Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2001-07-06
2004-11-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S018000
Reexamination Certificate
active
06822979
ABSTRACT:
BACKGROUND OF THE INVENTION
Telecommunication transmitter lasers are used in dense wavelength division multiplexing (DWDM) optical communication systems wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. A popular set by the International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows at least 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers.
Such telecommunication transmitter lasers typically use a grid generator that defines multiple, selectable transmission wavelengths that correspond to the ITU grid wavelengths or other selected grid. The grid generator used is typically a Fabry-Perot interference filter having transmission maxima that are evenly spaced apart in frequency. Through careful manufacturing of the grid etalon and alignment of the grid etalon with respect to the momentum axis of the optical beam passing therethrough, the spacing of the transmission peaks defined by the grid etalon can be aligned with a selected wavelength grid, such as a grid defined by the ITU standards. A thermoelectric controller may be coupled to the grid etalon, and careful thermal control of the grid etalon during laser operation provides a temperature lock for the grid etalon so that variation in the grid transmission peaks does not occur due to thermal fluctuations.
As optical communication networks evolve towards re-configurable architectures, increasingly sophisticated telecommunication transmitter lasers have become necessary. In particular, a need has arisen for a telecommunication transmitter laser that provides active tuning or adjustment of grid transmission peaks during laser operation to allow selection of different communication grids.
SUMMARY OF THE INVENTION
The invention provides a laser apparatus, as well as systems and methods, for active tuning of a grid generator during laser operation to provide selected grid transmission channel spacing. The apparatus of the invention, in its broadest sense, is a laser having a grid generator that is configured such that the grid transmission peaks can be changed or varied during laser operation to allow selection of different communication grids. The grid generator is tunable over at least a small range about a nominal grid setting. More specifically, the grid generator may be tunable over a range as great as, or exceeding its initial or nominal grid setting, such that any desired wavelength grid may be selected by appropriate tuning or adjustment of the grid generator. The laser may be of various configurations, and may be a tunable external cavity laser having a channel selector in the external cavity. The laser may further comprise a tuner or tuning assembly operatively coupled to the grid generator and configured to adjust the grid generator to the selectable communication grids.
The laser may further comprise a gain medium having a first and second output facets, with the gain medium emitting a coherent beam from the first output facet along an optical path to an end mirror in the optical path, such that the end mirror and the second output facet define an external cavity. A channel selector may be positioned in association with the external cavity, and operatively coupled to a channel selector tuner configured to tune the channel selector in the optical path to selectable transmission wavelengths corresponding to transmission bands in the selectable communication grids. The laser may further comprise an external cavity optical path length tuner operatively coupled to the external cavity and configured to adjust the optical path length for the external cavity.
The grid generator may be in the form of a grid etalon that defines a plurality of wavelength passbands or transmission bands that are spaced apart according to the free spectral range (FSR) of the grid etalon. The spacing of the passbands may correspond to, for example, the ITU grid spacing. In one embodiment of the laser, the grid generator tuner is configured to rotatably adjust the grid etalon in the optical path. The grid generator tuning assembly may be operatively coupled to the grid etalon to tune the grid by rotating or tilting the grid etalon to change the grid etalon's optical thickness or path length, thus changing the selected communication grid spacing. The grid generator tuning assembly may further comprise a controller having stored adjustment data for different selectable grids, with the grid etalon being adjusted to provide the selected wavelength grid.
In other embodiments the grid generator tuning assembly may alternatively, or additionally, tune the grid etalon by thermal control with a thermoelectric controller (TEC) operatively coupled to the grid etalon and configured to adjust the optical thickness of the grid etalon by selective heating or cooling to provide a selected grid spacing. Heating or cooling affects the size of a spacer controlling the etalon gap. In other embodiments, the etalon may comprise a non-vacuum gap filled with a material, the heating or cooling of which affects a change in the optical path length across the etalon gap. In still other embodiments, the etalon may comprise an electro-optic material and have an effective optical path length that is adjustable by application of voltage across the electro-optic material.
Various other tuning mechanisms are usable for adjustment of a grid etalon or grid generator in accordance with the invention. Such tuning may comprise, for example, pressure-tuning of a gas-filled etalon, piezoelectric tuning of the etalon gap, optical tuning via non-linear optical effects, voltage tuning of an electro-optic etalon material, micro-positioning using a MEMS (micro-electro-mechanical system) actuator, or other form of tuning.
Control of the grid generator may be carried out according to grid etalon adjustment parameters stored in a look-up table by the grid generator controller. The grid etalon adjustment parameters may comprise a list of temperatures for select wavelengths and a rule for interpolating the temperatures for non-listed wavelengths. Additional rules may be provided for adjusting the temperatures based on ambient temperature or knowledge of the system state. For temperature control of the grid etalon, the temperature may be achieved by placing a temperature sensor, for example a thermistor, in the region where heat is pumped by the TEC. The temperature sensor is preferably, but not necessarily, near the location of the grid to be controlled. A control mechanism can then adjust the current into the TEC to achieve the desired temperature at the thermistor. The control algorithm may comprise a PID loop. The control algorithm may alternatively comprise a state estimator and control laws for controlling the output states. The optical path length across the etalon is an output state, and the input states comprise the temperature sensor in the region where heat is pumped, ambient temperature sensors, and other sensors.
The laser may further comprise a detector positioned and configured to monitor laser output and provide a detector output indicative of the wavelength location of a transmission peak or peaks of the grid etalon. Error signals may be derived from the detector output and used by the controller to adjust laser wavelength with respect to the grid transmission peaks in order to zero out or null and error signal. The error signal may be obtained by providing a wavelength modulation to the grid generator transmission spectrum to allow detection of the transmission peaks, by wavelength modulating a laser transmitted through the grid generator,
Blakely , Sokoloff, Taylor & Zafman LLP
Intel Corporation
Ip Paul
Vy Hung T
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