I am trying to couple light from Laser with M^2=1, wavelength = 1030nm and beam diameter = 1.3mm. Please suggest a lens for efficient coupling into PCF (say 2um core). As far as I know, I have two options, either to use Aspheric lens or Microscope objective, however, I am not really sure which one would be a best choice. As I understood (Please advice if right or wrong)
For efficient coupling, these points should be considered.
1) N.A of lens should be equal or close to that of fibre (PCF)
2) Lens focus diameter should be equal to core size of the fibre (PCF)
3) Input beam should be parallel to Optical axis (Collimated)
Please advice.
Thanks
Abbas N. regarding your 3 points:
1) There is no need for the lens NA to equal that of the fibre. Usually, coupling will be more efficient if the lens NA exceeds that of the fibre, provided the lens aperture (entrance pupil) is sufficiently large to avoid vignetting the input beam.
2) Coupling is optimised when the beam is focussed to a spot whose field profile matches the field distribution of the fibre mode. Typically, both launch spot and mode profile are approximately Gaussian, and the matching criterion is for the launch spot diameter to equal the mode field diameter.
3) Launching the collimated input beam parallel to the optical axis of the lens which in turn is coincident with the optical axis of the fibre will minimise mode mismatch due to tilt errors and lens aberrations.
More specifically, you specify a beam quality factor M2 = 1, so the beam is close to an ideal Gaussian beam. When focussed by a lens, the radius of the beam waist at 1/e2 intensity, w0, is related to the wavelength, λ, and the divergence half-angle, θ, also at 1/e2 intensity by: sin θ = M2 λ / π w0
https://www.rp-photonics.com/m2_factor.html
Your collimated input beam width is W=0.65 mm. If the focal length of the lens is f, the radius of the beam at the lens principal plane is W = sin θ f = f λ / π w0
provided the lens NA > sin θ. https://www.newport.com/n/gaussian-beam-optics
The fibre mode field diameter will be comparable to the fibre core diameter, but need not be identical. The relationship depends on wavelength, and the extent to which the evanescent field extends into the cladding.
If we assume that the beam spot radius is equal to the fibre radius, w0 = a = 1 μm, then we require a lens focal length f = π W w0 / λ = 1.98 mm.
A larger mode field diameter will require a proportionately larger focal length. A 2.8 μm mode field diameter will require a focal length of approximately 2.8 mm. The exact value is not too critical. If the mode field radius is r0, the coupling efficiency (assuming Gaussian profile for both fibre mode and excitation) is:
η = 4 w02 r02 / (w02 + r02)2
A 40% mismatch between launch spot diameter and mode field diameter degrades the theoretical coupling efficiency by only 11% (to 89%).
Regarding the choice between microscope objective and aspheric lens, both are capable of diffraction-limited performance for a monochromatic beam aligned with the optical axis. The microscope objective will be more complex in construction, because it must deliver high resolution imaging over a relatively wide field of view, with illumination comprising a broad range of wavelengths. In contrast, a single element spherical lens (recommended in some answers) will suffer from spherical aberration, and will typically deliver poorer performance than a high quality aspheric.
The exact focal length of microscope objectives is not always stated. Magnification depends on the distance between objective and eyepiece. Tube lengths of order 160 mm are common, so a crude estimate for high power objectives is focal length f = 160 mm / magnification.
The 60x objective suggested by Suchita Yadav. would have a focal length around 2.7 mm, and would be a reasonable choice, especially if the fibre MFD is larger than 2 μm, but an 80x objective may be better. A 40x objective with 4 mm focal length is probably too long.
If the microscope objective is optimised for visible wavelengths, there may be some additional losses due to surface reflections at infra-red wavelengths.
Note that a weak supplementary lens can be used to modify the diameter of the beam as it enters the focussing lens, and adjust the spot size for optimum coupling.
Hope this helps.
According to my understanding, microscopic objective of 40x and 60x should work for you.
Dear Sir;
I think for each type their is a specific coupler... For example; the spherical one is better for longer wavelength...
