# Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

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## Abstract

**:**

## 1. Introduction

#### 1.1. Power Scaling Limitations of Fiber Lasers and Amplifiers

#### 1.1.1. Nonlinear Effects (NLEs)

- Stimulated Raman Scattering effect

^{−13}m/W, which is nearly three orders of magnitude smaller than the Brillouin gain coefficient discussed next. Therefore, SRS is more prominent in pulsed systems, however, can also occur in kW-level CW systems.

- Stimulated Brillouin Scattering effect:

^{−11}m/W and it has the lowest power threshold among the NLEs in single-frequency CW fiber amplifiers. The main approach to mitigating the SBS is broadening the spectral linewidth via external phase modulation [34,35]. To realize the SBS suppression, the change in phase modulation should be less than the phonon lifetime. This is also why SBS does not typically occur for pulsed systems with pulse durations below the ~10 ns phonon lifetime. Numerous theoretical and experimental studies on mitigating the SBS effect in fiber lasers and amplifiers has been conducted to prevent instability and physical damage to the laser system [36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53]. As a case in point, in 2020, Wang et al. demonstrated a 2.5 kW narrow linewidth linearly polarized MOPA by efficient suppressing of SBS-induced self-pulsing (SP) via implementing cascade phase modulation [54]. In the same year, they presented the first 3 kW all-fiber narrow linewidth linearly polarized MOPA via a bi-directional pumping scheme [55]. In the case of non-polarized MOPA, in 2019, a 3.7 kW monolithic narrow-linewidth single-mode fiber amplifier was reported by suppressing the NLEs [56].

- Self-phase modulation effect

- Self-focusing effect

- Four-wave mixing effect

- Transverse mode instability effect

#### 1.1.2. Thermal Issues

#### 1.1.3. Optical Damage

#### 1.1.4. Pumping Limitations

^{2}factor of ∼1.3 [11], and 10.6 kW with the beam quality M

^{2}factor of ∼2 [85]. Even if the technical challenges for fabricating high-power YDFLs could be overcome, the highest theoretical achievable output power through diode pumping is calculated as 28–38 kW via a diode pumping scheme [20]. Therefore the need for new power-scaling approaches beyond the fundamental limitation has been recognized and is an active field of research.

#### 1.2. Methods for Power Scaling

#### 1.2.1. Tandem Pumping

#### 1.2.2. Beam Combining

## 2. Incoherent Beam Combining

#### 2.1. Side-by-Side Beam Combining

#### 2.2. Passive Components

#### 2.3. Spectral Beam Combining (SBC)

## 3. Coherent Beam Combining (CBC)

^{2}factor) of the output laser beam. Combining efficiency is typically calculated by dividing the power of the combined beam by the sum of the output power of amplified laser beams. However, for CBC systems, two key metrics are employed for assessing the function of CBC, which are brightness (B) and Strehl ratio (S). The brightness of each optical beam, which considers the output power and the quality of the beam (${M}^{2}\mathrm{factor}$), can be defined as:

#### 3.1. The Geometry of Combining/Splitting in Space and Time

#### 3.1.1. Tiled Aperture (TA)

- Multicore fibers and photonic crystal fibers (MCFs and MC-PCFs);

#### 3.1.2. Filled Aperture (FA)

- Polarization beam combiners and thin-film polarizer

- Intensity beam combiners

- Diffractive optical elements

- Segment mirrors (SM)

#### 3.1.3. Mixed Aperture (MA)

#### 3.2. Laser Sources and Amplifiers

#### 3.2.1. Seed Lasers

#### 3.2.2. Laser Amplifiers

- Large mode area (LMA) fibers

- Photonic crystal fibers (PCFs)

- Taper double-clad fibers (T-DCF)

#### 3.3. Phase-Locking Systems

#### 3.3.1. Passive Phase Control

- Common resonator

- Optical phase conjugate

- Evanescent (leaky) wave coupling

- Self-organized

#### 3.3.2. Active Phase Control

- Hansch–Couillaud (HC) polarization detection

- Hill Climbing

- Optical Heterodyne Detection (OHD)

