Stabilized 1.5 microns dual-frequency laser prototype for optical generation of high purity microwave signals

Abstract

The generation of optically carried microwave signals is of great interest for a wide range of fields, from long-range transmission of RF to wideband radar signal processing. Among the available generation techniques, the potentialities of optical heterodyning are firmly established. However, it leads to severe constraints in optical phase-locked loop (OPLL) design. In this paper, we report on the realization of a compact and ruggedized Er,Yb:glass OFL at 1.5 11m stabilized with three servo-loops.The principle of our OFL is the same as the one presented in [1]. A linear phase anisotropy is inserted inside the laser cavity in order to lift the polarization eigenstates degeneracy. Adjusting the amount of phase anisotropy allows then the frequency difference between the two orthogonal polarizations to be tuned. Since the two optical oscillators share the same cavity, common first-order frequency fluctuations cancel in the resulting beatnote. The laser cavity (see Fig. la and Fig. Ib) is composed of four elements: the gain medium (co-doped phosphate glass) pumped at 975 nm, the birefringent medium (PLZT ceramics), an uncoated silica etalon (ensuring stable dual frequency operation), and the output coupler (2% transmission). PLZT ceramics and etalon are bonded to glass holders, which are pressed between active medium and output coupler in order to achieve the best mechanical stability possible. Under 1.3 W pump power level, the laser outputs a Gaussian beam with a 60 m W overall output power at 1.53 11m, (more than 30 mW are fiber-coupled). The ouput beam is composed by two cross-polarized optical frequencies. Their frequency difference depends on the voltage applied on PLZT element, varing from 800 MHz to 13 GHz with 650 V. When the two frequencies are projected along the same polarization axis, it results in an optical beatnote which frequency is the frequency difference. This beatnote is detected with a photo diode and this leads to the generation of a microwave electrical signal.

Widely Tunable Opto-Electronic Oscillator Based on a Dual-Frequency Laser

“…This stability of the microwave signal may not be sufficient for stringent applications such as the generation of a high-purity optically-carried frequency reference. We have been able to demonstrate its stabilization on an external RF source with a low additive phase noise [16]. To get rid of this external source and fully benefit from the tunability of the laser, we implement an optical frequency locked loop described in the next section.…”

Tunable opto-electronic oscillator based on a fiber-ring resonator and a dual-frequency laser

“…In particular, bulk solid-state DFLs have proven to provide, in a compact single cavity, powerful optical signals, modulated in the microwave range with a low phase noise plus a wide and continuous frequency tunability [7,[17][18][19].…”