Pockels laser is a hybrid integrated laser made by integrating the pockels effect into a semiconductor laser. This laser has high-frequency modulation speed and fast switching by the integration of the electro-optic (EO) effect. It is the first multi-color laser in which the lasing occurs at infrared and visible frequencies by frequency doubling. It has narrow linewidth and wide tunability. 

This laser has a III-V/external cavity structure having high stability. By using a lithium-niobate-on-insulator (LNOI) waveguide element as an external cavity, a III-V gain section is united with the pockels effect in an integrated laser.

Design of Pockels laser

Figure 1: Schematic of Pockels laser

The schematic of pockels laser is depicted in figure 1. This hybrid integrated laser is formed by edge-coupling a III-V reflecting semiconductor optical amplifier (RSOA) to an external cavity on a LNOI chip. There is a chance for mode mismatch causing insertion loss between the RSOA and LNOI which may result in power loss. This is solved by using a spot size converter at the edge of the RSOA. An anti-reflective layer is coated on the III-V facet to reduce the facet reflection. To achieve reduced reflectivity and match the angle of injected light, LNOI’s input-facet waveguide is angled by 10 degrees. For reducing further reflectivity, an AR coating is applied to the input facet of LNOI. By electro-optically modulating the phase shifter of the device, laser frequency modulation is achieved.

The LNOI external cavity is made up of two racetrack resonators forming a Vernier mirror structure. The racetracks and bus waveguides are designed to reduce the number of coupled mode families and prevent multi-mode lasing. The selection of coupling is done carefully by considering the lasing power, laser linewidth, and the tuning speed of the cavity. The free spectral range of the resonators is set at 70 GHz and there is a difference of 2 GHz between the two resonators.

Each resonator has a unique purpose. The first one contains a microheater for broad wavelength tuning using the thermo-optic effect. The second one is designed for high speed electro-optic tuning by integrating driving electrodes in it. The periodically-poled lithium niobate (PPLN) cavity section embedded inside the second resonator makes it compatible with the second harmonic generation process. To align the longitudinal laser cavity mode with the Vernier mode, a tunable phase control section is also implemented in the cavity and this section is operated using the EO effect as the EO Pockels factor in lithium niobate is very large. This method allows high-speed, energy-efficient, and independent control of individual functionalities. There is a sagnac loop ring placed at the end of the external cavity that acts as the output end mirror with 30% reflectivity.

Both racetrack resonators use different coupling structures. The first one uses a pulley coupler structure and the second one uses a straight waveguide for second harmonic generation at both telecom and near-infrared wavelengths. Both the bus waveguide structures are mainly designed to work for the fundamental quasi-TE mode.

The bandwidth of the lasing spectrum is improved by the pulley coupling structure, but the risk of multimode lasing is high. This problem is solved using the second coupling design by achieving single-mode lasing and suppressing the mode that is one vernier free spectral range away. The high-single-mode suppression ratio is above 50 dB. 

The tuning wavelength range for this laser is from 1576 nm to 1596 nm as shown in figure 2 and this is realized by thermo-optical tuning. The fundamental lasing wavelength of this laser is 1518.12 nm which is in the near IR range and the frequency-doubled wavelength is 790.56 nm which comes under the visible range and is shown in figure 3. The power of the second harmonic light is relatively low and limits the intracavity lasing power at the fundamental frequency. This can be improved by optimizing the coupling efficiency between the RSOA and the LN chip and by implementing longer sections of PPLN inside the cavity to enhance the SHG efficiency.

The Pockels laser has the advantage of incorporating a wavelength converter inside the lasing cavity offering faster configurability of the visible light which is important for atomic/ion trapping experiments to conduct optical pumping, controlling image light, and laser cooling steps.

Applications

Integrated pockels laser can be used for different applications like atomic physics, LiDAR, microwave photonics, etc. In atomic physics, switching speed is required upto MHz level for ion/atom manipulation at visible and near visible bands and LiDAR requires a narrow-linewidth laser with high linearity and speed above MHz range.