Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-05-12
2001-05-08
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06229632
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communication systems that transmit information over an optical fiber cable. More specifically this invention pertains to a method of improving the carrier to noise ratio of analog video transmission without impacting the linearity and fidelity of the transmission.
PRIOR ART
The need for communication bandwidth capacity has increased dramatically in the last decade and continues on an exponential growth path. It is forecast that the bandwidth requirements of telecommunication systems will increase by a factor of 400 (IEEE Spectrum, January 1995) when most of the personal computers in businesses and homes are connected over telephone networks. To fill this need communications companies have invested large sums into developing infrastructures to transmit all this information. One of the various methods of transmitting large quantities of information that has experienced much growth in the last decade utilizes optical fibers and transmits information in the form of modulated light through these fibers.
Optical fiber transmission systems fall into two categories: analog and digital systems. In analog transmission systems the amplitude of the light in the fiber directly represents the amplitude of the transmitted analog signal. The fidelity and linearity of all the components of such an analog system, such as the transmitter, receiver and any amplifiers in-between is of critical importance to the proper transmission of the signal. The transmitter usually consists of a laser and two options are available to modulate its output: direct modulation of the current through the laser or external modulation via an additional light modulator. Both techniques have been practiced and each suffers from particular drawbacks.
Lasers tend to have nonlinear current to output relationship and external modulators have a sinusoidal input output curve. It follows that in either modulation type the amplitude of the modulation must be kept low in order to maintain the linearity of the transmitter within the required system performance range. A direct result of the low modulation ratio is a low signal to noise ratio, thus severely impairing the operation range of the system.
As an example the Carrier to Noise Ratio (CNR) in Community Antenna Tele-Vision (CATV) distribution networks is required to be higher than 50 dB for proper reception. In such systems, up to 80 or more RF carrier channels are summed and transmitted over one light transmitter. The mixing of all the analog carriers due to nonlinearities in the system leads to the formation of second order and third order mixing terms in the frequency spectrum, referred to as second order and triple beat terms. The sum total of all the power in the second order terms is referred to as Composite Second Order (CSO), and the sum of all the power in the triple beats as Composite Triple Beat (CTB). These quantities are required to be 65 to 70 dB below the carrier power. In order to satisfy these requirements the optical modulation index (OMI), which is defined as the peak to peak modulation amplitude divided by the average light power, must be kept below 4 or 5% per carrier. As a result, the CNR at the transmitter is on the order of 55 dB and degrades from that value when the signal goes through amplifiers.
In order to improve the CNR while keeping nonlinearities low several approaches have been taken in the prior art. A first approach pre-distorts the electrical input to a Mach Zehnder optical modulator or to a directly modulated laser as in U.S. Pat. No. 4,992,754. Such pre-distortion linearized modulators are now commercially available from for example Uniphase Telecommunications Products in Bloomfield Conn. (model APE CATV). A second approach applies specific ratios of RF power to two modulators arranged optically in tandem as described by H. Skeie and R. Johnson in “Linearization of Electro-Optic Modulators By a Cascade Coupling of Phase Modulating Electrodes”, SPIE Vol. 1583, p. 153 (1991).
As mentioned above, in an optical analog vestigial side-band (AM-VSB) transmission system, it is desireable to increase the level of optical modulation per channel at the transmitter in order to achieve the highest carrier to noise ratio at the receiver. All the above methods suffer from limitations on the linearity that allow only a maximum of 4 to 5% modulation per TV channel RF carrier, before the CSO and CTB distortions start exceeding the required levels.
More recently, in the Optical Fiber Communications Conference Technical Digest, paper WN2, p. 196 (1996), Kuo et. al. described a differential detection approach to improving the CNR of an optical transmission by 3 dB. As described in
FIG. 1
of this disclosure, Kuo's approach requires two transmission fibers
10
and
10
′, one for each output of the Mach Zehnder optical modulator
5
.
The present invention aims to improve the CNR level without increasing the CSO and CTB and to use a single transmission fiber instead of two.
Furthermore in the optical transmission of digital signals, the ratio of the difference between the peak light power in the fiber, representing a 1 (one), and the minimum light power representing a 0 (zero), to the noise power directly affects the bit error rate of the transmission. The present invention can also serve to maximize this ratio and thus minimize the bit error rate.
SUMMARY OF THE INVENTION
Therefore this invention provides a method for improving the Carrier to Noise Ratio (CNR) of an analog transmission over optical fiber without increasing the nonlinear distortions such as Composite Second Order (CSO) and Composite Triple Beats(CTB) and without using more than one transmission fiber. This invention achieves this objective by utilizing two light wavelengths modulated by a single Mach Zehnder type optical modulator having two fiber outputs. At the first output fiber, the first wavelength is filtered out and at the second fiber output, the second wavelength is filtered out. Since the Mach Zehnder outputs are necessarily in phase opposition, said first wavelength light increases at said first fiber output by substantially the same amount that said second wavelength light decreases at said second fiber output. The two fiber outputs of said Mach Zehnder modulator are then re-combined into a single mode fiber for transmission. At the other end of the said transmission fiber, the said first and second wavelengths are separated into a first and second optical detectors by means of a wavelength demultiplexer. Thus the electrical output of the first detector is proportional to the power of the first wavelength light and the output of the second detector is proportional to the wavelength of the second wavelength light. By virtue of those said first and second light powers being equal in amplitude and opposite in phase, when the electrical outputs are summed in an RF combiner, the electrical signal output of the combiner is twice as high as any of the said first and second outputs of the detectors. Accordingly the optical modulation index of the transmitted light appears to have been doubled, resulting in a 3 dB increase in CNR. This is achieved without actually increasing the OMI at the Mach Zehnder and causing an increase in the nonlinear distortions such as CSO and CTB. Further any correlated electrical noise that is common mode to both detectors, such noise possibly originating from the transmitter, is eliminated by the combiner. A further improvement utilizes four detectors, two for each of the said wavelengths. The method of this invention can equally as well be applied to digital transmission where it enhances the signal to noise ratio at the receiver.
REFERENCES:
patent: 4992754 (1991-02-01), Blauvelt
patent: 5170273 (1992-12-01), Nishio
patent: 5491576 (1996-02-01), Bergano
patent: 5543952 (1996-08-01), Yonenaga et al.
Bello Agustin
Caserza Steven F.
Chan Jason
Ditech Corporation
Flehr Hohbach Test Albritton & Herbert LLP
LandOfFree
Broadband optical transmission system utilizing differential... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Broadband optical transmission system utilizing differential..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Broadband optical transmission system utilizing differential... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2446142