We consider an indoor multi-input single-output (MISO) visible light communication (VLC) system without channel state information at the transmitter. For such a system, an energy-efficient time-collaborative modulation (TCM) constellation is first designed by minimizing a total optical power subject to a fixed minimum Euclidean distance. Then, a new space–time transmission scheme is proposed. Comprehensive computer simulations indicate that our proposed design always has better average error performance within illumination coverage area than the currently available schemes for this application.

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.

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.

An advanced light source at blue-green waveband is the key to improve the link-distance and communication performance of an underwater wireless optical communication (UWOC) system. This work presents the underwater channel characteristics and UWOC system using a blue semiconductor disk laser that passively mode-locked by a semiconductor saturable absorb mirror for the first time. The attenuation coefficients of the continuous-wave and mode-locked lasers under different concentrations of Maalox solution are studied, and the results indicate that the attenuation coefficients of the mode-locked laser are obviously lower than that of the continuous-wave laser. Then an UWOC system based on an acousto-optic modulator are constructed, and the water tank communication experiments are carried out by using quasi-cyclic low-density-parity-check coding and 64-pulse-position-modulation. Experimental results show that the bit error rates of the mode-locked laser are significantly lower than that of continuous-wave laser under the same Maalox solution concentration and same signal-noise-ratio. These results provide an attractive light source for practical long-distance UWOC.