**2. Experimental setup**

The experimental setup for the Littrow external-cavity feedback system is schematically shown in **Figure 1**. The high-power GaN diode lasers (2 W for blue or 1 W for green devices) used in our experiment are broad-area diode lasers. The laser beam is TE-polarized for both devices. The laser beam emitted from the front facet is collimated by an aspherical lens of 4.0 mm focal length with a numerical aperture of 0.6. The collimated beam is incident on a bulk diffraction grating mounted in Littrow configuration and oriented with the lines in the grating parallel to the slow axis of the diode laser.

The first-order diffracted beam from the grating is the feedback beam, thus the external cavity is formed between the diode laser and the grating. A beam splitter is inserted in the external cavity and reflects part of the beam as a diagnostic beam in which the spectral bandwidth and tunability of the laser system are measured. The zeroth-order diffracted beam of the grating is the output beam of the ECDL system, and a power meter is used to record the power of the output beam. The ECDL system is tuned by rotating the grating along the groove direction.

Tunable High-Power External-Cavity GaN Diode Laser Systems in the Visible Spectral Range http://dx.doi.org/10.5772/intechopen.79703 5

**Figure 1.** Experimental setup of the ECDL system. BAL, broad-area diode laser; L, lens; BS1, BS2 and BS3, beam splitter; HWP, half-wave plate; G, bulk grating, PD1 and PD2, photodiode; DO, digital oscilloscope; OSA, optical spectrum analyzer; Am, amplifier; ESA, electrical spectrum analyzer; PM, power meter.
