Programmable High-Resolution Spectral Processor in C-Band Enabled by Low-Cost Compact Light Paths

hv photonics

Communication Programmable High-Resolution Spectral Processor in C-band Enabled by Low-Cost Compact Light Paths

Zichen Liu 1, Chao Li 2, Jin Tao 3 and Shaohua Yu 3,*

1 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology and Wuhan Research Institute of Post and Telecommunication, Wuhan 430074, China; [email protected] 2 Information Materials and Intelligent Sensing Laboratory of Anhui Province, Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, China; [email protected] or [email protected] 3 National Information Optoelectronics Innovation Center, Wuhan Research Institute of Posts and Telecommunications, Wuhan 430074, China; [email protected] * Correspondence: [email protected]

Received: 2 October 2020; Accepted: 3 December 2020; Published: 7 December 2020

Abstract: The flexible photonics spectral processor (PSP) is an indispensable element for elastic optical transmission networks that adopt wavelength division multiplexing (WDM) technology. The resolution and system cost are two vital metrics when developing a PSP. In this paper, a high-resolution 1 6 programmable PSP is investigated and experimentally demonstrated by using × low-cost compact spatial light paths, which is enabled by a 2 K (1080p) liquid crystal on silicon (LCoS) and two cascaded transmission gratings with a 1000 line/mm resolution. For each wavelength channel, the filtering bandwidth and power attenuation can be manipulated independently. The total insertion loss (IL) for six ports is in the range of 5.9~9.4 dB over the full C-band. The achieved 3-dB bandwidths are able to adjust from 6.2 GHz to 5 THz. Furthermore, multiple system experiments utilizing the proposed PSP, such as flexible spectral shaping and optical frequency comb generation, are carried out to validate the feasibility for the WDM systems.

Keywords: programmable; photonics spectral processor (PSP); liquid crystal on silicon (LCoS); high-resolution and low-cost; optical frequency combs (OFC)

1. Introduction With the exponential growth of communication traffic driven by high-definition TVs, online social networks, cloud computing and the Internet of things (IoT), there is an unprecedented requirement on high-capacity data transmission over optical transport networks [1]. The next generation wavelength division multiplexing (WDM) optical networks are required to be elastic. They must be capable of dynamically and efficiently allocating a flexible amount of optical spectra with different modulation baud rates to separated outputs [2]. To maximize the network efficiency with limited fiber bandwidths, a new G.694.1 standard has been established by the International Telecommunication Union. The new provision has a 12.5 GHz bandwidth slot width, which helps to reduce the optical protective band in order to improve the transmission efficiency in the previous G.692 specification. Moreover, advanced modulation techniques such as multi- carrier orthogonal frequency division multiplexing (OFDM) and single-carrier Nyquist shaping combined with high-order quadrature amplitude modulation (QAM) formats are expected. This scheme is expected to result in promising solutions for realizing highly-efficient future super-channel optical networking where multiple carriers are very close to each other with minimal guard-bands [3,4]. Both programmable high-resolution photonics spectral processors (PSPs) and wavelength selective switches (WSSs) can switch optical signals flexibly.

Photonics 2020, 7, 127; doi:10.3390/photonics7040127 www.mdpi.com/journal/photonics Photonics 2020, 7, 127 2 of 11