Regards ;
Dear Sir
According to the fibre specifications your choice between the two available lenses for better coupling is depend in addition to your light source properties , as suggestion investigate the spherical lens first as it commonly used in coupling of IR wavelengths
Regards
Abbas N. regarding your 3 points:
1) There is no need for the lens NA to equal that of the fibre. Usually, coupling will be more efficient if the lens NA exceeds that of the fibre, provided the lens aperture (entrance pupil) is sufficiently large to avoid vignetting the input beam.
2) Coupling is optimised when the beam is focussed to a spot whose field profile matches the field distribution of the fibre mode. Typically, both launch spot and mode profile are approximately Gaussian, and the matching criterion is for the launch spot diameter to equal the mode field diameter.
3) Launching the collimated input beam parallel to the optical axis of the lens which in turn is coincident with the optical axis of the fibre will minimise mode mismatch due to tilt errors and lens aberrations.
More specifically, you specify a beam quality factor M2 = 1, so the beam is close to an ideal Gaussian beam. When focussed by a lens, the radius of the beam waist at 1/e2 intensity, w0, is related to the wavelength, λ, and the divergence half-angle, θ, also at 1/e2 intensity by: sin θ = M2 λ / π w0
https://www.rp-photonics.com/m2_factor.html
Your collimated input beam width is W=0.65 mm. If the focal length of the lens is f, the radius of the beam at the lens principal plane is W = sin θ f = f λ / π w0
provided the lens NA > sin θ. https://www.newport.com/n/gaussian-beam-optics
The fibre mode field diameter will be comparable to the fibre core diameter, but need not be identical. The relationship depends on wavelength, and the extent to which the evanescent field extends into the cladding.
If we assume that the beam spot radius is equal to the fibre radius, w0 = a = 1 μm, then we require a lens focal length f = π W w0 / λ = 1.98 mm.
A larger mode field diameter will require a proportionately larger focal length. A 2.8 μm mode field diameter will require a focal length of approximately 2.8 mm. The exact value is not too critical. If the mode field radius is r0, the coupling efficiency (assuming Gaussian profile for both fibre mode and excitation) is:
η = 4 w02 r02 / (w02 + r02)2
A 40% mismatch between launch spot diameter and mode field diameter degrades the theoretical coupling efficiency by only 11% (to 89%).
Regarding the choice between microscope objective and aspheric lens, both are capable of diffraction-limited performance for a monochromatic beam aligned with the optical axis. The microscope objective will be more complex in construction, because it must deliver high resolution imaging over a relatively wide field of view, with illumination comprising a broad range of wavelengths. In contrast, a single element spherical lens (recommended in some answers) will suffer from spherical aberration, and will typically deliver poorer performance than a high quality aspheric.
The exact focal length of microscope objectives is not always stated. Magnification depends on the distance between objective and eyepiece. Tube lengths of order 160 mm are common, so a crude estimate for high power objectives is focal length f = 160 mm / magnification.
The 60x objective suggested by Suchita Yadav. would have a focal length around 2.7 mm, and would be a reasonable choice, especially if the fibre MFD is larger than 2 μm, but an 80x objective may be better. A 40x objective with 4 mm focal length is probably too long.
If the microscope objective is optimised for visible wavelengths, there may be some additional losses due to surface reflections at infra-red wavelengths.
Note that a weak supplementary lens can be used to modify the diameter of the beam as it enters the focussing lens, and adjust the spot size for optimum coupling.
Hope this helps.
Most important for effficient coupling is that the focus diameter matches the mode field diameter of the fiber. That is equvalent to the effective(!) NA matching the fiber NA. Effective NA results from the lens diameter actually used by the beam diameter. Gaussian diameter of the focus spot is 4*lambda*focal_length/(pi*beam_diameter).
A microscope objective is generally over-engineered for that purpose. It is corrected for off-axis spot size, field flatness, longitudinal and transverse achromaticity, distortion and so on, none of that is needed to focus a single axial monochromatic beam. The many lenses used inside will rather create reflections and cost light.
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