Hybrid external-cavity lasers (ECL) using photonic wire bonds as coupling elements

Abstract

Combining semiconductor optical amplifiers (SOA) on direct-bandgap III–V substrates with low-loss silicon or silicon-nitride photonic integrated circuits (PIC) has been key to chip-scale external-cavity lasers (ECL) that offer wideband tunability along with small optical linewidths. However, fabrication of such devices still relies on technologically demanding monolithic integration of heterogeneous material systems or requires costly high-precision package-level assembly, often based on active alignment, to achieve low-loss coupling between the SOA and the external feedback circuits. In this paper, we demonstrate a novel class of hybrid ECL that overcome these limitations by exploiting 3D-printed photonic wire bonds as intra-cavity coupling elements. Photonic wire bonds can be written in-situ in a fully automated process with shapes adapted to the mode-field sizes and the positions of the chips at both ends, thereby providing low-loss coupling even in presence of limited placement accuracy. In a proof-of-concept experiment, we use an InP-based reflective SOA (RSOA) along with a silicon photonic external feedback circuit and demonstrate a single-mode tuning range from 1515 to 1565 nm along with side mode suppression ratios above 40 dB and intrinsic linewidths down to 105 kHz. Our approach combines the scalability advantages of monolithic integration with the performance and flexibility of hybrid multi-chip assemblies and may thus open a path towards integrated ECL on a wide variety of integration platforms.

Sub-kHz-Linewidth External-Cavity Laser (ECL) With Si3N4 Resonator Used as a Tunable Pump for a Kerr Frequency Comb

“…The intensity distribution has an approximately circular shape with a 1/e²-diameter of 3.6 µm. The device is similar to the RSOA used in [1], and further details relating to the measurement techniques are described in the associated Supplementary Information. At a pump current of 100 mA, the measured maximum small-signal gain is 22 dB, and the saturated output power is sat 11.4dBm, P = both obtained at a wavelength of 1550nm.…”

“…1(b). The measured transmission can be fit to a model to estimate the resonator parameters such as the loaded Q-factors and the FSR, see Appendix A and [1] for more details. At a wavelength of 1525 nm, corresponding to the center of our tuning range, we extract FSR of 195.7 GHz and 202.0 GHz for R1 and R2, respectively, along with approximately equal loaded Q-factors of 20 000.…”

“…where (0) c  corresponds to the round-trip phase delay at the center frequency c  whereas (1) c  and (2) c  give the corresponding first-and second-order derivatives. Note that…”

“…In this context, hybrid external-cavity lasers (ECL) are particularly interesting, opening the possibility to combine optical amplification in direct-bandgap III-V-based gain elements with tunable optical feedback offered by advanced passive photonic integrated circuits (PIC). Such feedback circuits can be efficiently implemented on the silicon photonic (SiP) platform [1]- [8]. Alternatively, silicon nitride (Si3N4) waveguides can be used for the feedback circuits [9]- [16], offering much lower linear and nonlinear losses than their SiP counterparts.…”

Flip-chip-integrated silicon nitride ECL at 640nm with relaxed alignment tolerances

“…6 Further research efforts aim at hybrid III-V chip integration without requiring a dedicated micro-package, among them flip-chip integration [7][8][9][10] as well as laser integration via photonic wire bonds. 11,12 Flip-chip integration in particular has the potential for being a very high throughput integration scheme. However, difficulties then arise in maintaining a good coupling efficiency, since it is not readily compatible with active alignment and obtaining good mode matching between the III-V chip and SiP PIC can be challenging without interposed optics.…”