850-nm Vertical-cavity Surface-emitting Lasers Research Articles

Anomalous noise spikes beating from a fine-splitting mode of surface-emitting laser with imperfect true-roundness aspect

High-frequency discrete and distinct mode-beating noise (MBN) spectra are an anomalous phenomenon observed in the related intensity noise spectrum from the vertical-cavity surface-emitting laser (VCSEL), which reveals strong correlation with fine-splitting transverse modes caused by non-circular emission aspect of the VCSEL with imperfect true-roundness. Such a phenomenon is demonstrated for the first time, which seriously degrades the transmission performance of high-speed and broad-band encoding data carried by the multi-mode (MM) VCSEL biased under a large DC bias. In this work, the 4-ary quadrature-amplitude modulation (4-QAM) orthogonal frequency-division multiplexing (OFDM) and the non-return-to-zero on–off keying (NRZ-OOK) transmissions with the direct modulation of different 850-nm MM VCSELs, as suffered from the MBN-induced notches of the signal-to-noise-ratio (SNR) spectra, are characterized under their high-level lasing conditions with multi-mode numbers. The significant SNR degradation on the receiving and decoding performances caused by the cross-correlation between MBN and data spectra is also examined by tuning the lensed fiber along the transverse (X/Y)-axis of the VCSEL (perpendicular to the surface normal) when collimating the output beam, which finds the disorderly shifted MBN spectra, as attributed to the collection of light emitted from the overlapped gain region of different transverse modes. In contrast, the MBN reveals invariant spectral features while the lensed fiber is either back-and-forth tuning along the vertical (Z)-axis (parallel with the surface normal) or tilting its angles from the surface normal under the same horizontal position and distance concerning the MM VCSEL. Such MM VCSEL exhibits several MBN spikes to decay the SNRs of NRZ-OOK and QAM OFDM data encoded around 21 and 34 GHz. For other MM VCSELs with reducing numbers of their transverse lasing modes and deforming roundness of their circular emitting aspects, the corresponding MBN spectral peaks can also be respectively observed at 14/17/24 GHz and 4.5/10.5/13 GHz to degrade the SNRs of the encoded data at such frequencies. Even though pre-emphasized encoding technology is employed, such an MBN effect is strongly correlated with the asymmetric circular aspect of the VCSEL with a degraded roundness ratio, which still affects the data modulation performance of the MM VCSEL for data center communication.

Single-fundamental-mode cryogenic (3.6 K) 850-nm oxide-confined VCSEL

Cryogenic oxide-confined vertical-cavity surface-emitting laser (VCSEL) has promising application in cryogenic optical interconnect for cryogenic computing. In this paper, we demonstrate a cryogenic 850-nm oxide-confined VCSEL at around 4 K. The cryogenic VCSEL with an optical oxide aperture of 6.5 μm in diameter can operate in single fundamental mode with a side-mode suppression-ratio of 36 dB at 3.6 K, and the fiber-coupled output power reaches 1 mW at 5 mA. The small signal modulation measurements at 298 and 292 K show the fabricated VCSEL has the potential to achieve a high modulation bandwidth at cryogenic temperature.

40.1-GHz sub-freezing 850-nm VCSEL: microwave extraction of cavity lifetimes and small-signal equivalent circuit modeling.

We present an 850-nm vertical-cavity surface-emitting laser (VCSEL) constructed for a wide operating temperature range from 25°C to -50°C sub-freezing temperature, demonstrating 40.1-GHz at -50°C. The optical spectra, junction temperature, and microwave equivalent circuit modeling of a sub-freezing 850-nm VCSEL between -50°C and 25°C are also discussed. Reduced optical losses, higher efficiencies, and shorter cavity lifetimes at sub-freezing temperatures are the leading causes of the improved laser output powers and bandwidths. The e-h recombination lifetime and the cavity photon lifetime are shortened to 113 and 4.1 ps, respectively. Could potentially supercharge VCSEL-based sub-freezing optical links for applications in frigid weather, quantum computing, sensing, aerospace, etc.

