Mode-lock oscillator

The OPA should not be confiised with an optical parametric oscillator (OPO), a resonant-cavity parametric device that is syncln-onously pumped by a femtosecond, mode-locked oscillator. 14 fs pulses, tunable over much of the visible regime, have been obtained by Hache and co-workers [49, with a BBO OPO pumped by a self-mode-locked Ti-sapphire oscillator. 

Bade P, Bourvier M and Coe J S 1987 Nd YLF mode-locked oscillator and regenerative amplifier Opt. Lett. 12 319-21... 

In 1989 A. Wajid invented the mode-lock oscillator. He presumed that a connection existed between the fundamental wave and one of the anharmonics, similar to that ascertained by Benes between the fundamental oscillation and the third quasi-harmonic oscillation. The frequencies of the fundamental and the enharmonic oscillations are very similar and they solve the problem of the capacity of long cables. He found the necessary considerations for establishing this connection in works by Wilson (1954) as well as Tiersten and Smythe (1979). 

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

Figure B2.1.3 Output of a self-mode-locked titanium-sapphire oscillator (a) non-collinear intensity autocorrelation signal, obtained with a 100 pm p-barium borate nonlinear crystal (b) intensity spectrum.

Fig. 4. Temporal pulse characteristics of lasers (a) millisecond laser pulse (b) relaxation oscillations (c) Q-switched pulse (d) mode-locked train of pulses, where Fis the distance between mirrors and i is the velocity of light for L = 37.5 cm, 2L j c = 2.5 ns (e) ultrafast (femtosecond or picosecond) pulse.

M. Ebrahimzadeh, G. J. Hall and A. I. Ferguson, Singly resonant, all-solid-state, mode-locked LiBaOs optical parametric oscillator tunable from 652nm to 2650nm, Optics Lett. 17, 652-654 (1992). 

As early as 1982, a diode laser-pumped miniature NdtYAG laser with a linewidlh of less lhan 10 kHz. was demonstrated. The research in this area continued apace at Stanford University and by a numher of commercial electronics limis. w ith emphasis placed on ihe development of three-level lasers. Q-switched and mode-locked operation, single-frequency operation (monolithic nonplanar ring oscillator), visible radiation by harmonic generation, and array-pumped solid-slate lasers. See Fig. ft. 

Previously we have shown that the repetition rate of a mode locked laser equals the mode spacing to within the experimental uncertainty of a few parts in 1016 [26] by comparing it with a second frequency comb generated by an efficient electro-optic modulator [41]. Furthermore the uniform spacing of the modes was verified [26] even after further spectral broadening in a standard single mode fiber on the level of a few parts in 1018 [13]. To check the integrity of the femtosecond approach we compared the / 2/ interval frequency chain as sketched in Fig. 3 with the more complex version of Fig.4 [19]. We used the 848 nm laser diode of Fig. 4 and a second 848 nm laser diode locked to the frequency comb of the / 2/ chain. The frequencies of these two laser diodes measured relative to a quartz oscillator, that was used as a radio frequency reference for the frequency combs, are 353 504 624 750 000 Hz and 353 504 494 400 000 Hz for the / 2/ and the 3.5/ 4/ chain respectively. We expect a beat note between the two 848 nm laser diodes of 130.35 MHz which was measured with a radio frequency... 

Figure S> Schemaiic layout of an ultrafasl puaip-probe spectrometer. Top left panel A cw diode-pumped, Frequency-doubled, Nd YAO laser B mode-locked Ti-S oscillator C regenerative Ti-S amplilier D Q-switched, frequency-doubled Nd YLF amplifier pump laser E second/third-iiarmonic generators or OPA. Lower right panel F continuum generation G sample H CCD spectrograph. The double arrow indicates the optical delay stage, and the dashed tine indicates the pump-beam trajectory.

Transient spectroscopy experiments were performed with a pump-probe spectrometer [7] based on a home-made original femtosecond Ti saphire pulsed oscillator and a regenerative amplifier system operated at 10 Hz repetition rate. The Tirsaphire master oscillator was synchronously pumped with doubled output of feedback controlled mode-locked picosecond pulsed Nd YAG laser. The pulse width and energy of Ti saphire system after the amplifier were ca. 150 fs and 0.5 mJ, respectively, tunable over the spectral range of 760-820 nm. The fundamental output of the Ti saphire system (790 nm output wavelength was set for present study) splitted into two beams in the ratio 1 4. The more intense beam passed through a controlled delay line and was utilized for sample... 

The apparatus employs a passively mode locked Nd/YAG laser oscillator, a Pockel cell pulse extractor, and Nd/YAG laser amplifier to produce laser pulses at 1064 nm. Non-linear crystals convert 30% of this light to 355 nm, which is used for excitation of the sample. The optical path length of the 355 nm light is varied by a computer-controlled time delay stage. 

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