RIN measurement in GEPON system
发布时间：2005-06-30 17:36:35 热度：4215
06/30/2005,Jacob Cai,Sinclair Li, Douglas Cheng
Fiberxon Inc.
Introduction
With the development of optical access networks technology,
Passive Optical Networks (PON) is making rapid progress in the world's leading technology-adopter markets. Accompany with the Request For Proposal (RFP) of PON equipments from U.S. RBOCs and NTT, the promise of delivering last-mile optical connections to residential customers becomes more realizable. As the deployment of Gigabit Ethernet PON (GEPON) systems becomes increasingly important, the performance evaluation of the corresponding system is crucial. The relative intensity noise (RIN) is one of the limiting factors in the transmission of analog or digital signals over fiber and must be characterized. It may reduce the Signal-to-Noise ratio (SNR) and cause an obvious performance degradation of GEPON system. This paper presents a detailed analysis of the RIN in component level and system level respectively. The authors also describe an optimum measurement method of RINxOMA in GEPON system in great detail.
Theoretical analysis of RINlaser and RINsystem
The noise at the receiver output in GEPON system mainly results from three fundamental contributions: laser intensity noise, thermal noise and photonic shot noise.
Laser intensity noise, NL, refers to the noise generated by the laser. The intensity of semiconductor lasers fluctuates during operation due to the quantum nature of the lasing process. Intensity noise is caused by optical interference between the laser signal and spontaneous emission within the cavity. In addition, operating conditions, such as bias level and modulation frequency, also affect the noise level directly. The presence of external feedback or reflection will increase the Laser intensity noise as well.
Thermal Noise, Nth, limits the sensitivity of the receiver and restricts the transmission distance of both analog and digital signal in GEPON system.
Shot noise, Nq, is produced by the quantum nature of photons arriving at the detector, and related detection statistics. The noise level is directly related to the amount of incident light on the photodetector.
Thermal noise and laser intensity noise vary with frequency. Shot noise is a function of the intensity of incident light on the photodiode.
The total system noise, NT(f), is the linear summation of these three noise terms:
(1)
where NL(f) is the laser intensity noise power per Hz; Nq is the photonic shot noise power per Hz; Nth(f) is the contribution of thermal noise power per Hz.
RIN describes the amplitude fluctuations in the optical field. It is defined as the ratio of mean-square optical intensity noise to the square of the average optical power, the unit is dB/Hz:
(2)
where is the mean square intensity fluctuation spectral density of the optical signal and P is the average optical power.
As the ratio of optical powers squared is equivalent to the ratio of the detected electrical powers, RIN can be expressed in terms of detected electrical powers:
(3)
where N is the power spectral density of the photocurrent at a specific frequency, and Pave is the average power of the photocurrent.
If the dominant noise source in the system is laser intensity noise, the maximum realizable SNR increases as RIN decreases:
(4)
where m is the index of modulation and B is the noise bandwidth.
Distinguish between RINlaser and RINsystem
RIN is the ratio of noise power to average power. Equations (1) and (3) can be used to distinguish the value of RINLaser and RINsystem.
(5)
And RINLaser is only the ratio of laser intensity noise power to average power:
(6)
Figure 1. RINlaser calculated from the measured RINsystem, versus average power.
Figure 1 describes RINlaser against RINsystem to show the effect of subtracting the thermal and shot noise with varying average power and constant thermal noise (8 dB NF) and responsivity (0.8 A/W into 50 ohms). The figure shows that when the total measured noise is greater than the thermal and shot noise terms by about 5 to 10 dB, the values of RINsystem and RINlaser are essentially equal.
