**3.2 External-cavity tapered diode laser systems at 675 nm**

The external-cavity configuration is the same for both tapered gain device B and C as depicted in Fig. 6 (Chi et al., 2011). The detailed description on the external-cavity tapered diode laser system can be found in section 3.1. An aspherical lens is used to collimate the beam from the back facet. A bulk grating is mounted in the Littrow configuration, and the laser cavity is formed between the diffraction grating and the front facet of the tapered amplifier. A second aspherical lens and a cylindrical lens are used to collimate the beam from the output facet. 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, where the spectral bandwidth and the beam quality factor *M*2 are measured. The output power of the laser system is measured behind the second aspherical lens.

Fig. 6. Experimental setup of the external-cavity tapered diode laser system for SHG. BS, beam splitter; OI, optical isolator. Units are in millimeters.

The lasers are TE-polarized, i.e., linearly polarized along the slow axis. The temperature of the amplifiers is controlled with a Peltier element and the tapered amplifiers are operated at 20 ºC in the experiment. The emission wavelength of the laser systems is tuned by rotating the diffraction grating.

Fig. 7. The power/current characteristics for tapered diode laser system B (squares) and C (circles).

The external-cavity configuration is the same for both tapered gain device B and C as depicted in Fig. 6 (Chi et al., 2011). The detailed description on the external-cavity tapered diode laser system can be found in section 3.1. An aspherical lens is used to collimate the beam from the back facet. A bulk grating is mounted in the Littrow configuration, and the laser cavity is formed between the diffraction grating and the front facet of the tapered amplifier. A second aspherical lens and a cylindrical lens are used to collimate the beam from the output facet. 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, where the spectral bandwidth and the beam quality factor *M*2 are measured. The output power of the laser system is measured

Fig. 6. Experimental setup of the external-cavity tapered diode laser system for SHG. BS,

The lasers are TE-polarized, i.e., linearly polarized along the slow axis. The temperature of the amplifiers is controlled with a Peltier element and the tapered amplifiers are operated at 20 ºC in the experiment. The emission wavelength of the laser systems is tuned by rotating

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

Fig. 7. The power/current characteristics for tapered diode laser system B (squares) and C

Current (A)

beam splitter; OI, optical isolator. Units are in millimeters.

Optical power (mW)

**3.2 External-cavity tapered diode laser systems at 675 nm** 

behind the second aspherical lens.

the diffraction grating.

(circles).

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 current.

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 with output power higher than 460 mW.

Fig. 8. Tuning curves of the tapered diode laser system B (squares) at an operating current of 2.0 A and system C (circles) at an operating current of 1.8 A.

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

Red Tunable High-Power Narrow-Spectrum


output power of 930 mW.

waveguide section.




Power (dB)




0.05 and 0.07 nm throughout their tunable ranges, respectively.

External-Cavity Diode Laser Based on Tapered Amplifier 117

for laser system C at 675.04 nm with an output power of 930 mW is shown in Fig. 11. The figure shows that the diode laser system is operated in multiple longitudinal modes. The spectral bandwidth (FWHM) is 0.038 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 bandwidths of the output beams for diode laser system B and C are less than

I = 1.6 A, = 675.04 nm, pm

673 674 675 676 677 678

Wavelength (nm)

Fig. 11. Optical spectrum of the output beam from tapered diode laser system C with an

Two tunable high-power 675 nm external-cavity diode laser systems based on tapered semiconductor optical amplifiers are demonstrated in this subsection. The main parameters of these two diode laser systems are summarized in Table 2; the results of diode laser system A described in subsection 3.1 are also listed in the table for comparison. Diode laser system B can produce more output power than diode laser system C, but the spatial beam quality of diode laser system C along the slow axis is significantly better than that of diode laser system B, especially with high output power. The two tapered devices are from the same wafer, so the different behaviour of the two laser systems is originated from the different sizes of the ridge-waveguide section and the tapered section. Diode laser system B produces higher output power since device B has a longer tapered gain section, but this laser system has worse spatial beam quality due to insufficient filtering of the light in the shorter ridge-

Compared with the results from diode laser system A in subsection 3.1, much more output power around 675.5 nm is obtained from laser system B and C, i.e., 1.25 and 1.05 W vs. 0.83 W; and diode laser system A cannot reach wavelengths longer than 676 nm.

output beam with different output powers are located almost at the same position. This indicates that the change of the astigmatism of the tapered device with different injection currents is negligible.

Fig. 9. Beam width of the output beam for the slow axis from diode laser system B with the output power of 385 mW (circles and dotted curve) and 1000 mW (squares and solid curve). The curves represent the fits to the measured data using formula (1).

Fig. 10. Beam width of the output beam for the slow axis from diode laser system C with the output power of 390 mW (circles and dotted curve) and 930 mW (squares and solid curve). The curves represent the fits to the measured data using formula (1).

