Low-Energy Ion Implantation and Deep-Mesa Si-Avalanche Photodiodes with Improved Fabrication Process.

Since the avalanche phenomenon was first found in bulk materials, avalanche photodiodes (APDs) have been exclusively investigated. Among the many devices that have been developed, silicon APDs stand out because of their low cost, performance stability, and compatibility with CMOS. However, the increasing industrial needs pose challenges for the fabrication cycle time and fabrication cost. In this work, we proposed an improved fabrication process for ultra-deep mesa-structured silicon APDs for photodetection in the visible and near-infrared wavelengths with improved performance and reduced costs. The improved process reduced the complexity through significantly reduced photolithography steps, e.g., half of the steps of the existing process. Additionally, single ion implantation was performed under low energy (lower than 30 keV) to further reduce the fabrication costs. Based on the improved ultra-concise process, a deep-mesa silicon APD with a 140 V breakdown voltage was obtained. The device exhibited a low capacitance of 500 fF, the measured rise time was 2.7 ns, and the reverse bias voltage was 55 V. Moreover, a high responsivity of 103 A/W@870 nm at 120 V was achieved, as well as a low dark current of 1 nA at punch-through voltage and a maximum gain exceeding 1000.

Electrically driven supersymmetric semiconductor laser arrays with single-lobe far-field patterns.

Semiconductor laser arrays based on the third-order supersymmetric (SUSY) transformation are proposed to increase the mode discrimination between fundamental supermode and high-order supermodes. The distance between the edge waveguide of the main array and that of the superpartners is optimized. Then, the electric field distributions of different modes are also calculated, which show that, except for the fundamental supermode, the high-order supermodes penetrate deeper into the superpartner arrays, which accounts for the increased loss of high-order supermodes. The fabricated third-order SUSY laser array can emit light with a single-lobe far-field pattern under an injection current of 70 mA, which is a promising candidate for optical couplings between lasers and optical elements.

Optimized performance of 905†nm semiconductor lasers by using the high strain quantum well.

We propose and experimentally demonstrate that the lasing power and characteristic temperature (T0) of 905 nm semiconductor lasers can be optimized by use of the high strain quantum well (HSQW). To fix the lasing wavelength around 905 nm, HSQW with a higher ndium (In) content of the InGaAs gain material than that of the commonly used low strain quantum well (LSQW) requires a thickness-reduced quantum well. Thus, the HSQW has the following two advantages: stronger quantum size effects caused by the deep and thin quantum well, and higher compressive strain caused by a high In content of the InGaAs gain material. With the similar epitaxial structure, laser diodes with HSQW have a characteristic temperature T0 of 207 K and can deliver a higher lasing power with less power saturations. The high strain quantum well optimization method can be extended to other laser diodes with a wavelength near 900 nm with low In content InGaAs quantum wells and other similar low-strain gain material systems.

Doping profile architecture towards lower loss and higher efficiency for laser diodes.

A doping optimization model towards lower loss and higher efficiency at the target operating current is investigated. This model considers the effect of doping concentration on the series resistance and the internal loss. 780 nm lasers doped with a normal doping profile (Dop_normal) and an optimized doping profile (Dop_optimize) are both designed and fabricated. After doping optimization, the power loss decreased by 17%, the output power of the lasers increased by 26% and the electro-optical conversion efficiency increased by 22%. The model provides significant theoretical guidance for the optimization of the laser doping.

High-efficiency spectral beam combining of an individual laser diode bar with polarization multiplexing.

In this Letter, a new strategy for the spectral beam combining (SBC) of an individual laser diode (LD) bar based on a polarization multiplexing external cavity is proposed and demonstrated. The maximum combining efficiency is up to 95.51%, which leads to an output power of 76.6 W and an electro-optic conversion efficiency of 48.33% under continuous wave operation at a current of 100 A. Compared to the conventional SBC, the combining efficiency, the output power, and the electro-optical conversion efficiency present improvements of 12%, 10W, and 6%, respectively. The results show that this novel SBC method is a prospective technique for increasing the combining efficiency of LD bars.