Benefiting from the adoption of PSP and WSS, flexible-grid reconfigurable optical add/drop multiplexers (ROADM) realize the functionalities of the supporting programmable channel bandwidth and provide a fine wavelength tuning of sub-GHz, i.e., 6.25 GHz [5]. PSP provides a higher resolution than WSS, excluding the fact that they achieve the same functions of filtering and optical power manipulation. To achieve cost efficiency and a satisfactory performance, both WSSs and PSPs need to consider the factors such as cost, volume, and insertion loss (IL). It has been proposed that one replaces the expensive M N WSS with multiple 1 K WSSs for the partially × × directional high-degree ROADMs [6]. Generally, researchers hope to increase the number of ports and the degree of integration by sharing an expensive liquid crystal on silicon (LCoS) device and bulk optical components [7–9]. It is believed that designing a high-resolution 1 K PSP with lower cost and × compact size can be helpful in decreasing the total cost of an M N PSP, since it is compatible with the × multiple integration solution demonstrated in [8]. Due to its multiple functionalities, the programmable PSP can be implemented in future ultra-high capacity optical systems and networks. Generating the shape of the arbitrary spectrum: The generation of a variable bandwidth and arbitrary spectrum shape is important in the implementation of the system, such as channel shaping, channel selection and channel gain equalization. For example, by programming the shape of the filter into PSP, one can study the cascading effect of the filter on the transmission quality. Generating proper optical frequency combs: The generation of controllable optical combs is the key point in optical WDM transmission systems. It requires the PSP to have the high-resolution capability of attenuating the optical power arbitrarily and creating special spectral lines even if they have independent shapes with modulation. Currently, the spatial light modulator (SLM) utilizing LCoS (LCoS-SLM) is the most competing technique adopted in the design of multiple port WSSs [10–14]. LCoS device combing with gratings can be further developed as programmable PSP for processing optical spectrum resources [15]. Recently, demonstrations have been reported on generating a reconfigurable PSP [15–18]. In [15], Gao et al. proposed and demonstrated a flexible PSP based on a 4K LCoS with a minimum pixel spacing of 3.74 µm. However, they just developed a one-input one-output filter with an achieved minimum 3-dB bandwidth of 10 GHz. Moreover, the price of the employed high-resolution LCoS-SLM is quite expensive. In 2019, their group also demonstrated a reconfigurable PSP, that provided a minimum bandwidth of 12.5 GHz in the C-band by using digital micro-mirror devices. The major drawback is that the achieved tuning resolutions of the center wavelength and IL of this filter are large, being 4.125 GHz (0.033 nm) and 10 dB across the whole C-band, respectively [16]. Rudnick et al. proposed the use of a waveguide grating router (WGR) to implement a high-resolution PSP with a 0.8 GHz optical resolution [17]. However, the achieved free spectral range is just 200 GHz. Besides, to obtain such a high-resolution, large delays within the WGR are required while still maintaining phase accuracy at the output, which is beyond standard fabrication tolerances. Employing spatial light paths, WSS with a high input and output ports has been demonstrated in [8,19,20]. In order to better meet the future development of high-resolution PSP, the designed spatial light paths are required to be compact and have a low IL. In this paper, we propose low-IL and compact spatial light paths in order to develop a high-resolution low-cost programmable PSP. The concrete scheme is realized by using a 2K LCoS- SLM (The price of 2K LCoS-SLM is less than half of 4K LCoS-SLM) and two cascaded diffraction gratings with a 1000 line/mm resolution. We experimentally demonstrate a 1 6 PSP operating across the entire × C-band. The achieved minimum 3-dB bandwidth is 6.2 GHz and the largest coverage can be extended to the full C-band of 5 THz spectra by a tuning step of 3.1 GHz. The best and worst ILs for all six ports are less than 6 dB and 9.5 dB, respectively, with about a 1-dB wavelength-dependent loss (WDL). Moreover, arbitrary spectral amplitude manipulations with more than a 20-dB suppression ratio have been experimentally obtained. Compared with the optical signal generator based on commercial products, our proposed method has a better performance for the minimum bandwidth setting and spectral suppression ratio for optical frequency comb (OFC) generation. The achieved results reveal Photonics 2020, 7, x 127 FOR PEER REVIEW 33 of of 11 cheap devices and compact spatial light paths and to further implement them in a cost-efficient M × Nthat PSP. it is This feasible is because to develop the employed such a programmable optical comp andonents high-resolution and the designed PSP by compact employing light commercial paths help tocheap improve devices the and wavelength compact spatialseparation light ability, paths and while to furtherthis does implement not affect them the design in a cost-e of thefficient number M ofN × inputPSP. This and isoutput because ports the of employed the WSS.optical components and the designed compact light paths help to improve the wavelength separation ability, while this does not affect the design of the number of input 2.and Materials output ports and Methods of the WSS.

2.1.2. Materials Principle of and Operation Methods

2.1. PrincipleWhen designing of Operation an optical filter, the center wavelength tuning resolution 𝑅 and minimum spectral bandwidth 𝐵 are the two key specifications that need to be take into consideration. In R this paper,When designingby using two an optical cascaded filter, gratings, the center the wavelength proposed method tuning resolutionand light pathres and contribute minimum to achievingspectral bandwidth a wide dispersedBmin are spectrum the two key in space, specifications finally cover that need larger to working be take into length consideration. on the LCoS In chip. this Basedpaper, on by using[15], the two larger cascaded spot gratings,size across the the proposed y-axis, methodthe smaller and the light spectral path contribute bandwidth. to achieving Figure 1 illustratesa wide dispersed the operation spectrum principle in space, of the finally LCoS-bas cover largered programmable working length PSP. on The the near LCoS parallel chip. Basedinput beamon [15 is], theinjected larger onto spot the size transmission across the y -axis,gratings the. smallerThe employed the spectral gratings bandwidth. (PING-Sample-083) Figure1 illustrates with a 1000the operation line/mm principleresolution of thefeature LCoS-based a high efficiency programmable and high PSP. Thetolera nearnce parallelto the illumination input beam is angle injected of incidence.onto the transmission With a 90% diffraction gratings. Theefficiency, employed the in gratingscidence (PING-Sample-083)angle range can be tuned with afrom 1000 46° line to/mm 54° [21].resolution feature a high efficiency and high tolerance to the illumination angle of incidence. With a 90% diffraction efficiency, the incidence angle range can be tuned from 46◦ to 54◦ [21].

Transmission Φ Input beam grating 1 𝝀𝟏 𝝀𝒏

𝚯𝝀𝒊,𝟏

𝒚 Φ𝝀𝒊,𝟐 𝒙 𝒇 𝝀 𝟏 𝚯 L 𝝀𝒊,𝟐 Transmission 𝝀𝒏 grating 2

LCOS SLM Cemented lens

Figure 1. 1. OperationOperation principle principle of the ofliquid-on-crystal the liquid-on-crystal (LCoS) based (LCoS) programmable based programmable PSP using dual- PSP grating.using dual-grating. In the experiment, the incidence angle to the first grating is set at the optimal value of 49.9 , In the experiment, the incidence angle to the first grating is set at the optimal value of 49.9°◦, which means that Φ1 equals to 49.9◦. When the input optical beam passes through the transmission which means that Φ equals to 49.9°. When the input optical beam passes through the transmission grating 1, the wideband signals (λ1 = 1530 nm~λn = 1570 nm in the experiment) are spread into grating 1, the wideband signals (𝜆 = 1530 nm~𝜆 = 1570 nm in the experiment) are spread into angularly separated wavelengths. The first-order diffraction angle of the transmission grating 1 follows angularly separated wavelengths. The first-order diffraction angle of the transmission grating 1 the grating equation as follows [22] follows the grating equation as follows [22] λ + =λ i sinsinΘ Θλi,1 +sinΦsin Φ1 = (1)(1) , 𝑑d Θ 𝜆 where Θλi,,1 represents the diffraction diffraction angle of the input optical beam with the wavelength of of λi transmitted after after the the first first grating. grating. d disis the the groove groove width width of of the the gratings, gratings, which which is is1 μm 1 µ.m According. According to