Extrinsic Parasitics and Design Considerations on Modulation Bandwidth of 850-nm Vertical Cavity Surface Emitting Lasers

The extrinsic parasitics generally limit the modulation bandwidth of vertical cavity surface emitting lasers (VCSELs) for optical data links and interconnects. In this article, the extrinsic parasitics of 850-nm VCSELs are investigated by three different designs: oxide-only (design A), oxide-implant (design B), and oxide-implant with benzocyclobutene (BCB) (design C). The static and dynamic characteristics of all the designs with~7- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> aperture are measured and compared. The maximum modulation bandwidth of design A is only about 12 GHz mainly limited by the large parasitic capacitances. With a proper proton implantation, a thick nonconducting layer is created inside the device with design B, which dramatically reduces the parasitic capacitances, and thus the bandwidth is increased to about 18 GHz. Based on design B, a 4- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -thick BCB layer with low dielectric constant is employed underneath the bond pad of design C to further reduce the pad capacitance, leading to the highest bandwidth about 24 GHz and the highest modulation current efficiency factor (MCEF) of 10.8 GHz/mA <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . The microwave reflection coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{11}}$ </tex-math></inline-formula> ) parameters of the three designs are measured and compared. Results indicate that design C has the lowest parasitic capacitances and the highest modulation bandwidth, which is preferred for high-speed VCSELs to obtain good static and dynamic performance.

Optical Uplink, D2D and IoT Links Based on VCSEL Array: Analysis and Demonstration

In this paper, vertical cavity surface emitting laser (VCSEL) array is used to establish gigabits/second (Gbps) optical uplink, device-to-device (D2D), and Internet-of-thing (IoT) links, as a supplementary for visible light communication (VLC) and ultra-low latency near-field communication in a typical indoor scenario. The mathematical model based on a modified Monte-Carlo ray-tracing (MMCR) algorithm for VCSEL-based optical wireless communication (OWC) is presented, which takes into account both accuracy and time complexity for the calculation of the channel characteristics including power distribution, impulse response, error analysis, and signal-to-noise ratio (SNR). Simulation firstly takes a global approach to the optical uplink lens design method, compared between Lambertian and Gaussian sources, and then extended six typical line-of-sight (LOS) or non-line-of-sight (NLOS) VCSEL-based OWC models. For demonstration, we adopted a 940-nm VCSEL array and 850-nm single-pixel VCSEL to establish LOS and NLOS systems after measuring the optics-electronics and bandwidth characteristics, respectively. Furthermore, multiple multi-carrier schemes are adopted to improve the OWC performance system based on a 940-nm VCSEL array including uniform-loading orthogonal frequency division multiplexing (OFDM), channel-coded OFDM, bit-loading/ power-allocation OFDM, and OFDM access (OFDMA). Results show that OFDM can effectively decrease the inter-symbol interference (ISI) of the indoor channel and increase the data rate, and the bit/ power-loading method achieves the highest 7.2 Gbps transmission with the bit error rate (BER) within the forward error correction (FEC). All the theoretical and experimental results, for the first time, provide a comprehensive design and optimizing process of VCSEL-based indoor high-capacity OWC systems for future optical uplink, D2D, and IoT applications.

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.

High Speed VCSEL Technology and Applications

Historically optical links up to 100–300m distances are served by light emitting devices in the 850 nm spectral range in combination with multimode glass fibers (MMF). As the silicon scaling continues, a single channel data rate is to double each 24 months. Light-emitting diodes had to be replaced by vertical cavity surface emitting lasers (VCSELs) and the data rate increased from 100 Mb/s to 10 Gb/s. At higher data rates problems with further scaling evolved. To avoid the collapse an anti-waveguiding VCSEL cavity design was invented, applied, and presently serves data links operating up to 50–100 Gb/s per channel. Another requirement in data communication is the bandwidth density scaling, with the number of channels per link increasing approximately 5-fold each 10 years, while keeping a similar space for connectors. A coarse short-wavelength division multiplexing allowing 850 nm, 880 nm, 910 nm, and 940 nm wavelengths in a single MMF is introduced. The bandwidth density increase is also possible by using multicore fiber (MCF) coupled to on-chip VCSEL arrays. The data rates up to 224 Gb/s are already reached by 850nm VCSELs. At such data rates significant transmission distance over MMF is only possible by applying ultra-narrow spectrum VCSELs minimizing the chromatic dispersion effects. On-chip mini-arrays of oxide-confined VCSELs allow a high coupling efficiency to MMFs, a narrow spectrum, a high power, a significant transmission distance at high data rates. Coherent lasing in such arrays allow photon-photon resonance engineering aimed at modulation bandwidths ∼50–100 GHz.

