Performance improvement in high speed optical networks with low cost amplifier configuration

Abstract

In this paper, the application example of 8 × 100 Gbit/s over 463 km (76.9 dB) optical transport network is realized by using commercial bidirectional Raman amplifier and additional auxiliary laser without any remotely pump amplifier. The positions of the auxiliary laser in the commercial Raman amplifier are compared and optimized experimentally. The results show that 7.3 dB and 10.2 dB more power budget is obtained with the auxiliary laser in the backward direction and forward direction respectively comparing to the situation without any auxiliary laser. The results also show that forward second order Raman amplifier is a good choice in the condition without remotely pump amplifier.

Keywords

Commercial bidirectional Raman amplifier auxiliary laser remotely pump amplifier

1. Introduction

Unrepeatered transmission systems have proven to be a cost-effective solution to transmit high capacity over moderate distances of several hundred kilometers without any in-line active elements. Unrepeatered systems have also been proposed for longer reaches with relatively low transmission capacity. In recent years, commercial Raman amplifier has become a very common means to extend the span distance of ultra-long span transmission system in high speed optical networks [11]. In some cases the span distance is generally more than 350 km, so the commercial Raman amplifier cannot meet the requirements and thus the remotely pump amplifier (RPA) becomes a necessary choice [1,4,19]. However, RPA will encounter many problems in specific implementation process, since RPA use passive remotely gain unit (RGU) module, which was often wrapped in sealed box and placed some distance away from the receiver side. On the other hand, considering the construction complexity and long-term reliability of the RGU, sometimes the users are not willing to choose RPA. So bidirectional Raman amplifier becomes a safe and better choice [3,7,12,17]. However, the gain of the bidirectional Raman amplifier cannot be very high (usually less than 30 dB) because of double Rayleigh scattering (DRS) noise. In order to achieve a low intra-span signal power variation, second order Raman amplifier will be used to distribute the gain more evenly along the fiber, leading to better optical signal to noise ratio (OSNR) and noise figure (NF) performance [2,6,9,13,18]. The benefits of the second order pump have been demonstrated by Chi [2] in terms of extended the reach of transmission system. A. Iqbal [6] demonstrates a novel design of dual stage backward pumped broadband discrete Raman amplifier with distributed pumping architecture which improves the short wavelength noise performance by 3.3 dB and 1134 km reach extension. Although Raman amplifiers have been commercially used in ultra-long-distance transmission systems, in the past two years, there have been some new explorations and improvements in the existing Raman amplifier structure [5,8,10,14–16].

In this paper, we propose a low cost and practical second-order Raman amplifier. Only one auxiliary laser is added on the basis of current infrastructure-commercial Raman amplifier without any remotely pump amplifier. The addition of auxiliary laser actually acts as a second order Raman pump laser and constitutes a second order Raman amplifier, which greatly improves the system performance while extending the span distance. The performance improvement of the system is verified and the position of the auxiliary lasers is optimized. At the same time, the auxiliary pump laser can be controlled by commercial Raman amplifier without increasing the complexity of the control system. The application example of 8 × 100 Gbit/s over 463 km (76.9 dB) high speed optical networks is realized by using the combination of a commercial bidirectional Raman amplifier and an auxiliary laser, without remotely pump amplifier.

2. Theoretical analysis

The addition of auxiliary laser actually acts as a second order Raman pump and constitutes a second order Raman amplifier. Assume that three and high order stokes waves are ignored in the coupling equations, and other nonlinear effects such as four-wave mixing and cross-phase modulation are also ignored. The nonlinear coupled equations, including pump, the first, and the second Stokes. $\begin{array}{l} \frac{d p_{p}}{d z} = \pm \frac{g_{R}}{A_{eff}} \frac{ω_{p}}{ω_{s}} p_{p} p_{s 1} \pm α_{p} p_{p} \\ \frac{d p_{s 1}}{d z} = + \frac{g_{R}}{A_{eff}} p_{p} p_{s 1} - \frac{g_{R}}{A_{eff}} \frac{ω_{s 1}}{ω_{s 2}} p_{s 1} p_{s 2} + α_{s 1} p_{s 1} \\ \frac{d p_{s 2}}{d z} = - \frac{g_{R}}{A_{eff}} p_{s 1} p_{s 2} + α_{s 2} p_{s 2} \end{array}$ In the coupling equation, $g_{R}$ is the Raman gain coefficient and $A_{eff}$ is the effective fiber core area. For simplicity, the Raman gain coefficient $g_{R}$ is assumed constant in the wavelength range that includes the pump, first and second Stokes waves. P, α and ω are the wave power, fiber loss and optical frequency. Subscripts p, $s 1$ and $s 2$ denote the pump, the first Stokes and the second Stokes components, respectively. The parameter of the simulation is listed in Table 1.

