Wavelength-Flexible Thulium-Doped Fiber Laser Based on Digital Micromirror Array

Wavelength-tunable thulium-doped fiber laser is demonstrated employing a digital micromirror device (DMD) in combination with a fixed grating. The diffraction property of four typical models of DMDs and its steering efficiency for the laser system are analyzed based on two-dimensional grating theory. By spatially modulating reflective patterns on a DMD, the stable, fast, and flexible tuning of lasing wavelength from 1930 nm to 2000 nm is achieved with wavelength tuning accuracy of 0.1 nm. The side-mode suppression ratio is larger than 50 dB around the 2 μm band with 3 dB linewidth less than 0.05 nm. The wavelength drift and power fluctuation are lower than 0.05 nm and 0.1 dB within 1 h at the room temperature, respectively.

light modulator [25], digital micromirror device (DMD) [26][27][28], and so on. Up to now, a DMD chip as a semiconductor-based addressable micromirror array has drawn considerable attention due to its flexible and fast filtering functionality in optical switching, interconnecting, and lasers. The diffracted light from selected micro-mirrors on a DMD determines its spectral filtering characteristic. In this paper, we propose a 2 µm band tunable TDFL by using a DMD chip in combination with a fixed grating. The required architecture and the characteristics are experimentally demonstrated in the next section. Figure 1 depicts the configuration of the proposed tunable TDFL. The laser system consists of a fiber-ring resonator and an optical filter module. The fiber module includes a thulium-doped fiber amplifier (TDFA), a 90/10 coupler, a polarization controller, a fiber circulator, and a collimator. The TDFA emits the spontaneous emission spectrum (ASE) of 1910-2020 nm by adding Tm 3+ -fiber into a segment of double cladding pumped by 793 nm light. After a fiber coupler, 90% ASE light energy returns into a ring and then continues to be coupled into optical filter module via a collimator. Figure 1 depicts the configuration of the proposed tunable TDFL. The laser system consists of a fiber-ring resonator and an optical filter module. The fiber module includes a thulium-doped fiber amplifier (TDFA), a 90/10 coupler, a polarization controller, a fiber circulator, and a collimator. The TDFA emits the spontaneous emission spectrum (ASE) of 1910-2020 nm by adding Tm 3+ -fiber into a segment of double cladding pumped by 793 nm light. After a fiber coupler, 90% ASE light energy returns into a ring and then continues to be coupled into optical filter module via a collimator.

Operating Principle of TDFL
A diffraction grating and DMD are placed on the front and rear focal planes of a collimating lens, respectively. The ASE spectrum from a collimator is irradiated on a grating and then produces the 1st-order dispersion spectrum in a horizontal plane. After the lens, the collimated ASE spectrum images onto different portion of a DMD.
A DMD is an array of highly reflective aluminum micromirrors. It is an electrical input, optical output micro-electrical-mechanical system (MEMS) that allows performing high speed, efficient, and reliable spatial light modulation. During operation, the DMD controller loads each underlying memory cell with a '1' or a '0'. Next, a mirror reset pulse is applied, which causes each micromirror to be electrostatically deflected about a hinge to the associated +/− degree state. For example, a 0.7" DMD is composed of 1024 × 768 micromirrors with a pitch 13.68 μm and fill factor 92%. Each micromirror has +/−12° tilt angle corresponding to the 'on' and 'off' states. A typical hologram is demonstrated in the inset of Figure 1. The white bar on a hologram drives the corresponding mirrors to tilt +12°, so that the corresponding waveband of ASE spectrum landing on the white area return into the collimator while the others are dropped out with dramatic attenuation, thereby realizing the laser longitudinal mode selection and wavelength tuning. The selected waveband through the collimator into a ring cavity is amplified by the TDFA, leading, after several re-circulations, to high-quality lasing generation.  A diffraction grating and DMD are placed on the front and rear focal planes of a collimating lens, respectively. The ASE spectrum from a collimator is irradiated on a grating and then produces the 1st-order dispersion spectrum in a horizontal plane. After the lens, the collimated ASE spectrum images onto different portion of a DMD.
A DMD is an array of highly reflective aluminum micromirrors. It is an electrical input, optical output micro-electrical-mechanical system (MEMS) that allows performing high speed, efficient, and reliable spatial light modulation. During operation, the DMD controller loads each underlying memory cell with a '1' or a '0'. Next, a mirror reset pulse is applied, which causes each micromirror to be electrostatically deflected about a hinge to the associated +/− degree state. For example, a 0.7" DMD is composed of 1024 × 768 micromirrors with a pitch 13.68 µm and fill factor 92%. Each micromirror has +/−12 • tilt angle corresponding to the 'on' and 'off' states. A typical hologram is demonstrated in the inset of Figure 1. The white bar on a hologram drives the corresponding mirrors to tilt +12 • , so that the corresponding waveband of ASE spectrum landing on the white area return into the collimator while the others are dropped out with dramatic attenuation, thereby realizing the laser longitudinal mode selection and wavelength tuning. The selected waveband through the collimator into a ring cavity is amplified by the TDFA, leading, after several re-circulations, to high-quality lasing generation.

