Two-stage optical fiber amplifier

Abstract

The present disclosure relates to an optical fiber amplifier comprising a first amplification stage comprising a first active fiber configured to amplify light received from a seed laser or a pre-amplifier stage; a second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage, wherein the second active fiber is configured for supporting only a single mode of light; and an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier, said undesired light being generated from amplified spontaneous emission (ASE) and having a center wavelength of about 1 µm; wherein the first and second active fibers are Er / Yb co-doped double-clad fibers, wherein the optical fiber amplifier is configured for amplifying light having a center wavelength in the range from about 1535 nm to about 1580 nm The present disclosure further relates to a laser system comprising the optical fiber amplifier.

Description

[0001] TWO-STAGE OPTICAL FIBER AMPLIFIER

[0002] Technical field

[0003] The present disclosure relates to an optical fiber amplifier. In particular, the present disclosure relates to a two-stage optical fiber amplifier. The present disclosure further relates to a laser system comprising a seed laser and said optical fiber amplifier.

[0004] Background

[0005] Optical fiber amplifiers have become critical components in a wide range of laser-based applications, from telecommunications and data transmission to medical devices and industrial lasers. They amplify optical signals without the need for optical-to-electrical conversion, providing high efficiency and reduced signal loss. Among various types of optical amplifiers, doped fiber amplifiers, such as erbium-doped fiber amplifiers (EDFAs), have gained prominence for their ability to amplify signals in specific wavelength ranges with high gain and low noise.

[0006] Despite their widespread use, existing optical fiber amplifiers face several technical challenges that can limit their performance and applicability in certain contexts. Two significant issues that arise with fiber amplifiers are relative intensity noise (RIN) and amplified spontaneous emission (ASE). RIN refers to fluctuations in the output intensity of the amplified signal. It can be caused by various factors, including fluctuations in the pump laser, nonlinear interactions within the fiber, and spontaneous emission from the doped regions of the fiber. High RIN can degrade signal quality, leading to errors in data transmission, reduced precision in measurement systems, and instability in laser-based applications. ASE is generally an undesired byproduct of the amplification process in doped fiber amplifiers. It occurs when spontaneous emission from the doped medium is further amplified, leading to a broadband background noise in addition to the desired signal. ASE can reduce the signal-to-noise ratio (SNR) and contribute to heating and damage in laser systems.

[0007] Given these challenges, there is a need for new optical fiber amplifier and laser system that can effectively address RIN and / or ASE to improve signal quality and amplifier stability. Therefore, there is a desire to develop and provide an optical fiber amplifier that overcomes these issues, and thereby offers a reliable solution for various applications where low noise and high signal quality are critical. Summary

[0008] The above-mentioned challenges are solved by providing a two-stage optical fiber amplifier comprising: a first amplification stage comprising a first active fiber configured to amplify light, such as light received from a seed laser or a pre-amplifier stage; and a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage, wherein the optical fiber amplifier further comprises an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier. As an example, said undesired light may be generated from amplified spontaneous emission (ASE) in the fiber amplifier, such as from spontaneous emission of the Ytterbium (Yb) ions in the first and / or second active fiber.

[0009] In accordance with some embodiments, the presently disclosed optical fiber amplifier comprises a first amplification stage comprising a first active fiber configured to amplify light received from a seed laser or a pre-amplifier stage; a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage, wherein the second active fiber is configured for supporting only a single mode of light; and an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier, said undesired light having a center wavelength of about 1 pm; wherein the first and second active fibers are co-doped double-clad fibers, and wherein the optical fiber amplifier is configured for amplifying continuous wave (CW) light.

[0010] In accordance with some embodiments, the presently disclosed optical fiber amplifier comprises a first amplification stage comprising a first active fiber configured to amplify light received from a seed laser or a pre-amplifier stage; a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage, wherein the second active fiber has a core diameter of between 6 pm and 12 pm and a cladding diameter of between 80 pm to 150 pm; and an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier, said undesired light having a center wavelength of about 1 pm; wherein the first and second active fibers are Erbium / Ytterbium (Er / Yb) co-doped double-clad fibers, wherein the optical fiber amplifier is configured for amplifying continuous wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm.

[0011] In accordance with some embodiments, the presently disclosed optical fiber amplifier comprises a first amplification stage comprising a first active fiber configured to amplify light received from a seed laser or a pre-amplifier stage; a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage, wherein the second active fiber has a core diameter of between 6 pm and 12 pm and a cladding diameter of between 80 pm to 150 pm, and wherein the second active fiber is configured for supporting only a single mode of light; and an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier, said undesired light having a center wavelength of about 1 pm and being generated from amplified spontaneous emission (ASE) in the fiber amplifier; wherein the first and second active fibers are polarization-maintaining (PM) fibers, and further wherein the first and second active fibers are Erbium / Ytterbium (Er / Yb) co-doped double-clad fibers, wherein the optical fiber amplifier is configured for amplifying continuous wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm.

[0012] In accordance with some embodiments, the presently disclosed optical fiber amplifier comprises a first amplification stage comprising a first active fiber configured to amplify light received from a seed laser or a pre-amplifier stage; a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage, wherein the second active fiber is configured for supporting only a single mode of light; and a wavelength-division multiplexing device (WDM) configured for at least partially coupling out undesired light from the optical fiber amplifier, said undesired light being generated from amplified spontaneous emission (ASE) in the fiber amplifier, such as in the second amplification stage; wherein the first and second active fibers are polarization-maintaining (PM) fibers, and further wherein the first and second active fibers are Erbium / Ytterbium (Er / Yb) co-doped double-clad fibers, wherein the optical fiber amplifier is configured for amplifying continuous wave (CW) light. The fiber amplifier may further comprise a coreless fiber for attenuating the undesired light generated from ASE, wherein the coreless fiber is in optical communication with the WDM and configured for receiving the at least partially coupled out undesired light from the WDM.

[0013] The present disclosure further relates to a laser system comprising: a seed laser configured for providing light; and the optical fiber amplifier disclosed herein. The seed laser may be configured for providing coherent, continuous wave (CW), light at a center wavelength in the range from about 1535 nm to about 1580 nm. The laser system may further comprise a pre-amplifier arranged downstream of the seed laser. The pre-amplifier may be configured for amplifying the light from the seed laser to a power level of between 1 mW and 100 mW. Any of the embodied optical fiber amplifiers disclosed herein may form part of the laser system disclosed herein.

[0014] In accordance with some embodiments the laser system comprises a seed laser configured for providing coherent, continuous wave (CW), light at a center wavelength in the range from about 1535 nm to about 1580 nm, wherein the light from the seed laser has certain relative intensity noise (RIN) characteristic; a pre-amplifier arranged downstream of the seed laser, said pre-amplifier configured for amplifying the light from the seed laser to a power level of between 10 mW and 100 mW; and an optical fiber amplifier comprising a first amplification stage comprising a first active fiber configured to amplify light received from the pre-amplifier, wherein the optical fiber amplifier further comprises a second amplification stage comprising a second active fiber configured for further amplifying light from the first amplification stage, wherein the second active fiber is configured for being single-moded, and wherein the optical fiber amplifier is configured for amplifying light from the pre-amplifier to a power level of between 10 W and 20 W at the output of the fiber amplifier; and wherein the pre-amplifier and / or the optical fiber amplifier is configured for maintaining or improving the RIN characteristic of the seed laser.

