Vertical-cavity Surface-emitting Laser Output Research Articles

Chinese Herbal Medicine Recognition Using a VCSEL-Based Time-Delay Reservoir Computing System

We numerically investigate the performance of Chinese herbal medicine (CHM) recognition using a time delay reservoir computing (TDRC) system consist of a vertical-cavity surface-emitting laser (VCSEL) subjected to optical feedback. A database of 960 images from 12 CHM categories is used to evaluate the recognition performance, most of which contain many CHM samples and have different backgrounds. The CHM image feature vector is masked and optically injected into the VCSEL, and the VCSEL output in two polarization modes are sampled as the virtual node states for readout. After testing the CHM recognition performance of TDRC under injecting the same information into two polarization modes of the VCSEL, we further test the performance of parallel processing by injecting half of the image information into each of the two polarization modes. By concatenating the virtual node states collected from two modes for readout, the recognition speed is doubled. Meanwhile, the influence of key parameters on recognition performance are investigated and the relationship between system idle state and recognition performance is also examined. The results show that, under parallel processing and orthogonal optical feedback, the TDRC achieves the minimum recognition error of 1.7% at a reservoir processing rate of 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> images per second.

VCSEL-based optical transceiver with a 90°-bent graded-index core polymer waveguide coupler for 25.78-Gb/s transmission

In this paper, we propose a low-loss assembly of optical elements [vertical cavity surface emitting laser (VCSEL)/photodiode (PD)] and a 90°-bent graded-index (GI) core polymer optical waveguide for compact high-speed optical transceivers. The air gap between the 90°-bent GI-core waveguide and VCSEL/PD chips are filled with a high-index resin, and we demonstrate a reduction not only of the Fresnel reflection loss but also of the coupling loss due to the lower divergence angle of VCSEL and fiber output beams. In this paper, the core size and NA of the 90°-bent GI-core in the polymer waveguide are redesigned to match the specific launch conditions realized with the gap filled with resins, resulting in theoretical estimates of insertion losses as low as 0.82 dB and 1.38 dB for the waveguides on the transmitter and receiver sides, respectively. Such low insertion losses are obtained not by step-index core but GI core waveguides. After redesigning the structure, 90°-bent GI-core polymer waveguides with a bending radius of 1 mm are experimentally fabricated using the Mosquito method, and we experimentally demonstrate that the insertion loss of the waveguide can be reduced by about 4 dB when the 100-µm air gap is filled with an optical adhesive (na = 1.51). In addition, an error-free 25.78-Gb/s data transmission is successfully demonstrated over a 100-m MMF link using the fabricated VCSEL based transceivers with 90°-bent GI core polymer optical waveguides.

Coupling angle tolerance of the 850-nm single-mode VCSEL output collimated by lensed OM4-MMF or GI-SMF for a NRZ-OOK link.

By collimating the single-mode (SM) vertical-cavity surface-emitting laser (VCSEL) at 850 nm with either the OM4 multi-mode fiber (OM4-MMF) or the graded-index single-mode fiber (GI-SMF) with lensed end-face, the directly encoded non-return-to-zero on-off keying (NRZ-OOK) data transmission performance is characterized when tilting the coupling angle with respect to the surface normal of the SM-VCSEL. In comparison with the lensed OM4-MMF and lensed SMF coupling, the lensed OM4-MMF collimator shows a large coupling angle tolerance with the coupling efficiency only degraded by 5% when enlarging the tilted angle from 0° to 10°. In contrast, the lensed GI-SMF collimator attenuates the coupled SM-VCSEL output by more than 50% when tilting the coupling angle up to 10°. For the lensed OM4-MMF coupling, the receivable NRZ-OOK data rate in BtB and after 100-m OM4-MMF cases can achieve 50 Gbit/s with its corresponding BER degraded from 6.5 × 10-10 to 8.8 × 10-10 when enlarging its tilting angle ranged from 0° to 10°. By changing the collimator to the lensed SMF, the decoded BER significantly degrades from 5.8 × 10-5 to 1.2 × 10-1 when coupling and transmitting the NRZ-OOK data at 50 Gbit/s. Owing to the low coupling efficiency via the lensed SMF collimator, the error-free NRZ-OOK data rate under the lensed SMF coupling somewhat decreases to 35 Gbit/s in the BtB link and to 32 Gbit/s after the 100-m GI-SMF link with allowable coupling angle tilted from 0° to 4°. This work confirms the applicability of the lensed MMF or SMF collimator for coupling the SM-VCSEL output with a relatively large tolerance on the tilting angle with respect to the surface normal of the SM-VCSEL.