A Scalable 32–56 Gb/s 0.56–1.28 pJ/b Voltage-Mode VCSEL-Based Optical Transmitter in 28-nm CMOS

An optical transmitter (TX) data path is demonstrated in 28-nm CMOS which drives an 850-nm vertical-cavity surface-emitting laser (VCSEL) up to 56 Gb/s non-return-to-zero (NRZ). A dc-coupled single-ended voltage-mode driver employs area- and energy-efficient equalization techniques including slew-rate-based pulse-shaping at <40 Gb/s, active shunt peaking, and a three-tap feed-forward equalizer (FFE) at ≥40 Gb/s, for an optimum TX optical eye. At a 56 Gb/s data rate, the energy efficiency of 1.28 pJ/b from a 1.15 V supply is measured which is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&gt; 2\times $ </tex-math></inline-formula> more energy- and area-efficient than optical TXs reported to date. The highly digital circuit architecture enables the supply scalability to 0.7 and 0.85 V to operate the TX at 32 and 40 Gb/s at 0.56 and 0.77 pJ/b, respectively, which is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\ge 4\times $ </tex-math></inline-formula> more energy-efficient than the prior art.

Effect of Transmission-Line Contact Length on the 50-Gbit/s Data Encoding Performance of a Multimode VCSEL

Directly modulated 850-nm multimode vertical-cavity surface-emitting lasers (MM-VCSELs) with different oxide apertures and transmission microstrip lengths are compared on the transmission performance of the non-return-to-zero on-off keying (NRZ-OOK) and four-level pulse amplitude modulation (PAM-4) data formats. In this work, intrinsic and extrinsic responses of the MM-VCSEL are also discussed concurrently. By tuning the length of the transmission microstrip in VCSEL, the low reflection coefficient and the enhanced 3-dB modulation bandwidth are achieved. The inductance of the transmission microstrip in the series connection with the capacitance in the active region is optimized to reduce the power loss induced by imaginary impedance. The different oxide aperture sizes for MM-VCSEL are also studied to control the capacitance and photon density. More importantly, the 3-dB modulation bandwidth, impedance matching, slope efficiency, relative intensity noise (RIN), and mode partition noise (MPN) for the MM-VCSEL with various designs are discussed to determine the best device with the high-speed transmission capability. The optimal MM-VCSEL with a diameter of 7 µm oxide aperture and a length of 25 µm transmission microstrip successfully demonstrates 50-Gbit/s OOK and 84-Gbit/s PAM4 after using the pre-emphasis technique for future data-center applications.

1550-nm vertical-cavity surface-emitting laser with single-mode power of milliwatts

We report on a 1550-nm vertical-cavity surface-emitting laser (VCSEL) with single mode power of 0.97 mW. The quaternary AlGaInAs quantum well is designed to improve the gain level in an active region. The mesa structure with tunneling capability is designed and fabricated to achieve the efficient carrier injection and the transverse mode guiding. The distributed Bragg reflector (DBR) mirror of 1550 nm VCSEL consists of the semiconductor DBR and outer dielectric DBR. The central wavelength of VCSEL is 1547.6 nm. The maximum output power of 2.6 mW is achieved at 15 ℃, and the maximum single-mode output power is 0.97 mW. The side mode suppression ratio (SMSR) can reach more than 35 dB. The maximum output power decreases with operation temperature increasing. However, the maximum output power of more than 1.3 mW is also gained at 35 ℃. The shift coefficient of the central wavelength varying with the operation current is 0.13 nm/mA. And the wavelength shows a stable shift with the operation current in the single-mode working region, which indicates the application possibility in the field of gas detection.

Enhanced 850-nm SM VCSEL transmission by favorable chirp interaction with fiber dispersion

Single-mode (SM) vertical-cavity surface-emitting lasers (VCSELs), especially those operating around 850 nm, have been studied intensively in recent years for short distance transmission. Despite the demonstrations of increased data rate and system distance, the impact of frequency chirp that is commonly present in directly modulated lasers is an area that needs more detailed studies for 850 nm VCSEL-based systems. In this paper, we explore the interaction between a laser chirp and fiber chromatic dispersion using an 850-nm SM VCSEL over a standard SM fiber that is two-mode at the operating wavelength. Our transmission experiments show that the system can enjoy a benefit from negative fiber dispersion instead of a penalty compared to the back-to-back case, due to the favorable chirp–dispersion interaction, which is also supported by our system bandwidth measurements. Furthermore, we measure the chirp value of the SM VCSEL and conduct modeling using the time-domain pulse concept to illustrate the impact on the chirp–dispersion interaction and explore the optimal chirp parameters for different transmission data rates. Our study indicates a significant system benefit of using 850-nm SM VCSELs with a high bandwidth single-mode fiber around 850 nm due to the favorable chirp–dispersion interaction. Such a benefit can enable high data rate and longer distance system transmission for modern data center applications.