Equationto Equation (1), (1), the the diffraction diffraction angle angle difference difference for for the the wideband wideband signal signal can can be calculated byby ∆Θ∆Θ1 = Θ −ΘΘ =53.6°= 53.6 − 49.9°49.9 == 3.7°3.7. Then. Then the the angular angular separated separated lights lights are are transformed transformed into into the the second second λn,,1 − λ1,,1 ◦ − ◦ ◦ transmission grating. grating. The The optimal optimal incidence incidence angle angle of of49.9°49.9 ◦ is isassigned assigned to tothe the central central wavelength wavelength of 1550of 1550 nmnm. Thus,. Thus, the theincidence incidence angles angles for for1530 1530 nm nmandand 1570 1570 nm nmare areset setas as49.9°49.9 +◦ 3.7°/2+ 3.7◦=51.75/2 = 51.75° and◦ 49.9°and 49.9 − 3.7°/23.7=48.05/2 =°,48.05 respectively., respectively. The calculated The calculated diffraction diff angleraction difference angle di ffaftererence traveling after traveling through ◦ − ◦ ◦ thethrough second the grating second is grating ∆Θ =Θ is ∆Θ −Θ= Θ =55.7°Θ − 48.1°= 55.7 = 7.6°48.1. Therefore,= 7.6 . Therefore,compared comparedwith the single with ,2 ,λn,2 − λ1,2 ◦ − ◦ ◦ grating, the dispersed angle in space is doubled. The transmission grating 2 and LCoS chip are

Photonics 2020, 7, 127 4 of 11 the single grating, the dispersed angle in space is doubled. The transmission grating 2 and LCoS chip are arranged at the front and back focal planes of the cemented lens, whose focal length is f = 100 mm. The position distribution L along the y-axis on the LCoS chip is further expressed as

L = 2 f tan(∆Θ /2) (2) · 2 Thus, based on Equation (2), the theoretical spot size can be calculated as 13.28 mm, which is consistent with the measured result of 12.6 mm. This error of about 5% is acceptable caused by the optical path design and package. The main reason for the specific error is that in the process of packaging and debugging, in order to reduce the fine adjustment of the combined lens of the WDL, the imaging of the spot from the collimator on the LCoS-SLM is changed after passing through the optical subsystem. The input light spots from the collimator array are expanded by a 20-fold cylindrical beam expansion system, resulting in an enlarged elliptical image on the LCoS-SLM. The image is distributed by wavelength in the horizontal direction of the SLM (y-axis). The employed LCoS chip (Holoeye Pluto) includes an array of 1920 1080 pixels with a pitch × of 8 µm and effective length of 15.36 mm and can be addressed pixel-by-pixel. The reciprocal linear dispersion along the wavelength axis (y-axis) on the LCoS chip plane is 0.37 GHz/µm. That is to say, one pixel corresponds to a 3-GHz bandwidth. Thus, the minimum resolution of the central channel wavelength Rres_min is 3 GHz. If a 4K LCoS-SLM is employed, the minimum resolution can be further enhanced. Theoretically, the spectral bandwidth is an integer multiple of the minimum resolution, in proportion to the number of used pixels in the y-axis. One can note that when the number of cascaded gratings increases to m > 5, the diffraction angle difference ∆Θm = Θ Θ > m ∆Θ λn,m − λ1,m · 1 and the minimum resolution of the MHz scale can be theoretically achieved.

2.2. Implementation In this section, the implementation and characterization of the 1 6 programmable PSP is described. × 2.2.1. Optical Path Design Using Zemax In this paper, we first theoretically study the design of the compact light path based on the Gauss beam coupling equation [23]. The positions and parameters of the used optical elements are calculated. We further optimize them by using the optical path optimization and tolerancing software of Zemax as illustrated in Figure2. This operation ensures the good coupling e fficiency of the Gaussian beams from the input port couples into the multiple output ports and achieves lower values of IL. We then fix and solidify all the optical elements on an optical substrate according to the compact design in the experiment. No mechanical adjustments are needed. It is worth noting that in the optical design the beam spot size expanded by the combination of the cylindrical lenses (Components 2 and 3) matches the sizes of the two gratings, so that both gratings can be fully utilized. Meanwhile, no additional light leakage loss is introduced. The x-arises in Figure3a,b show the normalized pupil, whose function is to represent the normalized field of view, ranging from 1 to +1. As shown in Figure3a at the − wavelength direction (y-axis), the optical wavelengths of 1530 nm, 1550 nm, and 1570 nm are uniformly distributed on the LCoS plane and the simulated spot size from 1530 nm to 1570 nm is about 12.89 mm, which confirms the measured result of 12.6 mm. At the switching direction (x-axis), the spherical aberration of the optical system is optimized, and no chromatic aberration is caused for the entire C-band wavelengths, since the ray fan curves for 1530 nm, 1550 nm and 1570 nm completely coincide, as shown in Figure3b. Furthermore, the beam spots formed by di fferent wavelengths are well separated, as shown in Figure3a, and the respective ellipticity of the beam spot is quite large at the switching direction, which ensures that full use is made of the LCoS pixels (approximately 400 pixels). The elliptical spots on the LCoS plane make it easier to achieve a linear optimization and high control accuracy for the signal attenuation of each wavelength (the minimum power attenuation of 0.1 dB can be achieved). Photonics 2020, 7, 127 5 of 11 Photonics 2020, 7, x FOR PEER REVIEW 5 of 11

