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Yi et al. Vol. 27, No. 11 /November 2010 /J. Opt. Soc. Am. B 2211

Enhancement of gain recovery rate and cross-gainmodulation bandwidth using a two-electrodequantum-dot semiconductor optical amplifier

Yu Yi, Huang Lirong,* Xiong Meng, Tian Peng, and Huang Dexiu

College of Opto-electronics Science and Engineering, Wuhan National Laboratory for Optoelectronics,Huazhong University of Science and Technology, Wuhan 430074, China

*Corresponding author: lrhuang@mail.hust.edu.cn

Received June 30, 2010; revised August 29, 2010; accepted August 31, 2010;posted September 2, 2010 (Doc. ID 130948); published October 8, 2010

A two-electrode quantum-dot semiconductor optical amplifier (QD-SOA) is proposed to enhance gain recoveryrate and cross-gain modulation (XGM) bandwidth. In the theoretical model, electron and hole dynamics as wellas the carrier diffusion are accounted for in the quantum-dot rate equations, which are solved with forwardand backward propagation equations of signal and amplified spontaneous emission. The simulation resultsshow that two-electrode QD-SOA can distribute injection current density nonuniformly to maintain carriers incarrier reservoirs of quantum dot sufficient along the entire cavity length of the semiconductor optical ampli-fier, thus making gain saturation dynamics dominated by spectral hole burning at lower bias current thancommon QD-SOA. Besides, distributing more current density in the second section of the two-electrode QD-SOA at higher bias can greatly accelerate gain recovery as well as expand the XGM bandwidth. © 2010 Op-tical Society of America

OCIS codes: 250.5980, 230.5590, 320.7130.

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. INTRODUCTIONemiconductor optical amplifier (SOA) has been widelysed for linear amplification and signal processing in op-ical communication systems [1,2]. Quantum-dot semicon-uctor optical amplifier (QD-SOA) has attracted a greateal of attention in the recent decades [3–13]. Comparedith bulk and quantum well (QW) counterparts, QD-SOAas shown advantages such as broader gain spectra,igher saturation output power, and faster gain recovery5–7] which make it very suitable for the next-generationigh-speed broadband all-optical photonic networks. Theain saturation dynamics of QD-SOA generally consists ofwo recovery processes: a fast recovery process dominatedy spectral hole burning (SHB) and a slow recovery pro-ess dominated by total carrier density depletion [8–12].n bulk and QW SOA, SHB is hard to dominate gain satu-ation due to fast carrier-carrier intraband scattering,nd total carrier density depletion serves as the mainain saturation mechanism. However, in the QD-SOA,HB dominant gain saturation can be got much easierwing to the low state density and discrete QD energy lev-ls [10,11]. It has been reported that gain saturation dueo SHB can greatly accelerate gain recovery rate andake QD-SOA applicable for up to 40 Gbits/s pattern

ffect-free cross-gain modulation (XGM) wavelength con-ersion [12].

Figure 1 shows the schematic diagram of the two gainecovery processes of a QD ensemble. We consider thathe wavelength of incident light is in resonant with theround state (GS) of QD. When the bias current is high inig. 1(a), the carriers in two-dimensional wetting layer

WL) or excited state (ES) are sufficient, and they serve as

0740-3224/10/112211-7/$15.00 © 2

arrier reservoirs to replenish the carriers in GS. Thenhe gain recovery time is determined by the carrier relax-tion time from the carrier reservoir to GS. This gain re-overy is dominated by SHB which is on the order of pi-osecond. In contrast, when the bias current is low in Fig.(b), carrier reservoirs are nearly empty, and the gain re-overy consists of multiple carrier capture and escape pro-esses. Then the gain recovery is dominated by total car-ier density depletion which is on the order ofanosecond. So in order to make gain saturation of QD-OA SHB dominant, a high bias current density is neces-ary. However, enhanced bias current density not onlyroduces large thermal effect but also causes currentaste.The multi-electrode method has natural characteristic

f monolithic integration, and it has been used in variouselds like laser diodes for optical bistability [14], inuantum-dot (QD) semiconductor super-luminescent di-de for broad bandwidth [15], in SOA for increased satu-ation output power or pattern effect reduction [16,17], orn inline photodetector for enhanced reception responsiv-ty and bandwidth [18]. In our study, a two-electrode QD-OA is proposed to investigate the enhancement of gainecovery rate and the bandwidth of the XGM wavelengthonversion.

