Linewidth-power Product Research Articles

Linewidth Study of InAs–InGaAs Quantum Dot Distributed Feedback Lasers

The linewidth of laterally loss-coupled distributed feedback (DFB) lasers based on InAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) is investigated. Narrow linewidth operation of QD devices is demonstrated. A linewidth-power product less than 1.2 MHz /spl middot/ mW is achieved in a device of 300-μm cavity length for an output power up to 2 mW. Depending on the gain offset of the DFB modes from the QD ground state gain peak, linewidth rebroadening or a floor is observed at a cavity photon density of about 1.2-2.4×10/sup 15/ cm/sup -3/, which is much lower than in QW lasers. This phenomenon is attributed to the enhanced gain compression observed in QDs.

Bragg Section Effects on Linewidth and Lineshape in 1.55-<tex>$mu $</tex>m DBR Tunable Laser Diodes

We report linewidth and lineshape measurements on 1.55-μm distributed Bragg reflector tunable laser diodes using the heterodyne method. We measure Lorentzian linewidths ranging from 3.5 to 25 MHz without any passive section injection. Biasing the Bragg section enhances the residual linewidth up to 6.2 MHz, changes the linewidth-power product, and brings an additional Gaussian lineshape. The feedback sensitivity of the overall setup is also demonstrated.

Polarization-resolved linewidth-power product of a vertical-cavity semiconductor laser

We have studied the linewidth around threshold of the lasing and nonlasing polarization mode (x and y) in a TEM00 oxide-confined vertical-cavity semiconductor laser (VCSEL). We experimentally demonstrate: (i) that the nonlasing mode shows surprising behavior when the lasing mode passes through threshold, and (ii) that the polarization fluctuations in a VCSEL are limited by the same quantum noise that sets its finite Schawlow–Townes linewidth.

High-power 1.55-μm mass-transport-grating DFB lasers for externally modulated systems

High-output-power operation of 1.55-/spl mu/m-wavelength distributed-feedback (DFB) lasers with a novel mass-transport grating (MTG) structure which is composed of InAsP buried with InP are reported. To improve high output power characteristics, we have investigated the influence of the width of the active layer on the light output power and the spectral linewidth at high injection current. It is confirmed that the increase of the active layer width is effective to realize high output power and to reduce the linewidth power product. The fabricated lasers show high single-longitudinal-mode output power of 180 mW, which is the highest value reported for 1.55-/spl mu/m DFB lasers. They also exhibit narrow spectral linewidths less than 0.3 MHz and low noise characteristics of -159 dB/Hz. Moreover, we have obtained the mean time to failure of longer than 10/sup 5/ h with a lifetime test over 200 h at 50/spl deg/C.

Spectral linewidth of a free-running continuous-wave single-frequency external-cavity quantum-well InGaAs/AlGaAs diode laser

The linewidth of a room-temperature free-running 940-nm cw single-frequency 1-mm-long quantum-well InGaAs/AlGaAs diode laser in a 10-cm-long external cavity has been measured as a function of the output power by the heterodyne technique. The power-dependent component of the observed Lorentzian linewidth displays a linewidth–power product of 28 ± 4 kHz mW, which is 2 orders of magnitude larger than that predicted from the spontaneous emission model. The shot-noise model of Welford and Mooradian [Appl. Phys. Lett. 40, 560 (1982)] for the power-independent linewidth has been extended to account for the observed power-dependent linewidth.

Optimization of planar Be-doped InGaAs VCSEL's with two-sided output

Threshold current, output power, wall-plug efficiency and operating voltage of epitaxially grown InGaAs-AlGaAs planar vertical-cavity surface-emitting lasers (VCSEL's) are strongly influenced by the electrical and optical properties of the p-doped Bragg reflector. Here we study in some detail the dependence of output behavior on the composition, interface grading, and modulation doping of the Be-doped AlGaAs-GaAs Bragg reflector. Using optimized p-doped mirrors VCSEL's with low threshold current densities of 300 A/cm/sup 2/, low driving voltages of 1.6 V and high wall-plug efficiencies of 17.6% are obtained. Transverse single-mode emitting devices show a record low emission linewidth of 30 MHz and a linewidth-power product of 2.2 mW/spl middot/MHz. >

Continuous wave spectral width of surface-emitting lasers

Measurements of the continuous wave spectral width of GaAs gain-guided surface-emitting lasers are reported. The linewidth is found to vary inversely as the output power with a linewidth-power product of 92 MHz. This value agrees well with the calculated result.

