Modeling and analysis of single-mode widely tunable all-fiber Ho-doped CW master oscillator power amplifier system

We report the design of an all-fiber single-mode polarization maintaining widely tunable Ho-doped master oscillator power amplifier (MOPA) system operating in the wavelength range of 2005-2135 nm based on a pre-amplifier and two power amplification stages. The single clad Ho-doped active fibers in the ring cavity based master oscillator, pre-amplifier, and power amplifiers are pumped using 1950 nm semiconductor laser diode pump sources. The amplification of the MOPA system is investigated over the entire spectral range of 2005-2135 nm. The highest output power achieved by the continuous wave (CW) Ho-doped MOPA system is 43 W at a wavelength of 2028.6 nm using pump power of 20 W with power conversion efficiency (PCE) of 90% for second power amplifier while the total pump power of Ho-doped MOPA system is 45 W. Optical bandpass filters (OBPFs) are employed between different stages of MOPA system to enable amplification over the wider spectral range of 2005-2135 nm by suppressing the amplified spontaneous emission (ASE). Not using OBPFs results in amplification that is limited to the spectral range of 2045-2130 nm. Therefore, a penalty of 45 nm can be avoided at the expense of using the OBPFs between different stages of MOPA system. This flexible MOPA system can be used in multiple applications, especially for space-borne high power transmitters operating in atmospheric transmission windows.


