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Mismatch Considerations in Excitation of Single-Mode Circular Core Parabolic Index Fiber by Laser Diode via Upside Down Tapered Hemispherical Microlens on the Tip of the Fiber

  • Bishuddhananda Das , Tapas Ranjan Middya and Sankar Gangopadhyay EMAIL logo
Published/Copyright: November 21, 2017
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Abstract

We report the theoretical investigation of the coupling optics involving laser diode to single-mode circular core parabolic index fiber via upside down tapered hemispherical microlens on the tip of the fiber in the presence of possible transverse and angular mismatches. Using the relevant ABCD matrix for such tapered hemispherical microlens, we formulate analytical expressions for the coupling efficiencies in the presence of the said two mismatches. Further, the transmitted spot size of the source via the hemispherical lens and the tapered region should match with the spot size of the fiber for obtaining maximum coupling. The investigations have been made for two practical wavelengths, namely 1.3 and 1.5 μm in order to find the tolerance of this coupling device with respect to the said kinds of mismatches at the concerned wavelengths. Although our simple method predicts the concerned coupling optics excellently, the evaluation of the concerned efficiencies and associated losses involve little computations. Thus this user-friendly technique and also the results found thereof will benefit the designers and packagers who are working in the field of optical technology.

Appendix

The light beam input and output parameters of laser beam, namely q1 and q2, respectively, are mutually related as follows:

(18)q2=Aq1+BCq1+D

where

(19)1q1,2=1R1,2jλ0πw1,22n1,2

with R, n, w and λ0 representing the radius of curvature of wavefront, refractive index, spot size and the wavelength in free space, respectively.

The ray matrix M for the upside down tapered hemispherical microlens on the fiber tip can be given as [6, 20]

(20)M=A1B1C1D11u01=ABCD

where

(21)A1=R2(z)(nco1)ncoR0R1(z),B1=1ncoR1(z),C1=ncodR2(z)dz(nco1)R0dR1(z)dz,D1=dR1(z)dz,

Further, R1z,dR1zdz,R2zanddR2zdz are given by

(22)R1z=LηGz12SinηlnGzdR1zdz=1Gz12CosηlnGz+12ηSinηlnGzR2z=Gz12CosηlnGz12ηSinηlnGzdR2zdz=A02a02Ld2Gz12SinηlnGz

where z represents the axial length in the tapered region, L stands for the axial length from the end face of the fiber to the geometrical vertex of the tapered profile and d is the aperture radius.

Again, the parameters G(z), η, A0, z have been found as [6]

(23)Gz=1zLη=A02a02L2d214A0=d1zL1n2cln2co12a02z=Lda0d

Using eq. (20), one gets

(24)A=A1;B=A1u+B1;C=C1;D=C1u+D1

The upside down tapered hemispherical microlens transformed spot sizes w2x, 2y and radii of curvature R2x, 2yr can be found by using eqs (18), (19) and the ABCD matrix given by eq. (20) and those are as follows:

(25)w2x,2y2=A22w1x,1y2+(λ12B2)/(π2w1x,1y2)nA2DBC2
(26)1R2x,2y=A2C2w1x,1y2+λ12BD/(π2w1x,1y2)A22w1x,1y2+λ12B2/(π2w1x,1y2)

where λ1=λ0/n1,A2=A+B/R1;C2=C+D/R1andn=n2n1.

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Received: 2016-4-16
Accepted: 2016-6-10
Published Online: 2017-11-21
Published in Print: 2017-12-20

© 2017 Walter de Gruyter GmbH, Berlin/Boston

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