1. Introduction

Optical coherent technology is nowadays a mandatory choice for newly installed high-capacity long-haul transmission networks, due to the enormous advantages with respect to traditional systems based on direct detection [1]. The availability of high speed electronic digital signal processing (DSP) for electronic equalization allows the adoption of high order modulation formats such as quadrature phase shift keying (QPSK) and m-quadrature amplitude modulation (m-QAM) as well as polarization division multiplexing (PDM). Moreover, the enhanced spectral efficiency, flexibility, and robustness to transmission impairments brought by coherent technology can be beneficial also in metropolitan/regional optical networks. However, the costs (in terms of both capex and opex) and size of the required components need to be significantly reduced before extending coherent transmission to metro and (eventually) access networks. To this aim, photonic integration, which allows mass production of cost-effective high-performance photonic integrated circuits (PICs), is a key technology. In particular, the Indium Phosphide (InP) integration platform permits, in principle, the full integration of active and passive components, such as tunable laser, couplers, phase modulators and amplifiers. By exploiting two different InP integrated circuits, i.e. a distributed Bragg reflector (DBR) laser plus a semiconductor optical amplifier and an integrated nested Mach-Zehnder modulator (MZM), in [2] a dual-polarization coherent transmitter producing PDM-16-QAM at 32 Gbaud was demonstrated by employing DAC-generated multilevel driving signals. Extremely dense InP integration of multi-wavelength and polarization multiplexed coherent transmitters made by distributed feedback (DFB) lasers and a series of Nested-MZMs has been also recently reported [3].

In [4], we proposed a modified dual-drive IQ modulator, driven by equal-amplitude binary signals for generating offset-free QPSK and 16-QAM constellations. Indeed, schemes employing only binary electronics are preferable for high-speed multilevel modulations since they reduce the complexity of driving electronics [5].

In this communication, we report experimental results about the first implementation of the scheme firstly proposed in [4], where a tunable laser has been also monolithically integrated on the same PIC. The PIC has been designed and fabricated within PARADIGM [6, 7], the European advanced generic integration platform for the manufacturing of InP PICs. We report the characterization of the fabricated device and demonstrate the generation of PDM-QPSK signals: at 10 Gbaud, when using the DBR laser embedded in the transmitter; up to 32 Gbaud with the use of an external low phase noise laser.

2. Device description and characterization

The circuit implements the transmitter proposed in [4] with a valuable extension with respect to the original scheme. Indeed, a tunable laser has been monolithically integrated into the PIC as a continuous wave (CW) source. Figure 1 shows the diagram of the proposed circuit and a picture of the fabricated device. The tunable laser has been designed to be a sampled grating distributed Bragg reflector (SG-DBR). A simpler DBR test laser has been also included. Both lasers are connected to a 2 × 2 multi mode interference (MMI) coupler, feeding both a monitor output and a further 2 × 2 MMI coupler where the modulation section begins. An additional input to the same MMI allows the use of an external laser. Then, thermally tunable Mach-Zehnder Interferometers (MZI) are used as thermally tunable splitters. By adjusting the driving current of the phase shifter in the first MZI, the power splitting ratio between the upper and the lower arm of the modulator can be optimized. By properly choosing the driving current of the phase shifters in the following two MZI, one on the I-arm and the other on the Q-arm, the power splitting ratio of the signals feeding the subsequent Mach-Zehnder Modulators (MZMs) can be optimized. Each arm hosts a MZM composed of two 800-µm-long electro-optic phase modulators. Each of them can be modulated by applying two electrical input signals (binary data streams all with same amplitude levels) coupled to proper bias voltages by means of Bias Tees. Two 2 × 1 MMIs (one for each arm) couple the two signals from the corresponding arms. A phase shifter (PS) at the I-arm output is used to adjust the phase shift between the I- and the Q- components, which are then combined through another 2 × 1 MMI and sent to the output port.

 

Fig. 1 Scheme of the proposed circuit Picture of the fabricated chip. Footprint: 2 × 6 mm.

