- Email: [email protected]
White LED based Faraday current sensor using a quartz wavelength encoder
1 April 1999
Optics Communications 162 Ž1999. 44–48
White LED based Faraday current sensor using a quartz wavelength encoder G.D. Li, R.A. Aspey, G.R. Jones
1
Centre for Intelligent Monitoring Systems, Department of Electrical Engineering and Electronics, UniÕersity of LiÕerpool, LiÕerpool L69 3GJ, UK Received 2 November 1998; received in revised form 20 January 1999; accepted 11 February 1999
Abstract The use of polychromatic light with chromatic modulation has been shown to provide a different approach to Faraday current sensing. In this contribution the use of purely white LEDs along with a quartz rotary encoder and a multiple reflection Faraday sensing element is described for realizing a practical form of a chromatic modulation Faraday current sensor. It is shown that such a sensor embodiment leads to a system sensitivity, for a chromatic system, which is approximately a factor of three times higher than previously reported Ž0.65 nm Ay1 compared with 0.19 nm Ay1 .. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The use of polychromatic modulation techniques in conjunction with Faraday rotation effects have been surveyed by Jones et al. w1x. They are attractive in being largely independent of light intensity fluctuations and less sensitive to the temperature dependence of the Verdet constant and vibrational effects. Three main polychromatic schemes were identified by Jones et al. w1x, namely Ø referenced Faraday rotation in which the intensity change of polarized shorter wavelength Žvisible. light caused by Faraday rotation is compared with the intensity of unpolarised infrared light from the same tungsten halogen source. Ø waÕelength dependent Verdet constant in which chromatic modulation of polarized polychromatic light is produced by the wavelength dependence of the Verdet constant. Ø waÕelength dependent rotary power element in which the polarized polychromatic light is wavelength encoded by an element such as BSO which possesses a natural rotary power.
1
Corresponding author. E-mail: [email protected]
Although all three approaches perform well in principle and a referenced Faraday rotation unit has been successfully field tested w2x for transformer tap changer applications, further investigations are warranted of such systems. For instance one of the limitations to the referenced Faraday system would appear to be the temperature dependence of the extinction edge of the polarising filter Žalthough this may in principle be alleviated through the use of a notch filter at the expense of reduced sensitivity.. The wavelength dependent Verdet constant and rotary element approaches have hitherto been limited by the non-availability of suitable polychromatic light sources covering the shorter wavelength at the expense of the less affected longer wavelengths w1x. There has also been the need to address the problem of producing an appropriate optical path length within the Faraday element itself in order to maximize the Faraday rotation whilst minimising the attenuation w3x. The relative performance of these various schemes which have hitherto been identified with a tungsten halogen source are given in the first columns of Table 1. A number of these potential difficulties may in principle be alleviated through the use of the newly available white light emitting diodes Žwhite LEDs. which have recently become commercially available w4x. These LEDs
0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 0 7 8 - 4
Table 1 Comparison of various chromatic Faraday sensor systems. ŽChromatic sensors are primarily judged in terms of dominant wavelength changes Ž =l. w6x. A change of 0.01 nm in =l may be taken as a practically demonstrable minimum change which is measurable Žsee, e.g., Ref. w1x.
G.D. Li et al.r Optics Communications 162 (1999) 44–48
Dominant wavelength shift may be increased through the use of an aluminum reflecting film to reduce elliptical polarization effects at the expense of signal power. 45
46
G.D. Li et al.r Optics Communications 162 (1999) 44–48
have a high intensity at short wavelengths Ž; 450 nm. and a spectral width covering the visible part of the spectrum only, range from 420–700 nm, which falls well within the polarization range of the visible polarizers ŽF 850 nm. ŽFig. 1.. In this contribution we examine the use of such LEDs in conjunction with the wavelength dependent Verdet constant and the wavelength dependent rotary power techniques. Both theoretical and experimental results are presented for these two cases and their performances for measuring electric current are assessed and compared. Fig. 2. Structure of the Faraday current sensor with a quartz encoder and white LED.