[0015] The present disclosure further relates to method comprising the steps of: providing single-frequency laser light having a center wavelength; amplifying the laser light to a certain first optical power level in a first amplification stage comprising a first active fiber, whereby amplified first laser light is generated; amplifying the first laser light to a certain second optical power level, higher than the first optical power level, in a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for supporting only a single mode of light, whereby amplified second laser light is generated; coupling out at least a part of undesired light generated from amplified spontaneous emission (ASE) in the second amplification stage, such as in the second active fiber, wherein the undesired light has a wavelength in the Yb-band, such as a wavelength of about 1 pm, and wherein the undesired light is coupled out using a wavelengthdivision multiplexing device (WDM) that is wavelength selective and configured to selectively couple out light at a predefined wavelength, such as at about 1 pm; optionally directing the undesired light to a coreless fiber optically connected to the WDM, wherein the coreless fiber is suitable for attenuating the undesired light. The first and / or second active fibers may be polarization-maintaining (PM) fibers, and further may be Erbium / Ytterbium (Er / Yb) co-doped double-clad fibers.

[0016] The presently disclosed optical fiber amplifier and laser system is now further described in the following detailed description and accompanying drawings. Brief description of the drawings

[0017] Fig. 1 shows an embodiment of a first amplification stage that may form part of the presently disclosed optical fiber amplifier.

[0018] Fig. 2 shows an embodiment of a second amplification stage that may form of the presently disclosed optical fiber amplifier.

[0019] Fig. 3 shows an embodiment of a two-stage optical fiber amplifier according to the present disclosure.

[0020] Fig. 4 shows an embodiment of a laser system according to the present disclosure.

[0021] Detailed description

[0022] A better understanding of the features and advantages of the present disclosure will be obtained by reference to the following detailed description and the accompanying drawings.

[0023] Seed laser

[0024] The laser system may comprise a seed laser for generating laser light, such as continuous-wave (CW) light or pulsed light. The seed laser may be a solid-state laser or a fiber-based laser. The seed laser may comprise a gain medium, such as a doped fiber, said gain medium being doped with a rare-earth element such as erbium (Er), ytterbium (Yb), thulium (Tm), or holmium (Ho). In preferred embodiments, the seed laser is a fiber-based laser comprising an Er-doped fiber as the gain medium.

[0025] The seed laser may be configured for single-frequency operation. In particular, the seed laser may be configured to emit laser light having a center wavelength in the range from about 1535 nm to about 1580 nm, such as from about 1545 nm to about 1565 nm. The center wavelength may be understood as the center of the full width at half maximum (FWHM) of the laser light. Additionally, or alternatively, the peak wavelength of the emitted laser light may lie in the forementioned range. The peak wavelength may be understood as the wavelength at which the spectral distribution of the emitted laser light reaches its largest value. The seed laser may be further configured to, at least slightly, tune the wavelength in these ranges, such that the output wavelength can be adjusted. Alternatively, in some embodiments, the seed laser is configured to emit laser light having a wavelength, such as a center or peak wavelength, from about 950 nm to about 1050 nm, such as about 1 pm. The seed laser may be configured for providing coherent light and / or continuous-wave (CW) light. The seed laser may be configured for providing laser light, such as CW light, having a narrow linewidth. In some embodiments, the seed laser is configured to provide laser light having a linewidth of less than 1 kHz, such as less than 0.5 kHz, such as less than 0.1 kHz. In some embodiments, the laser light from the seed laser has a beam quality, M2, of less than 1.10, such as less than 1.05.

[0026] The doped fiber in the seed laser may be optically pumped using a suitable optical pump source, such as a laser diode or a semiconductor laser, providing energy (via pump light) to excite the rare-earth ions within the core of the fiber. Accordingly, the seed laser may comprise a pump source, such as a laser diode, configured for providing energy to excite the rare-earth ions within the doped fiber. The rare-earth ions may be selected from the group of erbium (Er), ytterbium (Yb), thulium (Tm), or holmium (Ho). The optical pump source is preferably chosen to substantially match the absorption band of the rare-earth ions for efficient energy transfer. However, there may be a difference between the wavelength of the pump light and the wavelength of the laser emission from the laser cavity.

[0027] The seed laser may further comprise an optical filter, such as an optical bandpass filter (BPF), configured for substantially blocking certain predefined wavelengths and transmitting other wavelengths through the filter. Preferably, the optical filter is configured for transmitting pump light from the optical pump source and further configured for blocking laser light from the cavity of the seed laser in the direction from the cavity to the optical pump source.

[0028] The seed laser may further comprise one or more gratings, such as one or more fiber- Bragg gratings, said grating(s) defining a laser cavity. The grating(s) may be written into the aforementioned doped fiber using ultraviolet (UV) light. The laser cavity may have a predefined and well-defined length. As an example, the doped fiber including the grating(s) may constitute a distributed-feedback laser (DFB). Thus, the distributed-feedback (DFB) fiber-based laser may comprise the aforementioned rare-earth-doped fiber serving as a gain medium. It may further comprise a fiber-Bragg grating (FBG) preferably configured as both a feedback mechanism and a wavelength-selective element. The FBG, inscribed within the doped fiber, is preferably configured for providing distributed feedback, thereby ensuring single-frequency operation. Additionally, the optical pump source may be configured for exciting the gain medium to initiate stimulated emission. The laser cavity design may integrate these components to achieve stable, single-mode emission having a narrow linewidth. This configuration enables precise control over the laser output, making the disclosed seed laser suitable for applications requiring high spectral purity and coherence, such as telecommunications, sensing, and spectroscopy.

[0029] The seed laser may further comprise an isolator arranged downstream of the laser cavity. The isolator may be configured for substantially blocking light reflected from the output and travelling backwards towards the doped fiber. The seed laser may further comprise an optical splitter for splitting the light output from the laser cavity into two or more optical paths. A first optical path may be utilized for monitoring purposes, i.e., it may include a monitor diode for monitoring laser light from the cavity. The monitored signal may be utilized to adjust one or more parameters of the optical pump source, e.g., in order to reduce relative intensity noise (RIN) of the laser. A second optical path may be utilized as the output of the laser system.

