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Abstract
Wavelength division multiplexing is a widely used monolithic device with modulating light sources at different wavelengths based on a designed configuration. In this paper, we report an in-chip demultiplexer with a simple design operating at 532/1064 nm in pure YAG crystal. The device is fabricated by femtosecond laser direct writing inside the transparent substrate with just a width of 36 µm. The compact structure is designed based on the principle of self-imaging, and the propagation features have been simulated by utilizing the beam propagation method. The performance of this wavelength demultiplexer has been investigated through an end-face coupling system, which proves the device can separate 532 nm and 1064 nm light into two discrete waveguides polarization-insensitively with an extinction ratio as high as 13 dB. These superior performances manifest this exquisite device can emerge into kinds of photonic applications in the future. Also, this work further proves that femtosecond laser direct writing technology has irreplaceable advantages in processing micro-nano devices in transparent materials for in-chip integration.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the past decades, the interest in photonic integrated circuits (PICs) has surged enormously due to the broad applications in different areas, including optical fiber communications, imaging, sensing, and quantum information processing, etc. [1–4]. Driven by the demand for manipulating light on chips, various of miniaturized functional devices, such as waveguide, gratings, and power splitters, have been researched for the highly integrated photonic circuits [5–7]. Wavelength division multiplexing is one solution to the requirement of high optical communication density and increased transmission rate. As a passive device, the wavelength demultiplexer can separate different wavelengths of light, which makes it important in the field of optical communication and information processing. At present, most of the researches on demultiplexing devices are concentrated in the near-infrared band [8,9], limiting the scope of applications. Furthermore, conventional wavelength demultiplexers conclude arrayed waveguide grating [10,11], echelle grating [12,13], and ring resonator array [14,15], etc., which always have complex structures. Heretofore, the development of PICs is hindered by the lack of suitable multiplexing or demultiplexing dimensions. Therefore, the multimode-interference-based (MMI) demultiplexer with a simple structure, compact size, and large processing tolerance shows great potential in different application scenarios.
Traditionally, the PICs are always fabricated by wafer-scale technology on transparent materials, such as silica, silicon, or crystal [16–18]. Monolithic integrated optical devices processed by traditional techniques are mainly 2D structures on the sample surface, which will reduce the material utilization efficiency and complicate the fabrication process. Thus, how to fabricate optical waveguide devices more flexibly and efficiently in chips is widely concerned. Femtosecond laser direct writing (FLDW), as a mature micro-nano processing technology, has several distinct advantages for the fabrication of flexibility, high resolution, and real 3D structural devices [19,20]. Besides, the FLDW technology can be applied in kinds of transparent materials, such as LiNbiO3 [18], polymer [21,22], diamond [23,24], YAG [25,26], etc. So far, plenty of monolithic optical components have been achieved in the related materials, for example, beam splitter, fiber Bragg grating, vortex laser generator, and wavelength demultiplexer, which proves that the FLDW technology is a powerful tool for precise processing [18,25–29].
Yttrium aluminum garnet (YAG) crystals are one of the most significant optical window materials, which possess cubic structure, high mechanical strength, and stable physical and chemical properties. Moreover, YAG crystal does not produce birefringence effect, so the refractive index of different polarization states when propagating inside it shows isotropy, which provides a favorable condition for realizing polarization independence performance, such as the waveguide laser oscillation [30].
In this work, we have fabricated an MMI-based demultiplexer by FLDW technology in a pure YAG crystal with the working wavelength at 532/1064 nm. The structure has been designed based on the principle of self-imaging, and the wavelength division performance has been simulated by the beam propagation method (BPM). The device was constructed with the structure of square-clad waveguide geometry. Then, the index change in the region of the input port end face was characterized and analyzed by a confocal Raman measurement. The output field intensity distributions have been detected by a CCD and the spectra of the output light has been measured by spectrometers. Compared with the previous report about the demultiplexer achieved in birefringent crystal LiNbO3 [18], the polarization-insensitive property of this device has been demonstrated owing to the advantages of its cladding geometry and the cubic symmetry of YAG crystal. Moreover, this demultiplexer shows outstanding characteristics for the wavelength division with an extinction ratio as high as 13 dB.
2. Experimental details
According to the self-imaging effect, the structure of the wavelength demultiplexer is designed as the MMI-based demultiplexer, which consists of an MMI coupler, one input port, and two output ports. With the principle of self-imaging [31], when the input field is propagating in the MMI coupler, multimode interference will occur, which makes the input field reproduce at specific positions with regular periodic intervals. And the period depends on the size of the structure, the effective refractive index, and the operating wavelength, it can be expressed as beat length Lπ of the two lowest-order modes:
In order to achieve the wavelength division at 532 nm and 1064 nm, the structure parameters have been designed by Eq. (2) combined with the BPM simulation. In YAG crystals, the refractive index corresponding to the incident light fields of 532 nm and 1064 nm wavelengths are 1.832 and 1.816 respectively, which are approximately equal enabling a polarization-independent feature compared with that in LiNbO3 crystal. Of course, this will lead to a slight difference in the beat length of the lasers. Excellent wavelength division performance can be achieved by properly adjusting the length of the MMI. Figure 1(b) is the structural parameter diagram of the device, the WMMI and LMMI are 36 µm and 6.8 mm, the distance x from the input port to the middle of the MMI coupler is set to be 6 µm, equal to the WMMI/6. Here, the value of W2 is set to be larger than W1, and this will increase the input cross-sectional area and reduce the coupling loss.
