OFC SYSTEM: Design Considerations BC Choudhary, Professor NITTTR, Sector 26, Chandigarh.
OFC point-to-point Link Transmitter Electrical to Optical Conversion Coupler Optical Fiber Coupler Optical to Electrical Conversion Receiver
System Measurements & Design Considerations Design & installation of an OFC system require measurement techniques for verifying the operational characteristics of the constituent components. Of particular importance are accurate & precise measurements of optical fiber, since this component cannot be readily replaced once it has been installed. Two groups of people interested in fiber measurements are: Manufacturers- concerned with the material composition and fabrication effects on fiber properties System Engineers- must have sufficient data on the fiber to perform meaningful design calculations and to evaluate systems during installation and operation.
Operational Wavelength
Systems Communication Requirements Mainly Two Parameters of concern Link Length Repeater less distance (50km, 100km, 150km) Maximum data transmission rate (Mbps, Gbps) These requirements will decide the type of input data, transmitter (launch power, modulation), optical fiber cable, receiver(sensitivity) etc.
Optical Transmitter Transmitter component serves two functions. Must be a source of the light coupled into the fiber optic cable. Must modulate this light so as to represent the binary data that it is receiving from the Source. Input Signal Coder or Converter Light Source Source-to-Fiber Interface Fiber-optic Cable
ILDs or LEDs? ILDs : LDs have advantages over LED's in the following ways. Can be modulated at very high speeds. They produce greater optical power. They have higher coupling efficiency to the fiber LEDs : LED's have advantages over LD's because they have Higher reliability Better linearity Lower cost
Semiconductor lasers for SM operation Febry - Perot Lasers Coupled Cavity Lasers Distributed Feedback Lasers DFB & DBR Quantum Well Lasers Vertical Cavity Surface Emitting Laser (VCSEL) All these configurations require a rigid control of the cavity parameters to achieve and maintain single mode operation.
Consideration Parameters Fiber Optic Cable How much light can be coupled into the core through the external acceptance angle? How much attenuation will a light ray experience in propagating down the core? How much time dispersion will light rays representing the same input pulse experience in propagating down the core?
Fiber optic cable can be one of two types Multi-mode or Single-mode. These provide different performance with respect to both attenuation and time dispersion. Glass fiber optic cable has the lowest attenuation and comes at the highest cost. Plastic fiber optic cable has the highest attenuation, but comes at the lowest cost.
Optical Receiver Receiver component serves two functions. Detect the light coupled out of the fiber optic cable then convert the light into an electrical signal. Demodulate the light to determine the identity of the binary data. Optical Fiber Coupler Optical to Electrical Conversion Receiver
Detectors There are two types of photodiode structures; Positive Intrinsic Negative (PIN) and Avalanche Photo Diode (APD). In most premises applications the PIN is the preferred element in the Receiver. This is mainly due to fact that it can be operated from a standard power supply, typically between 5 and 15 V. APD devices have much better sensitivity. In fact it has 5 to 10 db more sensitivity. They also have twice the bandwidth. However, they cannot be used on a 5V printed circuit board. They also require a stable power supply. This makes cost higher. APD devices are usually found in long haul communications links.
Fiber Connectors The connector must direct light and collect light. It must also be easily attached and detached from equipment. This is a key point. The connector is disconnectable. FC, FC/PC, SC, SMA, ST, Biconic, D4, Commonly Used Connectors
Designing A Fiber Optic System When designing a fiber optic system, there are many factors that must be considered all of which contribute to the final goal of ensuring that enough light reaches the Receiver. Without the right amount of light, the entire system will not operate properly.
Step-by-step procedure to be followed while designing any system Determine the correct optical transmitter and receiver combination based upon the signal to be transmitted (Analog, Digital, Audio, Video, RS-232, RS-422, RS-485 etc.) Determine the operating power available (AC, DC etc.) Determine the special modifications (if any) necessary (Impedances, Bandwidths, Special Connectors, Special Fiber Size, etc.) Calculate the total optical loss (in db) in the system by adding the cable loss, splice loss and connector loss. These parameters should be available from the manufacturer of the electronics and fiber.
