Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
1999-10-15
2001-08-28
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Tuning
C372S032000, C372S019000, C372S028000, C372S099000, C372S102000, C372S038060
Reexamination Certificate
active
06282215
ABSTRACT:
BACKGROUND OF THE INVENTION
Copyright Authorization
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
1. Field of Invention
The field of the present invention relates in general to tunable external cavity lasers, and particularly to an improved tuning system which suppresses mode-hopping and reduces unwanted feedback.
2. Description of the Related Art
Tunable external cavity diode lasers (ECDLs) are widely used in lightwave test-and-measurement equipment and are becoming recognized as essential components for the rapidly expanding fields of wavelength division multiplexed (WDM) optical voice and data communications. The many applications within these fields represent many different sets of performance specifications. However, the following requirements are typical: small size of the optomechanical assembly; phase-continuous tuning of the wavelength; servo control of the wavelength; controllable frequency modulation (FM) at audio rates (e.g., 100 Hz to 10 kHz) in order to broaden the linewidth; and ability to generate highly linear wavelength sweeps.
Commercial ECDLs come in a variety of tunable configurations. Generally, either a retroreflector, a diffraction element, or an interference element is utilized for controlling the feedback wavelength to the gain medium and thus the output wavelength of the laser. In a first of these tunable configurations, e.g., a “Littman” configuration, tuning is accomplished by motion of either a diffraction grating or a retroreflector positioned on an actuated pivot arm. The arm, which is rather long and massive, is required to provide phase-continuous tuning. The arm is driven by either a rotary stepper motor or a rotary DC motor. The stepper motor solution provides long-range travel and built-in position encoding, but its bandwidth is too low for FM and its discrete nature is unsuitable for linear FM sweeping. The DC motor can generate linear sweeps provided it has sufficient torque, but it cannot be used for FM. In a second configuration, the tuning is accomplished by an interference-filter which is rotated with respect to the optical path. A laser with this second configuration is described in the article by Dr. P. Zorabedian et al. entitled: “
Interference
-
filter
-
tuned, alignment
-
stabilized, semiconductor external
-
cavity laser”,
13 Optics Letters pp. 826-828 (1988). This laser configuration as well relies on actuators, such as stepper motors, which are not suitable for FM modulation. In both the first and second configuration, FM modulation is typically performed by modulating the injection current to the semiconductor optical gain medium, which has the highly undesirable effect of introducing amplitude modulation (AM) on the laser's output.
What is needed is a new type of external cavity laser that has wide-range continuous tuning, possesses a compact, robust, inexpensive form factor, and is FM tunable.
SUMMARY OF THE INVENTION
The present invention provides a continuously-tunable external cavity laser (ECL) with a compact form factor and precise tuning. A novel interference filter which may be used to tune the ECL provides an absence of mode-hopping and reduced feedback from both spurious interference and reflections in the external cavity. A novel tuning mechanism is disclosed which provides for mechanical FM tuning of ECL tuning elements such as: an interference filter, a diffraction element, and a retroreflector. A novel feedback circuit is disclosed which provides closed loop feedback for selecting output wavelength in a laser.
In an embodiment of the invention, a tunable external cavity laser with a gain medium, a retroreflector, a first and a second reflector and a positioner is disclosed. The gain medium emits a beam. The retroreflector lies in a path of the beam. The first and second reflectors are positioned opposite one another and at an angle to one another in the path of the beam between the gain medium and the retroreflector to provide feedback of a selected wavelength to the gain medium to tune the laser. The positioner positions at least one of said first reflector and said second reflector to vary a separation between the reflectors along the path of the beam, thereby tuning the laser.
In an embodiment of the invention, a tunable external cavity laser with a gain medium, a tuning element and a voice coil actuator is disclosed. The gain medium emits a beam. The tuning element is moveably positioned in a path of the beam to provide feedback of a selected wavelength to the gain medium. The voice coil actuator couples to the tuning element and responds to an electrical signal to position the tuning element in the path of the beam to provide feedback of a selected wavelength to the gain medium to tune the tunable laser.
In an alternate embodiment of the invention, a tunable external cavity laser with a gain medium, a tuning element, an encoder, a reader, and a positioner is disclosed. The gain medium emits a beam. The tuning element is movably positioned in a path of the beam to provide feedback of a selected wavelength to the gain medium. The encoder records along a length thereof indicia corresponding to at least one of the positions of the tuning element and selected wavelengths of the beam. The reader reads the indicia. Either the encoder or the reader is coupled to the tuning element. The positioner is coupled to the tuning element to position the tuning element in the path of the beam to provide feedback of a selected wavelength to the gain medium to tune the tunable laser. The positioner positions the tuning element such that the data read by the reader substantially corresponds with the selected wavelength.
In an alternate embodiment of the invention a method for controlling the output wavelength of an external cavity laser is disclosed. The method for controlling the output wavelength comprises the acts of:
moving the tuning element to successive positions across a position range;
measuring a corresponding output wavelength at each of the successive positions across the position range responsive to said moving act;
recording indications of the corresponding output wavelengths along a length of an encoding medium coupled to a selected one of the base and the tuning element responsive to said measuring act;
reading from the non-selected one of the base and the tuning element the indications of the corresponding output wavelengths recorded on the encoding medium in said act of recording; and
moving the tuning element to a selected position at which the indication of the corresponding output wavelength read in said act of reading corresponds with the selected wavelength.
REFERENCES:
patent: 6108355 (2000-08-01), Zorabedian
patent: 6205159 (2001-03-01), Sesko et al.
Chapman William B.
Jenq Michael Y.
Zorabedian Paul
Cary Charles C.
Cary & Kelly LLP
Jr. Leon Scott
New Focus Inc.
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