Low loss, single mode rib waveguides, based on PECVD deposited multi-layer amorphous silicon are fabricated. These waveguide are refractive index and mode-matched to III/V laser waveguides. Methods for monolithic integration of these passive amorphous silicon waveguides with InGaAsP/InP gain sections are demonstrated. Results of a multi-wavelength laser based on an amorphous silicon arrayed waveguide grating integrated on a single chip with InGaAsP gain sections are presented.
Enhanced electrooptic coefficient of GaInAsP three-step quantum wells (3SQW) for high power electrorefraction modulator applications is reported. Measured electrooptic coefficient of the 3SQW is nearly three times higher than the conventional rectangular quantum well (RQW) at lambda=1.55 um. Higher electrooptic effect, combined with a low optical absorption coefficient α<1 cm-1 in the 3SQW increased the modulator figure of merit by nearly 53 times, and decreased the power consumption by nearly one order of magnitude compared with a conventional RQW design.
We report optical frequency comb drift stabilization of an external cavity semiconductor laser hybridly modelocked at the 10 GHz cavity fundamental using the Pound-Drever-Hall frequency stabilization scheme. Laser longitudinal mode comb was locked to a Fabry-Perot reference cavity with a finesse value of 214. The frequency error signal was fed back to the bias current of the semiconductor gain medium to change the effective laser cavity length through the coupling between carrier density and refractive index. The peak-to-peak 2.4 GHz frequency drift of the comb of longitudinal modes was reduced to a RMS fluctuation of 30 MHz for up to 5 minutes. To the authors’ knowledge, this is the first optical frequency comb stabilization of a modelocked semiconductor laser. The intended application of the optical frequency stabilization is to keep the laser optical frequency comb locked to a WDM filter that is used for spatially separating the individual longitudinal modes of the laser for photonic arbitrary waveform generation.
Low-capacitance, two-section, curved-waveguide gain elements were packaged with lensed polarization-maintaining fiber within standard-sized butterfly-style packages and shown to produce low-jitter pulses when used within a harmonically modelocked sigma cavity laser (jitter = 25 fs; 10 Hz - 10 MHz). Incorporation of a high finesse etalon filter into the sigma-cavity loop resulted in greater than 25 dB suppression of the supermode spurs while maintaining low integrated phase noise (jitter = 30 fs; 10 Hz - 10 MHz). A module containing the in-line sigma-cavity modelocked laser source and packaged semiconductor optical amplifiers was developed to create a configurable low jitter pulse source.
We report the measurement of electric field correlations of a hybridly modelocked external linear cavity semiconductor laser as a function pulse delay. We also report the measurement of residual phase noise corner frequency and longitudinal mode linewidth as a function of laser cavity length. We find that the pulses in the modelocked pulsetrain are correlated at only multiples of the cavity roundtrip time. Excellent agreement between residual phase noise corner frequency and longitudinal mode linewidth measurements suggest that the corner frequency is the average longitudinal mode linewidth. This relationship leads to a fundamental limit in the timing jitter of modelocked lasers.
Current sate-of-the-art electronic analog-to-digital converters (ADCs) operating at multi gigahertz sampling frequencies are known to exhibit fairly limited resolution. These high-frequency restrictions stem primarily from the response time of the constituent transistors that make up the ADCs comparators. In an effort to improve the resolution of ADCs operating at ultrahigh sampling frequencies, several areas of investigation are currently underway regarding the capabilities of hybrid optoelectronic systems. High-power optical pulses can be used as sampling windows and high- bandwidth electro-optic modulators as voltage-to-intensity transducers to provide a means for digitizing ultrafast voltage waveforms with much greater accuracy than conventional ADCs. When optical sampling is employed, the primary limiting factors determining ADC conversion accuracy becomes the noise in the sampling pulse train and the extent of the sampling time. Detrimental pulse train noise is associated with either phase modulation or amplitude modulation, and recent measurements of AM and residual PM noise on our 10 Ghz ring laser show the best results to date for an actively-mode-locked semiconductor diode system. Carrier offset integration bands extending form 10 Hz to 10 MHz exhibit RMS levels of AM and PM noise as low as 0.12 percent and 43 fs, respectively. In addition, linear dispersion compensation has successfully reduced the optical pulsewidth from 13 ps to 1.2 ps. Based on these experimental numbers, this laser could form the front end for an optoelectronic ADC capable of a theoretical resolution as high as 8.6 bits.
A novel approach to residual jitter measurement examines the intensity cross correlation generated by two optical pulses with various relative delays. A relative delay of 25 pulses produces a residual jitter value of 26 fsec RMS for a 10 GHZ actively mode-locked ring laser. The phase noise measurement carried out to the Nyquist frequency offset gives 47 fsec RMS pulse-to-pulse timing jitter. The field correlation measurement obtains a 10 asec RMS pulse-to-pulse optical carrier jitter.
Optical analog to digital conversion schemes require a sampling source of high repetition rate, low temporal jitter, low amplitude noise, and short pulse duration to achieve the desired sampling rate and number of bits of resolution. We report on the development of an actively mode-locked semiconductor external cavity laser system where the emission is comprised of multiple wavelengths nominally centered around 1.55 microns. Cavity design includes an intra-cavity grating to produce a spatially dispersed optical spectral filtering plane. Amplitude filtering in this spectral plane serves to flatten the effective gain and a rectangular aperture array selects those wavelengths which are allowed to lase. Modelocked at 311 MHz and producing 8 spectral lines, the laser provides a sampling rate of approximately 2.5 GHz. Temporal interleaving of the pulse train by factor 4 increases the sampling rate to 10 GHz.
