Recent Developments in Integrated Comb and Tunable Laser Assemblies

Several important applications in optical communications require high power fiber coupled laser sources operating at 1.3 /spl mu/m, 1.48 /spl mu/m and 1.55 /spl mu/m. Semiconductor diode lasers for these wavelengths can be fabricated using the InP/lnGaAsP material system. High power operation requires careful selection of the active region, p and n doping and the lateral waveguide structure. Additionally, distributed feedback (DFB) lasers require appropriate choice of grating feedback. High power, narrow linewidth and low noise single mode diode laser sources operating at 1.3 /spl mu/m and 1.55 /spl mu/m wavelengths have applications in cable television transmission systems, microwave photonics, interferometric sensors and as an alternative to diode-pumped solid state lasers. High output power is desirable to overcome losses in external modulators and for large scale distribution systems. It is particularly important for 1.3 /spl mu/m applications as reliable and efficient fiber amplifiers are not commercially available at this wavelength. 1.3 /spl mu/m DFB lasers with 440 mW chip power, over 150 mW fiber coupled power and 100 kHz linewidth have been demonstrated.

We show that unscented Kalman filters can be used to mitigate local oscillator phase noise and to compensate carrier frequency offset in coherent single-carrier optical communication systems. A parallel processing architecture implementing the unscented Kalman filter is proposed, improving upon a previous parallelized linear Kalman filter (LKF) implementation.

The high speed rotation of state of polarization (RSOP) is a key issue for 16 quadrature amplitude modulation (QAM) coherent optical polarization division multiplexing (PDM) communication systems in strong lightning weather. To reduce the bit error rate (BER) caused by the high speed of RSOP, the historical information is first proposed to correct the demodulation process in coherent optical communication systems. By adding the time reverse error term into the cost function, a new equalization algorithm is proposed for coherent optical communication systems. Compared with the conventional Multi-modulus Algorithm (MMA), simulation results show that the average BER and steady-state error (SSE) of the new algorithm is reduced by 33.47% and 4.08%, respectively. Moreover, the average tolerance of RSOP of the new algorithm is increased by 28.57% compared with the MMA in 16 QAM coherent optical PDM communication systems.

Theory of Optical Communications. Fiber Devices for Optical Communication Systems. Integrated Devices for Optical Communication Systems. Direct Detection Optical Communication Systems. Coherent Systems: Structure and Ideal Performance. Performance Degradation Sources in Coherent Optical Systems. Multilevel Coherent Optical Systems. Multichannel Optical Systems. Present--Day Systems Implementations. Appendices. Index.

The huge amount of data traffic behind the rapid growth of cloud computing is putting pressure on the operation of mobile fronthauls and data center networks so there is a need to improve their power consumption and latency. We developed an electro-optically tunable laser diode employing a tunable filter that is practically tuned even with small refractive index change of the electro-optic effect. The laser shows a small tuning power dissipation of less than 10 mW for a practical tuning range of over 35 nm with a linewidth of about 350 kHz. We also achieved high-speed optical switching of less than 50 ns for 100 Gb/s coherent signals. In addition to its application in optical communications, the electro-optically tunable laser diode is also beneficial to laser sensing applications because its higher tuning speed increases the time resolution of the sensing system. Furthermore, a narrow linewidth, conventionally difficult to reconcile with high-speed tuning, can also enable a longer sensing distance and/or a higher signal-to-noise ratio when using coherent detection. Our result shows that we can use the electro-optic effect to overcome the limitations of conventional tunable laser diodes and drastically change optical communications and laser sensing systems.

This doctoral thesis focuses on the theoretical studies and experimental implementations of three types of tunable lasers using the microelectromechanical systems (MEMS) technology. A discretely tunable laser is first developed, including new theoretical models for the extremely short cavity and discrete wavelength tuning, and a MEMS prototype fabricated by deep etching on a silicon-on-insulator wafer. It can be tuned over 13.5 nm within 1 ms. Secondly, a continuously tunable laser is developed, including a modified theory and a fabricated MEMS laser. A continuous tuning range of 30.3 nm is obtained. Finally, an injection-locked laser subsystem is developed by integration of single MEMS laser components. A new generalized theory is established and a MEMS injection-locked laser is demonstrated. Locking states with a SMSR > 50 dB and all optical switching up to 50 MHz are achieved. In summary, the developed MEMS lasers exhibit many advantages such as small size, fast tuning speed, and high mechanical stability, and would find wide applications in optical communications, biomedical studies, and military defense.

