Free space LASERcommunication

7/27/2019 Free space LASERcommunication

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Free Space Laser CommunicationSeminar 2008

1. INTRODUCTION

Lasers have been considered for space communication since their realization in 1960.

For a number of applications, laser technology is being championed as the successor to radio-

frequency (RF) technology for meeting the bandwidth demands of the 21st century. With RF

carrier frequencies ranging from 106 to 1010 Hz and optical carrier frequencies ranging from

1013 to 1015, the switch from RF to optical should ultimately accommodate an increase in

bandwidth of five to six orders of magnitude.

Much of the technology and many of the components for achieving this kind of bandwidth

performance have already been developed during the past decade for military and space

programs, as well as for applications in fiber optic telecommunications.

Free space point to point communication systems are used extensively in the communication

field. A network point to point microwave system can carry messages across the country as part

of the public switched telephone network. Despite strong competition from fiber optic based

communication systems, microwave or other free space systems are often justified for shorter

routes, when right of way for cable system is not available, or when the high communications

capacity of a fiber optic system is not needed.

Laser communications systems in particular have become increasingly popular to provide a free

space communication link between two locations. Laser systems do not require extensive

frequency coordination as do microwave systems in common frequency bands. Laser systems

often are less expensive to install then conventional copper cable or fiber optic cable

communication systems because physical installation of a cable is unnecessary. For example, a

laser communication system may have application between two corporate locations in a campus

environment. Each laser communication terminal may be positioned on the building roof top or

even positioned adjacent a window and aligned to operate between buildings. A communication

link within a building may also be provided by a free space laser communication system. Modern

office automation also typically generates large amounts of data that must often be

communicated between different corporate locations. Accordingly, the demand for laser

communication links is increasing.


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Figure1.1 Figure showing practical use of laser communication


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Figure1.2 Schematic diagram of a 2-way laser communication system


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1.1. FIGURE DESCRIPTION

10. Laser communication system.

11, 11. Microprocessor.

12, 12. Laser radiation source.

13, 13. Photo detector.

14, 14. Analog laser control circuit.

15/15. Output laser beam from A/B.

16, 16. Digital input/output control and buffer circuit.

20, 20. Collecting lens.

21/21. Input laser beam for A/B.

22, 22. Laser transmitter housing.

23, 23. Beam shaping lens.

24, 24. Amplifier and threshold circuit.


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2. FEATURES OF LASER COMMUNICATIONS SYSTEM

A block diagram of typical terminal is illustrated in figure. Information, typically in the form of

digital data, is input to data electronics that modulates the transmitting laser source. Direct or

indirect modulation techniques may be employed depending on the type of laser employed. The

source output passes through an optical system into the channel. The optical system typically

includes transfer, beam shaping, and telescope optics. The receiver beam comes in through the

optical system and is passed along to detectors and signal processing electronics. There are also

terminal control electronics that must control the gimbals and other steering mechanisms, and

servos, to keep the acquisition and tracking system operating in the designed modes of operation.


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3. WORKING

The figure shows a laser communication system including a pair of communication terminals A

and B. The communication terminals A and B are separated by a free space distance which may

vary depending on the desired path, expected atmospheric conditions etc. the maximum

separation distance for a free space communication system operating at a predetermined

frequency is determined primarily by the desired signal-to-noise ratio and the desired information

transmission rate, or bit rate.

The system operates at either two power levels. At low power safety level, the output laser

power is reduced to meet safety regulations for a stationary laser beam. At the high power

normal level, the output laser power may be above the regulatory safety criteria for stationary

laser sources to thereby provide a greater signal-to-noise ratio or otherwise enhance system

performance.

The communication terminal A and B may preferably be the same except for a unique identifying

code associated with each terminal.

The terminal A includes a microprocessor operating under stored program control. Terminal A

also includes, a transmitter comprising a laser radiation source, and a receiver comprising a photo

detector. The microprocessor is connected to the laser and photo detector by conventionalinput/output methods. The laser is controlled by an analog laser control circuit.

The analog laser control circuit properly biases the laser and regulates its optical power output,

compensating for temperature, component ageing, and other variations. The analog laser control

circuit in cooperation with the microprocessor also modulates the laser output beam with the

desired information to be transmitted. The analog laser control circuit interfaces to the

microprocessor through the digital I/O control and buffer circuit.

