Optical FM receiver

Details Optical FM receiver Optical FM receiver

1999-10-20

2001-07-24

Chan, Jason (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200

Reexamination Certificate

active

06266173

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of communications, and, in particular, to optical devices for satellite communications systems.
BACKGROUND
Orbiting satellites are an important aspect of modern communication systems. Originally used for “single-bounce” communication, with a signal going up from one place on the surface of the earth and coming down in another, communication satellites are now being used to form complex networks in space, with each satellite in the network being able to communicate with many, but not all, of the other satellites. Optical inter-satellite links, with their high directionality, high energy efficiency, and tremendous information bandwidth, allow satellites to talk to one another, and to transmit a much larger amount of information. Optical Frequency Modulation (FM) links, a new concept, offer a way to transmit not only digital signals, but also analog signals, and to do this with a much higher signal quality than was heretofore possible. However, effective optical FM links have not yet been demonstrated primarily because of the lack of appropriate photonic components, such as FM optical sources, limiters, discriminators, and receivers.
The primary function of most of these systems is to transport analog information data from one point to another. It would be very beneficial to have an approach that provided for the transport of this data in its original format, without having to convert it first into a digitally encoded bit stream, and then reconverting upon reception. Conventional modulation schemes do not have sufficient Signal to Noise Ratio (SNR) to allow this, which is why the majority of this data is digitally encoded before transport. However, FM techniques offer a “processing gain” that can increase the SNR by 20 dB or more, thus allowing direct analog transmission of the data in many of these applications. This offers a considerable simplification in system hardware, an increase in system flexibility, and a reduction in cost.
Optical links using FM and Phase Modulation (PM) have been discussed in the article by R. F. Kalman, J. C. Fan, and L. G. Kazovsky, entitled “Dynamic Range of Coherent Analog Fiber-Optic Links”, J. Of Lightwave Technology, Vol. 12, p. 1263 (1994). Their approach uses a conventional technique employing a local oscillator offset from the signal frequency by a difference equal to the Intermediate Frequency (IF), together with a limiter, a filter, and an envelope detector.
Referring to prior art
FIG. 1
, input signal
10
is combined with light
12
from a local oscillator (lo) laser at directional coupler
14
. The combined signal is mixed at photodetector
16
, amplified at IF amplifier
18
, (optionally) filtered by filter
19
, limited by limiter
20
, split into a delayed and undelayed signal, with the two signals (delayed and undelayed) being mixed (multiplied together) in a final RF mixer
23
. To get an FM processing gain, the system shown in
FIG. 1
must have an IF that is much larger than the baseband modulation frequency. The square of the ratio of the IF frequency to the baseband bandwidth is the SNR improvement that one gets with the FM approach. Therefore, if 20 dB of noise suppression is desired, and if there is a baseband bandwidth of 20 GHz, an IF frequency of 200 GHz and a bandwidth for all the IF components of twice this, namely, 400 GHz, are needed. Amplifiers, limiters and envelope detectors that operate over a 400 GHz bandwidth simply do not now exist, so that such a system cannot be presently realized.
Therefore, there exists a need for an effective FM receiver to help further realize optical FM inter-satellite links. The present invention provides a unique solution for such need by providing an all-optical FM receiver that performs the same function in the optical domain that a conventional FM receiver performs in the Radio Frequency (RF) domain.
SUMMARY OF THE INVENTION
A preferred embodiment of the invention includes an optical amplifier, optical filter, optical limiter, an optical delay-line discriminator and dual-balanced photodetectors that are assembled to form a system that converts an incoming lightwave having frequency modulation into an electrical signal that is proportional to the degree of phase or frequency deviation. The system thus performs the same function in the optical domain that a conventional FM receiver performs in the RF domain without the use of an IF stage. This lack of an IF stage provides the difference between a practicable system, and one that is an unrealizable mathematical curiosity.
The present invention allows an FM optical communication link to have all of the benefits of wide-band FM without the need for an IF stage. The elimination of the IF stage allows the link to operate at the highest frequencies achievable with today's photodetectors and microwave amplifiers (e.g., 40 GHz), while simultaneously achieving a high degree of noise suppression. The invention further allows the discriminator to be operated in a way that leads to high linearity. The result is a system that will have superior noise properties and low distortion. It is thus an ideal technique for optical inter-satellite communication systems requiring high Spur Free Dynamic Range (SFDR).
In accordance with a preferred embodiment of the present invention a method and apparatus for converting a frequency modulated lightwave signal into an electrical signal that is proportional to the frequency deviation of the frequency modulated lightwave signal is provided. The frequency modulated lightwave signal is split into a first split lightwave signal and a second split lightwave signal by a 50:50 4-port directional coupler (such as any of the single-mode couplers manufactured by Gould Fiber Optics, Inc.). The second split lightwave is shifted 90° in phase (or an odd multiple thereof) with respect to the first split lightwave by the delay length &tgr;. The first split lightwave and the second delayed split lightwave are then recombined in a second 50:50 directional coupler, in which the fields combine to give the sum of the delayed and undelayed fields at one directional coupler output port, and the difference of the delayed and undelayed fields at the other directional coupler output port. The two output ports of this 4-port coupler are then directed to a first and second photodetector, respectively. The first photodetector and the second photodetector are connected in series with a common terminal therebetween. The common terminal provides a photodetector current proportional to an instantaneous frequency deviation of the carrier frequency. The frequency modulated lightwave is received by an optical amplifier that provides an amplified lightwave signal. Amplitude fluctuations and noise in the amplified lightwave signal can be reduced to provide an adjusted lightwave signal to be split into the first split lightwave signal and a second split lightwave signal. An input of a microwave amplifier is coupled to the common terminal to amplify the photodetector current proportional to the instantaneous frequency deviation. Thermally controlled mediums can be provided in which the first split lightwave signal and the delayed second split lightwave signal propagate.


REFERENCES:
patent: 5245461 (1993-09-01), Fitzmartin
patent: 5742714 (1998-04-01), Byron
patent: 5786913 (1998-07-01), Pfeiffer
patent: 0 473 873 (1992-03-01), None
patent: 0 591 866 (1994-04-01), None
patent: 00/51271 (2000-08-01), None
Kalman, R.F. et al., “Dynamic Range of Coherent Analog Fiber-Optic Links,” Journal of Lightwave Technology, vol. 12, No. 7, (Jul. 1994), pp. 1263-1277.
Hirano, A. et al., “All-optical limiter circuit based on four-wave mixing in optical fibres,” Electronics Letters, vol. 34, No. 14 (July 9, 1998) pp. 1410-1411.
Sorin, W.S. et al., “Frequency Domain Analysis of an Optical FM Discriminator,” Journal of Lightwave Technology, vol. 10, No. 6, (Jun. 1992), pp. 787-793.
Swanson, E.A. et al., “High sensitivity optically preamplified direct detection DPSK receiver with active delay-line stabiliza

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