IEEE Photonics Technology Letters

Published by Institute of Electrical and Electronics Engineers
Print ISSN: 1041-1135
We report a new method for fabricating an optofluidic waveguide that is compatible with polydimethylsiloxane (PDMS). The light path follows the microfluidic channels, an architecture that can maximize detection efficiency and make the most economic use of chip area in many lab-on-chip applications. The PDMS-based microfluidic channels are coated with Teflon amorphous fluoropolymers (Teflon AF) which has a lower refractive index (n = 1.31) than water (n = 1.33) to form a water/Teflon AF optical waveguide. Driven by a vacuum pump, the Teflon AF solution was flowed through the channels, leaving a thin (5-15 µm) layer of coating on the channel wall as the cladding layer of optical waveguides. This coating process resolves the limitations of spin-coating processes by reducing the elasticity mismatch between the Teflon AF cladding layer and the PDMS device body. We demonstrate that the resulting optofluidic waveguide confines and guides the laser light through the liquid core channel. Furthermore, the light in such a waveguide can be split when the fluid flow is split. This new method enables highly integrated biosensors such as lab-on-chip flow cytometers and micro-fabricated fluorescence-activated cell sorter with on-chip excitation.
We demonstrate a wide tuning range high-speed wavelength-swept semiconductor laser based on a polygon scanning filter that is common to two laser cavities. Linear wavelength tuning was achieved over 145 nm around 1310 nm at a tuning repetition rate of 20 kHz. The wavelength tuning filter is expandable to accommodate multiple semiconductor optical amplifiers for further widening of the laser wavelength tuning range.
We demonstrate a compact high-power rapidly swept wavelength tunable laser source based on a semiconductor optical amplifier and an extended-cavity grating filter. The laser produces excellent output characteristics for biomedical imaging, exhibiting >4-mW average output power, <0.06-nm instantaneous linewidth, and >80-dB noise extinction with its center wavelength swept over 100 nm at 1310 nm at variable repetition rates up to 500 Hz.
(a) CR spectrum with and without bandpass optical filters. The power of unfiltered CR is 0.54 mW. Inset: (right) autocorrelation and (left) far field saturated visible image of typical CR output. (b) Noise spectra solid lines: CR measured with and without bandpass optical filters with the same detected optical power; red dashed-dotted line: MOPA oscillator; blue dashed line: full MOPA; black line: noise floor of the photoreceiver.  
We investigate the noise properties of a femtosecond all-fiber Cherenkov radiation source with emission wavelength 600 nm, based on an Yb-fiber laser and a highly nonlinear photonic crystal fiber. A relative intensity noise as low as 103 dBc/Hz, corresponding to 2.48% pulse-to-pulse fluctuation in energy, is observed at the Cherenkov radiation output power of 4.3 mW, or 150 pJ-pulse energy. This pulse-to-pulse fluctuation is at least 10.6-dB lower compared to spectrally sliced supercontinuum sources traditionally used for ultrafast fiber-based generation at visible wavelengths. Low noise makes all-fiber Cherenkov sources promising for biophotonics applications such as multiphoton microscopy, where minimum pulse-to-pulse energy fluctuation is required. We present the dependency of the noise figure on both the Cherenkov radiation output power and its spectrum.
A narrow-band apodized fiber Bragg grating was fabricated with a phase mask method that incorporated a fluorescence monitoring technique. The grating length was 100 mm and the 3-dB reduced bandwidth was as small as 0.04 nm. The 20-dB reduced bandwidth was 0.1 nm at a center wavelength of 1554 mm. A single longitudinal oscillation mode of a 10-GHz pulse train from a mode-locked fiber laser was successfully extracted using the grating.
