BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to optical fiber amplification systems such as fiber amplifiers and fiber lasers, and more particularly to a gain fiber assembly that provides the gain media for such systems.

Description of the Related Art

Lasers are becoming more important in the fields of spectroscopy, microscopy, remote sensing, communications, medicine, industry, and military. As laser applications expand, many new applications require higher power, wider spectral coverage, and higher pump-to-signal conversion efficiency. To improve the laser and amplifier performance, researchers and commercial laser and amplifier manufacturers are adopting fiber lasers in which the gain medium is rare earth doped optical fiber. Rare earth doped fiber lasers and fiber amplifiers provide solutions that require less space, reduced input power, and simpler packaging.

Optical fiber amplification systems such as fiber lasers and fiber amplifiers both use a rare earth doped active fiber as the gain media. In a fiber amplifier, a seed signal (typically from a low power laser) is injected into the fiber and amplifier where the signal is amplified by the gain medium. In a fiber laser, a cavity (typically formed by mirrors at opposite ends of the active fiber confine light to the gain media) confines the light to the active fiber. A pump provides the population inversion (the “pump signal”) and the natural oscillations in the laser cavity are amplified and generate the laser signal that is coupled out of the cavity.

The typical structure of a commercial single-mode active fiber100used in construction of lasers and amplifiers is shown inFIG. 1. Signal light propagates primarily in a core102and is guided by a 1stcladding104. Pump light propagates primarily in core102and 1stcladding104and is guided by a 2ndcladding106. The pump light travelling in the core is absorbed and re-emitted at a longer wavelength by the gain media in the core increasing the signal power along the length of the fiber. For laser and amplifier designs, the following are key considerations, (1) an active fiber length which is shorter than the distance for the onset of parasitic non-liner effects such as self-phase modulation (SPM), four wave mixing (FWM), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS) or amplified stimulated emission (ASE), (2) achieving as high efficiency (Pout vs Ppump) as possible, and (3) integration with commercial fiber components. Simultaneously satisfying all three elements requires that the diameters of the core102, 1stcladding104and 2ndcladding106of the active fiber100be optimized and integrated into a laser or amplifier.

Suppression of the onset of parasitic non-linear effects is key to the successful design of high peak power pulsed systems or narrow-linewidth lasers and amplifiers. Both the signal and parasitic non-linear effects grow with increasing length of the active fiber but beyond a certain length the parasitic non-linear effects begin to dominate and the signal output plateaus, begins to decrease or acquires undesirable spectral properties.

The shorter wavelength pump causes a population inversion in the core and enables gain at the signal wavelength. The higher the population inversion, the greater the gain per unit length and the shorter the active fiber can be to reach the desired output power. The shorter active fiber avoids the onset of the parasitic non-linearities or ASE. To maximize the pump absorption and thus the population inversion in the core102, the amplifier or laser system needs to concentrate as much of the pump power in the core of the fiber as possible.

To a first order for multi watt-level systems, the majority of the pump light travels in the 1stcladding104and core102of the fiber. The amount of pump light overlapping with the core is proportional to the ratio of the area of core102to the area of 1stcladding104. Since the cross-sectional area of the fiber is proportional to the square of the diameter, the output power of the laser or amplifier per unit length is proportional to the square of the ratio of the core diameter (dcore) to the 1stcladding diameter (d1st cladding) as shown in Eqn 1.

Signal⁢⁢Power⁢⁢per⁢⁢unit⁢⁢length∝(dcored1⁢st⁢⁢cladding)2(1)

The highest population inversion can be achieved when the ratio of the diameter of the core to the diameter of the 1stcladding (the core-to-cladding ratio) is as large as possible.

There are two ways to maximize the core-to-cladding diameters ratio: make the core diameter as large as possible or make the 1stcladding diameter as small as possible. In single mode laser and amplifier applications, the maximum core diameter is determined by the size required for single mode operation.

dcore<2.405⁢λπ⁢⁢NA(2)
where λ is the wavelength and NA is the numerical aperture of the double clad fiber. The numerical aperture is a dimensionless number that characterizes the range of angles over which the fiber can accept or emit light.

