- Email: [email protected]
Pre-amp EDFA ASE noise minimization for optical receiver transmission performance optimization
Optics Communications 283 (2010) 2603–2606
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Pre-amp EDFA ASE noise minimization for optical receiver transmission performance optimization Akram Abu-aisheh, Saeid Moslehpour * Department of Electrical and Computer Engineering, University of Hartford, 200 Bloomfield Ave., West Hartford, CT 06117, USA
a r t i c l e
i n f o
Article history: Received 20 May 2009 Received in revised form 15 February 2010 Accepted 15 February 2010
a b s t r a c t Amplified spontaneous emission (ASE) noise mitigation from the pre-amp erbium-doped fiber amplifier (EDFA) to the photon detector (PD) in optical receivers can be reduced by optimizing the EDFA at the optical receiver level to achieve optimal optical receiver transmission performance. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Amplified spontaneous emission (ASE) Erbium-doped fiber amplifier (EDFA) Transmission performance and optical receivers
1. Introduction The basic design of an optical receiver consists of an EDFA, an optical band pass filter, a photon detector, and an electrical low pass filter. Pre-amp EDFAs are becoming an integral part of optical receivers since their performance is interrelated to the performance of the receiver photon detector. The photon detector used in optical receivers is either a PIN photodiode or an avalanche photodiode (APD). APDs have higher sensitivity than PIN diodes, but they exhibit excess noise that degrades the optical receiver transmission performance. PIN diodes have better noise performance than APDs. So the best optical receiver transmission performance can be obtained by using a combination of a pre-amp EDFA for good sensitivity and a PIN photon detector for low noise. The generation of noise in doped optical pre-amplifiers is an effect of the spontaneous de-excitation of the laser ions. As the electrons have a finite excited state lifetime, some of the electrons return spontaneously to the ground state-emitting a photon. This photon has no coherence characteristics with respect to the incoming light signal, as opposed to a photon generated by stimulated emission. The collection of such spontaneously generated photons, being multiplied by the fiber amplifier, forms a background noise. This background noise is known as amplified spontaneous emission [1]. This is the dominant noise element in pre-amp EDFAs. The optical noise elements of a pre-amp EDFA at different input
power, output power, and different signal wavelengths and their effects on the EDFA transmission performance can be analyzed by evaluating the contributions of the EDFA ASE noise generated in the process of signal amplification. Pre-amp EDFA operation is based on stimulated emission of optically pumped Er + 3 ions in silica. Erbium atomic structure and the 3-level atomic level rate equations, for the case of a single stage 980 nm pumped EDFA, were analyzed to help characterize the pre-amp ASE noise. This should help in analyzing the optical receiver transmission performance degradation under different pre-amp operating conditions. The most fundamental limitation to the gain of an erbium-doped fiber amplifier is the energy conservation principle. This principle can be expressed in terms of the photon flux. The photon flux of the output signal cannot exceed the photon flux of the input signal plus the laser pump photon flux [1]. Since modern receivers use a narrow optical band pass filter at the output of their pre-amp EDFAs, the pump shot noise is filtered in this filter. So pump noise does not play a factor in the pre-amp EDFA transmission performance; however, the pump power affects transmission performance from its contribution to ASE. The most basic treatment of erbium-doped fiber amplifier noise is by analyzing a 3-level erbium atomic system. So the erbium atomic structure needs to be understood to analyze pre-amp noise performance. 2. Erbium atomic structure
* Corresponding author. Address: University of Hartford, College of Engineering Technology and Architecture, Department of Electrical and Computer Engineering, UT 221 West Hartford, CT 06117, USA. Tel.: +1 860 768 4211; fax: +1 860 768 5073. E-mail addresses: [email protected] (A. Abu-aisheh), moslehpour@yahoo. com, [email protected] (S. Moslehpour). 0030-4018/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.02.038
