Dual-wavelength diode lasers with adjustable wavelength distance
Tunable diode laser sources emitting at two individually adjustable wavelengths are required for applications such as absorption or Raman spectroscopy, terahertz frequency generation, and non-linear frequency conversion. In absorption spectroscopy, for example, two wavelengths are needed to measure inside and outside of an absorption feature to determine the concentration of the substances under study. Also, in Raman spectroscopy, shifted excitation Raman difference spectroscopy (SERDS) can be used to separate between the wanted Raman signal and unwanted background signal from fluorescence or ambient light for an improved substance measurement.
FBH has been developing wavelength-stabilized light sources using DBR-RW lasers as well as dual-wavelength Y-branch DBR-RW with a fixed wavelength distance. The spectral distance of about 10 cm-1 was chosen for SERDS to address most solids and liquids. However, a flexible spectral distance between both excitation wavelengths was expected to improve the result of the SERDS technique, especially for biological material which can show Raman signals with significantly larger linewidths. Therefore, the laser concept was extended by implementing a common heater above the DBR section of the Y-branch DBR-RW laser as shown in Fig. 1.
The device has a total length of 3 mm and contains four individually addressable sections. Within the grating section, two 10th order DBR gratings with a spectral spacing of about 0.6 nm were manufactured using an i-line wafer stepper. Moreover, a heater implemented above the grating section allows spectral tuning. Typically, the gratings have a length of 500 µm, and the spatial distance between the gratings is about 80 µm. The RW section, the coupler section and partly the output section are formed using a sine-generated curve to form s-shaped bends over a length of about 2 mm.
The power-current characteristics of the two branches are shown in Fig. 2. The output section was operated at a current of 35 mA without connecting the coupler section. Laser operation for the two branches starts at a current of Ileft = Iright = 20 mA. The slope efficiency, determined between threshold and P = 50 mW, amounts to 0.58 W/A. At an injection current of 400 mA, the output powers were close to 180 mW. The maximal conversion efficiency amounts to 0.22 at about 50 mW.
At an optical power of 100 mW and a temperature of 25°C, the spectral behavior of the device when changing the heater current over the DBR gratings is shown in Fig. 3. The devices were operated in single mode with an emission width smaller than 13 pm, i.e., 0.2 cm-1 over the whole measurement range. With this narrow spectral width, the light sources meet the requirements for excitation sources used in Raman spectroscopy.
It can be seen that the spectral distance without heater current is 0.62 nm, as implemented within the process. When increasing the heater current up to 600 mA, the distance between the two wavelengths remains almost constant, though causing a temperature-induced wavelength shift of about 1.36 nm for both emission lines. By using two different heater currents, the spectral distance between the two branches can be adjusted to the needs of the substances under study up to a spectral distance of 2 nm. Within the measured heater current range, the output power showed only a weak decrease of about 5 % with rising heater current using constant currents through the branches and the output section. Here, a slight correction of the latter currents would be easily possible.
Results confirm that the two branches deliver the necessary two wavelengths for SERDS, and the implemented heater element allows a flexible adjustment of the excitation lines for SERDS according to the width of the Raman lines under study. Raman spectroscopic experiments on red wine and ethanol showed the applicability of the devices , which are described in detail in [2, 3].
This work was supported by the European Commission within the project MIB Multi-modal, Endoscopic Biophotonic Imaging of Bladder Cancer for Point-of-Care Diagnosis (MIB 667933-2).
 B. Sumpf, J. Kabitzke, A. Müller, M. Maiwald, J. Fricke, P. Ressel, G. Tränkle, “Dual-wavelength Y-branch DBR-RW diode laser at 785 nm with an electrically tunable wavelength distance up to 2 nm”, Conf. on Lasers and Electro-Optics/Europe and European Quantum Electronics Conf. (CLEO/Europe-EQEC 2017), Munich, Germany, ISBN: 978-1-5090-6736-7, cb-p.14 (2017).
 B. Sumpf, J. Kabitzke, J. Fricke, P. Ressel, A. Müller, M. Maiwald, G. Tränkle, “785 nm dual-wavelength Y-branch DBR-RW diode laser with electrically adjustable wavelength distance between 0 nm and 2 nm”, Proc. SPIE 10123, Photonics West, San Francisco, USA, 101230T (2017).
 B. Sumpf, J. Kabitzke, J. Fricke, P. Ressel, A. Müller, M. Maiwald, G. Tränkle, “Dual-wavelength diode laser with electrically adjustable wavelength distance at 785 nm”, Opt. Lett., vol. 41, no. 16, pp. 3694-3697 (2016).