Diffraction Performance of DMD
A DMD is adopted in TDFL as a wavelength-tunable component for selecting lasing wavelength. In experiment, we upload holograms onto a DMD by LabVIEW to control the micromirrors status. Those tilting micromirror array demonstates the diffraction effect, similar with a two-dimensional blazed grating. The diffraction performance depends on the mirror pitch, tilting angle and incident wavelength.
In the TDFL, the configuration of a DMD must satisfy two basic conditions: (1) the diffracted light by a DMD should meet the near-blazed condition to achieve the maximum diffraction efficiency and reduce the insertion loss; (2) the selected wavelength channels must route back into the system to build a closed loop. In order to choose a suitable DMD chip operating efficiently in 2 µm band, the diffraction features of four typical DMD models launched by Texas Instruments are analyzed in detail. Table 1 shows near-infrared DMD chipsets at TI website [29]. Figure 2 demonstrates the diffraction distribution of a DMD in 2 µm band according to the two-dimensional DMD grating model that was established in our previous work [30].  Figure 2(a1-d1) demonstrate the diffraction distribution on the bisecting plane when 2 µm wavelength light radiates on 0.2", 0.45", 0.65", and 0.7" DMDs at the incident angles 17 • in Figure 2(a1,a2) and 12 • in Figure 2(b1-d1,b2-d2), respectively. Based on two-dimensional DMD grating model [30], the principal maxima of multiple-pixel interference (blue peaks in Figure 2) are modulated by the single-pixel diffraction envelope (red curves). Figure 2(a2-d2) show the corresponding diffraction patterns. In Figure 2(c1,c2), when 2 µm light is incident on the 0.65" DMD, four distinct diffraction orders occur in space. Thus, the energy is not concentrated and the diffraction efficiency is lower than 14% on average, called off-blazed. In Figure 2(b1,b2,d1,d2), the diffraction energy is focused on the first order, and the diffraction efficiency is up to 60%, satisfying the ideal blazed condition. The diffracted beam from 0.45" DMD, however cannot backtrack because the corresponding diffraction angle is away from the original path, while the diffraction order from 0.7" DMD returns at 12 • , exactly along the original path into the fiber ring. So the 0.7" DMD is appropriate for the closed-loop 2 µm laser.

