Surface-emitting Laser Diodes Research Articles

Compact Optical Probe for Time-Resolved NIRS-Imaging

We present the design of a standalone optical probe integrated with a Time-Gated Single Photon Detection module and Pulsed Light Emission unit. The detection module is capable of detecting photons in specific time windows, thus enabling the measurement of very faint optical signals. The pulse repetition frequency extends up to 100 MHz with a gate-ON time range up to 35ns. The module is user-configurable via a PC interface and can be integrated in any optical setup. The pulsed light emission unit uses a 50 GHz GaAs Vertical Cavity Surface Emitting Laser diode. Experimental characterization was conducted for all the building blocks of the module to ensure its proper functioning. The module was also employed in a real-life application, heart rate monitoring by illuminating the index finger of a volunteer. The novelty of the proposed miniaturized optode lies in the development of a simplified interface for portable Brain-computer interfaces which can be effectively used in applications such as Near-Infrared Spectroscopy and functional brain imaging.

An electrically pumped surface-emitting semiconductor green laser.

Surface-emitting semiconductor lasers have been widely used in data communications, sensing, and recently in Face ID and augmented reality glasses. Here, we report the first achievement of an all-epitaxial, distributed Bragg reflector (DBR)-free electrically injected surface-emitting green laser by exploiting the photonic band edge modes formed in dislocation-free gallium nitride nanocrystal arrays, instead of using conventional DBRs. The device operates at ~523 nm and exhibits a threshold current of ~400 A/cm2, which is over one order of magnitude lower compared to previously reported blue laser diodes. Our studies open a new paradigm for developing low-threshold surface-emitting laser diodes from the ultraviolet to the deep visible (~200 to 600 nm), wherein the device performance is no longer limited by the lack of high-quality DBRs, large lattice mismatch, and substrate availability.

AlGaN nanocrystals: building blocks for efficient ultraviolet optoelectronics

AlGaN nanocrystals have emerged as the building blocks of future optoelectronic devices operating in the ultraviolet (UV) spectral range. In this article, we describe the design and performance characteristics of AlGaN nanocrystal UV light-emitting diodes (LEDs) and surface-emitting UV laser diodes. The selective-area epitaxy and structural, optical, and electrical properties of AlGaN nanocrystals are presented. The recent experimental demonstrations of AlGaN nanocrystal LEDs and laser diodes are also discussed.

Impact of distributed Bragg reflector on carrier and photon dynamics in GaN-based surface emitting diodes manifested by ultrafast transient absorption spectroscopy

Small signal modulation bandwidth characterized by relaxation oscillations (ROs) of GaN-based vertical cavity surface emitting laser diodes are of great importance in high-speed applications. In this work, we investigate the impact of distributed Bragg reflectors (DBRs) on ROs by developing a simulation framework, which can accommodate the reflected photons self-consistently in the rate equation model. Carrier and photon dynamics in the active region and at the transmitting end have been evaluated for a typical GaN/InGaN multiple quantum well heterostructure having a dielectric DBR. Temporal broadening in photon dynamics is observed due to the phase difference. Interestingly, an enhanced radiative recombination rate is also noticed due to the increased confinement, which leads to a reduced turn-on delay. Transient absorption spectroscopy has been performed to obtain the decay rates of captured carriers in the well and it turns out be in harmony with the simulation results. Photoluminescence spectroscopy also validates the increased radiative recombination.

Non-Hermiticity and exceptional points in coherently coupled vertical cavity laser diode arrays

Coherently coupled laser arrays can be described by the temporal coupled mode theory in which the array modal behavior can be classified according to the coupling matrix, M¯¯. Accounting for a nonuniform gain/loss distribution in a laser array makes M¯¯ a non-Hermitian matrix, and experimentally we find phase-front tuning (beam steering) of the coherent supermode as a result of the non-Hermiticity. We report the experimental characterization of the supermodes in coherently coupled vertical cavity surface emitting laser diode arrays and demonstrate control of non-Hermiticity by spatially varying injection currents. Exceptional points are identified in these electrically injected microcavity diode arrays.

1.6 MHz scanning rate direct absorption temperature measurements using a single vertical-cavity surface-emitting laser diode.