[0030] The seed laser may be configured for providing laser light that is substantially singlefrequency light, e.g., light having a narrow linewidth at a center wavelength selected in the range from about 1535 nm to about 1580 nm. The center wavelength may be understood as the center of the full width at half maximum (FWHM) of the laser light. The linewidth of the laser light emitted by the seed laser may be below 100 Hz, such as below 10 Hz, such as below 1 Hz. Thus, the seed laser may be configured for providing laser light having a sub-Hz linewidth. The seed laser may be configured for providing laser light having a low phase noise, such as a maximum phase noise of -90 dB ((RadA / Hz) / m) at 10 Hz, and / or a maximum phase noise of -110 dB ((RadA / Hz) / m) at 100 Hz, and / or a maximum phase noise of -130 dB((RadA / Hz) / m) at 20 kHz. Furthermore, the laser light provided by the seed laser may have a low relative intensity noise (RIN) level, such as a RIN level of below -100 dBc / Hz at a RIN peak of about 0.7 MHz, and / or a RIN level below -135 dBc / Hz at 10 MHz. The RIN may be measured relative to the carrier signal. The seed laser may have a RIN peak at about 0.7 MHz. In some embodiments, the optical fiber amplifier disclosed herein, is configured for largely maintaining said properties of the seed laser, e.g., in terms of phase noise and / or relative intensity noise. In some embodiments, the seed laser is configured to output laser light having a phase noise with an absolute value of less than 135 dB((RadA / Hz) / m) at frequencies at or below 20 kHz.

[0031] Pre-amplifier

[0032] The laser system may comprise a pre-amplifier. The pre-amplifier may comprise or constitute an optical amplifier for amplifying an optical signal. The pre-amplifier may be arranged downstream of the seed laser, such as arranged between the seed laser and the first amplification stage. The pre-amplifier may comprise a semiconductor optical amplifier (SOA) or a single-clad optical fiber. An advantage of utilizing a SOA is that it generally has a very fast response time, e.g., compared to an EDFA, whereby undesired relative intensity noise (RIN) is suppressed up to higher frequencies, such as up to frequencies in the MHz range. Another advantage of a SOA is that it is more easily saturated, which has the desired consequence that RIN is suppressed or lowered up to higher frequencies. In some cases, the RIN is suppressed with up to 15 dB, or even up to 20 dB, when using a semiconductor optical amplifier in the pre-amplifier. In case of a single-clad optical fiber, said fiber may be configured for being optically pumped by pump light from one or more pump sources, such as one or more laser diodes. The pump light may be co-propagating with the signal light to be amplified. Thus, the singleclad fiber may be core-pumped. In some embodiments, the pre-amplifier is configured for providing an optical power level from about 1 mW to about 100 mW, such as from about 20 mW to about 80 mW, such as from about 40 mW to about 60 mW, at the output. Thus, the optical signal, such as the 1.5 pm signal from the seed laser, may have a power level in the aforementioned ranges when measured at the output of the pre-amplifier.

[0033] Optical fiber amplifier

[0034] The optical fiber amplifier disclosed herein may comprise one or more amplification stages, such as at least two amplification stages: a first amplification stage and a second amplification stage. Accordingly, the optical fiber amplifier may be a two-stage optical fiber amplifier. The optical fiber amplifier may comprise or constitute a two-stage Erbium / Ytterbium co-doped fiber-based master oscillator power amplifier (MOPA) architecture. The optical fiber amplifier may be configured as a polarization-maintaining (PM) amplifier. The amplification stages may also be referred to as power amplification stages. The optical fiber amplifier may further comprise a pre-amplifier as described herein. The pre-amplifier may be arranged upstream of first and second amplification stages, i.e., the power amplification stages. The optical fiber amplifier may form part of the laser system described herein. The first and second amplification stages are further described in the following paragraphs.

[0035] First amplification stage

[0036] The optical fiber amplifier may comprise a first amplification stage comprising a first active fiber configured to amplify light received from the seed laser or from the pre-amplifier. Thus, the first amplification stage may be arranged downstream of said seed laser or preamplifier. The optical signal input to the first amplification stage may have a power level of about 1 mW to about 100 mW, such as about 20 mW to about 80 mW, such as about 40 mW to about 60 mW.

[0037] The first active fiber may be a co-doped fiber, such as an Erbium / Ytterbium (Er / Yb) codoped fiber. Furthermore, the first active fiber may be a double-clad fiber, such as a co-doped Er / Yb double-clad fiber. In such a fiber, the core of the fiber is doped with active ions, such as Erbium and Ytterbium ions, whereas the cladding is configured for supporting pump light for pumping said active ions. An advantage of utilizing a double-clad fiber is that it is better at handling high power levels, since more pump light can be sent into the double-clad fiber, compared to e.g., a single-clad fiber. During operation of the fiber amplifier or laser system, the double-clad fiber may be cladding-pumped by one or more pump sources, such as one or more laser diodes. An advantage of cladding-pumping the first active fiber is that it allows higher power levels. The first active fiber may be a polarization-maintaining (PM) fiber. The first active fiber may have a core diameter of between 8 pm and 15 pm, such as between 10 pm and 14 pm, such as about 12 pm. An advantage of utilizing a first active fiber having a core diameter of between 10 pm and 14 pm is that undesired stimulated Brillouin scattering (SBS) is minimized or ideally entirely avoided with such core diameters. For smaller core diameters, such as for core diameters below 8 pm, SBS can become more pronounced and thereby cause problems for the optical fiber amplifier. The first active fiber may be considered ‘few-moded’, i.e., it may be configured for supporting more than one mode. Alternatively, the first active fiber is single-moded, such that it is configured for supporting only a single mode of light. The first active fiber may have a cladding diameter of between 100 pm and 150 pm, such as between 120 pm and 140 pm, such as about 130 pm.

[0038] The first active fiber may be configured for receiving pump light from one or more first pump sources, such as one or more first laser diodes. The pump light may be counterpropagating pump light. The pump sources may be configured for providing pump light at a wavelength of between 905 nm and 925 nm, such as at about 915 nm. In some embodiments, the first amplification stage comprises two first pump sources, such as two laser diodes, wherein the two pump sources are configured to be operated concurrently. The pump sources may be power-controlled, such that the pump power is automatically adjusted in case the output power of the laser system drops. In some embodiments, the optical fiber amplifier is configured for operating the two pump sources at a power level less than their maximum specification, such as less than 75 % of the maximum specification, such as less than half of the maximum specification. This has the advantage of increasing the lifetime of the optical fiber amplifier. It has the additional benefit that less relative intensity noise (RIN) is introduced into the amplifier from the pump sources, such as from the laser diodes. The pump sources may be optically connected to the first active fiber via a first coupler or combiner, such as a first wavelength-division multiplexing (WDM) device. Accordingly, the first active fiber may be backward-pumped and / or cladding-pumped during operation of the optical fiber amplifier. Thus, the first amplification stage may be backward-pumped, such that the pump light propagates in a direction opposite to the signal light from the seed laser, during operation of the fiber amplifier. An advantage of utilizing backward-pumping is that it is more powerefficient, compared to e.g. forward-pumping.

