Abstract
We demonstrate that large mode area (LMA) photonic crystal fibers (PCFs) can be used as single-mode patch-cords for 250 nm laser light. We have studied the transmission of the 250 nm output beam of a frequency-quadrupled diode laser through a triangular structure LMA PCF with 10 microm core. We have achieved single-mode output with coupling loss of 1.8 +/- 0.6 dB and transmission loss of 1.5 +/- 0.2 dB/m. The critical bend loss radius is approximately 6 cm. The transmission loss is compared with published bulk silica measurements. Effects of optically induced damage were observed after prolonged operation and have been studied as function of laser power and time. The optical damage occurs primarily at the fiber input and can be partly ameliorated by cleaving the fiber input. For input power levels of < approximately 0.3 mW stable operation can be achieved for periods of >40 hours which is sufficient for many laboratory based applications. The results show the utility of these fibers for single-mode beam delivery in a spectral region where step-index single-mode fibers are not readily available.
Highlights
Single-mode delivery of ultraviolet (UV) light by optical fibers is desirable in many scientific applications requiring high beam quality
We demonstrate that large mode area (LMA) photonic crystal fibers (PCFs) can be used as single-mode patch-cords for 250 nm laser light
We have studied the transmission of the 250 nm output beam of a frequency-quadrupled diode laser through a triangular structure LMA PCF with 10 m core
Summary
Single-mode delivery of ultraviolet (UV) light by optical fibers is desirable in many scientific applications requiring high beam quality. Optical damage of silica by UV photons provides a practical impediment to fiber delivery in this wavelength range We have observed such effects in our experiment which, depending on laser power, lead to damage (reduced transmission) after 1s-10s of hours. These durations preclude use in long-term applications (e.g. communication links), but they are sufficient for many laboratory based experiments that critically require fiber optic delivery. A measurement campaign to study several thruster operating conditions requires approximately 1 hour In these experiments the laser must be kept outside the chamber and single mode fiber delivery is critical to allow precise and stable alignment of the beam to the high-finesse optical cavity within the chamber. In some cases pulsed lasers are currently used, but the use of single-mode fibers with continuous-wave lasers can be useful in these or similar experiments, for example to improve spatial resolution or detection limits
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