Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-06-04
2001-09-11
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S341400, C359S341500
Reexamination Certificate
active
06288835
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to optical amplifiers and light sources. By way of example, though not exclusively, the invention relates to single- or few-moded waveguiding lasers, superfluorescent sources, optical amplifiers, high pulse-energy devices, energy-storage devices, cladding-pumped devices, semiconductor signal amplifiers, and waveguiding saturable absorbers.
BACKGROUND OF THE INVENTION
The tightly confined modal fields of single- or few-moded waveguiding lasers, superfluorescent sources, and amplifiers lead to a very strong interaction between any waveguided light and the active medium in the waveguiding core. Therefore, a comparatively small amount of gain medium is sufficient for providing the gain in these devices. Specifically, the gain for a given stored energy, as well as for a given absorbed pump power, is high. This is often beneficial, since it means that the pump power requirements for a given desired laser output power or amplifier gain can be low.
However, for several devices, this efficient interaction between mode and gain medium can be detrimental. The following example refers to certain types of amplifiers and lasers, but of course the skilled man will realise that the same or similar problems can occur in, for example, superfluorescent sources.
In a laser or amplifier, the achievable single-pass gain is limited to, say, 50 dB. The reason is that at this gain, a significant fraction of the pump power is converted to amplified spontaneous emission (ASE). A 10 dB higher gain results in approximately 10 dB more ASE, so at these gains, the extra pump power required to increase the gain further will be prohibitively high. Since the ASE limits the gain of the device, it also limits the energy stored in the gain media. This in turn obviously limits the amount of energy that a pulse can extract from the device. Consequently, the pulse energy that can be obtained from waveguiding lasers and amplifiers is limited. Instead, bulk (i.e., not waveguiding) lasers and amplifiers for which the extractable energy for a given gain can be several orders of magnitude lower are often employed to provide much higher pulse energies. However, the robustness and stability of bulk lasers is often inferior to waveguiding ones.
Moreover, the gain limit can also be problematic for lasers and amplifiers irrespective of whether the stored energy is a major concern, if the high gain appears at another wavelength than the desired one. The reason is that ASE (or lasing) at the gain peak will suppress the gain achievable at the desired wavelength, possibly to a value below what is required for a good amplifier or laser. This applies to all types of amplifiers and lasers.
Furthermore, in optically pumped lasers and amplifiers, a suitable interaction between the gain medium and the amplified or generated signal beam is not enough; also the interaction between the pump beam and the gain medium must be appropriate. However, in some types of lasers and amplifiers (typically cladding-pumped ones), the interaction with the pump beam is significantly smaller than the interaction with the signal beam. Then, for a device that efficiently absorbs the pump, the interaction with the signal beam will be much stronger than what is required. Unfortunately, this excess interaction is often accompanied by excess losses for the signal beam, since:
1. The scattering loss of an active medium is normally higher than it can be for a passive medium. For instance, rare-earth-doped fibers have scattering losses of, e.g., several orders of magnitude higher than standard, passive, single-mode fibers.
2. A fraction of the active medium often has inferior properties. For instance, in Er-doped fibers, pairs of Er
3+
-ions can form. These result in an unbleachable loss. The strong interaction then leads to a high loss.
3. The active medium in its amplifying state can also absorb light (so-called excited-state absorption, ESA). Again, a stronger interaction leads to more power lost through ESA.
Moreover, a bleachable medium (e.g., an unpumped gain medium with a ground-state absorption) can be used as a saturable absorber. An efficient interaction leads to a low saturation power. A reduced interaction leads to a higher saturation power, which can be more suitable for some applications, especially if the interaction, and hence the saturation power, can be controlled.
Clearly, although often beneficial, the tight confinement of the guided light is a problem for some devices.
SUMMARY OF THE INVENTION
An aim of the present invention is to improve the interaction between light guided along a waveguide and rare-earth dopants within an active medium.
Accordingly in one non-limiting embodiment of the present invention, there is provided apparatus comprising a waveguide and an amplifying region wherein the waveguide comprises a core and a cladding and the amplifying region comprises rare-earth dopants and wherein the amplifying region comprises a ring around the core of the waveguide.
Various aspects of the invention are defined in the appended claims, and in passages throughout the present application.
According to a first embodiment of the present invention, there is provided an amplifying optical device comprising a first waveguiding structure comprising a first core and cladding and configured to guide optical radiation, at least one pump source configured to supply optical pump power, an amplifying region situated in the cladding; and wherein the pump source is optically coupled to the amplifying region; and wherein in use the optical radiation guided in the first waveguiding structure overlaps the amplifying region.
The invention also provides a method of pumping at least one optical fiber amplifier with a fiber laser, the method comprising providing a first waveguiding structure fabricated from at least one glass system and comprising a first core and cladding; providing a second waveguiding structure comprising a second core at least partly formed by the cladding and an amplifying region comprising Ytterbium; providing a source of optical pump power in optical communication with the second waveguiding structure and having a wavelength in the band from about 870 nm to about 950 nm; providing an optical feedback device; guiding optical radiation using the first waveguiding structure; guiding the optical pump power using the second waveguiding structure such that the amplifying region interacts with the optical radiation guided in the first waveguiding structure and the optical pump power guided in the second waveguiding structure to amplify the optical radiation guided by the first waveguiding structure; using the optical feedback device to ensure that a plurality of times a portion of the optical radiation guided by the first waveguiding structure is amplified more than once by the amplifying region; providing an amplifying region characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the amplifying region are arranged such that the fiber laser emits optical radiation at an emission wavelength in the region of about 970 nm to about 990 nm; and coupling the optical radiation at the emission wavelength in the region of about 970 nm to 990 nm into the at least one optical amplifier.
A second method provided by the invention is a method of amplifying optical pulses to energies exceeding the intrinsic saturation energy of an amplifying optical device, comprising: providing a first waveguiding structure comprising a first core and cladding; providing a source of optical pump power; providing a second waveguiding structure comprising a second core at least partly formed by at least part of the cladding, and an amplifying region; guiding optical radiation using the first waveguiding structure; and guiding the optical pump power using the second waveguiding structure such that the amplifying region interacts with the optical radiation guided in the first waveguiding structure and the optical pump power guided in the second w
Hanna David Colin
Minelly John Douglas
Nilsson Lars Johan Albinsson
Paschotta Ruediger Eberhard
Hellner Mark
Reid John S.
Reidlaw, L.L.C.
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