The erbium atomic structure has three energy levels that are of interest for the study of its amplification characteristic for communication use. In 3-level erbium atomic structure, population
2604
A. Abu-aisheh, S. Moslehpour / Optics Communications 283 (2010) 2603–2606
inversion can be achieved using laser pumping at 980 nm to excite electrons to the upper erbium atomic state. When excited to the upper state, electrons rapidly decay non-radioactively to the meta-stable state. If electrons in the meta-stable state are not stimulated within the electron lifetime in that state, electron transition to the lower states results in spontaneous emission. Spontaneous emission is a random emission that introduces noise. The behavior of this erbium-doped fiber atomic structure is described in the following level rate equations [2]:
dN 3 N3 þ ðN 1 N3 Þ rP SP ¼ dt s32 dN 2 N2 N3 þ ðN2 N1 Þ rS SS ¼ dt s21 s32 dN 1 N 2 ¼ ðN1 N3 Þ rP SP þ ðN2 N1 Þ rS SS dt s21
ð1Þ ð2Þ ð3Þ
where N is the population density at the given level [1/cm3], S is the photon flux [1/cm2 * s], s is the spontaneous lifetime [s], and a is the transition cross section [cm2]. The first equation describes the population change rate for the upper state, the second equation describes the population change rate for the meta-stable state, and the third equation describes the population change rate for the ground state. The steady state atomic populations N1 and N2 are functions of the pumping rate which represents the pump absorption rate, between levels 1 and 3, and of the absorption and stimulated emission rates between levels 1 and 2. Fig. 1 shows the 3-level erbium atomic structure, and it shows the level transitions when erbium is used in a single stage 980 nm pumped pre-amp EDFA [3]. The sum of the population in the three states of the erbium atomic structure is equal to the total population. The sum of the population in the three states is equal to the total population.
N ¼ N1 þ N2 þ N3
ð4Þ
Under a steady state condition, electron transition is given by
f 12 ¼
E1 E2 h
ð5Þ
L3 (Excited State)
L3 to L2 Atomic Transition (T 32 = 1us) L2 (Meta-Stable State)
980 nm Pumping hvp
Amplified Spontaneous Emission (Tsp =10 ms) Amplified Signal hvs L1(Ground State)
Fig. 1. Pumping (980 nm) in erbium atomic structure.
ð6Þ
Here, h is Plank’s constant = 6.626 10E-34 [J/s]. Stimulated photons are in coherence with the input signal, and that results in signal amplification. In free space, the radiation wavelength is given by
k21 ¼ hc=ðE2 E1Þ
ð7Þ
When this radiation interacts with a photon in the lower energy level, the photon is transformed into the upper atomic level. If a photon in the excited state is not stimulated within the 10 ms lifetime of the excited state, it will spontaneously decay to the ground state producing ASE. When this photon travels through the erbiumdoped fiber, it gets amplified, resulting in amplified spontaneous emission. All the excited ions can spontaneously relax from the upper state to the ground state by emitting a photon that is unrelated to the signal photons. This spontaneously emitted photon can be amplified as it travels down the fiber and stimulates the emission of more photons from excited electrons. Amplified spontaneous emission can occur at any frequency within the fluorescence spectrum of the amplifier transitions. The dominant noise source in any EDFA is amplified spontaneous emission. This spontaneous emission reduces the amplifier gain by consuming the photons that would otherwise be used for stimulated emission of the input signal. In single mode fiber, the noise output power resulting from amplified spontaneous emission is given by [5]
PASE ¼ 2 nsp ðG 1Þhv Dðv Þ
dN1 dN2 dN3 ¼ ¼ ¼0 dt dt dt
Input Signal
The basic principle of signal amplification in erbium-doped fiber is based on the fact that when an optical signal passes through erbium-doped fiber, the signal is amplified due to stimulated transition between electronic states in the presence of electromagnetic radiation at the correct wavelength to achieve population inversion. In order for signal amplification to happen [4], a frequency f12 is needed where
