**3. Leakage channel fibers**

7). The fiber had a d/Λ=0.19, again firmly in the multimode regime (19-cell PCF not shown in Figure 2). The pump guide was 175μm in diameter. The pump air-clad had a web thickness of 400nm, which was 10μm long. The measured pump NA was 0.6 at 975nm, fairly close to the 0.67 NA predicted by the slab model (see Figure 3). The pump absorption is ~30dB/m at 976nm. The rod diameter is ~1.5mm. A 58cm long fiber was used to demonstrate 55W/m power extraction. In the same paper, a passive 100μm core fiber was also demonstrated. The 19-cell fiber had a d/Λ=0.2, again in the multimode regime. The effective mode area was ~4500μm2

doping level was also increased compared with earlier fibers. The increased doping level and reduced pump guide led to a high pump absorption of ~30dB/m at 976nm. The fiber was used to demonstrate power extraction of

Figure 7 Microscope‐image of the extended‐mode‐area rod‐type photonic crystal fiber, SEM‐picture of the micro‐ structured region and measured near‐field intensity profile of the 60μm core fiber [30]. The demonstration of a 100m active 19‐cell PCF finally came in 2006 [6]. The pump guide had a diameter of 290m. The rod diameter was 1.5mm. A 90cm long fiber was used to amplify 1ns pulses at 9.6kHz to record peak

**Figure 7.** Microscope-image of the extended-mode-area rod-type photonic crystal fiber, SEM-picture of the micro-

The demonstration of a 100μm active 19-cell PCF finally came in 2006 [6]. The pump guide had a diameter of 290μm. The rod diameter was 1.5mm. A 90cm long fiber was used to amplify 1ns pulses at 9.6kHz to record peak power of 4.5MW and pulse energy of 4.3mJ with M2

Figure 7 Microscope‐image of the extended‐mode‐area rod‐type photonic crystal fiber, SEM‐picture of the micro‐ structured region and measured near‐field intensity profile of the 60μm core fiber [30]. The demonstration of a 100m active 19‐cell PCF finally came in 2006 [6]. The pump guide had a diameter of 290m. The rod diameter was 1.5mm. A 90cm long fiber was used to amplify 1ns pulses at 9.6kHz to record peak

=1.3.

Figure 8 SEM image of the single polarization PCF with an effective area of ~700m2 [31] (left) and 2300m2 (right) [32].

Figure 8 SEM image of the single polarization PCF with an effective area of ~700m2 [31] (left) and 2300m2 (right) [32].

**Figure 8.** SEM image of the single polarization PCF with an effective area of ~700μm2 [31] (left) and 2300μm2 (right)

Polarization-maintaining (PM) PCFs have also been developed by introducing stress elements. A PM 7-cell PCF with a mode area of ~700μm2 was demonstrated in 2005 [31] (see left figure in Figure 8). The pitch Λ was 12.3μm with a d/Λ of 0.2. In the weakly guided PCFs, the polarization modes on slow and fast axis have different bend losses. This enables singlepolarization operation where the polarization mode on the slow axis is still guided while polarization mode on the fast axis suffers high bend loss. Another single-polarization 19-cell

Polarization‐maintaining (PM) PCFs have also been developed by introducing stress elements. A PM 7‐cell PCF with a mode area of ~700m2 was demonstrated in 2005 [31] (see left figure in Figure 8). The pitch was 12.3m with a d/ of 0.2. In the weakly guided PCFs, the polarization modes on slow and fast axis have different bend losses. This enables single‐polarization operation where the polarization mode on the slow axis is still guided while polarization mode on the fast axis suffers high bend loss. Another single‐polarization 19‐cell PCF was demonstrated in 2008 with a mode area of ~2300m2 (right figure in Figure 8) [32]. The corner‐to‐corner distance

Polarization‐maintaining (PM) PCFs have also been developed by introducing stress elements. A PM 7‐cell PCF with a mode area of ~700m2 was demonstrated in 2005 [31] (see left figure in Figure 8). The pitch was 12.3m with a d/ of 0.2. In the weakly guided PCFs, the polarization modes on slow and fast axis have different bend losses. This enables single‐polarization operation where the polarization mode on the slow axis is still guided while polarization mode on the fast axis suffers high bend loss. Another single‐polarization 19‐cell PCF was demonstrated in 2008 with a mode area of ~2300m2 (right figure in Figure 8) [32]. The corner‐to‐corner distance

A 2D micro‐structured cladding, which is made possible by the stack‐and‐draw technique developed for photonic crystal fibers, enables new designs which do not possess the closed core‐and‐clad boundaries of conventional optical fibers. When a mode is guided in a conventional optical fiber, total internal reflection everywhere around the closed core‐and‐clad boundary, traps light entirely in the core, leading to zero waveguide

A 2D micro‐structured cladding, which is made possible by the stack‐and‐draw technique developed for photonic crystal fibers, enables new designs which do not possess the closed core‐and‐clad boundaries of conventional optical fibers. When a mode is guided in a conventional optical fiber, total internal reflection everywhere around the closed core‐and‐clad boundary, traps light entirely in the core, leading to zero waveguide

=1.3.