This paper is organized as follows. In Section 2, weresent the principle of SHB enhancement by the two-lectrode QD-SOA. Based on QD coupled carrier ratequation as well as both forward and backward propaga-ion equations of signal and amplified spontaneous emis-ion, a numerical model of two-electrode QD-SOA is de-cribed in Section 3. In Section 4, the corresponding

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2212 J. Opt. Soc. Am. B/Vol. 27, No. 11 /November 2010 Yi et al.

imulation results of optical gain, gain recovery time, andGM bandwidth of the two-electrode QD-SOA are dis-ussed. Finally, a brief conclusion is given in Section 5.

. PRINCIPLE OF SHB ENHANCEMENT BYWO-ELECTRODE QD-SOAigures 2(a) and 2(b) show the schematic view of a com-on QD-SOA and a two-electrode QD-SOA, as well as

heir gain saturations of a single pulse. The common QD-OA has only one electrode and a uniform distribution of

njection current, and its cavity length is set to be L. Inontrast, the two-electrode QD-SOA comprises two sec-ions with two separated electrodes that are electricallynsulated from each other by a narrow SiO2 insulatedhannel, and the length and bias current of the first sec-ion are L1 and I1, respectively. The second section of two-lectrode QD-SOA has a length of L2 and a bias current of2. Both sections share the same active region and othernternal structures. The current densities in each sectionre expressed as J1=I1 / �WL1�, J2=I2 / �WL2�, where W ishe stripe width of active region.

As to the common QD-SOA in Fig. 2(a), if the bias cur-ent is not high, the input signal makes the carriers inarrier reservoir insufficient near the rear part of SOA forarger light power, then the gain saturation consists ofwo different recovery processes: the ultrafast gain recov-ry dominated by SHB and the slow gain recovery domi-ated by total carrier density depletion. In order to accel-rate the gain recovery of QD-SOA, gain saturation due to

ig. 1. (Color online) Schematic diagram of the two gain recov-ry processes of a QD ensemble. (a) Recovery process at high biasurrent. (b) Recovery process at low bias current.

ig. 2. (Color online) Schematic view of QD-SOA. (a) CommonD-SOA. (b) Two-electrode QD-SOA.

HB should be enhanced. The two-electrode QD-SOA cane adjusted flexibly to meet various application require-ents by easily controlling injection currents into differ-

nt sections. In Fig. 2(b), when J1=J2, it corresponds to aommon QD-SOA. When J1�J2, it corresponds to a two-lectrode QD-SOA with nonuniform current density dis-ribution. Especially when J1�J2, this configuration canccelerate QD-SOA gain recovery rate. Since the lightower in the front part of the QD-SOA is smaller, we ap-ly a comparably smaller current density into the firstection, while a larger current density is biased on theecond section to compensate for the excessive carrieronsumption by the larger optical signal. With this non-niform distribution of the bias current, the injection cur-ent density increases in the direction along the SOA cav-ty length. This configuration can make the carriers inarrier reservoir sufficient in the entire active region ofOA, and then SHB can be enhanced as the only domi-ant gain saturation mechanism.

. THEORETICAL MODELn our simulation, to account for the longitudinal distri-ution of the carrier density, the QD-SOA is split into Kegments (k=1 to K) of equal length in the longitudinalirection; a uniform carrier density and an averaged pho-on density in each segment are assumed [10,13]. Therst section of the QD-SOA includes K1 segments, the sec-nd section includes K2 segments, the SiO2 insulatedhannel includes K3 segments, and K=K1+K2+K3.

. Coupled Carrier Rate Equationsue to the inhomogeneous broadening introduced by thetranski–Krastanow growth mode, QD ensemble is di-ided into 2M+1 subgroups (m=0 to 2M) of identical QDsith an energy interval of �Em. In our study, �Em1.5 meV [13]. The processes of carrier transfer betweenL, ES, and GS are considered. Carrier capture directly

rom WL into the GS is neglected due to the large energyeparation between the GS and the WL band edge andast intradot carrier relaxation [13]. The carrier ratequations as in [10,13] are considered for the position de-endent occupation probabilities in the WL, ES, and GS;nd both electron and hole dynamics are included sepa-ately. The resulting coupled rate equation system in theth segment is as follows:

