Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-07-19
2003-05-13
Swarthout, Brent A. (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06563623
ABSTRACT:
The invention relates to the field of transmitting digital data by optical means. It is more particularly concerned with transmission at high bit rates on long-haul fiber optic links.
BACKGROUND OF THE INVENTION
Such transmission uses an optical transmitter connected to an optical receiver by the fiber. The transmitter generally modulates the power of an optical carrier wave from a laser oscillator as a function of the information to be transmitted. NRZ or RZ modulation is very frequently used and entails varying the power of the carrier wave between two levels: a low level corresponding to extinction of the wave and a high level corresponding to a maximum optical power. The variations of level are triggered at times imposed by a clock rate and this defines successive time cells allocated to the binary data to be transmitted. By convention, the low and high levels respectively represent the binary values “0” and “1”.
The maximum transmission distance is generally limited by the ability of receivers to detect without error these two power levels after the modulated wave has propagated in the optical link. The usual way to increase this distance is to increase the ratio between the average optical power of the high levels and that of the low levels, this ratio defining the “extinction ratio” which is one of the characteristics of the modulation.
For a given distance and a given extinction ratio, the information bit rate is limited by chromatic dispersion generated in the fibers. This dispersion results from the effective index of the fiber depending on the wavelength of the wave transported, and it has the consequence that the width of the transmitted pulses increases as they propagate along the fiber.
This phenomenon is characterized by the dispersion coefficient D of the fiber, which is defined as a function of the propagation constant &bgr; by the equation D=−(2&pgr;c/&lgr;
2
)d
2
&bgr;/d&ohgr;
2
, where &lgr; and &ohgr; are respectively the wavelength and the angular frequency of the wave.
The value and sign of the dispersion coefficient D depend on the type of fiber and the transmission wavelength. For example, for the “standard” monomode fibers routinely used, and for &lgr;=1.55 &mgr;m, the coefficient D is positive and has a value of 17 ps/(nm.km). In contrast, the coefficient D is zero for &lgr;=1.30 &mgr;m. The coefficient D can generally be positive, zero or negative depending on the wavelength and the type of fiber used.
If the coefficient D has a non-zero value, to compensate the phenomenon of pulse widening in the case of NRZ or RZ modulation, it has already been proposed to modulate the phase &phgr; (and therefore the frequency or the angular frequency) of the carrier wave in a manner that correlates to the modulation of the power. The phase &phgr; corresponds to the convention whereby the electric field of the carrier wave is represented as a function of time t by a complex expression of the type: Ap exp (j&ohgr;
o
t) and the field of a transmitted wave S of amplitude A is represented by: S=A exp [j(&ohgr;
o
t+&phgr;)], where (&ohgr;
o
is the angular frequency of the carrier wave and &phgr; is the phase of the transmitted wave.
To be more precise, to compensate chromatic dispersion, and if the coefficient D is positive, the phase must decrease on the rising edges of the pulses and increase on their falling edges. The modulated wave is then said to feature a transient negative “chirp”. If, in contrast, the coefficient D is negative, the phase modulation must be reversed and the transient “chirp” is positive.
A transient “chirp” parameter &agr; is introduced to characterize this modulation, and is defined by the equation &agr;=2P(d&phgr;/dt)/(dP/dt), where P is the power of the modulated wave and &phgr; is its phase in radians.
For the previously mentioned standard fibers and for values of &lgr; close to 1.55 &mgr;m, for example, the value of the parameter &agr; must be constant and substantially equal to −1 if by approximation &agr; is regarded as constant.
Another approach proposes to reduce the bandwidth of the signal to be transmitted by appropriate encoding. One particular proposal is to use the “duobinary” code which is well-known in the field of electrical transmission. This code has the property of halving the bandwidth of the signal. According to the standard definition of this code, a signal is used with three levels respectively symbolized by 0, + and −. The binary value 0 is encoded by the level 0 and the value 1 is encoded either by the level + or by the level − with an encoding rule whereby the levels encoding two successive blocks of “1” around a respectively even or odd number of successive “0” are respectively identical or different.
Using the duobinary code for optical transmission is mentioned in the article “10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fiber”, X.Gu et al., ELECTRONICS LETTERS, Dec. 9, 1993, Vol.29, No.25. According to the above article, the three levels 0, +, − respectively correspond to three levels of optical power.
French Patent Application No. 94 047 32, publication number FR-A-2 719 175, also describes application of duobinary encoding to the optical field. In the above document, binary “0” always corresponds to a low level of the optical power and the symbols + and − correspond to the same high optical power level and are distinguished by a 180° phase-shift of the optical carrier.
The use of that phase inverting duobinary code is also mentioned in the article “Optical duobinary transmission system with no receiver sensitivity degradation”, K, Yonenaga et al., ELECTRONICS LETTERS, 16 Feb. 16, 1995, Vol.31, No.4.
In simulations and tests in which the experimental parameters were varied, it was found that an improvement is obtained provided that a phase shift of the carrier wave occurs within each “0” which precedes or succeeds each block of “1” or each isolated “1”. The absolute value of the phase shift can be approximately 180°. Also, the average optical power of the low levels which encode “0” must have a value relative to that of the high levels sufficient to create intersymbol interference favorable to compensating chromatic dispersion. This amounts to saying that the extinction ratio must have a finite value.
The above observations have lead to the definition of a new optical transmission method known as Phase-Shaped Binary Transmission (PSBT). This method is described in European Patent Application EP-A-0 792 036 (Application No. 97400345.1), for example.
The PSBT process requires a transmitter capable of applying an absolute phase shift in the order of 180° to the carrier wave within each cell that corresponds to logic “0” and which precedes or succeeds any cell containing a logic “1”.
A solution using a laser oscillator coupled to an electro-optical power modulator in turn coupled to an electro-optical phase modulator, for example, has the drawback of requiring complex and costly electronic control.
In reality, it is not at all inconvenient for the phase shifts to be effected systematically in each cell containing a logic “0”. This leads to a simpler implementation using a “Mach-Zehnder” interferometer modulator. A modulator of this kind comprises an interferometer structure with an input optical guide that splits into two branches that are combined to form an output guide. Electrodes apply respective electric fields to the two branches. When the input optical guide receives a carrier wave of constant power, two partial waves propagate in the two branches and then interfere at the output. The output guide then supplies a wave whose power and phase depend on the values of the electrical control voltages applied to the electrodes. Phase shifts of approximately 180° can be produced at the times when the instantaneous power of the transmitted wave is zero.
To satisfy the conditions for PSBT modulation, the electrical control system must firstly feature amplitude modulation at three
Bridoux Christophe
Penninckx Denis
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