A microstructured optical fiber was first used in 2000 for supercontinuum generation. Since then, enormous progress has been made in understanding, controlling, and marketing fiber-based supercontinuum sources. In particular, biomedical applications of such sources are revolutionizing the field of medical imaging. In this talk I review the recent progress in this area and describe how a supercontinuum can be employed for biomedical imaging using the techniques known as coherent anti-Stokes Raman scattering, stimulated emission-depletion microscopy, and optical coherence tomography.
We present an empirical thermal model for VCSELs based on extraction of temperature dependence of macroscopic VCSEL
parameters from CW measurements. We apply our model to two, oxide-confined, 850-nm VCSELs, fabricated with
a 9-μm inner-aperture diameter and optimized for high-speed operation. We demonstrate that for both these devices, the
power dissipation due to linear heat sources dominates the total self-heating. We further show that reducing photon lifetime
down to 2 ps drastically reduces absorption heating and improves device static performance by delaying the onset
of thermal rollover. The new thermal model can identify the mechanisms limiting the thermal performance and help in
formulating the design strategies to ameliorate them.
In this work we describe the nonlinear propagation of optical pulses through an array of silicon nanowires made with
the silicon-on-insulator technology. A generalized analysis of the nonlinear coupled system is given where we have
considered the vector nature of optical modes and the effects of two-photon absorption on various nonlinear processes.
The general theoretical model includes not only the effects of free-carrier absorption and free-carrier dispersion but also
linear and nonlinear losses, and it extends previous vector nonlinear models to the case where coupling of supermodes of
a waveguide array occurs in silicon waveguides. Analytical solutions are provided for the coupled-mode equations in
some cases in which the density of the free carrier is relatively low, and dispersive effects are relatively weak, assuming
that the nonlinear effects do not affect the waveguide modes significantly. The impact of two-photon absorption and
free-carriers effect on the properties of the nonlinear coupling effects is studied in detail together with the evolution of
optical power inside an array of silicon nanowires.
We observe unique dynamics of optical solitons formed when an optical pulse experiences initially normal GVD with a
monotonous dispersion slope. For negative (positive) third-order dispersion the blue (red) components of the spectrum
form a soliton which attracts (repels) the red (blue) components of the spectrum and forces them to travel with its own
When ultrashort optical pulses propagate as a soliton inside optical fibers, the presence of higher-order dispersion leads
to transfer of energy from the soliton to a narrowband resonance in the form of dispersive waves (DW). The frequency
of the radiation is determined by a phase-matching condition in the form of a polynomial whose coefficients depend on
the numerical values of the third- and higher-order dispersion coefficients. In this paper we show that there is a striking
correlation between the number of zero-dispersion points (ZDPs) and the generation of DW peaks. Detailed simulations
indicate that the number of ZDPs present in a specific dispersion profile is an excellent predictor of the number of
dispersive peaks created in the output pulse spectrum. A fiber with a single ZDP only has one DW peak, and a fiber
with two ZDPs always exhibits dual DW peaks. Moreover, no DW can be expected in a fiber that has no zero-dispersion
crossings over the entire range of wavelengths. We examine numerically dispersion profiles with as many as six ZDPs
and find that this criterion always holds. Another interesting feature we notice is that, if the frequency of the ZDP is
larger (smaller) than the operating frequency, DWs fall on the higher (lower) frequency side of the operating frequency.
Therefore there is a possibility to generate two DW peaks on in same side (blue or red side) of the output pulse spectrum
by tailoring the dispersion curve suitably.
This paper presents detailed numerical and experimental study of SPM in semiconductor optical amplifiers (SOAs) with
ultrafast gain-recovery times. These SOAs have a range of gain-recovery speed which is a function of drive current. At
increased drive current, the amount of internal ASE in the SOA increases, which causes the small signal gain to saturate
and reduces the gain-recovery time. Understanding pulse amplification in these SOAs is important for optimizing the
performance of SOA-based optical regenerators and wavelength converters. Our study addresses the full range of gain-recovery
times in commercial SOAs extending from less than 10 ps to >100 ps.
We show that optical soliton can be realized in very short waveguides (5 mm long) fabricated on silicon-on-insulator
(SOI) wafers. By tailoring their zero-dispersion wavelength and launching optical pulses at only sub
pico-joule energy level close to this wavelength, we have observed significant spectral narrowing due to the pulse
reshaping during the formation of optical soliton, which is in strong contrast to previous measurements. The
extent of spectral narrowing depends on the carrier wavelength of the input pulse in the normal dispersion region
and spectral broadening is observed in normal dispersion region. We simulate femto-second pulses' propagation
in such waveguide. Simulation results show the evolution of the pulse shape in both time domain and frequency
domain when the pulse energy is increased from very low level, when nonlinear effects are negligible, to the
energy level we used in our experiment. The simulation results agree well with our observation. To be best of
our knowledge, this is the first report on soliton in silicon waveguides.
Numerical simulations of the spatial dynamics of the light output of 2D arrays of VCSELs are presented. The cases presented include square and circular arrays of nine elements. For both configurations the spacing between elements is varied to study the effects on the interaction between elements. In addition, the effects of index guiding on the supermodes of the arrays will be shown. It was found that, with only a small amount f index guiding, the interactions between elements of the VCSEL array are effectively eliminated for all spacings between elements. The time evolutions of the spatial profiles of the laser intensity and carrier density are obtained by solving the Effective Semiconductor Bloch-Maxwell equations by a finite- different algorithm. The algorithm can handle devices with multiple active regions of nay shapes or pattern. There is no a priori assumption about the type or number of modes.
