![]() ![]() Due to the measurement range of the spectrum analyzer (Agilent E4440A), from 3 to 26.5 GHz, the beat signal near 0 Hz could not be detected or displayed by the analyzer, and therefore led to a deep dip near 0 Hz. The beat note signal was centered at a frequency of 0 Hz. The full power of the beat signal was almost concentrated on the frequency domain of 2.9 MHz, showing that the laser had a higher spectral purity than others with similar configurations. Results from the self-homodyne technique measured by an RF spectrum analyzer (Agilent E4440A) are displayed in Fig. (b) L– I curve for the ECDL.Īfter good alignment was no longer achieved, the linewidth of the ECDL was determined by a delayed self-homodyne linewidth measurement technique, that mixes the optical wave down to the RF range. The spectrum of the diode laser measured at 54 mA under a continuous wave condition at 28 ☌ is shown in the inset. (color online) (a) Light output power versus current ( L– I) for the diode laser without an external cavity. However, due to the insert loss introduced by the optical components in the cavity, the slope efficiency and output power decreased to ∼ 0.5 mW/mA and 32 mW, respectively.įig. 3(a) and 3(b), the threshold was reduced from 40 to ∼ 30 mA (the minimum threshold of the ECDL), indicating that a strong optical feedback was realized and a good alignment was accomplished. ![]() Optimum alignment was accomplished when the threshold was reduced to a minimum. After adding the external cavity to the diode laser, iterative adjustment of the focus and external cavity alignment were implemented to optimize the optical feedback. 3(a), which shows a center wavelength of 852.355 nm and a spectral linewidth of 0.31 nm. The spectrum of the diode laser measured at 54 mA under a continuous wave condition at 28 ☌ is also displayed in the inset in Fig. Here, the diode laser had a threshold of 40 mA, an output power higher than 50 mW, and a slope efficiency of 0.66 mW/mA. ![]() The overall external cavity length counted from the output facet of the diode laser to the front facet of the partially reflecting mirror was 85 mm.įigure 3(a) shows the curve of the light output power versus current ( L– I) of the diode laser without an external cavity measured by a power meter at room temperature. Such a cat’s eye reflector can decrease the sensitivity to the optical misalignment and maximize the feedback efficiency. At the end of the external cavity, a cat’s eye reflector consisting of an aspheric lens with 18.6 mm focal length and a partially reflecting mirror with 30% reflectivity was constructed to provide optical feedback. Then an interference filter was placed behind the collimating lens as the wavelength discriminator. An aspheric lens with 4.3 mm focal length and 0.55 numerical aperture was located in the front of the output facet to collimate the laser beam. The front facet of the diode was coated for anti-reflection and the rear facet was coated for high reflection. The F–P diode laser with a center wavelength of 852 nm was coupled to a heatsink, and the temperature was precisely controlled by a TEC combined with a thermistor. The configuration of the ECDL is depicted in Fig. The ECDL produced a narrow Lorentzian fitted linewidth of 95 kHz, spectral purity of 2.9 MHz, and long-term frequency stability of 5.59 × 10 − 12, exhibiting an excellent performance. In this paper, we use a readily available broad bandwidth (∼ 4 nm) interference filter to achieve single mode operation in an ECDL. Nevertheless, the interference filters used in these designs have an extremely narrow bandwidth (∼ 0.3 nm) comparable to the intrinsic mode spacing of the diode laser that are not readily available at a broad range of wavelengths, resulting in higher cost, restraining the popularization of this method. Īn alternative approach is to simultaneously employ a narrowband interference filter, placed in the linear cavity as the wavelength discriminator, and a mirror located at the end of the cavity as the optical feedback component, and has been proved to have a greater alignment tolerance and wider tunability. However, these designs are sensitive to the ambient pressure and optical misalignment induced by the mechanical and thermal deformation. One common method to construct an ECDL is using a diffraction grating as the optical feedback and wavelength discrimination component in either the Littrow or Littman–Metcalf configurations. In general, narrow linewidths can be effectively achieved by external cavity diode lasers (ECDLs). Moreover, lasers with narrow linewidths have great potential in Faraday anomalous dispersion optical filters. Narrow linewidths (< 1 MHz) are essential in a variety of laser applications, such as atom clocks, atomic physics, precise measurements, and coherent light communication. ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |