About This Webinar
FREE WEBINAR
Dr. Hamid Hemmati
Supervisor, Optical Communications Group
Jet Propulsion Laboratory
California Institute of Technology
In this talk, Dr. Hemmati will cover laser transmitter options for short-range and long-range fiber optic communications, using both direct detection and coherent detection. Lasers for free-space communications will also be described briefly.
Additional Questions & Answers from the Webinar
Q: What modulated devices are available @ <= 1micron?
You can either directly modulate semiconductor lasers, or externally modulate them at these wavelength by a Mach-Zehnder interferometer (commercial).
Q: What is electrical power requirement for under ocean amplifiers. How is it supplied? With intercontinental electrical lines?
These amplifiers are <15% efficent. An electrical power line along with the fiber supplies power.
Q: What happens with the energy when destructive interference happens within a mach-zehnder modulator? Where does it go? Heat?
It goes to the constructively generated fringes.
Q: Were any self-focusing effects during the process were observed?
Not sure what process you are talking about. At these power levels self-focusing is negligible.
Q: What's the ppm shift per degree celsius? is this wavelength dependent?
There is no PPM shift. If you are talking about wavelength shift, it s about 0.3nm per degrees C.
Q: What encryption techniques are in-situ within those free-space laser systems?
Many coding schemes are available. Google papers by Bruce Moision of JPL.
Q: Is the 40nm the full-width at half-max?
Typically at 1/e.
Q: Any self-focusing effect during the process?
Generally no.
Q: Particularly For short reach FSO communication, which laser transmitter is suitable?
Depends on how short and what data-rate.LEDs and diode lasers are typically used for <a few 100m.
Q: Over the years our bit-rate capability increased without really showing slow down in increase. Do we have an estimation of the bit-rate limit the physics will allow us to achieve?
Ultimately Shanon limit, and detection/detector and laser noise efficiency. I do not know what the ultimate limit is. A few papers have been written about Limits of Optical Communications.
Q: Is the temperature wavelength shift still 0.3nm/C at shorter wavelength diodes, such as in the blue?
Typically yes.
Q: In transmitter, the free optic and integrated optic circuit were used. What are the advantage and disadvantage for using free optic configuration?
You use fiber waveguide as much as possible. Only when what is needed is not available or impractical, use free-space.
Q: I think you mentioned Tb/s modulation speed capability with Mach-Zender modulators. Has that already been demonstrated?
Yes, I have seen papers. Search in "Google Scholar"
Q: How MOPA differs from EDFA.
EDFA is a version of MOPA
Q: How about using QCLs working in the range of 10 microns? Have You considered it? They will work better in hard weather conditions - fog, rain etc.
Fog attenuation is >300 dB. At 10 micron you may gain 10-20 dB over 1.5 micron. But that hardly makes a difference since you still have ~280dB attenuation to make up.
Q: Hello - excellent presentation - please can you provide a brief summary of the difference between wdm and coherent comm.
Coherent detection has to do with detection. For example AM radio (direct detetion), FM radio (coherent detection). WDM has to do with multiplexing of dierent wavelength at transmitter end.
Q: For Fabry-Perot laser, when is it better to use temperature tuning than cavity length to select a particular wavelength?
Temperature tuning changes cavity length also.
Q: Do you think that SOA and MZI-SOA will lead the all-optical processing systems in the future?
Yes, these are important ingredients of all-optical processing.
Q: Can you discuss polarization modulation a bit more? How often is it used? How many different states of polarization are generally implemented in such systems?
It used to be used in early days. Not as much now since much better modulators are now available. Usually two-state (but higher numbers are possible with greater degree of difficulty), which can be combined with other modulation schemes.
Q: What is the approximate cost difference between an electrical network and optical network??
It is comparing apples and oranges since the capabilities are widely different. If it is cost/delivered bit. I would guess, optical is thousands of times less expensive.
Dr. Hemmati's current research interests are in developing lasercom technologies and low mass/power/ size flight terminals for both interplanetary and satellite communications. Prior to joining JPL in 1986, he was a researcher at NASA's Goddard Space Flight Center and at NIST in Boulder, Colo. Dr. Hemmati is the editor and author of two books: "Deep Space Optical Communications" and "Near-Earth Laser Communications" and author of five other book chapters. He has published over 150 journal and conference papers, has nine patents granted and two patents pending. He is a Fellow member of SPIE and teaches lasercom at UCLA.
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