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High-power 150 mW extended cavity Si3N4 tunable narrow-linewidth
laser
Albert van Reesa, Wilson Tsonga, Ian van den Vlekkerta, Fathema Farjanaa, Rob E. M. Lammerinka,
Ilka Visscherb, Chris G. H. Roeloffzenb, Sami Musaa, Dimitri Geskus*a
aChilas B.V., Hengelosestraat 500, 7521 AN, Enschede, Netherlands
bLioniX International B.V., Hengelosestraat 500, 7521 AN, Enschede, Netherlands
*dimitri.geskus@chilasbv.com
ABSTRACT
Chilas develops off-the-shelf laser sources based on hybrid integration of Photonic Integrated Chips (PICs). Combining
the high optical powers of semiconducting optical amplifiers (SOAs) with low-loss wavelength tunable mirror structures
on Si3N4 PICs results in compact and robust tunable laser sources. These extended cavity diode lasers (ECDLs) exhibit
unique characteristics like wide tuning ranges (>100 nm), ultra-narrow linewidths (<1 kHz) and high output powers.
Here we present up to 162.5 mW of optical output power by combining two SOAs inside a single cavity, thereby scaling
the output power without the need of additional optical amplification on the output port. The presented laser operates
inside the telecom C-band, but the strategy can be tailored to other wavelengths like 850 nm, 780 nm and 690 nm, where
Si3N4 plays a key role. This new generation of hybrid integrated ECDLs, exhibiting high optical output powers, wide
wavelength tuning ranges and ultra narrow linewidths, opens up a wide range of applications.
Keywords: extended cavity lasers, tunable lasers, semiconductor lasers, silicon nitride
1. INTRODUCTION
Hybrid integration of semiconducting optical amplifiers (SOAs) with Photonic Integrated Chips (PICs) enables compact
and robust tunable laser sources with unique characteristics. Extending the laser cavity with a Si3N4 based feedback filter
enables wide tuning ranges and ultra-narrow intrinsic linewidths [1]. These tunable diode lasers have already found a
wide range of applications, from sensing and LIDAR to coherent communication and microwave photonics. Many
applications would benefit from increasing the output power, for example to overcome the threshold of nonlinear effects
or to increase the signal-to-noise ratio in sensing and RF signal transmission.
For the telecom C-band, these lasers have already matured and are commercially available. The output power is typically
in the order of tens-of-milliwatts. Adding an SOA outside the laser cavity has been shown to enable further light
amplification up to 220 mW [2]. However, adding a second SOA inside the laser cavity has some advantages. Two
identical SOAs can be used, both with a high reflection coated back facet, to form the end mirrors of the laser cavity. In
addition, increasing the intra-cavity power reduces the laser intrinsic linewidth, which scales with one over the intra-
cavity power. A hybrid-integrated with two SOAs has been shown to emit up to 117 mW of output power [1].
Here we present an integrated extended cavity laser by combining two SOAs with a Si3N4 circuit inside a single cavity.
This enables an increase of the output power up to 162.5 mW, as measured in a single-mode output fiber, without the
further need for amplification. The presented laser operates inside the telecom C-band, but the broad transparency
window of Si3N4 can be exploited with a similar design for lasers at other wavelengths, such as 850 nm, 780 and 690 nm.
2. DESIGN
The ultra-narrow linewidth, tunable dual-gain extended cavity laser comprises two optical amplifiers and a frequency-
selective feedback circuit, as schematically displayed in Fig. 1(a). The reflective semiconductor optical amplifiers
(RSOA) are based on indium phosphide (InP). Both RSOAs are electrically connected in series and can be electrically
pumped. The benefit of using two RSOAs over one is that the optical gain per cavity roundtrip is doubled in comparison
with a single RSOA. The high-reflective back-facets of both RSOAs are used as the mirrors of the laser cavity.
For frequency selection and cavity extension, the RSOAs are hybrid integrated with a photonic integrated circuit (PIC)
based on TriPleX® silicon nitride (Si3N4) waveguides embedded in a silicon oxide (SiO2) cladding [3]. This waveguide
structure allows for very low propagation losses (<0.1 dB/cm), which enables a high output power. At the interface of
the waveguide circuit and the RSOA, a waveguide taper is designed to match the mode field of the RSOA, which
reduces the interface loss when the light is coupled towards the waveguide circuit.
Figure 1. (a) Schematic design of the laser with two reflective semiconductor optical amplifiers (RSOA), coupled to a
feedback chip based on silicon nitride waveguides, shown with the red lines, and with platinum heaters on top, as shown in
green. (b) Photograph of the laser in a 14-pin butterfly mount. Both RSOA chips, the feedback chip and output fiber are all
assembled, and the heaters are wire bonded to the pins of the mount.
The feedback circuit consists of two micro-ring resonators (MRR) with a small difference in circumference to exploit the
Vernier principle so select one wavelength within the gain bandwidth of the RSOA. Two symmetrical Mach-Zehnder
interferometers are used, as tunable outcoupler and as coherent combiner, to couple light from the laser cavity towards
the output and to coherently combine the light in the laser output port. For phase tuning, platinum-based resistive heaters
are placed on top of several structures, indicated in Fig 1(a). When a voltage is applied over these heaters, the waveguide
refractive index changes locally. The heaters on top of the rings facilitate wavelength tunability. In addition, a heater is
placed on top of an intra-cavity waveguide, called the cavity phase section. This heater can be used to align the cavity
resonance with both ring resonances, for single-mode and high-power and laser operation. With the tunable outcoupler
the ratio of the light can be controlled, which is directed to the fiber output or back to the RSOAs. Finally, by the heaters
on the coherent combiner, the phase shifts can be optimized to obtain constructive interference of light at the laser
output, to maximize the output power.
