Femtosecond laser direct-writing plays a key role in optical waveguide device fabrication owing to its advantages of being maskless, true three-dimension, high precision, and flexibility. However, due to the lack of a mature theoretical model that can completely describe the interaction mechanism of the laser pulse with different materials, numerous experiments are required to determine the set of optimal processing parameters. Scanning electron microscopy or atomic force microscopy can provide high-resolution images of fabricated samples, but they generally need the sample to be destroyed, which is a time-consuming process. An optical microscope combined with a laser processing system can be used for in-process monitoring. However, the material to be processed needs to be transparent to illumination, and the resolution of the optical microscope is generally limited. Recently, methods based on optical coherence imaging have been proposed to obtain depth information in addition to a two-dimensional image. However, owing to the limited dynamic range detection, these methods cannot be used to measure the weak scattered signal induced by the small refractive index of the material in optical waveguide device processing. Therefore, in-process techniques for efficiently determining the processing parameters of femtosecond laser direct-writing optical waveguide devices are useful and significant.

The backward reflection signal induced by femtosecond laser direct-writing optical waveguide devices was used for the in-process monitoring of the key processing parameters. We developed coherence domain reflectometry (OCDR) (Fig. 1) with a large reflection dynamic range (-10 dB to -95 dB) and high accuracy (±1.0 dB) to measure the reflection signal. By directly writing micro-nano defects in the core of a single-mode optical fiber with different processing parameters, backward reflection signals were generated and subsequently detected by the OCDR connected to one port of the fiber (Fig. 2). The variation tendency of the measured reflection curve was used to determine the optimal processing parameters, and the reflection for a specific case was used to identify the type of change in the material (Fig. 5).

The influence of three key parameters, that is, the pulse energy, pulse frequency, and direct writing speed of the femtosecond laser, on the backward reflection signal in fiber micro-nano processing is studied using OCDR. By increasing the pulse energy (fixed pulse frequency of 1 kHz), the variation tendency of the measured reflectivity can be divided into four different regions A, B, C, and D (Fig. 4), where the abrupt change point (0.355 μJ) between A and B is identified as the pulse energy threshold, and B is the optimal region for optical waveguide device writing owing to its relatively low return loss and large reflectivity tuning range. A similar behavior is also observed when the pulse frequency is changed (fixed pulse energy 0.415 μJ) (Fig. 6). By scanning the pulse frequency at different pulse energies, the threshold points for different setups of the two key parameters are obtained (Fig. 7). The results show that the threshold energy gradually decreases with an increase in the pulse frequency. By the distributed monitoring of the reflection signal along the direct writing path, we find that the reflectivity decreases with an increase in the writing speed (Fig. 8), and the uniformity of the reflectivity curve can be improved by multiple writing (Fig. 9).

A distributed sensing technique based on OCDR for the in-process monitoring of direct-writing microstructures in optical waveguides with femtosecond pulses is proposed. The measured backward reflection signal is used to determine the optimal fabrication parameters. The influence of three key parameters, that is, pulse energy, pulse frequency, and direct writing speed, on the backward reflection signal in fiber micro-nano processing is studied by OCDR. The measured results show that the threshold conditions of the pulse energy and pulse frequency that cause material property changes can be determined quickly and accurately via abrupt changes in the reflection curve, and the type of material property change can also be identified according to the curve variation and reflectivity. In addition, the influence of the direct writing speed on the fabrication uniformity is determined by taking advantage of the distributed sensing ability of the OCDR. This work provides a non-destructive, efficient, and in-process solution for exploring the processing parameters of femtosecond laser direct-writing optical waveguide devices.