Gain-clamped Semiconductor Optical Amplifier Research Articles

SOA Enabled Self-Homodyne Coherent Bidirectional Transmission for Metro-Datacenter Interconnects

Self-homodyne coherent bidirectional (SHC-BiDi) scheme has gained much attention as a promising coherent-lite solution for datacenter interconnects (DCIs) recently. However, the twofold propagation loss of parallelly delivered signal and remote local oscillator (LO), as well as the power split loss of laser, severely restrict its power budget, making it inapplicable for metro-DCI networks until now. In this work, we propose a reach-extended, cost-efficient SHC-BiDi scheme suitable for metro-DCI applications. Thereinto, two bidirectionally operated semiconductor optical amplifiers (SOAs), with the signal gain clamped by counter-propagating remote LO, are utilized to simultaneously achieve linear signal amplification and remote LO regeneration. We comprehensively investigate the bidirectionally operated gain-clamped SOA and reach-extended transmission. The results show that the SOA can be transformed into a linear booster signal amplifier by clamping LO at proper power, while long-wavelength clamping reduces the nonlinear distortions of signal more efficiently. Besides, despite the potential cross-phase modulation (XPM) impairments on regenerated LOs, its induced Q <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> factor penalty can be kept below 0.570 dB, when the signal power and LO power injected into the SOA are below -10 dBm and above 0 dBm, respectively. Furthermore, by utilizing the built-in dithers of lasers to suppress the stimulated Brillion scattering (SBS) in LO propagation, successful 400G dual-polarization 16QAM transmissions are demonstrated over 50km and 75km links, with the corresponding transmitter laser power of 13.7 dBm and 18.5 dBm. Wherein, the digital signal processing (DSP) employing simple maximum likelihood (ML) phase recovery is also validated to be feasible under ∼1MHz laser linewidth.

Acceleration of Carrier Lifetime in Gain-Clamped Semiconductor Optical Amplifiers

A new method to accelerate the carrier lifetime in semiconductor optical amplifiers (SOAs) is experimentally demonstrated. The four-wave mixing (FWM) effect is induced without recourse to external optical light sources. A pair of fiber Bragg gratings is utilized to generate pre-spectral sliced seed light channels as FWM inputs, by reflecting the backward amplified spontaneous emission from the SOA in a gain-clamped configuration. The theory predicts a three times shorter carrier lifetime, compared to coherent inputs in a travelling-wave arrangement. Furthermore, this method can be a more accurate measure of the nonlinear parameters of SOAs rather than conventional techniques, owing to the polarization-independent nature of the corresponding incoherent products.

Effect of doping and energy detuning on the gain and crosstalk of quantum dot gain-clamped semiconductor optical amplifiers

The spectral characteristics of the linear gain and the small-signal crosstalk of positively detuned and negatively detuned quantum dot gain-clamped semiconductor optical amplifiers (QDGC-SOA) have been studied. Our analysis reveals that the linear gain of the ground-state in positively detuned QDGC-SOA can be enhanced by doping the dots by p-type concentration, while the linear gain of the excited-state in negatively detuned QDGC-SOA can be enhanced by doping the dots by n-type concentration. The small-signal crosstalk is found to be proportional to the amplification where we find that positively detuned QDGC-SOA exhibits less crosstalk (by at least 3 dB) than negatively detuned QDGC-SOA. In addition, we find that positively detuned QDGC-SOA is capable of providing larger optical gain.

Monolithic Adjustable Gain-Clamped Semiconductor Optical Amplifier

We report a monolithic adjustable gain-clamped semiconductor optical amplifier (AGC-SOA). The device consists of two tunable gratings and a gain section and enables the gain of the SOA to be regulated without loss of saturated output power. Gain control is achieved by adjusting the wavelength overlap of two Distributed Bragg Reflector gratings positioned at either side of the active region which can be wavelength tuned by carrier injection. Gain clamped operation with adjustable gain over a range of 4 dB has been demonstrated. A maximum saturated output power of 21 dBm was obtained.