Optimization of Bulk Optics
In bulk optics, it is necessary to optimize the optical system so as to maximum utilize the DMD workspace for higher tuning accuracy. The length of ASE dispersion covering a DMD is determined by the focal length of a collimating lens. Therefore, a collimating lens with appropriate focal length is important to improve the tuning accuracy.
According to the grating equation: d(sinα + sinβ = kλ), where k is the diffraction order (here only the first order is considered), d is a grating period of 459 lines/mm, λ is the wavelength, and α and β are the incident angle and diffraction angle, respectively. The laser operates between 1930 nm and 2000 nm and α = 17 • (the blazed angle of the grating), so that β=36.40 • @1930 nm and 38.72 • @2000 nm.
We choose the focal length to be 200 mm, 300 mm, and 400 mm for comparison. The calculation shows the dispersion length on a DMD after the collimating lens are 8.1 mm, 12.1 mm, and 16.2 mm, respectively. For the size of 0.7" DMD in the horizontal axis is 14 mm, when the focal length of a collimating lens is 300 mm, the utilization ratio of the DMD is up to 86%. Figure 3 shows schematic diagram of light path in bulk optics and the dispersion spectrum of 1930-2000 nm on a 0.7" DMD simulated by OpticStudio. The single pixel tuning accuracy of a DMD is defined as: p = δ ∆λ L , where ∆λ = 70 nm is the tuning range of TDFL, δ = 13.68 µm is the pitch size, and L = 12.1 mm is the dispersion length. So, p is calculated to be 0.079 nm/pixel.
Micromachines 2020, 11, x FOR PEER REVIEW 5 of 10 and the blue curves are multi-pixel interference.). (a2-d2) are corresponding diffraction patterns in space by simulation.

Optimization of Bulk Optics
In bulk optics, it is necessary to optimize the optical system so as to maximum utilize the DMD workspace for higher tuning accuracy. The length of ASE dispersion covering a DMD is determined by the focal length of a collimating lens. Therefore, a collimating lens with appropriate focal length is important to improve the tuning accuracy.
According to the grating equation: d(sinα + sinβ = kλ), where k is the diffraction order (here only the first order is considered), d is a grating period of 459 lines/mm, λ is the wavelength, and α and β are the incident angle and diffraction angle, respectively. The laser operates between 1930 nm and 2000 nm and α = 17° (the blazed angle of the grating), so that β=36.40°@1930 nm and 38.72°@2000 nm.
We choose the focal length to be 200 mm, 300 mm, and 400 mm for comparison. The calculation shows the dispersion length on a DMD after the collimating lens are 8.1 mm, 12.1 mm, and 16.2 mm, respectively. For the size of 0.7" DMD in the horizontal axis is 14 mm, when the focal length of a collimating lens is 300 mm, the utilization ratio of the DMD is up to 86%.

Characteristics of Tunable TDFL
The wavelength tunability of a DMD filter is important for the tuning performance of a laser. We upload the patterns onto the DMD to obtain signal output with different wavelength and bandwidth. Figure 4 demonstrates the tunable wavelength and flexsible bandwidth by a DMD filter in ASE spectrum of TDFA when the loop is open. The measured total loss of bulk optics from the input and output of a circulator is around 10.6 dB, which is mainly caused by the diffraction grating and DMD processor. The detailed loss of the components in the TDFL is shown in Table 2.

