We report the first demonstration of silica-based thulium-doped fiber amplifier (TDFA) working in the 1.65 - 1.7 µm waveband. Up to 29 dB small signal gain and noise figure as low as 6.5 dB are achieved

Alam, S.U.

Clarkson, W.A.

Daniel, J.M.O.

Heidt, A.M.

Jung, Y.

Li, Z.

Richardson, D.J.

Shardlow, P.C.

Simakov, N.

Southampton (e-Prints Soton)

Tu2C.1.pdf OFC 2015 © OSA 2015Extreme Short Wavelength Operation (1.65 – 1.7 µm) of Silica-Based Thulium-Doped Fiber Amplifier  Z. Li1, Y. Jung1,*, J. M. O. Daniel1,2, N. Simakov1,2, P. C. Shardlow1, A. M. Heidt1, W. A. Clarkson1, S. U. Alam1, and D. J. Richardson1 1Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK 2Laser Technology Group, Cyber and Electronic Warfare Division, Defence Science and Technology Organisation - Edinburgh, South Australia 5111, Australia ymj@orc.soton.ac.uk   Abstract: We report the first demonstration of silica-based thulium-doped fiber amplifier (TDFA) working in the 1.65 – 1.7 µm waveband. Up to 29 dB small signal gain and noise figure as low as 6.5 dB are achieved.  OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.2330) Fiber optics communications  1.  Introduction  The quest for increasing transmission capacity of optical communication networks has stimulated interest in radical new approaches to data transmission, e.g., space division multiplexing [1]. Recently, the 2 µm wavelength region has attracted growing attention as a potential new transmission window for optical communications [2]. This is underpinned by emerging technologies such as hollow-core photonic bandgap fibers (HC-PBGFs). These fibers offer a transmission medium with ultra-low nonlinearity, low latency, and potentially an ultra-low loss window covering both the conventional 1.5 µm and the emerging 2 µm wavebands [3]. As the background loss of HC-PBGFs is continually improving, it becomes possible to envisage a future optical communication network operating seamlessly from 1.5 µm to 2 µm. In order to realize such a system, high quality optical amplifiers are required. Silica-based thulium-doped fiber amplifiers (TDFAs) have recently demonstrated high gain, low noise amplification in the 1720 – 2050 nm region [4], leaving a ~100 nm gap between the TDFA and erbium doped fiber amplifier (EDFA) operating windows. It is therefore of immense interest to further extend silica-based TDFA gain to <1.7 µm and possibly to bridge the gap between the EDFA and TDFA. It is worth noting that a high performance small signal amplifier in this exotic waveband can also find applications in sensing, medicine, and material processing [5]. At these wavelengths, thulium-doped silica exhibits a strong three-level behavior and therefore requires a large inversion to achieve any gain. Moreover, the upper laser level lifetime is significantly reduced by multi-phonon non-radiative decay, further increasing the challenge of obtaining the very large population inversion required for operation at <1.7 µm. In addition, a high inversion will result in a substantial gain at 1.8 – 1.9 µm and management of the amplified spontaneous emission (ASE) at these longer wavelengths becomes essential to achieve shorter wavelength gain. Consequently prior to this work small signal amplification in the 1.65 – 1.7 µm range could only be obtained via nonlinear approaches (e.g., Raman amplifiers [6]), or using fluoride-based TDFAs with distributed loss at longer wavelengths [7]. It is interesting that a recently reported bismuth-doped fiber laser has shown emission in this band [8], however, to date there has not been a demonstration of any small signal amplifier based on such fiber. In this contribution, we present to the best of our knowledge the first demonstration of a silica-based TDFA operating in the 1.65 – 1.7 µm spectral region. A 50 nm gain extension compared with previously reported results [9] is achieved as a result of more effective ASE management.  2.  