Summary form only given. Wavelength conversion by four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) has several advantages, including transparency to the modulation format and bit rate. An important feature of the intensity and polarization of the FWM wavelength-converted signal is their dependence on the polarizations of the input signal and pump waves. In this paper, we discuss the polarization properties of the FWM susceptibility χ_(ijkl) of strained multiple quantum well SOAs and their potential for application to polarization-independent wavelength conversion and pump suppression

Hunziker, Guido

Koren, Uzi

Miller, Barry L.

Paiella, Roberto

Vahala, Kerry J.

Zhou, Jianhui

English

Caltech Authors - Main

504 1 CLEO'96 1 FRIDAY MORNING 
CFE6 Fig. 1 Experimental setup. The 
focal lengths of the objectives are 
respectively 15 mm, 100 mm, 60 mm for 
0 1 , 0 2  and 03.  For the afocal systems: 
/ /  (resp. I) indicates that the afocal 
works in the direction parallel (resp. 
perpendicular) to the junction of the 
diode. 
/ 
0 "pin( 
0 
I I I I l I I I  
0 50 llm 1s m 2s MO 3s 
A!mrW pump power (mW) 
CFE6 Fig. 2 Small signal pass gain 
under different pumping configurations 
for one pumping diode. The 
"extrapolated gain" corresponds to a 
small signal gain without upsonversion 
process and without thermal quenching 
of fluorescence. 
by polarization to the first one: the gain 
fell down to 4.8% principally due to 
quenching of fluorescence. 
Then we dewased the thermal load 
by pumping the crystal with one diode at 
each side (Fig. 1). We obtained a higher 
value of go equal to 10.2%. We used this 
last pumping configuration to produce 
nanosecond pulses. We added in the cav- 
ity an acousto-optic modulator to operate 
in a (2-switch regime. We obtained 7-pJ 
pulses with a duration of 230 ns at a rep- 
etition rate of 5 kHz at 850 nm. The en- 
ergy per pulse remained above 5 pJ be- 
tween 820 and 900 nm. The energy per 
pulse remained above 7 pJ up to 20-k.Hz 
repetition rate. 
We are currently investigating a the- 
oretical model describing the unsaturated 
gain that takes into account the influence 
of up-conversion and thermal quenching 
of fluorescence. 
In conclusion, owing to in situ small 
signal gain measurement we succeeded 
in the realization of an efficient Q- 
switched diode-pumped Cr:LiSAF laser 
operating at high repetition rate (up to 
- . - - - 
Y 
-e -
, I tract N"93/078. 
 20 kHz) producing tunable nanosecond 
pulses with an energy up to 7 pJ. 
These works have been supported by 
the DRET, division optique under con- 
1. 
- - 
- 
-~ _ _   M. A. Noginov, H. I?. Jensen, A. Cas- 
sanho, in Advanced Solid-state Lasers, 
Vol. 15, OSA Proceedings Series 
(Optical Society of America, Wash- 
ington, DC, 1993) pp 376-380. 
2. S. A. Payne, L. K. Smith, R. J. Beach, 
B. H. T. Chai, J. H. Tassano, L. D. 
Deloach, W. L. Kway, R. W. Solarz, 
W. F. Krupke, Appl. Opt. 33,5526- 
5536 (1994). 
3. M. Stalder, M. Bass, B. H. T. Chai, J. 
(1992). 
Opt. Soc. Am. B 9,2271-2273 
CFE7 9 4 5  am 
Alexandrite laser passive Q-switching 
and spectral output enhancing up to 
0.7 t 1.15 pm 
S. B. Mirov, A. Yu. Dergachev, V. B. 
Fleurov, V. A. Konyushkin," The University 
of Alabama at Birmingham, Department of 
Physics, 310 Campbell Hall, 1300 University 
Boulevard, Birmingham, Alabama 352941 170 
The nature of the absorption band at 780 
nm (at 300 K) in radiationally colored LiF 
crystals was investigated. It was shown 
that this absorption band formed under 
thermal annealing of LZF; crystals com- 
pletely coincides with the absorption 
band of N; centers that were created by 
recharging of the neutral N1 centers un- 
