W eierstraß-institut für Angew andte Analysis und Stochastik DFG Research Center MATHEON Mathematics for key technologies Tailoring the dynamics of multisection lasers for 4 Gb/s direct modulation Mindaugas Radziunas In cooperation with U. Bandelow, M. Wolfrum, A. Glitzky, R. Hünlich (WIAS), U. Troppenz, J. Kreissl (HHI) NUSOD 25, Berlin, 2.9.25 NUSOD 25 radziunas@wias-berlin.de www.wias-berlin.de September 2, 25
Outline Main goals; Basic methods to increase the speed potential; Finding the Photon-Photon resonance; Small and large signal analysis; Conclusions. NUSOD 25 September 2, 25 2(21)
Main goals NUSOD 25 September 2, 25 3(21)
Our aims modulation at 1 Gb/s 1 32 8 24 6 16 4 8 2 1 2 time, ns 3 modulation at 2 Gb/s 4 32 8 24 6 16 4 8 2 1 2 time, ps 1 2 time, ns 3 1 2 time, ps We seek to have a required laser performance at a current modulation with 4 Gb/s PRBS. NUSOD 25 1 September 2, 25 4(21) output power, mw 1111111111111111 current injection, ma 4 output power, mw current injection, ma Conventional high-speed lasers are mainly limited by:. interaction speed of carriers & photons (CP resonance frequency). and/or by insufficient damping of relaxation oscillations
System requirements: opened eye with good extinction P 1 _ P 1 P _ P + _ P 1 + _ P h τ _ h t t 1 T Main requirements for open eye: Opening height h > h/2 ; Width of digital 1 and layers P ± j P j < h/4 ; Opening duration τ > T/5 ; Extinction 1 log P 1 P 3(dB). NUSOD 25 September 2, 25 5(21)
Basic methods to increase the speed potential NUSOD 25 September 2, 25 6(21)
Fast transition between and 1 states To increase a bit transmission rate one needs to make transitions between P and P 1 layers faster Output power P+ 1 P 1 P- 1 P+ P P - T T 1 T 2 time overshooting Say, P (t) P 1 Re e (iω γ)t. γ 1 γ 2 γ 4 γ 1, ω 2 γ 1, ω 4 To achieve fast transitions between states one needs large damping γ or/and high relaxation frequency ω. However, if ω/γ is too big, overshootings are expected! NUSOD 25 September 2, 25 7(21)
Improving extinction 1 log( P 1 / P ) of the eye diagram Mean P and P 1 states correspond to the constant current injection at and 1 levels and are given by PI-charcateristic output power, mw 3 2 1 ext ~ 1.5 db ext ~ 5.5 db I mod. I mod. 25 ma 25 ma 2 4 6 8 1 12 current injection, ma 3 2 1 CP resonance frequency, GHz To achieve 3dB extinction one needs to have low mean injection or/and large enough modulation amplitude But, CP resonance frequency at low injections is not very high! NUSOD 25 September 2, 25 8(21)
Concept: to enhance resonance frequency Increase f CP g (I I thr ) (up to 3 GHz) by increase of g when optimizing transversal design (M.N. Akram..., Semicond. Sci. Technol., 19, 24) but: to get a required high f CP one still needs to apply large enough injection where extinction can be low! Increase of 3 db bandwidth of modulation response function by, e.g., optical injection locking (L. Chrostowski..., Proc. OFCC, OThM2, 25) exploiting intra-cavity or Photon-Photon (PP) resonance in multisection lasers (J.P. Reithmayer..., Proc IPRM, 25; U. Feiste, IEEE JQE, 34, 1998) but: is damping of low frequency components (CP resonance) high enough for good performance of the laser at current modulation with large amplitude signal? NUSOD 25 September 2, 25 9(21)
Concept: to improve damping Increase of damping of cw state by, e.g., injected beam at transparency wavelength of gain media (G. Morthier..., IEEE PTL, 16, 24) optical feedback from resonator (V. Tronciu..., submitted to Phys. Rev. A) but: resonance frequency remains the same (dominated by CP resonance). Is improvement of damping alone high enough for system applications at 4 Gb/s modulation rate? We shall show, that multisection laser concept allows both: enhancement of PP resonance and improvement of damping rate! NUSOD 25 September 2, 25 1(21)
