Laser diode to photonic crystal fiber coupling via cylindrical microlens on the fiber tip: study of coupling optics in the presence of possible misalignments

The theoretical approximation of the coupling mechanics between a laser diode (LD) and photonic crystal fiber (PCF), which has cylindrical microlens (CML) on its tip, in the context of probable angular and transverse mismatches is presented in this article. It is easy to fabricate reasonably efficient CML on the fiber and further, the single-mode area of PCF is vast. Thus, this coupling device stands out as a promising contender in the realm of optical coupling. The analysis accounts for the permissible aperture offered by the CML. Further, as the coupling in the horizontal plane performs poorly, we will just address the coupling in the vertical plane here. By employing a suitable ABCD matrix of refraction for the CML in the vertical plane, we provide analytical formulations for the coupling efficiencies (CE) when the said types of misalignments exist. Our analysis adopts Gaussian field distribution (GFD) of the fiber and laser fields. The investigation contains the use of two wavelengths namely 1.5 µm and 1.3 µm. In respect of maximum CE, we have used optimized distance between the LD and the microlens for a certain type of PCF with a given air-filling ratio and hole pitch. This allows us to obtain the greatest CE for the said kind of fiber in the existence of possible mismatches for any value of the radius of the CML. There is minimal computation involved in the process of evaluation on the basis of the suggested formulations. Hence, the assessment of the relevant CEs is highly elegant. The tolerance levels of the coupling devices are exposed in this investigation. The study reveals the degree of tolerance of the coupling device with respect to both kinds of misalignments at the wavelengths used. In fact, the coupling device has been found to be more tolerant with respect to angular mismatches. Therefore, the current work creates opportunities to adjust various PCF characteristics and the operating wavelength as well in order to improve CE. Accordingly, to design an effective coupler of this type, the current study will be quite beneficial.