Eric J. Korevaar

Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications

12 There is currently a misconception among designers and users of free space laser communication (lasercom) equipment that 1550 nm light suffers from less atmospheric attenuation than 785 or 850 nm light in all weather conditions. This misconception is based upon a published equation for atmospheric attenuation as a function of wavelength, which is used frequently in the free-space lasercom literature. In hazy weather (visibility > 2 km), the prediction of less atmospheric attenuation at 1550 nm is most likely true. However, in foggy weather (visibility < 500 m), it appears that the attenuation of laser light is independent of wavelength, ie. 785 nm, 850 nm, and 1550 nm are all attenuated equally by fog. This same wavelength independence is also observed in snow and rain. This observation is based on an extensive literature search, and from full Mie scattering calculations. A modification to the published equation describing the atmospheric attenuation of laser power, which more accurately describes the effects of fog, is offered. This observation of wavelength-independent attenuation in fog is important, because fog, heavy snow, and extreme rain are the only types of weather that are likely to disrupt short (< 500 m) lasercom links. Short lasercom links will be necessary to meet the high availability requirements of the telecommunications industry.

Availability of free-space optics (FSO) and hybrid FSO/RF systems

Free Space Optics (FSO) has become a viable, high-bandwidth wireless alternative to fiber optic cabling. The primary advantages of FSO over fiber are its rapid deployment time and significant cost savings. The disadvantage of FSO over fiber is that laser power attenuation through the atmosphere is variable and difficult to predict, since it is weather airports, the link availability as a function of distance can be predicted for any FSO system. These availability curves provide a good indication of the reasonable link distances for FSO systems in a particular geographical area. FSO link distances can vary greatly from desert areas like Las Vegas to heavy-fog cities like St. Johns NF. Another factor in determining FSO distance limitations is the link availability expectation of the application. For enterprise applications, link availability requirements are generally greater than 99%. This allows for longer FSO link ranges, based on the availability curves. The enterprise market is where the majority of FSO systems have been deployed. The carriers and ISPs are another potential large user of FSO systems, especially for last-mile metro access applications. If FSO systems are to be used in telecommunication applications, they will need to meet much higher availability requirements. Carrier-class availability is generally considered to be 99.999% (5 nines). An analysis of link budgets and visibility-limiting weather conditions indicates that to meet carrier-class availability, FSO links should normally be less than 140m (there are cities like Phoenix and Las Vegas where this 99.999% distance limitation increases significantly). This calculation is based on a 53 dB link budget. This concept is extended to the best possible FSO system, which would have a 10 W transmitter and a photocounting detector with a sensitivity of 1 nW. This FSO system would have a 100 dB link margin, which would only increase the 99.999% link distance to 286 m. A more practical solution to extending the high availability range would be to back up the FSO link with a lower data rate radio frequency (RF) link. This hybrid FSO/RF system would extend the 99.999% link range to longer distances and open up a much larger metro/access market to the carriers. It is important to realize that as the link range increases, there will be a slight decrease in overall bandwidth. To show the geographical dependence of FSO performance, the first map of FSO availabilities contoured over North America is presented. This map is the first step to developing an attenuation map for predicting FSO performance, which could be used in similar fashion to the International Telecommunication Union (ITU)/Crane maps for predicting microwave performance.

Wireless optical transmission of fast ethernet, FDDI, ATM, and ESCON protocol data using the TerraLink laser communication system

The TerraLink laser communication (lasercom) system was developed as a cost-effective, high-bandwidth, wireless alternative to fiber optic transmission. The advantages of lasercom over fiber optic cabling are primarily economic. However, free-space lasercom is subject to atmospheric effects, such as attenuation and scintillation, which can reduce link availability and may introduce burst errors not seen in fiber transmission. The TerraLink transceivers use large receive apertures and multiple transmit beams to reduce the effects of scintillation. By designing the lasercom link with sufficient margin for atmospheric attenuation and scintillation, a bit error rate (BER) of 1029 or better can be achieved. Since we designed the TerraLink transceivers to be eye-safe at the transmit aperture, each system is range-limited. Link power budgets for the TerraLink systems are presented, and link margin data are shown that quantitatively describe how the effective laser link range varies in different weather conditions. Since the TerraLink transceivers act as simple repeaters, they are protocol-independent. Examples of TerraLink installations transmitting wireless fast ethernet (125 Mbits/s), fiber distributed data interface (FDDI) (125 Mbits/s), asynchronous transfer mode (ATM) (155 and 622 Mbits/s), and Enterprise Systems Connection (ESCON) (200 Mbits/s) protocol data are presented.

