Design of a noncooled fiber collimator for compact, high-efficiency fiber laser arrays

Leonid A. Beresnev, R. Andrew Motes, Keith J. Townes, Patrick Marple, Kristan Gurton, Anthony R. Valenzuela, Chatt Williamson, Jony J. Liu, and Chris Washer

Author Affiliations

Leonid A. Beresnev,^{1,}^{*} R. Andrew Motes,^{2} Keith J. Townes,^{1} Patrick Marple,^{3} Kristan Gurton,^{1} Anthony R. Valenzuela,^{1} Chatt Williamson,^{1} Jony J. Liu,^{1} and Chris Washer^{2}

^{1}U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, USA

^{2}Schafer Corp., 2309 Renard Place SE, Suite 300, Albuquerque, New Mexico 87106, USA

^{3}University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA

Leonid A. Beresnev, R. Andrew Motes, Keith J. Townes, Patrick Marple, Kristan Gurton, Anthony R. Valenzuela, Chatt Williamson, Jony J. Liu, and Chris Washer, "Design of a noncooled fiber collimator for compact, high-efficiency fiber laser arrays," Appl. Opt. 56, B169-B178 (2017)

A high-power fiber laser collimator and array of collimators are described with optical architecture, allowing one to transmit almost 100% of the full power output from fiber facets. In the case of coherent beam combining, more than 70% of the full power can be focused into a diffraction limited spot determined by the diameter of the conformal aperture. The truncated-Gaussian beam tails are not trapped inside the array but are redirected through the output lenses and dispersed outside of the array along with the main collimated beam, thus eliminating the requirement for cooling the array. Detailed analysis is presented for the beam tail propagation geometry’s dependence on array optical parameters, including the interior redirecting lenses. The parasitic scattering from imperfections of the interior lenses is estimated to be as small as a few watts when 1.5–2 kW is emitted by each fiber facet.

Mikhail Vorontsov, Grigory Filimonov, Vladimir Ovchinnikov, Ernst Polnau, Svetlana Lachinova, Thomas Weyrauch, and Joseph Mangano Appl. Opt. 55(15) 4170-4185 (2016)

Svetlana L. Lachinova and Mikhail A. Vorontsov J. Opt. Soc. Am. A 25(8) 1960-1973 (2008)

References

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Dependence of Power (${\mathsf{P}}_{\mathsf{F}}$) in the DLS, Truncated Power ${\mathsf{P}}_{\mathsf{TL}}$, and Irradiance in the DLS on the Ratio $\mathsf{d}/{\mathsf{\omega}}_{\mathsf{0}}$ Between Lens Diameter $\mathsf{d}$ and Gaussian Radius ${\mathsf{\omega}}_{\mathsf{0}}$

$d/{\omega}_{0}$

${P}_{F}$, % of Full Power

${P}_{\mathrm{TL}}$, % of Full Power

${P}_{\mathrm{LL}}$, % of Full Power

Decrease of Irradiance in DLS

1

2.05

73.7

7

19.3

1

2

2.20

73.0

5

22.0

1.01

3

2.65

66.5

1.4

32.1

1.11

4

3.5

48.3

0.1

51.6

1.52

Table 2.

Parameters of the Dispersed Beam’s Dependence on the Distance $\mathsf{a}$ Between the Fiber Tip and the Interior Lens^{a}

$a$, distance to fiber facet (cm)

8

9

10

11

12

$l$, distance between lenses (cm)

27

26

25

24

23

$b$, distance to first image (cm)

−13.3

−16.4

−20

−24.4

−30

$h$, size of hole in interior lens (mm)

6.4

7.2

8.0

8.8

diameter of ${P}_{\mathrm{TL}}$ spot on output lens (mm)

29.7

28.5

27.6

$B$, focusing distance of ${P}_{\mathrm{TL}}$ from array (m)

2.65

2.014

1.58

1.26

1.03

${D}_{s}$, spot size of ${P}_{\mathrm{TL}}$ at safe distance ${B}_{S}=10\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{m}$ (cm)

8.2

11.2

14.8

20.2

Diameter of ${P}_{\mathrm{TL}}$ spot at 1 km (m)

11

14

17.5

22.6

The output lens’s focal length is $F=350\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$, its diameter is 43 mm, and the focal length of the interior lens is $f=200\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$. The Gaussian beam fill factor, ${d}_{\mathrm{TL}}/{\omega}_{0}=3.5$, is selected to produce redirected beam tails with a truncated fraction, ${P}_{\mathrm{TL}}=0.1\%$. See “white” point $\mathit{C}$ in the plot of Fig. 4 ($d/{\omega}_{0}=3.5$).

Table 3.

Calculation of Power Density on the Interior Lens in Dependence on the Distance $\mathsf{r}$ from the Center; $\mathsf{d}$ is the Output Lens’s Diameter as a Projection of a Circle with Diameter $\mathsf{2}\mathsf{r}$^{a}

Dependence of Power (${\mathsf{P}}_{\mathsf{F}}$) in the DLS, Truncated Power ${\mathsf{P}}_{\mathsf{TL}}$, and Irradiance in the DLS on the Ratio $\mathsf{d}/{\mathsf{\omega}}_{\mathsf{0}}$ Between Lens Diameter $\mathsf{d}$ and Gaussian Radius ${\mathsf{\omega}}_{\mathsf{0}}$

$d/{\omega}_{0}$

${P}_{F}$, % of Full Power

${P}_{\mathrm{TL}}$, % of Full Power

${P}_{\mathrm{LL}}$, % of Full Power

Decrease of Irradiance in DLS

1

2.05

73.7

7

19.3

1

2

2.20

73.0

5

22.0

1.01

3

2.65

66.5

1.4

32.1

1.11

4

3.5

48.3

0.1

51.6

1.52

Table 2.

Parameters of the Dispersed Beam’s Dependence on the Distance $\mathsf{a}$ Between the Fiber Tip and the Interior Lens^{a}

$a$, distance to fiber facet (cm)

8

9

10

11

12

$l$, distance between lenses (cm)

27

26

25

24

23

$b$, distance to first image (cm)

−13.3

−16.4

−20

−24.4

−30

$h$, size of hole in interior lens (mm)

6.4

7.2

8.0

8.8

diameter of ${P}_{\mathrm{TL}}$ spot on output lens (mm)

29.7

28.5

27.6

$B$, focusing distance of ${P}_{\mathrm{TL}}$ from array (m)

2.65

2.014

1.58

1.26

1.03

${D}_{s}$, spot size of ${P}_{\mathrm{TL}}$ at safe distance ${B}_{S}=10\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{m}$ (cm)

8.2

11.2

14.8

20.2

Diameter of ${P}_{\mathrm{TL}}$ spot at 1 km (m)

11

14

17.5

22.6

The output lens’s focal length is $F=350\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$, its diameter is 43 mm, and the focal length of the interior lens is $f=200\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$. The Gaussian beam fill factor, ${d}_{\mathrm{TL}}/{\omega}_{0}=3.5$, is selected to produce redirected beam tails with a truncated fraction, ${P}_{\mathrm{TL}}=0.1\%$. See “white” point $\mathit{C}$ in the plot of Fig. 4 ($d/{\omega}_{0}=3.5$).

Table 3.

Calculation of Power Density on the Interior Lens in Dependence on the Distance $\mathsf{r}$ from the Center; $\mathsf{d}$ is the Output Lens’s Diameter as a Projection of a Circle with Diameter $\mathsf{2}\mathsf{r}$^{a}