**2. Description of the gain media and the tapered devices**

Tapered diode amplifiers (TDAs) consist of an index-guided ridge-waveguide section and a gain-guided tapered section (Kintzer et al., 1993; Sumpf et al., 2009). The ridge-waveguide section works as a spatial filter, thus diffraction-limited laser beam is available from tapered diode amplifiers, and high output power can be obtained due to the amplification from the gain-guided flared section.

The tapered amplifiers used in the experiment were grown using metal organic vapor phase epitaxy. The epitaxial structure of these tapered diode amplifiers were the same. As active layer, a 5-nm-thick compressively strained single In1-xGaxP quantum well was used for all the devices. The gallium content x was selected for an emission wavelength in the range between 670 and 680 nm. The single quantum well was embedded in the 500 nm thick Al0.36Ga0.16InP *p*- and *n*-waveguide layers. For the 800 nm *n*-cladding layer Al0.52In0.48P was

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

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

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

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 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

Tapered diode amplifiers (TDAs) consist of an index-guided ridge-waveguide section and a gain-guided tapered section (Kintzer et al., 1993; Sumpf et al., 2009). The ridge-waveguide section works as a spatial filter, thus diffraction-limited laser beam is available from tapered diode amplifiers, and high output power can be obtained due to the amplification from the

The tapered amplifiers used in the experiment were grown using metal organic vapor phase epitaxy. The epitaxial structure of these tapered diode amplifiers were the same. As active layer, a 5-nm-thick compressively strained single In1-xGaxP quantum well was used for all the devices. The gallium content x was selected for an emission wavelength in the range between 670 and 680 nm. The single quantum well was embedded in the 500 nm thick Al0.36Ga0.16InP *p*- and *n*-waveguide layers. For the 800 nm *n*-cladding layer Al0.52In0.48P was

the beam quality factor *M*2 is 1.13 at an output power of 0.93 W.

**2. Description of the gain media and the tapered devices** 

power of 600 mW (Chi et al., 2009).

and compared.

is attained.

gain-guided flared section.

output power of 1.27 W.

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

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 book chapter.


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

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 vertical far field angles vert for the devices were about 30° (FWHM) and between 50° and 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 *I*th, the differential efficiency D, and the characteristic temperature of 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 threshold current were 110 and 120 K for gain medium A and B, respectively.

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 loss i = 1.8 cm-1 was larger in comparison to medium B with i = 1.2 cm-1. The internal efficiency for medium A was with i = 0.90 larger than i = 0.75 for medium B. The modal gain coefficient *g*0 remained constant for both gain media within the experimental uncertainty.

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

Red Tunable High-Power Narrow-Spectrum

External-Cavity Diode Laser Based on Tapered Amplifier 111

The grating is mounted in the Littrow configuration, this means the first-order diffraction beam of the grating propagates back towards the tapered amplifier. Therefore the laser cavity is formed between the diffraction grating and the front facet of the tapered amplifier. The tapered amplifier works as a gain medium in the laser cavity. When the injected current in the tapered device is higher than the threshold, the laser beam oscillates in the laser cavity. The emission wavelength of the laser system is tuned widely by rotating the diffraction grating because of the broad gain bandwidth (a few tens nanometers) of the tapered device. The emission spectrum of the laser system is narrowed significantly compared with the freely running tapered lasers (from a few hundreds picometers to a few tens picometers) due to the dispersion of the diffraction grating and the narrow aperture of

the tapered device in the fast axis direction (Sumpf et al., 2007; Chi et al., 2010, 2011).

experiment. The tapered device lases without the grating feedback.

output power of 1.38 W is obtained with an operating current of 2.0 A.

 freely running, around 667.13 nm grating feedback, around 668.35 nm

0

300

600

Optical power [mW]

external-cavity feedback.

running mode.

900

1200

1500

The laser is TE-polarized, i.e., linearly polarized along the slow axis. The temperature of the amplifier is controlled with a Peltier element and the amplifier is operated at 15 ºC in the

The power/current characteristics for laser system A with and without the external-cavity feedback are shown in Fig. 2. Without feedback, the threshold current is around 0.7 A, the slope efficiency is 0.63 W/A, the emission wavelength is around 667.1 nm, the roll-over takes place around 1.5 A, and an output power of 0.65 W is achieved with an injected current of 2.0 A. With the external-cavity feedback, the maximum power is obtained at a wavelength around 668.4 nm, the threshold current of the laser system is decreased to 0.5 A, the slope efficiency is increased to 1.05 W/A, the roll-over takes place around 1.7 A, and an

0,0 0,4 0,8 1,2 1,6 2,0

Fig. 2. The power/current characteristics for tapered diode laser system A with and without

The output power at different wavelengths is shown in Fig. 3 at an operating current of 1.8 A. A maximum output power of 1.27 W is obtained at the wavelength of 668.38 nm, the output power is higher than 0.8 W in the tuning range from 659 to 675 nm. The tunable range is narrower compared with that in Ref. (Chi et al., 2009), since if we rotate the diffraction grating further to tune the wavelength, the laser system will jump to the freely

Current [A]

C. The width of the ridge-waveguide section was 7.5 µm for all the three tapered devices. The tapered gain-guided section was defined by ion implantation, and had a length of 1.5 mm for device A and B, and 1.25 mm for device C. The flared angle was 4° for all the three devices, and the output apertures for tapered amplifier A, B and C were 110, 112 and 95 µm, respectively.

The tapered amplifier facets were passivated and antireflection coated to achieve mirror reflectivities of 1% for the front facet, and 5×10-4 for the rear facet, respectively. The tapered devices were mounted *p*-side down on copper tungsten (CuW) or chemical vapour deposited (CVD) diamond submounts using AuSn solder. These subassemblies were mounted on standard C-mounts.
