Extended-bandwidth frequency sweeps of a distributed feedback laser using combined injection current and temperature modulation

Abstract

This article details the generation of an extended-bandwidth frequency sweep using a single, communication grade distributed feedback (DFB) laser. The frequency sweep is generated using a two-step technique. In the first step, injection current modulation is employed as a means of varying the output frequency of a DFB laser over a bandwidth of 99.26 GHz. A digital optical phase lock loop is used to lock the frequency sweep speed during current modulation, resulting in a linear frequency chirp. In the second step, the temperature of the DFB laser is modulated, resulting in a shifted starting laser output frequency. A laser frequency chirp is again generated beginning at this shifted starting frequency, resulting in a frequency-shifted spectrum relative to the first recorded data. This process is then repeated across a range of starting temperatures, resulting in a series of partially overlapping, frequency-shifted spectra. These spectra are then aligned using cross-correlation and combined using averaging to form a single, broadband spectrum with a total bandwidth of 510.9 GHz. In order to investigate the utility of this technique, experimental testing was performed in which the approach was used as the swept-frequency source of a coherent optical frequency domain reflectometry system. This system was used to interrogate an optical fiber containing a 20 point, 1-mm pitch length fiber Bragg grating, corresponding to a period of 100 GHz. Using this technique, both the periodicity of the grating in the frequency domain and the individual reflector elements of the structure in the time domain were resolved, demonstrating the technique's potential as a method of extending the sweeping bandwidth of semiconductor lasers for frequency-based sensing applications.

Broadband High-Linear FMCW Light Source Based on Spectral Stitching

“…The high-linear swept light source ensures the accuracy of the ranging system, while the swept bandwidth directly affects the measurement resolution. Common realizations for achieving broadband frequency-swept light sources are categorized into external modulation methods [14][15][16][17][18] and internal modulation methods [19,20].…”

“…These techniques have been successfully demonstrated by various researchers with remarkable results. For example, Hefferman et al simultaneously modulated a single DFB laser with temperature and current and aligned the spectra using cross-correlations [19] to achieve a swept source with a total bandwidth of 510.9 GHz. Li et al used 10 DFBs with different output wavelengths for spectral concatenation, supplemented by equal optical frequency interval resampling to correct nonlinearity, and successfully increased the resolution of the system by 10 times [20].…”