Broadband near-to-shot-noise suppression of ... - OSA Publishing

1 downloads 0 Views 529KB Size Report
Apr 1, 2015 - Broadband near-to-shot-noise suppression of arbitrary cw-laser excess intensity noise in the gigahertz range. Ernest A. Michael1,* and Laurent ...
1334

OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015

Broadband near-to-shot-noise suppression of arbitrary cw-laser excess intensity noise in the gigahertz range Ernest A. Michael1,* and Laurent Pallanca1,2 1

Department of Electrical Engineering, FCFM, Universidad de Chile, Av. Tupper 2007, Santiago, Chile 2

ESO Paranal Observatory, II Region, Chile *Corresponding author: [email protected] Received December 16, 2014; revised March 23, 2014; accepted January 26, 2015; posted February 13, 2015 (Doc. ID 228326); published March 23, 2015 Broadband near-to-shot-noise suppression of the intensity noise from a continuous-wave (cw) fiber laser at 1550 nm is demonstrated at GHz-frequencies using feed-forward phase-matched destructive noise interference impressed onto the optical signal with a fiber electro-optic power modulator. The scheme is independent of the laser frequency and therefore is suitable for tunable lasers. It can be used with some modifications after an optical fiber-amplifier boosting a cw laser signal. A noise residual of down to 2 dB above the shot-noise was measured, which is about 2 dB below the prediction with a rigorous noise model. While the total laser noise can be removed, inclusive shot noise, because the latter is still 10 dB above the thermal noise, the power splitter introduces some partition noise above the shot level. In that case, a sub-shot-noise suppression scheme should be possible by replacing the photon anti-correlation of the power splitter by the co-correlation obtained from a paired photon or twin beam source. © 2015 Optical Society of America OCIS codes: (030.5290) Photon statistics; (060.3510) Lasers, fiber; (060.2320) Fiber optics amplifiers and oscillators; (040.5570) Quantum detectors; (230.1360) Beam splitters. http://dx.doi.org/10.1364/OL.40.001334

Laser intensity noise is desired to be at the shot noise limit for many precision applications, as in spectroscopy, metrology, and in quantum information processing (cryptography) [1]. In some cases, even sub-shot-noise squeezing is essential. For example, in laser interferometry, which relies on opto-mechanical interaction between the laser field and the mirror positions [2,3], the intensity fluctuations of the laser itself need to be as low as possible in order to detect the mirror position with the output intensity of the interferometer, while making just the detection scheme blind for the laser fluctuations, as e.g., in heterodyne detection with “balanced detection” [4], is not sufficient. Broadband laser noise reduction to the shot-noise limit by laser-external [5] or intrinsic means [6], or even the generation of photon-number squeezed light [3], is so far reported only for the (sub-)acoustic to MHz range. However, for some applications, laser relative intensity noise (RIN) reduction in the microwave range is required, e.g., in atmospheric research (radiometry, LIDAR, etc.) and astronomy, in heterodyne, or recently also in optomechanical receivers [2]. For infrared heterodyne receiver photodiode arrays it may be impractical to implement balanced detection. As intensity noise is equivalent with sidebands occurring [7], i.e., 2 sin2πνt · cos2πft  sin2πν  f t  sin2πν − f t, where f is the noise frequency and ν is the laser frequency, spectral filtering techniques have been applied based on fiber resonators [8], Bragg-gratings [9], or unbalanced Mach–Zehnder interferometers [10], but these techniques need a precise match to the laser frequency and are just slightly thermally tunable, therefore are not ideal for largely tunable lasers. ¯ the laser RIN can be defined With δPt ≔ Pt − P, R 2 ¯2 as RIN ≔ δP ∕P  S f df [11] with Sf  (unit: dBc/ Hz) the normalized power spectral density from the autocorrelation of the power fluctuations δPt integrated 0146-9592/15/071334-04$15.00/0

over the interesting noise frequency range [9]. Whereas in the excess noise power range, this is independent of the laser power, in the standard quantum (“shot”) noise ¯ which is equivalent to limit it becomes S SN f   2hν∕P, ¯ where n¯ is the mean photon number in a given δn2  n, ¯ where B is the noise bandtime, or to δP 2  2hνB · P, width. Excess RIN above this limit is expected from relaxation oscillations that peak in ranges from