Broadband gain-clamped linear quantum dash optical amplifiers

The linear optical gain of gain-clamped quantum dash semiconductor optical amplifiers (GCSOAs) has been investigated using the rate equation model. The gain spectrum of GCSOA for different wavelength detuning and different doping has been studied. Our analysis shows that the linear gain can be increased as the laser wavelength is detuned to high wavelength where the peak of the optical gain, which is found at wavelengths below the ground state wavelength, is shifted to lower wavelength as the laser wavelength is increased. We find that doping the dashes by either N-type or P-type enhances the linear optical gain and shifts the gain peak to lower wavelength. Moreover, we found that GCSOA with lightly N-type doping demonstrates large separation between the laser and the amplifier wavelength. Also we find that small inhomogeneous line broadening enhances the linear gain peak, shifts the gain peak to wavelength lower than the GS wavelength and widens the gain spectrum.

A broadly tunable fiber ring laser employing a gain-clamped semiconductor optical amplifier

A widely tunable fiber ring laser is demonstrated experimentally using a specially designed gain-clamped semiconductor optical amplifier (GC-SOA). The 3 dB bandwidth of the generated amplified spontaneous emission is increased by 10 nm using the GC-SOA. The lasing wavelength is continuously tunable in a range from 1522 nm to almost 1600 nm using a thin-film Fabry–Pérot tunable filter. A side-mode-suppression ratio of greater than 55 dB is achieved over the entire tuning range.

Remote-seeded WDM-PON upgrade using linear semiconductor opticalamplifiers

In this work we have assessed the capacity of a linear (gain-clamped) semiconductor optical amplifier to enhance the budget of WDM PON network links for their evolution from FTTC to FTTH access. A wavelength-seeded network architecture has been considered, evaluating the performance improvement obtained by the use of an amplifier for the cases of link reach extension and optical splitting to reach end users. The evaluation measurements have shown that the extra budget is enough to compensate for the losses of a passive splitter up to atleast 1:16 division rate or to highly increment reach of the network.

Multiwavelength fiber ring laser using a gain clamped semiconductor optical amplifier

We demonstrate a novel multi-wavelength fiber ring laser based on a gain clamped semiconductor optical amplifier. The number of lasing lines can be tuned by adjusting the loss inside the cavity. The wavelength interval between the wavelengths is 100GHz. The proposed laser shows a stable operation with total intensity fluctuation for a single laser line within ±0.02dB at room temperature for a period of 30-minutes.

Detailed Theoretical Model for Adjustable Gain-Clamped Semiconductor Optical Amplifier

The adjustable gain-clamped semiconductor optical amplifier (AGC-SOA) uses two SOAs in a ring-cavity topology: one to amplify the signal and the other to control the gain. The device was designed to maximize the output saturated power while adjusting gain to regulate power differences between packets without loss of linearity. This type of subsystem can be used for power equalisation and linear amplification in packet-based dynamic systems such as passive optical networks (PONs). A detailed theoretical model is presented in this paper to simulate the operation of the AGC-SOA, which gives a better understanding of the underlying gain clamping mechanics. Simulations and comparisons with steady-state and dynamic gain modulation experimental performance are given which validate the model.

The Dynamic Gain Modulation Performance of Adjustable Gain-Clamped Semiconductor Optical Amplifiers (AGC-SOA)

The growth in demand for high bandwidth services has stimulated the deployment of Passive Optical Networks (PONs), directly to the home or to the kerb. In many cases, particularly extended reach PONs which may cover distances of 100 km or more [1], there is the need for low cost reach extension technologies. Semiconductor Optical Amplifiers (SOAs) have a key role in this context, particularly because upstream traffic is commonly carried at 1.3 μm. Upstream traffic in a PON (from the Optical Network Unit, ONU to the Optical Line Terminal, OLT) is normally Time Division Multiplexed (TDM) with a wide variation in path loss arising from differences in transmission distances and splitting losses. The bursty nature of this traffic combined with a wide dynamic range of signal strength ( -15 dBm to -28 dBm-the difference between a very close ONU with a small split ratio and a distant ONU with a high split ratio), places severe demands on the burst mode receiver at the OLT. Conventional fibre amplifiers cannot adjust their gain with packet to packet variations due to their response time. Similarly, conventional SOAs suffer loss of linearity if their bias current and hence gain is rapidly reduced. The paper reports on an adjustable gain-clamped semiconductor optical amplifier (AGC-SOA) designed to maximize the output saturated power while adjusting gain to regulate power differences between packets without loss of linearity. Theoretical modeling predicts that this device is able to modulate gain at nanosecond rates. The analysis is validated experimentally.

