Experimental Study on a Broadband Erbium-doped Fiber Amplifier

Abstract: In this paper, we utilized optimized C-band and L-band amplifiers to constitute a broadband CA-L erbium-doped amplifier with a parallel structure. This amplifier has a simple configuration, high-flattened gain and low noise. Meanwhile, we have realized an amplification bandwidth of over 70 nm(1 524—1 602 nm)without any broadband gain flattening component. The C-band average gain is over 30 dB and L-band gain variation is less than 2 dB.

1. Introduction

In order to meet the growing demand for communication capacity, it is a general trend to extend the dense wavelength division multiplexing system (DWDM) from the traditional c-band (1 525 to 1 5 6 5 nm) to the L-band (1 570, 1 610 nm). . The development of C + L band ultra-wideband erbium-doped fiber amplifiers not only doubles the bandwidth of communication, but also increases the dispersion of the system compared to other methods of increasing the transmission capacity of a single channel or reducing the channel spacing. There is no need to worry about nonlinear effects such as four-wave mixing (FWM) that may be caused by narrow channel spacing, which effectively avoids system performance degradation.

As the research on C-band erbium-doped fiber amplifiers is becoming more and more mature, the research focus of current ultra-wideband amplifiers is how to achieve high-gain and low-noise amplification of L-band signals. There are three main methods: (1) development of erbium-doped fibers with different matrices. In this application, the connection problem between the doped fiber and the ordinary silicon-based fiber occurs in the practical application; (2) the traditional EDFA is used in combination with the fiber Raman amplifier (FRA), because the FRA requires multiple high-power pump lasers of different wavelengths. To obtain a wide flattening bandwidth, increase the cost; (3) using different structures of silicon-based erbium-doped fiber amplifiers, based on mature c-band technology is more practical, using this method

At present, the C + L band ultra-wideband EDFA can be divided into two types: tandem type and parallel type according to the connection mode of c-band and L-band [i, wow series-connected structure is more flexible, but c-band and L A band of signals together through the amplification of the fiber will inevitably affect each other. This paper introduces a method of amplifying two bands by parallel method, and sending the signals to the c-band and L-band amplifiers respectively for amplification. The amplified signals are combined to form a wide-band signal output through the combiner, which can effectively prevent the c-band. Interacting with the L-band signal. Only the independent c-band and L-band amplification techniques are used. It is simple and practical. As long as the amplification performance of the respective bands is improved, the performance of the entire ultra-wideband amplifier can be greatly improved. The development direction of the amplifier·

2. Experimental Principle and Device

Generally, the wavelength range of the erbium-doped fiber gain spectrum is in the C-band (1 525, 1 565 nm), which corresponds to the transition of the erbium ion 4113/2 to 4115 /2. In 1990, Massicott et al. found that by controlling the erbium-doped fiber. The length of the 铒 ion particle number distribution is stabilized to a low degree, and the gain spectrum of the erbium ion can be shifted to the L band. The gain spectrum of this displacement corresponds to the tail of the 41]3/2 to 4115/2 transition. The absorption and divergence coefficients are small, but relatively flat due to the low absorption and emission coefficients. To obtain the high gain spectrum of the L-band, it is necessary to increase the pump power and increase the length of the erbium-doped fiber, which is generally about the conventional C-band at the same doping concentration. 4 or 5 times [six not only increases the cost, but also increases the pump power, the low population number reversal rate of the signal input terminal will also cause waste of pump power and produce gain saturation effect. Currently, high doping is mainly used. Low-loss erbium-doped fiber reduces the length of the required fiber, reduces absorption loss and accumulation of back-amplified spontaneous emission spectrum (ASE) energy, and improves power conversion efficiency. In addition, various techniques are used to suppress backward ASE. Production , to improve pump efficiency; such as adding a reflector [9] at the pump input end; using a post-ASE secondary pump source to pump an unpumped fiber 10 '11]; and using multiple different wavelengths And the fiber grating of the bandwidth forms the pump source in multiple directions, and on the basis of this, a front end of the ASE reflection using the fiber loop mirror is proposed, so that the originally wasted ASE is reused. , improving power conversion efficiency,

The ultra-wideband EDFA structure is shown in Figure 1. It is mainly divided into two parts, C and L. Both are 1 480 nm LD backward pump and 980 nm LD forward pump.

