Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1999-07-07
2002-06-18
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S045013, C372S050121, C372S096000, C372S026000, C372S064000
Reexamination Certificate
active
06408014
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of high speed optical sources for dense wavelength division multiplexed (DWDM) transmission systems, and specifically to a monolithically integrated light source and frequency discriminator.
BACKGROUND OF INVENTION
The explosive growth in internet, multimedia and wireless traffic in recent years is rapidly exhausting capacity in public networks worldwide, forcing network service providers to aggressively install new lines and upgrade old ones. However, technological breakthroughs have made all-optical DWDM systems a cost-effective way to utilize the vast bandwidth already available in the embedded fiber plant.
Externally modulated lasers of either the monolithically integrated or hybrid type are high speed optical sources favored for DWDM transmission systems. DWDM based transmission systems increase the information carrying capacity of a transmission system by loading multiple channels of differing optical frequencies onto a single optical fiber. As the number of channels in a DWDM system increases in a given amplifier bandwidth, the channel spacing decreases. Over the last three years, the channel density of commercial DWDM systems has increased dramatically. As a result, the narrower frequency spacing between channels has become susceptible to long-term aging-induced frequency drifts of conventional optical sources as known in the art. Consequently to ensure that optical signals do not wander out of their allotted channel bands, and moreover, to ensure the proper performance of narrow band passive optical components which perform functions such as adding or dropping channels, optical sources must possess a high degree of frequency stability. For these reasons, frequency stabilization has become a necessary part of a DWDM transmission system.
There are several schemes for achieving the frequency stabilization of optical sources known in the art. These include the use of absolute reference cells containing vases with well defined atomic or molecular transitions, calibrated reference elements such as fiber-gratings. Fabry-Perot (FP) etalons or waveguide interferometers, and monolithically integrated reference elements such as a Distributed FeedBack (DFB) or Distributed Bragg Reflector (DBR) gratings. While these prior art schemes for the frequency stabilization of optical sources work for their intended purpose, they have significant drawback in that they are costly, not compact in size, not easy to use and do not provide the level of frequency stable, low chirp, optical signals that are required for DWDM transmission systems today and the future.
Monolithically integrated reference elements such as DUB or DBR gratings are capable of providing a single frequency optical output signal as known in the art. DFB lasers operate in a single optical mode in contrast to multi-longitudinal mode Fabry-Perot (FP) lasers. Reference is first made to
FIG. 1
which depicts the optical cavity
10
of a DFB laser constructed in accordance with the prior art. DFB laser optical cavity
10
is contained within a DFB back facet
15
and a DFB front facet
16
. A DFB waveguide
14
and a Bragg grating
13
extend from DFB back facet
15
to DFB front facet
16
. Bragg grating
13
is a diffraction grating that provides frequency selective feedback to photons in DFB optical cavity
10
. DFB optical cavity
10
is connected to a DFB signal source
11
which is generally used to pass a DC current through DFB optical cavity
10
so as to emit light in DFB waveguide
14
as known in the art. DFB optical cavity
10
is grounded by a DFB ground connection
12
. A DFB output signal
18
is produced from DFB front facet
16
as known in the art. Although, a DFB laser can provide a single frequency optical signal output, it does not provide a stable frequency that is desired in a DWDM transmission system today.
An electro-absorption modulated laser (EML) is capable of providing a single frequency, high speed, low chirp optical signal output as known in the art. Reference is now made to
FIG. 2
which depicts the body
20
of an EML constructed in accordance with the prior art. EMIT
20
has a modulator
19
which is connected to DFB optical cavity
10
. Specifically, modulator
19
has a modulator waveguide
24
which can be constructed of substantially the same material as that used for at DFB laser waveguide as known in the art. Modulator
19
also has a modulator back side
25
and a modulator front side
26
such that modulator waveguide extends from modulator back side
25
to modulator front side
26
. A modulator signal source
21
provides a signal to modulator
19
. EML optical cavity
20
is grounded by an EML ground connection
22
. Modulator back side
25
is connected to DFB front side
16
such that modulator waveguide
24
and DFB waveguide
14
are aligned and coextensive. An EML output signal
28
is produced from modulator front side
26
as known in the art. Although, an EML can provide a single frequency, high speed, low chirp optical signal output, it does not provide a stable frequency that is desired in a DWDM transmission system today. Thus, it is desirable to provide for a better scheme for the frequency stabilization of optical sources which is a balance of performance, reliability and cost which provides for a high speed, low chirp, single stable frequency optical source for DWDM transmission systems.
SUMMARY OF THE INVENTION
The present invention is directed at overcoming shortcomings in the prior art. Generally speaking, in accordance with the present invention, a monolithically integrated light source and frequency discriminator has a section which produces a single frequency optical signal and a section which senses the optical frequency. The light producing section comprises an active material in which an electrical signal is converted to an optical signal. A portion of the optical signal is coupled to the frequency discriminator which comprises at least two photodetectors arranged to sense the optical frequency. The outputs of the photodetectors are connected to a controller comprising a microprocessor, discrete electronics or some combination thereof, for stabilizing the optical frequency of the light source at a desired value.
In a preferred embodiment, a monolithically integrated electro-absorption modulated laser (EML) and frequency discriminator has a distributed feedback laser having a back side and a front side. A modulator is connected to the front side of the distributed feedback laser such that the modulator and the distributed feedback laser form an EML which has an output signal with an EML output frequency. Further, a two element photodetector array is connected to the back side of the distributed feedback laser such that the photodetector array can sense the EML output frequency. The photodetector is connected to controller to stabilize the EML output frequency.
Other objects and features of the present invention will become apparent from the following detailed description, considered in conjunction with the accompanying drawing figures. It is to be understood, however, that the drawings, which are not to scale, are designed solely for the purpose of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.
REFERENCES:
patent: 4751710 (1988-06-01), Yamaguchi et al.
patent: 5227648 (1993-07-01), Woo
patent: 5401957 (1995-03-01), Suzuki et al.
patent: 6064681 (2000-05-01), Ackerman
patent: 6222861 (2001-04-01), Kuo et al.
patent: 6233082 (2001-05-01), Johnson
patent: 6233263 (2001-05-01), Chang-Hasnain et al.
Ackerman David A.
Johnson John E.
Jr. Leon Scott
Stroock & Stroock & Lavan LLP
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