Strong tunable slow and fast lights using a gain-clamped semiconductor optical amplifier

Previously demonstrated slow light is still far from applications, particularly due to the limited bandwidth and control speed. Although semiconductor-based slow light has the high bandwidth and sub-nanosecond control speed, slow light was observed only in the absorption regime with attenuation, while fast light observed in the gain regime with amplification. The large power difference in two regimes makes the use of the optical delay impractical. We report novel slow light in the gain regime, with a high power comparable to that of fast light, utilizing the anomalous gain characteristic in a gain-clamped semiconductor optical amplifier. The slow light is tunable to fast light with the current as the only variable. Additional high speed operation, fast delay control, and wide range of operation wavelength make the present approach practical.

Analysis of high bit rate operation of a vertically gain-clamped semiconductor laser amplifier

A detailed theoretical model of the high bit rate operation of a vertically gain-clamped semiconductor optical amplifier with high bit rate return-to-zero and non-return-to-zero input signals is presented. It is shown that good eye opening at 10–40 GBit/s can be achieved at sufficiently high operating currents (hundreds of mA in a realistic construction). A resonance between the relaxation oscillations of the vertical laser and the bit rate of the input signal is detrimental to the amplifier performance and should be avoided. When designing the length of the amplifier, a trade-off exists between the eye opening and the average gain.

A &lt;formula&gt;&lt;tex&gt;$C/L$&lt;/tex&gt;&lt;/formula&gt;-Band Gain-Clamped SOA-Raman Hybrid Amplifier for CWDM Access Networks

We demonstrate an 83-nm flat gain-bandwidth gain-clamped semiconductor optical amplifier-Raman hybrid amplifier for coarse wavelength-division-multiplexing (CWDM) access networks. Four channels of 2.488-Gb/s CWDM signals were amplified and the input dynamic range for 1-dB bit-error-rate power penalty exceeds 17.3 dB.

Gain Control of Semiconductor Optical Amplifier Using a Bandpass Filter in a Feedback Loop

The authors present a simple configuration of a gain-clamped semiconductor optical amplifier (GC-SOA) based on automatic intensity control of a feedback light generated by amplified spontaneous emission, using a narrow bandwidth thin-film-tunable filter. Experimental results show that the proposed amplifier has good gain clamping characteristics and the feedback light dramatically reduces steady and transient gain variations. The feedback light operates satisfactorily with the channel's add-drop frequency up to 20.9 kHz. We also examined the performance of the GC-SOA by employing the feedback light at different wavelengths.

All-Optical Conversion From RZ to NRZ Using Gain-Clamped SOA

An all-optical converter from return-to-zero (RZ) pulses to the nonreturn-to-zero (NRZ) format is presented. The converter operates in two stages: the laser generated in a gain-clamped semiconductor optical amplifier (SOA) is modulated with the data signal; afterwards this signal is wavelength-converted by cross-gain modulation in a common SOA. The setup is noninverting and can feature wavelength conversion. Experimental error-free conversion from 5- and 40-ps RZ pulses to NRZ format is presented at 10 Gb/s using a 211-1 bit sequence

Theoretical investigation of gain-clamped semiconductor optical amplifiers using the amplified spontaneous emission compensating effect

