The section on Raman lasing in microspheres shows that efficient generation of nonlinear waves in a silica microsphere through the Raman effect is possible, efficient, and matches very well with a simple theoretical model. However, the presence of a multiline emission spectrum due to the closely spaced, nearly degenerate azimuthal mode structure in a spherical cavity is a significant drawback to practical utilization of these structures. The toroidal microresonator is capable of retaining the ultra high-Q present in the microsphere resonators, while improving the reproducibility of Q, parallel fabrication and forming a nondegenerate azimuthal mode structure. Thus, these structures should allow low-threshold and single-mode Raman oscillation.
Nonlinear wave generation has been investigated in this microcavity, with Figure 1 showing a typical Raman emission spectrum for a toroidal microcavity pumped above threshold.
Both first order and second order SRS are shown, with the pump near 1550 nm. Observation of the Raman spectrum reveals that only a single azimuthal mode is lasing, in contrast to microspheres. This occurred for a wide range of pump power and coupling conditions, suggesting that this is an intrinsic property of SRS in this toroidal cavity. This result is not surprising, as the toroid geometry does not possess the degenerate azimuthal modes present in the microsphere, due to the added vertical confinement.
A typical threshold graph of a toroidal microcavity is shown in Figure 2. It shows a threshold of approximately 120 microwatts with a differential efficiency of 22%. This value is consistent with that determined by theory for the experimental values used. Additionally, further investigation has obtained thresholds on the order of 70 microwatts, very close to the current record in microspheres (62 microwatts at a wavelength of 1550 nm). This demonstrates that toroidal cavities do in fact retain the low threshold behavior of microspheres.
More information can be found in the following papers:
B. Min, L. Yang, and K. J. Vahala,
"Controlled transition between parametric and Raman oscillations in ultrahigh-Q silica toroidal microcavities"
Applied Physics Letters, vol. 87, issue 18, October 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.