J.C. Livas

High sensitivity optically preamplified direct detection DPSK receiver with active delay-line stabilization

The authors present experimental results for a 3 Gb/s optically preamplified, optically demodulated, differential phase shift keyed communication link achieving 62 photons/bit receiver sensitivity and using an actively stabilized optical delay line demodulator.<<ETX>>

All-optical pulse width and wavelength conversion at 10 Gb/s using a nonlinear optical loop mirror

An all-optical pulse width and wavelength converter is demonstrated using a nonlinear optical loop mirror. The conversion of a 10 Gb/s, 8 ps pulse width data stream at 1551 nm to a 23 ps pulse width data stream at 1543 nm is demonstrated. The control pulse energy required for switching is 10 pJ. Bit-error-rate measurements are presented.<<ETX>>

Fabrication of two-sided anamorphic microlenses and direct coupling of tapered high-power diode laser to single-mode fiber

An anamorphic microlens has been developed to couple a tapered unstable-resonator laser directly to a single-mode fiber, and has demonstrated capability for simple, compact and efficient high-power diode laser systems. Far high collection and coupling efficiencies, the refractive microlens has been fabricated by utilizing both sides of a GaP substrate, in which the first side was used to remove the astigmatism of the laser output and the second side to focus the beam to a spot size comparable to the fiber mode. The microlenses have been accurately formed by using a recent technique of mass-transport smoothing of etched multimesa preforms. Initial fiber-coupling experiments showed powers as high as 360 mW at the fiber output, and coupling efficiency as high as 29.5% has been measured at a lower power.<<ETX>>

High-data-rate systems for space applications

High data rate communications systems will soon be needed for space applications. Technology and applications which support high data rates are already in place for ground- based telecommunications, or will be in the future. The advantages of an optical system over a traditional RF link for free space communications are particularly compelling for high data rates. We have been developing the necessary technology to demonstrate the feasibility of high rate free-space optical communications technology at 1.5 micrometers . The existence of a large, mature technology base at 1.5 micrometers developed for the telecommunications industry has allowed us to focus our development effort on two key technologies needed for space applications that have not been developed for the ground: a 1 Watt class optical power amplifier and a near quantum limited receiver. This paper will describe the overall system design for a high data rate optical communications system and present experimental results demonstrating < 50 photons/bit sensitivity at 10 Gbps with 1 Watt of optical power. The existence of a feasibility demonstration at this data rate enables downward scalability to data rates of 1 Gbps or less with small, inexpensive terminals.

Single-mode optical fiber coupling of high-power tapered gain region devices

A simple method for collimating the optical output of a tapered gain region amplifier or laser and coupling to a single-mode optical fiber is described, along with a technique for quantitatively assessing the expected coupling efficiency. By using a tapered laser, 840 mW at 980 mm was coupled into single-mode fiber with 44% efficiency measured from fiber input to available internal fiber power, in excellent agreement with the 48% predicted. When transmission losses of the collimating optics are included, the power coupling efficiency referred to the laser facet power is 32%.<<ETX>>

A one-watt, 10-Gbps high-sensitivity optical communication system

High data rate communications are of interest for many applications. In fiber-based broadcast systems, high receiver sensitivity and high transmitter power translate into the ability to reach more customers. For space applications, high receiver sensitivity and high optical power are essential since it is impossible to use amplifiers between the transmitter and receiver. We describe here the experimental demonstration of a 10-Gbps communications system with a receiver sensitivity of 77 photons/bit at a bit error rate of 10/sup -9/ and a one-watt optical transmitter based on an erbium-doped fiber amplifier pumped by tapered-gain-region semiconductor lasers.<<ETX>>

High-data-rate error correcting coding

Applications such as high resolution image transmission and the aggregation of multiple bit streams have increased the interest in very high speed (Gbit/s) data communications for space applications. The ability to perform error correction on a data link can yield significant improvements in channel efficiency and can mitigate the effects of various bit error rate (BER) impairments afflicting many communications systems. A codec integrated circuit (IC) has been constructed which is capable of supporting up to 2 Gbit/s of data throughput. The device has been demonstrated in two optical communications systems, a 1 Gbit/s binary optically preamplified OOK system at 1.5 µm and an 800 Mbit/s FSK MOPA system at 0.98 µm. At a BER of 10-9, this coding provided a 3.7 dB sensitivity improvement for the OOK system, and a 6.2 dB improvement with the FSK system. The measured sensitivity of the coded OOK system was 37 photons/bit, which is better than could be realized with an ideal uncoded system. The viability of applying error correcting coding to higher data rate systems is discussed and a method of utilizing these ICs to provide four Gbit/s operation is shown.

Gbps-class optical communications systems for free-space applications

For very high data rates, optical communications holds a potential performance edge over other technologies, especially for space applications where size, weight, and power are of prime importance. We report demonstrations of several Gigabit-per-second (Gbps) class all- semiconductor optical communications systems which have been developed for free-space satellite crosslink applications. These systems are based on the master-oscillator-power- amplifier (MOPA) transmitter architecture which resolves the conflicting requirements of high speed and high power on a single-laser coherent transmitter. A 1 Gbps, 1 Watt system operating at 973 nm with a frequency-shift-keyed (FSK) modulation format is the highest power coherent optical communications system using all semiconductor lasers reported to date. A 3 Gbps differential-phase-shift-keyed (DPSK) system uses a 2-stage injection-locked diode array as a power amplifier at 830 nm. At a wavelength of 1.5 micrometers , an optically- preamplified direct-detection on-off-keyed (OOK) receiver was demonstrated at both 3 and 10 Gbps. A 3 Gbps optically-preamplified direct-detection DPSK receiver was also demonstrated and represents, to our knowledge, the highest sensitivity DPSK receiver reported to date for data rates above 2 Gbps.

Recent progress with tapered-gain-region devices

Semiconductor laser devices with tapered gain regions have recently generated much interest because they promise high output power with near-diffraction-limited spatial beam quality and good electrical to optical conversion efficiency. We report recent progress on two specific applications: a ring laser and a high- power erbium-doped fiber amplifier (EDFA). The ring laser operates unidirectionally in a single longitudinal mode with an output power of 170 mW and without a Faraday isolator. The high- power EDFA has an output power of 520 mW at 1.55 micrometers , the highest power reported to dates for an erbium-doped fiber amplifier using all semiconductor pump lasers. The common theme for both of these applications is the development of optical systems that produce high power in near-diffraction-limited collimated beams and efficient coupling into single mode optical fiber. We present an experimental procedure for quantitatively predicting the optical fiber power coupling efficiency. We have measured 64% power coupling efficiency measure fiber fact to power in the single-mode fiber, or 51% laser facet to power in the fiber, in good agreement with the predictions.

Broadband Search Techniques for Periodic Sources of Gravitational Radiation

Full exploitation of the potential of a laser interferometric gravitational wave detector to detect sources emitting periodic gravitational waves will require the development of specialized search techniques and place unprecedented demands upon computer memory and speed. Traditional approaches to detecting small amplitude sinusoidal signals of unknown period in the presence of broadband noise must be modified to accomodate the unusual nature of the received signal from a gravitational wave source. This paper outlines two methods that have been developed and presents the results of an application of these methods to data taken with the M.I.T. 1.5 meter prototype. Based on these results, a combined hardware/software solution to the data analysis problem is needed before full scale gravitational wave astronomy with periodic sources becomes practical.