High Power Tunable Laser using CMOS Compatible Amplifier

Introduction

High power tunable lasers and amplifiers have numerous applications such as integrated LIDAR, coherent photonic RADAR, telecommunications, optical frequency combs and synthesizers, and photonic devices for space applications. However, the technology landscape of high-power laser systems is dominated by bulky benchtop solid-state and fiber systems. Integrated photonic circuits can provide compact and mass-producible solutions, but have traditionally been limited to output powers in the milliwatt range due to the tight optical confinement.

Recent advances in large mode area (LMA) waveguide technology have enabled watt-level output powers from integrated amplifiers. This tutorial covers the design and experimental demonstration of a widely tunable laser amplified to high powers up to 1.5W using a CMOS-compatible integrated LMA power amplifier.

Device Design

The amplifier consists of two key sections - a high confinement small footprint mode guiding section and a high-power amplification large mode area (LMA) section, as shown in Figure 1.

Figure 1: a) Device layout showing high confinement (red) and LMA (green) sections. b) Waveguide cross-section and mode profile of the signal in the LMA section.

In the high confinement section, the optical mode is tightly confined within a silicon nitride (SiN) waveguide layer. This allows efficient coupling of pump and signal light as well as tight waveguide bends for compact circuitry.

The LMA section consists of a stack of SiN, silica, and alumina layers designed for a large optical mode area of ~30 μm2 around 1850 nm. The LMA and high confinement sections are connected via adiabatic tapers.

The LMA waveguides are coated with an alumina layer doped with thulium ions at a concentration of 6 x 1020 /cm3 to provide optical gain. While thulium is used here, the concept can be scaled to other gain ions like erbium and ytterbium.

Experimental Results

The experimental setup is illustrated in Figure 2a. A tunable thulium:YLF laser provides the seed signal which is coupled into the chip along with a 1610 nm pump laser through wavelength division multiplexers (WDMs). The amplified output is collected through a 90% tap coupler.

Figure 2: a) Experimental setup. b) Measured (markers) and simulated (lines) net gain. c) Measured and simulated amplified signal power.

The key results are the net gain (Figure 2b) and amplified signal power (Figure 2c) as a function of seed power and wavelength. Maximum amplified powers between 1400-1510 mW were measured for seed powers of 80-120 mW over the 1830-1890 nm wavelength range. The net gain ranged from 11-12.8 dB.

Higher gains up to 15 dB are possible with higher pump powers before the onset of lasing due to facet reflections. The pump power requirement was higher than expected due to slightly elevated passive waveguide losses of 0.3 dB/cm.

The measured spectra in Figure 3a show the signal is amplified with a high signal-to-noise ratio, although some broadening is observed due to feedback from the chip facets into the seed laser cavity. An isolator at the seed laser output can mitigate this effect.

Over the wider 1600-1950 nm bandwidth (Figure 3b), no amplified spontaneous emission (ASE) pedestal is observed when operating near the gain peak at 1850 nm with a high-power seed. At the edges of the gain bandwidth, ASE pedestals can appear as expected.

Figure 3: a) Signal spectra at input and output showing amplification. b) Wide optical spectrum showing negligible ASE pedestal around 1850 nm.

Significance

The achieved power levels of 1.4-1.5 W are over 25 times higher than the best semiconductor amplifiers at 2 μm wavelengths demonstrated previously. The integrated LMA waveguide amplifier surpasses the performance of many benchtop fiber amplifier systems while offering a compact, mass-producible integrated photonic solution.

With further increases in mode area to match large mode area fibers, such CMOS-compatible integrated high-power lasers can truly achieve performance parity with traditional benchtop systems. Additionally, integrated pump laser diodes can be co-packaged to create self-contained integrated high-power laser systems, opening up transformative new application landscapes.

Conclusion

This work demonstrates a key milestone towards replacing bulky benchtop laser systems with compact and scalable integrated photonic solutions across a wide range of applications requiring high optical powers and wavelength tunability.

Reference

[1]   N. Singh et al., "High power >1.5W tunable laser based on CMOS compatible high-power amplifier," Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Germany; Integrated Optical Systems, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands; LIGENTEC SA, EPFL Innovation Par L, Ecublens, Switzerland, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.