Fig. 1. (a) Schematic diagram of the FLDW processing. (b) The structural parameters of the wavelength demultiplexer. The device consists of three parts: an input section (Port1), an MMI coupler, and output sections (Port 2 and Port 3). W1, and W2 are 7 µm, and 10 µm, X is 6 µm, WMMI, and LMMI is 36 µm, and 6.8 mm, respectively.
An optically polished pure YAG crystal with a size of 10 × 10 × 2 mm3 is used as the platform for the demultiplexer. Then, we use the FLDW technology to fabricate the device inside the crystal as displayed in Fig. 1(a). The femtosecond laser (1 kHz repetition rate, 90 fs pulse width, 6 mJ pulse energy, and the center wavelength at 800 nm) is generated by an amplified Ti:sapphire laser system (Astrella, Coherent Inc., USA). During the processing, the single pulse energy is adjusted to 0.28 µJ by a watt pilot motorized attenuator and focused on the sample by a long working distance objective (50×, N.A. = 0.55). The scanning speed is adjusted to 0.6 mm/s by setting the parameters of a six-dimensional translation stage. According to the structural characteristics of the device, we have fabricated each part of the device with a square depressed cladding geometry, which means the unmodified guiding region is surrounded by laser-induced low-refractive-index damage tracks. The induced tracks are with a vertical height of 8 µm and a horizontal width of 2.5 µm. Based on this condition, the distance between adjacent tracks is adjusted to 3 µm, and the distance between the top and bottom tracks is about 23 µm.
3. Results and discussions
Figures 2(a)–2(d) are the simulation diagrams of the incident field propagating in the demultiplexer corresponding to the laser wavelength at 532 nm (TE polarized), 532 nm (TM polarized), 1064 nm (TE polarized) and 1064 nm (TM polarized), respectively. It can be seen that the 532-nm laser and the 1064-nm laser output from Port 2 and Port 3, separately. Figures 2(e)-(f) show the distributions of light intensity propagating on the lines where the two output ports are located. For the situation of lasers at different polarizations, there are only slight differences between the output power. Comparing the intensities of the two output ports under different conditions, the extinction ratios reach as high as 13 dB (96%/4%). The calculated propagation losses according to the simulation results are 0.56 dB/cm (532 nm at TE polarization), 0.49 dB/cm (532 nm at TM polarization), 0.6 dB/cm (1064 nm at TE polarization), and 0.51 dB/cm (1064 nm at TM polarization), respectively.
Fig. 2. (a)-(d) Simulations of the different input fields propagating in the waveguide. (e)-(f) The intensity distribution along the straight line where the two output ports are located with different input fields. (a) and (e) 532 nm at TE polarization. (b) and (f) 532 nm at TM polarization. (c) and (g) 1064 nm at TE polarization. (d) and (h) 532 nm at TM polarization.
In order to further explore the effect of the laser writing induced tracks on the guiding area, the Raman properties of the Port 1 cross-section have been analyzed by a µ-Raman spectroscopy system (XperRam200, Nanobase, South Korea). A continuous laser (532 nm, 50 mw) is used as the probe light, which is focused by a microscope objective on the incident facet area, and the Raman spectra and the mapping information can be measured with the help of a galvo scanning system. Figure 3(d) shows the Raman spectra in different regions, the inset shows the micrograph of the input port region. It is obvious that the Raman intensity in the track region is lower than that in the bulk and waveguide regions, while there is only a slight difference between the bulk and the waveguide region. This indicated that the properties of the guiding region have not been modified obviously during the laser writing. Figures 3(a)-(c) are the two-dimensional scanning images of Raman intensity, peak full width at half maximum (FWHM), and peak frequency shift at the peak of 258 cm-1, respectively. It can be observed that the Raman intensity decreases, the FWHM of the peak broadens, and a blue-shift of the peak frequency is generated in the laser-irradiated region, namely in the tracks. These features suggest a damage and a crystalline lattice expansion are induced in the track region. As a consequence, the refractive index along the depressed cladding structure will be decreased. Meanwhile, the guiding core area and the substrate show the same performances, which means the laser writing nearly has no effect on the crystal material in that region. And thus, the light can be confined very well in the whole compact structure. Compared with the previous reports about the FLDW waveguides in YAG crystals [32,33], the results of the µ-Raman measurement demonstrate that the refractive index of the guiding has been barely changed. Thus, we can used the refractive index of the bulk substrate during the design of the total configuration.
Fig. 3. Results measured the cross-section of the input port (Port 1) by the Confocal Raman (µ-Raman) system. (a), (b) and (c) are the 2D mapping results at a characteristic peak of 258 cm-1 with channels of peak intensity, FWHM, and peak frequency shift, respectively. (d) Raman spectra corresponding to the guiding core area, the bulk substrate, and the track region. The inset shows the micrograph of Port 1.
