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Efficient, Compact Laser Generates 40 mW of Yellow Output

Photonics Spectra
Mar 2006
Breck Hitz

As more and more applications have turned to diode-pumped solid-state lasers, one important region of the visible spectrum has proved especially challenging. Applications requiring yellow light — such as ophthalmology and dermatology — have for the most part relied on bulky and inefficient dye or copper-vapor lasers. Yellow solid-state lasers, some entailing remarkable technical ingenuity, have been introduced, but they are complex devices intended for sophisticated users.

Recently, a research group at Stanford University’s Edward L. Ginzton Laboratory in Stanford, Calif., has demonstrated a yellow solid-state laser source that is compact and efficient, is simple to operate and that might, eventually, be suitable for integration into standard medical equipment (Figure 1).

Efficient, Compact Laser Generates 40 mW of Yellow Output
Figure 1. The 40-mW beam of 575-nm light — which here illuminates a lemon — emerged from the periodically poled lithium niobate oven at lower left.

The yellow source is based on a frequency-doubled, ytterbium-doped fiber laser. The challenging part of the demonstration was not the frequency doubling, but operating the Yb laser at a long enough wavelength to be doubled into the yellow.

Although the optical gain in Yb-doped fiber extends up to 1200 nm, laser lines at shorter wavelengths such as 1030 nm tend to saturate the gain and to prevent lasing at the longer wavelengths unless they are severely curtailed. Even the amplified spontaneous emission at shorter wavelengths can usurp enough gain to prevent the longer wavelengths from reaching threshold. The investigators took several steps to discriminate against the shorter wavelengths.

The system was entirely fiber-based, with no free-space optics (Figure 2). The 1150-nm output of the fiber laser was doubled in a periodically poled lithium niobate (PPLN) waveguide to generate 40 mW of 575-nm radiation. The scientists invoked a novel fiber layout as well as spectrally selective fiber Bragg gratings (FBGs) to suppress lasing and amplified spontaneous emission at wavelengths shorter than 1150 nm.

Efficient, Compact Laser Generates 40 mW of Yellow Output
Figure 2. The Yb-doped fiber laser was pumped from both ends with 980-nm laser diodes. A dichroic beamsplitter (D1) separated the yellow second harmonic from the infrared fundamental. Photodetector 1 (PD1) had an 1150-nm spike filter, and photodetector 2 (PD2) had a 575-nm spike filter. ©OSA.

The fiber gain medium consisted of a 2-m length of highly doped fiber, with a 4-m length of less-doped fiber spliced to each end. Most of the 1150-nm gain occurred in the less-doped fiber, while the heavily doped fiber suppressed the 1030-nm line by ground-state absorption. Ground-state absorption is much stronger at 1030 nm than at 1150 nm.

The design of this fiber layout was the result of a compromise among avoiding photodarkening, minimizing the intracavity loss at 1150 nm and maximizing the loss at 1030 nm. Photodarkening occurs over time, when the Yb-doped fiber is exposed to high levels of pump radiation. Ideally, the researchers would have constructed the laser from a short length of only heavily doped fiber, but this was not possible because that fiber is more sensitive to photodarkening. In the end, they arrived at the optimum fiber lengths through trial and error after determining the optimum total small signal absorption through simulations.

They first operated the laser in an unpolarized configuration, observing up to 121 mW of 1150-nm power (Figure 3). To polarize the laser, as required for frequency doubling, they relied on the polarization dependence of the FBGs. Both the Yb-doped fiber and the FBGs were polarization-maintaining, exhibiting stress-induced birefringence.

Efficient, Compact Laser Generates 40 mW of Yellow Output
Figure 3. The 1150-nm laser produced 121 mW of unpolarized and 89 mW of polarized power. The solid line is the mathematical simulation’s prediction. ©OSA.

The investigators spliced the output-coupling FBG, which had a reflectivity of ~0.5, at 90° to the Yb-doped fiber, while they spliced the maximum-reflecting FBG, which had a reflectivity of ∼0.995, in alignment to the gain fiber. They tuned the temperature of the output coupler so that its slow-axis peak wavelength was aligned with the high reflector’s fast-axis wavelength.

In this configuration, only light in the slow axis of the Yb-doped fiber was reflected from both FBGs, and, hence, only that polarization oscillated in the resonator. The scientists observed a polarized output of 89 mW (Figure 3) that was stable for several weeks, indicating the absence of photodarkening.

The 5.2-cm-long lithium-niobate waveguide chip — 3 cm of which was periodically poled — generated a 575-nm output of 40 mW. Coupling losses between the laser and the PPLN reduced the infrared power in the nonlinear crystal to ~65 mW.

The researchers observed photorefractive damage in the lithium niobate after several hours of operation at the highest power levels, but they predict that similar results, without the damage, could be obtained with nonlinear crystals such as LiTaO3 or Mg:LiNbO3.

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