**1. Introduction**

106 Selected Topics on Optical Amplifiers in Present Scenario

T.Marozsák, "Transmission Characteristics of All Semiconductor Fiber OpticLinks Carrying

pp.52-55.

Microwave Channels", European Microwave Conf., Paris, France, 2000, vol.2,

Diffraction-limited high-power narrow-spectrum red diode lasers are attractive for many applications, such as photodynamic therapy, laser display, and as a pump source to generate UV light by second harmonic generation (SHG). High-power, diffraction-limited diode lasers can be realized by the technology of lasers with a tapered gain-region (Kintzer et al., 1993; Donnelly et al., 1998; Wenzel et al., 2003; Paschke et al., 2005; Sumpf et al., 2009; Fiebig et al., 2010). The tapered laser devices can be used in applications where narrowspectrum is not needed such as photodynamic therapy, but for other applications such as a pump source for UV light generation, the spectral quality of these devices has to be improved.

In order to improve the spectral quality of a tapered laser, different techniques are applied, such as a monolithically integrated master oscillator power amplifier by forming Bragg gratings in the semiconductor material (O'Brien et al., 1993, 1997a), injection locking to an external single-mode laser (Goldberg et al., 1993; Mehuys et al., 1993b; O'Brien et al., 1997b; Wilson et al., 1998; Ferrari et al., 1999; Spieβberger et al., 2011), and different external-cavity feedback techniques (Jones et al., 1995; Cornwell & Thomas, 1997; Morgott et al., 1998; Goyal et al., 1998; Pedersen & Hansen, 2005; Chi et al., 2005; Lucas-Leclin et al., 2008; Tien et al., 2008; Sakai et al., 2009). Up to 1 W output power at 668 nm from a Fabry-Perot tapered diode laser was obtained with a beam quality factor of 1.7, and the spectral width was smaller than 0.2 nm (Sumpf et al., 2007). Around 670 nm, tunable narrow-linewidth diffraction-limited output was also achieved from an injection-locking tapered diode laser system seeded with a single-mode external-cavity diode laser (Häring et al., 2007); the output power was up to 970 mW. A 670 nm micro-external-cavity tapered diode laser system was demonstrated with a reflecting volume Bragg grating as a feedback element; in continuous wave (CW) mode, more than 0.5 W output power was obtained, and in pulse mode, 5 W peak power was obtained with a beam quality factor of 10 and a spectral width below 150 pm (Tien et al., 2008). Up to 1.2 W output power at 675 nm from a tapered laser

Red Tunable High-Power Narrow-Spectrum

book chapter.

loss 

gain coefficient

uncertainty.

vertical far field angles

described previously (Sumpf et al., 2007, 2011).

External-Cavity Diode Laser Based on Tapered Amplifier 109

used, the 1000 nm *p*-cladding layer was made of Al0.85Ga0.15As, which allowed carbon doping with concentrations in the range of some 1018 cm-3 and a standard AlGaAs process. These epitaxial structures were also used for the manufacturing of tapered lasers as

Here we should mention that two factors influence the wavelength, i.e., the spectrum, of a tapered amplifier: the composition of the materials of the quantum well (in the red tapered amplifier, the gallium content x in the In1-xGaxP quantum well) and the strain between quantum well and waveguide. The detailed design on the wavelength of a tapered device is based on the semiconductor physics on quantum well, and this is out of the scope of this

/ nm 668 675

vert / ° (FWHM) 31 30

vert / ° (95% power) 50 52 *I*th / mA 330 315

<sup>D</sup> 0.66 0.70 *T*0 / K 110 120

Table 1. Summary of the data for the gain media used in this study.

threshold current *I*th, the differential efficiency

efficiency for medium A was with

i / cm-1 (1.8 0.1) (1.2 0.1)

i (0.90 0.08) (0.75 0.02)

*g*0 / cm-1 (19.6 0.4) (19.8 0.4)