=1.3.

multimode regime. The effective mode area was ~4500m2 and MFD was ~75m.

230 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

multimode regime. The effective mode area was ~4500m2 and MFD was ~75m.

structured region and measured near-field intensity profile of the 60μm core fiber [30].

The rod‐type PCF was further developed with the demonstration of a 19‐cell PCF with a core diameter of 60m, effective mode area of ~2000m2 and MFD of 75m in 2006 [30] (see Figure 7). The fiber had a d/=0.19, again firmly in the multimode regime (19‐cell PCF not shown in Figure 2). The pump guide was 175m in diameter. The pump air‐clad had a web thickness of 400nm, which was 10m long. The measured pump NA was 0.6 at 975nm, fairly close to the 0.67 NA predicted by the slab model (see Figure 3). The pump absorption is ~30dB/m at 976nm. The rod diameter is ~1.5mm. A 58cm long fiber was used to demonstrate 55W/m power extraction. In the same paper, a passive 100m core fiber was also demonstrated. The 19‐cell fiber had a d/=0.2, again in the

The rod‐type PCF was further developed with the demonstration of a 19‐cell PCF with a core diameter of 60m, effective mode area of ~2000m2 and MFD of 75m in 2006 [30] (see Figure 7). The fiber had a d/=0.19, again firmly in the multimode regime (19‐cell PCF not shown in Figure 2). The pump guide was 175m in diameter. The pump air‐clad had a web thickness of 400nm, which was 10m long. The measured pump NA was 0.6 at 975nm, fairly close to the 0.67 NA predicted by the slab model (see Figure 3). The pump absorption is ~30dB/m at 976nm. The rod diameter is ~1.5mm. A 58cm long fiber was used to demonstrate 55W/m power extraction. In the same paper, a passive 100m core fiber was also demonstrated. The 19‐cell fiber had a d/=0.2, again in the

doping level was also increased compared with earlier fibers. The increased doping level and reduced pump guide led to a high pump absorption of ~30dB/m at 976nm. The fiber was used to demonstrate power extraction of

and MFD was ~75μm.

power of 4.5MW and pulse energy of 4.3mJ with M2

power of 4.5MW and pulse energy of 4.3mJ with M2

of the core was 70μm. The pitch was 11m with a d/ of 0.1.

of the core was 70μm. The pitch was 11m with a d/ of 0.1.

3 Leakage channel fibers

[32].

3 Leakage channel fibers

~250W/m.

~250W/m.

A 2D micro-structured cladding, which is made possible by the stack-and-draw technique developed for photonic crystal fibers, enables new designs which do not possess the closed core-and-clad boundaries of conventional optical fibers. When a mode is guided in a conven‐ tional optical fiber, total internal reflection everywhere around the closed core-and-clad boundary, traps light entirely in the core, leading to zero waveguide loss. In designs with an open cladding, light can leak out, leading to finite waveguide loss associated with each mode. The waveguide loss is mode-dependent, providing opportunities for mode control by minimizing loss of the desired mode while maximizing loss of the unwanted modes. *Leakage channel fibers* (LCF) takes advantages of these new opportunities made possible by open cladding designs. A LCF can be precisely engineered to have high confinement loss for all higher order modes and low confinement loss for the fundamental mode and can, therefore, significantly extend the effective mode area of the fundamental mode. LCFs essentially exploit the increased ability of higher order modes to leak through small gaps in the cladding while maintaining good fundamental mode confinement. loss. In designs with an open cladding, light can leak out, leading to finite waveguide loss associated with each mode. The waveguide loss is mode‐dependent, providing opportunities for mode control by minimizing loss of the desired mode while maximizing loss of the unwanted modes. *Leakage channel fibers* (LCF) takes advantages of these new opportunities made possible by open cladding designs. A LCF can be precisely engineered to have high

#### **3.1. Leakage channel fibers with air holes** confinement loss for all higher order modes and low confinement loss for the fundamental mode and can, therefore, significantly extend the effective mode area of the fundamental mode. LCFs essentially exploit the

loss penalty. This is a significant improvement over PCFs.

~1.3.

3.2 All‐glass leakage channel fibers

ease of fabrication and use, compared with fibers with air holes.