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Yi et al. Vol. 27, No. 11 /November 2010 /J. Opt. Soc. Am. B 2213

dfg,mc���

dt=

Nefe,mc���

Ng�egc���

�1 − fg,mc��� � −

fg,mc���

�gec���

�1 − fe,mc���� −

�fg,mc fg,m

�

�gr− Rg,m,

�3�

n which Bk is the bias current of the kth segment, for ex-mple, Bk �k�K1

=I1 /K1 for the first section of the two-lectrode QD-SOA, and Bk �k�K2

=I2 /K2 for the second sec-ion. In the insulated channel, Bk �k�K3

=0. The term q ishe electron charge, � is the injection efficiency, fw

c��� (c isor conduction band and � is for valence band) is the car-ier occupation probability in WL, and fe,m

c��� and fg,mc��� are

he carrier occupation probabilities in ES and GS of theth group, respectively. Ne=DeND and Ng=DgND [7,10],here Dg and De are the degeneracy of the GS and the ES

ncluding the spin. ND is the number of QDs in each seg-ent which can be expressed as ND= lDnDWLk, where lD

s the QD layer number, Lk �L /K� is the length of eachegment, W is the stripe width, and nD is the QD surfaceensity. The term Gm

c��� represents the fraction of the mthD electron (hole) state, which is determined by theaussian distribution [7,19]. �w

c��� is the number of statesf WL in each segment expressed as �w

c���

�mwc��� /��2�KBTlDWLk [13], where mw

c��� is the carrierass of WL, and mw

c =0.04 m0, mw� =0.06 m0 [20]. KB is

oltzmann’s constant, and T is the absolute temperature.he associated characteristic times in the absence of theelated energy level occupations are carrier capture timerom WL to ES ��we

c����, escape time from ES to WL ��ewc����,

elaxation time from ES to GS ��egc����, escape time from GS

o ES ��gec����, carrier recombination time in WL ��wr�, re-

ombination time in ES ��er� and GS ��gr�. The last termn the right hand side of Eq. (1) is the WL diffusion term,here Dw

c��� is the WL diffusion coefficient which can beerived from Einstein relation with InAs mobility w

c��� atoom temperature [21]. z is the distance in longitudinalirection (z=0 and z=L are for input and output facets ofhe QD-SOA, respectively). The terms Re,m and Rg,m in-lude both stimulated emission and spontaneous emissionnd are expressed as

Ri,m =Lk

Ni

gi,m�z,t,�s�Ps

��s+

Lk

Ni�sp

gi,m�z,t,�sp�Psp

��sp

�i = e,g�, �4�

n which is the optical confinement factor, Ps and �s arehe averaged optical power and angular optical frequencyf the input signal light in each SOA segment, Psp is theveraged optical power of spontaneous emission light in arequency interval d�sp centered at �sp in each SOA seg-

ent, and d�sp=�c / �nrL�. The summation over sp is car-ied out to account for all the spontaneous emissions atifferent frequencies �sp. The linear optical gain of theth group QDs at frequency � is defined as [7,19]

gi,m�z,t,�� =Di�q2�ND

nrc�0m02VEi

Gms �Menv�2�Mb�2L�Ei,m,����fi,m

c �z,t�

+ fi,m� �z,t� − 1�, �5�

n which n is the background refractive index, � is the

r 0

ermittivity of free space, m0 is the free electron mass,nd V is the volume of each segment of the active regionxpressed as V= lDWLkH, where H is the thickness of oneD layer. The term Ei,m is the resonant energies of theth group for ES or GS which can be expressed as Ei,mEi− �m−M��Em, where Ei is resonant energy of ES orS of the most probable size of QD. The term Gm

s repre-ents the fraction of the radiative electron–hole recombi-ation from the mth QD group which has a Gaussian dis-ribution with the full width at half-maximum (FWHM) of

s=c+� [10], where c��� is the FWHM of Gmc���. �Menv� is

he overlap integral over the electron and hole envelopeunctions. If we treat QD shape as a cylinder, this overlapntegral is nearly one between the electron and hole of theame quantum numbers. �Mb� is the momentum matrixerived by the first-order k ·p interaction [19]. L�Ei,m ,���s the Lorentzian homogeneous broadening function,

L�Ei,m,��� = /2�

�Ei,m − ���2 + � /2�2 , �6�

n which is the FWHM of the inhomogeneous broaden-ng. In our paper, the relation between relaxation and es-ape times is dictated by the principle of detailed balance22]. For example, �ge

c���=�egc����Dg /De�exp�sc����Eeg /KBT�,

ewc���=�we

c����Ne /�wc����exp�sc����Ewe /KBT�. �Eeg is the energy

ifference between ES and GS. �Ewe is the energy differ-nce between WL and ES. sc��� is the factor to transit thenergy difference into electron (hole) energy level separa-ion; in our study, we chose sc=0.8, s�=0.2 [22].

