Coherent light generators – Optical fiber laser
Reexamination Certificate
1998-09-01
2001-10-30
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Optical fiber laser
C372S003000, C372S099000
Reexamination Certificate
active
06310899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to optical amplifiers and lasers. More particularly, the invention relates to optical amplifier and laser apparatus and methods using cascaded Raman resonators.
2. Description of the Related Art
Optical amplifiers and lasers are used within optical communications systems to compensate for losses incurred throughout the system. Optical amplifiers often include a Raman amplifier or laser to pump light at a particular wavelength. See, e.g., U.S. Pat. No. 5,323,404, which is assigned to the instant assignee and hereby is incorporated by reference herein.
In general, Raman amplifiers and Raman lasers are based on stimulated Raman scattering, a non-linear optical process that involves converting light from an optical source to the vibrational modes of a non-linear optical transmission medium (e.g, an optical fiber, typically a silica-based optical fiber) and re-radiation at a different (typically longer) wavelength.
For example, a cascaded Raman laser typically is a Raman laser with a non-linear optical transmission medium that has, in addition to a pair of reflectors that defines an optical cavity for radiation of an output wavelength &lgr;
n
, at least one Raman-Stokes order reflector pair defining a corresponding optical cavity for radiation of wavelength &lgr;
n−1
<&lgr;
n
, where n≧2. The reflector pairs are, e.g., Bragg gratings, etched gratings or in-line refractive index gratings. When fused silica is used as the non-linear medium, the maximum Raman gain occurs at a frequency shift of 13.2 terahertz (THz), which corresponds to a wavelength shift of approximately 50-100 nanometers (nm) for pump wavelengths between approximately 1.0 and 1.5 microns (&mgr;m).
A cascaded Raman resonator (CRR) includes a non-linear optical transmission medium to generate Raman laser energy at a specific output wavelength (&lgr;
n
). More specifically, the cascaded Raman resonator converts light from an optical source such as a pump laser operating at a pump wavelength (&lgr;
p
) to the desired output wavelength (&lgr;
n
). Suitable applications of such cascaded Raman resonator include, e.g., remotely pumped erbium (Er) fiber amplifiers in repeaterless optical fiber communication systems.
However, conventional cascaded Raman resonators typically require optical sources that operate at a specific pump wavelength (&lgr;
p
) depending on the cascaded Raman resonator output wavelength (&lgr;
n
) desired. For example, a cascaded Raman resonator having an output wavelength (&lgr;
n
) of 1480 nm typically is useful only with an optical source such as a pump laser operating at a pump wavelength (&lgr;
p
) of 1117 nm, which corresponds to a series of resonators spaced at wavelengths corresponding to the maximum Raman gains or frequency shifts of about 13.5 THz. Similarly, a cascaded Raman resonator having an output wavelength (&lgr;
n
) of 1450 nm typically is useful only with an optical source such as a pump laser operating at a pump wavelength (&lgr;
p
) of 1100 nm.
Thus, it would be desirable to have available Raman laser devices that are power scaleable and more independent of the device input wavelength (&lgr;
p
). Such devices would be more versatile in that, e.g., the devices would not be limited to use with sources having only a specific pump wavelength (&lgr;
p
) that corresponds to a Raman-Stokes order that leads to the desired output wavelength (&lgr;
n
) of the device.
SUMMARY OF THE INVENTION
The invention is embodied in an apparatus for converting light within an optical fiber communications system. Embodiments of the invention provide a cascaded Raman resonator (CRR) or other suitable Raman frequency shifting device having an optical energy transmission medium with a series of Raman-Stokes order reflectors and an output reflector therein. One or more of the reflectors are written to provide conversion with less than maximum efficiency, but sufficient efficiency to allow different pump laser wavelengths to be converted thereby. For example, the reflectors within the cascaded Raman resonator are written at wavelengths that are not necessarily at the maximum of the Raman gain for the pump wavelength (&lgr;
p
) but still provide sufficient conversion efficiency
In one embodiment of the invention, a cascaded Raman resonator includes an optical fiber with an optical cavities defined by a pump reflector and a pair of highly reflective gratings whose maximum reflectance wavelength does not correspond to the wavelength where the theoretical maximum Raman gain occurs but is within an acceptable range for sufficient conversion efficiency. Alternatively, one or more of the reflectors in the series of intermediate Raman-stokes reflectors and the low reflectivity output reflector are not necessarily written at wavelengths that correspond to the theoretical maximum Raman gain but are within acceptable ranges thereof for sufficient conversion efficiency.
According to another alternative embodiment of the invention, the cascaded Raman resonator does not use a pump reflector. More specifically, when operating at sufficiently high pump powers, e.g., greater than approximately 4 watts, sufficient conversion exists over approximately the first 500 meters of fiber that the pump reflector, typically written within at the pump wavelength (&lgr;
p
), is not necessary.
Cascaded Ramam resonators designed in accordance with embodiments of the invention are useful with different pump lasers having different pump wavelengths (&lgr;
p
). For example, 1480 nm cascaded Raman resonators, which conventionally convert optical energy from 1117 nm pump lasers to an output signal wavelength of 1480 nm, also are useful for converting optical energy from, e.g., 1100 nm pump lasers to the output signal wavelength of 1480 nm.
Alternatively, when operated at low pump powers, e.g., less than approximately 4 watts, 1480 nm cascaded Raman resonators (CRR) include an additional high reflectance grating written at approximately 1100 nm to reflect the non-absorbed light. In this manner, 1480 nm cascaded Raman resonators are useful with, e.g., 1100 nm pump laser sources in addition to 1117 nm pump lasers. In conventional arrangements, 1100 nm pump lasers typically are used only with 1450 nm cascaded Raman resonators, which conventionally are designed to convert optical energy from 1100 nm pump lasers to an output signal wavelength of 1450 nm.
Also, in a similar manner, according to embodiments of the invention, 1450 nm cascaded Raman resonators, which conventionally convert optical energy from 1100 nm pump lasers to an output signal wavelength of 1450 nm, also are useful in converting optical energy from 1117 nm pump lasers to the output signal wavelength of 1450 nm. Alternatively, when operated at low pump powers, e.g., less than approximately 4 watts, 1450 nm cascaded Raman resonators include an additional high reflectance grating written at approximately 1117 nm to reflect the non-absorbed light. In conventional arrangements, 1117 nm pump lasers typically are used only with 1480 nm cascaded Raman resonators, which conventionally are designed to convert optical energy from 1117 nm pump lasers to an output signal wavelength of 1480 nm.
Cascaded Raman resonator (CRR) devices according to embodiments of the invention are power scaleable in a manner sufficient to provide adequate optical power for a variety of application at a desired output wavelength for a given plurality of pump wavelengths. Also, the devices according to embodiments of the invention are less complex and less expensive than conventional arrangements.
REFERENCES:
patent: 5323404 (1994-06-01), Grubb
patent: 5815518 (1998-09-01), Reed
patent: 5887093 (1999-03-01), Hansen
Jacobovitz-Veselka Gloria R.
Reed William Alfred
Arroyo Teresa M.
Harman John M.
Inzirillo Gioacchino
Lucent Technologies - Inc.
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