The first LCF was demonstrated in 2005 [7]. It has a simple cladding design with air holes in the cladding [7]. The LCF is shown in in the left figure in Figure 9. The LCF had an outer diameter of ~270μm. The two smaller holes had a diameter of d=39.6μm and pitch Λ=51.2μm. The four larger holes had a diameter of d=46.0μm and pitch Λ=51.1μm. The passive LCF provided robust single-mode operation with a measured mode area of ~1417μm2 (MFD=42.5μm). The most significant aspect of this work is that the LCF can be coiled down to 15cm with negligible loss penalty. This is a significant improvement over PCFs. increased ability of higher order modes to leak through small gaps in the cladding while maintaining good fundamental mode confinement. 3.1 Leakage channel fibers with air holes The first LCF was demonstrated in 2005 [7]. It has a simple cladding design with air holes in the cladding [7]. The LCF is shown in in the left figure in Figure 9. The LCF had an outer diameter of ~270m. The two smaller holes had a diameter of d=39.6m and pitch =51.2m. The four larger holes had a diameter of d=46.0m and pitch =51.1m. The passive LCF provided robust single‐mode operation with a measured mode area of ~1417m2 (MFD=42.5m). The most significant aspect of this work is that the LCF can be coiled down to 15cm with negligible

Figure 9 The LCF used in the first demonstration [7] (left), the first ytterbium‐doped LCF (center) [33] and the first ytterbium‐doped PM LCF (right) [34]. **Figure 9.** The LCF used in the first demonstration [7] (left), the first ytterbium-doped LCF (center) [33] and the first ytterbium-doped PM LCF (right) [34].

The first Ytterbium‐doped LCF was demonstrated in 2006 [33] (see the middle figure in Figure 9). The fiber outer diameter was ~350m and it was coated with a low index polymer to give a pump NA of 0.46. The average hole diameter was ~55m and average pitch was ~67m, giving an average d/=0.82. The effective mode area was 3160m2 (MFD=63.4m). Pump absorption was measured to be ~3.6dB/m at 976nm. Slope efficiency versus launched power was measured to be ~60% in a 5m long amplifier coiled at 40cm diameter. M2 was measured to be

The first ytterbium‐doped PM LCF was also demonstrated in 2006 [34] (see the right figure in Figure 9). A pair of boron‐doped silica stress rods was used to replace two opposing air holes. The hole diameter was ~37m. The effective mode area was ~1400m2 (MFD=42.2m). The birefringence was measured to be ~2.110‐<sup>4</sup> over 1010‐ 1080nm. The LCF had an outer diameter of ~245m and was coated with a low index polymer to give a pump NA of ~0.46. The pump absorption was ~2.6dB/m at 976nm. Slope efficiency versus launched power was measured to be ~60%. M2 was measured to be ~1.2. The PM LCF could be coiled to 12cm diameter with negligible bend loss.

High refractive index contrast is not necessary for large core fiber designs. Low refractive index contrast is sufficient and often advantageous for further limiting higher order mode propagation. Fluorine‐doped silica can be used to replace air holes in the LCFs described in the last section. The all‐glass LCFs can provide much improved

Despite the fact that air is a readily available ingredient, there are a number of drawbacks related to the use of air holes in fibers. The first one is the difficulty in precisely controlling the dimension of air holes in fiber fabrication. This is an intrinsic problem of a holey structure due to the air hole's tendency to collapse during fiber drawing. This is usually countered by a precise control of pressurization of the air holes, a process dependent on drawing conditions such as furnace temperature, feed rate, and drawing speed. When small air holes are desirable as in endless single‐mode PCFs, higher pressure is required to maintain air hole dimensions due to the significantly The first Ytterbium-doped LCF was demonstrated in 2006 [33] (see the middle figure in Figure 9). The fiber outer diameter was ~350μm and it was coated with a low index polymer to give a pump NA of 0.46. The average hole diameter was ~55μm and average pitch was ~67μm, giving an average d/Λ=0.82. The effective mode area was 3160μm2 (MFD=63.4μm). Pump absorption was measured to be ~3.6dB/m at 976nm. Slope efficiency versus launched power was measured to be ~60% in a 5m long amplifier coiled at 40cm diameter. M2 was measured to be ~1.3.

The first ytterbium-doped PM LCF was also demonstrated in 2006 [34] (see the right figure in Figure 9). A pair of boron-doped silica stress rods was used to replace two opposing air holes. The hole diameter was ~37μm. The effective mode area was ~1400μm2 (MFD=42.2μm). The birefringence was measured to be ~2.1×10-4 over 1010-1080nm. The LCF had an outer diameter of ~245μm and was coated with a low index polymer to give a pump NA of ~0.46. The pump absorption was ~2.6dB/m at 976nm. Slope efficiency versus launched power was measured to be ~60%. M2 was measured to be ~1.2. The PM LCF could be coiled to 12cm diameter with negligible bend loss.