Linewidth and α-factor in AlGaAs/GaAs vertical cavity surface emitting lasers

Measurements of the α-factor, the linewidth-power product, and the differential gain in AlGaAs/GaAs vertical cavity surface emitting lasers are presented. The linewidth power product of 95 MHz mW which results in the α-factor of 3.7 is obtained. The α-factor as a ratio of the refractive index and gain derivatives with respect to the carrier density is also estimated. From the small signal modulation measurements of the resonance frequency, a differential gain of 3.7×1016 cm2 is obtained. The estimate of differential effective index is made difficult by an anomalously strong dependence of the emission wavelength on injection current.

Linewidth enhancement factor of vertical-cavity surface-emitting laser diodes

The linewidth enhancement factor /spl alpha/ characterizes the performance of semiconductor lasers, such as spectral linewidth broadening for CW operation and wavelength chirping under high-frequency modulation. We have for the first time determined the /spl alpha/ factor of vertical-cavity surface-emitting lasers from the measured linewidth-power product and more precisely from the frequency to amplitude modulation ratio at high-frequency current modulation. Both methods provide the same /spl alpha/ factor of 3.7. The minimum linewidth was as narrow as 70 MHz and the linewidth-power product was 5.4 MHz/spl middot/mW neglecting the residual linewidth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Corrugation-Pitch-Modulated Distributed Feedback Lasers with Ultranarrow Spectral Linewidth

We demonstrate that the spectral linewidth of distributed feedback (DFB) semiconductor lasers can be reduced by suppressing the longitudinal spatial hole-burning (SHB) effect. We confirm that the minimum spectral linewidth of lattice-matched multiple-quantum-well λ/4-shifted DFB (lattice-matched MQW-λ/4-DFB) lasers is limited by the SHB effect and we show that a corrugation-pitch-modulated (CPM) grating structure reduces the SHB effect while maintaining stable single-mode oscillation: at an output power of 25 mW, a lattice-matched MQW-CPM-DFB laser gives a spectral linewidth of 56 kHz. We further introduced the strained MQW structure into the CPM-DFB laser to obtain a narrower spectral linewidth. Introducing a 1.0% compressively strained MQW active layer into a CPM-DFB laser gives a spectral linewidth of 3.6 kHz at 55-mW output power and a linewidth floor (residual linewidth for extrapolated infinite output power) of 2 kHz and results in a linewidth-power product of 140 kHz·mW.

The optical gain coupling saturation effect on the linewidth property of complex coupled DFB lasers

The cavity length dependence of the linewidth property of complex coupled (CC) DFB lasers is investigated. In CC-DFB lasers with a fixed geometrical structure of the active region, the cavity length strongly affects the ratio of index coupling strength to the gain coupling one, which results in the variation of the effective linewidth enhancement factor /spl alpha//sub eff/. It is also found that pure gain coupled (GC) DFB lasers have the saturation characteristic in the linewidth power product, with the extension of the cavity length, These results indicate the importance of the selection of the cavity length in CC-DFB lasers. >

Linewidth enhancement factor in InGaAsP/lnP modulation-doped strained multiple-quantum-well lasers

Reduction of the linewidth enhancement factor /spl alpha/ is studied in InP-based strained multiple-quantum-well (MQW) lasers. Theoretical analysis shows that the /spl alpha/-parameter is greatly reduced in modulation-doped strained MQW lasers and may be zero while keeping positive gain. The experimental evaluation exhibits a very small /spl alpha/-parameter of around 1 in InGaAsP/InP modulation-doped strained MQW lasers. As a result of the small /spl alpha/-parameter, the linewidth-power product is effectively reduced in 1.5-/spl mu/m DFB lasers with the modulation-doped strained MQW structure. A narrow spectral linewidth around 100 kHz was also obtained reproducibly in 1.5-/spl mu/m modulation-doped strained MQW DFB lasers. >

Strained multiquantum-well corrugation-pitch-modulated distributed feedback laser with ultranarrow (3.6 kHz) spectral linewidth

Introducing a 1% compressive strained multiquantum-well (MQW) active layer into a corrugation-pitch-modulated distributed feedback (CPM-DFB) laser reduces the linewidth floor (residual linewidth for extrapolated infinite output power) to 2 kHz and results in a linewidth-power product of 140 kHz mW. Strained MQW CPM-DFB lasers produced a 55 mW output with a spectral linewidth of only 3.6 kHz.