Dynamics of high-contrast grating vertical-cavity surface-emitting laser with lateral optical feedback by a heterostructure interface.

We investigate the dynamics of high-contrast grating vertical-cavity surface-emitting laser (HCG-VCSEL) with a lateral optical feedback cavity. The lateral optical feedback is realized by the reflection at the heterostructure interface between two different HCGs. The lateral optical feedback cavity possesses slow light which can be tuned by changing HCG parameters, and can control the dynamics of the HCG-VCSEL. The optical feedback can enhance the -3-dB bandwidth and enlarge the eye openings of diagrams of the HCG-VCSEL, and can also reduce the frequency chirp. The HCG-VCSEL with a lateral optical feedback cavity can achieve a -3-dB bandwidth of 37.7 GHz at 12 mA and eye diagrams at 60 Gbps (non-return to zero format) and 50 GBaud (4-level pulse amplitude modulation format) with sufficient openings.

Different phases in non-Hermitian topological semiconductor stripe laser arrays.

As a novel branch of topology, non-Hermitian topological systems have been extensively studied in theory and experiments recently. Topological parity-time (PT)-symmetric semiconductor stripe laser arrays based on the Su-Schreiffer-Heeger model are proposed. The degree of non-Hermicity can be tuned by altering the length of the cavities, and PT symmetry can be realized by patterned electrode. Three laser arrays working in different non-Hermitian phases are analyzed and fabricated. With the increasing degree of non-Hermicity, the peaks of output intensities move from the edge to the bulk. The proposed semiconductor stripe laser array can function as an active, flexible, and feasible platform to investigate and explore non-Hermitian topology for further developments in this field.

High-power laser diode at 9xx nm with 81.10% efficiency.

In this study, a low-resistance, low-loss, continuously gradual composition extreme double asymmetric (CGC-EDAS) epitaxial structure is designed to improve efficiency. The structure and facet reflectivity of the broad area (BA) lasers are optimized to maximize the power conversion efficiency (PCE). In the experiment, the peak PCE of 75.36% is measured at 25°C. At 0°C, a peak PCE of 81.10% is measured and the PCE can still reach 77.84% at an output power of 17.10 W, which, to the best of our knowledge, is the highest value to date for any BA lasers.

Single-chip hybrid integrated silicon photonics transmitter based on passive alignment.

A single-chip hybrid integrated silicon photonics transmitter based on passive alignment flip-chip bonding technology has been demonstrated. The transmitter is developed by the hybrid integration of a C-band slotted laser with 1 mm cavity length and a Mach-Zehnder modulator with 2 mm long phase shifter. A 3 dB bandwidth of the small signal response is 16.35 GHz at 5.99 VPP superimposed with a reverse bias voltage of 2.43 V. A 25 Gbps data transmission experiment of the hybrid integrated transmitter is performed at 25°C.

High-power tunable dual-wavelength diode laser with a composite external cavity.

A high-power tunable dual-wavelength composite external cavity architecture obtained by means of a holographic grating and a volume Bragg grating is proposed and demonstrated. The tunable frequency difference of the dual-wavelength output is from 0.41 THz to 3.89 THz. We obtain an output power of 2.1 W when the frequency difference is 1.86 THz. The side-mode suppression ratio of more than 29 dB is suppressed over the entire tunable dual-wavelength output range. The two corresponding wavelengths of the dual-wavelength output basically maintain the same intensity with the smallest power difference of only 0.10%.

High-brightness and high-efficiency diode laser module based on double wavelength division multiplexing external cavities.

