III-V-on-silicon heterogeneously integrated lasers (Conference Presentation)

New machine learning algorithms such as deep neural networks and the availability of large datasets have created a large drive towards new types of hardware capable of executing these algorithms with higher energy-efficiency. Recently, silicon photonics has emerged as a promising hardware platform for neuromorphic computing due to its inherent capability to process linear and non-linear operations and transmit a high bandwidth of data in parallel. At Hewlett Packard Labs, an energy-efficient dense-wavelength division multiplexing (DWDM) silicon photonics platform has been developed as the underlying foundation for innovative neuromorphic computing architectures. The latest research on our silicon photonic neuromorphic platform will be presented and discussed.

The knowledge of the exact frequency of an optical source has always been one of the ultimate goals in optics. Since the discovery of the laser, complex systems have been developed to address this challenge. That effort reached a significant milestone with the advent of the femtosecond laser frequency comb that reduced the system size from an entire lab down to the bench‐top. That spurred interest in the development of integrated optical frequency synthesizers that can generate precisely different optical frequencies on demand and can be deployed widely. In this work, such an optical frequency synthesizer using supercontinuum waveguide and second harmonic generator on silicon photonics platform is demonstrated. Integrated silicon photonics based tunable continuous wave laser is phase‐locked to a microwave reference, to synthesize absolute optical frequencies in the telecom band. A relative frequency instability of 1 × 10−12 at 1 s level is achieved by utilizing an integrated self‐referencing scheme that exploits the strong 3rd order and electric‐field‐induced 2nd order nonlinearities of silicon waveguides. With this work, an all on‐chip silicon photonics based frequency synthesizer seems promising for mass production of next generation broad‐band coherent optical communication systems, spectroscopic, detection, and ranging systems and future integrated quantum systems with Hz‐level precision.

Fiber-optic sensors have experienced great interest in research and development since their invention. Dedicated measurement systems are essential for utilization and development of these sensors. Instruments based on tunable lasers are established devices for the demodulation of such sensors. We introduce and demonstrate a tunable laser measurement system based on a widely tunable monolithic laser diode (TML). It is capable of demodulating different types of fiber-optic sensors, although we focus on the interrogation of fiber Bragg grating (FBG) sensors. The rapid tuning of the laser's wavelength is critical for achieving high measurement rates. However, the high tuning rate is demanding and requires careful characterization. We present a method for this and show, to our knowledge for the first time, data on the rapid tuning through the whole spectrum of this type of laser diode. We propose a modified centroid algorithm that can cope with nonequidistantly sampled spectra caused by the laser's functional principle. Finally, we demonstrate the dynamic performance of the new measurement system in an application: An FBG-based acceleration sensor is demodulated at a 5-kHz sample rate during a vibrational test showing the potential of the approach.

Proton radiation damage in laser diodes is investigated for several types of laser diodes with wavelengths from 650 to 1550 nm. Key parameters include slope efficiency, threshold current, and the transition characteristics between laser-emitting diode (LED) and laser operation. Some of the devices exhibited nonlinear relationships between threshold current and proton fluence. All of the lasers, including vertical-cavity surface-emitting lasers, were strongly affected by recombination-enhanced annealing, in contrast to double-heterojunction LEDs, which are only slightly affected by annealing. Analysis of laser characteristics after irradiation showed that the main effect of radiation damage is an increase in bulk recombination that increases loss within the laser cavity.

Photonic integrated circuits (PICs), implementing optical functions such as light generation, modulation, routing and detection on a single chip, are emerging as a powerful platform to realize miniaturized optical systems. These chips find applications in various fields, ranging from high-speed optical transceivers to disposable biosensors, LiDARs for detection and ranging, spectroscopic analytical sensors, etc. Silicon photonics is the field that is using silicon fabrication technologies, developed over the last decades for advanced electronic integrated circuits, to realize PI Cs. Using this approach advanced PICs can be realized on 200 mm or 300 mm wafers in high volume and at low cost. On the silicon photonics platform many device structures are readily available: Si or SiN waveguides, micro-heaters for tuning/switching, Si or Ge based modulators and photodetectors. However, other optical functions such as light generation require the integration of III-V semiconductors on the silicon wafers. This can be realized using different approaches ranging from hybrid assembly over die-towafer bonding to monolithic integration. Every approach has its advantages and disadvantages. An interesting approach that we are developing is the use of microtransfer- printing technology for the integration of III-V semiconductor devices on a silicon photonic wafer, which is a scalable and minimally-invasive approach.

A five-wavelength surface emitting laser diode array based on surface mode emission is reported. The wavelength control is achieved by postgrowth adjustment of the surface structure. Wavelength spacing between laser groups is 1.1 nm in average and the wavelength reproducibility is ±0.13 nm. The total change in the emission wavelength over the array is 4.78 nm. The single array elements show single mode emission with a full width at half-maximum of 0.08 nm in average and a side mode suppression ratio up to 20 dB. This type of laser diode is of high interest for wavelength division multiplexing applications.

