Wireless Laser communication link using array sensor and GPS/ electronic compass based aligner

Transcription

1 Wireless Laser communication link using array sensor and GPS/ electronic compass based aligner K.K. Sharma, Col. S.K. Razdan, Shammi Wadhwa, Rachna Deepanshu, R.K. Sharma CO 2 Laser Division, Laser Science and Technology Center, Defence R&D Organisation Metcalfe House, Delhi , INDIA ABSTRACT Laser Science and Technology Center has recently developed a rapidly deployable wireless two way laser communication link for transmitting audio/video and graphics data for military applications. The link is operational in simplex mode upto few hundred meters within the laboratory. It is proposed to extend the range to 2 Km and above, in duplex mode operation. The lab prototype model utilizes a single mode 2-3 mw, 780 nm diode laser with collimated optics and wide modulation capability electronics. The receiver incorporates a sufficiently wide FOV optics and a very high sensitive wideband APD detector ( Perkin Elmer C30954E). A narrow band IR filter is incorporated to reduce stray radiation and background noise. As the link is sensitive to alignment and beam positioning, GPS and IR array sensor based system has been developed for precision alignment of transmitter and receiver simultaneously. This technique of aligning transmitter and receiver simultaneously in bi-static communication link offers cost savings by drastically reducing setup time at the experimental site and providing incredible accuracy. Keywords: Free Space Laser Communication, Optical Wireless Communication, GPS, IR Sensor Array 1. INTRODUCTION Wireless Laser Communication is rapidly gaining popularity as an effective means of transferring data at high rates at long ranges. The system includes a Laser transmitter and a receiver along with control & driving electronics separated by several Kms. Laser beam propagating through the atmosphere carry the information from transmitter to the receiver. The system provides many advantages such as rapidly deployable, large bandwidth, secure communication link for jam resistant, negligible interference with full duplexed communication, spectral separation from existing communication systems, light weight and compact equipments, reduced power requirement, ECM capability and lower recurring cost over conventional systems. Notably, system facilitates high capacity communication without licensing fees and tariffs. Advantages gained in long range ground to ground, ground to air and air to air links have attracted the users both in civil and defence sectors. The data rate demands have also risen from hundreds of megabits per second to tens of gigabits per second. The users include commercial/business, educational and recreational establishments, government offices, boarder area defence and other utilities. The high data rate transmission can also be attained with optical fibers, which are distributed to connect cities and continents. However, often the last mile from the fiber backbone to the user premises presents a significant problem. In some cases, it may not be possible or practical to lay down optical fibers, and it is invariably costly and time consuming. The wireless communication offer solution to last mile problem, affording flexible and rapid connectivity. Prominent of them is Radio Frequency based wireless communication system. However, deployment of radio frequency system is a time consuming process and requires permission from government agencies. For military purpose, it is particularly troublesome when a temporary solution is sought, or an unexpected redeployment of premises calls for the provision of instant communication links. Furthermore, there is growing concern about the health hazards associated with extensive exposure to radio frequency radiation. On the other hand, if eye safety hazards are allayed, Laser communication may not be a dangerous proposition. Communication link may provide the transmission capacities of fiber links and even more using lightweight and compact equipments that can be installed in less than a day and also no licensing is required; which are absolute necessities of a military application. The IR laser link developed at LASTEC is jam resistant and has several additional features. It carries audio, video and graphics transmission electronics in a single package. The high security feature of the system is of paramount importance. The diode laser specifications such as wavelength, tunability and power are chosen depending upon the atmospheric conditions and range requirement. The extremely directional, invisible (IR Laser), and narrow