The optical frequency of a 1.5- mu m distributed-Bragg-reflector (DBR) laser is stabilized against that of a master laser by heterodyne-type frequency locking with a phase-locked loop (PLL). Despite the laser's wide linewidth of 16 MHz, stable PLL operation with an optical hold-in range of 26 GHz is realized, and residual frequency fluctuations are reduced to 0.04 Hz at an averaging time of 500 s. The combination of DBR laser and PLL is, therefore, suitable for frequency-controlled light sources. The offset error from the settled frequency caused by the bandlimited beat spectrum is discussed.< >
A uniform In/sub 0.05/Ga/sub 0.95/As ternary substrate was grown by using liquid encapsulated Czochralski (LEC) technique with a method of supplying GaAs source material at a constant temperature, and InGaAs/InGaAsP strained single quantum well (SQW) lasers were fabricated on the substrate for the first time. The lasers lased at 1.03 /spl mu/m and exhibited low threshold current density of 222 A/cm/sup 2/ and excellent characteristic temperature of 221 K, showing that the ternary substrate has a sufficient quality for laser fabrication.< >
An ultralow-threshold 1.3-/spl mu/m InGaAsP-InP 10-element monolithic laser array is achieved through careful optimization of a strained-MQW active layer. This array has a record-low threshold current, highly uniform threshold current characteristics (1.3/spl plusmn/0.09 mA and slope efficiency of 0.37/spl plusmn/0.01 W/A), and long-term reliability. This array is suitable as light sources for a parallel high-density optical interconnection system.< >
A large spontaneous emission factor of 0.1 was evaluated in a 1.56 /spl mu/m-GaInAsP microdisk injection laser operating under continuous-wave condition with a threshold current of 50 /spl mu/A. Some clear evidence of the large spontaneous emission factor, i.e., superlinear light-current characteristics and nonclamped carrier-current characteristics, were observed. It was confirmed that the matching of the lasing wavelength to the spontaneous emission allowed larger spontaneous emission factor.
We successfully fabricated channel waveguides utilizing photonic crystal (PhC) dispersion having a very low loss of ∼0.1 dB/mm by the autocloning method. We achieved the loss reduction by improving the fabrication condition and adjusting the core width. The result of propagation loss can be regarded as a "milestone," and it enables us to realize practical PhC circuits that contain functional components such as chromatic dispersion equalizers and high Q wavelength filters.
Waveguide losses as low as 0.1 dB/cm at a wavelength of /spl lambda/=1.3 /spl mu/m have been achieved in SOI rib waveguides fabricated by wet chemical etching. The single-mode waveguides have a large cross section yielding field mismatch losses to a standard single-mode fiber of only 0.17 dB/facet (both polarizations). Polarization independent fiber-chip-fiber insertion losses of 0.9 dB (best 0.5 dB) at a chip length of 60 mm have been measured.
Uniform avalanche gains of 40 to 80 are observed in InAs/sub y/P/sub 1-y/ (0.1<y<0.3) grown on p-type InP substrates, with lattice strains as high as 1%. These devices have a primary dark current as low as 200 pA, with the dark current increasing, in some cases, to only 1 nA at 90% of the breakdown voltage. The ionization coefficients for both electrons and holes are approximately equal, and the generation of secondary carriers is primarily via ionization of defect levels. The InAs/sub yx/P/sub 1-y/ layers have the same lattice parameter as compositions of In/sub x/Ga/sub 1-x/ which absorb light wavelengths /spl les/2.1 /spl mu/m, suggesting that this material combination has potential for use in avalanche photodiodes that detect moisture in LIDAR and gas-sensing applications in the mid-IR spectral range.< >
We introduce a new all-optical technique to transmit a spectrum-sliced incoherent channel with its optical bandwidth smaller than the conventional limit. As a demonstration, we reduce the optical bandwidth to only 0.1 nm for the 2.5-Gb/s incoherent channel transmission. Even though the signal-to-noise ratio (SNR) is poor during the transmission, sufficient SNR can be obtained through the intrachannel four-wave mixing at the receiver. With this slim bandwidth transmission technique, the maximum number of spectrum-sliced wavelength-division-multiplexed channels can be increased greatly and the dispersion penalty can be reduced simultaneously.
