- Email: [email protected]
Multiplexing and transmission of RF signals using an optical fiber
Ultrasonics 38 (2000) 542–545 www.elsevier.nl/locate/ultras
Multiplexing and transmission of RF signals using an optical fiber Jose´ Curpia´n Alonso a, *, Francisco Montero de Espinosa b, Manuel Lo´pez-Amo c a Universidad de Jae´n, Dpto. Electro´nica, E.U.P Linares, Alfonso X el Sabio 28, 23700 Linares, Spain b Instituto de Acu´stica, CSIC, Serrano 144, 28006 Madrid, Spain c Universidad Pu´blica de Navarra, Campus de Arrosadı´a s/n, Pamplona, Spain
Abstract Ultrasonic non-destructive testing systems designed to control huge structures normally use several transducers in the reception stage. To avoid increasing the cost of electronics, a multiplexer is used to send all received signals to the same processing module. Traditionally, transmission of such signals is carried out using copper cables. For special applications (i.e. continuous monitoring of nuclear plants) metallic cables are not suitable because of their high sensitivity to electromagnetic perturbations. Moreover, the multiplexing is made electronically. When the distance between the transducers and the reception unit is large and/or electromagnetic noise is important, signal degradation takes place. The proposed system implements the transmission and multiplexing of ultrasonic electrical signals obtained by means of broadband transducers (up to 1 MHz), using an optical fiber. Optical fibers are made of dielectric materials (silica or plastic) so they are inherently passive to electromagnetic noise. Wavelength division multiplexing is utilized for adding channels to the system by means of fiber optic couplers and different light sources. The wavelengths of the optical signals utilized are located far apart in the optical spectrum in order to avoid serious crosstalk in transmission. The limit to the number of multiplexed channels depends on the optical fiber selected, the spectrum of the light sources and the wavelength division multiplexers or couplers utilized. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Fiber; FM; Multiplexing; NDT; Ultrasonic
1. Introduction Non-destructive testing (NDT ) systems are used, as their name indicates, for the detection of defects, such as bubbles, splits, etc., inside materials without the need to destroy them. In the case of ultrasonic NDT, ultrasonic signals generated by piezoelectric transducers are used. These signals penetrate to the interior of the material, propagating inside it; the echoes from the interfaces are detected by the same or another transducer and converted into electrical signals to be transmitted and processed. From this detected or received signal, the information required for the of detection defects in the interior of the material can be extracted. The non-destructive inspection technique used in this work is a troughtransmission one in which one piezoelectric transducer transmits a broad-band signal, the pulse transmitted through the inspected structure and being received by * Corresponding author. Tel: +34-953-64-9554; fax: +34-953-64-9508. E-mail address: [email protected] (J.C. Alonso)
another piezoelectric transducer. The electrical signal generated in the receiving transducer is transmitted by an optical fiber after being processed and converted in an optical signal. Finally, the signal is converted back to electrical after being processed, with recovery of the initial pulse.
2. General characteristics and system architecture For an NDT system, the physical distances between the receiving transducers and the processing stage must be as short as possible. Increasing the distance implies the deterioration of the SNR. The piezoelectric sensors used are of a broadband commercial type with 1 MHz of central frequency and approximately 1.5 MHz of maximum bandwidth. This bandwidth value is the one taken for our base band, because of the analog nature of the transmitted signals. In the transmission medium, i.e. the optical fiber, the transmitted power has a very low level, so, to maintain an acceptable the SNR, an RF sub-carrier frequency modulation scheme is used. This sub-carrier intensity
0041-624X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 4 1 -6 2 4 X ( 9 9 ) 0 0 05 8 - X
543
J.C. Alonso et al. / Ultrasonics 38 (2000) 542–545
modulates the optical source. In addition, because of the modulation type, we can have several ultrasonic signals multiplexed on the same fiber. Multiplexing can be accomplished electrically or optically. Electrical multiplexing, which is widely known, is achieved by electrical addition. However, this implies that all the transducers must be located in the same area and very close to each other. In our case, using optical multiplexing, we can assume that all the transducers are independent of each other and can be installed in different sites far away from each other – see the system block diagram in Fig. 1 In Fig. 1 we can see a block diagram in which there are two different receiving sensors and their transmitters ( T1 and T2). The electrical signals generated at the sensors 1 and 2 are converted to light by electrical to optical converters ( E/O), and then coupled to a fiber. In the receiver stage, an optical to electrical converter (O/E ) is used, which may be a PIN photodiode. As optical sources, we use LEDs because their spectral emission width is very broad, and this permits that the optical signals coming from each source be added in power in the couplers, minimizing, or practically eliminating, the noise due to possible interferometrics effects. Fig. 2 shows the general design of each of the transmitters T1 and T2. In this figure we can deal with the different processing stages of the electrical signal from the electrical signal at the sensors until arriving to the optical source. The first stage consists of an impedance matching network and amplifier to match signal levels. This stage is necessary because the sensor exit signal level should be adapted to the input level needed by the modulator, so wide band frequency modulation ( WBFM ) will operate correctly. The modulator consists of a phase-locked loop (PLL)
Fig. 1. System block diagram.
