Optical Communications Research Group Department of Electronic and Computer Engineering University of Limerick, Ireland b

Transcription

1 Numerical Analysis of Pulse Pedestal and Dynamic Chirp Formation on Picosecond Modelocked Laser Pulses after Propaation throuh a Semiconductor Optical Amplifier Michael J. Connelly a Aislin M. Clarke b Prince M. Anandarajah b and Liam P. Barry b a Optical Communications Research Group Department of Electronic and Computer Enineerin University of Limerick Ireland b Research Institute for Networks and Communications Enineerin School of Electronic Enineerin Dublin City University

2 Outline. Introduction. Experiment 3. Numerical model 4. Results 5. Conclusions NUSOD-5

3 Introduction SOAs have attracted much interest both as basic amplifiers and also in all-optical sinal processin applications such as all-optical clock recovery and time division demultiplexin. Common pulse sources used in ultra-fast optical communications include: Mode locked lasers Gain switched lasers CW lasers followed by electro-absorption modulators. These sources can enerate hih quality pulses that exhibit low chirp jitter and hih temporal and spectral purity. NUSOD-5 3

4 Mode locked laser pulse sources can exhibit pedestals at either side of the main pulse *. The pedestals are usually at a power level > 4 db below the main pulse. When amplified by an SOA the pedestal power will increase relative to the main pulse due to dynamic SOA ain saturation. This can lead to interchannel crosstalk in optical TDM systems. We present experimental results and numerical simulations on the propaation of ps wide modelocked laser pulses after propaation throuh an SOA. * J.M. Dudley at al Quantum Semiclass. Opt. 8 (996) pp NUSOD-5 4

5 Compress pulses to ps Experiment Giatera FROG Source PC DCF SOA OA EDFA 5 ps wide pulses. Frequency- Resolved Optical Gatin FROG technique * allows measurement of the intensity and phase of an optical pulse. Commercial SOA (Kamelian) 9 μm lon. Tensile-strained bulk material. * R. Trebino Frequency-Resolved Optical Gatin: The Measurement of Ultrashort Laser Pulses ; Spriner. A. Clark M. Connelly et al IEEE Photon. Tech. Lett. Sept. 5. NUSOD-5 5

6 Numerical Model ( z ) Require equations to model propaation of the timedomain complex envelope of the optical pulse (z). Local time = t z v NUSOD-5 6

7 NUSOD-5 7 Modified Schrödiner equation * M.J. Hon et al. IEEE J. Quantum Electron April 994 & Booni at al IEEE J. Quantum Electron. 4. γ γ α α 4 z z ib i i i z z T T N N Δ = Use model based on a modified Schrödiner equation which includes nonlinearities induced by ain saturation two-photon absorbtion and carrier heatin *. = d e W s s N ) ( exp ) ( Carrier density induced non-linearly saturatin ain also leads to self phase modulation via α N (linewidth enhancement factor). unsaturated ain coefficient W s SOA saturation enery.

8 NUSOD-5 8 [ ] = Δ 4 int ) ( ) ( ) ( ) ( d e h h e s U ch s T Non-linear ain caused by carrier heatin due to combined effects of stimulated emission free carrier absorption (h ) and two-photon absorption (h ) [ ] B A = [ ] B A = Dynamically varyin slope and curvature of the ain coefficient. T N Δ = where

9 γ γ ib ( z) Material loss coefficient. Two photon absorption and the non-linear Kerr effect. The model contains a lare number of parameters. Obtain A B A B W s N T α α γ from theoretical SOA material ain calculations and usin a steady-state wideband model *. M.J. Connelly IEEE J. Quantum Electron.. NUSOD-5 9

10 The material ain calculations were carried out usin a 6x6 k. p Hamiltonian that includes the split-off band. Band-tail effects are included in the ain calculations. The material ain spectra were then used in the steady-state model to predict the SOA output amplified spontaneous emission spectra at different bias currents. Areement between predicted and measured ASE spectra was obtained by extractin the Auer recombination coefficient material loss coefficient and the trap recombination coefficient. NUSOD-5

11 It is then possible to determine the SOA carrier density for a iven bias current and hence A B A B W s α α N T The remainin parameters must be obtained by comparin the dynamic experimental results with the model. NUSOD-5

12 The modified Schrödiner equation is a non-linear p.d.e. which in eneral does not have an analytical solution. We use the split-step Fourier method fast because it uses the Fast Fourier Transform (FFT). The MSE can be written in the form = ( Dˆ Nˆ ) z NUSOD-5

13 NUSOD ˆ z ib z z B z z ib i i N T T N N γ α α Δ = With linear and non-linear operators 4 ˆ δ δ δ δ γ B A B A i D = The split-step Fourier method obtains an approximate solution by assumin that in propaatin the optical field over a short distance Δz the linear and nonlinear effects act independently. So we can write

14 ( z Δz ) exp( ΔzD) exp( ΔzN ) ( z ) ˆ ˆ The operator exp ΔzDˆ can be evaluated in the frequency domain accordin to exp ( ˆ ) ( ) { exp [ ˆ ( )] { ( ) } ΔzD y z = F ΔzD i F y z T T NUSOD-5 4

15 Numerical Alorithm Flowchart Synthesised pulse or experimentally determined. NUSOD-5 5

16 - Results mw Model input pulse profile 8 mw Experimental input pulse profile mw experimental 8 mw - experimental Intensity (A.U.) Time (ps) Pulse temporal profiles NUSOD-5 6

17 Pin = 8 mw Pulse evolution NUSOD-5 7

18 Chirp (GHz) mw 8 mw - experiment mw 8 mw Input chirp mw - experimental Time (ps) Pulse chirp NUSOD-5 8

19 Normalised power spectral density Input pulse spectrum mw 8 mw mw - experiment 8 mw - experiment Frequency deviation (THz) Pulse spectrum NUSOD-5 9

20 Pin = 8 mw Spectrum evolution NUSOD-5

21 Conclusions Experimentally pedestals present on mode-locked laser pulses can be sinificantly amplified with respect to the main pulse. Used a dynamic numerical model includin detailed calculation of the material ain spectra to enable reasonable prediction of the pulse amplification dynamic chirp and pulse spectrum. NUSOD-5

22 However the model contains a lot of parameters. Future work will use a parameter extraction technique and the measured pulse temporal profile and dynamic chirp to estimate more accurately the parameters that determine the relative importance of the various non-linear mechanisms in the SOA. Thank you for your attention. NUSOD-5