Nowadays, the coherent optical communication system plays an important role in communication field because of large capability and bandwidth. A coherent optical communication, based on high-order modulation and digital signal processing technologies, consists of optical transmitters, optical fiber lines, optical amplifiers and optical receivers. In the high-speed coherent optical communication system, the phase noise from the transmitter laser and the local oscillator laser can significantly degrade the performance of the signal transmission and detection, especially for the systems using high-order modulation format, such as m-ary phase shift keying (mPSK) and m-ary quadrature amplitude modulation (m-QAM). Therefore, investigations on laser phase noise compensation algorithm based on digital signal processing technologies has become more and more significant. In this work, a multi-ring carrier phase recovery algorithm is developed for compensating the laser phase noise in optical fiber communication systems using high-order modulation formats. Degradations on the performance of communication systems due to the laser phase noise have been investigated. The system performance using the proposed algorithm and the conventional Viterbi-Viterbi algorithm were also evaluated in 9-channel and 15- channel, 32-Gbaud, Nyquist-spaced QPSK, 16-QAM, 64-QAM and 256-QAM coherent transmission systems with considering the impact of the laser phase noise. It is found that the phase noise leads to stricter constraints on the linewidths of transmitter-side and receiver-side lasers, and it can greatly degrade the achievable information rates in communication systems. Besides, compared to the conventional Viterbi-Viterbi algorithm, which is usually applied in the QPSK system, our proposed algorithm can also well mitigate the laser phase noise in 16-QAM, 64-QAM and 256-QAM optical communication systems.
\n

Narrow linewidth fiber lasers are intensively studied during these years, for their wide use in coherent optical communication, optical fiber sensing, high-precision spectroscopy and many other industrial and military applications. A usual technique to suppress linewidth is narrow band filtering in the resonating structure of the laser. However, it is hard in fabrication for a tradition optical filter to achieve an ultra narrow pass band such as kilo-Hertz. In this paper, the parallel feedback structure is proposed and experimentally studied. A fiber laser with ultra narrow linewidth (15Hz detected) is achieved in laboratory, basing on the parallel feedback mechanism in a fiber ring cavity. In multimode fiber, each transverse mode has a different propagation constant. Equivalently, when a light beam propagates form single mode fiber to multimode fiber, it will split into a few parallel light paths with different propagation constants. The parallel feedback structure of a fiber ring laser is carried out by introducing one or more pieces of multi-mode fiber into its resonant cavity. Lasing light in the cavity must fit the restrictions of all light paths, thus linewidth of the laser output is suppressed. Ultra narrow linewidth can be achieved by carefully adjusting the amount and length of multi-mode fiber pieces. The narrowest linewidth achieved in laboratory is 15Hz detected by a delayed self-heterodyne interferometer with a 100km fiber delay line. In this work, we focus on pressuring linewidth, and mechanism of selecting and stabilizing mode isn't employed. Therefore, the narrow linewidth fiber laser isn't single-longitudinal-mode (SLM). A similar ring laser structure with bidirectional output is also experimental implemented, achieving a bandwidth of the same order. The bidirectional ring laser is the basic component of a laser gyroscope, and its linewidth is one key limiting factor of the gyroscope accuracy. This narrow linewidth bidirectional ring laser is a good candidate for accurate fiber laser gyroscope. Theory analysis and experiment shows the fiber ring laser based on parallel feedback structure is robust against environmental disturbance, achieving high stability and ultra narrow linewidth. The parallel feedback mechanism in optical circuits can also used in other applications demanding narrow linewidth.