[0039] The first amplification stage may be configured for amplifying continuous-wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm. The CW light may be substantially single-frequency light, such as laser light having a narrow spectral width. Furthermore, the CW light from the seed laser or pre-amplifier may be coherent light, such as light having a long coherence length. Advantageously, the single-frequency light and / or CW light from the seed laser has a low phase noise and / or relative intensity noise. The first amplification stage may further comprise an isolator arranged downstream of the first coupler, such as downstream of the first wavelength-division multiplexing (WDM) device. The isolator may be arranged between the first and second amplification stages.

[0040] Second amplification stage

[0041] The optical fiber amplifier may further comprise a second amplification stage. The second amplification stage may comprise a second active fiber configured to amplify light received from the first amplification stage. The second active fiber may be an Erbium / Ytterbium (Er / Yb) co-doped double-clad fiber. The double-clad fiber may be cladding- pumped by one or more pump sources, such as one or more laser diodes, during operation of the fiber amplifier and / or laser system. The pump sources may be configured for optically pumping the Yb ions in the Er / Yb double-clad fiber. Furthermore, the second active fiber may be a polarization-maintaining (PM) fiber. The second active fiber may have a core diameter of between 6 pm and 12 pm, such as between 8 pm and 11 pm, such as about 10 pm. The second active fiber may have a cladding diameter of between 80 pm and 150 pm, such as between 110 pm and 140 pm, such as about 125 pm. The second active fiber is advantageously configured for supporting only a single mode of light. Thus, the second active fiber may be a single-mode fiber. In some embodiments, both the first and second active fibers are configured to be single-moded.

[0042] The first active fiber may have a length of between 2 m and 5 m, such as between 2.5 m and 4.5 m, such as between 3 m and 4 m. The second active fiber may have a length of between 3 m and 6 m, such as between 3.5 m and 5.5 m, such as between 4 m and 5 m. The advantage of a length of the first and / or second active fibers selected in one of these ranges is that the signal-to-ASE noise is lowered. In some cases, the signal-to-ASE noise is less than 5 %, such as less than 2 %, such as less than 1 %. This may be measured at the output of the two-stage optical fiber amplifier. Generally, the Er / Yb co-doped fiber allows for a relatively short length of the first and second active fibers, since Yb ions are more efficient at a given pump light intensity.

[0043] The second active fiber may be configured for receiving pump light from one or more second pump sources, such as one or more second laser diodes. The pump sources may be arranged to provide counter-propagating pump light with respect to the signal light. The pump sources may be configured for providing pump light at a wavelength of between 905 nm and 925 nm. The pump sources may be optically connected to the second active fiber via a second coupler or combiner, such as a second wavelength-division multiplexing (WDM) device. Accordingly, the second amplification stage may be backward-pumped during operation of the optical fiber amplifier. In some embodiments, both the first and second active fibers are counter-pumped, i.e., backward-pumped, such that the signal light and pump light propagate in opposite directions.

[0044] The second amplification stage may be configured for amplifying continuous wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm. The CW light may be substantially single-frequency light having a wavelength, such as a center or peak wavelength, selected in the aforementioned range. Further, the CW light from the first amplification stage may be coherent light, such as light having a long coherence length. The second amplification stage may further comprise an isolator arranged downstream of the second WDM. The optical fiber amplifier may comprise an isolator arranged between the first and second amplification stages.

[0045] The second amplification stage may further comprise an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier. The undesired light may have a center wavelength in the range from about 0.9 pm to about 1.1 pm, such as at about 1 pm, and may be generated from amplified spontaneous emission (ASE) in the fiber amplifier. Such undesired light may also be referred to as ‘ASE light’. In some cases, the undesired light has a wavelength in the Yb-band, such as a wavelength in the range from about 960 nm to about 1060 nm. The optical device may comprise or constitute a coupler, such as a wavelength-division multiplexing (WDM) device, configured for at least partially coupling out undesired light resulting from ASE in the second active fiber. The optical device may be a fused coupler, such as a fused WDM. As an alternative to using a WDM for the purpose of coupling out ASE, a long-period grating (LPG) may be utilized. As an alternative to arranging the LPG between the first and second amplification stages, the LPG may be directly written into the second active fiber, e.g., using ultraviolet (UV) light. Alternatively, the LPG may be defined in another fiber arranged upstream of the second active fiber and in optical communication with said second active fiber. The ASE light may be directed to a coreless fiber, or another type of beam dump. The coreless fiber is configured for attenuating the undesired light generated from ASE, and preferably in optical communication with the WDM and configured for receiving the at least partially coupled out undesired light from the WDM. A technical effect of the coreless fiber is that it ensures a high return loss for the undesired light, such as the 1 pm ASE light. The optical device, such as the WDM or the LPG, may be configured for introducing a loss in an unwanted laser cavity, said cavity amplifying the undesired 1 pm ‘ASE light’. The loss is preferably specific for the undesired 1 pm light. The unwanted cavity may arise between two or more components in the amplifier, e.g., such that the cavity is defined by two or more reflecting components, e.g., isolators, pumps, splices, etc. Accordingly, the optical fiber amplifier may be configured for suppressing the lasing effect of the 1 pm ASE light. The WDM should preferably be configured to have a low loss for the wavelength of the seed light, e.g., light having a center wavelength in the range of from about 1535 nm to about 1580 nm, such as about 1.5 pm. In preferred embodiments, the optical device is not an ASE filter or a bandpass filter. A disadvantage of an ASE filter, e.g., arranged at the output of the amplifier, is that such a filter is typically reflective, thereby introducing reflections of light, such as light at about 1 pm. Another disadvantage of a filter-based solution is that it may introduce losses when the optical power is high.

[0046] In some embodiments, the second amplification stage further comprises a second optical device, such as a second wavelength-division multiplexing (WDM) device. Preferably, the second WDM is configured for at least partially coupling out undesired light resulting from ASE in the second active fiber. In particular, the second WDM may be arranged downstream of the second active fiber, such as at the output of the second amplification stage. The second WDM may advantageously be configured for at least partially coupling out forward-going ASE light from the second amplification stage. The ASE light that is coupled out may be directed into a coreless fiber, as explained in other embodiments herein. Thus, the optical fiber amplifier may comprise two WDMs; a first WDM for coupling out backward-going ASE and a second WDM for coupling out forward-going ASE. The first and / or second WDM may be fused WDMs.

[0047] Detailed description of the drawings

[0048] The presently disclosed fiber amplifier and laser system is described in more detail in relation to the following exemplary and non-limiting embodiments provided in connection with the figures.

[0049] Fig. 1 shows an embodiment of a first amplification stage (100) forming part of the presently disclosed optical fiber amplifier. In this embodiment, the first amplification stage (100) comprises a first active fiber (104) configured to amplify light received from a seed laser or a pre-amplifier (not shown). The first active fiber (104) may be an Erbium / Ytterbium (Er / Yb) co-doped double-clad fiber. An advantage of utilizing an Er / Yb co-doped active fiber is that the Ytterbium ions are generally better at absorbing pump light. Once the Yb ions are in an excited state they are able to provide energy to the Erbium ions, e.g., via fluorescence or other ion interactions. Since the Yb ions are more efficient at a given pump light intensity, it allows for a shorter interaction length, i.e., the length of the first active fiber can be made shorter when choosing an Er / Yb co-doped fiber, compared to e.g. an Yb-doped or Er-doped fiber.