Table 1
Simulation parameters

Parameters Value Units

Signal wavelength 192.6∼193.5 THz

Booster amp power 27 dBm

Pre amp Gain 30 dB

FRA pump power 1050 (Max) mW

BRA pump power 1050 (Max) mW

Raman pump wavelength 1425, 1457 nm

Auxiliary pump wavelength 1360 nm

Fiber length 463 km

Fiber loss (1550.12 nm) 0.167 dB/km

Dispersion coefficient <18 ps/nm/km

Fiber nonlinear coefficient 2.6e⁻²⁰ m²/W

Fiber $A_{eff}$ 80 um²

Raman coef 1.0 × 10⁻¹³ m/W

Parameters	Value	Units
Signal wavelength	192.6∼193.5	THz
Booster amp power	27	dBm
Pre amp Gain	30	dB
FRA pump power	1050 (Max)	mW
BRA pump power	1050 (Max)	mW
Raman pump wavelength	1425, 1457	nm
Auxiliary pump wavelength	1360	nm
Fiber length	463	km
Fiber loss (1550.12 nm)	0.167	dB/km
Dispersion coefficient	<18	ps/nm/km
Fiber nonlinear coefficient	2.6e⁻²⁰	m²/W
Fiber $A_{eff}$	80	um²
Raman coef	1.0 × 10⁻¹³	m/W

Fig. 1.

Power distribution of signals and pumps.

Figure 1 shows the power distribution of signals and pumps of simulation results. Here 1425 nm and 1457 nm are the pump wavelength of the first order Raman amplifier. 1360 nm is the auxiliary wavelength which could be served as a second order Raman pump, but it is still under control by the first order Raman amplifier. We can see that auxiliary laser acting as second order pump transfer its energy to the first order pump of 1425 nm and 1457 nm. The signal is amplified by the first order pump.

3. Experimental setup

Fig. 2.

Optical schematic of commercial Raman amplifier with the Auxiliary laser. Notes: BA – Booster amplifier; PA – Pre amplifier; FRA – Forward Raman amplifier; BRA – Backward Raman amplifier; VOA – Variable optical attenuator; TDCM – Tunable dispersion compensation module.

The investigated transmission system is illustrated in Fig. 2. The transmission fiber used in the experiment was Ultra low loss (ULL) fiber with a total length of 463 km in the average fiber loss of 0.16∼0.17 dB/km. The fiber length in the front section is 350 km, and the second fiber length in the back section is 113 km. The fiber link has a total loss of 76.9 dB. Eight 100G line cards are multiplexed using wavelength multiplexer (MUX). The wavelength of eight channels is 1546.94, 1548.53, 1549.35, 1550.14, 1553.37, 1557.39, 1559.00, 1560.63 nm respectively. The signals are RZ-PM-QPSK modulated at 100 Gb/s which accounts for the 15% over head of the Soft Decision Forward Error Correction (SD-FEC) code. The in-band enhanced forward error correction function of soft decision achieves a net coding gain of 11.2 dB. The SD-FEC can correct a BER of $1.9 \times 10^{- 5}$ to less than $10^{- 12}$ .

