Gain-clamped Semiconductor Optical Amplifier Research Articles

A variable-gain optical amplifier for metro WDM networks with mixed span losses: a gain-clamped semiconductor optical amplifier combined with a Raman fiber amplifier

We propose a variable-gain optical amplifier that exhibits excellent performance in metro wavelength-division-multiplexing networks containing mixed span losses. The amplifier consists of a variable-gain Raman fiber amplifier and a gain-clamped semiconductor optical amplifier. Each span of different loss is exactly compensated by adjusting the pump power of a Raman laser diode to have the maximum optical signal-to-noise ratio. Sixteen channels of 10-Gb/s signals were successfully transmitted over 170 km of mixed span losses (total, 87 dB) under dynamic additions/removals situations.

Gain clamping in semiconductor optical amplifiers with second-order index-coupled DFB grating

A systematic investigation of gain-clamped semiconductor optical amplifiers (GC-SOAs) based on the second-order index-coupled DFB gratings is carried out by way of simulation. In particular, we focus on the main effects of the radiation loss caused by the first-order diffraction of the gratings on the amplifier performance. The magnitude of the total complex coupling coefficient is the main factor to determine the level of gain clamping. We demonstrate that a high-performance GC-SOA can be realized by using purely loss-coupled second-order DFB gratings with more relaxed tolerance on grating strength and period. It is shown that, in the presence of weak reflection-related coupling, the parasitic radiation loss associated with the second-order grating always helps to expand the linear amplification region and to reduce the longitudinal spatial hole burning along the cavity. Further, we demonstrate through comparison that the GC-SOAs have higher saturation power and much shorter carrier lifetime than the conventional SOAs. An improved design by longitudinal variation of the grating duty cycle is proposed such that the noise performance of the amplifier can be enhanced without much sacrifice on the linear amplification regime.

2R limiter circuit with gain clamped SOA for XGM wavelength converter

The authors propose an optical limiter circuit with gain-clamped semiconductor optical amplifiers (GCSOAs) for performance improvement of an XGM wavelength converter. The 2R limiter circuit based on a Mach–Zehnder interferometer (MZI) and GCSOAs can reinvert and regenerate the XGM output signal. The signal reshaping is obtained by controlling the gain and phase conditions of GCSOAs in both arms of the MZI. Simulation results show that the 2R limiter circuit improves extinction ratio by 30 dB and the input power dynamic range can be increased by adjusting the parameters of the XGM wavelength converter.

Acceleration of a Gain-Clamped Semiconductor Optical Amplifier by the Optical Speed-Up at Transparency Scheme

The optical speed-up at transparency principle has been successfully applied to a gain-clamped semiconductor optical amplifier yielding a remarkable acceleration in the gain recovery of the device as well as to the disappearance of the typical relaxation oscillations stemming from the coupling between the internal lasing mode and the carrier density. A record gain recovery time of 13 ps has been measured with an available gain of still 16 dB. A small-signal analysis of the semiconductor laser amplifier rate equations properly reproduces the acceleration of the recovery time and the disappearance of the relaxation oscillations. These results apply to semiconductor lasers in general.

A Gain-Clamped SOA With Distributed Bragg Reflectors Fabricated Under Both Ends of Active Waveguide With Different Lengths

We demonstrate a gain-clamped semiconductor optical amplifier (GC-SOA) which has gratings under both end regions of an active waveguide. Two gratings of the new GC-SOA are fabricated in different lengths. Their lasing modes and gain characteristics are measured and analyzed with different combinations of grating lengths. It shows different gain and noise characteristics according to the measurement direction in asymmetric grating combinations.

160-Gb/s All-Optical Encoding Experiments by Four-Wave Mixing in a Gain-Clamped SOA With Assist-Light Injection

All-optical encoding at 160 Gb/s was demonstrated by using four-wave mixing in a gain-clamped semiconductor optical amplifier (SOA) with a continuous-wave assist-light injection. Clear eye opening was successfully observed with increasing the assist-light power due to suppression of pattern effect caused by carrier depletion in the SOA.

