Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2001-08-27
2003-12-30
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S098000, C372S099000
Reexamination Certificate
active
06671295
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of tunable diode lasers, and particularly to external cavity tunable diode lasers with high speed wavelength selection capability.
BACKGROUND OF THE INVENTION
Monochromatic light sources, such as lasers, have broad application in the spectroscopic and telecommunications industry due to their ability to be narrowly tuned to emit a specific wavelength. The applications within these industries are pushing the development of tunable lasers. Current desired characteristics include fast and broadband wavelength tuning, arbitrary wavelength selection, simultaneous selection of multiple wavelengths, no macroscopic mechanical motion, long-term amplitude and spectral stability, low cost, small size and capability for remote programming and control of the output spectrum. Most of these requirements are natural extensions of previous achievements in laser tuning. However, the combined requirements of long term amplitude and optical frequency stability and tunability is a challenging requirement in applications such as telecommunications.
In the telecommunications industry, a set of optical frequencies (wavelengths) has been allocated for optical channels by the ITU committee (an international organization headquartered in Geneva, Switzerland within which governments and the private sector coordinate global telecom networks and services). According to the current standards, the optical channels range from 186 THz (1611.79 nm) to 200.95 THz (1491.88 nm) with fixed channel spacings specified at 100 and 50 GHz covering three bands: C, L, and S. The resulting telecommunications standard, i.e., dense wavelength division multiplexing schema (and similar multiplexing schemas that may be later established as standards and utilized in the future) requires precise and stable laser sources and highly discriminating low loss filters to match to the ITU grid.
One of the popular methods of tuning a laser source is by implementing the Littman-Metcalf external laser cavity configuration. A Littman-Metcalf external cavity configuration is traditionally accomplished with a diffraction grating and a mechanically rotating single mirror used to select the specific wavelength. Using this approach, a high degree of precision in the mirror rotation mechanism is required for wavelength selection, and the tuning process is relatively slow. This original approach, as applied to dye laser technology, is described in detail in the following non-patent prior art: M. G. Littman and H. J. Metcalf, Applied Optics, vol. 17, no. 14, 2224-2227, Jul. 15, 1978, and P. McNicholl and H. J. Metcalf, Applied Optics, vol. 24, no. 17, 2757-2761, Sep. 1, 1985.
An intra-cavity etalon is an additional component that can be used in a laser source for enhanced spectral selectivity of the cavity. An additional Fabry-Perot cavity is provided with partially reflecting mirror surfaces, inserted into the laser resonator, and typically, slightly tilted from the normal to the optical axis of the laser cavity. High precision etalons may be maintained at a constant temperature to ensure dimensional stability. The etalon acts as a wavelength dependent filter restricting the wavelength(s) that can pass through it. As a Fabry-Perot cavity, the etalon imposes a set of longitudinal modes in addition to those of the original cavity. The peaks of transmission correspond to the conditions where constructive interference occurs at both surfaces of the etalon. This happens when the effective thickness of the etalon (i.e., the distance between its partially reflecting surfaces) is an integer multiple of ½ the wavelength of the light inside the etalon. The width of the peaks is a function of reflectivity of the etalon's surfaces; higher reflectivity produces narrower peaks. Fabry-Perot etalons can be designed to produce transmission peaks at specific constant intervals. Outside of the transmission peaks light is mostly reflected from the etalon. If the etalon is slightly tilted in the cavity, this reflection causes loss, thus suppressing any spectral components that are not coincident with the etalon's transmission peaks.
The use of an etalon for fine tuning of both dye and diode lasers is well known in the art. Use of Fabry-Perot etalons to tune dye lasers is documented in non-patent prior art such as Okada, et al. Applied Optics, vol. 15, 472, 1976; and Okada, et al. Applied Optics, vol. 14, 917, 1975. The effect of an etalon on spectral output of a dye laser is represented in
FIG. 1
in which the transmission peaks of the etalon
982
overlay the dye laser spectral gain curve
980
. A Fabry-Perot etalon would effectively narrow the laser output to the transmission peak of the etalon within the spectral gain curve of the dye laser.
Use of a Fabry-Perot etalon to increase the stability of a diode laser is documented for example in Applied Optics, vol. 28, 4251, 1989. The effect of an etalon on spectral output of a diode laser is represented in
FIG. 2
in which the transmission peaks of the etalon
982
overlay the diode laser spectral gain curve
984
. A Fabry-Perot etalon would effectively narrow the laser output to the transmission peaks of the etalon within the spectral gain curve of the diode laser.
The same effect can be implemented continuously over the spectrum and not limited to the etalon transmission peaks by replacing the etalon in the cavity with a Fabry-Perot interferometer (FPI). The FPI acts as an etalon with tunable (or variable) optical length.
However, in either of the situations illustrated by
FIGS. 1 and 2
, undesired multi-mode emission—defined by the multiple peaks within the gain curve—may occur, unless some means for additional wavelength selectivity is introduced as part of the laser cavity. Certain particular types of such selectivity, i.e., digital switching means, in combination with the tilted Fabry-Perot etalon, as disclosed in detail herein, do provide relatively coarse wavelength selection to ensure stable single-mode operation of the laser.
As diode lasers have come to replace dye lasers in many applications, a variety of techniques have been applied to tuning diode lasers for implementation in both spectroscopic and telecommunications applications. For example, a variety of U.S. Patents exist for laser tuning with alternative configurations of the mirror at the cavity end. U.S. Pat. No. 4,896,325 discloses an alternative cavity configuration in which a pair of mirrors with narrow discontinuities to provide reflective maxima bound the active cavity. These narrow bands of reflective maxima provide a means for wavelength tuning which is actively controlled by a vernier circuit. U.S. Pat. No. 4,920,541 discloses an external laser cavity configuration of multiple resonator mirrors used to produce multiple wavelength emission from a single laser cavity simultaneously or with a very fast switching time. U.S. Pat. No. 5,319,668 discloses a tunable diode laser with a diffraction grating for wavelength separation and a moveable mirror at the cavity end for wavelength selection. The pivot points are designed to provide an internal cavity length equal to an integer number of half wavelengths at three different wavelengths and an exceptionally close match at all other wavelengths within the tuning range. Alternative tuning arrangements are possible. U.S. Pat. No. 5,771,252 discloses an external cavity continuously tunable wavelength source utilizing a cavity end reflector moveable about a pivot point for simultaneous rotation and translation for wavelength selection.
In addition, several U.S. Patents disclose the use of alternative components in the laser cavity configuration in order to achieve wavelength tuning. U.S. Pat. No. 4,216,439 discloses a spectral line selection technique that utilizes a spectral line selection medium in the gain region of an unstable laser cavity. U.S. Pat. No. 4,897,843 discloses a microprocessor-controlled laser system capable of broadband tuning capabilities by using multiple tuning elements each with progressively finer linew
InterScience, Inc.
Ip Paul
Menefee James
Simkulet Michelle D.
Yablon Jay R.
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