This paper presents the case for the silicon Raman laser as a potential source for the technologically important midwave infrared (MWIR) region of the optical spectrum. The mid-IR application space is summarized, and the current practice based on the optical parametric oscillators and solid state Raman lasers is discussed. Relevant properties of silicon are compared with popular Raman crystals, and linear and nonlinear transmission measurements of silicon in the mid-IR are presented. It is shown that the absence of the nonlinear losses, which severely limit the performance of the recently demonstrated silicon lasers in the near IR, combined with unsurpassed crystal quality, high thermal conductivity and excellent optical damage threshold render silicon a very attractive Raman medium, even when compared to the very best Raman crystals. In addition, silicon photonic technology, offering integrated low-loss waveguides and microcavities, offers additional advantages over today's bulk crystal Raman laser technology. Using photonic crystal structures or microring resonators, the integrated cascaded microcavities can be employed to realize higher order Stokes emission, and hence to extend the wavelength coverage of the existing pump lasers. Exploiting these facts, the proposed technology can extend the utility of silicon photonics beyond data communication and into equally important applications in biochemical sensing and laser medicine

Prospects for Silicon Mid-IR Raman Lasers

We present a comprehensive overview of different types of parametric interactions in nonlinear optics which are associated with simultaneous phase-matching of several optical processes in quadratic nonlinear media, the so-called multistep parametric interactions. We discuss a number of possibilities of double and multiple phase-matching in engineered structures with the sign-varying second-order nonlinear susceptibility, including (i) uniform and non-uniform quasi-phase-matched (QPM) periodic optical superlattices, (ii) phase-reversed and periodically chirped QPM structures, and (iii) uniform QPM structures in non-collinear geometry, including recently fabricated two-dimensional nonlinear quadratic photonic crystals. We also summarize the most important experimental results on the multi-frequency generation due to multistep parametric processes, and overview the physics and basic properties of multi-color optical parametric solitons generated by these parametric interactions.

Multistep Parametric Processes in Nonlinear Optics

We report a high-efficiency and high-power mid-IR laser generation by a periodically poled (PP) MgLN-based optical parametric oscillator (OPO) working at room temperature, from which 9.23 W of output was obtained at 3.82 microm when pumped by a linearly polarized acousto-optic Q-switched Nd:YVO(4) laser of 48.2 W output. A semi-external-cavity structure was applied in the OPO system to avoid the conversion saturation caused by the thermal lens effect of the PPMgLN crystal and thus to maintain the high conversion efficiency and the high beam quality under the high power level. It was computed that 29.4% slope efficiency and 19.2% total efficiency were realized for the 1.063-3.82 microm conversion.

High-efficiency semi-external-cavity-structured periodically poled MgLN-based optical parametric oscillator with output power exceeding 9.2 W at 3.82 microm.

We report a synchronously pumped optical parametric oscillator that generates the sum frequency of the pump and the signal wavelengths. A single KTiOPO4 (KTP) crystal is used for both parametric generation and sum-frequency generation in which these two processes are simultaneously phase matched for the same direction of propagation. The parametric oscillator, pumped by a femtosecond Ti:sapphire laser at a wavelength of 827 nm, generates a blue output beam at 487 nm with 43% power-conversion efficiency. The polarization geometry of simultaneous phase matching requires rotation of the pump polarization before the cavity. Adjusting the group delay between the two orthogonally polarized pump components to compensate for the group-velocity mismatch in the KTP crystal increases the photon-conversion efficiency more than threefold. Angle tuning in conjunction with pump wavelength tuning provides output tunability in the 484–512-nm range. A plane-wave model that takes group-velocity mismatch into account is in good agreement with our experimental results.

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Single-crystal sum-frequency-generating optical parametric oscillator

A Ti:sapphire-pumped idler-resonant femtosecond tandem optical parametric oscillator is reported that is based on periodically poled lithium niobate and operates with wavelength tuning from 2.1 to 4.2 μm (idlers) and 1.25 to 1.40 μm (signal). The configuration uses two cascaded gratings arranged so that the nonresonant signal from the first grating acts as a pump for the second grating. Novel phase-matching behavior is described and explained, including full tandem operation, degenerate parametric downconversion of the signal pulses, and simultaneous tandem operation with seeded optical parametric amplification. Signal spectra show characteristic depletion profiles, implying substantial conversion from the signal to the tandem idler output.

Idler-resonant femtosecond tandem optical parametric oscillator tuning from 2.1 μm to 4.2 μm

We have observed the spectral and temporal characteristics of a 1.064-mm pumped, continuously tunable optical parametric oscillator (OPO) using a periodically poled lithium niobate nonlinear crystal. An efficient secondary OPO pumped by the resonant signal of the primary OPO was observed. Predictions obtained from theory agree with observed results and provide insight into the dynamics of pulsed parametric oscillation. 1997 Optical Society of America.

Timothy Patrick Grayson

Papers

Cascaded optical parametric oscillations