ð8Þ
The total amplified spontaneous emission at any point in the fiber is the sum of all amplified spontaneous emission power from the previous sections in the fiber and the amplified spontaneous emission at the given fiber point. In order to minimize ASE noise, the pump power should be just enough to achieve population inversion. Population inversion can be achieved when the population in the excited state, N2, is greater than the population in the ground state N1. The threshold pump power required to achieve population inversion can be obtained by setting the rate equation of level 2–0, and setting N1 to be equal to N2. A long meta-stable state lifetime and a large absorption cross section are needed to have a low pump threshold to achieve population inversion. A detailed analysis of EDFA and photodiodes noise elements was performed by different researchers [6,7]. When the EDFA ASE noise is converted to an electrical signal in the photodiode, it gives rise to signal-spontaneous beat noise and spontaneous–spontaneous beat noise. When pre-amps are used in the optical receiver, signal-spontaneous beat noise dominates all other noise elements in the photon detector [8]. 3. Optical receiver transmission performance Optical receiver transmission performance, commonly known as bit error rate (BER) performance, is the gauge by which optical receivers are characterized. It characterizes the ability of the receiver to perform up to the transmission performance specifications under the same test conditions as those where the receiver operates in the field [3]. So transmission performance will be used to analyze the optical receiver performance under different operating
2605
A. Abu-aisheh, S. Moslehpour / Optics Communications 283 (2010) 2603–2606
OSA
DCA
Optical Switch
EDFA
90/10
O/E
90/10
LPF
Optical Atten. #3
BPF
ED ED
CW Laser
Optical Atten. #1
Combiner
E/O Optical Atten. #2
PPG PPG
Noise Source
Fig. 2. Optical receiver transmission performance test setup.
Table 1 Optical receiver performance at different input and output powers. System BER (dBm)
Pin = 24 (dBm)
Pin = 22 (dBm)
Pin = 20 (dBm)
Pout = 11.5 Pout = 10.5 Pout = 9.5 Pout = 8.5 Pout = 7.5
4.30E-08 4.00E-09 3.60E-10 1.80E-10 1.40E-10
8.90E-09 7.40E-10 1.40E-10 7.60E-11 6.50E-11
3.10E-09 3.20E-10 6.20E-11 4.80E-11 4.60E-11
power, output power, and operating wavelength should be taken into account. This allows designers to choose the right erbiumdoped fiber length and pump power combination, and it helps minimize amplified spontaneous emission at the output of the pre-amp EDFA. Since the most important factor in pre-amp performance is how well it performs in the optical receiver, optical receiver optimal transmission performance analysis under different operating conditions is the ultimate method for optimizing pre-amp EDFA performance in the optical receiver. The pre-amp EDFA design needs
Pre-amp Performance at Different Pin and Put Values
-6.00 -7.00 L0g10(BER)
conditions. The setup of Fig. 2 was used to perform the optical receiver tests analyzed in this paper. This setup has the ability and flexibility to perform optical receiver transmission performance measurement, optical input and output measurements, optical signal and noise control, and optical input SNR control. Since the most important factor in the pre-amp performance is how well it performs in the optical receiver, the optical receiver optimal transmission performance analysis under different operating conditions is the ultimate method for characterizing pre-amp EDFA noise performance. The pre-amp EDFA design needs to be optimized at the pre-amp level and the EDFA level. Then the pre-amp performance should be determined by how well the pre-amp performs in the optical receiver. For optimal optical receiver transmission performance, the preamp EDFA design must be coordinated with the photon detector design in order to minimize amplified spontaneous emission noise mitigation from the pre-amp EDFA to the photon detector and the photon detector signal-spontaneous beat noise. The pre-amp input
-8.00 -9.00
Pin=-24dbm Pin=-22dbm
-10.00
Pin=-20dbm
-11.00
-11.5 -10.5 -9.5 -8.5 -7.5 Output Power dBm
Fig. 3. Optical receiver performance change at different input and output power levels.