Experimental Setup A schematic of the experiment is shown in Fig. 1(a). The amplifier employs a double-pass cavity design, whereby the signal to be amplified passes the gain region twice. The gain region comprises two 0.5 m lengths of thulium-doped fiber (OFS TmDF200) spliced to each end of an in-house fabricated ASE filter. The single-mode TDF has a ~6.5 µm mode-field diameter at 2 µm and a core absorption of ~20 dB/m at 1.56 µm. A relatively short combined length of TDF (1.0 m) is used to minimize reabsorption losses. The short-pass ASE filter is realized by exploiting the bend loss of a dispersion compensating fiber (DCF). This filter provides high loss for >1.7 µm wavelengths suppressing long wavelength ASE and the potential for parasitic lasing. The DCF was chosen because of its strong bend loss dependence on wavelength. The transmission loss for the ASE filter is shown in Fig. 1(b). It can be seen that the transmission curve is relatively flat with a 3 – 7 dB insertion loss for wavelengths <1700 nm. The loss increases dramatically for wavelengths >1700 nm, exhibiting over 20 dB transmission loss for 1720 nm, over 30 dB loss at 1730 nm, and effectively no light is guided at wavelengths >1750 nm. A second ASE filter and a fiber retro-Tu2C.1.pdf OFC 2015 © OSA 2015reflector were used to further filter any long wavelength ASE and to reflect the signal back to the gain region for the second pass amplification. An in-house built fiber laser operating at 1565 nm was used as a pump source to forward core-pump the TDF. Pump and signal wavelengths were combined using a custom-made filter-based 1550/2000 nm wavelength division multiplexer (WDM) coupler with ~1 dB insertion loss for both pump and signal wavebands. For characterization, the TDFA was seeded by either of the two narrow line-width (<0.1 nm) tunable laser sources (TLSs). One TLS is a commercially available laser (Tunics T100S-HP) covering 1500 – 1680 nm and the other one is an in-house built laser covering 1690 – 1800 nm. An attenuator was used to vary the power of the input signal. We chose input signal powers of -20 dBm and 0 dBm to represent the small signal and saturated signal scenarios throughout the paper. A circulator was used for in and out coupling of the input and amplified signals, respectively. The circulator also provides isolation for the input and output signals. This helps in suppressing parasitic lasing and any feedback to the seed lasers, and therefore increases the stability of the amplifier. A power meter (Ophir 3A-FS) and an optical spectrum analyzer (Yokogawa AQ6375) were used to measure the gain and noise figure (NF)   Fig. 1. (a) Schematic of the thulium-doped fiber amplifier (TDFA). TLS: tunable laser source; Atten.: attenuator; Cir.: circulator; WDM: wavelength division multiplexer; TDF: thulium-doped fiber; ASE: amplified spontaneous emission; Iso.: isolator; F.L.: fiber laser; P.D. power dump; M: Mirror. (b) Left: insertion losses of the circulator and WDMs; Right: transmission loss of the in-house built ASE filters. It is worth noting that the passive components used in the TDFA have relatively high insertion losses in the 1650 - 1700 nm region (see Fig. 1(b)). For instance, at the input coupling side of the signal, the circulator and WDM coupler each introduces ~1 dB insertion loss. The total round-trip insertion loss from the amplifier input to output adds up to 18 dB at 1650 nm and 26 dB at 1690 nm. 3.  Results and discussion The spectral characterization of the TDFA is shown in Fig. 2. This figure presents the wavelength dependence of the external small-signal gain (measured with an input signal power of −20 dBm), the saturated gain (measured with an input signal power of 0 dBm), as well as the external noise figure (NF) for both the gain curves.   Fig. 2. Gain and noise figure (NF) characteristics of the TDFA. Solid red curves and dashed blue curves represent gain and NF performances, respectively. Circle and triangle symbols correspond to the small signal (-20 dBm) and saturated signal (0 dBm) scenarios, respectively. All data points in the graph are measured external values. A small signal gain of up to 29 dB was achieved at 1690 nm – a wavelength never reached before by any kind of silica-based rare-earth doped fiber amplifier. The 20 dB small signal gain window covers a ~35 nm band (1670 – Tu2C.1.pdf OFC 2015 © OSA 20151705 nm), which corresponds to 3.7 THz bandwidth. The 15 dB small signal gain window spans from 1660 – 