1.

INTRODUCTION

The rapid growth of data traffic increases the demand for capacity in all types of transmission systems. Most of the transmission systems operate in the 1550 nm wavelength domain, where the fiber capacity is being utilized with maximum possible efficiency using advanced modulation formats and dense wavelength multiplexing (DWDM). A way to increase the transmission capacity of an optical fiber is the parallel utilization of different wavelength domains¹. This is a very promising solution for a legacy fiber system capacity upgrade, since it does not require very expensive installation of new fibers. In particular, the 1310 nm wavelength domain can be used in parallel to the 1550 nm one. The 1310 nm window has the advantages of reduced chromatic dispersion related distortions and wide availability of components, such as semiconductor optical amplifiers (SOAs) and electro-absorption modulators (EAMs)². Recently, 1310 nm transmission systems are drawing increased attention due to the introduction of the 100 G Ethernet standard³, where 4x25 Gbit/s 1310 nm transmission is used. As has been shown in Ref. 4, the 1310 nm wavelength domain can be used to deploy transmission systems with capacities of over 400 Gbit/s.

To fully explore the potential of 1310 nm transmission, efficient amplification techniques are needed. The 1310 nm SOA is relatively inexpensive, but introduces strong signal distortions, like noise and saturation effects. With the very high costs of the praseodymium-doped-fiber-amplifier (PDFA)⁵, the most attractive alternative or extension to the 1310 nm SOA is the 1310 nm Raman amplifier, offering a low noise figure, high saturation output power and easy implementation as an in-line amplifier. The recent development of high power quantum-dot (QD) pumping lasers at 1240 nm⁶ allows realization of the cost-effective 1310 nm Raman amplifiers.

The presented research on 1310 nm Raman amplification focused mainly on discrete Raman amplifiers, where special highly-nonlinear fibers are used^{7, 8}. In Refs. 9–11, the use of Raman amplifiers as the reach extenders in passive optical networks (PONs) is described. However, in Ref. 9, a complex cascaded pumping laser was used and in Ref. 10 only simulation results were provided. In Ref. 11, a Raman amplifier with two orthogonally polarized quantum-dot pumping lasers was used and a gain of 14.3 dB was achieved. However, the main scope of this article was the overall performance of the PON system and no detailed characteristics of the Raman amplifier were provided.

To fully explore the potential of the 1310 nm Raman amplifier, a thorough investigation of its design and performance is needed. In this paper we present detailed studies on the design and performance of the Raman amplifier based on the QD lasers. We evaluate the performance of the Raman amplifier in the numerical simulations and experimental measurements. We achieve a maximum gain of 19.5 dB and a minimum noise figure of 3.8 dB in a QD-laser pumped 1310 nm Raman amplifier.

2.

RAMAN AMPLIFICATION

Raman amplification is caused by the Raman scattering effect in the optical fibers¹². In the Raman scattering, the energy from an optical pump is transferred to the signal, resulting in signal amplification¹². In typical fibers used in telecommunication systems, the maximum gain is achieved when the difference between the pump and signal frequencies is 13,2 THz. In the 1310 nm wavelength domain, this corresponds to around 70 nm, so the optimum pump wavelength is around 1240 nm. In a Raman amplifier, a high-power optical pump is injected into the optical fiber in a co-propagating or counter-propagating direction with respect to the signal. Fig. 1 shows the basic setup of a Raman amplifier in the counter-propagating configuration. The 1240 nm signal from the Raman pump (P) is inserted into the transmission fiber (SSMF) through a coupler (C), amplifying the 1310 nm signal propagating from the transmitter (Tx) to the receiver (Rx).

Fig. 1

Scheme of the Raman amplifier in a counter-propagating configuration

Raman amplifiers can be divided into two types: discrete and distributed amplifiers. The discrete Raman amplifier is a lumped element, inserted into the transmission line to provide gain. In these devices, special, highly-nonlinear fibers are used, increasing the effect of Raman scattering. In the distributed Raman amplifier, optical amplification is realized in the transmission fiber. The use of the transmission fiber, as well as lower signal degradation, make this type of Raman amplifier a more attractive solution¹³.

In a Raman amplifier, the gain is described as the ratio of the signal power PS with and without pumping¹²,

where L is the length of the fiber, P_S is the signal power, P_P is the pump power, g_R(W⁻¹m⁻¹) is the Raman gain coefficient of the fiber and L_eff is the effective length of the fiber, defined as

where αP is the attenuation coefficient of the fiber at the pump wavelength. Raman gain varies with the polarization states of the signal and pump. However, as shown in Refs. 12, 14, the utilization of counter-propagating pumping decreases the polarization dependence of the Raman gain to values below 1 dB.