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Table 2 Ber at different input powers and signal to noise ratios. System BER (dBm)
SNR = 8 (dB)
SNR = 9 (dB)
SNR = 10 (dB)
SNR = 11 (dB)
SNR = 12 (dB)
Pin = 28 Pin = 27 Pin = 26 Pin = 25
2.5E-06 7.7E-07 2.3E-07 8.5E-08
3.9E-07 8.2E-08 2.0E-08 4.9E-09
7.6E-08 1.3E-08 1.6E-09 3.1E-10
1.2E-08 9.5E-10 9.0E-11 1.3E-11
2.3E-09 1.2E-10 7.6E-12 6.0E-13
Pre-amp Noise Loaded Performance -4 -28 dBm
-27 dBm
-26 dBm
-25 dBm
-6
Log10 (BER)
SNR=8 dB -8
SNR=9 dB SNR=10 dB
-10
SNR=11 dB -12
Table 2 show that the optical receiver transmission performance improves as the pre-amp input signal power is increased. A graphical representation of the system transmission performance, after normalizing BER, is given in Fig. 4. At the atomic structure level, an increase in the input power causes more stimulated emission of the excited electrons. This leaves fewer electrons to move to the ground state spontaneously. This means that the pre-amp is generating less amplified spontaneous emission, and this reduces the signal-spontaneous noise in the photon detector which results in improved optical receiver transmission performance.
SNR=12 dB
5. Conclusion -14
Input Power (dBm) Fig. 4. Optical receiver performance change at different input powers and different input signal to noise ratios.
to be optimized at two levels: the pre-amp/photon detector subsystem level and the optical receiver level. Several characterization experiments were performed to analyze the effects of changing the pre-amp operating conditions on the optical receiver transmission performance. Testing the preamp-based optical receiver at a fixed signal to noise ratio at 1550 nm, the transmission performance was recorded at different input/output combinations. The optical system transmission performance results are given in Table 1. 4. Different input and output powers A graphical representation of the optical system transmission performance results, after normalizing BER, is given in Fig. 3. From the results in Fig. 3, we see that the optical receiver transmission performance improves as the pre-amp output power is increased. This improvement is due to the fact that more output power requires more pump output, and more output power excites more electrons to the upper state. This excitation will result in the population inversion that is needed for the amplification process. Testing the pre-amp-based optical receiver at different input powers and at different signal to noise ratios at fixed output power and input signal wavelength, the transmission performance changes due to the changes in the operating conditions were monitored, and the results are given in Table 2. The results given in
Since the most important performance factor in pre-amp performance is how well it performs in the optical receiver, the optical receiver optimal transmission performance analysis under different operating conditions is the ultimate method for characterizing pre-amp EDFAs noise performance. The results of the tests performed for this work shows a need for designing the pre-amp EDFA and photon detector as one subsystem. Then the pre-amp EDFA needs to be fine-tuned at the optical receiver level to achieve optimal optical receiver transmission performance. For optimal optical receiver transmission performance, the preamp EDFA design must be coordinated with the photon detector design in order to minimize amplified spontaneous emission noise mitigation from the pre-amp EDFA to the photon detector. Doing so will minimize the photon detector signal-spontaneous beat noise. Both the pre-amp EDFA and the photon detector should be treated as one block during the design stage. References [1] E. Desurvire, Erbium-Doped Fiber Amplifiers, Principles and Applications, Wiley, NY, 1994. [2] J.T. Verdeyen, Laser Electronics, third ed., Prentice Hall, 1995. [3] A. Abu-aisheh, Pre-amp EDFA Noise Characterization for Optimal Optical Receiver Transmission Performance, Ph.D. Dissertation, Dept. of Elec. Eng. Florida Institute of Technology, Melbourne, FL, 2003. [4] J. Gowar, Optical Communication Systems, second ed., Prentice Hall Inc., 1993. [5] P.C. Becker, N.A. Olsson, J.R. Simpson, Erbium-Doped Fiber Amplifiers, Fundamentals and Technology, Academic Press, NY, 1999. [6] R. Tucker, H. Kingston, Optical Sources Detectors and Systems Fundamentals and Applications, Academic Press, Inc., 1995. [7] R.S. Tucker, D.M. Baney, Optical Noise Figure: Theory and Measurement, OFC, Anaheim, CA, 2001. [8] S. Soerensen, J. Light Wave Technol. 18 (10) (2000).
