Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-09-26
2003-11-18
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S237000, C385S002000, C385S003000, C385S009000
Reexamination Certificate
active
06650458
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to optical signal modulation and more particularly to electro-optic modulators incorporating phase modulation.
BACKGROUND OF THE INVENTION
Networks with topologies more complicated than simple point-to-point or ring architectures have variable amounts of fibre dispersion at each wavelength, depending on where the wavelength is being routed, and the distance, fibre type, and dispersion map present in each section of the link.
To make such networks capable of flexible and intelligent provisioning, especially with the use of a software operating system instead of hardware re-configurations, a method of compensating changes in dispersion caused by alterations in the route of a particular wavelength is required. In particular, it would be desirable for transmission systems set up to compensate particular dispersion conditions in the network to be flexible enough to adapt to changing conditions in the network.
Chromatic dispersion which occurs naturally in the single mode fibers of standard optical networks causes a frequency shift in the leading edge of an optical pulse and a different frequency shift in the trailing edge of an optical pulse due to different speeds of transmission of light with different frequencies. This kind of dispersion may be compensated for by adding a chirp to the optical signals at the transmission end.
A common technique used to modulate optical data signals is to split the optical signal, impart a phase modulation to one of the split portions of the optical signal, and recombine the phase modulated and unmodulated portions of the optical signal. With proper choice of phase modulation, constructive and destructive interference between the two portions will result in an amplitude modulated combined signal. One side effect of this method of amplitude modulation is signal chirp. The phase modulation may be described as a phase shift as a function of time &thgr;(t). A logical “0” and a logical “1” may be represented by different phase shifts. It should be noted that all phases described herein are defined with reference to the wavelength of the optical carrier signal to be modulated and are in units of radians. It also should be noted that whenever the frequency of an optical signal is referred to it should be understood as being a reference to the instantaneous frequency of the optical signal. One possible choice of phase shifts is a phase shift of 0 representing a logical “0” and a phase shift of &pgr; representing a logical “1”. As &thgr;(t) varies in time during the modulation from a logical “0” state to the logical “1” state, the phase shift &thgr;(t) varies from 0 to &pgr;, and during the modulation from a logical “1” state to the logical “0” state, the phase shift &thgr;(t) varies from &pgr; to 0. In the modulation of a logical “1” from a data pulse, since the phase of the resulting modulated optical signal has been increased during the rise of the data pulse (the leading edge), the frequency of the signal is slightly increased during the time of this rising edge. The increase in frequency is exhibited as a blue shift of the leading edge of an optical pulse in the combined optical signal. Since the phase of the resulting modulated optical signal has been decreased during the fall of the data pulse (the trailing edge), the frequency of the signal is slightly decreased during the time of this falling edge. The decrease in frequency is exhibited as a red shift of the trailing edge of an optical pulse in the combined optical signal. The blue shifting of the leading edge and the red shifting of the trailing edge is known as positive chirp. It should be noted that the particular values of 0 and &pgr; are not required for positive chirp, but in general it is the sign of the rate of change of the phase shift which determines the chirp. If &phgr;(t) is increasing (d&phgr;(t)/dt is positive) on a leading edge of an optical pulse, and if &phgr;(t) is decreasing (d&phgr;(t)/dt is negative) on the trailing edge, the optical signal is positively chirped.
Another possible choice of phase shifts is a phase shift of &pgr; representing a logical “0” and a phase shift 0 representing a logical “1”. As &thgr;(t) varies in time during the modulation from a logical “0” state to the logical “1” state, the phase shift &thgr;(t) varies from &pgr; to 0, and during the modulation from a logical “1” state to the logical “0” state, the phase shift &thgr;(t) varies from 0 to &pgr;. In the modulation of a logical “1” from a data pulse, since the phase of the resulting modulated optical signal has been decreased during the rise of the data pulse (the leading edge), the frequency of the signal is slightly decreased resulting in a red shift of the leading edge. Since the phase of the resulting modulated optical signal has been increased during the fall of the data pulse (the trailing edge), the frequency of the signal is slightly increased resulting in a blue shift of the trailing edge. The red shifting of the leading edge and the blue shifting of the trailing edge is known as negative chirp. As described above, in general it is the sign of the rate of change of the phase shift which determines the chirp. If &phgr;(t) is decreasing (d&phgr;(t)/dt is negative) on a leading edge of an optical pulse, and if &phgr;(t) is increasing (d&phgr;(t)/dt is positive) on the trailing edge, the optical signal is negatively chirped.
A third possible method of phase modulation creates a zero chirp amplitude modulated signal. In this case two modulated signals are used to create the amplitude modulated signal instead of combining a modulated and unmodulated signal. In this scheme one modulator varies from 0 to &pgr;/2, and the other from 0 to −&pgr;/2 when the data varies from a logical “1” to a logical “0”. The signals will be in phase during a logical “1”, and out of phase during a logical “0”. Since the respective phase changes in opposite directions the combined signal will have no chirp.
Since chromatic dispersion acts similar to signal chirp, insofar as the leading edge of a pulse is shifted in frequency and the trailing edge of a pulse is also shifted in frequency, compensation of chromatic dispersion may be made at the transmission end by transmitting signals with signal chirp opposite to the equivalent chirp caused by the chromatic dispersion. Due to the range of type and magnitude of dispersion and hence the type and amount of chirp that the optical signals will encounter for various conditions along various fibers, the more flexible the chirp characteristics of the transmitter, the more flexible the transmitter will be in terms of its ability to be used.
In each of the schemes described above however, the method of modulation is fixed as is the amount of chirp imparted to the optical signals making it difficult for the transmitter to adapt to different network conditions.
Alternative solutions have been tried in the past, and include having a dispersion compensation device on each channel. This approach, however, is bulky and expensive.
Another approach is to use a standard zero chirp MZ (Mach-Zehnder) amplitude modulator followed by a series phase modulator. This however requires an extra drive signal, adding to cost, size and complexity. In addition, the phase modulator would also require a variable gain broadband driver to adjust the chirp. Maintaining the RF phase timing of the variable gain broadband driver over the full dynamic range of its variable gain is difficult.
Another approach is a single-end-drive amplitude chirped modulator for example an MZ or EA (Electroabsorption) modulator followed by a series phase modulator. With this approach the resultant chirp profile may be non-ideal for highly dispersive links. The phase modulator would require a broadband variable gain amplifier to adjust the chirp, which as mentioned above is a custom part that has a number of drawbacks.
Another approach is a standard push-pull MZ modulator driven by a custom driver containing an XOR gate to invert or pass the signal, a differential pre-amplifier,
Betty Ian
Prosyk Kelvin
Bookham Technology PLC
Epps Georgia
Hasan M.
Lahive & Cockfield LLP
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