We report on the progress of our efforts to apply silicon nitride photonic integrated circuits (PIC) to the miniaturization of optical coherence tomography (OCT) with the goal of facilitating its widespread use in ophthalmology at the point of care. In particular, we highlight the design and optical characterization of photonic building blocks allowing the realization of a silicon nitride PIC-based multi-channel swept-source OCT system in the 1060 nm wavelength region. Apart from waveguide structures, these building blocks include 3D-printed microlenses on the PIC end facets for efficient light coupling to and from the PIC.
As demand towards cloud-based services and high-performance computations grows, it imposes requirements on data center performance, and efficiency. Taking advantage of the mature CMOS process technology, and the fact that silicon is the basic material of electronics industry, silicon photonics makes possible production photonic integrated circuits that satisfy these requirements.
Here we explore the short-cavity hybrid laser consisting of a III-V amplifier integrated with a silicon photonic crystal (PhC) cavity reflector by so-called butt-coupling approach. The laser possesses great stability characteristics meeting the criteria for data center interconnect applications. The PhC reflector having a Q-factor of 104 at the lasing wavelength 1535 nm can be considered as a narrow-bandwidth filter. The laser demonstrates single mode and eventless operation without any dynamics on the background, and smooth radiofrequency spectrum without evidence of relaxation oscillation frequency. The latter fact is beneficial for many applications, and indicates extremely high damping in PhC laser, where the photon cavity lifetime is greatly improved by the high-Q PhC cavity reflector.
We confirm our experimental observations by theory based on delay differential equation model for a single-section semiconductor laser. We reveal the effective damping of the laser, when the detuning between the filter peak and the laser cavity mode is small, and the imaginary parts of the model eigenvalues equal zero. It is possible to undamp the relaxation oscillations forcing self-Q-switched operation in the laser owing to the cumulative action of the alpha-factor and the narrow filter.
In conclusion, we experimentally and theoretically demonstrated that relaxation oscillations can be suppressed in the short-cavity semiconductor laser with a narrow intracavity frequency filter. Additionally, on the basis of our analysis we expect the undamping of relaxation oscillations, and self-pulsations when the cavity mode is detuned from the filter peak frequency. The results might be useful for applications in data communications.
The ever decreasing demand for bandwidth in optical communications has made silicon photonics one of the promising technologies as it can dramatically reduce energy consumption and footprint in photonic integrated circuits (PIC). Many research efforts have aimed to incorporate silicon into the PIC platform by using it as a resonant reflector in the form of a microdisk, racetrack resonator, ring resonator or photonic crystal (PhC) cavity. Tuning of these devices allow for modulation of the lasing frequency by means of the electro-optic or thermo-optic effect.
Our solution utilises a III-V hybrid laser with a reflective semiconductor optical amplifier (RSOA) and a PhC cavity resonant reflector. Current research shows electro-optical modulation of a PN junction on the Si-reflector as a means of tuning the reflectance wavelength. This work focuses on the thermo-optical effect in silicon to achieve modulation of the lasing frequency. Modulation of the current to the PN junction on the Si-reflector of the external cavity laser will change the refractive index which will tune the reflectance wavelength and hence modulate the lasing frequency. PhC cavities are smaller in area than a typical ring resonator and have larger free spectral range that results in less severe mode competition effects.
For trace gas detection a frequency modulated laser scanned across the absorption frequency of the target gas will result in change in the output power of the laser. The PhC laser we demonstrate shows to have a very small intensity modulation (IM) on the output offering it as an ideal candidate for this application.
Experimental results show the laser to have a threshold current of 15 mA with output optical power of 300 µW. With an applied heating power of 25 mW, a frequency shift of 10 GHz was observed. At a modulation frequency of 10 kHz, a modulation depth of 2 GHz was observed.
We demonstrate frequency modulation (FM) in an external cavity III-V/Silicon laser, comprising a Reflective Semiconductor Optical Amplifier (RSOA) and an SU8 polymer waveguide vertically coupled to a 2D Silicon Photonic Crystal (PhC) cavity. Laser FM was achieved by local heating of the PhC using a resistive element of Ni-Cr metal as a microheater to change the refractive index in the cavity hence changing the lasing frequency. Presented is a thermal study of the laser dynamics and observations of the shift in lasing frequency.
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