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Pre-amp EDFA ASE noise minimization for optical receiver transmission performance optimization Akram Abu-aisheh, Saeid Moslehpour * Department of Electrical and Computer Engineering, University of Hartford, 200 Bloomfield Ave., West Hartford, CT 06117, USA
a r t i c l e
i n f o
Article history: Received 20 May 2009 Received in revised form 15 February 2010 Accepted 15 February 2010
a b s t r a c t Amplified spontaneous emission (ASE) noise mitigation from the pre-amp erbium-doped fiber amplifier (EDFA) to the photon detector (PD) in optical receivers can be reduced by optimizing the EDFA at the optical receiver level to achieve optimal optical receiver transmission performance. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Amplified spontaneous emission (ASE) Erbium-doped fiber amplifier (EDFA) Transmission performance and optical receivers
1. Introduction The basic design of an optical receiver consists of an EDFA, an optical band pass filter, a photon detector, and an electrical low pass filter. Pre-amp EDFAs are becoming an integral part of optical receivers since their performance is interrelated to the performance of the receiver photon detector. The photon detector used in optical receivers is either a PIN photodiode or an avalanche photodiode (APD). APDs have higher sensitivity than PIN diodes, but they exhibit excess noise that degrades the optical receiver transmission performance. PIN diodes have better noise performance than APDs. So the best optical receiver transmission performance can be obtained by using a combination of a pre-amp EDFA for good sensitivity and a PIN photon detector for low noise. The generation of noise in doped optical pre-amplifiers is an effect of the spontaneous de-excitation of the laser ions. As the electrons have a finite excited state lifetime, some of the electrons return spontaneously to the ground state-emitting a photon. This photon has no coherence characteristics with respect to the incoming light signal, as opposed to a photon generated by stimulated emission. The collection of such spontaneously generated photons, being multiplied by the fiber amplifier, forms a background noise. This background noise is known as amplified spontaneous emission [1]. This is the dominant noise element in pre-amp EDFAs. The optical noise elements of a pre-amp EDFA at different input
power, output power, and different signal wavelengths and their effects on the EDFA transmission performance can be analyzed by evaluating the contributions of the EDFA ASE noise generated in the process of signal amplification. Pre-amp EDFA operation is based on stimulated emission of optically pumped Er + 3 ions in silica. Erbium atomic structure and the 3-level atomic level rate equations, for the case of a single stage 980 nm pumped EDFA, were analyzed to help characterize the pre-amp ASE noise. This should help in analyzing the optical receiver transmission performance degradation under different pre-amp operating conditions. The most fundamental limitation to the gain of an erbium-doped fiber amplifier is the energy conservation principle. This principle can be expressed in terms of the photon flux. The photon flux of the output signal cannot exceed the photon flux of the input signal plus the laser pump photon flux [1]. Since modern receivers use a narrow optical band pass filter at the output of their pre-amp EDFAs, the pump shot noise is filtered in this filter. So pump noise does not play a factor in the pre-amp EDFA transmission performance; however, the pump power affects transmission performance from its contribution to ASE. The most basic treatment of erbium-doped fiber amplifier noise is by analyzing a 3-level erbium atomic system. So the erbium atomic structure needs to be understood to analyze pre-amp noise performance. 2. Erbium atomic structure
* Corresponding author. Address: University of Hartford, College of Engineering Technology and Architecture, Department of Electrical and Computer Engineering, UT 221 West Hartford, CT 06117, USA. Tel.: +1 860 768 4211; fax: +1 860 768 5073. E-mail addresses: [email protected] (A. Abu-aisheh), moslehpour@yahoo. com, [email protected] (S. Moslehpour). 0030-4018/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.02.038
