Single-Wavelength Silicon Evanescent Lasers

The silicon evanescent device platform provides electrically pumped active device functionality on a low-loss silicon-on-insulator waveguide platform. We present here recent research in the area of single-wavelength silicon evanescent lasers that utilize distributed feedback, distributed Bragg reflector (DBR), and sampled grating (SG) DBR laser topographies.      

High efficiency widely tunable SGDBR lasers for improved direct modulation performance

We report on two novel approaches to improve the differential quantum efficiency (DQE) of widely tunable 1.55-/spl mu/m lasers: the bipolar cascade sampled grating distributed Bragg reflector (BC-SGDBR) laser and the gain-levered SGDBR (GL-SGDBR) laser. Each is fabricated on a robust InGaAsP/InP photonic integrated circuit platform. The lasers demonstrate improved direct modulation performance over conventional SGDBR lasers. The BC-SGDBR laser was also monolithically integrated with a semiconductor optical amplifier and photodetector receiver in order to perform wavelength conversion. Error free wavelength conversion at 2.5 Gb/s and improvements in conversion efficiency are demonstrated.     

Advanced Ti:Er:LiNbO3 waveguide lasers

This paper reviews the latest developments of diode-pumped Ti,Er:LiNbO₃ waveguide lasers emitting at wavelengths around 1.5 μm. In particular, harmonically mode-locked lasers, Q-switched lasers, distributed Bragg reflector (DBR)-lasers, and self-frequency doubling lasers are discussed in detail. Supermode stabilized mode-locked lasers have been realized using a coupled cavity concept; a side mode suppression ratio of 55 dB has been achieved at 10-GHz pulse repetition rate with almost transform limited pulses. Q-switched lasers with a high extinction ratio (>25 dB) intracavity electrooptic switch emitted pulses with a peak power level up to 2.5 kW and a pulsewidth down to 2.1 ns at 1-kHz repetition frequency. Numerical simulations for both lasers are in a good, almost quantitative agreement with experimental results. A DBR-laser of narrow linewidth (≈3 GHz) with a permanent (fixed) photorefractive grating and 5 mW output power has been realized. Self frequency doubling lasers have been fabricated with a periodic microdomain structure inside an Er-doped laser cavity; simultaneous emission at the fundamental wavelength, 1531 nm, and at the second harmonic wavelength, 765 nm, has been obtained     

Accuracy on emitted wavelengths in DFB laser arrays resulting fromthe longitudinal mode selection mechanism

We compare the merit of various longitudinal mode selection mechanism in distributed feedback laser for application as arrayed sources in the wavelength-division multiplexing context. Phase-shifted and complex-coupled laser arrays have been fabricated and characterized. We propose that the dispersion on the emitted wavelengths is strongly correlated to the field localization along the cavity. By comparison of the dispersion values measured within the laser arrays, we can conclude that the complex grating with gain is the best suitable mechanism for high wavelength accuracy and stability, due to the strong and distributed localization of the field in the gain sections of each period of the grating     

1.5-/spl mu/m wavelength narrow stripe distributed reflector lasers for high-performance operation

1.5 /spl mu/m-wavelength narrow stripe distributed reflector (DR) lasers consisting of first-order vertical grating (VG) and distributed Bragg reflector (DBR) mirrors were realized by deeply etching the as-grown wafer and passivating the etched surface by SiO/sub 2/. Design consideration, fabrication, and lasing performances were studied. A low threshold current of 2.8 mA and a differential quantum efficiency of 28% from the front facet were achieved for a 1.3 /spl mu/m stripe width and a 150 /spl mu/m cavity length under room temperature (RT) continuous wave (CW) operation. Details of threshold behavior of these lasers are presented. Lasing performances of FP and DBR lasers are also described for comparison.     

Inherently mode-hop-free distributed Bragg reflector (DBR) laser array

An inherently mode-hop-free tuning of 4.5 nm was achieved with a short-active-region distributed Bragg reflector (DBR) laser. The DBR laser contains no phase-adjustment electrode, which greatly simplifies the tuning. Fast tuning of several nanoseconds and error-free operation for 2.5-Gbit/s direct modulation were also observed. A 4-ch array of short-active-region DBR lasers achieved wavelength tuning of 12.5 nm.     

