1. Introduction

Mode division multiplexing (MDM) [1–4], utilizing few-mode fibers supporting multiple spatial modes, is currently under intense investigation as an efficient approach to overcome the current capacity limitations (~100Tbit/s per fiber) of high-speed long-haul transmission systems based on single mode optical fiber. In order to realize the potential energy and cost savings offered by MDM systems, the individual guided modes should be simultaneously amplified within a few-mode erbium doped fiber amplifier (FM-EDFA) [5–8] and switched within a few-mode compatible reconfigurable optical add-drop multiplexer (FM-ROADM) [9, 10]. In FM-EDFAs, mode dependent gain directly affects the system outage probability and minimizing the gain difference between the guided spatial modes is essential to guarantee robust performance of the overall MDM system. Two main strategies have been used to equalize the mode dependent gain: i) tailoring the radial erbium-doping concentration profile of the erbium doped fiber [11] and ii) controlling the pump field intensity distribution [7, 8]. In practice, a combination of both strategies is generally required for higher mode count amplifiers. To date, FM-EDFAs that simultaneously amplify up to 4 linearly polarized (LP) mode groups (i.e. 6 spatial modes) [12–14] have been demonstrated and a differential modal gain (DMG) of less than 2dB has been achieved using a ring-doped erbium doped fiber combined with bi-directional higher-order mode (LP₂₁) pumping [14]. However, as the number of modes is scaled up to 10 and beyond, it will become increasingly challenging to meet the associated pump power requirements with single-mode pump diodes. Moreover, even if this is technically possible, multiplexing a number of single-mode pump diodes to generate sufficient pump power is an expensive way of pumping such an amplifier. Cladding pumping represents a promising way to address these issues. High power, low cost (in terms of $/W) multimode pump diodes operating in the multi-watt regime are now readily available given the emergence and commercial success of the high power cladding pumped fiber laser. Cladding pumping has previously been shown as a viable approach to pumping both multi-core [15, 16] and multi-element fiber amplifiers [17], but as of yet has not been fully demonstrated for a few-mode fiber amplifier. Also cladding pumping may be the only way to efficiently pump hybrid SDM fiber amplifiers (e.g. few-mode multicore fibers and few-mode multi-element fibers) [18, 19], which essentially present the combination of different SDM approaches demonstrated and allow far greater levels of transmission capacity scaling.

Herein, we demonstrate for the first time the feasibility and detailed characterization of a cladding pumped FM-EDFA amplifying all 6 spatial modes of an erbium doped fiber, note provisional results were reported in [20]. The fiber was fabricated with a low-index cladding material to ensure compatibility with direct-diode cladding pumping through one end of the gain fiber while a phase plate based mode multiplexer/demultiplexer was employed to characterize the amplifier performance. The shape of the gain spectra of the 6M-EDFA was similar to that of the conventional cladding pumped single-mode EDFA and an average signal gain of 20dB and DMG of ~3dB were achieved across the C-band at an input signal power of −7.5dBm per mode. The detailed WDM gain spectra for various fiber lengths, pump powers and input signal powers was also investigated.