#### **3.2. All-glass leakage channel fibers**

High refractive index contrast is not necessary for large core fiber designs. Low refractive index contrast is sufficient and often advantageous for further limiting higher order mode propa‐ gation. Fluorine-doped silica can be used to replace air holes in the LCFs described in the last section. The all-glass LCFs can provide much improved ease of fabrication and use, compared with fibers with air holes.

Despite the fact that air is a readily available ingredient, there are a number of drawbacks related to the use of air holes in fibers. The first one is the difficulty in precisely controlling the dimension of air holes in fiber fabrication. This is an intrinsic problem of a holey structure due to the air hole's tendency to collapse during fiber drawing. This is usually countered by a precise control of pressurization of the air holes, a process dependent on drawing conditions such as furnace temperature, feed rate, and drawing speed. When small air holes are desirable as in endless single-mode PCFs, higher pressure is required to maintain air hole dimensions due to the significantly increased tendency for the air holes to collapse by surface tension in this regime. This can make air hole dimensions to become highly sensitive to drawing conditions. This delicate balance of pressurization and collapse can lead to issues of controll‐ ability and repeatability in PCF fabrication. Air holes can also disturb smooth fracture wave propagation during fiber cleaving, leading to a poor cleaved surface due to the appearance of deep fractures behind the air holes, a problem often aggravated by large air holes and high cleaving tensions. In addition, air holes often have to be thermally sealed at the fiber ends to minimize environmental contamination. Mode distortion can occur from the air holes collaps‐ ing during splicing. This is especially true for large-mode-area fibers, which are much more susceptible to small perturbations.

A detailed analysis of all-glass LCFs was reported in [35]. For a LCF formed by one layer of features as shown in the inset of Figure 10, the core of diameter 2ρ is formed by six features with diameter d and refractive index nf . Center-to-center feature spacing is Λ. The refractive

much more susceptible to small perturbations.

increased tendency for the air holes to collapse by surface tension in this regime. This can make air hole dimensions to become highly sensitive to drawing conditions. This delicate balance of pressurization and collapse can lead to issues of controllability and repeatability in PCF fabrication. Air holes can also disturb smooth fracture wave propagation during fiber cleaving, leading to a poor cleaved surface due to the appearance of deep fractures

The first Ytterbium-doped LCF was demonstrated in 2006 [33] (see the middle figure in Figure 9). The fiber outer diameter was ~350μm and it was coated with a low index polymer to give a pump NA of 0.46. The average hole diameter was ~55μm and average pitch was ~67μm, giving an average d/Λ=0.82. The effective mode area was 3160μm2 (MFD=63.4μm). Pump absorption was measured to be ~3.6dB/m at 976nm. Slope efficiency versus launched power

The first ytterbium-doped PM LCF was also demonstrated in 2006 [34] (see the right figure in Figure 9). A pair of boron-doped silica stress rods was used to replace two opposing air holes. The hole diameter was ~37μm. The effective mode area was ~1400μm2 (MFD=42.2μm). The birefringence was measured to be ~2.1×10-4 over 1010-1080nm. The LCF had an outer diameter of ~245μm and was coated with a low index polymer to give a pump NA of ~0.46. The pump absorption was ~2.6dB/m at 976nm. Slope efficiency versus launched power was measured to

High refractive index contrast is not necessary for large core fiber designs. Low refractive index contrast is sufficient and often advantageous for further limiting higher order mode propa‐ gation. Fluorine-doped silica can be used to replace air holes in the LCFs described in the last section. The all-glass LCFs can provide much improved ease of fabrication and use, compared

Despite the fact that air is a readily available ingredient, there are a number of drawbacks related to the use of air holes in fibers. The first one is the difficulty in precisely controlling the dimension of air holes in fiber fabrication. This is an intrinsic problem of a holey structure due to the air hole's tendency to collapse during fiber drawing. This is usually countered by a precise control of pressurization of the air holes, a process dependent on drawing conditions such as furnace temperature, feed rate, and drawing speed. When small air holes are desirable as in endless single-mode PCFs, higher pressure is required to maintain air hole dimensions due to the significantly increased tendency for the air holes to collapse by surface tension in this regime. This can make air hole dimensions to become highly sensitive to drawing conditions. This delicate balance of pressurization and collapse can lead to issues of controll‐ ability and repeatability in PCF fabrication. Air holes can also disturb smooth fracture wave propagation during fiber cleaving, leading to a poor cleaved surface due to the appearance of deep fractures behind the air holes, a problem often aggravated by large air holes and high cleaving tensions. In addition, air holes often have to be thermally sealed at the fiber ends to minimize environmental contamination. Mode distortion can occur from the air holes collaps‐ ing during splicing. This is especially true for large-mode-area fibers, which are much more

A detailed analysis of all-glass LCFs was reported in [35]. For a LCF formed by one layer of features as shown in the inset of Figure 10, the core of diameter 2ρ is formed by six features