5 4

10 9 8

x 2 x y 2

1 1 Figure 2.2. The proposed compact light pathpath usingusing dual-grating.dual-grating. The insets show the receiverreceiver beam

distribution on the LCoSLCoS planeplane atat aa 4545°◦ view.view. TheThe numbersnumbers 1–101–10 represent:represent: No.No. 1 isis thethe polarizationpolarization converterconverter withwith aa birefringentbirefringent crystalcrystal YVOYVO44 and half-wave plates; Nos. 2 and 3 are cylindrical mirrors withwith focal lengths lengths of of 4 4mm mm and and 60 60 mm, mm, respectively; respectively; Nos. Nos. 4 and 4 and5 are 5 transmi are transmissionssion gratings gratings with 1000- with 1000-lineline/mm /resolution;mm resolution; Nos. Nos. 6, 7, 6,and 7,and 9 are 9 arecylindrical cylindrical lenses lenses with with focal focal length length of of 141 141 mm; mm; No. No. 8 8 is a mirror; No.No. 10 is an LCoS chip, whose working wavelength is from 1520 nm to 1620 nm, pixel size is 88µm,µm, ﬁllfill factorfactor isis 87%87% andand resolutionresolution isis 19201920 × 1080. ×

8000 3000 (a) 1530 nm 3000 (a) 1530 nm (b) 1530 nm 1550 nm 6000 1550 nm 1550 nm 1570 nm 2000 1570 nm 2000 1570 nm 4000 1000 2000

0 0 0

-2000 𝟏𝟐. 𝟖𝟗 𝒎𝒎 𝟏𝟐. 𝟖𝟗 𝒎𝒎 -1000 -4000 -2000 -6000 Distribution at switching direction (μm) Distribution at wavelength direction (μm) direction wavelength at Distribution Distribution at switching direction (μm) Distribution at wavelength direction (μm) direction wavelength at Distribution -8000 -8000-1 -0.5 0 0.5 1 -3000 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 Normalized pupil Normalized pupil

Figure 3. 3. SimulationSimulation results results using using Zemax, Zemax, (a) at (thea) atwavelength the wavelength direction direction and (b) at and the ( bswitching) at the switchingdirection. direction.

2.2.2. Experimental Setup Figure4 4 shows shows the the experimental experimental site site of theof the assembled assembled programmable programmable PSP utilizingPSP utilizing a compact a compact light configuration,light configuration, including including a fiber a array, fiber aarray, micro-lens a micro-lens array for array the interactionfor the interaction of the light of the beam light between beam thebetween fiber andthe fiber free space,and free a polarization space, a polarization conversion conv unit,ersion collimating unit, collimating lenses, two lenses, transmission two transmission gratings, cementedgratings, cemented lenses and lenses a 2 K LCoS-SLM.and a 2 K LCoS-SLM. The dense Th WDMe dense optical WDM signal optical from signal one fiber from input one portfiber can input be switchedport can be to switched several di tofferent several output different ports output by the spectralports by dispersedthe spectral gratings dispersed addressed gratings on addressed the LCoS. Inon thethe paper,LCoS. In one the fiber paper, port one is picked fiber port as theis picked input, as and the six input, ports and with six a ports 500 µ withm pitch a 500 are µm selected pitch are as outputs.selected as The outputs. six output The portssix output are divided ports are into divi twoded groups, into two addressed groups, addre equallyssed on equally the upper on andthe upper lower partand oflower the input part port.of the This input design port. with This symmetrical design with distribution symmetrical helps distribution to reduce IL dihelpsfferences to reduce between IL thedifferences output ports,between because the output the closer ports, the because input portthe cl isoser to thethe center,input port the smalleris to the the center, switching the smaller angle andthe switching the smaller angle the lossand are.the Thesmaller beam the divergence loss are. Th half-anglee beam divergence is compressed half-a byngle the is micro-lens compressed array by fromthe micro-lens around 0.6 array◦ to 6from◦. To around maximally 0.6° decreaseto 6°. To ma theximally pointing decrease error, the the micro-lens pointing error, array the must micro-lens be very carefullyarray must aligned be very with carefully the input aligned fiber with array the during input the fiber design array process. during the Then, design the inputprocess. polarization Then, the statesinput polarization onto the LCoS states chip onto are adjustedthe LCoSto chip the are P-polarization, adjusted to the since P-polarization, the LCoS chip since only the manipulates LCoS chip theonly P-polarization manipulates lightthe P-polarization beams. A pair oflight cylindrical beams. lensesA pair are of employed cylindrical to collimatelenses are the employed beam spots to andcollimate expand the them beam into spots elliptical and expand beams. them The into input elliptical beamfurther beams. goesThe input through beam the further proposed goes light through path the proposed light path by using dual gratings. The cemented lens, corresponding to a lens with a

focal length of 100 mm, is composed of two cylindrical lenses (𝑓 =𝑓 = 141 mm). We add a mirror

Photonics 2020, 7, x FOR PEER REVIEW 6 of 11 with a 45° reflective angle in the design to make the light path compact and more conveniently adjustable. The collimated and expanded light is finally converged on the LCoS display and modulated by its phase grating. Different optical spectra are projected onto different pixels of the LCoS chip plane. The diffracted and reflected spectra are recombined again by the transmission gratings onto the individual output ports. Thus, light beams with different wavelengths are able to be routed to any desired output ports.