This paper presents 1.6MHz scan rate, non-intrusive, time-resolved temperature measurements of a normal shock reflection from a plane end wall within a shock tube. A vertical-cavity surface-emitting laser (VCSEL) was used to conduct tunable diode laser absorption spectroscopy with water vapor as the probe species. The results are compared with analytical predictions. Temperatures measured with this technique agree within a single-scan standard deviation of ±33 K with calculated temperatures at a VCSEL modulation frequency of 800kHz, which is sufficiently rapid enough to be used to investigate highly transient shock wave interaction processes.

High Dynamic Micro Vibrator with Integrated Optical Displacement Detector for In-Situ Self-Calibration of MEMS Inertial Sensors.

The scale factor drifts and other long-term instability drifts of Micro-Electro-Mechanical System (MEMS) inertial sensors are the main contributors of the position and orientation errors in high dynamic environments. In this paper, a novel high dynamic micro vibrator, which could provide high acceleration and high angular rate rotation with integrated optical displacement detector, is proposed. Commercial MEMS inertial sensors, including 3-axis accelerometer and 6-axis inertial measurement unit which is about 3 mm * 3 mm * 1 mm with 19 mg, could be bonded on the vibration platform of the micro vibrator to perform in-situ during the self-calibration procedure. The high dynamic micro vibrator is fabricated by a fully-integrated MEMS process, including lead zirconate titanate (PZT) film deposition, PZT and electrodes patterning, and structural ion etching. The optical displacement detector, using vertical-cavity surface-emitting laser (VCSEL) and photoelectric diodes (PD), is integrated on the top of the package to measure the 6-DOF vibrating displacement with the detecting resolution of 150 nm in the range of 500 μm. The maximum out-of-plane acceleration of the z-axis vibrating platform loaded with commercial 3-axis accelerometer (H3LIS331DL) achieves above 16 g and the maximum angular velocity achieves above 720°/s when the driving voltage is ±6 V.

Simultaneous Generation of Multiband Signals Using External Cavity-Based Fabry–Perot Laser Diode

Photonic generation of tunable microwave signal has gained the attention of many research communities. Various active components such as vertical-cavity surface-emitting laser diode, semiconductor optical amplifier, and Fabry–Perot laser diode have been used for generating microwave signals using different techniques including side mode injection, feedback injection, and frequency combs. In this paper, we propose and demonstrate a novel approach to generate millimeter wave, simultaneous microwave and millimeter wave, and microwave together with hopping of RF frequency using a single external beam. Furthermore, we demonstrate injecting multiple external beams in a spatial-mode Fabry–Perot laser diode (SMFP-LD) to generate simultaneous multiband signals with negative wavelength detuning. The generated signal ranges from microwave to terahertz. We inject four external beams in SMFP-LD to generate RF frequencies that lie in Ku-, K-, Ka-, V-bands and higher millimeter wave. The frequency of generated signals can be tuned from a few gigahertz to several terahertz. The effects of changing the dominant mode, injecting external beams to different side modes, wavelength and power stability of beating wavelengths, and harmonic generations are analyzed. The maximum linewidth of the generated RF signals with multiple input beams (up to 42.5 GHz) is found to be about 300 kHz.

Experimental Demonstration of 84 Gb/s PAM-4 Over up to 1.6 km SSMF Using a 20-GHz VCSEL at 1525 nm

We demonstrate 84-Gb/s four-level pulse amplitude modulation (PAM-4) over up to 1.6-km standard single mode fiber using a 20-GHz single mode short cavity vertical cavity surface emitting laser diode at a transmission wavelength of 1525 nm. Different equalizer approaches including a common feedforward equalizer, a nonlinear Volterra equalizer (NLVE), a maximum likelihood sequence estimator (MLSE) and their combinations are evaluated working either as an equalizer for a standard PAM-4 or a partial response PAM-4 signal with seven levels. It is demonstrated that a standard FFE is not enough for a transmission distance of >0.6 km, while the use of an NLVE or FFE + MLSE is able to improve the transmission distance towards 1 km. The use of partial-response PAM-4 FFE in combination with a short memory MLSE is able to efficiently equalize the bandwidth limitations, showing more than 10-times BER improvement compared to standard NLVE or FFE + MLSE at a transmission distance of 1.6 km. Using a partial-response NLVE instead of an PR-FFE further performance improvement is achieved, resulting in BERs below the KP4 FEC-threshold with a BER-limit of 2E-4 after 1.6-km transmission distance, allowing error free operation.

