Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-04-01
2002-10-29
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06473214
ABSTRACT:
REFERENCE TO RELATED APPLICATION
U.S. patent application Ser. No. 09/048,402 filed Mar. 25, 1998 in the names of K. B. Roberts et al. (corresponding to UK patent application No. 9802913.5 filed Feb. 11, 1998) entitled “Multiplexed Transmission Of Optical Signals” relates to high capacity optical transmission systems in which optical signals are multiplexed using a waveguide array to provide a relatively large number of transmission channels and hence a very high transmission capacity of the order of 1 Tb/s (one terabit, or 10
12
bits, per second). The entire disclosure of this United States patent application is hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
In an array transmission system as described in the application referred to above, optical pulses from a laser source are split into a large number of channels by a waveguide array, and optical signals of the channels are modulated by respective modulators and are combined by another waveguide array to be communicated over an optical fiber path. Such a system can provide high spectral density, but requires a relatively large number of modulators.
It is known that optical fiber chromatic dispersion is a limiting factor for transmission distance in high speed optical communications systems. Another important criterion in an array transmission system as discussed above is the implementation of the system, particularly in relation to the costs and technical complexity and risks associated with the modulators.
It would be desirable to avoid the need for external modulators by providing direct modulation of a semiconductor laser. Amplitude modulation (AM) of the intensity of the optical signal produced by the laser enables direct detection at an optical receiver to recover the original binary signal. However, direct AM of a semiconductor laser results in the optical signal having a spectral occupancy, or frequency chirp, that is not acceptable for long distance transmission due to the chromatic dispersion of the fiber.
This difficulty can be addressed by performing direct frequency modulation (FM) of the semiconductor laser, and converting the resulting FM optical signal to an AM optical signal using a bandpass optical filter, for example as described in “5 Gbit/s Optical FSK Modulation Of A 1530-nm DFB Laser” by R. S. Vodhanel et al., European Conference on Optical Communication, 1988. As described there, a DFB (distributed feedback) laser is modulated with a pseudo-random NRZ (non-return to zero) binary signal and the resulting FSK (frequency shift keyed, i.e. FM) optical signal is conducted via an etalon and an optical fiber to a p-i-n photodiode detector, the etalon serving to perform FSK demodulation.
While this provides improved performance compared with direct AM, it remains inadequate for long distance transmission. In particular, the above article recognizes that the laser has a non-uniform low-frequency FM response which can distort the optical FSK signal and produce errors in the communicated signal. This non-uniformity is understood to be a result of thermal frequency shift of the laser, and particularly affects that part of the spectrum of the signal being transmitted that is below a frequency of about 20 MHz.
“Bipolar Optical FSK Transmission Experiments at 150 Mbit/s and 1 Gbit/s” by R. S. Vodhanel et al., Journal of Lightwave Technology, Vol. 6, No. 10, October 1988, pages 1549-1553, mentions various modulation techniques, such as Manchester coding, proposed to eliminate unwanted thermal frequency modulation of semiconductor lasers, and proposes using a bipolar signal format for this purpose. The bipolar signal has a signal power or energy which decreases towards zero for low frequency signal components towards zero frequency, so that the undesired thermal frequency modulation of the laser at low frequencies is reduced. However, the optical receiver is much more complicated, in this case requiring a frequency discriminator for demodulation and using a Schmitt trigger to convert the demodulated signal from the bipolar format back to its original NRZ form. In addition, Manchester coding or this bipolar format increases the spectral occupancy of the resulting optical signal, which as discussed above results in reduced performance for long distance transmission due to chromatic dispersion.
It is known, for example from Yonenaga et al., “Dispersion-Tolerant Optical Transmission System Using Duobinary Transmitter and Binary Receiver”, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pages 1530-1537, and from Yonenaga et al. U.S. Pat. No. 5,543,952 issued Aug. 6, 1996 and entitled “Optical Transmission System”, to use duobinary code for a modulating signal supplied in push-pull manner to a dual-drive Mach-Zehnder (MZ) type optical intensity modulator in an optical communications system. The use of duobinary code in this manner reduces the signal bandwidth required for a given signal transmission rate, and permits direct detection to recover the original binary signal at an optical receiver. Such an arrangement again requires an external modulator and involves the costs and risks associated therewith especially in an array transmission system. For example, cross-talk of high voltage, high frequency signals among closely spaced electrical circuits presents a significant problem, and modulation using duobinary encoded signals as disclosed by Yonenaga et al. doubles the voltage swings of signals supplied to the modulators, thereby exacerbating this problem.
An alternative duobinary encoding technique is described in International patent application PCT/CA98/00275 by Northern Telecom Limited, published Oct. 8, 1998 under No. WO 98/44635 and entitled “Duobinary Coding And Modulation Technique For Optical Communication Systems”.
The article by Yonenaga et al. referred to above also refers to a dispersion-supported transmission (DST) technique, as disclosed by B. Wedding et al., “10-Gb/s optical transmission up to 253 km Via Standard Single-Mode Fiber Using the Method of Dispersion-Supported Transmission”, Journal of Lightwave Technology, Vol. 12, No. 10, October 1994, pages 1720-1727. The DST technique uses direct modulation of a laser diode with a NRZ binary signal to produce an FSK optical signal, and FM-AM conversion in the dispersive optical fiber with direct detection of the AM component at an optical receiver. Consequently, the DST technique requires the frequency deviation of the FSK optical signal to be adjusted, depending upon the chromatic dispersion of the fiber, to match the group delay between the FSK components to the bit duration. In addition, recovery of the NRZ binary signal from the detected AM component of the converted optical signal requires additional processing, for example by an integrator and a decision circuit.
This invention seeks to facilitate optical signal transmission of high speed signals over long distances, with relatively low technical complexity and cost, in a manner that can be suitable or advantageous for use for array transmission.
SUMMARY OF THE INVENTION
One aspect of this invention provides a method of producing an amplitude modulated optical signal representing a binary signal, comprising the steps of: encoding the binary signal to produce a three-level encoded signal, the encoded signal having two outer levels representing a first state of the binary signal and having an intermediate level representing a second state of the binary signal; producing an optical signal frequency modulated in accordance with the three-level encoded signal; and optically converting the frequency modulated optical signal in dependence upon its frequency to produce an amplitude modulated optical signal having first and second amplitudes representing the first and second states of the binary signal.
Preferably the step of producing the frequency modulated optical signal comprises direct modulation of a semiconductor laser by the encoded signal.
The step of encoding preferably encodes the binary signal in accordance with a polynomial having factors (1−D) and (1+
Habel Richard R.
O'Sullivan Maurice S.
Roberts Kim B.
Bello Agustin
Nortel Networks Limited
Pascal Leslie
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