**1. Introduction**

Since the first demonstration of a room temperature continuous wave (CW) GaN violet diode laser by Nakamura et al. [1], great progress has been achieved on the GaN-based diode lasers in the violet to green spectral range. Nowadays, high-power GaN diode lasers with CW output power of a few watts are commercially available in the blue to green spectral range [2].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 high-power GaN diode lasers.

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 narrow bandwidth, tunable blue and green diode laser systems.

**3. Experimental results**

**3.1. Blue GaN external-cavity diode laser system**

analyzer; Am, amplifier; ESA, electrical spectrum analyzer; PM, power meter.

spacing of the FP modes is around 28.5 pm.

In this subsection, the results of blue ECDL system are presented [15]. Two bulk diffraction gratings are used in the blue ECDL system, one is a holographic diffraction grating with a groove density of 2400 lines/mm (Thorlabs, GH13-24 U), and the zeroth- and first-order diffraction efficiencies are 78.8 and 8.3%. The other is a ruled diffraction grating that is ruled with 1800 lines/mm and has a blaze wavelength of 500 nm (Thorlabs, GR13-1850), the zeroth- and first-order diffraction efficiencies are 29.6 and 53.5%, respectively. The length of the external cavity is around 110 mm.

**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

Tunable High-Power External-Cavity GaN Diode Laser Systems in the Visible Spectral Range

http://dx.doi.org/10.5772/intechopen.79703

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In the freely running condition, the threshold current of the laser device is around 0.12 A, and the slope efficiency is around 1.62 W/A. With an injected current of 1.4 A, a 2 W output power is obtained. The diode laser is operating in multiple Fabry-Perot (FP) modes. The spectrum is centered around 456 nm with a spectral bandwidth around 1.2 nm. The longitudinal mode

The blue ECDL system is characterized by measuring the spectrum at different wavelengths with these two different gratings. **Figure 2** shows the spectra of the blue ECDL system with an output power around 85 mW for both gratings. For the ECDL system with the holographic diffraction grating at an injected current of 0.2 A, **Figure 2(a)** shows seven normalized spectra from 453.4 to 456.5 nm. The bandwidth of the spectrum (FWHM) is around 9 pm in the 3.1 nm tunable range; it is much less than the 28.5 pm mode space of the FP laser resonator; this means the laser is forced to operate in a single longitudinal FP mode by the external feedback. **Figure 2(b)** shows eight normalized spectra from 452.8 to 458.8 nm for the ECDL system with the ruled diffraction grating at an injected current of 0.3 A. The spectral bandwidth is less than 20 pm for wavelengths longer than 456 nm in the 6.0 nm tunable range; for shorter wavelengths, the spectral bandwidth is less than 42 pm, that is, two FP modes oscillate simultaneously. The amplified spontaneous emission (ASE) is more than 20 dB suppressed in the tunable ranges for both diffraction gratings.

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 is studied.
