New narrow-linewidth laser design and vibration testing

Narrow-linewidth lasers are used in a large range of applications, but many practical requirements remain unmet. They must be small, lightweight, highly insensitive to vibrations, demonstrate great performance related to power spectral density of frequency noise (PSDFN), and offer very good value for money. In this context, we have developed a new narrow-linewidth semiconductor-laser design that incorporates a linewidth-reduction system based on π-phase-shifted fiber-Bragg gratings (FBGs), a technology of interest for the ultranarrow-transmission notch they provide. (Phase-shift techniques are introduced in a uniform FBG structure to achieve a multibandpass filter with ultranarrow-transmission bandwidth.)

We designed a first version of our new laser source based on a 34mm-long π-phase-shifted FBG filter acting as a frequency discriminator.¹ This filter provides a transmission notch width of 15MHz (full width at half maximum). We observed that the FBG was mainly responsible for the narrowed laser's sensitivity to vibrations. Therefore, we decreased the FBG length for improved performance. Because the π-phase-shifted filter stores an important amount of energy at its center (see Figure 1: top row), decreasing its length while maintaining the filter width would increase the energy buildup. This could make the filter sensitive to optical power (heating effects). We therefore propose a π-phase-shifted FBG with a grating-free central region, which causes a spread of the intensity distribution along the FBG. It also allows us to decrease the FBG length to 8.5mm while keeping the buildup factor to an acceptable value (see Figure 1: bottom row). We adjusted the FBG's maximum index change to obtain the same 15MHz notch transmission width as for the 34mm FBG. The main characteristics of the 8.5 and 34mm filters are illustrated in Figure 1.

Figure 1. (left) Apodization and intensity profile, (center) reflection and transmission spectra, and (right) transmission and group-delay spectrum over the notch for a 34mm-long π-shifted fiber-Bragg grating (FBG: top row) and a reduced 8.5mm-long π-shifted FBG with grating-free central region (bottom row). T, R: Transmissivity, reflectivity.

The shorter 8.5mm FBG is easier to package and is inherently less sensitive to vibrations. In addition, with its central grating-free region, it can sustain incident optical powers of a few milliwatts without any problem. We fabricated this FBG filter in a polarization-maintaining fiber and characterized it using a frequency-scanned narrow-linewidth laser. The result (see Figure 2) illustrates good agreement with the simulated design.

Figure 2. Experimental reflection spectrum of the 8.5mm-long π-shifted FBG with grating-free central region.

The FBG was packaged and incorporated into a linewidth-reduction system (see Figure 3). This enables reduction of the frequency noise of the free-running laser within the bandwidth of the servo loop (several hundreds of kHz) by up to 2–4 orders of magnitude (see Figure 4: left). As a result, the linewidth of the laser decreases from approximately 300 to 1.8kHz (see Figure 4: right). These results are similar to those obtained with the original 34mm-long π-phase-shifted FBG.

Figure 3. Schematic of the linewidth-reduction system. DFB: Distributed feedback. Σ: Summator.

Figure 4. (left) Measured power spectral density of frequency noise (PSDFN) and (right) calculated optical spectrum, under (red) free-running conditions and (blue) negative electrical feedback, using the 8.5mm-long FBG (TeraXion's PS-NLL PureSpectrum^TM narrow-linewidth laser).

We characterized the laser's sensitivity to vibrations by mounting the FBG package onto a mechanical shaker. We imposed a random vibration profile (in the 5–200Hz range) on the shaker and measured its effect on the laser's PSDFN (see Figure 5). Reducing the filter length resulted in a dramatic improvement in the acoustic-isolation efficiency. The results clearly demonstrate a very low sensitivity of the linewidth-narrowed laser to external vibrations, close to that of the unperturbed laser (gray curve).

Figure 5. PSDFN of the laser submitted to a random vibration profile.

In summary, we qualified our semiconductor narrow-linewidth laser under a series of stringent vibration tests.² We compared the results against internal (previous designs) and external alternatives (distributed-feedback fiber lasers). Vibration insensitivity is the most important parameter for many industrial and energy applications, for instance for lidar (light detection and ranging) and wind energy, metrology, and interferometric fiber-sensing applications. We made our qualification testing methodology public and invite customers and other vendors to take a similar approach and compare their results to ours. Future work will focus on developing and testing a newly engineered narrow-linewidth laser source with reduced dimensions and decreasing the noise at high frequencies.