3. Experimental Results and Discussions We first measured the IL of each output port as a function of the input wavelength across the whole C-band. In the experiment, a scanning laser source module (Agilent, 81940A) with a sweeping step of 0.001 nm is employed as a transmitter whose wavelength range is set from 1530 nm to 1570 nm. At the receiver side, we use a high-sensitivity optical power meter module (Agilent, N7745A) to detect the optical spectra one by one. According to the measured results as shown in Figure 5, the total IL fluctuations for the six output ports are between 5.9 dB to 9.4 dB, corresponding to port 1 and port 4. The introduced IL can be mainly classified as static loss and dynamic loss [24]. The former one is mainly caused by optical components such as the two transmission gratings (1.9 dB), micro-lens array (1.5 dB), LCoS chip (0.9 dB), collimating lens (0.5 dB), polarization conversion unit (0.5 dB), and cylindrical lens (0.2 dB). These ILs are calculated according to the corresponding material parameters. Due to the input and output of the optical path and the record of passing twice, these losses are doubled in the calculation. The latter one is mainly induced by different responses of different wavelengths passing through the slanted cascaded transmission gratings and different distances betweenPhotonics 2020 the, 7principal, 127 optical axis and output ports, leading to fluctuation losses from 0.4 dB to6 of3.9 11 dB. For each port, the measured intrinsic WDL within the full C-band is around 1 dB as illustrated in Figure 5. by using dual gratings. The cemented lens, corresponding to a lens with a focal length of 100 mm, In order to facilitate the programming of the spectrum, we have written the corresponding is composed of two cylindrical lenses ( f = f = 141 mm). We add a mirror with a 45 reflective angle operation software, and its operation1 interface2 is similar to that of Finisar WaveShaper.◦ After in the design to make the light path compact and more conveniently adjustable. The collimated and calibrating the LCoS-SLM grayscale of the corresponding device port, and the accuracy of the wavelength expanded light is finally converged on the LCoS display and modulated by its phase grating. Different and pixel control bandwidth, our software can also directly read the .txt or .xlsx data files composed of optical spectra are projected onto different pixels of the LCoS chip plane. The diffracted and reflected the wavelength, port and attenuation along the whole C-band, so as to realize the programming on the spectra are recombined again by the transmission gratings onto the individual output ports. Thus, wavelength. light beams with different wavelengths are able to be routed to any desired output ports.

Transmission grating 2 Transmission grating 1

Cemented lens Collimating lens

LCOS Mirror

Polarization Micro Lens array converters Fiber array

Figure 4. Experimental site using our proposed low-cost compact spatial light paths. All the optical Figure 4. Experimental site using our proposed low-cost compact spatial light paths. All the optical elements are solidified on an optical substrate. elements are solidified on an optical substrate. 3. Experimental Results and Discussions We first measured the IL of each output port as a function of the input wavelength across the whole C-band. In the experiment, a scanning laser source module (Agilent, 81940A) with a sweeping step of 0.001 nm is employed as a transmitter whose wavelength range is set from 1530 nm to 1570 nm. At the receiver side, we use a high-sensitivity optical power meter module (Agilent, N7745A) to detect the optical spectra one by one. According to the measured results as shown in Figure5, the total IL fluctuations for the six output ports are between 5.9 dB to 9.4 dB, corresponding to port 1 and port 4. The introduced IL can be mainly classified as static loss and dynamic loss [24]. The former one is mainly caused by optical components such as the two transmission gratings (1.9 dB), micro-lens array (1.5 dB), LCoS chip (0.9 dB), collimating lens (0.5 dB), polarization conversion unit (0.5 dB), and cylindrical lens (0.2 dB). These ILs are calculated according to the corresponding material parameters. Due to the input and output of the optical path and the record of passing twice, these losses are doubled in the calculation. The latter one is mainly induced by different responses of different wavelengths passing through the slanted cascaded transmission gratings and different distances between the principal optical axis and output ports, leading to fluctuation losses from 0.4 dB to 3.9 dB. For each port, Photonics 2020, 7, x FOR PEER REVIEW 7 of 11 the measured intrinsic WDL within the full C-band is around 1 dB as illustrated in Figure5.

-5 Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 -6

-7

-8

Insertion Loss (dB) -9

1530 1535 1540 1545 1550 1555 1560 1565 1570 Wavelength (nm) Figure 5. Measured total insertion loss (IL) for six ports in the full C-band wavelengths. Figure 5. Measured total insertion loss (IL) for six ports in the full C-band wavelengths.