The theoretical model on gain-clamped semiconductor optical amplifiers (GC-SOAs) based on compensating light has been constructed. Using this model, the effects of insertion position and peak reflectivity of the fiber Bragg grating (FBG) on the gain clamping and noise figure (NF) characteristics of GC-SOA are analyzed. The results show that the effect of the FBG insertion position on gain clamping is slight, but the lower NF can be obtained for input FBG-type GC-SOA; when the FBG peak wavelength is designed to close the signal wavelength, the gain clamping and NF characteristics that can be reached are better. Further study shows that, with the increased peak reflectivity of the FBG, the critical input power is broadened and the gain tends to be varied slowly; the larger bias current is helpful to raise gain and decrease the noise figure but is harmful to a gain flatness characteristic.

Gain-clamped semiconductor optical amplifiers based on compensating light: Theoretical model and performance analysis

It is well known that the gain-clamped semiconductor optical amplifier (GC-SOA) based on lasing effect is subject to transmission rate restriction because of relaxation oscillation. The GC-SOA based on compensating effect between signal light and amplified spontaneous emission by combined SOA and fiber Bragg grating (FBG) can be used to overcome this problem. In this paper, the theoretical model on GC-SOA based on compensating light has been constructed. The numerical simulations demonstrate that good gain and noise figure characteristics can be realized by selecting reasonably the FBG insertion position, the peak reflectivity of FBG and the biasing current of GC-SOA.

Time and spectral domain properties of distributed-feedback-type gain-clamped semiconductor optical amplifiers

A comprehensive model for the analysis of distributed-feedback (DFB)-type semiconductor optical amplifiers (SOAs) both in time- and frequency-domain is developed by combination of a wideband field-based traveling-wave rate equation model and an extended transfer matrix formalism for corrugated structures derived in spectral domain. The numerical implementation of the model is successfully fulfilled by dividing the device into many short segments along the cavity length. Based on this unique model, some time- and spectral-domain properties, such as amplified spontaneous emission, laser action, and signal amplification of DFB-type gain-clamped SOAs are simulated and analyzed.

Experimental and Numerical Small-Signal Analysis of Two Types of Gain-Clamped Semiconductor Optical Amplifiers

The behavior of two types of gain-clamped semiconductor optical amplifiers under small-signal optical modulation is analyzed both numerically and experimentally. The small-signal gain as well as the crosstalk is investigated in two different injecting schemes. Dependence on mean input power and signal frequency is studied. The study reveals a strong dependence of the small-signal gain on the direction of the data signal as compared to the pump signal. The biggest difference between both amplifier types is found in the co-propagation scheme where the RF gain shows a clear resonance in the case the gain clamping is achieved by a longitudinal laser field which results in a decrease of the small-signal gain around the resonance frequency. The behavior of the crosstalk is similar in both cases, however still showing differences due to the origin of the gain clamping, which will be discussed. The experiments confirm the results found in the numerical study in all cases.

Time-domain simulation of channel crosstalk and inter-modulation distortion in gain-clamped semiconductor optical amplifiers

A time-domain model is implemented for gain-clamped semiconductor optical amplifiers (GC-SOAs) based on a combination of the separated traveling-wave equations and effective Bloch equations. The key feature of this model lies in its capability of handling the lasing–signal, signal–signal, and signal–noise interactions over a broad wavelength band. Therefore, various nonlinear phenomena such as the cross-gain saturation (XGS) and nondegenerate four-wave mixing (ND-FWM) can readily be captured. After being implemented and validated, this model is applied to the simulation of GC-SOA dynamic behaviors such as the channel crosstalk and intermodulation distortion (IMD). Simulation results show that the third-order IMD can be effectively suppressed by a gain-clamping lasing mode in GC-SOAs in comparison with that in conventional SOAs. The channel crosstalk can also be suppressed to some extent in GC-SOAs, but not as effectively. Other than a homogeneous reduction, the gain-clamping in GC-SOAs does not change the dependence of the channel crosstalk and IMD on the input signal power and channel spacing. It is also shown that the channel crosstalk, unlike the IMD, cannot be efficiently reduced by enlarging the channel spacing even in GC-SOAs.