Then, the wavelength division performances have been measured by an end-face coupling system. In the system, two CW lasers (at 532 nm and 1064 nm) on a coaxial optical path are coupled into the Port 1 of the demultiplexer through a half-wave plate and a microscope objective (40×, N.A. = 0.65). Collected by the imaging objective lens (50×, N.A. = 0.42) behind the sample, the near-field mode profiles can be detected by a CCD. As shown in Figs. 4(a) and 4(b), the wavelength of the incident laser light is 532 nm. Under different polarization states, a single-mode laser has been output from Port 2, and detected by the CCD. Meanwhile, Figs. 4(c) and 4(d) show that the intensity distribution when the incident laser wavelength is 1064 nm. Similarly, there is a single-mode Gaussian beam output from Port 3 imaged on the CCD regardless of TE or TM polarization. And for the light output from Port 2, the intensity is pretty weak. Thus, the experimentally observed near-field intensity distributions show reasonable agreement with the BPM simulation results, and proving the self-imaging principle and the simulation can be helpful in designing similar devices working at other wavelengths. Figures 4(e)–4(h) depict the intensity distribution under both TE and TM polarizations when the lasers at a wavelength of 532 nm and 1064 nm are incident at the same time. By now, two single-mode laser beams can be observed, which are located in the positions as same as the case of the laser incident alone.
Fig. 4. The output field intensity distribution measured by the CCD. (a) and (b) corresponding to the incident lasers at 532 nm with polarizations of TE and TM respectively; (c) and (d) corresponding to the incident lasers at 1064 nm with the polarization of TE and TM respectively; (e)-(h) the lasers at 532 nm and 1064 nm are incident coaxially at the same time, corresponding to 1064 nm (TE)-532 nm (TE), 1064 nm (TE)-532 nm (TM), 1064 nm (TM)-532 nm (TE), and 1064 nm (TM)-532 nm (TM), respectively.
To experimentally verify the wavelength of lasers output from Port 2 and Port 3, we use a Y-branch optical fiber connected with two spectrometers (NOVA-EX and NIR2500, Ideaoptics, China) to detect the output optical signal. Figure 5(a) shows the spectra in case of only one laser incident. When the laser beam at 532 nm (TE or TM polarized) is applied, a strong wavelength signal at 532 nm can be detected from Port 2, while the intensity detected from Port 3 is very weak. Similarly, when a laser at a wavelength of 1064 nm (TE or TM polarized) is incident, a wavelength signal with high intensity can be detected at Port 3, while the intensity detected at Port 2 is rather low. It should be noted that due to the characteristics of the spectrometer itself, the noise floor in the band near 1064 nm is higher. The spectra measured when the two lasers are incident together is shown in Fig. 5(b). It also can be seen that the two input lasers both at TE and TM polarizations have achieved efficient directional output at specific output ports, namely that the laser at 532 nm only outputs from Port 2, and the laser at 1064 nm still outputs from Port 3. More importantly, the extinction ratios are above 13 dB for both of these two lasers, which is consistent with the simulation results, further verifying the reliability of the device.
Fig. 5. The spectra measured at the positions of Port 2 and Port 3 when using different wavelength incident lasers at different polarizations The parameters are listed in the brackets.
It can be said that the device realizes the function of polarization-insensitive wavelength division with a high efficiency, and has good transmission characteristics with keeping a single mode. The insert losses calculated through the experiments are 8.17 dB (532 nm, TE polarized), 9.83 dB (532 nm, TM polarized), 8.39 dB (1064 nm, TE polarized), 9.25 dB (1064 nm, TM polarized), respectively. We simulated the process of the incident field coupling into the device through the BeamPROP module of Rsoft Photonics CAD Suite, and roughly calculate the coupling loss is 5.78 dB. Thus, the propagation losses are 2.39 dB/cm (532 nm, TE polarized), 4.05 dB/cm (532 nm, TM polarized), 2.61 dB/cm (1064 nm, TE polarized), 3.47 dB/cm (1064 nm, TM polarized), respectively. The losses are mainly due to the large coupling losses at output ports, and the increased propagation losses caused by the inhomogeneity of the guiding configuration fabricated in the laser writing process.
4. Conclusion
In summary, a novel MMI-based demultiplexer has been designed according to the principle of the self-imaging effect, and the device has been fabricated by the FLDW technology in a pure YAG crystal. Benefiting from the depressed cladding structure almost having no influence on the guiding region and the YAG crystal being a typical cubic symmetry crystal with isotropic refractive index distribution, the demultiplexer shows a polarization-insensitive feature. Based on the compact designed structure, our theoretical analysis and experimental results demonstrate that the output lasers are kept in single modes, and the extinction ratio is as high as 13 dB. The extremely promising, versatile and low-cost fabrication technology and the advances of this proposed high-efficiency MMI-based demultiplexer open up new avenues for realizing the wavelength division device on-chip and promoting the applications and the development of the PICs.
Funding
Natural Science Foundation of Shandong Province (ZR2021ZD02); National Natural Science Foundation of China (12274236, 12274271).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding author upon reasonable request.
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