Saturated Amplifier Research Articles

Interferometric method of suppressing the pattern effect in a semiconductor optical amplifier

A new idea of using change in index of refraction to suppress gain variation in a saturated semiconductor optical amplifier (SOA) is presented. This kind of gain compensation has the advantage of high speed because it involves two phenomena that always accompany each other. This compensation can be achieved with a nonsymmetrical Mach-Zehnder interferometer structure. Calculated results show that with this structure the input and output power of the SOA can be extended to nearly 10 dB from the former small-signal limit when less than 1-dB gain variation is permitted. Numerical simulations with an advanced dynamic model of the SOA agree with the calculated results.

On three-dimensional dissipative optical solitons: Collisions of laser bullets and topological solitons

Results of numerical simulation of collisions of two fundamental laser bullets—three-dimensional solitons formed in a medium with saturable amplification and absorption and frequency dispersion—are presented. A new collision regime resulting in the formation and the propagation of a switching wave is revealed. The existence of a metastable topological (with a wave front dislocation and an annular intensity distribution) three-dimensional dissipative optical soliton is demonstrated.

Review of collisional excitation neon-like and nickel-like soft X-ray lasers pumped by multiple infrared laser pulses at the Institute of Laser Engineering, Osaka University

We report on the soft X-ray laser developments performed at the Institute of Laser Engineering, Osaka University especially for the collisional excitation X-ray lasers at wavelengths between 25 nm and 4 nm. The performance of neon-like and nickel-like ion X-ray lasing pumped by a train of short laser pulses has been investigated. We have also demonstrated double pass amplification using a soft X-ray multi-layer mirror at wavelengths of 19.6 nm and 7.9 nm. Based on these results and experimental technique, two targets were placed in series to double the gain length. Two opposing laser beams irradiated the double targets with a suitable time difference for quasi-traveling wave pumping. The double target amplification was successfully demonstrated with two beam irradiation for Ag, Nd, Yb, Hf and Ta lasing. The estimated absolute photon flux from the saturated amplification Ag X-ray laser was ∼300 μJ. Based on the experimental results, we also discuss the applications of an X-ray laser as a super-high brightness soft X-ray source.

Application of enhancement mode FET technology for wireless subscriber transmit/receive circuits

Single-supply power amplifiers have become the new paradigm in portable phone handsets due to the recent availability of heterojunction bipolar transistor (HBT) and pseudo enhancement mode PHEMT technology. We have developed a true enhancement mode heterostructure insulated-gate FET device (HIGFET) which is suitable for use in both saturated and linear power amplifiers. A three-stage power amplifier designed for 1900-MHz NADC application delivered +30-dBm output power and 41.7% power-added efficiency with an adjacent channel power of -29.8 dBc and alternate adjacent channel power of -48.4 dBc. In addition to this, we have demonstrated excellent noise figure and linearity performance for small-signal applications. At 900 MHz and bias conditions V/sub DS/=1.0 V and I/sub DSQ/=1 mA, a single-stage amplifier achieved a noise figure of 1.17 dB with an associated gain of 18.5 dB. These results make the technology an ideal candidate for application in both transmitter and receiver circuits.

Suppression of interferometric crosstalk and ASE noise using a polarization multiplexing technique and an SOA

Noise suppression at 10 Gbit/s and 20 Gbit/s is demonstrated using a gain saturated semiconductor optical amplifier (SOA) and a polarization multiplexing technique, where no impairments like waveform distortion and extinction ratio degradation caused by the gain saturation of the SOA appear. Moreover, the method is bit rate transparent and the input power dynamic range is very large. Furthermore, the SOA can provide a high gain.

Two-color generation in a rare-earth-doped quasiphase-matched structure

A numerical study of two-color operation of quasiphase-matched structure in the lasing regime is presented for the case when a 980 nm pump produces the output both at 1550 and 775 nm. Successful operation of the two-color device relies on the use of a novel gain medium which consists of diode-pumped high-density rare-earth ions and on the utilization of a saturable amplification and absorption in the pumped and unpumped section of the device, correspondingly.

