A high-speed receiver optical subassembly (ROSA) based on silica hybrid-integrated technology for 400 G Ethernet applications is designed and manufactured. It consists of an eight-channel-arrayed waveguide grating (AWG) with a 2% refractive index difference, two 4-channel InP-based waveguide photodetector (PD) array chips, eight lenses for enhancing the coupling between AWG and PDs, two 4-channel trans-impedance amplifier array chips, and an eight-channel high-frequency flexible printed circuit. All the channels of the ROSA have achieved 3 dB bandwidths of above 27.05 GHz, and receiver sensitivities of below −8.5 dBm, supporting a receiving rate of 8×53 Gbps.
A silica-based 90 deg optical hybrid based on a 4 × 4 multimode interference coupler with a refractive index contrast of 0.36% is designed and fabricated for coherent receiver. The measured results of this device show that the common mode rejection ratios are better than 23 dB and excess loss including a coupling loss of −0.3 dB is less than −0.9 dB in C band (1530 to 1565 nm). The phase error is less than ±4 deg. The designed low-cost 90 deg optical hybrid is promising for further improving the sensitivity of quadrature phase shift keying coherent receivers.
Quantum key distribution can realize unconditional security of communication. Here, a quantum decoding chip based on silica-on-silicon planar lightwave circuit (PLC) platform, which is compatible for multiprotocol has been demonstrated. PLC technology provides stability, high integration, and practicality for the chip, whereas silica-on-silicon material makes it low loss and low cost. The chip is integrated with three variable optical splitters, two asymmetric Mach–Zehnder interferometers, and four variable directional couplers. The interference visibility is investigated by self-interfering method, and the results showed it is high to 98.2% under temperature control. The extinction ratio for the phase states is kept between 23 and 25 dB for 6 h without active phase correction.
A low-loss 1 × 256 optical power splitter is designed and fabricated using silica-based planar lightwave circuit technology on a 6-in. quartz substrate. The measured results show that the insertion loss, uniformity, and wavelength-dependence loss (WDL) of a 1 × 256 splitter are <26.2, 1.97, and 1.83 dB, respectively, in the wavelength range from 1.26 to 1.63 μm. The insertion loss, uniformity, and WDL of a 1 × 256 splitter are <25.62, 1.33, and 1.03 dB, respectively, in the normal communication wavelength range from 1.31 to 1.55μm. The polarization dependence loss is <0.16 dB in the wavelengths of 1.31 and 1.55 μm.
Silica-based planar lightwave circuit (PLC) devices can offer great potential for quantum key distribution (QKD) with the benefits of low-loss, low-cost, large-scale integration, miniaturization, stability and mass production. A quantum decoding integrated chip for QKD based on silica PLC technology was demonstrated, which consists of a variable optical splitter (VOS), an asymmetric Mach–Zehnder interferometer (AMZI) with a thermo-optic phase modulator (TOPM) and a delay line (DL) whose delay time is 400ps, and a variable directional coupler (VDC). The balanced pulsepairs of double channels with measured delay times of 396ps and 398ps respectively were obtained in the conditions of both classical optics and single-photon transmissions. The chip has advantageous to achieve high interference visibilities of double channels simultaneously for BB84 phase and time-bin protocols.
A receiver optical subassembly (ROSA) based on hybrid integrated technology is designed and fabricated. It consists of an 8-channel arrayed waveguide grating (AWG) with 2% refractive index difference, two 4-channel positive intrinsic-negative photodetector arrays, and two 4-channel transimpedance amplifier arrays. A flat-top optical spectrum was observed because the output waveguides have a multimode structure, and the average 1-dB bandwidth of AWG channels is 2.8 nm. The insertion loss of all channels is between −1.8 and −2.9 dB, and the cross talk between adjacent channel is less than −20 dB. The ROSA shows a maximum responsivity of 0.4 A / W and 3-dB bandwidth of more than 22 GHz for four channels, which can meet the data transmission rate of 25 Gb / s. In the future, we will improve the fabrication process to achieve a transmission rate of 8 × 25 Gb / s.
Based on silica arrayed waveguide grating technology, a hybrid integrated transmitter optical subassembly was developed. Four direct-modulating distributed feedback lasers and four focusing microlenses were integrated to a coarse wavelength-division multiplexer (CWDM) on a CuW substrate. The four-channel silica-on-silicon CWDM was fabricated with 1.5% refractive index difference and 20-nm wavelength spacing. The experimental results showed that the output optical power was >3 mW with 45 mA of injection current, the slope efficiency was >0.0833 W/A, and the 3-dB bandwidth was broader than 18.15 GHz. The 1-dB compress points were higher than 18 and 15.8 dBm for frequency of 10 and 18 GHz, respectively.