[0050] The first active fiber (104) may be a polarization-maintaining (PM) fiber. The first active fiber may have a core diameter of between 8 pm and 15 pm, such as about 12 pm, and a cladding diameter of between 100 pm and 150 pm, such as about 130 pm. The first active fiber (104) may be configured for receiving counter-propagating pump light from one or more first pump sources (110), such as one or more first laser diodes. The pump sources may be configured for providing pump light at a wavelength of between 905 nm and 925 nm. The pump sources (110) may be configured for optically pumping the Yb ions in the first active fiber (104). The pump sources (110) may be optically connected to the first active fiber (104) via a first coupler, such as a first wavelength-division multiplexing (WDM) device (108). Accordingly, the first amplification stage (100) may be backward-pumped during operation of the optical fiber amplifier.

[0051] The first amplification stage (100) may further comprise one or more splices (102, 106), such as fusion splices. As an example, the first amplification stage (100) may comprise at least a first splice (102) and a second splice (106). In this example, the first active fiber (104) is a double-clad fiber, whereas the fiber to the left of the first splice (102) is a single-clad fiber. Thus, during operation, when the pump light propagates from right to left (backward-pumped), the light experiences a transition from a double-clad fiber to a single-clad fiber. This transition causes the light to spread out at the first splice (102), which can cause said splice to heat up. To mitigate this problem, the first splice (102) may be provided with a tape, such that the fiber and first splice is taped down to a heat sink, such as a base plate of the fiber amplifier. The heat sink may be configured for transporting heat away from the first splice (102). In some embodiments, the tape used is a graphite tape, which improves the thermal conduction to the base plate. This solution improves the lifetime of the optical amplifier, since otherwise there would be a risk that the first splice (102) would be burned from the heat.

[0052] The first amplification stage (100) may be configured for amplifying continuous wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm. The CW light may be substantially single-frequency light having a wavelength, such as a center or peak wavelength, selected in the aforementioned range. Further, the CW light from the seed laser or pre-amplifier may be coherent light, such as light having a long coherence length. The first amplification stage (100) may further comprise an isolator (112) arranged downstream of the first coupler I first WDM (108). The isolator (112) may be provided to mitigate, or entirely avoid, backward reflected light from the second amplification stage into the first active fiber (104).

[0053] Fig. 2 shows an embodiment of a second amplification stage (200) forming part of the presently disclosed optical fiber amplifier. In this embodiment, the second amplification stage (200) comprises a second active fiber (206) configured to amplify light received from the first amplification stage. The second active fiber (206) may be an Erbium / Ytterbium (Er / Yb) co- doped double-clad fiber. Furthermore, the second active fiber (206) may be a polarizationmaintaining (PM) fiber. The second active fiber may have a core diameter of between 6 pm and 12 pm, such as about 10 pm, and a cladding diameter of between 80 pm and 150 pm, such as about 125 pm. The second active fiber (206) is advantageously configured for supporting only a single mode of light. Additionally, the second active fiber (206) may be configured for receiving counter-propagating pump light from one or more second pump sources (212), such as one or more second laser diodes. The pump sources (212) may be configured for providing pump light at a wavelength of between 905 nm and 925 nm. The pump sources (212) may be configured for optically pumping the Yb ions in the second active fiber (206). The pump sources (212) may be optically connected to the second active fiber (206) via a second coupler (210), such as a second wavelength-division multiplexing (WDM) device (210). Accordingly, the second amplification stage (200) may be backward-pumped during operation of the optical fiber amplifier.

[0054] The second amplification stage (200) may further comprise one or more splices (204, 208), such as fusion splices. As an example, the second amplification stage (200) may comprise at least a first splice (204) and a second splice (208). In this example, the first splice (204) of the second amplification stage may be largely similar to the first splice (102) of the first amplification stage. Thus, the splice (204) may be protected by a piece of tape, such as graphite tape such that the fiber and the splice (204) are taped down to a heat sink, such as a base plate of the fiber amplifier. The graphite tape improves the thermal conduction to the base plate. The second splice (208) may constitute a joint between two double-clad fibers. Generally, the second stage (200) is associated with higher power levels than the first stage (100). In particular, the power level may be quite high at the second splice (208) since the amplified signal light from the first stage meets the pump light from the second stage at this transition. Consequently, a lot of heat may be generated at this splice (208). In order to protect the second splice (208), it may be re-coated, such that a new coating is applied to the fiber after the splice. Furthermore, the fiber at the second splice may be taped down to a base-plate of the amplifier. The base-plate may aid to conduct heat away from the second splice.

[0055] The second amplification stage (200) may be configured for amplifying continuous wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm. The CW light may be substantially single-frequency light. Further, the CW light from the first amplification stage may be coherent light, such as light having a long coherence length. The second amplification stage (200) may further comprise an isolator (214) arranged downstream of a second coupler, such as a second WDM (210). The isolator (214) may be provided to mitigate, or entirely avoid, backward reflected light into the fiber amplifier. The second coupler or second WDM (210) may be configured to receive a double-clad fiber at the input and a single-clad fiber at the output.

[0056] The second amplification stage (200) may further comprise an optical device (202), such as a wavelength-division multiplexing (WDM) device or a long-period grating (LPG), for coupling out undesired light resulting from amplified spontaneous emission (ASE) in the second active fiber (206). The undesired light may also be referred to herein as ‘ASE light’, ‘1 pm ASE’, or Yb-ASE. The ASE light may be directed to a coreless fiber (216), or another type of beam dump. The coreless fiber may be suitable for, or configured for, attenuating the ASE light. The coreless fiber may have a predefined length from about 10 cm to about 100 cm. In this embodiment, the ASE light may have a center wavelength of about 1 pm. The ASE light may origin from spontaneous emission from the Yb-ions in the active fiber (206). In order to mitigate the effect of amplifying the ASE light, the optical device (202) may be configured to introduce a loss in the unwanted cavity; thus, the optical device may couple out at least a part of the ASE light, such that the unwanted lasing of about 1 pm is suppressed. As an alternative to using a WDM for the purpose of coupling out ASE, a long-period grating (LPG) may be utilized.

[0057] Fig. 3 shows an embodiment of a two-stage optical fiber amplifier (300) according to the present disclosure. In this embodiment, the optical fiber amplifier comprises a first amplification stage comprising a first active fiber (304) configured to amplify light received from a seed laser or a pre-amplifier (not shown); and a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber (314) configured for further amplifying light received from the first amplification stage. The first and second active fibers (304, 314) may be polarizationmaintaining (PM) fibers, and further both active fibers may be Erbium / Ytterbium (Er / Yb) codoped double-clad fibers. In particular, the second active fiber (314) may be configured for supporting only a single mode of light. To achieve this, the second active fiber (314) may have a core diameter of between 8 pm and 12 pm, such as a core diameter of between 9 pm and 11 pm, and a cladding diameter of between 100 pm to 150 pm, such as a cladding diameter of between 115 pm to 135 pm.