The digital I/O control and buffer circuit buffers an entire packet of data from the

microprocessor. The buffer is then emptied and the data is modulated onto the output laser beam.

The microprocessor may be connected to a data terminal which communicates with a

corresponding remotely positioned data terminal via the laser communication system.


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The microprocessor also controls the safety related and other functions for the terminal A. For

example, the microprocessor periodically inserts the terminal identification code into the datastream to be modulated onto the output beam. The identification code is recognized by the

remotely positioned terminal B. laser power and duty cycles are also controlled by the

microprocessor. A conventional Dead Man circuit, as used in laser scanning systems may

preferably be include in the laser control circuit to turn off the laser transmitter if it is determined

that microprocessor has malfunctioned.

A collecting lens collects the input laser beam from the remotely positioned terminal B. The

photo diode is positioned at the focal point of the collecting lens. The collecting lens may also

include an opening at the centre thereof to permit mounting of the laser transmitter housing and

its associated beam shaping lens.

The photo detector feeds the signal from the input beam to an amplifier and threshold circuit. The

amplifier and threshold circuit may include an automatic gain control circuit, or the gain may be

digitally controlled by the microprocessor. The threshold circuit converts the analog signal from

the photo detector to a digital pulse stream. The threshold circuit also preferably implements a

lockout criteria that reduces the digitization of noise.

The digital signal from the amplifier and threshold circuit is fed to a digital buffer in the digital

I/O control and buffer circuit. When the buffer is filled, an interrupt is sent to the microprocessor

which then receives the contents of the buffer. Thus, the microprocessor operates on large blocks

of data.

The communication terminal A includes detector means, cooperating with the receiver, for

detecting a blockage or misalignment of the output beam based upon the input beam. The

blockage or misalignment of the output beam is indicated if the remotely positioned terminal B

fails to properly receive the output beam. The remotely positioned terminal beam B produces a

confirmation signal periodically modulated onto the input beam in the confirmation data stream.

A blockage or misalignment of the output beam will cause the confirmation signal to stop being


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sent. A blockage or misalignment of the output beam indicates that the beam may be directed to

the eyes of an accidental or unintended observer.

The detector means preferably includes means for recognizing the confirmation signal. If theconfirmation signal is not detected for a predetermined time, a blockage or misalignment of the

output beam is indicated. In the interest of safety, the laser communication system requires

positive confirmation that the output beam is being received at the remotely positioned terminal

B before operation at the high power level is permitted.


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4. ACQUISITION AND TRACKING

There are three basic steps to laser communication: acquisition, tracking, and communications.

Of the three, acquisition is generally the most difficult; angular tracking is usually the easiest.

Communications depends on bandwidth or data rate, but is generally easier than acquisition

unless very high data rates are required. Acquisition is the most difficult because laser beams are

typically much smaller than the area of uncertainty. Satellites do not know exactly where they are

or where the other platform is located, and since everything moves with some degree of

uncertainty, they cannot take very long to search or the reference is lost. Instability of the

platforms also causes uncertainty in time. In the ideal acquisition method, the beam width of the

source is greater than the angle of uncertainty in the location of receiver. The receiver field of

includes the location uncertainty of the transmitter. Unfortunately, this ideal method requires a

significant amount of laser power.

It is possible to operate a number of laser types at high peak power and low duty cycle to make

acquisition easier. This is because a lower pulse rate is needed for acquisition than for tracking

and communications. High peak power pulses more easily overcome the receiver set threshold

and keep the false alarm rate low. A low duty cycle transmitter gives high peak power, yet

requires less average power, and is thus a suitable source for acquisition. As the uncertainty area

becomes less, it becomes more feasible to use a continues source of acquisition, especially if the

acquisition time does not have to be short.


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Figure 4.1 flow chart illustrating operation of a laser communication system


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4.1 DETAILS OF THE FLOW CHART

When the terminal A is first powered up the laser is operated at a low power safety level so that

the output beam is below the class 1 safety threshold. At block 34, the microprocessor generates

a unique terminal identification code which is transmitted on the output beam. The data on the

output beam is then monitored for an initial confirmation signal, such as a predetermined digital

code, indicating that the remotely positioned terminal B has received the output beam. If the

confirmation signal has not been received, the microprocessor again sends the terminal

identification code at block 34 at the low power safety level and monitors for the initial

confirmation signal.

