Coherent light generators – Particular resonant cavity – Plural cavities
Reexamination Certificate
2002-05-20
2004-08-03
Wong, Don (Department: 2828)
Coherent light generators
Particular resonant cavity
Plural cavities
C372S029020
Reexamination Certificate
active
06771687
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the stabilization of a laser, specifically a semiconductor diode laser of the type commonly used in opto-electronics, mostly as so-called pump lasers for fiber amplifiers in the field of optical communication, e.g. for Erbium-doped fiber amplifiers. Such lasers are designed to provide a narrow-bandwidth optical radiation with a stable power output in a given frequency band. In particular, the invention concerns an improved design of the external cavity exhibiting a significantly improved stability compared to prior art designs.
BACKGROUND AND PRIOR ART
Semiconductor lasers of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted which improves speed as well as reliability within such systems.
In one kind of optical fiber communication systems, the lasers are used for pumping Erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance are 980 nm lasers with a power output of 150 mW or more, which wavelength matches the 980 nm Erbium absorption line and thus achieves a low-noise amplification. InGaAs lasers have been found to serve this purpose well and are used today in significant numbers. However, the invention is in no way limited to InGaAs lasers.
There are other examples of lasers of other wavelengths and materials for which the present invention is applicable. Generally, laser diode pump sources used in fiber amplifier applications are working in single transverse mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot (or FP) lasers. Three main types are typically being used for Erbium amplifiers, corresponding to the absorption wavelengths of Erbium: InGaAsP and multiquantum-well InGaAs lasers at 1480 nm; strained quantum-well InGaAs lasers at 980 nm; and GaAlAs lasers at 820 nm.
One of the problems occurring when using semiconductor lasers for the above purpose is their wavelength and power output instability which, though small, still affects the amplification sufficiently to look for a solution to the problem. This problem is already addressed in Erdogan et al. U.S. Pat. No. 5,563,732, entitled “Laser Pumping of Erbium Amplifier”, which describes the stabilization of a pump laser of the type described above by use of a Bragg grating in front of the laser. This grating forms an external cavity with the laser. The laser bandwidth is broadened and stabilized by the reflection from the grating. It is believed that the laser operation in so-called “coherence-collapse” is obtained by providing sufficient external optical feedback, here from a fiber Bragg grating within the optical fiber into which the laser light is usually coupled. This grating is formed inside the guided-mode region of the optical fiber at a certain distance from the laser. Such a fiber Bragg grating is a periodic structure of refractive index variations in or near the guided-mode portion of the optical fiber, which variations are reflecting light of a certain wavelength propagating along the fiber. The grating's peak-reflectivities and reflection bandwidths determine the amount of light reflected back into the laser.
Ventrudo et al. U.S. Pat. No. 5,715,263, entitled “Fibre-grating-stabilized Diode Laser” describes an essentially similar approach for providing a stabilized laser, showing a design by which the laser light is coupled to the fiber by focussing it through a fiber lens. Again, a fiber Bragg grating is provided in the fiber's guided mode portion, reflecting part of the incoming light back through the lens to the laser. To summarize, when positioning a fiber Bragg grating beyond the coherence length of the laser and when the laser gain peak is not too far from the Bragg grating's center wavelength, it is understood that the laser in coherence collapse operation is forced to operate within the optical bandwidth of the grating and thus is wavelength-stabilized. Additionally, low-frequency power fluctuations seem to decrease by the effect of induced high-frequency multi-mode operation.
In general, the above-described prior art devices must have a length of the external cavity, i.e. the optical fiber, somewhere at least between 0.5 and 1 m, to definitely assure coherence collapse laser operation. For some even up to 2 m long optical fibers are required. This rather long fiber determines the size of the laser source and makes it comparatively bulky.
Some types of semiconductor lasers, especially others than those in the above mentioned patents, e.g. lasers having a narrow spectral gain width, are seen to exhibit instability at certain operating conditions, in particular undesirable switching from multi-mode to single-mode operation within the grating bandwidth. This mode switching (coherence collapse occurs in both cases) results in a fluctuation of the effective laser output which in turn produces noise, thereby negatively affecting or actually disturbing the amplification process. The mode-switching problem is aggravated by new generations of semiconductor laser diodes having at least twice as much output power than the lasers in the Ventrudo or Erdogan patent and the desire of the industry to have wavelength stabilization for all possible operating conditions of a laser.
Other techniques have been proposed to correct fiber amplifier output power fluctuations, e.g. active methods to control the variations in the fiber amplifier output by feedback of an electric signal effecting a correction of the laser power. A further solution is an electronic dithering circuitry forcing the laser to operate multimode, described by Heidemann et al. in U.S. Pat. No. 5,297,154, entitled “Fiber-Optic Amplifier with Feedback-Insensitive Pump Laser”. However, the need for active components for these solutions add complexity and cost.
For a quite different purpose, Fischer et al. describe in “High-dimensional Chaotic Dynamics of an External Cavity Semiconductor Laser”, Phys. Review Letters, Vol. 73, No. 16, October 1994, pp. 2188-2191, an experimental laser setup with an external T-shaped cavity comprising a beam splatter and high reflecting gold mirrors at each of the two ends of the cavity's two arms. Though this layout shows an external two-cavity arrangement, it is absolutely unsuitable for the purpose of the present invention, since the lengths chosen for the arms of the cavity and the reflectivities of the laser's exit facet and the above-mentioned gold mirrors are selected to avoid the coherence collapse just the opposite of the present invention, where coherence collapse is a prerequisite.
Also in a very different field, Wang Xianghyang et al. disclose a “Theoretical and Experimental Study on the Fabrication of Double Fiber Bragg Gratings” in the journal Optical Fiber Technology: Materials, Devices and Systems, Vol. 3, No. 2, pp. 189-193. Double gratings are provided at the same location within the fiber and this “chirped” grating is said to widen the transmission spectrum of the fiber. Again, this publication does nowhere address the problem that the invention intends to solve.
Thus, it is the main object of the invention to devise a simple and reliable laser source layout, especially for pump lasers in optical fiber communication systems, that provides a stable output under all operating conditions. A specific object is to avoid the detrimental mode switching of the laser, even for a laser output power of more than 150 mW, and thus increase the stability of the output of high power laser sources. Output stability shall be achieved for high optical power having reduced low frequency noise, wavelength stability and high side lobe suppression outside th
Bookham Technology plc.
Menefee James
Renner , Otto, Boisselle & Sklar, LLP
Wong Don
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