Finding the Photon-Photon resonance NUSOD 25 September 2, 25 11(21)
DFB laser with external cavity The simpliest of possible multisection lasers admitting dominance of the PP resonance with f P P 4 GHz is the single-mode laser with a short external cavity. (A. Tager..., IEEE JQE, 3, 1994).. AR. I+I (t) M DFB laser I ~ feedback phase E - E + Integrated external cavity HR. To simulate and to analyze the performance of this device we use the Traveling Wave model and its mode aproximation system (Tutorial MB1) (LDSL-tool, http://www.wias-berlin.de/software/ldsl) NUSOD 25 September 2, 25 12(21)
Photon-Photon resonance Suitably selecting optical field feedback strength and phase one can achieve dominance of the PP resonance. This resonance is induced by frequency separation of longitudinal modes (M. Radziunas..., in Optoel. Devices, Springer, 25). frequency (Im λ ), GHz rf power, 5 db/div. power, 2 db/div. 4 2-2 -4 (a) rf spectra 1 2 3 4 5 frequency, GHz -2-1 1 relative wavelength, nm 2 (c) f CP f CP (b) optical spectra damping CP ~f PP spectra of linearized model around considered cw state (stability analysis) f PP -5-4 -3-2 -1 gain (Re λ ), 1/ns f PP damping PP NUSOD 25 September 2, 25 13(21)
Finding intervals with dominant PP resonance on parameter axis max. output, mw 4 3 2 1 (a) Hopf bif. B A continuous wave state mode-beating pulsations tori & irregular cw frequency, GHz damping, 1/ns 4 2-2 -4 1.2 (b) CP resonance (c) CP damping PP resonance PP damping pulsations 1.8.6 phase tuning parameter CP damping of solitary DFB.4 ϕ/2π CP.2 NUSOD 25 September 2, 25 14(21)
Finding regions with dominant PP resonance in parameter space,8 PP ~75 GHz H tp feedback strength,6,4,2 pulsations PP at ~45-3 GHz 1,6 mode-beating H 1,4 TB transition frequencies Q-switching pulsations B Dominant 1,2 1,8 feedback phase, rad./2π f tp A GG PP resonance at ~45-3 GHz RO resonance md,6 mode-beating pulsations,4 TB RO transition frequencies Q-switching pulsations,2 NUSOD 25 September 2, 25 15(21)
Small and large signal amalysis NUSOD 25 September 2, 25 16(21)
Current modulation with periodic signal Small signal analysis Large amplitude modulation modulation responce, db 2-2 -4-6 -8 a) b) ϕ ϕ =B ϕ=a I M =4 ma I M =2 ma I M =1 ma 1 2 3 4 1 2 3 4 modulation frequency, GHz =A 3 2 1 amplitude power, mw NUSOD 25 September 2, 25 17(21)
Current modulation with 4 Gb/s 2 7 1 PRBS Laser response in time Eye diagram Histogram 8 a) b) c) 4 current injection, ma 6 4 2 3 2 1 output power, mw,5 1 time, ns 2 4 time, ps 1 2 3 number of events NUSOD 25 September 2, 25 18(21)
Areas of opened eyes in the parameter plane injection (gain section), ma 14 12 1 8 Freq. at PP resonance, GHz 25 6 2.2.4.6.8 1 14 12 1 8 6 (a) 1 lg(<pmax>/<pmin>), Extinction, db db (c) 5 45 4 35 3 1 8 6 4 2 RO supression w.r.t. PP, db CP 14 (b) 5 4 12 3 1 2 8 1 6.2.4.6.8 1 Open eye height, mw 14 (d) 5 4 12 3 1 2 8 1 6 Above: unmodulated laser. PP resonance frequency and relative suppression of CP. Below: directly modulated laser with 4 Gb/s PRBS. Open eye extinction and amplitude..2.4.6.8 1 feedback phase, rad./2 π.2.4.6.8 1 NUSOD 25 September 2, 25 19(21)
Conclusions NUSOD 25 September 2, 25 2(21)
Conclusions The PP resonance can dominate over the CP resonance in multisection lasers; For dominance of the PP resonance we need large enough feedback and a possibility to tune the feedback phase; Need to select the conditions where PP resonance dominates and the damping is improved; Opened eyes at 4 Gb/s current modulation with PRBS; Best extinction of eye diagrams at lower currents; Better performance of lasers with higher differential gain. Other multisection laser configurations are also possible! NUSOD 25 September 2, 25 21(21)