Laser modes

In 1991 a remarkable discovery was made, accidentally, with a Tp -sapphire laser pumped with an Ar+ laser. Whereas we would expect this to result in CW laser action, when a sharp jolt was given to the table supporting the laser, mode locking (Section 9.1.5) occurred. This is known as self-locking of modes, and we shall not discuss further the reasons for this and how it can be controlled. One very important property of the resulting pulses is that they are very short. Pulse widths of a few tens of femtoseconds can be produced routinely and with high pulse-to-pulse stability. Further modification to the laser can... 

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

Dye-Sensitized Photoisomerization. One technological appHcation of photoisomerization is in the synthesis of vitamin A. In a mixture of vitamin A acetate (all-trans stmcture) and the 11-cis isomer (23), sensitized photoisomerization of the 11-cis to the all-trans molecule occurs using zinc tetraphenylporphyrin, chlorophyU, hematoporphyrin, rose bengal, or erythrosin as sensitizers (73). Another photoisomerization is reported to be responsible for dye laser mode-locking (74). In this example, one metastable isomer of an oxadicarbocyanine dye was formed during flashlamp excitation, and it was the isomer that exhibited mode-locking characteristics. 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

The WGM laser mode structure is determined by the droplet size and shape (which can be influenced by optical trapping forces26), laser polarization, and dye concentration. ... 

Argon ion lasers, mode-locked to produce pulses in the picosecond domain, are in widespread use, producing power levels generally higher than diode lasers up to ca. 

In the case of a common lower level, the second absorption transition would show this narrowing effect when probed with a tunable monochromatic laser line. This example can be realized if atoms or molecules in a magnetic field are pumped by a laser, oscillating simultaneously on two cavity modes 324). if the Zeeman splitting of the probe equals the mode spacing of the laser, both transitions are pumped simultaneously and each laser mode selectively eats... 

With an appropriate (quadratic) field profile E(r), where r is the radial displacement, it might be exploited to focus an electron beam. Quite sufficient for this purpose would be, for example, the transverse laser mode TEMoi,... 

Fig. 22 Damping of two QCL laser modes at 997.8 and 999.09 cnr1 with increasing concentration of ethylene a 29 918 ppm ethylene, pure He...

Up to the scaling factors An this sum represents a periodic spectrum in frequency space. If the spectral width of the carrier wave Au>c is much smaller than the mode separation ojr, Eqn. 7 represents a regularly spaced comb of laser modes with identical spectral line shapes, namely the line shape of C(u>) (see Fig. 1). If C(oS) is centered at say uic then the comb is shifted from containing only exact harmonics of u)r by uic. The center frequencies of the mode members are calculated from the mode number n [23,24,21] ... 

The power outputs of the lasers will be actively stabilized and matched using a system of thermopile power meters (PM) and acousto-optic modulators (AOM s). Spatial filters (SF) will be used to ensure a well defined laser mode at the interaction region. A system of beam scanners will be used to accurately characterize the laser and ion beams at the interaction region and to measure the intersection angles 61. ... 

Time-correlated single photon counting A technique for the measurement of the time histogram of a sequence of photons with respect to a periodic event, e.g. a flash from a repetitive nanosecond lamp or a CW operated laser mode-locked laser). The essential part is a time-to-amplitude-converter (TAG) which transforms the arrival time between a start and a stop pulse into a voltage. Sometimes called single photon timing. 

The frequency-domain signal is measured in this case at the discrete frequencies at which laser mode beats occur. The time domain representation of these beat frequencies is... 

There are limits on this method which are imposed by the la e pacing of laser mode-noise peaks (130 MKz), setting an upper decay time limit of 7 ns, by detector noise background setting a lower Ifanit of 200 ps, of cost (of a spectrum analyser) and of difficulties in analysis of multi-component fluorescence. Since however any source could in principle be used to measure a noise spectmm, the method is worth further consideration. 

Figure 10.2. Spectra of benzene obtained one hour apart from a spectrometer with an unstable diode laser. Raman shift axis was not recalibrated between A and B, causing a 13 cm shift due to a laser mode hop. Both spectra and their difference use the same intensity scale.

We consider here for simplicity one chirped laser mode. Extension to multimode process is direct. The slow parameters of characteristic time r are the laser pulse envelope A(f) and frequency go(f). The time-dependent phase can be written as... 

This method has recently been utilized by the Konstanz quantum optics group, who has prepared the signal photon in a well-defined, highly pure spatiotemporal mode which can be matched with, coupled into, or caused to interfere with, a classical laser mode. Based on this achievement, we have demonstrated homodyne tomography of the single-photon Fock state [Lvovsky 2001]... 

The experimental setup rests on a marble table, insulated from the mechanical vibrations of the floor. The laser, the Plan Fabry-Perot (PFP), the sample and auxiliary optics are in the same table that insures the absence of relative movements between them. The source is a 2020 Spectra Physics laser working in monomode. The room temperature is controlled better than 0.1°C which insures good laser mode stability. The FP is insulated inside a plastic structure and covered by a black cloth that insures good protection from stray light and temperature fluctuations. The sample temperature is controlled by a... 

N photons present in the driving laser mode. When we include the interaction Vl the triplets recombine into new triplets with eigenvectors (dressed states)... 

In short note [23], Askar yan has pointed out two possible effects of oscillating surfaces on the electromagnetic field inside the (laser) resonator cavities. The first effect is the influence of oscillations on the generation and intensity of the laser radiation. It was extensively studied in many experiments, devoted, in particular, to such problems as the generation of optical pulses [24,25], phase locking of laser modes (where the frequencies of the mirror oscillations varied from 50 Hz [26] to 500 kHz [27,28] and 1 MHz [29,30] (see the review in Ref. 31), or modulation of the laser radiation [32,33] (in Ref. 33 the frequencies varied from 17 to 70 kHz). The theory of these phenomena was considered, for example in Refs. 34—36. 

We assume strong coherent pumping of laser modes and replace the operators aLj with corresponding complex amplitudes [Pg.558]

---