For precision displacement measurements, laser metrology is currently one of the most accurate measurements. Often, the measurement is located some distance away from the laser source, and as a result, stringent requirements are placed on the laser delivery system with respect to the state of polarization. Such is the case with the fiber distribution assembly (FDA) that is slated to fly aboard the Space Interferometry Mission (SIM) next decade. This system utilizes a concatenated array of couplers, polarizers and lengthy runs of polarization-maintaining (PM) fiber to distribute linearly-polarized
light from a single laser to fourteen different optical metrology measurement points throughout the spacecraft. Optical power fluctuations at the point of measurement can be traced back to the polarization extinction ratio (PER) of the concatenated components, in conjunction with the rate of change in phase difference of the light along the slow and fast axes of the PM fiber. Thermal variations are one of the major contributors to this change and can lead to tight spacecraft design requirements. In this presentation, we will discuss our experimentally-validated model which predicts
the polarization behavior for various distribution designs, as well as present the thermal performance of various PM components and how this levies thermal control requirements on the spacecraft.
The continued need for increased bandwidth is driving the pursuit of both increased speed in TDM and more channels in WDM for fiber optic communication systems. Multiwavelength arrays of monolithic mode-locked DBR lasers are an attractive source for future high bit rate (100 - 800 Gb/s) optical communication systems. Monolithic mode-locked lasers in the colliding-pulse mode-locked configuration have been fabricated, with DBR end mirrors for wavelength selection. A continuous gain region has been employed for ease of fabrication and the elimination of multiple reflections within the cavity. Arrays containing up to 9 wavelengths have been fabricated, with all the wavelengths within the erbium-doped fiber amplifier gain bandwidth. An RF signal is applied to the saturable absorber for synchronization to an external clock and reduction of the phase noise. For a 4.6 mm cavity, short (< 10 ps) optical pulses at high (approximately 18 GHz) repetition rates have been achieved. Low single side-band phase noise values (-107 dBc/Hz 100 kHz offset) have been demonstrated, nearly equal to that of the RF source.
The microwave optoelectronic oscillator (OEO) has been demonstrated on a breadboard. The future trend is to integrate the whole OEO on a chip, which requires the development of high power and high efficiency integrated photonic components. In this paper, we will present the design and fabrication of an integrated semiconductor laser/modulator using the identical active layer approach on InGaAsP/InP material. The best devices have threshold currents of 50-mA at room temperature for CW operation. The device length is approximately 3-mm, resulting in a mode spacing of 14 GHz. For only 5-dBm microwave power applied to the modulator section, modulation response with 30 dB resonate enhancement has been observed. This work shows the promise for an on-chip integrated OEO.
The emerging growth of wavelength division multiplexing for future long haul and local access networks has spurred interest in many novel components. Tunable and selectable wavelength sources are desired with high spectral stability, high modulation bandwidth and low chirp for long haul applications. Here we present two laser array designs for high speed application as low chirp NRZ transmitters and soliton pulse generators.
Wavelength division multiplexing (WDM) is attracting considerable interest in the field of fiber optic telecommunications since it provides the means to utilize the large optical bandwidth of optical fibers. A key issue to implementing a WDM system is the development of new lightwave components such as laser arrays that can transmit at predetermined optical frequencies, as well as discrete lasers that can be quickly switched between predetermined optical frequencies. In addition to transmitters, wavelength selective receivers and fast tunable photodetectors as well as passive wavelength multiplexers and routers are required. Photonic integrated circuits can potentially integrate several of these optical functions one a chip.
A dual back-illuminated photodetector for balanced coherent receivers applications is presented. The photodetectors have an InP/InGaAs/InP p-i-n structure. Several additional semiconductor layers have been added to fabricate the structure by selective wet etching. The material was grown by metalorganic vapor phase epitaxy (MOVPE) on a semiinsulating, Fe doped, InP substrate. The passivation and the insulation between contacts were obtained using polyimide. The contact structure is intended to connect the photodetector to the package by flip-chip. The measured photodetector capacitance was around 0.2 pF for a single photodetector (without series connection), the series resistance under 20 ohms, and the dark current under 10 nA.
System demonstrations employing soliton transmission and/or optical time division multiplexing have emphasized the need for stable and reliable pulse sources with repetition rates in the 1 GHz to 10 GHz range. For these applications, we have fabricated several photonic integrated laser devices that through gain-switching or mode-locking generate optical pulses with durations between 20 ps and 50 ps. In this talk, we will discuss gain-switched DBR laser integrated with high-power optical amplifiers or electro-absorption modulators and laser devices with long low-loss passive waveguides for active mode-locking.