Introduction
Wavelength tunable high output power laser sources operating in the vicinity of 2000 nm eye-safe optical spectrum are of practical interest for multiple applications, such as remote sensing, optical wireless communication, medical applications, material processing, chemical sensing and so on [1][2][3].The rare earth element Holmium has the ability to radiate light in this wavelength range [3].One of the prevalent utilization of these high power laser sources operating in the 2000-2200 nm wavelength range is deep-space optical wireless communication, that exploits their reduced scattering and atmospheric absorption compared to traditional 1550 nm laser sources [3,4].Therefore, power scaling of laser sources operating around 2000 nm has achieved a significant attention in the research field of lasers.The most suitable method of accomplishing the power scaling is the 'MOPA' (Master Oscillator Power Amplifier) technique in which the output power of a CW or pulsed laser can be significantly increased without altering the geometry, structure, or basis of operation [5].In general, a master oscillator produces the beam that is highly coherent, known as the seed, which is further amplified by preserving its properties through the use of optical amplifiers [5,6].The master oscillator is not required to have a very high power or to function at high efficiency as the efficiency is primarily calculated by the power amplifier [6].
Wavelength tunable high power CW and pulsed all-fiber MOPA systems have been extensively researched over the past decade.Owing to latest boost in research interest in the wavelength window around 2000 nm, a number of studies have been reported on tunable or single frequency high power Ho-doped MOPA systems.For instance, Qian et al demonstrated a Ho: YAG MOPA operating at 2100 nm for pumping the long-wave infrared ZnGeP 2 (ZGP) optical parametric oscillator (OPO) [2].The design was based on three power amplification stages, thus generating a highest average output power of 73.1 W with 17 ns of pulse width, at a repetition rate of 3 kHz.Jinwen et al demonstrated a Ho: YAP laser excited by a Tm-doped MOPA system generating optical power of 107 W and slope efficiency (SE) of 50.6% at 2117.1 nm, for an absorbed pump power of 215.4 W [7]. Qian et al proposed a mid-infrared pulsed ZGP MOPA system operating at repetition rate of 2 kHz, while generating an output energy of 52 mJ [8].The system was pumped by two-stage Ho: YAG MOPA laser with a highest average output power of 222 W at a wavelength of 2097 nm.Yao et al proposed a high-power pulsed allfiber integrated Tm-Ho MOPA system operating at 2116 nm, where a Ho-doped active fiber was pumped at a wavelength of 1980 nm [1].A combined power of 128.5 W was obtained from the hybrid MOPA system out of which an average power of 99.1 W was due to Ho emission at 2116 nm.Therefore, peak power of 1.91 kW and pulse width of 322 ns was achieved at the repetition rate of 161 kHz.Karsten and colleagues proposed a double pass laser system that uses Ho: YAG MOPA at 2090 nm and is pumped by Tm-doped fiber laser (TDFL) of 50 W [9]. Highest energy of 125 mJ and width of 20 ns were achieved at pulse repetition rate of 100 Hz.To provide a more detailed account of the literature survey, table 1 provides main findings of these studies, with missing information in corresponding studies being indicated by dashes.Moreover, the main results of the proposed work are also compared with these studies.
It may be inferred from the detailed literature survey discussed earlier that only a handful of studies related to high power Ho-doped MOPA systems are available so far.These studies typically demonstrate single frequency solid state pulsed MOPA systems where gain mediums were fabricated using synthetic crystalline materials such as Yttrium Aluminium Garnet (YAG), Yttrium Scandium Gallium Garnet (YSGG), Yttrium Lithium Fluoride (YLF) etc doped with Ho ions.To the best of our knowledge, we find no report related to the wavelength tunable all-fiber Ho-doped CW MOPA system.In this paper, we propose for the first time a widely-tunable all-fiber single-mode CW Ho-doped MOPA system, that is continuously tunable in the 2005-2135 nm wavelength range with ASE suppressed using OBPFs inserted between different stages.The proposed design is based on a preamplifier and two power amplifiers while short segments of single clad Ho-doped fibers are used in master oscillator and subsequent stages that are excited using semiconductor laser diode pump sources.A maximum output power of 43 W and PCE of 90% are obtained with a pump power of 20 W for second power amplifier at wavelength of 2028.6 nm exploiting a total pump power of 45 W. OBPFs are inserted among different stages of the MOPA to achieve the amplification over the entire wavelength range of 2005-2135 nm by suppressing the ASE.Based on the above discussion, the main contributions of this study are as follows: • Design of an all-fiber widely tunable (2005-2135 nm) CW Ho-doped MOPA system based on a pre-amplifier and two power amplifiers.
• Short segments of single clad Ho-doped active fibers are used as gain medium in ring cavity master oscillator and subsequent stages.
• The amplification of the MOPA system is investigated over the whole wavelength range of 2005-2135 nm.Maximum output power of 43 W has been achieved at 2028.6 nm using the pump power of 20 W with corresponding PCE of 90% for second power amplifier exploiting a total pump power of 45 W.
• The proposed MOPA system is flexible in terms of achieving tuning of 130 nm or 85 nm, depending upon inserting or eliminating the OBPFs between different stages.OptiSystem version 21 from Optiwave Inc. [3,10] is used in this work to design and analyze the overall performance of the Ho-doped MOPA system.The structure of the paper is as follows.Section-2 provides the theoretical knowledge, section 3 discusses the simulation set-up, section 4 discusses the results, and section-5 has conclusion of the paper.

Theoretical background
A group of important equations, known as rate and propagation equations alongwith absorption and emission cross section are used to explain lasing in a Ho-doped active fiber [11,12].These equations normally rely upon particular kind of laser and its settings.The time evolution of densities of photons and carriers in cavity is described by using the rate equations while the propagation equations outline the performance of the laser light because it propagates via active fiber and cavity [11,12].These equations are employed for modeling the performance of several laser structures, that include semiconductor, gas, and solid state lasers.
The normalized absorption and emission cross section of Holmium in glass fiber has been illustrated in figure 1(a).It is clear that Holmium has a wide absorption spectrum beginning at 1800 nm and ending at 2100 nm, with an absorption peak occurring close to 1950 nm.Holmium is typically excited exploiting in-band wavelengths of 1950 nm and 1840 nm [13,14].The ground energy level is marked as 5 I 8 manifold.The Holmium is pumped for 5 I 7 manifold by an in-band pump wavelength either at 1950 nm or 1840 nm.TDFLs are normally employed as pump sources to pump the Holmium.The transition executing the ground state absorption (GSA) and subsequent laser's emission around 2000 nm is 5 I 8 ↔ 5 I 7 [14].The GSA and laser emission are clearly marked by red arrows at plot of absorption and emission cross section as shown in figure 1(a).Figure 1(b) illustrates the energy diagram of an in-band pumped Ho-doped active fiber.It is clear that GSA and laser's emission occur between 5 I 8 and 5 I 7 levels whereas upconversion (UC) occurs between high energy manifolds, which are 5 I 5 and 5 I 6 [11,12].

