Optical: systems and elements – Optical modulator
Reexamination Certificate
2003-03-20
2004-10-26
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
C359S239000, C359S279000, C398S185000, C398S183000, C398S188000, C398S147000
Reexamination Certificate
active
06809849
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control technique for an optical modulator used in optical communications, and in particular, relates to a control technique for an optical modulator suitable for generating signal light corresponding to the Carrier-Suppressed RZ (Carrier Suppressed Return-to-Zero: to be referred to as CS-RZ below) modulation method.
2. Description of the Background Art
At present, optical transmission systems in which optical signals are transmitted at speeds of around 10 Gb/s are beginning to be in practical use, but due to the recent rapid increase in network usage, further increases in network capacity are sought, and in addition, demand for implementation over even longer distances is increasing.
In optical transmission systems with transmission speeds of 10 Gb/s or more, because the affect of wavelength dispersion on the waveforms is large and the optical spectrum is broadened, WDM transmission in which channel lights are arranged with a high level of density is difficult. Particularly, in 40 Gb/s optical transmission systems, wavelength dispersion is one of factors limiting the transmission distance.
Dispersion compensation technology in which a dispersion amount in the optical transmission path is accurately measured to compensate has been investigated as a method of solving the problems described above (Japanese Unexamined Patent Publication No. 11-72761 and Japanese Unexamined Patent Publication No. 2002-077053). Furthermore, in order to realize such an optical transmission system as described above, the development of a modulation method with an even slightly higher dispersion tolerance is essential. Specifically, in order to achieve a long distance optical transmission system, a modulation method in which an excellent optical S/N ratio can be ensured, in other words, a modulation method which is resistant to the self phase modulation (SPM) effect and for which the upper limit of the power of optical input to the optical transmission path can be made high, is required. In addition, in order to increase capacity, a modulation method with a narrow optical spectrum allowing high density WDM optical transmission is required.
Recently, research has been conducted into new modulation methods such as the carrier suppressed RZ (CS-RZ) modulation method (for example, Y Miyamoto et. al., “320 Gbit/s (8×40 Gbit/s) WDM transmission over 367 km zero-dispersion-flattened line with 120 km repeater spacing using carrier-suppressed return-to-zero pulse format”, OAA'99 PD, PdP
4
and the like). An advantage of this CS-RZ modulation method is that because, as described below, the optical spectrum width is ⅔ times that of the RZ modulation method, the waveform dispersion tolerance is broad, and the high density channel arrangement in WDM is possible. Furthermore, because waveform degradation due to the self phase modulation (SPM) effect is minimal, it becomes possible to ensure an optical S/N ratio suitable for long distance transmission.
FIG. 14
is a diagram showing a basic structure for generating a 40 Gb/s CS-RZ modulation signal.
In
FIG. 14
, a light source
100
generates continuous light. The continuous light output from this light source
100
is input, in sequence, to two LiNbO
3
modulators
110
and
120
(to be referred to as LN modulators below) connected in series, to thereby be modulated.
For example, a data signal, generated in a data signal generating section
111
, with a bit rate of 40 Gb/s and-corresponding to an NRZ modulation method is applied to a signal electrode (not shown in the figure) of the former LN modulator as a drive signal, and as a result, the former LN modulator
110
modulates the continuous light from the light source
100
according to the data signal, and outputs a 40 Gb/s NRZ signal light having a waveform as illustrated in (a) of
FIG. 15
to the latter LN modulator
120
.
As the latter LN modulator
120
, a Mach-Zehnder (MZ) modulator with two signal electrodes is used, for example. The latter LN modulator
120
further modulates the NRZ signal light received from the former LN modulator
110
as a result that a first drive signal and a second drive signal generated based on a clock signal with a frequency of ½ times the bit rate of the data signal are applied to the respective signal electrodes thereof, and outputs a 40 Gb/s CS-RZ signal light having a waveform as illustrated in (b) of FIG.
15
. Here, a clock signal with a frequency of 20 GHz which has a waveform such as a sine wave is generated in a clock signal generator
121
, and after being split into two in a splitter
124
, the split clock signals are adjusted in the phase shifters
125
A,
125
B, respectively so that a phase difference between the split clock signals is approximately 180°, and further, respective amplitudes of the clock signals are adjusted in amplifiers
126
A and
126
B, respectively, to become the first and second drive signals to be applied to each signal electrode of the LN modulator
120
.
Furthermore, a part of the clock signal generated in the clock signal generator
121
is split in a splitter
122
and sent to the data signal generating section
111
, and the phase difference between each signal is controlled by adjusting the phase of the clock signal by a phase shifter
123
, so that phases of the data signal and the clock signal are synchronized.
Here, the principle of generating a 40 Gb/s CS-RZ signal light is described simply using the optical intensity characteristics relative to the drive voltage of the LN modulator, as shown in FIG.
16
.
Generally, when generating a signal light corresponding to the NRZ modulation method or the RZ modulation method, using an optical modulator in which the optical intensity characteristics varies periodically relative to the drive voltage, modulation is performed by supplying, to the optical modulator, a drive voltage (hereafter, this drive voltage is referred to as V&pgr;) which corresponds to the “peaks and valleys” or the “valleys and peaks” which adjoin each other in the optical intensity characteristics. Here, the “peak” of the optical intensity characteristics refers to the apex of light emission, and the ‘valley’ refers to the apex of light extinction.
On the other hand, when generating signal light corresponding to the CS-RZ modulation method, the 40 Gb/s NRZ signal light which was modulated in accordance with the data signal in the former LN modulator
110
shown in
FIG. 14
, is further modulated in the latter LN modulator
120
, according to a 20 GHz clock signal with a frequency of ½ times the bit rate of the data signal. And as shown in the left of
FIG. 16
, a drive voltage (hereafter, this drive voltage is referred to as 2V&pgr;) which corresponds to the “peak, valley, peak” of the optical intensity characteristics relative to the drive voltage is supplied to this latter LN modulator
120
. This modulation of light is performed with each level −1, 0, 1 of the clock signal corresponding respectively to an on, off, on state of the light, and the generated CS-RZ signal light becomes a binary optical waveform as shown on the right of FIG.
16
. For the signal light in this CS-RZ modulation method, because the optical phase of each bit has a value of either 0 or &pgr;, then as shown in the calculation results of the optical spectrum in
FIG. 17
, the carrier component of the optical spectrum is suppressed in comparison with the signal light in the RZ modulation method.
As seen in the results of experiments on the optical spectrum and the optical waveform shown in
FIG. 18
, for example, the signal light in the CS-RZ modulation method generated in the manner described above has an optical waveform approximately equal in shape to the optical waveform obtained by the RZ modulation method, but the optical spectrum width is narrower than that in the RZ modulation method. Furthermore, as seen in the results of experiments relating to wavelength dispersion tolerance shown in
FIG. 19
, for example, the r
Akiyama Yuichi
Ishikawa George
Ben Loha
Fujitsu Limited
Staas & Halsey , LLP
LandOfFree
Control apparatus and control method for an optical method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Control apparatus and control method for an optical method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control apparatus and control method for an optical method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3289129