Mode-hop-free tuning over 140 GHz of external cavity diode lasers without anti-reflection coating

We report a home-built external cavity diode laser (ECDL), using a diode whose front facet is not antireflection (AR) coated, that has a mode-hop-free (MHF) tuning range greater than 140 GHz. We achieved this by using a short external cavity and by simultaneously tuning the internal and external modes of the laser. The general applicability of the method, combined with the compact portable mechanical and electronic design, makes it well suited for both research and industrial applications. Continuous-wave lasers exhibiting wide and continuous wavelength tunability are of considerable interest for a variety of applications including high resolution spectroscopy [1], cooling and trapping [2], as well as sensing [3]. Semiconductor lasers are often used because their lasing wavelength can be easily tuned by changing either the injection current or the chip temperature. Distributed Bragg reflector (DBR) and distributed feedback (DFB) lasers provide large tuning range along with narrow linewidth and excellent power and frequency stability. However, DBR/DFB lasers are relatively expensive and available only at a few selected wavelengths. Fabry-Perot (FP) diode lasers, on the other hand, are cheap and available in a variety of wavelengths, but their tunability can be limited. The performance of a FP diode laser can be significantly improved by placing it within an external cavity employing a diffraction grating, either in a Littrow [4] or a Littman [5] configuration. This arrangement substantially reduces the linewidth of the output beam, and allows coarse wavelength tuning; in the case of the Littrow configuration, through rotation of the grating. Several groups have addressed the issue of increasing the mode-hop-free (MHF) tuning range of Littrowtype lasers. It has been proposed [6,7] that MHF tuning range can be enhanced by rotating the grating about an optimal pivot point, located precisely at the intersection of the grating plane and plane of the diode’s rear facet. Several works [8,9] discussed the sensitivity of the behavior of the laser on the precise location of this pivot point. Later works focused on tuning the internal and external cavity lengths in the proper proportion, resulting in MHF tuning ranges as large as 110 GHz [10-12]. In this article, we report a simple Littrow-type ECDL with a 140 GHz MHF tuning range using “off the shelf” laser diodes without AR coatings. This, to the best of our knowledge, is the largest MHF tuning range among ECDLs using diodes without AR coating. Our observations lead us to conclude that the short external cavity length, in concert with careful scaling of the variations of the internal and external cavity lengths, are critical to achieving large MHF tuning range, while, the precise location of the pivot point of the grating is less important. We first discuss the principle behind continuous MHF tuning. This is followed by the details of the mechanical design of the ECDL and the method used to maximize the MHF tuning range. Measurements of the tuning range, including applications in spectroscopy, are then presented, followed by the conclusion. An ECDL tends to lase at the frequency with the greatest gain. Several factors are key in defining the frequency of this active mode [4,10,11]. The free running laser (FRL) has a broad gain profile (typical width ~ 10 nm). The internal cavity modes of the laser diode are typically spaced by around 100 GHz and the internal mode that experiences maximum gain determines the emission wavelength of the FRL. When the FRL is placed in an external cavity, the internal and external cavities are optically coupled. Frequencies of coupled cavities depend on the optical length of the two individual cavities and the reflectivity of the surfaces in a complex way [4,9,13]. In a simpler picture, one can assume that the wavelength of the ECDL is determined by the external cavity mode positioned most closely to the center of the internal cavity mode. For a Littrow-type ECDL, the wavelength λ that the grating feeds back to the diode chip is determined by