Transverse mode dynamics of a 20-micrometer-diameter vertical- cavity surface-emitting laser (VCSEL) undergoing gain switching by deep current modulation is studied numerically. The direct current (dc) level is set slightly below threshold and is modulated by a large alternating current (ac). The resulting optical pulse train and transverse-mode patterns are obtained numerically. The ac frequency is varied from 2.5 GHz to 10 GHz, and the ac amplitude is varied from one-half to four times that of the dc level. At high modulation frequencies, a regular pulse train is not generated unless the ac amplitude is large enough. At all modulation frequencies, the transverse spatial profile switches from single-mode to multiple-mode pattern as the ac pumping level is increased. Optical pulse widths vary in the range 5 - 30 ps, with the pulse width decreasing when either the frequency is increased or the ac amplitude is decreased. The numerical modeling uses an approximation form of the semiconductor Maxwell-Bloch equations. Temporal evolution of the spatial profiles of the laser (and of carrier density) is determined without any assumptions about the type or number of modes.
Propagation of partially coherent pulse trains in single- mode optical fibers is considered within the framework of the nonlinear Schrodinger equation. Statistical properties of chaotic modulated pulses are evaluated by modelling them after cyclostationary processes. Interesting spectra are obtained exhibiting the influence of self phase modulation on these random signals.
We experimentally measure the first-order spatio-temporal characteristics of filamentation and discover effects of the stripe width. We use an analytic theory to explain and reproduce these results through an expression for the filament gain, in which contributions of various mechanisms can clearly be seen. Through this model and computer simulations, we determine the stability boundaries of the material parameters for which the device will not exhibit filamentary tendencies. We then propose a new method of controlling filamentation using below-bandgap semiconductor nonlinearities. With simulations, we determine under what conditions this imposed nonlinearity can counteract the carrier-induced self-focusing inside the active region.
Most of the previous treatments of semiconductor lasers subject to optical feedback from a phase-conjugate mirror (PCM) have assumed the PCM responds instantaneously. Furthermore, the mechanism responsible for phase conjugation does not usually enter into the analysis. In this paper are derived the time-dependent reflectivity from a PCM created through non-degenerate four-wave mixing. The resulting laser dynamics are compared to the case of the ideal PCM, as a function of PCM mirror interaction depth, distance to the PCM, and laser current. The time-responsive PCM tends to suppress otherwise chaotic output and produces power pulses whose frequency is tunable by varying laser current or PCM reflectivity.
A novel analytic method for analyzing hybrid DFB structures containing multiple linear and nonlinear sections is presented. The technique is illustrated by considering uniform and phase- shifted nonlinear devices that play the roles of an amplifier, a filter, and an all-optical switch simultaneously.
Phase-conjugate feedback affects a laser in ways which are fundamentally different than conventional feedback. Notably, when the laser oscillates in more than one longitudinal mode, phase-conjugate feedback initiates a novel mode-coupling mechanism which can even lead to mode locking behavior. This paper explores these mode-coupling effects and also summarizes the simularities and differences between the two types of feedback.
In this paper we describe some of the effects of external optical feedback (OFB) on semiconductor lasers by simulation of the stochastic rate equations. Particular attention is paid to the laser's transition to optical chaos. In addition, we describe three techniques for avoiding this chaotic regime. The technique of high frequency injection, used in optical recording, can delay the onset of chaos till very high values of OFB. Experimental results are given and are in excellent agreement with the theory. A second technique called occasional proportional feedback can be used with some success to stabilize the chaotic output of semiconductor lasers. The final technique for controlling chaos consists of the optimization of various system and laser parameters so that the laser is least susceptible to OFB.
The usefulness of semiconductor lasers can be greatly limited when the laser is subjected to uncontrolled optical feedback (OFB). In particular, the laser intensity noise can be severely degraded when OFB is greater than 0.1%. Although the technique of high-frequency injection (HFI) can solve this problem, the proper modulation frequency and depth must be chosen empirically. We investigate this problem through cornputer simulations of the multimode stochastic rate equations, modified to include OFB and HFI. By providing the program with measurable laser and system parameters, the simulations predict the HFI modulation frequency and depth that optimize the laser behavior. The results of the simulations are compared with experiment, and good agreement is obtained.
Intrapulse stimulated Raman scattering (ISRS) is an important physical phenomenon responsible for the self-induced frequency shift of solitons in optical fibers. ISRS is generally treated as a perturbation term in the nonlinear Schrodinger equation, an approximation that breaks down for ultrashort optical pulses. The authors obtain the solitary-wave solutions of the nonlinear Schrodinger equation by including both ISRS and self-steepening. These solutions do not correspond to optical pulses but represent optical fronts or optical shocks. The properties of ISRS-supported optical shocks are discussed in detail.
This paper reviews the importance of nonlinear gain and its impact on the performance of semiconductor lasers. The physical mechanisms which can lead to an intensity dependence of the optical gain in the above-threshold regime are described briefly. A specific nonlinear-gain mechanism, referred to as intraband gain saturation, is discussed in detail by considering its effect on the important laser characteristics such as the modulation bandwidth, intensity noise, and the laser linewidth. Particular attention is paid to the effects of cross saturation in nearly single-mode semiconductor lasers. Even a weak side mode can lead to saturation and rebroadening of the main-mode linewidth due to mode coupling induced by the nonlinear gain.
Spontaneous emission is a major source of noise in semiconductor lasers. The noise phenomena such as relative intensity noise mode-partition noise and laser linewidth are discussed by using the Langevin rate equations. Particular attention is paid to the impact of intensity arid phase noise on the performance of optical communication systems.