Photon iterative numerical technique for steady-state simulation of gain-clamped semiconductor optical amplifiers

The photon iterative numerical technique, which chooses the outputs of the amplified spontaneous emission spectrum and lasing mode as iteration variables to solve the rate equations, is proposed and applied to analyse the steady behaviour of conventional semiconductor optical amplifiers (SOAs) and gain-clamped semiconductor optical amplifiers (GCSOAs). Numerical results show that the photon iterative method is a much faster and more efficient algorithm than the conventional approach, which chooses the carrier density distribution of the SOAs as the iterative variable. It is also found that the photon iterative method has almost the same computing efficiency for conventional SOAs and GCSOAs.

A gain-clamped-semiconductor-optical-amplifier combined with a distributed Raman-fiber-amplifier: a good candidate as an inline amplifier for WDM networks

A gain-clamped semiconductor optical amplifier (SOA) is used as an inline amplifier in combination with a distributed Raman fiber amplifier. The combined amplifier has 20 dB gain and a noise figure below 2.7 dB. The optical signal to noise ratios after five spans of 20 dB loss, equivalent to 5 × 80 km, are over 25.2 dB for eight-channel transmissions. In addition, the potentially compact amplifier shows negligible transients under dynamic add-drops.

Performance simulation and design optimization gain-clamped semiconductor optical amplifiers based on distributed Bragg reflectors

Performance of the gain-clamped semiconductor optical amplifier (GC-SOA) based on distributed Bragg reflector (DBR) structures is investigated by a comprehensive broad-band time-domain traveling wave model. Critical factors, e.g., the material gain profile, the waveguide grating structures, the longitudinal variation of the optical field, and the carrier density as well as the broad-band spontaneous noise emission are considered in the model. The effects of operation parameters (e.g., the electrical bias current and the input optical power) and design parameters (e.g., the normalized coupling coefficient /spl kappa/L and the lasing wavelength position /spl lambda//sub L/) on device performance are examined in detail. Based on the results of the performance simulation, guidelines for design optimization are discussed and, in particular, unbalanced grating configurations combined with nonuniform current injection schemes are proposed to reduce the noise figure without much sacrifice on other device performance.

Demonstration of 16 x 10 Gb/s WDM transmissions over 5 x 80 km using gain-clamped semiconductor optical amplifiers in combination with distributed Raman fiber amplifiers as inline amplifiers under dynamic add-drop situations

Sixteen channels of 10-Gb/s signals were successfully transmitted over five spans of 80 km using gain-clamped semiconductor amplifiers as inline amplifiers in combination with distributed Raman fiber amplifiers. The combined amplifier had a gain of 20 dB and a noise figure of less than 2.7 dB. No penalty was observed when 15 out of 16 channels were dynamically add-dropped in the 400-km transmission system.

On crosstalk and noise in an optical amplifier with gain clamping by vertical laser field

We have calculated the transient behavior and noise figure of a semiconductor optical amplifier (SOA) with the gain clamped by a vertical cavity laser (VCL). The characteristic behavior of the more conventional gain-clamped SOAs and SOAs with no gain-clamping is also studied and compared with the vertically gain-clamped amplifier. The calculations are based on a numerical stochastic rate equation model including several forward- and backward-propagating channels that are coupled to the vertical laser field through the active medium. The noise model takes into account the input noise, randomly amplified spontaneous emission, and random gain. Numerical simulations have been carried out to study the relaxation oscillations, crosstalk, and noise in a system with a strong input signal switched on and off while observing the output signals, VCL photon density, and carrier density. Results show that the VCL field captures most of the disturbances, in agreement with available experimental data.

Gain-clamped semiconductor optical amplifier based on compensating light generated from amplified spontaneous emission

A gain-clamped semiconductor optical amplifier (SOA) scheme based on automatic power control of a compensating light generated from amplified spontaneous emission by a reflector is proposed. Experimental demonstrations using an SOA with a fibre Bragg grating at the input fibre show the proposed amplifier has a low noise figure as well as good gain clamping characteristics.

All-Optical Decision Gate with an Adjustable Decision Threshold

We investigate the decision threshold adjustment in a proposed all-optical decision gate consisting of two gain-clamped semiconductor optical amplifiers (GCSOA's). The GCSOA discussed in this letter is constructed with a gain section and two passive distributed Bragg reflectors (DBR's) on each side. We derive a model for current-induced or temperature-dependent Bragg reflectance variation, with which the threshold current density and the oscillating power at DBR wavelength are described. This model allows a possible numerical prediction for decision threshold adjustment induced by current injection or temperature variation.

