A Viable Commercial Alternative to the Ti:Sapphire Laser?
Chromium-doped colquiriites can be pumped with inexpensive diodes.
The venerable Ti:sapphire laser has always been the workhorse of ultrafast technology, but it is an expensive beast, partly because it requires expensive green pump lasers. Cr3+-doped colquiriites (Cr:LiSAF, Cr:LiSGaF and Cr:LiCAF) also can be mode-locked to produce femtosecond pulses in the ∼800-nm region, and these lasers can be pumped with relatively inexpensive red diode lasers. Most of the work to date has been done with Cr:LiSAF, but that material’s low thermal conductivity has limited its output, in most cases to less than 100 mW. Using an unusual resonator design and a very thin laser crystal to facilitate heat removal, scientists in Switzerland have coaxed half a watt — apparently the world’s record — from a mode-locked Cr:LiSAF laser.
Figure 1. Pumped from both ends by an arrangement of multiplexed diode lasers (D, orange lines), the Cr:LiCAF laser could be configured to oscillate continuously (broken plus solid red lines) or mode-locked (solid red lines). DCM = double-chirped mirror; λ/2 = half-wave plate; M = mirror; OC = output coupler; f = focal length of lens; PBS = polarization beamsplitter. Images reprinted with permission of Optics Letters.
Now, a collaboration between scientists at Massachusetts Institute of Technology in Cambridge and at Koç University in Istanbul, Turkey, has achieved comparable power from a mode-locked Cr:LiCAF laser without resorting to special cavity designs or superthin crystals. These improvements were possible because LiCAF has a higher thermal conductivity than LiSAF, leading to lower thermal lensing, and also because fluorescence quenching of the ∼800-nm laser transition occurs at a much higher temperature in Cr:LiCAF (255 vs. 69 °C). Although mode-locked Cr:LiCAF lasers have been investigated in other laboratories, output powers have been well under 100 mW, and the mode-locking has been touchy because these experiments used a Kerr lens to mode-lock the laser.
The MIT/Koç scientists achieved stable, robust mode-locking by substituting a semiconductor saturable absorber mirror (SESAM) for the Kerr lens. They pumped the laser with no fewer than five nLight diode lasers, which were polarization- and wavelength-multiplexed to pump the laser longitudinally from two directions (Figure 1). The Cr:LiCAF crystal was 2 mm long with Brewster-cut faces and was doped with 10 percent chromium. Before mode-locking the laser, the scientists characterized its continuous-wave performance by aligning the resonator in the arrangement indicated by the broken lines in Figure 1.
Figure 2. In its continuous-wave configuration, the laser produced up to 580 mW with a 1.4 percent output coupler. The output with lower-transmission couplers (not shown here) was similar to that with the 1.4 percent mirror. The five pump diodes were turned on sequentially, as indicated by the legend at the top of the chart and by the alternation between solid and hollow data points.
By sequentially turning on, and then increasing the power of, each pump diode, they plotted the laser’s input/output transfer function for different values of output coupling (Figure 2). They calculated the intracavity loss to be about 1 percent plus the loss of the output coupler, so, predictably, the output mirrors with significantly higher transmission produced significantly less output power. Above about 3.5 W of pump power, thermal quenching started to diminish the output.
Figure 3. The laser generated 300 mW of output in a mode-locked train of 67-fs pulses.
Satisfied with the laser’s continuous-wave performance, the scientists inserted the SESAM and the other components indicated in the mode-locked configuration in Figure 1. When fully saturated, the SESAM added about 0.5 percent to the intracavity loss, so its insertion, together with the additional mirrors required for mode-locking, increased the laser’s threshold from 430 to 840 mW. The laser operated in continuous-wave mode for input power up to 1.5 W; then the SESAM both mode-locked and Q-switched the laser for pump powers between 1.5 and 2 W. At pump powers above 2 W, the laser became stably mode-locked without Q-switching.
At 4 W of pump power, the investigators observed a 300-mW output from their laser in a mode-locked train of 67-fs pulses (Figure 3). The 11-nm spectral width indicated that the laser was nearly transform limited. The resonator mode-locked at its fundamental frequency (i.e., only one pulse circulating inside) of 120 MHz, so the pulse energy was 2.5 nJ, and the peak power was ∼35 kW.
Optics Letters, Nov. 15, 2007, pp. 3309-3311.
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