X-band and Ku-band VCSEL-based optoelectronic oscillators using on-chip laser

We present the results of the implementation of two VCSEL-based optoelectronic oscillators (VBO) using one on-chip 850-nm vertical-cavity surface-emitting laser (VCSEL). The reported VBOs are implemented at 10 and 12 GHz using a direct-modulated VCSEL. The laser direct modulation bandwidth is 13.3 GHz. The VBOs performance is described through the phase noise and timedomain stability.

A laser source driver with adjustable amplitude and pulse-width in 130-nm CMOS technology for quantum key distribution experiments.

We present a laser source driver using a 130-nm complementary metal oxide semiconductor technology, named quantum laser source driver 2018 (QLSD2018). QLSD2018 drives the optical source with a current pulse signal, and the output of QLSD2018 has an adjustable pulse-width from 300 ps to 3.8 ns and an adjustable amplitude up to 70 mA. The data rate is up to 625 Mb/s, and the extinction ratio of the optical source (the 1550-nm distributed feedback laser or the 850-nm vertical-cavity surface-emitting laser) driven by QLSD2018 can reach 26 dB. The test results indicate that QLSD2018 can be used in quantum key distribution experiments. Using QLSD2018 on the transmitter side can significantly simplify the peripheral circuit of the optical source.

Stretchable, skin-conformal microscale surface-emitting lasers with dynamically tunable spectral and directional selectivity

Coherent, monochromatic light sources that can intimately integrate with human body and yet offer state-of-the-art optoelectronic performance will create new opportunities in wearable and implantable electronics for a wide range of applications from personalized health monitoring, light therapy, to three-dimensional sensing and security. Here, we report stretchable, electrically driven surface-emitting microlasers capable of being conformally integrated on soft, curvilinear surfaces of biological tissues and providing wafer-level performance under mechanical and thermal environments relevant to skin physiology. GaAs-based microscale 850-nm vertical-cavity surface-emitting lasers derived from epitaxially grown source materials are integrated on a thin, elastomeric membrane in stretchable and thermally robust configurations enabled by printing-based heterogeneous material assemblies. The resulting stretchable, electrically pumped microlasers offer a stable continuous-wave operation under both uniaxial and biaxial tensile strains up to ∼120% in air as well as on the human skin, where the synergistic choices of mechanical strain and underlying heat-transfer medium provide versatile routes to dynamically control the spectral and directional characteristics of lasing.

Performance Investigation of VCSEL-Based Voltage Probe and Its Applications to HPEM Effects Diagnosis of Embedded Systems

The achievable performances of bias-free vertical-cavity surface-emitting laser (VCSEL)-based voltage probe for high-power electromagnetic (HPEM) effects diagnosis of embedded systems are investigated. The minimum driving current needed by VCSEL is calculated to design the probes with high input impedance and high applicable data bit rate (BR) simultaneously. The VCSEL-based probes for 5 V/3.3 V logic family ICs are designed with a kind of 850-nm VCSEL and a family of high-frequency chip resistors. Measurement results show that the VCSEL-based voltage probe's input impedance can maintain higher than 1 kΩ until 300 MHz. The bit error ratio (BER) test shows that the voltage probe is applicable for monitoring digital signals with BR below 280 Mb/s, while the monitoring BER is better than 1E–6 for BR below 200 Mb/s. The processes of disturbance and breakdown effects induced to a minisized drone circuit are successfully captured by the voltage probe in a pulse injection test, demonstrating the compact and fiber output voltage probe's potential value in diagnosing embedded system's HPEM effects in system-level test.

An Analytical Design Method for High-Speed VCSEL Driver With Optimized Energy Efficiency

A universal analytical design approach for laser diode drivers (LDDs) is presented and verified. All design parameters are derived analytically, taking the vertical-cavity surface-emitting laser (VCSEL) characteristics into account. Two optimization strategies are proposed. First, the power consumption is minimized for the highest achievable optical modulation amplitude (OMA) and extinction ratio (ER). Second, the highest achievable data rate (DR) is considered to maximize the energy efficiency. As a result, a simple differential amplifier can be implemented as an LDD to achieve energy efficiencies and DRs comparable or even higher than the one of more complex designs utilizing preemphasis, feedforward equalization, and pulse amplitude modulation. Therefore, drivers designed with this holistic analytical approach accommodate well the demands for high DRs, high energy efficiency, high compactness, high reliability, and low latency in future optical data links. A compact LDD is designed with an active area of only $0.15 \times 0.12$ mm2 in a 130-nm SiGe BiCMOS technology. A DR of 40 Gbit/s with bit error rates $ and an energy efficiency of 2.245 pJ/bit are achieved for an OMA of 1.1 mW and an ER of 10 dB using a 20-GHz 850-nm common-cathode VCSEL. The achieved values deviate less than 10% from the predicted ones, which clearly illustrates the effectiveness of the presented analytical approach. Further improvements are achieved by optimizing the modulation levels and the performance to the link requirements. At 45 and 30 Gbit/s, 1.8 and 1.17 pJ/bit are achieved, respectively.

