Fig. 4: ECTL feedback measurements. | Nature Communications

Fig. 4: ECTL feedback measurements.

From: Hybrid integrated ultra-low linewidth coil stabilized isolator-free widely tunable external cavity laser

Fig. 4

a Schematic of the experimental setup for measuring the effect of optical feedback on the external cavity tunable laser (ECTL). The laser output power is initially split in two: the majority for feeding back to the laser and a small portion to measure the laser frequency noise (FN). A variable optical attenuator (VOA) sets the feedback power level that is monitored with a power meter. Polarization (POL) paddles ensure the returned optical field is aligned properly with the silicon nitride waveguide. The FN is measured with an optical frequency discriminator (OFD) and an optical spectrum analyzer (OSA) monitors for the onset of coherence collapse. b OSA trace of the ECTL output under the two highest feedback levels of −12 and −10 dB. c Frequency noise plots of the free-running ECTL operating at 1560 nm under different optical feedback levels ranging from −32 to −10 dB. The maximum feedback level is limited by the fiber-chip insertion loss and all fiber splitter and connector losses. The inset plots the OSA traces for ECTL output under the two highest optical feedback levels showing single-mode operation.

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