Abstract
A novel configuration of mutually injection-locked directly modulated lasers (DMLs) for generating in-phase quadrature (IQ) optical signals is proposed and numerically investigated by using rate equations. Two DMLs coupled with a high-Q ring resonator (Q = 2.2 × 10 5 ) are used for obtaining stable mutual-injection locking and suppression of the optical carrier component. First, conditions for mutual injection locking and modulation responses are investigated. Lasing frequencies are pulled toward the resonant frequency of the ring resonator due to the filtered mutual optical injections, and the pulling range is much broader than the transmission bandwidth of the ring resonator. Modulation bandwidth of the DMLs under the mutual-injection-locking condition is enhanced compared to that of the DMLs under the free-running condition. A resonant feature depending on the detuning condition appears in the modulation response, and it can be reproduced by the conventional injection-locking model. Next, optical-signal generations with quadrature phase shift keying (QPSK) format are tested under the assumption that the mutually injection-locked DMLs are driven by pseudo-random bit sequences. It is confirmed that 80-GBd QPSK modulation with back-to-back error vector magnitude of 32% can be achieved by using 8B/10B encoding.
Highlights
Recent rapid growth of data-center traffic has been attracting attention to short-reach digital coherent optical-communication systems [1], [2], which face challenges in terms of cost, power consumption, and size of optical devices
The frequency fluctuation of DML1 is relatively large compared with that in DML2. These fluctuations decay faster under conditions (ii) ( f1 = –10 GHz and f2 = 0 GHz) and (iv) ( f1 = 10 GHz and f2 = 0 GHz). This faster decay is attributed to a quasi-one-way strong optical injection from DML2 to DML1 that occurs in the initial stage of the optical injection, because DML2 is resonant with the ring resonator, while DML1 is significantly detuned (| f1| > 870 MHz)
Since the frequency fluctuation is converted into amplitude fluctuation by the ring resonator, the lasing frequencies of the directly modulated lasers (DMLs) are mutually modulated through the stimulated emission and carrier plasma effect in the laser cavity. φ0 of 1.42π rad and a phase inversion at around the resonant frequency of the ring resonator characterized these detuning-dependent transient responses
Summary
Recent rapid growth of data-center traffic has been attracting attention to short-reach digital coherent optical-communication systems [1], [2], which face challenges in terms of cost, power consumption, and size of optical devices. An array of injection-locked vertical-cavity surface-emitting lasers was used to demonstrate chirp-free optical phase modulation and QPSK modulation [14], [15]. These demonstrations verify a cost-effective and low-power-consumption optical IQ transmitter based on injection-locked DMLs. The use of DMLs has another advantage in developing an IQ transmitter operating in unexplored wavelength range in which external IQ modulators have not hitherto operated. Hereafter, using mutually injection-locked DMLs with a ring resonator for directly generating optical IQ signals without using an external master laser is proposed, and their novel characteristics are numerically investigated by using a rate-equation model.
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