Wavelength locked integrated optical source structure using...

Details Wavelength locked integrated optical source structure using... Wavelength locked integrated optical source structure using...

2002-03-11

2004-09-28

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular temperature control

C372S032000, C372S050121, C372S092000, C372S098000

Reexamination Certificate

active

06798799

ABSTRACT:

CLAIM OF PRIORITY
The present application makes reference to and claims all benefits accruing under 35 U.S.C. §119 from an application entitled, “Wavelength Locked Integrated Optical Source Structure Using Multiple Microcavity,” filed earlier in the Korean Industrial Property Office on Jun. 23, 2001 and thereby duly assigned Ser. No. 35999/2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength-locked, optical-signal source used in wavelength-division-multiplexing (WDM) system applications and, more particularly, to a temperature-independent wavelength-locked module using a multiple microcavity (MMC) structure.
2. Description of the Related Art
A demand for more speed and bandwidth in Internet applications has grown exponentially. As such, optical communication systems are a fast-growing constituent of communication networks. The expression “optical communication system,” as used herein, relates to any system that uses optical signals to convey information across an optical waveguide medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). While the need for communication services increases, the current capacity of existing waveguide media is limited. Although capacity may be expanded, i.e., by laying more fiber optic cables, the cost of such expansion is not economical. Accordingly, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Currently, the maximum transfer rate of optical signals has reached 10 Gbps or 40 Gbps, which substantially seems to be a technical limit applicable to the current state of the art. To overcome this problem, a wavelength-division-multiplexing (WDM) system has been widely used in the art, being capable of simultaneously transferring a multiplicity of carrier wavelengths in a single optical fiber. As a number of carrier wavelengths used in the WDM optical communication system increases, however, a more precise control of operating wavelengths is required to keep constant the space gap between those wavelengths to avoid cross-talk between optical channels. To this end, a distributed feedback semiconductor laser is used mainly as an optical source for many optical communication systems with a wavelength (i.e., about 0.1 nm/deg) which increases together with an increase in temperature. Some drawbacks in this type of laser are that its wavelength fluctuates as its internal components become degraded by aging and external environment changes. Therefore, there exists a need in the WDM optical communication system to maintain a constant wavelength through a successive measurement of the carrier wavelength and a suitable electrical control therefor.
A known prior art wavelength-locking technique uses etalon filters, dispersive gratings, fiber Gragg gratings, etc., in which the change in wavelength is detected by an optical detector, and the minute changes in wavelength are measured to keep a constant wavelength while taking into account an ambient temperature, a current amount, etc. Recent development in this field has proposed an improved method of measuring the changes in the light intensity of optical signals in optical channels according to the wavelengths by integrating a multi-mode interferometer (MMI) into optical components, but it is noted that the method still has a disadvantage in that the amount of such changes in the light intensity of optical signals according to the change in the wavelengths may be too small to be utilized for actual applications.
Referring now to
FIGS. 1 and 2
, a prior-art mechanism for measuring the wavelength changes using the etalon filter will be described briefly in the following figures. These figures show that the length of an optical channel in an etalon filter
14
changes depending upon the degrees of an incident angle of a laser beam incident on the etalon filter
14
from an optical source, i.e., a distributed feedback semiconductor laser
10
through a prism
12
. Here, it is noted that the light intensity of wavelengths in a first optical detector M
1
and a second optical detector M
2
measured with respect to multiple wavelengths is subject to some degree of fluctuation, as seen in
FIG. 2
of waveforms. The wavelength locking is carried out by using the measured values by these first and second optical detectors. Also, in such a system of using the characteristic of etalon filters, an incident light beam may be split into two beams, i.e., a first light beam measured by the first optical detector M
1
, and a second light beam passing through the etalon filter measured by the second optical detector M
2
, prior to measuring a change in the light intensity of the optical signal detected in comparison to a reference light beam.
As the above prior art wavelength-locking technique has mainly utilized distributed-feedback semiconductor laser diodes, etalon filters, and/or diffractive gratings for a dispersion of light beam, and its wavelength change could be determined by using the asymmetric property of dispersed light beam measured by multiple optical detectors, i.e., the aforementioned first and second optical detectors M
1
and M
2
(preferably, using a twin photodiode), many of specialized devices and components need to be integrated into one package. This requirement for an integrated package of an optical signal source inevitably results in considerable difficulty in its manufacturing process. As a result, this kind of high-precision, optical-alignment technique generally has a relatively low yield and a high-production cost owing to such difficulty in the manufacturing process, and has led to one of the obstacles to its wider use in the state of the art. Thus, an external type of a wavelength locker has been more generally used to date for implementation of the WDM optical communication systems.
Another known system used for keeping a constant optical wavelength is disclosed in the U.S. Pat. No. 5,299,212 entitled, “Article Comprising A Wavelength-Stabilized Semiconductor Laser,” to Koch et al., in which the wavelength of a tunable laser source is controlled using a fiber grating as a wavelength reference. In this system using a two-section distributed-Bragg-Reflector (DBR) laser diode, a launched laser beam is split into two beams by a beam splitter, one of which beams is directly measured by an optical detector and the other of which beams is subsequently applied to a fiber grating. Therefore, the comparison of the intensity of those two optical light beams in both of the optical detectors enables measuring the change in an operating carrier wavelength, by utilizing the typical physical property of a fiber optic cable with decreasing permeability relative to a particular wavelength. With such a measured wavelength, a feedback control current is fed to the Bragg section of a multi-segment DBR laser diode such that the wavelength of the laser is a function of the feedback control current, thereby regulating to maintain a constant wavelength.
While the above-known technique may be useful for controlling the wavelength of the optical signal source, the patent has some limitation in a range of wavelengths capable of regulation in accordance with the characteristic of the fiber grating, and its peak point of wavelength may drift away to some extent owing to the changes in the environment of the fiber gratings, although it could be somewhat regulated by control factors such as a grating period or reflectance. Furthermore, a relatively large number of constituent elements such as two beam splitters, optical fiber gratings, etc., has given birth to the technical difficulty in integration into a single optical module. Therefore, the above technique has been mainly used with providing the optical communication system with the length-control capability, irrespective of the DBR laser diode. In conclusion, the above-described construction of the prior art optical signal source inevitably involves the minute adjustment of wavele

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