Objective External-cavity tunable semiconductor laser (ETSL) has been widely studied and acted as a prior selected laser source for its prestigious characteristics such as broad wavelength tuning range, single mode, narrow linewidth, and compactness. However, limited by the intrinsic operation characteristics of currently available semiconductor lasers, it is difficult to obtain a wide-range tunable laser beam output with high spectral purity directly generated by traditional monolithic semiconductor lasers. Particularly, most applications require that the output wavelength of the ETSL can be scanned continuously over time. Consequently, it is critical to build and maintain an ETSL system with a wide mode-hopping free tuning range. For this purpose, a Littman-Metcalf external-cavity oscillation structure is designed in this paper.
Methods First, according to the principle and characteristics of the Littman-Metcalf external-cavity oscillation structure, a 900 grooves/mm blazed grating is used as the external-cavity feedback element, single-angled facet gain chip is served as the laser gain medium (Fig.1). Then, the threshold current performance of the ETSL system is characterized by measuring the output optical power at different lasing wavelengths to determine a minimum working current (Fig.3(a)). Finally, the linewidth of the ETSL system with a wide mode-hopping free tuning range at different lasing wavelengths are compared (Fig.6).
Results and Discussions The designed total physical lengths of the laser cavity are changed to obtain superimposed optical spectra for different resonance wavelengths. The injection current is fixed at 410 mA and the ambient temperature is adjusted at 25 ℃, and the tuning range results are highlighted (Fig.3(c)). The single-mode operation of different lasing wavelength can be clearly identified, and the side mode suppression ratio of the system satisfies the demand of optical frequency reflectormeter. Meanwhile, the peak output power of 16.95 dBm, full range power of better than 14.96 dBm are obtained (Fig.3(d)). In the current implementation, the overall physical length of the ETSL cavity is designed to be about 50 mm, namely from the gain chip rear (left) output facet to the tuning mirror front facet, and corresponds to an axial mode spacing of 24 pm operating at 1 550 nm. The mode-hopping performance of external-cavity semiconductor laser with blazed grating is characterized by using the wavelength difference measurement method, no mode-hopping can be observed in the wavelength range of 1 480-1 580 nm (Fig.4). Stability performance of the wavelength and output power are monitored using the commercial wavelength meter (Fig.5), within a 130 mins duration, the designed ETSL has good wavelength stability (±2.5 pm) and power stability (±0.035 dB). Based on the short delay self-heterodyne interferometry, the spectral linewidth is measured to be less than 98.27 kHz within the full tuning range, the minimum spectral linewidth is 64.11 kHz around lasing wavelength of 1570 nm (Fig.6).
Conclusions A wide mode-hopping free and narrow linewidth external-cavity tunable semiconductor laser is designed, which is based on a classical Littman-Metcalf configuration. Meanwhile, the tuning characteristics and spectral linewidth of the ETSL are investigated experimentally. A wide mode-hopping free continuous wavelength tuning range of about 100 nm (namely, 1 480-1 580 nm) with a side mode suppression ratio of more than 65.54 dB and an output power of more than 14.96 dBm over the whole tuning range can be achieved in a long-term free running. The spectral linewidth performance of the designed tunable laser source measured using short delay self-heterodyne interferometry is less than 98.27 kHz. With the help of this designed tunable laser source, it is helpful to promote its application in improving the measurement accuracy of optical frequency reflectormeter. Future work shall focus on the optimization of the length of the laser cavity design to further reduce the spectral linewidth.
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