FREQUENCY SHIFT KEYING LASER COMMUNICATION SYSTEM

• US 3,600,587 A
• Filed: 06/10/1969
• Issued: 08/17/1971
• Est. Priority Date: 06/10/1969
• Status: Expired due to Term

• Current Assignee: Donald A. Smith
• Original Assignee: Donald A. Smith
• Inventors: Smith, Donald A.
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Brodsky, James A.
• Application Number:
  US04/831,972
• Time in Patent Office: 798 Days
• Field of Search: 250/199 325/30,163 178/66,67
• US Class Current: 398/187
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/5563 Digital frequency modulation
• CPC Class Codes: H04L 27/10 Frequency-modulated carrier systems, i.e. using frequency-shift keying H04L27/32 takes precedence

Abstract

An optical frequency shift keying system for representing a first binary signal condition by a first optical frequency and a second binary signal condition by a second optical frequency by means including an optical modulator biased to provide optical transmission predominantly at one of said optical frequencies during one of said signal conditions and to provide transmission at the other of said optical frequencies during the second condition.

Claims

1. An optical frequency shift keying system for representing a first binary signal condition by a first optical frequency and a second binary signal condition by a second optical frequency comprising means for providing first and second optical beams of said first and second optical frequencies respectively, means for combining said optical beams, an optical modulator positioned in the path of said combined optical beams and having optical transmission characteristics which vary with a modulating signal, means for supplying a modulating signal of first and second level representing respectively said first and second conditions, and means for biasing said modulator to provide transmission therethrough predominantly of said first optical frequency during application to said modulator of the modulating signal of said first level and transmission predominantly at said second optical frequency during application to modulator of the modulating signal of said second level, wherein said biasing means includes a first n-quarter wave retardation plate in the path of said first optical beam and a second n-quarter wave retardation plate in the path of said second optical beam, the value of n for the two plates differing by an integral multiple of two.

2. An optical frequency shift keying system for representing a first binary signal condition by a first optical frequency and a second binary signal condition by a second optical frequency comprising means for providing first and second optical beams, of said first and second optical frequencies respectively, means for combining said optical beams, an optical modulator positioned in the path of said combined optical beams and having optical transmission characteristics which vary with a modulating signal, means for supplying a modulating signal of first and second levels representing respectively said first and second conditions, and means for biasing said modulator to provide transmission therethrough predominantly of said first optical frequency during application to said modulator of the modulating signal of said first level and transmission predominantly at said second optical frequency during application to the modulator of the modulating signal of said second level, wherein said biasing means includes a three quarter wave retardation plate disposed in the path of said first optical beam preceding said modulator and a quarter wave retardation plate disposed in the path of said second optical beam preceding said modulator.

3. An optical frequency shift keying system according to claim 1 wherein said biasing mEans comprises a source of unidirectional voltage of predetermined level applied to said modulator.

4. An optical frequency shift keying system for representing a first binary signal condition by a first optical frequency and a second binary signal condition by a second optical frequency comprising means for providing first and second optical beams of said first and second optical frequencies respectively, means for combining said optical beams, an optical optical modulator positioned in the path of said combined optical beams and having optical transmission characteristics which vary with a modulating signal, means for supplying a modulating signal of first and second levels representing respectively said first and second conditions, and means for biasing said modulator to provide transmission therethrough predominantly of said first optical frequency during application to said modulator of the modulating signal of said first level and transmission predominantly at said second optical frequency during application to the modulator of the modulating signal of said second level, wherein said modulator is of the electrooptical type comprising at least one pair of crystals having the crystallographic axes orthogonally arranged with respect to one another.

5. An optical frequency shift keying system comprising means for producing first and second optical beams of different frequency, and a modulator interposed in the path of said beams and having transmission characteristics which vary in accordance with an applied field, modulating means for applying a modulating signal of a first level to said modulator representative of a mark signal and a modulating signal of a second level to said modulator representative of a space signal, biasing means for biasing said modulator to provide optical beam transmission intensity vs modulating signal level characteristics of opposite slope for said first and second beams, said modulator responding to said modulating signal of first level and said biasing means for effecting optical transmission through said modulator predominantly that of said first beam said modulator responding to said modulating signal of said second level and said biasing means for effecting optical transmission through said modulator predominantly that of said second beam.

6. An optical frequency shift keying system according to claim 1 wherein said modulator is of the electrooptical type.

7. An optical frequency shift keying system according to claim 5 wherein said modulator is of the magneto-optical type.

8. An optical frequency shift keying system according to claim 1 each of said optical beams transmitted through said modulator consisting of two orthogonal components of elliptical polarization further including polarizing means following said modulator for transmitting only one of the two orthogonal components of polarization of said first and second optical beams emerging from said modulator.

9. An optical frequency shift keying system according to claim 8 further including beam splitting means following said polarizing means for transmitting the orthogonal component of said optical beam emerging from said polarizing means along one path and reflecting the orthogonal component of the second optical beam emerging from said polarizing means along a separate path.

10. An optical frequency shift keying system according to claim 5 further including detecting means responsive to the optical frequencies transmitted through said modulator for converting said beam intensities into electrical signals representing said first and second conditions.

11. An optical frequency shift keying system according to claim 10 wherein said detecting means includes first and second photodetectors each receptive of optical energy from one only of said beams and having a good response only to energy from said one beam for deriving a voltage variation and means for subtracting the voltage variations of said photodetectors to derive an output electrical signal which is a suBstantial replica of said modulation signal.

12. An optical frequency shift keying system according to claim 1 wherein said first and second optical beams originally are linearly polarized, said wave retardation plates operating upon a corresponding optical beam to provide optical beams therefrom of opposite elliptical polarization, said modulator acting upon said beams of opposite elliptical polarization to provide a pair of emergent beams each having predominant linearly polarized components which are orthogonal to one another.

13. An optical frequency shift keying system according to claim 5 each of said optical beams transmitted through said modulator consisting of two orthogonal components two orthogonal components of elliptical polarization, further including beam splitting means following said modulator for transmitting both orthogonal components of the first optical beam along one path and reflecting both orthogonal components of the second optical beam along a second path, and separate polarizing means disposed in each of said paths for transmitting one only of said orthogonal components of each of said beams.