Linearity and chirp investigations on SOA as an external modulator in SCM systems
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E UROPEAN M ICROWAVE A SSOCIATION
Linearity and chirp investigations on SOA as an external
modulator in SCM systems
Eszter Udvary1 and Tibor Berceli2
Abstract – It is shown by numerical simulation and measurement a sinusoidal modulation parts, hence the number of car-
that by using SOA as an external modulator, the device provides riers and photons are also time dependent and the shape
acceptable nonlinear distortion for SCM telecommunication sys- of these parameters are similar to the shape of the mod-
tems. It demands more advanced amplifier-modulator working
state planning. The temperature and the optical reflection in the ulation [6]. The intensity modulated optical signal can be
system have also important role in linearity. The frequency chirp- detected by traditional pin photodiode. The magnitude and
ing in external SOA modulator is treated for different operation purity of the signal depend on the modulation signal, the
conditions. bias current, the input power and the operation parameters
Index Terms – Amplitude and Phase Modulation, Chirp, Nonlin- [7]. The SOA modulator requires low modulation power,
earity, Semiconductor Optical Amplifier, Subcarrier Multiplex-
ing. the detected electrical power is high because of the optical
gain of the SOA in contrary to the optical insertion loss of
other external modulators. However the SOA has remark-
able optical noise.
I. Introduction
In this ever-increasing digital world, there is still need to II. Linearity investigation
send analogue RF, microwave and millimeter wave signals
over optical fibers. The SubCarrier Multplexed (SCM) op- Cascadability is critically important in optical SCM net-
tical systems are used for remote antenna feeding in radar works where several electrical subcarriers are transmitted
systems, microwave signal distribution in picocell commu- on the same optical signal. Degradation of the transmis-
nication systems, combined wireless-data communication sion system will occur due to the crosstalk between the
systems, multi-octave cable television distribution service, subcarriers (nonlinearity) and noise expansion (ASE). The
photonic switched networks with label on subcarrier, etc. second and third order intermodulations will be consid-
The Semiconductor Optical Amplifier (SOA) may be well ered, because of the crosstalk between the channels and the
used as a multifunctional device for the compensation of partial up-conversion of the baseband payload into the sub-
optical loss and addition of new channels in these SCM carrier. As the number of subcarriers increases the linearity
systems. becomes more and more serious problem because many
In the SCM systems the SOA can provide the branch- third order mixing products appear in the used band. The
ing function. The SOA operates as a modulator to add a traditionally used electro-optical modulator shows high
new channel, as a detector to drop the needed channel and nonlinearity, because it has a cosine type characteristic.
as an in-line amplifier to amplify the other channels, si- The photo-detector can be treated as linear device. The
multaneously. It realizes a compact, small size and cost- SOA-modulator can improve the nonlinear behavior of the
effective radio repeater for signal distribution [1], [2]. The system, if it provides lower nonlinear distortion than the
achieved functions are similar in Fiber-to-the-Home Net- electro-optical external modulators.
works, where simple optical network unit is needed for the
customer [3]. A) Simulation results
The compact SOA-modulator can solve the optical subcar-
rier label swapping problem in serial label packet switched The SOA model uses a pair of coupled partial differential
all optical systems. The wavelength conversion and all- equations. The model takes into account the detailed non-
optical regeneration can be achieved through cross-phase linear carrier recombination rate.
modulation (XPM) performed in a SOA based active (1) R(N ) = A · N + B · N 2 + C · N 3
Mach-Zehnder interferometer. Current modulation of the
Here N , A, B, and C are the z dependent carrier density,
SOA in one or both arms of the wavelength converter is
the nonradiative, the radiative and the Auger recombina-
used to add the new label [4].
tion coefficient, respectively.