Cross section of a PIN PD with DNW in an epi-CMOS process, showing one finger. The output is taken from cathode. Doping concentration numbers are approximate average values.
(a) Simulated potential contours and electric field vectors as well as (b) electric field magnitude inside the active region of the 70 2 70 m , 2-m finger spacing PD at 15-V bias. (c) Simulated small-signal frequency response of this PD at different bias voltages.  
Measured responsivity and dark current of the prototype PD at different bias voltages.  
(a) Impulse response and (b) its spectrum by DFT of the prototype PD. Measurements were done under 850-nm illumination for different bias condi- tions.  
(a) Measured small-signal frequency response of the prototype PD at different bias. (b) 3-dB bandwidth and gain variation with respect to bias.  
A new PIN photodiode (PD) structure with deep n-well (DNW) fabricated in an epitaxial substrate complementary metal-oxide-semiconductor (epi-CMOS) process is presented. The DNW buried inside the epitaxial layer intensifies the electric field deep inside the epi-layer significantly, and helps the electrons generated inside the epi-layer to drift faster to the cathode. Therefore, this new structure reduces the carrier transit time and enhances the PD bandwidth. A PD with an area of 70 times 70 mum<sup>2</sup> fabricated in a 0.18-mum epi-CMOS achieves 3-dB bandwidth of 3.1 GHz in the small signal and 2.6 GHz in the large signal, both with a 15-V bias voltage and 850-nm optical illumination. The responsivity is measured 0.14 A/W, corresponding to a quantum efficiency of 20%, at low bias. The responsivity increases to 0.4 A/W or 58% quantum efficiency at 16.2-V bias in the avalanche mode.
A six-stage transimpedance amplifier (TIA) was realized in a 0.18-mum complementary metal-oxide-semiconductor process. By adopting an effective gain-bandwidth product (GBW) enhancement technique, pi -type inductor peaking, the measured S <sub>21</sub>, transimpedance gain, and bandwidth are 41 dB, 75 dBldrOmega, and 7.2 GHz, respectively, in the presence of an on-chip photodiode capacitance of 450 fF at the input. The 10-Gb/s TIA can operate under a maximum output swing of 800 mV <sub>pp</sub> and achieve a recorded GBW per DC power of 441.1 GHzldrOmega/mW.
A hole-injection-type and an electron-injection-type Si avalanche photodiode (APD) were fabricated by a standard 0.18-μm complementary metal-oxide-semiconductor process. The avalanche amplifications are observed below 10 V of the bias voltage, and the maximum avalanche gains were 493 and 417 for the hole-injection-type and the electron-injection-type APDs, respectively. The maximum bandwidth is 3.4 GHz, and the gain-bandwidth products were 90 and 180 GHz for the hole-injection-type and the electron-injection-type APDs, respectively.
A high-power Er-Yb fiber amplifier for WDM applications has been constructed using a matched mid-stage gain shaping filter. Using precise measurements and careful design considerations, excellent gain flatness, with less than 0.2-dB variation, was obtained over a 14-nm spectral bandwidth. By simply adjusting the pump power to the amplifier, it was possible to maintain the flattened amplifier gain shape over a wide input signal power range from -11 dBm to 1 dBm. A low external noise figure of 5.2 dB at 1-dBm signal input and a high-output power up to 24.6 dBm has been measured.
