Improved linearization of laser source and erbium doped

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
– 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME
46
IMPROVED LINEARIZATION OF LASER SOURCE AND ERBIUM DOPED
FIBER AMPLIFIER IN RADIO OVER FIBER SYSTEM
Asish B. Mathews, Dr. Pavan Kumar Yadav
Optoelectronics Lab, B R A Bihar University
ABSTRACT
Radio over Fiber system is a heterogeneous network constituted by wireless and optical links.
This provides efficient provisioning for the simultaneous delivery of voice, data and video services
with mobility features to serve the fixed and mobile users in a unified networking platform. RoF
system jointly takes advantage of huge bandwidth offered by the optical communication systems and
flexibility provided by the wireless systems. But nonlinearity in RoF system influences negatively
the quality of signal by the introduction of harmonic and intermodulation distortions. Therefore it is
mandatory to reduce the distortion as low as possible. This paper discusses the reduction of nonlinear
distortion using the predistortion technique in optical source and feedforward technique in
succeeding erbium doped fiber amplifier. By introducing a predistortion circuit in front of the laser
source, the nonlinearity introduced by the laser diode can be reduced to a greater extent. Feedforward
technique is employed to reduce the nonlinear distortion in erbium doped fiber amplifier. These
effects are done individually and combined effect is also investigated in this paper.
Keywords: Linearization, Erbium Doped Fiber Amplifier, Radio over Fiber.
INTRODUCTION
The tremendous demand for broadband, interactive and multimedia services over wireless
media has been growing steadily for last few years. Over the last two decades, wireless
communications have gained enormous popularity. The proliferation of wireless services offers
attractive option for personal as well as organizational communication needs because of flexibility,
cost effectiveness and mobility. Wireless communication services are evolving rapidly in tandem
with developments and explosive growth of heterogeneous wireless access and network
infrastructures and their potential. Many new, next generation and advanced future services are being
conceived. In order to mitigate the bandwidth hungry on sophiscated wireless services, high
frequency carriers are used, thereby resulting in continuous increase in number of cells and
utilization of high frequency bands. This ultimately leads to large number of base stations to be
deployed; therefore, cost- effective base station is a key success in the market. In order to reduce the
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
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ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 4, Issue 5, July – August 2013, pp. 46-55
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system cost, Radio over fiber technology has been proposed since it provides functionally simple
base stations that are interconnected to a control station via. an optical fiber. The exciting features of
radio over fiber system has been listed as follows: (i) its transparency to bandwidth and modulation
techniques (ii) simple and cost effective base stations (iii) centralized operation is possible.
Radio over fiber is an innovative technology that provides efficient provisioning for a
communication technology that entails the use of optical fiber links to distribute radio frequency
signals from a central station to base station. In the current communication systems, the signal
processing functions such as frequency up-conversion, carrier modulation and multiplexing are
performed at the base stations and immediately fed to the antenna. RoF make it possible to centralize
all the radio frequency signal processing functions under one shared location (central station), and
then to use the optical fiber, which has the advantage of about 0.3 dB/km for 1550 nm and 0.5 dB/km
for 1330 nm wavelengths, to distribute the radio frequency signals to the base stations. As the base
stations are attributed to perform the operation of optical to electrical conversion and amplification
only, these are simplified significantly. Apart from this, the centralization of signal routing functions
enable operational flexibility, sharing and dynamic resource allocation and reduced power
consumption. That is, Radio over fiber consists of both radio link and optical link. Radio link exist
between base station and mobile station, while optical link exist between central station.
Fig: 1. Basic Radio over fiber system
The commonly employed methods for optically distributing RF signals are: (i) Intensity
Modulation (ii) Modulation through external means (iii) Remote heterodyning. In intensity
modulation, an electrical parameter of the light source is modulated by once modulated RF signal,
which bears the information. In practical optical links, this electrical parameter is the current of the
laser diode, serving as the optical transmitter. The second method make use of an unmodulated light
source and an external light intensity modulator such as Mach Zehnder or Electro-Absorption
modulator. In the third method, more than one optical signal is generated by the light source, one of
which is modulated by the information bearing signal and theses are heterodyned in an external
mixer to form the output transmitted signal. Direct intensity modulation is empolyed in this work.
However there exist non-linearity in laser source and succeeding semiconductor amplifier
stages, which may arise due to improper fixing of operating bias point, temperature variations,
Inhomogenity in active region and the spontaneous recombination of electrons and holes in the

