**3.1 External-cavity tapered diode laser system at 668 nm**

The external-cavity configuration employed for diode laser system A is depicted in Fig. 1 (Chi et al., 2010). An aspherical lens of 3.1 mm focal length with a numerical aperture (NA) of 0.68 is used to collimate the beam from the back facet in both fast and slow axes. The bulk grating is ruled with 1200 grooves/mm and has a blazed wavelength of 750 nm. The grating is mounted in the Littrow configuration and oriented with the lines in the grating parallel to the active region of the amplifier in order to obtain optimum spectral filtering by the narrow aperture of the tapered device in the fast axis direction. A second aspherical lens of 3.1 mm focal length with an NA of 0.68 is used to collimate the beam from the output facet in the fast axis. Together with a cylindrical lens of 60 mm focal length, these two lenses collimate the output beam in the slow axis and compensate the astigmatism simultaneously. All the lenses are antireflection coated for the red wavelength. A beam splitter behind the cylindrical lens is used to reflect part of the output beam of the tapered diode laser system as the diagnostic beam, the spectral width and beam quality factor *M*2 are measured in this beam. The output power of the laser system is measured behind the second aspherical lens.

Fig. 1. Experimental setup of tapered diode laser system A using external-cavity feedback. BS, beam splitter. Units are in millimeters.

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,

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

In this section, the tunable external-cavity diode laser systems based on the three tapered

The external-cavity configuration employed for diode laser system A is depicted in Fig. 1 (Chi et al., 2010). An aspherical lens of 3.1 mm focal length with a numerical aperture (NA) of 0.68 is used to collimate the beam from the back facet in both fast and slow axes. The bulk grating is ruled with 1200 grooves/mm and has a blazed wavelength of 750 nm. The grating is mounted in the Littrow configuration and oriented with the lines in the grating parallel to the active region of the amplifier in order to obtain optimum spectral filtering by the narrow aperture of the tapered device in the fast axis direction. A second aspherical lens of 3.1 mm focal length with an NA of 0.68 is used to collimate the beam from the output facet in the fast axis. Together with a cylindrical lens of 60 mm focal length, these two lenses collimate the output beam in the slow axis and compensate the astigmatism simultaneously. All the lenses are antireflection coated for the red wavelength. A beam splitter behind the cylindrical lens is used to reflect part of the output beam of the tapered diode laser system as the diagnostic beam, the spectral width and beam quality factor *M*2 are measured in this beam. The output power of the laser system is measured behind the second aspherical lens.

Fig. 1. Experimental setup of tapered diode laser system A using external-cavity feedback.

BS, beam splitter. Units are in millimeters.

respectively.

mounted on standard C-mounts.

**3. External-cavity tapered diode laser systems** 

amplifiers described in section 2 will be demonstrated.

**3.1 External-cavity tapered diode laser system at 668 nm** 

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).

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 experiment. The tapered device lases without the grating feedback.

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

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

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 running mode.

Red Tunable High-Power Narrow-Spectrum


output power of 1.26 W.

diode laser system.




Power (dB)



whole power range.

External-Cavity Diode Laser Based on Tapered Amplifier 113

Figure 4 shows the measured beam widths of the output beam and the fitted curves using formula (1) with the output power of 277 and 1275 mW. The *M*2 values are 1.39 ± 0.01 and 2.00 ± 0.01 with the output power of 277 and 1275 mW, respectively. For clarity, we have shifted the spatial position of the curves in the figure. In the experiments, the beam waists of the output beam with these two different output powers are located almost at the same position. This means that the astigmatism of the tapered device is rather stable over the

The optical spectrum characteristic of the output beam from diode laser system A is measured using a spectrum analyzer (Advantest Corp. Q8347). A typical result measured at 667.91 nm with an output power of 1260 mW is shown in Fig. 5. The figure shows diode laser system A is operated in multiple longitudinal modes. The spectral bandwidth (FWHM) is 0.034 nm (the resolution of the spectrum analyzer is 3 pm), and the amplified spontaneous emission intensity is more than 20 dB suppressed. We find the spectral bandwidth of the

