Broad-band semiconductor optical amplifiers

Future optical networks may require both wavelength conversion and bit shape regeneration in an all-optical domain. The possibility of pulse reshaping while providing wavelength conversion may support new demands over medium and large distances links (Kelly, 2001). Indeed, during propagation the optical data signal suffers deterioration due to the amplified spontaneous emission (ASE) from optical amplifiers, pulse distortion from intrinsic dispersion, crosstalk, and attenuation. All-optical regenerators may be important components for the restoration of these signals, providing complexity and cost reductions with the avoidance of optoelectronic conversions. The regeneration could be 2R (reamplification and reshaping) or 3R, which also provide retiming to solve jitter (Simon et al., 2008). Several 3R regenerators using the semiconductor optical amplifier (SOA) have been proposed, such as cascaded SOAs setups (Funabashi et al., 2006) or SOA based MachZehnder interferometers (MZI) (Fischer et al., 1999). However for small and medium distances systems, where the signal amplitude noise and distortions form the main problem and where jitter has fewer magnitudes, the simpler 2R processes can be adequate to keep signal quality (Simon et al., 1998). In addition, the SOA is a helpful device for both 2R-regeneration (Ohman et al., 2003) and wavelength conversion (Durhuus et al., 1996). Several techniques for 2R-regeneration based on SOAs have been proposed and tested, for example by using four-wave mixing (FWM) (Simos et al., 2004), cross-gain modulation (XGM) (Contestabile et al., 2005), integration within MZI (Wang et al., 2007), multimode interferometric SOA (Merlier et al., 2001), cross-phase modulation (XPM) with filtering (Chayet et al., 2004), and feed forward technique (Conforti et al., 1999). However, these techniques require complex designs and involve critical operation points, even the simplest ones based on XGM features. In addition, most of these techniques are not capable of wavelength conversion and regeneration simultaneously. Recently a regenerator based on cross-gain modulation was proposed using one SOA for wavelength conversion (in a counter-propagating mode) and another deeply saturated SOA (synchronized by an optical delay line) to achieve cross-gain compression (Contestabile 2005). This efficient approach has similarities with the all-optical feed-forward techniques. In addition, this regenerator could not done wavelength conversion if the wavelength of the

Summary form only given. An overview is presented of the progress in design and characteristics of semiconductor optical amplifiers (SOA) for 1310, 1400 and 1550 nm operation in fiber optical transmission systems. The technology and performance of SOAs will be discussed with respect to optical gain, noise figure, saturation output power and polarization (in-) dependent behavior. The use of both strained quantum wells and bulk tensile material will be discussed in view of obtaining polarization independent behavior. Gain clamping in SOAs has been shown to improve the linearity performance especially for analogue signal transmission. The SOA technology has been demonstrated for optical pre-amplification, in-line application and optical booster applications. DWDM experiments using SOAs in up to 20 Gbit/s systems in short- and medium-distance transmission will be highlighted. Also, SOAs have been used in optical demultiplexing from 40 Gbit/s down to 10 Gbit/s RZ signals using its phase properties and in integrated photonic devices like multiwavelength lasers.

Summary form only given. Summary form only given. Amplified spontaneous emission (ASE) noise is a major limit to the performance of modern day optical transmission systems. A way to alleviate system impairments due to ASE is the insertion of bandpass filters along the line. Soliton pulses have shown excellent stability to in-line narrow-band filtering, and also to sliding of the filter center frequency along the line. The dynamics of soliton signals in the presence of both fixed and sliding filters has been the subject of detailed studies. However, little attention has been given to the dynamics of the ASE growth in the presence of in-line filters. The analysis of the ASE noise growth in a chain of N amplifiers in the presence of in-line filters is the subject of this work. This analysis has been extended to the case of semiconductor optical amplifiers (SOA), where the noise growth, combined with the fast saturation of the amplifier gain, are more effective in degrading system performance. The analytical results have been compared with numerical simulations. Good agreement has been obtained for in-line erbium amplifiers, also considering a high inversion factor. For SOAs, on the other hand, the more complex amplifier dynamics makes the theory quantitatively less accurate, as expected. Even in this case, however, the analytical formulas can be taken as useful guidelines for the design of optical systems.

