Study of refractive index changes in Yb-doped fibers

O.L. Antipov, N.G. Zakharov, M.S. KuznetsovInstitute of Applied Physics of the Russian Academy of Science

Nizhny Novgorod, Russia, e-mail: antipov@appl.sci-nnov.ru

A.A. Fotiadi1,2, P. Megret2

1.Ioffe Physico-Technical Institute of Russian Academy of Sciences, St. Petersburg, Russia2.Faculté Polytechnique de Mons, SET, 31 Boulevard Dolez, B-7000 Mons, Belgium

OUTLINE1. Introduction: • motivation for the study.

2. Two main mechanisms of the index changes in the laser crystals and fibers – thermal and electronics.

3. Experimental schematic of investigations of the index changes in Yb-doped fibers and the main experimental results.

4. An application: using of the controllable index changes for the coherent beam combining of fiber lasers.

5. Conclusion.

Motivation for the study of the dynamic index change in the fiber lasers

The refractive index determines the mode structure of the fiber lasers; the mode parameters can be changed by the induced index changes.

The fiber Bragg gratings are created by the index change.

The dynamics of fiber laser generation depends on dynamic index changes.

The possibilities of use of the index change for: coherent beam combining,mode selection (narrow-band generation).

Two main mechanisms of the index changes in the intensively pumped laser crystal and glasses

1. The thermal index changes:

(∂n/∂T)T is the direct change of the refractive index (due to thermal change of the spectroscopic transition parameters and the thermal expansion of the crystal), (∂n/∂T)ε is the coefficient of the index change due to thermal-induced stress.2. The electronic index changes due to different polarizability ofthe excited and unexcited active ions (transition from electronlevel «i» to level «j»):

TTnTTnn TT Δ∂∂+Δ∂∂=Δ ε)/()/(

pij is the polarizability difference of the levels i and j, Nij isthe changes of population of the levels i and j, FL is theLorentz local field factor, n0 is the initial refractive index.

∑Δ=Δji

Le nFn,

ijij02 ΔN p)/(2π

Δ Δ

5d

4f

Ecm-1

2F5/2

0

~10000

4F7/2

~700

Charge-transferband

~50000

Energy levels structure of Yb3+ -ion in Yb-doped glasses and crystals.

The origin of the electronicrefractive index changes in Yb-

doped glasses and crystals

)(2)(0

2

νπν exgrexLe pN

nFn −ΔΔ=Δ

Δpgr-ex is due to the different probability of transitions (at a tasting wavelength) from the ground and excited states to the charge-transfer band and the 5d-electron shell.

'

0 22'

')1(')1( )(2)( ν

νννχν

πνχ dP im

re ∫∞

−Δ

=Δ

Kramers-Kronning relation:

Dispersion of the polarizability difference for Yb-doped material (example: Yb:YAG crystal)

5,0E-27

1,0E-26

1,5E-26

2,0E-26

2,5E-26

3,0E-26

900 910 920 930 940 950 960 970 980 990 1000 1010 1020 1030 1040 1050

Wavelength, nm

Pola

rizab

ility

diff

eren

ce,

cm^3

5E-27

1E-26

1,5E-26

2E-26

2,5E-26

3E-26

3,5E-26

4E-26

400 500 600 700 800 900 1000 1100

Wavelength, nm

Pola

rizab

ility

diff

eren

ce,

cm^3

Near absorption and emission bands on the wide frequency scale

The refractive index change is enhanced by wing of the strong UVtransition (the charge-transfer transition: Yb3+ to Yb2+).

Experimental setup

Pump980 nm

Residual980 nm1060 nm

Signal1060 nm

Pump:• Laser diode @980 nm• Up to 150 mW• ~10 µsec to CW

Signal:• Laser diode @1060 nm• Up to 25 mW• ~10 µsec to CW

CW Probe1550 nm

Probebalanceddetection

at 1550 nm

( )~ sinI φΔ

Coupler50/50

1550nm

Coupler50/50

1550nm

φΔ

SMF FlexCore 1060• Clad Ø: 125 µm• Core Ø: ~6 µm

Yb-doped fiber (DC)Inner clad: 125x125 µm2

Core Ø: ~6 µm~700 dB/m@976nm

WDM980/1550

WDM980/1550

Yb-dopedamplifier

Yb-doped fiber (SM)Inner clad Ø: 125 µm

Core Ø: ~3.6 µm~1200 dB/m@976nm

Yb-doped fiber (SM)Inner clad Ø: 125 µm

Core Ø: ~4.5 µm~245 dB/m@976nm

Our experiments:

The first experiments: the pulsed pump; the wave schematic in the fiber

amplifier

Pν Sν

Yb–doped fiber amplifier976,P in nmP

( )0

L

Gain g z dz= ∫

,g nδ

( ) ( )0 00

~ ,L

n z dzδϕ ν δ ν∫

976,P out nmP

δϕ1550,in nmϕ

Typical recorded trace and its processing

0 2000 4000 6000 8000 10000

0.0

0.5

1.0

Cur

rent

, a.u

.