der low-temperature electron reirradia- 
tion of the LiF crystal. 
The described "; crystals are very 
promising as passive Q-switches and 
mode-lockers for alexandrite laser reso- 
nators. N; centers feature a very high 
thermostability at temperatures up to 
200°C and their absorption bands ideally 
match spectral emission of tunable alex- 
andrite laser. Moreover, because of short 
lifetime of their excited level (determined 
only by a strong nonradiative decay), LiF: 
N; crystals can serve as classical passive 
mode-lockers for the alexandrite laser 
resonators. 
The nonlinear saturation properties of 
three color-center crystals (LSN;, E F ;  
and NaP:FZ*), promising for the alexan- 
drite laser resonator passive Q-switching 
and mode-locking have been experimen- 
tally investigated. The induced-transpar- 
ency measurements were performed us- 
ing an electro-optic Q-switched unfocused 
alexandrite laser radiation at 775 nm with 
a pulse duration of about 100 ns. Figure 
1 shows the intemal transmission as a 
function of the 775-nm pump-power den- 
sity. The induced-transmission measure- 
ments were analyzed using a four-level, 
fast-relaxing absorber model and the ab- 
sorption cross sections of the crystals in 
the maxima of the absorption bands were 
estimated as follows: C T ~ . ~ ,  (LZF;) = 
7.7 X lo-'' cm'; uLmmhUn (NaF:G*) = 2 X 
lo-" an2; LiF:N; crystal exhibit a very 
high magnitude of absorption cross sec- 
tion U B 1015 an2 at A = 775 nm. 
We have demonstrated an efficient, 
high-performance passive Q-switching 
for the alexandrite laser utilizing "; 
crystal yielding 50-mJ pulses of 80-ns 
(FWI-IM) pulse duration with efficiency 
(with respect to the free-running oscilla- 
tion) of about 50%. 
In addition to passive Q-switching of 
I UFN.' 
0.1 
0.0 0.1 0.2 0.3 0.4 0.6 0.6 0.7 
Power hnm, w/m2 
CFE7 Fig. 1 Induced transmission 
versus power density at 775 nm. 
alexandrite laser we performed the ex- 
periments on the enhancing of its spectral 
range of tunability by utilizing LiF crys- 
tals with F;* stabilized color centers. 
These F;* centers exhibit a wide absorp- 
tion band with A- = 63Onm with a half 
width of about 3500 an-' that overlaps 
with the alexandrite laser emission band 
For the first time, to our knowledge, a 
room-temperature stable operation of the 
LiF:F;* crystals pumped by the radiation 
of the alexandrite laser (740 nm) was re- 
alized in the region 0.82-1.15 pm with 
average output power of 0.7 W and effi- 
ciency of about 15%. The alexandrite 
color-center laser combination operated 
without a visible decrease in the output 
power at a pumping power density of 
about 50 MW/cmz. 
*General Physics Institute, Vavilov str, 38, 
Moscow, 11 7333, Russia 
(700-820 nm). 
~ -- - 
CFF 10:30 am 
Room 61 
Wave Mixing and Solid-state 
lasers 
D. D. Nolte, Purdue University, Presider 
CFFl 1030 am 
Using tensor properties of four-wave 
mixing in semiconductor optical amplifiers 
for polarization-independent wavelength 
conversion or pump suppression 
Roberto Paiella, Guido Hunziker, Jianhui 
Zhou, Kerry J. Vahala, Uzi Koren,' Barry I. 
Miller,* Department of Applied Physics, Mail 
Stop 128-95, California Institute of 
Technology, Pasadena, California 91125 
Wavelength conversion by four-wave 
mixing (FWM) in semiconductor optical 
amplifiers (SOAs) has several advan- 
tages, including transparency to the mod- 
ulation format and bit rate. An important 
feature of the intensity and polarization 
of the FWM wavelength-converted signal 
is their dependence on the polarizations 
of the input signal and pump waves. In 
this paper, we discuss the polarization 
