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We demonstrate a monolithically integrated quadrature coherent receiver photonic integrated curcuit on an InP substrate with a 90° optical hybrid and two balanced 56 GHz pin-photodetectors on chip level and as a packaged device. The presented devices enable the use of 56/64 Gbaud dual polarisation 16-QAM signals either in the C-band or the L-band.
© 2012 Optical Society of America
1. Introduction
In recent 100 G Ethernet long haul optical fiber transmission links 112 Gbits/s dual-polarisation quadrature phase-shift keying (DP-QPSK) is the commonly used modulation format. Typical receiver frontends of these transmission links are intradyne coherent receiver with a 90° hybrid converting the phase modulated signal into an amplitude modulation detectable by the photodiodes. There are several realisations of such a receiver, e.g. monolithic [1, 2] or hybrid implementations [3–5]. Besides good performance, a further demand of the industry to all these devices is low production cost and compact devices size.
In order to fulfill the still increasing demand of capacity in communication traffic, 400 G Ethernet is expected to be the data rate of the next generation. Moreover, it can be supposed that both modulation and baud rate will increase in order to achieve the required 448 Gbit/s. A possible candidate for the modulation format is dual-polarisation 16-quadrature amplitude modulation (16-QAM) with 56 Gbaud. In general, the 90° hybrids in combination with highly linear pin-photodiodes of the receivers photonic integrated circuit (PIC) can be used without major changes for 16-QAM [5]. Regarding the baud rate, the photodiode and the electrical interface have to be adapted without decreasing responsivity which is an important parameter for high sensitivity and high signal to noise ratios. The simultaneous increase of bandwidth and responsivity is challenging because the high responsivity requires a large active region area, which in turn increases the photodiode capacity, resulting in a reduced bandwidth of the photodiode. For the hybrid implementations of the receiver PIC, where the photodiodes are attached onto the waveguides [4, 5] the assembly becomes less tolerant due to smaller active areas and thus will increase the production costs. Although monolithically integrated waveguide photodiodes have been presented for silicon on insulator and silica substrates [1, 6, 7] the photodiodes either suffer from low responsivity in the C-band and L-band, or their dark current is increased because of the applied stressors, thus increasing the noise floor.
In this contribution, we demonstrate a monolithic InP based receiver chip with a 90° hybrid and 56 GHz photodiodes. The fabricated devices are supposed to operate with 56 Gbaud RZ-16-QAM signals in the C-band and L-band. Moreover, the performance of the packaged chips with a hybrid integrated polarisation beam splitter (PBS) is presented, extending the performance of the packaged receiver for polarisation diversity modulation schemes.
2. Design and fabrication of the PIC
A top-view SEM image of the integrated InP photonic chip is shown in Fig. 1. The receiver PIC includes two full spot-size fiber taper (one for the data signal S and one for the local oscillator LO), an optical 90° hybrid, and two balanced photodiodes.
A DC reverse voltage is applied to the array of photodiodes using an integrated bias network consisting of integrated capacitors and resistors (Fig. 2). As currently, no 56 Gbaud 4-arrays of transimpedance amplifiers are available, we chose a balanced detector scheme with 50 Ω terminated RF lines to guide the RF signal directly to devices outside of the package.
Fig. 2 Conceptual setup of the receiver PIC with 90° hybrid and photodiodes with biasts and 50 Ω terminated RF lines
Due to the balanced photodiode configuration, the fabricated chips have a size of only 3.5 mm2. The compact size results in a maximum skew between the different photodiode channels of only 0.2 ps.
A chip prototype has been fabricated on a 3 inch InP wafer. Semi-insulating InGaAsP/InP waveguide layers and the doped detector layers are grown by single-step MOVPE. The structuring of the photodiodes and the waveguides is done by etching. The waveguide integrated pin photodiodes comprise an InGaAs absorption layer and heterostructure contact layers in order to allow high responsivities and high bandwidths. The optical facet of the chips is AR coated to reduce insertion loss and to avoid back reflection into the local oscillator.
3. Measurement results of the PIC
For the following measurements the port definition and nomenclature from Fig. 2 will be used. The inphase component of the signal and the quadrature component of the signal are defined as I = IP − IN and Q = QP − QN. Important parameters for the envisaged system operation can already be obtained from single fibre input measurements:
- the minimum responsivities of the photodiodes
- the dark current of the photodiodes
- the small signal bandwidth of the photodiodes.
The imbalance can be regarded as figure of merit for the imperfection between the signal input and the local oscillator input. The uniformity represents the maximum pitch of the different optical paths and the CMRR is a figure of merit for the differential output signals (I, Q).
Figure 3 shows the responsivity as a function of the wavelength for a C-band device and a L-band device. For all four photodiodes responsivities of 0.12 A/W can be achieved at the centre of the C-band and the L-band, respectively.
Fig. 3 Measured responsivity for a C-band chip (left) and for a L-band chip (right); positive in-phase channel (IP), negative in-phase channel (IN), positive quadrature channel (QP), negative quadrature channel (QN)
The measurements mainly related to the performance of the 90° hybrid are presented in Fig. 4 and Fig. 5. An excellent imbalance, uniformity below 0.75 dB and a CMRR better 21 dB can be obtained over the entire C-band and L-band, respectively.
Fig. 4 Measured imbalance (left) and uniformity of the signal and the local oscillator input (right) for a C-band chip and a L-band chip; indices S and LO denote the signal input and the local oscillator input
In Fig. 6 the dark currents of the photodiodes of the fabricated chips are shown. Low dark currents of less than 1 nA for a reverse bias voltage of 3 V are obtained. The measured bandwidth of the photodiode is plotted in Fig. 6. For all four photodiodes bandwidths of 56 GHz are obtained.
Fig. 6 Dark current measurements (left) and frequency response measurements (right) for all four photodiodes of the receiver PIC
4. Design of the module
Packaging the receiver PIC and including a free space optic with a low loss polarisation beam splitter (PBS), the performance of the device is extended to dual-polarisation modulation formats. Figure 7 depicts the conceptual setup of such a module. The two hybrid integrated PBS split the signal and the local oscillator into their polarisation components so that they can be routed to their designated 90° hybrid.
A photograph of a package is presented in Fig. 7. A CCRx compatible footprint was chosen for the package to provide high comparability for nowadays applications. Nevertheless, due to the small chip size even smaller form factors of the package are possible. In order to route the RF output signals to a following device, the package has a GPPO® interface.
5. Measurement results of the module
The measured polarisation extinction ratio (PER) for the signal input of the packaged device is shown in Fig. 8. The excellent behaviour of more than 30 dB over the complete bands could be achieved for C-band and L-band modules, respectively.
The RF performance of the packaged device is characterised by the RF measurements presented in Fig. 9. Typically, a CMRR of more than 15 dB is obtained at 40 GHz. A differential frequency response with its 3 dB-limit of the power has also been measured at 47.5 GHz (Fig. 9).
Fig. 9 Measured CMRR over the excitation frequency (left) and differential frequency response measurement (right)
6. Conclusion
We proposed and realised a monolithic InP coherent receiver chips with optical 90° hybrid and photodiodes for C-band and L-band. The photodiodes show excellent performance regarding the responsivity, the dark current and the bandwidth. Thus, the performance of the receiver chips is sufficient for 56 GBaud QPSK data signals. Furthermore, the presented packaged devices have a hybrid integrated PBSs which extends the performance to dual polarisation modulation formats.
The presented coherent dual-polarisation optical front-ends can be regarded as a first step towards a fully integrated 400 G receiver. Moreover, the small chip size can result in smaller package sizes being also a step forward to next generation form factors.
References and links
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