Fig. 2. Transmitter.
with a current controlled oscillator, ICO, of the ‘charge bump’ type, which also generates the sub-carrier frequency. Because the modulating signal is of relatively high frequency, 1.5 MHz, and owing to the existence of higher frequency sensors, 25 MHz, the oscillator must be of the type indicated to accept these high-frequency modulating signals, since voltage controlled oscillators using varicap are not capable of accepting such signals. Subsequently, it is possible to filter the harmonics generated in the modulator through a bandpass filter, and finally the resulting signal is introduced in the LED’s driver. The driver must generate a current proportional to the input voltage with a transfer function as linear as possible to minimize intermodulation products generated and transmitted from the LED through the fiber. The receiver, in the case of multiplexing only two channels, is designed according to the Fig. 3. Some design constraints must be followed. First, the subcarrier frequencies must be carefully chosen to avoid channel interference. Also, if we are using WBFM,
Fig. 3. Receiver.
544
J.C. Alonso et al. / Ultrasonics 38 (2000) 542–545
channel spacing must be sufficient to accommodate each channel. According to Carson’s rule, channel bandwidth, BW, is: BW=2(Df+f ), (1) m where Df is the peak deviation frequency and f is the m higher modulating frequency. In order to achieve a good SNR, we should make Df as high as possible, bearing in mind that, if we want to transmit many channels, bandwidth is limited by the system bandwidth. For a frequency modulation–intensity modulation (FM–IM ) system [1,2], the post-detection SNR is: rms 3D2 P (RP )2A2 /2 0 c , SNR= f a (2) 2f N m 0 where D =Df/f , R is the photodiode responsivity, P f m 0 the incident optical power, A the sub-carrier amplitude, c and N the noise power denoted as: 0 4kTBF n. N =e(I +I )+ (3) 0 p d R L For all the above we choose 10 MHz as the channel bandwidth plus a 2 MHz band guard. Then the channel spacing must be at least 12 MHz.
3. Practical case In this section we present a practical one-channel system in accordance with Section 2. A graded index multimode optical fiber is used for the system, so we can couple enough power into the fiber with a surface emitting LED. As transmitter we used an LED diode of 50 MHz bandwidth. To accommodate two channels the subcarrier frequencies can be chosen as 25 and 40 MHz
Fig. 4. Sub-carrier spectrum.
Fig. 5. FM modulation.
respectively. The power coupled into fiber by this device is 40 mW with 80 mA forward current. As receiver we use a PIN photodiode, followed by a transimpedance preamplifier with automatic gain control. The automatic gain control is necessary because the number of sensors connected to the network can vary. The transmitted sub-carrier is sinusoidal but with a CW level superimposed, because the light can only be intensity modulated; for the different signals to be added, the modulation index of each sub-carrier will change, and also the mean of the continuous level. In this way, the automatic gain control ensures that the signal level that reaches each demodulator will be practically continuous. Once the signal has been preamplified, it is
Fig. 6. Time characteristic: upper plot, modulating signal; lower plot, received signal.