I=1.8 A, =667.91 nm, pm

665,7 666,5 667,3 668,1 668,9 669,7 670,5

Wavelength (nm)

Fig. 5. The optical spectrum of the output beam from tapered diode laser system A with an

Compared with the results obtained previously in the similar wavelength range (Chi et al., 2009), the output power and spatial beam quality of the tapered diode laser system described above are improved significantly. The reason is the tapered semiconductor amplifier used here has improved properties. The length of the ridge-waveguide section of the 2 mm tapered device is 0.5 mm instead of 0.75 mm in the previous device. It has been verified that the beam quality of the 650 nm tapered diode lasers with shorter ridgewaveguide section is better compared with the device with longer ridge-waveguide section (Adamiec et al., 2009). This may be also the reason for better beam quality obtained here. The internal loss of the present device is less than 2.0 cm-1, while the internal loss of the previous device is between 2.5 and 3.0 cm-1 (Sumpf et al., 2007). We believe that the lower internal loss is the reason for higher output power from the present external-cavity tapered

output beam is less than 0.07 nm in the 16 nm tunable range.

Fig. 3. Tuning curve of tapered diode laser system A at an operating current of 1.8 A.

The spatial beam quality of the output beam along the slow axis is determined by measuring the beam quality factor *M*2 for the tunable external-cavity diode laser system. A spherical lens with a 100 mm focal length is used to focus the diagnostic beam. Then the beam width, *W* (1/e2), is measured at various recorded positions *Z* along the optical axis – on both sides of the beam waist. The value of *M*2 is obtained by fitting the measured data with the formula (Siegman & Townsend, 1993; Chi et al., 2004):

$$\mathcal{W} = \left[ \mathcal{W}\_0^2 + \left( \frac{4\lambda M^2}{\pi \mathcal{W}\_0} \right)^2 \left( Z - Z\_0 \right)^2 \right]^{1/2} \,. \tag{1}$$

Here *W*0 is the beam waist width, is the wavelength of the laser system and *Z*0 is the beam waist position.

Fig. 4. Beam width of the output beam from tapered diode laser system A for the slow axis with an output power of 277 mW (circles and solid curve), and 1275 mW (squares and dotted curve). The curves represent the fits to the measured data using formula (1).

I=1.8A

658 662 666 670 674 678

Wavelength (nm)

 1 2 <sup>2</sup> <sup>2</sup>

is the wavelength of the laser system and *Z*0 is the beam

. (1)

The spatial beam quality of the output beam along the slow axis is determined by measuring the beam quality factor *M*2 for the tunable external-cavity diode laser system. A spherical lens with a 100 mm focal length is used to focus the diagnostic beam. Then the beam width, *W* (1/e2), is measured at various recorded positions *Z* along the optical axis – on both sides of the beam waist. The value of *M*2 is obtained by fitting the measured data with the formula

> 2 2 0 0 0

Circles: current 0.8 A, 277 mW Squares: current 1.8 A, 1275 mW


Position (mm) Fig. 4. Beam width of the output beam from tapered diode laser system A for the slow axis with an output power of 277 mW (circles and solid curve), and 1275 mW (squares and dotted curve). The curves represent the fits to the measured data using formula (1).

 

Fig. 3. Tuning curve of tapered diode laser system A at an operating current of 1.8 A.

<sup>4</sup> *<sup>M</sup> W W Z Z W* 

0

(Siegman & Townsend, 1993; Chi et al., 2004):

40

80

120

Beam width (m)

160

200

240

Here *W*0 is the beam waist width,

waist position.

300

600

Output power (mW)

900

1200

1500

Figure 4 shows the measured beam widths of the output beam and the fitted curves using formula (1) with the output power of 277 and 1275 mW. The *M*2 values are 1.39 ± 0.01 and 2.00 ± 0.01 with the output power of 277 and 1275 mW, respectively. For clarity, we have shifted the spatial position of the curves in the figure. In the experiments, the beam waists of the output beam with these two different output powers are located almost at the same position. This means that the astigmatism of the tapered device is rather stable over the whole power range.

