Ultrafast Laser Writing of In-Line Filters Based on MZI

Abstract

In mode-locked fiber lasers and optical sensors, in-line filters are essential components. Fiber-core Mach–Zehnder interferometer (MZI) technology has garnered a lot of research interest for the several manufacturing techniques for in-line MZI filters. Although multi-line inscription is frequently needed in existing methods to attain enough waveguide width, this approach adds complexity to production and may result in compromised waveguide quality. In this work, we present an improved single-line direct-writing method that attains similar MZI filtering results to multi-line scan. Additionally, the MZI filter created with the modified single-line direct-writing technique has a smaller insertion loss and requires less direct-writing energy than the previous single-line direct-writing technique. A 516 μm long MZI-based in-line filter was successfully constructed. The results of the characterization showed a central loss dip at 1089.82 nm, a free-spectral range (FSR) of 141.36 nm, an extinction ratio of 19.69 dB, and an insertion loss of 1.122 dB. This method decreased the insertion loss by a factor of 2.7 for an identical extinction ratio and improved the direct-writing efficiency by a factor of 9 for an equivalent FSR with multi-line scan. There was consistency between the experimental and simulation results. We also took measurements of the MZI’s temperature sensitivity. This work shows notable improvements in waveguide quality and ease of manufacture. This accomplishment lays the groundwork for further advancements in integrated mode-locked fiber laser technology.

1. Introduction

Mode-locked fiber lasers [1] and optical sensors [2,3,4,5,6] both depend on in-line filters. Fiber Bragg grating (FBG) filters [7,8,9], Long-period grating (LPG) filters [10,11,12], Lyot filters [13], Mach–Zehnder interferometer (MZI) filters [14,15], and birefringent plate filters [16] are among the many in-line filter technologies that have been created. Furthermore, multi-mode interferometers [17] and Fabry–Perot filters [18] have been widely used in optical sensing systems. The primary working principle of MZI filters is mode interference. Mode-field or core-mismatch fusion splicing [19], microfiber-based structures [20], paired long-period fiber gratings (LPGs) [21], specialty fiber segment fusion [22,23,24], and in-fiber air cavities [25] are some of the current MZI implementations. There are drawbacks to each method: LPGs limit the interference spectrum by their bandwidth and necessitate accurate manufacturing; specialty fibers are expensive; the manual assembly of fiber components creates issues with consistency; and microfiber and air-cavity devices are inherently fragile and unreliable. Notably, in these designs, core–cladding mode interference usually results in complicated, non-uniform spectra with significant insertion loss, and accurate control of the free spectral range (FSR) is still difficult to achieve.

Transparent solids like glass, crystals, and polymers can have their physical and chemical characteristics permanently changed using ultrafast laser direct writing [26,27,28,29,30,31,32]. Femtosecond laser micromachining of fiber in-line MZIs has been demonstrated in earlier work [33,34,35,36]. The majority of implementations still use multi-line scan techniques, which raise fabrication complexity and may impair device performance, even though Chen et al. [37] invented single-line inscription MZIs.

Three significant advancements in single-line scan technology are made in this research: (1) Initial use on HI1060 fiber, expanding usefulness to 1 μm wavelength mode-locked lasers; (2) improved waveguide position distribution to attain a free spectral range comparable to multi-line direct writing; and (3) reduced material damage risk and improved refractive index change by employing a lower direct-writing energy. These changes greatly improve MZI filter performance consistency and manufacturing efficiency. Using these developments as a foundation, we created a better single-line direct-writing method. We created a 516 μm MZI-based filter using this technique and contrasted it with results that had already been published [34]. Compared to multi-line scan, our method improved writing efficiency by 9 times for an equal FSR and decreased insertion loss by 2.7 times for an identical extinction ratio. This result lays the groundwork for the advancement of integrated mode-locked fiber lasers in the future.

2. Method

An off-axis positive refractive index-modified zone (PRIMZ) makes up the fiber core of the MZI’s basic structure, as shown in the schematic design (Figure 1).