2 beam optical radiation further makes eaves-dropping and jamming nearly impossible in real war scenario. A transient if it occurs due to a movement of an object across the laser path may momentarily block the path, the unwanted interferes are thwarted. The characteristic may be exploited especially in high security ground based and ground to air links, which prefer the optical communication modality because of its small footprints. 2. WIRELESS LASER COMMUNICATION SYSTEM The system comprises three basic components: the transmitter, the propagation channel and receiver. Fig.1 shows the photograph of complete transmitter and receiver assembly developed at LASTEC. 2.1 Transmitter The transmitter package consists of a IR Laser source that is modulated to carry the data. The laser is current modulated by an external modulating waveform in either voice video or graphic modulation format. The laser is chosen by its central wavelength, average power, and beam divergence angle. Ideally, its frequency spectrum is sufficiently narrow to allow optical analysis to relate to the central wavelength alone. Power received at the receiver at a distance R is given by Fig.1 Transmitter & Receiver Assembly P r = ( P t x T x e -2µR 2 ) x D r / (4R 2 ) (1) Where P t : Transmitted Laser Power T : Optical transmission factor µ : Atmospheric attenuation coefficient D r : Receiver telescope diameter The transmitter power P t is so chosen so that it produces significant S/N at the receiver. LED may also be used for the purpose. It is often preferred to the laser as a cheaper alternative, and is becoming increasingly attractive with the recent developments in superluminescent LEDs with relatively narrow bandwidths and up to 70 m W power outputs. The beam divergence angle determines the free space power loss, or how large the laser spot will be at a given distance from the source. The transmitter telescope has sufficient magnification to collimates the beam in the direction of the receiver and determines the beam diameter. The IR communication deployment may however pose a hazard if operated incorrectly; therefore, a laser safety standard has been established in which optical sources are classified in accordance with their total emitted power 1. The principal classifications are summarized in Table 1. It is therefore preferred to deploy an all purpose eye safe link. LASTEC is now engaged in the deployment of a long range eye safe communication link. The Outdoor point-to-point 650 nm 880 nm 1310 nm 1550 nm systems generally use high-power (visible) (Infrared) (Infrared) (Infrared) lasers that operate in the Class 3B band to achieve a good power Class 1 Up to 0.2 mw Up to 0.5 mw Up to 8.8 mw Up to 10 mw budget. The safety standard Class mw N/A N/A N/A recommends that these systems Class 3A 1 5 mw 1 5 mw mw mw should be located where the beam cannot be interrupted or viewed Class 3B mw mw mw mw inadvertently by a person. Rooftop locations and high Table 1. Laser safety classification for a point source emitter walls are desirable for this type of system in commercial link. The range performance is dependent on atmospheric conditions as depicted by the range equation (1). Even on an apparently clear day the atmosphere is pervaded by molecules and aerosols, which cause absorption and scattering of the light. Changes in temperature along the propagation path lead to scintillations in the received light due to the resultant turbulence.

3 The power budget and overall performance of a point-to-point free space link are strongly determined by atmospheric loss along the propagation path, which itself comprises free space loss, clear air absorption, scattering, refraction, and scintillation. Atmosphere has strong influence especially on spans greater than, say, 500 m. These systems must therefore have a good power budget to ensure error-free transmission, particularly if high-speed operation is required (e.g. 155 Mb/s or higher). A good power budget requires that the overall propagation loss be minimized. Little can be done to alleviate the various atmospheric losses; however, steps can be taken to minimize free space loss. It can be achieved by using large aperture telescope at each end which reduces free space losses. It was then possible to reduce the beam diameter, but then maintaining beam alignment became more difficult. The link channel developed at LASTEC as discussed above has the provision to install various IR diode lasers of different power levels and wavelengths as source according to distance, weather conditions and eye safety measures. For shorter distances within the laboratory single mode 2 3 and mw diode lasers at 780/808 nm are used. It is proposed to incorporate 500 mw or more power diodes at other λ for outdoor trials at longer ranges. Laser beam is collimated by the optics integral to the module. Transmitter electronics is capable of modulating laser beam for wide range of data for sound, audio, video, graphic to GHz rates. Laser beam is amplitude modulated and transmitted towards the receiver. Frequency modulation schemes may be incorporated for better reception. The schematic of system is shown in fig.2. Data from Modulating source Data Telescope Collecting APD receiver Telescope Atmosphere IR laser Matrix sensor control Fig.2. System schematic 2.2 Receiver At the receiver, a large area telescope collects the incoming light and focuses it onto the photodetector, which then converts it to electric current. The electrical signal is amplified and processed. A decision making device determines the nature of the signal according to its amplitude and arrival time. The quality of reception is measured by the probability of error, expressed in terms of bit error rate (BER). A 4 optical receiver is employed presently for link up to 2.0 Km or more. The optical receiver employed is a a transimpedance design because this affords a good compromise between bandwidth and noise, both of which are influenced by the capacitance of the photodiode. The photodiode employed has a significantly larger D* (Detectivity) and high efficiency (PerkinElmer 30954E). The operating parameters and other details are given below: Spectral Response : 400 nm 1100 nm Photo sensitive diameter : 0.8 mm Responsivity : nm : nm Dark Current (I d ) : 50 na Capacitance, C d : KHz Response time : 2 nsec NEP : 14 fw/(hz) 1/2 Operating Voltage : V A transimpedance design is used, in order to achieve a useful bandwidth in the presence of a high input capacitance, the feedback impedance need be very low, which in turn increases noise and reduces sensitivity. An improved approach common in optical wireless receivers combines transimpedance with bootstrapping, the later of