A novel ultra-compact high-efficiency broadband mode converter between silicon (Si) nanowire and silicon slot waveguide based on Silicon-on-Insulator (SOI) is proposed in this paper. By introducing a gradual-width structure between Si nanowire and slot waveguide, the favorable transition between nanowire mode (Gaussian-like mode) and slot mode (non-Gaussian-like mode) can be obtained and then the coupling efficiency is improved. The structure is simulated and optimized by using the three-dimension Finite-Difference Time-Domain Method (3D-FDTD). The coupling efficiency of over 90% within bandwidth of over 600nm can be achieved by only 200nm-length converter which is the smallest size to our knowledge. This presented mode converter can meet the demand of ultra-compact, wavelength-insensitive of monolithic integration.
A new design for polarization-insensitive silicon-on-insulator (SOI) arrayed waveguide grating (AWG) is proposed. In
arrayed waveguide region, Si nanowire waveguides and slot waveguides are combined to adjust the optical path
differences. For the slot waveguides, the refractive indices of the TE and TM polarizations satisfy nTM>nTE while in the
Si nanowire waveguides nTM<nTE, so the refractive indices of the TE and TM polarizations are complementary in these
two kinds of waveguides. By calculating the relationship of Si nanowire waveguides and slot waveguides' length
differences, optimizing the structure of slot waveguides and choosing the appropriate diffraction order, central channel
wavelength and channel spacing can both reduce the polarization sensitivity greatly. The design process is given in detail
and the simulation results demonstrate that the AWG demultiplexer can meet polarization insensitive demand.
A new triplexing filter based on a silica direction coupler and an arrayed waveguide grating is presented. Using a combination of a direction coupler and an arrayed waveguide grating, a 1310-nm channel is multiplexed and 1490- and 1550-nm channels are demultiplexed for fiber-to-the-home. The direction coupler is used to coarsely separate the 1310-nm channel from the 1490- and 1550-nm channels. Subsequently, an arrayed waveguide grating is used to demultiplex the 1490- from 1550-nm channel. The simulated spectra show the 1-dB bandwidth of 110 nm for the 1310-nm channel and 20 and 20.5 nm for the 1490- and 1550-nm channels. The insertion loss is only 0.15 dB for 1310 nm and 5 dB for 1490 and 1550 nm. The crosstalk between the 1490- and 1550-nm channels was less than −35 dB.
A 40-channel 0.8-nm-spaced flat-top silica-based arrayed waveguide grating (AWG) with a tapered multimode interferometer (MMI) at the end of its input waveguide has been experimentally demonstrated for the first time. By adding the MMI, the 1-dB and 3-dB bandwidths are increased to 0.45 and 0.62 nm, respectively. The insertion loss (IS) of the device ranges from 3.8 to 6.8 dB. The IS uniformity is better than 3.0 dB. The crosstalk is better than –25 dB. Compared to the AWG with a rectangular MMI, the AWG with a tapered MMI shows better IS, crosstalk, and ripple.
Oxidizing thick porous silicon layer into silicon dioxide is a timesaving and low-cost process for producing thick silicon dioxide layer used in silicon-based optical waveguide devices. The solution of H2O2 is proposed to post-treat thick porous silicon (PS) films. The prepared PS layer as the cathode is applied about 10mA/cm2 current in mixture of ethanol, HF, and H2O2 solutions, in order to improve the stability and the smoothness of the surface. With the low-temperature dry-O2 pre-oxidizations and high-temperature wet O2 oxidizations process, a high-quality SiO2 30 μm thickness layer that fit for the optical waveguide device was prepared. The SEM images show significant improved smoothness on the surface of oxidized PS thick films, the SiO2 film has a stable and uniformity reflex index that measured by the prism coupler, the uniformity of the reflex index in different place of the wafer is about 0.0003.
The transfer matrix method combined with the effective index method is adopted to model the silica-based channel waveguide patterned by UV writing. The effective indexes of the graded index channel waveguides with different dimension are calculated. The maximal error of the effective index is less than 3×10-5. By this method, the number of the guided mode and the dimension range to guide certain modes can be obtained easily. Finally, the dimension range to guide a single mode is presented.
The effective index method (EIM) was adopted to model the channel waveguide patterned by the UV in photosensitive silica film. The effective indexes of the different dimension symmetrical and asymmetrical channel waveguides were calculated, and the resource of the error of the method was pointed out. At last, the dimension rang to propagate single mode was presented.
The combination of the effective index method and the transfer matrix method is adopted to calculate the indices of quasi-TE and quasi-TM modes in a UV-written channel waveguide, and the difference between the indices is used to characterize its birefringence. The dimensions, the ratio of width to thickness, the original index of the core layer, the index of the cladding, and the index profile are all taken into account. The simulation results indicate that the birefringence decreases with increasing dimensions, ratio of width to thickness, and indices of the cladding; on the contrary, increases of the original index of the core layer and of the vertical index gradient intensified the birefringence.
A 32-channel 50-GHz spaced arrayed-waveguide grating with our innovative configuration has been designed and fabricated. The performance of the device has been fully tested by using a tunable laser light source, optical power meter, and polarization controller. The insertion loss (IS) of the device is 4.2~7.4 dB. The crosstalk is about –28 dB. The IS uniformity is less than 3.2 dB. With our configuration, the performance of the device has been enhanced effectively and the difficulty in alignment process has been decreased obviously.
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