High-Temperature Insensitivity of 50-Gb/s 16-QAM-DMT Transmission by Using the Temperature-Compensated Vertical-Cavity Surface-Emitting Lasers

In this study, a 50-Gb/s 16-quadrature amplitude modulation discrete multitone (16-QAM-DMT) was applied to an 850-nm oxide-confined vertical-cavity surface-emitting laser (VCSEL) at various temperature ranging from 25 °C to 85 °C. This study reports the highest bit rate for directly modulated VCSELs operating at 85 °C, in addition to demonstrating, for the first time, 16-QAM-DMT with less dependence on slope efficiency (SE) reduction. By using a temperature-compensated VCSEL, this study determined the degradation of the bit error rate (BER) in terms of the signal-to-noise ratio. The analysis results demonstrate that the transmission quality is dominated by optical bandwidth rather than the SE. Therefore, the preleveling technique was applied for error rate recovery to reduce the BER from $\text{6.3}\times \text{10}^{-3}$ to $\text{3.8}\times \text{10}^{-3}$ with a 0.2-dB/GHz preleveling slope. Thus, a low-bandwidth system (nearly 10 GHz) was realized for high data-rate transmissions (50 Gb/s) at 85 °C. The 16-QAM-DMT transmission provides an effective approach to alleviating thermal problems that occur in high-speed optical communication without the requirement of any external amplification circuits.

Theoretical Investigation of State Bistability Between Pure- and Mixed-Mode States in a 1550-nm VCSEL Under Parallel Optical Injection

Based on the spin-flip model, state bistability (SB) between pure- and mixed-mode states in a 1550-nm vertical-cavity surface-emitting laser under parallel optical injection is theoretically investigated. The simulated results show that two types of SB can be observed when the injection light frequency ( $v_{\textrm {inj}}$ ) is smaller than the dominant mode frequency of the free-running laser ( $v_{e}$ ). For type-I SB, which occurs through fixing $v_{\textrm {inj}}$ and scanning injection power ( $P_{\textrm {inj}}$ ) along different routes, there exist two hysteresis loops in which the laser operates at pure- or mixed-mode state, depending on the variation route of $P_{\textrm {inj}}$ , and the hysteresis loop width increases sharply for increasing $\vert \Delta \nu \vert $ ( $\Delta v = v_{\textrm {inj}}- v_{e}$ ) with relatively strong $P_{\textrm {inj}}$ whereas the loop width increases slowly for relatively weak $P_{\textrm {inj}}$ , in agreement with our recent experimental report. Furthermore, type-II SB is also investigated through fixing $P_{\textrm {inj}}$ and scanning $v_{\textrm {inj}}$ along different routes. There also exist two hysteresis loops in which the laser may operate at a pure- or mixed-mode state. With an increase of $P_{\textrm {inj}}$ , the hysteresis loop width located at lower $\vert \Delta \nu \vert $ increases sharply and then decreases after reaching a maximum. However, for the hysteresis loop located at higher $\vert \Delta \nu \vert $ , the loop width gradually increases with the increase of $P_{\textrm {inj}}$ .

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AbstractThe emission-line width for 850-nm single-mode vertical-cavity surface-emitting lasers based on InGaAs/AlGaAs quantum wells is studied. The width of the emission line for a laser with a 2-μm oxide current aperture attains it minimum (~110 MHz) at an output power of 0.8 mW. As the optical output power is further increased, anomalous broadening of the emission line is observed; this is apparently caused by an increase in the α-factor as a result of a decrease in the differential gain in the active region under conditions of increased concentration of charge carriers and of high internal optical losses in the microcavity. The α-factor is estimated using two independent methods.

Emission-Line Width and α-Factor of 850-nm Single-Mode Vertical-Cavity Surface-Emitting Lasers Based on InGaAs/AlGaAs Quantum Wells

The emission-line width for 850-nm single-mode vertical-cavity surface-emitting lasers based on InGaAs/AlGaAs quantum wells is studied. The width of the emission line for a laser with a 2-μm oxide current aperture attains it minimum (~110 MHz) at an output power of 0.8 mW. As the optical output power is further increased, anomalous broadening of the emission line is observed; this is apparently caused by an increase in the α-factor as a result of a decrease in the differential gain in the active region under conditions of increased concentration of charge carriers and of high internal optical losses in the microcavity. The α-factor is estimated using two independent methods.