Compare the loss figure obtained with the allowable optical loss budget of the receiver. Be certain to add a safely margin factor of at least few db to the entire system. Check that the fiber bandwidth is adequate to pass the signal as desired. After performing the above calculations, if it is discovered that the fiber bandwidth is inadequate for transmitting the required signal the necessary distance, it will be necessary either select a different transmitter/ receiver (wavelength) combination, or consider the use of a lower loss premium fiber.
System Design Check List Application (Brief description of intended use): Analog Signal Parameters: Input Voltage Input impedance Output Voltage Output impedance Signal/Noise Ratio DC or AC Coupling Bandwidth Digital Signal Parameters: Compatibility (RS-232, 485 etc.) Date Rate DC or AC Coupling Bit Error Rate Detector type/sensitivity (Minimum optical power required in dbm) Source type/optical power (Minimum optical power launched) Other Details Fiber Optic Requirements: Transmission Distance Optical Wavelength Required Loss Budget Optical Connectors Fiber Type Fiber Length Installation Environment Power Supply Requirements: Voltage Available Current Available General Requirements: Housing Size Mounting Method Environment Operating Temperature Range Storage Temperature Range Other Details
BUDGET CALCULATIONS Two analyses are usually carried out to ensure that the desired system performance can be met: Link Power Budget Rise-time budget Link Power Budget : Determines the power margin between the optical transmitter output and the minimum receiver sensitivity needed to establish a specified BER. This margin can then be allocated to connector, splice and fiber losses, plus any additional margin required for possible component degradation, transmission-line impairments, or temperature effects. If the choice of components did not allow the desired transmission distance to be achieved, the components might have to be changed or amplifiers might have to be incorporated into the link.
If P S is the optical power emerging from the end of the fiber attached to the light source, and P R is the receiver sensitivity, then P T = P S - P R = 2 l c + f L + system margin where l c is the connector loss, f is the fiber attenuation in (db/km) and L is the transmission length. System margin is normally taken 6dB for LED and 8 db for ILD.
Rise-time budget : Once the link power budget has been established, the designer can perform a system rise time analysis (dispersion limitations) to ensure that the desired overall system performance has been met. Total rise time t sys of the link is the root mean square of the rise times from each contributor (t i ) to the pulse rise-time degradation Four basic elements that may significantly limit system speed are Transmitter rise time, Group velocity dispersion (GVD) rise time of the fiber, Modal dispersion rise time of fiber and Receiver rise time. t N 2 sys t i i 1 Generally, the total transition-time degradation of a digital link should not exceed 70% of an NRZ bit period or 35% of a bit period for RZ data. 1 2
Link Performance Analysis A power budget example Link length of 5 km (premises distances). Data Rate of 50 Mbps at BER of 10-9. Transmitter LED: 850 nm, 3dBm, coupling loss 5 db. MM, SI, glass fiber optic cable 62.5/125 m Transmitter -fiber, fiber- receiver coupling loss; 1 db each. Fiber optic cable has 1 splice. Receiver- PIN sensitivity of -40 dbm at 50 Mbps.
Power Budget for a fiber optic data link LINK ELEMENT VALUE COMMENTS Transmitter LED output power Source coupling loss 3 dbm Specified value by vendor -5 db Accounts for reflections, area mismatch etc. Transmitter to fiber optic cable connector loss -1 db Transmitter to fiber optic cable with ST connector. Loss accounts for misalignment Splice loss Fiber Optic Cable Attenuation -0.25 db Mechanical splice -20 db Line 2 of Table 2-1 applied to 5 km Fiber optic cable to receiver connector loss Optical Power Delivered at Receiver -1 db -24.25 db Fiber optic cable to Receiver with ST connector. Loss Accounts for misalignment Receiver Sensitivity LOSS MARGIN -40 dbm Specified in link design. 15.75 db Surplus power available
Link Analysis Optical power at the Receiver is greater than that required by the sensitivity of the PIN to give the required BER. What is important to note is the entry termed Loss Margin? specifies the amount by which the received optical power exceeds the required sensitivity. In this example 15.75 db. Good design practice requires it to be at least 10 db. Why? Because no matter how careful the power budget is put together, entries are always forgotten, are too optimistic or vendor specifications may not be accurate.
