Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2001-10-04
2003-07-22
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular active medium
C359S333000
Reexamination Certificate
active
06597497
ABSTRACT:
FIELD
This patent specification relates to optical amplifiers. More specifically, it relates to a semiconductor optical amplifier capable of using transverse lasing to excite its gain medium.
BACKGROUND
As the world's need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are capable of very high-bandwidth operation.
Optical amplifiers are important components of optical communications links. Optical amplifiers are commonly used as (i) power amplifiers at the source end of an optical communications link, (ii) line amplifiers along the optical signal transmission path, and (iii) preamplifiers at the receiving end of the optical communications link, and have other uses as well.
In general, the two primary types of optical amplifiers are optical fiber based amplifiers, such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers, and semiconductor optical amplifiers (SOAs). EDFAs are widely used in line amplifiers and other applications requiring high output power, high data rates, and low noise. However, EDFAs are quite bulky, having a typical fiber length of about 30 feet, and require the presence of a separate pumping laser to operate. Accordingly, EDFAs are difficult to incorporate into confined spaces, and are not amenable to circuit-board-level or chip-level integration.
SOAs, on the other hand, are small in size and conveniently integrated into small devices. An SOA generally resembles a semiconductor laser structure, except that the end mirrors have been replaced by antireflection coatings. In such devices the product of the gain and the reflectivity is less than one so that the device does not oscillate. Rather, the device is used to amplify an incoming optical signal as it passes through the device. Such devices are often called traveling wave amplifiers, which highlights the fact that the optical signal does not pass back and forth within the device, but merely passes through it essentially only once. SOAs generally yield lower output power and higher noise levels as compared to EDFAs, and/or are restricted to lower data rates. Research continues toward improving the performance of SOAs, including making SOAs with higher power and lower noise characteristics, and/or that are capable of operating at higher data rates.
Crosstalk is one of the primary troublesome noise sources in conventional SOAs, with amplified spontaneous emission (ASE) being the other primary troublesome noise source. Crosstalk, or cross-channel modulation, involves data-dependent gain fluctuations at high output levels from the SOA, and can occur for either time-multiplexed or wavelength-multiplexed data. Crosstalk arises from gain saturation effects in an SOA. These effects can be understood by recalling that SOA devices rely on the phenomenon of stimulated emission to provide the necessary amplification. Stimulated emission, in turn, requires the establishment of a population inversion. In typical SOAs or lasers a population inversion is evidenced by the presence of a specified carrier density. When a sufficiently large optical signal is passed through the amplifier, the population inversion is substantially reduced or depleted, i.e. the gain of the SOA is saturated, and is reestablished only over some finite period of time. Consequently, the gain of the SOA will be reduced for some period of time following the passage of the signal through the amplifier, a time period commonly denoted as the amplifier gain recovery time.
When the gain medium becomes saturated due to a high signal level on a first channel, changes are induced in the signal level of a second channel because the saturated gain medium cannot properly amplify both channels. Since gain is modulated by the first signal, this modulated gain is impressed on the second signal. Thus, for wavelength division multiplexed (WDM) systems in which a plurality of channels at &lgr;
1
, &lgr;
2
, . . . , &lgr;
N
are present in a common optical signal, gain saturation induced by a first channel at &lgr;
1
can produce unwanted level changes (i.e., errors) in a second data channel at &lgr;
2
, and vice versa.
Crosstalk can be reduced by keeping the SOA out of gain saturation for the data rates, signal levels, and number of channels on the optical signal of interest. If the SOA is operated near gain saturation levels, crosstalk may be reduced by making the period of the data signals small in comparison to the gain recovery time, i.e., by slowing down the data rate. In general, an SOA will have reduced crosstalk effects if (i) its saturation power P
SAT
, i.e., the input optical power level for which the SOA gain is reduced to a predetermined percentage of its nominal value, is increased, and/or (ii) its gain recovery time is decreased. As used herein, an SOA has increased gain stability if (i) its saturation power P
SAT
is increased without a concomitant increase in gain recovery time, (ii) its gain recovery time is decreased without a concomitant decrease in saturation power P
SAT
, or (iii) both (i) and (ii) occur.
Several methods for dealing with crosstalk problems are discussed in U.S. Pat. No. 5,436,759, which is incorporated by reference herein. One strategy is to place a transverse laser across the SOA such that the laser's gain medium and the SOA's signal gain medium share an overlapping region. The lasing cavity is operated above threshold and the gain of the laser is clamped to overcome losses of the cavity. As used herein, a laser cavity is gain-clamped and lasing when it is excited by a bias current greater than a threshold current. When the transverse laser is gain-clamped, gain along the SOA signal path is stabilized. The transverse lasing enhances the establishment and maintenance of a population inversion in the overlapping region, resulting in both increased saturation power and a decreased gain recovery time. Advantageously, independent lasing only builds up in the transverse direction and does not corrupt the quality of the amplified signal.
The '759 patent supra discusses an SOA in which an input optical signal is amplified by a signal gain medium along a signal path, the signal path being intersected by a segmented optical cavity oriented off-axis (e.g., perpendicular) to the signal path. The optical cavity is a lasing cavity operated above threshold, and shares its gain medium with the signal gain medium at overlapping locations, thereby increasing gain stability. Certain segmentation and design techniques are proposed for dealing with parasitic lasing modes that can cause gain clamping at undesirably low levels, with some designs directed to suppressing the parasitic lasing modes (e.g., '759 patent, FIG. 1), and other designs directed to constructively using circulating modes (e.g., '759 patent, FIG. 2B) to increase the gain. To suppress parasitic lasing modes, the lasing cavity is segmented along the length of the amplifier with regions that are optically isolated, except at intersections with the gain medium/signal path. In some examples, the optical isolation is achieved by placing gaps between the cavities that include opaque barriers ('759 patent, FIG. 1), while in other examples angled trenches are used ('759 patent, FIG. 2C).
The proposed designs of the '759 patent supra can suffer from one or more shortcomings that can reduce the effectiveness of the device and/or cause difficulty in reliably fabricating the device. For example, the layers of the single active medium ('759 patent, FIG.
Wang Shih-Yuan
Zhu Miao
Cooper & Dunham, LLP.
Hellner Mark
LandOfFree
Semiconductor optical amplifier with transverse laser cavity... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor optical amplifier with transverse laser cavity..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor optical amplifier with transverse laser cavity... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3075014