Three-electrode germanium-on-silicon avalanche photodiode array.

In this Letter, we report a bridge-connected three-electrode germanium-on-silicon (Ge-on-Si) avalanche photodiode (APD) array compatible with the complementary metal-oxide semiconductor (CMOS) process. In addition to the two electrodes on the Si substrate, a third electrode is designed for Ge. A single three-electrode APD was tested and analyzed. By applying a positive voltage on the Ge electrode, the dark current of the device can be reduced, and yet the response of the device can be increased. Under a dark current of 100 nA, as the voltage on Ge increases from 0 V to 15 V, the light responsivity is increased from 0.6 A/W to 1.17 A/W. We report, for the first time to the best of our knowledge, the near-infrared imaging properties of an array of three-electrode Ge-on-Si APDs. Experiments show that the device can be used for LiDAR imaging and low-light detection.

Two-dimensional multi-layered SiN-on-SOI optical phased array with wide-scanning and long-distance ranging.

Silicon based optoelectronic integrated optical phased array is attractive owing to large-dense integration, large scanning range and CMOS compatibility. In this paper, we design and fabricate a SiN-on-SOI two-dimensional optical phased array chip. We demonstrate a two-dimensional scanning range of 96°×14.4° and 690 mW peak power of the main lobe. Additionally, we set up the time of flight (ToF) and frequency-modulated continuous-wave (FMCW) ranging systems by using this optical phased array chip, and achieve the objects detection at the range of 20 m in the ToF system and 109 m in the FMCW system, respectively.

60.4 W continuous-wave single-frequency laser output from a two-stage Nd:YAG single-crystal fiber amplifier.

A Nd:YAG single-crystal fiber amplifier for the amplification of continuous-wave single-frequency laser end-pumped by a laser diode (LD) is investigated. With a two-stage amplification configuration, an output power of 60.4 W under the total incident pump power of 200 W is achieved, which is, to our knowledge, the highest power from a continuous-wave single-frequency laser achieved with a single-crystal fiber scheme. The extraction efficiency reaches 41.6% in the second amplification stage, which is comparable with Innoslab amplifiers. The beam quality factors M2 at the maximum output power in the horizontal and vertical direction are measured to be 1.51 and 1.38, respectively. The long-term power instability for 1 hour is 0.97%.

Investigation and demonstration of a high-power handling and large-range steering optical phased array chip.

The optical power handling of an OPA scanning beam determines its targeted detection distance. So far, a limited number of investigations have been conducted on the restriction of the beam power. To the best of our knowledge, we for the first time in this paper explore the ability of the silicon photonics based OPA circuit for the high power application. A 64-channel SiN-Si based one-dimensional (1D) OPA chip has been designed to handle high beam power to achieve large scanning range. The chip was fabricated on the standard silicon photonics platform. The main lobe power of our chip can reach 720 mW and its peak side-lobe level (PSLL) is -10.33 dB. We obtain a wide scanning range of 110° in the horizontal direction at 1550 nm wavelength, with a compressed longitudinal divergence angle of each scanning beam of 0.02°.

Ultrafast fiber laser based on HfSe2 saturable absorber.

We demonstrate the HfSe2 saturable absorber (SA) for the generation of ultrafast pulse laser. The HfSe2 SA device is fabricated by integrating HfSe2 nanosheets (NSs) with a microfiber. The material and optical characteristics of HfSe2 NSs show their high quality. The nonlinear optical absorption of HfSe2 SA is measured with a modulation depth of 5.8%. Stable soliton mode-locked laser based on HfSe2 SA is realized at the central wavelength of 1561.43 nm with pulse duration of 297 fs and the maximum pulse energy of 2.68 nJ. Our soliton fiber laser has a maximum output power of 48.5 mW with a high slope efficiency of 12.8%, which indicate that HfSe2 is a good candidate of SA for high efficient ultrashort pulses generation.

Watt-level passively mode-locked Tm:YLF laser at 1.83 µm.

Passive, continuous-wave mode-locked (CWML) operation of a 1.83 µm Tm:YLF laser is experimentally demonstrated for the first time, to the best of our knowledge. Two specially selected output couplers are used to realize this operation. Stability of the CWML laser is obtained with a commercial semiconductor saturable absorber mirror. The maximum average output power is 1.04 W with a pulse duration of 107 ps and repetition rate of 54.1 MHz. Further, a 0.1 mm fused-quartz Fabry-Perot etalon is used to tune the central wavelength of the stable CWML laser at 1827.2 nm, 1829.5 nm, 1831.9 nm, and 1833.5 nm.

High-power and high-efficiency short wavelength operation of a Tm:YLF laser at 1.83 µm.

A high-power and high-efficiency diode-end-pumped Tm:YLF laser at 1.83 µm is demonstrated for the first time, to our best knowledge. To make the laser operate at 1.83 µm, a simple way of controlling the transmittance of the output coupler is used, and the criteria of the transmittance of the output coupler at the emission peaks of Tm:YLF are theoretically analyzed, which are verified by experimental results. Based on the theoretical analysis, laser oscillation at only 1.83 µm is realized. Maximum output power at 1833 nm is 8.5 W with corresponding slope efficiency of 53.4% regarding absorbed pump power. In addition, tunability of this laser from 1827 nm to 1837 nm is obtained. Laser beam quality at 1833 nm is measured to be 1.4 at maximum output power. The achieved laser performance represents considerable improvement compared to any other bulk laser emitting around 1.83 µm.

High-repetition-rate single-frequency Ho:YAG MOPA system.

