NOISES IN FIBER LASERS - Joint KSOP/ENSPS seminar -

Student : Besnier Alexandre ([email protected]) Supervisor : Fontaine Joel ([email protected]) Saturday, July 03, 2010

Contents 1.

Fiber lasers a) b)

2.

Sources of Laser Noises a) b) c)

3.

MOPA set-up Ring set-up

How to measure noise ? a) b)

5.

Phase noise Intensity noise and RIN Thermal noise

Solutions for noise reduction a) b)

4.

Loop cavity lasers Distributed Feedback Lasers (DFB)

RIN measure Phase noise measure

Bibliography

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1. FIBER LASERS 1.a Loop lasers - Basic principle - Reflectivity and transitivity

1.b Distributed Feedback lasers (DFB) - Principle - Reflectivity and wavelength selectivity Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 3/39

1. Fiber lasers • Based on doped optical fiber • A cavity is added to create laser effect – Fiber loop mirror – Bragg mirror (DFB) – External mirror

[3,4,16] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 4/39

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1.a Loop lasers Coupler

Optic fiber

• Mirror behavior :

   A1a  r  t  2 j K 1  K (1  ) exp   j l  A1   2

   A4a  t  t  (1  2 K )(1  ) exp   j l  A1   2

– K : coupling ratio – Γ : coupling loss

α : power loss coefficient β : propagation constant [16]

Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 5/39

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1.a Loop lasers R r Tt Transmitted intensity

t>0 → π shift

Reflected intensity

j factor → π /2 shift

phase is maintained Transmitted phase shift

Reflected phase shift

Fiber loop reflector performance with no losses in fiber and coupler Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 6/39

2 2

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1.a Loop lasers • Create a cavity : – 2 loop reflectors – Gain : G  Pout  Pin

T1T2 exp   ' l3  (1  R1R2 ) 2  4 R1R2 sin 2  ' Leff   / 2



– Where :  '    g ( , z )  '    



loss term propagation constant

– Effective cavity length Leff : Leff

l1  2l3  l2  2

Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 7/39

[16]

1.b Distributed FeedBack lasers (DFB)

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• Based on Bragg reflector : – Wavelength reflection selectivity  nH  nL     0 arcsin    nH  nL  4

[1,2] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 8/39

1.b Distributed FeedBack lasers (DFB)

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• Performance : – Very good reflectivity (possible > 99%) – Sharp cut-off & large bandwidth 100 nm

R > 99%

[1] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 9/39

2. SOURCES OF LASER NOISES 2.a Phase noise 2.b Intensity noise - Intensity noise spectrum - Relative Intensity noise (RIN)

2.c Thermal noise Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 10/39

2. Laser noise • Many noise sources : – – – –

Lasing process Spontaneous emission Pumping Thermal noise

• Effect on laser output fluctuations : – – – –

Amplitude / Intensity Phase Polarization Frequency [7,8,9]

Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 11/39

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2.a Phase noise • We can show that variance of phase fluctuations is : 2    2 DST t

– Where DST is the Schawlow-Townes scattering coefficient – Imply that we have a random motion (Brownian)

• We can also calculate spectral density of phase noise in the cavity : S ()  DST  2 • Moreover, we can calculate this density outside of the cavity : S out  DST  1 

2

Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 12/39

4 I out

[7,8,9]

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2.a Phase noise • We can plot those densities : S(Ω)

Sout () Standard quantum limit

S () Pulsation

Phase spectral densities into the cavity and at the output laser

[7,8,9] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 13/39

2.a Phase noise

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• Consequence on laser spectrum : – Line width change – Shawlow-Townes width : Increases with bandwidth

DST  2 h FWHM     2 Pout

= Schawlow-Townes width

Decreases with power [7,8,9] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 14/39

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2.b.1 Intensity noise

• The spectrum of intensity noise can be written as : S Iout ()  I

out

 2  1  2   1  ( p  1)  1  2 2 2       (1  1 R)  R  R 

– ζp : pumping noise – R : pumping ratio

κ : field decrease ratio in empty cavity

[7,8,9] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 15/39

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2.b.1 Intensity noise • We plot this spectrum for : – R=1,5 – ζp={0; 0,25; 0,5; 0,75; 1}