. Center-to-center feature spacing is Λ. The refractive

was measured to be ~1.2. The PM LCF could be coiled to 12cm diameter with

was measured

was measured to be ~60% in a 5m long amplifier coiled at 40cm diameter. M2

232 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

to be ~1.3.

be ~60%. M2

negligible bend loss.

with fibers with air holes.

susceptible to small perturbations.

with diameter d and refractive index nf

**3.2. All-glass leakage channel fibers**

Figure 10 Effect of index contrast on confinement loss and modal index difference for one‐cladding‐layer LCFs with d/Λ = 0.675 and 2ρ = 50μm (left) and effect of d/Λ on confinement loss and the loss ratio between the second‐ order mode and fundamental mode for LCFs with Δn=1.2×10<sup>−</sup><sup>3</sup> and 2ρ=50μm (right) [35]. **Figure 10.** Effect of index contrast on confinement loss and modal index difference for one-cladding-layer LCFs with d/ Λ=0.675 and 2ρ=50μm (left) and effect of d/Λ on confinement loss and the loss ratio between the second-order mode and fundamental mode for LCFs with Δn=1.2×10−3 and 2ρ=50μm (right) [35].

index of the background glass, usually silica, is nb. The fiber was studied by a multipole mode solver for the effect of index contrast Δn=nb−n<sup>f</sup> . For the simulation in Figure 10, the following parameters were used: 2ρ=50 μm, d/Λ=0.675, and nb=1.444. The wavelength of the simulations was at 1.05μm. It can be seen from the left figure in Figure 10 that confinement loss for both fundamental, αFM, and second-order modes, α2nd, increases with a reduction of index contrast Δn, with the loss of the second-order mode, α2nd, remaining over an order of magnitude higher than the loss of the fundamental mode, αFM. The modal index difference, the difference between the effective mode indices of the fundamental and second modes, decreases toward low index with Δn by just ∼40% over three orders of magnitude change in Δn. A detailed analysis of all‐glass LCFs was reported in [35]. For a LCF formed by one layer of features as shown in the inset of Figure 10, the core of diameter 2ρ is formed by six features with diameter d and refractive index nf. Center‐to‐center feature spacing is Λ. The refractive index of the background glass, usually silica, is nb. The fiber was studied by a multipole mode solver for the effect of index contrast Δn = nb−nf. For the simulation in Figure 10, the following parameters were used: 2ρ=50 μm, d/Λ=0.675, and nb = 1.444. The wavelength of the simulations was at 1.05μm. It can be seen from the left figure in Figure 10 that confinement loss for both fundamental, αFM, and second‐order modes, α2nd, increases with a reduction of index contrast Δn, with the loss of the second‐order mode, α2nd, remaining over an order of magnitude higher than the loss of the fundamental mode, αFM. The modal index difference, the difference between the effective mode indices of the fundamental and second modes, decreases toward low index with Δn by just ∼40% over three orders of magnitude change in Δn. The effect of normalized hole diameter d/Λ was also studied in [35] and is shown in the right figure in Figure

The effect of normalized hole diameter d/Λ was also studied in [35] and is shown in the right figure in Figure 10 for confinement losses and the ratio of the second-mode loss to the fundamental-mode loss. The confinement loss for both the fundamental and second modes increases toward small d/Λ with the loss ratio changing very little over the entire range of d/Λ, from 22 to 28. The normalized hole diameter d/Λ is typically chosen to give an acceptable fundamental-mode loss. For LCFs with one layer of features as shown in Figure 10, the loss ratio of all-glass LCFs is very similar to that of an LCF with air holes. Slightly larger d/Λ is, however, required for achieving a similar confinement loss. 10 for confinement losses and the ratio of the second‐mode loss to the fundamental‐mode loss. The confinement loss for both the fundamental and second modes increases toward small d/Λ with the loss ratio changing very little over the entire range of d/Λ, from 22 to 28. The normalized hole diameter d/Λ is typically chosen to give an acceptable fundamental‐mode loss. For LCFs with one layer of features as shown in Figure 10, the loss ratio of all‐ glass LCFs is very similar to that of an LCF with air holes. Slightly larger d/Λ is, however, required for achieving a similar confinement loss. LCFs with two layers of features can be used to further improve the differential confinement loss between the fundamental and second‐order modes at the expense of bending performance. Acceptable fundamental‐mode loss