Density clamping and longitudinal spatial hole burning in a gain-clamped semiconductor optical amplifier

We have directly measured, under operating conditions, the distributions of carrier densities and temperatures in a gain-clamped semiconductor optical amplifier designed for operation at 1.55 μm. As expected, longitudinal spatial hole burning is much smaller than in conventional semiconductor optical amplifiers and the effects of gain clamping are clearly evidenced. The amplifier nevertheless shows a sizeable temperature increase for both the lattice and the carriers at high currents, which are attributed to contributions of Auger recombination, intervalence band absorption, and Joule and recombination heating.

Comparison of gain clamped and conventional semiconductor optical amplifiers for fast all-optical switching

This paper reports computer simulation results to compare a Mach-Zehnder interferometer (MZI) when the two arms of it operate with gain-clamped and conventional semiconductor optical amplifiers (gcSOA and convSOA, respectively) for fast all-optical switching. The comparison is for demultiplexing optical pulses from 160 Gb/s to the base rate of 10 Gb/s or 40 Gb/s. The gain dynamics of the gcSOA is presented, the switching windows at the drop port of the MZI for both cases that were referred before and finally an evaluation of them is discussed.

Low input power wavelength converter using gain-clamped semiconductor optical amplifier

A wavelength converter based on direct optical modulation of lasing light in a gain-clamped semiconductor optical amplifier (GC-SOA) is proposed. A low input power of −13.5 dBm, which is the lowest reported to the best of our knowledge, due to good coupling of input data power into the GC-SOA and high lasing threshold, and negative power penalty operation, are experimentally demonstrated at 2.5 Gbit/s.

Demonstration of extinction ratio improvement from 2 to 9 dB and intensity noise reduction with the MZI-GCSOA all-optical 2R regenerator

In this letter, we experimentally demonstrate improved static and dynamic characteristics of the all-optical 2R regenerator based on a Mach-Zehnder interferometer with gain-clamped semiconductor optical amplifiers. A true digital-like optical decision characteristic with an adjustable decision threshold has been found. A tremendous noise suppression at the logic 0 and a large improvement in the extinction ratio have been obtained even for a very deteriorated input signal with small extinction ratio at a bit rate of 2.488 Gb/s. An 8-dB improvement in extinction ratio can be achieved for an input extinction ratio of 5- and 7-dB improvement for an input extinction ratio of 2 dB. Furthermore, a good regeneration is achieved over a broad wavelength range of 20 nm.

Experimental demonstration of an all-optical 2R regenerator with adjustable decision threshold and "true" regeneration characteristics

Experimental 2R regeneration characteristics of an optical decision circuit based on a Mach-Zehnder interferometer with gain clamped semiconductor optical amplifiers in both arms are reported for the first time. The decision circuit gives a true regeneration, i.e., a digital characteristic on a linear instead of a logarithmic (or dB) scale for the input and/or output power.

10 Gbit/s RZ transmission over 5000 km with gain-clampedsemiconductor 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.

Noise analysis of conventional and gain-clamped semiconductor optical amplifiers

We present a numerical study of the noise of conventional and gain-clamped semiconductor optical amplifiers (SOAs), using a detailed device model. The model makes use of a density-matrix gain calculation, and takes into account the forward and backward amplified spontaneous emission (ASE) spectra and the spatial carrier hole-burning. The device is longitudinally divided into M sections and a rate equation for averaged photon and carrier densities is used for each section. We demonstrate that the accuracy on the calculated noise figure strictly depends on the number of sections M. We obtain a good tradeoff between the results accuracy and the computational complexity with M=8. The model is then applied to study the noise in a distributed Bragg reflector (DBR)-type gain-clamped SOA for varying signal power, pump current, and lasing wavelength. We show that changes in the spatial carrier profile caused by the input signal significantly affect the noise figure, even when the gain is constant. A slight dependence of the noise figure on lasing wavelength is also foreseen, while the dependence on the pump current is negligible. A new method for gain-clamped SOA noise figure reduction is proposed, based on unbalanced Bragg reflectors. An improvement of noise figure (NF) as large as 2 dB is devised.