I.G. Cormack

Fluorescence suppression within Raman spectroscopy using annular beam excitation

This letter demonstrates how the use of an annular beam can significantly reduce the detected fluorescence from optical components within a Raman spectroscopic arrangement. The fluorescence from the excitation beam has a reduced generation and collection efficiency compared to the standard Gaussian beam. Crucially, the Raman signal from the annular beam is collected through the dark center of the excitation light field. This robust and simple technique may be immediately applied to all Raman configurations and may find broader applicability for any spectroscopic methodology which utilizes a high numerical aperture lens and requires the detected fluorescence to be suppressed.

Practical measurement of femtosecond optical pulses using time-resolved optical gating

We report a practical experimental implementation of time-resolved optical gating (TROG) using a dispersive pulse (DP) propagation geometry and a two-photon absorption nonlinear detector. A measurement of 50 fs pulses from a self-mode-locked Ti:sapphire laser is demonstrated and a general procedure for calibrating the dispersion axis of the TROG trace is presented. A complete iterative pulse retrieval algorithm is described and the limitations of the DP-TROG technique are discussed.

Broad tunability from a compact, low-threshold Cr: LiSAF laser incorporating an improved birefringent filter and multiple-cavity Gires-Tournois interferometer mirrors

We demonstrate prismless tuning of a compact femtosecond Cr:LiSAF laser. The employed technique, which uses a specially designed birefringent filter in combination with dispersion compensation from a pair of multiple-cavity Gires-Tournois interferometer mirrors, provides tuning over 20 nm. We give the results of theoretical modeling of the tuning velocity and the spectral width of the central passband. We show, both experimentally and theoretically, that a single birefringent plate can be used to control the oscillating bandwidth of the laser. The effect this has on the output-pulse duration has also been investigated.

Ultrashort pulse characterization using a scanning Fabry-Perot etalon enabling rapid acquisition and retrieval of a sonogram at rates up to 1.52 Hz

Rapid characterization of ultrashort pulses has been demonstrated by measuring a two-photon absorption sonogram trace. The use of a scanning Fabry–Perot frequency filter has made it possible to acquire and retrieve a sonogram at rates as high as 1.52 Hz. We discuss details of the experimental setup, including the synchronization electronics and the acquisition/retrieval software. Excellent agreement is obtained between retrieved pulse data and independent experimental pulse measurements.

Measurement of ultrashort pulse amplitude and phase by rapidly acquiring and retrieving a sonogram

Summary form only given. Over the past few years the techniques employed to characterise ultrashort pulses have matured greatly. Methodologies such as second harmonic generation frequency-resolved optical gating (SHG-FROG) have been successfully used to determine the amplitude and phase of pulses from mode-locked laser oscillators. SHG-FROG however has two fundamental disadvantages: it is insensitive to the sign of the pulse phase and has a limited measurement bandwidth because it uses a phase-matched SHG crystal. We have developed an alternative approach that avoids these limitations and is based on a measurement of the sonogram of the pulse.

Backward second-harmonic generation from starch for in-situ real-time pulse characterization in multiphoton microscopy

It is well known that the efficiency and selectivity of two-photon excited fluorescence (TPEF) process can depend on various parameters of the ultrashort pulses, such as the pulse intensity and phase, which interact with the specimen. In order to completely understand this dependence and to obtain optimal TPEF images, techniques like Collinear Frequency Resolved Optical Gating (CFROG) arrangement can be implemented in a microscope for complete pulse characterisation at the sample plane. However, this adds complexity that that additional forward collecting optics is required as well as a suitable frequency doubling crystal. Here we report a simple way to characterize the pulses within a multiphoton microscope that do not require forward collecting optics. This is achieved by taking advantage of the fact that backward propagating second harmonic generation (SHG) signal can be easily generated from starch granules. Since both the fluorescence and SHG signals can be collected using the same detection scheme the experimental arrangement is considerably simplified. Starch, being a non- toxic and non-soluble material does not affect living cells allowing the pulse characteristics to be measured in situ, without the need to move the sample. We obtained real-time SHG-autocorrelations traces by using a single starch granule that was placed alongside the living HeLa cells (GFP labeled) being imaged. Furthermore by placing a spectrometer at the output port of the microscope, a spectrally resolved SHG autocorrelation was acquired allowing complete characterisation of the pulse to be carried out. The temporal intensity and phase profile were retrieved using CFROG technique. Marginal analysis was carried out to ensure that the experimental data was successfully acquired.

Nonlinear microscopy pulse optimization at the sample plane using second-harmonic generation from starch

In this paper we report the use of a starch as a non-linear medium for characterising ultrashort pulses. The starch suspension in water is sandwiched between a slide holder and a cover-slip and placed within the sample plane of the nonlinear microscope. This simple arrangement enables direct measurement of the pulse where they interact with the sample.

A vertical extended cavity surface emitting diode laser as an active mirror for controlling mode-locking laser

Controllable mode-locking of a Ti:Sapphire oscillator is demonstrated using an extended vertical cavity surface emitting diode laser as an active-mirror. Modulated modelocking is observed for both reverse and forward pulsed current in the VECSEL.

Tuning of Compact Femtosecond Cr: LiSAF Lasers using a Novel Birefringent Filter Design

Smooth tuning over 30nm is demonstrated by a compact prismless femtosecond Cr:LiSAF laser using broadband negatively dispersive mirrors and a specially designed birefringent filter. 150fs pulses are generated with an electrical-to-optical efficiency exceeding 1%.

Single-shot sonogram: a real-time chirp monitor for ultrafast oscillators

We report a static single-shot configuration for measuring the sonogram trace of an ultrashort optical pulse. When it is combined with a CCD camera, the instrument provides an intuitive video-rate monitor of the pulse chirp, and it is equally compatible with a high-repetition-rate mode-locked oscillator and a low-repetition-rate amplified source. The performance of the instrument is described, and iterative retrieval of the measured sonogram trace, which yields complete and unambiguous pulse characterization, is demonstrated.