Abstract
We report on the experimental generation of broad and flat optical frequency combs (OFC) in a 1550 nm laser diode using gain switching with pulsed electrical excitation together with optical injection. The combination of both techniques allows the generation of high-quality OFCs at a repetition frequency of 500 MHz, showing a low-noise optical spectrum with unprecedent features in terms of width (108 tones within 10 dB) and flatness (56 tones within 3 dB) in comparison with those previously reported for this modulation frequency. The influence of the injection conditions on the OFC quality is studied. Using these two techniques, it has been possible to reduce the separation between tones, generating high spectral performance OFCs with a repetition rateof 100 MHz.
Highlights
In the same way as for other photonic technologies, the use of semiconductor sources for the generation of optical frequency combs (OFCs) is gaining increasing relevance due to the inherent advantages of the laser diodes in terms of high efficiency, low cost and small footprint, allowing to envisage prospects for the deployment of the technology outside the laboratory at a cost-effective price [1,2,3].Gain-switching (GS) has proven its convenience as a method for generating OFCs due to its easy implementation, robustness and stability, controllable repetition rate, and adaptability for integration using generic platforms [4,5,6]
As we reported in [11] at low repetition frequencies, broad, flat and well resolved OFCs cannot be generated by GS under sinusoidal electrical excitation and only relatively narrow and non-flat OFCs are generated when the laser is not switched-off, i.e., under direct modulation conditions
We have experimentally demonstrated that very broad and flat OFCs can be generated from a gain switched semiconductor laser at relatively low frequency by means of pulsed excitation in combination with optical injection
Summary
In the same way as for other photonic technologies, the use of semiconductor sources for the generation of optical frequency combs (OFCs) is gaining increasing relevance due to the inherent advantages of the laser diodes in terms of high efficiency, low cost and small footprint, allowing to envisage prospects for the deployment of the technology outside the laboratory at a cost-effective price [1,2,3].Gain-switching (GS) has proven its convenience as a method for generating OFCs due to its easy implementation, robustness and stability, controllable repetition rate, and adaptability for integration using generic platforms [4,5,6]. Out of this small range either the pulses are spectrally narrow when the laser is just modulated, or there is a switch-off period long enough for the coherence to be lost, resulting in a spectrum without discernible lines It is in this low repetition frequency range where OFCs generated by GS have recently found interesting advantages in dual-comb optical spectroscopy [9]. These advantages are simplicity of use, control of the line spacing, compactness and potential for integration, and capabilities for implementation outside the 1.5 μm spectral range. When higher resolution (lower repetition frequency) is required the approach is clearly hindered by the narrowing of the window of suitable conditions for the required spectral width of the comb, compromising the usefulness of the OFCs generated by GS for this application
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