Fiber Laser Offers New Approach to 488-nm Emission

Breck Hitz

Solid-state laser sources at the argon-ion wavelength of 488 nm find many applications in medicine, graphics, semiconductor inspection and elsewhere. Several commercial models are already available, and others are expected to be introduced to the market soon.

Figure 1. A frequency-doubled fiber-laser master oscillator power amplifier generates 18.1 mW of 488-nm light.

The current products are based on a frequency-doubled diode laser, but now researchers from the University of Southampton in the UK and from the Royal Institute of Technology in Stockholm, Sweden, have demonstrated a fiber laser master oscillator power amplifier (MOPA) frequency-doubled to 488 nm. Because fiber MOPAs are scalable to higher powers than individual diode lasers, this approach offers the enticing possibility of significantly higher blue powers.

The scientists amplified the output of a low-power, high-beam-quality master oscillator in a fiber amplifier and frequency-doubled its output with a periodically poled KTP (PPKTP) crystal (Figure 2). Yb-doped jacketed air-clad fiber (Figure 3) was in the oscillator and the amplifier, and in both cases, the fiber's inner cladding was pumped with the 915-nm output of diode lasers. The oscillator fiber was 35 cm in length and cleaved flat at one end to provide one of the laser's reflectors.

Figure 2. The wavelength-tunable oscillator (right) generates 183 mW at 977.4 nm, which is amplified to 2.7 W in the fiber amplifier (top left). The 2.7 W is focused into the periodically poled KTP crystal (bottom left), where it generates 18.1 mW at 488.4 nm. ©2004 IEEE.

A pair of external diffraction gratings reflected the light to a high-reflectance mirror at the other end of the resonator. The gratings polarized the laser and held its bandwidth to 0.6 nm, but introduced more than 50 percent round-trip loss. By adjusting the angle of the mirror, the researchers could tune the laser between 975 and 982 nm. For generating the blue output from the PPKTP crystal, they selected the precise wavelength of 977.4 nm. With 1.5 W of 915-nm power launched into the fiber's inner cladding, the oscillator produced 183 mW.

Figure 3. The jacketed air-clad fiber has a small inner-cladding diameter to suppress oscillation at 1.03 µm, but its numerical aperture is large because the effective index of the mostly air outer cladding is much smaller than the index of the inner cladding. ©2004 IEEE.

Several ytterbium laser lines share the same upper level, and it's important to take their dynamics into account when designing ytterbium lasers. The 980-nm line is a three-level transition, terminating on essentially the same level as the ground state. Longer wavelengths of ~1.03 to 1.06 µm are four-level transitions terminating on levels significantly above the ground state. Because the longer-wavelength lines are four-level transitions, they have lower thresholds and must be suppressed when, as in this case, oscillation on the 980-nm line is desired.

The scientists chose a fiber with a small inner cladding so that the pump radiation could be absorbed by the Yb-doped core in a relatively short length, suppressing emission at the longer wavelengths. Although the inner-cladding diameter was small, its numerical aperture was large (~0.5) because of the large index difference between the inner cladding and the outer cladding, the latter of which was mostly air.

And although both the oscillator and the amplifier used the same type of fiber, the amplifier fiber was 20 cm longer. Both ends of the amplifier fiber were cleaved at an oblique angle to prevent laser oscillation, and the IR radiation from the oscillator was coupled into the amplifier with 40 percent efficiency. A filter blocked 1.03-µm emission from the oscillator, which would have depleted the amplifier's population inversion. Sixty percent of the 15 W of incident 915-nm pump radiation was coupled into the amplifier's inner cladding, and the maximum output at 477 nm was 2.7 W, with an M² of 1.8.

The researchers focused this 2.7 W – which was 75 percent linearly polarized – to a 10-µm spot inside the 9-mm-long PPKTP crystal, generating slightly more than 18 mW at 488.7 nm. They measured the blue beam's M² to be 1.7. The blue output did not show any saturation or roll-off at the high-power end, suggesting that higher powers can be obtained with straightforward improvements in the MOPA design. But even at 18 mW, the scientists believe that they have achieved the highest 488-nm power yet reported from a fiber laser.