Long-term wavelength stability of 1.55-μm tunable distributedBragg reflector lasers

To investigate physical mechanisms involved in long-term wavelength drift of tunable distributed Bragg reflector (DBR) laser, the evolution of the tuning characteristics as well as the Bragg section intensity modulation response of several DBR lasers have been simultaneously assessed by current injection in the Bragg section only. Current versus voltage I(V) characteristics under aging have also been recorded to compare the role of leakage current and nonradiative recombination defect evolution. The wavelength drift as well as the carrier lifetime of the tuning section varies following an exponential law A+Bexp(-t_A/τ) versus aging time t_A. The time constant τ is aging temperature and Bragg current dependant. The carrier lifetime decreases with time indicating a wavelength drift mainly due to nonradiative recombination defect increase. Modeling of the I_B(V) and λ_B(I_B) characteristics is presented, that fits nicely the experimental data. The exponential form of the wavelength drift is used to propose novel and adequate burning conditions of DBR lasers     

Frequency plan and wavelength switching limits for widely tunablesemiconductor transmitters

As wavelength-division-multiplexing (WDM) channel spacing continues to decrease in size, and with the application of tunable lasers in dense wavelength-division-multiplexing (DWDM) systems, we demonstrate the ability of tunable semiconductor lasers to cope with demanding channel spacing and inevitable low frequency setting error. By finding the stable operating points of a single tunable laser at the desired frequencies, using advanced software a lookup table to drive the laser was generated. Once the drive currents to access 2000 channels with the laser are found, their frequency setting error and side mode suppression ratio (SMSR) were found. These results open up new possibilities for DWDM access networks as well as providing a limit of achievement for channel density in the network. Meanwhile, since the sampled-grating distributed Bragg reflector (SG-DBR) laser is among the most attractive sources for DWDM, it is important to investigate its wavelength switching characteristics. This behavior will affect wavelength routing and the capability limits for channel reallocation in future networks. We present new detailed experimental studies on a high-speed SG-DBR laser by using a Fabry-Perot interferometer technique adapted for the noncontinuous wave case. Measurements of fast intramodal (i.e., cavity mode) and intermodal (i.e., supermode) wavelength switching and insights into the device's dynamic behavior are obtained. Implications are given for transmitter design in dynamic wavelength routing and channel reallocation     

Characteristics of InGaN-AlGaN multiple-quantum-well laser diodes

We demonstrate room-temperature pulsed current-injected operation of InGaAlN heterostructure laser diodes with mirrors fabricated by chemically assisted ion beam etching. The multiple-quantum-well devices were grown by organometallic vapor phase epitaxy on c-face sapphire substrates. The emission wavelengths of the gain-guided laser diodes were in the range from 419 to 432 nm. The lowest threshold current density obtained was 20 kA/cm² with maximum output powers of 50 mW. Longitudinal Fabry-Perot modes are clearly resolved in the high-resolution optical spectrum of the lasers, with a spacing consistent with the cavity length. Cavity length studies on a set of samples indicate that the distributed losses in the structure are on the order of 30-40 cm^-1     

Resonators and materials for organic lasers based on energytransfer

Optically pumped lasers have been fabricated with organic/polymeric materials capable of charge transport. The active materials employed are doped films with small molecule hosts and dye, oligomer, and conjugated polymer emitters. In these materials, the excited states created in the host are transferred nonradiatively to the guest molecules which are the emitters. This energy transfer results in very low absorption losses at the emission wavelength and relatively low-threshold powers for the onset of stimulated emission. Such gain media have been successfully included in many types of resonators including whispering-gallery mode, photonic bandgap, and distributed Bragg reflector (DBR) based resonators. A number of novel patterning and fabrication procedures have been developed for organic-based lasers     

High-power semiconductor red laser arrays for use in photodynamic therapy

Semiconductor laser ridge arrays emitting 250 mW at a wavelength of 635 nm are designed for photodynamic therapy applications. Although ridge laser arrays are less efficient than broad area lasers, they are more reliable and can produce higher power from the same lateral width due to thermal considerations. An analytic expression for the active layer temperature of the laser array as a function of the ridge spacing, number of ridges, and width is derived and has excellent agreement with a finite-element analysis. This analytic expression allows optimization of the laser and the submount geometry to minimize the active region temperature with the constraint of a small submount, heat sink, and package.     

InGaAs angled-grating distributed Bragg reflector lasers integrated with grating outcoupler for collimation

An angled-grating distributed Bragg reflector (DBR) laser integrated with a grating outcoupler (GO) is proposed and demonstrated. It consists of an angled broad-area gain stripe surrounded by angled DBR (/spl alpha/-DBR) gratings parallel to the gain stripe and two DBR gratings for feedback at the stripe ends, and a GO for emitting a collimated output beam. Due to distributed Bragg reflection in the /spl alpha/-DBR gratings, filamentation can be suppressed. Single-mode lasing up to 69 mW peak power was obtained under pulse condition. A collimated output beam with divergence angles, close to the diffraction limit, was obtained. These results confirm the effectiveness of the /spl alpha/-DBR gratings.     