The design and performance of a traveling-wave 1*2 directional coupler modulator are described. A 0-20 GHz response (-4 dB/sub e/) was achieved for Ti:LiNbO/sub 3/ devices at the 1.3- mu m wavelength. From experimental determinations of coupling coefficient and interarm intrinsic phase-mismatch for several modulators, linear dynamic ranges of, on average, 78 dB and voltage sensitivities of down to 9 mu V are determined (for 1-mW detector power, 3-kHz bandwidth, 50- Omega detector load). The dependence of sensitivity on the coupling coefficient is described, and the effect of the buffer layer thickness on the optical bandwidth and sensitivity is demonstrated.< >
Very low threshold current density operation of triple quantum well vertical-cavity surface-emitting lasers (VCSELs) is reported. The active wells are strained In/sub 0.24/Ga/sub 0.76/As in GaAs. Devices from the same wafer operate at room temperature over a wavelength range of 958-1042 nm, with a minimum threshold current density of 366 A-cm/sup -2/ at 1018 nm. The dependence of threshold current on wavelength gives an insight into the optical gain spectrum of the quantum wells. It was shown that 50- mu m-diameter devices operate CW without heatsinking.< >
A 120-channel WDM demultiplexer was fabricated by etching a concave grating into a SiO/sub 2/-SiON-SiO/sub 2/-Si slab waveguide. The grooves of the grating reflected light by two instances of total internal reflection. The device was suitable for demultiplexing wavelength channels in the 1530-1565-nm range with a 0.29-nm (36 GHz) channel spacing. Each channel had a spectral width (full width at half maximum) of about 0.18 nm. The fiber-to-fiber insertion loss was between -20 dB and -30 dB for most channels. The loss was predominantly due to waveguide absorption and increased dramatically for wavelengths below 1539 nm.
Stable in-phase lateral- and single-longitudinal-mode operation up to four times the threshold (P/sub o/ approximately 100 mW) was achieved in a five-element 1.5- mu m-wavelength Ga/sub 0.3/In/sub 0.7/As/GaInAsP/InP compressive strained MQW grating filter laser array. The threshold current and external differential quantum efficiency under pulsed condition were 330 mA (emitter width of 18 mu m, active region length of 480 mu m) and 17%, respectively.< >
A room temperature CW operation of Ga/sub 0.3/In/sub 0.7/As/GaInAsP/InP GRINSCH compressive strained MQW lasers with 30 approximately 60 nm wide wire active region was achieved. This device was fabricated by two-step LP-OMVPE growths on p-type InP substrate and wet chemical etching. Threshold current as low as 53 mA (L=910 mu m, J/sub th/=2.9 kA/cm/sup 2/) was obtained at RT-CW condition. The spontaneous emission peak and the lasing wavelength of strained MQW wire lasers exhibited approximately 20-meV blue shift from those of MQW film lasers cut out from the same wafer.< >
The robust full-band dispersion compensation gratings (DCG) with <0. 3-dB power penalty in a 10-Gb/s nonreturn-to-zero (NRZ) optical communication system were discussed. The group delay with the modulation-phase shift method at a 63-MHz modulation frequency was measured. The grating in the diamond-like film (DLF)-encapsulated fiber was recoated with standard fiber acrylate coatings and then proof tested to 200 kpsi to ensure long-term reliability. The power penalty was determined across the full bandwidth of the DCG in 10-pm steps by decreasing the optical power impinging upon the receiver until a measurable bit error rate (BER) was achieved.
Temperature dependences of threshold current and differential quantum efficiency of Ga/sub 0.66/In/sub 0.34/As-InP tensile-strained quasiquantum-wire laser were measured, and were compared with those of quantum-well-film lasers fabricated on the same wafer. It was found that nonradiative recombination rate was not enhanced in the quasiquantum-wire laser compared with that of the quantum-well-film laser.< >
Fairly low threshold current operation was achieved with Ga/sub 0.66/In/sub 0.34/As/GaInAsP/InP tensile-strained (TS) single-quantum-well (SQW) lasers with 30- approximately 40-nm-wide and 70-nm-period wire active region fabricated by a combination of a wet chemical etching and two-step OMVPE (organometallic vapor phase epitaxy) growth. Threshold current as low as 16 mA and threshold current density of 816 A/cm/sup 2/ were obtained under a room temperature CW condition. In comparing the spontaneous emission peak wavelength of the TS-SQW wire laser with that of the TS-SQW film laser, an approximately 10-meV blue shift of the fundamental energy level was observed in the TS-SQW wire laser.< >
We demonstrate the first smart photodetector array designed to perform parallel error correction on an entire page of data and satisfy the sustained data rates of page-oriented optical memories. Our implementation integrates a differential photoreceiver with a cluster error correction code using 0.35-/spl mu/m complementary metal-oxide-semiconductor (CMOS). This approach provides high optical sensitivity (150-fJ switching energy), low electrical power (46.2 mW), and fast correction of 2/spl times/2 cluster errors to achieve corrected output data rates scalable to 102 Gb/s in the current technology increasing to 2.5 Tb/s in 0.1-/spl mu/m CMOS.