J.C. Alonso et al. / Ultrasonics 38 (2000) 542–545
545
For our system we used a 1.7 MHz and 0.7 mVpp sinusoidal signal to obtain this spectrum. Finally, we present the transmitted and received ultrasound signals and their spectra. Fig. 6 shows modulating and receiving signals. It can be seen that the effect of damping over a real ultrasonic signal is not important because such a signals do not show sharp changes. Hence with the optimum value for damping (j=0.707), a PLL will follow the signal correctly. Fig. 7 shows spectra for each signal. There is a slight loss in the bandwidth, mainly at higher frequencies. This is not important for NDT, because for a 1 MHz central frequency transducer we only need three times this frequency. 4. Conclusions
Fig. 7. Frequency characteristic: upper plot, modulating signal; lower plot, received signal.
again amplified, extracting each channel by filtering. Each one has its independent demodulator tuned by a PLL into the corresponding frequency. Fig. 4 shows the sub-carrier spectrum corresponding to the wave travelling through the fiber. As shown, the second harmonic level is lower and far from the second channel. From Fig. 4 it appears that the driver and the LED have an almost linear transfer function. This is good for our purpose, to transmit many channels by the same fiber. The frequency of the signal shown in Fig. 4 is 25 MHz, and the intensity modulation index is 80%. We can frequency modulate this sub-carrier with ultrasonic signals whose time and frequency characteristics are shown in the upper plots of Figs. 6 and 7, respectively. These signals are from a transducer of 1 MHz central frequency in reception mode, over a metachrilate of 1 cm depth. To adjust modulation index for FM, we modulate the sub-carrier with a sinusoidal tone whose frequency and amplitude are such as to obtain the spectrum shown in Fig. 5, where the sub-carrier level (central delta) is minimized. This indicates that the modulating signal has the maximum amplitude allowed for a 13% modulation index.
We have seen in this paper the possibility of ultrasonic signal transmission through an optical fiber. Because of the modulation method, we are able to transmit several signals over the same fiber. The number of multiplexed transducers is limited by the optical source, the fiber types used and the SNR required. As already mentioned, the intensity modulation index of the multiplexed signals decreases as they are summed on every coupler, so to achieve the required SNR we must to limit the number of transmitters, i.e. transducers. However, it is possible to multiply this number by three using wavelength multiplexing on each transmission window with sources at 850, 1300 and 1550 nm. The main advantage of this system is that it uses optical devices for transmission, showing all the advantages of such systems, mainly electromagnetic immunity and wide bandwidth. Acknowledgement This study has been carried out under a CICYT TAP98-0911-c03-01 project. References [1] J.M. Senior, Optical Fiber Communications, Prentice Hall, Hertfordshire, UK, 1992. [2] Gowar, Optical Communications Systems, Prentice Hall, New York, 1993.
Multiplexing and transmission of RF signals using an optical fiber Jose´ Curpia´n Alonso a, *, Francisco Montero de Espinosa b, Manuel Lo´pez-Amo c a Universidad de Jae´n, Dpto. Electro´nica, E.U.P Linares, Alfonso X el Sabio 28, 23700 Linares, Spain b Instituto de Acu´stica, CSIC, Serrano 144, 28006 Madrid, Spain c Universidad Pu´blica de Navarra, Campus de Arrosadı´a s/n, Pamplona, Spain
Abstract Ultrasonic non-destructive testing systems designed to control huge structures normally use several transducers in the reception stage. To avoid increasing the cost of electronics, a multiplexer is used to send all received signals to the same processing module. Traditionally, transmission of such signals is carried out using copper cables. For special applications (i.e. continuous monitoring of nuclear plants) metallic cables are not suitable because of their high sensitivity to electromagnetic perturbations. Moreover, the multiplexing is made electronically. When the distance between the transducers and the reception unit is large and/or electromagnetic noise is important, signal degradation takes place. The proposed system implements the transmission and multiplexing of ultrasonic electrical signals obtained by means of broadband transducers (up to 1 MHz), using an optical fiber. Optical fibers are made of dielectric materials (silica or plastic) so they are inherently passive to electromagnetic noise. Wavelength division multiplexing is utilized for adding channels to the system by means of fiber optic couplers and different light sources. The wavelengths of the optical signals utilized are located far apart in the optical spectrum in order to avoid serious crosstalk in transmission. The limit to the number of multiplexed channels depends on the optical fiber selected, the spectrum of the light sources and the wavelength division multiplexers or couplers utilized. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Fiber; FM; Multiplexing; NDT; Ultrasonic
1. Introduction Non-destructive testing (NDT ) systems are used, as their name indicates, for the detection of defects, such as bubbles, splits, etc., inside materials without the need to destroy them. In the case of ultrasonic NDT, ultrasonic signals generated by piezoelectric transducers are used. These signals penetrate to the interior of the material, propagating inside it; the echoes from the interfaces are detected by the same or another transducer and converted into electrical signals to be transmitted and processed. From this detected or received signal, the information required for the of detection defects in the interior of the material can be extracted. The non-destructive inspection technique used in this work is a troughtransmission one in which one piezoelectric transducer transmits a broad-band signal, the pulse transmitted through the inspected structure and being received by * Corresponding author. Tel: +34-953-64-9554; fax: +34-953-64-9508. E-mail address: [email protected] (J.C. Alonso)
another piezoelectric transducer. The electrical signal generated in the receiving transducer is transmitted by an optical fiber after being processed and converted in an optical signal. Finally, the signal is converted back to electrical after being processed, with recovery of the initial pulse.