The technical embodiment of the Huygens-Fresnel principle, an optical phased array (OPA) is an arrangement of optical emitters with relative phases controlled to create a desired beam profile after propagation. One important application of an OPA is coherent beam combining (CBC), which can be used to create beams of higher power than is possible with a single laser source, especially for narrow linewidth sources. Here we present an all-fiber architecture that stabilizes the relative output phase by inferring the relative path length differences between lasers using the small fraction of light that is back-reflected into the fiber at the OPA's glass-air interface, without the need for any external sampling optics. This architecture is compatible with high power continuous wave laser sources (e.g., fiber amplifiers) up to 100 W per channel. The high-power compatible internally sensed OPA was implemented experimentally using commercial 15 W fiber amplifiers, demonstrating an output RMS phase stability of λ/194, and the ability to steer the beam at up to 10 kHz.

The development of coherent free-space optical communications systems could provide an opportunity for significantly increased data rates compared to the traditional intensity-modulation-based communications. In this work, we present experiments demonstrating coherent free-space optical communications over an outdoor free-space link with data rates on the order of Gbit/s.

The low threshold current of 9 mA, the high side-mode suppression ratio of more than 45 dB, the extremely narrow spectral linewidth of 1.1 MHz, and the low chirping of 2.8 AA at -20 dB at 2 Gb/s nonreturn to zero (NRZ) modulation have been achieved in the multiple quantum well (MQW) distributed feedback (DFB) p-substrate partially inverted buried heterostructure (PPIBH) laser diode. The spectral linewidth of 1.1 MHz is the narrowest value among 300- mu m-length solitary laser diodes. These results suggest that the MQW-DFB laser diodes are a promising light source for longer distance and higher bit-rate optical communication systems and coherent optical communication systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Atmospheric turbulence and pointing errors are two major factors affecting satellite-to-ground coherent optical communication links. This study considers the effects of power scintillation and phase jitter caused by atmospheric turbulence, as well as active mode compensation for wavefront phase distortion. Additionally, residual pointing errors due to platform micro-vibrations are taken into account, and a statistical channel model is derived under the combined effects of atmospheric turbulence and residual pointing errors during tracking and acquisition. Based on this model, a closed-form approximate expression for the average bit error rate (BER) of 16QAM satellite-to-ground coherent optical communication is derived using the Meijer-G function. The results show that the derived BER expression matches well with numerical integration results, confirming its accuracy and applicability under various link conditions. Through numerical simulations, the effects of residual pointing error variance, turbulence strength, satellite zenith angle, and transceiver antenna aperture on the performance of 16QAM satellite-to-ground coherent optical communication are evaluated. The findings offer valuable theoretical insights for the design and optimization of future satellite-to-ground 16QAM coherent optical communication systems.

A compact single-frequency laser package is reported. In this device, a distributed feedback laser is coupled to a graded-index rod external cavity. hen light fed back into the laser cavity is at the proper phase, linewidth narrowing is obtained. The package consists of a laser source, the graded-index rod optics, a piezo-electric crystal, a milli-Kelvin temperature control element, and a microwave bias board, all in a 1 in thermally insulated enclosure. Miniaturization is made possible by a novel collinear optical assembly. A linewidth of 1.3 and a short-term frequency stability of +10 nliz have been measured. Direct frequency modulation with the injection current is possible with this short external coupled cavity arrangement. At 565 kb/s, a frequency modulation index of 1 was achieved for a 1010 pattern with negligible linewidth broadening. Error-free transmission was measured in a frequency-shift-keying heterodyne detection experiment for fixed word patterns at this data rate. The compact size and the FM properties of the unit are attractive for use either as a transmitter or as a local oscillator in coherent optical communication systems.

In this study, we proposed an improved Minimum Mean Square Error (MMSE) channel estimation algorithm for the coherent wireless underwater optical OFDM system aiming to improve parameters such as dispersion, attenuation and optical noise. The simulation results of the proposed algorithm was compared with the performance of Discrete Fourier Transform (DFT) based and time based Least Square (LS) and MMSE channel estimation techniques. The results demonstrated that the proposed approach improved the underwater wireless channel estimation performance and offers a promising alternative for the OFDM systems with less complexity and high efficiency.

Independent component analysis based digital signal processing in coherent optical fiber communication systems