Optical coherence tomography (OCT) has developed rapidly since its first realisation in medicine and is currently an emerging technology in the diagnosis of skin disease. OCT is an interferometric technique that detects reflected and backscattered light from tissue and is often described as the optical analogue to ultrasound. The inherent safety of the technology allows for in vivo use of OCT in patients. The main strength of OCT is the depth resolution. In dermatology, most OCT research has turned on non-melanoma skin cancer (NMSC) and non-invasive monitoring of morphological changes in a number of skin diseases based on pattern recognition, and studies have found good agreement between OCT images and histopathological architecture. OCT has shown high accuracy in distinguishing lesions from normal skin, which is of great importance in identifying tumour borders or residual neoplastic tissue after therapy. The OCT images provide an advantageous combination of resolution and penetration depth, but specific studies of diagnostic sensitivity and specificity in dermatology are sparse. In order to improve OCT image quality and expand the potential of OCT, technical developments are necessary. It is suggested that the technology will be of particular interest to the routine follow-up of patients undergoing non-invasive therapy of malignant or premalignant keratinocyte tumours. It is speculated that the continued technological development can propel the method to a greater level of dermatological use.

https://link.springer.com/content/pdf/bfm%3A978-3-319-06419-2%2F1.pdf

Optical Coherence Tomography

The theory of the diffraction grating pair is developed by expanding the frequency dependence of the phase shift as far as the quadratic frequency term. The analogy between pulse compression and Fresnel diffraction is treated. The effect of the cubic phase term is discussed for ultrashort pulses having appreciable fractional bandwidth.

Optical pulse compression with diffraction gratings

A new type of wavelength-modulation laser spectroscopy is accomplished by utilizing an external phase modulator driven at radio frequencies large compared to the width of the spectral feature of interest. The spectral feature is probed by a single frequency-modulated (FM) sideband, and the associated absorption and dispersion are measured by monitoring the resulting radio-frequency beat signal. Experimental results are presented for the measurement of Fabry–Perot resonances, I2 vapor absorption lines, and saturation holes in Na vapor.

Frequency-modulation spectroscopy: a new method for measuring weak absorptions and dispersions.

With well-defined coherent light pulses of several ${10}^{\ensuremath{-}12}$ sec duration we are in a position to investigate a variety of ultrafast vibrational processes in liquids and solids. Several new experimental techniques have been devised to study directly the dynamics of different vibrational modes and molecules in the electronic ground state. A first light pulse excites the vibrational system via stimulated Raman scattering or by resonant infrared absorption. A second interrogating pulse allows one to determine the instantaneous state of the excited system. Using a coherent probing technique one can measure the dephasing time of homogeneously broadened vibrational transitions and a collective beating of multiple isotope levels. In addition, one can investigate inhomogeneously broadened vibrational modes and observe the dephasing time of a small molecular subgroup. Different information is obtained when the coherent anti-Stokes Raman scattering of the probe pulse is measured. The population lifetime of known vibrational modes can be investigated and evidence for inter- and intra-molecular interactions is obtained. In a third probing technique, the vibrationally excited jolecules are promoted to the first electronic state by a second pulse and the fluorescence is measured. In this way it is possible to see the very rapid change of population of the primary excited vibrational mode. The article gives a detailed theoretical treatment of different excitation and probing processes. Several experimental techniques successfully applied in the authors investigations are outlined and a variety of results is presented and discussed. New information, not available from other experimental methods, is obtained.

Vibrational dynamics of liquids and solids investigated by picosecond light pulses

Curious wave phenomena that occur in optical fibres due to the interplay of instability and nonlinear effects are reviewed.

/pdf/instabilities-breathers-and-rogue-waves-in-optics-252ilwzt4i.pdf

Instabilities, breathers and rogue waves in optics

A simple technique is described for gating light on and off on the picosecond time scale. The gate is built in much the same fashion as traditional Kerr cells, the difference lying in the use of powerful optical pulses rather than electrical pulses to induce a birefringence in various liquids. The direct observation of the exponential decay (τ = 32 ± 6 psec) of the birefringence induced in nitrobenzene shows that orientational effects are largely responsible for the refractive index changes induced by light in this liquid.

An ultrafast light gate

Three new results have been obtained with a recently developed camera of 10-psec framing time: (1) The effect of the finite speed of light in photographing relativistic objects is experimentally demonstrated, by photographing a dumbbell-like entity formed by two packets of light. In contrast to material objects, which, theory predicts, should appear rotated, the light dumbbell appears sheared. (2) Photographs of the mode-locked Nd: glass laser radiation show numerous subsidiary pulses accompanying the main ultrashort pulses in the train. The latter have durations ranging from 7 psec to 15 psec. (3) The technique of gated picture ranging, previously used with nanosecond pulses, is extended to the picosecond range where a resolution of 1 cm is demonstrated. Some potentially useful applications are proposed.

Ultrahigh speed photography of picosecond light pulses and echoes.

A scheme, similar to one used in the chirp radar, is proposed to compress in time, without loss of energy, the light pulse generated by a mode-locked laser. On the basis of preliminary experimental results obtained with a mode-locked He-Ne laser and a LiNbO 3 phase modulator, it is predicted that compression of 50-ps pulses derived from a Nd:YAG laser to 0.4 ps or less is feasible.

Compression of optical pulses

The Nd:glass laser has become one of the most useful sources of light pulses a few picoseconds in duration. In this paper, we review the results of an extensive study of the time and spectral structure of the Nd:glass laser radiation. The time structure was studied by means of two-photon fluorescence (TPF) patterns: these were scanned by a very thin (28-μ) cell containing the fluorescent dye, the fluorescence being monitored by a photomultiplier. When the Nd:glass laser is Q switched by a rotating mirror or when it is free running, we find TPF patterns fully consistent with a model where the laser emission has the character of Gaussian noise (thermal light), i.e., a model where the modes are randomly phased. When the laser was simultaneously Q switched and mode locked we made two observations : 1) the TPF patterns show that the ultra-short pulses observed previously have an overall duration of ∼8 ps, but also possess an internal substructure containing peaks 0.4-0.8 ps in duration; 2) the spectral width of these pulses is Q -switched train and expands to ∼80 cm-1in the middle of the train. This rapid spectral broadening during pulse buildup is attributed to self-phase modulation in the laser glass matrix due to a nonlinear index n 2 which we evaluate as n{2}= (2 \pm 1) \times 10^{-22} m2/V2(or 1.8 \times 10^{-13} esu). Gain limiting due to this effect and self-focusing become very important at power densities above 1 GW/cm2, presenting a serious limitation on the energy density (J/cm2), which one may hope to extract from Nd: glass laser systems.

Study of the Nd: Glass laser radiation

Pulses of 500‐psec duration, emitted by a mode‐locked He–Ne laser, have been electro‐optically frequency swept and then compressed to 270 psec by multiple reflections from an interferometer proposed by Gires and Tournois.

M. A. Duguay

Papers

An ultrafast light gate

Ultrahigh speed photography of picosecond light pulses and echoes.

Compression of optical pulses

Study of the Nd: Glass laser radiation

Compression of Pulses from a Mode-Locked He-Ne Laser