Spectral conditioning system and method

Coherent light generators – Particular beam control device – Optical output stabilization

Reexamination Certificate

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Details

C372S032000

Reexamination Certificate

active

06785308

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
n/a
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
n/a
FIELD OF THE INVENTION
The present invention relates to fiber optic communications, and in particular to an system and method for spectrally conditioning the output of a fiber optic directly modulated laser transmitter to reduce the effects of dispersion and increase the span length between network elements.
BACKGROUND OF THE INVENTION
The proliferation of computing and networkable devices has created a need for increased bandwidth between locations, whether those locations are local, regional, national, or international. A technology extremely well suited to supporting high data rates over long distances is fiber optic communications. Typically, a fiber optic communication link includes a fiber optic transmitting device such as a laser, a fiber optic cable span, and a light receiving element. Fiber optic transmitters and receivers are typically quite extensive. As such, there is a desire to be able to increase the span length, i.e. increase the distance between network end points. However, the adverse effects of noise, attenuation and dispersion limit the distance between network elements. This impact is particularly seen as transmission rates increase, because as transmission rates increase, the sensitivity of the system to noise and dispersion also increases, effectively further limiting the span length as data rates increase. It is therefore desirable to have a method and system which increases dispersion-limited distance and permits the use of less expensive lasers in long distances.
FIG. 1
is a graph generally showing light output as a function of current input into a laser. As shown in
FIG. 1
, there exists a knee
10
at which point the slope increases, i.e. light output increases at a greater rate for a given amount of current input into the laser than at points below the knee
10
. It is therefore desired to operate a laser at a point just above knee
10
such that for a small amount of current, light output increases in an amount sufficient for a receiver to be able to detect a light existence, i.e. “1 bit” condition from a light off, i.e. “0 bit” condition. In operation, the light signal level for a “0 bit” is just above the knee and the light signal level for a “1 bit” is at the rated power output of the laser. The ratio of the “on” to “off” light for a 1 and 0 is referred to as an extinction ratio. It is desired to have a large extinction ratio number.
For directly modulated (“direct mod”) lasers, operating the last above knee
10
reduces the optical noise and signal distortion. However, the trade-off is extinction ratio which shows up as a sensitivity penalty at the receiver. As such, there is a trade-off between the distortions and noise caused by the high extinction ratio at or below knee
10
and a low extinction ratio receiver penalty. Further, operating a direct mod laser below the knee
10
results in unwanted noise, referred to as chirp.
Section
2

2
in
FIG. 1
corresponds to knee region
12
and is shown in exploded view in FIG.
2
. As shown in
FIG. 2
, knee region
12
is sub-divided into six sub-regions labeled a, b, c, d, e, and f, respectively. As is shown in
FIG. 2
, the slope of each successive sub-region increases. The relationship between the increasing slope and sub-regions a-f is explained with reference to FIG.
3
.
FIG. 3
is a chart showing optical spectrum emitted for each bias environment depicted in FIG.
2
. As shown in
FIG. 3
, the laser, when operating in sub segment f, has a high intensity about the laser wavelength p. This high intensity allows the receiver to clearly discern that a “1” has been transmitted. As sub-regions along the knee are traversed, the intensity decreases, and the spectrum of light emitted by the laser increases. The result is a dispersion in the energy transmitted by the transmitting laser as detected by the receiver, and further results in unwanted noise, i.e. spectral content far removed from point p. The resulting impact is that this unwanted noise, i.e. chirp, adversely impacts the transmission capabilities of the system.
Another factor which limits span distance and which is exacerbated by the existence of chirp is fiber dispersion. The wider the spectral output of the laser, the more differentiation in the dispersion of the fiber at the receiver. In other words, the wider the spectrum at the transmitting end, the more penalty is paid at the receiving end. As shown in
FIG. 4
, there are three main types of dispersion known to those of skill in the art. Multi-path (multi-modal) dispersion is illustrated in fiber
14
. Chromatic dispersion is illustrated in fiber
16
and polarization mode dispersion is shown in fiber
18
. Multi-path dispersion and polarization mode dispersion are not directly relevant to the subject invention and their discussion is therefore omitted.
Chromatic dispersion, shown in fiber
16
, results from a characteristic in which different wavelengths of light travel at different velocities in a fiber optic cable. As a result, a wider spectral content results in a wider differentiation in arrival times of the light pulses, thereby causing intersymbol interference. For example, referring to fiber
16
, a pulse transmitted at a given point which has a non-narrow spectral content results in a portion of the spectral content arriving at point x in a given time t, while other spectral portions of the same transmission only travel to point y in time t.
Eye diagrams
20
a
,
20
b
, and
20
c
show the adverse effects of the various types of distortion along a fiber optic cable. These effects are shown by the decrease in eye
22
a
,
22
b
, and
22
c
sizes along the distance of the fiber. The wider the eye, the easier it is for a receiver to detect the absence or presence of a bit. However, the longer the fiber, the more dispersion and the narrower the eye. Further, the shorter the bit period, the faster the effect impacts the receiver. As such, reducing the effects of dispersion along a fiber results in a wider eye, making reception easier. One way to accomplish this is by tightly controlling the transmission to, for example, reduce the effects of dispersion more effectively limiting the light spectrum transmitted by the laser. This can be accomplished by controlling chirp.
Chirp controlling technologies are expensive and are presently addressed by electrical regeneration, dispersion-compensating modules, or by generating a clean pulse shape. Regeneration is inefficient, because it requires the addition of network components due to limiting span length. Dispersion-compensating modules waste optical power and often require the addition of optical amplifiers. A clean pulse shape can be generated, thereby controlling chip by using externally-modulated lasers. However, externally-modulated lasers are larger in size than their directly-modulated counterparts and are significantly more expensive. It is desirable to have an arrangement which controls chirp, thereby reducing the effects of fiber dispersion in a manner which allows the use of an inexpensive directly-modulated laser without the need for additional external components such as dispersion-compensating modules, light-regenerating devices, and the like.
Standards such as those issued by the International Telecommunications Union (“ITU”) specify a grid which includes standard light wavelengths for different transmission bit rates. The grid sets forth center optical frequencies for a band pass filter mask inside of which the transmission frequencies must reside. This band pass filter mask becomes particularly important due to frequency drift experienced by lasers as they age as well as a change in the characteristics in filter/wavelength division multiplexing (“WDM”) coupling devices used to facilitate fiber optic communications.
Because the wavelength of light emitted by a laser is a function of the temperature of the laser, prior art devices have attempted to control the emitted light wavelength by monitoring the temper

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