Laser Communications


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Laser Communications

  2. 2. 2 Laser Communications A key technology to enabling small spacecraft missions is a lightweight means of communication. Laser based communications provides many benefits that make it attractive for small microlauncher spacecraft. First, because of the high frequency of light, the optics required to do effective communication is relatively small, much smaller than required for even microwaves. Second, laser communication is not regulated like radio bands. Third, highly efficient and small technology is readily available for use by microlauncher developers at very low prices. Introduction The basic principle of laser communication lies in the modulation of a laser source with information which is transmitted through free space then received and demodulated to reconstruct the original signal at the destination. Think of flashing a light on and off and detecting that flashing from a distance. It’s that simple. Obviously, the brighter the laser light, the farther it can be detected. However, at the distances being used for space-based laser communication, one might think that it would take a lot of energy. In reality, a lack of high energy at the transmitter side can be made up for by having really sensitive detectors. There are several very sensitive receiver technologies that lend themselves very well to space-based laser communications. Laser Diodes A laser diode is a semiconductor device able to convert electrical energy into coherent laser light. Modern laser diodes generally convert electricity to light with efficiencies near 25% and operate at low voltages near 2.5 volts. They can even be much more efficient, close to 70% efficient for more advanced technologies. Laser diodes produce light at a very narrow frequency; however, the frequency is usually specified in terms of the wavelength of the light. The relationship between light frequency and wavelength is:   c f Where c is the speed of light (186,282 miles/second or 299,792,458 m/s), λ is the wavelength in the appropriate units and f is the frequency in Hz. Knowing the frequency of light and the energy of the beam, we can also determine how many photons are being emitted. The equation for the photon generation rate is: n  p hc
  3. 3. 3 Where n is the number of photons generated per second, p is the power (in Watts), λ is the wavelength (say in meters), h is the Planck constant (6.62606957x10^-34 Joules/s), and c is the speed of light (299792458 meters/second). Therefore, if we have a laser that emits 30 mW of light at 650 nm, then the number of photons generated is: n  p hc n  0.030 0.000000650 6.62606957 10 34  299792458 1.95108 n  1.98655 1025 n  9.82 1016 photons per second Signal Modulation There are several methods by which you can carry data on the laser signal, some of which give much better performance than others. One basic modulation technique is known as On-Off Keying (OOK). In this technique, the laser is merely turned off and on consistent with the data to be transmitted. The signal can also modulate a carrier frequency to allow increased noise immunity. Pulse Position Modulation (PPM) is a different technique of transmitting data. In this case, the presence of a signal in a time slot is used to indicate the data. Using this technique, it might be possible to transmit more than one bit of data with each on time period.
  4. 4. 4 Although there are many other modulation techniques, these two represent two of the more commonly used modulation methods for laser communications. Beam Divergence And Energy Usually, the number of photons generated by the laser is confined to a small diameter near the outlet of the diode laser. However, over long distances, this beam spreads out and those photons get spread out over a larger area; this is called its divergence and is measured as the spread angle of the emitted beam. A typical value for low cost commercial lasers’ divergence is about 1 mRad (or milliRadian). The relationship between degrees and radians is: d  r  360 2   where r is in radians and d is in degrees. Therefore, a beam divergence of 1 mRad is equivalent to a divergence angle of:
  5. 5. 5 d  r  360 2  d  0.001 360 2 d  0.057   Another important aspect of understanding laser power levels is to be able to know how many photons exist in a given area. Looking at the previous diagram, we can see two different locations marked A1 and A2. If A1 is 5 inches away from the laser diode and the divergence is 1 mRad, then we can calculate how many photons exist per square inch. The equation to calculate the photons per unit of area for a diverging beam at range is: photons per unit area  n 2     l  tan    2   Where n is the number of photons generated by the source (per second), l is the distance in whatever units you choose and θ is the beam divergence angle. Looking at our earlier example we can calculate the number of photons receivable from a spacecraft some distance from Earth. If the spacecraft is 1,000,000 miles away and it has a 650 nm 30mW laser pointing with 0.1 mRad divergence pointed at Earth, then from our earlier equation: n  p hc n  0.030 0.000000650 6.626 1034  2.99 108 n  9.82 1016 photons/s Using the photons per unit area equation we just introduced, we can see that the area of the beam at Earth is: photons per unit area  n 2     l  tan    2   photons per unit area    9.82 1016 2    1106  tan 0.0001    photons per unit   area   2  9.82 1016 7.85103 photons per unit area 1.25 1013 photons per second / square mile If we had a telescope of 10 inches in diameter, we could determine how many photons