Frequency-modulated continuous-wave laser radar using dual vertical-cavity surface-emitting laser diodes for real-time measurements of distance and radial velocity

A frequency-modulated continuous-wave (FMCW) laser radar capable of real-time displaying the distance to a target object and its radial velocity as their corresponding frequency spectra is developed. The system employs a pair of oppositely frequency-swept vertical-cavity surface-emitting laser diodes (VCSELs). This makes possible simultaneous detection of beat signals induced by the increment (up-ramp) and decrement (down-ramp) in laser frequencies. By mixing these two beat signals, their sum and difference frequencies are directly obtained without arithmetic processing such as averaging and subtraction. Results of the test experiments adopting axially moving block gauges as target objects show that both the distance and given velocities are accurately determined from the spectrum of the frequency mixer.

Time-resolved temperature and number density measurements in a repetitively pulsed nanosecond-duration discharge

This paper presents a fast detection technique using diode laser absorption spectroscopy as an optical diagnostic tool to measure time-resolved temperature and number density in a repetitively pulsed nanosecond-duration discharge. Argon atoms in the 1s3 metastable state were optically probed by current scanning a vertical cavity surface emitting laser diode over the 1s3→2p4 transition at 794 nm. Temperature and number density measurements are presented at pulse energies from 20 μJ to 300 μJ, at a constant pressure of 2.67 kPa and 10 kHz repetition frequency. A time resolution of 2 ns was achieved for the measurements during and after the discharge pulse. We demonstrate the method used to make nanosecond resolution measurements, the precision of this technique and the effect of pulse energy on the translational temperature and number density of the metastable atoms. Our measurements show that, for small input pulse energies, the peak temperature of the argon atoms in the 1s3 state can exceed ambient room temperature by up to an order of magnitude.

1.6 kW Yb fiber amplifier using chirped seed amplification for stimulated Brillouin scattering suppression.

In a high power fiber amplifier, a frequency-chirped seed interrupts the coherent interaction between the laser and Stokes waves, raising the threshold for stimulated Brillouin scattering (SBS). Moving the external mirror of a vertical cavity surface-emitting diode laser 0.2μm in 10μs can yield a frequency chirp of 5×1017 Hz/s at a nearly constant output power. Opto-electronic feedback loops can linearize the chirp, and stabilize the output power. The linear variation of phase with time allows multiple amplifiers to be coherently combined using a frequency shifter to compensate for static and dynamic path length differences. The seed bandwidth, as seen by the counter-propagating SBS, also increases linearly with fiber length, resulting in a nearly-length-independent SBS threshold. Experimental results at the 1.6kW level with a 19m delivery fiber are presented. A numerical simulation is also presented.

Lateral carrier confinement of GaN-based vertical-cavity surface-emitting diodes using boron ion implantation

Boron ion implantation, which is used for confining carriers in gallium nitride (GaN)-based vertical-cavity surface-emitting laser diodes (VCSELs), was studied. Detailed analysis indicated that boron ion implantation of GaN increases GaN’s absorption coefficient from zero to 800 cm−1 and its refractive index from 2.45 to 2.51 at the surface of the wafer at a wavelength of 453 nm. The depth profile of boron obtained by secondary ion mass spectroscopy (SIMS) showed an exponential decrease toward the bottom of the wafer. Assuming that the changes in optical parameters caused by implantation are proportional to the concentration of boron in GaN, the boron ion implantation applied to GaN-VCSELs causes optical absorption of 0.04% per round trip in the cavity and extends the light path of the cavity by 2.2 nm, both of which apparently have negligible impact on the operation of GaN-VCSELs. The implanted boron ions pass through the active regions, introducing non-radiative recombination centers at the edges of those active regions made of InGaN multi-quantum wells, which, however, does not cause significant current injection loss.

Dynamics of dual-polarization VCSEL-based optical frequency combs under optical injection locking.

The present experimental work studies the dynamics of dual-polarization optical frequency combs (OFCs) based on gain switching (GS) vertical-cavity surface-emitting laser (VCSEL) diodes under optical injection locking (OIL). This study presents two main results. First, we have obtained an overall comb formed by two orthogonally polarized sub-combs with comparable span and power. The overall comb shows enhanced optical span and flatness and high coherence between its modes. The second result is that we have been able to control the polarization state of the overall comb by tuning the polarization state of the injected light by locking the same single teeth of the comb. This produces an overall comb with single polarization that is parallel or orthogonal. These are novel findings that provide for the development of efficient and compact OFCs based on GS VCSEL sources with versatile polarization dynamics.