Time, μsec

~980 or~1060 nm

0 2000 4000 6000 8000 10000

0

1

2

3

4

Pha

se d

iffer

ence

, Δφ/π

Time, μsec

( )tδϕ

( ) ( )0

~ ,L

t n z t dzδϕ δ∫

0 2000 4000 6000 8000 10000

-1.0

-0.5

0.0

0.5

1.0

Vol

tage

, a.u

.

Time, μsec

~1550 nm

( ) ( ) ( ) max min

max min

21 arcsink U t U U

t kU U

ϕ π− −⎛ ⎞

= − +⎜ ⎟−⎝ ⎠

Different pump pulse duration

0 2000 4000 6000 8000 10000

0

1

2

3

4

Pha

se d

iffer

ence

, Δφ/π

Time, μsec

20, 40, 80,

160, 320, 640,

1250, 2500 µsec

0 2000 4000 6000 8000 10000

0

1

Nor

mal

ized

pha

se d

iffer

ence

Time, μsec

rel ~ 850 sτ μ

The relaxation after transient pumping does not depend on the pump pulse duration, the relaxation time is equal to the population relaxation time of the upper laser level, indicating the electronic mechanism of index changes

Variation of the longer pump-pulse duration in a long fiber (2 m) (in dependence on pump

power)

( )2~ 1 expspP in

p p sp

tN t PAhτ

δϕ δ δη ν τ

⎛ ⎞⎛ ⎞= − −⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

%

0 2000 4000 6000 8000 10000

0

1

2

3

4

Pha

se d

iffer

ence

, Δφ/π

Time, μsec0 20 40 60 80 100 120 140

0

1

2

3

4

Pha

se d

iffer

ence

, Δφ/π

Pump, mW

Pump:0-145 mW~6 ms

( )2~ PP in

p p

N t PAhτδϕ δ δ

η ν=%

0 2000 4000 6000 8000 100000.00

0.05

0.10

0.15

0.20

0.25

0.30

Phas

e di

ffere

nce,

Δφ/π

Time, μsec 0 25 50 75 100 125 1500.00

0.05

0.10

0.15

0.20

0.25

0.30

Pha

se d

iffer

ence

, Δφ/π

Pump, mW

Pulsed pump, short fiber

( )26 30

220

9( ) 5.5 102 2

tgr ex p

p PP

np Ahc cmPn

λ δϕηλ δπ τ

−−Δ = ≈ ×

+

Pump:5-145 mWτ~200 μs

PPδϕδ

The estimation of the polarizability difference for the excited andunexcited Yb3+ ions:

The second experiments: CW pump + pulsed signal at 1060 nm

0 2000 4000 6000 8000 10000 Time, μsec

Pow

er, a

.u.

Pow

er, a

.u.

CW, 0 – 150 mW

0 – 25 mW10 μs - 5 ms

Yb–doped fiber amplifier

1060,S in nmP

976,P in nmP

1060,S out nmP

976,P out nmP

δϕ1550,in nmϕ

Yb–doped fiber amplifier

1060,S in nmP

976,P in nmP

1060,S out nmP

976,P out nmP

δϕ1550,in nmϕ

Pump cw: ~145mWPulse duration:~4 ms

0 5 10 15 20 250

1

2

Pha

se d

iffer

ence

, Δφ/π

Pump, mWSignal, mW

0 20 40 60 80 100

6 mW 12 mW 18 mW 24 mW

Pow

er

Time, μsec

The phase shift depends on the signal power at 1064 nm

The rise time of the phase shift was less than 1 µsec << thermal relaxation time; the fall time (after signal finish) was 500-800 µsec (in dependence on the signal power)

0 20 40 60 80 100

0.0

0.5

1.0

Pha

se d

iffer

ence

, Δφ/π

Time, μsec

Signal: 10-40 µsec

The pulsed signal, the long fiber

( ) ( )2~ 1 exp S in S outp p

T tN t P PAh T

δϕ δ δη ν

⎛ ⎞⎛ ⎞= − − −⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

6447448%

0 2000 4000 6000 8000 10000

0

1

2

3

4

Pha

se d

iffer

ence

, Δφ/π

Time, μsec

0 10 20 30 40 500.00

0.25

0.50

0.75

Phas

e di

ffere

nce,

Δφ/π

Pump, mW

Signal:2-25 mW~4 ms

0>

Abs

0>

0<

Ampl.