KEYWORDS: Signal to noise ratio, Semiconductor lasers, Diodes, Amplitude modulation, Signal detection, Phase modulation, Interference (communication), Information operations, Amplifiers, Oscillators
External-cavity, actively-modelocked semiconductor diode lasers (SDLs) have proven to be attractive candidates for forming the backbone of next-generation analog-to-digital converters (ADCs), which are currently being developed to sample signals at repetition rates exceeding several GHz with up to 12 bits of digital resolution. Modelocked SDLs are capable of producing waveform-sampling pulse trains with very low temporal jitter (phase noise) and very small fluctuations in pulse height (amplitude noise)--two basic conditions that must be met in order for high-speed ADCs to achieve projected design goals. Single-wavelength modelocked operation (at nominal repetition frequencies of 400 MHz) has produced pulse trains with very low amplitude noise (approximately 0.08%), and the implementation of a phase- locked-loop has been effective in reducing the system's low- frequency phase noise (RMS timing jitter for offset frequencies between 10 Hz and 10 kHz has been reduced from 240 fs to 27 fs).
Ramon Martinelli, Raymond Menna, Pamela York, Dmitri Garbuzov, Hao Lee, Joseph Abeles, Nancy Morris, John Connolly, S. Yegna Narayan, Jacobus Vermaak, Gregory Olsen, David Cooper, Clinton Carlisle, Haris Riris, Anthony Cook
We have fabricated single-frequency diode lasers from a number of III-V semiconducting compounds. These diode lasers were specifically designed for laser absorption spectroscopy. Their emission wavelengths span the internal of 0.76 to 2.7 micrometers . Water vapor, CO, CO2, NH3, CH4 HF, and O2 have been detected using them. After a brief review of their physical structure and principles of operation, we present representative output characteristics of these lasers, along with a discussion of several important applications.
KEYWORDS: Waveguides, Near field optics, Quantum wells, Waveguide lasers, Semiconductor lasers, Cladding, Continuous wave operation, Diodes, Near field, High power lasers
AlGaAs/GaAs graded-index separate-confinement heterostructure single quantum well (GRINSCH-SQW) lasers with different waveguide thickness have been analyzed experimentally and compared with results from modeling using transverse optical field distributions. We have found that for GRINSCH lasers the halfwidth of near-field and far-field patterns depends very weakly on the waveguide thickness due to the focusing of the optical field in the transverse direction by the graded-index waveguide. At the same time, the mode intensity in the cladding layers is reduced by two orders of magnitude as the waveguide thickness is increased from 40 nm to 1200 nm. As a result, a 20% improvement in the differential quantum efficiency ((eta) d) is realized, while the threshold current density remains unchanged. Differential quantum efficiency as high as 78% and output power exceeding 4 W cw have been obtained for broadened waveguide lasers.
Laser absorption spectroscopy using III-V semiconductor laser diodes has several advantages for gas sensing applications, as compared with traditional methods employing tunable dye laser and II-VI (e.g., lead salt) laser sources. These advantages include room-temperature operation, reduced cost, and compact size. Limited coverage of spectroscopy wavelengths by high-performance III-V lasers has prevented their widespread application to gas sensing. At those fixed wavelengths, performance of commercially available devices has been limited by multimode emission and/or inadequate wavelength tuning and mode hops. These spectra can, however, be greatly improved by incorporating frequency-selective structures. We have developed single-mode distributed-feedback (DFB) GaAs/AlGaAs quantum well lasers applicable to laser spectroscopy of molecules absorbing in the wavelength interval from 760 to 840 nm. These devices exhibit low threshold current (< 20 mA), high efficiency (> 40%), high output power (> 25 mW), and narrow linewidth (< 3.0 MHz). The lasers display smooth, continuous, single-mode wavelength tuning over 5 nm. Typical temperature and current wavelength-tuning coefficients are 0.065 nm/ degree(s)C and 0.0075 nm/mA (approximately -3.5 GHz/mA), respectively. In preliminary tests, they have been applied to the detection of H2O vapor and O2 gas.
Joseph Abeles, Robert Amantea, James Andrews, Pamela York, John Connolly, R. Rios, W. Reichert, Jay Kirk, T. Zamerowski, Dean Gilbert, So Liew, N. Hughes, Jerome Butler, Gary Evans, S. Yegna Narayan, Donald Channin
Monolithic fanned-out amplifier lasers (acronym: FOAL), are capable of producing high optical power, greater than 1 watt. They operate single wavelength and can be collimated to generate a nearly Gaussian beam useful for applications requiring an inexpensive compact source of coherent radiation. Examples are space communication, frequency doubling, thermal writing, optical sensing, optical interconnection, and optical computing.
The design and characteristics of active-grating surface emitting amplifiers are discussed. Performance projections indicate coherent power outputs of more than I W should be possible from devices that are 1 cm long. Preliminary experimental results on an AIGaAs/InGaAs active-grating MOPA are presented.
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