At present, ultraviolet light source has been widely used in medical sterilization, fluorescence spectroscopy, biological analysis/detection, water treatment, etc. The bactericidal properties of ultraviolet light source were discovered in the early 17th century, and the application of ultraviolet fluorescent tube technology began in the 1750s. The ultraviolet light sources used in these technologies are gas discharge lamps (such as low-pressure mercury lamps).
As a new type of military communication system, UV communication has the advantages of strong anti-interference ability, good confidentiality, non-line-of-sight communication and all-round communication, and has become the focus of research of military technicians at home and abroad. However, conventional ultraviolet light sources (low-pressure mercury lamps) have defects such as large volume, short life, low modulation rate, and fragility, which limit the development of ultraviolet light communication. In order to solve the problem of ultraviolet light communication source, the Defense Advanced Research Projects Agency (DARPA) launched a project to develop a variable wavelength transistor ultraviolet light emitter in 2002, and successfully developed a day with a wavelength of 274 nm. Blind-area UV light-emitting diodes (UVED. Compared with low-pressure mercury lamps, UV LEDs have the advantages of small size, long life, low-voltage power supply, and digital modulation.
The excellent characteristics of UV LEDs have been used in the field of UV communication since its inception. In 2005, MIT used a 274 nm UV LED manufactured by DARPA as a light source to develop a prototype of UV communication experiments, non-direct communication. In the range of 100 m, the communication rate is 200 b/s; other research institutes such as Israel's Ben Gurion University, British Aerospace Systems, and the University of California have also established UV-based ultraviolet communication systems. However, the specific conditions and technical details of their research work are highly confidential.
In 2010, China's first 280 nm deep ultraviolet light-emitting diode (UVED) production line was commercialized. In 2011, the blue-wavelength 280 nm deep-UV LED module was calibrated to output more than 32 mW. These research results promoted UV LEDs in UV light. Application in the field of communications.
In 2010, the modulation rate of the ultraviolet communication system set up by Chongqing University reached 7 Mb/s. In 2010, the Space and Applied Research Center of the Chinese Academy of Sciences built an ultraviolet light image transmission experiment system using ultraviolet LED arrays.
1 UV LED modulation rate characteristics
The technical indicators announced by the UV communication system development unit are system-level parameters, such as data transmission rate, transmission distance and bit error rate; UV LED manufacturers only test the DC parameters of the manufactured products, such as working voltage/current. , peak wavelength and half width. The light source of the ultraviolet light communication system can only realize the data transmission when it is working in the modulation state. The modulation characteristics such as the modulation rate and modulation spectrum of the ultraviolet LED are studied, which will promote the application of the ultraviolet LED in the field of ultraviolet light communication.
1.1 UV LED modulation rate test principle
The principle of ultraviolet LED modulation rate test is shown in Figure 1. The experimental equipment is described as follows:
(1) Function generator: Agilent's 33250A is used to generate a standard square wave signal for driving the UV LED.
(2) UV LED: single 280 nm UV LED with output power >0.6 mW.
(3) Detector (Si): Using PDA10A manufactured by THORLABS?
EC high-speed detector, suitable for wavelength range 200~1 100 nm, response time is 1 ns.
(4) Signal amplifier: The detector used in this experiment has signal filtering amplification effect. If the detector selects current output and has no amplification function, it needs to select the corresponding signal amplifier.
(5) Digital oscilloscope: Select the storage oscilloscope of Tektronix DP07054 model with a bandwidth of 500 MHz.
1.2 Drive signal test
The function generator 33250A produces a square wave with a rise/fall time of less than 5 ns and a duty cycle of 50%. The specification indicates a square wave that produces 80 MHz. The drive signals at different modulation rates are shown in Figure 2. The experimental results show that the square wave with a frequency of 30 MHz is close to a sine wave; the square wave with a frequency of 10 MHz has obvious protrusions on the rising/falling edge. The quality of the trigger signal will directly affect the modulation signal of the UV LED. Therefore, a square wave signal generator with high modulation rate and good signal quality must be developed later as a dedicated driving source for the UV LED.
1.3 High speed detector response
In this experiment, the PDA10A?EC model detector manufactured by THORLABS is used. The applicable wavelength range is 200~1 100 nm, and the response time is 1 ns. The theoretical response frequency is 500 MHz and the duty cycle is 50% square wave signal.
The experimental results are shown in Fig. 3. The curve with higher amplitude is the driving signal, and the curve with lower amplitude is the detector response. Figure 3(c) shows that the high-speed detector accurately detects the 10 MHz modulated signal of the UV LED. Therefore, the use of ultraviolet LEDs as a source of ultraviolet light communication can increase the data transmission rate to 10 Mb/s, and it is expected to obtain better results by using a dedicated ultraviolet LED driving power source .
2 ultraviolet LED modulation spectral characteristics
The half-wave width of the UV LED in the DC state is generally less than 12 nm. To verify the spectral characteristics of the UV LED under the modulation state, the 280 nm UV LED is modulated and spectroscopy. The test principle is shown in Figure 4. The main equipment parameters are as follows:
(1) Function generator, UV LED: Same as the device used in Figure 1.
(2) UV spectrometer: using iHR550 spectrometer produced by Horiba Jobin Yvon, the spectral range is 200~1 100 nm, the resolution of the spectrometer is 0.025 nm; the spectral accuracy is ±0.2 nm; the repeatability is ±0.075 nm.
The spectral characteristics of the UV LEDs at different modulation rates are shown in Figure 5. At 50 Hz and 80 Hz, the spectrum of the modulation state is quite different from that of the DC state because the sampling frequency of the UV spectrometer is greater than the UV LED modulation frequency.
When the modulation frequency of the ultraviolet LED (100 Hz) is greater than the sampling frequency of the ultraviolet spectrometer, the ultraviolet LED spectrum curve obtained by the ultraviolet spectrometer is substantially the same as that of the ultraviolet LED.
The experimental results show that the UV LED can maintain its spectral characteristics well under the modulation state, and ensure that the spectrum under DC working condition can be applied to the UV communication system in the range of the day blind zone.
3 Conclusion
UV LED has the characteristics of small size, long life, low voltage power supply, etc. It is suitable for use as a source of ultraviolet light communication. In this paper, the experimental research on the modulation rate and modulation spectrum of UV LED has been carried out. The results show that the modulation rate of UV LED can reach 10 MHz, and the modulation state can maintain its spectral characteristics well. These experimental data are UV LED in UV light. Applications in the communications field provide powerful data support. Ultraviolet LEDs will be widely used in the fields of ultraviolet radiation and ultraviolet communication in the near future.
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