properties of the FWM susceptibility x,jkl 
of strained multiple quantum well SOAs 
and their potential for application to po- 
larization-independent wavelength con- 
version and pump suppression. 
The nonlinear process described by 
xckl can be viewed as a two-step process 
as follows. Beating of the kth component 
of the pump and the lth component of the 
input signal induces dynamic gain and 
index gratings in the SOA. A converted 
signal along the ith direction is then gen- 
erated through scattering of the jth com- 
ponent of the pump from these gratings. 
Here, we focus on FWM by carrier-den- 
sity modulation and carrier heating, and 
show that the only nonzero components 
of x,jkl are those with k = 2 and i = j. Phys- 
ically, this implies that the dynamic grat- 
ings are only induced by beating of equal 
components of the pump and input sig- 
nal (either both TE or both TM), and the 
TE (TM) component of the pump can be 
scattered into a converted signal only 
along the TE (TM) direction. The con- 
verted signal field at the SOA output can 
then be expressed in terms of the input 
signal and pump fields E 4  and E P  
through a second-rank tensor M +  as fol- 
lows, 
2 
E;&) = E X O )  2 M+ERO)Ef(O)*. (1) 
This expression was experimentally 
verified by measuring the FWM conver- 
sion efficiency for different combinations 
of the input signal and pump polariza- 
tions. The experimental setup and the 
SOA used in these measurements are de- 
scribed in Refs. l and 2 respectively. The 
result of one such measurement is shown 
in Fig. 1, where we plot conversion effi- 
ciency versus angle of input signal polar- 
ization for a linearly polarized pump at 
-45" and 30" relative to the growth axis. 
Also shown in the figure is a theoretical 
fit based on Eq. (1). Notice that, unlike in 
the case of a perfectly isotropic medium, 
the conversion efficiency is finite even for 
orthogonally polarized input waves. We 
then use this result to discuss condition 
for the realization of FWM wavelength 
conversion with efficiency independent 
of the input signal polarization. 
As a second application, we propose 
and demonstrate a novel scheme for sup- 
pressing the pump wave from the SOA 
output. Given full knowledge of the ma- 
trix elements M a ,  one can set the input 
polarizations so that the converted signal 
polarization is orthogonal to that of the 
output pump. The pump can then be 
eliminated simply using a polarizer. We 
demonstrate this idea in Fig. 2, where the 
peaks shown are, from left to right, input 
signal, pump, and converted signal. The 
upper scan was taken directly at the SOA 
output, whereas for the middle scan a po- 
bl 
-"40 -45 0 45 90 
Probe Polarization Angle (degrew) 
CFFl Fig. 1 Converted signal power 
as a function of angle of linear 
polarization of the input signal relative 
to the TM direction, for linearly 
polarized pump at -45" and +30°, and 
1.5 nm detuning. The continuous lines 
are a fit to the magnitude squared of .E. 
as given by Eq. (l), with fitting 
parameters Ma .
CFFl Fig. 2 (left panel): illustration of 
our proposed pump suppression 
scheme: the input signal and pump 
polarizations are chosen so that, at the 
SOA output, the converted signal and 
pump are linearly polarized at +45" and 
-45", rrspectively. (right panel): SOA 
output spectrum with the above input 
polarizations and 4 nm detuning, before 
(upper trace) and after (middle trace) a 
linear polarizer at +45". The pump 
(center peak) is suppressed by more 
than 30 dB by the polarizer, whereas the 
converted signal (left peak) is essentially 
unattenuated. The lower trace is the 
ratio of the upper two. 