2. Sensor head The Faraday sensor head used in the present study was similar in structure to that described by Li et al. w3x consisting of a multiple reflection SF 57 glass Faraday element sandwiched in a gap in a B field concentrating iron core and optically addressed via input and output ball
lenses, input polarizer, output analyzer. However for the present work a quartz rotary power element was introduced along with an Oxley white LED. Photodetection was achieved with a PD150 double layer photodiode Žsee, e.g., Refs. w1,5x.. The structure of the sensing head is shown in Fig. 2.
3. Wavelength dependence of the optical elements The wavelength dependence of the Verdet constant of SF 57 glass decreases with wavelength from 90 at 400 nm to 15 at 800 nm w1x, so emphasizing the higher sensitivities at the shorter wavelengths. The effect is clearly discernible on the theoretical spectral curve for B s 0.2 T compared with the B s 0 curve for the white LED ŽFig. 1a. which shows a considerably higher attenuation at the shorter wavelengths. The attenuation of the light propagating through the SF 57 glass element due to multiple reflections and elliptical polarization effects has been discussed by Li et al. w3x. Instead of the BSO rotary element used by Jones et al. w1x a quartz rotary element is preferred because of its better robustness, stability and lower optical attenuation at shorter wavelengths. The angular rotation suffered by linearly polarized light of different wavelengths propagating through a 4.25 mm quartz plate is shown in Fig. 3. The higher sensitivity at the shorter wavelengths is apparent whereby the 450 nm light is rotated through 2008 whilst the 800 nm light is only rotated through 458. The transfer function for the sensing element, Ms , is given by Ms s Q w F6 R 5 F5 R 4 F4 R 3 F3 R 2 F2 R1 F1 ,
Fig. 1. Output spectra from the white LED for different B fields. Ža. Wavelength dependent Verdet constant effect. Žb. Quartz rotary element effect.
Ž1.
where Fi is the Jones matrix representing the Faraday rotation of the ith pass in the Faraday element, R i is the Jones matrix for reflection at point i and Q w is the Jones matrix for the natural rotation caused by the quartz plate. The effect of Faraday rotation on the broadband spectrum generated by the white LED and passed through the quartz and SF 57 glass combination is shown in Fig. 1b. By comparison with the spectra of Fig. 1a for the Faraday rotator alone, it is clear that the spectral changes with the
G.D. Li et al.r Optics Communications 162 (1999) 44–48
47
Faraday-quartz combination are greatly enhanced in that the decreased intensities at longer wavelengths Ž) 520 nm. are accompanied by an increase intensity at shorter wavelengths. Such a ‘push-pull’ characteristic is particularly attractive for chromatic sensing.
4. Chromatic monitoring of the polychromatic signals The spectral changes predicted theoretically in Figs. 1a and 1b may be addressed in practice using the chromatic modulation approach w1x. The simplest manifestation of such monitoring is to determine the dominant wavelength of each spectral signature from the ratio of the outputs from each of the diodes of the PD150 multilayered diode i.e.
ld A
I PD2 y aIPD1 I PD2 q bIPD1
,
Fig. 4. Dominant wavelength as a function of current in Amp-turns with and without a quartz rotary encoder.
Ž2.
where I PD1 and I PD2 are the output photocurrents of the photodiodes and a, b are constants. Theoretical calculations of the dominant wavelength shift using the output spectrum of the white LED ŽFig. 1., the responsiveness of the two PD150 photodiodes w3x, the wavelength dependence of the SF 57 Verdet constant w1x and the rotary power of the quartz element ŽFig. 3. show that for a B field change from y0.2 to q0.2 T, the dominant wavelength changes by 24 nm without the quartz element and by 40 nm with quartz. The implications are that the quartz element almost doubles the resolution and gives a more linear output. Results of experimental tests with the quartzrSF 57 system are shown in Fig. 4 in the form of dominant
Fig. 3. Rotation of plane of polarization produced by a 4.25 mm thick quartz plate at various wavelengths.
wavelength as a function of the B field producing current in the range y800 to q800 AT ŽAmp-turns.. The B-field for the experimental setup was produced by a 40 turn winding around the gapped iron core concentrator, with a current varied from y20 A to q20 A. The experimental results confirm the theoretically predicted trend that the quartz element system has a higher resolution Ž D ld ; 26 nm. than the system without quartz Ž D ld ; 15 nm..
