications networks,transmission systems, relay stations, tributary stations, and data terminalequipment (DTE) usually capable of interconnection and interoperation to forman integrated whole. The components of a communications system serve acommon purpose, are technically compatible, use common procedures, respondto controls, and operate in union.

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IntroductionHistory

-1854 John Tyndall

Tyndall knew light was trapped temporarilyinside the stream of water, but he couldn’texplain why. Today, using a combination ofmathematics and science, the explanation isvery straightforward. Tyndall’s 1870experiment demonstrated the principle knownas “total internal reflection.” Simply stated:Total internal reflection is a special opticalcondition in which optical rays cannot escapethe material in which they are traveling.

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IntroductionHistory

-1966CharlesK.Kao(1933-2018)

In 1965 Kao with Hockham concluded that the fundamental limitation for glasslight attenuation is below 20 dB/km (decibels per kilometer, is a measure of theattenuation of a signal over a distance), which is a key threshold value for opticalcommunications. However, at the time of this determination, optical fibrescommonly exhibited light loss as high as 1,000 dB/km and even more. Thisconclusion opened the intense race to find low-loss materials and suitable fibresfor reaching such criteria.

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IntroductionYear 1980 1985 1990 1996 2002

Generation First Second Third Fourth Fifth

Type Graded-indexfibers Single-mode fibers Single-mode lasers Opticalamplifiers Ramanamplification

Bitrate 45Mb/s100Mb/s

To1.7Gb/s

10Gb/s 10Tb/s40Gb/s

to160Gb/s

Repeaterspacing 10km 50km 100km 10,000km

24,000kmto

35,000km

Operatingwavelength 0.8um 1.3um 1.55um

1.45umto

1.62um

1.53umto

1.57um

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New-GenerationCommunications

Data Center

4th or 5th Mobile Networks (4G/5G) Triple Play Service(Communications)

• The speed of mobile network between 4G (1~50 Mbit/s) and 5G (1~10 Gbit/s) shows an improvement of 100times.

• Until now, the fiber network (1~50 Gbit/s) is widely used and needs to be upgrade.• The new-generation of 400 Gbit/s and 1 Tbit/s communication system have been discussed at 2019 OFC.

Fiber to the x (FTTx)

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OpticalFiberStructure

Glassopticalfibersarealmostalwaysmadefromsilica,butsomeothermaterials,suchasfluorozirconate,fluoroaluminate,andchalcogenideglassesaswellascrystallinematerialslikesapphire,areusedforlonger-wavelengthinfraredorotherspecializedapplications.Silicaandfluorideglassesusuallyhaverefractiveindicesofabout1.5,butsomematerialssuchasthechalcogenidescanhaveindicesashighas3.Typicallytheindexdifferencebetweencoreandcladdingislessthanonepercent.Anopticalfiberconsistsofthreebasicconcentricelements:thecore,thecladding,andtheoutercoating.

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OpticalFiberCharacteristic

1. Silica exhibits fairly good optical transmission over a wide range ofwavelengths. In the near-infrared (near IR) portion of the spectrum,particularly around 1.5 µm, silica can have extremely low absorption andscattering losses of the order of 0.2 dB/km. Such remarkably low lossesare possible only because ultra-pure silicon is available.

2. they permit transmission over longer distances and at higher bandwidths(data transfer rates) than electrical cables.

3. No electromagnetic interference.4. Inexhaustible materials(SiO2)

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OpticalFiberAttenuationofanopticalfiberasafunctionofthewavelength.

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FiberCommunicationSystem

O E S C L U

OH- ion absorption

OH- ion free

Low Loss Fiber Spectrum O

ESCLU

70nm

Fiber-to-the-home, FTTH

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300-nmCr-DopedFiberAmplifier

• Recently, Cr-doped fibers (CDFs) have demonstrated broadband emissions in the whole 1200-1600 nmrange. It is interesting to develop a single fiber amplifier to cover the bandwidth of 1300-1600 nm inlow-loss transmission.

• The bandwidth of Cr-doped fiber amplifier (CDFA) is 4.3 times than the commercial EDFA.

Er doped fiber(EDFA)

Cr doped fiber(CDFA)

C band(1530~1565nm: 35nm)

L band(1570~1605nm: 35nm)

Total : 70nm

1300~1600nm

( 300 nm )

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FabricationofMulti-modeCr-DopedFiber(MMCDF)

} MMCDF fabricationa. Bulk Cr:YAG crystal diameter from 500 µm was reduced

to 70 µm. Then, the crystal rod was put in a silica tube.

b. The LHPG system fabricated a double-clad MMCDF of14 µm core diameter and length of 6.7 cm.

