2. Principle of double-Littrow ECDL
In order to realize dual-mode operation from a diode laser,there are different monolithic [10] and external-cavity feedback based [1] techniques, such as double-Littmann configuration,double-Littrow configuration, and Fourier transform external cavity laser (FTECAL). The tuning ranges of the double-mode monolithic diode lasers are limited compared to the techniques with external-cavity feedback, whereas the stability of dual-mode operation of monolithic diode lasers is better. Different two color ECDL configurations show different performances [11] . In the present study, the Littrow setup is used to maximize the tuning range. Therefore, within the framework of this work it will be focused only on the double-mode Littrow configuration. Further