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Tapered double-clad fiber
Neil at PULSE Project: Added page on Tapered double-clad fibers
The patented technology <ref>V. Filippov, Yu. Chamorovskii, O. G. Okhotnikov and M. Pessa, US patent No.8,433,168 B2 "Active optical fiber and method for fabricating an active optical fiber".</ref> of active tapered double-clad fibers (T-DCF) was developed at Tampere University to overcome the nonlinear effects which are the major contraint for power-scaling of fiber lasers and amplifiers . T-DCF is a fiber in which outer and inner claddings and core diameters are varied smoothly with length.
The core at the narrow end of a T-DCF supports propagation of fundamental mode only, whereas at wide end the core can guide many modes. However, it has been shown experimentally <ref name=":0">Liquid error: wrong number of arguments (given 1, expected 2)</ref>, that the light launched into a narrow end of a T-DCF propagates in a wide core without any changes of a mode content. As a result, at the wide (substantially multimode) end of a T-DCF light propagates only in a fundamental mode with excellent beam quality. Thus, tapered fiber structure uniquely propagates and amlplifies a fundamental mode regime in a the multimode fiber.
Single-mode propagation has been achieved in T-DCF with a 120 µm core diameter <ref name=":0" /> and even 200 µm <ref name=":1">Liquid error: wrong number of arguments (given 1, expected 2)</ref> with core [[Numerical aperture|NA]]=0.11. The large core diameter of T-DCF has enabled record values of peak power and energy without significant non-linear distortions. Moreover, there are few practical limitations in the use of T-DCF, which can be wound in a 30 cm coil without compromising performance.
The main advantages of T-DCF are following:
# Allows use of a much larger fiber core (up to 200 µm) and cladding (up to 1.6mm) diameters;
# Increased pump absorption, per unit length compared to regular (cylindrical) active double-clad fiber;
# A built-in mechanism of suppressing of stimulated [[Brillouin scattering]] (SBS) and amplified [[spontaneous emission]] (ASE);
# Immunity to mode instability;
# High ratio of pump brightness enhancement;
# Simplicity of production comparing with other fiber amplifier technologies (rod type fibers, 3C fibers, HOMF and LCF).
These advantages are considered in details.
'''''Extremely large core (up to 200 µm) and cladding (up to 1.6mm) diameters''''';
Large active core diameter allows to reach a high stored energy/power and, simultaneously, significantly increase non-linear effects thresholds (SBS, SRS, SPM). Happy combination of these two circumstances helps to achieve efficient amplification of short pulses. Using T-DCF, we have demonstrated 60 ps pulses with 300 µJ energy <ref name=":1" /> - the best result so far for all-fiber [[Optical parametric oscillator|MOPA]]<nowiki/>s.
By virtue of the large cladding diameter T-DCF can be pumped by optical sources with very poor brightness factor such as laser diode bars or even VECSELs matrices significantly reducing the cost of fiber lasers/amplifiers.
'''''High absorption per unit length'''''
T-DCF has a better double clad pump absorption comparing to regular double clad fibers with similar level of core doping. This is a result of two main circumstances. Firstly, the absorption at the T-DCF is always better due to better clad mode mixing <ref name=":2">Liquid error: wrong number of arguments (given 1, expected 2)</ref> and secondly, the absorption per unit length in the wide end T-DCF is substantially higher for geometric reasons. Indeed, rare earth ions are located preferably at the wide end of T-DCF (proportional to the square of the core diameter). This feature allows the use of preforms with relatively low [[dopant]] concentration ([[Absorption (electromagnetic radiation)|absorption]] in the core of only 300-400dB/m), which avoids the [[photodarkening]] effect. The high absorption makes it possible to create very short-amplifiers (a few tens of cm), which is important for amplification of ultrashort pulses.