A rate-doubled 10-GHz fiducial comb generator for precision optical timing calibration.

Solid-state optical fiducial timing pulse generators provide a convenient and accurate method to include timing fiducials in a streak-camera image for time-base correction. Current commercially available vertical-cavity surface-emitting lasers (VCSELs) emitting in the visible range can be amplitude modulated up to 5 GHz. An optically passive method is utilized to interleave a time-delayed path of the 5-GHz pulsed light with itself, producing a 10-GHz pulsed fiducial, or comb. Comb pulse rates at 0.5, 1.0, and 2.5 GHz can also be selected. The instrument presented is a self-contained and portable generator with primary use for streak-camera temporal calibration. Applications can also be extended to many other optical timing needs. The VCSEL output is fiber optic coupled at a wavelength of 680 nm (visible red) with a nearly Gaussian pulse shape. The peak power of each ∼50-ps full width at half maximum (FWHM) comb pulse at 5-GHz operation, or picket, is approximately 5 mW. The low phase noise of the internal microwave modulation drive source provides low pulse to pulse jitter. An external reference frequency standard can be utilized to synchronize the output to external timing equipment. A selectable internal reference frequency crystal oscillator is incorporated for stand-alone operation.

Long-time dynamics of a vertical-cavity surface-emitting laser under optical feedback

External optical feedback in vertical-cavity surface-emitting laser (VCSEL) is found to influence its output intensity. We studied the effect of the amount of total output polarisation feedback and polarisation-selective feedback on the output intensities of a VCSEL for a low-resolution sampling and for long temporal duration. A $$40~\mu \hbox {s}$$ resolution time-series correlation analysis is performed for different feedback conditions and the characteristic dynamics is investigated. We found a correlated fluctuation in VCSEL output for a moderate amount of total feedback and polarised feedback. The period of such fluctuations is found to be reduced from total feedback to the polarised feedback of the system.

Reconfigurable dynamic all-optical chaotic logic operations in an optically injected VCSEL**Project supported by the National Natural Science Foundation of China (Grant No. 61475120) and the Innovative Projects in Guangdong Colleges and Universities, China (Grant No. 2015KTSCX146).

In a chaotic system of vertical cavity surface emitting laser (VCSEL) with external optical-injection, we propose a novel implementation scheme for reconfigurable dynamic all-optical chaotic logic operations (AOCLOs). Under different key parameters, such as the bias current, the injection strength and the frequency detuning of the injected light field and the VCSEL, we also explore the evolutions of the polarization-bistability with the amplitude of the injected light field when the output of VCSEL is chaotic wave. According to the dynamic evolutions, we find out the optimal value of the frequency detuning that is considered as a control logic signal, and further implement different AOCLOs, such as AND, NAND, OR, NOR, XOR, and XNOR, by controlling the logic operation of the control logic signal between two logic inputs. Moreover, the ability to reconstruct these logic operations is demonstrated under relatively low noise strength of the spontaneous emission.

All-Optical Shift Register Using Polarization Bistable VCSEL Array

We investigated all-optical shift register operations using 1.55-μm polarization bistable vertical-cavity surface-emitting lasers (VCSELs) formed in an array. The VCSEL array consisted of VCSELs that oscillated with single longitudinal and transverse modes at room temperature. The lasing wavelengths were tuned by adjusting the driving currents of each VCSEL. A VCSEL in the array exhibited polarization flip-flop operation by injection of the orthogonally polarized set-reset input pulses. By injection of the VCSEL output light into another VCSEL in the same array through a polarizer and an optical gate, the second VCSEL also exhibited polarization flip-flop operation according to the polarization state of the first VCSEL. The shift register operation has been successfully demonstrated using two adjacent VCSELs in the array. The optical power needed for polarization switching is usually lower than the VCSEL output power. Thus, this shift register operation was achieved without optical amplifiers.