Demonstration of negative dispersion fibers for DWDM metropolitanarea networks

We present a detailed experimental and theoretical study, showing that a novel nonzero dispersion-shifted fiber with negative dispersion enhances the capabilities of metropolitan area optical systems, while at the same time, reducing the system cost by eliminating the need of dispersion compensation. The performance of this dispersion-optimized fiber was studied using different types of optical transmitters for both 1310- and 1550-nm wavelength windows and for both 2.5-and 10-Gb/s bit rates. It is shown that this new fiber extends the nonregenerated distance up to 300 km when directly modulated distributed feedback (DFB) laser transmitters at 2.5 Gb/s are used. The negative dispersion characteristics of the fiber also enhance the transmission performance in metropolitan area networks with transmitters that use electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers, which are biased for positive chirp. In the case of 10 Gb/s, externally modulated signals (using either EA-DFBs or external modulated lasers using Mach-Zehnder modulators), we predict that the maximum reach that can be accomplished without dispersion compensation is more than 200 km for both 100- and 200-GHz channel spacing. To our knowledge, this is the first demonstration of the capabilities of a nonzero dispersion-shifted fiber with negative dispersion for metropolitan applications     

Recent advances in DFB lasers for ultradense WDM applications

State-of-the-art distributed feedback (DFB) laser modules integrated with a wavelength monitor are presented that provide excellent wavelength stability. By adopting unique and compact configuration, wavelength deviations of as small as a few picometers have been achieved. The laser modules are improved also in the scope of high power, high reliability, and wavelength tunability. Reliability test results of the DFB laser diodes and modules confirm a sufficiently long lifetime of more than 25 years and a small wavelength drift of less than /spl plusmn/3 pm. The developed laser modules are fully applicable to ultradense wavelength-division multiplexing applications with the current narrowest channel spacing of 25 GHz.     

Tapered-cavity surface-emitting distributed-Bragg-reflector semiconductor lasers: modeling and experiment

We report on the design and fabrication of tapered cavity grating coupled surface-emitting distributed Bragg reflector (DBR) lasers in the 980-nm regime. A curved second-order grating is used at the end of a tapered gain section to provide feedback as well as collimated surface out-coupling. A detailed numerical analysis shows that operation up to approximately twice the threshold is possible without significant degradation of the far field. Near diffraction-limited collimated surface-emitting output with moderate power of about 150 mW is obtained under continuous operation.     

Frequency range extension of actively mode-locked lasers integratedwith electroabsorption modulators using chirped gratings

We have fabricated actively mode-locked lasers integrated with electroabsorption modulators and chirped gratings. A chirped grating with a large chirp rate of 1.45 Å/μm can be realized by using multiphase-shifted patterns. Short pulses of 4-6 ps were generated over a wide frequency range from 18.9-19.8 GHz. We observed jumps in the wavelength during detuning. These jumps arise from multiple lobes in the reflectivity spectrum. It is found that the wavelength jumps cause increases in the intensity noise. We showed that by reducing the grating length from 150 to 100 μm the sidelobes were suppressed and the detuning frequency range of over 1% could be realized. A smaller pulsewidth was obtained for the negatively-chirped gratings when compared to the positively chirped gratings     

Nonlinear Effects in Widely Tunable ECSL With Sampled Fiber Grating

A theoretical study of nonlinear effects in the external cavity semiconductor laser (ECSL) with sampled fiber grating is presented; such effects are responsible for asymmetric side-mode suppression ratio or power characteristics, and hysteresis phenomena. Impacts of material and geometrical parameters on nonlinear effects of such lasers were analyzed and discussed. Apodization and interleaved technologies were demonstrated to be helpful with expanding tuning range and suppressing nonlinear effects. Based on this, a new ECSL with interleaved sampled grating has been proposed.      

Polymeric 16×16 arrayed-waveguide grating router usingfluorinated polyethers operating around 1550 nm

A 16×16 arrayed waveguide grating (AWG) router with a channel spacing of 0.8 nm (100 GHz) operating around the 1550-nm wavelength has been fabricated using newly synthesized fluorinated polyethers. It has a good processibility and a high thermal stability up to 510°C. The propagation loss of the buried-channel waveguide is about 0.4 dB/cm at the 1550-nm wavelength. The on-chip insertion loss ranges from 5.5 to 11 dB and the crosstalk is less than -27 dB. The AWG router shows good cyclic rotation property of the wavelength channels with an error smaller than 0.03 nm     

Monolithically integrated eight-channel WDM modulator with narrow channel spacing and high throughput

We demonstrate an eight-channel wavelength division multiplexing (WDM) modulator module that monolithically integrates arrayed waveguide gratings and semiconductor optical amplifiers and electroabsorption optical modulators arrays. The compact module can generate individual optical signals for each WDM channel with low optical and electrical crosstalk. We show two configurations for the narrow channel spacing of 25 GHz and high throughput of beyond 80 Gb/s. Combining this WDM modulator with a multi-wavelength light source is a promising approach to creating a compact WDM optical transmitter.     

Room temperature operation of InAs quantum-dot laser utilizing GaAs photonic-crystal-slab-based line-defect waveguide with optical pump

We have successfully fabricated InAs quantum dots (QDs) embedded in a line-defect waveguide in an air-bridge type GaAs photonic-crystal slab (PCS) and observed laser action from optical-pumping. This lasing is found to occur without any optical cavity, such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the triple-lines defect mode enables us to specify the lasing wavelength as that at the band edge. From this fact, it follows that the distributed feedback mechanism at the band edge with an infinitely small group velocity is responsible for the present lasing.