Tapered device A was made of gain medium A, and tapered device B and C were made of gain medium B. The gain media data were measured for uncoated broad-area devices (BADs) with a cavity length of 1 mm and a stripe width of 100 µm in pulsed mode. The

52° (95% power content). This relatively small vertical far field angle allows the use of standard optics with a moderate numerical aperture to collimate the output beam. The power-current characteristics and the spectra were measured for these BADs, and the

threshold current *T*0 were given in table 1. The threshold currents of gain medium A and B were 330 and 315 mA, respectively. The differential efficiency for gain medium A was slightly smaller compared to gain medium B. The characteristic temperatures of the

Assuming a logarithmic dependence of the gain on the current density, from the lengthdependence measurement of threshold current density *j*th and slope efficiency *S*, the gain medium data were obtained and given in Table 1. It showed that for medium A the internal

i = 0.90 larger than

Based on these gain media, tapered diode amplifiers were processed with total cavity length of 2 mm. The straight index-guided ridge-waveguide section manufactured by reactive ion etching had a length of 0.5 mm for tapered device A and B, and 0.75 mm for tapered device

threshold current were 110 and 120 K for gain medium A and B, respectively.

i = 1.8 cm-1 was larger in comparison to medium B with

Gain medium A Gain medium B

vert for the devices were about 30° (FWHM) and between 50° and

*g*0 remained constant for both gain media within the experimental

D, and the characteristic temperature of the

i = 1.2 cm-1. The internal

i = 0.75 for medium B. The modal

was obtained with a beam quality factor less than 1.3, the maximum conversion efficiency of 31% was reached at an output power of 1 W (Sumpf et al., 2011). External-cavity feedback based on a bulk diffraction grating in the Littrow configuration is a useful technique to achieve a tunable narrow-spectrum, high-power, diffraction-limited tapered diode laser system (Mehuys et al., 1993a; Jones et al., 1995; Goyal et al., 1997; Morgott et al., 1998; Chi et al., 2005). We have demonstrated such a tapered diode laser system around 668 nm with output power up to 810 mW; a beam quality factor of 3.4 was obtained with an output power of 600 mW (Chi et al., 2009).

In this chapter, three red tunable high-power narrow-spectrum diode laser systems based on three different tapered semiconductor optical amplifiers in Littrow external-cavity are demonstrated. Tapered device A is a 668 nm 2-mm-long tapered amplifier with a 0.5-mmlong index-guided ridge-waveguide section. Both tapered device B and C are 675 nm 2-mmlong tapered amplifier, the lengths of ridge-waveguide section are 0.5 mm for device B, and 0.75 mm for device C, respectively. The epitaxial structruce and the geometry of these tapered devices are described, and the data on the gain media of the devices are presented and compared.

Laser system A based on device A is tunable over a range of 16 nm centered at 668 nm. As high as 1.38 W output power is obtained at 668.35 nm. The emission spectral bandwidth is less than 0.07 nm throughout the tuning range, and the beam quality factor *M*2 is 2.0 with an output power of 1.27 W.

Laser system B based on device B is tunable from 663 to 684 nm with output power higher than 0.55 W in the tuning range, as high as 1.25 W output power is obtained at 675.34 nm. The emission spectral bandwidth is less than 0.05 nm throughout the tuning range, and the beam quality factor *M*2 is 2.07 at an output power of 1.0 W. Laser system C based on device C is tunable from 666 to 685 nm. As high as 1.05 W output power is obtained around 675.67 nm. The emission spectral bandwidth is less than 0.07 nm throughout the tuning range, and the beam quality factor *M*2 is 1.13 at an output power of 0.93 W.

The properties of the three tapered diode laser systems are summarized and compared. As an example of application, Laser system C is used as a pump source for the generation of 337.6 nm UV light by single-pass frequency doubling in a bismuth triborate (BIBO) crystal. An output power of 109 µW UV light, corresponding to a conversion efficiency of 0.026%W-1 is attained.
