Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2003-05-27
2004-02-10
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S098000, C372S099000, C372S107000, C372S108000
Reexamination Certificate
active
06690690
ABSTRACT:
TECHNICAL FIELD
The invention relates to tunable laser systems, particularly of the type having adjustable external cavities for varying the wavelength of laser output beams. One end of such cavities is formed by diffractive optics that are inclined to the laser beams under the control of adjustment mechanisms to provide controlled wavelength feedback.
BACKGROUND
External cavity lasers are well known in the art. The external cavity returns a portion of the radiation generated by the laser back into a primary laser cavity as a form of optical feedback that alters the laser radiation being amplified in the primary laser cavity. Modifications can be made to the external cavity to control properties of the light amplified in the primary laser cavity. This procedure has been used for several years to stabilize and tune the frequency output of dye lasers and is described in
Spectrally Narrow Pulsed Dye Laser Without Beam Expander, Applied Optics
, Vol. 17, No. 14, Jul. 15, 1978. External cavities are also commonly used with diode lasers and can be used to create narrowband, wavelength tunable diode laser systems for such applications as telecommunications, spectroscopy, and metrology.
A primary laser has a defined output beam direction and lasing wavelength. The lasing wavelength, which is the wavelength that exhibits maximum gain, is set by properties of the laser gain medium and the laser cavity. While it is possible for the primary laser to operate at any of a range of wavelengths, a single wavelength can be favored by prevailing conditions for which gain is maximized.
The external cavity allows for the alteration of the lasing wavelength of the primary laser by providing feedback to the laser in the form of a selected wavelength other than the one initially favored by the primary laser. In this manner, the primary laser is artificially caused to favor another wavelength, thus creating a means to tune the wavelength of the laser system.
Two types of external laser cavities are commonly used—the Littman cavity and the Littrow cavity. Specific configurations of Littman cavities have been the subject of U.S. Pat. Nos. 5,319,668; 5,802,085; and 5,867,512. An article in the March 1998 Review of Scientific Instruments, Volume 69, Number 3, pages 1236-1239, by Arnold et al. entitled “A Simple Extended-cavity Diode Laser” describes an exemplary Littrow cavity configuration. These patents and the above referenced articles are incorporated by reference.
Although both the Littman cavity and Littrow cavity configurations provide effective wavelength tuning, the Littrow cavity configuration is preferred for many commercial applications because it has a lower component cost. According to the Littrow configuration, a collimated output beam from the primary laser is directed toward a reflective diffraction grating that is oriented so that a first diffracted order of the output beam is returned to the primary laser on a path of retroreflection. The wavelength of the first diffracted order provides feedback to the laser to influence the lasing wavelength. The zero diffracted order of the output beam reflects from the grating for exiting the external cavity.
For returning a particular wavelength “&lgr;”, a Littrow orientation angle “&phgr;
pitch
” of the grating normal with respect to the beam direction of the primary laser is given by
φ
pitch
=
sin
-
1
(
λ
2
d
)
where “d” is the grating period. The external cavity thus provides feedback at wavelength “&lgr;”; and if this feedback is such that “&lgr;” becomes the wavelength for which the primary laser has maximum gain, then the laser system will lase at the feedback wavelength “&lgr;”.
A difficulty with the Littrow configuration, as shown in
FIG. 1
, is that the orientation of an output beam
18
from the external cavity
16
varies as the pitch angle &phgr;
pitch
of the diffraction grating
14
is changed. Thus, as the output beam
18
is tuned to different wavelengths, the orientation of the output beam changes, which produces significant alignment problems for optical devices intended for operation over multiple wavelengths.
SUMMARY OF THE INVENTION
The invention contemplates a wavelength tuning system for adjusting the external cavity of lasers and is intended to be particularly effective for varying output beam wavelength while maintaining the output beam from the external cavity at a fixed angular orientation. Improvements are made to the adjustment and alignment of components as well as to the components themselves, resulting in a robust structure that is easy to align and to maintain in alignment. The invention is especially useful for providing a tunable external cavity laser that operates optimally for various applications, such as multi-wavelength interferometry.
An exemplary wavelength tuning system for adjusting the external cavity of a laser in accordance with the invention includes a laser mount for supporting the laser that emits a wavelength-tunable output beam from the external cavity. A mounting arm supports both a diffractive optic and a reflective optic in a fixed orientation with respect to each other. A flexural member forms at least part of a connection between the mounting arm and the laser mount in a relative orientation that optically couples the diffractive optic to the laser within the external cavity. An actuator pivots the mounting arm with respect to the laser mount about a pivot axis for varying a wavelength of a diffracted portion of the output beam returned to the laser without significantly varying an angular orientation of the output beam with respect to the laser mount as reflected from the reflective optic. The flexural member supports a limited rotation of the mounting arm with respect to the laser mount about the pivot axis while providing resistance to a similar limited rotation of the mounting arm with respect to the laser mount about an orthogonal rotational axis.
Preferably, the tuning system also includes an adjuster to adjust the relative angular orientation of the mounting arm with respect to the laser mount about the orthogonal rotational axis. One such adjuster provides an adjustable mounting for the flexural member that adjusts the mounting of the flexural member between the mounting arm and the laser mount about the orthogonal rotational axis.
The orthogonal rotational axis is preferably a first of two orthogonal rotational axes that are orthogonal to the pivot axis, and the adjuster is preferably a first of two adjusters that adjust the relative angular orientation of the mounting arm with respect to the laser mount. A second of the adjusters can be used to adjust the relative angular orientation of the mounting arm with respect to the laser mount about the second of the orthogonal rotational axes. For example, the second adjuster can provide an adjustment of the flexural member between the mounting arm and the laser mount about the second orthogonal rotational axis. Similar adjustments can be made between two portions of the mounting arm, a first portion being connected to the flexural member and a second portion supporting both the diffractive optic and the reflective optic.
Alternatively, the second adjuster could engage the mounting arm remote from the flexural member for imparting a limited angular adjustment of the mounting arm about the second orthogonal rotational axis with respect to the laser mount through the flexural member. The mounting arm of this alternative preferably includes first and second ends—the first end being connected to the flexural member and a second end being connected to the second adjuster. The second adjuster permits motion of the mounting arm about the pivot axis while restricting further motion about the second orthogonal rotational axis from an established adjustment position. The flexural member preferably exerts a first preload torque on the mounting arm about the pivot axis for imparting a preload force on the actuator and preferably exerts a second preload torque on the mounting arm about the second orthogonal rotationa
Harter Secrest & Emery LLP
Ip Paul
LightGage, Inc.
Menefee James
Ryan Thomas B.
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