1710 nm, corresponding to a bandwidth of as much as 5.3 THz. The TDFA provided 8 dB of gain at wavelengths as short as 1650 nm. The saturated gain is above 10 dB in most of the waveband with up to 19 dB saturated gain achieved at 1690 nm. Both the small signal gain and saturated gain decrease rapidly at wavelengths >1.7 µm due to the rapidly increasing transmission loss imposed by the ASE filters. The small signal external NF was below 10 dB for almost the whole wavelength region with as low as 6.5 dB achieved at 1690 nm. The amplified signal spectrum is of high quality as shown in Fig. 3. The amplified small signal (Fig. 3(a)) has over 25 dB in-band optical signal-to-noise ratio (OSNR) in the 1660 – 1700 nm region and over 20 dB in-band OSNR at the edges of the amplification band. The amplified saturated signal (Fig. 3(b)) has over 40 dB in-band OSNR throughout the range of 1650 – 1700 nm.   Fig. 3. Amplified small (left) and saturated (right) signals. Measured with 0.5 nm optical spectrum analyzer (OSA) resolution. Taking account of the high insertion losses of the passive components, the gross small signal gain is estimated to be above 50 dB at the 1690 nm gain peak and above 25 dB at 1650 nm. The internal small signal NF values are estimated to be 1~2 dB less than the external NF. Significant improvement in the performance of the TDFA is envisaged with the future availability of lower loss passive components. Distributed ASE filtering at >1.7 µm is also expected to be more efficient than using discrete filters as we have used in the present case. This could be realized with a more complicated active fiber design. 4.  Conclusion In conclusion, we have presented the first demonstration and detailed characterization of a silica-based TDFA operating in the 1.65 – 1.7 µm waveband that was previously inaccessible by any kind of silica-based rare-earth doped fiber amplifier. Up to 29 dB net small signal gain and an external NF as low as 6.5 dB have been achieved at 1690 nm. The TDFA covers a 20 dB gain bandwidth of 3.7 THz and a 15 dB gain bandwidth of 5.3 THz, which are comparable with the 4.4 THz bandwidth of the C-band. Using the 20 dB small signal gain as a benchmark, we have successfully extended the working window of silica-based TDFAs by 50 nm from ~1720 nm in Ref [9] to 1670 nm, which corresponds to a ~5 THz gain bandwidth extension.  Such an amplifier could find applications in possible future optical communications systems as well as in areas such as medicine, optical sensing and material processing. This work was supported by the EU 7th Framework Program under grants 258033 (MODE-GAP), 287732 (ISLA) and by the UK EPSRC through grant EP/I01196X/1 (HYPERHIGHWAY). 5.  References [1] T. Morioka, et al., "Enhancing optical communications with brand new fibers," IEEE Commun. Mag. 50, s31–s42 (2012). [2] H. Zhang, et al., "81 Gb/s WDM transmission at 2 µm over 1.15 km of low-loss hollow core photonic bandgap fiber," in ECOC (2014), p.P.5.20. [3] F. Poletti, et al., "Towards high-capacity fibre-optic communications at the speed of light in vacuum," Nat. Photonics 7, 279–284 (2013). [4] A. M. Heidt, et al., "High power diode-seeded fiber amplifiers at 2 µm–from architectures to applications," IEEE J. Sel. Top. Quantum Electron. 20, 3100612 (2014). [5] J. M. O. Daniel, et al., "Ultra-short wavelength operation of a two-micron thulium fiber laser," in CLEO (2014), p.SW1N.2. [6] D. A Chestnut, et al., "Copropagating and counterpropagating pumps in second-order- pumped discrete fiber Raman amplifiers," Opt. Lett. 27, 1708–10 (2002). [7] M. Yamada, H. Ono, and J. Ono, "1.7 μm band optical fiber amplifier," in OFC (2014), p.Tu2D–3. [8] E. Dianov, et al., "A new bismuth-doped fibre laser, emitting in the range 1625-1775 nm," Quantum Electron. 44, 503–504 (2014). [9] Z. Li, et al., "90 nm Gain Extension Towards 1.7 µm for Diode-Pumped Silica-Based Thulium-Doped Fiber Amplifiers," in ECOC (2014), p.Tu.3.4.2. 

Extreme short wavelength operation (1.65 - 1.7 µm) of silica-based thulium-doped fiber amplifier

Extreme short wavelength operation (1.65 - 1.7 µm) of silica-based thulium-doped fiber amplifier

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