In the following measured results illustrated in Figures 6–9, we choose port 3 with neither the maximum nor the minimum IL of the C-band as the representative of the research object. Figure 6 shows the measured flexible 3-dB bandwidth adjustments from 6.2 GHz to 25.3 GHz by increasing the number of pixels on the LCoS chip one-by-one across the y-axis. The starting position is selected at the center of all the covered LCoS pixels, corresponding to the wavelength of 1550.8 nm. Without introducing extra IL, the achieved minimum 3-dB bandwidth is 9.8 GHz as shown in Figure 6. The maximum bandwidth can be extended to 5 THz across the entire C-band by adjusting a step of about 3.1 GHz, which is consistent with the theoretical minimum resolution of 3 GHz. The minimum filtering bandwidth of 6.2 GHz (corresponding to one pixel) can be obtained by introducing a 9-dB extra loss, since a fewer number of pixels results in a higher optical power loss. When increasing the 3-dB bandwidth to 8.1 GHz, the extra loss is decreased to 1.4 dB. We also measured the minimum 3- dB bandwidths in the C-band at the central wavelengths of 196 THz (1529.553 nm), 195 THz (1537.397 nm), 194 THz (1545.322 nm), 193 THz (1553.329 nm), 192 THz (1561.419 nm) and191 THz (1569.594 nm), respectively. As illustrated in Figure 7, the measured values are about 8.4 GHz, 8.1 GHz, 6.7 GHz, 6.59 GHz, 6.9 GHz and 7 GHz, corresponding to side band suppression ratios of 27 dB, 28 dB, 31 dB, 31.1 dB, 31.3 dB, and 29.3 dB, respectively. Furthermore, a programmable PSP needs to possess the capability to arbitrarily attenuate the optical power in order to meet the special spectral requirement. In the experiment, by using our proposed PSP, we generate three sets of continuous sine waves with repetition frequencies of 2.5 GHz (0.02 nm), 0.25 GHz (0.002 nm), and 0.125 GHz (0.001 nm), respectively. As shown in Figure 8, the achieved peak-to-peak power suppressions for 2.5 GHz, 0.25 GHz and 0.125 GHz sine waves are 23.4 dB, 18 dB and 6 dB, respectively. The experimental results show that when the frequency interval is greater than or equal to 0.25 GHz, our proposed system can already produce a peak-to-peak power suppression signal greater than 18 dB, and the performance is good. In addition, in order to further verify the application of our method in communications such as OFDM, we have also produced OFCs with spacing of 12.5 GHz and 25 GHz.

-10 6.2GHz 6.87GHz 8.12GHz -20 9.85GHz 12.68GHz 15.81GHz -30 18.8GHz 21.91GHz 25.3GHz

Insertion Loss (dB) -40 45

1550.8 1550.9 1551 1551.1 1551.2 1551.3 1551.4 Wavelength (nm)

Photonics 2020, 7, x FOR PEER REVIEW 7 of 11

-5 Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 -6

-7

Photonics 2020, 7, 127 -8 7 of 11

Insertion Loss (dB) -9 In order to facilitate the programming of the spectrum, we have written the corresponding

operation software, and its1530 operation1535 interface1540 is1545 similar1550 to that1555 of Finisar1560 WaveShaper.1565 1570 After calibrating the LCoS-SLM grayscale of the correspondingWavelength device port, (nm) and the accuracy of the wavelength and pixel control bandwidth, our software can also directly read the .txt or .xlsx data files composed of the wavelength,Figure port 5. Measured and attenuation total insertion along loss the (IL) whole for six C-band, ports in sothe as full to C-band realize wavelengths. the programming on the wavelength. InIn the the following following measured measured results results illustrated illustrated inin Figures Figures6 –6–9,9, we we choose choose port port 3 3 with with neither neither the the maximummaximum nor nor the the minimum minimum IL ofIL theof the C-band C-band as the as representative the representative of the of research the research object. object. Figure 6Figure shows 6 theshows measured the measured flexible 3-dB flexible bandwidth 3-dB bandwidth adjustments adjustments from 6.2 GHz from to 6.2 25.3 GHz GHz to by 25.3 increasing GHz by the increasing number ofthe pixels number on the of LCoSpixels chip on the one-by-one LCoS chip across one-by-one the y-axis. across The the starting y-axis. position The starting is selected position at the is center selected of allat thethe coveredcenter of LCoS all the pixels, covered corresponding LCoS pixels, to corre the wavelengthsponding to of the 1550.8 wavelength nm. Without of 1550.8 introducing nm. Without extra IL,introducing the achieved extra minimum IL, the 3-dBachieved bandwidth minimum is 9.8 3-dB GHz ba asndwidth shown inis Figure9.8 GHz6. Theas shown maximum in Figure bandwidth 6. The canmaximum be extended bandwidth to 5 THz can acrossbe extended the entire to 5 THz C-band across by the adjusting entire C-band a step of by about adjusting 3.1 GHz, a step which of about is consistent3.1 GHz, withwhich the is theoreticalconsistent minimumwith the theoreti resolutioncal minimum of 3 GHz. resolution The minimum of 3 GHz. filtering The bandwidth minimum offiltering 6.2 GHz bandwidth (corresponding of 6.2 GHz to one (corresponding pixel) can be to obtained one pixel) by introducingcan be obtained a 9-dB by introducing extra loss, since a 9-dB a fewerextra numberloss, since of a pixels fewer results number in of a higherpixels results optical in power a higher loss. optical When power increasing loss. theWhen 3-dB increasing bandwidth the to3-dB 8.1 GHz,bandwidth the extra to 8.1 loss GHz, is decreased the extra loss to 1.4 is dB.decreased We also to measured 1.4 dB. We the also minimum measured 3-dB the bandwidthsminimum 3- indB the bandwidths C-band at in the the central C-band wavelengths at the central of 196wave THzlengths (1529.553 of 196 nm), THz 195(1529.553 THz (1537.397 nm), 195 nm),THz 194(1537.397 THz (1545.322nm), 194 nm),THz 193(1545.322 THz (1553.329 nm), 193 nm), THz 192 (1553.329 THz (1561.419 nm), 192 nm) THz and191 (1561.419 THz (1569.594nm) and191 nm), THz respectively. (1569.594 Asnm), illustrated respectively. in Figure As7 illustrated, the measured in Figure values 7, are the about measured 8.4 GHz, values 8.1 GHz, are about 6.7 GHz, 8.4 6.59GHz, GHz, 8.1 6.9GHz, GHz 6.7 andGHz, 7 GHz,6.59 GHz, corresponding 6.9 GHz and to side7 GHz, band corresponding suppression to ratios side ofband 27 dB,suppression 28 dB, 31 ratios dB, 31.1 of 27 dB, dB, 31.3 28 dB,dB, and31 dB, 29.3 31.1 dB, dB, respectively. 31.3 dB, and 29.3 dB, respectively. Furthermore,Furthermore, aa programmableprogrammable PSPPSP needsneeds toto possess possess the the capabilitycapability toto arbitrarilyarbitrarily attenuateattenuate thethe opticaloptical powerpower inin orderorder toto meetmeet the special special spectral spectral requirement.requirement. InIn thethe experiment,experiment, byby using using our our proposedproposed PSP, PSP, we we generate generate three three sets sets of of continuous continuous sine sine waves waves with with repetition repetition frequencies frequencies ofof 2.5 2.5 GHz GHz (0.02(0.02 nm),nm), 0.250.25 GHzGHz (0.002 nm), and and 0.125 0.125 GHz GHz (0.001 (0.001 nm), nm), respectively. respectively. As Asshown shown in Figure in Figure 8, 8the, theachieved achieved peak-to-peak peak-to-peak power power suppressions suppressions for 2. for5 GHz, 2.5 GHz, 0.25 0.25 GHz GHz and and0.125 0.125 GHz GHz sine sine waves waves are are23.4 23.4dB, dB,18 dB 18 dBand and 6 dB, 6 dB, respectively. respectively. The The experimental experimental results results show show that that when when th thee frequency frequency interval interval is isgreater greater than than or or equal equal to to 0.25 0.25 GH GHz,z, our proposed system cancan alreadyalready produceproduce aa peak-to-peakpeak-to-peak powerpower suppressionsuppression signalsignal greater than than 18 18 dB, dB, and and the the performanc performancee is good. is good. In addition, In addition, in order in order to further to further verify verifythe application the application of our of method our method in communications in communications such suchas OFDM, as OFDM, we have we havealso alsoproduced produced OFCs OFCs with withspacing spacing of 12.5 of GHz 12.5 GHzand 25 and GHz. 25 GHz.