48
amplifier medium. These non linearities may reflect on the amplitude and phase of modulated signal
and hence are to be reduced below a particular threshold. To meet this stringent objective, two
innovative techniques are employed in this journal: (a) Predistortion technique in optical laser
source (b) Feedforward technique in Erbium Doped Fiber Amplifier. By employing frequency
profile shaping filters, The nonlinearity introduced by the lasing media can be reduced to a greater
extent. The use of feedforward technique in EDFA may improve the signal-to-noise ratio to a
remarkable level through the cancellation of error signals.
PREDISTORTION IN OPTICAL SOURCE
The stimulated emission allows laser source to produce intense high powered beam of
coherent light. Stable atoms occupy lowest energy levels. When an atom absorbs energy, it get
excited to higher energy level, where the atom is in a state of instability and quickly return back to
the ground state after releasing a photon.
Fig. 2: The General Structure of a Laser
Radio over Fiber (RoF) system shall be treated as a convenient method for the distribution of
radio frequency signal to a distant user. Today, Radio over Fiber technology has been established as
a primary solution in extending radio cellular coverage. For wireless communications, it offers
several advantages: capability of changing carrier frequencies and modulation formats, enhanced
micro-cellular coverage and low cost base station deployment. RoF technology offers the benefits of
versatility, flexibility, and scalability along with low cost [3]. RF signals are transmitted through
optical fiber by using three different methods: direct modulation, external modulation and remote
heterodyning. The direct modulation is the simplest solution and is used in this work. Here the input
current of laser is modulated by the information bearing RF signal. Distortions are introduced into
the laser due to the nonlinear relationship between input current and optical power of laser. It is
mandatory to keep the distortion below a certain level since linearity is an important constraint in the
design of analog light wave transmission system. Any device nonlinearity will create frequency
components in the output signal that were not present in input signal. If the input signal to a
nonlinear device is a simple sine wave that is given by x(t) = A sin(ωt), the output will be
y(t) = A0 + A1 sin(ωt) + A2 sin(2ωt) + A3 sin(3ωt)………………….. (6.1)
That is, the output signal will consist of a component at the input frequency and spurious
components at zero frequency, at the second harmonic frequency 2ω, at the third harmonic frequency

49
3ω and so on. This effect is known as Harmonic distortion. In order to determine the intermodulation
distortion, the modulating signal of a nonlinear device is taken to be the sum of two sine waves. This
signal includes all the harmonics of ω1 and ω2 plus cross product terms such as ω1±ω2, 2ω1±ω2,
2ω2±ω1 and so on. The sum and difference frequency give rise to intermodulation distortion.
The important causes of nonlinearity in laser diode are operating bias point, temperature
variations, Inhomogenity in active region, nonlinear energy exchanges and leakage current.
(i) Operating bias point: The operating bias point must be chosen in a linear region for precise RF
signal transfer into optical domain.
(ii) Temperature variations: Laser diodes are highly sensitive to temperature variations. An increase
in temperature will increase the threshold current and brings a drop in output power.
(iii) Inhomogenities in active region: Active region is the light emitting region of laser diode. The
size of active region is very small ranging from few micrometers to few nanometers. Therefore, high
precision is needed for the fabrication of active region.
(iv) Leakage current: Leakage current increases with temperature, causing nonlinearity in laser.
Nonlinear effects are introduced by the laser diode creates harmonic distortions (HD) and
intermodulation distortions (IM). As a consequence, the quality of the radiated signal at the remote
antenna can decrease significantly. Predistortion is an effective solution to linearize laser diode,
which is affected by the harmonic and intermodulation distortions. The amplitude and phase
nonlinearity of laser diode are compensated by the predistortion circuitry. The predistorter circuitry
is introduced before the laser diode to generate frequency components which are equal in amplitude,
but opposite in phase to the undesired frequency components caused by laser diode nonlinearity [13].
The predistorter has a transfer function with gain expansion that is inverse of laser diode
compression, and a phase rotation that is negative of laser diode phase rotation. The LD predistorter
should be integrated with RoF transmitter in order to reduce the complexity as low as possible. Fig.
6.1 shows the laser predistortion principle which is used to linearize the laser diode.
Fig. 3: Laser Predistortion Principle
To reiterate, the principle of predistortion is to create direct proportionality between input
signal and optical output. The fig.3 shows the basic principle. The basic principle is that when an
expansive block is cascaded with compressive block, the overall nonlinearity of the laser can be
reduced to an appreciable amount. For different frequencies, the nonlinear behavior also changes.
Therefore predistortion transfer function should be adjusted for different frequencies. The block
diagram of laser predistortion is shown in fig 4. The signal as such is passed through the top path
with a small time delay. X, X2, X3 blocks provide linear, second order and third order paths. A
suitable predistorter circuit topology, based on the scheme, is identified by choosing a configuration
characterized by two independent nonlinear channels that generate the second and third order
correction signals [14]. These are subsequently combined with the delayed transmit signal to
intensity modulate the laser diode. The compensation scheme is made such as to reproduce the laser
diode second and third order harmonic distortion frequency curves, while being in phase opposition.
This predistorter is applied to the LD model, simulating the entire system. The frequency profile
shaping filters G1 (s), G2 (s) and G3 (s) adjust each term to achieve the compression and time delay

