Optical transmission system

Details Optical transmission system Optical transmission system

1999-07-28

2003-05-06

Mullen, Thomas (Department: 2632)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200

Reexamination Certificate

active

06559996

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical transmission system, and a transmitter and/or receiver used in optical communication, in particular, relates to such a system which reduces optical signal bandwidth with return-to-zero (RZ) signal, and base-band electrical signal bandwidth in a transmitter.
Recently, an optical amplifier having high output power and wideband characteristics has been used in an optical transmission system, and input fiber launched power in an optical transmission line exceeds 10 dBm. As a result, the Kerr effect in which refractive index in an optical fiber is modulated by an input optical signal itself occurs, and therefore, an optical signal is phase modulated, the optical modulation spectrum is spread, and the waveform is subject to be distorted due to chromatic dispersion in an optical fiber. Further, in a wavelength division multiplex system, waveform and S/N ratio are degraded because of non-linear cross talk between channels.
It is recognized that above problems depend upon format of signals, and RZ (return-to-zero) signal in which each bit has equal pulse width with each other is preferable to NRZ (non-return-to-zero) signal since equalization of waveform distortion due'to non-linear effect after fiber transmission is easy in RZ signal.
For instance, in an inline-repeatered system in which the dispersion of 1.3 &mgr;m zero dispersion optical fiber line is compensated for each repeater section, it is estimated in a simulation that the regenerative repeater section for RZ signal may be three times as long as that for NRZ signal (reference; D. Breuer et al, “Comparison of NRZ and RZ-Modulation format for 40 Gbit/s TDM Standard-Fiber System”, IEEE Photon. Technol. Lett. vol 9, No. 3, pp. 398-400, 1997). Further, an experimental report (R. M. Jopson et al, “Evaluation of return-to-zero modulation for wavelength division multiplexed transmission over conventional single-mode-fiber”, Tech. Digest of Optical Fiber Comm. Conf, '98 FEI, pages 406-407, 1998) shows that RZ signal may have higher power for each channel than NRZ signal in 10 Gbit/s 8-waves WDM transmission system. Further, another experimental report (A.Sano et al, IEE Electronics Letters vol 30, pages 1694-1695, 1994) shows that phase modulation synchronized with transmission data effectively suppresses SBS (Stimulated Brillouin Scattering) higher fiber launched power.
Therefore, it is preferable to use RZ signal format in long distance optical transmission system.
A prior optical transmission system with RZ (return-to-zero) signal format is shown in
FIGS. 28 and 29
.
In
FIG. 28
, an input NRZ signal (non-return-to-zero) is applied to a NRZ/RZ converter
51
which converts NRZ electrical signal format to RZ electrical signal format. An output signal of the converter in RZ signal format is applied to an RZ optical modulator
50
through an amplifier
52
which amplifies a RZ electrical signal. The optical modulator
50
modulates CW (continuous wave) optical signal from an optical source
5
with a RZ electrical signal from the amplifier
52
, and provides modulated optical signal.
In
FIG. 29
, an input electrical NRZ signal is applied to a modulator driver
62
which amplifies the electrical signal. The output of the modulator driver
62
is applied to a first NRZ optical intensity modulator
60
which modulates continuous wave (CW) from an optical source
5
with the NRZ signal from the amplifier
62
. An output of the first modulator
60
is applied to a second optical intensity modulator
61
which modulates the input NRZ signal of the same with electrical sinusoidal wave of an output signal of a clock modulator driver
63
. The clock modulator driver
63
provides a clock signal with frequency B (Hz) (B; transmission symbol rate) which is synchronized with an input NRZ electrical signal. Thus, a final RZ optical signal is obtained at the output of the second modulator
61
. This prior art is shown in, for instance, A. Sano et al. IEEE electronics Letters vol. 30, pages 1694-1695, 1994.
Another prior art is shown in JP patent laid open 254673/1996 (which corresponds to U.S. Pat. No. 5,625,722; “Method and Apparatus for Generating data encoded Pulses in Return-to-zero Format”), in which periodical transmittance of a Mach-Zehnder type optical intensity modulator is used in full-wave rectifying characteristics using amplitude folding electrical-optical response of the Mach-Zehnder type optical intensity modulator, and binary NRZ electrical signal is converted to RZ optical signal. An input binary NRZ electrical signal is encoded in a pre-code circuit to produce coded NRZ electrical signal, then, two copies of the NRZ signal are produced, and one of the NRZ signal is logically inverted. RZ signal is generated by operating a Mach-Zehnder type optical intensity modulator with these differential coded NRZ electrical signal.
Further, the following three documents show how to produce optical clock pulses from clock electrical signal.
(1) K. Iwatsuki et al. “Generation of transform limited gain-switched DFB-LD pulses <6 ps with linear fiber compression and spectral window”, Electronics Letters vol. 27, pp 1981-1982, 1991. In this document, a gain switch semiconductor laser is used as a generation element.
(2) M. Suzuki, et al, “New application of sinusoidal driven InGaAsP electroabsorption modulator to in-line optical gate with ASE noise reduction effect”, J. Lightwave Technol. 1992, vol. 10 pp. 1912-1918. This document shows how to modulate CW optical signal generated by a semiconductor laser by using an electro-absorption type external modulator.
(3) K. Sato et al, “Frequency Range Extension of actively mode-locked lasers integrated with electroabsorption modulators using chirped grating” J. of selected topics in quantum electonics vol. 3, No. 2, 1997, pp. 250-255. This document shows an integrated mode-locked semiconductor laser. But, these three papers do not describe modulation means.
However, above prior art have the disadvantage that an output RZ optical signal has optical bandwidth larger than 4B when transmission rate is B (bit/s). That figure is twice as large as the bandwidth of NRZ optical signal. Therefore, an output RZ signal in prior art is subject to waveform distortion because of chromatic dispersion in an optical transmission fiber as compared with a NRZ optical signal.
FIG. 30
shows NRZ optical signal spectrum in the prior art, and
FIG. 31
shows RZ optical signal spectrum in the prior art. It should be noted in
FIGS. 30 and 31
that RZ optical signal has bandwidth twice as large as that of NRZ optical signal.
Further, in the prior art of
FIG. 28
, a NRZ/RZ converter
51
, an amplifier
52
and an optical modulator
50
must have the operational bandwidth twice (DC through 2B Hz) as large as the bandwidth (B) which is required for NRZ electrical signal. Thus, the higher the transmission rate is, the more difficult the design of a circuit is.
Further, in the prior art of
FIG. 29
, two optical modulators
60
and
61
are connected in series. Therefore, in order to keep S/N ratio of a resultant RZ signal output to be the same as that of NRZ optical signal, an output of an optical source
5
must be increased by 6-9 dB so that optical loss and modulation loss for one stage of an optical modulator are compensated, and therefore, an optical source must provide high output power. Further, a phase control circuit
64
is essential to adjust the modulation phase between the NRZ optical signal and the synchronization clock signal.
Further, in above prior art, an output RZ modulated optical signal has fine spectrum at fc±nxB (Hz) (n is an integer), where fc is carrier frequency of continuous wave light. Therefore, when signal power applied to an optical fiber exceeds 7 dBm, the input fiber launched power to a dispersion shifted fiber is limited because of Stimulated Brillouin Scattering (SBS). Therefore, an external linewidth modulation circuit
53
is necessary to enlarge linewidth of optical carrier for SBS suppression and increa

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