  6. 6. 6 per second would be received there. A 10 inch diameter telescope has an aperture area of 78.5 square inches. Since there are 4.0144896x10^9 square inches per square mile:
  7. 7. 7 78.5sqin  10"scope 1square_ mile 4.014 109 sqin 1.25 1013 photons/ s  1square_ mile 2.44 105 photons/ s  10"scope With a 10 inch diameter scope, there are 244000 photons per second (not counting atmospheric losses). Using the equation relating photons per second to the power in watts, we can determine how many watts to which this is equivalent. n  p hc p  nhc  2.44 105 6.626 1034 2.99 108 p  6.50 107 4.831020 p  6.50107 p  7.43 1014 Watts This is roughly equivalent to a 10th magnitude star in brightness. Is it possible to detect these photons? The answer is “yes” if you use a photon counting sensors like photomultiplier tubes or Avalanche Photo Diodes. Photon Counting Sensors There are several kinds of sensors that are useful for measuring weak light signals: Charged Coupled Devices (CCD’s), Photomultiplier Tubes (PMT's) and Avalanche Photo Diodes (APD's) are the ones most likely for microlauncher missions. Each has different benefits for the purposes of laser communications. Charge Coupled Devices Charge Coupled Devices (CCD’s) are familiar as the sensors for video cameras. They have a unique capability to build up an electrical charge on each picture element related to the intensity of light over time. By waiting one can allow sufficient charge buildup to count the number of arrived photons over longer durations of time. This can allow very weak signals to be detected and imaged. Photomultiplier Tubes Photomultiplier Tubes (PMT’s) are an older electron tube based technology which enables single photons to cause avalanches of electron generation within the tube; they can have gains as high as one million. Therefore, a single photon can generate possibly millions of electrons in the sensor and thereby make even small signals detectable. Avalanche Photodiodes
  8. 8. 8 Avalanche Photodiodes (APD’s) are solid-state diode-based sensors that operate similarly to PMT’s. Each photon that impacts the sensor can generate many electrons which can be detected. Therefore, they too have an in-sensor gain which enables the detection of very small signals. Detector Optics Given the availability of a suitable sensor (either PMT, CCD or APD), it is necessary to build a supportive system to make a suitable laser communications receiver. The first, obviously, is some sort of passive light amplifier like a telescope. The optics focuses the light impinging on a larger area to be focused to a smaller area and thereby collects more photons than a bare sensor would be able to detect. Another necessary component which is part of the sensing system is a suitable optical filter which removes almost all light except the desired transmitted laser light. This allows the signal to be more easily detected without too much extraneous light interfering. Typically, what are known as interference filters are used because they allow only a very narrow band of light to strike the photodetector. A filter like this greatly reduces the amount of noise seen by the sensor. An Integrated Transceiver It is possible to integrate the transmission and reception functions into one assembly. This allows a single amplifying/focusing telescope to be used for both transmission and reception. In this example, the downlink transmitter (out from the spacecraft) uses a 650 nm laser and a 780 nm signal wavelength is used for reception. By using a dichroic mirror/filter, the incoming 780 nm signal can be reflected towards the CCD sensor while the outgoing 650 nm transmission laser signal passes through the filter. A 780 nm filter in front of the CCD improves incoming signal discrimination.
  9. 9. 9 Laser Source Tracking Because the laser light source appears as such a small spot of light, and because the receiving telescope likely has a narrow receiving cone, it is necessary to be able to ensure that the receiver is pointed at the transmitter. The method used to track the transmitter source depends on the type of receiver used. In almost all cases, though, the principle of quadrant sensing is the likely mechanism. This consists of using four (or more) receiving elements in a 2x2 square array and using the pattern of signal intensity to indicate the direction of error and correction. If one is using a CCD as the receiver sensor, then some number of picture elements in the middle of the sensor can serve as both tracking sensors and signal sensors. There are APD's available in the quadrant layout which can be used to track the laser source. A similar but more complicated technique might use an optical chopper when one uses a photomultiplier tube. A slight variation of this approach might use two sensor sets: one a signal sensor and the other a CCD for tracking. By using a beam splitter, the incoming signal can be sent to both the signal sensor (of whatever kind) and also to the CCD so that tracking can be performed.
  10. 10. contact Blair Gordon COO /microlaunchers @mlaunchers [614]434-6027 /microlaunchers