Nonlinear absorption and carrier dynamics in slabcoupled optical waveguide amplifiers
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Motamedi, A.R. et al. “Nonlinear absorption and carrier dynamics
in slab-coupled optical waveguide amplifiers.” Lasers and
Electro-Optics, 2009 and 2009 Conference on Quantum
electronics and Laser Science Conference. CLEO/QELS 2009.
Conference on. 2009. 1-2. ©2009 IEEE.
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Institute of Electrical and Electronics Engineers
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Thu May 26 08:46:17 EDT 2016
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© 2009 OSA/CLEO/IQEC 2009
a1564_1.pdf
CMBB5.pdf
CMBB5.pdf
Nonlinear Absorption and Carrier Dynamics in SlabCoupled Optical Waveguide Amplifiers
Ali R. Motamedi and Erich P. Ippen
Research Lab of Electronics, Department of Electrical Engineering and Computer Science,
Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139
motamedi@mit.edu
Jason J. Plant, Joseph P. Donnelly, and Paul W. Juodawlkis
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA, 02420
Abstract: Limitations imposed on the saturation energy of high-power slab-coupled optical
waveguide amplifiers were studied for pulsed signal transmission. Loss due to the two-photon
absorption and free-carrier absorption processes becomes a dominant factor for ultrashort-pulse
amplification, leading to lower saturation energy. TPA and FCA coefficients are measured to be
65cm/GW and 7x10-17 cm2, respectively. Carrier-related gain recovery times vary from 160ps to
380ps at bias currents from 4A to 1A, respectively.
© Optical Society of America
OCIS codes: (320.0320) Ultrafast optics, (320.7100) Ultrafast measurements,(320.7110) ultrafast nonlinear optics
High saturation-energy semiconductor optical amplifiers (SOAs) operating in the 1.5μm range are in demand for
optical communication systems, remote sensing, and radar systems. The saturation energy of an SOA, Esat is
proportional to Atr Γ , where Atr is the transverse mode size, and Γ , the confinement factor, is the overlap of the
optical mode with the active region of the SOA. To achieve high saturation energies, either the transverse optical
mode needs to be increased and/or the confinement factor decreased. The low confinement factor (<1%) and large
transverse mode are combined in the design of slab-coupled optical waveguide amplifiers (SCOWA) to achieve high
saturation powers in the excess of 1W[1].
Two processes limit the maximum device saturation energy. One is the gain saturation which is a function
of the number of injected carriers and the bias current in the active region of the device. The other is the loss due to
the optical nonlinear processes of two-photon absorption (TPA) and free-carrier absorption (FCA) which are
functions of the intensity of the optical signal in the passive region of the device. Due to the large overlap between
the optical mode and passive structure, at high output powers TPA and FCA losses dominate the gain saturation,
further limiting the output saturation energy. Recently, the limitations imposed by TPA and FCA on the saturation
power of a SCOWA using CW signals were discussed [2,3]. In this paper, we report on our experimental and
theoretical studies of the saturation energy of an InGaAsP/InP SCOWA using pulsed optical signals.
The theoretical model is based on the solution to the pulse dynamics described by
dI ( z , t )
= I ( z , t ) ⎡⎣ Γg m ( z , t ) − α ( z , t ) ⎤⎦ ,
(1)
dz
where I ( z , t ) is the intensity of the optical signal, Γ is the confinement factor, g m ( z , t ) is the local dynamic material
gain, and and α ( z , t ) is the local loss coefficient. For pulses shorter than the recovery time of the device, the local
t
dynamic gain material is determined by g m ( z , t ) = g 0 e
−
∫ dτ I ( z ,τ ) Fsat
−∞
,where g 0 is the unsaturated small signal gain,
and Fsat is the pulse saturation fluence. The local loss α ( z , t ) is the sum of material loss, the TPA, and FCA induced
losses and is determined from
α ( z, t ) = α int +
β I ( z, t )
2
σβ I 2 ( z,τ )
dτ ,
3=ω
−∞
t
+
∫
(2)
where α int is the loss due to the constituent material of the device, and the next two terms are losses due to TPA and
FCA processes, respectively.
978-1-55752-869-8/09/$25.00 ©2009 IEEE
© 2009 OSA/CLEO/IQEC 2009
a1564_1.pdf
CMBB5.pdf
CMBB5.pdf
The SCOWA studied in this experiment is an InGaAsP/InP structure with a 5x7μm fundamental mode
output. The active region of the device consists of five 8-nm compressively strained (1%) InGaAsP quantum wells
with composition yielding a peak photoluminescence at 1530nm[2].
The experimental setup consists of a commercial Ti:sapph/OPO optical source producing 150fs pulses at
80MHz repetition rate. The output from the OPO is stretched to pulsewidths ranging from 1.4ps to 40ps. These
pulses were used to study the energy saturation as a function of the input pulsewidth and to study the recovery times
of the SCOWA at bias current from 1A to 4A using a degenerate double-modulated pump-probe technique.
14
10
Gain (dB)
8
Value
640
Γ (%)
0.5%
α int ( cm
No TP A
6
4
2
0
Fsat ( mJ cm
1 0 ps
5 ps
10
-1
10
0
10
1
10
2
)
0.5
7x10-17
β ( cm / GW )
1 .4 ps
-4
−1
σ ( cm2 )
4 0 ps
-2
-6 -2
10
Parameter
g 0 ( cm −1 )
12
10
2
65
)
1.4
3
E in (pJ)
Fig. 1. SCOWA gain as a function of the input pulse energy. Ibias=4A.
Solid lines are theoretical fits to experimental data (dashed
lines)
Table 1. Parameters extracted from the theoretical analysis
Fractional TPA induced loss
0.25
0.2
9 0 0 fJ
0.15
4 5 0 fJ
0.1
2 2 5 fJ
0.05
0
1 1 2 fJ
0
0.05
0.1
0.15
0.2
1/τ (ps -1 )
Fig. 2. Carrier recovery times of the SCOWA as a function of the input
bias current. Dashed lines are exponential fits to the measured
data (solid lines).
Fig. 3. Fractional TPA induced loss as a function of the inverse
pulsewidth. Ibias=4A. Solid lines are theoretical fit to the
measured data.
The close agreement between the experimental gain measurement and theoretical fit as shown in Fig. 1,
were obtained using the parameters of Table 1. From Fig. 1, we can see that the gain roll-off is reduced with shorter
input pulsewidths as the effects of the TPA and FCA losses dominate the material gain saturation. We have also
measured the carrier recovery time of the device as a function of the bias current which is shown in Fig. 2. The
recovery time of the device varies between 160ps at 4A to 390ps at 1A. These recovery times are important for the
design of mode-locked lasers as well as high-bit-rate signal amplification. Using the pump-probe measurements, we
have also determined the variation of the TPA induced loss as a function of 1 τ , as shown in Fig. 3. The additional
absorption of the pump due to FCA loss results in the deviation from the expected linear relation at 900fJ input pulse
energy.
We have demonstrated that nonlinear absorption processes limit the short-pulse saturation energy of a
SCOWA device, and we have investigated the recovery times of amplification as a function of the bias current.
The authors gratefully acknowledge the support of AFOSR under FA9550-07-0014. The MIT Lincoln
Laboratory portion of this work was sponsored by the DARPA MTO under Air Force contract number FA8721-05C-0002.
References
[1] P.W. Juodawlkis, et.al, Phot. Tech Lett. 17, 279-281, 2005.
[2] P.W. Juodawlkis, et.al, Opt. Exp. 16, 12387-12396, 2008.
[3] F.R. Ahmad,et.al, Opt. Lett. 33, 1041-1043, 2008.