Proposed architecture
The schematic of the designed Ho-doped CW MOPA system is shown in figure 2. The MOPA system consists of a ring cavity based master oscillator, pre-amplifier, and two power amplifiers.Four short segments of singlemode polarization maintaining Ho-doped single clad active fibers having the same specifications as iXblue's Hodoped fiber (Model # iXblue IXF-HDF-PM-8-125) [15] are used as gain media in the ring cavity master oscillator, pre-amplifier, and power amplifiers, as shown in figure 2. Length of Ho-doped active fibers and doping concentrations of Holmium ions must be optimized to achieve best performance [3,16,17].In this simulation study, we have considered already optimized values of Ho-doped active fiber length and doping concentration of Holmium ions from one of our previously published works [3].It is important to mention here that core radius, doping radius, numerical aperture (NA), doping concentration of Holmium ions, and length of all segments of active fibers used in the current study are exactly the same as in [3].The active fibers are pumped using 1950 nm semiconductor laser diode pump sources through pump and signal combiners using forward configuration in master oscillator and pre-amplifier while bidirectional configuration is used in both power amplifiers, as shown in figure 2.
The master oscillator consists of a short segment of Ho-doped active fiber acting as gain medium, a wavelength division multiplexer (WDM) to combine the pump with the active fiber, an optical isolator (ISO) to prevent back reflected light, a tunable optical filter (TOF) to select the operating wavelength inside the cavity in 2005-2135 nm wavelength range, and an output optical coupler (OC) for tapping out the lased signal from the cavity.The master oscillator provides seed at a particular wavelength as selected by the TOF.This seed is filtered using an OBPF to suppress the ASE generated from the master oscillator.The OBPF is tuned to the same wavelength as the TOF and is placed between the main oscillator and the pre-amplifier.The pre-amplifier is composed of another short segment of Ho-doped active fiber as gain medium, a pump combiner (PC) to combine the pump with the incoming seed and an optical ISO to prevent the back reflected light from later stages.The output signal from pre-amplifier is again filtered using another OBPF tuned at the same wavelength as TOF.The second OBPF is placed in between the pre-amplifier and the first power amplifier is used to suppress the ASE produced by the pre-amplifier.The pre-amplifier is followed by the first power amplifier, that is also composed of a short segment of Ho-doped active fiber as gain medium, two PCs to combine the pump with the input signal and Ho-doped active fiber, and an optical ISO to prevent the back reflected light from later stages.The output signal from the first power amplifier is filtered again using another OBPF placed between the first and second power amplifiers to suppress the ASE produced by the first power amplifier.The parameters of OBPFs used in the proposed architecture are exactly same.The second power amplifier has the same structure as the first power amplifier.An optical spectrum analyzer (OSA) and an optical power meter (OPM) are connected to the output of the second power amplifier to monitor and analyze the results.The main simulations parameters used in this study are shown in table 2.

