Ultra fast, robust, stable, and high precision: these are some of the characteristics of a new laser developed by an international research team. This ultra-small laser paves the way for a new generation of highly powerful, ultra-stable integrated lasers.
“We advanced a new approach to develop a laser that boasts as yet unparalleled stability and precision, allowing us to conduct new experiments and open up new realms of research,” said Professor Morandotti, who was elected a fellow by the Optical Society of America and by the International Society for Optics and Photonics (SPIE). “Plus, a multitude of applications may be created in biology, medicine, materials processing, IT, high speed communications, and metrology.”
Flexible and effective, this ultra-small laser stands out for its mode of operation. The researchers developed a ring resonator (a key laser key component) that has the unique feature of playing a dual role by acting both as a filter and a non-linear element. This is the first time researchers have successfully integrated a resonator and a micro-ring in the laser component that makes it possible to better control the light source. It is manufactured using a special glass capable of harnessing the nonlinear optical properties central to laser operation.
For the first time, the researchers tested the filter-driven four-wave mixing method, which presents a number of advantages. Notably the method makes it possible to increase the laser’s stability and resistance to external disruptions, increase the amplitude of light pulses while reducing their duration, and emit extremely high quality, high-repetition-rate pulses of up to 200 gigahertz or more, while maintaining a very narrow spectral bandwidth.
(a) Schematic of the central component—a monolithically integrated 4-port high-Q (Q=1.2 million) microring resonator (fibre pigtails not shown) (b) High repetition rate laser based on filter-driven FWM: the microring resonator in (a) is embedded in a loop cavity containing a gain fibre (EDFA), band-pass filter with the main purpose of controlling the central wavelength λ0, a delay line to control the phase of the main cavity modes with respect to the ring modes, an isolator to force the pulse-circulating direction, and a polarization controller to act on the pulse polarization, as the ring resonator is birefringent. The waveform at the output of the amplifier is monitored with an autocorrelator to measure the pulse duration, and an optical spectrum analyser (OSA) and a photodiode connected to an oscilloscope to respectively measure the optical and radio frequency spectrum of the waveform. An OSA is also employed to monitor the optical spectrum at the output of the ring resonator. (c) SEM picture of the ring cross-section before depositing the upper cladding of SiO2. The waveguide core is made of high index (n=1.7) doped silica glass. The scale bar represents 1 μm. (d) Electric field modal distribution for a TM polarized beam calculated through vectorial mode-solving. The scale bar represents 1 μm.
Ultrashort pulsed lasers, operating through the phenomenon of mode-locking, have had a significant role in many facets of our society for 50 years, for example, in the way we exchange information, measure and diagnose diseases, process materials, and in many other applications. Recently, high-quality resonators have been exploited to demonstrate optical combs. The ability to phase-lock their modes would allow mode-locked lasers to benefit from their high optical spectral quality, helping to realize novel sources such as precision optical clocks for applications in metrology, telecommunication, microchip-computing, and many other areas. Here we demonstrate the first mode-locked laser based on a microcavity resonator. It operates via a new mode-locking method, which we term filter-driven four-wave mixing, and is based on a CMOS-compatible high quality factor microring resonator. It achieves stable self-starting oscillation with negligible amplitude noise at ultrahigh repetition rates, and spectral linewidths well below 130 kHz.
Our ultimate objective is a versatile, fully monolithically integrated laser source. Because waveguide amplifiers have been demonstrated in silica glass platforms, we do not envisage any fundamental issues preventing the full integration of our scheme. This proof-of-concept device represents a key step in realizing a fully integrated, stable, high–performance, laser source operating at flexible and very high repetition rates.
In summary, we propose and demonstrate a novel mode-locked laser based on a monolithic high-Q resonator, capable of generating both picosecond and sub-picosecond transform-limited pulses at a repetition rate of 200.8 GHz and beyond. Our device operates via a new mechanism that enables stable mode-locked lasing with negligible amplitude noise, and extrinsically limited phase noise, not constrained by the repetition rate. We believe this work represents a key milestone in the generation of ultra-stable, high repetition rate, optical pulse sources, particularly because of its CMOS-compatible monolithic platform.