Stabilisation of emission in the first Stokes component of stimulated Raman scattering in the nonlinear regime

Equations of coherent stimulated Raman scattering were solved analytically for samples of length smaller than the wavelength of incident radiation, and numerically for extended media. The energy distribution function of Stokes pulses was determined. Transition from the linear regime of stimulated Raman scattering to the nonlinear regime was studied. The intensity of the first Stokes component was shown to be stable in the case of saturated amplification of spontaneous emission.

Modeling the electromagnetic response of hearing aids to digital wireless phones

Interference produced in hearing aids by the pulsed RF signal from digital wireless phones has become an increasingly important issue to wireless phone manufacturers and service providers, hearing aid manufacturers and users, and government regulatory agencies. Development and validation of a comprehensive model of the interaction would greatly benefit the efforts to achieve mutual electromagnetic compatibility (EMC). In order to develop reliable accurate methods to measure hearing aid immunity, an exact mathematical relationship must be demonstrated between the interference generated in hearing aids using a dipole with a standardized test signal [the input referenced interference level (IRIL)] and that produced by actual wireless phones with various signal formats [the overall input referenced interference level (OIRIL)]. A set of theoretical conversion factors has been developed and applied to predict OIRIL interference from the standard IRIL, measured immunity value. A square-law relationship exists within the linear response region of the hearing aid such that each 1 dB increase in RF power (or field strength in decibels V/m) results in a 2 dB sound pressure level (SPL) increase in acoustic power (or sound pressure level). Hence, the IRIL for any given field strength is obtained by doubling (in decibels) the change in field strength and adding the result to the reference IRIL (in decibels SPL). Subtracting 7.60 dB [for time-division multiple access (TDMA)-50 Hz] or 10.68 dB [for TDMA-217 Hz or code-division multiple access (CDMA)] from the IRIL predicts the corresponding OIRIL. The lower and upper limits of the predicted OIRIL are constrained by the measured ambient sound level and the amplifier saturation, respectively. The model predictions are valid when comparable field strength gradients and distributions, separation distances and orientations are maintained between the hearing aid and the RF emitter.

10 Gbit/s RZ transmission over 5000 km with gain-clamped semiconductor optical amplifiers and saturable absorbers

By combining a gain-clamped semiconductor optical amplifier (GC-SOA) and a saturable absorber-semiconductor optical amplifier module, 10 Gbit/s RZ transmission over 5000 km (200 × 25 km) has been demonstrated in a fibre loop setup. It is shown that a GC-SOA could compensate for the patterning introduced by a conventional SOA.

A transfer function approach to the small-signal response of saturated semiconductor optical amplifiers

A theoretical analysis of the small-signal frequency response (SSFR) of a wavelength converter based on cross-gain modulation in a semiconductor optical amplifier with a finite waveguide loss is presented. We use a transfer function formalism to explain the resonant behavior of the frequency response. The limitations to the magnitude of the spectral overshoot are also accounted for. Operating with the data and CW signals in a co-propagating configuration, we find that the resonance only exists for a finite waveguide loss. In a counter-propagating scheme, a resonance can exist regardless of the presence of loss.

Highly efficient degenerate four-wave mixing with multipass geometries in a polymer laser dye saturable amplifier

We demonstrate the use of highly efficient degenerate four-wave mixing for phase conjugation with multipass geometries in a polymer laser dye saturable amplifier. Phase-conjugate energy reflectivity of as much as 20,000% with a four-pass geometry in the longitudinally pumped polymer dye was obtained at 564 nm.