4 which reduces the effective capacitance of the photodiode. This allows relatively high feedback impedance to be used, which reduces noise and increases sensitivity. We have developed transimpedance receivers using high sensitivity wideband avalanche photodiodes (APDs). The receiver incorporates a sufficiently wide FOV optics AR coated receiving lens. The sensitivity improves (i.e. reduces in numerical value) as the photodiode area reduces because of the correspondingly lower capacitance. However, small area photodiodes incur a greater coupling loss due to the small aperture they present to the incoming beam, so a careful trade-off between these factors is necessary to optimize the final performance. APD receivers gives a 10 db sensitivity advantage over a corresponding PIN receiver, however, are more costly and require high operating voltage, and hence are predominantly used in specialist systems where performance is key. Systems in which economy is a priority, such as most indoor applications, favor PIN receivers Interference from Ambient Light Because of the wide field of view (i.e. aperture) stray light in addition to the wanted optical beam may reach the photodiode. Ambient light raises the level of photonic noise in the receiver and hence can impair performance. A narrowband interference infrared filter over the photodiode may be employed. The level of ambient light relative to the wanted beam will be significantly reduced. Consequently, photonic noise in the receiver will predominantly originate from the wanted signal, which is the optimum condition. Infrared filter used in the system was obtained from Advanced Laser System, Faridabad. The power spectral density of ambient induced noise extends from DC to typically a few tens of kilohertz, exceptionally a few hundreds of kilohertz, depending on the type of source. By conveying the data over the optical beam on a high-frequency sub carrier, or more commonly by applying a line code that contains no low-frequency components, interference is entirely avoided. The strong DC content of ambient light also means that the dynamic range of the receiver could be impaired. By using the above infrared filter and a receiver design which cancels or blocks any DC from the photodiode, this form of impairment is avoided. 2.3 System Alignment Line of sight is imperative for system, so beam alignment is a critical factor in long distance point-to-point systems because it will drift with temperature changes and fatigue in the anchorage assemblies 2. Some means of automatically maintaining alignment by mechanically steering the beams is desirable, particularly in defence installations where maintenance intervention must be kept to a minimum or even avoided altogether. Line of sight can be maintained with the help of a tracking and pointing system. Tracking is performed in two stages, coarse tracking and fine pointing. Coarse tracking may use GPS or other a priori knowledge. Fine pointing requires electro-optic mechanisms such as a quadrature or a matrix detector. Pointing involves a beam-steering device which may be mechanical, such as a galvo-mirror, or non-mechanical, such as acousto-optic crystals, electro-optic devices or optical phased arrays The (OPAs). in-house developed static communication link tested within the +12 V laboratory utilizes two precision adjustable (~1 o ) tripod stand and alignment is carried 10 K 680 Ω out with a red (He-Ne / diode) laser guiding Trim pot beam and IR sensor card. This works well for precision alignment at short ranges Photodetector LED within the laboratory. For larger ranges +V >>100 meters a GPS (Global Positioning System) / Electronic Compass based aligner system with a large number of IR array OP 37 sensor (matrix detector) is required for 33K -V precise alignment of the transmitter and receiver. Under test is the GPS Rino-130, make; Garmin Inc. This determines the 10 K Trim pot position as well as direction of the two 1 K assemblies (transmitter & receiver) within an accuracy of 3 meters. Fig.3 Circuit of IR sensor

5 The GPS incorporates electronic compass and therefore one unit each of GPS is mounted directly to transmitter and receiver. Transmitter (Receiver) GPS acquires the position of receiver (transmitter) GPS. This position information is processed and sends to electronic compass. Electronic compass directs towards the receiver (transmitter). This completes coarse tracking. For fine pointing matrix detector is being used. The laser beam from transmitter guided by GPS is made to fall on IR sensors/cluster placed alongside the receiver. Each IR sensor in the cluster is connected to a red LED that glows as the IR radiation falls on it. A 2x2 IR sensor cluster with red LED has been tested for this purpose. Circuit is shown Fig.3. A 20x20 cluster is under development. 3. TEST RESULTS The link has been tested for sound, audio and video signals within the laboratory. Now under test is low noise video and graphic transmission and reception. The results compare well with the theoretical results under clear weather conditions The results further show that laser power of few milli-watts is sufficient for setting up a low noise communication link up-to ranges of few hundred meters under clear weather condition. The choice of the laser power and wavelength is severely dependent on the range performance & atmospheric conditions. Also noise free audio, video and high bit rate data transmission is achievable simultaneously with a single IR diode laser. Successful demo of the system has been carried out from time to time. ACKNOWLEDGEMENT The authors wish to thank Director for the support extended in taking up the challenging program and also for allowing the work to be carried out in the laboratory. REFERENCES 1. Optical Wireless: The story so far, D.J.T. Heatley et al, IEEE Communication Mag., Dec.1998, pp Urban Optical Wireless Communication Networks: The Main Challenges And Possible Solutions, D. Kedar and S. Arnon, IEEE Optical Communication Mag., May 2004, pp. s2-s7.