Fig. 1 Experimental setup Of the broad band EDFA

Among them: the L-band amplification part adopts the high doping concentration erbium-doped fiber E-knife Fl: 25 m; the circulator is used to input the output L-band signal; the fiber loop mirror is connected by a fusion cone type with a coupling ratio of 5:5. The device is configured to reflect the spontaneous emission of the c-band. At the same time, the L-band signal is amplified and then double-amplified by the optical fiber ring mirror. Therefore, the structure fully utilizes the C-band ASE spectrum for EDFI secondary pumping. The L-band signal light is amplified twice, which greatly enhances the gain of the L-band signal and improves the power conversion efficiency.

The C-band amplifying part adopts the two-stage cascading EDFA. Due to the use of the intermediate isolator IS02, the amplification of the forward and reverse spontaneous radiation can be suppressed, so that the amplifier has low noise and high gain, and the total length of the erbium-doped fiber is 21 m. The position of the intermediate isolator is EDF2: 10 m, EDF3=11 m, and the output is equipped with a gain flatter. Two isolators ISOI, IS03 are used to prevent self-excited oscillation due to fiber end-face reflection. The amplified wideband signal is output through the ultra-wideband wavelength coupler. Since the L-band itself has good flatness, only the c-band gain flattener is used to achieve ultra-wideband gain flatness. This makes it possible to achieve low C and L bands. Loss gain and gain unevenness can also be improved

The c- and L-band narrowband signals are selected by the F-p waver on the basis of the broadband source, and the amplifier gain and noise are measured by the spectrometer.

3. Results and Discussion

Based on the broadband source, the c- and L-band signals selected by the F-p filter are selected at 2 nm and 1 602 nm apart by 2 nm. The input signal size is 40 dBm, and the error is between positive and negative 1 dB. Next, the gain curve after measuring the ultra-wideband EDFA is shown in Figure 2. The black squares represent the output gains corresponding to the respective wavelengths, where the c-band has a minimum of 1 530 nm at 28 · 75 dB' and a maximum of 1 at 562 nm. 37 dB, gain fluctuation less than 4 dB · L band minimum 1 602 nm at 20 · 39 dB, up to 1 594 nm at 23 · 08 dB, 3 dB bandwidth over 30 nm. 1 566, 1 570 nm for gain dead zone, mainly by c + L-wavelength division multiplexer performance determines that the actual measured gain is attenuated a lot and the noise is quite large. Generally, this wavelength interval is not used. Due to the instability of the C- and L-band signals selected by the F-p filter,

Both the c and L bands have individual wavelength gain output biased averages, but the overall trend is approximately flat. Considering the use of stable tunable fiber lasers, better experimental results should be obtained.

Figure 3 shows the ultra-wideband EDFA spontaneous emission spectrum, where the C-band power is 1 · 6 dB from 1 528 · 2 nm at 37 nm; the L-band is 2 · 3 dB from 25 GHz at 1 567 nm. In the experiment, the pump source was deliberately improved the ASE spectrum at the wavelength of 1 567 nm to 1 580 nm in the L-band, so that the L-band gain signal obtained was flatter. Several sets of L and C-band typical signal output line spectra were measured in the experiment. It can be seen from Fig. 4 that the L-band is pumped by the backward ASE and the L-band signal is amplified twice, and the gain is significantly enhanced. At a wavelength of 1 590 nm, the signal gain is 22 · 6 dB, and the noise figure is 9 · 5 dB. The measured noise is higher because the L-band amplifier uses a reflective structure, the L-band signal is amplified twice, and the L-band ASE is amplified twice, and the experimental equipment (mainly flanged and movable joints, etc.) has Reasons for non-negligible losses, etc. Figure 5 shows that at a wavelength of 1 554 nm, the signal gain is 31.2 dB, and the noise is 3 · 66 dB. It can be seen that since the C-band amplification part adopts a two-stage cascade structure, The gain is increased while the noise is reduced; by using the Bragg grating fiber grating filter, the gain flatness is also obtained on the C-band.

The gain fluctuation is less than 4 dB and the gain is not less than 22 dB at a bandwidth of nearly 70 nm from 1 524 to 1 602 nm.