A Simple High Capability C+ L Broad Bandwidth Erbium Doped Fiber Source

Abstract:  A novel C+ L band erbium doped fiber broadband light so urce w ith hig h power was introduced. In the ex periment, a fiber loop mirr or made fr om 3 dB coupler  was employed, mean while, power controlling circuit made fiber output steady. Single stage fiber and two pump LDs of 980 nm was used, and C band amplified spontaneous emission of backw ard again enhanced the efficiency of LD and stability o f output of fiber. Mean while, selecting appropriate Erbium doped fiber length simultaneously g ot output of C+ L band with power higher than 26.67 mW ( 14.26 dBm) , whose average wavelength was 1 550.887 nm.

1 Introduction

SFS (superfluorescent fiber sour ce) is a low-coherence, single-transverse broadband source. It is based on amplified spontaneous emission of erbium-doped fiber, with excellent temperature stability and wide fluorescence. The spectral width, the center wavelength is in the 1 550 nm band, and the output optical power is large, which has aroused great interest. More importantly, with the continuous development of fiber optic sensors and fiber optic detectors, the requirements for broadband sources with low temporal coherence are also increasing. The light source must have not only a large output power, but also a characteristic of wide frequency band and high average wavelength stability. Broadband sources also facilitate the development of wavelength division multiplexing (WDM) systems and communication networks. Ytterbium-doped light sources have been extensively studied in recent years. At present, the research on C-band amplified spontaneous emission broadband light source has been quite mature. In order to meet the requirements of capacity expansion, the gain band of EDFA will be adopted.

Widening from the C-band to the L-band, forming a C + L-band wideband EDFA has become a hot research topic.

The widely used broadband sources are super-radiation diodes, and the broadband source based on rare earth doped erbium-doped fiber autonomous emission (A SE) has an inherently broad emission spectrum, high output power, and easy coupling with fiber systems. Long life and other advantages make it the best choice for making non-coherent broadband sources. This makes the research of rare earth doped broadband light sources more urgent.

In this study, a single-stage dual-pumping C+ L-band source model was designed. The basic structure of the broadband EDFA was analyzed. The principle of the various bands produced by the erbium fiber, the fabrication principle of the fiber loop mirror, and a novel design scheme were proposed. In order to make the output light more stable, the power control circuit is added; and the design is optimized cost-effectively, reducing the pumping laser diode from two to one without affecting its performance. This design provides an experimental basis for the study of practical C+ L-band sources.

2 Fiber ring mirror fabrication and reflection principle

The fiber optic ring mirror fuses the two outputs of the 2 # 2 fused-taper coupler, which is a broadband mirror. The structure is shown in Figure 1. This structure is called a Sag nac interferometer in optical sensing (such as fiber optic gyroscopes). When the input optical signal is input from one end of the ordinary wideband coupler, it is divided into two beams of clockwise and counterclockwise opposite at the two output ports of the coupler. After transmission, the two beams are in the coupler. After the coupling region is coherent, the reflected light is output from the signal input end, and the transmitted light is output from the other end. The energy splitting ratio of the coupler is k. Under the premise of ignoring the loss of the coupler itself and the loss of the fiber, when the incident light power is Pin, the reflected light power P r and the light transmission power P t are respectively

Pr = 4k( 1 - k)P in      ( 1)

P t = ( 1 - 2k)²P in      ( 2)

From equations (1) and (2), the reflectivity R and the transmittance T of the fiber loop mirror can be expressed as

R = 4k ( 1- k)           ( 3)

T = ( 1 - 2k)²            ( 4)

From device symmetry, the device satisfies the relationship:

That is, the transmission matrix A of the device can be written as:

Figure 1 Schematic diagram of the fiber loop mirror structure

The solid and dashed lines in Figure 2 are the theoretical curves of R and T with k, respectively. Obviously, when the splitting ratio k is 0 or 1, R = 0, P r = 0, T = 1, P t = P in. In this case, the structure acts as a total transmission mirror; k ∃0. 147 or k ∃0. 853, that is, at the intersection of the two curves, R = T = 0.5, Pr = P t , at this time Structure

It acts as a semi-transmissive semi-reflection; when k = 1/ 2 (3 dB coupler), then R = 1, P r = P in , T = 0, Pt = 0, then the structure plays The role of a total reflection mirror. In practice, the fusion will bring a certain loss, the splitter ratio of the coupler is not completely consistent with its nominal value, or the broadband source used has certain fluctuations, environmental changes and other factors caused by human factors have a certain theoretical value. Deviation.