Atmospheric propagation characteristics of highest importance to commercial free space optics

There is a certain amount of disconnect between the perception and reality of Free Space Optics (FSO), both in the marketplace and in the technical community. In the marketplace, the requirement for FSO technology has not grown to even a fraction of the levels predicted a few years ago. In the technical community, proposed solutions for the limitations of FSO continue to miss the mark. The main commercial limitation for FSO is that light does not propagate very far in dense fog, which occurs a non-negligible amount of the time. There is no known solution for this problem (other than using microwave or other modality backup systems), and therefore FSO equipment has to be priced very competitively to sell in a marketplace dominated by copper wire, fiber optic cabling and increasingly lower cost and higher bandwidth wireless microwave equipment. Expensive technologies such as adaptive optics, which could potentially increase equipment range in clear weather, do not justify the added cost when expected bad weather conditions are taken into account. In this paper we present a simple equation to fit average data for probability of exceeding different atmospheric attenuation values. This average attenuation equation is then used to compare the expected availability performance as a function of link distance for representative FSO systems of different cost.

Measurement of scintillation for free-space laser communication at 785 nm and 1550 nm

An experiment comparing atmospheric scintillation for 785 nm and 1550 nm laser beam transmission is presented. Fluctuations in received optical power were recorded for both wavelengths at terrestrial ranges of 1.2 km and 2.2 km. The number of transmit apertures was also varied. The results indicated that scintillation fades are more of a problem at 1550 nm compared to 785 nm. This will require more scintillation fade margin built into the design of free-space laser communication systems operating at 1550 nm. As well, any advantage in decreased atmospheric attenuation margin at 1550 nm could be lost because of the need for greater fade margin. The overall reduction in scintillation with an increased number of transmit apertures was verified. A possible physical explanation will explain why more scintillation was observed at 1550 nm as compared to 785 nm.

2.4 km free-space optical communication 1550 nm transmission link operating at 2.5 Gb/s : experimental results

We describe a terrestrial free-space optical data link operating at 2.5 Gb/s using currently available 1.5 micrometers telecommunications electro-optic transmission components. The 2.4 km free-space optical data-link is characterized by bit-error-rate system performance. The optical link utilizes a 1.5 micrometers DFB laser device which is directly modulated and operating within the erbium amplification band.

Measurement of scintillation and link margin for the TerraLink laser communication system

AstroTerras TerraLinkTM 8-155 laser communications equipment is designed for a clear weather range of 8 km and a data rate of 230 Mb/s, and TerraLink 4-155 is designed for a 2 km range. The TerraLink equipment achieves a reduction in scintillation-induced intensity fluctuations by using large receive apertures and multiple transmit apertures. We present measurements of received intensity fluctuations at different ranges through 4 inch and 8 inch receive apertures. We also present link margin data, with its implications for use of lasercom equipment in various weather conditions. Scintillation measurements were made while a communications link was operating by placing a second receive telescope with a PIN photodiode next to one of the lasercom transceivers. By plotting the probability of intensity vs. intensity, the necessary link margin to achieve a desired burst error rate can be calculated. At the longest ranges, the TerraLink equipment requires a scintillation fade margin of about 10 dB to achieve a 10-9 bit error rate. The equipment is designed with an additional margin of 4 - 5 dB for atmospheric attenuation.

Optomechanical design of STRV-2 lasercom transceiver using novel azimuth/slant gimbal

For extremely high data rate communications between satellites, aircraft, and ground sites in situations where atmospheric interference is minimal, laser communications offers significant advantages over current radio frequency technologies in the areas of achievable data rate, size, weight, and power. Over the last two years, the Ballistic Missile Defense Organization (BMDO), contracting through the U.S. Army Space and Strategic Defense Command, has been funding the development of a laser communications terminal to be flown on the second Space Technology Research Vehicle (STRV-2) which is projected to launch in 1998. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low Earth orbit (LEO) satellite downlinks to the ground. The design of the terminal is based on direct modulation of semiconductor lasers, direct detection using avalanche photodiodes, separate acquisition/tracking and communications wavelengths, atomic line filter (ALF) technology for background light rejection, separate transmit and receiver apertures, and a hemispherical field-of-regard gimbal based upon a novel design. This paper discuses details of the optomechanical design of the terminal as presented at the programs critical design review.

Optical feedback locking of a diode laser using a cesium Faraday filter

Abstract We used weak filtered optical feedback to set the output frequency of a commercial single mode diode laser close to the peak of an atomic line filter passband. A Cs Faraday atomic line filter provided ultra-narrow optical feedback passbands near 852 nm. The locked laser power of 34 mW was nearly that of the unlocked laser. In-band laser operation was tolerant of laser diode current and temperature fluctuations and feedback path instabilities, important qualities in the design of matched filtered optical transceivers. We describe the feedback locked laser diode and present locked spectrum measurements. We show how fine tuning over a 100 MHz range is available by feedback mirror positioning.

Description of STRV-2 lasercom flight hardware

Laser communication hardware being built under funding from the Ballistic Missile Defense Organization will be flown on the Space Technology Research Vehicle 2 to be launched as part of Air Force mission TSX-5 in 1998. The flight hardware, which weighs 31.5 pounds and is designed for satellite-to-ground laser communications at data rates up to 1.24 Gb/s and ranges up to 2000 km, will be delivered to JPL for integration in March, 1997. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low Earth Orbit satellite downlinks to the ground. This paper describes the hardware with photographs.