[0058] The two-stage optical fiber amplifier (300) may further comprise an optical device (308), such as a wavelength-division multiplexing device (WDM) (308) configured for suppressing and / or coupling out amplified spontaneous emission (ASE) light from the second active fiber (314). Thus, the optical device (308) may be configured for coupling out ASE light, such as light having a center wavelength of about 1 pm. The center or peak wavelength of the ASE light may lie in the range from about 0.9 pm to about 1.1 pm, such as in the range from about 0.96 m to about 1.06 pm. Specifically, the optical device (308) may be configured to couple out ASE light at a predefined wavelength, such as at about 1 pm. In this example, the optical device (308) is configured to couple out backward-propagating ASE light from the second amplification stage. The optical device (308) may be configured for directing the ASE light into a coreless fiber (310), or another type of beam dump. The coreless fiber (310) may be configured to attenuate the ASE light. The coreless fiber (310) may be coiled to a certain predefined coil diameter, such as a coil diameter of between 1 cm and 10 cm. An advantage of such a tight coil diameter is that it further increases the attenuation of the undesired ASE light that has been coupled out. A further technical effect hereof is that it minimizes the risk of any reflections occurring; in particular, it is desired to minimize or avoid reflections re-entering the second amplification stage. In this embodiment, the optical device (308) is arranged between the first and second amplification stages. As an alternative arrangement, the optical device (308) may be arranged downstream of the second active fiber (314), such that it is arranged closer to the second pump sources (not shown). As an alternative to using a WDM for the purpose of coupling out ASE, a long-period grating (LPG) may be utilized. Thus, the optical device (308) may be embodied as a WDM or an LPG, or another suitable coupling device. In preferred embodiments, the optical device (308) is a fused WDM. The optical fiber amplifier (300) may be configured for amplifying continuous-wave (CW) light having a center or peak wavelength in the range from about 1535 nm to about 1580 nm, such as wavelengths from about 1545 nm to about 1565 nm. The optical fiber amplifier (300) may be configured for amplifying light to an average optical power in the range of 10 W to 20 W, such as 12 W to 18 W, at the output of the optical fiber amplifier. The optical fiber amplifier (300) may further comprise a pre-amplifier (not shown) arranged before the first amplification stage. The preamplifier may comprise and / or constitute a semiconductor optical amplifier (SOA) or a singleclad optical fiber. The embodied optical amplifier (300) of fig. 3 may comprise any one or more of the splices mentioned in connection with the embodiments of figs. 1-2.

[0059] Fig. 4 shows an embodiment of a laser system (400) according to the present disclosure. The laser system comprises a seed laser (402) for providing continuous-wave (CW) light. In this embodiment, the seed laser (402) is configured for providing CW light at a wavelength from about 1545 nm to about 1565 nm. The light is substantially single-frequency light having a narrow linewidth. The seed laser (402) may be configured to tune the output laser light in the aforementioned range. The laser system (400) further comprises a pre-amplifier (404) arranged downstream of the seed laser (402). The pre-amplifier is configured for amplifying light from the seed laser, such that the amplified light at the output of the pre-amplifier has a power level from about 40 mW to about 300 mW, such as from about 50 mW to about 100 mW. The laser system further comprises the two-stage optical fiber amplifier disclosed herein, said fiber amplifier comprising a first amplification stage (406) and a second amplification stage (410) arranged in succession, embodied here with an optical device (408) arranged in between the two stages (406, 410). The optical device (408) is configured for coupling out undesired ASE light. As an example, the optical device (408) may be embodied as a wavelength-division multiplexing device (WDM) or a long-period grating (LPG). The power level at the output of the second amplification stage (410) may be from about 1 W to about 15 W, such as from about 10 W to about 15 W Accordingly, the laser system may be configured to provide CW light at an average power at these power levels. As mentioned in relation to other embodiments described herein, both amplification stages (406, 410) may comprise Er / Yb co-doped double-clad fibers as the gain fibers. Furthermore, both gain fibers may be polarization-maintaining fibers (PM fibers). Advantageously, the active fiber of the second amplification stage may be configured for supporting only a single mode of light, i.e., such that the second active fiber is single-moded. Additionally, the first active fiber may be single-moded. The optical fiber amplifier (406, 408, 410) may comprise any one or more of the features described herein, such as in connection with other embodiments. In particular, the first amplification stage (406) may be embodied as described in connection with fig. 1 and the second amplification stage (410) may be embodied as described in connection with fig. 2, or elsewhere herein.

[0060] Further details of the disclosure

[0061] 1 . A two-stage optical fiber amplifier comprising:

[0062] - a first amplification stage comprising a first active fiber configured to amplify light received from a seed laser or a pre-amplifier stage; and

[0063] - a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage.

[0064] 2. The optical fiber amplifier according to item 1 , wherein the second active fiber has a core diameter of between 6 pm and 12 pm.

[0065] 3. The optical fiber amplifier according to any of the preceding items, wherein the second active fiber has a cladding diameter of between 80 pm to 150 pm.

[0066] 4. The optical fiber amplifier according to any of the preceding items, wherein the second active fiber is configured for supporting only a single mode of light. 5. The optical fiber amplifier according to any of the preceding items, wherein the first and / or second active fibers are polarization-maintaining (PM) fibers.

[0067] 6. The optical fiber amplifier according to any of the preceding items, wherein the first and / or second active fibers are Erbium / Ytterbium (Er / Yb) co-doped double-clad fibers.

[0068] 7. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier is configured for amplifying continuous wave (CW) light.

[0069] 8. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier is configured for amplifying light having a center wavelength in the range from about 1535 nm to about 1580 nm.

[0070] 9. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier is configured for amplifying continuous wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm.

[0071] 10. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier is configured for amplifying light to an average optical power in the range of 5 W to 25 W

[0072] 11. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier is configured for amplifying light to an average optical power in the range of 7.5 W to 25 W

[0073] 12. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier is configured for amplifying light to an average optical power of more than 10 W

[0074] 13. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier is configured for amplifying light to an average optical power of at least 15 W 14. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier further comprises an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier.

[0075] 15. The optical fiber amplifier according to item 14, wherein the optical device is configured to introduce a wavelength-dependent loss in one or more unwanted cavities formed in any of the first and / or second amplification stages.

[0076] 16. The optical fiber amplifier according to any of the items 14-15, wherein the optical device is not an optical filter, such as an ASE filter or a bandpass filter.

[0077] 17. The optical fiber amplifier according to any of the items 14-15, wherein the undesired light has a center wavelength of about 1 pm.

