Design of a high-power continuous source of broadband down-converted light

We present the design and experimental proof of principle of a low threshold optical parametric oscillator (OPO) that continuously oscillates over a large bandwidth allowed by phase matching. The large oscillation bandwidth is achieved with a selective two-photon loss that suppresses the inherent mode competition, which tends to narrow the bandwidth in conventional OPOs. Our design performs pairwise mode locking of many frequency pairs, in direct equivalence to passive mode locking of ultrashort pulsed lasers. The ability to obtain high powers of continuous and broadband down-converted light enables the optimal exploitation of the correlations within the down-converted spectrum, thereby strongly affecting two-photon interactions even at classically high power levels, and opening new venues for applications such as two-photon spectroscopy and microscopy and optical spread spectrum communication.

Figure 1

(Color online) A pairwise mode-locked OPO cavity configuration.

Figure 2

(Color online) The selection mechanism of the oscillating frequency pairs in the OPO cavity. (a) Assuming a single two-photon loss medium at a dispersion value ${\gamma }_1$ and (b) assuming two media for two-photon loss at two different dispersion values $({\gamma }_1,{\gamma }_2)$.

Figure 3

(Color online) Calculated conversion efficiency as a function of $N=I_p∕I_{\mathit{th}}$ for a narrow oscillation, ideal broadband oscillation and practical broadband oscillation (dashed). $I_{\mathit{th}}$ is taken to be the minimum threshold intensity among all possible narrowband oscillations.

Figure 4

(Color online) Threshold pump intensity $\left(I_{\mathit{th}}\right)$ as function of signal wavelength. (a) For a $10\mathrm{mm}$ long periodically polled KTP crystal with 4% loss in the cavity pumped by $532\mathrm{nm}$ in a broad phase matching configuration. (b) For a $14\mathrm{mm}$ long BBO crystal with 1% loss in the cavity pumped by $728\mathrm{nm}$ in an ultrabroad, zero dispersion, phase matching configuration.

Figure 5

(Color online) Experimental OPO cavity configuration.

Figure 6

(Color online) Measurements of pump depletion and SFG loss as a function of the differential OPO cavity length over a range of one pump wavelength. (a) Pump depletion and (b) the corresponding SFG loss when dispersion is not optimally compensated ($\sim 600\mathrm{f}{\mathrm{s}}^2$ residual dispersion). (c) Pump depletion and (d) corresponding SFG loss when dispersion is optimally compensated.

Figure 7

(Color online) A typical oscillation spectrum of the OPO configuration in Fig. 5.