One of the primary goals in the development of underwater wireless optical communication (UWOC) is to extend the link length. Meanwhile, the key to increasing the link length of UWOC is to decrease the attenuation of the optical signals underwater. This work demonstrates that a decreased underwater attenuation coefficient and improved communication performance can be achieved by using a pulsed laser. The pulsed blue laser used in the experiment is a self-Q-switched semiconductor disk laser, whose Q-switching is initiated by the nonlinear Kerr effect of the multiple quantum wells in the active region of the gain chip. The width of the laser pulse is about a few nanoseconds and the repetition rate of the output pulse train is in the order of tens of megahertz. According to the measurements of the attenuation coefficients at various concentrations of Maalox solution, the pulsed laser has a significantly smaller attenuation coefficient than the continuous-wave laser, and this drop in attenuation coefficient is particularly noticeable in turbid water with a higher concentration of Maalox solution. The attenuation coefficient of the pulsed laser was only 62% of the continuous-wave laser at a Maalox concentration of 2905 mg·m ⁻³ . Subsequent tests reveal that the lower attenuation coefficient of the pulsed laser is mainly resulted from the reduced scattering it encounters underwater, rather than the ignorable decrease in the attenuation coefficient raised from the water's absorption of the laser. A UWOC system based on the above pulsed laser has also been built and much better communication performance compared to the continuous-waves laser has been obtained. The bit error rate of the UWOC based on the pulsed laser is 1–2 orders of magnitude lower than that of the UWOC with a continuous-wave laser when the data rate is 10 Mb/s and the link length is 18 m.

Advanced light sources in the blue-green band are crucial for underwater wireless optical communication (UWOC) systems. Vertical-external-cavity surface-emitting lasers (VECSELs) can produce high output power and good beam quality, making them suitable for UWOC. This paper presents a 108 m distance UWOC based on a 100 mW 490 nm blue VECSEL and an acousto-optic modulator (AOM). The high-quality beam, which is near diffraction-limited, undergoes relatively small optical attenuation when using a conventional avalanche photodiode (APD) as the detector and employing 64-pulse position modulation (PPM). At the time-slot frequency of 50 MHz, the bit error rate (BER) of the UWOC was 2.7 × 10⁻⁵. This is the first reported AOM-based UWOC system with a transmission distance over 100 m. The estimated maximum transmission distance may be improved to about 180 m by fully utilizing the detection accuracy of the APD according to the measured attenuation coefficient of the blue VECSEL used. This type of UWOC system, composed of a high-beam-quality light source and a conventional detector, make it more closely suited to practical applications.

A typical self-mode-locked vertical-external-cavity surface-emitting laser must operate at the edge of the stable region of the resonant cavity. Its minimal pump spot on the gain chip is used as a soft aperture. By comparing with a continuous-wave (CW) laser, the pulsed laser is focused more tightly on the gain chip due to the Kerr-lens effect. This mitigates cavity loss of the pulsed laser in comparison with the CW laser, such that successive mode-locking can be initiated. The disadvantage of the above method is that the relatively small pump spot, producing relatively large thermal effect, limits the output power of the laser. To address this issue, we propose another method with the work point of the laser moved slightly from the edge of the stable region and the pump spot moderately extended, with a spot of the pulsed laser on the gain chip that could be smaller or larger than that of the CW laser. We achieve stable self-mode-locking with a record average output power of 8.18 W in a V-type resonator, limited by the applied pump power. The pulse repetition rate and width are 0.71 GHz and 1.92 ps, respectively, and the corresponding peak power is 5.6 kW.

Reducing the repetition rate is one of the effective ways to increase the peak-power of the mode-locked pulses. However, for a vertical-external-cavity surface-emitting laser (VECSEL), the carrier lifetime in the nanosecond regime limits the further reduction of the pulse repetition rate, or in other words, limits the average output power of the mode-locked laser at low repetition rates, and ultimately restricts the peak-power of the pulses. This work uses a specially designed saturable Bragg reflector to start the mode-locking, and both low repetition rate and high average power are achieved simultaneously in a passively mode-locked VECSEL. When the repetition rate of the pulses is 78 MHz, a peak power of 12.9 kW, twice the reported highest value, is achieved. The pulse repetition rate is further reduced to the lowest value of 67 MHz, and peak-powers of 2.33, 3.00 and 1.23 kW under the fundamental, second-harmonic, and fourth-harmonic mode-locking are demonstrated, respectively. In addition, a theoretical analysis of the influence of the saturation fluence of the saturable absorber on the evolution of the mode-locked pulses is also provided.

We report a widely tunable optically pumped vertical-external-cavity surface-emitting laser. The multiple quantum wells in the active region of the gain chip are generally designed to form the resonant periodic gain structure, and three different methods are used to tune the oscillating wavelength. The maximum wavelength coverage of 45 nm is obtained when a 2 mm thickness birefringent filter is introduced in a straight-line cavity, while the tuning range of 8 nm is performed by employing a 0.15 mm thickness uncoated Fabry–Perot etalon. For the first time, to the best of our knowledge, we present an 11 nm tuning range by the use of an inserted blade as the tuning element, and the related wavelength tuning mechanism is analyzed.