When an application requires precise control of the timing and frequency of a Q-switched laser’s pulses, active Q-switching is the only way to go. However, for the many applications that don’t need such temporal control, passive Q-switching is often simpler and less expensive.Pumped through an undoped fiber with blue (454 nm) light from an Ar-Kr-ion laser, a Bi-doped fiber emits in the infrared (1150 nm) and in the deep visible red (750 nm).Recently, researchers at the Russian Academy of Sciences in Moscow demonstrated an all-fiber approach to passive Q-switching that allows considerable flexibility in the laser’s output wavelength and temporal characteristics.Passive Q-switching involves an intracavity saturable absorber, which initially absorbs spontaneously emitted photons at the laser wavelength and prevents the laser from reaching threshold. As the population inversion builds, however, the flux of photons in the resonator increases until eventually it bleaches the saturable absorber. At that instant, the resonator switches from a low-Q condition to high-Q, and the energy in the population inversion is quickly emptied into the Q-switched pulse that emerges from the output mirror. Thus, to passively Q-switch a laser, the first step is to find a suitable medium for the saturable absorber — one that absorbs at the laser wavelength but that can be efficiently bleached when the photon flux reaches a critical level. The researchers had previously developed bismuth-doped fiber lasers that absorb pump light at approximately 1 μm and that lase at ~1.2 μm. It occurred to them that Bi-doped fiber might be a good saturable absorber in ytterbium-doped fiber lasers because the 1-μm absorption of Bi corresponds to the lasing wavelength of Yb-doped fibers.Figure 1. The Bi fiber laser inside the Yb fiber laser resonator acted as a saturable absorber to Q-switch the Yb laser. 1 = high-reflecting fiber Bragg grating (FBG) for the Yb wavelength; 2 = high-reflecting FBG for the Bi wavelength; 3 = output coupling FBG for the Bi laser; 4 = output coupling facet for the Yb laser. Images reprinted with permission of Optics Letters.The only problem was the excessive time it takes Bi to unbleach after it has been bleached by absorbing 1-μm photons. The upper-state relaxation time is ~1 ms, which would limit the Q-switched Yb fiber laser to an unacceptably slow repetition frequency of about 1 kHz. To overcome this problem, the researchers placed mirrors at each end of the Bi-doped fiber. In other words, they created a Bi-fiber laser inside an Yb fiber laser as shown in Figure 1 (and, actually, the mirrors were fiber Bragg gratings). Because stimulated emission depopulated the Bi upper state, its 1-ms spontaneous lifetime became moot, and the Yb fiber laser could Q-switch at rates well over 1 kHz.An additional benefit of this configuration was the ~1.2-μm output of the Bi laser, so that the combination Yb-Bi fiber laser lased at a number of wavelengths between 1050 and 1215 nm, considerably extending the ~1050- to 1080-nm spectral range of a normal Yb laser.Figure 2. The researchers observed stable Q-switched pulses from the Yb laser at 1066 nm.Operating on the 1066-nm transition of the Yb laser, the researchers observed stable trains of Q-switched, 1-μs-duration pulses with up to 100 μJ (and 140 μJ subsequent to publication of the Optics Letters paper) per pulse, at repetition frequencies well above the kilohertz limitation imposed by the Bi spontaneous relaxation time (Figure 2). They also obtained Yb laser outputs at 1050, 1064 and 1080 nm.Figure 3. To boost the 1160-nm output of the Bi fiber laser, the researchers added a Bi fiber amplifier to their configuration. (HR BG = highly reflective Bragg grating, L = length)To demonstrate the usefulness of the Bi laser transitions at 1160 and 1215 nm, they added a Bi fiber amplifier to their configuration (Figure 3). Operating the system in this configuration at the 1160-nm Bi transition, they observed 1.5-μs pulses containing 6.5 μJ at a repetition frequency of tens of kilohertz.Optics Letters, March 1, 2007, pp. 451-453.