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The PIC was fabricated by Oclaro Technology plc, UK, in a multi-project wafer run of the PARADIGM platform [6, 7]. Deep etched passive waveguides with a width of 1.5 μm and losses of about 5 dB/cm are used. The SG-DBR laser, designed to be tunable on the whole C-band, is made up of four different sections: a 342.5-μm-long front mirror, a 700-μm-long gain section, a 100-μm-long phase shifter and a 738-μm-long rear mirror. The front and the rear mirrors are realized by means of periodically sampled DBR gratings, with a pitch of 237.5 nm and different sampling period: 5 sections, 32-μm-long and 36.5-μm-spaced form the front mirror and 12 sections, 48 μm-long and 13.5-μm-spaced form the rear one. Variations in the current injected into the mirrors produce a shift in the peak wavelength of the laser. In Fig. 1(a) the overlapped spectra of the tunable laser are reported for a fixed current of the gain section (40 mA) and different values of the mirrors currents. The measured tuning range is about 10 nm, when the circuit temperature is stabilized at 24°C by means of a TEC controller. This limited tunability range is likely due to limitations in the multiuser fabrication process, in particular to a too much low value of the grating Kappa and to a limited thickness of the contrast deposition layer, which does not allow the sampled grating effect. Measured output power from the chip is 4 dBm so that the estimated output power from the laser is 11 dBm (considering the 3 dB integrated MMI coupler and around 4 dB coupling loss). Regarding the modulator, Fig. 2(b) and 2(c) show the static transfer function and the frequency response (electrical/electrical S21 parameter) as measured on a single Mach-Zehnder test structure on the same chip. The frequency response shows a −3dB bandwidth of 7.5 GHz and a significant dip at around 17 GHz. The bandwidth is limited because of lack of travelling wave electrical contacts which were not supported in the fabrication process. Indeed, InP nested Mach-Zehnder modulators with travelling wave electrical contacts clearly show larger modulation bandwidth [8]. The dip in the frequency response is likely due to the not impedance matched electrical driving circuit. However, it should be noted that, despite the limited bandwidth and as reported in the following, the mild response slope of the frequency response together with the use of DSP allowed device operation for signals up to 32 Gbaud (this fact is confirmed also by numerical simulations that will be reported in a separate publication).

 

Fig. 2 a) Overlapped spectra of the SG-DBR laser. b) Modulator static transfer function and c) bandwidth of a test Mach-Zehnder structure.

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All the electrical bias and the RF signals are provided to the whole structure by means of two 14-pin multi-contact wedge probes. The short facets of the PIC are anti-reflection coated, with the input and output waveguides exploiting a 7° angled spot size converter in order to reduce optical reflections. The output signal is collected from the InP device by means of a tapered fiber with a focused spot size of 2.5 μm. Including the fiber to PIC coupling loss of 4 dB per facet, overall input-output insertion loss of the PIC when using the external input port is 23 dB.

3. Experimental results

After device characterization, a transmission experiment in back to back with coherent detection has been set up as shown in Fig. 3. At first, an external laser (external cavity, linewidth = 150 KHz, injected power to the PIC = 12 dBm) has been used as optical carrier and fed to the transmitter PIC through the input port. For the QPSK transmission experiment, all the three tunable splitters have been set to balanced condition (i.e., with a symmetric split ratio 50/50), so that the modulator behaves as a conventional dual-drive IQ-MZM. The four RF input pads are driven by the data streams provided by a programmable bit pattern generator (BPG) coupled with proper DC voltages for optimum biasing, as explained above. Two separate data streams (PRBS length = 2¹¹-1) are fed to I and Q arms. Within each arm, the inner MZM is driven in push-pull configuration, that is, differential data D and D̅ are applied to the two phase modulators of the same nested MZM to attain opposite phase shift. Then, the IQ relative phase shifter is set equal to 90° to compose the QPSK constellation.

 

Fig. 3 Experimental setup. On the right side, generated optical spectrum at 32 Gbaud.