The erbium atomic structure has three energy levels that are of interest for the study of its amplification characteristic for communication use. In 3-level erbium atomic structure, population
2604
A. Abu-aisheh, S. Moslehpour / Optics Communications 283 (2010) 2603–2606
inversion can be achieved using laser pumping at 980 nm to excite electrons to the upper erbium atomic state. When excited to the upper state, electrons rapidly decay non-radioactively to the meta-stable state. If electrons in the meta-stable state are not stimulated within the electron lifetime in that state, electron transition to the lower states results in spontaneous emission. Spontaneous emission is a random emission that introduces noise. The behavior of this erbium-doped fiber atomic structure is described in the following level rate equations [2]:
dN 3 N3 þ ðN 1 N3 Þ rP SP ¼ dt s32 dN 2 N2 N3 þ ðN2 N1 Þ rS SS ¼ dt s21 s32 dN 1 N 2 ¼ ðN1 N3 Þ rP SP þ ðN2 N1 Þ rS SS dt s21
ð1Þ ð2Þ ð3Þ
where N is the population density at the given level [1/cm3], S is the photon flux [1/cm2 * s], s is the spontaneous lifetime [s], and a is the transition cross section [cm2]. The first equation describes the population change rate for the upper state, the second equation describes the population change rate for the meta-stable state, and the third equation describes the population change rate for the ground state. The steady state atomic populations N1 and N2 are functions of the pumping rate which represents the pump absorption rate, between levels 1 and 3, and of the absorption and stimulated emission rates between levels 1 and 2. Fig. 1 shows the 3-level erbium atomic structure, and it shows the level transitions when erbium is used in a single stage 980 nm pumped pre-amp EDFA [3]. The sum of the population in the three states of the erbium atomic structure is equal to the total population. The sum of the population in the three states is equal to the total population.
N ¼ N1 þ N2 þ N3
ð4Þ
Under a steady state condition, electron transition is given by
f 12 ¼
E1 E2 h
ð5Þ
L3 (Excited State)
L3 to L2 Atomic Transition (T 32 = 1us) L2 (Meta-Stable State)
980 nm Pumping hvp
Amplified Spontaneous Emission (Tsp =10 ms) Amplified Signal hvs L1(Ground State)
Fig. 1. Pumping (980 nm) in erbium atomic structure.
ð6Þ
Here, h is Plank’s constant = 6.626 10E-34 [J/s]. Stimulated photons are in coherence with the input signal, and that results in signal amplification. In free space, the radiation wavelength is given by
k21 ¼ hc=ðE2 E1Þ
ð7Þ
When this radiation interacts with a photon in the lower energy level, the photon is transformed into the upper atomic level. If a photon in the excited state is not stimulated within the 10 ms lifetime of the excited state, it will spontaneously decay to the ground state producing ASE. When this photon travels through the erbiumdoped fiber, it gets amplified, resulting in amplified spontaneous emission. All the excited ions can spontaneously relax from the upper state to the ground state by emitting a photon that is unrelated to the signal photons. This spontaneously emitted photon can be amplified as it travels down the fiber and stimulates the emission of more photons from excited electrons. Amplified spontaneous emission can occur at any frequency within the fluorescence spectrum of the amplifier transitions. The dominant noise source in any EDFA is amplified spontaneous emission. This spontaneous emission reduces the amplifier gain by consuming the photons that would otherwise be used for stimulated emission of the input signal. In single mode fiber, the noise output power resulting from amplified spontaneous emission is given by [5]
PASE ¼ 2 nsp ðG 1Þhv Dðv Þ
dN1 dN2 dN3 ¼ ¼ ¼0 dt dt dt
Input Signal
The basic principle of signal amplification in erbium-doped fiber is based on the fact that when an optical signal passes through erbium-doped fiber, the signal is amplified due to stimulated transition between electronic states in the presence of electromagnetic radiation at the correct wavelength to achieve population inversion. In order for signal amplification to happen [4], a frequency f12 is needed where
ð8Þ