5. Conclusions A practical realisation of a multiple reflection Faraday current sensor using polychromatic light from a white LED Žspectral range 420–700 nm. in conjunction with a quartz rotary element and a double diode detector has been achieved. The system has a dynamic range of 26 nm measured in dominant wavelength shift compared with a potential resolution capability of the double diode detector of 0.01 nm w6x. This dynamic range of 26 nm compares with a dynamic range of only 9.5 nm achieved with a tungsten halogen source and a plastic optical fibre w1x. The temperature susceptibility of the quartz rotator has been shown to be negligibly small. The performance of the present scheme is compared with achievements to date obtained with the other chromatic Faraday sensing schemes in Table 1 and shown to have improved the dominant wavelength range. Further improvements in performance may be anticipated from a better matching of the responsiveness of the photodetectors to the spectral signature change. The present system has only been used in the current range of "20 A, this governed by the B-field concentrator ŽFig. 2. and not the optical elements. Through the use of an appropriate field concentrator the optical system is currently being tested up to several kiloamperes.
48
G.D. Li et al.r Optics Communications 162 (1999) 44–48
Acknowledgements The authors wish to acknowledge the support provided by EPSRC without which this work could not have been undertaken.
References w1x G.R. Jones, G. Li, J.W. Spencer, R.A. Aspey, M.G. Kong, Optics Comm. 145 Ž1998. 203.
w2x K.G. Lewis, R.E. Jones, G.R. Jones, A tap changer monitoring system incorporating optical sensors, in: Proc. 2nd Int. Conf. Reliab. Trans. and Distri. Equip., Warwick, 1995, England. w3x G.D. Li, R.A. Aspey, M.G. Kong, J.R. Gibson, G.R. Jones, Measur. Sci. Technol. 10 Ž1999. 25. w4x Oxley Developments Company Ltd., Priory Park, Ulverston, Cumbria, LA12 9QG, UK. w5x Sharp Corp., PD150 Dual layer photodiode – Technical data sheet, International Business Group, Electronic Component Sales Department, 22 Nagaike-cho, Abernoku, Osaka 545, Japan, 1992. w6x G.R. Jones, P.C. Russell, Pure Appl. Optics 2 Ž1993. 87.
Optics Communications 162 Ž1999. 44–48
White LED based Faraday current sensor using a quartz wavelength encoder G.D. Li, R.A. Aspey, G.R. Jones
1
Centre for Intelligent Monitoring Systems, Department of Electrical Engineering and Electronics, UniÕersity of LiÕerpool, LiÕerpool L69 3GJ, UK Received 2 November 1998; received in revised form 20 January 1999; accepted 11 February 1999
Abstract The use of polychromatic light with chromatic modulation has been shown to provide a different approach to Faraday current sensing. In this contribution the use of purely white LEDs along with a quartz rotary encoder and a multiple reflection Faraday sensing element is described for realizing a practical form of a chromatic modulation Faraday current sensor. It is shown that such a sensor embodiment leads to a system sensitivity, for a chromatic system, which is approximately a factor of three times higher than previously reported Ž0.65 nm Ay1 compared with 0.19 nm Ay1 .. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The use of polychromatic modulation techniques in conjunction with Faraday rotation effects have been surveyed by Jones et al. w1x. They are attractive in being largely independent of light intensity fluctuations and less sensitive to the temperature dependence of the Verdet constant and vibrational effects. Three main polychromatic schemes were identified by Jones et al. w1x, namely Ø referenced Faraday rotation in which the intensity change of polarized shorter wavelength Žvisible. light caused by Faraday rotation is compared with the intensity of unpolarised infrared light from the same tungsten halogen source. Ø waÕelength dependent Verdet constant in which chromatic modulation of polarized polychromatic light is produced by the wavelength dependence of the Verdet constant. Ø waÕelength dependent rotary power element in which the polarized polychromatic light is wavelength encoded by an element such as BSO which possesses a natural rotary power.