In this Letter, a new, to the best of our knowledge, external cavity structure based on double wavelength division multiplexing external cavities is proposed and demonstrated. The electro-optical conversion efficiency is improved and the brightness of the spectral beam combining diode lasers is enhanced. One wavelength division multiplexing external cavity is placed on the rear-side of the laser emitters to provide the strong optical feedback for wavelength locking and the other wavelength division multiplexing external cavity is placed on the front-side of laser emitters to combine three emitter beams to one beam. A maximum output power of up to 7.5 W is obtained and the brightness of the laser diode is 100 MW cm-2 sr-1 with an electro-optical conversion efficiency of 46.5%. Compared with a standard cavity for spectral beam combining, the use of double wavelength division multiplexing external cavities results in an electro-optical conversion efficiency improvement of 6.5%. The whole structure provides a new technology to achieve high-brightness and high electro-optical conversion efficiency for a laser diode source.

Electrically injected supersymmetric semiconductor lasers with narrow vertical divergence angle.

Electrically injected supersymmetric (SUSY) semiconductor lasers are proposed and fabricated. Two successive SUSY transformations are applied to the main array arranged along the direction of epitaxial growth, which can remove the propagation constants of the fundamental mode and the leaky mode of the main array from the superpartner while keeping those of other high-order modes. The SUSY laser possesses an excellent mode discrimination and favors the lasing of the fundamental mode. The fabricated SUSY laser can emit light with a single-lobe vertical far-field pattern with the full width at half maximum of 16.87° under an injection current of 1.4 A.

Quasi-continuous tunable semiconductor laser based on the gain-lever effect with full C-band coverage fabricated by standard lithography.

A quasi-continuous tunable semiconductor laser covered full C-band is demonstrated. The quasi-continuous tuning range of the tunable semiconductor laser is significantly improved by optimizing the length of the phase section using the gain-lever effect, achieving a 36 nm range that covered the whole C-band. In the tuning range, 46 channels with 100 GHz spacing are achieved, and all channels exhibit a side mode suppression ratio above 30 dB. No regrowth or high-precision lithography is involved in the fabrication process of the tunable semiconductor laser, which has the potential to provide a cost-effective light source for dense wavelength division multiplexing systems.

Approaches to tuning the exceptional point of PT-symmetric double ridge stripe lasers.

Electrically injected Parity-time (PT)-symmetric double ridge stripe semiconductor lasers lasing at 980 nm range are designed and measured. The spontaneous PT-symmetric breaking point or exceptional point (EP) of the laser is tuned below or above the lasing threshold by means of varying the coupling constant or the mirror loss. The linewidth of the optical spectrum of the PT-symmetric laser is narrowed, compared with that of traditional single ridge (SR) laser and double ridge (DR) laser. Furthermore, the far field pattern of the PT-symmetric laser with EP below the lasing threshold is compared with that of the PT-symmetric laser with EP above the lasing threshold experimentally. It is found that when the laser start to lase, the former is single-lobed while the latter is double-lobed. when the current continues to increase, the former develops into double lobe directly while the latter first develops into single lobe and then double lobe again.

Low-coherence, high-power, high-directional electrically driven dumbbell-shaped cavity semiconductor laser at 635 nm.

An electrically driven dumbbell-shaped cavity semiconductor laser laterally confined by isolation and metal layers at 635 nm has been proposed. In the simulation, we systematically analyzed the Q-factors, mode intensity distributions, and directionality of the dumbbell-shaped cavity. A measured speckle contrast as low as 3.7%, emission divergence of 7.7°, and maximum output power of about 2.36 W were obtained in the experiment. Such a semiconductor laser with low coherence, high power, and high directivity may provide great potential application value in laser display and imaging.

Single-mode semiconductor lasers fabricated by standard photolithography for direct modulation.