Core: 14 µmInner Cladding: 95 µm

Outer Cladding: 320 µm

MMCDF

LHPG growth chamber

Laser heated pedestal growth (LHPG)

S.M. Yeh, et. al., J. Lightw. Technol., 2009. 14

GainMeasurementofMMCDF•Single pump

•Double pump

S.M. Yeh, et. al., J. Lightw. Technol, 2009. 15

GainPerformanceofMMCDF

0

1

2

3

4

0 1 2 3Pump power (W)

Gro

ss g

ain (d

B)

MMCDF of core diameter of 14 µm and length of 6.7 cm

Gross gain = 2.8 dB 1. The gross gain of Cr-doped fiber amplifier (CDFA) is defined as:

G (dB) = 10log [(Ps+p -Pp)/Ps]2. The insertion loss in dB unit is defined as:

IL (dB) = 10log (Pout / Pin)where the Pin and Pout are the input and output signal power at the facet of CDFA, respectively.

Insertion loss = 1.6 dB

3. The net gain of CDFAs is defined as:Gn(dB) = Gg(dB) – IL(dB)

Net gain = 2.8 dB – 1.6 dB = 1.2 dB

This is the first demonstration of a net gain in a broadband Cr-doped fiber amplifier (CDFA).

Double pump

S.M. Yeh, et. al., J. Lightw. Technol, 2009.

S.M. Yeh, et. al., J. Lightw. Technol, 2009. 16

40Gbit/sSingleChannelCommunicationSystem

• Four test conditions for eye diagram and bit error rate (BER) : (a) back-to-back (BTB), (b) CDFAwithout Pumping, (c) CDFA with 4.0 W Pumping, and (d) CDFA with 8.0 W Pumping.

• All the eye diagrams and BER performance showed successful. This result further verified that nopattern dependence was observed in CDFAs. 17

Structureof300-nmCDFAinFTTHSystem

10km

40 Gbit/s and 1550 nm signal:Power penalty was 0.4 dB withCDFA and 1.6 dB after 10 km.

40 Gbit/s and 1310 nm signal:Power penalty was 0 dB withCDFA and 0.7 dB after 10 km.

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ModeReductionofCDF(I)

S.M. Yeh, et. al., J. Lightw. Technol, 2009.

Lower Threshold of Few-mode/Single-mode

Advantages:

• Lower threshold pumping power

• Reduce the heat effect by pumping

• Increase the gain efficiency

M. J. F. Digonnet et.al., Appl. Opt., 1985.

Single-mode

Multi-mode

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ModeReductionofCDF(II)

Core diameter：14 µm

Few-mode or single-mode:

• Reduce the core diameter.

• Reduce core/clad index difference.

MMCDF

Cr:YAG index：1.82 @1064 nmSiO2/Silica index：1.45 @1064 nmNumerical aperture (NA)：1.1

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FabricationofFewModeCDF(FMCDF)

The few-mode Cr-doped fiber (FMCDF) was fabricated by LHPG system with multi-core reduction process. The FMCDF exhibited a core diameter of 2 µm and V-numberof 3.77 to provide a few-mode characteristics.

2µm

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ModeCharacteristicsofFMCDF

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GainPerformanceofFMCDF

Gross gain = 2.4 dB

FMCDF of core diameter of 2 µm and length of 4 cm

A gross gain of 2.4 dB at 1400-nm wavelength was obtained at 0.2 W pump power. Compared with theMMCDF of gross gain of 2.8 dB at 3 W pump power, the pumping efficiency of gross gain for theFMCDF showed an improvement of 10 times.

Single pump

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DifficultFabricationofLongerFMCDF

} Uniformity of core diameter: Difficult in fabrication (< 2 µm).} Hard to grow in long length (> 5 cm).} Lower Cr4+-ions.} Bad gain efficiency

2µm

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Single-modeCDF(SMCDF)FabricatedbyFiberDrawingTower(I)

Nextrom fiber drawing tower

Preform

Furnace

Capstan

First coating

First UV curing

Secondary UV curingSecondary coating

Capstan

Take-up

Laser gauge

Laser gauge

Preform pressure control

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PreformofCr:YAG CrystalRod(II)Ø Modified Rod-in-Tube (MRIT):

Ø 500 µm Cr:YAG crystal rod was reduced to 290 µm diameter rod by LHPGmethod.

Ø 290 µm crystal rod as a core was placed in 20/7mm silica tube as a claddingto form preform and then by drawing tower.

Y.C. Huang, J.S. Wang, Y.S. Lin, T.C. Lin, W.L. Wang, Y.K. Lu, S.M. Yeh, H.H. Kuo, S.L. Huang, and W.H. Cheng, “Development of BroadbandSingle-mode Cr-Doped Silica Fibers,” IEEE Photon. Technol. Lett., Vol. 22, No. 12, pp. 914-916, June 15, 2010. 26

PhotoandFFPofSMCDF(III)(a)1260nm (b)1310nm (c)1350nm

(d)1400nm (e)1550nm (f)1600nm

5-µm core diameter and 125-µm cladding diameter

A SMCDF was observed at wavelength > 1310 nm

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FluorescenceSpectrumofSMCDF(IV)

The fluorescence spectrum of SMCDF pumping by 1064-nm (solid-line) with peak at 1200-nm for Cr4+ ions andpumping by 532-nm (dashed-line) with peak at 1000-nm for Cr3+ ions.

