1、 TIA TELECOMMUNICATIONS SYSTEMS BULLETIN Fiber Optic Power Meters: Measurement and Application Issues TSB-143 July 2007TELECOMMUNICATIONS INDUSTRY ASSOCIATION The Telecommunications Industry Association represents the communications sector of NOTICE TIA Engineering Standards and Publications are des
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22、TAL ELEMENT OF THE USE OF THE CONTENTS HEREOF, AND THESE CONTENTS WOULD NOT BE PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TSB-143 iTSB-143 Fiber Optic Power Meters: Measurement and Application Issues Contents 1. Scope 1 2. Description . 2 3. Critical Performance Issues 4 3.1 Detector . 4 3.2 Collect
23、ing Optics . 10 3.3 Electronics 12 4. Calibration Issues 14 4.1 Environmental conditions 14 4.2 Absolute power calibration. 14 4.3 Optical linearity 19 4.3 Calibration versus verification 22 5. Conclusion . 23 6. References . 24 TSB-143 ii This Page is Blank TSB-143 iii_ Foreword _ This Standard was
24、 formulated under the cognizance of TIA Subcommittee FO-6.1 (later becoming FO-4.5). Future document and subject matter support will fall under TIA Subcommittee FO-4.5. Key words: fiber optics, optical power, dB, dBm TSB-143 iv This Page is Blank TSB-143 1TSB-143 Fiber Optic Power Meters: Measuremen
25、t and Application Issues _ 1. Introduction _ 1.1 Scope The proper calibration of an optical power meter requires the verification of two critical parameters: absolute accuracy at a given reference power over a wavelength range and linearity at a given wavelength over a power range. We discuss the ke
26、y elements involved in determining the uncertainty in the absolute accuracy and the non-linearity of the power response under what are termed “reference conditions of calibration”. We also discuss the underlying causes of erroneous measurements resulting from each of the three subsystems constitutin
27、g a power meter: detector element, collecting optics and electronics. TSB-143 2_ 2. Description _ Ever since the discovery of the photovoltaic effect in the late nineteenth century, optical scientists have struggled to accurately correlate the electrical current generated by a photosensitive surface
28、 (i.e. a detector) to the incident optical power. Rapid developments in fiber-optic telecommunications have pushed the requirements for optical measurement accuracy to ever more stringent levels. Consequently, it is still a challenge to calibrate a fiber-optic power meter if one seeks a high degree
29、of absolute accuracy and linearity over a wide range of input power levels and across a wide wavelength range such as: From -90 dBm (or 1 pW) to +30 dBm (or 1 W); and From 600 nm to 1700 nm. Ultimately, the establishment of a correspondence between the essentially thermodynamic concept of incident o
30、ptical energy and the detected electrical current requires a careful calorimetric determination of the optical power, generally first by means of a cryogenic radiometer. Such a measurement, normally painstakingly undertaken at a national standards laboratory, represents the first step in a rigorous
31、calibration chain. A typical example of this calibration chain is presented in Figure 1. The awkward and non-transportable primary reference standard, characterized by a very low absolute uncertainty, is generally used to calibrate in turn a slightly less accurate but more practical semiconductor th
32、ermopile or an electrically calibrated pyroelectric radiometer (ECPR). The national standards laboratory then performs an absolute calibration of a photoelectric detector serving as a “transfer” standard, that becomes a reference standard when used under what is known as the “reference conditions” a
33、ppropriate to the nature of the optical beam under test. Since the radiometer cannot provide absolute measurements at full rated accuracy below a relatively high optical power (typically 10 or 100 W), calibration of the photodetector transfer standard at lower powers should then proceed via a rigoro
34、us determination of the response linearity, to be discussed in more detail below. Although expensive, these national laboratory transfer standards, with an absolute accuracy of typically 1-% can be purchased and used as a reference for a commercial calibration system. TSB-143 3Figure 1. Typical Powe
35、r Meter Calibration Process When considered as a generic optical detection system, the two key issues characterizing the calibration of a fiber-optic power meter are: Absolute calibration of the response at one or more wavelengths (generally over a wavelength range) and at a reference power level (s
36、ame as the one used by the national standards laboratory); and Linearity of the response with varying input power level and at a reference wavelength such as 1550 nm. These issues should be considered for a number of well-defined reference conditions1, to be discussed in the following section. Natio
37、nal Laboratory Primary Standard: Cryogenic Radiometer National Laboratory Working Standard: Electrically Calibrated Pyroelectric Thermopile (ECPR)Thermopile Radiometer Commercial Reference Standard: Photoelectric-Based Calibrated Power Meter Commercial Working Standard: Photoelectric-Based Power Met
38、erCommercial Power Meter 950 1550 1650/20 mW Few mW Linearity Dependence on Power (P) P Density Total Input P P Density Surface Dependence of Response Uniformity Good Best Strong Polarization Dependent Loss (PDL) Good Good Strong TSB-143 113.2.2 Connector issues The uniformity of the response of the
39、 detector over its surface is also very important when calibrated with a multimode fiber illuminating a large portion of it and using it with a singlemode fiber producing a smaller spot. The use of an angle polished contact (APC) connector on the fiber end can also cause some problems. Even if the d
40、etector is calibrated with a singlemode fiber, the light emerges from a fiber with an APC connector with an angle impinging on the detector at a different location than at the calibration. The uniformity of the response is then a key point. Another problem is that the angle of 8 of the APC connector
41、 induces an intrinsic polarization sensitivity of about 0.02 dB (0.5%), which is non-negligible in some applications. 3.2.3 Detector issues A last point to consider in the design of the collection optics of a power meter is the reflection coming from the detector. This reflected light could come bac
42、k to the ferrule that usually terminates the fiber and back again on the detector causing a false increase of the power read. The detector should be as far as possible from the fiber to decrease this effect since the light is diverging more and more and the detector should have an angle with respect
43、 to the ferrule. But even then, the fiber-optic connector adapter covered with a black surface should cover most of the ferrule. This is especially important when metallic ferrules are used instead of ceramic ferrules. In summary, Figure 7 provides a list of the critical design factors affecting the
44、 proper calibration performance of fiber-optic power meter for different applications such as multimode (MM) and singlemode (SM). Figure 7. Optical Design Issues for Multimode (MM) and Singlemode (SM) Calibration of Fiber-Optic Power Meters Optical Design Issues MM SM Detector Size (minimize edge ef
45、fects) 5 mm (Large Area) 5 mm (Large Area) White Light Source (800-1700 nm) MM Fiber MM Fiber Laser (1310/1550 nm) - SM Fiber Detector/Source Distance - Minimize detector non-uniformity As Close as Possible As Close as Possible - Minimize laser coherence effects - As Far as Possible w Detector at an
46、 Angle - Minimize fiber-detector reflection As Far as Possible w Detector at an Angle As Far as Possible w Detector at an Angle APC Connector Never Never Minimize PDL Effects No Angle No Angle Ferrule Issue Better w Ceramic Better w Ceramic TSB-143 123.3 Electronics The electronics is an important p
47、art of an optical power meter. It should be perfectly adapted to the detector parameters such as its shunt resistance and its capacity to optimize the performance. A compromise has to be made to have low noise and high filtering without excessively reducing the bandwidth. For applications where very high speed is not required, a transimpedance amplifier is recommended. This configuration minimizes the noise keeping the detector near zero volt bias. As observed in Figure 8, the dark current is minimum when the bias voltage is m