The operation of the multifunctional SOA-modulator is
based on the following phenomenon. The electrical bias
Received May 3rd, 2007. Revised August 29th, 2007.
current of the SOA is modulated, therefore the material
gain is modulated, and consequently in case of CW in- Department of Broadband Infocommunications, Budapest Uni-
versity of Technology and Economics (BUTE).
put the intensity of the output power is also modulated H1111, Goldmann ter 3, Budapest, Hungary.
[5]. If small signal sinusoidal current modulation is con- Fax: +36 14633289; E-mail: 1 udvary@mht.bme.hu;
sidered, the electrical signal consists of an invariant and 2
berceli@mht.bme.hu
Proceedings of the European Microwave Association Vol. 3; September 2007; 217–222LINEARITY AND CHIRP INVESTIGATIONS ON SOA AS AN EXTERNAL MODULATOR IN SCM SYSTEMS
The carrier density is obtained by solving the spatial de-
pendent rate equation, and the propagation of the electro-
magnetic field inside the amplifier is governed by solving
the wave equation. The time dependent amplifier’s out-
put power is calculated by solving numerically the cou-
pled rate and wave equations. There are two types of the
nonlinear distortion of the SOA [8]. The static distortion is
caused by the nonlinearity of the amplifier output power-
current curve under the CW condition. The dynamic dis-
tortion is caused by signal-induced carrier density modu-
lation. During the simulation the nonlinearity is character-
ized by using a single tone modulation. The static distor-
Fig. 3. Second and third order harmonics.
tion is calculated directly from the Fig. 1. The main objec-
tive is to select the most linear region of the curve over a
wide bias current range, and then place the operating point power increases, because of the saturation effect. The input
at the middle of this region. It is strongly dependent on the optical power won’t affect the relative value of harmonic
input optical power. product in case of the unsaturated situation, when the level
of the input optical power is very low. It can be acceptable
to SCM telecommunication optical systems [9].
In the beforegoing we supposed that the velocity of the
traveling microwave signal was matched exactly with that
of the optical signal. The next model applies a more re-
alistic situation where the current modulation propagates
with a speed different from the optical signal. The phase
velocity of the microwave is in the range of 8-12% of the
velocity of the light in vacuum for the frequencies in the
range of 5-40GHz [10].
Fig.4 and Fig.5 show calculation for harmonic products in
Fig. 1. Static distortion, optimal bias point of SOA. case of the typical co-propagating effect (n μ = 10) com-
With the optimal operation conditions, the calculated val-
ues of the static nonlinear distortions are less than the dy-
namic distortions, hence static distortions will not be taken
into account. The dynamic nonlinearity is calculated by
numerical analysis of the output optical power. Fig. 2 rep-
resents the signal levels for the fundamental (P1), the sec-
ond (P2) and the third (P3) order harmonic products. The
modulation and distortion products depend on the bias cur-
rent and the input optical power. Fig. 3 shows the sec-
ond and third order harmonic distortion as a function of
the modulation frequency for various input optical powers.
The nonlinearity can be improved when the input optical
Fig. 4. Mismatch between the microwave and the light.
Fig. 5. Mismatch between the microwave and the light propaga-
Fig. 2. Dynamic distortion products versus bias point. tion velocities.
218 Proceedings of the European Microwave AssociationE. UDVARY AND T. BERCELI
pared with the matched situation. The mismatch leads to versus SOA working state. In the first part of the graph
dips in the modulation response and reduces the modula- the gain of the device increases, hence the IP3 and the
tion bandwidth, but the bandwidth remains in the range of SFDR improve. In the second part the optical gain does-
10 GHz. n’t change significantly but the noise level rises, hence
the SFDR decreases. Finally, the intermodulation products
B) Experimental results also start rise and the degradation of the SFDR is faster.
The device ensures efficient SFDR for the general opti-
In the two-tone intermodulation experiments the SOA was
cal networks, because in personal communication systems
biased and modulated by the sum of two microwave sig-
72 − 83 d B · H z 2/3 spurious free dynamic range is re-
nals. The output noise (Pnoise) and signal levels were
quired.
measured for the fundamental (P1), the second (P2) and
the third (P3) order mixing products. For characterizing
the level of third order nonlinearity the third order inter-
cept point, IP3, or the spurious suppression in dBc is used.