LCFs with two layers of features can be used to further improve the differential confinement loss between the fundamental and second-order modes at the expense of bending perform‐ ance. Acceptable fundamental-mode loss at smaller feature sizes can be realized in LCFs with two layers of features, while leakage of higher order modes is substantially increased by a reduction of feature size despite the additional layer of features. Higher differential loss between modes can therefore be realized. Since bending loss of the fundamental mode is very strongly dependent on feature size, a reduction of feature size increases the bend loss of the fundamental mode in LCFs. An LCF with two layers of features was studied in [35] and the results are shown in Figure 11. Both the fundamental mode and second-order-mode loss shows the characteristic increase at small d/Λ, while the loss ratio α2nd/αFM is increased by over an order of magnitude compared to the one-layer designs in Figure 10. At d/Λ ≈ 0.548, the at smaller feature sizes can be realized in LCFs with two layers of features, while leakage of higher order modes is substantially increased by a reduction of feature size despite the additional layer of features. Higher differential loss between modes can therefore be realized. Since bending loss of the fundamental mode is very strongly dependent on feature size, a reduction of feature size increases the bend loss of the fundamental mode in LCFs. An LCF with two layers of features was studied in [35] and the results are shown in Figure 11. Both the fundamental mode and second‐order‐mode loss shows the characteristic increase at small d/Λ, while the loss ratio α2nd/αFM is increased by over an order of magnitude compared to the one‐layer designs in Figure 10. At d/Λ ≈ 0.548, the fundamental‐mode loss αFM ≈ 0.1 dB/m, while the second‐order‐mode loss α2nd ≈ 48 dB/m, a loss ratio α2nd/αFM of ∼480. A very high loss ratio α2nd/αFM of ∼700 is possible at d/Λ = 0.62.

fundamental-mode loss αFM ≈ 0.1 dB/m, while the second-order-mode loss α2nd ≈ 48 dB/m, a loss ratio α2nd/αFM of ∼480. A very high loss ratio α2nd/αFM of ∼700 is possible at d/Λ=0.62.

**Figure 11.** Effect of d/Λ on confinement loss and the loss ratio between the second-order mode and fundamental mode for an LCF with two layers of features, Δn=1.2×10−3 and 2ρ=50μm. [35].

**Figure 12.** Some examples of fabricated all Glass LCFs. Core diameter is given above the fiber [35].

A wide range of all-glass LCFs were fabricated from core diameters from 35μm to well over 100μm [35] (see Figure 12). All fibers were made with silica glass as the background glass and slightly fluorine-doped silica glass as the cladding features and coated with standard coating with index of 1.54. The refractive index difference between the background and the low index feature wasΔ n=1.2×10−3. LCFs with both circular and hexagonal features were fabricated and tested. The conditions for the fabrication of LCFs with hexagonal features also created LCFs with a rounded hexagonal outline. Such a shape is known to be preferred for the pump mode mixing in a double clad fiber where the pump light propagates in a much larger cladding guide. All the fabricated LCFs in Figure 12 operated in the fundamental mode with a varying degree of bend loss performance. In general, bend loss increases rapidly with core diameter increase (see Figure 13). This effect is fundamentally related to the fact that the ability of guided modes to navigate a bend is related to how rapidly a mode can change its spatial pattern without breaking up while propagating, i.e. maintain adiabatic transition. As the mode gets larger, this ability to change diminishes very quickly due to larger Rayleigh range.

**Figure 13.** Cross section, measured mode and fiber details are given for the LCF with 101μm core, left inset, and the LCF with 183.3μm core, right inset. Measured bend loss for LCFs with various core diameters [36].

The LCF, shown on the top left inset in Figure 13, had 2ρ=101μm, and d/Λ=0.9. The effective mode area of the LCF was calculated to be 5117μm2 (MFD=80.7μm). A length of the LCF ~6m long was loosely coiled in a 1m coil and the measured M2 was M2 x=1.26 and M<sup>2</sup> <sup>y</sup>=1.29. The LCF, shown on the top right inset, had 2ρ=183.3μm, and d/Λ=0.8. A conventional single mode optical fiber of the same scale is also shown for comparison. The effective mode area of this LCF was calculated to be 15861μm2 (MFD=142.1μm), a record effective mode area for single-mode operation. The Measured M2 of a 1m straight fiber is M2 x=1.22 and M<sup>2</sup> <sup>y</sup>=1.23. Measured mode patterns at the output of the fibers are also shown in Figure 13.

#### **3.3. Polarization maintaining all-glass leakage channel fibers**

fundamental-mode loss αFM ≈ 0.1 dB/m, while the second-order-mode loss α2nd ≈ 48 dB/m, a loss ratio α2nd/αFM of ∼480. A very high loss ratio α2nd/αFM of ∼700 is possible at d/Λ=0.62.