Once the initial conformational signal is received at block 38, the microprocessor then operates

the laser at the high power normal level. While the system is operating at a high power level, the

system operating performance is enhanced. For example, the system may operate at a higher

signal-to-noise ratio, the bit rate may be increased, or a combination of these may be achieved.

At block 42 the microprocessor monitors the received data signal for the confirmation signal. At

block 44, if the confirmation signal is absent for a time greater than T then the laser is again

operated at the low power safety level. The absence of the confirmation signal indicates a

blockage or misalignment of the output beam. The time T is determined by the allowable

exposure time, based on the laser power.


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5. VERY NARROW BEAMS

12-Inch telescope (30.5 cm)

1.55 m wavelength

Beamwidth ~ 1.0 arc second

(5.1 rad)

(0.00028 degree)

From geosynchronous orbit

- 180 meter spot

- High directivity deliversstrong signal

- (Radio spot from a 12-inch

antenna

would be ~ 1800 km)

On the other hand must point

Correctly and hold steady to

~ 0.25-0.5 rad (10-20 meters)

Fig. 5.1 Boston Harbor


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Figure 5.2 Graph showing allowed exposure time versus elapsed time of exposure for variouspower laser beams


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Referring to the graph shown above we can see that the laser powers vary from 0.001 milli-watts

to 10 milli-watts. The limit L is indicated by the straight line extending diagonally across the

graph. Class 1 safety limits are complied with if the laser power and elapsed time are above the

line. If they are below the class 1 limits, then the beam is not safe.

The laser communication system controls the laser transmitter so that the power of the output

beam is maintained below the safety level threshold when a blockage or misalignment of the

beam is detected. At all other times during normal operation, the laser transmitter may be

operated at a higher average power level to improve system performance.

The laser communication system includes the confirmation signal within the communication data

stream. Accordingly, it requires no additional major components other than those necessary

without the safety features and enhanced performance of the present invention. Thus the laser

communication system is relatively simple, yet highly reliable.


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6. ADVANTAGES

Free space laser communications links eliminate the need for securing right of ways, and buried

cable installations. As the equipments operate within the near infrared spectrum, they are not

subject to government licensing and no spectrum fees have to be paid (according to Art. 7 in [3]

requires only the use of the frequency spectrum below 3000 GHz a license). Additionally, since no

radio interference studies are necessary, the systems are quickly deployable. The narrow laser beam

width precludes interference with other communication systems of this type. The advantages of

this wavelength range for Satellite-to-Earth communication include good atmospheric

penetration, the ability to use smaller optics than with longer wavelength technology, and the

ability to use sensitive detectors based on silicon avalanche photodiode technology


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7. DISADVANTAGES

Free space laser communications systems provide only interconnection between points that have

direct line-of-sight (LOS). They can transmit through glass, however, for each glass surface the

light intensity is reduced, due to a mixture of absorption and refraction, thus reducing the

operational distance of a system. Occasionally, short interruptions or unavailability events lasting

from some hours up to a few days can occur.

In addition to its bandwidth-improvement potential, free-space laser communication technology

also offers tangible benefits in satellite design.


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8. LIMITATIONS

Free space laser communication systems are considered stationary laser sources for regulatory

purposes, and as such, must comply with regulatory limits established to protect the eyes of an

accidental or unintended observer. An accidental observer may receive permanent damage from a

high power laser beam without experiencing any pain which might forewarn the observer of the

harmful exposure. In addition, the wavelengths used by the laser systems are often invisible.

Accordingly, standards have been put in place that establish safe limits for the power that may be

transmitted by a stationary laser source, such as a laser communication terminal. This maximum

permissible power limits the communication systems signal-to-noise ratio (SNR), bit rate, and/or

separation distance. Accordingly there is great need for a free space laser communication system

and method that complies with safety limits yet which has improved performance over existing

systems.