Spectral linewidth reduction in partly gain-coupled 1.55 μm strained multiple quantum-well distributed feedback laser

The effective linewidth enhancement factor αeff and the spectral linewidth Δυ are reported for the first time for a partly gain-coupled distributed feedback laser with a gain corrugation in a strained multiple quantum-well active region emitting at 1.55 μm wavelength. The αeff value is estimated to be as low as 1 and is independent of the laser cavity length L, while a decrease in the linewidth-power product ΔυPo with L has been observed, with a measured linewidth of 2 MHz at 10 mW optical output power for 300-μm-long devices. The experimental results demonstrate that a small αeff is obtained due to the strain and the gain coupling effects, while further reduction of the spectral linewidth with L is the result of longitudinal index coupling effects.

Very narrow-linewidth (70 kHz) 1.55 μm strained MQW DFB lasers

A linewidth as narrow as 70 kHz together with a linewidth-power product of 0.2 MHz.mW have been achieved on 1450 μm long compressively strained quantum well uniform DFB lasers. The results are achieved through a reduction of the phase-amplitude coupling factor, which is measured to be 1.7 times lower than in unstrained devices.

Numerical study of the linewidth of a semiconductor laser: Effect of saturation.

In this paper we calculate the intensity and carrier-density distribution inside a Fabry-P\'erot-type semiconductor laser with arbitrary facet reflectivities. We derive an expression for the linewidth of such a laser operated well above threshold, when the inversion saturates and the carrier-density distribution becomes nonuniform. Results are given for the frequency and carrier-density dependence of the linewidth-enhancement factor of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As lasers. We find that saturation results in substantial nonuniformity of the distributions of the spontaneous-emission factor and the linewidth-enhancement factor. Although these factors codetermine the laser linewidth, their combined effect on the linewidth-power product is found to be negligible in most cases of practical importance. However, a small power-independent contribution to the laser linewidth is predicted for large mirror asymmetries.

Low-threshold and narrow-linewidth 1.5 μm compressive-strained multiquantum-well distributed-feedback lasers

1.5 μm compressive-strained multiquantum-well distributed-feedback lasers have been fabricated and characterised. 5.5 mA threshold current, 1 MHz mW linewidth-power product, and 600 kHz minimum linewidth were measured on 500 μm long devices. Measured threshold current as low as 2.2 mA was also obtained on 150 μm long devices. Both low threshold and narrow linewidth are attributed to the reduced transparency current and linewidth enhancement factor due to the effect of strain.

Submilliamp threshold vertical-cavity laser diodes

We report for the first time room-temperature, continuous-wave operation of individual vertical-cavity laser diodes with submilliampere threshold currents. A single quantum well active region emitting at 979 nm surrounded by GaAs/AlAs Bragg reflector mirrors was used. Threshold currents were as low as 0.7 mA. A record low linewidth-power product of 5 MHz mW and a linewidth as narrow as 85 MHz was measured. High yield and good uniformity were demonstrated.

Corrugation-pitch-modulated MQW-DFB laser with narrow spectral linewidth (170 kHz)

A spectral linewidth of 170 kHz was achieved at a 1.5- mu m wavelength in a 1200- mu m long corrugation-pitch-modulated (CPM) distributed feedback (DFB) laser having multiple quantum wells (MQW). This is the narrowest linewidth ever reported for InGaAsP DFB lasers. The CPM structure is effective in maintaining the single longitudinal mode at a high output power by suppressing the spatial hole burning effect. The linewidth-power product is 400 kHz-mW, and the extrapolated residual spectral linewidth is 150 kHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Highly coherent long cavity GaAs/AlGaAs single quantum-well lasers

Measurements of the spectral properties of ridge waveguide graded index separate-confinement heterostructure single-quantum-well GaAs/AlGaAs lasers are discussed. Long cavity lasers (800 mu m) exhibit remarkably pure single-longitudinal mode spectra under continuous operation in spite of the short cavity mode spacing. At an output power of 5 mW, the sidemode suppression exceeds 24 dB and the linewidth is 1.5 MHz. The linewidth-power product is 6.4 MHz-mW. Measurements of the linewidth power product as a function of cavity length L gives an L/sup -2/ dependence in agreement with theory for lasers with small internal loss. The results are used to deduce the linewidth enhancement factor alpha at the gain peak wavelength ant its dependence on the excitation level. A decrease in alpha was observed for lasers operating at the second quantized state due to a recovery of the differential gain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>