234 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**Figure 11.** Effect of d/Λ on confinement loss and the loss ratio between the second-order mode and fundamental mode

A wide range of all-glass LCFs were fabricated from core diameters from 35μm to well over 100μm [35] (see Figure 12). All fibers were made with silica glass as the background glass and slightly fluorine-doped silica glass as the cladding features and coated with standard coating with index of 1.54. The refractive index difference between the background and the low index feature wasΔ n=1.2×10−3. LCFs with both circular and hexagonal features were fabricated and tested. The conditions for the fabrication of LCFs with hexagonal features also created LCFs with a rounded hexagonal outline. Such a shape is known to be preferred for the pump mode mixing in a double clad fiber where the pump light propagates in a much larger cladding guide. All the fabricated LCFs in Figure 12 operated in the fundamental mode with a varying degree of bend loss performance. In general, bend loss increases rapidly with core diameter increase (see Figure 13). This effect is fundamentally related to the fact that the ability of guided modes to navigate a bend is related to how rapidly a mode can change its spatial pattern without breaking up while propagating, i.e. maintain adiabatic transition. As the mode gets

**Figure 12.** Some examples of fabricated all Glass LCFs. Core diameter is given above the fiber [35].

larger, this ability to change diminishes very quickly due to larger Rayleigh range.

for an LCF with two layers of features, Δn=1.2×10−3 and 2ρ=50μm. [35].

A PM all-glass LCF was first reported in [37] (see Figure 14). The passive all-glass PM LCF had a core diameter of 50μm. A high d/Λ=0.9 was used for a smaller critical bend radius. The LCF had a refractive index difference between the background and the low index feature of Δn=1.2×10−3. The low index features were made of slightly fluorine-doped silica. Two stress elements with a refractive index of ~13 × 10−3 below that of the background silica glass were used instead of the regular features on either sides of the core to provide birefringence. The fiber had an outer diameter of ~885μm and was coated with standard acrylic coating. The near field image measured with a single lens is shown in Figure 14(c) for the 1.8m long sample and in Figure 14(d) for the 30m sample. Due to the much higher d/Λ=0.9 used for this fiber, some bending was necessary for fundamental mode operation in a short length of this fiber. The output was robustly single mode in a 30m long sample of this passive LCF coiled in 40cm diameter coils (see Figure 14(d)). The critical bend radius for 1dB/m loss, expected to be ~11cm by FEM simulation, matched very well to the 10.5cm measured. *Polarization extinction ratio* (PER) was characterized at the output of a 1.8m long sample to be >15dB over 1010-1100nm.

**Figure 14.** (a) Cross section of the passive PM LCF, (b) magnified cross section, (c) near field from the 1.8m long fiber in a 40cm-diameter coil, (d) near field from the 30m long fiber in a 40cm-diameter coil [37].

#### **3.4. Characterization of mode losses in all-glass leaky channel fibers**

Recently, fundamental and higher order mode losses have been characterized and compared to simulations based on the assumption of an infinite cladding [38]. A LCF with a ~50μm core diameter and a hexagonal cladding boundary (see Figure 15(a)) was spliced to a tunable source to ensure launch stability during the measurements. The passive LCF was coated with a lowindex polymer to simulate a double-clad fiber with a pump NA of ~0.45. Power in various modes at the output was measured using the S2 method [39]. The fiber was cut back several times to determine the propagation losses of various modes. The measurement was repeated at various coil diameters. The results are summarized in Figure 15(b) and show remarkable agreement between the measured and simulated losses of fundamental and higher-order modes. LP11 mode loss as high as ~20dB/m was measured, demonstrating the validity of the design. A similar LCF with a circular cladding boundary was also measured, showing significant less higher-order mode losses than those predicted by simulations. It is speculated that the coherent reflection at the circular cladding boundary played a significant role in enhancing the guidance of leaky higher-order modes in this case. It is, therefore, critical to have a non-circular cladding boundary to achieve the maximum possible higher-order mode losses.

#### **3.5. Ytterbium-doped all-glass leakage channel fibers**

An ytterbium-doped all-glass LCF with one layer of cladding features was also reported in [37]. The LCF was coated with a low index polymer, providing a pump NA of 0.45. This LCF had pump absorption of 11dB/m at ~976nm. The LCF also had 2ρ=52.7μm and d/Λ=0.8. This gives a simulated effective area of 1548μm2 at 1.05μm. The LCF has a rounded hexagonal shape and a flat-to-flat dimension of 254.2μm. The fiber was used to demonstrate amplification of 600ps pulses (with 600μJ pulse energy) to 1MW peak power.

by FEM simulation, matched very well to the 10.5cm measured. *Polarization extinction ratio* (PER) was characterized at the output of a 1.8m long sample to be >15dB over 1010-1100nm.