. Propagation Equations for Signal and Spontaneousmissionhe propagation of the optical signal and the amplifiedpontaneous emission along the QD SOA in the forwardnd backward directions are considered and can be de-cribed in the shifted time coordinate system [23],

�Ps±�z,�s�

�z= � �

i=e,g�m=1

M

gi,m�z,�s� − �inPs±�z,�s�, �7�

�Psp± �z,�sp�

�z= � �

i=e,g�m=1

M

gi,m�z,�sp� − �inPsp± �z,�sp�

+ ��sp

d�sp

2� �i=e,g

�m=1

M

gi,m�z,�sp�fi,mc fi,m

�

�fi,mc + fi,m

� − 1�,

�8�

n which �in is the internal loss of the cavity. The super-cript “�” denotes the signal propagating in the forwardnd backward directions. The second term in the rightand side of Eq. (8) represents the spontaneous emissionoupled into Psp

± . The boundary conditions for the propa-ating signal are as follows:

Ps+�0,�s� = �1 − R1�Ps,in + R1Ps

−�0,�s�, �9�

Ps−�L,�s� = R2Ps

+�L,�s�, �10�

n which Ps,in is the input signal power. R1 and R2 areower reflectivities at the input and output facets, respec-

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2214 J. Opt. Soc. Am. B/Vol. 27, No. 11 /November 2010 Yi et al.

ively. The boundary conditions for the propagating spon-aneous emission are Psp

+ �0,�s�=R1Psp− �0,�sp�, Psp

− �L ,�sp�R2Psp

+ �L ,�sp�.Here we use a typical InAs/GaAs QD-SOA as ourodel. The active layer of the investigated QD-SOA con-

ists of ten InAs QD layers, and the thickness of eachayer is 8 nm [19]. Simple constant characteristic timesre adopted as in [7,10,13] for simulation efficiency in thetudy; the parameters are from [4,10,13,21]: �we

c =3 ps,

egc =1 ps, �we

� =0.13 ps, �eg� =0.13 ps, �wr=1 ns, �er=1 ns,

gr=1 ns, c=40 meV, �=10 meV, =10 meV, De=4, Dg2, Dw

c =879 cm2/s, Dw� =13.7 cm2/s, R1=R2=10−4, nr

3.5, �=0.9, L=1 mm, W=4 m, K=40, M=40, nD=51010 cm−2, �in=5 cm−1, Eg=0.97 eV, Ee=1.04 eV, Ew1.14 eV, and =0.025. The ratio of the two section

engths of the two-electrode QD-SOA is fixed as 7:3. Be-ause the SiO2 insulated channel is too narrow (normallybout 10 m) compared with the SOA cavity length and itas little effect on the results, we neglect this segmentnd treat K3=0.

. CALCULATED RESULTS ANDISCUSSIONS

n our study, J1 :J2 is chosen to scale the nonuniformity ofnjection current density distribution. The electron andole occupation probabilities of WL along the two-lectrode QD-SOA cavity length with J1 :J2 varying from:2 to 1.5:1 are presented in Figs. 3(a) and 3(b), respec-ively. The input continuous light has the power of 3 mW,nd the bias current is 100 mA. As discussed in Section 2,t can be found in Fig. 3 that compared with the commonD-SOA �J1 :J2=1:1�, the two-electrode QD-SOA with

maller J1 :J2 can enhance the carrier density of carriereservoir in the second section of the SOA to make themufficient for larger light power.

. Acceleration of Gain Recovery by Two-Electrode QD-OAy passing a short Gaussian pulse at 1292 nm with peakower of 200 mW and FWHM of 2 ps through the two-lectrode QD-SOA, we first investigate its gain satura-ion. Figure 4 shows the simulated gain saturation at

ig. 3. (Color online) Carrier occupation probability of WL withrobability.

arge bias current of 200 mA with J1 :J2 varying from 1:2o 1.5:1. It can be obviously seen that the gain saturationomprises two recovery processes: an ultrafast gain recov-ry process within several picoseconds due to SHB and alow gain recovery process in several nanoseconds due tootal carrier density depletion. Compared with the com-on QD-SOA �J1 :J2=1:1�, the two-electrode QD-SOA

J1 :J2�1� can make SHB more significant, and the gainecovery rate is enhanced as J1 :J2 decreases. This is be-ause distributing larger carrier densities in the secondection of the two-electrode QD-SOA can supply carriereservoirs with much more carriers to compensate forarger stimulated light power.