- Frequency dithering

- Collective phase-intensity technique

- Collective phase measurement technique

- 2.
- Phase-intensity mapping (PIM)

#### 3.4. Optical Path Difference Control

#### 3.5. Channel Scaling

## 4. Coherent Beam Combining of Ultrafast Fiber Lasers

#### 4.1. Spatial CBC

#### 4.2. Temporal CBC

#### 4.3. Multidimensional (Spatial + Temporal) CBC

#### 4.4. Spectral CBC (Spectral Pulse Synthesis)

## 5. Coherent Beam Combining of CW Fiber Lasers

#### 5.1. Tiled Aperture

- Directed-energy applications

- Power scaling

#### 5.2. Filled Aperture

## 6. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**Schematic representation of four main methods of incoherent beam combining (

**a**) side-by-side beam combining, (

**b**) beam combining using all-fiber passive signal components, (

**c**) SBC with volume Bragg gratings (VBG) as a wavelength-dependent transmission element, and (

**d**) SBC with reflection diffraction grating as a dispersive optical element.

**Figure 4.**Intensity patterns of the coherent combination of seven beams in tiled-apperture geometry at different distances. The beam profiles are shown in the same frame scale.

**Figure 5.**Impact of fill factor on the far-field intensity patterns of the coherent combination of seven beams in tiled-apperture geometry. The 2D beam profiles shown in the insets are illustrated in the same frame scale.

**Figure 6.**Schematic of tiled aperture coherent beam combination; (

**a**) CBC of multiple laser array in far field due to propagation and (

**b**) CBC of multiple laser beam emitted from multicore fiber in far field by implementing an optical lens.

**Figure 7.**Schematic of filled aperture CBC systems by the four most popular optical elements; (

**a**) CBC with polarization beam splitter (PBS), (

**b**) CBC with intensity beam splitter (IBS), (

**c**) CBC with diffractive optical element (DOE), and (

**d**) and CBC with segment mirror.

**Figure 8.**Schematic concept of mixed aperture CBC based on MLAs as a splitting and combining sections along with spatial light modulator as a phase shifter.

**Figure 9.**Far-field intensity patterns of coherent beam combination of seven laser beams, (

**a**) when the phase-locking system is on and (

**b**) when the phase-locking system is off. The 2D beam profiles shown in the insets are illustrated in the same frame scale.

**Figure 13.**Schematic representation of spatial coherent beam combining; (

**a**) PCBC with PBSs as splitters, TFPs as combiners, and phase-locking system by implementing Hansch–Couillaud detectors; (

**b**) CBC using fiber coupler as a splitter, IBSs as combining elements, and phase-locking systems by utilizing a single detector.

**Figure 15.**Schematic representation of multidimentional coherent beam combination of ultrafast pulsed laser.

**Figure 16.**Schematic representation of spectral coherent beam combination of ultrafast pulsed laser.

Year | Channel | Geometry | Operation Mode | Combining Efficiency | Controlling System | Institution | Ref. |
---|---|---|---|---|---|---|---|

2006 | 48 | Tiled | CW | RMS erorr < λ/30 | SPGD | MIT | [293] |

2011 | 64 | Tiled | CW | RMS erorr < λ/10 | PIM | TRT | [148] |

2017 | 37 | Tiled | CW | 96% | PIM | UNILIM | [151] |

2020 | 61 | Tiled | Pulsed | 50%, RMS erorr < λ/10 | SPGD | IPP | [152] |

2020 | 81 | Filled | Pulsed | RMS error < 1% | PIM | LBNL | [153] |

2020 | 107 | Tiled | CW | 96% * | SPGD | NUDT | [154] |

Combining Configuration | Year | Average Power | Peak Power | Pulse Energy | Pulse Duration | Beam Quality (M^{2}) | Combining Efficiency (%) | Channel Number/Replicas | Configuration | Institution | Ref. |
---|---|---|---|---|---|---|---|---|---|---|---|

