Simultaneous measurement of patterns in the signal and idler near and far fields from a confocal optical parametrical oscillator with applications in quantum optics

We present the results of an experimental investigation of multimode intensity patterns from an optical parametric oscillator operating above threshold and show that it oscillates in 10–15 transverse modes strongly coupled through the nonlinear crystal, which makes this setup useful for future investigation of quantum correlations in the transverse plane. We describe the experimental setup for simultaneous measurements of signal and idler near- and far-field patterns and analyze the effects of various experimental complications such as walk-off and thermal index changes on the generated patterns. We also show that the oscillator can be stabilized by optical feedback, indicating a possible route for controlling the generated intensity patterns.

Figure 1

(Color online) The experimental setup.

Figure 2

(Color online) The Gauss stability as a function of cavity length change away from the exact confocal length. The thermal lens is $f=63\mathrm{mm}$ (pump power 1 W). The dashed curve is without thermal lens and the solid curve is with thermal lens. In the small box the deviation between the unpumped (without lens) and pumped (with lens) can be seen.

Figure 3

Multimode fields in a confocal, doubly resonant OPO. Cavity length $L=101.5\mathrm{mm}$.

Figure 4

(a) Simulation of a superposition of ten Gauss-Laguerre modes. (b) Idler NF consists of a central spot surrounded by concentric rings (Gauss-Laguerre-like modes). Cavity length $L=101.5\mathrm{mm}$.

Figure 5

(Color online) Fabry-Perot scan of OPO output showing the longitudinal modes.

Figure 6

Different transverse structures of the signal and idler. Cavity length $L=101.1\mathrm{mm}$.

Figure 7

(a) Central part of the idler near field and (b) central part of the idler far field. The near field and far field display different transverse patterns, which is an indication of multimode emission. Cavity length $L=101.5\mathrm{mm}$.

Figure 8

Changing the fine-tuning of the cavity slightly around the exact resonance length. (a) The idler looks like a Gauss-Ince mode. (b) For larger negative detuning the signal changes into a pattern that looks like a Gauss-Ince mode.

Figure 9

The evolution of the complex signal far-field patterns as a function of the measured cavity length. (a) $L=102.0\mathrm{mm}$. (b) $L=101.5\mathrm{mm}$. (c) $L=101.3\mathrm{mm}$. (d) $L=100.8\mathrm{mm}$. All images are recorded with a gray filter. Observed confocal length 101.6 mm.

Figure 10

Feedback experiment where the idler is forced into a pattern by feedback. (a) Without feedback. (b) With feedback.