Researchers [C. Jauregui, J. Limpert, and A. Tünnermann, Nat. Photonics 7(11), 861 (2013), X. Ma, C. Zhu, I-N. Hu, A. Kaplan, and A. Galvanauskas, Opt. Express 22, 9206 (2014), K. Bobkov, A. Andrianov, M. Koptev, S. Muravyev, A. Levchenko, V. Velmiskin, S. Aleshkina, S. Semjonov, D. Lipatov, A. Guryanov, A. Kim, and M. Likhachev, Opt. Express 25, 26958 (2017), K. S. Abedin, R. Ahmad, A. M. DeSantolo, J. W. Nicholson, P. S. Westbrook, C. Headley, D. J. DiGiovanni, Proc. SPIE 10512, 105121E (2018), L. Kotov, M. Akbulut, A. Chavez-Pirson, J. Zong, and N. Peyghambarian, Proc. SPIE 10897, 108970X (2019), M. E. Likhachev, S. S. Aleshkina, A. V. Shubin, M. M. Bubnov, E. M. Dianov, D. S. Lipatov, and A. N. Guryanov, in CLEO EUROPE/EQEC 2011 Conference Digest, paper CJ_P24 (2011)] have tried several approaches to make large core fiber-based systems operating in a single-mode regime but have not been able to increase the core diameter beyond the beyond the single mode core limit. Besides, such fibers cannot be easily spliced to standard double-clad fibers, thus making it difficult to manufacture all-fiber optical devices.

With the maximum achievable core diameter set by equation 2, the other approach to achieving a large core-to-cladding ratio is to decrease the 1stcladding diameter. The minimum diameter of a 1stcladding is set by the NA of the pump light source and the NA that can be supported by refractive indexes of the 1stcladding and 2ndcladding. According to the brightness conservation theorem (also called conservation of the Etendue, which characterizes how spread out light is in an area and angle), Eqn 3, attempts to launch pump light into the cladding of a fiber with a smaller 1stcladding diameter (by means of free space optics or by fiber tapering) leads to the increase of pump light's NA. Most fiber manufacturers use fused silica glass and low-index polymers for the 1stand 2ndcladdings. At a working pressure of 1 atm and ambient temperature at 1 μm, the RI for fused silica is 0.45 and for polymer is 0.37 resulting in fibers with an NA of 0.46 or less. At the same time, commercially available pigtailed pump sources operating near 1 μm typically have the smallest fiber 1stcladding diameter of 105 μm and NA of 0.15-0.22. Thus, light from these diodes theoretically can be coupled into an active fiber guiding 0.45 NA with cladding diameter 35-50 μm, while still conserving Etendue.
NA1*Area1≥NA2*Area2(3)

Where light travels from surface 1 to surface 2 and NA1and NA2are the numerical apertures of surface1and2and Area1and Area2are the areas of surface 1 (input fiber 1stcladding) and 2 (active fiber 1stcladding), corresponding to D1and D2of the 1stcladding respectively.

However, fabricating and scaling the power in all-fiber lasers and amplifiers requires integrating pump/signal combiners into the fiber chain composed of other fiber components. Commercially available combiners typically have output fibers with 125 μm or larger 1stcladding diameters with maximum pump guidance NA of 0.46. The typical integration of a pump combiner and active fiber100is shown inFIG. 2. The pump combiner has a passive double-clad fiber110(“fiber pigtail”). The pigtail has a core112, 1stcladding114and 2ndcladding116. The ends of fiber pigtail110and active fiber102are stripped of their 2ndcladding, cleaved, and spliced together at a splice118. The stripped portion of the fibers is recoated with a coating120. To minimize the power loss at the splice118, the active fiber should have the same or a larger 1stcladding diameter than the pump combiner fiber to avoid pump loss at the splice. An optical signal122propagates in pigtail core112and is coupled into the active fiber core102. An optical pump124is guided by 2ndcladding116and coupled into the active fiber100to activate the gained media.