The PRIMZ is created by femtosecond laser inscription, which alters the refractive index within a section of the core and cladding that is L in length and D in breadth. The original single-mode fiber segment becomes a few-mode portion upon the introduction of the PRIMZ. Consequently, when incident light travels through this region, the difference in effective refractive index between the different modes creates an optical path difference (OPD). An MZI filter is produced when these modes interfere with one another. Following transmission via the modified fiber of length L, the phase difference between the fundamental mode and a higher-order mode can be roughly described as follows:

$$\phi = {\displaystyle \frac{2·\pi ·OPD}{\lambda }}={\displaystyle \frac{2·\pi ·∆n_{eff}·L}{\lambda }}$$

(1)

where λ is the wavelength of light in a vacuum and $n_{eff}$ is the effective refractive index difference between the fundamental mode and the higher-order mode. The mode coupling efficiency is strongly affected by the distribution of waveguide sites. In our experiment, the core contained 80% of PRIMZ, with a 1.75 $\mathsf{\mu }$m center offset. The experimental setup for ultrafast laser direct-writing of MZI filters in a single-mode fiber (SMF, Corning HI1060, Corning Optical Communications LLC, Charlotte, NC, USA) is depicted in Figure 2.

Our ultrafast laser system (Qingdao Zimao Laser, TCR-FS-1030-20, Qingdao Free Trade Laser Technology Co., Ltd., Wuhan, China) has a repetition rate of 200 kHz, a pulse width of 900 fs, and a center wavelength of 1030 nm. By using an LBO crystal to double the frequency, 515 nm light was generated. To guarantee that frequency-doubled radiation dominated the writing process, a harmonic beam splitter filtered the light at the fundamental frequency. A 0.1 μm resolution three-dimensional translation stage (Newport, Inc., Franklin, MA, USA) was used to install a segment of stripped single-mode fiber. With the aid of an oil-immersion objective (Leica, 100x, NA 1.25, Leica Mikrosysteme Vertrieb GmbH, Wetzlar, Germany), the laser beam was concentrated into the fiber core. In order to reduce lensing effects at the fiber surface, the fiber was submerged in index-matching oil (SHINHO, n = 1.47, SHINHO OPTICS LIMITED, Shanghai, China). During the fabrication process, MZI filters were inscribed by scanning the ultrafast laser beam parallel to the fiber axis at 5 μm/s. The focused laser point had a pulse energy of 57.15 nJ and a diameter of about 3.5 μm. The laser polarization direction and the writing direction were parallel.

We used a gain fiber (LIEKKI, Yb 1200-6/125DC-PM, nLIGHT, Inc., Camas, WA, USA) and a single-mode pump (Mai Rui Optoelectronics, Hefei Mairui Optoelectronics Technology Co., Ltd., Hefei, China) at 976 nm to build a broadband source with an effective bandwidth of 200 nm in order to describe the spectrum response of the MZI filters. Real-time monitoring of output spectral changes was performed using an optical spectrum analyzer (Yokogawa AQ-6370D, Yokogawa Test & Measurement Corporation, Tokyo, Japan).

3. Results and Discussion

The input spectrum, the broadband source’s transmission spectrum, and the MZI filter’s transmission spectrum are shown in Figure 3.

The MZI filter’s transmission spectrum is shown in Figure 3 by the green solid line. The middle wavelength is 1089.82 nm. Its neighboring peak is 1019.14 nm. The summit and the valley are separated by 70.68 nm, about half of the FSR. The effective refractive index of PRIMZ can be computed by substituting it into Formula (2).

$$\mathrm{F}\mathrm{S}\mathrm{R}={\displaystyle \frac{{\mathsf{\lambda }}^2}{\Delta {\mathrm{n}}_{\mathrm{e}\mathrm{f}\mathrm{f}}\mathrm{L}}}$$

(2)

Substituting into Equation (2) gives a refractive index difference of 0.016, consistent with the 0.011 value reported in reference [34]. Figure 4 shows microscopic images of the MZI filter.

Physical pictures of the MZI filter made of HI1060 fiber are shown in Figure 4. Excellent pre- and post-inscription homogeneity and exact waveguide location within the core (1.75 μm offset) are confirmed in Figure 4a. Significant scattering from the MZI filter is observable under broadband illumination (Figure 4b), but the majority of the light remains contained within the waveguide, indicating outstanding guiding qualities. To validate the MZI design, we used commercial Rsoft software Version 2018.12 to run numerical simulations. Elliptical cylindrical waveguide with a major axis of 4 μm, a minor axis of 3.5 μm, and a length of 516 μm were among the parameters. A gaussian beam with a core diameter of 5.3 μm, a cladding diameter of 125 μm, a waveguide index of 1.48, a cladding index of 1.458, and a core index of 1.464 traveled through the fiber. BeamPROP was used in the simulations.