Finger photodiodes in PIN technology are introduced to enhance the responsivity for blue and ultraviolet light. A thick low doped epitaxial layer results in high responsivity and high bandwidth also for red and near-infrared light. Results of PIN finger photodiodes are compared to that of PIN photodiodes for 10- and 15-mum epitaxial intrinsic layer thickness. The cathode finger structure results in a high responsivity of 0.20 A/W (quantum efficiency 61%) for 410-nm light and a bandwidth of 1.25 GHz for 10- mum epi thickness at a reverse bias voltage of 3 V. The rise and fall times with an epitaxial layer thickness of 15 mum are below 1 ns for the wavelength range from 410 to 785 nm.
The authors examine the operating characteristics of short wavelength (617< lambda <640 nm) AlGaInP lasers containing three thin ( approximately 20 AA) compressively strained Ga/sub 0.4/In/sub 0.6/P/(Al/sub 0.6/Ga/sub 0.4/)/sub 0.5/In/sub 0.5/P quantum wells and Al/sub 0.5/In/sub 0.5/P cladding layers, grown by low pressure organometallic vapor phase epitaxy. At room temperature, wavelengths as short as 617 nm have been achieved, with pulsed threshold current density of 2.5 kA/cm/sup 2/. As a result of greater electron confinement at longer wavelengths, the threshold, and its temperature sensitivity, are improved.< >
A frequency offset locked loop between two different optical frequency combs spaced from 0.4 THz is reported. In a new experiment, a frequency offset locked loop between one comb and one laser diode as a way to link two combs together is realized. A spacing as wide as 0.6 THz is achieved between the linked combs.
The authors fabricated and packaged 1*4 arrays of In/sub 0.53/Ga/sub 0.47/As p-i-n photodetectors connected in a GaAs FET bias-switched, common cathode circuit. The bandwidth of the discrete (packaged) photodiodes in the arrays exceeds 11 GHz, and is 5 GHz for the packaged arrays. The on/off isolation ratio for fully packaged arrays using GaAs FET bias switches was measured at both low and high frequencies. It was found that the on/off isolation of the detectors in the array was 70 dB, and the bias switching times were less than 5 ns. The performance characteristics suggest that the p-i-n detector arrays used in a common cathode, current summing configuration have applications for high-freqeuncy digital and analog optical switching applications.< >
A monolithically integrated photoreceiver using an InAlAs/InGaAs HBT-based transimpedance amplifier has been fabricated and characterized. The p-i-n photodiode is implemented using the base-collector junction of the HBT. The 5 /spl mu/m/spl times/5 /spl mu/m emitter area transistors have self-aligned base metal and non-alloyed Ti/Pt/Au contacts. Discrete transistors demonstrated f/sub T/ and f/sub max/ of 54 GHz and 51 GHz, respectively. The amplifier demonstrated a -3 dB transimpedance bandwidth of 10 GHz and a gain of 40 dB/spl Omega/. The integrated photoreceiver with a 10 /spl mu/m/spl times/10 /spl mu/m p-i-n photodiode showed a -3 dB bandwidth of 7.1 GHz.< >
We demonstrate 2.33-mum-wavelength InP-based distributed feedback (DFB) lasers with InAs-In<sub>0.53</sub>Ga<sub>0.47</sub> multiple-quantum wells as the active region. The maximum output power is 20 mW at 25degC and the maximum operating temperature is as high as 95degC. Stable single-mode operation with a sidemode suppression ratio of 30 dB is obtained, and the emission wavelength of the laser is finely controlled from 2.335 to 2.348 mum by adjusting the injection current and the operating temperature. The current-tuning and temperature-tuning rates of the DFB wavelength are +0.007 nm/mA and +0.148 nm/K, respectively.