1
Corresponding author. E-mail: [email protected]
Although all three approaches perform well in principle and a referenced Faraday rotation unit has been successfully field tested w2x for transformer tap changer applications, further investigations are warranted of such systems. For instance one of the limitations to the referenced Faraday system would appear to be the temperature dependence of the extinction edge of the polarising filter Žalthough this may in principle be alleviated through the use of a notch filter at the expense of reduced sensitivity.. The wavelength dependent Verdet constant and rotary element approaches have hitherto been limited by the non-availability of suitable polychromatic light sources covering the shorter wavelength at the expense of the less affected longer wavelengths w1x. There has also been the need to address the problem of producing an appropriate optical path length within the Faraday element itself in order to maximize the Faraday rotation whilst minimising the attenuation w3x. The relative performance of these various schemes which have hitherto been identified with a tungsten halogen source are given in the first columns of Table 1. A number of these potential difficulties may in principle be alleviated through the use of the newly available white light emitting diodes Žwhite LEDs. which have recently become commercially available w4x. These LEDs
0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 0 7 8 - 4
Table 1 Comparison of various chromatic Faraday sensor systems. ŽChromatic sensors are primarily judged in terms of dominant wavelength changes Ž =l. w6x. A change of 0.01 nm in =l may be taken as a practically demonstrable minimum change which is measurable Žsee, e.g., Ref. w1x.
G.D. Li et al.r Optics Communications 162 (1999) 44–48
Dominant wavelength shift may be increased through the use of an aluminum reflecting film to reduce elliptical polarization effects at the expense of signal power. 45
46
G.D. Li et al.r Optics Communications 162 (1999) 44–48
have a high intensity at short wavelengths Ž; 450 nm. and a spectral width covering the visible part of the spectrum only, range from 420–700 nm, which falls well within the polarization range of the visible polarizers ŽF 850 nm. ŽFig. 1.. In this contribution we examine the use of such LEDs in conjunction with the wavelength dependent Verdet constant and the wavelength dependent rotary power techniques. Both theoretical and experimental results are presented for these two cases and their performances for measuring electric current are assessed and compared. Fig. 2. Structure of the Faraday current sensor with a quartz encoder and white LED.
2. Sensor head The Faraday sensor head used in the present study was similar in structure to that described by Li et al. w3x consisting of a multiple reflection SF 57 glass Faraday element sandwiched in a gap in a B field concentrating iron core and optically addressed via input and output ball
lenses, input polarizer, output analyzer. However for the present work a quartz rotary power element was introduced along with an Oxley white LED. Photodetection was achieved with a PD150 double layer photodiode Žsee, e.g., Refs. w1,5x.. The structure of the sensing head is shown in Fig. 2.
3. Wavelength dependence of the optical elements The wavelength dependence of the Verdet constant of SF 57 glass decreases with wavelength from 90 at 400 nm to 15 at 800 nm w1x, so emphasizing the higher sensitivities at the shorter wavelengths. The effect is clearly discernible on the theoretical spectral curve for B s 0.2 T compared with the B s 0 curve for the white LED ŽFig. 1a. which shows a considerably higher attenuation at the shorter wavelengths. The attenuation of the light propagating through the SF 57 glass element due to multiple reflections and elliptical polarization effects has been discussed by Li et al. w3x. Instead of the BSO rotary element used by Jones et al. w1x a quartz rotary element is preferred because of its better robustness, stability and lower optical attenuation at shorter wavelengths. The angular rotation suffered by linearly polarized light of different wavelengths propagating through a 4.25 mm quartz plate is shown in Fig. 3. The higher sensitivity at the shorter wavelengths is apparent whereby the 450 nm light is rotated through 2008 whilst the 800 nm light is only rotated through 458. The transfer function for the sensing element, Ms , is given by Ms s Q w F6 R 5 F5 R 4 F4 R 3 F3 R 2 F2 R1 F1 ,
Fig. 1. Output spectra from the white LED for different B fields. Ža. Wavelength dependent Verdet constant effect. Žb. Quartz rotary element effect.
Ž1.
where Fi is the Jones matrix representing the Faraday rotation of the ith pass in the Faraday element, R i is the Jones matrix for reflection at point i and Q w is the Jones matrix for the natural rotation caused by the quartz plate. The effect of Faraday rotation on the broadband spectrum generated by the white LED and passed through the quartz and SF 57 glass combination is shown in Fig. 1b. By comparison with the spectra of Fig. 1a for the Faraday rotator alone, it is clear that the spectral changes with the
G.D. Li et al.r Optics Communications 162 (1999) 44–48
47
Faraday-quartz combination are greatly enhanced in that the decreased intensities at longer wavelengths Ž) 520 nm. are accompanied by an increase intensity at shorter wavelengths. Such a ‘push-pull’ characteristic is particularly attractive for chromatic sensing.