Typical OFC link & Performance Parameters Performance-measurement parameters of users interest
Optical Test Equipments Basic pieces of test equipment for carrying measurements on optical fiber components and system include Optical power meters, Continuity testers, Visual fault locators, Talk sets, Spectrum analysers, OTDRs and BER-Testers. These comes with variety of capabilities, with sizes ranging from portable, handheld units for field use to sophisticated briefcase sized instruments for laboratory applications. Most of these units has reached a high degree of sophistication with automated microprocessor-controlled test features and computer-interface capabilities
Power Meters & Talk Sets
Continuity Testers & Visual Fault Locators
OTDR Models
Optical Spectrum Analyzers (OSA)
BER TESTERS
Testing System Performance z=0 z=l Attenuation z=0 z=l Dispersion Attenuation & Dispersion degradation as a function of distance
Bit-Error-Rate (BER) Measurements Performance of any communication system can be evaluated by one of the following methods: Eye Diagrams / Patterns. Histogram Generation Bit Error Rate Measurements.
Bit Error Rate (BER) Most significant performance parameter in any digital communications system. Indeed, it is often accepted as the primary performance figure of merit for a communication system. Defined as the ratio of the number of errors in a given time interval (N e ) to the number of bits in that time interval (N t ). BER N N e t It is simply the probability that an error will occur in a given bit period. For many applications the maximum specified BER is 10-9 implying that only one error in 10 9 received bits is tolerated. For telecommunication applications the specified maximum BER falls in the range 10-12 to 10-9.
BER Estimates Bit error rate (BER) : Predict the statistical likelihood of encountering an error during communications. Can be measured empirically by counting the number of errors over an adequately long span of transmission BER depends primarily on the S/N ratio of the received signal, which in turn determined by transmitted signal power, attenuation of the link and receiver noise. Many other factors besides SNR degrade the BER and in their presence the received SNR must be increased to yield the desired BER. The increase necessary to completely offset the degradation caused by a given mechanism is referred to as the power penalty for that mechanism.
Main factors leading to significant penalties are Intersymbol interference (ISI) Non zero extinction ratio and Pulse position jitter BER estimation one of the valuable ways of viewing parametric performance of digital communication systems at high speeds. Requires sophisticated and expensive equipment to achieve accuracy, particularly at high bit rates BER Tester. Can be investigated qualitatively and perhaps in a pseudo quantitative manner by generating Eye Diagram An intuitive way of viewing parametric performance
Threshold detection and BER To allow the system designers to determine SNR and threshold level required to achieve the specified bit error rate. Useful to calculate the probability of error (BER) Need to establish the noise statistics and compute the probability that the noise level at any given sampling point pushes the signal to the wrong side of the threshold for a 1 or 0 transmitted. Signal Probability distribution functions for 0 & 1 levels. P 0 p0dv v th & v th P1 p1dv BER = P e = a P 0 + b P 1 Fig.1: PDFs for levels of 0 and 1 in the presence of random (Gaussian) noise. Shaded region - For a 0 signal Hatched region - For a 1 signal
Threshold Detection. V 1 V TH V 0 Sampling Instants Bit period 1 2 3 4 Tx Bit 1 0 1 0 Rx Bit 1 0 0 1
Total Probability of Error (Pe) : BER = a P 1 + b P 0 In term of Error Functions : BER P e 1 2 1 erf Q 2 where Q (v v th 0 0 ) (v 1 v 1 th ) (v 1 v v N th ) Q-factor can be estimated from the measured noise voltages and hence BER can be determined For BER in the range of 10-9 to 10-12 ; Q should falls between 6 and 7. Waterfall Curves Fig.2: Error probability Pe versus error probability factor Q Small variations in the Q-factor lead to fairly dramatic changes in the BER. Cannot afford to let the received SNR R drop below specification.
Eye-diagram Test Setup Eye-pattern technique is a simple but powerful measurement method for assessing the data-handling ability of a digital transmission system. Method used extensively for evaluating performance of wired systems and can also be applied to OFC data links. Eye-pattern measurements are made in the time-domain and allow the effects of waveform distortion to be shown immediately on a DSO. Basic Equipment for Eye-diagram Measurements
Experimental Set Up & Measurements
Eye Patterns A visual method to assess the quality of the output of a transmitter or the input / output of a receiver. Although the technique is largely qualitative it can provide some useful quantitative information in terms of trends and whether or not a system is performing to specification. Distance 2km from transmitter Distance 6 km from transmitter
Eye Pattern Interpretation M N is a measure of noise margin. S T is measure of sensitivity-totiming error. Full width noise V 1 Jitte r 20-80% rise time V 0
RMS Noise and Jitter D A provides the following information: Jitter RMS noise can be estimated by a rule of the thumb that total noise on oscilloscope is 5 times the rms noise The mean 1 and 0 levels can also be calculated and hence Q can be estimated Q can be used now to find the BER. J T the range of amplitude differences of the zero crossing, is a measure of the timing jitter. Jitter introduces an uncertainty on the sampling position relative to the centre of the bit period and leads to an increase in error rate.
Power Penalty Significant levels of dispersion and ISI result in reduced received signal levels for 1s and increased level for 0s. This occurs due to the spread of power from 1 bit period into adjacent 0 bit period. This power penalty can be measured from eye diagram and is given as: Power Penalty = 10 log 10 [(V 1D V 0D )/ (V 1 - V 0 )]db
Noise Vs Distortion
Eye Diagram Analysis Often used for assessing the quality of received signal and indeed the quality and integrity of system transmitting it. Although qualitative; provides useful data in terms of trends and system operation as per specifications. Semi-quantitative information about the transmission quality Determination of Q -value and hence BER. Eye diagram showing sample measurements of 20-80% rise time, jitter, full width noise and the mean 0 & 1 levels.
Q-factor Analysis Software Softwares enable a DSO to sample the received signals in the centre of the bit period, transfer the sample to a PC and then to analyse them. The analyses algorithms enable the construction of signal level histogram (i.e. plot of the number of samples occurring in a narrow voltage range Vs voltage) which is essentially the probability distribution of the signal levels around 0 and 1 levels. Theoretical Gaussian distributions are curve fitted within the software to the measure distribution, signal level (noise) variance are extracted and Q-factor & BER are determined.
Histogram A histogram is a function which corresponds to the number of samples having a particular value (a) : Good reception. (b) : Poor reception
Sampling for Q-factor & BER Estimation
Factors affecting BER The main factors affecting BER are: Input Power. Signal to Noise Ratio (SNR). P e SNR
Signal maintenance using Optical Devices
Path Degradation/Engineering Fiber Fiber Original Signals Degraded & Dispersed Signals Amplified & Corrected Signals/Noise & Nonlinear gain Unusable Signal from Noise Generally amplifiers (Repeaters) are used to achieve the required SNR or depending on signal health, regenerators are used for amplification as well as shaping the signal to desired level. To compensate the dispersion (pulse broadening)- DCFs are used either in pre- or post-compensation scheme.
Dispersion and Power maps
High Capacity OFC System Experimental setup for 55 wavelengths WDM transmission using 1300nm optimized fibers and DCFs in the link