Experimental and theoretical characterization of a series of strained InGaAs/InGaAs quantum well heterostructures is presented. Measurements of X-ray diffraction on calibrated pairs of samples are in excellent agreement with simulated X-ray spectra and provide precise values for compositions and layer thicknesses. With these parameters, the authors have calculated the light- and heavy-hole to electron transition wavelengths, taking into account quantum confinement effects and the influence of strain on the strained-layer bandgap and on the band offset at the lattice-mismatched heterointerface. These calculated bandgaps are in good agreement with the measured room temperature photoluminescence spectra.< >
A high-speed waveguide In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As separate absorption, charge, and multiplication avalanche photodiode suitable for operation at 1.55 /spl mu/m has been demonstrated, a unity-gain bandwidth of 27 GHz was achieved with a gain-bandwidth product of 120 GHz.
An optical transimpedance receiver front-end that is adaptable to a wide range of bit-rates up to 3 Gb/s has been realized by monolithically integrating high efficiency p-i-n photodiodes with low noise InGaAs junction field effect transistors. The transimpedance-bandwidth product of the receiver is 2.8 THz Omega . The average equivalent input noise current for full circuit bandwidth is 4.0 pA/ square root Hz. The preamplifier for nonreturn to zero data transmission without equalization of the frequency response at 1.55 mu m offers a sensitivity of -41.5 dBm and -29.5 dBm at 140 Mb/s and 2.4 Gb/s, respectively. The dynamic range is 17 dB at 2.4 Gb/s and exceeds 30 dB at 500 Mb/s.< >
The InGaAsP-GaAs material system is an alternative to AlGaAs-GaAs for optoelectronics, but has the disadvantage of relatively low refractive index differences. This makes implementation of vertical-cavity Bragg reflector devices a greater challenge. The maximum index difference is obtained for In/sub 0.49/Ga/sub 0.51/P-GaAs; mirrors based on this combination are inefficient in the 800-880-nm wavelength region due to absorption in the GaAs layers. The mirror efficiency can be improved by exploiting band-filling (Burstein-Moss) effects in heavily doped GaAs. This possibility was demonstrated by fabrication of resonant-cavity metal-semiconductor-metal photodetectors operating near 800-nm wavelength. A responsivity of 0.34 A/W was obtained with a 410-nm-thick absorption layer.
AlGaInP-based flip-chip light-emitting diodes (LEDs) with geometric sapphire shaping structures were fabricated by sapphire chemical etching and glue-bonded techniques. Furthermore, a nanoscale rough texture was applied on the epiwafer surface. This novel structure improved the output light power, wall-plug efficiency, and reliability. The output power of this structure was enhanced 31.2% under 350-mA current injection as compared with the conventional AlGaInP flip-chip LEDs.