larizer at the converted signal polariza- 
tion was placed between the SOA and the 
detection stage; the pump is suppressed 
by more than 30 dB, while the signal is 
unaffected. These results also show how 
the model presented here can be used to 
control the polarization of the FWM sig- 
nal, which is of importance in optical 
switching applications. 
*AT&T Bell Laboratories, Holmdel, New Jersey 
07733 
1. J. Zhou, N. Park, J. W. Dawson, K. J 
Vahala, M. A. Newkirk, B. I. Miller, 
IEEE Photon. Technol. Lett. 6,50-52 
(1994). 
M. A. Newkirk, B. I. Miller, U. Ko- 
ren, M. G. Young, M. Chien, R. M. 
Jopson, C. A. Burrus, IEEE Photon. 
Technol. Lett. 4, 406-408 (1993). 
2. 
FRIDAY MORNING 1 CLEO'96 I 505 
1045 am CFFZ 
Four-wave mixing in semiconductor laser 
amplifiers: phase matching in 
configurations with three input waves 
S. Diez, R. Ludwig, E. Patzak, H. G. 
Weber, G. Eisenstein,* R. Schimpe,** 
Heinrich-Hertz-Institut f i r  
Nachrichtentechnik, Einsteinufer 37, 0-10587 
Berlin, Germany 
Four-wave mixing (FWM) in semicon- 
ductor laser amplifiers (SLA) is of interest 
for both theoretical and experimental in- 
vestigations. considerable progress has 
been made towards a better understand- 
ing of the basic physical phenomena,'-3 
as well as towards applications for fast 
optical signal processing."' Conventional 
FWM experiments use a pump wave E l  
(frequency 0,) and a signal wave E A  (aA) 
copropagating with the pump. To effi- 
ciently create mixing products, both 
waves are required to have the same po- 
larization, which is a major drawback for 
the application of FWM in optical com- 
munication networks. 
The addition of a second pumpwave 
El (02) allows for both, polarization-in- 
dependent frequency conversion4" as 
well as investigations on phase matching 
in optical amplifiers (for experimental 
setup see Fig. 1). Being mediated by var- 
ious dynamic processes such as carrier- 
density pulsations (CDP), carrier heating 
(CH) and spectral hole burning (SHB), 
the beating of both pump waves causes 
a modulation of the gain medium with 
Bol2 = 0~ - wl. This generates gain and 
index gratings (grating period 2nc/  
nAw12), which create additional waves at 
frequencies w1 - AuI2, o2 + A o I 2  and o,, 
2 AoI2. Figure 2 shows a schematic out- 
put spectrum, which is expected for E l ,  
E2 and E A  having the same polarization. 
The generated spectral components Ee 
and E ,  are frequency-converted replica of 
the signal wave and are almost indepen- 
dent of the polarization of E A  if IO,, - 
In our experiments we used a polari- 
zation-insensitive bulk InGaAsP-SLA. 
Optical powers were measured by an op- 
tical spectrum analyzer (OSA). By ex- 
changing the source for E A  and the OSA, 
01.21 >> IAoIzI. 
CFFZ Fig. 1 Schematic experimental 
setup for FWM with two pump waves 
optical signals are cw and are provided E- 
by extemal-cavity lasers. Polarization ?-- 
adjustment of the polarization for each 
wave. SLA-input powers were P,(O) = 
P2(0) = 1.66 mW and PA(0) = 0.16 mW. 
The wavelength of E,  was fixed at X2 = 
1564 nm whereas XI was tunable. The 
E ,  and E2 and a signal wave EA. All 
controllers (PC) allow for separate 
- 
- 
c- 
- 
L = 
R signal wavelength was chosen to be 
A A  = 1555 IUII. 


Using tensor properties of four wave mixing in semiconductor optical amplifiers for polarization independent wavelength conversion or pump suppression

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Using tensor properties of four wave mixing in semiconductor optical amplifiers for polarization independent wavelength conversion or pump suppression

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