'''''SBS and amplified spontaneous emission (ASE) suppression'''''
T-DCF owns embedded mechanisms of SBS and ASE suppression. It is well known from the literature that the diameter modulation leads to increasing of SBS threshold <ref>Liquid error: wrong number of arguments (given 1, expected 2)</ref>. This important feature is very useful for amplification of very narrow-band signals (MHz or kHz FWHM), for example, pulsed sources for LIDARs. ASE is suppressing at the T-DCF during propagation towards to a narrow end due to violation of the total internal reflection law. It allows to exploit a T-DCF for amplification of pulses with very low duty cycle (up to "on demand" mode).
'''''Immunity to the mode instability'''''
Earlier it was experimentally shown that spatial modulation of the core diameter along a fiber length leads to the increasing of the mode instability threshold <ref name=":2" />. This means that the T-DCF technique allows lasers and amplifiers to be built with higher output power in comparison to a regular [[Double-clad fiber|DCF]].
'''''High brightness magnification factor'''''
A [[laser]] or [[optical amplifier]] is a device which improves the brightness of a powerful pump source. Specifically, the low brightness light from a pump source is converted through the absorption process into output radiation with a longer wavelength and simultaneously with better brightness. The maximum launched pump power is determined by the clad diameter of [[Double-clad fiber|DCF]]. Since the diameter of a T-DCF's cladding is much higher (up to 1.6 mm <ref name=":2" />), it is possible to increase the brightness enhancement factor compared to regular DCFs. T-DCF lasers and amplifiers can also use low quality pump sources with very low [[brightness]], or [[M squared|M-Squared]] value which are unsuitable for pumping other fibers.
'''''Simplicity of production'''''
One of the significant advantages of T-DCF is the simplicity of production. The preform production for special high power fibers (microstructured rod type fibers, 3C or LCF fibers) involves complex technology and strict structural requirements. Conversely, T-DCF is made using standard fiber preforms. Simple production techniques of varying of the drawing speed during the pulling process leads to the fiber diameter changing along its length. T-DCF production is lhardly more complex than the production of a regular active fiber.
Thus, the simple production and special properties of T-DCF enables the realisation of a broad range of photonics devices with unique properties:
# · Ultrafast (ps and sub-ns) powerful (tens of watts) amplifiers;
# · SBS-free amplifiers for high coherent radiation;
# · Powerful actively Q-switched pulsed lasers;
# · Brightness converters: lasers or amplifiers pumped by inexpensive powerful pumps with low brightness (laser bar stuck, VECSELs, etc.).
The core at the narrow end of a T-DCF supports propagation of fundamental mode only, whereas at wide end the core can guide many modes. However, it has been shown experimentally <ref name=":0">Liquid error: wrong number of arguments (given 1, expected 2)</ref>, that the light launched into a narrow end of a T-DCF propagates in a wide core without any changes of a mode content. As a result, at the wide (substantially multimode) end of a T-DCF light propagates only in a fundamental mode with excellent beam quality. Thus, tapered fiber structure uniquely propagates and amlplifies a fundamental mode regime in a the multimode fiber.
Single-mode propagation has been achieved in T-DCF with a 120 µm core diameter <ref name=":0" /> and even 200 µm <ref name=":1">Liquid error: wrong number of arguments (given 1, expected 2)</ref> with core [[Numerical aperture|NA]]=0.11. The large core diameter of T-DCF has enabled record values of peak power and energy without significant non-linear distortions. Moreover, there are few practical limitations in the use of T-DCF, which can be wound in a 30 cm coil without compromising performance.
The main advantages of T-DCF are following:
# Allows use of a much larger fiber core (up to 200 µm) and cladding (up to 1.6mm) diameters;
# Increased pump absorption, per unit length compared to regular (cylindrical) active double-clad fiber;
# A built-in mechanism of suppressing of stimulated [[Brillouin scattering]] (SBS) and amplified [[spontaneous emission]] (ASE);
# Immunity to mode instability;
# High ratio of pump brightness enhancement;
# Simplicity of production comparing with other fiber amplifier technologies (rod type fibers, 3C fibers, HOMF and LCF).
These advantages are considered in details.