The authors present a simple technique for the fabrication of integrated optical channel waveguides that are prepared by indiffusion of an E-beam evaporated amorphous alloy of germanium and silicon into commercially available silicon with low dopant concentration, using only simple technological processes such as standard lithography, PVD, and diffusion. The waveguides are polarization independent and have waveguide losses as low as 0.3 dB/cm at wavelengths of lambda =1.3 mu m and lambda =1.55 mu m. The spot sizes are well suited for low-loss single-mode fiber device coupling, being on the order of a few microns in both horizontal and vertical directions.< >
We report the power scaling of a diode-pumped GaAs-based 850-nm vertical external-cavity surface-emitting laser, by use of an intracavity silicon carbide (SiC) heatspreader optically contacted to the semiconductor surface. To our knowledge, this is the first demonstration of bonding of SiC to a III-V semiconductor structure using the technique of liquid capillarity. High output power of >0.5 W in a circularly symmetric, TEM/sub 00/ output beam has been achieved with a spectral shift of only 0.6 nm/W of pump power. No thermal rollover was evident up to the highest pump power available, implying significant further output-power scaling potential using this approach.
We report demonstration of a new high-power semiconductor laser technology, the optically pumped semiconductor (OPS) vertical-external-cavity surface-emitting laser (VECSEL). Using diode laser pump, an OPS-VECSEL laser with a strain-compensated InGaAs-GaAsP-GaAs multiquantum-well (MQW) structure operated continuous-wave (CW) near /spl lambda//spl sim/1004 nm with record output power of 0.69 W in a TEM/sub 11/ mode, 0.52 W in a TEM/sub 00/ mode, and 0.37 W coupled to a single-mode fiber. It is feasible to produce greater than 1 W of power in a diffraction-limited circular beam from an efficient, compact, manufacturable and reliable OPS-VECSEL laser.
We report 670-nm native-oxide confined GaInP-(Al<sub>x</sub>Ga<sub>1-x</sub>)<sub>0.5</sub>In<sub>0.5</sub>P quantum-well ridge waveguide laser diodes. The devices are fabricated from a compressively strained GaInP-(Al<sub>x</sub>Ga<sub>1-x</sub>)<sub>0.5</sub>In<sub>0.5</sub>P quantum-well separate confinement heterostructure laser structure. Wet chemical etching and wet oxidation process are used to form native oxide confined ridge waveguides. The oxidation process converts the p-Al<sub>0.5</sub>In<sub>0.5</sub>P cladding layer into AlO<sub>x</sub> after ridge etch. Laser diodes of 3.5-μm-wide ridge waveguide operate with threshold currents below 13.5 mA and differential quantum efficiencies over 35%/facet
This letter assesses the performance of dispersion-supported transmission (DST) for three 10-Gb/s WDM channels separated 0.5 nm, using a three-mirror Fabry-Perot demultiplexer. It is shown that the use of three-mirror demultiplexers reduces the crosstalk penalty to less than 1 dB in the region of small linear increase of dispersion penalty of the DST method, while double-cavity Fabry-Perot demultiplexers are less suitable to operate at this channel spacing. Compared with published performance studies for WDM-DST systems with 1 nm of channel spacing, these results indicate the channel spacing may be cut in a half, if a three-mirror filter is used as demultiplexer.
TM-polarized laser emission is demonstrated at wavelengths longer than 650 nm, for (AlGa)/sub 0.5/In/sub 0.5/P-based laser diodes. These structures contain tensile-strained AlGaAsP or InGaAsP quantum well active regions, which are capable of spanning a wavelength range of roughly 650-850 nm, for TM-mode lasers on GaAs substrates. This represents an extension of the wavelength range available from typical GaInP-(AlGa)/sub 0.5/In/sub 0.5/P lasers, where the requirement for biaxial tension limits the TM-mode wavelengths to less than 650 nm. In addition, compared to AlGaAs confining structures, the high-bandgap (AlGa)/sub 0.5/In/sub 0.5/P confinement structure used here makes AlGaAs(P) active regions feasible at shorter wavelengths, with good performance maintained for 670</spl lambda/<700 mn. Likewise, the wavelength range 700</spl lambda/<750 nm, where AlGaAs laser characteristics are diminished, becomes accessible using these materials.< >
(a) Schematic diagram of the AlGaInP-based GSS-LED structure with surface textured and sandwich transparent conductive ITO layer. (b) Illustration of the different extracted light paths of the GSS-LEDs and the conventional GB-LEDs.