The simulation results are shown in Figure 5. In the waveguide region with high transmittance, Figure 5a, which corresponds to 1001.74 nm (passband region), exhibits notable modulation. At the stopband area of 1089.41 nm, shown in Figure 5b, effective mode blocking is evident. Wavelength-dependent transmission is shown in Figure 5c, where the wavelength of minimal transmittance corresponds to the experimental findings in Figure 3.

3.1. Performance Comparison

Table 1. Performance comparison of different direct-writing approaches for all fiber MZI.

Fiber Type	Waveguide Distribution	Length (mm)	Width ($\mathsf{\mu }\mathbf{m}$)	Strategic	Insertion Loss (dB)	Extinction Ratio (dB)	FSR (mm)	Pulse Energy (nJ)	Reference
SMF-28e	Core 100%	1.25	4	Multi-line (9 scan)	3.0	20	140	600	[34]
SMF-28e	Core 50% Clad 50%	5	4	Multi-line	5.0	-	34.5	84	[35]
G652.d	Core 100%	1.5	4	Single-line	5.0	10	112.5	200	[37]
HI1060	Core 80% Clad 20%	0.516	3.5	Single-line (1 scan)	1.1	19.7	141.4	57.15	This work

lists the current representative multi-line direct-writing and single-line direct-writing methods. In comparison to the multi-line direct-writing approach, our improved single-line direct-writing method is 2.7 times lower in terms of insertion loss and 9 times higher in efficiency, assuming the same free spectral range [34]. In comparison to the first single-line direct-writing method, there is a 4.5-fold decrease in insertion loss, a 1.97-fold gain in extinction ratio, and a 3.5-fold decrease in pulse energy [37]. Additionally, the distribution of waveguide positions now used differs from the approaches that are currently in existence. A 100% core fraction or a 50% core and cladding fraction are the two varieties of the multi-line direct-writing technique. We implemented a new position distribution that has a 20% cladding proportion and an 80% core fraction. This method lowers the insertion loss and efficiently increases the mode coupling efficiency.

3.2. Temperature Response

To describe the temperature response of the MZI filter, we created a unique measurement technique (Figure 6a). The temperature response curve is shown in Figure 6b. We took three observations at 10 min intervals for each temperature point to guarantee statistical validity. With a sensitivity of 10.5 pm/oC, the resonance wavelength moves toward longer wavelengths.

This MZI filter’s temperature sensitivity (10.5 pm/°C) is comparable to that of the conventional FBG sensor (9.18 pm/°C) [38], suggesting that it may find use in temperature sensing applications. The comparatively low temperature sensitivity of mode-locked fiber lasers aids in preserving steady mode-locking performance in the face of ambient temperature variations [9].

3.3. Waveguide Optimization

Figure 7 illustrates how the waveguide width and longitudinal length will affect the transmission rate at the central wavelength of 1089.41 nm at the present writing length. These two characteristics should therefore be crucial optimization goals when creating a single-line waveguide. Furthermore, the location of the core axis in Figure 7a serves as the waveguide width’s starting point. The percentage of the waveguide in the core location varies as the waveguide width rises. For instance, the transmission rate at the central wavelength is 0.57 and the core proportion is 100% when the waveguide width is 2 $\mathsf{\mu }$m; the core proportion is 80% and the transmission rate at the central wavelength is 0.1 when the waveguide width is 3.5 $\mathsf{\mu }\mathrm{m}$; and the core proportion is 60% and the transmission rate at the central wavelength is 0.53 when the waveguide width is 4 $\mathsf{\mu }\mathrm{m}$. This indicates that the current ideal waveguide distribution approach is an 80% core proportion.

4. Conclusions

An improved single-line scan direct-writing method that preserves high modulation performance while streamlining waveguide construction is presented in this study. We fabricated a 516 μm MZI filter using this method for the first time in HI1060 fiber. Commercial software simulations confirmed the MZI design by precisely controlling waveguide offset, and the results were in line with experimental characterization. In comparison to current methods, performance benchmarking validates benefits in terms of device footprint and insertion loss. Measurements of temperature response point to possible uses for mode-locked lasers and sensing.

Future research will concentrate on the following areas: (1) lengthening the waveguide to reach an FSR of less than 15 nm, which is appropriate for mode-locking applications; (2) examining pulse-burst laser processing to lower thermal damage thresholds; and (3) investigating integration paths for multi-functional photonic devices.