894 nm high temperature operating vertical-cavity surface-emitting laser and its application in Cs chip-scale atomic-clock system

In this study, an 894 nm high temperature vertical-cavity surface-emitting laser (VCSEL) is reported. Furthermore, a Cs chip-scale atomic clock (CSAC) system experiment based on this VCSEL is carried out.To achieve low threshold/power consumption under high temperature condition, the VCSEL epitaxial structure is optimized. Especially, the so-called gain cavity-mode detuning technology is utilized to improve the temperature sensitivity of the device output characteristics. The relationship between the structure of quantum well and the gain is simulated by using the commercial software PICS3D. In order to achieve high gain and low threshold properties, the thickness of the quantum well is optimized. Based on the theory of transmission matrix, the VCSEL cavity mode (etalon) is calculated. Finally, a -15 nm quantum well gain-cavity mode offset is utilized to achieve relatively stable cavity mode gain, which can guarantee the temperature-insensitivity of the VCSEL output characteristics.The output performance of the VCSEL device we fabricated is investigated experimentally. The VCSEL lightcurrent (L-I) characteristic is tested under different temperatures. It is found that benefiting from the gain-cavity mode offset design, the threshold can be maintained at 0.200.23 mA when the temperature increases from 20 ℃ to 90 ℃. Meantime, the output power of more than 100 W is achieved at different temperature levels. By comparing with the results at room temperature, No dramatic degradation of the VCSEL high temperature L-I characteristics is found, which means that the VCSEL output characteristic is relatively temperature-insensitive. The wavelength of the VCSEL is 890.4 nm at a temperature of 20 ℃. When the temperature increases up to 85.6 ℃, the VCSEL wavelength is red-shifted to 894.6 nm (Cs D1 line), corresponding to a red shift ratio of 0.064 nm/℃. According to the polarization requirement of CSAC applications, the polarization properties of the VCSEL are studied and the results are as follows: under an injected current of 1 mA and operation temperature of 20 ℃, Pmax = 278.2 W and Pmin = 5.9 W, corresponding to a polarization ratio of 47:1; at a temperature of 85.6 ℃, Pmax = 239.2 W and Pmin = 4 W, corresponding to a polarization ratio of 59:8:1.Using the VCSEL reported in this paper as a laser source, the CSAC experiment is carried out. At 4.596 GHz of modulated frequency, the output laser of the VCSEL is collimated and interacts with Cs atoms. Finally the closed-loop frequency locking atomic clock is demonstrated. The Cs laser absorption spectrum for laser frequency stabilization, as well as the CPT signal for Cs CSAC microwave frequency stabilization is obtained.

All-optical 2-bit header recognition and packet switching using polarization bistable VCSELs.

We propose and evaluate an all-optical 2-bit header recognition and packet switching method using two 1.55-µm polarization bistable vertical-cavity surface-emitting lasers (VCSELs) and three optical switches. Polarization bistable VCSELs acted as flip-flop devices by using AND-gate operations of the header and set pulses, together with the reset pulses. Optical packets including 40-Gb/s non-return-to-zero pseudo-random bit-sequence payloads were successfully sent to one of four ports according to the state of two bits in the headers with a 4-bit 500-Mb/s return-to-zero format. The input pulse powers were 17.2 to 31.8 dB lower than the VCSEL output power. We also examined an extension of this method to multi-bit header recognition and packet switching.

Bit Error Rate Measurements of All-Optical Flip-Flop Operations of a 1.55-μm Polarization Bistable VCSEL

We measured the bit error rate (BER) of all-optical flip-flop operations using a 1.55-μm polarization bistable vertical-cavity surface-emitting laser (VCSEL). Polarization bistable flip-flop operations were obtained by injecting optical set and reset pulses with independently adjusted wavelengths toward the two VCSEL lasing modes, whose polarizations are orthogonal to each other. Acquired waveforms showed clearly opened eyes, and BERs less than 1 × 10−9 were measured up to 1 Gb/s although the optical set and reset pulse power was much lower than the VCSEL output power. We also demonstrated all-optical flip-flop operations by injecting optical set and reset pulses with the same wavelength, which were generated by a single laser diode. In this case, we obtained polarization bistable flip-flop operations up to 500 Mb/s with almost the same BER values that were obtained when the wavelengths of the set and reset pulses were adjusted independently. The optical input pulse powers were higher than for the case using input pulses with two different wavelengths, but still lower than the VCSEL output power.