**Figure 14.** (a) Cross section of the passive PM LCF, (b) magnified cross section, (c) near field from the 1.8m long fiber in

Recently, fundamental and higher order mode losses have been characterized and compared to simulations based on the assumption of an infinite cladding [38]. A LCF with a ~50μm core diameter and a hexagonal cladding boundary (see Figure 15(a)) was spliced to a tunable source to ensure launch stability during the measurements. The passive LCF was coated with a lowindex polymer to simulate a double-clad fiber with a pump NA of ~0.45. Power in various

times to determine the propagation losses of various modes. The measurement was repeated at various coil diameters. The results are summarized in Figure 15(b) and show remarkable agreement between the measured and simulated losses of fundamental and higher-order modes. LP11 mode loss as high as ~20dB/m was measured, demonstrating the validity of the design. A similar LCF with a circular cladding boundary was also measured, showing significant less higher-order mode losses than those predicted by simulations. It is speculated that the coherent reflection at the circular cladding boundary played a significant role in enhancing the guidance of leaky higher-order modes in this case. It is, therefore, critical to have a non-circular cladding boundary to achieve the maximum possible higher-order mode losses.

An ytterbium-doped all-glass LCF with one layer of cladding features was also reported in [37]. The LCF was coated with a low index polymer, providing a pump NA of 0.45. This LCF had pump absorption of 11dB/m at ~976nm. The LCF also had 2ρ=52.7μm and d/Λ=0.8. This

and a flat-to-flat dimension of 254.2μm. The fiber was used to demonstrate amplification of

method [39]. The fiber was cut back several

at 1.05μm. The LCF has a rounded hexagonal shape

a 40cm-diameter coil, (d) near field from the 30m long fiber in a 40cm-diameter coil [37].

236 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**3.4. Characterization of mode losses in all-glass leaky channel fibers**

modes at the output was measured using the S2

**3.5. Ytterbium-doped all-glass leakage channel fibers**

600ps pulses (with 600μJ pulse energy) to 1MW peak power.

gives a simulated effective area of 1548μm2

**Figure 15.** (a) The hexagonal LCF used in the measurement and (b) simulated and measured mode loss in the hexago‐ nal Re-LCF [38].

Ytterbium double-clad all-glass LCFs with highly fluorine-doped silica as pump cladding were also demonstrated (see Figure 16) [37]. All-glass LCFs have no polymer in the pump path and have independent control of the fiber outer diameters and pump cladding dimensions. This, therefore, enables designs with smaller pump guides for higher pump absorption and, at the same time, with larger fiber diameters to minimize micro and macro bending effects, a much desired feature for large core fibers where intermodal coupling could be an issue due much increased mode density. Stress rods can also be added for PM LCFs (see Figure 16).

The LCFs had a refractive index difference between the background and the low index feature of Δn=1.2×10−3. The non-PM LCF (see the left figure in Figure 16) had an inner layer d/Λ=0.8 and outer layer d/Λ=0.7. It had a 47μm core diameter, a rounded hexagon pump guide with a dimension of 238μm by 256μm, a pump NA of 0.28, pump absorption of ~12dB/m, and an outer diameter of ~538μm coated with standard high index coating, shown as the outermost layer in Figure 16(a). A 3.5m fiber coiled in 53cm diameter was used to demonstrate a slope efficiency of 75% in an amplifier [37]. A single stage gain of 33dB was demonstrated using this fiber. It was also used to directly amplify 15ps pulses to a peak power of ~1MW.

The PM LCF (see Figure 16(b)) had a core diameter of 80μm and had a fluorine-doped pump cladding, providing a pump NA of ~0.28. Low index features with an inner layer d/Λ of 0.8 and an outer layer d/Λ of 0.7 were used. This active PM LCF had a pump guide diameter of ~400μm (flat-to-flat), a fiber outer diameter of ~835μm, and was coated with standard acrylic coating. Pump absorption was estimated to be ~12dB/m. The mode field diameter was measured to be ~62μm. The fiber was used as an amplifier in a single coil 76cm in diameter with a length of straight section at each end, demonstrating a slope efficiency of ~74% and a maximum single-path gain in excess of 30dB [37]. It demonstrated direct amplification of 14.2ps pulses to 190kW peak power with pulse energy of 2.74μJ and negligible SPM spectral broadening. M2 was measured to be below 1.35 for the entire output power range.

**Figure 17.** (a) The ytterbium-doped LCF with an index depression in the core center, (b) measured near field intensity of the guided mode, (c) measured mode intensity distribution, (d) simulated effective mode area versus the index de‐ pression, and (d) measured laser output and near field patters at various powers [40].

Recently, a flat-top mode has been demonstrated in an ytterbium-doped LCF with a ~50μm core by introducing an area ~30μm in diameter in the core center with a refractive index of ~2×10-4 lower than that of the background glass (Figure 17(a)) [40]. The flat-top mode (see Figure 17(b) and (c)) increased the effective mode area of the LCF from ~1200μm2 to ~1900μm2 , a ~50% increase (see Figure 17(d)). The LCF also demonstrated near quantumlimited efficiency (see Figure 17(e)). Lasing wavelength was 1026nm and the pump was at 976nm.
