Offset Locking of a Fully Integrated Optical Phase-Locked Loop Using On-chip Modulators (original) (raw)

We demonstrate an integrated heterodyne optical phase-locked loop for potential RF remoting. Offset-locking of the two on-chip lasers is achieved by applying a RF signal to an on-chip optical phase modulator and locking to an optical sideband. Over the years, microwave/millimeter wave photonic link technology, supporting the fiber optic remoting of RF signals based on remote heterodyne detection (RHD) [1], has attracted a great deal of attention for a wide range of applications including delay lines over phased-array antenna feeders and backbone networks for cellular phone systems [2]. Such a fiber-optic link at the transmitter or base station requires two widely-tunable lasers with slightly different wavelengths, the phases of which must be strictly correlated. The strict phase correlation between these two lasers can be achieved by using a heterodyne optical phase-locked loop (OPLL) [3] transmitter configuration. There are two techniques that could be adopted for such heterodyne locking. As a first technique, the RF signal can be applied to an electronic mixer following optical detection in such a coherent photonic receiver and the RF difference frequency used for offset locking [4,5]. Another technique is to apply the RF to an on-chip optical modulator monolithically integrated on the photonic receiver following the tunable laser and to achieve the locking using an optical sideband [6]. Use of higher-order sidebands are also possible enabling higher offset frequencies than available from the RF source or the electronics. In this work, we report the second technique using an InP-based photonic integrated circuit with an on-chip optical phase modulator following the LO laser for applying the offset. Outputs from the LO and signal lasers are combined into a pair of photodetectors that provide inputs to agile and highly-sensitive feedback electronics that control the phase section of the LO for locking. A net loop bandwidth of 500 MHz was obtained, and an offset locking frequency range ~16 GHz is achieved in the system, which can employ up to the third-order-harmonic optical sidebands for locking, yielding a locking range as high as 48 GHz. Figure 1(a) shows a schematic of the coherent optical receiver PIC used for OPLL. It includes two widely-tunable sampled-grating distributed Bragg reflector (SG-DBR) lasers that are to be offset locked, MMI couplers, semiconductor optical amplifiers (SOAs), photodetectors, and several RF-modulators. The chip size is 7 mm × 0.5 mm. As can be seen, light from each laser is first equally divided into two paths using 1 × 2 MMIs. One half from each laser is guided into a 2 × 2 MMI, which serves as a 180 degree hybrid to feed the two photodetectors for the feedback loop. Each input arm of the 2 × 2 MMI contains a phase modulator (M) that can be used for applying RF to generate optical sidebands. After adding the fields from these two lasers in the MMI coupler, light is detected in a pair of photodetectors (D) to provide a differential output. The other half from each laser travels through semiconductor-optical-amplifiers (SOAs) to increase their amplitude and modulators for applying possible information. They are then combined in a 2 × 2 MMI at the right side of the OPLL-PIC. In the current experiment this is used for monitoring of the interference between the two SG-DBR lasers by coupling into an optical fiber. An optical microscope photo of the fully-processed PIC on an InGaAsP/InP material platform is shown in Fig. 1(b). The process used to fabricate the devices is quantum-well intermixing (QWI) that creates self-aligned passive regions by intermixing the quantum-wells with their barriers and surrounding waveguide material by a patterned diffusion of implanted phosphorus ions after a first growth. Details of the processing steps for the well-established QWI-based material structure can be found elsewhere [7]. Two widely tunable on-chip SG-DBR lasers along with all of the other optical components were used to form the heterodyne OPLL. One of the lasers was used as a master (or signal) laser, while the other as a slave (or LO) laser to be offset phase-locked to the former. Prior to combining the outputs of these two lasers using a 2 × 2 optical coupler, the output of slave SG-DBR laser is phase-modulated for offset-locking using an integrated on-chip modulator and envelope detected using a pair of balanced on-chip photodetectors. The current output from the