High power and good beam quality all-solid-state lasers with circularly symmetric single transverse mode are in demand in a variety of applications like nonlinear optics and the optical communications. The diode-pumped, doped- dielectrics bulk solid-state or fiber lasers are developed to be a promising light source in these fields. However, the discrete energy level of the doped ions of these lasers limits the output wavelength coverage. In contrast, the semiconductor lasers enable the potential broad spectral range from ultra-violet to mid-infrared via the bandgap engineering. The traditional edge-emitting semiconductor lasers with high output power suffer from low beam quality which is mainly due to the extraction of light perpendicular to the growth direction. Alternatively, the surface-emitting scheme is applied in the well-known vertical-cavity surface-emitting lasers (VCSELs) to realize the single transverse mode output. The VCSELs are conventionally composed by a quantum-well (QW) active region clamped by two distributed Bragg mirrors (DBRs) with current-injection pumping. To obtain the single mode output beams the current aperture need to be smaller than 10 μm because of the short and flat-flat linear cavity.
As a result, the maximum output power of the VCSELs is restricted to be lower than 15 mW.
In 1993, a new type of surface-emitting semiconductor laser in which one of the DBRs fabrication is replaced by a curved external output mirror is first reported by Hadley et al [58]. The single mode pulsed 100 mW output power of this laser whish is also called the vertical-external-cavity surface-emitting laser (VECSEL) was obtained with an enlarged carrier injection aperture of 100 μm diameter in the InGaAs gain
region. In analogy to the diode-pumped solid-state lasers, the optically-pumped VECSELs, or named as optically-pumped semiconductor lasers (OPSLs), are demonstrated by Kuznetsov et al with further scaled output power greater than 0.5 W under single mode continuous-wave operation [59]. Nowadays, the OPSLs are confirmed to allow high power single mode operation in a wide spectral range via reliable semiconductor epitaxy design and growth and efficient thermal management [60-62].
The gain chips of the OPSLs are fabricated with QW structures grown on the latticed-matched substrate via the metal-organic chemical-vapor-deposition or molecular beam epitaxy. Several epitaxial growth configurations are presented depending on the ways of thermal management [63-65] which will be further discussed in section 3.1. In this section, a typical schematic of the OPSLs is shown in Fig. 1.3-1. The DBRs structure with 25-30 periods is first grown on the substrate with high reflection to the pump and lasing wavelengths to serve as the front mirror. Then the active region consisted of the QWs and the barrier structure is deposited on the top of DBRs. The designation that the QWs are separated by the pump absorption barriers with half lasing wavelength interval is the resonant periodic gain (RPG) structure.
Under this frame the QWs active region is located at the anti-node of the lasing standing waves to enlarge the gain exploitation [66,67]. Finally, a cap layer which is transparent to the pump and lasing wavelengths is integrated on the top of QWs to avoid the surface recombination and the oxidation. Besides, the thickness of the cap layer could be tuned to control the sub-cavity resonance due to the Fabry-Perot interference between the semiconductor-air interface and the DBRs. Then the total assembly, which is called the gain mirror, is mounted on a heat sink with substrate side to activate the heat dissipation. The gain chips are usually pumped by the
commercial laser diode array in pairs with the focusing optics with an angle of 45o to the surface normal [64,68]. But the oblique incidence makes the pump spot to be elliptical shaped. This is unfavorable to obtain single transverse mode output and will decrease the degree of mode matching. Alternatively, the end-pump schemes of OPSLs have been demonstrated in use of the modified DBR or high transmittance substrate under CW and pulsed operation [69,70].
To complete the laser cavity, an external curved mirror is added as the output coupler to ensure the high power single transverse mode output as shown in Fig. 1.3-1.
The radius of curvature and the distance to the gain mirror of the external mirror are assigned to let the laser cavity mode correspond to the pump mode. Because of the short active length, the semiconductor gain mirror is inherently a low gain device in comparison to the doped-crystals in the solid-state lasers. Consequently, the reflectivity of the output mirrors in the OPSLs is typically higher than 97% under CW-operation [64,71] and the RPG structure, as mentioned above, is applied to enhance the MQW gain. As an exception, the output reflectivity could be lowered to be 92-96% or even 70% under quasi-CW and pulsed pumping [72,73]. Using the separated external mirror makes the OPSLs capable of inserting various intra-cavity components such as the nonlinear elements, spectral filters and saturable absorbers.
Although many materials as the direct emitters at visible region like the AlInGaP and GaN based systems are presented, the absence of high power and high photon energy pump source hindered the developments of the OPSLs in this field. Therefore, the second harmonics generation of the mature and efficient NIR OPSLs draws lots of interests in producing visible OPSLs. So far, a variety of the frequency doubled OPSLs with blue, green and yellow-orange light emission [74-78] are realized with fundamental emission of 920-940 nm, 960-1100 nm and 1140-1250 nm, respectively.
Fig. 1.3-1 The typical gain mirror configuration of the OPSLs.
Substrate
.... DBRs
MQWs Cap layer
Cooper heat sink
Coupling lens Output mirror
Gain mirror
Pump beam Laser output
To improve the wavelength conversion efficiency and enlarge the spectrum coverage, single frequency operation [79,80] and wavelength tunable OPSLs [81,82] have also been preceded. The large gain-bandwidth and gain cross-section of the semiconductor QW gain mirror are helpful in producing high-repetition rate and ultra-short passively mode-locked pulsed laser. Several passively mode-locked OPSLs with semiconductor saturable absorber mirrors (SESAMs) have demonstrated and supply giga-Hertz repetition rate and pico- to femto-seconds output pulses [83,84]. With these additional intra-cavity elements the 3- or 4- mirror SDL cavity in contrast to the traditional linear cavity as shown in Fig. 1.3-1 are applied [61] to provide intensive and focusing beam spots on these components.