An Efficient Simulation Model of the Erbium-Doped Fiber for the Study of Multiwavelength Optical Networks

The first part of this paper reviews the spectrally resolved erbium-doped fiber model by Saleh, Jopson et al. (1990, IEEE Photon Technol. Lett.2, 714; 1991, Fiber Laser Sources and Amplifiers III, Vol. 1851, pp. 114–119, SPIE). This model is adequate for fast simulation of erbium-doped fiber amplifiers pumped at 980 or 1480 nm which are not self-saturated by amplified spontaneous emission noise. The second part of this paper reviews the wavelength-domain representation of optical signals and network components at the optical transport layer of multiwavelength optical networks. This representation stems from the spectrally resolved model of erbium-doped fiber amplifiers. Optical signals are represented by their carrier wavelength and average power exclusively and not by their temporal waveform, as is customary in simulation of analog and digital communication systems. In addition, network components are fully characterized by their loss or gain as a function of wavelength. The wavelength-domain representation is adequate for efficient steady-state and transient power-budget computations; i.e., it can be used to evaluate the optical signal, amplified spontaneous emission noise, and linear optical crosstalk average powers at all points in a multiwavelength optical network. To illustrate the capabilities of the spectrally resolved erbium-doped fiber model by Saleh, Jopson et al. and the wavelength-domain representation, transient power fluctuations caused by the dynamic interaction of saturated erbium-doped fiber amplifiers and servo-controlled attenuators in a bidirectional ring composed of four wavelength add–drop multiplexers are studied. The mechanisms responsible for this oscillatory behavior are identified and remedies are proposed.

Table-top soft x-ray lasers

This article reviews the progress in the development of practical table-top sources of soft x-ray laser radiation. The field is rapidly approaching the stage at which soft x-ray lasers sufficiently compact to fit onto a normal optical table will be routinely utilized in science and technology. This is the result of recent advances in the amplification of soft x-ray radiation in both compact laser-pumped and discharge-pumped devices. The use of excitation mechanisms that take full advantage of new ultrafast high power optical laser drivers and multiple pulse excitation schemes has resulted in the demonstration of saturated soft x-ray amplification at wavelengths as short as 14 nm using several Joule of laser-pump energy. Moreover, several schemes have demonstrated significant gain with only a fraction of a Joule of laser-pump energy. In addition, the demonstration of saturated table-top soft x-ray lasers pumped by very compact capillary discharges has shattered the notion that discharge-created plasmas are insufficiently uniform to allow for soft x-ray amplification, opening a route for the development of efficient, high average power soft x-ray lasers. Recently, a table-top capillary discharge laser operating at 46.9 nm has produced millijoule-level laser pulses at a repetition rate of several Hz, with a corresponding spatially coherent average power per unit bandwidth comparable to that of a beam line at a third generation synchrotron facility. This review summarizes fundamental and technical aspects of table-top soft x-ray lasers based on the generation of population inversions in plasmas, and discusses the present status of development of specific laser systems.

Effects of Spectral Broadening and Cross Relaxation on the Gain Saturation Characteristics of Quantum Dot Laser Amplifiers

Spectral broadening and cross relaxation of an aggregate of highly carrier-injected quantum dots have been studied experimentally by evaluating the gain saturation and optical-wave mixing characteristics of traveling-wave laser amplifiers that include self-assembled 0.98-µm InGaAs dots. Measured signal-gain spectra at room temperature showed that the gain decreases over the entire gain spectra when an intense and narrow-linewidth saturating signal is injected at a higher-order subband transition of the quantum dot. Differential-gain spectrum was calculated from the saturated and unsaturated signal gain spectra, and revealed asymmetric gain saturation with respect to the wavelength of the saturating signal. These results suggest that cross relaxation is so fast that the dot aggregate has a homogeneously broadened gain spectrum. Self-saturation and cross-saturation characteristics, and the detuning characteristics of highly nondegenerate four-wave mixing were also investigated. The latter revealed that the characteristic times of two cross-relaxation processes are about 1 ps and 100 fs.