Ring cavity master oscillator
The tuning of ring cavity master oscillator is achieved in 2005-2135 nm wavelength range, as exhibited in figure 3. A 1950 nm semiconductor laser diode pump having output power of 5 W is employed in forward configuration, enabling 130 nm wide tuning range.As mentioned earlier, the function of the master oscillator is to provide seed for power amplification stages at a particular wavelength, that is selected by the TOF.The TOF can be tuned to any of the wavelengths shown in figure 3. A minor fluctuation in powers of the lasing wavelengths is observed which is due to different absorption and emission cross sections of Holmium for different wavelengths, as presented in figure 1(a).The wavelength versus average output power and optical signal to noise ratio (OSNR) plots are shown in figure 4(a) with a pump power of 5 W for ring cavity master oscillator and output coupling ratio of 50%.It is clear that there is a variation in values of average output power and OSNR corresponding to lasing wavelength which may be associated to different absorption and emission cross sections of Holmium for different wavelengths as shown in figure 1(a).Similarly, wavelength versus SE plot is shown in figure 4(b) with a pump power of 5 W for ring cavity master oscillator and output coupling ratio of 50%.It may be noticed that different wavelengths have different SE values which is due to different absorption and emission cross sections of each wavelength.Moreover, highest value of output power is 4.6 W and SE is 40%, which are obtained at wavelength of 2028.6 nm with a pump power of 5 W for ring cavity master oscillator and coupling ratio of 50%.Therefore, output lasing power of around 4.6 W is available as seed at the output coupler of the cavity.

Pre-amplifier stage
The output spectrum of the pre-amplifier in the 2005-2135 nm wavelength range is shown in figure 5(a).This spectrum is obtained by employing a semiconductor laser diode pump centered at 1950 nm and having output power of 5 W in forward configuration, as shown in figure 2. As discussed earlier, small variation in powers of the lasing wavelengths are observed due to different absorption and emission cross sections of Holmium for different wavelengths.The wavelength versus average output power and PCE plots for a pump power of 5 W are  shown in figure 5(b).Again, the small variation in values of output power and PCE with respect to lasing wavelength may be attributed to different absorption and emission cross sections of Holmium for different wavelengths.Highest value of output power obtained is 9 W and PCE obtained is 88%, at wavelengths of 2028.6 nm and 2016.8 nm, respectively with a pump power of 5 W for pre-amplifier.Therefore, the seed of 4.6 W power at 2028.6 nm has been boosted to 9 W by using the pre-amplifier.

1st power amplifier
Output spectra of first power amplifier in the 2005-2135 nm wavelength range is shown in figure 6(a).Two semiconductor laser diode pumps, each centered a wavelength of 1950 nm and having an output power of 7.5 W are used in bidirectional configuration.The small variations in the lasing wavelengths are again visible in figure 6(a).The wavelength versus average output power and PCE plots are shown in figure 6(b) with a pump power of 15 W for first power amplifier.The highest value of output power obtained is 23.2 W at a wavelength of 2028.6 nm and the highest value of PCE obtained is 94% at a wavelength of 2016.8 nm with a pump power of 15 W for first power amplifier.Therefore, the input signal from the preceding stage has been boosted to 23.2 W from 9 W, after the first power amplifier.

2nd power amplifier
The spectrum at the output of the second power amplifier is shown in figure 7(a).The wavelength range for the spectrum is between 2005-2135 nm.Again, two semiconductor laser diodes each having a center wavelength of 1950 nm and output power of 10 W are employed as pumps in bidirectional configuration.It may be observed that the peak powers of the individual wavelengths shown in the spectral plot are higher compared to previous cases, shown in figure 5(a) and figure 6(a).The wavelength versus average output power and PCE plots are shown in figure 7(b) with a pump power of 20 W for second power amplifier.The highest value of output power obtained is around 43 W at a wavelength of 2028.6 nm and the highest value of PCE obtained is 90% at a wavelength of 2016.8 nm.These values are obtained with a pump power of 20 W for second power amplifier.Therefore, input signal from preceding stage of 23.2 W power at 2028.6 nm has been boosted to 43 W by the second power amplifier.