The optical spectrum characteristic of the output beam from diode laser system B and C is measured using a spectrum analyzer (Advantest Corp. Q8347). A typical result measured

output beam with different output powers are located almost at the same position. This indicates that the change of the astigmatism of the tapered device with different injection

> output power 385 mW output power 1000 mW

82 86 90 94 98 102 106 110

Position (mm)

 output power 390 mW output power 930 mW

76 81 86 91 96 101 106

Position (mm)

Fig. 10. Beam width of the output beam for the slow axis from diode laser system C with the output power of 390 mW (circles and dotted curve) and 930 mW (squares and solid curve).

The optical spectrum characteristic of the output beam from diode laser system B and C is measured using a spectrum analyzer (Advantest Corp. Q8347). A typical result measured

Fig. 9. Beam width of the output beam for the slow axis from diode laser system B with the output power of 385 mW (circles and dotted curve) and 1000 mW (squares and solid curve).

The curves represent the fits to the measured data using formula (1).

The curves represent the fits to the measured data using formula (1).

currents is negligible.

0

0

50

100

Beam waist (m)

150

200

250

50

100

Beam waist (m)

150

200

250

for laser system C at 675.04 nm with an output power of 930 mW is shown in Fig. 11. The figure shows that the diode laser system is operated in multiple longitudinal modes. The spectral bandwidth (FWHM) is 0.038 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 bandwidths of the output beams for diode laser system B and C are less than 0.05 and 0.07 nm throughout their tunable ranges, respectively.

Fig. 11. Optical spectrum of the output beam from tapered diode laser system C with an output power of 930 mW.

Two tunable high-power 675 nm external-cavity diode laser systems based on tapered semiconductor optical amplifiers are demonstrated in this subsection. The main parameters of these two diode laser systems are summarized in Table 2; the results of diode laser system A described in subsection 3.1 are also listed in the table for comparison. Diode laser system B can produce more output power than diode laser system C, but the spatial beam quality of diode laser system C along the slow axis is significantly better than that of diode laser system B, especially with high output power. The two tapered devices are from the same wafer, so the different behaviour of the two laser systems is originated from the different sizes of the ridge-waveguide section and the tapered section. Diode laser system B produces higher output power since device B has a longer tapered gain section, but this laser system has worse spatial beam quality due to insufficient filtering of the light in the shorter ridgewaveguide section.

Compared with the results from diode laser system A in subsection 3.1, much more output power around 675.5 nm is obtained from laser system B and C, i.e., 1.25 and 1.05 W vs. 0.83 W; and diode laser system A cannot reach wavelengths longer than 676 nm.

Red Tunable High-Power Narrow-Spectrum

beam from the second harmonic output beam.

0

measured data, and the curve is a quadratic fit.

20

40

60

Power of UV (

W)

80

100

120

theoretically calculated value is 0.040%W-1 (Steinbach et al., 1996).

W-1

T= 19.8oC

External-Cavity Diode Laser Based on Tapered Amplifier 119

A 30 dB optical isolator is inserted between the aspherical lens and the cylindrical lens in the output beam to avoid feedback from the optical components and the nonlinear crystal, as shown in Fig. 6. A biconvex lens of 75 mm focal length is used to focus the red fundamental beam into the BIBO crystal. The available output power of the fundamental beam in front of the crystal is 650 mW. The size of the focus *w*s × *w*f is around 70 µm × 35 µm, where *w*s and *w*f are the beam waists (diameters at 1/e2) in the slow and fast axes, respectively. The elliptical beam is used to reduce the effects of walk-off in the BIBO crystal. The walk-off angle in our crystal is 72.9 mrad, corresponding to a heavy walk-off parameter *B* of 15.1. The elliptical beam was proved to be optimum in the experiments, in good agreement with the theory of frequency doubling using elliptical beams (Boyd & Kleinman, 1968; Steinbach et al., 1996). The slight change in astigmatism with output power will cause the focusing conditions to vary slightly at different power levels. In the experiments, the astigmatism was corrected at maximum pump power. Two dichroic beam splitters separate the fundamental

The wavelength of the fundamental beam is tuned to 675.16 nm, and the temperature of the crystal is 19.8 ºC. Figure 12 shows the measured second harmonic power as a function of fundamental power. The curve represents a quadratic fitting. A maximum of 109 µW UV light is obtained with a fundamental pump power of 650 mW. The conversion efficiency *η* is 0.026%W-1, compared to a conversion efficiency of 0.019%W-1 for a single-pass frequency doubling through a 15 mm long LiIO3 bulk crystal (Knappe et al., 1998), and the

0 100 200 300 400 500 600 700

Fundamental power (mW)

Fig. 12. Second harmonic power as a function of fundamental power. The squares are


Furthermore, we compare the results from the three laser systems based on different tapered gain devices. This is important for us to choose tapered gain devices for different applications.

Table 2. Summary of the main parameters for diode laser system A, B and C.