Characteristics of Tunable TDFL
The wavelength tunability of a DMD filter is important for the tuning performance of a laser. We upload the patterns onto the DMD to obtain signal output with different wavelength and bandwidth. Figure 4 demonstrates the tunable wavelength and flexsible bandwidth by a DMD filter in ASE spectrum of TDFA when the loop is open. The measured total loss of bulk optics from the input and output of a circulator is around 10.6 dB, which is mainly caused by the diffraction grating and DMD processor. The detailed loss of the components in the TDFL is shown in Table 2.    Figure 5 shows the typical output signal of TDFL. The laser wavelength is 1997.21 nm and the 3 dB-width is less than 0.05 nm, limited by spectrometer resolution. The side-mode suppression ratio (SMSR) is more than 50 dB. Figure 6 shows nine different laser spectra by coarse tuning, that exhibits high uniformity and stability. By moving the reflective area on a DMD, the wavelength tuning range from 1930-2000 nm is achieved, limited by the ASE spectrum covering a 0.7" DMD. The SMSR in whole range is greater   Figure 5 shows the typical output signal of TDFL. The laser wavelength is 1997.21 nm and the 3 dB-width is less than 0.05 nm, limited by spectrometer resolution. The side-mode suppression ratio (SMSR) is more than 50 dB.
Micromachines 2020, 11, x FOR PEER REVIEW 7 of 10 than 50 dB. The power distribution between these wavelengths can be modified by changing the height of each reflective column on the DMD to control the wavelength dependent feedback efficiency. The tuning time of TDFL depends on the flip speed of micromirrors on a DMD chip. The switching time of this type DMD used in the system is around 80 μs.   Figure 7 demonstrates the fine tuning output signals of TDFL around 1939.5 nm by fine tuning. The tuning accuracy is less than 0.1 nm. The reflection loss of a DMD cover glass, as well as the self-phase modulation and nonlinear effect caused by high pump power affect the laser contour [16]. Figure 8 shows the drift of central wavelength at 1975 nm (dotted line) and the fluctuation of peak power (solid line) when the pump power is 5 W. At room temperature, the maximum power fluctuation is 0.1 dB, and the maximum wavelength drift is less than 0.05 nm within 1 h. This fluctuation is mainly caused by the fluctuation of TDFA pumping power and the doping uniformity of the gain fiber. 1920 1930 1940 1950 1960 1970 1980 1990 Figure 6 shows nine different laser spectra by coarse tuning, that exhibits high uniformity and stability. By moving the reflective area on a DMD, the wavelength tuning range from 1930-2000 nm is achieved, limited by the ASE spectrum covering a 0.7" DMD. The SMSR in whole range is greater than 50 dB. The power distribution between these wavelengths can be modified by changing the height of each reflective column on the DMD to control the wavelength dependent feedback efficiency. The tuning time of TDFL depends on the flip speed of micromirrors on a DMD chip. The switching time of this type DMD used in the system is around 80 µs.
Micromachines 2020, 11, x FOR PEER REVIEW 7 of 10 than 50 dB. The power distribution between these wavelengths can be modified by changing the height of each reflective column on the DMD to control the wavelength dependent feedback efficiency. The tuning time of TDFL depends on the flip speed of micromirrors on a DMD chip. The switching time of this type DMD used in the system is around 80 μs.   Figure 7 demonstrates the fine tuning output signals of TDFL around 1939.5 nm by fine tuning. The tuning accuracy is less than 0.1 nm. The reflection loss of a DMD cover glass, as well as the self-phase modulation and nonlinear effect caused by high pump power affect the laser contour [16].    The tuning accuracy is less than 0.1 nm. The reflection loss of a DMD cover glass, as well as the self-phase modulation and nonlinear effect caused by high pump power affect the laser contour [16].

Conclusions
Wavelength-tunable thulium-doped fiber laser is demonstrated based on a DMD chip. The laser achieves flexible and fast tuning of wavelength by modulating the reflective area on the DMD. The operating wavelength is continuously tunable from 1930 nm to 2000 nm with a wavelength selectivity accuracy of 0.1 nm. The 3 dB linewidth of output signals is less than 0.05 nm and the SMSR is over 50 dB. The proposed tunable TDFL has the potential applications in space communication, laser spectroscopy, medical treatment and mid-infrared systems.

Conclusions
Wavelength-tunable thulium-doped fiber laser is demonstrated based on a DMD chip. The laser achieves flexible and fast tuning of wavelength by modulating the reflective area on the DMD. The operating wavelength is continuously tunable from 1930 nm to 2000 nm with a wavelength selectivity accuracy of 0.1 nm. The 3 dB linewidth of output signals is less than 0.05 nm and the SMSR is over 50 dB. The proposed tunable TDFL has the potential applications in space communication, laser spectroscopy, medical treatment and mid-infrared systems.

Conclusions
Wavelength-tunable thulium-doped fiber laser is demonstrated based on a DMD chip. The laser achieves flexible and fast tuning of wavelength by modulating the reflective area on the DMD. The operating wavelength is continuously tunable from 1930 nm to 2000 nm with a wavelength selectivity accuracy of 0.1 nm. The 3 dB linewidth of output signals is less than 0.05 nm and the SMSR is over 50 dB. The proposed tunable TDFL has the potential applications in space communication, laser spectroscopy, medical treatment and mid-infrared systems.
Author Contributions: X.C. and D.D. conceived and designed the experiments; D.D., Y.Z., and H.W. performed the experiments; D.D., Y.Z., and Y.G. analyzed the data; G.C. and Y.W. participated in the discussion; D.D. and X.C. wrote the paper. All authors have read and agreed to the published version of the manuscript.