The brightness conservation theorem (Eqn 3) prohibits decreasing the diameter of the 1stcladding of the active fiber below a 125 μm diameter while keeping the pump at the same NA. To accommodate smaller 1stcladding diameters in the active fiber, researchers have tried approaches to increase the NA of the 1stcladding by changing the 2ndcladding to a Teflon coating or using an air hole 2ndcladding. These approaches were able to provide 1stcladding NAs as high as 0.6 [K. H. Ylä-Jarkko, R. Selvas, D. B. S. Son, J. K. Sahu, C. A. Codemard, J. Nilsson, S. A. Alam, and A. B. Grudinin, in Advanced Solid State Photonics, J. Zayhowski, ed., Vol. 83 of OSA Trends in Optics and Photonics, paper 103 (2003), L. V. Kotov, M. E. Likhachev, M. M. Bubnov, O. I. Medvedkov, M. V. Yashkov, A. N. Guryanov, J. Lhermite, S. Février, and E. Cormier, Opt. Lett. 38, 2230 (2013)]. In these cases, the 1stcladding diameter of an active fiber might be decreased to ˜95 μm. However, these techniques are still NA limited and make fiber manufacturing and handling more complicated.

For many applications current fiber components are not available that provide the correct combination of core-to-cladding ratio, 1stcladding diameter, single mode core diameter and NA to build a cladding-pumped high gain laser or amplifier.

To minimize the length of the active fiber, designers want to fabricate fibers with a high core-to-1stcladding diameter ratio because pump absorption, and thus the gain, of cladding-pumped fibers is proportional to approximately the square of core to cladding diameter ratio. A core-to-cladding ratio of 1:10 is the maximum value for the single mode commercial double cladding fibers designed for 1 micron operation. More typical values are 1:12 up to 1:20. The improved pump absorption leads to a high population inversion of the active ions, which is beneficial for gain media operating with three-level energy systems. For instance, in Yb-doped fiber lasers at 976 nm, or Nd-doped fiber lasers near 920 nm operation with a high inversion allows suppressing amplified spontaneous emission (ASE) above 1 μm and improves their performances [A. S. Kurkov, O. I. Medvedkov, V. M. Paramonov, S. A. Vasiliev, E. M. Dianov, V. Solodovnikov, V. Zhilin, A. N. Guryanov, A. Y. Laptev, and A. A. Umnikov, in Optical Amplifiers and Their Applications, paper OWC2, (2001), K. H. Ylä-Jarkko, R. Selvas, D. B. S. Son, J. K. Sahu, C. A. Codemard, J. Nilsson, S. A. Alam, and A. B. Grudinin, in Advanced Solid State Photonics, J. Zayhowski, ed., Vol. 83 of OSA Trends in Optics and Photonics, paper 103 (2003), M. Laroche, B. Cadier, H. Gilles, S. Girard, L. Lablonde, and T. Robin, Opt. Lett. 38, 3065 (2013), M. Leich, M. Jáger, S. Grimm, J. Dellith, D. Hoh, S. Jetschke, M. Becker, A. Hartung, T. Eschrich, J. Kobelke, H. Bartelt, Proc. SPIE 8961, 89610Z (2014), S. S. Aleshkina, M. E. Likhachev, D. S. Lipatov, O. I. Medvedkov, K. K. Bobkov, and A. N. Guryanov, Proc. SPIE 9728, 97281C (2016), T. Matniyaz, W. Li, M. Kalichevsky-Dong, Th. W. H., J. Parsons, G. Gu, and L. Dong, Opt. Lett. 44, 807 (2019), V. Kotov, M. E. Likhachev, M. M. Bubnov, O. I. Medvedkov, D. S. Lipatov, N. N. Vechkanov, A. N. Gur'yanov, Quantum Electron., 42(5), 432 (2012)]. In addition, it has been demonstrated that increasing the core-to cladding diameter ratio significantly improves pump conversion efficiency of cladding-pumped Yb-free Er-doped fiber lasers [L. V. Kotov, M. E. Likhachev, M. M. Bubnov, O. I. Medvedkov, M. V. Yashkov, A. N. Guryanov, J. Lhermite, S. Février, and E. Cormier, Opt. Lett. 38, 2230 (2013), D. A. Gaponov, S. Février, M. Devautour, P. Roy, M. E. Likhachev, S. S. Aleshkina, M. Y. Salganskii, M. V. Yashkov, and A. N. Guryanov, Opt. Lett. 35, 2233 (2010)].