Influence of optical filtering on transmission capacity in single mode fiber communications

This paper presents the design and analysis of optical filters that are placed at the output of directly modulated vertical cavity surface emitting laser (VCSEL) in the process of inexpensive transmitter’s implementation for upcoming generation optical access network. Generation of non return to zero (NRZ) optical signal from the transmitter for 110 km error-free single mode fiber (SMF) transmission at 10 Gb/s with bit error rate (BER) of 10−30 in the absence of the external modulator and encoder was proposed. Effects of super-Gaussian and Butterworth optical filters at VCSEL output were demonstrated to maximize performance of SMF optical systems without need of any dispersion compensation technique.

Power-stabilized tunable narrow-band source using a VCSEL and an EDFA for FBG sensor interrogation

Wavelength tuning with a current of a long-wavelength vertical-cavity surface-emitting laser (VCSEL) was studied for application to wavelength readout of fiber Bragg grating (FBG) sensors. 1.5 µm single-longitudinal-mode VCSELs with a spectral line width of 30 MHz were used. To decrease the variation in the output power with the current, saturated amplification of the VCSEL output by an erbium-doped fiber amplifier (EDFA) was proposed. In the experiment, the wavelength readout of an FBG by variation in the VCSEL current was studied first, and then amplification characteristics using a commercially available EDFA module were measured. Two VCSELs with different wavelengths and tuning ranges were used as input and compared. For a VCSEL with a wavelength tuning range of 2.5 nm, there was a threefold change in the VCSEL output power. However, the variation in the amplified output power was within ±14%. For another VCSEL with a wavelength tuning range of 3.9 nm, the variation in the amplified output power was within ±18% for variation in the VCSEL output power by 2.2 times. The power slope with respect to the wavelength was also decreased by the stabilization. Computer simulation was performed to study the wavelength shift in the measured Bragg wavelength induced by the power slope. The simulation showed that the wavelength shift can be reduced to 1/2.5–1/5 by the stabilization compared with that using a single VCSEL. Application to long-period grating sensors was also discussed.

All-Optical Header Recognition and Packet Switching Using Polarization Bistable VCSEL

We demonstrate an optical header processing system using a 1.55-μm polarization bistable vertical-cavity surface-emitting laser (VCSEL) that operates as an all-optical AND gate and a holding element. Depending on the state of 1 bit in the 4-bit header, the VCSEL held one of two-orthogonal polarization states. The light of the VCSEL output through a polarizer switched and held the output state of an optical switch. This enables the payload to be sent to the destination determined by the header. The characteristics of the polarization bistable VCSEL enable low-power consumption and lower latency operation. The system also has the potential to process multibit optical headers involved in ultrafast optical packets.

A chip scale lin⊥lin quasi-bichromatic laser scheme

We present a chip scale lin⊥lin quasi-bichromatic laser beam generator scheme and our experimental study results. Through modulating two vertical-cavity surface-emitting lasers (VCSELs) with the same microwave, both VCSELs output multichromatic lasers and every frequency component in their outputs contributes for the frequency mutual injection locking between VCSELs. The system outputs a lin⊥lin quasi-bichromatic laser beam in which the two wanted frequency components share more than 90% of the total laser power. It is promising to apply the chip device in the chip scale coherent population trapping (CPT) atomic clock and the chip scale CPT magnetometer.

Vertical-cavity surface-emitting laser vapor sensor using swelling polymer reflection modulation

Vapor detection using a low-refractive index polymer for reflection modulation of the top mirror in a vertical-cavity surface-emitting laser (VCSEL) is demonstrated. The VCSEL sensor concept presents a simple method to detect the response of a sensor polymer in the presence of volatile organic compounds. We model the physics as a change in the top mirror loss caused by swelling of the polymer upon absorbing the target volatile organic compound. Further we show how acetone vapors at 82 000 ppm concentration can change the polymer coated VCSEL output power by 20 μW.