[0078] 18. The optical fiber amplifier according to any of the items 14-17, wherein the undesired light has a center wavelength in the Ytterbium (Yb)-band.

[0079] 19. The optical fiber amplifier according to any of the items 14-18, wherein the undesired light is generated from amplified spontaneous emission (ASE) in the fiber amplifier.

[0080] 20. The optical fiber amplifier according to any of the items 14-19, wherein the optical device is arranged between the first and second amplification stages.

[0081] 21. The optical fiber amplifier according to any of the items 14-20, wherein the optical device is arranged downstream of the second active fiber.

[0082] 22. The optical fiber amplifier according to any of the items 14-21 , wherein the optical device forms part of the second active fiber.

[0083] 23. The optical fiber amplifier according to any of the items 14-22, wherein the optical device is selected from the group of wavelength-division multiplexing devices (WDM) or long-period gratings (LPG).

[0084] 24. The optical fiber amplifier according to any of the preceding items, wherein the optical device is a wavelength-division multiplexing device (WDM). 25. The optical fiber amplifier according to any of the preceding items, wherein the optical device is a fused wavelength-division multiplexing device (WDM) comprising two or more optical fiber fused together.

[0085] 26. The optical fiber amplifier according to any of items 24-25, wherein the WDM is configured to separate a predefined wavelength from the second amplification stage, such as a wavelength in the range from about 0.9 pm to about 1.1 pm.

[0086] 27. The optical fiber amplifier according to any of items 24-26, wherein the WDM is configured to ensure a lossy cavity at a predefined wavelength such as at about 1 pm.

[0087] 28. The optical fiber amplifier according to any of the items 24-27, wherein the WDM is configured to couple out ASE light at a predefined wavelength, such as at about 1 pm.

[0088] 29. The optical fiber amplifier according to any of the items 27-28, wherein the predefined wavelength corresponds to a wavelength at which undesired amplified spontaneous emission is generated within the optical fiber amplifier, such as within the second amplification stage.

[0089] 30. The optical fiber amplifier according to any of the items 24-29, wherein the WDM is configured to couple out backward-propagating ASE light from the second amplification stage.

[0090] 31. The optical fiber amplifier according to any of the items 24-30, wherein the WDM is configured to direct the ASE light to a coreless fiber connected to the WDM

[0091] 32. The optical fiber amplifier according to item 31 , wherein the coreless fiber is suitable for, or configured for, attenuating the ASE light.

[0092] 33. The optical fiber amplifier according to any of the items 31-32, wherein the coreless fiber is configured to provide attenuation of the ASE light, thereby preventing, or minimizing, residual reflections from re-entering the optical fiber amplifier. 34. The optical fiber amplifier according to any of the items 31-33, wherein the coreless fiber has a predefined length from about 5 cm to about 300 cm, such as from about 10 cm to about 200 cm, such as from about 25 cm to about 100 cm.

[0093] 35. The optical fiber amplifier according to any of the items 31-34, wherein the coreless fiber is coiled to a predefined coil diameter between 1 cm and 50 cm, such as between 1 cm and 25 cm, such as between 1 cm and 10 cm.

[0094] 36. The optical fiber amplifier according to any of the items 24-35, wherein the WDM is arranged in close proximity to the second active fiber, such as at a proximal end of the second active fiber.

[0095] 37. The optical fiber amplifier according to any of the items 24-36, wherein the WDM is arranged at an input to the second amplification stage.

[0096] 38. The optical fiber amplifier according to any of the preceding items, wherein the CW light has a wavelength from about 1545 nm to about 1565 nm.

[0097] 39. The optical fiber amplifier according to any of the preceding items, wherein the first active fiber has a core diameter of between 10 pm to 15 pm, and a cladding diameter of between 120 pm to 140 pm.

[0098] 40. The optical fiber amplifier according to any of the preceding items, wherein the second active fiber has a core diameter of between 8 pm to 11 pm.

[0099] 41. The optical fiber amplifier according to any of the preceding items, wherein the second active fiber is a single-mode fiber having a core diameter of more than 8 pm, such as more than 9 pm.

[0100] 42. The optical fiber amplifier according to any of the preceding items, wherein the second active fiber has a cladding diameter of between 120 pm to 130 pm.

[0101] 43. The optical fiber amplifier according to any of the preceding items, wherein the first active fiber is configured for receiving counter-propagating pump light from one or more first pump sources, such as one or more first laser diodes. 44. The optical fiber amplifier according to any of the preceding items, wherein the second active fiber is configured for receiving counter-propagating pump light from one or more second pump sources, such as one or more second laser diodes.

[0102] 45. The optical fiber amplifier according to any of the items 20-44, wherein the pump sources are configured for providing pump light at a wavelength of between 905 nm and 925 nm.

[0103] 46. The optical fiber amplifier according to any of the preceding items, wherein the optical fiber amplifier further comprises a pre-amplifier arranged before the first amplification stage.

[0104] 47. The optical fiber amplifier according to item 46, wherein the pre-amplifier comprises a semiconductor optical amplifier (SOA).

[0105] 48. The optical fiber amplifier according to any of the items 46-47, wherein the preamplifier comprises a single-clad optical fiber.

[0106] 49. The optical fiber amplifier according to item 48, wherein the single-clad optical fiber is configured for being optically pumped by co-propagating pump light from one or more pump sources, such as laser diodes.

[0107] 50. The optical fiber amplifier according to any of the items 46-49, wherein the preamplifier is configured for providing a gain to a power level of about 10 mW to about 100 mW.

[0108] 51. A laser system comprising:

[0109] - a seed laser configured for providing coherent, continuous wave (CW), light; and

[0110] - an optical fiber amplifier according to any of the preceding items.

[0111] 52. The laser system according to item 51 , wherein the laser system further comprises a pre-amplifier arranged downstream of the seed laser.

[0112] 53. The laser system according to item 52, wherein the pre-amplifier is configured for amplifying the light from the seed laser to a power level of between 1 mW and 100 mW. 54. The laser system according to item 52, wherein the pre-amplifier is configured for amplifying the light from the seed laser to a power level of between 20 mW and 80 mW.

[0113] 55. The laser system according to any of the items 52-54, wherein the pre-amplifier is selected from the group of semiconductor optical amplifiers (SOA), or core-pumped single-clad fibers.

[0114] 56. The laser system according to any of the items 51-55, wherein the CW light has a center wavelength in the range from about 1535 nm to about 1580 nm.

[0115] 57. The laser system according to any of the items 51-56, wherein the CW light from the seed laser has certain relative intensity noise (RIN) characteristic, and wherein the optical fiber amplifier, or pre-amplifier, is configured for maintaining or improving said RIN characteristic.

[0116] 58. The laser system according to any of the items 51-57, wherein the CW light from the seed laser has certain phase noise, and wherein the optical fiber amplifier is configured for maintaining or improving said phase noise.

[0117] 59. The laser system according to any of the items 51-58, wherein the optical fiber amplifier is configured for receiving an optical signal having a power level of between 10 mW and 100 MW.