SUMMARY OF THE INVENTION

The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.

The present invention provides a gain fiber assembly for use in a cladding-pumped optical fiber amplification system such as a fiber amplifier or fiber laser. The gain fiber assembly provides much higher pump conversion efficiencies, which allows for much shorter fiber lengths to achieve equivalent levels of output power. As such, the length of fiber can be selected to maximize output power while avoiding the onset of non-linear effects or ASE.

In an embodiment, a gain fiber assembly comprises a passive double-clad input fiber (“input fiber”) that is spliced to an active single mode fiber (“bare fiber”). The input fiber includes a passive core with diameter D1, a 1stglass cladding of diameter D2 and a 2ndcladding, typically polymer, of diameter D3 configured to receive and propagate an optical signal in the passive core and an optical pump that propagates in the passive core and the 1stglass cladding. The 2ndcladding is removed from an end section, and the input fiber (the core and 1stcladding) is tapered to a smaller diameter D4<D2. The bare fiber includes an active fiber core and only a 1stglass cladding having a diameter D4 less than 80 microns and preferably less than 60 microns and even less than 40 microns. The active single-mode fiber is spliced to the tapered diameter of the input fiber. A glass-fluid or glass-vacuum interface along the taper and the active single mode fiber provides guidance of the optical pump into and along the 1stcladding of the active single mode fiber.

The glass-fluid or glass-vacuum interface for the bare fiber can be formed by completely removing the 2ndcladding layer from the fiber leaving only the 1stglass cladding and core, hence the name “bare fiber” or by fabricating the fiber without the 2ndcladding. The high refractive index difference between glass and fluid or glass and vacuum produces a NA that is larger than 0.8 and typically greater than 1. The large NA allows the diameter of the bare fiber's 1stglass cladding to be reduced and increase the core-cladding ratio. The large NA enables the efficient coupling of light from commercial input fibers with large 1stcladding diameters into the bare fiber with a smaller 1stcladding diameter satisfying the conservation of Etendue. The 2ndcladding is stripped from an end of the commercial input fiber and the fiber's core and 1stcladding are tapered to match the smaller 1stcladding diameter of the bare fiber. The smaller 1stcladding diameter of the bare fiber increases the power density of the pump light and results in a higher population inversion of the rare earth ions. The higher population inversion increases the pump to signal efficiency and shortens the length of the gain medium suppressing the parasitic effects from non-linear processes such as stimulated Brillion and Raman scattering or self-phase modulation.

In an embodiment, the input fiber's 1stglass cladding diameter D2 is greater than 105 microns and the active single mode fiber's 1stglass cladding diameter D4 is less than 60 microns and possibly less than 40 microns.

In different embodiments, the input fiber's NA is less than 0.6. If the fluid is a gas in which 1<RI<1.01, the NA of the bare fiber is greater than 1. If the fluid is a liquid in which 1<RI<1.2, the NA of the bare fiber is greater than 0.8.

In an embodiment, the input and output fibers comprise fused silica having an RI of approximately 4.5. The active single mode fiber comprises a different glass having a RI greater than that of fused silica.

In an embodiment, an enclosure houses the tapered end section of the input fiber and active single mode fiber with the passive double-clad input and output fibers penetrating through the enclosure. Inlet and outlet ports in the enclosure are configured to receive, circulate and remove a fluid to remove heat from the active single mode fiber.

In an embodiment, the tapered end section of the input fiber and the active single mode fiber are housed in a vacuum enclosure. The glass-vacuum interface guides the optical pump along the tapered end section and the active single mode fiber

These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:

