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1、AVAILABLE Application Note: HFAN-02.2.2 Rev1; 04/08 Optical Modulation Amplitude (OMA) and Extinction Ratio Application Note HFAN-02.2.2 (Rev.1; 04/08) Maxim Integrated Page 2 of 5 Optical Modulation Amplitude (OMA) and Extinction Ratio 1 Introduction The optical modulation amplitude (OMA) of a sign
2、al is an important parameter that is used in specifying the performance of optical links used in digital communication systems. The OMA directly influences the system bit error ratio (BER). With an appropriate point of reference (such as average power), OMA can be directly related to extinction rati
3、o. The purpose of this application note is to define OMA and how it relates to other parameters such as extinction ratio and average power. Further, this application note will clarify the trade-offs between specifying OMA versus extinction ratio and explore appropriate specification ranges for each.
4、 2 Definitions and Relationships For bi-level optical signaling schemes, such as nonreturn-to-zero (NRZ), only two discrete optical power levels are used. The higher level represents a binary one, and the lower level represents a zero. We will use the symbol P1 to represent the high power level and
5、the symbol P0 to represent the low power level. Using these symbols we can mathematically define a number of useful terms and relationships. OMA is defined as the difference between the high and low levels, which can be written mathematically as: 01 PPOMA= (1) Average power is simply the average of
6、the two power levels, i.e., 2 01 PP PAVG + = (2) We will use re to represent the extinction ratio, which is the ratio between the high and low power levels: 0 1 P P re= (3) Through algebraic manipulation of equations 1, 2, and 3, we can derive the following relationships: ? ? ? ? ? ? + = 1 1 2 e e A
7、VG r r POMA (4) 1 0 += P OMA re (5) ? ? ? ? ? ? + =+= 1 2 2 1 1 e e AVGAVG r r POMAPP (6) ? ? ? ? ? ? + = 1 1 2 2 1 0 e AVGAVG r POMAPP (7) 3 Absolute Versus Relative Specs OMA and extinction ratio by themselves are relative quantities, since they only specify the difference or ratio of the power le
8、vels. In order to derive an absolute quantity from the OMA or extinction ratio, we must have an additional point of reference, such as PAVG, P1, or P0. The relationships of equations 4-7 all depend on one of these absolute points of reference. For example, an OMA of 100?W can correspond to an infini
9、te number of possible values for PAVG, P1, or P0: P1 could be 100?W with P0 equal to 0?W, or P1 could be 150?W with P0 equal to 50?W, or P1 could be 100mW with P0 equal to 99.9mW, etc., etc. In the alternate case of extinction ratio, a similar example using re=10 can correspond to an infinite number
10、 of possible values for PAVG, P1, or P0: P1 could be 100?W with P0 equal to 10?W, or P1 could be 150?W with P0 equal to 15?W, or P1 could be 100mW with P0 equal to 10mW, etc., etc. Application Note HFAN-02.2.2 (Rev.1; 04/08) Maxim Integrated Page 3 of 5 If, in addition to the OMA or extinction ratio
11、, we specify a reference point of PAVG = 100?W, for example, then the ambiguity is gone. With an OMA of 100?W and PAVG = 100?W, P1 can only be 150?W and P0 can only be 50?W. If the extinction ratio is 10 and PAVG = 100?W, then P1 can only be 182?W and P0 can only be 18.2?W. 4 Optical Attenuation Up
12、to this point in the discussion, it may seem apparent that OMA and extinction ratio are basically equivalent. Either can be computed with knowledge of the other and one reference point. Both can be quantified when the values of P1 and P0 are known, etc. There are differences, however, and one of the
13、se is how OMA and extinction ratio change as the signal propagates through an optical system. Assuming a system with linear attenuation between two points, the extinction ratio will stay constant even though the signal is attenuated, while the OMA will change by a factor equal to the attenuation. Fo
14、r example, over 10km of optical fiber with an attenuation of 0.3dB/km, the total attenuation over the length of the fiber is 3dB, which is equivalent to a factor of 2. If we transmit a signal through the fiber that starts with P1 = 1mW and P0 = 0.1mW, then re = 1/0.1 = 10 and OMA = 1 0.1 = 0.90mW at
15、 the fiber input. After passing through the fiber the signal levels are reduced by a factor of 2, so P1 = 0.5mW and P0 = 0.05mW. Therefore, at the fiber output, re = 0.5/0.05 = 10 (the same as at the input re) and OMA = 0.5 0.05 = 0.45mW (half of the input OMA). From this example we see that once th
16、e extinction ratio is known, a simple average power measurement anywhere in the system will yield enough information to calculate P1, P0, and even OMA. On the other hand, if we have knowledge of the OMA at one point in the system, we cannot determine its value after attenuation without knowing the magnitude of the attenuation or else measuring additional parameters (such as P0, P1, or PAVG). 5 Power-Level Effects on Transmitters and Receivers In theory, the system bit error