When the nonlinearity is investigated together with noise
the figure of merit is the spurious free dynamic range,
SFDR. The determination of SFDR, IP2 and IP3 are pre-
sented in (2) and Fig. 6.
Fig. 7. Nonlinear behavior of SOA modulator.
The nonlinear behavior depends on several parameters
[11]. It is temperature sensitive, because the operation of
semiconductor devices depends on the temperature. The
Fig. 8 shows the SFDR and the IP3 versus temperature.
From the measurement results it is clear, that the linear-
ity decreases, when the temperature increases. The change
of SFDR and IP3 are about 2d B/H z 0.5 and 3 dB for 10C
temperature fluctuation.
Fig. 6. Determination of SFDR, IP2, IP3.
(2)
I P2[d Bm] = 2 · P1 [d Bm] − P2 [d Bm]
1
I P3[d Bm] = · (3 · P1 [d Bm] − P3 [d Bm])
2
Pin (P3 = noise) P1 (P3 = noise)
SFDR = =
Pin (P1 = noise) Pnoise
2
S F D R[d B] = · (I P3[d Bm] − Pnoise [d Bm])
3
All the measurement instruments were carefully checked
to have higher dynamic range and better linearity than Fig. 8. Nonlinearity depends on the Temperature.
the value expected from the SOA-modulator. During the
calculations 7% modulation depth was applied, because The noise effect and the nonlinear distortion products are
the modulation indices are usually less than 0.1 in typical more significant in case of strong optical reflection level,
SCM systems. However, it was checked for a wide range i.e. without optical isolators. In case of short distance the
of modulation depth (3-30%). level of optical reflection is usually determined by the
In the linear regime the SOA modulator shows low, not optical detector. Typical optical connectors (FC/PC) have
measurable nonlinearity because the noise generated by more than 40 dB return loss (RL) and low reflection con-
the SOA will dominate in the system. The intermodulation nectors (FC/APC) provide RL>70dB. By using optical
products overcome the noise floor in case of high modu- isolator the problem can be eliminated but its price is in the
lation indices. Fig.7 shows the noise level, IP3 and SFDR range of the laser transmitter. The system will be more in-
Proceedings of the European Microwave Association 219LINEARITY AND CHIRP INVESTIGATIONS ON SOA AS AN EXTERNAL MODULATOR IN SCM SYSTEMS
stable in case of strong optical reflection, and larger SFDR represents one of the most severe limitations to the max-
degradation can be observed as seen in the Fig. 9. imum attainable value of the length-bit rate product in
communication system links working at 1550 nm, unless
dispersion-shifted optical fibers are employed.
When the pump current of the laser amplifier is modulated,
the optical gain is affected in both magnitude and phase via
the modulation of the complex refractive index caused by
the electron density. Consequently, in SOA the optical sig-
nal becomes amplitude modulated (AM) and phase mod-
ulated (PM) caused by carrier density change. It is funda-
mental to know the behavior of the refractive index within
the active region. It can be modeled using the linewidth
Enhancement Factor (LEF=Henry factor=αfactor) approx-
imation. The LEF was originally defined as the ratio of the
Fig. 9. SFDR depends on the optical reflection. changes of the real to the imaginary part of the material
refractive index. For practical situations the definition can
The change of the SFDR is caused by two different effects. also be expressed using the real part of the refractive index
First the noise level of the device increases as a function of (n) and the material gain (g). LEF is a differential parame-
the bias point, the degradation is more significant without ter, hence we can calculate the variation of the single-pass
optical isolator (Fig. 10). On the other hand the level of the phase. The carrier density and the optical intensity are de-
nonlinear product will fluctuate in case of strong optical termined by the rate equations [13].
reflection (Fig. 11). Assuming that the carrier density change (N) is uniform
in SOA, for a pure traveling-wave amplifier (the facet re-
flectivity is ignored) the AM response becomes indepen-
dent of LEF, the PM response becomes proportional to
LEF, and the ratio of PM to AM reduces to LEF /2 [14].
