**3. Experimental results**

The spectral bandwidth of the high-power devices is around 1.0 nm, and the devices are not tunable. Although single longitudinal mode GaN diode laser around 405 nm has been achieved with the laterally coupled distributed feedback (DFB) technique [3]; this technique is under development and not available for commercial devices, especially not available for

Tunable, high-power, narrow spectral bandwidth light sources based on semiconductor lasers from the violet to green spectral range are attractive for many applications, such as high-resolution spectroscopy, holographic data storage, laser cooling, laser holographic display, biophotonics and as pump sources for nonlinear frequency conversion and for titanium-sapphire lasers [4–10]. The broad emission bandwidth limits the usage of the high-power GaN diode lasers in some of these applications. There are two main techniques to achieve high-power, narrow-bandwidth blue and green laser emission based on semiconductor devices. The first one is based on nonlinear frequency conversion, including second harmonic generation and sum frequency generation of GaAs lasers emitting from 800 to 1100 nm [11, 12]. The second approach is external-cavity feedback technique used to improve the spectral quality of the high-power GaN diode lasers and make the lasers tunable [13, 14]. The laser systems developed based on the first method are relatively complex, and the laser systems are not tunable, or the tunable range is narrow. Thus, the second approach is applied in this chapter to achieve

In this chapter, we first demonstrate two tunable, narrow bandwidth high-power GaN diode laser systems; one system is emitting around 455 nm in the blue spectral region, the other is emitting around 515 nm in the green spectral region. Secondly, the tuning range and output power optimization of an external-cavity diode laser (ECDL) system is investigated based on the experimental results. Finally, the dynamics of the green external-cavity diode laser system

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

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.

narrow bandwidth, tunable blue and green diode laser systems.

high-power GaN diode lasers.

4 Laser Technology and its Applications

is studied.

**2. Experimental setup**

grating parallel to the slow axis of the diode laser.