2. Experimental setup for the cladding pumped 6M-EDFA

Figure 1(a) shows the schematic diagram of our cladding pumped 6-mode EDFA (6M-EDFA) that simultaneously amplifies the LP₀₁, LP_11a, LP_11b, LP_21a, LP_21b and LP₀₂ signal modes. To evaluate the gain performance of the 6M-EDFA under a wavelength division multiplexing (WDM) configuration, 9 external cavity lasers at distinct wavelengths across the C-band (1534-1560nm) were multiplexed to provide probe signals. As the shortest available wavelength was 1534nm we couldn’t measure the amplifier performance over the full C-band. An additional L-band tunable laser source (1565-1575nm) was combined to characterize the short wavelength L-band edge of the amplifier. The 10 combined lasers across the C + L band were split into four equally powered signal tributaries and fed to the mode multiplexer. The mode multiplexer was based on an arrangement of phase plates and beam splitters in order to selectively excite the individual transverse modes in a step-index 6-mode passive fiber (6MF, OFS Denmark) as depicted in Fig. 1(b). The 6MF was then spliced directly to a double clad 6-mode erbium doped fiber (6M-EDF), in which the erbium ion density was raised at the edges of the fiber core relative to the center of the core to help mitigate the DMG. The fiber was drawn from the same preform as in [5], but with the key difference that the current fiber has a low refractive index (n~1.375) polymer coating to guide pump light in the glass cladding. A detailed refractive index profile of the fiber is presented in [5]. The splice losses between the 6MF and 6M-EDF for the different modes were measured as 0.5 ± 0.2dB for all guided modes. The 6M-EDF has an outer cladding diameter of 97μm and a core diameter of ~26μm. The estimated effective NA of the core is ~0.10. The estimated absorption for 975nm pump light launched into the cladding is ~1.08dB/m. The fiber was pumped using a counter-directional pumping scheme incorporating a dichroic mirror which is highly reflective (>99%) at the pump wavelength and highly transmissive (~98%) at the signal wavelength. The multimode pump module can deliver an optical power of up to ~10W and was wavelength-stabilized at 975nm with a volume Bragg grating. The coupling efficiency of the pump power into the inner cladding was measured to be ~80%. To suppress any Fresnel reflections of the signal retro-reflecting back into the core, coreless end caps (diameter = 200μm, length~600μm) were spliced at both ends of the spliced active/passive fiber assembly. A polarization insensitive free-space optical isolator was used to further prevent unwanted reflections. The amplified output was de-multiplexed by a simplified phased plate based de-multiplexer, which is composed of three phase plates (i.e. LP_21a, LP_21b, LP₀₂ phase plates). By placing the amplified signal beam at different spatial positions of the binary LP_21a phase plate (e.g. uniform sector for LP₀₁ excitation, vertical half-sector for LP_11a, horizontal half-sector for LP_11b, and central quarter-sector for LP_21a) the desired mode can be converted into an LP₀₁ mode and coupled efficiently into a single mode fiber. Note that the two (nominally degenerate) orthogonal modes of the LP₁₁ and LP₂₁ mode groups experience continuous mode-mixing along the length of the fiber and the spatial orientation of the lobes can also be altered by external perturbations of the fiber. Therefore in order to minimize the uncertainty, the gain of LP₁₁ mode should be measured in two directions, perpendicular to each other, and the average of the two measurements then used to estimate the average modal gain. In a similar way, the gain of LP₂₁ mode can be estimated from two spatial orientations of the LP₂₁ phase plate, this time however separated by an angle of 45 degrees. To confirm clean amplification of the input signals, transverse mode images were taken by a CCD before and after amplification when the individual modes are amplified separately by the 6M-EDFA. As shown in Fig. 1(c) top, the mode profiles of the four input signals are well defined after propagating through 10m of passive 6MF. The quality of the individual input modes was largely preserved during amplification although some small degradation can be noticed by careful comparison of the images in Fig. 1(c) (top versus bottom).

 

Fig. 1 (a) Schematic diagram of the cladding pumped 6M-EDFA, (b) mode multiplexer based on binary phase plates and (c) signal mode profiles before (top) and after (bottom) amplification. BS: non-polarizing beam splitter, P1-P3: phase plates, DM: dichroic mirror, SMF: single mode fiber, MMF: multimode fiber, 6MF: passive 6-mode fiber, 6M-EDF: 6-mode erbium doped fiber, AMP: amplifier.