Silicon photonics has drawn increasing attention in the past few decades and is a promising key technology for future daily applications due to its various merits including ultra-low cost, high integration density owing to the high refractive index of silicon, and compatibility with current semiconductor fabrication process. Optical interconnects is an important issue in silicon photonic integrated circuits for transmitting light, and fiber-to-chip optical interconnects is vital in application scenarios such as data centers and optical transmission systems. There are mainly two categories of fiber-to-chip optical coupling: off-plane coupling and in-plane coupling. Grating couplers work under the former category, while edge couplers function as in-plane coupling. In this paper, we mainly focus on edge couplers in silicon photonic integrated circuits. We deliver an introduction to the research background, operation mechanisms, and design principles of silicon photonic edge couplers. The state-of-the-art of edge couplers is reviewed according to the different structural configurations of the device, while identifying the performance, fabrication feasibility, and applications. In addition, a brief comparison between edge couplers and grating couplers is conducted. Packaging issues are also discussed, and several prospective techniques for further improvements of edge couplers are proposed.

A method for rapid wavelength tuning of an extended cavity diode laser (ECDL) is presented providing for high resolution, narrow bandwidth output over limited spectral regions. The method permits tuning over isolated spectroscopic features at repetition rates of tens of kHz, greatly exceeding conventional ECDL tuning speeds. In this paper we present high repetition rate laser induced fluorescence (LIF) spectroscopy of the 52P1/2 to 62S1/2 transition in indium at 410 nm, to demonstrate the technique. The presented ECDL design is very easy to implement, cheap and robust, as it employs no moving parts and can be used over all wavelength regions where FP diode lasers are available. This extends the usefulness of standard FP diode lasers to high speed sensing applications. Advantages and disadvantages of the technique are discussed.

Design and fabrication of high power InGaN blue laser diode over 8 W

We demonstrate a coherent modulator and a receiver based on monolithically-integrated silicon photonic circuits, capable of modulating and detecting 224-Gb/s polarization-division-multiplexed 16-QAM. The high-degree photonic integration promises small-form-factor and low-power transceivers for future coherent systems.

In recent years, with the further ministration of the semiconductor device in integrated circuits, power consumption and data transmission bandwidth have become insurmountable obstacles. As an integrated technology, photonic integrated circuits (PICs) have a promising potential in the post‐Moore era with more advantages in data processing, communication, and diversified sensing applications for their ultra‐high process speed and low power consumption. Silicon photonics is believed to be an encouraging solution to realize PICs because of the mature CMOS process. The past decades have witnessed a huge growth in silicon PICs. However, there is still a demand for the development of silicon PICs to enable powerful chip‐scale systems and new functionalities. In this paper, a review of the photonic components, functional blocks, and emerging applications for PICs is offered. The common photonic components are classified into several sections, including on‐chip light sources, fiber‐to‐chip couplers, photonic resonators, waveguide‐based sensors, on‐chip photodetectors, and modulators. The functional blocks of the PICs mentioned in this review are photonic memories and photonic neural networks. Finally, the paper concludes with emerging applications for further study.

Kinoform for irregular space-variant nonparaxial optical interconnection (01) was calculated by the generalized error-reduction algorithm (ERA). Diffraction limited spots, easy to align fordetectors of 10 im width, and diffraction efficiency r 34 were obtained for the binary kmoform case. This approach is fast, takes into account the incident beam intensity distribution,and an equalization of the intensity between spots is easy. However, the resulted surface relief is complicated. NONPARAXIAL KINOFORM SYNTHESIS Free-space Ols for clock distribution will require a low f-number (F/i) holograms due to expected detector sizes of 10 m x 10 im and typical laser diode (LD) light divergence angles of 30 ° and iS °. Large field angles needed to obtain large fanouts and to deflect beams towards detectors located near the chip boundary also cause that paraxial conditions are not satisfied. Kinoforms, which are computer generated phase holograms, afford possibilities for Ols with complicated detector patterns and high .

We experimentally demonstrate a 10 GHz electrically wavelength tunable active mode-locked fiber laser with a phase modulator serving as both mode locker and wavelength selector. Tuning of 4nm with 1.2 V bias voltage is obtained.

We show that it is possible to make a hologram with true-height contours by using dispersion compensation provided by holographic gratings in the reference and picture beams. Holograms can be made with either a short-pulse, mode-locked laser or a tunable dye laser with 2N + 1 frequencies each spaced by Δν frequency. Experiments are presented to illustrate the high-contrast contours obtained with consecutive exposures from a tunable dye laser. Fine-scale vernier and coarse contours are obtained at intervals c/[2(2N + 1)Δν] and c/(2Δv), respectively.

A predictive model of scattering processes in semiconductor lasers is derived, enabling us to model relaxation processes starting from well-known parameters such as the dielectric constant. The resulting effective mode interaction terms are explicitly calculated for an (InGa)N quantum well using Coulomb scattering. In contrast to the method used so far to model mode competi- tion phenomena in Fabry--Pérot type laser diodes, the model correctly includes e.g. accelerated scattering at higher densities or temperatures and eliminates the scattering rate as an unknown parameter. The effective mode interaction term derived in this work can be used for the simulation of the mode dynamics in various laser diode types, for example broad area laser diodes, where multiple transversal and longitudinal modes are active. Thus, our model offers an increased pre- dictability and improved modelling of switch-on behavior.