In the Raman amplifier, signal distortions in the form of noise are generated due to the spontaneous Raman scattering and Rayleigh scattering effects¹³. An additional distortion cause is the fast reaction time of the Raman gain (< 1 ps). Because of this fast reaction, in a Raman amplifier, any pump fluctuations are instantly transferred to the signal¹⁵. The amount of the introduced distortions depends on the frequency of the fluctuations and the pumping configuration. In a counter-propagating amplifier, the pump fluctuations are averaged over the whole fiber length, significantly reducing the degradation of the signal.

The amount of signal degradation introduced in an optical amplifier is evaluated using the noise figure parameter (NF). A method of calculating the noise figure for a Raman amplifier was proposed in¹⁶. The noise figure is calculated based on the Raman gain value and is defined as

where G_A is the Raman gain (1) and α_S is the attenuation of the fiber at the signal wavelength.

3.

NUMERICAL INVESTIGATION OF THE 1310 NM RAMAN AMPLIFIER

The characteristics of the Raman amplifier in the 1310 nm wavelength domain were first investigated in the numerical simulations. The simulation setup was based on the Raman amplifier scheme from Fig. 1. The transmitter consisted of a continuous wavelength (CW) laser with an output power of 0 dBm at 1310 nm and the Raman pump consisted of a CW laser with an output power of 0 ÷ 1200 mW at 1240 nm. The polarization states of both lasers were aligned. The transmission line consisted of a standard single-mode fiber (SSMF). A two-way fiber model was used, based on two conjugate nonlinear Schroedinger equations¹⁷. The fiber model took into account attenuation, chromatic and polarization mode dispersion and nonlinear effects. The parameters of the SSMF were: attenuation at 1310 nm 0.4 dB/km, group refractive index 1.47, nonlinear index 2.6·10–20 m2/W, core area 80 μm2, chromatic dispersion coefficient D = 0 ps/nm•km, chromatic dispersion slope S = 0.080 ps/nm2•km. The pump laser power was changed from 0 mW to 1200 mW. The fiber length was 10 ÷ 100 km. The signal wavelength was changed from 1260 nm to 1340 nm.

The dependence of Raman gain (1) on fiber length and pump power is shown on Fig. 2.

Fig. 2

The Raman gain for different values of fiber length and pump power

The maximal gain for pump powers of 1200 mW, 800 mW and 400 mW is 27.4 dB, 18.3 dB and 9.2 dB respectively and the 1-dB gain fall value occurs at distances of 40 km, 32 km and 24 mW. From Fig. 2b, the linear dependence of Raman gain on pump power can be observed. The slope of the curve is higher for longer fibers, i.e. 0.014 dB/mW at 10km, 0.019 dB/mW at 20 km, 0.021 db/mW at 30 km, 0.022 dB/mW at 40 km and 0.023 dB/mW for fiber lengths of 50 km and more.

The dependence of Raman gain on the signal wavelength and the signal output power is shown in Fig. 3. Maximum gain is achieved for a signal wavelength of 1310 nm. The 1dB gain bandwidth is 8 nm for a pump power of 1200 mW and increases with a lower pump power to 10 nm at 800 mW and to 18 nm at 400 mW. The saturation output power (P_SAT) of the Raman amplifier is around 19-20 dBm and does not depend strongly on the pump power. A decrease of the pump power from 1200 mW to 600 mW lowers the P_SAT by only around 1 dB, from 20.3 dB to 19.1 dB.

Fig. 3

Raman gain versus signal wavelength and signal output power, at a 40 km fiber length

In Fig. 4, the dependence of the noise figure on the signal wavelength is shown. The noise figure was calculated using equation (3). The noise figure is maximal (4.58 dB) at 1310 nm and the pump power of 1200 mW. An increase of the noise figure with the pump power can be seen, from 3.0 dB at 200 mW to 4.3 dB at 800 mW and to 4.6 dB at 1200 mW.

Fig. 4

Noise figure versus signal wavelength for different pump powers and a fiber length of 40 kmNoise figure versus signal wavelength for different pump powers and a fiber length of 40 km

4.

1240 NM QD LASERS

As shown in the simulations, the performance of the Raman amplifier depends strongly on the Raman pump. Recently, the 1240 nm quantum dot pumping lasers with output powers of over 500 mW have been demonstrated⁶. The devices are fabricated from a InAs/GaAs compound, with indium arsenide as the quantum dot material and gallium arsenide as the substrate⁶. The lasers are available in 14-pin butterfly packages with a cooling element inside the package and a polarization maintaining output fiber. The optical spectra of two pumping lasers with a 1500 mA current are shown in Fig. 5. As can be seen in Fig. 5, the optical spectra of both lasers are almost identical. The 3-dB bandwidth is in both cases, around 1 nm. The peak output power is 14.6 dBm and 14.4 dBm at 1241 nm.