'''''Extremely large core (up to 200 µm) and cladding (up to 1.6mm) diameters''''';
Large active core diameter allows to reach a high stored energy/power and, simultaneously, significantly increase non-linear effects thresholds (SBS, SRS, SPM). Happy combination of these two circumstances helps to achieve efficient amplification of short pulses. Using T-DCF, we have demonstrated 60 ps pulses with 300 µJ energy <ref name=":1" /> - the best result so far for all-fiber [[Optical parametric oscillator|MOPA]]<nowiki/>s.
By virtue of the large cladding diameter T-DCF can be pumped by optical sources with very poor brightness factor such as laser diode bars or even VECSELs matrices significantly reducing the cost of fiber lasers/amplifiers.
'''''High absorption per unit length'''''
T-DCF has a better double clad pump absorption comparing to regular double clad fibers with similar level of core doping. This is a result of two main circumstances. Firstly, the absorption at the T-DCF is always better due to better clad mode mixing <ref name=":2">Liquid error: wrong number of arguments (given 1, expected 2)</ref> and secondly, the absorption per unit length in the wide end T-DCF is substantially higher for geometric reasons. Indeed, rare earth ions are located preferably at the wide end of T-DCF (proportional to the square of the core diameter). This feature allows the use of preforms with relatively low [[dopant]] concentration ([[Absorption (electromagnetic radiation)|absorption]] in the core of only 300-400dB/m), which avoids the [[photodarkening]] effect. The high absorption makes it possible to create very short-amplifiers (a few tens of cm), which is important for amplification of ultrashort pulses.
'''''SBS and amplified spontaneous emission (ASE) suppression'''''
T-DCF owns embedded mechanisms of SBS and ASE suppression. It is well known from the literature that the diameter modulation leads to increasing of SBS threshold <ref>Liquid error: wrong number of arguments (given 1, expected 2)</ref>. This important feature is very useful for amplification of very narrow-band signals (MHz or kHz FWHM), for example, pulsed sources for LIDARs. ASE is suppressing at the T-DCF during propagation towards to a narrow end due to violation of the total internal reflection law. It allows to exploit a T-DCF for amplification of pulses with very low duty cycle (up to "on demand" mode).
'''''Immunity to the mode instability'''''
Earlier it was experimentally shown that spatial modulation of the core diameter along a fiber length leads to the increasing of the mode instability threshold <ref name=":2" />. This means that the T-DCF technique allows lasers and amplifiers to be built with higher output power in comparison to a regular [[Double-clad fiber|DCF]].
'''''High brightness magnification factor'''''
A [[laser]] or [[optical amplifier]] is a device which improves the brightness of a powerful pump source. Specifically, the low brightness light from a pump source is converted through the absorption process into output radiation with a longer wavelength and simultaneously with better brightness. The maximum launched pump power is determined by the clad diameter of [[Double-clad fiber|DCF]]. Since the diameter of a T-DCF's cladding is much higher (up to 1.6 mm <ref name=":2" />), it is possible to increase the brightness enhancement factor compared to regular DCFs. T-DCF lasers and amplifiers can also use low quality pump sources with very low [[brightness]], or [[M squared|M-Squared]] value which are unsuitable for pumping other fibers.
'''''Simplicity of production'''''
One of the significant advantages of T-DCF is the simplicity of production. The preform production for special high power fibers (microstructured rod type fibers, 3C or LCF fibers) involves complex technology and strict structural requirements. Conversely, T-DCF is made using standard fiber preforms. Simple production techniques of varying of the drawing speed during the pulling process leads to the fiber diameter changing along its length. T-DCF production is lhardly more complex than the production of a regular active fiber.
Thus, the simple production and special properties of T-DCF enables the realisation of a broad range of photonics devices with unique properties:
# · Ultrafast (ps and sub-ns) powerful (tens of watts) amplifiers;
# · SBS-free amplifiers for high coherent radiation;
# · Powerful actively Q-switched pulsed lasers;
# · Brightness converters: lasers or amplifiers pumped by inexpensive powerful pumps with low brightness (laser bar stuck, VECSELs, etc.).
January 18, 2020 at 03:41AM