Figure 5 shows the calculated gain saturation at smallias current of 100 mA. Like the results in Fig. 4, as J1 :J2ecreases from 1.5:1 to 1:1.5, gain recovery is accelerated.owever, in Fig. 5, when J1 :J2 decreases further to 1:2,

he recovery rate decreases. This is because the highlyonuniform current density distribution of small J1 :J2 at

ow bias current results in over depletion of carrier reser-oirs in the first section of the two-electrode QD-SOA,hen the total carrier depletion of the first section is veryignificant which overwhelms the SHB in the second sec-ion.

10%–90% recovery time is an important parameter tovaluate the speed of gain recovery [24]. Figure 6(a)

nt J1 :J2. (a) Electron occupation probability. (b) Hole occupation

ig. 4. (Color online) Gain saturation of a single Gaussian pulsen two-electrode QD-SOA. The bias current is 200 mA.

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Yi et al. Vol. 27, No. 11 /November 2010 /J. Opt. Soc. Am. B 2215

hows the 10%–90% recovery time of the two-electrodeD-SOA versus J1 :J2 when the bias currents are 100 and00 mA, respectively. It can be found that compared withhe common QD-SOA (J1 :J2=1:1, which is indicated byash line), the two-electrode QD-SOA �J1 :J2�1� canreatly reduce the gain recovery time or, in other words,peed up the recovery rate. However, when the bias cur-ent is low, over small J1 :J2 increases the recovery timend may make the gain recovery even slower than theommon QD-SOA due to serious depletion of carrier res-rvoirs in the first section of the two-electrode QD-SOA,hile a higher bias guarantees sufficient carriers in car-

ier reservoirs along the entire SOA cavity length whichan enable higher nonuniformity of current density distri-ution to further enhance the gain recovery rate.As the Gaussian pulse passes through the SOA, its op-

ical gain evolves from the large value at small inputower to the minimum at its peak input power because ofain saturation. Figure 6(b) shows the optical gain of theaussian pulse at peak input power versus J1 :J2 when

he bias currents are 100 and 200 mA, respectively. It cane found that using the two-electrode QD-SOA cannotnly accelerate gain recovery rate but also enhance theptical gain due to sufficient carriers of GS supplied byighly occupied carrier reservoirs in the second section.ike the results in Fig. 6(a), this advantage becomes more

ig. 5. (Color online) Gain saturation of a single Gaussian pulsen two-electrode QD-SOA. The bias current is 100 mA.

ig. 6. (Color online) (a) Gain recovery time versus J1 :J2 with di:J with different bias currents.

1 2

bvious in smaller J1 :J2 as the bias current increases,hich is ascribed to that at smaller J1 :J2, a higher bias

urrent not only provides larger amount of carriers in GSn the second section of two-electrode QD-SOA, but alsorevents over depletion of carrier reservoirs in the firstection.

. Improvement of XGM Bandwidth by Two-ElectrodeD-SOAo investigate the optical gain dynamics of pulse trains,e study the XGM response of the two-electrode QD-SOA.ogether with a small continuous probe light at 1290 nmith light power Pprobe of 0.1 mW, a series of non-return-

o-zero saturating pump pulse train at 1292 nm is appliednto the two-electrode QD-SOA with the average powerpump of 3 mW and the amplitude (half of the differenceetween the maximum and the minimum of the power)pump of 0.3 mW. Through the XGM effect, the probe sig-al is modulated with the waveform polarity inverted tohat of the pump signal.

By adjusting J1 :J2 at fixed total bias, we study theGM efficiency. The XGM efficiency is defined as �XAprobe�L� /Apump�0� [10], where Aprobe�L� is the amplitudef the converted probe signal at the output facet of SOA;pump�0� is the amplitude of the pump signal at the input

acet of SOA. The XGM efficiency is calculated when theutput converted probe light gets stabilized.

Figure 7 shows the normalized XGM efficiency at dif-erent J1 :J2 when the bias currents are 145 and 80 mA,espectively. The XGM bandwidth is defined as the modu-ation frequency when the XGM efficiency drops by 3 dBrom its maximum (horizontal dashed line). In Fig. 7(a),he XGM bandwidth at 145 mA increases as J1 :J2 de-reases. Compared with the common QD-SOA, the two-lectrode QD-SOA can flatten the XGM trace and expandhe 3 dB modulation bandwidth. This is because the two-lectrode QD-SOA can make SHB significant, and high-peed XGM bandwidth of SHB is determined by QD car-ier relaxation time which is less than a few picoseconds;hus, SHB dominant gain recovery can support the modu-ation bandwidth up to several hundreds of gigahertz and

ake the XGM efficiency almost independent of theodulation frequency.

bias currents. (b) Optical gain of the Gaussian pulse peak versus

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2216 J. Opt. Soc. Am. B/Vol. 27, No. 11 /November 2010 Yi et al.