-10 6.2GHz 6.87GHz 8.12GHz -20 9.85GHz 12.68GHz 15.81GHz -30 18.8GHz 21.91GHz 25.3GHz

Insertion Loss (dB) -40 45

1550.8 1550.9 1551 1551.1 1551.2 1551.3 1551.4 Wavelength (nm)

Figure 6. Flexible bandwidth adjustment with a minimum 3-dB bandwidth of 6.2 GHz to 25.3 GHz.

Photonics 2020, 7, x FOR PEER REVIEW 8 of 11

Photonics 2020, 7, x FOR PEER REVIEW 8 of 11 Photonics 2020Figure, 7, 127 6. Flexible bandwidth adjustment with a minimum 3-dB bandwidth of 6.2 GHz to 25.3 GHz.8 of 11

Figure 6. Flexible bandwidth adjustment with a minimum 3-dB bandwidth of 6.2 GHz to 25.3 GHz. -10 -10 -10

-10 -20 -10 -20 -10 -20 8.4GHz 8.1GHz 6.7GHz -20 -30 -20 -30 -20 -30 8.4GHz 8.1GHz 6.7GHz -30 -40 -30 -40 -30 -40 Insertion Loss (dB) Loss Insertion Insertion Loss (dB) Loss Insertion

-40(dB) Loss Insertion -40 -40 -50 -50 -50 1530 1530.1 1530.2 1530.3 1530.4 1537.2 1537.3 1537.4 1537.5 1545.1 1545.2 1545.3 1545.4 1545.5 Insertion Loss (dB) Loss Insertion Insertion Loss (dB) Loss Insertion Insertion Loss (dB) Loss Insertion -50 -50 Wavelength (nm) -50 Wavelength (nm) 1530 1530.1 Wavelength1530.2 1530.3 (nm) 1530.4 1537.2 1537.3 1537.4 1537.5 1545.1 1545.2 1545.3 1545.4 1545.5 -10 Wavelength (nm) -10 Wavelength (nm) -10 Wavelength (nm)

-10 -10 -10 -20 -20 -20 -20 6.59GHz -20 6.9GHz -20 7GHz -30 -30 -30 6.59GHz 6.9GHz 7GHz -30 -30 -30 -40 -40 -40 -40 -40 -40 Insertion Loss (dB) Loss Insertion Insertion Loss (dB) Loss Insertion Insertion Loss (dB) Loss Insertion -50 -50 -50 1569.4 1569.5 1569.6 1569.7 Insertion Loss (dB) Loss Insertion

1561.2 1561.3 1561.4 1561.5 (dB) Loss Insertion Insertion Loss (dB) Loss Insertion 1553.1 1553.2 1553.3 1553.4 1553.5 -50 -50 -50 1569.4 1569.5 1569.6 1569.7 1553.1 1553.2 Wavelength1553.3 1553.4 (nm) 1553.5 1561.2 1561.3 1561.4Wavelength1561.5 (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) Figure 7. Measured minimum bandwidths in the C-band at the central wavelengths of 196 THz Figure 7. Measured minimum bandwidths in the C-band at the central wavelengths of 196 THz Figure(1529.553 7. Measured nm), 195 minimum THz (1537.397 bandwidths nm), in194 the THz C-band (1545.322 at the nm), central 193THz wavelengths (1553.329 of nm), 196 THz192 THz (1529.553 nm), 195 THz (1537.397 nm), 194 THz (1545.322 nm), 193THz (1553.329 nm), 192 THz (1529.553(1561.419 nm), nm) 195 and THz 191 (1537.397 THz (1569.594 nm), 194nm), THz respectively. (1545.322 nm), 193THz (1553.329 nm), 192 THz (1561.419(1561.419 nm) nm) and and 191 191 THzTHz (1569.594(1569.594 nm), respectively. respectively.