RINxOMA
(a) for various levels of RIN (b) with reduced extinction ratio
Figure 2 BER curves for various RIN
The curves presented in Figure 2(a) is achieved by assuming the laser is fully modulated. In order to achieve good performance in GEPON systems, designers prefer to operate the laser at reduced modulation amplitude. By reducing the optical modulation amplitude (OMA) while maintaining the same average power, the laser dynamic response characteristics is generally improved. The Fibre Channel specification is specifically written to permit lasers to be operated at reduced modulation (reduced extinction ratio). Instead of specifying minimum average power and extinction ratio, a specification is only given for the minimum amount of transmit OMA. This gives the designer freedom to select the optimum extinction ratio for best transmit eye quality. However, transmit RIN plays an important role in determining how low an extinction ratio can be used.
Figure 2(b) illustrates the impact of RIN on BER performance when the same laser employed in Figure 2(a) is operated at a reduced extinction ratio. Since the Extinction Ratio is less, the required average power for a given BER is higher as seen in the left most curve.
To permit the operation of optical sources at levels other than full modulation, but not experience the added RIN degradation in Figure 2(b), IEEE 802.3ah adopts a more challenging specification for transmitter RIN identified as RINxOMA. It requires the transmitter RIN ratio to be calculated using the same RMS modulation amplitude of the laser instead of an average DC level. RINxOMA can be expressed as:
(7)
where
RINxOMA is the Relative Intensity Noise referred to optical modulation amplitude measured with x dB reflection;
PN is the Electrical noise power in Watts with modulation off;
PM is the Electrical power in Watts with modulation on;
BW is the Low pass bandwidth of apparatus - high pass bandwidth of apparatus due to DC blocking capacitor [noise bandwidth of the measuring system (Hz)].
Equation 7 shows that as one attempts to reduce the extinction ratio of the laser by decreasing the modulation, the value of PM will decrease proportionally and result in an increment of RINxOMA.
Relationship between RINxOMA and RIN
Comparing the two definitions of RINxOMA and RIN, we can derive the following relationship between these two parameters:
(8)
where PER is the extinction ratio penalty given by
(9)
Where P1 and P0 are optical power for “1” and “0” levels of the optical signal respectively.
Measurement of RINxOMA
RIN can be measured by capturing the electrical output of a high-speed photodetector on an electrical spectrum analyzer. Alternatively, RIN can be measured using one lightwave signal analyzer only. A setup for measuring RIN in GEPON is shown in Figure 3. It contains the transmitter under test, optical splitter, attenuator, polarization rotator, Optical to Electrical converter O/E and measuring equipment.
Figure 3 RINxOMA measurement setup
The optical path and detector combination must be configured for a single dominant reflection with an optical return loss. The length of the fiber is not critical but should be in excess of 2 m.
The polarization rotator is capable of transforming an arbitrary orientation elliptically polarized wave into a fixed orientation linearly polarized wave. The purpose is to remove the polarization dependence of the devices, which used in the test system, like splitter and O/E converter.
The optical splitter permits the laser output to be monitored with the measurement equipment, while at the same time generating a controlled amount of reflection back to the laser.
Various link physical interface standards require that a portion of the light transmitted in the forward direction be reflected back towards the laser. This is to insure the transmitter RIN is being characterized under a more stressful, but realistic, link condition. Reflections can significantly degrade laser RIN performance as a portion of the light can couple back through the optics and into the laser die itself. This can create instabilities that appear RIN like in nature and increase the effective noise output of the laser. The potential exists in fiber optic links for back reflections to be present.
Both the OMA power and noise power are measured by AC coupling the O/E converter into the electrical power meter. If needed, an amplifier may be used to boost the signal to the power meter. A low pass filter is used between the photo detector and the power meter to limit the noise measured to the passband appropriate to the data rate of interest. In order to measure the noise, the modulation to the DUT is turned off.
The upper –3 dB limit of the measurement apparatus is as specified for the transmitter optical waveform test. The bandwidth used in the RIN calculation takes the low-frequency cutoff of the DC blocking capacitor into consideration. The low-frequency cutoff is recommended to be less than 1 MHz. The filter should be placed in the circuit as the last component before the power meter so that any high-frequency noise components generated by the detector/amplifier are eliminated. If the power meter used has a very wide bandwidth, care should be taken to ensure that the filter does not lose its rejection at extremely high frequencies.
The RMS electrical power meter should be capable of being zeroed in the absence of input optical power to remove any residual noise.
Equipment selection
Various measurement equipments can perform the task of RINxOMA test. Below is one recommended solution:
O/E converter:
Agilent 83440C, 81000FI
Polarization controller
Agilent 8169A
Varible refelector
Agilent 8163B, 81570A, 81000FI x2, 81000BR, 81000UM
Spectrum analyzer:
Agilent E4440A
It is worth to point out that Agilent 71400C or 71401C is combined with the spectrum analyzer and O/E converter. They are the preferred equipment in test platform setup.
Test procedure
Use the following procedure to test RIN optical modulation amplitude:
a) With the DUT disconnected, zero the power meter;
b) Connect the DUT, turn on the laser, and ensure that the laser is not modulated;
c) Operate the polarization rotator while observing the power meter output to maximize the noise read by the power meter. Note the maximum power, PN;
d) Turn on the modulation to the laser using the pattern specified for the PMD type and note the power measurement, PM. It may be necessary to change or remove the effective reflection to obtain an accurate reading;
e) Calculate RIN from the observed electrical signal power and noise power using equation 7.
RINxOMA parameter Requirement in IEEE 802.3ah
IEEE 802.3ah specifies the value of RINxOMA in detail, as following table describes:
As a globally leading vendor of GEPON transceiver, Fiberxon apply very strictly requirement of the measurement on RIN15OMA of GEPON OLT/ONU transceivers. Readers can refer to the product datasheets for more detail.
Conclusion
The level of relative intensity noise reflects the amplitude fluctuations in the optical field. These fluctuations propagate over fiber and limit SNR at the receiver. Fiberxon provides GEPON OLT/ONU transceivers with excellent RIN performance by strictly product design, manufacture and measurement. All of the related products are fully compliant with the requirement of GEPON application.
Fiberxon Inc.
Introduction
With the development of optical access networks technology,
Passive Optical Networks (PON) is making rapid progress in the world's leading technology-adopter markets. Accompany with the Request For Proposal (RFP) of PON equipments from U.S. RBOCs and NTT, the promise of delivering last-mile optical connections to residential customers becomes more realizable. As the deployment of Gigabit Ethernet PON (GEPON) systems becomes increasingly important, the performance evaluation of the corresponding system is crucial. The relative intensity noise (RIN) is one of the limiting factors in the transmission of analog or digital signals over fiber and must be characterized. It may reduce the Signal-to-Noise ratio (SNR) and cause an obvious performance degradation of GEPON system. This paper presents a detailed analysis of the RIN in component level and system level respectively. The authors also describe an optimum measurement method of RINxOMA in GEPON system in great detail.
Theoretical analysis of RINlaser and RINsystem
The noise at the receiver output in GEPON system mainly results from three fundamental contributions: laser intensity noise, thermal noise and photonic shot noise.
Laser intensity noise, NL, refers to the noise generated by the laser. The intensity of semiconductor lasers fluctuates during operation due to the quantum nature of the lasing process. Intensity noise is caused by optical interference between the laser signal and spontaneous emission within the cavity. In addition, operating conditions, such as bias level and modulation frequency, also affect the noise level directly. The presence of external feedback or reflection will increase the Laser intensity noise as well.
Thermal Noise, Nth, limits the sensitivity of the receiver and restricts the transmission distance of both analog and digital signal in GEPON system.
Shot noise, Nq, is produced by the quantum nature of photons arriving at the detector, and related detection statistics. The noise level is directly related to the amount of incident light on the photodetector.
Thermal noise and laser intensity noise vary with frequency. Shot noise is a function of the intensity of incident light on the photodiode.