Download Full Size | PDF

3. Measured gain and noise figure performance of the 6M-EDFA

The left hand plots of Fig. 2 show the measured gain spectra of the guided modes for three different lengths of 6M-EDF, i.e. 6m, 3.5m and 2m respectively, at a constant input signal power of −7.5dBm per mode. For the 6m length of EDF (Fig. 2a), the gain peaked at ~1565 nm and exhibited a relatively narrow 3dB gain compression bandwidth of ~10nm spanning from 1560nm to 1570nm. In order to shift the gain peak towards 1550nm to better align with traditional C-band operation, we reduced the EDF length to 3.5m. As shown in Fig. 2b, this resulted in a flatter gain spectrum with a 3dB gain compression bandwidth increased to ~20nm spanning from 1545nm to 1565nm. When the fiber length was further reduced to 2m (Fig. 2c) the gain was flattened over a 30nm bandwidth. Although the reduction in the EDF length reduced the overall pump absorption to ~2dB, the FM-EDFA still exhibited respectable modal gains of ≥20dB between 1534nm to 1565nm. Note that the maximum injected pump powers were different for different fiber lengths. For example, for L = 6m, the maximum injection pump power was about 1.3W but it was 3.1W for L = 2m. This behavior can be easily understood from the fact that input signal wavelengths are mostly situated in the C-band while the gain peak for the L = 6m EDF is located at 1565nm with a severe gain tilt, resulting in parasitic lasing when the amplifier was pumped harder. The variation of noise figure (NF) as afunction of fiber length is also shown in the right hand plots of Fig. 2. Figure 2(a) shows that the NF rises sharply for wavelengths shorter than 1565nm while it tends to decrease at longer wavelengths. The high NF at short wavelengths is mainly due to insufficient population inversion within the active medium due to the limitations on the maximum allowable pump power set by parasitic lasing and is clearly evident from the sharp drop in signal gain. The situation improved dramatically when the EDF length was shortened to 2m. The NF was improved from 13 to 14dB to 6-7dB for all guided modes due to the gain enhancement at the short wavelengths. The flattened gain spectra in Fig. 2(c) lead us to believe that the amplifier will have similar gain/NF performance around the short wavelength edge of the C-band. The NF of our cladding pumped 6M-EDFA is slightly higher compared to that of typical core pumped EDFAs but the values are still reasonable when compared to cladding pumped single mode EDFAs [15–17]. We estimate a measurement error of ± 0.5dB in all of our gain and NF measurements.

 

Fig. 2 Measured gain spectra (left) and noise figure (right) of the individual guided modes for three different fiber lengths; (a) 6m, (b) 3.5m and (c) 2m.

Download Full Size | PDF

We further investigated the performance of our cladding pumped 6M-EDFA as a function of the pump power and input signal power. As shown in Fig. 2, the optimum fiber length was 2m in terms of the flattened gain spectra over the C-band however we chose a 3.5m length of fiber for the detailed power performance characterization because it improved the overall pump absorption and thereby the signal gains. Firstly, Fig. 3 depicts the modal gain and noise figure plots for different pump powers at a fixed input signal power of −7.5dBm per mode. For low pump power (~1.1W), the amplifier gain spectrum experiences a severe gain tilt where the gain peak located at ~1565nm with very little gain available at short wavelengths due to the low inversion level. As the pump power was increased from 1.1W to 2.4W, the signal gains of each mode increased due to the increased population inversion level however the gain at the short wavelengths grows more quickly. The average DMG was measured to be ~3dB across the C-band. It is also evident that the noise figure of our cladding pumped 6M-EDFA improves with increasing pump power (inversion) and the measured NF was below 6~7dB for wavelengths longer than 1540nm.

 

Fig. 3 Measured modal gain (a) and noise figure (b) as a function of launched pump power. The input signal power is −7.5dBm per mode.

Download Full Size | PDF

Secondly, the mode dependent signal gain and NF were measured as a function of input signal power at a fixed pump power of 1.6W as shown in Fig. 4. The signal gain decreases with an increase in input signal power for all modes due to the amplifier being operated in the saturated gain regime. The DMG did not show much dependence on the input signal power and remains at ~3dB for all the input signal powers investigated. The measured NF remains almost constant, regardless of the changes in input signal powers.

 

Fig. 4 (a) Measured modal gain and (b) noise figure as a function of input signal power per mode at a fixed pump power of ~1.6W.

Download Full Size | PDF

4. Conclusion

We have demonstrated for the first time a cladding pumped few-mode EDFA supporting 6 spatial modes. A small signal gain of >20dB was achieved across the C-band with a differential modal gain of ~3dB amongst the mode groups while the average noise figure was measured to be between 6 and 7dB. Reduction in the EDF length from 6m to 2m widens the operating window of the FM-EDFA from 10nm to 30nm and ensures near full C-band operation. The amplifier performance could be further improved by optimizing the core dopant distribution and by reducing the core-to-clad area ratio. We consider this to be an important step in increasing the mode scalability of the few-mode EDFA, offering cost-effective and efficient amplification of a large number of spatial data channels in a single device.