FigureFigure 8. 8. Measured optical optical spectra spectra for for2.5 2.5GHz GHz (0.02 (0.02nm), nm),0.25 GHz 0.25 (0.002 GHz nm), (0.002 and nm), 0.125 and GHz 0.125 (0.001 GHz Figure 8. Measured optical spectra for 2.5 GHz (0.02 nm), 0.25 GHz (0.002 nm), and 0.125 GHz (0.001 (0.001nm) continuous nm) continuous sine waves. sine waves. nm) continuous sine waves. Figure9a shows the measured spectral results of 12.5 GHz and 25 GHz combs by using our designed 1 6 PSP,and the peak-to-peak power suppressions are more than 8 dB and 25 dB, respectively. × Figure9b shows the measured spectral results of 12.5 GHz and 25 GHz combs by using a commercial PSP, whose peak-to-peak power suppressions are about 3 dB and 18.7 dB, respectively. Obviously, the performance of our proposed system outperforms that of commercial PSP. This is due to the stronger ability by the proposed method to distinguish photons to the output ports.

Photonics 2020, 7, 127 9 of 11 Photonics 2020, 7, x FOR PEER REVIEW 9 of 11

(a)

(b)

Figure 9. Measured spectra of optical frequency combs (OFCs) with 12.5 GHz and 25 GHz space using ((aa)) ourour 11 × 6 PSP and ( b) commercial PSP. × 4. ConclusionsFigure 9a shows the measured spectral results of 12.5 GHz and 25 GHz combs by using our designedIn this 1 × paper, 6 PSP, we and have the developedpeak-to-peak a fine-resolution power suppressions programmable are more PSP than based 8 dB onand low-cost 25 dB, compactrespectively. light Figure paths, 9b enabled shows by the a 2Kmeasured LCoS-SLM spectral and dualresults cascaded of 12.5 GHz 1000 lineand/ mm25 GHz diff ractioncombs by gratings. using Ana commercial assembled PSP, experimental whose peak-to-peak system with power one-input suppressions and six-output are about has been 3 dB built and in18.7 order dB, torespectively. implement aObviously, flexible filtering the performance bandwidth of and our arbitrary proposed amplitude system outperforms transfer function. that of The commercial measured PSP. total This ILs foris due the sixto the ports stronger are around ability 5.9 by dB–9.4 the proposed dB over method the full to C-band. distinguish Theachieved photons to minimum the output 3-dB ports. bandwidth is 6.2 GHz, and the largest operating bandwidth can be extended to the full C-band from 1530 nm to 4. Conclusions 1570 nm (5 THz). Moreover, by using the proposed PSP, a high-resolution high-performance arbitrary opticalIn spectrumthis paper, can we be generated,have developed such asa afine-r sineesolution wave signal programmable with a 0.25-GHz PSP repetition based on frequency low-cost andcompact 18-dB light peak-to-peak paths, enabled power by suppression. a 2K LCoS-SLM Based and on dual the achievedcascaded experimental1000 line/mm results, diffraction developing gratings. a programmableAn assembled andexperimental high-resolution system 1 Nwith PSP one-inpu by usingt low-costand six-output compact has spatial been light built paths inis order feasible, to × andimplement is believed a flexible to further filtering the implementation bandwidth and of arbi antrary M Namplitude PSP. transfer function. The measured × total ILs for the six ports are around 5.9 dB-9.4 dB over the full C-band. The achieved minimum 3-dB Author Contributions: Z.L. performed the investigation and experiment, wrote the original draft; C.L. performed thebandwidth analytical is calculations 6.2 GHz, and revisedthe largest the manuscript; operating J.T.bandwidth discussed withcan be the extended experimental to the results full and C-band revised from the manuscript;1530 nm to S.Y. 1570 supervised nm (5 the THz). whole Moreover, project and approvedby using the the manuscript. proposed All PSP, authors a high-resolution have read and agreed high- to theperformance published versionarbitrary of theoptical manuscript. spectrum can be generated, such as a sine wave signal with a 0.25-GHz Funding:repetitionThis frequency work was supportedand 18-dB by National peak-to-peak Natural Sciencepower Foundation suppression. of China Based (61805184), on Hubeithe Provincialachieved Naturalexperimental Science results, Foundation developing of China a (2018CFB250),programmable Anhui and Provincialhigh-resolution Natural 1×N Science PSP Foundation by using low-cost of China (1808085MF186). compact spatial light paths is feasible, and is believed to further the implementation of an M×N PSP. Acknowledgments: The first author would like to thank Quan You, Ying Qiu and Xi Xiao from National Information Optoelectronics Innovation Center, Wuhan Research Institute of Posts and Telecommunications, Author Contributions: Z.L. performed the investigation and experiment, wrote the original draft; C.L. for useful discussions during the experiment. And also thank Fulong Yan from Eindhoven University of Technology forperformed valuable the suggestions. analytical calculations and revised the manuscript; J.T. discussed with the experimental results and revised the manuscript; S.Y. supervised the whole project and approved the manuscript. All authors have Conflicts of Interest: The authors declare no conflict of interest. read and agreed to the published version of the manuscript.

Funding: This work was supported by National Natural Science Foundation of China (61805184), Hubei Provincial Natural Science Foundation of China (2018CFB250), Anhui Provincial Natural Science Foundation of China (1808085MF186).

Photonics 2020, 7, 127 10 of 11

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