4. Chromatic monitoring of the polychromatic signals The spectral changes predicted theoretically in Figs. 1a and 1b may be addressed in practice using the chromatic modulation approach w1x. The simplest manifestation of such monitoring is to determine the dominant wavelength of each spectral signature from the ratio of the outputs from each of the diodes of the PD150 multilayered diode i.e.
ld A
I PD2 y aIPD1 I PD2 q bIPD1
,
Fig. 4. Dominant wavelength as a function of current in Amp-turns with and without a quartz rotary encoder.
Ž2.
where I PD1 and I PD2 are the output photocurrents of the photodiodes and a, b are constants. Theoretical calculations of the dominant wavelength shift using the output spectrum of the white LED ŽFig. 1., the responsiveness of the two PD150 photodiodes w3x, the wavelength dependence of the SF 57 Verdet constant w1x and the rotary power of the quartz element ŽFig. 3. show that for a B field change from y0.2 to q0.2 T, the dominant wavelength changes by 24 nm without the quartz element and by 40 nm with quartz. The implications are that the quartz element almost doubles the resolution and gives a more linear output. Results of experimental tests with the quartzrSF 57 system are shown in Fig. 4 in the form of dominant
Fig. 3. Rotation of plane of polarization produced by a 4.25 mm thick quartz plate at various wavelengths.
wavelength as a function of the B field producing current in the range y800 to q800 AT ŽAmp-turns.. The B-field for the experimental setup was produced by a 40 turn winding around the gapped iron core concentrator, with a current varied from y20 A to q20 A. The experimental results confirm the theoretically predicted trend that the quartz element system has a higher resolution Ž D ld ; 26 nm. than the system without quartz Ž D ld ; 15 nm..
5. Conclusions A practical realisation of a multiple reflection Faraday current sensor using polychromatic light from a white LED Žspectral range 420–700 nm. in conjunction with a quartz rotary element and a double diode detector has been achieved. The system has a dynamic range of 26 nm measured in dominant wavelength shift compared with a potential resolution capability of the double diode detector of 0.01 nm w6x. This dynamic range of 26 nm compares with a dynamic range of only 9.5 nm achieved with a tungsten halogen source and a plastic optical fibre w1x. The temperature susceptibility of the quartz rotator has been shown to be negligibly small. The performance of the present scheme is compared with achievements to date obtained with the other chromatic Faraday sensing schemes in Table 1 and shown to have improved the dominant wavelength range. Further improvements in performance may be anticipated from a better matching of the responsiveness of the photodetectors to the spectral signature change. The present system has only been used in the current range of "20 A, this governed by the B-field concentrator ŽFig. 2. and not the optical elements. Through the use of an appropriate field concentrator the optical system is currently being tested up to several kiloamperes.
48
G.D. Li et al.r Optics Communications 162 (1999) 44–48
Acknowledgements The authors wish to acknowledge the support provided by EPSRC without which this work could not have been undertaken.
References w1x G.R. Jones, G. Li, J.W. Spencer, R.A. Aspey, M.G. Kong, Optics Comm. 145 Ž1998. 203.
w2x K.G. Lewis, R.E. Jones, G.R. Jones, A tap changer monitoring system incorporating optical sensors, in: Proc. 2nd Int. Conf. Reliab. Trans. and Distri. Equip., Warwick, 1995, England. w3x G.D. Li, R.A. Aspey, M.G. Kong, J.R. Gibson, G.R. Jones, Measur. Sci. Technol. 10 Ž1999. 25. w4x Oxley Developments Company Ltd., Priory Park, Ulverston, Cumbria, LA12 9QG, UK. w5x Sharp Corp., PD150 Dual layer photodiode – Technical data sheet, International Business Group, Electronic Component Sales Department, 22 Nagaike-cho, Abernoku, Osaka 545, Japan, 1992. w6x G.R. Jones, P.C. Russell, Pure Appl. Optics 2 Ž1993. 87.