We demonstrate the cascading of broad-band semiconductor optical amplifier-Raman hybrid amplifiers which provide nearly flat gain over 70 nm. A coarse-wavelength-division-multiplexing transmission system consisting of three spans of 80 km shows uniform performance and <1-dB power penalty.

We have proposed a simple and convenient gain control scheme of the semiconductor optical amplifier (SOA) utilizing the amplified spontaneous emission (ASE) from the SOA itself. Based upon this scheme, an optical pulse active replicator using a SOA could be used for high-fidelity replication of short analog optical pulses. Taking into account the ASE noise, a numerical model of the SOA is developed to investigate the optical pulse recirculating propagation in the active replicator. Results indicate that the replicator is feasible and effective. Selecting appropriately operating parameters, such as the loss within the loop, the power of signal pulse and the front-end receiver electrical bandwidth, the replicator can generate a sequence of precise periodic optical pulse trains with low distortion and good SNR.

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Fiber optic amplifier is a field of new growing interest owing to its application to the optical metro and access networks for amplifying a weak optical signal. In order to design short-fiber-length and broadband optical amplifier, a new amplified fiber with semiconductor inner cladding has been presented. The semiconductor optical amplifier (SOA) is usually pumped by electric currents, but an optical pump is used to the semiconductor inner cladding fiber amplifier in our work. The rate equation is derived in the optical pumped amplifier, and the properties of the amplifier are analyzed. Comparison with conventional optical pumped amplifier, the semiconductor inner cladding fiber amplifier can be pumped by a wide-band optical pumped power, and high pump efficiency can be designed.

A mode-adapted semiconductor optical amplifier (SOA) has been fabricated and packaged. At the gain peak, 1500 nm, the fiber to fiber gain was measured to be 32.5 dB. Statistics for eight packaged devices indicate that a fiber-to-fiber gain of 26.3 dB /spl plusmn/ 1.3 dB and a saturation output power of 12.4 dBm /spl plusmn/ 0.4 dBm are typical at a bias of 500 mA for /spl lambda/ = 1550 nm. Polarization sensitivity at 1550 nm was measured to be 1.1 dB /spl plusmn/ 0.4 dB and the transverse electric (TE) polarization state noise figure (NF) was determined to be 7.0 dB /spl plusmn/ 0.5 dB. The coupling loss was 1.3 dB /spl plusmn/ 0.1 dB per facet. This SOA, with a 1.3-nm filter, was used as an optical preamplifier in a 10-Gb/s return-to-zero (RZ) system testbed with a pseudorandom binary sequence (PRBS) of 2/sup 31/ -1. A 14.5-dB improvement in receiver sensitivity was observed at a bit error rate (BER) of 10/sup -11/.

P-doped and undoped quantum dot (QD) semiconductor optical amplifiers (SOAs) having a similar chip gain of 22-24 dB are compared with regard to their static and dynamic characteristics. Amplified spontaneous emission (ASE) spectra reveal the influence of p-doping on the gain characteristics and the temperature stability. In contrast to QD lasers, p-doping does not significantly increase the thermal stability of QD SOAs. The static four-wave mixing efficiency is larger and more temperature stable in undoped devices, leading to a maximum chip conversion efficiency of -2 dB. Small-signal cross-gain modulation (XGM) experiments show an increase in the small-signal bandwidth from 25 GHz for the p-doped SOAs to 40 GHz for the undoped QD SOAs at the same current density. P-doped QD SOAs also achieve small-signal bandwidths beyond 40 GHz but at a larger bias. The XGM is found to be temperature stable in the range of 20°C to 40°C.