0<

The experiment 3: variation of the probe wavelength (1460-1580 nm)

0 20 40 60 80 100 120 140

0,0

0,5

1,0

1,5

Pha

se d

iffer

ence

,

Δφ/π 1460 nm

1522 nm1579 nm

Pump power, mW

ΛT

1450 1500 1550 16004

5

6

7

Pol

ariz

abilit

y di

ffer

ence

x10-26

Wavelength, nm

The result: the polarizability don’t depend on the probe wavelength (in the range of 1460-1579 nm), it is in a good accord with the UV-enhanced effect.

Calculation of Δp assuming the change of the mode diameter at change of the wavelength

POLARIZABILITY IS DETERMINED BY: UV -LINE,

RESONANT IR-LINE

Estimation of the electronic index changes in the Yb3+ -doped fiber amplifiers :

exgrexLe pN

nFn −ΔΔ=Δ0

22π

The thermal changes, for comparison:

ξν ××=Δ −5106.4)(en

TTnnT Δ

∂∂

=Δ )(

For the quartz fiber: (∂n/∂T)=1.2×10-6 K-1

Δpgr-ex = 5.5 ×10-26 cm3, n0=1.5, ΔNex= ξ×N0 (ξ - fraction of the inverted Yb3+ ions, N0 - the full concentration of the Yb3+ ions),N0=1020 см-3.

Coherent beam combining of multi-channel fiber lasers

.

By J. Anderegg, S. Brosnan, M. Webber et all, Proc. SPIE 4974, 2003.

Fundamental requirements for achieving ideal coherent beam combining:

1. Same spectra for all parallel amplifiers.2. Relative control of polarization of the each channel.

3. The phase control requirements?The bandwidth of the strong phase noise (multi-πmodulation) of 10-W Yb-doped fiber amplifier is determined manly by a laboratory environment, and limited by several kHz. The acoustic noise is more broadband but has sufficiently small depth.Hence: the phase control system must provide multi-π modulation, and have bandwidth more than 1 kHz.

Our principle of coherent beam combining - resonant optical control of refractive index of Yb-doped fiber in multi-channel fiber system.

The index of the Yb-doped fiber at different wavelength can be changed by the pumping beam control and under amplified signal.

The index changes of the laser fibers have the electronic origin: due to different polarizability of the excited and unexcited Yb3+ ions. The changes in 2F5/2-level population accompanies the index changes.

Experimental setup for beam combining

Detectionat 1550 nm

Pump980 nm

Coupler50/50

1550nm

Coupler50/50

1550nm

WDM980/1550

WDM980/1550

1.8-m Yb-dopedfiber

PC

500-mW Er-dopedamplifier

15-dB Er-dopedpreamplifier

500-mW Er-dopedamplifier

Balancefiber

Labview Controller, linewidth 3 MHz

Photodiode

Feedback

CW Laser diode

at 1550 nm (10-m

coherence length)

Current driver

Experimental results for the two-channel combining

0 200 400 600 800 10000

100

200

300

400

500

600

700

800

900

1000

1100

1 2

Out

put p

ower

, mW

Pump power, mW

The main result: theoutput power of the controlled system

increases in 2 times incomparison with

uncontrolled system.

1- the absence of control , 2 - the presence of control

Proposed laser system architecture: the multi-channel Er-fiber amplifier with Yb-doped fiber phase control.

Er-fibermasteroscillator FI

Multiplexer Multiplexer

Yb-dopedfibers Er - fiber amplifiers

Outputfiber

Pumping control

Summary

1). The pump and (/or) signal induced phase shifts in the Yb-doped single-mode fiber amplifiers are caused manly by the electronic index changes due to different polarizability of the excited and unexcited Yb3+-ions.

2). The electronic index changes in Yb-doped fibers (under pump and/or at resonant signal amplification) can be used for the coherent combining of muli-channel fiber system.

Publications1). Antipov O.L., Fotiadi A.A., Megret P., “Dynamics of pump/signal-induced index change in Yb-doped fiber amplifier.” In Technical Digest of “Conference on Lasers and Electro-Optics/Europe”(CLEO/Europe 2007), Munich, Germany, June 16 - 22, 2007, paper СJ3-4-THU.2). Antipov O.L., Fotiadi A.A., Megret P., in Technical Digest of “Conference on Lasers, Applications and Technologies” (LAT 2007), Minsk, Belarus, May 28 - June 1, 2007, paper LO/ IV – 5.

3). A.A. Fotiadi, N.G. Zakharov, O.L. Antipov, P. Mégret, “All-fibercoherent combining of Er-doped fiber amplifiers by active resonantly-induced refractive index control in Yb-doped fiber,” in Conference on Lasers and Electro-Optics, USA, 2008 (will be presented in May).