Fig. 2 Curve of reflectance and transmittance of fiber loop mirror with coupling ratio

3 Light source structure analysis

A variety of structures of erbium-doped fiber superfluorescent sources have been proposed and studied. Among these structures, the two-way backward structure is considered to be the most ideal structure, which has high output power, wide line width, and can eliminate the center of the light source caused by the change of pumping power by optimizing the length of the erbium-doped fiber. Wavelength instability. Through many experiments, we found that the double pumping structure has higher conversion efficiency than other two-way two-stage structure, can obtain higher output power, has better wavelength stability, and more importantly, the experimental device is simple. Easy to implement. Therefore, the experimental device was finally selected as a double pumping structure. A 3 dB wideband fiber coupler is used to form an approximately 100% total reflection mirror, which is used as a mirror in the light source. The fiber has a erbium-doped concentration of 0.77-10-3, and the cut-off wavelength is 853.5 nm. The background loss at 1 200 nm is less than or equal to 50 dB/km, and the peak absorption coefficient at 980 nm is 4. 5 毫米。 The mode diameter is greater than or equal to 0. 2, the mode field diameter is 6.68 ?m. In the experiment, a 980 nm laser diode was used as the pumping source with a center wavelength of 979.04 nm and a threshold current of 27.8 mA. The laser diode pigtail output optical power is substantially linear with the pumping current (from the threshold current point). In the design of the experimental setup, two 980 nm laser diodes were used as the pumping source, one as the forward pump and one as the backward pump. The experimental structure is shown in Fig. 3. In order to reduce the cost, a 980 nm LD is finally realized, and the forward and backward pumping optical signals are provided by the coupler to split the light. The simplified device is shown  in Fig.4.

Figure 3 Schematic diagram of C + L band source structure

Figure 4 Schematic diagram of the simplified structure of the C+ L-band source

4 Experimental results and analysis

Broadband source output light characteristics are typically characterized by three characteristic parameters: spectral bandwidth, center wavelength, and output power. The doping concentration of erbium  fiber is one of the factors affecting the gain. Choosing a highly doped erbium fiber to produce a superfluorescent source can achieve superfluorescence with higher output power. The output light characteristics of the double pumping light source are determined by two major parameters: one is the erbium-doped fiber material and the waveguide structure parameters, that is, the selected fiber type, including Er3+ concentration, co-doped ion concentration, absorption and emission cross-section spectrum, and numerical aperture. , cutoff wavelength, optical field overlap and fiber core area; the second is the operating mode parameters, including pump wavelength, pump efficiency, erbium fiber length, mirror reflectivity.

Usually the simple C-band light source adopts a single-pass backward structure; the simple L-band light source adopts the single-pass forward structure directly, and the utilization efficiency is low, or the two-stage two-way forward direction and other complicated structures are realized, the cost is increased, and the introduction is further introduced. Many uncertainties. Therefore, in the experiment, a double-pumping structure is adopted, a high-concentration erbium-doped fiber is used as a gain medium, and a fiber-optic total reflector made by a 3 dB coupler is used to redirect the backward-generated high-power C-band light back into the fiber. The amplified absorption of the erbium-doped fiber again improves the utilization efficiency of the light source and improves the stability of the light source.

In the experimental device structure, two 980 nm laser diodes are used as the pump source, and the output power is up to 26.67 mW ( 14.26 dBm) with an average wavelength of 1 when the optical power is pumped at both ends. 550. At 887 nm, the output spectrum is shown in Figure 5. Due to the high cost of the laser diode, from the perspective of cost, changing the two 980 nm laser diodes to one, through a coupler, the performance of the light source is not affected. Device on top

In the case of a pigtail with a length of about 4 meters, it is not necessary to connect the pigtails in the repeated test, directly connected to the output through the coupler, and the number of temporary joints during the test is reduced, effectively reducing The purpose of the attenuation of optical power is to improve the utilization of the light source. Add an isolator to the light output to avoid laser light.

Figure 5 Experimental output spectrum of C+ L-band source

The experimental results show that the C+ L-band amplified spontaneous emission broadband source can be realized by bidirectional pumping single-stage erbium-doped fiber structure. Firstly, the length of the erbium-doped fiber should be reasonably selected. The length of the fiber has great influence on the experimental results because of the flatness and blending of the output spectrum.erbium Fiber length selection has a close relationship. If the erbium-doped fiber is too short, no matter how the forward and backward pumping power is adjusted, the L-band spectrum and the C-band spectrum cannot be matched, so that the spectrally flat C+ L-band amplified spontaneous emission source cannot be obtained. When the length of the erbium-doped fiber exceeds a certain value, the L-band and C-band spectra can be matched by adjusting the control current of the forward-forward pumping laser diode to adjust the L-band and C-band spectra to obtain a flat C+L-band amplified spontaneous emission. Spectral output. Moreover, in the range of suitable fiber lengths, the flatness of the C + L band amplified spontaneous emission spectrum is basically equal, and the pumping conversion efficiency of the amplified spontaneous emission source and the flatness of the spectrum are not necessarily related.

In practical experiments, the connection of erbium-doped fiber with wavelength division multiplexer, laser diode pigtail and wavelength division multiplexer, erbium fiber and output pigtail is crucial for the development of light source. In the experiment, the fusion of the optical fibers was done with a high precision welding machine.

5 Conclusion

This paper analyzes the formation mechanism and interaction between C-band and L-band in erbium-doped fiber. The basic principle of simultaneous output of broadband light source in C+ L band is further analyzed. A simple and excellent single-stage double-pumped two-pass C+ L-band light source structure is designed and optimized without affecting its performance. In the case of changing two laser diodes to one, the cost is reduced and the device is simplified. The basic principle of erbium ion generating C+ L band while outputting broadband spectrum is analyzed. In the experiment, we optimized the length of the erbium fiber to make the output spectrum flatter and the output optical power higher. At the same time, we changed the concentration of different erbium and added the power control circuit to further improve the flatness of the light source to expand its application range.