Linear feedback shift registers (LFSRs) are essential components of communications systems such as pseudo random bit sequence (PRBS) generators, convolutional encoders, scramblers and randomizers [1, 2]. Fig.1 (left) shows a LFSR scheme in which the XOR gate is operated among two tapped bits i and i+k within the shift register. The result bit is fed to the shift register at the last position (n) and the sequence in the register is 1-bit shifted as well. The experimental setup for demonstrating an all-optical LFSR is shown in Fig.1 (middle). The main components of the scheme are an optical buffer [3], a bit selecting circuit, and a XOR gate. The data vector (A) is generated by modulating the continuous wave (CW) λ s with a 10Gbps non-return-to-zero (NRZ) electrical pattern whose period is T P =33.3168µs. Only 5 bits are used as the initial sequence for the shift register. The travelling time of the packet in the buffer is T L =1.85 µs, meaning that up to 18 round trips can be accommodated by the pattern's period T P but one of those should be preserved for erasing the buffer before it is fetched a new sequence. The initial sequence enters the buffer as well as propagates directly to the bit selecting circuit. Every round trip, a new sequence as a replica of the previous one plus the feedback bit comes out. At the last round trip, a pump signal inserted into the buffer through the circulator will strongly reduce the gain of the semiconductor optical amplifier (SOA). Both the circulating packet and the accumulating in-band amplified spontaneous emission (ASE) noise will be suppressed. The buffer is now completely cleaned for a new cycle. The first and the last bits of the 1-bit shifted sequence are selected for the XOR operation. Referring to Fig.1 (left), i=1 and k=4, n=5. With this setup, the bit selecting circuit requires 2 clock signals (C) for tap the proper bits from the incoming sequence. The sampling process is done by exploiting four-wave-mixing (FWM) in a 250m-long highly nonlinear fiber (HNLF). Since at every round trip the sequence must be 1-bit shifted, the sampling clocks must be accordingly delayed for 1 bit, resulting the clock period of T C =T L +T B where T B =100ps is the bit time. The outputs of the bit selecting circuit are fed as pump signals to port a and d of a SOA Mach Zehnder interferometer (MZI). They are synchronized with a clock at λ s used as the probe signal (F) at port b. The output of the XOR gate is then fed back into the buffer. The feedback bit should be precisely allocated at the end of the packet circulating inside the buffer, meaning that the feedback time should be T F =T L +T B +T seq in which T seq is the original sequence duration and equal to 500ps in our setup. Fig.1 (right) depicts the results at the 10th round trip for two different input initial sequences (10001) and (10011) taken at point (B). The equalization between bits in the sequence and the quality of the pulses is strongly limited by the accumulating ASE noise inside the buffer, the long fiber-based structure and the bit's polarization. The fiber-based setup also causes misalignment between the clocks, spoils the essential synchronism and degrades the performance of the FWM and the XOR operation. One possible solution for solving all of these problems and hence improving the performance is the integration approach. The bit selection could be implemented exploiting nonlinear effects into an SOA or a periodic-poled lithium niobate (PPLN) instead of using a long HLNF. Moreover the extinction ratio of the pulses at the SOA-MZI XOR output (&#60;13.5dB) could be improved, e.g. with a saturable absorber at its output, thus increasing the number of allowed round trips into the shift register and reducing the noise on the final bit sequence.

This study presents a time-stretched wavelength-swept laser source based on stretched-pulse mode-locking. A broadband semiconductor optical amplifier (SOA) technology is used as an optical gain element. The laser comprises a unidirectional ring cavity with matched positive and negative continuously chirped fiber Bragg gratings (FBG’s). One FBG generates a total positive dispersion of 454 ps/nm at 1275 nm and the other chirped FBG generates a total negative dispersion of -454 ps/nm at 1275 nm. A high-extension lithium-niobate intensity modulator (>30dB extinction at 1275 nm, 4.9 dB loss at maximum transmission) is driven with short pulses by a bit pattern generator providing approximately 0.235 ns full-width at half-maximum pulse profiles. These pulses are stretched, amplified, and compressed within the ring cavity, and the modulator pulsing is synchronized to a harmonic of the cavity round trip time. The laser output is provided from the cavity by a 25% coupler. The output light is amplified by another SOA. The laser source provides a sweeping range of approximately 90 nm centered at around 1275 nm at a repetition rate of ~5 MHz. This yields an estimated axial resolution of 8 μm in air.