The optical spectrum characteristic of the output beam from diode laser system A is measured using a spectrum analyzer (Advantest Corp. Q8347). A typical result measured at 667.91 nm with an output power of 1260 mW is shown in Fig. 5. The figure shows diode laser system A is operated in multiple longitudinal modes. The spectral bandwidth (FWHM) is 0.034 nm (the resolution of the spectrum analyzer is 3 pm), and the amplified spontaneous emission intensity is more than 20 dB suppressed. We find the spectral bandwidth of the output beam is less than 0.07 nm in the 16 nm tunable range.

Fig. 5. The optical spectrum of the output beam from tapered diode laser system A with an output power of 1.26 W.

Compared with the results obtained previously in the similar wavelength range (Chi et al., 2009), the output power and spatial beam quality of the tapered diode laser system described above are improved significantly. The reason is the tapered semiconductor amplifier used here has improved properties. The length of the ridge-waveguide section of the 2 mm tapered device is 0.5 mm instead of 0.75 mm in the previous device. It has been verified that the beam quality of the 650 nm tapered diode lasers with shorter ridgewaveguide section is better compared with the device with longer ridge-waveguide section (Adamiec et al., 2009). This may be also the reason for better beam quality obtained here. The internal loss of the present device is less than 2.0 cm-1, while the internal loss of the previous device is between 2.5 and 3.0 cm-1 (Sumpf et al., 2007). We believe that the lower internal loss is the reason for higher output power from the present external-cavity tapered diode laser system.

Red Tunable High-Power Narrow-Spectrum

with output power higher than 460 mW.

0

2.0 A and system C (circles) at an operating current of 1.8 A.

200

400

600

Output power (mW)

800

1000

1200

1400

current.

External-Cavity Diode Laser Based on Tapered Amplifier 115

The power/current characteristics for these two tunable diode laser systems are shown in Fig. 7. For diode laser system B, the maximum output power is obtained at the wavelength 675.34 nm, the threshold current is around 0.7 A, the slope efficiency is 1.03 W/A, the rollover takes place around 1.7 A, and an output power of 1.25 W is achieved with an injected current of 2.0 A. For diode laser system C, the output power is measured at the wavelength of 675.67 nm, the threshold current of the laser system is around 0.6 A, the slope efficiency is 0.99 W/A, the roll-over takes place around 1.4 A, an output power of 1.05 W is obtained with an operating current of 1.8 A, and the output power decreases with higher injected

The output power at different wavelengths for these two laser systems is shown in Fig. 8. For laser system B, at an operating current of 2.0 A, a maximum output power of 1250 mW is obtained at the wavelength of 675.34 nm. The output power is higher than 550 mW in the tuning range from 663 to 684 nm. For laser system C, a maximum output power of 1055 mW is obtained at the wavelength of 675.67 nm; the laser system is tunable from 666 to 685 nm

664 668 672 676 680 684

 system B, I=2.0 A system C, I=1.8 A

Wavelength (nm)

Fig. 8. Tuning curves of the tapered diode laser system B (squares) at an operating current of

For both diode laser system B and C, the spatial beam quality of the output beam along the fast axis is assumed to be diffraction-limited because of the waveguide structure of the tapered gain devices. The spatial beam quality of the output beam along the slow axis is determined by measuring the beam quality factor *M*2. A spherical lens with a 65 mm focal length is used to focus the diagnostic beam. Then the beam width, *W* (1/e2), is measured at various recorded positions along the optical axis – on both sides of the beam waist. The value of *M*2 is obtained by fitting the measured data with formula (1). Figure 9 shows the measured beam widths and the fitted curves for tapered diode laser system B at output powers of 385 and 1000 mW, where the *M*2 values are 1.28 ± 0.02 and 2.07 ± 0.02, respectively. Figure 10 shows the measured beam widths and the fitted curves for tapered diode laser system C with the output power of 390 and 930 mW. The *M*2 values are 1.20 ± 0.02 and 1.13 ± 0.02 at output powers of 390 and 930 mW, respectively. For clarity, we have shifted the spatial position of the curves in the figures. In the experiments, the waists of the