The total system noise, NT(f), is the linear summation of these three noise terms:
(1)
where NL(f) is the laser intensity noise power per Hz; Nq is the photonic shot noise power per Hz; Nth(f) is the contribution of thermal noise power per Hz.
RIN describes the amplitude fluctuations in the optical field. It is defined as the ratio of mean-square optical intensity noise to the square of the average optical power, the unit is dB/Hz:
(2)
where is the mean square intensity fluctuation spectral density of the optical signal and P is the average optical power.
As the ratio of optical powers squared is equivalent to the ratio of the detected electrical powers, RIN can be expressed in terms of detected electrical powers:
(3)
where N is the power spectral density of the photocurrent at a specific frequency, and Pave is the average power of the photocurrent.
If the dominant noise source in the system is laser intensity noise, the maximum realizable SNR increases as RIN decreases:
(4)
where m is the index of modulation and B is the noise bandwidth.
Distinguish between RINlaser and RINsystem
RIN is the ratio of noise power to average power. Equations (1) and (3) can be used to distinguish the value of RINLaser and RINsystem.
(5)
And RINLaser is only the ratio of laser intensity noise power to average power:
(6)
Figure 1. RINlaser calculated from the measured RINsystem, versus average power.
Figure 1 describes RINlaser against RINsystem to show the effect of subtracting the thermal and shot noise with varying average power and constant thermal noise (8 dB NF) and responsivity (0.8 A/W into 50 ohms). The figure shows that when the total measured noise is greater than the thermal and shot noise terms by about 5 to 10 dB, the values of RINsystem and RINlaser are essentially equal.
RINxOMA
(a) for various levels of RIN (b) with reduced extinction ratio
Figure 2 BER curves for various RIN
The curves presented in Figure 2(a) is achieved by assuming the laser is fully modulated. In order to achieve good performance in GEPON systems, designers prefer to operate the laser at reduced modulation amplitude. By reducing the optical modulation amplitude (OMA) while maintaining the same average power, the laser dynamic response characteristics is generally improved. The Fibre Channel specification is specifically written to permit lasers to be operated at reduced modulation (reduced extinction ratio). Instead of specifying minimum average power and extinction ratio, a specification is only given for the minimum amount of transmit OMA. This gives the designer freedom to select the optimum extinction ratio for best transmit eye quality. However, transmit RIN plays an important role in determining how low an extinction ratio can be used.
Figure 2(b) illustrates the impact of RIN on BER performance when the same laser employed in Figure 2(a) is operated at a reduced extinction ratio. Since the Extinction Ratio is less, the required average power for a given BER is higher as seen in the left most curve.
To permit the operation of optical sources at levels other than full modulation, but not experience the added RIN degradation in Figure 2(b), IEEE 802.3ah adopts a more challenging specification for transmitter RIN identified as RINxOMA. It requires the transmitter RIN ratio to be calculated using the same RMS modulation amplitude of the laser instead of an average DC level. RINxOMA can be expressed as:
(7)
where
RINxOMA is the Relative Intensity Noise referred to optical modulation amplitude measured with x dB reflection;
PN is the Electrical noise power in Watts with modulation off;
PM is the Electrical power in Watts with modulation on;
BW is the Low pass bandwidth of apparatus - high pass bandwidth of apparatus due to DC blocking capacitor [noise bandwidth of the measuring system (Hz)].
Equation 7 shows that as one attempts to reduce the extinction ratio of the laser by decreasing the modulation, the value of PM will decrease proportionally and result in an increment of RINxOMA.
Relationship between RINxOMA and RIN
Comparing the two definitions of RINxOMA and RIN, we can derive the following relationship between these two parameters:
(8)
where PER is the extinction ratio penalty given by
(9)
Where P1 and P0 are optical power for “1” and “0” levels of the optical signal respectively.
