Paul Narum

Effect of laser mode structure on stimulated Brillouin scattering

The gain and reflectivity of the stimulated Brillouin scattering (SBS) process used as a Stokes-wave generator are shown theoretically to be independent of the mode structure of the pump laser provided that the pump-laser mode spacing exceeds the Brillouin linewidth and that the laser coherence length exceeds the characteristic gain length of the SBS process. Under the same set of conditions, the gain of an SBS amplifier is found to depend on the degree of correlation between the laser and Stokes fields. However, due to nonlinear coupling, these two fields become correlated within several characteristic gain lengths, and the subsequent propagation of the two fields is governed by the same set of equations that apply for the case of a single-mode pump laser. These theoretical predictions are tested experimentally for an SBS generator using acetone, carbon disulfide, and methanol as the Brillouin-active media, and the results are in full agreement with the theoretical predictions.

Simple, compact, high-performance permanent-magnet Faraday isolator

The design of a Faraday isolator that uses a short glass rotator rod and produces highly uniform rotation across its clear aperture is presented. The rotator rod is 19.5 mm long, and at a wavelength of 633 nm the rotation angle is 45 deg and the isolation ratio is >45 dB.

Slow- and fast-light: fundamental limitations

We present an analysis of the propagation of light pulses through materials possessing extreme values of the group velocity. We begin with an analysis of the behaviour that occurs upon propagation through materials possessing simple Lorentzian gain or absorption lines or materials possessing sharp dips in gain or absorption features. We also describe how more complicated lineshapes can be used to tailor the dispersion of a slow-light system. We furthermore present an analysis of fundamental limitations to how many pulse widths a pulse of light can be delayed or advanced. We show how these mechanisms lead to different limits for slow and fast propagation media.

Non-frequency-shifted, high-fidelity phase conjugation with aberrated pump waves by Brillouin-enhanced four-wave mixing

The results of an experimental investigation of a new geometry for producing phase conjugation by Brillouin-enhanced four-wave mixing are presented. In this geometry, the four-wave mixing medium is carbon disulfide, and the backward-going pump wave is created from the transmitted forward-going pump wave by stimulated Brillouin scattering (SBS) in glycerol. The two pump waves are hence phase conjugates of each other, and the quality of the phase-conjugation process is not degraded even by the use of an aberrated pump wave. The probe wave is created by SBS in carbon disulfide, which has a Brillouin frequency half that of glycerol, and the conjugate wave is therefore generated at the same frequency as the probe. Since the pump and signal waves differ in frequency by the Brillouin frequency of the carbon disulfide four-wave mixing medium, high reflectivities (approximately 2000%) are obtained as a result of Brillouin resonance enhancement.

Saturated absorption and degenerate four-wave mixing in Nd 3+ beta″ alumina

Nd3+ beta″ alumina is one of a family of potentially useful nonlinear-optical materials based on rare-earth ions substituted into a beta″ alumina host. The saturation intensity of Nd3+ beta″ alumina has been measured to be in the range 16 to 35 kW cm−2. Phase conjugation by degenerate four-wave mixing has been demonstrated, and the phase-conjugate reflectivity has been measured as a function of both laser wavelength and intensity.

Non-frequency-shifted phase conjugation by Brillouin-enhanced four-wave mixing

We present a theoretical treatment of four-wave mixing (FWM) in a Brillouin-active medium for the case in which the pump waves differ in frequency by approximately twice the Brillouin frequency shift of the medium and in which the probe-wave frequency is approximately the arithmetic mean of the frequencies of the two pump waves. Under these conditions, the conjugate wave produced by the FWM process has the desirable property of being at the same frequency as the probe. We derive the coupled amplitude equations describing this interaction. We solve these equations analytically in the limit of negligible pump depletion and find that large phase conjugate reflectivities are readily achievable. The coupled amplitude equations are solved numerically for the general case, and it is found that large power transfer from the pumps to the output wave is possible. The output wave is shown to be a nearly perfect phase conjugate of the probe wave, even far into the regime where pump depletion effects are important. Our formalism predicts the existence of a parametric instability in the propagation of the pump waves, but good performance is predicted before the onset of this instability.

Causality in Superluminal Pulse Propagation

The theory of electromagnetism for wave propagation in vacuum, as embodied by Maxwell’s equations, contains physical constants that can be combined to arrive at the speed of light in vacuum c. As shown by Einstein, consideration of the space–time transformation properties of Maxwell’s equations leads to the special theory of relativity. One consequence of this theory is that no information can be transmitted between two parties in a time shorter than it would take light, propagating through vacuum, to travel between the parties. That is, the speed of information transfer is less than or equal to the speed of light in vacuum c and information related to an event stays within the so-called light cone associated with the event. Hypothetical faster-than-light (superluminal) communication is very intriguing because relativistic causality would be violated. Relativistic causality is a principle by which an event is linked to a previous cause as viewed from any inertial frame of reference; superluminal communication would allow us to change the outcome of an event after it has happened.

Experimental investigation of the transient dynamics of slow light in ruby

When a pulsed light beam propagates through ruby, it is delayed by a slow-light mechanism. This mechanism has been the subject of debate (Wisniewski-Barker et al 2013 New J. Phys. 15 083020; Kozlov et al 2014 New J. Phys. 16 038001; Wisniewski-Barker et al 2014 New J. Phys. 16 038002). To distinguish between the two main proposed mechanisms, we investigate the trailing edge of a squarewave pulsed laser beam propagating through ruby. Our observation of a pronounced tail on the trailing edge of the transmitted pulse cannot be explained solely by the effects of a time-varying absorber acting upon the incident pulse. Therefore, our observation of the creation of a tail at the trailing edge of the pulse provides evidence for a complicated model of slow light in ruby that requires more than pulse reshaping. The different delays of individual Fourier components of the pulse signal explain the pulse distortion that occurs upon transmission through the ruby and must be accounted for by any model that attempts to describe the effects of slow light in ruby.

STIMULATED BRILLOUIN SCATTERING WITH A MULTIMODE LASER

The dependence of stimulated Brillouin gain on the pump laser mode structure is studied both theoretically and experimentally. It is found that under conditions that are often fulfilled in practice, the gain and reflectivity are independent of the number of modes in the pump laser even for the case of strong pump depletion.

Phase Conjugate Mirror With Gain Based On Brillouin Scattering

We describe a new type of phase-conjugate mirror based on Brillouin enhanced four-wave mixing that has many attractive features distinguishing it from previous types of phase-conjugate mirrors. Chief among these features are its high reflectivity, ease of alignment, and its insensitivity to aberrations on the pump waves. Both theoretical and experimental results are presented.