**4. Conclusion**

230, 250, 340 and 420 mA are shown in **Figure 12**. The pulse packages first oscillate irregularly, as shown in **Figure 12(a**, **e)** with an injected current of 230 mA. The time series in **Figure 12(a)** shows the pulse packages consisting of the pulse train with the period of external-cavity delay time oscillate irregular. The intensity noise spectrum in **Figure 12(e)** shows the peak for LFO and peaks for external-cavity oscillation at multiples of *ν*EC. However, the peaks at *ν*EC ± *ν*LFO originating from the mixing of external-cavity frequency *ν*EC and LFO frequency *ν*LFO, which is the indication of regular PPO are not observed. The broad peaks for *ν*EC and *ν*LFO mean that the intensity noise is increased strongly, and the dynamics of the laser system is more complex. When the operating current is increased to 250 mA, the time series in **Figure 12(b)** shows irregular PPO, and the high-frequency external-cavity oscillation is not as clear as in the condition of low injected current. The average duration of the pulse package is less than 20 ns. The broad peaks for *ν*EC and *ν*LFO mean the dynamic behavior of the laser system is more complex. The pulse package is not clear with the operated current of 340 mA, and the pulses

**Figure 12.** Time series and intensity noise spectra of the output beam from the green ECDL with the injected current of

(a, e) 230 mA, (b, f) 250 mA, (c, g) 340 mA and (d, h) 420 mA.

16 Laser Technology and its Applications

Both blue and green high-power, tunable, narrow-bandwidth ECDL systems based on GaN broad-area diode lasers and external grating feedback are demonstrated. For the blue ECDL, two gratings are applied. The holographic grating is for obtaining high power, a 530 mW output power with a tunable range of 1.4 nm is obtained with this grating; the ruled grating is for achieving broad tunable range, an output power of 80 mW with a tunable range of 6.0 nm is achieved with the ruled grating. For the green ECDL, the laser system can be operated in two modes, for *p*-polarized mode operation, an output power of 50 mW with a tunable range of 9.2 nm is obtained; for *s*-polarized mode operation, an output power of 480 mW with a tunable range of 2.1 nm is achieved.

The tuning range and the output power optimization of an external-cavity diode laser system with grating feedback is investigated based on the experimental results on the blue and green ECDL systems and diode laser theory. The obtained results can be used as a guide to select grating for an ECDL system for different applications. The dynamic behavior of the green ECDL system operated in *p*-polarized mode is studied. As the increase of the injected current, different dynamic states, such as regular PPO, irregular PPO and chaos are observed.

than 900 mW for next-generation holographic displays. Optical Review. 2016;**23**:141-145.

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

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

19

[8] Müller A, Marschall S, Jensen OB, Fricke J, Wenzel H, Sumpf B, Andersen PE. Diode laser based light sources for biomedical applications. Laser & Photonics Reviews. 2013;**7**:605-

[9] Ruhnke N, Müller A, Eppich B, Maiwald M, Sumpf B, Erbert G, Tränkle G. Compact deep UV system at 222.5 nm based on frequency doubling of GaN laser diode emission. IEEE Photonics Technology Letters. 2018;**30**:289-292. DOI: 10.1109/LPT.2017.2787463 [10] Gürel K, Wittwer VJ, Hoffmann M, Saraceno CJ, Hakobyan S, Resan B, Rohrbacher A, Weingarten K, Schilt S, Südmeyer T. Green-diode-pumped femtosecond Ti: Sapphire laser with up to 450 mW average power. Optics Express. 2015;**23**:30043-30048. DOI:

[11] Fiebig C, Sahm A, Uebernickel M, Blume G, Eppich B, Paschke K, Erbert G. Compact second-harmonic generation laser module with 1 W optical output power at 490 nm.

[12] Hansen AK, Andersen PE, Jensen OB, Sumpf B, Erbert G, Petersen PM. Highly efficient single-pass sum frequency generation by cascaded nonlinear crystals. Optics Letters.

[13] Ruhnke N, Müller A, Eppich B, Maiwald M, Sumpf B, Erbert G, Tränkle G. 400 mW external cavity diode laser with narrowband emission at 445 nm. Optics Letters. 2014;**39**:3794-

[14] Chen YH, Lin WC, Chen HZ, Shy JT, Chui HC. Single-frequency external cavity green diode laser. IEEE Photonics Journal. 2017;**9**:1507207. DOI: 10.1109/JPHOT.2017.2776284

[15] Chi M, Jensen OB, Petersen PM. Tuning range and output power optimization of an external-cavity GaN diode laser at 455 nm. Applied Optics. 2016;**55**:2263-2269. DOI:

[16] Chi M, Jensen OB, Petersen PM. Green high-power tunable external-cavity GaN diode laser at 515 nm. Optics Letters. 2016;**41**:4154-4157. DOI: 10.1364/OL.41.004154

[17] Unger P. Introduction to power diode lasers. In: Diehl R, editor. High-Power Diode Lasers, Fundamentals, Technology, Applications. Berlin Heidelberg: Springer-Verlag;

[18] Conroy RS, Hewett JJ, Lancaster GPT, Sibbett W, Allen JW, Dholakia K. Characterisation of an extended cavity violet diode laser. Optics Communication. 2000;**175**:185-188. DOI:

[19] Lonsdale DJ, Willis AP, King TA. Extended tuning and single-mode operation of an antireflection-coated InGaN violet laser diode in a Littrow cavity. Measurement Science and

[20] Soriano MC, García-Ojalvo J, Mirasso CR, Fischer I. Complex photonics: Dynamics and applications of delay-coupled semiconductors lasers. Reviews of Modern Physics.

Technology. 2002;**13**:488-493. DOI: 10.1088/0957-0233/13/4/310

2013;**85**:421-470. DOI: 10.1103/RevModPhys.85.421

Optics Express. 2009;**17**:22785-22790. DOI: 10.1364/OE.17.022785

2015;**40**:5526-5529. DOI: 10.1364/OL.40.005526

3797. DOI: 10.1364/OL.39.003794

10.1364/AO.55.002263

2000. pp. 37-46

10.1016/S0030-4018(99)00742-7

DOI: 10.1007/s10043

10.1364/OE.23.030043

627. DOI: 10.1002/lpor.201200051