In Fig. 7(b), the XGM bandwidth at 80 mA increases as1:J2 decreases from 1.5:1 to 1:1.5. However, when J1 :J2rops further to 1:2, the XGM bandwidth reduces. This islso because the bias current is not large enough, andver high nonuniformity of current density distribution atmall J1 :J2 results in the insufficient carriers of carriereservoirs in the first section of the two-electrode QD-OA; then it makes total carrier density depletion signifi-ant.

Figure 8 shows the XGM bandwidth versus bias cur-ent with different J1 :J2. We can find that as the bias cur-ent increases, the XGM bandwidth increases in bothommon QD-SOA and two-electrode QD-SOA. This is as-ribed to that as the injection current increases, QD car-ier reservoirs are gradually occupied, and ultra-fastGM due to SHB becomes dominant which not only en-ances but also flattens the XGM efficiency. Besides, asor two-electrode QD-SOA in Fig. 8, when J1 :J2 is larger,he XGM bandwidth increases slowly. By contrast, when1:J2 is smaller, the XGM bandwidth increases much

aster as the bias current gets larger. This is becausehen the bias current increases, the QD-SOA with

maller J1 :J2 gets more carriers in the second section

ig. 7. (Color online) Normalized XGM efficiency versus the puent is 145 mA. (b) The bias current is 80 mA.

ig. 8. (Color online) XGM bandwidth versus bias current withifferent J :J .

1 2

han that with larger J1 :J2. Since the second section ofhe two-electrode QD-SOA plays a more important role inGM than the first section due to stronger stimulatedmplifications, the XGM bandwidth increases more rap-dly with smaller J1 :J2. This characteristic brings advan-ages that the same XGM bandwidth can be obtained at auch lower bias using the two-electrode QD-SOA com-

ared with the common QD-SOA, which is helpful to re-uce the heat produced by the whole device.As for a large bias current, the XGM bandwidth of SOA

s determined by SHB. We can find in Fig. 8 that becausef the efficient utilization of injection current by the two-lectrode QD-SOA, the larger is the bias current, theore advantageous the two-electrode QD-SOA becomes.owever, for a low bias current in Fig. 8, smaller J1 :J2oes not correspond to a wider XGM bandwidth. This isecause the carrier reservoirs are almost empty, and gainaturation dynamics is dominated by total carrier densityepletion. Thus the XGM response due to SHB cannot bechieved in any J1 :J2. So to increase the XGM bandwidthf the QD-SOA operated at low bias currents, current den-ity distribution should be adjusted to find an appropriatealue of J1 :J2 to balance the carrier consumption and torevent over carrier depletion in part of the QD-SOA.

. CONCLUSIONe have proposed a two-electrode QD-SOA to enhance the

ain recovery rate and XGM bandwidth. Compared withhe common QD-SOA, the two-electrode QD-SOA can bedjusted flexibly to get nonuniform injection current den-ity which makes carrier reservoirs like WL or ES ofuantum dot (QD) sufficient along the entire cavityength of the SOA; then SHB is enhanced to serve as theominant gain saturation dynamics mechanism to accel-rate the gain recovery, and the optical gain can be in-reased as well. The simulation results also show that dis-ributing more current density in the second section of thewo-electrode QD-SOA at large bias currents can greatlyxpand the XGM bandwidth, and because of the efficienttilization of injection current, the larger is the bias cur-ent, the more advantageous the two-electrode QD-SOA

nal modulation frequency with different J1 :J2. (a) The bias cur-

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2

2

Yi et al. Vol. 27, No. 11 /November 2010 /J. Opt. Soc. Am. B 2217

ecomes. This characteristic can enable the two-electrodeD-SOA to obtain same XGM bandwidth as in the com-on QD-SOA at much lower bias currents, which reduces

he heat produced by the whole device.

CKNOWLEDGMENTShis work was supported by the National High Technol-gy Research and Development Program of China (no.007AA03Z414) and National Natural Science Founda-ion of China (NSFC) (no. 60777019).

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