Influence of LED Optical Characteristic Variations on Performance of a Plastic Optical Fiber Transmission Link E.Moschim, H.Ding, A.Destrez and G.Attal Ecole Superieure d'Electricite, Service Radioelectricite et Electronique Plateau du Moulon, 91190, Gif sur Yvette, France Abstract Plastic optical fiber transmission link (POFTL) becomes an atractive option for short distance LAN's and industrial applications. In this work, we investigate the influence of LED optical characteristic variations due to its junction temperature changements on performances of a PMMA core POFTL. A selective loss As is introduced for this purpose. The value of As is found to be 0.330/km °C. Introduction Recently plastic optical fiber transmission link (POFTL) becomes an atractive option for short distance LAN's and industrial applications, because of its low installation and maintenance cost.] PMMA core plastic fiber, which is actually available on the market, has the following particular properties: its attenuation is 1) high and, 2) very sensitive to light wavelength. As a consequence, the maximal length of; POFTL without repeater is limited to low values and all LED spectral variations will change POFTL performance to a nonnegligible level. Our aim is to investigate this effect. We give out in section II some measured optical characteristic variations of a LED at 660nm due to its junction temperature changements. This kind of LED is well adapted for the POFTL applications.2 In section III, a simple POFTL model is proposed. The influence of LED optical characteristic variations on performances of POFTL is examined by introducing a selective loss As. Section IV is our conclusions. II. Optical characteristic variations of a LED at 660nm caused by junction temperature changement Any changement in the junction temperature of a LED will alter its principal optical characteristics: peak emission wavelength, spectrum, emission power and modulation linearity. Junction temperature changements may be caused by ambient temperature variations or by LED's bias current variations. Several works have been devoted to this subject.i Here we show only the measured results of a LED at 660nm, which are used in our study. The LED under test has GaA1As structure 4 and the experimental set up is shown in fig.1.5 All our results are represented as a function of the ambient temperature when the LED's bias current is kept at 50mA, and as a function of the LED's bias current when the ambient temperature is kept at 30 °C. The variation of the peak emission wavelength versus the ambient temperature ( opened circle ) and the LED's bias current ( solid circle ) is shown in fig.2. The LED's peak emission wavelength Ap(um) shifts to the near red region when its junction temperature Tj increases. This effect can be explained by the following relations 6 AP 1.24 Eg T2 Eg(Tj)=Eg(0)- oiTj +Tj ) (1) (2) where Eg is the energy bandgap (eV) and a, ß are two constants that depend on the LED's structure. The calculated Eg in using (1) is plotted in the same figure. We gee that Eg versus Tj relation is nearly linear this observation agrees with the theoretical result given by Another LED spectral property is its spectral form, whose variation versus the ambient temperature and bias current is shown in fig.3. The spectral form is determined by the electron -hole recombination process in the active region. The half power spectral width (FWHM) La is expressed bye dX=0.81 TJk (Oiñp) q 110 / SPIE Vol. 867 Optical Devices in Adverse Environments (1987) (3) Influence of LED Optical Characteristic Variations on Performance of a Plastic Optical Fiber Transmission Link E.Moschim, H.Ding, A.Destrez and G.Attal Ecole Superieure d'Electricite, Service Radioelectricite et Electronique Plateau du Moulon, 91190, Gif sur Yvette, France Abstract Plastic optical fiber transmission link (POFTL) becomes an atractive option for short distance LAN's and industrial applications. In this work, we investigate the influence of LED optical characteristic variations due to its junction temperature changements on performances of a PMMA core POFTL. A selective loss A s is introduced for this purpose. The value of A s is found to be 0.33dB/km°C. Introduction Recently plastic optical fiber transmission link (POFTL) becomes an atractive option for short distance LAN's and industrial applications, because of its low installation and maintenance cost.l PMMA core plastic fiber, which is actually available on the market, has the following particular properties: its attenuation is 1) high and, 2) very sensitive to light wavelength. As a consequence, the maximal length of; POFTL without repeater is limited to low values and all LED spectral variations will change POFTL performance to a nonnegligible level. Our aim is to investigate this effect. We give out in section II some measured optical characteristic variations of a LED at 660nm due to its junction temperature changements. This kind of LED is well adapted for the POFTL applications.2 In section III, a simple POFTL model is proposed. The influence of LED optical characteristic variations on performances of POFTL is examined by introducing a selective loss AS» Section IV is our conclusions* II. Optical characteristic variations of a LED at 660nm caused by junction temperature changement Any changement in the junction temperature of a LED will alter its principal optical characteristics: peak emission wavelength, spectrum, emission power and modulation linearity. Junction temperature changements may be caused by ambient temperature variations or by LED's bias current variations. Several works have been devoted to this subject.3 Here we show only the measured results of a LED at 660nm, which are used in our study. The LED under test has GaAlAs structure ^ and the experimental set up is shown in fig.1. All our results are represented as a function of the ambient temperature when the LED's bias current is kept at 50mA, and as a function of the LED's bias current when the ambient temperature is kept at 30°C. The variation of the peak emission wavelength versus the ambient temperature ( opened circle ) and the LED's bias current ( solid circle ) is shown in fig.2. The LED's peak emission wavelength Ap(um) shifts to the near red region when its junction temperature Tj increases. This effect can be explained by the following relations 6 1.24 (1) (2) where Eg is the energy bandgap (eV) and CX,^ are two constants that depend on the LED's structure. The calculated Eg in using (1) is plotted in the same figure. We see that Eg versus Tj relation is nearly linear this observation agrees with the theoretical result given by °* Another LED spectral property is its spectral form, whose variation versus the ambient temperature and bias current is shown in fig.3. The spectral form is determined by the electron-hole recombination process in the active region. The half power spectral width (FWHM) A\ is expressed by (3) 110 / SPIE Vol. 867 Optical Devices in Adverse Environments (1987) Downloaded From: http://spiedigitallibrary.org/ on 09/18/2012 Terms of Use: http://spiedl.org/terms