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9. SOLUTIONS

Moving laser beams present less of a hazard than do stationary beams as required for a free space

laser communication system. For example, laser scanning systems for reading bar codes

producing a moving, or non-stationary, laser beam. A spinning holographic disk produces a

series of facet pulses from the beam. If the facet pulses are not detected, it is assumed the

holographic disk is not spinning and thus the laser beam is stationary. The holographic scanner

then operates the laser at a low duty cycle until the facet pulses are again detected.


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10. HISTORY OF SPACE LASERCOM

Fig 10.1 History of space lasercom


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10.1. TECHNICAL CHALLENGES OF FIRST GENERATION SPACE

LASERCOM

Spatial acquisition and tracking

- Initial uncertainty may require

searching over 105 beam spots.

- Spacecraft vibrations may be

hundreds of beamwidths.

- Point-ahead requirements may be

1-10 beamwidths.

Rugged opto-mechanical-thermal

design.

Technology maturity

- Lasers.

- Amplifiers.

- Modulators.

- Efficient detection architectures.

Fig 10.2Spatialacquisition and tracking


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11. OPTIC vs RF

For a number of applications, laser technology is being championed as the successor to radio-

frequency (RF) technology for meeting the bandwidth demands of the 21st century.

With RF carrier frequencies ranging from 106 to 1010 Hz and optical carrier frequencies ranging

from 1013 to 1015, the switch from RF to optical should ultimately accommodate an increase in

bandwidth of five to six orders of magnitude.

Commercial activity has already begun in the space-to-space-link (SSL) arena, which is likely to

provide the overwhelming commercial pull for the time being as engineering firms endeavour to

design and build the optics required for low-Earth-orbit telecommunications systems, such as the

"Internet in the Sky" project by Teledesic (Kirkland, WA).


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12. APPLICATIONS

Depending on the climatic zone where the free space laser communications systems are used,

they can span distances up to 6 km at low bitrates or provide bitrates up to 1.25 Gbps at shorter

distances. The systems are protocol transparent allowing transmission of digital computer data

(LAN interconnect), video, voice over IP, multiplexed data, or ATM. They are suitable for

temporary connectivity needs such as at conventions, sporting events, corporate and university

campuses, disaster scenes or military operations.


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14. FURTHER REMARKS

Microwave systems have the ability to achieve a high link quality (error performance and

availability) for distances of up to 100 km. Measurements on a 600 m first generation free space

laser communications link carried out by Swiss Telecom in the area of Berne, showed a

significant lower quality. The degradation effects can be categorized as follows:

Propagation effects and

Mechanical insufficiencies.

The origin of the first category is quite obvious, e.g.: if heavy fog, snow or smoke blocks the

line-of-sight between the units or the sun is interfering the laser beam. Unfortunately, there are

not many countermeasures to improve the situation in such cases. The source for the second

category can be found in the narrow optical beam. Therewith the link performance is sensitive to

vibration, wind sway, and thermal expansion of the equipment. However, since the products

listed in Table 1 are second and third generation products, it can be expected that in the

meantime mechanical improvements have been made.

Last but not least it should be noted, that a microwave link designer has within limits - the

possibility to reduce the influence of the propagation effects and to optimize the desired link

quality and its costs by choosing the frequency, antenna diameters, diversity protection, etc. A

free space laser communication system has to be taken as it is.


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15. CONCLUSION

Anticipating a number of fixed launch schedules, the relevant departments are cooperating to

develop and test engineering models of satellite-mounted equipment that will be able to provide

high-capacity transmission despite its globally unparalleled small size.


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16. REFERENCES

1) M. Jeganathan, Development of the free-space optical communications analysis software,

Free-Space Laser Communications Technologies X, Proceedings of SPIE, [Ed. G.

Stephen Mecherle], Volume 3266, 90-98, 2005.

2) C. C. Chen and C. S. Gardner, Impact of random pointing and tracking errors on the

design of coherent and incoherent optical intersatellite communication links, IEEE

Trans. Commun.37 (3), 252260 (2004).

3) S. G. Lambert and W. L. Casey, Laser Communication in Space(Artech House, Boston,2002).

4) Henderson A. R., Bioptica: A Guide to Laser Safety ISBN 0-412-72940-7, UKFebruary 2005

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Free space LASERcommunication