The total amplified spontaneous emission at any point in the fiber is the sum of all amplified spontaneous emission power from the previous sections in the fiber and the amplified spontaneous emission at the given fiber point. In order to minimize ASE noise, the pump power should be just enough to achieve population inversion. Population inversion can be achieved when the population in the excited state, N2, is greater than the population in the ground state N1. The threshold pump power required to achieve population inversion can be obtained by setting the rate equation of level 2–0, and setting N1 to be equal to N2. A long meta-stable state lifetime and a large absorption cross section are needed to have a low pump threshold to achieve population inversion. A detailed analysis of EDFA and photodiodes noise elements was performed by different researchers [6,7]. When the EDFA ASE noise is converted to an electrical signal in the photodiode, it gives rise to signal-spontaneous beat noise and spontaneous–spontaneous beat noise. When pre-amps are used in the optical receiver, signal-spontaneous beat noise dominates all other noise elements in the photon detector [8]. 3. Optical receiver transmission performance Optical receiver transmission performance, commonly known as bit error rate (BER) performance, is the gauge by which optical receivers are characterized. It characterizes the ability of the receiver to perform up to the transmission performance specifications under the same test conditions as those where the receiver operates in the field [3]. So transmission performance will be used to analyze the optical receiver performance under different operating
2605
A. Abu-aisheh, S. Moslehpour / Optics Communications 283 (2010) 2603–2606
OSA
DCA
Optical Switch
EDFA
90/10
O/E
90/10
LPF
Optical Atten. #3
BPF
ED ED
CW Laser
Optical Atten. #1
Combiner
E/O Optical Atten. #2
PPG PPG
Noise Source
Fig. 2. Optical receiver transmission performance test setup.
Table 1 Optical receiver performance at different input and output powers. System BER (dBm)
Pin = 24 (dBm)
Pin = 22 (dBm)
Pin = 20 (dBm)
Pout = 11.5 Pout = 10.5 Pout = 9.5 Pout = 8.5 Pout = 7.5
4.30E-08 4.00E-09 3.60E-10 1.80E-10 1.40E-10
8.90E-09 7.40E-10 1.40E-10 7.60E-11 6.50E-11
3.10E-09 3.20E-10 6.20E-11 4.80E-11 4.60E-11
power, output power, and operating wavelength should be taken into account. This allows designers to choose the right erbiumdoped fiber length and pump power combination, and it helps minimize amplified spontaneous emission at the output of the pre-amp EDFA. Since the most important factor in pre-amp performance is how well it performs in the optical receiver, optical receiver optimal transmission performance analysis under different operating conditions is the ultimate method for optimizing pre-amp EDFA performance in the optical receiver. The pre-amp EDFA design needs
Pre-amp Performance at Different Pin and Put Values
-6.00 -7.00 L0g10(BER)
conditions. The setup of Fig. 2 was used to perform the optical receiver tests analyzed in this paper. This setup has the ability and flexibility to perform optical receiver transmission performance measurement, optical input and output measurements, optical signal and noise control, and optical input SNR control. Since the most important factor in the pre-amp performance is how well it performs in the optical receiver, the optical receiver optimal transmission performance analysis under different operating conditions is the ultimate method for characterizing pre-amp EDFA noise performance. The pre-amp EDFA design needs to be optimized at the pre-amp level and the EDFA level. Then the pre-amp performance should be determined by how well the pre-amp performs in the optical receiver. For optimal optical receiver transmission performance, the preamp EDFA design must be coordinated with the photon detector design in order to minimize amplified spontaneous emission noise mitigation from the pre-amp EDFA to the photon detector and the photon detector signal-spontaneous beat noise. The pre-amp input
-8.00 -9.00
Pin=-24dbm Pin=-22dbm
-10.00
Pin=-20dbm
-11.00
-11.5 -10.5 -9.5 -8.5 -7.5 Output Power dBm
Fig. 3. Optical receiver performance change at different input and output power levels.
