Wavelength tuning and polarisation control with an integrated tunable birefringent filter for quantum cascade lasers

Abstract

Summary form only given. Birefringent filters are routinely used as a wavelength tuning element in bulk optics, broadly tunable lasers such Ti:Sapphire [1] and dye lasers. A detailed description of birefringent filter operation can be found in ref. [2]. In this paper, we report the design, fabrication and characterisation of an integrated electro-optically tunable birefringent filter for wavelength and polarisation tuning of quantum cascade lasers (QCLs) [3].Our generic design for an integrated tunable birefringent filter (ITBF) for QCLs is shown in Fig. 1(a). It consists of 2 quarter waveplates with a differential phase shift (DPS) section between them. The integrated equivalents of waveplates are polarization mode convertors (PMC) [4] that converts the TM (Transverse Magnetic) polarised light from the gain section into circularly polarised light. In a QCL there is only gain for TM modes because of the intersubband transition selection rules. The DPS section adjusts the relative phase of the TM and TE (Transverse Electric) components of the circularly polarized light from the first PMC (PMC1 in figure 1(a)); this is because the birefringence of DPS section can be adjusted by injection current. The relative phase of the TE and TM components at the input into the second PMC (PMC 2 in figure 1(a)) determines the output polarization. In a QCL, only TM light will experience gain and the wavelengths that are rotated back to TM polarization are determined by the current in the DPS section. Thus we have a QCL that has its operating wavelength selected by an electro-optic tunable birefringent filter. For wavelength tuning the gain section was operated pulsed (100 ns pulse width at 60 kHz repetition rate) above threshold, at a current density of 6.8 kA/cm, and wavelength tuning was recorded (shown in Fig. 1(b)) for various values of DC injection current density into the DPS up to 24nm at 0.41kA/cm2. In our polarization control experiments at 0 kA/cm2 a maximum throughput polarisation angle of 37O was recorded and this angle increases up to 82O with an applied DC current of 0.97 kA/cm2 into the DPS and no further change in polarization angle recorded above 0.97 kA/cm2 current density (shown in Fig. 1(c)).In summary we have presented results on a QCL that includes an integrated birefringent filter. The results show that the output wavelength and polarisation can be electro-optically tuned by injecting a DC current into a differential phase shift section.