In the experiment, the output power of booster amplifier (BA) is 27 dBm. Forward Raman amplifier (FRA) has nominal gain of 8 dB, whose pump wavelength is 1425, 1439 and 1457 nm with the maximum power of 1.05 W. The auxiliary laser (acting as second order pump) wavelength is 1360 nm with maximum pump power of 2 W. The maximum transmission tolerance was determined by adjusting the attenuation value of attenuator (VOA) and observing the error code of the instrument, as well as the pump power of the first order Raman amplifier and auxiliary laser, and the output power configuration of BA. There is only one Auxiliary laser (1360 nm) in the experiment. So during the experiment, it is placed in the forward direction or backward direction respectively. In the experiment, the schemes of B1 and F1B1 without an auxiliary laser are also tested for comparison.

Fig. 3.

Control system of commercial Raman amplifier and the auxiliary laser.

Figure 3 shows that the auxiliary pump laser is controlled by the commercial first order Raman amplifier. The schematic in the blue box shows the existing first-order Raman control chart. Two first pump lasers are combined to form first-order Raman pump unit. The control system (control CPU) controls the switching of the pump lasers, pump power warning, reflecting power warning, signal power warning through pump detector PD, pump reflect PD and signal detector PD. The auxiliary laser is launched through an extra port of pump combiner and controlled through the control system of the first-order Raman amplifier. They share the control CPU and alarm PDs including pump alarm, pump reflection alarm and signal alarm which make the circuit control very simple. At the same time, the cost is also significantly reduced.

Fig. 4.

Gain and NF of without /with auxiliary laser in forward/backward direction.

4. Experiment results

4.1. Gain and NF results with different pump configuration

Figure 4 shows Gain and NF value of auxiliary laser in the forward and backward direction case. Also the case F1B1 without auxiliary laser is show for comparison. It can be seen that the average gain and NF are 31.7 dB and −9.0 dB for the only commercial bidirectional Raman amplifier without auxiliary laser. When the auxiliary laser is injected in the backward direction, the gain and NF are 38.9 dB and −17.2 dB. When the auxiliary laser is injected in the forward direction, the gain and NF are 40.16 dB and −18.7 dB. The addition of auxiliary laser improves gain and NF distinctly. Also it can be seen that placing auxiliary laser in the forward direction could achieve higher gain and lower NF, which is more advantageous to improve system performance. The corresponding power of commercial pump and auxiliary laser pump is listed in Table 2.

Table 2
Performance of different amplifier schemes and the pump power setup

Case no. Amplifier scheme Gain (dB) Power budget (dB) OSNR (dB) 1st Pump power (mW) Forward 1st Pump power (mW) Backward Auxiliary pump power (mW)

1 B1 20 59 17.89 210/150/380

2 F1B1 31.7 66.7 (400 km) 14.24 210/150/380 210/150/380

3 F1B1+AuBack 38.9 74 12.89 210/150/380 210/150/380 500/1000

4 F1B1+AuFor 40.16 76.9 (463 km) 10.38 210/150/380 210/150/380 500/1000

Case no.	Amplifier scheme	Gain (dB)	Power budget (dB)	OSNR (dB)	1st Pump power (mW) Forward	1st Pump power (mW) Backward	Auxiliary pump power (mW)
1	B1	20	59	17.89		210/150/380
2	F1B1	31.7	66.7 (400 km)	14.24	210/150/380	210/150/380
3	F1B1+AuBack	38.9	74	12.89	210/150/380	210/150/380	500/1000
4	F1B1+AuFor	40.16	76.9 (463 km)	10.38	210/150/380	210/150/380	500/1000

Notes: Four cases refer to Fig. 2. B1 – only backward Raman amplifier; F1B1 – forward and backward Raman amplifier (bidirectional Raman amplifier, no any auxiliary laser); F1B1+AuBack – bidirectional Raman amplifier plus Auxiliary laser in backward direction; F1B1+AuFor – bidirectional Raman amplifier plus Auxiliary laser in forward direction.