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Table 2 Ber at different input powers and signal to noise ratios. System BER (dBm)
SNR = 8 (dB)
SNR = 9 (dB)
SNR = 10 (dB)
SNR = 11 (dB)
SNR = 12 (dB)
Pin = 28 Pin = 27 Pin = 26 Pin = 25
2.5E-06 7.7E-07 2.3E-07 8.5E-08
3.9E-07 8.2E-08 2.0E-08 4.9E-09
7.6E-08 1.3E-08 1.6E-09 3.1E-10
1.2E-08 9.5E-10 9.0E-11 1.3E-11
2.3E-09 1.2E-10 7.6E-12 6.0E-13
Pre-amp Noise Loaded Performance -4 -28 dBm
-27 dBm
-26 dBm
-25 dBm
-6
Log10 (BER)
SNR=8 dB -8
SNR=9 dB SNR=10 dB
-10
SNR=11 dB -12
Table 2 show that the optical receiver transmission performance improves as the pre-amp input signal power is increased. A graphical representation of the system transmission performance, after normalizing BER, is given in Fig. 4. At the atomic structure level, an increase in the input power causes more stimulated emission of the excited electrons. This leaves fewer electrons to move to the ground state spontaneously. This means that the pre-amp is generating less amplified spontaneous emission, and this reduces the signal-spontaneous noise in the photon detector which results in improved optical receiver transmission performance.
SNR=12 dB
5. Conclusion -14
Input Power (dBm) Fig. 4. Optical receiver performance change at different input powers and different input signal to noise ratios.
to be optimized at two levels: the pre-amp/photon detector subsystem level and the optical receiver level. Several characterization experiments were performed to analyze the effects of changing the pre-amp operating conditions on the optical receiver transmission performance. Testing the preamp-based optical receiver at a fixed signal to noise ratio at 1550 nm, the transmission performance was recorded at different input/output combinations. The optical system transmission performance results are given in Table 1. 4. Different input and output powers A graphical representation of the optical system transmission performance results, after normalizing BER, is given in Fig. 3. From the results in Fig. 3, we see that the optical receiver transmission performance improves as the pre-amp output power is increased. This improvement is due to the fact that more output power requires more pump output, and more output power excites more electrons to the upper state. This excitation will result in the population inversion that is needed for the amplification process. Testing the pre-amp-based optical receiver at different input powers and at different signal to noise ratios at fixed output power and input signal wavelength, the transmission performance changes due to the changes in the operating conditions were monitored, and the results are given in Table 2. The results given in
Since the most important performance factor in pre-amp performance is how well it performs in the optical receiver, the optical receiver optimal transmission performance analysis under different operating conditions is the ultimate method for characterizing pre-amp EDFAs noise performance. The results of the tests performed for this work shows a need for designing the pre-amp EDFA and photon detector as one subsystem. Then the pre-amp EDFA needs to be fine-tuned at the optical receiver level to achieve optimal optical receiver transmission performance. For optimal optical receiver transmission performance, the preamp EDFA design must be coordinated with the photon detector design in order to minimize amplified spontaneous emission noise mitigation from the pre-amp EDFA to the photon detector. Doing so will minimize the photon detector signal-spontaneous beat noise. Both the pre-amp EDFA and the photon detector should be treated as one block during the design stage. References [1] E. Desurvire, Erbium-Doped Fiber Amplifiers, Principles and Applications, Wiley, NY, 1994. [2] J.T. Verdeyen, Laser Electronics, third ed., Prentice Hall, 1995. [3] A. Abu-aisheh, Pre-amp EDFA Noise Characterization for Optimal Optical Receiver Transmission Performance, Ph.D. Dissertation, Dept. of Elec. Eng. Florida Institute of Technology, Melbourne, FL, 2003. [4] J. Gowar, Optical Communication Systems, second ed., Prentice Hall Inc., 1993. [5] P.C. Becker, N.A. Olsson, J.R. Simpson, Erbium-Doped Fiber Amplifiers, Fundamentals and Technology, Academic Press, NY, 1999. [6] R. Tucker, H. Kingston, Optical Sources Detectors and Systems Fundamentals and Applications, Academic Press, Inc., 1995. [7] R.S. Tucker, D.M. Baney, Optical Noise Figure: Theory and Measurement, OFC, Anaheim, CA, 2001. [8] S. Soerensen, J. Light Wave Technol. 18 (10) (2000).