Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2001-06-27
2004-05-18
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Particular active medium
C359S337000
Reexamination Certificate
active
06738187
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same, and particularly to a novel wavelength-locked loop servo-control circuit for optimizing performance of semiconductor optical amplifiers.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Optical communication links in systems employing WDM or, optical networks in general, require amplification to extend their distances. For example, optical signal amplification are needed in optical links for applications such as disaster recovery in a storage area network or parallel sysplex. There are many types of amplifiers, however, for some wavelength ranges of interest, semiconductor optical amplifier devices (SOAs) have emerged as being extremely useful. An SOA functions much like an in-line semiconductor laser diode in that it is optically pumped for amplifying incoming optical signals without requiring optical/electrical conversions. However, the SOA also broadens the optical spectrum of the amplified light, which may induce undesired effects such as dispersion and modal noise that limit the effectiveness of this technology.
Particularly, as illustrated in
FIG. 1
, the basic SOA device
100
(also known as a semiconductor laser amplifier or “SLA”) is very similar in construction to a Fabry Perot semiconductor laser diode, comprising semiconductive layers
110
,
111
and an active layer
112
forming an optical cavity which receives an input optical signal
120
. Generally, when an electrical current
115
is pumped through the device, electrons are excited in the optical cavity
112
to effect gain of the input signal
120
in the direction of propagation. The output optical signal
130
is thus an amplified version of the input signal. It is understood that mirrors may be implemented in the optical cavity for increasing the effective path length through the gain medium, and hence increase the overall gain. The SOA offers potential advantages over other optical amplification technologies such as doped fiber amplifiers. In particular, the SOA can be monolithically integrated with other semiconductor devices on a common chip or substrate, e.g., GaAs or hybrid Si on insulator, and mass produced at low cost. SOAs can easily amplify light at various wavelengths, including 1300 nm and 850 nm which is a unique feature, since erbium doped fiber amplifiers (EDFAs) operate only at wavelengths near 1550 nm, and more exotic doped fiber amplifiers at other wavelengths are more expensive and difficult to manufacture. This is an important advantage, as the SOA is a low cost solution to amplify the 1300 nm and 850 nm windows most commonly used in data communication systems such as ESCON, Fibre Channel, and Gigabit Ethernet. The SOA is also a very compact and highly reliable device. However, an SOA differs from a laser diode in that the SOA operates below the threshold current required for laser action. (In a variant design, the traveling wave SOA, may be operated above threshold but has other design and manufacturing problems which have so far prevented its becoming a commercially available device). Due to this, the light emerging from an SOA has a very broad spectral width, around 20-50 nm and, in some cases, several hundred nanometers, as opposed to a typical narrowband laser which has about 2-3 nm spectral width. Thus, an optical signal entering the SOA will be amplified, but suffers a significant spectral broadening; the additional optical power is spread across a much wider frequency range. Not only is this an inefficient way to amplify the light, but the spectral broadening causes secondary effects such as increased dispersion, modal noise, and mode partition noise on the communication link; these noise sources can exhibit a noise floor, which means that the noise limits the maximum link distance regardless of the strength of the amplified signal. For this reason, SOAs have not been widely deployed in very long distance links, although they have found applications in shorter data and telecommunication systems.
Furthermore, if the SOA is operated at higher voltages or currents (still below threshold), the gain increases and the spectral broadening becomes worse. In principle, the SOA output may be optically filtered with a narrow band element such as an array waveguide grating or multilayer thin film interference filter, as these devices can be integrated onto the semiconductor substrate. However, such filters are very difficult to fabricate with their center wavelength exactly aligned to the peak of the SOA output spectrum, hence they have unacceptably high insertion loss (up to several dB) which cancels out the gain of the optical amplifier. Further complicating the problem, the SOA tends to have a high insertion loss, as well as high spontaneous emission noise due to random generation of photons at the amplified wavelengths. The SOA spectrum also drifts with changes in temperature or bias voltage, as well as with the aging of the SOA diode.
It would thus be highly desirable to provide a system and method for automatically compensating for the undesirable effects of an SOA, and particularly a system and method for overcoming the spectral broadening associated with SOA devices.
It would thus be highly desirable to provide a servo-control feedback loop for stabilizing the SOA output and tracking the center wavelength of the amplified signal to the peak of an optical filter passband with high accuracy to enable higher gains than currently achievable with SOAs.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a system and method for overcoming the spectral broadening associated with semiconductor optical amplifier (SOA) devices.
It is another object of the present invention to provide a servo-control loop for implementation in an SOA device that enables for dynamic tracking of the center wavelength of the amplified signal to the peak of an optical filter passband with high accuracy.
It is a further object of the present invention to provide a servo-control loop for implementation in an SOA device that provides stabilization of the SOA output and provides tracking of the center wavelength of the amplified signal to the peak of an optical filter passband to enable higher gains than currently achievable with SOAs.
It is another object of the present invention to provide a servo-control loop for implementation in an SOA device that is implemented on a common semiconductor substrate and thus may be integrated with the SOA diode design.
It is still another object of the present invention to provide a servo/feedback loop, referred to as a “wavelength-locked loop,” that provides stabilization of the SOA output and provides tracking of the center wavelength of the amplified signal to the peak of an optical filter passband to enable higher gains, thereby enabling significantly larger
DeCusatis Casimer Maurice
Jacobowitz Lawrence
Hellner Mark
Scully Scott Murphy & Presser
Townsend, Esq. Tiffany L.
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