Optical communications – Transmitter – Having particular modulation
Reexamination Certificate
2000-01-05
2003-12-09
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter
Having particular modulation
C398S185000, C375S301000
Reexamination Certificate
active
06661976
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a method and system for optical single-sideband data generation and transmission.
2. Description of Related Art
The ability of the Mach-Zehnder (MZ) intensity modulator to control both intensity and optical frequency has been demonstrated to improve the performance of long-haul fiber-optic systems such as those used by the telecommunications industry. See, F. Koyama and K. Iga, “Frequency chirping in external modulators,” Journal of Lighwave Technology, Vol. 6, pp. 87-93, 1988. In addition, the use of high-power diode-pumped YAG lasers operating at 1300 nano-meters (nm) and MZ external intensity modulators based on LiNbO
3
has found wide application in the cable TV (CATV) industry.
Conventionally, optical transmission systems may use either direct or external modulation of a laser. Ideally, the intensity of the light output from a modulated laser should be linearly proportional to the injected current, and the frequency of the optical carrier should be minimally influenced. However, modulation of the optical frequency occurs when a laser is directly modulated. For example, a data signal may be added directly to the laser current, so that the laser output, i.e., the optical carrier, is intensity modulated. In theory the bandwidth of the transmitted signal must be as broad as the bandwidth of the data signal. However, the resultant optical spectrum is more than twice as broad as the theoretical limit. Not only does the optical signal have double-sidebands, one below and one above the optical carrier, the intensity modulation also superimposes an unintentional frequency modulation, i.e., laser frequency chirp. Laser frequency chirp is modulation of the laser frequency caused by modulation of the refractive index of the laser cavity in response to current modulation
The interaction of chirp and chromatic dispersion in the fiber can cause system impairments. Therefore, to avoid the effects of laser frequency chirp, externally-modulated optical fiber links are conventionally recognized as the preferred choice. By using an MZ external modulator it is possible to modulate an optical carrier so that the resulting optical spectrum does not have any excess chirp. See, id. F. Koyama and K. Iga, “Frequency chirping in external modulators”. However, the optical spectrum will still have double-sidebands, and therefore, be twice as wide as the theoretical limit.
FIG. 1
shows schematics of radio frequency (RF) and optical spectra to illustrate these points.
FIG. 1
shows the data signal used to modulate the optical carrier, in this example the data signal is made of multiple RF subcarriers (CATV signals are a good example of this sort of signal).
FIG. 2
is a schematic of the spectrum of an externally modulated optical carrier. Note that it is twice as wide as the spectrum shown in FIG.
1
.
Conventional intensity modulation creates signals with two sidebands around the optical carrier frequency. These two sidebands contain the same information. Because of optical fiber dispersion, different frequency components will travel at different speeds, creating interference in the transmitted signals. Although the two sidebands contain the same information, they travel at different speeds in the optical fiber and arrive at the receiver at different times. The net result is a power penalty and limit in the transmission distance. The greater the frequency separation the higher the penalty.
Optical transmission systems employing baseband digital transmission, e.g., by on/off keying of the light, may also suffer from the effects of dispersion. In long-distance transmission systems, dispersion can interact with non-linearities in the optical fiber, further impairing transmission.
All conventional fiber optic communication systems employ double-sideband modulation. To reduce the effects of dispersion it is preferable to either operate at wavelengths corresponding to low-fiber dispersion, or include dispersion compensation. Some optical fibers also suffer from polarization-mode-dispersion, which may vary with time due to strain and temperature variations. It is difficult to compensate for this sort of dispersion. In addition, some optical non-linearities, such as self-phase modulation, are worse in transmission systems with low dispersion. Therefore; there is a need for optical modulation techniques that are tolerant of dispersion.
FIG. 4
shows a conventional fiber optic data transmission system
400
. In the system
400
, an optical carrier signal is emitted from optical source
410
. The carrier signal is modulated by optical modulator
412
, which is driven by a modulating signal
415
, to generate an optical signal consisting of an optical carrier signal with double-sideband, DSB
420
. However, when the DSB signal
420
is sent over fiber link
425
, chromatic dispersion causes each spectral component to experience a different time delay depending on the length of the fiber link
425
. The transmitted DSB signal
420
is received by a photodetector
435
coupled to the fiber link
425
. This photodetector
435
converts the incident optical DSB signal
425
into current. The photodetector
435
generates a current corresponding to the received optical power P
r
which has a direct current part corresponding the average received optical power and an alternating current part which corresponds to the instantaneous optical intensity change due to modulation.
However, if the phase difference between the two optical sidebands at optical frequencies (f
carrier
+f
RF
) and (f
carrier
−f
RF
) received at the photodetector
435
is an odd multiple of &pgr;, the received signal from the upper sideband and the lower sideband will destructively interfere with each other, canceling out all the information power in the signal received by the photodetector
435
at f
RF
. As the frequency f
RF
increases, the dispersion effect causes impairments at shorter transmission distances. As a result, the length of the fiber link
425
becomes severely limited. For example, when conventional single-mode fiber is employed, a 3-dB degradation in the detected RF power occurs in an externally modulated, 6 km link operating at 1.5 &mgr;m with a 20 GHz sub-carrier.
Therefore, chromatic dispersion can be a major factor limiting the maximum distance and/or bit rate of long haul fiber-optic systems that require relatively lengthy optical links. Dispersion compensation can mitigate these effects, but it adds to the system's complexity.
FIG. 3
shows an optical signal with single-sideband transmission. The transmission of single-sideband (SSB) signals has also been used in RF transmission systems to reduce the RF electromagnetic spectrum occupied by the signal by a factor of two. The use of optical SSB transmission also reduces the transmitted optical signal spectrum by a factor of two. The smaller the bandwidth used in transmission, the smaller the dispersion penalty in the transmitted signal. Therefore, because only half of the optical bandwidth is required, the dispersion suffered by an optical single-sideband signal is half of the same signal using double-sideband modulation. In an intensity modulated double-sideband optical transmission system, the detected signal is generated by mixing the two sidebands with the optical carrier in the transmitted spectrum shown in FIG.
2
. The down-converted signal has components from both the upper and lower sidebands.
However, the relative delay between different corresponding frequency components in the upper sideband and the lower sideband are different, although they represent the same information, making it difficult to compensate for optical fiber dispersion in the electrical domain. In contrast, in an optical single-sideband transmission system, the detected baseband signal is generated in the photodetector by mixing the optical carrier signal with only one sideband. Therefore, the relative arrival times of the various signal frequency components are preserved in the electrical output signal re
Gnauck Alan H.
Lam Cedric F.
Woodward Sheryl Leigh
AT&T Corp.
Leung Christina Y
Pascal Leslie
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