G dg
AM = = · L · N
G dN
dk
P M = = − · L · N =
dN
(3) LEF dg
= · · L · N
2 dN
2·π dn dn dg
LEF = −2 · · ·
λin dN dN dN
Fig. 10. Noise level depends on the optical reflection.
where k is the wave number.
Measurements of LEF can be found in the literature and
have shown that LEF is not a mere constant factor, but it
is for instance a function of bias current, wavelength and
input optical power. To obtain the total phase variation of
the beam in a long SOA, we have to take into account the
longitudinal variation of LEF. To solve it we can divide
the active region into a large number of short sections. It
means a quasi ideal situation: constant carrier density (N)
along the active region of the section length. It follows that
the total amplitude and phase modulation:
M
dg j
AM = · L j · N j
j=1
dNj
Fig. 11. Nonlinearity depends on the optical reflection. (4)
M
1 dg j
PM = · LEF j · · L j · N j
j=1
2 dNj
III. Unwanted phase modulation Where L j , LEF j and N j are the length, the linewidth
Frequency chirping, that is the change in the instantaneous enhancement factor for semiconductor material and carrier
frequency of the optical signal produced by semiconduc- density variation of the active region in section j, respec-
tor lasers under pulsed or modulated operating conditions, tively.
220 Proceedings of the European Microwave AssociationE. UDVARY AND T. BERCELI In the unsaturated region the LEF value ranges from 2 to 7 is suppressed. Beside frequency modulation, however, this for GaAs and GaInAsP conventional lasers and from 1.5 to method does also reduce the amplitude of intensity modu- 2 for quantum well lasers. Therefore the external modula- lation of the SOA. Thus, near-pure AM can be obtained. tor using the same process as lasers gives almost the same In intensity modulation systems, in fact, the meaning- frequency chirping as the direct modulation [15]. ful quantity which affects the system performances is the However, as the optical input power increases, carrier de- ratio between frequency and intensity modulation effi- pletion occurs in SOA and this induces gain saturation. In ciencies. This is the chirping to power ratio C P R = optical amplifiers under saturation conditions, an increas- f /P( f andPbeing, respectively, the frequency and ing input intensity causes a decrease in the amplifier gain power deviations). (dG/dPin
LINEARITY AND CHIRP INVESTIGATIONS ON SOA AS AN EXTERNAL MODULATOR IN SCM SYSTEMS
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Eszter Udvary received the M.Sc. degree Tibor Berceli graduated in electrical engi-
in Telecommunication Engineering from the neering at the Technical University of Bu-
Budapest University of Technology and dapest. Later he received the Doctor of
Economics (BUTE), Hungary, in 1997 and Technical Science (D.Sc.) degree. He is
did her Master thesis in the area of mi- now Professor of Electrical Engineering.
crowave oscillators. She is currently a Ph.D. His present field of interest is the com-
candidate and her research is focused in bined optical-microwave systems. He initi-
the study of multifunctional semiconductor ated a new lightwave-microwave phase de-
optical amplifier application techniques in tector, and new mixing processes. He sug-
the department of Broadband Infocommuni- gested new approaches for optical millime-
cations and Electromagnetic Theory at the ter wave generation and sub-carrier optical
BUTE. Her research interests include optical and microwave commu- reception. Prof. Berceli is Fellow of IEEE. He is the author of 126 papers
nication systems, optical and microwave interactions and applications of and 6 books published in English. He presented 86 papers at interna-
special electro-optical devices. tional conferences. He was visiting professor at universities in the USA,
UK, Germany, Japan, France, Finland and Australia.
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