2. General characteristics and system architecture For an NDT system, the physical distances between the receiving transducers and the processing stage must be as short as possible. Increasing the distance implies the deterioration of the SNR. The piezoelectric sensors used are of a broadband commercial type with 1 MHz of central frequency and approximately 1.5 MHz of maximum bandwidth. This bandwidth value is the one taken for our base band, because of the analog nature of the transmitted signals. In the transmission medium, i.e. the optical fiber, the transmitted power has a very low level, so, to maintain an acceptable the SNR, an RF sub-carrier frequency modulation scheme is used. This sub-carrier intensity
0041-624X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 4 1 -6 2 4 X ( 9 9 ) 0 0 05 8 - X
543
J.C. Alonso et al. / Ultrasonics 38 (2000) 542–545
modulates the optical source. In addition, because of the modulation type, we can have several ultrasonic signals multiplexed on the same fiber. Multiplexing can be accomplished electrically or optically. Electrical multiplexing, which is widely known, is achieved by electrical addition. However, this implies that all the transducers must be located in the same area and very close to each other. In our case, using optical multiplexing, we can assume that all the transducers are independent of each other and can be installed in different sites far away from each other – see the system block diagram in Fig. 1 In Fig. 1 we can see a block diagram in which there are two different receiving sensors and their transmitters ( T1 and T2). The electrical signals generated at the sensors 1 and 2 are converted to light by electrical to optical converters ( E/O), and then coupled to a fiber. In the receiver stage, an optical to electrical converter (O/E ) is used, which may be a PIN photodiode. As optical sources, we use LEDs because their spectral emission width is very broad, and this permits that the optical signals coming from each source be added in power in the couplers, minimizing, or practically eliminating, the noise due to possible interferometrics effects. Fig. 2 shows the general design of each of the transmitters T1 and T2. In this figure we can deal with the different processing stages of the electrical signal from the electrical signal at the sensors until arriving to the optical source. The first stage consists of an impedance matching network and amplifier to match signal levels. This stage is necessary because the sensor exit signal level should be adapted to the input level needed by the modulator, so wide band frequency modulation ( WBFM ) will operate correctly. The modulator consists of a phase-locked loop (PLL)
Fig. 1. System block diagram.
Fig. 2. Transmitter.
with a current controlled oscillator, ICO, of the ‘charge bump’ type, which also generates the sub-carrier frequency. Because the modulating signal is of relatively high frequency, 1.5 MHz, and owing to the existence of higher frequency sensors, 25 MHz, the oscillator must be of the type indicated to accept these high-frequency modulating signals, since voltage controlled oscillators using varicap are not capable of accepting such signals. Subsequently, it is possible to filter the harmonics generated in the modulator through a bandpass filter, and finally the resulting signal is introduced in the LED’s driver. The driver must generate a current proportional to the input voltage with a transfer function as linear as possible to minimize intermodulation products generated and transmitted from the LED through the fiber. The receiver, in the case of multiplexing only two channels, is designed according to the Fig. 3. Some design constraints must be followed. First, the subcarrier frequencies must be carefully chosen to avoid channel interference. Also, if we are using WBFM,
Fig. 3. Receiver.
544
J.C. Alonso et al. / Ultrasonics 38 (2000) 542–545
channel spacing must be sufficient to accommodate each channel. According to Carson’s rule, channel bandwidth, BW, is: BW=2(Df+f ), (1) m where Df is the peak deviation frequency and f is the m higher modulating frequency. In order to achieve a good SNR, we should make Df as high as possible, bearing in mind that, if we want to transmit many channels, bandwidth is limited by the system bandwidth. For a frequency modulation–intensity modulation (FM–IM ) system [1,2], the post-detection SNR is: rms 3D2 P (RP )2A2 /2 0 c , SNR= f a (2) 2f N m 0 where D =Df/f , R is the photodiode responsivity, P f m 0 the incident optical power, A the sub-carrier amplitude, c and N the noise power denoted as: 0 4kTBF n. N =e(I +I )+ (3) 0 p d R L For all the above we choose 10 MHz as the channel bandwidth plus a 2 MHz band guard. Then the channel spacing must be at least 12 MHz.
