Raman oscillation at low pump powers using ultrahigh-Q microcavities is possible even in extremely linear media such as silica. To investigate the possibility of Raman oscillation in solgel microcavities, microtoroids were fabricated from an undoped solgel layer.
The quality factor of the initial microdisk preform was 7.13x104, whereas after the CO2 laser reflow the microtoroid achieved quality factors as high as 2.53x107 at 1561 nm. This increase in Q was also shown previously in thermal silica microdisks. A comparison of the thermal silica and sol-gel microtoroids using an optical microscope is shown in Figure 1. SEM images of the sol-gel microdisks and microtoroids are in Figure 2.
A Q of 107 corresponds to a waveguide loss of 0.009 dB/cm, which is the lowest loss reported to date for solgel silica-on-silicon technology. These high quality factors allow the observation of Raman emission with threshold pump powers below a milliwatt. In this case, a single-frequency tunable external cavity laser operating around 1550 nm band was used to pump the microcavity.
Figure 3 shows a typical Raman laser spectrum taken on a microtoroid with a quality factor of 2.53x107 at 1561 nm. In the figure both pump wave at 1561 nm and Raman lasing at 1679 nm are visible. The inset in Fig. 3 presents a measurement of Raman lasing power as a function of absorbed pump power, which shows a submilliwatt threshold of 640 mW.
More information can be found in the following papers:
L. Yang, T. Carmon, B. K. Min, S. M. Spillane, and K. J. Vahala
“Erbium-doped and Raman microlasers on a silicon chip fabricated by the sol-gel process”
Applied Physics Letters, Volume 86, 091114, February 2005.
T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala
"Ultralow-threshold microcavity Raman laser on a microelectronic chip"
Optics Letters, Volume 29, No. 11, 1224-1227, June 2004.
D. K. Armani, T. J. Kippenberg, S. M. Spillane and K. J. Vahala
"Ultra-high-Q toroid microcavity on a chip"
Nature, vol. 421, pp. 925-929, 27 February 2003.