[0118] 60. The laser system according to any of the items 51-59, wherein the optical fiber amplifier is configured for amplifying said optical signal to a power level of between 10 W and 20 W, such as to a power level above 15 W

[0119] 61 . The laser system according to any of the items 51-60, wherein the seed laser is configured to provide CW laser light having a linewidth of less than 1 kHz.

[0120] 62. The laser system according to any of the items 51-61 , wherein the laser light from the seed laser has a beam quality, M2, of less than 1.10, such as less than 1.05.

[0121] 63. The laser system according to any of the items 51-62, wherein the seed laser is a fiber-based laser. 64. The laser system according to any of the items 51-63, wherein the seed laser is a single-frequency fiber-based laser. Although some embodiments have been described and shown in detail, the disclosure is not restricted to such details, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized, and structural and functional modifications may be made without departing from the scope of the present disclosure. Furthermore, the skilled person would find it apparent that unless an embodiment is specifically presented only as an alternative, different disclosed embodiments may be combined to achieve a specific implementation and such specific implementation is within the scope of the disclosure.

Claims

Claims1 . A two-stage optical fiber amplifier comprising:- a first amplification stage comprising a first active fiber configured to amplify light received from a seed laser or a pre-amplifier stage;- a second amplification stage arranged downstream of the first amplification stage, the second amplification stage comprising a second active fiber configured for further amplifying light received from the first amplification stage, wherein the second active fiber has a core diameter of between 6 pm and 12 pm and a cladding diameter of between 80 pm to 150 pm, and wherein the second active fiber is configured for supporting only a single mode of light; and- an optical device configured for at least partially coupling out undesired light from the optical fiber amplifier, said undesired light having a center wavelength of about 1 pm and being generated from amplified spontaneous emission (ASE) in the fiber amplifier; wherein the first and second active fibers are polarization-maintaining (PM) fibers, and further wherein the first and second active fibers are Erbium / Ytterbium (Er / Yb) co-doped double-clad fibers, wherein the optical fiber amplifier is configured for amplifying continuous wave (CW) light having a center wavelength in the range from about 1535 nm to about 1580 nm.

2. The optical fiber amplifier according to claim 1 , wherein the optical fiber amplifier is configured for amplifying light to an average optical power in the range of 5 W to 25 W.

3. The optical fiber amplifier according to any of the preceding claims, wherein the optical fiber amplifier is configured for amplifying light to an average optical power of at least 15 W.

4. The optical fiber amplifier according to any of the preceding claims, wherein the optical device is arranged between the first and second amplification stages.

5. The optical fiber amplifier according to any of the preceding claims, wherein the optical device is selected from the group of wavelength-division multiplexing devices (WDM) or long-period gratings (LPG).

6. The optical fiber amplifier according to any of the preceding claims, wherein the optical device is a wavelength-division multiplexing device (WDM).

7. The optical fiber amplifier according to claim 6, wherein the WDM is configured to couple out ASE light at a predefined wavelength, such as at about 1 pm.

8. The optical fiber amplifier according to claim 7, wherein the WDM is configured to direct the ASE light to a coreless fiber for attenuating said ASE light.

9. The optical fiber amplifier according to claim 8, wherein the coreless fiber has a predefined length from about 10 cm to about 200 cm, such as from about 25 cm to about 100 cm.

10. The optical fiber amplifier according to any of the claims 8-9, wherein the coreless fiber is coiled to a predefined coil diameter between 1 cm and 10 cm.

11. The optical fiber amplifier according to any of the claims 6-10, wherein the WDM is arranged at an input to the second amplification stage.

12. The optical fiber amplifier according to any of the preceding claims, wherein the optical fiber amplifier is configured for providing ASE extraction without requiring an injection of an auxiliary signal into the first or second amplification stages.

13. The optical fiber amplifier according to any of the preceding claims, wherein the optical fiber amplifier is configured for providing ASE extraction without requiring a narrow-band optical filter, such as an ASE filter.

14. The optical fiber amplifier according to any of the preceding claims, wherein the optical device is not an optical filter, such as an ASE filter or a bandpass filter.

15. The optical fiber amplifier according to any of the preceding claims, wherein the CW light has a wavelength from about 1545 nm to about 1565 nm.

16. The optical fiber amplifier according to any of the preceding claims, wherein the first active fiber has a core diameter of between 10 pm to 14 pm, and a cladding diameter of between 120 pm to 140 pm.

17. The optical fiber amplifier according to any of the preceding claims, wherein the second active fiber has a core diameter of between 8 pm to 11 pm, such as a core diameter of approximately 10 pm.

18. The optical fiber amplifier according to any of the preceding claims, wherein the second active fiber is a single-mode fiber having a core diameter of more than 8 pm, such as more than 9 pm.

19. The optical fiber amplifier according to any of the preceding claims, wherein the second active fiber has a cladding diameter of between 120 pm to 130 pm.

20. The optical fiber amplifier according to any of the preceding claims, wherein the first and / or second active fibers are configured for receiving counter-propagating pump light from one or more first pump sources, such as one or more first laser diodes.

21. The optical fiber amplifier according to claim 20, wherein the pump sources are configured for providing pump light at a wavelength of between 905 nm and 925 nm.

22. The optical fiber amplifier according to any of the preceding claims, wherein the optical fiber amplifier further comprises a pre-amplifier arranged before the first amplification stage.

23. The optical fiber amplifier according to claim 22, wherein the pre-amplifier comprises, or constitutes, a semiconductor optical amplifier (SOA).

24. The optical fiber amplifier according to any of the claims 22-23, wherein the preamplifier comprises a single-clad optical fiber configured for being optically pumped by co-propagating pump light from one or more pump sources, such as laser diodes.

25. The optical fiber amplifier according to any of the claims 22-24, wherein the preamplifier is configured for providing a gain to a power level of about 10 mW to about 100 mW, such as a power level of about 30 mW to about 80 mW.

26. The optical fiber amplifier according to any of the preceding claims, wherein the signal-to-ASE noise is less than 5 %, such as less than 2 %, such as less than 1 %, measured at the output of the two-stage optical fiber amplifier.

27. A laser system comprising:- a seed laser, such as a fiber-based laser, configured for providing coherent, continuous wave (CW), light at a center wavelength in the range from about 1535 nm to about 1580 nm; - a pre-amplifier arranged downstream of the seed laser, said pre-amplifier configured for amplifying the light from the seed laser to a power level of between 1 mW and 100 mW; and- an optical fiber amplifier according to any of the preceding claims.

28. The laser system according to claim 27, wherein the seed laser is configured to provide CW laser light having a linewidth of less than 1 kHz.

29. The laser system according to any of the claims 27-28, wherein the laser light from the seed laser has a beam quality, M2, of less than 1.10, such as less than 1.05.

30. The laser system according to any of the claims 27-29, wherein the laser system is configured to output laser light at an average power of at least 15 W

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