SEM images of (a), (b) schematic diagram of the AlGaInP-based GSS-LED structure with a surface textured and a sandwich transparent conductive ITO layer. (c), (d) Illustration of the wafer surface with a nano-roughed texture on the thick n-AlGaInP surface layer via natural cluster metal nano-mask and chemical etching processes.
Tilted cross section photomicrographs view of (a) GB-LED and (b) GSS-LED chip under a forward current 70-mA injection.
Beam patterns of the GB-LEDs and GSS-LEDs under 70-mA current injection.
AlGaInP-based light-emitting diodes (LEDs) with a transparent sapphire substrate were fabricated by the glue-bonding (GB) method. This transparent sapphire substrate is a geometric shaping structure by wet etching processes. Furthermore, the n-side-up surface has a nano-roughened texture by natural mask and chemical wet etching processes. The light output of this novel LED structure could be enhanced about 26.7% (at 350 mA) due to the higher light extraction as compared with the conventional GB-LEDs.
We describe the fabrication and characteristics of 15-/spl mu/m spaced dual-spot, 670-nm native-oxide confined GaInP-(Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P quantum-well ridge waveguide laser diodes. The devices are fabricated from a compressively strained GaInP-(Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P quantum-well separate confinement heterostructure laser structure. Wet oxidation of Al/sub 0.5/In/sub 0.5/P is used to form native-oxide-confined dual-ridge waveguides. The oxidation process converts part of the p-Al/sub 0.5/In/sub 0.5/P cladding layer into AlO/sub x/ after ridge etching. These diodes show excellent performance: uniform low threshold currents of 15 mA and differential quantum efficiencies over 35%/facet. The diodes show crosstalk less than 2%.
A monolithically integrated optical receiver circuit in a 0.6-μm silicon bipolar complementary metal oxide semiconductor technology with f<sub>t</sub>=25 GHz is presented. It incorporates a vertical p-i-n photodetector with a responsivity of 0.36 and 0.26 A/W at 660 and 850 nm, respectively. At these wavelengths, sensitivities of -23.5 and -21.2 dBm, respectively, at a bit rate of 2.5 Gb/s and a bit-error rate of 10<sup>-9</sup> are achieved. The transimpedance gain of the receiver is 18 kΩ and overall -3-dB bandwidths of 1.35 and 1.05 GHz at 660 and 850 nm, respectively, are observed.
An extension of the Claussius-Mossotti interpolation scheme is proposed so that the refractive index and material dispersion of GeO/sub 2/- and F-doped silica glasses (with doping concentrations different than those of published data) can be predicted in the 0.6-1.8- mu m wavelength region. The new interpolation expression provides a well-behaved functional relationship for use in computer models which analyse propagation in single-mode fibers. The technique proposed is particularly powerful because it can be applied to any glass, whether single or multicomponent, having any other single dopant.< >
Tapered lasers fabricated from a GaInAsSb-AlGaAsSb single-quantum-well structure are reported. The laser structure, grown by molecular beam epitaxy, has broad-stripe pulsed threshold current densities as low as 50 A/cm/sup 2/ at room temperature. Tapered lasers have exhibited diffraction-limited continuous-wave output power up to 600 mW.
We investigate Mach-Zehnder interferometric (MZI) space switches with quantum-well phase shifters. We find that the minimum phase shifter length is limited by additional crosstalk due to electroabsorption-induced imbalance in the MZI. This criterion also provides an optimal detuning between the bandgap and the operating wavelength of the MZI. Finally, we present a novel MZI with an ultrashort 0.64-mm phase shifter.
Top-cited authors
Jianping Yao
  • University of Ottawa
John Edward Bowers
  • University of California, Santa Barbara
Roel Baets
  • Ghent University
Jianjun Yu
  • Fudan University
L.A. Coldren
  • University of California, Santa Barbara