Measurement of RINxOMA
RIN can be measured by capturing the electrical output of a high-speed photodetector on an electrical spectrum analyzer. Alternatively, RIN can be measured using one lightwave signal analyzer only. A setup for measuring RIN in GEPON is shown in Figure 3. It contains the transmitter under test, optical splitter, attenuator, polarization rotator, Optical to Electrical converter O/E and measuring equipment.
Figure 3 RINxOMA measurement setup
The optical path and detector combination must be configured for a single dominant reflection with an optical return loss. The length of the fiber is not critical but should be in excess of 2 m.
The polarization rotator is capable of transforming an arbitrary orientation elliptically polarized wave into a fixed orientation linearly polarized wave. The purpose is to remove the polarization dependence of the devices, which used in the test system, like splitter and O/E converter.
The optical splitter permits the laser output to be monitored with the measurement equipment, while at the same time generating a controlled amount of reflection back to the laser.
Various link physical interface standards require that a portion of the light transmitted in the forward direction be reflected back towards the laser. This is to insure the transmitter RIN is being characterized under a more stressful, but realistic, link condition. Reflections can significantly degrade laser RIN performance as a portion of the light can couple back through the optics and into the laser die itself. This can create instabilities that appear RIN like in nature and increase the effective noise output of the laser. The potential exists in fiber optic links for back reflections to be present.
Both the OMA power and noise power are measured by AC coupling the O/E converter into the electrical power meter. If needed, an amplifier may be used to boost the signal to the power meter. A low pass filter is used between the photo detector and the power meter to limit the noise measured to the passband appropriate to the data rate of interest. In order to measure the noise, the modulation to the DUT is turned off.
The upper –3 dB limit of the measurement apparatus is as specified for the transmitter optical waveform test. The bandwidth used in the RIN calculation takes the low-frequency cutoff of the DC blocking capacitor into consideration. The low-frequency cutoff is recommended to be less than 1 MHz. The filter should be placed in the circuit as the last component before the power meter so that any high-frequency noise components generated by the detector/amplifier are eliminated. If the power meter used has a very wide bandwidth, care should be taken to ensure that the filter does not lose its rejection at extremely high frequencies.
The RMS electrical power meter should be capable of being zeroed in the absence of input optical power to remove any residual noise.
Equipment selection
Various measurement equipments can perform the task of RINxOMA test. Below is one recommended solution:
O/E converter:
Agilent 83440C, 81000FI
Polarization controller
Agilent 8169A
Varible refelector
Agilent 8163B, 81570A, 81000FI x2, 81000BR, 81000UM
Spectrum analyzer:
Agilent E4440A
It is worth to point out that Agilent 71400C or 71401C is combined with the spectrum analyzer and O/E converter. They are the preferred equipment in test platform setup.
Test procedure
Use the following procedure to test RIN optical modulation amplitude:
a) With the DUT disconnected, zero the power meter;
b) Connect the DUT, turn on the laser, and ensure that the laser is not modulated;
c) Operate the polarization rotator while observing the power meter output to maximize the noise read by the power meter. Note the maximum power, PN;
d) Turn on the modulation to the laser using the pattern specified for the PMD type and note the power measurement, PM. It may be necessary to change or remove the effective reflection to obtain an accurate reading;
e) Calculate RIN from the observed electrical signal power and noise power using equation 7.
RINxOMA parameter Requirement in IEEE 802.3ah
IEEE 802.3ah specifies the value of RINxOMA in detail, as following table describes:
As a globally leading vendor of GEPON transceiver, Fiberxon apply very strictly requirement of the measurement on RIN15OMA of GEPON OLT/ONU transceivers. Readers can refer to the product datasheets for more detail.
Conclusion
The level of relative intensity noise reflects the amplitude fluctuations in the optical field. These fluctuations propagate over fiber and limit SNR at the receiver. Fiberxon provides GEPON OLT/ONU transceivers with excellent RIN performance by strictly product design, manufacture and measurement. All of the related products are fully compliant with the requirement of GEPON application.
