Not that long ago, high-power mode-locked lasers covered tabletops and required extensive optical layouts. Recent advances using vertical external cavity surface-emitting lasers (VECSELs) have produced high-power ultrafast pulses from simple cavities built around these small lasers. These devices deliver high performance and the potential to be inexpensive when commercialized. "One day, optical clocking of integrated circuits or datacom transmission in optical networks may be accomplished using a productized version of this technology," says Ursula Keller of the ETH (Zurich, Switzerland). Keller's group developed these new VECSELs and collaborated with Anne Tropper at ORS/University of Southhampton on a paper for CLEO 2002 (1924 May; Long Beach, CA) discussing their work on a passively mode-locked VECSEL.
Clocking is an important requirement in all-optical switching applications, and these new VECSELs could play an important role. Another possible application is in RGB displays. Frequency conversion and doubling are much more efficient using a pulsed laser, so high brightness displays could be achieved using these small passively mode locked VECSELs.
Figure 1. In a side-pumped mode-locked VECSEL, the cavity is formed by the SESAM and output coupler, with the gain structure in between.
The architecture incorporates an external cavity and optical pumping of the VECSEL gain media to get the high-output powers (see figure 1). Passive mode locking allows the device to produce near transform-limited pulses and to be compact and simple. The high index of refraction in the semiconductor material is compatible with non-normal incidence pumping of the VECSEL. The group recently generated 950 mW of average power in 15 ps pulses at a 6 GHz rep rate. The power scales with mode size, so cavity parameters can be adjusted to optimize output power. Repetition rates are limited by mechanical design issues and space constraints, but in principle, the design can achieve very high rep rates.
A key element in this design is the semiconductor saturable absorbing mirror (SESAM). This semiconductor mirror modulates the gain in the cavity as a function of intensity, which mode locks the laser without any active control. The continuous-wave pump power is directly converted into pulsed power without control electronics or active elements in the cavity. Ultimately the SESAM could be integrated into the gain structure, which would make the devices simpler to manufacture. This approach could produce single wafers of VECSEL/SESAM devices with no post-processing required.
Figure 2. Autocorrelation of the pulse from the mode-locked VECSEL shows that the pulse is transform limited. The optical spectrum is shown in the upper right.
The cavity length is inversely proportional to the pulse repetition rate, so making the cavity small results in high rep rates in the GHz regime. Pulses are virtually transform limited (see figure 2).
The current output wavelength of these lasers is 980 nm, but band-gap engineering can yield customized designs that operate at desired wavelengths, unlike traditional diode-pumped solid-state lasers (DPSS). "We could go to 1.5 µm with indium phosphide or InGaAsN-based materials," says Keller, noting that 980 nm was a natural wavelength to start with because proven SESAMs were available. Next on the horizon is proving that the power and rep-rate scales and moving toward new wavelengths.
microspheres yield Raman laser
The illustration shows three silica-sphere Raman lasers along a fiber-optic taper, where the black arrow represents the input wave. Each microsphere may emit a different Raman wave (colored arrows). (Caltech)
A fiber-coupled Raman laser 10 times smaller than conventional semiconductor lasers has demonstrated a 62 µW threshold, 36% differential quantum efficiency, and operation throughout the near IR and all of the telecom bands, say researchers at the California Institute of Technology (Caltech; Pasadena, CA). Used to extend the wavelength range of conventional semiconductor lasers, which operate only in selected optical wavelength bands, Raman lasers allow a shifting of the pump wavelength by a prescribed amount. Through a process called cascade, it is possible to use the created Raman wave again as a pump to generate a secondary Raman wave.
The CalTech source consists of a high-Q (>100 million) silica microsphere with a mode volume near 1 fm3 and an optical fiber that has been stretched and thinned to micron scale transverse dimensions over a short portion of its overall length. The device shows extremely efficient coupling both to the micro-resonator pump mode and from the micro-resonator Raman mode. "High Q combined with small mode volume give the micro resonator the ability to achieve enormous optical intensities even when only weak optical signals are applied from the optical fiber," explains Kerry Vahala of the research group.
The researchers observed Raman lasing at extremely low input power levels, 1000 times lower than in previous work. "It is possible to generate appreciable amounts of output power (200 µW) with very little applied pump power," says Vahala.
Possible applications may range from spectroscopy to wavelength generation in bands not easily accessible at present. Future work will focus on fabricating high-Q planar disc or ring microcavities. These structures support fewer modes, hence would enable stable and single mode Raman lasing, paving the way for next-generation ultra-low threshold and compact Raman lasers.
Phillip B. Espinasse