3. Practical case In this section we present a practical one-channel system in accordance with Section 2. A graded index multimode optical fiber is used for the system, so we can couple enough power into the fiber with a surface emitting LED. As transmitter we used an LED diode of 50 MHz bandwidth. To accommodate two channels the subcarrier frequencies can be chosen as 25 and 40 MHz
Fig. 4. Sub-carrier spectrum.
Fig. 5. FM modulation.
respectively. The power coupled into fiber by this device is 40 mW with 80 mA forward current. As receiver we use a PIN photodiode, followed by a transimpedance preamplifier with automatic gain control. The automatic gain control is necessary because the number of sensors connected to the network can vary. The transmitted sub-carrier is sinusoidal but with a CW level superimposed, because the light can only be intensity modulated; for the different signals to be added, the modulation index of each sub-carrier will change, and also the mean of the continuous level. In this way, the automatic gain control ensures that the signal level that reaches each demodulator will be practically continuous. Once the signal has been preamplified, it is
Fig. 6. Time characteristic: upper plot, modulating signal; lower plot, received signal.
J.C. Alonso et al. / Ultrasonics 38 (2000) 542–545
545
For our system we used a 1.7 MHz and 0.7 mVpp sinusoidal signal to obtain this spectrum. Finally, we present the transmitted and received ultrasound signals and their spectra. Fig. 6 shows modulating and receiving signals. It can be seen that the effect of damping over a real ultrasonic signal is not important because such a signals do not show sharp changes. Hence with the optimum value for damping (j=0.707), a PLL will follow the signal correctly. Fig. 7 shows spectra for each signal. There is a slight loss in the bandwidth, mainly at higher frequencies. This is not important for NDT, because for a 1 MHz central frequency transducer we only need three times this frequency. 4. Conclusions
Fig. 7. Frequency characteristic: upper plot, modulating signal; lower plot, received signal.
again amplified, extracting each channel by filtering. Each one has its independent demodulator tuned by a PLL into the corresponding frequency. Fig. 4 shows the sub-carrier spectrum corresponding to the wave travelling through the fiber. As shown, the second harmonic level is lower and far from the second channel. From Fig. 4 it appears that the driver and the LED have an almost linear transfer function. This is good for our purpose, to transmit many channels by the same fiber. The frequency of the signal shown in Fig. 4 is 25 MHz, and the intensity modulation index is 80%. We can frequency modulate this sub-carrier with ultrasonic signals whose time and frequency characteristics are shown in the upper plots of Figs. 6 and 7, respectively. These signals are from a transducer of 1 MHz central frequency in reception mode, over a metachrilate of 1 cm depth. To adjust modulation index for FM, we modulate the sub-carrier with a sinusoidal tone whose frequency and amplitude are such as to obtain the spectrum shown in Fig. 5, where the sub-carrier level (central delta) is minimized. This indicates that the modulating signal has the maximum amplitude allowed for a 13% modulation index.
We have seen in this paper the possibility of ultrasonic signal transmission through an optical fiber. Because of the modulation method, we are able to transmit several signals over the same fiber. The number of multiplexed transducers is limited by the optical source, the fiber types used and the SNR required. As already mentioned, the intensity modulation index of the multiplexed signals decreases as they are summed on every coupler, so to achieve the required SNR we must to limit the number of transmitters, i.e. transducers. However, it is possible to multiply this number by three using wavelength multiplexing on each transmission window with sources at 850, 1300 and 1550 nm. The main advantage of this system is that it uses optical devices for transmission, showing all the advantages of such systems, mainly electromagnetic immunity and wide bandwidth. Acknowledgement This study has been carried out under a CICYT TAP98-0911-c03-01 project. References [1] J.M. Senior, Optical Fiber Communications, Prentice Hall, Hertfordshire, UK, 1992. [2] Gowar, Optical Communications Systems, Prentice Hall, New York, 1993.