Tunable optical transmitter and tunable light source

Details Tunable optical transmitter and tunable light source Tunable optical transmitter and tunable light source

2000-08-01

2001-12-25

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular beam control device

Tuning

C372S032000, C372S082000, C372S006000

Reexamination Certificate

active

06333941

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to optical systems, in general, and to wavelength tunable light sources for such systems, in particular. Even more particularly, the present invention pertains to tunable laser light sources and optical transmitters including tunable laser light sources.
BACKGROUND OF THE INVENTION
The term “optical system” as used herein refers to any system that utilizes light waves to convey information between one node and one or more other nodes.
Telecommunications carriers began installing optical fiber cable about 15 years ago. At the time the optical fiber cables were installed, it was expected that the optical fiber infrastructure would provide communications systems and networks with ample capacity for the foreseeable future. However, the phenomenal growth of data traffic on the Internet has taxed the capabilities of the optical fiber infrastructure. In addition, new high bandwidth applications are being developed and are being made available for corporate applications. The result of this increased usage of the fiber infrastructure is serious network congestion and exhaustion of the fiber infrastructure. In the past, optical fiber systems relied on time division multiplexing to route traffic through a channel. Time division multiplexed systems add more capacity by time multiplexing signals onto an optical fiber. A disadvantage of time division multiplex systems is that data must be converted from light waves to electronic signals and then back to light. The system complexity is thereby increased.
Wavelength Digital Multiplexing (WDM) and Dense Wavelength Digital Multiplexing (DWDM) are being used and/or proposed for use in long-haul telecom network applications for increasing the capacity of existing fiber optic networks. The advantage of both WDM and DWDM is that conversion to electrical signals is not necessary. The devices that handle and switch system traffic process light and not electrical signals. In WDM, plural optical channels are carried over a single fiber optic, with each channel being assigned to a particular wavelength. Each transmitter includes a laser that is intended to operate at only one of the allowed wavelength channel frequencies. Each laser is engineered to operate within the channel specifications for the life of the system. For repair purposes, an inventory of transmitters for each wavelength channel must be maintained. As the number of channels increases, the size of the inventory likewise increases. DWDM is a WDM system in which channel spacing is on the order of one nanometer or less. WDM and DWDM expand the capacity of an optical fiber by multiple wavelength channels into a single laser beam. Each wavelength is capable of carrying as much traffic as the original. The need for maintaining a large inventory of transmitters is likewise an issue for DWDM systems.
It is highly desirable to provide a tunable optical transmitter as well as a tunable light source.
SUMMARY OF THE INVENTION
In accordance with the principles of the invention, a tunable optical transmitter including a tunable light source is provided. The tunable light source includes a gain media connected in an optical loop and a tunable filter inserted in the loop. A modulator is coupled into the loop and provides for modulating tuned optical signals obtained from the loop. In accordance with one aspect of the invention, a Mach-Zehnder interferometer type modulator is utilized to modulate the optical signs. A wavelength sensor is coupled into the loop and provides wavelength indicative signals that a processing unit utilizes in accordance with a predetermined algorithm to determine the wavelength of optical signals in the loop. The processing unit utilizes the wavelength indicative signals to determine control signals to be provided to the tunable filter to adjust the filter such that the wavelength of optical signals in the loop are at a pre-selected wavelength.
In accordance with one aspect of the invention, the sensor is an acoustic optical tuning filter that is operated by the processing unit in accordance with a predetermined algorithm to determine the wavelength of optical signals. The processing unit is utilized to apply the algorithm to signals indicative of the wavelength of optical signals to determine the wavelength of optical signals in the loop, compare the wavelength to a desired wavelength and to control the tunable filter to adjust the wavelength of signals in the loop.
In one embodiment of the invention, the tunable filter is a two stage filter that includes a first stage acoustic optical filter and a second stage filter that is an unbalanced Mach-Zehnder interferometer. The first stage filter is a bandpass filter and the second stage filter is a comb filter. The combination of the two stages produces a narrow bandpass filter.
In another embodiment of the invention, the two-stage filter is provided by a bi-directional reflective tunable filter.
In yet another embodiment of the invention, a two-stage filter is inserted in cascade with the optical loop. In one embodiment of the invention, an Erbium doped fiber amplifier is utilized as the gain media. A light source provides pumping to the gain media.
An integrated optical device in accordance with the invention comprises a substrate that carries at least the tunable filter and sensor. The sensor comprises an acousto-optic tunable filter. Embodiments of the invention include a Mach-Zehnder interferometer on the integrated optic device. In addition, at least a portion of a gain media may also be integrated onto the same substrate as the integrated optic device. In accordance with another aspect of the invention, the integrated optical device is micro-machined to provide a reflective surface for the bi-directional tunable filter.


REFERENCES:
patent: 5867513 (1999-02-01), Sato
patent: 6222861 (2001-01-01), Kuo et al.
patent: 6233263 (2001-01-01), Chang-Haswnain et al.

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