Compact, rugged and all solid-state high peak power pulsed laser source is of great interests in many applications such as micro-processing, laser lidar, range finding and communication. To achieve the high peak power, high repetition rate and sub-nanosecond laser pulses, two intracavity modulation techniques have been extensively used. One way is using mode-locked laser in which the axial modes in the laser cavity are forced to be superimposed with a fixed phase. Consequently, the laser output will be forced to form a nearly delta function shaped pulse. The shortest duration of pulse is decided by the gain-bandwidth of the laser gain medium. The pulse-to-pulse period is equivalent to the cavity round-trip time. The other method is using the Q-switching techniques in which the population inversion density in cavity is accumulated by introducing additional loss and then release when the cavity Q value is restored to the usual large value at a suitable time interval. In general, the result of this process is the generation of a single short laser pulse with pulse duration typically several to a few tens of nanosecond long. Moreover, the practical modulation
methods of these two pulse formation techniques can be classified to active and passive modulation. The passive modulation technique is simple and no need of any electrical drivers and heat dissipation devices of the modulator. The primary method for passively Q-switched or mode-locked operation is to place a saturable absorber element inside the cavity to serve as a Q-switcher or phase modulator. The saturable absorber could be any materials with absorption band corresponding to the laser emission and has strong nonlinear saturable absorption properties. Although the organic dye solution is mostly used saturable absorber in the early stage due to its broad gain-bandwidth, the doped crystal and semiconductor saturable absorbers have taken place of it owing to the high toxicity and severely degradation under high power operation.
Mode locked laser is a promising method to produce high repetition rate and ultra-short laser pulse. But the pulse period of mode-locked laser which is equal to cavity round-trip time is too short and, as a result, the large number of pulse per second reduce the single pulse energy and could not produce pulses with high peak power even with high average output power [85,86]. Besides, the complexity and high expense and maintenance of the device configuration hinder the use in practical utilizations. Therefore, passive Q-switching of solid-state lasers which could provide high pulse energies with repetition rates in the kilo-Hz regime and nanosecond pulse width is a good alternation to produce high peak power pulsed lasers. The pulse formation process is depicted in Fig. 1.4-1. Although the pulse stabilities and periods are easy to control for the actively Q-switched lasers, the simple, low cost and monolithic fabrication and no need of high voltage electro- or acoustical-optical drivers and heat dissipation devices of passively Q-switches technique is more promising in applications requiring high peak power laser sources. Nowadays, the
bulk crystal saturable absorbers are extensively used in the passively Q-switched solid-state lasers such as Cr4+:YAG at 0.9-1.2 μm, V3+:YAG and Co2+:MgAl2O4 at 1.3 μm and Er3+:CaF and Co2+:ZnSe at 1.5 μm [87-90]. But the discrete energy level of the doped ion limited the applications of the bulk crystal saturable absorber in the more versatile spectral region. In 1996, the Nd:LSB microchip laser is firstly passively Q-switched by an antiresonant Febry-Perot SA (A-FPSA), or the so called semiconductor saturable absorber mirror (SESAM), by Braun et al. [91] with output pulse width shorter than 200 ps. In comparison to the bulk crystal saturable absorber, the advantage of SESAM is that high density of states makes the short absorption length and, therefore, the shorter cavity length even in several hundreds of microns.
The Q-switching efficiency is also better because of the lower nonsaturable loss due to the shorter action length. The most important is that the absorption band of the SESAM which is decided by the bandgap of the semiconductor materials could be adapted to the gain medium at variant lasing wavelengths. Finally, there is enough design freedom of the saturable absorber parameters such as saturation fluence, modulation depth and recombination lifetime to adjust independently. Typical device structure of A-FPSAs is demonstrated in Fig. 1.4-2. In section 1.1 we have mentioned that the InP-based material system is conventionally used in the 1.3 and 1.5 μm semiconductor lasers. Because the absorption and emission wavelengths are all related to the bandgap of the semiconductors, the InGaAsP/InP and AlGaInAs/InP multiple-quantum-wells are also suitable to be fabricated as SESAMs at spectral region between 1.2-1.6 μm. But the distributed Bragg mirrors (DBRs) of InP-based semiconductor systems suffer from low refractive index contrast, low thermal conductivity or complexity in fabrication. As an alternative, semiconductor saturable absorbers (SESAs) have been developed by using an external mirror to replace the functions of DBRs. So far, numerous passively Q-switched solid-state lasers in use of
Time gain
loss
pulse formation
Fig. 1.4-1 The pulse formation processes of typical passively Q-switched lasers.
Output coupler Doped crystal
gain chip
A-FPSA
Dichroic beam splitter Pump laser beam
Output laser beam
Fig. 1.4-2 Typical device structure of A-FPSA used in the passively Q-switched microchip laser.
Copper heat sink
Top reflectors
High reflection at pump wavelength &
partial reflection at lasing wavelength Substrate
MQW absorber layer DBRs
High reflection at lasing wavelength
SESAs with or without the DBRs have been demonstrated in the near-infrared spectral region [90-99].
Because the minimum pulse durations of the Q-switched lasers are inherently proportional to the resonator length [86,100,101], the microchip actively and passively Q-switched lasers have been developed with cavity length below 1 mm and, as a result, sub-nanosecond pulses output is obtained [86,102,103]. The ultra-short cavity length is beneficial to the single frequency operation in which the axial mode spacing is larger than the gain-banwidth. In the early stage, the Nd3+:YAG bonded to thin piece of Cr4+:YAG saturable absorber [102] and Nd, Cr-codoped YAG crystal [104,105]are applied in the passively Q-switched microchip lasers with output pulse durations of 337 and 290 ps around 1-μm, respectively. However, the limited doped concentration of bulk crystal SAs enlarged the cavity length and the restricted choices of the laser spectral region impede the further development as mentioned above.
Therefore, high peak power and short pulse duration single frequency microchip lasers passively Q-switched by SESAM with divergent wavelength at 1 μm and at the communication region of 1.34 and 1.5 μm have beet demonstrated [90-95]. By tuning reflectivity of the top reflector, the pulse width could be varied from tens of ps to several ns range and the repetition rate from kilo- to mega-Hz with changed modulation depth and saturation intensity. Recently, the passively Q-switched microchip lasers in use of SESAMs with shortest pulse duration of 22 ps [94] and peak power up to 20 kilo-watt [106] have been reported. However, the emission wavelengths of these lasers are mainly determined by the gain mediums which are limited to the discrete energy level of doped ions.
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