Generation of millijoule-level soft-x-ray laser pulses at a 4-Hz repetition rate in a highly saturated tabletop capillary discharge amplifier

Laser pulses with energies of as much as 1 mJ were generated at a wavelength of 46.9 nm by single-pass amplification in a 34.5 cm-long Ne-like Ar capillary discharge plasma. The large gain-length product of this plasma column allows for soft-x-ray amplification in a highly saturated regime, resulting in efficient energy extraction. Average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz. With an estimated peak spectral brightness of approximately 1x10(23)photons /(s mm(2)mrad> (2)0.01% bandwidth) this tabletop laser is one of the brightest soft-x-ray sources to date.

Numerical simulation of a passive twin-core fibre nonlinear coupler ring laser

Passive modelocking of a passive nonlinear directional coupler ring laser employing a near-half-beat-length passive twin-core fibre is numerically simulated by taking into account Raman self frequency shift and gain saturation. Raman self frequency shift is reduced by inserting two bandpass filters. Modelocking threshold condition and steady-state characteristics are investigated in extensive numerical experiments. It is found that, in addition to solitonic modelocking operation regime predicted by previous studies, non-soliton and chirped soliton operation regimes also exist in such a laser. The solitonic modelocking operation requires a proper balance of saturable amplification, bandpass filtering, nonlinear coupling and output coupling. The generation of transform-limited solitons with durations as short as 0.44 ps and pedestal extinctions of better than 40 dB is numerically demonstrated.

Measurement and calculation of the critical pulsewidth for gain saturation in semiconductor optical amplifiers

The nonlinear gain response of InGaAsP bulk optical amplifiers under ultrafast optical excitation at 1.53 μm is investigated. In particular, the dependence of the gain saturation energy on the pulse duration is measured in the range of pulse durations from 150 fs to 11 ps, for different bias currents and lengths of the amplifier. By comparison with a theoretical model, a critical pulsewidth is inferred below which nonlinear carrier dynamics like carrier heating and spectral hole burning dominate the gain saturation.

Unique Properties of Quadratic Solitons

Quadratic spatial solitons exist in media with second order nonlinearities near the phase-matching condition for frequency mixing processes involving two or three waves of different frequency. Discussed here are a number of properties of these special solitons which are different from those of other spatial solitons which rely on optically induced index changes for guiding. First, the self-guiding properties of quadratic solitons are shown to have completely different origins than solitons which rely on index changes. Second, it is shown that there exists a large variety of quadratic solitons which contain two or three distinct spectral components with relative amplitudes depending on the phase mismatch, dimensionality of the propagation geometry, the soliton power and the launching conditions. Third, under appropriate conditions, solitons can be formed even when the group velocity directions for the spectral components lead to walk-off under normal circumstances. Fourth, for type II phase-matching in bulk crystals, seeded interactions lead to saturating amplifier characteristics.

Impact of amplifier nonlinearities on OFDM transmission system performance

The power spectral density of an orthogonal frequency-division multiplexing (OFDM) signal after a saturated high-power amplifier (HPA) is analytically derived. The distortion of the HPA-processed OFDM signal is defined, and its power spectrum is computed. The spectra of the signal and of the distortion are used to get an accurate estimate of the bit-error rate of an OFDM transmission system and to derive compensation at the receiver, which leads to performance improvement.

Overview of `Super-ASHURA' KrF Laser Program

In the Super-ASHURA KrF Laser at ETL, a main amplifier with full pumping duration (270 ns) and 12 beam lines have been completed. At present, total output energy of 2.7 kJ has been extracted from the main amplifier by double-pass amplified 12 beams (20 ns×12) with an aperture filling factor of 74%. Local intrinsic efficiency has reached 10%. In the Raman amplification which is to be used for high peak power pulse generation, multi-pass forward amplification of nanosecond Stokes pulse showed an energy conversion efficiency of 73%. Also, a multi-joule short Stokes pulse (150 ps) has been obtained from 12.5 J, and 20 ns pulse by saturated backward amplification of a self-generated Stokes pulse. To investigate the effect of irradiation smoothness, the two-dimensional profile of the target rear surface displacement has been measured by using short (<100 ps) 268 nm probe pulse.