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AdvantagesofCladdingbyHighIndexGrass

• High-index glass：1.81 @1060 nm

• Cr:YAG index：1.82 @1060 nm

• Numerical aperture (NA)：0.19

Few-mode or single-mode:

• Reduce the core diameter

• Reduce the numerical aperture (NA)

If V-number is less than 2.405, it will be the single mode.

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FabricationofFMCDFandSMCDFbyHighIndexGlassCladding(HIGC)

PullCr4+:YAG single crystal

CO2 laser beam

ZnSe window

Planar mirror

Paraboloidal mirror

FMCDF by HIGC

N-SF57

Cr4+:YAG single crystal

W.L Wang, et. al., IEEE PTL, 2014.

SMCDF by HIGC

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CrystalCharacteristicsofFMCDFandSMCDFbyHIGCHRTEM images of SMCDF

The hexagonal sharp bright diffraction spots of selected area electron diffraction (SEAD) was verifiedthe Cr:YAG single crystal structure in the core and the amorphous structure of glass in the cladding.

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ModeCharacteristicsofFMCDFandSMCDFbyHIGC

25 30 35 40

2.5

3.0

3.5

4.0

V-n

umbe

rCore diameter ( m)

LP01

LP11

2.405

SMF

Thor Lab BP 104-IR

LTunable laserwith different λ

FMCDF/SMCDF

(a) 1400nm (b) 1550nm

FMCDF by HIGC

SMCDF by HIGC

At 1400 nm

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GainPerformanceofFMCDFsbyHIGC

0 100 200 300 4000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

3.8cm 5.4cm 7.4cm 9.8cm

Gro

ss g

ain

(dB

)

Pump power (mW)

Length Core diameter Gross gain 3.8 cm 41 µm 2.10 dB5.4 cm 34 µm 2.54 dB7.4 cm 34 µm 3.10 dB9.8 cm 34 µm 3.34 dB

FMCDF by HIGC

Single pump

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GainPerformanceofSMCDFsbyHIGC

Length Core diameter Gross gain 3.3 cm 25.6 µm 1.99 dB6.3 cm 25.0 µm 2.47 dB6.7 cm 25.7 µm 2.68 dB

10.6 cm 25.0 µm 3.90 dB

Single pumpSMCDF by HIGC

C.N. Liu, et. al., IEEE PTL., 2016. 34

GainComparisonofMMCDF,FMCDF,andSMCDF

Sample Core dia. Fiber length Gross gain Insertion loss Net gain

MMCDF (2009) 14.0 µm 6.7 cm 2.8dB(pump 3.0 W) 1.6 dB 1.2 dB

FMCDF (2012) 2.0 µm 4.0 cm 2.4dB(pump 0.25 W) 2.5 dB N.A

FMCDF (2014) 34.0 µm 9.8 cm 4.1dB(pump 0.40 W) 4.0 dB N.A

SMCDF (2015) 25.7 µm 6.7 cm 2.7dB(pump 0.32 W) 2.0 dB 0.7 dB

SMCDF (2016) 25.0 µm 10.6 cm 3.9 dB(pump 0.42 W) 2.0 dB 1.9 dB

SMCDF (2017) 25.5 µm 24.0 cm 6.4 dB(pump 0.52 W) 2.0 dB 4.4 dB

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GainImprovementbySeveralWays1. Optimized Molten-Zone Growth

The P = 10-8V1.82 is a key equation to control the laser power for the optimaland symmetrical molten zone during growth process with a longer length more than30cm to 60cm.

2. Fiber annealing processThe Cr4+-ions with tetrahedral sites were the major concentrations in the

Cr:YAG. However, the coexistence of the Cr3+- and Cr4+-ions in the Cr:YAG, theSMCDFs after multi-growth process may reduce the concentration of Cr4+ (tetra).

2Cr3+ (octa) + 1/2 O2 → 2Cr4+ (octa) + O2-

Cr4+ (octa) + A13+ (tetra) → Cr4+ (tetra) + A13+ (octa)

800-1200 nm 1200-1600 nm

3. Physical vapor deposiion (PVD)3-1. Cr2O3 and CaO layer：Enhance the Cr4+ and Ca2+-ions3-2. TiO2 and SiO2 layer：Bragg reflector

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GainSimulation

The simulation results indicate that with transmission loss and annealing process a higher gross gain morethan 10-dB may be achieved if a longer fiber length over 30-cm and a lower loss less than 0.2 dB/cm.

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Conclusion} The rapidly increasing capacity of fiber transmission calls for a new generation of

transport networks, including new active fibers for extended optical bandwidth.

} Currently, the capacity of optical transmission systems are limited to the usablegain bandwidth in rare earth-doped fiber amplifiers, which doesn't cover all thelow-loss bandwidth from 1.3-1.6 µm.

} Further studies on higher gain improvement of more than 10-dB gross gain of theSMCDFs are necessary to achieve the SMCDFs as broadband fiber amplifiers forusing in the next-generation fiber transmission systems and are currently underinvestigation.

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