4.2. Transmission performances

Table 2 shows the transmission performance and the pump power setup. Four amplifier schemes are compared in the table. They are normal backward Raman amplifier (B1, forward pump turn off), bidirectional Raman amplifier (F1B1), auxiliary in backward (F1B1+AuBack) and forward (F1B1+AuFor) direction. The corresponding Gain, power budget, OSNR, pump power and Auxiliary laser pump power are listed in the table. Although the bidirectional Raman amplifier has 7.7 dB more power margin than the backward Raman amplifier [17], if an auxiliary laser is placed in the backward/forward Raman amplifier to constitute a forward/backward second order Raman amplifier, a more power budget 7.3 dB and 10.2 dB is achieved, which will extend the span length of 43 km and 61 km, respectively. Moreover, it can be seen that the situation of the auxiliary laser placed in the forward direction achieve the best results. It would get the power budget of 76.9 dB. More 2.9 dB power budgets are achieved compared with auxiliary laser placed in the backward direction, but the Auxiliary laser in forward or backward direction has obtained better results than commercial bidirectional Raman amplifier (F1B1) or only backward Raman amplifier (B1). It is a pity that we have only one auxiliary laser. There is no opportunity to verify the situation that auxiliary pumps are placed at both forward and backward direction at the same time.

Figure 5 shows the OSNR (dB/0.1 nm) comparison at receiver side when Auxiliary laser in forward and in backward direction.

Fig. 5.

OSNR character in the transmission link with Auxiliary laser in forward and in backward direction.

Table 3

Cost and control complexity comparison

Case no.	Amplifier scheme	Power budget (dB)	Amplifier cost ($)	Construction cost and maintenance cost ($)	Construction difficulty	Control complexity
1	F1B1	66.7	4000	0	Easy	Easy
2	F1B1+AuBack	74	5000	0	Easy	Easy
3	F1B1+AuFor	76.9	5000	0	Easy	Easy
4	F1+Remotely amplifier	76.5	5000	>1000	Hard	Hard

Notes: Four cases refer to Fig. 2. Case 1 – F1B1 (no any auxiliary laser); Case 2 – F1B1+AuBack (without auxiliary laser 1); Case 3 – F1B1+AuFor (without auxiliary laser 2); Case 4 – F1+ remotely amplifier (just for cost comparison).

In Table 3, we show the cost and control complexity of different amplifier schemes. Case 4 is a configuration scheme of a forward Raman amplifier plus a remotely pump amplifier. From Table 3, we could see that case 3 and case 4 have similar power budget. Comparing amplifier cost of case 3 and case 4, the material costs are similar, but construction and maintenance costs must also be considered. On the other hand, remotely amplifier usually consists of a remote pump unit (RPU) and a remote gain unit (RGU, passive gain module). Since the passive gain modules must be buried or hung up in the real trial, this means high installation costs and high maintenance costs (more than 1000$). In contrast, case 3 with the auxiliary pump is very flexible. Also the control method is easy and practical. When field amplification shows that the there is no enough margin in high speed optical networks, we just add an auxiliary pump laser in the commercial backward Raman amplifier to solve the problem without any control and installation troubles.

5. Conclusions

In this paper, a flexible amplifier configuration scheme of commercial Raman amplifier plus auxiliary laser is proposed for a 463 km ultra-long distance transmission system without the help of remotely pump amplifier. Through the experiment, we found that in the absence of the remotely pump amplifier, the commercial bidirectional amplifier plus a second order auxiliary laser in forward direction could achieve transmission distance of 463 km in ULL fiber (76.9 dB loss). Also, the amplifier performance and transmission performance are compared with commercial Raman amplifier and auxiliary laser in forward and backward separately. The transmission power budget of 74 dB and 76.9 dB were obtained respectively with auxiliary laser in backward and forward direction, respectively. Additional auxiliary laser in both forward and backward direction could obtain better results than only commercial bidirectional Raman amplifier. It becomes a good choice for those people who don’t like remotely pump amplifier in the ultra-long unrepeated transmission system. Placing auxiliary lasers in both forward and backward direction could be a more flexible way and more margins could be obtained.

Footnotes

Acknowledgements

The authors would like to thank Wuxi Taclink Optoelectronics Technology Co., Ltd. (TACLINK) and Wuxi Hannuo Optoelectronic Technology Co., Ltd for the useful equipment used in the experiments. This work was supported by “333 projects” of Jiangsu province (RA2019148), six talent peaks project in Jiangsu Province (XYDXX-169) and Future Network Scientific Research Fund Project (FNSRFP-2021-YB-56).

Conflict of interest

None to report.

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