Transmitter Predistortion for Simultaneous Improvements in Bit Rate, Sensitivity, Jitter, and Power Efficiency in 20 Gb/s CMOS-Driven VCSEL Links

The effect of applying feed-forward equalization (FFE) on the transmitter side is studied for three different full optical links. In contrast to all previous works, the FFE settings are optimized for a complete link, rather than just the vertical-cavity surface-emitting laser output. The approach results in dramatic improvements in total link performance: >; 6 dB in sensitivity, 3X in timing margin, and 2X in power efficiency at 15 Gb/s, and a record 5.7 pJ/bit at 20 Gb/s.

Polarization and Time-Resolved Dynamics of a 1550-nm VCSEL Subject to Orthogonally Polarized Optical Injection

We report a first experimental study of the simultaneous temporal and polarization-resolved dynamics of a 1550-nm vertical-cavity surface-emitting laser (VCSEL) subject to orthogonally polarized optical injection. A novel technique is used to reveal the behavior of both polarizations at the VCSEL's output. In general, the same type of dynamics can be seen in both polarizations of the fundamental transverse mode but with unequal intensity/modulation depth. We show that both polarizations exhibit period one and chaotic dynamics and reveal antiphase dynamics. These results offer promise for the development of dual-channel, anticorrelated periodic and chaotic-signal generators with a single VCSEL for use as oscillators or chaotic sources in present and future optical networks.

Design of a 4 $\times$ 10 Gb/s VCSEL Driver Using Asymmetric Emphasis Technique in 90-nm CMOS for Optical Interconnection

This paper describes the design and experimental results of a 4 × 10 Gb/s vertical-cavity surface-emitting laser (VCSEL) driver using the asymmetric emphasis technique. Conventional symmetric emphasis techniques can compensate for the influences of parasitic capacitances; however, they cannot compensate for the nonlinear effects of a VCSEL. To overcome this problem, an asymmetric emphasis technique that can separately control the emphasis pulses at the rising and falling edges is proposed. This allows fast transition in VCSEL output waveform suppressing ringing. A driver circuit that has two separate emphasis circuits for the rising and falling edges is proposed in order to implement the asymmetric emphasis technique. This configuration enables us to separately control the height, width, and setup time of the emphasis pulses at the rising and falling edges. The test chip fabricated by using 90-nm CMOS technology generates a clearly open optical eye at a data rate of 10 Gb/s, and we can confirm the existence of a wide phase margin by a transmission experiment.

Concept of feedback-free high-frequency loss modulation in detuned duo-cavity vertical cavity surface-emitting laser

The authors propose and demonstrate a novel concept for ultrahigh-speed loss modulation of a vertical cavity surface-emitting laser (VCSEL). A duo-cavity device architecture is used to optically decouple the ac feedback component of a loss-modulation section from the VCSEL active region. The elimination of feedback allows modulation of the VCSEL output far beyond the optoelectronic relaxation frequency by eliminating both resonance and intrinsic high-frequency response roll-off. Precise detuning of the resonances of both coupled cavities is used to achieve ac feedback elimination by control of thickness during molecular beam epitaxy growth. Variation in the applied bias at the multiple-quantum-well modulator section allows adjustment of detuning to change coupling between the two sections, resulting in resonance features in the modulation response. In the ideal case, the resulting resonance-free high-frequency modulation response is limited only by parasitics of the modulator section. A flat (±3 dB) modulation response up to 20 GHz and data bandwidth up to 20 GB/s are demonstrated by using this modulation method.

VCSEL with reticular electrode and larger emitting aperture and its optoelectronic characteristics

The enlargement of the emitting aperture is usually one of the important methods of increasing verticalcavity surface-emitting laser (VCSEL) optical output power. However, in a VCSEL with a larger aperture, the inhomogeneity in the injected current often causes inhomogeneous or even no emission. To solve this problem and to increase VCSEL output power, as well as to improve its thermal characteristics, we develop a new type of injected VCSEL with a larger aperture and a reticular electrode, where the conventional circular injection electrode of the P side is turned into a reticular one, and the heat sink is on the N side. The tests of the new VCSEL show an improvement in homogeneity in not only the injected current but also the emission intensity. The optical output power is also considerably increased, and the device optoelectronic performance is improved.