Fig. 5

The optical spectra of the 1240 nm QD lasers

Fig. 6 shows the dependence of laser output power on the current, for both pumping lasers. The threshold current is around 140 mA for both lasers. For higher current values, the output power increases linearly with the current, at a rate of around 0.37 mW/mA. The maximum achieved output power is, respectively, 563.6 mW and 576.8 mW with a laser current of 1800 mA. The output power does not vary strongly with the temperature. Increasing the temperature from 20 °C to 40 °C changes the output power by only 30 mW and 55 mW respectively for the two lasers. The optimum temperature is in both cases 35 °C and this temperature setting was maintained during the rest of the measurements.

Fig. 6

Output power versus current for both 1240 nm QD lasers

5.

HIGH GAIN 1310 NM QD LASER BASED RAMAN AMPLIFIER

Using the 1240 nm QD pumping lasers, an experimental Raman amplifier was designed. The experimental setup was based on the Raman amplifier scheme from Fig. 1. The transmitter consisted of a tunable 1260-1350 nm laser with an output power of 9.6 dBm at 1310 nm. To ensure the safety of the transmitter, an optical isolator was placed before the transmission fiber, with an insertion loss of 0.6 dB at 1310 nm. The transmission line consisted of two SSMF fiber spans, with lengths of 25 km and 38 km and attenuation at 1310 nm of 0.33 dB/km and 0.32 dB/km respectively. The Raman pump consisted of two 1240 nm QD pumping lasers and output powers of 563.3 mW and 576.8 mW at 1240 nm. Signals from the lasers were combined in a polarization beam combiner, with a 1.2 dB insertion loss at 1240 nm and inserted into the transmission line through a 1%:99% coupler. The output of the power splitter was connected to the transmission fiber. The experimental setup is shown on Fig. 7.

Fig. 7

Experimental setup

In Fig. 8 the Raman gain versus signal wavelength characteristics are shown. The pump laser current values transferred directly into different pump power values. The total pump powers measured after the coupler were 489.6 mW at 1000 mA, 592.5 mW at 1200 mA, 684.2 mW at 1400 mA, 762.7 mW at 1600 mA and 842.4 mW at 1800 mA, respectively.

Fig. 8

Raman gain versus signal wavelength for different pump laser current and fiber lengths

The Raman gain profile corresponds to the characteristic obtained in numerical simulations (Fig. 3a). Maximum gain of 19.5 dB is achieved at 1313 nm. At 1310 nm, the maximum gain was 18.8 dB. This value matches the gain of 18.9 dB achieved in the simulations with similar parameters (40 km fiber length and 850 mW pump power). The 1dB gain bandwidth is higher at lower laser current values. For a current of 1800 mA, the 1dB gain bandwidth is 14 nm, at 1000 mA it increases up to 18 nm. The gain is higher for a longer fiber, for 25 km the gain at 1310 nm is 17.8 dB, for 38 km the gain increases to 18.8 dB. The gain bandwidth does not change with the fiber length.

On Fig. 9, the noise figure versus signal wavelength characteristics are presented. The noise figure value was calculated using equation (3), based on the obtained gain profile (Fig. 7). The maximum NF value of 4.4 dB is achieved at 1313 nm. The noise figure is higher for higher laser currents, increasing from 3.8 dB at 1000 mA to 4.3 dB at 1800 mA and 1310 nm. Again, the experimental results match the simulation results, where a noise figure of 4.3 dB was achieved with similar working parameters. The dependence of noise figure on the fiber length is negligible, only around 0.1 dB at 1310 nm.

Fig. 9

Noise figure versus signal wavelength for different pump laser current and fiber lengths

The saturation output power of the investigated Raman amplifier is, according to the simulations (Fig. 3b), over 19 dBm. The gain versus output power characteristic was measured up to 14 dBm of output power and had a perfectly linear shape, indicating the very high saturation output power of the Raman amplifier.

6.

CONCLUSIONS

1240 nm QD lasers can be used to realize high gain 1310 nm Raman amplification. In the paper, a record gain of 19.5 dB is achieved with a low noise figure of 4.4 dB. The 1-dB gain bandwidth of the Raman amplifier is around 14–18 nm and can be broadened by the use of additional pumps on different wavelengths, even up to 100 nm as shown in Ref. 8. As shown in Ref. 4, WDM transmission up to 400 Gbit/s is possible in the 1310 nm wavelength domain with wavelength channels between 1300 nm and 1320 nm, around the operating range of the presented Raman amplifier. The saturation output power of the Raman amplifier is very high, significantly over 14 dBm. The described 1310 nm Raman amplifier could be a strong candidate to compete with SOAs in 1310 nm tranmission systems.

A limiting factor in the utilization of the Raman amplifier could be its high cost. Therefore, several design optimizations will be presented in further studies, leading to a more cost-effective solution.

The potential applications of the investigated Raman amplifier are long-reach PON systems as well as transmission range extensions of high speed Ethernet in the 1310 nm wavelength domain.