The RSOAs and feedback circuit are all hybrid integrated, together with a fiber array, and placed in a 14-pin butterfly
package, as shown on the photograph in Fig 1(b).
3. EXPERIMENTAL RESULTS
To characterize the laser in terms of output power, we varied the pump current and measured the fiber-coupled output
power, as shown in Fig. 2(a). The laser power increases approximately linearly from the threshold current of 10 mA up
to currents of 250 mA. Above 250 mA, the power starts fluctuating with increasing current, probably due to mode hops.
The maximum output power of 134 mW in this experiment was reached at a pump current of 426 mA. For each current
setting, we fine-tuned the heaters for maximum power and confirmed single wavelength operation using an optical
spectrum analyzer (OSA). For currents above 400 mA, the laser started to heat up quickly, which prevented us from
further increasing the current in this experiment. In a separate measurement, 162.5 mW output power was measured at
438 mA gain current. We note that, at these high gain currents, the laser tends to operate multi-mode, probably due to the
high optical gain available in a single cavity roundtrip. It requires fine-tuning of all heaters to find a good setting for
single mode operation at high output power.
Figure 2. (a) Measured laser output power (purple) as function of amplifier current. The threshold current is about 10 mA.
The maximum output power of 134 mW in this measurement series was reached for 426 mA pump current. All heaters were
fine-tuned for maximum power and single-wavelength operation at 1538.4 nm for each current setting. The corresponding
amplifier voltage over both amplifiers, connected in series, is plotted in orange. (b) Laser spectra when the laser is tuned in
steps of about 5 nm over a range of 133 nm between 1466.7 and 1600.1 nm.
Because of the Vernier filter with heaters integrated on top of the ring resonators, the laser can be tuned over its full gain
bandwidth. To demonstrate that the laser is also widely tunable at high pump currents, we set the current to 350 mA for
this experiment. Figure 2(b) shows the tuning spectra, measured using an EXFO OSA20 optical spectrum analyzer, when
the laser is tuned in steps of about 5 nm, by varying the heater voltages on ring 1 and 2. The measured tuning range is
133 nm, between 1466.7 and 1600.1 nm.
Figure 3. Measured power spectral density (PSD) of the frequency noise as function of Fourier frequencies f, averaged over
10 measurements. The noise follows a 1/f dependence at low Fourier frequencies, as is typical for technical noise. At high
Fourier frequencies, the noise levels off to a white noise level of 67 Hz2/Hz, which corresponds to an intrinsic laser
linewidth of 212 Hz.
To characterize the frequency stability of the laser, we measured its frequency noise using a HighFinesse LWA 1k-1550.
For this measurement, we also set the pump current to 350 mA, to determine the frequency stability at a high pump
current. The ring resonators and phase section are tuned such that the laser operates in a single mode at a wavelength of
1549.2 nm, approximately at the center of its tuning range. The resulting power spectral density of the frequency noise,
as function of the Fourier frequency f, is shown in Fig. 3. At low Fourier frequencies, the noise follows a 1/f dependence,
which is typical for technical noise. At high Fourier frequencies, beyond 1 MHz, the noise level averages to a white
noise level of 67 Hz2/Hz. This noise level can be converted to an intrinsic (Schawlow-Townes) linewidth of 212 Hz.
4. CONCLUSION
We have demonstrated a compact and robust hybrid-integrated extended-cavity diode laser that delivers a maximum
output power of 162.5 mW, and operates over a broad wavelength range from 1466.7-1600.1 nm, while providing ultra-
narrow intrinsic linewidths down to 212 Hz. With these results, this new laser type has the potential to become the
workhorse for many quantum applications such as quantum sensing, quantum computing and quantum key distribution.
The wavelength agile character makes them suitable sources for spectroscopy and fiber sensor applications whereas the
high power and large side mode suppression ratio (>50 dB) makes them perfect candidates for LIDAR, coherent
communication and optical beamformer applications.
ACKNOWLEDGEMENTS
This project has received funding from the European Defense Fund (EDF) under grant agreement EDF-2021-DIS-RDIS-
ADEQUADE (n°101103417). Funded by the European Union. Views and opinions expressed are, however, those of the
authors only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting
authority can be held responsible for them.
REFERENCES
[1] Boller, K.-J., et al., “Hybrid Integrated Semiconductor Lasers with Silicon Nitride Feedback Circuits,”
Photonics, 7(1), 4 (2019).
[2] Chen, C., Wei, F., Han, X., Su, Q., Pi, H., Xin, G., Wu, H., Stroganov, A., Sun, Y., Ren, W., Chen, X., Ye, Q.,
Cai, H., and Chen, W., Hybrid integrated Si3N4 external cavity laser with high power and narrow linewidth.
Opt. Express, 31(16), 26078 (2023).
[3] Roeloffzen, C. G. H., et al., “Low-Loss Si3N4 TriPleX Optical Waveguides: Technology and Applications
Overview,” IEEE J. Sel. Top. Quantum Electron, 24(4), 4400321 (2018).