Spatial | 2014 | 230 W | 22 GW | 5.7 mJ | 200 fs | ≤1.3 | 88 | 4 | CPA, HC detection, PBS | Jena | [16] |

2020 | 10.4 kW | 0.5 GW | 130 µJ | 254 fs | ≤1.2 | 96 | 12 | CPA, LOCSET, IBS | Jena | [169] | |

2021 | 1 kW | 68 GW | 10 mJ | 120 fs | ≤1.2 | 94 | 16 | CPA, HC detection, PBS and TFP | Jena | [176] | |

Temporal | 2013 | 77 W | 1.3 GW | 430 µJ | 320 fs | ≤1.3 | 97 | 2 | CPA + DPA, TFP, passive | Amplitude &CNRS | [300] |

2014 | 37.5 W | 2.9 GW | 1.25 mJ | 380 fs | NA | 75 | 4 | DPA, LOCSET, PBS | Jena | [301] | |

Spatio-temporal | 2015 | 55 W | 3.1 GW | 1.1 mJ | 300 fs | ≤1.3 | 90 | 2×2 | CPA + DPA, passive | Amplitude &CNRS | [302] |

2015 | NA | NA | 37 µJ | 50 ps | NA | 75 | 2 × 4 | DPA, LOCSET, PBS | Jena | [304] | |

2016 | 700 W | 45 GW | 12 mJ | 262 fs | ≤1.2 | 78 | 8 × 4 | CPA + DPA, LOCSET, PBS and TFP | Jena | [173] | |

2019 | 674 W | 80 GW | 23 mJ | 235 fs | NA | 71 | 12 × 8 | CPA + DPA, LOCSET, PBS and TFP | Jena | [175] | |

Spectral synthesis | 2013 | 273 mW | NA | NA | 403 fs | NA | 85.8 | 3 | Spectral filters, LOCSET | UMich | [308] |

2013 | 10 W | 2 MW | 0.29 µJ | 130 fs | ≤1.4 | 86 | 3 | LMA fiber, dichroic mirror | CNRS | [309] | |

2013 | 370 mW | NA | NA | 290fs | NA | NA | 12 | MCF, Grating and MLA twin pulses with 1.75 ps separation | CNRS | [310] |

Tiled aperture | Directed Energy | Year | Distance | Channel number | Tip/Tilt Correction | Phase Control Method | Institution | Ref. |

2011 | 7 km | 7 | √ | SPGD | UD | [149] | ||

2015 | 7 km | 21 | √ | SPGD | UD | [166] | ||

Power scaling | Year | Power (kW) | Channel number | Combining efficiency (%) | Phase control method | Institution | Ref. | |

2011 | 1.08 | 9 | 85 * | SFD | NUDT | [279] | ||

2011 | 4 | 8 | 78 | SPGD | MIT | [147] | ||

2020 | 16 | 32 | >95 | OPA | CIVAN | [167] | ||

2021 | 7.1 | 7 | 86 * | SPGD | NUDT | [168] | ||

Filled aperture | Combining technique | Year | Power (kW) | Channel number | Combining efficiency (%) | Phase control method | Institution | Ref. |

DOE | 2016 | 5 | 5 | 82 | LOCSET | AFRL | [181] | |

PBS | 2017 | 2.16 | 4 | 94.5 | SFD | NUDT | [108] | |

AFPL | 2017 | 1.27 | 3 | NA | SPGD | MIT | [314] | |

RIW | 2010 | 0.1 | 4 | 80 | LOCSET | LM Corp | [315] |

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**MDPI and ACS Style**

Fathi, H.; Närhi, M.; Gumenyuk, R.
Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers. *Photonics* **2021**, *8*, 566.
https://doi.org/10.3390/photonics8120566

**AMA Style**

Fathi H, Närhi M, Gumenyuk R.
Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers. *Photonics*. 2021; 8(12):566.
https://doi.org/10.3390/photonics8120566

**Chicago/Turabian Style**

Fathi, Hossein, Mikko Närhi, and Regina Gumenyuk.
2021. "Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers" *Photonics* 8, no. 12: 566.
https://doi.org/10.3390/photonics8120566