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Both single and dual polarization transmission have been implemented. Noise loading is applied to evaluate performance under different OSNR conditions. The generated signal is then sent to the coherent receiver, where it is mixed by means of two 90° hybrid couplers with a local oscillator (LO) set at same nominal frequency as the data optical carrier. Then, frequency down-conversion and opto-electronic conversion are completed by means of four balanced photodetectors and four 50-GSamples/s, 20-GHz analog-to-digital converters (ADC). The acquired traces are processed offline through DSP. The DSP chain encompasses a re-sampling block to adapt the sampling rate to 2 Samples/symbol followed by an adaptive two-dimensional fractionally-spaced feed-forward equalizer (2D-FFE) with 11 taps and a symbol-by-symbol asynchronous detector. The 2D-FFE is able to compensate for linear distortions such as PMD and limited GVD and to recover the four signal quadratures. Blind equalization is applied in a first portion of the acquired block (80k samples) to help taps convergence, then a decision-based LMS algorithm processes the remaining samples. A symbol-by-symbol asynchronous detection strategy is implemented to better compensate for phase noise. BER measurements have been taken up to 32 Gbaud and are reported in Fig. 4 for both single and dual polarization, as a function of Eb/N0 (energy per bit to noise power spectral density ratio). In the dual polarization case, the reported BER is the worst case value of the two polarizations. At 10 Gbaud, about 3.5 dB penalty with respect to theory is measured at a BER = 10⁻³. We attribute this to the not ideal RF routing to the PIC and to the fact that modulators are not impedance matched with wedge probes in this test configuration. At higher rates, an additional penalty is measured (4 to 6 dB), due to the limited electrical bandwidth of the modulator and an error floor appears at BER = 10⁻⁵. However, correct operation (far below the limit of common FEC) is demonstrated for symbol rate as high as 32 Gbaud. An example of optical spectrum generated by the transmitter (at 32 Gbaud) is reported on the right hand side of Fig. 3, while the recovered constellations at BER = 10⁻⁴ are shown in the insets of Fig. 4.

 

Fig. 4 BER measurements for single (left) and dual polarization (right) transmission. Insets report two examples of recovered constellations at 32Gbaud for BER = 10⁻⁴.

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After the test of the modulator with an external laser, we tested the transmitter using the integrated one. Unfortunately, in this case, we find that the phase noise of the integrated laser affects the transmission performance, dominating other sources of degradation and limiting the operating baud rate to 10 Gbaud (with a measured BER not better than 10⁻³). Figure 5 on the left hand side shows the optical spectrum and a recovered constellation for this case. The impact of the phase noise is clearly visible through the typical distortions of the constellation reported in the inset. Indeed, we measured the linewidth of the integrated laser by beating the CW output signal with a reference ECL (linewidth 150 KHz) in a 90° hybrid, and receiving the beat signal through the coherent receiver. The spectrum of the acquired signal is reported on the right side of Fig. 5 and it is well fitted by a Lorentzian distribution with 76 MHz full-width-half-maximum computed through a least mean square error algorithm. This value is clearly too large for generating a reliable high speed coherent signal.

 

Fig. 5 (Left) Optical spectrum and constellations for 10 Gbaud PM-QPSK signals generated with the integrated laser. (Right) Laser linewidth measurement with the normalized Lorentzian fitting.

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In order to generate higher level m-QAM modulation formats, together with proper unbalance of the power splitters in the modulator, it is required to individually modulate each of the phase modulator in the nested interferometer structure [4]. The scheme also assumes a limited amount of spurious intensity modulation in the phase modulators. Unfortunately, we experimentally found that the undesired amplitude modulation in the InGaAsP multi quantum well electro-optic modulators [9] creates an amount of constellation off-set and distortion which does not allow the generation of 16-QAM signals that could be processed by the DSP engine in our coherent receiver. The effect of those distortions generated from the intensity modulation and maximum acceptable spurious modulation values will be presented and discussed in an aforementioned separate publication that will include simulations of the integrated modulator.

4. Conclusions

A novel InP PIC made by a DBR laser and a nested Mach-Zehnder with tunable input power splitters for the generation of phase-amplitude signals has been designed, fabricated and characterized. Results show the generation of 32 Gbaud PDM-QPSK signals when using an external low phase noise laser and external polarization multiplexing and 10 Gbaud operation when using the integrated DBR laser. The generation of m-QAM signals, which is possible in principle with this novel PIC, was not attainable because of spurious amplitude modulation found in the electro-optic modulators. For performance improvement it is required an alternative design for the integrated laser (giving an overall reduced phase noise) and a different material or geometry for the phase modulators giving reduced spurious amplitude modulation. As an example, a self-referencing technique for the frequency lock of DBR lasers based on an integrated asymmetric Mach-Zehnder interferometer and an electronic loop filter has been recently demonstrated showing a 27x reduction of laser linewidth [10].