50
for different frequencies. Due to the nonlinear nature, this third order predistortion circuit needs to
work up to three times the carrier frequency, which imposes great difficulty on circuit design. By
suitable shaping in a frequency range as wide as possible, the simulation showed that a broadband
compensation of both harmonic and intermodulation distortions (IM2 and IM3) can be obtained.
Fig. 4: Block diagram of Laser Predistortion
It is foreseeable that the frequency range where the distortions are effectively counteracted
can be extended at least to the entire GSM (900MHz) band. However, the approach and the
techniques here illustrated can be extended to the UMTS field, including the compensation of higher
odd-order distortions.
FEEDFORWARD TECHNIQUE IN ERBIUM DOPED FIBER AMPLIFIERS
In optical systems, a wide range of bandwidth can be used for communication; this makes
optical communication popular in modern communication systems. The most important necessities
of modern society are higher bandwidth, higher data rate, increased signal quality without
information loss, higher secrecy and low power requirement. This leads to the deployment of Radio
over Fiber system. In a long transmission system, optical amplifiers are needed to periodically
restore the power level after it has decreased due to attenuation in the fiber. Nonlinearity in optical
amplifiers degrades the performance of the system. In order to reduce this, feedforward technique is
used in optical amplifiers since there is a strong demand for a linearizer that has a simple optical
circuit configuration, a minimal control circuit and large improvement in the reduction of harmonic
and intermodulation distortions. In order to increase the linearity performance of the system, a
linearization technique is used. Feedforward has a better linearization performance and provides a
more wideband stable operation since there is no feedback path.
The basic block diagram of feedforward technique is shown in Fig. 5. The optical signal at
the input of optical amplifier is split into two identical paths. The signal at the top path is amplified
by the main power amplifier. Because of the nonlinearities present in the main power amplifier,
harmonic and intermodulation distortions are being added to the original signal. The block diagram
that is shown in fig.5 reveals the model of RoF system which utilizes feedforward technique at the
optical amplifier in order to linearize its output. The nonlinearities present in optical amplifiers
results in harmonic and intermodulation distortions [5].
The output of the main amplifier, which contains the distortions, is split by the first
directional coupler (C1) to reach the subtractor. The other end of subtractor is fed by the input signal
of optical amplifier which is referred to as reference signal that comes from the beam splitter that is

51
placed before the optical amplifier. The output of the subtractor will be the distortion caused due to
the nonlinearity in optical amplifier, which is referred to as error signal. This error signal is amplified
by means of an error amplifier and fed to the second coupler. The signal from the top end after a time
delay, reaches the other end of the coupler so that the distortion products are cancelled at the output
of the second coupler. The main path signal through coupler 1 is time delayed by an amount equal to
the time delay through error amplifier and fed to the output coupler. The amplified error signal is fed
in antiphase to the original input signal, so that amplified original input signal will be the result [7].
The optical feed forward linearization system is made up two loops: error-determination loop and
distortion cancellation loop. The unwanted distortion signals add in antiphase, resulting in an output
with suppressed distortion products [6]. Both error-determination and distortion cancellation loops
need amplitude and phase matching for cancellation of the distorted signal at the optical coupler to
yield better results.
The dominant noise generated in Erbium Doped Fiber Amplifier is Amplified Spontaneous
Emission (ASE) due to the spontaneous recombination of electrons and holes in the amplifier
medium [7]. The ASE noise generated during amplification process is added to the signal leading to
decrease the signal to noise ratio at the amplifier output. The other two noises arise from the mixing
of the different optical frequencies contained in the light signal and the ASE, which generates two
sets of beat frequencies. Because of the nonlinearities present in the amplifier, harmonic and
intermodulation products are being added to the original signal. The figure 6.5 shows the basic block
diagram of feedforward technique in EDFA.
Fig.5: Block diagram of Feedforward technique in EDFA
In this work, for measuring the harmonic distortion arising from the EDFA, a single-tone
signal of 2GHz is applied as the input. The EDFA output is detected by means of a photo detector.
The second and third order harmonic distortions can be measured practically by use of a spectrum
analyzer. The relation between the highest output power Psout, launched pump power
Pp-i-n and input signal power Psin can be analytically obtained as follows:

52
where, γ is the ratio of pump light frequency to that of signal, Pth is the threshold pump power
and αs and αp are the signal and pump absorption coefficients respectively [5]. The term
{(γ/ε)(Psin/Pth)} is the normalized input signal power, {(γ/ε)(Psout/Pth)} is the normalized highest
output signal power and {Pp-i-n/Pth} is the normalized launched pump power. From the three
parameters mentioned above, we can also estimate the possible output power using these figures,
even in the gain saturated region. The parameters used in simulations are αp = 0.076dB/km, αs =
0.175dB/km, Pth = 7.424mW, Pp-i-n = 50mW, ε = 0.6 and γ = 0.954. A basic design for EDFA which
require a high power output can be accomplished using these figures and the three basic parameters.
In order to obtain high output power, a large input signal power and a large pump power are
effective.
Combined Effect of both Predistortion and Feedforward Techniques in EDFA
The laser sources and optical amplifiers are nonlinear devices. Both of these introduce
distortions in the system. By suitable shaping in a given frequency range, predistorter circuitry
compensates both harmonic and intermodulation distortions in laser while feedforward technique in
optical amplifier. The combined effect reduces the overall nonlinearity to a greater extent.
Fig. 6: Block diagram of the combined effect of both the techniques.
RESULTS AND COMPARISON
1. Predistortion Technique in Laser
The laser in Radio over fiber is modeled and then a suitable predistorter circuit topology is
made by choosing a configuration characterized by two independent nonlinear channels that generate
second and third order correction signal. These are subsequently combined with the delayed transmit
signal to intensity modulate the Laser Diode. Here simulations are carried out with a QPSK
modulated RF signal of 2 GHz frequency, modulating the laser. The Fig 7 shows the result when a
simple sinusoidal signal is used as the input to the laser. There is an improvement of 40% to second
order distortion and 30% to third order distortion. The Fig.8 shows the result when a QPSK signal is
used as the input to laser. There is an improvement of 30% to second order distortion and 20% to
third order distortion.

53
Fig. 7: Simulation of Predistortion principle using single tone input
Fig. 8: Simulation of Predistortion principle using QPSK input

54
Feedforward technique in EDFA
Fig. 9: Simulation of Feedforward technique in EDFA using sinusoidal input
The effect of feedforward technique in EDFA is verified by applying a single tone signal of
2GHz as input. On simulation, there is an improvement of 39dBc for second order and 42dBc for
third order distortions.
7.4. Combined Effect of both the Techniques
Fig. 10: Distorted and corrected signal using the combined effect of Predistortion and Feedforward
in EDFA

55
CONCLUSION AND FUTURE WORK
The effect of predistortion in laser, feedforward in semiconductor optical amplifier and
feedforward in Erbium Doped Fiber Amplifier used in Radio over Fiber system has been investigated
individually. Applying predistortion alone, there is an improvement of more than 40% for second
order distortions and 30% for third order distortions. The effect of feedforward in EDFA are also
verified using single tone input. EDFA produced an improvement of 39dBc for second order and
42dBc for third order distortions. . The combined effect of predistortion in laser and feedforward in
EDFA showed a performance enhancement of better than 30% to second order and 10% to third
order distortions. On comparison of the techniques, it came to the conclusion that combined
technique is most suitable. Here, simulations are done with a QPSK modulated RF signal.
Future works may include GSM, and WLAN systems. In the future work, the frequency
range where the distortions are effectively counteracted can be extended at least to the entire GSM
(900MHz) band. However, the approach and the techniques here illustrated can be extended to the
UMTS field, including the compensation of higher odd order distortions.
REFERENCES
[1] P Shen, A. Nkansah., “Multilevel Modulated Signal Transmission for Millimeter-Wave Radio
over Fiber System,” University of Kent, UK, 2008 IEEE.
[2] Javier Harrera, Francisco Ramos, and Javier Marti, “Nonlinear distortion generated by
semiconductor optical amplifier booster in analog optical systems” OPTICS LETTERS /
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[3] Bao Linh Dang and Ignas Niemegeers, “Analysis of IEEE 802.11 in Radio over Fiber Home
Networks,” Proceedings of the IEEE Conference on Local Computer Networks, 2005.
[4] Eszter Udvary, Tibor Berceli, Marek Chacinski, Richard Schatz, Pierre-Yves Fonjallaz,
“Reduction of Dispersion Induced Distortions in Radio over Fibre links,” Proceedings of the
38th European Microwave Conference, October 2008.
[5] Tabassam Ismail, Chin-Pang Liu, John E. Mitchell, “High-Dynamic-Range Wireless-Over-
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November 2007.
[6] A. H. Coskun and S. Demir, “A mathematical characterization and analysis of a feedforward
circuit for CDMA applications,” IEEE Trans. Microw. Theory Tech., vol.14, no.3, pp.767
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[7] Optical Fiber Communication, 4th Edition, Gred Keiser, Tata Mc. Graw-Hill Ltd. 2008.
[8] N. Pothencary, “Feedforward Linear Power Amplifiers”, Norwood, MA: Artech House, 2000.
[9] Fiber Optic Communication, Govind P. Agrawal, New York, Willey, 2002.
[10] S.K Mohapatra, R. Bhojray and S.K Mandal, “Analog and Digital Modulation Formats of
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