A stable, 2.09 µm single-frequency Ho:YAG master oscillator and power amplifier (MOPA) system at a pulse repetition frequency of 1.25 kHz is demonstrated. The maximum output energy of the single-frequency injection-seeded laser is 13.76 mJ, and the corresponding pulse width is 178.9 ns. The seed laser is a continuous-wave Ho:YAG non-planar ring oscillator laser. The half-width of the pulse spectrum is measured to be 2.65 MHz by using the heterodyne technique, which is about 1.06 times Fourier transform limited.

1 kHz single-frequency 2.09 µm Ho:YAG ring laser.

In this paper, we report the experimental realization of a high-repetition-rate single-frequency Ho-doped yttrium aluminum garnet (Ho:YAG) ring laser at 2.09 µm. Single-frequency operation of the ring laser is achieved by injection-seeding with a continuous wave (CW) Ho:YAG non-planar ring oscillator (NPRO) laser. The output energy of the ring laser is 6.24 mJ with a pulse width of 172 ns and a repetition rate of 1 kHz. The beam quality M2-factor is measured to be ∼1.3 at the maximum output energy. The half-width of the pulse spectrum is measured to be 2.61 MHz by a heterodyne technique.

High-energy, stable single-frequency Ho:YAG ceramic amplifier system.

We demonstrate a 2090 nm single-frequency, high-energy, Q-switched Ho:YAG ceramic master oscillator and power amplifier system that contains two-stage amplifiers. The maximum single-frequency pulse energy is 55.64 mJ at a pulse repetition frequency of 200 Hz. The half-width of the pulse spectrum is measured to be 3.96 MHz by a heterodyne technique. To the best of our knowledge, 55.64 mJ is the highest single-frequency output energy obtained from the Ho:YAG laser.

44 mJ, 2.1 µm single-frequency Ho:YAG amplifier.

A single-frequency two-stage amplifier delivering up to 44 mJ at 2.1 µm is demonstrated. The first-stage amplifier is double-end-pumped by a 1.9 µm Tm:YLF laser, and is seeded with a 15 mJ single-frequency Ho:YAG laser operating at 200 Hz. The second-stage amplifier is end-pumped by another 1.9 µm Tm:YLF laser. The output pulse width from the second-stage amplifier is 113 ns, with a beam quality of M2∼1.32.

Single-frequency, injection-seeded Q-switched Ho:YAG ceramic laser pumped by a 1.91µm fiber-coupled LD.

A 2090 nm injection-seeded Q-switched Ho:YAG ceramic laser pumped by a 1.91 µm fiber-coupled LD is demonstrated in this paper. Single-frequency operation of Q-switched Ho:YAG ceramic laser is achieved by injection seeding technique. The maximum output energy of the single-frequency Q-switched Ho:YAG ceramic laser is 14.76 mJ, with a pulse width of 121.6 ns and a repetition rate of 200 Hz. The half-width of the pulse spectrum measured by heterodyne technique is 3.84 MHz. The fluctuation of the center frequency of the pulsed laser is 1.49 MHz (RMS) in 1 hour. As far as we know, this is the first time to report a single-frequency, injection-seeded Ho:YAG ceramic pulsed laser.

34 mJ Ho:YAG ceramic master oscillator and power amplifier laser at 2097 nm.

We demonstrated a 2097 nm Ho:YAG ceramic master oscillator and power amplifier system pumped by Tm:YLF lasers. Laser characteristics of 0.6 at.% and 0.4 at.% Ho:YAG ceramics were studied in CW and Q-switched modes. An output energy of 20.6 mJ at a pulse repetition frequency of 200 Hz was achieved from the master oscillator. The pulse energy was amplified to 34 mJ by the Ho:YAG ceramic amplifier with a pulse duration of 54 ns.

Measuring OAM states of light beams with gradually-changing-period gratings.

A method to measure orbital angular momentum (OAM) states of light beams by using gradually-changing-period gratings is reported. Two kinds of gradually-changing-period gratings were used for the measurement of OAM states. The OAM states of the incident beam can be measured easily from the Hermite-Gaussian-like diffraction patterns. The simulation results agree well with the experiments.

Generation of optical vortices by using spiral phase plates made of polarization dependent devices.

A method to construct spiral phase plates (SPPs) from quarter-wave plates and spatially variable half-wave plates (SVHWPs) is proposed. Optical vortices were experimentally generated by using SPPs made of polarization-dependent devices. The topological charge of the optical vortex can be also determined by the structure of the SVHWP and the polarization of the incident beam.

On-chip generation of Bessel-Gaussian beam via concentrically distributed grating arrays for long-range sensing.

Bessel beam featured with self-healing is essential to the optical sensing applications in the obstacle scattering environment. Integrated on-chip generation of the Bessel beam outperforms the conventional structure by small size, robustness, and alignment-free scheme. However, the maximum propagation distance (Zmax) provided by the existing approaches cannot support long-range sensing, and thus, it restricts its potential applications. In this work, we propose an integrated silicon photonic chip with unique structures featured with concentrically distributed grating arrays to generate the Bessel-Gaussian beam with a long propagation distance. The spot with the Bessel function profile is measured at 10.24 m without optical lenses, and the photonic chip's operation wavelength can be continuously performed from 1500 to 1630 nm. To demonstrate the functionality of the generated Bessel-Gaussian beam, we also experimentally measure the rotation speeds of a spinning object via the rotational Doppler Effect and the distance through the phase laser ranging principle. The maximum error of the rotation speed in this experiment is measured to be 0.05%, indicating the minimum error in the current reports. By the compact size, low cost, and mass production potential of the integrated process, our approach is promising to readily enable the Bessel-Gaussian beam in widespread optical communication and micro-manipulation applications.