(Invited) New Directions in GaN Photonics Enabled by Electrochemical Processes

We have developed a novel conductivity based selective electrochemical etching to introduce nanometer sized pores into GaN. The nanoporous (NP) GaN can be considered as a new form of GaN with an unprecedented tunability in optical index. The advantages of NP-GaN for both edge-emitting laser diodes and vertical surface-emitting laser diodes (VCSEL) are subsequently exhibited.

Frequency scanning interferometry with nanometer precision using a vertical-cavity surface-emitting laser diode under scanning speed control

Frequency scanning interferometry technique with a nanometer precision using a vertical-cavity surface-emitting laser diode (VCSEL) is presented. Since the frequency scanning of the VCSEL is linearized by the phase-locked-loop technique, the gradient of the interference fringe order can be precisely determined using linear least squares fitting. This enables a length measurement with a precision better than a quarter wavelength, and the absolute fringe number including the integer part at the atomic transition spectrum (rubidium-D2 line) is accurately determined. The validity of the method is demonstrated by excellent results of block gauge measurement with a root mean square error better than 5 nm.

Effect of Relief Aperture on Single-Fundamental-Mode Emission of 1.3-$\mu $ m GaInNAs GaAs-Based VCSELs

This paper investigates methods of suppressing higher order transverse modes in a GaInNAs/GaAs quantum-well GaAs-based vertical-cavity surface-emitting diode laser, with the aid of a 3-D self-consistent model. The inverted surface relief design, which creates the antiphase condition, proved efficient at suppressing higher order transverse modes, in spite of unfavorable thermal effects.

Experimental Study of VCSEL-Based Optical Frequency Comb Generators

In this letter, we explore the performance of an optical frequency comb generator (OFCG) sources based on vertical-cavity surface-emitting laser diodes. The direct gain switching (GS) technique, the indirect electro-optical (EO) technique, and a combination of both have been experimentally evaluated. We have observed that this later combination gives birth to an enhanced OFCG that offers a tunable improved comb in terms of frequency span, flatness, and coherence with respect to the sole EO approach, as it partly inherits the high-efficiency qualities of the GS-OFCG while offering a wider span.

200-W Operation of an Ion-Implanted Vertical-Cavity Surface-Emitting Laser Array

We have demonstrated a high-power 980-nm vertical-cavity surface-emitting laser diode (VCSEL) array with ion-implanted current apertures. The device was characterized on an exclusively developed water-cooled heat sink for the VCSEL array. A peak output power of over 200 W has been achieved under quasi-continuous-wave operation at a cooling water temperature of 10 °C. The VCSEL array consists of 635 emitters, which were defined by proton implantation and arranged in a closest packed arrangement with 175- \(\mu \) m-spacing in \(5~{\rm mm} \times 5\) mm square. The results provide a chance to the next step for a higher-power VCSEL array.

Performance comparison of 850-nm and 1550-nm VCSELs exploiting OOK, OFDM, and 4-PAM over SMF/MMF links for low-cost optical interconnects

We experimentally compare the performance of two commercially available vertical-cavity surface-emitting laser diodes (VCSELs), a multi-mode 850-nm and a single-mode 1550-nm, exploiting on–off keying/direct detection (OOK/DD), and orthogonal frequency division multiplexed (OFDM) quadrature phase-shift keying (QPSK)/16-ary quadrature amplitude modulation (16QAM) with direct detection, over SMF (100m and 5km) and MMF (100m and 1km) short-range links, for their potential application in low-cost rack-to-rack optical interconnects. Moreover, we assess the performance of quaternary-pulse amplitude modulation (4-PAM), for the 1550-nm transmitter over SMF and MMF links and we compare it to the data-rate equivalent NRZ-OOK. The extensive performance comparison under various transmission scenarios shows the superiority of 1550-nm single-mode VCSEL compared to its multi-mode 850-nm counterpart. Moreover, OFDM/DD and 4-PAM in conjunction with low-cost, inexpensive VCSELs as transmitters prove to be an enabling technology for next-generation WDM, point-to-point, short-reach, SMF/MMF optical interconnects and potential candidates to substitute NRZ-OOK. Nevertheless, the sensitivity requirements are higher in that case, whereas these advanced, spectrally-efficient modulation formats become severely degraded when transmitted over MMF links, especially, when employing the inexpensive 850-nm VCSELs as transmitter. Finally, we compare the performance of the point-to-point links under investigation to the performance of a semiconductor optical amplifier (SOA)- based, scalable permutation switch fabric, the Optical Shared MemOry Supercomputer Interconnect System (OSMOSIS).