The output characteristics, modal properties, far-field profiles, and dynamic modulation responses of semiconductor lasers with surface higher-order gratings fabricated by the standard photolithography are presented. Single-mode semiconductor lasers with 20th- and 37th-order gratings for the 1.55 µm wavelength range are realized. The single-mode semiconductor lasers with 20th-order gratings have lower threshold currents and higher slope efficiencies than those with 37th-order gratings. The surface higher-order grating placed closed to the output facet can deteriorate the vertical far-field profile of the semiconductor laser. However, the properties of the semiconductor laser's single-mode operation are not affected by the surface higher-order grating's position in the ridge waveguide. The -3 dB bandwidth of these single-mode semiconductor lasers can achieve 9 GHz at 100 mA, which is the highest, to the best of our knowledge, for such a kind of single-mode semiconductor laser with a surface higher-order grating.

High-power high-brightness 980 nm lasers with >50% wall-plug efficiency based on asymmetric super large optical cavity.

High-power high-brightness super large optical cavity laser diodes with an optimized epitaxial structure are investigated at the wavelength of 980 nm range. The thicknesses of P- and N-waveguides are prudently chosen based on a systematic consideration about mode characteristics and vertical far-field divergences. Broad area laser diodes show a high internal quantum efficiency of 98% and a low internal optical loss of 0.58 cm-1. The ridge-waveguide laser with 7 µm ridge and 3 mm cavity yields 1.9 W single spatial mode output with far-field divergence angles of 6.8° in lateral and 11.5° in vertical at full width at half maximum under 2 A CW operating current. The corresponding M2 values are 1.77 and 1.47 for lateral and vertical, respectively, and the corresponding brightness is 76.8 MW‧cm-2‧sr-1. The far-field divergence angles with 95% power content are in the range of 24.7° to 26.1° across the whole measured range.

Eight-channel laser array with 100 GHz channel spacing based on surface-slotted structures fabricated by standard lithography.

An eight-channel laser array with 100 GHz channel spacing based on surface-slotted structures is demonstrated. Wavelength selection is realized by using a group of micrometer-order slots fabricated by standard lithography technology. An output power of over 14 mW and a side-mode suppression ratio of better than 35 dB for each laser within the array are achieved. A laser array without tuning for 100 GHz channel spacing by standard lithography was realized for the first time, to the best of our knowledge. The laser array can be used to enhance the capacity of communication links and provides a promising light source for photonic-integrated circuits.

Efficient self-frequency-doubling Nd:GdCOB green laser at 545 nm pumped by a 796 nm laser diode.

A 796 nm laser-diode (LD)-pumped self-frequency-doubling Nd:GdCa4O(BO3)3 (Nd:GdCOB) green laser is first demonstrated. With 2.93 W of 796 nm LD pump power, a maximum power of 460 mW green laser at 545 nm has been achieved. The optical conversion efficiency of 15.8% is higher than that pumped with a 808 nm LD. As the pump wavelength shifts toward 796 nm, the output power and optical conversion efficiency increase.

Large-aperture subwavelength grating couplers.

Subwavelength nanostructure grating couplers fabricated on silicon-on-insulator substrates are used to simplify the fabrication process while maintaining high coupling efficiency. The main obstacle for their application in photonic integrated circuits is the small aperture size of the nanostructure when TE polarization is involved, since they are difficult to achieve with 193 nm deep-ultraviolet lithography and cause problems in inductively coupled plasma etching. A larger lateral period has been used to increase the aperture size. Here, we propose that decreasing the effective index of the nanostructure can also enlarge the aperture size. We analyze the two methods in detail with a rectangle-hole nanostructure and 220 nm thick waveguide layer, aiming at TE polarization centered at 1560 nm. We find performance degenerations for large lateral periods, and this can be simply compensated by adjusting the width of the rectangle hole. The minimum linewidth of the nanostructure can reach 240 nm, while the coupling efficiency is just slightly decreased. The backreflections of a large-aperture grating increase but stay in the same order with ordinary ones, and we also show that this can be overcome by apodizing the grating structure. Finally, we experimentally demonstrate the designed large-aperture grating couplers and the coupling efficiencies are higher than 35%, and reach a rectangle-hole width.