Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2002-05-03
2004-11-09
Vo, Tuyet (Department: 2821)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S092000, C372S020000
Reexamination Certificate
active
06816534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to lasers, and particularly to single frequency lasers such as those used for telecommunication purposes.
2. Description of Related Art
A single frequency, tunable laser with a narrow linewidth is useful for many applications. For example advanced sensors for defense applications require stable, highly single frequency lasers with as narrow a linewidth as possible. On the commercial front, optical networks can benefit from the added functionality that a tunable source can provide, and therefore the laser technologies required to support those networks continue to be a major area of focus of developers.
In order to provide single frequency operation, a variety of techniques have been used. One technique is to insert a Fabry-Perot etalon (FPE) into a laser cavity that is thin enough to restrict operation to a single mode within the gain-bandwidth of the laser material. However, this technique cannot be used effectively with broadband gain media due to the thinness that would be required to limit operation to a single frequency. Furthermore, the longitudinal modes (wavelengths) allowed by the etalon are set by its geometry, and therefore a laser with a conventional etalon is not tunable in any significant way.
It has been suggested to use birefringent materials in an intracavity filter configuration in order to reduce the number of longitudinal modes and to provide single frequency operation. In such conventional birefringent filters, a birefringent crystal is arranged within the laser cavity at Brewster's angle, oriented such that the polarization is at 45° between two differing dielectric axes. Problems with such conventional birefringent filters include lack of discrimination between adjacent modes; in other words, the peaks are not sharp enough to provide single frequency operation. In an attempt to improve discrimination, multiple birefringent filters (e.g. 2, 3, or more) may be used together to obtain single frequency operation; unfortunately this approach increases cost and complexity, and reduces reliability.
It may be noted that these two types of filters—the etalon and the birefringent filter—have different uses. The FPE is generally used with a narrowband gain medium in an attempt to restrict oscillation to a single (or at most a few) frequencies, whereas the birefringent filter is generally used with a broadband gain medium to restrict oscillation to a narrower portion of the gain spectrum. For conventional single frequency lasers, a FPE is not constructed of a birefringent material, as this would not result in single frequency operation except under unusual circumstances.
Although both an etalon and a birefringent filter may be used simultaneously in a laser cavity in an attempt to restrict the oscillation of a broadband gain medium to a single frequency, that approach is unlikely to be effective by itself. Particularly, such an arrangement is highly unlikely to operate effectively over a significant tuning range since it requires that, at some point within the gain bandwidth, both the FPE and the birefringent filter have some preferred frequency in common; i.e. there is a requirement of synchronism between the preferred frequency of the FPE and birefringent filter. The existence of this synchronism is a fortuitous occurrence, although it can be controlled to some extent by independent control of some of the filter parameters, such as the angle of incidence or temperature of the FPE. Unfortunately, this arrangement is highly sensitive to any disturbance or other variation in the local environment. Furthermore, such an arrangement is extremely difficult to tune over any significant range.
Although single frequency lasers can be useful in a wide variety of wavelengths and applications, when developing photonic systems for communications, it becomes advantageous to consider the 1.5 micron wavelength regime as the band of choice. Use of this band allows system designers to leverage developments in the optical communications arena, usually leading to wider availability and lower product costs. This allows the use of low-loss optical fiber, filters, optical amplifiers, and so forth; all of which have been developed for the commercial marketplace. However, despite explosive growth in the number of optical products, there remains a significant shortfall relative to laser transmitters that meet the technical requirements for advanced military and commercial applications.
Many high performance applications involving 1.5 micron laser sources call for narrow linewidth and single-frequency output. In this context, system designers wishing to use conventional technologies are often forced to utilize the available semiconductor-based sources at 1.5 microns. Unfortunately, performance limitations of such semiconductor-based sources often require substantial design-arounds to meet system design goals. Semiconductor laser manufacturers achieve single-frequency, narrow linewidth operation by incorporating either distributed feedback (DFB) or distributed Bragg reflector (DBR) configurations into their basic semiconductor laser design. In this way, the DFB and DBR sections of the device enforce single frequency behavior, also leading to line narrowing. Although there have been great strides in improving the performance of these structures, typical DFB linewidths are still large (e.g. in the 1-10 MHz regime), prohibiting their use in applications that require very narrow linewidth emission. Additionally, by their very nature, DFB and DBR lasers are fixed-wavelength devices, and, as a consequence, are unsuitable for applications requiring rapid tunability.
One method currently utilized for producing a tunable output in the 1.5 &mgr;m regime involves the use of conventional semiconductor-based lasers that have been incorporated into an external grating configuration. In such lasers, the external grating enforces single-frequency, narrow-linewidth performance of an otherwise multi-mode, broad-spectrum semiconductor laser. Tuning is achieved by mechanically tilting the grating. Although this method has been successfully implemented in commercial devices, the tuning rate is slow (on the order of seconds); a limitation which prevents their use in applications that require rapid tuning.
Another method being investigated to produce a tunable output employs a semiconductor-based vertical cavity surface emitting laser (VCSEL) gain region that is integrated with a micro-electro-mechanical (generically referred to as a MEMS device) mirror to provide one of the mirrors in the resonator. By moving the MEMS mirror along the VCSEL axis, the wavelength of the output can be tuned. However, this process also has relatively slow tuning (tens of microseconds). Furthermore, such devices have not been successfully brought to market.
To improve upon existing DFB and DBR laser capabilities by incorporating tunability, it has been suggested to incorporate additional sections into the DFB and/or DBR structures, such as disclosed in B. Mason, et al., IEEE Phot. Tech. Left., Vol. 10, No. 9, Sept. 1998 and in P. -J. Rigole, et al., Electron. Left., Vol. 32, No. 25, 1996. These devices integrate multiple frequency-selective sections into a common semiconductor laser structure. By varying the injection current into each of the independent sections, their frequency selective properties are slightly modified so as to produce wavelength tuning. Because of the relatively low amount of injection current required to tune across the desired wavelength range (typically on the order of 10 mA), the tuning speed can, theoretically, approach the tens-of-nanoseconds regime. In every case, however, the linewidths are wide which is typical of semiconductor DFB and DBR sources (on the order of one megahertz) with relatively low (<10 mW) output power. In summary, although the DFB and DBR technologies appear to be progressing toward the development of fast switching devices, no products have been developed that are fast and have a narrow linewidth suffi
Flint Graham W.
Pessot Maurice A.
Takeuchi Eric B.
Al-Nazer Leith A
General Atomics
Law Offices of James D. McFarland
Vo Tuyet
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