Controlling the ASE for wider amplification range
Achieving high optical output power and wide amplification range in any all-fiber MOPA system is extremely difficult.One of the main limiting factors is the ASE that severely affects the ability to achieve desired power scaling and wide amplification [18].A significant amount of ASE is usually introduced in the master oscillator ring cavity and amplified in the cascaded power amplifier stages.The ASE introduced by a certain stage gets amplified with the signal at each power amplifier [18].Therefore, it is necessary to mitigate the detrimental effects of ASE on the amplification process in MOPA systems [18].Various techniques have been proposed to suppress the ASE for achieving better signal quality and wider amplification range, such as specially designed noise suppression unit [18], signal feedback loop [19], diffraction-grating pair [20], double-seeding [21], and optimizing the various physical parameters [22].We exploit a relatively simple technique to suppress the ASE in the proposed MOPA system, enabling a wider amplification of 130 nm that is based on inserting OBPFs between different stages.The spectrum at the output of the second power amplifier in 2045-2130 nm wavelength range without using OBPFs is shown in figure 10(a) for the same pump settings as we have already used in section-4.4.The wavelength versus average output power plot in the wavelength range of 2045-2130 nm without OBPFs for the same pump settings as we have already used in section-4.4 is shown in figure 10(b).The highest value of output power obtained is around 41.6 W at a wavelength of 2050 nm.Small variations in the lasing wavelengths as well as in average output power of second power amplifier are again observed in figure 10.

Conclusion
Design of an all-fiber widely tunable continuous wave Ho-doped master oscillator power amplifier system operating in the wavelength range of 2005-2135 nm based on a pre-amplifier and two power amplification stages was proposed and demonstrated through numerical simulations.Single clad Ho-doped active fibers in the MOPA system are excited using 1950 nm semiconductor laser diode based pump sources.Output power and  power conversion efficiency of 43 W and 90%, respectively are achieved at 2028.6 nm with a pump power of 20 W for second power amplifier corresponding to 45 W total pump power of continuous wave Ho-doped master oscillator power amplifier system.Optical bandpass filters are placed between different stages of master oscillator power amplifier to enable amplification over a wide spectral range of 2005-2135 nm by suppressing the amplified spontaneous emission.In the absence of the OBPFs, the amplification is limited to the range of 2045-2130 nm.Therefore, a penalty of 45 nm can be avoided at the expense of using the OBPFs between different stages of master oscillator power amplifier.This flexible design can be exploited in a variety of applications, including space-borne high power transmitters operating in atmospheric transmission windows.

Figure 1 .
Figure 1.(a) Holmium's absorption and emission cross section (b) Energy diagram.The absorption and emission cross section data has been provided by commercial HDF vendor and used by OptiSystem.

Figure 3 .
Figure 3. Tuning of ring cavity master oscillator in 2005-2135 nm wavelength range for 50% coupling ratio of output optical coupler.This spectra is input to pre-amplifier.

Figure 4 .
Figure 4. Wavelength versus (a) Average output power and OSNR (b) SE.The plots are obtained with a pump power of 5 W for ring cavity master oscillator and 50% output coupling ratio.

Figure 5 .
Figure 5. (a) Output spectra of pre-amplifier in 2005-2135 nm wavelength range (b) Wavelength versus average output power and PCE plots of pre-amplifier.The plots are obtained with a pump power of 5 W for pre-amplifier.

Figure 6 .
Figure 6.(a) Output spectra of 1st power amplifier in 2005-2135 nm wavelength range (b) Wavelength versus average output power and PCE plots of first power amplifier.The plots are obtained with a pump power of 15 W for first power amplifier.

Figure 7 .
Figure 7. (a) Output spectra of 2nd power amplifier in 2005-2135 nm wavelength range (b) Wavelength versus average output power and PCE plots of second power amplifier.The plots are obtained with a pump power of 20 W for second power amplifier.

Figure 8 .
Figure 8. Spectra of different wavelengths at the output of second power amplifier without OBPFs (a) No amplification (b) Amplification starts at 2045 nm (c) Amplification at 2130 nm (d) No amplification at 2135 nm.

Figure 9 .
Figure 9. Spectra of different wavelengths at the output of second power amplifier with OBPFs (a) Amplification starts at 2005nm (b) Amplification at 2135 nm.

Figure 10 .
Figure 10.(a) Output spectra of 2nd power amplifier in 2045-2130 nm wavelength range (b) Wavelength versus average output power plot of second power amplifier.The plots are obtained with a pump power 20 W for second power amplifier without using OBPFs.

Table 2 .
Simulation parameters used in proposed CW Ho-doped MOPA system.