Pulsation

• R=20 • ζp={0; 0,25; 0,5; 0,75; 1} Pulsation Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 16/39

[7,8,9]

2.b.2 Relative Intensity Noise (RIN)

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• Common figure of merit • RIN = intensity noise spectrum compared with general intensity SF () h r  2  2 RIN ()   2 2 FON  cav Pout  2  (r  1)  cav 2  r 2  2  2





– τ : life time of pop inversion r : excitation strength – τcav : life time photon in the cavity – FON : number of photon for the stationary mode [7,8,9] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 17/39

2.b.2 Relative Intensity Noise (RIN) • There is a resonance for :

r 

(2)

r 1

 cav

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2.c Thermal noise

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• Thermal fluctuation have effect on frequency noise – We suppose : T ( , r ) variation of temperature – Spectrum :

1 2 2 S ( )  l q S T ( ) 2  kbT 2  4  S T ( )  ln1  4  2 8CT   ak D ( )   where 𝑆∆𝑇(𝜔) is the thermal noise Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 19/39

[11]

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2.c Thermal noise

Good simulation for high frequencies

1 S   2 q 2S T ( ) 2

Comparison of theoretical (RMS) and experimental thermal noise in DFB fiber Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 20/39

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3. SOLUTIONS FOR NOISE REDUCTION 2.a MOPA set-up - Characteristics - High power with MOPA

2.b Ring set-up - RIN performance - Other interest Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 21/39

3.a MOPA set-up • Master Oscillator Power Amplifier :

– Erbium doped fiber (g ≈ 10dB/m) – 2 Bragg gratings – Pumped by a laser diode (75mW, 1480nm) – Use an active feedback to reduce the noise [6] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 22/39

3.a.1 MOPA characteristics • Basic performance (without feedback) : – Power : 168 µW (with diodes turn off) – RIN : -75dB/Hz

• Increase performances using diode pump actively stabilized by feedback : – Compensate transfer function of laser and amplifier – Increase output power Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 23/39

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3.a.1 MOPA characteristics

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• Results : – High power (>60 mW) – Low RIN (< 110 dB/Hz) Diode 1 is kept started

Influence of diodes power on MOPA output power Remove resonance peak

RIN diagram with and without feedback control Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 24/39

[6]

3.a.2 High power with a MOPA • High power and low noise with a MOPA : – 5W for output power – RIN < -117dB/Hz

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3.b Ring set-up

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• The set-up :

• DFB performances : – Power output : 0.5mW – Power pump : 100mW – DFB RIN : -79dB/Hz [15] Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 26/39

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3.b Ring set-up • Ring performances : – RIN : cut the resonance peak

RIN measured with and without ring set-up

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3.b Ring set-up • Ring performances : – Output signal : constant – Relative frequency power : narrow

Output signal with and without ring set-up Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 28/39

Relative power with and without ring set-up

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4. HOW MEASURE NOISE ? 4.a RIN measure 4.b Phase noise measure

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4.a RIN measure • The system :

Variable Optical Attenuator

Optical-Electrical converter Output voltage : VO=4V

Gain : G=100

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Save data : frequency (f) and voltage (V) Resolution bandwidth : Rb=18Hz, f ~ 5kHz Rb=150Hz, f ~ 2MHz

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4.a RIN measure • Calculate the RIN V ( f )   Pr    GVO 

– Power ratio : – Linear RIN : RIN – Logarythm RIN :

L

2

 Pr Rb RIN dB  10 logRIN L 

• Plot RINdB :

Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 31/39

4.b Phase noise measure

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• The system :

Save Power Spectral Density (PSD) Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 32/39

4.b Phase noise measure • Calculate phase noise : – Phase noise spectrum : Nϕ20 = PSD x 1 rad/V – Normalise for 1m : Nϕ1 = Nϕ20 / 20

• Plot :

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Conclusion • We have 3 important sources : – Pumping, spontaneous emission, temperature

• Influence of noises : – Phase, intensity, frequency – Create a resonance

• Solutions – MOPA – Ring-type Besnier Alexandre - Joint KSOP/ENSPS seminar - Noise in Fiber Laser Saturday, July 03, 2010 34/39

5. BIBLIOGRAPHY

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5. Bibliography

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5. Bibliography

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5. Bibliography

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THANK YOU FOR YOUR ATTENTION Do you have any questions ?

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