All-optical wavelength conversion is demonstrated by using a spectrum-sliced amplified spontaneous emission (ASE) in the semiconductor optical amplifier (SOA) as the probe light and a delay interference configuration. Characteristics of the ASE output in terms of its spectral dependences under the co-propagating injection by a signal light are analyzed theoretically. Analytical expressions for the ASE output in terms of power and phase changes in the wavelength conversion are deduced. An experimental system based on this scheme is built and its operation principles are introduced. System's performances are investigated and several experimental results on measurements of bit error ratios, extinction ratios under different wavelengths and time waveforms as well as eye diagrams are presented.

Band-gap shrinkage is an important effect in semiconductor lasers and optical amplifiers. In the former it leads to an increase in the lasing wavelength and in the latter an increase in the gain peak wavelength as the bias current is increased. The most common model used for carrier-density dependent band-gap shrinkage is a cube root dependency on carrier density, which is strictly only true for high carrier densities and low temperatures. This simple model, involves a material constant which is treated as a fitting parameter. Strained InGaAsP material is commonly used to fabricate polarization insensitive semiconductor optical amplifiers (SOAs). Most mathematical models for SOAs use the cube root bandgap shrinkage model. However, because SOAs are often operated over a wide range of drive currents and input optical powers leading to large variations in carrier density along the amplifier length, for improved model accuracy it is preferable to use band-gap shrinkage calculated from knowledge of the material bandstructure. In this letter the carrier density dependent band-gap shrinkage for strained InGaAsP is calculated by using detailed non-parabolic conduction and valence band models. The shrinkage dependency on temperature and both tensile and compressive strain is investigated and compared to the cube root model, for which it shows significant deviation. A simple power model, showing an almost square-root dependency, is derived for carrier densities in the range usually encountered in InGaAsP laser diodes and SOAs.

In this paper, we investigate a SOA (semiconductor optical amplifier) preamplifier model by suitable choosing the material and device parameters for the SOA to reduce the amplified spontaneous emission (ASE) noise at low range of 22.3 &#956;W with 0.1 mW input power for PIN receiver. Our proposed SOA optical preamplifier is found to be more relaxed from optical alignment & antireflection coating and eliminate the need of optical filter and give large tolerance of an input light wavelength more than 100 nm. Also we investigate the receiver sensitivity for different bit rate. We show that receiver sensitivity is -69.9 dBm at bit error rate (BER) of 4.6×10^-10 for 10 Gb/s and for the 40 Gb/s bit rate the improved receiver sensitivity is -19.2 dBm with PIN receiver. We also observe the sensitivity of -40.5 dBm at 40 Gb/s with DPSK (differential phase shift keying) receiver. Further the impact of amplified spontaneous emission power, gain variation with input light wavelength & optical gain for TE & TM modes with input power for PIN & DPSK receivers has been illustrated. We show that optical gain of SOA is polarization independent and remain constant 30.06 dB up to gain saturation. We also show that DPSK receiver suffer less from ASE noise. The variation of material loss, length & bias current for our SOA preamplifier to optimize the receiver sensitivity is further illustrated.

A novel optical preamplifier using the optical modulation of amplified spontaneous emission (ASE) in a semiconductor optical amplifier (SOA) is demonstrated. The input signal is converted to the modulation of the ASE due to saturation characteristic in an SOA. The receiver sensitivity was improved by 10 dB at 1 Gbit/s using the proposed preamplifier. The proposed device can not only amplify optical signal but also suppress the intensity noise of the input signal under saturation regime. The intensity noise of 1 Gbit/s spectrum-sliced light was significantly suppressed using the nonlinear characteristic of the preamplifier.